D.C Current-Voltage Characteristics and Admittance Spectroscopy of an Al-Porous Si Barrier
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ABSTRACT We present the temperature and frequency dependence of the currentvoltage (I-V) characteristics of Al barriers deposited on porous Si grown on p-type Si substrates. These barriers exhibit a rectifying behaviour when the temperature is higher than 250 K. The I-V characteristics can be understood by a conduction in porous Si taking place via free electrons thermally excited from Pb centers associated with the existence of a SiO 2 -Si interface and via hopping between these Pb centers.
INTRODUCTION The incorporation of poirous Si (PSi) in an active device requires an understanding of the electrical characteristics of the structure to control its optical properties. Basically, the structure can be a metal-PSi barrier where PSi is grown on crystalline Si. The d.c. current-voltage (I-V) characteristics of such a structure have been already studied in the case of Al/PSi grown on p-type Si [1-5] and of Al and Au/PSi grown on n-type Si [6]. These studies have indicated the existence of 1) a rectifying Schottky-type barrier at the metal/ PSi interface [6] with pinning of the Fermi level at an energy similar to that of the (0/-) transition of the dangling bond at the Si-SiO 2 interface, the so-called Pb center [7]; 2) a surface conduction controlled by tunneling [4]; and 3) characterized by a Poole-Frenkel effect, attributed to disorder [1]. The aim of this communication is to analyze in detail some of the above aspects by looking at the temperature dependence of the I-V characteristics. We shall also verify the existence of a tunneling regime by performing an admittance spectroscopy and correlate this regime with the nature and the density of traps at the Si-SiO 2 interface. 619
Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
EXPERIMENTAL The studied structures are Al/SiP on p-type, , 10-2 Qcm Si. The porous layers are 9 mtrn thick with a porosity of 50 %, obtained by electrochemical dissolution in an HF (40 %)-ethanol (1:1) electrolyte under a current of 20 mA cm-2 . Aluminum is evaporated on top (area 8 x 10-3 cm 2 ) after the SiP surface has been cleaned by an HF treatment. The ohmic contact on the back side of the Si substrate is made prior to the growth of SiP by p+ doping. The measurements have been performed in the temperature range 250 400 K. The a.c. admittance, its real and imaginary parts (respectively the conductance and susceptance) has been measured in the range 5 Hz to 13 MHz.
RESULTS AND DISCUSSION D.c. measurements Typical I-V characteristics and their variation with temperature are given in Figure 1. It shows that the structure is characterized by a barrier when the temperature is high enough. The barrier is in series with a large resistance Rs. This resistance increases when the temperature decreases, which results in an ohmic characteristic below typically 250 K. Under reverse bias, the conductance varies exponentially with the inverse of the temperature T- 1 (Fig. 2). The associated activation energy Ea is about 0.4 eV and increases slightly with the applied bias V (Fig
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