Optical transitions in small band offset superlattices are studied within the framework of the nearly free electron approximation, in which the weak superlattice potential is treated as a perturbation. Interband selection rules are derived for transitions involving conduction and valence band states at the superlattice Brillouin zone center and the zone edge. It is found that a number of new transitions can occur in such small-offset superlattices due to wave function mixing of different subband states. The effect of the effective mass on the optical transitions is also discussed. The theory is used to explain the results observed in magneto-optical absorption experiment in ZnSe/Zn_{1-x}Mn_{x}Se small-offset superlattices. Furthermore, the nearly free electron formulation is found to be in excellent agreement with rigorous multi-band numerical calculation on superlattices involving small band offsets.
The dispersion and field structure of in-plane propagating p-polarized waves in n-GaAs/AlAs superlattice is studied in the frequency range below 10 THz in the presence of the steady magnetic field which is perpendicular both to the direction of propagation and periodicity. The existence of non-localized photon-phonon-plasmon modes as well as the magnetic-field-controlled mode localization at selected set of interfaces is predicted.
The paper presents the first experimental results on the in-plane propagation of millimeter waves in GaAs ...n-i-n-i-... type periodic layered structures in the Voigt geometry. The data are found to agree with theoretical calculations based on the effective medium approximation, showing the feasibility of contactless non-destructive probing of periodic layered structures.
Polarization anisotropies of luminescence spectra caused by a potential modulation in the quantum well plane are discussed. Any lateral potential induces heavy hole light hole mixing. This leads to lateral polarization dependence of the interband transition rates if the confinement potential is not symmetrical with respect to rotations by 90 degrees around the quantum well growth axis. We present some numerical results for quantum wire arrays and discuss to what extent the present theory can be compared to experimental results.
This paper presents results of investigation of the temperature dependence of visible luminescence in porous silicon layers prepared by anodization in hydrofluoric acid. Luminescence spectra were measured in the temperature range between 40 K and 350 K. Room temperature reflectivity spectra were also measured in vacuum ultraviolet radiation range from 4 eV to 12 eV.
The numerical dispersion calculations in long-period GaAs/AlAs super-lattice were performed using a local theory with retardation as well as an effective medium approximation. The splitting of the dispersion branches near the transverse optical phonon frequencies and phonon-like to photon-like modes transition were found to depend on the superlattice spatial period. The characteristic frequencies of bulk modes and crossing point of dispersion branches estimated analytically with the use of an effective medium approximation were found to agree with the results of rigorous solution.
The recent topic of visible luminescent porous Si is reviewed with an emphasis on luminescent mechanism. Among various mechanisms proposed, size quantization plays some important role and is experimentally observable but it would not be a key entity which emits visible light in dispute. The role of oxygen is found to be very important and this is a key constituent for the luminescent substance. A possible mechanism of localized electron-hole pair recombination nature for porous Si involving oxygen is discussed.
A perovskite-type cobalt oxide, Pr_{0.5}Ca_{0.5}CoO_3 (PCCO), shows photoinduced phase transition. In this study, we successively irradiated two laser pulses with different intensities to PCCO and probed the transient change of the reflection at 2.0 eV. Assuming propagation of the two different photoinduced metallic states, we could reproduce the time profiles as well as the magnitude in the reflectance change, indicating the fabrication of the photoinduced multilayered thin film in Pr_{0.5}Ca_{0.5}CoO_3.
Indium oxide (In_2O_3) thin films were deposited on glass substrate by varying substrate temperature in the range of 400-600C using the spray pyrolysis technique. In this research, physical properties of indium oxide thin films were studied and then nanocrystalline sizes at different substrate temperature were deeply compared and investigated. All films were characterized at room temperature using X-ray diffraction, scanning electron microscopy, atomic force microscopy, photoluminescence, the Hall effect and UV-visible spectrophotometer. The optimal substrate temperature to obtain films of high crystallographic quality was 575°C, for this temperature, the electrical resistivity was in the order of ρ=0.147 Ω cm. For comparing optical transmittance and electrical conductivity the best figure of merit of the films was achieved at 575C.
The authors demonstrate selective detection of terahertz radiation employing berylliumδ-doped GaAs/AlAs multiple quantum wells. The sensitivity up to 1 V/W within 4.2-7.3 THz range at liquid helium temperatures is reached. The Franz-Keldysh oscillations observed in photo- and electroreflectance spectra allowed one to estimate built-in electric fields in the structures studied. It was found that the electric field strength in the cap layer region could vary from 10 kV/cm up to 26 kV/cm, depending on the structure design and temperature.
Optical absorption measurements were exploited to study periodic InN:In structures grown by plasma-assisted molecular beam epitaxy with the thickness of the metallic inclusions varied from 2 to 48 monolayers. We demonstrate that the observed higher-energy shift of an effective absorption edge may be due to In depletion of the InN matrix via the coalescence of In into large clusters, accompanied by the respective higher-energy shift of the Mie resonance. The relevant uncertainty in the optical gap of InN is discussed.
Raman scattering, reflectivity and photoluminescence measurements of the porous silicon layers prepared on (001) p/p^{+} silicon epitaxial wafers by anodization method are presented. We have studied dependence of the frequency shift and halfwidth of LO mode in Raman spectra and shift of the luminescence peak in photoluminescence spectra vs. anodization conditions.
The interfaces between nonmagnetic CdTe quantum wells and semimagnetic barriers of Cd_{1-x}Mn_{x}Te were investigated for several well widths by low temperature photoluminescence and photoluminescence excitation spectroscopy. Specially designed Cd_{1-x}Mn_{x}Te/CdTe/Cd_{1-y}Mg_{y}Te structures enable us to distinguish the quality of the semimagnetic normal and inverted interfaces. The normal interface shows to have a better structural quality than the inverted interface.
We explore the possibility of using electron paramagnetic resonance (EPR) of Mn^{++} for measuring uniaxial strain in II-VI superlattices. This work is motivated by the fact that the EPR spectrum of Mn^{++} is very strongly affected by crystalline fields. Changes in a crystalline field which arise from strain are thus automatically expected to have a profound effect on the EPR spectrum. Consistent with this expectation, we have observed giant crystal field splittings of Mn^{++} EPR lines in ZnTe/MnTe, CdTe/MnTe, and ZnTe/MnSe superlattices. The EPR spectra observed in these systems are ascribed to isolated Mn^{++} ions diffused into the ZnTe or the CdTe layers from the respective MnTe or MnSe layers. In addition to providing precise information oii the magnitude and the sign of strain produced by lattice mismatch between the superlattice constituents, we show that the EPR spectrum also provides a direct measure of strain fluctuations in the layered medium.
We have studied the influence of ion implantation and post-implantation annealing regimes on the structural and optical properties of silicon matrix with ion-beam synthesized InAs nanocrystals. (100) Si wafers were implanted at 25 and 500°C, subsequently with high fluences of As and In ions. After implantation the samples were processed by furnace and rapid thermal annealing at 900, 950 and 1050°C. A part of the samples implanted at 25°C was additionally exposed to H_2^{+} ions (100 keV, 1.2 × 10^{16} cm^{-2} in terms of atomic hydrogen). This procedure was performed to obtain an internal getter. In order to characterize the implanted samples transmission electron microscopy and low-temperature photoluminescence techniques were employed. It was demonstrated that by introducing getter, varying the ion implantation temperature, ion fluences and post-implantation annealing duration, and temperature it is possible to form InAs nanocrystals in the range of sizes of 2-80 nm and create various concentration and distribution of different types of secondary defects. The last ones cause in turn the appearance in photoluminescence spectra dislocation-related D1, D2 and D4 lines at 0.807, 0.870 and 0.997 eV, respectively.
An approach is proposed to estimate separately parameters of homogeneous and inhomogeneous broadenings from an optical reflection line of a quasi-2D exciton. A phenomenological model is proposed to take into account statistically an inhomogeneous broadening of the exciton resonant spectra. The concept is applied to study a modulation-doped heterostructures with a single quantum well CdTe/CdMgTe. From exciton reflection lines taken in a magnetic field the temperature-dependent homogeneous and inhomogeneous broadening parameters as well as the exciton radiative decay rate are measured.
Photoreflectance spectroscopy has been used to study optical transitions in In_{0.045}Ga_{0.955}As/GaAs double quantum well at 80 K. The derivative nature of this contactless electromodulation technique allows for the observation of excited state transitions in the low-dimensional structure including the symmetry-forbidden ones. Excitonic symmetry-forbidden transitions can be observed due to the effect of mixing of heavy and light hole excitons and/or due to some asymmetry in the structure. We have shown that the built-in electric field in the region of double quantum well is weak enough (less than 0.5 kV/cm) not to cause any significant energetic shift of features due to quantum confined Stark effect, on one hand. On the other hand, it is sufficient to change strongly the oscillator strength of forbidden transitions. To change the internal electric field, we have used photoreflectance in the three-beam mode with a third beam continuously illuminating the sample and causing changes of the built-in electric fields due to the photovoltage effect. This method works as a contactless forward bias and allows for a change of the field down to the flat band conditions. We have shown that changes of built-in electric field by amount of a few tenths of kV/cm can modify the intensity of forbidden transitions significantly. We show that, although the mixing of excitons is still important, a very weak built-in electric field can be dominant in the observation of forbidden excitonic transitions in double quantum well.
Ultrafast spatio-temporal effects in optically excited semiconductors are investigated by solving the coupled semiconductor Maxwell-Bloch equations which include the relevant relaxation phenomena. The analysis is used to describe transport of electronic excitations on nano- to micrometer scales with a dynamic range on the femto- to picosecond time scale.
For a single GaAs/AlAs/GaAs type II pseudodirect double quantum well, as well as for superlattices it was predicted that the oscillator strength of the lowest optical transition has a periodic dependence on the number of AlAs monolayers. The oscillator strength depends on the coupling between theΓ and X electron states. We use samples containing a single GaAs/AlAs/GaAs double quantum well with thickness gradient to show experimental evidence of this effect. The results concerning theΓ-X coupling are obtained from the study of the ratio of photoluminescence intensities of the zero-phonon line and the phonon replica and from their time decay. They show the monolayer dependence of the Γ-X mixing potential. We extend the model describing the Γ-X coupling for ideal interfaces in the frame of the envelope approximation to the case of non-abrupt interfaces and exciton localization. The amplitude of variation of the radiative recombination time due to the Γ-X mixing is well reproduced within this model.
We discuss a theoretical model for the Zeeman splitting in dimension reduced structures consisting of semimagnetic semiconducting materials. The interplay of the magnetic field in different orientations with the confinement and strain induced symmetry reduction in quantum well structures is discussed. The coupling of valence band states of magnetic and nonmagnetic wells in asymmetric double quantum well structures is studied.
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