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.     

Doping of microcrystalline Si:H,Cl films in R.F. glow discharge

The deposition rate of n-doped μc-Si:H,Cl GD films has been found to increase with the gas phase phosphine content, while the opposite behaviour has been observed for p-doped specimens. The optical gap ranges between 1.8 ÷ 2.6 eV and its variation seems related to hydrogen and chlorine contents. The effectiveness of the doping has been tested by electrical conductivity measurements.     

Transport properties of hydrogenated amorphous silicon deposited by dc glow discharge decomposition

Conductivity and thermoelectric power measurements have been made as a function of temperature on a series of hydrogenated amorphous silicon samples. The samples were prepared by the dc glow discharge decomposition of silane and silane phosphine mixtures. The activation energy for conduction varied with the substrate temperature and discharge condition for undoped specimens. The difference in the activation energy for conduction as well as the dependence of photoconductivity and optical gap on the activation energy for conduction among undoped specimens can be explained by introducing centers acting as donors or by change transfer between the island and hydrogen rich interfacial region. The kinks in the log σ versus inverse temperature curves always appear at about 430 K for the undoped specimens prepared at 300°C, while they are absent for low substrate temperature specimens. The downward kinks with increasing temperature can be explained by a two-phase material model. A revised two-channel conduction path model including material heterogeneity is applied to interpret the conductivity and thermopower versus inverse temperature curves of doped a-Si:H films, and to determine the position of phosphorus donor levels. The levels are found to lie at about 0.47 eV below E_c, the mobility edge at the conduction band.     

Optical and electrical properties of chlorinated and hydrogenated amorphous silicon prepared by glow discharge

Chlorinated and hydrogenated amorphous silicon films were prepared by glow discharge of a SiCl₄/H₂ mixture. Infrared spectra of these films show that, in addition to the hydrogen induced bands, two new modes appear at 545 cm^?1 (SiCl stretching) and 500 cm^?1 (Si TO modes induced by chlorine). Observation of the 545 cm^?1 band proves that chlorine is able to act as a dangling bond terminator in an amorphous silicon matrix. A good agreement is found between the total amount of chlorine determined by electron microprobe analysis and the value estimated from the integrated strength of the SiCl stretching mode. The relatively high value of the optical band gap (1.80 eV) of our material containing only 5 at.% bonded hydrogen shows that chlorine plays a major role in the optical gap value. Electrical conductivity, photoconductivity and luminescence properties are qualitatively similar to that of a: SiH films.     

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3 cm²/(V s) respectively at 300 K and a thickness to electric field ratio of 1.8 × 10⁻⁷ cm²/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13 eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.     

Preparation of microcrystalline silicon solar cells on microcrystalline silicon carbide window layers grown with HWCVD at low temperature

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7 mA/cm² was achieved with an absorber layer thickness of 1 μm, which led to a cell efficiency of 9.2%.     

Influence of boron doping on roughness microcrystalline silicon

We have studied roughness of boron-doped microcrystalline Si (μc-Si) surfaces with an emphasis on the influence of heavy doping. μc-Si films were prepared using plasma-enhanced chemical vapor deposition (PECVD) with different boron concentrations in gas phase from 0% to 2%. Growth-induced roughening of μc-Si surfaces was monitored ex situ using an atomic force microscope (AFM). With an increase in the deposition time, the surface width (rms roughness), w, of undoped μc-Si surface exhibited usual behaviors; first, (a) w increased, (b) slightly dropped, (c) rose again, and then (d) gradually increased. In the case of B-doped μc-Si, w differently behaved; (a) w increased very soon, (b) slightly dropped, (a′) rose again, (b′) slightly dropped again, (c) rose, and finally (d) gradually increased. The quick increase in w indicates that boron doping promotes the nucleation, and the repeated nucleation is responsible for the behavior (a′)–(b′). Additionally, the nucleation density, that was derived using the lateral correlation length of surface heights, monotonically increased with an increase in the boron concentration. The effects of boron doping are discussed with the catalytic effects and the formation of the surface-covering layer.     

Relation of defects and grain boundaries to transport and photo-transport: Solved and unsolved problems in microcrystalline silicon

Defects and grain boundaries play a crucial role in the dark and photo-transport of charge carriers. Surprisingly, the transport (trapping and recombination) in microcrystalline silicon is better understood at low temperatures, while room-temperature operation is of interest for real-life devices. In the first part of this review, the advantages of photo-transport techniques, used for the defect density evaluation, will be recapitulated and commented on. The second part is devoted to the present understanding of the specific features of transport in microcrystalline silicon like anisotropy, dominant transport path and the role of H and O in the grain boundary formation. The results of macroscopic measurements on series of samples will be confronted with the results of local conductivity studies on a nanometer scale and finally, the influence of oxygen and the ability to explain it by our model of transport will be illustrated.     

Metastable dark and photoconductive properties of microcrystalline silicon

Metastable changes in the dark conductivity of microcrystalline silicon upon heat treatment at different temperatures obey the Meyer–Neldel rule. Dark conductivity variations are accompanied by changes in the photoconductivity or the majority-carrier mobility-lifetime product. The minority-carrier mobility-lifetime product is not affected. The observations can be related to Fermi-level induced changes of the excess carrier lifetimes.     

Characterization of microcrystalline silicon by Hall and photo-Hall measurements

Transport properties of microcrystalline silicon are studied by Hall and photo-Hall measurements. The temperature dependence of the mobility shows that there exist potential barriers for majority carrirs to jump over. Illumination increases the majority carrier mobility while the minority carrier mobility turns out to be very small (<0.1 cm²/Vs) compared with the majority carrier mobility (5–30 cm²/Vs).     

Transformation of microcrystalline state of hydrogenated silicon to amorphous one due to presence of more electronegative impurities

When a slight fraction (～ 2 mol %) of N₂ is added into H₂, sputtering atmosphere for microcrystalline hydrogenated Si films, without changing other fabrication parameters, the amorphous film rather than microcrystalline one forms. The stabilization mechanism of the amorphous film of Si is discussed through TEM and IR observations of this kind of transformation from microcrystal to amorphous state.     

Raman scattering in low wavenumber region as a new probe to structural properties of microcrystalline silicon

The structural properties of microcrystalline silicon (μc-Si) are studied by Raman scattering. It is found that the intensity of each Raman band closely correlates with the absorption coefficient in the interband region and that the Raman band at ca. 150 cm^?1 is a sensitive probe to randomness of Si-Si bonding structure in μc-Si.     

Effect of thermal annealing and hydrogen-plasma treatment in boron-doped microcrystalline silicon

We have investigated the effects of temperature (during film growth and post-deposition thermal annealing) and H₂-plasma treatment on the electronic and structural properties of p-type microcrystalline silicon films (p-μc-Si:H) for solar cell applications. The highest dark conductivity is obtained in the thermally annealed p-μc-Si:H prepared at low substrate temperature of 50 °C. This dark conductivity is decreased by two orders of magnitude when the film is exposed to H₂-plasma, being completely restored after thermal annealing. Namely, reversible dual-conductivity cycle is observed between thermally annealed state and H₂-plasma-treated state in p-μc-Si:H. The dual-conductivity cycle is accompanied with the reversible change in the infrared-absorption spectrum at around 1845 cm^? 1 assigned as SiHB complex in p-μc-Si:H network structure. Taking into account of the reversible structural change by H₂-plasma-exposure and thermal-annealing cycles, necessary process-procedure condition has been proposed for obtaining high photovoltaic performance in thin-film-Si solar cells with high quality p-μc-Si:H.     

Determination of Raman emission cross-section ratio in hydrogenated microcrystalline silicon

The determination of the crystalline volume fraction from the Raman spectra of microcrystalline silicon involves the knowledge of a material parameter called the Raman emission cross-section ratio y. This value is still debated in the literature. In the present work, the determination of y has been carried out on the basis of quantitative analysis of medium-resolution transmission electron microscopy (TEM) micrographs performed on one layer deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) close to the amorphous/microcrystalline transition. Subsequent comparison of these data with the crystallinity as evaluated from measured Raman spectra yields a surprisingly high value of y = 1.7. This result is discussed in relation to previously published values (that range from 0.1 to 0.9).     

Excimer laser crystallization of microcrystalline silicon for TFTs on flexible substrate

Microcrystalline silicon (μc-Si) films have been deposited on PDMS as well as on PEN substrate. Excimer laser annealing was used to improve the crystalline structure and so to obtain high mobility TFTs. The effect of the laser annealing on the crystalline structure of silicon films is studied using different characterization techniques and discussed. Mobility values of 60 cm²/V s with PDMS and 46 cm²/V s with PEN are obtained.     

Deposition of microcrystalline silicon films at ultrafast rate by using a new microwave induced plasma source

Synthesis of microcrystalline silicon (μc-Si) film at an ultrafast deposition rate over 100 nm/s is achieved from SiH₄ + He by using a high density microwave plasma source even without employing H₂ dilution and substrate heating techniques. Systematic deposition studies show that high SiH₄ flow rate and working pressure increase film deposition rate while high He flow rate decreases the rate. On the other hand, crystallinity of deposited Si film decreases with increasing SiH₄ or He flow rate and working pressure. Enhancements of gas phase and surface reactions during film deposition process are responsible for the achievement of high deposition rate and high film crystallinity.     

Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH₄–H₂ mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65 Å/s has been achieved for a SiH₄ concentration of 67% diluted in H₂ with a high Raman crystallinity of X_c > 65% and a low defect density of (1–2) × 10¹⁶ cm⁻³ by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH₃, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.     

Preparation and properties of microcrystalline silicon films using photochemical vapor deposition

microcrystalline silicon films have been prepared through mercury photosensitized decomposition of monosilane at low gas pressures. The dark and light conductivities of the silicon films tend to increase at reactant pressures lower than 65 Pa and become 10^?2Ω^?1· cm^?1 at 26 Pa. From the Raman scattering and x-ray diffraction, silicon films were found to consist of a mixed phase structure including both microcrystalline and amorphous regions.