Correlation between structure and photoluminescence in amorphous hydrogenated silicon nitride alloys

As-deposited a-SiN_x:H (0.1<x<0.9) thin films prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions exhibit visible photoluminescence at room temperature without any posttreatment. The nitrogen concentration was determined by X-ray photoemission spectroscopy. The structural characterization was performed with Fourier transform infrared absorption spectroscopy. The optical gap was obtained from transmission measurements in the ultraviolet–visible–near infrared range. These studies were correlated to the evolution of the photoluminescence properties.     

Dispersive processes of light-induced defect creation in hydrogenated amorphous silicon

The growing curve of light-induced dangling bonds under illumination has been observed for various intensities of illumination in a-Si:H. It is fitted to a stretched exponential function and then two parameters β and τ involved in the function are estimated as a function of saturated dangling bond density . The experimental values of β, τ, and are compared with those calculated based on our model of light-induced defect creation in a-Si:H.     

退火处理对掺铒氢化非晶硅悬挂键密度和光致荧光的影响

采用磁控溅射技术制备了铒掺杂的氢化非晶硅(a-Si∶H(Er))样品.进一步通过200—500℃温度递增的后退火处理，获得了不同的Si悬挂键(Si-DBs)密度，并在此基础上研究了Si-DBs密度改变对其Er光荧光(Er-PL)的影响.退火温度低于350℃时，Er-PL强度持续增加，但Si-DBs密度的变化显得较复杂；350℃以上时，Er-PL强度随Si-DBs密度的增加而减小.在200—250℃的退火温度范围内，Si-DBs是由于结构弛豫而减少；在250—500℃的退火温度范围内，可能由于Si—H键的断 关键词： 氢化非晶硅 铒掺杂 Si悬挂键 光荧光     

Large supercell molecular dynamics study of defect formation in hydrogenated amorphous silicon

We have performed molecular dynamics simulations of the defect formation associated with the Staebler-Wronski (SW) effect in a-Si:H using 224 and 231 atom supercells and employing semiempirical Si-Si and Si-H total energy functionals. The role of hydrogen in the defect formation within the bond breaking model of the SW effect has been investigated for both large supercells. The results suggest that, within this model, H can be important in weakening the normal Si-Si bonds which break to produce defects in the SW effect.     

Nitrogen doping in hydrogenated amorphous silicon

A direct evidence of substitutional doping in ion beam deposited amorphous hydrogenated silicon by nitrogen is presented. From the analysis of infrared (IR) absorption spectra and Si-2p core level shape, measured with X-ray photoelectron spectroscopy (XPS), the preferential tendency of nitrogen to go in for three-fold coordination at higher concentration and tetrahedral bonding at lower concentration (⩽4 at %) is established. XPS technique has been used for the first time to deduce the upper limit for substitutional solid solubility of the impurity.     

Persistent photoconductivity in hydrogenated amorphous silicon

The persistent photoconductivity (PPC) has been observed in undoped, phosphorus-doped, and boron-doped hydrogenated amorphous silicon (a-Si: H) films. As the annealing temperature is raised for these films the decay of residual conductivity is accelerated and the PPC disappears almost completely after annealing at 500°C. These experimental results rule out the mechanism requiring the phosphorus-boron complexes or the deep defects. The PPC is found to be related with the sample inhomogeneity from the experimental observation that the decay of residual conductivity is closely correlated with the microstructure. A model is proposed to explain the PPC in a-Si: H films.     

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si?:?H) and amorphous As₂Se₃ (a-As₂Se₃) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As₂Se₃ than for a-Si?:?H has been observed and this is interpreted in terms of the higher structural flexibility of a-As₂Se₃. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.     

Nanoindentation of hydrogenated amorphous silicon

Nanoindentation was carried out on thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition. The composite values of elastic (Young's) modulus, E _c, and hardness, H _c, of the film/substrate system were evaluated from the load–displacement curves using the Oliver–Pharr approach. The film-only parameters were obtained employing the extrapolation of the depth profiles of E _c and H _c. Scanning probe microscopy was employed to image the nanoindenter impressions and to estimate the effect of film roughness and material pile-up on the testing results. It was established that the elastic modulus of thin a-Si:H films is in the range 117–131 GPa, which is lower than for crystalline silicon. In contrast, the values of hardness are in the range 12.2–12.7 GPa, which is comparable to crystalline silicon and higher than for hydrogen-free amorphous silicon. It is suggested that the plastic deformation of a-Si:H proceeds through plastic flow and it is the presence of hydrogen in the amorphous matrix that leads to a higher hardness.     

Extended-state mobility in hydrogenated amorphous silicon

We use the results of time-of-flight experiments in conjunction with recent conclusions about the behavior of the density of localized states below the conduction-band mobility edge to calculate the mobility of electrons moving in extended states in a-Si:H. We find that the extended-state mobility is considerably larger than previous estimates, which were based on the assumption that the exponential behavior responsible for dispersive transport extends all the way to the mobility edge. Using a recent estimate for the density of localized states, we find that the extended state mobility in a-Si:H is about 500 cm²/V-s, a value consistent with the results deduced from high-level injection experiments on p-i-n structures.     

Radiative recombination in hydrogenated amorphous silicon

Hydrogenated amorphous silicon exhibits efficient optical transitions across a gap larger than that of crystalline Si. Hydrogen passivates the dangling bonds and endows the material with a reduced number of non-radiative recombination centers. A gap widening has been observed in other hydrogenated semiconductors.Research reported herein was supported by the Department of Energy, Division of Solar Technology, under Contract No. EY-76-C-03-1286 and by RCA Laboratories, Princeton, NJ 08540.     

Optically induced changes in the sub-band gap absorption of hydrogenated amorphous silicon

Measurements of infrared absorption in thin (120nm) films of amorphous hydrogenated silicon subjected to light soaking were performed by Fourier transform infrared spectroscopy. Light exposure increased absorption in the region 0.3 to 0.4 eV but reduced it between 0.40 and 0.43 eV. These effects, which were room temperature annealable, indicate that at least two kinds of electronic states are involved.     

Alternating space-charge-limited currents in hydrogenated amorphous silicon

Space-charge-limited currents in n⁺-i-n⁺ sandwich samples of hydrogenated amorphous silicon have been investigated under alternating current (AC) conditions. When the voltage sweeping rate exceeds the release rate of carriers from deep traps, the AC current-voltage characteristics differ markedly from the direct current (DC) characteristics. This is attributed to the remaining space charge in the samples which acts as a potential barrier and reduces the current. A simple model is proposed to describe the time-dependence of this injection-induced barrier.     

Dynamics of hydrogen in hydrogenated amorphous silicon

The problem of hydrogen diffusion in hydrogenated amorphous silicon (a-Si:H) is studied semiclassically. It is found that the local hydrogen concentration fluctuations-induced extra potential wells, if intense enough, lead to the localized electronic states in a-Si:H. These localized states are metastable. The trapping of electrons and holes in these states leads to the electrical degradation of the material. These states also act as recombination centers for photo-generated carriers (electrons and holes) which in turn may excite a hydrogen atom from a nearby Si-H bond and breaks the weak (strained) Si-Si bond thereby apparently enhancing the hydrogen diffusion and increasing the light-induced dangling bonds.     

Anomalous surface photoconductivity in hydrogenated amorphous silicon

The intensity and time dependence of the source-drain photocurrent is measured on hydrogenated amorphous silicon (a-Si:H) field effect transistors as a function of gate bias. The photocurrent increases rapidly, becomes weakly dependent on excitation intensity, and exhibits long decay times. A model in which the relaxation of the space charge dominates the photocurrent quantitatively predicts the experimental data. The results show that anomalous surface photoconductivity is often the dominant photoresponse of a-Si:H.