argon pressure ratio on the structural and optical properties of Mn-doped ZnO thin films prepared by magnetron pulsed co-sputtering

Accepted Manuscript Effects of oxygen/argon pressure ratio on the structural and optical properties of Mn-doped ZnO thin films prepared by magnetron pulsed co-sputtering

Xiaoxia Suo, Shujun Zhao, Yujing Ran, Haonan Liu, Zhaotan Jiang, Yinglan Li, Zhi Wang PII: DOI: Reference:

S0257-8972(18)31191-5 https://doi.org/10.1016/j.surfcoat.2018.10.084 SCT 23947

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

28 July 2018 24 October 2018 27 October 2018

Please cite this article as: Xiaoxia Suo, Shujun Zhao, Yujing Ran, Haonan Liu, Zhaotan Jiang, Yinglan Li, Zhi Wang , Effects of oxygen/argon pressure ratio on the structural and optical properties of Mn-doped ZnO thin films prepared by magnetron pulsed cosputtering. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.10.084

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ACCEPTED MANUSCRIPT Effects of oxygen/argon pressure ratio on the structural and optical properties of Mn-doped ZnO thin films prepared by magnetron pulsed co-sputtering Xiaoxia Suo, Shujun Zhao, Yujing Ran, Haonan Liu, Zhaotan Jiang, Yinglan Li, and Zhi Wang∗

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School of Physics, Beijing Institute of Technology,

Abstract

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Beijing 100081, People’s Republic of China

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Manganese doped ZnO (Zn0.97 Mn0.3 Oy ) thin films were deposited by magnetron pulsed cosputtering. The oxygen content of the films was controlled by the gas flow ratio of oxygen to argon

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r. The influence of r on the structure, surface morphology, optical properties and resistivity of the films was studied. A preferential growth along c axis was found in all films. As r increases, the grain size decreases, but lattice constant c, compressive lattice stress σ, and the dislocation δ

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density increase. The absorption edge shifts toward the shorter wavelength with r increasing, and the optical band gap is narrowed by lower r values. The resistivity of the films is also reduced

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by lower oxygen partial pressure. The results indicate a possibility to fabricate multifunctional devices using Manganese doped ZnO thin films.

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Keywords: Sputtering; Thin films; Mn-doped ZnO; Optical properties

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Electronic address: [email protected]

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ACCEPTED MANUSCRIPT I.

INTRODUCTION

The II-VI compound zinc oxide (ZnO), one kind of significant and adaptable oxide semiconductors, is considered as the most promising potential material for application because of its unique combination of electrical, optical, piezoelectric and acoustic properties [1, 2]. ZnO has low dielectric constant, high photoelectric coupling ratio, good chemical stability, and excellent piezoelectric and optical characteristics [3]. Furthermore, ZnO has a direct

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band gap and a large exciton binding energy at room temperature [4–6], thanks to which it has a considerable prospect of new photoelectric material in the ultraviolet and blue light

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emitting aspect [7, 8]. It is generally known that semiconductor properties of ZnO can be

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controlled by selective doping, such as Ag[9], Ho[10], Al[11], Ga[12], Cu[13–15], Ti[16], and Sn[17] doping, to meet different demands. The ability to control the fundamental electronic and optical properties of ZnO thin films through doping has been the central issue in devel-

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oping active electronic and optoelectronic devices [1]. For instance, ferromagnetism at room temperature can be induced in ZnO by doping with low concentrations of manganese (Mn)

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[18, 19]. Mn-doped ZnO has become a promising dilute magnetic semiconductor with an important application in spintronics [20, 21].

To realize the application of ZnO in optoelectronic devices, many growth techniques, such

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as pulse laser deposition (PLD), molecular-beam epitaxy, metal-organic chemical deposition, and magnetron sputtering, are performed to obtain highly performed ZnO materials [22].

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Among many methods, magnetron sputtering is one popular growth technique for films studies due to its low cost, simplicity and low operating temperature. The target material

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is bombarded by energetic ions, such as Ar+ , to release target atoms. These atoms are then deposited on a nearby substrate surface as a thin film. To realize doping in ZnO by magnetron sputtering, most previous studies used doped targets, or Zn/ZnO targets with Cu chips. Few groups used co-sputtering of two targets, in which the doping content in films can be easily modulated by the sputtering power of doping target[13, 14]. Though Zn1−x Mnx O film is attracting widespread interest in its modification of magnetic properties[23], its other characteristics, such as electrical and optical properties, still need to explore [24]. Among the methods used to prepare Zn1−x Mnx O films, magnetron sputtering is a suitable technique due to the inherent ease with which the deposition parameters can be controlled. The physical properties of Zn1−x Mnx O films prepared by magnetron sputter2

ACCEPTED MANUSCRIPT ing are generally dependent on deposition parameters including the oxygen partial pressure, sputtering power, substrate temperature, etc. The physical properties of the films relative to the substrate temperature have been investigated widely. However, only few researchers have studied the effect of oxygen partial pressure on the physical properties of Zn1−x Mnx O thin films. Li, et al, studied the influence of oxygen partial pressure on electrical and optical properties of Zn1−x Mnx O thin films [24]. They deposited their films by DC magnetron

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sputtering, in which a Zn metallic target, along with some smaller ruleless Mn slices laid on Zn disk, was used as the sputtering target. In our work, the thin films were prepared

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by magnetron pulsed co-sputtering of a ZnO target and a metallic Mn target. The oxygen content was controlled by the flow ratio of oxygen/argon (r). The effects of r on microstruc-

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ture, surface morphology and optical properties of the films were investigated. The aim of this research is to explore the effects of the oxygen content on the structural and opti-

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cal properties of Mn-doped ZnO (Zn1−x Mnx O) thin films. Because Zn1−x Mnx O is a dilute magnetic semiconductor, the modulation of electrical and optical can lead to its future of

EXPERIMENTAL DETAILS

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multifunctional devices.

The undoped ZnO and Mn doped ZnO (Zn1−x Mnx O) films were fabricated by magnetron pulsed co-sputtering of two commercially available sintered targets. A ZnO target was

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sputtered by a radio frequency power supply, and a metal Mn target was sputtered by a direct current power supply. The purity of ZnO and Mn target was 99.99% and 99.9%

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respectively. The fragile Mn target was bound by a copper back target. The distances between the targets and the substrates were 130 mm. The sputtered atoms off-normally deposited at room temperature on glass substrates, which were subjected to ultrasonic rinse in ethanol for 1 hour prior to deposition. During deposition, the substrates were rotated with a frequency of 15 revolutions per minute. To avoid excessive doping, a shutter was pulsedly used in front of the Mn target. The period of the shutter was 27 s, and the deposition time was 3 s in each period of the shutter. The base pressure of the sputtering chamber was 9×10−4 Pa, and the working pressure was 0.5 Pa. The sputtering power of the ZnO target and the Mn target was 100 W and 10 W respectively. The partial pressure ratio of O2 /Ar (r) was set as 0/1, 1/3, 1/1, 2/1 and 3/1, which was controlled by the flow ratio of O2 /Ar. 3

ACCEPTED MANUSCRIPT The total flow rate of O2 and Ar was kept stable at 20 sccm. The atomic content of elements was measured by an energy dispersive spectrometer (EDS, OXFORD, X-act) attached on a scanning electron microscope system of Zeiss Supra55. The Mn-doping content (x value of in molecular formula Zn1−x Mnx O) of the films lies between 3.1%-3.2%, which was calculated by EDS results. The uncertainty of the EDS measurement is above 0.1% and below 1%. The oxygen content was influenced by r value. However,

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because glass substrates were used in experiments, the measured oxygen content values were influenced by substrate and difficult to determine by EDS. So we can think the Mn

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content is constant around 3%, and all the film can be expressed as Zn0.97 Mn0.03 Oy . An X-ray diffraction (XRD) spectroscopy system (Bruker, D8 ADVANCE) with a Cu-Kα1 radiation

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(λ=0.15406 nm) was employed to examine growth orientation and microstructure of the films. The surface morphology and the roughness of the films were characterized by tapping

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mode of an atomic force microscope (AFM) system (Veeco, Nanoscope IIIa) with a silicon tip. The resistivity of the films was measured with four-probe method by a Keithley 4200-

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SCS system. The optical absorption/transmission characteristics were measured using an ultraviolet-visible (UV-Vis) spectrophotometer of Shimadzu UV-2401PC with a resolution of 0.5 nm and a measurement range of 300-800 nm. All the measurements were performed

Crystal structure

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A.

RESULTS AND DISCUSSION

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III.

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at room temperature.

FIG. 1: (a): XRD θ-2θ scan results of Zn0.97 Mn0.03 Oy films deposited with different O2 /Ar pressure ratio r; r dependence of lattice constant c (b) and stress σ (c) of the films.

The XRD θ-2θ scan results of the Zn0.97 Mn0.03 Oy films deposited with different r are shown in Fig. 1(a). The analysis of the XRD data is based on the comparison with JCPDS card (No. 36-1451). Only hexagonal (002) peaks are visible in the results of all films. This result indicates that the thin films grow in single phase hexagonal wurtzite crystal structure and have a strongly preferred orientation along c axis, which reflects the stable hexagonal 4

ACCEPTED MANUSCRIPT wurtzite ZnO structure was preserved though Mn atoms had been incorporated, and so Mn atoms are mainly in the form of substitutional atoms. Fig. 1(b) shows the lattice constant c calculated from the θ-2θ results by Bragg’s law: 2d cos θ = nλ

(1)

where n is the diffraction order, θ is the position of diffraction peak, and d is the crystal

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plane distance. For (002) peak, d corresponds to the lattice constant c. Fig. 1(c) shows the

(c − c0 ) c0

(2)

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σ = −233 × 109

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lattice stress value σ, which was calculated by the formula[4].

where c0 =0.52069 nm is the unstrained crystal lattice constant. Lattice stress directly

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affects the stability and reliability of thin film components. It plays an important role in the application of thin films. An undoped ZnO thin film was deposited with r=1/1 as a control,

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and the measured lattice constant c of which is 0.525 nm. The contrast of the undoped ZnO (0.525 nm) and the Zn0.97 Mn0.03 Oy (0.530 nm) films of r=1:1 tells that Mn2+ replacing Zn2+ causes bigger lattice parameter and compressive stress, since the radius of Mn2+ (0.083

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nm) is greater than Zn2+ (0.074 nm). The results show that the stress is compressive in all samples, which mainly results from the mismatch of thermal expansion coefficient between

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films and substrates. Another result is that lattice constant c and the compressive stress σ increase with r increasing, which can be explained by different oxygen-related defects

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in films. Oxygen vacancies and interstitial oxygen atoms lead to tensile and compressive stress, respectively. As r increases, because more oxygen atoms were ionized and entered the films, oxygen vacancies decrease and interstitial oxygen atoms increase, which enhances the compressive stress.

FIG. 2: FWHM of (002) peak (a), crystal grain size D (b), strain (c), and dislocation density (d) of Zn0.97 Mn0.03 Oy films deposited with different O2 /Ar pressure ratio r.

To further investigate the micro-structure of the films, we calculated the full width of half maximum (FWHM) of (002) peak of Zn0.97 Mn0.03 Oy films. Fig. 2 (a) shows that FWHM 5

ACCEPTED MANUSCRIPT value of the films increases monotonically with r. Based on FWHM, we can calculate crystal grain size D according to the Scherrer equation[16] D = 0.9λ/β cos θ

(3)

where λ is the wavelength of X-ray, β is FWHM value, and θ is the position of diffraction peak. The results of the grain size are also shown in Fig. 2 (b). As r increases, D value of Zn0.97 Mn0.03 Oy films decreases monotonically. The grain size of the undoped ZnO films in

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our previous work varies (versus oxygen partial pressure) with the same law [14]. This result can be explained by the effects of neutral oxygen atoms on energetic deposition particles[25].

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With increasing r, namely increasing oxygen partial pressure, the unionized neutral oxygen

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atoms increase. The neutral oxygen atoms impact on the sputtered particles and thus degrade the energy of the latter. Consequently, these particles are not so energetic as to

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move to low energy positions, which causes more defects, and reduces the grain size of the films.

Moreover, the strain (ε) and the dislocation density (δ) are calculated from the FWHM

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(β) and grain size (D) values, respectively[16]:

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ε = β cos θ/4

δ = 1/D2

(5)

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and

(4)

The strain (ε) and the dislocation density (δ) value of the films increase monotonically with

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r, as shown in Fig. 2 (c) and (d), which is related to the variation of grain size D.

Surface morphology

FIG. 3: 1×1µm2 AFM morphology of the Zn0.97 Mn0.03 Ox films deposited with different ratio of O2 /Ar.

AFM is used to observe the morphology and detect the surface roughness of the films. Fig. 3 shows an AFM morphology of the Zn0.97 Mn0.03 Oy films prepared at r value of 1/3, 6

ACCEPTED MANUSCRIPT 1/1, and 3/1. As can be seen from Fig. 3, the surface morphology and grain size of the Zn1−x Mnx Oy films are strongly affected by r. The grain size in Fig. 3(a) (r=1/3) is around 50 nm in the surface of the films, and the grains are relatively sparse. As r increases, the granular size decreases, which is consistent with the XRD results. And the surface becomes more homogenous with r value increasing. The grain size in Fig. 3(b) (r=1/1) and (c) (r=3/1) is around 20 nm. But the film of r=3/1 appears denser than that of r=1/1, though

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there are few bigger grains in Fig. 3(c). The results of the roughness reflected by RootMean-Square (RMS) are listed in Table I. With r value increasing, the surface roughness

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decreases. The variation of surface roughness is strongly related to the grain size. This result can be explained by nucleation and coalescence in growth process. As r increases, the

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oxygen becomes more sufficient, which results in more nucleus. Therefore the films become denser and smoother at higher r. Another possible factor affecting surface roughness is that

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the compress stress increases with increasing r[24], as indicated in Fig. 1(c). More or less, the stress is related to the mobility of atoms to lower energy sites on the surface, and results

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in different surface roughness.

TABLE I: Root-mean-square (RMS) of the films. 0

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O2 /Ar ratio

2/1

3/1

2.736

1.446

1.395

1.376

Optical absorption and band gap

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C.

2.813

1/1

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RMS (nm)

1/3

To confirm the optical characteristics of all the samples, the transmittance and band gap were analyzed, and the results are shown in Fig. 4. The band gap Eg =3.37 eV of ZnO at room temperature is larger than the maximum energy of visible photon (3.1 eV), and the irradiation of visible light can not cause intrinsic excitation, so the optical transmission of the films is very high in most of the visible range, which is depicted in transmission spectra in Fig. 4(a). The absorption coefficient α is determined by [26] α=

1 1 × ln( ) thickness T

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(6)

ACCEPTED MANUSCRIPT where T is transmittance. The absorption coefficient α and optical band gap Eg can be given by Tauc plot: (αhν)2 = A(hν − Eg )

(7)

where α is the absorption coefficient, h is the Planck’s constant, ν is the frequency of the incident light, and A is a constant. In such a direct-transition semiconductor like ZnO, the

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exponent of αhν is 2 [27]. Fig. 4(b) gives a connection between (αhν)2 and photon energy hν. The Eg values, obtained by extrapolating the absorption edge of the (αhν)2 vs hν curve

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to the hν (photon energy) axis, are shown in Fig. 4(c) as a function of r. Compared with ZnO (Eg =3.37 eV), the the optical absorption edge and band gap of the Zn0.97 Mn0.03 Oy

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film are narrowed. Such effect of the red shift of the absorption edge, i.e. the decrease in band gap with the increase in dopant concentration has already been reported for Mn-doped

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ZnO and was attributed to the increase of the sp-d exchange interaction between the band electrons and the localized d-electrons of the Mn2+ ions substituting the divalent Zn2+ ions

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[28, 29]. The s-d and p-d exchange interactions lead to a negative and a positive correction to the conduction-band and the valence-band energies, respectively, and lead to narrowing

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of the band gap [30].

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FIG. 4: Optical properties of the Zn0.97 Mn0.03 Oy films prepared at different r. (a): UV-Vis transmission patterns; (b): relation between (αhν)2 and photon energy hν; (c): optical band gap

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Eg .

Fig. 4(b) suggests that with r increasing, the optical absorption edge gradually shifts to the higher photon energy side. Furthermore, an increase in band gap of the Zn0.97 Mn0.03 Ox film with r increasing in Fig. 4(c) shows the role of oxygen vacancy. As one of main kinds of positively charged free carrier in oxides, oxygen vacancy is an important factor affecting Eg value. Oxygen vacancy can narrow the band gap because of the related carrier concentration. Band gap narrowing is possibly due to many-body effects of carriers on the conduction and valence bands[31]. The many-body effects shrinking the band gap originate from electron interaction and impurity scattering. It has been attributed to the merging of an impurity band into the conduction band, thereby narrowing the band gap. For high r value, the 8

ACCEPTED MANUSCRIPT Eg value is near to Zn0.97 Mn0.03 Oy that of ZnO, for the low oxygen vacancy concentration and low Mn-doping content. As r decreases, the concentration of oxygen vacancy increases, which narrows the band gap. For r below 1/3, Eg decreases dramatically with r . A Eg reduction of nearly 0.16 eV results from oxygen vacancy. The Eg reduction with increasing oxygen vacancies enhances the absorption efficiency. So we can also modulate the band gap

D.

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by oxygen partial pressure in a relatively wide range.

Resistivity

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The carrier concentration of the films can be reflected by the resistivity to some extent.

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Fig. 5 shows the resistivity ρ of the Zn0.97 Mn0.03 Oy films. ρ value of the films increases with oxygen partial pressure r. Since Mn2+ is an isovalent impurity in the ZnO matrix, no

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change in the carrier concentration resulting from Mn2+ can be expected. The conduction electrons in ZnO films are supplied from donor sites associated with oxygen vacancies or Zn interstitial atoms. The donor could be either an impurity or an intrinsic defect, since both

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zinc interstitials and oxygen vacancies can act as shallow donors. The strong dependence of resistivity on r implies that oxygen vacancies play a more important role in the conductivity

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of the films [24]. In Zn0.97 Mn0.03 Oy films, lower po causes more oxygen vacancies (Vo ), and hence enhances the carrier concentration. The results of the resistivity is in agreement with

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optical gap discussed above.

IV.

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FIG. 5: Resistivity of the Zn0.97 Mn0.03 Ox films prepared at different r.

CONCLUSIONS

In this work, single phase (002)-oriented Mn-doped ZnO thin films were deposited on glass substrates by magnetron pulsed co-sputtering method with different of oxygen/argon flow ratio r. The oxygen content was controlled by gas flow ratio of oxygen to argon r. XRD results confirmed Mn2+ substitution for Zn2+ lattice sites. Based on the EDS analysis, Mn concent was estimated to be approximately 3% for all films. With flow ratio increasing, lattice constant c and compressive lattice stress σ increase strongly. The grain size decreases 9

ACCEPTED MANUSCRIPT and the dislocation density increases with increasing oxygen content. The AFM images indicate that the change of r affects seriously on surface morphology, and high oxygen content makes films smoother and denser. The resistivity of the films increases with oxygen content increasing. All films exhibit a transmittance high in the visible region and a sharp fundamental absorption edge. The optical band gap of Zn0.97 Mn0.03 Oy films was smaller than that of bulk ZnO due to Mn doping. The optical band gap of the thin films increases

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with increasing the oxygen concentration, in which oxygen vacancies play an important role due to many-body effect. The different oxygen partial pressure can lead to the difference of

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0.16 eV in optical band gap. As a dilute magnetic semiconductor, its electrical and optical properties can be modulated by oxygen content, which reflects its possibility to fabricate

ACKNOWLEDGEMENTS

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multifunctional devices by Mn-doped thin films.

The research is financially supported by National Natural Science Foundation of China

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(Project 11774029).

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ACCEPTED MANUSCRIPT Highlights

• Manganese doped ZnO thin films were deposited by magnetron co-sputtering with different oxygen/argon pressure ratio r. • As r increases, lattice constant and compressive stress increase.

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• The optical band gap is narrowed by low r values.

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• The absorption edge shifts toward the shorter wavelength with increasing r.

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