Room-temperature ferromagnetism in amorphous In–Ga–Zn–O
films fabricated by using pulsed-laser deposition
Shiu-Jen Liu• Shih-Hao Su•Jenh-Yih Juang
Received: 3 December 2013 / Accepted: 15 January 2014 / Published online: 1 February 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Room-temperature ferromagnetism (RTFM) was observed in pulsed-laser deposited amorphous In–Ga– Zn–O (a-IGZO) films undoped with impurities containing unpaired d or f electrons. The presence of oxygen vacan-cies in the prepared a-IGZO films was verified by X-ray photoelectron spectroscopy and suggested to be responsible for the observed RTFM. The electrical and optical prop-erties of the a-IGZO films were also investigated.
1 Introduction
Diluted magnetic semiconductors (DMS) have attracted interest due to the potential application of both charge and spin of electrons [1–3]. Room-temperature ferromagnetism (RTFM) in DMS is typically realized via doping transition metals (TM) in wide bandgap oxide semiconductors as predicted by theoretical calculation [4, 5]. Actually, a number of studies on TM-doped oxide semiconductors exhibiting RTFM have been reported [6–11]. However, so-called d0 ferromagnetism is observed in oxide semicon-ductors doped with elements containing no unpaired d or f electrons such as N- and C-doped ZnO films [12, 13]. RTFM is also observed in ZnO nano-particles with defects [14], oxygen-deficient SnO2[15] and ZnO films [16]. On
the other hand, amorphous In–Ga–Zn–O (a-IGZO) with wide bandgap (Eg= 3.1–3.5 eV) [17] is a promising
material for fabricating optoelectronic devices such as thin-film transistor (TFT). Previous studies demonstrated that the Hall mobility of carriers in a-IGZO films is not sig-nificantly affected in amorphous crystal structure even if the films are under bending [18]. It reveals that the elec-tronic structure of a-IGZO is insensitive to chemical bond distortion. The unique feature is believed to be originated from the carrier transport path composed of extended spherical s orbitals of heavy metal cations [19] and has attracted much attention on fabricating flexible and trans-parent TFTs using a-IGZO films as the active channel material. Since a-IGZO has been proven to be suitable for fabricating oxide electronic devices, ferromagnetic a-IGZO would be possible to realize DMS devices. In this paper, we report the RTFM in a-IGZO films undoped with TM. The electrical and optical properties of the a-IGZO films were also investigated.
2 Experiments
The a-IGZO films used in this study were grown on double-side polished c-cut sapphires using pulsed-laser deposition (PLD) with a ceramic pellet as the target. The target with a composition ratio of In:Ga:Zn = 2:2:1 at.% was prepared by conventional solid state reactions from a mixture of high-purity (99.95 %) In2O3, Ga2O3, and ZnO powders.
KrF excimer laser (k * 248 nm) was used to ablate the target with an energy density of 1.5 J/cm2per pulse and a repetition rate of 5 Hz. The distance between the target and substrates was 4–5 cm. The growth temperatures (Ts) were
kept at room temperature, i.e., 25C, (Sample #1) and 150
C (Sample #2) during PLD. For Sample #2, the film was
naturally cooled down to room temperature after PLD. No post-annealing was adopted after the deposition of both S.-J. Liu (&)
Department of Mathematics and Science (Pre-college), National Taiwan Normal University, Linkou Dist., New Taipei, Taiwan e-mail: [email protected]
S.-H. Su J.-Y. Juang
Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
123
Appl. Phys. A (2014) 116:1473–1476 DOI 10.1007/s00339-014-8263-0
films. The chamber was evacuated to a base pressure of 10-6torr before the film deposition. The oxygen pressure in the chamber was 10-2torr during PLD. The growth rate is about 0.05 nm/pulse and the thickness of the films is about 130–150 nm. Except the peaks of the sapphire sub-strates, no signature from the films was observed in X-ray diffraction scans (not shown here), which indicates the amorphous structure of the prepared films. The room-temperature magnetization versus magnetic field M(H) curves were performed on a Quantum Design superconducting quantum interference device magnetom-eter. The X-ray photoelectron spectroscopy (XPS) analysis was carried out using the Thermo VG Scientific ESCALAB 250 system with a Al Ka X-ray source (1,486.6 eV). The analysis chamber is equipped with a flood gun used for charge compensation when necessary. The XPS spectra are referenced to the C 1s photoemission line of 284.8 eV. The Hall effect measurement and van der Pauw method were employed to obtain the electrical properties including resistivity (q), carrier density (n) and carrier mobility (l). The optical transmission and reflection spectra were recorded using an UV–Vis spectrometer.
3 Results and discussion
The field-dependent magnetization M(H) curves measured at room temperature are depicted in Fig.1. The M(H) curves were taken with fields applied parallel to the plane of films and corrected for the diamagnetism of sap-phire substrates. As seen from the M(H) curves, Sample #2
grown at 150C evidently exhibits ferromagnetism at room temperature. On the other hand, Sample #1 grown at 25C shows a diamagnetic behavior combined with paramagne-tism or weak ferromagneparamagne-tism. The films used in this study were deposited using the same target. And special cares were taken during the sample preparation and measurement procedures to exclude the contribution from magnetic contaminations. The RTFM exhibited by Sample #2 should not be attributed to impurities containing unfilled d or f shells. The RTFM in oxide compounds undoped with transition metals, i.e., the so-called d0 ferromagnetism, is generally related to lattice defects such as oxygen vacan-cies [15,16], cation interstitials [20] and cation vacancies [21,22].
Since oxygen vacancies, cation interstitials and cation vacancies were suggested to play important roles in the RTFM observed in oxide semiconductors, it is necessary to characterize the valence states of cations and oxygen vacancies in the a-IGZO films. Core level In 3d, Zn 2p, Ga 2p and O 1s XPS spectra were carried out at room temperature and respectively illustrated in Fig. 2a–d. For In 3d, Zn 2p and Ga 2p XPS spectra shown in Fig.2a–c, since no obvious difference was observed between the spectra for Sample #1 and #2, the RTFM observed here could not be resulted from the cations. On the other hand, both the O 1s XPS curves shown in Fig.2d can be fitted by combining two symmetric Gaussian peaks. The one with lower binding energy (*530.2 eV) is referred to as the low binding energy component (LBEC) and has been ascribed to the O 1s core peak of O2- bound to metal cations [23]. Perhaps, the more relevant peak centered at about 532.1 eV is the high binding energy component (HBEC), which has been suggested to directly relate to the concentration of oxygen vacancies [14, 24]. The HBECs indicate the presence of oxygen vacancy in the a-IGZO films, as revealed by previous studies [17]. Moreover, the areal increment of HBEC in Sample #2 grown at 150C indicates the more oxygen vacancies than Sample #1. Therefore, we would like to attribute RTFM observed here to oxygen vacancies which maybe positively charged monovalent (VO?) and induce local magnetic moments
[25].
Electrical properties q, n, and l of the two samples are listed in Table1. As shown in the table, the q of Sample #2 (4:5 10 3X cm) is much lower than that of Sample #1
(0.35 X cm) which is mainly resulted from the enhanced l of Sample #2 by growing at a higher temperature (150C) since the carrier densities of these two films are similar, 2.1 9 1019 and 5.9 9 1019 cm-3 for Sample #1 and Sample #2, respectively. The high carrier density in a-IGZO films is believed to be induced by oxygen vacan-cies [17].
Fig. 1 Room-temperature magnetization (M) as a function of mag-netic field (H) of the a-IGZO films. The magmag-netic fields were applied parallel to the plane of the films
1474 S.-J. Liu et al.
Although the XRD results reveal amorphous structures of both films, the low carrier mobility of Sample #1 (\ 1 cm2/V s) indicates that the crystal structure of Sample #1 is more disorder than that of Sample #2 owing to be deposited at a lower temperature, 25°C. To further explore the transport properties of these two samples, the temper-ature dependence of electrical resistivity was carried out and is depicted in Fig.3. Sample #2 shows a degenerate conduction behavior. However, although the carrier density of Sample #1 is as high as 2.1 9 1019cm-3, the resistivity of Sample #1 increases as the temperature is reduced. Moreover, as shown in the inset of Fig.3, the resistivity of Sample #1 follows q(T) * exp(T-1/4) relationship in the temperature range 80 \ T \ 290 K. This result is generally explained by the Mott variable-range hopping mechanism describing conduction in strongly disordered systems with localized charge-carrier states [26]. Nevertheless, another possible mechanism suggested by Takagi et. al. [17] for explaining the q(T) * exp(T-1/4) behavior in amorphous oxide semiconductor is the percolation conduction which
has been used to describe the carrier transport in highly doped polycrystalline Si [27].
The optical properties of the prepared a-IGZO films were investigated by measuring the transmission and reflection spectra at room temperature, as illustrated in Fig.4. First of all, as seen in the figure, the transmission of a-IGZO films is found to be obviously enhanced by raising the film growth temperature to 150°C. And the average transmission of Sample #2 exceeds 80 % in the visible
(a)
(c)
(b)
(d)
Fig. 2 Core level XPS spectra of a In 3d, b Zn 2p, c Ga 2p and dO 1s for Sample #1 and Sample #2 grown at 25 and 150
C, respectively. The dash lines
plotted in (d) are the Gaussian fitting. LBEC and HBEC mean low binding energy component and high binding energy component, respectively
Table 1 Growth temperature (Ts), resistivity (q), carrier density
(n) and carrier mobility (l) of the a-IGZO films used in this study Sample Ts(C) q (X cm) n (91019cm-3) l (cm2/V s)
#1 25 3.5 9 10-1 2.1 0.9
#2 150 4.5 9 10-3 5.9 23.7
Fig. 3 Temperature dependence of electrical resistivity of Samples #1 and #2. The inset shows the ln(q) vs T-1/4plot for Sample #1 Room-temperature ferromagnetism in amorphous In–Ga–Zn–O films 1475
range (400–700 nm). Furthermore, the optical bandgap (Eg)
of transparent conducting oxide films can be estimated by the relationship between absorption coefficient (a) and photon energy (hm) of the form (ahm) * (hm - Eg)r with
r = 2 suggested by Tauc for amorphous semiconductors [28, 29]. The Eg of Sample #2 is then estimated to be
3.1 eV by linear extrapolation of (ahm)0.5to the hm-axis, as depicted in the inset of Fig.4.
4 Conclusions
In conclusion, electrical, optical and magnetic properties of a-IGZO films grown on sapphires at 25 and 150C using pulsed-laser deposition were investigated. RTFM was observed in the a-IGZO film grown at 150 C, which undoped with impurity containing unpaired d or f electrons. On the other hand, the a-IGZO film grown at 25C exhibits a diamagnetism behavior combined with paramagnetism or weak ferromagnetism. The observed RTFM was attributed to the oxygen vacancies revealed by XPS measurements. The temperature dependance of electrical resistivity of the a-IGZO films grown at 25C follows a q(T) * exp(T-1/4) relationship. The a-IGZO film grown at 150 C shows a degenerate conduction behavior. The optical transmission of a-IGZO films can be remarkably enhanced by raising the growth temperature to 150C.
Acknowledgments This work was supported by the National Sci-ence Council of Taiwan, under Grant Nos. NSC 101-2112-M-003-007.
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