Electron Time-of-Flight Measurements in Porous Silicon
- PDF / 984,556 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 32 Downloads / 205 Views
ABSTRACT Transient photocurrent measurements are reported in an electroluminescent porous silicon diode. Electron drift mobilities are obtained from the data as a function of temperature. Electron transport is dispersive, with a typical dispersion parameter (x f 0.5. The range of mobilities is 10" - 104 cm 2 /Vs between 225 K and 400 K. This temperature-dependence is much less than expected for multiple-trapping models for dispersion, and suggests that a fractal structure causes the dispersion and the small mobilities.
INTRODUCTION Canham's discovery that porous, crystalline silicon photoluminesces efficently near roomtemperature [1] has led to enormous interest not only in the origins of this unexpected effect, but also in the possibility of preparing electroluminescent diodes from the same material. Substantial progress in this direction has been made by several groups, based on a variety of device designs [cf 2,3,4 for early examples]. Despite these device successes, practically nothing is known conclusively about the fundamental electrical properties of porous silicons; adding to the complexity of establishing these properties is the fact that porous silicons can be made with an enormous range of structures and passivation properties, presumably with widely varying transport. In this paper, we are concerned primarily with carrier mobilities, which determine carrier drift speeds inside electrical devices and can thus dominate the speed of the device itself The carrier mobilities of crystalline silicon, from which the porous silicon is prepared, are at best a starting point. Porous silicon's extremely fine and disordered topology would be expected to drastically reduce the drift velocity of a carrier, and the experiments published to date [5,6,7] are consistent with a qualitative change vis a vis the properties of the crystal. Indeed, one may speculate that it is precisely this drastic reduction in the diffusion of carriers which permits them to recombine radiatively in the porous structure. In the present paper, we report our progress in obtaining carrier mobilities using the photocarrier time-of-flight method. In its simplest implementation, the method involves launching a sheet of photocarriers in a sample using a short laser flash. The carriers then drift under the influence of an external bias field E, and the transit time tr required for the carriers to drift a distance L across the specimen is determined by measuring the transient current in the bias circuitry. The mobility is the ratio L/EtT. Ref. [8] may be consulted for additional information about procedures as implemented by our laboratories. In brief, we have observed transient photocurrents in highly porous silicon which we associate with carrier sweepout. We find mobilities in the range 10-5 - 10-4 cm2 iVs. These are comparable to those obtained previously using electroluminescence modulation [7], although the structures involved are quite different. The dispersion properties of the mobilities (the dependence of mobility on the displacement of the
Data Loading...
