⇦Chinese Physics Letters, 2019, Vol. 36, No. 4, Article code 047301 Effect of Post-Annealing on Structural and Electrical Properties of ZnO:In Films * Guo-Ping Qin (秦国平)1**, Hong Zhang (张红)2, Hai-Bo Ruan (阮海波)3, Jiang Wang (王江)1, Dong Wang (王冬)1, Chun-Yang Kong (孔春阳)1 Affiliations 1College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331 2College of Physics, Chongqing University, Chongqing 401331 3Research Center for Materials Interdisciplinary Sciences, Chongqing University of Arts and Sciences, Chongqing 402160 Received 2 December 2018, online 23 March 2019 *Supported by the National Natural Science Foundation of China under Grant Nos 51472038 and 51502030, the Natural Science Foundation of Chongqing City under Grant Nos CSTC2016jcyjA and 2018jcyjA2923, the Education Commission of Chongqing under Grant Nos KJ1500319, 1501112 and KJ1600314, and the PhD Scientific Research Fund under Grant No 16XlB002.
**Corresponding author. Email: qingp206@163.com Citation Text: Qin G P, Zhang H, Ruan H B, Wang J and Wang D et al 2019 Chin. Phys. Lett. 36 047301    Abstract Indium-doped ZnO (ZnO:In) films are deposited on quartz substrates by rf magnetron sputtering. The effects of post-annealing on structural, electrical, optical and Raman properties are investigated by x-ray diffraction, Raman scattering, Hall measurement and first-principles calculation. The results indicate that all of the ZnO:In films have excellent crystallinity with a preferred ZnO (002) orientation. It is found that the incorporation of In can dramatically increase the intensity of the 274 cm$^{-1}$ Raman mode. However, both post-annealing treatment and increasing O$_{2}$ partial pressure in the process of preparing thin films can reduce the intensity of the 274 cm$^{-1}$ mode or even eliminate it, and relax compressive stress of the ZnO:In film judged by analyzing the shifts of the (002) Bragg peaks and $E_{2}$ (high) mode. Finally, the origin of the 274 cm$^{-1}$ mode is inferred to be the vibration of Zn interstitial (Zn$_{\rm i}$) defects, which play a crucial role in the high electron concentration and low resistivity of ZnO:In films annealed in an appropriate temperature range (450–600$^{\circ}\!$C). DOI:10.1088/0256-307X/36/4/047301 PACS:73.61.Ga, 78.30.Fs, 31.15.A- © 2019 Chinese Physics Society Article Text ZnO has generated great interest in optoelectronic devices, solar cells, gas sensors and other applications because of its wide direct-bandgap of 3.37 eV and large exciton binding energy of 60 meV at room temperature.[1,2] The combination of high conductivity and transparency for visible light makes this material a possible candidate for the transparent conductive oxide (TCO) in thin film solar cells.[3] It has been confirmed that group III elements (B, In, Al, Ga) doping can improve the electrical conductivity of ZnO films by increasing the carrier concentration.[4-8] Meanwhile, many researchers reported that the crystallinity and native defects of ZnO can be impacted by doping group III elements, among which indium is the most promising dopant to enhance the photoelectric properties of ZnO thin films due to its suitable ionic radius (0.62 Å) and large electronegativity (1.7).[9,10] For polycrystalline ZnO thin films deposited by various preparation methods, post-annealing can modify the grain size, lattice strain, the defect density of the films, and the extent of orientation in ZnO.[11,12] Although some groups concentrated on the effect of post-annealing on structural, electrical and optical properties of ZnO:In films,[13-16] there are few reports on Raman properties of annealed ZnO:In films. The 274 cm$^{-1}$ Raman mode is often present in ZnO:N,[17] ZnO:Al,[18] ZnO:Sb[19] systems. However, the origin of this additional Raman mode has attracted controversy in the past decade. In the present work, the 274 cm$^{-1}$ Raman mode is observed in the ZnO:In films grown on quartz substrates by rf magnetron sputtering. Combined with x-ray diffraction (XRD), Raman spectra, Hall measurement, and first-principles calculation, effects of post-annealing on structural, Raman, and electrical properties are deeply investigated. Furthermore, a simple model is proposed to explain the formation of compressive stress in ZnO:In films. Finally, we determine the origin of the 274 cm$^{-1}$ Raman mode observed in ZnO:In films, and propose that shallow donor Zn$_{\rm i}$ defects play a crucial role in the high electron concentration and low resistivity in ZnO:IIIA (IIIA=Al, Ga and In) by the appropriate annealing. ZnO:In films were deposited on quartz glass substrates (4 mm $\times$ 4 mm) by rf magnetron sputtering. The sputtering chamber was first evacuated to a base pressure below $8.0\times 10^{-4}$ Pa with a turbo molecular pump, then filled with Ar (99.999% purity) up to 1.0 Pa. The target for ZnO:In films was prepared using a sintering mixture of ZnO (99.99% purity) and 1.5 wt.% In$_{2}$O$_{3}$ (99.99% purity) powders at 1000$^{\circ}\!$C for 10 h in air ambient. The sputtering power and time were 100 W and 50 min, respectively. Finally, the ZnO:In films were annealed at 450$^{\circ}\!$C, 600$^{\circ}\!$C, 700$^{\circ}\!$C and 750$^{\circ}\!$C, respectively, for 30 min in high purity Ar (99.999%). The thickness of the as-grown sample is measured to be approximately 800 nm using a step profiler (AMBIOS XP-1). The crystal structure of the films was characterized by applying x-ray diffraction (XRD) with Cu $K\alpha_{1}$ radiation ($\lambda=1.540598$ Å). Raman measurements were carried out at room temperature (RT) using the 514.5 nm excitation lines from an Ar ion laser and a Horiba HR800 spectrometer in the back scattering geometry. Finally, the electrical properties were obtained by the van der Pauw Hall effect measurement (Ecopia HMS-3000) at RT.
cpl-36-4-047301-fig1.png
Fig. 1. (a) XRD patterns, (b) variation of ZnO (002) peak position, stress, and FWHM versus annealing temperature.
Figure 1(a) illustrates the XRD patterns for as-grown and annealed ZnO:In films. All the films exhibit only ZnO (002) Bragg peaks, and no traces of In and its oxides can be observed, indicating that all the ZnO:In films have excellent crystallinity with a preferred $c$-axis orientation and the In element has been doped into the ZnO lattice. The variation of ZnO (002) peak position, stress, and FWHM versus annealing temperature is shown in Fig. 1(b). It can be seen that all 2$\theta $ values of ZnO:In films compared to bulk ZnO (34.4$^{\circ}$) shift to a lower angle, indicating the existence of compressive stress for all the films. Meanwhile, the full width at half maximum (FWHM) of the (002) peak becomes narrower, while the average crystal size $D_{\rm acs}$ of ZnO:In films estimated using the Scherrer formula[20] increases from 28.347 to 34.153 nm with the increase of annealing temperature ranging from RT to 750$^{\circ} \!$C, which indicates that post-annealing treatment can reduce grain boundary and thus effectively improve the crystalline quality of the ZnO:In films. Based on the biaxial strain model,[21] the stress $\sigma$ parallel to the film surface is also calculated (see Fig. 1(b)). The negative, positive numbers represent compressive and tensile stresses in the film, respectively. First, the $\sigma $ of the as-grown ZnO:In film is $-$0.3762 GPa, meaning strong compressive stress. Then, the residual stress of the ZnO:In film gradually changes from compressive stress to tensile stress with the increase of $T_{\rm ann}$. This may be attributed to numerous defects at interstitial sites either driven off the films or occupying lattice vacancy. A detailed discussion about the process of defect evolution in combination with Raman spectrum will be given in the following. Raman spectroscopy is an effective technique for monitoring the defects evolution and evaluating the stress in the ZnO host lattice. Figure 2(a) shows the Raman spectra of as-growth and post-annealed ZnO:In films, as well as the spectrum of quartz substrate for comparison. Raman mode at 489 cm$^{-1}$ (marked by Q1) results from quartz substrate. It can be seen that the $E_{2}$ (high) mode at about 437 cm$^{-1}$ closely related to the crystal quality of ZnO thin film appears in all the samples. Furthermore, the intensity of $E_{2}$ (high) mode increases with $T_{\rm ann}$, indicating that the crystal quality of ZnO:In films can be improved by post-annealing treatment. As we know, the existence of compressive stress makes the $E_{2}$ (high) mode move to the high frequency. In contrast, tensile stress will bring it to the low frequency.[21,22] Figure 2(b) shows the shift of $E_{2}$ (high) mode of annealed ZnO:In films. It is found that the frequency of $E_{2}$ (high) mode of the as-grown film is higher than that of the ZnO standard sample (437 cm$^{-1}$),[23] indicating that the compressive stress exists in the as-grown ZnO:In film. The $E_{2}$ (high) mode of the ZnO:In films annealed at 450$^{\circ}\!$C is basically equal to 437 cm$^{-1}$. As the annealing temperature increases, the frequency of the $E_{2 }$(high) mode of all annealed ZnO:In films is less than that of the ZnO standard sample, which suggests that the ZnO:In films have tensile stress that is consistent with the results of XRD.
cpl-36-4-047301-fig2.png
Fig. 2. (a) Raman spectra of as-grown and post-annealed ZnO:In films. (b) Raman shift of $E_{2}$ (high) and (c) variation of the integral intensities of Raman modes at 274 and 580 cm$^{-1}$ versus $T_{\rm ann}$.
Two additional modes appear at $\sim $274 and 578 cm$^{-1}$ for the as-grown ZnO:In film as shown in Fig. 2(a). Based on the previous Raman backscattering measurements with different excitation wavelengths, the vibrational mode at $\sim $578 cm$^{-1}$ mode from resonantly enhanced longitudinal optical phonons ($A_{1}$(LO)) is commonly assigned to the Zn$_{\rm i}$, $V_{\rm O}$, or defect complexes containing Zn$_{\rm i}$ and $V_{\rm O}$ in ZnO.[24,25] From Fig. 2(c), it can be seen that the intensity of $\sim $578 cm$^{-1}$ mode decreases with the increase of annealing temperature ($T_{\rm ann} < 700^{\circ}\!$C), which implies that post-annealing treatment can reduce the concentration of Zn$_{\rm i}$ and/or $V_{\rm O}$ defects. Although the origin of $\sim $274 cm$^{-1}$ mode has been controversial in the past decade,[26-28] recent isotope experiments investigating the influence of isotropically purified ZnO thin films on the frequency of the vibrational mode around 274 cm$^{-1}$ firmly indicated that the mode is attributed to Zn$_{\rm i}$-related defects or small Zn clusters.[29] Note that the Raman intensities of both $\sim $274 and $\sim $578 cm$^{-1}$ modes have the similar changing tendency as shown in Fig. 2(c), which suggests that the origin of $\sim $274 cm$^{-1}$ mode may be related to Zn$_{\rm i}$ and/or $V_{\rm O}$ defects.
cpl-36-4-047301-fig3.png
Fig. 3. Comparison between Raman spectra of the Zn-rich and O-rich ZnO/ZnO:In films.
To further clarify the origin of $\sim $274 cm$^{-1}$ mode, it is necessary to analyze Raman spectroscopy of the ZnO:In films grown under both Zn-rich and O-rich conditions. Commonly, the stoichiometry of the sputtered metal oxide films can be controlled by the oxygen partial pressure in various O$_{2}$/Ar gas mixtures and rf-power.[30] In this work, the ZnO and ZnO:In films deposited by rf magnetron sputtering in pure Ar sputtering gas mean oxygen deficient, namely, the formation of Zn-rich films, and the O-rich ZnO and ZnO:In films can be obtained by controlling O$_{2}$ to Ar ratio under a cover gas mixture of Ar and O$_{2}$. Figure 3 shows the Raman spectra of Zn-rich and O-rich ZnO and ZnO:In films. It should be pointed out that the frequency of $E_{2}$ (high) mode of O-rich ZnO:In films is lower than that of the Zn-rich one by 1.2 cm$^{-1}$, which indicates that the compressive stress can be decreased by increasing the O$_{2}$ partial pressure. Moreover, it is obviously observed that the 274 cm$^{-1}$ mode is present in both the Zn-rich ZnO and Zn-rich ZnO:In films, and the incorporation of In can dramatically increase the intensity of the 274 cm$^{-1}$ mode. However, the 274 cm$^{-1}$ mode disappears in Raman spectrum for both the O-rich ZnO and ZnO:In films in our work. Therefore, it is demonstrated that the origin of the 274 cm$^{-1}$ mode is Zn$_{\rm i}$-related defects. In addition, the intensity of the 578 cm$^{-1}$ mode of the ZnO:In film ($T_{\rm ann}>700^{\circ}\!$C) is enhanced because the lattice structure of ZnO films is deteriorated and a large amount of related $V_{\rm O}$ defects are induced to a certain extent. For a clearer understanding of the evolution of intrinsic Zn$_{\rm i}$ and $V_{\rm O}$ defects in the ZnO:In films with annealing, we perform density-functional theory calculations in conjunction with the climbing image nudged elastic band (CI-NEB) method to study the self-diffusion of intrinsic Zn$_{\rm i}$ and $V_{\rm O}$ defects in ZnO:In. According to Ref. [31], the highly symmetric octahedral interstitial Zn$_{\rm i}^{2+}$ is set as initial and final configurations. Our calculations of the migration barriers of intrinsic Zn$_{\rm i}^{2+}$ give values of $0.7\pm 0.15$ eV, which are equivalent to the results of interstitial Zn$_{\rm i}^{2+}$ in ZnO bulk (0.8 eV).[32] For O vacancy, the migration barriers are, however, as much as $1.60\pm 0.15$ eV in Fig. 4. Therefore, the migration barriers for interstitial Zn$_{\rm i}^{2+}$ are in general small and preferentially diffuse to the second nearest neighboring positions, which indicates that the appropriate annealing treatment ranging from 400 to 600$^{\circ}\!$C can make Zn$_{\rm i}^{2+}$ active and improve the electrical conductivity of ZnO:In films, while $V_{\rm O}$ in ZnO:In only can diffuse by annealing at higher temperature ($T_{\rm ann}>700^{\circ}\!$C).
cpl-36-4-047301-fig4.png
Fig. 4. Calculated migration energy of (a) Zn$_{i}$, (b) $V_{\rm O}$ along in-plane (//$c$-axis) and out-of-plane ($\bot$$c$-axis) in the 96-atom w-ZnO:In supercell.
cpl-36-4-047301-fig5.png
Fig. 5. Electrical properties of as-grown and annealed ZnO:In films with various temperatures.
As seen from Fig. 5, the as-grown ZnO:In film exhibits a good n-type behavior with carrier concentration in the order of 10$^{19}$ cm$^{-3}$, and the electrical conductivity of ZnO films can be improved by annealing at an appropriate temperature. Notably, the film annealed at 450$^{\circ}\!$C for 30 min keeps better n-type conductivity with a hole concentration of $4.37\times 10^{20}$ cm$^{-3}$, a resistivity of 2.5$\times 10^{-3}$ $\Omega\cdot$cm, and a Hall mobility of 10.6 cm$^{2}$V$^{-1}$s$^{-1}$. The excellent electrical conductivity of ZnO:In films is attributed to the existence of intrinsic donor defects together with In donor impurities. Moreover, combined with the analysis of compressive stress in the samples, it can be concluded that the main form of intrinsic defects in ZnO:In films with good n-type behavior is Zn$_{\rm i}$. However, when the annealing temperature rises to nearly 700$^{\circ}\!$C, the carrier concentration is reduced to 10$^{17}$ cm$^{-3}$ and the resistivity increases two orders in magnitude. Here, the carrier concentration decreases at the higher $T_{\rm ann}$ may be Zn$_{\rm i}$ escaping from the films according to the low diffusion energy barrier of Zn$_{\rm i}^{2+}$. Meanwhile, a large amount of deep donor $V_{\rm O}$ defects are induced in ZnO:In films, correspondingly the residual stress changes from compressive stress to tensile stress from the above analysis. In summary, ZnO:In films have been obtained on quartz substrates by rf magnetron sputtering. The effects of annealing treatment ranging from 450 to 750$^{\circ}\!$C on structure and electrical properties of films were systematically examined. The crystallization of ZnO:In films by annealing is greatly improved. It is found that the incorporation of In can dramatically increase the intensity of the 274 cm$^{-1}$ mode. However, the residual stress of the ZnO:In films changes from compressive stress to tensile stress by annealing treatment or increasing O$_{2}$ partial pressure. Combined with the analysis of self-diffusion of intrinsic defects based on the CI-NEB method, the origin of the 274 cm$^{-1}$ mode is firmly believed to result from Zn$_{\rm i}$ defects, which play an important role in the excellent n-type conduction of donor In-doped ZnO films. 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