Defects and structure of hydrogenated microcrystalline silicon films deposited by different techniques

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Abstract

The development of structure and paramagnetic defects of microcrystalline silicon films prepared by plasma enhanced chemical vapour deposition at very high frequency (VHFPECVD), at radio frequency and in the high pressure, high power regime (RFPECVD) and by the hot wire CVD (HWCVD) are studied. The relationship between paramagnetic defects and structural configuration of μc-Si:H films obtained under deposition conditions near the amorphous-microcrystalline structural transition is investigated using results from electron spin resonance (ESR) such as spin density, g-value, ESR peak to peak line width ΔHpp, and crystalline volume fraction as calculated from Raman spectroscopy. Similar relationships between the structure transition from crystalline to amorphous and the density and spectral shape of paramagnetic defects are observed for all deposition methods. Low substrate temperatures and deposition conditions close to the transition to amorphous growth are found beneficial for low spin densities in accordance with the optimum growth conditions for high efficiency solar cells.

Introduction

Microcrystalline silicon (μc-Si:H) has been used and extensively studied as material for thin films solar cells and large area electronics because of its properties such as material stability under illumination and considerably larger optical absorption in the infrared when compared to a-Si:H [1], [2], [3], [4], [5]. Promising achievements have been obtained in terms of device performance, deposition rates and the process up-scaling to large areas. Record efficiencies for as deposited (non-degraded) single junction and tandem junction μc-Si:H solar cells are beyond 10% and 14%, respectively [6]. For the deposition of `solar grade' microcrystalline films, very high frequency plasma enhanced chemical vapour deposition (VHFPECVD) is an established technique. Strong research efforts have also allowed the production of very high quality solar cells, with efficiencies similar to those obtained with VHF absorber layer, using the so-called hot wire (HW) or catalytic (CAT) chemical vapour deposition method [7], [8], [9], [10], [11] and RFPECVD (13.56 MHz) in the high pressure, high power regime [12], [13], [14]. When microcrystalline silicon is used as absorber layer for solar cells the best efficiencies are achieved for material prepared near the transition from microcrystalline to amorphous growth. Up to now it is not clearly understood which material properties, e.g. defects, electronic transport or optical absorption, determine this optimization point on approach to the transition in growth regime. In this context, an investigation of the structural transition from microcrystalline to amorphous, and the density and spectral shape of paramagnetic defects of μc-Si:H films deposited by these three different methods is of great relevance.

It is the topic of the present work to study and compare the development of structure and paramagnetic defects of μc-Si:H films prepared under deposition conditions near the microcrystalline to amorphous structural transition, by the deposition techniques cited above.

Section snippets

Experiment

The films were produced in multi-chambers systems designed for deposition of μc-Si:H. Great care was taken for purity standards of the vacuum and gas supply systems to prevent impurity contamination of the material which is known to be highly susceptible to low level doping. μc-Si:H samples were deposited on aluminium foil and rough glass substrates. The VHFPECVD (95 MHz) deposition was done at a substrate temperature of TS=200 °C, a discharge power density of 0.08 W/cm2 and a pressure of 40

Results

The Raman intensity ratio ICRS vs. silane concentration SC is shown in Fig. 1 for samples deposited under different temperature using HWCVD and VHFPECVD. Similar values of ICRS are obtained for highly microcrystalline films produced by both deposition methods within the low silane concentration regime (SC = 0–5%). With increasing SC the crystalline volume fraction decreases, indicating a structure change from microcrystalline to amorphous. Comparison of crystalline volume fractions of VHFPECVD

Discussion

Highly crystalline μc-Si:H films produced under very high hydrogen dilution consist of columnar clusters of coherent regions separated by crack like voids [15], [26]. This porous structure allows the in-diffusion of impurities or atmospheric gases along the column boundaries [20], [21]. The interaction of impurities with the column boundary can lead to both the formation of defects or the termination of surface states. Furthermore through electrostatic effects of adsorbents it can lead to the

Conclusions

In this work, the development of structure and paramagnetic defects was studied and compared for μc-Si:H films prepared by VHFPECVD, HWCVD and RFPECVD deposition techniques, under deposition conditions near the microcrystalline to amorphous structural transition. The material properties of microcrystalline films produced by these techniques are mainly determined by their structure and not by the particularities of these three distinct deposition methods. Films produced under highest hydrogen

Acknowledgments

We thank S. Michel for technical support. A.L.B.N. acknowledges the financial support of Cnpq Brazilian agency.

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