Photoconductivity and Carrier Transport in Porous Silicon
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PHOTOCONDUCTIVITY AND CARRIER TRANSPORT IN POROUS SILICON M.J. HEBEN AND Y.S. TSUO National Renewable Energy Laboratory, Golden, CO 80401.
ABSTRACT We present results from our investigations of the transport properties of p-type Si/porous silicon/Au devices. Current-voltage measurements were performed as a function of temperature from room temperature down to 23K and indicate that a tunneling mechanism governs the transport properties of these devices. Photocurrent spectroscopy measurements were performed as a function of temperature and excitation wavelength and support the conclusion that a tunneling mechanism is operative in these devices. The external quantum efficiencies of our porous-silicon-based structures can be greater than 20%, and the shape of the photocurrent spectra of porous-silicon-based devices, when compared to that of a p-type Si/Au diode, suggests that carriers photogenerated within porous silicon can be collected in an external circuit.
INTRODUCTION The optical emission behavior of porous silicon (PS) has been the subject of 1 2 considerable research activity since the appearance of two seminal publications., , To date, most work has been directed towards exploration of the relationships between preparation variables, post preparation treatments, and the structural, chemical and light emission 3 properties of porous silicon films. These experimental efforts have been mainly concerned with determining if quantum confinement effects, surface-resident chemical species, or entrapped amorphous silicon phases are responsible for the intense visible emission from the 4 material. In particular, results from time-resolved photoluminescence experiments, optical 5 absorption measurements,I and transmission electron microscopy support the theory that the visible photoluminescence (PL) is due, at least in part, to radiative recombination of carriers confined within 3-dimensionally quantized silicon particles in the PS network. PL emission data from nanoscopic silicon structures prepared by a variety of techniques including solution-7 6 phase synthetic methods, constrained recrystallization within multiquantum well structures, 8 and microwave plasma decomposition of SiF14 have established that significant visible radiative recombination yields can be observed from Si. In contrast to the level of effort dedicated to studies of the optical emission properties of porous silicon, relatively few investigations have focused on the electrical transport and photoconductivity properties of the material. A detailed knowledge of the transport properties of porous silicon in the dark and under illumination will assist in the development of new electroluminescent devices and determine the viability of porous silicon as a light-absorbing layer in photovoltaic cell applications. In addition, mechanistic information derived from transport measurements can aid in developing a thorough understanding of the unique properties of porous silicon. Data concerning the charge and potential distribution in AI/PS diodes has been obt
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