Probing Tunnel Barrier Shape and Its Effects on Inversed Tunneling Magnetoresistance at High Bias

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1 Journal of ELECTRONIC MATERIALS, Vol. 33, No. 11, 2004 Special Issue Paper Probing Tunnel Barrier Shape and Its Effects on Inversed Tunneling Magnetoresistance at High Bias WEN-TING SHENG, 1,2 W.G. WANG, 2 X.H. XIANG, 2 F. SHEN, 3 FEI-FEI LI, 1 T. ZHU, 3 Z. ZHANG, 3 ZHENG-ZHONG LI, 1 JUN DU, 1 AN HU, 1 and JOHN Q. XIAO 2 1. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing , Republic of China. 2. Department of Physics and Astronomy, University of Delaware, Newark, DE Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, Beijing , Republic of China. We have used an electron holography (EH) technique to directly probe the potential profile of tunnel barriers in magnetic tunnel junctions (MTJs). Barriers with under-, optimum-, or over-oxidized condition have been investigated. One important finding is that there is always slight oxidation of the top electrode because of film morphology. Sharp interfaces can be achieved in the bottom interface of optimally oxidized barrier or both interfaces in MTJs with under-oxidized barriers. We also demonstrate, theoretically and experimentally, how barrier shape affects the bias dependence and, in low barrier height case, result in inversed tunneling magnetoresistance (TMR) at high bias. The mechanism is very different from that responsible for inversed TMR in all biases. The finding leads to the possibilities of achieving better signals at high bias in real applications. Key words: Magnetic tunnel junction (MTJ), ZrO, inversed tunnel magnetoresistance (TMR), electron hologram INTRODUCTION The transport properties in magnetic tunnel junctions (MTJs) sensitively depend on the interface and barrier quality. 1,2 The common Al 2 O 3 barrier is typically formed by oxidizing around the 1.5-nm Al layer, and the barrier quality depends on the film roughness, grain size, and oxidation conditions. The barrier height and barrier thickness are often extracted from current-voltage curves using either Simmons or Brinkman et al. s model. 3,4 Erroneous data can sometimes be obtained because solutions to both models are not unique. In addition, only average barrier height can be obtained without any information on barrier shape. In this manuscript, we report the use of an electron hologram technique to measure the barrier potential profile. Over-, optimum-, and under-oxidized barriers were investigated. It is found that the barrier energy profile sensitively depends on the oxidation condition (Received May 14, 2004; accepted May 26, 2004) and film morphology. The technique is very useful in optimizing the insulating barrier to achieve the best magnetotransport properties. To study how barrier shape affects the transport behavior, we investigated inversed tunnel magnetoresistance (TMR) at high bias. Inversed TMR refers to the phenomenon of conductance in a parallel magnetization configuration (σ in MTJs is lower than that in an antiparallel magnetization configuration σ ). 5 7 The inversed TMR phenomenon is very rare and is only observed in Co/SrTiO/Co, 8 TiO/CrO, 9 and Co/AlO/TaO/Co 10 systems. It should also be noted that the first two material systems show inversed TMR in all bias voltages, and the last type of MTJs show inversed TMR at high bias. Based on the simplified Jullière model, 11 TMR 2P 1 P 2 /(1 P 1 P 2 ), where P 1 and P 2 are spin polarizations of the two ferromagnetic (FM) electrodes; inversed TMR can be observed if P 1 and P 2 have opposite signs. In this manuscript, we will point out that the energy-dependent density of states (DOS) is unlikely the only reason for inversed TMR at high bias. 1274

2 Probing Tunnel Barrier Shape and Its Effects on Inversed Tunneling Magnetoresistance at High Bias 1275 In addition, the mechanism for inversed TMR at high bias is also very different from that responsible for inversed TMR in all bias ranges. The inversed TMR at high bias is due to both DOS and barrier shape, which determine the slope of σ and σ, and cause the crossover between two conductances at high bias. EXPERIMENT Samples were prepared with a magnetron sputtering system. The barrier of one sample was fabricated by depositing a layer of 4.2 Å uniform aluminum layer and a layer of wedge-shaped zirconium. Another one was fabricated by depositing a wedgeshaped zirconium layer first and then a layer of 4.2 Å uniform aluminum. All samples were oxidized in an oxygen plasma at 60 mtorr for 45 sec. The typical structures of these samples are Si/FeNi (20)/FeMn (10)/Co (8)/AlO x /ZrO x /Co (16)/Cu (80) and Si/FeNi (20)/FeMn (10)/Co (8)/ZrO x /AlO x /Co (16)/Cu (80) (with numbers inside the parentheses being thickness in units of nanometers). Samples with FeNi electrodes were used for the electron holography (EH) study. The typical structure is Si/Ta (5)/FeNi (5)/FeMn (12)/FeNi (6)/AlO x /FeNi (6)/Ta (5), and here, the AlO x barrier is formed by oxidizing a wedge-shaped Al layer for 60 sec. All MTJs were patterned with a junction area of about µm 2. Transport properties were measured with a four-probe technique with positive bias referring to the current flowing from the top to bottom electrodes. A CM200 field-emission gun transmission electron microscope (FEM TEM) equipped with an electrostatic bi-prism and a multiscan charge device camera was used to characterize the microstructure. The off-axis EH was performed with a bi-prism voltage of 140 V. All holograms were recorded with the cross-sectional specimens plane perpendicular to the incident electron beam direction and were processed using Digital Micrograph (DM, Gatan, Inc., Pleasanton, CA) software including Holowoks package. As a special and complementary analytical method, the EH technique has been widely used. 12 The phase shift Φ is sensitive to electric and magnetic fields in the sample and, in the absence of dynamical diffraction and Fresnel contrast effects, is given by the following expression: 13 e Φ( xy, ) = CE Vxydz (, ) B n( x, y ) dxdz h Φ( xy, ) e = Btxy n (, ) x h (1) where C E is an interaction constant, V is the inner potential, and B n is the component of the magnetic field normal to the plane defined by the incident electron-beam paths, i.e., in the y direction. For a specimen of uniform composition and negligible thickness variation, the x gradient of the phase shift is given: (2) Therefore, the phase gradient will be zero if B n is zero. 13 Here, EH was used to study the barrier shape and other properties that are related to the inner potential variation across the MTJ multilayer. The contributions of the magnetostatic field must be removed carefully. Considering the design of the objective lens in our CM200-FEG TEM, the sample can be put where the sample plane was vertical to the optical axis in the experimental geometry; thus, the induced magnetic field (B n ) in the interesting sample area is zero. RESULTS AND DISCUSSION Figure 1 shows the room-temperature TMR ratio versus the nominal Al thickness in the FeNi/AlO x / FeNi achieved by oxidizing a wedge-shaped Al layer. With the Al thickness increases, the TMR ratio first increases, reaches a maximum, and then decreases again, indicative of junctions with over- to underoxidized barriers. 14 The maximum TMR corresponds to the optimum oxidization (marked by a solid arrow in the figure). Figure 2 shows the high-resolution transmission electron microscopy (HRTEM) cross-sectional image of an optimum-oxidized MTJ with a nominal Al thickness of about 1.3 nm. It can be seen that the bottom electrode has a very strong (111) texture, denoted by two pairs of white bars. Thus, the welltextured layer leads to a large average grain size. In contrast, the top electrode has a smaller average grain size with a random crystallographic orientation, also denoted by two pairs of rotated white bars. Figure 3a shows the hologram of a cross-sectional MTJ with an optimum-oxidized barrier. The bright, narrow band corresponds to the Al 2 O 3 barrier. The holographic fringes are shifted as they approach the two FeNi/Al 2 O 3 interfaces. The phase image shown in Fig. 3b was extracted from the hologram Fig. 1. The TMR ratio versus nominal Al thickness in the FeNi/ AlO x (t)/feni with a wedge-shaped Al layer oxidized by 60 sec.

3 1276 Sheng, Wang, Xiang, Shen, Li, Zhu, Zhang, Li, Du, Hu, and Xiao Fig. 2. The HRTEM cross-section image of the MTJ with an optimum-oxidized Al layer. The pairs of white bars denote the {111} planes of FeNi. A near layer-by-layer growth of the bottom electrode and a three-dimensional growth of the top electrode are shown. (Fig. 3a) using DM software. The average phase profile perpendicular to the barrier, as shown in Fig. 3c, can be achieved by averaging line profiles across the interfaces of FeNi/Al 2 O 3 /FeNi in the related phase image. Also in Fig. 4, the systematic phase-shift profiles were carried out on the MTJs with over-, optimum-, and under-oxidized barriers, corresponding to nominal Al thicknesses of 0.5 nm, 1.3 nm, and 1.7 nm, respectively. To convert the phase shift to the inner potential variation, the information on the sample thickness is crucial. The thickness profile across the interfaces has been characterized to be constant in our examined regions involved in EH. The phase gradient from the phase image in two vertical plane directions of the both FeNi layers in the MTJ is zero, indicating that the B n in Eq. 2 is zero. Then, the magnetostatic contribution can be neglected. Hence, the phase shift in Fig. 4 is proportional to the inner potential variation of MTJ, i.e., the phase shift represents the barrier shape. Defining the half maximum width in the phase shift profile as the barrier width, we obtained barrier widths of 2.6 nm, 1.7 nm, and 2.8 nm for over-, optimum-, and underoxidized barriers, respectively. The barrier width of 1.7 nm for the optimum-oxidized junction is the same as that extracted from the HRTEM image (Fig. 2). This is very reasonable by the 4/3 thickness expansion theory where a nominal Al thickness of 1.3 nm expands to an Al 2 O 3 thickness of 1.7 nm. In the case of the over-oxidized MTJ with a nominal Al thickness of 0.5 nm, the larger expansion is due to the fact that a part of both electrodes were oxidized with sufficient oxygen. For the under-oxidized MTJ with a nominal Al thickness of 1.7 nm, the barrier width of 2.8 nm is larger than the 2.1 nm extracted from HRTEM. This width difference is probably due to the charge accumulation effect that will be discussed later. From the EH results, we can find several features that are important to interpret transport properties. First, the top interface is less sharp than the bottom one in the optimum-oxidized MTJs. This phenomenon may arise from the different morphologies of the two electrodes revealed from HRTEM, as discussed before. The top electrode has smaller and randomly distributed grains, and thus, it is very easy for oxygen to back diffuse into the layer and subsequently oxidize a minuscule part of the top electrode. The second important finding is that both interfaces are actually sharp in MTJs with underoxidized barriers (Fig. 4c). This is related to the high electronegativity of Al that attracts oxygen very effectively. 15 When the Al layer thickness is less than 2 nm, a little oxygen can turn the whole Al layer into oxides with much less stoichiometry than Al 2 O 3, i.e., there is no metallic Al layer left, leading to the sharp bottom interface, and there is insufficient oxygen to back diffuse into the top electrode. a b c Fig. 3. (a) Hologram image of the MTJ with the optimum-oxidized barrier; (b) reconstructed image from hologram image (a); and (c) averaged phase-shift profiles perpendicular to the barrier layer.

4 Probing Tunnel Barrier Shape and Its Effects on Inversed Tunneling Magnetoresistance at High Bias 1277 Fig. 5. The variations of resistance (squares) and TMR (circles) at a bias of 4 mv as a function of nominal Zr-layer thickness in the Co/AlO x /ZrO x (t)/co MTJ. Fig. 4. Averaged phase-shift profiles perpendicular to the barrier layer in MTJs with (a) over-, (b) optimum-, and (c) under-oxidized barriers. Another finding is that a shoulder and a valley always appear at the sharp interface. This is possibly caused by the local charge redistribution resulting from the conduction mismatch between the metal and insulator. Because of the well-known electron screening effect, the electric field profile at the interface is modified, which consequently changes the electron phase shift. 16 To investigate inversed TMR, MTJs with AlO x /ZrO x were used. The resistance and TMR as a function of nominal Zr-layer thickness for Co/AlO x /ZrO x (t)/co are shown in Fig. 5, whose barrier was fabricated by depositing a layer of 4.2 Å uniform aluminum and a layer of wedge-shaped zirconium. Again, the optimum-oxidized junction shows maximum TMR (marked by a solid arrow in the figure). Below and above this thickness, the MTJs are over- and under-oxidized, respectively. The resistance shows exponential dependence on nominal Zr thickness. Figure 6 shows resistance as a function of applied field for an optimum-oxidized MTJ (d Zr nm) at three biases: (a) 500 mv, (b) 800 mv, and (c) 4 mv. Clearly, the TMR is inversed at high positive bias. Figure 7a shows the bias dependence of TMR of the optimum-oxidized junction (open circles) in which the barrier structure is AlO x /ZrO x. Apparently, the bias dependence is much stronger than the bias dependence in MTJs with a pure alumina barrier. Near the zero bias, the TMR is about 7%. With increasing positive bias, TMR first decreases to 0 at Vc 0.33 V and becomes negative at higher bias. With negative bias, TMR decreased to near zero but maintains positive values. Further increase of bias in both directions results in permanent damage of MTJs. If we reverse the deposition sequence between ZrO x and AlO x, as shown in Fig. 7b, the entire bias dependence curve is reversed, and the inversed TMR is observed at negative bias. This indicates that the inversed TMR is Fig. 6. Resistance versus applied magnetic field at biases of (a) 500 mv, (b) 800 mv, and (c) 4 mv.

5 1278 Sheng, Wang, Xiang, Shen, Li, Zhu, Zhang, Li, Du, Hu, and Xiao Fig. 7. The bias dependence of TMR in over- (square), optimum- (circle), and under- (triangle) oxidized MTJs with the structure of (a) Co/ZrO x /AlO x /Co and (b) Co/AlO x /ZrO x /Co. Fig. 8. The TMR as a function of the bias for MTJs with a square barrier. The parameters in the calculation are E F 5 ev, k F /k F 0.3, and barrier width d 1.5 nm. related to either the Co/ZrO x interface or ZrO x barrier properties. The inversed TMR could arise from the DOS at the Co/ZrO x interface if the spin polarization reversed the sign above the Fermi level 17 or from the barrier shape effect because ZrO x has a lower barrier height than that of Al 2 O 3. 18,19 By replacing the Co electrode with FeNi, we found very similar inversed TMR, which suggested that polarization inversion above the Fermi level is unlikely the reason. To use barrier height and shape effects to explain our results, we extended Slonczewski s model to high bias. 20 This model uses a free electron model with a parabolic electronic-band structure. We found that electron wavefunction matching at two interfaces are significantly affected by the barrier height and shape. This results in different bias dependences for σ and σ, such that σ is higher than σ for voltage biases above the critical bias, V c. This behavior also appears in MTJs with an Al 2 O 3 barrier (Fig. 8), except the V c is above the breakdown voltage of Al 2 O 3 and, thus, will never be observed. Although the barrier height and shape play an important roll in the inversed TMR, it should also be pointed out that energy-dependent DOS also have strong effect on bias dependence because of the parabolic band structure. The importance of the DOS effect can be seen for the parameters used to obtain Fig. 8. The values of k 2 F /k 2 0.7, 0.6, 0.5, and 0.4 correspond to barrier heights of 6.2 ev, 5.7 ev, 5.2 ev, and 4.7 ev, respectively. According to this trend, it is expected that, in the case of Al 2 O 3 whose barrier height is 2.5 ev, the TMR would switch sign at approximately 400 mv. The discrepancy comes from the DOS because an unrealistic parabolic DOS was used. The exchange splitting between spin-up and spin-down bands was assumed to be 4.55 ev in order to give a spin polarization of about 54%. Therefore, the bottom of the spin-up electron band is very close to the Fermi level (E F 5 ev). Consequently, the DOS have strong bias dependence, resulting in rapid decrease of TMR and, ultimately, a lower transition bias. The detailed calculation can be seen in our theoretical work. 20 It can be concluded that both the energy-dependent DOS and the barrier height determine the bias dependence of the TMR. The former has more influence on the slope of the conductance curves, and the later mainly affects the vertical shift of the curves and also modifies the slope. These two effects lead to the spin polarization changing sign at transition bias. SUMMARY The barrier shape profile in MTJs has been directly observed by the EH technique, which reveals several interesting phenomena. One in particular is that a sharp interface between the bottom FM electrode and the barrier can be achieved in the optimum-oxidized MTJs, whereas there is always slight oxidation of the top electrode; this is due to the smaller average grain size in the top electrode, allowing oxygen to diffuse in. Another observation is that both interfaces appear to be sharp in MTJs with an under-oxidized barrier. In addition, charge accumulation seems to exist at the sharp interfaces. We also observed inversed TMR at high bias in MTJ with the AlO x /ZrO x barrier and proposed a theoretical model based on electron wavefunction matching at the interface to explain this phenomenon. It is found that both DOS and barrier height contribute to the bias dependence of TMR, and the major contribution to the sign change of TMR is due to the low barrier height of ZrO x. ACKNOWLEDGEMENTS This work is supported by NSF Grant No. DMR Wenting Sheng thanks the support from the National Sciences Foundation of China under NNSF Grant No and the State Key Project of Fundamental Research 001CB

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