氢等离子体处理SiC表面技术的研究

对SiC表面进行传统湿法清洗之后,采用电子回旋共振(ECR)氢等离子体系统在200℃低温条件下对SiC表面进行了氢等离子体处理,对处理前后的SiC样品表面分别做了反射高能电子衍射(RHEED)分析和X射线光电子能谱(XPS)分析。通过RHEED分析发现,经过氢等离子体处理的SiC表面比传统湿法清洗的SiC表面平整度更高,而且实验发现氢等离子体处理SiC表面时间越长,SiC表面平整度越高。通过XPS分析发现,SiC表面氧的含量显著减少,C/C—H化合物被去除,从而显著提高了SiC的抗氧化能力。     

分子束外延用碲锌镉(211)B衬底湿化学预处理技术研究

主要分析了不同溴甲醇溶液腐蚀方法对分子束外延用碲锌镉(211)B衬底表面粗糙度及反射式高能电子衍射(RHEED)图样的影响。实验发现化学抛光后未使用溴甲醇腐蚀的CZT(211)B衬底虽然表面粗糙度较小(小于1.0 nm),但表面RHEED衍射图样无任何衍射点或者条纹;采用0.05%溴体积比的溴甲醇溶液腐蚀CZT(211)B衬底时,即使腐蚀极短的时间,衬底表面粗糙度也达到2.0 nm以上,且表面存在高密度的柱状物,衬底表面RHEED图样呈圆点状或条纹较粗且存在亮点;采用0.01%溴体积比的溴甲醇溶液腐蚀CZT(211)B衬底,表面粗糙度可控制在1.0 nm左右,同时RHEED图样为特征的CZT(211)B晶面短条纹衍射图样,条纹清晰,同时多片衬底重复实验结果一致。同时在其上分子束外延的未优化的中波碲镉汞材料半峰宽为(49.1±5.9)arcsec,组分为0.3083±0.0003,厚度为6.54±0.019 μm。     

GaAs(100)同质外延表面相变的动态过程研究

本文叙述了用CCD系统对MBE生长中RHEED图案强度变化进行实时监测，通过（00）级条纹的RHEED强度分析，直接给出了不同生长条件下表面相变的动态过程，得到了从C（4×4）到α（2×4）的连续相变过程，进一步给出了不同条件下的表面化学配比情况．     

表面氢化对SiC/金属接触的作用机理

提出 Si C表面氢化模型。以氢饱和 Si C表面悬挂键 ,减少界面态 ,从而制备理想的金属 /半导体接触 ,并将此模型用于 Si C器件表面处理 ,其优点在于避免了欧姆接触 80 0～ 1 2 0 0°C的高温合金 ,且肖特基接触整流特性较好。在 1 0 0°C以下制备了比接触电阻 ρc=5～ 8× 1 0 - 3Ω·cm2的 Si C欧姆接触和理想因子 n=1 .2 5～ 1 .3的肖特基结。与欧姆接触采用 95 0°C高温合金制备的 Si C肖特基二极管比较表明 ,表面氢化处理不仅能避免高温合金 ,降低工艺难度 ,而且能改善器件的电学特性。     

激光斜冲实验与理论研究

金属板材的激光冲击成形通常采用激光束沿法向作用于零件表面,产生垂直于表面的冲击波作用力,但在曲面零件冲击成形时很难满足每次都为法向垂直冲击,因此非常有必要进行斜冲击研究。用钕玻璃激光器斜冲击LD31(6063)板料,并使用千分尺对变形特征进行了测量,实验表明等离子体的迎光性以及等离子体与激光之间的相互作用使得等离子体逆着激光方向斜膨胀,产生的应力波则顺着激光方向斜冲击板料,造成了被冲击板料变形的偏心。在激光能量约为42 J,脉冲宽度约为23 ns,光斑直径为8 mm,入射角度约为0°,30°,60°时,板料的偏心依次为1.00 mm,1.68 mm,3.77 mm,深度方向变形依次为5.60 mm,5.00 mm,3.00 mm,变形最大点显微硬度提高率依次为50%,33.3%,25%。     

GaAs(111)B上MBE同法质外延生长中的表面再构和RHEED分析

采用分子束外延(MBE)技术,在正晶向GaAs(111)B衬底上进行同质外延生长时,利用反射式高能电子衍射(RHEED)绘制出静态和动态生长条件下的表面相图,研究了生长参数对表面再构的影响,并通过衍射图案强度分析确定了适合镜像外延生长的(√19×√19)再构区域。在生长过程中,通过RHEED强度振荡和表面形貌的分析优化了生长参数,成功地生长出无金字塔结构的镜像外延薄膜。     

Cl_2/Ar/BCl_3 ICP刻蚀对AlGaN的损伤研究

感应耦合等离子体(ICP)刻蚀在AlGaN基紫外探测器台面制作中起着重要作用,初步研究了Cl2/Ar/BCl3ICP刻蚀对AlGaN材料的损伤。运用X射线光电子能谱(XPS)对ICP刻蚀前后的n型Al0.45Ga0.55N表面进行了分析,并对刻蚀后AlGaN材料在N2气中快速热退火进行了研究。结果表明,在N2气中550°C退火3 min对材料的电学性能有明显的改善作用。     

玻璃衬底上低温沉积GaN薄膜研究

采用电子回旋共振.等离子体增强金属有机物化学气相沉积(ECR-PEMOCVD)方法在康宁7101型普通玻璃衬底上沉积了GaN薄膜,利用反射高能电子衍射(RHEED)、X射线衍射(XRD)、原子力显微镜(AFM)和霍尔测量系统对样品进行了检测,研究了其结晶性和电学特性随沉积温度的变化.结果表明,当沉积温度为250-430℃时,得到的GaN薄膜都呈现高度的C轴择优取向,结晶性较好;薄膜表面形貌较为平整且呈n型导电.     

高稳定波导介质稳频体效应振荡器

本文给出了一种k_u波段高频率稳定性的波导介质稳频体效应振荡器,结构新颖实用。通过选用具有适当温度系数的新型介质谐振器,得到小于±250KHz(-10°C～±60°C)和±60KHz(0°C～±60°C)的高频率稳定性。     

基于石墨烯Tamm等离子体激元的太赫兹强耦合效应

设计了一种全新的一维石墨烯-光子晶体(PhC)复合谐振结构,使用传输矩阵法(TMM)对其所支持的太赫兹Tamm等离子体激元(TPP)和腔体模式(CM)之间的强耦合效应进行理论研究。研究表明,用于表征耦合强度的Rabi分裂能随着光子晶体周期数的减小而增大;改变间隔层和腔体厚度以及石墨烯的费米能级时,也会影响Rabi分裂能。最后通过入射角度对耦合模式进行主动调控,当入射角处于0°~60°时,耦合模式的特性与偏振无关。本文的研究结果为太赫兹强耦合效应的研究和应用提供了新的思路。     

Modification of 4H-SiC and 6H-SiC(0001)<Subscript>Si</Subscript> surfaces through the interaction with atomic hydrogen and nitrogen

The interaction of 4H-SiC(0001)_Si and 6H-SiC(0001)_Si surfaces with atomic hydrogen and atomic nitrogen produced by remote radio-frequency plasmas is investigated. The kinetics of the surface modifications is monitored in real time using ellipsometry, while chemical modifications of the surface are characterized using x-ray photoelectron spectroscopy (XPS). Film morphological properties are assessed with atomic force microscopy (AFM). A two-stage substrate preparation procedure is described that effectively removes oxygen from the SiC surface at low (200°C) temperature. In the first step, the SiC surface is etched with an HCl/HF acid solution as an alternative to the conventional HF(1%)-H₂O solution. The HCl/HF etch provides effective hydrogen passivation of the SiC surface. In the second step, the SiC surface is exposed to atomic hydrogen that selectively interacts with residual oxygen. In addition, the temperature dependence of the nitridation of SiC surfaces has also been investigated. It is found that interaction of SiC surfaces with atomic hydrogen at 200°C provides clean, smooth, and terraced surfaces suitable for epitaxial growth. In contrast, SiC surface exposure at high temperature (750°C) to atomic hydrogen and nitrogen results in very rough and disordered Si-rich surfaces. Finally, we find that the 4H-SiC surface is more reactive than the 6H-SiC surface to both species studied, independent of temperature. Surface geometry and electronic factors responsible for the observed reactivities are discussed.     

Dopant activation and surface morphology of ion implanted 4H- and 6H-silicon carbide

The activation of ion-implanted B into 4H-SiC, and B, and Al into 6H-SiC is investigated. Complete activation of B implants into 4H-SiC is achieved by annealing at 1750°C for 40 min in an Ar environment. Significant activation (>10%) is not achieved unless the annealing temperature is 1600°C or greater. Sheet resistances of Al-implanted 6H-SiC annealed at 1800°C are 32.2 kΩ/□, indicating high activation of Al at this temperature. Annealing conditions which result in good acceptor activation are shown to be damaging to the surface of either 4H- or 6H-SiC. Atomic force microscopy and Nomarski differential interference contrast optical microscopy are applied to characterize the surfaces of these polytypes. Roughening of the surfaces is observed following annealing in Ar, with measured roughnesses as large as 10.1 nm for B-implanted 4H-SiC annealed at 1700°C for 40 min. Based on data obtained from these techniques, a model is proposed to describe the roughening phenomenon. The premise of the model is that SiC sublimation and mobile molecules enable the surface to reconfigure itself into an equilibrium form.     

Reaction mechanisms of tertiarybutylarsine on GaAs (001) surfaces and its relevance to atomic layer epitaxy and chemical beam epitaxy

We explore the use of tertiarybutylarsine (TBAs) as an alternative arsine source in atomic layer epitaxy (ALE) of GaAs. X-ray photoelectron spectroscopy (XPS), reflection high energy electron diffraction (RHEED), and reflectance difference spectroscopy (RDS) are used to characterize the surface reactions of TBAs on GaAs (001) Ga-rich surfaces. At a substrate temperature of 320° C and an exposure level of 90 L of TBAs, AsH_x (x = 1 or 2) is thought to be the adsorbed arsenic species. As the substrate temperature increases, As-rich surfaces are readily obtained with improved RHEED 2 x 4 patterns. No carbon related species are observed throughout the TBAs exposure experiments between 320° C and 540° C. It is suggested that AsH_x is the adsorbed species after TBAs decomposes on surface Ga atom. Interactions between AsH_x pairs form arsenic atoms by H₂ release. RDS allows anin-situ real time study of TBAs on GaAs (001) Ga-rich surfaces. It is found that the RDS results are consistent with those obtained from XPS and RHEED investigations and can provide information on the rates of reactions and the extent of surface reconstruction simultaneously. Implications for the growth of GaAs by atomic layer epitaxy and chemical beam epitaxy using TBAs are discussed.     

Very low defect remote hydrogen plasma clean of Si (100) for homoepitaxy

We discuss a remote hydrogen plasma cleaning technique which is effective at removing carbon, nitrogen and oxygen from a silicon surface that has been pretreated with a wet chemical clean and a final dilute HF dip. It has been found that for best results, air exposure of the wafer after the HF dip must be minimized, as the ability of the H plasma clean to remove oxygen seems to be reduced on wafers which have been exposed to air for several hours. The atomic H supplied by the clean also results in different hydrogen terminated surface reconstructions depending on the substrate temperature. We observe a (1 × 1) RHEED pattern at clean temperatures below 190° C which is believed to be due to disordered silicon monohydride and dihydride termination, a (3 × 1) pattern between 200 and 280° C, corresponding to ordered monohydride and dihydride coverage, transitioning to a (2 × 1) pattern around 300° C due to monohydride termination. A dilute HF dip produces a (1 × 1) pattern due to disordered (3 × 1) cells,i.e. disordered monohydride and dihydride coverage. The passivating effects of the dilute HF dip and of the remote hydrogen plasma clean have also been discussed. The two processes have been found to result in similar hydrogen coverage of the wafer surface which in turn results in similar surface passivation. Both the dilute HF dip and thein situ remote H plasma clean have been found to protect the wafer from contamination for up to 15 min in ambient air and for an indefinite period under vacuum.     

The use of atomic hydrogen for low temperature oxide removal from HgCdTe

Reflection high energy electron diffraction (RHEED) patterns of HgCdTe surfaces etched with bromine methanol are diffuse with a faint ring pattern indicative of an overlayer consisting of a mixture of oxides and amorphous Te. Exposure to an atomic hydrogen flux results in a RHEED pattern indicative of a high quality, two-dimensional surface. Atomic force microscopy (AFM) measurements indicate a rms surface roughness less than 1 nm. CdTe grown on this surface at 80°C maintains the streaky RHEED pattern and smooth surface as indicated by AFM. X-ray photoelectron spectroscopy measurements indicate that the etched surfaces contain both an oxide layer and a metallic Te overlayer which were removed by continued exposure to atomic hydrogen. Further exposure results in significant HgTe depletion, which appears to be a near-surface phenomenon. Preliminary device results indicate that use of atomic hydrogen is a viable approach for low temperature cleaning of etched HgCdTe surfaces.     

The electrical characteristics of 4H-SiC schottky diodes after inductively coupled plasma etching

The electrical characteristics of metal contacts fabricated on 4H-SiC have been investigated. Sputtered nickel ohmic contacts have been successfully produced on untreated 4H-SiC substrates and 4H-SiC surfaces cleaned with aggressive chemicals and ion sputtering. The current-voltage (I–V) characteristics of asdeposited contacts are observed to be nonohmic on all surfaces. After annealing at 1,000°C in a N₂ atmosphere, the contacts are seen to become ohmic with considerably less annealing time being required for the samples exposed to aggressive cleaning stages. Schottky diodes were then produced on the silicon carbide (SiC) surface after etching in an SF₆/O₂ inductively coupled plasma (ICP) for 3 min at varying substrate bias voltages and also on an untreated surface used as a control sample. The ideality factors of all diodes formed on the etched surfaces increased in comparison to the control sample. The highest ideality factor was observed for the diodes produced after etching at −0-V and −245-V bias voltage. A two-diode and resistor model was applied to the results that successfully accounted for the excess leakage paths. The defect density of each SiC surface was calculated using both the measured and the simulated ideality factors. An annealing stage was successful at reducing the ideality factors of all the diodes formed. The defect density calculated using the measured ideality factors of the annealed diodes was seen to have been reduced by an order of magnitude.     

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.     

Investigation of Ti/Al and TiN/Al thin films as the stable ohmic contact for p-type 4H-SiC

Interfacial reactions, surface morphology, and current-voltage (I-V) characteristics of Ti/Al/4H-SiC and TiN/Al/4H-SiC were studied before and after high-temperature annealing. It was observed that surface smoothness of the samples was not significantly affected by the heat treatment at up to 900°C, in contrast to the case of Al/SiC. Transmission electron microscopy (TEM) observation of the Ti(TiN)/Al/SiC interface showed that Al layer reacted with the SiC substrate at 900°C and formed an Al-Si-(Ti)-C compound at the metal/SiC interface, which is similar to the case of the Al/SiC interface. The I-V measurement showed reasonable ohmic properties for the Ti/Al films, indicating that the films can be used to stabilize the Al/SiC contact by protecting the Al layer from the potential oxidation and evaporation problem, while maintaining proper contact properties.     

SiC power DIMOS with double implanted Al/B P-well

Power 6H- and 4H-SiC DIMOS test structures have been fabricated using a new technology. P-well formation was optimised from simulation study and previous experiments on high-energy Al implantation. Reduction of Al presence at interface and reduced defect formation during implantation were the main objectives of this optimisation. Complementary surface implantation of boron was performed to adjust the interface (channel) doping. Power DIMOS were characterised at ambient temperature and up to 320°C. 4H-SiC DIMOS exhibits very poor characteristics while 6H-SiC device functionality is maintained at 300°C. Power DIMOS without complementary boron implantation also exhibits good performances up to 300°C.     

Cleaning and passivation of the Si(100) surface by low temperature remote hydrogen plasma treatment for Si epitaxy

This paper presents the results of a study of the hydrogen-passivated Si(100) surface prepared by a remote hydrogen plasma treatment which serves the dual purpose of cleaning and passivating the Si(100) surface prior to low temperature Si epitaxy by Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD). The remote hydrogen plasma treatment was optimized for the purposes of cleaning and passivation, respectively. To achieve a clean, defect-free substrate surface, the remote hydrogen plasma process was first optimized using Transmission Electron Microscopy (TEM) and Auger Electron Spectroscopy (AES). For hydrogen passivation, the substrate temperature was varied from room temperature to 250° C in order to investigate the degree of passivation as a function of substrate temperature by examining the amount of oxygen readsorbed on the substrate surface after air exposure. Low temperature Si expitaxy was subsequently performed on the air-exposed substrates without further cleaning to evaluate the effectiveness of the hydrogen passivation. It was found that better Si surface passivation is achieved at lower substrate temperatures as evidenced by the fact that less oxygen is observed on the surface using AES and Secondary Ion Mass Spectroscopy (SIMS) analyses. The amount of readsorbed oxygen on the H-passivated Si surface after a two hour air exposure was found to be as low as 0.1 monolayer from SIMS analysis. Using Reflection High Energy Electron Diffraction (RHEED) analysis, different surface reconstructions ((3 × 1) and (1 × 1)) were observed for H-passivated Si surfaces passivated at various temperatures, which was correlated to the results of AES and SIMS analyses. Epitaxial growth of Si films at 305° C was achieved on the air-exposed Si substrates, indicating a chemically inert Si surface as a result of hydrogen passivation. A novel electron-beam-induced-oxygen-adsorptiom phenomena was observed on the Hpassivated Si surface. Scanning Auger Microscopy (SAM) analysis was performed to study the reaction kinetics as well as the nature of Si—H bonds on the H-passivated Si surface. Preliminary results show that there is a two-step mechanism involved, and oxygen adsorption on the H-passivated Si surface due to electron beam irradiation may be due to the formation of O-H groups rather than the creation of Si—O bonds.