Growth Of Gallium Nitride Research Articles

Improving the GaN Growth Rate by Optimizing the Nutrient Basket Geometry in an Ammonothermal System Based on Numerical Simulation.

The low growth rate of bulk gallium nitride (GaN) when using the ammonothermal method is improved herein by optimizing the nutrient geometry. A numerical model considering the dissolution and crystallization process is developed. Heater powers are employed as thermal boundary conditions to match the real ammonothermal system. The flow field, thermal field, and mass concentration field in the autoclave are calculated with or without a central hole in the nutrient basket. Numerical results show that the mass transfer efficiency and supersaturation on the seed crystal surface can be improved with a hole in the nutrient basket in the center in spite of its diverse effect on the heat transfer, which can be overcome by adjusting the heater powers. As a result, the growth rate of the GaN crystal can be obviously increased.

Insight into the Step Flow Growth of Gallium Nitride Based on Density Functional Theory

Insight into the Step Flow Growth of Gallium Nitride Based on Density Functional Theory

Molecular beam epitaxy growth and characterization of AlGaN epilayers in nitrogen-rich condition on Si substrate

In this work, we investigate the molecular beam epitaxy growth and characterization of aluminum gallium nitride (AlGaN) epilayers in nitrogen (N)-rich condition on silicon using an aluminum nitride (AlN) buffer layer on a nanowire template. It is found that the growth condition of the AlN buffer layer plays an important role on the surface morphology of the AlGaN epilayers. Under the optimized growth temperature of the AlN buffer layer, a relatively smooth AlGaN epilayer can be formed, with a root-mean-square roughness of around 1.8 nm, comparable to the typical III-nitride thin films grown in N-rich conditions by molecular beam epitaxy. The epilayer is further confirmed to be N-polar from both the reflection high-energy electron diffraction 3 × 3 reconstruction and potassium hydroxide etching experiments.

The insertion of the ALD diffusion barriers: An approach to improve the quality of the GaN deposited on Kapton by PEALD

The influence of growth temperature (200–300 ℃) on the properties of gallium nitride (GaN) directly deposited on Kapton by plasma-enhanced atomic layer deposition (PEALD) has been systematically investigated. With increasing growth temperatures, the O content and surface roughness of the GaN thin films increase. Besides, the diffusion behavior of GaN in Kapton was found when the growth temperature exceeds 250 ℃. In this study, ultrathin ALD aluminum oxide (Al2O3) and aluminum nitride (AlN) films were proposed to see if they could alleviate and/or even suppress the negative effects of interface diffusion during high-temperature (≥250 ℃) GaN growth. It was found that oxygen impurities were significantly decreased within the as-deposited GaN. Furthermore, we observed a complete suppression of GaN diffusion into the Kapton throughout the entire growth process. Additionally, an introduction of either the Al2O3 or AlN before the GaN deposition could improve the surface morphology of the GaN deposited on Kapton.

The role of AlN thickness in MOCVD growth of N-polar GaN

The influence of the thickness of aluminum nitride (AlN) on the surface morphology, crystalline quality, and polarity of N-polar gallium nitride (GaN) grown by metal organic chemical vapor deposition (MOCVD) is investigated in this study. With a medium temperature (1000 ℃) AlN buffer layer of 150 nm, a hexagonal-defect-free N-polar GaN thin film with low dislocation densities (4.2 × 108 cm-2 for screw dislocation density and 3.7 × 109 cm-2 for edge dislocation density) is obtained on a 2-in. vicinal sapphire substrate (c off m plane 2°). With the reduction of the AlN thickness to 80 nm, the N-polar GaN surface becomes rough and the crystalline quality is exacerbated. While the thickness of AlN is increased to 200 nm, the polarity of GaN is changed to Ga-polar. The transmission electron microscopy (TEM) results reveal an undulated interface between GaN and AlN with a large density of pyramidal defects which may contribute to the polarity inversion of GaN. The controllable polarity inversion through changing the thickness of AlN provides a promising method for the polarity engineering of nitride-based devices.

High quality GaN tetrapodal structures hetero-integrated on 3D Si surfaces

Gallium nitride (GaN) light-emitting diodes (LEDs) on silicon (Si) substrates have been highlighted due to the potentials to achieve significant cost reduction, effective thermal management, and highly directional light emission, by replacing the conventional sapphire substrates with the opportunities to adopt the well-established Si-based semiconducting technologies. However, large mismatch of physical properties between Si and GaN considerably lowers the material quality of GaN and the corresponding device performances. Herein, we employ three-dimensional faceted Si surfaces (3D faceted Si) to effectively release the generated strain energy during the hetero-epitaxial growth of GaN on Si for fabricating high performance GaN-on-Si LEDs. The resulting outcome not only includes the unique morphology of 3D tetrapodal GaN layers on 3D faceted Si to form space-filling arrays for efficient device utilization, but also contains the excellent material quality of the hetero-integrated GaN layers, comparable to that grown on planar sapphire substrates. Consequently, those 3D tetrapodal GaN-on-Si LEDs exhibit the excellent light extraction characteristics due to their shapes to effectively suppress total internal reflection of generated light. In addition, the anisotropic formation of active layers in the 3D tetrapodal GaN-on-Si LEDs performs multi-color emission characteristics with tunable color capabilities.

High Internal Quantum Efficiency AlGaN Epilayer Grown by Molecular Beam Epitaxy on Si Substrate

We report the molecular beam epitaxy growth and characterization of aluminum gallium nitride (AlGaN) epilayer on an Si substrate. The AlGaN epilayer was grown on an AlN buffer layer through a coalescence process on a nanowire template. The AlGaN epilayer possessed a relatively smooth surface. The room-temperature internal quantum efficiency (IQE) was investigated using power-dependent photoluminescence experiments and theoretical analysis. A maximum IQE of around 50% is derived, with an estimated carrier density of 3 × 1018 cm−3. This IQE is significantly improved compared to the previously reported bulk AlGaN epilayers grown on sapphire and/or AlN-on-sapphire templates. The efficiency droop mechanism is also discussed, which could presumably be attributed to the carrier delocalization.

Selective area growth of cubic gallium nitride on silicon (001) and 3C-silicon carbide (001)

Selective area growth of cubic gallium nitride is investigated in a plasma assisted molecular beam epitaxy setup. 380 μm thick silicon (001) and 10 μm thick 3C-silicon carbide (001), grown on 500 μm silicon (001), were used as substrates and structured with silicon dioxide masks. Selective area growth on silicon and 3C-silicon carbide was tested for both thermal and plasma deposited oxides. Multiple growth series showed that gallium nitride coverage of silicon dioxide vanished at growth temperatures of 870 °C for silicon substrates and at a surface temperature of 930 °C for 3C-silicon carbide substrates. Whereas gallium nitride is grown in its hexagonal form on silicon substrates, phase pure cubic gallium nitride could selectively be grown on the 3C-silicon carbide template. The cubic phase is verified by high resolution x-ray diffraction and low temperature photoluminescence measurements. The photoluminescence measurements prove that gallium nitride condensed selectively on the 3C-silicon carbide surfaces uncovered by silicon dioxide.

Reduction of threading dislocations in GaN grown on patterned sapphire substrate masked with serpentine channel

Gallium nitride (GaN) is a promising semiconductor for the fabrication of a wide range of electronic and optoelectronic devices. For GaN grown on widely used sapphire substrate, bundles of threading dislocations (TDs) are induced by thermal and lattice mismatch. These TDs intersect the surface of GaN and degrade the internal quantum efficiency (IQE) of GaN-based devices. To address this issue, two innovative techniques are proposed for effective filtration of TDs. First, a novel design of serpentine channel pattern sapphire substrate (SCPSS) is applied for effective filtration of TDs at the interface. Transmission electron microscopy (TEM) indicated that SCPSS plays an effective role in the filtration of TDs; however, cathodoluminescence (CL) revealed that many TDs still intersect the surface of GaN. To further optimize the growth of GaN, the temperature was ramped down to the intermediate level and an interlayer (IL) of InGaN was introduced near the window of SCPSS. CL analysis indicated that additional use of InGaN-IL further reduced the TDs. Raman microscopy showed that incorporation of IL lead to drastic reduction in compressive stress. Atomic force microscopy (AFM) confirmed the achievement of excellent surface topography with InGaN-IL. Moreover, it is observed that growth of GaN by SCPSS is better than conventional growth because of improved surface morphology and low TDs. The reduction of TDs by SCPSS and InGaN-IL is an effective approach for the growth of high-quality GaN towards efficient devices.

Numerical Analysis of the High Pressure MOVPE Upside-Down Reactor for GaN Growth

The present paper focuses on the high-pressure metal-organic vapor phase epitaxy (MOVPE) upside-down vertical reactor (where the inlet of cold gases is below a hot susceptor). This study aims to investigate thermo-kinetic phenomena taking place during the GaN (gallium nitride) growth process using trimethylgallium and ammonia at a pressure of above 2 bar. High pressure accelerates the growth process, but it results in poor thickness and quality in the obtained layers; hence, understanding the factors influencing non-uniformity is crucial. The present investigations have been conducted with the aid of ANSYS Fluent finite volume method commercial software. The obtained results confirm the possibility of increasing the growth rate by more than six times through increasing the pressure from 0.5 bar to 2.5 bar. The analysis shows which zones vortexes form in. Special attention should be paid to the transitional flow within the growth zone as well as the viewport. Furthermore, the normal reactor design cannot be used under the considered conditions, even for the lower pressure value of 0.5 bar, due to high turbulences.

Laser‐Assisted Metal–Organic Chemical Vapor Deposition of Gallium Nitride

Ammonia (NH3) is commonly used as group V precursor in gallium nitride (GaN) metalorganic chemical vapor deposition (MOCVD). The high background carbon (C) impurity in MOCVD GaN is related to the low pyrolysis efficiency of NH3, which represents one of the fundamental challenges hindering the development of high purity thick GaN for vertical high power device applications. This work uses a laser-assisted MOCVD (LA-MOCVD) growth technique to address the high-C issue in MOCVD GaN. Carbon dioxide (CO2) laser with wavelength of 9.219 um was utilized to facilitate NH3 decomposition via resonant vibrational excitation. The LA-MOCVD GaN growth rate (as high as 10 um/hr) shows a strong linear relationship with the trimethylgallium (TMGa) flow rate, indicating high effective V/III ratios and hence efficient NH3 decomposition. Pits-free surface morphology of LA-MOCVD GaN was demonstrated for films with growth rate as high as 8.5 um/hr. The background [C] in LA-MOCVD GaN films decreases monotonically as the laser power increases. A low [C] at 5.5E15 cm-3 was achieved in LA-MOCVD GaN film grown with the growth rate of 4 um/hr. Charge transport characterization of LA-MOCVD GaN films reveals high crystalline quality with room temperature mobility >1000 cm2/Vs. LA-MOCVD growth technique provides an enabling route to achieve high quality GaN epitaxy with low-C impurity and fast growth rate simultaneously. This technique can also be extended for epitaxy of other nitride-based semiconductors.

Laser‐Assisted Metal–Organic Chemical Vapor Deposition of Gallium Nitride

Ammonia (NH3) is commonly used as group‐V precursor in gallium nitride (GaN) metal–organic chemical vapor deposition (MOCVD). The high background carbon (C) impurity in MOCVD GaN is related to the low decomposition efficiency of NH3, which represents one of the fundamental challenges hindering the development of high‐purity thick GaN for vertical high‐power device applications. This work uses a laser‐assisted MOCVD (LA‐MOCVD) growth technique to address the high‐C issue in MOCVD GaN. A carbon dioxide (CO2) laser with a wavelength of 9.219 μm is utilized to facilitate NH3 decomposition via resonant vibrational excitation. The LA‐MOCVD GaN growth rate (GR; as high as 10 μm h−1) shows a strong linear relationship with the trimethylgallium (TMGa) flow rate, indicating high effective V/III ratios and, hence, efficient NH3 decomposition. [C] in LA‐MOCVD GaN films decreases monotonically, as the laser power increases. A low [C] at 5.5 × 1015 cm−3 is achieved in the LA‐MOCVD GaN film grown with GR of 4 μm h−1. LA‐MOCVD GaN films reveal high crystalline quality with room‐temperature mobility of >1000 cm2 V−1 s−1. LA‐MOCVD provides an enabling route to achieve high‐quality GaN epitaxy with low‐C and fast GR simultaneously. This technique can be extended for epitaxy of other nitride‐based semiconductors.

Single-crystal GaN growth and polarity control using an E-beam evaporated aluminum layer

We report on a method for controlling the polarity of gallium nitride (GaN) using an E-beam evaporated aluminum (Al) layer on a sapphire substrate. A high-temperature nitridation process was designed to enable the amorphous Al layer to serve as a nucleation layer for single-crystal Ga-polar GaN growth. The Al layer also acts as a mask that prevents N-polar GaN growth. As a result, Ga-polar and N-polar GaN can be grown on the Al layer and sapphire surface, respectively. This method is not only advantageous for the selective polarity control but also to simplify the fabrication process of lateral polarity structures.

Metalorganic Chemical Vapor Deposition Gallium Nitride with Fast Growth Rate for Vertical Power Device Applications

The development of high‐quality gallium nitride (GaN) epitaxy with thick drift layer, low controllable doping, and high mobility is key for vertical high‐power devices. Herein, the effect of increasing trimethylgallium (TMGa) molar flow rate on the growth rate, impurity incorporation, charge compensation, surface morphology, and carrier mobility is systematically studied. An optimized metalorganic chemical vapor deposition GaN growth condition with a typical growth rate of 2 μm h−1 is used as the baseline. With significant suppression of background Si, other impurity concentrations, and a precise control of the doping precursor SiH4 flow, an electron concentration as low as 4 × 1015 cm−3 in n−‐GaN is achieved. Through increasing the TMGa flow rate, the GaN growth rate is increased to 5.2 μm h−1. Secondary ion mass spectroscopy results show that the background H, O, and Mg remain below detection limit, but C level is increased to 2 × 1016 cm−3. GaN growth on Mn‐doped semi‐insulating GaN substrate is performed to probe the transport properties of film with low dislocation densities. Hall measurement shows that an electron mobility decreases from 852 to 604 cm2 V−1 s−1 as the growth rate increases. Results from this work reveal the challenge and guidance for achieving GaN vertical high power devices.

High-Temperature Atomic Layer Deposition of GaN on 1D Nanostructures.

Silica nanosprings (NS) were coated with gallium nitride (GaN) by high-temperature atomic layer deposition. The deposition temperature was 800 °C using trimethylgallium (TMG) as the Ga source and ammonia (NH3) as the reactive nitrogen source. The growth of GaN on silica nanosprings was compared with deposition of GaN thin films to elucidate the growth properties. The effects of buffer layers of aluminum nitride (AlN) and aluminum oxide (Al2O3) on the stoichiometry, chemical bonding, and morphology of GaN thin films were determined with X-ray photoelectron spectroscopy (XPS), high-resolution x-ray diffraction (HRXRD), and atomic force microscopy (AFM). Scanning and transmission electron microscopy of coated silica nanosprings were compared with corresponding data for the GaN thin films. As grown, GaN on NS is conformal and amorphous. Upon introducing buffer layers of Al2O3 or AlN or combinations thereof, GaN is nanocrystalline with an average crystallite size of 11.5 ± 0.5 nm. The electrical properties of the GaN coated NS depends on whether or not a buffer layer is present and the choice of the buffer layer. In addition, the IV curves of GaN coated NS and the thin films (TF) with corresponding buffer layers, or lack thereof, show similar characteristic features, which supports the conclusion that atomic layer deposition (ALD) of GaN thin films with and without buffer layers translates to 1D nanostructures.

HVPE growth of bulk GaN with high conductivity for vertical devices

The electrical properties of gallium nitride (GaN) substrate are crucial to the performance of vertical power devices. Bulk GaN substrates with carrier concentrations in the range from 6.7 × 1017 to 1.7 × 1019 cm−3 are grown by hydride vapor phase epitaxy. All samples show no obvious tensile stress regardless of the carrier concentration. Moreover, the mobility of Si-doped high-quality bulk GaN is superior to the GaN template with higher dislocation density at the same carrier concentration. The influence of carrier concentration on the performance of ohmic contact on N-face of Si-doped GaN is also carefully studied by circular transfer length measurement and rapid thermal annealing methods. The specific contact resistivity decreases monotonically with increase of carrier concentration, while it increases with the annealing temperature. The N-face contact becomes non-ohmic when the annealing temperature exceeds the limit value, which increases with the carrier concentration. The sample with carrier concentration of 1.7 × 1019 cm−3 still showed ohmic behavior after annealing at 450 °C. These results are not only useful to improve the electrical properties of N-type bulk GaN substrate, but also provide a potential solution for improving the efficiency of vertical devices in the future.

(Invited) MOCVD GaN-on-GaN Towards Vertical Power Devices & MOCVD Development of β-Ga2O3

Gallium nitride (GaN) has been considered as a promising candidate for power device applications due to the wide band gap (3.4 eV), high critical electric field (3 MV/cm) and high electron mobility (>1000 cm2/Vs). Baliga’s figure of merit (BFOM) of GaN is more than 500 times higher than silicon (Si) and more than three time higher than silicon carbide (SiC). Due to the availability of native GaN substrates, it is feasible to develop high performance GaN vertical power devices on low-defect GaN substrate. GaN-on-GaN vertical PN diode with record high VBR ~5 KV was reported [1], presenting the potential of GaN as a high-performance material for power devices. Compared with SiC, the performance of GaN vertical PN diodes is still limited by its high background doping in n-type drift layer at mid-1015 to low-1016 cm-3 range. Therefore, it is of great importance to study the sources and incorporation mechanisms of unintentionally doped background impurities in metal-organic chemical vapor deposition (MOCVD) GaN growth, to control the impurity incorporation, in order to maximize device performance for GaN based vertical power devices. This talk will discuss the dependence of MOCVD growth conditions on the impurity incorporations such as C, Si, O, H, and Fe. Quantitative SIMS and CV measurements are used for material characterization.Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) with a band gap of 4.5-4.9 eV, represents an emerging semiconductor material with excellent chemical and thermal stability. The monoclinic β-phase Ga2O3 represents the thermodynamically stable crystal among the known five known phases (α, β, γ, d , ε). The breakdown field of β-Ga2O3 is estimated to be 6-8 MV/cm. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. Single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. MOCVD growth technique was used to develop high quality β-Ga2O3 thin films and its ternary alloy (AlxGa1-x)2O3. Control of background and n-type doping in β-Ga2O3 will be discussed. Record carrier mobilities of 194 cm2/V·s at room temperature and ~10,000 cm2/V·s at low temperature were measured for MOCVD β-Ga2O3 thin films [2]. MOCVD growth of β-AlGaO targeting for Al composition of > 40% will be discussed [3, 4]. Acknowledgement: The authors acknowledge the funding support from Advanced Research Projects Agency-Energy (ARPA-E) DE-AR0001036, U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office, FY18/FY19 Lab Call, the Air Force Office of Scientific Research FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir) and the National Science Foundation (1810041, 1755479). [1] H. Ohta, N. Asai, F. Horikiri, Y. Narita, T. Yoshida, and T. Mishima, Jpn. J. Appl. Phys. 58, SCCD03 (2019).[2] Z. Feng, A F M A. U. Bhuiyan, M. R. Karim, H. Zhao, Appl. Phys. Letts., 114, 250601, 2019.[3] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, Z. Chen, H. -L. Huang, J. Hwang, H. Zhao, Appl. Phys. Lett., 115, 120602, 2019.[4] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, H.-L. Huang, J. Sarker, M. Zhu, M. R. Karim, B. Mazumder, J. Hwang, and H. Zhao, APL Materials, 8, 031104, 2020.

Multiple epitaxial lateral overgrowth of GaN thin films using a patterned graphene mask by metal organic chemical vapor deposition

Single-crystal gallium nitride (GaN) thin films were grown using a graphene mask via multiple epitaxial lateral overgrowth (multiple-ELOG). During the growth process, the graphene mask self-decomposed to enable the emergence of a GaN film with a thickness of several hundred nanometres. This is in contrast to selective area growth of GaN using an SiO2 mask leading to the well known hexagonal-pyramid shape under the same growth conditions. The multiple-ELOG GaN had a single-crystalline wurtzite structure corresponding to the crystallinity of the GaN template, which was confirmed with electron backscatter diffraction measurements. An X-ray diffraction rocking curve of the asymmetric 102 reflection showed that the FWHM for the multiple-ELOG GaN decreased to 405 from 540′′ for the underlying GaN template. From these results, the self-decomposition of the graphene mask during ELOG was experimentally proven to be affected by the GaN decomposition rather than the high-temperature/H2 growth conditions.

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.

Impact of carbon nanotube pattern layers on gallium nitride-based light emitting diodes

Patterned sapphire substrate has been used extensively in the growth of gallium nitride (GaN) material and related light emitting devices (LEDs). Recently, carbon nanotube patterned sapphire (CNTPS) was utilized to improve the GaN material and LED devices. In this article, intrinsic analyzation of LEDs on CNTPS were studied. LEDs grown on three layers of CNTs showed highest radiative quantum efficiency and internal quantum efficiency, while LEDs on double layers of CNTs exhibited the best light output power and external quantum efficiency. The physics of carriers’ injection, radiative, non-radiative, Auger recombination and light extraction in CNT patterned LEDs were unraveled by the ‘ABC’ modelsimulation.