Excitation Time Dependence of Luminescence Decay in Thermally Oxidized Porous Si
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537 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
RESULTS AND DISCUSSION The temporal change in PL intensity for an excitation pulse width of 30 or 600 psec is shown in Fig. 1, where t=0 corresponds to the onset time of the excitation pulse. The initial PL intensity for 600#sec excitation is about 2.5 times larger than that for 30osec. It is likely that photogenerated carriers are captured by localized radiative centers [16, 17] and the carrier population at the states with a broad energy distribution increases as the excitation time increases. For evaluating the initial PL intensity enhancement the PL intensity ratio for 600psec excitation I6000,sec to 30psec I30o,, is plotted as a function of temperature at different emission energies as shown in Fig. 2. The increase of the intensity ratio with decreasing temperature indicates that the reduction of nonradiative recombination at lower temperatures enhances the cumulative number of photogenerated carriers trapped at localized radiative centers. At temperatures above 130K the PL intensity ratio for the lower energy emission is larger than that of the higher energy emission, while at temperatures below 77K it is reversed. The reason for this will be discussed later. The increase of excitation pulse width results in the increase of the PL decay time constant as shown in Fig. 3, where the decay curves at an emission energy of 1.63eV are displayed. The PL decay curve can be fitted to a stretched exponential function as IpL=10exp[-(t/T)0] [14]. Here, 10 is the 3.0 7= 2.5 2.0 ,• 1 .5
C 1.0
e.0 C
-0 0.
0.5 0
Time
(psec)
Fig. 1 The luminescence decay for different excitation pulse widths of 30 and 600psec. The onset of Ar+ ion laser (488nm) pulse is taken as time zero. 12.0 10.0
cc
8.0 C
I
6.0
LL
4.0 2.0 0
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200
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300
Temperature (K) Fig. 2 The temperature dependence of PL intensity ratio I/6o0,sec/l30,cc for different emission energies. The inset shows the definition of the luminescence intensity ratio. 538
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0.5
0 U)
-0.5
-B
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U)
-1.0 .•C
C
10 msec
a. -2.0
-2.0 At 1.63 eV
3
-2.5
A C
-3.0 -3.5
2.5 msec
("- -1.5
S-1.5 ,
A 0.5 msec
-0.5 B
1.1 msec
C
-1.0
1
0
51lIsec 145 psec
A
-2.5
At 1.63eV
-3.0
18K
B C
-3.5 ..............
0
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Time (psec)
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10
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Time (msec) (b)
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Fig. 3 The 1.63eV luminescence decay for different excitation pulse widths measured at 293K (a) and at 18K (b). PL intensity at t=0, -r the mean lifetime, and 13a parameter between 0 and 1, such that 13=1 corresponds to luminescence from a single radiative state and 13
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