The secondary electron emission flux in a scanning electron microscope is a powerful tool for delineation of electrically active dopant concentration, built-in potentials, and surface electric fields in semiconductor junctions. In all the secondary electron images of p-n junctions, the p-doped regions appear brighter than n-doped regions. We present a theory for the doping contrast in p-n junctions that is based on the secondary electron emission yield and surface band bending extracted from Kelvin probe force microscopy measurements. We show that the contrast is governed by the secondary electron escape depth, and their escape probability which is related to the secondary electron energy distribution and the effective electron affinity. It is found that the escape depth is the main factor determining the dopant contrast, and the escape probability has a smaller effect. In addition, our theory explains the logarithmic dependence of the measured contrast on the acceptor concentration in silicon reported by many groups.
Lattice constants of GaAs layers grown by molecular beam epitaxy were examined by using the high resolution x-ray diffractometer. For highly doped samples ͑up to 9ϫ10 18 cm Ϫ3 of free-electron concentration͒ we observed an increase of the lattice constant with respect to the undoped layers. Since substitutional silicon atoms decrease the lattice constant of GaAs, the results are explained by the influence of free-electrons via the deformation potential of the ⌫ minimum of the conduction band. The best fit to our diffractometric data was obtained for the band-gap deformation potential equal to Ϫ8.5 eV.
Temperature dependence of THz detection by field effect transistors was investigated in a wide range of temperatures from 275 K down to 5 K. The important increase of the photoresponse following 1/T functional dependence was observed when cooling from room temperature down to 30 K. At the temperatures below $30 K, the THz response saturated and stayed temperature independent. Similar behavior was observed for GaAs, GaN, and Si based field effect transistors. The high temperature data were successfully interpreted using recent theory of overdamped plasma excitation in field effect transistors. The low temperature saturation of the photoresponse was tentatively explained by the change of the transport regime from diffusive to ballistic or traps governed one. Our results clearly show that THz detectors based on field effect transistors may improve their responsivity with lowering temperature but in the lowest temperatures (below $30 K) further improvement is hindered by the physics of the electron transport itself. V
Phenomena of the radiation coupling to the field effect transistors based terahertz (THz) detectors are studied. We show that in the case of planar metal antennas a significant portion of incoming radiation, instead of being coupled to the transistors, is coupled to an antenna substrate leading to responsivity losses and/or cross-talk effects in the field effect based THz detector arrays. Experimental and theoretical investigations of the responsivity versus substrate thickness are performed. They clearly show how to minimize the losses by the detector/ array substrate thinning. In conclusion simple quantitative rules of losses minimization by choosing a proper substrate thickness of field effect transistor THz detectors are presented for common materials (Si, GaAs, InP, GaN) used in semiconductor technologies.
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