Quality and Growth Rate of Hot-wire Chemical Vapor Deposition Epitaxial Si Layers
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1066-A11-06
Quality and Growth Rate of Hot-wire Chemical Vapor Deposition Epitaxial Si Layers Charles W Teplin, Ina T. Martin, Kim M. Jones, David Young, Manuel J. Romero, Robert C. Reedy, Howard M. Branz, and Paul Stradins NCPV, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401 ABSTRACT Fast epitaxial growth of several microns thick Si at glass-compatible temperatures by the hot-wire CVD technique is investigated, for film Si photovoltaic and other applications. Growth temperature determines the growth phase (epitaxial or disordered) and affects the growth rate, possibly due to the different hydrogen coverage. Stable epitaxy proceeds robustly in several different growth chemistry regimes at substrate temperatures above 600°C. The resulting films exhibit low defect concentrations and high carrier mobilities.
INTRODUCTION Recently, new film silicon solar cell technologies are being developed to reduce the amount of Si used in solar cells and their manufacturing costs. Even with indirect-gap crystalline Si, absorber layers less than 10 micron thick are sufficient to achieve reasonable external quantum efficiencies. A promising approach is to start with high-quality c-Si seed layer on a relatively inexpensive substrate such as borosilicate glass and epitaxially thicken it up to 10 microns [1]. To achieve good cell performance, the epi-layer material quality should allow for photocarriers to be collected before they recombine via bulk defects. Roughly, this means the areal density of defects (such as bulk dislocations originating from seed interface) should be below 106 cm-2 for 10 micron film. The quality of the epitaxial interface and surfaces must be high because recombination in thin layers can be dominated by surfaces. In addition, the impurity levels in the layers should be low to reduce recombination and permit controlled doping at low levels. In past, we have demonstrated epitaxial growth by hot-wire chemical vapor deposition (HWCVD) above 620°C with defect densities ~108 cm-2 [2, 3], grew epitaxially on the difficult (111) Si surface [2] and successfully thickened a Si seed on borosilicate glass with this technique [3]. In this work, we report further systematic study and improvements of the technique leading to films that show promise for the future PV device work.
EXPERIMENTAL Epitaxial Si films were grown on (100) oriented, RCA-cleaned c-Si wafers in our hotwire chemical vapor deposition (HWCVD) reactor from silane gas decomposing on a W filament heated to 2100 °C. Diborane and phosphine gases were introduced during deposition through flow-controlled orifices to achieve controlled dopant levels in the 1016 – 1018 cm-3 range. Growth temperature, a critical epi-growth parameter (see below), was monitored by ellipsometery, pyrometry, and thermocouple response. Film structure was monitored in-situ by real-time spectral ellipsometry. Deposited films were analyzed by transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), X-ray diffraction, cathodoluminescence and
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