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Abstract
We report on the influence of temperature on the polarization behavior of highly oriented ZnO thin film. First, the investigation of crystal structure change is studied, providing supporting information on the macroscopic-scale polarization of the ZnO thin film. Here, the lattice distortion is investigated by using X-ray diffraction. Furthermore, the role of temperature on the polarization behavior of the ZnO thin film is comprehensively studied by using temperature dependent spectroscopic ellipsometry. Here, the temperature dependent dielectric function analysis and electronic excitation models are used to understand the mechanism of polarization. We found an interesting temperature dependence of electronic transition, where the red-shift absorption and exciton-phonon interaction are observed on the system. This interaction is responsible for the increase of polarization response, which is confirmed by dielectric susceptibility spectra. These results provide important understanding for the control of the polarization dependence on the working temperature of ZnO thin film, which is the essential key in the fabrication of switchable optical devices.
© 2017 Optical Society of America
1. Introduction
Wurtzite ZnO (w-ZnO) is an II-VI semiconductor which has a high stability at room temperature compared with rocksalt and zinc blende structures [1]. The w-ZnO has a hexagonal unit cell (P63mc no. 186) [2] with each Zn atom is surrounded by tetrahedra of O atoms, and vice versa, which can be tuned to modify the electrical properties [3, 4]. The w-ZnO exhibits high polarization due to its non-centrosymmetric (6mm) structure. Materials with 6mm point group show both polarization response and pyroelectricity under particular temperature. Here, polarization response is observed from its termination direction; polar termination (c-axis oriented) and non-polar termination.
The w-ZnO has a wide band gap (~3.3eV) and a high exciton binding energy (60meV) [5], which can be tuned through various processes, such as controlling the temperature [6] and doping with other atoms [7–10]. For practical application, the exciton engineering plays an important role in the ZnO-based optoelectronic devices, such as exciton UV lasers [11, 12], tunable UV photodetectors [13, 14], and LEDs [15]. The presence of exciton provides information on electron-hole polarization in the system, in which the electrons are excited from the valence band to the conduction band and it forms a Coulomb interaction with the hole. A new type of fast and sensitive self-powered ZnO based photodetector was successfully developed in metal-insulator-semiconductor (Pt/Al2O3/ZnO) by controlling the photo-generated electron-hole pair separation. Here, the high responsivity of 0.644 μA/W and recovery time less than 100 ms were obtained by introducing strain-induced polarization in the system [16]. A fast response and high transport efficiency were also reported in ITO/CsPbBr3:ZnO/Ag photodetector. The decoration of ZnO in CsPbBr3 contributes to the structural improvement and transfer enhancement of photo-generated electron-hole pairs by acting acts as the transition layer, which is effectively alleviating the large energy barrier between the CsPbBr3 and electrodes [17]. Other study revealed that high electron-hole polarization is a key factor for developing quantum memory storage [18]. Moreover, the ZnO play the important role in generating high photoemission in LEDs applications. The combination between microhole array and roughened ZnO structure can lead an increase in the light extraction efficiency by allowing more photons emission from the active region of the LEDs with maximum output power improvement of 58.4% [19].
In this work, we study the role of temperature on polarization behavior of highly oriented ZnO thin film by optical characterization using spectroscopic ellipsometry. To obtain good understanding in polarization mechanism, we investigated temperature dependence of excitonic interaction and electronic transfer by using dielectric function analysis and electronic excitation modeling. We reveal that the formation of temperature induced exciton-phonon interaction significantly changed polarization response of the ZnO thin film.
2. Experimental details
A single orientation of the ZnO thin film (~450nm) on the quartz substrate has been prepared by pulsed laser deposition (PLD). X-ray diffraction (XRD) measurement was used to investigate lattice distortion in the system. Furthermore, polarization behavior on the system was observed by using temperature dependent spectroscopic ellipsometry (SE) from J. A. Woollam Co., light source: Halogen-lamp (infrared), Xenon-lamp (UV-VIS), and Deuterium-lamp (deep UV), energy range [eV]: 0.5–6.5. The measurements were taken using the incident angle of 70° and polarization angle of 45° with temperatures variation of 77K, 300K, and 350K. Moreover, incident-angle dependent optical properties of the ZnO thin film have been analyzed as reported elsewhere [7, 20, 21].
3. Results and discussion
Figure 1 shows diffraction pattern of the ZnO thin film. Single orientation (002) of wurtzite ZnO was found on diffraction angle 2θ = 34.36°. By applying Le Bail method, we obtain the lattice constant of a = 3.2464Å and c = 5.2102Å. Here, the structural distortion is observed via lattice strain (ε) calculation [22]. We obtain the lattice strain of 0.003305 in the c-axis direction. The high orientation crystal with the existence of structural distortion will promote the system to have spontaneous polarization when the external field is applied. The study of structural modification and polarization response has been presented in detail elsewhere [3, 4].
In this work, the polarization of the ZnO thin film is studied by using temperature dependent dielectric function analysis obtained from SE characterization. Figure 2(a) shows the dielectric function spectra of a ZnO thin film obtained by fitting the SE data ( and ) using Drude-Lorentz model and Kramers-Kronig relation [23]. The real part and imaginary parts of the dielectric function are attributed to polarization and absorption on the system, respectively. The result shows that both of the and intensity increase accompanied by a red shift as the higher temperature. The red-shift is presumably due to the increase of dipole moment upon photoexcitation [24]. The increase of dipole moment is attributed to the increase of susceptibility (polarization) of the system. The high polarization can be generated by the system with weak electron-hole interaction. These conditions are supported by the fact that our system has high preferred in (002) orientation, which is polar in this direction. In order to obtain polarization response of the ZnO thin film, the frequency dependent susceptibility is plotted. Here, the susceptibility is obtained by using dielectric function equation below.
with n and k are the real part and imaginary part of the refractive index.
Fig. 2 Real part and imaginary parts of the dielectric function (a) and temperature- and frequency-dependent susceptibility of the ZnO thin film (b). The inset shows the difference polarization response induced by the presence of defects in the ZnO thin film, as presented in PL characterization.
Figure 2(b) shows the susceptibility as a function of photon energy at different temperatures. Relatively constant values of the susceptibility are observed in the energy range of 0.3eV - 1eV, where the highest value of dielectric susceptibility is shown at T = 350K. This result indicates that ZnO exhibits the paraelectric characteristic. The different response is observed in the energy range of 1eV - 3eV, as presented in inset Fig. 2(b). Here, susceptibility no longer represents a constant value and tends to have an exponential growth trend. This exponential curve indicates the presence of ferroelectric characteristic on ZnO. Paraelectric response is ascribed to the high orientation non-centrosymmetric structure of ZnO thin film. The previous study revealed that the ferroelectric response in undoped ZnO was associated with the existence of the defects [25]. To understand the origin of the different response in ZnO thin film, the photoluminescence measurement is performed. The result confirms that the ferroelectric response in ZnO thin film is related to the presence of oxygen vacancy (VO) defect. In wurtzite structure, the presence of the VO promotes the unbound Zn due to the missing oxygen. Under the electric field, the positively charged Zn move to the corresponding electric field direction. The susceptibility of the ZnO thin film increase as the higher temperature with the respective value of 7.45 (77K), 7.69 (300K), and 8.04 (350K). The reasonable explanation for this result is the increase in the electronic transfer. Under applied photon, the electrons excited from valence band to excitation state and further make electron-hole interaction. The higher temperature promotes the additional energy to the electrons and further increase number of excited electrons from valence band to excited band.
To understand the electronic transfer of the ZnO thin film, temperature dependent electronic excitation modeling is performed. Here, the critical point of electronic transfer is determined by using the second derivative of the real part of the dielectric function. Figure 3(a) shows the critical point of the temperature dependent dielectric function. Here, excitonic Eexc and valence to conduction band EVB-CB critical points are observed. Varshni equation is used to obtain the relation between temperature and exciton [26].
with E(T) is the temperature dependent critical point, E(0) is the critical point at a temperature of 0K, T is the temperature, A and B are the constants. As presented in Fig. 3(b), the Varshni equation does not fit with the data, which means that the model does not effectively represent the relation between temperature and energy gap of our system. Furthermore, another model is used to fitting the data. We use Bose-Einstein equation, which the involvement of phonon is considered in the system [27].with Eb, aB, θ and Τ indicate the exciton binding energy, exciton-phonon binding energy, mean frequency of the phonons involved and temperature, respectively. Here, Bose-Einstein equation yields the best agreement with data. This result reveals that the exciton-phonon interaction is observed in our system. As mentioned above, the red-shift is attributed to the increase in temperature. Based on this results, the red-shift profile is ascribed to the formation of the exciton-phonon interaction. Here, increasing of temperature promotes the additional kinetic energy to the system, which increases the electronic transfer and promotes lattice vibration.Fig. 3 Second derivative spectra of the real part of the dielectric function, with Eexc and EVB-CB represent the exciton critical point and the valence to the conduction band critical point (a) and exciton state of the ZnO thin film as a function of temperature (b).
Further analysis is presented to support the explanation of the electron excitation. Figures 4(a) and 4(b) show the absorption and spectral weight integral which provides important information of an effective number of electrons excited by photons from the valence band to conduction band [28]. The results show that the absorption is observed in band gap region, which is indicated by green shaded area. Here, two regions are investigated by temperature dependent partial spectral weight integral. Region I indicates band gap area and region II indicates valence band to conduction band transition. A mid-gap state is observed in band gap area, which is generally no electronic transfer due to insufficient energy to excite electrons to the excitation state. The mid-gap state is presumably due to the presence of VO, as mentioned above. The existence of VO acts as double donor [29], which provides an additional electron to the system. Furthermore, the increasing electronic transfer is observed in region II. The result shows that the electronic transfer increases as the higher temperature increase. We confirmed that values of the partial spectral weight integral are 3.68, 3.78, and 3.84 for the temperature of 77K, 300K, and 350K, respectively. The increasing of electronic transfer will promote the increase of total electron-hole polarization of the system. Aside from photons, electrons also receive kinetic energy from the heating process. Intriguingly, the applied temperature has the double function. The increasing of temperature not only provides the additional kinetic energy for electronic transfer but also contributes to lattice vibrations, which play a role in the enhancement of polarization response in the ZnO thin film.
4. Conclusion
Polarization behavior of the ZnO thin film has been studied by temperature dependent spectroscopic ellipsometry. The ZnO thin film shows high crystal orientation in c-axis direction with the relaxation structural distortion of 0.003305 was observed. The analysis of the temperature dependence of dielectric function revealed that the ZnO thin film showed the increase in electronic transfer due to the increase of kinetic energy, which plays the role in generating polarization responses. The ferroelectric characteristic was obtained due to the presence of the VO in the system. In addition, thermal treatment given in the system provides the additional kinetic energy and contributes to lattice vibrations, which further promotes the increase of polarization in the system.
Funding
PMDSU 2013 research program (794j/I1.C01/PL/2016); PUPT 2017 (009/SP2H/LT/DRPM/IV/2017:LPPM.PN-7-57-2017); Desentralisasi 2016 (585g/I1.C01/PL/2016) research program from Ministry of Research, Technology and Higher Education of the Republic of Indonesia.
Acknowledgments
The Authors wish to thank Dr. Daniel Smith for SE measurement.
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