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 temperature and illumination intensity on the formation of metastable states in a-Si:H

Nonmonotonic variation in the concentration of slow photoinduced metastable states and in the half-width of the distribution function for these states concerning their annealing times is observed in a-Si:H as temperature is increased in the range from 400 to 480 K. These nonmonotonic variations in the parameters of ensembles are determined by the temperature-dependent ratio between the rates of formation and annealing of the metastable states under study. It is also established that, as the film’s illumination intensity is decreased, a decrease in the concentration of metastable states and in the half-width of their annealing-time distribution sets in at higher temperatures. This behavior can be accounted for, in particular, by a substantial decrease in the rate of annealing of slow metastable states in comparison with a decrease in the rate of photoinduced formation of these states; according to the three-level model, this formation also involves a thermal process.     

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.     

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.     

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.     

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)     

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 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...     

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)     

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.     

Unusually Fast Optically Induced Birefringence in Polyoxetanes Bearing 4‐(N,N‐Diphenyl)amino‐4′‐nitroazobenzene Chromophores

Two new polyoxetane polymers bearing azobenzene‐type chromophore pendants were prepared, and their photoinduced molecular orientation and relaxation were studied. The two polymers differ in the mode of attachment of the chromophores, which are either laterally ( Pox 1 ) or perpendicularly ( Pox 2 ) bound to the polymer backbone. The two polymers reveal unusually fast (∼2 s to saturation at 0.4 W/cm²) photoinduced reorientation of chromophores and decay behavior at room temperature. Side‐on attachment of the chromophores ( Pox 1 ) results in a significantly faster response to polarized light and higher maximum birefringence than end‐on attachment. The kinetics of photoinduced orientation and relaxation decay were quantitatively analyzed and the results are discussed in relation to the structures of the polymers.     

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.     

非晶硅中的缺陷态光吸收谱

<正> 非晶硅薄膜的光学及电学性质与共电子局域态分布紧密相关。为了改进非晶硅薄膜器件的性质,如太阳能电池、薄膜晶体管等,需要低局域态密度的材料,因而测量并了解局域态的性质十分重要。局域态包括带尾态、缺陷态及亚稳缺陷态。本文着重讨论缺陷态及亚稳缺陷态。     

An investigation of the effects of radiation exposure on stability constraints in epitaxial SiGe strained layers

The thermodynamic stability of device-relevant epitaxial SiGe strained layers under proton irradiation is investigated using X-ray diffraction techniques, and compared with its stability constrain under high-temperature annealing. Irradiation with 63 MeV protons is found to introduce no significant microdefects into the SiGe thin films, regardless of starting stability condition of the SiGe film, and thus does not appear to be an issue for the use of SiGe HBT technology in emerging space systems. The strain relaxation of SiGe thin film under thermal annealing, however, is sensitive to the composition and thickness of the as-grown samples, as expected, with the subsequent lattice relaxation of the unstable samples occurring at a much higher rate than that of metastable samples.     

Kinetics of attaining the steady state of hot carrier thermoelectric power in a p-n junction with consideration for lattice heating

The lattice heating effect on the kinetics of attaining the steady state of hot carrier thermoelectric power in a p-n junction is theoretically studied. It is shown that lattice heating leads to an additional third stage in the process of attaining the steady state in thermal currents and the thermoelectric power of hot carriers with a relaxation time determined by the thermal conductivity and heat capacity of the sample. It is also shown that the relaxation rate of the third stage is lower in comparison with stages I and II observed by Veinger and Sargsyans.     

Structural and Biomimetic Chemical Kinetics: Kinetic Magnitudes Include Structural Information

Electrochemical reactions in films of conducting polymers involve reactive chains, exchanging ions and solvent with the electrolyte under current flow. The reaction induces structural changes: conformational movements in chains and swelling, shrinking, compaction, and relaxation processes in the reactive dense gel. Conformational movements and structural changes induced by reactions are not included by today's chemical kinetic models. Using different packed conformations as initial states for the study of the reaction kinetics in films of different conducting polymers, rate coefficients, activation energies, and reaction orders related to the concentration of active centers in the film change with the structure of the selected initial state. The kinetic magnitudes include and quantify conformational and structural energetic states. Therefore, the reaction activation energy includes the constant reaction activation energy and the conformational energy of the initial packed state. The electrochemically stimulated conformational relaxation model describes the empirical variation of the kinetic magnitudes.     

Photoresponsive Ferroelectric Liquid‐Crystalline Polymers

The photoresponse of ferroelectric smectic side‐chain liquid‐crystalline (LC) polymers containing a photoisomerizable azobenzene derivative as a covalently linked photochromic side group is investigated. By static measurements in different photostationary states, the effect of trans–cis isomerization on the material's phase‐transition temperatures and its ferroelectric properties (spontaneous electric polarization P_S and director tilt angle θ) are analyzed. It turns out that the Curie temperature (transition S_C* to S_A) can be reversibly shifted by up to 17 °C. The molecular mechanism of this “photoferroelectric effect” is studied in detail using time‐resolved measurements of the dye's optical absorbance, the director tilt angle, and the spontaneous polarization, which show a direct response of the ferroelectric parameters to the molecular isomerization. The kinetics of the thermal reisomerization of the azo dye in the LC matrix are evaluated. A comparison to the reisomerization reaction in isotropic solution (toluene) reveals a faster thermal relaxation of the dye in the LC phase.     

Interpulse kinetics in copper and copper halide lasers

The various rate processes that govern the interpulse relaxation in metal vapor and metal halide vapor lasers are considered. Computer calculations indicate that the rapid metastable levels relaxation observed in copper and copper halide laser experiments requires the existence of a relatively small resonance in the cross section for metastable excitation or deexcitation near threshold. The accurate calculation of interpulse relaxation requires knowledge of rate constants presently not well known; this is especially so for metal halide lasers.     

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.     

Reversible Photoinduced Phase Segregation and Origin of Long Carrier Lifetime in Mixed‐Halide Perovskite Films

Mixed‐halide hybrid perovskite semiconductors have attracted tremendous attention as a promising candidate for efficient photovoltaic and light‐emitting devices. However, these perovskite materials may undergo phase segregation under light illumination, thus affecting their optoelectronic properties. Here, photoexcitation induced phase segregation in triple‐cation mixed‐halide perovskite films that yields to red‐shift in the photoluminescence response is reported. It is demonstrated that photoexcitation induced halide migration leads to the formation of smaller bandgap iodide‐rich and larger bandgap bromide‐rich domains in the perovskite film, where the phase segregation rate is found to follow the excitation power‐density as a power law. Results confirm that charge carrier lifetime increases due to the trapping of photoexcited carriers in the segregated smaller bandgap iodide‐rich domains. Interestingly, these photoinduced changes are fully reversible and thermally activated when the excitation power is turned off. A significant difference in activation energies for halide ion migration is observed during phase segregation and recovery process. Additionally, the emission linewidth broadening is investigated as a function of temperature which is governed by the exciton–optical phonon coupling. The mechanism of photoinduced phase segregation is interpreted based on exciton–phonon coupling strength in both mixed and demixed (segregated) states of perovskite films.