Study on The Photoconductive Effect from a P/N Junction Structure Incorporated with Porous Silicon
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STUDY ON THE PHOTOCONDUCTIVE EFFECT FROM A P/N JUNCTION STRUCTURE INCORPORATED WITH POROUS SILICON C.C.YEH, KLAUS Y.J. HSU*, P.C. CHEN and H.L. HWANG Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC *IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA ABSTRACT We utilized the conventional planar fabrication technique and the electrochemical etching method to prepare porous Si layers in the p-type region of a p/n junction, which could make the study on the transverse transport property of this material possible. The junctions were fabricated by low energy ion-implantation, with porous Si formed perpendicular to the junction and between two metal contacts. This structure confines currents to the direction parallel to the surface. Distinct features on current-voltage (I-V) curves has been observed. INTRODUCTION After the report of the visible light emission out of porous Sill], this material regained a lot of attention in the hope that optoelectronics in group IV materials can someday be mature. However, many essential subjects about it, such as the formation mechanism, the structure, and the origin of visible light emission, remain controversial. While most work has focused on the optical properties of porous Si, not much effort was paid to the electrical aspects. Bilenko et al.[2]measured the electrical conductivity and the Hall coefficient of porous Si. Beale et al.[3J used two and four terminal measurements to obtain the resistivity of porous Si. Both work did not present I-V characteristics. Some recent work[4--6 reported the I-V curves of metal-porous Si junctions. In these work, the current flowed through the junction and the porous layer along the direction of electrochemical etching. Results showed that metal/p-type porous Si junctions have Schottky junction-like behavior while metal/n-type porous Si junctions tend to be ohmic. Despite these, detailed study on the I-V characteristics and the transport mechanism of carriers in porous Si is lacking. As a matter of fact, better understanding in the electrical transport properties of porous Si is not only important to electroluminescence and other photosensitive devices design but also helpful to understanding the structure of porous Si itself. In this work, we present a study on the electrical transport property of porous Si. The I-V characteristics for currents flowing perpendicular to the etching direction are shown. EXPERIMENTAL The p-type layers were formed on n-type Si wafers with resistivities of 4-7 --cm by using low energy boron-implantation, and thermal annealing at 9500C for 20 minutes. The junction depths were determined by the spreading resistance method as 0.52-0.61 pm. Gold with 500 pm separations were evaporated on the surface. Porous Si was formed by electrochemical anodization in HF solutions in the region between contacts without illumination. Porosity was difficult to be determined accurately in our case.2 The concentration of HF solution was 10%. The etching current was 5mA/cm and the
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