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。     

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.     

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.     

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.     

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)     

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.     

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.     

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.     

Photoluminescence and recombination luminescence in amorphous molecular semiconductors doped with organic dyes

Photoconductivity, photoluminescence (PL), and thermally stimulated luminescence of photoconductive poly-N-epoxypropylcarbazole and poly-N-vinylcarbazole films and non-photoconductive polyvinylbutyral, polyvinyl alcohol, polystyrene, and polyethylene films doped with cationic, anionic, and neutral dyes are studied. It is found that the PL of cationic dyes in photoconductive polymer films is enhanced in comparison to nonphotoconductive ones. The PL enhancement correlates with an increase in photoconductivity, with the quenching effect of an external electric field on the PL intensity, and with an increase in the intensity of the recombination luminescence. It is assumed that this enhancement is related to the presence of predimer traps for holes in the vicinity of dye ions in the films of carbazolyl-containing polymers. A model describing the trap formation upon the photoexcitation of holes into predimer states is suggested.     

Effect of iodine impurity on relaxation of photoexcited silver chloride

The time and temperature dependences of relaxation of excited AgCl and AgCl:I crystals is studied by the method of photostimulated flash of luminescence. The presence of iodine impurity in silver chloride gives rise to hole recombination (luminescence) centers and hole traps in the band gap. It is shown that the main contribution to the decrease in the concentration of electrons localized at deep traps is made by the recombination of electrons with holes released thermally from shallow localization levels (iodine-related centers). Estimation of activation energy for the relaxation process showed that these energies for the AgCl and AgCl:I samples under study are the same within the experimental error and are equal to E _{rel 1} = 0.01 ± 0.0005 eV for the initial stage of relaxation and E _{rel 2} = 0.09 ± 0.005 eV for the final state. This fact indicates that the majority of hole traps involved in the relaxation process in AgCl are related to iodine impurity. In the course of thermal relaxation in AgCl, relocalization of nonequilibrium charge carriers from shallow levels to deep levels is observed. The depth of the corresponding trap is E _arl = 0.174 ± 0.03 eV.     

Improved thermal stability in photochromism-based optically controllable organic thin film transistor

Thermally induced structural change in photoisomerization molecules is a serious obstacle to the development of optically controllable organic field-effect transistors (OFETs). This is because the thermal relaxation of molecular structures degrades photo-induced change in drain current and removes the memory function. To deal with this issue, a naphthopyran (NP) derivative, namely 3,13-dihydro-3-(4-triphenylaminyl)-3,13-diphenylbenzopyrano[5,6-a]carbazole (NP-TPAC) was tested that displays pseudo p-type photochromism at room temperature. The NP-TPAC-doped poly(triarylamine) (PTAA) film exhibited a reversible change in transistor properties; the drain current was reduced by ultraviolet (UV) and returned to its original value by visible (VIS) light irradiation. Importantly, no change in the drain current was observed at room temperature for more than 30 h under dark conditions. This was because the open-ring trans–trans (TT) isomer of NP-TPAC is thermally stable owing to the CH-π interaction and the steric force exerted by the phenyl ring of the carbazole unit onto the double bond responsible for the thermal back reaction. In other words, the thermal stability of photochromism-based optical devices can be greatly improved by adopting an appropriate molecular design.     

Absorption and photoionization of the donor level in CdF2 semiconductor crystals

A model of strong vibronic interaction is proposed to interpret the specific features of infrared absorption and photoionization in CdF₂ semiconductor crystals. The model takes into account the polaronic nature of the conductivity in these crystals and the profound configuration shift of the free and bound polaron states. It is shown that the intense infrared absorption band in the crystals is not due to the transitions of charge carriers from hydrogen-like donor levels to the conduction band, but is caused by the phonon replicas of intracenter transitions. The low-temperature photoconductivity (in the temperature range 0–70 K) is a result of tunneling transitions between the phonon states of bound and free polarons, since these states are separated by rather high potential barriers. Overcoming the barriers in both directions is responsible for equilibration in the polaron subsystem upon the photoexcitation of charge carriers. The tunneling character of this process is responsible for the slight variation in the equilibration time in the above-indicated temperature range.     

PbSnTe:In compound: Electron capture levels,galvanomagnetic properties,and THz sensitivity

A model of the Pb_1–x Sn _x Te:In compound, based on concepts of the theory of disordered systems is considered. The temperature dependences of the Fermi-level position and carrier concentration are calculated depending on the indium doping level and are compared with experimental data. The transient current–voltage characteristics are calculated in the mode of injection from the contact and current limitation by space charge at various voltage-variation rates. The data obtained are compared with the experiments. It is demonstrated that the shape of the characteristics is controlled by the parameters of electron capture at localized states. Photocurrent relaxation in a magnetic field is studied, and the mechanism of such relaxation is discussed under the assumption of the magnetic freezing of carriers.     

Relaxation kinetics of impurity photoconductivity in p-Si:B with various levels of doping and degrees of compensation in high electric fields

The relaxation of impurity photoconductivity in p-Si:B crystals subjected to pulsed optical excitation by a narrow-band continuously adjusted source of radiation in the range of “heating” (10–500 V/cm) electric fields is studied. A variation of dependence of the relaxation time on the electric field E at E > 75 V/cm due to the additional relaxation processes with the emission of an optical phonon is observed. The dependence of the rates of carrier relaxation on the intensity and wavelength of the excitation radiation indicates also that there is a long-lived excited state, which plays the role of a metastable trap level upon the relaxation of charge carriers.     

Thermally induced crystallization behavior and film microstructure alteration of N,N,N′,N′-tetraphenylbenzidine (TPB) and N,N,N′,N′-tetra-p-tolyl-benzidine (TTB)

In this paper, the thermally induced crystallization behavior of N,N,N′,N′-tetraphenylbenzidine (TPB) and N,N,N′,N′-tetra-p-tolylbenzidine (TTB) and their microstructure changes as well as π–π aggregation mode as film state were comprehensively studied. The influence of heating rate, annealing temperature and time on the non-isothermal crystallization behavior of TPB and TTB powder was evaluated by Jeziorny method. Cold crystallization with the transition of metastable state was observed for both TPB and TTB. The thermally induced microstructure alteration and aggregation pattern of TPB and TTB films were investigated using XRD, SEM and AFM. The results revealed that both TPB and TTB could form smooth amorphous thin film and effective π–π interaction with identical edge-on orientations. TTB was prone to pack with both edge-on and face-on orientations especially after annealing, which accounts for its relatively low hole mobility. Besides, a more ordered film microstructure was formed after annealing at 70 °C for 24 h, which results in nearly four times increase of the hole mobility.     

Temperature stability of contact systems for GaSb-based photovoltaic converters

The thermally induced degradation (temperature T ≈ 200°C) of Cr-Au, Cr-Au-Ag-Au, Ti-Pt-Au, and Ti-Pt-Ag contact systems deposited onto the surface of p-GaSb by magnetron sputtering and resistive evaporation are studied. It is found that photovoltaic (PV) converters with a contact grid based on Ti-Pt-Ag are characterized by the maximum thermal stability. PV cells with gold-containing contacts based on Cr-Au and Ti-Pt-Au exhibit a high rate of degradation with increasing temperature, which may require a more effective heat removal system for their operation.     

Strain distribution in InP grown on patterned Si: Direct visualization by cathodoluminescence wavelength imaging

InP has been grown on patterned Si substrates using a low temperature metalorganic chemical vapor deposition process which insures compatibility with integrated circuit technology. Two different patterns are investigated: wet chemically etched V-grooves and SiO₂-masked dry etched grooves. Reduction of feature size leads to drastic defect reduction and quantum efficiencies up to those of homoepitaxially grown InP. Strain relaxation and quantum efficiency are directly visualized by cathololuminescence wavelength imaging. On (001)-and {111}-facets of V-grooves distinct relaxation of the tensile thermally induced strain are found. Surprisingly, in the bottom of V-grooves, close to or even at the InP/Si interface, a high quantum efficiency is found with a recombination time constant typical for thick InP layers of high crystallographic quality.     

Classification of macroscopic defects contained in p-type EFG ribbon silicon

Recombination behavior of the grown-in defects contained in p-type ribbon silicon has been examined. Minority carrier lifetime at various defect sites was measured as a function of temperature by the electron beam induced current (EBIC) method. Based on the lifetime vs temperature characteristics, we classify the defects into three macroscopic (> 20 μm scale) categories: (i) plastically deformed single crystal region; (ii) dislocation arrays with associated impurity atmosphere; and (iii) crystallographic structural line boundaries including twins and grain boundaries. The recombination processes described by category (ii) include a set of shallow electron donor traps near the dislocation core with apparent activation energies E_c − E_t = 0.066−0.087eV. The recombination lifetime of category (iii) can be described by Shockley-Read-Hall statistics with a recombination level located at the lower half of the band gap. Under the measurement condition, the activation energies of the acceptor-like center in category (iii) were found to be E_t − E_v = 0.086−0.114 eV. The recombination properties of category (i) and (ii) defects are consistent with the presence of compensating donor states in the p-type ribbon. Thus, it is believed that these types of defects are primarily responsible for the observed minority carrier lifetime dependence on the photoexcitation level in the EFG ribbon silicon.     

Strain relaxation mechanisms in Si1−xGex layers grown by solid-phase epitaxy: Influence of the layer composition and growth temperature

The influence of the composition and growth temperature on the strain and defect structure of Si_1−xGe_x layers of 0.21≤x≤0.34 grown on (001) Si wafers by solid phase epitaxy is presented. The strain in the layers was measured by Raman spectroscopy and Rutherford backscattering spectrometry/channeling angular scans. The defects were analyzed using high resolution electron microscopy. Three different relaxation mechanisms have been identified and characterized. The first mechanism occurs at the layer-substrate interface of the samples by the introduction of isolated defects. It is found to be thermally activated with an activation energy of E_a=0.16 eV and a prefactor that depends on the Gecontent of the layer. This mechanism produces partial relaxation of the layers and hinders the growth of fully strained layers. The second relaxation mechanism emerges at a distance from the interface which depends on the stress in the crystallized portion of the layer. In this case, the strain relaxation is caused by stacking faults that nucleate when they are energetically feasible and propagate toward the surface of the sample during growth. At low growth temperatures, the defects are confined to the upper part of the epitaxial layers at a distance from the interface that agrees with the theoretical predictions based on the equilibrium critical layer thickness. The third relaxation mechanism is introduced at high growth temperature and is based on the gliding of the stacking faults toward the layer-substrate interface. As a result of this mechanism, the stress in the layers is reduced compared to the stress in the layers grown at lower temperatures and approaches the equilibrium value corresponding to the total layer thickness. This behavior indicates that the layers grown at low temperature, where the stacking faults are confined to the upper part, are to some extent metastable.     

Hopping polarization photoconductivity of silicon with the involvement of impurity pairs of groups III and V

The long-wavelength bands of absorption by the impurity pairs and photoconductivity in the microwave (8 mm) electric field with the impurity pulse photoexcitation are investigated for Si doped by B, Al, Ga, In, P, As, and Sb in concentrations 10¹⁶–10¹⁸ cm^?3. The correlation between the pair concentration and the emergence of a slow component of photoconductivity relaxation is ascertained. This component is associated with the polarization hopping photoconductivity that emerges due to the optical recharge of impurity states, namely, the ionization of isolated impurities, impurities in pairs, and dipoles (pairs of the majority and compensating impurity ions). The hopping transfer of ion charges during relaxation is analyzed. It is shown that the main contribution to the polarization photoconductivity at relatively low impurity concentrations is made by hopping transitions in impurity pairs; the contribution of hops with the involvement of isolated ions becomes dominant with increasing concentration.