ELECTROLUMINESCENCE AND PHOTOVOLTAIC EFFECTS OF ANODICALLY FABRICATEDMETAL POROUS SI/SI SANDWICH STRUCTURES BASED ON N-TYPE ULTRAVIOLET-POROUS SI/

The electroluminescence and photosensitive surface effects from metal/ porous silicon/Si sandwich structures based on n-type ultraviolet-poro us silicon were investigated in this study. Contacts were made by bond ing to semitransparent coatings of Au (deposited by a novel technique of thermal evaporation) via Al electrodes. In the case of electrolumin escence, studied by applying a current across the electrodes, orange l ight is emitted beneath the semitransparent gold film, and a reversibl e avalanche breakdown was observed at the interface at a reverse bias of around 22 V. The photosensitivity of the surface, which was coated with a thin gold film having an initial transmission of less than 60%, was observed to get worse when the above process was repeated several times, Furthermore, the avalanche breakdown voltage shifted slightly to higher values, possibly because of both structural fluctuations in the porous networks and/or Schottky conductive changes in between the metal-porous silicon interface resulting from the high current passed at breakdown and causing excess heat inside the device. Photovoltaic e ffects at the Schottky metal/porous Si barrier were also observed unde r a number of different types of illumination in the range from UV to visible, and under white light of different powers. The current-voltag e characteristics of various sandwich structures operating as solar ce lls were measured, dong with the power efficiency of the solar cells u nder a calibrated tungsten bulb, this being over 0.35% under an illumi nation power of 12 mW/cm(2) The power efficiency (=-a+bP(in)) is found to increase linearly with increasing illumination power up to a satur ation value that depends simply on preparation conditions. (C) 1996 Am erican Institute of Physics.