Cristopher C. Eugster

Quantum field‐effect directional coupler

A quantum field-effect directional coupler is described comprised of two quantum waveguides closely spaced apart with an adjacent gate electrode over the space between waveguides. The coupling of electron probability density between waveguides is controlled by the voltage applied to the gate electrode. The coupler implements a voltage-controlled current switch. Several couplers can be connected to perform multiplex/demultiplexing functions.

Far‐infrared photon‐induced current in a quantum point contact

Motivated by the recently developed theory of photon‐assisted quantum transport, we have studied transport properties of an antenna‐coupled quantum point contract under coherent far‐infrared (285 GHz) radiation. A pronounced photon‐induced drain/source current is observed. The amplitude of the photon‐induced current is about 10% of that corresponding to a quantized conductance step, and it oscillates with the gate voltage. Our analysis suggests that the observed photon‐induced current is mainly due to heating of the electron gas in the source and drain (a bolometric effect) rather than photon‐assisted quantum transport. Future improvement of the device parameters to enhance the effect of photon‐assisted transport will be discussed.

Split‐gate dual‐electron waveguide device

A split‐gate technology on an AlGaAs/GaAs heterostructure is used to implement a novel quantum‐effect device which allows two electron waveguides to come into very close proximity to each other over a certain length. The field‐effect action of a middle gate controls the height and width of the energy barrier between the waveguides. This allows a gradual transition from two isolated waveguides to two closely spaced waveguides and finally to the merging of both waveguides into a single broad waveguide. Two side gates can control the number of occupied subbands in each waveguide. This is confirmed by the observation of sharp 2e2/h conductance steps in each waveguide at 1.8 K as the side‐gate voltage is modulated.

ONE-DIMENSIONAL TO ONE-DIMENSIONAL TUNNELLING BETWEEN ELECTRON WAVEGUIDES

We report the observation of controlled electron tunnelling between two closely spaced one‐dimensional (1D) electron waveguides implemented using a split‐gate scheme on a high mobility AlGaAs/GaAs heterostructure. The 1D to 1D tunnelling current shows a distinct bumpy pattern when the electronic subband population in the two waveguides is modulated. These results are consistent with a picture in which tunnelling primarily occurs when the 1D subbands in both waveguides line up in energy as expected from energy and momentum conservation rules.

Split‐gate electron waveguide fabrication using multilayer poly(methylmethacrylate)

We report on techniques for fabricating 20‐nm scale ballistic electron devices and on techniques for imaging and characterizing these patterns in thin layers of poly(methylmethacrylate) (PMMA). A split‐gate fabrication approach with nanometer‐scale Schottky gates is used. Using a multiple layer PMMA resist technique, we have fabricated Au/Pd gates as narrow as 20 nm. In order to enhance the undercut profile a lower molecular weight PMMA is used as the bottom layer. We have also developed a resist stabilization technique which allows the viewing of 20 nm scale features in 0.1‐μm thick PMMA resist under high magnification in a scanning electron microscope. These techniques have been used to fabricate ballistic electron devices which demonstrate quantum interference effects.

FAR-INFRARED RADIATION-INDUCED THERMOPOWER IN A QUANTUM POINT CONTACT

We have observed a far‐infrared (0.3–2.5 THz) radiation‐induced photovoltaic signal in an antenna‐coupled quantum point contract, which oscillates with the gate voltage and peaks at the onset of each subband. The polarity of this photovoltaic signal can be reversed by shifting the far‐infrared beam from the drain to the source, or vice versa. This signal has been unambiguously attributed to thermopower generated by asymmetric heating of the drain and the source by the far‐infrared radiation. Quantitative agreement has been obtained between measurements and calculations based on ballistic transport in one‐dimensional electron systems and the electrical and thermal circuit elements in our experimental system.

An InAlAs/InAs MODFET

An InAs modulation-doped field-effect transistor (MODFET) using an epitaxial heterostructure based entirely on arsenides is reported. The heterostructure was grown by MBE on InP and contains a 30-AA InAs channel. A 2- mu m-gate-length device displays well-behaved characteristics, showing sharp pinch-off (V/sub th/=0.8 V) and small output conductance (5 mS/mm) at 300 K. The maximum transconductance is 170 mS mm with a maximum drain current of 312 mA/mm. Strong channel quantization results in a breakdown voltage of -9.6 V, a severalfold improvement over previous InAs MODFETs based on antimonides. Low-temperature magnetic field measurements show strong Shubnikov-de Haas oscillations which, over a certain range of gate voltage, strongly indicate that the electron channel resides in the InAs layer.<<ETX>>

A novel analog-to-digital conversion architecture using electron waveguides

An analog-to-digital (A/D) conversion architecture based on the quantized conductance of electron waveguides has been demonstrated. In the scheme, a dual electron waveguide (DWG) device implements a binary quantizer and encoder for one significant bit of resolution. The conductance values of the one and off states in the binary code are 2e/sup 2//h and zero, respectively. By cascading multiple DWG devices, higher order significant bits can be attained. Here the first significant bit and the second significant bit of the analog-to-digital conversion architecture using a DWG device fabricated in an AlGaAs/GaAs modulation-doped heterostructure are shown. >

Conductance quantization in a GaAs electron waveguide device fabricated by x-ray lithography

We report on the fabrication of AlGaAs/GaAs split‐gate electron waveguide devices of lengths between 0.1 and 2 μm using x‐ray lithography, and the measurements of these devices at liquid‐helium temperatures and up to 15 K. An x‐ray mask (parent mask) was fabricated using e‐beam lithography and replicated using proximity x‐ray lithography (λ=1.32 nm) to generate a replica (daughter) mask. The daughter mask was then aligned to patterns on a high‐mobility AlGaAs/GaAs sample and x ray exposed using a conformable mask fixture. The conductance of the electron waveguides was measured as a function of the split‐gate bias. Sharp 2e2/h conductance steps were observed in devices up to 0.75 μm long at T=2 K. The features in the conductance remain visible up to 15 K.

Transport in Novel Gated Quantum Wires: The Impact of Wire Length

Split-gate quantum wires of various lengths including 0.0 µm (constrictions), 0.5 µm, and 1.0 µm have been fabricated on an AlGaAs/GaAs modulation-doped heterostructure. Confinement is achieved using a split-gate formed by two 40 nm Au/Pd lines, resulting in small gate leakage and short electron-beam writing times. In a two-probe measurement, we have observed strong conductance steps at 4.2 K in 0.5 µm long split-gate quantum wires. These features, which are characteristic of near ballistic electron transport through the wire, are present at temperatures as high as 10 K. Transport through the 0.0 µm constrictions was dominated by tunneling and transport in the 1.0 µm long wires was of a diffusive nature.