Daniel Stichtenoth

High-performance ZnO nanowire field effect transistors

ZnO nanowires with high crystalline and optical properties are characterized, showing strong effect of the surface defect states. In order to optimize the performance of devices based on these nanowires, a series of complementary metal-oxide semiconductor compatible surface passivation procedures is employed. Electrical transport measurements demonstrate significantly reduced subthreshold swing, high on/off ratio, and unprecedented field effect mobility.

Finite size effect in ZnO nanowires

To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the “anomalous” blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one-dimensional materials.

Scalable Fabrication of Nanowire Photonic and Electronic Circuits Using Spin-on Glass

We present a method which can be used for the mass-fabrication of nanowire photonic and electronic devices based on spin-on glass technology and on the photolithographic definition of independent electrical contacts to the top and the bottom of a nanowire. This method allows for the fabrication of nanowire devices in a reliable, fast, and low cost way, and it can be applied to nanowires with arbitrary cross section and doping type (p and n). We demonstrate this technique by fabricating single-nanowire p-Si(substrate)-n-ZnO(nanowire) heterojunction diodes, which show good rectification properties and, furthermore, which function as ultraviolet light-emitting diodes.

Optical size effects in ultrathin ZnO nanowires

Single crystal ZnO wurtzite nanowires grown along the c-axis with diameters down to 4 nm were synthesized by a catalytic vapor transport technique. Photoluminescence spectra of these wires indicate a blue shift of the free exciton by 19 meV due to confinement. This result was obtained by analyzing the line shape of the blue-shifted LO phonon replica of the free exciton. In addition, a surface-related excitonic luminescence feature centered at 3.366 eV was observed with a strongly elevated thermal activation energy.

Alignment of Semiconductor Nanowires Using Ion Beams

Gallium arsenide nanowires are grown on 100 GaAs substrates, adopting the epitaxial relation and thus growing with an angle around 35 degrees off the substrate surface. These straight nanowires are irradiated with different kinds of energetic ions. Depending on the ion species and energy, downwards or upwards bending of the nanowires is observed to increase with ion fluence. In the case of upwards bending, the nanowires can be aligned towards the ion beam direction at high fluences. Defect formation (vacancies and interstitials) within the implantation cascade is identified as the key mechanism for bending. Monte Carlo simulations of the implantation are presented to substantiate the results.

P-type doping of GaAs nanowires

Gallium arsenide (GaAs) nanowires with diameters of 150nm have been grown via metal-organic vapor deposition and were subsequently implanted with Zn64 ions. The amorphized nanowires were annealed at 800°C under arsenic overpressure resulting into a full recrystallization of the nanowires as well as an activation of the implanted acceptors. Consequently, we observe a strong increase in conductivity of the GaAs:Zn nanowires, where a simple estimation of the activated acceptors matches the implantation concentration.

Catalyst–nanostructure interaction and growth of ZnS nanobelts

Details of the vapour-liquid-solid Au droplet catalysed growth of ZnS nanobelts are elucidated in this work. The inclination of the Au droplet after solidification shows that it is indeed in the liquid state during nanobelt growth. Numerous stacking faults are observed when (0001) wurtzite is the side surface of the nanobelt. Compressive stress at the droplet-nanobelt-atmosphere triple interface is the cause of the stacking faults. Sawteeth-like structures are observed on the Zn-terminated polar (0001) side surface only. These surfaces are chemically active, while S-terminated [Formula: see text] surfaces and non-polar surfaces are not. On these active surfaces, autocatalysed vapour-solid growth leads to the formation of the observed sawteeth.

Characterization of the donor-acceptor-pair transition in Nitrogen-implanted zinc oxide

Zinc oxide bulk crystals were doped with nitrogen by ion beam implantation. After postimplantation annealing, a luminescent transition appears at 3.230eV. Power-dependent photoluminescence studies and time-resolved measurements at several spectral positions within this band can be described by a model for donor-acceptor-pair (DAP) transitions. By tracing the luminescence in a temperature-dependent study, a connection to phonon replicas could be excluded. Based on these results, this luminescence line could be clearly assigned to a DAP transition. In order to increase the doping efficiency, various approaches are considered and discussed. A slight increase could be obtained by high-temperature implantation without postimplantation annealing.

Modeling the Carrier Mobility in Nanowire Channel FET

We report on the extraction of carrier type, and mobility in semiconductor nanowires by adopting experimental nanowire field-effect transistor device data to a long channel MISFET device model. Numerous field-effect transistors were fabricated using n-InAs nanowires of a diameter of 50 nm as a channel. The I-V data of devices were analyzed at low to medium drain current in order to reduce the effect of extrinsic resistances. The gate capacitance is determined by an electro-static field simulation tool. The carrier mobility remains as the only parameter to fit experimental to modeled device data. The electron mobility in n-InAs nanowires is evaluated to µ = 13,000 cm 2 /Vs while for comparison n-ZnO nanowires exhibit a mobility of 800 cm 2 /Vs.

Optical activation of implanted impurities in ZnS nanowires

Nanostructures of zinc sulfide (ZnS), a II-VI compound semiconductor with a direct band gap of 3.66eV in the cubic phase and 3.74eV in the wurtzite phase, show interesting optical properties, making it a promising candidate for optoelectronic devices. Single-crystalline nanobelts and nanowires were synthesized in a computer-controlled process according to the vapor-liquid-solid-mechanism. We investigated the morphology, structure, and composition by scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The optical properties were studied by low-temperature photoluminescence (PL) and cathodoluminescence. The synthesized ZnS nanowires were implanted with nitrogen and boron as potential donor and acceptor, respectively. The implanted nanowires were investigated directly after ion implantation and showed a high quantity of defects resulting in nonluminescent material. Annealing procedures recovered the crystal structure and the luminescence, and we found emerging and varying P...