C. P. Umbach

Quantum interference devices made from superconducting oxide thin films

We have fabricated superconducting quantum interference devices (dc SQUID’s) from thin films of the superconducting oxide YBa2Cu3Oy. The devices were made by first lithographically patterning an ion implant mask containing a 40 by 40 μm loop and two 17‐μm‐wide weak links over a ∼1‐μm‐thick oxide film. Ion implantation was then used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID’s. The SQUID’s were operated in the temperature range from 4.2 to 68 K. The superconducting flux quantum was measured to be h/2e in these materials.

Indications of a Luttinger liquid in the fractional quantum Hall regime

Abstract We have studied the transmission through a point contact in a gated GaAsAlGaAs heterostructure. At the v = 1 3 quantum Hall plateau measurements of the tunneling conductance exhibit non-Fermi liquid like resonance properties. The temperature dependence of the off-resonance conductances and resonance line shapes agrees with recent predictions of Luttinger liquid theory.

Nonlocal electrical properties in mesoscopic devices

The magnetoconductance of nominally identical, 2‐μm‐long, Au lines has been measured. A 0.7‐μm‐diam loop was attached to the end of one of the lines outside the probes in such a way that classically no current would flow through it. As expected, the line by itself showed only aperiodic conductance fluctuations. In contrast, the line with the attached loop also exhibited h/e periodic oscillations. The periodic oscillations are due to the quantum interference of electrons diffusing around the loop. These results clearly demonstrate that the voltage measured between contact probes separated by a distance on the order of an electron phase coherence length is strongly influenced by regions outside the contact probes.

Direct nanometer scale patterning of SiO2 with electron‐beam irradiation

Nanometer scale patterns have been fabricated in SiO2 by direct electron‐beam exposure. Two techniques have been developed to eliminate the surface contamination and enable the subsequent development of the patterns in HF based wet etches: (1) exposing the oxide through a sacrificial layer (previously reported) and (2) O2 reactive ion etching (RIE). The latter approach eliminates the need for a sacrificial layer and improves resolution by reducing the forward scattering of the beam. To determine the resolution of this process, patterns were fabricated with both 50‐ and 300‐kV electrons in thin SiO2 membrane samples and imaged in transmission. Transmission imaging avoids the resolution limit of secondary electron micrographs set by the lateral range of secondary electrons. At 300 keV with a line dose of 7.5 μC/cm, arrays of lines with a period down to 15 nm were achieved as opposed to the 21‐nm period previously reported using a sacrificial layer and secondary electron imaging of bulk substrates. A better ...

High‐gain lateral hot‐electron device

A lateral hot‐electron device has been fabricated in a plane of a two‐dimensional electron gas. The transfer ratio of the device, α, was studied for different geometrical configurations of the emitter barrier. The maximum transfer ratio was greater than 0.99 at 4.2 K, corresponding to a current gain greater than 100 for devices with base widths of 220 nm. An emission of a single longitudinal optical phonon, by the injected electrons, has been observed.

Nanolithography with an acid catalyzed resist

Poly(p‐t‐butyloxycarbonyloxystyrene) resist shows great potential for electron‐beam nanolithography, particularly as a negative resist. The resist has been used to fabricate structures with linewidths as narrow as 18 nm. The resist can be processed in both positive and negative modes depending upon the developing solvent, and linewidths <40 nm have been obtained in both cases. The exposure mechanism is based upon a new resist design principle incorporating an acid catalyst. The sensitivity of the resist can be at least six times higher than that of polymethylmethacrylate (PMMA) exposed under the same conditions (i.e., 50 kV, thin membrane substrates). The exposure distribution for the resist in the negative mode has been determined, confirming its resolution potential. These data indicate that the increase in sensitivity realized through incorporation of a gain mechanism in the resist chemistry is not achieved at a large loss in resolution. In its negative mode, the resist exhibits adequate ion etch resis...

Observation of h/e Aharonov–Bohm interference effects in submicron diameter, normal metal rings

Clear evidence of Aharonov–Bohm oscillations with respect to the flux quantum Φ0=h/e has been observed in the magnetoresistance of single, submicron diameter, normal metal rings. The rings were fabricated using contamination nanolithography in a personal computer controlled, high resolution, scanning transmission electron microscope. The magnetoresistance oscillations developed below T≂1 K, and, surprisingly, persisted without attenuation for more than 1000 periods.

HOT ELECTRON TRANSPORT IN TWO DIMENSIONAL ELECTRON GAS

Abstract We demonstrate ballistic transport of hot electrons in a high mobility two dimensional electron gas (2DEG) over distances as large as two micrometers and tunneling through a barrier induced in the 2DEG. Various inelastic scattering mechanisms are studied experimentally. For electrons with excess energy larger than the longitudinal optical (LO) phonon energy quanta we show sequential emission of up to three phonons. Below that energy we find a surprisingly long mean free path, approximately an order of magnitude longer than expected theoretically for electron-electron scattering. The combination of a very short mean free path for LO phonon emission and the long mean free path below the phonon energy leads to a new type of periodic oscillations in the maximal energy of the collected electrons. Finally we report on a study of the angular distribution of hot electrons injected from a point contact and demonstrate electrostatic steering of the electronic beam.

Proximity effect in electron beam patterned x‐ray masks

Electron beam exposures of 30 nm Au/5 nm Cr/2.2 μm Si x‐ray lithography mask structures were carried out at 25, 50, 75, and 100 keV to study the reduction in the proximity effect at higher beam voltages resulting from the loss of backscattered electrons out the backside of the masks. Surprisingly, the thin Au/Cr plating base was found to be the major source of backscattering over distances <1.0 μm. In addition, the plating base served to limit the decrease in the ratio of the backscattered to forward scattered exposure that resulted from increasing the beam voltage. These results indicate that simply increasing the beam voltage will not completely eliminate the proximity effect due to backscattering that arises during the electron beam patterning of x‐ray masks.

Selectively regrown contacts to field-effect transistors with two-dimensional electron-gas channels

The authors have fabricated for the first time heterostructure field-effect transistors where the two-dimensional electron gas (2-DEG) channel is directly contacted by selectively regrown epitaxial GaAs contacts. Both modulation-doped FETs (MODFETs) and semiconductor-insulator-semiconductor FETs (SISFETs) were fabricated. Contact resistances were low, as evidenced by high transconductances and improvements to the transconductance at low temperatures. The low resistance and shallow nature of the regrown contacts should permit scaling of these structures to very small dimensions.<<ETX>>