Temperature-Dependent Growth of Crystalline Silicon Quantum Dots Embedded in Silicon Nitride

The crystalline silicon quantum dots (Si QDs) depending on growth temperature were investigated using plasma enhanced chemical vapor deposition. The size of Si QDs was increased with increasing growth temperature and the ratio between silicon-related gas flow and nitrogen-related gas flow. This is because the growth rate of Si QDs decreases due to surface sites blocking by hydrogen. Hydrogen atoms dissociated from N-H and Si-H could promote the growth of crystalline phase silicon QDs.


Introduction
Since 1990 [1], silicon quantum dots (Si QDs) have been intensively studied because of the possibility of applications such as compatible metal-oxide-semiconductor (CMOS) devices [2], light source [3][4][5], fluorescent tags for biomedical applications [6], and their novel applications with electrical and optical functions. For accomplishment of various applications, we need to be trying to understand how to control the size of Si QD embedded in an insulating layer. According to the structural size of Si QD, this is because confined and/or recombined carriers in Si QD represent their different electrical and optical properties [7-10]. However, few papers were reported about the structural properties of Si QDs embedded in silicon nitride. It is necessary 10 W and 1 torr. Growth time of all samples was 10 min. 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 (FTIR) in the wave number range from 400 to 4000 cm -1 with a resolution of 4 cm - 1 . The structures of Si QD were investigated by a high-resolution transmission electron microscopy (TEM).

Results and Discussions
To investigate the effect of size distribution of Si QDs, we performed TEM and transmission electron diffraction (TED) analyses of Si QDs embedded in the silicon nitride film in Figure 1a, Figure 1b and Figure 1c and the size distribution of Si QDs was obtained from the TEM image in Figure 1d, Figure 1e and Figure 1f.  Figure 1. In previous study [11], we reported that the crystallization of Si QDs is enhanced by the hydrogen dissociated from the NH 3 gas. These results reveal that the growth temperature can control the size of Si QDs, while Si QDs are the crystalline phase even at the low temperature.

NH
3 at fixed growth temperature of 100 °C and the growth temperature of 100 and 300 °C. Figure 3b and Figure 3c is a plot of that peak intensities normalized with respect to the as-grown film at 100 °C with SH 4 flow rate of 100 sccm and NH 3 flow rate of 10 sccm, respectively. When the NH 3 flow rate was increased, the N-H and Si-H peaks diminished in intensity as the increased temperature promoted the release of hydrogen atoms from N-H and Si-H bonds. When the annealing temperature was increased, the N-H and Si-H peaks diminished in intensity as the increased temperature promoted the release of hydrogen atoms from N-H and Si-H bonds. The Si-N peak intensities decreased slight-PL spectra were red-shifted from 506.36 to 564.56 nm with decreasing the flow rate of NH 3 / SiH 4 at 100 °C as shown in #1 and #2 of Figure 2. Increasing the growth temperature has also made PL spectra red-shifted from 564.56 to 615.75 nm in the fixed flow rate of gas sources as shown in #2 and #3 of Figure 2. Furthermore, PL spectra of Si QDs are red-shifted with increasing growth temperature with fixed flow rate of NH 3 /SiH 4 ( Figure  2b). The shifted PL of Si QDs is in good agreement with TEM analyses grown at various temperatures shown in Figure 1. It is that the shifts of peak positions were reflected as the changed size of Si QD. clearly show that the shifts of peak positions were reflected as the temperature increase. Figure 4c and Figure 4d shows PL spectra of annealed samples #1 and #2 in the temperature from 100 to 400 °C during 10 min. In order to maintain the same growth circumstance, we used PECVD chamber as an annealing furnace. PL peak positions of annealed sample #1 were not changed from 564.56 to 566.6 nm, while those of annealed sample #2 were effectively changed from 506.36 to 547.38 nm. This is because the annealing effect on smaller Si QDs was more effective than that on larger Si QDs due to their surface area-to-volume ratio. In the range of temperature 100 to 400 C, hydrogen atoms between Si QDs and dielectric matrix may be out-diffused and then the formation of new Si-Si ly with increasing annealing temperature and the peak location was shifted from 841 to 835 cm -1 . The reduced Si-N peak intensity suggests that the Si and N atoms released from Si-H and N-H bonds do not rearrange to form Si-N bonds. The changes in the Si-N peak most likely result from decomposition of the silicon nitride matrix [12].
In order to obtain the effect of growth temperature of Si QDs, we performed the annealing of Si QDs grown at 100 °C in PECVD chamber. Figure 4a depicts the variation in PL spectra and peak positions for Si QDs embedded in silicon nitride films as the growth temperature was varied between 100 and 400 °C in PECVD chamber ( Figure 4b). As the growth temperature was increased, the PL peaks were shifted to lower energy values. These  lowered. This is because the growth rate of Si QD decreases due to surface site blocking by hydrogen. The growth rate of Si QDs suppressed more significantly than nucleation as temperature drops, thus one obtains the densest and smallest Si QDs at the lowest possible temperature. The selectivity, or relative ability of a precursor to stick to a surface of Si QDs versus a dielectric surface, largely determines density and size of Si QDs. Atomic silicon is nonselective, i.e. it sticks with unit probability to either surface, thus smooth films or very fine polycrystalline films can result [17]. Hydride gases such as silane prefer to adsorb on the Si QD surfaces as opposed to dielectrics, thus sparse nuclei form on the substrate and grow quickly, eventually producing a coarse poly crystalline film.

Conclusion
In this study, the effect on growth temperature of crystalline Si QDs grown using PECVD was investigated. The PL peak positions of Si QDs were red-shifted with increasing the growth temperature. This is attributed to the increase in the size of Si QDs because the Si QDs grown at higher temperature rapidly grew compared to the Si QDs grown at lower temperature due to surface site blocking by hydrogen. The presence of hydrogen atoms during the growth of Si QDs could promote the growth of crystalline phase Si QDs at the even low growth temperature.