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The optical and electrical properties of ZnO:Zr films
Accepted Manuscript The optical and electrical properties of ZnO:Zr films Haiqin Bian, Shuyi Ma, Guijin Yang, Haibin Zhu, Xiaoli Xu, Shaohui Yan, Jiming Gao, Zhengmei Zhang PII:
S0925-8388(16)30435-2
DOI:
10.1016/j.jallcom.2016.02.178
Reference:
JALCOM 36796
To appear in:
Journal of Alloys and Compounds
Received Date: 30 October 2015 Revised Date:
16 February 2016
Accepted Date: 19 February 2016
Please cite this article as: H. Bian, S. Ma, G. Yang, H. Zhu, X. Xu, S. Yan, J. Gao, Z. Zhang, The optical and electrical properties of ZnO:Zr films, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.02.178. 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.
ACCEPTED MANUSCRIPT
The optical and electrical properties of ZnO:Zr films Haiqin Bian∗, Shuyi Ma, Guijin Yang, Haibin Zhu, Xiaoli Xu, Shaohui Yan, Jiming Gao, Zhengmei Zhang Key Laboratory of Atomic and Molecular, Physics & Function Material of Gansu Province
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College of Physics and Electronics Engineering, Northwest Normal University, Lanzhou 730070, China
Abstract: Zirconium doped zinc oxide (ZnO:Zr) films at different Zr doping concentrations were deposited by radio frequency (RF) reactive magnetron sputtering. The effect of Zr contents on the crystalline structure, optical
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and electrical properties of the as-deposited ZnO:Zr films were systematically investigated. The results showed that ZnO:Zr film for 0.51 at.% had a stronger preferred orientation toward the c-axis and a better crystallinity
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which were be good for UV photosensitivity. The lowest resistivity of 31.8 Ω cm was obtained for 0.51 at.% Zr doped ZnO film. The UV photosensitivity showed that photodetectors based on ZnO:Zr film for 0.51 at.% had very fast response and a higher photocurrent under UV illumination, which was attributed to the release of trapped electrons from surface defects or adsorbed oxygen. The photosensitivity mechanism of ZnO:Zr films has been also discussed in present paper.
1. Introduction
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Keywords: Zirconium doped zinc oxide, X-ray diffraction, UltraViolet detector
Zinc oxide or various metals doped zinc oxide films have been widely studied because they are nontoxic, inexpensive and abundant. Moreover, they have been proved to be beneficial
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material for application to laser diodes (LDs), light-emitting diodes (LEDs) and ultraviolet (UV) detecting devices in the UV range due to large excition binding energy (~60 meV) [1-5].
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More effort is needed to improve the ZnO properties. ZnO films show interesting
optoelectronic properties by alternative dopants such as Al, Mg, Cu, Co, Ti, Zn and Zr[6-10] ect. F.M. Li et al. [4] observed that the UV photodetectors based on ZnO: Cu films had high sensitivity and fast response and recovery times. A Mahdavi et al. [10] found that Ti- and Zr-doped ZnO exhibit desirable the electrical conductivities. D. Scorticati et al. [11] pointed out that the electrical properties of Al:ZnO thin films were improved by thermal annealing using ultra-short laser pulses. The deposition of metals doped ZnO films have been reported by several methods [12-15].
∗
Corresponding author Tel.: +86 13919328797; fax:+86 09317971503 Email: [email protected] 1
ACCEPTED MANUSCRIPT As an extensively used technique of preparation films, radio frequency (RF) reactive magnetron sputtering can prepare uniform and large area films more easily than other preparation method, and can prepare films with higher quality than sol-gel method. Doping of higher concentration of Zr in ZnO lattice has been extensively reported [16-18]. Hence, it is necessary to study the
by RF reactive magnetron sputtering.
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structural and optoelectronic properties of the lower concentration Zr doped ZnO films prepared
In this paper, ZnO:Zr films at different concentration were prepared on Si substrates via RF
reactive magnetron sputtering technique. The crystal structures, surface morphologies, optical and
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electrical properties of ZnO:Zr films were systematically studied. 2. Experimental details
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Pure and ZnO:Zr films were deposited on glass substrates using radio frequency (RF) reactive magnetron sputtering. A high-purity Zn target (99.9999 % purity, 60 mm in diameter) and glass (Corning 7105) substrate were used in the experiments. The distance between target and substrate was 50 mm. Before putting into the deposition chamber, the glass substrates were ultrasonically cleaned in acetone and rinsed in deionized water. To conduct Zr doping, Zr foils
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(purity ∼99.9 %) were pasted to Zn target. The Zr concentration of the ZnO:Zr films were determined by energy dispersive spectrometer (EDS, JSM-6701F). The Zr doping concentrations in the ZnO:Zr films were 0, 0.38, 0.51 and 0.78 at.%, respectively. High-purity Ar and O2 were
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used as the sputtering and reactive gas. The deposition chamber was initially evacuated gas, respectively. The deposition chamber was initially evacuated down to a residual pressure of
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5×10−4 Pa, and then the O2 and Ar gas mixture were introduced into the chamber to maintain reactant pressure of 2 Pa. The Zn target was presputtered for 15 min to remove surface contamination and maintain system stability. Then the Zr doped ZnO films were deposited at substrate temperatures 200 °C with RF power of 100 W for 1 h and the O2: Ar ratio was for 10: 10 sccm. The samples were annealed in vacuum at temperature of 600 °C. The annealing time was kept at the same value of 1 h. The crystal structures were studied by X-ray diffraction (XRD, D/Max-2400) using the Cu K-radiation with λ = 0.15406 nm. The surface morphology was characterized by scanning electronic microscopy (SEM, JSM-6701F). The photoluminescence (PL) measurements were carried out by using the excitation of Xe Lamp (RF-5301, wavelength 250 nm). Photocurrent was 2
ACCEPTED MANUSCRIPT measured on a CHI600D electrochemical workstation using a 150-W xenon lampas the illumination source and a grating monochromator (1200 grooves/mm) to provide monochromatic light at wavelengths of 300-800 nm. All spectra were measured at room temperature in air.
3. Results and discussion
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3.1. Structural properties of ZnO:Zr films Fig. 1 shows the XRD patterns of ZnO:Zr films with different doping concentration. The samples are polycrystalline and a preferred orientation with the c-axis perpendicular to the substrates. All the diffraction peaks in the XRD spectrum can be well matched with a pure
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hexagonal wurtzite structure (JCPDS No. 36-1451). No other peaks are detected after Zr doping, indicating that there are no new chemical phases. As the doping concentration increase, the
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intensity of (002) peaks decreases first and then increases, reaching a maximum at doping concentration of 0.51 at.%. However, the intensity of ZnO (0 0 2) peak decreases with the doping concentration further increase. The result shows that the doping concentration plays an indispensable role in the sputtering process. The optimum Zr doping concentration is at about 0.51 at.%, which suggests the crystallinity is improved by moderate Zr doping in ZnO film. The
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crystallinity of ZnO films can be enhanced by doping moderate impurity has also been observed in the Mg doped ZnO films [19]. This may be due to that moderate quantity of Zr or Mg atoms could be considered to exist as interstitials that shared the oxygen with Zn atoms and hence improve the
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(0 0 2) intensity [20].
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The average grain size is calculated using the following Scherrer equation [21]:
D=
0.89 λ β cos θ
(1)
where λ, θ and β are the X-ray wavelength (1.5406 Å), diffraction angle, and the full-width at half-maximum (FWHM) of the ZnO (002) peak, respectively. Table 1 shows experimental results of the samples. As shown in Table1, with the increases of Zr doping concentration, the FWHM of the ZnO (002) diffraction peak decreases first and then increases, reaching a minimum of about 0.46˚ at Zr doping concentration for 0.51 at.%. It can be also found that the ZnO:Zr film for 0.51 at.% has biggest the mean grain size. Combined with the intensity of the (002) diffraction peak, the 0.51 at.% Zr doped ZnO film has the best crystal quality. This can be reflected the 3
ACCEPTED MANUSCRIPT improvement of the electrical properties of the films. According to the Bragg’s law: nλ = 2dsinθ, where n is the order of diffraction (usually n=1); λ is the X-ray wavelength (0.15406 nm); θ is the diffraction angle of the peak and d denotes the crystalline plane distance of indices (h, k, l), which are the Miller indices. The lattice constants of
(2)
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1 4 h 2 + hk + k 2 l2 = ( ) + d hkl2 3 a2 c2
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(002) peak can be calculated by the following formula [22].
We calculate the unit cell parameters (c) of the ZnO:Zr films and the values for these
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parameters are shown in Table 1. We can observe that the lattice parameters (c) increase after Zr doped, which suggests the additional interstitial incorporation of Zr or additional incorporation of Zr ions with lattice defects. This increment of lattice parameters may be due to the larger ionic radius of Zr4+ (0.084 nm) than that of Zn2+ (0.074 nm) [23, 24]. As we know that free charge is the first factor responsible for increasing the lattice constant and also point defects such as Zn antisites,
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O vacancies, also increase the lattice constant.
The lattice mismatch between ZnO film and substrate can result in varying degrees of stress during the deposition process of ZnO thin films [25]
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σ = −233
c − c0 c0
(3)
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where c0 is the lattice constant (c0=0.5205nm) of ZnO films without defects and c is the lattice constant of ZnO film in our experiments. According to Eq. (3), it can be calculated the strain in ZnO:Zr films at various doping concentration as shown in Table 1. Pure ZnO and 0.78 at.% Zr doped ZnO films have positive stress, which indicated a tensile stress. However, 0.38 at.% and 0.51 at.% Zr doped ZnO films have negative stress, which indicated a compressive stress. We considered that Zr4+ substitute Zn2+ and Zr4+ interstitial might be the main reasons which induce the compressive and tensile stress in the ZnO:Zr films, because the lattice constant (c) was changed by these defects. 3.2. Morphologies of ZnO:Zr films In order to further study the effect of doping concentration on the structural properties of 4
ACCEPTED MANUSCRIPT films, we measured the SEM images of ZnO:Zr films prepared at the different concentration: (a) 0 at.%,(b) 0.38 at.%, (c) 0.51 at.% and (d) 0.78 at.%, as shown in Fig. 2. As the Zr doping with 0.38 at.% and 0.78 at.%, a less uniform grain size is observed in ZnO:Zr films. The morphology of ZnO:Zr film is found to be denser, uniform grain size when Zr doping concentration is about for
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0.51 at.% . This indicates that the crystallinity of the film is improved. This result is consistent well with the XRD analysis mentioned above. The Zr doping concentration of ZnO:Zr films is investigated by an EDS. The EDS pattern of Zr doped ZnO films in Fig. 2(f) indicate that the as-prepared films are composed of Zn, O and Zr. The peaks of Zr detected are not obvious. This
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may because the Si peak is too strong. 3.3 Optical properties of ZnO:Zr films
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Optical properties of ZnO:Zr films were studied in terms of Raman spectra with different Zr doping concentrations and reported here with the effect on scattering of such yielded ZnO films. Raman scattering is known to be the most powerful nondestructive technique to investigate the crystalline quality, defects, and structural disorder in the doped semiconductor. It provides important information about the local structural changes in virtue of incorporation of highly
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mismatched doped ions into the ZnO lattice. ZnO is a hexagonal wurtzite structure with four atoms per unit cell and belongs to a C6v-4(P63mc) point group symmetry having six Raman active optical phonon modes at the center of the Brillouin zone [26]. The frequencies of the basic optical modes in ZnO are E2 (low) =101 cm-1, E2 (high) =437 cm-1, A1 (TO) =380 cm-1, A1 (LO) =574
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cm-1, E1 (TO) = 407 cm-1, and E1 (LO) =583 cm-1. Fig. 3 shows the Raman spectra of Zr doped ZnO films. The peaks at 303.6 cm-1, 522 cm-1
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(marked by P) and 620.1 cm-1 are due to scattering from the silicon substrate. The E2 (high) mode of ZnO:Zr films at about 437.9 cm−1 can be observed in all spectra. Comparing with the pattern of pure ZnO film, the E2 (high) mode strengthens in 0.51 at. % Zr doped ZnO film, and weakens in the others films. The intensity of E2 (high) is closely related with the crystal quality of ZnO. The crystal quality of ZnO:Zr film for 5.1 at. % is better than the others. According to the frequency of the E2 (high) mode in ZnO standard sample (437 cm-1) [27], we are observed Raman shift of 0.9 cm-1 in the all samples. This indicates a residual stress in ZnO:Zr films, which is due to the lattice mismatch between the film and the substrate. The results are consistent with the XRD. A smaller peak at 674.8 cm-1 is attributed to an E2 (low) + A1 (LO) (multiphonon) mode. 5
ACCEPTED MANUSCRIPT To study the influence of Zr doping on the luminescence of ZnO film, we measured the temperature PL spectra of ZnO films with different Zr doping concentrations, as is shown in Fig. 4. A blue emission centered at about 482 nm (2.57 eV) can be observed in the spectra. We can found that the intense of the blue emission is the strongest when Zr doping concentration is for 0.51 at.%.
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A.F. Kohan et al. [28] 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 interstitial Zn to Zn vacancies was about 2.54 eV, which were well consistent with the energy of the blue emission peaks at about 483 nm (2.57 eV) in our samples. Therefore, the blue emission
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was assigned to the electron transition from the energy levels of Zni to VZn. Concerning more details about Fig. 4, the blue emission position shifts to higher wavelength region (482 to 485 nm)
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after Zr doping. In addition, when Zr doping concentration is for 0.51 at.%, the intense of the blue emission is the strongest, and the UV emission peak located at 402 nm can be observed from PL spectra. 3.4. Electrical properties of ZnO:Zr films
The effect of doping concentration on the resistivity of ZnO:Zr films deposited on Si
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substrates. We measured resistivity of ZnO:Zr films prepared at the different concentration. The resistivity of ZnO:Zr films are 63.7 Ω cm , 490.5 Ω cm, 31.8 Ω cm and 284.7 Ω cm, correspondingly to doping concentration are 0 at.%, 0.38 at.%, 0.51 at.% and 0.78 at.%,
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respectively. It can be seen that the resistivity of the deposited films increases first, decreases and then increases with increase in doping concentration. The lowest resistivity of 31.8 Ω cm was
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obtained for 0.51 at. % Zr doped ZnO film. The conductivity of the deposited films greatly depends on the film crystallinity [16, 29, 30]. It has been observed that ZnO:Zr for doping concentration 0.51 at.% the crystilline quality films seems yielded with minimal resistivity. ZnO is a wide-band gap semiconductor and can be excited by UV irradiation for photocurrent
generation. The crystal quality of the film is a crucial parameter in UV photosensitivity [4]. According to the above analysis, the ZnO:Zr film for 0.51 at.% has the best crystal quality. So we measured the UV photosensitivity of the ZnO:Zr film for 0.51 at.% under periodic UV illumination at 380 nm, as shown in Fig. 5. It can be found that when the photocurrent quickly increases first and then slowly increased asymptotically to attain maximum value (7.18×10-5 A) at a particular level in the presence of UV illumination. When the UV light was turned off, the 6
ACCEPTED MANUSCRIPT photocurrent rapidly decreased to attain the minimum current value (6.05×10-5 A). Response time (the dark current to rise to 90 % of saturation value under UV illumination) and recovery time (the photo current falls to 10 % of the saturation value in the absence of UV illumination) for the film based photodetectors have been calculated to be 0.1 s and 0.5 s, respectively. Furthermore, the
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ZnO:Zr film for 0.51 at.% shows stable photocurrent growth and decay under periodic UV illumination at 380 nm, so the films are potential candidate materials for UV photodetectors. A comparison between the photosensitivity of the UV photodetectors and literature reports is summarized in Table 2. It is noteworthy that the UV photodetector fabricated in our work exhibits
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higher photocurrent compared with those reported in the literatures [4, 31, 32]. It is known that the photocurrent of UV photodetector depends on electron hole generation, surface adsorption and
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photodesorption processes. In the present case, the photocurrent mainly depends on the numbers of photocarriers generated between the conduction band and valence band. We believe that the enhanced photocurrent in our work can be due to the increase in electron density in the conduction band of ZnO. 3.6. The UV photosensitivity mechanism
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The ZnO films are n-type semiconductor due to donor defects, such as oxygen deficiencies and interstitial Zn ions [33]. Photosensitivity characteristic of a ZnO prepared UV photodetector originates from adsorption and desorption of oxygen molecules on ZnO film surface. The
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schematic presentation of photosensitivity mechanism is shown in Fig. 6. Oxygen molecules adsorb on the surface of ZnO films. Adsorbed oxygen molecules capture a free electron from the conduction band of ZnO in the absence of UV illumination. Oxygen becomes negatively charged
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oxygen ions ( O−2 ), and then it leaves a depletion region near the surface and the grain boundaries: O 2 (g) + e −1 → O −2
(4)
where oxygen ions ( O−2 ) are different from bulk-related oxide ions ( O2- ). Oxygen ions adhere to the surface and the crystallite interfaces of the film and form a chemically adsorbed surface state. Generally, the photoresponse of ZnO UV photodetector was composed of a rapid process of photogeneration and recombination of electron-hole pairs [34]. Under UV illumination, pairs of electron and hole are generated: hυ → h + + e -
(5) 7
ACCEPTED MANUSCRIPT where h is the Planck constant, h+ is the hole. Photogenerated holes release the negatively charged oxygen ions, and then the oxygen is desorbed: h + + O −2 → O 2 ( g )
(6)
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Increase in photocurrent of ZnO can mainly attribute to the enhanced carrier density and mobility on UV illumination [35]. First, a large amount of generated electrons increase the carrier density, resulting in an increase the photocurrent of the film, which is a prompt process. On the
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other hand, the negatively charged oxygen ions capture generated holes and release O2 gas. As the captured holes with the same density, the electrons of conduction band can be excess leading to
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the photocurrent increases. In this process, the most generated holes are depleted due to the oxygen desorption, forbidden holes to recombine with the electrons of conduction band resulting in raising the carrier density. Moreover, the oxygen desorption enhances the carrier mobility. This is because generated holes move along bending and these moved holes trap negatively charged oxygen molecules decreasing the barrier height. So the photocurrent of the film increases. The
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produce of electrons is accompanied with the recombination of electron-hole pairs. If the produce rate of electrons is larger than the electron-hole pair recombination rate, the photocurrent can increase. However, the rate of carrier recombination decreases when the chemisorbed oxygen increase. When the produce rate of the carriers is equal to the recombination rate, the photocurrent
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attains the peak value, which is in a stable dynamic equilibrium state. When the UV illumination is off, electron-hole recombination dominates, which is a rapid process, so the photocurrent
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decreases fast. With the oxygen adsorbed again, the surfaces of the film appear depletion regions and barriers again and between crystallites, leading to the decrease of the carrier density and mobility, the photocurrent of the film decreases.
4. Conclusions
ZnO:Zr films at different Zr doping concentrations were prepared by RF reactive magnetron sputtering. The effect of Zr contents on the crystalline structure, optical and electrical properties of the as-deposited ZnO:Zr films were studied. It was found that ZnO:Zr film for 0.51 at.% has a stronger preferred orientation toward the c-axis and a better crystallinity. The E2 (high) mode at about 437.9 cm−1 can be found in the Raman spectra of the films. The photoluminescence (PL) 8
ACCEPTED MANUSCRIPT spectra measured at room temperature revealed that the blue emission position shifts to higher wavelength region (482 nm to 485 nm) after Zr doping. At the Zr doping concentration for 0.51 at.%, the UV emission peak located at 402 nm can be observed from PL spectra, the ZnO:Zr film had the lowest resistivity for 31.8 Ω cm and excellent photosensitivity on UV detecting. So the
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ZnO:Zr films are potential candidate nanomaterials for UV photodetectors.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grant no. 10874140), Natural Science Foundation of Gansu Province (Grant no. 0710RJZA105).
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1
XRD patterns of ZnO and ZnO:Zr films with different doping concentration (a) 0 at.%, (b) 0.38 at.%, (c) 0.51 at.% and (d) 0.78 at.%.
Fig. 2
SEM patterns of ZnO and ZnO:Zr films with different doping concentration (a) 0 at.%, (b) 0.38 at.%, (c) 0.51 at.%, (d) 0.78 at.% and (f) EDS spectra of ZnO:Zr film with different doping concentration. Raman spectra of ZnO:Zr films with different doping concentration (a) 0 at.%, (b) 0.38 at.%, (c) 0.51 at.% and (d) 0.78 at.%.
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Fig. 3
Fig. 4
PL spectra of ZnO and ZnO:Zr films with different doping concentration.
Fig. 5
Photocurrent behavior for ZnO:Zr film with 0.51 at.% at periodic UV illumination at 380nm.
Fig. 6
The schematic illustration of the UV photosensitivity mechanism.
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a
250
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Intensity(a.u.)
300
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Figure 1
(002)
150 100 50
26 28 30 32 34 36 38 40 42 44 46 48 50
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EP
2θ(degree)
12
(100)
(101)
0 26 28 30 32 34 36 38 40 42 44 46 48 50
2θ(degree)
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Figure 2
AC C
EP
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Figure 2
(a)
(b)
13
ACCEPTED MANUSCRIPT Figure 3 (d)
(c)
P
(a)
1200
-1 437.9cm
-1 303.6cm
-1 620.1cm
600 300
200
300
400
500
-1 303.6cm
600
700
P
(c)
0 800 100 1500
-1 437.9cm
300
400
500
P
600
300
300
300
400
-1 303.6cm
900
600
200
200
1200
900
0 100
-1 674.8cm
-1 437.9cm
500
600
700
-1 437.9cm
0 800 100
200
300
600
700
800
(d)
-1 620.1cm
400
500
600
700
800
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Raman shift (cm-1)
-1 620.1cm
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Raman intensity (a.u.)
1200
E2 (low) + A1 (LO)
900
-1 303.6cm
300 0 100 1500
(b)
1200
900 600
P
1500
E2 (high)
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1500
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Figure 4
402nm
485nm
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Intensity (a. u.)
EP
482nm
0 at.% 0.38 at.% 0.51 at.% 0.78 at.%
400
425
450
475
500
525
Wavelength (nm)
14
550
575
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Figure 5
74
UV OFF
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70 68 66 64
60 58 56
UV ON
0
5
10
15
20
25
30
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62
35
40
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photocurrent(uA)
72
Time(sec.)
AC C
EP
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Figure 6
15
45
50
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Table 1 Parameters of the (002) diffraction peak of Zr doped ZnO films. FWMH(deg)
c (nm)
D (nm)
a (0 at.%)
34.48
0.50
0.5200
16.45
b (0.38 at.%)
34.44
0.49
0.5206
16.78
c (0.51 at.%)
34.42
0.46
0.5208
19.58
d (0.78 at.%)
34.46
0.48
0.5202
17.14
d (nm)
σ (GPa)
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2θ(deg)
0.2600
0.224
0.2603
-0.045
0.2604
-0.134
0.2601
0.134
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Samples
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Table 2 Comparison of UV photosensitivity based on various composites Maximum
Publication year
Reference
17
2013
[4]
5.3
2012
[31]
ZnO:Al NWs/Au
20
2012
[31]
CVD
ZnSe NRs
13
2016
[32]
RF
ZnO:Zr film
71.8
-
This work
Composition
RF
ZnO:Cu film
Vapor-liquid-solid
ZnO:Al NWs
Vapor-liquid-solid
photocurrent (µA)
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EP
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Synthesis route
16
S0925-8388(16)30435-2
DOI:
10.1016/j.jallcom.2016.02.178
Reference:
JALCOM 36796
To appear in:
Journal of Alloys and Compounds
Received Date: 30 October 2015 Revised Date:
16 February 2016
Accepted Date: 19 February 2016
Please cite this article as: H. Bian, S. Ma, G. Yang, H. Zhu, X. Xu, S. Yan, J. Gao, Z. Zhang, The optical and electrical properties of ZnO:Zr films, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.02.178. 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.
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The optical and electrical properties of ZnO:Zr films Haiqin Bian∗, Shuyi Ma, Guijin Yang, Haibin Zhu, Xiaoli Xu, Shaohui Yan, Jiming Gao, Zhengmei Zhang Key Laboratory of Atomic and Molecular, Physics & Function Material of Gansu Province
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College of Physics and Electronics Engineering, Northwest Normal University, Lanzhou 730070, China
Abstract: Zirconium doped zinc oxide (ZnO:Zr) films at different Zr doping concentrations were deposited by radio frequency (RF) reactive magnetron sputtering. The effect of Zr contents on the crystalline structure, optical
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and electrical properties of the as-deposited ZnO:Zr films were systematically investigated. The results showed that ZnO:Zr film for 0.51 at.% had a stronger preferred orientation toward the c-axis and a better crystallinity
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which were be good for UV photosensitivity. The lowest resistivity of 31.8 Ω cm was obtained for 0.51 at.% Zr doped ZnO film. The UV photosensitivity showed that photodetectors based on ZnO:Zr film for 0.51 at.% had very fast response and a higher photocurrent under UV illumination, which was attributed to the release of trapped electrons from surface defects or adsorbed oxygen. The photosensitivity mechanism of ZnO:Zr films has been also discussed in present paper.
1. Introduction
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Keywords: Zirconium doped zinc oxide, X-ray diffraction, UltraViolet detector
Zinc oxide or various metals doped zinc oxide films have been widely studied because they are nontoxic, inexpensive and abundant. Moreover, they have been proved to be beneficial
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material for application to laser diodes (LDs), light-emitting diodes (LEDs) and ultraviolet (UV) detecting devices in the UV range due to large excition binding energy (~60 meV) [1-5].
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More effort is needed to improve the ZnO properties. ZnO films show interesting
optoelectronic properties by alternative dopants such as Al, Mg, Cu, Co, Ti, Zn and Zr[6-10] ect. F.M. Li et al. [4] observed that the UV photodetectors based on ZnO: Cu films had high sensitivity and fast response and recovery times. A Mahdavi et al. [10] found that Ti- and Zr-doped ZnO exhibit desirable the electrical conductivities. D. Scorticati et al. [11] pointed out that the electrical properties of Al:ZnO thin films were improved by thermal annealing using ultra-short laser pulses. The deposition of metals doped ZnO films have been reported by several methods [12-15].
∗
Corresponding author Tel.: +86 13919328797; fax:+86 09317971503 Email: [email protected] 1
ACCEPTED MANUSCRIPT As an extensively used technique of preparation films, radio frequency (RF) reactive magnetron sputtering can prepare uniform and large area films more easily than other preparation method, and can prepare films with higher quality than sol-gel method. Doping of higher concentration of Zr in ZnO lattice has been extensively reported [16-18]. Hence, it is necessary to study the
by RF reactive magnetron sputtering.
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structural and optoelectronic properties of the lower concentration Zr doped ZnO films prepared
In this paper, ZnO:Zr films at different concentration were prepared on Si substrates via RF
reactive magnetron sputtering technique. The crystal structures, surface morphologies, optical and
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electrical properties of ZnO:Zr films were systematically studied. 2. Experimental details
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Pure and ZnO:Zr films were deposited on glass substrates using radio frequency (RF) reactive magnetron sputtering. A high-purity Zn target (99.9999 % purity, 60 mm in diameter) and glass (Corning 7105) substrate were used in the experiments. The distance between target and substrate was 50 mm. Before putting into the deposition chamber, the glass substrates were ultrasonically cleaned in acetone and rinsed in deionized water. To conduct Zr doping, Zr foils
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(purity ∼99.9 %) were pasted to Zn target. The Zr concentration of the ZnO:Zr films were determined by energy dispersive spectrometer (EDS, JSM-6701F). The Zr doping concentrations in the ZnO:Zr films were 0, 0.38, 0.51 and 0.78 at.%, respectively. High-purity Ar and O2 were
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used as the sputtering and reactive gas. The deposition chamber was initially evacuated gas, respectively. The deposition chamber was initially evacuated down to a residual pressure of
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5×10−4 Pa, and then the O2 and Ar gas mixture were introduced into the chamber to maintain reactant pressure of 2 Pa. The Zn target was presputtered for 15 min to remove surface contamination and maintain system stability. Then the Zr doped ZnO films were deposited at substrate temperatures 200 °C with RF power of 100 W for 1 h and the O2: Ar ratio was for 10: 10 sccm. The samples were annealed in vacuum at temperature of 600 °C. The annealing time was kept at the same value of 1 h. The crystal structures were studied by X-ray diffraction (XRD, D/Max-2400) using the Cu K-radiation with λ = 0.15406 nm. The surface morphology was characterized by scanning electronic microscopy (SEM, JSM-6701F). The photoluminescence (PL) measurements were carried out by using the excitation of Xe Lamp (RF-5301, wavelength 250 nm). Photocurrent was 2
ACCEPTED MANUSCRIPT measured on a CHI600D electrochemical workstation using a 150-W xenon lampas the illumination source and a grating monochromator (1200 grooves/mm) to provide monochromatic light at wavelengths of 300-800 nm. All spectra were measured at room temperature in air.
3. Results and discussion
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3.1. Structural properties of ZnO:Zr films Fig. 1 shows the XRD patterns of ZnO:Zr films with different doping concentration. The samples are polycrystalline and a preferred orientation with the c-axis perpendicular to the substrates. All the diffraction peaks in the XRD spectrum can be well matched with a pure
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hexagonal wurtzite structure (JCPDS No. 36-1451). No other peaks are detected after Zr doping, indicating that there are no new chemical phases. As the doping concentration increase, the
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intensity of (002) peaks decreases first and then increases, reaching a maximum at doping concentration of 0.51 at.%. However, the intensity of ZnO (0 0 2) peak decreases with the doping concentration further increase. The result shows that the doping concentration plays an indispensable role in the sputtering process. The optimum Zr doping concentration is at about 0.51 at.%, which suggests the crystallinity is improved by moderate Zr doping in ZnO film. The
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crystallinity of ZnO films can be enhanced by doping moderate impurity has also been observed in the Mg doped ZnO films [19]. This may be due to that moderate quantity of Zr or Mg atoms could be considered to exist as interstitials that shared the oxygen with Zn atoms and hence improve the
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(0 0 2) intensity [20].
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The average grain size is calculated using the following Scherrer equation [21]:
D=
0.89 λ β cos θ
(1)
where λ, θ and β are the X-ray wavelength (1.5406 Å), diffraction angle, and the full-width at half-maximum (FWHM) of the ZnO (002) peak, respectively. Table 1 shows experimental results of the samples. As shown in Table1, with the increases of Zr doping concentration, the FWHM of the ZnO (002) diffraction peak decreases first and then increases, reaching a minimum of about 0.46˚ at Zr doping concentration for 0.51 at.%. It can be also found that the ZnO:Zr film for 0.51 at.% has biggest the mean grain size. Combined with the intensity of the (002) diffraction peak, the 0.51 at.% Zr doped ZnO film has the best crystal quality. This can be reflected the 3
ACCEPTED MANUSCRIPT improvement of the electrical properties of the films. According to the Bragg’s law: nλ = 2dsinθ, where n is the order of diffraction (usually n=1); λ is the X-ray wavelength (0.15406 nm); θ is the diffraction angle of the peak and d denotes the crystalline plane distance of indices (h, k, l), which are the Miller indices. The lattice constants of
(2)
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1 4 h 2 + hk + k 2 l2 = ( ) + d hkl2 3 a2 c2
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(002) peak can be calculated by the following formula [22].
We calculate the unit cell parameters (c) of the ZnO:Zr films and the values for these
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parameters are shown in Table 1. We can observe that the lattice parameters (c) increase after Zr doped, which suggests the additional interstitial incorporation of Zr or additional incorporation of Zr ions with lattice defects. This increment of lattice parameters may be due to the larger ionic radius of Zr4+ (0.084 nm) than that of Zn2+ (0.074 nm) [23, 24]. As we know that free charge is the first factor responsible for increasing the lattice constant and also point defects such as Zn antisites,
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O vacancies, also increase the lattice constant.
The lattice mismatch between ZnO film and substrate can result in varying degrees of stress during the deposition process of ZnO thin films [25]
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σ = −233
c − c0 c0
(3)
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where c0 is the lattice constant (c0=0.5205nm) of ZnO films without defects and c is the lattice constant of ZnO film in our experiments. According to Eq. (3), it can be calculated the strain in ZnO:Zr films at various doping concentration as shown in Table 1. Pure ZnO and 0.78 at.% Zr doped ZnO films have positive stress, which indicated a tensile stress. However, 0.38 at.% and 0.51 at.% Zr doped ZnO films have negative stress, which indicated a compressive stress. We considered that Zr4+ substitute Zn2+ and Zr4+ interstitial might be the main reasons which induce the compressive and tensile stress in the ZnO:Zr films, because the lattice constant (c) was changed by these defects. 3.2. Morphologies of ZnO:Zr films In order to further study the effect of doping concentration on the structural properties of 4
ACCEPTED MANUSCRIPT films, we measured the SEM images of ZnO:Zr films prepared at the different concentration: (a) 0 at.%,(b) 0.38 at.%, (c) 0.51 at.% and (d) 0.78 at.%, as shown in Fig. 2. As the Zr doping with 0.38 at.% and 0.78 at.%, a less uniform grain size is observed in ZnO:Zr films. The morphology of ZnO:Zr film is found to be denser, uniform grain size when Zr doping concentration is about for
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0.51 at.% . This indicates that the crystallinity of the film is improved. This result is consistent well with the XRD analysis mentioned above. The Zr doping concentration of ZnO:Zr films is investigated by an EDS. The EDS pattern of Zr doped ZnO films in Fig. 2(f) indicate that the as-prepared films are composed of Zn, O and Zr. The peaks of Zr detected are not obvious. This
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may because the Si peak is too strong. 3.3 Optical properties of ZnO:Zr films
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Optical properties of ZnO:Zr films were studied in terms of Raman spectra with different Zr doping concentrations and reported here with the effect on scattering of such yielded ZnO films. Raman scattering is known to be the most powerful nondestructive technique to investigate the crystalline quality, defects, and structural disorder in the doped semiconductor. It provides important information about the local structural changes in virtue of incorporation of highly
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mismatched doped ions into the ZnO lattice. ZnO is a hexagonal wurtzite structure with four atoms per unit cell and belongs to a C6v-4(P63mc) point group symmetry having six Raman active optical phonon modes at the center of the Brillouin zone [26]. The frequencies of the basic optical modes in ZnO are E2 (low) =101 cm-1, E2 (high) =437 cm-1, A1 (TO) =380 cm-1, A1 (LO) =574
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cm-1, E1 (TO) = 407 cm-1, and E1 (LO) =583 cm-1. Fig. 3 shows the Raman spectra of Zr doped ZnO films. The peaks at 303.6 cm-1, 522 cm-1
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(marked by P) and 620.1 cm-1 are due to scattering from the silicon substrate. The E2 (high) mode of ZnO:Zr films at about 437.9 cm−1 can be observed in all spectra. Comparing with the pattern of pure ZnO film, the E2 (high) mode strengthens in 0.51 at. % Zr doped ZnO film, and weakens in the others films. The intensity of E2 (high) is closely related with the crystal quality of ZnO. The crystal quality of ZnO:Zr film for 5.1 at. % is better than the others. According to the frequency of the E2 (high) mode in ZnO standard sample (437 cm-1) [27], we are observed Raman shift of 0.9 cm-1 in the all samples. This indicates a residual stress in ZnO:Zr films, which is due to the lattice mismatch between the film and the substrate. The results are consistent with the XRD. A smaller peak at 674.8 cm-1 is attributed to an E2 (low) + A1 (LO) (multiphonon) mode. 5
ACCEPTED MANUSCRIPT To study the influence of Zr doping on the luminescence of ZnO film, we measured the temperature PL spectra of ZnO films with different Zr doping concentrations, as is shown in Fig. 4. A blue emission centered at about 482 nm (2.57 eV) can be observed in the spectra. We can found that the intense of the blue emission is the strongest when Zr doping concentration is for 0.51 at.%.
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A.F. Kohan et al. [28] 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 interstitial Zn to Zn vacancies was about 2.54 eV, which were well consistent with the energy of the blue emission peaks at about 483 nm (2.57 eV) in our samples. Therefore, the blue emission
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was assigned to the electron transition from the energy levels of Zni to VZn. Concerning more details about Fig. 4, the blue emission position shifts to higher wavelength region (482 to 485 nm)
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after Zr doping. In addition, when Zr doping concentration is for 0.51 at.%, the intense of the blue emission is the strongest, and the UV emission peak located at 402 nm can be observed from PL spectra. 3.4. Electrical properties of ZnO:Zr films
The effect of doping concentration on the resistivity of ZnO:Zr films deposited on Si
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substrates. We measured resistivity of ZnO:Zr films prepared at the different concentration. The resistivity of ZnO:Zr films are 63.7 Ω cm , 490.5 Ω cm, 31.8 Ω cm and 284.7 Ω cm, correspondingly to doping concentration are 0 at.%, 0.38 at.%, 0.51 at.% and 0.78 at.%,
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respectively. It can be seen that the resistivity of the deposited films increases first, decreases and then increases with increase in doping concentration. The lowest resistivity of 31.8 Ω cm was
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obtained for 0.51 at. % Zr doped ZnO film. The conductivity of the deposited films greatly depends on the film crystallinity [16, 29, 30]. It has been observed that ZnO:Zr for doping concentration 0.51 at.% the crystilline quality films seems yielded with minimal resistivity. ZnO is a wide-band gap semiconductor and can be excited by UV irradiation for photocurrent
generation. The crystal quality of the film is a crucial parameter in UV photosensitivity [4]. According to the above analysis, the ZnO:Zr film for 0.51 at.% has the best crystal quality. So we measured the UV photosensitivity of the ZnO:Zr film for 0.51 at.% under periodic UV illumination at 380 nm, as shown in Fig. 5. It can be found that when the photocurrent quickly increases first and then slowly increased asymptotically to attain maximum value (7.18×10-5 A) at a particular level in the presence of UV illumination. When the UV light was turned off, the 6
ACCEPTED MANUSCRIPT photocurrent rapidly decreased to attain the minimum current value (6.05×10-5 A). Response time (the dark current to rise to 90 % of saturation value under UV illumination) and recovery time (the photo current falls to 10 % of the saturation value in the absence of UV illumination) for the film based photodetectors have been calculated to be 0.1 s and 0.5 s, respectively. Furthermore, the
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ZnO:Zr film for 0.51 at.% shows stable photocurrent growth and decay under periodic UV illumination at 380 nm, so the films are potential candidate materials for UV photodetectors. A comparison between the photosensitivity of the UV photodetectors and literature reports is summarized in Table 2. It is noteworthy that the UV photodetector fabricated in our work exhibits
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higher photocurrent compared with those reported in the literatures [4, 31, 32]. It is known that the photocurrent of UV photodetector depends on electron hole generation, surface adsorption and
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photodesorption processes. In the present case, the photocurrent mainly depends on the numbers of photocarriers generated between the conduction band and valence band. We believe that the enhanced photocurrent in our work can be due to the increase in electron density in the conduction band of ZnO. 3.6. The UV photosensitivity mechanism
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The ZnO films are n-type semiconductor due to donor defects, such as oxygen deficiencies and interstitial Zn ions [33]. Photosensitivity characteristic of a ZnO prepared UV photodetector originates from adsorption and desorption of oxygen molecules on ZnO film surface. The
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schematic presentation of photosensitivity mechanism is shown in Fig. 6. Oxygen molecules adsorb on the surface of ZnO films. Adsorbed oxygen molecules capture a free electron from the conduction band of ZnO in the absence of UV illumination. Oxygen becomes negatively charged
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oxygen ions ( O−2 ), and then it leaves a depletion region near the surface and the grain boundaries: O 2 (g) + e −1 → O −2
(4)
where oxygen ions ( O−2 ) are different from bulk-related oxide ions ( O2- ). Oxygen ions adhere to the surface and the crystallite interfaces of the film and form a chemically adsorbed surface state. Generally, the photoresponse of ZnO UV photodetector was composed of a rapid process of photogeneration and recombination of electron-hole pairs [34]. Under UV illumination, pairs of electron and hole are generated: hυ → h + + e -
(5) 7
ACCEPTED MANUSCRIPT where h is the Planck constant, h+ is the hole. Photogenerated holes release the negatively charged oxygen ions, and then the oxygen is desorbed: h + + O −2 → O 2 ( g )
(6)
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Increase in photocurrent of ZnO can mainly attribute to the enhanced carrier density and mobility on UV illumination [35]. First, a large amount of generated electrons increase the carrier density, resulting in an increase the photocurrent of the film, which is a prompt process. On the
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other hand, the negatively charged oxygen ions capture generated holes and release O2 gas. As the captured holes with the same density, the electrons of conduction band can be excess leading to
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the photocurrent increases. In this process, the most generated holes are depleted due to the oxygen desorption, forbidden holes to recombine with the electrons of conduction band resulting in raising the carrier density. Moreover, the oxygen desorption enhances the carrier mobility. This is because generated holes move along bending and these moved holes trap negatively charged oxygen molecules decreasing the barrier height. So the photocurrent of the film increases. The
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produce of electrons is accompanied with the recombination of electron-hole pairs. If the produce rate of electrons is larger than the electron-hole pair recombination rate, the photocurrent can increase. However, the rate of carrier recombination decreases when the chemisorbed oxygen increase. When the produce rate of the carriers is equal to the recombination rate, the photocurrent
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attains the peak value, which is in a stable dynamic equilibrium state. When the UV illumination is off, electron-hole recombination dominates, which is a rapid process, so the photocurrent
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decreases fast. With the oxygen adsorbed again, the surfaces of the film appear depletion regions and barriers again and between crystallites, leading to the decrease of the carrier density and mobility, the photocurrent of the film decreases.
4. Conclusions
ZnO:Zr films at different Zr doping concentrations were prepared by RF reactive magnetron sputtering. The effect of Zr contents on the crystalline structure, optical and electrical properties of the as-deposited ZnO:Zr films were studied. It was found that ZnO:Zr film for 0.51 at.% has a stronger preferred orientation toward the c-axis and a better crystallinity. The E2 (high) mode at about 437.9 cm−1 can be found in the Raman spectra of the films. The photoluminescence (PL) 8
ACCEPTED MANUSCRIPT spectra measured at room temperature revealed that the blue emission position shifts to higher wavelength region (482 nm to 485 nm) after Zr doping. At the Zr doping concentration for 0.51 at.%, the UV emission peak located at 402 nm can be observed from PL spectra, the ZnO:Zr film had the lowest resistivity for 31.8 Ω cm and excellent photosensitivity on UV detecting. So the
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ZnO:Zr films are potential candidate nanomaterials for UV photodetectors.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grant no. 10874140), Natural Science Foundation of Gansu Province (Grant no. 0710RJZA105).
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ACCEPTED MANUSCRIPT and the Oxygen Storage Capacity on the Structural Properties, J. Catal. 151 (1995) 168-177. [25] C. Wang, P. Zhang, J. Yue, Y. Zhang, L. Zheng, Effects of annealing and supersonic treatment on the structure and photoluminescence of ZnO films, Phys. B 403 (2008) 2235-2240. [26] T.C. Damen, S.P.S. Porto, B. Tell, Raman Effect in Zinc Oxide, Phys. Rev.142(1966)570-574.
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[27] J.B. Wang, H.M. Zhong, Z.F. Li, W. Lu, Raman study of N+-implanted ZnO, Appl. Phys. Lett. 88(2006)101913-3.
[28] A.F. Kohan, G. Ceder, D. Morgan, First-principles study of native point defects in ZnO, Phys. Rev. D 61(2000) 15019-15027.
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[29] H. Kim, J.S. Horwitz, G. Kushto, A. Pique, Z.H. Kafafi, C.M. Gilmore, et al., Effect of film thickness on the properties of indium tin oxide thin films, J. Appl. Phys. 88(2000)6021-6025.
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[30] X.T. Hao, J. Ma, D.H. Zhang, T.L. Yang, H.L. Ma, Y.G. Yang, et al., Thickness dependence of structural, optical and electrical properties of ZnO:Al films prepared on flexible substrates, Appl. Surf. Sci. 183(2001)137. [31] Soumen Dhara, P.K. Giri, Improved fast photoresponse from Al doped ZnO nanowires network decorated with Au nanoparticles, Chem. Phys. Lett. 541 (2012) 39-43.
[32] D. Wu, Z.F. Shi, T.T. Xu, Y.T. Tian, X.J. Li, Gate-controllable photoresponse of nitrogen-doped p-type
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[34] X.G. Zheng, Q.S. Li, W. Hu, D. Chen, N. Zhang, M.J. Shi, J.J. Wang, L.C. Zhang, Photoconductive properties of ZnO thin films grown by pulsed laser deposition, J. Lumin. 122-123 (2007) 198-201.
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[35] Q. H. Li, T. Gao, Y.G. Wang, T.H. Wang, Adsorption and desorption of oxygen probed from ZnO nanowire films by photocurrentmeasurements, Appl. Phys. Lett. 86(2005)123117-123119.
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1
XRD patterns of ZnO and ZnO:Zr films with different doping concentration (a) 0 at.%, (b) 0.38 at.%, (c) 0.51 at.% and (d) 0.78 at.%.
Fig. 2
SEM patterns of ZnO and ZnO:Zr films with different doping concentration (a) 0 at.%, (b) 0.38 at.%, (c) 0.51 at.%, (d) 0.78 at.% and (f) EDS spectra of ZnO:Zr film with different doping concentration. Raman spectra of ZnO:Zr films with different doping concentration (a) 0 at.%, (b) 0.38 at.%, (c) 0.51 at.% and (d) 0.78 at.%.
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Fig. 3
Fig. 4
PL spectra of ZnO and ZnO:Zr films with different doping concentration.
Fig. 5
Photocurrent behavior for ZnO:Zr film with 0.51 at.% at periodic UV illumination at 380nm.
Fig. 6
The schematic illustration of the UV photosensitivity mechanism.
(002) 200
200
150
150
100
0 300
50
(002)
0
250
c
d
200
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50
(100) (101)
0 26 28 30 32 34 36 38 40 42 44 46 48 50 26 28 30 32 34 36 38 40 42 44 46 48 50 300
200
100
(002)
100
250
150
b
250
50
Intensity(a.u.)
300
a
250
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Intensity(a.u.)
300
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Figure 1
(002)
150 100 50
26 28 30 32 34 36 38 40 42 44 46 48 50
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2θ(degree)
12
(100)
(101)
0 26 28 30 32 34 36 38 40 42 44 46 48 50
2θ(degree)
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Figure 2
AC C
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Figure 2
(a)
(b)
13
ACCEPTED MANUSCRIPT Figure 3 (d)
(c)
P
(a)
1200
-1 437.9cm
-1 303.6cm
-1 620.1cm
600 300
200
300
400
500
-1 303.6cm
600
700
P
(c)
0 800 100 1500
-1 437.9cm
300
400
500
P
600
300
300
300
400
-1 303.6cm
900
600
200
200
1200
900
0 100
-1 674.8cm
-1 437.9cm
500
600
700
-1 437.9cm
0 800 100
200
300
600
700
800
(d)
-1 620.1cm
400
500
600
700
800
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Raman shift (cm-1)
-1 620.1cm
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Raman intensity (a.u.)
1200
E2 (low) + A1 (LO)
900
-1 303.6cm
300 0 100 1500
(b)
1200
900 600
P
1500
E2 (high)
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1500
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Figure 4
402nm
485nm
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Intensity (a. u.)
EP
482nm
0 at.% 0.38 at.% 0.51 at.% 0.78 at.%
400
425
450
475
500
525
Wavelength (nm)
14
550
575
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Figure 5
74
UV OFF
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70 68 66 64
60 58 56
UV ON
0
5
10
15
20
25
30
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62
35
40
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photocurrent(uA)
72
Time(sec.)
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Figure 6
15
45
50
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Table 1 Parameters of the (002) diffraction peak of Zr doped ZnO films. FWMH(deg)
c (nm)
D (nm)
a (0 at.%)
34.48
0.50
0.5200
16.45
b (0.38 at.%)
34.44
0.49
0.5206
16.78
c (0.51 at.%)
34.42
0.46
0.5208
19.58
d (0.78 at.%)
34.46
0.48
0.5202
17.14
d (nm)
σ (GPa)
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2θ(deg)
0.2600
0.224
0.2603
-0.045
0.2604
-0.134
0.2601
0.134
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Samples
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Table 2 Comparison of UV photosensitivity based on various composites Maximum
Publication year
Reference
17
2013
[4]
5.3
2012
[31]
ZnO:Al NWs/Au
20
2012
[31]
CVD
ZnSe NRs
13
2016
[32]
RF
ZnO:Zr film
71.8
-
This work
Composition
RF
ZnO:Cu film
Vapor-liquid-solid
ZnO:Al NWs
Vapor-liquid-solid
photocurrent (µA)
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EP
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Synthesis route
16