Photoluminescence Mechanism of Silicon Quantum Dots and Wells
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Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
because holes are necessary for the electrochemical etching process of Si. The Si nanocrystal size can be controlled by changing illumination wavelength. Figure 1 summarizes the size dependence of the observed PL peak energies in surfaceoxidized Si nanocrystals [1,6,9,10] and as-prepared porous Si [8,11]. The size dependence of the calculated band-gap energy in Si nanocrystals is also plotted in the figure [12-14]. The observed PL peak energy in as-prepared porous Si is sensitive to the nanocrystal size, and the observed size-dependence is consistent with the theoretical calculations, as shown in Fig. 1. However, after prolonged air exposure, the PL spectra of any of the samples are similar to each other and appear in the red spectral region [15]. The surface of as-prepared Si nanocrystals is mainly covered with hydrogen during etching, but after air exposure the surface is naturally oxidized. The surface oxidation of nanocrystals changes the PL wavelength. The luminescence spectrum and dynamics of Si nanocrystals are very sensitive to surface structures of Si nanocrystals, particularly with regard to the amount of oxygen and hydrogen on the surfaces. The size dependence of the PL peak energy in surface-oxidized Si nanocrystals is different from that in H-terminated Si nanocrystals. In smaller nanocrystals, the PL energy in surfaceoxidized Si nanocrystals is much lower than that in H-terminated Si nanocrystals. Furthermore, there is a large different between the PL peak energy and the theoretical band gap energy in small oxidized nanocrystals. These results suggest that excitons relax from the higher-energy absorption state to the lower-energy emission states in oxidized Si nanocrystals. In particular, in small nanocrystal exhibiting efficient PL in the visible spectral region, the exciton localization (the disorder-induced exciton localization and extrinsic self-trapping of excitons) is important in the electronic process from the light absorption to light emission [16]. Si Quantum Wells The Si single quantum wells were formed on SIMOX (separation by implanted oxygen) wafers. The detailed fabrication methods and TEM (transmission electron microscopy) images were shown in Ref. 17. The 2D Si layer was sandwiched between thin surface Si0 2 and thick buried Si0 2 layers. Good crystalline quality in the Si layers was confirmed by the lattice image of the TEM and the roughness of the Si layers was about a few monolayer. The asymmetric PL spectra were observed in the red and infrared spectral region, and can be fitted by two Gaussian bands, the weak PL band (denoted as Q) and the strong PL band (denoted as I) [18]. The PL peak energy of the I band is almost independent of the well thickness. In contrast, the peak energy of the Q band shifts to higher energy with a decrease of the Si well thickness. Thickness dependence of the PL peak energies of the I and the Q bands is plotted in Fig. 2. The peak energy of the main I band does not depend on the well
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