A vacuum power switch using diamond PIN junction cathodes have been developing, which make use of spontaneous electron emission through negative electron affinity (NEA) surfaces of hydrogen-terminated diamond. [1] Using unique properties of diamond such as NEA, carrier injection through the junction between band-conduction and hopping-conduction layers, and high-density free excitons as well as high thermal conductivity, 10kV d.c. switching has been achieved with the efficiency of 73% during room temperature operations.
Recently, the emission current density reached to 0.5 A/cm², estimating with the PIN diode mesa area of 1.9 × 10^-3 cm^-2. High electron emission efficiency, which is determined by a ratio from the emission current to the PIN diode current, in the range of a few percent has already been obtained. According to the system model, it gives the practical efficiencies of more than 90% could be achieved with 100kV system. We introduce such a high potential of this new device for high voltage power switches.
This research was supported by Industrial Technology Research Grant Program in 2008 from New Energy and Industrial Technology Development Organization (NEDO) of Japan, and a part of this work was conducted at the Nano-Processing Facility, supported by IBEC Innovation Platform, AIST.
[1] D. Takeuchi et al., Phys. Status Solidi A 210 (2013) 1961.

Diamond is the ultimate semiconductor for high power and high frequency applications due to his high breakdown electric field, elevated mobility and the outstanding thermal conductivity. Among such perspectives, diamond-based MOSFET and Schottky diodes are attractive architectures where the conducting or insulating behavior of the device is based on the electrostatic control of the band curvature at the oxide/semiconductor interface.
Recently, diamond-MOS structures showing outstanding electrical behavior have been achieved [1]. To verify the presence of the oxide layer, as well as to characterize several key parameters (like the oxide thickness, homogeneity and oxygen distribution) various transmission electron microscopy have been used. Among these techniques, HREM mode allows measuring thick oxide layers with atomic resolution that were previously identified using Electron Energy Loss Spectroscopy [2].
In this contribution, several diamond-MOS structures have been studied combining HREM/EELS techniques. Each diamond-MOS structure was previously electrically characterized, allowing to relate the measured electrical behavior with the physical nanostructure. Furthermore, MOS structures were annealed at different temperatures, showing a different electrical behavior. HREM/EELS observations were carried out in each anneal step, allowing not only to relate the electrical behavior with the nanostructure, but the evolution of this nanostructure with the annealing temperature.
[1] A. Traoré, P. Muret, A. Fiori, D. Eon, E. Gheeraert, and J. Pernot, Appl. Phys. Lett. 104, 052105 (2014).
[2] J. C. Piñero, D. Araujo, A. Traoré, G. Chicot, A. Maréchal, P. Muret, M. P. Alegre, M. P. Villar, and J. Pernot, Physica Status Solidi (a), n/a (2014)

Surface conductive layer by hole carriers near hydrogen-terminated (C-H) diamond surface is also induced after high temperature (450 #8451;) atomic layer deposition of Al₂O₃ [1]. Recently, we have produced a number of field effect transistors operating at 400 #8451; and 500 V breakdown [2] using this C-H diamond.
Here, we report the lateral MOSFETs with better breakdown characteristics. In this study, the gate-drain distance L_GD of MOSFETs was stepwise increased from 1 to 9 mu;m. The fabricated lateral MOSFETs showed good I_DS-V_DS characteristics, and their maximum breakdown voltages without field plate were 363 V with L_GD of 1 mu;m, 392 V with 2 mu;m, 498 V with 5 mu;m, 591 V with 6 mu;m, 691 V with 7 mu;m and 996 V with 9 mu;m, respectively. The 996 V breakdown voltage is the highest of diamond FET reported in the past[3]. Moreover, the results are comparable to SiC[4] and GaN[5] lateral power FET in terms of gate-drain averaged electric field (V_B/L_GD) of 1 MV/cm, which is one of the important indicators to estimate the quality of lateral power device. The best V_B/L_GD was 3.6 MV/cm, with L_GD of 1mu;m. This value exceeds the maximum V_B/L_GD of 1.7 MV/cm (AlGaN/AlGaN HEMT[6]) around lateral FETs and indicates excellent potential of a new lateral power FET using diamond. With field plate structure, higher breakdown voltage would be expected.
[Acknowledgment] This research was supported by research grants from ALCA (JST).
References
[1] A. Hiraiwa, H. Kawarada et al. J. Appl. Phys. 112, 124504 (2012).
[2] H.Kawarada, H. Tsuboi,T. Yamada, A.Hiraiwa et al., Appl. Phys. Lett, 105 (2014, in press).
[3] T. Iwasaki, M. Hatano et al., IEEE Elec. Dev. Lett, 35, 241-243 (2014)
[4] M. Noborio, T. Kimoto et al., IEEE Elec. Dev. Lett, 30, 831-833 (2009)
[5] S. L. Selvaraj, T. Egawa et al., IEEE Elec. Dev. Lett.33, 1375-1377 (2012).
[6] T. Nanjo, Y. Tokuda et al., IEEE Trans Elec. Dev, 60, 1046-1053 (2013)

Diamond is one of promising materials for next generation low-loss power electronics due to its outstanding physical properties exceeding 4H-SiC and GaN. We have developed lateral p-n junction diodes [1] and junction field effect transistors (JFETs) [2-3]. In this study, we report a correlation of interface defects and leakage current in lateral p-n junction diodes, and also demonstrate high voltage operation of diamond JFETs composed of lateral p-n junctions.
Diamond lateral p-n diodes and JFETs were fabricated by a unique technique called selective growth of n⁺-diamonds [4]. After sidewalls of a boron-doped p-type film was exposed by ICP etching, n⁺-type diamonds were grown only at the sidewalls of the p-diamond. For JFETs, the p-diamond was shaped to a bar, which works as a channel. Thus, the current is controlled by depletion layers between the p-channel and n⁺-gates. The contact electrodes (Ti/Pt/Au) were fabricated by EB deposition and annealed to improve the contacts. I-V characteristics and breakdown voltages were measured in a vacuum chamber. Interface structure of the lateral p-n diodes was observed by cross-sectional TEM.
By cross-sectional TEM observations, dislocations were found at the interface of p-n diodes with large leakage currents, while good devices with very low leakage currents (below the measurement limit) have defect-free interface. The amount of dislocations is likely to be related with the leakage current level. Then, we examined breakdown characteristics of diamond JFETs composed of lateral p-n junctions with low leakage currents. At room temperature (RT), a breakdown voltage of 566 V was obtained with a gate voltage of 20 V [5]. This value corresponds to an electric field of 6.2 MV/cm, which is the highest in reported diamond FETs. We found that the breakdown occurred at the drain edge of the p-n junctions. Then, we measured the breakdown voltage at 473 K. The same JFET showed a higher breakdown voltage of 608 V. This fact indicates that the mechanism is avalanche breakdown, by which a breakdown voltage increases at a higher temperature due to the increase of phonon-scattering.
In summary, we investigated a relation of dislocation and leakage current in lateral p-n junction diodes. With the good p-n interface structure, we obtained high voltage operation of diamond JFETs. The device showed high breakdown voltages of 566 V and 608 V at RT and at 473 K, respectively.
References
[1] Y. Hoshino, et al., Phys. Status. Solidi A 209, 1761, 2012.
[2] T. Iwasaki, et al., Appl. Phys. Express 5, 091301, 2012.
[3] T. Iwasaki, et al., IEEE Electron Device Lett. 34, 1175, 2013.
[4] H. Kato, et al., Appl. Phys. Express 2, 055502, 2009.
[5] T. Iwasaki, et al., IEEE Electron Device Lett. 35, 241, 2014.

Diamond is expected to be used for ultimate high-power high-efficient transistors because of its superior physical properties. H-terminated diamond FETs showed very excellent RF and power performance such as a power-gain cut-off frequency (f_MAX) of 125 GHz and a RF output power of 2.1 W/mm at 1GHz. However, thermal instability of electrical properties of H-terminated surface was a critical issue.
Recently, we found that the hole carriers are generated by NO₂ adsorption in air on the H-terminated surface. With this method, we have increased the hole concentrations by one order of magnitude from ~1x10¹³ cm^-2 to ~1x10¹⁴ cm^-2 [1]. Their phenomena were well explained in terms of the SOMO level of NO₂ molecules is positioned below the valence band maxima of H-diamond, and therefore, valence electrons transferred from H-diamond to the adsorbant [2].
We have also observed that Al₂O₃ passivation layer thermally stabilizes the H-diamond surface. After NO₂ exposure to the H-diamond surface, thin Al₂O₃ passivation layer was deposited. Hall measurements revealed that the room-temperature hole sheet density (4.5x10¹³ cm^-2) unchanged after repeated heating and cooling from -125 to 200^o C [3].
With technologies of NO₂ exposure and the Al₂O₃ passivation, we fabricated diamond MOSFETs, which showed excellent DC and RF charactertistics, and in which even after 200 ^o C heating in vacuum, maximum drain currents (I_DSmax) were unchanged [4].
This success of diamond FETs encourages us to investigate electronic properties of MOS structures. First, by Synchrotron Radiation XPS/UPS/XANES measurements, we determined band offsets [5]. Next, by C-V measurements and analysis, we clarified two-dimensional hole properties formed at the MOS interface.
Acknowledgements: I appreciate deeply collaboration with Dr. K. Hirama of NTT and fruitful discussion with Prof. K. Shiraishi of Nagoya Univ., Profs. K. Takahashi, T. Oishi of Saga Univ. This work was partially supported by the Kakenhi, NEDO, the Mazda Foundation, and the Murata Foundation.
[1] M. Kubovic and M. Kasu et al., Appl. Phys. Express 2 (2009) 086502.
[2] Y. Takagi, K. Shiraishi, M. Kasu, and H. Sato, Surface Science 609 (2013) 203-206.
[3] M. Kasu, H. Sato, K. Hirama, Appl. Phys. Express 5 (2012) 025701.
[4] K. Hirama, M. Kasu et al., IEEE Electron Dev. Lett. 33 (2012) 1111-1113.
[5] K. Takahashi, M. Imai, K. Hirama, M. Kasu, Appl. Phys. Lett. 104 (2014) 072101.

Microplasmas are electrical discharges wherein one of the critical dimensions is <1 mm. Hollow-cathode microdischarges can achieve high densities with only moderate power input, and as their size decreases, their working pressure typically increases; indeed, for cavities with dimensions ~100 µm, operation has been demonstrated at atmospheric pressure. Arrays of microplasmas have a variety of potential applications, including removing contaminants from air supplies in enclosed environments (submarines; spacecraft), as flat panel light sources (especially of near monochromatic light), as large-area UV sources, and as small-scale flow reactors for chemical processing.
We recently presented the first results from an all-diamond microplasma device, in which the electrodes and the insulating dielectric were fabricated from boron-doped and undoped diamond, respectively, with the cavity formed by laser drilling a hole through all three layers. These devices operated at about 1 W d.c. power, and exhibited a sustaining voltage of 300-400 V for operation in up to 1 atm pressure helium.
We now present data from the second generation of these devices, with improved design and more comprehensive electrical diagnostics. The new devices are able to operate over a very wide range of currents and at up to 10 atm in He, striking reliably and functioning stably for many hours. Electrical characterization was performed using a high-voltage operational d.c. power supply (±#8239;1000 V, 0-40 mA), driven in voltage and current mode (for Paschen curve and current-voltage characteristic measurements, respectively) with a 0.1-100 Hz triangular waveform. Device voltages V and currents I were measured using high-side (high voltage) and low-side voltage probes connected to an oscilloscope, plus a low side current shunt. Various discharge phenomena, such as initiation, propagation, and extinction, were studied as a function of time during the voltage ramp cycle.
We find that the device Paschen curves (p×d vs V) closely resemble those for a Townsend discharge between parallel plate electrodes, despite the hollow cathode-type geometry. Current-voltage (I-V) characteristics are almost flat for all pressures and regardless of cavity dimensions, with sustaining voltages being typically 250-300#8239;V over the range I = 4-15#8239;mA, and only small differences arising with variations of pressure and discharge current.
In summary, we have now demonstrated, high pressure, high power density non-equilibrium glow discharges in monolithic diamond devices, with asymp;#8239;30#8239;min continuous operation at 9.5#8239;atm being achieved for the smallest (asymp;#8239;50#8239;µm) cavities. Operation at lower pressure and discharge current extends the device lifetime to days, and potentially weeks. We have also demonstrated plasma formation in 2D slots (200 µm wide × 1 mm long) and small (2×2; 3×3) cavity arrays.

In the past half-decade, diamond Schottky barrier diodes (SBD) are studied intensively for realizing high-performance rectifiers fulfilling both high-voltage blocking in reverse operation and low on-resistance in forward operation, which is difficult to achieve using common semiconductor materials. Several interesting results have been reported so far, but such diamond-based SBD electrical properties were not as good as expecting. One of the important subjects that should be studied is the thermal stability of diamond-based SBD, especially during its operation at the temperature higher than 500 K.
In this study, thermal behavior of tungsten carbide/p-diamond SBDs has been investigated. Reliable estimations of the Schottky barrier height (#981;_B) and ideality factor (n) of theses SBDs were achieved by using a vertical SBD structure. Both #981;_B and n measured at 300 K have been reduced through vacuum annealing at 600 K. In particular, #981;_B decreased exponentially in time at 600 K, and stabilized at 1.4 eV after 90 minutes. The lowest n among SBDs examined was close to 1.0 at 600 K.

Diamond is a new attractive material in power device field because it has many exceptional characteristics such as wide-band gap, high thermal conductivity, high breakdown voltage, etc. So far, we have developed high performance field-effect transistors (FET) [1] by H-terminated (C-H) diamond surface where holes are accumulated densely. Since the C-H surface was affected by external environment, the surface protection is required. Recently, we applied atomic layer deposition (ALD) of Al₂O₃ [2,3] at 450#730;C [4] as passivation. Even after the annealing at 550°C in air the hole density were preserved [5]. Here we fabricated metal oxide semiconductor (MOS) FETs with temperature resistant Al₂O₃ film, and successfully evaluated C-H diamond MOSFETs electrical properties from 10K to 673K (#65293;263°C~400°C).
C-H diamond MOSFETs using Al₂O₃ film was produced by following method. First 500nm-thick undoped layer was deposited on (100) Ib diamond by PECVD and Ti/Au were deposited as source and drain electrode. Second the channel surface was H-terminated by remote plasma. Third Al₂O₃ as gate oxide and passivation were deposited by ALD (oxidation source: H₂O) at 450°C [4]. Al gate electrode was formed on the Al₂O₃ gate insulator.
We measured I_DS - V_DS and I_DS - V_GS characteristics of the diamond power devices at 10K-673K with two different Al₂O₃ film thickness 32nm and 200nm in vacuum. Drain current value decreased when the temperature is higher than 573K (300°C) or lower than 273K (0°C). However, even at near the absolute zero temperature (#65293;263#730;C), the current value degradation was less than 50% of both ALD-Al₂O₃ MOSFETs. The mobility degradation at high temperature and the contact resistance increase at low temperature caused the drain current decrease. The latter can be eliminated if higher workfunction metal such as Au can be used. In addition, good pinch-off of the drain current density is obtained at 673K (400#730;C). The on-off ratio of both ALD-Al₂O₃ MOSFETs is 10⁹, and 10⁸ at 10K and room temperature. Due to leakage current increases as the temperature rose to 673K (400#730;C), the on-off ratio of ALD-Al₂O₃ (32nm) MOSFET decreased to 10³, and the ALD-Al₂O₃ (200nm) MOSFET decreased to 10². At 10K-673K the change ratio of max transconductance in both ALD-Al₂O₃ MOSFETs performed excellent characteristic, only 50% in ALD-Al₂O₃ (32nm) MOSFET and 25% in ALD-Al₂O₃ (200nm) MOSFET. As a result, we confirmed both ALD-Al₂O₃ MOSFETs can stably operate at wide range temperature and low temperature dependence of current was obtained as compared with those of the boron-doped diamond MOSFET.
This research was supported by research grants from ALCA (JST).
[1] H. Kawarada et al., Appl. Phys. Lett. 65, 1563 (1994).
[2] D. Kueck, E. Kohn et al., Diam. Relat. Mat. 19 166 (2010).
[3] M. Kasu et al., APEX 5, 025701(2012).
[4] A.Hiraiwa, H.Kawarada et al. J. Appl. Phys. 112(12), 124504 (2012).
[5] A Daicho, A. Hiraiwa, H. Kawarada et al., J. Appl. Phys. 115,223711 (2014).

Due to its exceptional thermal conductivity, wide bandgap, high breakdown electric field strength and combined carrier mobilities, diamond has the potential to revolutionize high-power and high-frequency electronics. The vertical architecture Schottky Barrier diode (SBD) is a particularly promising high-power application for diamond. Although vertical architecture devices have the potential to significantly outperform pseudo-vertical structures [1], up until now primarily pseudo-vertical structures have been investigated because of the difficulties in fabricating the vertical device structures. However, much progress has been made recently toward the reliable fabrication of the heavily doped ( > 10²⁰ cm^-3), free standing ( > 300 mu;m thick) p-type substrates that are needed for these structures. In our previous work investigating heavily boron doped single crystal diamond [2-3] we studied the plasma gas-phase to grown solid-phase boron-to-carbon ratio, and have shown a temperature dependence to this doping efficiency. We have additionally shown a temperature dependence for the formation of unepitaxial crystallites, which is a significant result, since unepitaxial crystallites are known to adversely affect SBD performance [4]. The etch-pit density associated with crystalline dislocations has also been shown to be adversely related to the reverse characteristics of SBDs [4]. Strategies for the reduction of screw- and edge-type threading dislocations in CVD diamond have been demonstrated [5], but up to now such strategies have not been effectively employed for the improvement of diode characteristics.
This work expands upon our previous effort to grow and characterize high-quality diamond for electrical applications. An SBD architecture is proposed for the reduction of threading type dislocations in the active region of the fabricated diodes. SBD performance will be evaluated, and discussed in the context of the dislocation density. Single crystal boron doped diamond films of high and low doping levels are deposited homoepitaxially in a microwave plasma-assisted chemical vapor deposition (MPACVD) bell-jar reactor with feedgas mixtures including hydrogen, methane, and diborane. We report on the results of growth experiments, and characterize the grown diamond by electrical measurements, FTIR, and SIMS. The formation and effect of defects will additionally be analyzed using etch pit techniques and Birefringence measurements.
References
[1] J. Achard et al.,Diamond and Related Materials, 20 (2011) 145
[2] S.N. Demlow, R. Rechenberg and T.A. Grotjohn “The Effect of Substrate Temperature and Growth Rate on the Doping Efficiency of Single Crystal Boron Doped Diamond.” Diamond and Related Materials, (2014) In Press - Accepted
[3] S.N. Demlow, et al.,MRS Proceedings, 1395, (2012)
[4] N. Tatsumi, et al.,SEI Technical Review 68 (2009) 54
[5] I. Friel, et al.,Diamond and Related Materials,18 (2009) 808

Diamond has unique properties, for example, hole carriers are densely accumulated on a carbon-hydrogen bond of the diamond surface (2DHG). Using this conductive layer, we have developed high-performance field-effect transistors (FETs)[1]. Convenience to use the 2DHG, planar-type devices have been made on H-terminated diamond surface mainly. But it is required making vertical-type devices in order to create integrated power FETs that handle a large current. Other materials of the power device, GaN has been studied as an example. GaN devices are using two-dimensional electron gas (2DEG) in the AlGaN / GaN interface, and are common in utilizing 2DHG in diamond. However, when applying these materials to the vertical structure, 2DEG can&’t be formed in GaN because of lack of polarization on the sidewall of wurtzite structure, but 2DHG can be formed in diamond easily by C-H polarization. H-termination of diamond is advantageous in that it can be formed anywhere on the diamond. Although the H-termination is affected by surroundings, we solve the challenge by passivation technique using atomic layer deposition (ALD) Al₂O₃ at 450 #730;C [2]. We have developed an advanced passivation for the surface p-type conduction operated at high temperature (450#8451;) using atomic layer deposition of Al₂O₃ film and achieved the good breakdown characteristic of MOSFETs. This result is comparable to the lateral FET characteristics using SiC or GaN[3], and further improvement can be expected by applying this diamond FET structure to vertical FETs. In this work, we fabricated the 3D rod structure on mono-crystalline diamond and investigated conductivity due to the 2DHG of the side walls of the rods. This study will contribute to the vertical-type MOSFET structure using surface conduction of the diamond.
Processing of the surface shape of the diamond has been reported in the formation of nanowires [4,5]. These reports showed the fine shape control by inductively coupled plasma ion etching (ICP-RIE). In this study, we fabricated diamond rods with ICP-RIE with oxygen etchant. Patterned gold patches were deposited by electron-beam evaporation on the mono-crystalline diamond (001) surface using photo-lithography technique. In order to create 2DHG at sidewalls of diamond rods, surface hydrogenation was performed by remote plasma CVD method. Conductive AFM was utilized to measure the conductivity of sidewalls of small dimer diamond rods by possible contact with the fine area. The quantitative data is going to be shown in the symposium.
[Acknowledgment] This research was supported by research grants from ALCA (JST).
References
[1] H.Kawarada, Surf. Sci. Rep. 26 (1996) 205.
[2] A. Hiraiwa, H. Kawarada et al J. Appl. Phys. 112, 124504 (2012).
[3] M.Noborio, T.Kimoto et al., Electron Devices, IEEE Transactions. 54 (2007) 1216-1223.
[4] Y.Ando, A.Sawabe et al., Diam. Relat. Mat. 13 (2004) 633-637.
[5] B. J.M. Hausmann, M. Lon#269;ar et al., Diam. Relat. Mat. 19 (2010) 621-629.

We have developed micrometer-sized porous diamond spherical particle (PDSP) as a new porous diamond material. The PDSP has mesopores with a diameter of ca. 10 nm and its BET surface area is ca. 300 m²/g. Since diamond is a chemically inert material, the PDSP should be useful for a stable catalyst support. In this study, metal nanoparticle-embedded PDSPs were fabricated and their application to a stable catalyst was investigated. An aqueous slurry containing nanodiamond (ND), platinum nanoparticle (PtNP) and polyethylene glycol (PEG) was spray-dried to form an ND/PtNP/PEG composite spherical particle. After removal of PEG from the particle by air oxidation, the treated particle was subjected to microwave plasma-assisted chemical vapor deposition for a short time to obtain PtNP-embedded PDSP (PtNP@PDSP). From the STEM observation, PtNP was found to be dispersed in the PtNP@PDSP. The catalytic activity of the PtNP@PDSP was estimated with cyclohexane dehydrogenation reaction as a model reaction, and was found to be more active than that of PtNP-supported carbon black. Similarly, PdNP@PDSP was fabricated and its catalytic activity to Suzuki coupling reaction was also investigated.

We report the novel composite nanostructures based on TiO₂ nanotubes over-grown by thin boron-doped diamond film. The BDD-modified titania nanotubes nanostructures shows increase of sensitivity and performance when used as an electrodes in electrochemical environments.
The thin BDD films (~200-500 nm) were deposited using microwave plasma assisted chemical vapor deposition (MW PA CVD) onto anodically fabricated TiO₂ nanotube arrays. The influence of boron doping level, methane admixture and growth time on performance of the Ti/TiO₂/BDD electrode have been particularly studied.
Numerical analysis of scanning electron microscopy (SEM) was applied to investigate surface morphology and grains size distribution. Controlling of the grain size is key parameter that influence on greater surface development witch corresponding to increasing capacity. Moreover, the chemical composition of Ti/TiO₂/BDD electrodes was investigated by means of micro-Raman Spectroscopy.
Electrochemical measurements composite electrodes were performed by the potentionstat-galvanostat system AutoLab PGStat 302N in a standard three-electrode assembly at 295 K. Different types of Ti/TiO₂/BDD layers and as reference Ti/BDD stayed as a working electrode. The counter electrode consisted of Pt mesh and an Ag/AgCl/0.1 M KCl electrode was the reference electrode. Electrodes with geometrical surface area of 1 cm² were tested by cyclic voltammetry in solutions: 0.1M NaNO₃ without and with 1 mM K₃Fe(CN)_6. Prior to taking electrochemical measurement, a pretreatment was performed in which working electrode was held at -0.1 V for 60 s. Electrochemical impedance spectroscopy measurements were carried out at the formal redox potential +0.13V vs. Ag/AgCl/0.1M KCl in the frequency range of 0.1 to 1000 Hz and amplitude 10 mV rms. All the electrolytes were purged with argon for 50 min. before electrochemical test and during measurements there was an Ar-cushion above the electrolyte.
In general all the composite electrodes TiO₂/BDD are characterized by the significantly higher current than BDD layer deposited directly onto the Ti substrate. The registered current increases with the boron concentration that could be related to the higher surface area and increase in conductivity. Analysis of electrochemical impedance spectra according to proposed electric equivalent circuit: R_e[Q₁R₁][Q₂(R₂W)] allows to determine surface area on the basis of value of constant phase element.

Doped diamond for electronic devices requires p and n-type material with sufficient doping concentration. While p-type diamond can be readily realized through boron doping the preparation of n-type electronic grade diamond is still a challenge. One of the most suitable donors for n-type diamond is phosphorus. Several groups have focused their doping studies utilizing phosphine gas, a highly toxic compound comprised of hydrogen and phosphorus. We report on doping results of (100) single crystal diamond using trimethylphosphine, P(CH₃)₃, in a microwave plasma assisted CVD process. The substrates, type IIa (100) plates, were chemically cleaned and an initial undoped diamond layer was deposited using methane and hydrogen. For the phosphorus doped layer a trimethlyphosphine/hydrogen mixture and hydrogen was used without additional methane flow. At a microwave power of 1300W and a chamber pressure of 70 torr the substrate temperature approached 850 °C. The growth temperature was adjusted by controlling the pressure and the microwave power where we prepared films with microwave power up to 2500W, pressure up to 90 torr and growth temperature up to ~1130 °C. At the lower microwave power phosphorus incorporation evaluated through SIMS exceeded 10¹⁷ cm³. However, with increased microwave power the phosphorus incorporation slightly exceeded 10¹⁸ cm^-3. Increasing the growth temperature at higher microwave power through increased pressure resulted in a small increase in the phosphorus concentration in the film. These results evidence an advantageous effect of microwave power and growth temperature on the doping concentration. The doping results indicate that trimethylphosphine should be an efficient doping source for n-type diamond. Film morphology was described in terms of a step-bunch growth mode resulting in a homoepitaxial film with patterned roughness.
This research is supported by the Advanced Research Projects Agency - Energy (arpa-e).

We have used Al₂O₃ as gate insulator of diamond MOSFET and achieved high voltage (~500V)[1] and high temperature operation (~400#8451;)[1]. But SiO₂ gate insulator should be used for because of its high reliability. Diamond device is also expected to get high reliability by using SiO₂ gate insulator for practical application. Thermal oxidation of Si is the most suitable gate-insulator for Si MOSFET. So, we thermally oxidized Si film deposited in MBE apparatus which hardly affect the termination structure of diamond.
First, we formed p-type surface conductive layers (1×10⁴Omega;/sq) on (001)-oriented single crystalline diamond substrates using hydrogen plasma. Then, the samples were anneled at 300#8451;to clean surface thermally in MBE apparatus. After cooling down to room temperature, we deposited Si on diamond substrates in 50-100 nm in thickness. The deposition temperature was between room temperature-300#8451;.
At room temperature deposition, Si kept amorphous from the beginning to the end at room temperature deposition by RHEED pattern However, at 300#8451;deposition, Si changed amorphous to polycrystal in the middle of deposition of 56nm in thickness. This change of crystallinity is similar to Si-molecular beam deposition on Si substrates[2]. The XPS result of diamond surface,
which is exposed by removing Si thin layer (or SiO₂ layer formed by air-exposuring) deposited at room temperature by hydrofluoric acid had no Si peaks. Due to this result, C-Si bonds formed at 600#8451; did not appeared. Consequently, C-H bonds on diamond surface was not replaced by other bonds and preserved in the interface of Si/diamond. We will conduct same experiment on (111)-oriented crystalline diamond. The influence of thermal oxidation on electric properties are discussed in the presentation. .
Acknowledgements
This study was supported by research grants from ALCA (JST) and JSPS.
References
[1] H.Kawarada, H. Tsuboi,T. Yamada, A.Hiraiwa et al., Appl. Phys. Lett. 105 (2014, in press).
[2] M. Tabe, K.Arai, et al, Jpn J. App. Physic. 20, 703-708 (1981).

Chemical Vapor Deposited (CVD) diamond growth on (111) diamond surfaces has received increased attention lately because of the use of N-V related centers in quantum computing as well as application of these defect centers in sensing extremely low levels of magnetic field. This magnetic sensing is of interest in studying the properties of materials under extreme conditions in a diamond anvil cell. We have carried out a detailed study of homoepitaxial diamond deposition on (111) diamond surface using a 1.2 kW microwave plasma chemical vapor deposition system utilizing a methane/hydrogen/nitrogen/oxygen chemistry. We have utilized Type Ib (111) oriented diamond as seed crystals in our study. The homoepitaxial grown diamond films were analyzed by Raman spectroscopy, Photoluminescence spectroscopy, Atomic Force Microscopy X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy, and single crystal x-ray diffraction. The nitrogen concentration in the plasma was carefully varied between 0 and 1500 ppm while the ppm level of silicon impurity is present in the plasma from the quartz bell jar. The concentration of N-V defect center and Si-defect center was quantitatively studied through monitoring of the zero phonon lines with the varying growth conditions. Higher concentration of nitrogen in the plasma was observed to suppress silicon incorporation. The role of plasma chemistry and surface roughness of the seed crystals in growing a high quality homoexpitaxial diamond film with controlled incorporation of N-V defect centers on (111) diamond surface will be presented.
Our research results are based upon work supported by the National Science Foundation Partnerships for Innovation: Building Innovation Capacity (PFI: BIC) subprogram under Grant No. IIP-1317210.

The surface roughness evolution of growing thin films of metals or semiconductors provides much needed information about their growth kinetics and corresponding mechanism. While some systems show stages of nucleation, coalescence and growth, the others exhibit varying microstructures with different process conditions. In view of these classifications, we hereby report detailed analyses relating the atomic force microscopy (AFM) characterizations to extract the surface roughness and growth kinetics exponents of relatively lower boron-doped diamond (BDD) films by utilizing the analytical power spectral density (PSD) and autocorrelation function (ACF) mathematical function tools. The machining industry has applied PSD for a number of years for the tool design, wear and machined surface quality. Hereby, we present similar analyses at mesoscale to study the surface morphology as well as quality of BDD films grown using microwave plasma-assisted chemical vapor deposition (MPACVD) technique. PSD spectra as a function of boron concentration (in gaseous phase) are compared with those grown without boron. We find that relatively higher boron concentration yielded higher amplitudes in the longer wavelength power spectral lines with amplitudes decreasing in an exponential or power law fashion towards the shorter wavelengths determining the roughness exponent (a ~ 0.16±0.03) and growth exponent (b ~ 0.54), albeit indirectly. A unique application of the ACF function, widely used in signal processing, was also applied to one-dimensional or line analyses (i.e. along the x- and y- axes) in AFM images revealing surface topology data sets with varying boron concentration. Here, the ACF was used to cancel random surface ‘noise&’ and identify any spatial periodicity via repetitive ACF peaks or spatially correlated noise. Periodicity at shorter spatial wavelengths was observed for un-doped and low doping levels, while smaller correlations were observed for the relative higher boron concentration. These semi-quantitative spatial analyses may prove useful in comparing synthesis techniques and films grown with varying compositional makeup of diamond films and other technological important electronic materials. These findings in terms of ‘critical exponents&’ are also correlated with traditional Raman spectroscopy (RS) and X-Ray Diffraction (XRD) structural properties thus helping to provide an insight into the growth kinetics albeit in reverse manner.

Conductive diamond electrode has attracted attention as a highly sensitive and stable electrode material in a wide electrochemical application field. However, the shape that can be made is limited because it is deposited on a flat substrate material as a thin film. We have fabricated screen-printed diamond electrodes that can be made into an arbitrary shape with a lower temperature procedure, and can be applied to mass production. In this study, we prepared screen-printed diamond electrode with different binder/BDDP ratios, and the electrochemical properties were investigated. BDD powder (BDDP) was obtained by microwave plasma-assisted CVD method using diamond powder (DP) as a substrate material. An ink containing BDDP and polyester binder was used for preparation of the screen-printed electrode. For a large binder/BDDP ratio, the screen-printed diamond electrode showed a microelectrode-like voltammetric behavior, while an electrode with a small binder/BDDP ratio exhibited typical planar electrode behavior. This should be because that the micrometer-sized conductive domains were exposed on the electrode surface with a low density for the large binder/BDDP electrode. Based on the microelectrode effect, large signal-to-background ratios were obtained for electrochemical detection of ascorbic acid. Thus, the screen-printed diamond electrode with a large binder/BDDP should be useful for a disposable and sensitive electrode material.

The nitrogen-vacancy (NV) center is an individually addressable electronic spin in diamond that can be initialized and read out optically and coherently controlled with microwaves at room temperature. These properties require NV centers to be located within a few nanometers of the surface and within 2 nanometers of ¹³C for long term spin applications. Engineering NV centers near the surface have not been fully characterized. We report on a hot filament growth chemical vapor deposition (HF-CVD) delta doping method that is very promising for both the NV center and ¹³C doping. The doping is on the order of 10-100 nm. In-situ laser reflectance interferometry tool (LRI) is used for monitoring growth characteristics of diamond thin film materials. The grown epi-layers were characterized using scanning electron microscopy, Raman spectroscopy, Atomic force microscope PL, and hall measurements for electron mobility measurements

Over the past decade, diamond films have been studied for use energy conversion devices such as thermionic energy converters. One crucial property of diamond films for thermionic emission is its surface work function, which depends on the type and concentration of impurities used to dope the diamond as well as its surface termination. It is desirable to reduce the diamond work function which leads to an increase in emitted current and therefore to a higher power output on a potential device.
Several techniques are available to characterise the work function, namely Ultra-violet Photoelectron Spectroscopy, Kelvin Probe or extrapolation from thermionic emission data. However, these only provide an average value of the surface work function making it difficult to extract more detailed information about the material in question. Alternatively, kelvin probe force microscopy (KPFM) is an AFM technique that can map topography and surface contact potential difference (CPD) with a lateral resolution of a few nanometres and a energy resolution of tens of milivolts. Using a calibration process it is possible to convert surface CPD into work function values so that the work function variation between different crystals due to crystallographic orientation can be distinguished. If the surface is subjected to a particular surface termination procedure it is also possible to detect if this has had any effect on the local work function and whether the coverage is as uniform as expected.
Here, we report the effects on the diamond work function produced by introducing Li, B and N impurities into the diamond lattice to act as a novel co-dopant cluster offering an alternative to traditional dopants like B or N. For this task we performed KPFM under UHV conditions showing the work function of different co-doped diamond films. The effect of a number of different surface terminations on the work function will also be presented.

The use of nanodiamond (ND) for drug delivery and bio-imaging is grounded on chemical functionalization, and the key task to be addressed is the capability to simplify the process steps, to reduce the process times and to maximizing the drug/ligand uptake. The idea underlying the present research is to modulate loading capabilities of ND by controlled modification of their chemical surface potential. To this aim the ND samples are treated either by wet chemistry using medium-strong reducing agents or by tuneable H-plasmas produced in a custom- designed MW-RF reactor.
SERS-aided micro-Raman spectroscopy and FTIR allow us to precisely detect structural and chemical features of both core and surface of the NDIA after the various treatments .
A further important result is the quenching , obtained by specific processes, of the radical behaviour of the NDIA surfaces. This achievement reduces the possibility of uncontrolled reactions between NDIA and chemical species and helps in controlling the related biochemistry . Several chemical and physical methodologies are used in order to investigate how the properties imparted to NDIA surfaces affect the uptake of molecules .
The affinity of the treated surfaces for drugs has been probed by conjugating a series of natural antioxidant molecules and by testing in vitro the ND-drug adducts as chemioterapics.
The methodology developed to modify the ND surfaces and to precisely identify their chemistry, are offering practical solutions to the still open problems related to NDIA-based systems for bio-medical applications

The polycrystalline boron doped diamond (BDD) shows stable electrical properties and high tolerance for harsh environments (e.g. high temperature or aggressive chemical compounds) comparing to other materials used in semiconductor devices. In this study we have designed electronic devices fabricated from non-intentionally/low boron doped diamond (NiD/LBDD) films and highly boron doped diamond structures. Presented semiconductor devices consist of highly boron doped structures grown on NiD/LBDD diamond films. Created structures were analyzed by electrical measurements for use in harsh environment applications. Moreover, the boron-doping level and influence of oxygen content on chemical composition of diamond films were particularly investigated.
Microwave Plasma Enhanced Chemical Vapor Deposition (MW PE CVD) has been used for thin diamond films growth. Non-intentionally doped diamond and low boron doped diamond (0-2000 ppm [B]/[C]) films have been deposited on the Si/SiO₂ wafers with different content of carbon, boron and oxygen. In the next step, the surface conductivity introduced by hydrogen termination were removed. Then, the shape of the highly doped diamond structures were obtained by pyrolysis of SiO₂ on NiD film and standard lithography process. The highly doped structures were obtained for different growth time and [B]/[C] ratio (4000 - 10000 ppm). The narrowest distance between two highly doped structures was 5µm. The standard Ti/Al/Au ohmic contacts were deposited using physical vapor deposition for electrical characterization of NiD/BDD devices. The influence of diffusion boron from highly doped diamond into non-doped/low-doped diamond film was investigated. Semiconductor character of NiD/BDD structures was preserved with relatively high sp³/sp² band ratio. Surface morphology of designed structures was analyzed by Scanning Electron Microscope and Atomic Force Microscope. The relative sp³/sp² band ratios were determined by deconvolution of Raman spectra. The resistivity of the NiD and LBDD film was studied using four-point probe measurements. DC and high frequency studies were performed to test the operation of the NiD/BDD devices in temperature range up to 600^oC exposed on acidic environment.

Diamond-like carbon (DLC) has received great attention as a new material for application to thin film solar cells and other semiconducting devices. DLC can be produced a lower cost than amorphous silicon which is utilized for solar cells today. However, the electrical properties of DLC are insufficient. To solve this problem, we investigated the effects of DLC fluorination on its structural and electrical properties. We prepared five kinds of fluorinated DLC (F-DLC) thin films which had different amounts of fluorine on the polycarbonate (PC), transparent conductive oxide (TCO) grass and polyethylene naphthalate (PEN) substrates. The films were deposited by the radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) method. C₆H₆ and C₆HF₅ were used as source gases. After analyzing DLC properties, to manufacture the solar cell, we prepared nitrogen doped DLC (F-DLC) thin films using N₂ gas as a doping gas. Surface chemical compositions and carbon bonding structure were analyzed by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and electron spin resonance (ESR). The surface resistivity was measured by the dual-ring method. The current-voltage (I-V) characteristics of the nitrogen doped F-DLC films /p-Si structures were measured with a solar simulator under AM 1.5 condition. The fluorine had been incorporated from 0 to 20 atom % into the DLC films which was discovered by XPS. Moreover, XPS analysis of the carbon spectra after fluorine saturation at 10 atom % showed the majority of bonding was C-F bonding. At 20 atom % fluorine content, C-F₂ bonding was observed in the addition to C-F bonding. As the amount of fluorine increases, Raman spectra show the intensity of the D band increases with respect to the G band. The width of the ESR spectra became broader. The surface resistivity of the films changed from 3.57×10¹² to 4.55×10⁹. Its values dropped to a minimum at 10 atom % fluorine content. Moreover, the energy conversion efficiency reached the maximum level in this case. Fluorination of DLC affects electrical properties. At first the surface resistivity decreases, because of π electron delocalization according to ESR spectra. After that the surface resistivity increases because the chain structure domains of DLC becomes larger. There are two reasons for this. First, XPS shows C-F₂ bonding within F-DLC which prevent ring structures. Second, the Raman spectra indicated that the sp² domains decreased. With a larger chain structure domain, the surface resistivity typically increased. We succeeded in improving the energy conversion efficiency and its electrical conductivity. Our results indicate the necessity of the application of DLC fluorination to the semiconducting material for electrical devices such as solar cells.

Despite recent advances with phosphorus doping of diamond, the production of n-type semiconducting diamond with useful electronic characteristics remains elusive. Lithium has been suggested as a possible alternative n-type dopant due to its potential as a shallow donor in diamond. However, experimentally doping with lithium is difficult to achieve due to the low solubility of Li in diamond, and its relatively high mobility which allows it to diffuse through the lattice at fairly low temperatures.
It has been suggested that the unwanted Li diffusion can be prevented by adding substitutional nitrogen or boron together with interstitial Li, with the N or B acting as a trap to pin down the Li in the diamond lattice and reduce its mobility, while retaining its n-donor properties. To study this, we investigated the defect formation energy, the band structure and density of states (DOS) of diamond containing various Li-N/Li-B centres, using calculations based on periodic Density Functional Theory (DFT) as implemented in the CASTEP code. The calculations use the PBE functional with a cubic supercell containing 64 atoms. Interstitial lithium and/or a substitutional nitrogen/boron atom are included in the supercell and the effect of neighbouring vacancies is also taken into account. The band structures and DOS of the co-doped material are compared with those of the separate singly doped systems. The results of this computational study will provide guidance for developing practical routes to realise these dopant systems experimentally.

Exciting properties of diamond like extreme hardness, chemical inertness and wide band gap can be exploited for utilization in high power electronics and high power optical components in harsh conditions. The main obstacle in using single crystal diamond substrates for these applications has been the synthesis of large (>1 cm²), high purity and defect-free SCDs to overcome the problem of grain boundaries arising in polycrystalline diamond (PCD) substrates.
There are several approaches for large SCD synthesis. One is by using the mosaic method. By precisely aligning tiled clones of grown CVD SCD, it is possible to enlarge SCDs. Another approach is by using side-seed growth along <100> direction to increase the seed growth area and then grow along the original direction for a final large SCD. Both these approaches lead to stress along the boundaries of different growth directions or misalignment which thereby lead to defects in the substrates. A third approach is to use heteroepitaxial growth of SCD on large non-diamond substrates. This needs more work to remove defects created by diamond islands formed on the substrate.
Here, we investigate methods of increasing the area of SCD during synthesis. It is important to mitigate the growth of the PCD rim which reduces the required SCD surface area. We have conducted experiments using several variations of open and pocket holders. By adjusting the dimensions of the pocket holder, we can minimize the rim. It is observed that by using these holders, the PCD rim can be reduced by a factor of 2 or more in comparison to open holders. The difference in electric field concentration around the seed edges leads to different thicknesses of PCD rim. By designing novel substrate holders which allow the growth of diamond in both longitudinal and lateral directions, due to the absence of the rim, will help in obtaining thick large SCD substrates.
By using a series of pocket holders we have grown high quality (< 300ppb [N] impurities), thick (~ 2.2mm) SCDs on 3.5 x 3.5mm² type Ib HPHT seeds. High growth rates of 25 - 32µm/h were achieved. These substrates have been synthesized via microwave plasma assisted CVD in a microwave cavity plasma reactor [1] whose operating conditions have been optimized for an efficient process [2].
Threaded dislocations arising from the HPHT seeds propagate parallel to growth direction <100> [3]. After removing the HPHT seed and by flipping the grown CVD SCD by 90° for another synthesis step, the propagation of the dislocations can be minimized. By subsequent flipping of substrates, after removing the PCD rim and polishing the grown surface, it is possible to obtain high quality, large type IIa SCDs. The importance of laser cutting the rim and polishing the surface after each step will be discussed.
References
[1] J. Asmussen, et.al. US Pat. 8,316,797 (2012)
[2] J. Asmussen, J. Lu, Y. Gu, & S. Nad, US Prov. Pat. (filed May 23, 2014)
[3] I. Friel et.al. Diam. & Rel. Mat. 18 (2009) 808-815

Zirconium alloys are used to make the hollow tubular fuel rods which hold the nuclear fuel in nuclear reactors. However at high temperatures it can react with the water and corrode, releasing hydrogen as a by-product. Under exceptional circumstances (e.g. a meltdown event such as happened at Fukushima) enough hydrogen can be released that an explosion occurs, with potentially catastrophic results.
One possible solution to this is to coat the Zr with a layer of diamond to prevent the hot Zr coming into direct contact with water. Diamond is an ideal material for this as its high thermal conductivity will help transfer heat from the fuel-rods into the water efficiently, while it&’s low atomic number means that it will not absorb neutrons and affect the nuclear reaction. Diamond is also mechanically robust and radiation hard.
However, little has been reported yet about the growth of CVD diamond onto Zr or Zr-alloys. As such, we have studied the deposition of CVD diamond onto flat samples of pure Zr using various CVD growth conditions in a hot filament reactor. We find that although growth is straightforward, adhesion of the diamond layer onto the Zr is poor, with the diamond layer often delaminating upon cooling. SIMS depth profiles show this to be due to the presence of a strongly-bonded native oxide on the Zr surface which is not removed in the reducing H₂ atmosphere during CVD. This, plus the lack of any carbide interfacial layer to ‘glue&’ the diamond onto the surface, together with a poor thermal expansion mismatch between Zr and diamond, all lead to poor adhesion. Some of these difficulties can be reduced by depositing at lower temperature (<500°C) at the cost of poorer quality diamond.

Ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films have a specific structure, wherein a large number of diamond nanograins are embedded in an a-C:H matrix, It has been demonstrated that their p- and n-type conduction accompanied by enhanced electrical conductivity are possible by boron and nitrogen doping, respectively.^1,2) . In this study, heterojunction diodes comprising nitrogen-doped UNCD/a-C:H and p-type Si were fabricated by coaxial arc plasma deposition (CAPD) and their electrical properties were investigated at room temperature. Experimentally, 3.55 at.% nitrogen-doped UNCD/a-C:H films were deposited on p-type Si (100) substrates with an electrical resistivity of 10-20 Omega; cm by CAPD. Radio Frequency (RF) sputtering was employed to deposit Pd ohmic electrodes on both sides of the samples. The current-voltage (I-V) characteristics of the diodes were measured in the dark at room temperature. The I-V curve exhibited a rectifying action with a recti#64257;cation ratio between bias voltages of ± 5 V of more than 10⁴, which proves that the nitrogen-doped UNCD/a-C:H evidently acts as a n-type semiconductor. Capacitance-voltage (C-V) measurements were applied to study the nature of depletion regions in the diodes. The frequency and testing signal for measuring the capacitance was 100 kHz and 50 mV, respectively. The capacitance decreased with increasing reverse voltage, which evidently indicates the expansion of the depletion region with the reverse bias. The built in potential was estimated to be 0.68 V from the intercept of the plot of 1/C² versus V (with the x-axis). Moreover, by using known physical values such as the carrier concentration of the n-type Si substrate being 5 x 10¹⁵ cm^-3, the active carrier concentration of the UNCD/a-C:H #64257;lm was estimated to be 6.0 x10¹⁵ cm^-3. It was experimentally demonstrated that the heterojunctions comprising nitrogen-doped UNCD/a-C:H work as typical diodes and nitrogen-doped UNCD/a-C:H is applicable to electrical devices as a n-type semiconductor.
[1] S. Ohmagari et al., JJAP 51, 090123 (2012). [2] S. Al-Riyami et al., APEX 49, 115102 (2010).

Color centers in diamond, such as nitrogen-vacancy (NV) centers, are intensively studied for realizing high-performance devices utilizing these unique spin properties. In order to make these centers single photon sources, their concentration should be controlled in the range of <10¹³ cm^-3 (<0.1 ppb). This concentration is at least two orders of magnitude less than the minimum donor/acceptor concentration required for electric conduction control. Synthesis of such high chemical purity diamond is one of the challenging topics for diamond chemical vapor deposition (CVD). Carbon isotopic control is another demand together with improving crystalline quality for controlling nuclear spin density in diamond and/or increasing the spin coherent time.
In this study, ¹²C isotopically-enriched homoepitaxial diamond films were deposited on Ib(100) or IIa(100) single crystals by a home-built microwave plasma-assisted CVD. Oxygen was fed with rather high concentration of 2% toward total flow to inhibiting crystalline defect formation during growth. By employing this condition, rather thick (~20 mu;m) homoepitaxial layers were grown with better surface flatness (Ra <10 nm).
The highest ¹²C enrichment of 99.998% among the previous reports was achieved. Nitrogen concentration of the homoepitaxial diamond films was less than 1 ppb. No NV^0/- and SiV^- centers were detected from as-grown homoepitaxial films by using cathodeluminescence and confocal photoluminescence set-up. Single NV^0/- and SiV^- were, then, successfully created by introducing nitrogen or silicon source during growth.

Synthetic diamond powder is synthesized by a high pressure/high temperature method. SEM analysis shows that the diamond powder (45 µm particle size) is composed of single crystals, with well-defined cubo-octahedral faceting. On the other hand, XRD analysis displays a single crystal pattern with a high degree of peak broadening, as well as a broad low-angle peak, clearly indicating a high level of lattice defects in the crystalline material. EELS analysis confirms that the powder is composed of high purity carbon. A small sample of the diamond powder is crushed and ground into much finer particles, such that a few tapered edges are suitable for high resolution TEM. SAD analysis confirms the XRD results, indicating areas of well-defined crystallinity, associated with regions of no resolved crystallinity. A typical SAD pattern shows crystalline spots superimposed on broad diffraction rings. Dark-field imaging of the non-crystalline regions indicates the presence of a second phase or lattice disorder. No dislocations are observed in the lattice-fringe patterns. The measured lattice constants verify the diamond-cubic structure. The current work quantifies defect density from EELS, XRD, and SAD.

The recent growth in the technological applications of nanoparticles is fueling increased interest in understanding how nanoparticles interact with with the environment, including interactions with single- and multi-cellular organisms. Nanodiamond is of particular interest because the ability to functionalize the surface with a range of ligands in an ultra-stable manner provides a mechanism for understanding how properties such as surface charge impact the resulting biological response. We developed nanodiamond particles functionalized with different ligands and examined their interaction with supported lipid bilayers and with two model organisms: Shewanella oneidensis (a soil bacterium) and Daphnia magna (the water flea). A comparison of identically functionalized 4-nm nanodiamond and ~4-nm nano-gold shows significant differences in the biological response and in their interaction with supported lipid bilayers. Our results highlight that even with functionalized nanoparticles where the primary interactions are witih the exterior ligands, the composition and nature of the nanoparticle core still have a large effect. In this talk I will discuss some of the reasons behind these effects and the implications for understanding how nanodiamond and other nanoparticles impact the environment.

In recent years there has been a leap forward in the number of applications and methods to read out and measure biological markers [1,2]. Depending on the envisaged application, different materials are used for the sensor platform. In this work, an integrated diamond based temperature sensor is presented, simultaneously acting as a DNA immobilization platform capable to detect single nucleotide polymorphisms through the in-house developed heat-transfer resistance method [1]. The different fabrication steps will be discussed in detail, complemented with finite-element modelling describing the expected thermal behaviour.
To ensure reproducible high quality measurements a well delineated number of steps are required assuring a T-sensor capable of reaching an accuracy of 0,1°C. A Corning Eagle-glass substrate is overgrown with a thick heavily boron-doped nanocrystalline diamond (B-NCD) layer. After a seeding step, using a colloidal solution of ultra-dispersed nanodiamond, the growth is done in an ASTeX 6500 microwave plasma enhanced chemical vapour deposition reactor using 4 % methane with respect to hydrogen, ± 8330 ppm trimethyl boron-to-methane at an overall deposition temperature of ± 560°C. Subsequently, the layers are patterned using photo-lithography to form a double interdigital meandering structure that will form the base for both the heating and the sensing structure of the temperature sensor. To ensure a wide range of resistance values of the meandering structures, this being a key factor for the steering of the sensor, 9 different structures were designed. Once the developed patterns are imprinted on top of the B-NCD-layer, an O₂-plasma etching step assured selective removal of part of the B-NCD, leaving the described meander structures, which show a total resistance ranging from 0,7 k#8486; for the simplest and widest structure to 40 k#8486; for the finest and more complicated structures. To assure heat conduction through the complete structure whilst retaining electrical separation between the two interdigitated B-NCD meanders a thin layer of undoped NCD is selectively deposited on top using AlN as a local mask during the reseeding [3].
Finally, the thermal behaviour of the design is modelled using COMSOL Multiphysics, analysing the possible existence of hotspots on the sensor surface, experimentally verified through the use of thermal imaging.
[1] B. van Grinsven, et al., ACS Nano 6(3) 2712-2721 (2012)
[2] M. Peeters, et al., Analytical and Bioanalytical Chemistry 405(20) 6453-6460 (2013)
[3] P. Pobedinskas, et al., Applied Physics Letters 102 201609 (2013)

The negatively charged Nitrogen-Vacancy (NV) center in diamond is a versatile quantum emitter that can be used for various quantum information and sensing applications. However, it has been shown that a NV center can randomly switch between its negative and neutral charge states, with the latter not exhibiting the optical and spin properties desired for many applications. Despite recent advances, the factors affecting the charge state stability, especially for NV centers in nanodiamonds, are not well understood. Here, we present spectroelectrochemical studies of single NV centers in high-pressure high-temperature (HPHT) nanodiamonds. Specifically, we deposit nanodiamonds on an indium-tin-oxide (ITO) coated coverslip, which functions as the working electrode of an electrochemical cell. The transparent ITO layer allows for the continuous excitation and monitoring of the NV fluorescence as a function of the voltage applied between the ITO electrode and an aqueous electrolyte. Wide-field microscopy measurements show that a high fraction of the NV centers exhibits voltage-controllable fluorescence intensity. Spectral and time-resolved measurements confirm that the applied voltage alters the NV charge state and fluorescence dynamics. We discuss models for the physical origin of the observed voltage-dependent fluorescence intensity. In addition to charge state control of NVs for quantum applications, these results indicate that NV centers in nanodiamonds could be used as optical sensors of local electric fields and voltages. Specifically, we demonstrate that the NV fluorescence can be used to detect 100 mV potential increases using wide-field microscopy and 10 ms voltage pulses using confocal detection, illustrating the potential of NV centers as fluorescent voltage indicators.

The growth of ordered graphene films on metalized diamond at reduced temperatures has been previously demonstrated[1]. The focus to date has been on the epitaxial diamond (111)/Fe interface, however recent studies have also confirmed the growth of graphene on the more industrially common diamond (001) substrates. Synchrotron radiation techniques such as photoelectron spectroscopy/microscopy (PES)/(PEEM), as well as electron illumination techniques such as low energy electron diffraction (LEED) and low energy electron microscopy (LEEM) have been used to study the growth, structure and morphology of the sample. The process has been investigated from the deposition of a thin catalyst layer, through to the growth of graphene, in situ and in real-time. The ability to laterally resolve the surface has led to further progress in patterning the catalyst on defined regions of the surface and subsequent graphene growth. Graphene areas as small 10 mu;m² were fabricated, enabling the ability to produce patterned graphene electronic devices.
1. Cooil, S.P., F. Song, G.T. Williams, O.R. Roberts, D.P. Langstaff, B. Joslash;rgensen, K. Hoslash;ydalsvik, D.W. Breiby, E. Wahlström, D.A. Evans, and J.W. Wells, Iron-mediated growth of epitaxial graphene on SiC and diamond. Carbon, 2012. 50(14): p. 5099-5105.

Despite that nanodiamond (ND) particles were discovered more than 50 years ago and experienced mass production in the early 80s, for a long time they were in the shadow of their more famous sp² carbon cousins. Two recent major breakthroughs, the production of individual nanodiamond particles 4-5nm in size and nanodiamond particles containing colour centres exhibiting stable luminescence and unique spin properties have brought ND particles to the forefront of materials research and applications.¹ Besides well-known outstanding mechanical and thermal properties diamond particles have remarkable optical properties in combination with biocompatibility, high specific surface area, and tuneable surface structure.
The optical emission, scattering and attenuation of ND are finding unique applications in a variety of fields. In life science nanoparticles are increasingly used as fluorescent probes to monitor cellular interactions and to study cellular dynamics at the single molecular level. In one approach foreign atoms can be incorporated into the lattice of ND particles providing perfectly photostable particles as well as systems for quantum sensing applications that might be used to probe the nanoscale intracellular environment of cells. Development of multimodal imaging probes based on 5-10nm ND and doping of ND with different elements to generate photoluminescence at alternative wavelengths are future directions for this field. Carbon dot-decorated ND is an alternative means of generating photoluminescent nanoparticles with tuneable emission over the visible portion of the electromagnetic spectrum and into the near-infrared region.² The photoluminescent ND is increasingly being viewed as a means of drug delivery. Encapsulating the ND in a porous silica shell will be discussed as a means to achieve stable fluorescent imaging with high loading capacity of bioactive molecules.³
Challenges related to the large scale production of ND containing colour centres as well as material characterization and standardization toward development of a commercial product will be also discussed.
¹V.N. Mochalin, O. Shenderova, D. Ho, Y. Gogotsi, Nature Nanotechnology. 7 (2012) 11-23.
²O. Shenderova, S. Hens, I.I. Vlasov, S. Turner, Y.-G. Lu, G. Van Tendeloo, A. Schrand, S. Burikov, T. Dolenko, Particle & Particle Systems Characterization, 2014; 31(5): 580-590
³E. Von Haartman, H. Jiang, A. Khomich, Shenderova O. et al. , J. Mater. Chemistry B, 1, (2013) 2358-2366

Diamond particles smaller than 10 nm (ND) have remarkable physical properties in combination with biocompatibility, high specific surface area, and tunable surface chemistry [1]. They are the least toxic of all carbon nanoparticles and their properties make them a favorable platform for drug delivery, cellular labeling/imaging, composites, lubricants and many other applications. However, to harness the potential of ND in all those applications, purification, characterization, particle size control and functionalization of the diamond surface is needed. We have demonstrated control of nanodiamond surface chemistry by using various dry and wet chemical methods in order to improve its dispersion stability in solvents and polymers, as well as control its adsorption capacity and strength of adsorption. For example, we have also shown that decoupling of the oxidation reaction into the oxygen chemisorption and the CO and/or CO_x desorption steps allows layer-by layer oxidation of nanodiamond [2], which can be used to control ND particle size, as well as its surface chemistry . The diamond nanoparticles showed a continuous size decrease and increase in the specific surface area during layer-by-layer oxidation cycles, while simple oxidation in air leads to burn-off of the smallest ND particles and an increase in the average particle size. We show on an example of several drugs that various surface modifications allow control of the amount of drug adsorbed and the strength of adsorption.
References
V. Mochalin, O. Shenderova, D. Ho, Y. Gogotsi, The properties and applications of nanodiamonds, Nature Nanotechnology,7 (1) 11-23 (2012)
B.J.M. Etzold, I. Neitzel, M. Kett, F. Strobl, V.N. Mochalin, Y. Gogotsi, Layer-by-Layer oxidation for decreasing the size of detonation nanodiamond, Chemistry of Materials, 26, 3479-3484 (2014)
V. N. Mochalin, A. Pentecost, X.-M. Li, I. Neitzel, M. Nelson, C. Wei, T. He, F. Guo, Y. Gogotsi, Adsorption of Drugs on Nanodiamond: Towards Development of a Drug Delivery Platform, Molecular Pharmaceutics, 10 (10), 3728-3735 (2013)

The electrical conductivity and chemical reactivity of diamond surfaces are controlled by surface processing using high temperature annealing and metalization. Parallel, real-time X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy has been applied to the processing of low index faces of semiconducting diamond in a controlled vacuum environment. Annealing of the diamond (111) and diamond (001) surfaces to 1200 K while monitoring using electrons and photons reveals correlations between surface composition and surface conductivity with high temperature accuracy. Metalization using ultrathin Ag and Fe overlayers modifies the surface termination, changes the conductivity and enhances the surface light scattering.

Nanocrystalline Diamond (NCD) has many extreme properties making it attractive for use in applications from Micro-Electro-Mechanical Systems (MEMS) to thermal management. The unrivalled Young&’s modulus (>1100 GPa) and thermal conductivity (2000 W/mK) as well as the possibility of integration with CMOS and III-V heterostructures have bolstered significant research efforts in the growth and applications of NCD films.
However, NCD films have a roughness that evolvesinvolves with films thickness, which can adversely affect its integration into multi-layered devices where a smooth surface is important. In order to reduce this roughness the technique of Chemical Mechanical Polishing (CMP), commonly used for polishing of dielectrics and metal interconnects in the semiconductor industry, has been applied to the polishing of NCD films. This technique is important as it is able to compensate for wafer bow which is significantly greater than the film thickness.
NCD films were grown under low methane / high power density conditions in order to realisze optimiszed diamond properties. These films were then polished with a Logitech Tribo CMP tool, using polyurethane impregnated polyester felts doused with alkaline colloidal silica (Syton^TM) and characterisesied after various durations. The surface roughness was characterised over 5 x 5 µm area with Atomic Force Microscopy and the morphology with Scanning Electron Microscopy. The surface roughness was reduced from 20 nm RMS (for as grown films) to below 2 nm RMS (over 5x5 µm²) [1].
Finally, the polishing of (111) and (100) single crystal diamond surfaces will be demonstrated. A reduction in roughness from 0.92 to 0.23 nm RMS and 0.31 to 0.09 nm RMS for (100) and (111) samples respectively was observed.
1. Thomas, E.L.H., et al., Chemical mechanical polishing of thin film diamond. Carbon, 2014. 68(0): p. 473-479.

Recent years have seen a steep increase in possible applications of thin nanocrystalline diamond (NCD) films and particles. While more traditional functions, such as wear resistant coatings, make use of undoped films, most of the currently envisaged applications require electrically conductive material. The possibility to vary the conductivity over many orders of magnitude led to various advanced device concepts based on the unique extreme and tuneable properties of the material [1]. While boron doped NCD films have been available for quasi a decade, latest progress include the development of boron doped nanoparticles and phosphorus doped NCD [2,3].
Focus will be put on the fabrication and use of B-doped films and particles. Thin heavily doped films on glass provide an excellent platform for (electrochemical) functionalization of the diamond surface with dye molecules. The p-type conductivity induced by the boron acceptor turns such films into novel hole conducting electrodes for dye-sensitized solar cell applications, as confirmed by photo-electrochemical measurements [4]. By selectively etching away part of the substrate, ultra-thin NCD membranes can be fabricated, suitable as highly sensitive pressure sensor [5]. Through the application of a differential pressure, the membranes bulge, having direct influence on the electrical transport due to a piezoresistive effect thought to be originating in the grain boundaries. As the boron accepter is well embedded within the grains, thick films provide an excellent starting material for the realization of B-doped nanoparticles. Through a multistep milling process, particles of a few tens of nanometres in size can be obtained that still contain up to ~ 2 x 10²¹ cm^-3 boron. Their use as seeds for thin film deposition opens up a pathway to lower the resistance of the substrate-film interface. During the final part of the presentation, the latest results in the deposition, characterisation, and application of P-doped NCD will be given.
Financial support by the EU 7^th Framework Programme, the Research Foundation Flanders (FWO), and Hasselt University is greatly acknowledged.
References:
[1] R.J. Nemanich, J.A. Carlisle, A. Hirata, K. Haenen, MRS Bulletin 39/6 (2014), 490-494.
[2] S. Heyer, W. Janssen, S. Turner, Y.-G. Lu, W.S. Yeap, J. Verbeeck, K. Haenen, A. Krüger, ACS Nano (2014), DOI: 10.1021/nn500573x.
[3] W. Janssen, S. Turner, G. Sakr, F. Jomard, J. Barjon, G. Degutis, Y.-G. Lu, J. D&’Haen, A. Hardy, M.K. Van Bael, J. Verbeeck, G. Van Tendeloo, K. Haenen, Physica Status Solidi RRL (2014), DOI: 10.1002/pssr.201409235.
[4] W.S. Yeap, X.J. Liu, D. Bevk, A. Pasquarelli, L. Lutsen, M. Fahlman, W. Maes, K. Haenen, ACS Applied Materials & Interfaces (2014), DOI: 10.1021/am501783b.
[5] S.D. Janssens, S. Drijkoningen, K. Haenen, Applied Physics Letters 104/7 (2014), 073107.

Nitrogen-vacancy (NV) centers near diamond surface have potential applications in quantum information processing and nanomagnetometry. Deterministically creating NV centers at specific location and controllable quantity is important for practical device fabrication. Recently, we developed a new creation method based on scanning focused helium ion microscope to create NV centers in bulk diamond with a high spatial resolution and superior spin coherence properties, which is a promising approach to practical NV center based devices.
In this work, we investigated the distribution of NV centers created by focused helium ion beam implantation through experimental characterization using superresolution microscopy techniques and numerical modeling based on a lattice Monte Carlo algorithm. Confocal microscopy and stimulated emission depletion (STED) nanoscopy were used to characterize the intensity and sizes of NV center spots created at different helium ion implantation doses. The characterization results were compared with the numerical model to obtain in-depth understanding on the vacancy diffusion and capture process for forming NV centers.
Specifically, we implanted focused helium ions with an energy of 30 keV into a high-pressure high-temperature synthetic diamond plate with 100 ppm impurity nitrogen atoms. Arbitrary NV-center patterns can be formed thanks to the accurate and high-resolution steering of the sub-nanometer spot of focused helium ions using a pattern generator. In this work, we focus on the implantation patterns of single-pixel dot arrays with a period of 600 nm. After helium ion implantation, the diamond plate was annealed at 900 °C for 2 hours with H₂ and Ar as forming gas, and at 450 °C in O₂ to form negatively charged NV centers.
Through super-resolution imaging and numerical modeling, we found that the NV center spot size is dependent on a number of factors, including the initial nitrogen impurity concentration in the diamond, the implanted helium ion dose, the annealing time, etc. Generally, on our 100 ppm diamond sample, sub-150 nm large NV center spot patterns can be formed near the surface of the diamond plate with a high NV center density. Our new NV-center fabrication method and the investigation on the NV center distiribution will be useful to develop novel NV-center based devices for applications in photonics, electronics and biology.

Nitrogen-Vacancy (NV) centers have been considered as objects of interest for highly sensitive nano-scale sensing applications such as electric field [1], magnetic field [2], or temperature [3] sensing, using their optically detected spin resonance. These sensing capabilities are based on energy changes of the electron spin ground state triplet, and the changes can be readout optically in so-called optically detected magnetic resonance (ODMR). ODMR can also be used to deterministically control the fluorescence of the NV center, allowing for sub-diffraction localization of the center [4]. In this study, we propose and experimentally demonstrate a novel method for far-field super-resolution optical field sensing based on fluorescence detection of the NV centers. Through manipulation of the wavefront of an incident laser using a digital-micromirror device (DMD) [5] and ODMR control of fluorescence of NV centers, we are able to quantify with sub-diffraction resolution the amplitude and phase of the electromagnetic field at the location of NV centers. Our method will motivate applications in the field of bio-photonics that require both of super-resolution and deep tissue sensing and imaging.
Reference
1. F. Dolde, H. Fedder, M. W. Doherty, T. Nobauer, F. Rempp, G. Balasubramanian, T. Wolf, F. Reinhard, L. C. L. Hollenberg, F. Jelezko, and J. Wrachtrup, "Electric-field sensing using single diamond spins," Nat Phys 7, 459-463 (2011).
2. J. M. Taylor, P. Cappellaro, L. Childress, L. Jiang, D. Budker, P. R. Hemmer, A. Yacoby, R. Walsworth, and M. D. Lukin, "High-sensitivity diamond magnetometer with nanoscale resolution," Nat Phys 4, 810-816 (2008).
3. G. Kucsko, P. C. Maurer, N. Y. Yao, M. Kubo, H. J. Noh, P. K. Lo, H. Park, and M. D. Lukin, "Nanometre-scale thermometry in a living cell," Nature 500, 54-U71 (2013).
4. E. H. Chen, O. Gaathon, M. E. Trusheim, and D. Englund, "Wide-Field Multispectral Super-Resolution Imaging Using Spin-Dependent Fluorescence in Nanodiamonds," Nano Lett 13, 2073-2077 (2013).
5. D. B. Conkey, A. M. Caravaca-Aguirre, and R. Piestun, "High-speed scattering medium characterization with application to focusing light through turbid media," Opt Express 20, 1733-1740 (2012).

The remarkable properties of diamond combined with the possibility to exploit some of its point defects in quantum information processing, make this material extremely attractive for quantum computing applications. In particular, the interest is in characterizing and understanding the optical behaviour of paramagnetic colour centres that may be used to realize quantum bits.
In this work we focus our attention on the defects formed by the aggregation of two N atoms with a vacancy, N₂V, in its neutral charge state, also known as H3 photoluminescence (PL) centre. It carries a spin S=0 in its ground state and has C_2v symmetry. We use Density Functional Theory (DFT) to study its optical properties after embedding a single defect in 512-atom diamond supercell. We employ the VASP.5.3.3 code with PAW potentials for modelling the atomic cores and a 370 eV energy cut-off for the plane-wave basis set. The Brillouin zone is sampled at the Γpoint in all our calculations which has shown to provide a well converged charge density. We apply a range-separated, screened, nonlocal hybrid HSE06 density functional which has shown to reproduce a thorough description of levels, band gaps and transition energies of point defects.
According to experiments, the N₂V centre exhibits a prompt luminescence which takes place between the singlet ¹B₁ and ¹A₁ states. This corresponds to a zero-phonon line (ZPL) at 2.463 eV which is observed both in emission and absorption. Time-resolved measurements detect a second recombination channel characterized by about the same ZPL. The lifetime of this transition is found to be of the order of ms and it is temperature-dependent. We can explain this by the triplet ³A₂ where this triplet state is populated through non-radiative transitions from ¹B₁ itself. By means of our calculations, we describe this delayed luminescence as coming from the decay of an electron from the excited ³A₂ to the metastable ³B₂ triplet. In our model, this is equivalent to a transition from configuration b¹₁a₁²b₂¹ to b₁²a₁¹b₂¹ with a calculated ZPL of 2.46 eV, in close agreement with the experimental value.
The presence of the S=0 ground state and a metastable excited spin state make the N₂V centre potentially suited for solid-state quantum technology applications because it is among the sought-after defects that are free from an unwanted electron-spin induced decoherence in the ground state. The singlet ground state ¹A₁ and the metastable triplet ³B₂ state are found close in energy (~0.2eV). Group theory considerations lead to the conclusion that they are linked by spin-orbit interaction transitions. We calculate the zero-field splitting and hyperfine interaction with nitrogen and carbon isotopes with non-zero nuclear spins. We find good agreement with the already known signals, and we identified 13C isotopes for entanglement. We conclude that ODRM signal may be detected for this defect, and an interesting qubit candidate, particularly, as quantum memory.

The nitrogen-vacancy (NV) defect in diamond, consisting of a single-substitutional nitrogen in the carbon lattice with a nearest-neighbor-vacancy, has recently gained much attention. In its negatively-charged state the NV- center possesses an interesting combination of spin and optical properties that lead to remarkable properties that can potentially be exploited in applications such as solid-state qubits, highly sensitive electric and magnetic field probes and single-photon emitters.
The NV- defect forms a two-level quantum system within the bandgap of diamond with an optical transition energy of 1.94eV (637nm). Vibronic coupling to a localized vibrational mode with energy of ~67meV gives rise to a broad (~300meV) phonon sideband in the optical absorption spectrum. Thus, next to its purely electronic transition energy (zero-phonon line, ZPL), higher vibronic peaks following at 2.01eV (617nm, one-phonon line, 1PL) and 2.08eV (595nm, 2PL) are visible in the optical absorption spectrum, which are particularly pronounced at cryonic temperatures.
Most investigations on the opto-electronic properties of NV- have so far employed steady-state optical measurements, possibly combined with low-frequency modulation schemes, or pulsed lasers that provide time resolutions of nanoseconds at best. A great deal of information on the electronic structure not accessible by such techniques can however be gained by investigating the femtosecond response of the optical spectrum after electronic photo-excitation. Only very recently a publication has addressed this by measuring the ultrafast response of NV- using femtosecond four-wave-mixing spectroscopy.
We have performed femtosecond transient absorption measurements on the vibronic spectrum of NV- as a function of temperature (10K-300K) and excitation wavelength (using narrowband pulses to selectively excite the ZPL, 1PL and 2PL). The obtained femtosecond dynamics of the isotropic signal and the anisotropy provide valuable information on the electronic structure of the excited state and its relaxation behavior after photo-excitation.
In the isotropic signal, related to population dynamics, we observe vibrational relaxation in the excited electronic state within less than 50fs, a manifestation of the strong electron-phonon coupling.
The anisotropy shows bi-exponential dynamics in the stimulated emission, indicating that they are originating from the excited state. Both time constants increase with decreasing temperature, pointing to a temperature-deactivated mechanism behind it. Furthermore, the long decay component only emerges at temperatures below 150K and reaches time constants of several picoseconds.
We report on our observations and their implications concerning excited state related phenomena such as vibrational and electronic dephasing and electronic motion at the conical intersection caused by a Jahn-Teller distortion.

2 dimensional hole gas (2DHG) produced by surface hole accumulation at diamond surface can be applied to high frequency and high voltage power device. There are following two factors which satisfy surface hole accumulation, 1) Hydrogen-termination. 2) Surface or near surface site to induce holes. Both factors are thought to be unstable in air exposure. However, these factors can be preserved by appropriate passivation. Atomic layer deposition (ALD) of Al₂O₃ has been applied to the H-terminated (C-H) diamond surface without destroying two factors [1,2]. Recently, we have used the same method, but at much higher deposition temperature such as 450°C to form Al₂O₃ on H-terminated surface by alternative Al(CH₃)₃ and H₂O supply. The surface hole accumulation becomes so stable and reproducible that the sheet resistivity of 10⁴ ohm/square, the hole density of 10¹³cm^-2 and the hole mobility of 50cm²/Vsec, typical for 2DHG on diamond surface were observed up to 500°C at least [3]. The field effect transistor (FET) operation has been measured up to 400°C [4]. These temperature are not limited by the above two factors, but by measuring systems. One of the reasons for high temperature operation is the high temperature passivation stability of Al₂O₃ formed at 450°C compared with those deposited at lower temperatures.
1) The stability of C-H diamond surface. In UHV condition, C-H bond is stable up to 700°C. With a perfect passivation, H-termination is preserved up to the temperature in air. However, forming the oxide at high temperature without deteriorating C-H bond, some kind of preserving mechanism for C-H bond during oxidation of aluminum is needed. H₂O oxidation has a role for C-H preservation. Generally, CH₄ +H₂O > CO + 3H₂ (-142 kJoule/mol) is endothermic reaction, so the C-H bond can be preserved even at 450°C.
2) The stability or formation of active site for hole accumulation. Just before ALD starts our H-terminated diamond (001) surface suffers 450°C heat treatment without oxygen. The surface adsorbates proposed to be surface acceptor [2] can be removed from the surface. We speculate that Al₂O₃ is deposited on H-terminated surface without those adsorbatea and some special sites within Al₂O₃ has a role for hole accumulation. Generally ALD Al₂O₃ thin film tends to be negatively charged by fixed charges in the film. In silicon, electrons are repelled and holes are accumulated at Al₂O₃/Si interface. Field effect caused by those fixed charges could be an important role to affect the carrier density of surface channel.
[1] D. Kueck, E. Kohn et al., Diam. Relat. Mat. 19 166 (2010).
[2] M. Kasu et al., APEX 5, 025701(2012).
[3] A. Hiraiwa, H. Kawarada et al. J. Appl. Phys. 112, 124504 (2012).
[4] H.Kawarada, H. Tsuboi,T. Yamada, A.Hiraiwa et al., Appl. Phys. Lett. 105 (2014, in press).

The unique chemical and mechanical stability of diamond based materials can be exploited in a variety of applications including electrochemical sensing, energy conversion and water remediation. The performance of these dimensionally stable electrodes can be significantly improved by generating highly corrugated electroactive surfaces. This contribution describes the preparation and properties of high surface area boron doped diamond (BDD) and diamond-like carbon (DLC) films grown by chemical vapor deposition (CVD) onto arrays of vertically aligned carbon nanotubes. Scanning electron microscopy, conducting atomic force microscopy and dynamic electrochemical measurements clearly demonstrate that the controlled diamond growth over the nanotubes templates leads to effective surface areas over two orders of magnitude larger than conventional CVD grown diamond films.
Vertically aligned multiwall carbon nanotubes (VACNT) were grown by plasma enhanced CVD on either Si (100) wafers or Ti substrate in the presence of Ni catalysts. The length and number density of nanotubes can be varied according to the growth conditions. In these studies, we shall contrast the behavior of two types of arrays characterized by different mean tube lengths (5 and 40 mm) and surface number densities (1.0×10⁹ and 2.5×10¹⁰ cm^-2). The growth of BDD onto the VACNT arrays was performed in a hot filament CVD reactor with a gas mixture of 1% CH₄/H₂ in the presence of B₂H₆ for a period of 1.5 h. Prior to the BDD growth, a suspension of 5 nm diamond nanoparticle seeds were electrosprayed onto the VACNT arrays, leading to a partial loss of the nanotubes alignment. Depending on the average length of the nanotubes, the resulting BDD structures exhibit “teepee” shapes or extended ridges with a high degree of corrugation. Detailed electrochemical studies employing cyclic voltammetry and electrochemical impedance spectroscopy revealed that electroactive surface area of the BDD electrodes were between 150 and 450 times larger than in non-templated diamond electrodes grown under identical conditions [1].
The second system investigated involved DLC films obtained by plasma-enhanced CVD in the presence of hexane and Ar. The DLC:VACNT composite also showed a highly corrugated topography with extended ridges and partially collapsed portions of the film. The striking property of this material is the high electrochemical activity in the absence of any dopant. By contrast, DLC films grown under identical conditions on Si wafers led to essentially insulating layers. The potential impact of these novel high surface area diamond electrodes in areas such as energy conversion and sensors will be briefly assessed.
Acknowledgements. We are indebted to our partners Eduardo Corat (Brazil&’s National Institute for Space Research) and Sara Vieira (University of Cambridge) for kindly providing the VACNT arrays used in this work.
[1] H. Zanin et al., ACS Appl. Mater. Interfaces, 6 (2014) 990.

Diamond is an outstanding material for thermal management of electronic devices due to its large thermal conductivity (ge;2000W/mK for single-crystalline diamond), which is attracting an increasing attention from the semiconductor device industry. However, for enabling commercial use, polycrystalline rather than single-crystalline diamond needs to be employed; furthermore, the diamond should be grown as close as possible to the heat source in semiconductor devices, for example the electron channel in an AlGaN/GaN transistor. This makes the impact of the micro/ nanostructure of the diamond on thermal conductivity near the seeding site critically important, however extremely little is known about its properties. In this work, we demonstrate a new technique to measure the lateral and vertical thermal conductivity of the polycrystalline diamond near its seeding layer, showing the crucial role played by the grain boundaries in the thermal transport at nanoscale.
We developed a novel thermal conductivity measurement technique based on Raman thermography assisted by silicon nanowires as surface nano-thermometers. In combination with micro-heating structures, this technique allows us to characterize the pure lateral and vertical component of the thermal conductivity of the ultrathin nanocrystalline diamond. Nanocrystalline diamond (NCD) layers with columnar structure and thickness ranging from 320nm to 1mu;m, and average grain sizes at surface from 100nm to up 200 nm grown by hot filament were studied, with their internal structure fully characterised by TEM. We found a strong correlation between the lateral grain size and the lateral thermal conductivity. We determined lateral thermal conductivities in the range of 90-140 W/mK for NCD films of le;1 micrometer thickness, which is at least one order of magnitude lower than the bulk limit. In addition, we observed anisotropy in excess of 2:1 between the vertical thermal conductivity and the lateral thermal conductivity of the NCD layers, evidencing that the structural anisotropy in the grain boundary distribution leads in anisotropy in the physical properties of the diamond. The important role played by the phonon scattering at the grain boundaries allows to tune the lateral thermal conductivity by modifying the average lateral grain size. This was demonstrated by comparing samples deposited under different growth conditions. A difference of more than a 50% on lateral thermal conductivity was found by increasing the average grain size at surface by a 25%.

For many years there has been interest in creating n-type semiconducting diamond, and despite various theoretical possibilities being modelled, n-type diamond with useful electronic properties has remained elusive. The most successful candidate is phosphorus; this shows n-type semiconductivity but low carrier mobility, and this is a problem for its use in many devices.
An alternative n-type dopant is lithium, but little work has been reported on this. Recent results from our group have shown that high levels of Li can be incorporated into diamond using Li₃N as a solid source of Li. Co-doping with nitrogen is believed to prevent the lithium from aggregating into inactive clusters, however, even with high levels of both N and Li present in the film the diamond remained electrically inactive.
Due to its diagonal relationship with lithium on the Periodic Table, magnesium may also be a potential n-type dopant for diamond. We now show that Mg, too, may be incorporated into diamond using a solid source (Mg₃N₂) in significant quantities. Being larger than C, substitutional Mg can cause distortion or strain in the diamond lattice, hindering Mg incorporation. This strain can be partially relieved by simultaneous incorporation (co-doping) of a small atom (e.g. N or B) situated near the Mg to allow the lattice to relax.
Here, we report the results from studying the use of Li and Mg as potential dopants in diamond, whilst simultaneously co-doping with either N or B In order to alter the electrical properties of the films. Secondary ion mass spectrometry depth-profile analysis has been used to detect dopant concentrations and thickness of dopant layers within the film.

Hybridization of the optoelectronic properties achieved by an assembly process of gold and diamond nanoparticles can induce a significant enhancement of the electric field properties with far-reaching implications for the applications in plasmonic, biosensing, and nanomedicine. The essential requirement in tuning the optical properties is the ability to control the uniformity of the size, shape, and interfacial contact of the nano-hybrid-crystallite. Innovative synthesis method to fabricate gold-nanodiamond avoiding chemicals potentially toxic is reported.
The nanocrystalline diamond used in our laboratory is produced by explosions of carbon precursors and it is called “detonation nanodiamonds” (DND). In addition to the classical properties of bulk diamond, this nanostructured material, characterized by polyhedral shapes, remarkably small average sizes (4-6 nm), and a complex core-shell structure, exhibits a number of supplementary outstanding characteristics and it has recently emerged as the most promising nano materials that can find applications in biomedical researches.
A proper modification of DND surface makes it able to reduce gold complex in nanoparticles of metallic gold. The self-aggregated Au-NDs nanoparticles are the perfect candidate for real applications in current medical and biological research areas. In fact the established biocompatibility, the rich surface chemistry coupled with a very large surface area makes DND a suitable candidate for drugs delivery systems. Gold nanoparticles on their surface could allow to detect information on molecules states, mechanism release, chemistry interaction and more all. Finally Au-NDs could be our SERS probe in biological environment and at the same time could be able to shuttle molecules and detect mechanism.
In this presentation we will report some results regarding the SERS measurement of different kinds of Au-ND systems coupled with several chemicals: rhodamine, cyanide, cumarine and quercitine.

Nanodiamond (ND) has been considered as a biocompatible and feasible platform for efficient cancer drug delivery. The surface properties, chemical stability and biocompatibility make it a convenient base for drug loading and delivery. Examples of such bio/medical applications as well as ND&’s non-toxicity have been successfully demonstrated for wide number of cells, including different cancer cells and normal cell lines; or in a first order animal model. However, to date, very few or none studies had included the assessment on the efficiency in a quantitative fashion; and the transportation these ND-drug to the cancer/tumor sites are still in a less understood state.
In this work, an attempt to study the possible ND-drug transportation in the animal model is proposed and investigated. In vivo investigation of ND in Rat model was performed. Blood of rat was then drawn to observe the interaction of the ND in the blood circulatory system. The efficiency of the ND-drug as compared to pure drug is evaluated in the cellular model. To these, various sizes of ND were used. A chemical linking method was developed to conjugate the clinical use anticancer drug Paclitaxel (Taxol) on nanodiamond surface. Different methods were used to characterize the conjugation, including Raman and infrared spectroscopies. A Thermogravimetric Analyzer (TGA) was used to estimate the surface loading of the drug on ND. The apoptosis effect of 3-5 nm ND-Taxol conjugate on Melanoma cells was studied in vivo. Cell viability test revealed at the same concentration, ND-drug is more efficient (~20 times) to induce apoptosis than pure Taxol for Melanoma cells in the cellular model studies. A possible mechanism of the drug transportation and delivery will be discussed in the work.

The generation of negative ions from low work function surfaces can play a significant role in the direct conversion of heat into electricity through thermionic energy conversion. A negative ion current can then contribute to the thermionic electron current increasing the power output of a thermionic energy converter. We have prepared nitrogen incorporated ultra-nanocrystalline diamond, (N)UNCD, based thermionic emitters for surface ionization of atomic hydrogen and ammonia. The nanostructured diamond films were evaluated by thermionic electron emission and the emission current was described in terms of the Richardson - Dushman relation. This formalism allows extraction of the work function and the materials Richardson constant. A low work function induced by the negative electron affinity, defect and donor states allows efficient ionization of atomic hydrogen and ammonia. The ion current can then be described by the Saha - Langmuir formalism which relates the negative ion fraction of the desorbed species to the electron affinity of the molecule and the work function and temperature of the surface. The Richardson constant quantifies the materials ability to establish a certain electron emission current and can vary widely for various thermionic emitter materials. (N)UNCD based emitters with various Richardson constant have shown to affect the magnitude of the surface ionization current over a temperature range used for thermionic electron emission. An advantageous effect was observed for materials with higher Richardson constant. This effect was more pronounced at elevated temperatures where a lower Richardson constant material exhibited a greater deviation from the law of Saha -Langmuir. We will discuss the observed surface ionization effects in terms of the Richardson constant of the thermionic emitter. Its applicability will be demonstrated in a device for the direct conversion of heat into electricity.
This research is supported by the Office of Naval Research.

Diamond is a unique material in many respects. One of the extreme properties of diamond is its ultrahardness. This property of diamond turns out to have interesting consequences for charge transport, in particular at low temperatures. In fact, the strong covalent bonds that give rise to the ultrahardness results in a lack of short wavelength lattice vibrations which has a strong impact on both electron and hole scattering. In some sense diamond behaves more like a vacuum than other semiconductor materials. At low temperatures, we have observed an intriguing electronic transport phenomenon in high-purity CVD diamond^1,2. In electron drift velocity measurements and Hall angle measurements below 80 K the electrons appear in six different valley-polarized states with different transport properties. The reason for this is that electrons fall into one of the six conduction band valleys of diamond with very low intervalley scattering rate. This low scattering rate is unique to diamond, and is due to the lack of short wavelength lattice vibrations in the extremely rigid diamond lattice. This opens up the possibility to use diamond in "Valleytronics", i.e., electronic circuits where the valley polarization is used to carry information. In this paper we describe an electrostatically controlled “valley valve” or “valley field-effect transistor” that enables direct control of the degree of valley polarization. This was made possible by the development of an effective surface passivation to diamond that allows for lateral valley transport in diamond layers, making it possible to develop complex valley devices and to perform lateral transport experiments with valley polarized electrons.
1. J. Isberg, M. Gabrysch, J. Hammersberg, S. Majdi, K. K. Kovi, D. J. Twitchen, "Generation, transport and detection of valley-polarized electrons in diamond", Nature Materials 12, 760-764 (2013)
2. J. Hammersberg, S. Majdi, K. K. Kovi, N. Suntornwipat, Markus Gabrysch, D. J. Twitchen and J. Isberg, "Stability of polarized states for diamond valleytronics", Appl. Phys. Lett. 104, 232105 (2014)

Lithium deposition on oxygen-terminated diamond (100) followed by thermal activation has been shown to lead to true negative electron affinity, with the resulting surface robust and insensitive to contamination [1]. Consequently, lithiated diamond holds promise for novel electron emission applications outside ultra-high vacuum. In this talk I will detail the surface preparation methods and present high-resolution photoemission measurements that reveal the surface chemistry of lithiated diamond. Although the chemistry generally agrees with our theoretical predictions [2] the need for thermal activation is unexpected and I will discuss this in the context of Li-O-C complexes at the surface. I will also compare the properties of lithiated diamond to the better-known hydrogen-terminated diamond surface. For example, the much larger band bending of boron-doped lithiated diamond gives a window into the bulk behaviour of hot carriers in diamond.
[1] K.M. O'Donnell, M.T. Edmonds, J. Ristein, A. Tadich, L. Thomsen, Q.-H. Wu, C.I. Pakes, and L. Ley, Adv. Funct. Mater. 23, 5608 (2013).
[2] K.M. O'Donnell, T. Martin, N.A. Fox, and D. Cherns, Phys. Rev., B Condens. Matter 82, 115303 (2010).

Ultrananocrystalline Diamond (UNCD) films grown by microwave plasma chemical vapor deposition (MPCVD) exhibit a unique set of multifunctionalities and properties for potential applications to mechanical, electronic, chemical and bio-medical devices, respectively. These properties include: a Young&’s Modulus and hardness equivalent to single crystal diamond, the lowest coefficient of friction among known thin films, electrical conductivity with nitrogen incorporated in grain boundaries or boron substituting C atoms in the diamond lattice, and outstanding biocompatibility.
One of the important issues related to the synthesis of UNCD films for application to the development of a new generation of multifunctional devices is achieving thickness and nanostructure uniformity over large area substrates (ge; 4 inch in diameter) with 3-5 nm grain size. Here, we report advances on the synthesis of UNCD films over large area substrates using Ar-rich/CH₄/H₂ chemistry via hot filament chemical vapor deposition (HFCVD). We demonstrated that the ratio of Ar/H₂ in the range 70/30 sccm to 90/10 sccm yield films with grain size from 10-30 nm for NCD films to 3-5 nm for the UNCD films. The Ar content appears to be critical to achieve the characteristic UNCD film nanostructure with roughness and chemical bonding uniformity in the range of 3-5 nm, verified with HRTEM, Raman spectroscopy, and XPS. We also investigated the surface morphology using SEM and AFM, which revealed an extremely smooth surface (~ 3-5 nm rms) for films grown with Ar (90 sccm)/H₂ (10 sccm). The role of Ar, CH₄ and H₂ relative proportions, in the gas mixture used to grow HFCVD UNCD films as well as the effect of the substrate, will be discussed in relation to their impact to get the nanoscale grains characteristic of UNCD films produced by the MPCVD method. In addition, new results will be discussed, which indicate that electrically conductive UNCD films can also be grown using the HFCVD technique, presumably via nitrogen incorporation into the grain boundaries as previously demonstrated for MPCVD.
This development of both insulating and electrically conductive UNCD films on large area substrates, using the HFCVD process, can contribute to accelerate the practical applications of the UNCD films to a new generation of high-tech devices/systems and medical devices.

The crystal quality of CVD diamond is compromised by the presence of dislocations which leads to internal stresses. In heteroepitaxially grown diamond, mismatch between the thermal expansivities of the substrate and diamond will also contribute to thermally-induced stress. The restrictive palette of suitable lattice-matched substrates and buffer layers suitable for diamond growth has frustrated significant advances in structural perfection. Epitaxial lateral overgrowth (ELO) is frequently used as a means for impeding the propagation of threading dislocations into the growing material. A number of variants have been used for III-V semiconductor growth with significant success. However, it has not been widely studied in heteroepitaxial diamond growth owing to the aggressive chemical and thermal environment in a CVD reactor. We describe an ELO variant that utilizes a metal mask patterned on the surface of a thin diamond (100) layer. The layer is grown by microwave plasma CVD on 1 cm² a-plane sapphire with Ir (100) as buffer layer. The metal mask, a series of parallel stripes, is produced by vacuum evaporation and standard lift-off photolithography. The primary characterization tool is spectroscopy of the 1332 cm^-1 Raman-active vibrational mode. In the early stage of ELO growth, a 2 µm thick crystal with 20 µm spatial modulation shows a 30% reduction in linewidth as it overgrows the mask. The internal and thermal contributions to the stress are distinguished by studying a region of the film that delaminated from the substrate. The linewidth of overgrown material is essentially unchanged but the line center shifts to 1332 cm^-1, the value for unstressed diamond. In a 30 µm thick diamond crystal the linewidth of overgrown vs non-overgrown diamond is reduced by a factor of 2. In a 75 µm thick film the linewidth at the growth surface is reduced to within 1 cm^-1 of the best natural diamond. We shall discuss the prospects for further improvements in heteroepitaxial diamond crystal quality.

Diamond heteroepitaxially grown on iridium substrates by the CVD technique is a promising material for the next generation of electronic and radiation detection applications [1]. The upscaling possibilities of the growth process are particularly interesting for producing large diamond surfaces, compatible with microelectronic technologies. However, today the performances of devices are limited by some important problems such as the residual mosaicity and the presence of a high density of dislocations in the range of 10⁷-10¹⁰ cm^-2 [2]. These extended defects are known to induce strain in diamond and can affect the electron-hole mobility-lifetime product and the charge collection efficiency of detectors [3]. In fact, a careful control of the growth conditions is required to avoid the formation of these defects and then improve the quality of diamond.
The objective of this work is to study dislocations and strain in heteroepitaxial diamond films grown on iridium/SrTiO₃ (001) substrates using cathodoluminescence (CL) at 10K. The growth was performed in two steps: First, 60 nanometer-thick diamond layer was obtained on iridium after a bias-enhanced nucleation protocol [4]. Then the films were grown at high rate to achieve 100-200 µm self-standing diamond plates. Since dislocations are known as non-radiative centers, we performed CL images of the free exciton recombination intensity in order to detect them. Dark spots are observed in plane-view images and are attributed to emerging dislocations. Cross-sections were also prepared to follow the dislocation propagation along the growth axis. Furthermore, we observed a splitting of the free exciton peak which is associated to the residual strain in the diamond samples grown on iridium. This result was confirmed by X ray diffraction (XRD) measurements. The origin of dislocations and strain will be discussed.
[1] M. Schreck et al., in “CVD diamond: research, applications and challenges”, MRS Bulletin 39, 504-510 (2014).
[2] C. Stehl, et al., Applied Physics Letters 103, 151905 (2013)
[3] E. Berdermann, et al., Diamond and Related Materials 19, 358-367 (2010)
[4] N. Vaissiere, et al., Diamond and Related Materials 36, 16-25 (2013)

Single crystal CVD diamond has noticeable advantages over conventional semiconductor materials and allows devices with a higher operating temperature, electric power, and radiation resistance to be developed. However, one of the main factors preventing its wide application and the advent of diamond electronics is the small geometric dimensions of diamond substrates on which epitaxial growth occurs. Currently, the technology for fabricating electronic devices on silicon is commercialized for substrates up to 300 mm in diameter. Recently, a Japanese team succeeded in fabricating two inches mosaic crystal diamond wafers [1]. However, the dimensions of these, although large, wafers are still far from the dimensions of silicon wafers. Therefore the problem is how can a single-crystal diamond of the small geometric dimensions be used as a material for fabricating electronic devices in a large-scale technological process? We suggested the method of solving this problem [2]. It is well known that polycrystalline diamond wafers of six inches in diameter are successfully grown in microwave plasma-assisted reactors. Therefore the wafers of polycrystalline diamond with single-crystal diamond inclusions can be produced. Such combined diamond wafers will have a polycrystalline wafer diameter up to 150 mm and contain a large number of ingrown rectangular (or circular) small CVD diamond single crystals (with geometric dimensions from 1×1 mm to 3×3 mm). Processing lines which are already developed for silicon technology can be used to fabricate electronic devices on the surface of single crystals of such 200-500 mu;m thick diamond wafers.
In this paper the different methods of producing the combined wafer are discussed. The experimental results of wafer growth of 50 mm and 75 mm in diameter with 20-100 single crystals are presented. For producing the combined wafers the HPHT substrates with dimensions 1.5×1.5×0.5 mm³ were used. The wafer was synthesized in 2.45 GHz CVD reactor operating with H₂ and CH₄ gas mixture at high pressures (150-200 Torr) and with high microwave power densities. We determined the conditions under which high-quality single-crystalline and polycrystalline diamond films are deposited. The grown diamond was characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The morphology of the surfaces as well as the stress in the single crystals as grown and after annealing of the wafer were investigated.
[1] H. Yamada, A. Chayahara, Y. Mokuno, Y. Kato, S. Shikata, Appl. Phys. Lett., v.104, 102110 (2014).
[2] A. L. Vikharev, A. M. Gorbachev, M. P. Dukhnovsky, A. B. Muchnikov, A. K. Ratnikova, and Yu. Yu. Fedorov, Semiconductors, v.46, No. 2, 263-266 (2012).

Diamond surfaces may obtain a negative electron affinity (NEA) by exposure to a hydrogen plasma. This enables a low effective work function and high electron emissivity for n-type doped diamond films, which have led to applications in thermionic energy conversion (TEC) devices. However, removal of hydrogen from the diamond surface at temperatures above 800°C has limited the efficiency of diamond-based TECs. Termination of the diamond surface with a thin layer of metal or metal oxide has been found to affect the surface electron affinity, and these techniques are currently under investigation for the possibility of providing a thermally stable NEA. In this work we present an in-situ photoelectron spectroscopy study of the electronic structure of vanadium-oxide-terminated diamond surfaces. Thin layers of vanadium with a thickness of ~ 0.1nm were deposited on oxygen-terminated, boron-doped and nitrogen-doped polycrystalline diamond samples by employing an electron beam deposition system. Vanadium oxide was formed immediately after deposition. The resultant surfaces were examined through ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS) analysis. After oxide formation and 650°C thermal annealing, the effective work function was significantly decreased, which indicates a negative electron affinity on boron-doped diamond, while a 1.1±0.2 eV positive electron affinity was found on nitrogen-doped diamond. We argue that this is likely resulted from the difference in the interface barrier at the diamond-oxide interface.
This research is supported through the Office of Naval Research under grant number # N00014-10-1-0540, and the National Science Foundation under grant # DMR-1206935.

#12288;Large-area diamond substrates are needed to realize low-loss high-power devices and highly sensitive area sensors based on NV centers. Heteroepitaxial growth of diamond on large size substrates is important for this purpose. In particular, diamond growth on a buffer layer composed of abundant elements is the key to establish diamond technology over long periods of time. In this study, we focus on the heteroepitaxial growth of diamond films on 3C-SiC/Si substrates by using a unique antenna-edge microwave plasma chemical vapor deposition (AE-MPCVD) system. AE-MPCVD has the following features: (i) Highly dense plasmas can be obtained since the microwave power is concentrated at the tip of the antenna, leading to a fast nucleation rate. (ii) Pressure, temperature and microwave power can be independently controlled to allow for optimal film growth.
A Si(001) wafer with a buffer layer of a 3C-SiC(001) (4 mu;m in thickness) deposited by low-pressure CVD was used as the substrate [1]. We have developed a combination process of bias-enhanced nucleation (BEN) and 2 step diamond growth [2]. First, diamond crystals were nucleated by BEN with a negative voltage of -50 V. The nucleation region was 7 mm in diameter which corresponds to the spherical plasma area formed at the antenna tip. Then, diamond nuclei were preferentially grown in the <001> direction (α parameter > 1) (1^st step growth). The diamond grains possess a pyramidal shape with exposed (111) faces after the <001> preferential growth process. In this process, tilted diamond grains are buried, and thus we have only aligned diamond grains. Finally, the selective growth in the <111> direction (α parameter < 1) was performed to obtain a continuous film (2^nd step growth). During this process, diamond grains merged with neighboring grains. These growth directions were controlled by methane concentration. The methane concentrations were 1% and 0.5 % in the <001> and <111> preferential growth conditions, respectively.
A ring diffraction pattern was observed from RHEED after the BEN treatment. This means the diamond nuclei are not totally aligned on the 3C-SiC(001) surface. However, the RHEED pattern changed to diffraction spots from the ring diffraction pattern after the <001> preferential growth process, indicating that the surviving diamonds grains were aligned on the 3C-SiC(001) / Si(001) surface. Highly oriented diamond films were then obtained after the <111> preferential growth step, although grain boundaries were still observed. Longer growth under the <111> preferential growth will lead to higher quality films. The results of this process will be reported.
[1] M. Reyes, et al., Spring Materials Research Society Meeting Proceedings, Vol. 911, pp. 79 (2006).
[2] T. Suesada, et al., Jpn. J. Appl. Phys. 34 (1995) 4898.

Laser-assisted combustion chemical vapor deposition (CVD) of diamond was studied using a wavelength-tunable CO₂ laser. The CH₂-wagging mode of ethylene (C₂H₄) precursor molecules is strongly infrared active and perfectly matches the emission line of the CO₂ laser at 10.532 µm. On- and off-resonance excitations of molecules were performed via tuning the incident laser wavelengths centered at 10.532 µm. With the same amount of laser power absorbed, the on-resonance vibrational excitation allowed a largest fraction of the absorbed laser energy coupled directly into C₂H₄ molecules whereas energy coupling under off-resonance excitations is less efficient in influencing the combustion process. The diamond deposition rate was enhanced by a factor of 5.7 accompanied with an improvement of diamond quality index under the on-resonance excitation at 10.532 mm. The calculated flame temperature and the variation of excited species concentrations characterized using optical emission spectroscopy indicate that the resonant vibrational excitation is an efficient route to coupling energy into the reactant molecules to surmount the chemical reaction barrier and steering the combustion process to favor the diamond formation.

Ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) is a new candidate carbon semiconductor applicable to electrical devices. It has been investigated that nitrogen-doping for diamond is ineffective for realizing n-type conduction at room temperature, because nitrogen form a deep donor level of 1.7 eV below the bottom of a conduction band in diamond. For a-C:H, n-type conduction is realized by nitrogen doping, however it is difficult for the carrier density to be controlled widely. On the other hand, it has been reported that nitrogen doping is effective for UNCD/a-C:H prepared by chemical vapor deposition (CVD) and pulsed laser deposition (PLD)^1,2), the doping mechanism for UNCD/a-C:H seems to differ from those for diamond and a-C:H. In this study, UNCD/a-C:H films were prepared at different inflow ratios of nitrogen and hydrogen (I_N/H) by coaxial arc plasma deposition (CAPD), and the effects of nitrogen doping on the chemical bonding structure of the films were structurally investigated. X-ray photoemission spectra (XPS) measurements with Mg Kα line exhibited that N1s peaks strengthen with an increase in the I_N/H, which evidently indicates that the nitrogen content in the film increased with the I_N/H. The nitrogen contents of the films deposited at I_N/H of 0.3, 0.5, 1.0 and 1.5 were estimated to be 3, 5, 6 and 8 at. %, respectively. Near edge X-ray absorption fine-structure (NEXAFS) spectra of the nitrogen-doped films showed an obviously different spectral profile from those of the undoped films. With increasing nitrogen content, p*C=N peaks are strengthened while s*C-C peaks seem to be weakened, which might be because the electrical conductivity of the films is enhanced with increasing nitrogen content. The details will be reported at the conference.[1] Dai, et al.: Phys. Rev. B 71 (2005) 075421. [2] T. Yoshitake, et al.: Jpn. J. Appl. Phys 49 (2010) 015503.

Whereas diamond has been widely used in many industrial fields such as cutting tools and optical lenses, highly pure quality single crystal diamonds applicable for electronic applications and binder-less nano-polycrystalline diamonds (NPD) are relatively new and have unclear properties such as optical properties, thermal conductivities, and mechanical properties.
Recently we have synthesized chemically and isotopically pure, ultra-highly pure, single crystal diamonds with 99.995% ¹²C containing less than 1 ppm of other chemical impurities by a temperature gradient method under high pressure (5.5 GPa) and high temperature (1350 C) condition (HPHT). We used Fe-Co alloy as a solvent in which diamond will be synthesized, with 1.5% of Ti dissolved in it. Whereas an ultra-highly pure NPD with 99.995% ¹²C was directly converted from graphite with 99.995 % ¹²C containing less than 1 ppm of other chemical impurities into diamond under 15 GPa and 2300 C.
We have evaluated the crystalline quality of the single crystals by magnetic resonance of nitrogen center, P₁. Pulsed echo electron spin resonance indicated that the full width at half maximum ΔB_1/2#12288;of the P₁ (nitrogen) center of the synthetic diamond is 0.036 Gauss (ΔBpp#65374;0.02 Gauss), which is the finest value observed thus far of HPHT diamonds. Furthermore one of the ultra-highly pure diamonds were irradiated with electron beams followed by annealing, and showed long spin coherence time T₂ = 1.2 ms of NV^- center signal by an optically detected magnetic resonance measurement (ODMR) as a result of not only chemical and isotopic purification but also its good crystalline quality.
Although, the ultra-highly pure single crystal was colorless transparent, the ultra-highly pure NPD was yellowish transparent. Single grain of the ultra-highly pure NPD can be regarded as an ultra-highly pure single crystal of diamond. Therefore, we expected that the ultra-highly pure NPD has colorless transparent appearance. Despite the fact that the measured amount of impurities of the NPD was comparable to that of ultra-highly pure single crystal, the ultra-highly pure NPD showed broad absorptions near the band edge and at around 730 nm. The peak at around 730 nm is distinctive to highly purified NPDs and we assume that this is caused by defects and imperfections of bonding at grain boundaries.
Thermal conductivities of ultra-highly pure single crystal and NPDs were 20% higher than only chemically pure single crystal and NPDs for both. Young&’s modulus of ultra-highly pure single crystal and polycrystalline diamonds were about 5% higher than only chemically pure diamonds as well, resulting in 5-10% higher Knoop hardnesses indicating that isotope can be an origin of a deformation as well as chemical impurities. We also will discuss the relationship between hardness and grain sizes for NPDs converted from a variety of carbon sources.

The presence of defects in crystalline diamond may affect the device performances. The low breakdown voltage and leakage current are serious issues from the electronic point of view. X-ray topography of high pressure high temperature (HPHT) type-IIa diamond [1] and of CVD diamond [2] has been discussed; however, the types of dislocations are not well understood. Here, we investigate crystalline defects of HPHT type-IIa and CVD diamond single crystals.
HPHT-grown high-pure type-IIa (001) diamond single crystal and CVD (001) diamond single crystal were observed by synchrotron X-ray topography. For HPHT diamond, we took and compared X-ray topography projection images for different g= [±2±20], and analyzed in terms of the extinction rule, i.e., b#12539;g=0. Consequently we determined the dislocation&’s Berger&’s vector. Dislocations run in [001] or <112> direction. For CVD diamond, we observed bundles of dislocations which start from the epitaxial layer/ substrate interface and extend in the [001] direction.
[1] H. Sumiya and K. Tamasaku, Jpn. J. Appl. Phys. 51 (2012) 090102.
[2] Y. Kato, H. Umezawa, H. Yamaguchi, and S. Shikata, Jpn. J. Appl. Phys. 51 (2012) 090103.

Sub-bandgap photon illumination has been observed to modify the secondary-electron emission (SEE) measured from the hydrogenated surface of undoped CVD diamond films. However, no observable photo-effect is detected in SEE measurements taken from similar B-doped CVD diamond films. Moreover, in the case of undoped CVD diamond, the photo-enhanced SEE behavior differs significantly for single-crystal and polycrystalline diamond. The photo-enhanced SEE was first observed from an 8.3-micron-thick, high-purity, single-crystal CVD diamond flake. Upon illumination with blue light (3.06 eV), the SEE current increased with increasing laser power P_o, although the light did not produce photoemission (PE) directly. In energy distribution curves (EDCs), the emission-onset energy E_onset remained constant regardles of P_o or electron beam current I_o, suggesting that the surface Fermi level is pinned. However, the high-energy side of the emission peak changed in shape as a function of increasing P_o. Analysis of these changes in the energy spectra suggests that the sub-bandgap illumination causes the internal band levels to shift, thereby reducing upwards band bending near the pinned surface.
Similar measurements were next taken from an undoped, 650-nm-thick, polycrystalline CVD diamond membrane. Such a thin membrane was used to permit examination of the SEE characteristics in both reflection and transmission measurements. Upon photon illumination, the reflected SEE peak intensity increased with increasing P_o while E_onset remained constant, as observed with the single-crystal flake. However, in this case, a distinct (but very low intensity) PE peak was observed under photon illumination alone, with E_onset(PE) being ~ 0.5 eV above E_onset(SEE). In transmission, the situation was very different. Without photon illumination, a transmission peak was detected that was higher in energy than the PE peak. Specifically, E_onset was at least 1 eV above E_onset(PE), although the peak characteristics varied greatly with I_o. Upon photon illumination, the transmitted current increased but, unlike the reflection measurements, the emission peak shifted down in energy. In fact, as P_o increased, the low-energy side of the EDC grew in intensity such that a shoulder, plateau, or even additional peak emerged that extended down down to E_onset(PE). Measurements will be presented that show distinct emission trends as a function of incident beam energy, I_o, and P_o, and analysis will be used to correlate the emission behavior to changes in the surface and bulk electronic properties. Finally, analysis is underway to identify possible photon absorption mechanisms associated with these emission changes, and to better understand the material properties that may influence electron emission behavior.

The photocathode is a key component of the electron injectors in synchrotrons, free electron lasers, linear accelerators (linacs) and ultrafast electron systems for diffraction and imaging. Semiconductor photocathodes report the highest quantum efficiency (QE) in the literature. Activation of heavily doped p-Si or GaAs surfaces with Cs has resulted in photocathodes with negative electron affinity (NEA). Such NEA photocathodes are bright electron sources with high QE and low emittance. However, the drawback of such alkali metal photocathodes is the requirement of a base vacuum pressure < 10^-10 Torr for synthesis, handling and operation.
Wide bandgap (>5eV) semiconductors are another class of NEA materials. Particularly, as-grown CVD diamond generally has NEA as an inherent surface property or can be engineered by intentational hydrogen-termination after growth. High-purity H-terminated synthetic diamond has been found to be an excellent electron amplifier, where the primary electrons from a standard QE photocathode (e.g. Cu) accelerated to keV energies get multiplied upon transmission through a diamond thin film. The addition of impurities (P, N₂, S) allow for n-type doping of the diamond. In tandem with NEA by H-termination, the aforementioned n-type dopants in diamond, having activation energies <2eV, are capable of promoting photo-excitation in the visible range.
We report the results of QE measurements carried out on a 150nm-thick nitrogen-incorporated ultrananocrystalline diamond (UNCD) film, terminated with hydrogen (abbreviated NUNCD:H). To date, there is one report of NUNCD:H's sensitivity to the visible range [1]. Measurable quantum effects were reported at room temperature in the 400-480nm range. However, no QE values were reported. We present proof-of-concept QE measurements that suggest NUNCD:H is an emergent photocathode. NUNCD:H demonstrates a QE of ~10^-3 (~0.1%) at 254nm, which is ~3 orders of magnitude higher than that of the molybdenum or copper substrates onto which the NUNCD was deposited. Moreover, NUNCD:H is sensitive in visible light with a QE ~5x10^-8 at 405nm and ~5x10^-9 at 436nm. Samples were exposed to air for 2 hours in between deposition and QE measurements for transfer and loading. Our NUNCD:H film takes advantage of two key features: 1) N-incorporation induces n-type conductivity in intrinsically insulating UNCD and; 2) H-termination induces NEA on the NUNCD and stabilizes the surface against air exposure. Our results indicated NUNCD:H is a robust photocathode material and a promising candidate to replace exisiting photocathodes.
[1] T. Sun et al. Applied Physics Letters 99, 202101 (2011) http://dx.doi.org/10.1063/1.3658638
Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

The field of optics is benefiting from the recent advances in synthesis of large and pure crystals of diamond. Being the hardest, most thermally-conductive and most atomically dense substance known, and with a host of outstanding optical properties, the opportunity exists to make massive extensions to device capability that have been previously hampered by a lack of availability. One promising avenue that exploits diamond's high cross-section for Raman scattering is to create beam converters with potentially very wide performance range.
This talk will review the pertinent optical properties of synthetic diamond and recent achievements in diamond Raman laser technology. The paper will analyse the important parameters of the sythetically available materials that are crucial to efficient operation. It will be shown that diamond is well suited to beam conversion at power levels in excess of several hundred watts and to wavelengths spanning from the UV to the mid-infrared. Some promising applications areas will be highlighted.

The nitrogen vacancy (NV) center in diamond provides a robust solid-state single photon source for emerging applications in quantum information processing, field sensing, and biological imaging. The stability of these centers is limited by imperfections in the diamond surface, including graphitization, unpassivated surface states, and damage arising from nanofabrication. Here, we develop a robust inorganic surface passivation scheme based on epitaxial growth of cubic boron nitride (c-BN) using bias-assisted RF sputtering at high temperature. Characterization with transmission electron microscopy, IR spectroscopy, and atomic force microscopy indicates high-quality epitaxial c-BN layers without secondary phases. Analysis of individual shallowly implanted NV centers with scanning confocal photoluminescence microscopy before and after BN growth indicates enhanced photostability after c-BN passivation. Preliminary measurements also indicate recovery of luminescence from NV centers under surfaces damaged by reactive ion etching, suggesting a path to stabilization of defect-based quantum emitters in nanofabricated structures. In addition, we examine inorganic passivation of the related material 4H-SiC, which hosts similar single photon-emitting defect centers consisting of a carbon antisite defect paired with a vacancy (CV), with epitaxial aluminum nitride (AlN) grown by high-temperature RF sputtering. We observe similarly enhanced photostability of individual CVs centers after passivation with AlN and examine the roles of band alignments and spontaneous polarization in determining the charge state stability of these defect centers. Accumulation of holes due to the polarization mismatch between AlN and SiC is found to selectively stabilize the luminescent positive charge state on the Si face of SiC. This work represents the first application of inorganic passivation methods to solid-state defect centers and provides a new route to highly photostable and environmentally robust single photon emitters.

In diamond, phosphorus is the only donor reported so far to induce reproducible n-type conductivity. However, the phosphorus donor level is deep, at 0.6 eV below the conduction band minimum. n-type CVD diamond grown with phosphorus impurities is therefore almost insulating at room temperature, with free electron densities of only ~10¹¹ cm^-3 in the best samples. The deep donor character of phosphorus hinders the development of diamond-based electronic devices, highly desired to exploit the exceptional properties of diamond as a semiconductor. That is the reason why the search for shallower donors has been so active during the last decades, though without convincing experimental results.
The purpose of this work is to experimentally demonstrate that arsenic impurities can provide donors significantly shallower than phosphorus in diamond. The method is based on ion implantation followed by high temperature annealing in order to introduce arsenic dopants in diamond. Their location in substitutional sites is observed by bound exciton recombinations thanks to a high sensitivity cathodoluminescence system. The experimental conditions are first validated with phosphorus implantation, as evidenced by the well-known signature of phosphorus-bound exciton recombinations. In arsenic-implanted diamond, a set of new excitonic recombinations is observed [1]. It is composed of two groups of emissions at 5.355/5.361 eV and at 5.215/5.220/5.227 eV. They are respectively attributed to the no-phonon and transverse-optical phonon-assisted recombinations of excitons bound to neutral arsenic donors. From the Haynes rule, a ionization energy of 0.41 eV is deduced for arsenic donors in diamond [1], which shows that arsenic is a shallower donor than phosphorus (0.6 eV) in good agreement with theory [2].
[1] J. Barjon, F. Jomard, and S. Morata, Phys. Rev. B 89, 045201 (2014).
[2] J. Sque, R. Jones, J. P. Goss, and P. R. Briddon, Phys. Rev. Lett. 92, 17402 (2004).

Due to its exceptional thermal properties and its wide band gap, diamond has always been considered a prime candidate for advanced, high-powered electronics. To date however, although p-type doping has successfully been achieved using boron doping during CVD film growth, no equivalent n-type diamond has been produced. The interaction of dopant atoms within the diamond lattice is complicated and further characterisation work is needed to understand the reasons behind the inactivity of potential n-type dopants such as lithium or sodium.
Atom probe tomography (APT) is a technique that enables elemental characterisation on the atomic scale. APT requires specimens to be sharpened into a needle shape with a tip radius of less than 100nm. A standing voltage is applied between the specimen and a local electrode creating an intense electric field at the apex. A pulse of either additional voltage or laser energy then facilitates controlled field evaporation of ions from the tip into a position-sensitive time-of-flight detector. This information is used to reconstruct a three-dimensional chemically resolved atom by atom model of the tip. Although APT has been used to characterise metal alloys for many years, it is only in the last decade that atom probe instrumentation, in particular laser technology, has developed to the point that semiconductors can be reliably examined. This paper will discuss the steps taken to optimise specimen preparation and analysis for the characterisation of how dopants segregate in diamond films.
A series of CVD diamond films of thickness ~1µm were grown on top of a series of etched silicon microtip pillars. These were then sharpened using focused ion beam (FIB) milling to produce tips with the dimensions required for analysis. This method produces reliable specimens without the additional complications of performing a FIB liftout of CVD diamond. The etched specimens were then successfully analysed in a LEAP 3000X HR atom probe instrument. Carbon materials are particularly challenging in many forms of mass spectrometry due to the tendency of carbon to come off in clusters. In APT this is observed as several groups of peaks corresponding to increasing combinations of carbon and hydrogen. The optimisation of analysis conditions to observe dopant atoms will be discussed, in addition to results from heavily boron doped CVD films.

Diamond is a promising candidate for x-ray detectors due to its low x-ray absorption, high thermal conductivity, mechanical strength, and radiation hardness. We are developing an instrumented diamond window, which can simultaneously measure the x-ray flux, position and morphology in real time and also provide the vacuum-to-air interface. The prototype is being designed to meet the specific goals of the XFP beamline at NSLS-II but will have broader application. In this poster we will present the progress of a prototype instrumented window which has a goal of 1024 effective pixels capable of delivering 90W/mm² of focused x-ray white beam. The pixel density will be achieved by having 32 vertical stripes on the front and 32 horizontal stripes on the back of the diamond. The bias is rotated through the back horizontal stripes and the current is readout on the front vertical stripes at a rate of ~30 Hz. Specifically, we will discuss the results on our recent measurements at X28C of a 16 pixel test device using this novel readout scheme. The fabrication of the device, including surface preparation, lithographic patterning and assembly will also be described.

Thermionic emission from diamond is a promising route for making thermal energy convertors for use in solar power generation or energy harvesting devices. However, despite much recent progress, the production of high-current-density diamond thermionic emitters remains elusive. The most promising thermionic emission materials reported to date use nanocrystalline or even ultrananocrystalline diamond (UNCD) films, since it is believed that a high sp²/sp³-carbon ratio is essential in improving the electrical conductivity throughout the films. However, there is a trade off with the film thermal conductivity, a high value of which is necessary to transport heat from the heater at the base of the film to its emitting surface.
Electron emission can be further improved by using n-type diamond, doped with phosphorus or sulfur, or n-type UNCD. Lithium has been suggested as a possible alternative n-type dopant due to its potential as a shallow donor in diamond. However, experimentally doping with lithium is difficult to achieve due to the low solubility of Li in diamond, and its relatively high mobility which allows it to diffuse through the lattice at fairly low temperatures. It has been suggested that the unwanted Li diffusion can be prevented by adding substitutional nitrogen together with interstitial Li, with the nitrogen acting as a trap to pin down the Li in the diamond lattice while retaining its n-donor properties. Calculations suggest that the optimum ratio of Li-to-N is 1:4, with LiN₄ clusters acting as shallow donors. To study this, we investigated the incorporation of both lithium and nitrogen while growing microcrystalline diamond in a hot-filament chemical vapour deposition (HFCVD) system. Diamond thin films were grown using a methane/ammonia/hydrogen gas mixture with Li being added as solid LiN₃ and overgrown with diamond. Secondary ion mass spectrometry showed that high levels of Li and N were embedded in the diamond and were situated in close proximity to each other. The crystallinity and morphology of the diamond films were unchanged by the Li/N incorporation. The diamond surface was hydrogen terminated. Thermionic emission measurements were performed on these Li-N co-doped diamond films and the results showed that emission began at a temperature of 500°C and gave a current of 124 µA/cm² at 627°C, making it a promising thermionic material. We shall report the results of these studies, plus those from varying the Li:N ratio in a set of films to investigate the effect upon thermionic emission.

Superconducting boron-doped diamonds were fabricated by high pressure and high temperature (HPHT) method in 2004 [1], since then many groups have challenged to make superconducting diamonds by using several methods. Our group think that Micro-wave Plasma assisted Chemical Vapor Deposition (MPCVD) is a perfect method for growing high quality heavily boron-doped diamond thin film. Using this method, we have succeeded in making a lot of Tc(offset)>8 K diamonds, and some showed above10 K [2]. In this study, we reexamined boron transfer methods, such as spraying gas effect, gas rate and ratio, in order to reproduce thin films with similar characteristics. Then, we evaluated the relationship between the superconducting transition temperature, the Tc(onset)-(offset) width, and the crystallinity by using (113) plane asymmetric Reciprocal Space Mapping(RSM).
Our previous work shows that up to a certain thickness, boron-doped diamond films formed on the strained layer that extends only in the vertical direction, and above that thickness, relaxation layer that extends in both the vertical and horizontal direction is grown on the strained layer [2]. So in this work, two samples of boron-doped diamond thin films, one with only the strained layer and the other with both the strained and the relaxation layer, were grown on the HPHT single crystal diamond (111) using quartz tube type MPCVD. The peak of the thin film with only the strained layer in RSM is sharper than that of the thin film with the strained and the relaxation layer. The sample with only the strained layer showed Tc(onset)=10.24 K and Tc(offset)=10.02 K so the superconducting transition temperature Tc(onset)-(offset) was only 0.22 K. On the other hand, the sample with the strained and the relaxation layer showed Tc(onset)=8.85 K and Tc(offset)=7.78 K so the width of Tc(onset)-(offset) is1.07 K. One reason why the Tc width of sample with only the strained layer is narrower than that of the sample with the strained and the relaxation layer is the fallowing: The relaxation layer will not reach superconducting transition at the temperature in which the strained layer begins to transition. Since the current passes through both the strained and the relaxation layer, zero resistance was not be obtained at that temperature. Therefore, relaxation layer causes the transition to occur slowly so transition width becomes wider. On the other hand, in the case of only the strained layer, the thin film will immediately show zero resistance because the current passes only the strained layer, so transition width becomes homogeneous and sharper.
This study was supported by research grants from ALCA (JST).
[1] E.A.Ekimov et al, Nature 428, 542 (2004)
[2] Akihiro Kawano, Hiroshi Kawarada et al., Phys. Rev. B 82, 085318 (2010)