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As the demand for computing applications capable of processing large datasets increases, there is a growing need for new in-memory computing technologies. Oxide-based resistive random-access memory (RRAM) devices are promising candidates for such applications because of their industry readiness, endurance, and switching ratio. These analog devices, however, suffer from poor linearity and asymmetry in their analog resistance change. Various reports have found that the temperature in RRAM devices increases locally by more than 1000 K during operation. Therefore, temperature control is of paramount importance for controlling their resistance. In this study, scanning thermal microscopy is used to map the temperature of Au/Ti/HfOx/Au devices at a steady power state and to measure temperature dynamics of the top electrode above the filament location during both resistive switching loops and voltage pulsing. These measurements are used to verify the thermal parameters of a multiphysics finite elements model. The model is then used to understand the impact of thermal conductivities and boundary conductances of constituent materials on resistance change during the first reset pulse in RRAM devices. It is found that the resistance change can be reduced significantly when the temperature in the titanium capping layer is reduced. We find that the greatest temperature reduction and, therefore, the lowest resistance change in the device are afforded by capping layers with increased thermal conductivities. This work links thermal properties to the resistance change in RRAM devices, providing critical insights into engineering devices with improved switching dynamics.

Thermal properties of materials are often determined by measuring thermalization processes; however, such measurements at the nanoscale are challenging because they require high sensitivity concurrently with high temporal and spatial resolutions. Here, we develop an optomechanical cantilever probe and customize an atomic force microscope with low detection noise ≈1 fm/Hz

Thermal fluctuations often impose both fundamental and practical measurement limits on high-performance sensors, motivating the development of techniques that bypass the limitations imposed by thermal noise outside cryogenic environments. Here, we theoretically propose and experimentally demonstrate a measurement method that reduces the effective transducer temperature and improves the measurement precision of a dynamic impulse response signal. Thermal noise–limited, integrated cavity optomechanical atomic force microscopy probes are used in a photothermal-induced resonance measurement to demonstrate an effective temperature reduction by a factor of ≈25, i.e., from room temperature down as low as ≈12 K, without cryogens. The method improves the experimental measurement precision and throughput by >2×, approaching the theoretical limit of ≈3.5× improvement for our experimental conditions. The general applicability of this method to dynamic measurements leveraging thermal noise–limited harmonic transducers will have a broad impact across a variety of measurement platforms and scientific fields.

Formation and aggregation of metal carboxylates (metal soaps) can degrade the appearance and integrity of oil paints, challenging efforts to conserve painted works of art. Endeavors to understand the root cause of metal soap formation have been hampered by the limited spatial resolution of Fourier transform infrared microscopy (mu-FTIR). We overcome this limitation using optical photothermal infrared spectroscopy (O-PTIR) and photothermal-induced resonance (PTIR), two novel methods that provide IR spectra with approximate to 500 and approximate to 10 nm spatial resolutions, respectively. The distribution of chemical phases in thin sections from the top layer of a 19th-century painting is investigated at multiple scales (mu-FTIR approximate to 10(2) mu m(3), O-PTIR approximate to 10(-1) mu m(3), PTIR approximate to 10(-5) mu m(3)). The paint samples analyzed here are found to be mixtures of pigments (cobalt green, lead white), cured oil, and a rich array of intermixed, small (often << 0.1 mu m(3)) zinc soap domains. We identify Zn stearate and Zn oleate crystalline soaps with characteristic narrow IR peaks (approximate to 1530-1558 cm(-1)) and a heterogeneous, disordered, water-permeable, tetrahedral zinc soap phase, with a characteristic broad peak centered at approximate to 1596 cm(-1). We show that the high signal-to-noise ratio and spatial resolution afforded by O-PTIR are ideal for identifying phase-separated (or locally concentrated) species with low average concentration, while PTIR provides an unprecedented nanoscale view of distributions and associations of species in paint. This newly accessible nanocompositional information will advance our knowledge of chemical processes in oil paint and will stimulate new art conservation practices.

Photothermal induced resonance (PTIR), also known as AFM-IR, enables nanoscale infrared (IR) imaging and spectroscopy by using the tip of an atomic force microscope to transduce the local photothermal expansion and contraction of a sample. The signal transduction efficiency and spatial resolution of PTIR depend on a multitude of sample, cantilever, and illumination source parameters in ways that are not yet well understood. Here, we elucidate and separate the effects of laser pulse length, pulse shape, sample thermalization time (τ), interfacial thermal conductance, and cantilever detection frequency by devising analytical and numerical models that link a sample's photothermal excitations to the cantilever dynamics over a broad bandwidth (10 MHz). The models indicate that shorter laser pulses excite probe oscillations over broader bandwidths and should be preferred for measuring samples with shorter thermalization times. Furthermore, we show that the spatial resolution critically depends on the interfacial thermal conductance between dissimilar materials and improves monotonically, but not linearly, with increasing cantilever detection frequencies. The resolution can be enhanced for samples that do not fully thermalize between pulses (i.e., laser repetition rates ≳ 1/3τ) as the probed depth becomes smaller than the film thickness. We believe that the insights presented here will accelerate the adoption and impact of PTIR analyses across a wide range of applications by informing experimental designs and measurement strategies as well as by guiding future technical advances.

Superlattice (SL) structures have been used to reduce the stress in the GaN epilayer of high-electron-mobility transistors (HEMTs). This has led to an improvement in their properties such as the breakdown voltage. The increase in thermal resistance associated with these structures, however, causes elevated device temperatures which may outweigh the benefits of this approach. To verify this, the thermal performance of SL structures on HEMTs must, thus, be accurately characterized. Transient thermoreflectance imaging (TTI) is an optical technique that can map the temperature distribution across a surface. For materials, such as gold, TTI shows a high spatial and temporal resolution. Consequently, the technique has been primarily used to monitor the gate metal temperature distribution in HEMTs. The origin of the localized heating in HEMTs, however, is known to be in the active GaN layer and, thus, accurate thermal characterization of the channel is necessary. Using a UV LED excitation source with a wavelength near the bandgap of GaN, TTI of GaN channels in HEMTs is presented and verified by comparing the gate metal temperature. To ensure a strong thermoreflectance signal from the GaN surface, the importance of using an excitation wavelength near the bandgap of the GaN channel is highlighted. The effect of the bandgap on the magnitude and the linearity of the thermoreflectance coefficient is presented and discussed. Overall, the improvements to TTI discussed in this article make the technique an accurate method to measure the temperature distribution in GaN HEMT SL structures.

Oil paints comprise pigments, drying oils, and additives that together confer desirable properties, but can react to form metal carboxylates (soaps) that may damage artworks over time. To obtain information on soap formation and aggregation, we introduce a new tapping-mode measurement paradigm for the photothermal induced resonance (PTIR) technique that enables nanoscale IR spectroscopy and imaging on highly heterogenous and rough paint thin sections. PTIR is used in combination with mu-computed tomography and IR microscopy to determine the distribution of metal carboxylates in a 23-year old oil paint of known formulation. Results show that heterogeneous agglomerates of Al-stearate and a Zn-carboxylate complex with Zn-stearate nano-aggregates in proximity are distributed randomly in the paint. The gradients of zinc carboxylates are unrelated to the Al-stearate distribution. These measurements open a new chemically sensitive nanoscale observation window on the distribution of metal soaps that can bring insights for understanding soap formation in oil paint.

International audience; Abstract:This paper details an extraction procedure to fully model the transient self-heating of transistors from a GaN HEMT technology. Frequency-resolved gate resistance thermometry (f-GRT) is used to extract the thermal impedance of HEMTs with various gate widths. A fully scalable analytical model is developed from the experimental results. In the second stage, transient thermoreflectance imaging (TTI) is used to bring deeper insights into the HEMTs' temperature distribution by individually extracting the transient self-heating of each finger. TTI results are further used to successfully validate the f-GRT results and the modeling of the thermal impedance. Overall, f-GRT is demonstrated to be a fast and robust method for characterizing the transient thermal characteristics of a GaN HEMT. For the first time to the authors' knowledge, a scalable model of the thermal impedance is extracted fully from experimental results.

Several optical and electrical methods have been developed to thermally characterize GaN high-electron mobility transistors (HEMTs) under DC operation. Gate resistance thermometry (GRT) has proven to be an inexpensive and reliable method to estimate the peak temperature. The complex Joule heating profile of a HEMT, however, is highly bias dependent, and its temperature profile under RF conditions may vary compared to the profile generated under DC operation. Evaluating the thermal performance of HEMTs under RF operation is thus necessary to quantify any differences between these two modes of operation. Implementing a differential probe, this paper highlights the capability of using GRT to measure accurately the temperature of GaN/Si HEMTs under RF operation. A larger gate-to-gate spacing is shown to decrease the overall temperature rise, while a linear correlation is found between the power-added efficiency and the junction temperature. The effect of the baseplate temperature on the device’s thermal performance both under steady-state and RF operations is presented. Overall, at a given power density and baseplate temperature, the RF temperature rise is shown to appear lower than its equivalent temperature under DC biasing.

The development of steady-state thermal characterization techniques for AlGaN/GaN high-electron mobility transistors (HEMTs) has been used to measure the device’s peak temperature under DC conditions. Despite these methods enabling the accurate quantification of the device’s effective thermal resistance and power density dependence, transient thermometry techniques are necessary to understand the nanoscale thermal transport within the active GaN layer where the highly localized joule heating occurs. One technique that has shown the ability to achieve this is transient thermoreflectance imaging (TTI). The accuracy of TTI is based on using the correct thermoreflectance coefficient. In the past, alternative techniques have been used to adjust the thermoreflectance coefficient to match the correct temperature rise in the device. This paper provides a new method to accurately determine the thermoreflectance coefficient of a given surface and is validated via an electrical method: gate resistance thermometry (GRT). Close agreement is shown between the temperature rise of the passivated gate metal measured by TTI and the averaged gate temperature monitored by GRT. Overall, TTI can now be used to thermally map GaN HEMTs under pulsed conditions providing simultaneously a submicrosecond temporal resolution and a submicrometer spatial resolution.

Hyperbolic phonon polaritons (HPhPs) enable strong confinements, low losses, and intrinsic beam steering capabilities determined by the refractive index anisotropy—providing opportunities from hyperlensing to flat optics and other applications. Here, two scanning-probe techniques, photothermal induced resonance (PTIR) and scattering-type scanning near-field optical microscopy (s-SNOM), are used to map infrared (6.4–7.4 µm) HPhPs in large (up to 120 × 250 µm2) near-monoisotopic (>99% 10B) hexagonal boron nitride (hBN) flakes. Wide (≈40 µm) PTIR and s-SNOM scans on such large flakes avoid interference from polaritons launched from different asperities (edges, folds, surface defects, etc.) and together with Fourier analyses (0.05 µm−1 resolution) enable precise measurements of HPhP lifetimes (up to ≈4.2 ps) and propagation lengths (up to ≈25 and ≈17 µm for the first- and second-order branches, respectively). With respect to naturally abundant hBN, we report an eightfold improved, record-high (for hBN) propagating figure of merit (i.e., with both high confinement and long lifetime) in ≈99% 10B hBN, achieving, finally, theoretically predicted values. We show that wide near-field scans critically enable accurate estimates of the polaritons’ lifetimes and propagation lengths and that the incidence angle of light, with respect to both the sample plane and the flake edge, needs to be considered to extract correctly the dispersion relation from the near-field polaritons maps. Overall, the measurements and data analyses employed here elucidate details pertaining to polaritons’ propagation in isotopically enriched hBN and pave the way for developing high-performance HPhP-based devices.

In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.

International audience; The breakdown voltage of GaN/Si high electron mobility transistors (HEMTs) for power electronics has shown to be improved by removing the silicon substrate. The drawback to this approach is the increase in the device's thermal resistance which limits the power dissipation that the device can achieve before severe degradation. This study shows the ability to improve the thermal dissipation of these devices by depositing Copper (Cu) below Aluminum Nitride (AlN) filled etched back GaN-on-Si HEMTs. The device's channel temperature is measured via Raman thermometry. The device's transient thermal dynamics is investigated via transient thermoreflectance imaging and the temperature profile across the gate metal is monitored. In addition to the device's thermal properties, residual stress analysis of the GaN channel is performed via photoluminescence. A notable decrease in the tensile residual stress is observed with the removal of the substrate and the addition of the AlN and Cu layers. Overall, the backside copper is shown to decrease the gate temperature of the etched backed AlN filled devices while maintaining a high breakdown voltage.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Energy efficiency, long life, exceptional color, and performance of solid-state light sources have resulted in a rapidly increasing trend in a number of practical applications especially for general lighting after a long history of incandescent lamps. Besides, light-emitting diodes (LEDs) have thermal limitations that are vital for device quality and lifetime. Specifically, to improve the heat dissipation, one major parameter used to evaluate the LED performance is thermal resistance (R). Reducing the resistance can improve the heat flow from the p-n junction to ambient during operation. To quantify this parameter, the LED junction temperature (TJ) must be determined. In this paper, the junction temperatures are first measured with forward voltage method (FVM), Raman spectroscopy, and infrared (IR) imaging for a 465-nm bare blue LED chip (without any phosphor coating). Then, the same samples have been coated with a phosphor-particles added epoxy mixture (%13, 4300 CCT) to convert blue to white light, and the junction temperatures were measured again experimentally with the previously mentioned three methods and compared to each other. While IR imaging shows better capability on capturing the possible hotspots over the surface, Raman method and FVM were in reasonably good agreement on measuring the junction temperature for 465-nm blue (uncoated) LED chip. However, the measurements performed after coating have shown slightly different results with IR imaging and Raman methods, while FVM has shown consistent results for coated chips. EU FP7 Career Integration ; Istanbul Development Agency

The synthesis of few-layer tungsten diselenide (WSe2) via chemical vapor deposition typically results in highly non-uniform thickness due to nucleation initiated growth of triangular domains. In this work, few-layer p-type WSe2 with wafer-scale thickness and electrical uniformity is synthesized through direct selenization of thin films of e-beam evaporated W on SiO2 substrates. Raman maps over a large area of the substrate show small variations in the main peak position, indicating excellent thickness uniformity across several square centimeters. Additionally, field-effect transistors fabricated from the wafer-scale WSe2 films demonstrate uniform electrical performance across the substrate. The intrinsic field-effect mobility of the films at a carrier concentration of 3 × 10(12) cm(-2) is 10 cm(2) V(-1) s(-1). The unprecedented uniformity of the WSe2 on wafer-scale substrates provides a substantial step towards producing manufacturable materials that are compatible with conventional semiconductor fabrication processes.

The development of high-quality gallium nitride (GaN) high electron mobility transistors (HEMTs) has provided opportunities for the next generation of high-performance radio frequency and power electronics. Operating devices with smaller length scales at higher voltages result in excessively high channel temperatures, which reduce performance and can have detrimental effects on the device's reliability. The thermal characterization of GaN HEMTs has traditionally been captured from either the top or bottom side of the device. Under this configuration, it has been possible to map the lateral temperature distribution across the device with optical methods such as infrared and Raman thermometry. Due to the presence of the gate metal, however, and often also the addition of a metal air bridge and/or field plate, the temperature of the GaN channel under the gate is typically inferred by numerical simulations. Furthermore, measuring the vertical temperature gradient across multiple epitaxial layers has shown to be challenging. This study proposes a new cross-sectional imaging technique to map the vertical temperature distribution in GaN HEMTs. Combining advanced cross-sectioning processing with the recently developed near bandgap transient thermoreflectance imaging technique, the full transient thermal distribution across a GaN HEMT is achieved. The cross-sectional thermal imaging of the GaN channel is used to study the effects of biasing on the Joule heating profile. Overall, the direct measurement of the GaN channel, capturing both the vertical and lateral gradient, will provide deeper insight into the device's degradation physics and supply further experimental data to validate previously developed electrothermal models.

Shrinking features and growing device complexity in advanced microwave devices has increased the challenge of fully understanding device thermal behavior on the sub-micron scale. Predicting the device static and dynamic thermal behavior is essential for ensuring optimal tradeoffs between performance and device reliability. Thermal imaging based on the Thermoreflectance Principle can meet the challenges imposed by these compact, high power density RF devices by providing submicron spatial resolution and temporal resolution in the picosecond range. This technique overcomes the limitations of traditional thermal imaging techniques such as IR and Micro-Raman and some of the specific challenges in measuring GaN devices. In the past, Thermoreflectance Imaging has been shown to accurately estimate the temperature rise of metals using visible wavelength excitation sources. This paper presents a novel method to estimate the temperature directly on GaN surfaces using UV wavelengths. These UV thermoreflectance measurements were verified with measurement of an on-chip GaN mesa resistor. Temperature measurements on top of the field plate and inside the GaN channel were compared for a commercial GaN HEMT both on Si and SiC substrate. The advantages and disadvantages will be presented for the thermoreflectance technique for thermal imaging.

In this study, we perform electrical and thermal characterization of vertical GaN pn diodes. One set of diode devices was subjected to passivation through the use of a polyimide film, while the other set of devices was non-passivated. Electroluminescence (EL) was performed on both sets of devices, and it was found that non-passivated devices demonstrated increased EL intensity at the mesa edges, most likely due to enhanced surface recombination, and subsequently showed I-V characteristics that outperformed the passivated devices in terms of power handling capabilities at high forward voltages. Thermoreflectance imaging was used to evaluate the temperature distribution of the metal contact for 130 μm diodes, and it was found that a very uniform distribution was consistent even for very large forward voltage bias. Additional thermal characterization was carried out using time-domain thermoreflectance (TDTR) and depth dependent measurements demonstrated an inhomogeneity in the thermal conductivity of the diodes when measuring from the heavily doped p-type region.

Transient thermoreflectance imaging (TTI) is a thermometry technique employed to map the surface temperature distribution of GaN HEMTs. The accuracy of the technique is dependent on applying the correct thermoreflectance coefficient to the region of interest on the device surface. TTI has shown high accuracy when measuring the temperature rise of metals such as Schottky contacts but the technique has never been verified when applied to semiconductors. Using UV LED excitation sources, TTI of the GaN channel in GaN/Si HEMTs is presented and verified for the first time via the comparison of the gate metal temperature. A pixel by pixel calibration method is implemented to improve the spatial accuracy and account for any variability in the thermoreflectance coefficient across the device. The improvements of TTI discussed in this study make the technique an accurate and effective method to measure the temperature distribution of both the gate metal and GaN.

In this work, we investigate the thermal response of GaN PIN diodes grown on a sapphire substrate and compare the results to GaN PIN diodes grown on a free standing GaN substrate (FS-GaN). Until now, thermal characterization techniques have been developed to assess the temperature distribution across lateral devices. Raman thermometry has shown to accurately measure the temperature rise across the depth of the GaN layer. Implementing this technique to assess the temperature distribution across the depth of a vertical GaN device is more challenging since a volumetric depth average is measured. The use of TiO2 nanoparticles is shown to overcome this issue and reduce the uncertainty in the peak temperature by probing a surface temperature on top of the device. For the sapphire substrate, an additional temperature rise of about 15 K was seen on the surface of the PIN diode as compared to the average in the bulk. While the steady state thermal measurements show an accurate estimation of the device’s peak temperature, the PIN diodes are normally operated under pulsed conditions and the thermal response of these devices under periodic joule heating must be assessed. A recently developed transient thermoreflectance imaging technique (TTI) is used in this study to monitor transient temperature rise and decay of top metal contact. Under the same biasing conditions, the FS-GaN PIN diode is found to result in less than half the temperature rise obtained by the sapphire substrate diode. Extracting time constants, a longer rise and decay is also observed in the sapphire substrate diode.

In this study, pioneering research was performed on GaN p-i-n diodes for the first ever assessment of surface temperature distribution by incorporating the use of infrared (IR) thermography, thermoreflectance thermal imaging, Raman thermometry, and thermal simulations. Each technique was advanced in order to obtain self-consistent results with higher accuracy. A two-temperature emissivity calibration procedure was utilized for IR measurements to acquire a temperature map of the p-contact metallization. Higher spatial resolution thermoreflectance thermal imaging was performed with a diverse range of illumination wavelengths including 470 nm and 530 nm. To confirm the results of thermoreflectance, TiO2 thermal nanoprobes were deposited on the device surface which enabled Raman thermometry to be performed on the p-contact metallization. Coherence of the techniques was then validated through thermal modeling. The results suggest that IR thermography, when using the two-temperature emissivity correction procedure, gives reasonable results at high power dissipating conditions. Thermoreflectance and nanopowder assisted Raman thermometry are viable options for GaN vertical device temperature assessment. However, results from Raman thermometry possess relatively large uncertainties and thermoreflectance measurements require multiple illumination wavelengths to ensure the validity of the measured temperatures that are derived from the thermoreflectance calibration coefficient.

The ability to fabricate AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates has enabled the production of low cost high power electronics. To further enhance the performance of GaN electronics for high power conversion, the ability to maintain high off-state breakdown voltages with large electron densities is necessary. The use of Si substrates, however, limits the device's capabilities due to its weak electrical field strength. This limitation has been identified as the main cause for breakdown in HEMTs when the high electric field reaches the silicon substrate underneath the region between the gate and drain. To overcome this obstacle, removal of the Si substrate between the gate and drain region has shown to increase the device's breakdown voltage up to 3000 V. While removing the Si substrate extends the capabilities of GaN HEMTs for high voltage applications, the effects of the Si removal on the thermal performance during operation has not yet been investigated. Raman Thermometry, a well-developed technique, is used to compare the maximum temperature rise between a Local Substrate Removed (LSR) device and a non-LSR device. The application of nanoparticles (TiO2 and ZnO) for measuring surface temperatures via Raman spectroscopy is also investigated and applied to determine a more accurate temperature of the gate junction temperature. The LSR device was found to have a much higher thermal resistance than its non-LSR device counterpart limiting the maximum power dissipation the LSR device can achieve before severe degradation. Volumetric averaged residual stress mapping was also measured via Raman Spectroscopy and suggests the removal of the Si relaxes the stress in the GaN buffer layer and AlGaN barrier which can be exploited in designs to improve reliability. Methods to improve the thermal reliability of LSR devices are key to implementing such devices as future power switches.

Thermal management and planning is important for heterogeneous integration due to the introduction of a complex thermal path. Thermal measurement of operating devices provides necessary data points for future design as well as validation of models. In this paper, two methods for measuring thermal performance of DAHI (Diverse Accessible Heterogeneous Integration) GaN HEMTs are presented and contrasted: IR microscopy and micro Raman spectroscopy. The QFI IR system uses a per-pixel material emissivity flat temperature calibration when the device is in an off-state, and then calculates operating temperatures by CCD exposure. Two separate QFI systems with differing CCD resolutions were used to collect thermal data and are compared. Raman Thermometry by contrast, is a laser point measurement of the frequency shift in scattered photons due to phonon vibrational modes whose frequencies are temperature dependent. Differences in measurements between the two methods arising from the stack of materials used in the DAHI process and their transparency are discussed. A method for measuring the surface temperature of the devices through Raman by the use of TiO2 nanoparticles is also presented in conjunction with a profile of the HEMT. Measurements are presented alongside thermal simulation results using prototype software Mentor GraphicsTM Calibre®.

Integration of materials such as GaN, InP, SiGe, and Si is a natural extension of the 3D-IC perspective and provides a unique solution for high performance circuits. In this approach, application of a component is no longer dependent on semiconductor material selection. In this paper, preliminary results are presented which examine the thermal performance of the technology. A thermal analysis prototype solution in Mentor Graphics™ Calibre® provides surface heat maps based on IC layout, material property, and geometric configuration files. Chiplets are connected by heterogeneous interconnect (HIC). Differences in thermal performance of GaN and InP chiplets are explored by varying the number of HICs. Two methods for building up the model of a test chip are compared. One method uses custom scripts to place discrete blocks in the model to represent HICs, while the other uses thermal material properties extracted from the layout. Measurements presented confirm simulated results.