3:00 PM - EP6.1.02
Band Alignment of Hydrogen-Plasma Cleaned MBE CdTe on InSb (001)
Xingye Wang 1,Calli Campbell 1,Yong-Hang Zhang 1,Robert Nemanich 1
1 Arizona State Univ Tempe United States,
Show AbstractThe combination of II-VI cadmium telluride, with a bandgap of 1.5 eV, and III-V indium antimonide, with a reported narrow bandgap of 0.17 eV, is promising for both electronic and optoelectronic applications. InSb has a high electron mobility (77,000 cm2/V-s) and saturation velocity (5 ×107 cm/s), which could be used in ultra-fast, low power digital logic applications. A close lattice match exists between these two semiconductors and CdTe can be effectively deposited. These properties enable the combination of CdTe and InSb to be components of a quantum well transistor (QWFET). Thus, it is interesting to investigate the band offset of a CdTe-InSb heterojunction. In this research, the single crystal InSb (001) substrates were commercially obtained. A dual-chamber II-VI and III-V molecular beam epitaxy (MBE) system was used to thermally desorb native oxides, then deposit an InSb (001) buffer layer followed by 7 nm of epitaxial CdTe (001). After film deposition the sample was transferred to a multichamber system for photoemission analysis. A remote hydrogen plasma process was used to clean the surfaces, which were exposed to atmosphere during the transition process. Monochromatic x-ray photoemission spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS) were used to characterize the electrical properties of the samples. The result of this research indicates that remote hydrogen plasma is an efficient method to clean InSb and CdTe surfaces. In addition, a valence band offset of 0.9 eV is measured, which is between the value predicted by the Anderson electron affinity rule (0.96 eV) and that calculated by Tersoff (0.84 eV).
3:15 PM - EP6.1.03
Band Engineered Coherent “Hetero-crystalline” Bi2Se3/II-VI Heterostructures
Thor Garcia 2,Zhiyi Chen 2,Lia Krusin 3,Maria Tamargo 1
1 Department of Chemistry The City College of New York New York United States,2 The Graduate Center of CUNY New York United States,3 Department of Physics The City College of New York New York United States,2 The Graduate Center of CUNY New York United States3 Department of Physics The City College of New York New York United States1 Department of Chemistry The City College of New York New York United States
Show AbstractTopological Insulators (TI) are electronic materials that have a bulk bandgap and time reversal-symmetry protected conducting states at the edges or surfaces. In three-dimensional (3D) topological insulators, spin-orbit coupling is very strong and charge carrier spins are locked with momentum; this robust spin polarization is a feature that could lead to novel spintronic devices. Among 3D TIs, Bi2Se3 has attracted the most attention due to its relatively large bandgap (~0.3 eV) and the experimentally verified ideal Dirac cone at the G point in the Brillouin zone. However, a combination of its small bandgap and the low activation energy of Se vacancies has yielded material that is metallic making the measurement and use of the topological surface states difficult to achieve. Several approaches have been attempted to alleviate this problem with limited success, including improving the crystal quality, impurity doping, and alloying with Bi2Te3. Heterostructures of Bi2Se3 with conventional semiconductors allow us to use band engineering to realize novel properties as we have previously shown with Se based II-VI/TI superlattices. In this work we present the molecular beam epitaxial (MBE) growth of heterostructures of II-VI and Bi2Se3 as a platform for band engineering to access TI surface states. Heterostructures such as Bi2Se3 with ZnCdSe, ZnSe and CdTe have been grown and characterized. Issues related to the growth of the coherent materials with different crystal structures and compositions will be explored. Additionally, control of polymorphism utilizing temperature and interfacial layers in the overgrown II-VI layers is presented.
3:30 PM - *EP6.1.04
6.1Å III-V/II-VI Heterovalent Structures for Optoelectronics and Spintronics
Sergey Ivanov 1,Sergey Sorokin 1,Yakov Terent'ev 1,Victor Solov'ev 1,Alexey Semenov 1,Alexey Toropov 1
1 Ioffe Institute St.Petersburg Russian Federation,
Show AbstractHybrid III-V/II-VI pseudomorphic structures containing a low defect density heterovalent interface (HI) in an active region offer new opportunities in developing heterostructure physics and designing advanced semiconductor devices. Application of molecular beam epitaxy (MBE) possessing unique properties of growing semiconductor interfaces with a sub-monolayer control gives a good chance for practical realization of this idea. The first successful application of the concept of functional heterovalent structures in a spintronic LED device combining an optically active AlGaAs quantum well (QW) capped with a ZnMnSe-based DMS spin aligner [1], was originated from the well-developed MBE technology of the ZnSe/GaAs HI employed for II-VI green lasers. These studies were further continued by our demonstration of resonant electron coupling through the HI between non-magnetic GaAs/AlGaAs and DMS ZnMnCdSe/ZnSe QWs.
Similarly, there exists a large variety of 6.1Å III-V (AlGaInAsSb) and II-VI (CdMgMnSe, ZnMnTe) compounds, including DMS ones, which are suitable for fabrication of coherent heterovalent structures possessing any desirable band alignment (type I or type II), superior transport properties of 2DEG (InAs), optical activity in the mid-IR range and strong spin-orbit interaction (InAs, III-Sb), as well as reported signatures of ferromagnetic behavior (Zn(Mn,Cr)Te). Successful employing the n-type wide gap CdMgSe:Cl cladding in the mid-IR p-AlGaAsSb:Si/i-InAs/n-II-VI heterostructure laser diodes improved dramatically the hole confinement in the active region as well as the optical confinement in the InAs waveguide [2], paving the way to fabrication of high-power mid-IR lasers. Lately, the AlGaSb/ZnCdSeTe heterostructures based on ZnTe/GaSb HI were proposed as a basis for efficient multijunction solar cells [3].
The paper gives an overview of past and recent activity in fabrication by MBE and studies of various hybrid heterostructures containing the InAs/II-VI HI both in a bulk active region of mid-IR lasers and in single QWs [4]. Structural, luminescence, and electrical properties, as well as an electronic band structure are considered in details as relevant to the HI [5]. Finally, the existence of magnetized 2DEG, occurred owing to exchange interaction between electrons located in the heterovalent AlGaSb/InAs/ZnMnTe QW with Mn2+ ions in the DMS ZnMnTe barriers, is demonstrated by measuring the microwave-radiation-induced spin-polarized electric currents [6]. The prospects and challenges of employing such structures for optoelectronic and spintronic applications are discussed.
1. R. Fiederling et al., Nature (London) 402, 787 (2000).
2. S.V. Ivanov et al., Appl. Phys. Lett. 78, 1655 (2001).
3. S. Wang et al., J. Cryst. Growth 311, 2116 (2009).
4. S.V. Ivanov et al. Appl. Phys. Lett. 84, 4777 (2004).
5. S.V. Ivanov et al., phys. stat. sol. (c) 1, 1468 (2004).
6. Ya.V. Terent’ev et al., Appl. Phys. Lett. 99, 072111 (2011).
5:30 PM - EP6.1.07
Material and Interface Properties of an Optically-Addressed Visible/MWIR Two-Color Photodetector Based on Monolithically-Integrated CdTe nBn and InSb PIN Device Structures
Zhao-Yu He 1,Jacob Becker 1,Calli Campbell 1,Ying-shen Kuo 1,Maxwell Lassise 1,Shi Liu 1,Zhi-Yuan Lin 1,Yong-Hang Zhang 1
1 Arizona State University Tempe United States,
Show AbstractWe have very recently demonstrated CdTe and CdTe/MgCdTe heterostructures on lattice-matched InSb substrates with a minority carrier lifetime of 2.7 µs and an interface recombination velocity of 0.1 cm/s [1].These values are either very close to or better than the record values reported for the well-studied GaAs/AlGaAs and GaAs/GaInP heterostructures. Using the novel optical addressing approach [2], we have designed and demonstrated an optically-addressed two-color, single-polarity, and two-terminal photodetector consisting of monolithically-integrated and lattice-matched CdTe nBn and InSb PIN device structures grown using molecular beam epitaxy (MBE). The CdTe nBn structure (with an 820 nm cut-off) and the InSb PIN structure (with a 5.5 μm cut-off) are electrically connected through a highly conductive (
EP6.2: Poster Session
Session Chairs
Wednesday AM, March 30, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - EP6.2.01
GaN Nanowire Arrays by a Patterned Metal-Assisted Chemical Etching
Guodong Yuan 1,Kechao Wang 1,Ruiwei Wu 1,Jinmin Li 1
1 Research and Development Center for Solid State Lighting, Institute of Semiconductors Chinese Academy of Sciences Beijing China,
Show AbstractWe successfully develop an E-beam evaporated metal mask-assisted chemical etching method and produce self-organized GaN nanowire arrays. Metal-uncovered regions show nanowire arrays while GaN layer underneath metal keeps almost unetched with parts porous sidewalls. We propose a model that these porous nanostructures combination depends on etch rate, which is controlled by solution and photo-generated holes concentration. We believe that nanowires and porous sidewalls are tuned by a vertically sufficient and laterally limited etch rate, respectively. Two separated electrochemical reactions which take place simultaneously on the same surface are illustrated by the energy distribution of redox couple，metal work function and energy state of GaN. The scheme of potential relationship between bands in Si, GaN and standard hydrogen electrode potential of solution and metals shows distinctively different etching behavior of GaN, Si and the enhanced metal carrier transport ability.
9:00 PM - EP6.2.02
High-Performance Wrap-Gated InGaAs Nanowire Field-Effect Transistors with Sputtered Dielectrics and Surface Passivation
Lifan Shen 1,Sen Po Yip 1,Dapan Li 1,Edwin Pun 1,Johnny Ho 1
1 City Univ of Hong Kong Hong Kong Hong Kong,
Show AbstractAlthough wrap-gated or gate-all-around nanowire field-effect transistors (NWFETs) have been explored as an ideal electronic device geometry for low-power and high-frequency applications, further performance enhancement and practical implementation are still suffering from electron scattering on the nanowire surface and/or interface traps between the nanowire channel and gate dielectric as well as the complicated device fabrication scheme. Here, we report the development of high-performance wrap-gated InGaAs NWFETs using the conventional sputtered Al2O3 layer as the gate dielectric, instead of the typically employed atomic layer deposited counterparts [1]. Importantly, the surface passivation of NW channels by self-assembly sulfur-containing monolayers is performed right before the dielectric deposition, in which it is found to significantly alleviate the plasma induced surface/interface defect traps on the NW channel. Utilizing this passivation, the wrap-gated device exhibits superior electrical performances: a high ION/IOFF ratio of ∼2x106, an extremely low sub-threshold slope of 80 mV/decade, a small IOFF of 0.4 pA, and a peak field-effect electron mobility of ∼1600 cm2/(Vs) at VDS = 0.1 V at room temperature, in which these values are even better than the ones of state-of-the-art NWFETs reported so far. This superior capacitive gate coupling and the improved electrical performance, including low leakage current and steep sub-threshold slope, indicate the effectiveness of our hybrid approach combining sputtering and pre-deposition chemical passivation to achieve high-quality gate dielectrics for wrap-gated NWFETs for future high-speed, low-power and high-frequency electronic devices.
Reference:
[1] L.F. Shen, S.P. Yip, Z.X. Yang, M. Fang, T.F. Hung, E.Y.B. Pun and J.C. Ho, “High-Performance Wrap-Gated InGaAs Nanowire Field-Effect Transistors with Sputtered Dielectrics”, Scientific Reports, in press, 2015.
9:00 PM - EP6.2.03
High Hole Mobility of GaSb Nanowires for Next-generation Nanoelectronics
Zaixing Yang 1,Dapan Li 1,Sen Po Yip 1,Xiaoguang Liang 1,Johnny Ho 1
1 City Univ of Hong Kong Hong Kong Hong Kong,
Show AbstractIn the past decades, because of the unique physical properties, III-V semiconductor nanowires (NWs) have attracted extensive research interests and are recognized as promising fundamental building blocks for next-generation electronics, photonics, photovoltaics and so on, high-mobility GaSb NWs have received tremendous attention for high-performance p-type transistors even more; however, due to the difficulty in achieving thin and uniform NWs, there is limited report until now addressing their diameter-dependent properties and their hole mobility limit in this important one-dimensional material system, where all these are essential information for the deployment of GaSb NWs in various applications. In this work, by employing the newly developed surfactant-assisted chemical vapor deposition, high-quality and uniform GaSb NWs with controllable diameters, spanning from 16 to 70 nm, are successfully prepared, enabling the direct assessment of their growth orientation and hole mobility as a function of diameter while elucidating the role of sulfur surfactant and the interplay between surface and interface energies of NWs on their electrical properties [1]. The sulfur passivation is found to efficiently stabilize the high-energy NW sidewalls of (111) and (311) in order to yield the thin NWs (i.e., <40 nm in diameters) with the dominant growth orientations of <211> and <110>, whereas the thick NWs (i.e., >40 nm in diameters) would grow along the most energy-favorable close-packed planes with the orientation of <111>, supported by the approximate atomic models. Importantly, through the reliable control of sulfur passivation, growth orientation and surface roughness, GaSb NWs with the peak hole mobility of∼400 cm2V s-1 for the diameter of 48 nm, approaching the theoretical limit under the hole concentration of∼2.2x1018 cm-3, can be achieved for the first time. All these indicate their promising potency for utilizations in different technological domains and prospect for next-generation high-mobility nanoelectronics.
Reference
1. Zai-xing Yang, SenPo Yip, Dapan Li, Ning Han, Guofa Dong, Xiaoguang Liang, Lei Shu, Tak Fu Hung, Xiaoliang Mo, and Johnny C. Ho, ACS Nano 2015 9 (9), 9268-9275
9:00 PM - EP6.2.04
Fabrication and Energy Band Diagram of Al2O3/AlGaN/GaN MOS Capacitors
Min-Woo Ha 1,You Jin Jo 2,Kangmin Choi 1,Tae Joo Park 2
1 Department of Electrical Engineering Myongji University Yongin Korea (the Republic of),2 Department of Materials Science amp; Engineering Hanyang University Ansan Korea (the Republic of)
Show AbstractAlGaN/GaN MOS devices are promising high-performance power switches because they have a highly conductive two-dimensional electron gas (2DEG) and high breakdown field. AlGaN/GaN MOS can increase the gate swing and reduce the gate leakage current compared with Schottky-controlled GaN devices. An atomic layer deposition (ALD) is an excellent method growing a high-quality gate insulator in GaN devices. We have fabricated and measured Al2O3/AlGaN/GaN MOS capacitors using the ALD. TMA/H2O-based Al2O3 ALD was performed on the AlGaN/GaN. A thickness of Al2O3 was 13 nm using 100 cycles at 300°C. A gate contact was 100 nm-thick TiN. Research on energy band diagrams of the fabricated MOS capacitors is still needed for understanding device operations.
The purpose of this work was to report the energy band diagram of Al2O3/AlGaN/GaN MOS capacitors. A numerical simulator was used in this work [1,2]. A metal work function of TiN was 4.8 eV. Dielectric constants of Al2O3, AlGaN, and GaN were 9.3, 9.5, and 9.5, respectively. Energy band gap of Al2O3, AlGaN and GaN were 9, 3.75, and 3.4 eV, respectively. Electron affinity of AlGaN and GaN were 3.82 and 4.33 eV, respectively.
It was important to describe the polarization induced 2DEG at the interface between AlGaN and GaN [3]. There were two polarization on wurtzite material which were a spontaneous polarization with zero strain and a piezoelectric polarization with strain. The piezoelectric polarization came from lattice mismatch between AlGaN and GaN. The 20 nm-thick AlGaN barrier had both spontaneous and piezoelectric polarization. A polarization scale factor and our epitaxial structure used calculating polarization charges. Surface Fermi-level pinning was also a critical factor for analyzing the energy band diagram. Zero-voltage conduction and valence bands went down from AlGaN surface to 2DEG due to the polarization charges [3]. The total polarization sheet charge at the AlGaN and electron concentration at the 2DEG were 3.357 × 1019 /cm2 and 4.819 × 1019 /cm3 if the the scale factor was 1. A gap between the conduction band and Fermi-level at the 2DEG was 0.394 eV. It was difficult to satisfy both measured threshold voltage (-2.26 V) of Al2O3/AlGaN/GaN MOS capacitors and 2DEG concentration (1.45 × 1019 /cm3) from Hall measurement. We have analyzed the energy band diagrams of the Al2O3/AlGaN/GaN MOS capacitors under various conditions.
References [1] Atlas User’s Manual, Silvaco TCAD (2015), [2] Simulation Standard, Silvaco International 16, 250 (2006), [3] J. P. Ibbetson, et.al, Appl. Phys. Lett. 77, 250 (2000)
9:00 PM - EP6.2.05
Thermal Deactivation of Tellurium Doping in In.53Ga.47As Grown by MOCVD
Ethan Kennon 1,Tommaso Orzali 2,Yan Xin 3,Henry Aldridge 1,Klaus Vollmer 1,Van Truong 1,Aaron Lind 1,Caleb Barrett 1,Kevin Jones 1
1 University of Florida Gainesville United States,2 Sematech New York City United States3 Florida State University Tallahassee United States
Show AbstractIn.53Ga.47As is a contender to replace Si in the channels of future logic devices because of its high carrier injection velocity, which allows for high on/off current ratios and switching speeds. However to minimize the contact resistance to these materials it is critical to maximize the doping density. Previous studies have shown that the maximum doping concentration by implantation and annealing is around 1.5 x 1019/cm3, which is around an order of magnitude below the values required for sub 10 nm devices. Recently, Tellurium has shown promise as an alternative to Si for n-type doping with doping densities as high as 8 x 1019/cm3 by MOCVD. Since these concentrations are above what has been achieved by thermal annealing of implanted samples they are metastable. The goal of this work is to investigate the thermal stability of heavily doped Te in In.53Ga.47As. The thermal stability was investigated by a combination of furnace and RTA annealing. Heavily doped samples with an electrically active concentration of 4.4 x 1019/cm3 were grown by MOCVD on Si with a buffer layer structure consisting of an InAlAs isolation layer, InP and GaAs. Layer thicknesses were confirmed with cross-sectional Transmission Electron Microscopy (TEM). These were subsequently capped with ALD Al2O3 and annealed in a furnace or RTA from 550°C to 700°C. Across the temperature range, samples deactivated to an electrically active concentration of 6-7 x 1018/cm3. The deactivation process starts at temperatures as low as 500°C after 10 minutes. The activation energy for electrical deactivation was found to be 2.6 eV. Continued anneals beyond the point of full deactivation showed a slight increase in activation, consistent with diffusion of Te. TEM analysis on the aberration corrected JEOL ARM 200cf TEM showed no visible signs of precipitation. Energy Dispersive X-ray Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS) were unable to detect Te in the active layer, with the exception of the interface where Te dopant was first introduced during growth. This suggests that any clustering after deactivating anneals was below the detection limits of these methods. It is proposed that the charged group III vacancies believed to be responsible for saturation of Si in InGaAs may also be responsible for the deactivation of Te.
9:00 PM - EP6.2.06
Lateral Epitaxial Overgrowth of High Quality AlN on Patterned h-BN Using Metal Organic Chemical Vapor Deposition
Cuong Tran 2
1 School of Semiconductor and Chemical Engineering Jeonju Korea (the Republic of),2 Solid State Physics University of Science, Vietnam National University Ho Chi Minh City Ho Chi Minh City Viet Nam,
Show AbstractIt is necessary to improve the crystalline quality and reduce the dislocation density of AlN epitaxial layer in AlGaN based ultraviolet light emitting diodes (UV-LEDs) and photodetectors, which have great potential for various applications such as fire detection, water purification, sterilization, decontamination, and thin-film curing. In this context, we performed the lateral epitaxial overgrowth of AlN on patterned stripe h-BN using high temperature metal organic chemical vapor deposition. Herein, the h-BN pattern acts as nucleation sites and a mask layer to partially block the propagation of defects from the substrate. Consequently, the average dislocation density of a coalesced AlN epitaxially grown on h-BN pattern is as low as 1.81×108 cm-2, and a full width half maximum of both the symmetric (0002) and asymmetric (10-12) diffraction curves improve as compared to that of AlN grown on stripe patterned sapphire substrate. Our results suggested that h-BN pattern plays a promising role in the growth of high-crystalline quality AlN.
9:00 PM - EP6.2.07
Variation of Vertical Direction Breakdown Voltage of the AlGaN/GaN HEMTs on AlN/Si Template Substrate as a Function of the Growth Temperature of the Initial Al Layer
Yuya Yamaoka 2,Kazuhiro Ito 2,Takashi Egawa 2,Akinori Ubukata 1,Toshiya Tabuchi 1,Koh Mastumoto 1
1 Taiyo Nippon Sanso Corp. Tsukuba City Japan,2 Nagoya Institute of Technology Nagoya city Japan,2 Nagoya Institute of Technology Nagoya city Japan1 Taiyo Nippon Sanso Corp. Tsukuba City Japan
Show Abstract1. Introduction
AlGaN/GaN high-electron-mobility-transistors (HEMT) are expected to be the next-generation power devices; however, one drawback of the HEMT on Si substrate is that their vertical direction breakdown voltage (VDBV) is lower than the theoretical value. In addition, the crystallinity of GaN varies significantly depending on the growth conditions of the initial AlN layer [1]. In this study, the relationship between the growth conditions of the initial AlN layer and the VDBV of the AlGaN/GaN HEMT was investigated by growing an HEMT structure simultaneously on the identical structure AlN layer on Si substrates (AlN/Si template) manufactured with different growth conditions
2. Experiments
All samples used in this study were grown on 8-inch P-type Si substrates using the multiwafer metal organic chemical vapor deposition tool (Taiyo Nippon Sanso Corp., UR26K, 8 inch × 6 wafers).
Three types of AlN (thickness = 150 nm) / Si templates were prepared by the following steps. First, the initial Al layer that forms the interface between the Si substrate and AlN was grown by flowing only Triethylaluminum (TMA) without ammonia (NH3). Next, the AlN layer was grown by supplying TMA and NH3 simultaneously. Three temperature conditions were considered for the Al layer (Sample A: 622 °C, Sample B: 811°C, and Sample C: 1000°C). Growth conditions of the AlN layer are the same for all three samples. A scanning electron microscope was used to compare the surface conditions, and X-ray diffractometry was employed to determine crystal quality.
To grow an HEMT structure (AlGaN/GaN/SLS/AlGaN/AlN/Si template = 25/1000/1800/250/150 nm), each AlN/Si template was grown simultaneously. HEMT sample D was grown on the AlN template sample A; sample E, on sample B, sample F, on sample C. VDBV was measured using a device structure (isolation: mesa structure, electrode: ohmic metal).The VDBV is defined as the vertical voltage at which the leakage current is 1.0 × 10-2 A/mm2.
3. Results and discussion
Surface pit densities of the AlN/Si templates were as follows: for sample A, 2.52 × 1010 cm-2; sample B, 1.44 × 1010 cm-2; and sample C, 3.84 × 1010 cm-2. The surface pit density was minimal at a growth temperature of 811°C of the Al layer. The rocking curve of the AlN(0002) direction of the full width at half maximum (FWHM) of each templates were as follows. Sample A is 2471 arcsec; Sample B, 2224 arcsec; and Sample C, 2194 arcsec. The crystallinity of the AlN improved with an increase in the growth temperature of the initial Al layer. The results of the VDBV measurements were as follows: Sample D is 185.2 V; Sample E, 586.4 V; and Sample F, 633.3 V. VDBV was shown to vary significantly based on the growth conditions of the AlN/Si template. The AlN/Si interface plays an important role in improving the VDBV in the AlGaN/GaN HEMT on Si.
[1] H. Lahre‘che, et al. Journal of Crystal Growth, Volume 217, Issues 1–2, 11 July 2000, pp.13–25
9:00 PM - EP6.2.08
Migration Enhanced Molecular Beam Epitaxy Growth of Heterovalent Systems for High Speed Electronic Device Applications
Maxwell Lassise 2,Ernesto Suarez 2,Xinhao Zhao 3,Brian Tracy 4,Calli Campbell 3,David Smith 4,Yong-Hang Zhang 2
1 Center for Photonics Innovation Arizona State University Tempe United States,2 School of Electrical, Computer, and Energy Engineering Arizona State University Tempe United States,1 Center for Photonics Innovation Arizona State University Tempe United States,3 School for Engineering of Matter, Transport, and Energy Arizona State University Tempe United States1 Center for Photonics Innovation Arizona State University Tempe United States,4 Department of Physics Arizona State University Tempe United States
Show AbstractHeterovalent integration of group II-VI and III-V compound semiconductors is key to unlocking the full potential of semiconductor electronic devices. A sharp transition from one material to the other must be achieved in order to avoid defects in the crystal lattice that can act as non-radiative recombination sites. The crystal structure of these compounds is most stable when the atoms have eight bonds, but when these two material systems are combined, the interface will alter the crystal structure to minimize the buildup of charge from the missing/excess electrons created in the valence band by the II-V or III-VI bonds. Rarely will a sharp transition be energetically favorable, but instead a defective compound may form at the interface and create dislocations. In addition to the instability of the bonds at the interface, the two material systems have different melting points and optimal growth temperatures. Many of the III-V compounds have optimal growth temperatures that are much higher than the melting point of their lattice-matched II-VI counterparts. This makes growth of high quality heterovalent systems difficult to achieve. To overcome these obstacles, migration enhanced epitaxy has been used to grow thin, abrupt layers of GaSb on a ZnTe virtual substrate. The structural and optical characteristics are investigated for different growth conditions using photoluminescence, high-resolution x-ray diffraction, and cross-sectional electron microscopy.
9:00 PM - EP6.2.09
Optoelectronic and Stability Studies of CdTe/MgxCd1-xTe Double Heterostructures Featuring Barrier Layers with over 46% Mg Composition Grown by Molecular Beam Epitaxy on InSb(001) Substrates
Calli Campbell 3,Yuan Zhao 2,Xinhao Zhao 3,Xingye Wang 3,Maxwell Lassise 1,Shi Liu 1,Ernesto Suarez 1,Robert Nemanich 4,Yong-Hang Zhang 1
2 Center for Photonics Innovation Arizona State University Tempe United States,3 School for Engineering of Matter, Transport and Energy Arizona State University Tempe United States,1 School of Electrical, Computer and Energy Engineering Arizona State University Tempe United States,2 Center for Photonics Innovation Arizona State University Tempe United States3 School for Engineering of Matter, Transport and Energy Arizona State University Tempe United States2 Center for Photonics Innovation Arizona State University Tempe United States,1 School of Electrical, Computer and Energy Engineering Arizona State University Tempe United States4 Department of Physics Arizona State University Tempe United States
Show AbstractCadmium telluride (CdTe) and its ternaries are exciting material systems with applications in photodetection and solar energy. Molecular beam epitaxial growth of CdTe on lattice-matched InSb (001) substrates has enabled the demonstration of ultra-high single-crystal quality. MgCdTe alloys exhibit minimal lattice mismatch to CdTe, while exhibiting carrier confinement capabilities beneficial to optoelectronic and solar cell structures. A record carrier lifetime of 2.7 µs has been previously reported by utilizing a 30nm Mg0.46Cd0.54Te barrier on the top and bottom of the CdTe layer. In this study, CdTe/MgCdTe double heterostructures featuring MgxCd1-xTe barrier layers of various thicknesses and Mg compositions higher than 46% will be grown by MBE on top of InSb (001) substrates. Using a two-chamber MBE system wherein an ultra-high vacuum transfer chamber connects a III-V growth chamber and a II-VI growth chamber, first the InSb substrate oxide is thermally desorbed under high Sb flux followed by the growth of a 500nm InSb (001) buffer layer. Upon transfer into the II-VI chamber, CdTe/MgCdTe double heterostructures are subsequently grown. Since MgTe oxidizes readily in air, X-ray photoemission spectroscopy is employed for stoichiometric analysis of the MgCdTe surface. X-ray diffraction is used to assess the crystal quality and determine the Mg concentration in the barriers for a given Mg cell temperature. In addition, photoluminescence spectroscopy is conducted on each sample to assess optical quality and carrier confinement capabilities. Samples topped with thin, protective CdTe caps are grown and comparisons made to uncapped samples. Successful optimization of these high Mg composition double heterostructures prompts the investigation of integrating such structures into CdTe solar cell designs.
Symposium Organizers
Yong-Hang Zhang, Arizona State University
Jacek Furdyna, University of Notre Dame
Henri Mariette, Institut Néel-CNRS
Maria Tamargo, The City College of New York
EP6.3: Heterovalent II-VI/III-V Semiconductor Integration II
Session Chairs
Wednesday AM, March 30, 2016
PCC North, 200 Level, Room 228 B
9:30 AM - *EP6.3.01
Low Temperature Metalorganic Chemical Vapor Deposition of Semiconductor Thin Films for Surface Passivation of Photovoltaic Devices
Ishwara Bhat 1
1 Rensselaer Polytechnic Institute Troy United States,
Show AbstractPhotovoltaic devices like solar cells and IR detectors have gained immense popularity today, due to their wide range of applications. Surface passivation of such devices improves their electrical as well as chemical stability greatly. Several materials and techniques for the deposition of passivation films have been investigated over the years. This talk will describe various II-VI and II-V compound semiconductors for the purpose of surface passivation of different photovoltaic devices. First part of the talk will focus on the surface passivation of HgCdTe IR detectors. Cadmium telluride (CdTe) has been the preferred material for surface passivation of HgCdTe IR detectors. Deposition of CdTe using metalorganic chemical vapor deposition (MOCVD) ensures good conformal coverage on mesa-etched high aspect-ratio focal plane array (FPA) structures for modern day IR detectors. MOCVD of CdTe generally requires high temperatures (~350°C), but HgCdTe surfaces cannot be exposed to such high temperatures as Hg is volatile and may deplete from the surface. A new process was developed that involves depositing films at much lower temperature than the conventional processes used till now. A hot-wall reactor with two clam-shell heaters has been designed to facilitate cracking of the precursors at a high temperature (~600°C), while maintaining the substrate at a lower temperature (135°C - 170°C). Deposition rates of 40 – 70 nm/hr were recorded by varying the temperature from 135°C to 170°C. Favorable conformal coverage on high aspect ratio HgCdTe structures was obtained. Significant improvement in minority carrier lifetime was demonstrated in HgCdTe samples passivated with CdTe deposited at 135°C. Further modification in the reactor design was attempted that led to the increase in deposition rates of CdTe greatly from 70nm/hr to 420nm/hr. Other films such as CdS and/or ZnS deposited by atomic layer deposition process are also being studied for optimal conformal coverage of high aspect ratio structures. Surface passivation effect has also been demonstrated on another photovoltaic application, a novel double heterostructure solar cell. This structure uses a thin p-type Si bulk wafer (~200μm) as the absorbing layer with double hetero-junctions on both sides of the Si wafer using p-type zinc telluride (ZnTe) and n-type zinc selenide (ZnSe) on opposite surfaces. High quality p-ZnTe layers have been grown using MOCVD at 400°C with carrier concentration of 3×1 018 cm-3 and resistivity of 0.33 Ω-cm. Preliminary work showed a remarkable improvement in the minority carrier lifetime of Si light absorbing layer after passivation with a thin layer of ZnTe. This work is partially supported by NSF Award # DMR-1305293, a subcontract from EPIR and First Solar and a DOE-BAPVC subcontract from ASU.
10:00 AM - *EP6.3.02
CdSe/ZnSe Ultra-Thin Quantum Wells on GaAs(001) for Photovoltaic Applications
Isaac Hernandez-Calderon 1
1 Physics Department - Cinvestav Mexico City, DF Mexico,
Show AbstractDue to the limited availability of large area, high structural quality, and economically convenient II-VI substrates several II-VI semiconductor devices have been developed after their growth on GaAs(001) substrates. Because of this, the ZnSe/GaAs(001) heterovalent heterostructure has been the subject of numerous studies, in order to overcome the serious problems that arise from the lattice mismatch, differences in thermal expansion coefficients, and interface chemical imbalance, which are serious obstacles to achieve the required performance of II-VI devices. However, ZnSe/GaAs(001) heterostructures have their own importance in the elaboration of photovoltaic devices. Long ago, in 1972, Sahai and Milnes [1] predicted that efficiencies around 15% and photovoltages around 800 mV could be achieved for solar cells based on this kind of heterostructures. Initial studies of n-ZnSe/p-GaAs solar cells resulted in efficiencies below 5% and open-circuit voltages lower than 700 mV [2-4]. It is evident that systematic studies of this heterostructure are necessary in order to improve their electrical characteristics for photovoltaic applications. In this work we present the results of the optical and electrical characterization of ZnSe/GaAs(001) and n-ZnSe/p-GaAs(001) heterostructures where the ZnSe layer contains insertions of CdSe ultra-thin quantum wells (UTQWs) produced by atomic layer epitaxy (ALE). The CdSe UTQWs have thickness of 1 to 3 monolayers. The ZnSe layers were grown by molecular beam epitaxy (MBE). Due to the higher band gap of ZnSe (Eg = 2.7 eV at 300 K) the spectral response (SR) extends beyond 3 eV. Furthermore, we observe an increase of the spectral response of the GaAs (below the band gap of ZnSe); it is attributed to multiple reflections within the ZnSe layer. We show that the SR of the heterostructures that contain multiple CdSe UTQWs is much larger than that of the heterostructures without the quantum wells. For example, a heterostructure containing 30 CdSe UTQWs (3 ML thickness) presents a spectral response around two orders of magnitude larger than a similar structure with thicker ZnSe layer but without the UTQWs [5]. The results obtained so far indicate that heterovalent heterostructures of CdSe/ZnSe UTQWs on GaAs(001) appear very attractive for the elaboration of novel solar cells and photodetectors.
Work in collaboration with the NanoSem Group.
Partially supported by Conacyt-Mexico
[1] R. Sahai, A.G. Milnes, Solid State Electron. 13, 1289 (1970).
[2] U. Blieske, T. Kampschulte, M. Saad, J. Söllner, A. Krost, K. Shatke, M.Ch. Lux-Steiner, Proceedings of the 26th IEEE PVSC, p. 939, 1997.
[3] O. de Melo, G. Santana, M. Meléndez-Lira, and I. Hernández-Calderón, J. Cryst. Growth 202, 971 (1999).
[4] D. W. Parent, A. Rodriguez, J. E. Ayers, F. C. Jain, Solid State Electron. 47, 595 (2003).
[5] D. A. Valverde-Chávez, F. Sutara, and I. Hernández-Calderón, AIP Conf. Proc. 1598, 171 (2014).
10:30 AM - EP6.3.03
Heterovalent Interface GaAs/ZnSe: Effect of MBE Growth and Annealing on Chemical and Physical Properties
Tatiana Komissarova 1,Mikhail Lebedev 1,Sergey Sorokin 1,Irina Sedova 1,Grigorii Klimko 1,Sergey Gronin 1,Wolfram Calvet 2,Evgenii Evropeytsev 1,Kirill Komissarov 1,Alla Sitnikova 1,Mikhail Drozdov 3,Sergey Ivanov 1
1 Ioffe Institute St. Petersburg Russian Federation,2 Helmholtz-Zentrum Berlin (HZB) Berlin Germany3 Institute for Physics of Microstructures RAS Nizhny Novgorod Russian Federation
Show AbstractHeterovalent III-V/II-VI semiconductor heterostructures are the prospective materials for optoelectronic, spintronic and photovoltaic applications. Special features of the heterovalent interface (HI), in contrast to the isovalent one, provide ample opportunities for designing heterostructures with tuned electronic structure and band offsets. Study of possibilities to control the HI properties by variation of growth conditions and ex-situ treatment is necessary for realization of these opportunities.
This paper is devoted to study of chemical, structural and electrical properties of the GaAs/ZnSe HI as dependent on the growth mode and ex-situ annealing. Additionally, the HI influence on the electrical and optical properties of the (Al,Ga)As/Zn(Mn)Se heterovalent quantum well (QW) structures was investigated.
Two types of the heterovalent structures were fabricated by MBE on GaAs(100) substrates. The first type structures contained Si-doped GaAs and undoped or Cl-doped ZnSe layers and were used to study the HI properties by SXPS, TEM, SIMS and electrical measurements (I-V curves and impedance spectroscopy). The thickness of the ZnSe layers was varied from 2nm (for SXPS measurements) to 300nm for SIMS and electrical measurements. The electron concentration in GaAs and ZnSe layers was changed from 2×1017 to 8×1017cm-3. In addition to as grown structures, the ex-situ annealed (500°C) ones were studied. The structures of the second type consisted of the undoped or Be-doped GaAs/AlGaAs QW placed near the HI and II-VI part comprising a Zn(Mn)Se layer. These structures were intended for studying the influence of HI on their optical and electrical properties. The following growth parameters were varied for the structures of both types: the initial GaAs surface reconstruction ((2x4)As or c(4x4)As); the use of Se pre-exposure of the GaAs surface; the growth mode of the Zn(Mn)Se layer (high-temperature (HT) (280°C) conventional MBE; low-temperature (210°C) or HT (280°C) MEE).
It has been found that band offset formation depends on atomic configurations at the HI and occurs within a finite thickness of the near-interface layer d. Atomic configurations at the HI, d and △EV values, chemical structure of the near-interface layer (stoichiometry, atoms interdiffusion) can be controlled by the variation of the HI growth mode and the ex-situ annealing. Study of the vertical electrical conductivity through the HI has revealed that there exist different mechanisms of electron transport: transport through the potential barriers induced by △EC and additional dipole electric field and transport through the space charge in the near-interface layer, formed due to atoms interdiffusion and stoichiometry violation. In some cases the space charge and dipole electric field at the HI affect strongly the PL intensity of the near-interface undoped GaAs/AlGaAs QW and the electrical conductivity of the 2DHG in the Be-modulation-doped QW.
The work is partially supported by RFBR grants.
10:45 AM - EP6.3.04
Optical Properties of Strain-Balanced InAs/InAsSb Superlattices Grown With and Without Bi as a Surfactant
Preston Webster 2,Jing Lu 2,David Smith 2,Yong-Hang Zhang 2,Shane Johnson 2
1 Arizona State University Tempe United States,2 Center for Photonics Innovation Tempe United States,
Show AbstractThe heterovalent integration of pseudomorphic IV, II-VI and III-V semiconductors at the GaSb lattice constant affords high-quality optoelectronic direct bandgap materials that cover the entire optical spectrum from ultraviolet to far infrared and beyond to semimetallic material. Although this bandgap range can be potentially covered by II-VI materials alone, the integration of III-V strain-balanced superlattice materials into the mix offers improved performance, ease of growth, reduced cost, and highly tunable materials for mid and long infrared applications. In particular, III-AsSbBi superlattices offer an abundant, easy to handle, and relatively safe substitute for HgCdSeTe alloys.
Strain-balanced type-II InAs/InAsSb superlattices are grown using molecular beam epitaxy on GaSb substrates at growth temperatures from 425 to 475 °C using As/In flux ratios from 1.2 to 1.3, Sb/In flux ratios from 0.3 to 0.5, and Bi/In flux ratios from 0.0 to 0.1. The InAs layers grow with a (2×4) surface reconstruction with and without Bi when the As/In flux ratio is 1.2, whereas the surface reconstructions are (2×3) with and (2×1) without Bi for larger As fluxes. The InAsSb surface reconstructions are (2×4) with and (2×3) without Bi when the As/In flux ratio is 1.2, whereas the surface reconstructions are (2×3) with and without Bi for larger As fluxes. The structural properties are assessed using X-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy, and the optical properties are assessed using spectroscopic ellipsometry and photoluminescence spectroscopy. InAs/InAsSb superlattices grown at 430 °C using Bi as a surfactant throughout the entire active region demonstrate improved integrated photoluminescence intensity for Bi/In flux ratios up to 1.0%.
One characteristic of InAs/InAsSb superlattices is the InAs-on-InAsSb interface that is not as abrupt as expected due to the surface segregation of Sb that continues to incorporate into the InAs layers when the Sb flux is stopped. Since the presence of a Bi surfactant layer reduces the sticking coefficient of Sb, another sample set is grown only using Bi during the growth of the InAs layer to examine the impact of a Bi surface layer on the unintentional incorporation of Sb in InAs. The integrated photoluminescence intensity of these superlattices grown at 425 °C improves for Bi/In flux ratios from 0.5% to 1.0%. The photoluminescence efficiency improves further for samples grown up to 475 °C, with additional improvement for Bi/In flux ratios as large as 10%. These samples are designed for optimal absorption (96% electron-hole wavefunction overlap) with a wavelength cutoff of 4.0 μm at low temperature. Using spectroscopic ellipsometry, the ground state miniband transition absorption coefficient of these structures is determined to be 4750 cm-1, which is in agreement with that expected for such a large wavefunction overlap based on previous absorption measurements.
EP6.4: III-Nitride/Si Integration
Session Chairs
Wednesday PM, March 30, 2016
PCC North, 200 Level, Room 228 B
11:30 AM - *EP6.4.01
Transmission Electron Microscopy Investigation of the Microstructure of Group III-N Nanowires and Their Interfaces with Si Substrates
Esperanza Luna 1,Javier Grandal 2,Miguel Angel Sanchez-Garcia 2,Enrique Calleja 2,Achim Trampert 1
1 Paul-Drude-Institute Berlin Germany,2 Universidad Politécnica de Madrid Madrid Spain
Show AbstractThe epitaxial growth of planar group III-N films on dissimilar substrates is accompanied by the formation of a high density of threading dislocations and other extended defects affecting their physical properties and limiting their successful application in optoelectronic devices such as lasers, detectors or solar cells. An alternative approach that is discussed since several years is the realization of perfectly aligned and laterally ordered nanowire ensembles where Si is frequently the substrate of choice. In addition to the general interest on the integration of III-V semiconductors with Si, the main advantages of the use of Si substrates arise from their low cost, crystal quality, doping capabilities, thermal conductivity (three times larger than sapphire) and the low lattice and thermal mismatch among the common substrates used to grow III-nitrides.
Because of the small footprint to the substrate, the free-standing nitride nanowires are expected to grow with minimized defect density. Similar to the case of planar semiconductor heterostructures, the knowledge of the nanowire microstructure, i.e., defect type, density and spatial distribution, is prerequisite for the basic understanding of the mechanical and optoelectronic properties. The small scale as well as the three-dimensional shape of nanowires requires electron microscopy techniques with high spatial resolution to obtain detailed structural information in the desired quality.
Here I present an overview of our investigations using transmission electron microscopy (TEM) techniques of the structural properties of III-N nanowires grown by plasma-assisted molecular beam epitaxy on Si substrates. We find that the mismatch between the III-N nanowire and the substrate is accommodated via a misfit dislocation network located at the interface plane and a strain relaxation process that is confined at the nanowire base. As a consequence, threading dislocations are not a main feature of the nitride nanowire microstructures. In particular, in this work, the interface structure between III-N nanowires and Si or AlN-buffered Si substrates is systematically described in dependence on the amount of lattice mismatch. A coincidence site lattice model for large mismatched systems is proposed and its capability is illustrated by selected examples.
Finally, although III-N nanowires are frequently free of threading dislocations, stacking faults are often observed, being the predominant extended defect in these structures, which significantly affect the optical properties.
12:00 PM - EP6.4.02
Low-Temperature Template-Assisted Fabrication of Hollow GaN Nano-Cylinders on Si Substrates
Ali Haider 2,Sevde Altuntas 3,Mehmet Yilmaz 1,Petro Deminskyi 4,Ibrahim Yilmaz 5,Fatih Buyukserin 3,Necmi Biyikli 2
1 National Nanotechnology Research Center Bilkent University Ankara Turkey,2 Institute of Materials Science and Nanotechnology Bilkent University Ankara Turkey,3 TOBB University Ankara Turkey1 National Nanotechnology Research Center Bilkent University Ankara Turkey1 National Nanotechnology Research Center Bilkent University Ankara Turkey,4 Institute of Microdevices, Photonics Department National Academy of Sciences of Ukraine Kiev Ukraine5 Turgut Özal University Ankara Turkey
Show AbstractNanostructured materials have attracted significant attention due to their applications in the fields of photonics, energy storage, nanosensors, and nanoelectronics. GaN has been considered as one of the most important inorganic compound semiconductors due to its superior properties such as high carrier mobility, electrical breakdown field, wide material band gap, high melting point, good thermal conductivity, biocompatibility and nontoxicity. Besides its thin film counterparts, one-dimensional (1D) semiconducting GaN nanostructures have recently attracted considerable attention due to their applications in sensing, photocatalysis, and light emitting diodes. High aspect ratio GaN nanostructures in the form of nanobelts, nanowires and nanotubes were synthesized using different techniques including vapor-liquid-solid crystal growth, laser-assisted catalytic growth, template-assisted synthesis, and etching. Among them, template-assisted synthesis is a simple way of producing nanostructures with controlled geometries, and properties. However, it is essential to select a suitable thin film deposition technique in order to obtain conformally coated, high aspect ratio 1D nanotemplates. Atomic layer deposition (ALD) is a powerful technique for coating high aspect ratio nanotemplates as it yields high conformality, high uniformity, sub-nm scale thickness control, and low impurity films grown at low temperatures.
In this work, we have performed the fabrication, integration to silicon substrates, and material characterization of freestanding vertical GaN hollow nanotubular arrays using template-assisted ALD synthesis technique. Fabrication, synthesis and integration to silicon substrate strategy, consists of the following steps:
1. Electrochemical anodization of aluminum foil to obtain anodized aluminum oxide (AAO) membrane, followed by transfer and sticking of AAO membrane to Si substrate using triton,
2. Ar and CHF3 based reactive ion etching (RIE) of clogged top layer of AAO membrane,
3. Si patterning with Ar and CHF3 based RIE using AAO membrane as hard mask template to obtain nanocylinder network on Si substrate,
4. Conformal GaN coating of nanocylinder network on Si surface via hollow cathode plasma assisted ALD (HCPA-ALD) growth using TMGa and N2/H2 plasma as Ga and N sources,
5. Ar based RIE of HCPA-ALD coated GaN from top surface of Si,
6. SF6 based isotropic RIE of silicon to obtain free standing GaN hollow nanocylinder network.
Optical and structural characterization of template-assisted grown GaN hollow nanocylinder networks have been performed by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and photoluminescence (PL) measurements.
For future work, (1) a better Si etch recipe using AAO template as patterning mask, and (2) more conformal III-N nanotube/nanowire networks are under development.
12:15 PM - EP6.4.03
Elastic and Plastic Stress Relaxation in Highly Mismatched SiGe/Si Crystals
Fabio Isa 1,Marco Salvalaglio 2,Yadira Dasilva 3,Mojmir Meduna 5,Michael Barget 2,Arik Jung 1,Thomas Kreiliger 1,Giovanni Isella 6,Rolf Erni 3,Fabio Pezzoli 2,Emiliano Bonera 2,Philippe Niedermann 7,Kai Zweiacker 8,Antonia Neels 8,Alex Dommann 8,Pierangelo Gröning 3,Francesco Montalenti 2,Hans von Kaenel 1
1 Laboratory for Solid State Physics ETH Zurich Zurich Switzerland,2 Materials Science L-NESS and Università di Milano-Bicocca Milano Italy3 Electron Microscopy Center EMPA Dübendorf Switzerland4 Condensed Matter Physics Masaryk University Brno Czech Republic,5 CEITEC Masaryk University Brno Czech Republic6 Physics L-NESS and Politecnico di Milano Como Italy7 CSEM Neuchatel Switzerland8 Center for X-Ray Analytics EMPA Dübendorf Switzerland
Show AbstractIn this work we study both theoretically and experimentally a new concept applicable to the epitaxial growth of dislocation-free semiconductor micro-crystals on a mismatched substrate with a thickness far exceeding the conventional critical thickness for plastic strain relaxation. This innovative concept is based on the out-of-equilibrium growth of compositionally graded alloys on deeply patterned substrates. We obtain space-filling arrays of individual crystals[1] several micrometers wide in which the mechanism of strain relaxation is fundamentally changed from plastic to elastic. It has been experimentally proven for SiGe/Si(001) alloys linearly graded at a shallow rate of 1.5 at. % Ge/micrometer up to a final Ge content of 80 at. % by defect etching, transmission electron microscopy (TEM), Raman scattering and high-resolution X-ray diffraction. A two-dimensional model[2] based on linear elasticity theory solved by the finite element method indicates the results to be valid far beyond the experimentally tested range of composition, grading rate and feature sizes. When combined with state-of-the-art layer transfer techniques, the concept may find application for the monolithic integration of a wide range of microelectronic and opto-electronic devices made from lattice mismatched semiconductors with processed Si wafers.
For plastically relaxed SiGe heterostructures we performed extensive defect etching and TEM analyses demonstrating that the ratio between misfit and threading dislocations, as well as their distribution along the [001] growth direction are strongly influenced by the elastic stress relaxation mechanism.
[1] C. V. Falub, H. von Känel, F. Isa, R. Bergamaschini, A. Marzegalli, D. Chrastina, G. Isella, E. Müller, P. Niedermann, L. Miglio, Science 2012, 335, 1330.
[2] M. Salvalaglio, F. Montalenti, J. Appl. Phys. 2014, 116, 104306.
12:30 PM - EP6.4.04
Growth and Strain Engineering of Ge Nanowires in an InAlAs Host by Spontaneous Phase Separation
Daehwan Jung 1,Joseph Faucher 1,Austin Akey 2,Samik Mukherjee 3,Matthew Cabral 4,Xiahan Sang 4,Daniel Ironside 5,Seth Bank 5,James LeBeau 4,Oussama Moutanabbir 3,Tonio Buonassisi 2,Minjoo Lee 1
1 Yale University New Haven United States,2 MIT Cambridge United States3 Polytechnique Montreal Montreal Canada4 North Carolina State University Raleigh United States5 University of Texas at Austin Austin United States
Show AbstractStrain engineering of nanowires (NWs) can be used to enhance and tune their electrical and optical properties. To date, core-shell heterostructure growth has been the main approach to apply strain to NWs. Alternatively, individual NWs may be manipulated by external stressors, though extensive processing is typically required. Nanocomposites grown by surface-mediated phase separation can also provide control of strain in nanostructures. However, this growth method has been mainly investigated in oxide material systems (e.g. BiFeO3/Sm2O3)1 and rare-earth-V/III-Vs (e.g. ErAs/InGaAs)2. Here, we demonstrate growths and strain control of self-assembled Ge NWs embedded in an In0.52Al0.48As (InAlAs) host by phase separation.
Growth was conducted using a III-V molecular beam epitaxy system equipped with a Ge effusion cell. Lattice-matched InAlAs buffers were first grown on (001) InP substrates at 500 °C. Next, 300 nm nanocomposite layers consisting of tensile Ge NWs embedded in InAlAs, were grown by codeposition of Ge, In, Al, and As2 over a range of growth temperatures, growth rates and Ge fluxes.
High-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) images and corresponding Ge energy dispersive x-ray spectroscopy (EDX) maps reveal vertical Ge NWs phase-separated from the InAlAs host. The NWs are coherent, single crystalline and free of extended defects. For a sample grown at 500 °C and 1 µm/hr, planar-view HAADF-STEM showed that the Ge NWs are 4-10 nm in diameter with a density of ~4.7x1010 cm-2.
Substrate temperature and Ge % greatly affect the formation of Ge NWs. For example, 5.8% Ge/InAlAs layers grown at 500 °C or above exhibit Ge NWs. However, samples grown below 500 °C with the same % Ge do not show any clear phase separation. We also found from EDX linescans that lowering the growth rate from 1 µm/hr to 0.2 µm/hr increases the Ge concentration in the NWs from ~70% to ~95%.
Raman spectroscopy of the Ge NW/InAlAs sample exhibits a distinctive Ge-Ge peak at 285.2 cm-1, which is not observed from a Ge/InAlAs alloy sample. The 16.1 cm-1 shift from the Ge substrate peak at 301.1 cm-1 can be correlated to 3.66 % biaxial tensile strain, which is close to the 3.72 % lattice mismatch between Ge and InAlAs. An even higher strain of 5.11 % was also obtained by growing Ge NWs in a relaxed In0.76Al0.24As host, demonstrating the ability to control the strain in the NWs.
We have investigated the growth and strain control of Ge NWs embedded in an InAlAs host via phase separation. We believe that Ge NW/InAlAs nanocomposites are an exciting new platform to tune the properties of Ge NWs over a wide range of tensile strain values.
1. Sophie A. Harrington and et al. Nature nanotechnology, 2011, 6, 491-495
2. Driscoll D. C. and et al. Journal of crystal growth, 2003, 251, 243-247
12:45 PM - EP6.4.05
Transforming Common III-V and II-VI Semiconductor Compounds into Topological Heterostructures: The Case of CdTe/InSb Superlattices
Qihang Liu 1,Xiuwen Zhang 1,L. B. Abdalla 1,Alex Zunger 1
1 University of Colorado-Boulder Boulder United States,
Show AbstractCurrently known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. We show via first-principle calculations how a heterovalent superlattice made of common semiconductor building blocks can transform its non-TI components into a topological nanostructure, illustrated by III-V/II-VI superlattice InSb/CdTe. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. We reveal the relationship between the interfacial stability and the topological transition, finding a “window of opportunity” where both conditions can be optimized. Once a critical InSb layer thickness above ~ 1.5 nm is reached, both [111] and[100] superlattices have a relative energy of 5-14 meV/A2higher than that of the atomically exchanged interface and anexcitation gap up to ~150 meV, affording room-temperature quantum spin Hall effect in semiconductor superlattices. The understanding gained from this study could broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next-generation super-structured functional materials.
This work was supported by Basic Energy Science, MSE division (Grant DE-FG02-13ER46959) and National Science Foundation (Grant DMREF-13-34170).
EP6.5: Heterovalent Integration on Si for Device Application
Session Chairs
Wednesday PM, March 30, 2016
PCC North, 200 Level, Room 228 B
2:30 PM - *EP6.5.01
II-VI Material Integration with Silicon for Detector and PV Applications
Sivalingam Sivananthan 1,Timothy Gessert 2
1 Univ of Illinois-Chicago Chicago United States,2 National Renewable Energy Laboratory Golden United States
Show AbstractLarge area, low cost, high quality II-VI materials are needed for the next generation of infrared detector and solar photovoltaic systems. Current state-of-the-art infrared focal plane arrays (IRFPAs) are fabricated with HgCdTe absorbers. CdZnTe (CZT) substrates are the natural choice for the molecular beam epitaxy (MBE) growth of HgCdTe. However, the lack of large-area CZT substrates due to their high production cost, and the difference in thermal expansion coefficients between CZT substrates and Si readout integrated circuits (ROICs), are drawbacks. The thermal expansion mismatch between the detector substrate and the ROIC is of particular concern for large-area IRFPAs that require repeated thermal cycling. Silicon substrates provide advantages in terms of the area (up to 9 inch diameter), reliability during processing and thermal cycling, and reduced cost compared with CZT. MBE growth on silicon substrates makes it possible to fabricate infrared detector arrays with reduced cost and more dies per wafer. Although not exactly lattice-matched to HgCdTe, CdTe/Si composite substrates available today at EPIR have allowed the fabrication of low cost, short wavelength, mid-wavelength and long wavelength IRFPAs. We will present our work on the material growth, fabrication and demonstration of low noise equivalent difference temperature (NEDT), high operability IRFPAs fabricated with MBE-grown HgCdTe on Si-based substrates. We will also discuss MBE HgCdTe use in the fabrication of high density vertically integrated photodiode (HDVIP) mid-wavelength IRFPAs at DRS Technologies and the anticipated advantages of horizontal integration in the industry.
II-VI based semiconductors have also been proven to have great potential for solar photovoltaic applications, especially high efficiency concentrated photovoltaics (CPV). Currently, III-V based multijunction solar cells have reached over 44% efficiency, but their high cost prevents them from been deployed for large scale power generation. With the single crystal CdTe on Si technology developed by EPIR, and the potential of achieving high efficiency (21.5% for poly-CdTe single junction solar cell by First Solar in 2015), CZT-based multijunction solar cell fabricated on Si substrate could potentially reduce the cost by factor of five. CdTe has a near-ideal band gap for a single-junction solar cells, and Cd1−xZnxTe/Si with 0.45
3:00 PM - *EP6.5.02
Enabling High-Efficiency III-V/Si Tandem Cells through Dislocation Engineering
Minjoo Lee 1,Kevin Nay Yaung 1,Joseph Faucher 1,Jordan Lang 1
1 Yale Univ New Haven United States,
Show AbstractGaAsyP1-y / Si tandem cells represent a promising path to simultaneously achieving high efficiency and the requisite 25-year lifetime, while leveraging the Si solar knowledge base and low-cost infrastructure. However, dislocation densities exceeding 108 cm-2 in III-V cells on Si have historically hampered the efficiency of such approaches. We analyzed strain relaxation dynamics during the early phases of III-V on Si growth and demonstrate that threading dislocation densities (TDD) can be controlled by balancing dislocation nucleation and glide. Understanding these dislocation dynamics enabled GaAsyP1-y solar cells on GaP/Si with low TDD values of 6.4-7.8×106 cm-2, only 20% higher than those co-grown on bulk GaP wafers and comparable to current record-breaking metamorphic solar cells. A promising bandgap-voltage offset of 0.55 V was obtained for GaAsyP1-y solar cells on GaP/Si, among the best reported for devices grown on GaP/Si. In addition, proof-of-concept Si subcells with high peak internal quantum efficiency values were fabricated after III-V deposition steps, a critical requirement for high-efficiency tandem cells. The results in this work show a realistic path towards dual-junction GaAsyP1-y/Si cells with efficiencies exceeding 30%.
3:30 PM - EP6.5.03
Ultrafast and Valence Sub-Band Dependent Auger Recombination in InGaN Quantum Wells
Kristopher Williams 1,Nicholas Monahan 1,Daniel Koleske 2,Mary Crawford 2,Xiaoyang Zhu 1
1 Columbia University New York United States,2 Sandia National Laboratories Albuquerque United States
Show AbstractThe quantum efficiency of InGaN quantum well (QW) based light emitting diodes is believed to be limited by Auger recombination at high injection currents, leading to the phenomenon known as efficiency droop. Identification of the Auger mechanism as the dominate source of efficiency loss in these devices has been hampered by the indirect nature of luminescence-based experimental techniques in quantifying carrier loss from the QW. Here, we directly observe carrier loss from a single InGaN quantum well on the sub-picosecond timescale using time-resolved two photon photoemission (TR-2PPE) spectroscopy. This technique enables us (i) to quantify electron population at or near the conduction band minimum (CBM) without interference from electrons above the CBM that may undergo leakage into the GaN region, (ii) to directly search for Auger electrons that are predicted to lie at ≥ 2 eV above the CBM, and (iii) to preferentially excite from particular valence sub-bands based on light polarization and optical selection rules. This allows us to present, for the first time, direct experimental observation of the time-dependent electron population and band-resolved Auger recombination in InGaN QWs. The ultrafast loss of conduction band electrons is quantitatively accounted for by a third-order process consistent with Auger recombination. Selective excitation of different valence sub-bands reveals that the Auger rate constant can decrease by two orders of magnitude as the effective hole mass decreases. These observations establish the Auger mechanism as the dominant loss channel at transient carrier densities > 2x1018 cm-3 and suggest the feasibility of minimizing the Auger recombination rate via band structure engineering.
3:45 PM - EP6.5.04
InGaN-Based Strained Quantum Well Laser with Etched Structure Analized by X-Ray Diffaction and Fluorescence of Indium Using High-Resolution Micro-Beam
Toshiya Yokogawa 1,Yasuhiko Imai 2,Shigeru Kimura 2
1 Yamaguchi University Ube Japan,2 JASRI Hyogo Japan
Show AbstractIII–V nitrides are promising materials for blue and UV lasers and light emitting diodes. The lasers and LEDs consist of hetero-structures of lattice-mismatched materials, such as AlGaN and InGaN, which cause lattice strain. Complicated strain distribution in real devices should exist because microfabrication was performed by ethching to obtain laser oscillation and high optical extraction efficency. It is important to clarify the micro-area strain distribution in real devices to improve device performance and reliability. In this paper, we investigate the micro-area strain distribution by the simultaneous analysis with the X-ray diffraction and fluorescence of Indium using high-resolution X-ray micro-beam in lasers and LEDs with etched structure.
The epi-structure of 2 μm thick n-type Al0.05Ga0.95N cladding layer, 3 nm thick InGaN three-quantum well active-layer, and 0.4 μm thick p-typeAl0.05Ga0.95N cladding layer were grown on GaN (0001). After the growth, the ethched structure of ridge stripe with the width of 1.5 μm and the height of 0.4 μm was formed parallel to the [1-100] direction by dry etching. For micro-area strain analysis, we used the Super Photon ring-8 GeV (SPring-8). Focused X-ray beam produced by using a phase zone plate for the photon energy of 10 keV and a refractive lens for 30 keV was used. The size of the focused beam is about 0.5 μm and 1 μm at the photon energy of 10 keV and 30 keV, respectively. Micro-area mapping of the reciprocal space map and the X-ray fluorescence of Indium were simultaneously taken at 0.5 μm intervals by scanning the measuring position across the ridge stripe region.
By this mapping, we clearly confirmed the micro-area change of the strain distribution around 1.5 μm-wide ridge stripe region due to the reduction of biaxial tensile strain of p-AlGaN layer. The reciprocal space map of the symmetric (0004) reflection at a photon energy of 30 keV of dry etched region and ridge stripe region. We confirmed that, in the ridge stripe region of the laser, the c-axis lattice parameter of p-AlGaN was elongated by the reduction of the biaxial tensile strain of p-AlGaN perpendicular to [0001] direction. The peak originated from p-AlGaN layer shifts towards smaller qz side, which means the elongation of the lattice of p-AlGaN towards [0001] direction. Also the peak from InGaN slightly shifts towards smaller qz side, although the lattice relaxation was not observed. It is supposed that this InGaN peak shift is induced by the p-AlGaN lattice elongation. The X-ray fluorescence of Indium were simultaneously taken at 0.5 μm intervals, at the exact same position where the reciprocal space map was obtained. We observed an increase in the intensity of X-ray fluorescence of Indium around the 1.5 μm-wide ridge stripe region. This increase should be related to the micro-area change of the strain distribution in the ridge stripe region. We also report on the results of ethched LEDs.
EP6.6: II-VI/IV-VI Semiconductor and Oxide/Semiconductor Integration
Session Chairs
Wednesday PM, March 30, 2016
PCC North, 200 Level, Room 228 B
4:30 PM - *EP6.6.01
Two Dimensional Electron Gas in CdTe/PbTe Heterojunction
Huizhen Wu 1,Yong Zhang 2
1 Department of Physics and State Key Laboratory for Silicon Materials Zhejiang University Hangzhou China,2 Department of Electrical Engineering University of North Carolina at Charlotte Charlotte United States
Show AbstractThe lattice-structure-mismatched CdTe/PbTe heterostructures are emerging materials with unique properties and promising applications in mid-infrared optoelectronics and spintronics. High density two-dimensional electron gas (2DEG) at the interface of CdTe/PbTe heterojunction has been investigated experimentally and theoretically. CdTe thin-films grown on PbTe(111) using molecular beam epitaxy are found to have high electron mobilities and carrier concentrations by low temperature Hall effect measurements and SdH oscillations. Density functional theory modeling reveals that the epitaxially grown CdTe/PbTe heterojunction forms twisted interfaces that exhibit unusual structural and electronic properties: The CdTe eplilayer on the PbTe(111) forms spontaneously a high density 2DEG over 1013 cm-2 near the interface, without the need for doping, which explains the experimentally observed high carrier density and mobility. CdTe/PbTe is a much simpler heterostructure yet able to offer high electron mobility comparable to and one or two order magnitude higher sheet carrier density than the best achieved values for those of Si and II-VI based quantum well structures relying on modulation doping. Abnormal enhancement of mid-infrared light emission and phonon blockage effect in the CdTe/PbTe heterostructures are also observed. The quantum oscillations observed in the 2DEG suggest that the interface of the CdTe/PbTe is a Dirac electron system with a non-zero Berry phase, indicating an interesting topological nature in this novel heterojunction.
References
(1) Shuqiang Jin, et al, Two-dimensional electron gas at the metastable twisted interfaces of CdTe/PbTe (111) single heterojunctions, Phys. Rev. B 87, 235315 (2013)
Bingpo Zhang, et al, Quantum Oscillations in a Two-Dimensional Electron Gas at the Rocksalt/Zincblende Interface of PbTe/CdTe (111) Heterostructures, Nano Lett., 15, 4381 (2015)
(3) Bingpo Zhang, et al, Phonon blocking by two dimensional electron gas in polar CdTe/PbTe heterojunctions, Appl. Phys. Lett. 104, 161601 (2014)
5:00 PM - EP6.6.02
Epitaxial Integration of Ni/VO2 Heterostructures on Si (001)
Srinivasa Rao Singamaneni 1,Gabrielle Foley 1,John Prater 1,Jagdish Narayan 1
1 North Carolina State Univ Raleigh United States,
Show AbstractVO2 is a strongly electron correlated oxide, which undergoes a first order metal-insulator (MIT) and structural phase transitions (SPT) (rutile to monoclinic) much above the room temperature 340K, making it attractive for high temperature sensor applications. Previous works1-3 have shown that the stress associated with structural changes associated with the MIT in VO2 can produce significant changes in magnetic properties of a ferromagnetic over layer such as Ni. This control of the magnetic properties could be important for many technological applications. However, the current use1-3 of r-sapphire and α-Al2O3 as substrates can be restrictive in the microelectronics industry. In addition, all the previous works1-3 focused their studies on polycrystalline Ni and VO2 films, which do not allow the precise controlling of the associated properties due to poor reproducibility of polycrystalline films. To address the above issues, for the first time, we have attempted to investigate the magnetic and electronic properties of Ni/VO2 films that are epitaxially integrated on Si (001) -a technologically important and CMOS compatible substrate. These films were grown onto YSZ-buffered Si (001) substrate by pulsed laser deposition (PLD) using domain matching epitaxy (DME) paradigm. Ni was grown both in nanoscale islands and layered form. The XRD results showed that the Ni, VO2 and YSZ layers were grown epitaxially with out-of-plane orientations of (111), (020) and (002), respectively. We found that the hysteresis in resistance vs. temperature curves for the VO2 thin films was relatively unaffected by the Ni and the heterostructures revealed the ferromagnetic features characteristics of Ni. We will present and discuss our comprehensive experimental findings. The current study represents a significant step forward in the epitaxial integration of Ni/VO2 heterostructures on a computer chip.
1 J. de la Venta, S. Wang, J. G. Ramirez, and I. K. Schuller, Appl. Phys. Lett., 102, 122404 (2013).
2J. de la Venta, S. Wang, T. Saerbeck, J. G. Ramirez, I. Valmianski, and I. K. Schuller, Appl. Phys. Lett., 104, 062410 (2014).
3T. Saerbecka, J. de la Venta, S. Wang, T. Saerbeck, J. G. Ramirez, M. Erekhinsky, I. Valmianski, and I. K. Schuller, J. Mater. Res.,29, 2353 (2014).
5:15 PM - EP6.6.04
Insight into Group V Dopant Incorporation in Polycrystalline CdTe via 3-D TOF-SIMS Tomography
Steven Harvey 1,Eric Colegrove 1,Ji-Hui Yang 1,Su-Huai Wei 2,Wyatt Metzger 1
1 National Renewable Energy Lab Golden United States,2 Beijing Computational Science Research Center Beijing China
Show AbstractTo enable continued development of CdTe photovoltaics with open-circuit voltage (Voc) approaching 1V, dopants and processing are being studied to increase acceptor concentrations and minority carrier lifetime. Using phosphorous as a dopant in CdTe single crystals and very large-grain poly-crystals, we have achieved lifetimes and doping densities exceeding 10 ns and 1016 cm-3, respectively.[1,2] In the current study we have investigated in detail the diffusion of phosphorous dopants into CdTe via TOF-SIMS high resolution 3-D tomography.[3]
A set of CdTe single crystals and polycrystalline thin- films were prepared for this study by being subjected to phosphorous diffusion annealing treatments with a range of diffusion times and temperatures in a Cd-rich ambient. The specimens were then analysed via TOF-SIMS under different measurement modes to allow the maximum amount of diffusion information to be extracted, standard depth profiling provides a high data-density to enable extraction of the diffusion coefficients from the profiles, while high resolution tomography (100 nm lateral resolution) provides a 3-D representation of the distribution of the phosphorous dopant to a depth greater than 10 microns. Analysis of the tomography data allows extraction of the grain core and grain-boundary dopant concentrations as a function of depth. The extracted diffusion coefficients for grain core and grain boundary diffusion agree well with literature data for phosphorous diffusion in CdTe, however the grain core diffusion data requires an additional fast diffusion mechanism in order to explain the high dopant concentrations which were measured deep into the films. The diffusion profiles from single-crystal specimens also show both a fast and slow diffusion mechanism, indicating a fast diffusion mechanism which is not limited just to polycrystalline material.
[1] E. Colegrove, D. S. Albin, H. Guthrey, J. Burst, H. Moutinho, S. Farrell, M. Al-Jassim, W. Metzger, S. Harvey “Phosphorus doping of polycrystalline CdTe by diffusion,” in Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd, New Orleans.
[2] J. Burst, D. Albin, J. Duenow, M. Reese, S. Farrell, D. Kuciauskas and W. Metzger, "Advances in control of doping and lifetime in single-crystal and polycrystalline CdTe," in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, Denver.
[3] E. Colegrove, S. Harvey, W. Metzger, “Phosphorus diffusion in CdTe single crystals and polycrystalline thin-films” Manuscript in Preperation