A new buffer layer for MOCVD growth of GaN on sapphire

High quality GaN films have been grown on sapphire substrates (C face and A face) by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) using a new buffer layer. With our reactor configuration and growth parameters, a GaN film grown on a single GaN buffer layer appears opaque with high density of hexagonal pits. Using a single A1N buffer layer results in extremely nonuniform morphology with mirror-like areas near the edge of the substrates and opaque areas in the center. The double buffer layer we report here, with GaN as the first layer and A1N as the second, each with an optimized thickness, leads to mirror-like films across the entire substrate. Scanning electron microscopy, photoluminescence, x-ray diffraction, and van der Pauw geometry Hall measurement data are presented to establish the quality of our films. The mechanism for this new buffer layer is also discussed.     

Nucleation and growth behavior for GaN grown on (0001) sapphire via multistep growth approach

Formation and coalescence of GaN truncated three dimensional islands (TTIs) on (0001) sapphire are observed during growth of GaN using a close spaced metalorganic chemical vapor deposition reactor. To encourage formation of TTIs to occur uniformly over the buffer layer, growth conditions are chosen under which thermal desorption and/or mass transport of the buffer layer can be suppressed. During coalescence of TTIs, growth conditions that favor higher desorption of species on the GaN (0001) surface and incorporation on other planes are beneficial. Therefore, changing the growth conditions as the growth mode changes is effective to obtain both good crystallinity and flat surface morphology.     

Metalorganic chemical vapor deposition growth of high optical quality and high mobility GaN

Hall mobilities as high as 702 and 1230 cm²/Vs at 300 and 160K along with low dislocation densities of 4.0 × 10⁸ cm^-2 have been achieved in GaN films grown on sapphire by metalorganic chemical vapor deposition. High growth temperatures have been established to be crucial for optimal GaN film quality. Photoluminescence measurements revealed a low intensity of the deep defect band around 550 nm in films grown under optimized conditions.     

Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes

We investigated the electrical and structural qualities of Mg-doped p-type GaN layers grown under different growth conditions by metalorganic chemical vapor deposition (MOCVD). Lower 300 K free-hole concentrations and rough surfaces were observed by reducing the growth temperature from 1,040°C to 930°C. The hole concentration, mobility, and electrical resistivity were improved slightly for Mg-doped GaN layers grown at 930°C with a lower growth rate, and also an improved surface morphology was observed. In_0.25Ga_0.75N/GaN multiple-quantum-well light emitting diodes (LEDs) with p-GaN layers grown under different conditions were also studied. It was found from photoluminescence studies that the optical and structural properties of the multiple quantum wells in the LED structure were improved by reducing the growth temperature of the p-layer due to a reduced detrimental thermal annealing effect of the active region during the GaN:Mg p-layer growth. No significant difference in the photoluminescence intensity depending on the growth time of the p-GaN layer was observed. However, it was also found that the electroluminescence (EL) intensity was higher for LEDs having p-GaN layers with a lower growth rate. Further improvement of the p-GaN layer crystalline and structural quality may be required for the optimization of the EL properties of long-wavelength (∼540 nm) green LEDs.     

Properties of GaN epitaxial layers grown at high growth rates by metalorganic chemical vapor deposition

GaN epitaxial layers were grown at high growth rates by increasing the input trimethylgallium (TMG) flow rate while keeping the NH₃ flow rate constant in metalorganic chemical vapor deposition. The electrical and optical properties of the grown layers have been investigated. With the increasing TMG flow rate, the electron concentration tends to decrease gradually and the Hall mobility decreases significantly. Considering the temperature dependence of the Hall mobility and the correlation between the Hall mobility and the electron concentration, it has been indicated that the more acceptors are incorporated and consequently the compensation ratio becomes higher with increasing the TMG flow rate. Photoluminescence measurements have revealed that the intensity ratio of the bound exciton emission to the 2.2 eV band emission, which is assumed to correlate to carbon or Ga vacancies, was decreased with increasing the TMG flow rate. It might be reasonable to take a lot of acceptor incorporation to explain the degradation of the electrical and optical properties in the samples grown at high growth rates by increasing the TMG flow rate.     

High-Quality GaN heteroepitaxial films grown by metalorganic chemical vapor deposition

In this paper, we describe the growth and characterization of high-quality GaN heteroepitaxial films grown on basal-plane sapphire substrates using metalorganic chemical vapor deposition. The quality of these films is analyzed by a variety of methods, including high-resolution x-ray diffraction, optical transmission spectroscopy, transmission electron microscopy (TEM), room temperature photoluminescence, and room-temperature Hall measurements. The x-ray diffraction full width at half maximum value of ΔΘ ～37 arc s is the narrowest reported to date for any III-V nitride film on any substrate. The x-ray rocking curves for ～0.48 μm thick GaN/Al₂O₃ heteroepitaxial layers exhibit Pendellösung fringes, indicating that even relatively thin films can be of high quality. High-resolution TEM lattice images further attest to the excellent structural quality, showing the films to be completely free of stacking faults. Furthermore, no evidence of columnar growth is observed.     

AlGaN缓冲层结构对Si基GaN材料性能的影响

Si衬底与GaN之间较大的晶格失配和热失配引起的张应力使GaN外延层极易产生裂纹,如何补偿GaN所受到的张应力是进行Si基GaN外延生长面临的首要问题.采用金属有机化合物化学气相沉积(MOCVD)技术在4英寸(1英寸=2.54 cm)Si (111)衬底上制备了GaN外延材料并研究了不同AlGaN缓冲层结构对Si基GaN外延材料性能的影响,并采用高分辨X射线衍射仪(HRXRD)、原子力显微镜(AFM)、喇曼光谱以及光学显微镜对制备的GaN材料的性能进行了表征.采用3层A1GaN缓冲层结构制备了表面光亮、无裂纹的GaN外延材料,其(002)晶面半高宽为428 arcsec,表面粗糙度为0.194 nm.结果表明,采用3层A1GaN缓冲层结构可以有效地降低GaN材料的张应力和位错密度,进而遏制表面裂纹的出现,提高晶体质量.     

A microstructural comparison of the initial growth of AlN and GaN layers on basal plane sapphire and sic substrates by low pressure metalorganic chemical vapor deposition

The initial growth by low pressure metalorganic chemical vapor deposition and subsequent thermal annealing of A1N and GaN epitaxial layers on SiC and sapphire substrates is examined using high resolution transmission electron microscopy and atomic force microscopy. Growth under low pressure conditions on sapphire substrates is significantly different from that reported for conventional (atmospheric pressure) conditions. Smooth, single crystal A1N and GaN layers were deposited on sapphire in the initial low temperature (600°C) growth step. Interfacial bonding and not lattice mismatch was found to be the determin ing factor for obtaining good crystallinity for the epitaxial layers as indicated by the growth results on SiC substrates.     

Characterization of very high purity InAs grown using trimethylindium and tertiarybutylarsine

The growth of high purity InAs by metalorganic chemical vapor deposition is reported using tertiarybutylarsine and trimethylindiμm. Specular surfaces were obtained for bulk 5-10 μm thick InAs growth on GaAs substrates over a wide range of growth conditions by using a two-step growth method involving a low temperature nucleation layer of InAs. Structural characterization was performed using atomic force microscopy and x-ray diffractometry. The transport data are complicated by a competition between bulk conduction and conduction due to a surface accumulation layer with roughly 2–4 × 10¹² cm⁻² carriers. This is clearly demonstrated by the temperature dependent Hall data. Average Hall mobilities as high as 1.2 x 10⁵ cm²/Vs at 50K are observed in a 10 μm sample grown at 540°C. Field-dependent Hall measurements indicate that the fitted bulk mobility is much higher for this sample, approximately 1.8 × 10⁵ cm²/Vs. Samples grown on InAs substrates were measured using high resolution Fourier transform photoluminescence spectroscopy and reveal new excitonic and impurity band emissions in InAs including acceptor bound exciton “two hole transitions.” Two distinct shallow acceptor species of unknown chemical identity have been observed.     

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.     

The impact of nitridation and nucleation layer process conditions on morphology and electron transport in GaN epitaxial films

A systematic study has been performed to determine the characteristics of an optimized nucleation layer for GaN growth on sapphire. The films were grown during GaN process development in a vertical close-spaced showerhead metalorganic chemical vapor deposition reactor. The relationship between growth process parameters and the resultant properties of low temperature GaN nucleation layers and high temperature epitaxial GaN films is detailed. In particular, we discuss the combined influence of nitridation conditions, V/III ratio, temperature and pressure on optimized nucleation layer formation required to achieve reproducible high mobility GaN epitaxy in this reactor geometry. Atomic force microscopy and transmission electron microscopy have been used to study improvements in grain size and orientation of initial epitaxial film growth as a function of varied nitridation and nucleation layer process parameters. Improvements in film morphology and structure are directly related to Hall transport measurements of silicon-doped GaN films. Reproducible growth of silicon-doped GaN films having mobilities of 550 cm²/Vs with electron concentrations of 3 × 10¹⁷ cm⁻³, and defect densities less than 10⁸ cm⁻² is reported. These represent the best reported results to date for GaN growth using a standard two-step process in this reactor geometry.     

MOCVD生长高反射率AlN/GaN分布布拉格反射镜

利用金属有机物化学气相沉积(MOCVD)方法在蓝宝石c面衬底上制备出高反射率AlN/GaN分布布拉格反射镜(DBR).利用分光光度计测量,在418 nm附近最大反射率达到99%.样品表面显微照片显示,有圆弧形缺陷和少量裂纹出现;在缺陷和裂纹以外的区域,DBR具有较为平坦的表面,其粗糙度在10μm×10μm面积上为3.3 m左右.样品的截面扫描电镜(SEM)照片显示,DBR具有良好的周期性.对反射率和表面分析的结果表明,该样品达到了制备GaN基垂直腔面发射激光器(VCSEL)的要求.     

Mass spectroscopy study of GaN metalorganic chemical vapor deposition

Metalorganic chemical vapor phase deposition of GaN on (100) GaAs has been studied using mass spectroscopy. With increasing substrate temperature, the amount of decomposed trimethylgallium (TMGa) was observed to increase exponentially with a characteristic energy of 1.5 eV. The presence of NH₃ was found to suppress the production of CH₃ in the gas phase. This implies that CH₃ of TMGa reacts with the hydrogen atom of NH₃, forming CH₄ as a main gas product. Studies of nitrogen evaporation from the growth surface when TMGa flow was off lead to the conclusion that increased growth rate could result in decreased background electron concentration due to nitrogen vacancy. The presence of NH₃ significantly promotes the decomposition of TMGa. Desorption of excess Ga atoms from the growth surface at low NH₃ flow rates takes place as suggested by the increased ratio of peak intensity of Ga (m/e = 69) to that of DMGa ((CH₃)₂Ga, m/e- 99) with decreasing NH₃ flow rate.     

Comparison of GaN and In<Subscript>0.04</Subscript>Ga<Subscript>0.96</Subscript>N <Emphasis Type="Italic">p</Emphasis>-Layers on the Electrical and Electroluminescence Properties of Green Light Emitting Diodes

We have compared the effects of Mg-doped GaN and In_0.04Ga_0.96N layers on the electrical and electroluminescence (EL) properties of the green light emitting diodes (LEDs). To investigate the effects of different p-layers on the LED performance, the diode active region structures were kept identical. For LEDs with p-InGaN layers, the p-In_0.04Ga_0.96N/GaN polarization-related EL peak was dominant at low current levels, while the multiple-quantum-well (MQW) peak became dominant at higher current levels, different from LEDs with p-GaN layers. Also, LEDs with p-InGaN exhibited slightly higher turn on voltages (V _on ) and forward voltages (V _f ) compared to LEDs with p-GaN layers. However, the MQW related EL intensity was much higher and diode series resistance lower for LEDs with p-InGaN layers compared with LEDs with p-GaN, showing possible improvements in output power for LEDs with p-InGaN layers. The diodes with p-GaN layers typically showed V _f of ∼3.1 V at a drive current of 20 mA, with a series resistance of ∼24.7 Ω, while diodes with p-InGaN showed V _f of ∼3.2 V, with a series resistance of ∼18.5 Ω, for device dimensions of 230 μm by 230 μm.     

Optical third-harmonic investigations of gallium nitride nucleation layers on sapphire

The magnitude of the χ _xxxx ⁽³⁾ element of the third-order optical susceptibility was measured in a series of wurtzite phase GaN nucleation layers (～450Å) deposited on (00.1) sapphire at 540°C and annealed to various temperatures up to 1050°C. The nonlinear optical response exhibited a significant increase in films that were annealed to temperatures in the range of 1015 to 1050°C. In addition, the correlation between the magnitude of χ _xxxx ⁽³⁾ with both the maximum value of the linear absorbance gradient and the residual homogeneous strain in the overlayer suggests that variations in the crystalline content of the film and the bonding distance between the Ga and N atoms are primary factors in determining the third-order nonlinearity in GaN.     

Characteristics of Green Light-Emitting Diodes Using an InGaN:Mg/GaN:Mg Superlattice as p-Type Hole Injection and Contact Layers

Mg-doped InGaN/GaN p-type short-period superlattices (SPSLs) are developed for hole injection and contact layers of green light-emitting diodes (LEDs). V-defect-related pits, which are commonly found in an InGaN bulk layer, can be eliminated in an InGaN/GaN superlattice with thickness and average composition comparable to those of the bulk InGaN layer. Mg-doped InGaN/GaN SPSLs show significantly improved electrical properties with resistivity as low as ∼0.35 ohm-cm, which is lower than that of GaN:Mg and InGaN:Mg bulk layers grown under optimized growth conditions. Green LEDs employing Mg-doped InGaN/GaN SPSLs for hole injection and contact layers have significantly lower reverse leakage current, which is considered to be attributed to improved surface morphology. The peak electroluminescence intensity of LEDs with a SPSL is compared to that with InGaN:Mg bulk hole injection and contact layers.     

锥形图形衬底上氮化镓薄膜生长和表征

GaN films are grown on cone-shaped patterned sapphire substrates(CPSSs)by metal-organic chemical vapor deposition,and the influence of the temperature during the middle stage of GaN growth on the threading dislocation(TD)density of GaN is investigated.High-resolution X-ray diffraction(XRD)and cathodeluminescence(CL)wereusedtocharacterizetheGaNfilms.TheXRDresultsshowedthattheedge-typedislocation density of GaN grown on CPSS is remarkably reduced compared to that of GaN grown on conventional sapphire substrates(CSSs).Furthermore,whenthegrowthtemperatureinthemiddlestageofGaNgrownonCPSSdecreases,the full width at half maximum of the asymmetry(102)plane of GaN is reduced.This reduction is attributed to the enhancement of vertical growth in the middle stage with a more triangular-like shape and the bending of TDs.The CL intensity spatial mapping results also showed the superior optical properties of GaN grown on CPSS to those of GaN on CSS,and that the density of dark spots of GaN grown on CPSS induced by nonradiative recombination is reduced when the growth temperature in the middle stage decreases.     

The growth of AlInSb by metalorganic chemical vapor deposition

We have grown Al_xIn_1−xSb epitaxial layers by metalorganic chemical vapor deposition using tritertiarybutylaluminum (TTBAl), trimethylindium (TMIn), and triethylantimony (TESb) as sources in a high speed rotating disk reactor. Growth temperatures of 435 to 505°C at 200 Torr were investigated. The V/III ratio was varied from 1.6 to 7.2 and TTBAl/(TTBAl+TMIn) ratios of 0.26 to 0.82 were investigated. Al_xIn_1−xSb compositions from x=0.002 to 0.52 were grown with TTBAl/(TTBAl+TMIn) ratios of 0.62 to 0.82. Under these conditions, no Al was incorporated for TTBAl/(TTBAl+TMIn) ratios less than 0.62. Hall measurements of Al_xIn_1−xSb showed hole concentrations between 5×10¹⁶ cm⁻³ to 2 × 10¹⁷ cm⁻³ and mobilities of 24 to 91 cm²/Vs for not intentionally doped Al_xIn_1−xSb.     

红橙光InGaN/GaN量子阱的结构与光学性质研究

采用金属有机物化学气相沉积(MOCVD)技术生长了具有高In组分InGaN阱层的InGaN/GaN多量子阱(MQW)结构,高分辨X射线衍射(HRXRD)ω-2θ扫描拟合得到阱层In含量28%。比较大的表面粗糙度表明有很大的位错密度。室温下光致荧光(PL)研究发现该量子阱发射可见的红橙光,峰位波长在610 nm附近。变温PL(15～300 K)进一步揭示量子阱在低温下有两个发光机制,对应的发射峰波长分别为538 nm和610 nm。由于In分凝和载流子的局域化导致的载流子动力改变,使得量子阱PL发光峰值随温度增加呈明显的"S"变化趋势。     

Effect of Se-doping on deep impurities in AlxGa1−xAs grown by metalorganic chemical vapor deposition

We have studied the effect of Se-doping on deep impurities in Al_xGa_1−xAs (x = 0.2∼0.3) grown by metalorganic chemical vapor deposition (MOCVD). Deep impurities in various Se-doped Al_xGa_1−xAs layers grown on GaAs substrates were measured by deep level transient spectroscopy and secondary ion mass spectroscopy. We have found that the commonly observed oxygen contamination-related deep levels at E_c-0.53 and 0.70 eV and germanium-related level at E_c-0.30 eV in MOCVD grown Al_xGa_1−xAs can be effectively eliminated by Se-doping. In addition, a deep hole level located at E_y + 0.65 eV was found for the first time in Se-doped Al_xGa_1-xAs when Se ≥2 × 10¹⁷ cm⁻³ or x ≥ 0.25. The concentration of this hole trap increases with increasing Se doping level and Al composition. Under optimized Se-doping conditions, an extremely low deep level density (N_t less than 5 × 10¹² cm⁻³, detection limit) Al_0.22Ga_0.78As layer was achieved. A p-type Al_0.2Ga_0.8As layer with a low deep level density was also obtained by a (Zn, Se) codoping technique.