Capacitive properties of zinc oxide thin films by radiofrequency magnetron sputtering
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The study focused on the production of zinc oxide (ZnO) thin films as a dielectric material for use in metal–insulator–semiconductor capacitors. The objective of this study has demonstrated the frequency dependence of conductivity and capacity. Zinc oxide (ZnO) was thin films deposited by a silicon wavelength sputtering\magnetron cathode sputtering. The capacitive properties of ZnO zinc oxide were studied at room temperature. The frequency dependence of the conductivity, capacitance and the measured current–voltage (I–V) characteristics of ZnO zinc oxide thin films were studied in the frequency range from 5 kHz to 13 MHz. It is shown that the conductivity is total. Indeed, the measurement of the conductivity in alternating regime obeys the Arrhenius equation. In addition, the measured I–V characteristics of the structures studied at 10 kHz and 10 MHz clearly revealed areas of accumulation, depletion and inversion in the plots. It has been observed that AC conductivity and capacity in the ZnO thin films (ZnO) are frequency dependent. This dependence indicates that the conduction is thermally activated and maintains the correlated barrier of the charge carrier on the localized states as a function of the experimental data. The FBAR (expand) devices with the ZnO films exhibited a pronounced resonance peak centered at 537 MHz with a k2 coupling coefficient of 7%. It found therefore that the impedance matching of the FBAR could be easily achieved simply by controlling the resonance the resonator.
KeywordsZinc oxide Thin films Sputtering Current–voltage study Capacity properties FBAR device
Currently, zinc oxide as a dielectric material is the subject of particular attention in the development of micro- and nanotechnologies. Indeed, the determination of its electrical properties in metal–insulator–semiconductor (MIS) capacitors as a dielectric material is much studied [1, 2]. Indeed, AC conductivity measurements provide information about the interior of the insulation or semiconductor which is a region of relatively low conductivity, even when the conduction process is limited to the electrode [3, 4]. The identification of the conduction mechanism depends on both frequency and temperature dependence of electrical conductivity, and Mahmood et al.  studied the electrical properties of ZnO thin films by RF sputtering. It has been found that the electrical conductivity has been thermally activated and dominated by the correlated barrier at low temperature .
Indeed, ZnO has the potential for many applications such as gas sensors. [7, 8, 9]. This material is also interesting because of its simple and economical photocatalytic activity. Wide-band-gap semiconductor, ZnO exhibits interesting optoelectronic properties and spintronic applications [10, 11] because of its direct band-gap and large exciton binding energy (60 meV at room temperature) that can be used for UV–blue lasers and diodes [10, 12]. UV and visible band emission provides useful information about the electronic properties of ZnO. The UV emissions are usually attributed to the free excitons and donor bound excitons, whereas deep level emissions usually arise from intrinsic defects in ZnO. The zinc oxide usually shows n-type conductivity because of intrinsic defects such as oxygen vacancies and zinc interstitials. Through these mechanisms, ZnO acts as an excellent antibacterial agent . ZnO materials have been synthesized through numerous methods, which include chemical precipitation, sol–gel, pulsed laser deposition technique, microwave radiation and hydrothermal methods . The doping of ZnO materials with different types of metallic ions [11, 15, 16] enhances its optical, magnetic and conducting properties. This modified ZnO is used as a base material in photocatalysts, dosimeters and biological systems . Furthermore, doping with rare earth elements provides many interesting ZnO properties, including lanthanum-doped ZnO with excellent gas sensitivity and photocatalytic activity . Functional dual TMs doping into ZnO nanostructures could influence the charge carrier recombination rate, optical, structural and interfacial electron-transport rate performance .
Also, the determination of the electrical properties of ZnO as a dielectric material in metal–insulator–semiconductor capacitors is more than necessary in the development of nanotechnology devices.
In the literature, the authors have shown that certain properties are determined by the orientation of the crystallites on the substrate. That is, the FWHM width of the ZnO peak along the orientation (002) of the X-ray inclination curve should be less than 0.58 to achieve an effective electromechanical coupling [18, 19]. Other studies have shown the frequency dependence of the electrical properties of ZnO sandwich structures . The σAC test based on the electrical properties of the ZnO thin films showed that in the frequency range of 10 kHz to 10 MHz, electrical conduction is performed by tunneling between the quantum wells. The resistivity of the dielectric material being high, the electrical conductivity of the material can only be achieved by transporting the blasting charge between the particles. In this work, the characteristics of metal–insulator–semiconductor (MIS) capacitors with zinc oxide (ZnO) as a dielectric material were studied. ZnO thin film was analyzed by X-ray diffraction, and the electromechanical coupling coefficient k2 was measured with network analyzer.
The RF magnetron sputtering was used to deposit the thin films of ZnO. Indeed, a target of zinc (99.99%) with a diameter of 51 mm and 6 mm of thickness allowed us to make these deposits. The silicon substrate used is of the p-type with orientation (100). The deposits were carefully cleaned with organic products. The magnetron sputtering is carried out at a gas/argon mixture atmosphere. A power at frequency of 13.56 MHz is supplied to the system. The RF power was about 50 W. The flow rates of argon and oxygen were monitored using a flowmeter (ASM, AF 2600). The spraying pressure was maintained at 3.35 × 10−3 Torr controlled by a Pirani gauge. Deposition rates covered the range of 0.35–0.53 μm/h. All films were annealed in an atmosphere of helium at 650 °C for 15 min. Indeed, we found that the titanium layer in a Pt/Ti/Si structure annealed at 650 °C for 15 min under an oxygen flow was completely oxidized, while under helium and under the same conditions, the structure exhibited unoxidized titanium layer.
We have previously deposited Ti/Pt as thin inking films on the silicon substrate to facilitate adhesion of the ZnO films. It has been found that a substrate temperature of 100 °C, target/substrate distance of about 70 mm, very low gas pressures of 3.35 × 10−3 Torr in a mixed argon gas atmosphere and for an oxygen content of 20%, giving to ZnO thin films a good homogeneity and high crystallinity. The thickness measurements of the samples were taken using a DEKTAK_ST high-resolution profilometer. We measured 2.8–3 μm ZnO thin films on samples. We confirmed these thicknesses by the determination of the optical properties. Indeed, the optical properties are a function of the thickness. To do this, we consider that the two envelopes of the measured optical transmittance form a nonlinear system consisting of continuous functions that can be solved by iteration. These continuous functions are the refractive index and the absorption percentage. After deposition of the zinc oxide, silver dot electrodes were evaporated on the Pt/ZnO/Ti/Pt/Si sample using an electron gun evaporation system to render the structure metal–insulator–semiconductor useful for electrical measurements. These were carried out in a vacuum chamber evacuated at about 10−3 Torr. Measurements of frequency and temperature dependencies of conductivity and capacity were taken using a laboratory configuration for the AC-based electrical test properties. The samples obtained were tested using an alternating current in the frequency range from 5 Hz to 13 MHz (using a capacitance bridge technique) using measurement temperatures of Tp = 30–300 °C, with a Hewlett-Packard LF 4192 A impedance analyzer between silver points and Pt electrode bottom. The temperature was measured with a Doric thermometer (Trendicator 400 K/ °C). To appreciate the nature of our piezoelectric ZnO layers, we determined the coupling coefficient of the complete structure based on frequencies for which the conductance is maximum and the minimum susceptance using a network analyzer (HP 8752 A Network Analyzer) whose pass-band ranges from 300 kHz to 1.3 GHz. This type of device is one of the instruments to define with precision the characteristics of an electric circuit (impedance measurement, electrical admittance, reflection coefficient, etc.).
The absolute value of this slope decreases as the temperature decreases. This slope also depends on the frequency. For a given temperature, this slope increases when ω decreases.
The comparative analysis of the dielectric permittivity and conductivity of the direct current indicates that the activation energy of the conductivity is higher than the activation energy of the permittivity. Indeed, we obtain ΔEε = 3.23 meV (Fig. 5) for the activation energy of the dielectric permittivity and ΔEσ = 91.4 meV (Fig. 6) for the activation energy of the low-frequency conductivity.
This means that one can calculate the coupling coefficient k2 by simply raising the maximum of the real parts of the impedance and admittance, i.e., raise the maximum values of the impedance and admittance at the resonant frequency when the imaginary parts are zero. Using the relation (16), we plotted the theoretical evolution of the real and imaginary parts of the electric admittance of the resonator free, thick 5.7 μm, depending on the frequency. Since the modeling was done for the propagation of longitudinal waves of volume, we find a good coupling coefficient k2 = 7.8%, k = 0.28 for a pure longitudinal mode .
It has been observed that AC conductivity in ZnO films follows a σ(ω) ∝ ωs dependence where s ≤ 1 such behavior appears to indicate that hopping is the predominant conduction process. At low temperature and at high frequencies, observed values of s approach those predicted by the literature that explains the behavior of AC conductivity of amorphous materials. The temperature and frequency dependences of the capacity and conductivity have been tested for a ZnO thin films prepared at a substrate temperature of 100 °C. Frequency dependencies of capacity and conductivity indicate an appreciable overlap of the polaron hopping distortion which could occur (OLPT), thereby reducing the value of the polaron hopping energy. The measured current–voltage characteristics of the Pt/ZnO/Ti/Pt/Si structures studied at 10 kHz and 10 MHz clearly revealed regions of accumulation, depletion and inversion in the plots. This offset showed that there was a fixed dielectric charge in the ZnO thin films with a density of the order of 1017 cm−2, hence a low tendency to increase with increasing temperature. The FBAR devices with the ZnO films exhibited a pronounced resonance peak centered at 537 MHz with a k2 coupling coefficient of 7%.
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