Elsevier

Applied Surface Science

Volume 464, 15 January 2019, Pages 562-566
Applied Surface Science

Full Length Article
UV laser annealing of Diamond-Like Carbon layers obtained by Pulsed Laser Deposition for optical and photovoltaic applications

https://doi.org/10.1016/j.apsusc.2018.09.085Get rights and content

Highlights

  • Production of a pure carbon-based alternative to indium tin oxide (ITO).

  • Hydrogen free DLC layers with a high level of sp3 hybridization are obtained by PLD.

  • UV-laser surface annealing of DLC leads conductive layers.

  • Transparency and conductivity performances reaches ITO values.

Abstract

One of the biggest challenge in optoelectronic devices is the necessity to provide a viable and reliable alternative to Transparent Conducting Oxide (TCO) and especially to Indium Tin Oxide (ITO). Graphene is a widely studied material and one of the best alternative to be used as conductive and transparent electrodes. It is well known that the difficulty to transfer graphene on another substrate is a serious limitation for its use on large scale devices. In this work, we explore Diamond-like Carbon (DLC) thin films prepared by Pulsed Laser Deposition (PLD) to be used as substrate for graphene-like layers. DLC thin films are excellent candidates due to their visible-range transparency being also a very good electrical insulator. Transmission measurements show the UV opaque character of the DLC layers, independently to the experimental parameters used to produce them. Thus, top-hat UV laser surface annealing can strongly modify the DLC thin film structure in order to bring conductivity to the first atomic layers. Raman spectroscopy and X-ray photoemission spectroscopy permit to confirm the graphitic character of the DLC surface. Optimizing PLD as well as laser annealing parameters is explored in detail in order to obtain comparable performances (conductivity and transparency) to ITO typical properties. Moreover, using a full-based laser process offers a complete compatibility with all technical steps of the microelectronic domain.

Introduction

The most common transparent conductive material used nowadays in optoelectronic and photovoltaic devices is Indium Tin Oxide (ITO). This one owns very high transparency in the visible range and very high conductivity over large areas [1], [2]. In the forthcoming years, we will face an increasing problem due to the indium rarefaction. Moreover, the impossibility of correctly recycling ITO is doomed to be a problem for the optoelectronic and photovoltaic components. A suitable alternative to this metallic oxide is needed. The hereby proposed solution is fully compatible with all the materials and processes used in micro and optoelectronics.

The Diamond-Like Carbon (DLC) is a material introduced nowadays in photovoltaic modules as encapsulation and protective anti-reflexion coating [3], [4]. DLC is an amorphous form of carbon used as a low-cost substitute to diamond. This material is a very good electrical insulator due to kinship with diamond [5], [6], [7]. DLC is constitute by a mixture of sp2 and sp3 bounded carbon atoms [8]. Sp2 is the stable binding corresponding to graphitic material whereas the sp3 is the metastable binding related to the adamant character which is formed under special conditions of temperature and pressure. The amount of each carbon hybridisation and correlated properties are directly related to the process used to elaborate DLC [9], [10], [11]. Another important property that DLC share with diamond is a high visible and infrared range transparency. Combined to its insulating property drives to use DLC as an insulating transparent substrate. We use Pulsed Laser Deposition (PLD) of carbon to grow DLC thin films. Those films possess advantageous properties directly depending on the deposition process parameters [8], [12], [13], [14]. Moreover, PLD allows while working under high vacuum, to obtain high purity, oxygen and hydrogen-free DLC layers. As we previously demonstrated, the sp2 and sp3 amount ratio is highly depending on the laser wavelength and the energy density used to ablate the carbon. Those parameters condition the optical and electrical DLC properties [15].

Graphene is another form of carbon. Its two-dimensional structure only constituted by a single layer of sp2 bounded carbon atoms, is organized in a honeycomb pattern. Well studied in the last years, graphene has a very high electron mobility assuming an excellent electrical conductivity [16], [17]. Moreover, graphene has a very low optical absorption approaching 2.3% per layer and can also be considered as a transparent electrical conductor. Despite many elaboration methods like chemical vapour deposition, graphite exfoliation or aerosol, it is nowadays very hard to transfer a graphene layer or large graphenic areas for electronic applications. Combining the properties of DLC and graphene could be an innovative solution to obtain a transparent conductive material directly set on an insulated transparent substrate.

Our original approach to obtain a pure carbon transparent conductive material is based on a two steps process. In first, we use PLD to grow a high purity DLC thin film. In a second step, the layer is exposed to an UV laser surface treatment. Indeed, compared to crystalline diamond, DLC present a very high UV opacity. Therefore, the use of deep UV excimer laser annealing at very low energy drives to modify only the DLC top structure by interacting only with the very first atomic layers of the film. In addition, laser annealing is only possible with PLD produced DLC due to the absence of hydrogen in the layers. Surface treatments will force carbon atoms to be reorganized in pure sp2 forming top graphitic layers. Those few surface layers will bring the requested conductivity. This full process, only based on laser technology, offers therefore a complete compatibility with standard microelectronic and photovoltaic processes.

Section snippets

Pulsed laser deposition of DLC

We use a standard PLD setup to perform our DLC depositions. A laser is focused with a 45° incidence angle on a high purity graphite target (99.999%) placed into a high vacuum chamber (residual pressure less than 10−8 mbar). When the laser hits the target, the matter is ablated perpendicularly to its surface and collected on a substrate set parallel in front of it. The target-substrate distance is kept constant and set to 5 cm in order to obtain a homogeneous sample coverage over 1 × 1 cm2. The

DLC as deposited

We have deposited 20 nm of DLC at a fluence of 5 J/cm2. A previous study [15] has shown that for these parameters, DLC layers contains 60.9% sp3 bounded atoms and 39.1% sp2, homogeneously distributed in the entire layer. Transmittance measurements reveal a good transparency in the visible range (approaching 80% at 800 nm for a 20 nm thick layer). This value is sinking down for shorter wavelengths (reduced to 33% at 248 nm). Measurements are in accordance with the light brown colour of the

Discussion

Electrical measurements help to reduce the optimal parameters range, in the way that for a 0.1 J/cm2 annealing, conductivity is clearly not satisfying. Withal a very high conductivity obtained for the 0.2 J/cm2 treatment and for a very high number of 10,000 shots, the DLC presents some sparsely small local surface defects. As discussed for air-made treatments, we suspect that for high annealing fluences combined with a high number of laser shots, the surface remaining residual oxygen (captured

Conclusion

We have demonstrated the possibility to obtain high conductive transparent DLC structures (approaching ITO performances) using a full laser combined process. High performance DLC deposited by PLD allows to obtain a very good transparent layer. A post UV-laser annealing is applied on the samples bringing surface conductivity while keeping a high value of transparency. Impacts of both fluence and number of shots on the electrical conductivity has also been studied. We show that increasing these

Acknowledgement

We specially want to acknowledge Mr ROQUES Stephane, Mr DIETRICH Florent and Mr MUGLER Florian for the technical support to carry on this study.

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