Mizuta, Hiroshi

Oda, S

Tsuchiya, Yoshishige

Uchida, K

Yamahata, G

English

Gento Yamahata

Ken Uchida

Shunri Oda

Yoshishige Tsuchiya

Hiroshi Mizuta

Crossref

Enhanced tunnel conductance due to QCA cotunneling processes observed for silicon serial triple quantum dots

Southampton (e-Prints Soton)

IT1(pA)(a)(c)(b)(d)IT4(pA)IT3(pA)IT2(pA)Fig. 4. The contour plots of IT1 (a), IT2 (b), IT3 (c), and IT4 (d) as afunction of VG1 and VG4, where VT1, VT2, and VT3 are -6 mV and VT4is 0 mV.VT2, and VT3 are -6 mV and VT4 is 0 mV. We observedcomplicated current peaks with the various charge stabilityregions as indicated by the red doted lines in Fig. 4(c). Wethen simulated the electron transport through the MQDADs byusing the equivalent circuit as shown in Fig. 3 with the MonteCarlo circuit simulator. Some circuit parameters are carefullyextracted from the experimental results. The simulated stabilitydiagrams as shown in Fig. 5 show an excellent agreementwith the experimental results as shown in Fig. 4. The chargestability regions as indicated by the red dotted lines shown inFig. 5(c) completely corresponds to those shown in Fig. 4(c).Since the electron transport through QD4 is almost suppressedin this bias region, the transport through the serial triplequantum dots (TQDs) which consist of QD1, QD2, and QD3,determines the entire transport characteristics. This is the ﬁrstdemonstration of the electron transport through the siliconserial TQDs. In the following, we will show the detail analysisof the transport through the TQDs.In the case of serial DQDs [6], the electron stability dia-grams are determined by the electrochemical potentials of thetwo quantum dots. There are two types of charging lines whichhave the different slopes, where the electrochemical potentialfor one particular electron conﬁguration is zero. As the twocharging lines cross, the intersection is avoided due to theelectrostatic coupling and the pair of triple points appears,where the three charging conﬁgurations meet and the tunnelcurrents ﬂow. As a result, the charge stability regions form thehexagonal shapes (the honeycomb diagram).On the other hand, in serial TQDs [7], one more electro-chemical potential for third quantum dot must be taken intoaccount, leading to the third kinds of charging lines. If two ofthe three charging lines meet, the avoided crossing and the pairof triple points appear in a two-dimensional stability diagramIT2(a.u.)IT1(a.u.)(a)(c)(b)(d)IT4(a.u.)IT3(a.u.)+ ++ +----Fig. 5. The simulated contour plots of IT1 (a), IT2 (b), IT3 (c) ,and IT4 (d) as a function of VG1 and VG4, where VT1 = VT2 =VT3 = ¡6 mV and VT4 = 0 mV. The circuit parameters are as follows:t1 = t3 (C = 16 aF; R = 500 kΩ), t2 (C = 16 aF; R = 5 MΩ),t4 = t5 (C = 5:5 aF; R = 500 kΩ), t6 (C = 1 aF; R = 1 MΩ),t7 (C = 0:5 aF; R = 1 MΩ), CG1D1 = 1:258 aF, CG1D2 = 0:0178 aF,CG1D3 = 0:409 aF, CG1D4 = 0:05 aF, CG4D1 = 0:546 aF, CG4D2 =0:0136 aF, CG4D3 = 1:097 aF, CG4D4 = 0 aF (because of very weakcoupling), and CD1D2 = 1 aF. Since IT3 is very small in the experimentresults, we use high tunnel resistance for t2. The Gate capacitances areestimated from experimental results. The offset gate voltages of 1.6 V forVG1 and 1.7 V for VG4 are used because of the background charges.just as in the case of the DQDs. To obtain the full chargingconﬁgurations in the TQDs, we need a three-dimensionalstability diagram with three gates (the beehive diagram), wherecharging lines and triple points turn into charging plains andtriple lines, respectively. In the crossing point of the two triplelines, which is referred to the quadruple point, the four electronconﬁgurations meet and the electron transports are allowed.However, the quadruple points are rarely observed becausethey are the distinct points in a three-dimensional space.Our experimental and simulated results correspond to thetwo-dimensional slice of the beehive diagram. Since the mea-surements are performed at the nonlinear transport region, allthe electrochemical potentials of QD1, QD2, and QD3 areallowed to enter the bias window not only at the quadruplepoints but also at the triple points in the vicinity of thequadruple points. Therefore the current peaks in Fig. 4(a) areattributed to be a few quadruple points and a lot of triplepoints. Far from quadruple points, the tunnel current peaksdisappear [the right hand side of Fig. 4(a)] because of theelectrochemical potentials in QD2 far from the bias window.To discuss the electron conﬁgurations in the red squareregion in Fig. 5(a), we simulated the electron numbers N1,N2, and N3 corresponding to those in QD1, QD2, and QD3,respectively, as shown in Fig. 6. Although our system has alot of electrons because of the heavy doping concentration,we use the simple notation of the excess electron numbers as

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https://eprints.soton.ac.uk/266688/1/cpaper_159.pdf

Enhanced tunnel conductance due to QCA cotunneling processes observed for silicon serial triple quantum dots

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Southampton (e-Prints Soton)