ICT2014 Abstracts

Mg₂Si can be a promising candidate for thermoelectric devices in middle-high temperature ranges, but there have been few reports on its fundamental electrical properties and their correlation with structural and optical properties. Mg films with a thickness of 100-500nm were deposited on p-type Si(111) substrates at room temperature by RF magnetron sputtering using a Mg target (purity 99.9%). Then, these samples were annealed at 400 ^oC for 5-20 hrs with an infrared ray oven in Ar for solid phase growth (SPG) of Mg₂Si films. A thickness of SPG-Mg₂Si layers was theoretically estimated to be 146nm when a thickness of Mg layer (t_Mg) was 100nm. These samples were characterized by XRD, Raman spectroscopy, SEM/Energy dispersive X-ray spectrometry (EDS), optical transmittance and reflectance, Hall effect measurements and Current (I)-Voltage (V) measurement.

All samples showed Mg₂Si(220) XRD peaks and Mg₂Si Raman peaks at 256 cm^-1. Si Raman peaks were observed at 520 cm^-1 especially for a t_Mg of 100nm. The molar ratio of Mg/Si was measured to be around 2.0 in the middle depth of Mg₂Si layers, while the metallic Mg layers are likely to remain on the top of the surface layers, judging from hexagonal patterns observed by SEM. The optical band gaps for Mg₂Si layers with t_Mg of 200-500 nm were 0.61-0.68eV.  All samples exhibited n-type conductivity. At annealing time for 5hrs, with increasing t_Mg, electron density was decreased slightly from 8.6×10¹⁶ to 6.8×10¹⁶ cm^-3, while the electron mobility was increased remarkably from 30 to 109 [cm²-V^-1-sec^-1]. For samples with a t_Mg of 500nm, longer time annealing for 20hrs enhanced the electron mobility from 109 to 185 [cm²-V^-1-sec^-1], which was about 7 times larger than the reported values of 24 [cm²-V^-1-sec^-1][1]. Rectification properties of n-Mg₂Si/p-Si(111) diodes were observed from I-V measurements.

[1]Takashi Kato et al., J. Appl. Phys., 110 (2011) 63723.