Numerical simulation of a low- and a high-electric-field photocurrent decay in a-Si:H

We report a numerical simulation of the photocurrent decay (PCD), from the steady state, in two different structure configurations based on the a-Si:H. The standard DOS of the a-Si:H is used. The high-electric-field PCD is considered in a structure configuration based on a metal/a-Si:H junction. Poisson’s and the two continuity equations are numerically solved in a one-dimensional space to calculate the current density. Two different boundary conditions of the a-Si:H film are considered. The low electric field PCD, which may occurs in a coplanar configuration, is calculated from the solution of a system of two non linear coupled rate equations which govern the free carriers concentrations and the different charges on the localized states in the gap. The calculated PCD versus time curves, for the two configurations, show a shoulder around 1 μs which separate two main regions. We can see that the initial current decay is dominated by the electron emission from the conduction-band tail and the recombination via the dangling bonds states. The second current decay is mainly due to the electrons emission from the dangling bonds. We show also that the PCD curve tends towards the PCD of the coplanar configuration when the electric field decreases.     

The interpretation of the electric and optical properties of a-Si:H films produced by rf glow discharge through dark conductivity,photoconductivity and pulse controlled capacitance-voltage measurements

This paper deals with the interpretation of transport properties of amorphous silicon hydrogenated films (a-Si:H) through dark conductivity, photoconductivity and pulse controlled capacitance-voltage measurements. a-Si:H films were produced by rf glow discharge coupled either inductively or capacitively to a 3% SiH₄/Ar mixture at different crossed electromagnetic static fields. The data concerned with the dark activation energy, photoactivation energy, variation of the density of localized states and photosensitivity, (σ_ph/σ_d)_25°C, of a-Si:H films can account for their optoelectronic properties which are strongly dependent on the deposition parameters. We also observed that crossed electromagnetic static fields applied during film formation influences hydrogen incorporation in a different manner than previously proposed.     

Recombination and transport through localized states in hydrogenated amorphous and microcrystalline silicon

Electrical transport and recombination mechanisms in hydrogenated amorphous silicon, a-Si:H, are determined by localized band-tail states and deep defects. At low temperatures (T < 100 K) the photoluminescence originates from tunneling recombination between localized band-tail states and the photoconductivity arises from hopping in the band tail. This review describes the present understanding of transport and recombination mechanisms in this low-temperature regime with a focus on two aspects: (i) the kinetics of carrier recombination and the competition between geminate and non-geminate recombination, and (ii) the microscopic identification of recombination paths by magnetic resonance techniques and the proof of excitonic recombination. Inspite of its complex nanocrystalline morphology, hydrogenated microcrystalline silicon, μc-Si:H, behaves in many respects similarly to a-Si:H in that the low-temperature properties are also determined by disorder-induced localized band-tail states.     

Photoconductivity and photoluminescence of a-Si:H at low temperature

We have studied the temperature dependence of the photoluminescence steady state and transient photoconductivity of a-Si:H down to 4 K. Below 50 K we observe photoconductivity with $\text{ημτ ? 10}{}^{\mathrm{?11}}\text{cm}{}^2\text{/}\text{V}$, which is independent of temperature and varies only little with the defect density in the films. We propose that this conduction arises predominantly from the drift of the photoexcited electrons and holes during thermalization in the extended states prior to the localization in states below the mobility edge. An important implication of the present data is that, even at helium temperatures, a considerable part of the carriers recombines in a non-geminate process.     

Study of anomalous behavior of steady state photoconductivity in highly crystallized undoped microcrystalline Si films

The photoconductivity (σ_ph) of highly crystallized dense undoped hydrogenated microcrystalline silicon (μc-Si:H) films was measured as a function of light illumination over a wide temperature range (∼15–325 K). A thermal quenching behavior in σ_ph was observed at ∼240 K. The photoconductivity exponent (γ) was found to be sublinear with γ as low as 0.13. A density of states (DOS) profile having a steep conduction band tail, and valence band tail with two distinct distributions was found to be necessary to understand the electronic transport behavior in the inherently heterogeneous μc-Si:H films.     

Local bonding of hydrogen in a-Si:H,a-Ge:H and a-Si,Ge:H alloy films

This paper reviews the local bonding of hydrogen (and deuterium) in a-Si:H(D), a-Ge:H(D) and a-Si, Ge:H(D) alloy films. We specify the types of atomic environments that have been identified through vibrational spectroscopy, primarily infrared (IR) absorption. We emphasize local modes and discuss the atomic motions that are responsible for the various spectral features. We discuss correlations between the occurrence of specific local bonding groups, e.g., polysilane and polygermane configurations, and the deposition techniques and parameters, including the substrate temperature (T_s), the gas mixtures and the RF power into a glow discharge, etc. We include a discussion of the theoretical approaches that have been used to treat vibrational modes in these materials. We emphasize the approximations that are valid because of the relatively light mass of the hydrogen and deuterium atoms compared with those of the silicon and germanium atoms. Finally we highlight the effects of neighboring alloy or impurity atoms on the frequencies of hydrogen vibrations in a-Si:H, and point out the differences between oxygen atom incorporation in a-Si and a-Ge alloys.     

Hole mobilities and the physics of amorphous silicon solar cells

The effects of low hole mobilities in the intrinsic layer of pin solar cells are illustrated using general computer modeling; in these models electron mobilities are assumed to be much larger than hole values. The models reveal that a low hole mobility can be the most important photocarrier transport parameter in determining the output power of the cell, and that the effects of recombination parameters are much weaker. Recent hole drift-mobility measurements in a-Si:H are compared. While hole drift mobilities in intrinsic a-Si:H are now up to tenfold larger than two decades ago, even with recent materials a-Si:H cells are low-mobility cells. Computer modeling of solar cells with parameters that are consistent with drift-mobility measurements give a good account for the published initial power output of cells from United Solar Ovonic Corp.; deep levels (dangling bonds) in the intrinsic layer were not included in this calculation. Light-soaking creates a sufficient density of dangling bonds to lower the power from cells below the mobility limit, but in contemporary a-Si:H solar cells degradation is not large. We discuss the speculation that light-soaking is ‘self-limiting’ in such cells.     

Gap states in a-Si:H by photoconductivity and absorption

The spectral dependence of photoconductivity in a series of a-Si:H samples has been analyzed in order to derive the absorption coefficient α. It has been found that the exponent β which relates photoconductivity and generation rate varies with photon energy and chopping frequency. Its critical influence upon the value of α is established. The Urbach tail in the region 1.4–1.8 eV has been studied and found to be independent of extrinsic gap states and hydrogen content. Finally an inverse correlation has been determined between the photoconductivity lifetime and the extrinsic gap-state density, as measured from the corresponding integrated area of the absorption coefficient.     

热丝辅助MW ECR CVD技术高速沉积高质量氢化非晶硅薄膜

氢化非晶硅薄膜具有优异的光电特性,在制备薄膜太阳能电池中有重要的应用.本文采用热丝辅助MWECR CVD技术,通过调整各种工艺参数,制备了高沉积速率(DR＞2.5nm/s)及高光敏性(σph/σD＞105)的氢化非晶硅薄膜.实验表明,在衬底表面温度的分布中,热丝辐射和离子轰击引起的温度对薄膜的光敏性影响较大;在薄膜沉积的最后几分钟适当加大H2稀释率,有利于薄膜光电特性的改善.     

Ellipsometry characterization of a-Si:H layers for thin-film solar cells

In order to determine microscopic structures of hydrogenated amorphous silicon (a-Si:H) layers incorporated in a-Si:H-based thin-film solar cells, the spectroscopic ellipsometry (SE) analysis of a-Si:H layers prepared by plasma-enhanced chemical vapor deposition has been performed. In particular, we have characterized the a-Si:H layers by applying a new dielectric function model that allows the evaluation of the SiH₂ bond densities in a-Si:H networks. This model is based on our finding that the a-Si:H dielectric functions in the visible/ultraviolet region vary systematically with the formation of SiH₂-clustered microvoids. We have applied this model to estimate the SiH₂ content in a-Si:H layers fabricated on glass substrates, on which the characterization of the SiH₂ bonding is generally difficult. The validity of the SE analysis has been confirmed from the direct characterization of the SiH_n local structures using infrared ellipsometry.     

Temporal electric conductivity variations of hydrogenated amorphous silicon due to high energy protons

Electric conductivity variations of undoped, n-type and p-type hydrogenated amorphous silicon (a-Si:H) thin films irradiated with various energy protons are systematically investigated. Dark conductivity (DC) and photoconductivity (PC) of the undoped samples increase at first due to proton irradiation and then decrease dramatically with increasing proton fluence. The increase in DC and PC becomes greater with increased proton energy. However, this increase is metastable and gradually decreases with time at room temperature. Similar results are observed in the n-type a-Si:H, whereas only a monotonic decrease is observed in DC and PC for the p-type samples. The increase of both DC and PC due to proton irradiation is attributed to metastable donor center generation. On further irradiation both the DC and PC decrease by the accumulation of radiation-induced defects, which act as deep traps and compensate carriers. The decrease in DC and PC becomes less pronounced as the proton energy increase and can be fitted along a universal line when the proton fluence is converted into displacement per atom (dpa).     

Thirty years trajectory of amorphous and nanocrystalline silicon materials and their optoelectronic devices

A review is given on research trajectory of hydrogenated amorphous and nanocrystalline silicon (a-Si:H and nc-Si) materials with their device applications ongoing since the period of 1970. A brief explanation on the motivation to start amorphous semiconductors research is given to produce a new kind of synthetic semiconductor having continuous energy gap controllability with valency electron controllability. Due to the result of some basic research on the film quality improvement of a-Si:H and nc-Si, some innovative devices had been developed since middle of 1980s in R&D phase such as a-SiC/a-Si heterojunction solar cells, a-Si/a-SiGe and also a-Si/nc-Si tandem type solar cells. Finally, the state of the art on the industrialization of the new devices is introduced and discussed.     

Transient photoconductivity studies of band tails states in compensated a-Si:H

The density of conduction band tail states has been determined for a series of compensated a-Si:H films using transient photoconductivity measurements. Relative to undoped material, the energy width of shallow tail states lying within 0.45 eV of the conduction band edge are found to be insensitive to the level of compensated doping for doping levels up to 1000 vppm. An observed reduction in photocurrent magnitude as the doping is increased is consistent with a corresponding reduction in the electron extended state mobility. The magnitude of the mobility reduction is found to be quantitatively consistent with potential fluctuations which arise from ionized dopants for compensated doping levels up to about 100 vppm.     

Metastability effects in hydrogenated microcrystalline silicon thin films investigated by the dual beam photoconductivity method

Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σ_D, photoconductivity, σ_ph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O₂ gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied.     

Network structure and dynamics of hydrogenated amorphous silicon

In this paper we discuss the application of current ab initio computer simulation techniques to hydrogenated amorphous silicon (a-Si:H). We begin by discussing thermal fluctuation in the number of coordination defects in the material, and its temperature dependence. We connect this to the ‘fluctuating bond-center detachment’ mechanism for liberating H bonded to Si atoms. Next, from extended thermal MD simulation, we illustrate various mechanisms of H motion. The dynamics of the lattice is then linked to the electrons, and we point out that the squared electron-lattice coupling (and the thermally-induced mean square variation in electron energy eigenvalues) is robustly proportional to the localization of the conjugate state, if localization is measured with inverse participation ratio. Finally we discuss the Staebler–Wronski effect using these methods, and argue that a sophisticated local heating picture (based upon reasonable calculations of the electron-lattice coupling and molecular dynamic simulation) explains significant aspects of the phenomenon.     

Influence of the amorphous/crystalline silicon heterostructure properties on planar conductance measurements

We report a quasi-analytical calculation describing the heterojunction between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) at equilibrium. It has been developed and used to determine the carrier sheet density in the strongly inverted layer at the a-Si:H/ c-Si interface. The model assumes an exponential band tail for the defect distribution in a-Si:H. The effects of the different parameters involved in the calculation are investigated in detail, such as the Fermi level position in a-Si:H, the density of states and the band offsets. The calculation was used to interpret temperature dependent planar conductance measurements carried out on (n) a-Si:H/ (p) c-Si and (p) a-Si:H/(n) c-Si structures, which allowed us to confirm a previous evaluation of the conduction band offset, ?E_C = 0.18 ± 0.05 eV, and to evaluate the valence band offset: ?E_V = 0.36 ± 0.05 eV at the a-Si:H/ c-Si heterojunction. The results are placed in the frame of recent publications.     

The nanostructural analysis of hydrogenated silicon films based on positron annihilation studies

Due to the complex nature of hydrogenated amorphous and microcrystalline silicon (a-Si:H; μc-Si:H) a profound understanding of the Si:H nanostructure and its relation to the Staebler–Wronski effect (SWE) is still lacking. In order to gain more insight into the nanostructure we present a detailed study on a set of Si:H samples with a wide variety of nanostructural properties, including dense up to porous films and amorphous up to highly crystalline films, using Doppler broadening positron annihilation spectroscopy (DB-PAS) and Fourier Transform infrared (FTIR) spectroscopy. The results obtained from these material characterisation techniques show that they are powerful complementary methods in the analysis of the Si:H nanostructure. Both techniques indicate that the dominant type of open volume deficiency in device grade a-Si:H seems to be the divacancy, which is in line with earlier positron annihilation lifetime spectroscopy (PALS), Doppler broadening (DB) PAS and FTIR studies.     

Properties of a-Si:H prepared by the photochemical decomposition of Si2H6

High quality a-Si:H films have been prepared by the direct photolysis of disilane at a substrate temperature below 350 °C. The growth rate is independent of substrate temperature for both undoped and phosphorus doped films, while it is thermally activated and dramatically enhanced by boron doping. The hydrogen content decreases from 7 to 2 at.%, as deposition temperature is varied from 200 to 300 °C. The photoconductivity as high as 3.7 × 10^?4 Ω^?1cm^?1 (AM1 100 mW/cm²) has been obtained and no light soak degradation was observed.     

Modulated photocurrent as a powerful method to reveal predominant transport by the majority carriers of disordered semiconductors and to resolve all the kinds of probed gap states

A basic difficulty in the interpretation of photoconductivity measurements may arise from possible mixed contributions of both carriers to the photocurrent. In this work it is demonstrated that a universal behavior in the simulated spectra of the out of phase modulated photocurrent signal is observed in cases where the majority carriers dominate. In these cases, a general formula can be used to evaluate the densities of various species of states using the data from all frequencies. Deviations from the universal behavior can be observed when there are contributions from both carriers. The applicability of our analysis is demonstrated in a-Si:H.     

Experimental evidence for extended hydrogen diffusion in silicon thin films during light-soaking

We have used mass spectrometry to detect hydrogen effusing from silicon thin films exposed to light. Our results indicate a long range diffusion of hydrogen through the whole film, which ends with its release into the vacuum system. The changes in the film properties are characterized by dark and photoconductivity and hydrogen exodiffusion measurements. From the evolution of dark conductivity measurements after turning off the light, we show that this long range motion of hydrogen is not due to the heating of the sample. A comparison of hydrogen exodiffusion spectra of as-deposited and light-soaked samples shows that the weakly-bonded hydrogen content decreases by 30% for a-Si:H films and that the tightly-bonded hydrogen migrates to grain boundaries of crystalline regions in the case of pm-Si:H films. These results clearly demonstrate the long range motion of hydrogen during light soaking.