Using light and heat to controllably switch and reset disorder configuration in nanoscale devices

Abstract

Quantum dots exhibit reproducible conductance fluctuations at low temperatures due to electron quantum interference. The sensitivity of these fluctuations to the underlying disorder potential has only recently been fully realized. We exploit this sensitivity to obtain a novel tool for better understanding the role that background impurities play in the electrical properties of high-mobility AlGaAs/GaAs heterostructures and nanoscale devices. In particular, we report the remarkable ability to first alter the disorder potential in an undoped AlGaAs/GaAs heterostructure by optical illumination and then reset it back to its initial configuration by room temperature thermal cycling in the dark. We attribute this behavior to a mixture of C background impurities acting as shallow acceptors and deep trapping by Si impurities. This 'alter and reset' capability, not possible in modulation-doped heterostructures, offers an exciting route to studying how scattering from even small densities of charged impurities influences the properties of nanoscale semiconductor devices.Disorder is an important issue in nanoelectronics; as a device is reduced in size it becomes more sensitive to temporal fluctuations and spatial inhomogeneities in the charged impurity distribution.1 The temporal fluctuations cause decoherence 2,3 , noise 4 and device irreproducibility. 5,6 These are troublesome for the development of quantum applications for semiconductor devices. Spatial inhomogeneities interrupt electron flow 7 adversely affecting 8 practical realization of concepts such as topological quantum computation using the 5/2 fractional quantum Hall state 9 . These barriers to applications have fueled research to reduce, control and better understand disorder in nanoscale electronic devices.Approaches to disorder reduction for nanoscale devices include modulation-doping 10 , short period superlattice doping 11,12 , and undoped heterostructures where the carriers are induced using either a metal 13,14 or degenerately-doped semiconductor gate 15,16 . While modulation-doped structures still provide the highest mobilities, undoped heterostructures provide two distinct advantages. First, short range neutral disorder dominates over long range Coulombic disorder. This brings benefits such as enhanced robustness of the 5/2 fractional quantum Hall state, 8 and improved experimental access to the metallic state generated by electron-electron interactions in 2D systems.17 Second, the absence of intentional ionized impurities gives electrical properties that are remarkably robust to thermal cycling 18 , in stark contrast to modulation-doped heterostructures 6 . Both features demonstrate there is much more to disorder than the popular metric of mobility alone.19 They also strongly motivate the quest for a deeper understanding of the nature of disorder in undoped heterostructure devices. a) Electronic mail: adam.micolich@nanoelectronics.physics.unsw.edu.auWe recently developed a novel approach to studying disorder in nanoscale devices. It relies on the fact...

Effect of Split Gate Size on the Electrostatic Potential and 0.7 Anomaly within Quantum Wires on a Modulation-DopedGaAs/AlGaAsHeterostructure

“…A potential avenue for future work is to perform measurements on undoped heterostructures where the absence of Si donors results in a reduction of background disorder [45], leading to more reproducible behavior both between split gate devices [46,47] and after thermal cycling [48]. Measuring an array of devices on an undoped structure may also give insight on the degree to which disorder affects nanostructures fabricated on undoped heterostructures [49].…”

Effects of biased and unbiased illuminations on dopant-free GaAs/AlGaAs 2DEGs

“…Second, it becomes more difficult to induce electrons in the GaAs channel after illumination, irrespective of insulator type or MBE growth chamber, requiring significantly higher turn-on threshold topgate voltages V th , defined as the extrapolated n 2d = 0 intercept on the topgate voltage axis[44]. This is contrary to what was observed in previous studies of illumination on 2DEGs in SISFETs[9,12,32], where V th was lower to achieve the same electron density after illumination. However, in SISFETs, the gate used to induce a 2DEG is a degenerately-doped GaAs layer.…”

“…[12], and 185 nm in Ref. [32]). Their observations of increased post-illumination mobility on dedicated Hall bars are consistent with the data shown here.…”

“…So the question remains: what type of impurity would, upon illumination, transition from a charged state to a neutral state in both GaAs and AlGaAs? See et al [32] have proposed that charge neutralization of a shallow acceptor carbon impurity, a − → a 0 , could fit this requirement. Indeed, carbon atoms are common background impurities in MBE chambers [49,50], and illumination of carbon modulation-doped GaAs/AlGaAs twodimensional holes gases (2DHG) can cause a persistent reduction of the hole carrier density, depending on MBE growth conditions [51].…”

“…Relative to their modulation-doped counterparts, dopant-free devices have exceptional reproducibility and low disorder [25,28,31], potentially making them suitable to study fragile fractional quantum Hall states [18,22]. Illumination has been studied in SISFETs, but with conflicting reports [9,12,32]. The effect has not been studied in HIGFETs.…”