Hydride Vapor Phase Epitaxy Growth Research Articles

(Invited) Scalable 2-Step Fabrication of Core-Shell GaN Micropillars

Group III-Nitride three-dimensional structures have gained substantial interest for application in optoelectronic, energy, and sensor devices with non-planar geometries [1-4]. The benefits of 3D structures, such as GaN-based nano-/micro-rods, compared to planar films include large surface area, high structural quality, non-polar-surface utilization, small footprint, mechanical flexibility, and enhanced light absorption/extraction efficiency. To advance the technology of non-planar III-Nitride semiconductors, it is necessary to optimize and scale up the fabrication of such structures with controlled geometries and desired electronic and optical properties. Here, we report the fabrication of wafer-scale periodic arrays of vertically aligned GaN core-shell micropillars using a combination of top-down etch and epitaxial overgrowth.The two-step process consists of inductively coupled plasma (ICP) etching of lithographically patterned GaN-on-Si substrate to produce an array of micropillars followed by selective growth of GaN shells over these pillars using Hydride Vapor Phase Epitaxy (HVPE). The most significant aspect of the study is the demonstration of the sidewall facet control in the shells, ranging from truncated hexagonal pyramids with the (1-101) semi-polar sidewalls to hexagonal prisms with the (1-100) non-polar sidewalls, by employing a post-ICP wet chemical etch and by tuning the HVPE growth temperature. Optimization of both ICP etching and epitaxial overgrowth reduced dislocation density in the GaN shells, thus enhancing the transport and optical characteristics of these device platforms. Photo- and cathodoluminescence showed a substantial reduction of parasitic yellow luminescence as well as strain-relaxation in the core-shell structures, supported by Raman scattering, electron-backscatter-diffraction (EBSD), and X-ray-diffraction measurements. Transmission electron microscopy revealed improved crystal quality with reduced dislocation density in the epitaxially grown shells. This work demonstrates the feasibility of selective epitaxy on micro/nano-engineered 3D templates for realizing high-quality GaN-on-Si devices such as LEDs and p-i-n photodetectors.

Influencing the surface quality of free-standing wurtzite gallium nitride in ultra-high vacuum: Stoichiometry control by ammonia and bromine adsorption

Gallium nitride (GaN), a wide bandgap semiconductor with absorption and emission in the ultraviolet/visible range, is proposed as an alternative to metallic surfaces for assembling organic molecular structures aiming at optoelectronic applications. However, the formation of a persistent surface oxide layer in air considerably limits the use of GaN for well-defined interfaces. In this work, we have investigated, characterized and processed n-type free-standing c-plane hexagonal wurtzite GaN crystals grown by hydride vapor phase epitaxy and ammonothermal growth methods. Surface cleaning and full removal of the oxide layer on GaN surfaces could be reproducibly achieved via sputtering and annealing cycles, as evidenced by X-ray photoelectron spectroscopy and low-energy electron diffraction. Scanning tunneling microscopy, however, indicated substantial roughening of the GaN surface and the formation of unwanted Ga-rich islands and clusters. Although ammonia (NH3) and bromine (Br) treatments compensated the N/Ga atoms ratio reduced by sputtering, the surface morphology remained rough, exhibiting randomly shaped and distributed hillocks. In addition, we studied the effect of electron bombardment on the surface quality of GaN during NH3 annealing, on-surface debromination and polymerization of 1,3,5-tris(4-bromophenyl) benzene on GaN, and the removal of Ga atoms by Br atoms during the desorption.

Status of h-BN quasi-bulk crystals and high efficiency neutron detectors

III-nitrides have fomented a revolution in the lighting industry and are poised to make a huge impact in the field of power electronics. In the III-nitride family, the crystal growth and use of hexagonal BN (h-BN) as an ultrawide bandgap (UWBG) semiconductor are much less developed. Bulk crystals of h-BN produced by the high-temperature/high-pressure and the metal flux solution methods possess very high crystalline and optical qualities but are impractical to serve as substrates or for device implementation as their sizes are typically in millimeters. The development of crystal growth technologies for producing thick epitaxial films (or quasi-bulk or semi-bulk crystals) in large wafer sizes with high crystalline quality is a prerequisite for utilizing h-BN as an UWBG electronic material. Compared to traditional III-nitrides, BN has another unique application as solid-state neutron detectors, which however, also require the development of quasi-bulk crystals to provide high detection efficiencies because the theoretical efficiency (ηi) relates to the detector thickness (d) by ηi=1−e−dλ, where λ denotes the thermal neutron absorption length which is 47 μm (237 μm) for 10B-enriched (natural) h-BN. We provide an overview and recent progress toward the development of h-BN quasi-bulk crystals via hydride vapor phase epitaxy (HVPE) growth and the attainment of thermal neutron detectors based on 100 μm thick 10B-enriched h-BN with a record efficiency of 60%. The thermal neutron detection efficiency was shown to enhance at elevated temperatures. Benchmarking the crystalline and optical qualities of h-BN quasi-bulk crystals with the state-of-the-art mm-sized bulk crystal flakes and 0.5 μm thick epitaxial films identified that reducing the density of native defects such as vacancies remains the most critical task for h-BN quasi-bulk crystal growth by HVPE.

(Invited) Better GaN Substrates Are the Key to Advancing GaN Device Technology

Gallium nitride (GaN) is a semiconductor that has an unusual combination of extreme values of fundamental physical and chemical properties. Its wide bandgap (3.4 eV at room temperature) results in a low intrinsic carrier density and thus low leakage and dark currents. These properties combined with its direct gap are crucial requirements for photo-detectors and high-temperature electronic applications. Despite the relatively large carrier effective masses, which lead to lower carrier mobilities, the high electron saturation velocity and high breakdown field make possible the fabrication of high frequency electronic devices. In addition, its relatively high thermal conductivity opens the possibility to realize a number of high-power device applications. These unique characteristics have made possible the fabrication of a variety of optoelectronic and electronic devices capable of performing at extreme operation conditions of current, voltage, temperature, and in harsh environments. GaN and its alloys and heterostructures with InN and AlN have resulted in devices with superior performance for a variety of applications. Despite this unprecedented and not fully developed semiconductor materials success, the realization of high-performance and high-yield devices will require native GaN substrates with well-controlled physical properties.Presently, Hydride Vapor Phase Epitaxy (HVPE) and Ammonothermal (Ammono-GaN) bulk growth are the only two methods that have successfully demonstrated the manufacture of GaN boules. The first, a fast-growth non-equilibrium process, only reproduces the starting substrate dimensions, while the latter, a slow-growth equilibrium process, allows increasing boule dimensions under controlled growth conditions. Despite the impressive accomplishments recently achieved by these growth methods, much work still needs to be done to establish a fully satisfactory bulk growth method for this material. Heteroepitaxial HVPE-GaN suffers from high dislocation densities, while Ammono-GaN suffers from high oxygen incorporation. Recent results suggest that Ammono-GaN wafers can be successfully used as seeds to grow thick freestanding GaN wafers by HVPE [1]. This approach resulted on HVPE-GaN wafers with orders of magnitude reduced extended defect and lower oxygen impurity concentrations. However, the relatively high background concentration of pervasive impurities is still preventing the realization of high performance devices. In this work, we illustrate how defect-sensitive optical techniques can be used to assist the optimization procedures at various stages of growth [2]. A brief review of defect-sensitive optical spectroscopic techniques employed to evaluate structural, optical, and electronic properties of the state-of-the-art bulk and thick-film (quasi-bulk) Nitride substrates and homoepitaxial films is presented. Defects control the performance of devices and feeding back knowledge of defects to growth efforts is key to advancing technology.References J. A. Freitas, Jr., J.C. Culbertson, N.A. Mahadik, T. Sochacki, M. Iwinska, and M.S. Bockowski, J. Crystal Growth 456 (2016) 113.J. A. Freitas, Jr., J.C. Culbertson, and E. R. Glaser, Crystals, 12 (2022) 1294.

Investigation of Growth Sectors in Gallium Nitride Substrate Wafers from Ammonothermal and Patterned Hvpe Growth Methods

Gallium Nitride (GaN) is a wide band gap semiconductor material that has the potential to replace silicon (Si) in power electronics. However, the quality and availability of bulk GaN substrates is currently limiting its use in high-performance power electronics. Several methods have been developed to grow bulk GaN crystals, including physical vapor transport (PVT) [1, 2], Na-flux [3], ammonothermal [4, 5], and hydride vapor phase epitaxy (HVPE) [6]. Additionally, patterned HVPE growth methods have been developed to produce GaN substrates with reduced dislocation densities [7]. Ammonothermal and patterned HVPE growth methods have shown promise in producing high-quality GaN crystals for use in power devices [8-10]. Ammonothermal-grown GaN has an overall low dislocation density of 1000 cm-2, while patterned growth can concentrate dislocations into small areas, leaving the rest of the crystal relatively dislocation-free (< 102 cm-2). Besides dislocations, the overall homogeneity of these wafers is also important for devices. Recent synchrotron X-ray topography studies revealed domain features in both ammonothermal and patterned HVPE GaN wafers. Using synchrotron X-Ray Plane-Wave Topography (SXPWT, previously known as synchrotron X-Ray Rocking Curve Topography), these domain features that have enclosed diamond or quadrilateral shapes are found to have relative compressive strain values (~ 10-5) (Figure 1b). These features are visible on SEM micrographs using a Robinson detector (RBSD) (Figure 1c). The origin of these strains is likely the different impurity incorporation rates in different growth sectors.In the patterned HVPE growth method, growth sectors have been reported to exist due to growth along different directions [11]. It is likely that a similar situation exists for ammonothermal substrates, although no studies have been reported on this. Understanding the nature of these domain areas is important for device fabrication as the inhomogeneities will likely be replicated into active regions. SIMS measurements are currently being conducted to characterize the differences in impurity concentrations in these domains. These results will be used to confirm the origin of the strain.Figure 1. (a) Synchrotron monochromatic beam X-ray topograph of a selected area on a patterned HVPE grown GaN substrate; (b) Cropped strain map of the same area obtained by SXPWT; (c) SEM image of the same area with RBSD contrast. All three images show contrast from the domains defined by the growth sectors.[1] H. Wu, J. Spinelli, P. Konkapaka, M. Spencer, MRS Online Proceedings Library Archive 892 (2005).[2] D. Siche, D. Gogova, S. Lehmann, T. Fizia, R. Fornari, M. Andrasch, A. Pipa, J. Ehlbeck, Journal of crystal growth 318 (2011) 406-410.[3] M. Aoki, H. Yamane, M. Shimada, S. Sarayama, F.J. DiSalvo, Journal of crystal growth 242 (2002) 70-76.[4] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, Journal of Crystal Growth 310 (2008) 3911-3916.[5] T. Hashimoto, F. Wu, J.S. Speck, S. Nakamura, Journal of Crystal Growth 310 (2008) 3907-3910.[6] H.P. Maruska, J. Tietjen, Applied Physics Letters 15 (1969) 327-329.[7] T. Nakamura, K. Motoki, GaN substrate technologies for optical devices, IEEE, vol 101, 2013, p. 2221.[8] B. Raghothamachar, Y. Liu, H. Peng, T. Ailihumaer, M. Dudley, F.S. Shahedipour-Sandvik, K.A. Jones, A. Armstrong, A.A. Allerman, J. Han, H. Fu, K. Fu, Y. Zhao, Journal of Crystal Growth 544 (2020) 125709.[9] Y. Liu, B. Raghothamachar, H. Peng, T. Ailihumaer, M. Dudley, R. Collazo, J. Tweedie, Z. Sitar, F.S. Shahedipour-Sandvik, K.A. Jones, Journal of Crystal Growth 551 (2020) 125903.[10] Y. Liu, H. Peng, T. Ailihumaer, B. Raghothamachar, M. Dudley, Journal of Electronic Materials 50 (2021) 2981-2989.[11] K. Motoki, T. Okahisa, S. Nakahata, N. Matsumoto, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, M. Ueno, Y. Kumagai, Journal of Crystal Growth 237 (2002) 912-921. Figure 1

Probing Boron Vacancy Complexes in h-BN Semi-Bulk Crystals Synthesized by Hydride Vapor Phase Epitaxy

Hexagonal BN (h-BN) has emerged as an important ultrawide bandgap (UWBG) semiconductor (Eg~6 eV). The crystal growth technologies for producing semi-bulk crystals/epilayers in large wafer sizes and understanding of defect properties lag decades behind conventional III-nitride wide bandgap (WBG) semiconductors. Here we report probing of boron vacancy (VB)-related defects in freestanding h-BN semi-bulk wafers synthesized by hydride vapor phase epitaxy (HVPE). A photocurrent excitation spectroscopy (PES) was designed to monitor the transport of photoexcited holes from deep-level acceptors. A dominant transition line at 1.66 eV with a side band near 1.62 eV has been directly observed, which matches well with the calculated energy levels of 1.65 for the VB-H deep acceptor in h-BN. The identification of VB complexes via PES measurement was further corroborated by the temperature-dependent dark resistivity and secondary ion mass spectrometry measurements. The results presented here suggested that it is necessary to focus on the optimization of V/III ratio during HVPE growth to minimize the generation of VB-related defects and to improve the overall material quality of h-BN semi-bulk crystals. The work also provided a better understanding of how VB complexes behave and affect the electronic and optical properties of h-BN.

Ultra-High-Speed Growth of GaAs Solar Cells by Triple-Chamber Hydride Vapor Phase Epitaxy

In photovoltaic (PV) power generation, highly efficient III-V solar cells are promising for emerging mobile applications, such as vehicle-integrated PVs. Although hydride vapor phase epitaxy (HVPE) has received attention due to its lower fabrication costs, realization of high throughput performance while maintaining solar-cell characteristics using this growth method is essential. In this study, the effect of atmospheric-pressure triple-chamber HVPE growth conditions on GaAs solar-cell properties were carefully investigated in conjunction with defect analysis using deep-level transient spectroscopy (DLTS). Based on the analysis on GaAs reaction processes, the suppression of arsine thermal cracking in the HVPE hot-wall reactor was important to achieve fast GaAs growth using a low input V/III ratio. Moreover, the DLTS results revealed that the reduced input V/III ratio was effective in suppressing the generation of EL2 traps, which is a common GaAs midgap complex defect involving arsenic antisites. Although the EL2 trap density increased with the growth rate, the performance of GaAs solar cells that were grown under reduced arsine thermal cracking exhibited almost no considerable cell parameter deterioration at a growth rate of up to 297 μm/h. Consequently, a conversion efficiency of 24% with a high open-circuit voltage of 1.04 V was achieved for the cells that were grown at 200 μm/h.

Realization of all-crystalline GaN/Er:GaN/GaN core-cladding optical fiber structures

Erbium-doped gallium nitride (Er:GaN) is a promising gain material for solid-state high-energy lasers operating in the 1.5 μm wavelength window due to the superior optical properties and extremely high thermal conductivity of a GaN host crystal that permit high-power and high-temperature applications. We report the realization of all-crystalline GaN/Er:GaN/GaN embedded waveguide fiber structures using the hydride vapor phase epitaxy growth and re-growth technique, along with chemical–mechanical polishing processes. The Er:GaN core layer possesses an Er doping concentration of 1.7×1020 atoms/cm3, confirmed by secondary ion mass spectrometry measurements. X-ray diffraction measurements confirm, respectively, c-, a-, and m-plane orientations for top/bottom, side, and front/back cross-sectional cladding layers of the fiber structure with good single-crystalline quality. The 1.5 μm Er3+ emission was detected from each surface of the fiber structures via 980 nm resonant excitation. The effect of 1.54 μm light guiding by the fiber structure has been demonstrated. This work laid the foundation toward achieving all-crystalline core-cladding fibers based on GaN wide bandgap semiconductor with potential applications in the harsh environments of high powers, power densities, and temperatures.

(Invited) Advancements in Vertical GaN p-n Junction Structures Via p-Type Ion Implantation

Vertical GaN power devices have emerged to become promising candidates for next-generation high power applications due to superior material properties such as high breakdown voltage, low on-resistance, and high mobility compared to devices based on Si and SiC. GaN-based p-n junction switching devices enable higher voltage power with significantly higher efficiencies with added advantages of reduced size and weight systems. A technological limitation of GaN, however, has been the inability to achieve high p-type doping in a planar, vertical device. Here, we will focus on recent developments to achieve high p-type efficiency though ion implantation, novel high temperature annealing schemes, and the importance of defects and morphology in native substrates and epitaxial layersGaN epitaxial layers for subsequent p-type ion implantation can be grown on sapphire substrates but vertical devices are optimal when using GaN homoepitaxial structures. Unlike most other semiconductor substrates, GaN substrates are not grown from the melt. Hydride vapor phase epitaxy or ammonothermal growth are predominantly used to produce substrates and these substrates will experience higher temperatures during subsequent epitaxy and ion implant activation annealing than occurs during substrate formation. In both types of substrates, the threading dislocation distribution can vary by orders of magnitude – these variations correlate well with leakage currents in vertical devices.The activation of the implanted p-type dopants requires high temperature annealing. For most semiconductors, an annealing temperature of ~ 2/3 the melting point leads to a high activation fraction (> 95%) of the implanted species. The implantation process introduces large concentrations of native defects as well as the p-type implant (typically Mg), so it is necessary to remove the defects and for the dopants to locate on substitutional sites (Ga sites for Mg). We demonstrate how the formation of inverted domain defects at temperatures below 1400 °C correlate with Mg-compound formation, whereas annealing temperatures 1400 °C and above do not lead to the formation of such compounds but rather high Mg activation efficiencies.These recent developments, partly through the ARPA-E PNDIODES program as well as international efforts, have brought understanding of the key processing steps and substrate requirements to achieve high activation efficiency p-type doping for planar, vertical device structures in a scalable framework.

Effect of oxygen on defect states of Al0.4Ga0.6N layers grown by hydride vapor phase epitaxy

The defect states and electrical properties of AlxGa1-xN (x = 0.4) grown by hydride vapor phase epitaxy (HVPE) were investigated. To identify the effect of incorporation of elemental O in AlxGa1-xN crystals, HVPE growth of AlxGa1-xN crystals was conducted with and without the flow of O2. The crystal quality and electrical properties of the AlxGa1-xN layer was analyzed by X-ray diffraction and deep level transient spectroscopy (DLTS). Schottky devices for I–V, C–V and DLTS measurement were formed using Ni/Au metal and Ti/Al metallization. Capacitance DLTS spectra showed two types of deep traps of H1 and H2 in Al0.4Ga0.6N grown without oxygen, while H1’ traps were observed in Al0.4Ga0.6N grown with oxygen. All traps were hole-like traps with activation energies of 1.3 eV(H1), 0.59 eV(H2), and 1.2 eV(H1ʹ). These results show that the oxygen atoms can improve the crystal quality and suppress the defect states in AlxGa1-xN crystals.

Separation of AlN layers from silicon substrates by KOH etching

In this work, the AlN/Si(111) epitaxial structures grown consistently by plasma assisted molecular beam epitaxy (PA MBE) and hydride vapour phase epitaxy (HVPE) methods were studied. The PA MBE AlN buffer layers were synthesized via coalescence overgrowth of self-catalyzed AlN nanocolumns on Si(111) substrates and were used as templates for further HVPE growth of thick AlN layer. It was shown that described approaches can be used to obtain AlN layers with sufficiently smooth morphology. It was found that HVPE AlN inherited crystallographic polarity of the AlN layer grown by PA MBE. It was demonstrated that the etching of such AlN/Si(111) epitaxial structures results in partial separation of the AlN epilayers from the Si(111) substrate and allows to form suspended structures. Moreover, the avoidance of surface damage and backside overetching was achieved by use thin Cr film as surface protective coating and by increasing the layer thickness accordingly.

(Invited) Developments in Vertical GaN p-n Junction Structures Via p-Type Ion Implantation

Vertical GaN power devices have emerged to become promising candidates for next-generation high power applications due to superior material properties such as high breakdown voltage, low on-resistance, and high mobility compared to devices based on Si and SiC. Silicon is now used for most power switches, but silicon-based devices lose efficiency under high power demands. GaN-based p-n junction switching devices enable higher voltage power with significantly higher efficiencies with added advantages of reduced size and weight systems. A technological limitation of GaN, however, has been the inability to achieve high p-type doping in a planar, vertical device. Here, we will focus on recent developments to achieve high p-type efficiency though ion implantation, novel high temperature annealing schemes, and the importance of defects and morphology in native substrates and epitaxial layers with an emphasis on x-ray scattering and electron microscopy measurements.GaN epitaxial layers for subsequent p-type ion implantation can be grown on sapphire substrates but vertical devices are optimal when using GaN homoepitaxial structures. Unlike most other semiconductor substrates, GaN substrates are not grown from the melt. Hydride vapor phase epitaxy or ammonothermal growth are predominantly used to produce substrates and these substrates will experience higher temperatures during subsequent epitaxy and ion implant activation annealing than occurs during substrate formation. We have developed x-ray topography techniques and combined these with electron microscopy and x-ray diffraction to understand how strain and defects in the substrates and epitaxial layers evolve during subsequent high temperature processing steps.The activation of the implanted p-type dopants requires high temperature annealing. For most semiconductors, an annealing temperature of ~ 2/3 the melting point leads to a high activation fraction (> 95%) of the implanted species. The implantation process introduces large concentrations of native defects as well as the p-type implant (typically Mg), so it is necessary to remove the defects and for the dopants to locate on substitutional sites (Ga sites for Mg). We demonstrate how high resolution x-ray scattering and transmission electron microscopy are used to understand the defect-induced strain recovery process and to mitigate the formation of defects such as dislocation loops, stacking faults, and other non-equilibrium defect structures that cause a degradation in dopant activation efficiency.These recent developments, partly through the ARPA-E PNDIODES program as well as international efforts, have brought understanding of the key processing steps and substrate requirements to achieve high activation efficiency p-type doping for planar, vertical device structures in a scalable framework.

Surface chemistry models for GaAs epitaxial growth and hydride cracking using reacting flow simulations

Hydride vapor phase epitaxy (HVPE) is a promising technology that can aid in the cost reduction of III-V materials and devices manufacturing, particularly high-efficiency solar cells for space and terrestrial applications. However, recent demonstrations of ultrafast growth rates (∼500 μm/h) via uncracked hydrides are not well described by present models for the growth. Therefore, it is necessary to understand the kinetics of the growth process and its coupling with transport phenomena, so as to enable fast and uniform epitaxial growth. In this work, we derive a kinetic model using experimental data and integrate it into a computational fluid dynamics simulation of an HVPE growth reactor. We also modify an existing hydride cracking model that we validate against numerical simulations and experimental data. We show that the developed growth model and the improved cracking model are able to reproduce experimental growth measurements of GaAs in an existing HVPE system.

Long catalyst-free InAs nanowires grown on silicon by HVPE

We report for the first time on the hydride vapor phase epitaxy (HVPE) growth of long (26 μm) InAs nanowires on Si(111) substrate. The thermodynamic and kinetic mechanisms involved during the growth of such long nanowires are identified.

(Invited) GaN Homoepitaxial Growth and Substrate-Dependent Effects for Vertical Power Devices

Vertical GaN power switch technology is expected to be utilized in next-generation medium to high voltage power converters due to the low ON-resistance and high breakdown voltage enabled by the improved critical electric field and mobility compared to Si and SiC-based devices. As large area substrates have become available by hydride vapor phase epitaxy (HVPE) and ammonothermal growth, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth, allowing recent realization of several fundamental vertical power devices, including diodes with edge termination, trench MOSFETs, and CAVETs. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow vertical GaN power devices to achieve their full potential, notably the realization of repeatable thick drift layers with low background doping. To enable this, a deeper understanding of substrate preparation and the effects of the substrate and growth initiation on the characteristics of the epitaxial layers is required for MOCVD growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation by growing simultaneously on wafers from different vendors. The goal of this work is to detect and identify defects in GaN substrates with a series of quick, non-destructive, inexpensive techniques with capabilities of mapping whole wafers. We have characterized multiple substrates from over eight suppliers and the homoepitaxy grown on those substrates. All the substrates had nominally similar as-received specifications (resistivity, thickness, off-cut angle, bow, surface finish). The substrates were evaluated with a variety of techniques including Raman spectroscopy, photoluminescence, white light interferometry, and Nomarski imaging, enabling to detection of different concentrations of grain boundaries, sample impurities, point defects, v-shaped pits, polishing defects, crystal stress damage, and non-uniform insulating and conducive regions. The substrates can be characterized in two different categories: those with uniform characteristics, including carrier concentration, and those without. Comparing these results to those from homoepitaxial growth on the same wafers, the effects are both subtle and overt. Macroscopic surface morphology, which has shown a direct correlation to leakage current, copies and exaggerates that of the underlying substrate. Photoluminescence of the homoepitaxial surface along with Raman spectroscopy show that non-uniformities in the substrate carrier concentration can continue into the epitaxy. Following optical characterization, vertical Schottky diodes were fabricated to quickly evaluate device performance. While most of the films showed the ability to withstand high electric fields, those with more uniform properties in the substrate also showed more uniform electrical properties. These results show that the bulk substrates enable the path to high voltage vertical devices, but also show the significant influence that substrates can play in device performance.

Simulation and analysis of III–V heterostructure solar cells for a continuous HVPE process

A continuous hydride vapor phase epitaxy (HVPE) growth system has the potential to realize large-scale production of low-cost III–V solar cells. To aid the process development, this work integrates HVPE reactor model with III–V solar cell simulation to intimately tie the device performance to the process design and operation. This modeling approach sets a potential to connect the control of a growth system directly to the device performance without intervening ancillary models. The approach of direct process-to-device level simulation is demonstrated through simulating the continuous HVPE fabrication of a model solar cell structure as a function of critical process parameter. Critical reactor design features as well as sensitive material parameters in affecting device performance are discussed. An illustrative case study is carried out to investigate the influence of gas curtain velocity on the solar cell efficiency using the integration of reactor and solar cell models. Through the correlations determined by the process-to-device simulations, the sensitivity of critical reactor variables in affecting cell efficiency can be established and optimized through the impact on solar cell performance directly. This integrated device performance–reactor design approach allows for the design of the process and its real-time control with direct knowledge of the expected dependence of device performance on reactor design trade-offs.

Effect of hydride vapor phase epitaxy growth conditions on the degree of atomic ordering in GaInP

We report a systematic study of CuPt-type ordering in hydride vapor phase epitaxy (HVPE)-grown Ga0.5In0.5P. Selected-area electron diffraction reveals ordering in samples grown on (001) GaAs substrates offcut toward (11¯1)B. The ordering is single-variant, occurring only on (11¯1)B planes and not on (1¯11)B. Quantitative analysis of the order parameter by high-resolution x-ray diffraction (HRXRD) indicates that ordering increases with deposition temperature in samples grown at 600–700 °C with a constant gas-phase V/III ratio ∼3. Ordering increases with V/III ratio in the range of 1.3–6.7 at a constant deposition temperature of 650 °C. Photoluminescence measurements correlate the order parameter with Ga0.5In0.5P bandgap contraction, though the contraction is larger than expected based on the magnitude of order parameters measured by HRXRD. A possible reason for this discrepancy is that the photoluminescence emission occurs in the lower bandgap ordered domains, which are small and evenly dispersed throughout the material. We also show that the degree of ordering decreases with growth rate, disappearing at ∼60 μm/h. The HVPE-grown material exhibits a generally weaker ordering than organometallic vapor phase epitaxy (OMVPE)-grown material, likely due to the moderate V/III ratios employed, in contrast to the V/III ratios in the 100s typical of OMVPE. However, the tendency for ordering to increase with V/III ratio suggests that the same dimer-induced stress mechanism used to explain the occurrence of ordering in OMVPE-grown material also applies to HVPE. The tendencies for ordering to increase with deposition temperature and decrease with growth rate show that kinetics limit the degree of ordering, also in agreement with OMVPE trends.

Evaluation of GaAs solar cells grown under different conditions via hydride vapor phase epitaxy

In this study, we characterize the GaAs solar cells grown under different conditions using a custom-built atmospheric-pressure hydride vapor-phase epitaxy (HVPE) reactor. Under typical HVPE growth involving the decomposition of AsH3 to Asx, the growth rate is considerably dependent on the temperature and is limited to ~10 μm/h at a low deposition temperature of 660 °C because of the large kinetic barrier of 198 kJ/mol. Herein, we grow GaAs with a considerably lower kinetic barrier of 7 kJ/mol by suppressing the decomposition of AsH3 in the reactor. Further, we grew GaAs solar cells at an extremely high growth rate of 120 μm/h using uncracked AsH3. However, the open-circuit voltage (VOC) was reduced from 1.0 V for the cells grown at 8 μm/h under typical growth using Asx to 0.95 V for the cells grown at 120 μm/h using hydride-enhanced growth. The reduction in VOC was attributed to the modulation of both the doping profile and the abruptness of heterointerfaces. A small amount of residual gases, which presents at the growth surface after the growth of the p-InGaP back surface field layer, is prominently incorporated during the growth of the p-GaAs base layer upon hydride-enhanced growth due to the fast kinetics.

AlN nanostructures and flat, void-less AlN templates formed by hydride vapor phase epitaxy on patterned sapphire substrates

Severe growth irregularity usually encountered in AlN growth on patterned sapphire substrates (PSSs) was completely suppressed in hydride vapor phase epitaxial (HVPE) growth on a PSS having small cone-pattern with a height of 220 nm. At low temperature, the AlN growth on the PSS led to a formation of regular array of hexagonal AlN nano-rods on the cone-tops. At high temperatures, the growth mode turned into a layered fashion and a void-less AlN template with an atomically smooth surface and high crystal quality were realized with an HVPE growth thickness less than 1 μm.

Reduction of surface roughness by modification of step-bunched aluminum nitride layers towards step-flow morphology

Conventional aluminum nitride (AlN) template fabrication techniques, like hydride vapour phase epitaxy or AlN growth on patterned sapphire substrates, usually lead to step-bunched template surfaces. The resulting macrosteps cause emission broadening or even multiple-peak characteristics of ultraviolet light-emitting diodes (UV LEDs) fabricated on such AlN templates. In order to reduce these macrosteps and to provide a smoother surface, even without the need of a thick AlN deposition, a two-layer growth procedure is reported here. A three-dimensional (3D) – twodimensional (2D) sequential growth initiates a significant modification of the previous AlN surface morphology and simultaneously limits the evolving tensile strain. A primarily step-bunched surface with a surface roughness root mean square of 1.8 nm is successfully reduced by the two-layer growth procedure down to 0.8 nm, without any film cracking. This distinct roughness reduction of more than 50% is achieved within an AlN thickness of only 1.3 µm. With a smoother surface, the electroluminescence characteristic of a UV LED structure is substantially improved. Instead of a double-peak emission, typical for LEDs grown on step-bunched templates, a single-peak emission and lower spectral width were achieved, indicating the high potential of the suggested two-layer technique for improving performance.