High Resolution Impedance and Potential Imaging using a Mavericks Cr:Forsterite Laser
Steffi Krause, Department of Materials, Queen Mary University of London, London E1 4NS

Photocurrent measurements at field-effect structures such as electrolyte-insulator-semiconductor or metal-insulator-semiconductor structures have been used to measure local electrical potentials, local concentrations such as pH or hydrogen and the local impedance of thin films (Figure 1) [1,2]. Local concentration and potential measurements are known as Light Addressable Potentiometric Sensors while local impedance measurements are carried out using Scanning Photo-induced Impedance Microscopy. In both techniques electron-hole pairs are generated by a laser focused into the space charge region of the semiconductor. If the field-effect structure is biased towards depletion or inversion the photo-generated charge carriers separate in the field of the space charge region causing a current to flow. Modulation of the laser beam intensity results in an ac-photocurrent. As the current is limited to the illuminated area of the structure measurement can be carried out with spatial resolution.

Figure 1. Field-effect structures used for local impedance and potential measurements.

The dependence of the photocurrent on the dc voltage applied is shown in Figure 2. With increasing depletion an increase in the photocurrent is observed reaching a plateau when the structure is biased towards inversion. A shift of the photocurrent curve along the voltage axis indicates a change in the local potential; a change in the maximum photocurrent can be translated into a change in the local impedance of materials deposited onto the insulator. This technique has potential applications in the characterization of heterogeneous materials or the local electrical properties of living cells or biological membranes.

Figure 2. Changes in the maximum photocurrent and the position of the photocurrent curve on the dc voltage axis can be used to measure local impedance or electrical potentials

Lateral resolution of photocurrent measurements
The lateral resolution of photocurrent measurements is determined by the properties of the semiconductor substrate, the quality of the focus of light and the wavelength employed. Charge carriers generated in the bulk of the semiconductor substrate do not only diffuse to the space charge layer where they cause a current but they also diffuse laterally resulting in a loss of resolution. Recent experiments have shown that the lateral diffusion length of charge carriers can be reduced to less than one micrometer by using a thin epitaxial layer of silicon on a sapphire substrate (SOS) or a semiconductor with a short diffusion length of charge carriers such as amorphous silicon [4]. However in both cases the low quality of the insulator limits the application of these semiconductor substrates.
To avoid the problems encountered using thin silicon layers and amorphous silicon, it would be advantageous if bulk silicon could be employed. However, if a laser beam is focused into the space charge region from the back of the semiconductor substrate, light has to travel through the bulk of the material where it generates charge carriers resulting in a loss of resolution (Figure 3). If light with energy smaller than the bandgap is used, no charge carriers are generated in the bulk of the semiconductor. Electron-hole pairs are generated only in the focus near the space charge layer at the semiconductor/insulator interface due to two-photon absorption. The possibility of high- resolution SPIM/LAPS measurements using a two-photon effect in bulk silicon will be investigated.

Figure 3. In case of a single photon effect charge carriers are produced throughout the bulk of the material. In case of a two photon effect charge carrier generation is confined to the focus near the space charge region of the semiconductor
 

References
1. S. Krause, H. Talabani, M. Xu, W. Moritz, and J. Griffiths, Electrochim. Acta, 47, 2143-2148 (2002).
2. W. Moritz, T. Yoshinobu, F. Finger, S. Krause, et al., Sens. Actuator B-Chem., 103, 436-441 (2004).
3. S.N. Jayasinghe, M.J. Edirisinghe, D.Z. Wang, Nanotechnology, 15, 1519-1523 (2004)
4. S. Krause, W. Moritz, H. Talabani, M. Xu, A. Sabot, G. Ensell, Electrochim. Acta, in press
5. E. Ramsay, N. Pleynet, D. Xiao, R. J. Warburton, and D. T. Reid, Opt. Lett., 30, 26-28 (2005).



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