arXiv subjects

Digbijoy N. Nath

Publications and source records attributed to Digbijoy N. Nath.

At least 19 recordsLinked to original sources

arXiv2023

Laser Etch Enabled Active Embedded Microfluidic Cooling in $β$-Ga$_2$O$_3$

We demonstrate active embedded microfluidic cooling in $β$-Ga$_2$O$_3$. We employ a cost-effective infra-red laser etch setup to achieve controlled etching of micro-channels in 500 um thick $β$-Ga$_2$O$_3$ substrate. The micro-channels are about 210 um deep and 340 um wide. Resistive heating is used as proof-of-concept. At a water flow rate of 50 ml/min, a 50% reduction in surface temperature from ~140$^\circ$C to ~72$^\circ$C is achieved for 3.5 W of input power. The experimental observations are backed by thermal simulation. This work is expected to lead to a new paradigm in thermal management in emerging $β$-Ga$_2$O$_3$ devices.

physics.app-ph↗

arXiv2020

UV/Near-IR dual band photodetector based on p-GaN/α-In2Se3 heterojunction

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 ~10^11 Jones and ~10^10 Jones corresponding to illumination at 365 nm and 850 nm respectively. A good linearity of ~0.4 was observed in power dependent analysis of spectral responsivity at 365 nm. The device performance, post annealing was also studied. This study is expected to pave way for new type of optoelectronic devices by integrating direct bandgap layered material like α-In2Se3 and wide bandgap semiconductors.

physics.app-ph↗

arXiv2019

Deep sub micron normally off AlGaN/GaN MOSFET on silicon with VTH > 5V and On Current > 0.5 A/mm

We report on the demonstration of a deep sub-micron normally-off AlGaN/GaN HEMT with high on-current and high threshold voltage (VTH). The high-performance device was realized by utilizing a gate recess with length and depth of 200 nm and 124 nm respectively. The recess etched region had a roughness of 0.7 nm. Various recess etch depths and dielectric annealing conditions were used to tune the VTH. The optimized device exhibited on-current and VTH of 500 mA/mm and 5 V respectively. The measured breakdown characteristics of the devices and their limitations were investigated using 2D-TCAD device simulation. The penetration of the residual electric field in most of the recess region could be the reason for the premature breakdown of deeply scaled recess-gate e-mode HEMTs.

physics.app-ph↗

arXiv2019

Meandering gate edges for breakdown voltage enhancement in AlGaN/GaN HEMTs

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.

physics.app-ph↗

arXiv2018

Investigation of Ta2O5 as an alternative high \k{appa} dielectric for InAlN/GaN MOS HEMT on Si

We report on the demonstration and investigation of Ta2O5 as high-\k{appa} dielectric for InAlN/GaN-MOS 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 conditions (PDA). The gate leakage was 16nA/mm at -15 V which was ~ 5 orders of magnitude lower compared to reference HEMT. The 2-dimensional electron gas (2DEG) 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 (VFB) of 0.4 V and interface fix charge (Qf) of 3.98x10^13 cm-2. XPS (X-ray photoelectron spectroscopy) spectra of as deposited and annealed Ta2O5 film were analyzed for Ta and O compositions in the film. The sample annealed at 500° C has shown Ta:O ratio of 0.41.XRD (X-ray diffraction) analysis was done to check the evolution of poly-crystallization of the Ta2O5 film at higher annealing temperatures.

physics.app-ph↗

arXiv2018

Multi-layer MoS2/GaN UV-Visible photodetector with observation of MoS2 band edge in spectral responsivity

We report on the demonstration of MoS2/GaN UV-visible photodetectors with high spectral responsivity both in UV and in visible regions as well as the observation of MoS2 band-edge in spectral responsivity. Multi-layer MoS2 flakes of thickness ~ 200 nm were exfoliated on epitaxial GaN-on-sapphire, followed by fabrication of detectors in a lateral Metal-Semiconductor-Metal (MSM) geometry with Ni/Au contacts which were insulated from the GaN layer underneath by Al2O3 dielectric. Devices exhibited distinct steps in spectral responsivity at 365 nm and at ~ 685 nm with a corresponding photo-to-dark current ratio of ~4000 and ~ 100 respectively. Responsivity of 0.1 A/W (at 10 V) was measured at 365 nm corresponding to GaN band edge, while the second band edge at ~ 685 nm is characterized by a spectral responsivity (SR) of ~ 33 A/W when accounted for the flake size, corresponding to the direct band gap at K point of multi-layer MoS2.

physics.app-ph↗

arXiv2018

A performance comparison between \b{eta}-Ga2O3 and GaN High Electron Mobility Transistors

In this letter, we report on the quantitative estimates of various metrics of performance for \b{eta}-Ga2O3 based High Electron Mobility Transistor (HEMT) for radio frequency (RF) and power applications and compare them with III-nitride devices. It is found that despite a lower cut-off frequency, \b{eta}- Ga2O3 HEMT is likely to provide higher RF output power compared to GaN-HEMT in the low-frequency regime although a poor thermal conductivity will impose limitations in heat dissipation. On the other hand, a much lower electron mobility will limit the DC switching performance in terms of efficiency and loss although their blocking voltage can be much higher than in GaN.

physics.app-ph↗

arXiv2018

MBE grown Self-Powered \b{eta}-Ga2O3 MSM Deep-UV Photodetector

We demonstrate self-powered \b{eta}-Ga2O3 deep-UV metal-semiconductor-metal (MSM) photodetectors (PD) with 0.5% external quantum efficiency (EQE) at zero bias. 150 nm thick (-201)-oriented epitaxial \b{eta}-Ga2O3-films were grown on c-plane sapphire using plasma-assisted MBE. Ni/Au and Ti/Au metal stacks were deposited as contacts to achieve asymmetric Schottky barrier heights in interdigitated finger architecture for realizing self-powered photodetectors. Current-voltage characteristics (photo and dark), time-dependent photocurrent and spectral response were studied and compared with conventional symmetric MSM PD with Ni/Au as the Schottky metal contact, fabricated on the same sample. The asymmetric, self-powered devices exhibited solar-blind nature and low dark current < 10 nA at 15 V with high photo-to-dark current ratio of ~ 103. The dark and photocurrents were asymmetric with respect to the applied bias and the responsivity in the forward bias was characterized by gain. The detectors (asymmetric-MSM) were found to exhibit a responsivity of 1.4 mA/W at 255 nm under zero-bias condition (corresponding to an EQE ~ 0.5 %), with a UV-to-Visible rejection ratio ~ 102 and ~105 at 0 V and 5 V respectively.

physics.app-ph↗

arXiv2017

Microwave Irradiation Assisted Deposition of Ga2O3 on III-nitrides for deep-UV opto-electronics

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.

cond-mat.mtrl-sci↗

arXiv2017

Dielectric Engineering of HfO2 Gate Stacks Towards Normally-ON and Normally-OFF GaN HEMTs on Silicon

We report on the interfacial electronic properties of HfO2 gate dielectrics both, with GaN towards normally-OFF recessed HEMT architectures and the AlGaN barrier for normally-ON AlGaN/GaN MISHEMTs for GaN device platforms 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. Conductance measurements on HfO2/GaN MOSCAPs reveal an interface trap state continuum with a density of 9.37x1012 eV-1cm-2 centered at 0.48 eV below EC. The forward and reverse current densities are shown to be governed by Fowler-Nordheim tunneling and Poole-Frenkel emission respectively. Normally-ON HfO2/AlGaN/GaN MISHEMTs exhibit negligible shifts in threshold voltage, transconductances of 110mS/mm for 3 μm gate length devices, and three-terminal OFF-state gate leakage currents of 20 nA/mm at a VD of 100 V. Dynamic capacitance dispersion measurements show two peaks at the AlGaN/GaN interface corresponding to slow and fast interface traps with a peak Dit of 5.5x1013 eV-1cm-2 and 1.5x1013 eV-1cm-2 at trap levels 0.55 eV and 0.46 eV below EC respectively. The HfO2/AlGaN interface exhibits a peak Dit of 4.4x1013 eV-1cm- 2 at 0.45 eV below EC.

cond-mat.mtrl-sci↗

arXiv2017

High Responsivity in Molecular Beam Epitaxy (MBE) grown \b{eta}-Ga2O3 Metal Semiconductor Metal (MSM) Solar Blind Deep-UV Photodetector

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.

cond-mat.mes-hall↗

arXiv2016

Surface States Engineering of Metal/MoS2 Contacts Using Sulfur Treatment for Reduced Contact Resistance and Variability

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

cond-mat.mes-hall↗

arXiv2015

Optical Phonon Limited High Field Transport in Layered Materials

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

cond-mat.mtrl-sci↗

arXiv2015

Electron mobility in InxGa1-xN channel HEMTs

In this letter, we report on the theoretical investigations of electron mobility in practically viable designs of InxGa1-xN channel high electron mobility transistors (HEMT). Carriers in such devices are expected to exhibit a higher velocity and hence higher cut-off frequencies (fT) for highly scaled architectures. We estimate that the mobility of two dimensional electron gas (2DEG) is limited by alloy scattering rather than phonon scattering unlike in conventional GaN-channel HEMTs. For indium composition of 0.30, the mobility and sheet resistance are found to be 500 cm2/Vs and 700 ohm per sq. respectively, which can severely affect the parasitic voltage drop in access regions. The results presented here are believed to significantly guide the practical exploration of InxGa1-xN channel HEMTs towards next-generation electronics by enabling careful design of device layouts in highly scaled transistors to minimize parasitic access region voltage drop which results due to significant degradation of 2DEG mobility.

cond-mat.mtrl-sci↗

arXiv2014

Epitaxial Growth of Large Area Single-Crystalline Few-Layer MoS2 with Room Temperature Mobility of 192 cm2V-1s-1

We report on the vapor-solid growth of single crystalline few-layer MoS2 films on (0001)-oriented sapphire with excellent structural and electrical properties over centimeter length scale. High-resolution X-ray diffraction scans indicated that the films had good out-of-plane ordering and epitaxial registry. A carrier density of ~2 x 1011 cm-2 and a room temperature mobility of 192 cm2/Vs were extracted from space-charge limited transport regime in the films. The electron mobility was found to exhibit in-plane anisotropy with a ratio of ~ 1.8. Theoretical estimates of the temperature-dependent electron mobility including optical phonon, acoustic deformation potential and remote ionized impurity scattering were found to satisfactorily match the measured data. The synthesis approach reported here demonstrates the feasibility of device quality few-layer MoS2 films with excellent uniformity and high quality.

cond-mat.mtrl-sci↗

arXiv2014

Growth and Electrical Characterization of 2D Layered MoS2/SiC Heterojunctions

The growth and electrical characterization of a heterojunction formed between 2D layered p-MoS2 and nitrogen-doped 4H-SiC is reported. Direct growth of p-type MoS2 films on SiC was demonstrated using chemical vapor deposition, and the MoS2 films were found to be crystalline based on x-ray diffraction measurements. The resulting heterojunction was found to display rectification and current-voltage characteristics consistent with a p-n junction diode. Capacitance voltage measurements were used to determine the built-in voltage for the p-MoS2/n-SiC heterojunction p-n diode. The demonstration of heterogeneous material integration between 2D layered semiconductors and 3D SiC enables a new class of heterostructures.

cond-mat.mtrl-sci↗

arXiv2013

Large Area Single Crystal (0001) Oriented MoS2 Thin Films

Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, and potential to open up new areas of physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area electronic grade single crystal Molybdenum Disulfide (MoS2) thin films with the highest mobility reported in CVD grown films so far. Growth temperature and choice of substrate were found to critically impact the quality of film grown, and high temperature growth on (0001) orientated sapphire yielded highly oriented single crystal MoS2 films for the first time. Films grown under optimal conditions were found to be of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy, and Raman measurements, approaching the quality of reference geological MoS2. Photoluminescence and electrical measurements confirmed the growth of optically active MoS2 with a low background carrier concentration, and high mobility. The CVD method reported here for the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D chalcogenide semiconductors and their heterostructures.

cond-mat.mtrl-sci↗

arXiv2013

Ohmic Contact Formation Between Metal and AlGaN/GaN Heterostructure via Graphene Insertion

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.

cond-mat.mtrl-sci↗