U. Wieser

Preparation of electron waveguide devices on GaAs/AlGaAs using negative-tone resist calixarene

We present a technique for the preparation of positively defined multiply connected electron waveguides on modulation-doped GaAs/AlGaAs heterostructures. This technique is based on a mix-and-match combination of electron-beam lithography (EBL) with standard photo lithography. Low-energy EBL on high-resolution negative-tone resist calixarene allows a nearly proximity-free positive definition of nanostructures with a minimum line width of about 25 nm. Subsequent to the EBL process the device leads and contacts are defined in photoresist with standard lithographic techniques. A single-step wet-chemical etch transfer enables the low-damage formation of isolated and multiply connected electron waveguides as well as large-area reservoirs. A 150 nm wide and 0.3 µm (1.2 µm) long quantum wire prepared by this technique shows quantized conductance with a maximum energy separation of 9.8 meV (10.9 meV) between the lowest one-dimensional subbands.

Ballistic rectification in an asymmetric mesoscopic cross junction

Ballistic rectification is demonstrated in a nanoscale waveguide cross junction consisting of a straight voltage stem channel and current-injecting branches which oppositely merge into the stem under an angle ϕ<90°. According to a simple billiardlike picture for both current directions, the injected electrons are deflected at the opposite boundary, thus charging one arm of the stem according to their preferential momentum component. The rectification efficiency has a maximum value of 2.6% and shows a characteristic voltage dependence, which is explained by the availability of unoccupied states in the voltage stem at low voltage and by carrier heating at high voltage.

Nanoscale patterning of Si/SiGe heterostructures by electron-beam lithography and selective wet-chemical etching

We present a low-damage fabrication technique for lateral sub-100 nm patterning of Si (13.5 nm)/Si0.76Ge0.24 (74 nm)/Si(001) heterostructures, which is based on successive selective wet-chemical etching. An oxide layer of 1-2 nm thickness on the Si top layer, which was formed well below the growth temperature of the heterostructure, was used as a sacrificial layer for transferring the resist pattern. Electron-beam lithography was done at 15 kV with a scanning electron microscope equipped with a field-emitter source. Transfer of the resist pattern is done by selective wet-chemical etching of the (i) oxide, (ii) Si and (iii) SiGe layers. The selectivity of the anisotropic Si etchant (25% w/w aqueous solution of tetramethyl ammonium hydroxide (TMAH) at 70 °C) is 20:1 for SiGe and better than 4200:1 for SiO2. The etch rate and selectivity of the SiGe etchant (buffered hydrofluoric acid, hydrogen peroxide and acetic acid) both depend on the waiting time between mixing and use. Due to the high anisotropy of TMAH the minimum width of grooves etched into the Si layer along the [110] direction of about 30 nm is mainly determined by the dimension of the oxide mask. Further transfer into the SiGe layer results in an undercut of the Si edge of about 70% of the etch depth, if the depth is less than the SiGe layer thickness. The profile of the SiGe grooves reveals a weak etching anisotropy. This patterning technique was applied to fabricate a geometric constriction in the epitaxial layers realized as a broken groove with the gap ranging from 15 to 300 nm width. Sufficiently long etching of the SiGe leads to complete underetching of the Si layer in the gap. The resulting suspended Si bridge is mechanically stable for 20 nm width and up to 360 nm length.

Thermopower-enhanced efficiency of Si/SiGe ballistic rectifiers

Injection-type ballistic rectifiers on Si/SiGe are studied with respect to the influence of gate voltage on the transfer resistance RT (output voltage divided by input current) for different positions of a local gate electrode. The rectifiers are trifurcated quantum wires with straight voltage stem and oblique current-injecting leads. Depending on the gate configuration, thermopower contributions arise from nearly pinched stem regions, which either cancel each other or impose upon the ballistic signal with same or opposite polarity. At best, this enhances RT to a maximum value of 470 Ω close to threshold voltage.

Phonon-induced breakdown of negative bend resistance in an asymmetric Si∕SiGe cross junction

An asymmetric nanoscale cross junction is fabricated from a high-mobility Si∕SiGe heterostructure. At T=4.2K, the four-terminal current-voltage characteristics reveal a polarity-dependent breakdown of the negative bend resistance. The breakdown is accompanied by negative differential conductance found in the two-terminal current-voltage characteristics of the orthogonal current leads. We attribute this behavior to phonon emission by hot electrons. From gate-voltage-dependent measurements, we determine a phonon threshold of 19meV.

Fabrication of Si/SiGe quantum point contacts by electron-beam lithography and shallow wet-chemical etching

Abstract We present a low-damage processing technique for fabricating quantum point contacts (QPCs) in the two-dimensional electron gas (2DEG) of a strained Si/Si0.7Ge0.3 heterostructure. The result is a QPC conductance characteristic which at T=4.2 K exhibits a series of smeared step-like features. The QPCs were fabricated with a novel technique which combines electron-beam lithography with low-damage selective wet-chemical pattern transfer. The constriction of the QPC is formed by an etched groove with a break w positioned at the mesa center of a field-effect device perpendicular to the transistor channel. After subtracting a series resistance of 800 Ω five quantum steps can be observed in the differential conductance each of them contributing 4e2/h. We attribute the steps to the quantum-ballistic 1D-transport through the constriction.

Quantum-ballistic transport in an etch-defined Si/SiGe quantum point contact

Ballistic constrictions are fabricated on a high-mobility Si/SiGe strained-layer heterostructure which exhibit conductance quantization in units of 4e2/h at T=4.2 K. Under finite drain voltage a half-plateau develops at 2e2/h and a series of oscillations appear which enable us to extract the energy separation ΔEN+1,N between successive one-dimensional subbands. The result is ΔE2,1=2.0 meV and ΔE3,2=1.4 meV.

Full-wave rectification based upon hot-electron thermopower

The hot-electron thermopower of a quantum point contact (QPC) is exploited for full-wave rectification at low temperatures. In a nanoscale AlGaAs/GaAs cross junction with orthogonal current and voltage leads the QPC is embedded into one voltage lead. The transfer resistance RT, given by the output voltage divided by input current, exhibits a distinct maximum at finite current and at gate voltages close to the QPC pinch-off voltage. Values in excess of RT=7 kΩ and output voltages up to 60% of the input voltages indicate an efficient ballistic rectification process.

Ballistic rectification in four‐terminal fork‐shaped Si/SiGe junctions

We present a new type of a ballistic rectifier where electrons are injected under zero injection angle into a straight central voltage stem of a four‐terminal Si/SiGe junction. For small input current a sign reversal of the rectified output signal is found which depends reversibly on the voltage applied to two side gate electrodes.

Ballistic transport and rectification in mesoscopic GaAs/AlGaAs cross junctions

We report on the observation of inertial-ballistic and mode-controlled rectification in mesoscopic GaAs/AlGaAs cross junctions. The cross junctions are composed of two current injecting branches which oppositely merge under an injection angle φ (90° ≥ φ ≥ 30°) into a straight central voltage stem. Ballistic electron transport is indicated by negative bend resistance which develops for φ = 90° in bend resistance configuration. Rectification is observed at both ends of the voltage stem. The mode-controlled signal at the upper end of the stem is found to be nearly independent from φ. The inertial-ballistic signal obtained from the potential difference between both ends of the stem vanishes for φ = 90° and increases with decreasing φ. Rectification is studied for different top-gate voltages and temperatures.