Region Of Interband Transitions Research Articles

Above-bandgap magneto-optical Kerr effect in ferromagnetic Ga1−xMnxAs

We performed a systematic magneto-optical Kerr spectroscopy study of Ga${}_{1\ensuremath{-}x}$Mn${}_{x}$As with varying Mn densities as a function of temperature, magnetic field, and photon energy. Unlike previous studies, the magnetization easy axis was perpendicular to the sample surface, allowing us to take remanent polar Kerr spectra in the absence of an external magnetic field. The remanent Kerr angle strongly depended on the photon energy, exhibiting a large positive peak at $~1.7$ eV. This peak increased in intensity and blueshifted with Mn doping and further blueshifted with annealing. Using a 30-band $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model with antiferromagnetic $s,p$-$d$ exchange interaction, we calculated the dielectric tensor of Ga${}_{1\ensuremath{-}x}$Mn${}_{x}$As in the interband transition region, assuming that our samples are in the metallic regime and the impurity band has merged with the valence band. Our modeling successfully reproduces the observed spectra.

Properties of Si Nanowires Synthesized by Galvanic Cell Reaction

A galvanic cell reaction is used to synthesize Si nanowires (SiNWs) in AgNO3/HF solution at 30 °C for 60 min. The AgNO3 concentration ise varied from M=0.0001 to 0.6 mol/L in aqueous HF solution (5 mol/L). Vertically well-aligned SiNW arrays are found to be formed in the limited AgNO3 concentration range of M∼0.02–0.08 mol/L. The maximum nanowire length is ∼35 µm (M∼0.05 mol/L). The photoluminescence (PL) spectra of the SiNWs exhibit a broad peak centered at ∼1.8 eV. Below M∼0.02 or above ∼0.08 mol/L, the solutions produce only roughened surfaces. No PL emission is observed from such surfaces. Passive HF etching of the SiNWs leads to the removal of the surface oxide overlayer and changes the wettability from highly hydrophilic (∼5°) to superhydrophobic (∼135°). Optical absorption and Fourier-transform infrared spectroscopy show that the SiNWs have an extremely large optical absorbability not only in the interband-transition region but also in the far-infrared spectral region.

Effect of Sn addition on the optical gap and far-infrared reflectivity spectra of amorphous Sb–Se films

This paper reports the spectroscopic studies on the thermally evaporated Sn x Sb 20Se 80 − x (2 ≤ x ≤ 8) films. Optical gap and band tailing parameter decreases except for x = 8, where it increases. The red shift in peak reflectivity spectrum in the interband transition region (200–800 nm) has been observed. The position and shape of the reflectivity bands (80–378 cm − 1 ) indicate the change in the local structure of the films. The Kramer–Kronig transformations have been used to evaluate the dielectric constants from the far-infrared reflectivity spectrum. Splitting of the longitudinal optic and transverse optic mode characterizes the partial ionic character for Sn–Sb–Se system. The relationship between the optical properties and structure has been revealed.

Creation of storage centers in CsBr:Eu2+ needle image plates by vacuum ultraviolet radiation

In this study, the photostimulated luminescence of the CsBr:Eu2+ needle image plates (NIPS) after vacuum ultraviolet (VUV) irradiation and their photoluminescence under VUV and UV excitation were investigated. It was shown that the photostimulable storage centers arise almost exclusively due to irradiation of the CsBr:Eu NIPs in the spectral lines of creation of the 4p55s anion excitons, and not in the region of interband transitions of CsBr. The explanation of the results is based mainly on the radiative and nonradiative decay of the self-localized e+Br2− excitons.

Determination of optical constants of sol‐gel derived Zn0.9Mg0.1O films by spectroscopic ellipsometry with various models

AbstractThe Zn0.9Mg0.1O thin films were prepared on Si (100) substrates by the sol‐gel method. The structural and the optical properties of Zn0.9Mg0.1O thin films, submitted to an annealing treatment in the 400–700 °C ranges, are studied by X‐ray diffraction (XRD) and ultraviolet (UV)‐visible spectroscopic ellipsometry (SE). The thickness, refractive index, and extinction coefficient of these films have been determined by analyzing the SE spectra using parameterized dispersion model. Moreover, we made a detailed comparison among various dispersion models and found that the Sellmeier model was superior to others in fitting the ellipsometric spectra in the transparent region. In the interband transition region, point‐by‐point fit was used. The spectral dependence of the refractive index and extinction coefficient was obtained in the photon energy range of 1.5–4.71 eV. The influence of annealing temperature on the refractive index, the extinction coefficient, and the optical bandgap energy was also discussed. We found that the refractive index and the extinction coefficient increase with increasing the annealing temperature, meaning the optical quality of Zn0.9Mg0.1O films is improved by annealing. Copyright © 2009 John Wiley & Sons, Ltd.

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li6Gd(BO3)3 doped with Eu3+ and Ce3+ and Li6Eu(BO3)3 crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd3+ → Ce3+ and Gd3+ → Eu3+ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce3+-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.

Si- and SiGe-Based LEDs

Single crystal Si, Si0.948Ge0.052 and Si0.66Ge0.34 diodes as well as Ge transistor structures with high electroluminescence (EL) intensities in the region of interband transitions at room temperature were fabricated by different techniques and their luminescence properties were studied. By varying the Ge content in the solid solution, one can control the wavelength at the emission maximum in the range of 1.1 - 1.8 μm. The integrated EL intensity varies by a factor of less than two in the temperature ranges of 80 - 500 and 80 - 300 K for Si and SiGe LEDs, respectively. Si LEDs can effectively operate, at least, up to ~200°C. The data analysis shows that recombination involving excitons is the dominant mechanism of near-band-edge radiative recombination in all the light-emitting structures at room temperature. Some of the structures have record values of EL intensity and/or quantum efficiency, so they can be used as effective light emitters in Si optoelectronics. In particular, Si LEDs were designed with a small p-n junction area of 8x10-3 mm2 and a radiation power of 0.3 mW. The record total emission power of 46 mW was achieved in solar cell LEDs with an emitting surface area of 3 сm2. The internal quantum efficiencies of 0.5% and 0.3% were recorded in Si0.948Ge0.052 and Si0.66Ge0.34 LEDs at the wavelengths of 1.15 and 1.3 μm, respectively. Room temperature near-band-edge EL was first observed in Ge structures.

Silicon light-emitting diodes with strong near-band-edge luminescence

Electroluminescence (EL) in the region of interband transitions from silicon light-emitting diodes (LEDs) fabricated by cutting a solar cell with an area of 21 cm2 and external quantum efficiency ηext of EL up to 0.85% has been studied at room temperature. Despite the considerable decrease in ηext because of the cutting and Auger recombination, record-breaking values of the total power emitted by a diode (up to W = 8 mW) and emitted power per unit area (up to P0 = 65 mW/cm2) were achieved at pulse currents of up to 10 A and structure areas in the range S = 0.1–0.9 cm2. The EL decay kinetics was measured for LEDs with different areas. The emission pattern of a Si LED with a textured surface and the emission intensity distribution along different directions in the plane of the emitting area of the LED were measured.

Light-Emitting Structures with Near-Band Edge Luminescence for Si Optoelectronics

Single crystal Si, Si0.948Ge0.052 and Si0.66Ge0.34 diode as well as Ge transistor structures with high electroluminescence (EL) intensities in the region of interband transitions at room temperature were fabricated by different techniques and their luminescence properties were studies. The analysis of the experimental data shows that recombination involving excitons is the dominant mechanism of near-band edge radiative recombination in all the light-emitting structures at room temperature. Some of the structures are characterized by record values of EL intensity and/or external quantum efficiency, so they can be used as effective light emitters for Si optoelectronics.

Structural and optical properties of Bi3.25Nd0.75Ti3O12 ferroelectric thin films

Ferroelectric Bi3.25Nd0.75Ti3O12 (BNT) thin films were grown on (111)Pt/Ti/SiO2/Si substrates by a chemical solution method. The films were composed of large rod-like grains. XRD and Raman spectroscopy measurements showed they were polycrystalline perovskite structure with a good crystallinity. Pt/BNT/Pt capacitors had been fabricated and showed good ferroelectricity. The optical constants (n, k) of BNT thin films in the wavelength ranges of 0.2–1.7 μm and 2.5–11.4 μm were obtained by spectroscopic ellipsometry measurements. The dispersion of the refractive index in the interband transition region followed the single electronic oscillator model. The optical band gap was found to be about 3.61 eV.

Luminescence VUV spectroscopy of cerium-and europium-doped lithium borate crystals

The paper reports on a study of the luminescence of lithium borate crystals (Li6Gd(BO3)3 doped by Eu3+ and Ce3+ ions, Li5.7Mg0.15Gd(BO3)3: Eu, and Li6Eu(BO3)3) initiated by selective excitation by synchrotron radiation at excitation energies of 3.7–27 eV at 10 and 290 K. Efficient energy transfer between the rare-earth ions Gd3+ → Ce3+ and Gd3+ → Eu3+ was found to proceed by the resonance mechanism, as well as by electron-hole recombination. Fast decay kinetics of luminescence of the Ce3+ activator centers was studied under intracenter photoexcitation and excitation in the interband transition region. The mechanisms involved in luminescence excitation and radiative relaxation of electronic states of rare-earth ions are analyzed, and the energy transfer processes operating in these crystals are discussed.

Electroluminescence in the region of interband transitions in a high-efficiency silicon light-emitting diode with a small area of the rectifying contact

Electroluminescence in the region of interband transitions is studied in a Si diode at room temperature with the densities of current (J) as high as 36 kA/cm2 through the p-n junction with an area of 0.008 mm2. The kinetics of the electroluminescence decay is represented by exponential function with the time constant τd ≈ 35 μs in the linear portion of the dependence of the electroluminescence intensity on current. At J > 2.5 kA/cm2, the current dependence of the electroluminescence intensity becomes sublinear, while the decay kinetics is described by the exponential function with the same τd, however, only after the initial portion of a more rapid decay of electroluminescence. The specific features observed at J > 2.5 kA/cm2 can be related to an appreciable contribution of the Auger recombination (in addition to the Shockley-Read-Hall mechanism) to nonradiative recombination. It is shown that it is feasible to fabricate Si-based light-emitting diodes with the emission power as high as ∼50 mW at the emitting-surface area of about 6 mm2.

Optical properties of Quantum Disks: Real density matrix approach

Abstract We show how to compute the optical response of a Quantum Disk (QDisk) to an electromagnetic wave as a function of the incident wave polarization, in the energetic region of interband transitions. Both the TM and TE polarization in guided-wave geometry are analyzed. The method uses the microscopic calculation of Quantum Disk eigenfunctions and the macroscopic real density matrix approach to compute the effective QDisk susceptibility, taking into account the valence band structure of the QDisk material and the Coulomb interaction between the electron and the hole. Analytical expressions for the QDisk susceptibility are obtained for a certain model electron — hole potential. Using these expressions, all optical functions can be computed. Results for the absorption coefficient are computed for InAs/GaAs QDisks. Fair agreement with experiments is obtained.

Dose dependences of the optical properties of fullerene films subjected to the electron irradiation

The optical conductivity σ( E) in the energy range of interband transitions in the C 60 films subjected to the high-energy electron irradiation (E e = 1,8 MeV) was studied by the method of spectral ellipsometry for the different doses of absorption ( Q = 1,5; 4 and 8 MGy). A change in the crystal and vibrational structure of these samples was investigated by the X-ray diffraction method and Raman spectroscopy. It was shown that the noticeable transformations of electron structure are observed depending on the dose of absorption. An increase of electronic concentration in the entire region of interband transitions with the σ( E) nature retention, peculiar to the nonirradiated samples, occurs under the small doses ( Q = 1,5 and 4 MGy). The σ( E) changes under these doses are explained as the consequence of the transfer of charges from the displaced carbon atoms to the fullerene molecules as a result of their radiation damages. An increase in the dose of absorption to Q = 8 MGy indicates the beginning of the degradation of C 60 molecule structure.

Second harmonic generation from Ag(1 1 1) electrochemical interfaces at the interband transition region: Effects of the presence of self-assembled monolayers

We have investigated the wavelength and potential dependence of the second harmonic response from Ag(1 1 1) surfaces covered by self-assembled monolayers (SAMs) obtained from two different α-functionalised terthiophenes in comparison with bare surfaces in the spectral region where interband transitions can occur. We have analysed both isotropic and anisotropic components of the second harmonic signal at different directions of the polarisation. A strong dependence on the potential, the wavelength and the nature of the film has been observed. Our results are consistent with those of Bradley et al. for bare surfaces and give additional evidence that the enhancement of the second harmonic process observed is mainly caused by a resonance with some elements of the surface susceptibility tensor.

Mechanism of Radiative Recombination in the Region of Interband Transitions in Si–Ge Solid Solutions

The electroluminescence of Si-Ge diodes (with a Ge content of 5.2% in the corresponding solid solutions) in the region of interband transitions has been studied at the temperatures T = 82 K and 300 K. The emission spectra, the linear dependence of the electroluminescence intensity on current, and the exponential decay of the intensity suggest an exciton mechanism of radiative recombination with and without the involvement of phonons during radiative transitions.

Optical properties of Bi3.25La0.75Ti3O12 and Bi3.25Nd0.75Ti3O12 thin films prepared by a chemical solution method

The optical properties of Bi3.25La0.75Ti3O 12 (BLT) and Bi 3.25Nd0.75Ti3O12 (BNT) thin films de posited on (111)Pt/Ti/ SiO2/Si substrates by a chemical solution method were investigated. B oth BLT a nd BNT thin films showed single phase of bismuth-layered structure. The BNT fil m composed of the homogeneous and large rod-like grains, while the grains in BLT film were small. The optical constants (refractive index and extinction coeffic ient) in the wavelength range of 200—1700nm and the thickness of the films we re obtained by spectroscopic ellipsometry measurements. The optical band gaps o f BLT and BNT were estimated to be 430 and 361eV, respectively. The disper sion of the refractive index in the interband transition region was analyzed by using the single electronic oscillator model.

The mechanism of radiative recombination in the region of interband transitions in single crystal silicon

Bimolecular description of the mechanism of radiative recombination in the region of interband transitions fails to explain the linear dependence of the electroluminescence (EL) intensity on the current in single crystal silicon at 300 K for a high level of injection and the exponential decay of emission after switching off the current. The results of experiments can be adequately interpreted within the framework of an exciton mechanism of the radiative recombination in silicon.

Ferroelectric and optical properties of Bi3.25Nd0.75Ti3O12 thin films prepared by a chemical solution method

The ferroelectric and optical properties of Bi3.25Nd0.75Ti3O12 thin films deposited on (111)Pt/Ti/SiO2/Si substrates using a chemical solution method were investigated. Bi3.25Nd0.75Ti3O12 thin films were polycrystalline and had (117) random orientation with large rod-like grains. They showed good ferroelectricity with a remnant polarization of 13.8 µC cm−2 and a coercive field of 120 kV cm−1. The dielectric constant and the dissipation factor were about 200 and 0.040, respectively, at a frequency of 10 kHz. No polarization fatigue was observed for the Pt/Bi3.25Nd0.75Ti3O12/Pt capacitor after 109 switching cycles. The optical constants (refractive index and extinction coefficient) in the wavelength range of 200–1700 nm were obtained by spectroscopic ellipsometry measurements. The optical band gap was estimated to be about 3.61 eV. The dispersion of the refractive index in the interband transition region followed the single electronic oscillator model.

Spectral photoresistive effect of the field in CdS crystals at low temperatures

The influence of the surface electric field on the low-temperature (T=77 K) photoconductivity spectra of CdS crystals in the region of exciton and interband transitions is experimentally studied by the field-effect method. The photoconductivity spectra of a semiconductor are numerically calculated in the framework of a model allowing for the dependence of the surface recombination rate of nonequilibrium charge carriers on the surface electric field. It is demonstrated that the surface electric field plays a decisive role in the formation of the fine spectral structure associated with the excitons. A correlation between the type of fine structure and the surface bending of the energy bands is revealed. It is shown that the surface electric field can be evaluated from the shape of the low-temperature photoconductivity spectrum of the semiconductor.