In phase change memory cells, the majority of heat is lost through the electrodes during the programming process, which leads to significant drops in the performance of the memory device. In this Letter, we report on the thermal properties of thin film carbon nitride with a modest electrical resistivity of 5–10 mΩ cm, a low thermal conductivity of 1.47 ± 0.09 W m−1 K–1, and a low interfacial thermal conductance between carbon nitride and phase change material for length scales below 40 nm. The thermally insulating property of carbon nitride makes it a suitable thermal barrier, allowing for less heat loss during Joule heating within the memory unit. We compare the thermal properties of carbon nitride against the commonly used electrodes and insulators such as tungsten and silicon nitride, respectively, to demonstrate the promise of carbon nitride as a potential material candidate for electrode applications in phase change memory devices.
Gallium nitride (GaN) has emerged as a quintessential wide band-gap semiconductor for an array of high-power and high-frequency electronic devices. The phonon thermal resistances that arise in GaN thin films can result in detrimental performances in these applications. In this work, we report on the thermal conductivity of submicrometer and micrometer thick homoepitaxial GaN films grown via two different techniques (metal-organic chemical vapor deposition and molecular beam epitaxy) and measured via two different techniques (time domain thermoreflectance and steady-state thermoreflectance). When unintentionally doped, these homoepitaxial GaN films possess higher thermal conductivities than other heteroepitaxially grown GaN films of equivalent thicknesses reported in the literature. When doped, the thermal conductivities of the GaN films decrease substantially due to phonon-dopant scattering, which reveals that the major source of phonon thermal resistance in homoepitaxially grown GaN films can arise from doping. Our temperature-dependent thermal conductivity measurements reveal that below 200 K, scattering with the defects and GaN/GaN interface limits the thermal transport of the unintentionally doped homoepitaxial GaN films. Further, we demonstrate the ability to achieve the highest reported thermal boundary conductance at metal/GaN interfaces through in situ deposition of aluminum in ultrahigh vacuum during molecular beam epitaxy growth of the GaN films. Our results inform the development of low thermal resistance GaN films and interfaces by furthering the understanding of phonon scattering processes that impact the thermal transport in homoepitaxially grown GaN.
Dielectric amorphous multilayers (AMLs) play a critical role in a wide array of technologies such as optical coatings, nanoelectronics, energy harvesting, and recovery devices. However, despite their wide applications, a robust understanding of the effect of the interplay between chemical and structural disorder on the thermal properties of AMLs is still lacking. Therefore, in this paper, we experimentally and numerically investigate the effects of composition and interface density on the sound speed and thermal conductivity of a series of amorphous aluminum nitride and aluminum oxide multilayers grown via plasma-enhanced atomic layer deposition. To systematically change the composition, the oxygen content of the AMLs is proportionally varied with interface density during growth. We find that the longitudinal sound speed of these AMLs is dictated by the oxygen content instead of the number of interfaces. The thermal conductivity, in contrast, is dictated by both interface density and oxygen content. The interfaces act to decrease the thermal conductivity, whereas the oxygen content increases the thermal conductivity. Due to the competing influence of the interfaces and oxygen content, the thermal conductivity of the AMLs remains nearly constant as a function of interface density. Our study provides crucial insights into the effect of the interplay of composition and interfaces on the sound speed and thermal conductivity of AMLs.
In this paper, a double-glazed solar air heater (SAH) using paraffin wax as phase change material (PCM) was designed, fabricated, and tested under the climatic condition of Mashhad, Iran (latitude, 37° 28′ N and longitude, 57° 20′ E) during three typical days in the summer. The PCM stores solar radiation of the sun as latent and sensible heat during daytime and then restores such stored energy during the night. Exploitation of both first and second laws of Thermodynamics, the energy and exergy efficiencies of this system are assessed. According to the experiments undertaken, it is found that the daily energy efficiency of the system varies between 58.33% and 68.77%, whereas the daily exergy efficiency varies from 14.45% to 26.34%. Eventually, the economic analysis shows that the cost of 1 kg of heated air utilizing double-glazed SAH would be 0.0036$.
Chalcogenide materials such as Ge 2 Sb 2 Te 5 (GST) which undergo structural transition between amorphous and crystalline phases with applied thermal load, have emerged as potential material candidates for new memory technologies due to prospective gain in speed, device lifetime, and capacity. In these devices, each memory cell is composed of various components with different material compositions and functionalities. Therefore, a solid understanding of how heat transfers between each component is pivotal in the enhancement of performance and minimization of power consumption. In this study using time-domain thermoreflectance, we measure thermal properties relevant to device operation, at material length scales (< 40 nm) similar to those used in actual devices, such as sound speed, thermal conductivity and thermal boundary conductance (TBC) for a temperature range from 25 °C to 400 °C. According to acoustic echoes obtained from picosecond acoustic measurements, the speed of sound in GST is calculated to be around 2,900 m/s. Moreover, we report the thermal boundary resistance (TBR) when different spacer compositions (W, SiO 2 , SiN x ) are introduced to separate GST from the other components where SiN x /GST interface showed the highest TBR compared to both W and SiO 2 interlayers. Additionally, the temperature dependent results indicate that the GST change phase from amorphous to cubic structure at 150 C and again from cubic to hexagonal at approximately 340 C. The thermal conductivity of GST experiences a significant jump at the transition temperature of 150 from 0.15 W/m/K to 0.30 W/m/K and continue to linearly increase by raising the temperature until its crystal structure completely transforms into the hexagonal where the thermal conductivity flattens out to the value of 1.4 W/m/K.
Colloidal crystals provide a versatile platform for designing phononic metamaterials with exciting applications for sound and heat management. New advances in the synthesis and self-assembly of anisotropic building blocks such as colloidal clusters have expanded the library of available micro- and nano-scale ordered multicomponent structures. Diamond-like supercrystals formed by such clusters and spherical particles are notable examples that include a rich family of crystal symmetries such as diamond, double diamond, zinc-blende, and MgCu2. This work investigates the design of phononic supercrystals by predicting and analyzing phonon transport properties. In addition to size variation and structural diversity, these supercrystals encapsulate different sub-lattice types within one structure. Computational models are used to calculate the effect of various parameters on the phononic spectrum of diamond-like supercrystals. The results show that structures with relatively small or large filling factors (f > 0.65 or f < 0.45) include smaller bandgaps compared to those with medium filling factors (0.65 > f > 0.45). The double diamond and zinc-blende structures render the largest bandgap size compared to the other supercrystals studied in this paper. Additionally, this article discusses the effect of incorporating various configurations of sub-lattices by selecting different material compositions for the building blocks. The results suggest that, for the same structure, there exist multiple phononic variants with drastically different band structures. This study provides a valuable insight for evaluating novel colloidal supercrystals for phononic applications and guides the future experimental work for the synthesis of colloidal structures with desired phononic behavior.
Reconfigurable or programmable photonic devices are rapidly growing and have become an integral part of many optical systems. The ability to selectively modulate electromagnetic waves through electrical stimuli is crucial in the advancement of a variety of applications from data communication and computing devices to environmental science and space explorations. Chalcogenide-based phase change materials (PCMs) are one of the most promising material candidates for reconfigurable photonics due to their large optical contrast between their different solid-state structural phases. Although significant efforts have been devoted to accurate simulation of PCM-based devices, in this paper, we highlight three important aspects which have often evaded prior models yet having significant impacts on the thermal and phase transition behavior of these devices: the enthalpy of fusion, the heat capacity change upon glass transition, as well as the thermal conductivity of liquid-phase PCMs. We further investigated the important topic of switching energy scaling in PCM devices, which also helps explain why the three above-mentioned effects have long been overlooked in electronic PCM memories but only become important in photonics. Our findings offer insight to facilitate accurate modeling of PCM-based photonic devices and can inform the development of more efficient reconfigurable optics.
Chalcogenide optical phase change materials (PCMs) have garnered significant interest for their growing applications in programmable photonics, optical analog computing, active metasurfaces, and beyond. Limited endurance or cycling lifetime is however increasingly becoming a bottleneck toward their practical deployment for these applications. To address this issue, we performed a systematic study elucidating the cycling failure mechanisms of Ge$_2$Sb$_2$Se$_4$Te (GSST), a common optical PCM tailored for infrared photonic applications, in an electrothermal switching configuration commensurate with their applications in on-chip photonic devices. We further propose a set of design rules building on insights into the failure mechanisms, and successfully implemented them to boost the endurance of the GSST device to over 67,000 cycles.
Abstract Chalcogenide optical phase change materials (PCMs) have garnered significant interest for their growing applications in programmable photonics, optical analog computing, active metasurfaces, and beyond. Limited endurance or cycling lifetime is however increasingly becoming a bottleneck toward their practical deployment for these applications. To address this issue, a systematic study elucidating the cycling failure mechanisms of Ge 2 Sb 2 Se 4 Te (GSST) is performed, a common optical PCM tailored for infrared photonic applications, in an electrothermal switching configuration commensurate with their applications in on‐chip photonic devices. Further a set of design rules building on insights into the failure mechanisms is proposed, and successfully implemented them to boost the endurance of the Ge 2 Sb 2 Se 4 Te (GSST) device to over 67 000 cycles.
