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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.

Despite being widely investigated for their memristive behavior, ferroelectrics are barely studied as channel materials in field-effect transistor (FET) configurations. In this work, we use multilayer α-In2Se3 to realize a ferroelectric channel semiconductor FET, i.e., FeS-FET, whose gate-triggered and polarization-induced resistive switching is then exploited to mimic an artificial synapse. The FeS-FET exhibits key signatures of a synapse such as excitatory and inhibitory postsynaptic current, potentiation/depression, and paired pulsed facilitation. Multiple stable conductance states obtained by tuning the device are then used as synaptic weights to demonstrate pattern recognition by invoking a hidden layer perceptron model. Detailed artificial neural network (ANN) simulations are performed on binary scale MNIST data digits, invoking 784 input (28 × 28 pixels) and 10 output neurons which are used in the training of 42 000 MNIST data digits. By updating the synaptic weights with conductance weight values on 18 000 digits, we achieved a successful recognition rate of 93% on the testing data. Introduction of 0.10 variance of noise pixels results in an accuracy of more than 70% showing the strong fault-tolerant nature of the conductance states. These synaptic functionalities, learning rules, and device to system-level simulation results based on α-In2Se3 could facilitate the development of more complex neuromorphic hardware systems based on FeS-FETs.

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.

This letter demonstrates heterogeneous stacking through postfabrication transfer of a thin layer of AlGaN/GaN high-electron-mobility transistors (HEMTs) and Si metal–oxide–semiconductor field-effect transistors (MOSFETs) onto a copper heat spreader. Both devices were fabricated on silicon substrate using separate process flows. Transfer of ~3- $\mu \text{m}$ GaN layer grown on silicon, however, leads to cracks due to the high-stress gradients arising from large lattice and thermal mismatch. Electroplated copper was used to improve mechanical strength, allowing transfer of the GaN layer using Cu–In bonding. Use of electroplated copper ensures high thermal conductivity to the bonded heat spreader. Next, an ultrathin silicon layer ( $\sim 1.5~\mu \text{m}$ ) with functional NMOS transistors was stacked above the GaN devices using a cost-effective epoxy bonding approach. The Cu–In bonding not only improved thermal dissipation but also led to enhanced performance of the GaN device due to the compressive stress induced by the bonding process. This compressive stress will also provide reliability against coefficient of thermal expansion (CTE) mismatch at higher operating temperatures.

Despite being widely investigated for their memristive behavior, ferroelectrics are barely studied as channel materials in field-effect transistor (FET) configurations. In this work, we use multilayer α-In

We report on the quantitative estimates of various metrics of performance for $\beta $ -Ga2O3-based high electron mobility transistors (HEMTs) for RF and power applications and compare them with III-nitride devices. Device parameters such as electron velocity and current density are estimated based on an optical phonon model reported earlier. 2-D simulation using an appropriate velocity-field relationship was employed to study the device characteristics and to assess the RF performance. It is found that despite a lower cutoff frequency, $\beta $ -Ga2O0 HEMTs are likely to provide higher RF output power compared to GaN HEMTs in the low-frequency regime. However, the thermal resistance (TR) and the channel temperature of $\beta $ -Ga2O3 HEMTs are expected to be significantly higher than those of GaN HEMTs which will pose serious limitations on heat dissipation. $\beta $ -Ga2O3 modulation doped field effect transistor on extremely thinned substrates will have similar TR s as compared to GaN devices on GaN substrates. The cutoff frequency was found to drop by 50% as the power dissipation increases from 1 to 7 W/mm. On the other hand, for estimates of dc power switching performance, we estimate the net losses as a function of device periphery and find that $\sim {8}\times $ – ${10}\times $ lower electron mobility in Ga2O3 devices compared to that in AlGaN/GaN HEMTs will limit its dc switching as well as its ON- state performance in terms of efficiency, loss, and current carrying capability although the blocking voltage can be much higher than in GaN.

We report on record high zero-bias external quantum efficiency (EQE) of 92% for back-illuminated Al0.40Ga0.60N p-i-n ultra-violet (UV) photodetectors on sapphire. The zero-bias responsivity measured 211 mA/W at 289 nm, which is the highest value reported for solar-blind, p-i-n detectors realized over any epitaxial wide band-gap semiconductor. This is also the first report for a p-i-n detector, where a polarization-graded Mg-doped AlGaN layer is utilized as the p-contact layer. The devices exhibited a ten-orders of magnitude rectification, a low reverse leakage current density of 1 nA/cm2 at 10 V, a high $\text{R}_{{0}}\text{A}$ product of $1.3\times 10^{{11}} \Omega $ .cm2 and supported fields exceeding 5 MV/cm. The light-to-dark current ratio and the UV-to-visible rejection ratio for the detectors exceeded six-orders of magnitude and the thermal noise limited detectivity (D*) measured $6.1\times 10^{{14}}$ cmHz1/2W−1. The state-of-the-art performance parameters can be attributed to a high crystalline quality absorbing AlGaN epi-layer resulting from the use of an AlN/AlGaN superlattice buffer and an improved p-contact via polarization grading.

In this letter, we demonstrate $1 \times 8$ linear metal–semiconductor–metal arrays of UV-C photodetectors based on optical-float-zone grown bulk (100)-oriented $\beta $ -Ga2O3. The as-grown polished bulk samples exhibited an RMS roughness of 1.4 nm and the full-width at half-maxima of 0.19°. The average peak responsivity (257 nm), dark current, photo-to-dark current ratio, and UV-to-visible rejection ratio of the devices in the array were measured to be 4 AW $^{-1}$ , 0.23 nA, $\sim 10^{2}$ , and $\sim 10^{3}$ , respectively, at a bias of 40 V. The uniformity as well as the variability of the detector parameters across the devices in the array was investigated before and after wire bonding.

We report on the demonstration of visible/near-IR high-performance photodetector based on exfoliated $\beta $ -Indium selenide (In2Se3) on sapphire with a clear signature of band edge in spectral responsivity at a wavelength of ~850–900 nm. Room temperature photoluminescence (PL) measurements also indicated a peak at ~900 nm confirming the band-edge. Devices with inter-digitated metal-semiconductor-metal (MSM) geometry exhibited a responsivity of 3.8 A/W (normalized to device area). A low dark current of 0.80 nA and a photo to dark current ratio of ~52 were measured when illuminated with 650 nm A specific detectivity of $1 \times 10^{10}$ cm Hz $^{0.5}\,\,\text{W}^{-1}$ at 650 nm and $6 \times 10^{8}$ cm Hz $^{0.5}\,\,\text{W}^{-1}$ at the band-edge of 900 nm were estimated. These results indicate the promise of $\beta $ -(In2Se3) for visible/near-IR detector applications.

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.

We investigate the vertical leakage mechanism in metal–organic chemical vapor deposition-grown carbon (C)-doped AlGaN/GaN High Electron Mobility Transistors (HEMTs) on 6-in silicon wafer. Substrate bias polarity-dependent ${I}$ – ${V}_{s}$ , temperature-dependent fitting, and band diagram analysis pointed to the Poole–Frenkel (P–F) type of conduction mechanism for vertical transport in the devices with breakdown as high as 580 V for a buffer of $\textsf {4}~\mu \text{m}$ . Trap activation energy of 0.61 eV was estimated from the P–F fitting which matches well with values reported in the literature. We propose that higher dislocation density leads to shallower traps in the buffer and build an analytical model of dislocation-mediated vertical leakage around this. The variation in leakage as a function of dislocation density at a given field is predicted and is found to be the most abrupt in the range from $\sim 10^{\textsf {7}}$ to $\sim 10^{\textsf {9}}$ cm $^{-\textsf {2}}$ of dislocation density. This can be attributed to a sharp decrease in trap activation energy in the above range of dislocation density, possibly due to complex formation between point defects and dislocations.

This Perspective seeks to understand and assess why ultrawide bandgap (UWBG) semiconductor-based deep-UV photodetectors have not yet found any noticeable presence in real-world applications despite riding on more than two decades of extensive materials and devices’ research. Keeping the discussion confined to photodetectors based on epitaxial AlGaN and Ga2O3, a broad assessment of the device performance in terms of its various parameters is done vis-à-vis the dependence on the material quality. We introduce a new comprehensive figure of merit (CFOM) to benchmark photodetectors by accounting for their three most critical performance parameters, i.e., gain, noise, and bandwidth. We infer from CFOM that purely from the point of view of device performance, AlGaN detectors do not have any serious shortcoming that is holding them back from entering the market. We try to identify the gaps that exist in the research landscape of AlGaN and Ga2O3 solar-blind photodetectors and also argue that merely improving the material/structural quality and device performance would not help in making this technology transition from the academic realm. Instead of providing a review, this Perspective asks the hard question on whether UWBG solar-blind detectors will ever find real-world applications in a noticeable way and whether these devices will be ever used in space-borne platforms for deep-space imaging, for instance.

We report on the growth of crystalline κ-Ga2O3 using mist-chemical vapor deposition (CVD) on (0001) sapphire. The κ-phase was confirmed using high-resolution x-ray diffraction (XRD) θ-2θ scan and pole figure-scan of (122) reflex, respectively, while an on-axis full width at half maximum (FWHM) of 104 arcsec was measured in symmetric rocking curve scan. Further, the heteroepitaxial film was analyzed using a mosaic crystal model employing reciprocal space maps (RSM) and a series of asymmetric rocking curve scans. A bandgap of 5.1 eV and excitonic binding energy of 85 meV were estimated from absorption measurements. Built-in field, depletion width, and doping density level were extracted from parabolic WKB model fit. Cathodoluminescence (CL) spectroscopy revealed a defect peak at 329 nm, which was found to be blue-shift with increasing excitation energy, indicating a possible donor-acceptor pair (DAP) transition. Temperature-dependent Current–voltage (I–V) study was performed to extract Schottky barrier height ( Φ B ) and ideality factor (η), which were found to be correlated with temperature. Photodetectors fabricated on the sample exhibited pure solar-blind characteristics.

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.

In this work, we show that a bilayer SiN x passivation scheme which includes a high-temperature annealed SiN x as gate dielectric, significantly improves both ON and OFF state performance of AlGaN/GaN metal insulator semiconductor high electron mobility transistors (MISHEMTs). Surface and bulk leakage paths were determined from devices with different SiN x passivation schemes. Temperature-dependent mesa leakage studies showed that the surface conduction could be explained using a 2D variable range hopping mechanism; this is attributed to the mid-gap interface states at the GaN(cap)/SiN x interface generated due to the Ga–Ga metal like bonding states. It was found that the high temperature annealed SiN x gate dielectric exhibited the lowest interface state density and a two-step C–V indicative of a superior quality SiN x /GaN interface as confirmed from conductance and capacitance measurements. High-temperature annealing helps form Ga–N bonding states, thus reducing the shallow metal-like interface states. MISHEMT measurements showed a significant reduction in gate leakage and a four-orders of magnitude improvement in the ON/OFF ratio while increasing the saturation drain current (I DS) by a factor of 2. Besides, MISHEMTs with two-step SiN x passivation exhibited a relatively flat transconductance profile, indicating lower interface states density. The dynamic R on with gate and drain stressing measurements also showed about 3× improvements in devices with bilayer SiN x passivation.

The talk will describe the defect-to-device performance correlations for optoelectronic and power devices on hetero-epitaxially grown III-nitride epi-layers on silicon and sapphire. While UV detectors and power electronic devices might seem far apart, we see similarities between the defect origins of dark current in the former and leakage currents in the latter. Both have their origins in crystal growth. The first part of the work focuses on the correlation between performance parameters of vertical deep-ultraviolet photodetectors on c-plane sapphire and the density of extended defects vis-a-vis screw and edge dislocations. Through a careful optimization of nucleation density on the growth surface, state-of-the-art crystalline quality AlN epi-layers were grown. This led to the realization of record performance 289 nm p-i-n photodiodes with zero-bias quantum efficiency of 92 %, leakage current below 1 nA at 100 V and breakdown field in excess of 6 MV/cm. Performance of III-nitride power devices on silicon, on the other hand, was found to be more sensitive to structural defects such as surface pits as compared to dislocations. By utilizing a two-temperature growth technique which involved an AlN layer grown at high temperatures, a significant reduction in the pit density could be achieved. This resulted in three-orders of magnitude reduction in lateral as well as vertical leakage in AlGaN/GaN high electron mobility transistors (HEMTs) on silicon. To achieve a further reduction in the reverse leakage current and an enhancement in the device breakdown voltage, point defect control also becomes important. Carbon doping to reduce the background n-carrier concentration in the GaN epilayers is discussed as one of the possible approaches. Reduction of pit density and C-incorporation cumulatively led to the realization of 2 MV/cm vertical breakdown field in AlGaN/GaN HEMTs on silicon.

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.

Atlas silvaco simulation has been performed to explore the effectiveness of buried channel architecture on AlGaN/GaN HEMTs for normally-off operation. The buried channel operation is achieved by adding a p-n junction in the GaN buffer. Thicknesses and doping concentration of the p-type and n-type region are optimized for the formation of the conduction channel away from the gate oxide-GaN interface, thereby reducing interface scattering and enhancing the field effect mobility. Better breakdown voltage is also demonstrated for buried channel device owing to the presence of p-n junction depletion region in the buffer. Three times increase in the on current and four order of magnitude reduction in the off state leakage current are demonstrated for the buried channel device compared to a reference device. HEMT devices are experimentally realized on an optimized GaN stack on Si and excellent agreements with simulation results are demonstrated.

In this work, we have investigated the material properties of PECVD (Plasma Enhanced Chemical Vapor Deposition) amorphous Silicon nitride (SiN) films and the influence of deposition conditions on gate-leakage increase of AlGaN/GaN HEMT’s (High Electron Mobility Transistor) after passivation. We have studied the effect of gas flow ratio (SiH4/NH3) on the structural and compositional properties of SiNx deposited on crystalline Silicon (Si). Based on the inference, electrical properties of the SiNx films were examined by depositing on AlGaN/GaN HEMT as a passivation layer. The optimized SiNx which is N-rich with a refractive index of 1.83, tensile stress of 681 MPa and NH3/SiH4 ratio of 18 shows only a 1.2x variation in gate leakage, 1.4x increase in gm, minimum left shift of 0.03 V in Vth and 1.26x increase Ion/Ioff ratio after passivation. We believe that the 3x reduction in Si-H bonds resulting in reduced interface traps at SiN/AlGaN interface is the reason for the minimal increase in gate leakage after passivation.

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.

We report on the study of step-graded AlGaN transition layers (TLs) in metalorganic chemical vapor deposition-grown GaN HEMT-on-silicon toward improving the breakdown field while minimizing buffer-induced current dispersion. The transition layers include three AlGaN epi-layers of 75%, 50%, and 25% Al-content, downgraded from bottom to top. The growth temperature and carbon doping are varied independently to assess the transition layer's role in reducing current collapse and leakage current. We observe that the introduction of High Temperature (HT) AlGaN increases the lateral but decreases the vertical leakage, the latter being attributed to the reduction of V-pit density. Temperature-dependent data indicate that the increased lateral (mesa) leakage current in HT AlGaN layers is due to space charge limited current, the activation energy of which yields the positions of the defect states within the bandgap. The increase in mesa leakage current in HT AlGaN layers is attributed to the formation of point defects such as oxygen in nitrogen site (ON) and VGa–ON complexes. The introduction of C-doping in the top AlGaN transition layer with 25% Al-content helps reduce lateral leakage in both mesa and 3-terminal configurations. The combination of HT AlGaN (75% Al-content) with C-doped AlGaN (25% Al-content) is found to be the optimal TL design that yielded a minimum buffer-induced current dispersion with a 65% channel recovery when the substrate was swept to −300 V and back; moreover, it also enabled a vertical breakdown field of 2.05 MV/cm defined at 1 A/cm2 for a buffer thickness of 1.65 μm.

In this chapter, we investigate vertical transistors based on hot electron transport—tunneling hot electron transfer amplifier (THETA). As compared to lateral transport devices such as HEMTs, electron transport can be defined by heterojunction growth at a scale shorter than 10 nm, and output conductance can be controlled through doping and epitaxial engineering. Furthermore, the power dissipation in a vertical device occurs over a volume rather than in a 2D sheet; the local temperature rise is not as significant as in the lateral case. THETA had been previously demonstrated in GaAs systems, and current gain in excess of 10 had been achieved with wide bandgap AlSbAs emitter at room temperature. GaN THETA has been reported in recent years, but the current gain in these devices has remained relatively low.

An overview of β-Ga2O3-based UV photodetectors (PD) as well as their status and promises are presented in this chapter. UV detectors and their applications are highlighted in the introduction followed by a brief discussion on β-Ga2O3 and its many potentials and challenges. The figures of merit for a PD are touched upon, especially from the point of view of UV detection. β-Ga2O3 UV detector configurations such as metal-semiconductor-metal (MSM) and Schottky diodes are discussed, and their recent advances and state-of-the-art results are presented in parallel. A comparison in terms of device performance between β-Ga2O3 and the competing III-nitride technology for UV detection is also highlighted. Finally, the future outlook and challenges associated with this emerging UV technology are summarized.

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.

We report on a multi‐layer MoS2‐based low power synaptic transistor using Ta2O5 as a back‐gate dielectric for mimicking the biological neuronal synapse. The use of high‐k dielectric allows for a lower voltage swing compared to using conventional SiO2, thus offering an attractive route to low‐power synaptic device architectures. Exfoliated MoS2 is utilized as the channel material and the hysteresis in the transfer characteristics of the transistor is exploited to demonstrate Excitatory and Inhibitory postsynaptic currents, long term potentiation and depression (LTP/LTD), indirect Spike Timing Dependent Plasticity (STDP) based on single and sequential gate (Vg) pulses respectively. The synapse had achieved a 35% weight change in channel conductance within 15 electrical pulses for negative synaptic gate pulse and 28% change for positive synaptic gate pulse. We also demonstrate complete tunability of weight in the synapse by Spike Amplitude dependent plasticity (SADP) at a low voltage of 4V This article is protected by copyright. All rights reserved.

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.

The authors study the effect of etch chemistry and metallization scheme on recessed Au-free Ohmic contacts to AlGaN/GaN heterostructures on silicon. The effect of variation in the recess etch chemistry on the uniformity of Ohmic contact resistance has been studied using two different etch chemistries (BCl3/O2 and BCl3/Cl2). Experiments to determine the optimum recess etch depth for obtaining a low value of contact resistance have been carried out, and it is shown that near-complete etching of the AlGaN barrier layer before metallization leads to the lowest value of contact resistance. Furthermore, two metal schemes, namely, Ti/Al and Ti/Al/Ti/W, are investigated, and it is found that the Ti/W cap layer on Ti/Al leads to low contact resistance with a smooth contact surface morphology. The effect of maintaining unequal mesa and contact pad widths on the extracted values of contact resistance and sheet resistance using the linear transfer length method (LTLM) has been studied. This is important as LTLM structures are used as monitors for process control during various steps of fabrication. It is shown that the extracted contact resistance and sheet resistance values are reliable when the mesa width is equal to the contact pad width. Finally, a possible mechanism for carrier transport in the Ohmic contacts formed using this process has been discussed, based on temperature dependent electrical characterization, and the field emission mechanism is found to be the dominant mechanism of carrier transport. A low Ohmic contact resistance of 0.56 Ω mm, which is one of the lowest reported values for identical metal schemes, and good contact surface morphology has been obtained with moderate post-metal annealing conditions of 600°C.

We report on crystalline quality-to-device performance correlation for self-powered Al0.40Ga0.60N, p-i-n ultraviolet (UV) photodetectors on c-plane sapphire. The active p-i-n detector stack was grown over an AlN buffer. Careful optimization of the nucleation density on growth surface helped achieve a two-orders and one-order of magnitude reduction in the screw and edge dislocation density in the buffer layer, respectively. This resulted in a nine-orders of magnitude reduction in the reverse leakage current from 4.3 mA to 4.2 pA (at 10 V). Correspondingly, a thirteen-fold enhancement in the zero-bias external quantum efficiency (EQE) from 3.4 to 45.5, when measured under 289 nm front-illumination was also observed. The detector epi-stack grown over the optimal AlN buffer layers led to the realization of high-performance p-i-n detectors with a dark current density below 4 nA cm-2 at 10 V and a zero-bias EQE of 74.7 under back-illumination. This is one of the highest zero-bias EQE reported for solar-blind detectors realized on template-free and mask-free III-nitrides grown using metal organic chemical vapor deposition on any substrate. The deep-UV-to-visible rejection ratio exceeded 106 while the deep-UV-to-near UV rejection exceeded 103. The thermal-noise limited detectivity was estimated to be 4 1014 cm Hz1/2 W-1. Hopping conduction along screw dislocation-mediated localized trap states was found to be the dominant carrier transport mechanism in the samples exhibiting high reverse leakage. For these samples, the responsivity (photocurrent) exhibited an exponential variation with reverse bias and a nonlinear variation with input optical power. This is explained using a hole-trapping associated gain mechanism and its impact on the transient characteristics of the detectors is investigated. A 6 1 linear array of the highest EQE detectors was realized and detector performance parameters were found to be comparable before and after wire bonding. This study is expected to enhance the understanding of III-nitrides-based vertical, self-powered detectors and benefit the development of high-performance, focal plane arrays using less complicated growth techniques. © 2020 IOP Publishing Ltd.

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.

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.

High photon absorption and photoconductive gain observed in 2D Transition Metal Dichalcogenides make them exciting candidates for high performance photodetectors and photovoltaics [1]. However, slow response times and low $\mathrm{V}_{\mathrm{oc}}$ (open circuit voltage) have hindered the development of reliable optoelectronic devices [1]. Most attempts to mitigate these disadvantages involve fabrication of complex heterostructures [2]–[4], which may render them unfit for large area production. Besides, almost all reports on MeSi-bascd opto-electronics exploit its lateral transport while studies on vertical transport towards detectors and photovoltaics are at an embryonic stage. In this work, we demonstrate that a facile vertical metal/multilaycr-Mo'Sc/rnctal device can be engineered to enable both excellent photodetectors (with high responsivity $> 3\mathrm{A}/\mathrm{W}$ & good speed ~ 1 ms) OR high $\mathrm{V}_{\mathrm{oc}}(\sim 0.5\mathrm{V})$ photovoltaics. We adopt a conscious design strategy of employing vertical instead of lateral transport through multilayer Mos2 as this provides both larger area for photon absorption, as well as short charge transfer length (enabling effective separation) [Fig l(a)].

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.

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.

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.

Common-emitter operation was demonstrated in an N-polar tunneling hot electron transistor. Under collector-emitter bias of 7 V, small signal current gain $\sim 1.3$ , and voltage gain $\sim 4$ were simultaneously obtained for transistors with 27.5-nm base. This is the first report of a III-nitride hot electron transistor with small signal current gain and intrinsic voltage gain both greater than unity. The result shows such III-nitride vertical transistors are promising for the next generation of high-frequency amplifiers.

We report on a new approach to achieve nonalloyed ohmic contacts with record low resistance in GaN-based transistors. We use upward and reverse grading of Al composition in a polarization-graded field-effect transistor structure to eliminate abrupt heterojunction band offsets and create direct contact to the channel. Total contact resistance of 73 mQ · mm and interfacial resistance of 29 mQ · mm to the 2-D electron gas were obtained. The method adopted here could enable to ultralow-resistance contacts without ohmic recess.