Optical nonlinearity of ZnO microcrystallite enhanced by interfacial state Y. B. Han1, J. B. Han1, S. Ding1, D. J. Chen1, Q. Q. Wang1, 2 1 Department of Physics, Wuhan University, Wuhan 430072, People’s Republic of China 2 Center of Nanoscience & Nanotechnology Research, Wuhan University, Wuhan 430072, People’s Republic of China
[email protected]
Abstract: A series of ZnO microcrystallite films deposited on quartz substrates were annealed at the temperature of 600~1050 ºC. A well c-axis grown wurtzite ZnO film was obtained at the annealing temperature of 850 ºC. For the samples annealed above this temperature, the empirical parameter E0 increased calculated from transmittance spectra, which indicated the changes of the interface of ZnO microcrystallite. Measured by Z-scans, the nonlinear absorption coefficient βeff increased from 1.2×102 cm/GW to 1.1×103 cm/GW when the annealing temperature rose from 950 ºC to 1050 ºC, mainly due to the interfacial state enhancement. © 2005 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (190.3970) Microparticle nonlinear optics; (240.4350) Nonlinear optics at surfaces.
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1. Introduction ZnO as a compound semi-conductor has draw considerable attention for its excellent piezoelectric and optical properties. It can be made for integrated acousto-optic devices and ultraviolet photonic devices [1, 2]. ZnO films grown on silicon [3, 4], sapphire [4-7], LiNbO3 [8], GaAs [9] and quartz [10-12] substrates have been studied and the electrics [13], photoluminescence [3, 14], optical absorption [4, 7, 10, 12] and optical nonlinearities [6, 11, 15-17] have been investigated. Laser ablation [4], laser deposition [11], electron beam evaporation [7], molecular-beam epitaxy [6] and sputtering technique [3, 5, 9, 10, 12] have been applied to prepare ZnO films. For sputtering technique, the substrate temperature [18], sputtering power [5, 19], oxygen partial pressure [5, 20] and post-treatments [10, 12, 21] may significantly influence the structure and optical properties of the films, however, the interfacial state effect of ZnO microcrystallite was seldom reported, since the electrical and the optical properties are strongly affected by the interface and the microstructure of the films. In this letter, we investigated the influences of the microcrystallite structure and interfacial state effect on the linear and nonlinear optical properties of the ZnO films on quartz substrates annealed at the temperature of 600~1050 ºC. 2. Experimental The ZnO microcrystallite films were deposited at room temperature by using radio frequency (r.f.) reactive sputtering technique with a zinc target (purity: 99.99%, radius: 50mm), the target-substrate distance was about 60 mm. The sputtering gas was the mixture of Ar and O2 with the partial pressure ratio of 5:1 and the sputtering pressure was about 3.5×10-2 Torr. The sputtering power was 150W. After deposition, the series of samples were annealed at the #8829 - $15.00 USD
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temperatures of 600 ºC, 750 ºC, 850 ºC, 950 ºC and 1050 ºC, respectively. The samples were held at each temperature for an hour in air atmosphere and then cooled to room temperature slowly. The structure and the microcrystallite size of the films were investigated by X-ray diffraction (XRD), the transmittance spectra measurements were carried out using a UV-VISNIR spectrophotometer (Varian, Cary 5000), and the nonlinear absorption and refraction were investigated by using Z-scan technique at the wavelength of 790 nm, in which a femtosecond (fs) Ti:Sapphire laser (Mira 900, Coherent) with pulse duration of 150 fs and repetition rate of 76 MHz was employed. 3. Results and discussion Figure 1 is the XRD patterns of the films annealed at the temperatures of A: 600 ºC, B: 750 ºC, C: 850 ºC, D: 950 ºC and E: 1050 ºC, respectively. The highest peaks corresponding to (002) plane of wurtzite ZnO locate at the diffraction angle (2θ) of 34.37º~ 34.76º in each curve. For samples A, B, D and E, peaks of (100), (002), (101), (102), (103) and (110) planes of wurtzite ZnO are found, but for sample C, only peak of (002) plane at 34.46º is found, which indicates that the highest c-axis oriented wurtzite ZnO films is obtained. For samples D and E, peaks of (107) and (228) planes of tridymite are found, this can be explained that ZnO films turned to be thinner at some areas and silicon oxide turned from normal amorphous body to tridymite above the annealing temperature of 870 ºC [22]. It also can be seen that the (002) peak of wurtzite ZnO does not rise but fall with the rise of annealed temperature above 850 ºC, which indicates a structure change of the films, this will be discussed in the following text. The ZnO microcrystallite size increases from about 20nm to 50nm with the rise of the annealing temperature from 600 ºC to 1050 ºC calculated by using the Debye–Scherrer formula.
Z n O (0 0 2 )
1000 S iO
2
(01 7 ) S iO
600
(2 2 8 )
E
400
D 1050
C B A 30
35 2 θ ( o)
40
45
750 600
)
0
900 (o C
200 25
2
T
Intensity (a.u.)
800
50
Fig. 1. XRD patterns of the samples annealed at different temperatures: A: 600ºC, B: 750ºC, C: 850ºC, D: 950ºC and E: 1050ºC.
Figure 2(a) is the optical transmittance spectra of the as-deposited and annealed films. The samples annealed below 850ºC exhibit a high transmittance (>75%) in visible region and show a sharp fundamental absorption edge at the wavelength about 380nm, but for samples annealed at 950 ºC and 1050 ºC, the transmittance gradually falls with the decreasing of the wavelength in the visible region and show relatively smaller gradient absorption edges. The linear absorption coefficient α0 near the band edge in the energy region of hν
