Electronic Transport and the Density of States Distribution in a-(Si,Ge):H,F Alloys
- PDF / 391,551 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 26 Downloads / 181 Views
ELECTRONIC TRANSPORT AND THE DENSITY OF STATES DISTRIBUTION IN a-(Si,Ge):H,F ALLOYS*
S. ALJISHI, Z E. SMITH, D. SLOBODIN, J. KOLODZEY, V. CHU, R. SCHWARZ and S. WAGNER Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544
ABSTRACT The electronic and optical properties of amorphous silicon-germanium alloys produced by d.c. and r.f. glow discharge are reported. Data on the sub-gap absorption, dark and photo conductivities, drift mobilities and drift mobility-lifetime products are used to propose a density of states model.
INTRODUCTION The major factor determining the electonic transport properties of amorphous silicon-germanium alloys is the density of gap states. While transport properties of a-SiGe:H alloys as a function of Ge content have been reported by several groups [1-41, it has been difficult to isolate effects fundamentally due to Ge from those due to extraneous factors such as gas feedstock choice and purity , structural inhomogeneity and deposition techniques. Several models of the density of states in a-Si,Ge:H have been put forth to explain the variation of electronic properties with alloying [4-71. The models apply to specific energy ranges within the gap and account for some of the observed behavior. A comprehensive density of states model requires a complete series of measurements on films made under similar preparation conditions. We report a variety of measurements on a-Sil_,Ge0 :H,F as a function of XGe. Conductivities and mobility-lifetime products are used to deduce information about the deep states, while sub-gap absorption and mobility measurements yield information regarding the band edges. The results are combined to construct a density of states model.
EXPERIMENTAL TECHNIQUES Alloy Preparation. The alloys were prepared by both d.c. and r.f. plasma glow discharge (GD) decomposition of SiF 4 , GeF 4 and H2 or of SiH 4 , GeF 4 and H2 gas mixtures. The d.c.-GD deposition station utilized a proximity mode electrode configuration [8,9]. Gas flow rates were 28 sccm for SiF 4 , 0 to 1 sccm for GeF 4 and 7 sccm for H2 ; or 28.5 to 32.5 sccm for Sill4 , 0 to 4 sccm for GeF 4 and 10 2to 12 sccm for H2 . The deposition pressure was 0.3 to 0.4 Torr, the power density 50 to 100 mW-cm- and the substrate temperature 250 C. The r.f. depositions were carried out in a UHV-grade deposition system with a diode electrode arrangement [10,11]. Gas flow rates were 28 sccm for SiF 4 , 0 to 0.7 sccm for GeF 4 and 4.6 sccm for H2 . The deposition pressure was 0.108 Torr (14.4 Pa), the power density 250 mW-cm-2 and the substrate temperature 270"C. The films were grown on Corning 7059 glass substrates for electrical conductivity and optical measurements and on glass substrates with 50 nm Cr back contacts for time of flight (TOF) experiments. The film thickness, d, lay between 1 and 3 pm . The composition of the films was determined by nuclear elastic scattering, electron microprobe and infrared spectroscopy. The H and F contents were 10-15% and 1-5%, respectively, for the films stud
Data Loading...
