Post-annealing effects on the structural and optical properties of vertically aligned undoped ZnO nanorods grown by radio frequency magnetron sputtering

Abstract

We report the nature of point defects associated with the visible transitions and X-ray photoelectron emissions of post-growth annealed ZnO nanorods under vacuum and air atmospheres. The ZnO nanorods are vertically aligned along the c-axis with a hexagonal cross section. The compressive strain in the as-grown ZnO nanorods has been completely relaxed by the post-growth annealing under vacuum. The relative quantity of oxygen deficiencies in the as-grown and post-annealed ZnO nanorods is calculated from the X-ray photoelectron spectra. Despite high oxygen deficiencies, the intense bi-donor bound exciton emission with narrow full width at half maximum reflects good optical quality of the vacuum annealed ZnO nanorods. The additional green and red emissions are attributed to electron transitions owing to the oxygen mediated defects in the nanorods.

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1. Introduction

Zinc oxide (ZnO) is one of the most morphologically rich materials and crystallizes in manifold structures such as nanoribbons,¹ nanotetrapods,² nanofibres,³ nanosheets,⁴ nanowires,⁵ nanobelts⁶ and nanoflowers.⁷ Various deposition techniques such as thermal evaporation,⁴ chemical bath deposition,⁸ metal–organic chemical vapor deposition,⁹ spray pyrolysis¹⁰ and sputtering¹¹ have been employed for the fabrication of various types of ZnO nanostructures. However, sputtering is one of the least investigated techniques for the fabrication of ZnO nanostructures since it is typically engaged for the deposition of thin films. However, one can easily grow nanostructures by fine tuning of the growth parameters, particularly pressure and temperature.

Recently, one-dimensional (1D) nanostructures of ZnO have received considerable attention as fundamental building blocks for optoelectronic devices such as light emitting and laser diodes in the short wavelength region¹² due to their wide and direct band gap of 3.37 eV with a large exciton binding energy (60 meV) at room temperature.¹³ Their large exciton binding energy in comparison with the thermal energy at room temperature (26 meV) allows an ultraviolet (UV) lasing action to occur even at room temperature.^14,15 The lasing efficiency of ZnO depends on the quality of the material. In general, ZnO commonly exhibits different defect mediated emissions in the visible region in addition to a dominant UV emission owing to its band edge. However, the correlation of visible emissions with the point defects is unclear, especially the origin of green luminescence in ZnO has been attributed to several intrinsic point defects such as oxygen vacancies (V_O),^16–18 oxygen interstitial sites (O_i),^19,20 zinc vacancies (V_Zn),^21,22 zinc interstitial sites (Zn_i)²³ and antisites of zinc and oxygen.²⁴ Furthermore, an earlier report suggested that the visible emissions in the photoluminescence (PL) spectrum of ZnO may be attributed to the various types of point defects in the same peak positions.²⁵ Hence, a detailed investigation of the point defects is essential to identify the nature of the visible emissions. It is obvious that temperature dependent photoluminescence (TDPL) spectroscopy can be used to analyze the point defects and exciton recombinations in ZnO along with a quantitative analysis using X-ray photoelectron spectroscopy (XPS).

In the present work, we have investigated the origin of the visible emissions and correlated these with the point defects in the as-grown (AG), vacuum annealed (VA) and air annealed (AA) ZnO nanorods. The relative oxygen deficiency for the VA sample is very high compared with the AG and AA ZnO nanorods. The neutral donor bound exciton transition is found to be independent of the oxygen mediated point defects in ZnO nanorods.

2. Experimental section

Vertically aligned ZnO nanorods were fabricated on n-type silicon (111) substrates by a radio frequency (rf) magnetron sputtering technique under a pure argon atmosphere using a high purity ZnO target. The growth conditions were previously described in detail.^26,27 In short, the deposition was carried out under pure argon with a deposition pressure of 0.01 mbar for 60 min with a fixed rf power of 60 W. The substrate temperature and target–substrate distance were kept constant at 650 °C and 50 mm, respectively. In order to correlate the point defects with the visible emissions, the AG ZnO nanorods were subjected to post-annealing at 550 °C for 60 min under vacuum (1 × 10⁻⁵ mbar) and open air atmospheres.

The surface morphology was examined by a field emission scanning electron microscope (FESEM, Carl Zeiss, ΣIGMA) with a maximum resolution of 1.2 nm. The crystalline nature of the ZnO nanorods was analyzed by a Rigaku X-ray diffractometer with Cu K_α radiation of wavelength λ = 1.5406 Å. The binding state of the ions was characterized by XPS (Omicron Nanotechnology Inc., Germany) studies using dual X-ray sources. Al K_α radiation with a photon energy of 1486.6 eV was used as an excitation source. The point defects in the ZnO nanorods were investigated by PL measurements in the temperature range between 10 and 300 K using a closed cycle helium cryostat. The luminescence signal from the sample excited with a 325 nm He–Cd laser was collected by a charge coupled device through a HORIBA JOBIN YVON monochromator (0.55 m) with an appropriate optical arrangement.

3. Results and discussion

Fig. 1 shows the FESEM images of the vertically aligned ZnO nanorods grown on silicon (111) substrates by rf-magnetron sputtering. The average diameter and length of the nanorods are ∼124 nm and ∼1 μm, respectively. The inset of Fig. 1(a) shows the diameter distribution of the AG ZnO nanorods. The FESEM images reveal that the nanorods are vertically aligned with a hexagonal cross section having smooth surfaces along the axial direction. In addition to that, the images clearly depict that the nanorods are densely packed with ∼1.5 × 10⁹ nanorods per cm². No noticeable changes were observed in the surface morphology of the post-growth annealed ZnO nanorods. The elemental analysis of the vertically aligned ZnO nanorods shows that the nanorods consist purely of zinc and oxygen (not shown).

Fig. 1 FESEM images of the vertically aligned ZnO nanorods grown by rf-magnetron sputtering. (a) Top view and (b) 45° tilted view. The inset of (a) shows the diameter distribution of the vertically aligned ZnO nanorods.

Fig. 2(a) shows the X-ray diffraction (XRD) patterns of the AG and post-growth annealed ZnO nanorods grown on silicon (111) substrates. All the diffraction peaks are indexed according to the hexagonal wurtzite structure of ZnO (JCPDS no. 89-0510). The dominant and weak peaks at around 34.38° and 72.6° correspond to the (002) and (004) reflections of ZnO, respectively, and indicate that the ZnO nanorods are oriented along the c-axis with a hexagonal wurtzite crystal structure. In addition to this, a weak peak is also observed at ∼62.8°, assigned to the (103) reflection of ZnO. The presence of this peak implies that a small number of nanorods are off-oriented from the c-direction. The (002) reflection of the AG (34.376°) ZnO nanorods is shifted to a lower angle by about ∼0.05° from its bulk value (34.430°), indicating uniform compressive strain. As shown in Fig. 2(b), the (002) peak positions of the AA (34.403°) and VA (34.429°) ZnO nanorods are shifted ∼0.025° linearly towards the bulk value from the (002) peak position of the AG ZnO nanorods. The observed (002) peak position of the VA ZnO nanorods is in good agreement with the standard bulk value. This indicates that the compressive strain in the ZnO nanorods is completely relaxed by the post-growth annealing under vacuum. Furthermore, the XRD intensity of the (002) peak in the VA ZnO nanorods is significantly enhanced compared with the AG ZnO nanorods and the relative intensity of the (103) peak is quenched, which can be attributed to the consequences of the recrystallization of the ZnO lattice during the vacuum annealing process. This indicates that the crystalline nature and orientation of the ZnO nanorods are considerably enhanced by the post-growth annealing under vacuum, which is substantiated by the narrow full width at half maximum (FWHM) of 685 arcsec for the (002) reflection of ZnO. The FWHM of the AG and AA ZnO nanorods are ∼720 and 700 arcsec, respectively.

Fig. 2 (a) XRD patterns of the ZnO nanorods grown on silicon substrates by rf-magnetron sputtering and (b) (002) peak positions of the AG, AA and VA ZnO nanorods.

XPS analysis was used to investigate the chemical compositions and valence states of the zinc and oxygen ions in the ZnO nanorods. Fig. 3(a)–(c) show the survey, Zn 2p and O 1s XPS spectra of the AG and post-growth annealed ZnO nanorods, respectively.

Fig. 3 XPS spectra of the AG and post-growth treated (VA and AA) ZnO nanorods. (a) Survey scan spectra, (b) Zn 2p spectra and (c) O 1s spectra.

The XPS survey spectra shown in Fig. 3(a) were recorded over a spectral range up to 1100 eV and reveal the presence of Zn, O and C in the samples. The element C is mainly ascribed to hydrocarbon contamination that occurs with XPS.²⁸ The peaks centered at 1022.8 and 1045.9 eV correspond to the Zn 2p_3/2 and Zn 2p_1/2 levels, respectively, as shown in Fig. 3(b), demonstrating that the Zn ions are in the +2 valence state. Furthermore, there is no noticeable change in the Zn 2p peak position with respect to the post-growth annealing under air and vacuum atmospheres. O 1s XPS spectra can be deconvoluted into two peaks by Lorentz fitting at 531.3 and 532.8 eV as shown in Fig. 3(c). The dominant peak at 531.3 eV is assigned to the O–Zn bond in the ZnO lattice²⁹ and the additional broad peak at 532.8 eV is associated with O²⁻ ions in the oxygen deficient regions within the matrix of ZnO.³⁰ It is obvious that oxygen and zinc vacancies are easily formed due to their small formation energy³¹ and that their formation will also depend on the growth conditions. Here, the growth of ZnO nanorods is carried out under a pure argon atmosphere at an elevated temperature of 650 °C. Consequently, the formation of oxygen vacancies is unavoidable for the above growth condition, which is favourable for the growth of ZnO nanorods. Therefore, the observed oxygen deficient peaks in the XPS spectra of the ZnO nanorods can be ascribed to oxygen vacancies. For the VA ZnO nanorods, the intensity of the oxygen deficient peak at 532.8 eV is increased in comparison with the AG and AA ZnO nanorods, indicating that a greater number of oxygen vacancies are created by annealing the nanorods under vacuum. The observed results agree with the earlier reports.³² Furthermore, the ratio of the oxygen deficient (O_D) peak area [O_D/O_Total] can be used to represent the relative quantity of oxygen vacancies in ZnO.³³ From the results, the ratios of the O_D peak area for the AG, VA and AA ZnO nanorods are 54.3%, 65.1% and 56.9%, respectively. This indicates that the VA ZnO nanorods have a lower oxygen content since vacuum annealing introduces more oxygen vacancies in the ZnO lattice.

Fig. 4(a) shows the TDPL spectra of the AG and post-growth annealed (VA and AA) vertically aligned ZnO nanorods grown on silicon substrates. The observed peaks at around 3.362, 3.266, 3.190 and 3.113 eV are similar for all the ZnO nanorods and correspond to the D⁰X, 1LO, 2LO and 3LO phonon replicas of two electron satellite (TES) emissions, respectively.³⁴ It is reported that the longitudinal optical phonon replicas of ZnO are separated from one another by 71–73 meV.^35,36 In addition to the D⁰X emission at 3.362 eV, one more peak was observed at 3.357 eV for the VA ZnO nanorods and is clearly shown in the inset of Fig. 4(a-2). The appearance of biexcitons at 3.362 and 3.357 eV for the VA ZnO nanorods indicates the high optical quality compared with the AG and AA ZnO nanorods. Furthermore, the peaks at 3.451, 3.307, 3.241 and 3.163 eV in the AA ZnO nanorods can be assigned to the free exciton (FX), 1LO, 2LO and 3LO phonon replicas of FX emissions, respectively.²⁴ The observation of weak FX emissions indicates good optical quality of the materials. The intensities of the green and red emissions are increased for the VA and AA ZnO nanorods, respectively, which indicate a greater number of oxygen vacancies and interstitial sites. However, the observation of biexciton and FX emissions in the VA and AA ZnO nanorods, respectively, reveals the greater optical quality of these nanorods compared with the AG ZnO nanorods.

Fig. 4 (a) TDPL spectra of the vertically aligned ZnO nanorods grown by rf-magnetron sputtering, (b) defects-mediated visible emission spectra of the ZnO nanorods and (c) integrated PL intensity of the D⁰X transitions vs. the inverse temperature for the ZnO nanorods.

Fig. 4(b) shows the TDPL spectra of the AG and post-growth annealed ZnO nanorods in the visible region of the electromagnetic spectrum. The observed peak at ∼2.28 eV for the AG and post-growth annealed ZnO nanorods corresponds to green luminescence, which is attributed to electron transitions from the shallow donor level of the oxygen vacancies (V_O) to a shallow acceptor level induced by the zinc vacancies (V_Zn) within the band gap.^26,37,38 This observed transition confirms the existence of the oxygen and zinc vacancies in the nanorods. It is obvious that point defects such as oxygen vacancies, zinc vacancies, oxygen interstitial sites, zinc interstitial sites and antisites of zinc and oxygen are expected to be formed depending on the growth conditions. In ZnO, oxygen and zinc vacancies are commonly observed due to their small formation energies compared with other point defects.³¹ Furthermore, the formation of oxygen vacancies is unavoidable with the above deposition conditions since the deposition is carried out under a pure argon atmosphere. However, for the AA ZnO nanorods, the intensity of the green emission (GE) is slightly quenched due to the compensation of oxygen vacancies from atmospheric oxygen gas molecules. However, with the VA ZnO nanorods, the intensity of GE at 2.28 eV is increased quantitatively compared with the AG and AA ZnO nanorods, which means that the VA ZnO nanorods have a greater number of oxygen vacancies. The XPS analysis pertaining to oxygen vacancies correlates well with the PL measurements. In addition to GE, the additional peak at around 1.8 eV in the AA ZnO nanorods corresponds to the red emission and is linked to oxygen mediated point defects such as oxygen interstitial sites.³² When annealing samples in an open air atmosphere, the oxidation of the surface as well as diffusion of oxygen molecules/atoms into the samples become imperative. Due to the greater number of oxygen molecules/atoms in the atmosphere, they will easily diffuse into the samples and compensate for the oxygen vacancies as well as create the interstitial sites. Consequently, GE is quenched and the red emission is enhanced for the AA ZnO nanorods. The ascribed structural and optical properties of the as-grown and post-annealed ZnO nanorods are displayed in Table 1.

Table 1 Comparison of the optical and structural properties of the AG and post-annealed ZnO nanorods

Sample	D^0X at 10 K (eV)	FWHM at 10 K (meV)	I(D^0X/GE) at 10 K	Red emission at 10 K (eV)	(002) peak position (Deg)	FWHM of (002) reflection (arcsec)	Interplanar spacing “d” (Å)	Lattice parameter “c” (Å)
AG	3.3617	23	34.7	—	34.376	720	2.6067	5.2134
VA	3.3619 3.3572	19 26	58.6	—	34.429	685	2.6027	5.2053
AA	3.3613	31	6.2	1.800	34.403	700	2.6047	5.2093

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Fig. 4(c) shows the variation of the integrated PL intensity of the D⁰X emission as a function of the inverse temperature for the AG and post-growth annealed (VA and AA) ZnO nanorods. The integrated PL intensity decreases with increasing temperature due to thermal quenching and is described by the following equation.³⁹

where I(T) and I(0) are the integrated PL intensities at temperature T and 0 K, respectively, A is the proportionality constant, E_a is the activation energy of the thermal quenching process, k is the Boltzmann constant and T is a thermodynamic temperature. The activation energies of the AG and post-growth treated (VA and AA) vertically aligned ZnO nanorods from the theoretical fittings are 6.5, 6.3 and 32.3 meV, respectively. These values are in good agreement with the earlier reports.^39,40 The activation energy is inversely proportional to the intensity quenching, which means that small values of the activation energy indicate high optical quality of the nanorods and vice versa. The small activation energy of the AG and VA ZnO nanorods (6.5 and 6.3 meV) indicates greater optical quality compared with the AA ZnO nanorods (32.3 meV). The activation energy of the AA ZnO nanorods is high due to the consequences of more point defects such as oxygen interstitial sites. In order to substantiate further, the line broadening of the D⁰X emission is analyzed and it was found that the PL line widths at 10 K for the AG and post-growth annealed (VA and AA) ZnO nanorods are ∼23, 19 and 31 meV, respectively. The small line width for the VA ZnO nanorods indicates high optical quality of the nanorods. The oxygen deficiency of the VA ZnO nanorods does not hinder the optical transitions; particularly D⁰X, as indicated by the narrow FWHM of ∼19 meV for the ensembles of VA ZnO nanorods.

4. Conclusions

ZnO nanorods have been successfully grown on silicon substrates by rf-magnetron sputtering under a pure argon atmosphere. The structural studies reveal that the AG ZnO nanorods experience a uniform compressive strain, which is relaxed by the post-growth annealing under vacuum. The morphological study reveals the high density (∼1.5 × 10⁹ nanorods per cm²) and vertical alignment of the ZnO nanorods with a hexagonal cross section. The oxygen deficiencies in the ZnO nanorods are calculated from the intensity variation of the O²⁻ and oxygen deficient peaks. Vacuum annealing significantly increases the oxygen deficiency compared with AG and AA ZnO nanorods. The biexciton peak for the VA ZnO nanorods signifies the high optical quality and the visible emissions at 2.28 and 1.8 eV are attributed to the defect mediated electron transitions, which substantiate the existence of oxygen mediated point defects in the nanorods.

Acknowledgements

KJ thanks the Department of Science and Technology (DST), Govt. of India for the financial support to develop the facility infrastructure under the schemes of Fund for Improvement of Science and Technology Infrastructure in Universities and Higher Educational Institutes (FIST) and Nanomission (Contract no. SR/NM/NS-77/2008). PSV acknowledges the Council of Scientific and Industrial Research (CSIR), Govt. of India for the award of a senior research fellowship (SRF). PSV also would like to thank Mr V. Purushothaman and Mr P. Dharmaraj for their kind technical assistance and fruitful discussions with the photoluminescence studies.

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