The photoreflectance method, i.e., a contactless version of optical modulation spectroscopy is applied to the study of the band structure features of single-crystal semiconductors, their doping ...level, composition of alloys, surface and interface energy band bendings. By the example of high-quality GaAs, the possibility of describing the photoreflectance spectral line shape within the one-electron and exciton models is demonstrated. An oscillating structure well described by exciton effects is detected in the spectra of ultrapure samples of this material. For III–V alloys, the results obtained by the photoreflectance method on the effect of the composition and temperature on the band gap and spin–orbit splitting are reviewed. The problem of Fermi-level determination (pinning) on the III–V crystal surface is considered. The currently developed method for measuring the photoreflectance in the mid-infrared region, i.e., the photomodulation Fourier transform infrared spectroscopy of reflectance is described in detail. It is shown that the crucial role in similar measurements is played by phase correction. Original results demonstrating the capability of this method in a wide wavelength range are presented.
Thin GaTe films were grown by molecular beam epitaxy (MBE) on GaAs(001) substrates. X-ray powder diffraction confirmed the coexistence of the
h
- and
m
-GaTe phases in all grown layers. A ...quantitative correlation between the MBE growth conditions and the phase composition of the grown films is established, and the upper limit of the MBE growth temperature for thin GaTe/GaAs(001) films is experimentally determined. New data are presented confirming the defect-related origin of the broad emission line with an energy maximum of ~1.45–1.46 eV, which dominates the low-temperature photoluminescence spectra of the grown GaTe/GaAs(001) layers.
High-efficiency semiconductor lasers and light-emitting diodes operating in the 3–5 μm mid-infrared (mid-IR) spectral range are currently of great demand for a wide variety of applications, in ...particular, gas sensing, noninvasive medical tests, IR spectroscopy etc. III-V compounds with a lattice constant of about 6.1 Å are traditionally used for this spectral range. The attractive idea to fabricate such emitters on GaAs substrates by using In(Ga,Al)As compounds is restricted by either the minimum operating wavelength of ∼8 μm in case of pseudomorphic AlGaAs-based quantum cascade lasers or requires utilization of thick metamorphic InxAl1-xAs buffer layers (MBLs) playing a key role in reducing the density of threading dislocations (TDs) in an active region, which otherwise result in a strong decay of the quantum efficiency of such mid-IR emitters. In this review we present the results of careful investigations of employing the convex-graded InxAl1-xAs MBLs for fabrication by molecular beam epitaxy on GaAs (001) substrates of In(Ga,Al)As heterostructures with a combined type-II/type-I InSb/InAs/InGaAs quantum well (QW) for efficient mid-IR emitters (3–3.6 μm). The issues of strain relaxation, elastic stress balance, efficiency of radiative and non-radiative recombination at T = 10–300 K are discussed in relation to molecular beam epitaxy (MBE) growth conditions and designs of the structures. A wide complex of techniques including in-situ reflection high-energy electron diffraction, atomic force microscopy (AFM), scanning and transmission electron microscopies, X-ray diffractometry, reciprocal space mapping, selective area electron diffraction, as well as photoluminescence (PL) and Fourier-transformed infrared spectroscopy was used to study in detail structural and optical properties of the metamorphic QW structures. Optimization of the growth conditions (the substrate temperature, the As4/III ratio) and elastic strain profiles governed by variation of an inverse step in the In content profile between the MBL and the InAlAs virtual substrate results in decrease in the TD density (down to 3 × 107 cm−2), increase of the thickness of the low-TD-density near-surface MBL region to 250–300 nm, the extremely low surface roughness with the RMS value of 1.6–2.4 nm, measured by AFM, as well as rather high 3.5 μm-PL intensity at temperatures up to 300 K in such structures. The obtained results indicate that the metamorphic InSb/In(Ga,Al)As QW heterostructures of proper design, grown under the optimum MBE conditions, are very promising for fabricating the efficient mid-IR emitters on a GaAs platform.
Built-in electric fields appear during the molecular-beam epitaxy of GaSe on a GaAs(001) substrate at the GaSe/GaAs interface, the presence of the fields is evidenced by Franz–Keldysh oscillations ...observed in photoreflectance spectra. The varying values of the strength of these fields (from 9.8 to 17.6 kV/cm) can be associated both with the diffusion of Se atoms into the GaAs substrate (buffer layer) and with the formation of transient submonolayers during epitaxial growth. Built-in fields are not observed at the interface of structures grown on GaAs(111)B and GaAs(112) substrates, which can be explained by the lower propagation efficiency of Se into the substrate with mentioned orientations when compared with GaAs(001).
Abstract
Infrared photoreflectance (PR) spectra of In(Ga,Al)As/GaAs metamorphic heterostructures have been obtained using a novel photomodulation FTIR spectroscopy technique. An analysis of the PR ...spectra features allowed us to estimate the critical point energies corresponding to the direct interband transitions in various regions of the In(Ga,Al)As heterostructures, and distinguish the PR signals originating from Fabry-Perot interference. Observation of Franz-Keldysh oscillations originating from the InAlAs virtual substrate and an InGaAs waveguide layer has enabled determination of the built-in electric field intensities within the heterostructures. The obtained results open up possibilities for contactless control of free carrier concentration in In(Ga,Al)As/GaAs metamorphic heterostructures developed for growth of emitters of mid-IR spectral range.
Single manganese and carbon δ-layers in undoped GaAs are analyzed by photoreflectance spectroscopy. The strength of built-in electric fields, determined by this method, is shown to increase with ...increasing layer concentration of introduced impurities and well correlate with technological and Hall data. The use of phase-sensitive photoreflectance makes it possible to measure the surface field and the field induced by δ-doping separately. This fact allows one to determine without contact the fraction of electrically active Mn impurity and reveal the contribution of carbon δ-layers to the characteristics of GaAs-based heterostructures.
A photoluminescence (PL) measurement method using boxcar integration in the active baseline subtraction mode has been implemented. It allows for infrared PL measurements with a low duty cycle of the ...pump pulse, thereby minimizing the uncontrolled heating of the structure under study. The implemented method was compared to phase-sensitive detection by signal-to-noise ratio of the resulting spectra, indicating its superior performance at low duty cycles. The decrease in uncontrolled heating of the structure under study was confirmed by observing a change in the ratio of temperature-sensitive PL peaks of a test InSb/InAs/InGaAs/InAlAs/GaAs nanoheterostructure.
The results of analyzing the photoluminescence spectra of metamorphic In0.75Al0.25As/In0.05-0.8Al0.95-0.2As/GaAs heterostructures grown by molecular-beam epitaxy method are presented. In the spectra ...of low-temperature photoluminescence, three emission peaks have been detected, which correspond to carrier recombination in different areas of the structure. These include a part of the In0.75Al0.25As "virtual substrate" grown at high temperature, and the area of a triangular potential well formed by the meta-morphic buffer layer and the "virtual substrate". Photoluminescence related to defects in the high-temperature part of the "virtual substrate" was also measured.
This work investigates the luminescence properties of pseudomorphic nanostructures with Ge1-x-ySixSny/Si superlattices (SL) grown on silicon substrates by molecular beam epitaxy. It was shown that ...the addition of Sn (y = 0.07) to the alloy layers within the structures results in a significant shift of the photoluminescence (PL) spectra towards longer wavelengths (2.0-3.5 μm) compared to similar Ge0.7Si0.3/Si superlattices. A series of experiments involving etching the structures to different depths have shown that the observed photoluminescence occurs exactly in the Ge1-x-ySixSny/Si SL region, with the sublinear nature of the PL power dependence indicating the probable participation of defects in radiative recombination. A significant increase in low-temperature PL was observed with a decrease in the thickness of narrow-gap SL layers from 6 to 2 nm, as well as with an increase in the number of superlattice periods from 10 to 30. The obtained structures exhibit relatively good temperature stability of luminescence, which is preserved up to 160 K. Thus, the current work demonstrates the possibility of creating mid-wave IR emitting epitaxial structures on silicon substrates.
The GaAs/AlGaAs superlattice grown by metal-organic chemical vapour deposition (MOCVD) was investigated by photoreflectance (PR) spectroscopy. Optical transitions over the whole band structure were ...measured at room temperature. Different laser pump beams (632 and 532 nm) were used to clarify the nature of the PR signal formation. Charge carriers transport over minibands was revealed by using long-wave modulation. An acceptable correlation of theoretical and experimental results was achieved for low-energy transitions. Optical transitions relating to the edges of the first and the second minibands for electrons, heavy and light holes were clearly observed experimentally.
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