Hot Wall Epitaxy Research Articles

Thermal Conductivity Measurement of Thermoelectric Thin Films by a Versatility-Enhanced 2ω Method

The 2ω method is a technique to measure the cross-plane thermal conductivity, κ, of a film sample based on sinusoidal Joule-heating of a metal film deposited on the sample surface and its thermoreflectance (TR) measurement of surface temperature cooling due to the heat dissipation. The 2ω method is being paid attention because it is more cost-effective and easier to use than the conventional time domain thermoreflectance (TDTR) method or the 3ω method. In some cases, however, it is difficult to apply the conventional 2ω method to the κ measurement of high thermal resistance films such as general thermoelectric films due to its non-linear TR signal response to (2ω)−0.5. Here, we present a 2ω method based on a versatile TR signal analysis which enables the κ measurement of high thermal resistance film more explicitly than the conventional analysis based on a linear TR signal response. This method determines explicitly the thermal conductivities of PbTe films and PbTe/GeS superlattices grown on BaF2(111) substrates by hot wall epitaxy: κ = 2.1 ± 0.13 Wm−1 K−1 and κ = 0.71 ± 0.05 Wm−1 K−1, respectively. Furthermore, a significant impact of PbTe film crystallinity on thermal conductivity is demonstrated by comparative measurements between polycrystalline PbTe film and epitaxial PbTe film grown on the BaF2(111) substrates. These results demonstrate that our method can be a powerful tool to measure the thermal conductivity of thermoelectric films.

Reduction of threading dislocation density for AlN epilayer via a highly compressive-stressed buffer layer

Crystalline qualities of three AlN films grown by cold-wall high temperature hydride vapor phase epitaxy (CW-HT-HVPE) on c-plane sapphire substrates, with different AlN buffer layers (BLs) deposited either by CW-HT-HVPE or by hot-wall low temperature hydride vapor phase epitaxy (HW-LT-HVPE), have been studied. The best film quality was obtained on a 500-nm-thick AlN BL grown by HW-LT-HVPE at 1000℃. In this case，the AlN epilayer has the lowest full-width at half-maximum (FWHM) values of the (0002) and (10–12) x-ray rocking curve peaks of 295 and 306 arcsec, respectively, corresponding to the screw and edge threading dislocation (TD) densities of 1.9×108cm−2 and 5.2×108cm−2. This improvement in crystal quality of the AlN film can be attributed to the high compressive-stress of BL grown by HW-LT-HVPE，which facilitate the inclination and annihilation of TDs.

High-Quality Bi2Te3 Single Crystalline Films on Flexible Substrates and Bendable Photodetectors**Supported by the National Basic Research Program of China under Grant No 2012CB619200, the National Natural Science Foundation of China under Grant Nos 61290304, 11074265 and 11174307, the Natural Science Foundation of Shanghai under Grant No 16ZR1441200, and the Frontier Science Research Project (Key Programs) of Chinese Academy of Sciences

Recently, great efforts have been made in the fabrication of arbitrary warped devices to satisfy the requirement of wearable and lightweight electronic products. Direct growth of high crystalline quality films on flexible substrates is the most desirable method to fabricate flexible devices owing to the advantage of simple and compatible preparation technology with current semiconductor devices, while it is a very challenging work, and usually amorphous, polycrystalline or discontinuous single crystalline films are achieved. Here we demonstrate the direct growth of high-quality Bi2Te3 single crystalline films on flexible polyimide substrates by the modified hot wall epitaxy technique. Experimental results reveal that adjacent crystallites are coherently coalesced to form a continuous film, although amounts of disoriented crystallites are generated due to fast growth rate. By inserting a quartz filter into the growth tube, the number density of disoriented crystallites is effectively reduced owing to the improved spiral interaction. Furthermore, flexible Bi2 Te3 photoconductors are fabricated and exhibit strong near-infrared photoconductive response under different degrees of bending, which also confirms the obtained flexible films suitable for electronic applications.

Spectral behavior of the temperature-dependent photoresponse of BaAl2S4 layers grown by using hot-wall epitaxy

The characteristic photocurrent (PC) property of BaAl2S4 layers grown by using hot-wall epitaxy was investigated. With decreasing temperature, the PC peaks position shifted toward the short-wavelength region and its intensity decreased dramatically. The energy variation of the PC peaks shift at different temperatures could be matched well by using E g (T) = E g (0) − 7.556 × 10−4 T 2/(T + 523), where E g (0) was 4.0596, 4.1053, and 4.1094 eV corresponding to Γ4(z), Γ5(y), and Γ5(x) in the valence band in order of increasing energy, respectively. Thus, by the selection rule, the crystal field and the spin-orbit splitting were found to be 47.5 and −5.9 meV, respectively, through PC spectroscopy measurement. From the negative result for the spin-orbit parameter, the middle Γ5(x) and the lowest Γ5(y) levels in the valence band were found to be interchanged. By using the relation between the logarithm of the PC density and the reciprocal temperature, we extracted the dominant trap level as 23.2 in the high-temperature region and 5.4 meV in the low-temperature region. Consequently, we found that the dramatic decrease on the PC intensity was caused by these trapping centers.

Hot-wall-epitaxy growth and electrical/optical characterization of epitaxial BaAl2S4/GaAs layers

The BaAl2S4 layers, which were identified to be a cubic structure in space group Pa3, were grown using the hot wall epitaxy method attached with the reservoir tail. Also, the coincidence lattice mismatch of the grown layers was shown to be −2.52% due to the influence of the compressive strain. The Hall effect measurement showed the different temperature-dependent decrease of mobility at a temperature above 100K. One was T−1/2 at the temperature range of 100<T<180K and the other was T−3/2 at the temperature of T>180K. The mobility decreased in proportion to T1 at a low temperature range of T<100 K. Three PC peaks obtained from the photocurrent (PC) spectra were corresponding to band-to-band transitions, which were observed over the temperature range. These PC peaks were caused by the transition of electrons from the three valence band states in order of increasing energy to the conduction band states, respectively. By analyzing absorption and PC results, the band gap variation has been compared and matched well with Eg(T)=Eg(0)−7.556×10−4T2/(T+523), where Eg(0) is estimated to be 4.0596, 4.1053, and 4.1094eV.

Amorphous/epitaxial superlattice for thermoelectric application

An amorphous/epitaxial superlattice system is proposed for application to thermoelectric devices, and the superlattice based on a PbGeTeS system was prepared by the alternate deposition of PbS and GeTe using a hot wall epitaxy technique. The structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray analysis, and it was found that the superlattice consists of an epitaxial PbTe-based layer and a GeS-based amorphous layer by the reconstruction of the constituents. A reduction in thermal conductivity due to the amorphous/epitaxial system was confirmed by a 2ω method. Electrical and thermoelectric properties were measured for the samples.

The composition and the structure of thin films based on metal porphyrin complexes

Thin films based on CuTPP, ZnTPP and FeClTPP (ТРР – TetraPhenylPorphyrin) complexes and manufactured by vacuum deposition under quasi-equilibrium conditions have been investigated. The hot-wall epitaxy method was chosen for the preparation of samples. The structure of objects and their composition were examined by scanning electron microscopy (SEM) and energy dispersive microanalysis (EDX). The CuTPP film was established to form whiskers about 20nm in diameter and 5µm in length. The structural features and morphology of ZnTPP and FeClTPP films’ surface were revealed. The theoretical interpretation of obtained results was proposed that made it possible to relate the film structure with its composition.

Hot Wall Epitaxy(HWE)법에 의한 BaIn&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;4&lt;/sub&gt; 단결정 박막 성장과 광전도 특성

A stoichiometric mixture of evaporating materials for <TEX>$BaIn_2S_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, <TEX>$BaIn_2S_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$420^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The carrier density and mobility of <TEX>$BaIn_2S_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method are <TEX>$6.13{\times}10^{17}cm^{-3}$</TEX> and <TEX>$222cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The temperature dependence of the energy band gap of the <TEX>$BaIn_2S_4$</TEX> obtained from the absorption spectra was well described by the Varshni's relation, <TEX>$E_g(T)=3.0581eV-(3.9511{\times}10^{-3}eV/K)T^2/(T+536K)$</TEX>. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$BaIn_2S_4$</TEX> have been estimated to be 182.7 meV and 42.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$BaIn_2S_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{24}$</TEX>-exciton peaks for n = 24. A stoichiometric mixture of evaporating materials for <TEX>$BaIn_2S_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, <TEX>$BaIn_2S_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$420^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The carrier density and mobility of <TEX>$BaIn_2S_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method are <TEX>$6.13{\times}10^{17}cm^{-3}$</TEX> and <TEX>$222cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The temperature dependence of the energy band gap of the <TEX>$BaIn_2S_4$</TEX> obtained from the absorption spectra was well described by the Varshni's relation, <TEX>$E_g(T)=3.0581eV-(3.9511{\times}10^{-3}eV/K)T^2/(T+536K)$</TEX>. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$BaIn_2S_4$</TEX> have been estimated to be 182.7 meV and 42.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$BaIn_2S_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{24}$</TEX>-exciton peaks for n = 24.

Complex Behavior of Caffeine Crystallites on Muscovite Mica Surfaces.

Defined fabrication of organic thin films is highly desired in technological, as well as pharmaceutical, applications since morphology and crystal structure are directly linked to physical, electrical, and optical properties. Within this work, the directed growth of caffeine deposited by hot wall epitaxy (HWE) on muscovite mica is studied. Optical and atomic force microscopy measurements reveal the presence of caffeine needles exhibiting a preferable alignment in the azimuthal directions with respect to the orientation of the defined mica surface. Specular X-ray diffraction and X-ray diffraction pole figure measurements give evidence that the β-polymorphic form of caffeine forms on the mica surface. All results consent that caffeine molecules have an edge-on conformation i.e. minimizing their interaction area with the surface. Furthermore, the azimuthal alignment of the long caffeine needle axis takes place along the [11̅0], [100], and [110] real space directions of mica; needles are observed every 60° azimuthally. While mica has a complex surface structure with mirror planes and lowered oxygen rows, the slightly disturbed 3-fold symmetry dictates the crystal alignment. This is different to previous findings for solution cast caffeine growth on mica. For HWE the needles align solely along the mica main directions whereby solution cast needles show an additional needle splitting due to a different alignment of caffeine with respect to the surface.

Step-bunched Bi2Te3 and Bi2Se3 layers epitaxially grown on high-index InP substrates

Bi2Te3 and Bi2Se3 layers are grown on InP with various substrate orientations by hot wall epitaxy. The layers develop step bunching on high-index substrates as a consequence of the alignment of the c axis to the InP[111] direction. The bunched Bi2Te3 steps are ragged due to significant reevaporation. Pronounced porosity is caused in Bi2Te3 layers on InP(114) as the c axis is aligned also to the [001] direction of InP as well as to the [111] direction. We compare the Seebeck coefficient in the step-bunched and (0001)-oriented Bi2Te3 layers, where the anisotropy of the thermoelectric property is shown to be small. In addition, we demonstrate for Bi2Se3 layers on InP(115) that a (0001)-oriented minority component is incorporated as submicrometer-size islands in the bunched steps as it is also lattice-matched to the substrates.

Growth and Hall-effect/photocurrent analysis on BaAl2Se4 layers grown by hot wall epitaxy method

The epitaxial growth of photoconductive BaAl2Se4 layers, which have a tetragonal structure, was first achieved through the hot wall epitaxy method. From the Hall effect result, in the high temperature range of T>230K, the mobility decreased as a function of T−3/2 and its scattering was mainly due to the acoustic phonon mode of lattice vibrations through a deformation potential. Thus, at the intermediate temperature range (100<T<230K), the mobility gradually decreased as a function of T−1/2. This behavior was attributed to the piezoelectric potential scattering. In contrast, the mobility decreased in proportion to T1 in a low temperature range of T<100K and its decrease was caused by dislocation scattering. Through the relationship between the reciprocal temperature and the carrier concentration, three donor levels were found to be 0.3, 14.7, and 95.3meV below the conduction band. From the photocurrent (PC) spectra, three PC peaks A, B, and C corresponding to band-to-band transitions were observed at all temperatures. According to the selection rule, the crystal field and spin orbit splitting were found to be 0.2029 and 0.2154eV, respectively, through the direct use of PC spectroscopy. Based on the analysis of optical absorption and PC spectra, the optical band gap was well expressed by Eg(T)=Eg(0)−4.39×10−4T2/(T+250), where Eg(0) is estimated to be 3.4205, 3.6234, and 3.8388eV for the transitions corresponding to the valence band states Γ3(A), Γ4(B), and Γ5(C), respectively.

Hot Wall Epitaxy(HWE)법에 의한 MnAl2S4단결정 박막 성장과 광전도 특성

수평 전기로에서 <TEX>$MnAl_2S_4$</TEX> 다결정을 합성하여 HWE(Hot Wall Epitaxy)방법으로 <TEX>$MnAl_2S_4$</TEX> 단결정 박막을 반절연성 GaAs(100)기판에 성장시켰다. <TEX>$MnAl_2S_4$</TEX> 단결정 박막의 성장 조건은 증발원의 온도 <TEX>$630^{\circ}C$</TEX>, 기판의 온도 <TEX>$410^{\circ}C$</TEX>였고 성장 속도는 <TEX>$0.5{\mu}m/hr$</TEX>였다. 이때 <TEX>$MnAl_2S_4$</TEX> 단결정 박막의 결정성의 조사에서 이중결정 X-선 요동곡선(DCRC)의 반폭치(FWHM)도 132 arcsec로 가장 작아 최적 성장 조건임을 알 수 있었다. <TEX>$MnAl_2S_4$</TEX>/SI(Semi-Insulated) GaAs(100) 단결정 박막의 광흡수를 293 K에서 10 K까지 측정하였다. 광흡수 스펙트럼으로부터 band gap <TEX>$E_g(T)$</TEX>는 Varshni 공식에 따라 계산한 결과 <TEX>$E_g(T)=3.7920eV-(5.2729{\times}10^{-4}eV/K)T^2/(T+786K)$</TEX>였다. <TEX>$MnAl_2S_4$</TEX> 단결정 박막의 응용소자인 photocell로 사용할 수 있는 pc/dc 값이 가장 큰 광전도셀은 S 증기분위기에서 열처리한 셀로 <TEX>$1.10{\times}10^7$</TEX>이었으며, 광전도 셀의 감도(sensitivity)도 S 증기분위기에서 열처리한 셀이 0.93로 가장 좋았다. 또한 최대 허용소비전력(MAPD)값도 S 증기분위기에서 열처리한 셀이 316 mW로 가장 좋았으며, S 증기분위기에서 열처리한 셀의 응답시간은 오름시간 14.8 ms, 내림시간 12.1 ms로 가장 빠르게 나타나, <TEX>$MnAl_2S_4$</TEX> 단결정 박막을 S 분위기에서 <TEX>$290^{\circ}C$</TEX>로 30분 열처리한 photocell이 상용화가 가능할 것으로 여겨진다. A stoichiometric mixture of evaporating materials for <TEX>$MnAl_2S_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, <TEX>$MnAl_2S_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$630^{\circ}C$</TEX> and <TEX>$410^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by the photoluminescence and double crystal X-ray diffraction (DCXD). The temperature dependence of the energy band gap of the <TEX>$MnAl_2S_4$</TEX> obtained from the absorption spectra was well described by the Varshni's relation, <TEX>$E_g(T)=3.7920eV-5.2729{\times}10^{-4}eV/K)T^2/(T+786 K)$</TEX>. In order to explore the applicability as a photoconductive cell, we measured the sensitivity (<TEX>${\gamma}$</TEX>), the ratio of photocurrent to dark current (pc/dc), maximum allowable power dissipation (MAPD) and response time. The results indicated that the photoconductive characteristic were the best for the samples annealed in S vapour compare with in Mn, Al, air and vacuum vapour. Then we obtained the sensitivity of 0.93, the value of pc/dc of <TEX>$1.10{\times}10^7$</TEX>, the MAPD of 316 mW, and the rise and decay time of 14.8 ms and 12.1 ms, respectively.

Element substitution from substrates in Bi2Se3, Bi2Te3 and Sb2Te3 overlayers deposited by hot wall epitaxy

In depositing Bi2Se3, Bi2Te3 or Sb2Te3 layers on certain substrates by hot wall epitaxy, the Bi and Sb atoms in the layers are replaced by the atoms supplied from the substrates. We extend our exploration on this substitution phenomenon for a number of combinations of the layer and the substrate to infer the factors that determine the occurrence of the substitution. Using a series of Ga- and In-based III–V substrates, it is evidenced that the group III atoms substitute the group V overlayer atoms when the bonds in the substrates are weak. We demonstrate that Ag triggers the substitution more effectively than Cu as a catalyst. The competition between the catalyst-induced substitutions on ternary alloy substrates shows that the dependence on the bond strength is not as strong as to be exclusive. Additionally, defectiveness around the interface between a semicoherently grown α-In2Se3 layer produced by the substitution and the InAs substrate is demonstrated. The cathodeluminescence properties are also provided focusing on the dependence on the phase of In2Se3.

GROWTH AND MICROSTRUCTURES OF ULTRATHIN Bi2Te3 NANOPLATES BY MODIFIED HOT WALL EPITAXY

Ultrathin Bi 2 Te 3 nanoplates have been grown on an oxidized silicon substrate by a modified hot wall epitaxy (HWE) method, in which a quartz plate with holes was employed. The microstructures and optical properties of Bi 2 Te 3 nanoplates were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscope (AFM) and micro-Raman spectroscopy. The results show that ultrathin Bi 2 Te 3 nanoplates with the thickness of about six quintuple layers (QLs) are obtained, which is difficult for the traditional HWE technique. The Raman vibration mode [Formula: see text] from the nanoplates exhibits an obviously red shift with decreasing thickness. The thickness variation of one nanoplate was obtained by the Raman map derived from the vibration frequency of [Formula: see text] mode and is in good agreement with the AFM result, which indicates that Raman map is an effective method to characterize the thickness difference of ultrathin Bi 2 Te 3 nanoplates.

Growth and photocurrent characteristics of the photoconductive MnAl2S4 layers grown by hot-wall epitaxy method

The epitaxial growth of the photoconductive MnAl2S4 with a layered-type rhombohedral structure was first achieved by means of the hot-wall epitaxy method. From the Hall effect measurement, two dominant scattering mechanisms on the mobility were extracted. One was an acoustic phonon scattering caused by lattice vibrations at the high-temperature range of T>100K and the other one was an impurity ion scattering at the low-temperature range of T<100K. Also, from the relation between the reciprocal temperature and the carrier concentration, two activation energies were evaluated to be the deep level of 83.1 and a shallow level of 14.1meV at high-temperature and middle-temperature ranges, respectively. From the photocurrent (PC) measurement, the A, B, and C peaks were observed at whole temperature range. The electronic origin of these three peaks were caused by the band-to-band transitions from the valence band states of Γ4(z), Γ5(x), and Γ5(y) to the conduction band state of Γ1(s), respectively. Thus, the parameters of the crystal-field and the spin–orbit splitting were directly extracted by means of PC spectroscopy, and those values were 3.5 and 39.9meV, respectively. The band gap energies estimated from PC and absorption measurements were well expressed by Eg(T)=Eg(0)−2.80×10−4T2/(T+287), where Eg(0) were found to be 3.7920, 3.7955, and 3.8354eV at the valence band states of Γ4(z), Γ5(x), and Γ5(y), respectively.

Multifractal analysis of areas of spatial forms on surface of ZnxCd1−xTe–Si (111) heterocompositions

A multifractal (MF) method is used for analysis of areas of spatial forms on the surface of thin layers of ZnxCd1−xTe solid solution grown on the Si (111) substrate by a method of the hot-wall epitaxy. AFM images of the film surface are considered as input information for the MF analysis. The parameters of the MF spectra of the film surface are obtained. It is shown that the MF parameters correspond to their canonical forms, and the developed computational approach can be applied to describe and analyze the spatial fractal structures formed on a layer surface. The quantitative relationships between the MF spectrum parameters of a film surface and its growth conditions are found.

Semicoherent growth of BiTe layers on InP substrates by hot wall epitaxy

We search for optimum growth conditions to realize flat BiTe layers on InP(111)B by hot wall epitaxy. The substrate provides a relatively small lattice mismatch, and so (0001)-oriented layers grow semicoherently. The temperature window for the growth is found to be narrow due to the nonzero lattice mismatch and rapid re-evaporation of BiTe. The crystalline qualities evaluated by means of x-ray diffraction reveal deteriorations when the substrate temperature deviates from the optimum not only to low temperatures but also to high temperatures. For high substrate temperatures, the Bi composition increases as Te is partially lost by sublimation. We show, in addition, that the exposure of the BiTe flux at even higher temperatures results in anisotropic etching of the substrates due, presumably, to the Bi substitution by the In atoms from the substrates. By growing BiTe layers on InP(001), we demonstrate that the bond anisotropy on the substrate surface gives rise to a reduction in the in-plane epitaxial alignment symmetry.

Surface states of charge carriers in epitaxial films of the topological insulator Bi2Te3

The galvanomagnetic properties of p-type bismuth telluride heteroepitaxial films grown by the hot wall epitaxy method on oriented muscovite mica substrates have been investigated. Quantum oscillations of the magnetoresistance associated with surface electronic states in three-dimensional topological insulators have been studied in strong magnetic fields ranging from 6 to 14 T at low temperatures. The cyclotron effective mass, charge carrier mobility, and parameters of the Fermi surface have been determined based on the results of analyzing the magnetoresistance oscillations. The dependences of the cross-sectional area of the Fermi surface S(kF), the wave vector kF, and the surface concentration of charge carriers ns on the frequency of magnetoresistance oscillations in p-type Bi2Te3 heteroepitaxial films have been obtained. The experimentally observed shift of the Landau level index is consistent with the value of the Berry phase, which is characteristic of topological surface states of Dirac fermions in the films. The properties of topological surface states of charge carriers in p-type Bi2Te3 films obtained by analyzing the magnetoresistance oscillations significantly expand fields of practical application and stimulate the investigation of transport properties of chalcogenide films.

Hot Wall Epitaxy(HWE)법에 의한 BaIn2Se4에피레어 성장과 가전자대 갈라짐에 대한 광전류 연구

A stoichiometric mixture of evaporating materials for BaIn2Se4 epilayers was prepared from horizontal electric furnace. To obtain the single crystal thin films, BaIn2Se4 mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were 620 o C and 400 o C, respectively. The crystalline structure of the epilayers was investigated by the photoluminescence and double crystal X-ray diffraction (DCXD). The carrier density and mobility of BaIn2Se4 epilayers measured from Hall effect by van der Pauw method are 8.94×10 17 cm -3 and 343 cm 2 /vs at 293 K, respectively. The temperature dependence of the energy band gap of the BaIn2Se4 obtained from the absorption spectra was well described by the Varshni's relation, Eg(T)=2.6261 eV-(4.9825×10 -3 eV/K)T 2 /(T+558 K). The crystal field and the spin-orbit splitting energies for the valence band of the BaIn2Se4 have been estimated to be 116 meV and 175.9 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the Δso definitely exists in the states of the valence band of the BaIn2Se4/ GaAs epilayer. The three photocurrent peaks observed at 10 K are ascribed to the A1-, B1-exciton for n = 1 and C21-exciton peaks for n=21.

Suppressing Al memory effect on CVD growth of 4H-SiC epilayers by adding hydrogen chloride gas

The Al memory effect during the growth of Al-doped 4H-SiC by the hot-wall chemical vapor phase epitaxy method was investigated. To suppress unintentional incorporation of Al impurities during succeeding growth, a technique was developed by employing HCl-assisted “site-competition” growth. Three methods of introducing HCl, namely, HCl flushing before growth, HCl addition during growth, and the combination of the two preceding methods, have been performed and the Al suppression effects corresponding to the input C/Si ratio were studied separately. It is found that lowering the C/Si ratio reduces Al incorporation for all methods of introducing HCl and using the combination of HCl flushing and HCl addition is highly effective. Optimizing growth rate, temperature, and pressure can further improve Al suppression efficiency; thus, a highly abrupt change in Al distribution between Al-doped and undoped epilayers with Al concentration differences of more than five orders of magnitude was obtained, e.g., a steep change from 1020 to 1014 cm−3.