Thin Solid Films 519 (2011) 5078–5081

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Thin Solid Films

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Effect of dopant concentration on the structural, electrical and optical properties of

Mn-doped ZnO films
H.B. Ruan a,b, L. Fang a,⁎, D.C. Li a, M. Saleem a, G.P. Qin b, C.Y. Kong b
a
Department of Applied Physics, Chongqing University, Chongqing 400030, China
b
Optical Engineering Key Lab, Chongqing Normal University, Chongqing 400047, China

a r t i c l e i n f o a b s t r a c t

Available online 20 January 2011 The Mn-doped ZnO (Zn1 − xMnxO) thin films with manganese compositions in the range of 0–8 at.% were
deposited by radio-frequency (RF) magnetron sputtering on quartz glass substrates at room temperature
Keywords: (RT). The influence of Mn concentration on the structural, electrical and optical properties of Zn1 − xMnxO
Mn-doped ZnO films has been investigated. X-ray diffraction (XRD) measurements reveal that all the films are single phase
Magnetron sputtering and have wurtzite structure with (002) c-axis orientation. The chemical states of Mn have been identified as
Dopant concentration
the divalent state of Mn2+ ions in ZnO lattice. As the content of Mn increases, the c-lattice constant and the
Optical property
Electrical property
optical band gap of the films increase while the crystalline quality deteriorates gradually. Hall-effect
measurements reveal that all the films are n-type and the conductivity of the films has a severe degradation
with Mn content. It is also found that the intensity of RT photoluminescence spectra (PL) is suppressed and
saturates with Mn doping.
© 2011 Elsevier B.V. All rights reserved.

1. Introduction present work, we report the fabrication of Mn-doped ZnO thin films
with manganese compositions in the range of 0–8 at.% using radio-
As a direct wide band gap (3.37 eV) semiconductor with large frequency (RF) magnetron sputtering, and the influences of Mn
exciton binding energy of 60 meV at room temperature (RT), ZnO has concentration on the structural, electrical and optical properties of
been extensively studied for several decades due to its many films are discussed in detail.
important applications, especially in short-wavelength optoelectronic
devices such as ultraviolet light emitting and laser diodes [1–3]. 2. Experiments
Currently, in the quest for materials which involve the charge and spin
degrees of freedom of electrons in a single substance, some have also Zn1 − xMnxO thin films were deposited on quartz glass substrates
paid considerable attention on ZnO-based diluted magnetic semi- by RF magnetron sputtering. The sputtering targets were prepared
conductors (DMS) realized through transition-metal (TM) doping. using the mixed powder of high purity ZnO (99.99%) and MnO
Several theoretical calculations have predicted that the Mn-doped ZnO (99.99%) with prescribed molar ratio (Mn/(Zn + Mn) = 0, 0.02, 0.04,
would show ferromagnetism with Curie temperature (TC) well above 0.06, 0.08). The sputtering chamber was first evacuated to a base
RT [4–7]. Besides, ZnO is transparent in the visible region and the pressure below 8 × 10−4 Pa with a turbo molecular pump, and then
thermal equilibrium solubility of Mn in ZnO is larger than 10 mol% [8]. filled with Ar (99.995%) up to 2 Pa. All the samples were deposited at
Thus these advantages also make Zn1 − xMnxO ideal for the fabrication RT. The sputtering power and time were 120 W and 120 min
of transparent spintronics devices. For this purpose, many efforts have respectively. The thickness of the samples was measured approxi-
been made to investigate Zn1 − xMnxO systems. Up to now, Mn-doped mately 500 nm by using a step profiler (AMBIOS XP-2). The crystal
ZnO bulk and thin films have been synthesized successfully by various structure of the samples was characterized by X-ray diffraction
methods such as molecular beam epitaxy [9], chemical vapor (Philips MRD system with Cu Κα radiation). The electrical properties
deposition [10], sputtering [11], pulsed laser deposition [12] and were carried out by Van der Pauw Hall effect measurement (Ecopia
sol–gel [13]. Among these methods, the magnetron sputtering is a HMS-3000) at RT. Chemical bonding states and chemical composi-
flexible and effective technique to deposit doped films. In our previous tions of the films were analyzed by X-ray photoelectron spectroscopy
work, for example, this technique has been used to prepare ZnO:Ga (Thermo ESCALAB 250, Al K radiation source hv = 1486.6 eV). The
and ZnO:In films with different dopant concentrations [14,15]. In the optical transmission spectra were recorded using UV–visible spec-
trometer in the spectral range of 300–800 nm. The RT photolumines-
cence spectra (PL) measurements were performed on a fluorescence
⁎ Corresponding author. Tel.: +86 23 65105870. spectrometer with the excitation wavelength of 325 nm produced by
E-mail address: fangliangcqu@yahoo.com.cn (L. Fang). a Xe lamp.

0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2011.01.132
 H.B. Ruan et al. / Thin Solid Films 519 (2011) 5078–5081 5079

3. Results and discussion elastic stiffness constants (C11 =2.1×1011 N/m2, C12 =2.1×1011 N/m2,
C13 =2.1×1011 N/m2 and C33 =2.1×1011 N/m2). So the stress derived
3.1. Structural properties from XRD can be obtained by the following numerical relation:
 
The XRD patterns of the Zn1 − xMnxO films with different Mn doping 11 2
σ = −4:5 × 10 ðc−c0 Þ = c0 N = m : ð2Þ
content are depicted in Fig. 1. Only one peak corresponding to the ZnO
(002) plane appears, and no other diffraction is observed, suggesting
that the Mn doping does not change the wurtzite structure of ZnO and The calculated results are listed in Table 1. The negative sign indicates
all these samples are preferentially oriented along the c-axis with that the ZnO:Mn films are in a state of compressive stress. The total stress
polycrystalline structure. Within the XRD detection limit, no secondary in the film commonly consists of two components. One is the intrinsic
phases related to oxides of manganese or manganese clusters are stress introduced by impurities and defects in the crystal, and the other is
detected. As the Mn content increase, the diffraction intensity from ZnO the extrinsic stress introduced by the lattice mismatch between the films
(002) peaks of the samples is getting weaker, which indicates that the and substrate. For the undoped ZnO film in the present work, the obtained
film crystallinity is deteriorated with more Mn incorporation. In (002) peak is 34.41°, which is very close to that of the bulk ZnO (34.42°).
addition, a shift of (002) peak positions related to the change of lattice Thus the latter component can be negligible. The increased strain is mainly
spacing was clearly observed. This is probably due to the substitution of due to the incorporation of Mn.
the relatively large ionic radii Mn2+ (0.080 nm) ions at the smaller radii
Zn2+ (0.074 nm) sites expands the lattice parameter, leading to a small 3.2. Chemical bonding states and components analysis
increase in the c-axis lattice constant with the Mn content. The
diffraction peak position of (002) peak and the calculated c-axis lattice Chemical bonding states in the Zn1 − xMnxO (x = 0.04, 0.08) films
constant of the samples with various Mn content is shown in Fig. 1(b). were characterized by XPS measurements, as shown in Fig. 2. The high
For hexagonal crystals with a highly c-axis preferred orientation, this in- resolution spectra for the Zn 2p, O 1s, and Mn 2p peaks were observed.
plane stress (σ) can be calculated based on the biaxial strain model [16]: The XPS spectra have been charge corrected to the adventitious C1s
peak at 284.6 eV. The binding energy of the Zn2p3/2 and O 1s states
σ = ½2C13 −C33 ðC11 + C12 Þ = C13 ðc−c0 Þ = c0 ; ð1Þ are located at around 1021.6 eV and 530.2 eV respectively, as
illustrated in Fig. 2(a) and (b). The intensity ratios between Zn and
Where c is the lattice constant obtained from the (002) reflection in the O were almost unaffected by Mn content. The O 1s peak is broad and
XRD profile, c0 is the corresponding bulk value (0.5206 nm), and Cij are asymmetric. The stronger peak at 530.2 eV can be attributed to the
Zn–O and Mn–O bands formation, while another at 532.2 eV is usually
associated with the loosely bound oxygen chemisorbed on the
(a)
surface/or grain boundary of polycrystalline film [17,18]. For the
ZnO (002) core level spectra of Mn 2p, the Mn 2p3/2 peak appears at 640.8 eV
16000
x=0.08 and has the satellite structure on the higher binding-energy side, in
x=0.06
good agreement with the reports of H.Y.Xu et al. [17] and Z.L.Lu et al.
XRD Intensity (a.u.)

12000
[18], which indicate that the divalent state of Mn ions is dominant in
Zn1 − xMnxO films. Moreover, by XPS quantitative analysis, the
x=0.04
concentration of 4 at.% and 8 at.% Mn doped ZnO films are 3.56 at.%
8000 and 6.72 at.%, respectively, indicating the Mn content in the films is a
little lower than that in the target. The same phenomenon was also
x=0.02 observed by H.K.Yadav et al. in their films, and they proposed that this
4000 difference is the result of the lower sputtering rate of Mn than Zn [11].

x=0.00 3.3. Electrical properties

0
30 32 34 36 38 40 Electrical properties of Zn1 − xMnxO films with different Mn doping
2θ (degree) contents are listed in Table 1. Good Ohmic contacts are confirmed before
Hall-effect measurement. All the measured samples show n-type
conduction with an overall increase in resistivity and decrease in carrier
(b) 5.25 concentration and mobility with the increase of Mn content. It is well
Deffiction peak poistion
known that ZnO always exhibits high levels of unintentional n-type
Diffraction peak position (deg.)

Lattice parameter c
34.4 conductivity with an electron concentration of the order of 1018 cm−3,
Lattice parameter c (nm)

5.24 and certain intrinsic point defects acting as donors have been
often invoked to explain this behavior. After Mn incorporation, the
34.3 5.23

5.22 Table 1
34.2 Electrical properties and the stress of Zn1 − xMnxO films.

Target Carrier Mobility Resistivity Type Stress

5.21 (ZnO + MnO) concentration (cm2 V−1 s−1) (Ω cm) (N/m2)
x= Mn/(Zn + Mn) (cm−3)
34.1
0 2 4 6 8 0 8.70 × 1018 16.2 0.044 n −9.40 × 109
2 3.21 × 1018 6.37 0.30 n −4.32 × 1010
Mn content (at.%)
4 1.75 × 1018 6.36 0.56 n −5.71 × 1010
6 6.22 × 1017 4.30 2.33 n −8.49 × 1010
Fig. 1. (a) XRD patterns of the Zn1 − xMnxO films. (b) Dependence of the diffraction peak
8 1.47 × 1015 2.23 1842 n −9.54 × 1010
position and the c-axis lattice constant on Mn content in the Zn1 − xMnxO films.
5080 H.B. Ruan et al. / Thin Solid Films 519 (2011) 5078–5081

(a) there will be other explanations for the strong electrical compensation
in the material and carrier localization.
2p 3/21021.6 eV
40000 3.4. Optical properties

The optical transmission spectra of Zn1 − xMnxO thin films are

Intensity (arb. units)

2p 1/2
shown in Fig. 3. The samples are transparent in the visible optical region,
30000 and the transmittance maxima decrease with the increase of Mn
doping. Compared to the spectra of undoped ZnO film, an additional
broad absorption around 420 nm was also observed after Mn
x=0.04
incorporation. The absorption coefficient α can be calculated by using
20000 the following equation α = ln(1/T)/d, where T is the transmittance index
and d is the film thickness. The optical band gap of the Zn1 − xMnxO films
x=0.08 was calculated using the relation α2 ∝ (hv − Eg). As shown in inset of
Fig. 3, the value of optical band gap of Zn1 − xMnxO films was found to
10000 increase approximately linearly with the Mn concentration and is in
1010 1020 1030 1040 1050 agreement to the results reported on Mn doped ZnO films prepared
Binding energy (eV) by other techniques [8.9.20]. In the present study the band gap of Zn1
− xMnxO films was found to follow Eg = 3.28 + 0.65x eV, the relationship
is consistent with the equation Eg = 3.30+ 0.625x eV (0% ≤ x ≤ 6.8%)
(b)
obtained by Yadav et al. [11].
16000
530.2 eV Fig. 4 shows the RT PL spectra of samples with different Mn doping
concentrations. Each sample has a very broad asymmetrical emission
peak in the range of 350–550 nm. Taking the sample (x = 0.02) as an
14000 532.2 eV
example its PL spectra can be fitted with five Gaussian curves located
Intensity (arb. units)

at around 385 nm (3.22 eV), 415 nm (2.99 eV), 430 nm (2.88 eV),
12000 455 nm (2.72 eV) and 488 nm (2.54 eV) respectively, as shown in the
x=0.04 inset in Fig. 4. The violet emission peak located at around 385 nm is
generally ascribed to the near band edge emission (NBE), while the
10000
other deep level emissions in the visible region are usually related
to Zinc interstitials (Zni) and oxygen vacancies (VO) defects in ZnO
8000 [21,22]. As the Mn concentration increases, the intensity of all emis-
x=0.08 sion peaks has a great suppression, and the suppression is going to
saturate with high Mn content in the samples. It has been reported
6000
that the PL emission characteristics of ZnO films are strongly
522 525 528 531 534 537 540 dependent on both the crystal quality and the film stoichiometry
Binding energy (eV) [23]. In general, the luminescence efficiency of the light emission can
be described by the following formula:
(c)
IR
η= ; ð3Þ
IR + INR
4000 640.8 eV 2p 3/2 2p 1/2
where η is the luminescence efficiency, IR and INR are radiative and
non-radiative transition probabilities respectively. By analysed the
Intensity (arb. units)

3800 XRD data, we have known that the film crystallinity is deteriorated
x=0.04 with increasing Mn doping concentration. It implies a large number of

100
3600
x=0.08
80
Transmittance (%)

3400 (a)(b)(c)(d)(e)
60
3.34

630 635 640 645 650 655 660 3.33

Band gap (eV)

Binding energy (eV) 40 3.32

3.31
3.30
Fig. 2. (Color online) XPS spectra of (a) Zn 2p, (b) O 1s, and (c) Mn 2p core levels for
3.29
Zn1 − xMnxO (x = 0.04, 0.08) thin films. 20
3.28
0 2 4 6 8
Mn content (at.%)
conductivity of ZnO films has a significant degradation with increasing 0
300 400 500 600 700 800
the Mn content in the films. The decrease in electron mobility is
probably related to the increased impurity scattering centers by doping Wavelength (nm)
more Mn, which was also observed in other works [19]. While for the Fig. 3. (Color online) Optical transmission spectra of Zn1 − xMnxO thin films. (a) x = 0,
decrease in carriers, since Mn2+ is an isovalent impurity in the ZnO (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, (e) x = 0.08. The inset shows the increase of
matrix, no change in the carrier concentration can be expected, hence, optical bandgap energy due to Mn doping.
 H.B. Ruan et al. / Thin Solid Films 519 (2011) 5078–5081 5081

Zn1-x MnxO 430 nm x=0.02 increases into the ZnO films, the strain and the optical band gap of the
1000
films have a slight increase, while the conductivity and the intensity of
415 nm
PL intensity (arb. units)
the RT PL spectra decrease remarkably. All these results indicate that
800 the Mn doping concentration has great influences on the structural,
electrical and optical properties of ZnO films.
600 455 nm

Acknowledgments
400 x=0
x=0.02 This work was partly sponsored by the National Natural Science
385 nm 488 nm x=0.04
200 x=0.06
Foundation of China under Grant nos. 11074314, 50942001 and
x=0.08 50975301, and the Third Stage of “211” Innovative Talent Training
Project (No. S-09109 and No. CDJXS10102207) of Chongqing University.
0
350 400 450 500 550 600
Wavelength (nm) References
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