Wide-gap a-C:H prepared by dc glow discharge of CH4: photoluminescence and electroluminescence in the visible region

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Abstract

We report on visible electroluminescence from metal–semiconductor–insulator–semiconductor–metal heterostructures employing a wide-gap insulating amorphous carbon (a-C:H) film. The a-C:H layers were prepared by dc glow discharge decomposition of methane (CH4). The electroluminescence is due to combined effects of surface and volume phenomena, namely to the radiative decay of surface plasmons excited at the positively biased electrode, and the radiative recombination of electron–hole pairs across the forbidden gap of the active layer. The surface- and bulk-related emissions are peaked at ∼750 and ∼600 nm, respectively. Thermal annealing of the samples, up to ∼200 °C, increases significantly the total electroluminescence efficiency, by one order of magnitude, due to an improvement of volume phenomenon in the emission process.

Introduction

Hydrogenated amorphous carbon (a-C:H) thin films with a wide range of structure and physical properties can be prepared by different deposition techniques and parameters [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. The a-C:H thin films with a wide optical band gap exhibit room temperature photoluminescence (PL) in the visible range [1], [3], [4], [5], [6], [7], [15], [17] and therefore are very promising for electroluminescence purposes [4], [6], [15], [17].

In recent studies [18], [19], we have observed an electroluminescence phenomenon from metal–insulator–metal structures constituted of a semiconducting a-C:H film with an optical band gap located in the infrared region. In these structures, the light emission is simply due to a surface phenomenon, namely to the radiative decay of surface plasmons excited by hot carriers on the positively biased electrode, and is not related to radiative recombination of electron–hole pairs in the forbidden gap of the material. We investigated wide-gap a-C:H thin films to observe an electroluminescence (EL) emission in the visible region.

Section snippets

Film synthesis

The a-C:H films were prepared in a flow-through dc glow discharge apparatus via the decomposition of methane (CH4) at a frequency of 20 kHz. The substrate temperature was kept around the room temperature. The film’s structure and composition have been thoroughly studied [20]. Briefly, the a-C:H layers are composed of small graphitic clusters (∼10 nm in size) embedded in an amorphous carbon matrix. The content of graphitic clusters can vary from 0 to ∼40%, depending on reactor pressure (pd) and

Photoluminescence spectra

Fig. 2 displays the room temperature PL spectra for both semiconducting (Eg=0.8 eV) and insulating (Eg=2.2 eV) films. For the semiconductor, no PL signal arises from the optical set-up, whereas the insulator exhibits an intense and broad PL emission covering almost the whole visible range (1.4–2.4 eV), with a maximum in the 1.8–2.0 eV range. This result is similar to other PL measurements on a-C:H [5], [6], [7]. Therefore, we can attribute the PL emission from the wide-gap (Eg=2.2 eV) a-C:H film to

Discussion

In order to understand the emission process in the MSISM heterostructure, our electroluminescence data were compared to other measurements on metal–semiconductor–metal structures using the semiconductor film alone [15]. In these ITO–semiconductor–Al structures, the light emission is due to an electrode effect, namely, to the radiative decay of surface plasmons excited by hot electrons at the positive electrode. The emission threshold in these structures, i.e. the minimum current intensity

Conclusions

Wide-gap (Eg=2.2 eV) a-C:H films prepared by dc glow discharge decomposition of methane (CH4) were investigated in metal–semiconductor–insulator–semiconductor–metal (MSISM) heterostructures. The EL emission detected in trough the MSISM structure is due to combined effects of radiative decay of surface plasmons excited on the positively biased electrode, and radiative recombination of electron–hole pairs generated in the insulator matrix.

Annealing of the samples at 200 °C increases the total EL

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