(Invited) Pb UPD ML As Universal Surfactant for Electrochemical Thin Film Growth
Authors: Brankovic Stanko , Wu Dongjun , Dordi Yezdi , Joi Aniruddha 
Published: February 2020

In this talk we will briefly review the progress in the electrochemical thin film growth assisted by UPD ML. First, our focus is examining the epitaxial growth of Cu on Ru(0001) assisted by Pb UPD ML serving as a surfactant. Cu and Ru represent the overlayer/substrate system which has significant epitaxial strain (-5.5%). This fact together with room temperature growth conditions favor a 3D growth. However, (Figure 1), with convenient use of Pb UPD ML serving as surfactant, a 2D growth of Cu on Ru is induced. In situ STM study indicates that Pb significantly enhances mobility of Cu thus improving inter- and intra-layer transport which leads to evolution of flat epitaxial Cu deposit. An uncommon for Cu growth at room temperature, a surface evolution was characterized by step flow indicating significantly reduced barrier for adatom step-down process. XPS study indicates that amount of Pb in Cu deposit is a function of the solution design and experiment control. Characterization of 2D Cu overlayers indicates that their quality is comparable to MBE grown films which qualifies our approach as relevant for many high-tech applications. In the second part of the talk we focus on electroless growth of Co ultra-thin films on highly textured Cu substrate assisted by Pb ML serving as surfactant. The Co/Cu system has been already studied before using different deposition approaches [[i]]. Although layer-by-layer growth was initially reported [[ii],[iii]] it is now known that, at room temperature, the growth of cobalt on Cu(111) is dominated by island formation due to kinetically limited interlayer transport [[iv]].
Pb ML is deposited either as UPD ML or using our new e-less approach. To improve the interlayer transport of Co during the growth, the monolayer (ML) of Pb is pre-deposited on Cu surface before the Co growth. The benefit of Pb ML presence on Co interlayer kinetics during the growth is evident by high quality of Co films obtained. They are characterized by large 2D grains having the width to height ratio in excess of 500. The ultrathin Co films are continuous, highly conformal and very smooth yielding a 2D-like appearance. The saturation roughness measured over the range of 10 x10 microns for 10 nm thick Co films is only 3 nm. The Co films show extremely soft properties and very high electrical conductivity which indicates a very small concentration of defects and high quality of the grain boundaries. The benefit and application of this Co deposition protocol is discussed through the analysis of Cu/Co GMR stacks deposited with and without assistance of the Pb ML as surfactant. The practicality of our e-less growth of Cu/Co GMR stacks is emphasized due to its simplicity, fidelity of control and low capital cost of equipment.
In conclusion, our results also re-confirm the fact that Pb surfactnat-assisted electrochemical growth could be used to produce smooth 2D films of various metal overlayers even in situations where both thermodynamics and kinetics conditions do not favor positive outcome.
The authors acknowledge the support from Lam Research Corporation and NSF Chemistry division under the contract # 0955922.
References
[i] M.T. Kief and W.F. Egelhoff, Phys. Rev. B
47 10 785 (1993)
[ii] Gonzalez, R. Miranda, M. Salmeron, J.A. Verges, and F. Yndurain, Phys. Rev. B
24, 3245 (1981).
[iii] 3Q. Chen, M. Onellion, and A. Wall, Thin Solid Films
196, 103 (1991).
[iv] J. de la Figuera, J.E. Prieto, C. Ocal, and R. Miranda, Phys. Rev.
B 47, 13 043 (1993).