Characterization of 4H-SiC substrates and epilayers by Fourier transform infrared reflectance spectroscopy

The infrared reflectance spectra of both 4H–SiC substrates and epilayers are measured in a wave number range from 400 cm 1 to 4000 cm 1 using a Fourier-transform spectrometer. The thicknesses of the 4H–SiC epilayers and the electrical properties, including the free-carrier concentrations and the mobilities of both the 4H–SiC substrates and the epilayers, are characterized through full line-shape fitting analyses. The correlations of the theoretical spectral profiles with the 4H–SiC electrical properties in the 30 cm 1 –4000 cm 1 and 400 cm 1 –4000 cm 1 spectral regions are established by introducing a parameter defined as error quadratic sum. It is indicated that their correlations become stronger at a higher carrier concentration and in a wider spectral region (30 cm 1 –4000 cm 1 ). These results suggest that the infrared reflectance technique can be used to accurately determine the thicknesses of the epilayers and the carrier concentrations, and the mobilities of both lightly and heavily doped 4H–SiC wafers.     

Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement

The free carrier density and mobility in n-type 4H-SiC substrates and epilayers were determined by accurately analysing the frequency shift and the full-shape of the longitudinal optic phonon--plasmon coupled (LOPC) modes, and compared with those determined by Hall-effect measurement and that provided by the vendors. The transport properties of thick and thin 4H-SiC epilayers grown in both vertical and horizontal reactors were also studied. The free carrier density ranges between 2×10¹⁸ cm^-3 and 8×10¹⁸ cm^-3 with a carrier mobility of 30--55 cm²/(V·s) for n-type 4H-SiC substrates and 1×10¹⁶--3×10¹⁶ cm^-3 with mobility of 290--490 cm²/(V·s) for both thick and thin 4H-SiC epilayers grown in a horizontal reactor, while thick 4H-SiC epilayers grown in vertical reactor have a slightly higher carrier concentration of around 8.1×10¹⁶ cm^-3 with mobility of 380 cm²/(V·s). It was shown that Raman spectroscopy is a potential technique for determining the transport properties of 4H-SiC wafers with the advantage of being able to probe very small volumes and also being non-destructive. This is especially useful for future mass production of 4H-SiC epi-wafers.     

SiC epitaxial layers grown by chemical vapour deposition and the fabrication of Schottky barrier diodes

This paper presents the results of unintentionally doped 4H-SiC epilayers grown on n-type Si-faced 4H-SiC substrates with 8° off-axis toward the [11\overline 2 0] direction by low pressure horizontal hot-wall chemical vapour deposition. Growth temperature and pressure are 1580~°C and 10⁴~Pa, respectively. Good surface morphology of the sample is observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and x-ray diffraction (XRD) are used to characterize epitaxial layer thickness and the structural quality of the films respectively. The carrier concentration in the unintentional 4H-SiC homoepitaxial layer is about 6.4×10¹⁴~cm^-3 obtained by c--V measurements. Schottky barrier diodes (SBDs) are fabricated on the epitaxial wafer in order to verify the quality of the wafer and to obtain information about the correlation between background impurity and electrical properties of the devices. Ni and Ti/4H-SiC Schottky barrier diodes with very good performances were obtained and their ideality factors are 1.10 and 1.05 respectively.     

Comments on a peak of AlxGa1−xN observed by infrared reflectance

Al_xGa_1−xN epilayers, grown on c-plane oriented sapphire substrates by metal organic chemical vapour deposition (MOCVD), were evaluated using FTIR infrared reflectance spectroscopy. A peak at ∼850 cm⁻¹ in the reflectance spectra, not reported before, was observed. Possible origins for this peak are considered and discussed.     

n-SiC的电子拉曼散射及二级拉曼谱研究

研究了利用离子注入法得到的掺氮n-SiC拉曼光谱. 理论线形分析表明,与4H-SiC相比,6H-SiC中LO声子等离子体激元耦合模(LOPC模)拉曼位移随自由载流子浓度变化较小. 5145nm激发光下得到的电子拉曼散射光谱表明,k位处由1s(A₁)到1s(E)的能谷轨道跃迁带来的拉曼谱6H-SiC中有四条,4H-SiC中有二条;高频6303及635cm^-1处观察到的谱线被认为与深能级缺陷有关. 最后,利用纤锌矿型结构二级拉曼散射选择定则指认了6 关键词： 碳化硅 电子拉曼散射 轨道能谷分裂 倍频谱     

Single crystal growth of SiC and electronic devices

Single crystal growth of silicon carbide (Sic) and application to electronic devices are reviewed. In the crystal growth, bulk and homoepitaxial growth are picked up, and crystal quality and electrical properties are described. For electronic devices, various device processes are argued. Power devices based on Sic are stressed in this review.

Bulk single crystals of SiC can be grown by a sublimation method, and large-area 6H-SiC and 4H-SiC single crystals are obtained. The occurrence of SiC polytypes is affected by the growth condition, and can be controlled successfully by optimizing these conditions. 6H-SiC is grown on 6H-SiC (0001) Si-faces, and 4H-SiC on 6H-SiC (0001) C-faces. The crystallinity of bulk crystals is investigated by reflection high-energy electron diffraction (RHEED) and X-ray analysis, and characterization is carried out in detail by optical and electrical measurement.

Successful homoepitaxial vapor phase growth of SiC can be realized using off-axis (0001) substrates prepared by a sublimation method called “step-controlled epitaxy”. Since the crystallinity of epilayers is improved during the step-controlled epitaxy, this growth technique is a key for getting high-quality crystal surfaces. Impurity doping is controlled during homoepitaxial growth by employing impurity gases, such as N₂, trimethylaluminum (TMA), and B₂H₆. A wide-range of carrier concentrations of 5 × 10¹³～3 × 10¹⁸ cm^?3 for n-type and 5 × 10¹⁶～3 × 10²⁰ cm^?3 for p-type are realized. The impurity-incorporation mechanism in the step-controlled epitaxy is discussed based on the C/Si ratio dependence of impurity doping.

Electrical properties of SiC grown by step-controlled epitaxy are determined precisely. A high electron mobility of 720 cm²/Vs is obtained in an undoped 4H-SiC epilayer with an electron concentration of 2.5 × 10^l6 cm^?3 at 300 K. This electron mobility is about two times higher than that of 6H-Sic (～380 cm²/Vs). High breakdown fields of 1～5 × 10⁶ V/cm are obtained for both 6H- and 4H-SiC, one order of magnitude higher than those for Si. A high saturation electron drift velocity of 1.6 × 10⁷ cm/s is obtained in 4H-Sic, which may make possible high performance of high-frequency 4H-SiC power devices. Impurity levels and deep levels are investigated by Hall effect, admittance spectroscopy, and DLTS measurement. Metal/4H-SiC Schottky barrier heights are characterized and a strong dependence on metal work function without strong “pinning” is elucidated.

Device processes are described for ion implantation. Interface properties of SiO₂/SiC are characterized in detail using metal-oxide-semicond.     

Study on Infrared Spectral Radiance by Remote Fourier Transform Infrared Spectroscopy

Abstract

Infrared emission spectra emitted by high luminosity infrared pyrotechnics have been observed remotely using the Fourier transform infrared spectroscopy. The primary purpose of the study is to determine infrared spectral radiance distribution, their time—resolved spectra and integrated emission energy. The spectra have been recorded between 4000 – 800cm^?1 region with spectral resolution of 4cm^?1. The study is very important for many applications.     

Growth and characterization of DyP/GaAs and DyAs/GaAs-based heterostructures and superlattices

Details of the structural and electrical properties of epitaxial DyP/GaAs and DyAs/GaAs is reported. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01%. DyAs, on the other hand, has a mismatch of nearly 2.4%. Both DyP and DyAs have been grown by solid source MBE using custom designed group V thermal cracker cells and group III high-temperature effusion cells. High-quality DyP and DyAs epilayers, as determined by XRD, TEM, and AFM analysis, were obtained for growth temperatures ranging from 500°C to 600°C with growth rates between 0.5 and 0.7 μm/h. The DyP epilayers are n-type with measured electron concentrations of the order of 3×10²⁰ to 4×10²⁰ cm⁻³, with room temperature mobilities of 250–300 cm²/V s, and with a barrier height of 0.75 eV to GaAs. The DyAs epilayers are also n-type with concentration of 1×10²¹ to 2×10²¹ cm⁻³, with mobilities between 25 and 40 cm²/V s. DyP is stable in air with no apparent oxidation taking place, even after months of ambient exposure to untreated air.     

Effect of surface morphology on the electron mobility of epitaxial graphene grown on 0° and 8° Si-terminated 4H-SiC substrates

Graphene with different surface morphologies were fabricated on 8° -off-axis and on-axis 4H-SiC(0001) substrates by high-temperature thermal decompositions. Graphene grown on Si-terminated 8° -off-axis 4H-SiC(0001) shows lower Hall mobility than the counterpart of on-axis SiC substrates. The terrace width is not responsible for the different electron mobility of graphene grown on different substrates, as the terrace width is much larger than the mean free path of the electrons. The electron mobility of graphene remains unchanged with an increasing terrace width on Siterminated on-axis SiC. Interface scattering and short-range scattering are the main factors affecting the mobility of epitaxial graphene. After the optimization of the growth process, the Hall mobility of the graphene reaches 1770 cm 2 /V·s at a carrier density of 9.8.×10 12 cm 2 . Wafer-size graphene was successfully achieved with an excellent double-layer thickness uniformity of 89.7% on a 3-inch SiC substrate.     

Diffuse Reflectance Spectra of V(CP)2 + on Silica

Abstract

Diffuse reflectance spectra of vanadocene adsorbed on silica were recorded in the spectral region 4000-30 000 cm^?1. A d-d band around 13 000 cm^?1 indicated, that vanadocene formed V(cp)₂ ⁺ on the support when the number of silanol groups was decreased. This cation is suggested to be stabilized by the negative charge of deprotonated silanol groups.     

Solution-deposited carbon nanotube layers for flexible display applications

We have investigated two possible fields of application for carbon nanotube (CNT) networks in flexible displays. Transparent and conductive layers of CNTs were spray coated onto glass and plastic substrates. The spectral transmission of the produced layers is almost even for all wavelengths in the visible regime. A sheet resistance of 400 Ω/□ at a transmittance of 80% was achieved.Thin-film transistors (TFT) were created on silicon wafers and glass substrates using low-density CNT networks as a semiconducting layer. The process used for device fabrication on glass substrates is fully compatible to application on plastic foils. The transistors reach on/off ratios of more than five orders of magnitude and show device charge carrier mobilities in the order of 1 cm²/Vs. These values promise an application in active matrix liquid crystal displays (AMLCD). Issues that need to be addressed are the homogeneity and reproducibility of the device properties.     

Polarized reflectance spectrum of β-(BEDT-TTF)2PF6

The polarized reflectance spectrum of β-(BEDT-TTF)₂PF₆ was measured over the spectral range from 720 cm^?1 to 25, 000 cm^?1 at 293 K (semiconductive phase) and at 318 K (metallic phase). The infrared band found in the conductivity spectrum, which was obtained by the Kramers-Kronig transformation of the reflectance data, is interpreted as the one associated with inter-band transition.     

The influence of the band structure of epitaxial graphene on SiC on the transistor characteristics

We fabricated high-mobility field-effect transistors based on epitaxial graphene synthesized by vacuum graphitization of both the Si- and C-faces of SiC. Room-temperature field-effect mobilities >4000 cm²/V s for both electrons and holes were achieved, although with wide distributions. By using a high-k gate dielectric, we were able to measure the transistor characteristics in a wide carrier density range, where the mobility is seen to decrease as the carrier density increases. We formulate a simple semiclassical model of electrical transport in graphene, and explain the sublinear dependence of conductivity on carrier density from the view point of the few-layer graphene energy band structure. Our analysis reveals important differences between the few-layer graphene energy dispersions on the SiC Si- and C-faces, providing the first evidence based on electrical device characteristics for the theoretically proposed energy dispersion difference between graphene synthesized on these two faces of SiC.     

Infrared spectroscopy of oxide layers on technical Si wafers

Measurements of the reflectance and transmittance in the infrared part of the spectrum (400...4000cm^–1) are carried out on oxidized Si wafers. Intrinsic dielectric properties of the oxide layers prepared by various methods are derived from experimental data. The dielectric functions of the oxides can be fitted by oscillator models. The related model parameters are compared with those of other SiO₂ samples, as crystals and glasses. Optical arrangements to detect and characterize layers of thicknesses down to 3 nm are discussed. In particular, it is shown that experiments with polarized light at oblique incidence up to 80° are a powerful tool to characterize those layers.Experiments are reported to determine, in addition, extrinsic properties such as B and P atoms in the oxide layer or defects due to an ion implantation treatment.Conditions are discussed which should be applied to obtain the best quantitative analysis of the defect concentration.     

Vibrational Assignments of Some Monosubstituted Pyridines

Abstract

The infrared spectra of 2-(ortho) and 4-(para) vinyl- and ethyl-pyridines have been analysed in the region 400–4000 cm^?1 in liquid phase. The bands obtained are discussed assuming the molecules under C_s and C_2v point groups. The group sensitive fundamentals are compared with those of similar molecules reported in the literature.     

Investigation of the manganese stress on wheat plant by attenuated total reflectance Fourier transform infrared spectroscopy

ABSTRACT

The present paper deals with the investigation of the changes in chemical contents of the leaf of wheat seedling stressed by excess manganese using attenuated total reflectance Fourier transform infrared spectroscopy (FTIR) technique. The infrared spectra of the leaves of control and manganese-treated wheat plants have been recorded in the spectral region 485–4000?cm^?1 at a resolution of 4?cm^?1. The recorded spectrum has been analyzed with the help of curve-fitting method for the quantitative estimation of chemical contents and conformational changes. The study indicated changes arising in the polysaccharide, lignin, amino acid, secondary structure of protein, and lipid in the leaves of control and manganese-treated wheat plants. Manganese treatment increased the amount of cellulose, lignin, and amide II till 200?μM concentration, while a decrease was observed at 1000?µM concentration. Manganese induced conformational changes in the secondary structure of protein which was indicated by shifting of the bands to the higher wavenumber and change in the β sheet/α helix ratio. In addition, manganese stress decreased lipid content in the leaves of the wheat seedlings. The study demonstrates the potential of attenuated total reflectance FTIR for the non-invasive and rapid monitoring of the plants stressed with heavy metals.     

Spectral Study of Some Samarium Complexes in the Region 750–250 cm−1

Abstract

Rare earth β -diketonates and diketo-ester complexes are promising laser materials due to narrow line width of the internal 4fⁿ transitions and weak crystal-field interactions¹. The electronic energy states², spectral intensities^3–5, bonding^6,7 and infrared spectra^8–11 of some of these complexes have been recently reported by the authors. However, little information regarding their structure and strengths of various bonds are available. The infrared spectra of rare earth complexes in the spectral region 4000 – 750 cm^?1 are characteristic of the ligand, while those in the region 750 – 250 cm^?1 characterize the metal-ligand bonding. The present communication reports the values of force constants computed from the observed infrared absorption spectra of four ethyl 1-methyl acetoacetate (EMA) and ethylbenzoylacetate (EBA) complexes of trivalent samarium in the region 750 – 250 cm^?1.     

N型4H-SiC同质外延生长

利用水平式低压热壁CVD (LP-HW-CVD) 生长系统,台阶控制生长和衬底旋转等优化技术,在偏晶向的4H-SiC Si(0001) 晶面衬底上进行4H-SiC同质外延生长,生长温度和压力分别为1550℃和10⁴ Pa,用高纯N₂作为n型掺杂剂的4H-SiC原位掺杂技术,生长速率控制在5μm/h左右.采用扫描电镜(SEM)、原子力显微镜(AFM),傅里叶变换红外光谱(FTIR)和Hg/4H-SiC肖特基结构对同质外延表面形貌、厚度、掺杂浓度以及均匀性进行了测试.实验结果表明,4H-SiC同质外延在表面无明显缺陷,厚度均匀性1.74%, 1.99% 和1.32%(σ/mean),掺杂浓度均匀性为3.37%,2.39%和2.01%.同种工艺条件下,样品间的厚度和掺杂浓度误差为1.54%和3.63%,有很好的工艺可靠性. 关键词： 4H-SiC 同质外延生长 水平热壁CVD 均匀性     

Control of acceptor doping in MOCVD HgCdTe epilayers

The acceptor doping of mercury cadmium telluride (HgCdTe) layers grown by MOCVD are investigated. (111)HgCdTe layers were grown on (100)GaAs substrates at 350°C using horizontal reactor and interdiffused multilayer process (IMP). TDMAAs and AsH3 were alternatively used as effective p-type doping precursors. Incorporation and activation rates of arsenic have been studied. Over a wide range of Hg_1−xCd_xTe compositions (0.17 < x < 0.4), arsenic doping concentration in the range from 5×10¹⁵ cm⁻³ to 5×10¹⁷ cm⁻³ was obtained without postgrowth annealing. The electrical and chemical properties of epitaxial layers are specified by measurements of SIMS profiles, Hall effect and minority carrier lifetimes. It is confirmed that the Auger-7 mechanism has decisive influence on carrier lifetime in p-type HgCdTe epilayers.     

厚度对MOCVD生长InN薄膜位错特性与光电性质的影响

研究了金属有机物化学气相沉积法制备的不同厚度InN薄膜的位错特性与光电性质.基于马赛克微晶模型,通过X射线衍射非对称面摇摆曲线测量,拟合出样品刃型位错密度分别为4.2×10¹⁰cm^-2和6.3×10¹⁰cm^-2,并发现样品的微晶扭转角与位错密度随薄膜厚度增加而减小.通过室温霍尔效应测量得到样品载流子浓度分别为9×10¹⁸cm^-3和1.2×10¹⁸cm关键词： 氮化铟 位错 载流子起源 局域态