Effect of Multilayer Conguration on Spectral Response of Thin Film TiO2/NiO Heterojunction Photodetector

In this work, a multilayer design for the TiO 2 /NiO heterojunction device is proposed. In this design, layers of TiO 2 nanostructures are consecutively deposited on layers of NiO nanostructures deposited on ITO substrates. This design was compared to the conventional design of thin lm TiO 2 /NiO heterojunction device by measuring the spectral response of both designs in the spectral range of 200–1200 nm. The proposed design showed increased response intensity by 14%, narrowing spectral width by 23% and single peak of response at 440nm. deposited on layers of NiO nanostructures deposited on ITO substrates. This design is based on the ne control of deposition rate of TiO 2 and NiO lms. This design was compared to the conventional design of thin lm TiO 2 /NiO heterojunction device by measuring the spectral response of both designs in the spectral range of 200–1200 nm. The proposed design showed increased response intensity by 14%, narrowing spectral width by 23% and single peak of response at 440nm. The proposed design is simple, low cost and ecient for heterojunction devices.


Introduction
Thin lms still have their outstanding position in photonics and optoelectronics systems and networks due to the irreplaceable characteristics to be provided by the bulk structures [1][2][3]. Therefore, the research interests to thin lms are continuously increased to achieve better characteristics those would develop the performance of the devices based on thin lms [4][5][6].
As thin lms are prepared by several different methods and techniques, many options are available for the designers of thin lm devices. However, no single method or technique could produce thin lms with the best characteristics. Therefore, continuous evaluation of thin lms prepared by any method or technique may provide useful data for designers to choose the optimum for their requirements and applications [1,3].
More than six decades when rst thin lm heterojunctions were fabricated. Despite, thin lm heterojunctions are the basis of the most effective photonic and optoelectronic devices used in many modern technologies, mainly optical communications and biomedical engineering [7][8][9][10]. The combination of thin lm physics and engineering has established such basis. However, new aspects of this combination are explored to produce new photonic and optoelectronic devices [11,12].
Titanium dioxide and nickel oxide are among the most common metal oxide semiconductors due to their excellent characteristics clearly shown in the devices fabricated from them [13]. Research works are motivated to use them because of the excellent energy outline formed due to their energy band gaps (3.20 eV for TiO 2 and 3.85 eV for NiO) [14,15]. The different types of electrical conductivity allowed to fabricate pn heterojunctions of very attractive characteristics. Furthermore, they uniquely show some features making them unbeatable by other metal oxide or compound semiconductors. Photocatalytic activity of titanium dioxide and electrochromic behavior of nickel oxide are examples [14].
Nanotechnology has revolutionized the thin lm technology as the designers are making use of nanostructures and their features in addition to those of thin lms. So, the ultra-fast response, ultra-high reliability, ultra-high e ciency, ultra-high compactness, high reproducibility and low production cost could be achieved at once [7,8]. Nevertheless, relatively simple ideas and inexpensive developments -those might not be seen before -still possible to design and produce new photonics and optoelectronics [9].
In this work, the multilayer con guration of TiO 2 /NiO thin lm heterojunction is proposed to enhance the spectral response of photonic devices based on such heterostructure.

Experimental Part
A dc reactive magnetron sputtering system was used in this work to deposit titanium dioxide (TiO 2 ) and nickel oxide (NiO) thin lms on indium-doped tin oxide (ITO) substrates. A closed-eld unbalanced dualmagnetron assemblies were employed at both electrodes of plasma discharge system. Such assemblies enables highly homogeneous sputtering of titanium and nickel targets as well as homogeneous deposition of thon lms on the substrates. Experimental details on the sputtering system used in this work can be found elsewhere [16][17][18][19][20][21][22][23][24].
The structural characterization tests, mainly x-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy-dispersive x-ray (EDX) spectroscopy, have con rmed the formation of nanostructures from both materials (TiO 2 and NiO). They have also con rmed the high structural purity of the prepared samples as no traces for other elements and compounds were found in the nal products. The minimum nanoparticle size was 20 nm for TiO 2 and 15 nm for NiO. The deposition rates of TiO2 and NiO lms shown in Fig. (1) were nely controlled due to the sputtering system stability.
Two multilayer structures shown in Fig. (2) were prepared in this work and their spectral response were compared to introduce the advantages of the proposed design as a heterojunction device. In the rst structure, a TiO 2 thin lm of 150 nm thickness was deposited on the ITO substrate after deposition time of 105 minutes. Then, a NiO thin lm of 150 nm thickness was deposited on the TiO 2 layer after deposition time of 90 minutes.
In the second structure, a NiO thin lm of 80 nm thickness was deposited on the ITO substrate after deposition time of 30 minutes. Soon after, two consecutive layers of NiO of 105 and 120 nm were deposited after deposition time of 45 and 60 minutes, respectively. Then, three layers of TiO 2 (105, 75 and 40 nm) were consecutively deposited after deposition times of 60, 45 and 30 minutes, respectively.
Thin lms of aluminum were deposited on the top surface of the prepared samples to act as electrodes for the electrical measurements. The optoelectronic measurements were carried out using different light sources with different wavelengths and intensities. The spectral response of the prepared samples was measured in the spectral range 200-1200 nm using a monochromator equipped with ceramic heater light source and chopper. shows two peaks at 400 and 460 nm with spectral width of 300 nm, while the other con guration containing of three layers of NiO deposited consequently on three layers of TiO 2 shows single peak at 440 nm. There three observations in these spectra, they are (1) disappearance of the two peaks at 400 and 460 nm and appearance of new peak at 440 nm, (2) increase in the intensity by 14%, and (3) narrowing of spectral width by 23%.

Results And Discussion
As the TiO 2 layers are receiving the incident radiation before NiO layers, then they may act as ltration layers to transmit the photons whose wavelengths are within the absorption band of TiO 2 , which is narrower than that of NiO. Therefore, the absorption peak of NiO and corresponding response at 400 nm disappeared from the spectral response spectrum of the multilayer con guration. As the absorption cross section of the TiO 2 nanoparticles is highly increased with decreasing particle size, then the absorption peak and hence the spectral response at 460 nm is shifted towards shorter wavelengths (440nm) as photons of higher energies are e ciently absorbed in the TiO 2 layers as well as in the heterojunction region.
When the TiO 2 and NiO lms are deposited as in the conventional con guration with thickness of 220 and 300 nm, respectively, the opportunity for the nanoparticles to get bigger and aggregate is reasonably available. Therefore, the radiation should traverse throughout these regions of aggregated particles before reaching the heterojunction. Consequently, this would limit the number of photons to be absorbed.
In the multilayer con guration, the thickness of each layer is smaller than that in the conventional con guration. Therefore, the opportunity for the nanoparticles to get bigger and aggregate is relatively lower. The radiation traverses faster throughout the TiO 2 layers to reach the heterojunction with larger number of photons and hence higher absorbance. This may interpret the higher intensity of spectral response in the multilayer con guration.
The narrowing observed in the spectral width may be attributed to the high homogeneity in the absorption of incident photons by the nanoparticles that normalizes the absorption in the heterojunction region.

Conclusion
In concluding remarks, a multilayer design for the TiO 2 /NiO heterojunction device is proposed. In this design, layers of TiO 2 nanostructures are consecutively deposited on layers of NiO nanostructures deposited on ITO substrates. This design is based on the ne control of deposition rate of TiO 2 and NiO lms. This design was compared to the conventional design of thin lm TiO 2 /NiO heterojunction device by measuring the spectral response of both designs in the spectral range of 200-1200 nm. The proposed design showed increased response intensity by 14%, narrowing spectral width by 23% and single peak of response at 440nm. The proposed design is simple, low cost and e cient for heterojunction devices.

Declarations
Availability of data and materials Page 5/9 The author declares that all data and materials included in this work are available.

Funding
The author declares that this work was not supported by any other entity or institution.

Con ict of Interest
The author declares that he does not have any con ict of interest related to this work.  The two multilayer structures prepared in this work