From Superlattices to Quantum Dots:
A Perspective of Nanostructure Devices
榊 裕之 教授
（豊田工業大学 学長、独立行政法人 物質・材料研究機構グループリーダー）
Thanks to advanced epitaxial technology, one can form a variety of layered nanostrcutures, such as quantum wells (QWs), superlattices (SLs), and selectively-doped heterojunctions. Two-dimen- sional (2D) carriers confined in such structures have been widely studied, because of their impor- tance both in 2D physics and advanced device applications, such as QW lasers and high-speed FETs.
To explore further potentials of nanostructures, the use of 1D and 0D electrons in quantum wires (QWRs) and quantum dots (QDs) was proposed to make such new devices  as planar SLs , QWR FETs , and QD lasers . Though QWRs and QDs could not be initially made, these proposals spurred material scientists to develop new methods of nanostructure fabrication.
While electron-beam technique was used initially to form QWRs and QDs of around 100nm in size, several new methods have been developed to form 10nm-scale QWRs and/or QDs; they include the overgrowth of an n-AlGaAs on the cleaved edge of GaAs QWs and the facet-selective growth on patterned substrates . It has been also found that 10nm-scale QDs can be formed by self-assem- bled Stranski-Krastanow (SK) growth on lattice-mismatched substrates and also by the droplet epitaxy, in which metallic droplets are formed and then transformed to QDs of intermetallic com- pounds . The SK growth has been used to make QD devices, such as QD lasers of excellent tem- perature stability , single-photon emitters, and interband /intersubband QD photodectors .
It has been also shown that 10nm-scale QWRs and related nanostructures can be formed by such methods as the stacking of multiple SK QDs and the vapor-liquid-solid growth of nanowires on catalytic nanoparticles; these methods have been successfully used to make various QWR/QD based devices, such as QWR FETs, single-electron transistors, LEDs, photodetectors and so on. In this talk, we review recent advances in QD/QWR growth  and discuss their device prospects.
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