ZnSe/ZnSxSe1−x heterojunction valence band discontinuity measured by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was used to measure the valence band discontinuity at the ZnSe/ZnS_xSe_1−x interface as a function of alloy composition. Due to the large lattice mismatch, the experimental values have to be corrected in order to account for strain in the ZnSe layers. We find the valence band discontinuity to vary between 0 and 0.76 eV as a function of S content x. This result is in excellent agreement with theoretical calculations. It also shows that the conduction band discontinuity is not negligibly small.     

Si衬底上无坑洞3C-SiC的外延生长研究

在冷壁式不锈钢超高真空系统上 ,利用低压化学气相淀积 (LPCVD)方法在直径为 5 0 mm的单晶 Si(1 0 0 )和 Si(1 1 1 )晶向衬底上生长出了高取向无坑洞的晶态立方相碳化硅 (3 C-Si C)外延材料 ,利用反射高能电子衍射 (RHEED)和扫描电镜 (SEM)技术详细研究了 Si衬底的碳化过程、碳化层的表面形貌及缺陷结构 ,获得了界面平整光滑、没有空洞形成的 3 C-Si C外延材料 ,并采用 X-射线衍射 (XRD)、双晶 X-射线衍射 (DXRD)和霍尔(Hall)测试等技术研究了外延材料的结构和电学特性     

Micro-raman investigation of the n-dopant distribution in lateral epitaxial overgrown GaN/sapphire (0001)

We have investigated the n-dopant distribution in the overgrown and window regions of lateral-epitaxial overgrown GaN/sapphire (0001) using room-temperature micro-Raman spectroscopy in the backscattering configuration. From a fit to the high energy-coupled longitudinal optical (LO) phonon-plasmon mode (LPP⁺), we have evaluated n ≈ (6.5±0.6) × 10¹⁷ cm⁻³ in the overgrown region; a value considerably higher than that previously reported by Pophristic et al.⁵ The spectrum from the window region was harder to interpret because of the considerable overlap of the A₁(LO) mode and E_g (750 cm⁻¹) sapphire line with the LPP⁺ trace. The implications of our findings for the overgrown region on the measured thermal conductivity as well as other parameters will be discussed.     

Mechanism investigation of NiO<Subscript>x</Subscript> in Au/Ni/p-type GaN ohmic contacts annealed in air

Recently, Au/Ni/p-type GaN ohmic contacts annealed in an air ambient have been widely investigated. However, to obtain a low specific-contact resistance, the annealing window is limited. In this study, to understand the oxidation function of metallic Ni, the Au/Ni/p-type GaN structure was annealed in an air ambient for 10 min at various temperatures. Using x-ray photoelectron spectroscopy (XPS) analysis, the metallic Ni was oxidized into NiO and NiO_1.3 compositions at annealing temperatures of 500°C and 600°C, respectively. However, metallic Ni still existed on the interface of the Ni/p-type GaN annealed at 400°C. The associated barrier heights of 0.42 eV, 0.21 eV, and 0.31 eV were obtained with p-type GaN for the Ni, NiO, and NiO_1.3 contacts, respectively. The hole concentrations of p-type NiO and p-type NiO_1.3 were 2.6×10¹⁶ cm⁻³ and 2.0×10¹⁸ cm⁻³, respectively. The lower hole concentration of the p-type NiO would lead to reducing the valence-band bending of the p-type GaN, as well as the barrier height for holes crossing from the p-type NiO to the p-type GaN. The formation of NiO was thus an important issue for lowering the specific-contact resistance of the Au/Ni/p-type GaN ohmic contacts annealed in an air ambient.     

Schottky barrier height studies of Au/4H-SiC(0001) using photoemission and synchrotron radiation

The Schottky barrier height (SBH) of Au on 4H-SiC(0001) has been studied using photoemission and synchrotron radiation. The Au was deposited in-situ on clean and well-ordered √3×√3 R30° reconstructed SiC surfaces prepared by in situ heating at ∼950°C. The SBH was determined from the shift observed in the Si 2p core level, in addition to the initial band bending determined for the clean surface. The results were compared with values obtained by electrical, capacitance-voltage (C-V), and current-voltage (I-V) characterization methods. A favorable comparison between the three independent, SBH determination methods was found.     

GaN/Al_2O_3(0001)的匹配机制及氮化的作用

在Ａｌ２Ｏ３（０００１）衬底上用ＭＯＣＶＤ（金属有机物气相沉积）方法进行了ＧａＮ的外延生长，通过Ｘ射线衍射（同步辐射源）研究了ＧａＮ和Ａｌ２Ｏ３（０００１）的匹配关系。结果表明，经充分氮化的衬底上，ＧａＮ以单一的匹配方式沿［０００１］方向生长；在Ａｌ２Ｏ３（０００１）衬底未经氮化或氮化不充分时，不同程度地出现了其它三种绕〈１１２０〉晶带轴倾斜一定角度的匹配位向。指出了ＧａＮ／Ａｌ２Ｏ３（０００１）的几种匹配方式的晶体学规律。ＧａＮ绕〈１１２０〉晶带轴倾斜的匹配方式是其外延生长过程中降低和Ａｌ２Ｏ３（０００１）的晶格失配、释放界面应变的重要机制之一。     

利用二次谐波产生研究Al2O3/Al(111)表面的动力学变化

主要介绍了当激光入射到单晶金属Al2O3／Al(111)表面时将发生二次谐波产生(SHG)现象的实验装置和实验方法,利用该装置可以研究SHG和从Al2O3／Al(111)表面反射的SHG在空间各向异性的变化情况,从而可以探测Al2O3／Al(111)表面结构对称性。实验中发现,当脉冲激光功率在2×106 W／cm2-9．6×106W／cm2范围内变化时,没有检测到SHG信号的各向异性变化;当使用P-极化泵浦激光时,发现Al2O3／Al(111)样品绕法线旋转360度时,P-极化的SHG信号在空间三个方向上呈现最大值相等;当使用脉冲强度为12×100 W／cm2的1064 nm P-极化泵浦激光时, Al2O3／Al(111)表面被损坏,损坏后的表面其SHG信号并不呈现对称的各向异性变化,当使用脉冲强度为11×106W／cm2的1064 nm P-极化泵浦激光时产生表面退火现象,从退火表面所产生的反射532 nm P-极化SHG信号中发现,SHG信号呈现衰减的各向异性成分。     

Nucleation of ZnTe on the As-Terminated Si(112) Surface

We employ a suite of surface analysis techniques that probe the outermost ZnTe/As-Si(112) surface to generate an understanding of the initial stages of the heteroepitaxial HgCdTe/CdTe/ZnTe/As-Si(112) layer formation. Ion scattering spectroscopy (ISS), reflection-high energy electron diffraction (RHEED), along with nondestructive depth profiles by angle-resolved x-ray photoelectron spectroscopy (XPS) are successfully applied to clarify and support the nucleation stages of ZnTe formation on the As-terminated Si(112) substrate. Data indicate a slow growth of the first ZnTe layer. In addition, no evidence of thick ZnTe island formation exists. The current ZnTe formation process generates full coverage on the Si(112) surface after six to nine MBE cycles. In order to fully understand the details of the ZnTe nucleation process on the Si(112) substrate, we present an inelastic background analysis with the Tougaard method to study surface morphology.     

Effects of H2/NH3 flow-rate ratio on the luminescent, structural, and electrical properties of GaN epitaxial layers grown by MOCVD

GaN epitaxial layers were grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. The flow-rate ratio of H₂ on the upper stream to NH₃ on the bottom stream is varied from 0.5 to 2. The growth condition and characterization of the GaN epitaxial layers are investigated in detail. The H₂ flow rate of the upper stream strongly affects the reactant gas flow pattern near the substrate surface and thus influences the quality of epitaxial layers. At the optimum H₂/NH₃ flow ratio of 1.0, we can obtain a good quality of GaN epitaxial layers which exhibit a strong near band-edge emis-sion in the 20 K photoluminescence (PL), a full width at half maximum of 66 meV for the 300 K PL, an electron mobility of 266 cm²/V-s and concentration of 1 × 10¹⁸ cm⁻³ at 300 K.     

CHF3-O2 reactive ion etching of 4H-SiC and the role of oxygen

Reactive ion etching of 4H-SiC was performed using a CHF₃-O₂ plasma. The etch rate and mean surface roughness were investigated as a function of the ratio of the O₂ flow rate to the total gas flow rate. It was found that oxygen plays an indirect role in contributing to the etching of SiC. An optimum O₂ fraction of 20% was found to give a maximum etch rate of 35 nm/min. On the other hand, the root-mean-square (RMS) surface roughness was found to increase from 1.31 to 2.34 nm when the O₂ fraction increased from 0% to 80%. Auger electron spectroscopy results for the samples etched at the optimum condition of 20% O₂ fraction revealed the presence of oxygen on the etched surface in a form of an oxide-like layer (SiO_x). No carbon residue (carbon rich-layer) and aluminum were found. Based on our results, the role of O₂ in the reactive ion etching of 4H-SiC will be presented.     

MIS characterization and modeling of the electrical properties of the epitaxial Caf2/Si(111) interface

CaF₂ layers have grown by molecular beam epitaxy on bothn-type andp- type Si(111). Capacitance-voltage (C-V) and current-voltage (I-V) measurements have been made on metal-insulator-semiconductor (MIS) capacitors to characterize these structures. For devices onp-type Si, C-V characteristics show only the insulator value of the capacitance over a wide range of applied voltages, indicating an accumulated surface. At room temperature, C-V characteristics ofn-type devices show the minimum value of the capacitance (corresponding to inversion) for small voltages, but modulation of the capacitance at larger voltages. At 200 K, this modulation is no longer present in the C-V curves. I-V curves show a rapid increase in the leakage current at relatively low fields at room temperature, and this leakage decreases dramatically with temperature. These results are largely independent of cleaning procedure, growth temperature, and the degree of misalignment of the substrate. The characteristics are modeled by assuming the Fermi level to be pinned at the valence-band edge, and the modulation in the C-V characteristics ofn-type samples to be driven by leakage currents. Work done at GE while a graduate student in the School of Electrical Engineering, Cornell University. Present address: Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109.     

Characterization of the silicon nitride–thermal oxide interface in oxide–nitride–oxide structures by ELS, XPS, ellipsometry, and numerical simulation

This work studies the properties of the SiO₂-Si₃N₄ interface in oxide-nitride-oxide (ONO) structures by using energy loss spectroscopy, X-ray photoelectron spectroscopy, ellipsometry measurements and numerical simulation. By oxidation the as-deposited Si₃N₄, silicon-silicon bonds at Si₃N₄-thermal SiO₂ interface are found. These excess Si-Si bonds are produced by replacing nitrogen with oxygen during the oxidation of Si₃N₄. We further propose that the Si-Si bonds are the major trap center at the Si₃N₄-SiO₂ interface. With MINDO/3 numerical simulation, we have found that the Si-Si bonds can capture both electrons and holes at the top Si₃N₄-SiO₂ interface. These bonds are proposed to be the responsible candidate for the positive charge accumulation in re-oxidized nitrided oxide.     

Influence of arsenic on the atomic structure of the Si(112) surface

The surface science techniques of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) have been used to characterize the clean Si(112) surface and the influence of an As monolayer on the properties and structure of the surface. In agreement with previous studies, the clean surface is found by both LEEd and atomically resolved STM images to be unstable with respect to faceting into other stable planes. Procedures for in-situ deposition of As onto clean Si surfaces were devised and XPS results show that approximately one monolayer of As can be deposited free of any contamination. The As/Si(112) surface is characterized by a sharper LEED pattern than for the clean surface and by STM images characterized by long rows along the direction with a regular width of 1.9 nm. This is consistent with a doubling of the periodicity in the direction of the bulk-terminated unit cell. This implies that As yields a stable but reconstructed Si(112) surface.     

In-situ spectroscopic study of the As and Te on the Si (112) surface for high-quality epitaxial layers

A detailed analysis of the As-exposed Si (112) and subsequent Te exposure was performed. X-ray photoelectron spectroscopy shows that the Te- and As-exposed Si (112) surface had 70% As and 27% Te coverage, respectively. Direct surface coverage measurement with ion scattering spectroscopy (ISS) shows that the Si (111) surface is completely covered by As, and that of the Si (112) had about 78% and 20% coverage of As and Te, respectively. Finally, using ISS shadowing effects, it was found that the Te atoms were positioned mainly on the step edges.     

Correlation of biaxial strains, bound exciton energies, and defect microstructures in gan films grown on AlN/6H-SiC(0001) substrates

Biaxial strains resulting from mismatches in thermal expansion coefficients and lattice parameters in 22 GaN films grown on A1N buffer layers previously deposited on vicinal and on-axis 6H-SiC(0001) substrates were measured via changes in the c-axis lattice parameter. A Poisson’s ratio of ν = 0.18 was calculated. The bound exciton energy (E_BX) was a linear function of these strains. The shift in E_BX with film stress was 23 meV/GPa. Threading dislocations densities of ~10¹⁰/cm² and ~10⁸/em² were determined for GaN films grown on vicinal and on-axis SiC, respectively. A 0.9% residual compressive strain at the GaN/AIN interface was observed by high resolution transmission electron microscopy (HRTEM).     

电离辐射对Si_3N_4/SiO_2/Si中SiO_2禁带的影响

根据X光激发电子能谱(XPS)中元素各个态的位置与价带顶、导带底的位置关系,提出了对一个已有唯像模型的修正,由这个修正模型能够利用XPS数据考察异质结的禁带在经历某些过程后是否有变化。将这个方法应用于经历60Co辐照的Si3N4/SiO2/Si,结果表明:从SiO2到Si存在SiO2禁带的弯曲,而辐照将SiO2禁带变薄;同时,SiO2禁带的变化明显依赖于辐照条件。就实验现象的机制进行了探讨。     

原位XPS分析Al2O3作为势垒层的Er2O3/Si结构

在超高真空条件下,通过脉冲激光沉积(PLD)技术制作了Er2O3/Al2O3/Si多层薄膜结构,原位条件下利用X射线光电子能谱(XPS)研究了Al2O3作为势垒层的Er2O3与Si界面的电子结构.XPS结果表明,Al2O3中Al的2p芯能级峰在低、高温退火前后没有变化;Er的4d芯能级峰来自于硅酸铒中的铒,并非全是本征氧化铒薄膜中的铒;衬底硅的芯能级峰在沉积Al2O 3时没有变化,说明Al2O3薄膜从沉积到退火不参与任何反应,与Si界面很稳定;在沉积Er2O3薄膜和退火过程中,有硅化物生成,表明Er2O3与Si的界面不太稳定,但随着Al2O3薄膜厚度的增加,其硅化物中硅的峰强减弱,含量减少,说明势垒层很好地起到了阻挡扩散的作用.     

复合物Mg+-S2(CH3)2光解离光谱

在230～440 nm波段,研究了复合物Mg -S2(CH3)2的光解离光谱.此波段内的复合物光诱导产物的质谱揭示,存在着非反应猝灭产物Mg 和反应产物Mg -SCH3.反应产物来源于Mg 离子插入S-S化学键,导致复合物解离的过程.复合物的光解离光谱由三个对应于离子Mg (32P←32S)跃迁的谱峰构成.由量化计算中的CIS方法所获得的吸收谱理论值与实验吻合地较好.