Pressure dependence of photoconductivity for hydrogenated amorphous silicon

Abstract

Measurements of steady-state photoconductivity for hydrogenated amorphous silicon (a-Si:H) have been carried out at pressures up to 14 GPa and at room temperature. The ratio of photoconductivity to darkconductivity [sgrave]p/[sgrave]D decreases with increasing pressure. The activation energy for photoconductivity Ep, which is 0.25 eV at ambient pressure, decreases with increasing pressure at the rate -7 meV/GPa. These resultls are discussed with change in density of states.     

Photogeneration,optical absorption and transport in hydrogenated sputtered amorphous silicon

By combining direct optical transmission with steady state photocurrent and photoconductivity gain measurements we evaluated the optical absorption constant of sputtered hydrogenated a-Si between 10⁵ to 10^?1cm^?1. The photogeneration process involves excitation from the valence band or from defect states in the middle of the gap to the conduction band, with the electrons making the major contribution to the photocurrent. The electron drift mobility, determined from the photoconductivity studies, is considerably smaller than that determined from the time of flight technique, due to trapping at deep centers.     

Method of low-temperature optical measurements with diamond anvil cells

Abstract

Constructions of a cryogenic diamond anvil system with mechanical clamping press and helium pressure medium for microscopic optical studies are described. Low temperature nonmagnetic cells ø40 and ø20 mm have been developed. GaP samples doped with S, Te and isoelectronic impurities N, NN have been investigated up to 20 GPa at 1.5-300K.     

Tunable absorption and electroluminescence in GaAs doping superlattices

Highly tunable electroluminescence is observed in GaAs doping superlattice (n-i-p-i crystal) at room temperature with peak energies shifted more than 600 meV below the bulk bandgap (λ > 1.55 μm). Peak efficiency is about 2 %. Tunability of the optical absorption spectrum with p-n junction bias is also demonstrated by both photoconductivity and direct transmission measurements. A change of transmission of about 9% is obtained at 0.89 μm wavelength through a 1.95 μm thick n-i-p-i crystal by varying the p-n junction bias between −0.5 V and 0.5 V.     

Observation of metastable effects in pseudomorphic heterostructures InCaAs/InAlAs

Abstract

The results of electrical transport measurements (Hall effect and resistivity) performed on In_.65Ga_.35As-In Al_.48 As modulation doped heterostructure (with Si-doped InAlAs donor layer) and on In _.52Al _.48As thick layer are presented as a function of temperature (77–300K range) and hydrostatic pressure (up to 1300 MPa). At low temperature, the persistent photoconductivity (PPC) and metastable donor states occupation effects were observed. Due to the pressure induced decrease of the two-dimensional electron gas concentration the pronounced increase in the amplitude of observed effects was obtained. The above results are discussed in terms of DX-like centers and/or interface/surface states in the QW structure.     

Photoconductivity of lead telluride-based doped alloys in the submillimeter wavelength range

Persistent photoconductivity in a Pb_0.75Sn_0.25Te(In) alloy initiated by monochromatic submillimeter-range radiation at wavelengths of 176 and 241 µm was observed at helium temperatures. This photoconductivity is shown to be associated with optical excitation of metastable impurity states.     

Magnetic order effect on optical absorption and photoconductivity in EuO thin films

The comparison of optical absorption with photoconductivity with left and right circularly polarized light versus temperature, shows similar behaviours. The transition responsible for photoconductivity has been assigned to a 4f-5d transition. Arguments are proposed to explain the relation between absorption which occurs between the 4f correlated levels and the 5d levels more delocalized and photoconductivity which has the same optical cause but whose electron is delocalized in a conduction band.     

Application of optical and luminescent techniques to the characterization of oxide thin films

The interaction between light and electrons in oxide compounds forms the basis for many interesting and practical effects, which are related to microstructure, energy band, traps, carrier transport and others. Thin films of oxides like WO₃, Ga₂O₃, Y₂O₃ and SrTiO₃ were investigated using various improved optical and luminescent techniques. The home-made systems for optical and luminescent measurements were described in detail. The facilities of photo-Hall and photoconductivity transients have been proven to be powerful tools in the studies, which allow us to perform photoinduced process and relaxation measurements over a wide time range from 10⁻⁸ to 10⁴ s. Furthermore, we extended the measurement capabilities of the commercial luminoscope by using an interferometer system with optical fiber and illuminance meter instead of an optical microscope. The cathodoluminescent measurements can be performed at a relative high pressure (20-60 mTorr) compared to ultra-high-vacuum condition of most commercial products. Luminescent characterization was employed as a probe to study doping ions, oxygen vacancies, trap and/or exciton levels in oxide thin films. Our results suggest that various traps and/or excitons in thin films of WO₃, Ga₂O₃ and SrTiO₃ involve in the process of photoconductivity relaxation and emission.     

Selective photoconductivity induced in PbTe(Ga) by a local phonon mode

Selective photoconductivity at frequency ω=155 cm⁻ was discovered in PbTe(Ga) narrow-gap semiconductors at liquid-nitrogen temperatures. The corresponding energy is much lower than all characteristic energies of the electronic spectrum of the semiconductor. The effect is attributed to optical excitation of a local vibrational mode of an impurity center leading to delocalization of the electrons. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 342–346 (10 March 1996)     

Evaluation of local photoconductivity of solar cells by microwave near-field microscopy technique

Local photoconductivity in surface layer of solar elements has been studied by the method of microwave near-field microscopy (MW NFM). Dependences of photoconductivity of a solar cell on the intensity and wavelength of the incident optical radiation are determined by measuring the coefficient of reflection of MW NFM resonator at the frequency 4.1 GHz.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

Effects of pressure on the structure and the thermal decomposition kinetics of RDX

Abstract

High pressure and temperature structural changes for RDX were investigated to 7.0 GPa and 570 K in a diamond anvil cell apparatus using FTIR absorption, optical microscopy, and energy-dispersive powder x-ray diffraction techniques. Three distinct solid phases were observed. The effects of pressure on the thermal decomposition kinetics as a function of RDX pressure were investigated using an infrared absorption technique. Solid phase I was found to have a pressure enhanced reaction rate with an energy and volume of activation of 51 Kcal/mole and -5.6 cc/mole respectively. Solid II was not observed to react and the observed reaction rate of Solid III decreased with increasing pressure.     

Hydrogen Degree of Dissociation in a Low Pressure Tandem Plasma Source

ABSTRACT

Hydrogen dissociation degree in an inductively-driven tandem plasma source, operating at low pressure, is measured by applying optical actinometry method with an argon gas as actinometer. The gas temperature, a parameter in the investigations, is obtained from the rotational temperature, analyzing the intensity distribution of the Fulcher-α band. Several actinometric pairs are used for examining the dissociation degree and its pressure dependence. The influence?on the results?of the difference in the excitation cross sections for argon, commonly accepted, is analyzed. Two suitable actinometric pairs are proposed, one of which can also be applied for fast monitoring of the dissociation degree.     

Giant burst of photoconductivity in semiconductors upon an increase in the concentration of recombination centers

It is shown in the approximation of a single recombination level that the intrinsic photoconductivity of semiconductors with extrinsic recombination of carriers can increase by several orders of magnitude upon an increase in the concentration of recombination centers in the presence of weak optical radiation. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 655–660 (10 May 1998)     

Features of laser-induced luminescence and photoconductivity of layered Cu<Subscript>3</Subscript>In<Subscript>5</Subscript>S<Subscript>9</Subscript> crystals

Luminescence and photoconductivity of layered Cu₃In₅S₉ crystals at high levels of optical excitation are studied experimentally. A pulsed nanosecond Nd:YAG laser with built-in second and third harmonic generators to generate 1064-, 532-, and 355-nm radiation is used as a light source. It is found that the photoluminescence spectra exhibit two emission bands due to zone–acceptor level and impurity donor–impurity acceptor transitions. It is shown that the photoconductivity in Cu₃In₅S₉ is monopolar. The waveform of the photoconductivity consists of fast and slow components associated with two channels of recombination.     

Characterization of oxide thin films using optical techniques

Thin films of oxide materials are playing a growing role as critical elements in optoelectronic devices and nanoscale devices. In this work, thin films of some typical oxides such as WO₃, Ga₂O₃ and SrTiO₃ were investigated. We present measurements of those films, using various optical techniques like photoconductivity transients over a wide time range and photo-Hall measurements. Analysis of the photo-Hall and photoconductivity data permits the determination of the contribution to the photoconductivity made by the carrier mobility and concentration. A model for dispersive carrier transport was proposed to explain the relaxation of the photoconductivity in oxide thin films. In addition, photoluminescence characterization was used to study microstructures and energy band in oxide thin films. The broad emission from oxide host, consisting of several band peaks, was likely due to a recombination process with several possible paths. The dependence of the luminescent intensity on the annealing atmosphere was associated with the presence of oxygen vacancies. It is suggested that our optical analysis efforts have improved the understanding of oxide thin films, and this should lead to the necessary advancements in a variety of devices.     

Observation of a shift of the mobility threshold in the D − band of silicon in an electric field according to the photoconductivity spectra

We have discovered that the extrinsic photoconductivity spectrum of doped, uncompensated crystalline Si at liquid-helium temperatures is qualitatively different in electric fields E above a critical value E _c . Specifically, the red edge of the photoconductivity, associated with photoionization of a neutral impurity, is shifted strongly to lower frequencies. This result is explained by the appearance of a mobility threshold in the D ⁻-band (upper Hubbard band) and the shift of this threshold as E increases. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 89–94 (25 January 1996)     

Poly(organylsilylene)s — perspective materials for optoelectronics

Poly(organylsilylene)s with their uninterrupted chains of silicon atoms are a new class of materials with significant delocalization of electrons along the polymer chain. Their electronic structure, optical properties, photoconductivity, electroluminescence, and photorefractivity are discussed on the model compound poly[methyl(phenyl)silylene]. Their unusual electrical and optical properties, such as high quantum generation efficiency, high charge-carrier mobility, efficient luminescence, and optical non-linearity, can be utilized in some optoelectronic devices. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was suported by the Grant Agency of the Academy of Sciences of the Czech Republic (grant No. A1050901) and by the Grant Agency of the Czech Republic (grant No. 106/98/0700).