Electrical control of magnetism in oxide semiconductor devices

Abstract

The use of nanotechnologies has allowed the creation of the memristor, the fourth two-terminal electronic component, besides the resistor, the capacitor and the inductor. The memristor is a bipolar device whose conductance can be changed by flowing charge through it. The first application of this electronic device is expected to be in data storage. This device would represent the elementary bit-cell in non-volatile memories based on Silicon technology, and even bears striking resemblance with a biological synapse. In this thesis, we have developed memristors based on oxygen-deficient semiconductor oxides. We first carried out a systematic characterization on several sets of oxide films grown in different conditions by pulsed laser deposition (PLD). This is because the electrical, optical, and magnetic properties of these compounds are strongly dependent on the stoichiometry and the crystal quality. The studied system for memristive switching consists of semiconductor film sandwiched between a top and a bottom conductive electrode to allow the current to flow uniformly perpendicular to the film. A flow of electrical current produces a change in the resistance, as well as in the magnetic moment of the film. Micro- and nano-devices were also fabricated by lithography and patterning. The effect of current flowing on the adaptive electrical and magnetic properties of the devices was investigated. In particular, the characterization of zinc oxide (ZnO) films and nickel oxide (NiO) films suggests that two different mechanisms can dominate in different systems, namely interface-type and filamentary-type conduction, both involving oxygen vacancy migration. In the first chapter a review is given of the literature of the memristive systems. The natural candidates for magnetically-active ionic memristors are ferromagnetic oxide semiconductors, which are low-cost and suitable for fast integration with the current CMOS technology. In particular, NiO, which is an antiferromagnetic oxide, and the wide band-gap semiconductor ZnO, that can be doped with low concentrations of 3d transition metals to become ferromagnetic. These oxides all show memristive behavior, either filamentary-type or interface-type. In the second chapter, we report on the growth and characterization of undoped, 2% and 4% Mn-doped ZnO films. A systematic comparison is presented among three sets of films grown by pulsed laser deposition from targets prepared under the same conditions. We show that ferromagnetic ordering in these compounds is directly related to the concentration of oxygen vacancies. The incorporation of Mn dramatically increases the magnetic moment in highly oxygen-deficient films. By analyzing the photoluminescence spectra and the structural properties of the films, we can exclude that magnetic ordering in our films originates from unintentional doping. In the third chapter we report our study on resistive memory switching with high endurance in 2% Mn-doped ZnO films and devices. Devices were fabricated, with lateral size ranging from 400 nm to 5 mm, that show a bipolar switching with a threshold current of ~ 102 A/cm2, independent on the device size, and linear area dependence of the resistance. By fitting the current-voltage characteristics we conclude that in Schottky contacts to oxide semiconductor films with a very high degree of crystallinity, oxygen vacancies can be trapped at a Schottky interface by the built-in electric field, which results in an interface-type memristive switching. Similar results were also obtained by using pure ZnO and 4% Mn doped ZnO films. In the fourth chapter we show that, in magnetic dilute ZnO, the change of electrical conductance is accompanied by a change of the magnetic moment. Our system behaves as a memristor, but the flow of electric charge induces a change of the magnetic proprieties, besides, or in alternative to, a change of the electrical conductivity. The fifth chapter focuses on bipolar resistive memory switching with high endurance and long retention in NiO films. Devices were fabricated, with lateral size ranging from 16 um to 200 um, that show a threshold current density ranging from ~102 A/cm2 to ~104 A/cm2, dependent on the device size, and nearly the same resistance, which is consistent with the conducting filament formation/rupture mechanism. Like for doped-ZnO, we found that the bistable switching of the resistive state was accompanied by a bistable switching of the magnetic moment at room temperature. Finally, the last chapter deals with the magnetic control of the conductance of ionic memristors. The last functionality is added to our ionic memristors: the adaptation to an externally applied magnetic field, which is an important function of a synapse. Memristors based on oxide semiconductors showed a change of conductance when an external magnetic field is applied. Magnetoresistance (MR) in ZnO films with different Mn doping was measured at different field values and temperatures. The MR is positive at low fields and negative at high fields at low temperatures, which might be used to model the behavior of a synapse in magnetic field. In summary, we report memristors that can change their conductance in response to a magnetic field, as well as a charge flow. The devices still make use of low-cost oxide semiconductors that can easily be integrated with the current Silicon-based technology. The devices have been fabricated and characterized. The effect of an externally applied magnetic field on the adaptive electrical properties of the devices has been investigated.

Date of Award	2 Oct 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Antonio RUOTOLO (Supervisor)