Structural and magnetotransport study of SrTiO3-δ/Si structures grown by molecular beam epitaxy
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SrTiO3 (STO) films were grown on p-Si (001) substrates using molecular beam epitaxy (MBE). Oxygen vacancies were introduced by controlling the Oxygen Pressure (PO2) that varied from 10-8 to 10-7 torr during growth, resulting in SrTiO3-δ with δ ~ 0.02% for the lowest pressure. The single phase STO/Si films were of high crystalline quality as verified by x-ray diffraction (XRD), transmission electron microscopy (TEM), and were atomically flat as verified by atomic force microscopy (AFM), with rms roughness of less than 0.5nm measured by AFM. The thickness was measured by x-ray reflectivity. Transport measurements were performed on the STO/Si structures in a Van der Pauw configuration. We measured resistance as a function of temperature for a range of T = 3K to 300K and as a function of an applied magnetic field, H=0 to ±9T. The resistivity decreased from 1 Ohm cm to 3x10-2 Ohm cm as the film thickness increased (3nm-60nm) at all temperatures. To identify the origin of the resistivity thickness dependence, we consider several competing effects in STO/Si such as 1.7% compressive strain induced by lattice mismatch with Si, strain and defects due to oxygen vacancies, an antiferrodistortive phase transition at 105K in bulk STO, and structural dislocations. We find that a charged space effect may explain the thickness dependence of resistivity, resulting from the interface of STO with Si and the Si doping type and concentration. The magnetic field dependence shows a reproducible trend of a positive magnetoresistance (MR) at 300K, which turns to a negative MR effect between 200K and 100K, and turns up again to a positive MR at 3K. Interestingly, at 3K, the MR starts to turn down again at high fields.