Figure 1.(a) Experimental setup and (b) cross-section of Photo-EMF sensors structure.The photo electromotive forces between top and bottom contacts of samples were studied in a special measuring chamber at room temperature [Figure 1(a)]. We used voltmeter-electrometer of B7-30 type for Photo-EMF registration with double screening. Composition of gas atmosphere was changed by gas generator of GR-645 type by dynamical mixing clean nitrogen and ammonia gases [Figure 1 (a)]. The surface of samples was illuminated at different intensity and wavelengths of light. Illumination level was measured by IL Luminance meter T-10 (Konica Minolta). For spectral measurements a spectrophotometer SF26 was used.3.?Results and DiscussionElectrical voltages (Photo-EMF) between top and bottom contacts [Figure 1 (a)] were registered under illumination of samples.

Their magnitudes depended on concentration of ammonia in the measurement chamber. Figure 2 shows spectral dependence of Photo-EMF at an illumination level 200 l�� at different concentrations of ammonia in the chamber. A maximal magnitude of Photo-EMF was observed near the wavelength of light of approximately 730 nm (Figure 2). An increase of ammonia concentration in the measurement camber leads to a decrease of the Photo-EMF magnitude. Usually photo-detectors have a maximum photo-response at a wavelength of light corresponding to band gap energy Eg [15]. In our case the band gap energy of a porous silicon layer equals approximately 1.7 eV (energy of light quantum at wavelength 730 nm [16]).Figure 2.

Spectral dependences of Photo-EMF on nitrogen and on different ammonia concentrations.Manufactured porous silicon layer had red (��max �� 780 nm) luminescence under laser illuminations at a wavelength of 441.2 nm. This red luminescence had a half-width at half maximum of approximately 0.2 eV. The luminescence in visible range of spectrum is related to quantum wires [1]. Broad band of luminescence peak demonstrates a different thickness of quantum wires. It means that porous silicon have quantum wires with greater band gap than crystalline silicon. Band gaps of our nanowires system are in the ranges of 1.7 �� 0.2 eV. In our case the heterojunction should be formed between porous silicon and silicon wafer. The electrical contacts had ohmic properties. The light Leukemia induced heterojunction can be the single reason of occurrence of the photo electromotive force on contacts.Figure 3 shows the experimental dependences on illumination level of the maximal magnitudes of Photo-EMF under different concentrations of ammonia. The magnitude of Photo-EMF increases with growing illumination level (Figure 3). Adsorption of ammonia in porous silicon layer affected the photo-EMF magnitudes appreciably.Figure 3.