R. E. Behringer

Submicron, vacuum ultraviolet contact lithography with an F2 excimer laser

An F2 excimer laser at 157 nm has been used for the first time as an exposure source for high resolution photolithography. At this short wavelength, conventional glass and quartz mask substrates are opaque, and therefore alkaline‐earth halides and sapphire were used as mask substrates. The masks were patterned by electron beam lithography, and mask features as narrow as 150 nm have been replicated and represent the smallest features yet produced by contact photolithography.

Using light as a stencil

A new method for laterally manipulating the morphology of a thin film is presented, which uses the force exerted by light to deflect neutral atoms in an atomic beam during deposition. We have evaluated the dependence of the thickness of a thin metal film on the frequency, intensity, and the spatial structure of the light field, and find that the stimulated component of the force is suitable for laterally organizing atoms from centimeter to submicron dimensions.

Deviation of the quantum hall effect from exact quantization in narrow GaAs-AlxGa1−xAs heterostructure devices

Abstract We report the first observation of deviaton of the i=4 quantum Hall plateau from its quantized value, in narrow GaAs-AlxGa1−xAs quasi-1-d wires of width 2000A. The deviation arises in the form of aperiodic fluctuations as the magnetic field is varied, even though a deep minimum developes in longitudinal resistance. The fluctuation size grows with decreasing temperature and can be as large as 250Ω at 50mK. We suggest the observations arise from a combination of localization and Aharonov-Bohm quantum interference effects.

Quantum transport in one-dimensional GaAs/AlGaAs microstructures

We have observed the Aharonov-Bohm effect in the magnetoresistance of doubly connected geometries fabricated in high mobility GaAs/AlGaAs heterostructures. Periodic oscillations in the resistance associated with the magnetic fluxes θ0 = hc/e, θ0/2, and θ0/3 penetrating the annulus are suppressed for fields above √hc/eH ≈ W/2 where W is the width of the wire comprising the annulus, corresponding to ωc⊥ >1 where ωc is the cyclotron frequency and ⊥ ist the scattering time. Correlated fluctuations in the resistance as a function of magnetic field associated with the flux penetrating the wires comprising the annuli are observed for 0<ωc⊥<300, concomitantly with Shubnikov-de Haas oscillations and the Hall effect.

Fabrication of ultrahigh resolution structures in compound semiconductor heterostructures

Techniques have been developed to make silicon metal‐oxide‐semiconductor field‐effect transistor (MOSFETs) with minimum dimensions as small as 25 nm for fundamental electron transport studies. These devices have been used to study a variety of quantum conduction and electron scattering effects. At a given size scale, these quantum effects can be increased by replacing silicon in these devices with a high mobility, low electron mass semiconductor like GaAs. To make such devices, we have extended our technology to the fabrication of III–V semiconductor nanostructures. As an example of this, we describe the fabrication of 0.1 μm rings in modulation‐doped GaAs using electron beam lithography and reactive‐ion etching.

Laser focused atomic deposition: A new lithography tool

We have used the force exerted on a neutral atom by an optical standing wave to focus a neutral atomic beam into a grating structure that is deposited on a substrate. We have made gratings with pitch of 294 nm, linewidths of δ=40–45 nm and contrast ≳10:1, over an area of 7.6 mm2. We discuss the conditions needed to make the narrowest structures, and how these conditions affect the performance of this technique as a lithography tool. We also show depositions that have lower resolution, δ≊80 nm, but could cover an area greater than 3×104 mm2.

Observation of Aharonov–Bohm effect in quasi‐one‐dimensional GaAs/AlGaAs rings

Small structures in high‐mobility semiconductor heterojunctions are expected to show a variety of quantum mechanical phenomena associated with electron interference. At low temperatures in the high‐mobility two‐dimensional electron gas at a GaAs/AlGaAs heterojunction, both the elastic and inelastic electron scattering lengths are several microns, making it possible to fabricate complete devices contained within these dimensions. In ring shaped devices we have observed periodic magnetoconductance oscillations (Aharonov–Bohm effect) caused by electron interference. The magnitude of this effect is as much as 10% of the total conductance, much larger than seen in previously studied metal systems. High‐resolution electron (e)‐beam lithography and reactive ion etching were used to fabricate rings of 1, 2, and 2.5 μm mean diameters with linewidths of 0.5 μm. The electrical channel width is smaller than the physical channel width because of edge depletion. Samples with conducting channels varying from about 0.6 t...

Nanolithography using a laser focused neutral atom beam

Drawing on techniques developed for laser cooling and trapping of neutral atoms, we demonstrate a novel lithographic tool where near‐resonant laser light is used to directly control the position of neutral atoms during deposition. The power of this technique is demonstrated by focusing sodium atoms into narrow lines as they traverse a one‐dimensional standing wave. We have studied the dependence of the linewidth and contrast on the light field parameters, such as detuning, intensity, and interaction length. The dependencies follow the predictions from a simple model and, under optimal conditions, we obtain high contrast lines with a resolution of 45 nm over an area of 0.2×6 mm2. Numerical simulations indicate that further improvement below 10 nm is likely in the near future.

Magneto-quantum transport in a quasi-one-dimensional GaAs/AlxGa1−xAs ring

Abstract We study the magneto-transport properties of a quasi-ID GaAs/Al x Ga 1− x As ring of mobility 40 000 cm 2 /V·a at 4 K. The inelastic scattering length is about 4000 A while the sample length is of the order of 3 μm, so that we are in the diffusive regime. A variety of quantum interference phenomena are observed, including negative magneto-resistance, hole Aharonov-Bohm effect, and aperiodic/quasiperiodic resistance fluctuations. This rich variety enables us to estimate the sample conducting width and electron phase coherence length, l θ , so that we may attempt to obtain a consistent picture of these related phenomena. In addition, we obtain a magnetic field scale on which the hole oscillations disappear.

One‐dimensional ballistic transport in AlGaAs/GaAs electron waveguides

Recently, there has been growing interest in exploring the limits to scaling of semiconductor devices, and in understanding their characteristics in the regime where quantum effects and ballistic transport dominate. Using high‐resolution fabrication techniques on high‐mobility, modulation‐doped AlGaAs/GaAs, it is possible to confine the two‐dimensional electron gas (2DEG) to structures that are smaller than both the inelastic and elastic scattering lengths, and are comparable in size to the electron wavelength. The conduction of such a device should be one dimensional and ballistic. Unlike large samples, the resistance of these ballistic devices does not follow classical equations; it is primarily caused by electron interference and scattering from the geometry of the sample, not by impurities. On this scale, these structures behave as electron waveguides not as diffusive conductors. We have used electron‐beam lithography and high‐resolution reaction ion etching to produce samples with well controlled, co...