Substrate For GaN Epitaxial Growth Research Articles

Suspended-ultrathin Si membrane on SOI: a novel structure to reduce thermal stress of GaN epilayer

A 30nm-thick suspended-ultrathin silicon membrane based on SOI (silicon-on-insulator) substrate for GAN epitaxial growth is fabricated and analysed. We demonstrate that thermal stress of GaN in the effective area on this novel substrate can be reduced 44% more than that in normal SOI substrate, which can reduce the dislocation and crack density in GaN. The absence of BOX (buried oxide) layer are confirmed as main factors to result in reduction of thermal stress in GaN layer. This work provides a promising approach to obtain high-quality GaN by utilizing mechanical structure.

Lattice-matched GaN(0002)/LSAT(111) hetero-structures grown by PLD and analyzed by first-principles calculations

La0.3Sr1.7AlTaO6(LSAT), a desired substrate for GaN epitaxial growth with a lattice mismatch as small as 1%, has not been applied in growth of GaN films due to hetero-interface rotation in epitaxial growth. In this paper, high-quality GaN epitaxial films with a small FWHM for GaN(0002) X-ray rocking curve of 0.06° have been grown by pulsed laser deposition (PLD) with a lattice mismatch as small as 1% between GaN and LSAT, and the corresponding mechanism avoiding interface rotation has also been investigated by first-principles calculation. On the one hand, we found that GaN grown on LSAT by PLD shows an in-plane relationship of GaN[2-1-10]//LSAT[1-10], revealing a small lattice mismatch of 1%. On the other hand, the first-principles calculation based on density function theory reveals that GaN growth along GaN[2-1-10]//LSAT[1-10] shows a higher interface energy barrier compared with that along GaN[1-100]//LSAT[1-10] with a lattice mismatch of 12.5%. The high interface energy barrier can only be provided by PLD. Moreover, this work is of paramount importance for understanding the hetero-interface formation mechanism in a GaN/substrates system and will be beneficial to the development of GaN-based devices that require high-quality GaN epitaxial films.

Properties of single-phase gallium nitride (GaN) films formed using a Ga(S2CN(CH3)2)3 nanoparticle solution

Ga(S2CN(CH3)2)3 (Ga(DmDTC)3) nanoparticles (diameter: ∼28.5 nm) were synthesized using an ultrasonic spray method, and Ga(DmDTC)3 films were deposited using a spin coating technique. The GaN films were formed by the pyrolytic transformation of Ga(DmDTC)3 films under a NH3 atmosphere at 850 °C for 10 min. The as-formed GaN films exhibited a hexagonal Wurtzite structure with a (0002) preferred orientation and showed strong photoluminescence centered at 2.95 eV. The Ga to N elemental ratio was 1:0.9 with sulfur detected in small amounts in the XPS spectrum. This result showed that almost all of the sulfur atoms in the Ga2S3 were replaced with nitrogen atoms, and the remaining sulfur atoms formed sulfur-doped GaN (GaN:S) under the NH3 environment. Furthermore, epitaxial GaN films on the pyrolytic GaN films, which were formed by MOCVD, exhibited lateral growth, whereas the GaN films on bare sapphire substrates were grown upward toward the z-axial direction. The developed method in this study is a simple technique to form single-phase GaN films using a liquid precursor and can be useful as a semi-homo substrate for GaN epitaxial growth.

HF 크리닝 처리한 코발트실리사이드 버퍼층 위에 PA-MBE로 성장시킨 GaN의 에피택시

실리콘 기판에 GaN 에피성장을 확인하기 위해, P형 Si(100) 기판 전면에 버퍼층으로 10 nm 두께의 코발트실리사이드를 형성시켰다. 형성된 코발트실리사이드 층을 HF로 크리닝하고, PA-MBE (plasma assisted-molecular beam epitaxy)를 써서 저온에서 500 nm의 GaN를 성막하였다. 완성된 GaN은 광학현미경, 주사탐침현미경, TEM, HR-XRD를 활용하여 특성을 확인하였다. HF 크리닝을 하지 않은 경우에는 GaN 에피택시 성장이 진행되지 않았다. HF 크리닝을 실시한 경우에는 실리사이드 표면의 국부적인 에칭에 의해 GaN성장이 유리하여 모두 GaN <TEX>$4\;{\mu}m$</TEX> 정도의 두께를 가진 에피택시 성장이 진행되었다. XRD로 GaN의 <0002> 방향의 결정성 (crsytallinity)을 <TEX>$\omega$</TEX>-scan으로 판단한 결과 Si(100) 기판의 경우 2.7도를 보여 기존의 사파이어 기판 정도로 우수할 가능성이 있었다. 나노급 코발트실리사이드를 버퍼로 채용하여 GaN의 에피성장이 가능할 수 있었다. We fabricated 10 nm-thick cobalt silicide(<TEX>$CoSi_2$</TEX>) as a buffer layer on a p-type Si(100) substrate to investigate the possibility of GaN epitaxial growth on <TEX>$CoSi_2/Si(100)$</TEX> substrates. We deposited 500 nm-GaN on the cobalt silicide buffer layer at low temperature with a PA-MBE (plasma assisted-molecular beam epitaxy) after the <TEX>$CoSi_2/Si$</TEX> substrates were cleaned by HF solution. An optical microscopy, AFM, TEM, and HR-XRD (high resolution X-ray diffractometer) were employed to determine the GaN epitaxy. For the GaN samples without HF cleaning, they showed no GaN epitaxial growth. For the GaN samples with HF cleaning, they showed <TEX>$4\;{\mu}m$</TEX>-thick GaN epitaxial growth due to surface etching of the silicide layers. Through XRD <TEX>$\omega$</TEX>-scan of GaN <0002> direction, we confirmed the cyrstallinity of GaN epitaxy is <TEX>$2.7^{\circ}$</TEX> which is comparable with that of sapphire substrate. Our result implied that <TEX>$CoSi_2/Si(100)$</TEX> substrate would be a good buffer and substrate for GaN epitaxial growth.

β-Ga2O3 单晶NH3中氮化的研究

Nitridated beta-Ga2O3 (100) substrate was investigated as the substrate for GaN epitaxial growth. The effects of nitridation temperature and surface roughness of beta-Ga2O3 wafers on the formation of GaN were studied. It was found that the most optimized nitridation temperature was 900 centigrade, and hexagonal GaN with preferred orientation was produced on the well-polished wafer. The nitridation mechanism was also discussed.

Fabrication of hexagonal GaN on the surface of β-Ga 2O 3 single crystal by nitridation with NH 3

The fabrication of GaN on the surface of a bulk β-Ga 2O 3 single crystal by nitridation with NH 3 was investigated for the purpose of using it as a substrate for GaN epitaxial growth. A β-Ga 2O 3 single crystal was prepared using a floating zone furnace with double ellipsoidal mirrors, and its polished (100) plane was nitridated in NH 3 atmosphere at 850 °C for 5 h. It was found that hexagonal GaN with preferred in-plane orientation was produced on the surface of β-Ga 2O 3, and the thickness of nitride layers was approximately 50 nm. High resolution transmission electron microscopic observation indicated that the synthesized GaN was composed of the aggregation with single crystalline GaN particles, whose size ranged from ∼ 5 nm to ∼ 50 nm, and dislocation or defect was not observed in a GaN particle. This method could be expected as a new route to fabricate a substrate for epitaxial growth of III-nitride materials instead of using a bulk GaN single crystal.

Growth and characterization of the perovskite-type oxides (NdSr)(AlNb)O 3 and (LaSr)(GaNb)O 3 as substrates for GaN epitaxial growth

New compositions for perovskite-type oxides (Nd 0.15Sr 0.85)(Al 0.575Nb 0.425)O 3 (NSAN) and (La 0.15Sr 0.85)(Ga 0.575Nb 0.425)O 3 (LSGN) were investigated from solid state reactions and grown by the Czochralski method as substrates for GaN epitaxial growth. Both compounds crystallized in the cubic system. The lattice mismatch with GaN and the linear expansion coefficients were investigated. Both crystals have a fairly small lattice mismatch with GaN. Therefore, they are expected to be good substrates for the growth of high-quality GaN epitaxy.

New oxide crystal (La,Sr)(Al,Ta)O 3 as substrate for GaN epitaxy

A new candidate single crystal (La,Sr)(Al,Ta)O 3 (LSAT) as a substrate for GaN epitaxial growth was found. Mixed-perovskite (1 1 1) oriented crystals were successfully grown by conventional Czochralski method. Excellent lattice matching with gallium nitride makes this material specially promising. Determination of the lattice parameters and the phase identification obtained crystals by X-ray diffraction analysis was carried out. Thermal expansion, thermal conductivity and electrical conductivity were measured. GaN epitaxial layers have been grown on (1 1 1) LSAT substrate and the results are presented.

Growth of perovskite-type oxides (RE,Sr)(Al,Ta)O3 as substrates for GaN epitaxial growth (RE=La,Nd)

Abstract New compositions for perovskite-type oxides (RE, Sr)(Al, Ta)O3 (RE=La, Nd) were investigated from solid-state reactions and grown by Czochralski method as substrates for GaN epitaxial growth. These compounds crystallize in a cubic system. The lattice mismatch with GaN is in the range of 1.24–1.70%. The linear expansion coefficients are around 10×10−6 K−1. Hydride vapor phase epitaxy growth of GaN was performed on (La, Sr)(Al, Ta)O3 substrates. Free-standing GaN wafers ∼300 μm thick were obtained.

Growth and Characterization of (La,Sr)(Al,Ta)O3 Single Crystals: a Promising Substrate for GaN Epitaxial Growth

Experimental results on the growth and characterization of single crystals of mixed perovskite (La,Sr)(Al,Ta)O 3 (LSAT), a promising material to prepare substrates for GaN epitaxial layers, growth by the Czochralski method, are reported. The single crystals obtained along orientation up to 20 mm in diameter and 50 mm in length were free of inclusions and macroscopic defects. The optical transmission was measured in the region 200 nm- 3000 nm. For the of oriented LSAT single crystals, its measured unit cell parameter (a = 7.730 A) matches well to the GaN lattice. The chemical composition was checked by electron microprobe analysis. It was found that it is close to the stoichiometric one. With Lang transmission topography growth striations and dislocations were examined. Data on Vickers microhardness, thermal expansion coefficient, etch pit density and temperature dependence of electrical conductivity and capacitance are reported.

Growth of Ca 8La 2(PO 4) 6O 2 single crystals as substrates for GaN epitaxial growth

〈0 0 0 1〉 oriented Ca 8La 2(PO 4) 6O 2 (CLPA) single crystals with the apatite structure are grown by the Czochralski method. Compositional and lattice parameter uniformity of CLPA is discussed in relation to the growth conditions. Since their lattice constant have excellent matching with three times that of GaN, they are candidates as new substrates for the growth of high-quality GaN epitaxial layers.

Growth and characterization of (La,Sr)(Al,Ta)O 3 single crystals as substrates for GaN epitaxial growth

We have successfully grown 〈1 1 1〉 oriented (La,Sr)(Al,Ta)O 3 (LSAT) mixed-perovskite single crystals by the Czochralski method. Compositional uniformity and coloring phenomena are discussed in relation to the growth conditions. Since LSAT single crystals have excellent lattice matching with GaN, they are promising as new substrates for the growth of high-quality GaN epitaxial layers.

ECR-MBE growth of GaN on LiGaO2

GaN thin films were grown at relatively low temperatures on multi-domain LiGaO2 substrates by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE). LiGaO2 is one of the most promising substrates for GaN epitaxial growth because of its small lattice mismatch of around 0.9%. The multi-domain LiGaO2 substrate has two different polarity domains in the (001) surface. Two different GaN surface morphologies depending on the surface polarities were observed by atomic force microscopy (AFM). The difference was also observed in the shift of photoluminescence (PL) spectra. Each peak corresponding to two inversion domains was shifted in parallel around 2nm. One possible explanation for the cause of the peak wavelength shift is due to the difference of the strains in GaN thin films.

GaN epitaxial growth on neodium gallate substrates

We have investigated the feasibility of neodium gallate (NdGaO 3, NGO) as substrates for GaN epitaxial growth for the first time. GaN films deposited on neodium gallate substrates by the hydride vapor phase epitaxy technique have been found to be single-crystalline with the epitaxial relationship of GaN(0001)|NGO(011) and GaN[10 1 0]|NGO[100], where the lattice mismatch between the film and the substrate is less than 2%. Photoluminescence (PL) of GaN NGO films at room temperature showed a strong band edge emission with little emission in the longer wavelength region. The carrier concentration and the electronmobility of GaN NGO films were 7 × 10 19 cm −3 and 45 cm 2/Vs, respectively. These results are superior to those of GaN/sapphire films deposited simultaneously under the same conditions.

Epitaxial Growth and Orientation of GaN on (1 0 0) γ-LiAlO2

The (1 0 0) face of γ-LiAlO2 has attracted attention as a possible substrate for GaN epitaxial growth. This is partly because this face has an excellent lattice and structural match to (1 0 0) GaN. This orientation would have a misfit of only −1.4% along the c-direction and −0.1% along the b-direction of LiAlO2. We find that in practice this orientation relationship does not occur; instead, (0 0 0 1) oriented GaN grows with a small tilt (0.6° towards the c-direction) between the film and substrate. Although the misfit along the substrate b direction is large (−6.3%) for this orientation, the tilt perfectly accommodates the −1.4% misfit in the c direction. We present characterization of these films by RHEED, X-ray diffraction, and TEM. We propose that the tilt is driven by a reduction of interface energy which occurs in polar, incoherent interfaces.