Alexandre Satrapinski

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.

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.

Thin-film cryogenic current comparator

We analyze a thin-film cryogenic current comparator (CCC) intended to be used in ac measurements. Our analysis shows that by calibrating the dc ratio error of the thin-film CCC against a conventional CCC, it can be used up to 10 kHz as a sensitive and highly accurate current comparator. We have fabricated a practical 1:1 planar CCC integrated with a low-noise SQUID magnetometer. The preliminary experiments obtained from the 1:1 comparator are described.

Atomic-Layer-Deposited Alumina

Metal-alloy (Ni-Cr-Cu-Al-Ge) thin-film resistors were coated with alumina (Al2O3) using the atomic layer deposition (ALD) technique. The electrical properties of the thin-film resistors were studied in the temperature range of 4.2-300 K. It was experimentally demonstrated that the protective dielectric alumina coating improves the long-term stability and repeatability of high-value, thin-film resistors (100-500 kOmega). The drift rate of the resistance due to the native oxidation at room temperature was reduced from -2.45 times10-6 h-1 for a nonaged uncoated resistor to 0.03 times10-6 h-1 for an alumina-coated resistor. It was shown that the additional 15-nm-thick alumina coating does not significantly change the thermoelectrical properties of the metal-alloy, thin-film resistors.

(\hbox{Al}_{2}\hbox{O}_{3})

Series connection of four quantum Hall effect (QHE) devices based on epitaxial graphene films was studied for realization of a quantum resistance standard with an up-scaled value. The tested devices showed quantum Hall plateaux RH,2 at filling factor i = 2 starting from relatively low magnetic field (between 4 T and 5 T) when temperature was 1.5 K. Precision measurements of quantized Hall resistance of four QHE devices connected by triple series connections and external bonding wires were done at B = 7 T and T = 1.5 K using a commercial precision resistance bridge with 50 microA current through the QHE device. The results showed that the deviation of the quantized Hall resistance of the series connection of four graphene-based QHE devices from the expected value of 4*RH,2 = 2h/e^2 was smaller than the relative standard uncertainty of the measurement (< 1*10^-7) limited by the used resistance bridge.

Coating on Thin-Film Cryoresistors

Thin film resistors, based on the NiCr alloys, have been fabricated and studied at temperatures down to 50 mK. Resistivity of the films varied within (7-11) Ohm*square depending on additions of Cu, Al and Ge. The resistors demonstrate Kondo minimum at 20 K - 30 K. Power dependence measurements at sub Kelvin temperatures showed an electron overheating at the power level above 10 pW for the Ni75Cr20Cu2.5Al2.5 (Evanohm) 500 kOhm film resistor.

Mini array of quantum Hall devices based on epitaxial graphene

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.

NiCr based thin film cryo resistors

We have carried out a resistance comparison of four QHR systems using a 100-/spl Omega/ resistor in a temperature-controlled and hermetically sealed enclosure, the comparison was performed in November and December 1998, the participants were Bureau International des Poids et Mesures (BIPM), Bureau National de Metrology-Laboratoire Central des Industries Electroniques (BNM-LCIE) France, Physikalisch-Technische Bundesanstalt (PTB) Germany, and Valtion Teknillinen Tutkimuskeskus (VTT Automation) Finland, acting as a pilot laboratory. The comparison results agree within 3 parts in 10/sup 9/ with a relative combined standard uncertainty of /spl plusmn/1/spl times/10/sup -8/.

Temperature dependence of Pd thin film cryo resistors

Improved magnetic materials open the perspective of operating room-temperature current comparators at lower frequency without compromising metrological accuracy. Two prototypes of low-frequency current comparator (LFCCs) have been successfully tested in precision resistance ratio measurements at drive frequencies of 0.5 Hz and 1 Hz using the BIPMs bridge setup with a recently realized frequency extension. These measurements have been performed using standard resistors of 10 kΩ, 1 kΩ and 100 Ω nominal values as well as the quantized Hall resistance (QHR) of GaAs and graphene devices.

Comparison of four QHR systems within one month using a temperature and pressure stabilized 100-/spl Omega/ resistor

Two prototypes of a current comparator for operation at low frequencies in the 0.1 Hz to 1 Hz range and at room temperature have been realized. Experimental tests include the determination of the frequency-dependent sensitivity and a comparative measurement of the ratio of two standard resistors with nominal values of 1 kΩ and 100 Ω. The realized current comparator is the main ratio standard in a precision ac resistance bridge operated without need of liquid helium which will be one of the main measurement instruments for a future graphene-based quantum Hall resistance standard.