Structure and Optical Properties of ZnO Thin Films Prepared by the Czochralski Method

MA Zhanhong, REN Fengzhang, YANG Zhouya

(1. School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471023, China; 2. Provincial and Ministerial Co-construction of Collaborative Innovation Center for Non-ferrous Metal New Materials and Advanced Processing Technology,Luoyang 471023, China)

Abstract: The zinc oxide seed film was coated on conductive glass (FTO) substrate by the Czochralski method, Zinc acetate and hexamethylenetetramine were used as raw materials to prepare growth solution, and then ZnO film was prepared by a low-temperature solution method. The effects of annealing temperature on the morphology, structure, stress and optical properties of ZnO films were studied. The thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible absorption spectra (UV - vis),photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). The results show that the films are ZnO nanorods. With the increase of annealing temperature, the diameter of the rod increases, and the nanorods tend to be oriented. The band gap of the sample obtained from the light absorption spectra first increases and then decreases with the increase of annealing temperature. When the annealing temperature is 350 ℃, the crystallinity of zinc oxide film is the highest, the band gap is close to the theoretical value of pure ZnO.

Key words: Czochralski method; ZnO film; annealing temperature; optical properties; micromorphology; internal stress

1 Introduction

Direct band gap semiconductor ZnO is semiconductor material with wide band gap (3.37 eV) and large exciton binding energy(60 meV)[1-3]. The energy band position of ZnO is close to that of TiO2, and it can be widely used in solar cells, photocatalysis, photoelectric devices and other fields[4-9]. There are many methods for preparing ZnO thin films, such as chemical vapor deposition (CVD)[10], molecular beam epitaxy (MBA)[11],pulsed laser deposition (PLD)[12], sol-gel method[13,14],hydrothermal method[15],etc.Compared with other methods, Czochralski method has the advantages of simple process and low cost, and has a crystalline layer without a conventional spin coating method, which is less than the temperature of the hydrothermal, and is therefore a new type of preparation ZnO method. In this paper, ZnO thin films were prepared by Czochralski method, and the effects of annealing temperature on the micro-morphology, internals stress and optical properties of ZnO films were studied.

2 Experimental

The raw materials used in the experiment are hexamethylenetetramine (HMT), zinc acetate[(CH3COO)2·2H2O], anhydrous ethanol (CH3CH2OH),acetone (CH3COOH), deionized water and detergent.

The FTO (15 mm×15 mm) was cleaned in deionized water, acetone, and then put the FTO in the air,and the FTO was performed perpendicularly in 0.025 mol / L of zinc acetate seed liquid for 5-10 s, and then pull out with the pull film machine (3 cm/s). After drying at room temperature, it was annealed in the furnace (250-400 ℃) for 2 h to give the ZnO seed layer.The 0.025 mol/L growth solution was formulated with zinc acetate and six methyltrialine as raw materials.The FTO of ZnO seed layer is vertically placed in the growth solution, heated in constant temperature water bath at 80 ℃ for 4.5 hours, then taken out, washed with deionized water, and dried to obtain ZnO thin film.

X- ray diffractometer D8 Advance (XRD, Cu target,Kα, tube voltage 40 kV, tube current 40 mA, λ =0.154 18 nm, step size 0.02, 2θangle range 20-80) was used to detect the phase composition of samples,The microstructure and elements of the samples were observed by JSM-5610LV scanning electron microscope(SEM, 20kV). The photoluminescence (PL) properties of the samples were tested by F-280 fluorescence spectrophotometer. UV-2600 UV-vis spectrometer was used to test the light absorption properties of the films. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of doped ZnO.

3 Results and analysis

3.1 Phase structure

Fig.1 shows the XRD patterns of ZnO film prepared by Czochralski method. the SnO2peak of XRD comes from FTO, and other diffraction peaks show hexagonal wurtzite ZnO. It can be seen from Fig.1 that when the annealing temperature is 250 ℃, (103)characteristic peak is missing, which is due to the low annealing temperature, zinc acetate is not completely decomposed, and the crystal quality of ZnO seed layer is poor, thus affecting the film. When the annealing temperature is 300 ℃, the diffraction peak intensity of (002) plane of ZnO thin film is higher than (101),which indicates that the film shows obviousc-axis orientation. When the annealing temperature is 350℃, the diffraction peak intensity of (002) of ZnO thin film is the highest, which indicates that zinc acetate has been completely decomposed into ZnO, and the quality of high-quality ZnO seed layer makes the ZnO thin film better. When the annealing temperature is 400 ℃,the diffraction peak intensity of the (002) plane of ZnO thin films is lower than that of the (101), which shows that with the increase of annealing temperature, the driving force of atomic diffusion will be too large, and the grain growth will be different, which will affect the film quality and deteriorate the orientation[16,17].

It can be seen from Fig.1 that when the annealing temperature changes, the angle of each diffraction peak has a slight shift, and there should be stress, so we have an in-depth analysis of the XRD. The stress in parallelc-axis is calculated according to Eq. (1)[16]:

In whichc11,c12, c13and c33arec-axis elastic constants of single crystal ZnO, and their values arec11=208.8 GPa,c12=119.7 GPa,c13=104.2 GPa andc33=213.8 GPa respectively.cis the lattice constant in the presence of stress.c0is the lattice constant without strain, and its value is 0.520 5 nm.

The lattice constant of doped ZnO can be obtained from XRD data by deducting background andKα, phase retrieval, smoothing and full spectrum fitting,and cell refinement calculation. The 2θis about 34.5,which corresponds to the (002) plane. The cell parametersaandccan be calculated according to Eqs.(2)[18]and (3)[19], and the Zn-O bond lengthLcan be calculated according to Eqs.(4)[20]and (5)[20]. The interplanar spacingdcan be obtained by Eq.(6).

where,aandcare lattice constants, λ is the wavelength andθis the diffraction angle.dis (002) planes spacing,andλis the excitation wavelength of X-ray.The results obtained by the XRD are shown in Table 1, Fig.2 and Fig.3.

Table 1 Lattice constant of ZnO film

It can be seen from Table 1 that with the change of annealing temperature, the diffraction angle of (002)shifts slightly, which indicates that annealing temperature affects the structure of ZnO. With the increase of annealing temperature, the width at half maximum of(002) plane first increases and then decreases, and the intensity of diffraction peak first increases and then decreases, which indicates that with the increase of annealing temperature, the orientation of the films increases first and then decreases. Fig.2 is the variation curve of cell parametersaandcof ZnO thin films. It can be seen from Fig.2 that with the increase of annealing temperature, the cell parametersafirst decrease and then increase, whilecfirst increases and then decreases. It can be seen from Fig.2 that when annealing temperature is 350 ℃, the cell volume is the smallest.

Fig.2 Cell parameters of ZnO thin films

Fig.3 Stress of ZnO thin film

It can be seen from Fig.3 that the stress of ZnO thin films after annealing at four temperatures is negative, indicating that ZnO thin films are subjected to tensile action on the (002) plane, showing tensile stress.With the increase of annealing temperature, the tensile stress first decreases, then increases and then decreases,and the stress values at each annealing temperatures is very close. When the annealing temperature is low,there are point defects in ZnO thin films, which make lattice mismatch and produce stress. With the increase of annealing temperature, the particle diffusion ability is enhanced, and the defects and stress are reduced.When the annealing temperature is 350 ℃, the crystal grows and stretches in the direction of (002) plane,showing greater tensile stress. When the annealing temperature is 400 ℃, the particle migration ability is further enhanced, and the transverse dimension of the(002) plane increases and the longitudinal dimension decreases, so the tensile stress decreases[14,21]. The change of stress is consistent with the change of lattice constant.

Fig.4 XPS spectra of ZnO: (a) Survey spectra; (b) Zn 2p; (c)O 1s

The element of surface of ZnO film annealing at 350 ℃ was studied by X-ray photoelectron spectroscopy(XPS). It can be seen from Fig.4(a) that the peak of 1 044.18 eV corresponds to Zn 2p1/2, and the binding energy corresponding to the characteristic peak of Zn 2p3/2 is 1 020.98 eV, which corresponds to the characteristic peak of Zn2+in ZnO, so Zn exists in the form of+2 valence in the prepared ZnO thin film[22]. Fig.4(c)shows the XPS spectra of O-1s. It can be seen from Fig.4(c) that the spectral line of O-1sis asymmetric,indicating that there are point defects related to O in the sample, which are fitted into two peaks by Gaussian fitting, which are located at 529.88 and 531.18 eV respectively. The peak of 529.88 eV corresponds to O in ZnO lattice, and the peak of 531.88 eV corresponds to adsorption of O or O vacancy[23].

Fig.5 SEM of ZnO: (a) 250 ℃; (b) 300 ℃; (c) 350 ℃; (d) 400 ℃

SEM morphologies of ZnO thin films are shown in Fig.5. It can be seen from Fig.5 that all ZnO thin films prepared by Czochralski method are rod-shaped,because zinc acetate is dissolved in absolute ethanol in the experiment, and a uniform liquid film is formed on FTO glass by Czochralski method; With the volatilization of absolute ethyl alcohol, zinc acetate molecules without solvent protection will shrink correspondingly, adhere to the substrate surface under the action of evaporation driving force, and slowly diffuse on the substrate in the subsequent drying process. Due to the limitation of the substrate surface, zinc acetate forms dendritic distribution in the crystallization process.When decomposed at high temperature, the corresponding dendrite ZnO is formed on the substrate, and its reaction is shown as Eq.(7):

2Zn(CH3COO)2= ZnO + CO2+CH3COCH3(7)

When FTO covered with ZnO seed layer is put into the growth solution made of zinc acetate and hexamethylenetetramine, epitaxial growth is easy to occur due to the induction of seed crystal. No matter whether the surface of ZnO seed layer is positively or negatively charged, it can adsorb Zn2+or OH-to form columnar ZnO nanorods.

Fig.5(a) is the SEM image of zinc oxide nanorod array prepared at an annealing temperature of 250 ℃.From the cross-sectional view, it can be seen that rodlike substances are formed on FTO substrate, but it can be seen that the growth density of nanorods is sparse and the orientation ofcaxis is poor, and the diameter of zinc oxide rods is about 90 nm. When the annealing temperature is 300 ℃, the density of the arrangement of the rods increases, and the size of the rods does not change much. When the annealing temperature is 350℃, the arrangement of rods is dense and the orientation is the best, at which time the diameter of rods is 200 nm, which is almost twice as low as the annealing temperature. When the annealing temperature is 400℃, the arrangement of rods is similar to 350℃, but the diameter of rods is larger, about 300 nm. Combined with the stress change curve, it can be seen that the defect content is less at this stage, and the tensile stress parallel to thecaxis is smaller, which will prevent the increase of rod length, but is conducive to the increase of the transverse size of the bars, so the diameter of the bars changes greatly at this stage.

3.2 Optical characteristic analysis

Fig.6 is photoluminescence spectra of ZnO films annealing at different temperatures. It can be seen from Fig.6 that there are two emission peaks in the four curves, one is between 400-420 nm, which is not prominent and belongs to the intrinsic peak of ZnO. It can be seen that the peak is the highest when annealing temperature is 350 ℃, indicating that the crystallization degree of ZnO is the highest at this temperature. The other peak is near 480 nm, which is caused by point defects. With the increase of annealing temperature, the wavelength corresponding to the peak shifts slightly,which indicates that the structure caused by defects changes with the increase of annealing temperature. In addition, no luminescence peaks related to Znior VOwere found, which indicated that its deep level emission (green band in ZnO) was suppressed[24].

Fig.6 PL of ZnO films

Fig.7 is a light absorption curve of ZnO thin film.It can be seen from Fig.7 that with the increase of annealing temperature, the light absorption of ZnO nano-films in ultraviolet region increases at first and then decreases. The edge of the corresponding absorption band first moves to the short wave direction and then to the long wave direction, resulting in the phenomenon of blue shift first and then red shift. Pure ZnO without defects has a wide band gap and low utilization rate of visible light. When there are defects in ZnO, the impurity energy level will narrow the band gap, which is beneficial to absorb more visible light. It can be seen that the ZnO thin films prepared in this paper may have defects, and the concentration of defects changes with the change of annealing temperature, which affects the absorption of visible light, resulting in the phenomenon that the band edge shifts first to blue and then to red.The polycrystalline thin film prepared in this paper has grain boundaries, and the atomic structure at the grain boundaries is different from that inside the grains,which leads to the existence of charges and potential barriers at the grain boundaries, the formation of electric fields, the change of band gap width and the corresponding movement of absorption band edges[25].

The relation between band gapEg(eV) and wavelengthλ(nm) of zinc oxide is shown in the Eq.(8)[26]:

wherehis Planck constant,νis photon frequency, andcis the speed of light in vacuum.

Fig.7 UV-Vis spectra of ZnO film

Fig.8 Band gap of ZnO film

The cross-axis intercept corresponding to tangent line is λ in Fig.7, and the relationship between the calculated band gap and annealing temperature is shown in Fig.8 .

It can be seen from Fig.8 that with the increase of temperature, the band gap first increases and then decreases. When the annealing temperature is 350 ℃,the maximum band gap at this time is 3.26 eV, which is very close to 3.27 eV in literature[27,28]. This shows that with the increase of annealing temperature, zinc oxide crystals gradually become complete, and point defects are gradually eliminated with the increase of temperature, resulting in less influence of impurity energy levels formed by defects. Therefore, the band gap gradually approaches the band gap of zinc oxide. When the annealing temperature reaches 400 ℃, the temperature is higher, the diffusion speed is accelerated, and new defects are formed again, forming impurity energy levels and reducing the band gap. This is consistent with the previous XRD analysis results.

4 Conclusions

The ZnO thin films with rod structure were successfully prepared by Czochralski method. With the increase of annealing temperature, the rods gradually tend to be oriented, and the stress in the films after annealing is tensile stress. With the increase of annealing temperature, the cell parameterafirst decreases and then increases,cdecreases then increases, while the stress first decreases and then increases and then decreases. The optimum annealing temperature is about 350 ℃, the ZnO rod is arranged, and the edge of absorption band moves first to short wave direction and then to long wave direction, resulting in blue shift first and then red shift. The band gap first increases and then decreases with the increase of annealing temperature.When the annealing temperature is 350 ℃, the maximum band gap is 3.26 eV.