Quantum Wires in Shallow Heterostructures

[摘要] In order to maximise the temperature at which the quantisation of the conductance is resolved in quasi-one dimensional GaAs-AlxGa1-x As structures, the induced sub-bands must be widely spaced in energy. For surface gated structures, this requires that feature sizes are below 100nm and that the two dimensional electron gas (2DEG) is formed near the surface (~30nm). Achieving sub 100nm feature sizes makes strong demands on electron beam lithography processes but suitable techniques, described in Chapter 2 have made it possible to routinely fabricate such structures. Developing a heterostructure which can exploit these feature sizes is a much more difficult task, but this too has been successful. Capacitance and magneto-transport measurements which have helped in this development process are described in detail in Chapter 4. In addition, problems in understanding various parameters such as carrier concentration and threshold voltages are analysed closely. It becomes clear when analysing the data that the characteristics of GaAs-Al0.3Ga0.7As heterostructures are explicable in terms of simple electrostatic models. It is found that applying the same model to heterostructures which include spacer layers of GaAs-AlAs in place of the conventional Al0.3Ga0.7As is complicated by evidence of free charge in the donor region at low temperatures. The transport experiments also show strong evidence of such charge accumulation. A comprehensive investigation of the smearing of the conductance quantisation with increasing temperature and source-drain is presented in Chapter 5. The sub-band spacing and the temperatures at which the quantisation smears are compared for various devices fabricated on the optimised heterostructure i.e. where the 2DEG is formed 28nm below the surface. Comparisons are also made with similar measurements carried out on two other heterostructures where the 2DEG is formed at depths of 40nm and 107nm. The data is used to determine the experimental sub-band spacing in the devices and they are found to be consistent with smaller sub-band spacings in heterostructures where the 2DEG is formed at a greater depth. The experimental sub-band spacings also compare fairly well to theoretical calculations using the actual device geometry. An equivalence between the thermal and electric smearing measurements is also discussed but no evidence is found that anything other than smearing due to broadening of the differential Fermi function is responsible for the washing out of the sub-band structure. Finally in Chapter 6 experiments are presented which map out the distance over which the conductance quantisation is robust to scattering in the optimised heterostructure. In this particular structure the donors are only separated from the 2DEG by 11nm and as such, scattering is expected to be strong. It is therefore surprising that quantisation persists in wires up to 400nm long. Conventional modelling of the donors as a fully ionised random distribution of charge cannot explain why this is the case. Similar discrepancies are also found when the mobility in the 2DEG is compared with the theoretical prediction. The possibility that this is evidence for correlations in the position of ionised donors is discussed.

[发布机构] University:University of Glasgow

[关键词] Condensed matter physics, Quantum physics