Multi-band structure of amorphous carbon luminescence
Introduction
The room temperature photoluminescence (PL) of hydrogenated amorphous carbon (a-C:H) layers is considered mostly as a wide band without structure, with a peak position in the energy range of 2.0–2.3 eV [1], [2], [3]. Additional luminescence bands with peak energies at 2.65 eV and 2.95 eV have also been observed [4]. On samples prepared from xylene, four PL peaks have also been reported, the appearance of these was found to change with substrate temperature [5].
The inhomogeneous broadening of the visible PL spectra have gained negligible attention in models, proposed to account for the observed PL properties in this material. Experiments show that the PL band is independent of the excitation photon energy until it is outside the PL band, and both the position and half-width of the peak changes by intruding the band [1], [6], [7]. These changes can be explained by the selective suppression of the independent components of the inhomogeneous PL band closer to the excitation light, while the lower energy components remain unchanged [8].
This inhomogeneous broadening indicates the strong excitation energy dependence of the contribution of different light emitting components to the a-C:H PL. Thus, the experiments performed at large excitation photon energies present an excellent method to specify the whole range of light emitted by a-C:H layers and contribute significantly to a better understand of the luminescence process in this material.
Section snippets
Experimental details
The a-C:H samples were deposited in a capacitively coupled r.f. (2.54 MHz) plasma enhanced (PE) CVD system by decomposition of pure benzene. The base pressure (300–400 mtorr) and negative self-bias (30–200 V) was varied to prepare samples at different conditions (see Table 1). The temperature of the substrate placed on the powered electrode was held below 70 °C. For PL and infrared (IR) measurements a-C:H layers were prepared on crystalline silicon wafer and for optical study a-C:H layers of
Results
Three-dimensional emission–excitation spectra of a C:H samples prepared at low self-bias (sample A) is shown in Fig. 1 in a wide excitation wavelength region. The PL band with peak position of approximately 2.48 eV dominates the whole range of light emitted by the sample and this corresponds to PL generally observed in a-C:H layers. Change of maximum position of this band starts with decreasing excitation wavelength as the excitation becomes resonant. Another feature of this band is the
Discussion
Important results of our photoluminescence measurements are the observation of seven characteristic luminescence bands with peak positions and relative intensities, influenced by deposition parameters. These experimental results support the intrinsic origin of this multi-band character and the existence of some type of radiative centers. The dominant light emission in the green wavelength region of sample A (Fig. 1) and the shift of the main emission into the violet region in sample C (Fig. 3),
Conclusions
Structured PL spectra of a-C:H characterized by 7 overlapping emission bands were presented. A close relation between luminescence and local bonding configurations was established, based on the IR spectra. The multi-band structure of luminescence is connected to some types of radiative centers, intrinsic to a-C:H structure. This sort of emission–excitation three-dimensional spectra might be a useful tool to study the fine structure of the electron DOS.
Acknowledgments
The Hungarian Science Foundation supported this work under contract numbers OTKA-T-026073, OTKA-T025540 and NATO under contract number SfP 976913. The authors are grateful to A. Watterich for her contribution to the measurements.
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