• PDF / 200,354 Bytes
• 7 Pages / 612 x 792 pts (letter) Page_size
• 96 Downloads / 235 Views

DOWNLOAD

REPORT

---

Thickness and Interface Layer Effects on the Amorphous Silicon Film Property Studied by Various Photoluminescence Excitation Wavelengths Guozhen Yue, Daxing Han*, Jeffrey Yang1, and Subhendu Guha1 Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA. 1United Solar Systems Corp., 1100W Maple Road, Troy, Michigan 48084, USA. ABSTRACT We have studied the structure of highly hydrogen-diluted a-Si:H films by using Raman and photoluminescence (PL) spectroscopies. Raman spectra show a typical broad a-Si:H peak in the films with different substrate surfaces or i-layer thicknesses, except for one 1.4 µm film deposited directly onto a stainless steel (ss) substrate that shows the c-Si peak. An apparent µcSi component was characterized by a low energy PL enhancement in a 0.5 µm film deposited directly onto the ss substrate. When a thin n-layer was inserted between the substrate and the a-Si film, no µc-Si growth was found in the first 0.5-1.0 µm thick layer but there was a µc-Si component in the top surface layer when the film was grown to a 1.5 µm thickness. The nonuniformity structure along the growth direction was characterized by PL spectroscopy using varied laser excitation wavelengths of 325, 488, 514.5, and 632.8 nm. We find that PL spectroscopy is a sensitive tool to characterize the microcrystallinity of the film. INTRODUCTION Significant progress has been made in improving the performance and stability of a-Si:Hbased p-i-n solar cells using intrinsic a-Si:H layers prepared with hydrogen dilution of silane or disilane[1,2]. It has been suggested that an improvement in the structure of the material with increasing hydrogen dilution contributed to the improved stability[3]. H2 dilution is also known to promote growth of microcrystals, and the most stable a-Si:H solar cells have been obtained at a H dilution ratio just before the onset of microcrystallinity[3]. A film thickness and substrate dependence of microstructure for high H diluted films have been examined by using Raman and X-ray diffraction (XRD) [3,4]. It was found that 0.5 µm-thick H2-diluted a-Si:H films are partially microcrystalline when deposited directly on stainless steel (ss) substrates but are fully amorphous when a thin n-layer is first deposited on the ss substrate. In the latter case, partial microcrystallinity develops as the films grow thicker (1.5-2.5 µm)[5]. However, near the onset of microcrystallinity, the film microstructure is very sensitive to the film growth conditions. A highly sensitive characterization tool for the film microcrystallinity is needed to determine optimum growth conditions. In this work, we use PL and Raman as spectroscopic tools to study three groups of samples: (a) films prepared with the same deposition conditions as above, (b) films obtained by first inserting a textured Ag/ZnO between the substrate and film to avoid interference effects, and (c)a 1.4 µm thick film in which the c-Si peak has been observed by XRD.

A5.9.1

EXPERIMENTAL DETAIL The films were dep