Investigation of Electronic Properties of Porous Silicon by the Pulsed Surface Photovoltage Technique
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Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
The measured photovoltage Uph can be written Uph = Y - Yo + UD. A simple experimental arrangement for photovoltage measurements is shown in fig. 1. The sample, a mica interlayer (about 50 pm thick) and a transparent electrode (for example, doped SnO 2) form a capacitor with the capacitance C. A light pulse is used to generate a photovoltage transient measured via a large resistance R. The value of RC determines the time scale in which a photovoltage transient can be correctly measured. Iulight I SPV transient
transparent electrode
CI
miIca| sample
R RC = 7 ms
L
Fig. 1: Experimental arrangement for photovoltage measurements. Fig. 2a shows schematically a photovoltage transient (for a p-Si surface) during which Yo remains constant. It is characterized by a fast photovoltage build-up due to excess carrier generation and by a slow photovoltage decay due to recombination processes. Typical decay times for photovoltage transients are in the range of 10.. 100 ps.
Y0=cnstCOI=W]°t C
C:
time
•l, I > .~
excess carrier ' generation •.~~~ 4-recombination
D
dep .0 1.":,eectron C trpig p trapping ocl
a Fig.2:
time
b
Photovoltage transients schematically drawn for a p-Si surface when the surface potential remains constant (a) and for a por-Si/p-Si interface when electron trapping occurs and the surface potential isalso time dependent (b).
Fig. 2 b shows schematically a photovoltage transient (for a por-Si/p-Si interface) during which YO does not remain constant due to trapping. Trapping and detrapping of charge at and from interface states is the reason for non-monotonous photovoltage transients. For large Ap(n) the surface quasi Fermi-levels shift strongly towards the conduction and valence bands and reach energetic positions of trap states which will be fastly filled. The equilibrium surface potential decreases and the photovoltage drops in the case of electron trapping at a por-Si/p-Si interface. Further, recombination processes decrease the surface quasi Fermi-levels below the energy of trap states and detrapping is going on. The equilibrium surface potential increases and consequently the photovoltage transient shows an increase at a time characteristic for bulk carrier lifetimes in the case of electron detrapping from a por-Si/p-Si interface.
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EXPERIMENTAL P- and n-type Si wafers with resistivity of 15 and 70 fEcm, respectively, were anodized in HF:ethanol (1:1) solution (anodization current 30 mA/cm 2,anodization time 5 min). N-type Si samples were illuminated with a 100 W lamp during anodization.The thickness of the por-Si layers was controlled by Scanning Electron Microscopy and ranged from 5 pm for por-Si on p-Si to 20 pm for por-Si on n-Si. The samples were aged in air for several months and some of them were annealed in N 2 atmosphere at 150 and 250'C or heat treated in N 2/saturated H20 atmosphere at 800C. The SPV transients were measured with a Tektronix RTD 710A digitizer minimal time resolution 5 ns) for times up to I ms. F
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