The studies on structure and electrical characteristic of Nb-doped SrTiO 3 (SNTO) substrates by XRD, Hall measurements, R-T and SEM, show that the SNTO substrates with the same Nb-doping concentration have different electrical properties because of the nonuniformity of Nb-doping. The uniformly doped high-quality substrates are extremely important for fabricating multilayer structure and devices. In addition, the experimental results also provide us with SEM method to judge the uniformity of conducting material.Keywords: Nb-doped SrTiO 3 , laser molecular beam epitaxy, electrical properties, doping uniformity.Strontium titanate (SrTiO 3 ) has been widely used as substrates for growing perovskite oxides thin films because SrTiO 3 is chemically and compositionally stable, and has small lattice mismatch with many perovskite oxides [1][2][3] . It is known that SrTiO 3 itself is an insulator with a bandgap of 3.2 eV. The electrical conductivity of SrTiO 3 is easily controlled from n-type semiconductor to metal-like conduction (up to 10 −4 Ωcm), and even to superconductivity at low temperature (T c <0.3 K) when Ti is partly substituted with Nb or La [4][5][6][7][8] . The conductive Nb-doped SrTiO 3 (SNTO) substance with variable conductivity is not only used as the bottom electrode for the deposition of functional perovskite oxides [9][10][11] , but also provides the possibility to fabricate some fascinating and novel devices such as multilayer structure [12,13] , superlattice [14] , tunnel junctions [15] , field effect transistor [16] , schottky junction [17] and p-n junction [18,19] . The substrate quality is very importance for the investigation of new structures and properties of functional perovskite oxide devices.In this work, we studied the surface morphology, crystal structure and electric property of two sorts of SNTO, labeled as sample SNTO1 and SNTO2, respectively, which were doped with the same Nb concentration but fabricated by different derivations. The results indicate that the Nb-doping uniformity in SNTO substrates is extremely crucial for the electric properties of devices especially for p-n junctions and heterostructures.The samples of SNTO1 and SNTO2 we measured are with 1% doped niobium bought from different companies. The surface morphology and structure were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) with Cu Kα 1 radiation. Electric properties were studied by Hall measurement and temperature dependence of in-plane resistance. In order to further investigate the effect of different substrates for devices, two p-n junctions with the SNTO1 and SNTO2 as substrates were fabricated by a computer-controlled laser molecular-beam epitaxy (laser-MBE) [20] . 3500 Å p-type La 0.9 Sr 0.1 MnO 3 (LSMO) thin films were epitaially grown on the n-type SNTO1 and SNTO2 substrates, respectively. The current-voltage (I-V) characteristics of the two p-n junctions were also measured at room temperature.Figs. 1 (a) and (b) show the XRD θ -2θ scan curves of SNTO1 and SNTO2, respectively. Bes...
