I describe the fabrication and performance of three-dimensionally structured backcontact thin film photovoltaic devices. The devices are fabricated on a backbone of two coplanar electrodes in an interdigitated comb structure. The widths of the thousands of individual wires in each of the comb electrodes, as well as the spacing between adjacent wires on the two electrodes, are of micrometer order. Functional devices are fabricated by first electrodepositing an n-type semiconductor on one comb electrode and then depositing a p-type semiconductor absorber over the entire structure. The geometry mimics the unobstructed absorber of the highest efficiency commercial silicon back contact devices. However, it requires only a single patterning process because electrodeposition with different applied potentials permits selective deposition, and thus differentiation, of the two interdigitated combs in the electrodes-first, semiconductors-last processing scheme.
I detail device performance as characterized through standard current density vs voltage (J-V) measurements under AM1.5 illumination and through External Quantum Efficiency (EQE) measurements of the spectral dependence of photon conversion to current extraction. Devices exhibit open circuit voltages between 400 and 500 mV and short circuit current densities exceeding 13 mA/cm^2 in J-V measurements. EQE exceeds 40% at energies above the CdTe bandedge of ~850 nm (bangap ~1.45 eV),
As with planar devices, processing includes annealing and device performance is strongly influenced by the post-processing quality of the absorber layer, which also manifests in a dependence of the short circuit current of the devices on the electrode wire pitch and gap. Because the devices do not have the top surface transparent conducting oxide, metal finger electrodes or n-type semiconductor window layer that block light in standard 1-D geometry devices the EQE decreases only a small fraction even at 300 nm, a wavelength well beyond the 510 nm absorption edge at which the CdS window layer in standard devices would be absorbing light and preventing it from reaching the CdTe absorber (or the 710 nm value for CdSe that replaces the CdS in some of the devices in this study).
Device performance is compared to quantitative modeling, the model predictions capturing trends observed with variation of electrode comb dimensions as well as semiconductor type and quality.
D.U. Kim, C.M. Hangarter, R. Debnath, J.Y. Ha, C. R. Beauchamp, M.D. Widstrom, J.E. Guyer, N. Nguyen, B.Y. Yoo, and D. Josell, Backcontact CdSe/CdTe Windowless Solar Cells, Solar Energy Materials and Solar Cells, Submitted.
C.M. Hangarter, B.H. Hamadani, J.E. Guyer, H. Xu, R. Need, and D. Josell, Journal of Applied Physics 109, 073514 (2011).

The conventional approach to achieving high efficiency photovoltaic solar energy conversion beyond the single junction limit of ~33% requires integrating multiple subcells with different bandgaps into a single monolithic, epitaxially grown, series-connected multijunction cell. This approach has resulted in solar cell efficiency as high as 44% [1], but it is limited to 3-5 subcells due to lattice and current matching constraints. However, carrier thermalization can be significantly mitigated, relative to current designs, by a very different ‘full spectrum&’ multijunction architecture with 6-12 subcells, yielding realistic limiting efficiencies in the 50-70% range [2]. In this work, we report on the design and fabrication of an optimal set of eight subcells for a full spectrum module, with bandgaps chosen in order to maximize efficient collection across the entire solar spectrum, maximizing intrinsic device efficiency and minimizing thermalization and transparency losses. The eight subcells are operated optically in parallel and electrically independently and form the receiver for one of several solar spectrum splitting photonic structures, including i) a light-trapping filtered concentrator, ii) a polyhedral specular reflector, and iii) a holographic spectrum-splitting concentrator. The eight subcells utilize appropriate III-V semiconductor materials for each bandgap range: three subcells have compound absorber layers lattice-matched to InP (In_0.53Ga_0.47As at 0.74 eV, In_0.71Ga_0.29As_0.62P_0.38 at 0.94 eV, and In_0.87Ga_0.13As_0.28P_0.72 at 1.15 eV), four subcells have compounds lattice-matched to GaAs (GaAs at 1.42 eV, Al_0.1Ga_0.9As at 1.58 eV, Ga_0.51In_0.49P at 1.84 eV, and Al_0.20Ga_0.32In_0.48P at 2.15 eV), and one compound is on GaN (Ga_0.85In_0.15N at 2.61 eV). We choose lattice-matched III-V materials to maximize crystal quality and leverage epitaxial lift-off to minimize cost. The design of each solar cell heterostructure is uniquely tailored to maximize energy conversion for a narrow slice of the solar spectrum, with slice spectral bandwidths ranging from 88 nm to 357 nm, allowing for particular design advantages in choosing the window and absorber layer materials. Six of these subcells have been grown, fabricated, and tested, and results for cell integration into full spectrum module testbeds will be presented. These subcell results are used, in combination with spectrum splitting optical efficiency, to estimate total module efficiency for such spectrum splitting photovoltaic structures.
[1] Solar Junction NREL-verified record efficiency, at 947 suns; www.sj-solar.com (October 2012).
[2] A. Polman and H.A. Atwater, Nature Materials, 11 pp. 174-177 (2012).

In order to make high efficiency and low cost solar cell modules, the concept of third generation of photovoltaic modules have been provided. The first generation solar cell: Crystal Si solar cell including single crystal and poly-crystal Si solar cell; The second generation solar cell: Thin film solar cell including Si base thin film, CIGS, CdTe and III-V thin films; The third generation solar cell is the future high efficiency and low cost solar cell modules, such as low cost quantum dots solar cell, thin film tandem and triple cell modules, Si base III-V solar cell and nanotechnology with no vacuum technique such as printable technologies and etc. This paper reviewed the advantages and disadvantages of each generation of the solar cell modules and technologies and discussed the research and development of the third generation of photovoltaic modules including the detail technology developments.

Solar cells based on III-V materials provide the highest power conversion efficiency. However, the cost of III-V materials and cells is too high. This talk is on our development of III-V Nanostructures in order to enable low-cost thin, flexible, freestanding single crystalline III-V film solar cells. We have developed advanced nanocone photon management to enable nearly complete absorption of above bandgap photons with very thin III-V materials and demonstrated good power efficiency.

For the past 15 years, dilute nitrides were the holy grail of the high efficiency solar cell field. Historically, the InGaP/InGaAs/Ge solar cell structure produced the highest efficiencies, but its band gaps were not ideal, motivating research into new materials and structures. Dilute nitrides, specifically GaInNAs alloys, had a band gap that was tunable in the range near 1 eV that could not be reached by the more traditional phosphide and arsenide materials. Further, they could be lattice-matched to the standard GaAs and Ge substrates used for high efficiency, multijunction solar cells. The efficiency of multijunction solar cells, which stack individual subcells of increasing band gap, depends on the number of subcells and their respective band gaps, as well as the quality of their material. It was well known that dilute nitrides could provide more ideal band gaps, but previous research had failed to produce material of sufficient quality. Recently, Solar Junction has developed dilute nitride material that has enabled the long-sought goal of multijunction solar cells with dilute nitride subcells. Solar Junction&’s 3 junction solar cells, using a dilute nitride bottom cell, set a new world record for solar cell efficiency of 44.0% in October, 2012. In this presentation, the improvements in dilute nitride material quality that enabled these high efficiencies are discussed, including the use of antimony as both a surfactant and a constituent element. Antimony is found to inhibit the incorporation of nitrogen-related defects, however material quality degrades with too much antimony; the antimony range for optimal minority carrier transport properties is discussed. Tailoring the composition to achieve small amounts of compressive strain, and reducing the concentration of impurities, especially hydrogen and carbon, are also shown to be important for attaining high material quality. With these improvements, dilute nitride solar cells no longer need to rely on wide depletion widths for current collection. Among other advantages, this enables lower resistivity, allowing the material to maintain high performance under concentrations of sunlight equivalent to several thousands of suns. In addition, for the first time, it is shown that dilute nitride material with excellent minority carrier transport properties has been achieved across a very wide band gap range, from 0.8 eV - 1.3 eV, using the quintary GaInNAsSb alloy. With this range of band gaps, solar cell efficiencies of 50% are possible with 6 junction solar cells. Designs for solar cells with 4, 5 and 6 junctions that can achieve efficiencies above the current world record are presented, including designs that utilize more than one dilute nitride subcell. Current-voltage curves and efficiencies are shown for 4 junction solar cells that have 2 dilute nitride subcells, demonstrating further the high material quality that has been achieved with GaInNAsSb.

Future ultrahigh efficiency multijunction solar cells may incorporate many more subcells (4-20) than current designs (2-4). By coupling spectrum-splitting optics with electrically independent III-V compound semiconductor subcells, 50-70% cell efficiency should be feasible [1]. As the number of subcells increases, the spectral bandwidth and photon flux incident on each subcell decreases. For compound semiconductor subcells with high radiative efficiencies, photon recycling can occur between subcells, increasing the photon flux to some subcells. The combined effects of dramatically narrowed spectral bandwidth and the extent of radiative recoupling have been largely neglected in previous cell efficiency models, but they significantly affect the efficiency of such multijunction cells. We report a method to calculate the consequences of these effects and experimentally validate our method with measurements for high efficiency GaAs cells to gain insight into the challenges for approaching the limit of solar energy conversion.
We consider two subcells in a multijunction architecture to illustrate how photon flux influences subcell performance. For a GaAs subcell with radiative recoupling from a higher bandgap subcell, decreasing spectral bandwidth induces a significant decrease in the open circuit voltage (V_oc). Only in the case of perfect radiative recoupling between subcells is the open circuit voltage equivalent to the value defined by detailed balance calculations [2]. We also examine the voltage at the maximum power point (V_max) and the subcell efficiency. V_max declines sharply with decreasing spectral width, but unlike for V_oc, V_max declines even for perfect radiative recoupling. Radiative recoupling has the largest impact on efficiency for very small spectral bandwidths where the radiation of the GaAs subcell plays a significant factor. When there is radiative recoupling between subcells, subcell efficiency increases for smaller spectral widths. Because the subcell receives more photon flux from reradiation of adjoining subcells in this regime than from the photon flux in its own spectral width, the efficiency increases. In the absence of radiative recoupling, there is a sharp decline in efficiency towards zero for small spectral widths. Although a subcell can more efficiently convert photons closer to its bandgap as the spectral bandwidth decreases, a logarithmic decline in voltage overpowers this effect, reducing subcell efficiency. We experimentally verify these trends in the absence of radiative recoupling, showing a decrease from over 37% to 15% subcell efficiency when decreasing the spectral width from 0.55 to 0.04 eV for a GaAs subcell. Broadly, these results inform the question of how many subcells are needed for optimal efficiency from a multijunction solar cell with and without radiative recoupling, and how this efficiency depends on subcell quality.
1. Polman and Atwater, Nat Mater, 11 (2012).
2. Abrams, et al, Spring MRS (2012).

Vapor-liquid-solid grown Cu-catalyzed Si microwire arrays have shown great promise as a solar absorber material. Single wire devices have demonstrated diffusion lengths greater than 100 microns, and wire arrays have been shown to have absorption exceeding an equivalent volume of planar material. Furthermore, the arrays can be infilled with a polymer and peeled off of the Si substrate to yield a flexible device, after which the substrate can be reused. However solar conversion efficiency for Si homojunction diffused, radial junction wire array solar cells is limited by the use of a single Si absorber layer. To exceed this limit, we are exploring the growth of III-V/Si microwire tandem cells with III-V growth directly on Si microwires to more efficiently convert the solar spectrum while retaining all of the benefits of the wire array substrate.
Strained and compositionally graded GaInP layers were grown with metallorganic chemical vapor deposition on chemically cleaned, catalyst-free, oxide masked Si microwire arrays. We use a two step growth process, whereby a thin conformal nucleation layer is deposited at 460°C before high temperature growth at 610°C, both at a reactor pressure of 100 mbar and V/III molar flow ratio of 50. We use an oxide mask patterned along the length of the wire to tune the contact area and geometry of the GaInP layer from a conformal coating to wire tip constrained growth via selective epitaxy only on the exposed surface of the Si wire. High resolution x-ray diffraction (XRD) confirmed the growth of epitaxial layers aligned to the growth axis of the wires and the underlying Si(111) wire growth substrate. A compositionally graded stack of ~200 nm thick layers was grown with each layer targeted to have 0.4% lattice mismatch, up to a final composition of Ga0.66In0.34P, a direct gap material, as confirmed by XRD and time-integrated photoluminescence measurements. Transmission electron microscopy of focused ion beam prepared radial and axial cross sections along single wires is used to quantify the defect propagation and strain relief along the facets of the wires.
Solar cell device stacks were grown on degenerately doped p+ Si microwires, with heavily doped p+ GaInP buffer layers, p-type Ga0.66In0.34P base, n-type emitter and a lattice matched AlGaInP window layer. The performance of single wire devices is evaluated under simulated AMG1.5G sunlight and monochromated light to evaluate photovoltaic performance and spectral response.

Multi-junction (MJ) solar cells have enabled very high efficiencies due to the effective utilization of the solar spectrum. These MJ solar cells are usually composed of complicated multi-layers on GaAs or Ge wafers using elaborative growth techniques. Therefore, these cells tend to be expensive and lack material flexibility. On the other hand, the MJ solar cells based on mechanical stacking have been considered as an alternative approach to produce MJ solar cells [1-2]. The mechanical stacking enables high efficiency and low-cost because of the flexible combinations of individually processed cells. We have recently reported an advanced bonding method using conductive nanoparticle alignments with simple mechanical stacking process [3]. In this report, we demonstrate a high-efficiency GaInP/GaAs/InGaAsP three-junction solar cell (eta; = 22.5%) fabricated by this method.
Our bonding method is the integration of conductive nanoparticle alignments with a classic bonding technique (van der Waals bonding). Conductive Pd nanoparticles were aligned on a bottom cell through the use of self-assembled block copolymer (polystylene-block-poly-2-vinylpyridine) templates. The typical domain size and concentration were 50 nm and 1×10^10 /cm^2, respectively. A top cell was separated from the growth substrate by epitaxial lift-off (ELO). Finally, two cells were stacked through van der Waals bonding. Because of the formation of ohmic-contacts between semiconductors and nanoparticles, two cells were interconnected electrically. In addition, light can pass through the interface without absorption loss because of the extremely thin nanoparticle thickness and low surface coverage (<20%). Therefore, low bonding resistance (~2 Omega;cm^2) as well as low interfacial optical loss (<2%) were possible. Using this method, we fabricated a GaInP/GaAs/InGaAsP three-junction solar cell. A two-junction GaInP (Eg-1.89 eV)/GaAs (Eg-1.42eV) top cell was stacked onto a single junction InGaAsP (Eg-1.15 eV) bottom cell through the Pd nanoparticle alignment. The photovoltaic performance was investigated under AM1.5 solar spectrum illumination (1 sun, 100 mW /cm^2). It was revealed that the total efficiency was 22.5% with open-circuit voltage, short-circuit current density and fill factor of 2.86 V, 10.6 mA /cm^2 and 0.75, respectively. The obtained efficiency was in good agreement with the theoretically predicted value (25.4%). These results suggested that this bonding method is highly useful to achieve high-efficiency mechanically stacked multi-junction solar cells. The efficiency would be further improved (>30%) by appropriate combinations of solar cells under current matching. Reference: [1] K. Tanabe, et. al., Appl. Phys. Lett., 89, 102106(2006) [2] K. Makita, et. al., 21st International Photovoltaic Science and Engineering Conference, 2B-4O-10 (2011). [3] K. Makita, et. al., 27th European Photovoltaic Solar Energy Conference, 1AO.9.6 (2012).

Achieving high conversion efficiencies has a very beneficial impact on the costs of photovoltaic systems since both system costs are inversely proportional to the conversion efficiency. Although the best energy conversion efficiencies are being attained with III-V semiconductors, the high costs of III-V semiconductor substrates are recognized as the most critical obstacle to the wide deployment of III-V based solar cells. Single-crystalline III-V nanowires can be synthesized on lower-cost, lattice-mismatched substrates such as silicon or polysilicon, and thereby constitute a promising pathway to efficient, yet affordable solar cells. But even with single-crystalline materials quality the performances of III-V nanowire solar cells have so far been inferior to those of planar, bulk III-V devices. The fundamental challenge of nanowire solar cells has been to obtain high open-circuit voltages, which is due to low external fluorescence yields that nanowires most often suffer from, leading to sizeable voltage penalties. In nanowires, it is typically the large surface-to-volume ratios that lead to enhanced non-radiative surface recombination of photogenerated carriers because of which fluorescence yields become very low. Here, we report on photoluminescence measurements on single InP micropillars that are nominally undoped and grown on silicon. InP is a material that exhibits exceptional surface recombination velocities that can be orders of magnitude lower compared to GaAs surfaces. Since the pillars exhibit the wurtzite crystal structure rather than the more common zincblende structure, the bandgap of the InP material is 70 mV higher at 1.42 eV. Utilizing a core-shell growth mode the diameter of pillars can be scaled to micrometer-dimensions while the single-crystallinity of the material is preserved. Using the intimate relationship between the Fermi-level split within a light-absorbing material and its external fluorescence, a current-voltage (I-V) characteristic of the material can be constructed without the need to realize diodes or electrical contacts. This method is particularly useful for the characterization of material systems in very early development stages. At room temperature, the InP pillars exhibit Fermi-level splits larger than 0.9V under illumination intensities comparable to 1 sun, which is just 480mV below the material&’s bandgap. By comparing photoluminescence intensities obtained at room temperature and 4 Kelvin we can extract the internal fluorescence yield of single InP pillars, which can exceed 30% under highly concentrated illumination intensities. These results suggest that InP could be indeed a promising material system for nanowire photovoltaics.

Semiconductor nanowires (NWs) are promising candidates for high-performance, low-cost solar cells since they allow the epitaxial growth of high-performance III-V materials on Si substrates[1]. We will report on the growth, processing and characterization of nanowire array-based solar cells with 13.8% efficiency [2]. First, gold particles were patterned on InP substrates with a 500 nm pitch, using nanoimprint lithography. Then, about 1.5 mu;m long InP nanowires were grown using DEZn and TESn as doping precursors, to create an axially defined p-i-n junction. HCl was used to prevent radial overgrowth. The nanowires were processed as-grown with a transparent conductive oxide as top contact to create 1x1 mm solar cells, with 4 million nanowires per cell.
The solar cells were investigated using a sun simulator at Fraunhofer ISE CalLab reference setup. Although the 180 nm-diameter NWs only covered 12% of the surface, the photocurrents were 71% of the theoretical maximum for an InP solar cell. This is six times the limit in a simple ray optics description, and comparable to the record planar InP cell. To understand the absorption, we used full three-dimensional electromagnetic optical modeling [3, 4]. We find excellent agreement between the spectra of modeled absorption and experimentally measured external quantum efficiency.
[1] M. T. Borgström et al., IEEE Journal of Selected Topics in Quantum Electronics 17, 1050 (2011).
[2] J. Wallentin et al., Science In press http://dx.doi.org/10.1126/science.1230969 (2013).
[3] N. Anttu, and H. Q. Xu, Physical Review B 83, 165431 (2011).
[4] J. Kupec, R. L. Stoop, and B. Witzigmann, Opt. Express 18, 27589 (2010).

New semiconductor functionalities imply mainstream Si technology to be extended to other semiconducting materials with optical and electrical properties beyond those of Si. In order to exploit the advantages of the III-V/Si system, significant challenges will need to be met. Defects due to the lattice or thermal expansion coefficients mismatch (4.1 % and 120 %, respectively at 300 K for GaAs/Si), and anti-phase domains (APDs) must be avoided.
In order to solve the inherent issues related to the integration of GaAs on Si, the most promising III-V/Si combination, we chose to adapt the highly successful concept of 3-dimensional (3D) hetero-epitaxy recently reported [1]. Our approach involves Ge towers on top of Si pillars as a substitute to Ge substrates, characterized by a very small lattice mismatch (~0.07%) and comparable thermal expansion coefficients (~15%) with respect to GaAs.
Si(001) substrates off-cut 6 degrees towards [110] were patterned by conventional photolithography and deep reactive ion etching. Patterns with uniformly spaced square pillars with widths from 2 µm to 15 µm were used. Two microns tall Ge towers were grown on patterned Si by low-energy plasma-enhanced chemical vapor deposition (LEPECVD). GaAs regrowth was performed by metal organic vapor phase epitaxy (MOVPE). In order to investigate the viability of the 3D epitaxy method to incorporate optoelectronic devices on Si, similar structures were grown having embedded 2.5, 5 and 10 nm thick In0.13Ga0.87As quantum wells (QWs) into the GaAs matrix.
Scanning electron microscopy revealed that GaAs deposition on Ge/Si towers exceeding 5x5 mu;m2 led to the formation of GaAs truncated pyramids exhibiting well developed (111) facets and a (001) top facet. However, growth on smaller patterns led to full pyramidal shaped crystal formation. Surprisingly, GaAs crystals grown on off-cut Si substrates present a rotation of the growth axis with respect to Ge towers below. The excellent structural quality of GaAs/Ge/Si grown by MOVPE was confirmed by high-resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM).
Low temperature (T=5 K) photoluminescence (PL) integrated intensity of excitonic recombination on GaAs pyramids is similar to the one collected on homoepitaxial GaAs. PL recombination energy in patterns up to 5x5 mu;m2 is centered in the GaAs band gap (Eg=1.519 eV), showing no effect of either misfit or thermal strain. GaAs grown on larger patterns exhibits a redshift of the excitonic emission which agrees with the strain measured by HRXRD.
For the strained InGaAs QWs embedded in GaAs strain relaxation of the matrix was again observed by PL of QWs on patterned areas as opposed to planar ones.
Financial support by the Swiss federal program Nano-Tera through project COSMICMOS is gratefully acknowledged.
[1] C. V. Falub, H. von Känel, F. Isa, R. Bergamaschini, A. Marzegalli, D. Chrastina, G. Isella, E. Müller, P. Niedermann , L. Miglio, Science 335, 1330 (2012).

Large, direct bandgap materials directly grown on silicon substrates are highly desirable for low-cost, visible light sources and multi-junction solar cells. We demonstrate single crystalline, wurtzite-phase InGaP and InGaAs/InGaP micropillar structures grown on silicon substrates without catalysts. High internal quantum efficiency is achieved despite of large lattice mismatch.
A new metastable growth mode of InGaAs/GaAs core/shell nanopillars on silicon substrates was reported using low temperature MOCVD without catalysts. We showed optically pumped laser and light emitting diodes on these high quality material. In this paper, we show that InGaAs/InGaP and InGaP micropillars can be grown on silicon using the same growth techniques. With this growth mode, high quality micropillars can be grown in presence of not only a large mismatch with the substrate but also between the core and shell. We show that micron-diameter InGaAs core remains single wurtzite-phase with InGaP capping layer as thick as 200 nm. We also demonstrate pure InGaP micropillars growth on silicon with nominal Indium composition around 66 %. Micro-photoluminescence measurements are performed in a continuous-flow liquid-helium cryostat with both 660 nm continuous wave laser diode and 760 nm Ti:Sapphire femtosecond laser.
Photoluminescence intensity is enhanced by near two orders of magnitude with changing InGaP capping layer thickness from 0 to 50nm, indicating that InGaP provides good passivation to inhibit electron-hole recombination at the interface. The InGaP layer shows very good wurtzite crystalline phase and does not induce any stacking faults into the InGaAs core. Low threshold room temperature lasing oscillation was achieved by optically pumping with a Ti:sapphire femtosecond laser.
With nominal 66 % of Indium composition, the bandgap of pure InGaP micropillars is around 1.47 eV at room temperature. By performing excitation-intensity dependent photoluminescence measurements, we plot integrated photoluminescence intensity versus pump power density of a single as-grown micropillar in logarithmic scale, the slope equals to one for all excitation levels. This can be interpreted as integrated intensity is proportional to the excitation intensity, which indicates radiative recombination dominates inside the micropillar. Compared with best results of InGaAs/InGaP pillar, the photoluminescence intensity of pure InGaP pillar is about two orders of magnitude higher, testifying the high quality material and its promise for solar cell application. Increasing Indium composition to reach even higher bandgap would be a promisng pathway to integrate lattice-mismatched InGaP directly on silicon substrate.

The direct conversion of solar energy into electricity with various solar cell devices has been actively pursued for an alternative energy resource to replace fossil fuels. Among the materials used in current solar cells, III-V compound semiconductors have shown very attractive energy conversion efficiency; for example, single junction GaAs or InP solar cells could have efficiency of more than 20% by using single crystalline GaAs or InP wafers. However, the high fabrication cost of such single crystalline wafers has prohibited the wide application of those materials for significant amount of energy generation.
In this work, we have explored vapor-liquid-solid (VLS) growth technique to obtain high quality InP thin films. Specifically, indium films were first deposited on molybdenum foils through vacuum evaporation or electroplating and then heated above the indium melting temperature in phosphorous vapor environment. The dissolution and supersatuation of phosphorous in liquid indium resulted in the precipitation of thin film InP crystals. The film thickness can be well controlled from 100nm to above 10 um by the initial indium amount. These films have grain size above 100 microns, “intrinsically” n doping with impurity concentration at around 2xE16 /cm^3 and electron hall mobility above 1000 cm^2/V-s. The films can also be controllably p doped at growth by including magnesium or zinc metal into the indium films.
The thin film VLS growth method produces high quality III-V semiconductors without the use of expensive epitaxial substrates. Meanwhile, the precursor (indium and phosphorus) utilization reaches above 90%. Those advantages could significantly reduce the fabrication cost of III-V semiconductors for large scale solar panel application.

Bismuth containing III-V semiconductors have recently attracted interest, creating prospects for a new class of long wavelength and terahertz devices. Incorporation of Bi in GaAs causes a large reduction of the bandgap, allowing 1 eV bandgaps and beyond to be reached on GaAs with less strain than for In, Sb and N (0.7% mismatch for 1 eV). This property along with the fact that GaAs1-xBix shows strong photoluminescence (PL) indicates that this material has promise for applications in optical devices such as multi-junction solar cells and long wavelength emitters and detectors [1,2].
The GaAs1-xBix epilayers used in this study were grown by molecular beam epitaxy (MBE) at substrate temperatures of 200-350°C [3]. We report the p-type hole conductivity in un-doped GaAs1-xBix epilayers for a wide range of Bi-content (0The bandgap energy and optical absorption coefficient of epilayers with 0Annealed samples with 2.2% Bi were tested as THz photoconductive switches. The results show THz emission is increased compared to similar LT-GaAs based devices. The thermal annealing also improved the photoluminescence (PL) intensity for temperatures up to 630°C and then decreased for higher temperatures, without significantly affecting the PL peak wavelength.
[1] T. Tiedje et al., Int. J. Nano. 5 ,p. 963 (2008).
[2] R.R. King et al., Prog. Photovolt: Res. Appl. 20, (2012).
[3] R.B. Lewis et al., Appl. Phys. Lett. 101, 082112 (2012).

Low concentrations of Bi alloyed with GaAs has been reported to result in a large band gap reduction (~80meV/ x Bi in GaAs1-xBix). Theoretical and experimental studies -confirm that this new semiconductor alloy possesses interesting (opto)electronic properties. GaAsBi semiconductors have a relatively temperature insensitive band gap energy. Moreover, a large spin-orbit splitting makes GaAsBi potentially a suitable candidate for spintronic applications.
In this work, we have studied the growth of GaAsBi heterostructures using metal-organic vapor phase epitaxial (MOVPE) growth. The films were characterized using Atomic Force Microscopy, Scanning Electron Microscopy, Energy-dispersive X-ray spectroscopy, High resolution X-ray Diffraction, Raman Spectroscopy and photoluminescence in order to assess the films quality and the Bi content in the GaAsBi/GaAs heterostructures grown on GaAs substrates.
A comparative study was made between various sequencing of reactants into the reactor. The conventional simultaneous feeding of precursors was compared with a pulsed MOVPE method. Pulsed growth resulted in the incorporation of Bi into the GaAs epitaxial layer with a reduced x-ray line width and more well-defined superlattice structure as determined by X-ray rocking curves; compared to those layers grown using the simultaneous introduction of reactants, no-pulsed growth process.

Moreover, the effect of growth temperature
(360oC-430oC) and precursors flow rate on the film properties and Bi incorporation was determined. A narrow range of growth conditions were determined, outside of which, no incorporation of Bi into the films was found or Bi droplets formed on the surface.
Outside of this temperature range, the materials properties degraded, particularly at higher temperatures. The growth rate of GaAs typically decreases with decreasing growth temperature. Surprisingly, the GaAsBi growth rate was almost temperature insensitive. This can be rationalized by considering Bi (TMBi) serving as a catalyst or enhancing the deposition rate when present. Through careful control of the reactant flows and temperature, ‘bulk&’ GaAsBi films could be grown with acceptable material properties at these lower temperatures. The highest Bi concentration in our GaAsBi heterostructures was about 4% as evaluated by XRD measurements.

The AlInP material system has the largest direct-bandgap accessible with non-nitride III-V semiconductors finding applications in yellow-green optoelectronics (560nm-590nm). These devices can be used in solid-state lighting, projectors and displays. AlInP is 1% lattice-mismatched to GaAs at these wavelengths thus preventing direct-growth of devices. We have grown high-quality AlInP films on relaxed compositionally graded InGaAs buffers via MOCVD to achieve threading dislocation densities below 1e6/cm2 suitable for device fabrication. AlInP was found be susceptible to phase-separation into Al-rich and In-rich regions, the microstructure of such sub-micron regions being very dependent on substrate offcut. CuPt-B type atomic ordering and phase-separation were simultaneously seen in the microstructure of AlInP and linked by surface-driven processes. We will show that such phase-separated and ordered films have broad room-temperature yellow-green emission but increasing substrate offcut can narrow the emission peak, important for LED fabrication. Both growth temperature and doping can eliminate phase-separation as well as atomic ordering. We have also observed surface composition modulation on the micron-scale in these AlInP films via elemental and bandgap mapping. We believe that this phenomenon might be linked to the cross-hatch morphology arising from the underlying network of misfit dislocations in the graded buffer. The effects of substrate offcut, strain grading rate and temperature on composition modulation will be discussed along with a possible mechanism. We will also present on the effects of threading dislocation glide on the AlInP surface during growth. Our results together show very interesting features of this material system and its interaction with the graded buffer, ultimately important in the design of light emitting devices in this technologically relevant wavelength range.

III-V compounds such as InGaAs, InAs, InSb have great potential for future low power high speed devices (such as MOSFETs, QWFETs, TFETs and NWFETs) application due to their high carrier mobility and drift velocity.The development of good quality high k gate oxide as well as high k/III-V interfaces is prerequisite to realize high performance working devices. Besides, the downscaling of the gate oxide into sub-nanometer while maintaining appropriate low gate leakage current is also needed since devices are getting smaller and smaller. The lack of high quality III-V native oxides has obstructed the development of implementing III-V based devices on on Si template. In this presentation, we will discuss our efforts to improve high k/III-V interfaces as well as high k oxide quality by using chemical cleaning methods including chemical solutions, precursors and high temperature gas treatments. The electrical properties of high k/InSb, InGaAs, InSb structures and their dependence on the thermal processes are also discussed. Finally, we will present the downscaling of the gate oxide into sub-nanometer scale while maintaining low leakage current and a good high k/III-V interface quality.

Understanding of defects in semiconductors is a fundamental factor to determining the material characteristics and its further technological applications. In the case of orientation patterned Gallium Arsenide (OP-GaAs), which are periodic gratings of inversion domains separated by anti-phase boundaries (APBs), the quality of the crystals is crucial for high conversion efficiency in the mid-infrared and terahertz laser sources. Defects with electro optical signatures contribute to optical losses and therefore, understanding them is a step toward to the improvement of the OP-GaAs crystals; in particular, the domain boundaries seem to play a major role in the efficiency conversion [1]. Therefore, a better understanding of their structure will be a step forward to reduce the optical losses.
Transmission electron microscopy including aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular dark field (HAADF) was used to investigate the structural perfection of such domains. Since images in STEM are proportional to the square of the atomic number (Z2) of the atomic columns this allowed us to resolve the Ga-As dumbbells leading to a direct determination of the growth polarity of particular domains. We found that these boundaries are formed on (110) planes with alternating Ga-Ga and As-As bonds. These boundaries are not always perfect and led to the formation of the rotation microtwins in each domain. These microtwins are referred to as an orthotwins with low-energy Ga-As bonds at the matrix/twin boundaries. The growth polarity within the orthotwin is reversed in comparison to the matrix, therefore, formation of these defects can influence the performance of the devices built on such orientation-patterned GaAs and can explain the reduced emission in earlier cathodoluminescence studies [1].
[1] O. Martinez, M. Avella, V. Horteano, J. Jimenez, C. Lynch, and D. Bliss, Journal of Electronic Materials, Vol. 39(6), 806-810 (2010).
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors appreciate the use of the TEM facility at the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory.

We report a tunable lattice parameter epitaxial template for high quality lattice-matched III-V growth [1]. This design enables access to lattice-matched band gap ranges not available at or near the single-crystal wafer lattice constants of GaAs, Ge, InP or Si. This “virtual substrate” consists of a large-area thin epitaxial film removed from its growth substrate and allowed to coherently relax strain until achieving its composition-dictated bulk lattice parameter. Supported on a reactor-compatible handle, it acts as template for epitaxial growth of any thickness of material directly at the desired lattice parameter without needing graded buffer layers to accommodate mismatch.
We have fabricated virtual substrates 50 mm in diameter from 40 nm films of InxGa1-xAs and InxGa1-xP grown by MOCVD on InP and GaAs substrates respectively. The films are grown with a composition-dictated mismatch to the substrate ranging from -1.5% (compressive) to 1.2% (tensile) and are elastically strained as verified by high resolution x-ray diffraction (XRD) measurements showing relaxation <1%. After growth a viscoelastic wax applied to the film surface provides support during removal of the substrate. The viscoelastic deformation of the wax allows the film strain to relax coherently while maintaining a near-planar configuration. Bonded to a handle substrate and wax removed, the III-V film can serve as epitaxial template for future growth. XRD symmetric and asymmetric rocking curve measurements verify template lattice constants are dictated by the film composition, specifically 5.62, 5.70, 5.80 and 5.83 Å. These give full access to the wide range of energy band gaps in the GaInAlAsP material family in lattice-matched designs and remove the layer thickness restriction imposed by strain-induced dislocation formation. In particular, the 5.80 Å virtual substrate allows fabrication of a lattice-matched three junction InGaAs/InGaAsP/InAlAs multijunction solar cell (MJSC) with detailed balance efficiency of 56.2% improving over current MJSC designs both by using more optimal band gaps and by eliminating the high dislocation density of lattice-mismatched growth. Additionally, using a silicon wafer for mechanical support enables integration of III-V and silicon devices on the same chip.
Virtual substrate growth performance was tested through fabrication of lattice-matched InxGa1-xAs single junction solar cells at 5.80 (x = 0.36) and 5.83 Å (x = 0.43). XRD measurements verify epitaxial growth at the designed lattice constant. Transmission measurements indicate absorption up to the expected band edge. The devices exhibit photoluminescence. TEM characterization of the growth interface will be presented, along with characterization of defect type and density and comparison to similar devices grown lattice mismatched to InP.
1. M. S. Leite et al., Adv. Materials 23, 33, p. 3801 (Sep, 2011).

Bi-containing III-V compounds are of significant interest for optoelectronic devices working in the Near Infrared Range (NIR) and Terahertz. The incorporation of Bi is a challenge, however, and synthesis conditions must balance trade-offs in Bi incorporation and device-relevant properties, such as light emission. Very low temperature growth during MBE is an effective means of creating kinetic limitations and therefore useful for synthesizing compounds wherein clustering, phase separation, or segregation are thermodynamically driven.
Herein, we report on a study aimed at determining the trade-offs in structural and optical characteristics as determined by x-ray diffraction and photoluminescence (PL) intensity, respectively. Samples with Bi concentration up to 4.9% were achieved over a range of growth temperatures from 290°C to 380°C and growth rates from 0.15mu;m/h to 0.81mu;m/h.
Over this range of conditions, when samples are grown with conditions reducing kinetic limitations (low growth rate and high growth temperature), their morphologies are characterized by a distinct surface undulation along [0-11], increasing the surface roughness from 1nm to around 30nm. Moreover, we clearly observe a trade-off between the structural and optical quality as MBE growth conditions are varied from more to less kinetically limited. When samples are grown with reduced kinetic limitations, they show higher 300K PL intensity but poorer structural quality as determined by high resolution x-ray diffraction HRXRD (004) omega;-2theta;. The full width at half maximum (FWHM) is used to assess the film&’s structural quality. Our result shows that samples with higher PL intensity have larger (004) omega;-2theta; FWHM which increases from approximately 50 arc seconds to 110 arc seconds. This behavior is the same for HRXRD (004) omega; scans and asymmetric (224) Reciprocal Space Maps (RSM) showing a larger FWHM along the Δomega; direction.
In addition, the reduction in crystalline quality is confirmed by a signature observed in the Raman spectrum. The GaAs TO (Γ) phonon mode (270 cm^(-1)) is forbidden due to site symmetry for an ideal zinc-blende structure. We observe that the intensity of this peak increases with the PL intensity, showing that the symmetry of the crystal is broken in less kinetically limited samples. [1][2] We speculate that the trade-off is related to the formation of Bi clusters. Bi clusters readily form during epitaxy due to Bi&’s low solubility and therefore conditions are favorable for formation as kinetic limitations are reduced during MBE.
[1] X. Lu, D. Beaton, R. Lewis, T. Tiedje, and Y. Zhang, "Composition dependence of photoluminescence of GaAs1-xBix," Applied physics letters, vol. 95, pp. 041903-041903-3, 2009.
[2] S. Imhof, A. Thranhardt, A. Chernikov et al. "Clustering effects in Ga (AsBi)," Applied physics letters, vol. 96, pp. 131115-131115-3, 2010.

GaAs metal-oxide-semiconductor (MOS) devices suffer from Fermi-level pinning, which is mainly due to the high trap density of states at the oxide/GaAs interface. These interface trap states mainly come from the dangling bonds at the oxide/GaAs interface, since the dielectric oxide is usually either amorphous or polycrystalline. In this work, we develop an atomic layer epitaxy (ALE) process to grow an epitaxial layer of high-k dielectric oxide, La_2-xY_xO₃, on GaAs(111)A. The heteroepitaxial structure of oxide/GaAs should reduce the density of the interface dangling bonds, since a perfect epitaxial interface is supposed to have no dangling bonds, and therefore the interface trap density of states (D_it) should be small.
Both of cross-sectional transimission electron microscopy (TEM) and high-resolution X-ray diffraction (HRXRD) confirm that high-quality epitaxial La_2-xY_xO₃ thin films are achieved on commerical GaAs substrates by our ALE process. Moreover, GaAs MOS capacitors made from this epitaxial structure show very good interface quality with small frequency dispersion and low interface trap densities. In particular, the La₂O₃/GaAs interface, which has a lattice mismatch of only 0.04%, shows very low D_it in the GaAs bandgap of below 3×10¹¹ cm^-2eV^-1 near the conduction band edge. The La₂O₃/GaAs capacitors also show the lowest frequency dispersion of any dielectric on GaAs. This is the first achievement of such low trap densities for oxides on GaAs.

When faced with bandwidth, power, and signal integrity issues on conventional metal interconnects in silicon microelectronics, industry has committed to adopt high-capacity fiber-optic technology to build high-performance optical interconnect systems. Recently developed hybrid silicon platform has emerged as one of the most promising device platform for practical applications. Among a variety of demonstrated hybrid silicon lasers, microring lasers were designed for their intrinsic advantages of small footprint, low power consumption and flexibility in wavelength division multiplexing (WDM), etc. Here we review recent progress in unidirectional microring lasers and device thermal management.
Directional bistability, the ability of a laser to operate either in the clock-wise (CW) or counter-clock-wise (CCW)) mode due to structural symmetry, is a unique characteristic of ring lasers. While useful for some applications, bistability is undesirable for ring lasers used in optical interconnects. We demonstrate an unidirectional microring laser by without additional power consumption and chip complexity for the first time. Unidirectional emission is achieved by integrating a passive reflector that feeds laser emission back into laser cavity to introduce extra unidirectional gain. We show that the length of the passive reflector is a critical parameter in determining the lasing behavior.
To overcome the device heating bottleneck in hybrid silicon lasers, we also demonstrated the design and experimental evidence of device heating reduction by employing a metal thermal shunt. By etching through the buried oxide layer in silicon-on-insulator substrate and filling with high thermal conductive metal, device joule heating can be “shorted” to the silicon substrate. Simulation and Thermal reflectance measurement show over 3X improvement. Direct comparison in devices with and without thermal shunt indicates the clear device performance improvement as well.

The continuing development of a wide range of mutually compatible single-crystal III-V absorber materials has led to 44% conversion efficiency in three-junction concentrator solar cells, with the promise of ~50% when a forth junction is successfully integrated. At the same time, the material quality (at least of epitaxial GaAs) is so nearly ideal that the radiative (Shockley-Queisser) limits of single-junction efficiency are being approached. As a consequence of the reciprocal nature of light absorption and emission in solar cells, optical design considerations of light emission becomes extremely important as this limit is approached. While light emitted from the front of the device is inevitable and desirable (as it must also be collected from the front), other optical pathways can be managed for optimal efficiency. Light emitted and subsequently reabsorbed within the same junction is known as photon recycling, while light emitted in one junction and absorbed in another is known as luminescent coupling. All other optical pathways, such as parasitic absorption in a substrate, merely result in energy loss.
Optical reflectors at the back of a junction promote photon recycling and can reduce dark current, thus increasing open-circuit voltage and efficiency. We demonstrate and model this effect in single-junction GaAs solar cells with one-sun efficiencies over 28%. We have systematically varied the effective optical reflectance at the back of the cell using metal reflectors with adjustable parasitic absorber layers to demonstrate these trends. I will discuss the challenging requirements and possibilities for omnidirectional, selective reflectors between junctions in multijunction solar cells to increase photon recycling in a top junction without starving lower junctions of transmitted light.
We have quantitatively modeled and measured luminescent coupling in multijunction solar cells. I will show luminescent coupling efficiencies up to 40% and how the effect can be non-linear with recombination current in non-ideal materials. This phenomenon has profound implications for measurement techniques, design considerations, and spectral sensitivity. The consideration of luminescent coupling is thus essential for the proper measurement of individual junction quantum efficiencies and short circuit currents. Furthermore, while multijunction solar cells are typically optimized for a standard spectrum, total energy yield calculations must consider the variation of the actual solar spectrum over a day and year. Solar cells designed with strong luminescent coupling are much less sensitive to these spectral variations.

Iron pyrite (cubic β-FeS2), known as an earth abundant and nontoxic semiconductor, has been attracting resurgent attention as a promising candidate for solar energy conversion thanks to its suitable band gap (0.95 eV indirect, 1.03 eV direct), high absorption coefficient (~6×10^5 cm^-1), excellent resistance to photocorrosion for photoelectrochemical applications. However, the application of pyrite for solar cells has been hindered by its low open circuit voltage (up to 200mV) and thus low efficiency (~3%), which is likely the results of phase impurities, and rich bulk and surface defects. Therefore, it is pivotal to understand their properties, and uncover the electrical conduction mechanism arising from defects at present. Pyrite nanorods (NRs) and nanoplates (NPs) offer a versatile platform to study their physical properties. Here, we report physical properties of phase-pure pyrite NRs and NPs that we have been synthesized using a vapor phase synthesis. Field effect transistor study of single as-grown pyrite NR or NP devices shows the p-type conduction behaviors that suggests holes as the majority carriers, whereas Hall measurements show signature of mixed conduction, which is strongly related to the defect state conduction. Combing these studies, we will reveal the mystery of pyrite conduction mechanism arising from bulk and surface defects. This understanding will allow us to enhance the solar cell performance of pyrite nanostructures and thin films.

Photon conversion material absorbs photon energy and converts into photons with different energies. One of applications of photon conversion materials is white LED lighting where photon conversion materials convert blue LED light into photons which have a wide spectrum of energy, i.e., white light. This work reviews a variety of photon conversion materials and technologies as well as their applications in a variety of areas. General requirements for materials design and processing are discussed. Future development in advanced photon conversion materials is proposed.

Due to recent advances, organic light-emitting devices, based on alternating current field-induced polymer electroluminescence (FIPEL), have gained attention as an alternative to conventional organic light-emitting diodes (OLEDs) for lighting applications. In this work, we investigate different ways of achieving effective charge generation and injection through the use of bulk heterojunction morphologies involving various blends of nanoparticles, small molecules, and carbon nanotubes (CNTs) with the light-emitting polymer. Discrete charge generation layers (CGLs) and charge transport layers (CTLs) are used to further enhance device performance. Devices consisting of an ITO electrode/polymer dielectric/CGL&CTL/polymer blend/CGL&CTL/metal electrode structure are used. We show that the bulk heterojunction blending of a nanophase with the luminescent polymer, in addition to discrete CGLs and CTLs, significantly improves brightness and efficiency of the devices. We will present a model for charge injection in this talk. The combination of solution processability and high performance in FIPEL devices may represent an exciting new approach to large panel lighting applications.

A study about the achievement of dichromatic white light-emitting diodes (LEDs) was performed. A series of dual wavelength LEDs with different last quantum-well (LQW) structure were fabricated. The bottom seven blue light QWs (close to n-GaN layer) of the four samples were the same. The LQW of sample A was 3 nm, and that of sample B, C and D were 6 nm, a special high In content ultra-thin layer was inserted in the middle of the LQW of sample C and on top of that of sample D. The XRD results showed In concentration fluctuation and good interface quality of the four samples. PL measurements showed dual wavelength emitting, the peak wavelength of blue light of the four samples were almost the same, sample A with a narrower LQW showed a emission peak at 495 nm and sample B, C, D showed weaker yellow light emission with peak wavelength longer than 560 nm. EL measurement was done at an injection current of 100 mA. Sample A only showed LQW emission due to holes distribution. Because of wider LQW, the emission peak wavelength of sample B, C and D was longer and peak intensity was weaker. Sample D with insert layer on top of LQW showed strongest yellow light emission with a blue peak. As the injection current increased, sample A showed highest output light power due to narrower LQW. Of the other three samples with the same thickness of LQW, sample D showed highest output power. Effective yellow light emission has always been an obstacle to the achievement of dichromatic white LED. Sample D with insert layer close to p-GaN can confine the hole distribution more effectively hence the recombination of holes and electrons was enhanced, the yellow light emission was improved and dichromatic white LED was achieved.

Zn₃P₂ is considered an ideal candidate for scalable photovoltaics, with a reported direct band gap of 1.5 eV and a long minority-carrier diffusion length (>5µm). However, much remains to be studied regarding the electronic properties of heterojunction devices incorporating Zn₃P₂ as a thin-film absorber. In this work, we have determined the energy-band alignment of epitaxial zb-ZnS(001)/α-Zn₃P₂(001), w-ZnO(0001)/α-Zn₃P₂(001) heterojunctions using high resolution X-ray photoelectron spectroscopy measurements. To improve the accuracy of the measurements, the position of the valence-band maximum for bulk ZnS, ZnO, and Zn₃P₂ films was estimated using density functional theory calculations of the valence-band density-of-states. The ZnS/Zn₃P₂ heterojunction was observed to be Type-I, with a valence-band offset, ΔE_V, of -1.19 ± 0.07 eV, resulting in a large conduction band spike at the interface. On the other hand, the ZnO/Zn₃P₂ heterojunction demonstrated a Type-III band alignment, with a ΔE_V of 3.55 ± 0.1 eV resulting in a tunnel junction band configuration. Both heterojunctions demonstrated band offsets which were significantly different from the Type-II alignments based on electron affinities that are predicted by Anderson theory.
In order to study the device properties of the ZnS/Zn₃P₂ and ZnO/Zn₃P₂ heterojunctions, current-voltage measurements were performed under dark and simulated Air Mass (AM) 1.5, 1-Sun illumination. As expected, the ZnO/Zn₃P₂ heterojunctions primarily showed ohmic behavior due to the tunnel junction alignment of the valence and conduction bands. However, the n⁺-ZnS/p-Zn₃P₂ heterojunctions demonstrated open-circuit voltages of >750 mV, indicating passivation of the Zn₃P₂ surface due to the introduction of the ZnS overlayer. Carrier transport across the heterojunction device was inhibited by the large conduction-band offset, which resulted in short-circuit current densities of <0.1 mA cm^-2 under 1 Sun simulated illumination. Hence, constraints on the current density will likely limit the direct application of the ZnS/Zn₃P₂ heterojunction to photovoltaics. However, these results suggest that ZnS can provide a good surface passivation layer for Zn₃P₂ and may be useful as a thin, intrinsic layer in metal-insulator-semiconductor (MIS) or semiconductor-insulator-semiconductor (SIS) photovoltaic devices.

Owing to the difficulty of producing GaN substrates, III-nitride-based devices are usually heteroepitaxially grown on lattice-mismatched substrates, which always leads to substantial defects and dislocations in the films. Threading dislocation is one of the most conspicuous crystal imperfections because it can propagate from the interface to the surface. For example, in light-emitting diodes (LEDs), they can penetrate though the multiple quantum wells (MQWs) and serve as nonradiative recombination centers that limit the internal quantum efficiency (IQE). In this study, we used hydrogen (H2) etch to enhance the IQE of a GaN-based LED epitaxial layer by removing dislocations in the MQWs. Transmission electron microscopy (TEM) examination verified that the dislocation sites can be etched to form cavities by H2. The LED epi-layers were etched by H2 under various temperature and H2 flow rate conditions. We found that when the temperature is equal to or higher than the growth temperature, the surface of the LED epi-layers can be etched to form numerous cavities, and that the cavity densities are comparable to the dislocation density, which implies that most of the dislocations have been removed. The Photoluminescence (PL) measurement shows that the IQE are enhanced after H2 etch. The IQE of the LED epi-layer etched at the optimal temperature was found to be larger than that of the unetched sample by an absolute value of about 40%. Thus, this study shows that hydrogen etch could be an efficient approach to enhance IQE. The concept may somehow be applied to other GaN-based devices.

The ZnO has attracted extended attention because of the superior material characteristics in optoelectronics and piezoelectronics. Furthermore, the physical properties of ZnO greatly resemble GaN, which include the lattice constant and thermal expansion coefficient, crystal structure and energy bandgap. Since the lower lattice mismatches between the wurtzite structure of GaN and ZnO are 0.4% for the a-axis direction and 1.9% along the c-axis direction. Based on their similar lattice constant, the related GaN/ZnO -based heterojunctions are suitable to realize high performance optoelectronic devices. Therefore, it has the potential to grow along the non-polar orientation direction such as a-plane (11-20) and m-plane (1-100) for the growth of GaN epilayer on ZnO layer. However, the growth mechanism and the optical properties of the GaN/ZnO interface are still not yet clear.
This work investigates the growth and characteristics of a-plane GaN/ZnO/GaN epitaxial structures, which can be applied to various optoelectronic devices. From the XRD results, we observed the formation of ZnGa2O4 and new crystal orientation of semipolar GaN (10-13) on GaN/ZnO/GaN heterostructures, due to the thermal decomposition and diffusion effects of ZnO. The in-plane orientation of ZnGa2O4 (220) have 55° twisted and the in-plane orientation of GaN (10-13) have 32° twisted. Moreover, the unique optical transitions intrinsic to heterovalent interfaces were found and analyzed. The full width at half maximum (FWHM) of photoluminescence spectra was decreasing with increasing excitation power in the power-dependent photoluminescence measurement. The carrier localization effect in the GaN/ZnO heterointerface is clearly observed. The carrier localization also results in strong luminescent intensity and dominates the emission spectrum at room temperature.

The high density of dislocations present in heteroepitaxial AlN and AlGaN films is known to limit the internal quantum efficiencies of III-nitride UV light emitting diodes (LEDs). Dislocations also play an important role in mediating the relaxation of the biaxial stresses that develop during and after the growth of these films. If stresses are not relaxed sufficiently during film growth, dislocation multiplication can occur within the active light-emitting region, and/or a dense network of cracks can form in the device. Therefore, prediction and control of dislocation behaviour is extremely important for developing better UV LEDs.
Recent experimental results indicate that dislocation movement by climb plays a significant role in the development of dislocation microstructure in III-nitride materials. In order to predict dislocation behaviour and enable the design of optimised dislocation reduction techniques, we have therefore developed a 3D dislocation dynamics model to simulate dislocation movement during growth of III-nitride films. In contrast to previous models, our model includes anisotropic elasticity and incorporates both dislocation climb and glide quantitatively. Dislocation climb is driven by in-plane biaxial stresses and fuelled by ‘pipe&’ diffusion along the dislocation core, consistent with experimental data.
Here, we report a transmission electron microscopy (TEM) investigation of the dislocation microstructures produced by stress relaxation of AlGaN-based heterostructures grown under conditions intended to either promote or minimise dislocation climb. The TEM data are compared to the dislocation microstructure predicted by the dislocation dynamics simulations. In device heterostructures grown under conditions designed to promote dislocation climb, the microstructure matches the simulations extremely well. In heterostructures grown under conditions designed to minimise climb, dislocation glide can occur instead, which is found to relieve stress more effectively. We can therefore tailor the stress relaxation mechanisms and dislocation microstructures within the films by controlling the growth conditions. This work allows us to propose new techniques for stress relaxation and dislocation reduction, which are directly relevant to improving the performance UV light-emitting devices.

Quantum well diode laser characteristics are dependent on many parameters used during growth of laser materials and fabrication of diode lasers. For example, composition, thickness, strain in the quantum wells, quantum barriers and cladding layers affect the emission wavelength of the diode laser[1]. The threshold current density and resistance of the diode depend strongly on the doping levels in the cladding layers and the thickness of the undoped core. Increase in resistance gives rise to heating, which leads to increase in Auger loss [2] and thus reduction in laser output power. Therefore, the output power of the laser depends on the doping in the cladding layers. For GaSb based III-V semiconductor lasers emitting in the 2-5 mu;m range, Al0.9Ga0.1As0.06Sb0.94 is used as cladding layers. Te is used as n-type dopant in the cladding layers. Optimization of the output power of the diode laser requires calibrations of incorporated dopant density and corresponding carrier concentration in the cladding layers. There has, however, been limited work reported on the Te dopant incorporation in AlGaAsSb [3, 4].
In this work, we present new data on carrier concentration versus Te dopant incorporation in epitaxially grown Al0.9Ga0.1As0.06Sb0.94 layer. 2 mu;m thick Te-doped Al0.9Ga0.1As0.06Sb0.94 layers were grown by molecular beam epitaxy on undoped GaAs (100) substrates. Incident Te dopant flux was varied by using different GaTe source temperatures. A 100 nm thick undoped Al0.3Ga0.7As cap layer was grown on the Al0.9Ga0.1As0.06Sb0.94 layer to reduce oxidation and errors in Hall measurements. Hall bar samples were processed where the Au contact pads were annealed so as to diffuse through the undoped cap layer and provide electrical contact to the doped layer. Carrier concentrations and mobilities of the doped Al0.9Ga0.1As0.06Sb0.94 layers were obtained from room temperature Hall measurements, and information on dopant incorporation was provided by secondary ion mass spectrometry depth profiling. The relation between incorporated Te dopant density and corresponding carrier concentration for the Al0.9Ga0.1As0.06Sb0.94 epitaxial layers will be presented. Optimized carrier concentration in the cladding layers can lead to higher output power from the diode laser.
References
[1] E. Tournié, A.N. Baranov, Advances in Semiconductor Lasers, (2012) 183.
[2] R.G. Bedford, G. Triplett, D.H. Tomich, S.W. Koch, J. Moloney, J. Hader, J Appl Phys, 110 (2011) 073108-073106.
[3] H. Ehsani, N. Lewis, G.J. Nichols, L. Danielson, M.W. Dashiell, Z.A. Shellenbarger, C.A. Wang, J. Crystal Growth, 291 (2006) 77-81.
[4] A.Z. Li, J.X. Wang, Y.L. Zheng, G.P. Ru, W.G. Bi, Z.X. Chen, N.C. Zhu, J. Crystal Growth, 127 (1993) 566-569.

Gallium arsenide is used as the absorbing layer in the highest efficiency solar cells today due to its high absorptivity and ideal band-gap for sunlight absorption. Unfortunately, the native GaAs surface under atmospheric conditions has mid-gap defects which make the fabrication of simple, low-cost GaAs devices difficult. To use GaAs as the absorbing layer in a low-cost inorganic-organic PV architecture we need to understand how to passivate the surface states during device fabrication. Here we report the fabrication and characterization of GaAs photovoltaic devices which have undergone a variety of chemical treatments to passivate the surface. In this study we use the conductive polymer PEDOT:PSS as a p-type soft contact to form a pn junction with n-GaAs. We characterize the devices by measuring the current-voltage response, open circuit voltage, ideality factor, and fill factor. We also perform AC impedance measurements and Mott-Schottky analysis to measure the built-in voltage and series resistance of the devices. Initial results show that by changing the pH of the PEDOT:PSS solution prior to deposition from 2.3 to 7 we can increase the photovoltage at 10 mA cm^-2 from 0.67 V to 0.73 V, indicating that the more basic solution is better at passivating the GaAs surface. It has been shown that thiol solutions can be used to form self-assembling monolayers on the GaAs surface. We demonstrate the addition of a thiol monolayer in this system and report no significant change in photovoltaic properties except for a decrease in fill factor and a large increase in series resistance. Depending on progress we will report; more on thiol passivation studies, the pH dependence of PEDOT:PSS passivation, and the effect of introducing a dipole into the junction via an ionic thiol monolayer. In addition to the specific application of this research to GaAs-polymer contacts the general principles of passivation and functionalization of the GaAs surface will apply to a broad range of hybrid inorganic-organic devices.

The heteroepitaxy of GaAs on Si substrates is great interest because of its applicability to monolithic integrated optoelectronic devices, such as in optoelectronic integrated circuits, quantum effect devices, and solar cells. In GaAs on Si, a system with a large area can be made of low-cost material system by exploiting the physical durability of mature technology associated with Si substrate. However, growing a high-quality GaAs on Si substrates is difficult because it exhibits a large lattice mismatch and quite different thermal expansion coefficients. Accordingly, the appearance of antiphase domains (APDs) between Ga and As atoms reverse their positions crystallographically during the growth of the polar semiconductor on the nonpolar substrate. We have studied the dependence of the growth of GaAs/Si on the flow rate of source for GaAs seed. We have investigated the formations of GaAs buffer and seed layer grown on Si substrate by changing the V/III ratio and the growth rate of GaAs seed layer. Also, we investigated the improvement of crystal quality in the GaAs buffer layer grown on Si substrate by the means of the thermal cycle annealing (TCA). Structural and optical properties of GaAs buffer grown on Si substrates were investigated by scanning electron microscope (SEM), double crystal X-ray diffraction (DCXRD), and photoluminescence (PL).

Ultra-violet photodetector assembled by ZnO and GaN semiconductors has taken center stage due to similar applicable properties such as lattice constant, wide band gap, optical transmittance and high breakdown voltage. The ZnO with p-type conductivity for making a homojunction has been challenging problem because of its low reproducibility. Therefore, the p-type GaN is one of most promising candidates for ZnO-based p-n junction detector. Despite of these efforts, however, the ZnO-based photodetector was still limited by low responsivity in UV region. The ZnO-based photodiode with high responsivity at UV region fabricated by physical vapor deposition method was not presented yet. Previously reported responsivity of n-ZnO/p-Si heterojunction photodetector was 0.5 and 0.3 A/W for 310 and 650 nm, respectively.
In this study, we present an n-ZnO/p-GaN heterojunction photodiode with high UV response. The n-type ZnO thin films were deposited on p-type GaN substrate by using an ultra-high vacuum magnetron sputter at room temperature. Deposition time, sputtering power was 60 min and 70 W, respectively. During the deposition, Ar/O2 ratio was maintained at 10/5 sccm. Short time post-annealing process at 580 oC for 1 min was carried out for enhancement of the crystallinity of ZnO film. After deposition, oxidized Ni/Au and Al metals for Ohmic contacts were deposited on p-GaN and n-ZnO, respectively, by using e-beam and thermal evaporators. The oxidation condition of Ni/Au gate was 500 oC for 10 minutes under ambient oxygen gas. The dark current and photocurrent measurements were conducted in the wavelength range of 260-800 nm using a HP4156A semiconductor parameter analyzer combination with monochromator. The optical characteristics were analyzed by photoluminescence. In addition, X-ray diffraction was used to analyze the crystallinity of ZnO thin film. The fabricated n-ZnO/p-GaN photodiode shows a high photoresponsivity and external quantum efficiency at UV region. Also, we will discuss the possible mechanism of high UV-sensing properties.

In this paper, we reported the fabrication of nitride-based hexagonal pyramids array (HPA) vertical-injection light emitting diodes (V-LEDs) by N-polar wet etching. The N-polar (000-1) surface of n-type GaN exposed by laser lift off was wet etched to form isolated hexagonal pyramids with separated active regions. It was revealed that wet etching process to separate the active regions was a simple but effective way to improve the crystal quality and light extraction of GaN based LEDs. The influence of N-polar wet etching on performence of GaN based V-LEDs were investigated as below.
1.The TEM images were collected with diffraction vector of g=000-2 and g=-2110, demonstrating that density of both edge and screw threading dislocations were reduced due to wet etching process.
2.The CL images showed the preserved function of isolated active regions of hexagonal pyramids array. No obvious dark point or other wavelength was detected.
3.The IQE was estimated by temperature dependence of photoluminescence (PL), which showed 30% increase for HPA V-LEDs compared with broad area (BA) V-LEDs.
4.The improved extraction efficiency was verified by finite difference time domain (FDTD) simulation, which was 20% higher than that of roughened BA V-LEDs. The simulated far-field pattern showed that the light escape angle is significantly enlarged for HPA VLEDs.
5.Based on conductive atomic force microscopy (CAFM) measurements, HPA V-LEDs showed much lower leakage current due to the improved cystal quality. The light output power for HPA V-LEDs showed 35% increase, and the relative EQE droop for HPA V-LEDs was only one sixth of the droop for BA V-LEDs at 350 mA.
In conclusion, N-polar wet etching process not only excluded detrimental processes and artificial mask adopted in common approaches but also improved the crystal quality and extraction efficiency during the fabrication of of GaN based LEDs.

white polymer light emitting diodes (WPLEDs) are considered as a very important corner stone for achieving energy efficient lighting. However, though the Polyfluorene have high quantum yield in solution state the thin film emission are very weak due to aggregation and resulting in concentration quenching. There are many approaches suggested by different research works to prevent the eximer formation in solid state; Aggregation indiced emission enhancement(AIEE) is a newest strategy and simplest solution to this problem. Herein, we discuss about a novel fluorenone based oligomer that exhibits low quantum yield in solution state, but a very high quantum yield in the thin film state. This system is different from the prominent silole and tetraphenylethylene and stilbene structures showing aggregation induced emission enhancement (AIEE) and thus presents a new example of AIEE fluorophore for introducing AIEE in fluorenes. A detailed study of mechanism operating in the system is done using experimental and theoretical methodologies. A study of fluorescence of solutions containing varying amount of water also showed that the emission is initially quenched and later increased with addition of 60% water to the THF solution, an observation typical in AIEE systems. Further a study on fluorescence life time of the AIEE fluorophore indicates that the aggregates showed a reduced emission life time with addition of water to the THF solution in contrast to eximers showing longer life time of emission. A study of thin film morphologies of the two derivatives show that, the fluorenone-based system with AIEE character exhibit unique bundled morphology on Indium tin oxide (ITO) substrate. Thus the system can be a potential candidate to offset difficulties arising from emission quenching in thin film of polyfluorenes with a white light emission.

The talk will provide a discussion on novel substrates for the growth of III-nitride nanowires. Results of fabrication and characterization of light emitting structures will be presented.

The implementation of nanowires (or nanorods) has been limited due to many issues despite the outstanding performances in nano-sensors and nano-optoelectronics. One of the issues is a position control of the nanowires (or nanorods), which have commonly been fabricated by electron-beam lithography or focused-ion beam methods, which are very time-consuming and expensive.
Dielectrophoretic (DEP) force can align the nanostructures (nanowires, nanorods, and nanoparticles) precisely by the use of the non-uniform electric field. We demonstrated various p-n junction diodes by DEP force without the need for any additional processes. Typical rectifying behaviors with a bright electroluminescence were observed from n-nanostructures (GaN and ZnO) / p-GaN thin film and from n-nanostructure (GaN and ZnO) / p-Si thin film p-n diodes. High aspect ratio structures, such as nanowires ( or nanorods), were easier to be controlled in position and direction by DEP, which is simple, inexpensive and suitable tool compatible with current semiconductor fabrication processes. Also, a facile method for fabricating uniform nanorod-LED devices was demonstrated by the combination of nanospheres lithography and dry-etch process.

Compound semiconductor nanostructures fabricated on graphene layers have recently been investigated extensively for various kinds of device applications in the field of optoelectronics. A single layer or multilayers of graphene, along with well-known distinctive features such as excellent electrical, thermal, and mechanical properties, could serve as novel substrates for fabricating semiconductor nanostructures, making it possible to be exploited in a flexible, stretchable, and/or transferable form. [1-3] Moreover, the nanostructures of semiconductors are preferred in terms of optoelectronic device applications, since they can provide systematic exploitation of quantum confinement effects and better device geometry for higher performance. However, the previous reports showed that those nanostructures were formed on the graphene layers in a random manner, preventing further application processes. It is well known that the precise control of positions and shape of the individual nanostructures is necessary for the nanostructures to be exploited in various kinds of nanodevices.
Here, the position- and morphology-controlled ZnO nanostructures are grown on the graphene layers using metal-organic vapor-phase epitaxy. With the selective oxygen plasma treatment with the aid of e-beam lithography, we could control nucleation sites for the ZnO nanostructures even without the use of growth mask, making it possible to form various kinds of ZnO nanostructures such as one-dimensional nanotubes or two-dimensional nanowalls in the form of letters. Structural and optical properties of those ZnO nanostructures were also investigated by using electron microscopy and optical spectroscopy analysis. Furthermore, in order to demonstrate the feasibility of the ZnO nanostructures on the graphene layers as building blocks for fabricating various kinds of nanodevices in the field of optoelectronics, electronics, and photovoltaics, we fabricated coaxial nanostructure light-emitting diodes by achieving heteroepitaxy of GaN on the ZnO nanotubes on the graphene layers.[4]
[1] K. Chung, C.-H. Lee, and G.-C. Yi, Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices, Science 330, 655 (2010)
[2] C.-H. Lee, Y.-J. Kim, Y. J. Hong, S.-R. Jeon, S. Bae, B. H. Hong, and G.-C. Yi, Flexible Inorganic Nanostructure Light-Emitting Diodes Fabricated on Graphene Films, Adv. Mater. 23, 4614 (2011)
[3] H. Yoo, K. Chung, Y. S. Choi, C. S. Kang, K. H. Oh, M. Kim, G.-C. Yi, Microstructures of GaN thin films grown on graphene layers, Adv. Mater. 24, 515 (2012)
[4] Y.-J. Kim, H. Yoo, C.-H. Lee, J. B. Park, H. Baek, M. Kim, and G.-C. Yi, Position- and morphology-controlled ZnO nanostructures grown on graphene layers, Adv. Mater. 24, 5565 (2012)

Compound semiconductors play an important role for generating, emitting and manipulating energy. The state-of the-art techniques for the fabrication of compound semiconductors are mostly vacuum-based physical or chemical deposition processes. Solution-based techniques offer opportunities to fabricate compound semiconductors at lower cost. Microreactor-assisted depostion process combine the merits of microreaction technology with solution-phase synthesis, purification, functionalization, and deposition. This technique offres a flexible and versatile platform for materials synthesis and deposition. It can be implemented in various ways for the manufacturing of functional materials including compound semiconductors. For example, Microreactor-Assisted Solution Deposition (MASD) involves the use of microreactor technology to produce reactive fluxes of short-life, intermediate molecules for heterogeneous growth on a temperature-controlled substrate. Another variant is Microreactor-Assisted Nanoparticle Deposition (MANpD) involving the use of microreactor technology to implement real-time nucleation, growth, purification, and functionalization of NPs for deposition and assembly of NP films and structures. In this paper, examples of fabricating compound semiconductors using microreactor-assisted solution deposition will be presented.

We studied electroluminescence properties of n-type polycrystalline CdSe nanowire arrays fabricated by the lithographically patterned nanowire electrodeposition (LPNE) process followed by thermal annealing.
Electroluminescence (EL) spectra of CdSe nanowires under various biases exhibited broad spectral range centered at 750 nm close to the band gap of CdSe (1.7eV). To enhance the intensity of emitted light and external quantum efficiency (EQE), we reduced distance between the contacts from 5 µm to less than1 mu;m which increased the efficiency by an order of magnitude. Also, increasing the annealing temperature of nanowires from 300^oC to 450^oC enhanced grain growth confirmed by structural characterization including x-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. Correspondingly the light emission intensity and EQE improved due to this grain growth.
We utilized Kelvin probe force microscopy (KPFM) to understand electroluminescence behavior in CdSe nanowires. According to KPFM results surface potential distribution along the nanowire showed potential drop at the nanowire-electrode interface which explains the tunnel injection of holes from the metal to semiconductor and subsequent radiative recombination of electron and holes at El spots on these nanowires.

Due to the superior electronic transport properties and light-coupling effects, GaAs nanowires are widely used as fundamental building blocks for the next generation electronics, optoelectronics, and photovoltaics. The synthesis of those NWs usually adopts a well-established vapor-liquid-solid (VLS) and/or vapor-solid-solid (VSS) mechanism, where metal particles, like the most commonly explored gold (Au) nanoclusters, are utilized for the catalytic growth. During the NW nucleation and growth, precursor atoms are diffused through these Au catalysts inducing NW formation at the Au/NW interface; therefore, chemical bonds are formed in this abrupt junction yielding nanoscale metal/NW contacts, which are in distinct contrast to “bulk-contacts” with the weak physical interaction between post-growth deposited metal electrodes and NWs. In this study, we utilize this nanoscale contact scheme on GaAs NWs with Au-Ga alloy catalytic tips to fabricate Schottky barrier photovoltaic devices. Current-voltage measurements were performed under simulated Air Mass 1.5 global illumination with the best performance delivering an overall energy conversion efficiency of ~ 2.8 % for a nanowire of 70 nm in diameter. As compared with metal contacts directly deposited on top of the nanowire, this nanoscale contact is found to alleviate the well-known Fermi-level pinning to achieve effective formation of Schottky barrier responsible for the superior photovoltaic response. All these illustrate the potency of these versatile nanoscale contact configurations for future technological device applications.
Reference:
1. N. Han, F. Y. Wang, S. P. Yip, J. J. Hou, F. Xiu, X. L. Shi, A. T. Hui, T. F. Hung, and J. C. Ho, Appl. Phys. Lett. 101, 013105 (2012).
2. N. Han, F. Y. Wang, J. J. Hou, F. Xiu, S. P. Yip, A. T. Hui, T. F. Hung, and J. C. Ho, ACS Nano 6, 4428 (2012).
3. N. Han, F. Y. Wang, A. T. Hui, J. J. Hou, G. C. Shan, F. Xiu, T. F. Hung, and J. C. Ho, Nanotechnology 22, 285607 (2011).

Nowadays, the nature of the non radiative recombination centers (NRRCs) in ZnO is a matter of controversy; they have been related to extended defects, zinc vacancy complexes, surface defects, among other possible candidates. We present herein the optical characterization of catalyst-free ZnO nanorods grown by atmospheric-MOCVD by micro-Raman and cathodoluminescence (CL) spectroscopies. The Raman spectra were acquired along individual ZnO nanorods grown under different precursor flows either with or without a buffer layer. The Raman spectra along the nanorods grown without the buffer layer present local modes associated with different defects and impurities, the contribution of the local modes to the Raman spectrum increases along the nanorod, resulting in strong modes at 275 cm-1 and 580 cm-1 as the top of the nanorod is approached. These Raman modes have been related to excess Zn. Simultaneously, the CL image of the NWs show a significant evolution of the light emission internal quantum efficiency, with a significant quenching of the emission towards the nanorod top. The correlation between the local Raman modes and the CL distribution along the nanorods permits to establish a relation between the NRRCs and the defects responsible for the local modes. The nanorods grown using a buffer layer did not exhibit the same behavior, which reinforces the role of the defects giving the local Raman modes as the NRRCs.

Nanostructures such as nanowires, nanorods and nanoparticles have been intensively studied because of their unique properties and potential applications in photovoltaics, sensors, transistors and optic devices. Especially, solar cells that are composed of nanowires have received significant attention due to its potentials to further enhance charge collection efficiency and light trapping efficiency. In our experiments, CdTe was chosen due to its high absorption coefficient and optimal direct bandgap (~1.5 eV) for photovoltaics. Because maximum efficiency of CdTe-based solar cells still remained below theoretical efficiency, a lot of efforts have been made to improve the efficiency in CdTe-based solar cells. Nanowire-based solar cells are expected to overcome current limitations in thin film-based solar cells. In our experiments, the CdTe nanowires were grown by using closed-space sublimation (CSS) method and fabricated to demonstrate optical sensors by using electric-field-assisted dielectrophoresis (DEP) force.
Various CdTe nano-/micro-structures were grown by using CSS method. For catalytic growth, 5 nm thick Au film was deposited on sapphire substrate. To form Au nanoparticles, rapid thermal annealing was carried out for 60 seconds at 800 celsius degree. Then, diverse CdTe nano-/micro-structures were grown using CSS method by controlling the growth time and the temperature gradient between substrate and CdTe powder source. SEM, XRD and micro-Raman spectroscopy were used to characterize optical and structural properties of the various CdTe nanostructures. Au-catalyst droplets were found at tips of CdTe nanowires by high resolution SEM characterizations, which confirmed that the CdTe nanowires were grown by VLS mechanism. The solution that contains CdTe nanowires was dropped on pre-patterned SiO2/Si substrate to fabricate the devices. CdTe nanowires were aligned by electric field-assisted method. The details of our experiments and device characteristics will be discussed.

Composite rods consisting of Alumina (Al2O3) and Silicon Carbide whiskers (SiCw) are used to fabricate microwave cooking racks because they effectively act as a microwave intensification system,1 that allows cooking at much faster rates than conventional microwave ovens. The percolation behavior, electrical conductivity and dielectric properties of these materials have been reported previously .2 and refs. However, it has been observed that the electrical response of the extruded bars is a function of the rod length and that long rods show substantially different behavior than thinner disks cut from them. A percolation model has been proposed that describes the effect of the alignment of the semiconducting SiC whiskers and the quality of the interfaces present in the composite rods: SiC-SiC and SiC-Al2O3-SiC for example. 2 This study was undertaken with the goal of testing out whether the response of the individual sections could be used to generate the response of the full length rods and to assess the importance of the homogeneous distribution of the SiC fillers.
With the help of impedance spectroscopy and analysis using equivalent circuits, it may be possible to individually identify the different processes present in these composite samples. Therefore, long rods, approximately 25 cm in length, were cut into halves, fourths, eighths, and so on and so forth, and they were measured in the same sequence so that the impedance values throughout the bar could be added and then compared to the response of the intermediate steps at the different lengths. It was found that this procedure worked well when the extruded rods were equally homogeneous at different positions along the length of the rods. However, it was quite easy to quickly determine if a section of a rod contained a more or less inhomogeneous distribution of the semiconducting fillers thus proving that the impedance measurements are superb at distinguishing the response of the insulating alumina and the semiconducting SiC. In addition to the measurements made with the electric field parallel to the length of the rods and thus parallel to the majority of the aligned SiC whiskers, additional measurements were also conducted on specimens that were cut parallel to the length of the rods and thus measured perpendicular to the extrusion direction. Not surprisingly, the electrical response was found to be anisotropic. Our results show that fitting the complex impedance alone does not accurately represent the sample due to the fact that there are multiple ways to fit the same impedance data. Admittance, permittivity, and modulus need to be used to obtain the equivalent circuit elements that will most accurately represent the sample microstructure and electrical properties.

It is well-established that wide bandgap (WBG) semiconductors are poised to greatly advance power electronics across a wide voltage range from ~100V through several kV. At 0-900V, representing approximately two-thirds of the total power electronics market, key applications include power supplies for IT and consumer products. At higher voltages, the U.S. Dept. of Energy has detailed the potential role of WBG devices in various energy-related applications such as the next generation electric grid [1], with forecast need for modules switching hundreds of kV at tens of kHz. Devices fabricated from gallium nitride (GaN) have long been targeted for such power device applications due to inherent material properties such as high breakdown field, thermal stability, and high mobility of heterostructures. Appropriately designed GaN high electron mobility transistors (HEMTs) have demonstrated performance figures-of-merit ten times greater than those attainable from modern Si-based MOSFETs. As compared to Si devices, GaN HEMTs can deliver higher blocking voltage, lower on-resistance, and/or faster switching with less loss. Furthermore, fabrication of GaN HEMTs on industry-standard Si substrates has been demonstrated by various groups [2-4] and presents a promising pathway for integration of GaN devices into mature Si processing lines. The reduced substrate cost, re-use of Si backside processing, and large wafer diameters allow for economical manufacture of high power GaN devices on Si. A primary impediment to practical realization of GaN-on-Si HEMT products is the current lack of widespread commercially availability of high quality, low background concentration, thick GaN epilayers formed on Si substrates.
This presentation will review challenges associated with growth of GaN-on-Si and will present MOCVD growth techniques and layer schemes optimized for power electronics applications. Concepts/results from both the buffer layers (typically associated with the off-state characteristics) and the HEMT barrier layers (typically associated with the on-state characteristics) will be discussed. Materials and device characteristics will be presented, and the forecasted role of the commercial epitaxial wafer foundry in the adoption of GaN power electronics will be briefly addressed.
References
1. U.S. Dept. of Energy, Power Electronics Research & Development Plan, April 2011. (http://energy.gov/oe/technology-development/power-electronics)
2. P. Rajagopal, J.C. Roberts, J.W. Cook, Jr., J. Brown, J., E. Piner, and K. Linthicum, Material Research Society Symposium Proceedings, 798, 61-66 (2004).
3. A. Dadgar, T. Hempel, J. Bläsing, O. Schulz, S. Fritze, J. Christen, and A. Krost, Phys. Status Solidi C, 8, 1503-1508 (2011).
4. P. Drechsel and H. Riechert, J. Crystal Growth, 315, 211-215 (2011)

Each year, several government and commercial satellites prematurely reach the end of their mission life because they have run out of fuel, need minor repair or were unintentionally launched into the wrong orbit [1,2]. An evolving endeavor by NASA Goddard Space Flight Center and partners is to develop the capability to restore the desired operation of defunct satellites using robotic manipulation while on-orbit [3]. In order to mitigate the complex contact dynamics involved with novel robotic rendezvous and docking of space vehicles which were never intended to be contacted after launch, a high-precision imaging system is being developed at West Virginia University and the WV Robotic Technology Center (WVRTC). Included in this proposed system are group-III nitride based LEDs, photodiodes (PDs) and high electron mobility transistors (HEMTs). The group III-nitride family of materials was chosen because of its radiation hardness [4-6] and ability to be used for all three devices of interest. For this work, III-nitride device structures were grown via metal organic vapor phase epitaxy. A selective area growth technique was employed to realize a planar strips of electronic and optoelectronic devices on the same substrate. Preliminary results indicate that strip width is critical for uniform growth across the substrate. LEDs and HEMTs were fabricated on the different regions of growth and connected by wire bonding. Performance of devices is reported and discussed.
[1] B.R. Sullivan, PhD. Thesis, Technical and Economic Feasibility of Telerobotic On-Orbit Satellite Servicing, University of Maryland, 2005.
[2] Notional Robotic Servicing Mission, NASA Goddard Space Flight Center, Accessed Nov. 1, 2012. < http://ssco.gsfc.nasa.gov/robotic_servicing_mission.html>
[3] On-Orbit Satellite Servicing Study Project Report, NASA Goddard Space Flight Center, Oct. 2010.
[4] A. Ionascut-Nedelcescu, C. Carlone, A. Houdayer, H. J. von Bardeleben, J.L. Cantin, and S. Raymond, IEEE Transactions on Nuclear Science, 49(6) 2733 (2002).
[5] S.M. Khanna, D. Estan, A. Houdayer, H.C. Liu, R. Dudek, Nuclear Science, IEEE Transactions on Nuclear Science, 51(6) 3585 (2004).
[6] R. Gaska, International Semiconductor Device Research Symposium (ISDRS), Dec. 7-9, 2011. doi: 10.1109/ISDRS.2011.6135383

Significant progress has been made in the field of III-nitride semiconductor electronic devices over the past decade. In particular, III-nitride based high electron mobility transistors (HEMTs) have advantages over conventional silicon-based devices in high-power, high-frequency, and high-temperature applications, including high temperature gas sensors, base stations for wireless communication systems, weather forecasting systems, and space communication systems. The outstanding radiation resistance of III-nitrides can make them a potential candidate for space applications, where the electronic devices have to tolerate incident high-energy protons up to a few hundred mega electron volts. There is a need to ensure the robustness of employing III-nitride based HEMTs for a satellite-based communication system, avionics, and missile control electronics to high-energy radiation. In semiconductors, it is empirically observed that the atomic displacement threshold energy inversely depends on the lattice constant. Because the lattice constants of Wurtzite GaN (a = 3.19 Å, c=5.19 Å) are much smaller than Si (5.43 Å) and GaAs (5.65 Å), III-nitrides should demonstrate exceptional tolerance under high-energy proton fluxes. InAlN/GaN material system offers an attractive alternative to more conventional AlGaN/GaN based HEMT structures. InAlN alloy can be grown lattice-matched to GaN with a large refractive index contrast and sheet charge density roughly twice that of typical AlGaN/GaN HEMTs. This enhanced sheet charge density is due to the more than 4x increase in spontaneous polarization of In0.17Al0.83N/GaN as compared to a traditional Al0.2Ga0.8N/GaN HEMT. In this talk, the effects of proton irradiation energy and doses on InAlN/GaN HEMT performance and reliability will be presented.

Power electronics approximately represents 20% of the entire semiconductor industry. High voltage switches are used in a wide variety of applications, from power adaptors in consumer electronics, to motor drives in electric vehicles or solid state transformers, building blocks of the future Smart Grid. However, although the great majority of these systems operate with Si-based devices, the intrinsic material properties of Si are currently seriously limiting the performance of power electronics along different dimensions. First, the operating temperature is typically limited to 120-150C, which requires the use of forced cooling in many applications. Second, the relatively low switching frequency of high voltage Si devices increases the overall size of the system. Finally, the specific resistance of Si power switches increases rapidly at high voltages, increasing the ohmic losses. It has been predicted that a new generation of power electronics that overcomes these issues could have a tremendous impact, potentially save 10-20% of the world&’s energy consumption.
This talk will describe how the use of wide bandgap semiconductors, specifically Gallium Nitrides (GaN), has the potential to significantly improve the performance of power electronics. The large critical electric field of this material (~3 MV/cm), in combination with its high electron mobility (~1700 cm2/Vs), allow the fabrication of GaN power transistors with, theoretically, x1000-fold lower specific on-resistance than Si devices for a given blocking voltage. This lower resistance can be traded-off for higher switching frequency, which enables a significant reduction in the footprint of GaN-based power electronics and important energy savings.
We will focus on describing new solutions to some of the challenges that need to be overcome to make GaN power electronics an engineering and commercial success. First, the fabrication technology needs to be compatible with standard 8” Si fabrication technologies, to make it cost competitive with today&’s solutions. It is also important to develop new transistor structures to fabricate normally-off (enhancement-mode) switches, highly preferable due to safety and legacy issues. Finally, the change of on-resistance with switching frequency and long-term degradation needs to be corrected.
Acknowledgements.- This work has been partially funded by the ONR Young Investigator Program, the DOE GIGA program, the ARPA-E ADEPT program and the MIT GaN Energy Initiative.

GaN-based semiconductors offer numerous advantages for high-speed and high-power devices. In addition to wide bandgap and high mobility, GaN has a polarized crystal structure due to strong spontaneous and piezoelectric contributions that promote 2DEG formation at GaN-based interfaces. The modulation of carrier density in the 2DEG offers a means for the generation and controlled emission of acoustic energy through dynamic screening of the piezoelectric field. Experimental evidence for this coupling has recently been measured in the form of coherent phonon generation [1] and surface acoustic wave (SAW) emission [2] from GaN-based HEMTs under typical operating conditions. Acoustic functionality in GaN-based devices could enable a range of new applications; for example, because acoustic loss coefficients are sensitive to thin-film and interfacial epitaxial quality, acoustic pulses emitted by a device could be used as a real-time diagnostic to detect defects or degradation in a device&’s 2DEG or epitaxial layers. Acoustic emitter/detector pairs based on HEMTs could furthermore provide a multi-channel, high-speed, amplified detection scheme for sensing applications with significantly better performance than existing SAW devices.
In this work we provide a detailed study of acoustic transduction by 2DEG modulation in GaN-based materials. Our previous work has used an integrated interdigital transducer (IDT) to detect SAW emission from a GaN HEMT [2]. Because the IDT has a single periodicity and is also relatively insensitive to certain SAW modes due to its geometry, it is unable to measure the full acoustic spectrum emitted by a modulated 2DEG. Here we use a reflective optical probe at normal incidence to the 2DEG active region and detect acoustic emission by the piezo-optic effect, providing a full picture of the emitted spectrum. We find four prominent emission peaks corresponding to Rayleigh (R), Love (L), and 1st and 2nd pseudo-bulk modes (PB1 and PB2), which propagate with different polarizations and at different depths within the epitaxial layer structure. When the device is modulated near its bias point of maximum transconductance, maximum time variation of the 2DEG carrier density occurs, and R and L modes (which are tightly confined to the surface) are strongly emitted while PB1 and PB2 modes are weak. When the device DC bias is moved away from this point, the modulated voltage drop no longer primarily occurs in the 2DEG, and PB1 and PB2 modes become strongly emitted while R and L modes weaken. We also find that R and L modes shift to higher frequencies as pinch-off occurs and the 2DEG channel length shrinks.
In conclusion, we find that a three-terminal 2DEG device is able to selectively turn on or off the emission of different acoustic modes based on applied bias conditions. This switchable source functionality is unique to the acoustic coupling provided by a GaN 2DEG.
1 Song et al., APL 83, 1023 (2003)
2 Shao et al., APL 99, 243507 (2011)

Power MOSFET devices can now be fabricated on 4H-SiC with high power-handling capability, and good reliability. Gate oxides thermally grown on SiC, followed with a nitric oxide (NO) passivation anneal, have high breakdown field strength. However, these thermal oxides have relatively low peak channel mobility (~20-35 cm2/Vs, dopant-dependent), and there remains much room for further device improvements. To produce substantial improvements, the present standard gate processing scheme may have to be radically changed. To enable greater flexibility with respect to device processing of the gate and channel interface regions, it would be helpful if deposited oxides could be integrated into the device processing flow. This would eliminate or minimize the issues of: varying SiC consumption rates with implant dopant concentrations; varying thermal oxide growth rates on different SiC crystal faces (especially important in trenched-channel devices); and the issue of the evolving C as oxidation proceeds.
In this report we present results comparing MOS capacitor and lateral MOSFET properties fabricated with thermally grown oxides, to those having CVD or PECVD deposited oxides. We observe that MOSFET field-effect mobility (25-34 cm2/Vs), and threshold voltage (~1 V) at low drain bias (50 mV) is very similar for the two cases. Regarding dielectric strength, it is clear that deposited oxides are inferior unless they receive high temperature post deposition processing. With appropriate anneal treatments, the deposited oxides approach the quality of thermally grown oxides. For example, the Fowler-Nordheim (FN) barrier height for electron tunneling at the SiO2/SiC interface is 2.4 and 2.5 eV for PECVD and CVD oxides, respectively; while it is 2.65 eV for thermally grown oxides. This gives an onset of FN leakage at ge;5 MV/cm for all cases, well above the gate fields experienced during normal device operation (3~4 MV/cm on-state field). It is thus apparent that deposited oxides should be suitable for gate oxides for advanced SiC MOS-based devices.
Further results demonstrating the effects of various anneal treatments, and the properties of devices as a function of operating temperature (mobility, threshold voltage, and threshold stability with gate bias), will also be presented.

The performance of high voltage (>10kV) SiC PiN diodes and transistors (BJT, GTO, IGBT, and any other switch requiring bipolar conduction) is currently limited by the relatively low carrier lifetime of the thick 4H-SiC epitaxial layers that are used to fabricate the devices. The low carrier lifetime results in a high on-state resistance and a high forward voltage drop of the device, hence significantly increasing the conduction power loss of the switch. Epitaxial growth techniques used to improve carrier lifetime, such as lower temperature growth or lower silicon-to-carbon ratio, all lead to higher defect densities in the epitaxial layer. The higher defect density in turn makes large area (high current) devices impractical. Post epitaxy processing of lower lifetime but lower defect density layers to improve carrier lifetimes is a relatively new concept. In this work, we have developed a technique that significantly improves the carrier lifetime by means of producing free Carbon interstitials for filling the unwanted Carbon vacancies in the thick 4H-SiC epi-layers after multiple thermal oxidations and anneals at high temperatures. As evidence, correlation of the minority carrier lifetime to forward voltage drop of the high-voltage 4H-SiC PiN diodes from room temperature to 200 degree C will be discussed and presented at the conference.

AlGaN/GaN based heterostructures possess an interfacial two-dimensional electron gas (2DEG) density of the order of 1013 cm-2 generated by the strong polarization charges in the AlGaN barrier layer. The source of these electrons is considered to be the surface donor states distributed in the forbidden gap of the barrier. We have previously shown how the bare surface barrier height (SBH) relates to polarization, thickness, and Al content of the barrier, as well as the surface state distribution. This model applies well to bare surface AlGaN/GaN, but the understanding of what happens when a gate metal is deposited is still lacking. Since metal contacts are required for efficient gate control in practical devices, the understanding of such contacts on AlGaN/GaN is quite important.
Experimentally, it has been shown that deposition of metals to form Schottky contacts reduces both the 2DEG and the SBH by partially neutralizing the surface states, but it also causes a redistribution of the surface states. Other experiments show an large unexplained difference between SBH measured on bare Al0.29Ga0.71N surfaces (about 2.4 eV) and on surfaces with deposited metal (1.4-1.6 eV found using internal photoemission spectroscopy). An interfacial oxide is usually etched away in AlGaN devices to improve reliability and therefore there shouldn&’t be any interfacial oxide. The effect of the metal on this post-deposition distribution is found to depend on the metal work-function, which, in effect, rules out the existence of high-density surface states that would otherwise have pinned the Fermi level. On the other hand, the Mott theory (Phi;b = Phi;m - chi;) applies to semiconductor surfaces with no surface states and therefore gives inaccurate results for practical semiconductors.
To address the above challenges, we present a new analytical model, which partly relies on experimental observations, to describe/predict the reduction in SBH, the effective donor level, and the density of surface states upon depositing metal on bare AlGaN surfaces. This new model is based on our previous work on bare SBH for both unrelaxed and relaxed barrier layers. The presented model is in satisfactory agreement with reported experimental data.

GaN-based UV LEDs were recently highlighted for their applications in sterilization, air/water purification and disinfection. However, UV LEDs have some critical problems in internal quantum efficiency and transparent conductive electrodes. Generally, indium tin oxide (ITO) layer has been widely used for transparent conductive electrode in GaN-based blue LEDs. However, ITO has very low transmittance in UV region and also chemical/mechanical instability. Graphene has a potential in transparent conductive electrodes as an alternative of ITO because graphene has very high thermal and electrical conductivity, good chemical/mechanical stability with high transmittance in UV spectral region.
Few layer graphene (FLG) was synthesized by chemical vapor deposition (CVD) method, then transferred on p-GaN layer in UV LEDs, which consisted of AlGaN/GaN/AlGaN single quantum well (SQW). UV LEDs at a peak wavelength of 372nm were fabricated by conventional photolithography processes. The thickness of FLG was determined by micro-Raman spectroscopy. The transmittance of 4-layer (4L) graphene and ITO (150nm) at 372nm was 89% and 68%, respectively. Successful current-spreading was observed from our graphene-based transparent conductive electrodes in UV LEDs, which was confirmed by electroluminescence images and current-voltage characteristics. However, there was trade-off between sheet resistance and transmittance of graphene layer, where we used 4L graphene as an optimal thickness. Undoped 4L graphene has very high sheet resistance above 600 Omega;/sq. Therefore, we employed p-type doping in graphene layer by using AuCl3 solution. Sheet resistance of graphene layer was drastically decreased via 20mM AuCl3 doping from 600 Omega;/sq to 120 Omega;/sq. Also, we observed the burning effect from the graphene-based transparent conductive electrodes in UV LEDs. During the continuous operation of our UV LEDs, the emitted area was drastically decreased owing to the oxidation of graphene layer, which is attributed to the heat and ozone by UV light. Therefore, we used SiNx passivation layer with the thickness of 20nm to prohibit the direct contact between air and graphene layer. As a result, the burning effect of graphene transparent conductive electrode was largely decreased even for the continuous operation. The details about experiments and results will be presented.

Since their inception [1,2], micro-size light emitting diode (µLED) arrays based on III-nitride semiconductors have emerged as a promising technology for a range of applications, including single-chip high voltage AC-LEDs for solid-state lighting [3], light sources for optogenetic neuromodulation [4] and self-emissive microdisplays [2,5,6]. This talk provides an overview on recent progresses on single-chip AC-µLEDs that can be plugged directly into standard high AC voltage power outlets. These single-chip AC-LEDs integrate arrays of mLEDs on-chip for high voltage AC operation. The number of linked emitters is chosen so that the sum of the voltage drops across the individual emitters adds up to the voltage of the AC supply. We also report the successful integration of III-nitride µLED array with Si CMOS to accomplish a high-resolution solid-state self-emissive microdisplay operating in an active driving scheme. The fabricated blue and green video graphics array (VGA) microdisplays (640 x 480 pixels) have a pixel size of 12 µm, a pitch distance of 15 µm and are capable of delivering real time video graphics images [6]. The luminance level of III-nitride microdisplays is much higher than those of liquid crystal and organic-LED displays. The present study indicates that III-nitride microdisplays are a favorable competing technology for ultra-portable products such as next generation pico-projectors, wearable displays, and head-up displays.
[1] S. X. Jin, J. Li, J. Z. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 76 631 (1999) and H. X. Jiang, S. X. Jin, J. Li, J. Shakya, and J. Y. Lin, ibid 78 1303 (2001).
[2] H. X. Jiang, S. X. Jin, J. Li, and J. Y. Lin, US patent 6410940.
[3] H. X. Jiang, et al US Patents 6957899; 7221044; 7535028.
[4] V. Poher, N. Grossman, G. T. Kennedy, K. Nikolic, H. X. Zhang, Z. Gong, E. M. Drakakis, E. Gu, M. D. Dawson, P.M.W. French, P. Degenaar, and M. A. Neil, J. Phys. D: Appl. Phys. 41, 094014 (2008).
[5] J. J. D. McKendry, B. R. Rae, Z. Gong, K. R., Muir, B. Guilhabert, D. Massoubre, E. Gu, D. Renshaw, M. D. Dawson,and R. K. Henderson, IEEE photonics Technology Letters, 21, 811 (2009).
[6] Jacob Day, J. Li, D.Y.C. Lie, Charles Bradford, J.Y. Lin, and H.X. Jiang, Appl. Phys. Lett. 99 031116 (2011).

InGaN/GaN Multi-Quantum Well (MQW) structure is commonly used for blue-Light Emitting Diodes (LEDs). Even its high defects (threading dislocations) density, light emission efficiency of LEDs that have InGaN active layer is very high. Many researchers have explained this high efficiency of devices as compositional fluctuations of In element in InGaN layer. In other words, band-gap energy may be not uniform in the InGaN layer because the In concentration is different from region to region in the InGaN layer. So, carriers can be easily gathered and recombined at the region that has locally minimum band-gap energy. However, any rigid evidences of the non-uniformity of band-gap energy of the InGaN MQW have not been reported.
In this study, we used cathodoluminescence (CL) analysis technique in a transmission electron microscope (TEM) to measure the band-gap energy of InGaN MQW with high spatial resolution. To do so, we developed a light collectable TEM specimen stage (TEM-CL stage) that has optical components such as mirror, lens and optic fiber light guide. Using the TEM-CL stage, we could get the CL spectrum of each region of the InGaN MQW as scanning electron beam in a TEM. We could make CL spectral map of MQW and also get the band-gap energy distribution map of MQW by converting the peak wavelength of CL spectra to photon energy.
From these maps, we could know that there is fluctuation of band-gap energy of InGaN MQW. So, we can see the local minima of band-gap energy of MQW and that may be the one of evidences for the reason of the high efficiency of GaN-based LEDs.

Blue light-emitting diodes (LEDs), utilizing InGaN-based multi-quantum well (MQW) active regions epitaxially deposited by metal-organic chemical vapor deposition (MOCVD), are one of the fundamental building-blocks for current solid-state lighting applications. Many studies have previously been conducted to explore the optical and physical properties of the active MQWs over a wide range of MOCVD growth conditions. However, the conclusions of these studies have often been contradictory, likely due to a lack of understanding of the fundamental gas-phase chemistry that occurs during the deposition process.
In this talk, the optical (photoluminescence) and physical (microscopy) characterization of MQWs grown over a range of pressures from typical low-pressure production processes at 200 torr up to near-atmospheric growth conditions at 700 torr will be compared. At all growth pressures, clear trends of reduced gas-phase chemical reactions occurring between NH3 and TMIn for reduced gas residence time (faster gas speeds from the injector flange & higher rotation rates) and reduced V/III ratios (lower NH3) can be seen. Conversely, greater MQW PL intensities are observed with increased V/III ratio. As the growth pressure is increased, the gas-phase chemical reactions are amplified, resulting in a reduced parameter space to achieve high-quality MQW layers. Reducing the gas residence time appears to lower the effect of gas phase reactions for high V/III ratio material at high growth pressures. Optimized MQWs grown at high pressure exhibit higher PL intensities than lower pressure samples. Results on LED structures indicate that this improvement in MQW optical quality at high pressures similarly translates to higher output power for a 20 mA drive current.
Confocal microscopy and time-resolved photoluminescence (TRPL) have been employed on these MQW structures to investigate the carrier lifetime characteristics. Near band-edge emission measurements show spatially-separated bright and dark regions in a 20 x 20 µm confocal image. The bright regions are red-shifted in wavelength relative to the dark regions, suggesting Indium-rich localization. As the growth pressure and V/III ratio is increased, a larger percentage of bright-to-dark regions, a greater difference in band-gap between bright and dark regions, longer lifetimes, and higher average PL intensities are observed, indicating higher optical quality can be realized.

Currently, GaN-on-Si is considered to be the most promising material for high power electronic applications as DC or AC current converters or inverters needed, e.g., for electro mobility or solar power applications. Besides that high frequency applications are feasible. For FET devices commercialization of GaN-on-Si has already been achieved. A major application for GaN-on-Si will be high-power LEDs for general lighting. An external quantum efficiency around 60 % has been reported recently and high power white LEDs suitable for general lighting are in a pilot stage in industry now*. Several groups have already demonstrated that high quality GaN on Si can be grown by MOVPE on 100, 150 and also 150 mm diameter substrates and that growth is easier than on large diameter sapphire. For such purpose thick, high-quality structures with highly conducting n-type layers are required which is a major challenge because of tensely induced stress upon Si-doping. Using in-situ curvature measurement technique we observed the strain state of group-III-nitride layers during growth by metal organic vapor phase epitaxy and established a process for growing thick, crack-free GaN layers with a quality comparable to that on sapphire. Latest results on the growth of semipolar GaN on Si(11h) surfaces will be reported, too.
*W. Bergbauer et al., Poster TuP-PR-19, IWN 2012, Sapporo, Japan, Oct. 2012

AlInN ternary alloy for ~17% InN molar fraction is lattice matched to GaN relieving strain at the interface between AlInN and GaN. This feature, along with the wide band gap and the strong electric field at the AlInN/GaN interface makes the heterostructure attractive for high electron mobility transistors (HEMTs). However, contrary to InGaN and AlGaN, AlInN has been explored and exploited to a much lesser extent. Primarily, this is related to technological difficulties: the mismatch between the bond length in AlN and InN, and the large difference in AlN and InN MOCVD growth temperatures make it difficult to grow defect-free AlInN layers. In addition, basic material parameters, such as the band gap and the bowing parameter are not well established, and the band potential fluctuations are not well understood. Since the band potential fluctuations directly influence carrier scattering in HEMTs, the origin of the sub-band edge states should be fully comprehended before any large scale AlInN applications can take effect.
In this work, localization potentials and carrier dynamics in AlInN were studied by time-resolved photoluminescence (PL) and photoreflectance, and scanning near-field optical microscopy (SNOM) on AlInN/GaN heterostructures with ~100 nm thick AlInN layers of 14% and 18% In content. Carrier dynamics has been studied throughout the whole spectrum of AlInN sub-band gap states by tuning excitation and probe wavelengths from the AlInN uniform alloy band gap to the band gap of GaN. SNOM measurements allowed mapping variations of GaN and AlInN PL spectra with a 100 nm spatial resolution.
The experimental data have shown that carrier lifetimes in the extended AlInN band gap states are of sub-picosecond duration. The ultrafast carrier transfer to the sub-band gap states, the large (0.9 eV) Stokes shift between the uniform alloy absorption edge and the emitting states, and the broad (0.4 eV FWHM) PL spectrum in the near- and far-field indicate that the sub-band edge states are dense and have an energetically broad spectrum. Most probably, they are caused by the nonuniform cation arrangements around nitrogen atoms, since valence band states in In clusters are known to have higher energies compared to the states in the uniform alloy. Interestingly, even though the sub-band edge states in AlInN are deep, the carriers located in these states are mobile and rapidly diffuse into the GaN layer. This efficient transfer suggests that, apart from the In clusters, the dislocations and V-type defects participate in the forming of the sub-band edge states as well.
SNOM measurements have also revealed spatial variations of the peak energy of the interface well PL. This shows that the local band gap variations due to the sub-band edge states in AlInN cause fluctuations of the built-in electric field at the GaN/AlInN interface influencing the width of the conduction band potential well and affecting the interface scattering in the HEMT channel.

Free-standing GaN substrates are currently cost-prohibitive, so commercially available GaN-based high electron mobility transistors (HEMTs) are commonly grown on sapphire, SiC, or Si. These substrates are not lattice-matched to GaN, and this leads to high threading dislocation densities (TDDs) in the range of 10^8 to 10^10 cm^-2. When considering the lattice distortion that surrounds dislocations and the potential for Coulombic interaction between electrons and charged dislocation lines, it would seem that threading dislocations could have a significant effect on two-dimensional electron gas (2DEG) mobility. Dislocation scattering in an AlGaN/GaN 2DEG has already been treated theoretically, but a thorough experimental study has yet to completed.
In this study, Al(x)Ga(1-x)N/GaN (x = 0.06, 0.12, 0.24) and AlGaN/AlN/GaN heterostructures were grown on 6H-SiC, GaN-on-sapphire, and free-standing GaN, resulting in heterostructures with threading dislocation densities of ~2 x 10^10, ~5 x 10^8, and ~5 x 10^7 cm^-2, respectively. All growths were performed under Ga-rich conditions by plasma-assisted molecular beam epitaxy. Temperature-dependent Hall measurements indicated the dominant scattering mechanisms with variations in threading dislocation density and sheet concentration and with the inclusion of an AlN interlayer. Dislocation scattering contributed to reduced mobility in these heterostructures, especially when sheet concentration was low or when an AlN interlayer was present. With alloy disorder scattering essentially eliminated in the AlGaN/AlN/GaN heterostructures, 2DEG mobility at 42 K improved from 2333 to 12,326 cm^2/Vs with a reduction in TDD from ~2 x 10^10 to ~5 x 10^7 cm^-2. With alloy disorder scattering greatly reduced in the Al(0.06)Ga(0.94)N/GaN heterostructures, mobility at 52 K improved from 1620 cm^2/Vs with ~2 x 10^10 cm^-2 TDD to 19,500 cm^2/Vs with ~5 x 10^7 cm^-2 TDD. Charged dislocation lines were not effectively screened with low sheet carrier density.

The epitaxial growth of GaN on Si substrates opens the possibility to integrate GaN and Si technologies. In order to have high efficiency light emitting diodes and increase lifetime of CW lasers based on the III-nitrides, a low defect density in GaN/AlN based materials is required. Recently we have shown using Transmission Electron Microscopy that misfit dislocations formed at the AlN/Si interface can interact with dislocation loops formed around He bubbles created by He implantation into Si [1, 2]. In this high fraction of threading dislocations created at the interface can move into the Si substrate instead of into the epi-layer. The optimal implantation dose and the distance of the He bubbles from the surface were determined experimentally. The growth temperature of AlN was used as the annealing temperature. He implantation into Si substrates, through a pseudomorphic Si-Ge layer and subsequent annealing was used earlier [3, 4], and has lead to defect free Si-Ge layers for Ge content up to 30%. However, in the case of GaN a pseudomorphic growth on Si is impossible due much too large lattice mismatch, therefore the earlier described procedure would not work. We developed a new procedure and implanted He into Si before the growth. Understanding of physical basis of strain relaxation at the AlN/Si interface can lead to the development of techniques leading to a substantial decrease of the density of the threading dislocations in the GaN/AlN layers grown on Si substrate. The total density of the threading dislocations estimated from the plan-view micrographs in the 1 mu;m thick GaN grown on the He implanted Si is in the low (2-3) x 10-9 cm-2, which makes this material competing with other technologies [5, 6] and encouraging for the implementation.
[1] Z. Liliental-Weber, R.L. Maltez, J. Xie, H. Morkoc, J. Cryst. Growth 310, 3917 (2008).
[2] US patent # 8, 008, 181
[3] H. Trinkaus et al., Appl. Phys. Letters, 76 (2000) 3552.
[4] M. Luysberg et al., J. Appl. Phys. 92 (2002) 4290.
[5] A. Dadgar et al J. Cryst. Growth 248, 556 (2003).
[6] US patent # 7,825, 432
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors appreciate the use of the TEM facility at the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory.

InAlN/GaN heterostructures have attracted great attention for HFETs since InAlN can be lattice-matched to GaN, and the channel two-dimensional-electron-gas (2DEG) density is higher than that of AlGaN/GaN structures. However, the surface electronic properties of InAlN epitaxial films have not been characterized to determine the position of the surface Fermi energy and, therefore, band bending in relation to the 2DEG concentration. We have synthesized and characterized InAlN/GaN and InAlN/AlN/GaN heterostructures in order to determine how the 2DEG concentration is related to the surface band bending. Our interpretation of the data reveals the interplay of surface donor states and electron traps in InAlN in determining the 2DEG density. Lattice-matched InAlN films with varying thickness were grown by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) at 450 °C under N-rich conditions. The surface (~2.5nm) band bending of the heterostructures was determined using x-ray photoelectron spectroscopy (XPS).
We find that the surface band bending is a function of the InAlN thickness and surprisingly shows a transition from depletion and upward band bending for thinner surface layers (2.5nm) to the unexpected downward band bending for larger thicknesses (15nm). 7 nm layers are found to be near flat band. Adding an AlN insertion layer does not modify the band bending, indicating that it is a characteristic of the InAlN barrier layer. By using a buffered-oxide-etch (BOE) to remove the surface oxide, the value of the surface potential decreases, but the band bending follows the same trend. We speculate that the surface band bending is controlled by the occupation of surface donor states and electron traps in the InAlN layer. These surface states are not predominantly a result of the surface oxide and are therefore native to InAlN surface. For the thin InAlN layer, the Fermi level is above the top of the surface states energy distribution, and therefore the band bending is upward. As the InAlN thickness is increased, the Fermi level reaches that of the surface states and electrons are available for the 2DEG as well as occupying electron traps in the barrier. The occupation of the traps creates a potential barrier at the surface leading to downward band bending.
In order to characterize the distribution and density of the InAlN surface states, bare surface barrier heights are measured by XPS and spectroscopic ellipsometry is used to determine the absorption edge. The 2DEG density is simulated using the measured band gap and surface potential of InAlN. Our data shows that the bare surface barrier height increases with surface barrier thickness before saturating beyond a critical thickness. The trend is very similar to that of the 2DEG concentration, indicating that the surface donor states are distributed in energy. The fitting shows that the surface donor states are distributed between ~0.7 to 1.2 eV and the density is almost constant at 1.09 x 10¹³ cm² eV^-1.

Zinc oxide (ZnO) is one of most promising material for those applications. However, it is difficult to make the p-type ZnO when the low-temperature process on the glass substrate etc. is demanded. On the other hand, the NiO film shows p type conduction, formed at the low temperature process and the possibility of the hetero-junction of n type ZnO and p type NiO is examined.
In this work, we study ZnO-NiO mixed crystal thin film as the hole carrier injection layer for the ZnO film luminescence and the absorption layer for solar cell, focused on the band diagram ( band offset for ZnO ), electrical and optical properties .
As for the issue of the hetero-junction of the ZnO and the NiO, the mismatch of the crystallographic structure and the lattice parameter and the band offsets of the valence band are thought. We made the ZnO-NiO mixed crystal thin film in all composition range, using magnetron sputtering process and acquired the basic data for the change of
1) valence band offset, electrical conductivity with conduction type,
2) optical absorption extending to all spectral regions of sunlight.
On the former, the offset of the valence band decreases as the ZnO composition increases and the p type conduction showed even in the composition of ZnO composition 85% and NiO composition 15% film with the exponential decrease of the conductivity, compared with NiO film. On the latter, the optical absorption of the NiO is observed in the energy region below the band gap energy of NiO, and it is assigned that those absorption is related to the localized state of 3d electrons. Those absorptions are observed the change with an increase in the composition of ZnO, and discuss the possibility as the intermediate band as the absorption layer of the solar cell in the ZnO-NiO thin film system.
We are going to carry out a more detailed analysis for band diagram by XPS and optical properties by the measurement of photoconductivity on ZnO-NiO thin film system.

We have set out to meet the need for high quality, low dislocation density, and low cost bulk gallium nitride substrates for GaN based LEDs, lasers, and power electronics. Our approach is a novel, scalable, and cost-effective electrochemical solution growth (ESG) method which avoids high temperature and pressure conditions often required for GaN growth. Furthermore, our approach is amenable to producing GaN as large boules.
In the approach discussed here, nitrogen gas is reduced to nitride ions at the cathode in a molten LiCl/KCl eutectic mixture. Gallium cations are simultaneously formed at the anode and both ions are drawn toward a GaN seed crystal attached to a rotating susceptor. In this fashion, steady-state GaN growth can occur for an extended period as long as gallium and nitrogen precursors are replenished.
We will report on the development of a GaN growth reactor capable of growing boules up to 50 mm in diameter. The impact of electrode design, preparation, and placement on the electrochemical results will be discussed. Additionally, the impact of LiCl/KCl quality on the electrochemical results and GaN produced will be reported along with a discussion of methods developed to purify salts for ESG.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nuclear transmutation doping (NTD) is known as an excellent doping method for silicon that gives highly uniform dopant distributions. However it has never applied on oxides. In this work, ZnO was successfully doped with Cu via NTD and Cu acceptors were optically and electrically characterized. The great advantage of NTD is the ability of controlling the dopant locations in the lattice, which is a significant problem in ZnO doping.
We also report on the development of simple luminescence method to measure the energy level of donors and acceptors in ZnO and other luminescent semiconductors. It could offer great benefits for the study of optoelectronic semiconductor materials.

ZnO is a versatile wide band gap semiconductor with a great potential for fabrication of semiconductor devices and optoelectronic devices. ZnO single crystal is native substrate for epitaxial growth of high-quality thin films of ZnO-based Group II-oxides (e.g. ZnO, ZnMgO, ZnCdO) for variety of devices, such as UV and visible-light emitting diodes (LEDs), UV laser diodes and solar-blind UV detectors. Because of the tremendous opportunities for ZnO single crystals in such device applications, development of ZnO single crystal substrates has attracted a lot of attention in recent years. ZnO bulk single crystals of large diameters (from 1 inch to 3 inches) with a good crystalline quality have been demonstrated through using the following three major techniques: (1) a hydrothermal technique, (2) a high-pressure melt growth technique, and (3) a chemical-assisted vapor transport (CVT) technique. Currently, commercially available ZnO single crystal wafers of 2-3 inches in diameter of (0001) orientation are produced using a hydrothermal technique. However, there are several drawbacks in hydrothermal growth technique for ZnO single crystals. The main drawback of hydrothermal growth technique is that the ZnO crystals contain large amounts of alkaline metals, such as Li and K. These alkaline metal elements are electrically active and hence can be detrimental to device performances. Another drawback of hydrothermal growth of ZnO crystals is the high anisotropy of growth rates inherent in hydrothermal growth technique, making it difficult to grow large-diameter ZnO crystal boules with an orientation other than the [0001] orientation. Therefore, we believe that there is a great need to develop a crystal growth technique that can produce Li-free ZnO single crystals and eliminate these drawbacks in hydrothermally grown ZnO single crystals.
In this presentation, we report results from a recently developed novel growth technique for ZnO bulk crystals. We demonstrated lithium-free ZnO single crystal boules of up to 1 inch in diameter. We also fabricated ZnO single crystal wafers in sizes up to 1 inch in diameter. Chemical purity, crystalline defects and electrical resistivity of the ZnO single crystals were analyzed using variety of techniques. Results from crystal growth and crystal characterization will be presented and discussed. The research results suggest that the novel crystal growth technique is a viable volume production technique for producing high-quality, lithium-free ZnO single crystal boules and wafers suitable for fabrication of high-performance semiconductor and optoelectronic devices.

We present results on a new type of stable field emitter capable of electron emission at levels comparable to thermal sources. Such an emitter potentially enables disruptive advances in several important technologies which currently use thermal electron sources. These include communications through microwave electronics, and more notably imaging for medicine and security where new modalities of detection may arise due to variable-geometry x-ray sources. Stable emission of 6 A/cm² is demonstrated in a macroscopic array, and lifetime measurements indicate these new emitters are robust. The emitter is a monolithic structure, and is made in a room-temperature process. It is fabricated from a silicon carbide wafer, which is formed into a highly porous structure resembling an aerogel, and further patterned into an array. The emission properties may be tuned both through control of nanoscale morphology and the macroscopic shape of the emitter array.
The silicon carbide wafer is first electrochemically etched into a structure with nanoscale porosity, and subsequently patterned into an emitting array. We chose silicon carbide as it is a refractory material and capable of withstanding high current densities. It also possesses as wide band-gap and these wafers are highly n-doped. Combined, these lead to an enhanced tunneling probability by reducing the effective work function (electron affinity). In these structures, field emission is controlled by altering the field enhancement factor through a two-level hierarchy: local nanostructure morphology and larger scale geometry. At the local nanostructure level, field enhancement is defined by the shape of the local nanostructure such as wall thickness and porosity. At the larger scale, the geometry of the emitting array determines field enhancement and is accomplished by conventional semiconductor processing. Through control of structure at both levels, we demonstrate stable emission in excess of 6 A/cm² at 7.5 V/mu;m, a level comparable with the standard thermal sources. Several lifetime and reliability measurements have been undertaken as to ascertain the robustness of these structures. The results show the emission is stable over an hour&’s operational time, with dc levels in excess of 1 A/cm².
We are cautiously optimistic performance characteristics may be further enhanced through improvements of electrochemistry conditions and geometric design, towards realization of an effective cold cathode technology capable of producing high current.

Thin film InAlN is a promising material for a range of applications including high electron mobility transistors, solid-state light emitters and photovoltaics due to the large spontaneous polarization charge density and a bandgap tunable from infrared to deep ultraviolet. However, InAlN grown by molecular beam epitaxy (MBE) is accompanied by a nano-columnar microstructure wherein an In-deficient core is surrounded by In-rich grain boundaries with deteriorated structural quality. [1] Size of these domains is around 10nm.[2][3]
Herein, we report on the modification of the nano-columnar structure as a result of MBE growth conditions. InAlN thin films were grown at temperature from 400°C to 540°C on GaN templates by plasma assisted molecular beam epitaxy. Nano-columnar features were identified using high resolution x-ray diffraction at (002) omega scan, in which a best fit model of the scattering is the superposition of domains with differing microstructure. While the main peak has the FWHM of 180s, the underlying peak is much broader with FWHM of around 3600s. We relate these domains to the nano-columnar structure with the higher quality being the core.
By deconvolving the scattering signal from core and boundary, the volume of nano-columnar boundary was shown to decrease with high growth temperature and low growth rate. The results were confirmed by transmission electron microscopy (TEM). Samples tested under plan view TEM showed nano-columnar structure with core diameters consistent with literature [1][2][3], but the sample grown at high temperature showed the reduced contrast of nano-columnar boundaries. We speculate the modulation of composition in the grain boundary was reduced in such case. In addition, for the near lattice matched InAlN grown at high temperature of 540°C and low growth rate of 8Å/min, the epilayer is shown under cross sectional TEM to be composed by two layers with different nano-columnar characteristics. The 20nm thick bottom layer show relatively uniform compositional distribution and the indium rich nano-columnar features initiated during growth after the first 20nm. In contrast, for the sample grown at low temperature nano-columnar structure initiated at the InAlN/GaN interface. This suggests an initial layer of InAlN with good structural quality by suppressing the formation of nano-columnar is possible.
[1] S. Choi, F. Wu, R. Shivaraman, E. C. Young, J. S. Speck, Appl. Phys. Lett. 100, 232102 (2012)
[2] L. Zhou, D. J. Smith, M. R. McCartney, Appl. Phys. Lett. 90, 081917 (2007)
[3] S. L. Sahonta, G. P. Dimitrakopulos, T. Kehagias, etc., Appl. Phys. Lett. 95, 021913 (2009)