Contains description of one research project.Joint Services Electronics Program Contract DAAL03-89-C-0001National Science Foundation Grant ECS 88-1325

Antoniadis, Dimitri A.

Foxman, Ethan D.

Kastner, Marc A.

McEuen, Paul L.

Meirav, Udi E.

Odoardi, Angela R.

Smith, Henry I.

Wind, Shalom

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Chapter 4. Single Electron TransistorsChapter 4. Single Electron TransistorsAcademic and Research StaffProfessor Marc A. Kastner,1 Professor Henry I. Smith, Professor Dimitri A. Antoniadis, Paul McEuenVisiting Scientists and Research AffiliatesShalom Wind 2Graduate StudentsEthan D. Foxman, Udi E. MeiravTechnical and Support StaffAngela R. Odoardi4.1 Project DescriptionSponsorsJoint Services Electronics ProgramContract DAAL03-89-C-0001National Science FoundationGrant ECS 88-13250Two years ago we discovered, completelyunexpectedly, new behavior in very small SiMOSFETs. Whereas conventional transistors turnfrom the off state to the on state only once as thegate voltage increases, these small transistors turnon and off periodically, in some cases as many asone hundred times. We have since learned how tocontrol the period of these oscillations in GaAsfield effect devices and have shown that a nm-sizetransistor turns on and off once for every electronadded to it. Because of this, we call this kind ofdevice a Single Electron Transistor. Having estab-lished that each cycle corresponds to the additionof one electron, the period in gate voltage providesa very precise measure of the capacitance of ourdevices. Because it is very small, _ 10-16 F, suchtransistors may eventually find application in ultra-sensitive charge detection. Different applicationsutilizing the multistate characteristics of thedevices can be imagined. Unfortunately, so far thesingle electron behavior has only been observedbelow about 1 K. Whether this behavior can beobserved at higher temperatures in the futuredepends on the details of the mechanism thatcauses it. Our objective is, therefore, to betterunderstand that mechanism.Our initial discovery was made 3 with inversionlayers in Si which were about 30 nm wide andseveral mm long. These were dual gate MOSFETswith a narrow slot, - 600 nm wide, in the lowergate, fashioned with x-ray lithography. This lowergate was made of refractory metals allowing a hightemperature anneal after all unconventional litho-graphy was complete. In the end, these deviceswere the narrowest transistors ever made, butwhile most previous narrow devices had lowmobility, these had mobilities comparable to widedevices. The successful fabrication of these ultra-narrow devices was the result of a close collab-oration between H.I. Smith, D.A. Antoniadis, M.A.Kastner and their students.Soon after these devices were first fabricated, itwas discovered that their conductance oscillatedperiodically as a function of the density of elec-trons in the conducting channel, proportional tothe gate voltage, V,. An example of this distinctivebehavior is shown in figure 1.1 Donner Professor of Physics2 IBM Thomas J. Watson Research Center, Yorktown Heights, New York.3 J.H.F. Scott-Thomas, S.B. Field, M.A. Kastner, H.I. Smith, and D. A. Antoniadis, "Conductance Oscillations Peri-odic in the Density of a One-Dimensional Electron Gas." Phys. Rev. Lett. 62: 583 (1989); S.B. Field, M.A. Kastner,U. Meirav, J.H.F. Scott-Thomas, D.A. Antoniadis, H.I. Smith, and S.J. Wind. "Conductance Oscillations Periodicin the Density of One-Dimensional Electron Gases." Phys. Rev. B 42: 3523 (1990).125Chapter 4. Single Electron TransistorsFigure 1.The foremost problem posed by our discovery wasto find out what determined the period of the con-ductance oscillations. We soon discovered thatthe period varied at random from device to device,indicating that it was determined by the randomspacing of interface charges that happened to fallover the conducting channel. This suggested thatpotential barriers resulting from the interfacecharges were isolating a segment of the channeland that the conductance underwent one cycleevery time the same number of electrons wasadded to the isolated segment. To prove this, andto ascertain what that number of electrons was,new devices had to be made.The first step was to produce devices close in theiroperation to a MOSFET but in GaAs where theinfluence of interface charges is much weaker.4This was accomplished through a remarkably suc-cessful collaboration with M. Heiblum of IBM.The new structure was created by growing AIGaAson a conductive substrate, followed by a layer ofundoped GaAs. The electron density at theGaAs/AIGaAs interface, inverted from the usualconfiguration was varied by applying a voltage tothe substrate. Heiblum was able to grow suchinverted heterojunctions with mobilities of~ 500,000 cm 2/V-s.Next, the conducting channel is defined by depos-iting a metal gate on top of the GaAs, and a gap ispatterned in the gate by electron beam lithography.Because a depletion region is created under thegate, the electrons accumulate only under the gap.In order to emulate the effect of two chargedimpurities, two constrictions are patterned in thegap, as illustrated in figure 2. The electron beamlithography was done at IBM with the help of S.Wind.Devices of the kind depicted in figure 2 workedexactly as we hoped they would.5 Periodic oscil-lations of the conductance were seen again, butnow they were controlled: The period was thesame for different devices with the same spacingbetween constrictions and was larger when thespacing was shorter, which is consistent with theidea that the same number of electrons is addedfor one period in all devices. This is illustrated infigure 3. Calculations of the capacitance of suchdevices are only consistent with each period corre-sponding to one electron added per oscillation.This is an amazing result: Figure 3 shows that forthe shortest structures the conductance consists ofperiodic, narrow, well-separated resonances. Theconductance rises and falls by a factor - 100 witha variation of gate voltage that corresponds to only- 1/10 of an electron.4 U. Meirav, M.A. Kastner, M. Heiblum, and S.J. Wind, "One-Dimensional Electron Gas in GaAs: Periodic Conduc-tance Oscillations as a Function of Density," Phys. Rev. B (Rapid Comm.) 40: 5871 (1989).5 U. Meirav and S. J. Wind, "Single Electron Charging and Periodic Conductance Resonances in GaAsNanostructures," Phys. Rev. Lett. 65: 771 (1990).126 RLE Progress Report Number 133Chapter 4. Single Electron TransistorsFigure 2.We are not yet sure what causes the Single Elec-tron Transistor to work the way it does. The con-ductance appears to result from resonanttunneling, but the nature of the states that are res-onant is still unclear. Patrick Lee and hiscollaborators believe that the states arise fromsingle particle quantum states (the Fabry-Perotstates induced by the two barriers, the same asexpected for any "quantum dot"), but that theenergies of the levels are dominated by theCoulomb interaction between the electrons in theisolated segment. Van Houten and Beenakkerargued that the Coulomb interaction alone couldexplain the phenomena we see, and that the effectwas the same as the Coulomb blockade recentlypopularized by Likharev. Some of our measure-ments suggest, however, that electron-electroncorrelation, in addition to the simple Coulombinteraction, may be important.127Chapter 4. Single Electron Transistors128 (a)412T 8 (b)c:I- (c)O 4, 12044 120 4 8 12(mV)V - VIFigure 3.128 RLE Progress Report Number 133

Single Electron Transistors

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Single Electron Transistors

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