Surface excitons on a ZnO (000-1) thin film

Abstract

Elementary excitations at the polar (000-1) surface of a 20 nm pseudomorphically grown ZnO thin film are examined by steady state and time-resolved photoluminescence spectroscopy at low temperature. We control the density of emission centers through the deposition of prototypical organic molecules with a carboxylic acid anchor group by the Langmuir-Blodgett technique. Knowledge of the precise film thickness, defect concentrations and number density of deposited molecules leads us to associate the surface exciton emission to defect-related localization centers that are generated through a photochemical process.

Фотоиндуцированные эффекты в спектрах люминесценции ZnO

“…В низкотемпературных спектрах люминесценции монокристаллов CdS, CdSe и ZnO на-блюдалась полоса излучения, приписанная поверхност-ному экситону. В спектрах фотолюминесценции (ФЛ) тонких пленок и других наноструктур ZnO с разви-той поверхностью при низких температурах полоса излучения поверхностного экситона (SX) может быть доминирующей структурой [4]. При интерпретации по-лосы SX в основном принимается модель, предложенная в [3,4] -локализация экситонов в неоднородном потен-циале, создаваемом акцептороподобными поверхностны-ми центрами.…”

Nanoscale transport of surface excitons at the interface between ZnO and a molecular monolayer

“…Using an analytical solution for a Gaussian initial distribution n(r, t=0)~e -r²/σ² with σ the half width at 1/e [14], and assuming that η coll (r) ~ n(r, t=0), we arrive at: an NSOM scan line of 1 µm in 50 nm steps. The PL spectra display two prominent bands identified as the DX band at 3.361-3.363 eV and the SX band at 3.365-3.369 eV [12]. The spectra reveal a spatial inhomogeneity in the DX signal amplitude and energy.…”

“…SX PL has been detected from nanostructures at low temperature [9][10][11], and from ZnO bulk single crystals or epitaxial layers [8,12]. Recently, we have observed a distinct SX PL band from a ZnO thin film covered with a molecular monolayer [12]. Based on time-resolved SX PL measurements, a model involving a population exchange between less localized and strongly localized states in a heterogeneous potential has been proposed [9].…”

“…The emission linewidth of ~4 meV observed even at T<10 K [8,12] could 3 result from homogeneous broadening by dephasing processes or an inhomogeneous distribution of emission energies in combination with spectral diffusion processes. The latter may involve diffusive SX motions between a multitude of sites.…”

“…Different types of excitons have been distinguished in the prototype wide gap semiconductor ZnO: free excitons (FX) with a binding energy of ~60 meV, excitons bound to different donor and acceptor atoms (DX), and surface excitons (SX) [5][6][7][8][9][10][11][12]. SX PL has been detected from nanostructures at low temperature [9][10][11], and from ZnO bulk single crystals or epitaxial layers [8,12].…”