Excitons, biexcitons, and phonons in ultrathin CdSe/ZnSe quantum structures

Abstract

The optical properties of CdSe nanostructures grown by migration-enhanced epitaxy of CdSe on ZnSe are studied by time-, energy-, and temperature-dependent photoluminescence and excitation spectroscopy, as well as by polarization-dependent four-wave mixing and two-photon absorption experiments. The nanostructures consist of a coherently strained Zn 1Ϫx Cd x Se/ZnSe quantum well with embedded islands of higher Cd content with sizes of a few nanometer due to strain-induced CdSe accumulation. The local increase in CdSe concentration results in a strong localization of the excitonic wave function, in an increase in radiative lifetime, and a decrease of the dephasing rate. Local LO-phonon modes caused by the strong modulation of the Cd concentration profile are found in phonon-assisted relaxation processes. Confined biexcitons with large binding energies between 20 and 24 meV are observed, indicating the important role of biexcitons even at room temperature. ͓S0163-1829͑99͒16935-2͔

Lifetime and polarization of the radiative decay of excitons, biexcitons, and trions in CdSe nanocrystal quantum dots

“…The ratio of the two lifetimes approaches 2 for the smallest dot and is close to 1 for R=19.2Å. In self-assembled CdSe/ZnSe dots the ratio τ(X B ):τ(XX B B B ) ~ 2:1 was found both in ensemble [24] and in single dot [5] measurements (for dots with exciton emission between 2.25 and 2.38 eV). However in the latter system, the decay of both exciton and biexciton occurs on a sub-ns time scale at low temperature.…”

Exciton dephasing and biexciton binding in CdSe/ZnSe islands

“…27 The beats are rather weak in the ͑↑→͒ configuration due to the small contribution of the unbound biexcitons. The beating period corresponds to a biexciton binding energy of E XX ϭ22.4Ϯ0.3 meV, which is by far larger than the bulk CdSe biexciton binding energy of about 4 meV, 18 indicating a strong three-dimensional quantum confinement in the islands. This is in agreement with experiments on similar structures showing biexciton binding energies of about 20 meV ͑Refs.…”

“…[1][2][3] The optical properties of such islands have been mainly studied by incoherent methods. [4][5][6][7] Driven by the search for blue-green-emitting semiconductor materials, the fabrication [8][9][10][11][12][13] and optical characterization [14][15][16][17][18] of self-assembled II-VI islands have recently been the subject of intensive research. Only little is known about the coherent properties of these structures, giving information about exciton scattering and biexciton formation.…”

Передача электронного возбуждения из органической матрицы в нанокристаллы CdS, полученные методом Ленгмюра--Блоджетт

“…Перенос электронного возбуждения из матрицы в НК активно исследовался для НК, находящих-ся в полупроводниковой матрице [12][13][14][15], а так-же для НК в проводящих органических соединени-ях poly [2-methoxy,5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) [5,9,16]. В последнем случае наблюда-ли как перенос электрона или дырки, так и экситона.…”