Deposition of Magnesium Silicide Nanoparticles by the Combination of Vacuum Evaporation and Hydrogen Plasma Treatment

The radio frequency plasma enhanced chemical vapor deposition (PECVD) of the hydrogenated amorphous silicon (a-Si:H) combined with the vacuum evaporation of magnesium followed by the hydrogen plasma treatment has been successfully applied for the in-situ deposition of magnesium silicide nanoparticles embedded in hydrogenated amorphous silicon thin layers. The homogeneous coverage of a-Si:H surface by magnesium silicide nanoparticles with diameter below 10 nm has been confirmed by the scanning electron microscopy (SEM) and X−ray photoelectron spectroscopy (XPS).


Introduction
Silicon layers with embedded nano-crystallites of semiconductor silicide have been studied recently important for designing new silicon based semiconductor devices [1].The Plasma Enhanced Chemical Vapour Deposition (PECVD) is based on possibility to transport volatile chemical compounds in the form of gases or vapors of convenient liquids [2].Most hydrogenated amorphous silicon (a-Si:H) is deposited in the form of thin films by PECVD from silane diluted in hydrogen [3].Although the principle of PECVD is quite simple, the running physical and chemical processes are exceedingly complex.Moreover, it is not easy in the gaseous phase to transport alkaline earth metals needed for silicide growth with convenient semi-conductive quality.For those elements it is necessary to use another transport method such as vacuum evaporation from thermally heated metal source.Our previous results were based on the ex-situ reactive deposition epitaxy (RDE) of Mg2Si nanoparticles (deposited under UHV conditions at the Institute of Automation and Control Processes of Far Eastern Branch of RAS, Vladivostok, Russia) on thin film a-Si:H (prepared at the Institute of Physics of the ASCR, v. v. i., Praha , Czech Republic) [4].The RDE was used to form silicide nanoparticles of different metals (Fe, Cr, Ca, Mg) with semiconducting properties and convenient band gap.Morphology of the deposited Mg2Si nanoparticles was observed by SEM and the cubic structure of prepared magnesium silicide has been confirmed by the Raman spectroscopy.The formation of Mg2Si nanoparticles on a-Si:H surface have been also proved by the changes of optical reflection and absorption.Increased near infrared absorption of the hydrogenated a-Si:H layers with embedded Mg2Si nanoparticles has been related to the formation of strong direct transitions in the Mg2Si nanoparticles inside amorphous silicon.The multi-structure was successfully integrated into p-i-n a-Si:H diode structure showing good rectifying I-V characteristics.In this work we realize a-Si:H as well as silicide nanoparticle deposition in-situ.For this purpose we developed a special deposition chamber in-situ combining PECVD with Vacuum Evaporation.Moreover, we can deposit thin film multi-structures periodically changing both processes.The reaction between elements in the nanostructures can be stimulated by thermal annealing.For it we usually need high temperatures which in many cases change a quality of thin films.It is case of amorphous or microcrystalline form of hydrogenated silicon we also study processes such as hydrogen plasma treatment of the surface for formation of compounds.
Fig. 1 The stainless-steel reactor KIII used for the combination of PECVD, vacuum evaporation and hydrogen plasma treatment.The vacuum evaporation is controlled by the current thermally heating the tungsten boat located below the chamber.

Experimental
The PECVD was performed in the capacitive coupled radio frequency plasma reactor stimulated by 13.56 MHz generator with the output power up to 100 W, see Fig. 1.In PECVD the 8% silane in the mixture of silane and hydrogen is decomposed at pressure in 70 Pa and the substrate temperature 180°C by the RF plasma with the 0.25 W/cm 2 power and deposited as a-Si:H in the form of thin film on glass substrate.The magnesium vacuum evaporation was realized from heated tungsten boat and is under control by the boat temperature and time.Deposition of magnesium silicide was done by overlapping a-Si:H layer at 180°C by the very thin evaporated Mg interlaced by about 20 nm thin a-Si:H and followed by the hydrogen plasma treatment with the surface density of power 1.5 W/cm 2 .The scanning electron microscopy (SEM) images were done by MAIA3 TESCAN − Scanning Electron Microscope.The chemical composition of the samples was analyzed by X−ray photoelectron spectroscopy (XPS) equipped with multi-channel hemispherical electrostatic analyser (Phoibos 150, Specs) and Al Kα X-ray source (1486.6 eV, Specs).Survey spectra were acquired at 40 eV pass energy with the step of 0.5 eV and the dwell time of 0.1 s.High resolution spectra were measured at 10 eV pass energy with the step of 0.05 eV and the dwell time of 0.1 s.The XPS spectra were acquired at a constant take-off angle of 90⁰.All XPS spectra were referenced to 99.3 eV, which corresponds to Si 2p bulk signal.

Results and Discussion
The Fig. 2 shows the morphology of Mg2Si nanoparticles formed on a-Si:H surface.The sample KIII-5 shows significantly bigger nanostructures of the size of about 30−50nm, whereas the sample KIII-6, prepared under optimized hydrogenated plasma treatment, shows homogeneous surface coverage with the nanoparticles with diameter below 10 nm.The results of XPS measurements reveal that the sample KIII-6 have rather high ratio of Mg/Si which exceeds the value of other sample KIII-5 in 7 times (see Table I).Thus, the Table I shows the crucial role of the hydrogen plasma treatment on the magnesium silicide formation.The lower Mg ratio in KIII-5 can be explained by the longer hydrogen plasma treatment etching of the magnesium layer.The comparison of high resolution Si 2p spectra of the samples presented on Fig. 3 shows the formation of Mg2Si in the case of the sample KIII-6.The peak at 100.2 eV on Fig. 2c) can be attributed to sub-stoichiometric silicon oxide peaks which appear as the result of Mg incorporation with amorphous Si substrate.The other peak at 98 eV corresponds to magnesium silicide which is consistent with the results reported by Casey at el. [5].The comparison of high resolution Si 2p spectra of the samples integrated over large area and presented on Fig. 3 shows the formation of Mg2Si in the case of the sample KIII-6.The peak at 100.2 eV on Fig. 3c) can be attributed to sub-stoichiometric silicon oxide peaks which appear as the result of Mg incorporation with amorphous Si substrate.The other peak at 98 eV corresponds to magnesium silicide which is consistent with the results reported by Casey at el. [5].Thus, Fig. 2 identifies the formation of the nanoparticles on the a-Si:H surface and Fig. 3 shows the existence of Mg2Si, therefore we conclude the particles on a-Si:H surface being Mg2Si.The optimized multilayer silicide nanostructures integrated into the diode structures based on the hydrogenated silicon promise the enhanced performance for energy conversion applications such as the solar cells or silicon integrated near infrared LED.

Conclusion
The plasma enhanced chemical vapor deposition (PECVD) and vacuum evaporation with subsequent hydrogen plasma treatment for stimulation of surface reaction and crystallographic configuration of silicide nanoparticles have been successfully used as in-situ process.The homogeneous coverage of a-Si:H surface by magnesium silicide nanoparticles with diameter below 10 nm has been confirmed by the scanning electron microscopy (SEM) and X−ray photoelectron spectroscopy (XPS).

Fig. 2 Fig. 3
Fig. 2 SEM images of a-Si:H surface covered by nanoparticles.KIII-5 sample (left) with about 50 nm in diameter nanoparticles and the optimized KIII-6 sample (right) with homogeneous coverage by Mg2Si nanoparticles with diameter below 10 nm.

Table I .
The Mg/Si ratio of atom concentration on the sample surfaces measured by XPS and compared to the reference a-Si:H layer with no evaporated Mg.