U. Kunze

Improved morphology and charge carrier injection in pentacene field-effect transistors with thiol-treated electrodes

The influence of chemical surface modifications of gold electrodes on the morphology and the electrical properties has been studied for pentacene based thin-film transistors with channel lengths of L⩽4μm. Self-assembled monolayers (SAMs) of various aliphatic and aromatic organothiols have been used to selectively modify the metallic source and drain electrodes and are further compared with reference samples with untreated electrodes. For all SAM-treated devices a reduced roughness of the pentacene film is observed which is accompanied by a reduction of the threshold voltage from about VT=2V for untreated transistors to −0.9V for transistors with SAM modified electrodes. Using aliphatic SAMs a poor on/off ratio of about 102 was obtained which is attributed to their low conductivity. In contrast, the on/off ratio is enhanced by four orders of magnitude if the surface is modified by an aromatic SAM. In this case a subthreshold swing as low as 0.55V/decade is achieved which corresponds to a trap density reduc...

Nanolithography with an atomic force microscope by means of vector-scan controlled dynamic plowing

We present a nanolithography technique based on an atomic force microscope. A thin resist layer on the sample surface is plastically indented by a vibrating tip. Controlling of the vibration amplitude and tip movement enables one to plow a narrow furrow along line segments of arbitrary length and direction. Different line segments which form a complex pattern can be plowed at a scan speed up to 5 μm/s. The geometric distortion of the resist pattern is less than 50 nm, where at scan speed in excess of 1 μm/s an interrupt of at least 10 ms is necessary between the line segments. The minimum offset error in positioning a pattern with respect to existing features is less than 4% of the scanning field. The patterns are transferred into SiO2, Si, GaAs, Ti, and Au by wet-chemical etching. Minimum linewidth is 25 nm in 1.5 nm oxide layers, 75 nm in 10 nm Ti film and 40 nm in 10 nm Au. On semiconductor surfaces smooth and perfectly shaped V grooves of 55 nm width are obtained.

Size distribution of coherently strained InAs quantum dots

The influence of the InAs coverage on the size and density of coherently strained InAs islands was investigated. At moderate InAs coverages the photoluminescence signal reflects the Gaussian size distribution of small coherently strained islands. However, before the coherently strained islands transform into dislocated ones the Gaussian line shape of their photoluminescence signal changes and a narrow peak appears on the low-energy tail. We attribute this change to an accumulation of coherently strained islands at a maximum size before dislocated island transformation occurs. Effects of luminescence from dislocated islands, size-dependent relaxation processes, capture efficiencies, and dot-dot coupling are also discussed. However, our calculations and the magnetophotoluminescence, as well as the photovoltage experiments, confirm our interpretation of a size accumulation process of coherently strained islands.

Influence of anthracene-2-thiol treatment on the device parameters of pentacene bottom-contact transistors

In pentacene-based bottom-contact field-effect transistors, the authors study the influence of anthracene-2-thiol-modified gold electrodes on the morphology, the contact and sheet resistance, the trap density, and the charge-carrier activation energy. The data are compared to reference samples with untreated gold electrodes. Anthracene-2-thiol treatment leads to a reduced sheet resistance, a reduced activation energy, and an improved film morphology.

Nanoscale devices fabricated by direct machining of GaAs with an atomic force microscope

We demonstrate a lithography wherein the tapping mode of an atomic force microscope the Si tip is used as a chiseling tool for direct machining of a GaAs surface. Single-groove drawing movements in a vector-scan mode result in approximately 3-4 nm deep and 30 nm wide furrows, which can be combined to arbitrary noncontiguous polygon patterns. Beneath such a groove a barrier arises in the electron channel of a GaAs/A1GaAs modulation-doped field effect transistor (MODFET). Using appropriate sub-100 nm line patterns we prepared quantum point contacts and single electron devices. At T = 4.2 K the transconductance characteristics of these nanoscale MODFETs exhibit structures, which represent signatures of either the quantized conductance or Coulomb-blockade effects.

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.

Fabrication of a quantum point contact by the dynamic plowing technique and wet-chemical etching

We have fabricated extremely confined ballistic constrictions using a nanolithography technique based on an atomic force microscope. Vector-scan controlled dynamic plowing with the vibrating tip enables to plastically indent a thin resist layer along a prearranged path. Transfer of the resist pattern into the semiconductor substrate is achieved by a strongly diluted aqueous etchant. In this way approximately 30 nm deep gooves were etched in the channel area of a modulation-doped GaAs/GaA1As field-effect transistor. The quantum point contacts were defined by a broken line whose 60 nm width represents the length and the sub-100 nm gap determines the width of the constriction. At liquid-helium temperature the conductance as a function of gate voltage shows a stepwise increase in units of 2e2/h. Signatures of the conductance quantization persist up to 50 K, which indicates a large subband spacing.

Low-temperature ballistic transport in nanoscale epitaxial graphene cross junctions

We report on the observation of inertial-ballistic transport in nanoscale cross junctions fabricated from epitaxial graphene grown on SiC(0001). Ballistic transport is indicated by a negative bend resistance of R12,43≈−170 Ω, which is measured in a nonlocal, four-terminal configuration at 4.2 Κ and which vanishes as the temperature is increased above 80 K.

Influence of processing parameters on the transport properties of quantum point contacts fabricated with an atomic force microscope

We describe a reliable technique for fabricating ballistic quantum point contacts (QPCs) with large energy separation between one-dimensional subbands. The technique is based on lithography with an atomic force microscope and wet chemical etching of a GaAs/AlGaAs heterostructure. The high-mobility two-dimensional electron gas located 55 nm below the surface is laterally confined by 20 nm or 50 nm deep grooves with separations ranging between 65–105 nm or 100–185 nm, respectively. The conductance characteristics at T = 4.2 K exhibit clear quasi-plateaux at multiples of 2e2/h. Both the conductance threshold voltage and the plateau widths are directly related to the QPC geometry. The energy separation ΔE1,2 of the lowest subbands is determined from the conductance under nonzero dc drain voltage. Upon reducing the QPC width, ΔE1,2 varies from 6 meV to 15 meV.

Nonlocal Aharonov–Bohm conductance oscillations in an asymmetric quantum ring

We investigate ballistic transport and quantum interference in a nanoscale quantum wire loop fabricated as a GaAs/AlGaAs field-effect heterostructure. Four-terminal measurements of current and voltage characteristics as a function of top gate voltages show negative bend resistance as a clear signature of ballistic transport. In perpendicular magnetic fields, phase-coherent transport leads to Aharonov–Bohm conductance oscillations, which show equal amplitudes in the local and the nonlocal measurement at a temperature of 1.5 K and above. We attribute this observation to the symmetry of the orthogonal cross junctions connecting the four quantum wire leads with the asymmetric quantum wire ring.