Electrode distance dependence of photo-induced degradation in hydrogenated amorphous silicon

Electrode distance between cathode and anode is one of the important parameters for fabricating hydrogenated amorphous silicon (a-Si:H) using plasma-enhanced chemical-vapor-deposition system with parallel plate electrodes. In this work, we have investigated the relationship between electrode distance and stability of a resulting a-Si:H. The stability is improved with decreasing electrode distance. At shorter electrode distance, formation of higher silane-related-reactive species is suppressed by heating effect of gas molecules near the cathode due to a proximity to the heated anode. Using cathode heating method, the stability of a-Si:H is improved even at long electrode distances.     

Electrical properties of Cl incorporated hydrogenated amorphous silicon

The electrical properties of chlorine incorporated hydrogenated amorphous silicon films were studied. The samples were deposited with dichlosilane/silane mixtures. The conductivity of these chlorine-incorporated hydrogenated amorphous silicon films decreases with increasing dichlosilane to silane ratio. The Fermi level shifts toward the valence band with increasing chlorine content, resulting in a corresponding conductivity activation energy and room temperature conductivity decrease. However, the defect density and Urbach energy are noot significantly changed in films containing chlorine.     

Hydrogenated amorphous silicon films with significantly improved stability

A new regime of plasma-enhanced chemical-vapor deposition (PECVD), referred to as “uninterrupted growth/annealing” method, has been proposed for preparation of high-quality hydrogenated amorphous silicon (a-Si:H) films. By using this regime, the deposition process no longer needs to be interrupted, as done in the chemical annealing or layer by layer deposition, while the growing surface is continuously subjected to an enhanced annealing treatment with atomic hydrogen created in the hydrogen-diluted reactant gas mixture at a relatively high plasma power. The intensity of the hydrogen plasma treatment is controlled at such a level that the deposition conditions of the resultant films approach the threshold for microcrystal formation. In addition, a low level of B-compensation is used to adjust the position of the Fermi level close to the midgap. Under these conditions, we find that the stability and optoelectronic properties of a-Si:H films have been significantly improved.     

Crystalline silicon surface passivation by amorphous silicon carbide films

This article reviews the surface passivation of n- and p-type crystalline silicon by hydrogenated amorphous silicon carbide films, which provide surface recombination velocities in the range of 10 cm s⁻¹. Films are deposited by plasma-enhanced chemical vapor deposition from a silane/methane plasma. We determine the passivation quality measuring the injection level (Δn)-dependent lifetime (τ_eff(Δn)) by the quasi-steady-state photoconductance technique. We analyze the experimental τ_eff(Δn)-curves using a physical model based on an insulator/semiconductor structure and an automatic fitting routine to calculate physical parameters like the fundamental recombination velocities of electrons and holes and the fixed charge created in the film. In this way, we get a deeper insight into the effect of the deposition temperature, the gas flow ratio, the doping density of the substrate and the film thickness on surface passivation quality.     

Effect of hydrogen on stability of amorphous silicon thin films

This paper presents results of the investigation of hydrogen influence on the stability of low pressure chemical vapour deposition a-Si films. We measured boron- or phosphorus-doped films post-hydrogenated by ion implantation with different hydrogen doses. The dark conductivity after fast quenching and slow cooling and the isothermal relaxation were measured at different annealing temperatures. It was found that higher hydrogen concentration causes greater metastable changes but shorter relaxation time of defects.     

Steady-state and transient photoconductivity in hydrogenated amorphous silicon nitride films

Amorphous SiN_x:H films were prepared by the rf glow-discharge decomposition of ammonia/silane gas mixture with varying nitrogen content. The steady-state photoconductivity and its dependence on light intensity have been investigated in a-SiN_x:H as a function of temperature between 100 and 420 K. The electron drift mobility of a set of SiN_x:H samples has been determined from their steady-state photoconductivity and response time measurements. The results suggest that electron drift mobility of the samples was nearly unchanged for a low nitrogen content. Two samples containing lowest nitrogen showed higher photoconductivity than that of unalloyed sample within a temperature range including the room temperature.     

Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Current-voltage-temperature (I-V-T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I-V-T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J_sc and V_oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J_sc which improved by an order of magnitude. The treatment also affected the lower temperature I-V-T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.     

Process development of amorphous silicon/crystalline silicon solar cells

We have already investigated some crucial limiting process steps of the amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell technology and some specific characterization tools of the ultrathin amorphous material used in devices. In this work, we focus our attention particularlyon the technology of the ITO front contact fabrication, that also is used as an antireflective coating. It is pointed out that this layer acts as a barrier layer against the diffusion of metal during the annealing treatments of the front contact grid. The criteria of the selection of the metal to be used to obtain good performance of the grid and the deposition methods best suited to the purpose are shown. We were able to fabricate low temperature heterojunction solar cells based p-type Czochralski silicon, and a conversion efficiency of 14.7% on 3.8 cm² area was obtained without back surface field and texturization.     

Structural and optical studies on hot wire chemical vapour deposited hydrogenated silicon films at low substrate temperature

Thin films of hydrogenated silicon are deposited by hot wire chemical vapour deposition technique, as an alternative of plasma enhanced chemical vapour deposition technique. By varying the hydrogen and silane flow rate, we deposited the films ranging from pure amorphous to nanocrystallite-embedded amorphous in nature. In this paper we report extensively studied structural and optical properties of these films. It is observed that the rms bond angle deviation decreases with increase in hydrogen flow rate, which is an indication of improved order in the films. We discuss this under the light of breaking of weak Si-Si bonds and subsequent formation of strong Si-Si bonds and coverage of the growing surface by atomic hydrogen.     

Effect of electrical contact configuration on gap-states absorption spectra by photocurrent methods in hydrogenated amorphous silicon alloys

Sub-bandgap optical absorption in the low photon energy range (0.7–1.5 eV) has been measured on the same undoped hydrogenated amorphous silicon alloys, using both CPM and DBT techniques in gap-cell and Schottky barrier (direct and reverse bias) electrical contacts configurations. Significant differences in results, sometimes larger than those noticed in the literature, are observed between spectra. These discrepancies are interpreted in terms of photocurrent equations theory, density of gap states model and light-induced or Staebler–Wronski effect.     

Deposition of hydrogenated amorphous silicon (a-Si:H) films by hot-wire chemical vapor deposition (HW-CVD) method: Role of substrate temperature

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited from pure silane (SiH₄) using hot-wire chemical vapor deposition (HW-CVD) method. We have investigated the effect of substrate temperature on the structural, optical and electrical properties of these films. Deposition rates up to 15 Å s⁻¹ and photosensitivity 10⁶ were achieved for device quality material. Raman spectroscopic analysis showed the increase of Rayleigh scattering in the films with increase in substrate temperature. The full width at half maximum of TO peak (Γ_TO) and deviation in bond angle (Δθ) are found smaller than those obtained for P-CVD deposited a-Si:H films. The hydrogen content in the films was found <1 at% over the range of substrate temperature studied. However, the Tauc's optical band gap remains as high as 1.70 eV or much higher. The presence of microvoids in the films may be responsible for high value of band gap at low hydrogen content. A correlation between electrical and structural properties has been found. Finally, the photoconductivity degradation of optimized a-Si:H film under intense sunlight was also studied.     

Mechanisms of metastability in hydrogenated amorphous silicon

We survey theoretical approaches to understanding the diverse metastable behavior in hydrogenated amorphous silicon. We discuss a recently developed network-rebonding model involving bonding rearrangements of silicon and hydrogen atoms. Using tight-binding molecular dynamics we find non-radiative recombination can break weak silicon bonds with low activation energies, producing dangling bond–floating bond pairs. The transient floating bonds annihilate generating local hydrogen motion and leaving behind isolated dangling bonds. Charged defects are also observed. Major experimental features of metastability including electron-spin resonance, t^1/3 kinetics, dangling-bond H anti-correlation, and hysteretic annealing are explained. In the second part we focus on large metastable structural changes observed in a-Si:H. We find H atoms have a local metastability involving the flipping of the H to the backside of the Si–H bond that results in a local increase of strain and increase of dipole moments. This naturally explains the larger infrared absorption found after light soaking, and may be related to other large structural changes in the network. Directions for future research are surveyed.     

High-rate deposition of hydrogenated amorphous silicon films using inductively coupled silane plasma

Inductively coupled plasma (ICP) generated at 13.56 MHz has been employed for high-rate deposition of device-quality hydrogenated amorphous silicon (a-Si:H). It has been shown that an increase in the flow rate of a monosilane gas enhances the generation rate of deposition precursors, while the ion flux decreases and becomes saturated. The defect density reaches the minimum at a deposition rate of 2.3 nm/s. It has also been demonstrated that even at deposition rates around 4 nm/s, a-Si:H deposited at 150°C exhibits a subgap defect density lower than 6×10¹⁶ cm⁻³ after 12 h AM1 (100 mW/cm²) light soaking.     

Advances in amorphous silicon alloy cell and module technology

The low material cost and proven manufacturability of amorphous silicon (a-Si) alloy solar panels make them ideally suited for low-cost terrestrial application. A major challenge for the researchers has been how to improve the light-to-electricity conversion efeiciency. Extensive R & D efforts have resulted in a significant improvement in stable cell and module efficiencies with the achievement of 12.8% active-area cell efficiency and 10.4% module efficiency using a spectral-splitting triple-band gap, triple-cell approach. Further gains in efficiency are expected through an improved understanding of plasma chemistry and growth kinetics. In this paper, we shall discuss the progress in science and technology of a-Si alloy photovoltaics with special emphasis on the opportunities and the challenges that exist.     

Light-induced defects in hydrogenated amorphous silicon germanium alloys

A selected survey of the phenomenon of light-induced deep defect creation in the hydrogenated amorphous silicon–germanium is presented. First a general review of the early studies that established the key salient features of light-induced degradation in a-Si,Ge:H is given. This is followed by a discussion of a couple of complicating issues that have more recently come to light; namely, the possibility that charged defects play a more central role in the alloys, and that both Si and Ge metastable dangling bonds may be playing a significant role in the alloys with germanium fractions below 20 at%. Following this, the results of some recent studies are summarized that have been focusing on the details of degradation in the low Ge fraction alloys to gain insight into the fundamentals of degradation of amorphous silicon materials in general. This review concludes with an overall assessment of the level of our understanding of degradation in the a-Si,Ge:H alloys and where some key issues are still remaining to be resolved.     

Thermal conductivity of amorphous silicon thin films

The thermal conductivity of amorphous silicon thin films is determined by using the non-intrusive, in situ optical transmission measurement as well as by the 3ω method. The temperature dependence of the film complex refractive index is determined by spectroscopic ellipsometry. The acquired transmission signal is fitted with predictions obtained by coupling conductive heat transfer with multi-layer thin film optics in the optical transmission measurement. The results of the two independent methods are in close agreement.     

Medium-range order in amorphous silicon measured by fluctuation electron microscopy

Despite occasional experimental hints, medium-range structural order in covalently bonded amorphous semiconductors had largely escaped detection until the advent of fluctuation electron microscopy (FEM) in 1996. Using FEM, we find that every sample of amorphous silicon and germanium we have investigated, regardless of deposition method or hydrogen content, is rich in medium-range order. The paracrystalline structural model, which consists of small, topologically ordered grains in an amorphous matrix, is consistent with the FEM data, and is rendered diffraction amorphous by strain effects. We present measurements on hydrogenated amorphous silicon deposited by different methods, some of which are reported to have greater stability against the Staebler–Wronski effect. The matrix material of these samples is relatively similar, but the order changes in different ways upon both light soaking and thermal annealing. Some materials are inhomogeneous, with either nanocrystalline inclusions or large area-to-area variation in the medium-range order. We discuss the implications of and future directions for understanding medium-range order.     

High quality amorphous silicon materials and cells grown with hydrogen dilution

Hydrogen dilution of the active gas during deposition has been found to be a very effective way to improve the quality of amorphous silicon-based materials and solar cells. With increasing hydrogen dilution, the material is characterized by an improved order, and at a certain threshold dilution, the amorphous to microcrystalline transition takes place. The best material is grown just below the threshold and is heterogeneous consisting of tiny crystallites embedded in an amorphous matrix of improved order. In this paper, we discuss the effects of hydrogen dilution on the material and cell properties of amorphous silicon-based alloys and provide an explanation for their improved stability against light-induced degradation. We also discuss some special properties of the on-the-edge materials that are not seen in the conventional amorphous or microcrystalline alloys.     

Substrate temperature and hydrogen dilution: parameters for amorphous to microcrystalline phase transition in silicon thin films

Amorphous to microcrystalline phase transition in hydrogenated silicon (Si:H) is realized separately with the variations of substrate temperature and hydrogen dilution. The Raman spectroscopy reveals structural transformations and marks the transition. It occurs at 450°C with 10% silane concentration, whereas that is noted at 250°C with a silane concentration of 4.5%. The material evolved in the transition region is a well-developed amorphous matrix containing a small fraction (12%) of crystallites. A uniform distribution of small (100 Å) crystallites in the films is observed by transmission electron microscopy. The transition material is photosensitive.     

Tin induced crystallisation of hydrogenated amorphous silicon thin films

Abstract

The present article focuses on the effect of annealing temperatures about the Sn induced crystallisation of hydrogenated amorphous Si (a-Si:H) thin films, which are used to fabricate polycrystalline Si (poly-Si) film. The a-Si:H thin films are coated onto Sn metal thin film and subsequently annealed from various temperatures. These are crystallised by annealing for 1 h at 300°C and identified by XRD spectroscopy for the investigation of each phase. Process temperature for crystallisation should not be high because a eutectic temperature of Si–Sn is ～232°C. Si crystal patterns of the prepared samples showed the tendency of changing from (111) to (002) with increasing temperature. It indicates that the crystal phases depend strongly on the annealing temperature of Sn induced a-Si:H thin films for the preparation of poly-Si film. Semiconducting type of Sn induced poly-Si films were shown in n-type through Hall effect measurement.