Correlation Between Freeze-In Temperature of Defect Density and Hydrogen Concentration in a-Si:H
- PDF / 281,059 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 19 Downloads / 176 Views
CORRELATION BETWEEN FREEZE-IN TEMPERATURE OF DEFECT DENSITY AND HYDROGEN CONCENTRATION IN a-Si:H X. Xu, M. Isomura, J. H. Yoona) and S. Wagner Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 and J. R. Abelson Department of Materials Science and Engineering and the Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ABSTRACT We measured the freeze-in temperature of the danglingbond density in a-Si:H in nine samples with hydrogen concentrations ranging from 7.0 to 31 at.%. The measurements were made by determining the defect density of samples quenched from successively higher temperature. We determined the defect densities with the constant photoconductivity method. The freeze-in temperature is 211±10 'C, and is independent of hydrogen concentration. INTRODUCTION A key characteristic of device-quality hydrogenated amorphous silicon (a-Si:H) is the reversible change of its density of dangling bonds with temperature, as long as the temperature is sufficiently high [i]. When the temperature (it is lowered, the defect density becomes constant freezes). In undoped material the freeze-in temperature Tf is 195 - 200 °C [2]. The freezing of the dangling-bond density reflects a slowdown in the process which removes dangling bonds ("annealing") . Far from equilibrium, where the rate of defect generation can be neglected, the rate of annealing r per site is determined by the attempt-to-anneal rate v 0 , the activation energy of annealing AE, and the temperature T: r
= voexp(-AE/kT).
(1)
During the cooling of a sample of a-Si:H, the defect density is defined to freeze when the rate of cooling, Rc = -dT/dt, becomes equal to the reciprocal value of the temperature dependence of the annealing time, dT/d(I/r) [3]. The freezein temperature then can be expressed as Tf = AE/[kln(VokTf Thus, the freeze-in proportional to AE.
2
(2)
/AER&)].
temperature
is
approximately
Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society
70
Hydrogen has been proposed to participate in the making, annealing and migration of the dangling bonds [4,5]. Therefore, hydrogen is likely to influence the freeze-in temperature. Indeed, two experiments have been reported which show that Tf depends on the concentration of hydrogen cH in phosphorus-doped a-Si:H. One group found that raising cH raised Tf [6], another saw a drop in Tf [7]. No experiments have yet been reported about dependence of the freeze-in temperature on hydrogen concentration in undoped a-Si:H. EXPERIMENTAL With these questions in mind we determined Tf (N3 ) for a-Si:H with cH ranging from 7 to 31 at.%. Five samples were deposited by dc maanetron reactive sputtering [8], a technique in which cH can be over a wide range by changing the hydrogen pressure without changing other deposition parameters. Three samples were prepared by glow-discharge deposition from mixtures of SiF4 and H2 in a DC triode, and one from SiH4 in an RF diode [9]. The hydrogen content was measured by infrared ab
Data Loading...
