Relaxation of photoinduced metastable states in a-Si:H films deposited at high temperatures

The results of an experimental study of dark-conductivity kinetics in a-Si:H after short-term and long-term illumination, are presented. The films were deposited at temperatures in the range T _s=300–390 °C. Data on relaxation of the photoinduced metastable states were found to correlate with the Fermi-level position. Fiz. Tekh. Poluprovodn. 32, 1269–1271 (October 1998)     

Metastability and relaxation processes in hydrogenated amorphous silicon

The kinetics of structural relaxation in hydrogenated amorphous silicon (a-Si:H) deposited by various methods is investigated by differential scanning calorimetry. The experimental results are used to analyze the nature of the metastable states in a-Si:H and to investigate the relationship between structural relaxation and light-induced metastability (the Staebler-Wronski effect). Fiz. Tekh. Poluprovodn. 31, 1449–1454 (December 1997)     

Effect of illumination time on the annealing of optically created metastable defects in p-type a-Si:H

The effect of illumination time on the relaxation of the concentration of optically created metastable defects in films of boron-doped a-Si:H in the temperature range 360–400 K is discussed. The concentration relaxes according to a stretched exponential law ~exp[\t-(t/σ _r ) ^β . In the region of temperatures under study and for illumination times of 0.1–7.0 s, the coefficient β is equal to 0.55–0.65; furthermore, the temperature dependence of the effective time τ _r was activated, with an activation energy E _a of 0.97–1.07 eV. As illumination increased, a weak increase in E _a and β is observed. The quantity τ _r increases as the illumination time increases, in accordance with a law that is close to logarithmic. The experimental results obtained are compared with the existing microscopic models for the formation and annealing of metastable defects in a-Si:H films. Fiz. Tekh. Poluprovodn. 31, 347–349 (March 1997)     

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.     

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)     

Light-induced relaxation of the metastable conductivity of undoped <Emphasis Type="Italic">a</Emphasis>-Si:H films illuminated at elevated temperatures

The kinetics of relaxation of the light-induced (at a temperature above 140°C) dark conductivity of undoped a-Si:H films is studied. The calculated time dependences of the relaxation rate of the dark conductivity are analyzed under the assumption that the thermal rates of the generation and relaxation of metastable defects formed by preliminary illumination are independent of illumination. It is shown that the features of the kinetics of the relaxation rates of dark conductivity under illumination are determined by the presence of light-induced processes of the relaxation and generation of slowly relaxing metastable defects whose energy levels are located in the upper half of the band gap.     

Charging of deep-level centers and negative persistent photoconductivity in modulationdoped AlGaAs/GaAs heterostructures

The relaxation kinetics of persistent photoconductivity in AlGaAs/GaAs modulation-doped heterostructures due to charging of EL2-and DX-centers is investigated over a wide range of temperatures and excitation photon energies. The light-induced charging of these deep centers was found to lead to accumulation of positive and negative localized charges, which give rise to positive and negative persistent photoconductivities, respectively. These positive and negative charges are accumulated in different parts of the heterostructure. Their different characteristic times, and the different temperature dependences of these times, result in nonmonotonic time and temperature dependences of the persistent photoconductivity. Charging of EL2-centers in the GaAs buffer layer leads to negative persistent conductivity in the temperature range 180–300 K, while the negative photoconductivity observed at the temperatures below 180 K is caused by excited states of DX-centers in the n ⁺-AlGaAs. Fiz. Tekh. Poluprovodn. 33, 68–74 (January 1999)     

Gamma-induced metastable states of doped, amorphous, hydrated silicon

The temperature dependence of the electrical conductivity of B-and P-doped a-Si:H films before and after irradiation with γ rays from a ⁶⁰Co source has been investigated. The irradiation dose was 10¹⁷–10¹⁸ photons/cm². The behavior of the n-type films was observed to be substantially different from that of the p-type films. The electrical conductivity of the p-type films increases slightly (by a factor of 2–3) and that of the n-type films decreased sharply (by 2–3 orders of magnitude). The observed difference is explained by the different character of the γ-irradiation-induced charge redistribution on the broken silicon bonds D⁺ and D⁻. Comparing the results with the published data shows that the γ rays induce metastable states of a-Si:H which are due to the motion of bound hydrogen. Fiz. Tekh. Poluprovodn. 32, 245–248 (February 1998)     

Transport properties of microcrystalline silicon at low temperatures

The dark conductivity and photoconductivity along with pulsed electron spin resonance have been measured over a wide temperature range with a high crystallinity hydrogenated microcrystalline silicon (μc-Si: H) sample. The transport mechanism in μc-Si: H is discussed on the basis of these measurements. Striking similarities in the temperature dependences of the dark conductivity and photoconductivity between μc-Si: H and some well-studied materials, such as hydrogenated amorphous silicon, suggest that at low temperatures hopping of carriers between localized states dominates the transport properties of μc-Si: H. Fiz. Tekh. Poluprovodn. 32, 905–909 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Conductivity relaxation in coated porous silicon after annealing

A study has been carried out of the effect of a brief anneal at 450–550 °C on the conductivity of porous silicon coated with a metallic film. Porous silicon, formed on p-and n-type substrates, had porosities of 16–40% and 5–10%, respectively. It has been shown that for anneals at 500 and 550 °C porous silicon on p-type Si is converted to the highly conducting state. The conductivity relaxation in coated porous silicon layers on p-Si after annealing is described. The results are analyzed from the point of view of a model of passivation of the impurity atoms by hydrogen. It is shown that after annealing an aluminum-〈porous silicon〉 junction possesses a rectifying property. The potential barrier parameters for Al-〈porous silicon〉 junctions on p-and n-type substrates are determined. Fiz. Tekh. Poluprovodn. 33, 476–480 (April 1999)     

Formation of order in a system of localized charges in disordered layers of solid solutions of cadmium telluride and cadmium sulfide

Relaxation processes stimulated in layers of the solid solutions CdTe_xS_1−x (x<0.2) by a change in an external electric field, temperature, and illumination have been studied. Polarization effects and maxima of the inversion current of photostimulated and thermally stimulated polarization were found. It is shown that all characteristic features of the relaxation processes can be explained in a quasidipole model, and the inversion maxima of the current can be interpreted as being due to a photostimulated and thermally stimulated order-disorder transition. Fiz. Tekh. Poluprovodn. 31, 286–290 (February 1997)     

Contact-free determination of the parameters of a 2D electron gas in GaAs/AlGaAs heterostructures

The parameters of a 2D electron gas in a GaAs/AlGaAs heterostructure — density, conductivity, and mobility in zero magnetic field, transport and quantum (one-particle) relaxation times, Dingle temperature, and spacer width — have been determined by a contact-free (acoustic) method. Fiz. Tekh. Poluprovodn. 31, 1092–1094 (September 1997)     

Low-frequency noise in n-GaN

Low-frequency noise has been investigated in the hexagonal polytype of n-type gallium nitride (GaN) with equilibrium electron concentration at 300 K n ₀⋍7×10¹⁷ cm⁻³. The frequency and temperature dependence of the noise spectral density S _I/I² was studied in the range of analysis frequencies f from 20 Hz to 20 kHz in the temperature range from 80 to 400 K. Over the entire temperature range the frequency dependence of the dark noise is close to S _I/I²∼1/f (flicker noise). The rather weak temperature dependence of the noise level is characterized by very high values of the Hooge constant α⋍5–7. These large α values indicate a rather low level of structural quality of the material. The effects of infrared and band-to-band illumination on low-frequency noise in GaN are studied here for the first time. The noise level is unaffected by illumination with photon energy E _ph<E _g (E _g is the band gap) even for a relatively high value of the photoconductivity Δσ⋍50%. Band-to-band illumination (E _ph⩾E _g) influences the low-frequency noise level over the entire investigated temperature range. At relatively high temperatures the influence of illumination is qualitatively similar to that of band-to-band illumination on low-frequency noise in Si and GaAs. At relatively low temperatures the influence of illumination on the noise in GaN is qualitatively different from the results obtained earlier for Si and GaAs. Fiz. Tekh. Poluprovodn. 32, 285–289 (March 1998)     

Electronic properties of a GaAs surface treated with hydrochloric acid

Processing in HCl is found to stabilize the system of surface electronic states of the (100) surface of n-GaAs in the temperature range 100–300 K. Distributions for the effective density of surface electronic states in the band gap of GaAs, which are obtained from the electric-field dependence of the surface photovoltage, depend on the measurement temperature. This is because the electronic states that affect the electric-field measurements are located both at the boundary between GaAs and the surface film and in the films themselves. Processing in HCl decreases the density of electronic states of both types, and also decreases the concentration of deep and shallow traps for nonequilibrium holes. These effects are even more pronounced when the processing in HCl is followed by washing in water. Fiz. Tekh. Poluprovodn. 33, 1196–1200 (October 1999)     

Thermally stimulated currents and instabilities of the photoresponse in PbTe(In) alloys at low temperatures

The photoelectric properties of Pb_1−X−Y Sn_XGe_YTe(In) epitaxial films (0.06<X<0.2, 0.08<Y<0.12) with In concentrations of 0.5–1 at.% on BaF₂ substrates are investigated in the temperature range 4.2 K<T<30 K. Thermally stimulated single current (TSC) peaks are observed at T∼6–14 K. It is established that a magnetic field up to 5 T slightly shifts the peaks essentially without affecting their profile. The TSC peaks and the electrothermal instabilities observed at T<15 K are attributed to the excitation of electrons from metastable states into the conduction band. Fiz. Tekh. Poluprovodn. 33, 9–12 (January 1999)     

Frenkel’-Poole effect for boron impurity in silicon in strong warming electric fields

A method based on measurement of the thermally stimulated conductivity of a weakly compensated semiconductor, which is doped with a deep impurity and which contains an impurity component that is shallower than the main component, has been developed for investigating the Frenkel’-Poole effect. The results of an investigation of the thermally stimulated conductivity of Si:Ga samples with gallium density N _A =(2–3)×10¹⁸ cm⁻³ and low accompanying impurity content (⩽10¹³ cm⁻³) are reported. The conductivity was measured after extrinsic photoexcitation of samples heated at a rate β=0.6 K/s in the temperature range T=4.2–24 K in electric fields E=20–1000 V/cm. It is shown that the maximum on the curves of the thermally stimulated conductivity is due to the thermally stimulated emptying of the boron impurity and shifts to lower values of T as E increases. The decrease of the ionization energy of impurity B in an electric field, which turns out to be somewhat weaker than the field according to the Frenkel’-Poole model for singly charged Coulomb centers, is found from the shift of the maximum. Fiz. Tekh. Poluprovodn. 31, 777–780 (July 1997)     

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.     

Properties of p-PbTe (Ga) based diode structures

Single crystals of p-PbTe(Ga) with gallium density too low for complete compensation of uncontrollable impurities and intrinsic lattice defects and realization of Fermi level (FL) pinning were investigated for the purpose of producing diode structures In contact-[p-PbTe(Ga)]-Pt contact. It was found that the properties of the structures obtained have a number of features that distinguish them from In-[p-PbTe] Schottky barriers. The current-voltage characteristics (IVCs) of the experimental structures are not described by the standard relation of the Schottky theory not only in the region of reverse bias but also in the region of direct bias. Residual photoconductivity (PC) is observed under illumination with a thermal radiation source in the temperature range T<80 K; after the illumination is switched off, the IVCs are linear. Under constant illumination a photo-emf appears and the branches of the IVCs in the region of reverse biases rectify. The experimental results are discussed on the basis of the assumption that regions with n-type conductivity form and FL stabilizes near the nonohmic contact as a result of band bending. It has not been ruled out that n-PbTe(Ga) regions are initially present in the sample, but they are not manifested under ohmic contact conditions. Fiz. Tekh. Poluprovodn. 31, 1431–1435 (December 1997)     

Study of optical absorption in thin films of a-As2Se3 by photocapacitance spectroscopy

Photocapacitive spectroscopy is used for the first time to study optical absorption in thin films of a-As₂Se₃ for optical photons with energies below the gap width. Photocapacitance spectra are obtained over energies of 0.83–1.94 eV and used to characterize the density of localized states in the tail of the valence band and in deep states of charged defects. Aging and illumination of the film are found to affect the level of optical absorption by deep centers. Fiz. Tekh. Poluprovodn. 32, 490–493 (April 1998)     

Effect of selenium on the galvanomagnetic properties of the diluted magnetic semiconductor Hg1−x MnxTe1−y Sey

The galvanomagnetic properties of single crystals of the diluted magnetic semiconductor Hg_1−x Mn_xTe_1−y Se_y (0.01<y<0.1) with x=0.05 and 0.14 have been investigated in the temperature range 4.2–300 K. Peculiarities of the temperature dependence of the Hall coefficient R _H and its complicated behavior in a magnetic field are quantitatively explained by the existence of three groups of current carriers: electrons and two types of holes with different mobilities for which the temperature dependence of concentration and mobility was obtained. A transition from p-type to n-type conductivity was observed with increase of Se content, alongside with a simultaneous change of sign of the magnetoresistance from negative to positive. Fiz. Tekh. Poluprovodn. 32, 57–60 (January 1998)