Ultraviolet photoluminescence and its relation to atomic bonding properties of hydrogenated amorphous carbon

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Abstract

We have observed photoluminescence in hydrogenated amorphous carbon (a-C:H) samples at substantially higher energy than the visible band. The emitted light is in the ultraviolet (UV) region, the spectrum consists of three bands with peak positions of ∼4.46 eV, ∼4.01 eV and ∼3.63 eV, depending slightly on sample properties. The UV photoluminescence (PL) has been observed in samples of different band gaps which show either high or low PL efficiency in the visible region. The excitation spectrum of UV luminescence exhibits high efficiency in the photon energy range of 5.6–6.2 eV and a strong decrease at excitations below this energy range. The experimental fact, that, the peak energies of UV bands exceed the optical gap energy of the studied samples, supports the light emission via radiative recombination of localised geminate electron–hole pairs. Strong localisation is expected for the excitation of π–π* transitions in conjugated double bonded fragments of small sizes. An infrared study of the UV light emitting a-C:H films confirms the presence of conjugated double bonds with aromatic and olefinic local configurations as well, however, the unambiguous relation between UV luminescence and small aromatic structures cannot be established. It is more probable that the olefinic fragments with chain lengths of 2–4 give π electronic levels through which UV light emission takes place. The role of twofold co-ordinated carbon sites cannot be excluded yet.

Introduction

Photoluminescence offers a sensitive method for the study of electronic levels near mobility edges and deep in the gap. Carbon atoms, in sp1, sp2 and sp3 hybridised states and different amounts of hydrogen are present in a-C:H layers. The density of states (DOS) shows a clear partition into the σ band further away from the Fermi level, and into the π band close to it. The π and π* bands control the band gap behaviour [1], [2]. The π–π* splitting and thus the size of the gap depends on the nature of π-bonded clusters as well as on the properties of the remaining matrix, both of which are in close correlation with mass density and hydrogen content. The observed small band gaps are not related to the formation of separated more or less extended aromatic ring groupings, they are, rather, a consequence of embedding π-bonded atomic arrangements in a strained bonding environment of a rigid cross-linked network, in which ‘mixed’ bonds dominate [1]. The a-C:H films of low density, which contain a small atomic percent of threefold co-ordinated carbon sites and large amounts of hydrogen, are wide band gap materials. These layers exhibit intense PL in the visible spectral region.

Current interest in developing new electro-luminescent materials stimulate researchers to understand PL of a-C:H and to produce thin layers of high luminescence efficiency. There is a fairly good agreement concerning the visible PL in a-C:H layers being related to π bonding states of sp2 co-ordinated phase [3], [4], [5], [6], [7], and the localisation or spatial confinement of geminate electron–hole pairs, which recombine radiatively, is more or less also accepted [4], [7], [8], [9], [10], [11], [12]. Recombination models in a-C:H suppose different mechanisms both for the radiative and for the non-radiative transitions. The one electron band tail model [12] assumes tail-to-tail radiative recombination, similarly to the PL mechanism of a-Si:H, and the wider distribution of band tails, as well as the stronger localisation of electron–hole pairs, compared to amorphous silicon, could then explain the peculiarities of a-C:H luminescence. Tunnelling the excited carriers to non-radiative centres (defect sites) was considered as the main mechanism, which quenches PL efficiency. It was also shown that the localisation radius could not be considered as a constant value because it depends on optical gap and hydrogen content as well [13]. The other mechanism, proposed very recently [14], suggests the radiative recombination of trapped exciton-like pairs and dissociation of excitons via electron tunnelling to the distorted π bonded sites, which are assumed to be non-radiative centres, is considered as dominant non-radiative transition. From the quench of the PL intensity with a decrease of optical gap of polymeric a-C:H, the Bohr radius of ∼6 Å was determined.

Recombination models for a-C:H luminescence, described briefly before, consider a single broad luminescence band in the visible region, and a crucial point both of band tail and exciton-like recombination is the quench of the PL intensity with a decreasing optical gap of a-C:H. In this report we present experimental evidence for the complex structure of a-C:H luminescence and show that the emitted light covers a very broad photon energy range with emission peaks up to the UV region. This high-energy luminescence can be explained with radiative recombination of localised geminate electron–hole pairs; therefore it supports the exciton-like recombination model. Our infrared study performed on light emitting samples makes it possible to relate UV luminescence and bonding properties of the a-C:H layers.

Section snippets

Experimental

The a-C:H layers were prepared using a 2.4 MHz capacitively coupled r.f. chemical vapour deposition (PECVD) with pure methane and benzene as source gases. Substrate material was placed on the powered electrode and the negative self-bias, developed on it, was controlled by the r.f. power. The bombardment of the layer depends on mean ion energy and therefore the structure of the layers is influenced largely by the self-bias and by the pressure. The self-bias from −10 to −200 V, and the pressure

Results

Fig. 1 shows the high-energy region of the luminescence spectra measured at an excitation of 5.63 eV on samples prepared from methane. Preparation conditions and optical gap values of samples studied in PL measurements are summarised in Table 1. Three well-defined bands have been observed in the UV region (<350 nm) with similar spectral distribution for each sample. However, the highest energy peak (labelled by c in Fig. 1) is weak in sample B compared to the other samples. Peak positions and

Discussion

The peak energies of the UV luminescence bands exceed considerably the E04 gap values for both sample series. Recombination via emission of these photon energies can be expected if the electron–hole pairs following photo-generation are localised, mostly on the place of generation, either because of low hopping mobility in localised π bonding states, or due to a deep potential well, which separates carriers from the extended σ states. Thus, the UV light emission can be explained by radiative

Conclusion

We have analysed UV photoluminescence parallel with IR spectra of a-C:H sample series prepared from methane and benzene at a different self-bias and pressure, that control mean ion energy and therefore sample structure, to investigate the relation between UV luminescence and bonding properties. Peak energies of the three UV bands and their excitation spectra strongly suggest that π electronic levels can be localised at much larger energies than the optical gap. The UV luminescence is explained

Acknowledgements

This work was supported by the Hungarian Science Foundation under contract, numbers OTKA-T-026073, OTKA-T025540, AKA-98-30 2,2 and NATO SfP 976913.

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