Preparation And Characterization Of All Oxide PN Junction And Multiferroic Tunnel Junction

Si microelectronics are continuously downscaling toward their physical limits with miniaturization in the past decades. It is urgent to develop a new generation of materials and technologies for the information industry. Perovskite oxides show a wide spectrum of physical properties that can be employed in novel devices. The unique structure characteristics provide advantages for property enhancement in ease. Therefore, they have attracted much attention from both industrial and scientific community. Specifically, the heterojunction interface composed of two different perovskites offers numerous opportunities in both fundamental science and device appications. In this work, we foucs on two perovskites heterostructures:all oxide PN junction and multiferroic tunnel junction.Fe:SrTiO3 thin films have been depositedon single-crystallineNb:SrTiO3 (001) substrate by pulsed laser deposition to form Fe:SrTiO3/Nb:SrTiO3 PN junctions. The Ⅰ-Ⅴ curves of the PN junction are measured as a function of temperature. The transport characteristics of the all-oxide PN junction observed can be explained by taking into account the temperature and bias dependence of the dielectric constant of SrTiO3. La0.7Sr0.3Mn0.8Ru0.2O3/BaTiO3/La0.7Sr0.3MnO3 all oxide multiferroic tunnel junctions have been fabricated on SrTiO3 (001) substrates by photolithography and ion milling. Tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) characteristics are measured to demonstrate four-state memory functions. SrTiO3/BaTiO3 composite barrier is used to improve the device performance. Results achieved are summarized below,(1) A clear rectifying characteris observed in Fe:SrTiO3/Nb:SrTiO3 PN junction at room temperature. However, this rectifying character weakens with decreasing temperature as the reverse current increases dramatically. The key feature is the decrease in width of the depletion region at low temperatures as the dielectric constant of SrTiO3 increases dramatically at low temperature. Under forward bias, diffusion and recombination currents are suppressed at low temperatures. But the decreased depletion width makes it easier for electrons to recombine through tunneling into defect levels in the band gap of Fe:SrTiO3. This results in an increase of ηwith decreasing temperature. Under reverse bias, the decreased depletion width may facilitate direct tunneling from the valence band of Fe:SrTiO3 into the conduction band of Nb:SrTiO3, similar to that in a backward diode.(2) Magnetic oxides, La0.7Sr0.3Mn0.8Ru0.2O3 and La0.7Sr0.3MnO3, are deposited by pulsed laser deposition. The coercive fields of La0.7Sr0.3Mn0.8Ru0.2O3 and La0.7Sr0.3MnO3 films at 10K are 600 and 60 Oe, respectively. The ferroelectricity of a 10 unit-cell (u.c.) BaTiO3 is demonstrated by piezoresponse force microscopy. Domains patterned by applying +/- 4 V voltage through the conductive tip keep stable for at least 10 hours.(3) In La0.7Sr0.3Mn0.8Ru0.2O3/BaTiO3/La0.7Sr0.3MnO3 tunnel junction, four-state tunneling resistance is observed with a tunneling electroresistance (TER) of 0.7% and a tunneling magntoresistance (TMR) of 4.9%. By using a SrTiO3/BaTiO3 composite barrier, the four-state memory function can be greatly improved. The TER value is increased to 103 % (←←,�'�') and 128% (�'←,←�') while the TMR value is increased to 15% ↑ and 30% ↓. The tunneling electromagnetoresistance (TMER) is 100% in our multiferroic tunnel junction. This indicates the high sensitivity of tunneling spin-transport to the ferroelectric polarization direction.