R. Schwarz

Closed-chamber chemical vapor deposition : new cyclic method for preparation of microcrystalline silicon films

A new cyclic chemical vapor deposition (CVD) process for low-temperature preparation of microcrystalline silicon and its alloys is proposed. The cycle includes an a-Si:H layer deposition step and a hydrogen-radical treatment step. The H-treatment step is carried out under closed-chamber CVD (CC-CVD). It provides conservation of Si mass at an equilibrium between H-etching and redeposition. Thus, films of high crystallinity can be achieved. The advantages are a high deposition rate, high reactive gas utilization and precise control of the film structure. In situ monitoring of the plasma emission spectrum has been used to investigate the CC-CVD process features. The films are characterized by Raman spectroscopy, scanning electron microscopy, temperature-dependent dark conductivity, and infrared transmission spectroscopy.

High‐temperature annealing behavior of μτ products of electrons and holes in a‐Si:H

The mobility‐lifetime products of electrons and holes [(μτ)e and (μτ)h] in undoped hydrogenated amorphous silicon samples have been studied by photoconductivity and ambipolar diffusion length measurements. The density of dangling bonds Nd in the samples is changed over a range of 3×1015–2×1018 cm−3 by annealing at high temperatures. Nd and the Urbach tail slope Eov have been determined by the constant photocurrent method. In addition, the optical gap, the activation energy of dark conductivity, and the exponent governing the intensity dependence of σpc have been measured. The results show that there is a correlation between Nd and Eov which is consistent with equilibrium theory. (μτ)e and (μτ)h change in quite different ways as Nd increases, namely, (μτ)e decreases as a linear function of the inverse of Nd. However, (μτ)h remains almost constant when Nd≤5×1016 cm−3, then decreases fast for higher Nd. The asymmetric dependence of transport properties of electrons and holes on Nd suggests that for electrons...

High band‐gap hydrogenated amorphous silicon‐selenium alloys

Hydrogenated amorphous silicon‐selenium alloy thin films were prepared by the decomposition of SiH4 and H2Se gas mixtures in a radio‐frequency plasma glow discharge at a substrate temperature of 250u2009°C. The alloy composition was varied by changing the gas volume ratio Rv = {[H2Se]/[SiH4]}. Infrared and Raman spectroscopies were used to probe the bonding structure of the material. In addition to the hydrogen induced bands normally observed in a‐Si:H, a new selenium induced band at 390 cm−1, assigned to the stretching mode of the Si—Se bond, was observed. Analysis of the vibrational SiH stretching region reveals the presence of a significant level of (Si)xSe3−xSiH and (Si)ySe2−ySiH2 configurations. Optical and electrical measurements show that increasing the selenium content results in an increase in the optical (Tauc) gap and a decrease in the dark conductivity (σD) and photoconductivity (σph). However, the photosensitivity (σph/σD) remains high for the entire composition range. The Urbach energy and defec...

Correlation between structural, optical and electrical properties of μc-Si films

Abstract A series of microcrystalline silicon films (μc-Si:H) containing different crystalline volume parts was investigated. The optical absorption of one film was studied in detail and its spectrum has been simulated as superposition of a crystalline (c-Si) and an amorphous phase (a-Si) corrected by a scattering coefficient. The carrier transport along the crystallites is discussed.

Characterization of interface properties in a-Si:H/a-Si x C1−x:H multilayers by the constant photocurrent method

Abstract In this work the constant photocurrent method (CPM) has been extended to characterize the interface properties of a-Si:H/a-Si x C1−x :H multilayers. The experimental results show that the interface defect density (N di) and the width of the interface region (l) strongly affect the apparent Urbach energy (Ecpm u) and the apparent defect absorption coefficient (α cpm d ) of CPM spectra in multilayers. Based on the assumption that the defect density decreases exponentially away from the interfaces, the CPM spectra of the multilayers have been analytically described. The calculated results show that the CPM spectra do not follow the absorption coefficient spectra in the multilayers, which is different from the case of a-Si: H bulk material. CPM spectra not only depend on N di , but also strongly on l, in contrast to the absorption coefficient spectra. The parameters l and N di can be obtained by fitting the measured CPM spectra to our theoretical expression. With this method it is found that, as the ...

Asymmetric degradation of electron and hole μτ-products in a-Si:H/a-SiC:H multilayers under illumination

In a series of a-Si:H/a-SiC:H multilayers with different well layer thicknesses d Si we have determined the electron and hole transport properties before and after a 24 hour exposure to AM1 light. Both the well layer thickness dependence in the initial state and the degradation behaviour show an asymmetry between electrons and holes. For the annealed samples the μτ -product of electrons, ( μτ ) e decreases rapidly with decreasing d Si whereas ( μτ ) h (derived from the steady-state photocarrier grating method) changes only slightly. During light soaking ( μτ ) e changes little for small d Si , whereas for ( μτ ) h this is true for large d Si . A qualitative explanation is given by the analytical dependence of ( μτ ) e and ( μτ ) h on the density of dangling bond defects. Since the samples cover a large range of sublayer thicknesses we can estimate the degradation of bulk and interface regions separately.

Numerical analysis of the transient response in amorphous silicon

Using our program, TRansient Amorphous DEvice Simulator (TRADES), for the simulation of transient phenomena in amorphous silicon devices, the transient response after turning the light off is numerically calculated. Parameters for the numerical analysis are obtained by fitting the two families of the measured steady-state characteristics: light-intensity and temperature dependence of the secondary photocurrent. Using these parameters, both temperature and light-intensity dependence of the transients are calculated. It is shown that the transient is faster at higher temperatures and at higher illumination levels. Results are compared with measured transients at different temperatures.

Trace of an interface layer between buffer and i-layer in the spectral response of a-SiC:H/a-Si:H solar cells

The comparison of numerical modelling and measurements of the internal collection efficiency q of a solar cell in the annealed and degraded state (state A and B) delivers information about the buffer/i-layer interface defect layer and the position dependent degradation. The plot of the measured internal collection efficiency shows two characteristic features: 1. The variation of the q-curves of blue light with increasing voltage at the crosspoint of the q-curves is like an “S”-shape in state A and B. 2. The q-values at zero voltage show a large spread in state B. The “S”-shape occurs in the voltage range, in which the positive charge of the defect layer is decreasing. The decreasing positive charge reduces the effect of the increasing applied voltage. The large spread of the q-values at zero voltage is the direct consequence of an enhanced buffer/i-interface layer degradation. The comparison of the modelled and the measured q-curves in state B leads to the following conclusion: Degradation occurs by a strong damage of the buffer/i-layer interface region and a weak damage of the i-layer.

Characterization of optoelectronic properties of a-Si1−xCx:H films

Abstract The optoelectronic properties of a-Si 1− x C x :H and their stability under illumination have been investigated by the constant photocurrent method, the steady state photocarrier grating technique, the response time ( τ R ) and photoconductivity ( σ pc ) measurements. The results show that σ pc and the ambipolar diffusion length decrease as the methane content ( c f ) in the gas mixture increases. The mobility-lifetime product of the electrons (( μτ ) e ) shows different behavior from that of holes (( μτ ) h ) for increasing c f or during light soaking. The drift mobility is influenced strongly by c f in contrast to τ R . μτ-products and τ R decrease when the generation rate increases. These results can be understood consistently in terms of (a) the increase of the defect density and the broadening of the band tail states with c f , (b) the asymmetric distribution of the valence and conduction band tail states.

Light and current degradation of a-Si:H pin, nin and pip diodes detected with CPM

Abstract Light and current degradation of nin, pip and pin diodes were investigated using CPM and SCLC. We found that degradation can be induced by recombination as well as by hole injection. Excess electrons do not cause any degradation.