Characterization of Light Emitting Porous Polycrystalline Silicon Films
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Technology, Clear Water Bay, Hong Kong ** Department of Electronic Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong ABSTRACT Polycrystalline silicon (poly-Si) thin films (-700nm) were deposited by LPCVD, doped with 950'C phosphorous diffusion, and rendered porous by anodization and stain etching. From x-ray photoelectron spectroscopy, poly-Si films have atomic concentration of C(ls):O(ls):Si(2p) = 6%: 15%:79%. However, porous poly-Si (PPS) films with weak photoluminescence (PL) have C:O:Si of 20%:38%:42%. For PPS films with strong PL, C:O:Si is 11%:38%:51%. From microRaman, scattered spectra for 632nm laser source has peak at 735nm and full wave half maximum (FWHM) of 76nm, and is similar to the PL spectra excited by 400nm uv laser source. High resolution transmission electron microscopy (TEM) study shows that PPS film is of complex structure and composes of numerous Si nano-crystals (1-l10nm) surrounded by amorphous materials. INTRODUCTION Efficient light emission from porous silicon (Si) has drawn considerable interest since it was first reported by Canham'. In recent years, visible photoluminescence (PL) has also been found to be emitted from porous Si (PS) layers formed by anodized or stain-etched polycrystalline Si (poly-Si) 2-12 The new luminescent porous poly-Si (PPS) film can be formed on Si or non-Si substrates such as oxide, metal or glass, and hence has numerous novel practical applications in future Si-based optoelectronic devices, such as sensors, photodetectors and displays. We have reported recently new nanostructures and formation mechanisms of luminescent PPS films using high resolution SEM and AFM studies2 . In this article, we present more characterization results on the nanostructure and composition of this novel material. EXPERIMENT Undoped poly-Si thin films with a thickness of 670 nm were deposited onto 5-10 Qcm p-type crystalline-Si (c-Si) substrates. Poly-Si films of around 450 nm in thickness were also deposited onto 95 nm-thick thermal oxide or 200 nm-thick low pressure chemical vapor deposition (LPCVD) nitride layers formed on Si substrates. The poly-Si films were formed by thermal decomposition of silane gas (SiH 4) in a LPCVD reactor at 6250C for 50 mrin. The flow rate and pressure of silane were 150 sccm and 300 mTorr, respectively, and the poly-Si deposition rate was 10 nm//min. Phosphorus was then diffused into the samples at 9500C for 20 min to form 12 a/sq n-
415 Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
type poly-Si films. To form the PPS layers, poly-Si films were anodized in dilute HF s9 lution (49%HF :ethanol (C2H5OH)=1: 1) with a Pt electrode at a current density of 10 mA/cm for 5-10 min in room light and at room temperature. P-type Si wafers were also anodized under the same conditions for comparison. Poly-Si films were also stain etched in HF:HNO 3:DI 1:3:5 solution for 0.5-2 min in room light and at room temperature. The incubation time for the stain-etched PPS film was about 20-30 sec. For passivation tests, some PPS samples
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