• PDF / 63,293 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 70 Downloads / 179 Views

DOWNLOAD

REPORT

---

Microwave Plasma Assisted VHF-PECVD of Micro-Crystalline Silicon Wim Soppe1, Julien Bailat, Corinne Droz, Urs Graf, Ulrich Kroll, Johannes Meier, Arvind Shah Institute of Microtechnology (IMT), Rue A-L. Breguet 2, CH-2000 Neuchâtel, Switzerland 1 Present adress: ECN Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands.

ABSTRACT Growth of intrinsic micro-crystalline silicon layers by means of VHF-PECVD, assisted by remote microwave (MW) plasma has been investigated. The aim of the MW plasma is to enhance the deposition rate by introducing excited hydrogen and Ar atoms from the MW plasma in the VHF deposition zone. For this purpose a remote microwave plasma source was constructed in which a H2/Ar plasma is generated in a 20 mm diameter quartz tube. A gasshower has been constructed for homogeneous distribution of the flow of excited gas species from the microwave source into the deposition zone of the VHF-PECVD reactor where the dissociation of silane takes place. In a first series of experiments we applied high microwave power (> 500 W) and pure hydrogen in the MW source. This resulted in a larger deposition rate, but all layers - even grown at low silane concentrations - were amorphous and had a high oxygen content. The oxygen contamination was partly due to reduction of the quartz tube by the hydrogen plasma. In a second series of experiments Ar dilution and reduced MW power were used to eliminate the effect of etching of the tube by the microwave hydrogen plasma. In this series of experiments an increase of the growth rate of micro-crystalline silicon by about 15 % due to assistance of the microwave plasma was found. Optical emission spectroscopy indicates that – in these experiments – the main mechanism for the increased dissociation of silane is through molecular quenching reaction of Ar* metastables.

INTRODUCTION Micro-crystalline silicon is a promising material for application in film silicon solar cells. A very attractive application is the so-called micromorph cell, as developed at IMT [1], consisting of an a-Si:H top cell and a µc-Si:H bottom cell. Stable efficiencies of more than 11 % have been obtained by IMT, by research groups of Jülich [2], Canon Corp. [3] and Kaneka Corp. [4] for such tandem cells. These efficiencies already approach the values of wafer based mc-Si solar cells, but still have large potential for improvement if the best individual values for Voc, Jsc and fill factors could be combined [5]. A bottleneck for large scale production of film silicon solar cells based on µc-Si:H is the rather low deposition rate by which this material – with an appropriate, ‘solar grade’ quality – can be grown so far. Commonly, µc-Si:H is grown by a silane-based glow discharge technique with parallel-plate reactors in which the silane is diluted by hydrogen (and sometimes Ar). In the early days the standard industrial frequency of 13.56 MHz was used and this allowed for deposition rates up to approximately 0.1 nm/s. Later, the Very High Frequency Glow Discharge (VHF-GD) process was introduced by IMT, u