Bixcitons and Trions in cylindrical shaped ZnO nanorods of with first and second type band
alignment and their influence on emission spectra
Bachana Beradze,
Tamar Tchelidze, Tamaz Kereselidze, Zaal Machavariani
Ivane Javakhishvili Tbilisi State University, Faculty of Exact and Natural Sciences,
Tchavtchavadze Ave 1. 0179 Tbilisi, Georgia
E-mail
Optical response of quantum-confined particles have been increasingly investigated over the past
decades due to the superior efficiency and tunability of their emission wavelength from the
ultraviolet to the near infra red regions. Shape and size effects also govern the optical response of
quantum structures in the multiexcitonic regime, where potential applications such as optical gain
are envisaged, and laser emission is expected. These effects are mainly expressed in strong
enhancement of Coulomb interaction. Two characteristic Coulomb energies are: (i) the exciton
binding energy, which provides a measure of the electron-hole ((e-h)) interaction strength, and (ii)
the biexciton binding energy, which is a measure of the strength of the exciton-exciton interaction.
While Coulomb interaction increases with, size reduction some nonradiative processes also become
more intensive. Competition between radiative and nonradiative processes crucially affects optical
gain. In nanocrystal quantum dots/rods, nonradiative carrier losses are dominated by surface
trapping and multiparticle Auger relaxation.
Auger recombination is a process in which the (e-h) recombination energy is not emitted as a photon
but is transferred instead to a third particle (an electron or a hole) that is re-excited to a higher
energy state. This state can be either inside the QS or outside of it. Auger recombination has a
relatively low efficiency in bulk semiconductors, for which significant thermal energies are required
to activate the effect. However, Auger decay is greatly enhanced in quantum-confined systems . The
point of importance is that Auger recombination is a nonradiative multiparticle effect, which
strengthen with size reduction. Since it has the same activation threshold as optical gain, it is
unavoidable in the regime of optical amplification, imposing an intrinsic limit on optical gain
lifetimes. That is why at high excitation optical gain is possible if relaxation down to the emitting
state is much faster that the Auger process time.
In this paper we investigate theoretically trions and biexcitons in ZnO quantum structures of
cylindrical shape. The calculations are carried out in the frame of configuration-interaction method.
Binding energies and radiation lifetime are calculated versus rod and barrier sizes. We investigated
both as type I as well as type II quantum structures. ZnO possesses very low laying valence band
and with the most II-VI compounds forms type II structures, where only elections can be localized
inside quantum dot/rod area. The exception is MgO. That is why we investigate ZnMgO/ZnO
systems with different Mg composition and distribution, when investigated type I quantum
structures on the base ZnO. Auger recombination rates are also calculated and the optimal rod
geometry at which radiative recombination form trion and biexciton states dominate over Auger
recombination is revealed.
PS2 29
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