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For the first time, we obtained the photoreflectance spectra in single-layer samples of porous silicon with 13 μm thick in the 550–1000 nm spectral region. To describe the observed reflection and photoreflectance spectra, we use a theory of multiple-beam interference, taking into account the strong absorption of Si in this spectral region. We show that, under the action of a laser radiation with a wavelength 532 nm of 30 mW power and pulse duration 3 ms, the change in the refractive index δn reaches values of the order of 10−5, and this takes place due to the thermal nonlinearity of the refractive index. We show that photoreflectance spectroscopy can be used to measure the thermo-optic coefficients of porous silicon.

The internal electric fields of InGaN/GaN-based green-emission LED heterostructures with various numbers of quantum wells in the active region are investigated by electrotransmission spectroscopy. The frequencies of the observed spectral lines are attributed to possible types of interband transitions. An increase in the number of interband transitions of the “quantum well—quantum barrier” type with an increase in the number of quantum wells is found. This is explained by the nonidentical degree of segregation of In atoms in different GaN barriers layers. The strength of internal electric fields in quantum wells is calculated for various values of the bias of the p–n junction using a series of electrotransmission spectra. It is found that the strength of the internal piezoelectric field decreases from 3.20 to 2.82 MV/cm with an increase in the number of quantum wells.

InGaN/GaN-based LED heterostructures with different numbers of quantum wells are investigated by photocurrent spectroscopy in the wavelength range of 350–500 nm. As a result of the analysis of a series of spectra obtained at various p–n-junction biases, the effect of changing photocurrent direction when varying the excitation wavelength (photoreversible effect) is discovered. The range of p–n-junction biases for which this effect is observed in structures with different numbers of quantum wells in the active region is established.

The article is devoted to the study of the mechanism of piezoelectric scattering of charge carriers in an InGaN/AlGaN/ GaN heterostructures with one filled quantum-confinement subband. The aim of the article is to create a mathematical model that can be used to estimate the kinetic parameters of a two-dimensional electron gas (2DEG) in the structure under study. The results of the calculation of the matrix scattering potentials demonstrated the reliability and consistency on the used model of industrial LED heterostructures with InGaN/GaN quantum wells. It is shown that mechanical stresses in the InGaN/GaN layers lead to an uneven distribution of the total electric field along the active layer. It has been established that the lifetime limiting the mobility of a two-dimensional electron gas in piezoelectric scattering is of the order of 10–9 s.

Electric-field nonuniformity in the active region of a LED (light-emitting diode) heterostructure based on five identical GaN/InGaN quantum wells is investigated by the electroreflectance method. The energies of band-to-band transitions in the quantum wells and barriers are determined from the analysis of electroreflectance spectra using the Kramers–Kronig transforms. The procedure for estimating the electric-field strength in separate quantum wells of the active region by the position of spectral lines is proposed. It is found that the energies of the main transition in quantum wells of the active region with zero bias of the p–n junction differ by a magnitude on the order of 140 meV, which corresponds to a difference in the electric-field strengths of 0.78 MV/cm. It is shown that the nonuniformity of the electric field in the active region depends on the p–n junction bias.

The mechanical strains and densities of surface charge states in GaAs layers grown by low-temperature (LT) molecular-beam epitaxy on Si(100) and GaAs(100) substrates are investigated by photoreflectance spectroscopy. Lines corresponding to the fundamental transition (E g ) and the transition between the conduction band and spin-orbit-split valence subband (E g + Δ SO ) in GaAs are observed in the photoreflectance spectra of Si/LT-GaAs structures at 1.37 and 1.82 eV, respectively. They are shifted to lower and higher energies, respectively, relative to the corresponding lines in GaAs/LT-GaAs structures. Comparing the spectra of the Si/LT-GaAs and GaAs/LT-GaAs structures, it is possible to estimate mechanical strains in LT-GaAs layers grown on Si (by analyzing the spectral-line shifts) and the density of charge-carrier states at the GaAs/Si heterointerface (by analyzing the period of Franz–Keldysh oscillations).

Nitride-based heterostructures with InGaN/GaN multiple quantum wells in active region were investigated by electroreflectance spectroscopy. Two spectral lines with the stable difference between their phase parameters and slightly different energies were observed in the electroreflectance spectra under various bias voltages. Using models taking into account interference effects it was shown that electroreflectance signal at low amplitude of modulation voltage originates from the first and the last quantum wells in active region. The difference between built-in electric field strengths in these two quantum wells were estimated as 270–320 kV/cm.

InP (100) crystals implanted with Be+ ions with an energy of 100 keV and doses of 1013–1015 cm–2 are studied by Raman spectroscopy before and after thermal annealing at temperatures of 300–850°C. It is found that, as the implanted ion dose is increased, the surface region of InP is partially amorphized; in this case, spectral lines related to longitudinal lattice vibrations exhibit a shift to lower frequencies and inhomogeneous broadening, which is indicative of the formation of a nanocrystalline phase. Thermal annealing results in recovery of the crystal structure of InP. At annealing temperatures of >700°C, scattering at phonon–plasmon coupled modes is detected in the Raman spectra. This is attributed to electrical activation of the impurity. From the frequency of the phonon–plasmon mode, the concentration of heavy holes is estimated in the context of the model of a two-oscillator dielectric function.

A line at E = 2.77 eV (with a width of Γ = 88 meV) related to interband transitions in the region of multiple quantum wells in the active region is detected in the electroreflectance spectra of the GaN/InGaN/AlGaN heterostructure. As the modulation bias is reduced from 2.9 to 0.4 V, the above line is split into two lines with energies of E 1 = 2.55 eV and E 2 = 2.75 eV and widths of Γ1 = 66 meV and Γ2 = 74 meV, respectively. The smaller widths of separate lines indicate that these lines are caused by interband transitions in particular quantum wells within the active region. The difference between the interband transition energies E 1 and E 2 in identical quantum wells in the active region is related to the fact that the quantum wells are in an inhomogeneous electric field of the p–n junction. The magnitudes of the electric-field strengths in particular quantum wells in the active region of the heterostructure are estimated to be 1.6 and 2.2 MV/cm.

We present a method of an optical nonlinearity measurement in the photonic crystal by means of the photo-reflectance spectroscopy. We propose that the photoreflectance is a promising, non-destructive and sensitive technique for a measurement of optical nonlinearities of photonic band gap structures.

GaAs/AlGaAs quantum-well heterostructures with well widths from 20 to 35 nm are investigated by photoreflectance spectroscopy. The broadening of spectral lines related to band-to-band transitions increases with the transition energy and decreases with the well width. The observed decrease in the broadening parameter with increasing well width is explained in terms of spatial inhomogeneity of the heterointerfaces. According to the experimental data, the interface inhomogeneity in the structures under study is 0.34–0.39 nm (1.3–1.4 monolayers).

A series of AlGaAs/InGaAs/AlGaAs quantum-well heterostructures with different quantum-well depths and approximately the same concentrations of two-dimensional electrons is grown by molecular-beam epitaxy. The built-in electric field in the grown samples is determined from the photoreflectance data and, on this basis, the energy-band structure in the quantum-well region is calculated. It is found that the highest mobility μe of two-dimensional electrons is attained in the sample with a barrier-layer thickness of Lb = 11 nm. Measurements of the photoluminescence spectra and the band-structure calculations demonstrate that, as the quantum well becomes closer to the surface, the doping profile broadens due to diffusion and segregation processes. The nonmonotonic dependence of μe on the distance between the surface and the quantum well is explained.

In recent works one could propose that the periodicity of the interference fringes in the reflectance and electroreflectance spectra is the same. Detailed study of the GaN based LEDs multilayered heterostructures by means of reflectance and electroreflectance spectroscopy show that this proposal is not correct. The possible explanation of such difference has been shown in this paper. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Delta-doped GaAs layers have been studied by photoreflectance spectroscopy. The built-in electric fields and interband transition energies in the semiconductor structure have been evaluated from analysis of Franz-Keldysh oscillations. The delta-doped region is found to have an increased interband transition energy, which is interpreted in terms of the Burstein-Moss effect and carrier photogeneration.

We have studied in detail the coupled phonon-plasmon mode Raman spectra of n-InxGa1 − xAs with n in the range 1017 to 1019 cm−3. The results indicate that the behavior of the high-frequency mode L+ can be described in terms of coupled modes in the Drude approximation. The proposed theory and experimental data are used to estimate the carrier concentration in the solid solution and its composition.

The spectrum of electron-hole states in a GaAs/In0.5Ga0.5As quantum well with a width graded in the range from 1.1 to 3.6 nm is studied by photoreflectance spectroscopy. The energies of the size-quantization levels of electrons and holes are calculated taking into account the strain-induced changes in the band structures of the quantum well. It is shown that the best fit of the experimental data to the results of calculations is attained if the ratio between the offset of the conduction band and that of the valence band at the heterojunction is Q = ΔEc/ΔEv = 0.62/0.38. A photoreflectance signal is detected in the region of the shadow of modulating radiation beam at a spacing between the spots produced by probing and modulating radiation shorter than 6 mm.

Interference effects in InGaN/AlGaN/GaN light-emitting-diode heterostructures of blue emission are studied by spectroscopy of electroreflectance and electroluminescence. The periodic bands observed in the electroreflectance and electroluminescence spectra in a blue spectral range are caused by interference effects in the structure in general. The emergence of interference fringes in the electroreflection spectra is explained by modulation of built-in electric fields in the active region of the heterostructure. The long-period interference fringes observed in the electroreflectance spectra in a wide spectral range from infrared to ultra-violet allows one to determine the location of the active region of the heterostructure with respect to different reflecting surfaces in the cavity.

p−n InGaN/AlGaN/GaN heterostructures with InGaN/AlGaN multiple quantum wells are studied by electroreflectance spectroscopy. The structures are grown by metal—organic epitaxy and arranged with the p region in contact with the heat sink. Light is incident on and reflected from the structures through the sapphire substrate. To modulate the reflectivity, rectangular voltage pulses and a dc reverse bias are applied to the p−n junction. A line corresponding to interband transitions in the region of InGaN/AlGaN multiple quantum wells is observed in the electroreflectance spectra. The peak of this line is shifted to shorter wavelengths from the peak of injection luminescence of the light-emitting diode structures. The low-field model developed by Aspnes is used to describe the electroreflectance spectra. By choosing the parameters of the model to fit the experimental data, the effective band gap of the active region of the structure, E g * , is determined at 2.76–2.78 eV. The experimental dependence of E g * on the applied voltage is attributed to the effect of piezoelectric fields in the InGaN quantum wells. In the electroreflectance spectra, an interference pattern is observed in the wide spectral range from 1.4 to 3.2 eV. The interference is due to the dependence of the effective refractive index on the electric field.

The carrier concentrations in Si-doped n-GaAs films have been determined by Raman and light reflection spectroscopies. The data obtained are in good agreement with the results of Hall measurements. It is shown that the light reflection and Raman spectroscopies supplement each other in determination of carrier concentrations in the range 1017−1019 cm−3.

We have studied light scattering in synthetic opal crystals infiltrated with ferroelectric sodium nitrite, NaNO2, and have analyzed simple models for the energy band structure of photonic crystals. Expressions have been derived for the group velocity of photons whose energy is close to the photonic band gap. Our results indicate that the infiltration of photonic crystals with NaNO2 markedly increases the Raman scattering intensity.

Photoreflectance spectra of AlGaAs/GaAs/AlGaAs wide quantum wells doped by Si up to Nd = 2 x 1018cm-3 were investigated at room temperature. Three kinds of spectral peculiarities were observed in photoreflectance spectra: a spectral line due to the GaAs band gap (1.42 eV), short-period Frantz?Keldysh oscillations originating from the barrier band gap (1.71 eV) and lines of subbands in the quantum well. The energies of optical transitions are determined by means of the least-squares approximation of experimental data by the sum of Aspnes relations. The experimental results are in good agreement with the self-consistent subband structure calculation. It is shown that with an increase of the doping level the energies of the interband transitions in the quantum well are changed.

Energies of band-to-band transitions with the involvement of the quantum-confinement subbands are determined from the photoreflection spectra of the strained short-period GaAs/GaAs0.6P0.4 superlattice. Strains caused by the mismatch of the crystal lattice in the GaAs and GaAs0.6P0.4 layers are calculated on the basis of the observed shift of the fundamental-transition energy in GaAs0.6P0.4. Positions of minibands in the superlattice are simulated in relation to the potential jump at the heteroboundary; the Kronig-Penney model is used in the calculations. Comparison of the results of simulation with experimental data shows that the studied superlattice is of type I with weakly localized electrons and light holes. The potential jump at the heteroboundary in the conduction band amounts to ΔE c /ΔE g =0.15.

The Raman and Brillouin spectra in bismuth vanadate crystals near the ferroelectric phase-transition point are studied. The intensity maximum corresponding to the soft mode in light scattering spectra is found. This mode is responsible for the instability of the bismuth vanadate crystal lattice in the vicinity of the ferroelectric phase-transition point. Strong interaction of the soft optical mode with an acoustic mode of the corresponding symmetry is revealed by an analysis of the Raman and Brillouin spectra observed. The theory of light scattering by coupled lattice modes based on the model of two strongly coupled oscillators is developed. The introduction of an additional oscillator strongly interacting with the fundamental soft mode is shown to result in a temperature shift of the structural phase transition to higher temperatures. The results obtained confirm the possibility of changing the phase-transition temperature by modification of the vibrational spectrum through introduction of additional degrees of freedom or comminution of the macroscopic sample with formation of the ordered superdispersed structure (globular crystal). These results are general and can be subsequently used to increase the phase-transition temperature in ferroelastics, ferroelectrics, and superconductors.

Room-temperature photoreflectance spectra of coupled-quantum-well heterostructures with a built-in electric field are measured. The optical transition energies are determined and their dependences on the well width and the barrier thickness are examined. The experimental results are compared with the calculated energies of electron-hole transitions. Good agreement between calculations and experiment is found for narrow wells; in the case of wide wells, optical transitions are associated with groups of several closely spaced electron-hole transitions.

Photoreflectance spectra are measured in heterostructures with coupled quantum wells at room temperature. The energies of the optical transitions are determined, and their variation with the well width and barrier thickness is studied. The experimental results are compared with the theoretically calculated electron-hole transition energies. Good agreement is obtained for narrow wells.

The influence of built-in piezoelectric field in the light-emitting diodes based on InGaN/AlGaN/GaN heterostructures on the electroreflectance spectra have been studied. The structures were grown by MOCVD technology and «flip-chip» mounted. The spectral line connected with the InGaN/GaN multiple quantum wells region are observed in the electroreflectance spectra. The blue shift of this spectral line with the increasing reverse bias voltage has been explained as the result of decreasing of the electric field in the quantum well. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure in the Raman scattering spectra of near-surface n-GaAs layers (n=2×1018 cm−3) implanted with 100 keV B+ ions in the dose range 3.1×1011–1.2×1014 cm−2 is investigated. The qualitative and quantitative data on the carrier density and mobility and on the degree of amorphization of the crystal lattice and the parameters of the nanocrystalline phase as a result of ion implantation are obtained using a method proposed for analyzing room-temperature Raman spectra.

Semi-insulating GaAs(100) substrates subjected to ion plasma etching in different regimes have been investigated by photoreflectance spectroscopy. The photoreflectance spectra of the processed samples exhibit Franz-Keldysh oscillations, which indicate a decrease in the defect density in the surface region. On the basis of the experimental data and the results of simulation of the photoreflectance spectra, optimal regimes of sample etching have been found.

The electron-hole states in the mo lecular beam epitaxy grown GaAs/In 0.5 Ga 0.5 As quantum well, placed into space charge region, have been studied by photoreflectance spectroscopy. The energies of electrons and holes have been calculated in the envelope function model including deformation-induced changes in the band structure of quantum well. It is shown that the best accordance between experimental and theoretical data is achieved in the case of band offset Q = ' Ec /' Ev = 0.62/0.38 at GaAs/In 0.5 Ga 0.5 As heterojunction. The most intense transition is observed in this case between 1 electron and 1 light hole energy. This fact is connected with the indium segregation in the GaAs/In 0.5 Ga 0.5 As quantum well. In this case one could obtain the flat bands in the quantum well and realize th e parity selection rules for the rectangular potential. The model of the nonsymmetrical rectangular potential is applied to describe the energies of electronic levels in the quantum well. The segregation parameters have been calculated from the segregation-induced shift of the energies of interb and transitions in the GaAs/In

Electroreflectance (ER) spectra of InGaN/AlGaN/GaN p-n- heterostructures with multiple quantum wells (MQW) are studied. Structures with MQW InGaN/GaN were grown for blue LEDs by MOCVD technology and “flip-chip” mounted. The ER spectral maxima correspond to the high energy side of electroluminescence spectral line. The ER spectra caused by Franz-Keldysh effect are approximated by Aspnes theory. The ER spectra in a range 400 ÷ 800 nm have interference bands caused by the change of refraction index in the structure.

A study is reported of the crystallization of amorphous hydrogenated silicon films under the influence continuous radiation from an argon laser. The structure was investigated by Raman scattering of light. The radiation power density during the annealing process was 1.5–4.5 kW/cm2 and the exposure was 1/125 sec. The Raman spectra were recorded for power densities below 0.1 kW/cm2. The power density threshold for the appearance of crystallites was found to be 3.0 kW/cm2. The phonon localization model was used to show that the size of the crystallites produced for power densities of 3 kW/cm2 was 40 A.

The room temperature photoreflectance (PR) investigation of optical transitions in Al 0.2 Ga 0.8 As/GaAs/Al 0.2 Ga 0.8 As single and coupled quantum wells is presented. The structures were grown by molecular beam epitaxy for different barrier thickness and quantum well width. Three kinds of spectral features were observed in PR spectra: sharp line connected with GaAs band gap (1.42 eV), Frantz-Keldysh oscillations near Al 0.2 Ga 0.8 As band gap (1.71 eV) and features originated from electron-hole transitions in quantum well. The energies of observed transitions have been compared with the results of envelope function calculations.

Raman spectra (RS) of GaAs implanted with 140 keV Si+ ions, both before and after annealing, were obtained at room temperature, in x(yz)x backscattering geometry, using the 514.5 nm line of Ar laser. The implantation-induced amorphous bands and LO-line shape changes were observed with increasing fluences from 1013 to 5 1014 cm-2. We explained the modifications of the spectra via disorder-induced selection rule breakdown and estimated nanocrystallite size for different fluences according to the mode of reduction of the spatial correlation length. For annealing studies implanted samples were caped with Si3N4 layer and annealed at 900 C for 20 min in hydrogen. After annealing, the amorphous bands have disappeared, and the new spectral features were observed. We explained the evolution of the spectra by the dopant activation, resulting in RS by phonon-plasmon coupled modes and RS by LO phonon, originating from the surface-depletion layer. We interpreted shifts of the coupled modes and LO intensity change as evidence of the carrier concentration variation with fluences. Experimental results were described by the Lindhard-Mermin dielectric function including the non-parabolicity of the conduction band, evaluated for the case, when Drude approximation is not available. From Raman line-shape analysis we have obtained that the value of carrier concentration were 1018 cm-3 for ion fluences 1013-5 1014 cm-2.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Raman spectra (RS) of (100) GaAs implanted with 140 keV Si + ions, both before and after annealing, were obtained at room temperature, in x(yz)x backscattering geometry, using the 514.5 nm line of Ar laser. The implantation-induced amorphous bands and LO-line shape changes were observed with increasing fluences from 10 13 to 5 X 10 14 cm -2 . We explained the modifications of the spectra via disorder-induced selection rule breakdown and estimated nanocrystallite size for different fluences according to the model of reduction of the spatial correlation length. For annealing studies implanted samples were capped with Si 3 N 4 layer and annealed at 900 C for 20 min in hydrogen. After annealing, the amorphous bands have disappeared, and the new spectral features were observed. We examples the evolution of the spectra by the dopant activation, resulting in RS by phonon- plasmon coupled modes and RS by LO phonon, originating from the surface-depletion layer. We interpreted shifts of the coupled modes and LO intensity change as evidence of the carrier concentration variation with fluences. Experimental result were described by the Lindhard-Mermin dielectric function including the non-parabolicity of the conduction band, evaluated for the case, when Drude approximation is not available. From Raman line-shape analysis we have obtained that the value of carrier concentration were 10 18 cm -3 for ion fluences 10 13 -5 X 10 14 cm -2 .

The linear electro-optic effect in InGaN/AlGaN/GaN pn-heterostructures for light emitting diodes, grown by metal–organic chemical vapor deposition on sapphire substrates and flip-chip mounted, was studied by electroreflectance spectroscopy. Interference fringes, whose parameters depend on the DC voltage applied on the pn-junction, were observed in electroreflectance spectra. Data analysis, based on a calculation of the built-in electric field in the depletion layer and linear electro-optic effect, yielded the linear electro-optic coefficient r 13 of 22±6 pm/V for hexagonal In0.12Ga0.88N.

Raman scattering and time-resolved reflectivity measurements were used to monitor the kinetics of the crystallinity recovering process in P+-amorphized silicon, irradiated by pulse ruby-laser with the variable power density W. Below the melting threshold (W=0.4 J/cm2) only the amorphous phase structural relaxation up to the maximal ordered state with the bond angle disorder Δθ=8.7° has been observed. Above the melting threshold W-gradual increasing leads to the following structural transformations: the fine grains (of 50–100 A), included in an amorphous matrix (at W~0.4–0.6 J/cm2); the coarse-grained polycrystal (at W~0.6–1.5 J/cm2) and the monocrystal (at W~1.5–2.2 J/cm2).

Photoreflection spectroscopy is used to study the activation of impurity in InP crystals implanted with 100-keV Be+ ions at a dose of 1013 cm−2 and then subjected to thermal annealing for 10 s. After annealing at temperatures no higher than 400°C, lines characteristic of crystalline InP are not observed in the photoreflection spectrum, which indicates that the crystal lattice has become disordered as a result of the ion implantation. If the annealing temperature is in the range from 400 to 700°C, the lines related to the fundamental transition in InP (1.34 eV) and the transition between the conduction band and the subband, which has split off from the valence band owing to a spin-orbit interaction (1.44 eV), are observed in the spectrum, which indicates that the InP crystal structure is restored. The dopant is activated in samples annealed at 800°C, as indicated by the Franz-Keldysh oscillations observed in the corresponding photoreflection spectra. Free-carrier concentration is determined from the oscillation period and is found to be equal to 2.2 × 1016 cm−3.

An experimental setup for studying semiconductor structures by photoreflectance spectroscopy is designed. The double-monochromator-based optical scheme of the setup makes it possible to depress uncontrolled heating of the sample and diminishes a bending of the energy bands due to charge carrier photogeneration. Accordingly, the photoreflectance spectra are detected with a minimal influence of the modulating and probe radiations on the sample. With this setup, the room-temperature photoreflectance spectra from GaAs/GaAsP superlattices are taken and the interband transition energies, as well as a potential step in the conduction band of these superlattices, are measured.

The influence of built in piezoelectric field in the light-emitting diodes based on InGaN/AlGaN/GaN heterostructures on the electroreflectance spectra have been studied. The structures were grown by MOCVD technology and «flip-chip» mounted. Light was emitted or reflected through sapphire substrate. The built in electric field in the structure was modulated by pulses of reverse bias from -6 to +1 V applied to the contacts of diode. Observed blue shift of spectral line from InGaN/GaN multiply quantum wells region with the increasing reverse bias voltage has been explained as the result of lowering of the electric field in the quantum well.