Two-dimensional ambipolar transport and coupled one-dimensional conductors in undoped GaAs/AlGaAs heterostructures

In this talk, I will present my PhD research which was largely focussed on device development and engineering, together with low-temperature measurement, of novel low-dimensional devices based on undoped GaAs/AlGaAs heterostructures, with a view to study ambipolar transport in the strongly interacting regime. Undoped systems allow the fabrication of ambipolar devices, where electrons and holes can be ‘induced’ in the same conduction channel [1,2]. Additionally, the absence of disorder from dopant impurities in these systems enables the achievement of higher mobilities at low densities, the regime of strong inter-particle interactions [3]. Another system which facilitates strong carrier interactions is the coupled low-dimensional bilayer system, consisting of two separate layers of charge carriers placed in close proximity, and probed through Coulomb drag measurements [4,5,6]. Interlayer interactions in these bilayers can lead to the formation of novel phases, which could never be achieved in a single layer [7].
Through a systematic study and optimisation of various fabrication parameters, we have developed a simple and reliable, front-sided-only fabrication method for n-type ohmic contacts to two-dimensional electron gases (2DEGs) in undoped GaAs/AlGaAs quantum wells (QWs). Utilising this technique, we have demonstrated a front-side-gated ambipolar field effect transistor (FET), where both 2D electron and hole gases can be induced in the same QW [8]. We have developed a device design that can enable Coulomb drag measurements in ambipolar vertically coupled one-dimensional (1D) quantum wires defined in undoped QWs. The fabrication of these structures is extremely challenging. It requires independent contacts to the electron and hole wires; 1D confinement for both electrons and holes at the same time and vertical coupling for lowest spacing between charge carriers. Our devices contain independent n-type and p-type ohmic contacts to both layers of an undoped double quantum well (DQW) structure, allowing us to induce either layer with electrons or holes. The 1D channels are defined by wet chemical etching to provide hard-wall confinement to electrons and holes coupled in a vertical geometry. We have investigated both electron and hole transport through the upper wire and confirmed its 1D behaviour. Independent hole transport in the bottom QW has also been studied. Furthermore, we have established the existence of at least a coupled hole-hole wire system through preliminary tests at a temperature of 4 K. Further measurements at lower temperatures are being planned.

References:
[1] J. C. H. Chen et al. Appl. Phys. Lett. 100, 052101 (2012).
[2] A. F. Croxall et al. Appl. Phys. Lett. 102, 082105 (2013).
[3] R. H. Harrell et al. Appl. Phys. Lett. 74, 2328 (1999).
[4] U. Sivan et al. Phys. Rev. Lett. 68, 1196 (1992).
[5] A. F. Croxall et al. Phys. Rev. Lett. 101, 246801 (2008).
[6] D. Laroche et al. Nat. Nanotech. 6, 793 (2011).
[7] S. De Palo et al. Phys. Rev. Lett. 88, 2064014 (2002).
[8] D. Taneja et al. Semicond. Sci. Technol. 31, 065013 (2016).