C Chiel Smit

Determining the material structure of microcrystalline silicon from Raman spectra

An easy and reliable method to extract the crystalline fractions in microcrystalline films is proposed. The method is shown to overcome, in a natural way, the inconsistencies that arise from the regular peak fitting routines. We subtract a scaled Raman spectrum that was obtained from an amorphous silicon film from the Raman spectrum of the microcrystalline silicon film. This subtraction leaves us with the Raman spectrum of the crystalline part of the microcrystalline film and the crystalline fraction can be determined. We apply this method to a series of samples covering the transition regime from amorphous to microcrystalline silicon. The crystalline fractions show good agreement with x-ray diffraction (XRD) results, in contrast to crystalline fractions obtained by the fitting of Gaussian line profiles applied to the same Raman spectra. The spectral line shape of the crystalline contribution to the Raman spectrum shows a clear asymmetry, an observation in agreement with model calculations reported previo...

Fast deposition of microcrystalline silicon with an expanding thermal plasma

Abstract Microcrystalline silicon has been deposited using an expanding thermal plasma. High deposition rates are achieved, which is attractive for solar cell production. A first survey of the influence of the deposition parameters on the optical, electrical and structural material properties is performed. SEM analyses show columnar growth and infrared absorption shows varying oxygen content. For some of the deposition conditions the dark and photoconductivities approximate 10−7 and 10−5 S/cm, respectively. Also the deposition plasma has been studied by means of mass spectrometry. It is concluded that the expanding thermal plasma is well suited for the deposition of microcrystalline silicon at growth rates up to 3.7 nm/s.

Combined optical and electrical measurements on pulsed corona discharges

To produce very intense coronas, a pulsed high voltage is applied. The peak voltage can be far above the breakdown value, if the pulse width remains below a few microseconds. Aiming at high charge production during the pulse we found that the pulse risetime is a sensitive parameter1. A DC bias voltage below the inception voltage is added to transport the charges to the wall2. The application of short pulses on top of a DC voltage instead of pure DC allows a much wider voltage range for corona operation. This wider voltage range is very advantageous in situations of serious contamination or at high operating temperatures, in the range of (400–800) °C3. Pulsing also permits a separate control of production (pulse) and transport (bias). Thus very high charge production rates can be achieved. Our work further aims at improving this charge production and at an understanding of these very high corona intensities. The absence of a full breakdown and the very interesting high corona current (we observed up to 600 A/m) are not fully understood however, a possible explanation might be connected to the ‘bright spot’ regime described later in this paper.

Integration of expanding thermal plasma deposited hydrogenated amorphous silicon in solar cells

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The role of the silyl radical in plasma deposition of microcrystalline silicon

Expanding thermal plasma chemical-vapor deposition has been used to deposit microcrystalline silicon films. We studied the behavior of the refractive index, crystalline fraction, and growth rate as a function of the silane (SiH4) flow close to the transition from amorphous to microcrystalline silicon. It was found that the refractive index, a measure for film density, increases when the average sticking probability of the depositing radicals decreases. Furthermore, we studied the influence of the position at which SiH4 is injected in the expanding plasma on the film density. It was found that the film density becomes higher when the SiH4 is injected closer to the substrate. Both findings strongly suggest that the film density benefits from a high contribution of the SiH3 radical to the growth of microcrystalline silicon.

Electron beam induced fluorescence measurements of the degree of hydrogen dissociation in hydrogen plasmas

The degree of dissociation of hydrogen in a hydrogen plasma has been measured using electron beam induced fluorescence. A 20 kV, 1 mA electron beam excites both the ground state H atom and H2 molecule into atomic hydrogen in an excited state. From the resulting fluorescence the degree of dissociation of hydrogen is determined. In addition, the absolute atomic hydrogen density can be determined if the gas temperature and pressure are known without any additional calibration. To check the consistency of the method the fluorescence from the first four Balmer transitions is measured. It is demonstrated that this technique can be applied in hydrogen and argon–hydrogen plasmas with a pressure of up to 1 mbar and 0.2 mbar, respectively.

Material properties and growth process of microcrystalline silicon with growth rates in excess of 1 nm/s

The expanding thermal plasma (ETP) has been used to deposit microcrystalline silicon (µc-Si:H) with rates up to 2.7 nm/s. Typical material properties of well crystallised material are crystallite sizes of 20 nm, photo- and dark conductivity of 2x10 -5 and 2x10 -7 S/cm respectively, and an activation energy of 600 meV. The radical densities of SiH3, SiH, and Si present in the gas phase have been quantified. In conditions where µc-Si:H is deposited the atomic hydrogen flux towards the surface is of the same magnitude or higher as the flux of deposited radicals. Furthermore, the abundance of radicals such as SiH and Si is large and may contribute several tens of percent to the deposition rate.

High-rate microcrystalline silicon for solar cells

In order to produce thin silicon films for solar cells at high growth rates we deposited films with a cascaded arc expanding thermal plasma. We demonstrate the power of this technique by applying amorphous films deposited at rates up to 1.4 nm/s in solar cells. We used the same deposition technique to produce microcrystalline silicon films. Growth rates up to 3.7 nm/s are achieved. The material structure is analyzed using Raman spectroscopy and XRD. We see that the crystalline fraction increases with the H/sub 2/ flow, whereas the amorphous and the void fraction decrease.

Temperature dependence at various intrinsic a-Si:H growth rates of p-i-n deposited solar cells

With a cascaded arc expanding thermal plasma, intrinsic solar grade amorphous silicon can be deposited at growth rates varying from 2 to 100 /spl Aring//s. The temperature above which good material is obtained becomes higher for higher growth rates. Higher deposition temperatures affect the p-layer within p-i-n grown solar cells, which will result in other optimum deposition temperatures of the i-layer. In this paper, the authors address the dependence of the p-i-n solar cell performance on the deposition rate and deposition temperature.