Lateral Surface Superlattice Devices

A fabrication process was developed to realize lateral surface superlattice (LSSL) devices on high-mobility modulation-doped GaAs/AlGaAs heterostruc-tures for low-temperature transport studies. The process involved the use of high-resolution electron beam lithography to define an interdigitated array of metal gates, with periodicities down to 230 nm and a 1:1 mark space ratio. The two sets of interdigitated gate arrays, defined by metallization and liftoff, could be independently biased so that an independent control of the the average electrostatic potential and of the periodic potential amplitude was possible. Two different heterostructures were used: a conventional HEMT GaAs/AlGaAs layer, where the 2DEG was formed 90 nm below the surface and a novel, delta-doped heterostructure where the 2DEG was only 28 nm deep. The devices were characterized at low temperatures (~ 70 mK) by means of magnetoresistance measurements. The effect of the periodic gate potential on the 2DEG was reflected in oscillations of the magnetoresistance due to commensurability resonances between the cyclotron orbit and the period of the superlattice potential. A strong first harmonic of the commensurability oscillations was observed in the magnetoresistance traces measured on the shallow-2DEG device, suggesting an enhanced contrast of the superlattice potential due to the proximity of the gates to the 2DEG. The strength of the harmonic signal was quantified using Fourier analysis and it was found that the high first harmonic contents could not be explained by simple electrostatic models. An alternative source of modulation, involving the strain field caused by the differential contraction of the metal gates, is discussed. The potential profile obtained from the strain model is in better agreement with experimental results and confirms that the enhanced contrast of the potential is due to the proximity of the gates to the 2DEG. An identical device fabricated on conventional HEMT material exhibited purely sinusoidal commensurability oscillations A study of the commensurability oscillations as a function of gate bias was carried out, and some information on the causes of device performance degradation was extracted. In particular, it was found that a parallel sheet of parasitic charge in the shallow-HEMT layer was responsible for its relatively high mobility but also for strongly reducing the electrostatic modulation of the 2DEG by the gate potential. Nevertheless, some indication of period doubling due to differential bias was observed in the Fourier tranform of the experimental data. In this work we demonstrated the feasibility to fabricate laterally gated devices on shallow HEMT layers and the enhanced contrast of the potential at the 2DEG, achieved because of the proximity of the gates to the 2DEG. However, other problems have been identified that limit the advantages of the increased potential contrast: o an important component of the potential modulation arises from the strain field generated by the differential contraction of the metal gates on the surface of the semiconductor; the strain couples to the 2DEG via the deformation potential and gives rise to a residual potential modulation that cannot be controlled by the gate bias. o the presence of a parasitic sheet of electrons in the novel shallow-HEMT layer strongly reduces the efficiency of the gates in electrostatically modulating the 2DEG potential Further work should be directed to improve the design of the shallow-HEMT layers so that its full potential in relation to laterally gated devices could be achieved