Mg2Si Layer Research Articles

Silicon overgrowth atop low-dimensional Mg2Si on Si(111): structure, optical and thermoelectrical properties

The growth of silicon cap layers atop Mg2Si nanocrystallites (NCs) and Mg2Si two-dimensional layer with structure (2/3√3×2/3√3)-R30° were studied by methods of AES, EELS, AFM, optical and Raman spectroscopy. It was established that method of solid phase epitaxy (SPE) ensures the embedding of Mg2Si NCs in polycrystalline quality silicon cap layer at temperatures not higher than 920 K. The increase of the temperature higher than 970 K results to the moving of silicide NCs toward the surface between silicon grains in cap layer, following their destruction and Mg desorption from the surface. The MBE method with temperatures 430–485 K was used to the embedding of continuous 2D Mg2Si layer with structure (2/3√3×2/3√3)-R30° in silicon top layer (9–20 nm) with monocrystalline grains. Investigations of thermoelectrical properties of grown nanoheterostructures have shown that a Seebeck coefficient (α∼130μV/K) increases in ten times as compared with undoped silicon substrate (α=10–15μV/K).

Magnesium Induced Superstructural Changes in High Index Si (5 5 12) Surface: Formation of Quasi One Dimensional Structures

This is the first report of the initial formation of Mg/Mg2Si/Si interface at room temperature on high index Si (5 5 12) surface. The work describes the initial Mg2Si formation on the silicon surface, which forms a silicide layer after a critical coverage, and then metallic magnesium forms on top of the Mg2Si layers. The studies have been performed in situ in ultra high vacuum conditions (5 x 10(-11) Torr) and analyzed by surface sensitive techniques like Auger electron spectroscopy, electron energy loss spectroscopy, and low energy electron diffraction. The Auger uptake curve obtained by AES, shows the layered growth of Mg while the LEED and EELS shows the formation of surface phases, which are identified as the quasi 1D structure, with different electronic natures indicated by EELS studies. The growth process occurs in the following way that firstly the adsorption of Mg reconstructs the Si (5 5 12)-2 x 1 surface and forms two stable phases-Si (337) and Si (113) below the coverage of 1 ML, while there is Mg2Si formation at the coverage of 1 ML, further increase in the coverage of magnesium results in the metallic magnesium on top of the Mg2Si layer. Thus, with the Mg adsorption at RT not only the Si (5 5 12)-2 x 1 surface is reconstructed to give the stable phases but it also demonstrates the formation of Mg2Si at RT, which is the interfacial layer in between metallic Mg and Si.

Kinetic limitations of the Mg2Si system for reversible hydrogen storage

Despite the promising thermodynamics and storage capacities of many destabilized metalhydride hydrogen storage material systems, they are often kinetically limitedfrom achieving practical and reversible behavior. Such is the case with theMg2Si system. We investigated the kinetic mechanisms responsible for limiting the reversibility of theMgH2+Si system using thin films as a controlled research platform. We observed that thereaction is limited by the mass transport of Mg and Si into separate phases. Hydrogen readily diffuses through theMg2Si material andnucleating MgH2 phase growth does not result in reaction completion. By depositing and characterizing multilayer films ofMg2Si and Mgwith varying Mg2Si layer thicknesses, we conclude that the hydrogenation reaction consumes no more than 1 nm ofMg2Si, making this system impractical for reversible hydrogen storage.

Formation of transition-metal-based ohmic contacts to n-Mg2Si by Plasma Activated Sintering

AbstractElectrode materials consisting of Cu, Ti and Ni were formed on Bi-doped n-type Mg2Si by means of a monobloc plasma-activated sintering (PAS) technique. Due to the difference in thermal expansion coefficients between Ti and Mg2Si, rather high residual thermal stresses gave rise to the introduction of cracks, which were mainly located in the Mg2Si layer, when Ti was used as the electrode material. In the case of the Cu electrodes, monobloc sintering could not be performed in a reproducible manner because Cu melts abruptly and effuses at around 973K, which is 100 K lower than the sintering temperature that is required for Mg2Si of good crystalline quality. When compared with the results for Cu and Ti, the monobloc PAS process for Ni was both stable and reproducible. The room-temperature I-V characteristics of Ni electrodes were considered to be adequate for practical applications, with durable Mg2Si-electrode junction properties being realized at a practical operating temperature of 600 K with ΔT = 500 K. The highest open circuit voltage (VOC) observed was 41 mV at ΔT = 500 K (between 873 K and 373 K) for Ni electrodes fabricated using the monobloc PAS process. The voltage (V) and current (I) values with a 10 Ohm load were ∼ 48 mV and ∼ 2 mA at ΔT = 500 K.

Surface electronic structure of pure and oxidized non-epitaxial Mg2Si layers on Si(111)

Non-epitaxial magnesium silicide (Mg2Si) films of 100 Å thickness were grown on Si(111). The formation of Mg2Si is identified by characteristic shifts of the Mg 2p and Si 2p peaks in X-ray photoelectron spectroscopy (XPS). Information on the electronic structure of this surface is obtained by metastable impact electron spectroscopy (MIES). The surface density of states obtained with MIES is dominated by strong emission below the Fermi level (EF) displaying a characteristic double peak structure. The oxidation of these silicide surfaces is performed at room temperature. The electronic structure of these surfaces is investigated with MIES, UPS (HeI) and XPS. From the comparison with oxidized Mg films it is concluded, that the surface is terminated by an insulating MgO layer. Subsurface oxidation of the silicide does not take place. Furthermore, no formation of silicon oxides is observed.

Thin film growth of semiconducting Mg2Si by codeposition

Ultrahigh vacuum evaporation of magnesium onto a hot silicon substrate (⩾200 °C), with the intention of forming a Mg2Si thin film by reaction, does not result in any accumulation of magnesium or its silicide. On the other hand, codeposition of magnesium with silicon at 200 °C, using a magnesium-rich flux ratio, gives a stoichiometric Mg2Si film which can be grown several hundreds of nm thick. The number of magnesium atoms which condense is equal to twice the number of silicon atoms which were deposited; all the silicon condenses while the excess magnesium in the flux desorbs. The Mg2Si layers thus obtained are polycrystalline with a (111) texture. From the surface roughness analysis, a self-affine growth mode with a roughness exponent equal to 1 is deduced.

Mg2Si buffer layers on Si(100) prepared by a simple evaporation method

The formation of Mg2Si(100), ao= 6.39A, on Si(100) substrates has been investigated. Mg was first evaporated onto Si(100) surfaces and Mg2Si (100) films were formed in a subsequent annealing process. The Mg2Si layers were characterized by x-ray diffraction and transmission electron microscopy analysis. Optical and scanning electron microscopy analysis show the surface morphology to be smooth. The films are stable under thermal cycling and exhibit low resistivity. Epitaxial films of Mg2Si on Si(100) could be an ideal substrate for mercury cadmium telluride and antimonide based III-V semiconductor for mid-infrared devices because of its close lattice matching (the lattice misfit factor is less than 1.5%).