Identification of extended defect and interface related luminescence lines in polycrystalline ZnO thin films grown by sol–gel process

Abstract

The luminescence lines related to extended defects and interfaces in polycrystalline ZnO thin films grown by sol–gel process are deeply investigated by combining temperature-dependent photoluminescence and cathodoluminescence imaging with high-resolution transmission electron microscopy. A typical broad emission band is shown in the range of 3.316 to 3.333 eV and mainly consists of two distinct contributions. At high energy, a 3.333 eV line is associated with interfaces (i.e., free surfaces and grain boundaries) and predominates for small ZnO nanoparticles owing to their high density. The intensity ratio of the excitonic to interface-related transitions is low in this first configuration and the 3.333 eV line is characterized by an activation energy of 12.0 ± 1.2 meV and a Huang-Rhys factor of 0.54 ± 0.05 at 12 K. At low energy, a 3.316 eV line is attributed to basal plane stacking faults that are mostly of I₁-type and prevail for large ZnO nanoparticles. The 3.316 eV line is characterized by an activation energy of 6.7 ± 0.8 meV and a Huang Rhys constant of 0.87 ± 0.03 at 12 K. Basal plane stacking faults are most likely formed as the coalescence process proceeds with the decomposition and crystallization processes during annealing. As shown by low-temperature monochromatic cathodoluminescence imaging, the luminescence corresponding to the 3.316 eV line is, in this second configuration, limited to some specific area (i.e., large nanoparticles), and the relative intensity ratio of the excitonic to interface-related transitions is increased due to the smaller free surface area and density of grain boundaries.

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Introduction

The identification of extended defects and interfaces in polycrystalline ZnO thin films as well as the understanding of their effects on the optical properties are of primary importance for their efficient integration into a large variety of electronic, optoelectronic and photovoltaic devices.^1–7 Over the last decade, it has been stated by photoluminescence (PL) measurements that the radiative transitions involving extended defects and interfaces typically lie in the range of 3.31 to 3.33 eV and involve three main distinct lines located at 3.31, 3.329, and 3.33 eV. The 3.31 eV line has been reported in various kinds of ZnO materials including single crystals, thin films, and nanostructures grown by different vapour phase and solution deposition techniques.^8–13 It has been assigned to different types of radiative transitions involving two-electron-satellites (TES) and related phonon replica (PR) transitions, impurity-bound excitons, defect complex-bound excitons, free-electrons from the conduction band with holes bound to an acceptor state (e,A₀), or free-holes from the valence band with electrons bound to a donor state (D₀,h) (see for instance ref. 9 and therein). More recently, the structural extended defects located in the bulk^9,11 or surface contributions^10,12 have been emphasized to play a major role in this specific luminescence. Typically, by combining temperature-dependent PL measurements, low-temperature cathodoluminescence (CL) imaging, and transmission electron microscopy (TEM), Schirra et al. have attributed the 3.31 eV line to (e,A₀) radiative transitions located at basal plane stacking faults (BSFs).⁹ Nevertheless, the predominance of the so-called A-line in ZnO nanostructures with a high crystalline quality and a large surface-to-volume ratio may indicate that the main contribution to the 3.31 eV line is related to the surface through radiative transitions involving (e,A₀) or defect complex-bound excitons.^8,10,12 At higher energy, Khranovskyy et al. have revealed the presence of the 3.329 eV line in ZnO nanowires with a high density of BSFs.¹⁴ BSFs have been suggested to form zinc blend inclusions acting as quantum wells through a type II band alignment. The indirect recombination of electrons confined in the quantum wells with holes located in their vicinity may account for the 3.329 eV line. Nevertheless, since the same line has been pointed out from 3.319 to 3.324 eV, more complex mechanisms may be involved.^15–17 Eventually, a 3.333 eV line (sometimes referred to as Y₀) has been more and more discussed in the past years.^18–21 Wagner et al. have ascribed the Y₀ line to radiative transitions involving excitons bound to extended structural donor defect complexes that would be located in the vicinity of extended defects such as dislocations.¹⁸ In the same time, the Y₀ line has also been strongly correlated with the presence of interfaces such as grain boundaries and free surfaces, still through an excitonic-type transition.^19–21 Typically, it has been shown that the relative intensity of the 3.333 eV line with respect to the free-exciton line at 3.37 eV is increased by reducing the diameter of ZnO NPs in thin films grown by sol–gel process, namely by increasing the density of grain boundaries.²¹ The low activation energy of the Y₀ line may be due to the two following considerations: (i) the excitons might be bounded to the defect as two particles with different binding energies rather than as a quasi-particle in the case of a bulk model hypothesis¹⁸ or (ii) the interface barrier near surfaces might be decreased due to the local band bending in the case of an interface model hypothesis.²⁰

In this paper, the different contributions to the 3.31–3.33 eV broad emission band in polycrystalline ZnO thin films deposited by sol–gel process using dip coating with varying growth conditions are disentangled and identified by combining temperature-dependent PL measurements and low-temperature monochromatic CL imaging correlated with high resolution TEM (HRTEM) imaging.

Experimental

Polycrystalline ZnO thin films were grown on top of (001) silicon substrates by dip coating. An equimolar concentration of zinc acetate dihydrate (Merck) and monoethanolamine (JT Baker) in the range of 0.09 to 0.75 M were prepared by dissolving these two precursors in absolute ethanol. A pre-heat treatment was performed on a first hot plate at 300 °C for 10 minutes and then a post-heat treatment was immediately achieved on a second hot plate operating at high temperature (typically, 500 °C) for 1 h. The physical properties of the polycrystalline ZnO thin films were investigated by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), temperature-dependent PL, low-temperature CL, and TEM/HRTEM imaging. The FESEM images and XRD patterns were recorded using a ZEISS Ultra+ microscope and a Bruker D8 Advance diffractometer according to the Bragg–Brentano configuration, respectively. A JEOL-JEM 2010 microscope operating at 200 kV with a 0.19 nm point-to-point resolution was used for TEM and HRTEM imaging. The samples were polished on both sides using a series of plastic diamond lapping films with grains of decreasing sizes (30 μ, 15 μ, 6 μ, 1 μ, 0.5 μ) according to the tripod polishing method. Low-angle ion beam milling was used for their final perforation. PL measurements were performed under a closed circuit cryostat using a diode pumped continuous wave solid state laser operating at 266 nm (CryLaS FQCW 266-10), giving rise to a maximum excitation energy density of 5–10 W cm⁻². A monochromator (Jobin Yvon HR640) equipped with a 600 lines per mm grating blazed at 500 nm was used and associated with a GaAs photomultiplier (Hamamatsu H5701 50) for detection. Monochromatic CL images were recorded at 5 K using an Inspect F50 FEI FESEM equipped with a liquid helium cooled stage. For CL, the detection system was composed of a parabolic mirror collecting the light generated and focusing it on the entrance slit of a spectrometer (Jobin Yvon iHR550), and of a single channel detector (Hamamatsu multi-alkali PMT R928) placed at the exit of the spectrometer. A low SEM electron beam acceleration voltage of 5 keV was used to avoid charge effects and to reach a high lateral resolution (typically in the range of a few tens of nm).

Results and discussion

Structural and optical analysis on the macroscopic scale

The structural and optical properties of ZnO thin films are presented in Table 1 and Fig. 1. The ZnO thin films are polycrystalline and composed of one monolayer of ZnO NPs. The film thickness is typically found to decrease from about 94 ± 5 to 24 ± 3 nm, as the solution concentration is decreased from 0.75 to 0.09 M, while the average diameter of ZnO NPs is reduced from 60 ± 5 to 25 ± 3 nm. The relative evolution of the average diameter of ZnO NPs with respect to film thickness indicates that their quasi-spherical shape elongates towards a columnar shape, as the solution concentration is increased. The texture is strongly pronounced along the c-axis and drastically improved until reaching an optimum around 90% at 0.28 M.⁷ The texture mechanisms are governed by particle/particle interactions. The reduction of internal forces in the gel, when the primary cluster size in the sol is decreased, accounts for the stronger texture at low solution concentration. Interestingly, the thin film thickness as deduced from XRD measurements using Scherrer formula is significantly smaller than the film thickness as determined by cross-sectional FESEM images for the solution concentration of 0.75 and 0.37 M. The dense thicker films that are composed of one monolayer of large ZnO NPs are thus characterized by the presence of a sub-structure of grains or, more likely, of extended defects in their center.

Table 1 Morphological, structural, and PL properties of polycrystalline ZnO thin films composed of NPs and deposited by sol–gel process using dip coating with a solution concentration in the range of 0.09 to 0.75 M

Solution concentration	NP average diameter	C0002	Thin film thickness (SEM)	Thin film thickness (XRD)	Spectral position of the extended defect line
0.75 M	60 ± 5 nm	70%	94 ± 5 nm	55 nm	3.316 eV
0.37 M	44 ± 5 nm	79%	50 ± 5 nm	34 nm	3.319 eV
0.28 M	32 ± 3 nm	90%	38 ± 3 nm	34 nm	3.321 eV
0.09 M	25 ± 3 nm	88%	24 ± 3 nm	26 nm	3.333 eV

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Fig. 1 (a)–(d) Near-band-edge PL-spectra at 12 K of polycrystalline ZnO thin films presenting a decreasing NP average diameter of 60 ± 5, 44 ± 5, 32 ± 3 and 25 ± 3 nm, respectively. The insets are the corresponding top-view FESEM images. (e) and (f) Temperature-dependent PL spectra of polycrystalline ZnO thin films presented in (a) and (d), respectively. The insets are the energy shifts of the excitonic line (square full dots) and of the extended defect line (circle full dots) as a function of the temperature.

The corresponding 12 K PL spectra of the ZnO thin films, as presented in Fig. 1(a)–(d), are focused on the near band edge (NBE) emission, since no significant visible band emission arises, pointing out the good optical quality of the ZnO thin films. Whatever the film morphology, the NBE emission is dominated by two major lines. The first line at 3.368 eV is composed of radiative transitions involving free excitons and neutral/ionized donor-bound excitons.¹ Native point defects and impurities atoms, such as aluminium and/or hydrogen, may typically be involved since they are easily formed and incorporated in ZnO.^1,22 The second line lies in the range of 3.316 to 3.333 eV and may be related to radiative transitions involving extended defects. The remaining lines are the longitudinal optical (LO) phonon replicas of the second line since they are all separated by 72 ± 10 meV, which is equal to the energy of LO phonons in ZnO at low temperature.¹ Importantly, the second line undergoes a clear blue-shift from 3.316 to 3.333 eV, and its relative intensity with respect to the excitonic line at 3.368 eV is increased, as the NP average diameter is decreased from 60 ± 5 to 25 ± 3 nm. Since (i) the ZnO thin films were all deposited using the same process and (ii) the PL-spectra were all recorded using the same experimental conditions, it is deduced that the nature of the involved radiative transitions depends on the thin film morphology. In order to investigate these radiative transitions in more detail, temperature-dependent PL measurements of ZnO thin films grown with a solution concentration of 0.75 and 0.09 M are presented in Fig. 1(e) and (f), respectively. In the first case, when a 0.75 M solution concentration is used, it is shown that the intensity ratio between the extended defect line and its phonon replicas is not drastically decreased when the temperature is raised and that the second line vanishes above 180 K. This indicates that the extended defect line pointed at 3.316 eV cannot likely be attributed to the A-line.^8–13 Additionally, it should be noted that the annealing process applied here for crystallizing the ZnO thin film is achieved at 500 °C under ambient atmosphere. In such conditions, surface impurities such as surface adsorbed H species, which have been shown to play a significant role in the A-line luminescence, are not involved (see ref. 13 and therein). Importantly, the 3.316 eV line is found to undergo a strong blue-shift when the temperature is raised from 12 to 60 K, as shown in the inset of Fig. 1(e). This unusual behaviour, associated with a strong phonon coupling (S = 0.87 ± 0.03 at 12 K) and with the presence of tail states according to Yang et al.,¹⁷ is typical of radiative transitions involving BSFs.^14–17 By fitting the line intensity as a function of the temperature in an Arrhenius plot, a low activation energy of 6.7 ± 0.8 meV is determined. In the second case, when a 0.09 M solution concentration is used, the evolution of the extended defect line position with the temperature follows a classical excitonic behaviour, as seen in the inset of Fig. 1(f). The associated Huang-Rhys factor at 12 K is reduced down to 0.54 ± 0.05.²¹ Also, by fitting the 3.333 eV line intensity as a function of the temperature, an activation energy of 12.0 ± 1.2 meV is found. The 3.333 eV line observed here should thus be distinguished from the line involving BSFs and is proposed to be the Y₀ line.

Fig. 2 (a) Cross-sectional HRTEM image of a ZnO polycrystalline thin film grown with the solution concentration of 0.37 M with a special focus on one ZnO NP. (b) and (c) FFT respectively recorded on the NP and its neighbouring one as shown in (a). (d) and (e) HRTEM images respectively recorded at the NP free surface and at grain boundary as shown in (a) revealing the local disorder associated with the presence of these interfaces. (f) 101̄0 Bragg filtered images of the four BSFs crossing the NP from one side to the other side. (g) HRTEM image of the double I₁-type BSFs located in the center of the NP as shown in (a). All TEM images are collected along the 〈112̄0〉 zone axis.

TEM/HRTEM/CL analysis on the local scale

The presence of extended defects in polycrystalline ZnO thin films was investigated by cross-sectional TEM and HRTEM imaging on the film deposited by using a solution concentration of 0.37 M, as revealed in Fig. 2. The strong texture of the associated thin film along the c-axis leads to a low vertical misalignment between NPs: fast-Fourier-transforms (FFT) on the studied and adjacent NPs in Fig. 2(b) and (c), respectively, show only 6° of vertical misalignment. The free surface of the ZnO NP is not atomically sharp, but it is instead fairly rough leading to the occurrence of surface defects, as revealed in Fig. 2(d). Also, the grain boundary, as presented in Fig. 2(e), is expected to be of relatively high angle since the ZnO thin film does not present any in-plane orientation.⁷ More importantly, the NP is crossed from one side to the other side by a double defect line close to its center and by two single ones respectively located in its upper- and lower-parts. All these defects are identified as BSFs from HRTEM and 101̄0 Bragg filtered imaging (see Fig. 2(f)), which reveals a shift of the (101̄0) planes characterizing the BSFs in the wurtzite crystalline phase. The three BSFs located in the center and upper part of the NP are characterized as I₁-type BSFs, namely as defined by the plane stacking sequence …ABABABCACACAC… (see Fig. 2(g)). This type of BSFs is not related to strain relief, but it is instead nucleated typically during growth owing to its low formation energy. The double BSFs in the center of the NP consist of two consecutive I₁-type BSFs with a 1/3 [101̄0] component that is along the same direction as seen in Fig. 2(f). As a result, those neighboring I₁-type BSFs does not form an I₃-type BSF due to the fact that their in-plane components are not opposite.²³ In contrast, the BSF located in the lower part of the NP exhibits a larger translation vector along the [101̄0] direction and is most likely of I₂-type. It should further be noted that any associated partial dislocation bounding the BSFs are not identified since all the BSFs are terminated at the interfaces (i.e., free surfaces and/or grain boundaries). Interestingly, the formation of I₁-type BSFs has been reported in the center of GaN nanowires grown by molecular beam epitaxy and a direct relationship with a coalescence process has been pointed out.²⁴ Similarly, a coalescence process occurs as decomposition and crystallization processes proceed during annealing to form polycrystalline ZnO thin films by dip coating.⁷ Its magnitude may be more pronounced by increasing the primary cluster size in the sol, which is larger at higher solution concentration. I₂-type BSFs might additionally be nucleated, especially when the film texture along the c-axis is lost, as a consequence of stress effects generated at grain boundaries.^7,24 In both processes, the film porosity is expected to play a significant role. Denser films, as investigated in the present case, are, on the one hand, much more adapted for their integration into nanoscale engineering devices, but, on the other hand, strengthen coalescence and related stress effects. This is supported by the results previously published in ref. 21, where the spectral position of the extended defect line in porous ZnO thin films is not significantly influenced by the NP average diameter. Following this discussion, the formation of BSFs in ZnO thin films deposited by dip coating is an inhomogeneous process: some NPs contain BSFs while other NPs are BSF-free. By recording monochromatic CL images on the ZnO thin film deposited by using a solution concentration of 0.75 M, it is clearly revealed that the luminescence associated with BSFs is highly localized: the luminescence corresponding to the 3.316 eV line (i.e., recorded at about 3.32 eV) originates from specific NPs whereas the luminescence corresponding to excitonic transitions (i.e., recorded around 3.37 eV) is uniform, as shown in Fig. 3(a) and (b), respectively. The combination of these results with the ones discussed above shows that several distinct contributions are involved in the extended defect line of polycrystalline ZnO thin films grown by sol–gel process using dip coating. Typically, when the film is composed of small NPs, a high density of grain boundaries is formed together with a high free surface area and the coalescence process is limited. The extended defect line is, in this case, dominated by the presence of interfaces, such as free surfaces and grain boundaries, through the line at 3.333 eV. In contrast, when the film is composed of large NPs associated with a strong coalescence process, the contribution of grain boundaries and free surfaces is strongly reduced while the density of BSFs is drastically increased. The extended defect line is most likely the convolution of a major contribution at 3.316 eV related to I₁-type BSFs and of minor additional ones in its shoulders related to interfaces and/or I₂-type BSFs.

Fig. 3 5 K monochromatic CL images recorded on a ZnO polycrystalline thin film grown with the solution concentration of 0.75 M at (a) 3.32 eV and (b) 3.37 eV.

Conclusion

In summary, by combining temperature-dependent PL measurements and low-temperature CL imaging with HRTEM image analysis on polycrystalline ZnO thin films deposited by sol–gel process using dip coating, the structural origin of the extended defect and interface luminescence lines lying in the range of 3.31 to 3.33 eV has been clarified. Two independent single lines, which have been shown to be more or less pronounced depending on the thin film morphology, have clearly been identified: the 3.333 eV line, with an activation energy of 12.0 ± 1.2 meV and with a Huang-Rhys factor of 0.54 ± 0.05, is associated with the presence of extended defects located in the vicinity of interfaces (i.e., free surfaces and/or grain boundaries), while BSFs give rise to the 3.316 eV line with an activation energy of 6.7 ± 0.8 meV and a Huang-Rhys factor of 0.87 ± 0.03. The BSFs in these ZnO thin films are most probably the result of coalescence process and are mainly of I₁-type, as shown by HRTEM imaging. The present findings open the way for the control of the optical properties in the range of 3.31 to 3.33 eV by the specific treatment of the extended defects and interfaces involved.

Acknowledgements

Funding from la Région Rhône-Alpes via the Research Cluster Micro–Nano as well as from Grenoble INP via a Bonus Qualité Recherche grant through the CELESTE project is acknowledged. Sophie Guillemin held a doctoral fellowship from la Région Rhône-Alpes.

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