| Random access memory (RAM) technology has been developing fast, propelled by the progressing of semiconductor technology, information technology and the never-stopping human needs. For RAM exchange data with CPU directly, the property of RAM has great influence on the data processing speed of CPU, then the performance of entire computer. Currently, RAM of computer is based on capacitor technology, and for the reason of leakage current, RAM needs refresh to against data loss. This kind of easily volatile RAM can’t fulfill human needs anymore. Non-volatile RAM draws more and more attention. Indeed, in 2010, the International Technology Roadmap for Semiconductors (ITRS), Emerging Research Devices and Emerging Research Materials Working Groups identified spin transfer torque magnetic random access memory (STTMRAM) and resistive random access memory (RRAM) as two emerging memory technologies recommended for promising non-volatile RAM.In a sandwich structure made by a fixed magnetic layer, a free magnetic layer and a middle non-magnetic layer, when the density of current circuiting from fixed magnetic layer to free magnetic layer exceed a critical value, the magnetic moment of free magnetic layer flips to the direction of magnetization of the fixed magnetic layer; when current flows backward, the electrons scattered back by fixed magnetic layer make free magnetic layer magnetize antiparallel to fixed magnetic layer. This effect is spin transfer torque (STT). The sandwich structure with different magnetization configuration of these two magnetic layers has different resistance:when two layers magnetized parallelly, the resistance of the sandwich structure is smaller than that when two layers magnetized antiparallelly. As a result, STT effect could be used to achieve logic states switching. Furthermore, magnetizations of two magnetic layers don’t change when the current is turned off. Therefore, STTMRAM is non-volatile.Since the critical current density of STT is proportional to the Gilbert damping factor of the free magnetic layer, a free layer with low Gilbert damping factor is important to make the STT device more energy-saving and higher integration. Co based full-Heusler compounds Co2XY (X is 3d transition element, and Y is main element) show prominent prospects in spintronics, because of their theoretical half-metal states, experimental high tunneling magnetic resistance applications, and high Curie temperature. Besides, these compounds have a low Gilbert damping constant. We deposited Co2MnSi thin film on high temperature MgO substrate with magnetron sputtering, and increased Co2MnSi crystal structure by manipulating depositing temperature and post annealing temperature. Fittings to ferromagnetic resonance (FMR) data demonstrate that the Gilbert damping factor of sample deposited at 300℃ and subsequently annealed at 550℃ is 0.00015, which is the smallest value among the metals.It is self-evident that precise measurement of Gilbert damping factor is important to both fundamental physics and applied physics. FMR is a common way to measure Gilbert damping factor by analyzing linewidth with respect to measuring frequency. Spin wave excited by FMR has wave vector k=0. These spin wave, on one hand, directly dissipate energy to lattice system though magnon-electron interaction, called Gilbert damping, and on the other hand it could also be scattered to degenerate k≠0 spin wave, namely, two-magnon scattering, by magnetic anisotropy, voids, second phase, surface defects, etc., and then dissipate energy to lattice system at last. Therefore, FMR linewidth contains Gilbert damping contribution and two-magnon scattering contribution at least. Many reference fitted out Gilbert damping factor without considering two-magnon scattering contribution, which is advisable only when measured linewidth is a linear function of measuring frequency in a broad band measurement. Linewidth non-monotonic behavior with respect to measuring frequency is rarely reported, and none are reported in broad band measurement. We found linewidth non-monotonic behavior in samples deposited at 300℃,400℃ and subsequently annealed at 550℃ in broad band measurement of 5-40 GHz. Based on previous work by Sangita et al., we developed two-magnon scattering model, and fitted different type linewidth of Co2MnSi thin film, from non-monotonic to linear with respect to measuring frequency, by numerical calculation and multiple linear regression without fixed parameters. Fitting results show that two-magnon scattering play a key role in non-linear linewidth, while in the case of linear linewidth, two-magnon scattering is so weak that direct fitting with Gilbert damping only is valid.Organic materials, because of the advantages such as low cost fabrication, printability, high mechanical flexibility and environment friendly, are being successfully used in organic light-emitting display (OLED), field effect transistor and solar cell fields. Recently, the organic memory devices have inspired more and more attention for their potential use in substituting inorganic ones to satisfy the demand for inexpensive and high density data storage media, which could be used in RRAM. Many organic materials, including small molecular, polymer and composite materials, have achieved memory effect. Organic semiconductor, not like their counterpart inorganic semiconductor which had a long-range order and strong coupling between the lattices, had a weak Van Der Waals interaction between unit cells leading to the strong localization of the charge carriers. The charge carriers could not contribute to the conduction until they hopped to another unit cell by absorbing enough phonon energy, which resulted in the low mobility of organic materials. Normally, the conduction mechanism in organic materials is explained by intrinsic charge carriers which are generated by organic materials itself and charge carriers injected to organic materials from electrodes. A lot of conduction mechanisms have been proposed to explain the conduction process in organic materials such as Ohmic conduction, thermionic emission, Schottky emission, tunneling current, space charge-limited current, hopping conduction, ionic conduction, and impurity conduction, etc. Resistance switching mechanisms based on these models are also of diversity. A uniform resistance switching mechanism may not exit for all kind of device structure and materials, indicating this field still needs further study.Copper phthalocyanine (CuPc), a prototypical conjugate p-type organic semiconductor, and is widely used in OLED, organic thin film transistor, and organic photovoltaic cell, while little attention has been paid on resistance switching and memory effect. We have found high ON/OFF ratio multi-polar resistance switching effect in CuPc junction with different electrodes fabricated by magnetron sputtering and thermal vaporization. When top and bottom electrodes both are Co, the resistance switching is unipolar, and the stability of low resistance state is poor and it will switch to high resistance state after a period of time, depending on the thickness of CuPc. But with Au being bottom electrode, the resistance switching is bipolar, and low resistance state is much more stable, and no evident resistance degrading is noticed in measuring time of 10000 seconds. The resistance-temperature (R-T) measurement of both low resistance state and high resistance state demonstrate that conduction mechanism doesn’t change much between them. The resistance of both states increases with the decreasing of temperature. Both R-T curves fit well with Mott variable-range hopping with γ=1/3 at low temperature, while deviate much at high temperature. Fitting results indicate that the difference of density of states at Fermi level between low resistance state and high resistance state leads to discrepancy of resistance. R-T measurement and the varying pattern of resistance of low resistance state with respect to junction size rule out filament resistance switching mechanism. Base on measurement results and the properties of CuPc thin film, we believe that trapping and detrapping of holes in CuPc trap states leads to resistance switching. Although Al2O3 between CuPc and top electrode and dipole states between CuPc and Au have no contribution to resistance switching, they do mean a lot to the stability of low resistance state. |
PDF Full Text Request