Pulsed Laser Deposition Technique

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Fabrication of Y2O3 and Y1.94Yb0.05Er0.01O3 thin films by pulsed laser deposition technique

Vinča Institute

Djordje Veljović,a Natalia Mihailescu,b Angela Stefan,b G. E. Stan,c Catalin Luculescu,b Djordje Janaćković,a Vesna Đorđević d, Miroslav D.Dramićanin,d Radenka Krsmanović Whiffen,d Carmen Ristoscu,b Serban Georgescu,b Ion N. Mihailescu,b aFaculty

of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia, e-mail: [email protected] b National Institute of Lasers, Plasma and Radiation Physics, Magurele, Ilfov, Romania; e-mail: [email protected] c National Institute of Materials Physics, Magurele, RO-077125, Romania d Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia, e-mail: [email protected]

Ultraviolet photons generated by a KrF*-laser source (λ=248 nm) were used for the fabrication by Pulsed Laser Deposition (PLD) of nanometric Y2O3 and Y1.94Yb0.05Er0.01O3 thin films with controlled thickness, stoichiometry and photoluminescence properties. The expulsed material was collected onto SiO2 and Si (100) wafers. We studied the effects of substrate temperature (500 °C and 600 °C) and Oxygen pressure (1 Pa and 10 Pa) on the structural properties of the films. On the basis of XRD analysis we selected the regime 10 Pa/500°C and on the basis of FTIR analysis the regime 1Pa/600°C as the best ones for the deposition of doped and undoped Y2O3 layers. The deposited structures were characterized from a physical-chemical point of view by Scanning Electron Microscopy in top- and cross-view modes, X-Ray Diffraction, FTIR and Energy Dispersive X-Ray Spectroscopy investigations. The optical activity of Y1.94Yb0.05Er0.01O3 films was proved by photoluminescence and NIR spectroscopies. This material has significant potential as a phosphor for up-conversion luminescence thermometry in the temperature range from 10 K to 300 K where it provides excellent sensitivity and clear spectral resolution [1] .

Y2O3 and Y1.94Yb0.05Er0.01O3 Nanopowders

Pulsed Laser Deposition Technique With PLD method, thin films are prepared by the ablation of one or more targets illuminated by a focused pulsed-laser beam.

Advantages of PLD • Accurate control of the stoichiometry of the deposited material (congruent ablation); • Reduced film contamination due to the use of laser light; • Promotion of the growth of crystalline structures for desired applications; • Energy source independent of the deposition environment; • Relative simplicity of the growth facility offering great experimental versatility (multilayers, doping); • Control of the film thickness (with a precision of > 10-2 Å/pulse).

PLD system set-up

Experimental set-up For each target we used 2 grams of nanopowder that were pressed at 5 MPa in a 2 cm diameter mold. X-ray diffraction measurements were performed with a Bruker D8 Advance diffractometer using Cu Kα1 radiation in Bragg-Brentano configuration. In case of the films deposited on quartz, even though a Ni filter has been used, peaks of some parasitic diffraction (K radiation) can be seen. These peaks belong to the single crystal substrate (peaks much narrow compared with the film peaks).

Observed (dots), calculated (solid red line) and difference (solid blue line) XRD patterns, SEM and TEM images of Y2O3 powder sample and histogram of nanoparticles size distribution. The diffraction peaks are indexed according to the JCPDS card no. 41-1105 of cubic Y2O3.

Pure Y2O3 nanopowder and doped with Er and Yb of (Y1.94Yb0.05)2O3 composition, have been prepared with the polymer complex solution (PCS) method i.e. polymer modified standard combustion method, and used as the starting material for producing thin films. Polyethylene glycol (PEG), with average molecular weight 200, served both as a fuel and dispersing medium for nanocrystals formation.

Fourier Transform Infrared (FTIR) Spectrometry analysis was carried out in transmission mode with a Shimadzu 8400S instrument within the range of 400 - 4000 cm-1.

Target

Subs.

Y2O3 Y2O3 Y2O3:Er-Yb Y2O3:Er-Yb

Substrate Temp. (ᴼC) 600

SiO2 and Si (100)

500 600 500

Pressure (Pa) 10-2 (1 Pa: O2) 10-1 (10 Pa: O2) 10-2 (1 Pa: O2) 10-1 (10 Pa: O2)

Energy mJ @ 5 Hz

200

Distance target-substrate (cm)

4

Spot area (mm2)

10

• Average crystalite size  35 nm • Dense aggregates made up of 20–30 nm NPs

PLD targets analysis: The X-ray diffraction analysis of non-ablated side of the target showed that the target is single phase. All peaks have been indexed as Y2O3 in a cubic structure, according to ICDD 01-071-5970. The ablation process does not affect the target (no secondary phases or target surface metallization). The drift of the 2 theta position and intensity decrease for the ablated side is attributed to the nonuniform surface of the target (figures not presented here).

Doped TFs: 500 ᴼC, 1 Pa (above) / 10 Pa (below) – BLACK; 600 ᴼC, 1 Pa (above) / 10 Pa (below) – RED; 700 ᴼC, 1 Pa (above) / 10 Pa (below) – BLUE

Thin films analysis: • Films deposited at lower ambient pressure (1 Pa) (see figure above) show a higher degree of crystallinity compared with the ones deposited at 10 Pa (see figure below). • Films deposited at higher pressure (10 Pa) are randomly oriented and the crystallinity improves by increasing the substrate temperature (figure below). These films show a relative texturing (100), in addition to the present of the (400) line, and the line (222) of slightly lower intensity (same figure).

No. of pulses

• Films deposited at lower pressure (1 Pa) shows identical crystallographic features, crystallinity is better in the case of film deposited at 600 ° C (figure above). These films show a strong texture (111), with dominant presence of the line (222), characteristic to yttrium set of lines (same figure).

10.000

On the basis of this analysis we selected the regime 10 Pa/500°C for the TF deposition of doped/undoped Y2O3.

Microstructural Investigations (SEM/EDX)

• Cubic bixbyte structure – space group Ia-3

Structural Investigation (XRD)

Morphological investigations of the thin films were performed using a FEI Inspect Scanning Electron Microscope (SEM) operating at an acceleration voltage of 20 kV, in high vacuum, in top-view mode. To estimate the films thickness, cross-sectional SEM micrographs were also recorded on twin samples deposited on Si substrates. All SEM specimens were coated with a thin layer of Au (~10 nm thick) before examination in order to prevent electrical charging. Energy Dispersive X-ray Spectroscopy (EDS) studies were carried out on all specimens using a SiLi detector (model EDAX Inc.) operated at 20 kV. The measurements were conducted in duplicate, on different, relatively large regions of (250 × 250) µm2.

• Polymer (PEG200) complex solution method

• We can conclude that the used pressure play a decisive role on crystallite orientation during film growth, but both types of structures (deposited at 1 Pa or 10 Pa) show significant structural changes compared to the target.

FTIR spectroscopy FTIR spectra demonstrated a congruent transfer of material from the target to the substrate. FTIR spectra of thin films deposited at 1Pa (left) show that after laser transfer the pollutants (C-O; O-H) disappear. Peak at 460 cm−1 and the sharp peak at 560 cm−1, in all cases belong to the strong metal-oxygen (Y-O) vibrations, indicating the formation of Y2O3. In the case of thin films deposited at 10 Pa (right), peaks are very intensive, but peaks of pollutants are also visible.

Top-view SEM images (left) of the selected thin films and one EDX spectrum of doped sample (right).

FTIR analysis shows that the best conditions for the deposition of doped/undoped Y2O3 samples is 1Pa/6000C and it has been added to previously selected 10 Pa/5000C regime.

SEM images of cross-section from the selected thin films: 1Pa-600°C (left) and 10Pa-500°C (right). TFs average thickness is ̴ 300 nm.

These two sets of samples were studied further by SEM, and optical spectroscopies.

Optical Measurements To record the IR luminescence the samples were illuminated with a LDC 1000 diode emitting at 980 nm, @6.8-6.9 A from a monochromator Jarrell-Ash, 1 m. Detector: Ge, 77 K with a resolution of 3 nm.

IR photoluminescence spectra for Y2O3:Er-Yb (500ᴼC, 10 Pa – BLUE colour), and Y2O3:Er-Yb (600ᴼC, 1 Pa – RED colour), thin films deposited on Si substrates by PLD method.

To record the PL luminescence the samples were illuminated with an Ar laser (488 nm), model (Mellw Cirist 35LA-P431-230). The experimental set-up for luminescence measurements contains a Horiba Jobin-Yvon monochromator (model 1000MP), an S-20 photomultiplier and the SR830 lock-in amplifier on line with a computer.

One PL spectrum (λexc = 488 nm) taken from the Y2O3:Er-Yb thin film deposited on Si substrate by PLD method (600 C, 1 Pa). The spectra from all doped samples were identical.

Reference [1] V. Lojpur, M.G. Nikolić, M.D. Dramićanin, Luminescence thermometry below room temperature via up-conversion emission of Y2 O3:Yb3+,Er3+ nanophosphors, Journal of Applied Physics (2014), 115(20), 203106/1-203106/7.

Energy level scheme of Er3+ and Yb3+.