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Physica E 40 (2008) 1950–1951

www.elsevier.com/locate/physe

Electron transport study of a lateral InGaAs quantum dot

M. Larsson, D. Wallin, H.Q. Xu�

Division of Solid State Physics, Lund University, P.O. Box 118, S-221 Lund, Sweden

Available online 12 September 2007

Abstract

We report on fabrication and electron transport measurements of lateral InGaAs quantum dots defined by wet chemical etching. The

tunneling barriers and dot potential are tuned using both etching-defined in-plane gates and a local top gate. Transport measurements performed

at low temperature show Coulomb diamonds with excited states in the bias spectroscopy indicating the effects of quantum confinement.

r 2007 Elsevier B.V. All rights reserved.

PACS: 74.40.Xy; 71.63.Hk

Keywords: EP2DS-17; MSS-13; Quantum dot; InGaAs

1. Introduction

Quantum dots fabricated in semiconductor materialsenable us to study fundamental low-dimensional physicsinteresting for both quantum information processing andspintronics applications. Recently progress has been madewith both measuring and controlling single electron spinsin GaAs-based quantum dots [1,2]. Lateral InGaAsquantum structures are interesting to study because thesystems have large, electrical tunable spin–orbit interaction[3–5]. The spin–orbit interaction can be employed as animportant means for spin manipulation and for spindecoherence in spin qubits. Here we report on transportmeasurements of a single lateral quantum dot fabricated inan In0:75Ga0:25As=InP heterostructure two-dimensionalelectron gas (2DEG). We define our quantum dot by usingelectron beam lithography (EBL) and wet chemicaletching. The tunneling barriers and the number of electronsin the dot are controlled using etching-defined in-plane2DEG gates and a local top metal gate.

2. Fabrication

Our device was fabricated from a modulation dopedIn0:75Ga0:25As=InP heterostructure grown by metal-organic

e front matter r 2007 Elsevier B.V. All rights reserved.

yse.2007.09.011

ing author.

ess: Hongqi.Xu@ftf.lth.se (H.Q. Xu).

vapor phase epitaxy. The layer sequence was the following:50 nm thick not intentionally doped (NID) InP bufferlayer, 9 nm thick InGaAs quantum well, 20 nm thick NIDInP cap layer, 1 nm thick Si-doped (doping concentration5� 1018 cm�3) InP layer, and finally a 20 nm thick NIDInP cap layer (for more details on material growth, seeRef. [6]). 2DEG Mesas and ohmic contacts were fabricatedusing standard UV lithography. The quantum dots and thegates were defined using EBL and subsequent wet chemicaletching in a solution of H2O:HBr:HNO3 and saturatedbromic water. The depth of the trenches were measuredwith an atomic force microscope and were found to bearound 120 nm deep. The inset in Fig. 1(a) shows a deviceafter etching. The quantum dot has a diameter of around440 nm with six surrounding in-plane gates. The samplewas covered with a 250 nm thick layer of PMMA whichwas cross-linked over the mesa using EBL. Finally a localTi/Au gate was fabricated over the quantum dot againusing EBL. The position of the top gate is indicated by thesquare with the yellow dashed-line boundaries in the insetof Fig. 1(a).

3. Measurement results

Electron transport measurements were performed at300mK in a 3He based cryostat with a DC bias voltageapplied symmetrically to the source and drain contacts.Negative voltages were applied to the in-plane gates and

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Fig. 1. (a) Coulomb blockade oscillations in the current through the quantum dot as a function of top gate voltage VTG measured at a DC bias of 25mVand 300mK. The inset shows a device after etching without top gate and isolation layers. The top gate position is indicated by the square with boundaries

defined by the yellow dashed lines. (b) Charge stability diagram showing the differential conductance through the dot as a function of bias and top gate

voltages. Excited states are visible in the plot.

M. Larsson et al. / Physica E 40 (2008) 1950–1951 1951

top gate to tune the tunneling barriers between the dot and2DEG reservoirs and the number of electrons in the dot.Fig. 1(a) shows the current through the dot as a function oftop gate voltage, VTG, with a DC bias of 25mV. ClearCoulomb blockade oscillations indicating the effect ofsingle electron charging can be seen with an separation ofDVTG � 31mV. Fig. 1(b) shows the charge stabilitymeasurements of the device by sweeping the source–drainvoltage and voltage applied to the top gate. Inside the‘‘diamond’’ the total charge of the dot is constant andtransport is blocked, while at the boundaries of the‘‘diamond’’ transport is allowed. A pronounced excitedstate spectrum is also seen outside the diamonds indicatingthe effects of zero-dimensional confinement in the dot. Wehave also performed the charge stability measurements ofthe device by sweeping the source–drain voltage and thevoltage applied to a pair of side gate. Similar differentialconductance spectra were observed.

4. Conclusion

We have developed a fabrication scheme for makinglateral quantum dots in an InGaAs/InP heterostructure

using wet chemical etching. A combination of in-planegates and a top gate can be used to tune the coupling to thedot and the number of electrons in the dot. We believe it ispossible to further improve our fabrication techniques andscale down the dot size to allow for realization of few-electron multiple-dot systems for quantum informationprocessing.

Acknowledgements

This work was supported by the Swedish ResearchCouncil (VR), the Swedish Foundation for StrategicResearch (SSF), and the EU program SUBTLE.

References

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[5] G. Engels, J. Lange, Th. Schapers, H. Lth, Phys. Rev. B 55 (1997)

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