Possible Mechanism of the 30-100 Picoseconds Fast and Efficient Photoluminescence From nc-Si/a-SiO 2 Doped with Transiti
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Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
desirable energy transfer. Figure 1 shows the theoretical calculations of the dependence of the band gap on the crystallite size by Delley and Steigmeier [12] and by Delerue et al. [13]. It is well known that the PL from nc-Si/a-SiO 2 films shows no blue shift with decreasing crystallite size and that the shift observed in PS or isolated Si-nanocrystals is much smaller than the calculated increase of the band gap. This is illustrated for our films in Fig. I as well [15]. Recently we have measured the increase of the band gap by the excitation spectra of the red PL [ 14]. In such a case, the intensity of the red PL at a constant energy (here the peak energy of 1.5 eV) is measured as a function of the energy of the excitation light E,.. Because the crystallite sizes have a certain distribution which is difficult to determine, the onset of the PL excitation spectra is somewhat broadened. Therefore we use the energy corresponding to a 20% increase of the PL intensity IpL with respect to the maximum (see symbols A in Fig. 1) and the value obtained by extrapolation of the dependence of (IpL-ECX)1/2 on Eex to IPL=O, which is linear for an indirect band gap (see full symbols in Fig. 1). One can see that a very good agreement with the theo-
retical calculations is obtained in both cases. Thus, for the W"ý-stimulated PL one has to use nc-Si/a-Si0 2 films with an average crystallite size of•_ 2 nm.
i
S
5
1
extrapolation of (IpLEex) 2 to zero 5)
A 20 %point
4".1
c
maximum of red PL
4
(D
3 "C
UJ)
130 (-
CD
1
2
A,
Delley &Steigmeier
0
Delerue et al.
C)
• ,.•_,~~--------
0
10
20
30
40
*-------.--..-4. ..-.
50
60
70
80
crystallite size (A)
0
9,
Fig. 1: Dependence of the band gap of nc-Si on the crystallite size calculated by Delley and Steigmeier [12] and by Delerue et al [13] and experimentally determined from the PL excitation spectra [14]. For comparison, the peak position of the broad PL band which remains unchanged [15] is shown as well.
EXPERIMENTAL The sample preparation was described in our earlier papers [1,2,15-17]. Thus only a brief summary is given here. The nc-Si films of desirable crystallite size were prepared either by chemical transport of silicon in hydrogen glow discharge or by depositing a-Si followed by its controlled recrystallization. Afterwards, the surfaces of the crystallites are passivated by oxidation of the grain boundaries. The oxidation also allows us to control the average separation between the crystallites which is very important for obtaining an efficient red PL [ 18]. A subsequent annealing in forming gas further decreases the concentration of dangling bonds (which act as non-radiative recombination centers) thus resulting in a further increase of the red PL. Afterwards, the films are doped with tungsten by evaporating a desirable amount of W0 3 at the surface and diffusion of the tungsten into the films at about 870°C in forming gas. This
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preparation procedure is rather tri
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