Tuning structural and magnetic properties of Fe films on Si substrates by hydrogenation processing

https://doi.org/10.1016/j.mseb.2013.11.004Get rights and content

Highlights

  • Fe films have been grown on Si(0 0 1) substrates and subsequently hydrogenated.

  • As deposited films present soft magnetic character and a strong magnetic texture.

  • Structural and magnetic properties can be tuned via hydrogenation treatments.

  • Ferromagnetic/semiconductor interfaces might be manipulated via hydrogenation.

Abstract

In order to study specific phenomena at ferromagnetic/semiconducting interfaces, of potentially high interest in spintronics and information technology, structural aspects and magnetic properties of Fe thin films grown on Si(0 0 1) substrates by RF sputtering have been investigated using 57Fe conversion electron Mössbauer spectroscopy (CEMS) and magneto-optic Keer effect (MOKE). Films of different thicknesses have been deposited either directly on crystalline Si substrates or on Cu buffer layers. An inherent Fe oxide layer is observed in all as prepared films, with a relative thickness decreasing drastically with the deposition time. The Cu buffer layer does not diminish either the interfacial diffusion or the oxidation process. An efficient method to prepare sharper oxygen- and silicon-free interfaces for an improved spin injection, via thermal treatment in hydrogen atmosphere, is proposed. Accordingly, the hydrogenation treatments are very efficient in the modification of the ferromagnetic film structure, phase composition, magnetic properties and interfacial mixing.

Introduction

An important topic of the present research in spintronics is related to the efficient injection of a spin-polarized current from an electrode into a semiconductor [1], [2], with numerous applications related to novel spin dependent electron transport phenomena (e.g. giant magneto-resistance [3], [4], tunneling magneto-resistance [5], etc.) or spin based electronic devices (e.g. spin wave logic devices [6], spin transfer torque [7], [8], [9] devices, etc.). Although magnetic semiconductor electrodes would be in principle more suitable in this respect, their main inconvenient is related to their relatively low Curie temperature [10]. An alternative solution consists in using an ordinary ferromagnetic film as electrode, with high remanent magnetization if possible, although the main breakdown is the “conductivity mismatch” [11] between the ferromagnetic (FM) electrode and the interfaced semiconductor (SC). The situation can be partially improved by using a high spin dependent resistance at the FM/SC interface [12], but the problem remains open for further achievements. Therefore, many efforts have been done in the last years for finding suitable FM/SC interfaces [13], [14], [15], [16], [17], [18], [19], [20], requiring both convenient material combinations and negligible atomic intermixing. Fe films grown on crystalline Si (c-Si) substrates have been intensively studied [21], [22], [23] since it can represent an attractive solution, providing opportunities for integration of the interface in the conventional Si-based electronics. However, in the well-studied system of Fe films grown on c-Si substrates, the involved interface plays a key role, determining the overall structural and magnetic properties. Depending on the preparation conditions, atomic diffusion and intermixing processes can take place during the growth process leading to the so-called reaction of silicization [24], [25], [26]. Magnetically dead silicide layers [27], [28], [29], [30] with low coercivity or even with non-magnetic character can be formed at different extends depending on the deposition method, with an expected negative impact not only on the magnetic properties of the ferromagnetic film but also on the quality of the FM/SC interface. One of the most common silicide phases formed at depositing Fe on c-Si at room temperature is the magnetic Fe3Si compound [31], but other phases can also be formed [32].

A solution to reduce the Fe–Si intermixing process has came out observing that metallic Fe layers can be grown epitaxially, with negligible intermixing and with induced magnetic texture, on Ge(0 0 1) substrates [33]. Therefore, the growing of Fe layer on Si(0 0 1) substrates via a Si1−xGex buffer layer has been successfully applied [34], by using the plasma enhanced chemical vapor deposition method.

The RF sputtering deposition method, although it is one of the most effective for large scale production, leads unfortunately to a strong interfacial atomic intermixing and consequently, new methods to reduce this process would be of large interest. To solve the aforementioned problem, we have considered to reduce the intermixing at the Fe/Si interface by the deposition of a buffer layer containing elements with lower diffusion coefficients relative to iron (e.g. Sb, CrSi2, Au or Cu buffer layers have been mentioned in [35], [36], [37], [38]). Moreover we propose a more efficient method, related to the post annealing of the RF sputtered iron films in hydrogen atmosphere at convenient temperature. Hydrogen is known as a reducing agent when reactions at relatively high temperatures (300–400 °C) occur. Previous studies revealed that the oxidation degree in thin films is considerably reduced and their crystallinity is improved via hydrogenation treatments [39], [40] and hence, ferromagnetic/semiconducting interfaces more suitable for applications in spintronics would be expected.

The aim of this paper is the investigation of structural and magnetic properties of iron films deposited by RF sputtering on c-Si substrates. A detailed study of the changes induced on the deposited Fe films, by either Cu buffer layers or a subsequent annealing under high pressure of hydrogen is presented. The experimental investigation has been performed using X-ray reflectometry (XRR), conversion electron Mössbauer spectroscopy (CEMS), atomic force microscopy (AFM), magnetic force microscopy (MFM) and magneto-optic Kerr effect (MOKE) measurements.

Section snippets

Experimental details

Fe films enriched in the 57Fe Mössbauer isotope have been deposited on c-Si(0 0 1) substrates. The samples have been prepared via RF sputtering in Ar atmosphere of a high purity Fe target, partially covered by small plackets of metallic Fe 95% enriched in the 57Fe isotope and keeping the substrate at about room temperature (the substrate holder was maintained at constant temperature by flowing water at about 20 °C). A bottom-up configuration (bottom horizontal 2″ Fe target and top substrate at

Results and discussions

The thickness, density and roughness of the hydrogenated samples Si/Fe(9 nm)_H and Si/Cu(24 nm)/Fe(9 nm)_H have been estimated via X-ray reflectometry (XRR). The mass density profiles obtained from the fitting of the XRR spectra of the above samples are shown in Fig. 1. The corresponding XRR patterns (dotted line-experimental and solid curve – theoretical fit) are shown in the insets. The best fit was obtained in the frame of the following structural models: Si substrate, a thin SiO2 layer, a Fe

Conclusions

Fe films enriched in the 57Fe isotope have been deposited by means of RF sputtering either directly on the c-Si(0 0 1) substrate, or on a Cu buffer layer. In order to reduce the oxidation and to improve the crystallinity of the films, treatments in hydrogen atmosphere have been subsequently applied. The films have been analyzed both as-deposited and after treatment. Structural investigations have been performed by using AFM, XRR and CEMS, while magnetic measurements were done via MFM and MOKE.

The

Acknowledgement

The financial support through the Core Program PN09-450103 of the Romanian Ministry of Education, Youth and Sport and the project BS-M-5-EUROATOM are highly acknowledged.

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