Analyses of HF/NH4F buffer-treated Si(111) surfaces using XPS,REM and SIMS

The effect of different cleaning procedures on Si(111) wafers has been studied. A three-step cleaning process was used. The first two steps (thermal oxidation followed by RCA cleaning) were common to all samples. The final step involved rinsing in one of a set of HF/NH4F buffer solutions with a wide range ofpH values. Three different surface techniques were used for characterizing the chemical condition and morphology of the treated surfaces: XPS (X-ray Photoemission Spectroscopy), REM (Reflection Electron Microscopy) and SIMS (Secondary-Ion Mass Spectroscopy). It has been found that thepH value of an HF solution does significantly affect the etching rate and morphology of the Si(111) surface: For the same type of solution, the smaller thepH value, the higher the etching rate. Basic solutions withpH values larger than eight have a much weaker etching effect on the surface, which is contradictory to some previous reports. The most effective solutions for the etching of the Si(111) surface are the solutions of HF buffered by NH₄F, with thepH in the range of 2–6. REM images indicate that the surface morphology after etching in the HF solution is strongly affected by the length of the etching time: Overetching will roughen the surface. The SIMS data show that water rinsing in air during the cleaning process does speed up oxidation, but it is necessary to use water to clean off the residuals from the HF solutions.     

Quasiparticle spectrum and optical excitations of semiconductor surfaces

I investigated the spectra of well-ordered semiconductor surfaces within an ab-initio framework. Both the quasi-particle spectrum of electron and hole states and the optical differential reflectivity spectrum were addressed. As examples, I discuss the spectra of three surfaces: Si(111)-(2×1), hydrogenated H:Si(111)-(1×1), and Si adatom-terminated 6H-SiC(0001)-(×). I studied a number of physical features beyond the single-particle band-structure picture. In the case of Si(111)-(2×1), the dangling-bond surface states give rise to a surface exciton which dominates the differential reflectivity spectrum. In the case of 6H-SiC(0001)-(×), a Mott-Hubbard metal-insulator transition is observed. All calculations were performed within many-body perturbation theory, employing single- and two-particle Green functions. The solutions of the corresponding equations of motion yielded the observable excitations, i.e., single-particle electron and hole excitations, as well as bound and resonant electron-hole pair excitations. Received: 28 April 2000 / Accepted: 19 June 2000 / Published online: 7 March 2001     

Chemical etching investigation of polycrystalline p-type 6H-SiC in HF/Na2O2 solutions

In this work, an experimental study on the chemical etching reaction of polycrystalline p-type 6H-SiC was carried out in HF/Na₂O₂ solutions. The morphology of the etched surface was examined with varying Na₂O₂ concentration, etching time, agitation speed and temperature. The surfaces of the etched samples were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) Fourier transform infrared spectroscopy (FT-IR) and photoluminescence. The surface morphology of samples etched in HF/Na₂O₂ is shown to depend on the solution composition and bath temperature. The investigation of the HF/Na₂O₂ solutions on 6H-SiC surface shows that as Na₂O₂ concentration increases, the etch rate increases to reach a maximum value at about 0.5 M and then decreases. A similar behaviour has been observed when temperature of the solution is increased. The maximum etch rate is found for 80 °C. In addition, a new polishing etching solution of 6H-SiC has been developed. This result is very interesting since to date no chemical polishing solution has been developed on the material.     

密度泛函理论方法研究6H-SiC(0001)表面对氧分子和水分子的吸附

本文通过密度泛函方法计算6H-SiC(0001)表面对氧分子和水分子的吸附. 在6H-SiC(0001)表面上吸附的O₂分子自发地解离成O^*,并被吸收在C与Si原子之间的空位上. 吸附的H₂O自发地分解成OH^*和H^*,它们都被吸附在Si原子的顶部,OH^*进一步可逆地转化为O^*和H^*. H^*可以使Si悬键饱和并改变O^*的吸附类型,并进一步稳定6H-SiC(0001)表面并防止其转变为SiO₂.     

Vicinal and on-axis surfaces of 6H-SiC(0001) thin films observed by scanning tunneling microscopy

Surfaces of 6H-SiC(0001) homoepitaxial layers deposited on vicinal (3.5° off (0001) towards [11 0]) and on-axis 6H---SiC wafers by chemical vapour deposition have been investigated using ultra-high vacuum scanning tunneling microscopy. Undulating step configurations were observed on both the on-axis and the vicinal surfaces. The former surface possessed wider terraces than the latter. Step heights on both surfaces were 0.25 nm corresponding to single bilayers containing one Si and one C layer. After annealing at T>1100°C for 3–5 min in UHV, selected terraces contained honeycomb-like regions caused by the transformation to a graphitic surface as a result of Si sublimation. A model of the observed step configuration has been proposed based on the observation of the [ 110] or [1 10] orientations of the steps and energetic considerations. Additional deposition of very thin (2 nm) SiC films on the above samples by gas source molecular beam epitaxy was performed to observe the evolution of the surface structure. Step bunching and growth of 6H---SiC layers and formation of 3C---SiC islands were observed on the vicinal and the on-axis surfaces, respectively, and controlled by the diffusion lengths of the adatoms.     

氢氟酸刻蚀对Ni/6H-SiC接触性质的作用

采用浓度为10%的氢氟酸（HF）刻蚀6H-SiC单晶片,研究了HF刻蚀时间对Ni/6H-SiC接触性质的影响．经24?h刻蚀的SiC基片在溅射Ni层后,其接触表现良好线性的电流-电压（I-V）曲线．低于这个腐蚀时间的接触具有明显的势垒,但在大于1000℃快速退火后,也得到了良好线性的I-V曲线．X射线衍射（XRD）和俄歇能谱（AES）深度元素分析表明Ni₂Si和C是快速退火后的主要产物．XRD和低能反射电子能量损失谱表明表层的C 关键词： 欧姆接触 SiC 富碳层 互扩散     

Silicon interaction with the (0001) surface of La and Gd layers

A study is reported on a system consisting of a Si layer on the surface of rare-earth metals (REMs), which is the reverse of a rare-earth metal on silicon, the system of current widespread interest. Interaction of silicon with the (0001) surface of trivalent La and Gd single-crystal layers grown on a W(110) surface is studied by Auger spectroscopy combined with layer-by-layer argon-ion etching of the system and photoelectron spectroscopy. It is found that silicon interacts with the La(0001) and Gd(0001) surfaces even at room temperature with the formation of silicide, but no mutual mixing of the silicon and substrate atoms occurs. When the Si/La(0001) and Si/Gd(0001) systems are heated at 400°C, silicon does not diffuse into the bulk of the metal substrate or to the REM/W(110) interface.     

Self-ordering of nanofacets on vicinal SiC surfaces

Vicinal 4H and 6H-SiC(0001) surfaces have been investigated using atomic force microscopy and cross-sectional high-resolution transmission electron microscopy. We observed the characteristic self-ordering of nanofacets on any surface, regardless of polytypes and vicinal angles, after gas etching at high temperature. Two facet planes are typically revealed: (0001) and high index (112;n) that are induced by equilibrium surface phase separation. A (112;n) plane may have a free energy minimum due to attractive step-step interactions. The differing ordering distances in 4H and 6H polytypes imply the existence of SiC polytypic dependence on nanofaceting. Thus, it should be possible to control SiC surface nanostructures by selecting a polytype, a vicinal angle, and an etching temperature.     

Adsorption of water molecule on (001) and (110) surfaces of MgH2

First principle calculations have been performed to explore the adsorption characteristics of water molecule on (001) and (110) surfaces of magnesium hydride. The stable adsorption configurations of water molecule on the surfaces of MgH₂ were identified by comparing the total energies of different adsorption states. The (110) surface shows a higher reactivity with H₂O molecule owing to the larger adsorption energy than the (001) surface, and the adsorption mechanisms of water molecule on the two surfaces were clarified from electronic structures. For both (001) and (110) surface adsorptions, the O p orbitals overlapped with the Mg s and p orbitals leading to interactions between O and Mg atoms and weakening the O–H bonds in water molecule. Due to the difference of the bonding strength between O and Mg atoms in the (001) and (110) surfaces, the adsorption energies and configurations of water molecule on the two surfaces are significantly different.     

Etchant anisotropy controls the step bunching instability in KOH etching of silicon

STM investigations of vicinal Si(111) surfaces etched in KOH solutions under controlled flow conditions show that step bunching instability is due to inhomogeneities that develop in the etchant as the result of highly step-site-specific etching reactions. Other previously postulated mechanisms for step bunching, including anisotropic surface diffusion, surface strain, and impurity deposition, are conclusively ruled out. The inhomogeneities locally accelerate etching near surface steps. Kinetic Monte Carlo simulations of this process qualitatively reproduce the observed morphologies.     

稀氢氟酸中硅表面氢终端的原位拉曼光谱研究

本文用共焦显微拉曼系统原位观察了Ｓｉ（１００）表面氢终端原子在稀氢氟酸中的变化过程。研究表明：在硅片浸入氢氟酸溶液的初期,表面主要被硅和三个氢原子的结合体（Ｓｉ Ｈ３）以及硅和两个氢原子的结合体（Ｓｉ Ｈ２）所覆盖。随着腐蚀过程的延长,Ｓｉ Ｈ３越来越少,Ｓｉ Ｈ２的信号不断增强,并且,硅和单个氢原子的结合体（Ｓｉ Ｈ）的信号也开始出现。最终,硅表面主要被Ｓｉ Ｈ２所覆盖,有少量Ｓｉ Ｈ３和Ｓｉ Ｈ键。本文还表明,拉曼光谱用来原位观察半导体材料表面终端原子键在溶液中的变化是很有用的工具     

6H-SiC(0001)表面graphene逐层生长的分子动力学研究

利用经典分子动力学方法和模拟退火技术分析研究了6H-ＳiC（0001）表面graphene的逐层生长过程及其形貌结构特点．研究表明,经过高温蒸发表面硅原子后,6H-ＳiC（0001）表面的碳原子能够通过自组织过程生成稳定的局部单原子层graphene结构．这种过程类似于6H-ＳiC（0001）表面graphene的形成,其生长和结构形貌演化主要取决于退火温度和表面碳原子的覆盖程度． 研究发现,当退火温度高于1400K时,6H-ＳiC（0001）表面碳原子能形成局部的单原子层graphene结构．这一转变温 关键词： graphene 碳化硅 分子动力学     

Thermal etching of sodium chloride III. Etching of non-cubic surface

Previous work on the thermal etching of (100) surfaces of NaCl single crystals is extended to the other surface orientations by using polycrystalline samples. The results obtained with (110), (111), (520) and (144) surfaces are used to corroborate the correctness of the mechanisms proposed in a previous paper to explain the thermal etching in air of (100) surfaces; it is also shown that these mechanisms are only dependent on crystal structure and not on the surface orientation.When such surfaces were treated with wet silk, concentrated HC1, and Moran's reagent, a very different behaviour was observed for each orientation. The reactivity is very sensitive to crystallographic orientation, being very low in the case of the (144) surface.     

Passivation effects of phosphorus on 4H-SiC(0001) Si dangling bonds: A first-principles study

The effect of phosphorus passivation on 4H-SiC(0001) silicon(Si) dangling bonds is investigated using ab initio atomistic thermodynamic calculations. Phosphorus passivation commences with chemisorption of phosphorus atoms at high-symmetry coordinated sites. To determine the most stable structure during the passivation process of phosphorus, a surface phase diagram of phosphorus adsorption on SiC(0001) surface is constructed over a coverage range of 1/9–1 monolayer(ML). The calculated results indicate that the 1/3 ML configuration is most energetically favorable in a reasonable environment. At this coverage, the total electron density of states demonstrates that phosphorus may effectively reduce the interface state density near the conduction band by removing 4H-SiC(0001) Si dangling bonds. It provides an atomic level insight into how phosphorus is able to reduce the near interface traps.     

Applications of porous silicon formed by electrochemical etching using an electrolyte based on HF:formaldehyde

In this work, we report the experimental results on the formation of porous silicon (PSi) monolayers by electrochemical etching using a formaldehyde based electrolyte. The results were compared with PSi monolayers obtained with the traditional electrolyte (HF:ethanol). Both electrolytes facilitate the removal of H₂ generated as a subproduct during the electrochemical etching process in the surface of the c-Si substrate. Formaldehyde presents a good affinity to surfaces and interfaces and the excess of water in the electrolyte reduces the pore sizes of PSi samples. The porosity and etching rate values are similar than those obtained using HF:et solutions. The refractive index values are the same in both cases at the same porosity in the visible range. The results have shown that the chemical characteristics of the ethanol and formaldehyde can give some different advantages to the PSi process and its applications.     

ATLEED研究6H-SiC(0001)-(3×3)R30°重构表面

低能电子衍射(LEED)对6H-SiC(0001)-(3×3)R30°表面的研究结果表明,该表面有1/3单层的Si原子吸附在T4空位上与第一个SiC复合层中的三个Si原子键接,它们之间的垂直距离为0.171nm.通过对该表面10个非等价垂直入射衍射束的自动张量低能电子衍射(ATLEED)计算,得到“最佳结构”由于表面SiC复合层堆积顺序不同而产生的三种表面终止状态(surface termination)的混合比例为S1∶S2∶S3=15∶15∶70,理论计算与实验I-V曲线比较得到可靠性因子RVHT=0.165,RP=0.142,表明表面生长符合能量最小化的台阶生长机制.     

Investigation of porous silicon carbide as a new material for environmental and optoelectronic applications

In this paper, we present an experimental study on the chemical and electrochemical etching of silicon carbide (SiC) in different HF-based solutions and its application in different fields, such as optoelectronics (photodiode) and environment (gas sensors). The thin SiC films have been grown by pulsed laser deposition method. Different oxidant reagents have been explored. It has been shown that the morphology of the surface evolves with the etching conditions (oxidant, concentration, temperature, etc.). A new chemical polishing solution of polycrystalline 6H-SiC based on HF:Na₂O₂ solution has been developed. Moreover, an electrochemical etching method has been carried out to form a porous SiC layer on both polycrystalline and thin SiC films. The PL results show that the porous polycrystalline 6H-SiC and porous thin SiC films exhibited an intense blue luminescence and a green-blue luminescence centred at 2.82 eV (430 nm) and 2.20 eV (560 nm), respectively. Different device structures based on both prepared samples have been investigated as photodiode and gas sensors.     

Surface structure of GaAs(211)

The atomic structures of the two inequivalent (211) surfaces of GaAs have been investigated by LEED. Both surfaces, prepared by etching and heat-cleaning or ion-sputtering and annealing, are unstable and develop large (110) facets which exhibit the atomic geometry of the (110) GaAs surface. These facets entirely cover the surface. Three sets of facets, making 30°, 30° and 54° angles with the (211) plane, are detected on one surface. Only two sets, making 30° angles with the (211) plane, are detected on the other. The reasons for this difference are not understood at this time. The LEED study of Si(211) and Ge(211) shows that the Si surface is flat whereas the Ge surface exhibits reconstructed (311) facets. The structural difference between the (211) surfaces of GaAs and Ge and the facetting of the compound are invoked to explain the problems encountered in the MBE growth of GaAs on Ge(211).     

Atomic,electronic, and vibronic structure of semiconductor surfaces

A brief review on recent progress in the theory of electronic, structural, and vibronic properties of semiconductor surfaces is presented with particular emphasis on the empirical and selfconsistent scattering theoretical method for semiinfinite systems. The current knowledge of the Si(001) (2×1) surface is discussed in detail. The Ge(001) (2×1) surface, as well as, the clean and the Ge-covered GaAs(110) surfaces are addressed, in addition. In the discussion of the results it is shown, that the scattering theoretical method is an extremely versatile tool for calculating electronic surface properties unambiguously with high spectral resolution concerning energy, wavevector, layer-index and orbital type. Currently used approaches for calculating the total energy, Hellmann-Feynman forces and optimal structure models are summarized. Using the total energy as a starting point, the calculation of atomic force constants and surface phonon spectra is exemplified.     

Investigation of the Hydrogen Etching Effect of the SiC Surface on the Formation of Graphene Films

Technical Physics - We have studied the effect of temperature and etching duration of the 4H-SiC (0001) surface in hydrogen on the structural perfection of graphene films grown by thermal...