Density of States Spectroscopy in p-Type a-Si:H and a-SiC:H
- PDF / 304,403 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 11 Downloads / 170 Views
DENSITY OF STATES SPECTROSCOPY IN p-TYPE a-Si:H AND a-SiC:H
RICHARD S. CRANDALL, STANLEY J. SALAMON, AND YUEQIN XU Solar Energy Research Institute, 1617 Cole Blvd., Golden, CO 80401
ABSTRACT We derive a closed form expression for differential junction capacitance applicable when the density of states (DOS) varies exponentially with energy. Using this expression, we analyze p/n junction capacitance measurements that probe the DOS in boron doped hydrogenated amorphous silicon and silicon carbide alloy. In both materials we find that the p-layer DOS is described by an exponential increase with energy above the Fermi level. INTRODUCTION The electronic properties of hydrogenated amorphous silicon, a-Si:H, depend critically on the density of states (DOS) in the band gap. Various techniques, such as photothermal deflection spectroscopy, total yield photoemissioni and capacitance can measure the DOS. The first two methods are surface sensitive whereas the latter is a true bulk measurement permitting a spatial determination of the DOS. Furthermore, photoemission is not readily available in the average laboratory. Capacitance measurements are straightforward 'and readily available. Glade et. a12 , using the frequency and temperature dependence of the capacitance and Lang et. a13 using transient admittance techniques each determined the DOS in undoped and lightly doped n-type material. However, there is little information on p-type a-Si:H. In this paper we describe a simple method to measure and analyze capacitance versus temperature measurements. By making the assumption that the DOS near the Fermi level (EF) varies exponentially with energy (E), we obtain a closed form expression for the capacitance. We use this approach to measure the DOS in p-type a-Si:H and a-SiC:H. ANALYSIS OF THE EXPERIMENT The frequency (f = w/27t) temperature and band bending in the sample determine the ac junction capacitance. Only those states whose thermal emission rate (r) is faster than I/0o can respond during a cycle of the ac applied voltage and contribute to the measured capacitance. These states lie between a demarcation level (Ed.) and the valence band mobility edge, Evb. This level is determined by setting r = ) to give Ede = kBT ln(v/co) - Evb.
(1)
Here kB is Boltzmann's constant, T is the absolute temperature, and v is the "attempt-to-escape" frequency. These ideas are represented in Fig. 1, which shows the potential (I) as a function of distance (X) measured from the n/p interface. The hatched region contains states that are empty of holes owing to the band bending. These contribute to the negative space charge in the p layer. A small change in applied voltage on the device causes thermal emission, redistribution and retrapping of holes at EF. The point X6) separates the region of the p layer containing those states at EF that can not follow the ac signal from those farther into the p layer that do follow the signal. Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
558
The region 0 < X < XO functions as a die
Data Loading...
