Numerical Simulation and Experimental Investigation of the Time-of-Flight Technique Applied to a-Si:H/a-SiGe:H-Heterojun
- PDF / 448,524 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 78 Downloads / 187 Views
NUMERICAL SIMULATION AND EXPERIMENTAL INVESTIGATION OF THE TIME-OF-FLIGHT TECHNIQUE APPLIED TO a-Si:H/a-SiGe:H-HETEROJUNCTIONS RUDI BRUGGEMANNt, NORBERT BERNHARD, CHARLES MAIN* AND GOTTFRIED H. BAUERt Institut fur Physikalische Elektronik, Universitit Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart 80, Federal Republic of Germany ' Department of Electronic & Electrical Engineering, Dundee Institute of Technology, Bell Street, Dundee DD1 1HG, Scotland, UK ABSTRACT We report on a comparative study, employing the TOF-technique for the characterization of the properties of a-Si:H/a-SiGe:H-heterojunctions. Both the simulated and experimental electron current transients exhibit a pronounced difference depending on the direction of movement of the excess carriers (from the a-Si:H into the a-SiGe:H or vice versa). For the movement from the low bandgap a-SiGe:H into the a-Si:H we find a characteristic increase in the transient current at room temperature, which, as the simulation can reveal, is attributed to the higher drift mobility in the a-Si:H. A drop in the current is observed when the direction for the transit is from a-Si:H to a-SiGe:H. The post-transit behaviour is dominated by the large amount of trapped carriers which remain on the SiGe side of the sample. The situation for a high barrier between a-Si:H and a-SiC:H and the influence of various parameters on the shape of the current transient are discussed. INTRODUCTION The determination of drift mobilities and the density-of- Ec states (DOS) from TOF-experiments has led to the extraction of a great amount of important information on the materials i-SiGe:H n-Si:H studied. We applied the TOF-technique to heterojunctions E0-Ef=0.3eV based on amorphous hydrogenated silicon (a-Si:H) and its alloys with C or Ge. These structures have been proposed for EC A, i-Si:lH use in bandgap engineered amorphous pin solar cells in order i-SiGe:H E0-Ef=0.75eV to improve cell performance and the knowledge of the precise influence of a particular materials parameter on the overall Figure 1: Schematic shape behaviour of the device is of course of great importance. The of the conduction band of excess carriers cross the junction during the transit and also junctions between intrinfrom one material with its distinct electronic properties to the sic a-SiGe:H and intrinother with different properties, and the aim was to extract in- sic/doped a-Si:H after data formation on the nature of the junction and its influence on from [8] the overall transport properties and to compare the materials by analysis of the TOF current and related measurements [1]. By means of computer simulation we systematically studied the influence of parameters such as the DOS in the tail region on the transient current, giving further insight into the processes during the transit. The experimental results are discussed in the light of the simulation study (see also [2] and [3] for related work). SIMULATION Heterojunction model We employ a numerical finite difference method that has been developed and used for several
Data Loading...
