Evidence for excitonic behavior of photoluminescence in polymer-like a-C:H films

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Abstract

To understand the dynamics of energy transfer and randomization of photoluminescence polarization in hydrogen-rich polymer-like amorphous carbon a-C:H films, time-resolved investigations of intensity and anisotropy decays have been performed recently. The intensity decay rates increase exponentially as a function of emission energy with a behavior very similar to that observed in wide band-gap C-rich a-Si1–xCx:H. In addition, in polymer-like carbon, the observation of a plateau of PL anisotropy in the 100–1000 ps range, is taken as strong evidence for the existence of a finite density of excitonic species in radiative recombination phenomena; it does not fit the phonon-assisted depolarization models proposed earlier. Polarization anisotropy decays and steady-state values are consistently interpreted using a dipole–dipole non-radiative energy transfer mechanism (Förster mechanism) with a characteristic depolarization time of 50 ps rather independent of the emission energy. The latter value is likely to be related to the density of radiative centers distribution estimated independently in the constant exciton radius approximation, rather than the result of hopping in an exponential distribution of tail states.

Introduction

As a consequence of the allotropic properties of carbon, both nanostructure and optoelectronic properties of carbon-based films are strongly dependent on the deposition conditions. Hydrogen incorporation introduces additional degrees of freedom into the atomic structure, leading to low density carbon films. The π-bonding favors a segregation between sp2 and sp3 sites, so that sp2 sites can pair up to form chain or ring configurations. This medium range organization related to conjugation of π-bonds confers a high versatility to optical and transport properties of carbon thin-films [1].

Hydrogenated amorphous carbon (a-C:H) films are also potentially interesting for the deposition of large area electronic devices at low temperatures, compatible with polymer substrates. However, so far, the development of light emitting diodes [2], heterojunction photovoltaic devices [3], thin-film transistors [4] and field emission devices [5] remains far from expectations. The achievement of a good quality electronic material cannot be separated from a detailed understanding of the relationship between growth mechanisms, film nanostructure and the resulting electronic density of states (DOS).

In amorphous carbon films containing both sp2- and sp3-hybridized C atoms, the weaker π-bonding and the orientation dependence of the π interaction cause the localization of the entire π- and π*-bands within the σσ*-gap [6]. Strong site-to-site potential fluctuations result from the large difference between π- and σ-bond energies, which in turn lead to low carrier hopping mobilities [4], [7] and, in some cases, strong luminescence efficiency.

In polymer-like a-C:H films, characterized by high H content, low C atom density, low dielectric constant and wide band-gap, the strong confinement of photo-generated electron-hole pairs inside π-bonded ‘grains’ is supported experimentally by the intense PL in the visible spectrum at the ambient temperature. The lower PL efficiency observed for denser a-C:H films has been tentatively explained by a decrease of the pair confinement due to stress-induced σπ orbital mixing [8].

Further indications of strong confinement have been reported such as: (i) the lack of thermal quenching of PL intensity [9]; (ii) the anisotropy (polarization memory) of photoluminescence obtained using linearly polarized excitation light [10], [11], [12]; (iii) the fast (subnanosecond) decay of PL intensity [12], [13]; and (iv) the constant decay time as a function of generation rate [13].

As far as intensity decay rates are concerned, they are found to increase exponentially as a function of emission energy over a broad spectral range. Interestingly, this behavior is found to be very similar for different types of carbon-based thin-films with wide band-gap (E04≈4.0 eV) such as ECR-grown C-rich a-Si1–xCx:H [14] and dual-plasma (r.f.-assisted microwave plasma) a-C:H [12]. This common behavior shown in Fig. 1 might indicate an intrinsic property of π-bonded materials but it has also been found in silicon nitride insulating films and multilayers [15]. It has been suggested that non-radiative (NR) lifetimes govern the experimental decay times, through the dissociation of excitonic electron-hole pairs mediated by the electron tunneling towards a vacant site at the same energy [8].

In a recent report [12], anisotropy decays and steady-state values have been consistently interpreted using a dipole–dipole non-radiative energy transfer mechanism (Förster mechanism [16], [17] with a characteristic depolarization time of 50 ps). For emission energies Eem between 1.8 and 3.5 eV, anisotropy decreases within 100 ps and reaches a plateau within 1 ns. This observation has been explained by the transfer of electronic excitation towards a finite density of luminescent centers but it is not consistent with previous models based on phonon-assisted depolarization [10], [18] which should occur on a much shorter (sub-picosecond) time scale. However, in this paper, we will first consider the role of thermalization of photoexcitation in disordered insulators in the frame of the ‘trapped exciton’ model proposed by Kivelson and Gelatt [19].

In the second part, we will analyze in more detail the photoluminescence intensity and anisotropy decays in the picosecond–nanosecond range reported recently [12] for polymer-like a-C:H, excited using a linearly polarized UV photon beam. The polarization spectroscopy results will be discussed in terms of exciton-like radiative recombination and the characteristic depolarization time constant of 50 ps (nearly constant for emission energies in the range 1.5–3.5 eV) will be related to the density of radiative centers, estimated by two independent methods: (a) the Förster intensity decay for primarily excited chromophores, and (b) the spectral density method using a constant exciton radius approximation.

Section snippets

Energy relaxation

In this section, we consider thermalization mechanisms which occur before and during radiative recombination. In particular, we address the dynamics of the hopping motion of either a single charged carrier or a neutral exciton through a band of localized states and we discuss their respective relaxation rates.

A number of photoluminescence investigations of polymer-like carbon films, including excitation spectra and polarization memory properties, indicate that radiative recombination arises

Polarization spectroscopy

Polarization spectroscopy is a powerful experimental tool for investigating structural and kinetic properties of complex molecular systems. In any system, whether anisotropic or isotropic, the absorption of photons from a polarized beam confers a degree of anisotropy, which persists until it is lost through molecular motion or energy randomization. Typical time scales depend on the randomization process itself, ranging from 10−14–10−12 s for carrier thermalization (phonon exchange), 10−11–10−8

Energy distribution of radiative centers

Polarization anisotropy decays and steady-state values have been consistently interpreted using a dipole–dipole non-radiative energy transfer mechanism (Förster mechanism) leading to characteristic depolarization times (50 ps) rather independent of the emission energy. In order to understand the latter values, we estimate the density distribution of the radiative centers, using two independent methods: (a) the Förster intensity decay for primarily excited chromophores; and (b) the spectral

Conclusion

PL characteristics of polymer-like carbon films indicate that excitonic species are efficiently photo-generated using visible and UV excitation energies. This polarization spectroscopy study indicates that excitonic luminescent centers have a broad energy distribution, as obtained by two independent methods, which does not follow the overall distribution of π and π* states, as measured by optical absorption techniques [25]. However, the latter sites are available for exciton dissociation by

Acknowledgements

The authors wish to thank T. Heitz, J.E. Bourée (Ecole Polytechnique, Palaiseau), J.P. Conde and A. Fedorov (IST, Lisbon) for stimulating discussions and participation in the experimental characterizations of carbon films.

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