Localisation of Carriers in Porous Silicon
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V
thermal
activation'
,.
X
tunnel effect
BC B
NR center
Fig 1I A schematic representationofporous silicon, in real
•
space on right and in energy scale above, for a given direction as indicated by the arrow. Also
shown: non-radiativecenters as black spots and two examples of carrierescape by tunneling and thermal
activation in hydrofluoric acid (HF)[1] where, starting with a low porosity (and therefore a low resistivity) and a weak PL emission, the thinning leads to an increase in porosity along with a strong enhancement of the PL. Aging, has in general a positive effect on the luminescence efficiency of porous silicon. This process is not well characterized at present but in many respects can be seen as a "soft" oxidation similar to the anodic oxidation. The same correlation can be found but with the temperature as parameter. As shown by Koyama et a]. [2] and Ben-Chorin et a]. [3], the conductivity (in the dark) as well as the photoconductivity strongly increase for increasing temperatures while, for the same range of temperatures, the PL intensity is reduced. These various examples point out that there exists a strong relation between the ability of carriers to reach extended states (or on the contrary to be localized) and the efficiency of the luminescence. This can be understood in an extension of the model previously presented in ref [4] and shown in figure 1 where the porous structure is made up of slightly interconnected quantum crystallites. Electron-hole pairs are then supposed to recombine radiatively when they are present in "bright crystallites" but are dissociated and recombine non-radiatively when a carrier wavefunction is extended to "dark" crystallites . This process reduces the PL efficiency and can contribute to the free carrier population i.e. enhance the conductivity. One can imagine that a modification either via oxidation or dissolution of the interconnections between the crystallites will certainly affect this process. The temperature can also activate this extension. It is this "scenario" which will be now analyzed more carefully through the various post-formation treatments. EXPERIMENTAL PROCEDURES In this paper, an extensive use of porous silicon anodic oxidation and chemical dissolution will be done together with temperature variation. Carrier population decay times are monitored by the mean of time resolved optical measurements. We will focus on the « red >>luminescence whose decay is in the microsecond range. Porous films formation, chemical dissolution and anodic oxidation have been presented in previous papers. Excitation of luminescence is provided by the 366 nm line of a mercury arc lamp or by a pulsed Nitrogen laser at 337 nm and the temporal resolution of the analysis chain was about 5ns. Temperature variation above and below room temperature were obtained by a o Linkam )) cooling and heating stage.
500
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A"
10'
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=
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0.3 S.a
Dissolution time in mn.
0
Increasing dissolution time
100
10.1
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1'6.•
0.5 0.4 Oxidation level Q/Q
0.6
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