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This paper details developments in the use of room temperature current comparators operating with low-frequency (

Gas sensor operating in part-per billion (ppb) range is required for environmental monitoring of nitrogen dioxide. Currently there is a lack of cheap sensors, which can operate in this concentration range. In this work, an ultrasensitive gas sensor based on epitaxial graphene on SiC has been proposed. The sensor exhibits a strong and reproducible response to nitrogen dioxide (NO 2 ) for concentrations in air down to 1 ppb. A prototype of a portable device for environmental monitoring which utilizes a combination of sample gas exposure at room temperature and sensor’s recovery at elevated temperature has been made. Prototype allows fast and reproducible measurements of NO 2 concentration in the typical range for environmental pollution (5 ppb–50 ppb).

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) x 1011 cm-2, at which the dissipation in QHR regime is reduced and the full quantization of Hall resistance is expected.

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 BIPM's 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.

We report preliminary results of precision measurements of the graphene quantum Hall resistance plateau value with respect to magnetic field, temperature and applied current. Resistance value of SiC graphene Hall device was modulated by chemical doping, heating, and UV irradiation at the ambient condition. Precision measurements for Rh (filing factor v = 2) were then performed with a cryogenic current comparator resistance bridge. Relative deviation from RH (v = 2) less than 30 parts in 109 can be achieved down to 7 T of magnetic field and up to 6 K of temperature with driving current of 19. 37 μA.

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., 11 pages, 5 figures

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 10 6 /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.

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.

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. NO2concentration 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.

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 metalization process. The tested devices had an initial carrier concentration of (0.6 - 10)*10^11 1/cm^2 and showed half-integer quantum Hall effect at a relatively low (3 T) magnetic field. 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 (QHR) in graphene and GaAs devices at moderate magnetic field strengths (, 6 pages, 9 figures, 3 tables

This paper describes a low-frequency AC resistance bridge based on a Cryogenic Current Comparator (CCC). A low-noise dc SQUID is used as a null detector. The bridge is the main tool to trace resistance to 10 k/spl Omega/, 100 /spl Omega/ and 1 /spl Omega/ levels from the quantum Hall resistance (QHR). The ratio of the resistors ranging from 1 /spl Omega/ to 10 k/spl Omega/ can be measured with an uncertainty of the order of 1 ppb. The bridge is typically operated with a PC generating a sinusoidal ac signal ranging from 0.1 Hz-2 Hz. Using a vector analyzer as a signal source the bridge is used up to 1 kHz. Both sinusoidal and pseudorandom signals have been used to verify the frequency dependence of the standard resistors against the QHR. At present, the bridge can be used to perform accurate calibrations only up to 10 Hz.

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.

A new joint research project (JRP) integrating metrology institutes and universities from nine countries is aimed at realization of a new generation of standards for quantum resistance metrology. The project exploits graphene's properties to simplify operation of standards without compromising the unprecedented precision delivered by semiconductor quantum Hall devices. Higher operating temperatures (above 4.2 K, and up to 8 K) and together with lower magnetic fields (below 5 T, and potentially down to 2 T) will lead to a significantly improved and cost-saving dissemination of intrinsically referenced resistance standards to all end-users relying on electrical measurements.

Precision measurements of the quantum Hall resistance with alternating current (ac) in the kHz range were performed on epitaxial graphene in order to assess its suitability as a quantum standard of impedance. The quantum Hall plateaus measured with alternating current were found to be flat within one part in 107. This is much better than for plain GaAs quantum Hall devices and shows that the magnetic-flux-dependent capacitive ac losses of the graphene device are less critical. The observed frequency dependence of about -8?×?10-8/kHz is comparable in absolute value to the positive frequency dependence of plain GaAs devices, but the negative sign is attributed to stray capacitances which we believe can be minimized by a careful design of the graphene device. Further improvements thus may lead to a simpler and more user-friendly quantum standard for both resistance and impedance.

Precision quantum Hall resistance (QHR) measurements were performed on large-area epitaxial graphene device at temperature T = 1.5 K and at magnetic fields B from 8 T down to 2.5 T, that is much lower than typically used in precision QHR measurement. QHR was measured using cryogenic current comparator resistance bridge with relatively large biasing current I sd = 41 µA to reduce measurement uncertainty. The results showed that at B = 8 T, the relative deviation of Hall resistance from the expected quantized value h/2e 2 is within experimental uncertainty of 3.5 parts in 108 and remained below 0.35 parts per million (ppm) down to B = 3 T.

Graphene is the promising material for the gas sensing application. High sensitivity to the nitrogen dioxide and ozone allows application of the simple graphene based devices for the environmental monitoring. The aims of the work were the fabrication of reliable graphene devices and the comparison of epitaxial and CVD-based graphene sensors for their sensing abilities. In order to increase sensitivity and reliability of graphene sensors the optimization of fabrication technology as well as operation parameters was done. Results demonstrated ultra high sensitivity of the fabricated epitaxial sensors upon exposure to NO2 and ozone gases.

This paper describes the frequency dependence of three 100-/spl Omega/ standard resistors in the frequency range from 0.1 Hz to 1 Hz. The measurements were carried out with an automated AC bridge based on the cryogenic current comparator (CCC) and a DC SQUID null detector. A GaAs Quantum-Hall resistance (QHR) cooled down to 1.4-1.8 K was used as a reference. The error sources related to the AC bridge and the quantum-Hall component are analyzed in detail. The kinetic inductance in the QHR is found to have an important role in ac measurements. The ac bridge has a very high sensitivity, providing a resolution of 1 ppb for a current of 40 /spl mu/A at 1 Hz, with a 5 min averaging time. The plateau i=2 was obtained at 7.4 T. Some resistors, such as an NBS type 100 /spl Omega/ (L&N), reveal a strong frequency dependence below 1 Hz. The frequency dependence of the Wilkins type 100 /spl Omega/ (Tinsley) is reasonably small, and a film resistor (ZIP) shows no significant frequency dependence. Thus the frequency dependence of the standard resistors cannot, in general, be neglected, and an accurate measurement of the dependence is necessary for the determination of the value of the resistance. In addition, all the precise resistance measurements should be performed with a sinusoidal current in order to make the interpretation of the results less ambiguous. >

Possibilities and limitations of using measurement system based on two sampling multimeters (DVM) for the ratio measurement of two impedances was investigated in MIKES. Calibration of a 1 µF capacitor from the quantized Hall resistance (QHR) was made at low frequencies. A frequency dependence of ceramic 1 µF capacitor in frequency range 12 Hz – 1600 Hz was measured against different reference resistors at low voltage level with the ratios of 1∶1 to 1∶100. A 1 µF capacitor can be measured at low frequency directly against the QHR by using precision DVM with proper estimation of applied corrections.

Epitaxial 4H-SiC graphene films were fabricated and tested for sensing ambient gases. Sensitivity response to nitrogen dioxide was optimized by varying operation temperature and humidity. At elevated temperature the response was found to be −10.0 % of the resistance change when the gas mixture with NO 2 concentration of 10 parts per billion (10 ppb) was applied. Optimisation of sensitivity can be done by control and adjustment of the operation temperature and humidity level.

Control electronics and software for Cryogenic Current Comparator (CCC) Resistance Bridge operated with sinusoidal biasing currents at ultra low frequencies is upgraded in MIKES and the details of its operation are presented. Results of comparative measurement with the previously used control units showed that developed electronics and software allows simplification of measurement process and make automation of resistance calibration more convenient.

A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz. >

Four high-accuracy LCR meters have been compared with the use of Metrosert and MIKES impedance standards. Specifications of LCR meters have been verified at frequency of 1 kHz for the resistance range from 1 Ω to 100 kΩ, for the capacitances in the range from 100 pF to 1 µF and for 100 mH inductor. The performance of LCR meters have been compared also at frequencies from 1 kHz to 1 MHz with ac resistors of different types, at 1 kHz and 100 kHz with 100 pF - 1 µF capacitance standards, and at 1 kHz with 300 µH – 300 mH inductance standards. Results of measurements presented in this summary confirm the specified uncertainty of the tested LCR meters

Epitaxial Graphene layers were grown on SiC substrate in ultra-high vacuum chamber and in Ar filled furnace. Measurement of the Hall resistance in the magnetic fields up to 11 T and at the temperatures down to 1.4 K presently show some fine features and minimums in magnetic field dependence, suggesting the visibility of observation of quantization effects after further optimization of technology of fabricationis.

Series resonance (SR) method was investigated for the realization of 100 mH inductance at 1 kHz in VNIIM and MIKES. Practical details of the application of this method and comparison with the results of realization by the Maxwell-Wien Bridge method are presented. Results showed that 1 s uncertainty of the 100 mH realization by SR method at 1 kHz can be at the level of parts in 106.

A comparison of inductance realisation at 100 mH was carried out in October –November 2009 at MIKES and SP. The measurand was a four terminal pair toroidal inductor measured at three frequencies 400 Hz, 1000 Hz and 1591 Hz. Realisations by series and parallel resonance methods and by a 2 DVM sampling method were used. The results showed agreement within combined expanded uncertainty.

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.

Ni-Cr-based thin-film resistors have been fabricated and studied at temperatures of down to 50 mK. The resistivity of the films varied within (14-25)Omega*sq, depending on the additions of Cu, Al, Ge, and Mn. The minimum temperature coefficient (TC) at 4.2 K (TC = -50 middot 10 -6 /K) is obtained for Ni 75 Cr 20 CU 2.5 AI 2.5 (Evanohm alloy) doped with 2.5% Ge. At the 50-150 mK range, the TC of the alloy increases to -4.15 middot 10 -6 /mK. The resistors demonstrate the Kondo minimum at 20-30 K. The power coefficient of the 560-kOmega sample, which was measured at 4.2 K, was found to be les -0.008 middot 10 -6 /muW. Power dependence measurements at subkelvin temperatures showed an electron overheating at the power level of above 10 p W for a 500-k film resistor.

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.

Metal alloy (NiCrCuAlGe) thin film resistors were coated with atomic layer deposited (ALD) alumina (Al2O3) in order to improve the stability and repeatability of the high value resistors (100 kOmega - 500 kOmega) in the temperature range from 4.2 K to 300 K. The drift rate of the resistance due to the native oxidation at room temperature was reduced from -2.45 ppm/h for an uncoated resistor to 0.03 ppm/h for an alumina-coated resistor. It was shown that the additional alumina coating does not significantly change the thermoelectrical properties of the metal alloy thin film resistors.

In this paper, plans and progress of a Finnish collaboration towards direct closure of the quantum metro- logical triangle are presented. The experimental concept and the present status of the main components, including two possibilities of the single charge transport device, Bloch oscillating transistor (BOT) based null detector, and a thin-film cryoresistor, are described.

Results on development of the measurement setup for representation of the farad from DC QHR in MIKES are presented. Measurement chain and designed equipment used in the link between QHR and capacitance are described. Expected uncertainty is parts in 107. The important components of that chain, the standard combining 100 kOhm resistors and 1 nF capacitors and AC 10:1 self-calibrated coaxial resistance bridge, are described in this paper

An automated measurement system for resistance scaling is developed in MIKES. The system was compared with our Cryogenic Current Comparator (CCC) resistance bridge. The results of the measurements in the range 1 Ohm-10 kOhm agreed within 0.03 ppm. The same CCC bridge is used for the measurements of the 10 mOhm resistor with a combined standard uncertainty less than 1 ppm. Temperature hysteresis was observed in some of the standard resistors during measurements of their temperature coefficients.

This paper describes the maintenance and traceability of resistance in VTT. The bank of the primary standard resistors consists of four Thomas-type 1 /spl Omega/ resistors in an oil bath (T=20/spl deg/C), a Thinsley 100 /spl Omega/ resistor (100 /spl Omega//sub Thinsley/) in an air bath (T=25/spl deg/C or 31/spl deg/C) and a ESI 10 k/spl Omega/ resistor (10 k/spl Omega//sub ESI/) located in an air bath (T=23/spl deg/C). Both 100 /spl Omega//sub Thinsley/ and 10 k/spl Omega//sub ESI/ can be calibrated directly against the quantum Hall resistance (QHR) using a low-frequency cryogenic ac resistance bridge based on the Cryogenic Current Comparator (CCC). The cryogenic ac bridge is also used to trace 1 /spl Omega/ standard resistors from 100 /spl Omega/. All these standard resistors can be traced to the QHR within 10 ppb. The calibrations are mainly performed at 1 Hz but the frequency dependence of the resistors are first determined in the frequency range from 0.1 Hz to 1 Hz.

The paper describes frequency dependence of some 100 /spl Omega/ standard resistors in the frequency range from 0.1 Hz to 1 Hz. The measurements are carried out with an automated ac bridge based on the cryogenic current comparator (CCC) and a dc SQUID null detector. A GaAs quantum-Hall resistance cooled down to 1.4 K-1.8 K was used as a reference. The plateau i=2 was obtained at 7.5 T. Some resistors such as a N.B.S. type 100 /spl Omega/ (L&N) reveal a strong frequency dependence below 1 Hz while the frequency dependence of the Wilkins type 100 /spl Omega/ (Tinsley) is barely visible. >

This paper describes improvements in the realisation of the quantum Hall effect (QHE) resistance standard at VTT and presents the results of the latest measurements. A new superconducting magnet and a new QHE sample were used to calibrate standard resistors in terms of R/sub K-90/, (R/sub K-90/=25812.807 /spl Omega/ is the conventional value of the von Klitzing constant R/sub K/). A 100 /spl Omega/ standard resistor was calibrated directly against the quantum Hall resistance (QHR) on i=2 plateau using a low-frequency cryogenic ac resistance bridge based on the cryogenic current comparator (CCC). The same cryogenic ac bridge was used to trace the 1 /spl Omega/ and 10 k/spl Omega/ standard resistors from the 100 /spl Omega/ standard.

A resistance comparison carried out with a temperature and pressure stabilized 100 /spl Omega/ standard resistor was performed in November and December 1998. The participants were BIPM, BNM-LCIE (France), PTB (Germany) and VTT (Finland)-pilot. The comparison results agree within 4/spl middot/10/sup -9/ with a combined standard uncertainty of 11/spl middot/10/sup -9/.

A new 100 /spl Omega/ transfer standard has been built and placed in service at VTT Automation. The temperature of the resistor is maintained at 30.72/spl deg/C where the temperature coefficient is zero. The daily variation in temperature is less than /spl plusmn/0.005/spl deg/C. The temperature during transportation is about 0.06/spl deg/C less than the nominal value and it can be hold for about 40 hours. The behavior of the standard was verified in the resistance comparison between VTT (Finland), PTB (Germany), BIPM (International) and BNM-LCIE (France) in November-December 1998. The result indicates that the variation of the standard during the transportation is less than 6.10/sup -9/.

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/.

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.

Thin film resistors, based on the Evanohm (Ni75%Cr20%Cu2.5%Al2.5%) alloy, have been investigated at cryogenic temperatures. The objective of the study is the development of the high value resistor for precision electrical measurements at low temperature and particularly for metrological triangle experiments. Thin film resistors of different configurations have been designed and fabricated by the thermal evaporation process. The resistivity of investigated resistors is 110 × 10−8 Ω m; the resistance exhibits a Kondo minimum at a temperature near 30 K and increases with further reduction of temperature. In the temperature range 50–65 mK, the temperature coefficient reaches −20 × 10−3 K−1. Power dependence measurements at subkelvin temperatures demonstrate that noticeable electron overheating takes place only at the power level above 10 pW for a 500 kΩ resistor. The electron–phonon coupling constant for the fabricated Evanohm thin films has been derived from experimental results.