Claudia Bock

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.

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.

Valence-band structure of self-assembled InAs quantum dots studied by capacitance spectroscopy

Hole transport into self-assembled InAs quantum dots (QDs) embedded in a GaAs/AlAs matrix was studied by capacitance spectroscopy. From the differential capacitance, a Coulomb blockade energy of EC0h≈22 meV for holes in the ground state was extracted. When the front barrier between the dot layer and the Schottky contact is precisely reduced by selective wet chemical etching, the QD ground state signal shifts to lower gate voltages according to a simple leverage law. From the linear fit of the voltage shift versus the front barrier thickness the hole binding energy of E0h≈194 meV was determined.

High-Quality Solution-Processed Silicon Oxide Gate Dielectric Applied on Indium Oxide Based Thin-Film Transistors

A silicon oxide gate dielectric was synthesized by a facile sol-gel reaction and applied to solution-processed indium oxide based thin-film transistors (TFTs). The SiOx sol-gel was spin-coated on highly doped silicon substrates and converted to a dense dielectric film with a smooth surface at a maximum processing temperature of T = 350 °C. The synthesis was systematically improved, so that the solution-processed silicon oxide finally achieved comparable break downfield strength (7 MV/cm) and leakage current densities (<10 nA/cm(2) at 1 MV/cm) to thermally grown silicon dioxide (SiO2). The good quality of the dielectric layer was successfully proven in bottom-gate, bottom-contact metal oxide TFTs and compared to reference TFTs with thermally grown SiO2. Both transistor types have field-effect mobility values as high as 28 cm(2)/(Vs) with an on/off current ratio of 10(8), subthreshold swings of 0.30 and 0.37 V/dec, respectively, and a threshold voltage close to zero. The good device performance could be attributed to the smooth dielectric/semiconductor interface and low interface trap density. Thus, the sol-gel-derived SiO2 is a promising candidate for a high-quality dielectric layer on many substrates and high-performance large-area applications.

In-plane and perpendicular tunneling through InAs quantum dots

Abstract A Schottky diode with InAs dots in the intrinsic GaAs region was used to investigate perpendicular tunneling (in growth direction) through InAs quantum dots (QDs). At forward bias conditions electrons tunnel from the ohmic back contact into the metal Schottky gate. Peaks appear in the differential conductance when a QD level comes into resonance with the Fermi-level of the n-doped region. The observed tunneling features are attributed to electron transport through the s- and p-shell of the InAs islands. In our in-plane tunneling experiments the islands were embedded in the channel region of an n-doped GaAs/AlGaAs HEMT-structure. In order to study tunneling through single InAs islands, a quantum point contact was defined by lithography with an atomic force microscope and subsequent wet-chemical etching. In contrast to unpatterned devices sharp peaks appear in the I – V characteristic of our samples reflecting the transport of electrons through the p-shell of a single InAs QD.

Ultrawide electrical tuning of light matter interaction in a high electron mobility transistor structure

The interaction between intersubband resonances (ISRs) and metamaterial microcavities constitutes a strongly coupled system where new resonances form that depend on the coupling strength. Here we present experimental evidence of strong coupling between the cavity resonance of a terahertz metamaterial and the ISR in a high electron mobility transistor (HEMT) structure. The device is electrically switched from an uncoupled to a strongly coupled regime by tuning the ISR with epitaxially grown transparent gate. The asymmetric potential in the HEMT structure enables ultrawide electrical tuning of ISR, which is an order of magnitude higher as compared to an equivalent square well. For a single heterojunction with a triangular confinement, we achieve an avoided splitting of 0.52 THz, which is a significant fraction of the bare intersubband resonance at 2 THz.

Single-Electron Tunneling through Individual InAs Quantum Dots within a Saddle Point Potential

InAs quantum dots were embedded in a two-dimensional electron gas of a modulation doped heterostructure. A constriction was lithographically defined to allow electron transport through at maximum three quantum dots. Sharp resonances appear at the onset of the conductance which are attributed to electron tunneling through the first excited state of a single dot within the constriction. A Coulomb blockade energy of 12 meV was estimated for this energy level which is in good agreement with capacitance data measured on a dot ensemble. When the source bias is varied Coulomb blocked regimes can be observed which are typical for single electron devices.

Capacitance and tunneling spectroscopy of InAs quantum dots

A Schottky diode type sample (Au/i-GaAs/InAs/i-GaAs/n+-GaAs) with InAs quantum dots (QDs) embedded in the intrinsic GaAs region between the Schottky contact and the n-doped GaAs-back contact was used to investigate the electron transport into and through InAs QDs. According to a simple leverage law the QD ground state resonance shifts to higher gate voltages when the thickness of the tunneling barrier t2 between the QDs and the Schottky contact is reduced from t2=90 nm to t2=10 nm. Additionally, the transition from a pure capacitive to a predominant conductive signal of the QD ground state is observed. The gate voltage offset between the charging and the tunneling signal of the s-shell is explained by QDs of different size dominating the ohmic and the capacitive I–V traces, respectively. This interpretation is confirmed by different Coulomb blockade energies as well as different confinement energies of the QD ground state determined from both types of signal.

Influence of structural properties on ballistic transport in nanoscale epitaxial graphene cross junctions

In this paper we investigate the influence of material and device properties on the ballistic transport in epitaxial monolayer graphene and epitaxial quasi-free-standing monolayer graphene. Our studies comprise (a) magneto-transport in two-dimensional (2D) Hall bars, (b) temperature- and magnetic-field-dependent bend resistance of unaligned and step-edge-aligned orthogonal cross junctions, and (c) the influence of the lead width of the cross junctions on ballistic transport. We found that ballistic transport is highly sensitive to scattering at the step edges of the silicon carbide substrate. A suppression of the ballistic transport is observed if the lead width of the cross junction is reduced from 50 nm to 30 nm. In a 50 nm wide device prepared on quasi-free-standing graphene we observe a gradual transition from the ballistic into the diffusive transport regime if the temperature is increased from 4.2 to about 50 K, although 2D Hall bars show a temperature-independent mobility. Thus, in 1D devices additional temperature-dependent scattering mechanisms play a pivotal role.

Electronic structure of self-assembled InAs quantum dots

Abstract Electroluminescence (EL) is always related to carrier transport (i.e. current flow) into an active region. We used EL and transport measurements to study in detail the electronic structure in InAs quantum dots (QDs) embedded in the intrinsic GaAs region of a double hetero p–i–n diode. According to the position of the dot layer with respect to the n- and p-doped regions we independently investigated the QD levels of electrons and holes. From the differential capacitance the Coulomb blockade energy for electrons in the QD ground state and the energy splitting between the ground and first excited state in the conduction band system was extracted. Additionally the energy separation of the electron ground state from the GaAs conduction band edge was determined by the same technique. The energetic distance between the hole ground and first excited state can be estimated from the electroluminescence signal as well as from the differential conductance.