Transport properties of SrTiO$_{3-\delta}$ thin films grown by Molecular Beam Epitaxy on p-Si(001) substrates*

SrTiO$_{3}$ (STO) films were grown on p-Si(001) and STO(001) bulk substrates using molecular beam epitaxy (MBE). Oxygen vacancies were introduced by controlling the Oxygen pressure during growth (P(O$_{2})$: 4 $\times$ 10$^{-8}$ - 8 $\times$ 10$^{-7})$ resulting in SrTiO$_{3-\delta}$ with $\delta $ $\sim$ 0.02{\%} for the lowest P(O$_{2})$. The single-phase STO/Si films were of high crystalline quality as verified by x-ray diffraction, transmission electron microscopy, and atomically flat. Transport measurements were performed on the STO/Si structures in a Van der Pauw configuration. The P(O$_{2})$ during growth determines the conduction behavior which changes from strongly localized transport that fits a Variable Range Hopping (VRH) model (low P(O$_{2})$-high disorder) to thermally activated transport (high P(O$_{2})$-low disorder). The resistivity of the strongly disordered STO/Si films decreased from 1 Ohm$\cdot$cm to 3x10$^{-2}$ Ohm$\cdot$cm as the film thickness increased (3nm-60nm). The perpendicular magnetoresistance (MR) is positive at 300K and becomes negative at T$=$3-20K. We consider competing effects on the STO/Si heterostructure such as 1.7{\%} compressive strain induced by lattice mismatch to Si, defects due to oxygen vacancies, the bulk STO antiferrodistortive phase transition at 105K, and structural dislocations.