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The conductance of metal-oxide-silicon field-effect transistors with ~ 70-nm-wide inversion layers exhibits nonmonotonic variations with electron density below 15 K. The variations are largest at low electron concentrations and are the result of variations of the activation energy E A . When E A is largest the current is found to be limited by spatial barriers which contain tunneling channels at discrete energies, as in the model of Azbel.PACS numbers: 73.40.Qv, 71.55.Jv Several groups 1 " 4 have observed nonmonotonic conductance variations with electron concentration in ultranarrow Si inversion and accumulation layers. These variations, seen at cryogenic temperatures, are the result of the quasi onedimensional (Q-1D) structure of the devices, and are not, therefore, found in wide inversion layers. For devices which are narrow but are more than 100 nm wide, the variations are small and have been interpreted 2 ' 3 in terms of the theories of weak localization and interaction. On the other hand, for devices less than 100 nm wide the variations can be very large, and it has been suggested 1 ' 2 that they may result from statistical variations of resistance associated with strong localization. To investigate these large variations we have studied an inversion layer whose mean width is ~ 70 nm. By measuring the current as a function of both the temperature T and voltage drop along the inversion layer V Dy we find that the current at low electron density is limited by tunneling through spatial barriers.Our device is a metal-oxide-silicon field-effect transistor (MOSFET) on a (100) Si surface with a gate that is a narrow aluminum wire. When a gate voltage V G is applied, the electric field confines the electrons to a narrow potential well directly under the gate (see Fig. 1). The narrow gate is created by first reactive-ion etching a 50-nm step down into the 100-nm-thick gate oxide using photoresist as the mask, and then evaporating Al into the step at a glancing angle to the surface. Residual A1 is removed by a liquid etch. The resulting gate is 70 nm wide and 55 nm high with a ~ 20% variation in width. Computer simulations suggest that the inversion layer has about the same width as the gate. As sketched in Fig. 1, wide gates overlap the n + regions so that electrical contact to the narrow inversion layer is made through ~l-mm-wide inversion layers, Electrical continuity of the gate is con-firmed by measuring the resistance between the two wide parts of the gate that overlap the narrow one and are separated by 7 Mm. Conventional (wide) devices were prepared simultaneously on the same wafer over an oxide which was also reactive-ion etched. These wide devices are found to have a fairly high mobility (8000 cm 2 / V-s at 4.2 K).For our narrow devices, increasing V G , which is proportional to the electron concentration, causes the conductance to increase almost linearly at T £ 15 K. This is the same behavior seen at all temperatures in wide MOSFETs. However, as the temperature is reduced below ~15 K the conductance near t...

One goal of this work was to develop a reproducible method of preparing high quality Y-Ba-Cu-O superconducting films and to study their properties versus thickness. This was accomplished by rf diode sputtering from a single target. Twenty-seven depositions were made using a target containing 8.9-at. % Y, 37.3-at. % Ba, and 53.8-at. % Cu. Film thicknesses ranged from 0.09 to 2.4 μm. The film compositions obtained were 15.6±1.0-at. % Y, 35.8±1.0-at. % Ba, and 48.7±1.7-at. % Cu for the mean and standard deviation. The films were amorphous as-deposited and crystallized by annealing in O2 at 915 °C. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy indicated that, on (100)SrTiO3 substrates, films with thickness less than ∼0.25 μm were epitaxially oriented with their c axis perpendicular to the substrate. Films on (110)SrTiO3 were oriented with their c axis parallel to the substrate. On (100)SrTiO3, zero resistance was achieved in 30 samples from 27 runs at 85.6±1.4 K with a transition width (10%–90%) of 1.8±1.1 K independent of thickness. Results for deposition on a variety of other substrates were more variable. Diamagnetic shielding of up to 38% was calculated from the initial slope of M vs H. This low value was attributed to the presence of second phases and a significant diffusion layer thickness, both observed by transmission electron microscopy. Critical currents, measured by transport using a four-point probe, reached 8.1×105 A/cm2 at 77.35 K for a 0.2-μm film deposited on (100)SrTiO3 . Comparable values were obtained by calculation from the M-H hysteresis loop, from which we infer that there are either continuous epitaxial sheets of c axis oriented 123 or, if there are grain boundaries, the coupling across them is strong.