Tuning the electrical parameters of p-NiO x based Thin Film Transistors (TFTs) by pulsed laser irradiation

: We utilized laser irradiation as a potential technique in tuning the electrical performance of NiO x /SiO 2 thin film transistors (TFTs). By optimizing the laser fluence and the number of laser pulses, the TFT performance is evaluated in terms of mobility, threshold voltage, on/off current ratio and subthreshold swing, all of which were derived from the transfer and output characteristics. The 500 laser pulses irradiated NiO x /SiO 2 TFT exhibited an enhanced mobility of 3 cm 2 /V-s from a value of 1.25 cm 2 /V-s for as-deposited NiO x /SiO 2 TFT. The laser-irradiated NiO x material likely has a significant concentration of defect gap states, which could also be involved in light absorption processes. Second, and more importantly, the excess energy that the photogenerated charge carriers possess (due to the significant difference between the photon energy and the bandgap of NiO x ), combined with the very high light intensity, would result in complex thermal and photo thermal changes thus resulting in an enhanced electrical performance of p-type NiO x /SiO 2 TFT structure.


Introduction
Over the last few years, metal oxide semiconductors have received considerable attention for the applications in thin-film transistors (TFTs) because of their superior physical properties [1]. Extensive research on n-type oxide semiconductors {for example, Indium Gallium Zinc Oxide (InGaZnO), Zinc Oxide (ZnO), and Indium Oxide (In2O3)} resulted in industry-specific high-performance oxide TFTs. It is imperative to develop p-type substitutes for the development of next-generation transparent electronics which would enable the development of complementary metal-oxide semiconductor (CMOS) circuits [2], [3]. Few p-type semiconductors such as compounds, Tin Oxide (SnO), Copper Oxide (CuxO), and Nickel Oxide (NiOx), have been demonstrated and incorporated into transistors as the p-type channels. Meanwhile, high performance p-type oxide remains to be a challenging work due to the localized 2p orbitals in the valence band maximum (VBM), the deep VBMs (5-8 eV), and self-compensation by donors [4]. It is well known that stoichiometric deviation alters the electronic structure, and oxygen vacancy causes the formation of sub-gap defects and donor states in many oxides. Therefore, a trade-off between the stoichiometry and the mechanisms employed for defect termination processes are determine the extent of electrical performance of the p-type oxide transistors.
Among various p-type oxide candidates, NiOx is an interesting and promising deposition [17], spray pyrolysis [18,19]. Sputtering, which is a type of physical vapor deposition technique, is preferred due to its industrial scalability. However, the composition and electrical conductivity of sputtered films are far from equilibrium [20,21]. Lattice defects are not well-defined in NiO films with high oxygen content [22]. Some studies used working gas and thermal annealing to adjust the film composition and found that the lattice parameter increased with increasing oxygen content, thus resulting in oxygen interstitials as the dominant defects. However, the lattice parameter may not be appropriate to determine the dominant mechanism. On the other hand, the dynamics of interstitials/vacancies created due to laser irradiation are little discussed regarding non-stoichiometric NiO films [23].
Similarly, laser irradiation has been used for modifying intrinsic properties of metallic [24,25], semiconducting [26][27][28][29], superconducting [30], multiferroic [31] and ceramic [32] thin films. In [33], the most frequently used high dielectric material, hafnium oxide (HfO2) was subjected to irradiation by continuous wave laser with wavelength 355nm to analyze the temporal behavior of absorption annealing. In [34], post-deposition annealing of tin oxide (SnO2) thin films by ultra-short laser pulses resulted in a change in refractive index and conductivity of the films. Significant modification in the stoichiometry, desorption of dopant atoms and adsorption of hydrogen atoms from the atmosphere were also observed. Crystallization of amorphous titanium oxide (TiO2) by pulsed laser irradiation using an excimer laser is studied in [35]. Cadmium oxide (CdO) thin films deposited by sol-gel coating method were laser irradiated using a Q-switched Nd:YAG laser operating at its first and second harmonic wavelengths in [36]. An agglomeration of nanoparticles and variation in the bandgap, photoluminescence spectra with laser irradiation were observed. The structural, optical, luminescent and vibrational properties of zinc oxide (ZnO) under the influence of continuous-wave CO2 laser irradiation were studied in [37]. In this work, we explore the effect of ultra-violet (UV) laser irradiation in tuning the properties of NiO thin films, thus enhancing the electrical performance of NiO based TFTs.
In [38], the low-temperature solution-processed p-type nickel oxide thin films along with an aqueous high-k aluminium oxide Al2O3 gate dielectric significantly improved the electrical performance of NiOx TFT compared to those based on SiO2 dielectric. The hole mobility was reportedly enhanced by 60 times from 0.07 to 4.4 cm 2 /Vs. Similarly, ink-jet printed p-type NiOx TFTs annealed at 280°C in [39], gave the best electrical performance with field-effect mobility of 0.78 cm 2 /Vs, SS of 1.68 V/dec, on/off current ratio (Ion/Ioff) of 5.3 x 10 4 with a 50nm Al2O3 insulator layer. By optimizing the annealing temperature, precursor concentration, source/drain electrodes, and dielectric material, the authors in [40] successfully achieved p-type NiO TFTs with remarkable mobility of 6.0 cm 2 /Vs, excellent on/off current modulation ratio of 10 7 , as well as a good subthreshold swing of 0.13 V/decade at a low processing temperature of 250°C. Hence, from our previous works Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 16 March 2021 [5-7, 23, 29], we propose the non-contact laser irradiation on sputter-deposited NiOx/SiO2 based TFTs as a potential technique to enhance the electrical parameters by tuning the laser fluence and a number of laser pulses. In this work, the relation between the number of laser pulses to the mobility of NiOx, threshold voltage, on/off ratio and subthreshold swing of NiOx/SiO2 TFT device is also studied extensively.

Experimental Details
Nickel oxide films were deposited on SiO2/Si substrate by reactive radio frequency (RF) magnetron sputtering from a 99.99% pure Ni target in an 80:20 oxygen-argon gas mixture at 300 0 C. We used multiple cleaning steps for our substrate. The cleaning procedure is as follows: wash in cleaning solution (Micro-90), then thoroughly rinse with     on the values of VTH being negative or positive, the p-type TFT devices typically operate in enhancement or depletion modes respectively. As the number of laser pulses increases from 0 to 500, then the value of threshold voltage increases slightly more than 12 V.

Results and Discussion
The subthreshold swing (SS) is an important parameter that indicates the switching efficiency of a transistor. The SS is directly related to the quality of the dielectric/semiconductor interface and defined as the inverse of the maximum slope of the transfer characteristic and reflected by the VGS needed to increase the IDS by one decade in the sub-threshold region. When the SS value is low, then the operation speed is high, and the power consumption is also low. As the number of laser pulses increases from 0 to 500, then the value of SS decreases from 3.8 to 0.65 V/decade (Figure 3(d)).
Furthermore, the associated (IDS) ½ and IGS as a function of the VGS were shown in Figure 4(a, b). It was found that the saturation drain-source current and the gate leakage current

Conclusions
In this work, we studied the effects of laser irradiation in enhancing the electrical performance of p-type NiOx TFT with SiO2 as high-k dielectric layer. The mobility Data Availability Statement: In this section, please provide details regarding where data supporting reported results can be found, including links to publicly archived datasets analyzed or generated during the study. Please refer to suggested Data Availability Statements in section "MDPI Research Data Policies" at https://www.mdpi.com/ethics. You might choose to exclude this statement if the study did not report any data.