Speakers

Thermal investigation of highly mismatched semiconductor alloy PIN diodes

Abstract:

Highly mismatched semiconductor alloys (HMAs) offer greater options for designers of electrical and optical devices by expanding the range of material band gaps and lattice constants from those found in traditional semiconductor alloys. For example, high concentrations of boron (> 20%) have been incorporated into GaAs, decreasing both the lattice constant and the bandgap, creating an enticing III-V platform for photovoltaics, emitters, and detectors. Here we demonstrate PIN photodiodes created from BGa(In)As containing 1-6% boron (2-13% indium) grown by MBE on GaAs substrates. The external quantum efficiency (EQE) is characterized from fabricated circular mesa structures. Steady-state thermoreflectance is used to measure the thermal resistance of the epitaxial film and extract the thermal conductivity.

Phonon heat transport in ion beam modified Ga2O3 thin films.

Abstract:

Ion implantation has been extensively employed to customize the band structure and electrical characteristics of materials for applications in semiconductor technologies. While research on the electronic properties of ion-implanted materials continues to be a field of active research, there is relatively less emphasis on its impact on heat transfer within the material. Ion irradiation introduces defects into the lattice of a material, acting as scattering centers for phonons, which consequently impacts thermal conductivity. However, various factors of the implantation process such as ion mass, energy and temperature create different kinds of defects inside the material when exposed to irradiation. Very recently, we learned that the phase stability of Ga2O3 polymorphs is readily controllable by the accumulation of radiation damage. Thus, using ion beams, different Ga2O3 phases may be stabilized at different spatial locations of ion beam penetration, which can be exploited for thermal energy management.

In this work, we investigate the thermal transport properties of different Ga2O3 polymorphs and particularly the double polymorph γ/β-Ga2O3 structures, fabricated by a self-organized γ-polymorph transformation induced in β-Ga2O3 by ion irradiation. The thermal conductivity was measured by time-domain (TDTR) and frequency-domain thermoreflectance (FDTR), effectively obtaining depth profiles of the thermal conductivity across different Ga2O3 phases. The molecular dynamics simulations with machine-learned potential are also presented to validate the experimental results. Importantly, the formation of such double γ/β-Ga2O3 polymorph structures features an unprecedented sharpness of the γ/β interface leading to abrupt changes in thermal properties across homo-interfaces.

Time Domain Thermoreflectance: Tips, Tricks, and a few Extensions (workshop)

Abstract:

Time Domain Thermoreflectance (TDTR) is one of the most sought-after techniques for conducting nanoscale thermal measurements. Including electron-phonon coupling, picosecond acoustics, thermal conductivity, and thermal boundary conductance measurements all at once with built in normalization and noise correction is an outstanding prospect. Over the years many researchers have streamlined TDTR systems to attempt to simplify an otherwise complex system. On the flip side, many researchers have extended TDTR to enhance various capabilities and sensitivity (honestly, most of us have done both). In this talk, we’ll discuss a few major options and opportunities with TDTR that can be a great help to TDTR users to help tailor their optical setup, analysis process, and even their design of experiments with samples and controls. In particular, we’ll spend some time discussing Asynchronous Optical Sampling Systems (ASOPS) with TDTR, a technique that effectively removes path-length matching and optical delay stages from TDTR designs (but introduces a new set of tips, tricks, and considerations).

Exploring Plasma-Induced Non-Equilbrium in Thin Gold Films

Abstract:

Plasmas have long been used for the synthesis and manipulation of materials because of their unique ability to deliver both energy and chemically active species to the surface of materials – a feature that separates them from other materials processing techniques. Further, plasma irradiation drives the surface out of thermal equilibrium with the bulk material thus enabling local physicochemical processes that can be harnessed to establish unique material properties. Traditionally, our understanding of energy delivery from plasmas is developed using a variety of temperature measurements, models, and post-irradiation, ex situ surface characterizations to analyze energy deposition and absorption. However, none of these approaches provide a direct measure of the localized, transient response associated with the energy flux at the surface.

In this study, with the use of thermoreflectance based methods, we resolve the influence of the various energetic species in an atmospheric plasma on the resulting transient thermal response of thin gold films. We measure the change in the optical response of these films due to various types, intensities, and temporal profiles of atmospheric pressure plasma excitations. Thus, by using these in-situ thermoreflectance measurements we directly measure the highly non-equilibrium states and the influence of the plasma on the thermal properties of thin film gold during plasma irradiation.

Considerations in the measurement of high thermal conductivity materials with steady-state thermoreflectance

Abstract:

In this presentation, we consider the challenges in the measurement of high thermal conductivity materials with themoreflectance techniques. We focus upon the steady-state thermoreflectance (SSTR) method and and compare with other common opto- and electrothermal techniques. An analysis is performed on polycrystalline CVD diamond substrates with thermal conductivity in excess of 2000 W m-1 K-1. Variants of the SSTR technique including lock-in amplification (LIA) and periodic waveform analysis (PWA) are compared.

Characterizing the Anisotropic Thermal Properties in Irradiated 2D hexagonal-Boron Nitride (h-BN) Flakes

Abstract:

2D-materials, such as hexagonal boron nitride (h-BN), are promising candidates for deployment in radiative environments, such as space instrumentation (e.g., spacecraft shielding, studying solar flares), nuclear reactors, long-distance quantum communication, etc. The ability for precise transport measurements and a subsequent investigation into the resulting structural damage mechanisms as a function of ion irradiation is, thus, of critical importance. In this talk, we focus on using time domain thermo-reflectance (TDTR), in both its concentric and beam offset configurations, to resolve the in-plane and cross-plane thermal conductivities of h-BN flakes, as a function of radiation exposure. We experimentally demonstrate a significant reduction in the in-plane thermal conductivity of h-BN (by up to an order of magnitude) upon increased ion dosage, whereas the cross-plane thermal conductivity is reduced by only up to a factor of 2. This dramatically different orientational thermal property response to irradiation in h-BN, as opposed to other conventional bulk materials such as silicon, presents an interesting opportunity to explore various unique forms of radiation-induced damage such as ballistic displacements caused by quasi-elastic scattering (knock-on damage), radiolysis, and other de-ionizing effects in 2D materials.

Temperature-Dependent Thermal Conductivity of Rutile Germanium Dioxide (GeO₂) Measured by Time-Domain Thermoreflectance

Abstract:

The demand for power electronics devices is experiencing rapid growth across various sectors, driven by their enhanced functionality, energy efficiency, and system reliability. A crucial factor in meeting these demands is the advancement of ultra-wide bandgap emiconductors. In this context, rutile germanium dioxide (r-GeO₂) is gaining attention as a promising semiconductor material with an ultra-wide bandgap of 4.68 eV [1] comparable to materials of significant interest like β-Ga₂O₃. However, unlike β-Ga₂O₃, r-GeO₂ may provide both n- and p-type conductivity. Additionally, r-GeO₂ has a higher and less anisotropic thermal conductivity compared to β-Ga₂O₃, making it an attractive semiconductor for power electronic devices where heat management is crucial. So far, experimental studies have only reported the thermal conductivity of r-GeO₂ at room and elevated temperatures [2].

For the first time to our knowledge, we performed temperature-dependent measurements of the thermal conductivity of r-GeO₂ in the [001] and [110] directions in the temperature range between 80 to 300 K. For the measurements, we used samples prepared from bulk r-GeO₂ single crystals grown by the Top-Seeded Solution Growth method at Leibniz-Institut für Kristallzüchtung, as described in detail elsewhere [3]. The thermal conductivity was measured utilizing our in-house developed two-color time-domain thermoreflectance setup. Our findings closely align with the theoretical results previously published by Chae et al. [2], showing higher thermal conductivity in the [001] direction as compared to the [110] direction.

This work was funded by the Leibniz-Gemeinschaft (Senatsausschuss Wettbewerb – SAW, Germany) project under grant number K417/2021. It was partly performed in the framework of GraFOx, a Leibniz-Science Campus partially funded by the German Leibniz Association.

Thermal Transport in Materials with Dynamic Conformational Disorder

Abstract:

Alkyl chains that undergo phase transitions between ordered and dynamically disordered states have been the centerpoint of numerous thermal energy applications, notably solid-state cooling through barocaloric effects. Understanding how heat flows along these chains can offer valuable insights for device engineering and efficiency. In this talk, I will discuss thermal transport in layered crystals whose alkyl chains undergo sharp order–disorder transitions, including hybrid perovskites and organic salts. As these materials offer tunable interchain chemistries, we measure the thermal conductivities of systematically varied structures through frequency-domain thermoreflectance (FDTR) to assess the role of non-covalent interactions and chain confinement. Quasielastic and inelastic neutron scattering measurements further enable us to understand how conformational disorder influences the type of thermal carriers and the amount of heat they carry. Our results demonstrate a molecular-level picture of thermal transport in these materials and suggest the critical structural features that dictate heat conduction.

ITCC Workshop: Arbitrary geometries and confined transducers

Abstract:

In this talk, I will discuss the integration of a numerical, least-squares fitting routine into conventional frequency-domain thermoreflectance (FDTR) measurements to extract the thermal properties of materials and the thermal boundary conductance across interfaces in samples with complex geometric features. The goal of this work is to demonstrate the utility of FDTR for (1) measuring the thermal properties of materials in-situ (i.e., in actual device architectures) and (2) improving the sensitivity of the technique to so-called “buried” interfaces. Critical insights into measurement sensitivity and uncertainty will be discussed, as well as practical limitations associated with such experiments. The presentation will end with a discussion of challenges associated with these measurements and the need for further refinement in future work.

Thermal and Optical Properties of Rare Earth Oxides for Thermal Barrier Coatings

Abstract:

Rare earth oxides (REOs) show promising thermal properties required of next-generation thermal barrier coatings (TBCs) for ultrahigh temperature applications (1500 K+). TBCs mitigate conductive heating between hot components by reducing thermal conductivity through increased phonon-phonon scattering. New research aims to explore the fundamental mechanisms for this reduction and explore other properties of interest such as melting temperature and emissivity.

In this work, we perform a series of thermal and optical studies via pump-probe thermoreflectance, laser radiometry, and spectroscopic ellipsometry to elucidate the temperature-dependent thermal properties of REOs. With novel laser-based metrology, thermal conductivities, melting temperatures and emissivities of ceramics over 2000 K can be measured nondestructively. Understanding these trends is of utmost importance in choosing REOs that can endure cycling to operating temperatures.

Additionally, through ellipsometry, the lifetimes of optical phonons can be understood. Anharmonic scattering dominates thermal transport at high temperatures so measuring lifetimes with changing temperatures is important to understand fundamental energy transport in REOs. By investigating pertinent physical scattering mechanisms at relevant temperatures, we deconvolute key design considerations for next-generation TBCs.

Topic 1: Exploring Thermal Transport in Liquid Metal and Eutectic Gallium-Indium Alloys: Implications for Flexible Electronics

Abstract:

Liquid metal and eutectic gallium-indium (EGaIn) alloys represent a promising avenue in the realm of next-generation electronic devices and wearable electronics, owing to their notable advantages such as flexibility, stretchability, and compatibility with unconventional substrates. Leveraging the combined thermal and electrical properties of these materials opens up avenues for novel applications, including flexible sensors, stretchable circuits, and wearable energy harvesters. Given that the efficiency and reliability of electronic devices crucially depend on effective heat dissipation into heat sinks, a thorough understanding of the thermal conductivity of liquid metal, particularly as influenced by processing parameters, becomes imperative for achieving optimized performance.

Traditionally, the Wiedemann-Franz (WF) Law, incorporating a constant Lorenz number (L0 = 2.45 × 10−8 WΩ K−2), has been utilized for determining the thermal conductivity of metallic systems. However, recent scrutiny has cast doubt on the applicability of the Sommerfeld value of the Lorenz number, particularly in thin metallic interconnects and length scales pertinent to contemporary technology. Therefore, a systematic investigation into the thermal conductivity of eutectic gallium-indium Ga0.858In0.142 (14.2 wt. % In), henceforth referred to as EGaIn, employing methods not reliant on electrical resistivity, assumes significance both technologically and scientifically. Moreover, the impact of processing history on the formation and growth of the surface oxide layer of liquid metal and its subsequent effects on thermal boundary resistance remain unexplored territories.

In this study, we present direct measurements of the thermal conductivity of EGaIn utilizing the laser-based pump-probe technique, specifically time-domain thermoreflectance (TDTR). Complementary to these measurements, we infer the electrical resistivity of EGaIn from existing literature, thereby verifying the validity of employing the constant Lorenz number in the WF Law to predict the thermal conductivity of EGaIn. Furthermore, by systematically characterizing the thermal boundary conductance of liquid metal in response to cycling and processing variations, we provide invaluable insights into the intricate interplay between processing conditions, surface oxidation, and thermal transport properties.

Our findings not only contribute to a deeper understanding of the thermal transport characteristics of liquid metal and EGaIn alloys but also inform the refinement of theoretical frameworks such as the WF Law for predicting the behavior of these materials. Moreover, elucidating the influence of processing history on thermal boundary resistance sheds light on potential strategies for optimizing heat dissipation in electronic devices utilizing liquid metal components. Overall, this work underscores the importance of interdisciplinary approaches in advancing the field of thermal management in flexible electronics and engineering for emerging electronic applications.

Topic 2: Measurements of electron and phonon transport and scattering in ferroelectric thin films using ultrafast and infrared spectroscopy

Ferroelectric materials feature a spontaneous electrical polarization that originates from their unit-cell structure, with a spatial orientation that can be switched between stable states when subjected to an external electric field. This characteristic sets them apart as a unique class of materials with enhanced functionality among piezoelectrics. The push toward the miniaturization of piezoelectric sensors and actuators, particularly in microelectromechanical systems (MEMS), along with the integration of ferroelectric properties into integrated circuit (IC) technology and the rise of new applications relying on polarization control, has sparked significant scientific and commercial interest in ferroelectric thin films. Many key ferroelectric materials are oxide perovskites, which often suffer from limitations such as low paraelectric transition temperatures, non-linear displacements, and poor compatibility with technologies like complementary metal-oxide-semiconductor (CMOS) or III-nitride systems. These challenges currently hinder the widespread adoption of ferroelectric functionality in microtechnology.

In this study, we focus on ferroelectric wurtzite nitrides, which exhibit unique electronic and phononic properties that enable dynamic tunability and control over light, charge, spin, and heat. We present characterizing the phonon transport, sound speed, and phononic lifetimes of Al1-xBxN ferroelectric thin films using ultrafast spectroscopy. Additionally, when combined with infrared variable angle spectroscopy ellipsometry (IR-VASE), these methods allow for precise monitoring of scattering rates. Temperature-dependent measurements provide further insights into the changes in optical phonon energies. Understanding these changes will enhance our knowledge of phonon dynamics and could lead to improved ferroelectric materials for next-generation microtechnology applications.

Investigating size effects atf the thermal conductivity plateau of amorphous silica

Abstract:

The future of modern technology relies on understanding and developing materials useful for the quantum computing revolution. Silica (SiO2) is an ever present interlayer in most electronic devices due to the ubiquity of silicon transistors. However, at the low temperature extremes of next generation quantum qubits, the thermal dissipation and isolation capabilities of these thin silica films is not fully agreed on. Due to the amorphous nature of Silica, the thermal conductivity at low temperatures (<20K) exhibits a plateau, where the thermal conductivity becomes constant with respect to temperature. There has been extensive work on the theoretical and experimental investigation of this regime, however the effects of film thickness, or size effects are often overlooked. In this work, we utilize thermal reflectivity techniques in the frequency domain as well as in the steady state, to understand how size effects impact the thermal conductivity plateau of amorphous SiO2 at ultralow temperatures.

Current state of the art optical thermometry platforms for measuring these thermal properties are the pump-probe techniques time- and frequency-domain thermoreflectance, TDTR and FDTR, respectively. These techniques are often claimed not to be suitable at ultralow temperatures due to the fact that the measurement relies on modulated temperature rises. Our proposed approach of low duty cycle FDTR and low-power steady state thermoreflectance (SSTR) will overcome these limitations. SSTR is also a pump-probe technique, but unlike TDTR, it uses continuous wave laser sources to heat and probe the surface temperature change of a material as a function of laser power in a low frequency, “nearly-DC” regime. By limiting the pump powers used in SSTR as well as the duration of the heating even in FDTR we are able to restrict the thermal perturbation required for measurement to nearly 3K. Further, since SSTR is a steady-state technique, it is a direct measure of thermal conductivity, and is not subjected to uncertainties and errors due to transient heating of thermal masses and heat capacity assumptions, and the combined approach with FDTR allows for isolation of these thermal parameters which obfuscate the low temperature trends of materials.

Nano- and micro-scale phonon-mediated thermal transport in swift heavy ion irradiated crystalline insulators

Abstract:

Understanding heat conduction on micro- and nano- meter spatial scales in solid-state materials is very important for thermal management encountered in numerous applications involving, but not limited to, nuclear and fission energy, thermoelectric, electronic and opto-electronic devices. We will review our recent results on phonon-mediated thermal transport in swift heavy ion (SHI) irradiated single crystalline metal oxides [1 – 4] and alkali halides [5], as well as in polycrystalline nitride ceramics for heat management in nuclear energy applications. SHI-induced nano-patterning can be employed for making novel thermal functional materials. Heat conduction measurements were implemented by advanced thermal metrology techniques probing heat propagation in SHI irradiated crystalline insulators at nanoscale and microscale depths by picosecond time-domain thermoreflectance with MHz modulated rates and by continuum wave laser-based frequency- and spatial-domain thermoreflectance with kHz modulation rates, respectively. Measured thermal conductivity values were found to be in agreement with molecular dynamics and semi-analytical phonon thermal transport modeling. New results revealed intriguing heat propagation phenomena associated with the presence of point and extended (latent track) radiation defects associated with electronic and nuclear energy losses encountered by impinging SHIs.