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Optical and electrical properties of Al doped ZnO thin film with preferred orientation in situ grown at room temperature
Ceramics International 45 (2019) 14347–14353
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Optical and electrical properties of Al doped ZnO thin film with preferred orientation in situ grown at room temperature
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Hongyan Liua,b, Xiaoqiang Wanga,b,∗, Mingya Lia,b, Shujin Yua,b, Rongxu Zhenga,b a b
Key Laboratory of Dielectric and Electrolyte Functional Materials Hebei Province, Qinhuangdao, 066004, China School of Natural Resources and Materials Science, Northeastern University at Qinhuangdao, Qinhuangdao, 066004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: AZO films Magnetron RF sputtering Room temperature Argon flow rate
To optimize the process and obtain highly conducting and transparent Aluminum-doped zinc oxide (AZO) thin films, AZO films were deposited on glass substrates at room temperature by Radio-frequency (RF) magnetron sputtering with various Argon flow rates. The influences of Argon flow rate on structure, morphology, optical, electrical and photoluminescence properties of AZO films were investigated by varying the Argon flow rate from 36 to 68 sccm. The best quality AZO film with resistivity 1.39 × 10−3 Ω cm, sheet resistance 8.2 Ω/sq and 84.2% average visible transmittance was prepared at 44 sccm for 30 min. Also, the self-heating effect of target was investigated by preparing AZO films for 10 min and 20 min at 44 sccm, 180 W and 1.0 Pa. The influence of increasing structural quality actually affected by Argon flow rate was more prominent on carrier concentration than mobility. The schematic illustration of microstructural evolution was proposed. The average growth rate of around 60 nm/min demonstrated the self-heating effect of target was weak and could be ignored.
1. Introduction The transparent conducting oxide (TCO) thin films are broadly used in organic light emitting devices, plasma displays, thin film solar cells, and flat-panel Displays [1], due to its high electrical conductivity and transmittance in visible and near-infrared spectral wavelengths [2]. Indium tin oxide (ITO) is the most common commercial TCO material owing to its low resistivity (≤10−3 Ω cm) and high transmittance (≥80%) in visible wavelengths [3]. However, indium is a high price rare metal and the high production cost will limit the production scale of ITO. Furthermore, the toxicity and instability in hydrogen plasma have restricted its application [4]. Recently, aluminum-doped zinc oxide (AZO) has attracted numerous interests owing to its outstanding characteristics such as a direct wide band gap, rich raw materials, low price, nontoxicity, high stability in hydrogen plasma, the relatively low growth temperature and capability to be produced on a large scale [5]. High quality AZO thin films can be prepared by different methods, including spray pyrolysis, sol-gel, pulsed laser deposition, atomic layer deposition, molecular beam epitaxy (MBE) and magnetron sputtering [3,6–12]. Among them, magnetron sputtering is widely used for largearea manufacturing in the industry due to high growth rate, high surface uniformity, high packing density, strong adherence and controllable parameters [11,13]. The properties of films depend strongly on the deposition ∗
conditions, including working pressure, the thickness of films, sputtering power, gas flow rate, temperature and deposition time [9,11,14–20]. It is believed that relatively high deposition temperature or post annealing is indispensable to gain the AZO films with excellent properties [4,11,17]. Nevertheless, high substrate temperature or annealing temperature will restrict the application of films. For example, high working temperature is not appropriate for electrochromic devices, CIGS solar cells and polymer substrates [9,19]. In this work, AZO films with approaching optical and electrical properties were prepared in situ by RF magnetron sputtering at room temperature. Luminescent characterizes are possibly related to the different native point defects which may be induced by the incorporation of Al in AZO films [15], and therefore the measurement of room temperature photoluminescence spectra is essential. The influences of Argon flow rate on microstructure, optical, electrical and photoluminescence properties of films were investigated in detail. 2. Experimental details 2.1. Deposition of samples AZO films were deposited on glass and quartz substrates (for photoluminescence spectra) by radio-frequency (RF) sputtering with an AZO ceramic target (ZnO:Al2O3, 98:2 wt%, 60 mm diameter). JGP-
Corresponding author. Key Laboratory of Dielectric and Electrolyte Functional Materials Hebei Province, Qinhuangdao, 066004, China. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.ceramint.2019.04.149 Received 26 March 2019; Received in revised form 16 April 2019; Accepted 17 April 2019 Available online 18 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 14347–14353
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Table 1 Deposition parameters of AZO thin films. Parameters
Value
Base pressure (Pa) Working pressure (Pa) Target-substrate distance (mm) Sputtering power (W) Deposition time (min) Film thickness (nm) Argon flow rate (sccm)
8.0 × 10−4 1.0 65 180 30 approx 1600 36, 44, 52, 60, 68
450A. The flow rate of Argon gas was varied from 36 sccm (standard cubic centimeters per minute) to 68 sccm. The deposition parameters were listed in Table 1. 2.2. Characterization of AZO films The physical morphologies of thin films were observed by the electron microscopy (SEM, SUPRA55, ZEISS, Germany). The structural properties were analyzed by X-ray diffraction (XRD, Cu Kα, 0.154056 nm, Rigaku, Japan) technique. To estimate the average grain size of AZO films, the Debye-Scherrer equation is quoted as follows [19]: D = kλ/β cos θ
Table 2 Structural parameters of (002) peak of films prepared at different Argon flow rates.
(1)
Where k is 0.94, λ is the wavelength (0.154056 nm) of Kα line of copper, θ is the Bragg's diffraction angle of (002) peak, β is full width at half maximum (FWHM) of the (002) plane peak (radians) and FWHM value of diffractometer was ignored when particles were considered spherically symmetrical. Electric characterizations were determined by Hall Effect measurements (ET9103). Optical transmission spectra were measured by UV–Vis spectrophotometer (Shimadzu UV-1800, Japan) in the wavelength range between 350 nm and 1100 nm. In order to obtain a balance between optical and electrical properties, the figure of merit (FOM) proposed by Haacke was quoted [11] and the FOM was calculated by the following equation: ϕ = T10/Rsheet
Fig. 1. XRD patterns of AZO films at different Argon flow rates.
(2)
Where ϕ is the figure of merit, T is the average optical transmittance and Rsheet is sheet resistance. The room temperature photoluminescence spectra (PL) of films were recorded by F-7000 Fluorescence Spectrophotometer and the excitation wavelength (λex) was 375 nm.
Argon flow rate (sccm)
2θ of (002) (°)
FWHM of (002) (degree)
Average grain size (nm)
36 44 52 60 68
34.240 34.340 34.359 34.340 34.300
0.525 0.500 0.531 0.483 0.456
16.5 17.4 16.4 18.0 19.1
(53pm), it's likely that ZnO lattice dimensions will diminish if Al ions replace the site of Zn ions on a large scale. Accordingly, the location of diffraction will shift in the direction of large degree. On the contrary, compared with the standard 2θ value from JCPDS data, a shift toward low 2θ values is detected in the location of (002) peak for all films, as shown in Table 2. The shift implies the existences of macro residual stress [9] or compressive stress which may originate from the interstitial Al atoms [14]. 3.2. Optical properties Fig. 2 displays the optical transmittance of samples prepared at different Argon flow rates. The average and maximum transmittance from 400 nm to 800 nm are depicted in the inset of Fig. 2. An average transmittance higher than 81% in visible wavelength is exhibited in all films. The tendency of structural quality is consistent with the tendency of average optical transmittance, which indicates the transmittance of
3. Results and discussion 3.1. Structural properties The X-ray diffraction patterns and structural parameters of the AZO films prepared at different Argon flow rates are presented in Fig. 1 and Table 2, respectively. In Fig. 1, an obvious diffraction peak is detected in all films around 2θ = 34.4°, which is matched with the (002) plane of ZnO (Card No. 36–1451). The hexagonal wurtzite structure and preferred C-axis orientation are obtained. No crystalline Al2O3 or metallic Al phase is found in the XRD pattern of film deposited at 68 sccm, which indicates that the preferable possibility of Al atoms substituted Zn sites in ZnO lattice. Nevertheless, it's still possible that the interstitial sites of ZnO lattice were occupied by Al ions or Al ions were segregated to the amorphous region in grain boundaries and Al-O bond came into being [14]. Additional peaks at around 2θ = 31° (marked ▲) and 2θ = 33° (marked ○) are shown in the XRD patterns of the others. The two peaks are inferred to ZnO phase (Card NO. 21–1486) and Zinc Aluminum oxides phase (Card NO. 22–1034) respectively. The presence of Zinc Aluminum Oxides indicates the presence of Al-O bond as mentioned above. Owing to the ionic radius of Zn2+ (72pm) is bigger than Al3+
Fig. 2. Variations of optical transmittance of AZO films with Argon flow rates.
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Fig. 3. Variations of the resistivity, carrier concentration and mobility with Argon flow rates.
films is relevant to the crystalline quality. With the approximately equal thickness of films, the enhancement of the absorption and scattering is caused by the increasing surface roughness, and the increasing surface roughness can be ascribed to the deterioration of crystalline quality [1,14]. 3.3. Electrical properties The electrical properties of films deposited at different Argon flow rates are shown in Fig. 3 and the variations of sheet resistance and figure of merit are shown in Fig. 4. The preferable carrier concentration of 2.99 × 1020 cm−3, the highest mobility of 15.02 cm2/V s and the lowest resistivity of 1.39 × 10−3 Ω cm are obtained for the film prepared at 44 sccm. The variation of resistivity is consistent with the relation:1/ρ = nqμ, where the q is a constant [4]. The tendency of carrier concentrations is in accordance with the tendency of structural qualities. This finding may be ascribed to the increasing carrier concentration which benefits from the enhancement of structural quality [9] and trap probability [15]. The increasing trap probability results from doping by substitutional Al and consequent interstitial Zn atom, while substitutional Al and interstitial Zn atom both play the role of donors [15]. The lowest mobility is obtained for the film prepared at 52 sccm, according to the literature reported by Prashant Misra [9], which is inferred that owing to the enhancement of surface roughness and grain boundaries affected by structural quality. The influence of increasing structural quality actually affected by Argon flow rate is more prominent on carrier concentration than mobility. As shown in Fig. 4, the lowest sheet resistance of 8.2 Ω/sq and the highest FOM of 2.184 × 10−2 Ω−1 are obtained for the film prepared at 44
Fig. 4. The average transmittance, figure of merit and sheet resistance of films as the. function of Argon flow rate.
Fig. 5. (a)The photoluminescence spectra of AZO films as the function of Argon flow rates corresponding to an excitation wavelength of 375 nm. (b)The PL emission spectra of AZO. films deposited at 44sccm after deconvolution and Pearson fitting.
sccm, which implies the best optical and electrical balance.
3.4. Photoluminescence properties Generally, the PL spectra of the deep-level (DL) emission in visible region are relevant to various lattice defects [15]. A common opinion agrees that oxygen vacancies (VO) and interstitial Zn (Zni) play the role of main donor center, meanwhile, zinc vacancies (VZn) play the role of acceptor center [15]. The photoluminescence spectra of AZO films deposited at different Argon flow rates are shown in Fig. 5(a). Fig. 5(b) displays the PL spectra after deconvolution and Pearson fitting, and the corresponding percentage of areas are marked as S1, S2, S3, S4, S5 and S6, which show the relative abundance of different defects in six bands and they are shown in Table 3. The first violet emission (405 nm) may be due to electron transition between conduction band and VZn state [21]. Meanwhile, the secondary violet emission (420 nm) may arise from the electron transitions from Zni to valence band [21] and it lies in around 2.96 eV [22]. Usually, the occurrence of blue emission is the result of transition from conduction band to VZn which lay around 2.75 eV [21,23]. It is also reported that if the electrons in Zni achieved the transition to VZn, the blue emission
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Table 3 The relative abundance of different defects in six bands after deconvolution at different Argon flow rates. Argon flow rate (sccm)
36 44 52 60 68
Relative abundance (%) S1
S2
S3
S4
S5
S6
1.43 6.90 6.67 3.42 3.56
34.94 38.90 36.64 24.85 42.26
43.93 34.45 31.26 47.05 33.58
16.44 15.14 23.37 21.68 18.35
2.23 4.04 0.67 0.98 2.08
1.34 0.87 1.80 2.40 0.46
Fig. 7. The cross-section SEM image of film deposited at 120 W for 30 min.
will be produced [21]. The green emissions at 482 and 492 nm are assigned to occur through potential electron transition from the VO to valence band [21], in addition, the VO are reported at around 2.5 eV [23]. Besides, the green emission may be led by both VO and VZn [23]. In general, shallow donor levels due to VO lie at 0.5–0.7 eV under the conduction band [24]. Meanwhile, Shallow acceptor levels due to Oi and VZn are at the energy state 0.4 and 0.3 eV above the valence band, respectively [25]. Maybe carrier transport benefits from shallow traps on surface oxygen vacancies or defects through the relatively inefficient and slow diffusion [26]. According calculation, it is found that the tendency of the abundance of VO is in accordance with the tendency of carrier mobility. Therefore, it is inferred that the green emissions at 482 and 492 nm are mainly related to the shallow donor levels due to VO. Finally, the PL transition energy diagram is proposed as shown in Fig. 6. From Fig. 6, the different PL emissions induced by different defects are demonstrated by this schematic band diagram. 3.5. Self-heating effect of target Energy is essential for the growth of the films. Plasma energy is also accumulated over time during the deposition process. The accumulation is assumed to be the self-heating effect of target in this paper. The influence of RF power on properties of films has been studied in previous work with the constant parameters of 44 sccm, 1.0 Pa, which is not shown in this paper. Surprisingly, the cross-sectional SEM image of film deposited at 120 W for 30 min is different from the others shown in Fig. 7. On the basis of the excellent optical and electric properties of film obtained at 180 W, 44 sccm and 30 min, and in order to initially explore whether self-heating of target has affected the properties of AZO films,
Fig. 6. The PL transition energy diagram related to different defects.
films are deposited for 10 min and 20 min at 44 sccm and 180 W, and the morphology, structure, optical and electrical properties are studied. The surface morphologies of AZO films deposited for 10 min, 20 min and 30 min are shown in Fig. 8. Films are uniform, dense and continuous and the surfaces of films present hilly and valley-like patterns. Similar hill-like structure can be obtained with a RF power above 99 W, which can be explained by the sufficient kinetic energy and successful diffusion of sputtered atoms into the equilibrium sites of surface during the film growing period [27]. With the increasing thickness, the grains become larger and the density of grain boundaries decreases. In Fig. 9, it is obvious that the crosses-section image of film deposited for 10 min is different from 20 min and the latter is likely to possess a dense columnar structure with increased thickness, which is inferred to be related to the island growth mode. Furthermore, the dense columnar structure of film deposited for 30 min is strengthened. Generally, the microstructural evolution of many sputter deposited films can be described by the Volmer-Weber (VW) mode which also known as island growth [28]. In VW island growth mode, the morphology evolves from nucleation and growth of isolated islands, impingement, coalescence which is associated with the formation of grain boundaries and channel filling, and eventually to thickening to a continuous film [29]. Meanwhile, the stress also evolves from an initial compressive stress in the early stage to a tensile stress in the second stage and finally to a tensile state or compressive stress condition [28]. The pre-coalescence compressive stress is proposed that is generated from the Laplace pressure in relation to the surface stress induced by the formation of initial islands, or in connection with adatom-surface interactions [30]. The tensile stress originates from the impingement of islands and associated with elastic strains as a result of islands merging to minimize the free surface energy caused by the individual islands [31]. In the final stage of island growth, elemental films with low adatom mobility have tendency to hold a tensile state such as Cr and Fe [32], and film with high adatom mobility like Ag and Cu can return to a compressive stress condition [33]. At the beginning of deposition, the incident adatoms arrived at the substrate surface and were absorbed, and in order to minimize the surface area, the hemispheric nuclei formed by the diffusion of adatoms and adatoms would be predominately captured by grown nuclei rather come into being a new nucleus [34]. The three-dimensional growth of nuclei induced the inter-islands impingement which caused the production of grain boundaries, meanwhile, the size and coverage of islands have a rapid increase [35]. With the continuous deposition, islands grew to coalesce and then formed a continuous film, and film thickened without further nucleation [34]. Interestingly, Fig. 7 shows that the growth of dense columnar structure in AZO film is not from the bottom up. Maybe this phenomenon can be explained by the molecular dynamics simulation of Xuyang Zhou [28] et al., who has proposed the opinion that the evolution of microstructure is influenced by different islands geometry, and the inter-islands partition distances of square and cylindrical islands will be predominantly resolved by zipping which means the elastic straining bridged the gaps, while the hemispherical islands will be mainly resolved by filling which means incoming adatoms filled the gaps. Then the schematic illustrations of microstructural evolution of different
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Fig. 8. The surface morphology images of films deposited at different time for (a) 10 min, (b) 20min, (c) 30min.
island morphologies are shown in Fig. 10. Furthermore, J.A. Thornton [36] reported the reasonability that a columnar grain structure will be adopted by films under different processing condition. It is found that compared with the film deposited for 10 min, film deposited for 20 min exhibits the presence of Al-O bond in the latter XRD analysis, which may indicate the more complex tensile impact [28]. After above analysis and discussion, a pictorial representation of the microstructural evolution which might have occurred with the growth of AZO film deposited on glasses at room temperature is proposed, as shown in Fig. 11. The XRD patterns and FWHM of (002) peak of films deposited for 10 min and 20 min are shown in Fig. 12 and Table 4, respectively. It can be estimated that with the increasing thickness, the crystalline quality has increased by comparing the FWHM of films. In Fig. 12, the XRD pattern of film deposited for 20 min shows an additional peak at around 2θ = 33° (marked ○) vs 10 min, and this peak was inferred to Zinc Aluminum oxides (Card NO.22–1034). In addition, the XRD pattern of film deposited for 30 min is different from the 10 and 20 min, which is shown in Fig. 1. The calculated average optical transmittance is shown in Table 4. Compared with the sample deposited for 30 min, it is obvious that the optical transmittance of films decreased with the thickness increased. The resistivity, sheet resistance and FOM of films deposited for 10 min and 20 min are shown in Table 4. The FOM of 20 min is extremely close to 30 min, which is 2.185 × 10−2 Ω−1 and 2.184 × 10−2 Ω−1 respectively, meaning the extensive choices for different applications. The growth rates of films deposited for 10 min, 20 min and 30 min are 59.0, 60.6, 56.6 nm/min respectively. The nearly equal results have highlighted the weak dependence of electrical properties on the selfheating effect of target. As for the decreased resistivity can be explained by the better crystalline properties of film deposited for 20 min than 10 min, which may be influenced by the thickness of film. More direct and deeper evidence of this effect on plasma energy accumulation remains to be further studied.
Fig. 10. The microstructural evolution of different island morphologies with time (1200 picosecond).
Fig. 11. The pictorial representation of the microstructural evolution with growth stages.
4. Conclusions AZO thin films with low resistivity, sheet resistance and high optical transmittance were deposited on glass substrates in situ by RF magnetron sputtering without any additional treatment. The Argon flow rate dependence of the structure, morphology, optical, electrical and photoluminescence properties, and the self-heating effect of target were investigated. Film deposited at 44 sccm for 30 min shows a polycrystalline structure in nature. The lowest resistivity of 1.39 × 10−3 Ω cm, the sheet resistance of 8.2 Ω/sq, the highest figure of merit of 2.184 × 10−2 Ω−1 and excellent average optical
Fig. 12. The XRD patterns of films at different deposition time.
transmittance of 84.2% were obtained simultaneously. The PL spectra imply various defect levels exist in AZO films. The schematic illustration of microstructural evolution is proposed. Considering the average growth rate of around 60 nm/min, the self-heating effect of target can be ignored. It indicates the procedure is suitable for the processing Fig. 9. The cross-section SEM images of films deposited at different time for (a) 10 min, (b) 20min, (c) 30min.
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Table 4 The FWHM, resistivity, sheet resistance, figure of merit and average transmittance of films deposited for 10 min and 20 min. Sputtering time (min)
FWHM (degree)
Resistivity ( × 10−3 Ω cm)
Resistance (Ω/sq)
Average transmittance(T%)
FOM ( × 10−2 Ω−1)
10 20
0.527 0.467
2.86 1.52
48.6 12.6
90.2 87.9
0.734 2.185
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Optical and electrical properties of Al doped ZnO thin film with preferred orientation in situ grown at room temperature
T
Hongyan Liua,b, Xiaoqiang Wanga,b,∗, Mingya Lia,b, Shujin Yua,b, Rongxu Zhenga,b a b
Key Laboratory of Dielectric and Electrolyte Functional Materials Hebei Province, Qinhuangdao, 066004, China School of Natural Resources and Materials Science, Northeastern University at Qinhuangdao, Qinhuangdao, 066004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: AZO films Magnetron RF sputtering Room temperature Argon flow rate
To optimize the process and obtain highly conducting and transparent Aluminum-doped zinc oxide (AZO) thin films, AZO films were deposited on glass substrates at room temperature by Radio-frequency (RF) magnetron sputtering with various Argon flow rates. The influences of Argon flow rate on structure, morphology, optical, electrical and photoluminescence properties of AZO films were investigated by varying the Argon flow rate from 36 to 68 sccm. The best quality AZO film with resistivity 1.39 × 10−3 Ω cm, sheet resistance 8.2 Ω/sq and 84.2% average visible transmittance was prepared at 44 sccm for 30 min. Also, the self-heating effect of target was investigated by preparing AZO films for 10 min and 20 min at 44 sccm, 180 W and 1.0 Pa. The influence of increasing structural quality actually affected by Argon flow rate was more prominent on carrier concentration than mobility. The schematic illustration of microstructural evolution was proposed. The average growth rate of around 60 nm/min demonstrated the self-heating effect of target was weak and could be ignored.
1. Introduction The transparent conducting oxide (TCO) thin films are broadly used in organic light emitting devices, plasma displays, thin film solar cells, and flat-panel Displays [1], due to its high electrical conductivity and transmittance in visible and near-infrared spectral wavelengths [2]. Indium tin oxide (ITO) is the most common commercial TCO material owing to its low resistivity (≤10−3 Ω cm) and high transmittance (≥80%) in visible wavelengths [3]. However, indium is a high price rare metal and the high production cost will limit the production scale of ITO. Furthermore, the toxicity and instability in hydrogen plasma have restricted its application [4]. Recently, aluminum-doped zinc oxide (AZO) has attracted numerous interests owing to its outstanding characteristics such as a direct wide band gap, rich raw materials, low price, nontoxicity, high stability in hydrogen plasma, the relatively low growth temperature and capability to be produced on a large scale [5]. High quality AZO thin films can be prepared by different methods, including spray pyrolysis, sol-gel, pulsed laser deposition, atomic layer deposition, molecular beam epitaxy (MBE) and magnetron sputtering [3,6–12]. Among them, magnetron sputtering is widely used for largearea manufacturing in the industry due to high growth rate, high surface uniformity, high packing density, strong adherence and controllable parameters [11,13]. The properties of films depend strongly on the deposition ∗
conditions, including working pressure, the thickness of films, sputtering power, gas flow rate, temperature and deposition time [9,11,14–20]. It is believed that relatively high deposition temperature or post annealing is indispensable to gain the AZO films with excellent properties [4,11,17]. Nevertheless, high substrate temperature or annealing temperature will restrict the application of films. For example, high working temperature is not appropriate for electrochromic devices, CIGS solar cells and polymer substrates [9,19]. In this work, AZO films with approaching optical and electrical properties were prepared in situ by RF magnetron sputtering at room temperature. Luminescent characterizes are possibly related to the different native point defects which may be induced by the incorporation of Al in AZO films [15], and therefore the measurement of room temperature photoluminescence spectra is essential. The influences of Argon flow rate on microstructure, optical, electrical and photoluminescence properties of films were investigated in detail. 2. Experimental details 2.1. Deposition of samples AZO films were deposited on glass and quartz substrates (for photoluminescence spectra) by radio-frequency (RF) sputtering with an AZO ceramic target (ZnO:Al2O3, 98:2 wt%, 60 mm diameter). JGP-
Corresponding author. Key Laboratory of Dielectric and Electrolyte Functional Materials Hebei Province, Qinhuangdao, 066004, China. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.ceramint.2019.04.149 Received 26 March 2019; Received in revised form 16 April 2019; Accepted 17 April 2019 Available online 18 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Table 1 Deposition parameters of AZO thin films. Parameters
Value
Base pressure (Pa) Working pressure (Pa) Target-substrate distance (mm) Sputtering power (W) Deposition time (min) Film thickness (nm) Argon flow rate (sccm)
8.0 × 10−4 1.0 65 180 30 approx 1600 36, 44, 52, 60, 68
450A. The flow rate of Argon gas was varied from 36 sccm (standard cubic centimeters per minute) to 68 sccm. The deposition parameters were listed in Table 1. 2.2. Characterization of AZO films The physical morphologies of thin films were observed by the electron microscopy (SEM, SUPRA55, ZEISS, Germany). The structural properties were analyzed by X-ray diffraction (XRD, Cu Kα, 0.154056 nm, Rigaku, Japan) technique. To estimate the average grain size of AZO films, the Debye-Scherrer equation is quoted as follows [19]: D = kλ/β cos θ
Table 2 Structural parameters of (002) peak of films prepared at different Argon flow rates.
(1)
Where k is 0.94, λ is the wavelength (0.154056 nm) of Kα line of copper, θ is the Bragg's diffraction angle of (002) peak, β is full width at half maximum (FWHM) of the (002) plane peak (radians) and FWHM value of diffractometer was ignored when particles were considered spherically symmetrical. Electric characterizations were determined by Hall Effect measurements (ET9103). Optical transmission spectra were measured by UV–Vis spectrophotometer (Shimadzu UV-1800, Japan) in the wavelength range between 350 nm and 1100 nm. In order to obtain a balance between optical and electrical properties, the figure of merit (FOM) proposed by Haacke was quoted [11] and the FOM was calculated by the following equation: ϕ = T10/Rsheet
Fig. 1. XRD patterns of AZO films at different Argon flow rates.
(2)
Where ϕ is the figure of merit, T is the average optical transmittance and Rsheet is sheet resistance. The room temperature photoluminescence spectra (PL) of films were recorded by F-7000 Fluorescence Spectrophotometer and the excitation wavelength (λex) was 375 nm.
Argon flow rate (sccm)
2θ of (002) (°)
FWHM of (002) (degree)
Average grain size (nm)
36 44 52 60 68
34.240 34.340 34.359 34.340 34.300
0.525 0.500 0.531 0.483 0.456
16.5 17.4 16.4 18.0 19.1
(53pm), it's likely that ZnO lattice dimensions will diminish if Al ions replace the site of Zn ions on a large scale. Accordingly, the location of diffraction will shift in the direction of large degree. On the contrary, compared with the standard 2θ value from JCPDS data, a shift toward low 2θ values is detected in the location of (002) peak for all films, as shown in Table 2. The shift implies the existences of macro residual stress [9] or compressive stress which may originate from the interstitial Al atoms [14]. 3.2. Optical properties Fig. 2 displays the optical transmittance of samples prepared at different Argon flow rates. The average and maximum transmittance from 400 nm to 800 nm are depicted in the inset of Fig. 2. An average transmittance higher than 81% in visible wavelength is exhibited in all films. The tendency of structural quality is consistent with the tendency of average optical transmittance, which indicates the transmittance of
3. Results and discussion 3.1. Structural properties The X-ray diffraction patterns and structural parameters of the AZO films prepared at different Argon flow rates are presented in Fig. 1 and Table 2, respectively. In Fig. 1, an obvious diffraction peak is detected in all films around 2θ = 34.4°, which is matched with the (002) plane of ZnO (Card No. 36–1451). The hexagonal wurtzite structure and preferred C-axis orientation are obtained. No crystalline Al2O3 or metallic Al phase is found in the XRD pattern of film deposited at 68 sccm, which indicates that the preferable possibility of Al atoms substituted Zn sites in ZnO lattice. Nevertheless, it's still possible that the interstitial sites of ZnO lattice were occupied by Al ions or Al ions were segregated to the amorphous region in grain boundaries and Al-O bond came into being [14]. Additional peaks at around 2θ = 31° (marked ▲) and 2θ = 33° (marked ○) are shown in the XRD patterns of the others. The two peaks are inferred to ZnO phase (Card NO. 21–1486) and Zinc Aluminum oxides phase (Card NO. 22–1034) respectively. The presence of Zinc Aluminum Oxides indicates the presence of Al-O bond as mentioned above. Owing to the ionic radius of Zn2+ (72pm) is bigger than Al3+
Fig. 2. Variations of optical transmittance of AZO films with Argon flow rates.
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Fig. 3. Variations of the resistivity, carrier concentration and mobility with Argon flow rates.
films is relevant to the crystalline quality. With the approximately equal thickness of films, the enhancement of the absorption and scattering is caused by the increasing surface roughness, and the increasing surface roughness can be ascribed to the deterioration of crystalline quality [1,14]. 3.3. Electrical properties The electrical properties of films deposited at different Argon flow rates are shown in Fig. 3 and the variations of sheet resistance and figure of merit are shown in Fig. 4. The preferable carrier concentration of 2.99 × 1020 cm−3, the highest mobility of 15.02 cm2/V s and the lowest resistivity of 1.39 × 10−3 Ω cm are obtained for the film prepared at 44 sccm. The variation of resistivity is consistent with the relation:1/ρ = nqμ, where the q is a constant [4]. The tendency of carrier concentrations is in accordance with the tendency of structural qualities. This finding may be ascribed to the increasing carrier concentration which benefits from the enhancement of structural quality [9] and trap probability [15]. The increasing trap probability results from doping by substitutional Al and consequent interstitial Zn atom, while substitutional Al and interstitial Zn atom both play the role of donors [15]. The lowest mobility is obtained for the film prepared at 52 sccm, according to the literature reported by Prashant Misra [9], which is inferred that owing to the enhancement of surface roughness and grain boundaries affected by structural quality. The influence of increasing structural quality actually affected by Argon flow rate is more prominent on carrier concentration than mobility. As shown in Fig. 4, the lowest sheet resistance of 8.2 Ω/sq and the highest FOM of 2.184 × 10−2 Ω−1 are obtained for the film prepared at 44
Fig. 4. The average transmittance, figure of merit and sheet resistance of films as the. function of Argon flow rate.
Fig. 5. (a)The photoluminescence spectra of AZO films as the function of Argon flow rates corresponding to an excitation wavelength of 375 nm. (b)The PL emission spectra of AZO. films deposited at 44sccm after deconvolution and Pearson fitting.
sccm, which implies the best optical and electrical balance.
3.4. Photoluminescence properties Generally, the PL spectra of the deep-level (DL) emission in visible region are relevant to various lattice defects [15]. A common opinion agrees that oxygen vacancies (VO) and interstitial Zn (Zni) play the role of main donor center, meanwhile, zinc vacancies (VZn) play the role of acceptor center [15]. The photoluminescence spectra of AZO films deposited at different Argon flow rates are shown in Fig. 5(a). Fig. 5(b) displays the PL spectra after deconvolution and Pearson fitting, and the corresponding percentage of areas are marked as S1, S2, S3, S4, S5 and S6, which show the relative abundance of different defects in six bands and they are shown in Table 3. The first violet emission (405 nm) may be due to electron transition between conduction band and VZn state [21]. Meanwhile, the secondary violet emission (420 nm) may arise from the electron transitions from Zni to valence band [21] and it lies in around 2.96 eV [22]. Usually, the occurrence of blue emission is the result of transition from conduction band to VZn which lay around 2.75 eV [21,23]. It is also reported that if the electrons in Zni achieved the transition to VZn, the blue emission
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Table 3 The relative abundance of different defects in six bands after deconvolution at different Argon flow rates. Argon flow rate (sccm)
36 44 52 60 68
Relative abundance (%) S1
S2
S3
S4
S5
S6
1.43 6.90 6.67 3.42 3.56
34.94 38.90 36.64 24.85 42.26
43.93 34.45 31.26 47.05 33.58
16.44 15.14 23.37 21.68 18.35
2.23 4.04 0.67 0.98 2.08
1.34 0.87 1.80 2.40 0.46
Fig. 7. The cross-section SEM image of film deposited at 120 W for 30 min.
will be produced [21]. The green emissions at 482 and 492 nm are assigned to occur through potential electron transition from the VO to valence band [21], in addition, the VO are reported at around 2.5 eV [23]. Besides, the green emission may be led by both VO and VZn [23]. In general, shallow donor levels due to VO lie at 0.5–0.7 eV under the conduction band [24]. Meanwhile, Shallow acceptor levels due to Oi and VZn are at the energy state 0.4 and 0.3 eV above the valence band, respectively [25]. Maybe carrier transport benefits from shallow traps on surface oxygen vacancies or defects through the relatively inefficient and slow diffusion [26]. According calculation, it is found that the tendency of the abundance of VO is in accordance with the tendency of carrier mobility. Therefore, it is inferred that the green emissions at 482 and 492 nm are mainly related to the shallow donor levels due to VO. Finally, the PL transition energy diagram is proposed as shown in Fig. 6. From Fig. 6, the different PL emissions induced by different defects are demonstrated by this schematic band diagram. 3.5. Self-heating effect of target Energy is essential for the growth of the films. Plasma energy is also accumulated over time during the deposition process. The accumulation is assumed to be the self-heating effect of target in this paper. The influence of RF power on properties of films has been studied in previous work with the constant parameters of 44 sccm, 1.0 Pa, which is not shown in this paper. Surprisingly, the cross-sectional SEM image of film deposited at 120 W for 30 min is different from the others shown in Fig. 7. On the basis of the excellent optical and electric properties of film obtained at 180 W, 44 sccm and 30 min, and in order to initially explore whether self-heating of target has affected the properties of AZO films,
Fig. 6. The PL transition energy diagram related to different defects.
films are deposited for 10 min and 20 min at 44 sccm and 180 W, and the morphology, structure, optical and electrical properties are studied. The surface morphologies of AZO films deposited for 10 min, 20 min and 30 min are shown in Fig. 8. Films are uniform, dense and continuous and the surfaces of films present hilly and valley-like patterns. Similar hill-like structure can be obtained with a RF power above 99 W, which can be explained by the sufficient kinetic energy and successful diffusion of sputtered atoms into the equilibrium sites of surface during the film growing period [27]. With the increasing thickness, the grains become larger and the density of grain boundaries decreases. In Fig. 9, it is obvious that the crosses-section image of film deposited for 10 min is different from 20 min and the latter is likely to possess a dense columnar structure with increased thickness, which is inferred to be related to the island growth mode. Furthermore, the dense columnar structure of film deposited for 30 min is strengthened. Generally, the microstructural evolution of many sputter deposited films can be described by the Volmer-Weber (VW) mode which also known as island growth [28]. In VW island growth mode, the morphology evolves from nucleation and growth of isolated islands, impingement, coalescence which is associated with the formation of grain boundaries and channel filling, and eventually to thickening to a continuous film [29]. Meanwhile, the stress also evolves from an initial compressive stress in the early stage to a tensile stress in the second stage and finally to a tensile state or compressive stress condition [28]. The pre-coalescence compressive stress is proposed that is generated from the Laplace pressure in relation to the surface stress induced by the formation of initial islands, or in connection with adatom-surface interactions [30]. The tensile stress originates from the impingement of islands and associated with elastic strains as a result of islands merging to minimize the free surface energy caused by the individual islands [31]. In the final stage of island growth, elemental films with low adatom mobility have tendency to hold a tensile state such as Cr and Fe [32], and film with high adatom mobility like Ag and Cu can return to a compressive stress condition [33]. At the beginning of deposition, the incident adatoms arrived at the substrate surface and were absorbed, and in order to minimize the surface area, the hemispheric nuclei formed by the diffusion of adatoms and adatoms would be predominately captured by grown nuclei rather come into being a new nucleus [34]. The three-dimensional growth of nuclei induced the inter-islands impingement which caused the production of grain boundaries, meanwhile, the size and coverage of islands have a rapid increase [35]. With the continuous deposition, islands grew to coalesce and then formed a continuous film, and film thickened without further nucleation [34]. Interestingly, Fig. 7 shows that the growth of dense columnar structure in AZO film is not from the bottom up. Maybe this phenomenon can be explained by the molecular dynamics simulation of Xuyang Zhou [28] et al., who has proposed the opinion that the evolution of microstructure is influenced by different islands geometry, and the inter-islands partition distances of square and cylindrical islands will be predominantly resolved by zipping which means the elastic straining bridged the gaps, while the hemispherical islands will be mainly resolved by filling which means incoming adatoms filled the gaps. Then the schematic illustrations of microstructural evolution of different
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Fig. 8. The surface morphology images of films deposited at different time for (a) 10 min, (b) 20min, (c) 30min.
island morphologies are shown in Fig. 10. Furthermore, J.A. Thornton [36] reported the reasonability that a columnar grain structure will be adopted by films under different processing condition. It is found that compared with the film deposited for 10 min, film deposited for 20 min exhibits the presence of Al-O bond in the latter XRD analysis, which may indicate the more complex tensile impact [28]. After above analysis and discussion, a pictorial representation of the microstructural evolution which might have occurred with the growth of AZO film deposited on glasses at room temperature is proposed, as shown in Fig. 11. The XRD patterns and FWHM of (002) peak of films deposited for 10 min and 20 min are shown in Fig. 12 and Table 4, respectively. It can be estimated that with the increasing thickness, the crystalline quality has increased by comparing the FWHM of films. In Fig. 12, the XRD pattern of film deposited for 20 min shows an additional peak at around 2θ = 33° (marked ○) vs 10 min, and this peak was inferred to Zinc Aluminum oxides (Card NO.22–1034). In addition, the XRD pattern of film deposited for 30 min is different from the 10 and 20 min, which is shown in Fig. 1. The calculated average optical transmittance is shown in Table 4. Compared with the sample deposited for 30 min, it is obvious that the optical transmittance of films decreased with the thickness increased. The resistivity, sheet resistance and FOM of films deposited for 10 min and 20 min are shown in Table 4. The FOM of 20 min is extremely close to 30 min, which is 2.185 × 10−2 Ω−1 and 2.184 × 10−2 Ω−1 respectively, meaning the extensive choices for different applications. The growth rates of films deposited for 10 min, 20 min and 30 min are 59.0, 60.6, 56.6 nm/min respectively. The nearly equal results have highlighted the weak dependence of electrical properties on the selfheating effect of target. As for the decreased resistivity can be explained by the better crystalline properties of film deposited for 20 min than 10 min, which may be influenced by the thickness of film. More direct and deeper evidence of this effect on plasma energy accumulation remains to be further studied.
Fig. 10. The microstructural evolution of different island morphologies with time (1200 picosecond).
Fig. 11. The pictorial representation of the microstructural evolution with growth stages.
4. Conclusions AZO thin films with low resistivity, sheet resistance and high optical transmittance were deposited on glass substrates in situ by RF magnetron sputtering without any additional treatment. The Argon flow rate dependence of the structure, morphology, optical, electrical and photoluminescence properties, and the self-heating effect of target were investigated. Film deposited at 44 sccm for 30 min shows a polycrystalline structure in nature. The lowest resistivity of 1.39 × 10−3 Ω cm, the sheet resistance of 8.2 Ω/sq, the highest figure of merit of 2.184 × 10−2 Ω−1 and excellent average optical
Fig. 12. The XRD patterns of films at different deposition time.
transmittance of 84.2% were obtained simultaneously. The PL spectra imply various defect levels exist in AZO films. The schematic illustration of microstructural evolution is proposed. Considering the average growth rate of around 60 nm/min, the self-heating effect of target can be ignored. It indicates the procedure is suitable for the processing Fig. 9. The cross-section SEM images of films deposited at different time for (a) 10 min, (b) 20min, (c) 30min.
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Table 4 The FWHM, resistivity, sheet resistance, figure of merit and average transmittance of films deposited for 10 min and 20 min. Sputtering time (min)
FWHM (degree)
Resistivity ( × 10−3 Ω cm)
Resistance (Ω/sq)
Average transmittance(T%)
FOM ( × 10−2 Ω−1)
10 20
0.527 0.467
2.86 1.52
48.6 12.6
90.2 87.9
0.734 2.185
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