The early stages in the nucleation process and residual stress of electrodeposited ${\mathrm{Co}}_x{\mathrm{Fe}}_{100-x}$ on Si(111)

Highlights

• Electrodeposition of Co–Fe alloys directly on n-type silicon substrates.

• The first stages of the nucleation process of ${\mathrm{Co}}_{70}{\mathrm{Fe}}_{30}$ and ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$.

• The films of Co–Fe were electrodeposited on silicon under compressive stress.

Abstract

In this work we investigated the early stages in the nucleation process and the residual stress of thin films of ${\mathrm{Co}}_x{\mathrm{Fe}}_{100-x}$ grown directly on n-Si(111) substrates. The films were prepared potentiostatically at the composition of $x=70$ and $x=90$ atomic %. The morphology and growth were investigated by scanning electron microscopy at deposition time ranging from 0.5 s to 5 s. Our findings reveal for both the compositions hemispherical nuclei growing from hemispherical centers. The crystallographic characterization and residual stress analysis by X-ray diffraction reveal structural phase transition from a body-centered cubic (bcc) to face-centered cubic (fcc) structure by increasing the amount of Co in the alloy and a compressive regime of strain in the deposits, respectively.

Introduction

Growth of ferromagnetic thin films on semiconductors are widely discussed in the literature for the last decades, it is being constantly highlighted the growth of thin films of ${\mathrm{Co}}_x{\mathrm{Fe}}_{100-x}$ alloys. There are reports on thin films prepared by electrochemistry, molecular beam epitaxy (MBE) [1], [2], RF/DC Magnetron Sputtering, Chemical Vapor Deposition [3]. These efforts are due to their possible technological applications in Tunnel Magnetoresistance (TMR) [4] devices, Magnetoimpedance Devices(MI), Tunnel Magnetic Junctions (TMJ) and most recently, applications of Co–Fe alloys in spin caloritronics [5]. Concerning spintronic applications, it is reported that there is a possibility to inject high spin-polarized current on semiconductors (Si, Ge, GaAs) [6] even at room temperature. Additionally, the possibility of using Co–Fe alloys on thermal spin injectors in magnetic tunnel junction, was recently investigated pointing out that such alloys are a useful and a fascinating material to produce spin-polarized current paving the way toward advances in nano-spintronic devices [7].

The electrochemical deposition of metals on semiconductors it is a challenge due to the complexity of the semiconductor-electrolyte interface. It is long stand known that the deposition occurs via the conduction band in n-type semiconductors, following the Wolmer–Weber mechanism (3D) due to the weak interaction between semiconductors and metals [8], [9]. In the beginning, the formation small thermodynamically stable 3D clusters is followed by the film growth, exhibiting a dependence with the applied potential. In other words, the difference between the chemical potential of the solid and the liquid is proportional to the magnitude of overpotential, which controlled by the applied potential [10]. A critical point related to metal growth on semiconductor, it is that the nucleation mechanism affects the morphology, hence to obtain a uniform film, a large number of nuclei is required. In fact, there is a lack of literature on the growth and nucleation process of Co–Fe directly on silicon i.e without a seed layer, existing a few works focused on electrodeposition of Fe, Co and Co–Fe on silicon substrates [8], [11], [12], [13], [14].

Electrodeposition of soft magnetic Co–Fe alloys can be a useful tool to minimize costs, once the procedure is widely studied in a very well established technique. The electrodeposits show the same interesting magnetic properties when compared to other techniques above-mentioned. Regarding the Co–Fe binary alloy, the magnetic properties are tuned into the composition of the alloy. It is reported Co–Fe thin films alloy exhibiting a high magnetic moment, reaching up to 2.4 T for Co amount from 50 to 70 at.%, which is considerable for applications in the magnetic record. To date, it is found a maximum coercive field of 50 Oe at 70 at.% of Co [15], [16] and the magnetostriction at 90 at.% Co it is very low in thin films [17]. Besides, the regime of residual stress in electrodeposited magnetic alloys can leads to larger changes in the magnetic properties [18], [19], [20], being an important parameter to be investigated. Thus, it is very important to investigate the early nucleation process, which is an important parameter to evaluate the characteristics of the deposit, as well as it may pave the way to enhance some characteristics, i.e density, morphology, and magnetic properties.

Here, we focus our studies on electrodeposition of ${\mathrm{Co}}_x{\mathrm{Fe}}_{100-x}$ thin films alloys directly on n-Si (111) aiming to understand the growth mechanism and how does the growth regime can affect the morphology and microstructure, giving an important contribution in face of the lack of data about nucleation process and residual stress of Co–Fe films growth directly on silicon substrates. We describe the first stages of the nucleation and growth and the regime of residual stress for thin films at 70 at.% and 90 at.% of Co.