GaN外延层中的缺陷研究

采用阴极射线致发光法观察金属有机物汽相外延法生长的具有不同表面形貌的GaN外延层中黄色发光带的强度分布.结果表明六角金字塔形表面形貌对发光强度分布的测量有很大影响.测量和比较表面镜面加工样品的黄色发光带强度分布、原子序数衬度和X射线波谱发现,黄色发光带的强度在含有O和C等杂质缺陷附近较强.高分辨透射电子显微镜观察表明,杂质缺陷区的晶格结构不同于GaN基质,以及位错和裂缝等由应力引起的缺陷.认为此类缺陷可能是生长过程中,杂质在结晶小丘合并处的V形凹角中的沉积所产生. 关键词：     

Si基GaN外延层光致发光光谱与二次离子质谱的研究

报道了在Si基上用简便的真空反应法制备出GaN外延层．光致发光光谱测试结果表明不同的生长温度和退火工艺会对GaN外延层的发光特性产生影响,在1050℃下生长的GaN外延层的发光强度高于其他温度下生长的发光强度,退火可以使GaN外延层的发光强度增强．二次离子质谱(SIMS)测试结果表明外延层中Ga和N分布均匀,在表面处Ga发生了偏聚,同时外延层中还存在Si,O等杂质,这使得外延层中背景电子浓度高达1.7×10¹⁸/cm³. SIMS测试结果还表明,在外延生长前采用 关键词：     

氢化物气相外延生长高质量GaN膜生长参数优化研究

系统研究了低温成核层生长时间、高温生长时的V/Ⅲ 比以及生长温度对氢化物气相外延生长GaN膜晶体质量的影响. 研究发现合适的低温成核层为后续高温生长提供成核中心, 并能有效降低外延膜与衬底间的界面自由能, 促进成核岛的横向生长; 优化的V/Ⅲ比和最佳生长温度有利于降低晶体缺陷密度, 促进横向生长, 增强外延膜的二维生长. 利用扫描电子显微镜、原子力显微镜、高分辨X射线衍射、 低温光致发光谱和室温拉曼光谱对优化条件下生长的GaN外延膜进行了结构和光电特性表征. 测试结果表明, 膜表面平整光滑, 呈现二维生长模式表面形貌; (002)和(102)面摇摆曲线半高宽分别为317和343 arcsec; 低温光致发光谱中近带边发射峰为3.478 eV附近的中性施主束缚激子发射峰, 存在11 meV的蓝移, 半高宽为10 meV, 并且黄带发光强度很弱;常温拉曼光谱中E₂ (high) 峰发生1.1 cm^-1蓝移.结果表明, 优化条件下生长的GaN外延膜具有良好的晶体质量和光电特性, 但GaN 膜中存在压应力. 关键词： 氮化镓 氢化物气相外延 低温成核层     

侧向外延法生长的高质量GaN及其外延缺陷的观察

在有条状SiO2图形的GaN“模板”上,侧向外延方法生长了高质量的GaN。荧光显微镜的结果表明在SiO2掩膜区有成核过程发生。原因可能是SiO2的质量不高,为GaN的生长提供了一些成核中心。在GaN层的厚度达到4．5μm后,侧向的融合开始发生。侧向生长的速度与垂直生长速度几乎相同。在所有的SiO：掩膜上方都形成了空洞。样品在240℃熔融的KOH中腐蚀13min。在SiO2掩膜区生长的GaN,其腐蚀坑密度(相当于穿透位错密度)减少到几乎为零。而在窗口区生长的GaN,腐蚀坑密度仍然很高,达到10^8cm^-2量级。同时,我们发现具有不同窗口尺寸的样品在SiO2掩膜区上侧向生长的CaN的晶体质量基本相同,与窗口区的宽度几乎无关。室温光荧光结果表明侧向外延法生长的CaN中的晶格失配应力已被部分释放。     

表面预处理对石墨烯上范德瓦耳斯外延生长GaN材料的影响

基于范德瓦耳斯外延生长的氮化镓/石墨烯材料异质生长界面仅靠较弱的范德瓦耳斯力束缚,具有低位错、易剥离等优势,近年来引起了人们的广泛关注.采用NH_3/H_2混合气体对石墨烯表面进行预处理,研究了不同NH_3/H_2比对石墨烯表面形貌、拉曼散射的影响,探讨了石墨烯在NH_3和H_2混合气氛下的表面预处理机制,最后在石墨烯上外延生长了1.6μm厚的GaN薄膜材料.实验结果表明:石墨烯中褶皱处的C原子更容易与气体发生刻蚀反应,刻蚀方向沿着褶皱进行;适当NH_3/H_2比的混合气体对石墨烯进行表面预处理可有效改善石墨烯上GaN材料的晶体质量.本研究提供了一种可有效提高GaN晶体质量的石墨烯表面预处理方法,可为进一步研究二维材料上高质量的GaN外延生长提供参考.     

MOCVD侧向外延GaN的结构特性

侧向外延（EL0）GaN的原子力显微镜（AFM）表面形貌图像表明,ELO—GaN相当平整,而掩膜区中央有线状分布的小丘群,这是由ELO—GaN在掩膜区中央接合时晶向不一致而产生的缺陷造成的。观察ELO—GaN扫描电子显微镜（SEM）截面图,侧向接合处存在着三角空洞,并且侧向接合导致上述小丘群。X射线衍射（XRD）曲线表明,掩膜边界处的GaN晶面有倾斜。拉曼散射谱表明,掩膜区GaN质量相对于窗口区大为提高。ELO—GaN和普通外延GaN的拉曼散射谱比较表明,ELO—GaN中的应力较小,晶体质量较高。     

GaN外延片中载流子浓度的纵向分布

采用适合宽禁带半导体材料的电化学电容电压(ECV)分析仪,对掺硅GaN外延片用硫酸逐层进行了精密腐蚀后,在此基础上得到了在进口MOCVD设备上生产的GaN基外延片的载流子浓度纵向分布.探讨了该分布与晶体生长过程及晶体质量的关系,测量分析结果可为生长工艺参数的优化提供参考.还采用主扩散模型对测量结果进行高斯拟合,得出了高温时(1030℃)硅在GaN中的扩散系数,并由此估算了硅在GaN外延片中的扩散宽度.该结果可为GaN外延层结构设计提供参考.     

氢化物气相外延生长的GaN膜中的应力分析

对在c面蓝宝石上用氢化物气相外延法(HVPE)生长的六方相纤锌矿结构的GaN膜中的应力进行了分析。高分辨X射线衍射(002)面和(102)面摇摆曲线扫描(半高宽数值分别为317和358角秒)表明生长的GaN膜具有较好的晶体质量。利用高分辨X射线衍射技术准确测量了制备的GaN膜的晶格常数,并计算得到GaN膜中面内双轴应变和面外双轴应变分别为3.37×10-4和-8.52×10-4,等静压应变为-7.61×10-5。拉曼光谱和激光光致发光谱测试表明HVPE-GaN外延膜具有较好的光学特性,利用拉曼光谱的E₂模式特征峰和激光光致发光谱中近带边发射峰的频移定量计算了外延膜中的面内双轴压应力和等静压应力。两种方法得到的面内双轴压应力较为相符。     

三甲基镓流量对GaN外延层和GaN缓冲层生长的影响

采用金属有机物气相外延法在蓝宝石衬底上生长了以GaN为缓冲层的GaN薄膜.研究了不同三甲基镓流量下所生长缓冲层对GaN外延层质量的影响.对样品采用X线双晶衍射法测试其结晶质量,光致发光法测试其光学特性.实验结果显示高三甲基镓流量下生长的缓冲层可以提高GaN外延层的质量.对缓冲层进行的原子力显微镜测试分析表明不同三甲基镓流量会显著地影响缓冲层的生长模式.根据试验结果构造了一个GaN缓冲层的生长模型.     

Effect of Si doping in wells of A1GaN/GaN superlattice on the characteristics of epitaxial layer

An A1GaN/GaN superlattice grown on the top of a GaN buffer induces the broadening of the full width at half maximum of （102） and （002） X-ray diffraction rocking curves. With an increase in the Si-doped concentration in the GaN wells, the full width at half maximum of the （102） rocking curves decreases, while that of the （002） rocking curves increases. A significant increase of the full width at the half maximum of the （002） rocking curves when the doping concentration reaches 2.5 × 10^19 cm-3 indicates the substantial increase of the inclined threading dislocation. High level doping in the A1GaN/GaN superlattice can greatly reduce the biaxial stress and optimize the surface roughness of the structures grown on the top of it.     

Temperature-dependent photoluminescence spectra of GaN epitaxial layer grown on Si(111) substrate

In this paper, the temperature-dependent photoluminescence(PL) properties of Ga N grown on Si(111) substrate are studied. The main emission peaks of Ga N films grown on Si(111) are investigated and compared with those grown on sapphire substrates. The positions of free and bound exciton luminescence peaks, i.e., FX A and D0 X peaks, of Ga N films grown on Si(111) substrates undergo red shifts compared with those grown on sapphire. This is attributed to the fact that the Ga N films grown on sapphire are under the action of compressive stress, while those grown on Si(111) substrate are subjected to tensile stress. Furthermore, the positions of these peaks may be additionally shifted due to different stress conditions in the real sample growth. The emission peaks due to stacking faults are found in Ga N films grown on Si(111) and an S-shaped temperature dependence of PL spectra can be observed, owing to the influence of the quantum well(QW) emission by the localized states near the conduction band gap edge and the temperature-dependent distribution of the photo-generated carriers.     

Raman spectroscopy of optical phonons as a probe of GaN epitaxial layer structural quality

We report on Raman scattering measurements of all Raman-active phonons in wurtzite and zinc blende structure GaN epilayers grown on GaAs (001), GaAs (111)A, and GaAs (111)B oriented substrates by means of molecular beam epitaxy (MBE). Raman spectra are taken from these epilayers at room temperature and 77 K in backscattering geometry. The measured values of the phonon frequencies are in agreement with other studies and with lattice dynamic calculations of phonon modes in GaN zinc blende and wurtzite structures. We show that crystal quality is much better in samples grown on GaAs (111) substrates than in samples grown on GaAs (001) substrates. The observation of disorder-activated modes gives information about sample quality. Comparison of the spectra from different thickness epilayers shows that the GaN is more highly disordered close to the substrate, particularly for the (001) substrates. Received 16 July 1999     

Study of thermal and optical properties of SiO2/GaN opals by photothermal deflection technique

In this paper the thermal and optical properties of the SiO₂/GaN synthetic opals are studied by photothermal deflection technique. This technique, used in different configurations, allows to determine the effective thermal diffusivity and the absorption spectra.     

The influence of strain-reducing layer on strain distribution and ground state energy levels of GaN/AlN quantum dot

This article deals with the strain distributions around GaN/AlN quantum dots by using the finite element method. Special attention is paid to the influence of Al_0.2Ga_0.8N strain-reducing layer on strain distribution and electronic structure. The numerical results show that the horizontal and the vertical strain components are reinforced in the GaN quantum dot due to the presence of the strain-reducing layer, but the hydrostatic strain in the quantum dot is not influenced. According to the deformation potential theory, we study the band edge modifications and the piezoelectric effects. The result demonstrates that with the increase of the strain reducing layer, the transition energy between the ground state electron and the heavy hole increases. This result is consistent with the emission wavelength blue shift phenomenon observed in the experiment and confirms that the wavelength shifts toward the short wavelength range is realizable by adjusting the structure-dependent parameters of GaN/AlN quantum dot.     

Effect of Si doping in wells of AlGaN/GaN superlattice on the characteristics of epitaxial layer

An AlGaN/GaN superlattice grown on the top of a GaN buffer induces the broadening of the full width at half maximum of (102) and (002) X-ray diffraction rocking curves. With an increase in the Si-doped concentration in the GaN wells, the full width at half maximum of the (102) rocking curves decreases, while that of the (002) rocking curves increases. A significant increase of the full width at the half maximum of the (002) rocking curves when the doping concentration reaches 2.5 × 1019 cm-3 indicates the substantial increase of the inclined threading dislocation. High level doping in the AlGaN/GaN superlattice can greatly reduce the biaxial stress and optimize the surface roughness of the structures grown on the top of it.     

碳化硅基上3UCVD淀积二氧化硅及其C-V性能测试

利用3UCVD技术在p型4H-SiC上制备了栅氧化层，其正的氧化物电荷密度仅有1.6×10¹¹cm^-2,这一结果优于传统的热氧化工艺.为检验氧化层质量所做的高频C-V测试采用了正面接地的新的测试结构，克服了常规测试结构的缺点. 关键词： 淀积 高频C-V测试 二氧化硅 碳化硅     

Influence of thermal stress on the relative permittivity of the AlGaN barrier layer in an AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on AlGaN/GaN heterostructures were fabricated. Some samples were thermally treated in a furnace with N₂ ambience at 600 °C for different times (0.5 h, 4.5 h, 10.5 h, 18 h, 33 h, 48 h, and 72 h), the others were thermally treated for 0.5 h at different temperatures (500 °C, 600 °C, 700 °C, and 800 °C). With the measured current—voltage (I—V) and capacitance—voltage (C—V) curves and by self-consistently solving Schrodinger's and Poisson's equations, we found that the relative permittivity of the AlGaN barrier layer was related to the piezoelectric and the spontaneous polarization of the AlGaN barrier layer. The relative permittivity was in proportion to the strain of the AlGaN barrier layer. The relative permittivity and the strain reduced with the increased thermal stress time until the AlGaN barrier totally relaxed (after 18 h at 600 °C in the current study), and then the relative permittivity was almost a constant with the increased thermal stress time. When the sample was treated at 800 °C for 0.5 h, the relative permittivity was less than the constant due to the huge diffusion of the contact metal atoms. Considering the relation between the relative permittivity of the AlGaN barrier layer and the converse piezoelectric effect, the conclusion can be made that a moderate thermal stress can restrain the converse piezoelectric effect and can improve the stability of AlGaN/GaN heterostructure devices.