Structural, optical, and electrical properties of Mo-doped ZnO thin films prepared by magnetron sputtering

Applied Surface Science 324 (2015) 791–796

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Structural, optical, and electrical properties of Mo-doped ZnO thin films prepared by magnetron sputtering Muying Wu a,∗ , Shihui Yu b , Guihua Chen a , Lin He a , Lei Yang a , Weifeng Zhang c a b c

School of Electronic Engineering, Dongguan University of Technology, Guangdong Dongguan 523808, China School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Photovoltaic Materials of Henan Province and School of Physics and Electronics, Henan University, Kaifeng 475004, China

a r t i c l e

i n f o

Article history: Received 19 July 2014 Received in revised form 31 October 2014 Accepted 10 November 2014 Available online 15 November 2014 Keywords: MZO Magnetron sputtering Preferred orientation TCO

a b s t r a c t Molybdenum doped zinc oxide thin films have been prepared by RF magnetron sputtering. The influence of the film thickness (120–500 nm) on the structural, electrical, and optical properties of the films is investigated respectively. X-ray diffraction (XRD) studies reveal that with an increase in the film thickness, the crystallinity of the film improves. The obtained film with thickness of 500 nm exhibits the best electrical properties with the lowest resistivity of around 9.6 × 10−4  cm. The mobility varied from 7.8 to 14.7 cm2 V−1 s−1 without reducing the achieved high carrier concentration of ∼4.5 × 1020 cm−3 . Optical band gaps extracted from transmission spectra shows irregular changes due to the Burstein–Moss shift modulated by many-body effects. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Transparent conductive oxide (TCO) films are widely used in flat panel displays, transparent thin film transistors, solar cells, and optoelectronic devices, where high luminous transmittance and good electrical conductivities are required [1–3]. At present, commercially employed TCO materials are indium–tin oxide (ITO) thin films due to their low resistivity and high optical transmittance [4]. However, high cost, the scarcity of indium and toxicity of indium compound (including ITO) powders make researchers develop substitutive materials [5]. Among various alternatives, impurity doped zinc oxide (ZnO) has been attracting a great deal of attention as an alternative for ITO due to its high electrical conductivity, high optical transmittance and high infrared reflectance in the visible spectrum [6,7]. Moreover, ZnO is non-toxic, inexpensive, abundant, and mechanically strong, which is important for the fabrication and operation of solar cells. Presently, many dopants, such as aluminium (Al) [8,9], gallium (Ga) [2,7], molybdenum (Mo) [10], fluorine (F) [6], tungsten (W) [11], niobium (Nb) [12], manganese (Mn) [13], indium (In) [1], titanium (Ti) [14], zirconium (Zr) [15], phosphorus (P) [16] and chlorine (Cl) [17], have been studied to improve the electrical and optical properties of ZnO thin films. Among these, Mo-doping seems to be successful and promising due

to its smaller ionic radius of 0.62 A˚ [18] (0.72 A˚ [19]), as compared with corresponding values of Zn given in parentheses. Furthermore, Mo has high thermal stability that is enhanced even further when it is doped with ZnO. As a result, ZnO-based TCO thin films have a great potential in numerous applications. For the preparation of the ZnO-based structures, various techniques have been employed such as sputtering [9,10,13], sol–gel method [20], chemical vapour deposition [17], and pulsed laser deposition (PLD) [16,19]. Among these techniques, magnetron sputtering is considered to be the most favorable deposition method to obtain highly uniform films even on polycrystalline substrates at high deposition rates [13]. Also, the surface of the thin films prepared by magnetron sputtering can be very smooth. The thin films with smooth surface are advantageous for optoelectronic devices [21]. Until now, various experimental conditions have been employed, resulting in films with diverging properties. Therefore, further investigations are required to clarify the influence of the growth parameters on the properties of pure and doped ZnO films. In this paper, transparent conducting Mo:ZnO (MZO) films deposited on glass substrates by RF magnetron sputtering are reported. The thickness dependence of structural, electrical and optical properties for the MZO film is investigated in detail. 2. Experimental

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Wu). http://dx.doi.org/10.1016/j.apsusc.2014.11.039 0169-4332/© 2014 Elsevier B.V. All rights reserved.

MZO thin films were prepared on glass substrates by radio frequency magnetron sputtering. The MZO targets were prepared

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M. Wu et al. / Applied Surface Science 324 (2015) 791–796

Fig. 1. XRD patterns of MZO thin films with different film thicknesses.

from ZnO powder (purity 99.99%) and MoO3 powder (purity 99. 99%) in a mass proportion of 98:2. The materials are mixed in an agate mortar for 5 h, pressed into 60 mm diameter and 4 mm thickness pellet in 20 tons of pressure and then sintered at 1150 ◦ C for 8 h in air. The glass substrates were cleaned in an ultrasonic bath with alcohol for 30 min. The distance between the target and the substrate was kept at 6 cm. Before sputtering, the vacuum chamber was evacuated down to a base pressure of 5.0 × 10−4 Pa. High purity (99.999%) Ar and O2 were introduced through a mass flow controller. The total pressure during sputtering was maintained at 0.5 Pa, and the O2 /Ar ratio was 1:20. The thin films were deposited at a substrate temperature of 350 ◦ C. The MZO thin films with diverse thickness are obtained by varying the deposition time. The crystal structure of the thin films was characterized by Xray diffraction (Rigaku D/MAX-RB, Akishima, Tokyo, Japan). The morphology was characterized using a field emission scanning electron microscope (FE-SEM, FEI Nanosem 430). The thicknesses of the thin films were measured by Alpha-Step D-100 profilometer (KLA-Tencor, California, USA). Room temperature Hall effect measurements were carried out using a Hall effect measurement system (Ecopia HMS-3000). The transmittance of the thin films in the wavelength range of 200–800 nm was measured using an ultraviolet–visible–near infrared (UV–Vis–NIR) spectrophotometer (Varian Cary 5000). The chemical state of Mo in MZO thin films was examined by X-ray photoelectron spectroscopy (XPS ESCALAB 250).

3. Results and discussions Fig. 1 shows the XRD patterns of the MZO thin films with different thicknesses deposited by RF magnetron sputtering. For all the MZO thin films, a highly preferred oriented (0 0 2) and a small (0 0 4) peaks are observed, revealing that the films have hexagonal wurtzite structure and a preferred orientation along the c-axis perpendicular to the substrate. No MoO3 phase is found from the XRD patterns, which implies that Mo substitutes Zn in the hexagonal lattice or Mo probably segregates to the non-crystalline region in the grain boundaries and forms Mo O bond. The intensity of the (0 0 2) peak increases evidently as the thickness increases, indicating that crystallinity increases along with the thickness. The inset shows the full-width at half-maximum (FWHM) values of (0 0 2) peak of the MZO films with different thicknesses. It is seen that the FWHM decreases from 0.37 to 0.33 as the thickness increases from 120 to 500 nm, the position and the FWHM of the (0 0 2) peak do not change significantly, the average crystallite dimension of the MZO thin films is about 22–25 nm estimated from the XRD pattern

according to Scherrer’s formula [22], indicating the grain size of MZO thin films in this experiment is not sensitive to film thickness. Surface morphology of the MZO thin films prepared at different thickness has been studied by field emission scanning electron microscope. The FE-SEM images in Fig. 2 indicate that the surface morphology is strongly dependent on the thickness. The average particle size of the MZO film is small when it prepared in a thin thickness. With the increasing of thickness, the average particle size of the MZO film increased. This consists well with the above XRD analysis results. Fig. 3 shows the resistivity, carrier concentration and carrier mobility as functions of thickness for the MZO thin films. As thickness increases from 120 to 350 nm, the resistivity decreases from 19.1 × 10−4 to 9.6 × 10−4  cm and then increases as the thickness further increases. Generally, the electrical conductivity of a semiconductor is determined by the concentration and Hall mobility of carriers as follows: =

1 ne

(1)

where  is the resistivity, n is the number of charge carriers, e is the charge of the carrier, and  is the carrier mobility [23,24]. The resistivity is inversely proportional to the carrier concentration and carrier mobility, closely related to the film structure. However, as can be seen in Fig. 4 at a carrier concentration around 4.5 × 1020 cm−3 , the values of carrier mobility are scattered in the range from approximately 7.7 to 14.7 cm2 V−1 s−1 . The result implies the carrier mobility might be the main factor which affected the resistivity, and the contribution of carrier concentration might be relatively minor. For MZO thin films, the carrier mobility is determined by a variety of scattering mechanisms, which are ionized impurity scattering, neutral impurity scattering and lattice vibration scattering [25]. It is necessary to analyze various scattering mechanisms not only for understanding the causes of this decrease in mobility but also for providing useful information to improve the mobility by decreasing the number of scattering centers in films. The films scattering mechanisms may be expressed: 1 1 1 1 1 + + + +  g i l n

(2)

where  is the carrier mobility, g is for grain boundary scattering mobility, i is for ionized impurity scattering mobility, l is for lattice vibration scattering mobility, and n is for neutral impurity scattering. The grain boundary scattering is dominant only when the grain sizes of the film are comparable to the mean free path of the carriers. The free-electron’s mean free path in the films is calculated by the following formula [26]:

l=

 h   3n 1/3 e





(3)

where l is the mean free path (nm) of the free-electron, h is the Plank constant (6.63 × 10−34 J s), e is the electron charge (1.60 × 10−19 C), n is the carrier concentration (cm−3 ), and  is the carrier mobility (cm2 V−1 s−1 ). The mean free path l, which is determined according to Eq. (4), are 2.4, 3.6, 4.5 and 4.5 nm for the film thickness of 120, 220, 350 and 500 nm, respectively. This mean free path is considerably shorter than the grain sizes of the present films (tens to hundreds nanometers). Therefore, carrier mobility is unlikely to be limited by the grain boundary scattering.

M. Wu et al. / Applied Surface Science 324 (2015) 791–796

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Fig. 2. FE-SEM image of MZO thin films with different film thicknesses.

e3 NiZ 2

ln

1+

εEF 1/3

Ni

Ze2

(4)

where EF , ε, e and Z are the Fermi level, static dielectric constant, electron charge, and ion valence, respectively. Also, mc * = 0.38m0 is the electron effective mass in conduction band, Ni is the concentration of the scattering centers, which can be considered to be approximately equal to the electron concentration (ne ) for the

-3

Carrier Concentration (10 cm )

Resistivity (10

-4

18

20

•cm)

20

16 14 12 10 8

100

200

300

400

500

Thickness (nm) Fig. 3. The resistivity for MZO thin films with different film thicknesses.

5.0

16

4.8

14

-1 -1

m∗c

2 ⎤−1 ⎦

2

i =

⎡   2 1/2 ε1/2 E3/2 F ⎣

degenerate MZO thin films. The ionized impurity scattering is the dominant scattering mechanism in the range of ne > 1020 cm−3 for MZO thin films. It was reported before that a carrier mobility controlled by ionized impurity scattering enhances as the carrier concentration reduces. However, for the MZO thin films with different thicknesses, the present results showed the same carrier concentration. Therefore, it can be inferred that ionized impurity scattering is not the dominant scattering change mechanism of mobility change and there should be another primary scattering mechanism in the MZO thin films. Lattice scattering is result from thermal oscillation of atoms and their interference with the moving electrons. The (0 0 2) orientation

Carrier Mobility (cm V s )

For the MZO thin film, the contribution of ionized impurity scattering is given by the following formula [27]:

4.6

12

4.4

10

4.2 8 4.0

100

200

300

400

500

Thickness (nm) Fig. 4. Hall mobility and carrier concentration of the MZO thin films with different film thicknesses.

M. Wu et al. / Applied Surface Science 324 (2015) 791–796

120 nm 220 nm 350 nm 500 nm

2

α ( arb. units)

794

2.6

2.8

3.0

3.2

3.4

3.6

3.8

Energy (eV) Fig. 6. The band gap for MZO thin films with different film thicknesses.

Fig. 5. Optical transmittance as a function of wavelength for MZO thin films with different film thicknesses.

has a low atomic density [28,29]. Lattice scattering along (0 0 2) plane is weak and thus leads to high mobility. That is to say, the larger the peak strength of (0 0 2) is, the larger the carrier mobility is. The variation of lattice scattering may result from a change of crystallinity. According to Yanfeng et al. [30], oxygen atoms were prone to form Na zeolite structure which could create neutral impurity scattering center in the thin films. In addition, the Mo atoms may form the neutral Mo-based defect complexes in the film which not only contribute the carriers in the materials but also act as the neutral impurity scattering center. The mobility of the MZO thin films can be affected by the neutral impurity scattering. For photovoltaic device applications, it is important to factor of the intensity distribution of the solar spectrum in discussing transparency so that transmittance across the spectrum is proportionately represented. To determine the average transmittance Tav , the following relationship is used:

Tav =

T ()d



(5)

d

where T() is the transmittance. The transmission spectra of the MZO thin films with different thicknesses in the visible region (300–800 nm) are shown in Fig. 5. In the range (380–780 nm), the transmission spectra show fluctuations due to the interference of light on the film/air and film/substrate interfaces. The appearance of interference fringes indicates that the thickness of MZO films is uniform [31]. It shows that the average transmittance in the visible range from 380 to 780 nm is 84.5%, 87.0%, 88.4% and 86.3% for the thickness of 120, 220, 350 and 500 nm, respectively. The increase of initial average transmittance with the film thickness is mainly due to the smaller light scattering in the thicker films with better crystallinity [32]. With further increase of the film thickness, the decrease of the average transmittance can be attributed to the absorption of the MZO thin film. In order to further investigate the influence of the thickness on the absorption of the MZO thin films, the recorded transmission spectra are employed to determine the optical energy gap Eg. The fundamental absorption, which corresponds to the electron excitation from valance band to conduction band, is usually used to determine the value of optical band gap. As a direct band gap semiconductor, the optical band gap Eg of the thin films can be determined from the absorption coefficient (˛) of the films using the relation for parabolic bands [33]: ˛hv = C(hv − Eg)

1/2

(6)

where C is a constant, hv is the incident radiation energy. The absorption coefficient (˛) can be determined from the film transmission (T) as follows [34]: ˛=

ln(1/T ) d

(7)

where d is the film thickness. Fig. 6 shows the curves of ˛2 versus photon energy. The optical band-gap is determined by extrapolating the straight regions of the plots of ˛2 versus h to ˛2 = 0 (i.e., ˛h = 0) [33]. The Eg value can be obtained by extrapolating the linear portion of this plot to the energy axis. The values of Eg are 3.31, 3.33, 3.37 and 3.41 eV for the thickness of 120, 220, 350 and 500 nm, respectively. With increasing thickness from 120 to 500 nm, the band gap of MZO thin films increases, which coincides with the highest electron concentration. This movement of the band gap can be explained by the cause of the Burstein–Moss (B–M) shift [35,36]. An energy band widening (blue shift) effect is resulting from the increase of the Fermi level in the conduction band of degenerate semiconductors. If we assume that the Fermi surface is spherical, the following well-known formula is given as [35,37]: Eg = E0 + EBM = E0 +

h2 2/3 (32 n) 8m∗ 2

(8)

The Burstein–Moss shift EBM is given by EBM =

h2 2/3 (32 n) 8m∗ 2

(9)

where EBM is the shift of doped semiconductor compared with undoped semiconductor, h is the Planck’s constant, m* is the electron effective mass, E0 is the band gap of undoped ZnO. The optical band gap of the doped MZO thin films as a function of two-thirds power of the carrier concentration (n2/3 ) is shown in Fig. 7. It is found that the change of band gap does not increase linearly with n2/3 in the present work. A similar derivation toward a low value was reported in other groups [37–39], showing that the exponent is in the range of 1/3–2/3. At a high carrier concentration above the Mott critical concentration >1020 cm−3 in ZnO, the electronic states of the material are modified because of the electron–electron and electron–impurity interactions [40,41]; that is, the many-body effects such as exchange and Coulomb interactions make the band gap narrow. This phenomenon is in competition with the B–M shift for a semiconductor. Thus, the exponent of n obtained from experimental value is not exactly equal to, and always lower than that in Eq. (9). This is the Burstein–Moss shift with the modulation of many-body effects. In order to further understand the doping behavior of Mo, XPS measurements on the MZO films were performed to investigate the local chemical bonding of Mo in ZnO thin films. Fig. 8 shows the XPS of Mo 3d5/2 for the 350 nm thick MZO thin film. Since the energy

M. Wu et al. / Applied Surface Science 324 (2015) 791–796

795

References

3.42

Eg (eV)

3.39 3.36 3.33 3.30 13

5.7x10

13

5.8x10

13

5.8x10

2/3

13

5.9x10

13

5.9x10

-2

n (cm )

Photoelectron Intensity (a. u)

Fig. 7. Dependence of the optical band-gap on the electron concentration in MZO thin films.

Mo 3p5/2

+6 +5 +4

238

236

234

232

230

228

Bingding Energy (eV) Fig. 8. XPS narrow scan spectrum of Mo 2p5/2 for the MZO film with thickness of 350 nm.

spectrum of Mo 3d5/2 shows an unsymmetrical characteristic, this means the Mo element is formed by many ionic states; therefore, there may be Mo6+ , Mo5+ , and Mo4+ existing in the thin films. 4. Conclusions The MZO thin films with different thicknesses were deposited on glass substrates by RF magnetron sputtering. With film thickness increasing, the crystallinity of the films improves. It is shown that the resistivity of the thin films decreases slowly with the thickness increasing and achieves a minimum value of 9.6 × 10−4  cm at 500 nm. A slight increase in the resistivity occurs by increasing the thickness to 500 nm. Hall effect measurement further reveals that carrier mobility has obvious relations with the resistivity. The mobility can be varied from 7.8 to 14.7 cm2 V−1 s−1 without reducing the high carrier concentration of ∼4.5 × 1020 cm−3 . Optical band gaps extracted from transmission spectra show irregular changes due to the Burstein–Moss shift modulated by many-body effects. Acknowledgements This project is supported by the Major National Development Project of Scientific Instrument and Equipment of China (Grant No. 2012YQ1400511) and The National Basic Research Program of China (973 Program) (Grant No. 2013CB834305).

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