Contacts to P-doped GaAs Nanowires by Fabrication of Electrodes using Metals and Graphene

摘要

P-type GaAs semiconducting nanowires (NWs) with NW/electrode contacts have been examined by fabricating single NW devices to investigate their electrical properties. Both NWs grown with Au-assisted and Ga-assisted vapor-liquid-solid (VLS) method have been examined, with great emphasis on Ga-assisted NWs because these show superior contact properties. The NWs were grown using molecular beam epitaxy (MBE) and electrical contacts were fabricated using electron beam litography, electron beam evaporation for metallization and annealing to enhance ohmic behaviour of the contacts. Two different types of contacts were tested. One was metal contacts where various combinations of metal layers were tested, including such metallizations as Pt/Ti/Pt/Au and Pd/Zn/Pd/Au. The second electrode type was graphene which is expected to show very interesting electric and optoelectronic properties.It is shown in this thesis that ohmic contact with linear I-V characteristics has been achieved by two different methods. The first being annealing of the NW contact after contact fabrication. The second is by increasing doping concentration during NW growth. It is also shown that Pt/Ti/Pt/Au metallization is the layer configuration that exhibits the best ohmic contact and the most consistency in electrical measurements. 4-probe measurements were also carried out to measure the intrinsic resistivity of the NW, which makes it possible to estimate its doping concentration and carrier mobility.NW/graphene contact was also tried.Mechanical exfoliation of kish graphite was performed to make high-quality graphene, which was selctively placed to make NW contact. However it is found that heating of the EBL-resist during e-beam evaporation caused the graphene to wrinkle and dissolve. CVD-grown graphene on Cu-foil was also tested. Inspection after the graphene transfer proved that the graphene that was used was incontinous and flaky making it difficult to obtain proper contacts to NWs. In addition the NWs connected to graphene probably suffered from over-etching due to multiple etching steps. Since a lot of efforts was put into obtaining a suitable measure of making NW/graphene contacts only preliminary measurements were conducted, so these results has yet to be verified.