Spectroscopic Ellipsometry Studies of Nanocrystalline Silicon in Thin-Film Silicon Dioxide
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Spectroscopic Ellipsometry Studies of Nanocrystalline Silicon in Thin-Film Silicon Dioxide Gerald E. Jellison, Jr.,1 Stephen P. Withrow,1 Supriya Jaiswal,2 Christopher M. Rouleau,1 John T. Simpson,2 Clark W. White,1 and C. Owen Griffiths3 1 Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 378316030 2 Engineering Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6004 3 Hinds Instruments, Inc. 3175 N. W. Aloclek Drrive, Hillsboro, OR 97124-7135 ABSTRACT Nanocrystalline silicon (n-Si) is formed in a silicon dioxide thin-film matrix by ion implantation followed by thermal annealing in forming gas at 1100 °C for 1 hour. The ion implantation is performed using multiple implants with different implantation energies and doses to create a quasi-flat concentration of silicon atoms throughout the silicon dioxide film. These samples are then analyzed using spectroscopic ellipsometry to characterize their linear optical properties. Implantations with small doses (5 x 1020 Si atoms/cm3) increase the refractive index by a small amount (∆n~0.006 at 600nm), while implantations with moderate dose (5 x 1021 Si atoms/cm3) have a larger increase in refractive index and exhibit optical absorption above ~1.9 eV (650 nm). INTRODUCTION There are many techniques for forming silicon nanocrystals with dimensions in the 3-10 nm range. One of the most intriguing utilizes ion implantation of silicon atoms into silicon dioxide followed by a high-temperature anneal to precipitate the excess silicon, resulting in embedded silicon nanocrystals.1-4 These silicon nanocrystals have an intense, broad room-temperature photoluminescence (PL), peaking at 750-800 nm, with a full width at half-maximum of ~200 nm. Silicon nanocrystals can be formed either in bulk SiO2 or in thin-film SiO2 grown on a silicon wafer. Recently, there has been renewed interest in this material because several workers have shown evidence of optical gain.1,2 Since SiO2 is an essential material of the microelectronic industry, this discovery could have enormous economic impact in that it makes an all-silicon laser a distinct possibility. In this paper, we discuss the spectroscopic ellipsometry results from a series of nanocrystalline silicon samples made in thin-film SiO2. From these measurements, it is possible to determine film and interface thicknesses as well as spectroscopic refractive indices and extinction coefficients. This is particularly important for possible waveguide applications, where surface roughness, interface roughness, and small amounts of optical absorption are significant.
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EXPERIMENTAL DETAILS Sample preparation
Concentration (10
21
1/cm 3)
Samples were prepared from oxidized silicon wafers fabricated using high-temperature wet oxidation, where the Table 1: Samples examined in this study final oxide thicknesses were ~550 nm and ~1060 nm as determined by spectroscopic ellipsometry. These samples SiO2 Si conc. were ion-implanted using several different ion implantation Sample th
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