Transport of carriers in thin Metal/PS/c-Si device structures based on porous silicon
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Transport of carriers in thin Metal/PS/c-Si device structures based on porous silicon Leonid A. Balagurov*, Sue C. Bayliss**, Elena A. Petrova*, Bayram Unal**, and Dmitrii G. Yarkin* *State Institute of Rare Metals, B.Tolmachevsky 5, Moscow, 109017 Russia **Faculty of Applied Sciences, De Montfort University, Leicester, LE1-9BH, England ABSTRACT Thin porous silicon (PS) based metal/PS/p-c-Si structures were prepared on moderateand high- resistivity substrates. Measurements of current-voltage (I-V) dependencies and impedance at various temperatures were used for the investigation of carrier transport in these structures. The exponential forward bias I-V dependencies for both types of structures spread over several orders of magnitude with a low value of quality factor (close to 2) and have activation temperature dependencies with an activation energy equal to half the c-Si band gap. The reverse current has a square root dependence on the reverse bias voltage and the activation energy is equal to half the c-Si band gap. Therefore, it was concluded that the reverse and forward currents in thin PS-based device structures were determined by the generation and recombination of carriers in the depletion region of the c-Si substrate. It was shown that a large area spreading current exists in structures made on highly resistive substrates, which appears to be due to a highly conductive inverse (n-type) layer formed in the c-Si substrate at the PS/c-Si heterojunction. INTRODUCTION Since Canham discovered efficient luminescence from PS at room temperature, much attention has been paid to its optoelectronic applications. From this point of view, it is of great importance to study the mechanisms of charge carrier transport, because they play an important role in the development and further performance of PS based devices such as photodetectors, solar cells, light–emitting devices, etc. However, only a little work has been done on electrical transport in PS device structures as compared to the optical properties [1-5]. These studies were mainly performed on device structures with a relatively thick PS layer. In this work, we investigated the I-V dependencies and the impedance of Al/PS/p-c-Si structures prepared on moderately and (for the first time) on highly resistive substrates. It was found that the reverse and forward currents in PS device structures with thin and relatively highly porous PS layers were determined by the generation and recombination of carriers in the depletion region of the c-Si substrate. It was also shown that highly conductive inverse layer appears in the c-Si substrates at the heterojunction, which strongly influences the impedance and I-V dependencies. EXPERIMENTAL For the fabrication of device structures, the p-type substrates were used with resistivities of 10, 1000 and 20000 Ω cm. A p+ layer was formed on the back side of the substrates using ion implantation with boron atoms. The Al/PS/(10- Ω cm)c-Si structures were made with 58-80% porosity and 1-10 µm thickness (d) of the PS layer. The anodization
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