Ammonothermal Gallium Nitride Research Articles

(Invited) Effect of Seed Crystal Orientation on Ammonothermal Growth of GaN

Gallium nitride (GaN) is a wide bandgap semiconductor which has received widespread attention as it has enabled the development of short wavelength (UV, blue) optical emitters and promises a new generation of high efficiency power electronic devices. To enable widespread use of high power, high efficiency devices, the availability of large-area, inexpensive, low-defect-density GaN substrates is necessary.The ammonothermal method is a bulk, single crystal growth method which has demonstrated growth of high structural quality GaN boules greater than 2 inches in diameter. Despite these demonstrations, challenges remain with regards to improving growth rates to reduce cost of boules and understanding the growth behavior of GaN in these environments to further improve upon crystal quality. In particular, the general trend and affect of the GaN seed crystal on growth rates and crystal quality has not been studied in detail.This talk will discuss our current understanding of how the initial seed crystal orientation affects the resulting ammonothermal GaN growth, surface morphology, crystal quality, and impurity incorporation. Seed crystals were sourced from hydride vapor phase epitaxial (HVPE) grown GaN boules that were sliced and polished to atomically smooth crystallographic facets of varying orientations. Three series were created resulting in incrementally changing orientations ranging from +c-plane to m-plane to -c-plane, from +c-plane to a-plane to -c-plane and from m-plane to a-plane.

Ultra-high pressure annealing of Mn-implanted HVPE-GaN

Manganese diffusion in ion implanted gallium nitride crystals was investigated. The ions were implanted into a sample composed of a GaN layer grown using halide vapor phase epitaxy on an ammonothermal gallium nitride substrate. The sample was polished into an atomically flat state prior to the implantation. Manganese ions were implanted into the surface of a (0001) plane (also known as the c-plane) using the beam energy of 230 keV and ion fluence of 5E15 cm−2. The beam was tilted relative to the sample’s normal to prevent channeling. After the implantation, ultra-high pressure annealing method was used to both, remove the post-implantation damage and induce the diffusion of implanted Mn ions. Manganese profiles were analyzed along the c-direction using secondary ion mass spectrometry after 5 and 30 h of annealing in temperatures ranging from 1473 K to 1753 K. Two diffusion mechanisms were observed. The diffusion parameters, activation energy and temperature-independent diffusion coefficients, are presented for each mechanism. Additionally, the structural quality of the samples is assessed after 5 and 30-hour annealing in the aforementioned temperature range through the means of X-ray diffractometry.

Progress in Ammonothermal Crystal Growth of Gallium Nitride from 2017–2023: Process, Defects and Devices

Gallium nitride continues to be a material of intense interest for the ongoing advancement of electronic and optoelectronic devices. While the bulk of today’s markets for low-performance devices is still met with silicon and blue/UV LEDs derived from metal–organic chemical vapor deposition gallium nitride grown on foreign substrates such as sapphire and silicon carbide, the best performance values consistently come from devices built on bulk-grown gallium nitride from native seeds. The most prominent and promising of the bulk growth methods is the ammonothermal method of high-pressure solution growth. The state-of-the-art from the last five years in ammonothermal gallium nitride technology is herein reviewed within the general categories of growth technology, characterization and defects as well as device performance.

Europium diffusion in ammonothermal gallium nitride

Europium doping of gallium nitride using a novel ultra-high pressure annealing method was investigated. Ammonothermal gallium nitride substrates (n-type) were used as europium ion implantation targets using beam energy of 490 keV and ion fluences ranging from 1 × 1015 to 1 × 1016 Eu/cm2. The implanted samples were annealed at temperatures between 1473 K and 1753 K in high nitrogen pressure. Europium concentration profiles were analyzed along the [0001] crystallographic direction. The morphology and structural quality of implanted and annealed samples were examined. The “out-diffusion” phenomenon was noted. The presence of optically active EuGa-X defect complexes was suggested based on the analysis of photoluminescence spectra measured at low temperature (20 K). Limitations of diffusion-based europium doping of ammonothermal gallium nitride are discussed.

(Invited) Progress in Elucidating the Growth Process for Basic Ammonothermal GaN

Gallium nitride (GaN) is an attractive materials platform for the pursuit of high-power, high-efficiency (opto-)electronic devices. Key to allowing this wide band gap material system to achieve its full potential is the availability of low-cost, large-area, high-crystal-quality native substrates. While important advances have been made to this effect, continued progress and breakthroughs are required to truly meet this objective.The ammonothermal method, which utilizes supercritical ammonia to preferentially dissolve and crystalize GaN on a native seed, offers a true bulk crystal growth process to yield large, inexpensive GaN substrates. To further enhance the growth rate and quality of GaN, much still needs to be learned and gained from an improved understanding of the chemical processes and growth mechanisms occurring in the ammonothermal environment. A key challenge to achieving this, is the difficulty in obtaining in-situ information during growth of GaN due to the use of thick-walled autoclaves containing the corrosive environment at temperatures up to ~ 650 °C and pressures up to ~ 2500 atm.This talk will present the progress made in our understanding of the underpinnings of the basic ammonothermal growth of GaN. Specifically, it will discuss advances made to allow us to understand the thermodynamic foundation of the solvent, for the first time. This is achieved by the creation and application of a new, simple equation of state for supercritical ammonia under ammonothermal conditions. Additionally, by combining this solvent knowledge with in-situ temperature, total system measurements, and GaN single crystal growth, various aspects of the crystal growth process and solute behaviors are elucidated and will be presented. Consequences towards our ability to enhance the growth rate of basic ammonothermal GaN will then be discussed.

Investigation of the Area Defects in Ammonothermal Grown Gallium Nitride Substrate Wafers

Gallium Nitride (GaN) is one of the most promising wide band gap semiconductor materials that are replacing silicon (Si) in power electronic applications. However, the quality of the material is limiting its application in high performance power electronic materials. Apart from the threading dislocations reported in our previous studies [1, 2], some area defects are observed in both ammonothermal GaN substrates by synchrotron X-ray topography (XRT) (Figure 1). Information on the nature of these defects is important to prevent the formation of such defects and improve the crystal quality.In this study, synchrotron X-ray rocking curve topography (SXRCT), Nomarski optical microscopy (NOM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) are adopted to investigate the nature of the area defects. Apart from XRT images, these domain features are visible in the NOM images (Figure 2). SXRCT results show that these areas have a different strain level from the rest of the wafer area. SEM images in RBSD mode show that the area defects are observed as a diffraction contrast. To investigate the atomic configuration of these domains, HRTEM of specimens sliced from the domains are being compared with those from outside the domains. The characterization results from SXRCT, SEM and TEM analysis in conjunction with observations from other techniques like XRT and NOM will be discussed to determine the nature of the area defects and their formation mechanisms will be deduced.[1] 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.[2] Y. Liu, H. Peng, T. Ailihumaer, B. Raghothamachar, M. Dudley, Journal of Electronic Materials 50 (2021) 2981-2989. Figure 1

On Stress-Induced Polarization Effect in Ammonothermally Grown GaN Crystals

The results of basic ammonothermal crystallization of gallium nitride are described. The material is mainly analyzed in terms of the formation of stress (called stress-induced polarization effect) and defects (threading dislocations) appearing due to a stress relaxation process. Gallium nitride grown in different positions of the crystallization zone is examined in cross-polarized light. Interfaces between native ammonothermal seeds and new-grown gallium nitride layers are investigated in ultraviolet light. The etch pit densities in the seeds and the layers is determined and compared. Based on the obtained results a model of stress and defect formation is presented. New solutions for improving the structural quality of basic ammonothermal gallium nitride crystals are proposed.

Synchrotron X-ray topography characterization of high quality ammonothermal-grown gallium nitride substrates

Ammonothermal growth of bulk gallium nitride (GaN) crystals is considered the most suitable method to meet the demand for high quality bulk substrates for power electronics. A non-destructive evaluation of defect content in state-of-the-art ammonothermal substrates has been carried out by synchrotron X-ray topography. Using a monochromatic beam in grazing incidence geometry, high resolution X-ray topographs reveal the various dislocation types present. Ray-tracing simulations that were modified to take both surface relaxation and absorption effects into account allowed improved correlation with observed dislocation contrast so that the Burgers vectors of the dislocations could be determined. The images show the very high quality of the ammonothermal GaN substrate wafers which contain low densities of threading dislocations (TDs) but are free of basal plane dislocations (BPDs). Threading mixed dislocations (TMDs) were found to be dominant among the TDs, and the overall TD density (TDD) of a 1-inch wafer was found to be as low as 5.16 × 103 cm−2.

Recent progress in basic ammonothermal GaN crystal growth

Progress in basic ammonothermal gallium nitride crystallization is reported. In order to prevent the appearance of cracks in growing GaN a new shape of native seeds is proposed. Crystallization results on irregular (old) and round (new) shapes are compared in terms of growth rate and structural quality. The new type of seeds allowed to grow crystals twice as fast without losing the very high structural quality. Etch pit density in the new-grown material is examined and compared to that in the seeds. Material deposited in vertical <000–1> and lateral <11–20> directions is analyzed. In the latter case the etch pit density is as low as 102 cm−2. An influence of growth process parameters and crystal growth configuration on the structural quality of crystallized GaN is discussed.

Investigation of diffusion mechanism of beryllium in GaN

Diffusion mechanism of beryllium in gallium nitride was investigated by analyzing temperature-dependent diffusion profiles from an infinite source. Beryllium atoms were implanted into a high structural quality gallium nitride layer crystallized by halide vapor phase epitaxy on an ammonothermal gallium nitride substrate. Post-implantation annealing was performed at different temperatures, between 1000 °C and 1400 °C, under high nitrogen pressure. Beryllium profiles were analyzed in the as-implanted and annealed samples by secondary ion mass spectrometry. It was shown that the diffusion of the dopant results from the combination of two mechanisms: rapid interstitial and slow interstitial-substitutional diffusion. The pre-exponential factor as well as activation energy for both diffusion paths were determined. Moreover, from the characteristic features of beryllium depth profiles, the formation energies of gallium vacancy and beryllium in interstitial position were calculated and compared to the theoretical values.

Thermal annealing effects on SCAATTM substrate grown toward the c- and m-directions

The effect of the thermal annealing on an acidic ammonothermal gallium nitride (GaN) crystal grown toward the c- and m-directions was investigated. The specific annihilation of hydrogenated Ga vacancies was confirmed for the GaN crystal grown toward the m-direction but not for that grown toward the c-direction. A transparent and yellow luminescence-free ammonothermal GaN substrate was realized. The annihilation of the hydrogenated Ga vacancies was explained by the difference in the diffusion of F incorporated as a mineralizer between the c- and m-directions.

Growth kinetics of basic ammonothermal gallium nitride crystals

The ammonothermal method has been extensively studied for the last two decades because of its potential to produce high-quality bulk gallium nitride (GaN) boules at low cost. Currently, the ammonothermal literature lacks in situ, internal fluid temperature measurements during the crystal growth process. This study reports on internal fluid temperature measured simultaneously in the dissolution and growth zones for twelve basic ammonothermal GaN growth experiments (pressure range = 180–261 MPa; molar NH3:Na fill ratio = 20:1) on {0 0 0 1} and {1 0 1¯ 0}-orientation GaN seeds. Knowledge of the fluid temperature combined with mass-change measurements of the polycrystalline source and seed crystals has enabled the distinction of three growth regimes: a mass transport-limited regime at internal fluid density differences below 1.2 mol/L between dissolution and growth zones; a surface reaction-limited regime above the aforementioned critical fluid density difference and below growth zone fluid temperatures of ∼570 °C; and an apparent loss of solvent-limited regime above growth zone fluid temperatures of ∼570 °C. Analysis of growth in [0 0 0 1], [0 0 0 1¯], and [1 0 1¯ 0] has enabled the determination of activation energies of each respective GaN crystallographic orientation in the surface reaction-limited regime.

Power Electronic Devices and Systems Based on Bulk GaN Substrates

Wide-bandgap power semiconductor devices offer enormous energy efficiency gains in a wide range of potential applications. As silicon-based semiconductors are fast approaching their performance limits for high power requirements, wide-bandgap semiconductors such as gallium nitride (GaN) and silicon carbide (SiC) with their superior electrical properties are likely candidates to replace silicon in the near future. Along with higher blocking voltages wide-bandgap semiconductors offer breakthrough relative circuit performance enabling low losses, high switching frequencies, and high temperature operation. ARPA-E’s SWITCHES program, started in 2014, set out to catalyze the development of vertical GaN devices using innovations in materials and device architectures to achieve three key aggressive targets: 1200V breakdown voltage (BV), 100A single-die diode and transistor current, and a packaged device cost of no more than ȼ10/A. The program is drawing to a close by the end of 2017 and while no individual project has yet to achieve all the targets of the program, they have made tremendous advances and technical breakthroughs in vertical device architecture and materials development. GaN crystals have been grown by the ammonothermal technique and 2-inch GaN wafers have been fabricated from them. Near theoretical, high-voltage (1700-4000V) and high current (up to 400A pulsed) vertical GaN diodes have been demonstrated along with innovative vertical GaN transistor structures capable of high voltage (800-1500V) and low RON (0.36-2.6 mΩ-cm2). The challenge of selective area doping, needed in order to move to higher voltage transistor devices has been identified. Furthermore, a roadmap has been developed that will allow high voltage/current vertical GaN devices to reach ȼ5/A to ȼ7/A, realizing functional cost parity with high voltage silicon power transistors.

Observation of Threading Dislocations in Ammonothermal Gallium Nitride Single Crystal Using Synchrotron X-ray Topography

Synchrotron monochromatic-beam x-ray topography observation has been performed on high-quality ammonothermal gallium nitride single crystal to evaluate threading dislocations (TD) in a nondestructive manner. Asymmetric diffractions with six equivalent g-vectors of 11–26, in addition to a symmetric diffraction with g = 0008, were applied to determine the Burgers vectors (b) of dislocations. It was found that pure edge-type TDs with \( {\varvec b} = \left\langle {11 - 20} \right\rangle /3 \) did not exist in the sample. A dominant proportion of TDs were of mixed type with \( {\varvec b} = \left\langle {11 - 20} \right\rangle /3 + \left\langle {0001} \right\rangle \), i.e., so-called c + a dislocations. Pure 1c screw dislocations with \( {\varvec b} = \left\langle {0001} \right\rangle \) and TDs with c-component larger than 1c were also observed.

(Invited) GaN Wafers with Dramatically Improved Crystalline and Electrical Properties

Gallium Nitride (GaN) and its alloys with Indium and Aluminum, part of third semiconductors generation, are revolutionizing both the optoelectronic and electronic device industries. Despite a previous lack of native substrates, a number of devices exceeding the performance of those fabricated from well-developed semiconductors were realized. However, the fabrication of higher performance devices with higher yields requires higher crystalline quality substrates with fully controlled electronic transport properties. Bulk ammonothermal GaN substrates, recently developed and commercialized, are characterized by low dislocation density, a single crystal nature, and high flatness; but they lack full control of the electrical properties. It is expected that films deposited by vapor transport techniques, typically with low impurities background, would allow the growth of materials with high crystalline quality and improved electrical properties. More recently, it was demonstrated that thick GaN films deposited by Hydride Vapor Phase Epitaxy (HVPE, a fast deposition process) on ammonothermal GaN substrates reproduce the high crystalline quality of those substrates, and have free carrier concentrations that are several orders of magnitude lower that of the substrates [1]. We have verified that such HVPE GaN wafers can be used for further homoepitaxial growth. This is extremely important, because it demonstrates the usefulness of this new type of substrate to fabricate highly efficient optoelectronic and electronic devices. An ~800 mm thick HVPE film deposited on an (0001) epi-ready Ammono substrate (~0.3 degree toward “m”) was removed from the reactor and its surface was chemical-mechanically polished (CMP). This template was put back in the HVPE reactor and an additional ~800 mm was deposited. The final crystal consisted of two sequentially grown HVPE-GaN layers on the Ammono substrate. The top HVPE layer was sliced off and three samples were diced from different regions of this wafer. The surfaces of these samples were mechanically polished and CMP finished. A combination of techniques was employed to investigate the structural, optical, and electrical properties of the crystals. High resolution XRD was measured on two samples using a Rigaku Smartlab diffractometer with a four bounce Ge (220) monochromator. Lattice parameters were measured using symmetric and asymmetric scans: sample A: c=5.1856Å and a=3.1824Å; sample B were, c=5.1856Å and a=3.1839Å. These values are very close to the bulk lattice constants. The rocking curves for these samples were measured and had full widths at half maximum of ~16 arcsecs, indicating superior crystalline quality. High lateral and depth resolution micro-Raman scattering (RS) measurements were carried out on the front and back surfaces of two samples. The E2 2 phonon frequency was mapped on both faces of two samples; the distribution of Raman shifts was 567.45±0.05 cm-1, indicating stress free GaN. The A1(LO) phonon frequencies and the linewidths measured on the Ga-polar face were larger than those measured on the N-polar face; this is consistent with a larger incorporation of donor impurities as the growth proceeds. Low temperature, high spectral resolution photoluminescence (PL) measurements carried out on the Ga- and N-polar faces of these samples were also consistent with more doping as the growth proceeds [2]. SIMS depth profile measurements also confirm this observation. Hall measurements provided information about the free-carrier concentration and carrier mobility. [1] T. Sochacki, et al., J.J. Appl. Phys., 53 (2014)05FA04. [2] J.A. Freitas, Jr., et al., Cryst. Growth & Design, 15 (2015) 4837.

Defects in Single Crystalline Ammonothermal Gallium Nitride

Native bulk gallium nitride (GaN) has emerged as an alternative for sapphire and silicon as a substrate material for III‐N devices. While quasi‐bulk GaN substrates are currently commercially available, single crystal GaN substrates are considered essential for future high performance light emitters and power devices. The ammonothermal method is currently considered one of the most feasible methods to grow large truly bulk crystals of GaN at low cost and high structural quality. High crystalline quality GaN substrates sliced from ammonothermally grown crystals have been demonstrated and utilized in homoepitaxy for III‐N devices. However, despite the high crystalline quality the properties of as‐grown ammonothermal GaN crystals and substrates are affected by the presence of impurities and other defects that hinder their use for device applications. Here, the main developments of ammonothermal growth of GaN and the effects of impurities, native point defects, and dislocations on the material properties are summarized. Additionally, measurement techniques that enable the evaluation of point defect concentration and low dislocation density distribution over a large area on bulk GaN substrates are reviewed.

Heat, mass, and crystal growth of GaN in the ammonothermal process: A numerical study

ABSTRACTThis paper presents a computational model for the ammonothermal gallium nitride (GaN) crystal growth process, including fluid flow, heat transfer, dissolution and crystallization rates, GaN metastable phase transport, and crystal interface advancement. The presented article solves the Navier–Stokes equations along with the Brickman–Darcy–Forchheimer extensions for nutrient porous medium and Boussinesq approximation for free convection. Piecewise Linear Interface Calculation (PLIC) method is adopted to construct and advance the crystal interface. Simulations, in particular, were performed for a common research autoclave with a retrograde ammonothermal system. Special attention is given to the regions close to the crystal interface.

Basic ammonothermal GaN growth in molybdenum capsules

Single crystal, bulk gallium nitride (GaN) crystals were grown using the basic ammonothermal method in a high purity growth environment created using a non-hermetically sealed molybdenum (Mo) capsule and compared to growths performed in a similarly designed silver (Ag) capsule and capsule-free René 41 autoclave. Secondary ion mass spectrometry (SIMS) analysis revealed transition metal free (<1×1017cm−3) GaN crystals. Anomalously low oxygen concentrations ((2–6)×1018cm−3) were measured in a {0001} seeded crystal boule grown using a Mo capsule, despite higher source material oxygen concentrations ((1–5)×1019cm−3) suggesting that molybdenum (or molybdenum nitrides) may act to getter oxygen under certain conditions. Total system pressure profiles from growth runs in a Mo capsule system were comparable to those without a capsule, with pressures peaking within 2 days and slowly decaying due to hydrogen diffusional losses. Measured Mo capsule GaN growth rates were comparable to un-optimized growth rates in capsule-free systems and appreciably slower than in Ag-capsule systems. Crystal quality replicated that of the GaN seed crystals for all capsule conditions, with high quality growth occurring on the (0001) Ga-face. Optical absorption and impurity concentration characterization suggests reduced concentrations of hydrogenated gallium vacancies (VGa−Hx).

Free electron concentration dependent sub-bandgap optical absorption characterization of bulk GaN crystals

Optical transmission measurements were performed on high quality bulk gallium nitride (GaN) crystals grown by sodium flux, hydride vapor phase epitaxy, and the ammonothermal method with varying free electron concentrations ranging from 4×1016cm−3 to 9×1018cm−3. The quality of the crystals was analyzed by x-ray diffraction, threading dislocation density determination, impurity concentrations, and Hall mobility measurements. The sub-bandgap absorption coefficient and index of refraction was determined based on illumination wavelengths ranging from 360nm to 800nm. Phonon-assisted free carrier absorption was determined to be the dominant absorption mechanism above approximately 0.1cm−1. The absorption coefficient at 450nm varied linearly from 0.1cm−1 to 5cm−1 for free electron concentrations ranging from 1×1017cm−3 to 9×1018cm−3. The ammonothermal GaN samples exhibited a strong defect related onset of absorption above 2.9eV which can be explained by the presence of appreciable hydrogenated gallium vacancies having defect states close to the valance band within the electric bandgap of GaN. The presence of hydrogenated gallium vacancies was experimentally confirmed by Fourier transform infrared absorbance measurements and double hydrogenated gallium vacancy defect are speculated to be prominent in ammonothermal GaN.

Linear piezoelectricity material constants for ammonothermal gallium nitride measured by bulk acoustic waves

Measurements of linear elastic and piezoelectric constants of gallium nitride by bulk acoustic waves are reported. All material properties, including static dielectric constants by capacitance method and density by x-ray method, were determined, using semi-insulating ammonothermally grown w-GaN. The obtained values: c11 = 366.9 ± 0.4 GPa, c12 = 135.0 ± 0.6 GPa, c13 = 97.5 ± 0.2 GPa, c33 = 398.1 ± 0.6 GPa, c44 = 99.1 ± 0.2 GPa, e15 = −0.36 ± 0.03 C m−2, e31 = −0.41 ± 0.04 C m−2, e33 = 0.86 ± 0.04 C m−2, = (9.3 ± 0.3), and = (10.3 ± 0.3) are proposed as the new standard, confirming only some results from the very sparse and ambiguous published data.