Growth of high-quality thick GaN using a tungsten interlayer

A high-quality GaN film was (W-GaN) grown by hydride vapor phase epitaxy (HVPE) on metalorganic chemical vapor deposition (MOCVD) GaN templates with a tungsten (W) interlayer.A sample without interlayer was also grown at the same time for comparison.Significant reductions of dislocation density in W-GaN film is confirmed by the result of high-resolution X-ray diffraction and transmission electron microscope (TEM) observation.The improvement of optical properties of the W-GaN is confirmed by photoluminescence (PL) result.A shift of PL peak suggests that the strain is lower in the W-GaN than the film without W interlayer.This technique offers a potential path to obtain high-quality GaN film as free-standing substrate.     

A study on the minority carrier diffusion length in n-type GaN films

The minority carrier diffusion length of n-type GaN films grown by metalorganic chemical vapor deposition (MOCVD) has been studied by measuring the surface photovoltaic (PV) spectra. It was found that the minority carrier dif- fusion length of undoped n-type GaN is considerably larger than that in lightly Si-doped GaN. However, the data suggested that the dislocation and electron concentration appear not to be responsible for the minority carrier diffusion length. It is suggested that Si doping plays an important role in decreasing the minority carrier diffusion length.     

Developing nitride-based blue LEDs on SiC substrates

Blue light-emitting diodes (LEDs) have consistently increased in brightness as devices have evolved from the homojunction SiC device to the double heterojunction GaN-based LED on SiC substrates. These LEDs are used in a wide range of applications requiring blue, white, and/or a combination of colors. The technology to develop the nitride devices involves growing single-crystal thin films with compositions from AlN-InN-GaN via metalorganic chemical vapor deposition on single-crystal 6H-Sic substrates. In this study, AlGaN containing high and low fractions of aluminum was grown directly on the SiC for use as a buffer layer. Subsequent epitaxial layers of GaN and AlGaN were doped with magnesium and silicon to achieve p-type and n-type conductivity, respectively. N-type InGaN layers with indium compositions up to ～50% were also achieved.     

Engineering Design of High Quality GaN Epitaxy on Silicon Substrate Using Varying GaN/AlGaN Composite Buffer Structures

High quality GaN epitaxy thin films have been desired for the energy-efficient,solid-state semiconductor illuminating devices.Silicon substrates offer high crystal quality,low wafer cost,large wafer size,and potential integration with the well-established silicon processing technologies.However,due to the large mismatch in lattice constants and thermal expansion coefficients,it is still challenging to grow high quality GaN on silicon substrates.In this study,high quality GaN epitaxy has been engineering designed to grow on Si(111)substrate using varying GaN/AlGaN composite buffer structures by an Axitron 200 metal-organic vapor phase epitaxy deposition system.A thin AlN seed layer of 25 nm was firstly grown at 720℃.AlGaN layer of different thickness was then grown at 1050℃with subsequent GaN thin film until the total thickness reached 500 nm.The thickness of the subsequent GaN thin film could be increased by reducing the AlGaN thickness in the composite buffer structures.The results have shown that the lower GaN/AlGaN thickness ratio would decrease the dislocation density and provide crack-free,mirror-like upper GaN crystal thin film.On the other hand,the GaN/AlGaN thickness ratio could be designed to be 2-6 to balance the processing cost and the thin film quality for engineering applications.The dislocation density has been about 2×10 9 cm-2.In addition,dislocation close loop was observed near the GaN/AlGaN interface.The annihilation mechanism could be depicted by the reduction in dislocation strain energy.     

MOCVD生长GaN力学性能研究

采用MOCVD技术在蓝宝石衬底（0001）面上生长了GaN外延膜，利用原子力显微镜AFM、扫描电镜SEM分析了薄膜表面形貌，利用纳米压痕仪和UMT试验机考察了GaN膜的硬度、临界载荷以及摩擦学性能等。结果表明，薄膜以二维模式均匀生长，表面平整，硬度达22．1MPa，弹性模量为299．5GPa，与衬底结合紧密，临界载荷达1．6N，与GCr15钢球对磨时摩擦系数仅为0．13，与Si3N4陶瓷球摩擦时膜很快就磨穿。     

ZnO／Ga2O3膜的氨化温度对制作硅基GaN纳米材料的影响

通过在不同温度下氨化ZnO／Ga2O3膜，在Si衬底上成功制备了GaN纳米结构材料。氨化前，ZnO层和Ga2O3膜分别通过射频磁控溅射法依次溅射到Si衬底上。用X射线衍射(XRD)、红外傅里叶变换光谱(FTIR)分析了GaN晶体的结构和组分，利用扫描电子显微镜(SEM)观察了样品的形貌。通过对测试结果的分析可知在Si衬底上由ZnO挥发辅助生长出六方纤锌矿GaN纳米结构晶体，并且ZnO／Ga2O3的氨化温度对形成GaN纳米材料具有明显的影响。     

Growth behaviors of GaN/Si heteroepitaxy with various terrace widths grown by the LEPS method

Epilayers of GaN were grown on patterned Si (111) substrates of various terrace widths by means of metal organic chemical vapor deposition. The technique of lateral epitaxy on a patterned substrate used the growth of GaN epilayers from the periodic and parallel stripes that form as a result of the substrate etching. Silicon substrates were patterned for various terrace widths of 3 μm, 8 μm, and 18 μm. A low temperature AlN was used as a seed layer for the growth of a 1.5 μm thick GaN epilayer. The as-grown samples were characterized by using double-crystal X-ray diffractometry (DCXRD), photoluminescence and atomic force microscopy (AFM). From the DCXRD spectra, the full width at half maximum (FWHM) of the samples was found to decrease as the terrace width decreased. This behavior indicates that there is an improvement in the crystalline quality of the GaN epilayers as the terrace width decreases. The photoluminescence spectra reveal a decrease in the FWHM and an increase in the peak intensity as the terrace width decreases. This behavior indicates that there is an improvement in the optical quality of the GaN epilayer as the terrace width decreases. The atomic force micrographs reveal a dislocation-free homogeneous surface in the trench region compared to the terrace region with defects such as pits and dislocations. The results clearly show that GaN epilayers grown on a patterned Si substrate with a terrace width 3 μm have a good crystalline quality with minimal threading dislocation and excellent band edge emission.     

An experimental study about the influence of well thickness on the electroluminescence of InGaN/GaN multiple quantum wells

The influence of well thickness on the electroluminescence (EL) of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition is investigated. It is found that the peak wavelength of EL increases with the increase of well thickness when the latter is located in the range of 3.0–5.1 nm. The redshift is mainly attributed to the quantum confined Stark effect (QCSE). As a contrast, it is found that the EL intensity of InGaN/GaN MQWs increases with the increase of well thickness in spite of QCSE. The result of X-ray diffraction demonstrates that the interface become smoother with the increase of well thickness and suggests that the reduced interface roughness can be an important factor leading to the increase of EL intensity of InGaN/GaN MQWs.     

Effect of AlN intermediate layer on growing GaN film by hydride vapor phase epitaxy

Thick GaN layer deposited by hydride vapor phase epitaxy (HVPE) on a metalorganic chemical vapor deposition (MOCVD) GaN template with a thin low temperature (LT) AlN intermediate layer was investigated.High resolution X-ray resolution diffraction (HRXRD) shows that the crystalline quality of thick GaN layer was improved compared with the template.As confirmed by atomic force microscopy (AFM) observations, the surface morphology of AlN intermediate layer helps to improve the nucleation of GaN epilayer.Photoluminescence (PL) spectra measurement shows its high optical quality and low compressive stress, and micro Raman measurement confirms the latter result.Thus, the deposition of the LT-AlN interlayer has promoted the growth of an HVPE-GaN layer with an excellent crystalline quality.     

Effect of the substrate and alignment on the strain distribution in periodic QD structures

In this study we have investigated theoretically the strain fields of cubic GaN quantum dot superlattices embedded in zinc-blend AlN induced by the relative position of quantum dots and by the substrate composition. We have produced atomistic structures that model nanostructures grown along [0 0 1] direction on a GaN/AlN alloy substrate. First we found that it is energetically favorable for the quantum dots to be vertically aligned in consecutive dot layers. Also there is a similar but less intense preference for the dots of the same layer to form a square two-dimensional lattice. These two results are in accordance with experimental data for GaN/AlN quantum dots but also with different compounds. We found that due to the inclusion of QDs, the AlN matrix expands but to a lesser degree compared to a random alloy with the same atomic composition. With the energetically favorable quantum dots arrangement, an effective wire-like strained AlN structure is created, parallel to the growing axis, which might have influence in the electronic structure and optical properties of the quantum dots array. This structure depends on the substrate composition and disappears when the content of GaN in the substrate is increased.     

热壁化学气相沉积Si基GaN晶体膜的研究

采用热壁化学气相沉积工艺在Si(111)衬底上生长GaN晶体膜，并对其生长条件进行研究。用X射线衍射(XRD)、扫描电镜(SEM)、荧光光谱(PL)对样品进行结构、形貌和发光特性的分析。测试结果表明：用此方法得到了六方纤锌矿结构的GaN晶体膜。实验结果显示：采用该工艺制备GaN晶体膜时，选择H2作反应气体兼载体，对GaN膜的形成起着非常有利的作用。     

Synthesis of GaN nanorods on Si substrates with assistance of the volatilization of ZnO middle layers

GaN nanorods have successfully been synthesized on Si(111) substrates via ammoniating ZnO/Ga2O3 films at 950℃. Ga2O3 thin films and ZnO middle layers were deposited in turn on Si(111) substrates by r.f. magnetron sputtering system. ZnO volatilized at 950℃ in the ammonia ambience and Ga2O3 reacted to NH3 to fabricate GaN nanorods in the later ammoniating process. The volatilization of ZnO layers played an important role in the fabrication. The structure and composition of the GaN nanorods were studied by X-ray diffraction (XRD) and Fourier transform infrared spectrophotometer (FTIR). The orphology ofGaN nanorods was investigated using scanning electron microscopy (SEM) and transmission electronic microscope (TEM). The analyses of measured results revealed that GaN nanorods with hexagonal wurtzite stxucture were prepared by this method.     

The luminescence characteristics of GaN heteroepitaxial films

The III–V compound semiconductors are widely used throughout the world (as well as in space) for a wide variety of electronic and optoelectronic devices. Recently, a strong commercial interest has developed in the use of the III–V nitrides for high-temperature electronics and visible (i.e., blue and green) light- emitting diodes and injection lasers. The luminescent properties of these materials are of critical importance. This article reports the results of a study on the room-temperature cathodoluminescence and photoluminescence for nominally undoped GaN films grown by low-pressure metalorganic chemical vapor deposition on (0001) oriented sapphire substrates.     

Surface and in-depth characterization of InGaN compounds synthesized by plasma-assisted molecular beam epitaxy

InGaN layers with multiple quantum wells are widely used as active layers in advanced optoelectronic devices. In the present work, surface properties of some InGaN layers grown on GaN/sapphire substrates by plasma-assisted molecular beam epitaxy were examined. The total indium content incorporated in the crystalline lattice of In_0.165Ga_0.835N and In_0.353Ga_0.647N layers grown with a thickness of 70-200 nm was controlled by the growth temperature, and was determined from X-ray diffraction. Auger electron spectroscopy and X-ray photoelectron spectroscopy analysis reveal relatively smaller concentration of In within the surface area than in the bulk of the InGaN layers. The Ar⁺ XPS depth profile analysis shows the thick InGaN layers to be chemically homogeneous within an analytical area. To determine the electron inelastic mean free path in the layers within the 500-2000 eV range, relative elastic-peak electron spectroscopy measurements with Ni and Au standards were performed. The measured IMFPs were considerably larger than those predicted from the TPP-2M formula. The smallest root-mean-square-deviation and the mean percentage deviation of 9.9 Å and 44.5%, respectively, were found between EPES IMFP data and those predicted for the In_0.353Ga_0.647N layer with respect to the Au standard. This work provided the detailed compositional and chemical changes of InGaN thick layers, and could be useful in solving key issues associated with the growth of high-quality layer with much higher In content.     

自支撑GaN厚膜制备及其光学性能研究

采用镀Ti插入层在氢化物外延设备中制备了高质量自支撑GaN厚膜。X射线衍射测试发现(0002)峰摇摆曲线的半高宽为260 arcsec;5 K下样品带边发光峰的半高宽为3 meV,室温下样品的带边发光峰也只有20 meV,并且在室温的PL谱中观察不到黄光带;扫描电子显微镜观察显示,腐蚀后的自支撑GaN厚膜表面有位错延伸形成的六角坑,并估算出样品位错密度约为2.1×l07 cm-2。这些结果说明镀Ti插入层有助于提高GaN外延层的晶体质量。通过Raman和低温荧光分析,可以看出自支撑GaN厚膜表面应力已经完全释放。研究了不同温度下样品的荧光特性,证明得到的无应力自支撑GaN厚膜具有很好的晶体质量和光学质量     

Influence of GaCl carrier gas flow rate on properties of GaN films grown by hydride vapor-phase epitaxy

The influence of GaCl carrier gas flow rate on GaN films grown by hydride vapor-phase epitaxy (HVPE) was investigated. The symmetric (0 0 0 2) and asymmetric (10-12) ω scans were detected to estimate the quality of GaN films. Optical properties were studied by room temperature photoluminescence spectra. Raman spectroscopy was employed to analyze the residual stress in the samples. The surface morphology of the GaN films was investigated by atomic force microscopy (AFM). On the basis of process optimization the optimal GaCl carrier gas flow rate for growth of high quality GaN films in our system was obtained as 1.3 L/min.     

GaN/C60多层膜结构及光学特性的理论分析

使用转移矩阵方法计算了GaN／C60多层膜一维光子晶体的带隙结构。计算结果表明,由厚度分别为21nm、49nm的GaN、C60薄膜组成的多层膜结构,在中心带隙为6．46eV处有一不完全的光子带隙存在,反射率最高可达64．3％。     

Crystal Perfection in GaP Films Heteroepitaxially Grown on GaAs by Low-pressure Metalorganic Chemical Vapor Deposition

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Spatial stress distribution and optical properties of GaN films grown on convex shape-patterned sapphire substrate by metalorganic chemical vapor deposition

μ-Raman and μ-photoluminescence methods have been employed to investigate microscopic spatial stress distribution and optical properties of GaN films grown on the convex shape-patterned sapphire substrate (CSPSS). By comparison of the μ-Raman and μ-PL spectra, we found that significantly large difference, Δσ_xx ∼0.46 GPa, in biaxial compressive stress between the flat trench and convex regions in the side facet of the GaN film, around ∼2 μm below the surface whereas on the GaN surface, little difference with large residual stress was observed in both regions compared to those from the side facet. Temperature dependent and time-resolved photoluminescence spectra have shown that the GaN film grown on the CSPSS has improved crystal purity through the reduction of intrinsic point defects.