Structural, electrical, and dielectric properties of Cr doped ZnO thin films: Role of Cr concentration

Accepted Manuscript Title: Structural, electrical, and dielectric properties of Cr doped ZnO thin films: Role of Cr concentration Author: Osman Gurb ¨ uz ¨ Mustafa Okutan PII: DOI: Reference:

S0169-4332(16)31343-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.06.114 APSUSC 33484

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Please cite this article as: Osman Gurb ¨ uz, ¨ Mustafa Okutan, Structural, electrical, and dielectric properties of Cr doped ZnO thin films: Role of Cr concentration, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.06.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structural, electrical, and dielectric properties of Cr doped ZnO thin films: Role of Cr concentration

Osman Gürbüz a,* and Mustafa Okutan a

a

Department of Physics, Yıldız Technical University, Davutpaşa 34220, İstanbul, Turkey

*Corresponding author. Tel: +90 212 3837070 Ex: 7268; Fax: +90 212 3834011 E-mail address: [email protected] (O. Gürbüz).

GRAPHICAL ABSTRACT

Highlights:    

Magnetic material of Cr and semiconductor material of ZnO were grown by the magnetron sputtering co-sputter technique. Perfect single crystalline structures were grown. DC and AC conductivity with dielectric properties as a function of frequency (f = 5Hz-13MHz) at room temperature were measured and compared. Cr doped ZnO can be used in microwave, sensor and optoelectronic devices as the electrical conductivity increases while dielectric constant decreases with the Cr content.

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Abstract An undoped zinc oxide (ZnO) and different concentrations of chromium (Cr) doped ZnO (CrxZnO1-x (x= 3.74, 5.67, 8.10, 11.88, and 15.96) thin films were prepared using a magnetron sputtering technique at room temperature. These films were characterized by X-ray diffraction (XRD), High resolution scanning electron microscope (HR-SEM), and Energy dispersive Xray spectrometry (EDS). XRD patterns of all the films showed that the films possess crystalline structure with preferred orientation along the (100) crystal plane. The average crystallite size obtained was found to be between 95–83 nm which was beneficial in high intensity recording peak. Both crystal quality and crystallite sizes decrease with increasing Cr concentration. The crystal and grain sizes of the all film were investigated using SEM analysis. The surface morphology that is grain size changes with increase Cr concentration and small grains coalesce together to form larger grains for the Cr11.88ZnO and Cr15.96ZnO samples. Impedance spectroscopy studies were carried out in the frequencies ranging from 5Hz to 13 MHz at room temperature. The undoped ZnO film had the highest dielectric value, while dielectric values of other films decreased as doping concentrations increased. Besides, the dielectric constants decreased whereas the loss tangents increased with increasing Cr content. This was considered to be related to the reduction of grain size as Cr content in ZnO host material increased. Furthermore, by increasing the Cr concentration, the improved electrical performance was observed. The electrical resistivity of samples decreased from 3.98x10-2 Ω.cm to 4.03x10-4 Ω.cm with the increase in Cr content. For these reasons, Cr doped ZnO (Cr:ZnO) thin films may be used in microwave devices as the electrical conductivity increases while dielectric constant decreases with the Cr content.

Keywords: Cr doped ZnO; RF Magnetron sputtering; dielectric constant; electrical conductivity

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1. Introduction Metal oxides semiconductors (MOSs) have been used in various implementations, in particular light emitting diodes (LED) in the near-UV region, gas sensor, solar cells, nonvolatile memories, and optoelectronic devices because of their wide band gap [1-6]. Furthermore, as semiconductors have distinguished electrical and optical properties when doped with metal atoms, they have gained significance popularity in recent years. Among several semiconductors, ZnO is a well-known transparent and conductive material because of its unique physical properties such as wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature (RT) and it crystallizes in hexagonal Würtzite structure with lattice constants of a = 3.25 Å and c = 5.12 Å [7, 8]. Dielectric studies and alternating current (AC) conductivity measurements of transparent semiconducting oxides play a significant role on the fabrication of novelty devices [9, 10]. These measurements are useful in the determination of structure and defects in material and also in understanding the nature of conduction mechanism [11, 12]. The dielectric properties for the development of transducers dielectric coatings and transparent conducting electrodes for photovoltaic cells have been investigated in the literature [10, 13-16]. However, there is still a limited amount of research on the dielectric properties, which have a fundamental effect on its applications in microelectronic fields In line with this, we have doped various concentrations chromium (Cr) in ZnO host material. The goal of this study was to evaluate and validate the influence of Cr content on structural, electrical and dielectric properties of Cr doped ZnO (Cr:ZnO) thin films. Recently, many researchers have focused on the fact that the presence of dopant can modify the dielectric behavior of ZnO [17–19]. The results of the studies show that the dielectric constant values of metal doped ZnO were smaller than that of undoped-ZnO bulk materials and decreased with the increase in doped metal content [20-24]. Several techniques, including electron beam evaporation technique [25], chemical processes [1], sputtering method [7, 23], sol-gel [24], hydrothermal method [2], and spray pyrolysis method [26], have been used to obtain Cr:ZnO thin films. The magnetron sputtering method stands out amongst others because of its features such as enabling growing uniform films with large area, having no toxic gas emissions, dense structure and high single crystallinity [5, 27].

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A lot of work has been done to improve the structural, optical, and magnetic properties of Cr doped ZnO thin films [22-26]. Nevertheless, only few studies reported about the conducting and dielectric properties of Cr:ZnO thin films. In this study, we mainly focus on producing single crystalline pure ZnO and Cr:ZnO thin films via RF magnetron sputtering . In order to achieve a better understanding of the influence of the Cr concentration on such properties, higher knowledge of the interrelation between electrical and dielectric studies of Cr:ZnO films is required to the electronic devices. The structure, electrical conductivity and dielectric studies of fabricated films were investigated by XRD, Four Point Probe (FPP) method, and Impedance Spectroscopy respectively. In this context, the purpose of the present work is to fill this gap in the literature by studying the effect of various Cr concentrations on the structural, conducting and dielectric properties of Cr:ZnO thin films. 2. Experimental details

An undoped ZnO and Cr:ZnO thin films were deposited on Si (100) substrates by magnetron sputtering technique with different concentrations of Cr. Before the deposition, the substrates were ultrasonically cleaned, and then dried with nitrogen gas. Ultra-pure Ar and O2 mixed gases with the flow rate ratio 1:1 (15 sccm for each) were introduced by mass flow controllers (MFCs). The ZnO (99.99 % purity, 2 inch diameter) and metallic Cr (99.99 % purity, 2 inch diameter) ceramic targets were used. The deposition pressure of base chamber was 3.2 × 10−2 Torr. The RF power of ZnO target was held fixed at 100 W while the DC power of Cr target was changed between 70 to 95W to have different composition rates of Cr. The deposition temperature was maintained at room temperature (27 0C), and deposition time was maintained at 15 minutes. Elemental concentration rates of the films were investigated by Energy Dispersive X-ray Spectroscopy (EDS). The crystalline nature was determined by Rigaku (D/MAX 2500) XRD diffractometer with CuKα radiation source of wavelength (λ=0.1541 nm). The electrical properties of films were measured by four point probe (FPP) technique. Dielectric properties were examined by HP 4192A impedance analyzer in the frequency range 5 kHz -13 MHz and all the experiments were carried out at room temperature. 5

3. Results and discussion

3.1. Structural characterization

The elemental concentration rates of Cr:ZnO films were measured by EDS as 3.74, 5.67, 8.10, 11.88, and 15.96 at. %, respectively. The spectrum recorded from Cr8.10 ZnO film that includes 8.10 at. % of Cr and 91.90 at. % of ZnO is seen in Fig. 1.

In Fig.1, the major peak belongs to Si (100) substrate while other peaks belong to Cr, Zn and O elements, whose intensities are related to their composition rates. Table 1 demonstrates the existence of Cr within the samples as well as the increase in concentration of Cr in Cr:ZnO thin films and growth conditions (Ar:O2 ratio, RF and DC powers) and thickness of the films. While the DC power of Cr target was changed between 70 to 95W to have different composition rates of Cr, with the concentration of Cr content increased in the lattice, the thickness of films increased for the same optimized deposition.

XRD patterns of pure and Cr:ZnO thin films that were fabricated on Si (100) substrates at room temperature are shown in Fig.2. It can be seen that all films revealed single diffraction peak observed at 2θ measurements between 32.40-32.80. The peak originates from the (100) plane of hexagonal Würtzite lattice and this result matches with the literature [28, 29]. In the pure ZnO film, the peak originating from the (100) plane is much more intense with respect to the peak originating from the (002) plane. This case reveals that one of the orientations is strongly preferred during crystallization [30]. Moreover, no diffraction peaks corresponding to metallic Cr and any other impurities such as CrO, Cr2O3, CrO2 and CrO3 exist in these films.

Briefly, according to the EDS and XRD results, the diffraction patterns registered from Cr doped ZnO films confirm a hexagonal Würtzite structure, which includes either the substitution of Zn by Cr atoms in the lattice. 6

Moreover, the intensity of XRD peak depends on many factors such as growth conditions, crystallinity and thickness of films [6]. The narrowing of the diffraction peaks demonstrated the formation of nano-crystallites and is related to the crystal quality of the film. However, the decreasing intensity of peaks implies that the crystal quality decreases when Cr concentration increases. It means that the crystal structure of pure ZnO is better than that of Cr:ZnO films [6].

The average crystallite size, D, was calculated from full width at half maximum (FWMH) of more intense XRD peaks using Scherrer’s formula:

=



(1)



In this equation, λ represents the wavelength of the radiation, k is the shape factor, θ is half the scattering angle (the Bragg diffraction angle), and β is the peak FWHM intensity of the X-ray diffraction line in radians. Table 2 shows the XRD spectral data (i.e., 2θ, β, d (the interplanar spacing between atoms), a (lattice constant) and D in m) for the films that have one preferred orientation.

According to XRD results, the crystallite size decreased from 95 nm to 83 nm, which can be attributed to the increase in doping concentrations. This confirms the existence of Cr in ZnO host semiconductors, which matches the XRD results. Moreover, the observed average size of the Cr:ZnO thin films inclusion was approximately 70 nm, as measured by using Scanning electron microscopy (SEM). Scanning electron microscopy (SEM) was employed to observe the morphology and crystallite size of the films. The surface and cross-sectional view SEM images of undoped ZnO and Cr:ZnO are depicted in Fig.3. The surface images provide general information regarding the microstructure, surface morphology, crystallite size, and porous structures of the surfaces. It 7

was clearly observed in the SEM films that regular and rounded grains were equally distributed over the surface. Fig.3 (a) shows the SEM images of undoped ZnO film. The top and cross views reveal the long range uniformity of ZnO film without any remarkable defects in the structure [28]. The influence of various concentrations of Cr doping on the morphology of ZnO thin films can be seen in Fig.3 (b)-(f). The figure shows the presence of hexagonal and spherical aggregates of smaller individual particle sizes of Cr:ZnO thin films in different concentrations [29]. In addition, the thickness of films extended with increasing Cr concentration. Based on these results, it can be inferred that Cr doping has a positive effect on the structure of undoped ZnO thin films. Furthermore, one can say that the surface morphology that is grain size changes with increase Cr concentration and small grains coalesce together to form larger grains for the Cr11.88ZnO and Cr15.96ZnO samples.

3.3. Electric Conductivity

Fig.4 shows the change in resistivity depending on the concentrations of Cr:ZnO thin films. To investigate the various concentrations of Cr with regard to the electrical properties of Cr:ZnO thin films, the resistivity, ρ, is calculated by using the following formula [8];

ρ=

π V x x t ln2 I (2)

8

In this equation, I is the current, V is the voltage, and t is the thickness of the film. It was calculated that the conductivity of Cr:ZnO thin films improved after Cr doping. Firstly, the resistivity increased from 3.98x10-2 to 8.54x10-2 Ω.cm as Cr concentration increased from 3.74 to 5.67 at. %; then, it decreased dramatically to 4.03x10-4 Ω.cm for 15.96 at. % Cr concentration. The decrease in resistivity can be due to Cr ions acting as donors [31] or larger grain size, which can reduce grain boundary scattering and lengthen carrier lifetime, [23, 32].

Furthermore, according to the XRD spectra, the narrower the FWHM values of the diffraction peaks indicate the better the crystal structure, thus the fine structure contributed the better the conducting properties [23, 29, 31]. Besides, metal behavior of Cr doped into the semiconductor material of ZnO increases the conductivity of new Cr:ZnO thin films because each Cr atom donates one electron to the conduction band of ZnO. Therefore, we say that the electrical transport properties of Cr:ZnO thin films are primarily related to the change in grain size of crystal structure and crystallinity because of with increasing Cr concentration, the Cr ions create an abundant number of free electrons into the ZnO lattice. Hence, the Cr ions in the ZnO lattice act as charge carriers reservoir and acceptor impurities [33].

3.3. Dielectric studies

The dielectric constant (ε′) of Cr doped ZnO thin films with different concentrations of Cr as a function of frequency was investigated by Impedance Spectroscopy. To determine the effect of crystallite structure of films with different concentrations of Cr on the dielectric properties of thin films, the following formula is used, which describes the relation between capacitance and dielectric constant [11, 34]. Cp . t

ε'= Ɛ . A 0

(3)

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In this equation,Cp is the capacitance of the film in Farad (F), t is thickness of the film, ε0 is the permittivity of free space (8.854x 10-12 F/m), and A is the effective area of the thin film. After that, the dielectric constant can be determined by the following formula [29]; ε*= ε′+iε′′

(4)

In this equation, ε′ and ε′′ are real and imaginary parts of dielectric constant.

Fig.5 (a) and (b) demonstrate the real and imaginary parts of dielectric dispersion for undoped ZnO and Cr:ZnO thin films with various Cr concentrations, measured at room temperature over the frequency range of 5 Hz-13 MHz . The undoped ZnO film had the highest dielectric value, while dielectric values of other films decreased as doping concentrations increased.

As a result of the common features of semiconducting films, ε′ decreases with the increase in doping concentration in the lower frequency range and remains almost constant at higher frequency ranges [35-37]. Moreover, either the free charge carrier concentrations or the amount of defects is increased by Cr doped in ZnO [38-40].

Therefore, it can be inferred that the dielectric properties of Cr:ZnO thin films depends on several parameters such as crystallite size, thickness of the samples, frequency of applied electric field.

According to the Eq.(4), the dielectric loss (tan δ) is obtained by calculating the ratio of the imaginary part of the dielectric constant to the real part of the dielectric constant. The formula for the dielectric loss is as follows;

ε′′= ε′ tan ()

(5)

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Like dielectric constant, tan () also decreases with frequency as depicted in Fig.6. As it is observed from the data, the behavior of (tan δ) follows the same trend as imaginary part of dielectric constant (ε′′). Moreover, the dielectric loss in the low frequency region may be due to high defect charge concentrations in ZnO [35, 41, and 42] or the decreasing ability of Cr ions to respond to the field.

According to Eq. 5, the dielectric loss of the Cr:ZnO thin films was greater than that of the undoped ZnO thin films. Dielectric loss primarily stems from the interfacial polarization at the low-frequency range, which is referred to as the Maxwell–Wagner (M-W) effect [29, 43]. According to the M-W effect, the charge carriers easily move within the grains cannot migrate between grains due to the resistive interfacial. Therefore, few charge carriers tunneling from one grain to another at low frequencies, however, the more charge carrier tunneling let to the ac conductivity with increase frequencies, thus the charge carriers get sufficient energy to overcome the barrier at higher frequencies [44, 45]. Briefly, one can say that the each grain interfacial act as potential barrier and behaves like charge in the potential well [45].

Dielectric loss may also be explained by Koops phenomenological theory, which expresses that the dielectric constant of thin films may decrease as the frequency increases, thus a dielectric medium is assumed to be made of well conducting grain boundaries [13]. In such a case, the dielectric loss is high and has a strong frequency dependence. Because of its low dielectric constant and dielectric loss at high frequency range, we may deduce that Cr:ZnO can qualify as a suitable material for high frequency applications. The AC conductivity (σ ac) is calculated with the following equation:

σac = ε′ ε0 ω tan ()

(6)

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In this equation, ω is the angular frequency, ε0 is the permittivity of free space (8.854x 10-12 F/m), ε′ is the real part of dielectric constant, and tan () is dielectric loss. It can be noted that the AC conductivity of all films exhibit rising trends with the increase of frequency. The frequency dependence of the AC conductivity at room temperature is shown in Fig. 7.

It is seen from Fig.7 that the conductivity increased slowly at lower frequencies and increased rapidly at higher frequencies for undoped and Cr doped ZnO thin films. The AC conductivity became more frequency dependent as the frequency increased. The real part of dielectric constant (ε′) which is related to the AC conductivity consists of two parts: one from DC conduction at low frequency, and the other from AC conduction at high frequency. When doping concentration increases, the electrical conductivity increases too, leading to a high increase of the dielectric constant. This phenomenon is can be explained via the jump relaxation model, according to which, at low angular frequencies an ion jumps from one site to its adjacent vacant site, successfully contributing to DC conductivity [29]. In order to better explain the relation between conductivity and frequency, the ln σ versus ln f plots at room temperature were calculated and the results are shown in Fig.8. .

The AC conductivity of the various concentrations of Cr doped ZnO thin films exhibits the universal power-law behavior in Eq.7: σ = σ dc +Aωs

(7)

In this equation, σ dc is the DC conductivity, s is the exponent of the law, between 0
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materials is characterized by s parameter. By taking the logarithm of both sides of Eq.7, the s value is determined in Eq.8:

s =

(8)

The s values decreased with increasing Cr concentration as depicted in Fig.9. It was found that exponent s decreased from 1.71 to 1.53 with increasing Cr concentration from 0 to 11.88 at. %. In the literature, this kind of behavior can be explained with Correlated barrier hopping (CBH) model, according to which, charge carriers do not tunnel through the barrier that separates them but hop between sites over the potential barrier [46, 47].

Nevertheless, some studies report that conductivity is related to the rate of hopping between Cr2+ to Cr3+, not to the number of charge carriers [46-48]. That is why, the thin films with higher Cr content also display higher conductivity at higher frequencies.

4. Conclusion

An undoped ZnO and Cr:ZnO with different concentrations of Cr thin films were deposited on Si (100) substrates by magnetron sputtering technique, and an experimental investigation was carried out on structural, conducting and dielectric properties at room temperature. XRD patterns of all the films showed that the films possess crystalline structure with preferred orientation along the (100) crystal plane. SEM images of the thin films had surface and cross sectional views with nano-sized grains and were well dispersed on the surface without any cracks or impurities. The average crystallite size obtained was found to be between 95–83 nm which was beneficial in high intensity recording peak. The electrical resistivity of samples decreased from 3.98x10-2 Ω.cm to 4.03x10-4 Ω.cm with increasing Cr content. Therefore, it can 13

be said that the conductivity was increased by increasing Cr concentrations with the enhanced transition of charge carriers between the grains.

When the dielectric properties of Cr:ZnO thin films were analyzed with impedance analyzer, it was determined that the dielectric constant decreased while the dielectric loss increased, which was due to the increase in frequency or dopant concentration. According to the results of the present study, Cr:ZnO thin films are suitable for high frequency applications, sensors, optoelectronic and microwave devices due to the conducting and dielectric properties of the material.

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[48] M. M. Hassan, A.S. Ahmed, M. Chaman, W. Khan, A.H. Naqvi, A. Azam, Mater, “Structural and frequency dependent dielectric properties of Fe+3 doped ZnO nanoparticles”, Res. Bull. 47 (2012) pp. 3952-3958.

19

Fig. 1. EDS spectrum recorded from virgin Cr8.10 ZnO.

20

Fig. 2. θ-2θ XRD measurements of undoped and Cr doped ZnO thin films.

21

A-)

0%

a-)

B-)

3.74 %

b-)

C-)

5.67 %

c-)

Fig. 3. SEM micrographs of top view (capital letters) and cross-sectional view (small letters) undoped and Cr doped ZnO with various concentrations thin films.

22

D-)

8.10 %

d-)

E-)

11.88 %

e-)

F-)

15.96 %

f-)

Fig. 3. Continued.

23

Fig. 4. Variation of electrical resistivity of Cr doped ZnO thin films.

24

Fig. 5. Variation of (a) real and (b) imaginary part of dielectric constant with frequency.

25

Fig. 6. Variation of dielectric loss for all samples.

26

Fig. 7. Variation of ac conductivity for all samples.

27

Fig. 8. Variation of ac conductivity dependence on the frequency.

28

Fig. 9. Variation of frequency exponent (s) with different Cr concentration.

29

Table 1. Cr, Zn and O concentration rates, flow rate of mix gases, RF and DC power. Sample

Cr

Zn

O

Name

(at. %)

(at.%) (at.%)

Ar-O2

ZnO

ratio 1:1 RF

Cr

Thickness

power DC power (nm)

(sscm)

(W)

(W)

ZnO

-

36.39

63.61

15

100

70

90±10

Cr3.74 ZnO

3.74

29.99

66.27

15

100

75

139±10

Cr5.67 ZnO

5.67

26.23

68.11

15

100

80

146±10

Cr8.10 ZnO

8.10

24.72

67.19

15

100

85

191±10

Cr11.88 ZnO

11.88

29.54

58.58

15

100

90

195±10

Cr15.96 ZnO

15.96

27.63

55.41

15

100

95

225±10

30

Table 2. Calculated parameters for undoped ZnO and Cr doped ZnO thin films using XRD analysis. Cr

2θ (degrees)

Concentration

FWHM,β

d-spacing

Lattice

Crystallite

(degrees)

(nm)

constant,a

size, D (nm)

(at. % )

(nm)

0

32.66

0.080

0.2739

0.5478

95

3.74

32.68

0.082

0.2738

0.5476

92

5.67

32.69

0.085

0.2738

0.5476

89

8.10

32.68

0.087

0.2738

0.5476

87

11.88

32.69

0.090

0.2739

0.5478

84

15.96

32.69

0.091

0.2739

0.5478

83

31