Photoconductivity In Semiconductors Research Articles

Time-resolved terahertz time-domain near-field microscopy.

We demonstrate a novel method for measuring terahertz (THz) photoconductivity of semiconductors on length scales smaller than the diffraction limit at THz frequencies. This method is based on a near-field microscope that measures the transmission of a THz pulse through the semiconductor following photoexcitation by an ultrafast laser pulse. Combining back-excitation of the sample using a Dove prism, and a dual lock-in detection scheme, our microscope design offers a flexible platform for near-field time-resolved THz time-domain spectroscopy, using fluences available to typical laser oscillators. Experimental results on a thin film of gallium arsenide grown by metal organic chemical vapor deposition are presented as a proof-of-concept, demonstrating the ability to map the complex conductivity as well as sub-ps dynamics of photoexcited carriers with a resolution of λ/10 at 0.5 THz.

Features of Stationary Photoconductivity of High-Ohmic Semiconductors Under Local Illumination

Photoconductivity has been thoroughly studied for a long time. However, most researchers have examined photoconductivity of semiconductors while illuminating the entire surface of samples. The present paper examines the effect of local exposure that ensures a high level of injection of free charge carriers upon the conductivity of high-ohmic cadmium telluride and semi-insulating gallium arsenide samples and upon the properties of ohmic contacts to samples. The authors found that regardless of the exposure area the value of transition resistance of ohmic contacts decreases and the concentration of the main charge carriers increases in the sample in proportion to radiation intensity. This research uncovered a number of previously unknown effects that are interesting from the physical point of view. This paper focuses on discussing these effects.

Paradoxes of photoconductive target and optical control of secondary ion yield

This study of the photoconductivity of semiconductors, in particular, cadmium chalcogenides as materials for targets of vacuum image converters followed the path of overcoming paradoxes. The concepts developed by the classics of photoelectricity also help to understand the paradoxes of the new secondary-ion photoelectric effect, especially, its spectral characteristic. The optical channel of secondary ion yield control via a photoconductive target opens the way to a new branch of nanotechnology, i.e., optoionics.

Low frequency modulated photoconductivity in semiconductors having multiple species of traps

Modulated photoconductivity has proved to be an excellent tool to probe the density of states of semiconductors. However, though a great deal of attention has been devoted to the high frequency regime for its simplicity of application, the modulated photoconductivity in the low frequency (LF-MPC) regime has been the object of researches only recently. In the case of a dominant species of states present in the forbidden gap, it was shown that LF-MPC is a good complementary technique to the high frequency method to extract different transport parameters. In this paper, we present a complete theoretical analysis of the LF-MPC experiment for the case when different species of traps are present. We solve the complete system of equations that describe the experiment, and we apply simplifying assumptions to deduce a simple formula relating the photocurrent phase shift to the density of states (DOS) at the majority carriers’ quasi-Fermi level. By means of numerical calculations, we discuss the accuracy of our development, and of previous approximate formulas, to reconstruct a given DOS. Finally, we present experimental and theoretical evidence that, under sensitization conditions, the sign of the phase shift of the modulated photocurrent reverses and the steady-state photoconductivity depends superlinearly on the generation rate.

Suppression of numerically induced chaos with nonstandard finite difference schemes

It has been previously shown that despite its simplicity, appropriate nonstandard schemes greatly improve or eliminate numerical instabilities. In this work we construct several standard and nonstandard finite-difference schemes to solve a system of three ordinary nonlinear differential equations that models photoconductivity in semiconductors and for which it has been shown that integration with a conventional fourth-order Runge-Kutta algorithm produces numerical-induced chaos. It was found that a simple nonstandard forward Euler scheme successfully eliminates these numerical instabilities. In order to help determine the best finite-difference scheme, it was found useful to test the local stability of the scheme by direct inspection of the eigenvalues dependent on the step size.

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.

Long-decay-time photoconductivity in highly disordered carbon fibers.

A photoconductivity with a rather long decay time (\ensuremath{\simeq}0.3 s at 290 K) is observed in a carbon material that is characterized by its huge specific surface area (1000 ${\mathrm{m}}^{2}$/g). The decay time is temperature dependent at low temperatures (100 K) with a small activation energy (3--5 meV), and is also dependent on the illumination power. A decrease in the mobility of the carriers during the illumination is strongly suggested. The relaxation of dangling bonds on the large surface area is proposed as the mechanism to screen the Coulomb attraction among the photocarriers and to build a recombination barrier, resulting in the long decay time. This photoconductivity is considered to be an effect analogous to the persistent photoconductivity in semiconductors, although some apparent differences are also observed.

Determination of optical constants near the fundamental absorption edge in semiconductors

Abstract The photoconductivity of semiconductors was considered under the following general conditions: the surface recombination velocity was not equal to zero, the space charge near the surface is taken into account, the value of absorption coefficient was not limited. Solving both the continuity and Poisson equations for an illuminated thick specimen, we obtained a solution which suggests a new combined measurement in order to determine various optical constants. The absorption coefficient and the diffusion length of electrons in CZ grown n-type Si (N a = 1012-1013 cm−3) were measured at room temperature, using the new technique.

Characteristics of recombination in semiconductors with intercrystallite barriers

A model of nonuniform potential relief (modulated by adsorption of oxygen and an applied electric field) of a film containing centers of slow recombination distributed over the volume of a crystallite and centers of fast recombination localized on their boundaries is studied. The characteristic features of the current-voltage characteristics (CVC) of the photocurrent and the effect of adsorption of oxygen on the photoconductivity in semiconductors with intercrystallite barriers were studied. The saturation of the CVC is determined by the transformation of the surface trap levels into centers of fast recombination under the action of an electric field. The proposed model permits explaining from a unified viewpoint the sensitization and desensitization of a semiconductor as a result of oxygen chemisorption.

Persistent photoconductivity in semiconductors with defect clusters.

Defect clusters in semiconductors may cause persistent photoconductivity. A model is developed based on the spatial separation of the photogenerated carriers by the local potential barriers of the clusters. The buildup and decay of the defect-cluster-induced persistent photoconductivity depend on the geometry of the cluster.

Decay kinetics of persistent photoconductivity in semiconductors.

Kinetic studies of the decay of persistent photoconductivity can decide between competing interpretations. This paper is concerned with the temporal decay of excess conductivity after illumination for low temperatures and spatial carrier separation. The initial decay is rapid since closely spaced carriers recombine; later decay is retarded. Analytic expressions are derived for various trap profiles. Epitaxial GaAs and interfaces of Al/sub x/Ga/sub 1-x/As-GaAs heterostructures are investigated; the results agree with theory, which predicts a decay essentially logarithmic in time.

A stress tunable gallium doped germanium infrared detector system

A simple partable cryostat system for studying photoconductivity in semiconductors under variable uniaxial stress at liquid helium temperatures is described. It has been used with a Fourier transform spectrometer to determine the spectral response of the photoconductivity of gallium doped germanium between 10 and 140 cm−1 as a function of stress and electric field. Details are also presented of the variation of breakdown voltage and dynamic resistance as a function of stress under low background conditions.

Large-signal photoconductivity in semiconductors

A simple formulation based on only particle conservation, continuity of occupation of imperfection levels, and continuity of free carriers, is applied to several large signal photoconductivity problems in semiconductors with isolated imperfections with discrete energy levels. The validity of the method is demonstrated by comparison with a variety of large signal predictions of the Shockley-Read recombination model. The model is then applied to several problems of interest: (1) the lifetime expected for two competing recombination centers, (2) the phenomenon of superlinearity of majority carriers encountered in two and three center models, and (3) the effect of several different trap distributions on superlinearity and the expected light intensity dependence of photoconductivity.

Breit-Wigner-Fano resonances in the photoconductivity of semiconductors: theory

We present a theory of the Breit-Wigner-Fano (BWF) resonances in photoconductivity due to the interference of bound states of the hole plus optical phonons with unbound hole states. The theory is found to be in qualitative agreement with experiment. Differences between the BWF resonances in absorption and photoconductivity are explored and explained.

Breit-Wigner-Fano resonances in the photoconductivity of semiconductors: Experiment

We report the observation of new structure in the photoconductivity spectra of the single acceptors (B, Al, Ga, and In) in Si. This structure occurs at frequencies corresponding to an energy equal to the excitation energy of bound Psu 3 2 states from the ground state plus the energy of a near zone center optical phonon. Based on this observation the structure is attributed to Breit-Wigner-Fano resonances due to the mixing of the excited bound states of the acceptor plus an optical phonon with the continuum of unbound states.

Exchange Impurity Levels and Photoconductivity of Semiconductors

Exchange Impurity Levels and Photoconductivity of Semiconductors

Spin physics of recombination centers in amorphous silicon

Spin-dependent photoconductivity in semiconductors results from angular momentum conservation during the recombination of injected carriers. A model, based on the existence of trapped election-hole pairs, predicts a strong correlation between the spins of the recombining pairs, a “pumping” of triplet spin-states being produced by the allowed recombination of the singlet pairs. This effect not only provides a very sensitive method of resonant spectroscopy for the study of recombination centers, but also explains a magnetic field variation of photoconductivity as large as a few percent at room temperature, as well as a very low field magnetoresistence effect which both have been observed in thin films of hydrogenated amorphous silicon.

Photoconductivity of semiconductors at low temperatures

Low-temperature oscillations in the photoconductivity of semiconductors are discussed. It is shown that oscillations will be observed if the probability for energy relaxation with optical phonons exceeds both the probability of carrier recombination and the energy-relaxation time for acoustic phonons. The oscillation shape is studied for momentum relaxation with acoustic phonons and with ionized impurities for the case in which interelectronic collisions can be neglected and when an account of these collisions leads to the establishment of an electron temperature.

Measurement of the photoconductivity of semiconductors at ultrahigh frequencies

Relations are derived which permit the absolute value of the photoconductivity of a semiconductor to be determined from UHF measurements. The cases considered include various placements of a sample within a cylindrical resonator for the H011 and E010 modes, and the H01p modes in a rectangular resonator. The results derived are correct if we consider the random character of light absorption and neglect the diffusion of photo-excited carriers. Working formulas are given for calculating the design of a measuring setup with a re-entrant resonator. All conclusions are accurate in the small-perturbation approximation.

Interpretation of field effect on CdS for linear and superlinear photocurrent

Weber's theoretical calculations concerning photoconductivity of semiconductors in a transverse electric field are extended to two particular models for CdS single crystals. An agreement with experiment is found when photoexcitation is weak; the presence of surface states is pointed out.