**4.3 Investigation of conduction mechanism by ac complex impedance spectroscopy for PLD binary oxides ZnO/MoO3 thin films**

The complex impedance (Z" vs. Z') plots of pulsed laser deposited binary oxides ZnO/MoO3 (ZMO) thin films are displayed in **Figure 8**. The response of a measurement in a complex impedance plot enables us to separate two contributions which appear in the form of semicircles arcs. We know that, for a semiconducting material having interfacial boundary layers (grain-boundary) and the figure exhibits

**53**

**Figure 8.**

**Figure 7.**

*various deposition conditions.*

*Binary Metal Oxides Thin Films Prepared from Pulsed Laser Deposition*

*XRD spectra of pulsed laser deposited Orthorhombic MoO3 and monoclinic MoO2 thin films prepared at* 

*The complex impedance (Z" vs. Z') plots of ZMO thin films (a) 298 K (b) 623 K (c) 773 K and (d) 923 K.*

*DOI: http://dx.doi.org/10.5772/intechopen.96161*

*Binary Metal Oxides Thin Films Prepared from Pulsed Laser Deposition DOI: http://dx.doi.org/10.5772/intechopen.96161*

**Figure 7.**

*Practical Applications of Laser Ablation*

450 °C under the argon gas atmosphere, near the unfilled region, neatly arranged monoclinic molybdenum dioxide (MoO2) structures. Finally, the sample deposited at 450 °C using the argon gas atmosphere, which shows uniformly arranged mono-

*Field Emission Scanning Electron Microscopy (FESEM) images of molybdenum oxides thin films deposited by* 

**Figure 7** shows the XRD spectra of the pulsed laser deposited molybdenum oxide thin films at the various conditions. The sample deposited at the room temperature was the as-deposited thin film which exhibits the amorphous nature. Then the sample was deposited at a substrate temperature of 450 °C and 600 °C, which exhibit orthorhombic structure with 020 and 040 peaks and are confirmed with the ICSD 80577. Then the deposition duration was increased from 2 minutes to 20 minutes, with the O2 atmosphere, which results in orthorhombic structures and hexagonal structures and the peaks are found at 100 and 211 respectively. Then the next sample when deposited with same temperature and duration under the Ar atmosphere, the XRD spectra shows the monoclinic structures and confirms as in **Figure 4** the presence of MoO2 with 110, 020 and 220 peaks with the ICSD 23722 and JCPDS 65–5787. When the sample deposition temperature increased from

clinic MoO2 structures is shown in **Figure 6d** [10].

*PLD. (a) and (b) MoO3 at 450* °C *(c) and (d) MoO2 at 450* °C*.*

450 °C to 600 °C, the structure crystallization takes place [11].

**4.3 Investigation of conduction mechanism by ac complex impedance spectroscopy for PLD binary oxides ZnO/MoO3 thin films**

The complex impedance (Z" vs. Z') plots of pulsed laser deposited binary oxides ZnO/MoO3 (ZMO) thin films are displayed in **Figure 8**. The response of a measurement in a complex impedance plot enables us to separate two contributions which appear in the form of semicircles arcs. We know that, for a semiconducting material having interfacial boundary layers (grain-boundary) and the figure exhibits

**52**

**Figure 6.**

*XRD spectra of pulsed laser deposited Orthorhombic MoO3 and monoclinic MoO2 thin films prepared at various deposition conditions.*

#### *Practical Applications of Laser Ablation*

semicircles that are deformed and depressed with their centres below the real axis as the temperature range investigated, complex curve consists only one depressed semi-circle and its centre lies below the real axis. Furthermore, depressed arc is typical for a dipolar system involving distribution of relaxation time. Moreover, it is noted that the diameter of semi-circles decreases with increase in temperature which also refers to the decrease in the resistivity. The equivalent circuit for this sample is a series Resistor-Capacitor (RC) circuit. Whereas the semi-circled impedance samples possess the characteristics which are attributed to the semiconductor behaviour, in which the electrical conduction process is thermally activated.

The equivalent circuit for these depressed semi-circles of the ZMO thin films may be described by a parallel connection of an ohmic resistor R and a capacitor C, also known as Randles circuit. Here the capacitor is replaced with a constant phase element (CPE) associated with both the resistors and capacitors [12]. Indeed, in the present study, the complex plane plot can be described by the Nyquist plot, which is given by

$$Z = \frac{R}{\left[1 + \left(j o \sigma \right)^{a}\right]} \tag{2}$$

where ω is angular frequency, τ = RC is the relaxation time and α is a parameter that characterizes the distribution of relaxation times with values ranging from 0 to 1. When α is zero, the relaxation is said to be Debye relaxation and when α is greater than zero, the relaxation times are said to have distribution.

**Figure 9.**

*Frequency dependence of the real part (Z') of the complex impedance. (a) As deposited ZMO, (b) ZMO deposited at 623 K, (c) ZMO deposited at 773 K and (d) ZMO deposited at 923 K.*

**55**

**5. Conclusions**

**Figure 10.**

*Binary Metal Oxides Thin Films Prepared from Pulsed Laser Deposition*

**Figure 9** shows the frequency dependence of the real part (Z') of the complex

*Imaginary part of impedance (Z") as a function of frequency. (a) As deposited ZMO, (b) ZMO deposited at* 

It is observed that as there is a rise in the temperature and the frequency, the real impedance magnitude Z' decreases and hence the materials show a negative temperature coefficient of resistance (NTCR) and are attributed to a semiconductor behaviour. It is also noticed that the value of Z' for all the temperatures merges towards the high frequency due to the space charge dependent behaviour. The charge carriers are settled on the portion of the grain boundaries with sufficient energy to overcome the barrier with the increase in the temperature and hence we can emphasize as in conductivity. It can be seen that the peak position shifts from lower to higher frequencies with increasing temperatures. But the maximum value of the imaginary part Z"max values decreases due to thermally activated dielectric relaxation process as shown in **Figure 10** [4]. The same investigation carried out for the binary oxides ZnO/V2O5 and ZnO/ TiO2 and its performance as dielectric material with respect to wide frequency range and wide temperature range. The behaviour is compared and studied with respect

The nanostructured thin films metal oxides such as ZnO, MoO3, MoO2, binary oxides ZnO/MoO3, ZnO/TiO2 and ZnO/V2O5 were prepared using pulsed laser deposition technique. The ZnO thin film nanostructures are seen through the SEM/ FESEM at the different deposition temperature. Through the increase of deposition

impedance Z\* = Z'-jZ" for temperatures ranging from 298 K to 923 K [4].

*623 K, (c) ZMO deposited at 773 K and (d) ZMO deposited at 923 K.*

to various deposition temperatures [13].

*DOI: http://dx.doi.org/10.5772/intechopen.96161*

*Binary Metal Oxides Thin Films Prepared from Pulsed Laser Deposition DOI: http://dx.doi.org/10.5772/intechopen.96161*

#### **Figure 10.**

*Practical Applications of Laser Ablation*

semicircles that are deformed and depressed with their centres below the real axis as the temperature range investigated, complex curve consists only one depressed semi-circle and its centre lies below the real axis. Furthermore, depressed arc is typical for a dipolar system involving distribution of relaxation time. Moreover, it is noted that the diameter of semi-circles decreases with increase in temperature which also refers to the decrease in the resistivity. The equivalent circuit for this sample is a series Resistor-Capacitor (RC) circuit. Whereas the semi-circled impedance samples possess the characteristics which are attributed to the semiconductor behaviour, in which the electrical conduction process is thermally activated.

The equivalent circuit for these depressed semi-circles of the ZMO thin films may be described by a parallel connection of an ohmic resistor R and a capacitor C, also known as Randles circuit. Here the capacitor is replaced with a constant phase element (CPE) associated with both the resistors and capacitors [12]. Indeed, in the present study, the complex plane plot can be described by the Nyquist plot, which is given by

> <sup>1</sup> ( ) <sup>=</sup> <sup>+</sup> *<sup>R</sup> <sup>Z</sup>*

where ω is angular frequency, τ = RC is the relaxation time and α is a parameter that characterizes the distribution of relaxation times with values ranging from 0 to 1. When α is zero, the relaxation is said to be Debye relaxation and when α is greater than

*Frequency dependence of the real part (Z') of the complex impedance. (a) As deposited ZMO, (b) ZMO* 

*deposited at 623 K, (c) ZMO deposited at 773 K and (d) ZMO deposited at 923 K.*

zero, the relaxation times are said to have distribution.

*j* α

ωτ

(2)

**54**

**Figure 9.**

*Imaginary part of impedance (Z") as a function of frequency. (a) As deposited ZMO, (b) ZMO deposited at 623 K, (c) ZMO deposited at 773 K and (d) ZMO deposited at 923 K.*

**Figure 9** shows the frequency dependence of the real part (Z') of the complex impedance Z\* = Z'-jZ" for temperatures ranging from 298 K to 923 K [4].

It is observed that as there is a rise in the temperature and the frequency, the real impedance magnitude Z' decreases and hence the materials show a negative temperature coefficient of resistance (NTCR) and are attributed to a semiconductor behaviour. It is also noticed that the value of Z' for all the temperatures merges towards the high frequency due to the space charge dependent behaviour. The charge carriers are settled on the portion of the grain boundaries with sufficient energy to overcome the barrier with the increase in the temperature and hence we can emphasize as in conductivity. It can be seen that the peak position shifts from lower to higher frequencies with increasing temperatures. But the maximum value of the imaginary part Z"max values decreases due to thermally activated dielectric relaxation process as shown in **Figure 10** [4].

The same investigation carried out for the binary oxides ZnO/V2O5 and ZnO/ TiO2 and its performance as dielectric material with respect to wide frequency range and wide temperature range. The behaviour is compared and studied with respect to various deposition temperatures [13].

### **5. Conclusions**

The nanostructured thin films metal oxides such as ZnO, MoO3, MoO2, binary oxides ZnO/MoO3, ZnO/TiO2 and ZnO/V2O5 were prepared using pulsed laser deposition technique. The ZnO thin film nanostructures are seen through the SEM/ FESEM at the different deposition temperature. Through the increase of deposition temperature to 450 °C, the orthorhombic MoO3 and monoclinic MoO2 are prepared at the O2 and Ar gas deposition atmosphere and confirmed using the FESEM and XRD. The binary oxides ZnO/MoO3, ZnO/TiO2 and ZnO/V2O5 shows amorphous nature even at high deposition temperature. The thin films dielectric properties, electric modulus and impedance properties were analysed using ac complex impedance spectroscopy. The dielectric relaxation process is thermally activated for all the samples suitable for channel applications in FET devices. They confirm them to possess the nature of semiconducting property by the deformed semi-circle for the impedance Nyquist plots.
