Stebic Revisited
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Abstract We re-examine the technique of Scanning Transmission Electron Beam Induced Conductivity and report on the acquisition of remote contact EBIC images at 200keV showing variations in electrical activity within P-doped CdTe. Introduction The procedure of electron beam induced conductivity (EBIC) is most commonly associated with bulk specimens observed in an SEM, even though improved resolution may be afforded by EBIC imaging of an electron transparent foil due to minimisation of beam spreading effects. The problem with imaging the electrical activity within a thin foil, however, in addition to the practicality of contacting and handling, primarily has been the low electrical signal due to the small generation volume and surface recombination effects. Improved technology and the wide availability of TEMSCAN instruments with consequent (relative) ease of loading a sample with electrical contacts as compared with STEM suggests that scanning transmission EBIC (STEBIC) deserves re-evaluation, particularly with the availability of electron sources with increased brightness to increase the generated signal and the possibility of tailored growth of device structures incorporating buried p-n junctions to maximise charge collection within a TEM foil. We briefly comment on the techniques of EBIC and REBIC, and then report on the acquisition of images showing electrical activity within thin foils of CdTe in a TEMSCAN, using accelerating voltages up to 200keV and cooling down to liquid nitrogen temperatures. EBIC AND REBIC
The principle of EBIC, or charge collection microscopy, is that charge carriers generated by a focused electron beam are drifted by an electric field (created either by a p-n junction or a Schottky contact) within a sample and collected as a current in an external circuit. An 'EBIC' image is formed by displaying the collected specimen current as a function of the position of the incident beam as it is scanned across the specimen. Thus if dislocations act as recombination centres then they appear as dark lines in the image because of the reduced specimen current when the beam is incident at a dislocation. The resulting contrast is dependent on recombination strength, the range of incident electrons within the crystal, defect position and width of the depletion region associated with the collecting junction and the minority carrier diffusion length. The information content of EBIC images includes the location of p-n junctions, recombination sites such as dislocations and precipitates and the presence of doping level inhomogeneities. The signal detected is dependent on the electrical properties of the collecting junction, the series resistance of the circuit and the size of the induced current. Provided the induced current is less than the diode saturation current, then the EBIC contrast measurement will be independent of the value of circuit series resistance and hence quantitative measurements can be made of recombination activity in semiconductors [1]. Since a specimen and contacts are likely to be non-
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