Investigation of Sprayed Lu₂O₃ Thin Films Using XPS

Towhid Adnan Chowdhury

Department of Electrical & Electronic Engineering, Ahsanullah University of Science & Technology, Dhaka, Bangladesh.

DOI: 10.4236/ampc.2023.1311014                

Abstract

Spray pyrolysis method was used to deposit Lutetium Oxide (Lu₂O₃) thin films using lutetium (III) chloride as source material and water as oxidizer. Annealing was carried out in argon atmosphere at 450°C for 60 minutes of the films. To investigate the composition and stoichiometry of sprayed as-deposited and annealed Lu₂O₃ thin films, depth profile studies using X-ray photoelectron spectroscopy (XPS) was done. Nearly stoichiometric was observed for both annealed and as-deposited films in inner and surface layers.

Keywords

Lu₂O₃, Depth Profiling, X-Ray Photoelectron Spectroscopy, Thin Films

1. Introduction

Lutetium oxide (Lu₂O₃), a rare earth metal oxide, is a promising material in microelectronics field due to its wide bandgap about 5.5 eV [1] [2] [3] [4] , good thermal stability [5] , high k dielectric constant (k = 11 - 13) [6] [7] [8] and highest lattice energy (−13,871 kJ/mol) [9] . Eu³⁺ doped Lu₂O₃ was investigated as a promising scintillator material because of its high density of about 9.4 g∙cm⁻³ and high absorption coefficient in X-ray detectors [10] [11] . Amorphous Lu₂O₃ also possesses promising unipolar resistive switching (RS) behavior [12] . Lutetium oxides superior switching characteristics make it a possible material for flexible electronics.

Lu₂O₃ thin films were deposited by different methods like pulsed laser deposition [13] , atomic layer deposition [14] [15] , electron beam deposition [16] [17] [18] , physical and chemical vapor deposition (PVD–CVD) [19] [20] and sol–gel method [21] . These methods need complex experimental procedure, high vacuum and high temperature.

In this work Lu₂O₃ thin films were fabricated by spray pyrolysis method using lutetium (III) chloride as source material of lutetium. Spray pyrolysis is a low-cost non-vacuum technique to fabricate thin films over large areas and easy to implement. The deposited films can be used as scintillators and in laser applications. Due to immense effect on device performance it is necessary to determine stoichiometry of bulk and surface of Lu₂O₃ films. In the present work, X-ray photoelectron spectroscopy (XPS) depth profiling was used to analyze Lu₂O₃ thin films to determine its stoichiometry.

2. Experimental Details

All the glassware in our experiment has been cleaned by first washing and scrubbing with Alconox, followed by a 20 min. sonication in acetone, methanol, and then washed by isopropanol and DI water. Afterwards, the glassware was dried using N₂ gas. An aqueous solution of (Conc.) 0.050 M Lutetium (III) Chloride (LuCI₃∙6H₂O) has been used for precursor solution spray deposition. The precursor solution was added in 100 mL of deionized water. Substrate temperature was controlled by a hot plate with which a thermocouple was attached within ±5˚C of 350˚C.

Spraying was accomplished using Iwata CM-C precision atomizing spray nozzle and compressed air at 40 PSI was used as the carrier gas. After film deposition, to avoid any thermal stress the substrates were cooled down slowly to room temperature. Substrates were annealed at 450˚C for 60 min. in argon atmosphere. A change in morphology was observed of the Lu₂O₃ thin films after the annealing process.

Composition of the Lu₂O₃ thin film was studied using XPS. The XPS spectra were obtained by using monochromatic Al Kα radiation (1486.6 eV) through a Kratos AXIS Ultra DLD XPS system at a base pressure of 5 × 10⁻¹⁰ Torr, equipped with an electronic neutralization gun to eliminate the charge effect on the sample surface. All binding energy values were calibrated by using contaminant carbon (C 1s 284.6 eV) as a reference. The sample was then ion sputtered with Ar⁺ at 4000 eV and 15 mA for 10 minute.

XPSPeak software version 4.1 was used to obtain all the spectra. A mixture of Lorentzian-Gaussian type peaks was used to deconvulate the spectra.

3. Results and Discussion

The composition and chemical purity of as-deposited Lu₂O₃ thin films were analyzed by XPS analysis. The typical XPS survey spectrum of as-deposited Lu₂O₃ is showed in Figure 1(a). The peaks arising from Lu 4d, 4f, 4p, 5p, C 1s and O 1s are clearly seen in the spectrum. No foreign impurities are identified in the spectrum. It is impossible to eliminate carbon contamination in almost all the preparations. The Lu 4d intensity is very large compared to the other Lu peak intensity, and that is why we have reported just Lu 4d spectra of Lu compounds. High resolution spectra of Lu 4d and O 1s core level are shown in the

Figure 1. (a) XPS survey spectrum of as-deposited Lu₂O₃ film; (b) High resolution XPS spectra of the Lu 4d core level of as-deposited Lu₂O₃ film; (c) High resolution XPS spectra of the O 1s core level of as-deposited Lu₂O₃ film.

Figure 1(b) and Figure 1(c) respectively. The lower binding energy value 195.5 eV of Lu 4d is characteristic of Lu ions in the oxide thin film. The higher binding energy value 198.4 eV of Lu 4d is for Lu₂O₃ [22] . The presence of Lu ionic peak in Lu 4d core level can be due to the existence of chemical defects, most probably the oxygen vacancies. The lower binding energy value 530 eV of O 1s is for Lu₂O₃. The higher binding energy value 532 eV of O 1s is due to nonlattice oxygen ions or oxide defects [22] [23] .

The XPS survey spectrum of as-deposited Lu₂O₃ thin film after 10 min. Ar⁺ ion sputtering is showed in Figure 2(a). The peaks arising from Lu 4d, 4f, 4p,5p, C 1s and O 1s are clearly seen in the spectrum. Carbon contaminations were reduced to a low level after 10 min. Ar⁺ ion sputtering. High resolution spectra of Lu 4d and O 1s core level are shown in the Figure 2(b) and Figure 2(c) respectively. No chemical shift was observed in Lu 4d for Lu ionic peak and O 1s for Lu₂O₃. A shift of 0.1 eV in O 1s core level for oxide defects and 0.2 eV in Lu 4d for Lu₂O₃ were observed after 10 min. of Ar⁺ ion sputtering.

The XPS survey spectrum of annealed Lu₂O₃ thin film is showed in Figure 3(a). The peaks arising from Lu 4d, 4f, 4p, 5p, C 1s and O 1s are clearly seen in the spectrum. High resolution spectra of Lu 4d and O 1s core level are shown in the Figure 3(b) and Figure 3(c) respectively. The lower binding energy value 195.5 eV of Lu 4d is characteristic of Lu ions in the oxide thin film. The higher binding energy value 197.9 eV of Lu 4d is for Lu₂O₃ [22] . The presence of Lu ionic peak in Lu 4d core level can be due to the existence of chemical defects, most probably the oxygen vacancies. The lower binding energy value 529.5 eV of O 1s is for Lu₂O₃.The higher binding energy value 531.7 eV of O 1s is due to nonlattice oxygen ions or oxide defects [22] [23] . No chemical shift was observed in Lu 4d for Lu ionic peak. A shift of 0.3 eV in O 1s core level for oxide defects, 0.5 eV in O 1s core level for Lu₂O₃ and 0.5 eV in Lu 4d for Lu₂O₃ were observed in annealed Lu₂O₃ thin film as compared to as deposited Lu₂O₃ thin film.

The XPS survey spectrum of annealed Lu₂O₃ thin film after 10 min. Ar⁺ ion sputtering is showed in Figure 4(a). The peaks arising from Lu 4d, 4f, 4p, 5p, C 1s and O 1s are clearly seen in the spectrum. Carbon contaminations were reduced to a low level after 10 min. Ar⁺ ion sputtering. High resolution spectra of Lu 4d and O 1s core level are shown in the Figure 4(b) and Figure 4(c) respectively. No chemical shift was observed in Lu 4d for Lu ionic peak. A shift of 0.6 eV in O 1s core level for oxide defects, 0.5 eV in O 1s core level for Lu₂O₃ and 0.3 eV in Lu 4d for Lu₂O₃ were observed in annealed Lu₂O₃ thin film after 10 min. of Ar⁺ ion sputtering as compared to annealed Lu₂O₃ thin film.

4. Conclusion

XPS study in this work presents that composition of surface layers and the inner layers of Lu₂O₃ thin films is almost stoichiometric. The XPS analysis reveals that as-deposited and annealed film contains the elements Lutetium, oxygen, and carbon. After 10 min. Ar⁺ ion sputtering the amount of carbon was reduced to a

Figure 2. (a) XPS survey spectrum of as-deposited Lu₂O₃ film after 10 min. Ar⁺ ion sputtering; (b) High resolution XPS spectra of the Lu 4d core level of as-deposited Lu₂O₃ film after 10 min. Ar⁺ ion sputtering; (c) High resolution XPS spectra of the O1s core level of as-deposited Lu₂O₃ film after 10 min. Ar⁺ ion sputtering.

Figure 3. (a) XPS survey spectrum of annealed Lu₂O₃ film; (b) High resolution XPS spectra of the Lu 4d core level of annealed Lu₂O₃ film; (c) High resolution XPS spectra of the O1s core level of annealed Lu₂O₃ film.

Figure 4. (a) XPS survey spectrum of annealed Lu₂O₃ film after 10 min. Ar⁺ ion sputtering; (b) High resolution XPS spectra of the Lu 4d core level of annealed Lu₂O₃ film after 10 min. Ar⁺ ion sputtering; (c) High resolution XPS spectra of the O 1s core level of annealed Lu₂O₃ film after 10 min. Ar⁺ ion sputtering.

small value. A small chemical shift in Lu 4d for Lu₂O₃ was observed in both as-deposited and annealed Lu₂O₃ thin film after 10 min. of Ar⁺ ion sputtering. XPS studies show presence of oxide defects in surface layers and the inner layers of Lu₂O₃ thin films. Hence it is concluded that there is no evidence of the formation of any other Lu-related compounds other than Lu₂O₃ on the surface and in the bulk. So Lu₂O₃ thin films fabricated by low cost spray pyrolysis technique can be applied to fabricate devices that can be used for high temperature environment.

Acknowledgements

The work was supported by the Advanced Support Program for Innovative Research Excellence—(ASPIRE-I), grant number 15530-E404 and Support to Promote Advancement of Research and Creativity (SPARC), grant number 15530-E413 of the University of South Carolina, Columbia, USA.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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