In situ low temperature growth of poly-crystalline germanium thin film on glass by RF magnetron sputtering

• Published in: Solar energy materials and solar cells 2010-09, Vol.94 (9), p.1501-1505
• Main Authors: Tsao, Chao-Yang, Weber, Jürgen W., Campbell, Patrick, Conibeer, Gavin, Song, Dengyuan, Green, Martin A.
• Format: Article
• Language: English
• Subjects: Absorption coefficient; Applied sciences; Cross-disciplinary physics: materials science; rheology; Deposition; Deposition by sputtering; Electronics; Energy; Energy gaps (solid state); Exact sciences and technology; Germanium; Glass; Magnetron sputtering; Materials science; Methods of deposition of films and coatings; film growth and epitaxy; Natural energy; Optical properties; Optoelectronic devices; Photovoltaic conversion; Physics; Poly-crystalline germanium; RF sputtering; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Solar cells; Solar cells. Photoelectrochemical cells; Solar energy; Thin film
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2010.02.035

Structural and optical properties of germanium thin films deposited on silicon nitride coated glass are investigated with the aim to develop a material for the bottom cells of low cost monolithic tandem solar cells. The films were deposited by radio-frequency magnetron sputtering at various substrate temperatures ( T s )≤450 °C. X-ray diffraction spectra reveal the structural evolution from amorphous to crystalline phase with increasing T s We find that the film sputtered at 450 °C is poly-crystalline with strong (1 1 1) preferential orientation, confirmed by cross-sectional transmission electron microscopy. Optical band gaps of these films derived from Tauc plots, using absorption coefficient values derived from both reflectance/transmittance measurements and spectroscopic ellipsometry data, are in a reasonable agreement. Optical band gap values decrease from ∼0.88 to 0.68 eV over the transition from the amorphous to poly-crystalline phase. The absorption coefficient of the poly-crystalline Ge film is higher than that of bulk Ge over a wide waveband and exhibits an absorption tail. The optical properties upon substrate temperature are correlated with the structural properties of Ge films.