Exciton–phonon interaction in epitaxial CdSe/ZnSe nanostructures

Abstract

The low-temperature optical properties of submonolayer CdSe/ZnSe nanostructures have been investigated with nominal layer thickness of 0.15 ML and 0.58 ML CdSe. In photoluminescence excitation spectroscopy (PLE) we observe more than 10 equidistant peaks separated by the ZnSe LO-phonon energy even at energies high above the ZnSe-band edge. The peak heights and line shapes are extremely sensitive to the monolayer coverage, have strongly different intensities in emission and PLE and deviate from the Poissonian intensity distribution expected within the concept of a constant, material specific Huang-Rhys parameter S. A bound polaron model is proposed to explain the optical properties of quantum structures for which the approach of a pure electronic quantum confinement is not appropriate. Intrinsic coherence of joint exciton-phonon modes is very promising, e.g. for application in quantum information processing or coherent population transfer.1 Introduction Epitaxial techniques allow to grow high-quality semiconductors with extremely small structural elements. The concept of quantum confinement is a very effective tool for the description of self-assembled CdSe quantum dots grown by means of deposition of a few monolayers (ML) [1] or even submonolayers of guest material [2]. Further decrease of epitaxial coverage down to values of the order of 0.1 ML results in nanoobjects which comprise only a small number of atoms rather than 3D-islands. In such nanostructures coherent coupling with lattice vibrations can have even more significant effects on the energy states and coherent dynamics of localized eh-pairs compared to semiconductor quantum dots where excitonic polarons [3][4][5], non-adiabatic exciton-phonon interaction [6], enhanced phononassisted absorption [7], etc. are currently discussed.In this contribution we present experimental results on the optical properties of CdSe/ZnSe heterostructures with nominal thickness of CdSe equal to 0.15 ML and 0.58 ML. To explain the optical properties of such nanoinclusions, we propose a bound polaron concept [8,9] which goes beyond the usual quantum-dot models. We consider the bound-polaron picture as basic requirement for the understanding of coherent properties while relaxation processes would be described here as a polaron decay.

Preparation and photoluminescence of water-dispersible ZnSe nanocrystals

“…Currently, we can prepare nanocrystals in vapor [1][2][3], solid [4][5][6] and liquid phases [7][8][9]. The advantage of the liquid-phase preparation is the easiness of size and shape control because mass transfer between nanocrystal and surrounding solvent continues intensively and homogeneously during preparation.…”

Luminescence of epitaxial sub-monolayer CdSe nanostructures in ZnSe

“…The 0.58 ML coverage is sufficient for the formation of quantum dots [5,12]. The occurrence of tightly localized dot-like states is confirmed by the low-energy shift and the broadening of the PL spectrum (and agrees with the appearance of slower, nanosecond components in the PL decay kinetics of the 0.58 ML samples [11]). …”

“…This process involves the exciton transition from heavy-to light-hole state. The effect of the host LO-phonons is more obvious at low excitation level and can be interpreted in terms of a ''polaron in a potential'' model [9][10][11]. LO-phonon-assisted luminescence enables qualitative estimation of the exciton localization degree.…”

“…An advanced model of the exciton-LO-phonon interaction was proposed based on the polaron approach [9,10]. As was shown in [10,11], the concept of bound polarons is suitable for the description of characteristic phononic features in the absorption and excitation (PLE) spectra of sub-ML epitaxial polar semiconductor nanostructures. This report is concentrated on the exciton-LO-phonon coupling in sub-ML CdSe/ZnSe heterostructures with relaxed quantum confinement and on the multi-exciton effects monitored by means of the photoluminescence (PL) technique.…”

Relaxation dynamics of bimodally distributed CdSe quantum dots

“…22 On the other hand electron-phonon interactions have been found to achieve much larger energy transfer in semiconductor QDs. [23][24][25] In contrast resonant and nonresonant tunnel processes have been suggested to increase the carrier mobility in QD structures. [26][27][28][29][30][31] The transfer processes in semiconductor CdSe QDs are usually characterized by their direction: from initial state at higher energies to final states at lower energies.…”