arXiv subjects

Siddharth Rajan

Publications and source records attributed to Siddharth Rajan.

At least 73 records · Page 4Linked to original sources

arXiv2016

Low-resistance GaN tunnel homojunctions with 150 kA/cm^2 current and repeatable negative differential resistance

We report GaN n++/p++ interband tunnel junctions with repeatable negative differential resistance and low resistance. Reverse and forward tunneling current densities were observed to increase as Si and Mg doping concentrations were increased. Hysteresis-free, bidirectional negative differential resistance was observed at room temperature from these junctions at a forward voltage of ~1.6-2 V. Thermionic PN junctions with tunnel contact to the p-layer exhibited forward current density of 150 kA/cm^2 at 7.6 V, with a low series device resistance of 1 x 10^-5 ohm.cm^2.

cond-mat.mtrl-sci↗

arXiv2016

Phonon lasing as a likely mechanism for density-dependent velocity saturation in GaN transistors

We show that density-dependent velocity saturation in a GaN High Electron Mobility Transistor (HEMT) can be related to the stimulated emission of longitudinal optical (LO) phonons. As the drift velocity of electrons increases, the drift of the Fermi distribution in reciprocal space produces population inversion and gain for the LO phonons. Once this gain reaches a threshold value, the avalanche-like increase of LO emission causes a rapid loss of electron energy and momentum and leads to drift velocity saturation. Our simple model correctly predicts both the general trend of the saturation velocity decreasing with increasing electron density and the values of saturation velocity measured in our experiments.

cond-mat.mes-hall↗

arXiv2015

Enhanced Light Extraction in Tunnel Junction Enabled Top Emitting UV LEDs

The efficiency of ultra violet LEDs is critically limited by the absorption losses in p-type and metal layers. In this work, surface roughening based light extraction structures are combined with tunneling-based top-contacts to realize highly efficient top-side light extraction efficiency in UV LEDs. Surface roughening of the top n-type AlGaN contact layer is demonstrated using self-assembled Ni nano-clusters as etch mask. The top surface roughened LEDs were found to enhance external quantum efficiency by over 40% for UV LEDs with a peak emission wavelength of 326 nm. The method described here can enable highly efficient UV LEDs without the need for complex manufacturing methods such as flip chip bonding.

cond-mat.mtrl-sci↗

arXiv2015

Simulation of Enhancement Mode GaN HEMTs with Threshold > 5 V using P-type Buffer

A high threshold voltage enhancement-mode GaN HEMT with p-type doped buffer is discussed and simulated. Analytical expressions are derived to explain the role of buffer capacitance in designing and enhancing threshold voltage. Simulations of the proposed device with p-type buffer show threshold voltages above 5 V, and a positive shift in threshold voltage as the oxide capacitance is reduced, thus enabling threshold voltage tunability over an unprecedented range for GaN-based HEMTs. The electric field profiles, breakdown performance, on-resistance and delay tradeoffs in the proposed pGaN back HEMT device are also discussed.

physics.ins-det↗

arXiv2015

Density-Dependent Electron Transport and Precise Modeling of GaN HEMTs

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

cond-mat.mtrl-sci↗

arXiv2015

Layer-Transferred MoS2/GaN PN Diodes

Electrical and optical characterization of two-dimensional/three-dimensional (2D/3D) p-molybdenum disulfide/n-gallium nitride (p-MoS2/n-GaN) heterojunction diodes are reported. Devices were fabricated on high-quality, large-area p-MoS2 grown by chemical vapor deposition (CVD) on hexagonal sapphire substrates. The processed devices were transferred onto GaN/sapphire substrates, and the transferred films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). On-axis XRD spectra and surface topology obtained from AFM scans were consistent with previously grown high-quality, continuous MoS2 films. Current-voltage measurements of these diodes exhibited excellent rectification, and capacitance-voltage measurements were used to extract a conduction band offset of approximately 0.2 eV for the transferred MoS2/GaN heterojunction. This conduction band offset was confirmed by internal photoemission (IPE) measurements. The energy band lineup of the MoS2/GaN heterojunction is proposed here. This work demonstrates the potential of 2D/3D heterojunctions for novel device applications.

cond-mat.mtrl-sci↗

arXiv2015

Interband Tunneling for Hole Injection in III-Nitride Ultra-violet Emitters

Ultra-violet emitters have several applications in the areas of sensing, water purification, and data storage. While the III-Nitride semiconductor system has the band gap region necessary for ultraviolet emission, achieving efficient ultraviolet solid state emitters remains a challenge due to the low p-type conductivity and high contact resistance in wide band gap AlGaN-based ultra-violet light emitters. In this work, we show that efficient interband tunneling can be used for non-equilibrium injection of holes into ultraviolet emitters. Polarization-engineered tunnel junctions were used to enhance tunneling probability by several orders of magnitude over a PN homojunction, leading to highly efficient tunnel injection of holes to ultraviolet light emitters. This demonstration of efficient interband tunneling introduces a new paradigm for design of ultra-violet light emitting diodes and diode lasers, and enables higher efficiency and lower cost ultra-violet emitters.

cond-mat.mtrl-sci↗

arXiv2014

Modeling of High Composition AlGaN Channel HEMTs with Large Threshold Voltage

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 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 two-dimensional electron gas (2DEG) 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.

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

InGaN/GaN Tunnel Junctions For Hole Injection in GaN Light Emitting Diodes

InGaN/GaN tunnel junction contacts were grown on top of an InGaN/GaN blue (450 nm) light emitting diode wafer using plasma assisted molecular beam epitaxy. The tunnel junction contacts enable low spreading resistance n-GaN top contact layer thereby requiring less top metal contact coverage on the surface. A voltage drop of 5.3 V at 100 mA, forward resistance of 2 x 10-2 ohm cm2 and a higher light output power are measured in tunnel junction LED. A low resistance of 5 x 10-4 ohm cm2 was measured in a MBE grown tunnel junction on GaN PN junction device, indicating that the tunnel junction LED device resistance is limited by the regrowth interface and not by the intrinsic tunneling resistance.

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

Low frequency noise in chemical vapor deposited MoS2

Inherent low frequency noise is a ubiquitous phenomenon, which limits operation and performance of electronic devices and circuits. This limiting factor is very important for nanoscale electronic devices, such as 2D semiconductor devices. In this work, low frequency noise in high mobility single crystal MoS2 grown by chemical vapor deposition (CVD) is investigated. The measured low frequency noise follows an empirical formulation of mobility fluctuations with Hooge' s parameter ranging between 1.44E-3 and 3.51E-2. Small variation of Hooge's parameter suggests superior material uniformity and processing control of CVD grown MoS2 devices than reported single-layer MoS2 FET. The extracted Hooge's parameter is one order of magnitude lower than CVD grown graphene. The Hooge's parameter shows an inverse relationship with the field mobility.

cond-mat.mes-hall↗

arXiv2013

GdN Nanoisland-Based GaN Tunnel Junctions

We show that GdN nanoislands can enhance inter-band tunneling in GaN PN junctions by several orders of magnitude, enabling low optical absorption low-resistance tunnel junctions (specific resistivity 1.3 X 10-3 Ω-cm2) for various optoelectronic applications. We exploit the ability to overgrow high quality GaN over GdN nanoislands to create new nanoscale heterostructure designs that are not feasible in planar epitaxy. GdN nanoisland assisted inter-band tunneling was found to enhance tunneling in both of the polar orientations of GaN. Tunnel injection of holes was confirmed by low temperature operation of GaN p-n junction with a tunneling contact layer, showing strong electroluminescence down to 20K. The availability of tunnel junctions with negligible absorption could not only improve the efficiency of existing optoelectronic devices significantly, but also enable new electronic and optical devices based on wide band gap materials.

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↗

arXiv2013

Low Resistance GaN/InGaN/GaN Tunnel Junctions

Enhanced interband tunnel injection of holes into a PN junction is demonstrated using P-GaN/InGaN/N-GaN tunnel junctions with a specific resistivity of 1.2 X 10-4 Ω cm2. The design methodology and low-temperature characteristic of these tunnel junctions is discussed, and insertion into a PN junction device is described. Applications of tunnel junctions in III-nitride optoelectronics devices are explained using energy band diagrams. The lower band gap and polarization fields reduce tunneling barrier, eliminating the need for ohmic contacts to p-type GaN. This demonstration of efficient tunnel injection of carriers in III-Nitrides can lead to a replacement of existing resistive p-type contact material in light emitters with tunneling contact layers, requiring very little metal footprint on the surface, resulting in enhanced light extraction from top emitting emitters.

cond-mat.mtrl-sci↗

arXiv2011

Electrostatic carrier doping of GdTiO3/SrTiO3 interfaces

Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately 1/2 electron per surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities in all structures containing more than one unit cell of SrTiO3 are independent of layer thicknesses and growth sequences, indicating that the mobile carriers are in a high concentration, two-dimensional electron gas bound to the interface. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental results, insights into interfacial band alignments, charge distribution and the influence of different electrostatic boundary conditions are obtained.

cond-mat.mtrl-sci↗

arXiv2011

A heterojunction modulation-doped Mott transistor

A heterojunction Mott field effect transistor (FET) is proposed that consists of an epitaxial channel material that exhibits an electron-correlation-induced Mott metal-to-insulator transition. The Mott material is remotely (modulation) doped with a degenerately doped conventional band insulator. An applied voltage modulates the electron transfer from the doped band insulator to the Mott material and produces transistor action by inducing an insulator-to-metal transition. Materials parameters from rare-earth nickelates and SrTiO3 are used to assess the potential of the "modulation-doped Mott FET" (ModMottFET or MMFET) as a next-generation switch. It is shown that the MMFET is characterized by unique "charge gain" characteristics as well as competitive transconductance, small signal gain and current drive.

cond-mat.mtrl-sci↗