Hydride Vapor Phase Epitaxy GaN Research Articles

Evaluation of defect density in bulk gallium nitrides by photothermal deflection spectroscopy and steady-state photocapacitance methods

Bulk GaN samples were characterized by both photothermal deflection spectroscopy (PDS) and steady-state photocapacitance (SSPC) methods. The PDS signal intensity in the bandgap was found to correlate quantitatively with the defect density estimated by the SSPC method. The defect density of GaN bulks fabricated by hydride vapor phase epitaxy (HVPE) was decreased by controlling the incorporation of carbon and silicon impurities. Differences in the reciprocal of the slope near the valence band maximum and the signal intensity in the bandgap among HVPE GaN bulks could be detected by PDS, although they had the same crystalline quality. PDS can be used to evaluate the GaN bulks that have been improved with a highly insulative property caused by Fe- doping or low carbon incorporation.

Elastic constants of GaN grown by the oxide vapor phase epitaxy method

Oxide vapor phase epitaxy (OVPE) has attracted much attention as a highly efficient method for synthesizing high-quality bulk GaN crystals, but the mechanical properties of OVPE GaN have not been clarified. We measured the five independent elastic constants of the OVPE GaN by resonant ultrasound spectroscopy. The in-plane Young modulus E 1 and shear modulus C 66 of the OVPE GaN are smaller than those of the hydride vapor phase epitaxy GaN by 1.8% and 1.3%, respectively. These reductions agree with predictions by density functional theory calculations. We also calculated the Debye temperature, revealing that oxygen impurity decreases its magnitude.

(Invited) Application of Synchrotron X-Ray Topography Techniques to the Analysis of Defect Microstructures in Bulk GaN Substrates and Epilayers for Power Devices

The study of defects and strain in GaN substrates and epilayers for vertical device development using synchrotron X-ray topography techniques is presented. Synchrotron monochromatic beam X-ray topography (SMBXT) studies show that ammonothermal-grown GaN substrate wafers have the lowest dislocation densities while patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities and HVPE GaN substrates contain high dislocation densities. Epitaxial growth does not nucleate new defects but interactions of dislocations with point defects can cause changes in dislocation configurations. Rocking curve Analysis by Dynamical Simulation (RADS) is shown to be effective in analysis of implantation by correlating the depth profile of strain with the doping profile. Removal of lattice damage induced by implantation requires annealing with proper capping or under high pressures to limit loss of material and lattice distortion. Inductively coupled plasma (ICP) etching may introduce strain in GaN material that can be partially recovered by TBCl etching. Diffusion doping does not introduce new strains in GaN material while neutron transmutation doping at low levels can lead to curing of defects while higher levels lead to irradiation damage and nucleation of new defects.

Origins of Electrical Compensation in Si‐Doped HVPE GaN

Herein, the results of positron lifetime and X‐ray absorption spectroscopy obtained in Si‐doped GaN crystals, grown by the hydride vapor phase epitaxy (HVPE), are reported. Pushing the Si doping to high concentrations leads to surprisingly strong compensation effects. Positron experiments show that the concentrations of Ga vacancies are not high enough to be efficient compensation centers. Other acceptor‐like impurities are present in concentrations orders of magnitude lower than the Si content in the samples. X‐ray absorption shows that the local environment of Si dopants in compensated samples is different from the fully activated case. Simulated spectra of X‐ray absorption near edge structure strongly suggest that in compensated spectra Si is likely to have more Si atoms in the nearest local environment. Hence, autocompensation of Si dopants appears as a likely compensation mechanism at high Si contents GaN samples grown by HVPE.

Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN: Understanding Nucleation and Design of Growth Strategies

Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high-quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low-temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 × 1015 cm–3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.

Size of dislocation patterns induced by Vickers indentation in hydride vapor-phase epitaxy GaN

3D dislocation structures induced by Vickers indentation depending on the imprint size are precisely investigated using an alternation of cathodoluminescence and chemical mechanical polishing (CMP), multiphoton excitation photoluminescence, and (scanning) transmission electron microscopy under a load range within a constant Vickers hardness. The dislocation structures are composed of a rosette pattern, a flower pattern, and a triangular area. The flower pattern distribution is dispersive along the z direction. The determined penetration depth of the dislocations by practical CMP is almost the same as the length of the imprint diagonal (d1). The dimensions of the individual patterns in the dislocation structure can be described as multiples of d1. That is, the geometric similarity between the imprint size and the dimensions of the dislocation structure is confirmed. This suggests that the dimensions of the dislocation structures induced under scratch can be estimated by measuring the width of the scratch. This shows that a simple method may be developed to determine the maximum thickness of the affected layer over the entire wafer area and the ability to develop low-cost GaN wafers free of affected layers.

Mechanism and enhancement of anti-parasitic-reaction catalytic activity of tungsten-carbide-coated graphite components for the growth of bulk GaN crystals

The working mechanism of the anti-parasitic-reaction (APR) catalyst of tungsten carbide (WC) coating on graphite in hydride vapor phase epitaxy GaN growth were examined. During NH3 annealing, the surface of WC is reduced as well as nitrided. The W2N topmost layer was found to work as an APR-active catalyst to suppress the formation of GaN polycrystals during high-rate HVPE-GaN growth, while the regions covered with thick pyrolytic graphite residues were catalytically inert. The formation of an additional W2C top layer on the WC underlayer was demonstrated to exhibit superior APR activity, i.e. complete suppression of GaN polycrystal formation.

Propagation of threading dislocations and effects of Burgers vectors in HVPE-grown GaN bulk crystals on Na-flux-grown GaN substrates

The propagation behavior of threading dislocations (TDs) and the effects of Burgers vectors in hydride vapor phase epitaxy (HVPE) GaN bulk crystals generated on Na-flux-grown GaN and in a commercially available HVPE-grown GaN bulk crystal were investigated. Analyses based on chemical etching and transmission electron microscopy (TEM) revealed a close correlation between the etch pit sizes and the Burgers vector of these TDs. The existence of TDs with the unique Burgers vector b = 1m + 1c was observed for the first time ever using a large-angle convergent-beam electron diffraction technique and plan-view bright-field scanning TEM. Multi-photon excitation photoluminescence microscopy observations showed that TDs with b = 1c had a meandering morphology in contrast to the linear morphology of TDs with b = 1a, 1a + 1c, or 1m + 1c in both types of HVPE-grown GaN crystals. The inclinations of TDs with b = 1a and 1a + 1c in HVPE-grown GaN on Na-flux-grown GaN were greatly affected by large symmetrical hexagonal hillocks. The TDs with b = 1a were inclined in the slope directions of the hillock planes, while those with b = 1a + 1c were inclined in the a directions parallel or antiparallel to the a component in their Burgers vector. These inclinations were readily explained by the isotropic elastic theory of an individual TD. The hillocks were produced around pairs of parallel mixed TDs for which the a components were opposite to one another. This phenomenon was attributed to inclusions at the interface between the Na-flux-grown GaN substrate and the HVPE-grown layer. The origins of other TDs including unusual ones having b = 1m + 1c are also discussed herein.

Role of threading dislocations and point defects in the performance of GaN-based metal-semiconductor-metal ultraviolet photodetectors

The role of threading dislocations and point defects is investigated by comparing the performance of metal-semiconductor-metal ultraviolet photodetectors (PDs) fabricated on GaN epilayers grown by hydride vapour phase epitaxy (HVPE) and metal organic vapour phase epitaxy (MOVPE) techniques. It is found that the density of threading dislocations is higher in HVPE GaN epilayers, however, the devices fabricated on them show a higher photo response, lower leakage current and faster transient response when compared to those fabricated on MOVPE GaN epilayers. It is noticed that a high density of threading dislocations of HVPE grown GaN epilayers doesn't always restrict their usefulness in the development of specific devices. On the other hand, an inferior performance of PDs fabricated on MOVPE GaN epilayers is observed despite their low dislocation density, which is explained by considering the presence of point defects. Further, the room temperature electronic transport is found to be dominated by thermionic emission (thermionic field emission) mechanism in devices fabricated on GaN epilayers grown by HVPE (MOVPE) technique. In case of HVPE based devices, a switching of dominant transport mechanism is seen at ~200 K during cooling down whereas no such behaviour is observed for MOVPE based devices. Key factors affecting the performance of ultraviolet PDs fabricated on GaN epilayers grown by the two techniques and associated charge transport mechanisms are discussed. • Crystalline quality of MOVPE GaN templates is found to be better than HVPE GaN. • HVPE GaN based MSM devices turn out to be better than those fabricated on MOVPE GaN. • A high dislocation density of HVPE GaN doesn't always restrict their usefulness. • Fundamental physical reasons for a better performance of MSM devices are discussed. • Transport mechanism switches at ~200 K for HVPE based GaN MSM devices.

Cathodoluminescence and electrical study of vertical GaN-on-GaN Schottky diodes with dislocation clusters

Electrical properties of vertical Schottky diodes fabricated on GaN regrown on hydride vapor phase epitaxy GaN substrates are investigated. The deposition of Ni frames makes it possible to designate the dislocation locations by cathodoluminescence before the fabrication of diodes. Since the distribution of dislocations is inhomogeneous and arranged into clusters over the GaN substrates, diodes made on either areas free of dislocation-clusters or areas with dislocation-clusters have been studied. Both forward and reverse characteristics of diodes fabricated on GaN with different doping levels and dislocation densities have been investigated. It is shown that the reverse leakage is not sensitive to the presence of dislocation clusters. On the other hand, there is a dispersion of reverse leakage which increases in diodes grown on substrates having larger mean dislocation density. The results also demonstrate that a critical maximum distance of around 100 µm between active defects triggers larger reverse leakage currents.

Screw dislocations on pyramidal planes induced by Vickers indentation in HVPE GaN

The dislocation structure around the Vickers indentation on a hydride vapor-phase epitaxy GaN wafer was observed by multi-photon photoluminescence imaging and cross-sectional scanning transmission electron microscopy. Straight dislocation lines parallel to the six equivalent directions, i.e. a rosette pattern, were observed in the subsurface. Below the rosette pattern, a flower-like pattern similar to dislocation loops expanded in the six equivalent directions. Under the flower-like pattern, an inverted triangular dark area composed of dislocations on pyramidal planes was observed. This structure did not vary with load (20–980 mN) or crack formation. The inverted triangular dark area contained screw dislocations with a slip system and an estimated density of 2 × 109 cm−2. The penetration depth of the screw dislocation on the pyramidal plane was found to be proportional to the length of the diagonal indentation line.

Surface potential imaging and characterizations of a GaN p-n junction with Kelvin probe force microscopy

We applied Kelvin probe force microscopy (KPFM) to characterize the p-n junction grown on hydride vapor-phase epitaxy GaN wafers with three different doses of the p-type dopant Mg. The distributions of the contact potential difference (CPD) were visualized to observe the abrupt changes in the CPD across the p-n junction. Based on this result, we attempted to evaluate the electrostatic potential distributions across the GaN p-n junction, which consequently provide the dopant concentrations in the p-type region (NA) and unintentionally doped regions (NUID). The obtained values of NA in this study were two orders of magnitude smaller than doped Mg concentrations, while those of NUID were consistent with the results of secondary ion mass spectroscopy. We demonstrate the potential of KPFM in the evaluation of GaN p-n junctions.

Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds

Abstract GaN crystals were grown on non-polar and semi-polar HVPE GaN seeds by basic ammonothermal method. Stress distributions were investigated in cross-section of (1 1 −2 0) plane, (1 0 −1 0) plane, (2 0 −2 1) plane and (1 0 −1 1) plane GaN crystal. The cathodoluminescence (CL) images show cross-section information clearly and each examined object consisted of hydride vapor phase epitaxy (HVPE) seed and ammonothermal GaN (Am-GaN). The impurity concentration and free carrier concentration were estimated by secondary-ion mass spectroscopy (SIMS) and Hall. Moreover, Raman spectroscopy was used for studying stress distribution in the cross section. Shifts of E2(high) phonon lines were analyzed to determine stress. Our results indicate that the stress is about 35 MPa in bulk GaN and the stress is less than 60 MPa in the interface of Am-GaN.

Fabrication of Pyramid Structure Substrate Utilized for Epitaxial Growth Free-Standing GaN

Metal–organic chemical vapor deposition (MOCVD)-grown GaN on sapphire substrate was etched by hot phosphoric acids. Pyramid structures were obtained in the N-polar face of the MOCVD–GaN. Details of the formation process and morphology of the structures were discussed. The crystallographic plane index of the pyramid facet was calculated dependent on the symmetry of the wurtzite crystal structure and the tilt angle. The substrates with pyramid structures were utilized in subsequent hydride vapor phase epitaxy (HVPE) growth of GaN. Free-standing crystals were obtained, while HVPE-grown GaN achieved a certain thickness. Raman spectroscopy was employed to obtain the stress conditions of the HVPE–GaN without and with sapphire substrate. The mechanism of the self-separation process was discussed. This facile wet etching method may provide a simple way to acquire free-standing GaN by HVPE growth.

Room-temperature infrared photoluminescence in GaN doped with various impurities

The steady-state infrared-photoluminescence spectra (IR-PL) emitted from about 400 μm thick, free-standing GaN wafers, grown by the ammono-thermal and hydride vapour-phase epitaxy GaN, and containing carbon, magnesium, manganese and iron doping have been examined. The room-temperature IR-PL spectra are correlated with pulsed-photo-ionization spectra using van Roosbroeck-Schockley approach for spectrum conversion. It has been revealed that iron and carbon dopants appear as the most efficient impurities for the room temperature of infra-red emission from GaN grown using different technologies.

Effect of Hydrogen-Ion-Implantation on Self-Separation of Free-Standing Gallium Nitride Wafers Grown By Hvpe

In recent years, free-standing Gallium Nitride (FS-GaN) wafers were utilized as substrates for high-brightness light-emitting-diodes (HB-LED), LASER diodes (LD) and power devices due to their characteristics, such as 3 times higher thermal conductivity than sapphire substrates and higher breakdown voltage than conventional Si substrates. The FS-GaN wafers grown by hydride vapor phase epitaxy (HVPE) demonstrated lower dislocation density (DD) than that of GaN wafers grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates (template GaN wafer). However, one of big challenges for their commercializing is the crack free FS-GaN wafers during the HVPE GaN growth, which resulted in lowering the productivity and increasing the fabrication cost. The main cause of the cracking is the stress induced by the differences in thermal expansion coefficients (TEC) and lattice constants of sapphire and GaN. To overcome this issue, the design of GaN growth process with self-separation is the key technology. The self-separation technique consists of two key points, one is separation enhancement and the other is blocking edge-poly-GaN growth. In this study, we investigated the effect of hydrogen-ion-implantation on self-separation of FS-GaN wafers. In addition, the blocking effect of edge grinding of template GaN wafers was also investigated. Two-inch-diameter template GaN wafers with the GaN layer thickness of 5 μm were prepared and the hydrogen-ions were implanted with various dose conditions to enhance self-separation of FS-GaN wafers from the sapphires. The edges of the implanted template GaN wafers were then ground to eliminate the polished edge surfaces to deter the poly-GaN growth. 800-μm-thick GaN layers were grown by HVPE on the template GaN wafers, and the FS-GaN wafers were successfully self-separated during the cool-down process. Fig. 1 shows the edge images of the GaN on sapphire substrates after 300-μm-thick GaN layer was grown on the template GaN substrates. When the edge-grinding was utilized to template GaN substrate, the poly-GaN was grown less than that of the substrate without edge-grinding. Also, the poly-GaN was easily removed at the edge of the substrate by edge-grinding without additional process. Fig. 2 shows the hydrogen-ion-band inside the MOCVD-GaN layer with three different hydrogen-ion dose conditions. The sample with hydrogen-ion dose of 2.25 E17 H+/cm2 showed larger size of the hydrogen cavity than the samples with hydrogen-ion dose of 1.75 E17 and 2.0 E17 H+/cm2. Fig. 3 shows the wafer images after 800-μm-thick GaN grown on the edge-ground template GaN with three different hydrogen-ion-implantation conditions. The FS-GaN wafers with hydrogen-ion dose of 1.75 E17 and 2.0 E17 H+/cm2 were successfully self-separated from the sapphire substrates, while the sample with hydrogen-ion-dose of 2.25 E17 H+/cm2 was self-separated around 63% of the area. In our presentation, we will report how we achieved the self-separation of FS-GaN wafers without cracking by hydrogen-ion-implantation into the template GaN substrates, and discuss about the crystal quality of the grown FS-GaN samples. * This work was financially supported by the Brain Korea 21 Plus Program in 2018. Figure 1

GaN drift-layer thickness effects in vertical Schottky barrier diodes on free-standing HVPE GaN substrates

Vertical Schottky barrier diodes (SBD) with different drift-layer thicknesses (DLT) of GaN up to 30 μm grown by metalorganic chemical vapour deposition (MOCVD) were fabricated on free-standing GaN grown by hydride vapour phase epitaxy (HVPE). At room temperature, SBD’s exhibited average barrier heights (ΦB) in the range of 0.73 eV to 0.81 eV. The effective barrier heights (ΦBeff) of SBDs with different DLT also exhibited a similar range of ΦB measured at room temperature. The measured reverse breakdown voltages (VBD) of SBDs increased from 562 V to 2400 V with an increase in DLT. The observation of high VBD of SBDs could be due to the lower effective donor concentration (7.6×1014 /cm3), which was measured from SIMS analysis. The measured VBD of 2400 V is the highest value ever reported for a 30 μm DLT vertical GaN SBD without additional edge termination or field plate (FP).

Electrical characterization of HVPE GaN containing different concentrations of carbon dopants

A comprehensive study of the electrical characteristics in Schottky diodes made of GaN:C grown by hydride vapour phase epitaxy (HVPE) technology has been reported. The Schottky junctions were made of an Ni/Au (25/200 nm) metal stack and ohmic contacts were fabricated by Ti/Al/Ni/Au (30/90/20/100 nm) e-beam deposition of the metal thin-films followed by rapid thermal annealing. A good quality of the fabricated Schottky diodes has been proved by considering the transient shape using a pulsed technique of barrier evaluation under linearly increasing voltage (BELIV). The concentrations of equilibrium carriers of 1 × 1011 cm−3 and of 2 × 1010 cm−3 have been evaluated for relatively low and high carbon density doped samples, respectively, using photo-capacitance characteristics dependent on excitation intensity. The effective mobility of carriers of μeff = 610 cm2/Vs has been estimated by considering the serial resistance variations dependent on excitation density, through analysis of delay times that appeared in the formation of peaks within BELIV transients. Optical deep level transient spectroscopy has shown that thermal emission from the rather shallow centres prevails. The centres, ascribed to vacancies as well as to carbon on Ga sites, have been identified. It has been demonstrated that Schottky junctions made of heavy carbon doped (NC ≥ 1018 cm−3) HVPE GaN material are able to withstand voltages of ≥300 V.

Evaluation of Hvpe GaN Layers Grown on Ammonothermal GaN Substrates By Synchrotron X-Ray Topography

For obtaining low defect density GaN substrates with controllable electronic properties for high performance electronic and optoelectronic devices, quasi-bulk growth by hydride vapour phase epitaxy (HVPE) on ammonothermal grown GaN substrates appears to be a feasible approach [1]. Synchrotron X-ray topography (white beam and monochromatic beam) has been employed to characterize dislocation configurations in such GaN substrates that were grown by HVPE on ammonothermal GaN substrate, and then slicing the substrate. Threading dislocations and basal plane dislocations (BPDs) are observed in these GaN substrates. Compared with GaN grown on sapphire and silicon carbide substrates [2], HVPE GaN on ammonothermal GaN [3] is obviously of higher crystalline quality with lower defect densities. Threading screw dislocation (TSD) density was measured to be about 880 cm-2 while threading edge dislocation (TED) density was about an order higher (~ 5824 cm-2). The distribution of BPDs which are induced by deformation, was found to be highly non-uniform with most regions of the wafer nearly BPD-free as well as high densities concentrated near one edge. Also, besides the surface features, like the surface scratches, arrays of threading dislocations can be seen from the transmission and grazing incidence topographs, which likely originate from scratches on the surface of the ammonothermal substrate. Correlation of the defect distributions with the HVPE growth process and their implication for devices will be discussed.

The investigation of stress in freestanding GaN crystals grown from Si substrates by HVPE

We investigate the stress evolution of 400 µm-thick freestanding GaN crystals grown from Si substrates by hydride vapour phase epitaxy (HVPE) and the in situ removal of Si substrates. The stress generated in growing GaN can be tuned by varying the thickness of the MOCVD AlGaN/AlN buffer layers. Micro Raman analysis shows the presence of slight tensile stress in the freestanding GaN crystals and no stress accumulation in HVPE GaN layers during the growth. Additionally, it is demonstrated that the residual tensile stress in HVPE GaN is caused only by elastic stress arising from the crystal quality difference between Ga- and N-face GaN. TEM analysis revealed that the dislocations in freestanding GaN crystals have high inclination angles that are attributed to the stress relaxation of the crystals. We believe that the understanding and characterization on the structural properties of the freestanding GaN crystals will help us to use these crystals for high-performance opto-electronic devices.