Electron emission from deep traps in hydrogenated amorphous silicon and silicon-germanium: Meyer-Neldel behavior and ion
- PDF / 286,553 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 22 Downloads / 142 Views
Electron emission from deep traps in hydrogenated amorphous silicon and silicon-germanium: Meyer-Neldel behavior and ionization entropy Qi Long1, Steluta Dinca1, Eric A. Schiff1, Baojie Yan2, Jeff Yang,2 and Subhendu Guha2 1 Department of Physics, Syracuse University, Syracuse, New York 13244-1130, U.S.A. 2 United Solar Ovonic LLC, Troy, Michigan 48084, U.S.A.
ABSTRACT We have measured electron drift in amorphous silicon-germanium nip photodiodes using the photocarrier time-of-flight technique. The samples show electron deep-trapping shortly after photogeneration, which is generally attributed to capture by a neutral dangling bond (D 0) to form a negatively charged center (D-). An unusual feature is that electron re-emission from the trap is also clearly seen in the transients. Temperature-dependent measurements on the emission yield an activation energy of about 0.8 eV and the remarkably large value of 1015 Hz for the emission prefactor frequency. We also compiled results on electron emission from deep traps in a-Si:H, a-SiGe:H, and a-SiC:H from six previous publications. Collectively, these measurements exhibit "Meyer Neldel" behavior for electron emission over a range of activation energies from 0.2-0.8 eV and a prefactor range extending over nine decades, from 106 to 1015 Hz. The Meyer-Neldel behavior is consistent with the predictions of the multi-excitation entropy model. We extract a ionization entropy of 20kB from the measurements, which is very large compared to crystal silicon. We discuss this result in terms of a bond charge model. INTRODUCTION The capture and emission of photogenerated electrons has been studied in amorphous silicon for over thirty years, typically with transient photocurrent techniques such as time-offlight [1]. In this paper we focus on measurements of the emission time ݐா of electrons from deep levels in a-SiGe:H (i.e. the (0/-) transition of a dangling bond). These obey the temperature dependence of the general Arrhenius equation: ͳൗ ൌ ߥ ሺെ ܧȀ݇ ܶሻ . ݐா
(1)
Several aspects of these measurements were unusual. Deep level emission is often difficult to observe in time-of-flight measurements. We see it readily because in our samples the activation energies are unusually large (about 0.8 eV), as are the emission prefactors ߥ (1015 s-1). When we place these Arrhenius parameters on a scatter plot with all other measurements for amorphous silicon based materials, we find a single "Meyer-Neldel" line extending from 0.20.8 eV and from 106 and 1015 s-1. The slope is consistent with the multiple-excitation entropy model proposed by Yelon and Movaghar [2,3], which has been applied with some success to a remarkably wide range of processes and materials [4]. We speculate that our measurements also address the huge range of defect energies and emission rates that are simply assigned to a single defect, the dangling bond. Such variability may be unique to amorphous silicon and related
329
EXPERIMENTAL RESULTS
10
-8
10
-9
tE 370 K 340 K
2
Q(t)d/Q0E (cm /V)
materials; it i
Data Loading...
