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.     

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.     

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     

Si（111）衬底上生长GaN晶环的研究

利用热壁化学气相沉积在Si(111)衬底上获得GaN晶环，采用扫描电镜(SEM)、选择区电子衍射(SAED)、X射线衍射(XRD)，光致发光(PL)谱和傅里叶红外吸收谱(FTIR)对晶环的组成、结构、形貌和光学特性进行分析。初步结果证明：在Si(111)衬底上获得择优生长的六方纤锌矿结构的GaN晶环。SEM显示在均匀的薄膜上出现直径约为10μm的5品环，由XRD和SAED的分析证实晶环呈六方纤矿多晶结构，FTIR显示GaN薄膜的主要成分为GaN，同时含有少量的C污染，PL测试表明晶环呈现不同于GaN薄膜的发光特性。     

镍改性层增强铜基底沉积金刚石膜的形核(英文)

在铜基底溅射约100nm厚的镍改性层,然后置入纳米金刚石悬浮液中超声震荡加载籽晶,随后在热丝化学气相沉积设备中制备出晶体颗粒接近热力学平衡形态的高质量金刚石膜,其中sp2碳相含量低于5.56%。分别采用激光拉曼光谱、扫描电镜与X射线衍射对金刚石膜的形核与生长进行研究。实验结果表明:在溅射有镍改性层的铜基底上,金刚石的形核密度比在无改性层的铜基底上的形核密度高10倍。镍改性层的增强机制主要来源于两个方面:镍改性层的纳米级粗糙表面增强金刚石籽晶颗粒的吸附;镍改性层的强催化效应加速铜基底上金刚石形核生长所需的石墨过渡层的形成,从而促进金刚石的快速形核。     

Synthesis of one-dimensional GaN nanorods by Tb intermediate layer with different thicknesses

GaN nanorods were synthesized by magnetron sputtering and ammonification system, and the thickness of Tb intermediate layer was changed to study the effect on GaN nanorods. The resultant was tested by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra. The results show that the thickness of Tb layer has an evident effect on the modality, quality, and luminescence properties of GaN nanorods. PL spectra at room temperature show a very strong emission peak at 368 nm and a weak emission peak at 387 nm, and the intensities of the peak for the produced samples reach the maximum when Tb layer is 20 nm. Finally, the optimal thickness of 20 nm of Tb intermediate layer for synthesizing GaN nanostructures is achieved.     

Etching Effect on CMP of Different GaN Layers

Chemical mechanical polishing (CMP) was used to etch various GaN materials, such as GaN layers on sapphire and silicon carbide substrates grown by metal-organic chemical vapor deposition and thick GaN layers grown by physical vapor transport. It was found that CMP could reveal the dislocations in GaN surfaces due to a selective etching component. After the optimization of CMP condition, the surface finish improved and the subsurface damage was almost completely removed, demonstrated by atomic force microsco...     

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.     

Surface and defect microstructure of GaN and AlN layers grown on hydrogen-etched 6H–SiC(0001) substrates

Hydrogen-etching of 6H–SiC(0001) substrates removed mechanical polishing damage and produced an array of parallel, unit cell high steps. The initial stage of AlN deposition on these etched substrates occurred via island nucleation, both on step edges and on terraces. Coalesced AlN films did not show scratch-induced undulations observed on the surfaces of AlN films deposited on as-received substrates. The films also had a lower density of growth pits. The majority of threading dislocations (TDs) observed in these films were of a type. Jagged networks of misfit dislocations were seen on the terraces in the 15 nm thick AlN/hydrogen-etched SiC composite. GaN islands nucleated primarily at undulations in AlN layers and at hillocks on the AlN surface of as-received and hydrogen-etched substrates, respectively. Complete coalescence of these islands occurred at thicknesses close to 20 nm, and subsequent growth occurred via the step-flow mechanism. Strain measurements showed more strain relaxation in GaN films grown on the hydrogen-etched substrate. On- and off-axis X-ray rocking curves revealed statistically similar full width at half maximum values for both on- and off-axis reflections, indicating similar densities of TDs in the two types of films. The majority of TDs in GaN epi-layers resulted from defective regions observed contiguous to the GaN/AlN interfaces.     

Growth of Gallium Nitride Films on Multilayer Graphene Template Using Plasma-Enhanced Atomic Layer Deposition

In this work,the GaN thin films were directly deposited on multilayer graphene(MLG) by plasma-enhanced atomic layer deposition.The deposition was carried out at a low temperature using triethylgallium(TEGa) precursor and Ar/N_2/H_2 plasma.Chemical properties of the bulk GaN and GaN-graphene interface were analyzed using X-ray photoelectron spectroscopy.The sharp interface between GaN and graphene was verified via X-ray reflectivity and transmission electron microscope.The microstructures and the nucleation behaviors of the GaN grown on graphene have been also studied.The results of grazing incidence X-ray diffraction and Raman spectrum indicate that the as-deposited sample is polycrystalline with wurtzite structure and has a weakly tensile stress.Optical properties of the sample were investigated by photoluminescence(PL) at room temperature.The successful growth of GaN on MLG at a low temperature opens up the possibility of ameliorating the performance of electronic and optical devices based on GaN/graphene heterojunction.     

Aligned multi-walled carbon nanotubes on different substrates by floating catalyst chemical vapor deposition: Critical effects of buffer layer

Aligned multi-walled carbon nanotubes (MWCNTs) were synthesized by floating catalyst chemical vapor deposition on two types of substrates, with emphasis on the effects of an aluminum buffer layer. It has been revealed that the presence of the aluminum buffer layer on insulating/semiconducting silicon oxide/silicon wafer resulted in a higher growth rate, narrower diameter distribution, neater morphology and improved crystalline quality of MWCNTs. When an aluminum buffer layer is deposited on electrically conductive carbon paper, high yield CNTs can be achieved while no CNTs can be observed without this buffer layer. Morphology, structure and chemical states of the products were examined by field-emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The generation of alumina from the aluminum buffer layer is thought to play an important role in promoting the nanotube growth. Detailed growth mechanism of MWCNTs was also discussed.     

High temperature chemical vapor deposition of AlN/W1−xRex coatings on bulk SiC

The residual porosity of structural silicon carbide (SiC) composites limits their use in advanced nuclear systems. The use of thick coatings of high-Z materials like tungsten (W) or tungsten alloys (W_1−xRe_x) is a promising solution to overcome such problems. However, solid-state reactions occur between SiC and metals at high temperatures. An intermediate layer is therefore selected, based on thermodynamic computation. It is shown that aluminum nitride (AlN) could limit the interface reactivity at temperatures close to 1000 °C. Duplex AlN/W_1−xRe_x coatings were fabricated in two steps by chemical vapor deposition on bulk silicon carbide to verify experimentally the theoretical material solution approach. Electron probe micro-analyses showed that, at the micrometer level, there was no interface reaction during the growth process. It is the first time that such a material stack has been fabricated, and it seems promising for the high-temperature use of SiC with tungsten alloys.     

反应溅射制备AlN薄膜中沉积速率的研究

通过对浅射过程中辉光放电现象及薄膜沉积速率的研究，发现随着氮浓度的增大，靶面上形成一层不稳定的AlN层，由于AlN的溅射速率远小于Al，从而使薄膜的沉积速率显著下降。同时还研究了其它溅射参数对薄膜沉积速率的影响。结果表明：随靶基距的增大靶功率的减小，不同程度引起沉积速率的下降；随着溅射气压的增大，最初沉积速率不断增大，当溅射气压增大到一定程度时，沉积速率达到最大值，之后随溅射气 压的增大，又不断减小。     

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.     

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。     

定向金属氮化纯铝熔体表面Mg蒸气传质行为

从热力学角度分析并模拟了Mg蒸气在定向金属氮化法制备AlN材料中的相转变过程,证实了在高纯氮气气氛下Mg蒸气层的存在.按照气体扩散传质动力学原理,求出了Mg蒸气层的厚度关系式.研究表明:熔液距坩埚上沿距离、反应前沿的氧分压和初始Mg含量均影响着Mg蒸气层厚度.氧分压降低,初始镁含量的增加,Mg蒸气层厚度增加,有利于铝熔液的渗透氮化.     

钢基渗铝过渡层上沉积金刚石薄膜的研究

在微波等离子体化学气相沉积(MPCVD)装置中,以45钢钢板上的渗铝层作为过渡层,制备金刚石薄膜。研究了基体表面不同的铝含量对金刚石膜质量的影响。扫描电子显微镜(SEM)、能谱和Raman谱测试表明,渗铝层中的FeAl等金属间化合物在低温沉积时能减弱碳向基体的扩散,防止非金刚石相碳的出现,从而有利于高质量的金刚石薄膜的沉积。而试样表面过低的铝含量及过高的沉积温度不利于金刚石薄膜的形核与生长。     

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.     

以磁控溅射＋自组装反应方式制备GaN薄膜

为了用简单的方法得到GaN薄膜，以射频磁控溅射方法将Ga2O3薄膜沉积到Si（111）衬底上的SiC中间层上，通过其同NH3的自组装反应形成了GaN薄膜。同样，利用磁控溅射方法把SiC层沉积到Si衬底。其目的是为了缓冲由GaN外延层和Si衬底的晶格失配造成的应力。为了比较中间层的作用，对按照两种方案（使用中间层和不使用中间层）实验样品进行了测试和比较。实验结果证实了SiC中间层提高了GaN薄膜的质量。     

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.