Natalia Lebedeva

Fabrication and Study of Large-Area QHE Devices Based on Epitaxial Graphene

Quantum Hall effect (QHE) devices based on epitaxial graphene films grown on SiC were fabricated and studied for development of QHE resistance standard. The graphene-metal contacting area in the Hall devices has been improved and made using double metallization process. The tested devices has initial carrier concentration of 3 1011 cm-2 and showed half-integer quantum Hall effect at relatively low (4 T) magnetic field.

Formation of (Ga,Mn)As nanowires and study of their magnetic properties

The molecular-beam epitaxy technique is used to synthesize arrays of (Ga,Mn)As nanowire crystals on a GaAs (111)B surface in the growth-temperature range 480–680°C. It is established that the formation of (Ga,Mn)As nanowires can be described in the context of a vapor-liquid-crystal mechanism. It is shown that the growth of (Ga,Mn)As nanowires must occur in conditions stabilized with respect to Ga. It is found that the field and temperature dependences of the static magnetic susceptibility for samples produced at the temperature 660°C exhibit paramagnetic behavior.

Ultrasensitive NO2 Gas Sensor Based on Epitaxial Graphene

We report about technology of fabrication and optimization of a gas sensor based on epitaxial graphene. Optimized graphene/metal contact configuration exhibited low contact resistance. Complementary annealing of graphene sensor after each gas exposure led to significant improvement in the sensing performance. The response of the annealed sensor to the nitrogen dioxide (NO2) was tenfold higher than that of an as-fabricated graphene sensor. NO2 concentration as low as 0.2 parts per billion (ppb) was easily detectable. Devices have high signal-to-noise ratio. The detection limit of the graphene sensor was estimated to be 0.6 ppt (parts per trillion). The present technology with additional annealing improves the performance of the graphene based sensor and makes it suitable for the environmental nitrogen dioxide gas monitoring.

Temperature dependence of Pd thin film cryo resistors

Cryo resistors based on Pd thin films were designed and investigated in temperature range 50 mK – 300 K. The resistors in the range 100 kΩ – 1.3 MΩ were fabricated using thermal evaporation technique. Minimum temperature coefficient, −0.4 (37)·10{−6}/K, in temperature range 50 mK – 100 mK has been obtained for 20 nm thin film 560 kΩ resistor. Binary and decimal configuration of connection of resistive elements were used for flexible adjustment of resistance value.

Fabrication and study of large area QHE devices based on epitaxial graphene

Quantum Hall effect (QHE) devices based on epitaxial graphene films grown on SiC were fabricated and studied for development of the QHE resistance standard. The graphene-metal contacting area in the Hall devices has been improved and fabricated using a double metallization process. The tested devices had an initial carrier concentration of (0.6-10) · ~1011 cm-2 and showed half-integer QHE at a relatively low (3 T) magnetic field. The application of the photochemical gating method and annealing of the sample provides a convenient way for tuning the carrier density to the optimum value. Precision measurements of the quantum Hall resistance in graphene and GaAs devices at moderate magnetic field strengths (≤7 T) showed a relative agreement within ·10-9.

Temperature Dependence of Pd Thin-Film Cryoresistors

Cryoresistors based on Pd thin films were designed and investigated in the temperature range 50 mK-300 K. The resistors in the range 100 kΩ-1.3 MΩ were fabricated by thermal evaporation technique. Resistivity of Pd film at 293 K varies in (21-53) · 10 8 Ωm range for 15-40 nm films. Temperature co efficient of resistance at low temperatures depends on the thickness of the film. The lowest temperature coefficient in the range 3 K-4 K of about few parts in 106/K was obtained for 20-nm-thick film. Current coefficient measured on 20-nm-thin film 1.002-MΩ resistor does not exceed 2 · 10 5/μA in 0.7 K-1 K temperature range.

Experimental study of carrier density tuning in epitaxial graphene QHR devices

Adjustment of the charge carrier density in quantized Hall resistance (QHR) devices based on epitaxial SiC graphene films can be performed using photochemical and chemical gating. Our experiments show that the carrier concentration in fabricated graphene devices can be reversibly varied by either treatment with UV light or using molecules from aqua ammonia. Treatment of SiC graphene QHR devices in such a way provides tuning of carrier concentration to optimum values, in the range (1-3) × 1011 cm-2, at which the dissipation in QHR regime is reduced and the full quantization of Hall resistance is expected.

New method of determining the young’s Modulus of (Ga,Mn)As nanowhiskers with a scanning electron microscope

Arrays of (Ga,Mn)As nanowhiskers have been grown by molecular-beam epitaxy. The scanning electron microscopy study of the surface morphology of the samples has revealed the appearance of mechanical vibrations of individual nanowhiskers. To describe the vibrations, a model has been developed for the determination of the Young’s modulus of (Ga,Mn)As nanowhiskers.

Tests of series quantum Hall arrays based on epitaxial graphene

Epitaxial graphene based quantum Hall resistance devices with four and eight series-connected Hall bars have been fabricated and tested for realization of quantum resistance standards with up-scaled values. Triple series connection with external bonding wires was used. Most of the tested devices had relatively low longitudinal resistance and showed half-integer quantum Hall effect at filling factor 2 starting from magnetic field of about 6 T, but the quantized Hall resistance deviated from the expected value much more than in our earlier experiments with four series-connected Hall bars.

First ac measurements of the quantum Hall effect in epitaxial graphene

We present first ac measurements of the quantized Hall resistance in graphene. Remarkably, the first results are very similar to that of conventional, unshielded GaAs devices. The future exploration of the detailed loss mechanisms in graphene by established ac analysis techniques thus gives a good chance to understand and to eliminate the ac losses, enabling a future graphene-based impedance standard.