Photoluminescence of silicon quantum dots in silicon nitride grown by ...

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Feb 25, 2005 - Baek-Hyun Kim, Chang-Hee Cho, Tae-Wook Kim, Nae-Man Park,a) Gun Yong Sung,a) and Seong-Ju Parkb). Nanophotonic Semiconductors ...
APPLIED PHYSICS LETTERS 86, 091908 共2005兲

Photoluminescence of silicon quantum dots in silicon nitride grown by NH3 and SiH4 Baek-Hyun Kim, Chang-Hee Cho, Tae-Wook Kim, Nae-Man Park,a兲 Gun Yong Sung,a兲 and Seong-Ju Parkb兲 Nanophotonic Semiconductors Laboratory, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 500-712, Republic of Korea

共Received 28 September 2004; accepted 7 January 2005; published online 25 February 2005兲 The photoluminescence 共PL兲 property of crystalline silicon quantum dots 共Si QDs兲 in silicon nitride grown by using ammonia and silane gases is reported. The peak position of PL could be controlled in the wavelength range from 450 to 700 nm by adjusting the flow rates of ammonia and silane gases. The PL intensity of Si QDs grown by ammonia was more intense compared to that of Si QDs grown by nitrogen gas. To investigate the role of hydrogen in the PL enhancement, the Si QDs grown by nitrogen gas were postannealed under hydrogen ambient. The enhancement in PL intensity was attributed to the hydrogen passivation of dangling bonds related to silicon and/or nitrogen at the interface of Si QDs and silicon nitride. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1872211兴 Bulk silicon has a limited efficiency as a light emitter because, as the main semiconductor in microelectronic circuitry, it has indirect transition characteristics.1 However, when silicon is in the form of low-dimensional systems,2–7 such as porous silicon, silicon nanocrystals, and superlattices, light emission from silicon has been shown to be possible at room temperature. Such studies have led to many claims of a future role for silicon in photonic applications, in which silicon-based light-emitting diodes 共LEDs兲 represent promising candidates for the next generation of full-color flat panel displays, optical interconnections, telecommunications, and lasers. The advantages of silicon-based LEDs include complementary metal-oxide-semiconductor compatibility, system feasibility, and their low cost of fabrication. Although a variety of emission colors from silicon, such as porous silicon2 and nanocrystalline silicon,4 shows a sufficiently high efficiency for applications, the tuning of emission color, particularly in the short-wavelength region, continues to be a challenge because the silicon/dielectric interface is also thought to play an important role in the formation of radiative states.8 Silicon nitride films containing amorphous silicon 共a-Si兲 quantum dots 共QDs兲 grown by using SiH4 and N2 as gas sources have excellent optical properties even in the short-wavelength region due to the quantum size effect.9 However, the external quantum efficiency of an LED using these films is still low because of nonradiative defects and poor carrier injection into a-Si QDs in the active region.10 Therefore, one of the methods for improving the quantum efficiency of an LED is to reduce nonradiative defects due to dangling bonds. In this letter, we report on the enhancement in photoluminescence 共PL兲 from silicon QDs 共Si QDs兲 by replacing N2 which was used in the previous studies,9,10 with NH3 which contains hydrogen atoms to passivate the nonradiative defects. Silicon nitride films were grown by plasma-enhanced chemical vapor deposition 共PECVD兲, where nitrogen-diluted a兲

Future Technology Research Division, Electronics and Telecommunications Research Institute, Daejeon, 305-350, Republic of Korea. Author to whom correspondence should be addressed; electronic mail: [email protected]

b兲

5% SiH4 and additional NH3 or N2 with the purity in excess of 99.9999% were used as the reactant gas sources. A lowdoped p-Si wafer 共100兲 was employed as a substrate. In the preparation of the silicon nitride films, the flow rate of SiH4 was varied from 200 to 800 sccm and an additional flow rate of NH3 gas was used in the range from 10 to 100 sccm. The pressure and growth temperature were maintained at 1.0 Torr and 300 °C, respectively. The plasma power was in the range from 10 to 20 W. For comparison, silicon nitride films were fabricated by replacing NH3 with N2 under a flow rate of SiH4 that ranged from 6 to 3 sccm and an additional flow rate of N2 gas was used in the range from 100 to 900 sccm.9,10 The pressure and growth temperature were maintained at 0.5 Torr and 300 °C, respectively. The sample grown by using SiH4 and N2 was postannealed at 700 °C for 30 min under H2 ambient to study the effect of hydrogen passivation of Si QDs in the film. A spectral analyzer system was used for the PL measurements at room temperature and a He–Cd 325 nm laser was used as an excitation source. Chemical bonds in the film were examined by a Fourier transform infrared spectroscopy 共FTIR兲 in the wave number range from 400 to 4000 cm−1 with a resolution of 4 cm−1. The structure of the Si QDs was investigated by a high-resolution transmission electron microscopy 关共HRTEM兲 Phillips CM20T/STEM Electron Microscope兴 operated at 200 kV. The gas flow rates of SiH4 / NH3 corresponding to three primary colors of blue, green, and red were observed to be 100 sccm/ 90 sccm 共450 nm兲, 200 sccm/ 30 sccm 共546 nm兲, and 800 sccm/ 30 sccm 共670 nm兲, respectively. The PL intensity was very strong and could be observed with the naked eye in a bright room. The change in PL peak position depending on the gas flow rate is shown in Fig. 1共a兲. This clearly shows that the PL peaks are shifted toward the lowerenergy side when the flow rate of SiH4 is increased from 100 to 800 sccm at a fixed NH3 flow rate of 30 sccm. The PL peaks, however, are shifted toward the higher-energy side when the flow rate of NH3 is increased from 10 to 90 sccm at a fixed SiH4 flow rate of 400 sccm as shown in Figs. 1共a兲 and 1共b兲. Two types of luminescent mechanisms, such as radiative defects in the film and the quantum confinement effect

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FIG. 2. FTIR spectra as a function of NH3 flow rate at a fixed SiH4 flow rate of 800 sccm. Four absorption peaks are observed and the position was not changed for any of the samples.

FIG. 1. 共a兲 PL peak positions as functions of the flow rates of SiH4 and NH3, and 共b兲 PL spectra of samples grown by changing the flow rate of NH3 at a fixed SiH4 flow rate of 400 sccm.

共QCE兲 in Si QDs, have been proposed to explain the origin of light emission from nanosized silicon. In defect-related models, the PL peak position cannot be easily controlled because the peak position should be observed only near the defect energy levels. Three main radiative defect energy levels of 1.8, 2.4, and 3.0 eV have been reported for the defect model. The 1.8 eV peak is due to recombination between the N+4 and N02 levels, the broad 2.4 eV peak is attributed to recombination processes at the silicon dangling bond, and the 3.0 eV peak is due to recombination either from the conduction band to the N02 level or the valence band to the N+4 level.11 However, our samples did not reveal any specific PL peaks corresponding to defect energy levels. Because the PL peak position of our samples was freely controllable by the adjustment of the gas flow rates, we exclude the possibility of PL emission from the radiative defects in the silicon nitride film. Another model is the QCE in which the silicon clusters exist in the silicon nitride film and the position of PL peak is dependent on the size of Si QDs as shown in Refs. 9, 10, and 12, where the PL peak position could be tunable by controlling the size of Si QDs via modulation of N2 gas flow rate. The full width at half maximum and the control of PL spectra observed in this work are also very similar to results reported by Park et al.9,10,12 Therefore, based on these results, the luminescent origin of the samples as shown in Figs. 1共a兲 and 1共b兲 is considered to be QCE in Si QDs embedded in the silicon nitride film. The formation of Si QDs in the silicon nitride was further studied by FTIR measurements. The FTIR spectra shown in Fig. 2 show the change of chemical bonding configurations in the silicon nitride film as a function of NH3 flow rate. The intense absorption band at 840–879 cm−1 is assigned to the Si–N stretching mode. The weak band in the 1170–1190 cm−1 range is generally assigned to the N–H rocking mode and the band at 3350 cm−1 corresponds to the N–H stretching mode. The band at

2180–2200 cm−1 can be attributed to the Si–H stretching mode. Figure 2 shows that four absorption peaks and the peak intensities vary with the NH3 flow rate, 30, 60, 90 sccm, respectively, and a SiH4 flow rate was fixed at 800 sccm. Figure 3 shows the FTIR peak intensities of the four absorption peaks as shown in Fig. 2. These peak intensities were normalized with respect to the peak intensity values at a NH3 flow rate of 30 sccm. Figure 3 shows that the N–H peaks increase and Si–H peaks decrease as the NH3 flow rate is increased. The increase in NH3 flow rate appears to promote the dissociation of Si–H bonds, resulting in an increase in silicon atoms having dangling bonds. The increase in dangling bonds of silicon atoms is believed to facilitate the creation of nucleation sites and the formation of silicon clusters in the silicon nitride film during the growth process.9,12 Because the flow rate of SiH4 gas is maintained at a constant level and the number of nucleation sites of the silicon clusters is increased, the size of the silicon clusters decreases with increasing NH3 flow rate. Therefore, the blueshift of the PL peak with increasing NH3 flow rate or decreasing SiH4 flow rate as shown in Figs. 1共a兲 and 1共b兲 can be explained by a mechanism in which the increase in silicon dangling bonds can facilitate the nucleation of silicon clusters to produce small Si clusters in the silicon nitride film. It is noteworthy that the absorption peaks of Si–H and N–H bonds of the silicon nitride film grown by using SiH4 / NH3 as shown in Fig. 3 are stronger than those for samples grown by using SiH4 / N2.9,12 The enhancement in the absorption of hydrogen

FIG. 3. Relative FTIR absorption peak intensities as a function of NH3 flow rate. All peak intensities are normalized with respect to the peak intensity values at a NH3 flow rate of 30 sccm. Downloaded 17 Mar 2005 to 203.237.48.250. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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FIG. 5. Comparison of PL spectra of silicon nitride films grown by using SiH4 / NH3 共filled circle lines兲 and SiH4 / N2 共filled square lines兲 and a silicon nitride film grown by using SiH4 / N2 and subsequently annealed at 700 °C for 30 min under H2 共filled triangle lines兲.

FIG. 4. HRTEM image of c-Si QDs embedded in a silicon nitride film: 共a兲 Dark spots represent c-Si QDs with a dot density of about 1.5⫻ 1012 / cm2, 共b兲 high-resolution lattice image, and 共c兲 ring patterns for the transmission electron diffraction from c-Si QDs.

related peaks is attributed to NH3 gas that can introduce a larger amount of hydrogen species into the film than N2 gas. Figure 4共a兲 shows the HRTEM images of Si QDs embedded in the silicon nitride film. The average size of Si QDs was about 4.5 nm and the Si QD density was about 1.5 ⫻ 1012 / cm2. Most of the Si QDs were essentially in a crystalline state, as revealed by a HRTEM image of the lattice fringe which corresponds to the 共111兲-lattice planes of Si as shown in Fig. 4共b兲 and the electron diffraction patterns showing ring patterns as shown in Fig. 4共c兲. This result is very different from the samples grown by using SiH4 / N2, where most of the Si QDs were in an amorphous state.12 This suggests that the crystallization of Si QDs is enhanced by the hydrogen dissociated from the NH3 gas.13 Figure 5 shows a comparison of the PL spectra of two as-grown samples grown by using SiH4 / NH3 and SiH4 / N2, and the SiH4 / N2 grown sample annealed at 700 °C for 30 min under H2. The sample grown by using SiH4 / NH3 shows a more intense PL peak compared to that grown by using SiH4 / N2, and the SiH4 / N2 grown sample annealed under H2 as shown in Fig. 5. The PL intensity of the SiH4 / N2 grown sample annealed under H2 is significantly increased compared to that of the sample grown by using SiH4 / N2 as shown in Fig. 5. The enhancement in PL intensity of the SiH4 / N2 grown sample by thermal annealing under H2 indicates that the thermal diffusion of hydrogen into the sample grown by using SiH4 / N2 is responsible for the passivation of dangling bonds which are related to silicon and/or nitrogen in silicon nitride film and at the Si QD-silicon nitride interface. The enhancement in PL intensity of the SiH4 / NH3 grown sample compared to that of SiH4 / N2 sample as shown in Fig. 5 can be also attributed to the hydrogen passivation of dangling bonds because more hydrogen atoms are produced from NH3 compared to the N2 gas source and the radiation rate of crystalline silicon quantum dots 共c-Si QDs兲 is lower than that of a-Si QDs.14–16 The hydrogen passivation of silicon nanocrystals has been also shown to have a significant effect on the PL emission intensity to increase it by up to an order of magnitude.17,18 This result was attributed to the passivation of nonradiative defect centers either within the sili-

con nanocrystals or at the silicon nanocrystal-SiO2 interface.17–19 In summary, a strong PL intensity was observed from the silicon nitride films containing high-density c-Si QDs which were grown by PECVD using SiH4 and NH3. The PL emission energy could be controlled in the wavelength range of 450–700 nm by varying the flow rates of SiH4 and NH3 gases. The significant enhancement in PL intensity of Si QDs grown by using SiH4 / NH3 and the SiH4 / N2 grown Si QDs annealed under H2 ambient, compared to that of Si QDs grown by using SiH4 / N2, is attributed to the hydrogen passivation of nonradiative defects, such as silicon and/or nitrogen dangling bonds. This work was supported by the Ministry of Information and Communications and the National Research Laboratory program for nanophotonic semiconductors in Korea. 1

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