Re: Metal Semiconductor Contacts Rhoderick Pdf Download
Achmat Das
Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.
Metal-silicon junctions are crucial to the operation of semiconductor devices: aggressive scaling demands low-resistive metallic terminals to replace high-doped silicon in transistors. It suggests an efficient charge injection through a low Schottky barrier between a metal and Si. Tremendous efforts invested into engineering metal-silicon junctions reveal the major role of chemical bonding at the interface: premier contacts entail epitaxial integration of metal silicides with Si. Here we present epitaxially grown EuSi2/Si junction characterized by RHEED, XRD, transmission electron microscopy, magnetization and transport measurements. Structural perfection leads to superb conductivity and a record-low Schottky barrier with n-Si while an antiferromagnetic phase invites spin-related applications. This development opens brand-new opportunities in electronics.
Integrated-circuit scaling faces fundamental restrictions in the areas of design, manufacturing, energy and physical space1. Information technologies have entered the era of material limited device scaling2: basic materials of electronics have been extended to their performance limits. Likewise, emerging computing technologies pose further challenges to materials research. In particular, front end processes shift focus onto contacts in nanoscale devices2, thus emphasizing the importance of precise control over the structure and composition of the metal-semiconductor (MS) interface3.
Nowadays, metal silicides are an integral part of microelectronics being used as ohmic and Schottky barrier (SB) contacts, interconnects, gate electrodes or diffusion barriers6. Compatibility with Si technology, low resistance, suppression of electromigration, good contacts to other materials ensure numerous applications of silicides. In particular, the self-aligned silicide (SALICIDE) technology free of lithographic patterning processes is commonly employed for manufacturing ultra-large-scale integration devices7. Mid-gap silicides TiSi2, CoSi2 and NiSi are most popular materials due to low resistivity but scaling to ultra-small gate lengths and junction depths is challenging: issues like phase purity and Si consumption become increasingly important5,6,8. When reduced to nanoscale, silicides make a frontier research subject: they form nanodots and nanowires with appealing properties5,6,9,10, are employed as contacts to Si nanowires11,12,13 and integrate Si technology with prospective materials like graphene14.
Additional requirements are imposed on the contact material if it is designed to be compatible with spintronic applications. Silicon spintronics is an emerging set of energy-efficient information technologies implementing spin functionality in Si21,22. Injection of spin-polarized carriers into a semiconductor is demonstrated from magnetic semiconductors23, half-metals24 and metals through insulating tunnel barriers25,26,27,28,29 or Schottky-tunnel-barrier contacts30. Ohmic contacts are not functional without spin pumping31. Thus, SBH is a crucial parameter for metal/semiconductor spin injection32,33. Although transient femtosecond spin current can be induced in a non-magnetic material34, spin injection requires magnetic contacts. In general, any magnetic order suppresses spin scattering. Ferromagnetic silicides MnSi35 and Fe3Si36 are effective spin injectors. On the other hand, antiferromagnetic (AFM) contacts are also functional in spintronics applications as they support spin currents and better suited for using as spin detectors than ferromagnets37. AFM buffer layers may enhance spin transfer efficiency from a ferromagnet38.
Notice that our procedure is not very different from that employed in ref. 42 but the remarkable change of the outcome (single crystal instead of polycrystalline film) originates from a meticulous optimization of the growth conditions. The stability of the EuSi2 structure probably plays a great role in the easy formation of the epitaxial silicide film: similar reaction of Sr with Si substrate results in polycrystalline SrSi2 despite Sr being an equally active metal and very similar ionic radii of Eu(II) and Sr. Since the synthesis requires relatively mild conditions a reduced thermal budget is expected for its technological implementation. Although non-stoichiometry is often observed in rare-earth disilicides the problem is significant for hexagonal and orthorhombic structures while the tetragonal phase (like EuSi2) is characterized by a composition close to stoichiometric. As for our particular system EuSi2/Si(001), a synchrotron radiation study of EuSi2 nanoislands and polycrystalline films45 shows that the product of reaction between Eu and the Si(001) substrate has the EuSi2 stoichiometry.
In Si-based nanoelectronics the resistivity of a silicide is coupled with another major characteristic, consumption of Si in its reaction with a metal. Low silicon consumption is a most important technological requirement constraining applications of metal silicides to ultra-shallow junctions and silicon-on-insulator films. Low sheet resistance requires the silicide thickness to be maximized but correspondingly increased Si consumption leads to local junction penetration. It is a major factor that hinders applications of CoSi2 and elevates NiSi among other mid-gap silicides. Si consumption is characterized by the ratio of the resulting silicide thickness to the thickness of consumed Si, required to be as large as possible. Typical values are close to 1: 1.10 for TiSi2, 0.97 for CoSi2, 1.20 for NiSi and 1.27 for stoichiometric ErSi2. The same parameter calculated for EuSi2 is very large (1.58) making this material highly attractive for nanoscale applications. However, EuSi2 is not designed to compete with transition-metal low-resistivity silicides; instead, it is suggested as a prospective material for the SB-MOSFET technology (see below).
In summary, taking into account technological advantages of metal silicides and their full compatibility with Si technology we propose europium disilicide as a prospective junction material. In the course of our work we optimized conditions for manufacturing EuSi2/Si contacts. Epitaxial films are grown by reaction of Eu with silicon substrate. The synthesis is robust, easy to implement and what is most important is free from unwanted side products. Moreover, electron microscopy shows that the EuSi2/Si interface is atomically abrupt despite a significant lattice mismatch.
Apart from the superb structural quality of the EuSi2/Si interface and EuSi2 film, europium silicide exhibits a combination of properties which respond to demands of modern electronics: At low temperature EuSi2 becomes antiferromagnetic which may assist applications employing spin-related phenomena. Rather low resistivity and very low Si consumption are among other advantages of the material. Most importantly, the EuSi2/n-Si junction exhibits the lowest among silicides Schottky barrier height. Overall, EuSi2 is the most promising material for the SB-MOSFET technology.
D.V.A. and V.G.S. synthesized the EuSi2 films. C.G.K., I.A.K. and A.L.V. carried out T.E.M. experiments. G.V.P. performed X-ray studies. A.N.T. carried out magnetization measurements. O.E.P. performed transport experiments. A.M.T., E.F.L. and V.G.S. carried out the analysis. A.M.T. and V.G.S. wrote the paper with contributions from D.V.A., A.L.V., A.N.T. and O.E.P. All authors reviewed the manuscript.
The structure of a metal-semiconductor junction is shown in Figure 1. It consists of a metal contacting a piece of semiconductor. An ideal Ohmic contact, a contact such that no potential exists between the metal and the semiconductor, is made to the other side of the semiconductor. The sign convention of the applied voltage and current is also shown on Figure 1.
7fc3f7cf58