Polarization photosensitivity of a-Si:H/c-Si heterojunctions

The photosensitivity of heterojunctions formed by depositing thin amorphous films on silicon crystalline substrates is investigated. It is found that heterojunctions exhibit polarization photosensitivity, which is observed at an oblique incidence of the linearly polarized radiation on their receiving plane. The induced photopleochroism of heterojunctions increases quadratically with an increase in the angle of incidence θ and reaches 60% for θ=80°. It is concluded that the heterojunctions obtained can be used as the broadband photodetectors of the linearly polarized radiation.     

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.     

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.     

Anodic oxidation of a-Si:H films

It is shown that thick native oxide layers (up to 2500 ?) can be grown on a-Si:H films at room temperature by anodic oxidation. Ellipsometric measurements indicate uniformity of the oxide layer with a refractive index of 1.46. The voltage growth rate is found to be 5.3 ?/V.     

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)     

Hopping ε2 conductivity of boron-doped a-Si:H films annealed in hydrogen at high temperature

It is shown that the high-temperature annealing in hydrogen flow results in substantial modification of the temperature dependence of the conductivity of boron-doped and undoped a-Si:H films. For doped films subjected to annealing, the ε₂ conductivity related to hopping between localized states near the valence-band edge appears in the intermediate temperature range, along with the contributions of the band conductivity and the variable-range hopping conductivity. The ε₂ conductivity becomes possible due to an increase in the concentration of electrically active boron atoms and to the appreciable shift of the Fermi level after high-temperature annealing of doped films in hydrogen atmosphere. The experimentally measured parameters of the ε₂ conductivity make it possible to determine the width of the energy region, in which the density of localized states near the valence band falls off nonexponentially.     

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)     

Silicon network in a-Si:H films containing ordered inclusions

a-Si:H films with inclusions of (SiH₂)_n clusters or Si nanocrystals have been grown by magnetron-assisted SiH₄ decomposition (dc-MASD). The films were characterized by the microstructural parameter R=0.7–1.0. Ultrasoft X-ray emission spectroscopy was applied to establish the effect of these inclusions on the increasing ordering of Si network. It is shown that, irrespective of the nature of the inclusions, their effect is strongest for films of intrinsic material deposited at high temperatures (up to 400°C).     

Study of light-induced annealing effects in a-Si:H thin films

Light-induced metastability was examined in hydrogenated amorphous silicon thin films using a 500 W xenon lamp and a 10 mW HeCd laser. Positron beam annihilation spectroscopy (PAS) and fourier transform infrared spectroscopy (FTIR) were examined to investigate the effects of light on the structural properties of the films. The experimental results exhibited significant decrease in the S-parameter of the PAS, indicating marked reduction in the defect density of the films. The FTIR spectroscopy showed significant reduction in the transmission coefficient of IR radiation at frequencies corresponding to Si–H and Si–H₃ phonon modes, indicating that the observed annealing effects were due to light-induced formation of Si–H and Si–H₃ bonds. A second thermal annealing process conducted after the light exposure experiment resulted in a further substantial decrease in defect density for the sample exposed to HeCd laser. The experimental results are explained by a competing, light induced, dangling bond creation/annealing process, in which the incoming photons caused the annealing of dangling bonds, particularly those at around the voids. However, in the bulk region, the photons caused both the breaking of weak Si–Si bonds as well as the annealing of dangling bonds.     

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...     

Defects in a-Si:H films induced by Si ion implantation

Undoped a-Si:H films implanted with silicon ions (dose 10¹²–10¹⁴ cm⁻², mean energy ɛ=60 keV) at room temperature have been studied. The following results of the interaction of such films with ion beams have been established: formation of defects (dangling Si-Si bonds) in the neutral state (D ⁰), change in the charge state D ⁰→D ⁻, the a-Si:H→a-Si transition, and growth of inhomogeneity of the structure. It is shown that these effects depend on the initial structures and electronic characteristics of the films. Fiz. Tekh. Poluprovodn. 33, 464–468 (April 1999)     

Structure and properties of a-Si:H films grown by cyclic deposition

Amorphous hydrogenated silicon films obtained by cyclic deposition with intermediate annealing in hydrogen plasma were studied. a-Si:H films deposited under optimal conditions are photosensitive (photoconductivity to dark conductivity ratio σ_ph/σ_d is as high as 10⁷ under 20 mW cm^?2 illumination in the visible region of the spectrum) and have an optical gap (E _g ) and activation energy of conductivity (E _a ) of 1.85 and 0.91 eV, respectively. Electron microscopy studies revealed a clearly pronounced layered structure of a-Si:H films and the presence of nanocrystalline inclusions in the amorphous matrix.     

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.     

High field-effect-mobility a-Si:H TFT based on high deposition-ratePECVD materials

The hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFT's) having a field-effect mobility of 1.45 ±0.05 cm² /V·s and threshold voltage of 2.0±0.2 V have been fabricated from the high deposition-rate plasma-enhanced chemical vapor deposited (PECVD) materials. For this TFT, the deposition rates of a-Si:H and N-rich hydrogenated amorphous silicon nitride (a-SiN_1.5 :H) are about 50 and 190 nm/min, respectively. The TFT has a very high ON/OFF-current ratio (of more than 10⁷), sharp subthreshold slope (0.3±0.03 V/decade), and very low source-drain current activation energy (50±5 meV). All these parameters are consistent with a high mobility value obtained for our a-Si:H TFT structures. To our best knowledge, this is the highest field-effect mobility ever reported for an a-Si:H TFT fabricated from high deposition-rate PECVD materials     

气体压强对非晶硅薄膜光学特性的影响

采用RF-PECVD工艺在20～100 Pa范围内改变a-Si:H薄膜沉积工作气体压强.傅立叶红外光谱仪(FTIR)测试KBr衬底上的薄膜红外光谱峰随工作气体压强的变化情况,结合红外光谱峰的理论分析确定薄膜中氢含量随工作气体压强变化规律.光谱式椭偏仪中用Forouhi Bloomer(FB) 模型拟合得到薄膜的折射率(n),消光系数(k),厚度及光学禁带Eg,并用扫描电镜(SEM)断面分析对椭偏仪测试结果的准确性进行验证.根据Tauc公式推出薄膜的光学禁带宽Eg和截止波长,并和FB模型得到结果进行了比较,Eg(FB)和Eg(Tauc)的差值在11 meV内.原子力显微镜(AFM)和X射线衍射(XRD)表征了工作气体压强对a-Si:H薄膜微结构的影响.     

a-SiH 热处理过程中纳米硅粒尺寸的控制(英文)

报道了控制热处理过程中含氢非晶硅中纳米硅颗粒大小的一种新方法。用喇曼散射、X射线衍射和计算机模拟,发现在非晶硅中所形成的纳米硅颗粒的大小,随着热退火过程中升温速率的变化而变化。在退火过程中,若非晶硅薄膜升温速率较高(～100℃/s),则所形成纳米硅粒的大小在1.6～15nm;若非晶硅薄膜升温速率较低(～1℃/s),则纳米硅粒大小在23～46nm。根据晶体生长理论,讨论了升温速率的高低与所形成的纳米硅颗粒大小的关系。     

Nanostructured a-Si:H films obtained by silane decomposition in a magnetron chamber

Nanostructured a-Si:H films grown by the MASD method at various deposition temperatures (T _s =300–390°C) were studied. Among these films, those “on the verge of crystallinity” are of particular interest, because they tend to crystallize. In addition, although their electron-transport parameters are slightly inferior to those of conventional device-grade a-Si:H, they are preferable because of the higher stability of their photoconductivity under exposure to light.     

a-Si:H热处理过程中形成的纳米硅粒及其光致发光

报道了氢化非晶硅薄膜在600～620℃温度下快速退火10 s可以形成纳米晶硅,其拉曼散射表明,所形成的纳米晶硅在薄膜中的分布是随机的,其直径在1.6～15 nm内.根据晶体生长理论和计算机模拟,讨论了升温快慢与所形成纳米硅颗粒大小之间的关系,并且在强光照射下观察了纳米晶硅在薄膜中的结晶和生长情况.经退火形成的nc-Si可见光辐射较弱,不能检测到它们的光致发光,但用氢氟酸腐蚀钝化后则可检测到较强的红PL,并且钝化后的nc-Si在空气中暴露一定时间后,其辐射光波长产生了蓝移.就表面钝化和量子限制对可见光辐射的重要性作了讨论.     

On the quality of contacts in a-Si:H staggered electrode thin-film transistors

Amorphous silicon thin-film field-effect transistors have been made with a staggered electrode structure. In this structure we distinguish two separate contributions to the total contact resistance, namely, the Al/a-Si:H barrier itself and the bulk resistance of the underlying a-Si:H layer. Concerning the first contribution it was found that a P-implantation forming n⁺regions followed by post-metallization annealing (PMA) at a moderate temperature of 200°C is very efficient in reducing the resistance of the Al contacts to negligibly small values. The second contribution, i.e., the bulk resistance, implies a variable series resistance in field-effect (FE) measurements. Thin-film transistors (TFT's) with different gate lengths were used for the first time to determine this residual series resistance Rres.