Strongly capacitively coupled quantum dots
I. H. Chan, R. M. Westervelt, K. D. Maranowski, A. C. Gossard
2002
Applied Physics Letters
Double quantum dots were formed in a gated GaAs/AlGaAs heterostructure with negligible interdot tunneling; strong capacitive coupling was provided by a floating interdot capacitor. The interdot capacitance was measured to be 0.28C ⌺ , where C ⌺ is the single-dot capacitance. Coulomb blockade conductance images for both dots versus side gate voltages at 70 mK show a hexagonal pattern of peaks; the double dot acts as a single-electron current switch. For weak tunneling, the conductance peaks of
more »
... th dots fit thermally broadened line shapes. Charge fluctuations produced by strong tunneling on one dot are induced on the second, filling in its peak splitting. Quantum dots show a range of important phenomena including the Coulomb blockade, single-electron transport, and individual electronic states. 1-3 Circuits of quantum dots are being studied now to provide approaches to sensors and electronics. Coupling dots together via electron tunneling mimics molecular bonds formed between atoms. 4 -9 Tunnelcoupled quantum dots have opened avenues of study, including proposed implementations of quantum information processing. 10 Coupling two or more quantum dots via interdot capacitors provides an additional way to correlate electrons and build circuits for logic and quantum information processing. In this letter, we describe the use of an interdot capacitor to form a strongly coupled double quantum dot with no interdot tunneling. In conventional laterally defined double quantum dots formed from a two-dimensional electron gas ͑2DEG͒ in a GaAs/AlGaAs heterostructure, the edge-on capacitance between dots is small, and an excess electron on one dot only has a linear effect on the conductance of the adjacent dot. 11 In the present work, capacitor plates were added over each dot to increase the interdot capacitance significantly. Because the interdot capacitor is electrically floating and comparable in size to the dots, it can rapidly transfer small induced charges from one dot to the other. In Coulomb blockade measurements at He dilution refrigerator temperatures, a single excess electron on one dot switched the other dot on or off its Coulomb blockade peak, changing its electrical conductance by two decades. In addition, charge fluctuations caused by lifetime broadening in one dot are induced on the second dot kept in the weak tunneling regime. The conductance peaks of the two dots fit a combination of thermally broadened and lifetime broadened line shapes. A scanning electron microscope image of the double-dot device used in this experiment is shown in Fig. 1͑a͒ with a schematic diagram in Fig. 1͑b͒ . The two quantum dots were formed in a 2DEG inside a GaAs/AlGaAs heterostructure with the following layers ͑surface down͒: 50 Å GaAs, 300 Å Al 0.3 Ga 0.7 As, 8ϫ10 12 cm Ϫ2 Si ␦-doping layer, 220 Å Al 0.3 Ga 0.7 As, and 10 000 Å GaAs on a GaAs substrate. The 2DEG has mobility ϭ5ϫ10 5 cm 2 /V s and density n s ϭ3 ϫ10 11 cm Ϫ2 at liquid-He temperatures. The two rectangular quantum dots shown in Fig. 1͑a͒ are defined on three sides by Cr-Au gates ͑light regions͒ and on the fourth side by a 50-nm-deep trench ͑very dark region͒ down the center of the device. A special feature of this device is the floating interdot capacitor that consists of two capacitor plates, one over each dot, connected by a thick bridge of Cr-Au ͑very light center bar͒. The trench down the center of the device prevents tunneling between the two dots (Rӷ10 M⍀), so that interactions between the dots arise from capacitive coupling and not tunneling. Both dots are nominally identical with lithographic dimensions 500ϫ800 nm 2 . The left-hand dot will be referred to as dot 1 and the right hand dot as dot 2; their respective side gate voltages are V G1 and V G2 . Electrical measurements of the device were made in a He dilution refrigerator held at the base temperature 10 mK. The measured electron temperature 70 mK was determined from the widths of the Coulomb blockade conductance peaks. Two leads were connected to each dot by quantum point contacts. The conductances G D1 and G D2 of the two dots were measured simultaneously using two lock-in amplifiers at different frequencies. a͒ Electronic mail: westervelt@deas.harvard.edu b͒ Current address: Cielo Communications, 325 Interlocken Parkway, Bldg. A, Broomfield, CO 80021. FIG. 1. ͑a͒ Scanning electron micrograph of a capacitively coupled double quantum dot formed in a GaAs/AlGaAs heterostructure. Light areas are metal gates; the dark vertical trench prevents tunneling between dots; the bright bar in the center connects two capacitor plates above each dot, forming an interdot capacitor. ͑b͒ Schematic diagram of the device.
doi:10.1063/1.1456552
fatcat:5ihx4lcxbnhuxnuoe2ujpbms6u