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Effect of 50 MeV Li3+ irradiation on structural and electrical properties of Mn doped ZnO

The present work aims to study the effect of ion irradiation on structural and electrical properties and their correlation with the defects in Zn1-xMnxO type system. Zn1-xMnxO (x = 0.02, 0.04) samples have been synthesized by solid-state reaction method and have been irradiated with 50 MeV Li3+ ions. The concomitant changes have been probed by x-ray diffraction (XRD), temperature dependent electrical resistivity and positron annihilation lifetime (PAL) spectroscopy. XRD result shows single phase wurtzite structure for Zn0.98Mn0.02O, whereas for Zn0.96Mn0.04O sample an impurity phase has been found apart from the usual peaks of ZnO. Ion irradiation dissolves this impurity peak. Grain size of the samples found to be uniform. For Zn0.98Mn0.02O, the observed sharp decrease in room temperature resistivity (RhoRT) with irradiation is consistent with the lowering of FWHM of the XRD peaks. However for Zn0.96Mn0.04O, RhoRT decreases for initial fluence but increases for further increase of fluence. All the irradiated Zn0.98Mn0.02O samples show metal-semiconductor transition in temperature dependent resistivity measurement at low temperature. But all the irradiated Zn0.96Mn0.04O samples show semiconducting nature in the whole range of temperature. Results of room temperature resistivity, XRD and PAL measurements are consistent with each other.


Introduction
Dilute magnetic semiconductors (DMS) have attracted much of research interest because of their potential application in the field of spintronics [1,2]. Ferromagnetism (FM) has been achieved both in II-VI and III-V semiconductors by addition of 3d transition metal (TM) elements [3,4]. The euphoria started following the prediction of room temperature ferromagnetism (RTFM) in Mn doped ZnO by Dietl et al. [5]. Thereafter, very often researchers reported intrinsic FM in TM doped ZnO [6][7][8][9][10]. However, the results are quite contradictory regarding the origin of FM in the host semiconductor [1,[10][11][12][13]. It is also conjectured that defects plays a crucial role in controlling the magnetic properties of such systems [7,8,[14][15][16]. Apart from magnetic properties, structural, optical and electrical properties of TM doped ZnO is also attracting lot of research interest [15,[17][18][19].
Energetic ion beam irradiation is an efficient tool for introducing defect states in solid materials.
Consequently, it is an important technique for controlled modifications of structural, optical and magnetic properties of semiconductors [20,21]. There are reports of irradiation studies on ZnO, both with light [22][23][24] and heavy [25,26]  implanted with Fe and observed that oxygen vacancies and/or Zn interstitials are introduced into the system due to irradiation [27]. Fukuoka et al. [28] and Sugai et al. [29] investigated the effect of high energy Xe and Ni ion irradiation on the electrical, optical and structural properties of Al doped ZnO films. They observed an increase in conductivity of the Al doped films with ion irradiation and suggested that the irradiation induced band gap modification has close relation with the conductivity increase. Formation of single phase Co-implanted ZnO thin films using swift heavy ion (SHI) irradiation has been reported by Angadi et al. [21] and R. Kumar et al. [30].
They found a decrease in the electrical resistivity of the irradiated samples and observed close interplay between electrical and magnetic properties. Also, very recently Sunil Kumar et al. reported SHI induced modifications in Co doped ZnO thin films and concluded that SHI irradiation can be used to improve the quality of the thin films by intrinsically modifying the structural and optical properties [20]. To the best of our knowledge, there is no such report on irradiation induced modification of electrical transport in Mn doped ZnO system and so a systematic investigation has been carried out.

Experiments
The Zn 1-x Mn x O (x = 0.02, 0.04) samples were synthesized by conventional solid-state reaction method [6,16,31]. Stoichiometric amount of ZnO and MnO 2 powders (each of purity 99.99%; Sigma-Aldrich, Germany) have been weighed, mixed and ground together. The samples were initially milled for 32 h followed by annealing at 400 0 C for 8 h. The resulting powder was again milled for another 64 h. All the milling was performed in "Fritsch planetary mono mill" machine (Model no: Pulverisette 6) using agate ball and container.
In order to avoid large grain size reduction (and hence to avoid the grain size related effects), the samples have been milled at ball to mass ratio of 1:1. The powder thus obtained was then pressed into pellets, followed by final sintering at 500 0 C for 12 h. The reasons for choosing 500 0 C as the final sintering temperature has been discussed earlier [31]. The phase characterization of the Zn 1-x Mn x O (x = 0.02, 0.04) samples before and after irradiation has been carried out using powder x-ray diffractometer [Philips, Model: PW1830] with Cu-K  radiation. All X-Ray Diffraction (XRD) measurements were carried out in the range of 20 0 < 2 < 80 0 in -2 geometry. The electrical resistivity as a function of temperature of all the samples was measured using conventional two-probe technique.
A Keithley electrometer (model 6514) was employed to measure the resistance. Positron annihilation lifetime (PAL) measurement at RT was performed on Zn 0.98 Mn 0.02 O sample with 0, 1 × 10 12 , 5 × 10 13 ions/cm 2 irradiation fluence. For PAL study, a 10-Ci 22 Na positron source (enclosed in 2 micron thin mylar foil) was sandwiched between two identical plane faced pellets of the samples. The PAL spectra were measured with a fast-slow coincidence assembly having 182 + 1 ps time resolution [14]. Measured spectra were analyzed by computer program PATFIT-88 [32] to obtain the possible lifetime components  i and their corresponding intensities I i .  [31]. Interestingly, this impurity peak disappeared just after irradiation with lowest fluence (1×10 12 ions/cm 2 ), as shown in inset of figure 2. This indicates that the impurity phase has been dissolved and the sample become single phased, at least within the detection limit of XRD.
Irradiation induced dissolution of impurity phase has also been observed earlier for Ag ion irradiated Co and Fe doped ZnO thin films [21,27,30]. But for Mn doped ZnO system, probably this is the first report of impurity phase dissolution using comparatively lighter ion, Li beam irradiation. The irradiated particles after entering the target suffers a number of collisions both elastic and inelastic and loses its energy via these interactions viz., electronic energy loss (S e ) and nuclear energy loss (S n ) respectively [20,21,27]. The evaluation using the simulation program SRIM-2008 [33] showed that the mean electronic energy loss S e and nuclear energy loss S n of 50 MeV Li 3+ -ions in our synthesized Mn doped ZnO samples are 13.69 eV/Å and 7.65 × 10 -3 eV/Å, respectively. Thus irradiated Li ion loses its energy mainly by electronic energy loss process. Transfer of energy to the lattice, locally, can modify the impurity phase to more stable doped ZnO structure. Thus, in concurrence to the earlier reports of obtaining single phase TM doped ZnO thin films by ion beam irradiation [20,21,27,30], our study also lead to possibility of single phase formation in 4 at % Mn doped ZnO with Li ion irradiation.
Furthermore, we have monitored the variation of peak intensity as well as FWHM of the most dominant (101) peak with irradiation fluence. Figure 3 shows that both the intensity and FWHM of the (101) peak of Zn 0.98 Mn 0.02 O, decreases strongly with initial irradiation fluence and gradually saturates at higher fluence. This decrease in peak intensity along with its decreasing FWHM appears to be contradictory in nature, as far as crystallinity of the sample is concerned. But this apparent contradicting behavior has also been reported earlier in 100 KeV Ne and 1.2 Mev Ar irradiated ZnO [34,35]. It is noteworthy to mention that, we observed some peculiar behavior in temperature dependent resistivity data of irradiated Zn 0.98 Mn 0.02 O samples, which may have some correlation with the strange features obtained in XRD for the same samples. In general, polycrystalline samples are rich in defects. Also the grain boundaries (GB) are much more defective compared to the grain interiors [36]. During the passage of energetic projectiles the re-organization of defects in the GB region is more likely due to larger abundance of defects there. Thus, with increasing fluence up to the dose of 1x10 13 ion/cm 2 , the distribution of the grain orientation becomes sharper with increase in grain size, which is reflected in the decrease in FWHM of the most intense (101) peak [35,37]. Whereas in the grain interiors, before irradiation which was less defective region, the energy lost by the high energy ion beam also creates some defects. Thus within each grain, the crystalline quality degrades with increasing ion dose, as also reported by Matsunami et al [35]. This could explain the decrease of the XRD intensity of the most dominant (101) peak with initial doses of fluence. Thus high energy ion beam irradiation affect the GB regions and grain interior regions in two mutually opposite direction, as obtained earlier by Matsunami et al [35]. With further increase of doses of irradiation, the competition between these two effects leads to homogenization of defects to some extent. Also a saturation of defects is expected in such case [34,38]. This is reflected in figure 3, where with irradiation dose the FWHM as well as peak intensity initially deceases and then gradually saturates for the irradiated Zn 0.98 Mn 0.02 O samples.
We have also monitored the variation of peak intensity as well as FWHM of the most dominant (101) host ZnO matrix increases with dissolution of impurity phase. Thus the observed shift of (101) peak towards lower angle (enhancement of lattice parameter) seems to quiet justified as ionic radii of Mn 2+ (0.67 Å) is higher than that of Zn 2+ (0.60 Å) [40]. There must be some higher angle shifting of (101) peak due to release of residual strain in the system with irradiation in case of Zn 0.96 Mn 0.04 O samples also. But lower angle shifting of (101) peak due to more Mn incorporation with irradiation predominates over the earlier strain release effect. MeV Ar irradiated ZnO [34]. Huge resistance loss due to irradiation by light/heavy energetic ions has also been observed by other groups [42]. The reason for change in resistivity due to irradiation, particularly in doped ZnO systems, is a matter of investigation till date [17]. The decrease of resistivity is due to increase of donors or deactivation of compensating acceptors or both. In polycrystalline samples, most of the vacancy clusters reside near the GB region as mentioned earlier. The region is devoid of carriers (depletion region) and act as a potential barrier during the transport of carriers between the grains. Increased donor density can reduce the height of the potential barrier in n type ZnO. On the other hand, recovery of a fraction of GB defects can also lower the carrier scattering at GB. At the same time, Dong et al [17] proposed that the presence of large vacancy clusters and huge oxygen vacancies are the source of reduced resistance in ion irradiated ZnO. It should be mentioned here that, electronic energy deposition can excite (and also ionize) the atoms and after de-excitation within few ps, a reorganization of local defect structure is possible. This process is more effective near the highly defective  [27,21,43]. It is noteworthy to mention that, our observation is slightly different. We observed that the sample is metallic in low temperature regime and semiconducting in higher temperature. Also the sample irradiated with lowest fluence shows multiple transitions but for the samples irradiated with higher fluences (1×10 13 ions/cm 2 to 1×10 14 ions/cm 2 ), only one transition has been observed. Angadi et al [21] and Nistor et al [43] have shown that, the presence of oxygen vacancies possibly give rise to metal-semiconductor transition in ZnO thin film. Further we observe that the transition temperature shifts towards lower temperature with increasing doses of irradiation The possible impurity phase related problem may complicate the PAL data for 4 at% Mn doped samples and so the measurement for this sample is avoided. The results of PAL spectrum analysis are shown in table 1. All the PAL spectra are found to be best fitted with three lifetime components. The longest lifetime component (   1400 ps with intensity 3-5%) originates due to the annihilation of positron from orthopositronium atoms. Decay of orthopositronium into parapositronium through pickoff annihilation gives rise to such a large lifetime. In polycrystalline samples, there always exist micro voids where orthopositronium formation is favourable [14]. As the first lifetime component ( 1 ) and the intermediate one (  ) have changed significantly with irradiation fluence, we feel that both have defect related origin.   and   provide a qualitative indication about the spatial extension of the defects (i.e, defect size). The corresponding intensities (I 1 and I 2 respectively) reflect the relative abundance of such defect sites. For unirradiated sample the value of   is close to the positron lifetime at zinc vacancies in ZnO. Indeed, Wang et al [44] have attributed the origin of   from the diffused zinc vacancies at the interface of the grains. The value of   indicates that there exist large vacancy clusters in the sample, possibly of type V Zn-O divacancies near the GB region [14]. High concentration of open volume defects and related disorder can produce foam like structure near the GB as predicted by Straumel et al. [8]. Such grain boundary disorder is drastically modified due to Li 3+ irradiation. Compared to unirradiated sample,   sharply decreases after initial fluence of 1 × 10 12 ions/cm 2 .  RT also shows decrease with increasing irradiation fluence. This is due to formation of increasing neutral oxygen vacancies with irradiation fluence. As decreasing trends of   as well as RT with irradiation indicate increasing presence of free electrons in the system. It is possible that a fraction of the injected positrons are now annihilated in the bulk of the sample. So   becomes closer (but lower) [45] to the bulk lifetime of positrons in ZnO after irradiation. It is indeed an interesting observation.   also shows significant reduction due to initial dose of irradiation. Hence, we feel that there is no evidence of increase in vacancy clusters with initial dose of irradiation. Also, we observe that,   and   shows a minute increase with further irradiation fluence. However,   and   are fitting dependent parameters to some extent. In such complex systems, statistically more accurate parameter is average positron lifetime,  av = (  I 1 +   I 2 )/(I 1 +I 2 ), which reflects the overall defective nature of the sample [14,46].  av decreases due to irradiation with 1 × 10 12 ions/cm 2 flunce and remains unaffected due to further increase of irradiation.

Conclusion
The un-irradiated and 50 MeV Li 3+