Blue-green and red luminescence from non-polar ZnO:Pb films

Applied Surface Science 270 (2013) 467–472

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Blue-green and red luminescence from non-polar ZnO:Pb films X.B. Li, S.Y. Ma ∗ , F.M. Li, F.C. Yang, J. Liu, X.L. Zhang, Q. Zhao, X.H. Yang, C.Y. Wang, J. Zhu, C.T. Zhu, X. Wang College of Physics and Electronic Engineering, Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, Northwest Normal University, Lanzhou, Gansu 730070, China

a r t i c l e

i n f o

Article history: Received 15 October 2012 Received in revised form 27 December 2012 Accepted 9 January 2013 Available online 17 January 2013 Keywords: Non-polar Chemical state Optical band gap White light I–V characteristics

a b s t r a c t Pure zinc oxide (ZnO) and lead (Pb) doped zinc oxide (ZnO:Pb) films with different Pb doping concentrations were deposited on glass substrate by using radio frequency reactive magnetron sputtering technique. X-ray diffraction spectroscopy measurements showed that all samples with the (1 0 0) preferential orientation were growth of the non-polar. The results of X-ray photoelectron spectroscopy analysis suggested that the Pb ions were successfully doped into lattice of ZnO and the valence of Pb in the ZnO films was a mixed state of +2 and +4. Optical band gaps of the ZnO:Pb were 3.24, 2.92, 2.86 and 2.74 eV with the increase of Pb doping concentration, it could attribute this red shift phenomenon to the decrease of carrier concentration. Photoluminescence measurements showed that a broad emission band including the two blue emission peaks are about at 437 nm and 470 nm, one green and red emission peaks are about at 510 nm and 710 nm, which may compound white light. Moreover, growth of non-polar ZnO enhanced enormously the luminous efficiency of photoluminescence in our experiment. The current–voltage measurements between two surface electrodes showed the increase in resistance with increase of Pb doping concentration. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction In recent years, many attention have focused on light emitting diode (LED) based white light sources for lighting purposes. Among various semiconductor nanomaterials, ZnO films have attracted great scientific and technological interest owing to its direct wide band gap (3.37 eV), large exciton binding energy (60 meV). As we known, many elements have been doped into ZnO films in order to adjust its properties in the recent years, such as Co, Ga and Ag [1–3]. Although, to the best of our knowledge, there are some literatures [4,5] that investigated the properties of ZnO:Pb nanowires, but the achieved properties from ZnO:Pb films are very excellent and unique. We have obtained full non-polar ZnO films by suitable Pb doping concentration that grew along the direction of the vertical (1 0 0) rather than the (0 0 2) direction [6,7], that is to say the films grow along the direction of the (1 0 0) and (1 1 0) in our experiment. The ZnO film with (0 0 2) preferred orientation is the polar film that generated internal electric field and reduces the luminous efficiency of the ZnO-based light emitting device. However, the non-polar ZnO film is able to eliminate the adverse effect of internal electric field. Meanwhile, the blue, green and red emission peaks appeared at same time in our non-polar ZnO films. The

∗ Corresponding author. Tel.: +86 13893422608; fax: +86 9317971503. E-mail address: [email protected] (S.Y. Ma).

red, green and blue emission, the three primary colors of light, could compound white light. So the ZnO:Pb films were excellent candidate for possible luminescence applications. In this paper, the influence of different Pb doping concentrations on the microstructure, optical and electrical properties of ZnO films are investigated via X-ray diffraction, X-ray photoelectron spectroscopy, UV/vis, fluorescence spectrophotometry, electrochemisty workstation and 4-point Probes Resistivity Measurement System, respectively.

2. Experiments The ZnO:Pb films were deposited on glass substrates by using radio frequency (RF) reactive magnetron sputtering under different Pb doping concentration. In order to control the Pb doping concentration, the effective sputtering area ratios of Pb and Zn were varied 0%, 0.75%, 1.5% and 2.5% corresponding to samples (a), (b), (c) and (d), respectively. The Pb concentrations of ZnO:Pb films were 0, 1.17, 1.53, and 2.24 at.% corresponding to samples (a), (b), (c) and (d), respectively, which were determined by energy dispersive spectrometer (EDS, JSM-6701F). Argon and oxygen (purity 99.99%) were used as the sputtering and reactive gas. All the films were deposited with RF power of 100 W under pressure of 1.0 Pa for 1 h, the O2 :Ar ratio was 12:8 sccm. All films were annealed in vacuum at 500 ◦ C for 1 h.

0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.057

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0.7

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(c)

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Fig. 2. Variation of TC(1 0 0) , TC(0 0 2) , TC(1 0 1) and TC(1 1 0) of ZnO:Pb films with increase of Pb doping concentration.

50

60

2 (deg.) Fig. 1. XRD patterns of ZnO:Pb films with different Pb doping concentration: (a) 0%, (b) 1.17 at.%, (c) 1.53 at.% and (d) 2.24 at.%.

The microstructure, optical and electrical properties of ZnO films are investigated via X-ray diffraction (XRD) (D/Max-2400) using the Cu K␣ radiation with  = 0.15406 nm, X-ray photoelectron spectroscopy (XPS) (ESCA LAB 220-XL), Lambda 35UV/VIS, fluorescence spectrophotometry (PL) the excitation of Xe lamp (RF-5301, wavelength 360 nm), electrochemisty workstation (CHI660D) and 4-point Probes Resistivity Measurement System (RTS-8). All spectra were measured at room temperature in air. 3. Results and discussion 3.1. Structure properties

films generally showed (0 0 2) preferred orientation that perpendicular to the substrate due to the lower surface free energy of (0 0 2) plane [5]. But in our experiment (1 0 0) plane had the lowest surface free energy because of all samples have exhibited (1 0 0) preferred orientation. Therefore, Pb doping increases the surface free energy of (0 0 2) plane, which leads to the (0 0 2) diffraction peaks vanished, and decreases the surface free energy of (1 0 0) plane. Moreover, the diffraction angles of (1 0 0) diffraction peaks are 31.70◦ , 31.42◦ , 31.32◦ and 31.06◦ corresponding to the samples of (a), (b), (c) and (d), respectively. It was obvious that the diffraction angles of ZnO:Pb films (1 0 0) diffraction peaks shifted to lower angle with the increment of the Pb doping concentration. Fig. 3 was an enlarged view of (1 0 0) diffraction peaks and obviously showed the change of diffraction angles of (1 0 0) diffraction peaks. Ma et al. [10] reported that when Cu+ ions (radius is 0.096 nm) substitute Zn2+ ions (radius is 0.074 nm) at their lattice sites, this resulted in the increase of the crystalline plane distance, which would lead to the decrease of the diffraction angle compared with pure ZnO film. That is consistent with our results. The valence of Pb could be

Fig. 1 shows the XRD patterns of ZnO:Pb films that were fabricated under different Pb doping concentration. It is observed from the figure that all films show dominant peak corresponding to the (1 0 0) direction of ZnO and other weak peaks corresponding to the (0 0 2), (1 0 1) and (1 1 0). All samples grew along to the non-polar direction and full growth of non-polar was obtained in the sample (d) (Fig. 1(d)). In order to further explain non-polar growth, the texture coefficient (TC) which can describe the preferential orientation is calculated by using the following expression [8]: I(h k l) I(1 0 0) + I(0 0 2) + I(1 0 1) + I(1 1 0)

(1)

where TC(h k l) is the texture coefficient of (h k l) plane, I(h k l) is the intensity of the diffraction peak. As can be seen in Fig. 2, the TC(1 0 0) decreases slightly at first and then increases sharply, meanwhile the TC(1 1 0) increases gradually with the increase of Pb doping concentration. The TC(0 0 2) decreases to zero in the end. This phenomenon further indicated that the increase of Pb doping concentration could promote the non-polar growth and inhibit the polar growth. It was obvious that preferential orientation of ZnO films were strongly affected by Pb doping. The most important is that Pb doping have successfully achieved the transform of preferential orientation of ZnO films from blended growth to complete non-polar growth, which are able to eliminate the adverse effect of internal electric field that exist in the polar ZnO films and improve luminous efficiency. Waltereit et al. [9] have reported the luminous efficiency of non-polar film (GaN-based LED) has significantly improved. ZnO

250 200 150 100 50 0 400

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2 (deg.) Fig. 3. Enlarged figures of (1 0 0) diffraction peaks with different Pb doping concentration: (a) 0%, (b) 1.17 at.%, (c) 1.53 at.% and (d) 2.24 at.%.

Zn 2p3 Zn 2p1

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0

Zn 2p3/2

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Fig. 4. Survey (a), Zn2p3/2 (b), O1s (c) and Pb4f (d) XPS spectra of ZnO:Pb film that Pb doping concentration is 2.24 at.%.

assumed to be +2 and +4 in the ZnO:Pb films, the radius of Pb2+ and Pb4+ ions were 0.119 and 0.084 nm, respectively. ZnO crystal structure is more open, the metal ions with smaller radius cannot only become substitution ions but also easily become interstitial ions, while the ions with larger radius than the Zn2+ ions is easier to become substitution ions. Therefore, it could be obtained that the Pb2+ and Pb4+ substituting Zn2+ at lattice in the ZnO:Pb films leads to the shift of the diffraction angle of (1 0 0) diffraction peak to lower angle. 3.2. Elemental composition In order to further characterize the microstructure of ZnO:Pb films and the chemical states of every elements, we measured the XPS spectrum of the ZnO:Pb film that Pb doping concentration is 2.24 at.%. The survey spectrum and high resolution XPS spectrum of Zn2p3/2, O1s and Pb4f are shown in Fig. 4. The survey spectrum included the binding energy peaks corresponding to Zn2p3/2, O1s and Pb4f and any impurity, besides carbon, were not found in the spectrum of in Fig. 4(a). Typically, the binding energy peak of Zn2p3/2 at 1021.7 eV which correspond to standard value [11] shows high degree of symmetry in Fig. 4(b), which indicates that majority of Zn ions mainly exist in the form of ZnO [12]. The XPS spectrum of O1s shows a little asymmetry in Fig. 4(c), which can be divided into three components by Gaussian fitting method: a low binding energy peak located at 529.89 eV, a middle binding energy peak located at 531.66 eV and a high binding energy peak located at 532.54 eV. The low binding energy peak is attributed to O2− ions in the ZnO structures, the high binding energy peak is usually attributed to the presence of loosely bound oxygen on the surface of ZnO film, which belonging to a specific species, e.g.

–CO3 or adsorbed H2 O [13]. The middle binding energy peak is associated with O2− ions in the oxygen deficient regions [14]. The middle and low binding energy peaks is comparatively strong, this phenomenon indicates that the majority of oxygen exists at the intrinsic sites and the oxygen deficient regions. As can be seen in Fig. 4(d), with the spin–orbit split, Pb4f peak exhibited two binding states, one at Pb4f7/2 = 138.1 and Pb4f5/2 = 140.4 eV, and the other at Pb4f7/2 = 139.4 and Pb4f5/2 = 142.8 eV. The Pb4f7/2 = 138.1 eV and Pb4f5/2 = 140.4 eV correspond to Pb4+ , Pb4f7/2 = 139.4 eV and Pb4f5/2 = 142.8 eV correspond to Pb2+ [15,16], which indicate Pb in ZnO film was a mixed state of +2 and +4 valence. This is consistent with the result of XRD. 3.3. Optical properties Fig. 5 shows optical absorption and transmission spectra of four samples. As can be seen in Fig. 5, the ZnO:Pb films with relatively high and broad optical absorption band compared with pure ZnO that has very low absorbance in the visible range, and the absorption edge shifted to a longer wavelength with increase of Pb doping concentration. The inset of Fig. 5 shows optical transmission of pure ZnO film and ZnO:Pb films were prepared with various Pb doping concentration. It can be clearly seen that pure ZnO film had a higher transmittance about 70%, but it quickly decreased with increasing of Pb doping concentration. Tominaga et al. [17] reported that transmittance of the Al doped ZnO films decreased obviously in the wavelength range of 320–500 nm. He attributed it to the incorporation of metallic Al in the films. Chakraborti et al. [18] found that the Zn0.9 Co0.05 Cu0.05 O film had a very low transmittance relative to Cu nanoclusters were formed in the film. In our experiment XRD measurements combined with XPS results demonstrate that Pb ions

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1.0

Absorbance

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d

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0.2

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Wavelength (nm) Fig. 5. Optical absorption and transmission (the inset) spectra of ZnO:Pb films with different Pb doping concentration: (a) 0%, (b) 1.17 at.%, (c) 1.53 at.% and (d) 2.24 at.%.

have been successfully incorporated into ZnO films. Therefore, we think that the low transmittance may be due to the incorporation of Pb related charged defects in the ZnO:Pb films. The optical band gap is not only an important theoretical problem in condensed matter physics but also an important basis parameter for the design of semiconductor devices and prediction their properties. In order to calculate the optical band gap of ZnO and ZnO:Pb films, we used the Tauc relationship [19] as follows: ˛hv = A(hv − Eg )

n

(2)

where ˛ is the optical absorption coefficient, A is the constant, h is the Planck’s constant, v is the photon frequency, Eg is the optical band gap and n is the 1/2 for direct band gap semiconductors. The Eg value can be obtained by extending the linear part of the curves toward the x-axis, the photon energy at the intersection of the extension line and the x-axis is Eg . The plot of (˛hv)2 versus hv was shown in Fig. 6. The values of Eg were 3.24, 2.92, 2.86 and 2.74 eV corresponding to samples (a), (b), (c) and (d), respectively. The inset of Fig. 6 shows the change trend of Eg value with the increase of Pb doping concentration. It is obvious that the Eg of ZnO:Pb films red shift markedly compared with the pure ZnO film and almost linearly decrease with increase of Pb doping concentration. Therefore, we can change the value of optical band gap by changing the amount of incorporation of Pb. It is well known that only the shallow acceptors or donors could increase the car25

3.3

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hv)2( m-1ev)2

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0 1.8

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hv (ev) 2

Fig. 6. Plot of (˛hv) versus hv of ZnO:Pb films with different Pb doping concentration: (a) 0%, (b) 1.17 at.%, (c) 1.53 at.% and (d) 2.24 at.%. The inset is the change trend of Eg with increase of Pb doping concentration.

rier concentration. Trolio et al. [20] have reported that blue shift of optical band gap of Al-doped ZnO films due to Al doping has been largely attributed to the Burnstein–Moss effect. The shallow donor Al atoms provide additional carriers which cause the Fermi level move into the conduction band lead to the optical band gap becomes larger. Lee et al. [21] have reported that Cu doping decrease the optical band gap of ZnO and carrier concentration due to that Cu generally behaves as an acceptor level with a deep energy. The Pb atoms often behave as a deep acceptor, however, the high concentration of Pb introduces a large number of charged defects which act as recombination centers in the semiconductors and could decrease the carrier concentration. Therefore, we attributed significant decreasing of optical band gap to the decreasing of carrier concentration that could increase the resistivity. ZnO nanocomposites [22,23] have attracted much attention in recent years due to important potential application for white light sources of LED. People generally use multilayer film to achieve the white light emitting, such as the combination of mesoporous substrate and nanomaterials or multilayer nanomaterials. But ZnO:Pb films could achieve the white light emitting through monolayer nanomaterials. PL spectra of pure ZnO and ZnO:Pb films under various Pb doping concentration were showed in Fig. 7. Two blue emission peaks with stronger intensity are about at 430 nm (437 nm, 423 nm) and 470 nm (463 nm) have appeared in the all samples, one green emission peak with weak intensity is about at 510 nm has appeared and vanished in Fig. 7(d) and one red emission peak is about at 710 nm that appeared with the increase of Pb doping concentration, which could use for ZnO:Pb based LED white light sources. The most significantly change is gradual increase of PL emission intensity with increase of Pb doping concentration and showed in the inset of Fig. 7. We attributed this phenomenon to the enhancing of non-polar growth and inhibition of polar growth due to the growth of non-polar are able to eliminate the adverse effect of internal electric field and improve luminous efficiency [9]. It is well known that there are many different intrinsic defects in ZnO films, such as oxygen vacancies (Vo ), zinc vacancies (VZn ), interstitial oxygen (Oi ), interstitial zinc (Zni ) and so on [24]. In our experiment, blue emission peaks centered at about 430 nm that corresponding phonon energy was about 2.88 eV, Ding et al. [25] reported that a blue emission peak at 433 nm (2.9 eV) was observed in Al-doped ZnO films, which fully supported our results. They explained this peak by the electrons transition from the Zni levels to top of the valence band. Kohan et al. [26] have calculated the energy levels of defects in ZnO films by the full-potential linear muffin-tin orbit method, and they have shown that the energy interval from the Zni to VZn was about 2.54 eV, which were consistent with the energy of the blue emission peaks at about 470 nm (2.64 eV) in our samples. The green emission peaks at about 510 nm corresponding phonon energy was about 2.43 eV. According to the theoretical analysis, we concluded that the green emission at 510 nm might due to the electron transition from deep oxygen vacancies level to the top of valance band [27]. Kanti et al. reported the energy interval is 1.08 eV between the top of valence band and Oi levels [28]. So we can calculate that the energy interval are 1.78 eV and 1.66 eV between bottom of conduction band and Oi levels in the sample (c) (Eg is 2.86 eV) and (d) (Eg is 2.74 eV), respectively. The results were almost consistent with the energy of the red emission peaks at about 710 nm (1.75 eV). So the red emissions are related to Oi defects [29]. 3.4. Electrical properties Electrical I–V characteristics between two surface copper electrodes on films were measured and shown in Fig. 8. It is observed that as soon as Pb is introduced in the ZnO lattice, the current magnitude falls drastically compared to the undoped one for the

X.B. Li et al. / Applied Surface Science 270 (2013) 467–472

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Fig. 7. PL spectra of ZnO:Pb films with different Pb doping concentration: (a) 0%, (b) 1.17 at.%, (c) 1.53 at.% and (d) 2.24 at.%. The inset shows the increase of PL intensity with increase of Pb doping concentration.

same bias voltage. With increase in the Pb concentration, the film becomes highly resistive. The decrease in the current level due to Pb doping, which indicates that Pb is incorporated into the lattice of ZnO and leads to the increase of resistance in ZnO:Pb films. This phenomenon is also consistent with the resistivitys that are 12.82, 26.84, 39.04 and 50.46 k cm corresponding to samples (a), (b), (c) and (d), respectively. The resistivity increased with increase of Pb concentration. Our results are similar to the earlier electrical results on Li doped ZnO films. It has been reported that most of the Li doped ZnO films show lower conductivity [30,31]. In the Cu doped ZnO

0.06 a

Current (mA)

0.04

b c d

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0.02 0

Film

-0.02 -0.04 -0.06 -6

Cu

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Glass

b a

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-2

0

2

4

6

Voltage (V) Fig. 8. Electrical I–V characteristics of ZnO:Pb films with different Pb doping concentration: (a) 0%, (b) 1.17 at.%, (c) 1.53 at.% and (d) 2.24 at.%. The inset shows the schematic arrangements for the I–V measurements.

films, the increase in the resistivity is assigned to the capture of free electrons in ZnO by the empty lower energy Cu3d states and [CuZn + Zni ]x complex defects [32]. Therefore, the increase in the resistivity with the increase of Pb concentration, it was attributed to the decrease of carrier concentration and formation of more carrier trap centers related to PbZn defects. 4. Conclusions Non-polar ZnO films were successfully deposited on glass substrates by using RF reactive magnetron sputtering technique. XRD measurements revealed that the ZnO:Pb films showed polycrystalline structure that belonged to the ZnO hexagonal wurtzite type. All ZnO films with a strong (1 0 0) preferred orientation realized the non-polar growth. The result of XPS analysis showed that the Pb successfully incorporated into the lattice of ZnO by mixed state of +2 and +4. Our results indicate that large red shift of the optical band gap is related to the increase of resistivity. The three main emission peaks at about 430 nm (470 nm), 510 nm and 710 nm that may constitute white light are observed from PL spectra. Moreover, the non-polar growth of ZnO films has greatly improved the luminous efficiency of photoluminescence. The increase in the resistivity with the increase of Pb concentration, it was attributed to the decrease of carrier concentration and formation of more carrier trap centers related to PbZn defects. Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant No. 10874140), the College Basic Scientific Research Operation Cost of Gansu province (the manufacture and

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characteristic research of the optical gas sensing film and the Y series superconducting materials) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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