Effect Of High Concentration Of Defect States At PS/c-Si Heterointerface On Transport Properties Of Al/PS/c-Si Photodiod
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Al/PS/c-Si and n-PS/p-PS/c-Si structures and fabricated photodetectors with a photosensitivity spectrum (S(X)) similar to that of c-Si photodetectors and a quantum yield close to unity. The Al/n-PS/p-PS/c-Si structures had noise-equivalent power (NEP) values of the order of 6*10-13 W/IHz/ 2 and photoconductivity relaxation time of t=2 ns [4], which are close to the values known for the commercially available fast p-i-n silicon detectors. But NEP of the Al/PS/c-Si structures was much higher due to high reverse current. Belyakov et al. [5] and Ozaki et al. [6] reported, that the photosensitivity spectrum of metal/PS/c-Si structures is blueshifted compared to that of c-Si based photodetectors. In this paper we report the properties of improved Al/PS/c-Si photodetector structures. EXPERIMENT Photodiode structures were manufactured from p-type (10 fQ cm) c-Si wafers having an Al ohmic contact fused at 5500C on the rear side. The porous silicon layers of 0.2 - 10 pm thickness (d) and 60% and 80% porosity were obtained by anodic etching of the wafers in a solution of HF (48%): C2H5OH = 3 :1 at a current density of 10 mA/cm2 and HF (48%): C2H5OH = 3 : 5 at a current density of 13 mA/cm 2, respectively. Finally, Al contacts deposited on PS layers with total area (A) equal to 6.5 mm2 composited of a contact pad with an area of 3.5 mm 2 (with thick
Al layer) and a semitransparent (transmission of about 50%) contact with an area 3 mm2 for 305
Mat. Res. Soc. Symp. Proc. Vol. 486 © 1998 Materials Research Society
photoelectric measurements. Porosity of the samples was determined from the values of the refraction index calculated using interference fringes in optical transmission spectra. The spectral depen-
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dence of the photoluminescence (PL) for our PS layers induced by laser radiation with a wavelength of 0.34 pm at room temperature has two maximums of 0.9 and 0.65 pm (see [7] for details).
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RESULTS
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Figure 1shows typical I - V curves for the 10°10 Al/PS/c-Si structure with a 0.2 and 2 pm thick 80% porosity PS layers. The positive 3 1 3 5 axis (forward bias) corresponds to the case when c-Si substrate is connected to the V (V) positive output of the power source. It is seen that dark I-V characteristics of strucFig. 1. Current-voltage characteristics of the tures with different thickness of PS are very Al/PS/c-Si structure with thickness of PS close. It means that reverse current is independent on PS thickness and is determined layers 2 (,2) and 0.2 pm (3,4) measured in by Al/PS barrier or PS/c-Si heterojunction. dark and under the light irradiation. The activation energy of reverse current was equal 0.5-0.6 eV for measured structures. For structures with thickness > 2 pm at large forward bias, the I - V curves exhibit an I-V 2 dependence. We have also observed an I-d 3 dependence and a very weak temperature dependence of the current at a large forward bias for structures with 2 - 6 pm thickness of PS layers. These results indicate that a region of space-charge limited current
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