Increase In Sheet Resistance Research Articles

Effect of temperature on the performance of ScAlN/GaN high-electron mobility transistor

We have studied the operation of the ScAlN/GaN high-electron mobility transistor (HEMT) at high temperatures up to 700 K (423 °C). A maximum drain current density of ∼2 A/mm and an on-resistance of ∼1.5 Ω·mm was measured at room temperature (RT). The epi-structure exhibited a very high two-dimensional electron gas (2DEG) density of 6 × 1013 cm−2 at RT using Hall measurement. The Sc0.15Al0.85N barrier, nearly lattice matched to the GaN channel, showed a drain current reduction of ∼50% at 700 K. The decrease in 2DEG mobility, which leads to an increase in sheet resistance, is mostly responsible for this reduction in drain current. However, an excellent electrostatic control was achieved at 700 K with the drain current value exceeding 1 A/mm, which is 2 times higher compared to that of AlGaN/GaN HEMTs reported previously. These results indicate that ScAlN/GaN HEMTs are a promising candidate for high-temperature and high-power electronic applications.

Dose-Dependent Performance of Boron-Implanted Self-Aligned Bottom-Gate IGZO TFTs

This work provides an interpretation of donor activation in self-aligned bottom-gate (SA-BG) Indium-Gallium-Zinc Oxide (IGZO) TFTs with ion-implantation of boron (11B+) species as the source/drain treatment. The effect of implant dose on device operation was investigated. Transfer characteristics appeared similar with a boron dose of 1 and 2×1015 cm-2 however, a 45% decrease in max current was observed for devices implanted with a 4×1015 cm-2 dose. Van der Pauw measurements displayed a similar trend; an increase in boron dose resulted in an increase in sheet resistance. A left-shift in transfer characteristics was observed with a greater shift in the 4×1015 cm-2 dose devices, following thermal stability treatments. The existence of two separate boron species is hypothesized, (1) an electrically active donor species involving a relatively small fraction of the total boron concentration and (2) additional boron interstitials and/or other associated defects. A detailed discussion on developed arguments arrived at through analysis of TFT electrical characteristics is presented.

Recent Advancements on Slot-Die Coating of Perovskite Solar Cells: The Lab-to-Fab Optimisation Process

Perovskite solar cells (PSCs) are the most rapidly advancing photovoltaic technology in terms of power conversion efficiency. An efficiency of 26.1% was achieved in a decade, which is on par with the efficiency of very mature silicon panels. However, PSC commercialisation is partly hindered by the difficulty of scaling these devices without efficiency loss, mostly due to the increasing sheet resistance of the transparent conductive layer substrates and the nonuniformity of the layers when deposited across large areas. Therefore, it is crucial for the commercialisation of PSCs to implement easily scalable deposition processes with low material waste and compatibility with roll-to-roll (R2R) processes to reduce manufacturing costs. Slot-die coating can meet all these requirements, allowing for great uniformity over large areas. The most recent developments in PSC upscaling using slot-die coating as the main deposition process, along with its extension to the R2R process, are reviewed, including a thorough discussion of the slot-die coating process and the theory behind its operating limits. In fact, R2R coating is a very promising strategy for PSC industrialisation, since all processing steps use low-cost materials and scalable processes at temperatures lower than 120 °C, allowing the cost-effective and high-throughput production of PSC devices.

Intrinsic doping and ageing of indium oxide thin films

Indium oxide (In2O3) is one of the most used materials for the synthesis of transparent electronics devices and currently tin-doped indium oxide (ITO) dominates the field of Transparent Conductive Oxides for most applications, from ICT to photovoltaics. In this work we present a study on the responses of indium oxide upon the formation, and stabilization, of point defects induced with different methods. After the deposition of In2O3 thin films via RF magnetron sputtering, treatments such as Ion Implantation (I.I.), Ultraviolet (UV) irradiation and Sun exposure were employed to generate oxygen vacancies as intrinsic dopants. Both I.I. and UV irradiations significantly improved the electrical conductivity of films, increasing the carrier density by up to 3 orders of magnitude. Moreover, after I.I. also the crystalline quality of the films was enhanced.To monitor the stability of oxygen vacancies upon exposure to ambient atmosphere, the ageing of samples was followed and recorded for a couple of weeks, reporting a fast and strong degradation of electrical properties. As a facile strategy to counteract the sheet resistance increase, samples were encapsulated in an ultra-thin (≈10 nm) SiO2 layer deposited via sputtering. Irradiated and encapsulated films retained a much lower sheet resistance throughout the observation time, preserving the samples conductivity.

High temperature stability of regrown and alloyed Ohmic contacts to AlGaN/GaN heterostructure up to 500 °C

This Letter reports the stability of regrown and alloyed Ohmic contacts to AlGaN/GaN-on-Si high electron mobility transistors (HEMTs) for high temperature applications up to 500 °C. Transfer length method (TLM) measurements from 25 to 500 °C in air show that the regrown contacts appear to be stable up to 500 °C during short term (approximately 1 h) testing, while alloyed contacts appear to decrease in contact resistance from 300 to 500 °C though increases in the error bounds due to increase sheet resistance make it difficult to conclude definitely. Additionally, longer term testing shows both technologies remain stable at least up to 48 h at 500 °C, after which the large increase in sheet resistance makes the measurement uncertainty too large to conclude definitively. Advanced microscopy images indicate both the regrown and alloyed contact regions remain structurally intact after prolonged high temperature exposure with no visible degradation in crystallinity or metal composition.

A comparison of physical properties of dielectric/metal/dielectric film on polyethylene terephthalate and optical glass substrates

AbstractMolybdenum trioxide/indium/silver/indium/molybdenum trioxide layers were evaporated on the polyethylene terephthalate and glass substrates using thermal evaporation coating machine. In this study, we compare the electrical, optical, and topological properties of dielectric/metal/dielectric fabricated on the polyethylene terephthalate substrate and glass substrate. In this way, the influence of dielectric and metal thicknesses on the electroptical behavior and topological properties of the transparent conductive film are studied. Furthermore, the influence of the substrate temperature on the physical properties of prepared thin films is investigated. The results show that by increasing molybdenum trioxide film thickness, the sheet resistance increases. We find that the prepared thin films on the glass substrate has high optical transmittance with respect to its on the polyethylene terephthalate substrate. We also find that by increasing the substrate temperature, figure of merit decreases. It means that the maximum figure of merit is achieved at the room substrate temperature.

Disorder enhanced relative intrinsic detection efficiency in NbTiN superconducting nanowire single photon detectors at high temperature

The intrinsic detection performance of superconducting nanowire single photon detectors (SNSPDs) is highly dependent on the superconducting properties of underlying thin films. This report outlines the enhancement of detection performance for single telecom wavelength photons in disordered NbTiN SNSPD at 4.2 K. By increasing the nitrogen content and deposition pressure, the NbTiN films show suppression in critical temperature and an increase in sheet resistance. Notably, the resulting SNSPDs display a broader saturation plateau at 2.2 K, leading to superior detection performance at 4.2 K. With the disordered 7-nm-thick NbTiN films, we fabricated SNSPDs with system detection efficiency up to 83% for 1550 nm photons at 4.2 K. Moreover, these devices also show saturated intrinsic detection efficiency for 2000 nm photons. With the features outlined, the devices can be integrated into the idle 4.2 K stage of the dilution refrigerator for applications in optical quantum information processing or utilize for detecting laser radar signals in airborne platforms.

Effect of thermal shock on ZnO/Ag/ZnO nano-multilayers: Photoelectric property degradation

We investigated the degeneration at photoelectric properties of typical multilayer transparent conductive electrodes (TCEs) of ZnO/Ag/ZnO under thermal shock by experiments and simulation. The experimental data indicated a reduction in the visible region transparency and an increase in sheet resistance for ZnO/Ag/ZnO after undergoing thermal cyclic shock. This resulted in a significant decrease in the Haacke figure of merit (FOM) from 30.7 × 10-3 Ω−1 to 7.8 × 10-3 Ω−1. We identified oxygen diffusion into Ag at the ZnO/Ag interface using molecular dynamics (MD) simulations. Concurrently, density functional theory (DFT) simulations showed that interstitial oxygen creates impurity charge centers within Ag, indirectly causing the deterioration of the photoelectric properties of ZnO/Ag/ZnO. This finding highlights the challenge of maintaining optoelectronic stability in TCO/metal/TCO (OMO) thin films for practical applications and provides both experimental and theoretical support for designing highly reliable optoelectronic devices.

A two-step junction welding technique for achieving high-performance broadband silver nanowire transparent conductive films.

Transparent conductive films (TCFs) are critical components of various optoelectronic and photovoltaic devices. The performance of TCFs is mainly ascribed to their superior transparency and low sheet resistance. However, the welding strategy for silver nanowires (AgNWs), the primary material used in TCFs, can significantly affect the optical and electrical properties of the films. Herein, we report a simple and effective method for preparing AgNWs TCFs with superior broadband transparency and conductivity. By applying pressure to the surface of graphene, we successfully enhanced the tight connections between loose nanowires. Based on this, we developed a high-performance, wide-spectrum TCF and employed ultraviolet welding as its post-treatment method. The sheet resistance of the electrode decreased to 27.1 Ω sq-1 with the addition of graphene films, and subsequent UV light welding further reduced the resistance value by 25.1%. Moreover, the incorporation of graphene films provides excellent protection for AgNWs against oxidation. One month later, compared to AgNWs without a protective coating, the increase in sheet resistance was reduced by 414.6%. This study provides a new method for preparing TCFs with wide spectrum and high performance.

Chemically-stable flexible transparent electrode: gold-electrodeposited on embedded silver nanowires

Silver nanowires (AgNWs) with a low diameter, high aspect ratio, stable suspension, and easy synthesis have recently attracted the optoelectronic industry as a low-cost alternative to indium tin oxide transparent conductive films. However, silver nanowires are not chemically stable, and their conductivity diminishes over time due to reactions with atmospheric components. This is a bottleneck for their wide industrial applications. In this study, we aim to address this issue by synthesizing silver nanowires with an average diameter of approximately 65 nm and a length of approximately 13 µm. The prepared Ag nanowires are then applied to fabricate transparent, flexible, and chemically stable conductive films. The fabrication includes spraying of silver nanowires suspension on a glass substrate followed by Dr. blade coating of polystyrene (PS) solution and delamination of the PS-AgNWs film. The resulting film exhibits an optimum sheet resistance of 24 Ω/□ and transmittance of 84%. To further enhance the stability of the transparent conductive film, the facial and scalable double pulse electrodeposition method is used for coating of gold on the exposed surface of the AgNWs embedded in PS. The final transparent film with gold coating demonstrates a remarkable stability under harsh conditions including long exposure to UV light and nitric acid solution. After 100 min of UV/Ozone treatment, the increase in sheet resistance of the optimal PS-AgNW@Au sample is 15.6 times lower than the samples without gold coating. In addition, the change in sheet resistance after 2000 bending cycles in the optimal PS-AgNW@Au electrode is measured and it showed an increase of only 22% of its initial sheet resistance indicating its good flexibility. The proposed electrode performs an excellent chemical stability, good conductivity, transparency, and flexibility that makes it a potential candidate for various optoelectronic devices.

Site‐Selective Enhancement of Superconducting Nanowire Single‐Photon Detectors via Local Helium Ion Irradiation

AbstractAchieving homogeneous performance metrics between nominally identical pixels is challenging for the operation of arrays of superconducting nanowire single‐photon detectors (SNSPDs). Here, local helium ion irradiation is utilized to post‐process and tune single‐photon detection efficiency, switching current, and critical temperature of individual devices on the same chip. For 12 nm thick highly absorptive SNSPDs, which are barely sensitive to single photons with a wavelength of 780 nm prior to He ion irradiation, an increase of the system detection efficiency from <0.05% to (55.3 1.1)% is observed following irradiation. Moreover, the internal detection efficiency saturates at a temperature of 4.5 K after irradiation with 1800 ions nm−2. Compared to 8 nm SNSPDs of similar detection efficiency, a doubling of the switching current (to 20 µA) is observed for irradiated 10 nm thick detectors, increasing the amplitude of detection voltage pulses. Investigations of the scaling of superconducting thin film properties with irradiation up to a fluence of 2600 ions nm−2 revealed an increase of sheet resistance and a decrease of critical temperature towards high fluences. A physical model accounting for defect generation and sputtering during helium ion irradiation is presented and shows good qualitative agreement with experiments.

Highly doped graphene on ion-exchanged glass

Engineering the doping level in graphene is essential to realizing functional electronic and optoelectronic devices. While achieving strong p-doping is relatively straightforward, electrostatic or chemical approaches to negatively dope graphene have yielded electron densities (ns ) of −9.5 × 1012 cm−2 or below. In this work, we demonstrate very high ns (−1013 to −1014 cm−2) in graphene, on an ion-exchanged glass substrate, which is widely used in touch screen displays (e.g. smart phones). Moreover, the proposed method, which is easy to implement and scalable, leads to relatively stable graphene doping, with about a 40% increase in sheet resistance over 5 months at ambient conditions.

Effect of blocking layer on achieving low sheet resistances in low-E multilayer structures

Low-emissivity films consisting of a metal oxide/Ag/metal oxide structure improve the thermal insulation performance of windows. Deposition of an oxide layer on top of the Ag layer requires either the insertion of a blocking layer (BL) or the sputtering deposition of an oxide target under an argon atmosphere. In this study, we carried out a sequential sputtering deposition method for inserting a BL to achieve both high visible-light transmittance and low sheet resistance. The effects of metallic and oxide BLs on the sheet resistances were investigated, and metallic BLs were confirmed to cause an increase in sheet resistance due to the diffusion of metal atoms into the Ag layer. Oxide BLs deposited under low oxygen concentrations maintained the low sheet resistance. The most significant result was obtained with a TiO x BL deposited under 2.9% oxygen, which prevented an increase in sheet resistance even after the deposition of the top layer (TL). By optimizing the thickness of the TL, high visible-light transmittance and low sheet resistance were achieved in a glass/TiO2/ZnO/Ag/TiO x /ZnO multilayer. Thus, high-performance multilayers could be fabricated by selecting an appropriate BL, which is beneficial for the industrial fabrication of low-E films.

Conformal Passivation of Self-Cleanable, Flexible, and Transparent Polytetrafluoroethylene Thin Films on Two-Dimensional MXene and Three-Dimensional Ag Nanowire Composite Electrodes

Despite the excellent performance of MXene–Ag nanowire (MA) composite transparent conductive electrodes (TCEs), they rapidly and severely degrade in ambient and humid environments. To address this critical issue, we developed conformal polytetrafluoroethylene (PTFE) passivation thin films prepared using magnetron sputtering at room temperature. Compared to the PTFE passivated electrode, the nonpassivated bare MA-composite electrode exhibited a considerably increased sheet resistance, and the electrode flexibility and electromagnetic interference (EMI) shielding efficiency (SE) degraded rapidly over time because the moisture and impurities severely oxidized the MXene layer in the harsh testing environment. However, the MA-composite electrode with PTFE passivation shows constant sheet resistance, transmittance, flexibility, and EMI SE, even after the 85 °C–85% relative humidity (RH) test. Compared to the nonpassivated MA-electrode-based thin film heaters (TFHs), the PTFE-passivated MA-composite electrode-based TFHs exhibited improved stability and reliability, indicating that the sputtered PTFE film effectively prevented the moisture and impurity penetration of the MA-composite electrodes.

Disorder-tuned conductivity in amorphous monolayer carbon.

Correlating atomic configurations-specifically, degree of disorder (DOD)-of an amorphous solid with properties is a long-standing riddle in materials science and condensed matter physics, owing to difficulties in determining precise atomic positions in 3D structures1-5. To this end, 2D systems provide insight to the puzzle by allowing straightforward imaging of all atoms6,7. Direct imaging of amorphous monolayer carbon (AMC) grown by laser-assisted depositions has resolved atomic configurations, supporting the modern crystallite view of vitreous solids over random network theory8. Nevertheless, a causal link between atomic-scale structures and macroscopic properties remains elusive. Here we report facile tuning of DOD and electrical conductivity in AMC films by varying growth temperatures. Specifically, the pyrolysis threshold temperature is the key to growing variable-range-hopping conductive AMC with medium-range order (MRO), whereas increasing the temperature by 25 °C results in AMC losing MRO and becoming electrically insulating, with an increase in sheet resistance of 109 times. Beyond visualizing highly distorted nanocrystallites embedded in a continuous random network, atomic-resolution electron microscopy shows the absence/presence of MRO and temperature-dependent densities of nanocrystallites, two order parameters proposed to fully describe DOD. Numerical calculations establish the conductivity diagram as a function of these two parameters, directly linking microstructures to electrical properties. Our work represents an important step towards understanding the structure-property relationship of amorphous materials at the fundamental level and paves the way to electronic devices using 2D amorphous materials.

Effects of adsorbed molecular ordering to the superconductivity of a two-dimensional atomic layer crystal

The effect of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) adsorption on the physical properties of the two-dimensional (2D) atomic layer superconductor (ALSC) In/Si(111)-($\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$) has been studied by angle-resolved photoelectron spectroscopy, transport measurements, and scanning tunneling microscopy. Hole doping from the adsorbed molecules has been reported to increase the superconducting transition temperature ${T}_{\mathrm{c}}$ of this ALSC, and the molecular spin tends to decrease it. Owing to its large electron affinity and its nonexistent spin state, the adsorption of PTCDA was expected to increase ${T}_{\mathrm{c}}$. However, the PTCDA adsorption dopes only a small number of holes in the In layers and causes a suppression of ${T}_{\mathrm{c}}$ with a sharp increase in the normal-state sheet resistance followed by an insulating transition. Taking the disordering of the arrangement of PTCDA into account, we conclude that the increase in resistance is due to the localization effect originating from the random potential that is induced by the disordered PTCDA molecules. The present result also indicates the importance of the crystallinity of a 2D molecular film adsorbed on ALSCs.

Influence of Al doping and annealing on the microstructures and electrical properties of CrSi films prepared by magnetron co-sputtering

In this work, we prepared CrSi films with different Al contents by DC magnetron co-sputtering, and studied the effect of Al content and annealing temperature on the microstructure and electrical properties of the films. The temperature coefficient of resistance (TCR) of the films increases with increasing Al content. Furthermore, the TCR gradually changes from negative to positive value with the increase of the annealing temperature, and reaches zero at a critical temperature. When the Al content is lower than 3.06 at.%, the TCR is always negative after annealing. An excellent combination of high sheet resistance of 5.36 kΩ/sq with near zero TCR of −10.07 ppm/°C were achieved in the films with an Al content of 3.87 at.% after annealing at 520 °C. The stability and crystallinity of the film are improved by the addition of Al element. The mixture of amorphous and crystalline phases results in a near zero TCR. Moreover, the coarsening of the crystalline phase and the increase in spacing of the crystalline phase induces the increase of sheet resistance.

Combined effects of residual stress and microstructure on degradation of Cu thin films on Si

In this paper, combined effects of residual stress and microstructure on degradation of Cu thin films on Si have been investigated. Magnetron sputtering with different processing parameters produced cracked and non-cracked Cu films with various grain sizes and residual stresses. Phase transformation and the change in electrical resistance of the Cu thin films have been analyzed for different annealing conditions. It has been revealed that non-cracked and large-grained (25.2±8.9nm) Cu films with high tensile residual stress (186 MPa) result in the most significant degradation (sheet resistance increase of 894%) upon annealing at 300°C. Specimens with smaller grains (17.3±6.4 nm) and low residual stress (41.6 MPa) or cracks showed no increase in sheet resistance after annealing at 300°C. The microstructural evaluation showed that residual stress of Cu affects the grain growth rate during annealing, which is closely related to the Cu/Si interdiffusion behavior.

Thermal Stability of Sputtered Tungsten Nitrides for Solar Thermal Applications

In this work, tungsten nitrides sputtered at different powers supplied to a W target (300 W, 500 W, 700 W) and proposed for solar thermal applications as part of solar absorbers, as active and robust materials for capacity energy storage and as plasma-facing materials were annealed in vacuum at medium-high temperatures (470 °C, 580 °C) and characterized by means of X-ray diffraction (XRD), AFM, micro-Raman, FTIR, UV–VIS–-NIR, sheet, surficial energy and wetting angle measurements. From the overall set of analyses, some important modifications and differences between samples after annealing emerged (which will be useful for selecting them for specific applications) and have been correlated to sputtered W metallic clusters’ ability to adsorb, form complexes with and react with the strong N2 triple bond under the various plasma conditions of a reactive sputtering process. In particular, the 300 W film of poor crystalline quality as deposited, after annealing released entrapped nitrogen and retained its W2N structure up to a temperature of 580 °C. Despite there being no phase transition, there was an increase in sheet resistance, which is detrimental because the preservation of metallic character is an important requisite for the proposed applications. The 500 W film had a stable crystalline structure and a metallic character unmodified by increasing temperature. The 700 W film, whose structure as deposited was almost amorphous, underwent the most severe modification after annealing: crystallizing, disproportioning and giving rise to a composite and porous nature (W + WNx) not ideal for spectrally selective coating applications, but useful for tailoring capacitive energy storage devices, or for catalysts for hydrogen evolution reactions (as an alternative to platinum) in alkaline water electrolysis.

Anomalous electrical conductivity change in MoS2 during the transition from the amorphous to crystalline phase

Transition metal dichalcogenides exhibit unique properties, which make them interesting for fundamental studies and for applications in many devices. Here, we report on the anomalous electrical behavior during the amorphous-to-crystal phase transition of MoS2. While crystallization typically results in an increase in conductivity, the situation in MoS2 is opposite. Amorphous MoS2 shows a sheet resistance of 3.2 × 103Ω□ with the value remaining nearly constant until 200 °C, MoS2 samples annealed above 200 °C exhibit an unexpected two-step increase in sheet resistance. The first abrupt increase takes place after annealing at temperatures between 300 and 400 °C while the second increase occurs after heating to 700 °C with typical sheet resistance values of 1.2 × 105 and 8 × 106Ω□ after heating to 500 °C and 900 °C, respectively. Using a combination of X-ray photoelectron spectroscopy and X-ray diffraction studies and ab-initio modeling, we argue that the large electrical conductivity observed in amorphous MoS2 is associated with the existence of a large quantity of Mo–Mo homopolar bonds that exceed the percolation threshold. The dramatic increase in the sheet resistivity in the first step is accompanied by the formation of the Mo–S bonds upon the disassociation of homopolar Mo–Mo bonds for a further increase in the sheet resistance upon annealing temperatures of 700 °C that can be attributed to the grain boundary formation during crystallization of MoS2 into the 2H layered structure.