Effects of Si-Layer-Thickness Ratio on UV-Light-Emission Intensity from Si/SiO2 Multilayered Thin Films Prepared Using Radio-Frequency Sputtering ()
1. Introduction
Various works on silicon (Si)-based luminescent materials utilizing the quantum-confinement effect, such as porous Si [1] [2] and Si nanocrystal (Si-nc) [3] -[5] , have been reported. Ultraviolet (UV) light emission can be obtained from such materials fabricated using various methods such as laser ablation [6] and sputtering [7] [8] . UV photoluminescence (PL) spectra having a peak wavelength around 370 nm were obtained from sputtered SiO2 films including Si-ncs after annealing [7] . The UV light emission might originate from emission centers of interface layers (SiOx) between Si-ncs and SiO2 matrices [7] .
Furthermore, Si/SiO2 multilayers produced by alternately stacking several-nanometer-thick Si and SiO2 layers using the sputtering method are very important structures from which we can controllably and effectively produce such Si-ncs and SiOx [8] . In these multilayered structures, Si-ncs seem to be produced from nanometer- order-thick Si layers, and SiOx can clearly be formed after annealing (see Figure 1) [8] . In our previous study, we fabricated Si/SiO2 multilayered films having nanometer-order-thick Si and SiO2 layers using radio-frequency (RF) magnetron sputtering and observed UV PL having a sharp peak at a wavelength of 370 nm after annealing at 1150˚C to 1250˚C [8] . Such UV-light-emitting materials may be used in future higher-density optical disk systems.
However, the UV peak intensities from the Si/SiO2 multilayered films should be stronger than those in our previous study in order to be used as light-emitting materials. We considered that the Si-layer-thickness ratio of the film was a very important parameter for enhancing the peak intensity because the ratio was concerned with the size of Si-nc, which might affect the intensity of the UV light emission [7] . Therefore, in this study, we investigated effects of the Si-layer-thickness ratios on the UV peak intensity of the Si/SiO2 sputtered and multilayered films.
2. Fabrication
A RF magnetron sputtering apparatus (ULVAC, SH-350-SE) was used in our fabrication processes. Si and SiO2 plates having diameters of 100 mm were used as sputtering targets, and Si/SiO2 multilayered films were deposited by alternately supplying RF power to the Si and SiO2 targets. The flow rate of Ar gas introduced into the chamber was 10 sccm, and the RF powers supplied to the Si and SiO2 targets were 50 and 100 W, respectively. Commercial fused-silica substrates (ATOCK Inc., 11.5 mm × 24.5 mm × 1 mmt) were used and not heated during deposition. One hundred pairs of Si and SiO2 layers were alternately stacked on the substrates. The deposition rates of Si and SiO2 thick films were 0.05 and 0.12 nm/s, respectively. Switching of sputtering targets and opening or closing of two shutters positioned above the targets were automatically controlled by digital timers. The total thicknesses of multilayered films were measured using a surface profiler (ULVAC, Dektak 3ST). We can estimate the Si and SiO2 layer thicknesses (dSi and
, see Figure 1) from the measured total thicknesses of the films and the ratio of their deposition rates (0.5:1.2). In this study, we prepared seven samples with estimated Si-layer-thickness ratios dSi/(dSi +
) of 0.15, 0.23, 0.24, 0.26, 0.27, 0.29, or 0.30 (Table 1). Four specimens were prepared from one as-deposited sample by cutting it using a diamond-wire saw. We subsequently annealed the specimens in ambient air at 1150˚C, 1200˚C, or 1250˚C for 25 min using a Siliconit electric furnace after sputter deposition.
3. Evaluation
PL spectra from the annealed samples were measured at room temperature with excitation using a He-Cd laser
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Table 1. Seven samples prepared in this study. The Si and SiO2 layer thicknesses (dSi and
) were estimated from the measured total thicknesses of the Si/SiO2 multilayered films and the ratio of their deposition rates. The deposition rates of Si and SiO2 thick films were 0.05 and 0.12 nm/s, respectively.
(Kimmon, IK3251R-F, wavelength l = 325 nm). A monochromator (Nikon, P250), a photomultiplier (Hamamatsu, R2658), and a lock-in amplifier (NF, LI-572B) were used in our measurements. The intensities of PL spectra were calibrated by using a white-light spectrum measured with an optical spectrum analyzer (Anritsu, MS9701C + MS9030A) at wavelengths from 350 to 1750 nm because sensitivities of the monochromator and the photomultiplier depend on input light wavelengths.
Figure 2 presents typical PL spectra of specimens prepared from the sample with a Si-layer-thickness ratio of 0.29 (sample 6) after annealing at 1150˚C, 1200˚C, and 1250˚C. The total thickness of the multilayered film was 463 nm, and the thickness of the Si and SiO2 layer can be estimated to be 0.9 and 2.2 nm, respectively. UV PL peaks around a wavelength of 370 nm were observed from all the specimens regardless of annealing temperature. The maximum arbitrary intensity of the UV PL peak was 8.45.
Figure 3 plots the maximum intensities of the UV PL peaks versus the Si-layer-thickness ratios dSi/(dSi +
) of the samples. The data in our previous work (plotted as a rectangle in Figure 3) [8] agreed well with the present graph. From the result, we can estimate that the proper Si-layer-thickness ratio to obtain the strongest UV peak from our Si/SiO2 multilayered films is around 0.29.
4. Summary
We investigated the effects of the Si-layer-thickness ratios on the UV peak intensities of Si/SiO2 multilayered films produced by alternately stacking several-nanometer-thick Si and SiO2 layers using RF sputtering for the first time. The proper Si-layer-thickness ratio to obtain the strongest UV peak from our Si/SiO2 multilayered films was estimated to be around 0.29. The sizes of Si-ncs seem to be controllable by selecting both the amounts
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Figure 1. Schematic figure of a Si/SiO2 multilayered film and the forming process of Si-ncs and interface layers (SiOx) in the film [8] .
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Figure 2. Measured PL spectra of specimens prepared from the sample with a Si-layer-thickness ratio of 0.29 (sample 6) after annealing at 1150˚C, 1200˚C, and 1250˚C for 25 min.
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Figure 3. Dependence of UV PL peak intensities on Si-layer- thickness ratios of Si/SiO2 multilayered films.
of Si and SiO2 deposition and the annealing conditions [8] . We will try to optimize the conditions (annealing temperature and time) in order to further increase the UV light-emission intensity of the Si/SiO2 multilayered films.
Such sputtered films can be used as constituent materials of autocloned photonic crystals that can be applied to novel light-emitting devices [9] -[11] . UV-light-emitting materials and devices are expected to be used in future higher-density optical-disk systems.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 26390073. Part of this work was conducted at the Human Resources Cultivation Center (HRCC), Gunma University, Japan.
NOTES
*Corresponding author.