INVESTIGATION OF HYDROGENATED AMORPHOUS SILICON FILMS AND SCHOTTKY BARRIER JUNCTIONS FABRICATED BY THE GLOW DISCHARGE TECHNIQUE
The goal of this research work is to test experimentally, how the parameters of the d.c. glow discharge silane process influence the electronic parameters of the resultant a-Si:H films and the metal a-Si:H junctions. The films were deposited in a two parallel plate electrode system (cathodic films) and in a three electrode system, where the substrate was held on an electrode biased positively with respect to the cathode. An objective for employing the latter system was to determine if polarization of the substrates influence the properties of the deposited a-Si:H films.
Undoped a-Si:H films were deposited on single crystal silicon substrates in a range of substrate temperatures and the discharge environment pressures.
The structure of the films was studied using x-ray diffraction and SEM techniques. Infrared spectroscopy was employed to examine different hydrogen environments in the films. The temperature dependence of the dark- and photo-conductivity has been investigated. Gold and platinum Schottky barrier photovoltaic cells were fabricated employing discharge-deposited a-Si:H films. Characterization of these cells in terms of dark and illuminated current-voltage measurements was performed. A theoretical model of the amorphous silicon photovoltaic cell, which allowed the development of a computer simulation program for the solar cell operation, has been derived.
The films deposited in the three electrode system under bias revealed different composition and, apparently associated with it, better semiconductor quality, than the films deposited in the two electrode system. The films deposited under bias revealed predominately monohydride phase when deposited at 250(DEGREES)C under pressure 250 mTr in contrast to dihydride and trihydride phase found in the cathodic films.
The dark conductivity of the films was found to be 2.10('5) (ohm cm)('-1) with activation energy (DELTA)E = 0.33 eV. The low value of the activation energy resulted probably from oxygen contamination found in the films.
The carrier transport in the films was found to be due to electrons in the extended states at temperatures above 240 - 280(DEGREES)K. At low temperatures phonon assisted hopping transport was significant. The lifetime of electrons was estimated to be 10('-6) sec. at room temperature. The films deposited in the three electrode system were characterized by the recombination mechanism, which reflected a quasi-exponential distribution of the trapping centers in the mobility gap.
The photocells made on the films deposited under the substrate bias revealed both dark and illuminated current-voltage characteristics superior to those of photocells made on the cathodic films. The junction ideality factor was close to unity and the rectification ratio was (TURN)10('4). The barrier potential of Au-a-Si:H junction was found to be 0.87 V. The values of the short circuit current 1.8 mA/cm('2) and the open circuit voltage 420 mV have been measured in photocells with gold film (50 percent transparent for illumination 120 mW/cm('2) of the tungsten light). The short circuit current of 4 mA/cm('2) has been obtained in Pt-a-Si:H photocell with platinum film, 25 percent transparent, under illumination 140 mW/cm('2) of the tungsten light.
The experimental results of the photocell measurements have been compared with a theoretical data obtained from numerical calculations of the photocell model.