Nonlinear Electrical Functions of Porous Silicon Light-Emitting Diodes
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Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
samples were transferred into a vacuum chamber, and semitransparent Au films were deposited onto the PS layers. The active area of the PS diodes was 5 mm in diameter. Measurements The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of PS diodes were measured under the forward and reverse bias conditions at room temperature and low temperatures. The EL intensities are also measured at the same time. For the PS diodes fabricated on p-type Si substrates, the forward bias condition corresponds to the case in which a negative voltage is applied to the Au electrode with respect to the substrate, and vice versa in the case of ntype substrates. RESULTS Electrical Properties as a MIS Diode Typical I-V curves of the PS diode with sample A at room temperature and 50 K are shown in Fig. 1 by the solid and dashed curves, respectively. The experimental PS diode under the forward bias condition is also schematically shown as the inset of Fig. 1. The diodes exhibit a definite rectifying behavior at room temperature. At low temperatures of 50 K, a significant decrease in the diode current is clearly observed at low reverse bias voltages. The reverse diode current, however, shows a rapid increase at a critical reverse bias voltage in a similar way in conventional MIS tunnel diodes. To investigate these peculiar characteristics of the experimental PS diodes, a band model is schematically shown in Fig. 2. The PS layer includes a great number of quantum-sized silicon crystallites surrounded by some electronic barriers. Under the threshold reverse bias voltage Vth, the inversion effect occurs at the PS/p-Si interface, and consequently a number of electrons are generated there. The generated electrons are injected into PS by tunneling. The observed value of Vth is close to the estimated value in this system. The calculated curve of the diode current in accordance with this model is shown in Fig. 3 by the solid curve. The inset shows the potential barrier diagram, where Ad and cD,are the barrier width and the barrier height, respectively. We assumed that the generation-recombination current is dominant below Vth. The calculated result at Ad = 6.0 nm and ( = 0.3 eV explains well the experimental behavior indicated by open circles. These values of Ad and (D,used for the fit in Fig. I are reasonable in view of the structural property of PS sample A and the previously observed activation energy (=0.3 eV) [6] in the PS conductivity near room temperature. Au PS
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Fig. 1. The I-V curves of a PS diode with sample A at room temperature and 50 K. The inset is a schematic illustration of the sample under the reverse bias condition.
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Fig. 2. Schematic model of a PS diode under the condition that a sufficiently positive bias voltage is applied to the Au electrode with respec
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