Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Film growth of TM-doped ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Characterization techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Local structure and ferromagnetism of TM-doped ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Effect of doping concentration on structure and magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.1. Effect of doping concentration on magnetic ordering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.2. Formation of a secondary phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 and Cu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Materials Science and Engineering R 62 (2008) 1–35 TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier- mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics. � 2008 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Materials Science and Engineering R 3.8. Comparison of ZnO doped with Sc, Ti, V, Cr, Mn, Fe, Co, Ni 4. Origin of ferromagnetism in TM:ZnO . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Substrate effects on magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.1. Substrate-dependent local structure and magnetic behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.2. Substrate orientation-induced distinct magnetization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3. Effects of substrate temperature and film deposition rate on magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4. Influence of oxygen partial pressure on magnetic ordering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.5. Strain-induced ferromagnetism enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.6. Influence of post-annealing on magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.7. Effect of co-doping on magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 ferromagnetic ordering of Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films F. Pan *, C. Song, X.J. Liu, Y.C. Yang, F. Zeng Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China A R T I C L E I N F O Keywords: Diluted magnetic oxide ZnO Local structure Defects Spintronics A B S T R A C T This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent journal homepage: www.elsevier .com/locate/mser 4.1. Computational work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.2. Experimental work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.2.1. Secondary phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.2.2. Carrier exchange interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 * Corresponding author. Tel.: +86 10 62772907; fax: +86 10 62771160. E-mail address: panf@mail.tsinghua.edu.cn (F. Pan). 0927-796X/$ – see front matter � 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mser.2008.04.002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 aterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 junctions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 d ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 >300 K). Second, it is a major advantage if there is existing technology for the material in other applications [14]. Fortunately, mecha spintronics, but also owing to the excellent physical properties. So far, RTFMhas commonly been obtained in ZnO systems dopedwith TM elements such as Sc [23,24], Ti [23,25], V [26–28], Cr [11,23,29,30], Mn [31–40], Fe [41–43], Co [8,23,44–61], Ni [62,63], and Cu [64–68], and co-doping such as CoFe [69,70] and and Engineering R 62 (2008) 1–35 intensively investigated to achieve magnetic ordering above RT and promising magneto-transport properties [11,14,16–20]. As a low-cost, wide-band gap (Eg = 3.37 eV) semiconductor, ZnO itself has been the focus of renewed research for applications such as UV light-emitters, transparent high-power electronics, surface acous- tic wave devices, piezoelectric transducers and window materials for display and solar cells [21,22]. Recently, a surge in research into TM:ZnO DMOs has been observed, as shown in Fig. 1, indicating that this area is attracting Fig. 1. Publications per year on TM:ZnO DMOs according to the Web of Science: increa tical prediction of RTFM ascribed to a carrier-mediated nism in ZnO-based DMOs [9], TM-doped ZnO has been TM-doped ZnO satisfies these two criteria. Following the theore 4.2.3. Defect-based BMP . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Effect of bandgap on the Curie temperature . . . . . . . . . . . . . 5. TM-doped ZnO-based prototype spintronics and multi-functional m 5.1. Co-doped ZnO-based magnetic tunnel junctions . . . . . . . . . 5.1.1. Spin-polarized transport in (Zn,Co)O/ZnO/(Zn,Co)O 5.1.2. Anomalous TMR in (Zn,Co)O-based junctions . . . . 5.2. TM-doped ZnO-based spin field-effect transistors . . . . . . . . 5.3. Ferroelectricity and giant piezoelectric d33 in V and Cr-dope 6. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction A diluted magnetic semiconductor (DMS) is a compound with properties intermediate between a non-magnetic semiconductor and amagnetic element, and is obtained by doping a non-magnetic semiconductor with transition-metal (TM) elements (e.g., Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu), including TM-doped III–V (GaAs and InAs) [1–4], II–VI (CdTe) [5] or group IV (Ge and Si) types [6]. However, most DMSs have a low Curie temperature (TC), which limits their use in practical applications [7,8]. In 2000, Dietl et al. [9] theoretically predicted that the TC of DMSs could be increased above room temperature (RT) for p-type DMSs, and ferromagnet- ism (FM) was stable in DMSs based on wide-bandgap semicon- ductors, i.e., ZnO and GaN. Using first-principle calculations, Sato and Katayama-Yoshida [10] theoretically demonstrated that a ZnO matrix doped with TM atoms such as V, Cr, Fe, Co and Ni exhibited FM ordering, whereas doping with Ti and Cu resulted in a paramagnetic state, which opened a window for experimental attempts to prepare DMSswith room temperature ferromagnetism (RTFM). FM semiconductors with O2� anions, such as TM-doped ZnO, are also termed diluted magnetic oxides (DMOs). Soon after that, consecutive experimental reports revealed that TM-doped ZnO and TiO2 exhibited intrinsic RTFM [11,12]. These observations prompted extensive experimental work and theore- tical studies on RTFM DMOs, driven both by an urge to understand the mechanisms involved and by the demand for better materials. In fact, interest in TM-doped ZnO has increased because of promising applications in the field of semiconductor spintronics, which seeks to extend the properties and applications of established electronic devices by using the spin of electrons in addition to their charge [7,13–15]. Electron spin can potentially be used to provide efficient injection of spin-polarized carriers for spintronics and to fabricate transparent magnetic materials for spintronics [16–22]. There are two major criteria for selecting the most promising materials for semiconductor spintronics. First, the intrinsic FM should be retained at practical temperatures (i.e. F. Pan et al. /Materials Science2 sing interest not only because of promising applications in MnCo [71,72], for which Co- and Mn-doped ZnO are the most popular systems. Many groups have reported that TM-doped ZnO films exhibit FM ordering at a TC much lower than RT, such as 83 K [73] and 110 K [74] forMn-doped ZnO.Many previous publications indicate that TM ions at Zn sites is a necessary but not sufficient condition for FM in true TM:ZnO DMOs. Some ZnO films doped with Ti [75], V [27], Mn [42], Co [76,77] and Cu [78] only exhibited paramagnetism or superparamagnetism as well as spin glass behavior [79]. Although a considerable amount of experimental data and corresponding mechanisms has been accumulated since the first report of RTFM [11,16–20], the origin of FM ordering in DMOs, including TM-doped ZnO, is still a matter of debate, i.e., whether carrier-mediated exchange based on TM2+ replacement of Zn2+ [16,17] or the formation of secondary phases [18,69,80] is involved. Thus, it is critical to distinguish true FM semiconductors from those that merely show magnetic hysteresis. Even more ambiguously, distinct results are frequently obtained in rather similar TM-doped ZnO films in the case of Co2+ substitution for Zn2+ [11,81,82], indicating that the magnetization of TM-doped ZnO is strongly dependent on the preparation parameters, and that the preparation process is not easily reproduced from one growth run to another. In this rather contradictory context, several experiments and calculations have been performed to elucidate the nature of RTFM http://apps.isiknowledge.com/. and [8,31,44,83]. Mechanism proposed include a bound magnetic polaron (BMP) mechanism involving defects, including oxygen vacancies (VO) and Zn interstitials (Zni) [8,44,57], hole doping involving Zn vacancies [34,49,84], and the formation of FM secondary phases (e.g., Mn2�xZnxO3�d via Zn diffusion into the Mn oxide [31]) and TM-rich nanocrystals [83]. Moreover, ab initio calculations predicted that competition between FM and anti-FM coupling existed in DMOs [84]. However, there is ongoing debate about these systems. In addition, a trace amount of an external pollutant would also lead to a hysteresis loop during magnetiza- tion measurements, further adding to the difficulties in determin- ing true DMOs [20,85,86]. So far, publications, including reviews [16–20], have commonly focused on a summary and comparison of DMOs obtained by different groups and the different experimental conditions used (e.g., preparation methods and parameters). The resulting macroscopic magnetic properties, which are critically dependent on growth and processing conditions, are diverse without any common characterizations. It is therefore difficult to identify a consistent mechanism to effectively understand the origin of FM ordering merely by comparison of experimental results achieved by different groups. Half a decade has already passed since the first report of RFTM for TM-doped ZnO. Efforts are increasingly shifting not only to the rational design of methods to evaluate this system, but also to the development of TM-doped ZnO-based prototype spintronics [14,85]. When successfully combined with semiconductor func- tionalities, spintronics will have a considerable impact on future electronic device applications [15]. Spin injection from a TM-doped ZnO layer to pure ZnO and other conventional semiconductors is an important long-term goal of the continuing research into ZnO DMOs [87,88]. This concept has been realized in a representative spintronics application, TM:ZnO-based magnetic tunnel junction (MTJs), which exhibit superior spin-polarized transport, such as a large half voltage (V1/2, at which the tunnel magnetoresistance ratio becomes half of the maximum) [88,89], compared to conventional magnetic metals and magnetic TM-oxide-based MTJs with AlOx and MgO barriers [90]. Another typical application is the spin field effect transistor (spin-FET) proposed by Datta and Das [91]. The core idea of this device is to induce spin precession by the Rashba spin–orbit interaction in a two-dimensional electron gas and to use spin-dependent materials. Although a working prototype of the Datta–Das spin-FET has not yet been fabricated, the relevant conditions necessary for the desired spin-FET operation and simplified devices have been obtained [92,93]. In this review, to study the nature of FM ordering at high TC, our main aim was to extract a single variable from a series of experiments. An example is substrate variation [94], which is of great interest owing to its potential to provide direct information regarding the factors that affect FM ordering. Moreover, since ZnO is a multi-functional material, recent significant advances for this system are described, including ferroelectric, piezoelectric, optical, and electric properties, which play important roles in spintronics applications of TM-doped ZnO [21].We summarize recent progress in TM-doped ZnO-based prototype spintronics, such as MTJs and spin-FETs. It should be pointed out that it is almost impossible to describe all the work performed around the world owing to the rapid pace of studies in this field, and therefore some references may have been overlooked. 2. Experimental procedure 2.1. Film growth of TM-doped ZnO TM-doped ZnO films are commonly deposited by pulsed laser F. Pan et al. /Materials Science deposition (PLD) [11,33,41,46–49,95], magnetron co-sputtering, including direct current (dc) reactive [44,45,94] and radio- frequency (rf) [32,96–98], combinatorial laser molecular-beam epitaxy [50,51], chemical vapor deposition (metal–organic [34,52,53,99,100], ultrasonic-assisted solution [57] and plasma- enhanced [101]), ion-beam implantation [35,42], ion-beam sputtering [54,55], and sol–gel methods [39,56]. Although the magnetic behavior of TM-doped ZnO films is sensitive to the deposition conditions [16–18], there is no conclusion on which deposition method is best for FM ordering of the films. For example, Co-doped ZnO films deposited by PLD [11,77] or magnetron sputtering [74,94], two widely used deposition methods, could alternatively exhibit RTFM and RT paramagnetism. Therefore, magnetron sputtering is increasingly popular for growing TM-doped ZnO films owing to its low cost, high efficiency, and easy control, and its production of uniform films of large size. The substrate generally used is Al2O3(0 0 1) for low mismatch between the film and the substrate (2%) after a 308 in-plane rotation [21], which is suitable for ZnO films exhibiting high crystallinity. Some other substrates suitable for the deposition of TM-doped ZnO films are Si [37,58,62,67,102], fused quartz [33], glass [54,68,103,104], ZnO [51,105], ScAlMgO4 [106], LiNbO3 [44], LiTaO3 [107], MgO [40], SiO2 [108] and NaCl [94]. Fig. 2 presents typical cross-sectional high-resolution transmission electron microscopy (HRTEM) images of Co-doped ZnO films grown by dc reactive magnetron co-sputtering on Al2O3(0 0 1), Si(1 1 1), SiO2(1 0 1) and LiNbO3(1 0 4) substrates. In contrast to the epitaxial growth of Co-doped ZnO film on Al2O3 substrate (Fig. 2a), the films deposited on Si, SiO2 and LiNbO3 exhibit an (0 0 2) preferred orientation with structural defects (e.g., edge dislocations, marked by ?) in Fig. 2b–d. Large- and small-angle grain boundaries are observed in the films on Si (Fig. 2b)/LiNbO3 (Fig. 2d) and SiO2 (Fig. 2c), respectively. Moreover, the (0 0 2) planes are not parallel to the surface of the LiNbO3 substrate; instead, the pillar-like grains tend to grow in two directions, as highlighted by the arrows in Fig. 2d. These observations reveal that the substrate affects the microstructure and the formation of defects in TM-doped ZnO film, which are expected to play an important role in FM ordering in this system [94]. For PLD and rf magnetron sputtering techniques, films were deposited using a ceramic target prepared using standard ceramic techniques fromZnO and TMOor TM2O3 powders of 4N purity [23]. For example, ZnO and MnO2 were mixed together, ground and calcined for 8 h at 400 8C and sintered at 600–900 8C for 12 h in air to obtain