Gallium oxide as an insulating barrier for spin-dependent tunneling junctions

Abstract

Spin-dependent tunneling has been shown to occur through Ga 2 O 3 as the insulating tunnel barrier. Magnetic tunnel junctions of the type Co/Ga 2 O 3 /Ni 80 Fe 20 were prepared by oxidizing thin metal layer of Ga in oxygen plasma and characterized. The highest junction magnetoresistance observed was 18.2% at room temperature, increasing to 27.6% at 77 K. The average barrier height was estimated to be about 2 eV, as opposed to over 3 eV for junctions with Al 2 O 3 barrier of comparable quality. Otherwise these junctions behave similar to those with Al 2 O 3 . This shows the feasibility of obtaining lower resistance junctions with Ga 2 O 3 as the barrier for magnetic storage applications. © 2000 American Institute of Physics. ͓S0003-6951͑00͒04348-5͔Tunneling studies in magnetic tunnel junctions ͑MTJs͒ are being actively pursued in the last few years for their fundamental complexity as well as their application potential in data storage industry.1-3 It has been shown that the quality of the tunnel barriers plays a critical role in the success of spin tunneling in MTJs.1 Aluminum oxide is widely used as the insulating barrier since Al metal film can be reproducibly grown as ultrathin layer. [4][5][6][7] The excellent insulating properties of ultrathin Al 2 O 3 as well as its large forbidden gap, in general, lead to high resistance-area ͑RA͒ product, which is unfavorable from the application point of view. Recently there are reports wherein MTJs having lower RA values were obtained, using ultrathin Al 2 O 3 barriers.6-8 Also, from the fundamental physics viewpoint, there are theoretical predictions and some experimental interpretations of barrier dependence of spin polarized tunneling. 9 The motivation and necessity to study alternative tunnel barrier materials that show good spin tunneling properties is thus apparent. In this letter, our successful study of spin tunneling with Ga 2 O 3 as tunnel barrier, in MTJs, is described. Ga 2 O 3 is an insulator with a forbidden gap of 4.8 eV and is the only reported stable phase.10-12 However, there have been studies about the nonstoichiometric compound formation. Ga 2 O 3Ϫx becomes n-type semiconductor due to oxygen vacancies, which act as shallow donors with activation energy of 30-40 meV.11 Ga metal can be readily oxidized, relatively slower than Al 2 O 3 , having a heat of formation energy ͑kJ/mol͒ of 1080.2 compared to 1675.6 for Al 2 O 3 . 10MTJs were prepared in a high vacuum system ͑base pressure of 10 Ϫ8 Torr͒ thermal evaporation system. 1 Initially the LN 2 cooled glass substrate was covered with a 1 nm thick Si seed layer, followed by an 8-nm-thick Co electrode of long strips. Shadow metal masks were used to define the film and the junction pattern. A gallium film with the thickness ranging from 0.4 to 1.8 nm was deposited to cover the entire Co layer. After the substrate was warmed to room temperature ͑RT͒ the Ga layer was oxidized in oxygen glow discharge plasma at a pressure of 80 mTorr and a dc voltage of 1.6-1.8 kV for varying time ͑5-110 s͒. After pumping dow...

High-pressure behavior of β-Ga2O3 nanocrystals

“…[1][2][3][4][5] Recently, considerable effort has been devoted to the study of nanostructured ␤-Ga 2 O 3 because of the potential for applications in nanodevices. [6][7][8][9][10] In ambient conditions, monoclinic ␤-Ga 2 O 3 is the thermodynamically stable form and there are other four metastable structures: ␣-, ␥-, ␦-, and -Ga 2 O 3 .…”

Origin of temperature dependence in tunneling magnetoresistance

“…The contribution from these channels grows with the number of thermally excited magnons, more so for the antiparallel (AP) configuration, with the consequent overall increase in conductivity and corresponding loss in TMR with increasing temperature [7,8]. In a different approach, Julliere's model was extended [9] to include both a temperature-dependent surface spin polarization, and a spin-independent current channel, assumed to arise from hopping within the barrier [10][11][12]. Other proposed mechanisms include barrier defect and/or magnetic impurity scattering [13][14][15][16] and band structure effects [17].…”

Deposition of Ga2O3−x ultrathin films on GaAs by e-beam evaporation

“…The Ga 2 O 3 /GaAs interface seems to be robust. 3 It has been suggested that optimal tunneling properties may be found with thinner junctions and upon annealing 1,7 in ferromagnet/insulator/ferromagnet structures. This is due to competing effects, namely oxidation of the neighboring layers and the formation of metallic paths in the junction layer.…”

“…Ultrathin films of Ga 2 O 3 have attracted interest for use in applications including spin-dependent magnetic tunnel junctions 1 and compound semiconductor passivation. 2,3 Fabrication of these and similar Al 2 O 3 films has been accomplished by oxidation of metallic layers in air 4 and by plasma.…”