Mar./2015
(STRATEGIC PROPOSALS)
Nanoscale Thermal Management for Device Innovation - Phonon Engineering -/CRDS-FY2014-SP-04
Executive Summary

The understanding and control of thermal properties in nano space and very short time scale are becoming more and more indispensable for solution of the information explosion, as well as for highly efficient utilization of energy which are required in the future society, since the innovation of devices for information processing and storage, and for thermoelectric conversion are confronting a limit due to heat problems. The present proposal relates to the establishment of a new academic field concerning the nanoscale heat management based on the concepts of phonons and the promotion of R&D activities toward the innovation of devices in terms of phonon engineering.

It is well recognized that the convenience of our lives has been greatly improved by higher performance of devices in the recent information-intensive/network society. On the other hand, however, the amount of newly generated information has been increasing dramatically, and is forecasted to reach 40 zettabytes (1021) by 2020, about ten times the present amount. To deal with the information explosion, continued technical innovation in information processing and the achievement of substantially higher performance and power saving by data storage devices are strongly required for the future. However, in a semiconductor integrated circuits, the problem of heat generation and dissipation by miniaturized devices on a nanoscale constitutes the limiting factor against advanced performances. In addition, the hard disk storage devices are also confronting a capacity expansion limit due to the thermal fluctuation of magnetized area in nanoscale. To resolve such problems, the development of nanoscale heat control methods and devices featuring a new operating principle which proactively utilizes the heat generation on a nanoscale is strongly expected. Under such circumstances, understanding the nanoscale behaviors of heat transport, and controlling and using the characteristics thereof will become strongly important.

In the nanoscale, the heat transport in a material should be treated in terms of the transport of phonons, which are quanta of lattice vibrations. The concept of phonons is rather old one since it is discovered around the beginning of the 20th century. Nevertheless, the understanding and the control technologies of heat based on phonons were much delayed compared to electronic properties and optical properties, since deep understanding and control have rarely been necessary for device development to date. Now that the miniaturization of electronic devices, optical devices, and magnetic devices proceeds to the nanoscale level, less than the mean free path of phonons, the correct understanding of device operations and designing is impossible unless electrons, photons, spins, and phonons are handled in a unified manner. Therefore, in the present proposal, the purpose is set out to ensure the establishment of a new academic field and the innovation of materials and devices by deepening the understanding of heat in the nanoscale region from the perspective of nanoscience, thereby establishing heat control and utilization technologies. Here, the new academic discipline to manipulate the transport of phonons and control the transport of heat by handling the transport of heat with the concept of phonons and using artificial structures will be referred to as phonon engineering. Proposals will be made regarding the subjects of R&D and the promotion system thereof that will be required for this purpose.

The research issues include heat measurement, theory/simulation of phonon transport, and phonon transport control by manufacturing materials and structures, and it is necessary to establish a new academic field that should be referred to as the thermal nano-science and the thermal nano-engineering. Furthermore, it is also important to create revolutionary technologies for materials and devices by understanding the quantum systems, including phonons, electrons, photons, and spins, in a unified manner and controlling the nanoscale physical phenomena where the quantum systems are intricately intertwined. More specifically, it is necessary to execute the following research and development activities.

Correct understanding of the phenomena regarding thermal properties at the nano-level requires a development of new methods and equipment capable of measuring the actual temperatures and heat conduction on a nanoscale with a high level of accuracy. It is necessary to improve the measurement accuracy in the conventional optical thermo-reflectance method that have been used for evaluation of heat conduction for a long time; expand the measurable range in time, space, structure, etc.; and develop new evaluation methods and equipment, including the scanning thermal probe microscope that is capable of measuring the nanoscale local structure with a high level of accuracy. In addition, it is also necessary to develop an evaluation method that will enable to obtain not only the steady state but also the transient state information of heat.

Establishment of the theory of heat conduction on the nanoscale taking into account the aspects of surfaces, interfaces, impurities, structural defects, etc., as well as development of the simulation algorithms are required. Here, it is also necessary to handle the heat transport on a nanoscale not only in terms of solely the size, but also taking account the scattering of phonons on the structure/material of low-dimensional systems, ie., ultra-thin films or ultra-fine wires, surfaces of materials, and interfaces of dissimilar materials. For the simulation, it is necessary to establish the art of computation that can easily calculate the parameters of basic thermal properties of materials in a theoretical manner by expanding the scale, increasing the accuracy, and improving the operability of the programs that calculate the behaviors at the atomic and molecular levels from the fundamental principles of physics. Furthermore, it is also necessary to develop the simulation algorithms for phonon transport in the actual materials and device structures by skillfully executing theoretical computation methods using the parameters of thermal properties, in particular, the multi-scale simulation that makes simulation algorithms at different scales to work interactively. It is important to investigate the control methods based on the understanding on the nanoscale heat transport, according to the phonon transport concept, thereby systematizing the methods as an integrated technology. More specifically, the materials and the device structures should be manufactured by introducing the interfaces, impurities, structural defects, dissimilar materials, microstructures, periodic structures, etc., thinner films and lower dimensions, thereby understanding the effects thereof on heat transport in an experimental manner. In addition, the comprehensive research and development activities to confirm the effects theoretically and through simulations and incorporate the knowledge obtained in the processes in the material design and device design are required. Furthermore, it is also important to work on new control methods through the formulation of artificial structures, such as a phononic crystal structure that controls the diffusion of phonons by utilizing the properties of wave motions of phonons. For such control approaches of heat transport at nanoscale, the knowledge on the manufacturing technologies of artificial structures to control electrons and photons which are established in the electronic devices and optical devices for a long period of time are expected to be utilized proactively. Researchstudies where the scenes to apply the heat control technologies through the nanoscale phonon transport and the operations thereof to actual application fields are assumed are also important. Here, it is necessary to give simultaneous consideration also to the control of phonons and other quanta including photons, electrons, and spins. Along with the development of the simulation method that is capable of handling such quantum systems in an integrated manner, it is important to proceed with modelling for easy handling and proceed with R&D work on the material/device design methods where the models are utilized. Through such arrangements, it is expected that the characteristics of the semiconductor integrated circuits, power semiconductors, next-generation hard disk drives, thermoelectric conversion elements, etc., where the nanoscale heat transport is a bottleneck in terms of performance and function, can be improved. Furthermore, the technologies can be developed for use with the new devices, such as memory devices and sensors where nanoscale heat control is utilized The most important thing concerning the promotion method of R&D work is that researchers and engineers involved on nanoscale heat conduction should make efforts while sharing the goals of R&D work, extending the boundaries of academic fields and application fields. The nanoscale heat control that has been overlooked so far is a tough proposition, and achievement will be unlikely unless the goals are shared between scientists, engineers, and researchers among industry, academia, and government. In addition, for the promotion of R&D work, the formation and development of community will be very important. The reason is that the nanoscale heat control cannot be handled with the expert knowledge and the technical territory region of a single field, and the opportunities where researchers and engineers get together, across the boundaries between different academic fields or between application fields, to make discussions and the network environment enabling close information exchange at any time will be required. At the same time, participants from different fields will have to assume the role of cooperating and executing joint research, develop apparatuses, and cultivate human resources. In this regard, it is necessary that, for example, the researchers of materials and devices, and the researchers involved in thermal property measurement and heat conduction theory/simulation will execute research work under one roof. Furthermore, for the present research and development, it is recommended to establish and operate a knowledge base on heat properties that can be shared widely by researchers involved in R&D work. Regarding the thermal properties of materials and devices in nanoscale, there has been no systematically organized knowledge base to date, and such heat properties have not been established as an academic field, which constitutes the barrier for researchers to enter the area anew. It is important to build a detailed database concerning the nanoscale heat properties and establish and organize an environment for usage and tools to which related researchers can access freely for use. From a global perspective, although the United States is in the most advanced position concerning individual research and conceptual representation, intensive approaches that include programming work according to the policy have not been implemented yet in any country. Therefore, now is the right time to design and implement a policy