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.     

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.     

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.     

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...     

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.     

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.     

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)     

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。     

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 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)     

磁约束铜蒸气放电中亚稳态铜原子密度的测量

利用自吸收方法测量了磁约束铜蒸气放电余辉初期的铜２Ｄ５／２亚稳态原子密度。对亚稳态原子的弛豫情况，同普通铜蒸气激光器进行了对比，分析了磁约束放电条件下亚稳态原子的弛豫机制。为建立重复频率运转的磁约束铜蒸气激光器提供了基础     

用光热偏转谱拟合计算研究非晶硅缺陷态及其稳定性

通过光热偏转谱（ＰＤＳ）研究了衬底温度对用超高频辉光放电（ＶＨＦ－ＧＤ）制备的ａ－Ｓｉ：Ｈ光学特性的影响，考虑了电子在缺陷态的相关统计，数字拟合各样品在不同亚稳状态下ＰＤＳ次带吸收谱（０．８￣１．７ｅＶ），获得带尾态，光能隙，缺陷态分布及相关能等电子态结构参数。结果表明，缺陷态分布随光照时间向能隙深处移动，相关能增加，在电中性条件下，用迭代法求出费米能级与电导率，与实验结果吻合较好，分析讨论所用计     

Aggregation of interstitial copper atoms in silicon

The formation energy of metastable Cu aggregates consisting of one to four interstitial Cu atoms is examined using ab initio electronic structure theory. Localized vibration energies are calculated for two aggregates. These are a substitutional and interstitial Cu pair and three interstitial Cu atoms. In the latter aggregate there is extensive atomic relaxation with a Si atom moving into a nearby interstitial position and leaving a Si vacancy and Si interstitial. There is no bond breaking. The relaxation stabilizes the complex to have trigonal symmetry with an almost exact tetrahedral geometry. The results highlight that several Cu slightly metastable aggregates may be possible in silicon having strong localized modes at similar energies.     

Numerical simulation of the effect of the free carrier motilities on light-soaked a-Si:H p-i-n solar cellL. Ayat1, A.F. Bouhdjar2, AF. Meftah2*, N. Sengouga2, AM. Meftah2

Using a previous model, which was developed to describe the light-induced creation of the defect density in the a-Si:H gap states, we present in this work a computer simulation of the a-Si:H p-i-n solar cell behavior under continuous illumination. We have considered the simple case of a monochromatic light beam nonuniformly absorbed. As a consequence of this light-absorption profile, the increase of the dangling bond density is assumed to be inhomogeneous over the intrinsic layer (i-layer). We investigate the internal variable profiles during illumination to understand in more detail the changes resulting from the light-induced degradation effect. Changes in the cell external parameters including the open circuit voltage, V_oc, the short circuit current density, J_sc, the fill factor, FF, and the maximum power density, P_max, are also presented. This shows, in addition, the free carrier mobility influence. The obtained results show that V_oc seems to be the less affected parameter by the light-induced increase of the dangling bond density. Moreover, its degradation is very weak-sensitive to the free carrier mobility. Finally, the free hole mobility effect is found to be more important than that of electrons in the improvement of the solar cell performance.     

闪锌矿GaN(110)表面原子和电子结构的理论计算

在密度泛函理论的基础上,采用平面波赝势方法计算了立方GaN(110)表面的原子和电子结构。结构优化表明最表层原子都向体内弛豫,且金属Ga原子弛豫幅度比非金属N原子大,同时各层层间距呈交错分布。表面弛豫后,最表层原子发生键长收缩的弛豫特性,表面Ga原子趋于形成sp2杂化得到的平面型构形,而表面N原子趋于形成p3型锥形结构。另外,理想立方GaN(110)表面在带隙中有两个明显的表面态,经过弛豫后,分别向价带和导带方向移动,并解释了导带底附近的表面态移动的幅度比价带顶附近的表面态大的原因主要由于表面Ga、N原子弛豫幅度不同引起的。此外,弛豫后,表面电荷重新分布,Ga原子周围的部分电子转移到N原子上。     

Kinetics of polarization current in a Pb<Subscript>3</Subscript>O<Subscript>4</Subscript> wide-gap photoconductor

The results of the study of isothermal polarization of Pb₃O₄ high-resistivity semiconductor layers in a dc electric field of various intensities with measurements without and with illumination are presented. To interpret the data obtained, the relay mechanism of charge transport over local states in the band gap is introduced. The values of the parameters describing the relaxation phenomena such as contact capacitance, width of the charge accumulation region, and effective charge mobility are obtained.     

Structural characterization of GaAs<Subscript>1−x</Subscript>Bi<Subscript>x</Subscript> alloy by rutherford backscattering spectrometry combined with the channeling technique

Rutherford backscattering spectrometry (RBS) combined with the channeling technique has been applied to a GaAs_1−xBi_x epilayer to investigate concentration and lattice location of Bi atoms and crystalline quality of the epilayer. The metastable GaAs_1−xBi_x alloy layer was grown on a GaAs substrate at a temperature as low as 365°C. The GaBi mole fraction obtained was 2.6 ± 0.2%. Angular scans for [100] and [111] crystal directions reveal that the incorporated Bi atoms exactly occupy substitutional sites in the GaAs crystal lattice. Crystal perfection of the GaAs_1−xBi_x metastable alloy is fairly good in spite of the low growth temperature.     

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.     

Metastable population of self-organized InAs/GaAs quantum dots

In the present work, we report on the investigation of a p-n heterostructure with InAs/GaAs quantum dots (QD) by capacitance-voltage and deep level transient spectroscopy. We have observed controllable and reversible metastable population of the energy states of quantum dots and interface in the structure containing one plane of InAs QDs as a function of temperature of isochronous annealing as well as under bias-on-bias-off cooling conditions and white light illumination. This effect was attributed to the change in the Fermi level position due to the hole capture on self-trapped defects similar to the DX center in GaAs after isochronous annealing and white light illumination.