Resonant Excitation Spectroscopy of Light-Emitting Silicon Nanostructures
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Mat. Res. Soc. Symp. Proc. Vol. 486 0 1998 Materials Research Society
Si/SiO 2 wells and nanocrystals suggest that Si/Si0 2 nanostructures are inhomogeneous systems and the inhomogeneously broadened PL spectrum will be due to the size fluctuations of wells and nanocrystals and strains and structural variations at the interface between Si well and Si0 2 barrier layers [8,9]. In the study of inhomogeneous materials, resonant excitation spectroscopy is a powerful method to extract intrinsic properties from inhomogeneously broadened spectra and provides a detailed information on the luminescence mechanism and exciton dynamics [10-12]. In this review-style paper, we summarize our recent spectroscopic studies on c-Si/Si0 2 dots and wells and discuss the electronic structures of c-Si nanostructures. Si NANOCRYSTALS Sample Preparation Isolated Si0 2 -capped Si nanocrystals were produced by laser-breakdown of SiI-4 gas [3] and plasma decomposition of SiI-4 gas [8]. By the decomposition of SiH 4 gas, Si nanocrystals were fabricated on quartz or c-Si substrates. Si nanocrystals with SiO 2 surface layer were formed after thermal oxidation at 800 0C. The size of the crystalline Si core was varied from 2.5 to 6 nm, by changing the oxidation time from 0 to 30 min. Very small Si nanoparticles in Si0 2 glasses were fabricated by Si' ion-implantation into fused silica glass of 1 mm thickness [13]. The samples were prepared by implanting at room temperature a dose of 2x10' 7 cm"2, 200 keV 2 1Sil ions followed by lamp annealing [900cC in forming gas (10% H + 90 % N ) 2 2 for 3 min]. The porous Si samples were prepared as follows. The substrates were (100)-oriented p-type Si wafers with resistivities of '-0.9 or -3-5 0 cm. Thin Al films were evaporated on the back of the wafers to form a good ohmic contact. The anodization (in dark) was carried out in H-ethanol solution (H:H 20:C2HsOH=1:1:2) at a constant current density of 10 mA/cm 2. The porous layer thickness of - 1 ii m was chosen to provide a uniform optical excitation within the porous layer and a uniform native oxidation within the layer after prolonged air exposure [14]. The Fourier transform infrared (FTIR) absorption spectrum of as-prepared porous Si shows that strong absorption peaks are clearly observed around 625, 665, 910, and 2100 cm". These peaks are due to Si-H, bending modes, Si-I-, deformation modes, Si-H 2 scissors mode, and Si-HI, stretching modes. The surface of nanocrystals in as-prepared porous Si is completely covered by Si hydrides (SiH, Sill 2 , and Sill 3). As-prepared porous Si can be used as H-passivated Si nanocrystals. After prolonged air storage at room temperature, the Si-O bending and the Si-OSi stretching modes are only observed in the FTIR spectrum. The surface of oxidized porous Si is covered by the amorphous Si0 2 layer [14]. Size Dependence of the Luminescence Peak Energy The size-dependence of the PL peak energy has been studied in surface-oxidized Si nanocrystals [3,8,15,16] and H-passivated porous Si [17-19]. Figure 1 summarizes the size dep
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