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SIMS profiles for highly aluminum-doped samples VLS93 and VLS98, with aluminum concentrations of about for sample VLS93 and of about for sample VLS98.
(a) Doping dependence of the activation energy on the aluminum level in . Experimental values of Ivanov et al. (Ref. 12) Matsuura et al. (Ref. 13) and Pernot et al. (Ref. 2) are included. The dashed line represents the theoretical concentration dependence following with and ; (b) Temperature dependence of the sheet resistance (logarithmic scale) for VLS93, showing a clear insulating behaviour. (c) Temperature dependence of the sheet resistance (linear scale) of sample VLS98. A metallic behaviour is clearly seen. The appearance of a drop in resistance at very low temperatures probably indicates the onset of a superconducting transition. Attention should be payed to the different scales of the resistance.
Comparison of Raman spectra taken at room temperature and for two aluminum-doped samples VLS93 and VLS98. All samples show a clear Fano resonance at room temperature. Only for sample VLS98 does the Fano resonance persist down to , indicating no significant change in the carrier concentration and, therefore, suggesting metallic behavior.
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