Study of GaSb Junction Devices by Cathodoluminescence and Scanning Tunneling Spectroscopy
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form a p-n junction. The diffusion temperature was around 500 *C. The diffusion profiles were measured by using SIMS in a set of samples similar to that used for the microscopy characterization. Diffused samples and one undiffused wafer, used as reference sample, were investigated by CL-SEM and by CITS. Plane-view CL measurements were carried out using a Hitachi S-2500 SEM at 77K at beam energies of 10-25 keV, with a cooled ADC germanium detector. The details of the experimental set up for recording images and spectra are described elsewhere [6]. For the scanning tunneling microscopy (STM) measurements a combined SEMISTM based on a Leica 440 SEM operating under a vacuum of 10-6 Torr was used. The small size of the STM enabled it to be mounted on the SEM specimen holder. The main features of this system are similar to those described in ref [7]. Electrochemically etched or mechanically sharpened Pt-Ir and Au wires were used as probe tips. For CITS measurements the constant current image was obtained in a 128x128 pixel grid, the feedback loop interrupted for 2 ms, and the voltage digitally ramped from the tunnel voltage to a set of 44 predetermined values while the current was sampled. This provides 44 tunneling current files at different voltages. To analyze the data, the normalized differential conductance, (dl/dV)/IIV, spectra were recorded. This quantity provides a rather direct measure of the surface density of states [8]. The STM samples were cleaned in hydrochloric acid and acetone, dried in a flow of dry nitrogen, and immediately mounted in the microscope chamber. The lateral surface of the samples was oriented to the STM tip to enable cross-sectional measurements of the Zn diffused layer. CITS images and spectra were recorded at different points of the layer, including the p-n junction region. RESULTS AND DISCUSSION Profile in Figure 1 shows the SIMS data of the diffused sample. The diffusion behaviour, showing kinks in the diffusion profile, is qualitatively similar to that previously described [9], which was interpreted using an interstitial-substitutional diffusion model. For the CL study, plane-view measurements were carried out. The spatial resolution of our CL-SEM system, in the micron or sub-micron -. 1E20 range, does not enable a cross-sectional '% investigation of the p-n junction. The CL image of the Te doped reference 1E19.
sample shows, in some regions, series of parallel straight lines, probably related to dopant fluctuations, as well ME8, as isolated extended defects and 1 precipitates (Figures 2a and 2b). The CL spectrum of the reference sample is IE17 .. shown in Figure 3. An emission band 0 200 400 600 800 1000 peaked at about 745 meV is observed Depth (m) in spectrum (a), in addition to the emission corresponding to the bandFig 1.- SIMS profile of zinc diffused inTe-doped GaSb band transition. The presence of a CL substrate. band at about 745 meV in Te doped GaSb crystals has been previously discussed [10] and attributed to transitions from the conduction band to an acceptor state involvin
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