Low Room-temperature Resistivity Research Articles

Conductive single-phase SrMoO3 epitaxial films synthesized in pure Ar ambience via plasma-assisted radio frequency sputtering

ABSTRACT The cubic perovskite SrMoO3 with a paramagnetic ground state and remarkably low room-temperature resistivity has been considered as a suitable candidate for the new-era oxide-based technology. However, the difficulty of preparing single-phase SrMoO3 thin films by hydrogen-free sputtering has hindered their practical use, especially due to the formation of thermodynamically favorable SrMoO4 impurity. In this work, we developed a radio frequency sputtering technology enabling the reduction reaction and achieved conductive epitaxial SrMoO3 films with pure phase from a SrMoO4 target in a hydrogen-free, pure argon environment. We demonstrated the significance of controlling the target-to-substrate distance (TSD) on the synthesis of SrMoO3; the film resistivity drastically changes from 1.46 × 105 μΩ·cm to 250 μΩ·cm by adjusting the TSD. Cross-sectional microstructural analyses demonstrated that films with the lowest resistivity, deposited for TSD = 2.5 cm, possess a single-phase SrMoO3 with an epitaxial perovskite structure. The formation mechanism of the conductive single-phase SrMoO3 films can be attributed to the plasma-assisted growth process by tuning the TSD. Temperature-dependent resistivity and Hall effect studies revealed metal-like conducting properties for low-resistive SrMoO3 films, while the high-resistive ones displayed semiconductor-like behavior. Our approach makes hydrogen-free, reliable and cost-efficient scalable deposition of SrMoO3 films possible, which may open up promising prospects for a wide range of future applications of oxide materials.

Flexible Positive Temperature Coefficient Composites (PVAc/EVA/GP-CNF) with Room Temperature Curie Point.

Polymeric positive temperature coefficient (PTC) materials with low switching temperature points are crucial for numerous electronic devices, which typically function within the room temperature range (0-40 °C). Ideal polymeric PTC materials for flexible electronic thermal control should possess a room-temperature switching temperature, low room-temperature resistivity, exceptional mechanical flexibility, and adaptive thermal control properties. In this study, a novel PTC material with a room-temperature switching temperature and superb mechanical properties has been designed. A blend of a semi-crystalline polymer EVA with a low melting temperature (Tm) and an amorphous polymer (PVAc) with a low glass transition temperature (Tg) was prepared. Low-cost graphite was chosen as the conductive filler, while CNF was incorporated as a hybrid filler to enhance the material's heating stability. PVAc0.4/EVA0.6/GP-3wt.% CNF exhibited the lowest room temperature resistivity, and its PTC strength (1.1) was comparable to that without CNF addition, with a Curie temperature of 29.4 °C. Room temperature Joule heating tests revealed that PVAc0.4/EVA0.6/GP-3wt.% CNF achieved an equilibrium temperature of approximately 42 °C at 25 V, with a heating power of 3.04 W and a power density of 3.04 W/cm2. The Young's modulus of PVAc0.4/EVA0.6/GP-3wt.% CNF was 9.24 MPa, and the toughness value was 1.68 MJ/m3, indicating that the elasticity and toughness of the composites were enhanced after mixing the fillers, and the mechanical properties of the composites were improved by blending graphite with CNF.

Metal–insulator–superconductor transition in nickelate-based heterostructures driven by topotactic reduction

The discovery of superconductivity in doped infinite-layer nickelates has attracted great interest recently. Here, a metal–insulator–superconductor transition is demonstrated by engineering the process of topotactic reduction. By employing topotactic reduction, a superconducting Nd0.8Sr0.2NiO2 layer is obtained from high-quality Nd0.8Sr0.2NiO3, which is characterized by layer-by-layer growth and low room-temperature resistivity, rather than the low-quality Nd0.8Sr0.2NiO3 with mixed phases. Moreover, an insulating intermediate state is uncovered within the transition from metallic Nd0.8Sr0.2NiO3 to superconducting Nd0.8Sr0.2NiO2, corresponding to a non-monotonic modulation of resistivity driven by topotactic reduction. In the incompletely reduced Nd0.8Sr0.2NiO2+δ, residual oxygen atoms in the Nd/Sr plane disrupt the long-range order of the infinite-layer structure, resulting in suppressed superconductivity with a low transition temperature and non-zero residual resistivity. On the other hand, the superconductivity is optimized in the fully reduced sample, where a sign change in the Hall coefficient is always observed at low temperatures. Our study highlights the diverse electronic states achievable by controlling topotactic reduction, providing valuable insight into the understanding and manipulating of superconductivity in infinite-layer nickelates.

Mechanism of high qualified Mn-Co-Ni-O thin films grown at low temperature

High quality of Mn-Co-Ni-O (MCNO) thin films were prepared by radio frequency (RF) magnetron sputtering deposition at a very low temperature of ≤450 °C. The structure and electrical transport of the films were investigated under different growth and annealing conditions. It was found that the film can be transformed from a mixed conduction mechanism to a single variable range hopping conduction mechanism under appropriate condition. The optimized Mn3+/Mn4+ ratio in the film was close to 1, which was corresponding to the best probability of small polaron hopping, lowest activation energy, lowest room temperature resistivity, large negative resistance temperature coefficient, and the specific preferred (400) orientation. The electrical properties of the film were similar to that prepared under high temperature condition, which promotes the integration of MCNO to the present matured silicon technology. Our results provide an important avenue for the development of next generation of uncooled infrared focal plane arrays.

Low temperature electrical transport in thin carbon films deposited on SiO2/Si substrates by pulsed laser deposition

In this paper electrical transport studies are performed on thin carbon films deposited on SiO2/Si substrates by pulsed laser deposition (PLD) applying laser ablation of micro-crystalline graphite target. Experiments were carried out on 320 - 420 nm thick SiO2 on Si substrates as well as on hydrogenated diamond-like carbon (DLC) films deposited on SiO2/Si. Structural studies by means of XPS, SEM and Raman spectroscopy revealed that the films can be characterized as nano-sized carbon phases possessing different phase composition (i.e. the ratio sp3/sp2 hybridized carbon, etc.). The electrical conductivity/resistivity of the films was measured in the temperature range 10 K < T < 300 K. Four-contact Van der Pauw method as well as two contact schemes have been applied. Some films have low room temperature resistivity in the range ρ = (0.1–1.5)×10-3 Ω.·m and consist predominantly of sp2 hybridized carbon with Raman spectra, which resemble that of nano-sized graphene depending on the deposition conditions and substrates used. The thinnest only 0.5 nm layer deposited directly on SiO2 exhibits relatively low specific resistance (~10-3 Ω. m), which can be taken as an indication of good deposition conditions of graphene-like layers. The current flow mechanism was explored at temperatures from 300 K down to 10K. The temperature dependence reveals non-metallic behavior - the conductivity decreases at decreasing temperature as opposed to typical metal behaviour. A model of variable range hopping (VRH) mechanism is applied to explain the low temperature conductivity drawn from transport in nanocrystalline disordered systems.

Epitaxial Grown VO2 with Suppressed Hysteresis and Low Room Temperature Resistivity for High-Performance Thermal Sensor Applications

The ability of VO2 to undergo semiconductor-to-metal phase transition (SMT) upon heating makes it a very attractive material for uncooled bolometers. The SMT of VO2 represents a large temperature coefficient of resistance, which is an important parameter for the development of highly responsive microbolometers. However, other characteristics of the SMT of VO2 such as its high transition temperature (341.2 K), the sharpness of the transition, its hysteresis, and the high room temperature resistivity limit the performance of this material in microbolometers. In this work, we grow a high-quality epitaxial ultrathin film VO2 on c-plane Al2O3 by pulsed laser deposition. The low deposition temperature and tuning the oxygen partial pressure during the growth process enable control over the grain size and oxygen vacancy concentration. This allowed controlling the SMT parameters of the samples. In particular, we show that the high density of grain boundaries associated with nanosized grains suppresses the thermal hysteresis of the SMT. Simultaneous control over the density of oxygen vacancies and the size of grains enables the adjustment of the temperature coefficient of resistance, room temperature resistivity, SMT temperature, sharpness, and thermal hysteresis toward suitable values for the fabrication of efficient VO2-based uncooled bolometers. Compared with other VO2 fabrication methods, this approach can be viewed as a simpler alternative for VO2 fabrication with favorable properties for practical bolometer applications.

Polymer positive temperature coefficient composites with room-temperature Curie point and superior flexibility for self-regulating heating devices

Polymer PTC (positive temperature coefficient) materials with low switching temperature point are important for many electronic devices since the electronic devices usually work under room temperature range (0–40 °C). The desired polymer PTC materials for the application in thermal control of flexible electronics should also include room-temperature switching temperature, low room-temperature resistivity, good mechanical flexibility and adaptive thermal control performances. This work designed two new types of PTC materials with room-temperature switching temperature and excellent mechanical properties. They are prepared by mixing the semi-crystalline polymer EVA with low melting temperature (Tm) with the amorphous polymer (PS or PVAc) with low glass transition temperature (Tg), while the low-cost graphite is selected as conductive filler. The PTC materials were obtained with low switching temperature point (<30 °C) and low room temperature resistivity (102 Ω m), high PTC repeatability and flexibility, short heating response time and good adaptive thermal control performances.

Growth of highly conducting MoS2-xNx thin films with enhanced 1T' phase by pulsed laser deposition and exploration of their nanogenerator application

SummaryHigh-quality growth of MoS2-xNx films is realized on single-crystal c-Al2O3 substrates by the pulsed laser deposition (PLD) in ammonia rendering highly stable and tunable 1Tʹ/2H biphasic constitution. Raman spectroscopy reveals systematic enhancement of 1Tʹ phase component due to the incorporation of covalently bonded N-doping in MoS2 lattice, inducing compressive strain. Interestingly, the film deposited at 300 mTorr NH3 shows ∼80% 1Tʹ phase. The transport measurements performed on MoS2-xNx films deposited at 300 mTorr NH3 display very low room temperature resistivity of 0.03 mΩ-cm which is 100 times enhanced over the undoped MoS2 grown under comparable conditions. A triboelectric nanogenerator (TENG) device containing biphasic MoS2-xNx film as an electron acceptor exhibits a clear enhancement in the output voltage as compared to the pristine MoS2. Device architecture, p-type N doping in MoS2 lattice, favorably increased work-function, multiphasic component of MoS2, and increased surface roughness synergistically contribute to superior TENG performance.

Stabilizing Metastable Polymorphs of van der Waals Solid MoS2 on Single Crystal Oxide Substrates: Exploring the Possible Role of Surface Chemistry and Structure

In this work we address the question of growth of thin films of a soft van der Waals solid (2D MoS2) on a hard, crystalline metal oxide substrates. Thus, MoS2 thin films are grown on different single-crystal metal oxide substrates (SrLaAlO4, c-Al2O3, SrTiO3, LaAlO3) by pulsed laser deposition (PLD) and characterized by different techniques to confirm and quantify their phase constitution (2H, 1T′). We observe that, on the SrLaAlO4 (001) substrate, a mixed 1T′+2H phase of MoS2 grows with a dominant (∼75%) 1T′ phase, while on c-Al2O3 (0001) pure 2H phase grows. On the SrTiO3 and LaAlO3 substrates also the mixed phase grows, but with a much lesser component of the 1T′. Higher 1T′ contribution in SLAO and STO points to the chemical role of strontium in the early growth. It is also noted that, with increased film thickness on SrLaAlO4, the contribution of the pure 2H phase is enhanced, indicating the pivotal role of lattice strain in stabilizing the initial layer(s). The mixed 1T′+2H phase in the ultrathin film shows significantly lower room temperature resistivity (∼2 mΩ cm) with respect to that of the pure 2H phase (∼14 mΩ cm). The substrate selective polymorphism with distinct electronic features could invite multiple application interests.

Mn-Co-Ni-O thin films prepared by sputtering with alloy target

The thin film of heat-sensitive materials has been widely concerned with the current trend of miniaturization and integration of sensors. In this work, Mn1.56Co0.96Ni0.48O4 (MCNO) thin films were prepared on SiO2/Si substrates by sputtering with Mn–Co–Ni alloy target and then annealing in air at different temperatures (650–900 °C). The X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis indicated that the main crystalline phase of MCNO thin films was spinel crystal structure; the surface of the thin films was very dense and uniform. The electrical properties of the thin films were studied in the temperature range of–5–50 °C. The MCNO thin film with a low room temperature resistance R25 of 71.1 kΩ and a high thermosensitive constant B value of 3305 K was obtained at 750 °C. X-ray photoelectron spectroscopy (XPS) analysis showed that the concentration of Mn3+ and Mn4+ cations in MCNO thin films is the highest when annealing temperature is 750 °C. The complex impedance analysis revealed internal conduction mechanism of the MCNO thin film and the resistance of the thin film was dominated by grain boundary resistance.

High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application

It is a great challenge to prepare chip thermistors with high positive temperature coefficient of resistivity (PTCR) jump and simultaneously low room temperature resistivity by reduction-reoxidation method. In this work, the influence of Mn doping on Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3 based chip thermistors were carefully investigated and Mn doping was proved to be an effective way in preparation of high-performance PTCR ceramics with low resistivity. The results showed that Mn can't create a high enough potential barrier in un-oxidized samples but can obviously increase the value of potential barrier height after reoxidation. The trap activation of Mn d states near Curie point helps to reduce room temperature resistivity and increase PTCR jump in reoxidized samples. As a result, samples with extremely low resistivity and high PTCR jump of 103.4 were reported for the first time.

Low temperature chemical vapor deposition of superconducting vanadium nitride thin films

Superconducting thin films of vanadium nitride have been grown by low temperature (250–300 °C) chemical vapor deposition from tetrakis(dimethylamido)vanadium (TDMAV) and ammonia. For example, films grown from TDMAV (1 sccm Ar as carrier gas) and 7 mTorr ammonia at 300 °C are nanocrystalline (cubic δ-phase) with an average crystal size of 20 nm, have relatively low room temperature resistivities of 250 μΩ cm, and are superconducting with critical temperatures as high as 7.6 K (versus a bulk value of 9 K). The films have a V:N ratio of 1:1, with a carbon content of &amp;lt;5 at. % and an oxygen content of &amp;lt;3 at. % (as determined by high resolution XPS). The V 2p3/2 and N 1 s XPS binding energies of 513.5 and 397.3 eV, respectively, are consistent with the presence of a nitride phase. In contrast, films grown at lower temperatures &amp;lt;200 °C show carbon incorporation, have a much higher resistivity of ∼3000 μΩ cm, and are not superconducting. The results suggest that, at low temperatures, the thermally activated transamination reaction with ammonia becomes too slow to remove dimethylamido groups from the surface, resulting in carbon-rich films (10–15 at. % carbon). The conformal step coverage of the VN films depends on the growth conditions. For thermal growth of nonsuperconducting films at 150 °C, the step coverage is &amp;gt;95% in trenches of an aspect ratio of 4:1; for superconducting films grown at 250 °C, the step coverage is 65% for an aspect ratio of 3:1. At 150 °C, near-stoichiometric films with &amp;lt;2 at. % carbon and &amp;lt;3 at. % oxygen can be deposited if the gaseous ammonia is precracked by a remote plasma source; the resulting films have low resistivities of 320 μΩ cm but are not superconducting down to 4 K.

Study on positive temperature coefficient of resistivity of co-doped BaTiO3 with Curie temperature in room temperature region

As a temperature self-regulating heater material, the doped BaTiO3 exhibits an attractive application perspective in the thermal management of electrical devices. However, the high Curie temperature does not meet the requirement in the thermal control application. In this work, (Ba0.997−xCe0.003Srx)(TiNb0.002)O3 (x=0.2, 0.3, 0.35, abbreviated as BCSTNs) ceramics were prepared by the solid-state reaction method. The purpose of doping different content of strontium is to shift the Curie temperature of BaTiO3-based ceramic to the ambient temperature region, maintaining excellent PTC effect and low room-temperature resistivity by codoped cation ions in Ba- and Ti-site. The influences of sintering temperature and soak time on the microstructure as well as electrical properties of BCSTNs ceramics have been studied. The X-ray diffraction reveals that the composition with x=0.35 exhibits the coexistence of tetragonal and cubic lattice symmetries, confirmed by the Rietveld structure refinement. The dense microstructure with average grain sizes 0.84–7.87 μm was observed for BCSTN ceramics. Temperature-resistivity measurements demonstrate that TC of the ceramics with x=0.35 shifted to the room temperature region. Additionally, the BCSTN ceramic with heavy doping Sr exhibits relative low room-temperature resistivity and the resistance jump greater than 2.0 orders of magnitude.

High Carrier Mobility, Electrical Conductivity, and Optical Transmittance in Epitaxial SrVO3 Thin Films

AbstractThe urgent need for more performant transparent conducting electrodes is stimulating intensive research on oxide thin films based on early transition metals (e.g., V, Nb, Mo, etc.), where it is expected that the partially occupied (i.e., nd1, nd2…) conduction band will give rise to metallic conductivity. Growing thin films of these oxides typically requires an extremely low oxygen pressure. However, in growth methods involving hyperthermal kinetics (such as pulsed laser deposition), this may have severe detrimental effects on the electrical and optical properties of the film. Here, it is shown that the use of a nonreactive gas during a pulsed laser deposition process allows epitaxial SrVO3 films to be obtained with low room temperature resistivity (ρ ≈ 31 μΩ cm), large carrier mobility (μ ≈ 8.3 cm2 V−1 s−1), and large residual resistivity ratio (RRR ≈ 11.5), while improving optical transparency in the visible range. It is argued that the success of this growth strategy relies on the modulation of energetics of plasma species and a concomitant reduction of defects in the films. These findings may find applications in other oxide‐based thin film technologies (i.e., ferroelectric tunnel memories, etc.) where growth‐induced point effects may compromise functionality.

Low-temperature sintering of BaTiO3 positive temperature coefficient of resistivity (PTCR) ceramics

Donor–acceptor-codoped and Ti-excess barium titanate ceramics BaTi1.007−x−yNbxMnyO3 with positive temperature coefficient of resistivity (PTCR) behavior were prepared via the mixed oxide route. Resistivity versus temperature characteristics of samples sintered in air at 1350 °C were investigated as a function of dopant concentrations. A low room-temperature resistivity ρRT = 80 Ωcm and large resistivity change ρmax/ρmin of four orders of magnitude at the Curie temperature was observed for x = 0.002 and y = 0.0003. To reduce the sintering temperature and enable low-temperature sintering of PTCR ceramics, several additives inducing liquid-phase sintering were tested. We report on the effect of BaB2O4 and LiF–SrCO3 additives on the sintering behavior, microstructure, and ρ(T) characteristics. It is shown by dilatometry that the sintering temperature is drastically reduced. PTCR ceramics with resistivity change ρmax/ρmin of two orders magnitude were obtained with addition of 3 wt% BaB2O4 and sintering at 950 °C, or with a combined addition of 3 wt% BaB2O4 and 0.25 wt% LiF–SrCO3 and sintering at 900 °C.

Synthesis, doping and electrical bulk response of (Bi1/2Na1/2)xBa1-xTiO3 + CaO –based ceramics with positive temperature coefficient of resistivity (PTCR)

(Bi1/2Na1/2)xBa1-xTiO3 samples were prepared using the mixed-oxide route; their tetragonal to cubic phase transition temperature increases from 125 °C for x = 0–190 °C for x = 0.2. (Bi1/2Na1/2)0.10Ba0.90TiO3 (x = 0.10) ceramics with CaO addition sintered in air exhibit a positive temperature coefficient of resistivity (PTCR) behavior. Dense PTCR thermistor ceramics with low room-temperature resistivity and resistivity change ρmax/ρmin of 3.5 orders of magnitude at the Curie temperature were prepared using 3.5 mol% CaO, 0.05 wt% SiO2 addition as sintering aid, and Mn+Fe acceptor doping. Sintering at 1235 °C with two hours holding time were found as optimum synthesis conditions for achieving a good electrical thermistor performance. Impedance spectroscopy confirms that the resistance change is a grain boundary phenomenon, since the grain interior exhibits semiconducting behavior across the phase transition.

The influence of calcination temperature on the PTCR effect in (Bax-0.002Sm0.002) TiO3 ceramics prepared by the reduction sintering-reoxidation method

We investigate the effect of calcination temperature on the positive temperature coefficient of resistivity (PTCR) characteristics in (Bax-0.002Sm0.002)TiO3 ceramics that were fired at 1250 °C for 30 min in a reducing atmosphere and reoxidized at 800 °C for 1 h in air. The results indicate that the room-temperature resistivity first decreased and then increased as calcination temperature increased from 1000 °C to 1200 °C. Moreover, the critical calcination temperature of 1150 °C is experimentally determined. The samples have a low room-temperature resistivity of 38.4 Ω·cm and exhibit a significant PTCR effect with a resistance jump of 2.1 orders of magnitude. Furthermore, the room-temperature resistivity of the S7 samples doped with a small amount of Mn(NO3)2 is much higher than that of the S3 specimens not doped with Mn(NO3)2 in our work.

Influence of Reoxidation Time on Electrical Properties of Y2O3-doped BaTiO3 Ceramics Sintered in a Reducing Atmosphere

The influence of the reoxidation time on electrical properties and the positive temperature coefficient of resistance (PTCR) effect in multilayer Y2O3-doped BaTiO3 (BTO) ceramics were investigated. The BTO ceramics were sintered at 1160 °C−1270 °C for 2 h in a reducing atmosphere and then reoxidized at 500 °C–900 °C for 0 h−4 h. Results indicated that the room-temperature resistance of BTO ceramics reoxidized at 750 °C for 2 h after sintering at 1160 °C−1270 °C for 2 h decreased with the increasing of the sintering temperature. As the reoxidation time increased, the room-temperature resistance of the BTO samples slightly increased. In addition, the effect of the reoxidation time on the PTCR effect of the ceramics was also studied. The BTO ceramics had a low room-temperature resistance of 0.3 Ω and a resistance jump greater than 3.0 orders of magnitude.

Electrical properties of graphene nanoplatelets/ultra-high molecular weight polyethylene composites

Graphene nanoplatelets (GNPs)/ultra-high molecular weight polyethylene (UHMWPE) composites with a segregated structure had been fabricated using ethanol-assisted dispersion and hot compression at 180 °C. A percolation threshold of 3.5 wt% was achieved because of the formation conductive network. The positive temperature coefficient (PTC) and the negative temperature coefficient (NTC) effects of GNPs/UHMWPE composites had been investigated. The PTC behavior enhanced with increasing GNPs content but this was not always the case. The maximum PTC effect was observed in GNPs/UHMWPE composites (GNPs, 3.8 wt%) with the relatively low room temperature resistivity and the relatively high peak resistivity. The structure for GNPs/UHMWPE composites was examined by the SEM. The fact revealed that the slight interaction between GNPs and UHMWPE matrix may be changed by thermal cycles, and this can explain why thermal cycles could increase PTC and NTC intensity.

Effects of BaCO3 addition on the microstructure and electrical properties of La-doped barium titanate ceramics prepared by reduction-reoxidation method

Effects of trace BaCO3 addition were investigated on microstructure and electrical properties of La-doped BaTiO3 ceramics prepared via reduction-reoxidation method. The addition of BaCO3 could effectively facilitate the densification and grain growth of ceramics. SEM images suggested that numerous pores presented in ceramics due to the decomposition of BaCO3 to expel generated CO2 gas. The presence of pores promoted reoxidation of the reduced ceramics. As a result, lowering room-temperature (RT) resistivity and improving PTCR jump were simultaneously achieved as suitable amount of BaCO3 was added. An optimal addition of 0.3mol% BaCO3 led to a low RT resistivity of about 28Ωcm and a PTCR jump up to 103.7 after ceramics were fired at 1100°C in high-purity N2 and reoxidized at 800°C in air.