Special features of relaxation of metastable states induced thermally and by photoexcitation in (a-Si:H):P films

Kinetics of relaxation of metastable states that are induced thermally or by photoexcitation and cause an increase in dark electrical conductivity of (a-Si:H):P films is discussed. It is established that the relaxation is described by expanded exponential functions with the parameters τ and β depending differently on temperature in the cases of thermal excitation and photoexcitation. Thus, the relaxation of photo-induced states is characterized by a decrease in β with temperature, whereas the parameter β is almost temperature-independent for thermally induced states. It is shown that these dissimilar temperature dependences of β correlate with temperature variations of the half-width of annealing-energy distribution for these states. The observed features of relaxation of thermally induced and photo-induced metastable states are caused by different mechanisms of their formation. The origin of these states can be the same and related to activation of hydrogen-passivated phosphorus atoms.     

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.     

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.     

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.     

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)     

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 metastable modification of optical properties in CH3NH3PbI3−xBrx perovskite films: Two-step mechanism

We have used photoluminescence (PL) and photomodulation (PM) spectroscopy to investigate the reversible spectral changes of PL in CH₃NH₃PbI_3−xBr_x films, where x is 1.7. In an as-prepared film, the peak of PL spectra shifts from ∼640 nm near bandedge to ∼750 nm after excitation by a continuous wave (CW) or a pulsed laser with high repetition rate, but keeps at 640 nm excited by same pulsed laser with the repetition rate smaller than 500 Hz. The PM spectroscopy also shows the formation of sub bandgap states after illumination which is responsible for the red shift of PL. The light induced modification of optical properties is reversible after keeping the film out of illumination for several hours at room temperature. We analyze the photoinduced modification to be two-steps processes: the temporary sub bandgap states were first photogenerated in perovskite film, if those states interacting with more coming photons within their lifetimes, light induced metastable states responsible for red-shift of PL will be formed. This instability reduces the electronic bandgap and generates more traps which will degrade the performance of the related photovoltaic devices.     

Photoinduced annealing of metastable defects in boron-doped a-Si:H films

Semiconductors - The effect of illumination on the isothermal relaxation of slow photoinduced metastable defects (metastable electrically active impurity atoms) in boron-doped a-Si:H films has been...     

Formation of electrically active centers in silicon irradiated with electrons and then annealed at temperatures of 400–700°C

The effect of irradiation with electrons on the formation of quenched-in donors in silicon is studied. It is found that n-and p-type regions are formed in the bulk of single-crystal silicon as a result of irradiation with electrons and subsequent annealing at a temperature of 450°C. The concentration of charge carriers in the regions of both types increases as the radiation dose and the annealing duration increase, which indicates that not only quenched-in donors but also quenched-in acceptors are formed. Nonuniformity in the distribution of the acceptor and donor centers correlates with fluctuations of the oxygen concentration in silicon.     

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.     

Redistribution of Al in implanted SiC layers as a result of thermal annealing

The experimental Al concentration profiles formed on implantation of Al into SiC at room temperature with subsequent high-temperature annealing are analyzed. It is shown that, at doses above the amorphization threshold, the profiles exhibit a number of specific features: a shift of the maximum of the distribution, accumulation of dopants at the surface, and formation of box-shaped profiles. To describe quantitatively the redistribution of Al dopants in the SiC layers implanted with high doses, the segregation-diffusion model is suggested for the first time. The model takes into account segregation of dopants between the α and c phases during solid-state epitaxial crystallization followed by diffusion of dopants and their evaporation from the surface The formation of box-shaped Al profiles as a result of short-term thermal annealing is attributed to the origination of highly damaged single-crystal and polycrystalline SiC layers in the recrystallized region, with a high diffusion coefficient of dopants, and to the suppression of the enhanced transient diffusion in the remaining single-crystal part of the implanted layer.     

New oxygen donors formed at 700 °C in silicon

Donor formation at 700 °C was studied by infrared absorption, etching, transmission electron microscope, resistivity, and spreading resistance measurements in Czochralski grown silicon. The donor concentration is related to the oxygen-precipitate density, oxygen reduction, and carbon reduction by annealing at 700 °C. The donor distribution corresponds to the distribution of oxygen precipitates observed after annealing. The proposed donor is an oxygen precipitate nucleated at a carbon site. The oxygen-related donor formation not only occurs in the bulk of samples but also in the denuded zone. Donor-related microdefects do not seriously influence the threshold voltage in metal-oxide-silicon field-effects-transistors, but are expected to decrease carrier lifetime at the surface of the denuded zone.     

A model of photoinduced annealing of intrinsic defects in hexagonal CdS<Subscript>x</Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> quantum dots

A possible mechanism of photoinduced annealing of intrinsic defects in quantum dots with a hexagonal crystal structure is justified on the basis of the studies of the kinetics of photoinduced decay of luminescence of CdS_xSe_1?x quantum dots synthesized in a glass matrix and ab initio calculations of chemical bond energies at the interface in the n(CdSe)-SiO_x-type cluster. The model proposed implies that photoinduced Se-O bond breaking at the anionic face results in an increase in electric field inside the quantum dot; this field stimulates cadmium vacancy diffusion to the surface. This model accounts for the degradation of luminescence and of the parameters of nonlinear optical devices observed during photoinduced annealing.     

Effect of annealing on the spectra of nuclear quadrupole resonance in gallium-indium selenides and characteristics of structures based on these materials

The results of studying the effect of low-temperature annealing (at temperatures no higher than 250°C) on the spectra of nuclear quadrupole resonance in layered GaSe and InSe single-crystal semiconductors are reported. The electrical and photoelectric characteristics of the p-GaSe-n-InSe structures have also been studied. It is shown that, as the annealing temperature is lowered to room temperature, the quality of the samples improves due to a decrease in the defect concentration in the crystals and due to ordering in a system of polytypes. The temperature conditions for the heat treatments, at which the main parameters of the heterojunction are improved, are determined. Mechanisms affecting the behavior of the p-GaSe-n-InSe structure during the course of annealing are discussed.     

Electrical and photoelectric properties of a-Si:H layered films: The influence of thermal annealing

The electrical and photoelectric properties of layered a-Si:H films obtained by cyclic plasmochemical deposition and the effect of thermal annealing on these properties have been studied. Unannealed films demonstrate high photosensitivity, with a photoconductivity to dark conductivity ratio of K=3.4×10⁶. Increasing the annealing temperature causes the film photosensitivity to fall because of a considerable decrease in the photoconductivity and increase in the dark conductivity. For films annealed at temperatures above 500°C, the conductivity is the sum of the band conductivity and the hopping conductivity via states at the Fermi level.     

Towards understanding recovery of hot-carrier induced degradation

This article treats the recovery of hot-carrier degraded nMOSFETs by annealing in a nitrogen ambient. The recovery rate is investigated as a function of the annealing temperature, where the recovery for increasing temperatures is in agreement with the passivation processes. At the original post-metal anneal temperature of T = 400 °C, the device's original performance is fully restored. Higher temperatures induce a permanent, unrecoverable change to the devices, manifested in a gradual V_T shift. The recovery rate is found to be independent of both the transistor gate length and the cooling rate (quench, slow and stepped cooling) upon annealing. These findings are used to gain further understanding of the mechanisms behind the recovery of hot-carrier damage. The recovery rate exhibits Arrhenius behavior and the recovery data are consistent with Stesmans' recovery model.     

The influence of oxidation rate and heat treatment on the Si surface potential in the Si-SiO2system

Thermal oxidation of silicon results in a nonuniform distribution of defects in the oxide and in a Si-SiO2interface characterized by a certain disorder. The Si surface potential is determined by the defect structure of the oxide and phase boundary. High temperature postoxidation treatment leads to a redistribution of the oxide defects and rearrangement of the interface. To study these effects, p- and n-type Si was oxidized in dry oxygen at 1145°C. The oxidation was followed by a heat treatment in a dry helium atmosphere at the oxidation temperature for various periods of time (2 minutes to 65 hours). The density of the charged oxide and interface states was determined by MOS capacitance measurements. It was found that the density of the donor states decreases logarithmically with the annealing time. Since such a behavior is typical of adsorption phenomena that are associated with modifications of the surface structure, we attribute this change to a rearrangement of the phase boundary. Further evidence for this is the fact that the density of donor states for quenched samples increases with the oxidation rate within the regime of parabolic growth, n-type specimens also exhibited donor states but their distribution was different from that for p-type units. In the presence of slight traces of water vapor, redistribution of hydroxyls can become the predominant mechanism, especially for longer annealing times.     

Metastable-state formation as a possible mechanism for the conductance anomalies in mesoscopic structures

We study metastable-state formation in the one-dimensional model of a quantum ballistic contact, in which the contact is represented by a potential barrier with electron-electron interaction localized therein. It is shown that when the interaction parameter exceeds a critical value, a metastable state with spontaneous spin polarization of the barrier is formed. The difference between the grand potentials of the metastable and globally stable states tends to zero at the critical point; therefore, the metastable state manifests itself in transport even at low temperatures and its effect gradually increases with temperature. The main effect is a decrease in the conductance with increasing temperature, which occurs in a certain range of the barrier-region potential, similar to that observed during the formation of the well-known 0.7 conductance anomaly.     

Thermal annealing of defects in InGaAs/GaAs heterostructures with three-dimensional islands

A report is presented on the investigation of the influence of in situ annealing of the InGaAs layer in p-n InGaAs/GaAs structures grown by the metalloorganic chemical vapor deposition upon the formation of coherently strained three-dimensional islands. The structures were studied by the methods of capacitance-voltage measurements, deep-level transient spectroscopy, transmission electron microscopy, and photoluminescence. It is established that three-dimensional islands with misfit dislocations are formed in the unannealed structure A, while quantum dots are formed in the annealed structure B. The deep-level defects were investigated. In structure A, defects of various types (EL2, EL3 (I3), I2, HL3, HS2, and H5) are present in the electron-accumulation layer. Concentrations of these traps are comparable to the shallow donor concentration, and the number of hole traps is higher than that of the electron traps. On the in situ annealing, the EL2 and EL3 defects, which are related to the formation of dislocations, disappear, and concentrations of the other defects decrease by an order of magnitude or more. For structure A, it is established that the population of the quantum states in the islands is controlled by the deep-level defects. In structure B, the effect of the Coulomb interaction of the charge carriers localized in the quantum dot with the ionized defects is observed.