Growth and characterization of GaN thin films on SiC SOI substrates

SiC semiconductor-on-insulator (SOI) structures have been investigated as substrates for the growth of GaN films. The SiC SOI was obtained through the conversion of Si SOI wafers by reaction with propane and H₂. (111) SiC SOI have been produced by this carbonization process at temperatures ranging from 1200 to 1300°C. X-ray diffraction (XRD) and infrared spectroscopy (FTIR) are used to chart the conversion of the Si layer to SiC. Under our conditions, growth time of 3 min at 1250°C is sufficient to completely convert a 1000? layer. XRD of the SiC SOI reveals a single SiC peak at 2θ = 35.7° corresponding to the (111) reflection, with a corrected full width at half-maximum (FWHM) of ~590±90 arc-sec. Infrared spectroscopy of SiC SOI structures obtained under optimum carboniza-tion conditions exhibited a sharp absorption peak produced by the Si-C bond at 795 cm⁻¹, with FWHM of ∼ 20–25 cm⁻¹. Metalorganic CVD growth of GaN on the (111) SiC SOI was carried out with trimethylgallium and NH₃. The growth of a thin (≤200?), low temperature (500°C) GaN buffer layer was followed by the growth of a thick (∼2 μm) layer at 1050°C. Optimum surface morphology was obtained for zero buffer layer. XRD indicates highly oriented hexagonal GaN, with FWHM of the (0002) peak of ~360±90 arc-sec. Under high power excitation, the 300°K photoluminescence (PL) spectrum of GaN films exhibits a strong near band-edge peak (at λ_p~371 nm, with FWHM = 100–150 meV) and very weak yellow emission. Under low power excitation, the 370 nm PL emission from the GaN/SiC SOI structure increases rapidly with SiC carbonization temperature, while the yellow band (∼550–620 nm) correspondingly decreases.     

可用于Ⅲ族氮化物生长的50mm 3C-SiC/Si(111)衬底的制备

利用新研制出的垂直式低压CVD(LPCVD)SiC生长系统,获得了高质量的50mm 3C-SiC/Si(111)衬底材料.系统研究了3C-SiC的n型和p型原位掺杂技术,获得了生长速率和表面形貌对反应气体中SiH4流量和C/Si原子比率的依赖关系.利用Hall测试技术、非接触式方块电阻测试方法和SIMS,分别研究了3C-SiC的电学特性、均匀性和故意调制掺杂的N浓度纵向分布.利用MBE方法,在原生长的50mm 3C-SiC/Si(111)衬底上进行了GaN的外延生长,并研究了GaN材料的表面、结构和光学特性.结果表明3C-SiC是一种适合于高质量无裂纹GaN外延生长的衬底或缓冲材料.     

Insulating substrates for cubic GaN-based HFETs

The growth of cubic group III-nitrides is a direct way to eliminate polarization effects, which inherently limits the fabrication of normally off heterojunction field-effect transistors (HFETs) in the GaN technology. However, for the achievement of electronic devices with cubic nitrides an important precondition is the availability of a high-resistive substrate or GaN buffer layer with zinc-blende crystal structure. We investigated the applicability of carbonized high resistance Si (0 0 1)-substrates and thick conductive free-standing 3C-SiC (1 0 0) substrates with an Ar⁺-ion-damaged surface layer for this purpose and studied the use of carbon-doped GaN buffer layers for electrical insulation. We found that Ar-implantation of 3C-SiC is an appropriate alternative to fabricate insulation layer for cubic GaN (c-GaN) growth and that C-doped GaN buffers introduce non-linear I-V characteristics. The structural properties of c-GaN on Ar-implanted 3C-SiC are comparable to GaN on untreated 3C-SiC whereas on carbonized Si substrates an increase of dislocation density and surface roughness is observed.     

Pendeo-epitaxial growth of gallium nitride on silicon substrates

Pendeo-epitaxy (PE)¹ from raised, [0001] oriented GaN stripes covered with silicon nitride masks has been employed for the growth of coalesced films of GaN(0001) with markedly reduced densities of line and planar defects on Si(111)-based substrates. Each substrate contained previously deposited 3C-SiC(111) and AlN(0001) transition layers and a GaN seed layer from which the stripes were etched. The 3C-SiC transition layer eliminated chemical reactions between the Si and the NH₃ and the Ga metal from the decomposition of triethylgallium. The 3C-SiC and the GaN seed layers, each 0.5 μm thick, were also used to minimize the cracking and warping of the GaN/SiC/silicon assembly caused primarily by the stresses generated on cooling due to the mismatches in the coefficients of thermal expansion. Tilting in the coalesced GaN epilayers of 0.2° was confined to areas of lateral overgrowth over the masks; no tilting was observed in the material suspended above the trenches. The strong, low-temperature PL band-edge peak at 3.456 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on AlN/6H-SiC(0001) substrates.     

Growth of zinc- blende GaN on GaAs (100) substrates at high temperature using low-pressure MOVPE with a Low V/lll molar ratio

Zinc-blende GaN films were grown on GaAs (100) substrates by low-pressure metalorganic vapor phase epitaxy using trimethylgallium or triethylgallium and NH₃. Films grown at lower temperatures contained considerable amounts of carbon, but the carbon concentration was reduced in high temperature growth. When the film was grown at 950°C using triethylgallium and NH₃, its carbon concentration was on the order of 10¹⁷ cm⁻³. The crystalline and optical quality of zinc-blende GaN crystal also improved with high-temperature growth at a low V/III ratio using a thin buffer layer. The films exhibited only one sharp photoluminescence peak at 3.20 eV with a full width at half maximum as low as 70 meV at room temperature.     

Effect of In Situ Thermal Cycle Annealing on GaN Film Properties Grown on (001) and (111) GaAs, and Sapphire Substrates

The effect of in-situ thermal cycle annealing (TCA) has been investigated for GaN growth on GaAs(lOO), GaAs(111) and sapphire substrates. X-ray diffractometry (XRD) and surface morphology studies were performed for this purpose. Enhanced cubic phase characteristics were observed by employing annealingfor GaN layers grown on (001) GaAs. The thickness of the layer subject to annealing is critical in determining the phase of the subsequently grown layer. Thin initial layers appear to permit maintenance of the cubic phase characteristics shown by the substrate, while hexagonal phase characteristics are manifested for thick initial layers. Higher temperature of annealing of thick pre-annealed layers results in changes from mixed cubic/hexagonal phase to pure hexagonal phase. Growth on GaAs(111) substrates showed single cubic phase characteristics and similar enhancement of crystal quality by using TCA as for layers on GaAs(OOl). Micro-cracks were found to be present after TCA on GaAs(lll) substrates. Thermal cycling also appears to be beneficial for layers grown on sapphire substrates.     

采用ECR-PEMOCVD方法进行立方和六方GaN单晶薄膜生长的准热力学模型和相图

基于热力学平衡理论,对在电子回旋共振等离子体增强金属有机化学气相沉积系统中的GaN薄膜生长给出了一个化学平衡模型.计算表明,GaN生长的驱动力Δp是以下生长条件的函数:Ⅲ族输入分压,输入Ⅴ/Ⅲ比,生长温度.计算了六方和立方GaN的生长相图,计算结果和我们的实验结果显示出一定的一致性.通过分析,解释了高温和高Ⅴ/Ⅲ比生长条件适合六方GaN的原因.上述模型可以延伸到用于GaN单晶薄膜生长的类似系统中.     

Scanning electron microscopy and cathodoluminescence study of the epitaxial lateral overgrowth (ELO) process for gallium nitride

Traditional epitaxial growth of GaN by metalorganic vapor phase epitaxy (MOVPE) on mismatched substrates such as sapphire or SiC produces a columnar material consisting of many hexagonal grains ∼0.2–1.0 μm in diameter. The epitaxial-lateral-overgrowth (ELO) process for GaN creates a new material: single-crystal GaN. We have studied the ELO process for GaN grown by MOVPE in a vertical flow rotating substrate reactor. Characterization consisted of plan-view SEM and vertical-cross-section TEM studies, which revealed a large reduction in dislocation density in the overgrown regions of the GaN. Panchromatic and monochromatic cathodoluminescence images and spectra were used to study the spatial variation of the optical properties within the GaN ELO samples. The effects of growth temperature and stripe material on the overgrown layers were examined. Through the use of a higher substrate temperature during growth and the use of a SiN_x stripe material, the overgrown crystal shape has a smooth 2D top surface with vertical sidewalls. Applying a second ELO step, rotated by 60°, over a fully coalesced ELO layer yields a further reduction of defects in GaN overgrown surfaces.     

Interfacial effects during GaN growth on 6H-SiC

GaN growth on 6H-SiC was investigated for heterojunction device applications. Dopant diffusion and surface reactions were discovered at the GaN/SiC heterojunction. A systematic study was therefore conducted focusing on: 1) SiC substrate preparation, 2) SiC nitridation; the effect of flowing ammonia (NH₃) at 1050°C on the SiC, and 3) the conductivity type and carrier concentration of the SiC substrate. Atomic force microscopy measurements revealed that the SiC substrates became smoother after the nitridation process possibly due to nitrogen chemisorption and etching. Current-voltage and capacitance-voltage measurements on Cr-Schottky diodes made on SiC revealed evidence for an increased potential barrier in the nitrided samples that can be explained by an interfacial monolayer ofSiN_x. Furthermore, we compared GaN/SiC heterojunction n-n and n-p diodes made from direct and selective GaN growth. Capacitancevoltage measurements on GaN/SiC n-p heterojunctions indicate that the effective doping in the junction increases as the growth temperature increases. Secondary ion mass spectrometry measurements exposed a tail of Al in the GaN due to acceptor out-diffusion from the p-SiC.     

Characterization of AlGaN/GaN structures on various substrates grown by radio frequency-plasma assisted molecular beam epitaxy

The structural properties and surface morphology of AlGaN/GaN structures grown on LiGaO₂ (LGO), sapphire, and hydride vapor phase epitaxy (HVPE)-grown GaN templates are compared. AlGaN grown on LGO substrates shows the narrowest x-ray full width at half maximum (FWHM) for both symmetric 〈00.4〉 and asymmetric 〈10.5〉 reflections. Atomic force microscopy (AFM) analysis on AlGaN surfaces on LGO substrates also show the smoothest morphology as determined by grain size and rms roughness. The small lattice mismatch of LGO to nitrides and easily achievable Ga-polarity of the grown films are the primary reasons for the smoother surface of AlGaN/GaN structure on this alternative substrate. Optimizations of growth conditions and substrate preparation results in step flow growth for an AlGaN/GaN structure with 300 Å thick Al_0.25Ga_0.75N on 2.4 μm thick GaN. A high III/V flux ratio during growth and recently improved polishing of LGO substrates aids in promoting two dimensional step flow growth. The GaN nucleation layer directly on the LGO substrate showed no evidence of mixed phase cubic and hexagonal structure that is typically observed in the nucleation buffer on sapphire substrates. Cross-sectional high-resolution transmission electron microscopy (HRTEM) was performed on an AlGaN/GaN heterostructure grown on LGO. The atomic arrangement at the AlGaN/GaN interface was sharp and regular, with locally observed monolayer and bilayer steps.     

The growth and properties of mixed group V nitrides

Bearing in mind the problems of finding a lattice-matched substrate for the growth of binary group III nitride films and the detrimental effect of the large activation energy associated with acceptors in GaN, we propose the study of the alloy system AlGaAsN. We predict that it may be possible to obtain a direct gap alloy, with a band gap as wide as 2.8eV, which is lattice-matched to silicon substrates. The paper reports our attempts to grow GaAsN alloy films by molecular beam epitaxy on either GaAs or GaP substrates, using a radio frequency plasma source to supply active nitrogen. Auger electron spectra demonstrate that it is possible to incorporate several tens of percent of nitrogen into GaAs films, though x-ray diffraction measurements show that such films contain mixed binary phases rather than true alloys. An interesting observation concerns the fact that it is possible to control the crystal structure of GaN films by the application of an As flux during growth. In films grown at 620°C a high As flux tends to increase the proportion of cubic GaN while also resulting in the incorporation of GaAs. Films grown at 700°C show no evidence for GaAs incorporation; at this temperature, it is possible to grow either purely cubic or purely hexagonal GaN depending on the presence or absence of the As beam.     

Quantitative analysis of small amounts of cubic GaN phase in GaN films grown on sapphire

Thin GaN films, grown by metal organic chemical vapor deposition on the basal plane of sapphire substrates, were characterized by x-ray pole figures, high-resolution x-ray diffraction and transmission electron microscopy. This combination was found sensitive to small amounts (down to 0.1%) of cubic GaN phase in specimens subjected to surface nitridation treatment prior to epitaxial growth. The presence of the cubic phase and its orientation relations to the hexagonal GaN matrix was established by means of pole figures and selected area electron diffraction. The amount of cubic phase was determined by comparing the integrated x-ray diffraction intensities of the (311) cubic GaN and the (11.2) hexagonal GaN reflections. Optimum nitridation duration was found, which corresponds to almost complete suppression of the cubic phase formation.     

Growth of GaN on porous SiC and GaN substrates

We have studied the growth of GaN on porous SiC and GaN substrates, employing both plasma-assisted molecular-beam epitaxy (PAMBE) and metal-organic chemical-vapor deposition (MOCVD). For growth on porous SiC, transmission electron microscopy (TEM) observations indicate that the epitaxial-GaN growth initiates primarily from surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. The dislocation density in the GaN layers is similar to, or slightly less than, that observed in layers grown on nonporous substrates. For the case of GaN growth on porous GaN, the overgrown layer replicates the underlying dislocation structure (although considerable dislocation reduction can occur as this overgrowth proceeds, independent of the presence of the porous layer). The GaN layers grown on a porous SiC substrate were found to be mechanically more relaxed than those grown on nonporous substrates; electron-diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature.     

GaSb衬底上分子束外延生长的低温GaSb薄膜的低缺陷表面

本论文系统的研究了,随着GaSb薄膜生长温度的降低,V/III比的变化对薄膜低缺陷表面质量的影响。为了获得良好表面形貌的GaSb外延层,生长温度与V/III比均需要同时降低。当Sb源裂解温度为900℃时,生长得到低缺陷表面的低温GaSb薄膜的最佳生长条件是生长温度为在再构温度的基础上加60℃且V/III比为7.1。     

Si衬底GaN外延材料六角形缺陷分析

通过改变AlN形核层的生长温度分别在Si(111)衬底上生长了两个GaN样品,并对GaN外延材料表面的六角形缺陷进行了分析研究。通过显微镜和扫描电镜(SEM)观测发现,AlN形核层在高温下生长时,GaN材料表面会产生大量六角形缺陷。通过电子能谱(EDS)分析得出GaN六角形缺陷中含有大量的Si元素以及少量的Ga和Al元素,其中Si元素从Si衬底中高温扩散而来。在降低AlN形核层的生长温度后,GaN材料表面的六角形缺陷随之消失。表明AlN形核层在较低的温度下生长时可以有效地抑制Si衬底表面Si原子的扩散,减少外延层中由于衬底Si反扩散引起的缺陷。     

MOVPE production reactors for high temperature electronics

For the fabrication of epitaxial films of silicon carbide or the Group III nitrides, high growth temperatures (up to 1700°C) and fast heating and cooling of the growth environment have been found to be necessary. A range of production systems meeting these requirements has been designed with different loading capacities. In this paper, we present results from various machines showing the high quality and excellent homogeneity obtainable for 3C-SiC on Si and on 6H-SiC, as well as GaN on sapphire.     

Si上外延的n型3C-SiC欧姆接触研究

用LPCVD在Si(111)上异质外延了n型3C-SiC,并在所外延的3C-SiC上蒸发Au/Ti,通过不同温度下的RTA(快速热退火)形成欧姆接触。用两种不同的传输线模型对Ti/3C-SiC欧姆接触的ρc(比接触电阻率)进行测量,在750°C退火后Ti/3C-SiC的ρc达到了最低值为3.68×10-5Ω·cm2,这满足了应用的要求。AES分析结果还表明由于Ti的氧化,更高温度下的退火会使ρc增大。     

基于76.2 mm圆片工艺的GaN HEMT可靠性评估

报道了利用76.2 mm圆片工艺实现了SiC衬底GaN HEMT微波功率管的研制,并对其进行了多项试验以评估其可靠性.器件工艺中通过引入难熔金属作器件肖特基势垒,有效提高了GaN HEMT器件肖特基势垒的热稳定性,经过500℃高温处理30 s后器件肖特基特性依然保持稳定.随后的高温工作寿命试验表明,该GaNHEMT能够...     

Structural and optical characterization of GaN grown on porous silicon substrate by MOVPE

GaN was grown on porous silicon (PS) substrates by Metalorganic Vapour Phase Epitaxy at temperature of 1050 °C. An additional AlN buffer layer is used between GaN and PS. The crystalline quality and surface morphology of GaN films were studied by X-ray diffraction and scanning electron microscope (SEM), respectively. Preferential growth of hexagonal GaN with 〈00.1〉 direction is observed and is clearly improved when the thickness of AlN buffer layer increases. Morphological changes in PS layer appearing after growth have been also discussed.GaN optical qualities were determined by photoluminescence at low and room temperature (RT).     

The effect of GaN and ain buffer layers on GaN film properties grown on both C-plane and A-plane sapphire

This paper presents a comparative study of the properties of GaN grown by organometallic vapor phase epitaxy, using both a GaN and A1N buffer layer, as a function of sapphire orientation (c-plane vs a-plane). Results are presented for varying the thickness of the buffer layer, varying the growth temperature of the GaN film, and also varying the ammonia/trimethylgallium mass flow ratio. The electron Hall mobilities of GaN films grown on an A1N buffer layer were, in general, higher compared to films grown using a GaN buffer layer. In addition, growth on a-plane sapphire resulted in higher quality films (over a wider range of buffer thicknesses) than growth on c-plane sapphire. The room temperature electron mobilities were also found to be dependent on, not only the growth temperature, but also the ammonia/trimethylgallium mass flow ratio.