Low Temperature Fabrication of Microcrystalline Silicon Germanium Films by RF Reactive Magnetron Sputtering
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0910-A13-05
Low Temperature Fabrication of Microcrystalline Silicon Germanium Films by RF Reactive Magnetron Sputtering Isao Nakamura, Toru Ajiki, and Masao Isomura Department of Electrical & Electronic Engineering, Tokai university, 1117 Kitakaname, Hiratsuka, Kanagawa, 259-1292, Japan ABSTRACT Microcrystalline silicon-germanium films (µ c-SiGe) were fabricated on Corning #7059 glass substrates by the RF reactive magnetron sputtering method. The µc-SiGe films with Ge fraction of 0.7-0.8 could be crystallized at 200 ˚C by H2 introduction into the sputtering gases. The absorption coefficients of the films decrease in long wavelength region corresponding to the photon energies below the energy gap and become close to those of single crystal Si0.25Ge0.75 by the decrease in the substrate temperature. The dark conductivities show lower values of 10-7 S/cm at 200 ˚C and 300 ˚C with H2 introduction. Besides, the photosensitivity is observed in these samples. These results indicate that the H2 introduction into the sputtering gases has two important effects to decrease the crystallization temperature of the µc-SiGe films and to improve the film properties by reducing the dangling bond defects.
INTRODUCTION Thin film solar cells are expected for the next generation solar cells from the viewpoint of higher efficiency, lower cost and larger area. However, the thin film solar cells are not enough to satisfy these requirements. Multi-junction solar cells are investigated to make more efficient use of the sunlight. Hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline Si (µc-Si:H) films fabricated by plasma-enhanced CVD method were conventionally used for two-junction solar cells [1,2]. However, these films absorb the sunlight below the wavelength of 1000 nm. In order to improve the efficiency, we proposed microcrystalline silicon germanium (µc-SiGe) films as bottom cell materials of the multi-junction solar cells. The µc-SiGe films have a sensitivity in the longer wavelength region and enable us to utilize the sunlight more efficiently [3,4]. In this study, we investigated fabrication of the µc-SiGe films by the RF reactive magnetron sputtering method. The sputtering method is a safety process without toxic gases such as SiH4 and GeH4. We used the Ar and H2 mixture for the sputtering gases [5], and expected that H radicals reduce the crystallization temperature and to improve the film properties by terminating the dangling bonds. The effects of H2 introduction into sputtering gases on the crystallinity and properties of the µc-SiGe films are discussed. EXPERIMETAL DETAILS The µc-SiGe films were fabricated on Corning #7059 glass substrates by an RF reactive magnetron sputtering method. The sputtering apparatus has two targets of Si and Ge. The film composition can be controlled by changing RF powers applied to these targets. The RF powers to
Si and Ge targets were 50 W and 113 W, respectively. Ar or Ar and H2 mixture were used for the sputtering gases. H2/Ar gas flow rate ratios were varied from 1 to 10 in
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