Elsevier

Diamond and Related Materials

Volume 14, Issues 3–7, March–July 2005, Pages 1108-1111
Diamond and Related Materials

Correlation of optical absorption and density of paramagnetic centers in a-C:H films

https://doi.org/10.1016/j.diamond.2004.10.026Get rights and content

Abstract

We investigated a-C:H films deposited by dual electron cyclotron resonance radio frequency PECVD by optical absorption and electron spin resonance (ESR) spectroscopy. We show that the variation of the density of paramagnetic centers obtained from electron spin resonance (ESR) spectroscopy can be directly correlated to the low energy part of the optical absorption spectrum revealed by photothermal deflection spectroscopy (PDS). A simple analysis of the low energy part of the optical absorption spectrum is proposed which allows an assessment of the density of states at the Fermi level. We interpret this result in terms of intraband transitions of localized defects situated far from the π−π* band tails which dominate the higher energy part of the optical absorption spectrum.

Introduction

Hydrogenated amorphous carbon (a-C:H) films are known to contain a high number of midgap defects with typical concentrations—usually determined by ESR—in the range of 1018−1021 cm−3 [1]. The spins are generally attributed to sp2 coordinated C atoms but the exact microscopic nature of these defects remains unclear. While in amorphous silicon (a-Si:H) it is now well established that by far most of the detected spins are due to dangling bonds (DB) which can be correlated with the low energy absorption tail determined for instance by PDS or constant photocurrent methods (CPM) the situation is more complex in a-C:H. There are several reasons for this : first the origin of the spins detected by ESR is hidden by the importance of the exchange interaction due to the high spin concentration [2]. Some results obtained using high frequency ESR suggest that there are several distinct paramagnetic centers [2], [3] while recent studies on powder samples only exhibit single resonance lines [4]. Some authors suggested that apart from DB other configurations such as odd numbered rings may generate midgap states [5], [6], [7]. Finally, it is difficult to correlate optical data with ESR measurements since the absorption spectra of a-C:H films usually do not show a clear linear Urbach tail that could a priori help to identify the contribution of deep defects to absorption [8].

It is the purpose of this paper to show that in the low energy domain the absorption spectrum can be related to a midgap density of states that correlates with the spin density provided that contributions from transitions involving the π−π* tail states can be neglected. In order to illustrate this we investigated a-C:H films with various spin densities obtained by annealing polymer like films.

Section snippets

Experimental

The a-C:H films were deposited by plasma enhanced chemical vapour deposition of pure methane at low pressure (0.35 Pa) in a dual electron cyclotron resonance radiofrequency glow discharge system. The plasma was excited by feeding microwave at a constant power of 150 W. More details about this technique can be found in refs. [9] and [10]. The samples were grown at low RF bias (−30 V). The initial film thicknesses (before annealing) were about 1.1 μm. Sample annealing was carried out in

ESR

Fig. 1 exhibits the ESR spectra of the PLC series as a function of the anneal temperature. Before annealing the spin concentration Ns is ∼1×1019 cm−3 (Table 1). NS remains constant up to T=350 °C and then strongly increases up to ∼1.2×1020 cm−3 for T=600 °C. The variation of the ESR linewidth is summed up in Table 1. All spectra exhibit a Lorentzian profile. It has been shown in the past that polymeric samples with high hydrogen content often exhibit a broad linewidth due to unresolved

A simple model for analyzing the low energy sector

The usual expression for the absorption coefficient readsα(ω)=KωEFEF+ωG1(Eω)G2(E)dEwhere K is proportional to the squared dipolar moment matrix elements. It describes transitions between states of the bands 1 and 2 associated with densities of states G1 and G2. The case of intraband transitions corresponds to the situation G1=G2. It shows clearly that at “low” energy, the transitions take place between states close to the Fermi energy. As this expression relies on the approximation that

Conclusion

We developed a simple model which can account for the shape of the absorption spectra in a-C:H samples in the low energy range where intraband transitions related to localized midgap defects dominate. The model provides values of the density of states G(EF) near the Fermi level in good agreement with ESR data. It is expected that our approach is valid as long as G(EF) is not much affected by the band tails and the overlapping of the wavefunctions associated to the defects is high enough to

Acknowledgements

The authors are grateful to Dr. M. Zarrabian for sample preparation.

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