J Mater Sci: Mater Electron (2015) 26:734–741 DOI 10.1007/s10854-014-2457-2

Optical, electrical and structural properties of aluminum-doped nano-zinc oxide thin films deposited by magnetron sputtering Jin Hua Gu • Lu Long • Zhou Lu • Zhi You Zhong

Received: 31 August 2014 / Accepted: 25 October 2014 / Published online: 4 November 2014 Ó Springer Science+Business Media New York 2014

Abstract Nano transparent conducting aluminum-doped zinc oxide (AZO) thin films were deposited on glass substrates by the magnetron sputtering technique. The thin films were characterized with X-ray diffractometer, scanning electronic microscopy, four-point probe and UV– Visible spectrophotometer. The dependence of structural, morphological, optical and electrical properties on substrate temperature was investigated. The results show that all the thin films have hexagonal wurtzite structure with highly c-axis orientation. The structural and optoelectrical properties of thin films are observed to be subjected to the substrate temperature. The AZO thin film deposited at the substrate temperature of 370 °C possesses the best optoelectronic properties, with the lowest resistivity of 6.12 9 10-4 X cm, the minimum microstrain of 0.92 9 10-3, the highest average visible transmittance of 85.1 % and the maximum figure of merit of 1.03 9 104 X-1 cm-1. The optical bandgap of thin films was estimated from Tauc’s relation and observed to be an increasing tendency with the increment of the substrate temperature. Furthermore, the optical constants such as refractive index, extinction coefficient, dielectric constant, dissipation factor and optical conductivity were determined by the pointwise unconstrained optimization method, and

J. H. Gu (&) Optical Laboratory, Center of Experiment Teaching, SouthCentral University for Nationalities, Wuhan 430073, People’s Republic of China e-mail: [email protected] L. Long  Z. Lu  Z. Y. Zhong (&) Plasma Research Institute, South-Central University for Nationalities, Wuhan 430073, People’s Republic of China e-mail: [email protected]

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the dispersion behaviour was studied by the Wemple–DiDomenico single-oscillator model.

1 Introduction Indium-tin oxide (ITO) has been most widely used as a transparent conducting oxide (TCO) electrode in photovoltaic solar cells [1–5], organic light-emitting diodes (OLEDs) [6–10], liquid crystal displays (LCDs) [11], plasma display panels (PDPs) [12] and gas sensors [13], since it has high visible transmittance, low resistivity and relatively high work function [14]. However, indium (In) is rare metal and reserve volume is scare, which limits its application. Impurity-doped (such as B, Al, Ga and Zr, etc.) zinc oxide (ZnO) semiconductor materials have attractive characteristics owing to its wide direct bandgap, the abundant raw materials, environmental friendliness and high radiation resistance. Based on many reported investigations of the impurities effect on ZnO films, the aluminum-doped ZnO (AZO) thin films not only have the similar optical and electrical properties to ITO, but also possess many advantages of inexpensiveness, nontoxicity, easy production procedure, high thermal stability and chemical stability. Among the impurity-doped ZnO films, the AZO thin film is considered an excellent candidate material [15]. It has been reported that the AZO thin films can be prepared by various techniques including magnetron sputtering [16–20], sol–gel [21], ultrasonic spray pyrolysis [22], hydrothermal method [23], atomic layer deposition [1] and pulsed laser deposition [24]. Among the deposition techniques, rf magnetron sputtering is considered to be a suitable technique due to its advantages such as low cost, high deposition speed, easy control of deposition parameters and simplicity of the deposition system required [25]. To our

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knowledge, although many experimental studies have been conducted on the synthesis, structural and optoelectrical properties of AZO thin films, there are no detailed studies on their refractive index and dispersion behaviour, which are crucial to the structure design and performance improvement of optoelectronic devices. In this present work, a series of nano-AZO thin films were prepared by rf magnetron sputtering at different substrate temperatures and the structural, optical and electrical properties of the deposited films were analyzed in detail.

2 Experimental details The AZO thin films were deposited on the glass substrates using the rf magnetron sputtering system (KDJ-567) with a basic pressure of 5.2 9 10-4 Pa. Prior to the deposition, the glass substrates (30.0 9 30.0 9 1.1 mm3, from CSG Holding Co. Ltd.) were routinely cleaned by rubbing in a detergent, rinsing in deionized water, successive ultrasonification with acetone and ethyl alcohol each for 20 min, and finally dried in high purity nitrogen gas stream. A sintered ceramic sputter target with a mixture of ZnO (4N purity) and Al2O3 (4N purity) was employed as the source material. The content of Al2O3 added to the sputter target was 2 wt%. The distance between the target and substrate was set at 70 mm, and pre-sputtering is conducted for 15 min to attain stability and to remove impurities. During the process of deposition, the Ar pressure and the rf power were controlled at 0.3 Pa and 110 W, respectively. In order to investigate the influence of substrate temperature on the properties of AZO thin films, the substrate temperature was controlled from 230 to 440 °C. The sputtering time for the deposited films was varied from 20 to 30 min to get the film thickness about 430 nm in average for all the samples. The film thickness was measured by a stylus surface roughness detector (Alpha-step 200). The structural properties of thin films were studied by an X-ray diffractometer (Rigaku D/Max-A) using the Cu Ka source with the wavelength (k) of 0.1541 nm. The elemental composition was analyzed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) equipped with Al Ka radiation source (hm = 1,486.60 eV). The operating pressure in the XPS analysis chamber was maintained at approximately 10-7 Pa, and Ar ion etching was performed for 10 min with an etching rate of 2 nm min-1. The surface morphology was characterized by a scanning electronic microscopy (SEM, JSM-6700F). The optical transmission spectra were recorded using a double beam UV–Visible spectrophotometer (TU-1901). The electrical properties were investigated with a four-probe meter (SZ-82) at room temperature. Based on the measured transmittance spectra,

Fig. 1 XRD patterns of the standard ZnO (JCPDS No. 36-1451) and the AZO thin films deposited at different substrate temperatures. The inset gives the dependence of orientation factor P(002) for the thin films on the substrate temperature

the refractive index and extinction coefficient of the deposited films were determined using the pointwise unconstrained optimization method.

3 Results and discussion 3.1 Structural and morphological properties Figure 1 shows X-ray diffraction (XRD) patterns of the standard ZnO (JCPDS No. 36-1451) and the AZO thin films deposited at different substrate temperatures. For all the AZO samples, the diffraction peak positions of 2h located at about 31.8°, 34.3° and 36.3° are associated with the (100), (002) and (101) plane of hexagonal phase according to the JCPDS No. 36-1451 card (ZnO) [26, 27]. Obviously, the intensity of (002) peak is much stronger than the others. The result indicates that the AZO thin films have hexagonal wurtzite structure with highly c-axis orientation. The degree of preferred orientation of each film was estimated with the orientation factor P(002) [28]: Pð002Þ ¼

Ið002Þ  100 %; Ið100Þ þ Ið002Þ þ Ið101Þ

ð1Þ

where I(100), I(002) and I(101) are the intensity of (100), (002) and (101) diffraction peaks, respectively. The value of P(002) is 100 % for a perfectly (002) oriented film. As shown in the inset of Fig. 1, with the increase of the substrate temperature from 230 to 370 °C, the orientation factor P(002) of (002) direction increases from 91.12 to 99.61 %. However, when the substrate temperature is over

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angular peak width at half maximum in radian along (002) plane, and h is the Bragg’s diffraction angle [32]. The microstrain (e) can be estimated using the following relation [33]: e¼

Fig. 2 The values of 2h, B, D and e for the AZO thin films deposited at different substrate temperatures

370 °C, the orientation factor P(002) of (002) direction of the thin films tends to decease. From Fig. 1, it is also worth noting that no peak related to aluminum oxide is found in the XRD patterns, which implies that aluminum atoms replace zinc in the hexagonal lattice and/or aluminum segregates to the non-crystalline region in grain boundary. Similar results have been reported by Chen et al. [28] and Ayadi et al. [29]. Figure 2a, b displays the peak position (2h) and fullwidth at half-maximum (B) of (002) plane for the AZO thin films deposited at different substrate temperatures. It is clear from Fig. 2a that the 2h value continuously increases with the increment of substrate temperature. When the substrate temperature is 370 °C, the 2h value is 34.403°, approaching the value (34.421°) of the standard ZnO (JCPDS No. 36-1451). From Fig. 2b, we note that the B decreases significantly with the substrate temperature up to 370 °C, and then it increases slightly above 370 °C. Based on the values of 2h and B of the (002) peak, the average grain sizes (D) were calculated using Debye–Scherrer’s formula [30, 31]: D¼

Kk ; B cos h

ð2Þ

where the constant K is the shape factor (0.90 value is used in this work), k is the wavelength of X-rays used, B is the

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B cos h : 4

ð3Þ

The results of D and e for the AZO thin films deposited at different substrate temperatures are plotted in Fig. 2c and d. The D values in the range of 15.9–37.6 nm are observed with the variation of the substrate temperature from 230 to 440 °C, and the maximum D value of 37.6 nm is obtained at the substrate temperature of 370 °C. From Fig. 2d, the e values are about 2.17 9 10-3, 1.84 9 10-3, 0.92 9 10-3 and 1.04 9 10-3 for the AZO thin films prepared at the substrate temperatures of 230, 300, 370 and 440 °C, respectively. Obviously, as the substrate temperature increases from 230 to 440 °C, the microstrain e decreases firstly and then increases. The minimum e is obtained for the thin films deposited at the substrate temperature of 370 °C. The results suggest that the average grain size and microstrain of the thin films are subjected to the substrate temperature. The XPS spectra were measured for studying the elemental composition of AZO thin films. All of the binding energies were calibrated by taking the carbon C 1s peak (284.60 eV) as reference in XPS spectroscopy. For the calculation of the atomic concentrations, a linear background correction was done, the peak areas were corrected with empirical sensitivity factors, the instruments transmission function and the specific mean free path lengths [34, 35]. According to XPS measurements, the Al contents are obtained to be about 2.03, 2.14, 2.32 and 2.36 at.% for the AZO thin films deposited at 230, 300, 370 and 440 °C, respectively, indicating that the composition is affected slightly by the substrate temperature. Figure 3 shows SEM images of the AZO thin films deposited at different substrate temperatures. Note that the substrate temperature greatly affects the surface structure of the deposited films. The morphology of AZO thin film deposited at the lower substrate temperature of 230 °C is observed to be continuous and dense. With the substrate temperature increasing to 370 °C, the crystallinity quality of deposited film is improved and the grain size evidently becomes larger than that of the sample of 230 °C, which is in agreement with the XRD results. At the substrate temperature of 370 °C, small grains are found to coalesce together to form larger grains. The evolution of microstructure could be attributed to the migration of surface atoms during the deposition process. The sputtered atoms migrate distantly, and thus a denser film with larger grains and lower defects can be obtained. Similar results for the rf sputtered ZnO thin films have been reported in the literature [36].

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Fig. 3 The SEM images of the AZO thin films deposited at (a) 230 °C and (b) 370 °C

Fig. 4 The transmittance spectra of the AZO thin films deposited at different substrate temperatures. The inset shows the average visible transmittance of the thin films deposited on the glass substrates as a function of substrate temperature

3.2 Optical properties Figure 4 depicts the room temperature transmittance of the AZO thin films deposited on the glass substrates (film/ substrate system) prepared at different substrate temperatures, using air as reference. Obvious interference phenomenon can be observed in each spectrum, indicating the AZO thin films have a very smooth and homogeneous surface in our present work [37, 38]. The system of film/ substrate exhibits 79.2–85.1 % average optical transmission in the visible range, which is important for applications. As seen from this figure the optical transmittance increases with the increment of substrate temperature from 230 to 370 °C, then slightly decreases when the substrate temperature is over 370 °C. This increase in the optical transmittance is related to the increase in the grain size of the films when the substrate temperature increases from

Fig. 5 Refractive index (a) and extinction coefficient (b) of the AZO thin films deposited at different substrate temperatures

230 to 370 °C. In addition, it is observed that the absorption edge shifts towards shorter wavelength, suggesting a widening of the optical bandgap with the substrate temperature increasing. This blue-shift in the absorption edge can be explained by the Burstein-Moss effect [39, 40] in which the absorption edge shifts towards higher energy with an increase of carrier concentration. From the measured transmittance spectra, the refractive index (n) and extinction coefficient (k) of the deposited films were determined using the pointwise unconstrained optimization method [41, 42]. Figure 5 shows the n and k values of all the thin films deposited at different substrate temperatures. The n decreases with the increase of wavelength k, indicating that the deposited films possess the normal dispersion characteristics in the visible region. Meanwhile, the n is closely related to the substrate temperature. For the AZO samples deposited at the substrate temperature of 230, 300, 370 and 440 °C, the n values are 2.207, 2.172, 2.057 and 2.102 respectively, when the

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dispersion energy, which is a measure of the strength of the interband optical transitions. The curves of (n2 - 1)-1 versus (hm)2 for the AZO thin films are plotted in Fig. 6 and the data are fitted into straight lines, indicating the W–D dispersion model is applicable to the AZO thin films in the present work. The values of Eo and Ed can be determined directly from the slope (EoEd)-1 and the intercept Eo/Ed on the vertical axis. Using the single oscillator parameters Eo and Ed, the oscillator strength (f), the M-1 and M-3 moments of the optical spectra can be readily derived from the following relations [46]: f ¼ E o Ed ;

Fig. 6 Experimental values and single-oscillator model fit for the AZO thin films deposited at different substrate temperatures

wavelength k = 400 nm. From Fig. 5b, the k is observed to be very small in the visible region, which indicates that the AZO thin films are almost transparent in the visible region. At the wavelength k = 400 nm, for the AZO thin films deposited at 230, 300, 370 and 440 °C, the k values are 3.981 9 10-2, 3.715 9 10-2, 1.299 9 10-2 and 9.722 9 10-3, and the corresponding values of absorption coefficient (a) determined by the formula a = 4pk/k [43] are 1.251 9 104, 1.167 9 104, 4.081 9 103 and 3.054 9 103 cm-1, respectively. The results in our present work are comparable to the results of previous studies. AlHardan et al. [44] reported that the n values of ZnO thin films were 1.94–2.20 obtained by Swanepole method, and the n was about 1.97–2.08 measured by spectroscopic ellipsometry by Hwang et al. [45] for Ga-doped ZnO thin films. The refractive index dispersion of the deposited films was evaluated according to the Wemple–DiDomenico (W– D) single-oscillator model. It is well known from the W–D dispersion theory that the refractive index n in the region of low absorption can be expressed by the following relationship [46, 47]: n2

1 Eo 1 ¼  ðhmÞ2 ;  1 Ed Eo Ed

ð4Þ

where h is Planck’s constant, m is the frequency, hm is the photon energy, Eo is the oscillation energy and Ed is the Table 1 The optical bandgaps and single-oscillator parameters of the AZO thin films deposited at different substrate temperatures

M1 ; M3

Ed2 ¼

3 M1 M3

ð5Þ

From Fig. 6 we can calculate the values of Eo, Ed, f, M-1 and M-3 for all the AZO thin films and these values are summarized in Table 1. The obtained values strongly agree with Wemple and DiDomenico [46]. It is clear from the table that the measured physical parameters are strongly dependent on the substrate temperature. The oscillation energy Eo is an average optical bandgap gap (Eg) as pointed out in many literatures [48–50]. We found that the Eo value of the thin films is related empirically to the direct bandgap Eg by Eo & (1.90–1.97) 9 Eg, which is consistent with the relation Eo & 2.0 9 Eg obtained from the single-oscillator model [48–50]. The fundamental electron excitation spectra of the thin films were described by means of a frequency dependence of the complex dielectric constant (e = e1 - ie2). The real (e1) and imaginary (e2) parts of the dielectric constant (e) are related to the values of n and k. These constants of the thin films were determined by the following formulae [51]: e1 ¼ n2  k2 ; e2 ¼ 2nk

ð6Þ

The e1 and e2 values dependence of wavelength k for all the thin films are shown in Fig. 7. Note that both e1 and e2 tend to decrease with increasing wavelength k, and the e1 values are higher than the e2 for all the samples. Also, the complex dielectric constants e are observed to be 4.91 - i0.17, 4.72 - i0.16, 4.44 - i0.056 and 4.26 - i0.039 for the AZO thin films deposited at the substrate temperatures of 230, 300, 370 and 440 °C, respectively, when the wavelength k = 400 nm. The dissipation factor (tand) can be calculated according to the following formula [51]:

Temperature (°C)

Eg (eV)

Eo (eV)

Ed (eV)

M-1

M-3 (eV-2)

f (eV2)

230

3.32

6.34

16.12

2.52

6.13 9 10-2

1.03 9 102

2.69

6.57 9 10

-2

1.11 9 102

5.52 9 10

-2

0.93 9 102

5.64 9 10

-2

0.95 9 102

300 370 440

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Eo2 ¼

3.34 3.42 3.45

6.59 6.60 6.54

17.28 14.52 14.83

2.27 2.31

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Fig. 7 Real (a) and imaginary (b) parts of the dielectric constant for the AZO thin films deposited at different substrate temperatures

Fig. 9 Optical conductivity of the AZO thin films deposited at different substrate temperatures

ropt ¼

c na; 4p

ð8Þ

where c is the velocity of light and n is the refractive index. Figure 9 shows the variation of optical conductivity ropt as a function of photon energy hm. The increase of optical conductivity at high photon energies is due to the high absorbance of AZO thin films and also may be due to the electron excited by photon energy [52]. Near the absorption edge or in the strong absorption zone of the transmission spectra, the absorption coefficient a is related to the optical bandgap gap Eg following the power-law behaviour of Tauc [53, 54]:
ð9Þ ðahmÞ2 ¼ C0 hm  Eg ; Fig. 8 Dissipation factor of the AZO thin films deposited at different substrate temperatures

tan d ¼

e2 ; e1

ð7Þ

and the dependence of tand on wavelength k for all the thin films is plotted in Fig. 8. Obviously, the tand is closely related to the substrate temperature and the value of tand is observed to decrease with increasing the substrate temperature. The tand values are found to be in the range of 3.65 9 10-4–4.66 9 10-3 with the variation of substrate temperatures from 230 to 440 °C, when the wavelength k = 450 nm. The absorption coefficient a can be used to calculate the optical conductivity (ropt) as follows [51]:

where hm is the photon energy, C0 is an energy-independent constant. The optical bandgaps of the thin films can be calculated using Tauc’s plot by plotting (ahm)2 versus hm (shown in Fig. 10) and by extrapolating the linear portion of the absorption edge to find the intercept with energy axis [55]. The dependence of optical bandgap on substrate temperature is shown in the inset in Fig. 10. The Eg values are 3.32, 3.34, 3.42 and 3.45 eV for the films deposited at the substrate temperatures of 230, 300, 370 and 440 °C, respectively. It is clearly seen that with increasing substrate temperature, the Eg values of the thin films are broadened. A similar behaviour was observed in Ga-doped ZnO thin films prepared by the chemical spray technique [56]. 3.3 Optoelectronic properties Figure 11a presents the resistivity (q) of the thin films deposited at different substrate temperatures. As the

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where q is the resistivity and Tav is the average transmittance in the visible range of the thin films [58]. Figure 11b shows the variation of FTC values for the thin films deposited at different substrate temperatures. The FTC increases from 2.88 9 102 to 1.03 9 104 X-1 cm-1 with the substrate temperature increasing from 230 to 370 °C, yet, after which it decreases to 6.96 9 103 X-1 cm-1 at higher substrate temperature (440 °C). The increase in FTC with the substrate temperature is due to the increase in optical transmittance and the decrease in resistivity. It is known that the higher the FTC, the better quality of the TCO thin film such as AZO thin film [59, 60]. Thus, in this study, it can be concluded that the most effective substrate temperature is 370 °C, where the FTC is the highest.

Fig. 10 Tauc’s plots of the AZO thin films deposited at different substrate temperatures. The inset is optical bandgap of the thin films as a function of substrate temperature

Fig. 11 Resistivity and figure of merit of the AZO thin films as a function of substrate temperature

substrate temperature increases from 230 to 440 °C, the resistivity initially decreases dramatically and then increases slightly. When the substrate temperature is 370 °C, the lowest resistivity of 6.12 9 10-4 X cm is achieved. The decrease in resistivity can be attributed to the improvement of crystallinity and the increase of grain size, which is confirmed by the results of XRD discussed above. In order to quantify the optoelectronic properties of the deposited AZO thin films, the figure of merit (FTC) is introduced and defined as [57] FTC ¼

1 ; q lnð1=Tav Þ

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ð10Þ

4 Conclusions In this study, highly conductive AZO transparent thin films were deposited on the glass substrates by the rf magnetron sputtering technique, and the influence of substrate temperature on the structural, morphological and optoelectronic properties of the thin films were studied. It is found that all the AZO samples have highly c-axis preferred orientation. As the substrate temperature increases, the microstrain and resistivity decrease initially and then increase, but the tendency in the change of the average visible transmittance and figure of merit is observed to be opposed to the microstrain and resistivity. The minimum microstrain (0.92 9 10-3), the lowest resistivity (6.12 9 10-4 X cm), the maximum average visible transmittance (85.1 %) and the highest figure of merit (1.03 9 104 X-1 cm-1) are obtained when the substrate temperature is 370 °C. The optical constants were determined using optical characterization methods and the dispersion of refractive index was investigated by the W–D single-oscillator theory. The result shows that the refractive index dispersion curve of thin films obeys the singleoscillator model. In addition, the optical bandgap of thin films was evaluated by extrapolation method, and the obtained value increases monotonically from 3.32 to 3.45 eV with increasing the substrate temperature. Acknowledgments The authors gratefully acknowledge the financial supports from the Fundamental Research Funds for the Central Universities (Grant No. CZW14019) and the Academic Team Project (Grant No. CTZ13004), South-Central University for Nationalities.

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