Growth Of High-quality GaN Research Articles

Fabrication of low thermal resistance 3C-SiC/diamond structure for GaN epitaxial layer growth

Integrating diamonds with GaN presents a promising solution for enhancing heat dissipation. However, a critical challenge lies in developing an engineering substrate that facilitates the growth of high-quality GaN epitaxial layers, enabling the production of GaN-based devices with superior thermal management on a large scale. In this study, we have successfully developed an engineering substrate conducive to high-quality GaN growth. This accomplishment was achieved through the direct transfer of a 3 C-SiC thin film, originally grown on Si, to a diamond substrate at room temperature using the surface-activated bonding technique. After the transfer process, we observed the presence of a SiC amorphous layer at the interface, measuring approximately 18.5 nm in thickness. Notably, this thickness reduced to 14.5 nm after annealing at 1100 °C, indicating a transformation from amorphous to polycrystalline SiC due to recrystallization. Furthermore, it is crucial to highlight that no exfoliation of the 3 C-SiC/diamond bonding interfaces was observed even after subjecting them to annealing at 1100 °C. This observation is pivotal for ensuring the high-quality growth of GaN crystals on diamonds. The thermal boundary conductance of the 3 C-SiC/diamond interface reached nearly 93 MW/m2·K, demonstrating significant efficiency for device cooling. These compelling results highlight the potential of the 3 C-SiC/diamond engineering substrate to enable high-performance GaN-based devices, providing significantly enhanced thermal management capabilities. This breakthrough represents a major stride forward in the field.

Study of High-Quality GaN Growth and Fracture Mechanisms: Insights from Molecular Dynamics

Study of High-Quality GaN Growth and Fracture Mechanisms: Insights from Molecular Dynamics

Spalling induced van der Waals lift-off and transfer of 4-in. GaN epitaxial films

Epitaxial lift-off (ELO) of high-quality GaN layers allows for integration with a variety of materials enabling improved performance, reduced costs, and development of new electronics. Of the ELO technologies, two-dimensional (2D) material-based lift-off offers great promise but is still in the early stages of development and has yet to demonstrate the scale and yield of other ELO technologies. Here, we demonstrate the potential of this process's scalability, speed, and yield through epitaxial growth and lift-off of 4-in. GaN films using a 2D boron nitride (BN) van der Waals (vdW) buffer layer. Since the BN layer acts as the growth template and the mechanical release layer, both the quality and adhesion of the GaN layer are correlated with the BN morphology and uniformity. Detailed spectroscopic mapping demonstrates excellent BN uniformity, which translates into growth of high-quality GaN as shown in mapping of the x-ray rock curves (XRCs), atomic force microscopy, and photoluminescence. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal abrupt chemically distinct interfaces between the sapphire, BN, and AlN/GaN layers essential for efficient lift-off. Combined with the BN/GaN vdW heterostructure, Ni spalling is used to efficiently lift-off and transfer a full 4-in. GaN layer. Post transfer characterization of a 1.9 μm thick GaN layer transferred to a SiO2/Si wafer shows a very minimal change in the XRC and photoluminescence. Strain measurements before and after transfer show that the process fully relaxes residual strain formed in the GaN during high-temperature growth. This work highlights the potential for industry scalability of an exciting 2D material-based lift-off technology, which can facilitate higher power and more efficient radio frequency devices.

Threading dislocation and lattice stress modulation of Si based GaN material with AlPN nucleation layer

An innovative AlPN nucleation layer is proposed for the growth of high quality GaN on Si. AlPN nucleation layer is prepared on Si substrate by metal organic chemical vapor deposition to explore the stress and dislocation adjustment mechanism. It is found that there are stress-related antisite defects in the P-doped lattice. In addition, the incorporation of P makes the dislocation densities of screw dislocations and edge dislocations have different trends, which can realize the independent control of the different types of dislocation densities. A hexagonal-cubic-phase mixed structure model is proposed to analyze and explain the variation trends of dislocation densities and stress. This work not only puts forward a new type of nitride material, which provides an idea for the growth of high-quality GaN on Si. On the other hand, it is of great significance to perfect the independent regulation mechanism of dislocations.

Growth of high quality GaN on (0001) 4H-SiC with an ultrathin AlN nucleation layer

AlN nucleation layers (NLs) with different thicknesses were grown by a pulsed atomic-layer epitaxial method on (0001) 4H-SiC. The surface morphology evolutions of AlN NLs and the subsequent GaN buffer layers (BLs) were characterized. The GaN grown on 5 nm AlN NL showed a strong 3D growth mode and didn’t coalescence until 1.2 um. But GaN on 12 nm AlN coalesced at the thickness 300 nm, as well as the GaN BLs on 25 nm or 50 nm AlN NL. It denoted that AlN needed to reach certain thickness to completely screen the effect of the SiC substrates and provided a superior wetting template for GaN. The (0002) full width at half maximum (FWHM) and (10–12) FWHM of 1.5 um GaN on 12 nm AlN NL were 122 arcsec and 196 arcsec respectively. No pits were observed in the STEM images of GaN/AlN interfaces.

Growth of High Quality GaN on Si (111) Substrate by Using Two-Step Growth Method for Vertical Power Devices Application

A crack-free GaN film grown on 4-inch Si (111) substrate is proposed using two-step growth methods simply controlled by both III/V ratio and pressure. Two-step growth process is found to be effective in compensating the strong tensile stress in the GaN layer grown on Si substrate. The high-resolution X-ray diffraction (XRD) rocking curves of (002) and (102) planes for the GaN epitaxial layer with two-step growth method are 317 and 432 arcsec, while the corresponding values for the reference sample without two-step growth method are 550 and 1207 arcsec, respectively. The reduced threading dislocation of GaN film with two-step growth method is obtained to be ~2 × 108/cm2, which is attributed to effectively annihilate and bend threading dislocation.

Growth of high-quality GaN by halogen-free vapor phase epitaxy

We report the electrical and optical properties of gallium nitride grown by halogen-free vapor phase epitaxy (HF-VPE). The electron mobility of the HF-VPE-GaN layers was found to be comparable to or better than the GaN layers obtained using MOCVD. The positron annihilation spectroscopy analyses revealed that the density of the electroneutral or negatively charged vacancy-type defects in the HF-VPE-GaN layers was below the detection limit (≤1015 cm−3), equivalent to that of a defect-free hydride vapor phase epitaxy (HVPE)-GaN reference sample. Our study shows that the HF-VPE technique can be employed to achieve high-quality and cost-effective bulk crystal and epitaxial layer growth for GaN devices.

Scalable fabrication of GaN on amorphous substrates via MOCVD on highly oriented silicon seed layers

Abstract The integration of gallium nitride with low-cost, transparent substrates such as glass and crystallographically incompatible substrates such as Si(0 0 1) has been long desired for III-N electronics. Here the authors demonstrate how GaN growth on uniformly 〈1 1 1〉 oriented silicon seed layers fabricated by aluminum-induced crystallization provide a straightforward method for realizing large area GaN growth on these substrate by adopting well-developed GaN on Si(1 1 1) growth processes. Critical factors for promoting high quality GaN growth are identified, and remaining steps necessary for realizing GaN devices are discussed.

Titanium Carbide MXene Nucleation Layer for Epitaxial Growth of High-Quality GaN Nanowires on Amorphous Substrates.

Growing III-nitride nanowires on 2D materials is advantageous, as it effectively decouples the underlying growth substrate from the properties of the nanowires. As a relatively new family of 2D materials, MXenes are promising candidates as III-nitride nanowire nucleation layers capable of providing simultaneous transparency and conductivity. In this work, we demonstrate the direct epitaxial growth of GaN nanowires on Ti3C2 MXene films. The MXene films consist of nanoflakes spray coated onto an amorphous silica substrate. We observed an epitaxial relationship between the GaN nanowires and the MXene nanoflakes due to the compatibility between the triangular lattice of Ti3C2 MXene and the hexagonal structure of wurtzite GaN. The GaN nanowires on MXene show good material quality and partial transparency at visible wavelengths. Nanoscale electrical characterization using conductive atomic force microscopy reveals a Schottky barrier height of ∼330 meV between the GaN nanowire and the Ti3C2 MXene film. Our work highlights the potential of using MXene as a transparent and conductive preorienting nucleation layer for high-quality GaN growth on amorphous substrates.

Review— Toyoda Gosei's History of R&D on Blue LEDs with Professor Isamu Akasaki

Toyoda Gosei (TG) started joint R&D with Prof. Akasaki and Prof. Amano (collectively, the Akasaki Group) in 1986, immediately after the Akasaki Group succeeded in the growth of high-quality GaN single crystals for the first time in the world. Then, the Akasaki Group and TG concluded an entrustment R&D agreement with Japan Science and Technology Agency (JST) in 1987 to promote R&D through the investment of JST. Under these circumstances, the Akasaki Group achieved the growth of high-quality p-type GaN and n-type GaN, believed impossible at that time. These breakthrough technologies were of fundamental importance for developing blue light-emitting diodes (LEDs). Namely, (1) high-quality GaN single crystals, (2) p-type GaN, and (3) n-type GaN were found to be applicable to industrial products. Thanks to the Akasaki Group, TG started the mass production of blue LEDs in 1995. However, at that time, these LEDs were not particularly bright, and no one believed that they would be used in general lighting. In spite of this prediction, the brightness of blue LEDs was improved markedly by industry-based developments, leading to their widespread adoption in many applications.

Study on the nitridation of β-Ga2O3 films**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0404201), the Six-Talent Peaks Project in Jiangsu Province, China (Grant No. XCL-107), the State Key Research and Development Program of Jiangsu Province, China (Grant No. BE2018115), the Fund from the Solid-state Lighting and Energy-saving Electronics Collaborative Innovation Center, PAPD, and

Single-crystal GaN layers have been obtained by nitriding β-Ga2O3 films in NH3 atmosphere. The effect of the temperature and time on the nitridation and conversion of Ga2O3 films have been investigated. The nitridation process results in lots of holes in the surface of films. The higher nitridation temperature and longer time can promote the nitridation and improve the crystal quality of GaN films. The converted GaN porous films show the single-crystal structures and low-stress, which can be used as templates for the epitaxial growth of high-quality GaN.

Growth of high-quality GaN on (1 0 0) Ga2O3 substrates by facet-controlled MOVPE

We report a comprehensive study of the growth of GaN layers on (1 0 0)-oriented Ga2O3 substrates by metalorganic vapour phase epitaxy (MOVPE), especially the improvement on using an intermediate facet-control layer between the low-temperature buffer and the high-temperature GaN layer compared to the usual two-step growth process. We examined in detail the effects of substrate nitridation, carrier gas, and variation of the thickness of the low-temperature buffer GaN layer, and the thickness and growth temperature of the facet layer. This optimization allowed us to obtain GaN layers on (1 0 0)-oriented Ga2O3 with high structural and optical quality, as shown by high-resolution X-ray diffraction, electron microscopy, optical absorption, and luminescence measurements.

Defect generation by nitrogen during pulsed sputter deposition of GaN

Pulsed sputter deposition has been demonstrated to be a viable process for the growth of high quality GaN and InGaN/GaN LEDs. It enables the fabrication of nitride LEDs with a red emission wavelength at large areas. In this study, we explore details on the epitaxial sputter deposition of GaN with a particular emphasis on ion damage. By changing the argon to nitrogen ratio, we adjust the growth mode from island to layer growth. TEM revealed speckles in the epitaxial GaN, which could be identified as isolated basal stacking faults, acting as non-radiative recombination centers. Using Monte Carlo methods, we modeled the energies of backscattered and sputtered atoms in order to get information on the ion damage mechanisms. Considering the collisions on the way from the target to the substrate, we found energetic nitrogen to induce the speckles. A shielding mechanism based on metallic gallium has been identified, leading to the strongly increased luminescence quality in comparison to the non-shielded material.

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and Inx Ga1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.

Evolution of threading dislocations in GaN epitaxial laterally overgrown on GaN templates using self-organized graphene as a nano-mask

Growth of high-quality GaN within a limited thickness is still a challenge, which is important both in improving device performance and in reducing the cost. In this work, a self-organized graphene is investigated as a nano-mask for two-step GaN epitaxial lateral overgrowth (2S-ELOG) in hydride vapor phase epitaxy. Efficient improvement of crystal quality was revealed by x-ray diffraction. The microstructural properties, especially the evolution of threading dislocations (TDs), were investigated by scanning electron microscopy and transmission electron microscopy. Stacking faults blocked the propagation of TDs, and fewer new TDs were subsequently generated by the coalescence of different orientational domains and lateral-overgrown GaN. This evolution mechanism of TDs was different from that of traditional ELOG technology or one-step ELOG (1S-ELOG) technology using a two-dimensional (2D) material as a mask.

Compensation of native donor doping in ScN: Carrier concentration control and p-type ScN

Scandium nitride (ScN) is an emerging indirect bandgap rocksalt semiconductor that has attracted significant attention in recent years for its potential applications in thermoelectric energy conversion devices, as a semiconducting component in epitaxial metal/semiconductor superlattices and as a substrate material for high quality GaN growth. Due to the presence of oxygen impurities and native defects such as nitrogen vacancies, sputter-deposited ScN thin-films are highly degenerate n-type semiconductors with carrier concentrations in the (1–6) × 1020 cm−3 range. In this letter, we show that magnesium nitride (MgxNy) acts as an efficient hole dopant in ScN and reduces the n-type carrier concentration, turning ScN into a p-type semiconductor at high doping levels. Employing a combination of high-resolution X-ray diffraction, transmission electron microscopy, and room temperature optical and temperature dependent electrical measurements, we demonstrate that p-type Sc1-xMgxN thin-film alloys (a) are substitutional solid solutions without MgxNy precipitation, phase segregation, or secondary phase formation within the studied compositional region, (b) exhibit a maximum hole-concentration of 2.2 × 1020 cm−3 and a hole mobility of 21 cm2/Vs, (c) do not show any defect states inside the direct gap of ScN, thus retaining their basic electronic structure, and (d) exhibit alloy scattering dominating hole conduction at high temperatures. These results demonstrate MgxNy doped p-type ScN and compare well with our previous reports on p-type ScN with manganese nitride (MnxNy) doping.

Reduction of crack density in ammonothermal bulk GaN growth

The growth of high quality GaN by the ammonothermal method is appealing due to the potential to scale and achieve very high crystal quality. Several applications could benefit from the supply of very high quality GaN such as high power light emitting diodes, laser diodes, and high power electronics. Despite steady advancement by the few groups developing ammonothermal growth technology, high quality ammonothermal GaN wafers have yet be manufactured in great quantities.This paper reviews the current progress of ammonothermal growth at SixPoint Materials. Growths were performed at T<600°C and P<300MPa on GaN seed crystals produced by hydride vapor phase epitaxy (HVPE). For thin boules, <1mm growth thickness, no cracking is observed. Historically however, SixPoint Materials’ ammonothermal growth on HVPE seeds eventually experiences a curvature flip giving extremely high radius of curvature at a critical thickness. As the growth continues the radius of curvature degrades and cracking is observed. Since IWBNSVIII, SixPoint Materials has improved the crack free area for 5mm thick boules from 5 to 80mm2 to the complete seed area. This result is repeatable in multiple reactors. Careful selection of the HVPE seeds led to the greatest reduction in cracking. Seed selection combined with an additional technique has allowed boules to be grown crack free. X-ray diffraction was carried out on an ammonothermally grown boule at 90 points along a 44mm line providing a mean (002) and (201) full width half max (FWHM) reflection of 29 and 35″ respectively using a beam spot of 0.3mmx0.3mm and an open detector. The radius of curvature is typically between 3 and 20m across the sample. Dislocation densities are routinely low 105cm−2 .

Growth of GaN/InGaN Films and Heterostructures Via Super-Atmospheric MOCVD

The beneficial material properties of Wide-Bandgap semiconductors have allowed them to find utility in a number of modern industries. GaN based devices particularly have found acceptance in both the solid-state lighting[1] and power-electronics[2]. In the realm of lighting applications, III-Nitride based devices theoretically have a bandgap capable of extending over the range of 0.7 eV to 3.39eV. However, it has proven difficult to realize devices with high crystalline quality and bandgaps that fall within the 2.25 to 2.1 eV range. This problem is partially due to the mismatch in lattice constant between GaN and Indium-Rich GaN crystals. Additionally, during the growth process, Indium desorbs from the crystal surface at the temperatures necessary for high-quality GaN growth[3]. This research chronicles the on-going efforts to utilize super-atmospheric growth conditions to alleviate these, among other, fundamental issues in the growth of Indium-Rich GaN heterostructures. Experiments are conducted within a unique reactor with internal geometry optimized for high-pressure growth. The reactor features an input geometry intended to reduce gas-phase reactions via physical separation of the group III and group V precursors. Furthermore, the chamber walls and exhaust are optimized to encourage laminar flow throughout the reactor.[4] Experiments are constructed to analyze the effect of several growth parameters on the resulting multiple-quantum-well heterostructure. Results are analyzed in terms of internal quantum efficiency measured by the temperature dependence of photo luminescent emission. To reach an acceptable baseline of MQW performance several early growth modifications were analyzed. Among these were the introduction of growth-interruptions at Barrier/Well interfaces, Indium pre-deposition and modification of TMIn waterbath temperature. Initial experimentation indicated that the PL peak red-shifts with increasing TMIn temperature as would be expected. PL intensity is also observed to increase at room temperature when growth interruptions are included, presumably due to an enhanced abruptness of QW/Barrier interfaces. See Figure 1. Furthermore, CFD simulations indicated that susceptor rotational speed could lead to an increase in gas flow velocity in the vicinity of the sample, possibly leading to a decreased boundary layer thickness. Experimentation showed a consistent red-shift of PL with increasing rotation rate that is not readily accountable by the very slight change in growth rates between samples. See Figure 2. Additional ongoing experiments and their results are presented and discussed in the complete body of work. [1] P. Smallwood, in Strategies in Light Conference, Las Vegas, NV, February 2015 [2] Mishra, Umesh K., et al. "GaN-based RF power devices and amplifiers."Proceedings of the IEEE 96.2 (2008): 287-305. [3] Koukitu, A., et al. "THERMODYNAMIC ANALYSIS OF THE MOVPE GROWTH OF IN X GA 1-X N." Journal of Crystal Growth 170 (1997): 306-311. [4] Melton, Andrew G., et al. "Superatmospheric MOCVD Reactor Design for High Quality InGaN Growth." ECS Transactions 45.7 (2012): 73-77. Figure 1

An ultra-thin compliant sapphire membrane for the growth of less strained, less defective GaN

An ultra-thin (26nm) sapphire (Al2O3) membrane was used as a compliant substrate for the growth of high quality GaN. The density of misfit dislocations per unit length at the interface between the GaN layer and the sapphire membrane was reduced by 28% compared to GaN on the conventional sapphire substrate. Threading dislocation density in GaN on the sapphire membrane was measured to be 2.4×108/cm2, which is lower than that for GaN on the conventional sapphire substrate (3.2×108/cm2). XRD and micro-Raman results verifed that the residual stress in GaN on the sapphire membrane was as low as 0.02GPa due to stress absorption by the ultra-thin compliant sapphire membrane.

Optimization of nitrogen plasma source parameters by measurements of emitted light intensity for growth of GaN by molecular beam epitaxy

A comprehensive analysis of operating parameters of Addon RF nitrogen plasma source was made in order to determine how a ratio of different active nitrogen species depends on operating parameters of the source such as supplied power and nitrogen flow. We show that output signal of the optical sensor that measures intensity of the light emitted by the plasma is a direct measure of the amount of active nitrogen available for growth. Results of optical emission spectroscopy and measurements of growth kinetics show that nitrogen excited metastable molecules are the species mainly contributing to the growth of GaN under Ga-rich conditions. A procedure is presented allowing to find an optimal conditions of the plasma cell for high-quality GaN growth. Under these conditions the nitrogen flux contains maximum amount of excited metastable molecules and minimal amount of ionic and atomic nitrogen species to minimize GaN lattice damage, even at high growth rates.