Fabrication and Characterization of Single Doped GaAs Nanowire Devices
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In the work with this master thesis, electron beam lithography (EBL) and other cleanroom methods was used to develop a reproducible process for making ohmic contactsto nanowires (NWs), with the aim of investigating different dopants influence ontheir physical properties. After optimizing the EBL process, good alignment of thecontact patterns to the NWs was achieved by doing a thorough alignment procedure.A detailed write field alignment close to the sample position prior to the 3-point alignment, along with consistent use of alignment positions, was found the be especially important to reduce misalignment.For surface treatment it is shown how a diluted HCl solution is used to removenative oxide on the NW surface, and how a solution of citric acid and hydrogen peroxidecan be used to effectively etch the AlGaAs shell of core-shell nanowires, thoughselective etching of AlGaAs on GaAs is difficult. Further, an ammonium sulphidesolution is used to passivate the surface prior to metal deposition.Metalization was done using an electron beam evaporator, however optimal thinfilms could not be achieved due to the presence of strain. This is believed to originate in the evaporation of electron beam resist (ER) during metal deposition due to high local heating of the samples. While this was remedied by, among other, depositing thinner films at high deposition rates, it could not be fully avoided. Nevertheless, satisfactory lift-off was still achieved in most cases.IV characterization of the different samples was done using a probing station. Thesamples generally display asymmetric and non-linear characteristics, indicating thatohmic contacts have not been achieved. Annealing tests was also done on a numberof samples, and while this generally resulted in more linear IV curves, the currentwas also reduced in most cases.