arXiv subjects

Srinivasan Raghavan

Publications and source records attributed to Srinivasan Raghavan.

31 records · Page 2Linked to original sources

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

Thickness Dependent Parasitic Channel Formation at AlN/Si Interfaces

The performance of GaN-on-Silicon electronic devices is severely degraded by the presence of a parasitic conduction pathway at the nitride-substrate interface which contributes to switching losses and lower breakdown voltages. The physical nature of such a parasitic channel and its properties are however, not well understood. We report on a pronounced thickness dependence of the parasitic channel formation at AlN/Si interfaces due to increased surface acceptor densities at the interface in silicon. The origin of these surface acceptors is analyzed using secondary ion mass spectroscopy measurements and traced to thermal acceptor formation due to Si-O-N complexes. Low-temperature (5K) magneto-resistance (MR) data reveals a transition from positive to negative MR with increasing AlN film thickness indicating the presence of an inversion layer of electrons which also contributes to parasitic channel formation but whose contribution is secondary at room temperatures.

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

An Early In-Situ Stress Signature of the AlN-Si Pre-growth Interface for Successful Integration of Nitrides with (111) Si

The integration of MOCVD grown group III-A nitride device stacks on Si (111) substrates is critically dependent on the quality of the first AlN buffer layer grown. A Si surface that is both oxide-free and smooth is a primary requirement for nucleating such layers. A single parameter, the AlN layer growth stress, is shown to be an early (within 50 nm), clear (<0.5 GPa versus >1 GPa) and fail-safe indicator of the pre-growth surface, and the AlN quality required for successful epitaxy. Grain coalescence model for stress generation is used to correlate growth stress, the AlN-Si interface and crystal quality.

cond-mat.mtrl-sci↗

arXiv2016

Making Consistent Contacts to Graphene: Effect of Architecture and Growth Induced Defects

The effect of contact architecture, graphene defect density and metal-semiconductor work function difference on resistivity of metal-graphene contacts have been investigated. An architecture with metal on the bottom of graphene is found to yield resistivities that are lower, by a factor of 4, and most consistent as compared to metal on top of graphene. Growth defects in graphene film were found to further reduce resistivity by a factor of 2. Using a combination of method and metal used, the contact resistivity of graphene has been decreased by a factor of 10 to 1200 +- 250 Ohm-um using Palladium as the contact metal. While the improved consistency is due to the metal being able to contact uncontaminanted graphene in the metal on the bottom architecture, lower contact resistivities observed on defective graphene with the same metal is attributed to the increased number of modes of quantum transport in the channel.

cond-mat.mtrl-sci↗

arXiv2015

Bright-field Nanoscopy: Visualizing Nano-structures with Localized Optical Contrast Using a Conventional Microscope

Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags. Here, we report a method for visualization of nanostructures down to a few nanometers using a conventional bright-field microscope without requiring additional molecular tags such as fluorophores. The technique, Bright-field Nanoscopy, is based on the strong thickness dependent color of ultra-thin germanium on an optically thick gold film. We demonstrate the visualization of grain boundaries in chemical vapour deposited single layer graphene and the detection of single 40 nm Ag nanoparticles. We estimate a size detection limit of about 2 nm using this technique. In addition to visualizing nano-structures, this technique can be used to probe fluid phenomena at the nanoscale, such as transport through 2D membranes. We estimated the water transport rate through a 1 nm thick polymer film using this technique, as an illustration. Further, the technique can also be extended to study the transport of specific ions in the solution. It is anticipated that this technique will find use in applications ranging from single-nanoparticles resolved sensing to studying nanoscale fluid-solid interface phenomena.

physics.optics↗

arXiv2015

Estimation of background carrier concentration in fully depleted GaN films

Buffer leakage is an important parasitic loss mechanism in AlGaN/GaN HEMTs and hence various methods are employed to grow semi-insulating buffer layers. Quantification of carrier concentration in such buffers using conventional capacitance based profiling techniques is challenging due to their fully depleted nature even at zero bias voltages. We provide a simple and effective model to extract carrier concentrations in fully depleted GaN films using capacitance-voltage (C-V) measurements. Extensive mercury probe C-V profiling has been performed on GaN films of differing thicknesses and doping levels in order to validate this model. Carrier concentrations as extracted from both the conventional C-V technique for partially depleted films having the same doping concentration, and Hall measurements show excellent agreement with those predicted by the proposed model thus establishing the utility of this technique. This model can be readily extended to estimate background carrier concentrations from the depletion region capacitances of HEMT structures and fully depleted films of any class of semiconductor materials.

cond-mat.mtrl-sci↗

arXiv2015

Spotting 2-D Atomic Layers on Aluminum Nitride Thin Films

The availability of large-area substrates imposes an important constraint on the technological and commercial realization of devices made of layered materials. Aluminum nitride films on silicon are shown to be promising candidate materials as large-area substrates for such devices. Herein, the optical contrast of exemplar 2D layers - MoS2and graphene - on AlN films has been investigated as a necessary first step to realize devices on these substrates. Significant contrast enhancements are predicted and observed on AlN films compared to conventional SiO2films. Quantitative estimates of experimental contrast using reflectance spectroscopy show very good agreement with predicted values.

cond-mat.mtrl-sci↗

arXiv2015

Growth Stress Induced Tunability of Dielectric Constant in Thin Films

It is demonstrated here that growth stress has a substantial effect on the dielectric constant of zirconia thin films. The correct combination of parameters - phase, texture and stress - is shown to yield films with high dielectric constant and best reported equivalent oxide thickness of 0.8 nm. The stress effect on dielectric constant is twofold, firstly, by the effect on phase transitions and secondly by the effect on interatomic distances. We discuss and explain the physical mechanisms involved in the interplay between the stress, phase changes and the dielectric constant in detail.

cond-mat.mtrl-sci↗

arXiv2013

A nearly relaxation-free opto-electronic memory from ultra-thin graphene-MoS$_2$ binary hybrids

Ultra-thin planar heterostructures of graphene and other two-dimensional crystals have recently attracted much interest. Very high carrier mobility in a graphene-on-boron nitride assembly is now well-established, but it has been anticipated that appropriately designed hybrids could perform other tasks as well. A heterostructure of graphene and molybdenum disulphide (MoS$_2$) is expected to be sensitive to photo illumination due to the optical bandgap in MoS$_2$. Despite significant advances in device architectures with both graphene and MoS$_2$, binary graphene-MoS$_2$ hybrids have not been realized so far, and the promising opto-electronic properties of such structures remain elusive. Here we demonstrate experimentally that graphene-on-MoS$_2$ binary heterostructures display an unexpected and remarkable persistent photoconductivity under illumination of white light. The photoconductivity can not only be tuned independently with both light intensity and back gate voltage, but in response to a suitable combination of light and gate voltage pulses the device functions as a re-writable optoelectronic switch or memory. The persistent, or `ON', state shows virtually no relaxation or decay within the the experimental time scales for low and moderate photoexcitation intensity, indicating a near-perfect charge retention. A microscopic model associates the persistence with strong localization of carriers in MoS$_2$. These effects are also observable at room temperature, and with chemical vapour deposited graphene, and hence are naturally scalable for large area applications.

cond-mat.mtrl-sci↗

arXiv2012

High contrast imaging and thickness determination of graphene with in-column secondary electron microscopy

We report a new method for quantitative estimation of graphene layer thicknesses using high contrast imaging of graphene films on insulating substrates with a scanning electron microscope. By detecting the attenuation of secondary electrons emitted from the substrate with an in-column low-energy electron detector, we have achieved very high thickness-dependent contrast that allows quantitative estimation of thickness up to several graphene layers. The nanometer scale spatial resolution of the electron micrographs also allows a simple structural characterization scheme for graphene, which has been applied to identify faults, wrinkles, voids, and patches of multilayer growth in large-area chemical vapor deposited graphene. We have discussed the factors, such as differential surface charging and electron beam induced current, that affect the contrast of graphene images in detail.

cond-mat.mes-hall↗

arXiv2012

Electrochemical integration of graphene with light absorbing copper-based thin films

We present an electrochemical route for the integration of graphene with light sensitive copper-based alloys used in optoelectronic applications. Graphene grown using chemical vapor deposition (CVD) transferred to glass is found to be a robust substrate on which photoconductive Cu_{x}S films of 1-2 um thickness can be deposited. The effect of growth parameters on the morphology and photoconductivity of Cu_{x}S films is presented. Current-voltage characterization and photoconductivity decay experiments are performed with graphene as one contact and silver epoxy as the other.

cond-mat.mtrl-sci↗

arXiv2010

1/f noise as a probe for investigating band structure in graphene

A distinctive feature of single layer graphene is the linearly dispersive energy bands, which in case of multilayer graphene become parabolic. Other than the quantum Hall effect, this distinction has been hard to capture in electron transport. Carrier mobility of graphene has been scrutinized, but many parallel scattering mechanisms often obscure its sensitivity to band structure. The flicker noise in graphene depends explicitly on its ability to screen local potential fluctuations. Here we show that the flicker noise is a sensitive probe to the band structure of graphene that vary differently with the carrier density for the linear and parabolic bands. Using devices of different genre, we find this difference to be robust against disorder in the presence or absence of substrate. Our results reveal the microscopic mechanism of noise in Graphene Field Effect Transistors (GraFET), and outline a simple portable method to separate the single from multi layered devices.

cond-mat.mes-hall↗