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1.
Optical studies of residual strain in cadmium telluride (CdTe) films grown using molecular beam epitaxy on gallium arsenide (GaAs) substrate have been performed using photoreflectance techniques. Measurements have been conducted to determine the fundamental transition energy, heavy-hole and light-hole transition energy critical-point parameters in a range of temperatures between 12 and 300 K. There are problems inherent in the fabrication of optoelectronic devices using high-quality CdTe films, due to strain effects resulting from both the lattice mismatch (CdTe: 14.6%) and the thermal expansion coefficient difference. The CdTe film exhibits compressive stress causing valence-band splitting for light and heavy holes. We have used different models to fit the obtained experimental data and, although the critical thickness for the CdTe has been surpassed, the strain due to the lattice mismatch is still significant. However, the strain due to the thermal expansion is dominant. We have found that the fundamental transition energy, E0, is affected by the compressive strain and the characteristic values are smaller than those reported. In addition, the total strain is compressive for the full measured range, since the strain due to the lattice mismatch is one order of magnitude higher than that calculated from the thermal expansion. 相似文献
2.
Absorption and photoreflectance spectroscopy of zinc phthalocyanine (ZnPc) thin films grown by thermal evaporation 总被引:1,自引:0,他引:1
Optical properties of the as-deposited and annealed ZnPc layers have been investigated using absorption, reflectance and modulated photoreflectance methods. The absorption coefficient of ZnPc layers was directly determined from the transmission and reflection spectra. The absorption spectra were analyzed in terms of the mixed Lorentz–Lorenz model. We found that annealing thin layers at 580 K caused a structural transformation, which results in the decrease of the absorption coefficient and the shifting of all peak position to lower energies except for the peak of the N-band. Photoreflectance spectroscopy confirmed that there exist three transitions in the Q-band region of the studied material. Complex refractive index and dielectric constants of the ZnPc layer were directly found from the spectral data. 相似文献
3.
The electromodulation method of photoreflectance (PR) is becoming an important tool for the characterization of semiconductors,
semiconductor interfaces and semiconductor microstructures such as superlattices, quantum wells, multiple quantum wells and
heterojunctions. Since PR is contactless, requires no special mounting of the sample and can be performed in a variety of
transparent ambients it can be utilized for in-situ monitoring of growth at elevated temperatures, in-situ elevation at 300K
before the samples are removed from the growth/processing chamber as well as convenient ex-situ characterization. This invited
article discusses some recent uses of PR to measure (a) the direct gap (and spin-orbit split component) of InP up to 600°
C, (b) strains in Si at the Si/SiO2 interface, (c) changes in the surface Fermi level of GaAs caused by photowashing, (d) quantized intersubband transitions
in a GaAs/Gao0.82Al0.18As multiple quantum well and (e) two-dimensional electron gas effects in selectively dopedn-Gao.7Alo.3As/ GaAs heterostructures. 相似文献
4.
C. Mejía-García A. Caballero-Rosas A. Winter J.L. López-López 《Thin solid films》2010,518(7):1825-1829
Al0.3Ga0.7As/GaAs Quantum Well structures were grown by molecular beam epitaxy (MBE) on a 500 nm thick GaAs buffer layer subjected to the following surface processes: a) in-situ Cl2 etching at 70 °C and 200 °C, b) air-exposure for 30 min. The characteristics of these samples were compared to those of a continuously grown sample with no processing (control sample). We obtained the quantum wells energy transitions using photoreflectance spectroscopy as a function of the temperature (8-300 K), in the range of 1.2 to 2.1 eV. The sample etched at 200 °C shows a larger intensity of the quantum well peaks in comparison to the others samples. We studied the temperature dependence of the excitonic energies in the quantum wells (QWs) as well as in GaAs using three different models; the first one proposed by Varshni [4], the second one by Viña et al. [5], and the third one by Pässler and Oelgart [6]. The Pässler model presents the best fitting to the experimental data. 相似文献
5.
N. Bottka D. K. Gaskill R. S. Sillmon R. Henry R. Glosser 《Journal of Electronic Materials》1988,17(2):161-170
Modulation spectroscopy is an optical characterization tool that can be of great utility to the materials scientist. We present
here numerous examples where a simple photo-reflectance and electroreflectance setup is used in our laboratory to determine
such important material parameters as alloy composition and carrier concentration in a very short time. For determining alloy
composition in semiconductors, contactless room temperature photoreflectance is nearly as sensitive as low temperature photoluminescence.
Examples will be given on how to determine: the effects of surface preparation and implant damage; alloy composition and carrier
homogeneity for large area wafers to better than 1%; the segregation coefficient of isoelectronic impurities in bulk semiconductors;
the sub-band energies in quantum well structures; and the presence and homogeneity of built-in electric fields in MODFET structures.
Particular emphasis will be placed on band edge and exciton effects on the photoreflectance and on the criteria used to distinguish
between them. Materials studied included Si doped GaAs, AlxGa1-xAs for variousx grown by OMVPE and MBE, bulk InP doped with iso-electronic As and Sb, and MODFET structures. 相似文献
6.
Highly strained In0.28Ga0.72As1−xNx/GaAs single quantum well (SQW) structures grown by molecular beam epitaxy for different N mole fractions have been investigated by photoreflectance (PR) spectroscopy at room temperature. The influence of nitrogen on the electronic structure and on the optical properties of the quantum well has been analysed. The observation of excited state transitions allowed demonstrating a type I band line-up for both heavy and light holes, for such indium and nitrogen mole fractions. All the observed optical transitions have been identified on the base of the results of the envelope function calculation including strain effects. The dependence of the optical transitions on the nitrogen content has been derived and compared with the experiment. 相似文献
7.
The E1 and E1+Δ1 energy bands of metal–organic chemical vapor deposition grown AlxGa1−xAs, with x in the range 0–0.55, have been determined using photoreflectance technique. The aluminum composition for each sample was determined using the energy of the room-temperature photoluminescence compensated peak value and a suitable fundamental band gap formula. The positions of the E1 and E1+Δ1 peaks were determined from curve-fitting an appropriate theoretical model to our experimental data by a modified downhill simplex method. Using the results, we propose new E1 and E1+Δ1 cubic expressions as functions of the aluminum composition, x, and compare them with the available reported expressions. 相似文献
8.
AlGaN/GaN heterostructures with a two dimensional electron gas (2DEG) at the interface have been investigated by contactless electroreflectance (CER) and photoreflectance (PR) spectroscopies. It has been shown that the 2DEG effectively screens the GaN layer and hence no signal related to a bandgap transition in the GaN layer is observed in CER spectra whereas the CER signal related to a bandgap transition in the AlGaN layer is very strong. The screening phenomenon is unimportant for PR spectroscopy due to different mechanism of the electromodulation. As a result both GaN and AlGaN related transitions are clearly observed in PR spectra. It has been proposed that the screening phenomena observed in CER can find application in contactless detection of the 2DEG in AlGaN/GaN heterostructures. 相似文献
9.
The optical properties of hetero-polarization GaN/AlGaN (12% Al and 16.5% Al) are studied by photoluminescence (PL), photoreflectance (PR) and photoluminescence excitation (PLE) at low temperature. This material system is known to exhibit very strong internal electric fields and we show that PR spectroscopy is one of the best techniques to get a quantitative measurement of such fields, as it is a direct all-optical measurement. Electric field strength derived from FKO is approximately 120 kV/cm in AlGaN (12% Al) and 850 kV/cm in a 4.2 nm GaN QW. These values are in good agreement with other indirect determinations realized on the same samples by PL and PLE spectroscopy, but which involve computer simulations that need an accurate knowledge of geometrical and composition parameters of the samples. 相似文献
10.
H. Shen Fred H. Pollak J. M. Woodall R. N. Sacks 《Journal of Electronic Materials》1990,19(3):283-286
We have studied the photoref lectance (PR) spectra from a MBE grown heterostructure consisting of 200 nm of Ga0.83Al0.17As, a 800 nm GaAs buffer layer on a semi-insulating (100) LEC GaAs substrate. By varying both the pump beam wavelength and
modulation frequency (up to 100 kHz) we are able to identify the component layers, their quality and the properties of the
various interfaces. In this study we find evidence for a low density of interface states between the GaAs buffer layer and
GaAlAs layer and a relatively large density of interface states between the substrate and buffer regions. These states, previously
observed by Deep Level Transient Spectroscopy of doped structures, are presumably associated with the interface produced by
MBE growth on etched and air exposed substrates. However, in our material, since the substrate is semi-insulating and the
buffer layer is undoped, it is difficult to resolve these states spatially by C-V techniques. Our results show that the PR
technique can be used to characterize low conductivity or semi-insulating structures such as enhancement mode MESFET and HEMT
type devices and it may be useful for the in-situ characterization of epigrown surfaces and interfaces 相似文献