arXiv subjects

Sriram Krishnamoorthy

Publications and source records attributed to Sriram Krishnamoorthy.

At least 55 records · Page 3Linked to original sources

arXiv2021

N-type doping of LPCVD-grown \b{eta}-Ga2O3 thin films using solid-source germanium

We report on the growth and characterization of Ge-doped \b{eta}-Ga2O3 thin films using a solid germanium source. \b{eta}-Ga2O3 thin films were grown using a low-pressure chemical vapor deposition (LPCVD) reactor with either an oxygen or gallium delivery tube. Films were grown on 6 degree offcut sapphire and (010) \b{eta}-Ga2O3 substrates with growth rates between 0.5 - 22 μm/hr. By controlling the germanium vapor pressure, a wide range of Hall carrier concentrations between 10^17 - 10^19 cm-3 were achieved. Low-temperature Hall data revealed a difference in donor incorporation depending on the reactor configuration. At low growth rates, germanium occupied a single donor energy level between 8 - 10 meV. At higher growth rates, germanium doping predominantly results in a deeper donor energy level at 85 meV. This work shows the effect of reactor design and growth regime on the kinetics of impurity incorporation. Studying donor incorporation in \b{eta}-Ga2O3 is important for the design of high-power electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

COMET: A Domain-Specific Compilation of High-Performance Computational Chemistry

The computational power increases over the past decades havegreatly enhanced the ability to simulate chemical reactions andunderstand ever more complex transformations. Tensor contractions are the fundamental computational building block of these simulations. These simulations have often been tied to one platform and restricted in generality by the interface provided to the user. The expanding prevalence of accelerators and researcher demands necessitate a more general approach which is not tied to specific hardware or requires contortion of algorithms to specific hardware platforms. In this paper we present COMET, a domain-specific programming language and compiler infrastructure for tensor contractions targeting heterogeneous accelerators. We present a system of progressive lowering through multiple layers of abstraction and optimization that achieves up to 1.98X speedup for 30 tensor contractions commonly used in computational chemistry and beyond.

cs.MS↗

arXiv2020

Growth and Characterization of Metalorganic Vapor-Phase Epitaxy-Grown \b{eta}-(AlxGa1-x)2O3/\b{eta}-Ga2O3 Heterostructure Channels

We report on the growth and characterization of metalorganic vapor-phase epitaxy-grown \b{eta}-(AlxGa1-x)2O3/\b{eta}-Ga2O3 modulation-doped heterostructures. Electron channel is realized in the heterostructure by utilizing a delta-doped \b{eta}-(AlxGa1-x)2O3 barrier. Electron channel characteristics are studied using transfer length method, capacitance-voltage and Hall measurements. Hall sheet charge density of 1.06 x 1013 cm-2 and mobility of 111 cm2/Vs is measured at room temperature. Fabricated transistor showed peak current of 22 mA/mm and on-off ratio of 8 x 106. Sheet resistance of 5.3 kΩ/Square is measured at room temperature, which includes contribution from a parallel channel in \b{eta}-(AlxGa1-x)2O3.

physics.app-ph↗

arXiv2020

Defect states and their electric field-enhanced electron thermal emission in heavily Zr-doped beta-Ga2O3 crystals

Performing deep level transient spectroscopy (DLTS) on Schottky diodes, we investigated defect levels below the conduction band minima (Ec) in Czochralski (CZ) grown unintentionally-doped (UID) and vertical gradient freeze (VGF)-grown Zr-doped beta-Ga2O3 crystals. In UID crystals with an electron concentration of 10^17 cm-3, we observe levels at 0.18 eV and 0.46 eV in addition to the previously reported 0.86 (E2) and 1.03 eV (E3) levels. For 10^18 cm-3 Zr-doped Ga2O3, signatures at 0.30 eV (E15) and 0.71 eV (E16) are present. For the highest Zr doping of 5*10^18 cm-3, we observe only one signature at 0.59 eV. Electric field-enhanced emission rates are demonstrated via increasing the reverse bias during measurement. The 0.86 eV signature in the UID sample displays phonon-assisted tunneling enhanced thermal emission and is consistent with the widely reported E2 (FeGa) defect. The 0.71 eV (E16) signature in the lower-Zr-doped crystal also exhibits phonon-assisted tunneling emission enhancement. Taking into account that the high doping in the Zr-doped diodes also increases the electric field, we propose that the 0.59 eV signature in the highest Zr-doped sample likely corresponds to the 0.71 eV signature in lower-doped samples. Our analysis highlights the importance of testing for and reporting on field-enhanced emission especially the electric field present during DLTS and other characterization experiments on beta-Ga2O3 along with the standard emission energy, cross-section, and lambda-corrected trap density. This is important because of the intended use of beta-Ga2O3 in high-field devices and the many orders of magnitude of possible doping.

cond-mat.mtrl-sci↗

arXiv2020

The anisotropic quasi-static permittivity of single-crystal beta-Ga2O3

The quasi-static anisotropic permittivity parameters of electrically insulating gallium oxide (beta-Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200 GHz to 1 THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c, and b, and reciprocal lattice direction a*. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio-frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high frequency contributions, and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of beta-Ga2O3 in high-frequency electronic devices

cond-mat.mtrl-sci↗

arXiv2020

Impurity Band Conduction in Si-doped \b{eta}-Ga2O3 Films

By combining temperature-dependent resistivity and Hall effect measurements, we investigate donor state energy in Si-doped \b{eta}-Ga2O3 films grown using metal-organic vapor phase epitaxy (MOVPE). High magnetic field Hall effect measurements (H = +/-90 kOe) showed non-linear Hall resistance for T < 150 K revealing two-band conduction. Further analyses revealed carrier freeze-out characteristics in both bands yielding donor state energies of ~ 33.7 and ~ 45.6 meV. The former is consistent with the donor energy of Si in \b{eta}-Ga2O3 whereas the latter suggests a residual donor state, likely associated with a DX center. This study provides a critical insight into the impurity band conduction and the defect energy states in \b{eta}-Ga2O3 using high-field magnetotransport measurements.

cond-mat.mtrl-sci↗

arXiv2020

Oxygen annealing induced changes in defects within beta-Ga2O3 epitaxial films measured using photoluminescence

In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, UV', blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy (MOVPE) induced by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find an overall activation energy for the generation of non-radiative defects of 0.69 eV in the bulk but 1.55 eV near the surface. We also deduce activation energies for the green emission-related defects of 0.60 eV near the surface and 0.89-0.92 eV through the films, whereas the blue-related defects have activation energy in the range 0.43-0.62 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 oC. These observations are consistent with the formation and diffusion of VGa and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.

cond-mat.mtrl-sci↗

arXiv2020

Delta-doped \b{eta}-Ga2O3 Films With Low FWHM Charge Profile Grown By Metalorganic Vapor-Phase Epitaxy

We report on low-temperature MOVPE growth of silicon delta-doped \b{eta}-Ga2O3 films with low FWHM. The as-grown films are characterized using Secondary-ion mass spectroscopy, Capacitance-Voltage and Hall techniques. SIMS measurements show that surface segregation is the chief cause of large FWHM in MOVPE-grown films. The surface segregation coefficient (R) is observed to reduce with reduction in the growth temperature. Films grown at 600 °C show an electron concentration of 9.7 x 1012 cm-2 and a FWHM of 3.2 nm. High resolution scanning/transmission electron microscopy of the epitaxial film did not reveal any significant observable degradation in crystal quality of the delta sheet and surrounding regions. Hall measurements of delta-doped film on Fe-doped substrate showed a sheet charge density of 6.1 x 1012 cm-2 and carrier mobility of 83 cm2/V. s. Realization of sharp delta doping profiles in MOVPE-grown \b{eta}-Ga2O3 is promising for high performance device applications.

physics.app-ph↗

arXiv2020

FPDetect: Efficient Reasoning About Stencil Programs Using Selective Direct Evaluation

We present FPDetect, a low overhead approach for detecting logical errors and soft errors affecting stencil computations without generating false positives. We develop an offline analysis that tightly estimates the number of floating-point bits preserved across stencil applications. This estimate rigorously bounds the values expected in the data space of the computation. Violations of this bound can be attributed with certainty to errors. FPDetect helps synthesize error detectors customized for user-specified levels of accuracy and coverage. FPDetect also enables overhead reduction techniques based on deploying these detectors coarsely in space and time. Experimental evaluations demonstrate the practicality of our approach.

cs.DC↗

arXiv2020

Analytical Modeling and Design of Gallium Oxide Schottky Barrier Diodes Beyond Unipolar Figure of Merit Using High-k Dielectric Superjunction Structures

This work presents the design of beta-Ga2O3 schottky barrier diode using high-k dielectric superjunction to significantly enhance the breakdown voltage vs on-resistance trade-off beyond its already high unipolar figure of merit. The device parameters are optimized using both TCAD simulations and analytical modeling using conformal mapping technique. The dielectric superjunction structure is found to be highly sensitive to the device dimensions and the dielectric constant of the insulator. The aspect ratio, which is the ratio of the length to the width of the drift region, is found to be the most important parameter in designing the structure and the proposed approach only works for aspect ratio much greater than one. The width of the dielectric layer and the dielectric constant also plays a crucial role in improving the device properties and are optimized to achieve maximum figure of merit. Using the optimized structure with an aspect ratio of 10 and a dielectric constant of 300, the structure is predicted to surpass the b-Ga2O3 unipolar figure of merit by four times indicating the promise of such structures for exceptional FOM vertical power electronics.

physics.app-ph↗

arXiv2020

Low Temperature Homoepitaxy Of (010) $β$-Ga$_2$O$_3$ By Metalorganic Vapor Phase Epitaxy : Expanding The Growth Window

In this work, we report on the growth of high-mobility $β$-Ga$_2$O$_3$ homoepitaxial thin films grown at a temperature much lower than the conventional growth temperature window for metalorganic vapor phase epitaxy. Low-temperature $β$-Ga$_2$O$_3$ thin films grown at 600$^{\circ}$C on Fe-doped (010) bulk substrates exhibits remarkable crystalline quality which is evident from the measured room temperature Hall mobility of 186 cm$^2$/Vs for the unintentionally doped films. N-type doping is achieved by using Si as a dopant and controllable doping in the range of 2$\times$10$^{16}$ - 2$\times$10$^{19}$ cm$^{-3}$ is studied. Si incorporation and activation is studied by comparing silicon concentration from secondary ion mass spectroscopy (SIMS) and electron concentration from temperature-dependent Hall measurements. The films exhibit high purity (low C and H concentrations) with a very low concentration of compensating acceptors (2$\times$10$^{15}$ cm$^{-3}$) even at this growth temperature. Additionally, abrupt doping profile with forward decay of $\sim$ 5nm/dec (10 times improvement compared to what is observed for thin films grown at 810$^{\circ}$C) is demonstrated by growing at a lower temperature.

physics.app-ph↗

arXiv2020

An Abstraction-guided Approach to Scalable and Rigorous Floating-Point Error Analysis

Automated techniques for rigorous floating-point round-off error analysis are important in areas including formal verification of correctness and precision tuning. Existing tools and techniques, while providing tight bounds, fail to analyze expressions with more than a few hundred operators, thus unable to cover important practical problems. In this work, we present Satire, a new tool that sheds light on how scalability and bound-tightness can be attained through a combination of incremental analysis, abstraction, and judicious use of concrete and symbolic evaluation. Satire has handled problems exceeding 200K operators. We present Satire's underlying error analysis approach, information-theoretic abstraction heuristics, and a wide range of case studies, with evaluation covering FFT, Lorenz system of equations, and various PDE stencil types. Our results demonstrate the tightness of Satire's bounds, its acceptable runtime, and valuable insights provided.

cs.PL↗

arXiv2020

Design of a $β$-Ga$_2$O$_3$ Schottky Barrier Diode With p-type III-Nitride Guard Ring for Enhanced Breakdown

This work presents the electrostatic analysis of a novel Ga$_2$O$_3$ vertical Schottky diode with three different guard ring configurations to reduce the peak electric field at the metal edges. Highly doped p-type GaN, p-type nonpolar AlGaN and polarization doped graded p-AlGaN are simulated and analyzed as the guard ring material, which forms a heterojunction with the Ga$_2$O$_3$ drift layer. Guard ring with non-polar graded p-AlGaN with a bandgap larger than Ga$_2$O$_3$ is found to show the best performance in terms of screening the electric field at the metal edges. The proposed guard ring configuration is also compared with a reported Ga$_2$O$_3$ Schottky diode with no guard ring and a structure with a high resistive Nitrogen-doped guard ring. The optimized design is predicted to have breakdown voltage as high as 5.3 kV and a specific on-resistance of 3.55 m$Ω$-cm$^2$ which leads to an excellent power figure of merit of 7.91 GW/cm$^2$.

physics.app-ph↗

arXiv2020

Theoretical Investigation of Optical Intersubband Transitions and Infrared Photodetection in $β$-(AlxGa1-x)2O3/Ga2O3 Quantum Well Structures

We provide theoretical consideration of intersubband transitions designed in the ultrawide bandgap Aluminum Gallium Oxide ((AlxGa1-x)2O3)/Gallium Oxide (Ga2O3)) quantum well system. Conventional material systems have matured into successful intersubband device applications such as large area quantum well infrared photodetector(QWIP) focal plane arrays for reproducible imaging systems but are fundamentally limited via maximum conduction band offsets to mid and long wavelength infrared applications. Short and near infrared devices are technologically important to optical communications systems and biomedical imaging applications, but are difficult to realize in intersubband designs for this reason. In this work, we use a first principles approach to estimate the expansive design space of monoclinic $β$(AlxGa1x)2O3/Ga2O3 material system, which reaches from short wavelength infrared (1 to 3μm) to far infrared (greater than 30μm) transition wavelengths. We estimate the performance metrics of two QWIPs operating in the long and short wavelength regimes, including an estimation of high room temperature detectivity (about 10^11 Jones) at the optical communication wavelength λp 1.55μm. Our findings demonstrate the potential of the rapidly maturing (AlxGa1-x)2O3/Ga2O3 material system to open the door for intersubband device applications.

physics.optics↗

arXiv2020

Highly tunable polarization-engineered two-dimensional electron gas in $ε$-AlGaO3 / $ε$-Ga2O3 heterostructures

We report on the modeling of polarization-induced two-dimensional electron gas (2DEG) formation at $ε$-AlGaO3 / $ε$-Ga2O3 heterointerface and the effect of spontaneous polarization (Psp) reversal on 2DEG density in $ε$-Ga2O3 /$ε$-AlGaO3 / $ε$-Ga2O3 double heterostructures. Density-functional theory (DFT) is utilized to calculate the material properties of $ε$-Ga2O3 and $ε$-AlGaO3 alloys. Using Schrodinger-Poisson solver along with DFT calculated parameters, the 2DEG density is calculated as a function of barrier type and thickness. By optimizing the layer thicknesses of $ε$-Ga2O3/$ε$-AlGaO3/$ε$-Ga2O3 heterostructures, charge contrast ratios exceeding 1600 are obtained. This computational study indicates the high potential for $ε$-Ga2O3-based heterostructure devices for non-volatile memories and neuromorphic applications.

cond-mat.mtrl-sci↗

arXiv2020

Delta-doped \b{eta}-Ga2O3 thin films and \b{eta}-(Al0.26Ga0.74)2O3/\b{eta}-Ga2O3 heterostructures grown by metalorganic vapor-phase epitaxy

We report on silicon delta doping of metalorganic vapor-phase epitaxy-grown \b{eta}-Ga2O3 thin films using silane precursor. Delta-doped \b{eta}-Ga2O3 epitaxial films are characterized using capacitance-voltage profiling and secondary-ion mass spectroscopy. Electron sheet charge density in the range of 2.9e12 cm-2 to 8e12 cm-2 with a half width at half maximum ranging from 6.2 nm to 3.5 nm is measured. We also demonstrate a high density (6.4e12 cm-2) degenerate two-dimensional electron gas using a delta-doped \b{eta}-(Al0.26Ga0.74)2O3/\b{eta}-Ga2O3 heterostructure.The total charge could also include a contribution from a parallel channel in the \b{eta}-(Al0.26Ga0.74)2O3 alloy barrier.

cond-mat.mtrl-sci↗

arXiv2020

Degenerate doping in \b{eta}-Ga2O3 Single Crystals through Hf-doping

N type conductivity of \b{eta}-Ga2O3 grown from the melt is typically achieved using Sn and Si. In this paper, we experimentally and computationally investigate Hf doping of \b{eta}-Ga2O3 single crystals using UV-Vis-NIR absorption and Hall Effect measurements and hybrid functional calculations. Unintentionally-doped and Hf-doped samples with a nominal concentration of 0.5at% were grown from the melt using vertical gradient freeze (VGF) and Czochralski method in mixed Ar+O2 atmosphere. We demonstrate Hf dopants, predicted to incorporate on the octahedral GaII site as a shallow donor, achieve degenerate doping in \b{eta}-Ga2O3 with a measured electron concentration 2 x 10^19 cm^-3 , mobility 80-65 cm^2 /Vs, and resistivity down to 5 mOhm-cm in our samples. The concentration of Hf was measured to be 1.3 x 10^19 atoms/cm^3 using glow discharge mass spectroscopy (GDMS) on doped samples, confirming Hf to be the cause of n-type conductivity (electron concentration ~2 x 10^19 cm-3).

cond-mat.mtrl-sci↗

arXiv2020

Schottky Barrier Height Engineering In $β$-Ga$_2$O$_3$ Using SiO$_2$ Interlayer Dielectric

This paper reports on the modulation of Schottky barrier heights (SBH) on three different orientations of $β$-Ga$_2$O$_3$ by insertion of an ultra-thin SiO$_2$ dielectric interlayer at the metal-semiconductor junction, which can potentially lower the Fermi-level pinning (FLP) effect due to metal-induced gap states (MIGS). Pt and Ni metal-semiconductor (MS) and metal-interlayer-semiconductor (MIS) Schottky barrier diodes were fabricated on bulk n-type doped $β$-Ga$_2$O$_3$ single crystal substrates along the (010), (-201) and (100) orientations and were characterized by room temperature current-voltage (I-V) and capacitance-voltage (C-V) measurements. Pt MIS diodes exhibited 0.53 eV and 0.37 eV increment in SBH along the (010) and (-201) orientations respectively as compared to their respective MS counterparts. The highest SBH of 1.81 eV was achieved on the (010)-oriented MIS SBD using Pt metal. The MIS SBDs on (100)-oriented substrates exhibited a dramatic increment ($>$1.5$\times$) in SBH as well as reduction in reverse leakage current. The use of thin dielectric interlayers can be an efficient experimental method to modulate SBH of metal/Ga$_2$O$_3$ junctions.

physics.app-ph↗