In this paper we present the results of a systematic study on the magnetic field penetration depth of superconducting niobium thin films. The films of thicknesses ranging from 8 to 300 nm were deposited on a Si substrate by dc magnetron sputtering. The values of the penetration depth ͑0͒ were obtained from the measurements of the effective microwave surface impedance by employing a sapphire resonator technique. Additionally, for the films of thickness smaller than 20 nm, the absolute values of ͑0͒ were determined by a microwave transmission method. We found that the reduction of the film thickness below 50 nm leads to a significant increase of the magnetic field penetration depth from about 80 nm for 300 nm thick film up to 230 nm for a 8 nm thick film. The dependence of the penetration depth on film thickness is described well by taking into account the experimental dependences of the critical temperature and residual resistivity on the thickness of the niobium films. Structural disordering of the films and suppression of superconductivity due to the proximity effect are considered as mechanisms responsible for the increase of the penetration depth in ultrathin films.

Trapping-detrapping processes in nanostructures are generally considered to be destabilizing factors. However, we discovered a positive role for a single trap in the registration and transformation of useful signal. We use switching kinetics of current fluctuations generated by a single trap in the dielectric of liquid-gated nanowire field effect transistors (FETs) as a basic principle for a novel highly sensitive approach to monitor the gate surface potential. An increase in Si nanowire FET sensitivity of 400% was demonstrated.

Source/drain electrodes contact effect on the stability of bottom-contact pentacene field-effect transistors AIP Advances 2, 022113 (2012) All-metallic lateral spin valves using Co2Fe(Ge0.5Ga0.5) Heusler alloy with a large spin signal Appl. Phys. Lett. 100, 052405 (2012) Contact transport of focused ion beam-deposited Pt to Si nanowires: From measurement to understanding Appl. Phys. Lett. 100, 053503 (2012) Ab initio quantum transport simulation of silicide-silicon contacts J. Appl. Phys. 111, 014305 (2012) Impact of fluorine treatment on Fermi level depinning for metal/germanium Schottky junctions Appl. Phys. Lett. 99, 253504 (2011) Additional information on J. Appl. Phys. A new mechanism of contact resistance formation in ohmic contacts with high dislocation density is proposed. Its specific feature is the appearance of a characteristic region where the contact resistance increases with temperature. According to the mechanism revealed, the current flowing through the metal shunts associated with dislocations is determined by electron diffusion. It is shown that current flows through the semiconductor near-surface regions where electrons accumulate. A feature of the mechanism is the realization of ohmic contact irrespective of the relation between the contact and bulk resistances. The theory is proved for contacts formed to III-V semiconductor materials as well as silicon-based materials. A reasonable agreement between theory and experimental results is obtained.

No abstract

Abstract-We have developed a surface resistance ( ) measurement technique for large-area high-temperature superconducting (HTS) films using quasioptical dielectric resonators (QDR) with HTS endplates (quasioptical Hakki-Coleman resonators). In this technique, the highest modes, namely whispering-gallery modes, in sapphire disk sandwiched between HTS films or between one HTS film and one Cu endplate are excited at K-band frequencies. The authors report on measurement results of surface resistance of 52 mm diameter high-quality YBCO thin films. The measurement results revealed that the technique is feasible for accurate -measurements of large-area thin films. The method is appropriate for standard measurement of at millimeter wave frequencies by analogy with classic DR-based microwave technique, although QDR-based technique has some fundamental differences.

The origin of the interface formation appearing due to the realization of contacts to ultrathin gold nanowire devices is revealed. Such interfaces play an important role in transport mechanisms in nanowire structures and can determine the electrical and operating parameters of a nanodevice. Based on experimental results, the specific electrical properties of bundles of ultrathin gold nanowires fabricated by wet chemical synthesis and subsequently assembled and contacted with gold electrodes are reported. It is demonstrated that these properties are strongly affected by the monolayers of organic molecules inevitably present on the surface of the nanowires due to synthetic conditions. In particular, such layers form a potential barrier to tunneling of the electrons from contacts to the nanowires. The electric transport behavior of the investigated nanowire structures in the temperature range from 500 mK to 300 K obeys the model of thermal fluctuation-induced tunneling conduction through the nanowire-metal electrode molecular junction. Application of this model allows calculation of the parameters of the molecular potential barrier. The formation of such a molecular barrier is verified by scanning tunneling microscope (STM) and transmission electron microscope (TEM) measurements performed using a supporting graphene layer. These findings are important for designing novel nanodevices for molecular electronics on the basis of ultrathin nanowires.

PACS 73.40.Gk, 81.05.Ea, 81.15.Hi, 85.30.Mn GaN/AlGaN double barrier resonant tunnelling structures grown by molecular beam epitaxy on GaN templates have been studied. Peaks in the I(V) characteristics are observed, which are similar to resonant peaks seen in conventional III-V based devices. However, the behaviour of the peaks in I(V) depend upon the previous charge-state of the device produced by electrical bias. Current instabilities are also observed at low voltages. The possible origin of the peaks in the I(V) at room temperature and 4 K is discussed. . Doublebarrier RTDs are the basic benchmark for quantum tunnelling devices. They have been made in a variety of materials systems, including AlGaAs, Si/Ge, and InAs/GaSb. If RTDs could be produced in the group III-Nitrides, a number of novel possibilities exist for development and exploitation. Due to the large band offsets, it should be possible to observe quantum behaviour at much higher temperatures than in other III-Vs. However for group III-Nitride tunnel barrier devices, two problems remain: the structural quality of the heterojunctions and the scattering by impurities due to the high background doping level. Both can destroy the translational invariance that leads to the conservation of electron momentum necessary for successful operation of RTDs.Conventional III-V RTDs are grown mainly by molecular beam epitaxy (MBE). For the group III-Nitrides, because no bulk GaN substrates were available, all structures were initially grown by hetero-epitaxy on sapphire or SiC substrates. Now, due to the availability of GaN templates, grown by hydride vapour phase epitaxy (HVPE) or metal organic vapour phase epitaxy (MOVPE), the structural and optical properties of MBE grown GaN films have been dramatically improved.The effective mass for GaN is about three times larger than for GaAs, so the quantum wells (QWs) in RTDs need to be thinner than in the arsenides [6]. For the tunnel barriers, the transmission coefficient (T) is given by T ~ exp (-2kb), where k = [2m*(V -E)]

PACS 72. 70.+m, 73.40.Kp, 73.50.Td γ-ray radiation effect has been studied on transport and noise properties of high electron mobility transistors (HEMTs) with gate lengths in the range from 350 to 150 nm at room temperature. Current-voltage (I -V) characteristics of the devices demonstrate higher radiation hardness to 60 Co γ-rays up to doses of 10 9 Rad at larger gate lengths. This confirms the very important role of surface passivation for channel transport of the HEMTs. The deviation of the I -V characteristics parameters saturated current, transconductance, channel conductance, and threshold voltage does not exceed 20% at highest radiation dose. The noise spectra of pre-irradiated devices and after γ-irradiation show different frequency dependences corresponding to different fluctuation processes in the HEMTs. The results are confirmed by dynamic current measurements of the channel conductivity.