Sublayer-Enhanced Growth of Highly Ordered Mn5Ge3 Thin Film on Si(111).

Mn5Ge3 epitaxial thin films previously grown mainly on Ge substrate have been synthesized on Si(111) using the co-deposition of Mn and Ge at a temperature of 390 °C. RMS roughness decreases by almost a factor of two in the transition from a completely polycrystalline to a highly ordered growth mode. This mode has been stabilized by changing the ratio of the Mn and Ge evaporation rate from the stoichiometric in the buffer layer. Highly ordered Mn5Ge3 film has two azimuthal crystallite orientations, namely Mn5Ge3 (001) [1-10] and Mn5Ge3 (001) [010] matching Si(111)[-110]. Lattice parameters derived a (7.112(1) Å) and c (5.027(1) Å) are close to the bulk values. Considering all structural data, we proposed a double buffer layer model suggesting that all layers have identical crystal structure with P63/mcm symmetry similar to Mn5Ge3, but orientation and level of Si concentration are different, which eliminates 8% lattice mismatch between Si and Mn5Ge3 film. Mn5Ge3 film on Si(111) demonstrates no difference in magnetic properties compared to other reported films. TC is about 300 K, which implies no significant excess of Mn or Si doping. It means that the buffer layer not only serves as a platform for the growth of the relaxed Mn5Ge3 film, but is also a good diffusion barrier.

Protein biosensor based on Schottky barrier nanowire field effect transistor.

A top-down nanofabrication approach involving molecular beam epitaxy and electron beam lithography was used to obtain silicon nanowire-based back gate field-effect transistors with Schottky contacts on silicon-on-insulator (SOI) wafers. The resulting device is applied in biomolecular detection based on the changes in the drain-source current (IDS). In this context, we have explained the physical mechanisms of charge carrier transport in the nanowire using energy band diagrams and numerical 2D simulations in TCAD. The results of the experiment and numerical modeling matched well and may be used to develop novel types of nanowire-based biosensors.

Structural, Optical, and Electronic Properties of Cu-Doped TiNxOy Grown by Ammonothermal Atomic Layer Deposition.

Copper-doped titanium oxynitride (TiNxOy) thin films were grown by atomic layer deposition (ALD) using the TiCl4 precursor, NH3, and O2 at 420 °C. Forming gas was used to reduce the background oxygen concentration and to transfer the copper atoms in an ALD chamber prior to the growth initiation of Cu-doped TiNxOy. Such forming gas-mediated Cu-doping of TiNxOy films had a pronounced effect on their resistivity, which dropped from 484 ± 8 to 202 ± 4 µΩ cm, and also on the resistance temperature coefficient (TCR), which decreased from 1000 to 150 ppm °C-1. We explored physical mechanisms causing this reduction by performing comparative analysis of atomic force microscopy, X-ray photoemission spectroscopy, X-ray diffraction, optical spectra, low-temperature transport, and Hall measurement data for the samples grown with and without forming gas doping. The difference in the oxygen concentration between the films did not exceed 6%. Copper segregated to the TiNxOy surface where its concentration reached 0.72%, but its penetration depth was less than 10 nm. Pronounced effects of the copper doping by forming gas included the TiNxOy film crystallite average size decrease from 57-59 to 32-34 nm, considerably finer surface granularity, electron concentration increase from 2.2(3) × 1022 to 3.5(1) × 1022 cm-3, and the electron mobility improvement from 0.56(4) to 0.92(2) cm2 V-1 s-1. The DC resistivity versus temperature R(T) measurements from 4.2 to 300 K showed a Cu-induced phase transition from a disordered to semimetallic state. The resistivity of Cu-doped TiNxOy films decreased with the temperature increase at low temperatures and reached the minimum near T = 50 K revealing signatures of the quantum interference effects similar to 2D Cu thin films, and then, semimetallic behavior was observed at higher temperatures. In TiNxOy films grown without forming gas, the resistivity decreased with the temperature increase as R(T) = - 1.88T0.6 + 604 µΩ cm with no semimetallic behavior observed. The medium range resistivity and low TCR of Cu-doped TiNxOy make this material an attractive choice for improved matching resistors in RF analog circuits and Si complementary metal-oxide-semiconductor integrated circuits.

Asymmetric Interfaces in Epitaxial Off-Stoichiometric Fe3+xSi1-x/Ge/Fe3+xSi1-x Hybrid Structures: Effect on Magnetic and Electric Transport Properties.

Three-layer iron-rich Fe3+xSi1-x/Ge/Fe3+xSi1-x (0.2 < x < 0.64) heterostructures on a Si(111) surface with Ge thicknesses of 4 nm and 7 nm were grown by molecular beam epitaxy. Systematic studies of the structural and morphological properties of the synthesized samples have shown that an increase in the Ge thickness causes a prolonged atomic diffusion through the interfaces, which significantly increases the lattice misfits in the Ge/Fe3+xSi1-x heterosystem due to the incorporation of Ge atoms into the Fe3+xSi1-x bottom layer. The resultant lowering of the total free energy caused by the development of the surface roughness results in a transition from an epitaxial to a polycrystalline growth of the upper Fe3+xSi1-x. The average lattice distortion and residual stress of the upper Fe3+xSi1-x were determined by electron diffraction and theoretical calculations to be equivalent to 0.2 GPa for the upper epitaxial layer with a volume misfit of -0.63% compared with a undistorted counterpart. The volume misfit follows the resultant interatomic misfit of |0.42|% with the bottom Ge layer, independently determined by atomic force microscopy. The variation in structural order and morphology significantly changes the magnetic properties of the upper Fe3+xSi1-x layer and leads to a subtle effect on the transport properties of the Ge layer. Both hysteresis loops and FMR spectra differ for the structures with 4 nm and 7 nm Ge layers. The FMR spectra exhibit two distinct absorption lines corresponding to two layers of ferromagnetic Fe3+xSi1-x films. At the same time, a third FMR line appears in the sample with the thicker Ge. The angular dependences of the resonance field of the FMR spectra measured in the plane of the film have a pronounced easy-axis type anisotropy, as well as an anisotropy corresponding to the cubic crystal symmetry of Fe3+xSi1-x, which implies the epitaxial orientation relationship of Fe3+xSi1-x (111)[0-11] || Ge(111)[1-10] || Fe3+xSi1-x (111)[0-11] || Si(111)[1-10]. Calculated from ferromagnetic resonance (FMR) data saturation magnetization exceeds 1000 kA/m. The temperature dependence of the electrical resistivity of a Ge layer with thicknesses of 4 nm and 7 nm is of semiconducting type, which is, however, determined by different transport mechanisms.