HVPE生长基座结构优化

GaN氢化物气相外延(HVPE)生长过程中,衬底表面的温度分布对其生长质量有着重要影响。我们采用计算机模拟,研究了GaN生长过程中所使用的不同结构石墨基座上的温度分布。根据所得不同结构下基座上的温度分布,选择衬底上温度分布最均匀的结构与普通实验使用的基座结构进行实验对比。结果发现,在实际的大流量载气的GaN生长中,采用优化的圆弧凹面结构石墨基座实验组的GaN晶体外延层的生长速率、生长质量和均匀性等比普通方法生长的更好。本文得到的不同HVPE生长环境下的基座结构优化设计方案,为HVPE炉体的设计和氮化物生长具有很好的指导意义。     

GaN单晶的HVPE生长与掺杂进展

相比于第一代和第二代半导体材料,第三代半导体材料具有更高的击穿场强、电子饱和速率、热导率以及更宽的带隙,更适用于制备高频、大功率、抗辐射、耐腐蚀的电子器件、光电子器件和发光器件。氮化镓(GaN)作为第三代半导体材料的代表之一,是制作蓝绿激光、射频微波器件和电力电子器件的理想衬底材料,在激光显示、5G通信、相控阵雷达、航空航天等领域具有广阔的应用前景。氢化物气相外延(Hydride vapor phase epitaxy, HVPE)方法因生长设备简单、生长条件温和和生长速度快而成为制备GaN晶体的主流方法。由于普遍使用石英反应器,HVPE法生长获得的非故意掺杂GaN不可避免地存在施主型杂质Si和O,使其表现出n型半导体特性,但载流子浓度高和电导率低限制了其在高频大功率器件中的应用。掺杂是改善半导体材料电学性能最普遍的方法,通过掺杂不同掺杂剂可以获得不同类型的GaN单晶衬底,提高其电化学特性,从而满足市场应用的不同需求。本文介绍了GaN半导体晶体材料的基本结构和性质,综述了近年来采用HVPE法生长高质量GaN晶体的主要研究进展;对GaN的掺杂特性、掺杂剂类型、生长工艺以及掺杂原子对电学性...     

基于模糊控制的HVPE生长设备温度控制系统

氢化物气相外延（hydride vapor phase epitaxy,HVPE）工艺的关键是确保加热炉的温场恒定和高精度控制。由于HVPE生长设备温度控制过程涉及多个加热温区,以及温度测量元件和电阻加热炉温度传导引起的延迟,其温度控制存在超调过大、控制精度低和调节时间过长等问题。为实现HVPE生长设备反应室内温度的精准调控,将模糊逻辑应用到PID （proportion integration differentiation,比例积分微分）控制中,设计系统各温区的模糊自适应整定PID控制器。依据实际设备与相应技术要求,设计研发了一套基于PLC （programmable logic controller,可编程逻辑控制器）、温度控制电路以及模糊自适应整定PID控制的HVPE生长设备温度控制系统。Simulink仿真结果与实测结果表明,模糊自适应整定PID控制器可以应用于HVPE生长设备的温度控制系统,且控制效果较好。研究表明,所设计的温度控制算法与温度控制系统能够很好地满足GaN材料生长的工艺要求,具有一定的实用价值。     

我国在非极性GaN材料研究中取得进展

在国家863项目和中国科学院创新工程的支持下,中科院半导体研究所曾一平研究员带领的课题,采用自行研制的HVPE氮化物生长系统,通过在m面蓝宝石上磁控溅射生长薄层的ZnO缓冲层,进而外延生长获得了非极性GaN厚膜材料,该材料具有较低的位错密度,适合开发用于LED、LD等氮化物发光器件的衬底材料,同时对比实验表明,薄层的ZnO对于形成非极性GaN起到了至关重要的作用,     

Si衬底上横向外延GaN材料的微结构和光学性质的研究

用氢化物气相外延 (HVPE)方法在Si(III)衬底上成功横向外延生长出晶体质量较好的GaN薄膜材料。透射电子显微镜 (TEM )的研究结果表明 ,横向外延区域GaN的位错密度明显减小。由于SiO2 掩膜腐蚀角的不同 (分别为 90°和 6 6°) ,导致了横向外延GaN材料一些独特的微观形貌。微区拉曼光谱由数百条拉曼散射曲线组成 ,每一条曲线有三个振动模 ,分别对应Si振动模式 (5 2 0cm- 1) ,E2 模式 (5 6 6cm- 1)和E1(LO)模式(732cm- 1)。在垂直条纹方向 ,峰位和峰宽没有明显的变化 ,而峰强约 5 μm会发生周期性变化     

水平HVPE反应器中气流动力学模拟与GaN生长

建立了GaN HVPE系统的流体动力学模型，研究了反应气体在反应室内的浓度场，讨论了反应室内GaCl和NH3管道空间配置对气体在衬底表面浓度分布的影响，并对HVPE系统反应室的设计进行了优化。材料生长结果表明，厚度均匀性良好，直接在蓝宝石衬底上生长的GaN外延层摇摆曲线半宽为660aresee。     

连续挤压铜扁排扩展腔及模具的结构优化设计

为了研究扩展腔结构和定径带长度对模具出口处金属流动的影响,从而优化扩展腔及模具的结构,采用有限元数值模拟方法对扩展腔结构参数以及定径带长度进行正交实验,并对挤压过程中模具入口处的金属流速、应变场和温度场进行有限元分析.结果表明:当扩展腔采用中间宽、边部窄的结构时,其模具出口处的金属流速比扩展腔厚度方向更均匀;影响模具出口处金属流速的扩展腔结构参数中,扩展腔中间厚度(H)最为显著,扩展腔中间宽度(l)的影响次之,其两边厚度h的影响较不明显.本实验条件下生产3 mm×100 mm的扁排,扩展腔组合结构的最佳方案如下:扩展腔中间宽度(l)为62 mm,中间出口厚度(H)为38 mm,两侧厚度(h)为24 mm.在其他条件不变的情况下,选择7 mm的定径带不仅有利于金属成形,还有利于提高扁排质量.     

半限域层次孔炭三维锂负极的构筑及性能EI

金属锂负极是锂电池极具发展潜力的高能二次电池负极材料,但是锂枝晶生长、界面不稳定、循环稳定性差和体积膨胀大等问题限制了锂负极的应用。针对枝晶生长和体积膨胀的问题,本工作通过模板法构筑了一种具有较大比表面积的半限域式层次孔炭(HPC)材料,HPC电极材料的高比表面积可降低局部电流密度,丰富的孔道结构可将锂限制在其内部沉积,从而达到抑制枝晶生长和缓解体积膨胀的目的。Li‖HPC电池在电流密度为1.0 mA·cm^(-2)、沉积电量为1.0 mAh·cm^(-2)条件下可以循环超过250周次,其库仑效率保持在97.6%。采用此负极与磷酸铁锂(LiFePO_(4))正极匹配制备的Li@HPC‖LiFePO_(4)全电池,在0.5 C下循环100周次后的正极放电比容量为93.6 mAh·g^(-1),较相同条件下的Li@Cu‖LiFePO_(4)全电池(60.8 mAh·g^(-1))提升了32.8 mAh·g^(-1)。     

半限域层次孔炭三维锂负极的构筑及性能EI北大核心

金属锂负极是锂电池极具发展潜力的高能二次电池负极材料,但是锂枝晶生长、界面不稳定、循环稳定性差和体积膨胀大等问题限制了锂负极的应用。针对枝晶生长和体积膨胀的问题,本工作通过模板法构筑了一种具有较大比表面积的半限域式层次孔炭(HPC)材料,HPC电极材料的高比表面积可降低局部电流密度,丰富的孔道结构可将锂限制在其内部沉积,从而达到抑制枝晶生长和缓解体积膨胀的目的。Li‖HPC电池在电流密度为1.0 mA·cm^(-2)、沉积电量为1.0 mAh·cm^(-2)条件下可以循环超过250周次,其库仑效率保持在97.6%。采用此负极与磷酸铁锂(LiFePO_(4))正极匹配制备的Li@HPC‖LiFePO_(4)全电池,在0.5 C下循环100周次后的正极放电比容量为93.6 mAh·g^(-1),较相同条件下的Li@Cu‖LiFePO_(4)全电池(60.8 mAh·g^(-1))提升了32.8 mAh·g^(-1)。     

酸性液／固双相流介质中材料冲刷腐蚀的阴极保护研究

研究了阴极保护对四种材料X60、AISI 32l、316L和F5在酸性液/固双相流10％H_2SO_4+15％刚玉砂介质中冲刷腐蚀的保护效果,重点研究了阴极保护度随流速的变化规律。结果表明:阴极保护一般能有效降低材料的冲刷腐蚀速率,保护度达32％～100％。对同一材料而言,阴极保护率随流速提高而减小,而同一流速下,四种材料的阴极保护率依X60、AISI 321、316L和F5的次序减小。对于钝化材料(如F5)在低流速条件下施加阴极保护可能出现负效应。     

Nonuniformities in free-standing GaN substrates

In this study, we report that free-standing GaN substrates grown by the hydride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions with low crystal and optical quality located close to the top (near as-grown surface) and bottom (near interface between GaN/sapphire) regions of substrate cross-section. We considered that the origins of these nonuniformities were surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. We used cathodoluminescence imaging and spectroscopy for analyzing these nonuniform regions. The low quality regions with high electron concentration are easily visualized using cathodoluminescence (CL). The coexistence of regions with low- and high quality allows us to explain the concurrent evidence of high substrate quality in double crystal X-ray diffraction and photoluminescence (PL).     

Formation of GaN nanostructures with various morphologies by photo-assisted electroless chemical etching

Interaction of GaN crystal faces with chemicals is crucial to understand why various nanostructures are formed during the etching process. We have prepared GaN nanostructures by a photo-assisted electroless chemical etching method in solutions containing KOH and K₂S₂O₈. Morphology nanostructure GaN layers grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied. For the GaN layers grown by MBE, the etching reaction process starts at grain boundaries and dislocation domains on the surface and inverted hexagonal pyramids are eventually formed. For the GaN layers grown by HVPE, scattered etch pits with well-defined hexagonal facets are observed after the etching process.     

Cathodoluminescence and Raman research of V-shape inverted pyramid in HVPE grown GaN film

Thick GaN films with high quality have been grown on (0001) sapphire substrate in a home-made vertical HVPE reactor. Micron-size hexagonal pits with inverted pyramid shape appear on the film surface, which have six triangular {10-11} facets. These {10-11} facets show strong luminescence emission and are characteristic of doped n-type materials. Broad red emission is suppressed in {10-11} facets and is only found at the flat region out of the pit, which is related with the decreasing defects on {10-11} facets. Low CL emission intensity is observed at the apex of V-shape pits due to the enhanced nonradiative recombination. Raman spectra show that there are higher carrier concentration and low strain in the pit in comparison to the flat region out of the pit. The strain relaxation may be the main mechanism of the V-shape pits formation on the GaN film surface.     

GaN single crystals grown on HVPE seeds in alkaline supercritical ammonia

Ammonothermal growth (synthesis in supercritical (sc) ammonia fluid) has the promise of producing large low defect gallium nitride crystals through the application of techniques similar to those used in hydrothermal growth. Retrograde solubility of GaN greater than 5% by weight using group I amides as mineralizers is demonstrated in high nickel content autoclaves at pressures of one to three kilobars and temperatures between 300 to 600°C. The above conditions were optimized to grow single-crystal GaN at rates up to 40 μm per day on one cm² seeds. Gallium nitride Hydride Vapor Phase Epitaxy (HVPE) seeds are placed in the higher temperature zone below the nutrient basket employing the same configurations used in reverse gradient hydrothermal growth of berlinite (AlPO₄). GaN single crystals grown by the ammonothermal technique were characterized by X-ray diffraction, photoluminescence, scanning electron microscopy (SEM), atomic force microscopy (AFM), and chemical etching. The nitrogen-terminated face tends to exhibita flatter surface morphology than the gallium-terminated face, which is made up of a series of hexagonal columns. Major impurities in the crystal include potassium from the mineralizer, metals from the autoclave, and oxygen. The nitrogen-terminated face incorporated a lower level of metallic impurities in comparison with the gallium-terminated face. Finally, several process phenomena such as ammonia decomposition, parasitic nucleation of GaN on the autoclave walls, impurity incorporation, and defect generation in single-crystal GaN layers grown on HVPE seeds are identified and their possible mechanisms are discussed.     

Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.     

Synthesis of GaN nanochestnuts by hydride vapour phase epitaxy

GaN nanochestnuts with numerous nanorods and nanoneedles were synthesized on AlN/Si(111) substrate using hydride vapour phase epitaxy (HVPE) method under constant N₂ carrier gas flow rate. The formation process of nanochestnuts was systematically investigated and discussed on the basis of the experimental results. The nanochestnuts were analyzed by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and cathodoluminescence (CL). GaN nanochestnuts were revealed as the composition of core, circular stacking layers, and surrounded with nanorods or nanoneedles on all sides. The resultant nanochestnuts may be a promising structure for omnidirectional nano device applications in the future.     

Sequential structural characterization of layers in the GaN/AlN/SiC/Si(111) system by X-ray diffraction upon every growth stage

X-ray diffraction and transmission electron microscopy techniques have been used to study the dynamics of variation of the structural characteristics and deformation state in SiC, AlN, and GaN epilayers sequentially grown on a Si(111) substrate. In this system, the SiC layer has been grown by solid-phase epitaxy, while the AlN and GaN layers have been deposited by chloride-hydride vapor-phase epitaxy (HVPE) using argon as a carrier gas.     

Preparation of large area free-standing GaN substrates by HVPE using mechanical polishing liftoff method

In this study, 30×30 mm² free-standing GaN substrates were fabricated from 400–450 μm thick GaN films grown on (0001) sapphire by hydride vapor phase epitaxy (HVPE). The thick films were separated from the substrate by mechanical polishing liftoff method, using a diamond slurry. After liftoff, the bow is only slight or absent in the resulting free-standing GaN wafers.     

Chloride vapor-phase epitaxy of gallium nitride at a reduced source temperature

Gallium nitride (GaN) layers on sapphire substrates have been grown by hydride-chloride vaporphase epitaxy (HVPE) at a Ga-source temperature reduced from 890°C (standard value) to 600°C. All epilayers are transparent and have smooth surfaces. The structural quality of layers was evaluated by measuring the X-ray rocking curve width (FWHM) using a triple-crystal X-ray diffractometer. For the best samples, this quantity amounted to 8 arc min. Analysis of the dependence of the crystal structure quality and photoluminescent properties of GaN layers on the Ga-source temperature showed that a decrease in this parameter to 600°C did not significantly affect the HVPE growth of GaN despite a considerable change in the vapor phase composition (ratio of GaCl and GaCl₃ concentrations).     

Electrical transport properties of III-nitrides

Excellent n-type GaN layers have been grown by all of the major epitaxial techniques: MBE, MOCVD, and HVPE. In this work, we analyze the band conduction in such samples by temperature-dependent Hall-effect measurement and theory, and determine quantitative information on donor and acceptor concentrations, as well as donor activation energies. In HVPE layers it is necessary to take account of a degenerate n-type layer at the GaN/sapphire interface in order to correctly analyze the bulk material. We also investigate hopping conduction, which occurs at low temperatures in conductive material, and at both low and high temperatures in semi-insulating material. Finally, we show by analysis of electron-irradiation data that both the N vacancy and the N interstitial are electrically active, demonstrating donor and acceptor character, respectively.