Hydride Vapor Phase Epitaxy Technique Research Articles

High responsivity solar-blind metal-semiconductor-metal photodetector based on α-Ga2O3

α-Ga2O3, with an ultra-high energy bandgap of 5.1 eV, is an attractive material for applications in solar-blind photodetectors and high-power electronics. We fabricated an α-Ga2O3 metal-semiconductor-metal (MSM) interdigitated solar-blind photodetector, where an epitaxial α-Ga2O3 thin film was grown on a sapphire substrate using a hydride vapor-phase epitaxy technique. Excellent optoelectronic device properties including high responsivity (4.24 × 104 A/W), detectivity (1.77 × 1011 Jones), and external quantum efficiency (2.07 × 105) were demonstrated in response to ultraviolet (UV) C wavelength. Under sunlight, the α-Ga2O3 MSM photodetector exhibited stable solar-blind sensitivity to UVC wavelength without interference from the incoming solar spectrum. Our work proposes that α-Ga2O3 has great potential as a next-generation high-performance solar-blind photodetector.

Erbium energy levels in GaN grown by hydride vapor phase epitaxy

Erbium doped GaN (Er:GaN) is a promising candidate as a novel gain medium for solid-state high energy lasers (HELs) due to its superior physical properties over a synthetic garnet such as Nd:YAG. Er:GaN emits in the 1.5 µm region, which is retina-safe and has a high transmission in the air. We report photoluminescence (PL) studies performed on Er:GaN epilayers synthesized by the hydride vapor phase epitaxy (HVPE) technique. The room temperature PL spectra of HVPE grown Er:GaN epilayers resolved as many as 11 and seven emission lines in the 1.5 µm and 1.0 µm wavelength regions, respectively, corresponding to the intra-4f shell transitions between Stark levels from the first (4I13/2) and the second (4I11/2) excited states to the ground state (4I15/2) of Er3+ in GaN. The observed peak positions of these transitions enabled the construction of the detailed energy levels in Er:GaN. The results agree well with those of the calculation based on a crystal field analysis. Precise determination of the detailed energy levels of the Stark levels in the 4I11/2, 4I13/2, and 4I15/5 states is critically important for the realization of HELs based on Er:GaN.

(Invited) Halide Vapor Phase Epitaxy of III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting devices comprising III-V semiconductors alloys hold record solar-to-hydrogen efficiencies, achievable only by complex architectures involving tandem cells i.e an inverted metamorphic GaInAsP/GaInAs solar cell with an efficiency of 32.6% under 1-sun and the GaInAs/p-GaInP PV/PEC with a solar to hydrogen conversion efficicncy close to 17% under 1-sun as well; nanostructures and buried junctions, which leads to high manufacturing complexity. These approaches have been necessary to circumvent the shortcomings towards unassisted water splitting of state-of-the art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, such as a) inadequate band edge alignment with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) charge recombination. An alternative path to overcome these limitations while maintaining device simplicity and competitive solar fuels costs is to develop novel III-V photoabsorbers with adequate band energetics.First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb into GaN or GaP forms GaSbxN1-x and GaSbyP1-y alloys with narrowed band gap in the former case and changes the electronic band structure from indirect to direct in the latter. Theoretical computations predict that with band gaps in the order of 2 eV, these materials could be able to straddle the electrochemical potentials for water oxidation and proton reduction in acidic electrolytes.Metal organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE) have been used to synthesize GaSbxN1-x and GaSbyP1-y alloys, respectively, achieving single crystalline quality in a wide range of homogeneous Sb incorporation. Experimental measurements confirm the significant band gap reduction from 3.4 eV in pure GaN to 1.5 eV in GaSbxN1-x as determined by Tauc plot analysis of diffuse reflectance data and low temperature photoluminescence spectroscopy; and suggest reduction of the direct transition of GaP at 2.68 eV in the case of GaSbyP1-y alloys.The photoelectrochemical performance towards water splitting of GaSbxN1-x and GaSbyP1-y alloys has been assessed in 2- and 3-electrode standard methods to determine their characteristic attributes, e.g. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situ fluorescence probing. Furthermore, photoelectrochemical impedance measurements have shown the absence of charge transfer resistance at the interface between electrode and electrolyte in the case of GaSbyP1-y alloys that is clearly observed in pure GaP, suggesting antimony can also modify the charge transport properties of the material.These promising results have also motivated our efforts for better understanding of the synthesis processes, particularly for the HVPE technique given its fast growth rate is both an advantage and a challenge for precise thickness and composition control. A computational model using Chemkin Prowas used to validate the previous experimental results for the growth of Gallium Antimonide Phosphide (GaSbyP1-y) films. The model was developed for a reactor having one heating zone for precursor formation and a second zone for film deposition. The first zone was assumed to be in thermodynamic equilibrium. The second (deposition) zone was modeled using a chemistry set describing various gas phase and gas-solid kinetics. The substrate temperature and system pressure were successfully correlated with the observed experimental values for the growth rate and other important features such as the crystalline quality and degree of Sb incorporation. The results also point out the major gas phase chemical species involved in the growth, such as GaCl, SbCl, P2 and P4, which opens the door for future optimization of the synthesis process oriented towards a better precursor utilization and higher antimony incorporation at elevated substrate temperatures, which are required for single crystalline material quality. Acknowledgements: Financial support from Conn Center for Renewable Energy Research and NSF EPSCoR Program (1355438)

Growth of Freestanding Gallium Nitride (GaN) Through Polyporous Interlayer Formed Directly During Successive Hydride Vapor Phase Epitaxy (HVPE) Process

The progress of nitride technology is widely limited and hindered by the lack of high-quality gallium nitride (GaN) wafers. Therefore, a large number of GaN epitaxial devices are grown on heterogeneous substrates. Although various additional treatments of substrate have been used to promote crystal quality, there is still plenty of room for its improvement, in terms of direct and continuous growth based on the hydride vapor phase epitaxy (HVPE) technique. Here, we report a three-step process that can be used to enhance the quality of GaN crystal by tuning V/III rate during successive HVPE process. In the growth, a metal-organic chemical vapor deposition (MOCVD) grown GaN on sapphire (MOCVD-GaN/Al2O3) was employed as substrate, and a high-quality GaN polyporous interlayer, with successful acquisition, without any additional substrate treatment, caused the growth stress to decrease to 0.06 GPa. Meanwhile the quality of GaN improved, and the freestanding GaN was directly obtained during the growth process.

(Invited) Understanding Interfaces in Homoepitaxial GaN Growth

The availability of high quality, free-standing GaN substrates opens windows for new device applications in III-nitrides, especially in vertical structures. With the introduction of these native substrates, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow homoepitaxial devices to achieve their full potential. In order to enable device-quality epitaxial layers, 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 metal organic chemical vapor deposition (MOCVD) growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation at the interface and in the films. Although the initial substrate factors influencing the epi can be subtle, they can have far reaching impact on device performance. Additionally, the interface between substrate and epitaxy is examined to enable reduction of unintentional impurity incorporation, especially Si, at this surface. By studying these effects using wafers from several different vendors, with substrates from both hydride vapor phase epitaxy (HVPE) and ammonothermal techniques, an understanding of the requirements for device quality MOCVD homoepitaxy can be determined.

(Invited) Homoepitaxial GaN Growth on Free-Standing Substrates

The availability of high quality, free-standing GaN substrates opens windows for new device applications in III-nitrides, especially in vertical structures. With the introduction of these native substrates, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow homoepitaxial devices to achieve their full potential. In order to enable device-quality epitaxial layers, 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 metal organic chemical vapor deposition (MOCVD) growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation at the interface and in the films. Although the initial substrate factors influencing the epi can be subtle, they can have far reaching impact on device performance. By studying these effects using wafers from several different vendors, with substrates from both hydride vapor phase epitaxy (HVPE) and ammonothermal techniques, an understanding of the requirements for device quality MOCVD homoepitaxy can be determined.

Study of Charge Carrier Transport in GaN Sensors.

Capacitor and Schottky diode sensors were fabricated on GaN material grown by hydride vapor phase epitaxy and metal-organic chemical vapor deposition techniques using plasma etching and metal deposition. The operational characteristics of these devices have been investigated by profiling current transients and by comparing the experimental regimes of the perpendicular and parallel injection of excess carrier domains. Profiling of the carrier injection location allows for the separation of the bipolar and the monopolar charge drift components. Carrier mobility values attributed to the hydride vapor phase epitaxy (HVPE) GaN material have been estimated as μe = 1000 ± 200 cm2/Vs for electrons, and μh = 400 ± 80 cm2/Vs for holes, respectively. Current transients under injection of the localized and bulk packets of excess carriers have been examined in order to determine the surface charge formation and polarization effects.

Epitaxial growth of three dimensionally structured III-V photonic crystal via hydride vapor phase epitaxy

Three-dimensional (3D) photonic crystals are one class of materials where epitaxy, and the resultant attractive electronic properties, would enable new functionalities for optoelectronic devices. Here we utilize self-assembled colloidal templates to fabricate epitaxially grown single crystal 3D mesostructured GaxIn1−xP (GaInP) semiconductor photonic crystals using hydride vapor phase epitaxy (HVPE). The epitaxial relationship between the 3D GaInP and the substrate is preserved during the growth through the complex geometry of the template as confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy. XRD reciprocal space mapping of the 3D epitaxial layer further demonstrates the film to be nearly fully relaxed with a negligible strain gradient. Fourier transform infrared spectroscopy reflection measurement indicates the optical properties of the photonic crystal which agree with finite difference time domain simulations. This work extends the scope of the very few known methods for the fabrication of epitaxial III-V 3D mesostructured materials to the well-developed HVPE technique.

Preparation of Ag Schottky contacts on n-type GaN bulk crystals grown in nitrogen rich atmosphere by the hydride vapor phase epitaxy technique

Electrical properties of Schottky contacts on n-type GaN grown in nitrogen rich atmosphere with different N/Ga ratios by hydride vapor phase epitaxy were investigated. We show that tunneling of electrons from the conduction band of GaN to the metal is dominant in our samples. The quality of Schottky contacts does not only depend on surface preparation but also on the growth conditions of the crystals. Schottky contacts on these crystals show an increasing deterioration when higher N/Ga growth ratios are used. We correlate our results with the presence of negatively charged gallium vacancies in the samples. These charges compensate the positively charged donors and lead to a significant increase in series resistance.

GaN-based substrates and optoelectronic materials and devices

In order to solve the problems of GaN heteroepitaxy on sapphire substrate, some techniques were explored. Freestanding GaN substrates have been made by hydride vapor phase epitaxy (HVPE), laser lift-off (LLO), and chemical mechanical polishing techniques. Wafer bending and cracking in the HVPE growth were partly settled by pulsed flow modulation method. High-crystal quality was established for 1.2 mm thick GaN substrate by X-ray diffraction measurement, in which the full width of half maximum values were 72, 110 arcsec for (102), (002) peaks. A novel micro-size patterned sapphire substrate (PSS) and a nano PSS were also fabricated. High-power vertical structure light emitting diodes (VSLEDs) have been developed by Au–Sn eutectic wafer bonding, homemade micro-area LLO, and light extraction structure preparation. The high-injection-level active region with low temperature GaN sandwiched layers was used for low-efficiency droop. The light output power of VSLED was achieved as 400 mW driven at 350 mA, and the dominant wavelength is about 460 nm. The structures and properties of strain modulated superlattices (SLs) and quantum wells as well as advanced simulation of carriers transport across the electron blocking layer were investigated in laser diodes. The hole concentration was achieved as high as 1.6 × 1018 cm−3 in AlGaN/GaN SLs:Mg by inserting an AlN layer. High-quality AlGaN epilayers and structures were grown by MOCVD. Some device structures of UV LEDs and detectors were demonstrated. The emission wavelength of 262 nm UV LED has been successfully fabricated. At last, high-quality InN and InGaN materials for solar cell were grown by boundary-temperature-controlled epitaxy and growth-temperature-controlled epitaxy. Hall-effect measurement showed a recorded electron mobility of 3,280 cm2/(V s) and a residual electron concentration of 1.47 × 1017 cm−3 at 300 K.

Ultralong and Defect-Free GaN Nanowires Grown by the HVPE Process

GaN nanowires with exceptional lengths are synthesized by vapor-liquid-solid coupled with near-equilibrium hydride vapor phase epitaxy technique on c-plane sapphire substrates. Because of the high decomposition frequency of GaCl precursors and a direct supply of Ga through the catalyst particle, the growth of GaN nanowires with constant diameters takes place at an exceptional growth rate of 130 μm/h. The chemical composition of the catalyst droplet is analyzed by energy dispersive X-ray spectroscopy. High-resolution transmission electron microscopy and selective area diffraction show that the GaN nanowires crystallize in the hexagonal wurzite structure and are defect-free. GaN nanowires exhibit bare top facets without any droplet. Microphotoluminescence displays a narrow and intense emission line (1 meV line width) associated to the neutral-donor bound exciton revealing excellent optical properties of GaN nanowires.

Development of the Hybrid Conjugated Polymer Solar Cell Based on GaN Quantum Dots

We report the hybrid p–n junction based on GaN quantum dots (QDs) as an electron transport layer and poly(3-hexylthiophene-2,5-diyl) (P3HT) as a hole transport layer, which has not been tried for the solar cell until now. The growth of GaN QDs was achieved by the hydride vapor phase epitaxy (HVPE) technique and P3HT film sequentially was coated on the top of QDs. The overall performance of P3HT/GaN QDs hybrid heterojunction was analyzed by current density–voltage (J–V) characteristics and finally exhibited an open-circuit voltage, short-circuit current density, and fill factor of 160 mV, 3.6 mA/cm2, and 0.25, respectively. Also, its efficiency was shown up to 0.14% in an active area of 0.04 cm2 under AM1.5G illumination with an intensity of 100 mW cm-2. In this paper, we discuss the factors which affect the power conversion efficiency for future works.

Metalorganic vapor phase growth of quantum well structures on thick metamorphic buffer layers grown by hydride vapor phase epitaxy

The hydride vapor phase epitaxy technique (HVPE) can deposit uniform materials at high growth rates, making it suitable for the formation of metamorphic buffer layers (MBLs). HVPE was used to form InxGa1−x As-based MBLs as substrates in the metalorganic vapor phase epitaxy (MOVPE) of superlattice (SL) structures. Samples were grown over a range of deposition temperatures and reactant flows and their effect on the ternary alloy composition and growth rate was determined. Over the compositional range of xInAs<0.4, the alloy composition varied almost linearly with indium chloride pressure (PInCl). A compositionally step-graded MBL consisting of nine intermediate steps followed by a thick, constant composition layer of xInAs∼0.23 was grown. The MBL was characterized by high-resolution x-ray diffraction (HRXRD) and reciprocal space mapping. The topmost layer was highly relaxed, exhibiting a residual strain of −0.0011±0.0002. Chemical-mechanical polishing (CMP) was used to reduce the surface cross hatching, affecting a decrease in rms roughness from 2.10 to 1.39nm. A 20 period, In0.33Ga0.67As (3.1nm)/Al0.90In0.10As (7.2nm) strain-balanced superlattice (SL) structure was grown by MOVPE on the as-grown and CMP-prepared MBL and characterized by HRXRD. The diffraction pattern of the SL grown on the CMP-prepared MBL was significantly more intense with narrower x-ray diffraction peaks when compared with the SL on as-grown MBL.

Epitaxial gallium nitride thin films grown on silicon substrates utilizing gallium nitride seed-layer formed by liquid source precursor

Gallium nitride (GaN) epitaxial thin films were deposited on Si substrates by a modified hydride vapor phase epitaxy (MHVPE) technique utilizing the GaN seed-layer formed from liquid source precursor. Tris N,N-dimethyldithiocarbamato gallium(III) (Ga(mDTC)3) powder was dissolved in chloroform (CHCl3) to prepare the liquid source precursor for seed-layer formation. The developed method was found to be suitable for the epitaxial growth of GaN on Si in spite of the large mismatch in lattice constants and thermal expansion coefficients, resulting in device-quality epitaxial films with fairly smooth surface morphology. The epitaxial GaN films obtained in this study had a hexagonal structure with (0002) preferred orientation with the FWHM value of 428.6 arcsec of the (0002) GaN XRD peak. Photoluminescence spectra of GaN films exhibited a strong and sharp peak at 3.41 eV with the FWHM value of 107meV.

HVPE법으로 성장시킨 GaN substrate 제작과 특성 평가

Bulk GaN single crystal with 1.5 mm thickness was successfully grown by hydride vapor phase epitaxy (HVPE) technique. Free-standing GaN substrates of mm size were fabricate after lift-off of sapphire substrate and their optical properties were characterized properties for device applications. X-ray diffraction patterns showed (002) and (004) peak, and the FWHM of the X-ray rocking curve (XRC) measurement in (002) was 98 arcsec. A sharp photoluminescence spectrum at 363 nm was observed and defect spectrum at visible range was not detected. The hexagonal-shaped etch-pits are formed on the GaN surface in at 5 minutes. The defect density calculated from observed etch-pits on surface was around . This indicates that the fabricated GaN substrates can be used for applications in the field of optodevice, and high power electronics.

GaN epitaxial layers grown by the solution-cast seed layer HVPE technique: Effect of reactor heating method on structural and optical quality

Thin high crystallinity GaN epitaxial layers were grown on (0001) Al 2O 3 substrates by solution-cast seed layer formation process using a precursor material, tris( N, N-dimethyl-dithiocarbamato) gallium(III) [(Ga(mDTC) 3)]. The solution-cast seed layer formation process was found compatible with the modified hydride vapor phase epitaxy (HVPE) technique, when the reactor heating was properly managed. XRD and PL studies evidenced that the GaN growth with continuous ramp-up of reactor temperature produces highly crystalline GaN thin films with negligible defects at relatively lower growth temperature compared to other existing growth techniques and further simplifies the growth process.

The influence of free-carrier concentration on the PEC etching of GaN: A calibration with Raman spectroscopy

Large areas of electrically active regions of in-homogeneities have been revealed by the electroless photo-etching (PEC) method in GaN layers grown by the hydride vapor phase epitaxy (HVPE) technique. Variations in the local etch rate have been correlated with the variations in the free-carrier concentrations as determined by micro-Raman spectroscopy. The etch rate decreased linearly with the log of the carrier concentration. The latter could change by more than two orders of magnitude on the same sample.

Roughness Analysis of GaN Surfaces at Different Annealing Temperatures for an AlN Buffer Layer

The surface morphologies of AlN and GaN on Si(111) and Si(100) were studied using atomic force microscopy. The AlN buffer layer was deposited using RF sputtering for 30 minutes on Si(111) and Si(100) substrates, and then the specimens were annealed at 850 degrees C, 950 degrees C and 1050 degrees C for 60 minutes in an NH3 atmosphere. It was found that the grains of AlN became larger by increasing the annealing temperature. GaN was grown on AlN/Si(111) and AlN/Si(100) substrates by using a hydride vapor phase epitaxy technique. The surface of GaN grown on the AlN/Si substrate showed thick- thread and fine-thread patterns. The microstructure of GaN grown on the buffer-GaN/Al2O3 had a step-flow pattern. These phenomena could be explained by a lattice mismatch and the growth temperature. A power spectral density analysis was performed on the GaN surfaces and the results were compared with the experimental data.

Surface preparation of substrates from bulk GaN crystals

Large gallium nitride (GaN) crystals were grown using a hydride vapor phase epitaxy (HVPE) technique and were processed into substrates for device applications. Polishing procedures were developed for GaN substrates to produce surfaces prepared for epitaxial growth. Surface preparation of (0 0 0 1) and (0 0 0 1¯) substrates was examined, along with preparation of (1 1 2¯ 0) and (1 1¯ 0 0) non-polar surfaces. For all surfaces, chemical mechanical polishing (CMP) resulted in an average root mean square (RMS) surface roughness on a 5 μm×5 μm scanning probe microscope (SPM) image of <0.2 nm. Characterization of the surfaces of polished substrates by cross-sectional transmission electron microscope (TEM) showed no sub-surface damage and no epitaxial defects generated at the substrate/epi interface during homoepitaxial growth. Cathodoluminescence (CL) imaging was used to verify the defect density and that no defects related to polishing were present. The average dislocation density of the substrates was <5×10 6 cm −2.

Extended microtunnels in GaN prepared by wet chemical etch

It is demonstrated in GaN that microtunnels extended beyond hundreds of microns can be easily achieved using wet chemical etch. To obtain this result, specially designed structures of GaN layers are first grown on sapphire substrates with metal-organic chemical vapor deposition and subsequently with hydride vapor phase epitaxy techniques. The prepared samples are then chemically etched in molten KOH. With the designed structure of GaN layers, extended microtunnels with triangular cross sections are formed. The crystallographic planes of the triangular bevels belong to the {112¯2} family. The etch rate of the tunnel can be as high as 10μm∕min at proper etching conditions.