New design of nozzle structures and its effect on the surface and crystal qualities of thick GaN using a horizontal HVPE reactor

High-quality thick GaN films without cracks were achieved by using a new nozzle structure in the reactor grown by the hydride vapor phase epitaxy on sapphire substrates. Optical contrast microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray double diffraction (XRD) and cathodoluminescence (CL) were carried out to reveal the surface, crystal and optical properties of the GaN epilayer. It was found that the nozzle structure in the reactor has a large effect on the growth rate, surface flat, crystal quality, and the uniformity of the growth. Compared with the old one, the new nozzle structure (denoted as multi-layers nozzle) can improve dramatically the properties of thick GaN. Mirror, colorless and flat GaN thick film was obtained and its (0 0 0 2) FWHM results were reduced from 1000 to 300 arcsec when the new nozzle was used. AFM result revealed a step flow growth mode for GaN layer with the new nozzle. Room-temperature CL spectra on the GaN films showed a strong near-band-edge peak for the new nozzle, but there is only weak emitting peak for the old nozzle. New nozzle structure can improve the uniform of flow field near the surface of substrates compared with the old one, which leads to the improvement of properties of GaN thick film by hydride vapor phase epitaxy (HVPE).     

Effect of nanoporous GaN templates with different pore diameters on the subsequent thick GaN layers by HVPE

A set of GaN films were overgrown by hydride vapor phase epitaxy (HVPE) on nanoporous GaN templates with different pore diameters. These samples have various properties as seen from the measurements of X-ray diffraction (XRD) and photoluminescence (PL). Cross-sectional observations under a scanning electron microscopy (SEM) reveal that the overgrowth mechanism and process are strongly related to the dimension of nanopores, indicating that an optimum diameter exists for the properties of subsequent HVPE–GaN layers. When the diameters of nanopores are less than the optimum value, the pores on top of GaN templates can be left, and the properties of HVPE–GaN films show significant improvement. In contrast, the pores are almost stuffed with HVPE–GaN films, which obviously limit the improvement degree of HVPE–GaN films.     

Electrochemical liftoff of freestanding GaN by a thick highly conductive sacrificial layer grown by HVPE

Separation technology is an indispensable step in the preparation of freestanding GaN substrate. In this paper, a largearea freestanding GaN layer was separated from the substrate by an electrochemical liftoff process on a sandwich structure composed of an Fe-doped GaN substrate, a highly conductive Si-doped sacrificial layer and a top Fe-doped layer grown by hydride vapor phase epitaxy(HVPE). The large difference between the resistivity in the Si-doped layer and Fe-doped layer resulted in a sharp interface between the etched and unetched layer. It was found that the etching rate increased linearly with the applied voltage, while it continuously decreased with the electrochemical etching process as a result of the mass transport limitation. Flaky GaN pieces and nitrogen gas generated from the sacrificial layer by electrochemical etching were recognized as the main factors responsible for the blocking of the etching channel. Hence, a thick Si-doped layer grown by HVPE was used as the sacrificial layer to alleviate this problem. Moreover, high temperature and ultrasonic oscillation were also found to increase the etching rate. Based on the results above, we succeeded in the liftoff of ～ 1.5 inch GaN layer. This work could help reduce the cost of freestanding GaN substrate and identifies a new way for mass production.     

Optical and Structural Properties of Cr-Doped GaN Grown by HVPE Method

Single crystalline Cr-doped GaN fihns are successfully grown by hydride vapor phase epitaxy. The structure analysis indicates that the film is uniform without detectable Cr precipitates or clusters and the Cr atoms are substituted for Ga sites. The impurity modes in the range 510 530cm^-1 are observed by the Raman spectra. The modes are assigned to the host lattice defects caused by substitutional Cr. The donor-aeceptor emission is found to locate at Ec - 0.20 eV by analyzing the photoluminescence spectrum obtained at different temperatures, and the emission is attributed to the structural defects caused by CrGa-VN complex. The superconductor quantum interference device results show that the Cr-doped GaN film without detectable Cr precipitates or clusters exhibits paramagnetic properties.     

Effect of nitrogen gas flow and growth temperature on extension of GaN layer on Si

The effect of nitrogen flow and growth temperature on extension of GaN on Si substrate has been studied. By increasing the nitrogen flow whose outlet is located in the center of the MOCVD (metal-organic chemical vapor deposition) gas/particle screening flange and by increasing the growth temperature of HT-AlN and AlGaN buffer layers near the primary flat of the wafer, the GaN layer has extended more adequately on Si substrate. In the meantime, the surface morphology has been greatly improved. Both the AlN and GaN crystal quality uniformity has been improved. X-ray diffraction results showed that the GaN (0002) XRD FWHMs (full width at half maximum) decreased from 579 arcsec～ 1655 arcsec to around 420 arcsec.     

Characterization of Thick GaN Films Directly Grown on Wet-Etching Patterned Sapphire by HVPE

Thick GaN films of high quality are directly grown on wet-etching patterned sapphire in a vertical hydride vapour phase epitaxy reactor. The optical and structural properties of GaN films are studied using scanning electronic microscopy and cathodoluminescence. Test results show that initial growth of hydride vapour phase epitaxy GaN occurs not only on the mesas but also on the two asymmetric sidewalls of the V-shaped grooves without selectivity. After the two-step coalescence near the interface, the GaN films near the surface keep on growing along the direction perpendicular to the long sidewall. Based on Raman results, GaN of the coalescence region in the grooves has the maximum residual stress and poor crystalline quality over the whole GaN film, and the coalescence process can release the stress. Therefore, stress-free thick GaN films are prepared with smooth and crack-free surfaces by this particular growth mode on wet-etching patterned sapphire substrates.     

Columnar Structures and Stress Relaxation in Thick GaN Films Grown on Sapphire by HVPE

Thick GaN films with high quality are directly grown on sapphire in a home-built vertical hydride vapour phase epitaxy （HVPE） reactor. The optical and structural properties of large scale columnar domains near the interface are studied using cathodoluminescence and micro-Raman scattering. These columnar domains show a strong emission intensity due to extremely high free carrier concentration up to 2 × 10^19 cm^-3, which are related with impurities trapped in structural defects. The compressive stress in GaN film clearly decreases with increasing distance from interface. The quasi-continuous columnar domains play an important role in the stress relaxation for the upper high quality layer.     

晶体生长角和凝固速率对贵金属电子束区域熔炼晶体生长的作用

基于电子束区域熔炼中熔区上力的平衡关系式, 计算获得了基座法、等径区熔法两种工艺下稳定成形熔区高度的表达式, 探讨了试样尺寸、晶体生长角和凝固速率等参数对六种贵金属稳定成形熔区高度的影响. 结果发现, 区熔相同尺寸试样时, 六种贵金属能够稳定成形熔区高度大小依次排序为 Ru> Pd> Ir> Pt> Ag> Au. 同时获得了这六种贵金属的晶体生长角在8.4°-10.7°之间, 而实际的晶体生长角与界面生长机制有关. 在基座法中, 连续生长机制所能支撑的熔区高度最小, 而等径区熔法中连续生长机制支撑的熔区高度大于位错生长机制和小面生长机制. 这三种晶体界面生长机制中连续生长方式对晶体生长角和区熔熔区高度影响较小, 有利于贵金属区熔单晶制备. 另外当凝固速率达到2.4 mm·min^-1, 位错和小面生长机制对区熔熔区高度的影响也变得很小, 预测的工艺参数与Ir和Ru单晶区熔实验报道结果基本符合.     

反射式高能电子衍射实时监控的分子束外延生长GaAs晶体衬底温度校准及表面相变的研究

以反射式高能电子衍射(RHEED)作为实时监测工具,根据GaAs(100)表面重构相与衬底温度、As₄等效束流压强之间的关系,对分子束外延(MBE)系统中衬底测温系统进行了校准,这种方法也适用于其他的MBE系统.为生长高质量的外延薄膜材料、研究InGaAs表面粗糙化及相变等过程提供了实验依据. 关键词： 分子束外延 反射式高能电子衍射 表面重构 温度校准     

Morphology evolution of hierarchical ZnO nanostructures modulated by supersaturation and growth temperature

Three kinds of ZnO hierarchical structures, nanocombs with tube- and needle-shaped teeth and hierarchical nanorod arrays, were successfully synthesized through the chemical vapor deposition method. Combining the experimental parameters, the microcosmic growing conditions (growth temperature and supersaturation) along the flux was discussed at length, and, based on the conclusions, three reasonable growth processes were proposed. The results and discussions were beneficial to further realize the relation between the growing behavior of the nanomaterial and microcosmic conditions, and the hierarchical nanostructures obtained were also expected to have potential applications as functional blocks in future nanodevices. Furthermore, the study of photoluminescence further indicated that the physical properties were strongly dependent on the crystal structure.      

Influence of substrate temperature,deposition rate and surface conditions on the epitaxial growth of copper on mica

In this experimental investigation copper was deposited on mica substrates under UHV conditions. Both air-cleaved and vacuum-cleaved mica substrates were used. Evaporation rate and substrate temperature have been varied systematically over a wide range. The structure of the films was studied with X-ray methods (Laue-transmission photographs and rocking curves). It was observed that the polar orientation of the crystallites depends mainly on evaporation rate and substrate temperature. The azimuthal orientation, however, is strongly dependent upon the surface conditions of the substrate, i.e. a surface gas layer improves the azimuthal orientation significantly. On vacuum-cleaved substrates an improvement of the azimuthal orientation can only be achieved by high evaporation rates (> 2500 Å/s). These effects are explained by the dependence of the rotation jump frequency on the size of nuclei and clusters.     

Dependence of the growth rate of crystal whiskers on their diameter

The growth kinetics of silicon crystal whiskers is investigated in this paper. An explanation is given of the results obtained on the basis of the kinetic equation of crystal whisker (CW) growth. An equation is proposed to describe CW growth under the condition that seed formation is the limiting stage of the growth process.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 18–22, December, 1986.     

The mass transfer process and the growth rate of NaCl crystal growth by evaporation based on temporal phase evaluation

The mass transfer process and the crystal growth rate have been proved to be very important in the study of crystal growth kinetics, which influence the crystal quality and morphological stability. In this paper, a new method based on temporal phase evaluation was presented to characterize the mass transfer process in situ and determine the crystal growth rate. The crystallization process of NaCl crystal growth by evaporation was monitored in situ by a Mach-Zehnder interferometer, and the absolute concentration evolution, the evaporation rate and the real-time supersaturation of solution were obtained using temporal phase analysis, which acted as a novel technique to extract phase variation along time axis recently. Based on the evaporation rate and the absolution concentration, a new method to calculate mass transfer flux during the crystal growth without the knowledge of the mass transfer coefficient was proposed, and then the crystal growth rate could also be retrieved under the hypothesis of cubic crystals. The results show that the crystal growth rate increases with the supersaturation linearly. It is in agreement with the diffusion theories, which presume that matter is deposited continuously on a crystal face at a rate proportional to the difference in concentration between the points of deposition and the bulk of solution. The method is applicable to the research of crystallization process based on evaporation or vapor diffusion of which the precise conditions of nucleation and supersaturation are usually unknown because of the complexity of the evaporation rate and crystal growth rate.     

Effects of FeNi-phosphorus-carbon system on crystal growth of diamond under high pressure and high temperature conditions

This paper reports the crystal growth of diamond from the Fe Ni–Carbon system with additive phosphorus at high pressures and high temperatures of 5.4–5.8 GPa and 1280–1360°C. Attributed to the presence of additive phosphorus,the pressure and temperature condition, morphology, and color of diamond crystals change obviously. The pressure and temperature condition of diamond growth increases evidently with the increase of additive phosphorus content and results in the moving up of the V-shape region. The surfaces of the diamonds also become coarse as the additive phosphorus added in the growth system. Raman spectra indicate that diamonds grown from the Fe Ni-phosphorus-carbon system have more crystal defects and impurities. This work provides a new way to enrich the doping of diamond and improve the experimental exploration for future material applications.     

The effect of temperature on the recombination rate of AlGaN/GaN light emitting diodes

Nitride semiconductors and their alloys recently have versatile applications as high-power and high-efficiency electro optical devices duo to their high thermal stability, direct transition and wide bang-gap. Nanostructure light emitting diodes of these materials have an emission spectrum from infrared to ultraviolet. In this paper, besides simulating a nanostructure nitride semiconductor LED, such as multi quantum well nitride LEDs, the effect of temperature on the recombination rate has been investigated.     

各向异性界面动力学与各向异性表面张力的相互作用对定向凝固过程中深胞晶生长的影响

通过应用匹配渐近展开法和多变量展开法研究了各向异性界面动力学与各向异性表面张力的相互作用对定向凝固过程中深胞晶生长的影响.结果表明:当各向异性界面动力学与各向异性表面张力的偏好方向之间相差角度为θ_0时,θ_0会对深胞晶生长形态产生影响;当0≤θ_0≤π/4时,随着θ_0的增大,深胞晶的指状界面全长减小,深胞晶根部的深度减小,根部附近界面的曲率减小,而曲率半径增大;当π/4≤θ_0≤π/2时,随着θ_0的增大,深胞晶的指状界面全长增大,深胞晶根部的深度增大,根部附近界面的曲率增大,而曲率半径减小.     

铜管表面水合物晶体生长特性研究

实验研究了浓度分别为5 wt%、15 wt%、20 wt%、30 wt%的四丁基溴化铵(TBAB)溶液在外径为21mm的铜管外表面的结晶特性.实验首先以15 wt%TBAB溶液为例,观察其降温结晶过程,得到了TBAB水合物晶体在不同阶段生长的特征.然后对不同浓度、不同过冷度条件下的TBAB溶液进行降温结晶实验,观察并测...     

碳气凝胶薄片的制备及表面密度致密层去除工艺

研究了不同密度和厚度的碳气凝胶薄片的制备及其表面致密层去除工艺。在以间苯二酚、甲醛为原料制备有机及碳气凝胶块体材料的基础上,结合自制活动式微模具成型工艺,制备了厚度在80~350 μm,密度在50~600 mg·cm-3范围内变化的碳气凝胶薄片。采用场发射扫描电镜、X射线相衬成像和表面轮廓仪-台阶仪等手段对其表面和内部微观结构进行了表征。测试结果表明,碳气凝胶薄片与块体的内部结构相同,但薄片表面存在一层和内部结构截然不同的致密层。采用不同粗糙程度的材料对薄片进行了表面微处理,成功去除该致密“皮”层。