GaN薄膜大型Ⅴ形表面坑的形成和光学性质

研究了用MOCVD设备在高温和低Ⅴ／Ⅲ条件下生长的GaN薄膜表面存在的与位错相连的大型Ⅴ形表面坑,并提出了一个有关质量疏运机制的模型以解释其形成机理．由衬底扩散上出来的Al原子对大型坑的形成具有辅助作用,并阻止了深能级杂质或空位缀饰与坑相连的位错．GaN内的位错是非辐射复合中心,但对深能级发光不起作用．     

KOH热湿腐蚀法准确估算GaN的位错密度

实验目的在于解决GaN外延层中六方形腐蚀坑起源问题存在的分歧,并利用腐蚀法准确地估计GaN的位错密度.大量对熔融KOH腐蚀GaN过程中表面形貌的演化以及温度和时间对腐蚀结果影响的实验结果表明,位错类型与腐蚀坑三维形状相对应,而与腐蚀坑大小无关,极性是GaN不同种类位错的腐蚀坑具有不同形状的决定性因素.使所有缺陷都显示出来所需的腐蚀温度和时间呈反比关系.腐蚀法估算GaN位错密度的准确性取决于优化的腐蚀条件和合理的微观观测方法.     

KOtt热湿腐蚀法准确估算GaN的位错密度

实验目的在于解决GaN外延层中六方形腐蚀坑起源问题存在的分歧,并利用腐蚀法准确地估计GaN的位错密度。大量对熔融KOH腐蚀GaN过程中表面形貌的演化以及温度和时间对腐蚀结果影响的实验结果表明,位错类型与腐蚀坑三维形状相对应,而与腐蚀坑大小无关,极性是GaN不同种类位错的腐蚀坑具有不同形状的决定性因素。使所有缺陷都显示出来所需的腐蚀温度和时间呈反比关系。腐蚀法估算GaN位错密度的准确性取决于优化的腐蚀条件和合理的微观观测方法。     

Si基外延GaN的结构和力学性能

采用MOCVD技术在Si衬底(111)面上生长了GaN外延膜,分析了薄膜表面形貌和Si基GaN的临界载荷,研究了表面发光性能和GaN晶体质量随深度的变化.结果表明,外延层的表面比较平整,多组超晶格插入层可以进一步降低位错密度,提高晶体质量.膜的表面有许多颗粒状的发光中心,除了强的带边峰外,还有弱的黄光带和红光带,这可能是ON与VGa所产生的深能级跃迁产生的.GaN的晶体质量具有梯度变化,GaN外延层的上层晶体质量比较好,界面附近比较差,但是外延层与衬底的结合强度较高,临界载荷达到2.05 N.     

氮化镓材料中的位错对材料物理性能的影响

氮化镓材料中的位错是制约GaN发光器件及电子器件的性能的一个关键因素。目前对于氮化镓材料中的位错的研究是一大热点。扼要综述了位错对于材料及器件的物理性能的影响：非辐射复合作用、造成器件的漏电流、缩短器件的寿命。并简要介绍了减少GaN外延层中的位错密度的几种方法。     

异质外延GaN薄膜中缺陷对表面形貌的影响

通过对异质外延GaN薄膜中各类结构缺陷进行系统的研究,发现材料内部的各种体、面和线缺陷,包括沉淀物、裂纹、反向边界、局部立方相、小角晶界和位错,都会对表面形貌产生影响,并具有对应的特征形貌.GaN薄膜中缺陷与表面形貌的这种对应关系,可以通过MOCVD生长机理和缺陷间相互作用机制加以解释,同时也提供了一种简单而有效的研究与检测GaN材料内部缺陷的方法.     

碳素钢强流脉冲电子束处理后的表面火山口状凹坑研究

利用强流脉冲电子束(HCPEB)技术对碳素钢进行了表面改性处理.利用扫描电镜(SEM)和X射线衍射(XRD)对改性试样的表面性质进行了分析,结果表明:样品表面出现了"火山坑"状的凹坑,淬火态T8凹坑数量比45#钢少,凹坑深度比45#钢深.凹坑的密度与输入的能量密度成正比关系;此外,凹坑的分布密度先随轰击次数(n=1～5)的增加而增加,随后(n=6～10)略有降低;分析表明凹坑的形成及分布与位错密度密切相关.     

蓝宝石衬底的斜切角对GaN薄膜特性的影响

利用金属有机物化学气相沉积技术在具有斜切角度的蓝宝石衬底(0～0.3°)上生长了非故意掺杂GaN薄膜,并采用显微镜、X射线双晶衍射、光荧光及霍尔技术对外延薄膜的表面形貌、晶体质量、光学及电学特性进行了分析.结果表明,采用具有斜切角度的衬底,可以有效改善GaN外延薄膜的表面形貌、降低位错密度、提高GaN的晶体质量及其光电特性,并且存在一个衬底最优斜切角度0.2°,此时外延生长出的GaN薄膜的表面形貌和晶体质量最好.     

结构缺陷对电子束诱发纯铝表面熔坑的影响

用Nadezhda-2强流脉冲电子束轰击不同预变形量的纯铝表面,对铝表面的结构缺陷和轰击后产生的表面熔坑进行了系统的表征,研究了表面熔坑与结构缺陷之间的关系.结果表明,结构缺陷对表面熔坑的形成有重要的影响,熔坑的数量随着结构缺陷数量的增加而增加,并且有沿晶界和高位错密度区择优形成的趋向,结构缺陷是表面熔坑形成的萌芽.另外,强流脉冲电子束还能在晶界和滑移带上诱发出大量由于空位聚集而产生的孔洞.     

高质量半绝缘GaN的制备

GaN作为宽禁带半导体材料之一，因其良好的热稳定性、较高的击穿电压，在高功率高频率电子器件方面有较大发展前途。通常非掺杂GaN因Ga空位和残余氧表现为n型。掺入某些金属元素（如铁）作为深能级受主，利用补偿作用可改善GaN电学性质。     

Optical processes of red emission from Eu doped GaN

A single crystalline Eu-doped GaN was grown by gas-source molecular beam epitaxy and photoluminescence (PL) properties were studied. The PL spectra show red-emission at 622 nm originating from intra 4f–4f transition of Eu³⁺ ion without band-edge emission of GaN. The peak shift of the red-emission with the temperature variation from 77 K to room temperature is less than 1.6 meV, and thermal quenching of the luminescence was found to be small compared with the band-to-band transition. Fourier transform infrared spectra showed an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the intensity was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu³⁺ ion in GaN and the optical process for the luminescence at 622 nm was discussed with the relation to the defect.     

Deep level centers in InGaN/GaN heterostructure grown on sapphire and free-standing GaN

Deep level transient spectroscopy has been used to characterize the deep levels in InGaN/GaN grown on sapphire substrate as well as on free-standing GaN. The deep levels at E_c − E_t ∼ 0.17-0.23 eV and E_c − E_t ∼ 0.58-0.62 eV have been detected in our samples which are present in GaN samples reported by others. These two deep levels have been attributed by us to threading dislocations as they exhibit logarithmic capture kinetic behavior and are found to be substantially reduced in its trap concentration (∼ from 10¹⁴ to 10¹² cm^− 2) in GaN grown on free-standing GaN template. Other than the two deep levels, an additional level at E_c − E_t ∼ 0.40-0.42 eV has been identified in both samples, which is believed to be related to In segregation. AFM image shows region of pits formation in InGaN epilayer for sample grown on u-GaN using sapphire substrate while the latter gives a much smoother morphology. From the X-ray diffraction space mapping, the mosaicity of the sample structure for both samples were studied. Dislocations do not play a significant role in the structural properties of InGaN grown on free-standing GaN since the FWHM based on the Δ ω is relatively small (± 0.15°) in the case of InGaN/GaN on free-standing GaN substrate as compared to that on sapphire (± 0.35°). The wider spread in Δω-2θ value for InGaN layer on free-standing GaN also suggested the effect of compositional pulling with increasing InGaN layer thickness.     

The effect of geometrical misfit dislocation on formation of microstructure and photoluminescence of Wurtzite GaN/Al2O3 (0001) films grown by low pressure metal-organic chemical vapor deposition

The incoherent GaN/sapphire interface and microstructure of GaN were observed by high resolution transmission electron microscopy. The most mobile 60° mixed-type dislocation is related to a structural metastability of the Wurtzite GaN film. In spite of the same feature of interband absorption, the photoluminescence mechanism is sensitive to deep level. A strong light emission from the bound exciton of Wurtzite GaN at 358 nm was observed in an n-type GaN sample with the GaN buffer layer. The donor–acceptor pair recombination at 380 nm with LO phonon replicas at 390 and 403 nm and the deep level at 559 nm were observed in both an undoped GaN sample with GaN buffer layer and an n-type GaN sample with AlN buffer layer. This optical behavior is sensitive to the Si doping and the type of buffer layer materials. The deep level emission along the dislocation line is suggested by the local band bending model providing the potential barrier of 0.63 eV by the space charge.     

The improvement of GaN p-i-n UV sensor by 8-pair AlGaN/GaN superlattices structure

GaN with pairs of AlGaN/GaN superlattices (SLs) structure for p-i-n UV photo detector are fabricated on sapphire by metal organic chemical vapor deposition (MOCVD). For 8-pair AlGaN/GaN SLs not only eliminates cracking through this strain management, but it also significantly decreases the threading dislocation density by acting itself as an effective dislocation filter. The related structure has exhibited excellent film qualities such as enhanced crystallinity, lower specific contact resistance, lower etching pit density or mean roughness in the film. GaN p-i-n diode fabricated with 8-pair SLs, the dark current of device is reduced by two orders of magnitude than that without SLs structure at reverse bias of −3 V. Moreover, the peak UV responsivity is 0.12 A/W, which is higher than that without SLs is 0.07 A/W at 360 nm. The rejecting ratio is also by two orders of magnitude higher than that without SLs structure.     

Effect of GaN doping on ZnO films by pulsed laser deposition

The present study reveals the codoping of Ga and N into ZnO films on sapphire substrates by pulsed laser deposition (PLD) with GaN doped ZnO targets. The glow discharge mass spectroscopy (GDMS) spectra confirm the presence of Ga and N in the doped films. The XRD measurements show that for GaN concentration up to 0.8 mol%, the full width at half maximum (FWHM) is almost unchanged thereby maintaining the crystallinity of the films, while for 1 mol% the FWHM increases. The PL spectra only show the strong near band edge (NBE) emission, whereas the deep level emissions are almost undetectable, indicating that they have been considerably suppressed. Our Hall measurements indicate that all the GaN doped ZnO films are of n-type. However, as the GaN concentration is greater than 0.6 mol%, the film shows a decrease in the carrier concentration, suggesting that N acceptors are not sufficient to compensate the native donor defects.     

Investigation on the role of indium in the removal of metallic gallium from soft and hard sputtered GaN (0001) surfaces

Cleaning of GaN by argon sputtering and subsequent annealing introduces metallic gallium on the GaN surface. Once formed, this metallic gallium can be difficult to remove. It has a strong influence on the Fermi level position in the band gap and poses a problem for subsequent epitaxial growth on the surface. We present a method of removing metallic gallium from moderately damaged GaN surfaces by deposition of indium and formation of an In-Ga alloy that can be desorbed by annealing at ~ 550 °C. After the In-Ga alloy has desorbed, photoemission spectra show that the Ga3d bulk component becomes narrower indicating a smoother and more homogeneous surface. This is also reflected in a sharper low energy electron diffraction pattern. On heavily damaged GaN surfaces, caused by hard sputtering, larger amount of metallic gallium forms after annealing at 600 °C. This gallium readily alloys with deposited indium, but the alloy does not desorb until a temperature of 840 °C is reached and even then, traces of both indium and metallic gallium could be found on the surface.     

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.     

Structural,optical, and electrical characteristics of 70 Mev Si<Superscript>5+</Superscript> ion irradiation-induced nanoclusters of gallium nitride

We report the formation of nanoclusters on the surface of gallium nitride (GaN) epilayers due to irradiation with 70 MeV Si ions with the fluences of 1 × 10¹² ions/cm² at the liquid nitrogen temperature (77 K). GaN epilayers were grown using a metal organic chemical vapor deposition system. Omega scan rocking curves of (002) and (101) plane reflection shows irradiation-induced broadening. Atomic force microscopy imagery revealed the formation of nanoclusters on the surface of the irradiated samples. X-ray photoelectron spectroscopy confirms that the surface features are composed of GaN. The effects of ion-beam-produced lattice defects on the surface, electrical, and optical properties of GaN were studied and possible mechanisms responsible for the formation of nanoclusters during irradiation have been discussed.     

Investigation of V-defects formation in InGaN/GaN multiple quantum well grown on sapphire

In the growth of InGaN multiple quantum well structure, V-pits has been observed to be initiated at the threading dislocations which propagate to the quantum well layers with high indium composition and substantially thick InGaN well. A set of samples with varying indium well thickness (3-7.6 nm) and composition (10-30%) are grown and characterized by photoluminescence (PL), X-ray diffraction, transmission electron microscopy and atomic force microscopy. The indium content and the layer thicknesses in InGaN/GaN quantum well are determined by high-resolution X-ray diffraction (XRD) and TEM imaging. With indium composition exceeding 10%, strain at the InGaN/GaN interface leads to the generation of V-pits at the interlayers of the MQW. Higher indium composition and increase in thickness of a period (InGaN well plus the GaN barrier) appear to enhance pits generation. With thicker InGaN well and reduction in thickness of GaN to InGaN (or the R ratio), pit density is substantially reduced, but it results in greater inhomogeneity in the distribution of indium in the InGaN well. This leads to a broadened PL emission and affect the PL emission intensity.     

Morphological, structural and optical properties of GaN grown on porous silicon/Si(100) substrate

In the present paper, we report results of GaN layers grown at 800 °C by metal organic vapour phase epitaxy (MOVPE) on porous silicon (PS) formed on Si(100) substrates. The surface morphology and the crystallinity of the GaN films were characterized by scanning electron microscope and X-ray diffraction. It was shown that GaN grows on PS preferentially on hexagonal polycrystalline form. The SEM observation reveals roughly surface textured by disoriented GaN grains having different shapes and sizes. The surface coverage and the wetting of GaN to PS are improved when the thickness of GaN layer increases. The optical properties of GaN layers were examined by PL and CL at low and room temperatures. Besides, the near edge-band (BE) emission, shows yellow (YL) and deep localized excitons bands at approximately 2.2 and 3.3-3.36 eV respectively. The depth CL analysis shows a spatial variation of the dominating YL and BE emissions as the electron beam energy rises from 3 to 25 kV.