Multifunctional oxide materials for memory applications

Abstract

Non-volatile memory technology in Si-based electronics dates back to 1990s. It is radically new storage technology that combines the performance and byte addressability of DRAM with the persistence of traditional storage devices like SSD. Emerging memory technologies promise new memories to store more data at less cost than the expensive-to-build silicon chips used by popular consumer gadgets. Emerging nonvolatile memory technologies such as magnetic random-access memory (MRAM), spin-transfer torque random-access memory (STT-RAM), ferroelectric random-access memory (FeRAM), phase-change memory (PCM), and resistive random-access memory (RRAM) combine the speed of static random-access memory (SRAM), the density of dynamic random-access memory (DRAM), and the nonvolatility of Flash memory and so become very attractive as another possibility for future memory hierarchies. Multiferroics (MF) magnetoelectric (ME), generally a combining properties of MRAM and FeRAM are another exciting NVM which are getting intensive scientific attraction recently. Most existing MF-ME have some drawbacks; (i) They have either very low or too high transition temperature, (ii) They possess very low FE and/or FM responses and (iii) have low ME coupling coefficient, that hinder the practical applications. MF-ME materials with ferroelectric and magnetic transition temperatures near room temperature are of current interest of research due to their technological importance and potential applications. This work provides the detail study of single phase compounds gallium ferrite (GFO) along with some dopants at its various compositions. Polycrystalline samples with modifying Fe and Mn contents on GFO, Ga_2-x(Fe_1-yMn_y)_xO₃, 0 ≤ y ≤ 0.02 and 1 ≤ x ≤ 1.4 were prepared by solid state reaction method, stabilizes in orthorhombic phase (space group C⁹_2v). Temperature dependent magnetization behavior studied in a wide temperature range of 5 - 395 K at both field-cooled (FC) and zero-field-cooled (ZFC) conditions using different static magnetic fields shows the lower magnetic transition temperature (T_C) for pure GaFeO₃ (GFO), i.e., 220 K, which increases to 345 K above room temperature (RT)) for 2% Mn-doped Ga_0.6Fe_1.4O₃ (GFMO3). Higher remanant magnetization (~ 16 emu/g) with a lower coercive field (~ 4 kOe) is obtained for Mn-doped Ga_0.6Fe_1.4O₃. Evidence of spin-glass like magnetic ordering is observed at low temperature. Temperature dependent Raman spectroscopic studies on pure GFO down to 82 K showed 31 Raman active modes in the spectral range 90 - 900 cm^-1. Some of the phonon mode frequencies exhibit anomalies close to T_C. At elevated temperatures no significant changes in Raman spectra were observed that can be attributed to the absence of any structural phase transition. Instead of expected anharmonic behavior, several librational and stretching modes of rigid BO6 units were found to be hardening below T_C, in the low temperature ferrimagnetic phase, suggesting the significant magnetoelastic coupling contributions to phonon frequencies. Magnetic excitation induced phonon renormalization is evident in the low temperature magnetic phase. The asymmetric stretching mode at 753 cm^-1 is found to have larger spin-phonon coupling contribution (λ ~ 2.93) while the lattice mode at 153 cm^-1 and external librational mode at 240 cm^-1 had the lowest effect (λ ~ 0.88). On Mn-doping, we observed exciting results with reduced leakage current. Current density has been reduced on doping Mn. The observation of multiferroicity in these single-phase materials at RT make them potential candidates for memory, multifunctional, and spintronics device applications. Highly oriented thin films of Ga_2-x(Fe_1-yMn_y)_xO₃, 0 ≤ y ≤ 0.02 and 1 ≤ x ≤ 1.4 were prepared by pulse laser deposition method on SrRuO₃ (SRO) buffered SrTiO₃ (111) substrates. The magnetic transition temperature (T_C) of thin films were found to be higher than corresponding ceramics. Ferroelectric properties were studied by PFM and P-E loops. Remnant polarization (P_r) of ~ 35 µC/cm² on pure GFO were observed which decreased on Mn and Fe substitution. The clear and reversible out-of-plane phase-contrast seen in the Piezoresponse Force Microscopy (PFM) phase and amplitude images measured within positive and negative poling confirms the polarization reorientation and hence piezoelectric nature of the compound. The temperature and frequency dependence of permittivity studies demonstrate the relaxor behavior of these thin films. The observed near room temperature (RT) magnetic phase transition with RT piezoelectric nature of these thin films elucidates the possible potential candidates for a multiferroic world with spintronics device applications. To study the optical properties of the compound, thin films were deposited on fused silica. Both direct bandgap and indirect bandgap were present and reduction in band-gap with increase in Fe concentration were observed. These observations of reduction in E_g with Fe addition and also observation of indirect band gap shows the potential application of the material in optical devices. We demonstrate the hybrid fabrication process of a graphene integrated highly transparent resistive random access memory (TRRAM) device. The indium tin oxide (ITO)/Al₂O₃/graphene nonvolatile memory device possesses a high transmittance of > 82 % in the visible region (370 - 700 nm) and exhibits stable and non-symmetrical bipolar switching characteristics with considerably low set and reset voltages (± 1 V). The vertical two-terminal device shows an excellent resistive switching behavior with a high on-off ratio of ~ 5 x 10³. We also fabricated ITO/Al₂O₃/Pt device and studied its switching characteristics for comparison and a better understanding of the ITO/Al₂O₃/graphene device characteristics. The conduction mechanisms in high and low resistance states were analyzed and the observed polarity dependent resistive switching is explained based on electro-migration of oxygen ions.