Ordering distance of surface nanofacets on vicinal 4H-SiC(0001)

After high-temperature H2 etching, vicinal SiC(0001) surfaces showed periodically ordered nanofacet structures consisting of pairs of (0001) and (112n). Here, we found that the characteristic ordering distance of approximately 10nm is independent of the vicinal angle (4 degrees -8 degrees off). However, fluctuation in the ordering distance is dependent on the vicinal angle. The 5.7 degrees off surface showed superior periodicity. The classical elastic theory of a surface predicted the characteristic (constant) ordering distance but not the fluctuation in ordering periodicity. By introducing "quantized step bunching" due to periodic surface energy, which is unique to polymorphic SiC, the fluctuation is described.     

Regular step formation on concave-shaped surfaces on 6H–SiC(0 0 0 1)

A concave-shaped surface has been prepared in a 6H–SiC(0 0 0 1) substrate by mechanical grinding. As a consequence, the different crystallographic planes building up the 6H–SiC polytype are cut under continuously changing polar angles in all azimuthal directions. Through hydrogen etching, this curved surface breaks up into a whole set of surfaces vicinal to the initial 6H(0 0 0 1) orientation. The local structural reorganisation after hydrogen etching has been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Two types of local bond environments are present at the step edges leading to a strong anisotropy in the surface etching with hydrogen. As a result, the distribution of the terrace width and the step heights varies with the azimuthal angle and reflects the sixfold symmetry of the bulk crystal. For most azimuthal directions, an alternation of large and small terraces, separated by steps of 0.75 nm heights (height of half the 6H polytype, three bilayers) is observed and only for well defined azimuthal directions, equally spaced terraces separated by steps of 1.5 nm height (one unit cell of 6H–SiC, six bilayers) are found. In addition, the polar variations have been studied by taking various line-scans along the concave-shaped surface with AFM. It seems that for polar angles above 3°, step bunching of several SiC steps occurs whereas below 3° the bimodal terrace width distribution is observed.     

Exciton related photoluminescence stimulation in SiC nanocrystals

The paper presents the results of porous SiC characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence spectroscopy techniques. XRD study shows the investigated porous 6H-SiC layers contain inclusions of 4H-SiC and 15R-SiC polytypes as well as the amorphous graphite phase. Photoluminescence study of PSiC layers with different thicknesses and SiC NC sizes reveals the intensity stimulation for exciton and defect-related PL bands. The intensity stimulation of defect-related PL bands is due to the increase, at the etching process, of the concentration of surface defects, apparently to deal with carbon. The intensity enhancement for exciton-related PL bands is attributed to the exciton recombination rate increasing due to the realization of exciton confinement and exciton–polariton effects in big size SiC NCs of different polytypes (6H-PSiC with inclusions of 15R- and 4H-PSiC).     

Surface state on the SiC(0001)-(√3 × √3) surface

An angle resolved photoemission study of a surface state on the SiC(0001)-(√3 × √3) surface is reported. Experiments carried out on the 6H and 4H polytypes give essentially identical results. A surface state band with semiconducting occupation is observed, centered around 1.0 eV above the valence band maximum (VBM) and with a width of about 0.2 eV. Recently calculated results for a Si-adatom-induced √3 × √3 reconstruction give a metallic surface state band centered about 1.2 eV above the VBM and with a width of 0.35 eV. The dispersion determined experimentally is smaller than calculated but exhibits the same trend, the surface state disperses downwards towards the VBM with increasing parallel wavevector component along both the qG--- and qG--- directions of the √3 × √3 surface Brillouin zone. The VBM is determined to be located at about 2.3(±0.2) eV below the Fermi level. The results indicate that Si adatoms on top of an outermost Si---C bilayer may be an inadequate structural model for explaining recent experimental findings for the SiC(0001)-(√3 × √3) surface.     

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.     

Step-controlled strain relaxation in the vicinal surface epitaxy of nitrides

On-axis and vicinal GaN/AlN/6H-SiC structures grown under identical conditions have been studied by x-ray diffraction and transmission electron microscopy to demonstrate the distinctive features of vicinal surface epitaxy (VSE) of nitrides on SiC. In VSE, the epilayers are tilted from the substrate due to the out-of-plane lattice mismatch (Nagai tilts), and the in-plane mismatch strains are more relaxed. The majority of misfit dislocations (MDs) at the vicinal AlN/6H-SiC interface are found to be unpaired partial MDs that are geometrically necessary to correct the stacking sequences from 6H to 2H. This mechanism indicates that it is possible to develop "step-controlled-epitaxy" strategies to control strain relaxation by adjusting the substrate offcut angles.     

iC多型体X射线定量分析的Rietveld方法

鉴于SIC多型体的主要衍射线完全重叠，用常规X射线粉末衍射方法确定SiC陶瓷材料中多型体含量的分布是非常困难的．提出以X射线粉末衍射全谱拟合的Rietveld方法进行SiC多型体定量分析，阐述了原理及方法．对含3C，4H，6H和15R4种多型体衍射数据的定量分析结果表明：Rietveld方法可对SiC材料中常见多型体的定量分析给出准确的结果．还给出了各自的标准偏差，并估计了该方法对各多型体能给出精确结果的最低含量     

Structural and optical properties of thin films porous amorphous silicon carbide formed by Ag-assisted photochemical etching

In this work, we present the formation of porous layers on hydrogenated amorphous SiC (a-SiC: H) by Ag-assisted photochemical etching using HF/K₂S₂O₈ solution under UV illumination at 254 nm wavelength. The amorphous films a-SiC: H were elaborated by d.c. magnetron sputtering using a hot pressed polycrystalline 6H-SiC target. Because of the high resistivity of the SiC layer, around 1.6 MΩ cm and in order to facilitate the chemical etching, a thin metallic film of high purity silver (Ag) has been deposited under vacuum onto the thin a-SiC: H layer. The etched surface was characterized by scanning electron microscopy, secondary ion mass spectroscopy, infrared spectroscopy and photoluminescence. The results show that the morphology of etched a-SiC: H surface evolves with etching time. For an etching time of 20 min the surface presents a hemispherical crater, indicating that the porous SiC layer is perforated. Photoluminescence characterization of etched a-SiC: H samples for 20 min shows a high and an intense blue PL, whereas it has been shown that the PL decreases for higher etching time. Finally, a dissolution mechanism of the silicon carbide in 1HF/1K₂S₂O₈ solution has been proposed.     

Temperature‐depending Raman line‐shift of silicon carbide

Silicon carbide (SiC) is often used for electronic devices operating at elevated temperatures. Spectroscopic temperature measurements are of high interest for device monitoring because confocal Raman microscopy provides a very high spatial resolution. To this end, calibration data are needed that relate Raman line‐shift and temperature. The shift of the phonon wavenumbers of single crystal SiC was investigated by Raman spectroscopy in the temperature range from 3 to 112°C. Spectra were obtained in undoped 6H SiC as well as in undoped and nitrogen‐doped 4H SiC. All spectra were acquired with the incident laser beam oriented parallel as well as perpendicular to the c‐axis to account for the anisotropy of the phonon dispersion. Nearly all individual peak centers were shifting linearly towards smaller wavenumbers with increasing temperature. Only the peak of the longitudinal optical phonon A₁(LO) in nitrogen‐doped 4H SiC was shifting to larger wavenumbers. For all phonons, a linear dependence of the Raman peaks on both parameters, temperature and phonon frequency, was found in the given temperature range. The linearity of the temperature shift allows for precise spectroscopic temperature measurements. Temperature correction of Raman line‐shifts also provides the ability to separate thermal shifts from mechanically induced ones. Copyright © 2009 John Wiley & Sons, Ltd.     

基于蒙特卡罗方法的4H-SiC(0001)面聚并台阶形貌演化机理

针对SiC外延生长中微观原子动力学过程,建立了一个三维蒙特卡罗模型来研究偏向■或■方向4H-SiC(0001)邻晶面上台阶形貌演化过程,并且利用Burton-Cabera-Frank理论分析了其形成机理.在蒙特卡罗模型中,首先建立了一个计算4H-SiC晶体生长过程的晶格网格,用来确定Si原子和C原子晶格坐标以及联系它们之间的化学键;其次,考虑了原子在台阶面上的吸附、扩散,原子在台阶边上的附着、分离以及传输等过程;最后,为了更加详细地捕捉微观原子在晶体表面的动力学过程信息,该模型把Si原子和C原子分别对待,同时还考虑了能量势垒对吸附原子影响.模拟结果表明:在偏向■方向的4H-SiC(0001)邻晶面,有一个晶胞高度的聚并台阶形貌形成,而对于偏向■方向的邻晶面,出现了半个晶胞高度的聚并台阶形貌,该模拟结果与实验中观察到的结果相符合.最后,利用Burton-Cabera-Frank理论对聚并台阶形貌演化机理进行了讨论.     

Impact of surface step heights of 6H-SiC (0 0 0 1) vicinal substrates in heteroepitaxial growth of 2H-AlN

Impact of step height of silicon carbide (SiC) substrates on heteroepitaxial growth of aluminum nitride (AlN) was investigated. Step-and-terrace structures with various step heights, 6 monolayer (ML), 3ML and 1ML, were formed on 6H-SiC (0 0 0 1) vicinal substrates by high-temperature gas etching. 2H-AlN layers were grown on the substrate by plasma-assisted molecular-beam epitaxy (MBE) and then these layers were characterized by atomic-force microscopy (AFM) and X-ray diffraction (XRD). High-quality AlN can be grown on SiC substrates with 6ML- and 3ML-height step, while AlN grown on SiC substrates with 1ML-height step exhibited inferior crystalline quality. A model for high-quality AlN growth on SiC substrates with 3ML-height step is proposed.     

Highly-doped p-type few-layer graphene on UID off-axis homoepitaxial 4H–SiC

In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H–SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 10¹⁵ cm⁻². We find that the material's electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.     

GROWTH MODE AND SURFACE RECONSTRUCTION OF GaN(0001) THIN FILMS ON 6H-SiC(0001)

Two-dimensional growth of GaN thin films on an atomically flat C-face 6H-SiC(0001) surface prepared by ultra-high vacuum Si-etching is observed when using an AlN buffer layer in N plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy and reflection high energy electron diffraction observations reveal a series of Ga-stabilized reconstructions which are consistent with those reported for an N-polar GaN(0001) film. The result, including the effect observed previously for GaN thin film on Si-terminated 6H-SiC(0001), agrees with the polarity assignment of heteroepitaxial wurtzite GaN films on polar 6H-SiC substrates, i.e., GaN film grown on SiC(0001) is <0001> oriented (N-face) while that on SiC(0001) is <0001> oriented (Ga-face).     

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.     

SiC多型体几何结构与电子结构研究

采用平面波超软赝势法和范数不变赝势法对几种SiC多型体的几何结构、能带结构等进行了系统的研究.结果表明:6HSiC导带最低点在ML线上U点,用平面波超软赝势法计算时U点在(0000,0500,0176)点附近;而用范数不变赝势法计算时在导带最低点附近能带呈现不连续点,不连续点出现在(0000,0500,0178)点附近.两种赝势法计算结果相比,用平面波超软赝势法得到的导带最低点位置更靠近布里渊区M(0,05,0)点.在平面波超软赝势下,随着六角度的增加,cp,cpa增大的趋势较为明显,能隙和价带宽度变宽的趋势也较为明显.在计算极限内,绝对零度下4HSiC系统能量最低、最稳定,而Ewald能量显示3CSiC最稳定. 关键词： 密度泛函理论 电子结构 SiC     

Synthesis of α -SiC nanocrystals by carbothermal reduction of spherical nanoparticles of amorphous silicon dioxide

The method for carbothermal reduction of spherical particles of amorphous silicon dioxide is developed, and hexagonal α-SiC polytype nanocrystals are synthesized. The prepared samples are characterized by X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, and electron microscopy. The silicon carbide nanocrystals prepared have sizes in the range 5–50 nm depending on the diameter of initial silicon dioxide particles. A detailed analysis of the positions of the lines in the Raman spectra, their broadening, and shift makes it possible to reliably establish that the samples under investigation predominantly contain the 6H and 4H silicon carbide polytypes and insignificant amounts of the 2H and 3C phases. The 15R and 21R polytypes in the samples are absent. It is noted that the samples are characterized by a substantial size effect: the luminescence intensity of small silicon carbide nanocrystals is more than three times higher than that of large SiC nanocrystals.     

属特殊结构系列的若干碳化硅多型体的晶体结构

碳化硅(SiC)是典型的层状结构化合物。到目前为止,已发现了150种以上的多型体。作者曾发展了一种特殊的劳厄法可有效地鉴定碳化硅多型体。这种方法,我们发现了85种碳化硅新多型体。为了测定其中一些多型体的结构,拍摄了回摆和魏森堡照相作结构分析,但没有成功。因为这些照相中只有一些基本类型6H,15R和8H的衍射斑点而没有高层多型体的斑点,这是由于新多型体在晶体中含量很少且这些薄晶体多型体处于基本类型6H,8H,15R中间的缘故。本文提出了一种测定碳化硅多型体晶体结构的劳厄法。提出了计算劳厄斑点衍射强度的方法。对结构系列[(33)_m32]₃,[(33)_m34]₃,[(22)_m23]₃和[(44)_m43]₃多型体的结构因子F_hkl的计算方法作了简化。利用这种方法对9种碳化硅新多型体的晶体结构作了测定,其结构用z字形堆垛(Жданов符号)表示时为231R:[(33)₁₂32]₃,249R:[(33)₁₃32]₃,321R:[(33)₁₇32]₃, 339R:[(33)₁₈32]₃,237R:[(33)₁₂34]₃,417R:[(33)₂₂34]₃, 453R:[(33)₂₄34]₃,93R:[(44)₃43]₃,261R:[(44)₁₀43]₃。 关键词：     

Surface saturation control on the formation of wurtzite polytypes in zinc blende SiC nanofilms grown on Si-（100） substrates

We investigate the formations of wurtzite （WZ） SiC nano polytypes in zinc blende （ZB） SiC nanofilms hetero-grown on Si-（100） substrates via low pressure chemical vapor deposition （LPCVD） by adjusting the Si/C ratio of the introduced precursors. Through SEM, TEM, and Raman characterizations, we find that the nanofilms consist of discrete WZ SiC nano polytypes and ZB SiC polytypes composed of WZ polytypes （WZ ＋ ZB） and disordered ZB SiC polytypes, respectively, according to Si/C ratios of 0.5, 1.5, and 3. We attribute the WZ polytype formation to being due to a kinetic mechanism based on the Si/C surface saturation control.     

SiC polytypes process affected by Ge predeposition on Si(111) substrates

Structural and optical measurements were performed on silicon carbide (SiC) samples containing several polytypes. The SiC samples investigated were grown on (111) Si substrates by solid source molecular beam epitaxy (SSMBE). Several quantities of Ge were predeposited before the growth procedure. The influence of Ge on the SiC polytypes formation was studied by X-Ray, FTIR and μ-Raman characterizations methods. The spectra of the samples with less than one Ge monolayer exhibit a mixture of 2H, 15R and 3C–SiC polytypes. This mixture is due to the mismatch between the heterostructure layers. We propose that the Ge predeposition in the heterostructure can be used to stabilize and unify the polytypes formation.     

Phonons in 3C-, 4H-, and 6H-SiC

Silicon carbide epilayers of cubic (3C) and hexagonal (4H and 6H) polytypes were investigated by Auger electron spectroscopy, high-resolution electron energy-loss spectroscopy and Raman spectroscopy to determine the excitation energies of the optical Fuchs-Kliewer surface phonons and their relation to bulk phonon frequencies. The surfaces were treated in a buffered hydrofluoric acid solution. Loss structures attributed to excitation of Fuchs-Kliewer phonons were clearly resolved. Their energies were found at 115.9 ± 1 meV irrespective of the SiC polytype. The experimental data agree with values calculated from the experimental bulk phonon frequencies and tabulated dielectric constants.