GaN薄膜生长实时监控系统的实现

分析了在ECR-MOCVD装置上外延生长GaN单晶薄膜的工艺过程特点和在此过程中影响GaN结晶质量的主要因素.在此基础上,设计了一套监控系统,用于GaN外延生长的工艺流程监控,并提出了一种合适的工艺流程监控策略.     

GaN生长工艺流程实时监控系统

分析了在ECR-MOCVD装置上外延生长GaN单晶薄膜的工艺过程特点和在此过程中影响GaN结晶质量的主要因素.在此基础上,设计了一套以80C31单片机为核心的光电隔离电路和PC机组成的两级系统,用于GaN薄膜外延生长的工艺流程监控,并提出了一种合适的工艺流程监控策略.     

采用铟束流保护下的调制中断生长技术改善(0001)GaN表面形貌

使用分子束外延方法,采用In束流保护下的调制中断生长技术,在(0001)蓝宝石衬底上生长GaN薄膜.利用反射式高能电子衍射(RHEED)对生长进行实时监控,并用扫描电子显微镜(SEM)、原子力显微镜(AFM)和X射线衍射(XRD)法对GaN外延薄膜的表面形貌和晶体质量进行分析.实验结果表明:采用该技术生长的Ga极性GaN外延薄膜中的晶体表面残留Ga滴密度大大降低,GaN外延薄膜的表面形貌得到改善,其均方根粗糙度(RMS)由3nm降低为0.6nm,同时XRD双晶摇摆曲线测试的结果表明,GaN外延层的晶格质量也得到改善.     

采用铟束流保护下的调制中断生长技术改善(0001)GaN表面形貌

使用分子束外延方法,采用In束流保护下的调制中断生长技术,在(0001)蓝宝石衬底上生长GaN薄膜.利用反射式高能电子衍射(RHEED)对生长进行实时监控,并用扫描电子显微镜(SEM)、原子力显微镜(AFM)和X射线衍射(XRD)法对GaN外延薄膜的表面形貌和晶体质量进行分析.实验结果表明:采用该技术生长的Ga极性GaN外延薄膜中的晶体表面残留Ga滴密度大大降低,GaN外延薄膜的表面形貌得到改善,其均方根粗糙度(RMS)由3nm降低为0.6nm,同时XRD双晶摇摆曲线测试的结果表明,GaN外延层的晶格质量也得到改善.     

SiNx插入层对Si衬底上生长GaN薄膜晶体质量和表面形貌的影响

在Si(111)衬底上采用金属有机化合物化学气相沉积(MOCVD)技术外延生长GaN薄膜,对外延生长所得GaN薄膜的晶体结构和表面形貌进行表征,并研究SiNx插入层对GaN薄膜的晶体质量和表面形貌的影响.结果表明,在Si衬底上生长GaN薄膜过程中引入SiNx插入层可使GaN薄膜的(10-12)面的X-射线回摆曲线的半峰宽(FWHM)值从974.01减小到602.01arcsec;表面凹坑等缺陷减少、表面平整度提高.可见,SiNx插入层对在Si衬底上外延生长GaN薄膜的晶体质量和表面形貌有着重要的影响.     

额外HCl和氮化对HVPE GaN生长的影响

在氢化物气相外延(HVPE)生长GaN过程中,发现了一种在成核阶段向生长区添加额外HCl来改善GaN外延薄膜质量的方法,并且讨论了额外HCl和氮化对GaN形貌和质量的影响.两种方法都可以大幅度地改善GaN的晶体质量和性质,但机理不同.氮化是通过在衬底表面形成AlN小岛,促进了衬底表面的成核和薄膜的融合;而添加额外HCl则被认为是通过改变生长表面的过饱和度引起快速成核从而促进薄膜的生长而改善晶体质量和性质的.     

脉冲激光沉积GaN薄膜的研究进展

脉冲激光沉积(PLD)技术凭借其低温生长优势,逐步在GaN薄膜外延领域得到广泛应用。回顾了近年来PLD技术外延生长GaN薄膜的研究进展,包括新型衬底上的GaN薄膜外延研究进展,以及作为克服异质外延的重要手段——缓冲层技术的发展现状。从目前的研究进展可以看出,应用PLD技术制备GaN薄膜及其光电器件具有广阔的发展前景。     

硅衬底GaN基LED外延生长的研究

采用在AlN缓冲层后原位沉积SiN掩膜层,然后横向外延生长GaN薄膜.通过该法在硅衬底上获得了1.7 μm无裂纹的GaN薄膜,并在此基础上外延生长出了GaN基发光二极管(LED)外延片,其外延片的总厚度约为1.9 μm.采用高分辨率双晶X-射线衍射(DCXRD)、原子力显微镜(AFM)测试分析.结果表明,GaN薄膜(0002)面的半峰全宽(FWHM)降低到403 arcsec,其表面平整度得到了很大的改善;InGaN/GaN多量子阱的界面较平整,结晶质量良好.光致发光谱表明,GaN基LED峰值波长为469.2 nm.     

AlN缓冲层厚度对脉冲激光沉积技术生长的GaN薄膜性能的影响

在蓝宝石(Al2O3)衬底上应用脉冲激光沉积技术(PLD)生长不同厚度的AlN缓冲层后进行GaN薄膜外延生长。采用高分辨X射线衍射仪(HRXRD)和扫描电子显微镜(SEM)对外延生长所得GaN薄膜的晶体质量和表面形貌进行了表征。测试结果表明:相比直接在Al2O3衬底上生长的GaN薄膜,通过生长AlN缓冲层的GaN薄膜虽然晶体质量较差,但表面较平整;而且随着AlN缓冲层厚度的增加,GaN薄膜的晶体质量和表面平整度均逐渐提高。可见,AlN缓冲层厚度对在Al2O3衬底上外延生长GaN薄膜的晶体质量和表面形貌有着重要的影响。     

AlN缓冲层厚度对脉冲激光沉积技术生长的GaN薄膜性能的影响

在蓝宝石(Al2O3)衬底上应用脉冲激光沉积技术(PLD)生长不同厚度的AlN缓冲层后进行GaN薄膜外延生长。采用高分辨X射线衍射仪(HRXRD)和扫描电子显微镜(SEM)对外延生长所得GaN薄膜的晶体质量和表面形貌进行了表征。测试结果表明: 相比直接在Al2O3衬底上生长的GaN薄膜, 通过生长AlN缓冲层的GaN薄膜虽然晶体质量较差, 但表面较平整; 而且随着AlN缓冲层厚度的增加, GaN薄膜的晶体质量和表面平整度均逐渐提高。可见, AlN缓冲层厚度对在Al2O3衬底上外延生长GaN薄膜的晶体质量和表面形貌有着重要的影响。     

The role of the low temperature buffer layer and layer thickness in the optimization of OMVPE growth of GaN on sapphire

In agreement with previous work,¹² a thin, low temperature GaN buffer layer, that is used to initiate OMVPE growth of GaN growth on sapphire, is shown to play a critical role in determining the surface morphology of the main GaN epilayer. X-ray analysis shows that the mosaicity of the main GaN epilayer continues to improve even after several μm of epitaxy. This continuing improvement in crystal perfection correlates with an improvement in Hall mobility for thicker samples. So far, we have obtained a maximum mobility of 600 cm²/V-s in a 6 μm GaN epilayer. Atomic force microscopy (AFM) analysis of the buffer layer and x-ray analysis of the main epilayer lead us to conclude that the both of these effects reflect the degree of coherence in the main GaN epitaxial layer. These results are consistent with the growth model presented by Hiramatsu et al., however, our AFM data indicates that for GaN buffer layers partial coherence can be achieved during the low temperature growth stage.     

Lattice‐Symmetry‐Driven Epitaxy of Hierarchical GaN Nanotripods

Lattice‐symmetry‐driven epitaxy of hierarchical GaN nanotripods is demonstrated. The nanotripods emerge on the top of hexagonal GaN nanowires, which are selectively grown on pillar‐patterned GaN templates using molecular beam epitaxy. High‐resolution transmission electron microscopy confirms that two kinds of lattice‐symmetry, wurtzite (wz) and zinc‐blende (zb), coexist in the GaN nanotripods. Periodical transformation between wz and zb drives the epitaxy of the hierarchical nanotripods with N‐polarity. The zb‐GaN is formed by the poor diffusion of adatoms, and it can be suppressed by improving the ability of the Ga adatoms to migrate as the growth temperature increased. This controllable epitaxy of hierarchical GaN nanotripods allows quantum dots to be located at the phase junctions of the nanotripods and nanowires, suggesting a new recipe for multichannel quantum devices.     

The growth and properties of mixed group V nitrides

Bearing in mind the problems of finding a lattice-matched substrate for the growth of binary group III nitride films and the detrimental effect of the large activation energy associated with acceptors in GaN, we propose the study of the alloy system AlGaAsN. We predict that it may be possible to obtain a direct gap alloy, with a band gap as wide as 2.8eV, which is lattice-matched to silicon substrates. The paper reports our attempts to grow GaAsN alloy films by molecular beam epitaxy on either GaAs or GaP substrates, using a radio frequency plasma source to supply active nitrogen. Auger electron spectra demonstrate that it is possible to incorporate several tens of percent of nitrogen into GaAs films, though x-ray diffraction measurements show that such films contain mixed binary phases rather than true alloys. An interesting observation concerns the fact that it is possible to control the crystal structure of GaN films by the application of an As flux during growth. In films grown at 620°C a high As flux tends to increase the proportion of cubic GaN while also resulting in the incorporation of GaAs. Films grown at 700°C show no evidence for GaAs incorporation; at this temperature, it is possible to grow either purely cubic or purely hexagonal GaN depending on the presence or absence of the As beam.     

Analysis of the growth of GaN epitaxy on silicon

Due to the great potential of GaN based devices,the analysis of the growth of crack-free GaN with high quality has always been a research hotspot.In this paper,two methods for improving the property of the GaN epitaxial layer on Si (111) substrate are researched.Sample A,as a reference,only has an AlN buffer between the Si substrate and the epitaxy.In the following two samples,a GaN transition layer (sample B) and an AlGaN buffer (sample C) are grown on the AlN buffer separately.Both methods improve the quality of GaN.Meanwhile,using the second method,the residual tensile thermal stress decreases.To further study the impact of the two introduced layers,we investigate the stress condition of GaN epitaxial layer by Raman spectrum.According to the Raman spectrum,the calculated residual stress in the GaN epitaxial layer is approximately 0.72 GPa for sample B and 0.42 GPa for sample C.The photoluminescence property of GaN epitaxy is also investigated by room temperature PL spectrum.     

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.     

Hydride vapor phase epitaxy for gallium nitride substrate

Due to the remarkable growth rate compared to another growth methods for gallium nitride(GaN)growth,hydride vapor phase epitaxy(HVPE)is now the only method for mass product GaN substrates.In this review,commercial HVPE systems and the GaN crystals grown by them are demonstrated.This article also illustrates some innovative attempts to develop homebuilt HVPE systems.Finally,the prospects for the further development of HVPE for GaN crystal growth in the future are also discussed.     

GaN基材料及其外延生长技术研究

介绍了GaN基材料的基本特性、三种主要外延生长技术(MOCVD、MBE、HVPE)、衬底材料的选择及缓冲层技术;分析得出目前存在的GaN体单晶技术不完善、外延成本高、衬底缺陷及接触电阻大等主要问题制约了研究的进一步发展;指出今后的研究重点是完善GaN体单晶材料的生长工艺,以利于深入研究GaN的物理特性及有效地解决衬底问题,研究缓冲层的材料、厚度、组分等以提高GaN薄膜质量。     

ECR-PEMOCVD生长GaN RHEED图像研究

通过在自制的电子回旋共振等离子体增强金属有机物化学气相沉积 (ECR -PEMOCVD)系统上装配反射高能电子衍射仪 (RHEED) ,对外延GaN生长过程进行原位监测。通过分析研究RHEED图像确定衬底清洗、氮化、缓冲层生长、外延层生长的合适条件