Heat-smart Optoelectronics &Phonon Engineering (HOPE) Laboratory
We have accumulated intensive research results and research know-how in the field of compound semiconductors and optoelectronics, and have built infrastructures such as clean rooms, cutting-edge experimental equipment and related research expertise. However, the manipulations of electrons and photons has reached technological saturation as revealed in the limitation of Moore's Law since the 2010’s.
What do we need to do then? In recent decades, major technological innovations such as the transistors and LEDs have been based on the manipulation of electrons and photons. Phonons, or quanta of atomic vibrations, are responsible for heat conduction but remain poorly understood, and this lack of understanding impedes efforts to improve device performance such as speed, lifetime, and quantum yields.
HOPE (Heat-smart Optoelectronics &Phonon Engineering) has proposed and pioneered new methodologies for integrated manipulations of electrons, photons, and phonons, to further enhance the efficiency and lifetime of semiconductor devices and systems. Especially, we note the heat-related technology follows the traditional diffusion-based thermal conduction and particle-oriented statistics; In this case, phonons (or, heat carriers in solids) could be treated only at large scale, while at the nanoscale, phonons were not only required to have new theoretical formulations according to their wavy nature, but also the newly engineered heat manipulation structures must be developed. For such purposes, we innovate the concept of integrated controllability beyond the conventionally separated electronics, photonics, and phononics to further utilize thermal properties and pioneer revolutionary heat manipulations.
The purpose of these HOPE-proposed work is thus impactful both in engineering and scientific viewpoints; to newly integrate engineered heat manipulation structures into optoelectronics, and to understand the microscopic heat transport mechanisms in nanoscale heat dissipation materials, thermoelectricity, and the artificially designed thermal metamaterials for next-generation semiconductor devices.
More detailed research topics are as follows:
> Optoelectronics, efficient in photonic and thermal perspectives
Since the revolutionary developments of blue light-emitting devices (LEDs, LDs) in Mid-90’s, the thrust to replace all lighting tools by those based on semiconductors has become an extremely active area of research. We approach this topic along two major directions: the integration of therally efficient nano-materials or even thermal meta-structures into optoelectronics based on the unique wavy manipulation schemes desinged in nanoscale and quantum efficiency improvement by utilizing novel nanomaterials.
> Terahertz devices
This electromagnetic spectrum from 100 GHz to 20 THz is a new technical frontier in demand for 2020. Especially for next generation wireless communications (beyond traditional RF range) to improve the transmission speed is already standardized in IEEE as of 2010. Another application as 21st century technology is harmless medical imaging systems because the THz is nondestructive source well adaptable into 2-D and 3D bio-medical targets. We are developing various emission sources, fabrication technics, imaging schemes beyond the traditional limit, in this regard
> Ultrafast devices
As the Moore’s law of transistor integration density already reached its limit, the alternative has risen up; the most important strategy here is either using cloud computing or making each transistor faster. To make it faster, now you need to have NT or faster materials such as graphene rather than traditional Si. small size as in nanostructures, then we need to be ultrafast. To characterize those ultrafast devices, we need to be able to see something very fast and that is where ultrafast probing techniques by using femtosecond lasers comes into play. If we do this in a raster style fashion, we can further build up a time resolved “image” of some property of the materials. We also study the emerging devices using ideal nanomaterials such as graphene (where the electrons therein are massless) as future alternative to Si.
> Novel materials and concepts
Novel concepts utilizing semiconductor nanostructures such as phonon transistors, thermal circuitry, metasurfaces and matamaterials. Concomittantly, we develope the relevant characterization and evaluation systems in nanoscale.
Main research directionality
> Semiconductor devices with better thermal management
This is one of the key topics with uniqe ideas and original mathematical platform developed in our group. The purpose of task is impactful both in engineering and scientific viewpoints; to newly integrate engineered heat manipulation structures into the devices such as light emitting diodes (LEDs) and transistors which are currently optimized only in terms of electronic and photonic efficiency, and to understand the microscopic heat transport mechanisms in nanoscale where the particle picture of heat often fails. Phonons, or quanta of atomic vibrations, are responsible for heat conduction but remain poorly understood in terms of wavy contribution into thermal transport, and this lack of understanding impedes efforts to improve device performance.
In this regard, we design and grow various thermally manipulated optoelectonic materials and devices, then use ultrafast schems to trace the generation, propagation, and manipulations of electrons and phonons in nanoscale and multiple boundaries. We often test various novel nanomaterials and directly observe the dynamics in those structures in conjunction with heat dissipation layers such as graphene, transistion metal dichalcogenides, and other novel 2D nanomaterials.
On the basis of the transmission speed trends which continue to expand as summarized in Fig. below, there is a need to supplement present microwave and millimeter bands with THz carriers in 10 to 15 years to come. The developments of communication schemes and related devices (sources, detectors, filters, modulators etc) are in demanding need; this is why we are working for various THz fusion technology developments.
<Yearly transmission speed trends based on standardization, compared with laboratory-based wireless demonstration. [Source: H. Takahashi et al. NTT Tech Rev. 7, 1 (2009).]>
As Moore’s empirical law on semiconductor device scale is reaching its limit as shown in the figure below, the nanotechnology has emerged as one of the most important area both in scientific research and in economic productions. In this standpoint, nanospec has placed particular emphasis on how we can understand and utilize the nanostructures for optoelectronic devices in UV to IR and in THz ranges.
<Moore's Law is a self-fulfilling prophecy financed by Intel, his company, as long as they can afford it.[Source: http://milwaukee.indymedia.org ]>