Contains research summary.Joint Services Electronics Program (Contract DAALO3-86-K-0002

Kastner, Marc A.

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Ultralow Temperature Studies of ElectronicConduction in Submicron Silicon Inversion Layers11. Ultralow Temperature Studies of ElectronicConduction in Submicron Silicon InversionLayersAcademic and Research StaffProf. M.A. Kastner, Dr. J. MelngailisGraduate StudentJ.C. LiciniJoint Services Electronics Program (Contract DAALO3-86-K-0002)Marc A. KastnerField effect transistors have been fabricated, with widths as narrow as -50 nm, in theSubmicron Structures Laboratory. The narrow gate of these MOSFET's is created byglancing-angle evaporation of tungsten into a 50-nm step etched in the 100-nm oxide on theSi(100) surface. The tungsten wires are more uniform than those fabricated previously of Al,presumably because of the smaller grain size.During the past year we have focused attention on the regime of gate voltage VG well abovethreshold, in which the fluctuations of conductance with VG are much smaller. In experimentscarried out in collaboration with D.J. Bishop of AT&T Bell Laboratories, we measured themagnetoresistance of our MOSFET's at ultralow temperatures. We discovered aperiodicmagnetoresistance oscillations similar to those seen previously in ultranarrow metal wires andnarrow strips of GaAs. The magnitude of the magnetoresistance fluctuations is about the same asthat of the fluctuations of resistance with gate voltage. The latter have been ascribed by othergroups to structure in the density of states resulting from one-dimensional subbands. If thisassignment is correct, then the magnetoresistance structure would result from a shift of theenergy of these subbands with magnetic field. To explore whether field-induced energy shifts ofthe density of states could be the origin of the structure in the magnetoresistance, we measuredthe magnetoresistance at closely spaced values of VG. If the effect of the magnetic field were toshift the energies of states relative to the Fermi energy, then small changes in VG should causethe structure in the magnetoresistance to shift in field. On the contrary, we found that theoscillations in magnetoresistance remain unchanged for small changes in VG below a criticalvalue and become uncorrelated for changes in VG larger than this value.This provides strong evidence that the oscillations do not result from energy shifts of electronicstates but rather from field-induced changes in the phase of the wavefunctions. We believe thatthe fluctuations in resistance with VG and magnetic field are an example of theRLE P.R. 128Ultralow Temperature Studies of ElectronicConduction in Submicron Silicon Inversion Layersquantum-interference effect predicted for all disordered conductors.1Our experiments have provided another example of the importance of ultralow temperaturemeasurements in understanding quantum mechanical transport phenomena in submicronstructures. In order to carry out such experiments at M.I.T. we have ordered a dilution refrigeratorand 10T superconducting magnet from Oxford Instruments. The funds to pay for this came, inpart, from a grant from the AT&T foundation.References1. P.A. Lee and A.D. Stone, Phys. Rev. Lett. 55, 1622 (1985).RLE P.R. 128

Ultralow Temperature Studies of Electronic Conduction in Submicron Silicon Inversion Layers

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Ultralow Temperature Studies of Electronic Conduction in Submicron Silicon Inversion Layers

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