The effect of photoconductivity localization in macroporous silicon

Abstract

The surface photoconductivity in macroporous silicon with an oxide coating with thickness of 3 to 30 nm with a strong surface light absorption, the absorption coefficient is varied from 330 to 105 cm^–1 (wavelength is from 0.93 to 0.4 micron) has been investigated.The starting material consisted of n-type silicon with [100] orientation and 4.5 Ω∙cm resistivity. Macropores with diameter Dp = 4 μm and depths hp = 100 μm were formed by electrochemical etching. At the wavelengths of 0.4 and 0.57 microns, and the thickness of the oxide layer of 3 to 15 nm the surface monopolar negative photoconductivity was manifested, due to the inversion band bending in the surface layers of the semiconductor and due to capture of majority carriers on the so-called «slow» surface levels. When the thickness of the oxide layer is 30 nm, a positive photoconductivity  with anomalous spectral dependence takes peace with an increase in the absorption coefficient from 330 to 105cm ^-1 (decreasing wavelength from 0.93 to 0.4 microns) photoconductivity  is not decreasing, but increases – localization effect of the surface photoconductivity in the space charge region was manifested. The experimental results are explained as follows. With the increasing of oxide coating thickness its positive charge also increases and compensates the negative charge of the slow surface levels. The inversion band bending at the same time becomes depletion band bending. In general, the semiconductor photoconductivity, taking into account surface effects (space charge region) consists of three components: bipolar photoconductivity of the space charge region, monopolar photoconductivity of the space charge region and bipolar photoconductivity of the quasi-neutral volume. Effect of localization photoconductivity occurs in all cases where the bipolar photoconductivity of the space charge region is more than bipolar photoconductivity quasi-neutral volume. This occurs in the case of the depleting band bending on the semiconductor surface.