Quantum Dots and Their Impact on Optoelectronics


Yasuhiko Arakawa

Professor, Research Center for Advanced Science and Technology

University of Tokyo

There have been lots of research recently devoted to realizing the predicted potential of zero-dimensional (0D) quantum-confined structures, or quantum dots (QDs). Because of their unique electronic states (i.e., atomic-like discrete states with a d-function density of states), QDs are expected to have many interesting and useful properties for optolecetronic device applications. It was 1982 that the concept of the "quantum dots" was proposed as artificial atoms for semiconductor laser applications by Arakawa and Sakaki[1]. In this presentation, we discuss evolution and perspective of quantum dot research as an impact of the nanotechnologies on photonic and electronic devices which are indispensable for broad-band and wireless network systems toward ubiquitous communications.


The semiconductor laser with a QD active region promises ultra low and temperature independent threshold current, high-frequency modulation with negligible chirping effect, and non linear gain effect. Since it was almost impossible to fabricate the QD lasers in 1980's, quantum confinement effects on lasing characteristics were first demonstrated by using high magnetic fields in which Lorentzian force eliminates the 2D freedom of motions of electrons. The reduced temperature dependence, narrower spectral line width, and enhanced modulation frequency were demonstrated in semiconductor double heterostucture lasers and quantum well lasers placed in the high magnetic fields up to 30T.


The most straightforward technique to produce array of QDs is to fabricate suitably sized mesa-etched quantum wells grown by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). However, the nonradiative defects produced during etching procedure leads to degradation the material quality, which results in unsuitable structures for lasers. In 1990's, both selective growth and self-assembled growth techniques which can avoid nonradiative defects were well developed. Particularly, the Stranski-Krastanow growth mode is very successful for InGaAs/GaAs systems.


This self-assembed growth for making nanoscale islands led to a breakthrough to QD devices. Using the SAQDs, lasers, detectors for both inter-band and inter-subband transitions have been demonstrated. InGaAs/GaAs QD lasers have been successful to demonstrate high performance lasing characteristics. Low threshold current density of 21A/cm2 and high T0 up to 385K, and high differential gain at room temperature. Furthermore, 1.3mm lasing wavelength has been achieved, which is significant for low-cost lasers for access network communication systems utilizing GaAs substrates. Vertical cavity surface emitting lasers (VCSELs) with QDs have been also achieved. The material has been also extended from InGaAs/GaAs to other III/V and II/IV semiconductors. Particularly, GaN-based III-V semiconductors have received great attention for application to blue light emitting lasers. Recently we have succeeded in growing InGaN QDs on GaN epitaxial layer and operated an InGaN/GaN QD laser at room temperature.

(1) Y. Arakawa and H. Sakaki, "Multidimensional Quantum Well Lasers and Temperature Dependence of Its Threshold Current ", Appl. Phys. Lett., vol. 40, pp. 939-941(1982).


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