Spectroscopy of Single Semiconductor Quantum Dots

Gerhard Abstreiter

Walter Schottky Institut, Technische Universität München, D-85748 Garching

New phenomena are observed when charge carriers are confined in three dimensional potential wells with length scales in the range of 5 to 50 nm. The conventional optical spectra, probing many of such quantum dots at the same time, are, however broadened inhomogeneous due to variations in size, shape, and composition. Sharp spectral lines in absorption and emission are observed when individual quantum dots are studied.

First photoluminescence (PL) experiments on embedded single quantum dots have been performed using local intermixing [1] and probing thickness variations of narrow GaAs quantum wells [2, 3, 4]. These pioneering works revealed already all the interesting features of single dot spectroscopy like narrow line-width due to the exclusion of inhomogeneous broadening as well as a defined population with 2 electron hole pairs (biexcitons) and the occupation of higher dot levels, demonstrating the discrete density of electronic states in such quantum dots. These new observations were possible due to the development of optical spectroscopy with high spatial resolution as there are confocal microscopy [1-4], near-field optics [5], shadow-mask techniques [6] and spatial separation of dots by e-beam lithography and etching [7]. New epitaxial growth techniques like cleaved edge overgrowth and self-assembly led to the fabrication of arbitrary types of quantum dots in semiconductors and a controlled coupling of such dots. In special electrically tunable semiconductor structures it is also possible to study the influence of individual charging of a dot. The applications of magnetic field yield interesting spin dynamics in single dots. A large number of publications appeared in the past 6 years, dealing with various aspects of optical properties of individual quantum dots of different types [8].

In this contribution I will present an overview of the exciting development of the area of single dot spectroscopy during the past decade.

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[2] K. Brunner, G. Abstreiter, G. Böhm, G. Tränkle, and G. Weimann, Appl. Phys. Letters 64, 3320 (1994)

[3] A. Zrenner, L. V. Butov, M. Hagn, G. Abstreiter, G. Böhm, and G. Weimann, Phys. Rev. Letters, 72, 3382 (1994)

[4] K. Brunner, G. Abstreiter, G. Böhm, G. Tränkle, and G. Weimann, Phys. Rev. Letters 73, 1183 (1994)

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[8] For a recent review with references therein see: A. Zrenner, J. Chem Phys. 112, 7790 (2000)