Type Of Optical Transition Research Articles

CdSe/ZnSe quantum dot structures grown by molecular beam epitaxy with a CdTe submonolayer stressor

A procedure for formation of CdSe quantum dots (QDs) in a ZnSe matrix is suggested. The procedure is based on the introduction of a CdTe submonolayer stressor deposited on the matrix surface just before deposition of the material of the QDs. (For CdTe/ZnSe structure, the relative lattice mismatch is Δa/a ≈ 14%.) The stressor forms small strained islands at the ZnSe surface, thus producing local fields of high elastic stresses controlling the process of the self-assembling of the QDs. According to the data of transmission electron microscopy, this procedure allows a considerable increase in the surface density of QDs, with a certain decrease in their lateral dimensions (down to 4.5 ± 1.5 nm). In the photoluminescence spectra, a noticeable (∼150 meV) shift of the peak to longer wavelengths from the position of the reference CdSe/ZnSe QD structure is observed. The shift is due to some transformation of the morphology of the QDs and an increase in the Cd content in the QDs. Comprehensive studies of the nanostructures by recording and analyzing the excitation spectra of photoluminescence, the time-resolved photoluminescence spectra, and the cathodoluminescence spectra show that the emission spectra involve two types of optical transitions, namely, the type-I transitions in the CdSeTe/ZnSe QDs and the type-II transitions caused mainly by the low cadmium content (Zn,Cd)(Se,Te)/ZnSe layer formed between the QDs.

OPTICAL AND PHOTOCONDUCTION IN PbI2 POLYCRYSTALLINE DEPOSITED FROM SOLUTION

A deposited layer of PbI 2 has been prepared by the solution technique. The yellow layers are made of hexagonal platelets parallel to the substrate plane. The photoconductive property has been investigated in the visible and near-IR bands using current measurements at different temperatures. The measurements show that an increase in the sample temperature leads to a remarkable decrease in photocurrent. The bandgap energy and type of optical transition were determined from transmission spectra, and an optical bandgap of Eg=2.32 eV for direct transition was estimated. Using the dependence of photoconductivity from temperature, a bandgap of Eg=2.32 eV was determined as well.

Influence of high-energy X-ray irradiation on the optical properties of tetraphenylporphyrin thin films

Tetraphenylporphyrin (TPP) thermally evaporated films were irradiated by different doses (0.5–2.5 kGy) of X-ray with energy 6 MeV. The optical properties for TPP were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range from 200 to 1100 nm. The absorption spectra recorded in the UV-VIS region of spectra showed different absorption bands, namely four Q-bands in the visible region of the spectrum and a more intense band termed as the Soret band in the near-UV region of the spectrum. Two other bands labeled N and M appear in the UV region. The Soret band showed Davydov splitting. Increasing X-ray irradiation dose influences the optical properties of TPP films. All absorption bands show a continuous blue shift in position and a decrease in intensity with increasing X-ray dose. At 2.5 kGy the B, N, and M bands disappeared. The reduction in the absorbency was calculated as a function of X-ray dose. The energy gap was determined and the type of optical transition was found to be an indirect allowed transition.

Effect of bath temperature on the electrodeposition of copper tin selenide films from aqueous solution

The effect of bath temperature on Cu 2SnSe 4 films prepared by electrodeposition was studied in this work. The structure, morphology and composition of the films were analysed by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis of X-rays (EDAX). The band gap energy and type of optical transition were determined from optical absorbance data. The results showed that electrodeposition of Cu 2SnSe 4 films was most suitable to be obtained at room temperature. Increasing the bath temperature did not improve the crystallinity of Cu 2SnSe 4 compound, but to formation of binary phase, CuSe. EDAX studies showed that increasing the temperature resulted in drastic increase in Cu content and decrease in Sn content. The optical absorption studies showed the film deposited at room temperature has direct transition with band gap energy E g of 1.2 eV.

Doping and irradiation effects on the optical band gap of potassium tetrachlorozincate single crystals

Optical transmission and reflection measurements for crystals of potassium tetrachlorozincate (KZC) doped with Mn2+ or Ni2+ in different concentrations and irradiated with different γ-doses have been carried out along the three crystallographic axes. The results were analyzed to determine the absorption coefficient (α), type of optical transition and the optical energy gap (E g ). Different absorption bands along the three axes have been observed. Doping with Mn2+ or Ni2+ affected the intensity and position of these bands. Irradiation by γ either created or inhibited some of these bands. The nature of the transition involved was found to be the indirect allowed one. Doping with Mn2+ or Ni2+ up to 0.41 wt% and irradiating with γ-doses up to 150 kGy did not change the type of transition. The optical energy gap decreased considerably after doping and irradiating KZC.

Optical and photoconductive properties of SnS thin films prepared by electron beam evaporation

Abstract Thin films of SnS have been prepared by electron beam evaporation. The films represent Herzbergite orthorhombic structure, established by their XRD patterns. The band gap energy and type of optical transitions were determined from transmission spectra and an optical band gap of E g (tr) =1.23 eV for indirect transitions and E g (tr) =1.38 eV for direct transitions were estimated. Using the dependence of photoconductivity from wavelength, a band gap of E g (ph) =1.2 eV was determined as well. A thermal band gap of E g (T) =1.29 eV was evaluated from the temperature dependence of the dark resistivity, and admixture level with activation energies (0.25 and 0.36 eV) were found. Roughness of the surface of SnS thin films was evaluated using atomic force microscopy.

Influence of strontium doping on the indirect band gap and optical constants of ammonium zinc chloride crystals

Optical transmittance measurements near the absorption edge in the energy range 3.4–6.4 eV have been carried out on (NH 4) 2ZnCl 4: xSr 2+ single crystals with x=0.000%, 0.020%, 0.039%, 0.087% or 0.144%. The introduction of Sr 2+ in low concentration, x=0.02% or 0.039%, little bleach the intense charge transfer band observed for the undoped sample with a maximum at 5.30 eV. Doping with Sr 2+ in a little bit higher concentration, x=0.087% or 0.144%, led to the disappearance of this band. While doping with Sr 2+ has no effect on the indirect allowed type of optical transition in AZC, the optical energy gap decreased with increasing Sr 2+ concentration. Measuring reflectance of AZC permitted the calculation of the refractive index n, the extinction coefficient K and both the real ε r and imaginary ε i components of the dielectric permittivity as functions of photon energy. The validity of Cauchy–Sellimaier equation was checked in the wavelength range 5.45–5.8 eV and its parameters were calculated. Applying the single-effective-oscillator model, the moments of ε( E) could be estimated. Electric susceptibility, as a measure of intraband transition, was calculated and its dependence on photon energy was considered.

Optical transitions in hydrogenated amorphous silicon

We study how the various kinds of optical transitions which occur within hydrogenated amorphous silicon contribute to the spectral dependence of the complex component of the dielectric function. While the optical transitions from the valence band band states to the conduction band band states are found to play a dominant role in determining the magnitude of this function for photon energies in excess of the optical gap, other types of optical transitions are also found to be important. We suggest that these other types of optical transitions should not be neglected when interpreting the optical properties of hydrogenated amorphous silicon.

Cathodic electrodeposition of Cu2S thin film for solar energy conversion

Cathodic electrodeposition in the presence of EDTA in aqueous solution was used to prepare Cu 2 S thin film deposited on Ti substrate. The effect of deposition potential, concentration and deposition time was studied to determine the optimum condition for electro-deposition process. Cyclic voltammetry was performed to elucidate the electrodic processes that occur while potentials for electrodeposition were applied to determine the optimum potential for electrodeposition. The thin films are characterised by X-ray diffractometry. The photoactivity of the deposited films and their conduction types were evaluated using photoelectrochemical technique. The band gap energy and type of optical transitions were determined from optical absorbance data.

The microstructure and physical properties of thin SnO2 films

Thin polycrystalline SnO2 films were deposited on glass substrates by magnetron sputtering. Electrical, optical, and gas-sensing properties, as well as the structure and phase composition of the films, were studied. The electrical resistance of the films and the concentration and mobility of free charge carriers were determined by the four-point-probe and van der Pauw methods. The band gap and the type of optical transitions in the films were derived from optical absorption spectra. The sensitivity to toxic and explosive gases was measured. The composition, morphology, and crystal structure of the films annealed at 600°C were examined by X-ray diffraction and electron microscopy. The films were found to contain only a tetragonal SnO2 phase and have good crystallinity. The average grain size in the annealed films is 11–19 nm. A model of the electrical conduction in the polycrystalline SnO2 films is discussed.

Atom-scale optical determination of Si-oxide layer thickness during layer-by-layer oxidation: Theoretical study

Optical anisotropy spectra of SiO2/Si(001) interfaces were theoretically investigated based on the sp3s* tight-binding calculation. In the spectra, we found three types of optical transitions originating from the E1 and E2 transitions of bulk Si, the interface Si–Si bonds, and the dangling-bond states at the interface. It was shown that the sign of these transitions oscillates during the layer-by-layer oxidation, which indicates that by counting the oscillation one can determine the layer thickness of oxidized Si layers in an atomic scale.

Dipole matrix elements of semiconductor intersubband quantum structures

A method for determining the dipole matrix element for an intersubband optical transition in multi-layered semiconductor quantum heterostructures is presented. The single-band effective-mass Schrödinger equation is solved by employing the argument principle method (APM) to extract the bound (B) and quasibound (QB) eigenenergies of the quantum heterostructure. The major types of optical transitions involving bound and QB states are defined and the corresponding dipole matrix elements are calculated for each type. The method presented incorporates the energy-dependent effective mass of electrons arising from conduction-band nonparabolicity. The performance and the accuracy of the method are evaluated for an asymmetric Fabry–Perot electron wave interference filter. The physical dimensions of the filter are varied to show their effect on the dipole matrix elements. Results with and without nonparabolic effects are presented and compared. Dipole matrix elements are also calculated for the filter with an applied electric field bias. In this case the eigenstate wavefunctions can be expanded as linear combinations of Airy and complementary Airy functions. In addition, results from the present method are compared to a Kronig–Penney and a multi-band model. The dipole matrix element values calculated by the present method are shown to be in excellent agreement with the values obtained from these models. Further, the present model is numerically efficient and easily implemented.

Polarization effects in the interaction between light and multilevel quantum systems

We develop a theory that describes the interaction of radiation and multilevel particles on the basis of the classical description of the orientation of angular momentum. For examples of application of this theory we take the problems of stimulated Raman scattering and of parametric generation of light in four-wave mixing. We find that the efficiency of the radiation processes largely depends on the polarization of the pump waves and on the types of optical transitions (ΔJ) and to a much lesser extent on the value of the angular momentum J. We also explain some polarization phenomena observed in experiments on four-wave mixing.

Quantum-sized effects and the nature of the primary photoprocesses in copper(I) and copper(II) sulfide nanoparticles

The spectral and semiconductor properties of copper(I) and copper(II) sulfides in aqueous solution were studied. The colloids studied display quantum-sized effects. Two types of optical transitions, namely, indirect band-band and intraband transitions, are responsible for light absorption by the nanoparticles in the visible range.

New possibilities for producing photon echoes from a homogeneous ensemble of atoms and with the use of a single light pulse

In investigations of predicted new types of photon echoes — an echo from a homogeneous ensemble of atoms and an echo of a single light pulse — have established that the magnitude and form of these new types of photon echoes are completely determined by the type of optical transition and by the “area” and polarization of the exciting light pulses (and the form can be controlled by varying the magnetic field). It is also established that the echo amplitude decreases for both small (compared to 1) and large light-pulse areas, and the optimal areas for which the maximum echo is obtained have been found. Investigations show that such photon echoes can also appear under conditions when an ordinary photon echo is absent (in atomic or molecular gases at high pressure, in the far-IR region of the spectrum, from cooled trapped atoms or ions, and so on).

Far-infrared absorption spectra of three electrons in a square-well quantum dot

We determined the far-infrared absorption spectra of a square-well quantum dot in which three electrons are confined by a heterostructure composed of GaAs AlGaAs . There are many types of optical transitions, some induced by electron-electron interaction, although there are no many-body effects on far-infrared absorption spectra in a quantum dot with parabolic confinement. As the size of the square-well quantum dot increases, electron correlation becomes strong, resulting in considerable changes in the far-infrared absorption spectra. Thus, far-infrared absorption spectroscopy enables us to investigate correlated electrons confined in zero-dimensional nanostructures.

The Significance of Charged Defects in Understanding the Light-Induced Degradation of Hydrogenated Amorphous Silicon-Germanium Alloys

ABSTRACTWe have characterized the defect state structure in a series of device quality glow discharge intrinsic, n-type doped, and p-type doped a-Si,Ge:H alloys with Ge content ranging from 20 at.% to 35 at.%. Our experimental methods include capacitance profiling, transient junction photocurrent and photocapacitance measurements. These methods have allowed us to identify one type of thermally induced defect transition plus two types of optical transitions from deep defects. Our results indicate that these transitions must involve at least two distinct defect sub-bands. Comparison of the magnitudes of these sub-bandsfor the intrinsic, n-type, and p-type alloys has allowed us to confirm that one of the optical transition belongs to D+ defect sub-band. All the optically and thermally induced bands of defect transitions are present with similar magnitudes for the most intrinsic a-Si,Ge:H alloys, which implies that charged defects play a significant role in glow discharge a-Si,Ge:H alloys. We then examined the changes in these defect densities, along with the changes in the hole mobility-lifetime products, that result from prolonged light exposure. By comparing the annealed state and light soaked state of each sample, we have been able to correlate the relative changes of the identified defect sub-band with the measured hole mobility-lifetime products. These data indicate that charged defects probably play a dominant role in determining the degradation of these a-Si,Ge:H alloys in device applications.

Cathodic electrodeposition of SnS thin films from aqueous solution

SnS thin films were prepared by cathodic electrodeposition on ITO/glass and Ti substrates from a solution containing SnCl 2 and thiosulphate ions. Cyclic voltammetry experiments were performed to elucidate the electrodic processes occurred when potentials were applied and to determine the optimum potential for electrodeposition. The photoactivity of the deposited films and their conduction types were evaluated using the photoelectrochemical technique. The bandgap energy and type of optical transitions were determined from optical absorbance data. Structural and compositional analysis were accomplished using X-ray diffractometry, electron dispersive analysis of X-ray, and X-ray photoelectron spectroscopy. The morphology of the films were examined using scanning electron microscopy.

Langmuir-Blodgett films of Cu(II)-tetrakis (3,3-dimethylbutoxycarbonyl) phthalocyanine: a spectrophotometric and TEM analysis of their structure and morphology

Langmuir-Blodgett (LB) films of copper(II)-tetrakis (3,3-dimethylbutoxycarbonyl) phthalocyanine [Cu(dmbc)Pc] have been prepared from toluene solutions; multilayers (up to 80 layers) were deposited onto hydrophobic quartz substrates. The refractive index n and extinction coefficient κ at normal incidence have been determined from both transmission and reflection measurements carried out in the 400–800 nm spectral range. The optical absorption spectrum as a function of the incident photon energy was also registered in order to determine the optical transition type in Cu(dmbc)Pc LB films by using a band model approach. Linearly polarised light absorbance measurements were performed at room temperature in the same spectral range. The average orientation of the phthalocyanine molecular rings in the LB film, with respect to the normal to the substrate and the dipping direction, has been evaluated. We have found that such orientation differs from that determined for LB films deposited from ethyl acetate solutions. Structural and morphological analysis were also carried out by transmission electron microscopy techniques. Small ordered domains in a low range order matrix were observed by high resolution images. The structure of the material in the substrate plane has been determined by electron diffraction performed on single ordered domains.

Optical properties of Cd 3As 2 polycrystalline thin films

The transmittance T and reflectance R have been measured at normal incidence of light for a set of vacuum deposited Cd 3As 2 thin films of different thickness in the wavelength range of 0.5–10 μm. The measured values T and R in conjunction with the film thickness were used to determine the optical constants (the refractive index n, the absorption index k and the absorption coefficient α) of the films. The lattice dielectric constant was found to be 17.6. Graphical representation of log (αhν) versus log ( 1 λ ) yielded three distinct linear regions. The first region had the photon energy range up to 0.3 eV. The second region had the photon energy range of 0.3–0.9 eV. The third region had the photon energy range of 0.95–2.1 eV. It was found that the only allowed type of optical transitions in the first region was indirect with an energy gap E d ing = 0.05 eV. In the second region, the only allowed type of optical transition was direct with energy gap E g d = 0.72 eV. In the third region, the only allowed type of optical transition was direct with energy gap E g d = 1.56 eV. The last two values were confirmed throughout the graphical representation of- lm( 1 ε ) = f(hv) . The existing allowed optical transitions in the above mentioned three regions were interpreted according to the LinChung model.