• PDF / 302,937 Bytes
• 6 Pages / 420.48 x 639 pts Page_size
• 20 Downloads / 197 Views

DOWNLOAD

REPORT

---

A THERMAL

GERHARD

EQUILIBRIUM MODEL FOR THE DOPANT AND DEFECT STRUCTURE IN

MOLLER,

Messerschmitt-B6lkow-Blohm GmbH, Postfach 80 8000 M~nchen 80, Fed. Rep. of Germany.

11

a-Si:H

09,

ABSTRACT Our previous thermal equilibrium approach, based on the assumption of an intrinsic relaxation mechanism, has been evaluated on a quantitative scale. The predictions of this model are compared to experimental defect density data and a microscopic description of the thermalization process is presented.

INTRODUCTION Recently (1), we have proposed that the a-Si:H lattice is able to support an intrinsic relaxation mechanism that adjudsts the density of dangling bond states near mid-gap to the densities of charge carriers in localized band tail states according to: Si(4/0) + e(-) Si(4/O) + e(+) and:

v- Si(3/-)

(Ia)

Si(3/+)

(Ib)

•

Si(3/+) + Si(3/-)

*

.-

2Si(3/0)

(Ic)

Here, Si(4/0) stands for normally coordinated network atoms, Si(3/+,O,-) for 3-fold coordinated Si-atoms that give rise to dangling bond states near midgap and e(-) and e(+) for charge carriers in localized band-tail states. Supplementing these reactions by the doping reactions P(3/0) and:

B(3/O)

-

PP(4/+) + e(-)

(IIa)

B(4/-) + e(+)

(IIb)

we have shown that a thermal equilibrium formalism can be derived that, in its nature, is a generalization of Street's auto-compensation model of doping (4) and that, in addition, allows doping, compensation and various degradation phenomena to be treated within a single and unifying approach (1). In the present paper we should like to briefly review some of the results that have been obtained from a quantitative elaboration of the formalism (2) and to discuss the kinetics of the thermalizastion process in more detail (3).

EQUILIBRIUM PROPERTIES OF a-Si:H FILMS Assuming that equilibration proceeds through reactions Ia-c, our model predicts a low defect density in those situations in which the electronic occupation of the band tail states is negligible. Specifically, this should

"Mat. Res. Soc. Symp. Proc. Vol. 118. , 1988 Materials Research Society

286

apply to undoped a-Si:H at low temperature. When the carrier density in the doping or alloying band tail states is enhanced by illumination, annealing, with foreign impurities, reactions Ia and b are driven to the right-hand-side inducing an enhanced defect density. Analyzing these situations in more detail (2) we have obtained the results summarized in figs. 1-4. In fig.l, the variation of Nd, the equilibrium density of dangling bonds, is plotted as a function of (Ec-Ef) with the magnitude of the equilibration temperature as a parameter. Also included in this figure are experimental defect density data that have been obtained on differently doped aSi:H material. Comparing both data sets, it is seen that reasonable agreement is obtained provided that the comparison is made at the experimentally determined equilibration temperatures (5) and the appropriate Fermi-energy posithe experimental data are seen to deviate At room temperature, tions (6). more or less m