Elsevier

Vacuum

Volume 83, Issue 6, 12 February 2009, Pages 1001-1006
Vacuum

Studies on the target conditioning for deposition of LiCoO2 films

https://doi.org/10.1016/j.vacuum.2008.12.002Get rights and content

Abstract

Deposition of good quality thin films of Lithium Cobalt Oxide (LiCoO2), by sputtering is preceded by target conditioning, which dictates the surface composition, morphology and electrochemical performance of the deposited film. Sputtering from a virgin target surface, results in films with excess of the more reactive elements. The concentration of these reactive elements in the films decreases until the system reaches a steady state after sufficient sputtering from the target. This paper discusses the deposition kinetics in terms of target conditioning of LiCoO2. The composition, morphology and texturing of deposited film during various hours of sputtering were analyzed using X-ray photoelectron spectroscopy (XPS) and Field Emission Scanning electron microscopy (FESEM). The compositional stability is not observed in the films formed during the initial hours of sputtering from the fresh target, which becomes stable after several hours of sputtering. The Li and Co concentration in the films deposited subsequently is found to be varying and possible causes are discussed. After the compositional stability is reached, electrochemical analysis of LiCoO2 thin films was performed, which shows a discharge capacity of 129 μAh/cm2.

Introduction

Lithium cobalt oxide (LiCoO2) thin film electrodes have been found favorable for a reliable power source due to the superior properties in terms of cycle stability and energy density. RF-magnetron sputtering from a LiCoO2 target results in preferential (110) out-of-plane crystallographic orientated thin films [1]. For high temperature stable phase of LiCoO2 thin films, the films have to be deposited either with substrate biasing or heating during film deposition [2], [3], or post-deposition annealing in oxygen ambient at about 700 °C [4], [5]. HT-LiCoO2 exhibits an ideal layered α-NaFeO2 structure with alternating layers of Li and Co cations occupying in the octahedral interstices between the hexagonal stocked close-packed oxygen anions layers. The deposition parameters like Rf power, substrate-to-target distance, Ar:O2 ratio influence the structural, morphological and electrochemical performance of the deposited LiCoO2 thin films.

A new aspect of conditioning of a fresh multi-elemental target is identified as an important growth parameter for depositing stoichiometric thin films. For a compound target, since the sputter yield of all constituent elements is different, conditioning of target takes several hours of sputtering to reach compositional stability. Earlier reports on the growth of High Tc thin films of YBa2Cu3O7 indicate that a preconditioning of the target is essential before realizing stoichiometric films [6]. Also, the surface of a new target is non-stoichiometric, if reactive elements like lithium are present. The top layers would be lithium rich, forming impurities with oxides and hydroxides making a barrier layer over the surface. Impurities like Na, F and Cl are surface contaminations, which must be removed before a functional film deposition. The surface analysis of deposited thin films directly reflects the target surface elemental composition. The present study deals with the conditioning of the LiCoO2 target during Rf sputtering. The films have been deposited at fixed intervals of time and the characteristics of the films analyzed. Major interest has been in studying the chemical composition variation in the deposited films as the target is being conditioned. XPS analysis of films performed on these films reveals the elemental composition and phase formation for being electrochemically active. Along with this, the morphological features of the deposited films have been studied and discussed.

Section snippets

Experimental

Lithium cobalt oxide (LiCoO2) thin films were prepared using a custom designed high vacuum chamber, equipped with Rf-magnetron cathode in sputter-up geometry. A base vacuum of 5 × 10−7 mbar was obtained with a turbo molecular pump backed by a rotary pump. LiCoO2 target was made by cold pressing and sintering powder of pure LiCoO2 (Sigma Aldrich, 99%) at 800 °C for 20 h. The target size is 75 mm in diameter and 5.5 mm thick. Thin films have been deposited onto Si substrates held at a distance of 6 cm

Results and discussion

The aspect of conditioning of a new target has been analyzed by means of adsorbed impurity removal from the target surface and stoichiometric variations of elements involved. The compositional and morphological variation in the deposited films from a fresh LiCoO2 target was analyzed by a series of depositions at fixed conditions. The deposited films were immediately transferred to XPS analyzing chamber with minimal surface contamination. Samples 1–8 represent the films deposited during every 15 

Conclusions

The conditioning of a new LiCoO2 sputtering target at various hours of sputtering is studied by analyzing surface composition, and morphology of the deposited film. Applying 400 Wh of power and Ar:O2 ratio of 9:1 leads to erosion of the top most layers from the target leaving the stoichiometric LiCoO2 target surface. In XPS analysis, the surface contaminations and carbonaceous impurities are systematically seem to be reducing after each sputtering duration, which confirms the need of target

Acknowledgements

The authors thank TCE, Madurai for providing the LiCoO2 targets.

References (16)

  • P.J. Bouwman et al.

    Solid State Ionics

    (2002)
  • H.Y. Park et al.

    Materials Chemistry and Physics

    (2005)
  • J. Pracharová et al.

    Journal of Power Sources

    (2002)
  • C.N. Polo da Fonseca et al.

    Journal of Power Sources

    (1999)
  • G.K. Muralidhar et al.

    Vacuum

    (1993)
  • E. Rossen et al.

    Solid State Ionics

    (1993)
  • Woo-Seong Kim

    Journal of Power Sources

    (2004)
  • J.C. Dupin et al.

    Thin Solid Films

    (2001)
There are more references available in the full text version of this article.

Cited by (22)

  • Influence of Ti and Zr dopants on the electrochemical performance of LiCoO<inf>2</inf> film cathodes prepared by rf-magnetron sputtering

    2016, Materials Science and Engineering: B
    Citation Excerpt :

    In the present investigation, we have doped LiCoO2 films by partial substitution of the trivalent cobalt ion for a tetravalent transition metal, Ti and Zr, during deposition of the thin film employing a mosaic target. Due to the interest of having LiCoO2 under the form of thin films for many applications, a variety of techniques has been used to fabricate them, including RF sputtering [7,33–36], pulsed laser deposition (PLD) [37–40], electrostatic spray [41], chemical vapor deposition [42]. PLD is a remarkable technique that allows control for layer by layer growth.

  • Synthesis and electrochemical properties of non-stoichiometric Li-Mn-spinel (Li <inf>1.02</inf>M <inf>x</inf>Mn <inf>1.95</inf>O <inf>4-y</inf>F <inf>y</inf>) for lithium ion battery application

    2012, Electrochimica Acta
    Citation Excerpt :

    The battery properties depend on three important factors, the nature of the cathode, the nature of the anode, and the type of electrolyte used. The cathode materials used in Li-ion batteries are presently LiCoO2, LiNiO2, LiMnO2, and LiVO2 with layered structure, and their ramifications, spinel LiMn2O4, olivine LiFePO4, etc. [1–4]. Among them, lithium manganese oxide (LiMn2O4) based materials are of considerable interest as cathode materials due to their low cost and high working voltage.

  • Growth and microstructural features of laser ablated LiCoO<inf>2</inf> thin films

    2010, Journal of Crystal Growth
    Citation Excerpt :

    The growth of LiCoO2 thin films with preferred orientation is known to be crucial. Several thin film deposition techniques such as RF sputtering [1,3–6], pulsed laser deposition [3,7–11], electrostatic spray deposition [12] and chemical vapour deposition [13,14] were employed for the growth of LiCoO2 thin films. A brief literature survey reveals that it is difficult to grow stoichiometric and stable c-axis oriented LiCoO2 thin films by several physical vapour deposition methods due to many growth kinetic processes, which occur in vacuum or at low oxygen partial pressures.

View all citing articles on Scopus
View full text