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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.

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.

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.

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 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 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.

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.

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.

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.

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.

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.

We demonstrate an efficient approach to engineer the dielectric/AlGaN positive interface fixed charges by oxygen plasma and post-metallization anneal. Significant suppression of interface fixed charges from 2 × 1013 to 8 × 1012 cm-2 was observed. Experimental and theoretical electron mobility characteristics and the impact of remote impurity scattering were investigated. The reduction in oxide/semiconductor interface charge density leads to an increase of electron mobility, and enables a positive threshold voltage.

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.

DC current gain greater than one was demonstrated in GaN tunneling hot electron transistors. At an emitter-base bias of 8 V, dc current gain is increased to 2.8. The results show the promise of GaN tunneling hot electron transistors for the next generation of high-frequency amplifiers. This work is funded by Office of Naval Research under the DATE MURI project (Program manager: Dr. Paul Maki).

We investigate the effects of quantum capacitance in an N-polar GaN/AlGaN/GaN heterostructures by directly measuring quantum displacement of the electron wavefunction Δd. A comparison between electrically and microscopically measured thicknesses showed negative quantum displacement effects in the inverted high-electron-mobility-transistor (HEMT) structure. As a result of the quantum capacitance effects, a quantum displacement Δd of ~ -4nm was extracted from the measurements. Further analysis using 1-D self-consistent Schrodinger-Poisson solver has been done to validate the measured data. Our simulation results, including multiple-subband occupancy, explain the increasing capacitance in the measured C-V profile in N-polar GaN-based HEMTs.

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.

We report N-polar GaN-based tunneling hot electron amplifier transistors (THETA) with common-emitter modulation, ac current gain up to 1.35 operating at collector current density ~50 kA/cm 2 , which are highest values reported to date in GaN-based hot electron transistors. Achieving higher frequency operation in the III-nitride system requires an alternative to highly-scaled GaN-based HEMTs whose transconductance and frequency are limited by electron saturation velocity. In vertical devices, channel lengths below 10nm can be achieved through heterostructure engineering. While bipolar junction transistors have been very successful in other material technologies (InP and SiGe HBTs), the poor hole doping efficiency and mobility in GaN make it difficult to achieve bipolar vertical devices. In this work, we circumvent the challenge of p-type conduction in III-nitrides by demonstrating a highly scaled vertical unipolar tunnel injection transistor. We show transistor action in an ultrascaled device with base thickness ~10 nm, which is the first experimental demonstration of tunneling hot electron transistor with N-polar orientation.

In this work, we demonstrate 2D/3D heterojunction using large-area crystalline p-MoS 2 epitaxially grown on n-doped 4H-SiC. The diode exhibited excellent rectification with an ideality factor of 1.5, as well as C-V characteristics typical of an anisotype p-n junction. Besides being promising for enabling novel device topologies, this realization of a p-type 2D/n-type 3D heterostructure holds potential for circumventing challenges associated with p-doping in wide bandgap (WBG) semiconductors which has otherwise held back the successful development of bipolar devices in WBG systems.

In this work, we have used electrostatic engineering of the ALD dielectric/III-nitride interface to do lateral band engineering in a III-nitride HEMT. Due to challenges related to dopant activation and damage anneal, traditional ion implantation and diffusion techniques for lateral band engineering that are commonly used in other semiconductors, cannot be applied easily in the III-nitride system. We have developed an alternate method that uses surface fixed charges to engineer lateral energy band profiles, and used this to demonstrate an enhancement-mode AlGaN/GaN HEMT without any gate recess. Metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) based on the III-Nitride system can efficiently suppress gate leakage enabling lower gate-channel spacing for high frequency transistors, and low off-state leakage for power switching devices. Conventional normally-off MISHEMTs require precise etching control for recess gate [1] or heavy p+ doping for the junction gate [2]. However, plasma etching may induce variation of electrical characteristics caused by surface damage while p-doping can cause hysteresis. In this work, we show a new technique to achieve normally off AlGaN/GaN transistors. Our method exploits the interface properties of dielectric/III-nitride, where a high density of fixed charges of the order of 1 μC/cm 2 can be formed the interface of atomic layer deposited (ALD) dielectrics on GaN and AlN[3-5]. In this work, we use the combination of oxygen plasma and post metallization anneal (PMA) treatments to engineer the Al 2 O 3 /AlGaN (AlN) interface fixed charges. Based on this technology, lateral energy band engineering by patterning ALD Al 2 O 3 is demonstrated. This technology provides a new approach to recess-free and doping-free normally-off MOSFETs /MISHEMTs.

Atomic layer deposited Al2O3/III-nitride interfacial charge properties were investigated experimentally. Substantial amount of positive interface fixed charges (> 1.8×1013 cm-2) were observed on all the orientations of GaN and Ga-polar AlGaN. These interface charges bring large impact on electron transport especially in vertically scaled devices. Post-metallization anneals is one of the methods which can significantly alter this sheet charge in the case of Al2O3 on GaN. Oxygen plasma is another approach to efficiently reduce the interface charges at Al2O3/AlGaN. The interface charge density engineering described here can be used to improve performance of device such as gate leakage and electron transport, and also provides a new design of normally-off MISHEMTs.

The density of two dimensional electron gas (2DEG) in the channel of AlGaN/GaN HEMT is often altered by trapped charges on the surface or in the bulk of the heterostructure, limiting the device performance at high frequencies. The existing methods, such as photoionization spectroscopy, Deep Level Transient Spectroscopy (DLTS) etc., mainly focus on deep level traps in GaN, providing little information about traps in the AlGaN layer. In this work, we use Inelastic Electron Tunneling Spectroscopy (IETS), in conjunction with the Frenkel-Poole (F-P) trap energy extraction method developed by Yeh at el, to examine electrically active traps in the AlGaN barrier layer. The results indicate that traps exist in the AlGaN bulk, as well as at the AlGaN/GaN interface. The trap energy levels are within a 0.5eV band below the conduction band edge (Ec) of AlGaN in the sample that we studied.

A simple method for the creation of Ohmic contact to 2-D electron gas (2DEG) in AlGaN/GaN high electron-mobility transistors (HEMTs) using Cr/Graphene layer is demonstrated. A weak temperature dependence of this Ohmic contact observed in the range 77 to 300 K precludes thermionic emission or trap-assisted hopping as possible carrier-transport mechanisms. It is suggested that the Cr/Graphene combination acts akin to a doped n-type semiconductor in contact with AlGaN/GaN heterostructure, and promotes carrier transport along percolating Al-lean paths through the AlGaN layer. This new use of graphene offers a simple and reliable method for making Ohmic contacts to AlGaN/GaN heterostructures, circumventing complex additional processing steps involving high temperatures. These results could have important implications for the fabrication and manufacturing of AlGaN/GaN-based microelectronic and optoelectronic devices/sensors of the future.

In this letter, we report on unipolar vertical transport characteristics in c-plane GaN/AlGaN/GaN heterostructures. Vertical current in heterostructures with random alloy barriers was found to be independent of dislocation density and heterostructure barrier height. Percolation-based transport due to random alloy fluctuations in the ternary AlGaN is suggested as the dominant transport mechanism. This hypothesis is supported by simulations using drift-diffusion transport model incorporating statistical fluctuations of Al-composition and confirmed through experiments showing that non-random or digital AlGaN alloys and polarization-engineered binary GaN barriers can eliminate percolation transport and reduce leakage significantly. The understanding of vertical transport and methods for effective control proposed here will greatly impact III-nitride unipolar vertical devices.

III-Nitride based tunneling devices have generated a great deal of interest due to their ability to operate at very high powers/current and elevated temperatures. Additionally, with the increasing interest for making GaN-based photovoltaics, the need for efficient interband tunneling in multi-junction solar cells will be a necessity too. But, to date, due to the nitride's large bandgap and lack of very high bulk doping capabilities, it has proved very difficult to create III-Nitride interband tunnel diodes operating at high current densities. One solution is to use a fairly high concentration of indium in the central tunnel junction to decrease the bandgap and thereby exponentially increase the tunneling probability, while also making use of the spontaneous and piezoelectric polarization charge available in the nitrides to supplement the creation of two quantum wells adjacent the tunnel junction, thereby creating a resonant interband tunneling diode (RITD).

We report on Gallium Nitride-based tunneling hot electron transistor amplifier with common-emitter current gain greater than 1. Small signal current gain up to 5 and dc current gain of 1.3 were attained in common-emitter configuration with collector current density in excess of 50 kA/cm2. The use of a combination of 1 nm GaN/3 nm AlN layers as an emitter tunneling barrier was found to improve the energy collimation of the injected electrons. These results represent demonstration of unipolar vertical transistors in the III-nitride system that can potentially lead to higher frequency and power microwave devices.

We report on the potential of high electron mobility transistors (HEMTs) consisting of high composition AlGaN channel and barrier layers for power switching applications. Detailed two-dimensional (2D) simulations show that threshold voltages in excess of 3 V can be achieved through the use of AlGaN channel layers. We also calculate the 2D electron gas mobility in AlGaN channel HEMTs and evaluate their power figures of merit as a function of device operating temperature and Al mole fraction in the channel. Our models show that power switching transistors with AlGaN channels would have comparable on-resistance to GaN-channel based transistors for the same operation voltage. The modeling in this paper shows the potential of high composition AlGaN as a channel material for future high threshold enhancement mode transistors.