Temperature-dependent Current-voltage Measurements Research Articles

Magnetoresistance properties in nickel-catalyzed, air-stable, uniform, and transfer-free graphene

A transfer-free graphene with high magnetoresistance (MR) and air stability has been synthesized using nickel-catalyzed atmospheric pressure chemical vapor deposition. The Raman spectrum and Raman mapping reveal the monolayer structure of the transfer-free graphene, which has low defect density, high uniformity, and high coverage (>90%). The temperature-dependent (from 5 to 300 K) current–voltage (I–V) and resistance measurements are performed, showing the semiconductor properties of the transfer-free graphene. Moreover, the MR of the transfer-free graphene has been measured over a wide temperature range (5–300 K) under a magnetic field of 0 to 1 T. As a result of the Lorentz force dominating above 30 K, the transfer-free graphene exhibits positive MR values, reaching ∼8.7% at 300 K under a magnetic field (1 Tesla). On the other hand, MR values are negative below 30 K due to the predominance of the weak localization effect. Furthermore, the temperature-dependent MR values of transfer-free graphene are almost identical with and without a vacuum annealing process, indicating that there are low density of defects and impurities after graphene fabrication processes so as to apply in air-stable sensor applications. This study opens avenues to develop 2D nanomaterial-based sensors for commercial applications in future devices.

Solution‐based fabrication of copper oxide thin film influence of cobalt doping on structural, morphological, electrical, and optical properties

In this study, Cobalt (Co) doped Copper Oxide (CuO) films at different concentrations were deposited on glass substrates, using the Chemical Bath Deposition (CBD) method. The films were characterized by Field Emission Scanning Electron Microscopy (FESEM), X-Ray Diffraction (XRD), Ultra Violet-Visible Spectroscopy (UV-Vis.) and two-point contact method. The FESEM images showed that nanoplates formed increased in size and voids on the films surface decreased with increasing Co concentration. The XRD patterns revealed an increase in crystallite size with increasing (from 14.40 to 18.60 nm) Co concentration and no secondary phase was formed. The Energy-dispersive X-ray spectroscopy (EDS) spectra showed the presence of Co in the film composition with increasing concentration. The results of UV-Vis. spectroscopy showed that band gap values could be changed with Co doping and thus the CuO band gap could be adjusted with the Co doping. The temperature-dependent current-voltage measurement results obtained with the two-point contact method showed that activation energy levels increased (from 0.134 to 0.232 eV) with increasing Co concentration. It was also observed that the conductivity increased with increasing temperature.

Electrical characteristics of in situ Mg-doped β-Ga2O3 current-blocking layer for vertical devices

The lack of p-type doping has impeded the development of vertical gallium oxide (Ga2O3) devices. Current blocking layers (CBLs) using implanted deep acceptors have been used to demonstrate vertical devices. This paper presents a pioneering demonstration of in situ Mg-doped β-Ga2O3 CBLs grown using metal–organic chemical vapor deposition. The Mg-doping density during growth was calibrated by quantitative secondary ion mass spectroscopy. Electrical test structures were designed with in situ Mg doped layers with various targeted Mg doping concentrations. The effectiveness of the CBL is characterized by using temperature-dependent current–voltage measurements using n-Mg-doped-n structures, providing crucial insight into the underlying mechanisms. Pulsed measurements show similar blocking characteristics as DC. To further validate the experimental results, a TCAD simulation is performed, and the electrically active effective doping is found to be dependent on the Mg-doping density, offering an alternate perspective on the optimization of CBL performance. Breakdown measurements show a peak 4 MV/cm field strength.

Investigation of high-performance Schottky diodes on a Ga2O3 epilayer using Cu with high barrier height, high temperature stability and repeatability

This work reports on high-performance Schottky barrier diodes (SBDs) fabricated on Ga2O3 epilayers using Cu as Schottky contacts (SCs). The fabricated SBDs exhibited high Schottky barrier heights (SBHs) with values greater than 1.0 eV, near-unity ideality factors, and a high rectification ratio (RR) of 1012 at 300 K. Temperature-dependent current–voltage (I–V–T) and capacitance–voltage (C–V–T) measurements were performed up to 500 K. The SBHs’ , calculated from the IVT characteristics, initially increased with temperature and then decreased with near-unity ideality factors. The increase in with temperatures up to 410 K indicated the spatial inhomogeneity at the metal–semiconductor interface. The observed decrease in above 410 K indicated the enhancement of barrier homogeneity at higher temperatures (⩾410 K). The decrease in above 410 K and decrease in , calculated from the CVT characteristics, with increasing temperature were assigned to the bandgap narrowing of Ga2O3. The IVT measurements were repeated many times, and the SBDs exhibited excellent thermal stability and showed high SBHs >1.0 eV, a high RR, and near-unity ideality factors. All these findings were attributed to the formation of high-work-function copper oxide thin film at the interface between Cu and Ga2O3. X-ray diffraction and x-ray photoelectron spectroscopy revealed the formation of a thin layer of high-work-function copper oxide. These findings provide the possibility to fabricate an SC with a high and homogeneous barrier and thermal stability using cheap, abundant Cu material, enabling low-cost mass production of future power semiconductor devices based on oxide semiconductors.

Interface-Engineered Organic Near-Infrared Photodetector for Imaging Applications.

We report a high-speed low dark current near-infrared (NIR) organic photodetector (OPD) on a silicon substrate with amorphous indium gallium zinc oxide (a-IGZO) as the electron transport layer (ETL). In-depth understanding of the origin of dark current is obtained using an elaborate set of characterization techniques, including temperature-dependent current-voltage measurements, current-based deep-level transient spectroscopy (Q-DLTS), and transient photovoltage decay measurements. These characterization results are complemented by energy band structures deduced from ultraviolet photoelectron spectroscopy. The presence of trap states and a strong dependency of activation energy on the applied reverse bias voltage point to a dark current mechanism based on trap-assisted field-enhanced thermal emission (Poole-Frenkel emission). We significantly reduce this emission by introducing a thin interfacial layer between the donor: acceptor blend and the a-IGZO ETL and obtain a dark current as low as 125 pA/cm2 at an applied reverse bias of -1 V. Thanks to the use of high-mobility metal-oxide transport layers, a fast photo response time of 639 ns (rise) and 1497 ns (fall) is achieved, which, to the best of our knowledge, is among the fastest reported for NIR OPDs. Finally, we present an imager integrating the NIR OPD on a complementary metal oxide semiconductor read-out circuit, demonstrating the significance of the improved dark current characteristics in capturing high-quality sample images with this technology.

Optimization of Ohmic Contact to Ultrathin-Barrier AlGaN/GaN Heterostructure via an ‘Ohmic-Before-Passivation’ Process

Non-recessed ohmic contact resistance (Rc) on ultrathin-barrier (UTB) AlGaN(<6 nm)/GaN heterostructure was effectively reduced to a low value of 0.16 Ω·mm. The method called the ‘ohmic-before-passivation’ process was adopted to eliminate the effects of fluorine plasma etching, in which an alloyed Ti/Al/Ni/Au ohmic metal stack was formed prior to passivation. The recovery of 2-D Electron Gas (2DEG) adjacent to the ohmic contact was enhanced by composite double-layer dielectric with AlN/SiNx passivation. It is found that the separation between the recovered 2DEG and the ohmic contacting edge can be remarkably reduced, contributing to a reduced transfer length (LT) and low Rc, as compared to that of ohmic contact to the AlGaN(~20 nm)/GaN heterostructure with a pre-ohmic recess process. Thermionic field emission is verified to be the dominant ohmic contact mechanism by temperature-dependent current-voltage measurements. The low on-resistance of 3.9 Ω·mm and the maximum current density of 750 mA/mm with Vg = 3 V were achieved on the devices with the optimized ohmic contact. The non-recessed ohmic contact with the ‘ohmic-before-passivation’ process is a promising strategy to optimize the performance of low-voltage GaN-based power devices.

Highly Sensitive MoS2 Photodetectors Enabled with a Dry-Transferred Transparent Carbon Nanotube Electrode.

Fabricating electronic and optoelectronic devices by transferring pre-deposited metal electrodes has attracted considerable attention, owing to the improved device performance. However, the pre-deposited metal electrode typically involves complex fabrication procedures. Here, we introduce our facile electrode fabrication process which is free of lithography, lift-off, and reactive ion etching by directly press-transferring a single-walled carbon nanotube (SWCNT) film. We fabricated Schottky diodes for photodetector applications using dry-transferred SWCNT films as the transparent electrode to increase light absorption in photoactive MoS2 channels. The MoS2 flake vertically stacked with an SWCNT electrode can exhibit excellent photodetection performance with a responsivity of ∼2.01 × 103 A/W and a detectivity of ∼3.2 × 1012 Jones. Additionally, we carried out temperature-dependent current-voltage measurement and Fowler-Nordheim (FN) plot analysis to explore the dominant charge transport mechanism. The enhanced photodetection in the vertical configuration is found to be attributed to the FN tunneling and internal photoemission of charge carriers excited from indium tin oxide across the MoS2 layer. Our study provides a novel concept of using a photoactive MoS2 layer as a tunneling layer itself with a dry-transferred transparent SWCNT electrode for high-performance and energy-efficient optoelectronic devices.

Conductance anisotropy of MOCVD-grown α-Ga2O3 films caused by (010) β-Ga2O3 filament-shaped inclusions

A significant anisotropy in the electrical conductance was observed in Si-doped α-Ga2O3/sapphire samples grown via liquid-injection metal organic chemical vapor deposition. Additionally to epitaxial α-Ga2O3 diffraction, x-ray diffraction (XRD) revealed maxima related to (010) β-Ga2O3. Transmission electron microscopy, atomic force microscopy (AFM), and Raman spectroscopy of undoped samples with similar XRD pattern confirmed the formation of (010) β-Ga2O3 filaments in the epitaxial α-Ga2O3 film. In Si-doped samples, conductive AFM and temperature-dependent current–voltage measurement on transmission line measurement resistors confirmed, that β-Ga2O3 filaments were responsible for significantly higher conductivity along the α-Ga2O3 direction compared to [0001] α-Ga2O3 direction. Current–voltage measurements of transistor structures suggested semiconducting behavior of the β-Ga2O3 filaments embedded in the highly resistive α-Ga2O3 films.

Charge Transport Across Au-P3HT-Graphene van der Waals Vertical Heterostructures.

Hybrid van der Waals heterostructures based on 2D materials and/or organic thin films are being evaluated as potential functional devices for a variety of applications. In this context, the graphene/organic semiconductor (Gr/OSC) heterostructure could represent the core element to build future vertical organic transistors based on two back-to-back Gr/OSC diodes sharing a common graphene sheet, which functions as the base electrode. However, the assessment of the Gr/OSC potential still requires a deeper understanding of the charge carrier transport across the interface as well as the development of wafer-scale fabrication methods. This work investigates the charge injection and transport across Au/OSC/Gr vertical heterostructures, focusing on poly(3-hexylthiophen-2,5-diyl) as the OSC, where the PMMA-free graphene layer functions as the top electrode. The structures are fabricated using a combination of processes widely exploited in semiconductor manufacturing and therefore are suited for industrial upscaling. Temperature-dependent current–voltage measurements and impedance spectroscopy show that the charge transport across both device interfaces is injection-limited by thermionic emission at high bias, while it is space charge limited at low bias, and that the P3HT can be assumed fully depleted in the high bias regime. From the space charge limited model, the out-of-plane charge carrier mobility in P3HT is found to be equal to μ ≈ 2.8 × 10–4 cm2 V–1 s–1, similar to the in-plane mobility reported in previous works, while the charge carrier density is N0 ≈ 1.16 × 1015 cm–3, also in agreement with previously reported values. From the thermionic emission model, the energy barriers at the Gr/P3HT and Au/P3HT interfaces result in 0.30 eV and 0.25 eV, respectively. Based on the measured barriers heights, the energy band diagram of the vertical heterostructure is proposed under the hypothesis that P3HT is fully depleted.

Electrical properties of Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT heterostructures

In this study, we analyze gold-free Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT (high electron mobility transistor) heterostructures. The silicide layers are fabricated through magnetron sputtering deposition using a stoichiometric Pd2Si target and are subjected to post-gate annealing at different temperatures. After annealing at 500 °C the leakage currents of the planar diodes are reduced below 3.2 mA/cm2. The microstructures of the diodes are analyzed through transmission electron microscopy (TEM). The post-gate annealing increases lateral dimensions of silicide grains. The electrical properties of the contacts are determined by performing temperature-dependent current-voltage (I-V-T) measurements. The forward current exhibits a specific voltage dependence, which is interpreted based on the two-diode model, reflecting the properties of the Pd2Si/heterostructure interface along with those of the heterostructure itself. The model is fitted to extract the parameters of the Schottky contacts. The barrier height of 0.91 eV and ideality factor n = 1.5 are obtained after post-gate annealing at 500 °C. The effects of post-gate annealing on the forward conduction and leakage mechanisms are then discussed. A simple method is proposed to estimate the polarization charge density, which is required to calculate the electric fields in the heterostructure in the reverse bias. It is observed that post-gate annealing affects the I-V characteristics, likely by reducing the density of the unintentional donors and trap states, leading to an increase in the apparent barrier height along with a reduction in the density of defects contributing to the leakage current.

Monolithic integration of a 10 μm cut-off wavelength InAs/GaSb type-II superlattice diode on GaAs platform

At room temperature, a 10 µm cut-off wavelength coincides with an infrared spectral window and the peak emission of blackbody objects. We report a 10 µm cut-off wavelength InAs/GaSb T2SL p-i-n diode on a GaAs substrate with an intentional interfacial misfit (IMF) array between the GaSb buffer layer and GaAs substrate. Transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed that the heterostructure on GaSb-on-GaAs is epitaxial, single-crystalline but with a reduced material homogeneity, extended lattice defects and atomic segregation/intermixing in comparison to that on the GaSb substrate. Strain-induced degradation of the material quality is observed by temperature-dependent current–voltage measurements. The T2SL with the IMF array appears as a potentially effective route to mitigate the impact of the lattice mismatch once its fabrication is fully optimized for these systems, but additional strain compensating measures can enable a low cost, scalable manufacturing of focal plane arrays (FPA) for thermal imaging cameras for spectroscopy, dynamic scene projection, thermometry, and remote gas sensing.

DX center formation in highly Si doped AlN nanowires revealed by trap assisted space-charge limited current

Electrical properties of silicon doped AlN nanowires grown by plasma assisted molecular beam epitaxy were investigated by means of temperature dependent current–voltage measurements. Following an Ohmic regime for bias lower than 0.1 V, a transition to a space-charge limited regime occurred for higher bias. This transition appears to change with the doping level and is studied within the framework of the simplified theory of space-charge limited current assisted by traps. For the least doped samples, a single, doping independent trapping behavior is observed. For the most doped samples, an electron trap with an energy level around 150 meV below the conduction band is identified. The density of these traps increases with a Si doping level, consistent with a self-compensation mechanism reported in the literature. The results are in accordance with the presence of Si atoms that have three different configurations: one shallow state and two DX centers.

Shallow donor and DX state in Si doped AlN nanowires grown by molecular beam epitaxy

Si doping of AlN nanowires (NWs) grown by plasma assisted molecular beam epitaxy was investigated with the objective of fabricating efficient AlN based deep ultra-violet light-emitting-diodes. The Si concentration ranged from 1016 to 1.8 × 1021 cm−3. Current–voltage measurements performed on nanowire ensembles revealed an Ohmic regime at low bias (below 0.1 V) and a space charge limited regime for higher bias. From temperature dependent current–voltage measurements, the presence of Si donors is evidenced in both shallow and deep DX states with an ionization energy of 75 and 270 meV, respectively. The role of Fermi level pinning on NWs sidewalls is discussed in terms of near surface depletion, inducing a favorable formation of shallow Si donors.

Influence of current conduction paths and native defects on gas sensing properties of polar and non-polar GaN

The CO gas sensing characteristics of polar GaN (P–GaN) and non-polar GaN (NP–GaN) thin-films grown by RF–plasma assisted molecular beam epitaxy on c–Al2O3 and r–Al2O3 substrate is analyzed. The temperature-dependent (27–300 °C) current-voltage (I–V) measurements were performed for both (P–GaN & NP–GaN) Schottky barrier diodes having identical device dimensions with Au as the metal contact. The Schottky barrier height increases with the increment in temperature, while vice versa was perceived for the ideality factor and series resistance. The I–V characteristics dictated that the terminal current for P–GaN is 25 times that of NP-GaN, further corroborated by simulation results. The I-V curve fitting suggests the initial emission of charge carrier from a trapped state to a continuum of electronic state, following the Frenkel-Poole emission model for P-GaN. The NP–GaN demonstrated a higher surface-to-volume ratio and native (shallow and deep level) defects corresponding to VGa and ON as compared to the P–GaN. The sensing response obtained for the NP–GaN for 100 ppm CO gas at 300 °C is ~ 33%, which is about seven times the response in the case of P–GaN. The role of native defects and the potentiality of the fabricated NP–GaN over P–GaN films in providing a better sensing characteristic by reducing the current conduction paths are elaborated.

Near surface defects: Cause of deficit between internal and external open‐circuit voltage in solar cells

AbstractInterface recombination in a complex multilayered thin‐film solar structure causes a disparity between the internal open‐circuit voltage (VOC,in), measured by photoluminescence, and the external open‐circuit voltage (VOC,ex), that is, a VOC deficit. Aspirations to reach higher VOC,ex values require a comprehensive knowledge of the connection between VOC deficit and interface recombination. Here, a near‐surface defect model is developed for copper indium di‐selenide solar cells grown under Cu‐excess conditions. These cell show the typical signatures of interface recombination: a strong disparity between VOC,in and VOC,ex, and extrapolation of the temperature dependent q·VOC,ex to a value below the bandgap energy. Yet, these cells do not suffer from reduced interface bandgap or from Fermi‐level pinning. The model presented is based on experimental analysis of admittance and deep‐level transient spectroscopy, which show the signature of an acceptor defect. Numerical simulations using the near‐surface defects model show the signatures of interface recombination without the need for a reduced interface bandgap or Fermi‐level pinning. These findings demonstrate that the VOC,in measurements alone can be inconclusive and might conceal the information on interface recombination pathways, establishing the need for complementary techniques like temperature dependent current–voltage measurements to identify the cause of interface recombination in the devices.

Analysis of Current Transport Mechanism in AP-MOVPE Grown GaAsN p-i-n Solar Cell

Basic knowledge about the factors and mechanisms affecting the performance of solar cells and their identification is essential when thinking of future improvements to the device. Within this paper, we investigated the current transport mechanism in GaAsN p-i-n solar cells grown with atmospheric pressure metal organic vapour phase epitaxy (AP-MOVPE). We examined the electro-optical and structural properties of a GaAsN solar cell epitaxial structure and correlated the results with temperature-dependent current-voltage measurements and deep level transient spectroscopy findings. The analysis of J-V-T measurements carried out in a wide temperature range allows for the determination of the dominant current transport mechanism in a GaAsN-based solar cell device and assign it a nitrogen interstitial defect, the presence of which was confirmed by DLTFS investigation.

Electrical properties of a single Ni-contact SnO2 nanowire field-effect transistors

Single crystalline SnO2 nanowires were grown using chemical vapor deposition (CVD) method and fabricated into a single-nanowire field-effect transistor (FET) with Ni contact electrodes by photolithography. Investigations show good field-effect electrical performance was achieved using the Ni-contact single-SnO2 nanowire device. Field effect mobility, on/off ratio and carrier species were 73.3 ± 4.17 cm2/V-s (for 10 devices), ~106, and electrons, respectively. Furthermore, a low barrier height of 45.6 meV between the Ni contact and as-grown SnO2 nanowire was achieved based on temperature dependent current-voltage measurement. It is thought this low barrier height improved field-effect transistor performance making SnO2 nanomaterial devices practicable.

Thermophysical Characterization of Efficiency Droop in GaN-Based Light-Emitting Diodes.

An efficiency droop in GaN-based light-emitting diodes (LED) was characterized by examining its general thermophysical parameters. An effective suppression of emission degradation afforded by the introduction of InGaN/GaN heterobarrier structures in the active region was attributable to an increase in the capture cross-section ratios. The Debye temperatures and the electron–phonon interaction coupling coefficients were obtained from temperature-dependent current-voltage measurements of InGaN/GaN multiple-quantum-well LEDs over a temperature range from 20 to 300 K. It was found that the Debye temperature of the LEDs was modulated by the InN molar fraction in the heterobarriers. As far as the phonons involved in the electron–phonon scattering process are concerned, the average number of phonons decreases with the Debye temperature, and the electron–phonon interaction coupling coefficients phenomenologically reflect the nonradiative transition rates. We can use the characteristic ratio of the Debye temperature to the coupling coefficient (DCR) to assess the efficiency droop phenomenon. Our investigation showed that DCR is correlated to quantum efficiency (QE). The light emission results exhibited the high and low QEs to be represented by the high and low DCRs associated with low and high injection currents, respectively. The DCR can be envisioned as a thermophysical marker of LED performance, not only for efficiency droop characterization but also for heterodevice structure optimization.

Charge Transport in and Electroluminescence from sp3-Functionalized Carbon Nanotube Networks.

The controlled covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp3 defects leads to additional narrow and tunable photoluminescence features in the near-infrared and even enables single-photon emission at room temperature, thus strongly expanding their application potential. However, the successful integration of sp3-functionalized SWCNTs in optoelectronic devices with efficient defect state electroluminescence not only requires control over their emission properties but also a detailed understanding of the impact of functionalization on their electrical performance, especially in dense networks. Here, we demonstrate ambipolar, light-emitting field-effect transistors based on networks of pristine and functionalized polymer-sorted (6,5) SWCNTs. We investigate the influence of sp3 defects on charge transport by employing electroluminescence and (charge-modulated) photoluminescence spectroscopy combined with temperature-dependent current–voltage measurements. We find that sp3-functionalized SWCNTs actively participate in charge transport within the network as mobile carriers efficiently sample the sp3 defects, which act as shallow trap states. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states that can be tuned by the defect density.

Carrier recombination mechanism and photovoltage deficit in 1.7-eV band gap near-stoichiometric Cu(In,Ga) S2

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_{2}$ is a promising semiconductor that offers excellent prospects for photovoltaics. However, the performance has been plagued mostly due to large photovoltage deficit. Here, we investigate defects and optoelectronic properties of 1.7-eV band gap near-stoichiometric $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_{2}$ (CIGS). We have estimated quasi-Fermi-level splitting of 921 meV from steady state photoluminescence (PL) measurements at 1 sun. Detailed analysis of temperature and excitation dependent PL reveals the behavior of a strongly compensated semiconductor. We show that spatially varying energetic disorder, described by electrostatic potential fluctuations, causes band-tail recombination and strongly affects the carrier recombination in compensated CIGS. Apart from band-tail transitions, we also observe two deep defects at about 0.3 and 0.45 eV below the band edge. The defects are also discerned by admittance measurements. Temperature dependent current-voltage measurements show that the open-circuit voltage is further limited by interface recombination. Thus, the performance improvement lies in the mitigation of deep defect states and absorber/buffer interface optimization.