A two-colour heterojunction unipolar nanowire light-emitting diode by tunnel injection

We present a systematic study of the current-voltage characteristics and electroluminescence of gallium nitride (GaN) nanowire on silicon (Si) substrate heterostructures where both semiconductors are n-type. A novel feature of this device is that by reversing the polarity of the applied voltage the luminescence can be selectively obtained from either the nanowire or the substrate. For one polarity of the applied voltage, ultraviolet (and visible) light is generated in the GaN nanowire, while for the opposite polarity infrared light is emitted from the Si substrate. We propose a model, which explains the key features of the data, based on electron tunnelling from the valence band of one semiconductor into the conduction band of the other semiconductor. For example, for one polarity of the applied voltage, given a sufficient potential energy difference between the two semiconductors, electrons can tunnel from the valence band of GaN into the Si conduction band. This process results in the creation of holes in GaN, which can recombine with conduction band electrons generating GaN band-to-band luminescence. A similar process applies under the opposite polarity for Si light emission. This device structure affords an additional experimental handle to the study of electroluminescence in single nanowires and, furthermore, could be used as a novel approach to two-colour light-emitting devices.Comment: 9 pages, 11 figure

Radiation induced zero-resistance states in GaAs/AlGaAs heterostructures: Voltage-current characteristics and intensity dependence at the resistance minima

High mobility two-dimensional electron systems exhibit vanishing resistance over broad magnetic field intervals upon excitation with microwaves, with a characteristic reduction of the resistance with increasing radiation intensity at the resistance minima. Here, we report experimental results examining the voltage - current characteristics, and the resistance at the minima vs. the microwave power. The findings indicate that a non-linear V-I curve in the absence of microwave excitation becomes linearized under irradiation, unlike expectations, and they suggest a similarity between the roles of the radiation intensity and the inverse temperature.Comment: 3 color figures; publishe

Probing energy barriers and quantum confined states of buried semiconductor heterostructures with ballistic carrier injection: An experimental study

A three-terminal spectroscopy that probes both subsurface energy barriers and interband optical transitions in a semiconductor heterostructure is demonstrated. A metal-base transistor with a unipolar p-type semiconductor collector embedding InAs/GaAs quantum dots (QDs) is studied. Using minority/majority carrier injection, ballistic electron emission spectroscopy and its related hot-carrier scattering spectroscopy measures barrier heights of a buried AlxGa1-xAs layer in conduction band and valence band respectively, the band gap of Al0.4Ga0.6As is therefore determined as 2.037 +/- 0.009 eV at 9 K. Under forward collector bias, interband electroluminescence is induced by the injection of minority carriers with sub-bandgap kinetic energies. Three emission peaks from InAs QDs, InAs wetting layer, and GaAs are observed in concert with minority carrier injection.Comment: 11 pages, 4 figures, submitted to Physical Review

Weak Localization and Mobility in ZnO Nanostructures

We conduct a comprehensive investigation into the electronic and magnetotransport properties of ZnO nanoplates grown concurrently with ZnO nanowires by the vapor-liquid-solid method. We present magnetoresistance data showing weak localization in our nanoplates and probe its dependence on temperature and carrier concentration. We measure phase coherence lengths of 50–100 nm at 1.9 K and, because we do not observe spin-orbit scattering through antilocalization, suggest that ZnO nanostructures may be promising for further spintronic study. We then proceed to study the effect of weak localization on electron mobility using four-terminal van der Pauw resistivity and Hall measurements versus temperature and carrier concentration. We report an electron mobility of ∼100 cm2/V s at 275 K, comparable to what is observed in ZnO thin films. We compare Hall mobility to field-effect mobility, which is more commonly reported in studies on ZnO nanowires and find that field-effect mobility tends to overestimate Hall mobility by a factor of 2 in our devices. Finally, we comment on temperature-dependent hysteresis observed during transconductance measurements and its relationship to mobile, positively charged Zn interstitial impurities.Engineering and Applied Science

Resonant thermal transport in semiconductor barrier structures

I report that thermal single-barrier (TSB) and thermal double-barrier (TDB) structures (formed, for example, by inserting one or two regions of a few Ge monolayers in Si) provide both a suppression of the phonon transport as well as a resonant-thermal-transport effect. I show that high-frequency phonons can experience a traditional double-barrier resonant tunneling in the TDB structures while the formation of Fabry-Perot resonances (at lower frequencies) causes quantum oscillations in the temperature variation of both the TSB and TDB thermal conductances $\sigma_{\text{TSB}}$ and $\sigma_{\text{TDB}}$.Comment: 4 pages. 4 figure.

Quantum Phonon Optics: Coherent and Squeezed Atomic Displacements

In this paper we investigate coherent and squeezed quantum states of phonons. The latter allow the possibility of modulating the quantum fluctuations of atomic displacements below the zero-point quantum noise level of coherent states. The expectation values and quantum fluctuations of both the atomic displacement and the lattice amplitude operators are calculated in these states---in some cases analytically. We also study the possibility of squeezing quantum noise in the atomic displacement using a polariton-based approach.Comment: 6 pages, RevTe

Electroluminescence from single nanowires by tunnel injection: an experimental study

We present a hybrid light-emitting diode structure composed of an n-type gallium nitride nanowire on a p-type silicon substrate in which current is injected along the length of the nanowire. The device emits ultraviolet light under both bias polarities. Tunnel-injection of holes from the p-type substrate (under forward bias) and from the metal (under reverse bias) through thin native oxide barriers consistently explains the observed electroluminescence behaviour. This work shows that the standard p-n junction model is generally not applicable to this kind of device structure.Comment: 6 pages, 6 figure

Hall carrier density and magnetoresistance measurements in thin film vanadium dioxide across the metal-insulator transition

Temperature dependent magneto-transport measurements in magnetic fields of up to 12 Tesla were performed on thin film vanadium dioxide (VO2) across the metal-insulator transition (MIT). The Hall carrier density increases by 4 orders of magnitude at the MIT and accounts almost entirely for the resistance change. The Hall mobility varies little across the MIT and remains low, ~0.1cm2/V sec. Electrons are found to be the major carriers on both sides of the MIT. Small positive magnetoresistance in the semiconducting phase is measured