Low-temperature performance of semiconducting asymmetric nanochannel diodes

We present our studies on fabrication and electrical and optical characterization of semiconducting asymmetric nanochannel diodes (ANCDs), focusing mainly on the temperature dependence of their current–voltage (I–V) characteristics in the range from room temperature to 77 K. These measurements enable us to elucidate the electron transport mechanism in a nanochannel. Our test devices were fabricated in a GaAs/AlGaAs heterostructure with a twodimensional electron gas layer and were patterned using electron-beam lithography. The 250-nmwide, 70-nm-deep trenches that define the nanochannel were ion-beam etched using the photoresist as a mask, so the resulting nanostructure consisted of approximately ten ANCDs connected in parallel with 2-μm-long, 230-nm-wide nanochannels. The ANCD I–V curves collected in the dark exhibited nonlinear, diode-type behavior at all tested temperatures. Their forward-biased regions were fitted to the classical diode equation with a thermionic barrier, with the ideality factor n and the saturation current as fitting parameters. We have obtained very good fits, but with n as large as ~50, suggesting that there must be a substantial voltage drop likely at the contact pads. The thermionic energy barrier was determined to be 56 meV at high temperatures. We have also observed that under optical illumination our ANCDs at low temperatures exhibited, at low illumination powers, a very strong photoresponse enhancement that exceeded that at room temperature. At 78 K, the responsivity was of the order of 104 A/W at the nW-level light excitation

Cost-effective fabrication of polycrystalline TiO2 with tunable n/p response for selective hydrogen monitoring

In this report, we demonstrate a simple and cost-effective strategy to prepare polycrystalline rutile TiO2 based gas sensors with tunable n/p type response inversion depending on gas concentration, operating temperature, and applied voltage. The effect of surface modification by Ag and Ni thin film on structural, morphological, and gas sensor characteristics is studied in detail. The sensors show excellent sensing performance in terms of sensitivity (sensor response∼25 for 0.1 vol. % H2 diluted in technical air), selectivity (selectivity factor for 0.1 vol. % H2 is about ∼24 against NH3, CH4, and NO2 and ∼8 for CO in the same concentration range), stability (both long-term and short-term), and reaction times (∼0.7 min for 0.1 vol. % H2). The aforementioned performance is recorded at 300 °C with applied voltage of 0.1 V. Excluding the power consumption of sensor heater (typically ∼5 × 10−5 watt), this applied voltage 0.1 V can reduce the power consumption to 3 × 10−10 watt. For the first time, we found a critical point, defined with critical-concentration (Cc), critical-temperature (Tc) and criticalvoltage (VC), at which the response inverses from one kind to another, something intriguing to novel sensing phenomena that can be exploited to tailor the selectivity of the sensors. A physical-chemical sensing model is presented to understand the aforesaid peculiar occurrence

Point contact spectroscopy of superconductors via nanometer scale point contacts formed by resistive switching

Point contact spectroscopy is commonly used to investigate electronic properties of superconductors. Here we show that nanometer scale point contacts, which enable to study the superconductor properties locally, can be created by means of the resistive switching phenomenon. Our experiments were performed on sandwiched MgB2/Al/TiO2/Pt structures, where multiple bipolar resistive switching cycles were conducted. The differential conductance as a function of voltage was measured at temperatures below the critical temperature of the MgB2 superconductor. In the low-resistance state the MgB2 and Pt electrodes are connected by an ultrathin metallic filament which creates at the MgB2 electrode the Sharvin point contact with diameter below 10 nm. In this case the differential conductance data demonstrate the Andreev reflections due to the carrier transport between the superconducting MgB2 electrode and filament. From these data the two-gap superconductivity of MgB2 is clearly visible which also confirms the fit by the Blonder-Tinkham-Klapwijk model. If the bottom electrode is made of a superconductor with known gap, our approach allows us to estimate from the Andreev reflection spectrum the resistance of both the filament and point contact. We can then determine from the Sharvin formula the cross-section size of the point contact and thus also the filament cross-section size. In the high resistance state when the filament is ruptured, the differential conductance data demonstrate the spectrum typical for tunneling between two normal metals, with a zero-bias anomaly due to the Altshuler-Aronov effect. This suggests that the filament is not ruptured at the superconducting MgB2 electrode but elsewhere

Simulation of Collective Excitations in Long Josephson Junction Stacks

The phase dynamics of a stack of long Josephson junctions has been studied. Both inductive and capacitive couplings between Josephson junctions have been taken into account in the calculations. The IV-curve, the dependence on the bias current of the radiation power and dynamics of each JJs of the stack have been investigated. The coexistence of the charge traveling wave and fluxon states has been observed. This state can be considered as a new collective excitation in the system of coupled Josephson junctions. We demonstrate that the observed collective excitation leads to the decrease of radiation power from the system

Simulation of Collective Excitations in Long Josephson Junction Stacks

The phase dynamics of a stack of long Josephson junctions has been studied. Both inductive and capacitive couplings between Josephson junctions have been taken into account in the calculations. The IV-curve, the dependence on the bias current of the radiation power and dynamics of each JJs of the stack have been investigated. The coexistence of the charge traveling wave and fluxon states has been observed. This state can be considered as a new collective excitation in the system of coupled Josephson junctions. We demonstrate that the observed collective excitation leads to the decrease of radiation power from the system