Real Time Optics of p-Type Microcrystalline Silicon Deposition on Specular and Textured ZnO-Coated Glass
- PDF / 273,632 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 28 Downloads / 180 Views
REAL TIME OPTICS OF p-TYPE MICROCRYSTALLINE SILICON DEPOSITION ON SPECULAR AND TEXTURED ZnO-COATED GLASS Pablo I. Rovira,* Andre S. Ferlauto,* Randy J. Koval,* Christopher R. Wronski,* Robert W. Collins,* and Gautam Ganguly** * Materials Research Laboratory and Center for Thin Film Devices, The Pennsylvania State University, University Park, PA ** BP Solarex, Thin Film R&D, Toano, VA ABSTRACT In this study, we optimize the plasma-enhanced chemical vapor deposition (PECVD) process to achieve high-density nucleation of single-phase microcrystalline silicon (µc-Si:H) p-type layers on zinc oxide (ZnO) surfaces at 200 °C for applications in amorphous silicon (a-Si:H) based p-i-n solar cells. The phase evolution of the Si:H p-layers on specular ZnO-coated glass substrates is characterized using real time spectroscopic ellipsometry (RTSE). The resulting evolutionary phase diagram depicts the accumulated film thickness at which the amorphous-tomicrocrystalline (a→µc) transition occurs versus the H2-dilution ratio, with all other parameters fixed. Guided by this diagram, we find that high-density microcrystallite nucleation and fullycoalesced µc-Si:H p-layers ~100 Å thick can be obtained on specular ZnO at 200 Å using a B(CH3)3 doping gas flow ratio of D=[B(CH3)3]/[SiH4]=0.02 and an optimized H2-dilution ratio of R=[H2]/SiH4]=200. Lower H2-dilution levels (R200) generate longer induction periods, low-density nucleation, and incomplete coalescence of microcrystallites even after ~100 Å. The time evolution of the microstructure and the resulting dielectric functions as determined by RTSE are similar for optimized µc-Si:H p-layers ~200 Å thick prepared on specular and textured ZnO surfaces, indicating that the substrate texturing does not necessitate process reoptimization. INTRODUCTION Previous real time spectroscopic ellipsometry (RTSE) studies have identified plasmaenhanced chemical vapor deposition (PECVD) processes for immediate nucleation of p-type microcrystalline silicon (µc-Si:H) layers at high density on H2-plasma-treated intrinsic amorphous silicon (a-Si:H) surfaces at 200 °C using diborane (B2H6), trimethyl boron [B(CH3)3], and boron trifluoride (BF3) as dopant source gases [1]. Such i/p structures with ~120 Å thick players have been designed for incorporation into a-Si:H n-i-p solar cells. The authors of Ref. [1] concluded that BF3 is preferable for this application in part because it presents a wide deposition parameter window for single-phase µc-Si:H p-layers. When BF3 is used, however, a higher plasma power density is required to generate a sufficient concentration of BFx (x
Data Loading...
