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Controlled defects in ZnO by low energy Ar irradiation

We report interesting observations in 1.2 MeV Ar8+ ion irradiated ZnO which, to the best of our knowledge, have not been published earlier and will be useful for the scientific community engaged in research on ZnO. Irradiation with the initial fluence 1 X 10^15 ions/cm^2 changes the colour of the sample from white to orange while the highest irradiation fluence makes it dark reddish brown that appears as black. Such changes in colour can be correlated with the oxygen vacancy type defects. No significant change in the grain size of the irradiated samples, as revealed from the x-ray diffraction (XRD) line width broadening, has been observed. Increase of surface roughness due to sputtering is clearly visible in scanning electron micrographs (SEM) with highest fluence of irradiation. Room temperature Photoluminescence (PL) spectrum of the unirradiated sample shows intense ultra-violet (UV) emission (~ 3.27 eV) and less prominent defect level emissions (2-3 eV). The overall emission is largely quenched due to initial irradiation fluence. But with increasing fluence UV emission is enhanced along with prominent defect level emissions. Remarkably, the resistivity of the irradiated sample with highest fluence is reduced by four orders of magnitude compared to that of the unirradiated sample. This indicates increase of donor concentration as well as their mobility due to irradiation. Oxygen vacancies are deep donors in ZnO, but surely they influence the stability of the shallow donors (presumably zinc interstitial related) and vice versa. This is in conformity with recent theoretical calculations.

Defects in ZnO and their role on structural, electronic and optical properties are being widely discussed at present. The extreme complexity of defects in ZnO has added merit to this agenda [1][2][3][4][5][6][7] . In one way, defect-induced tuning of material properties according to the technological need is becoming more and more prospective. For example, fabrication of p-type ZnO 8 (in general, ZnO bears intrinsic n-type conductivity) or developing UV-visible luminescent and lasing device with ZnO 9 or realizing ferromagnetic behavior in diamagnetic ZnO lattice 10  Therefore, controlled defect production in ZnO, defect characterization by suitable techniques and understanding the defect-property correlation is to be prime focus as of now. Controlled incorporation of defects generally involves high temperature annealing 14 , substitution 15 , mechanical milling 16 and irradiation by energetic particles 4,17 .
With the first three techniques, one can monitor the equilibrium defect configuration (from requirement of the free energy minimization) in the system. Energetic ion beam irradiation is an efficient way to induce non-equilibrium defect states in solid materials 18 .
In this paper, we have studied the effect of 1.2 MeV Ar 8+ ion irradiation on polycrystalline ZnO (Purity 99.99%, Sigma-Aldrich, Germany) from XRD, SEM, room temperature resistivity and photoluminescence (PL) measurements. SEM reveals the surface morphology of the samples whereas PL characteristics bear signature of few tens of nanometers (taking absorption coefficient 1.6 × 10 5 cm -1 at 325 nm 3,19 ) below the surface. XRD penetrates (1.54 Å Cu K α radiation) down to ~ 2 m 19 in ZnO while resistivity is purely a bulk property. All the prepared ZnO pallets are annealed in air at 500 o C. The pre-annealing before irradiation has been carried out to make the sample free from residual organic materials 10,20  leads to excitation and ionization of target atoms and is known as electronic energy loss (S e ). As the ion moves deeper inside the target its velocity becomes much lower than the Bohr velocity (0.22 cm/ns). In this regime another factor contributing to its energy loss becomes prominent. The ion suffers series of elastic collisions (with target atom nuclei) near the end of its trajectory. This energy loss of the ion due to elastic collision is known as nuclear energy loss (S n ). The total energy loss is the sum of electronic and the nuclear energy losses. The relative contribution of S e and S n depends on the projectile mass, velocity, charge state and on the target itself 27 31 have not ruled out the possibility of the occurrence of colour due to Zn I .
Recently, it has been confirmed by Selim et al. 32 and Halliburton et al. 33 that red colouration in ZnO is due to V O s. This is further evident from the fact that the red coloured ZnO becomes colourless transparent by annealing in oxygen atmosphere 32,34 . In the present case, the white samples turns out orange and finally dark reddish brown (appears as black) with increase of irradiation fluence (Fig. 3). It is understood that absorption in the blue green region 33 gives rise to such coloration in ZnO. We have m) 19 . So, here, only defects due to irradiation are being probed by XRD and no sign of amorphisation has been observed 7 . As far as XRD results are concerned, in the present work, high fluence (5 × 10 16 ions/cm 2 ) of 1.2 MeV Ar ions (penetrating less than the probing x-rays) does not produce any detectable chemical effect. Here, the initial fluence (1 × 10 15 ions/cm 2 ) produces two apparently opposite effects in XRD. The FWHM of the (101) peak is lowered as well as the peak intensity (Fig. 6). Such an effect has also been found 38  destruction of long range lattice order may lead to amorphisation with no XRD peak 41 . If the size of the defect clusters (isolated) is not very large but they are closely spaced then both FWHM as well as peak intensity may reduce 38 as is seen in this study. In case of such defect clusters placed in a relatively higher distance, peak FWHM may decrease with increase of coherent scattering leading to higher peak intensity 43  this condition is valid up to some low critical fluence. In a non-equilibrium condition during high fluence of irradiation, a saturation of defect concentration is more likely because the spatial separation between generated V Zn and Zn I is close enough to recombine immediately. A competition between defect generation and recovery (ionization induced recovery contributes to some extent) leads to such saturation of defects with high fluence regime 24 . We have also plotted the ratio (R) of peak intensities of (002) and (101) peak ( Fig. 7) with fluence. This also illustrates close similarity with the variation of peak FWHM with irradiation fluence (Fig. 7). The intensity of (101) peak in ZnO is believed to indicate 46 the oxygen deficiency to some extent. The variation of R shows dominant defect types at initial fluence and at higher fluence are different. A saturation of defect is also prominent at high fluence regime. The variation of R is not large enough for a conclusive understanding. However, to note, at least 50 % of total region probed by XRD is unaffected by the Ar beam. The variation of the position (2) of the (101) peak with fluence also follows the features of the variation of R (Fig. 7).
Lowering 2 is indicative of little expansion of lattice most probably due to increase of  Finally, we want to note a difference in the effect of annealing and ion irradiation of ZnO.
In case of annealing in air, the production of thermally generated Zn I requires very high temperature at which some Zn vapour also gets released. In such case, lowering of resistivity occurs without any prominent colouration 14  The origin of the 3.27 eV peak in the PL spectrum has also been debated. Most of the reports favor the excitonic origin 56 of this peak. However, the room temperature UV peak at such energies in granular ZnO may be an admixture of free exciton as well as some defect related transitions 53,61 . According to Qui et al. 61 such defects energy states are shallow and they reside near the grain boundaries. They assigned this defect giving rise to 3.28 eV peak as Zn I related defects. However, they favor such recombination is non-excitonic in nature. Fonoberov et al. have investigated 54 the thermal evolution of this UV peak in ZnO nanoparticles (~ 20 nm) with grain size similar to that of ours (~ 35 nm).
They attributed the origin of this peak is from donor bound excitons at room temperature.
The origin of defect level emissions (2-3 eV) in ZnO is even more It is to be noted that the overall emission is largely quenched with irradiation 1 × 10 15 ions/cm 2 . This is surely due to appearance of some non-radiative defect centres.
Interestingly, XRD peak FWHM is little lowered for this sample. This is due to the different probing region by XRD and PL. XRD probes down to ~ 2 m below the surface of the sample whereas PL comes from within first 100 nm or so. In the PL scan region ionization induced defect recovery is very high, particularly near the highly defective GBs (discussed earlier). Original GB related defect structure of the unirradiated sample