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We report a novel Sc/Al/Ni/Au metal scheme for Ohmic contacts to InAlN/GaN HEMT structures on silicon. A contact resistance of $0.39~\Omega $ -mm with a low surface roughness of 20± 3 nm of the annealed contact has been achieved using this metal scheme. The microstructure of the region under the contacts revealed the formation of ~60 nm deep ScGaN inclusions in the GaN channel layer. A thin (~3-5 nm) non-uniform layer of ScInAlN is observed on top of InAlN barrier. Field-emission is found to be the dominant conduction mechanism. Polarization mismatch arising due to the structural modifications is used to explain the possible mechanism related to Ohmic contact formation.

We demonstrate the influence of surface terminated V-pits in tuning dark current and spectral responsivity of Al0.25Ga0.75N-based UV-B photodetectors with metal–semiconductor–metal geometry on Si (111) substrate. We show that the V-pit morphological defects contribute to a large internal gain in these photodetectors, thereby leading to a substantial enhancement in external quantum efficiency (EQE) at relatively low applied biases. For photodetectors fabricated on metal organic chemical vapor deposition grown Al0.25Ga0.75N epilayers with a surface pit density of $2 \times 10^{8}$ cm−2, an EQE of 100% was measured at a meager bias of 1.7 V, which increased significantly with bias. The EQE, photo-to-dark current ratio, and UV-to-visible rejection ratio measured $5 \times 10^{4}$ %, $1.2 \times 10^{4}$ , and $2 \times 10^{3}$ , respectively, at 5 V. The evidence of localized enhancement of photoresponse at the surface terminations of V-pits is exemplified by UV-assisted conductive atomic force microscopy. Temperature-dependent carrier transport analysis under dark and UV illumination revealed cumulative contributions of pit-induced thermionic field emission and hole-trapping-induced gain to the observed large EQE. This work presents the highest value of responsivity for III-nitride UV-B detectors at a given bias.

This article reports on the experimental and analytical determination of the optimum carbon concentration in GaN to achieve enhanced breakdown in AlGaN/GaN high-electron mobility transistors (HEMTs). The lateral breakdown voltage increases when carbon doping is increased from $3\times 10^{{18}}$ to 1019 cm−3 beyond which it decreases, whereas there is no substantial enhancement in the vertical breakdown voltage with carbon doping. We invoke carrier statistics in a compensated semiconductor vis-a-vis the formation energy of carbon-occupying Ga (or N) vacancies to explain the observed buffer leakage. Temperature-dependent data indicate that the buffer leakage current is due to hopping transport, the activation energy of which yields the positions of the defect states within the bandgap. The increase in buffer leakage beyond optimum C concentration is attributed to the formation of shallow donor traps by carbon atoms occupying Ga vacancies (CGa). The observations correlated with the relative intensities of the defect-mediated peaks in the cathodoluminescence (CL) data of the samples. Based on our findings, a C doping beyond 1019 cm−3 is not recommended for GaN buffers in order to achieve high breakdown voltages.

We report on the demonstration and investigation of Ta2O5 as high-k dielectric for InAlN/GaN-MOS high-electron mobility transistor (HEMT)-on-Si. Ta2O5 of thickness 24 nm and dielectric constant ~30 was sputter deposited on InAlN/GaN HEMT and was investigated for different post deposition anneal (PDA) conditions. The gate leakage was 16 nA/mm at −15 V which was ~five orders of magnitude lower compared to reference HEMT. The 2-D electron gas (2-DEG) density was found to vary with annealing temperature suggesting the presence of net charge at the Ta2O5/InAlN interface. Dispersion in the capacitance–voltage ( ${C}$ – ${V}$ ) characteristics was used to estimate the frequency-dependent interface charge, while energy band diagrams under flat band conditions were investigated to estimate fixed charge. The optimum anneal condition was found to be 500 °C which has resulted into a flat band voltage spread ( $\Delta {V}_{\text {FB}}$ ) of 0.4 V and interface fix charge ( ${Q} _{\text {f}}$ ) of ${3.98} \times {10}^{{13}}$ cm $^{-2}$ . X-ray photoelectron spectroscopy spectra of as-deposited and annealed Ta2O5film were analyzed for Ta and O compositions in the film. The sample annealed at 500 °C has shown Ta:O ratio of 0.41. X-ray diffraction analysis was done to check the crystallization of amorphous Ta2O5 film at higher annealing temperatures.

In this letter, we demonstrate high-performance vertical solar-blind Schottky photodetectors on MBE-grown homoepitaxial (010)-oriented $\beta $ -Ga2O3 films. The structure, consisting of (100 nm) $\beta $ -Ga2O3/(60 nm) $\text{n}^{++} \beta $ -Ga2O3, was grown on a Fe-doped insulating (010) $\beta $ -Ga2O3 substrate. Ni/Au and indium were used as the Schottky and Ohmic contacts, respectively. The devices exhibited a rectification ratio of ~107 with turn-on voltage ~1 V and an ideality factor of 1.31. The extracted Schottky barrier height was 1.4 eV. The photodetectors showed low dark current of 0.3 nA at 5 V with a photo-to-dark current ratio of ~102 at 0 V. The devices exhibited a zero-bias responsivity of 4 mA/W at 254 nm corresponding to an external quantum efficiency ~3 %, with a UV-to-visible rejection ratio >103, showing true solar-blind operation. The transient response of the devices indicated rise/fall times of ~100 ms. Temperature-dependent current–voltage characteristics agree with the thermionic emission model.

We report on the band offset and interfacial electronic properties of e-beam evaporated HfO2 gate dielectrics on III-nitride device stacks on Si. A conduction band offset of 1.9 eV is extracted for HfO2/GaN along with a very low density of fixed bulk and interfacial charges for optimally annealed samples. Normally-ON HfO2/AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors exhibit negligible shifts in threshold voltage, transconductances of 120 mS/mm for 3- $\mu \text{m}$ gate length devices, and three-terminal off-state gate leakage currents of 55 nA/mm at a ${V}_{D}$ of 100 V. Dynamic capacitance dispersion measurements in the temperature range of 25 °C–200 °C show two peaks at the AlGaN/GaN interface corresponding to slow and fast interface traps with a peak ${D}_{\text {it}}$ of ${5.5}\times {10}^{13}$ and ${1.5} \times {10}^{13}$ eV−1cm−2 respectively as a function of Fermi level position above ${E}_{C}$ . The HfO2/AlGaN interface exhibits a reasonably constant peak ${D}_{\text {it}}$ of ${2} \times {10}^{13}$ – ${4.4} \times {10}^{13}$ eV−1cm−2 at trap levels of 0.42–0.72 eV below ${E}_{\text {C}}$ . Hysteretic pulsed $I_{D}$ – $V_{G}$ measurements revealed a negative shift in threshold voltages indicative of unoccupied donor-like trap states at the HfO2/AlGaN interface and comparable ${D}_{\text {it}}$ to that inferred from conductance measurements.

We report on a billion-fold reduction in reverse-bias leakage current density (11.3 A/cm(2) to 8.5 nA/cm(2 )at 20 V) across Schottky contacts made to Al0.50Ga0.50N epilayers grown on sapphire. Interdigitated back-to-back Ni/Au Schottky contacts were made to unintentionally doped Al0.50Ga0.50N epilayers grown by metal organic chemical vapor deposition to realize photodetectors with metal-semiconductor-metal geometry. Testing on a self-con-sistent series of samples grown under different conditions revealed that a two-order reduction in screw dislocation density is primarily responsible for the significant reduction in the lateral leakage (dark) current. This observation is validated by conducive atomic force microscopy experiments. Analytical modelling by temperature-dependent current-voltage measurements confirm a screw dislocation mediated carrier transport mechanism. The anomalously high reverse-bias leakage current (mu A-mA) in the highly defective samples is found to be dominated by thermionic field emission (TFE) at low biases and Poole-Frenkel emission (PFE) from a deep donor level at high biases. With a significant improvement in the crystalline quality, a solar-blind photodetector with an EQE of 47%, photo/dark current ratio of 1.4 x 10(5) and transient characteristics of < 20 ms is demonstrated. This study towards understanding the leakage in Al0.50Ga0.50N is expected to benefit the development of various deep-UV devices and GaN-AlGaN based power transistors.

We report on the estimation of trap capture cross section in AlGaN/GaN HEMTs as a function of bias and temperature. Conductance dispersion technique was employed to study the AlGaN/GaN interface of the devices with a carbon-doped GaN buffer grown on 6-in silicon. While a negligible shift in the threshold voltage ( $V_{\mathrm {{\textsf {TH}}}}$ ) was observed in temperature-dependent $I_{\mathrm {{\textsf {DS}}}}$ – $V_{\mathrm {{\textsf {GS}}}}$ sweeps, we observed a spread in the capacitance-voltage ( $C$ – $V$ ) measurements, indicating a contribution of interface traps. When biased near depletion, G/ $\omega $ versus frequency plot for AlGaN/GaN interface exhibits two peaks which correspond to a pair of trap density ( $D_{\mathrm {{\textsf {it}}}}$ ) and trap time constant ( $T_{\mathrm {{\textsf {it}}}}$ ) values. This was explained using a circuit model in conjunction with energy band diagram. The $D_{\mathrm {{\textsf {it}}}}$ and $T_{\mathrm {{\textsf {it}}}}$ values for one peak were in the range from ~ 0.3– $7\times 10^{12}$ /eV $\cdot $ cm2 and 0.6– $10~\mu \text{s}$ while for the other peak, $D_{\mathrm {{\textsf {it}}}}$ – $T_{\mathrm {{\textsf {it}}}}$ were in the range of ~0.1– $35\times 10^{12}$ /eV $\cdot $ cm2 and ~0.06– $0.3~\mu \text{s}$ at 25 °C. From the $T_{\mathrm {{\textsf {it}}}}$ values, electron capture cross section ( $\sigma )$ for both the traps was extracted and was found to be decreasing with increasing temperature in the range of $1.1\times 10^{-20}$ – $1\times 10^{-19}$ cm $^{\mathrm {{\textsf {2}}}}$ and $4.5\times 10^{-20}$ – $1\times 10^{-17}$ cm2 for slow traps and fast traps, respectively. A multiphonon emission effect was invoked to explain the temperature dependence of capture cross section.

We report on the characterization of the interfaces of Al2O3/InAlN/GaN HEMT structure grown on 200 mm diameter silicon using conductance dispersion technique. Irreversible threshold voltage (VTH) shift of up to +similar to 2.5 V was observed due to the gate stress induced activation of acceptor states. Further, frequency dependent VTH shift during capacitance voltage measurements were also recorded due to the presence of slow traps at InAlN/GaN interface. The conductance dispersion indicated the presence of acceptor traps of the order of similar to 4 x 10(12) to 7 x 10(13) cm(-2) eV(-1) with a time constant of similar to 10 to 350 ls at the InAlN/GaN interface. Trap density at the Al2O3/InAlN was found to be in similar range but with a time constant of similar to 2 ms. The presence of high density of traps at InAlN/GaN interface is attributed to the unavoidable growth interruption before the start of InAlN growth. (C) 2017 Elsevier Ltd. All rights reserved.

In this report, we demonstrate dual band vertical heterojunction photodetector realized by integrating α-In2Se3 with p-type GaN. Flakes of ∼ 110 nm thickness were exfoliated on MOCVD grown p-GaN on silicon substrate. Devices showed two distinct detection peaks in spectral responsivity, one at 365 nm and another at 850 nm, corresponding to band edges of GaN and α-In2Se3 respectively, with considerable rejection in visible spectrum. Normalised responsivity values were found out to be ∼70 mA/W at both 365 nm and 850 nm for the bias of -3V along with photo-to-dark current ratio of ∼665 and ∼75 in that order. The Devices also showed fast transient response with no persistent photoconductivity (PPC). The specific detectivity values estimated were ∼1011 Jones and ∼1010 Jones corresponding to illumination at 365 nm and 850 nm, respectively. Device showed ∼0.4 dependence on incident power at 365 nm. The device performance, post annealing was also studied. This study is expected to pave the way for new type of optoelectronic devices by integrating direct bandgap layered material like α-In2Se3 and wide bandgap semiconductors.

A new method for the separation of contact resistance ( $R_{\mathrm {contact}})$ into Schottky barrier resistance ( $R_{\mathrm {SB}})$ and interlayer resistance ( $R_{\mathrm {IL}})$ is proposed for multilayered MoS2 FETs. While $R_{\mathrm {SB}}$ varies exponentially with Schottky barrier height ( $\Phi _{\mathrm {bn}})$ , $R_{\mathrm {IL}}$ essentially remains unchanged. An empirical model utilizing this dependence of $R_{\mathrm {contact}}$ versus $\Phi _{\mathrm {bn}}$ is proposed and fits to the experimental data. The results, on comparison with the existing reports of lowest $R_{\mathrm {contact}}$ , suggest that the extracted $R_{\mathrm {IL}}$ (1.53 $\text{k}\Omega ~\cdot ~\mu \text{m}$ ) for an unaltered channel would determine the lower limit of intrinsic $R_{\mathrm {contact}}$ even for barrierless contacts for multilayered exfoliated MoS2 FETs.

We report on the demonstration of (InxGa1-x)2O3 (InGaO)-based UV photodetectors realized using a low temperature (∼200 °C) microwave irradiation-assisted deposition technique. By irradiating a solution of the substituted acetylacetonate (acac) complex, namely In0.6Ga0.4(acac)3, employed as the “single-source precursor”, InGaO film was deposited on sapphire substrate, and found to be poly(nano)crystalline, with root mean square (r.m.s). roughness of 8.9 nm. However, the indium content of the film (0.26 mol fraction) was considerably less than in the metal complex (0.6 mol fraction). The optical band gap of the film was found to be 4.5 eV from Tauc’s plot, indicative of a low indium mole fraction. This was confirmed using X-ray photoelectron spectroscopy measurements, from which the indium mole fraction was found to be 0.26. Further, the nature of band gap was determined and defect analysis was carried out using, respectively, Tauc’s plot and cathodoluminescence (CL) measurements. A planar, interdigitated metal-semiconductor-metal (MSM) photodetector fabricated with the InGaO film exhibited a high responsivity of 16.9 A/W at a bias of 20 V, corresponding to a band edge at ∼ 276 nm, with a high photo-to-dark current ratio of ∼105.

Limiting buffer layer current leakage is essential for the realization of high breakdown fields in GaN-on-Si high electron mobility transistors (HEMTs). In this report, we demonstrate the importance of controlling the surface morphology of the AlN nucleation layer (NL) in limiting this leakage. Testing on a self-consistent series of samples grown under two different AlN NL conditions revealed the presence of leakage paths within the epilayers grown using a single temperature AlN NL owing to the presence of surface pits. The introduction of a higher temperature AlN in the NL drastically reduced the pit density and led to a large reduction (>103) in the lateral and vertical buffer leakage in HEMT structures. Using conductive atomic force microscopy, secondary ion mass spectroscopy, and temperature-dependent carrier transport measurements, we confirm that these pits—which originate in the AlN NL, thread vertically, and propagate into the device structures—are associated with leakage paths, thus reducing the field that can be dropped across the epilayers. This is explained by invoking preferential oxygen segregation at their side-facets. It is shown that when a pit-free surface is maintained, a vertical field of 1.6 MV/cm can be achieved for HEMTs. This study is expected to benefit the development of high-performance GaN HEMTs in moving toward the theoretical breakdown field of III-nitrides.

Herein, device demonstration based on vertical transport in multilayer I±-In2Se3 is reported. Photodetectors realized using a metal/I±-In2Se3/indium tin oxide (ITO) vertical junction exhibit clear signature of the band edge in spectral responsivity. The wavelength at 680 nm corresponding to an ultrahigh responsivity of 1000 A Wâ��1 and a detectivity of >1013 cm Hz0.5 Wâ��1 at a bias of 0.5 V. The variation of responsivity and detectivity with optical power density is studied, and a transient response of 20 ms is obtained for the devices (instrument limitation). In addition, an asymmetric barrier height arising out of ITO and Au contacts to a vertical I±-In2Se3 junction resulted in a photovoltaic effect with VOC â��0.1 V and ISC â��0.4 I¼A under an illumination of 520 nm.

In this letter, we report on a unique device design strategy for increasing the breakdown voltage and hence Baliga Figure of Merit (BFOM) of III-nitride HEMTs by engineering the gate edge towards the drain. The breakdown of such devices with meandering gate-drain access region (M-HEMT) are found to be 62% more compared to that of conventional HEMT while the ON resistance suffers by 76%, leading to an overall improvement in the BFOM for by 28%. 3D-TCAD simulations show that the decrease in the peak electric field at the gate edge was responsible for increased breakdown voltage.

A buried channel normally-off AlGaN/GaN MOS-HEMT with gate recess etching is reported. The buried channel operation is achieved by in situ doping of GaN to introduce a p-n junction in the GaN buffer. The fabricated buried channel MOS-HEMT with 12.5 nm atomic layer deposition (ALD) Al2O3 gate dielectric featured a threshold voltage of 1.3 V with a drain saturation current of 287 mA mm(-1) for a device with 3.5 mu m long gate length and 11 mu m sourcedrain spacing. The field effect mobility improved from 25 cm(2)/Vs for a reference device to 142 cm(2)/Vs for the buried channel device. Due to the presence of the p-n junction depletion region in the GaN buffer, the leakage in the off state decreased by about 4 orders of magnitude (4 nA mm(-1) compared to 76 uA mm(-1) for the reference device). The buried channel device also gives better breakdown characteristics, with a breakdown voltage of 158 V compared to 98 V for the reference device.

Leakage mediated by GaN buffer traps is identified and studied using a novel characterization technique. Through back-gating measurement, the effect of buffer trap states on the lateral leakage is determined by probing mesa-isolated Ohmic pads. Time-dependent leakage measurements are carried out to study the extent of the increase in buffer leakage due to the traps. It is observed that the mesa leakage is more prominent at very slow sweep rates and high substrate bias. The temperature-dependent measurements show that the mesa leakage and the substrate leakage are characterized by thermionic emission from the traps with an activation barrier of 0.34 and 0.2 eV, respectively.

Low cost high-power electronics era has begun with the fabrication of Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMTs) on Silicon substrates. Silicon is however not an optimum material from the viewpoint of the final substrate. Having GaN based devices on other substrates would enable better performing RF and high-power devices. Here we report the process development of the transfer technology, of a fabricated GaN HEMT transistors to a glass wafer using temporary bonding material. Epoxy has been used to finally bond the GaN stack and the glass wafer. Transfer and output characteristics of the HEMT and the capacitance measurements of the capacitors were performed on the identical devices before and after the transfer process. Also, to understand the stress before and after the transfer process Raman measurements were carried out.

We report on the deposition of gallium oxide using microwave irradiation technique on III nitride epi layers. We also report on the first demonstration of a gallium oxide device, a visible blind deep UV detector, with GaN based heterostructure as the substrate. The film deposited in the solution medium, at less than 200 C, using a metalorganic precursor, was nanocrystalline. XRD confirms that as deposited film when annealed at high temperature turns polycrystalline beta gallium oxide. SEM shows the as deposited film to be uniform, with a surface roughness of 4 to 5 nm, as revealed by AFM. Interdigitated metal semiconductor metal MSM devices with Ni,Au contact exhibited peak spectral response at 230 nm and a good visible rejection ratio. This first demonstration of a deep-UV detector on beta-gallium oxide on III nitride stack is expected to open up new possibilities of functional and physical integration of beta gallium oxide and GaN material families towards enabling next generation high performance devices by exciting band and heterostructure engineering., Main manuscript 5 pages, 4 figures Supplementary information 2 pages, 2 figures

We demonstrate an InAlN/GaN-on-Si HEMT based UV detector with photo to dark current ratio > 107. Ti/Al/Ni/Au metal stack was evaporated and rapid thermal annealed for Ohmic contacts to the 2D electron gas (2DEG) at the InAlN/GaN interface while the channel + barrier was recess etched to a depth of 20 nm to pinch-off the 2DEG between Source-Drain pads. Spectral responsivity (SR) of 34 A/W at 367 nm was measured at 5 V in conjunction with very high photo to dark current ratio of > 10^7. The photo to dark current ratio at a fixed bias was found to be decreasing with increase in recess length of the PD. The fabricated devices were found to exhibit a UV-to-visible rejection ratio of >103 with a low dark current < 32 pA at 5 V. Transient measurements showed rise and fall times in the range of 3-4 ms. The gain mechanism was investigated and carrier lifetimes were estimated which matched well with those reported elsewhere., Comment: 12 pages

This work reports the very first systematic study on the physics of avalanche instability and SOA concerns in AlGaN/GaN HEMT using sub-μs pulse characterization, post stress degradation analysis, well calibrated TCAD simulations and failure analysis by SEM and TEM. Impact of electrical, as well as thermal effects on SOA boundary and avalanche instability are investigated. Trap assisted cumulative nature of degradation is studied in detail, which was discovered to be the root cause for avalanche instability in AlGaN/GaN HEMTs. Post failure SEM/TEM analysis reveal distinct failure modes in presence and absence of carrier trapping.

In this report, we demonstrate high spectral responsivity (SR) in MBE grown epitaxial \b{eta}-Ga2O3-based solar blind MSM photodetectors (PD). (-2 0 1)-oriented \b{eta}-Ga2O3 thin film was grown by plasma-assisted MBE on c-plane sapphire substrates. MSM devices fabricated with Ni/Au contacts in an interdigitated geometry were found to exhibit peak SR > 1.5 A/W at 236-240 nm at a bias of 4 V with a UV to visible rejection ratio > 105. The devices exhibited very low dark current < 10 nA at 20 V and showed no persistent photoconductivity (PPC) as evident from the sharp transients with a photo-to-dark current ratio > 103. These results represent the state-of-art performance for MBE-grown \b{eta}-Ga2O3 MSM solar blind detector.

We report on the demonstration of UV/visible and UV/near-IR photodetectors of high spectral responsivity (SR) in a non-conventional heterojunction, realized by combining multi-layered materials with wide band gap Gallium Nitride (GaN). Multi-layer MoS2 and β-In2Se3 flakes were exfoliated separately on epitaxial GaN-on-sapphire, followed by fabrication of photodetectors in a lateral inter-digitated metal semiconductor metal geometry with Ti/Au contacts. Devices exhibited distinct steps in SR graph at 365 nm with responsivity value of 127 A W−1 and at ~685 nm with responsivity value of 33 A W−1 for MoS2/GaN heterostructure. Whereas, similar steps exhibited at 365 nm with responsivity value of 1.6 A W−1 and at ~850 nm with responsivity value of 0.03 A W−1 in case of β-In2Se3/GaN heterostructure. The wavelength dependent I–V characteristics showed photo-to-dark current ratio of ~30 at 685 nm in case of MoS2/GaN heterostructure and ratio of ~2 at 850 nm for β-In2Se3/GaN heterostructure. The reasons which limit the performance of the devices were also investigated using transient analysis and power dependent responsivity analysis. In summary, current work paves the way for futuristic layered-materials/3D heterostructure devices.

We demonstrate zero-bias spectral responsivity in MBE-grown β-Ga2O3 planar UV-C detector with good linearity up to optical power density of 4.6 mW cm−2. Devices with asymmetrical metal contacts were realized on 150 nm thick β-Ga2O3 films on sapphire. The device exhibited a spectral responsivity of 1.4 mA W−1 at 255 nm under zero-bias condition, dark current

We demonstrate epitaxial β-Ga2O3/GaN-based vertical metal–heterojunction-metal (MHM) broadband UV-A/UV-C photodetectors with high responsivity (3.7 A/W) at 256 and 365 nm, UV-to-visible rejection >103, and a photo-to-dark current ratio of ~100. A small (large) conduction (valence) band offset at the heterojunction of pulsed laser deposition (PLD)-grown β-Ga2O3 on metal organic chemical vapor deposition (MOCVD)-grown GaN-on-silicon with epitaxial registry, as confirmed by X-ray diffraction (XRD) azimuthal scanning, is exploited to realize detectors with an asymmetric photoresponse and is explained with one-dimensional (1D) band diagram simulations. The demonstrated novel vertical MHM detectors on silicon are fully scalable and promising for enabling focal plane arrays for broadband ultraviolet sensing.

In this paper, a epitaxial design of InAlN/AlN/InGaN/GaN high electron mobility transistors (HEMTs), considering the vital aspects related to its device exploration is proposed. Mobility of two dimensional electron gas in the proposed, practically viable structures of InxGa1-xN-channel (0.1 0.3.

Variability and lack of control in the nature of contacts between metal/MoS2 interface is a major bottleneck in the realisation of high-performance devices based on layered materials for several applications. In this letter, we report on the reduction in Schottky barrier height at metal/MoS2 interface by engineering the surface states through sulphur treatment. Electrical characteristics for back-gated MoS2 field effect transistor structures were investigated for two high work-function metal contacts Ni and Pd. Contacts on MoS2 treated with sulphur exhibited significant improvements in Ohmic nature with concomitant reduction in variability compared to those on untreated MoS2 films leading to a 2x increase in extracted mobility. X-ray Photoelectron Spectroscopy (XPS) measurements, Raman Spectroscopy and comparison of threshold voltages indicated absence of additional doping or structural changes due to sulphur treatment. The Schottky barrier heights were extracted from temperature-dependent transfer characteristics based on the thermionic current model. A reduction in barrier height of 80 and 135 meV extracted for Ni/MoS2 and Pd/MoS2 contacts respectively is hence attributed to the increase in surface states (or stronger Fermi level pinning) due to sulphur treatment. The corresponding charge neutrality levels at metal/MoS2 interface, were extracted to be 0.16 eV (0.17 eV) below the conduction band before (after) Sulphur treatment. This first report of surface states engineering in MoS2 leading to superior contacts is expected to significantly benefit the entire class of devices based on layered 2D materials., 13 pages, 5 figures

An optical phonon limited velocity model has been employed to investigate high-field transport in a selection of layered 2D materials for both, low-power logic switches with scaled supply voltages, and high-power, high-frequency transistors. Drain currents, effective electron velocities and intrinsic cut-off frequencies as a function of carrier density have been predicted thus providing a benchmark for the optical phonon limited high-field performance limits of these materials. The optical phonon limited carrier velocities of a selection of transition metal dichalcogenides and black phosphorus are found to be modest as compared to their n-channel silicon counterparts, questioning the utility of these devices in the source-injection dominated regime. h-BN, at the other end of the spectrum, is shown to be a very promising material for high-frequency high-power devices, subject to experimental realization of high carrier densities, primarily due to its large optical phonon energy. Experimentally extracted saturation velocities from few-layer MoS2 devices show reasonable qualitative and quantitative agreement with predicted values. Temperature dependence of measured vsat is discussed and found to fit a velocity saturation model with a single material dependent fit parameter., Comment: 8 pages, 6 figures

We report on the highest responsivity for III-nitride Metal Semiconductor Metal solar-blind photodetectors on sapphire. Devices on unintentionally doped AlGaN epilayers grown by Metal Organic Chemical Vapor Deposition exhibited sharp absorption cut-off in the range of 245–290 nm. Very high responsivity >5 A/W at 10 V bias was achieved with visible rejection exceeding three orders of magnitude for front illumination. Compared to the responsivity values reported in the literature for state-of-the-art solar-blind photodetectors, this work presents the highest values of responsivity at a given bias and up to sub-250 nm detection threshold. The high responsivity is attributed to an internal gain mechanism operating on these devices. The reverse-bias leakage current across these samples was found to be dominated by thermionic field emission at low biases and Poole-Frenkel emission from a deep trap level (0.7 eV from the conduction band-edge for Al0.50Ga0.50 N) at high biases.

Hysteresis in channel conductance is commonly observed on graphene field effect transistors. Although consistent and repeatable hysteresis could possibly be attractive for memory based applications, it is detrimental to the deployment of graphene in high speed electronic switches. While the origin of such hysteresis has been variously attributed to graphene-insulator interface traps, adsorbed molecules and bulk charges in the dielectric, its dependence on the quality of the graphene has been largely unexplored. Since, CVD is the most promising synthesis route for large area graphene and defects in such a growth process are inevitable, it is important to understand the influence of the quality of graphene on hysteresis. Here we demonstrate, for the first time, the effect of graphene growth defect density on device hysteresis. By intentionally tailoring the defect densities in the growth phase, we demonstrate a linear correlation between the film defect density and conductance hysteresis. The trap charge density calculated from the observed hysteresis in the electrical transfer characteristics was found to both follow the same qualitative trend, and give reasonable quantitative agreement with the defect density as extracted from Raman spectroscopy. Most importantly, by extrapolation from the observed behavior, we identify the intrinsic limits of hysteresis in graphene-SiO2 system, demonstrating that the defects in graphene contribute to traps over and above the baseline set by the SiO2 surface trap charge density.