Photovoltaic Characterization of Trapping in Porous Silicon
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polarity will be opposite. Light The magnitude of this --- S-l R4 (488 voltage may be up to several •Porous Silicon Voltmeter millivolts. In the case of ntype, the contact nearest the light spot will be positive; for p-type, the contact nearest the light spot is negative. A plot of the photovoltage as a function of light spot position is Figure 1. The measurement configuration showing a light spot shown in Fig. 2. In the swept across the sample from one contact to the other. region between the contacts, the photovoltage varies quite 0.4 linearly with the position of
the light spot. The p-type sample described in Fig. 2 is a three inch wafer over which nearly the entire surface area is anodized. This sample exhibits a linear region over 4.5 cm across. These devices are sensitive throughout the visible spectrum; the excitation used for the data presented here is the 4880A line of an Argon laser, unless otherwise noted. Figure 3 shows the photovoltaic response of the p-type sample mentioned above as the light spot is scanned in two dimensions over the porous surface between the contacts. The peak and valley are located at the contacts. The photovoltage is zero when the light spot is equidistant from the two contacts, since this preserves symmetry and one contact cannot be distinguished from the other. But when the light spot nears one contact more than the other, symmetry is broken and a voltage develops. This response is similar to the conventional lateral photoeffect, which arises under the nonuniform illumination of a junction between two layers of differing conductivities.
0.2 5ý (D 0 0C CL
-0.2
-0.4
Figure 2. Photovoltage vs. position of light spot for an n-type and a p-type sample. Photovoltage (mV)
0.
Y-axis light spot position (cm)
X-axis light spot position (cm) Figure 3. Photovoltage vs. position of light spot, as the light spot scans the wafer in two dimensions.
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This effect was first12observed by Schottky 1I in 1930 and was first applied for use in positionsensitive detectors in 1957. Devices based on this effect are commercially available from companies such as Hamamatsu, which sells silicon-based position-sensitive detectors that can determine the position of a light spot in two dimensions in areas up to 27 mm square. Due to the slow time response of the lateral photovoltaic effects, practical applications are limited to special cases. However, the time response provides clues to the origin of the effect in to the slow time constant of the light-induced porous silicon. The time response is comparable photoluminescence degradation.1,2,4,5,13-1 6 Interestingly, different samples can show different photovoltage time constants, ranging from several seconds to several hundred seconds, depending on the substrate doping and etching conditions. The time constant dispersion is a topic of continuing research and will be reported later in a more detailed study of this effect. In Fig. 4, we show the lateral photovoltage and the photoluminescence intensity measured simultaneously for one device. Note th
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