arXiv subjects

Samuel Graham

Publications and source records attributed to Samuel Graham.

At least 19 recordsLinked to original sources

arXiv2026

Extremely Low Thermal Resistance Architectures for AlxGaN1-x Semiconductor Devices

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.

physics.app-ph↗

arXiv2025

Scaled Ultra-Wide Bandgap AlGaN Polarization-Graded FET with Ultra-thin Buffer Layer

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.

physics.app-ph↗

arXiv2025

Phonon selection and interference in momentum-resolved electron energy loss spectroscopy

As momentum-resolved Electron Energy Loss Spectroscopy (q-EELS) becomes more widely used for phonon measurements, better understanding of the intricacies of the acquired signal is necessary. Selection rules limit the allowed scattering, which may prohibit the appearance of specific phonon branches in some measurements. Simultaneous sampling of the lattice across all basis atoms also warrants a coherent treatment of phonons, which yields a reciprocal-space repeating unit cell that is larger than the standard Brillouin zone. We thus introduce the concept of the ``interferometric Brillouin zone'' where phonons are observed, which is closely related to the Dynamic Structure Factor. These effects follow from our new mathematical formalism for waves and vibrations, and we demonstrate calculations of q-EELS experiments using molecular dynamics and lattice dynamics. Results are compared to established q-EELS simulation methods in well-studied material systems, including the utilization of scattering selection rules to acquire a polarization-selective vibrational density of states. Finally, we note the analysis involved is directly applicable to any wave phenomena, such as plasmons or polaritons.

cond-mat.mtrl-sci↗

arXiv2024

Experimental observation of ballistic to diffusive transition in AlN thin films

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.

physics.app-ph↗

arXiv2024

Low Thermal Resistance of Diamond-AlGaN Interfaces Achieved Using Carbide Interlayers

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.

cond-mat.mtrl-sci↗

arXiv2022

High Thermal Conductivity in Wafer Scale Cubic Silicon Carbide Crystals

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.

cond-mat.mtrl-sci↗

arXiv2022

Measurements and Numerical Calculations of Thermal Conductivity to Evaluate the Quality of \beta-Gallium Oxide Thin Films Grown on Sapphire and Silicon Carbide by Molecular Beam Epitaxy

We report a method to obtain insights into lower thermal conductivity of \beta-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.

cond-mat.mtrl-sci↗

arXiv2021

Experimental Observation of Localized Interfacial Phonon Modes

Interfaces impede heat flow in micro/nanostructured systems. Conventional theories for interfacial thermal transport were derived based on bulk phonon properties of the materials making up the interface without explicitly considering the atomistic interfacial details, which are found critical to correctly describing thermal boundary conductance (TBC). Recent theoretical studies predicted the existence of localized phonon modes at the interface which can play an important role in understanding interfacial thermal transport. However, experimental validation is still lacking. Through a combination of Raman spectroscopy and high-energy resolution electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope, we report the first experimental observation of localized interfacial phonon modes at ~12 THz at a high-quality epitaxial Si-Ge interface. These modes are further confirmed using molecular dynamics simulations with a high-fidelity neural network interatomic potential, which also yield TBC agreeing well with that measured from time-domain thermoreflectance (TDTR) experiments. Simulations find that the interfacial phonon modes have obvious contribution to the total TBC. Our findings may significantly contribute to the understanding of interfacial thermal transport physics and have impact on engineering TBC at interfaces in applications such as electronics thermal management and thermoelectric energy conversion.

cond-mat.mes-hall↗

arXiv2021

Thermal Visualization of Buried Interfaces by Transient and Steady-State Responses of Time-Domain Thermoreflectance

Thermal resistances from interfaces impede heat dissipation in micro/nanoscale electronics, especially for high-power electronics. Despite the growing importance of understanding interfacial thermal transport, advanced thermal characterization techniques which can visualize thermal conductance across buried interfaces, especially for nonmetal-nonmetal interfaces, are still under development. This work reports a dual-modulation-frequency TDTR mapping technique to visualize the thermal conduction across buried semiconductor interfaces for beta-Ga2O3-SiC samples. Both the beta-Ga2O3 thermal conductivity and the buried beta-Ga2O3-SiC thermal boundary conductance (TBC) are visualized for an area of 200 um x 200 um. Areas with low TBC values ( smaller than 20 MW/m2-K) are successfully identified on the TBC map, which correspond to weakly bonded interfaces caused by high-temperature annealing. The steady-state temperature rise (detector voltage), usually ignored in TDTR measurements, is found to be able to probe TBC variations of the buried interfaces without the limit of thermal penetration depth. This technique can be applied to detect defects/voids in deeply buried heterogeneous interfaces non-destructively, and also opens a door for the visualization of thermal conductance in nanoscale nonhomogeneous structures.

cond-mat.mtrl-sci↗

arXiv2021

Phonon heat conduction in Al1-xScxN thin films

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.

cond-mat.mtrl-sci↗

arXiv2021

Quasi-Ballistic Thermal Conduction in 6H-SiC

The minimization of electronics makes heat dissipation of related devices an increasing challenge. When the size of materials is smaller than the phonon mean free paths, phonons transport without internal scatterings and laws of diffusive thermal conduction fail, resulting in significant reduction in the effective thermal conductivity. This work reports, for the first time, the temperature dependent thermal conductivity of doped epitaxial 6H-SiC and monocrystalline porous 6H-SiC below room temperature probed by time-domain thermoreflectance. Strong quasi-ballistic thermal transport was observed in these samples, especially at low temperatures. Doping and structural boundaries were applied to tune the quasi-ballistic thermal transport since dopants selectively scatter high-frequency phonons while boundaries scatter phonons with long mean free paths. Exceptionally strong phonon scattering by boron dopants are observed, compared to nitrogen dopants. Furthermore, orders of magnitude reduction in the measured thermal conductivity was observed at low temperatures for the porous 6H-SiC compared to the epitaxial 6H-SiC. Finally, first principles calculations and a simple Callaway model are built to understand the measured thermal conductivities. Our work sheds light on the fundamental understanding of thermal conduction in technologically-important wide bandgap semiconductors such as 6H-SiC and will impact applications such as thermal management of 6H-SiC-related electronics and devices.

cond-mat.mtrl-sci↗

arXiv2021

Thermal Resistance at a Twist Boundary and Semicoherent Heterointerface

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.

cond-mat.mtrl-sci↗

arXiv2020

Understanding Phonon Transport Properties Using Classical Molecular Dynamics Simulations

Predictive modeling of the phonon/thermal transport properties of materials is vital to rational design for a diverse spectrum of engineering applications. Classical Molecular Dynamics (MD) simulations serve as a tool to simulate the time evolution of the atomic level system dynamics and enable calculation of thermal transport properties for a wide range of materials, from perfect periodic crystals to systems with strong structural and compositional disorder, as well as their interfaces. Although MD does not intrinsically rely on a plane wave-like phonon description, when coupled with lattice dynamics calculations, it can give insights to the vibrational mode level contributions to thermal transport, which includes plane-wave like modes as well as others, rendering the approach versatile and powerful. On the other hand, several deficiencies including the lack of vibrationally accurate interatomic potentials and the inability to rigorously include the quantum nature of phonons prohibit the widespread applicability and reliability of Molecular Dynamics simulations. This article provides a comprehensive review of classical Molecular Dynamics based formalisms for extracting thermal transport properties: thermal conductivity and thermal interfacial conductance and the effects of various structural, compositional, and chemical parameters on these properties. Here, we highlight unusual property predictions, and emphasize on the needs and strategies for developing accurate interatomic potentials and rigorous quantum correction schemes.

cond-mat.mtrl-sci↗

arXiv2020

Simultaneous Evaluation of Heat Capacity and In-plane Thermal Conductivity of Nanocrystalline Diamond Thin Films

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.

physics.app-ph↗

arXiv2020

Wafer-scale Heterogeneous Integration of Monocrystalline \b{eta}-Ga2O3 Thin Films on SiC for Thermal Management by Ion-Cutting Technique

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.

physics.app-ph↗

arXiv2019

Substrate Dependent Resistive Switching in Amorphous-HfOx Memristors: An Experimental and Computational Investigation

While two-terminal HfOX (x<2) memristor devices have been studied for ion transport and current evolution, there have been limited reports on the effect of the long-range thermal environment on their performance. In this work, amorphous-HfOX based memristor devices on two different substrates, thin SiO2(280 nm)/Si and glass, with different thermal conductivities in the range from 1.2 to 138 W/m-K were fabricated. Devices on glass substrates exhibit lower reset voltage, wider memory window and, in turn, a higher performance window. In addition, the devices on glass show better endurance than the devices on the SiO2/Si substrate. These devices also show non-volatile multi-level resistances at relatively low operating voltages which is critical for neuromorphic computing applications. A Multiphysics COMSOL computational model is presented that describes the transport of heat, ions and electrons in these structures. The combined experimental and COMSOL simulation results indicate that the long-range thermal environment can have a significant impact on the operation of HfOx-based memristors and that substrates with low thermal conductivity can enhance switching performance.

cond-mat.mtrl-sci↗

arXiv2019

Experimental Observation of High Intrinsic Thermal Conductivity of AlN

AlN is an ultra-wide bandgap semiconductor which has been developed for applications including power electronics and optoelectronics. Thermal management of these applications is the key for stable device performance and allowing for long lifetimes. AlN, with its potentially high thermal conductivity, can play an important role serving as a dielectric layer, growth substrate, and heat spreader to improve device performance. However, the intrinsic high thermal conductivity of bulk AlN predicted by theoretical calculations has not been experimentally observed because of the difficulty in producing materials with low vacancy and impurity levels, and other associated defect complexes in AlN which can decrease the thermal conductivity. This work reports the growth of thick AlN layers by MOCVD with an air-pocketed AlN layer and the first experimental observation of intrinsic thermal conductivity from 130 K to 480 K that matches density-function-theory calculations for single crystal AlN, producing some of the highest values ever measured. Detailed material characterizations confirm the high quality of these AlN samples with one or two orders of magnitude lower impurity concentrations than seen in commercially available bulk AlN. Measurements of these commercially available bulk AlN substrates from 80 K to 480 K demonstrated a lower thermal conductivity, as expected. A theoretical thermal model is built to interpret the measured temperature dependent thermal conductivity. Our work demonstrates that it is possible to obtain theoretically high values of thermal conductivity in AlN and such films may impact the thermal management and reliability of future electronic and optoelectronics devices.

cond-mat.mtrl-sci↗

arXiv2019

Interfacial Thermal Conductance across Room-Temperature Bonded GaN-Diamond Interfaces for GaN-on-Diamond Devices

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.

physics.app-ph↗