Crystalline Si Films Grown Epitaxially at Low Temperatures by ECR-PECVD
- PDF / 393,060 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 100 Downloads / 204 Views
Crystalline Si Films Grown Epitaxially at Low Temperatures by ECR-PECVD J. Platen, B. Selle, S. Christiansen*, M. Nerding*, M. Schmidbauer**, and W. Fuhs Hahn-Meitner-Institut, Abteilung Silizium-Photovoltaik, Kekuléstr.5, D - 12489 Berlin, Germany * Universität Erlangen-Nürnberg, Institut für Werkstoffwissenschaften, Mikrocharakterisierung, Cauerstr. 6, D - 91058 Erlangen, Germany ** Humboldt-Universität Berlin, Institut für Physik, D - 10117 Berlin, Germany ABSTRACT Electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) is used to grow thin epitaxial films on Si(100) wafers. We report on systematic variations of deposition parameters like substrate temperature, substrate dc bias voltage, and gas composition. The structural quality was significantly improved by increasing the substrate temperature from 325 to 500 °C. Simultaneously, compressive lattice strain tends to increase. A negative dc bias voltage resulted in highly disordered films and increased surface roughness due to enhanced ion damage. In contrast positive bias voltages decreased the defect creation by reducing the ion bombardment of the surface during growth. Under so far optimized conditions the remaining disorder is given by two-dimensional, extended defects running parallel to the growth direction and forming grain boundaries with a lateral spacing of 500 – 700 nm. The single grains are essentially free of oneand two-dimensional defects and show the same orientation as the substrate. By reducing the H2 dilution and adding Ar to the excitation gas the deposition rate increased from 5.3 to 16.2 nm/min. This resulted in inferior structural quality which might be attributed to the reduced etching effect, the enhanced ion bombardment and/or the increased growth rate. INTRODUCTION Epitaxial growth of Si is mostly performed at high temperatures. However, ion assisted deposition techniques have been shown to enable (quasi-)epitaxial growth at low temperatures with reasonable high deposition rates [1,2]. Technologically such a process would be desirable for applications such as thin film solar cells on glass substrates, where the process temperature is limited by the softening temperature of glass (550 °C). Quasi-epitaxial thickening of seed layers performed on glass is an attractive approach. However, epitaxial growth is not only affected by the deposition parameters like temperature or ion bombardment. Doping of the films and the crystallographic orientation of the substrates play an important role. (Quasi-)epitaxial growth has been obtained on c-Si substrates with different surface normal orientations, but the defect density strongly depends on the substrate orientation. Highly phosphorous and boron doped films on Si(100) substrates turned out to be of high quality with densities of extended defects of less than 106 cm-2 [3]. But the structural order so far is inferior in case of undoped Si films. In this paper we report on a study relating the most important deposition parameters (substrate temperature, substrate dc bias volt
Data Loading...
