We present a multiplexing scheme for the measurement of large numbers of mesoscopic devices in cryogenic systems. The multiplexer is used to contact an array of 256 split gates on a GaAs/AlGaAs heterostructure, in which each split gate can be measured individually. The low-temperature conductance of split-gate devices is governed by quantum mechanics, leading to the appearance of conductance plateaux at intervals of 2e 2 /h. A fabrication-limited yield of 94% is achieved for the array, and a 'quantum yield' is also defined, to account for disorder affecting the quantum behaviour of the devices. The quantum yield rose from 55% to 86% after illuminating the sample, explained by the corresponding increase in carrier density and mobility of the twodimensional electron gas. The multiplexer is a scalable architecture, and can be extended to other forms of mesoscopic devices. It overcomes previous limits on the number of devices that can be fabricated on a single chip due to the number of electrical contacts available, without the need to alter existing experimental set ups.
We present a method of forming and controlling large arrays of gate-defined quantum devices. The method uses a novel, on-chip, multiplexed charge-locking system and helps to overcome the restraints imposed by the number of wires available in cryostat measurement systems. Two device innovations are introduced. Firstly, a multiplexer design which utilises split gates to allow the multiplexer to divide three or more ways at each branch. Secondly we describe a device architecture that utilises a multiplexer-type scheme to lock charge onto gate electrodes. The design allows access to and control of gates whose total number exceeds that of the available electrical contacts and enables the formation, modulation and measurement of large arrays of quantum devices. We fabricate devices utilising these innovations on n-type GaAs/AlGaAs substrates and investigate the stability of the charge locked on to the gates. Proof-of-concept is shown by measurement of the Coulomb blockade peaks of a single quantum dot formed by a floating gate in the device. The floating gate is seen to drift by approximately one Coulomb oscillation per hour.Motivation for the measurement of large numbers of quantum devices arises both from interest in the associated physical properties such as the formation of minibands [1] and from the drive to up-scale and integrate quantum phenomenon, such as spin physics[2], into future technology and quantum information processing [3]. Much of the physics of interest is only observable using cryogenic systems and the number of coupled devices is limited by the number of available contacts. Recent work has shown the use of multiplexing to greatly increase the number of isolated quantum devices available for measurement on a single chip and single cool-down [4,5] and frequency multiplexing, for the readout of spin qubits [6], has been demonstrated as a potential up-scaling route. The significant challenges presented by the need to upscale however are far from surmounted.The measurement of many individually addressable quantum devices has led to initial studies on yield[4], reproducibility [7] and statistical analysis of complex quantum phenomena [8]. The split gate[9-11] can be considered as the building block for more complex gatedefined devices, such as quantum dots [12]. Tuneable quantum dots require stable charge on several surface gates simultaneously in order to function. The multiplexing architecture presented in [4] doesn't allow the simultaneous use of multiple gates. We therefore present two innovations that facilitate the fabrication and measurement of large interacting quantum device arrays.We firstly show how a split gate can be used within a multiplexer-type addressing system, to enable the multiplexer to divide three or more ways at each node rather than two. Figure 1 shows a schematic of a single node of a 3-way multiplexer. A semiconducting two dimensional electron gas (2DEG), shown in blue, divides into three
The properties of conductance in one-dimensional (1D) quantum wires are statistically investigated using an array of 256 lithographically-identical split gates, fabricated on a GaAs/AlGaAs heterostructure. All the split gates are measured during a single cooldown under the same conditions. Electron many-body effects give rise to an anomalous feature in the conductance of a one-dimensional quantum wire, known as the '0.7 structure' (or '0.7 anomaly'). To handle the large data set, a method of automatically estimating the conductance value of the 0.7 structure is developed. Large differences are observed in the strength and value of the 0.7 structure [from 0.63 to 0.84 × (2e 2 /h)], despite the constant temperature and identical device design. Variations in the 1D potential profile are quantified by estimating the curvature of the barrier in the direction of electron transport, following a saddle-point model. The 0.7 structure appears to be highly sensitive to the specific confining potential within individual devices. arXiv:1407.7441v1 [cond-mat.mes-hall]
Ninety eight one-dimensional channels defined using split gates fabricated on a GaAs/AlGaAs heterostructure are measured during one cooldown at 1.4 K. The devices are arranged in an array on a single chip, and individually addressed using a multiplexing technique. The anomalous conductance feature known as the "0.7 structure" is studied using statistical techniques. The ensemble of data show that the 0.7 anomaly becomes more pronounced and occurs at lower values as the curvature of the potential barrier in the transport direction decreases. This corresponds to an increase in the effective length of the device. The 0.7 anomaly is not strongly influenced by other properties of the conductance related to density. The curvature of the potential barrier appears to be the primary factor governing the shape of the 0.7 structure at a given T and B.
We utilize a multiplexing architecture to measure the conductance properties of an array of 256 split gates. We investigate the reproducibility of the pinch off and one-dimensional definition voltage as a function of spatial location on two different cooldowns, and after illuminating the device. The reproducibility of both these properties on the two cooldowns is high, the result of the density of the two-dimensional electron gas returning to a similar state after thermal cycling. The spatial variation of the pinch-off voltage reduces after illumination; however, the variation of the one-dimensional definition voltage increases due to an anomalous feature in the center of the array. A technique which quantifies the homogeneity of split-gate properties across the array is developed which captures the experimentally observed trends. In addition, the one-dimensional definition voltage is used to probe the density of the wafer at each split gate in the array on a micron scale using a capacitive model.
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