Modification of the thermal relaxation kinetics of the photoinduced (at <Emphasis Type="Italic">T</Emphasis> = 425 K) metastable dark conductivity of <Emphasis Type="Italic">a</Emphasis>-Si:H films by weak illumination during the initial stage of relaxation

The effect of weak illumination during the initial stage of relaxation of the dark metastable conductivity of an undoped a-Si:H film, photoinduced at T = 425 K, on the rate of its subsequent thermal relaxation is studied. It is found that the kinetics of relaxation upon illumination or in the absence of illumination is described by stretched exponents with the parameters τ₀ and β, which are smaller in the case of illumination. It is shown that a decrease in these parameters increases the rate of thermal relaxation of the dark conductivity of the film. Because the temperature and the illumination intensities at which the study is carried out are low, the changes in the relaxation rate of the metastable conductivity are unlikely associated with significant restructuring of the amorphous network. This may be due to changes in the system of hydrogen bonds, which can result, in particular, from the generation and relaxation of slow photoinduced defects under the influence of illumination.     

Effect of illumination on the rate of relaxation of light-induced metastable states in <Emphasis Type="Italic">a</Emphasis>-Si:H(B)

Time dependences of the rates of relaxation of light-induced metastable states of electrically active impurity atoms in boron-doped a-Si:H films have been studied in the dark and under illumination. It has been established that under illumination the rate of relaxation of light-induced metastable states of boron atoms grows in the initial stage when the rate of light-induced relaxation of these states exceeds that of their light-induced generation.     

Relaxation of light-induced metastable state of boron-doped p-type a-Si:H

Relaxation of the dark conductivity of boron-doped a-Si: H films after illumination in the temperature range 360–470 K has been studied. It is shown that the measuring conductivity relaxation after illumination under different conditions (illumination time and temperature) makes it possible to separately investigate relaxation of the concentration of light-induced metastable defects of the “dangling-bonds” type and relaxation of the concentration of metastable states associated with impurity atoms. In both cases the relaxation obeys a stretched-exponential law. The main parameters of both relaxations and their temperature dependence have been measured. The experimental results can be explained within the framework of a model of the annealing activation energy distribution for light-induced metastable states. Fiz. Tekh. Poluprovodn. 32, 117–120 (January 1998)     

Photoinduced relaxation of metastable states in (<Emphasis Type="Italic">a</Emphasis>-Si:H):B

The kinetics of thermal relaxation of an ensemble of photoinduced metastable electrically active B atoms in (a-Si:H):B films is studied after partial relaxation of the ensemble in the dark and under illumination of various intensities and duration. The parameters of a stretched exponential function that describes the ensemble kinetics are determined. It is found that photoinduced relaxation of metastable states manifests itself under conditions in which its rate exceeds the rate of the states’ photoinduced generation. It is shown that the variations in the relaxation-time distribution function of metastable states caused by thermal and photoinduced relaxation are similar.     

Kinetics of light-induced degradation in a-Si:H films investigated by computer simulation

In this work we investigated the stability of a-Si:H films under illumination and following recovery in darkness at different temperatures. The a-Si:H films were fabricated with 55 kHz PECVD and with conventional rf 13.56 MHz PECVD. We measured the steady-state photocurrent and the dark-current after switching off the light source as a function of time. We observed photocurrent degradation and the following recovery of the dark current. The kinetics of the photocurrent degradation as well as the dark-current recovery demonstrated stretched-exponential behavior. The results of these straightforward measurements in combination with computer simulation were used to determine the effect of light-induced degradation and thermal recovery on the density of states distribution in the band gap of a-Si:H. We have found that the photocurrent degradation and the corresponding increase in the total defect concentration have different kinetics. The different kinetics were also determined for the dark-current recovery and the corresponding decrease in the total defect concentration. The results point out that slow and fast types of defects in a-Si:H films control the kinetics of light-induced changes of the defect distribution in the band gap. A model is proposed that relates the origin of the fast and slow metastable defects with the distribution of Si-Si bond lengths.     

Kinetics of light-induced degradation in compensated boron-doped silicon investigated using photoluminescence and numerical simulation

We use photoluminescence to observe light-induced degradation in silicon in real time. Numerical simulations are used to match our results and lifetime decay data from the literature with theoretical models for the generation of the light-induced boron–oxygen defects. It is found that the existing model of the slowly generated defect SRC, where its saturated concentration is a function of the majority carrier concentration, does not explain certain results in both p- and n-type samples. A new model is proposed in which the saturated SRC concentration is controlled by the total hole concentration under illumination.     

Influence of hydrogen sulfide on electrical and photoelectric properties of Al-<Emphasis Type="Italic">p</Emphasis>-Si-SnO<Subscript>2</Subscript>:Cu-Ag heterostructures

I-V characteristics, spectral photosensitivity, and the dependence of the photocurrent on bias, as well as the effect of a 1%-H₂S/N₂ gas mixture, were studied for an Al-p-Si-SnO₂:Cu-Ag heterostructure. The charge transport for carriers in the dark and under illumination was found to obey the law J ∝ U². Exposure to hydrogen sulfide results in a spectral shift of the photovoltage curve to shorter wavelengths. The kinetics of the decrease in photocurrent in a hydrogen sulfide medium is characterized by long relaxation times.     

Light-induced processes in a-Si:H films at elevated temperatures

At temperatures T>120 °C the kinetics of the dark conductivity (σ _d) of undoped and borondoped a-Si:H films during and after the cessation of illumination is observed to be nonmonotonic, with fast and slow processes of variation of σ _d of opposite sign. A fast or slow σ _d relaxation process described by a stretched exponential function can be isolated by varying the duration and intensity of illumination or the film temperature. The nonmonotonic relaxation of σ _d is described by a sum of two stretched exponentials, whose parameters τ and β depend on the film characteristics and on the temperature, exposure time, and intensity of illumination. The nature of the nonmonotonic relaxation is discussed. Fiz. Tekh. Poluprovodn. 31, 1455–1459 (December 1997)     

Features of long-term relaxation of capacitance in rectifying structures based on <Emphasis Type="Italic">n</Emphasis>-ZnP<Subscript>2</Subscript>

Results of studies of electrical properties of structures including metal-conducting oxide (ITO), and β-ZnP₂ semiconductor of n-type conductivity are reported. It is established that the total resistance of the structures under study is determined by deep levels of intrinsic defects in the band gap of semiconductor. Long-term relaxation of capacitance and conductivity is observed after switching off the reverse bias at low temperatures, which is associated with restoration of the nonequilibrium charge in the space-charge region. Models of processes in which experimental results are interpreted satisfactorily are constructed.     

Different mechanisms affecting the inversion layer transient response

The transient response of the MOS capacitance after the application of a large depleting voltage can be caused by three different mechanisms depending on the distribution of the electric field between the silicon and the oxide. The three different cases were distinguished by measurements of the temperature dependence and the voltage dependence of the relaxation behavior. Relaxation due to thermal generation of carriers prevails at low electric fields, relaxation via oxide states occurs at medium fields, and relaxation by avalanche effects predominates at high fields. From the case where thermal relaxation is predominant the effective lifetime of minority carriers in the depletion region and the surface recombination velocity could be determined. At low temperatures (&sim100°K) where most frequently no relaxation occurs in the dark, light-induced relaxation phenomena were studied. Only light with energies larger than the Si band gap causes relaxation.     

On the photoinduced effect in undoped a-Si:H films

The temperature dependences of the dark conductivity and photoconductivity of annealed and preliminarily illuminated undoped a-Si:H films are studied in different modes of temperature variation. Also, the variation kinetics of the photoconductivity and dark conductivity of the films during and after their exposure to light at different temperatures are analyzed. It is shown that the anomalous nature of the dependences obtained may be due to the formation of two kinds of photoinduced defects that have different energies of formation and thermal annealing and energy levels situated in different parts of the band gap of the films under study.     

Isothermal polarization in thin-film Al-As<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Al metal-dielectric-metal structures

Results of the study of the kinetics of isothermal polarization current in As₂Se₃ films are presented. In the samples under study, intense relaxation processes related to the accumulation of bulk charge in the metal-semiconductor contact area occur. The experimental results are interpreted using the model of the relay conductivity mechanism.     

Specific temperature-related features of photoconductivity relaxation in PbSnTe:In films under interband excitation

The time dependences of variations in the photoconductivity of PbSnTe:In films in the range of T ≈ 19—25 K upon interband excitation are studied. It is found that the character of conductivity relaxation after switch-off of illumination depends on the duration and intensity of the preceding illumination. In this case, the characteristic times of relaxation for various modes of illumination can differ by more than an order of magnitude. The obtained results are discussed in the context of a model assuming the presence of a quasicontinuous spectrum of capture levels in the band gap of PbSnTe:In and also a possible effect on the parameters of these levels of the ferroelectric phase transition, the temperature of which is found to be in the temperature range under study     

Photoconductivity of sulfur-doped silicon near 10.6 µm

The extrinsic photoconductivity of Si〈S〉 under short-wavelength (10.6 μm) illumination was investigated in the pulsed regime. It was found that sensitivity can be increased by 2–3 orders of magnitude by short-wavelength illumination. It was established that increasing the degree of compensation of the impurity levels of sulfur by γ-ray-induced acceptors decreases both the dark conductivity and the photoresponse due to pulsed illumination with a CO₂ laser. Fiz. Tekh. Poluprovodn. 31, 1425–1427 (December 1997)     

Accelerated Light-Induced Degradation (ALID) for Monitoring of Defects in PV Silicon Wafers and Solar Cells

In crystalline silicon, above-bandgap illumination can transform defects into strong recombination centers, degrading minority-carrier lifetime and solar cell efficiency. This light-induced degradation (LID) is due primarily to boron–oxygen and iron–boron defects, and can be reversed using thermal treatments that are distinctly different for each type of defect. Combining illumination and thermal treatment, we have designed an accelerated light-induced degradation (ALID) cycle that, within minutes, transforms defects into the distinct states needed to isolate individual contributions from boron–oxygen dimers (BO_2i) and interstitial iron (Fe_i). In this cycle, the concentrations of BO_2i and Fe_i are determined using surface photovoltage (SPV) diffusion length measurement. The ALID cycle uses reversible defect reactions and gives very good repeatability of wafer-scale mapping of BO_2i and Fe_i in photovoltaic (PV) wafers and final solar cells.     

Electrical and galvanomagnetic properties of cadmium telluride films synthesized under highly nonequilibrium conditions

The results of experimental studies of electrical and galvanomagnetic properties of CdTe films synthesized under highly nonequilibrium conditions via vapor condensation on a substrate cooled with liquid nitrogen are reported. The temperature dependences of dark conductivity, current-voltage characteristics with and without illumination, temperature dependences of the Hall coefficient R _H and effective Hall mobility μ_H in the planar geometry, and dark current-voltage characteristics in the sandwich geometry are reported. Anisotropy of conductivity is revealed. It is shown that the electrical and galvanomagnetic properties of the films are consistently described by a percolation model of charge transport, according to which, at high temperatures, the charge transport takes place over the percolation level of the valence band, and at low temperatures, over the percolation level of the impurity band.     

Effect of substrate inhomogeneity on extrinsic photoconductivity of n-type GaAs thin-film structures under backgating

The extrinsic photoconductivity of n-type GaAs thin-film structures under backgating was studied. It is shown that the change of the spatial inhomogeneity of the substrate under illumination is responsible for a new mechanism of extrinsic photoconductivity under backgating. This mechanism occurs through an illumination-induced variation in the backgating threshold voltage and may result in a transition from positive to negative photoconductivity even if all deep levels in the substrate are electron traps. Experiments have shown that the onset of backgating and of negative differential conductivity coincide both in the dark and under illumination thus supporting the proposed mechanism.     

Optical effect in InAlAs/InGaAs/InP MODFET

Analytical results have been presented for an optically illuminated InAlAs/InGaAs/InP MODFET with an opaque gate. Partial depletion of the active region is considered. The excess carriers due to photo generation are obtained by solving the continuity equation. The energy levels are modified due to the generation of carriers. The surface recombination effect has also been taken into account. The results of I-V characteristics have been compared under dark conditions, since under illumination experimental results are not available. The offset voltage, sheet concentration, I-V, and transconductance have been presented and the effect of illumination discussed     

Insulator-to-metal phase transformation of VO2 films upon femtosecond laser excitation

Light-induced insulator-to-metal phase transition of vanadium dioxide films was studied by ultrafast optical pump-probe spectroscopy. The transient optical reflection measurement shows that both heating and laser illumination contribute to the phase transition of VO₂. Within 10⁻¹¹–10⁻⁹ sec, these two mechanisms are competitive. Excited-state dynamics were found to be strongly dependent on the concentration of structural defects and the pump laser power as well. Comparison of the transient reflection of VO₂ films deposited on different substrates suggests that the excitonic-controlled light-induced insulator-to-metal phase transition in VO₂ proceeded through an intermediate state. The transient reflection measurement of VO₂ in metallic phase shows a three-stage relaxation process. A polaron excitation model is introduced to describe the dynamical process for metallic VO₂.     

Simulation of the effects of defects in low temperature Ge buffer layer on dark current of Si-based Ge photodiodes

The influence of defects in low temperature Ge layer on electrical characteristics of p-Ge/i-Ge/n-Si and n-Ge/i-Ge/p-Ge photodiodes (PDs) was studied. Due to a two-step growth method, there are high defect densities in low-temperature buffer Ge layer. It is shown that the defects in low-temperature Ge layer change the band diagrams and the distribution of electric field, leading to the increase of the total dark current for p-Ge/i-Ge/n-Si PDs, whereas these defects have no influence on the dark current for n-Ge/i-Ge/p-Ge PDs. As a complement, a three-dimensional simulation of the total current under illumination was also performed.