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Extremely Low Thermal Resistance Architectures for AlxGaN1-x Semiconductor Devices
Authors:
Kidus Guye,
Davide Orlandini,
Seungheon Shin,
Andy Allerman,
Damena Agonafer,
Siddharth Rajan,
Samuel Graham
Abstract:
Next-generation high-power radio-frequency (RF) devices increasingly demand transistors that operate efficiently with high gain at high frequencies. High-aluminum-content ultra-wide-bandgap (UWBG) AlGaN alloys have shown great potential for enabling such high-frequency RF technologies. However, the widespread adoption of AlGaN-based RF devices is limited by thermal-management challenges arising fr…
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Next-generation high-power radio-frequency (RF) devices increasingly demand transistors that operate efficiently with high gain at high frequencies. High-aluminum-content ultra-wide-bandgap (UWBG) AlGaN alloys have shown great potential for enabling such high-frequency RF technologies. However, the widespread adoption of AlGaN-based RF devices is limited by thermal-management challenges arising from the intrinsically low thermal conductivity of AlGaN, which leads to higher device thermal resistance for a given geometry compared to GaN RF devices. As a result, these next-generation devices are highly susceptible to self-heating. This study investigates the thermal behavior of UWBG AlGaN devices, focusing on the effects of AlGaN channel thickness, substrate technology, and high-k material integration on reducing device thermal resistance to enable high-power operation. Experimental results demonstrate a record-low thermal resistance of 3.96 mm$\cdot$K/W when an AlN substrate is employed and the AlGaN channel thickness is reduced to 5 nm. These findings provide valuable insights into mitigating thermal limitations in UWBG devices through device-level engineering and the strategic integration of high-k materials.
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Submitted 21 February, 2026;
originally announced February 2026.
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High-Resolution Electron Paramagnetic Resonance
Authors:
Colin J. Stephen,
Anton Tcholakov,
Maik Icker,
Stuart M. Graham,
Xiaoming Zhao,
Robert Day,
Jeanette Chattaway,
T. John S. Dennis,
Wolfgang Harneit,
Gavin W. Morley
Abstract:
Electron paramagnetic resonance (EPR) is a valuable tool for physics, chemistry, biology and medicine, providing complementary spectroscopic information to NMR. It has long been known that EPR at high magnetic fields offers greater spectral resolution, but limitations in the THz instrumentation have prevented the full realization of these opportunities. Here we describe an EPR spectrometer at the…
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Electron paramagnetic resonance (EPR) is a valuable tool for physics, chemistry, biology and medicine, providing complementary spectroscopic information to NMR. It has long been known that EPR at high magnetic fields offers greater spectral resolution, but limitations in the THz instrumentation have prevented the full realization of these opportunities. Here we describe an EPR spectrometer at the high magnetic field of 14 T using 396 GHz excitation, which adapts techniques from liquid-state NMR to obtain sharp EPR resonances with a width of 210 ppb (full-width half-maximum). We use this to measure resonance positions, and hence g-factors, with a precision that reaches $\pm$16 ppb. Our use of in-situ liquid-state NMR of our solvent within the same sample improves the accuracy of these measurements: it allows us to reference our EPR measurement back to dilute gas 3He NMR for which quantum calculations are accurate. We measure the g-factor of N@C60 in deuterated toluene as g = 2.002 099 09 (3), where the 3 in brackets means that the uncertainty on the last digit is $\pm$3.
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Submitted 19 January, 2026;
originally announced January 2026.
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High Field Diamond Magnetometry Towards Tokamak Diagnostics
Authors:
S. M. Graham,
C. J. Stephen,
A. J. Newman,
A. M. Edmonds,
M. L. Markham,
G. W. Morley
Abstract:
Nitrogen vacancy centres (NVC) in diamond have been widely used for near-dc magnetometry. The intrinsic properties of diamonds make them potential candidates for tokamak fusion power diagnostics, where radiation-hard magnetometers will be essential for efficient control. An NVC magnetometer placed in a tokamak will need to operate within a $\geq$ 1 T magnetic field. In this work, we demonstrate fi…
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Nitrogen vacancy centres (NVC) in diamond have been widely used for near-dc magnetometry. The intrinsic properties of diamonds make them potential candidates for tokamak fusion power diagnostics, where radiation-hard magnetometers will be essential for efficient control. An NVC magnetometer placed in a tokamak will need to operate within a $\geq$ 1 T magnetic field. In this work, we demonstrate fibre-coupled ensemble NVC optically detected magnetic resonance (ODMR) and magnetometry measurements at magnetic fields up to 1.2 T. Sensitivities of approximately 240 to 600 nT/$\sqrt{\textrm{Hz}}$ and 110 nT/$\sqrt{\textrm{Hz}}$ are achieved in a (10-150) Hz frequency range, for non-degenerate and near-$\langle$111$\rangle$ field alignments respectively.
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Submitted 19 January, 2026;
originally announced January 2026.
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Scaled Ultra-Wide Bandgap AlGaN Polarization-Graded FET with Ultra-thin Buffer Layer
Authors:
Yinxuan Zhu,
Ashley Wissel-Garcia,
Kidus Guye,
Chandan Joishi,
Can Cao,
Seungheon Shin,
Kyle Liddy,
Emils G. B. Jurcik,
Agnes Maneesha Dominic Merwin Xavier,
Andrew A. Allerman,
Brianna A. Klein,
Andrew Amrstrong,
James S. Speck,
Samuel Graham,
Siddharth Rajan
Abstract:
We report on the design and demonstration of ultra-wide bandgap AlGaN polarization-graded field effect transistors with ultra-thin channels to enable excellent current density and high-frequency performance while significantly reducing thermal resistance. We use polarization-graded AlGaN layers and ultra-thin pseudomorphic AlGaN buffer layers to enable low thermal resistance and excellent structur…
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We report on the design and demonstration of ultra-wide bandgap AlGaN polarization-graded field effect transistors with ultra-thin channels to enable excellent current density and high-frequency performance while significantly reducing thermal resistance. We use polarization-graded AlGaN layers and ultra-thin pseudomorphic AlGaN buffer layers to enable low thermal resistance and excellent structural quality. The polarization-graded field effect transistors (PolFETs) demonstrated here show Imax over 800mA/mm and current/power gain cutoff frequency (fT/fmax) of 26/28 GHz. Small signal modeling and analysis were used to determine parasitic/transit delays, and gate-resistance thermometry was implemented to thermally characterize AlGaN PolFET and benchmark against state-of-the-art AlGaN HEMTs. The ultra-thin AlGaN PolFET showed thermal resistance of 12 K.mm/W, representing a significant reduction from typical AlGaN transistors. These results show state-of-art combination of high current density, excellent fT-LG product for ultra-wide bandgap AlGaN transistors, and superior thermal performance, and highlight the promise of AlGaN transistors for future RF and mm-wave applications.
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Submitted 19 December, 2025;
originally announced December 2025.
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A portable LED-based diamond magnetometer for outreach and teaching labs
Authors:
Hollis Williams,
Alex Newman,
Stuart Graham,
Colin Stephen,
Gavin Morley
Abstract:
We present a compact, low-cost version of an NV center diamond magnetometer which replaces the standard green laser with a high-power LED. This modification improves safety, reduces cost, and allows the green excitation and red photoluminescence to be viewed directly during demonstrations. The device is simple to assemble and suitable for outreach activities and undergraduate laboratories. We show…
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We present a compact, low-cost version of an NV center diamond magnetometer which replaces the standard green laser with a high-power LED. This modification improves safety, reduces cost, and allows the green excitation and red photoluminescence to be viewed directly during demonstrations. The device is simple to assemble and suitable for outreach activities and undergraduate laboratories. We show that it can produce ODMR spectra and respond to nearby magnetic objects, with a sensitivity on the order of 1 $μ$T/$\sqrt{\text{Hz}}$. Supplementary material provides details of the construction and suggestions for student investigations to support use in teaching laboratories.
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Submitted 6 February, 2026; v1 submitted 1 December, 2025;
originally announced December 2025.
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A Spin-Based Pathway to Testing the Quantum Nature of Gravity
Authors:
Sougato Bose,
Anupam Mazumdar,
Roger Penrose,
Ivette Fuentes,
Marko Toroš,
Ron Folman,
Gerard J. Milburn,
Myungshik Kim,
Adrian Kent,
A. T. M. Anishur Rahman,
Cyril Laplane,
Aaron Markowitz,
Debarshi Das,
Ethan Campos-Méndez,
Eva Kilian,
David Groswasser,
Menachem Givon,
Or Dobkowski,
Peter Skakunenko,
Maria Muretova,
Yonathan Japha,
Naor Levi,
Omer Feldman,
Damián Pitalúa-García,
Jonathan M. H. Gosling
, et al. (30 additional authors not shown)
Abstract:
A key open problem in physics is the correct way to combine gravity (described by general relativity) with everything else (described by quantum mechanics). This problem suggests that general relativity and possibly also quantum mechanics need fundamental corrections. Most physicists expect that gravity should be quantum in character, but gravity is fundamentally different to the other forces beca…
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A key open problem in physics is the correct way to combine gravity (described by general relativity) with everything else (described by quantum mechanics). This problem suggests that general relativity and possibly also quantum mechanics need fundamental corrections. Most physicists expect that gravity should be quantum in character, but gravity is fundamentally different to the other forces because it alone is described by spacetime geometry. Experiments are needed to test whether gravity, and hence space-time, is quantum or classical. We propose an experiment to test the quantum nature of gravity by checking whether gravity can entangle two micron-sized crystals. A pathway to this is to create macroscopic quantum superpositions of each crystal first using embedded spins and Stern-Gerlach forces. These crystals could be nanodiamonds containing nitrogen-vacancy (NV) centres. The spins can subsequently be measured to witness the gravitationally generated entanglement. This is based on extensive theoretical feasibility studies and experimental progress in quantum technology. The eventual experiment will require a medium-sized consortium with excellent suppression of decoherence including vibrations and gravitational noise. In this white paper, we review the progress and plans towards realizing this. While implementing these plans, we will further explore the most macroscopic superpositions that are possible, which will test theories that predict a limit to this.
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Submitted 1 September, 2025;
originally announced September 2025.
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Endoscopic fiber-coupled diamond magnetometer for cancer surgery
Authors:
A. J. Newman,
S. M. Graham,
C. J. Stephen,
A. M. Edmonds,
M. L. Markham,
G. W. Morley
Abstract:
Interoperative measurements using magnetic sensors is a valuable technique in cancer surgery for finding magnetic tracers. Here we present a fiber-coupled nitrogen-vacancy (N-V) center magnetometer capable of detecting iron oxide suspension (MagTrace from Endomagnetics Ltd.) used in breast cancer surgeries. Detection of an iron mass as low as 0.56~mg has been demonstrated, 100 times less than that…
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Interoperative measurements using magnetic sensors is a valuable technique in cancer surgery for finding magnetic tracers. Here we present a fiber-coupled nitrogen-vacancy (N-V) center magnetometer capable of detecting iron oxide suspension (MagTrace from Endomagnetics Ltd.) used in breast cancer surgeries. Detection of an iron mass as low as 0.56~mg has been demonstrated, 100 times less than that of a recommended dose at a maximum distance of 5.8~mm. Detection of an iron concentration as low as 2.8 mg/ml has also been demonstrated, 20 times less than a recommended dose. The maximum working distance from the sensor can be as large as 14.6~mm for higher concentrations. The sensor head has a maximum diameter of 10~mm which would allow it to be used for endoscopy, laparoscopy and interoperative surgery.
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Submitted 8 April, 2025;
originally announced April 2025.
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Experimental observation of ballistic to diffusive transition in AlN thin films
Authors:
Md Shafkat Bin Hoque,
Michael E. Liao,
Saman Zare,
Zeyu Liu,
Yee Rui Koh,
Kenny Huynh,
Jingjing Shi,
Samuel Graham,
Tengfei Luo,
Habib Ahmad,
W. Alan Doolittle,
Mark S. Goorsky,
Patrick E. Hopkins
Abstract:
Bulk AlN possesses high thermal conductivity due to long phonon mean-free-paths, high group velocity, and long lifetimes. However, the thermal transport scenario becomes very different in a thin AlN film due to phonon-defect and phonon-boundary scattering. Herein, we report experimental observation of ballistic to diffusive transition in a series of AlN thin films (1.6 - 2440 nm) grown on sapphire…
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Bulk AlN possesses high thermal conductivity due to long phonon mean-free-paths, high group velocity, and long lifetimes. However, the thermal transport scenario becomes very different in a thin AlN film due to phonon-defect and phonon-boundary scattering. Herein, we report experimental observation of ballistic to diffusive transition in a series of AlN thin films (1.6 - 2440 nm) grown on sapphire substrates. The ballistic transport is characterized by constant thermal resistance as a function of film thickness due to phonon scattering by defects and boundaries. In this transport regime, phonons possess very small group velocities and lifetimes. The lifetime of the optical phonons increases by more than an order of magnitude in the diffusive regime, however, remains nearly constant afterwards. Our study is important for understanding the details of nano and microscale thermal transport in a highly conductive material.
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Submitted 22 September, 2024;
originally announced September 2024.
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Low Thermal Resistance of Diamond-AlGaN Interfaces Achieved Using Carbide Interlayers
Authors:
Henry T. Aller,
Thomas W. Pfeifer,
Abdullah Mamun,
Kenny Huynh,
Marko Tadjer,
Tatyana Feygelson,
Karl Hobart,
Travis Anderson,
Bradford Pate,
Alan Jacobs,
James Spencer Lundh,
Mark Goorsky,
Asif Khan,
Patrick Hopkins,
Samuel Graham
Abstract:
This study investigates thermal transport across nanocrystalline diamond/AlGaN interfaces, crucial for enhancing thermal management in AlGaN/AlGaN-based devices. Chemical vapor deposition growth of diamond directly on AlGaN resulted in a disordered interface with a high thermal boundary resistance (TBR) of 20.6 m^2-K/GW. We employed sputtered carbide interlayers (e.g., $B_4C$, $SiC$, $B_4C/SiC$) t…
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This study investigates thermal transport across nanocrystalline diamond/AlGaN interfaces, crucial for enhancing thermal management in AlGaN/AlGaN-based devices. Chemical vapor deposition growth of diamond directly on AlGaN resulted in a disordered interface with a high thermal boundary resistance (TBR) of 20.6 m^2-K/GW. We employed sputtered carbide interlayers (e.g., $B_4C$, $SiC$, $B_4C/SiC$) to reduce thermal boundary resistance in diamond/AlGaN interfaces. The carbide interlayers resulted in record-low thermal boundary resistance values of 3.4 and 3.7 m^2-K/GW for Al$_{0.65}$Ga$_{0.35}$N samples with $B_4C$ and $SiC$ interlayers, respectively. STEM imaging of the interface reveals interlayer thicknesses between 1.7-2.5 nm, with an amorphous structure. Additionally, Fast-Fourier Transform (FFT) characterization of sections of the STEM images displayed sharp crystalline fringes in the AlGaN layer, confirming it was properly protected from damage from hydrogen plasma during the diamond growth. In order to accurately measure the thermal boundary resistance we develop a hybrid technique, combining time-domain thermoreflectance and steady-state thermoreflectance fitting, offering superior sensitivity to buried thermal resistances. Our findings underscore the efficacy of interlayer engineering in enhancing thermal transport and demonstrate the importance of innovative measurement techniques in accurately characterizing complex thermal interfaces. This study provides a foundation for future research in improving thermal properties of semiconductor devices through interface engineering and advanced measurement methodologies.
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Submitted 15 August, 2024;
originally announced August 2024.
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On the Road with a Diamond Magnetometer
Authors:
S. M. Graham,
A. J. Newman,
C. J. Stephen,
A. M. Edmonds,
D. J. Twitchen,
M. L. Markham,
G. W. Morley
Abstract:
Nitrogen vacancy centres in diamond can be used for vector magnetometry. In this work we present a portable vector diamond magnetometer. Its vector capability, combined with feedback control and robust structure enables operation on moving platforms. While placed on a trolley, magnetic mapping of a room is demonstrated and the magnetometer is also shown to be operational in a moving van with the m…
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Nitrogen vacancy centres in diamond can be used for vector magnetometry. In this work we present a portable vector diamond magnetometer. Its vector capability, combined with feedback control and robust structure enables operation on moving platforms. While placed on a trolley, magnetic mapping of a room is demonstrated and the magnetometer is also shown to be operational in a moving van with the measured magnetic field shifts for the x, y, and z axes being tagged with GPS coordinates. These magnetic field measurements are in agreement with measurements taken simultaneously with a fluxgate magnetometer.
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Submitted 31 January, 2024; v1 submitted 29 January, 2024;
originally announced January 2024.
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Tensor gradiometry with a diamond magnetometer
Authors:
A. J. Newman,
S. M. Graham,
A. M. Edmonds,
D. J. Twitchen,
M. L. Markham,
G. W. Morley
Abstract:
Vector magnetometry provides more information than scalar measurements for magnetic surveys utilized in space, defense, medical, geological and industrial applications. These areas would benefit from a mobile vector magnetometer that can operate in extreme conditions. Here we present a scanning fiber-coupled nitrogen vacancy (NV) center vector magnetometer. Feedback control of the microwave excita…
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Vector magnetometry provides more information than scalar measurements for magnetic surveys utilized in space, defense, medical, geological and industrial applications. These areas would benefit from a mobile vector magnetometer that can operate in extreme conditions. Here we present a scanning fiber-coupled nitrogen vacancy (NV) center vector magnetometer. Feedback control of the microwave excitation frequency is employed to improve dynamic range and maintain sensitivity during movement of the sensor head. Tracking of the excitation frequency shifts for all four orientations of the NV center allow us to image the vector magnetic field of a damaged steel plate. We calculate the magnetic tensor gradiometry images in real time, and they allow us to detect smaller damage than is possible with vector or scalar imaging.
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Submitted 11 July, 2023;
originally announced July 2023.
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Fiber-coupled Diamond Magnetometry with an Unshielded 30 pT/$\sqrt{\textrm{Hz}}$ Sensitivity
Authors:
S. M. Graham,
A. T. M. A. Rahman,
L. Munn,
R. L. Patel,
A. J. Newman,
C. J. Stephen,
G. Colston,
A. Nikitin,
A. M. Edmonds,
D. J. Twitchen,
M. L. Markham,
G. W. Morley
Abstract:
Ensembles of nitrogen vacancy centres (NVCs) in diamond can be employed for sensitive magnetometry. In this work we present a fiber-coupled NVC magnetometer with an unshielded sensitivity of (30 $\pm$ 10) pT/$\sqrt{\textrm{Hz}}$ in a (10 - 500)-Hz frequency range. This sensitivity is enabled by a relatively high green-to-red photon conversion efficiency, the use of a [100] bias field alignment, mi…
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Ensembles of nitrogen vacancy centres (NVCs) in diamond can be employed for sensitive magnetometry. In this work we present a fiber-coupled NVC magnetometer with an unshielded sensitivity of (30 $\pm$ 10) pT/$\sqrt{\textrm{Hz}}$ in a (10 - 500)-Hz frequency range. This sensitivity is enabled by a relatively high green-to-red photon conversion efficiency, the use of a [100] bias field alignment, microwave and lock-in amplifier (LIA) parameter optimisation, as well as a balanced hyperfine excitation scheme. Furthermore, a silicon carbide (SiC) heat spreader is used for microwave delivery, alongside low-strain $^{12}\textrm{C}$ diamonds, one of which is placed in a second magnetically insensitive fluorescence collecting sensor head for common-mode noise cancellation. The magnetometer is capable of detecting signals from sources such as a vacuum pump up to 2 m away, with some orientation dependence but no complete dead zones, demonstrating its potential for use in remote sensing applications.
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Submitted 23 March, 2023; v1 submitted 16 November, 2022;
originally announced November 2022.
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High Thermal Conductivity in Wafer Scale Cubic Silicon Carbide Crystals
Authors:
Zhe Cheng,
Jianbo Liang,
Keisuke Kawamura,
Hidetoshi Asamura,
Hiroki Uratani,
Samuel Graham,
Yutaka Ohno,
Yasuyoshi Nagai,
Naoteru Shigekawa,
David G. Cahill
Abstract:
High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as either active functional materials or thermal management materials. We report an isotropic high thermal conductivity over 500 W m-1K-1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals…
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High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as either active functional materials or thermal management materials. We report an isotropic high thermal conductivity over 500 W m-1K-1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals (only surpassed by diamond). Furthermore, the corresponding 3C-SiC thin films are found to have record-high in-plane and cross-plane thermal conductivity, even higher than diamond thin films with equivalent thicknesses. Our results resolve a long-lasting puzzle that the literature values of thermal conductivity for 3C-SiC are perplexingly lower than the structurally more complex 6H-SiC. Further analysis reveals that the observed high thermal conductivity in this work arises from the high purity and high crystal quality of 3C-SiC crystals which excludes the exceptionally strong defect-phonon scatterings in 3C-SiC. Moreover, by integrating 3C-SiC with other semiconductors by epitaxial growth, we show that the measured 3C-SiC-Si TBC is among the highest for semiconductor interfaces. These findings not only provide insights for fundamental phonon transport mechanisms, also suggest that 3C-SiC may constitute an excellent wide-bandgap semiconductor for applications of power electronics as either active components or substrates.
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Submitted 11 July, 2022;
originally announced July 2022.
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Measurements and Numerical Calculations of Thermal Conductivity to Evaluate the Quality of β-Gallium Oxide Thin Films Grown on Sapphire and Silicon Carbide by Molecular Beam Epitaxy
Authors:
Diego Vaca,
Matthew Barry,
Luke Yates,
Neeraj Nepal,
D. Scott Katzer,
Brian P. Downey,
Virginia Wheeler,
Luke Nyakiti,
David J. Meyer,
Samuel Graham,
Satish Kumar
Abstract:
We report a method to obtain insights into lower thermal conductivity of β-Ga2O3 thin films grown by molecular beam epitaxy (MBE) on c-plane sapphire and 4H-SiC substrates. We compare experimental values against the numerical predictions to decipher the effect of boundary scattering and defects in thin-films. We used time domain thermoreflectance (TDTR) to perform the experiments, density function…
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We report a method to obtain insights into lower thermal conductivity of β-Ga2O3 thin films grown by molecular beam epitaxy (MBE) on c-plane sapphire and 4H-SiC substrates. We compare experimental values against the numerical predictions to decipher the effect of boundary scattering and defects in thin-films. We used time domain thermoreflectance (TDTR) to perform the experiments, density functional theory and the Boltzmann transport equation for thermal conductivity calculations, and the diffuse mismatch model for TBC predictions. The experimental thermal conductivities were approximately 3 times smaller than those calculated for perfect Ga2O3 crystals of similar size. When considering the presence of grain boundaries, gallium and oxygen vacancies, and stacking faults in the calculations, the crystals that present around 1% of gallium vacancies and a density of stacking faults of 106 faults/cm were the ones whose thermal conductivities were closer to the experimental results. Our analysis suggests the level of different types of defects present in the Ga2O3 crystal that could be used to improve the quality of MBE-grown samples by reducing these defects and thereby produce materials with higher thermal conductivities.
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Submitted 4 May, 2022;
originally announced May 2022.
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Pattern Formation in Quantum Ferrofluids: from Supersolids to Superglasses
Authors:
J. Hertkorn,
J. -N. Schmidt,
M. Guo,
F. Böttcher,
K. S. H. Ng,
S. D. Graham,
P. Uerlings,
T. Langen,
M. Zwierlein,
T. Pfau
Abstract:
Pattern formation is a ubiquitous phenomenon observed in nonlinear and out-of-equilibrium systems. In equilibrium, quantum ferrofluids formed from ultracold atoms were recently shown to spontaneously develop coherent density patterns, manifesting a supersolid. We theoretically investigate the phase diagram of such quantum ferrofluids in oblate trap geometries and find an even wider range of exotic…
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Pattern formation is a ubiquitous phenomenon observed in nonlinear and out-of-equilibrium systems. In equilibrium, quantum ferrofluids formed from ultracold atoms were recently shown to spontaneously develop coherent density patterns, manifesting a supersolid. We theoretically investigate the phase diagram of such quantum ferrofluids in oblate trap geometries and find an even wider range of exotic states of matter. Two-dimensional supersolid crystals formed from individual ferrofluid quantum droplets dominate the phase diagram at low densities. For higher densities we find honeycomb and labyrinthine states, as well as a pumpkin phase. We discuss scaling relations which allow us to find these phases for a wide variety of trap geometries, interaction strengths, and atom numbers. Our study illuminates the origin of the various possible patterns of quantum ferrofluids and shows that their occurrence is generic of strongly dipolar interacting systems stabilized by beyond mean-field effects.
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Submitted 25 March, 2021;
originally announced March 2021.
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Supersolidity in Two-Dimensional Trapped Dipolar Droplet Arrays
Authors:
J. Hertkorn,
J. -N. Schmidt,
M. Guo,
F. Böttcher,
K. S. H. Ng,
S. D. Graham,
P. Uerlings,
H. P. Büchler,
T. Langen,
M. Zwierlein,
T. Pfau
Abstract:
We theoretically investigate the ground states and the spectrum of elementary excitations across the superfluid to droplet crystallization transition of an oblate dipolar Bose-Einstein condensate. We systematically identify regimes where spontaneous rotational symmetry breaking leads to the emergence of a supersolid phase with characteristic collective excitations, such as the Higgs amplitude mode…
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We theoretically investigate the ground states and the spectrum of elementary excitations across the superfluid to droplet crystallization transition of an oblate dipolar Bose-Einstein condensate. We systematically identify regimes where spontaneous rotational symmetry breaking leads to the emergence of a supersolid phase with characteristic collective excitations, such as the Higgs amplitude mode. Furthermore, we study the dynamics across the transition and show how these supersolids can be realized with standard protocols in state-of-the-art experiments.
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Submitted 17 March, 2021;
originally announced March 2021.
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Phonon heat conduction in Al1-xScxN thin films
Authors:
Chao Yuan,
Mingyo Park,
Yue Zheng,
Jingjing Shi,
Rytis Dargis,
Samuel Graham,
Azadeh Ansari
Abstract:
Aluminum scandium nitride alloy (Al1-xScxN) is regarded as a promising material for high-performance acoustic devices used in wireless communication systems. Phonon scattering and heat conduction processes govern the energy dissipation in acoustic resonators, ultimately determining their performance quality. This work reports, for the first time, on phonon scattering processes and thermal conducti…
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Aluminum scandium nitride alloy (Al1-xScxN) is regarded as a promising material for high-performance acoustic devices used in wireless communication systems. Phonon scattering and heat conduction processes govern the energy dissipation in acoustic resonators, ultimately determining their performance quality. This work reports, for the first time, on phonon scattering processes and thermal conductivity in Al1-xScxN alloys with the Sc content (x) up to 0.26. The thermal conductivity measured presents a descending trend with increasing x. Temperature-dependent measurements show an increase in thermal conductivity as the temperature increases at temperatures below 200K, followed by a plateau at higher temperatures (T> 200K). Application of a virtual crystal phonon conduction model allows us to elucidate the effects of boundary and alloy scattering on the observed thermal conductivity behaviors. We further demonstrate that the alloy scattering is caused mainly by strain-field difference, and less by the atomic mass difference between ScN and AlN, which is in contrast to the well-studied Al1-xGaxN and SixGe1-x alloy systems where atomic mass difference dominates the alloy scattering. This work studies and provides the quantitative knowledge for phonon scattering and the thermal conductivity in Al1-xScxN, paving the way for future investigation of materials and design of acoustic devices.
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Submitted 24 February, 2021;
originally announced February 2021.
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Roton Excitations in an Oblate Dipolar Quantum Gas
Authors:
Jan-Niklas Schmidt,
Jens Hertkorn,
Mingyang Guo,
Fabian Böttcher,
Matthias Schmidt,
Kevin S. H. Ng,
Sean D. Graham,
Tim Langen,
Martin Zwierlein,
Tilman Pfau
Abstract:
We observe signatures of radial and angular roton excitations around a droplet crystallization transition in dipolar Bose-Einstein condensates. In situ measurements are used to characterize the density fluctuations near this transition. The static structure factor is extracted and used to identify the radial and angular roton excitations by their characteristic symmetries. These fluctuations peak…
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We observe signatures of radial and angular roton excitations around a droplet crystallization transition in dipolar Bose-Einstein condensates. In situ measurements are used to characterize the density fluctuations near this transition. The static structure factor is extracted and used to identify the radial and angular roton excitations by their characteristic symmetries. These fluctuations peak as a function of interaction strength indicating the crystallization transition of the system. We compare our observations to a theoretically calculated excitation spectrum allowing us to connect the crystallization mechanism with the softening of the angular roton modes.
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Submitted 2 February, 2021;
originally announced February 2021.
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Thermal Resistance at a Twist Boundary and Semicoherent Heterointerface
Authors:
Ramya Gurunathan,
Riley Hanus,
Samuel Graham,
Anupam Garg,
G. Jeffrey Snyder
Abstract:
Traditional models of interfacial phonon scattering, including the acoustic mismatch model (AMM) and diffuse mismatch model (DMM), take into account the bulk properties of the material surrounding the interface, but not the atomic structure and properties of the interface itself. Here, we derive a theoretical formalism for the phonon scattering at a dislocation grid, or two interpenetrating orthog…
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Traditional models of interfacial phonon scattering, including the acoustic mismatch model (AMM) and diffuse mismatch model (DMM), take into account the bulk properties of the material surrounding the interface, but not the atomic structure and properties of the interface itself. Here, we derive a theoretical formalism for the phonon scattering at a dislocation grid, or two interpenetrating orthogonal arrays of dislocations, as this is the most stable structure of both the symmetric twist boundary and semicoherent heterointerface. With this approach, we are able to separately examine the contribution to thermal resistance due to the step function change in acoustic properties and due to interfacial dislocation strain fields, which induces diffractive scattering. Both low-angle Si-Si twist boundaries and the Si-Ge heterointerfaces are considered here and compared to previous experimental and simulation results. This work indicates that scattering from misfit dislocation strain fields doubles the thermal boundary resistance of Si-Ge heterointerfaces compared to scattering due to acoustic mismatch alone. Scattering from grain boundary dislocation strain fields is predicted to dominate the thermal boundary resistance of Si-Si twist boundaries. This physical treatment can guide the thermal design of devices by quantifying the relative importance of interfacial strain fields, which can be engineered via fabrication and processing methods, versus acoustic mismatch, which is fixed for a given interface. Additionally, this approach captures experimental and simulation trends such as the dependence of thermal boundary resistance on the grain boundary angle and interfacial strain energy.
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Submitted 13 April, 2021; v1 submitted 4 January, 2021;
originally announced January 2021.
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Density Fluctuations across the Superfluid-Supersolid Phase Transition in a Dipolar Quantum Gas
Authors:
J. Hertkorn,
J. -N. Schmidt,
F. Böttcher,
M. Guo,
M. Schmidt,
K. S. H. Ng,
S. D. Graham,
H. P. Büchler,
T. Langen,
M. Zwierlein,
T. Pfau
Abstract:
Phase transitions share the universal feature of enhanced fluctuations near the transition point. Here we show that density fluctuations reveal how a Bose-Einstein condensate of dipolar atoms spontaneously breaks its translation symmetry and enters the supersolid state of matter -- a phase that combines superfluidity with crystalline order. We report on the first direct in situ measurement of dens…
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Phase transitions share the universal feature of enhanced fluctuations near the transition point. Here we show that density fluctuations reveal how a Bose-Einstein condensate of dipolar atoms spontaneously breaks its translation symmetry and enters the supersolid state of matter -- a phase that combines superfluidity with crystalline order. We report on the first direct in situ measurement of density fluctuations across the superfluid-supersolid phase transition. This allows us to introduce a general and straightforward way to extract the static structure factor, estimate the spectrum of elementary excitations and image the dominant fluctuation patterns. We observe a strong response in the static structure factor and infer a distinct roton minimum in the dispersion relation. Furthermore, we show that the characteristic fluctuations correspond to elementary excitations such as the roton modes, which have been theoretically predicted to be dominant at the quantum critical point, and that the supersolid state supports both superfluid as well as crystal phonons.
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Submitted 18 September, 2020;
originally announced September 2020.
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New states of matter with fine-tuned interactions: quantum droplets and dipolar supersolids
Authors:
Fabian Böttcher,
Jan-Niklas Schmidt,
Jens Hertkorn,
Kevin S. H. Ng,
Sean D. Graham,
Mingyang Guo,
Tim Langen,
Tilman Pfau
Abstract:
Quantum fluctuations can stabilize Bose-Einstein condensates (BEC) against the mean-field collapse. Stabilization of the condensate has been observed in quantum degenerate Bose-Bose mixtures and dipolar BECs. The fine-tuning of the interatomic interactions can lead to the emergence of two new states of matter: liquid-like selfbound quantum droplets and supersolid crystals formed from these droplet…
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Quantum fluctuations can stabilize Bose-Einstein condensates (BEC) against the mean-field collapse. Stabilization of the condensate has been observed in quantum degenerate Bose-Bose mixtures and dipolar BECs. The fine-tuning of the interatomic interactions can lead to the emergence of two new states of matter: liquid-like selfbound quantum droplets and supersolid crystals formed from these droplets. We review the properties of these exotic states of matter and summarize the experimental progress made using dipolar quantum gases and Bose-Bose mixtures. We conclude with an outline of important open questions that could be addressed in the future.
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Submitted 22 September, 2020; v1 submitted 13 July, 2020;
originally announced July 2020.
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Simultaneous Evaluation of Heat Capacity and In-plane Thermal Conductivity of Nanocrystalline Diamond Thin Films
Authors:
Luke Yates,
Zhe Cheng,
Tingyu Bai,
Karl Hobart,
Marko Tadjer,
Tatyana I. Feygelson,
Bradford B. Pate,
Mark Goorsky,
Samuel Graham
Abstract:
As wide bandgap electronic devices have continued to advance in both size reduction and power handling capabilities, heat dissipation has become a significant concern. To mitigate this, chemical vapor deposited (CVD) diamond has been demonstrated as an effective solution for thermal management of these devices by directly growing onto the transistor substrate. A key aspect of power and radio frequ…
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As wide bandgap electronic devices have continued to advance in both size reduction and power handling capabilities, heat dissipation has become a significant concern. To mitigate this, chemical vapor deposited (CVD) diamond has been demonstrated as an effective solution for thermal management of these devices by directly growing onto the transistor substrate. A key aspect of power and radio frequency (RF) electronic devices involves transient switching behavior, which highlights the importance of understanding the temperature dependence of the heat capacity and thermal conductivity when modeling and predicting device electrothermal response. Due to the complicated microstructure near the interface between CVD diamond and electronics, it is difficult to measure both properties simultaneously. In this work, we use time domain thermoreflectance (TDTR) to simultaneously measure the in plane thermal conductivity and heat capacity of a 1 um thick CVD diamond film, and also use the pump as an effective heater to perform temperature dependent measurements. The results show that the in plane thermal conductivity varied slightly with an average of 103 W per meter per K over a temperature range of 302 to 327 K, while the specific heat capacity has a strong temperature dependence over the same range and matches with heat capacity data of natural diamond in literature.
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Submitted 22 June, 2020;
originally announced June 2020.
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Wafer-scale Heterogeneous Integration of Monocrystalline \b{eta}-Ga2O3 Thin Films on SiC for Thermal Management by Ion-Cutting Technique
Authors:
Zhe Cheng,
Fengwen Mu,
Tiangui You,
Wenhui Xu,
Jingjing Shi,
Michael E. Liao,
Yekan Wang,
Kenny Huynh,
Tadatomo Suga,
Mark S. Goorsky,
Xin Ou,
Samuel Graham
Abstract:
The ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates make \b{eta}-Ga2O3 promising for applications of next-generation power electronics while its thermal conductivity is at least one order of magnitude lower than other wide/ultrawide bandgap semiconductors. To avoid the degradation of device performance and reliability induced by the localized Joule-heating,…
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The ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates make \b{eta}-Ga2O3 promising for applications of next-generation power electronics while its thermal conductivity is at least one order of magnitude lower than other wide/ultrawide bandgap semiconductors. To avoid the degradation of device performance and reliability induced by the localized Joule-heating, aggressive thermal management strategies are essential, especially for high-power high-frequency applications. This work reports a scalable thermal management strategy to heterogeneously integrate wafer-scale monocrystalline \b{eta}-Ga2O3 thin films on high thermal conductivity SiC substrates by ion-cutting technique. The thermal boundary conductance (TBC) of the \b{eta}-Ga2O3-SiC interfaces and thermal conductivity of the \b{eta}-Ga2O3 thin films were measured by Time-domain Thermoreflectance (TDTR) to evaluate the effects of interlayer thickness and thermal annealing. Materials characterizations were performed to understand the mechanisms of thermal transport in these structures. The results show that the \b{eta}-Ga2O3-SiC TBC values increase with decreasing interlayer thickness and the \b{eta}-Ga2O3 thermal conductivity increases more than twice after annealing at 800 oC due to the removal of implantation-induced strain in the films. A Callaway model is built to understand the measured thermal conductivity. Small spot-to-spot variations of both TBC and Ga2O3 thermal conductivity confirm the uniformity and high-quality of the bonding and exfoliation. Our work paves the way for thermal management of power electronics and \b{eta}-Ga2O3 related semiconductor devices.
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Submitted 26 May, 2020;
originally announced May 2020.
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Robust, accurate, and efficient: quantum embedding using the Huzinaga level-shift projection operator for complex systems
Authors:
Daniel S. Graham,
Xuelan Wen,
Dhabih V. Chulhai,
Jason D. Goodpaster
Abstract:
Wave function (WF) in density functional theory (DFT) embedding methods provide a framework for performing localized, high accuracy WF calculations on a system, while not incurring the full computational cost of the WF calculation on the full system. In order to effectively partition a system into localized WF and DFT subsystems, we utilize the Huzinaga level-shift projection operator within an ab…
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Wave function (WF) in density functional theory (DFT) embedding methods provide a framework for performing localized, high accuracy WF calculations on a system, while not incurring the full computational cost of the WF calculation on the full system. In order to effectively partition a system into localized WF and DFT subsystems, we utilize the Huzinaga level-shift projection operator within an absolutely localized basis. In this work, we study the ability of the absolutely localized Huzinaga level-shift projection operator method to study complex WF and DFT partitions, including partitions between multiple covalent bonds, a double bond, and transition metal-ligand bonds. We find that our methodology can accurately describe all of these complex partitions. Additionally, we study the robustness of this method with respect to the WF method, specifically where the embedded systems were described using a multiconfigurational WF method. We found that the method is systematically improvable with respect to both the number of atoms in the WF region and the size of the basis set used, with energy errors less than 1 kcal/mol. Additionally, we calculated the adsorption energy of H$_2$ to a model of an iron metal organic framework (Fe-MOF-74) to within 1 kcal/mol compared to CASPT2 calculations performed on the full model while incurring only a small fraction of the full computational cost. This work demonstrates that the absolutely localized Huzinaga level-shift projection operator method is applicable to very complex systems with difficult electronic structures.
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Submitted 27 November, 2019;
originally announced November 2019.
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Absolutely Localized Projection-Based Embedding for Excited States
Authors:
Xuelan Wen,
Daniel S. Graham,
Dhabih V. Chulhai,
Jason D. Goodpaster
Abstract:
We present a quantum embedding method that allows for the calculation of local excited states embedded in a Kohn-Sham density functional theory (DFT) environment. Projection-based quantum embedding methodologies provide a rigorous framework for performing DFT-in-DFT and wave function in DFT (WF-in-DFT) calculations. The use of absolute localization, where the density of each subsystem is expanded…
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We present a quantum embedding method that allows for the calculation of local excited states embedded in a Kohn-Sham density functional theory (DFT) environment. Projection-based quantum embedding methodologies provide a rigorous framework for performing DFT-in-DFT and wave function in DFT (WF-in-DFT) calculations. The use of absolute localization, where the density of each subsystem is expanded in only the basis functions associated with the atoms of that subsystem, provide improved computationally efficiency for WF-in-DFT calculations by reducing the number of orbitals in the WF calculation. In this work, we extend absolutely localized projection-based quantum embedding to study localized excited states using EOM-CCSD-in-DFT and TDDFT-in-DFT. The embedding results are highly accurate compared to the corresponding canonical EOM-CCSD and TDDFT results on the full system, with TDDFT-in-DFT frequently more accurate than canonical TDDFT. The absolute localization method is shown to eliminate the spurious low-lying excitation energies for charge transfer states and prevent over delocalization of excited states. Additionally, we attempt to recover the environment response caused by the electronic excitations in the high-level subsystem using different schemes and compare their accuracy. Finally, we apply this method to the calculation of the excited state energy of green fluorescent protein and show that we systematically converge to the full system results. Here we demonstrate how this method can be useful in understanding excited states, specifically which chemical moieties polarize to the excitation. This work shows absolutely localized projection-based quantum embedding can treat local electronic excitations accurately, and make computationally expensive WF methods applicable to systems beyond current computational limits.
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Submitted 26 September, 2019;
originally announced September 2019.
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Interfacial Thermal Conductance across Room-Temperature Bonded GaN-Diamond Interfaces for GaN-on-Diamond Devices
Authors:
Zhe Cheng,
Fengwen Mu,
Luke Yates,
Tadatomo Suga,
Samuel Graham
Abstract:
The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications such as wireless communication and radar systems. However, the performance and reliability of GaN-based HEMTs are limited by the high channel temperature induced by Joule-heating in the device channel. High thermal conductivity substrates…
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The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications such as wireless communication and radar systems. However, the performance and reliability of GaN-based HEMTs are limited by the high channel temperature induced by Joule-heating in the device channel. High thermal conductivity substrates integrated with GaN can improve the extraction of heat from GaN based HEMTs and lower the device operating temperature. However, heterogeneous integration of GaN with diamond substrates is not trivial and presents technical challenges to maximize the heat dissipation potential brought by the diamond substrate. In this work, two modified room temperature surface activated bonding techniques are used to bond GaN and single crystal diamond with different interlayer thicknesses. TDTR is used to measure the thermal properties from room temperature to 480 K. A relatively large TBC of the GaN-diamond interfaces with a 4nm interlayer was observed and material characterization was performed to link the structure of the interface to the TBC. Device modeling shows that the measured GaN-diamond TBC values obtained from bonding can enable high power GaN devices by taking the full advantage of the high thermal conductivity of single crystal diamond and achieve excellent cooling effect. Furthermore, the room-temperature bonding process in this work do not induce stress problem due to different coefficient of thermal expansion in other high temperature integration processes in previous studies. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling especially for GaN-on-diamond devices.
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Submitted 4 September, 2019;
originally announced September 2019.
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Integration of Atomic Layer Epitaxy Crystalline Ga2O3 on Diamond for Thermal Management
Authors:
Zhe Cheng,
Virginia D. Wheeler,
Tingyu Bai,
Jingjing Shi,
Marko J. Tadjer,
Tatyana Feygelson,
Karl D. Hobart,
Mark S. Goorsky,
Samuel Graham
Abstract:
Ga2O3 has attracted great attention for electronic device applications due to its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is significantly lower than that of other wide bandgap semiconductors, which will impact its ability to be used in high power density applications. Thermal management in Ga2O3…
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Ga2O3 has attracted great attention for electronic device applications due to its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is significantly lower than that of other wide bandgap semiconductors, which will impact its ability to be used in high power density applications. Thermal management in Ga2O3 electronics will be the key for device reliability, especially for high power and high frequency devices. Similar to the method of cooling GaN-based high electron mobility transistors by integrating it with high thermal conductivity diamond substrates, this work studies the possibility of heterogeneous integration of Ga2O3 with diamond for thermal management of Ga2O3 devices. In this work, Ga2O3 was deposited onto single crystal diamond substrates by ALD and the thermal properties of ALD-Ga2O3 thin films and Ga2O3-diamond interfaces with different interface pretreatments were measured by TDTR. We observed very low thermal conductivity of these Ga2O3 thin films due to the extensive phonon grain boundary scattering resulting from the nanocrystalline nature of the Ga2O3 film. However, the measured thermal boundary conductance (TBC) of the Ga2O3-diamond interfaces are about 10 times larger than that of the Van der Waals bonded Ga2O3 diamond interfaces, which indicates the significant impact of interface bonding on TBC. Furthermore, the TBC of the Ga-rich and O-rich Ga2O3-diamond interfaces are about 20% smaller than that of the clean interface, indicating interface chemistry affects interfacial thermal transport. Overall, this study shows that a high TBC can be obtained from strong interfacial bonds across Ga2O3-diamond interfaces, providing a promising route to improving the heat dissipation from Ga2O3 devices.
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Submitted 23 August, 2019;
originally announced August 2019.
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Thermal Conductance Across Harmonic-matched Epitaxial Al-sapphire Heterointerfaces
Authors:
Zhe Cheng,
Yee Rui Koh,
Habib Ahmad,
Renjiu Hu,
Jingjing Shi,
Michael E. Liao,
Yekan Wang,
Tingyu Bai,
Ruiyang Li,
Eungkyu Lee,
Evan A. Clinton,
Christopher M. Matthews,
Zachary Engel,
Yates,
Tengfei Luo,
Mark S. Goorsky,
William Doolittle,
Zhiting Tian,
Patrick E. Hopkins,
Samuel Graham
Abstract:
A unified understanding of interfacial thermal transport is missing due to the complicated nature of interfaces which involves complex factors such as interfacial bonding, interfacial mixing, surface chemistry, crystal orientation, roughness, contamination, and interfacial disorder. This is especially true for metal nonmetal interfaces which incorporate multiple fundamental heat transport mechanis…
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A unified understanding of interfacial thermal transport is missing due to the complicated nature of interfaces which involves complex factors such as interfacial bonding, interfacial mixing, surface chemistry, crystal orientation, roughness, contamination, and interfacial disorder. This is especially true for metal nonmetal interfaces which incorporate multiple fundamental heat transport mechanisms such as elastic and inelastic phonon scattering as well as electron phonon coupling in the metal and across the interface. All these factors jointly affect thermal boundary conductance (TBC). As a result, the experimentally measured interfaces may not be the same as the ideally modelled interfaces, thus obfuscating any conclusions drawn from experimental and modeling comparisons. This work provides a systematic study of interfacial thermal conductance across well controlled and ultraclean epitaxial (111) Al parallel (0001) sapphire interfaces, known as harmonic matched interface. A comparison with thermal models such as atomistic Green s function (AGF) and a nonequilibrium Landauer approach shows that elastic phonon scattering dominates the interfacial thermal transport of Al sapphire interface. By scaling the TBC with the Al heat capacity, a nearly constant transmission coefficient is observed, indicating that the phonons on the Al side limits the Al sapphire TBC. This nearly constant transmission coefficient validates the assumptions in AGF and nonequilibrium Landauer calculations. Our work not only provides a benchmark for interfacial thermal conductance across metal nonmetal interfaces and enables a quantitative study of TBC to validate theoretical thermal carrier transport mechanisms, but also acts as a reference when studying how other factors impact TBC.
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Submitted 23 September, 2019; v1 submitted 13 June, 2019;
originally announced June 2019.
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High Thermal Boundary Conductance across Bonded Heterogeneous GaN-SiC Interfaces
Authors:
Fengwen Mu,
Zhe Cheng,
Jingjing Shi,
Seongbin Shin,
Bin Xu,
Junichiro Shiomi,
Samuel Graham,
Tadatomo Suga
Abstract:
GaN-based HEMTs have the potential to be widely used in high-power and high-frequency electronics while their maximum output powers are limited by high channel temperature induced by near-junction Joule-heating, which degrades device performance and reliability. Increasing the TBC between GaN and SiC will aid in the heat dissipation of GaN-on-SiC power devices, taking advantage of the high thermal…
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GaN-based HEMTs have the potential to be widely used in high-power and high-frequency electronics while their maximum output powers are limited by high channel temperature induced by near-junction Joule-heating, which degrades device performance and reliability. Increasing the TBC between GaN and SiC will aid in the heat dissipation of GaN-on-SiC power devices, taking advantage of the high thermal conductivity of the SiC substrate. However, a good understanding of the TBC of this technically important interface is still lacking due to the complicated nature of interfacial heat transport. In this work, a lattice-mismatch-insensitive surface activated bonding method is used to bond GaN directly to SiC and thus eliminating the AlN layer altogether. This allows for the direct integration of high quality GaN layers with SiC to create a high thermal boundary conductance interface. TDTR is used to measure the thermal properties of the GaN thermal conductivity and GaN-SiC TBC. The measured GaN thermal conductivity is larger than that of GaN grown by MBE on SiC, showing the impact of reducing the dislocations in the GaN near the interface. High GaN-SiC TBC is observed for the bonded GaN-SiC interfaces, especially for the annealed interface whose TBC (230 MW/m2-K) is close to the highest values ever reported. To understand the structure-thermal property relation, STEM and EELS are used to characterize the interface structure. The results show that, for the as-bonded sample, there exists an amorphous layer near the interface for the as bonded samples. This amorphous layer is crystallized upon annealing, leading to the high TBC found in our work. Our work paves the way for thermal transport across bonded interfaces, which will impact real-world applications of semiconductor integration and packaging.
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Submitted 10 May, 2019;
originally announced May 2019.
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Significantly Reduced Thermal Conductivity in Beta-(Al0.1Ga0.9)2O3/Ga2O3 Superlattices
Authors:
Zhe Cheng,
Nicholas Tanen,
Celesta Chang,
Jingjing Shi,
Jonathan McCandless,
David Muller,
Debdeep Jena,
Huili Grace Xing,
Samuel Graham
Abstract:
Beta-Ga2O3 has emerged as a promising candidate for electronic device applications because of its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is at least one order of magnitude lower than that of other wide bandgap semiconductors such as SiC and GaN. Thermal dissipation in electronics made from beta-…
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Beta-Ga2O3 has emerged as a promising candidate for electronic device applications because of its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is at least one order of magnitude lower than that of other wide bandgap semiconductors such as SiC and GaN. Thermal dissipation in electronics made from beta-Ga2O3 will be the bottleneck for real-world applications, especially for high power and high frequency devices. Similar to GaN/AlGaN interfaces, beta-(AlxGa1-x)2O3/Ga2O3 heterogeneous structures have been used to form a high mobility two-dimensional electron gas (2DEG) where joule heating is localized. The thermal properties of beta-(AlxGa1-x)2O3/Ga2O3 are the key for heat dissipation in these devices while they have not been studied before. This work reports the first measurement on thermal conductivity of beta-(Al0.1Ga0.9)2O3/Ga2O3 superlattices from 80 K to 480 K. Its thermal conductivity is significantly reduced (5.7 times reduction) at room temperature comparing with that of bulk Ga2O3. Additionally, the thermal conductivity of bulk Ga2O3 with (010) orientation is measured and found to be consistent with literature values regardless of Sn doping. We discuss the phonon scattering mechanism in these structures by calculating their inverse thermal diffusivity. By comparing the estimated thermal boundary conductance (TBC) of beta-(Al0.1Ga0.9)2O3/Ga2O3 interfaces and Ga2O3 maximum TBC, we reveal that some phonons in the superlattices transmit through several interfaces before scattering with other phonons or structural imperfections. This study is not only important for Ga2O3 electronics applications especially for high power and high frequency applications, but also for the fundamental thermal science of phonon transport across interfaces and in superlattices.
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Submitted 30 April, 2019;
originally announced May 2019.
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Direct Visualization of Thermal Conductivity Suppression Due to Enhanced Phonon Scattering Near Individual Grain Boundaries
Authors:
Aditya Sood,
Ramez Cheaito,
Tingyu Bai,
Heungdong Kwon,
Yekan Wang,
Chao Li,
Luke Yates,
Thomas Bougher,
Samuel Graham,
Mehdi Asheghi,
Mark Goorsky,
Kenneth E. Goodson
Abstract:
Understanding the impact of lattice imperfections on nanoscale thermal transport is crucial for diverse applications ranging from thermal management to energy conversion. Grain boundaries (GBs) are ubiquitous defects in polycrystalline materials, which scatter phonons and reduce thermal conductivity. Historically, their impact on heat conduction has been studied indirectly through spatially-averag…
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Understanding the impact of lattice imperfections on nanoscale thermal transport is crucial for diverse applications ranging from thermal management to energy conversion. Grain boundaries (GBs) are ubiquitous defects in polycrystalline materials, which scatter phonons and reduce thermal conductivity. Historically, their impact on heat conduction has been studied indirectly through spatially-averaged measurements, that provide little information about phonon transport near a single GB. Here, using spatially-resolved time-domain thermoreflectance (TDTR) measurements in combination with electron backscatter diffraction (EBSD), we make localized measurements of thermal conductivity within few μm of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in conductivity from ~1000 W/m-K at the center of large grains to ~400 W/m-K in the immediate vicinity of GBs. Furthermore, we show that this reduction in conductivity is measured up to ~10 μm away from a GB. A theoretical model is proposed that captures the local reduction in phonon mean-free-paths due to strongly diffuse phonon scattering at the disordered grain boundaries. Our results provide a new framework for understanding phonon-defect interactions in nanomaterials, with implications for the use of high thermal conductivity polycrystalline materials as heat sinks in electronics thermal management.
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Submitted 29 July, 2018; v1 submitted 11 April, 2018;
originally announced April 2018.
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Stable spin domains in a non-degenerate ultra-cold gas
Authors:
S. D. Graham,
D. Niroomand,
R. J. Ragan,
J. M. McGuirk
Abstract:
We study the stability of two-domain spin structures in an ultra-cold gas of magnetically trapped $^{87}$Rb atoms above quantum degeneracy. Adding a small effective magnetic field gradient stabilizes the domains via coherent collective spin rotation effects, despite negligibly perturbing the potential energy relative to the thermal energy. We demonstrate that domain stabilization is accomplished t…
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We study the stability of two-domain spin structures in an ultra-cold gas of magnetically trapped $^{87}$Rb atoms above quantum degeneracy. Adding a small effective magnetic field gradient stabilizes the domains via coherent collective spin rotation effects, despite negligibly perturbing the potential energy relative to the thermal energy. We demonstrate that domain stabilization is accomplished through decoupling the dynamics of longitudinal magnetization, which remains in time-independent domains, from transverse magnetization, which undergoes a purely transverse spin wave trapped within the domain wall. We explore the effect of temperature and density on the steady-state domains, and compare our results to a hydrodynamic solution to a quantum Boltzmann equation.
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Submitted 2 March, 2018;
originally announced March 2018.
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Retardation effects in induced atomic dipole-dipole interactions
Authors:
S. D. Graham,
J. M. McGuirk
Abstract:
We present mean-field calculations of azimuthally averaged retarded dipole-dipole interactions in a Bose-Einstein condensate induced by a laser, at both long and short wavelengths. Our calculations demonstrate that dipole-dipole interactions become significantly stronger at shorter wavelengths, by as much as 30-fold, due to retardation effects. This enhancement, along with inclusion of the dynamic…
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We present mean-field calculations of azimuthally averaged retarded dipole-dipole interactions in a Bose-Einstein condensate induced by a laser, at both long and short wavelengths. Our calculations demonstrate that dipole-dipole interactions become significantly stronger at shorter wavelengths, by as much as 30-fold, due to retardation effects. This enhancement, along with inclusion of the dynamic polarizability, indicate a method of inducing long-range interatomic interactions in neutral atom condensates at significantly lower intensities than previously realized.
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Submitted 18 October, 2016;
originally announced October 2016.
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A New Equation of State for CCS Pipeline Transport: Calibration of Mixing Rules for Binary Mixtures of CO2 with N2, O2 and H2
Authors:
Thomas A. Demetriades,
Richard S. Graham
Abstract:
One of the aspects currently holding back commercial scale deployment of carbon capture and storage (CCS) is an accurate understanding of the thermodynamic behaviour of carbon dioxide and relevant impurities during the pipeline transport stage. In this article we develop a general framework for deriving pressure-explicit EoS for impure CO2. This flexible framework facilitates ongoing development o…
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One of the aspects currently holding back commercial scale deployment of carbon capture and storage (CCS) is an accurate understanding of the thermodynamic behaviour of carbon dioxide and relevant impurities during the pipeline transport stage. In this article we develop a general framework for deriving pressure-explicit EoS for impure CO2. This flexible framework facilitates ongoing development of custom EoS in response to new data and computational applications. We use our method to generalise a recent EoS for pure CO2 [Demetriades et al. Proc IMechE Part E, 227 (2013) pp. 117] to binary mixtures with N2, O2 and H2, obtaining model parameters by fitting to experiments made under conditions relevant to CCS-pipeline transport. Our model pertains to pressures up to 16MPa and temperatures between 273K and the critical temperature of pure CO2. In this region, we achieve close agreement with experimental data. When compared to the GERG EoS, our EoS has a comparable level of agreement with CO2 -N2 VLE experiments and demonstrably superior agreement with the O2 and H2 VLE data. Finally, we discuss future options to improve the calibration of EoS and to deal with the sparsity of data for some impurities.
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Submitted 2 July, 2015;
originally announced July 2015.
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Longitudinal spin diffusion in a nondegenerate trapped $^{87}$Rb gas
Authors:
D. Niroomand,
S. D. Graham,
J. M. McGuirk
Abstract:
Longitudinal spin diffusion of two pseudo-spin domains is studied in a trapped $^{87}$Rb sample above quantum degeneracy, and the effect of coherence in the domain wall on the dynamics of the system is investigated. Coherence in a domain wall leads to transverse-spin-mediated longitudinal spin diffusion that is slower than classical predictions, as well as altering the domains' oscillation frequen…
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Longitudinal spin diffusion of two pseudo-spin domains is studied in a trapped $^{87}$Rb sample above quantum degeneracy, and the effect of coherence in the domain wall on the dynamics of the system is investigated. Coherence in a domain wall leads to transverse-spin-mediated longitudinal spin diffusion that is slower than classical predictions, as well as altering the domains' oscillation frequency. The system also shows an instability in the longitudinal spin dynamics as longitudinal and transverse spin components couple, and a conversion of longitudinal spin to transverse spin is observed, resulting in an increase in the total amount of coherence in the system.
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Submitted 29 May, 2015; v1 submitted 15 January, 2015;
originally announced January 2015.
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A Comparison of Experimental Measurements and Computational Predictions of a Deep-V Planing Hull
Authors:
Thomas C. Fu,
Toby Ratcliffe,
Thomas T. O'Shea,
Kyle A. Brucker,
R. Scott Graham,
Donald C. Wyatt
Abstract:
In order to support development of computational fluid dynamics codes for high-speed, small-craft applications, laboratory experiments were performed on a representative of Deep-V planing craft model. The measurements included resistance, sinkage and trim, hull pressure measurements, longitudinal wavecuts, and bow-wave and stern-wake topologies. The model was towed in calm water over a speed range…
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In order to support development of computational fluid dynamics codes for high-speed, small-craft applications, laboratory experiments were performed on a representative of Deep-V planing craft model. The measurements included resistance, sinkage and trim, hull pressure measurements, longitudinal wavecuts, and bow-wave and stern-wake topologies. The model was towed in calm water over a speed range of 1.78 to 14.2 m/s (5.8 to 46.6 ft/s) corresponding to a Froude number range of 0.31 to 2.5. At planing speeds, +8 m/s (+26.2 ft/s), the model was run with the addition of trim tabs set at two different angles, 7 and 13 degrees. Photographic documentation, including still photographs and video, was recorded during the collection of all the data. Numerical simulation of the flow field was performed utilizing the Numerical Flow Analysis (NFA) code and compared to the model test results.
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Submitted 7 October, 2014;
originally announced October 2014.