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Annealing temperature dependence of the structures and properties of Co-implanted ZnO films
Accepted Manuscript Title: Annealing temperature dependence of the structures and properties of Co-implanted ZnO films Author: Bin Chen Kun Tang Shulin Gu Jiandong Ye Shimin Huang Ran Gu Yang Zhang Zhengrong Yao Shunming Zhu Youdou Zheng PII: DOI: Reference:
S0169-4332(14)01218-5 http://dx.doi.org/doi:10.1016/j.apsusc.2014.05.177 APSUSC 27996
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-2-2014 4-5-2014 25-5-2014
Please cite this article as: B. Chen, K. Tang, S. Gu, J. Ye, S. Huang, R. Gu, Y. Zhang, Z. Yao, S. Zhu, Y. Zheng, Annealing temperature dependence of the structures and properties of Co-implanted ZnO films, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.177 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Annealing temperature dependence of the structures and properties of Co-implanted ZnO films
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Bin Chen, Kun Tang, Shulin Gu*, Jiandong Ye*, Shimin Huang, Ran Gu, Yang Zhang, Zhengrong Yao, Shunming Zhu, and Youdou Zheng
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School of Electronic Science and Engineering, Nanjing University, Nanjing210093, PR China
The effects of thermal annealing treatment on the structural, electrical, optical and magnetic
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properties of Co-implanted ZnO (0001) films have been investigated in detail. The crystalline structure of the damaged region caused by ion implantation has been recovered via the thermal
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annealing at the temperature of 900oC and above, and no Co clusters or its related oxide phases have been observed. The electrical and optical properties of the annealed films have shown strong
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dependence on the annealing temperature. The zero field cooling magnetization curves of the annealed films have varied from concave shape to convex one as the annealing temperature
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increased from 750oC to 1000oC, which are possibly tuned by the changes of the ratio of the
itinerant carriers over the localized spin density. The low temperature magnetic hysteresis loops have indicated paramagnetic behavior for the annealed films with weak ferromagnetic characteristics. The ferromagnetism is attributed to the substituted Co2+ ions and vacancy defects,
while the paramagnetism could be induced by ionized interstitial Zn defects.
*Email address of corresponding authors: [email protected] and [email protected]
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I.INTRODUCTION Diluted magnetic semiconductors (DMS) are semiconductors in which transition-metal ions
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substitute cations of the host materials. In recent years, ZnO-based DMS materials have received considerable attention due to their potential capability for developing spintronic devices operated
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at room temperature [1, 2, 3, 4]. Much effort has been made to study the transitional metal (TM)
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doped ZnO-based DMS owing to their theoretically predicted high Tc above room temperature [5]. Cobalt (Co) is one of the promising dopants in ZnO films for high-Tc DMS due to its high
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solubility in ZnO and high theoretical saturated magnetic moment (Ms). For instance, high quality
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Zn1-xCoxO epitaxial films grown by low-temperature metal organic chemical vapor deposition (MOCVD) method exhibited ferromagnetic behavior at room temperature [6], and the
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Zn0.895Co0.1Al0.005O thin films grown by a pulsed laser deposition technique were also
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ferromagnetic without any indication of Co clusters [7]. On the other hand, Park et al. found that
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the Zn1-xCoxO films produced by a sol-gel-coating route were superparamagnetic for x<0.12 and the observed ferromagnetism originated from the nanometer-sized Co clusters [8]. All these studies caused a controversy over the origin of magnetism in Co-doped ZnO DMS. In fact, the TM doping induced secondary phase, carrier-mediated ferromagnetism [5, 9], bound magnetic polarons [10, 11], and even intrinsic defects such as oxygen vacancies [12] have been proposed to explain the observed ferromagnetic phenomena. The magnetic properties of the TM doped ZnO films have been found to strongly depend on the growth techniques [6, 7, 8], possibly due to their significant differences in structures and properties of the employed films. Generally, high quality films tend to show the behaviors of paramagnetism and spin-glass, while ferromagnetic phase is likely favorable in the deteriorated 2
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films [13]. In other words, the ferromagnetic behavior is suggested to correlate with the non-uniformity of transition metals (such as magnetic clusters [8]), unintentional impurities (such
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as hydrogen [14]), or/and native defects (such as vacancies [12]). Such complicated cases hindered people to investigate the physical mechanism behind the ferromagnetic phenomenon observed in
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TM doped ZnO films.
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Ion implantation method is a well-known and mature technique with the capability of precise control of depth profile and doses of different dopants beyond their equilibrium solubility limits.
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ZnO films implanted with Co ions have been previously investigated. It was found that the magnetic property changed from paramagnetism to ferromagnetism as the Co ion doses increased
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from 2.5×1016 to 2×1017cm-2 [15]. ZnO single crystals have been doped by Co ion implantation with fluences of 2×1016 and 1×1017 cm-2 and implantation energy of 100 keV. These samples
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annealed in vacuum led to the formation of Co clusters with the size of 15 nm, which were found to be responsible for the ferromagnetic behavior observed above 300 K [16]. The magnetic
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properties of Sn-doped ZnO substrates implanted with Co (fluences of 3×1016 and 5×1016 cm-2 and implantation energy of 250keV) have been contributed by the presence of Co nanocrystals [17]. However, these studies cannot exclude the contribution of Co substitution on Zn site to the observed magnetism [16, 17]. On the other hand, Yang et al. reported the saturated magnetization of the ZnO: Co thin films increased dramatically when the free electron carrier concentration exceeded ~1019 cm−3, indicative of the carrier-mediated ferromagnetism. In this case, Co ions were
implanted into as-grown ZnO and ZnO: Ga thin films with a dose of 3×1016 cm−2 and implantation energy of 50keV [9]. It should be noted that high-dose implantation inevitable creates great lattice distortion with the additional strain and high density of defects [18-22], which strongly 3
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deteriorates the optical, electrical and magnetic properties. The crystalline structures of damaged layer can only be partially recovered through high temperature annealing procedures. The
interpretation for the observed magnetic properties in Co-doped ZnO films.
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structural deterioration induced by heavy bombardment hampers one to make the precise
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In our work, to avoid the forming of Co clusters and explore the origin of the magnetism, we
carried out detailed investigation on the properties of the Co-implanted ZnO films with a rather
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low dose of 8×1015 cm-2 and high implantation energy of 1MeV. High temperature annealing at
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900oC or above led to the recovery of the crystalline structures of the Co-implanted ZnO films, with no Co clusters detected. The optical and electrical properties of the annealed films have
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shown strong dependence on the annealing temperature. The electrical and magnetic properties were investigated to establish the correlation between the carrier concentration and magnetic
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mechanisms. The possible origin of the observed ferromagnetic and paramagnetic behavior in the
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annealed Co-implanted ZnO samples will be discussed as well.
II.EXPERIMENTAL DETAILS
High quality ZnO films were grown on sapphire (0001) substrates using MOCVD technique.
The detail and growth process have been described elsewhere [23]. A 0.4 µm-thick ZnO buffer layer was grown at 450°C followed by the growth of a 2 µm undoped ZnO layer at 920°C. The deposited ZnO layers always exhibit a Zn-face polarity with excellent crystal quality and layer-by-layer surface morphology [24]. Co ions were implanted into the above as-grown ZnO films with fluence of 8×1015 cm-2. The implantation energy was 1 MeV at the current of 0.12 μA. The lower fluence of ions and high implantation energy used in this work are expected to cause 4
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less damage to the crystalline structure. For every energy of ion used for the implantation experiments, the expected Co projected range together with the vacancies/ interstitials density was
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determined by Stopping and Range of Ions in Matter (SRIM) program [25]. The estimated projected range of the ion in this work is 494.6 nm, resulting in an ion implantation damage depth
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about 1 µm. The implanted films appeared light brown, indicating the presence of lattice disorders induced by Co implantation. Subsequently, the samples were cut into small pieces for the
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post-annealing process at different temperatures ranging from 450oC to 1000oC for 5 minutes in a
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vacuum ambient.
High resolution x-ray diffraction (HRXRD) was performed in a double crystal diffractometer
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equipped with a position sensitive detector placed on the 2θ arm. The Cu K radiation was selected by a flat Ge (444) monochromator. The Raman scattering experiments were carried out in a
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backscattering geometry with an Ar+ laser (514.5nm) as the excited source at room temperature. The scattered light was analyzed with a multichannel detector worked at the temperature of 202K
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with accuracy better than 0.65 cm−1. Photoluminescence (PL) spectra were recorded at 300K and
10K using a He-Cd laser (325nm) as the excitation source. Hall effect measurement was carried out in a van der Pauw geometry using indium as the ohmic contact metal, and the employed magnetic field was about 0.2 T and the measured temperature ranged from 10 K to 300 K. Magnetic hysteresis (M vs H) curves were measured using a vibrating sample magnetometer (VSM) integrated in a physical property measurement system (PPMS-9, Quantum Design) up to 1T at 5 K. Zero field cooled (ZFC) and field cooled (FC) magnetic curves were measured at the magnetic field of 0.2T from 5 K to 300 K.
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III. EXPERIMENTAL RESULTS Fig. 1 (a) shows the XRD patterns of the as-implanted and annealed samples. No Co-related
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diffraction peaks were observed within the detection limit of XRD measurement, indicating that the ZnO: Co thin films were free of secondary phase. In fact, the doping density is about 0.4% in
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this work and it is hard to form Co clusters especially after thermal diffusion at high temperature.
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The enhanced intensity of the (0002) diffraction peaks indicates the improved crystalline quality as the annealing temperature increased. It is also supported by the decrease of the
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full-width-at-half-maximum (FWHM) of the (0002) rocking curves of the ZnO: Co thin films annealed at different temperatures, as shown in Fig.1 (b). It is noted that the peak (0002) rocking
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curve for 1000oC annealed sample is almost as narrow as that of as-grown ZnO epilayer. It implies that the induced damage created by ion implantation in the ZnO host material has been gradually
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removed by the subsequent annealing, and the crystalline structure of damaged region has been
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almost recovered through high temperature annealing over 900oC.
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It has been reported that ion implantation into ZnO would lead to the formation of a large
number of Frenkel pairs [22], i.e. interstitials and vacancies with nearly equal concentrations. The extended defects, such as dislocations or stacking faults, can also be formed during ion
implantation [21]. The concentrations of such defects mainly rely on the bonding type and structure of the host material, the mass of the impinging ion and the implantation temperature [18]. The microstructural changes of Co-doped ZnO before or after implantation are reflected by the strain evolution, as investigated through the high-resolution XRD patterns shown in Fig.2. For the as-implanted sample, an additional peak appears at the low-angle side of the main peak of ZnO (0002) with fringe features, indicating a compressively strained layer with a lattice constant c 6
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larger than that of the as-grown film [19]. The behavior can be interpreted as an increased concentration of point defect clusters and dislocation loops. The compressive strain is induced due
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to the generated interstitials. Co atoms may also contribute to the lattice expansion, but their concentration is two orders of magnitude lower than the Frenkel-pair concentration calculated by
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SRIM program. For the annealed samples, the implanted Co atoms tend to diffuse into the buried
layers and the whole ZnO films become Co doped via thermal annealing processes. After
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annealing at 450oC for 5mins, the main diffraction peak from the buried ZnO layers disappears,
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suggesting the structure degradation of the buried layers induced by Co diffusion. The additional peaks with fringes approach to the main peak as the annealing temperature increase from 450oC to
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750oC, which reflects the reduction of compressive strain and the density of interstitials and vacancies in the deformed region. The remaining fringes observed on the sample annealed at
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750oC indicate that the thermal energy is insufficient to remove point defects generated by ion-implantation, such as residual Zn or O interstitials and vacancies from the deformed strained
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layer.
As the annealing temperature increase to 900°C, most of the fringes disappear, which
indicates that the lattice deformation caused by implantation is significantly reduced by the annealing treatment. It is known that the Co element has a large solubility as high as 40 at. % in ZnO films owing to its very similar size to Zn2+ ions [26]. Jang et al. reported the complete
diffusion of Co ions into ZnO matrix even at the low deposition temperature of 100°C [27]. As a result, upon annealing in this work, the implanted Co ions would diffuse throughout the whole ZnO epilayer rather not being confined in the damaged region [28]. Two well-resolved diffraction peaks in the annealed samples at 900 °C and 1000 °C are located at both sides of ZnO (0002) peak 7
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of the as-grown sample. The peak at higher angle side is corresponding to the ZnO buried layers formed by Co thermal diffusion and the lower one is from the implanted layer after annealing. It
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suggests that thermal treatment at 1000 °C for 5 mins is also insufficient to fully remove the lattice deformation caused by non-uniform Co distribution in ZnO films. That is to say, although the
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crystalline structures for both layers are improved via the high temperature annealing processes,
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the non-uniform distribution of residual strain still exist in the high temperature annealed samples. Lattice defects and microstructural changes induce significant modifications to Raman
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scattering spectrum, which allows nondestructively monitoring of the evolution of lattice disorders. Fig. 3 shows the off-resonant Raman spectra of the as-implanted ZnO: Co films and annealed ones.
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The dominant peak at 437 cm-1 is assigned to E2high mode, which shows a reciprocal ratio to the concentration of oxygen vacancies [28] and can be used to evaluate the structural quality and
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strain involved in the materials. It is observed that this mode increases in intensity and becomes
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narrower for the samples annealed at higher temperature. The enhanced E2high mode with a small
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FWHM (full width at half maximum) value indicates a significant reduction of oxygen vacancy density. The E2high peak positions of implanted samples exhibit a slight blue-shift, approaching to the peak position of ZnO as-grown sample, which is a sign of reduction of strain in the damaged layer. Besides the E2high modes, the forbidden LO mode around 575 cm-1 accompanied with a
distinct broad mode around 516-580 cm-1 has also been observed in the samples annealed at the temperature of 750°C and below. These vibration modes are corresponding to the disorder-active LO phonons, originating from high density of phonon states of high-energy phonon branch [29, 30]. As the annealed temperature is as high as 900°C or above, these disorder-induced phonon modes disappear, which suggests the crystalline recovery with great reduction of defect density. In 8
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addition, no new vibration modes except for ZnO host materials are observed, indicative of no secondary phases formed at the doping level of 0.4% in ZnO: Co films. The evolution of Raman
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spectra shows an excellent consistence with the above-mentioned XRD analyses. The optical properties of Co-implanted ZnO films have been also recovered as shown in
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Fig.4. In Fig.4 (a), the PL spectra recorded at room temperature exhibit distinguished differences
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in UV emission efficiency and deep level emissions. The annealed samples show sharp band-edge emission and week deep level emissions (DLE), while optical emissions quench in the samples
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annealed below 750°C due to the presence of a large amount of nonradiative recombination centers. It is found that the energy position of visible deep level emissions has a consistent blue
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shift from 2.08eV to 2.5eV with the annealing temperature increasing from 750°C to 1000°C. It has been reported that the presence of a green emission was a sign of the recovery of crystalline
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quality for the implanted ZnO films. Low temperature photoluminescence spectra shown in Fig. 4(b) provide more solid evidences on the microstructural changes by annealing process. The
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as-grown ZnO film with high crystalline quality exhibits distinct emissions of neutral donor-bound exciton (D0X, 3.364eV), free exciton A (FXA, 3.371eV), free exciton C (FXC, 3.380eV) and their phonon replicas [31]. After 750oC annealing, the lattice disorder remains and only a broad
emission with weak efficiency is observed. As the annealing temperature increases, this broad emission develops into two well-resolved emissions: D0X and a broad emission located at 3.368eV. The latter one close to the FXA is assigned to the recombination of surface excitons (SX) due to the
trapping of FX at the near-surface states induced by lattice distortion [32]. Noting that the relative intensity ratio of SX to D0X decreases with the increment of annealing temperature from 750oC to 1000oC and it clearly implies that higher annealing temperature leads to higher degree of 9
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crystalline recovery with great reduction of nonradiative recombination centers in damaged regions, which is consistent with the structural properties as investigated by XRD and Raman
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scattering spectra. Besides the structural recovery upon annealing, the electrical properties also show a
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significant improvement. Fig. 5 shows the Hall carrier concentration and mobility recorded at room temperature as a function of annealing temperature. The as-implanted ZnO: Co films
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exhibits insulating, and the annealing at 750oC or below lead to the electronic activation of
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shallow donor-like impurities. However, it is still far below the threshold of an effective carrier-mediated magnetic coupling [33]. Once the structural damages are almost removed at high
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temperature of 900oC, the carrier concentration drops greatly down to 2.5×1018 cm-3, which is close to that of as-grown ZnO epilayer. On the other hand, the carrier mobility monotonically
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increases with the annealing temperature due to the consistent improvement in the crystalline structures. Comparing to the evolution of structural and optical properties, the transition point of
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electrical properties also occurs at 900oC. Temperature-dependent Hall measurement shown in Fig. 6 provides more information on carrier transport. The carrier concentration for 750oC-annealed
sample is much higher than the higher-temperature annealed ones within the whole temperature range from 10K to 300K. It could be ascribed to the crystalline improvements with less structural defects such as zinc interstitial and oxygen vacancies, both of which are considered as donors in ZnO films. The carrier concentration usually decreases as the measuring temperature decreases due to the carrier-freezing effect. However, for the 9000C or above annealed samples, the low temperature carrier concentrations do not change with the measured temperature, indicating the transportation characteristics of a high conductive layer, which has been previously considered to 10
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locate at the interface of the ZnO buffer layer and sapphire substrate [34]. Figure 7 shows the magnetic hysteresis loops of the samples annealed at 750oC, 900oC and
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1000oC at the temperature of 5 K. Paramagnetism is observed for all the measured samples. Once the magnetization curves are boosted as shown in the inset of Fig. 7, we can see a magnetic
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coercive force of 100Oe for the ZnO: Co sample annealed at 900oC, indicative of ferromagnetic
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behavior in this sample. Owing to the large amount of defects and weak ferromagnetism in terms of low doping concentration, the samples show large paramagnetic background at low temperature,
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which has also been reported in other research works [39, 40]. In Fig.8, zero field cooling (ZFC) magnetization curves are found to vary from concave to convex shapes with the annealed
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temperature increased from 750oC to 1000oC. The magnetization curves may be tuned by the variation of ratio of itinerant carrier density (c*) and localized spin density (c). Sun et al.
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theoretically proposed that the crossover of magnetization curvatures from concave to convex shapes occurred and the maximum FM transition temperature was reached with an optimal ratio of
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c*/c=0.05 [36]. In our research, the decrease of the carrier concentrations (c*) and the defect density is supported by the Hall measurements, Raman scattering spectra and PL spectra. The localized spin density is possibly contributed by substituted Co2+ ions and/or unpaired O 2p electrons surrounding zinc vacancy defects [40]. For Co-doped ZnO with dose of 8×1015 cm-2, density of vacancies/interstitials defects induced by ion implantation is expected as high as 1021
cm-3 calculated by SRIM program, two orders of magnitude higher than that of Co atoms. Upon annealing at 750oC, the crystalline is not recovered at all, and the localized spin density (c) is mainly contributed by vacancies/interstitials defects. As a result, the ratio of c*/c is still less than 0.01 (5.9×1018 cm-3/1021 cm-3). Therefore the shape of magnetization curve is concave with low 11
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critical temperature attained, well supported by the theoretical calculation [36]. In contrast to the usual convex curvature in Weiss mean-field theory, this concave shape is often used to be an indication of localization in the presence of disorder. As discussed above, the defect density
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reduced greatly along with the crystalline recovery at 900oC. In this case, Co2+ ions are supposed
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to contribute to the localized spin density (c), and the doping density of the implanted sample is
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4×1019 cm-3, on the assumption that all implanted Co2+ ions have diffused into the whole 2µm-thick ZnO layers. So the ratio of c*/c is given to be about 0.06 (2.5×1018 cm-3/4×1019 cm-3).
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The changes of c*/c ratio is the possible origin of the evolution of the magnetization curves varying from concave to convex shapes with an increased critical temperature. Furthermore, as the
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annealing temperature further increased to 1000oC, the substitution of Zn by Co on well-ordered sites would not change anymore, but the improvement on the crystalline structure leads to the
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further reduction in localized spin density c, resulting in a higher ratio of c*/c, regardless of a
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smaller carrier concentration (c*). It hence, exhibits a smaller magnetic moment and a rather lower
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transition temperature [37]. Actually, the magnetic curve of the intrinsic ZnO film also shows the mixture of the paramagnetic and ferromagnetic characteristics as shown in Fig. 7 and the weak ferromagnetism have been reported induced by intrinsic defects [38, 39]. Besides the ferromagnetic character contributed by substituted Co2+ ions and vacancy defects, the observed paramagnetism could be resulted from the defects such as ionized interstitial Zn formed in the films during the growth and annealing processes. More detailed modeling of the defects and their dynamics are needed to ascertain whether the scenarios can account for the observed phenomena.
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V.SUMMARY We have demonstrated that thermal annealing of Co-implanted ZnO films has led to complex changes in its structures and physical properties, which showed strong dependence on the
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annealing temperature. Annealing over 900oC is required to recover the deteriorated crystalline
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structure. The observed changes in the electrical, optical and magnetic properties of the Co-doped
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ZnO films have been attributed to the evolution of the ion implantation induced defects with various annealing temperature. The change of magnetization curves of ZFC from concave to
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convex shapes have been explained and discussed from the viewpoint of the relative ratio of itinerant carrier and localized spin density. This study indicates that the structural disorder and
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intrinsic defects play a critical role in the observed magnetic properties in the ion implanted ZnO
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films.
Acknowledgements：Research supported by the State Key Program for Basic Research of China under Grant No. 2011CB302003 and National Natural Science Foundation of China (Nos. 61025020, 60990312, 61274058 and 61322403), Basic Research Program of Jiangsu Province (BK2011437 and BK20130013) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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[24] J. D. Ye, S. Pannirselvam, S. T. Lim, J. F. Bi, X. W. Sun, G. Q. Lo, K. L. Teo, Two-dimensional electron gas in Zn-polar ZnMgO/ZnO heterostructure grown by metal-organic vapor phase epitaxy, Appl. Phys. Lett. 97 (2010) 111908. [25] J. F. Ziegler, J. P. Biersack, U Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [26] B. B. Straumal, A. A. Mazilkin, S. G. Protasova, A. A. Myatiev, P. B. Straumal, B. Baretzky, 16
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Increase of Co solubility with decreasing grain size in ZnO, Acta Mater. 56 (2008) 6246-6256. [27] J.H. Jang, Y. Cheng, Y. Liu, X. Ding, Y.X. Wang, Y.J. Zhang, H.L. Liu, Structure and
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room-temperature ferromagnetism of Co-doped ZnO DMS films, Solid State Commun. 149 (2009) 1164-1167.
cr
[28] J. S. Thakur, G. W. Auner, V. M. Naik, C. Sudakar, P. Kharel, G. Lawes, R. Suryanarayanan, R.
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Naik, Raman scattering studies of magnetic Co-doped ZnO thin films, J. Appl. Phys. 102 (2007) 093904.
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[29] J. D. Ye, S. Tripathy, F.F. Ren, X.W. Sun, G. Q. Lo, K. L. Teo, Raman-active Fröhlich optical phonon mode in arsenic implanted ZnO, Appl. Phys. Lett. 94 (2009) 011913.
Appl. Phys. Lett. 91 (2007) 111903.
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[30] F. Friedrich, N. H. Nickel, Resonant Raman scattering in hydrogen and nitrogen doped ZnO,
te
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[31] B. K. Meyer, H. Alves, D. M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J. Christen, A. Hoffmann, M. Strabburg, M. Dworzak, U. Haboeck, A. V. Rodina, Bound exciton and
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donor–acceptor pair recombinations in ZnO, phys. stat. sol. (b) 241No. 2 (2004) 231-260. [32] Y. Yang, B. K. Tay, X. W. Sun, J. Y. Sze, Z. J. Han, J.X. Wang, X.H. Zhang, Y.B. Li, S. Zhang, Quenching of surface-exciton emission from ZnO nanocombs by plasma immersion ion implantation, Appl. Phys. Lett. 91 (2007) 071921. [33] H. Chou, C. P. Lin, H. S. Hsu, S. J. Sun, The role of carriers in spin current and magnetic coupling for ZnO:Co diluted magnetic oxides, Appl. Phys. Lett. 96 (2010) 092503. [34] K. Tang, S.L. Gu, S.Z. Li, J.D. Ye, S.M. Zhu, H. Chen, J. G. Liu, R. Zhang, Y. Shi, Y. D. Zheng, Influence of thermally diffused aluminum atoms from sapphire substrate on the properties of ZnO epilayers grown by metal-organic chemical vapor deposition, J. Vac. Sci. Technol. A 29 17
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(2011) 03A106. [35] A. Barla, G. Schmerber, E. Beaurepaire, A. Dinia, H. Bieber, S. Colis, F. Scheurer, J.P.
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Kappler, P. Imperia, F. Nolting, F. Wilhelm, A. Rogalev, D. Müller, Paramagnetism of the Co sublattice in ferromagnetic Zn1−xCoxO films, J. J. Grob, Phys. Rev. B76 (2007) 125201.
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[36] J. S. Lee, J. D. Lim, Z. G. Khim, Y. D. Park, S. J. Pearton, S. N. G. Chu, Magnetic and
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structural properties of Co, Cr, V ion-implanted GaN, J. Appl. Phys.93 (2003) 4512-4516.
[37] S. J. Sun, H. H. Lin, Diluted magnetic semiconductor at finite temperature, Phys. Lett. A 327
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(2004) 73-77.
study, Phys. Rev. B 74 (2006) 144432.
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[38] C. H. Patterson, Role of defects in ferromagnetism in Zn1−xCoxO: A hybrid density-functional
[39] J. D. Ye, S. T. Lim, M. Bosman, S.L. Gu, Y.D. Zheng, H. H. Tan, C. Jagadish, X.W. Sun, K. L.
Rep. 2 (2012) 533.
te
d
Teo, Spin-polarized Wide Electron Slabs in Functionally Graded Polar Oxide Heterostructures, Sci.
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[40] Q. Wang, Q. Sun, G. Chen, Y. Kawazoe, P. Jena, Vacancy-induced magnetism in ZnO thin
films and nanowires, Phys. Rev. B 74 (2008) 205411.
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Figure captions Fig.1 (a) X-ray diffraction (XRD) patterns of the Co-implanted and their annealed ZnO films on
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sapphire substrate; (b) The full-width at half maximum of ZnO (0002) rocking curves of the ZnO: Co thin films annealed at different temperatures.
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Fig. 2 Triple-axis XRD patterns of the Co-implanted ZnO films annealed at different temperatures
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and the as-grown ZnO film for reference.
Fig. 3 Room-temperature Raman scattering spectra of ZnO as-grown film and the Co-implanted
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ZnO films annealed at different temperatures.
Fig. 4 Photoluminescence spectra of ZnO: Co films and ZnO as-grown film recorded at (a) 300K
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and (b) 10K, respectively.
Fig. 5 Hall carrier concentration and mobility of Co-implanted ZnO films as a function of
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annealing temperature.
Fig. 6 The temperature-dependent carrier mobility (a), concentration (b) and resistivity (c) of
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Co-implanted ZnO annealed at 750oC, 900oC and 1000oC respectively. Fig.7 M-H hysteresis curves of the ZnO as-grown film and Co-implanted ZnO films annealed at 750oC, 900oC and 1000oC, respectively at 5K.The inset shows the magnetic coercive force of ZnO:
Co sample annealed at 900oC.
Fig.8 The field cooling and zero field cooling magnetization curves of the ZnO as-grown film (a) and Co-implanted ZnO films annealed at 750oC (b), 900oC (c) and 1000oC (d), respectively.
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Highlight The paper includes nineteen pages and eight figures and it is about the effects of thermal annealing on the structural, electronic and optical and magnetic properties of Co‐implanted ZnO (0001) films grown on c‐plane sapphire substrates.
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To avoid the forming of Co clusters and explore the origin of the magnetism, detailed investigation on the properties of the Co‐implanted ZnO films with a rather low dose of
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8×1015 cm‐2 and high implantation energy of 1MeV were carried out.
The crystalline structure of the damaged region caused by ion‐implantation has been
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recovered via the thermal annealing treatment at the temperature of 900oC and above. The low temperature magnetic hysteresis loops have indicated paramagnetism for the
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annealed films with weak ferromagnetic characteristics.
The zero‐field cooling (ZFC) magnetization curves of the Co‐implanted ZnO samples have
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varied from concave shape to convex one as the annealing temperature increased from 750oC to 1000oC.
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S0169-4332(14)01218-5 http://dx.doi.org/doi:10.1016/j.apsusc.2014.05.177 APSUSC 27996
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-2-2014 4-5-2014 25-5-2014
Please cite this article as: B. Chen, K. Tang, S. Gu, J. Ye, S. Huang, R. Gu, Y. Zhang, Z. Yao, S. Zhu, Y. Zheng, Annealing temperature dependence of the structures and properties of Co-implanted ZnO films, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.177 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Annealing temperature dependence of the structures and properties of Co-implanted ZnO films
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Bin Chen, Kun Tang, Shulin Gu*, Jiandong Ye*, Shimin Huang, Ran Gu, Yang Zhang, Zhengrong Yao, Shunming Zhu, and Youdou Zheng
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School of Electronic Science and Engineering, Nanjing University, Nanjing210093, PR China
The effects of thermal annealing treatment on the structural, electrical, optical and magnetic
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properties of Co-implanted ZnO (0001) films have been investigated in detail. The crystalline structure of the damaged region caused by ion implantation has been recovered via the thermal
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annealing at the temperature of 900oC and above, and no Co clusters or its related oxide phases have been observed. The electrical and optical properties of the annealed films have shown strong
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dependence on the annealing temperature. The zero field cooling magnetization curves of the annealed films have varied from concave shape to convex one as the annealing temperature
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increased from 750oC to 1000oC, which are possibly tuned by the changes of the ratio of the
itinerant carriers over the localized spin density. The low temperature magnetic hysteresis loops have indicated paramagnetic behavior for the annealed films with weak ferromagnetic characteristics. The ferromagnetism is attributed to the substituted Co2+ ions and vacancy defects,
while the paramagnetism could be induced by ionized interstitial Zn defects.
*Email address of corresponding authors: [email protected] and [email protected]
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I.INTRODUCTION Diluted magnetic semiconductors (DMS) are semiconductors in which transition-metal ions
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substitute cations of the host materials. In recent years, ZnO-based DMS materials have received considerable attention due to their potential capability for developing spintronic devices operated
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at room temperature [1, 2, 3, 4]. Much effort has been made to study the transitional metal (TM)
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doped ZnO-based DMS owing to their theoretically predicted high Tc above room temperature [5]. Cobalt (Co) is one of the promising dopants in ZnO films for high-Tc DMS due to its high
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solubility in ZnO and high theoretical saturated magnetic moment (Ms). For instance, high quality
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Zn1-xCoxO epitaxial films grown by low-temperature metal organic chemical vapor deposition (MOCVD) method exhibited ferromagnetic behavior at room temperature [6], and the
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Zn0.895Co0.1Al0.005O thin films grown by a pulsed laser deposition technique were also
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ferromagnetic without any indication of Co clusters [7]. On the other hand, Park et al. found that
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the Zn1-xCoxO films produced by a sol-gel-coating route were superparamagnetic for x<0.12 and the observed ferromagnetism originated from the nanometer-sized Co clusters [8]. All these studies caused a controversy over the origin of magnetism in Co-doped ZnO DMS. In fact, the TM doping induced secondary phase, carrier-mediated ferromagnetism [5, 9], bound magnetic polarons [10, 11], and even intrinsic defects such as oxygen vacancies [12] have been proposed to explain the observed ferromagnetic phenomena. The magnetic properties of the TM doped ZnO films have been found to strongly depend on the growth techniques [6, 7, 8], possibly due to their significant differences in structures and properties of the employed films. Generally, high quality films tend to show the behaviors of paramagnetism and spin-glass, while ferromagnetic phase is likely favorable in the deteriorated 2
Page 2 of 20
films [13]. In other words, the ferromagnetic behavior is suggested to correlate with the non-uniformity of transition metals (such as magnetic clusters [8]), unintentional impurities (such
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as hydrogen [14]), or/and native defects (such as vacancies [12]). Such complicated cases hindered people to investigate the physical mechanism behind the ferromagnetic phenomenon observed in
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TM doped ZnO films.
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Ion implantation method is a well-known and mature technique with the capability of precise control of depth profile and doses of different dopants beyond their equilibrium solubility limits.
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ZnO films implanted with Co ions have been previously investigated. It was found that the magnetic property changed from paramagnetism to ferromagnetism as the Co ion doses increased
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from 2.5×1016 to 2×1017cm-2 [15]. ZnO single crystals have been doped by Co ion implantation with fluences of 2×1016 and 1×1017 cm-2 and implantation energy of 100 keV. These samples
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annealed in vacuum led to the formation of Co clusters with the size of 15 nm, which were found to be responsible for the ferromagnetic behavior observed above 300 K [16]. The magnetic
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properties of Sn-doped ZnO substrates implanted with Co (fluences of 3×1016 and 5×1016 cm-2 and implantation energy of 250keV) have been contributed by the presence of Co nanocrystals [17]. However, these studies cannot exclude the contribution of Co substitution on Zn site to the observed magnetism [16, 17]. On the other hand, Yang et al. reported the saturated magnetization of the ZnO: Co thin films increased dramatically when the free electron carrier concentration exceeded ~1019 cm−3, indicative of the carrier-mediated ferromagnetism. In this case, Co ions were
implanted into as-grown ZnO and ZnO: Ga thin films with a dose of 3×1016 cm−2 and implantation energy of 50keV [9]. It should be noted that high-dose implantation inevitable creates great lattice distortion with the additional strain and high density of defects [18-22], which strongly 3
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deteriorates the optical, electrical and magnetic properties. The crystalline structures of damaged layer can only be partially recovered through high temperature annealing procedures. The
interpretation for the observed magnetic properties in Co-doped ZnO films.
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structural deterioration induced by heavy bombardment hampers one to make the precise
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In our work, to avoid the forming of Co clusters and explore the origin of the magnetism, we
carried out detailed investigation on the properties of the Co-implanted ZnO films with a rather
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low dose of 8×1015 cm-2 and high implantation energy of 1MeV. High temperature annealing at
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900oC or above led to the recovery of the crystalline structures of the Co-implanted ZnO films, with no Co clusters detected. The optical and electrical properties of the annealed films have
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shown strong dependence on the annealing temperature. The electrical and magnetic properties were investigated to establish the correlation between the carrier concentration and magnetic
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mechanisms. The possible origin of the observed ferromagnetic and paramagnetic behavior in the
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annealed Co-implanted ZnO samples will be discussed as well.
II.EXPERIMENTAL DETAILS
High quality ZnO films were grown on sapphire (0001) substrates using MOCVD technique.
The detail and growth process have been described elsewhere [23]. A 0.4 µm-thick ZnO buffer layer was grown at 450°C followed by the growth of a 2 µm undoped ZnO layer at 920°C. The deposited ZnO layers always exhibit a Zn-face polarity with excellent crystal quality and layer-by-layer surface morphology [24]. Co ions were implanted into the above as-grown ZnO films with fluence of 8×1015 cm-2. The implantation energy was 1 MeV at the current of 0.12 μA. The lower fluence of ions and high implantation energy used in this work are expected to cause 4
Page 4 of 20
less damage to the crystalline structure. For every energy of ion used for the implantation experiments, the expected Co projected range together with the vacancies/ interstitials density was
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determined by Stopping and Range of Ions in Matter (SRIM) program [25]. The estimated projected range of the ion in this work is 494.6 nm, resulting in an ion implantation damage depth
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about 1 µm. The implanted films appeared light brown, indicating the presence of lattice disorders induced by Co implantation. Subsequently, the samples were cut into small pieces for the
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post-annealing process at different temperatures ranging from 450oC to 1000oC for 5 minutes in a
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vacuum ambient.
High resolution x-ray diffraction (HRXRD) was performed in a double crystal diffractometer
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equipped with a position sensitive detector placed on the 2θ arm. The Cu K radiation was selected by a flat Ge (444) monochromator. The Raman scattering experiments were carried out in a
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backscattering geometry with an Ar+ laser (514.5nm) as the excited source at room temperature. The scattered light was analyzed with a multichannel detector worked at the temperature of 202K
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with accuracy better than 0.65 cm−1. Photoluminescence (PL) spectra were recorded at 300K and
10K using a He-Cd laser (325nm) as the excitation source. Hall effect measurement was carried out in a van der Pauw geometry using indium as the ohmic contact metal, and the employed magnetic field was about 0.2 T and the measured temperature ranged from 10 K to 300 K. Magnetic hysteresis (M vs H) curves were measured using a vibrating sample magnetometer (VSM) integrated in a physical property measurement system (PPMS-9, Quantum Design) up to 1T at 5 K. Zero field cooled (ZFC) and field cooled (FC) magnetic curves were measured at the magnetic field of 0.2T from 5 K to 300 K.
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III. EXPERIMENTAL RESULTS Fig. 1 (a) shows the XRD patterns of the as-implanted and annealed samples. No Co-related
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diffraction peaks were observed within the detection limit of XRD measurement, indicating that the ZnO: Co thin films were free of secondary phase. In fact, the doping density is about 0.4% in
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this work and it is hard to form Co clusters especially after thermal diffusion at high temperature.
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The enhanced intensity of the (0002) diffraction peaks indicates the improved crystalline quality as the annealing temperature increased. It is also supported by the decrease of the
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full-width-at-half-maximum (FWHM) of the (0002) rocking curves of the ZnO: Co thin films annealed at different temperatures, as shown in Fig.1 (b). It is noted that the peak (0002) rocking
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curve for 1000oC annealed sample is almost as narrow as that of as-grown ZnO epilayer. It implies that the induced damage created by ion implantation in the ZnO host material has been gradually
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removed by the subsequent annealing, and the crystalline structure of damaged region has been
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almost recovered through high temperature annealing over 900oC.
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It has been reported that ion implantation into ZnO would lead to the formation of a large
number of Frenkel pairs [22], i.e. interstitials and vacancies with nearly equal concentrations. The extended defects, such as dislocations or stacking faults, can also be formed during ion
implantation [21]. The concentrations of such defects mainly rely on the bonding type and structure of the host material, the mass of the impinging ion and the implantation temperature [18]. The microstructural changes of Co-doped ZnO before or after implantation are reflected by the strain evolution, as investigated through the high-resolution XRD patterns shown in Fig.2. For the as-implanted sample, an additional peak appears at the low-angle side of the main peak of ZnO (0002) with fringe features, indicating a compressively strained layer with a lattice constant c 6
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larger than that of the as-grown film [19]. The behavior can be interpreted as an increased concentration of point defect clusters and dislocation loops. The compressive strain is induced due
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to the generated interstitials. Co atoms may also contribute to the lattice expansion, but their concentration is two orders of magnitude lower than the Frenkel-pair concentration calculated by
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SRIM program. For the annealed samples, the implanted Co atoms tend to diffuse into the buried
layers and the whole ZnO films become Co doped via thermal annealing processes. After
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annealing at 450oC for 5mins, the main diffraction peak from the buried ZnO layers disappears,
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suggesting the structure degradation of the buried layers induced by Co diffusion. The additional peaks with fringes approach to the main peak as the annealing temperature increase from 450oC to
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750oC, which reflects the reduction of compressive strain and the density of interstitials and vacancies in the deformed region. The remaining fringes observed on the sample annealed at
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750oC indicate that the thermal energy is insufficient to remove point defects generated by ion-implantation, such as residual Zn or O interstitials and vacancies from the deformed strained
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layer.
As the annealing temperature increase to 900°C, most of the fringes disappear, which
indicates that the lattice deformation caused by implantation is significantly reduced by the annealing treatment. It is known that the Co element has a large solubility as high as 40 at. % in ZnO films owing to its very similar size to Zn2+ ions [26]. Jang et al. reported the complete
diffusion of Co ions into ZnO matrix even at the low deposition temperature of 100°C [27]. As a result, upon annealing in this work, the implanted Co ions would diffuse throughout the whole ZnO epilayer rather not being confined in the damaged region [28]. Two well-resolved diffraction peaks in the annealed samples at 900 °C and 1000 °C are located at both sides of ZnO (0002) peak 7
Page 7 of 20
of the as-grown sample. The peak at higher angle side is corresponding to the ZnO buried layers formed by Co thermal diffusion and the lower one is from the implanted layer after annealing. It
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suggests that thermal treatment at 1000 °C for 5 mins is also insufficient to fully remove the lattice deformation caused by non-uniform Co distribution in ZnO films. That is to say, although the
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crystalline structures for both layers are improved via the high temperature annealing processes,
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the non-uniform distribution of residual strain still exist in the high temperature annealed samples. Lattice defects and microstructural changes induce significant modifications to Raman
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scattering spectrum, which allows nondestructively monitoring of the evolution of lattice disorders. Fig. 3 shows the off-resonant Raman spectra of the as-implanted ZnO: Co films and annealed ones.
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The dominant peak at 437 cm-1 is assigned to E2high mode, which shows a reciprocal ratio to the concentration of oxygen vacancies [28] and can be used to evaluate the structural quality and
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strain involved in the materials. It is observed that this mode increases in intensity and becomes
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narrower for the samples annealed at higher temperature. The enhanced E2high mode with a small
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FWHM (full width at half maximum) value indicates a significant reduction of oxygen vacancy density. The E2high peak positions of implanted samples exhibit a slight blue-shift, approaching to the peak position of ZnO as-grown sample, which is a sign of reduction of strain in the damaged layer. Besides the E2high modes, the forbidden LO mode around 575 cm-1 accompanied with a
distinct broad mode around 516-580 cm-1 has also been observed in the samples annealed at the temperature of 750°C and below. These vibration modes are corresponding to the disorder-active LO phonons, originating from high density of phonon states of high-energy phonon branch [29, 30]. As the annealed temperature is as high as 900°C or above, these disorder-induced phonon modes disappear, which suggests the crystalline recovery with great reduction of defect density. In 8
Page 8 of 20
addition, no new vibration modes except for ZnO host materials are observed, indicative of no secondary phases formed at the doping level of 0.4% in ZnO: Co films. The evolution of Raman
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spectra shows an excellent consistence with the above-mentioned XRD analyses. The optical properties of Co-implanted ZnO films have been also recovered as shown in
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Fig.4. In Fig.4 (a), the PL spectra recorded at room temperature exhibit distinguished differences
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in UV emission efficiency and deep level emissions. The annealed samples show sharp band-edge emission and week deep level emissions (DLE), while optical emissions quench in the samples
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annealed below 750°C due to the presence of a large amount of nonradiative recombination centers. It is found that the energy position of visible deep level emissions has a consistent blue
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shift from 2.08eV to 2.5eV with the annealing temperature increasing from 750°C to 1000°C. It has been reported that the presence of a green emission was a sign of the recovery of crystalline
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quality for the implanted ZnO films. Low temperature photoluminescence spectra shown in Fig. 4(b) provide more solid evidences on the microstructural changes by annealing process. The
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as-grown ZnO film with high crystalline quality exhibits distinct emissions of neutral donor-bound exciton (D0X, 3.364eV), free exciton A (FXA, 3.371eV), free exciton C (FXC, 3.380eV) and their phonon replicas [31]. After 750oC annealing, the lattice disorder remains and only a broad
emission with weak efficiency is observed. As the annealing temperature increases, this broad emission develops into two well-resolved emissions: D0X and a broad emission located at 3.368eV. The latter one close to the FXA is assigned to the recombination of surface excitons (SX) due to the
trapping of FX at the near-surface states induced by lattice distortion [32]. Noting that the relative intensity ratio of SX to D0X decreases with the increment of annealing temperature from 750oC to 1000oC and it clearly implies that higher annealing temperature leads to higher degree of 9
Page 9 of 20
crystalline recovery with great reduction of nonradiative recombination centers in damaged regions, which is consistent with the structural properties as investigated by XRD and Raman
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scattering spectra. Besides the structural recovery upon annealing, the electrical properties also show a
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significant improvement. Fig. 5 shows the Hall carrier concentration and mobility recorded at room temperature as a function of annealing temperature. The as-implanted ZnO: Co films
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exhibits insulating, and the annealing at 750oC or below lead to the electronic activation of
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shallow donor-like impurities. However, it is still far below the threshold of an effective carrier-mediated magnetic coupling [33]. Once the structural damages are almost removed at high
M
temperature of 900oC, the carrier concentration drops greatly down to 2.5×1018 cm-3, which is close to that of as-grown ZnO epilayer. On the other hand, the carrier mobility monotonically
te
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increases with the annealing temperature due to the consistent improvement in the crystalline structures. Comparing to the evolution of structural and optical properties, the transition point of
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electrical properties also occurs at 900oC. Temperature-dependent Hall measurement shown in Fig. 6 provides more information on carrier transport. The carrier concentration for 750oC-annealed
sample is much higher than the higher-temperature annealed ones within the whole temperature range from 10K to 300K. It could be ascribed to the crystalline improvements with less structural defects such as zinc interstitial and oxygen vacancies, both of which are considered as donors in ZnO films. The carrier concentration usually decreases as the measuring temperature decreases due to the carrier-freezing effect. However, for the 9000C or above annealed samples, the low temperature carrier concentrations do not change with the measured temperature, indicating the transportation characteristics of a high conductive layer, which has been previously considered to 10
Page 10 of 20
locate at the interface of the ZnO buffer layer and sapphire substrate [34]. Figure 7 shows the magnetic hysteresis loops of the samples annealed at 750oC, 900oC and
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1000oC at the temperature of 5 K. Paramagnetism is observed for all the measured samples. Once the magnetization curves are boosted as shown in the inset of Fig. 7, we can see a magnetic
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coercive force of 100Oe for the ZnO: Co sample annealed at 900oC, indicative of ferromagnetic
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behavior in this sample. Owing to the large amount of defects and weak ferromagnetism in terms of low doping concentration, the samples show large paramagnetic background at low temperature,
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which has also been reported in other research works [39, 40]. In Fig.8, zero field cooling (ZFC) magnetization curves are found to vary from concave to convex shapes with the annealed
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temperature increased from 750oC to 1000oC. The magnetization curves may be tuned by the variation of ratio of itinerant carrier density (c*) and localized spin density (c). Sun et al.
te
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theoretically proposed that the crossover of magnetization curvatures from concave to convex shapes occurred and the maximum FM transition temperature was reached with an optimal ratio of
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c*/c=0.05 [36]. In our research, the decrease of the carrier concentrations (c*) and the defect density is supported by the Hall measurements, Raman scattering spectra and PL spectra. The localized spin density is possibly contributed by substituted Co2+ ions and/or unpaired O 2p electrons surrounding zinc vacancy defects [40]. For Co-doped ZnO with dose of 8×1015 cm-2, density of vacancies/interstitials defects induced by ion implantation is expected as high as 1021
cm-3 calculated by SRIM program, two orders of magnitude higher than that of Co atoms. Upon annealing at 750oC, the crystalline is not recovered at all, and the localized spin density (c) is mainly contributed by vacancies/interstitials defects. As a result, the ratio of c*/c is still less than 0.01 (5.9×1018 cm-3/1021 cm-3). Therefore the shape of magnetization curve is concave with low 11
Page 11 of 20
critical temperature attained, well supported by the theoretical calculation [36]. In contrast to the usual convex curvature in Weiss mean-field theory, this concave shape is often used to be an indication of localization in the presence of disorder. As discussed above, the defect density
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reduced greatly along with the crystalline recovery at 900oC. In this case, Co2+ ions are supposed
cr
to contribute to the localized spin density (c), and the doping density of the implanted sample is
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4×1019 cm-3, on the assumption that all implanted Co2+ ions have diffused into the whole 2µm-thick ZnO layers. So the ratio of c*/c is given to be about 0.06 (2.5×1018 cm-3/4×1019 cm-3).
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The changes of c*/c ratio is the possible origin of the evolution of the magnetization curves varying from concave to convex shapes with an increased critical temperature. Furthermore, as the
M
annealing temperature further increased to 1000oC, the substitution of Zn by Co on well-ordered sites would not change anymore, but the improvement on the crystalline structure leads to the
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further reduction in localized spin density c, resulting in a higher ratio of c*/c, regardless of a
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smaller carrier concentration (c*). It hence, exhibits a smaller magnetic moment and a rather lower
Ac ce p
transition temperature [37]. Actually, the magnetic curve of the intrinsic ZnO film also shows the mixture of the paramagnetic and ferromagnetic characteristics as shown in Fig. 7 and the weak ferromagnetism have been reported induced by intrinsic defects [38, 39]. Besides the ferromagnetic character contributed by substituted Co2+ ions and vacancy defects, the observed paramagnetism could be resulted from the defects such as ionized interstitial Zn formed in the films during the growth and annealing processes. More detailed modeling of the defects and their dynamics are needed to ascertain whether the scenarios can account for the observed phenomena.
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V.SUMMARY We have demonstrated that thermal annealing of Co-implanted ZnO films has led to complex changes in its structures and physical properties, which showed strong dependence on the
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annealing temperature. Annealing over 900oC is required to recover the deteriorated crystalline
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structure. The observed changes in the electrical, optical and magnetic properties of the Co-doped
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ZnO films have been attributed to the evolution of the ion implantation induced defects with various annealing temperature. The change of magnetization curves of ZFC from concave to
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convex shapes have been explained and discussed from the viewpoint of the relative ratio of itinerant carrier and localized spin density. This study indicates that the structural disorder and
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intrinsic defects play a critical role in the observed magnetic properties in the ion implanted ZnO
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te
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films.
Acknowledgements：Research supported by the State Key Program for Basic Research of China under Grant No. 2011CB302003 and National Natural Science Foundation of China (Nos. 61025020, 60990312, 61274058 and 61322403), Basic Research Program of Jiangsu Province (BK2011437 and BK20130013) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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Figure captions Fig.1 (a) X-ray diffraction (XRD) patterns of the Co-implanted and their annealed ZnO films on
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sapphire substrate; (b) The full-width at half maximum of ZnO (0002) rocking curves of the ZnO: Co thin films annealed at different temperatures.
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Fig. 2 Triple-axis XRD patterns of the Co-implanted ZnO films annealed at different temperatures
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and the as-grown ZnO film for reference.
Fig. 3 Room-temperature Raman scattering spectra of ZnO as-grown film and the Co-implanted
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ZnO films annealed at different temperatures.
Fig. 4 Photoluminescence spectra of ZnO: Co films and ZnO as-grown film recorded at (a) 300K
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and (b) 10K, respectively.
Fig. 5 Hall carrier concentration and mobility of Co-implanted ZnO films as a function of
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annealing temperature.
Fig. 6 The temperature-dependent carrier mobility (a), concentration (b) and resistivity (c) of
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Co-implanted ZnO annealed at 750oC, 900oC and 1000oC respectively. Fig.7 M-H hysteresis curves of the ZnO as-grown film and Co-implanted ZnO films annealed at 750oC, 900oC and 1000oC, respectively at 5K.The inset shows the magnetic coercive force of ZnO:
Co sample annealed at 900oC.
Fig.8 The field cooling and zero field cooling magnetization curves of the ZnO as-grown film (a) and Co-implanted ZnO films annealed at 750oC (b), 900oC (c) and 1000oC (d), respectively.
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Highlight The paper includes nineteen pages and eight figures and it is about the effects of thermal annealing on the structural, electronic and optical and magnetic properties of Co‐implanted ZnO (0001) films grown on c‐plane sapphire substrates.
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To avoid the forming of Co clusters and explore the origin of the magnetism, detailed investigation on the properties of the Co‐implanted ZnO films with a rather low dose of
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8×1015 cm‐2 and high implantation energy of 1MeV were carried out.
The crystalline structure of the damaged region caused by ion‐implantation has been
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recovered via the thermal annealing treatment at the temperature of 900oC and above. The low temperature magnetic hysteresis loops have indicated paramagnetism for the
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annealed films with weak ferromagnetic characteristics.
The zero‐field cooling (ZFC) magnetization curves of the Co‐implanted ZnO samples have
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varied from concave shape to convex one as the annealing temperature increased from 750oC to 1000oC.
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