Background

A quantum dot is a 20-100-Angstrom, 3-dimensional structure involving 1,000 to 100,000 atoms. This size is interesting because it is smaller than the de Broglie wavelength of a slow electron, and thus involves various quantum phenomena. Another important point is the presence of surface-related phenomena, because the surface-to-volume ratio of a dot is large.

Quantum dots are also interesting because, while in a bulk conductor or semiconductor the energy states of the individual atoms are combined and spread across the entire material, in quantum dots they are restricted to the dot and there are a discrete number of them. These states are affected by the size and surface of a dot, the surrounding material, and any external fields. The energy levels of a quantum dot also change when it is excited by a photon of light.

The Masumoto single quantum dot project was aimed at understanding the basic physics of quantum dots, especially their optical properties.

Research Results.

Quantum dot arrays: One-dimensional In0.45Ga0.55As quantum dot (QD) arrays have been successfully fabricated on a GaAs (311) B substrate by metal-organic vapor-phase epitaxy (MOVPE). The spontaneous QD alignment along the [01-1] direction presumably originates from the periodic distribution of the strain field in a plane during In0.45Ga0.55As/GaAs multi-layer growth. The photoluminescence spectra show a clear polarization dependence.

J.-S. Lee, M. Sugisaki, H.-W. Ren, S. Sugou, and Y. Masumoto: "Spontaneous lateral alignment of multistacked In0.45Ga0.55As quantum dots on GaAs(311)B substrate" J. Cryst. Growth 200,77 (1999).

In situ ellipsometry: The first real-time observation of the formation of a QD by means of MOVPE was achieved in situ by an ellipsometry method. First, a thin layer is formed on the surface of a substrate. The layer then reorganizes into QDs. The new ellipsometry method was used to observe the thickness and composition of the layer as it is formed. Since the angle of polarization is determined by the refractive index of the surface, changes in the monolayer could be observed by selecting a wavelength sensitive to the material. An inflection point indicated the formation of QDs.
J.-S. Lee, S. Sugou, and Y. Masumoto: "Real-Time Observation of Ellipsometry Oscillation during GaAs Layer by Layer Growth by Metalorganic Vapor-Phase Epitaxy" Jpn. J. Appl. Phys. Part 2 38,L614 (1999).
J.-S. Lee, H.-W. Ren, S. Sugou, and Y. Masumoto: "In situ ellipsometric study of the formation process of metalorganic vapor-phase epitaxy-grown quantum dots" J. Vac. Sci. Technol. B 17, 1341 (1999).

Spectroscopy of a single QD: The photoluminescence from a single InP QD within a GaInP layer was observed by using a microscope within a spectrometer. Results showed sharp emission lines coming from excitons confined within a QD. A correlation was found between the confined excitons and the energy in the QD. The energy states in multi-exciton states were clarified.
M. Sugisaki, H.-W. Ren, S.V. Nair, K. Nishi and Y. Masumoto: "Many carrier effects in self-assembled InP quantum dots" Solid State Commun. 117, 435 (2001).

Interface/Surface Effects of a QD: It was shown that the luminescence of a InP QD embedded in a GaInP layer varied with the observation direction, due to interface effects. Exposing the interface of the QD to different level of stress caused optical anisotropy. It is therefore possible to control the electron state of a QD by altering the symmetry of the layer covering it.
M. Sugisaki, H.-W. Ren, S.V. Nair, K. Nishi, S. Sugou, T. Okuno and Y. Masumoto: "Optical anisotropy in self-assembled InP quantum dots" Phys. Rev. B 59, R5300 (1999).

Blinking of a QD: One of several hundred dots (in the above-mentioned experiment) blinked at an interval of between milliseconds and seconds due to defects near to the QD. The blinking mechanism has been clarified.
M. Sugisaki, H.-W. Ren, K. Nishi and Y. Masumoto: "Fluorescence Intermittency in Self-Assembled InP Quantum Dots" Phys. Rev. Lett. 86, 4883 (2001).

Carrier relaxation by phonons: Electron relaxation by longitudinal optical (LO) phonon and acoustic phonons has been viewed as clear structures in the luminescence spectra of site-selectively excited InP as well as InGaAs QDs under the electric field. Acoustic phonon mediated relaxation has been found to be faster than expected by the theory, which shows the minor contribution of phonon bottleneck effect in QDs.
I.V. Ignatiev, I.E. Kozin, S.V. Nair, H.-W. Ren, S. Sugou and Y. Masumoto: "Carrier relaxation dynamics in InP quantum dots studied by artificial control of nonradiative losses" Phys. Rev. B 61, 15633 (2000).
I.V. Ignatiev, I.E. Kozin, V.G. Davydov, S.V. Nair, J.-S. Lee, H.-W. Ren, S. Sugou and Y.Masumoto: "Phonon resonances in photoluminescence spectra of self-assembled quantum dots in an electric field" Phys. Rev. B 63, 075316 (2001).

Phonon renormalization in QDs: Sharp LO phonon sideband structure has been observed in the absorption spectrum of CuCl QDs using persistent hole-burning spectroscopy. The LO phonon frequency in the excited state of the QD is reduced by about 10% from the free value because of mixing with the excitons. When the mixing becomes resonant, the phonon sideband anticrosses with the excited exciton state. A theory of phonon renormalization in QDs has been developed which is in excellent agreement with the experimental results.
L. Zimin, S.V. Nair and Y. Masumoto: "LO Phonon Renormalization in Optically Excited CuCl Nanocrystals" Phys. Rev. Lett. 80, 3105 (1998).

Biexciton and triexciton states: Antibonding biexciton and triexciton states in CuCl QDs have been discovered by a time-resolved size-selective pump-and-probe technique. A clear induced absorption band has been observed at the high-energy side of the excitation photon energy. The induced absorption is assigned to a transition from the exciton ground state to an antibonding biexciton state which were theoretically predicted to exist. Under a high-density or two-color excitation condition, a triexciton state in QDs has been observed for the first time.
M. Ikezawa, Y. Masumoto, T. Takagahara and S.V. Nair: "Biexciton and Triexciton States in Quantum Dots in the Weak Confinement Regime" Phys. Rev. Lett. 79, 3522 (1997).

Perspectives

The Masumoto Quantum Dot project has been able to greatly extend our understanding of the basic properties of quantum dots, while opening up this field to a variety of new experimental and theoretical investigations. Quantum dots have thus been established as a fundamental and important phenomenon that will certainly find important applications in the future while also extending our view of basic physics and materials science.