Increase In Sheet Resistance Research Articles

A novel flexible heating element using graphene polymeric composite ink on polyimide film

In this study, a simple, large and flexible electronic heat generator with graphene polymeric composite ink on a polyimide film was fabricated. A 47-μm thin layer was fabricated using wire bar coating. The dispersion of graphene, electrical properties, flexibility, sheet resistance and temperature increase of the layer were studied. Uniform resistance and graphene dispersion were determined to be the keys to uniform heating. This study demonstrated the feasibility of flexible rapid heating elements with graphene polymer composites.

Picosecond pulsed laser irradiation of silver nanowire films coated on the polyethylene terephthalate substrates

ABSTRACTWe investigated the effect of picosecond pulsed laser irradiation on the sheet resistance and microstructure of the silver nanowire films coated on PET substrates. The sheet resistance of the silver nanowire film abruptly increases from ∼102 to ∼108 ohm/square at a certain range of laser power. The sheet resistance of the picosecond laser annealed silver nanowire film strongly depends on the microstructure. The silver nanowires are ablated from the substrate and the surface of the substrate are seriously damaged above certain range of laser power. We analyzed the abrupt increase of sheet resistance and the damage of substrate surface in terms of temperature rising during the laser irradiation.

Tc depression and superconductor-insulator transition in molybdenum nitride thin films

We have studied that the Tc depression and the superconductor-insulator transition (SIT) in molybdenum nitride (MoN) thin films. Thin films were fabricated by reactive DC magnetron sputtering method onto (100) MgO substrates in the mixture of Ar and N2 gases. Several dozen MoN thin films were prepared in the range of 3 nm < thickness d < 60 nm. The resistance was measured by a DC four-probe technique. It is found that Tc decreases from 6.6 K for thick films with increase of the normal state sheet resistance and experimental data were fitted to the Finkel’stein formula using the bulk superconducting transition temperature Tc0 = 6.45 K and the elastic scattering time of electron τ = 1.6 × 10−16 s. From this analysis the critical sheet resistance Rc is found about 2 kΩ, which is smaller than the quantum sheet resistance RQ. This value of Rc is almost the same as those for 2D NbN films. The value of τ for MoN films is also the similar value for NbN films 1.0 × 10−16 s, while Tc0 is different from that for NbN films 14.85 K. It is indicated that the mechanism of SIT for MoN films is similar to that of NbN films, while the mean free path ℓ for MoN films is larger than that for NbN films.

Establishment of a reliable transfer process for fabricating chemical vapor deposition-grown graphene films with advanced and repeatable electrical properties.

Graphene films grown by the chemical vapor deposition (CVD) method have attracted intensive attention due to their native advantages of both high quality and large quantity for commercial applications. However, previously reported graphene films have exhibited uncertain and conflicted electrical properties that greatly hinder them from being used to build reliable electrical devices because of incompatibility during the complex and multifarious transfer process. Herein, the relationship between the transfer parameters and electrical performance was systematically studied. It demonstrates that cracking during the transfer process causes significant loss of carrier mobility and hence an increase in sheet resistance. Additionally, unstable doping plays a key role in the carrier density and hence greatly influences the sheet resistance. By introducing HCl as a doping agent, graphene films with repeated sheet resistance of approximately 300 ohm sq−1 can be realized. This work establishes a facile and reliable route to fabricate graphene films with advanced and repeatable electrical properties, which is significant and essential for fair evaluation of CVD-grown graphene films and further practical applications.

Channel-to-Channel Coupling in Normally-Off GaN Double-Channel MOS-HEMT

A previously reported normally-off GaN double-channel (DC-) MOS-HEMT with a gate recess into the upper channel layer has achieved a remarkably low ${R}_{\text {on}}$ . In this letter, we found that the double-channel structure itself does not guarantee a low ${R}_{\text {on}}$ without careful consideration of the electrical coupling between the two channels. A strong channel-to-channel (C2C) coupling between the two channels is critical to reduce ${R}_{\text {on}}$ in the DC-MOS-HEMT by exploiting both channels at the access region while allowing currents to converge into the heterojunction lower channel at the gate region. Otherwise, with a weak C2C coupling, the conduction through the upper channel is comprised due to the high-resistivity MOS-channel section at the gate region. The fabricated DC-MOS-HEMT with C2C distance ( ${t}_{{\textsf {C2C}}}$ ) of 7.5 nm exhibits an appreciably lower ${R}_{\text {on}}$ than that with ${t}_{{\textsf {C2C}}} = \textsf {11.5}$ nm. Further analysis reveals the latter suffers from an increased sheet resistance at access region compared with the sheet resistance of the DC-heterostructure itself. A modified TLM characterization is proposed to analyze the C2C coupling in the DC-heterostructure. It is confirmed the DC-heterostructure with ${t}_{{\textsf {C2C}}} = \textsf {7.5}$ nm boasts a strong C2C coupling, while that with ${t}_{{\textsf {C2C}}} = \textsf {11.5}$ nm features relatively weak C2C coupling.

Effects of diffusion process on potential induced degradation of silicon solar cells.

Potential induced degradation (PID) has recently been identified as one of the most important degradation mechanisms for silicon solar cells. It is widely considered that PID is closely related with the manufacture and application period of solar modules. In this study, the effects of diffusion sheet resistance on PID were verified and explained by testing the emitter doping profile, the minority carrier lifetime, the emitter saturation current, the electrical performance of different cells, and the PID process. With increasing sheet resistance of cells, the depth and saturation current density of the emitter both decreased, and the cell efficiency increased, whereas the PID phenomenon became serious. It was found that higher sheet resistance or thinner P-N junction could lead to higher PID sensitivity. Therefore, more attention should be paid to PID phenomenon as the photovoltaic industry develops in the direction of high sheet resistance.

Graphene—vertically aligned carbon nanotube hybrid on PDMS as stretchable electrodes

Stretchable electrodes are a critical component for flexible electronics such as displays, energy devices, and wearable sensors. Carbon nanotubes (CNTs) and graphene have been considered for flexible electrode applications, due to their mechanical strength, high carrier mobility, and excellent thermal conductivity. Vertically aligned carbon nanotubes (VACNTs) provide the possibility to serve as interconnects to graphene sheets as stretchable electrodes that could maintain high electrical conductivity under large tensile strain. In this work, a graphene oxide (GO)-VACNT hybrid on a PDMS substrate was demonstrated. Here, 50 μm long VACNTs were grown on a Si/SiO2 wafer substrate via atmospheric pressure chemical vapor deposition. VACNTs were directly transferred by delamination from the Si/SiO2 to a semi-cured PDMS substrate, ensuring strong adhesion between VACNTs and PDMS upon full curing of the PDMS. GO ink was then printed on the surface of the VACNT carpet and thermally reduced to reduced graphene oxide (rGO). The sheet resistance of the rGO-VACNT hybrid was measured under uniaxial tensile strains up to 300% applied to the substrate. Under applied strain, the rGO-VACNT hybrid maintained a sheet resistant of 386 ± 55 Ω/sq. Cyclic stretching of the rGO-VACNT hybrid was performed with up to 50 cycles at 100% maximum tensile strain, showing no increase in sheet resistance. These results demonstrate promising performance of the rGO-VACNT hybrid for flexible electronics applications.

Numerical simulation of the impact of surface traps on the performance of InP‐based high electron mobility transistors

In this paper, the DC and RF characteristics of our own InP‐based HEMTs were investigated comprehensively with surface traps concentration varying among 0 cm−2, 1 × 1012 cm−2, 3 × 1012 cm−2. Reasonable physical models were used to describe the device performances, including hydrodynamic transport model, density gradient model, recombination models and so on. Moreover, surface Shockley–Read–Hall model and improved Possion's equation were introduced to implement for the impact of surface traps. The DC and RF characteristics have shown compatible trends between computation and experiment. With the increasing surface traps concentration over the gate‐recess region, the channel current, transconductance, current gain cut‐off frequency (fT) and maximum oscillation frequency (fmax) exhibit apparent decrease trends. The observed obvious changes of the DC and RF properties with surface traps could be accounted for carrier sheet density reduction, which resulted from the surface potential modulated by negatively charged surface traps. Additionally, the charged surface traps could deplete the electrons in channel region, which leads to an increase in channel sheet resistance, and thus, further decreasing the fmax.

Low Energy Phosphorus Plasma Implantation for Isolation of MoS2 Devices

Device isolation is a critical but overlooked challenge in the effort to assess MoS2 as a possible material for post-Si technology nodes. Although reactive ion etching or chemical etching can be used to create isolated channels in large-area films, these techniques are problematic due to the high density of dangling bonds that they create. In this study, radiation damage is presented as an alternative to physical isolation of MoS2. Large-area ~4 layer MoS2 films were implanted by a low energy phosphine plasma at a bias of 200 V and a dose of 1 x 1014 cm-2. The results of this experiment indicate that there is greater than a 104 increase in sheet resistance of the MoS2 without measurable etching of the films. Annealing studies show that the resistance increase is stable up to at least 525°C, suggesting that radiation damage is an alternative to physical isolation for MoS2 devices.

Effect of gate voltage polarity on the ionic liquid gating behavior of NdNiO3/NdGaO3 heterostructures

The effect of gate voltage polarity on the behavior of NdNiO3 epitaxial thin films during ionic liquid gating is studied using in situ synchrotron X-ray techniques. We show that while negative biases have no discernible effect on the structure or composition of the films, large positive gate voltages result in the injection of a large concentration of oxygen vacancies (∼3%) and pronounced lattice expansion (0.17%) in addition to a 1000-fold increase in sheet resistance at room temperature. Despite the creation of large defect densities, the heterostructures exhibit a largely reversible switching behavior when sufficient time is provided for the vacancies to migrate in and out of the thin film surface. The results confirm that electrostatic gating takes place at negative gate voltages for p-type complex oxides while positive voltages favor the electrochemical reduction of Ni3+. Switching between positive and negative gate voltages therefore involves a combination of electronic and ionic doping processes th...

Microcavity‐Enhanced Semitransparent Electrodes for Oligothiophene Small‐Molecule Organic Solar Cells

Transparent electrodes (TEs) with high optical transparency (&gt;85%) are usually considered to be prerequisite for realizing high‐efficiency solar cells. On the contrary, here an indium‐free TE is reported which is semitransparent with an average visible transparency of 66.7% only. The electrode consists of Ag 9 nm with TiO2 (30 nm) undercoat layer designed specifically for small molecule organic solar cell (SM‐OSC) based on methylated oligothiophene derivatives. The electrode also contains a polyethyleneimine (PEI) wetting layer which promotes the growth of continuous, ultrasmooth, and highly conductive Ag films on the TiO2 layer even at very low thicknesses. The developed TiO2/PEI/Ag (TPA) electrode exhibits much better mechanical properties with only &lt;8% increase in sheet resistance after 200 bending cycles when a tensile strain of 2.1% is applied. The role of TiO2 undercoat layer has been investigated in detail and is shown to enhance the microcavity effect, leading to increased light coupling especially in the lower wavelength regime. With comparable photocurrent, and high fill‐factor values owing to much better electrical properties, the semitransparent TPA‐TE based SM‐OSC outperforms the state‐of‐art indium tin oxide (ITO) based reference device with photon conversion efficiency of 8.1% compared to 7.7% despite having lower transmittance (≈21%) relative to ITO.

Further Studies on the Effect of SiNx Refractive Index and Emitter Sheet Resistance on Potential-Induced Degradation

We present the impacts of silicon nitride (SiN x ) antireflection coating refractive index and emitter sheet resistance on potential-induced degradation of the shunting type (PID-s). Previously, it has been shown that the cell becomes more PID-s-susceptible as the refractive index decreases or the emitter sheet resistance increases. To verify the effect of refractive index on PID-s, we fabricated cells with varying SiN x refractive index (1.87, 1.94, 2.05) on typical p-type base solar cells with ∼60 Ω/sq emitters. However, none of these cells showed output power degradation, regardless of the refractive index. Further investigation of the emitter showed that the PID-s was suppressed at ∼60 Ω/sq due to the extremely high surface phosphorus concentration (6 × 1021 cm−3), as measured by secondary ion mass spectrometry. Furthermore, PID-s was observed on cells possessing a high emitter sheet resistance (∼80 Ω/sq). The emitter surface phosphorus concentration plays an important role in determining PID-s susceptibility.

Proton implantation for electrical insulation of the InGaAs/InAlAs superlattice material used in 8–15 μm-emitting quantum cascade lasers

We demonstrate the conversion of lattice-matched InGaAs/InAlAs quantum-cascade-laser (QCL) active-region material into an effective current-blocking layer via proton implantation. A 35-period active region of an 8.4 μm-emitting QCL structure was implanted with a dose of 5 × 1014 cm−2 protons at 450 keV to produce a vacancy concentration of ∼1019 cm−3. At room temperature, the sheet resistance, extracted from the Hall measurements, increases by a factor of ∼240 with respect to that of an unimplanted material. Over the 160–320 K temperature range, the activation energy of the implanted-material Hall sheet-carrier density is 270 meV. The significant increase in room-temperature sheet resistance indicates that upon implantation deep carrier traps have been formed in the InAlAs layers of the superlattice. Fabricated mesas show effective current blocking, at voltages ≥10 V, up to at least 350 K. Thus, the implanted InGaAs/InAlAs superlattices are highly resistive to at least 350 K heat sink temperature. Such implanted material should prove useful for effective current confinement in 8–15 μm-emitting InP-based single-emitter QCL structures as well as in resonant leaky-wave coupled phase-locked arrays of QCLs.

Interface Electrical Properties of Al2O3 Thin Films on Graphene Obtained by Atomic Layer Deposition with an in Situ Seedlike Layer.

High-quality thin insulating films on graphene (Gr) are essential for field-effect transistors (FETs) and other electronics applications of this material. Atomic layer deposition (ALD) is the method of choice to deposit high-κ dielectrics with excellent thickness uniformity and conformal coverage. However, to start the growth on the sp2 Gr surface, a chemical prefunctionalization or the physical deposition of a seed layer are required, which can effect, to some extent, the electrical properties of Gr. In this paper, we report a detailed morphological, structural, and electrical investigation of Al2O3 thin films grown by a two-steps ALD process on a large area Gr membrane residing on an Al2O3-Si substrate. This process consists of the H2O-activated deposition of a Al2O3 seed layer a few nanometers in thickness, performed in situ at 100 °C, followed by ALD thermal growth of Al2O3 at 250 °C. The optimization of the low-temperature seed layer allowed us to obtain a uniform, conformal, and pinhole-free Al2O3 film on Gr by the second ALD step. Nanoscale-resolution mapping of the current through the dielectric by conductive atomic force microscopy (CAFM) demonstrated an excellent laterally uniformity of the film. Raman spectroscopy measurements indicated that the ALD process does not introduce defects in Gr, whereas it produces a partial compensation of Gr unintentional p-type doping, as confirmed by the increase of Gr sheet resistance (from ∼300 Ω/sq in pristine Gr to ∼1100 Ω/sq after Al2O3 deposition). Analysis of the transfer characteristics of Gr field-effect transistors (GFETs) allowed us to evaluate the relative dielectric permittivity (ε = 7.45) and the breakdown electric field (EBD = 7.4 MV/cm) of the Al2O3 film as well as the transconductance and the holes field-effect mobility (∼1200 cm2 V-1 s-1). A special focus has been given to the electrical characterization of the Al2O3-Gr interface by the analysis of high frequency capacitance-voltage measurements, which allowed us to elucidate the charge trapping and detrapping phenomena due to near-interface and interface oxide traps.

The effect of the second annealing step on the Ni1-xPtx:Si film thermal stability

In this paper, the influence of the formation history of a thin Ni1-xPtx:Si film on its thermal stability is investigated. Film degradation manifests itself as a transformation of the continuous film into physically separated islands, resulting in a sheet resistance increase. Higher silicide peak formation temperatures were observed to result in a reduced thermal stability. As a result an improved silicide thermal stability is demonstrated for conductive heating as compared to laser/flash msec anneal. This improved thermal stability is discussed in terms of stress, grain size and Pt distribution. The observed thermal stability differences can be explained in terms of the residual stress in the film.

Nitrogen enhanced thermal stability of nickel monosilicide

The effect of annealing ambient during rapid thermal processing on thermal stability of NiSi has been investigated. Nickel films deposited onto Si(100) were annealed in vacuum, Ar and N2 at pressures up to 40 Torr. Rapid thermal annealing in vacuum or in Ar at 750 °C resulted in the formation of NiSi2 with concomitant increase in sheet resistance and surface roughness. However, annealing in N2 increases the NiSi2 nucleation temperature and thus the thermal stability of NiSi. Higher the N2 pressure, higher was the annealing temperature required to initiate NiSi2 nucleation. Experimental results were analyzed using a simple thermodynamic model. It suggests dissolved nitrogen impurities lower the chemical potential and increase thermal stability of NiSi phase. Phase stability observed experimentally was in agreement with that predicted by the model.Threshold nitrogen pressure exists for stabilization of nickel monosilicide during rapid thermal annealing.

Planar Ohmic Contacts to Al0.45Ga0.55N/Al0.3Ga0.7N High Electron Mobility Transistors

We present a low resistance, straightforward planar ohmic contact for Al0.45Ga0.55N/Al0.3Ga0.7N high electron mobility transistors. Five metal stacks (a/Al/b/Au; a = Ti, Zr, V, Nb/Ti; b = Ni, Mo, V) were evaluated at three individual annealing temperatures (850, 900, and 950°C). The Ti/Al/Ni/Au achieved the lowest specific contact resistance at a 900°C anneal temperature. Transmission electron microscopy analysis revealed a metal-semiconductor interface of Ti-Al-Au for an ohmic (900°C anneal) and a Schottky (850°C anneal) Ti/Al/Ni/Au stack. HEMTs were fabricated using the optimized recipe with resulting contacts that had room-temperature specific contact resistances of ρc = 2.5 × 10−5 Ω cm², sheet resistances of RSH = 3.9 kΩ/■, and maximum current densities of 75 mA/mm (at VGATE of 2 V). Electrical measurements from −50 to 200°C had decreasing specific contact resistance and increasing sheet resistance, with increasing temperature. These contacts enabled state-of-the-art performance of Al0.45Ga0.55N/Al0.3Ga0.7N HEMTs.

Degradation Characteristics of AlGaN/GaN MOS-Heterostructure FETs by Alpha-Particle Irradiation

AlGaN/GaN metal-oxide-semiconductor heterostructure field effect transistors (MOS-HFETs) and schottky HFETs were irradiated with 18 MeV alpha-particles. Irradiated devices showed degradation in current-voltage (I-V) and capacitance-voltage (C-V) characteristics. After irradiation, both devices exhibited the positive shift of threshold voltage (Vth) and the increase of on-resistance (Ron). We also observed the increase of contact (RC) and sheet resistance (Rsh) as irradiation fluence increased. Post-irradiation characteristics were attributed to irradiation-induced displacement damage with strong dependence on fluence. Hall pattern measurement confirmed significant degradation of carrier transport property by alpha-particle irradiation. Extracted density of oxide interface states was also increased after irradiation.

Localization and pair breaking parameter in superconducting molybdenum nitride thin films

We have investigated the superconductor-insulator transition in molybdenum nitride films prepared by deposition onto MgO substrates. It is indicated that the Tc depression from for thick films with increase of the normal state sheet resistance was well explained by the Finkel’stein formula from the localization theory. Present analysis suggests that the superconducting-insulator transition occurs at a critical sheet resistance . It is found that the above shows different characteristics of and in the regions and , respectively, where is the classical residual resistance and A is a constant. The excess conductance due to thermal fluctuation has been analyzed by the sum of the Aslamazov–Larkin and Maki–Thompson correction terms with use of the pair breaking parameter in the latter term. The sum agrees well with the data, although the experimental results of the dependence of , that is, shows the disagreement with a linear relation derived from the localization theory.

The effects of UV radiation on the structure and properties of AZO thin films deposited on quartz glass by magnetron sputtering

For many devices using AZO thin films as electrodes, such as thin film solar cells, the properties of AZO thin films have important influence on the life and performance of devices. The property stability of AZO thin films after long-term illumination is a problem that deserves attention. Two groups of ZnO:Al thin films were deposited on quartz glass substrates by magnetron sputtering at the same conditions. One group of the films was irradiated with ultraviolet (UV) light for 168h at room temperature and another group was just stored in the sample box at room temperature without irradiation. The structures and optical and electrical properties of the AZO films were investigated by XRD, FESEM, UV–Visible spectrophotometer and Four-point Probes. The results reveal that all the AZO films have hexagonal structure and highly preferred c-axis orientation. The mean grain size decreases and the diffraction angles move to larger angles after a long-time UV radiation. At the same time, the transmittance in visible region decreases and the sheet resistance increases. The optical bandgaps of AZO thin films are 3.28–3.38eV. The optical bandgap decreases a little bit after UV radiation. The transmittance of the AZO film deposited at higher pressure decreases more obviously.