Study of the magneto- and electro- properties in an all-oxide junction device

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Study of the magneto- and electro- properties in an all-oxide junction device


Author: Li, Ming-kei
Title: Study of the magneto- and electro- properties in an all-oxide junction device
Degree: M.Phil.
Year: 2011
Subject: Diodes, Semiconductor.
Metal oxide semiconductors.
Thin films -- Electric properties.
Thin films -- Magnetic properties
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xiv, 103 leaves : ill. (some col.) ; 30 cm.
InnoPac Record:
Abstract: This project attempts to realize an all-oxide junction device that integrates the electric field modulation of a p-n junction diode with the magnetic field modulation of a spin valve in the same unit. This study envisages manipulation of the spin variable of electrons to control their transport in a p-n junction, in contrast to conventional electronics, which use only the charge property. Theoretically, there are certain advantages of spintronics over electronics, some of which emanate from longer spin relaxation lifetimes and diffusion lengths as compared to charge momentum relaxation times and lengths. These advantages, however, can only be harnessed if the corresponding devices can be realized in practice. The physical characteristics of oxide systems cover a broad range of interesting properties, such as ferroelectricity, high permittivity, piezoelectricity, photoelectricity, superconductivity and magnetoresistance, which have attracted the attention of technologists for quite some time. Many oxides are intrinsic semiconductors or can become good semiconductors by proper doping or by variation of their composition. In our study, we focus primarily on the exploration of thin films and device configurations involving ferromagnetic oxides with high spin polarization. Indeed, ferromagnetic oxides such as manganates have been used to make spin valves in recent years. At the same time, there is considerable emphasis in materials research on tailoring the magnetic properties of functional nonmagnetic materials by introducing a dilute concentration of magnetic impurities therein. This material is thereby a potential candidate in spintronics with the benefits of magnetic response and the application bearing attractive properties of the non-magnetic host. At the beginning of our project, La₀.₇Sr₀.₃MnO₃ (LSMO) and cobalt doped titanium dioxide were identified as suitable materials for our study. Heteroepitaxial junctions formed by p-type strontium doped lanthanum manganite and n-type cobalt doped titanium dioxide were fabricated on LAO (100) substrates by pulsed laser deposition. The layers were grown at 650℃ and under 150 mTorr ambient oxygen pressure. The CoxTi₁-xO₂ [x = 0.05 and 0.1] (CTO), which, at anatase phase, was reported as a wide-band-gap dilute magnetic semiconductor, were deposited on the LSMO film surface at 600oC with an ambient oxygen pressure of 20 mTorr. The as-grown CTO films exhibited pure anatase crystalline phase and semiconductor-like conduction. Under optimized fabrication conditions the CTO/LSMO junction revealed a heteroepitaxial relationship of (004)CTO││(001)LSMO││(001)LAO. Electrical characterization of these p-n junctions yielded excellent rectifying characteristics with a current rectifying ratio over 1000 at room temperature. The electrical transport across these diodes was dominated by diffusion at low current (low bias voltage) regime and by recombination at high current (high bias voltage) regime.
Our results have demonstrated an all-oxide junction diode with good transport property. The simultaneous of electrical and magnetic modulation in a diode junction is therefore potentially realizable. LMSO showed room temperature ferromagnetism and metallic-like electrical conduction with a resistivity of 0.015 ohm cm at 300 K. Nevertheless, since the magnetization of CTO is very close to the detection limit of our equipment, we have yet to demonstrate the simultaneous electric and magnetic modulation with this combination. Therefore, we have explored other n-type ferromagnetic oxide as an alternative to CTO. Carbon doped TiO₂ was one of the candidates investigated in our study. It is predicted that ferromagnetism at room temperature can occur in titanium dioxide with this non-magnetic dopant, carbon. As we have prepared specimen with different concentration of carbon, it was confirmed that they all showed ferromagnetic property under the vibrating sample measurement. On the other hand, the conductivity of carbon doped TiO₂ was improved at least two orders of magnitude when compared with pure TiO₂. A further step is to look into the possibility of formation of p-n junction by using carbon doped TiO₂ and LSMO. Other than diluted magnetic semiconductors, cobalt ferrite CoFe₂O₄(CFO) has also been studied successfully. We have demonstrated that CFO can be epitaxially grown on LSMO. Double coercivity of CFO/LSMO was observed. It indicated that the junction was magnetized in two stages. The magnetoresistance response of this combination has been evaluated. As the magnetic field was decreased from 5000 to -80 G, the resistance of these heteroepitaxial junctions increased MR ratio = 6%. These changes are corresponded to the switching of LSMO magnetization, which gives result in the change of parallel to anti-parallel magnetization in the CFO and LSMO layer.

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