Tuning the Electrical Parameters of p-NiO_x-Based Thin Film Transistors (TFTs) by Pulsed Laser Irradiation

Abstract

We utilized laser irradiation as a potential technique in tuning the electrical performance of NiO_x/SiO₂ thin film transistors (TFTs). By optimizing the laser fluence and the number of laser pulses, the TFT performance was 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/SiO₂ TFT exhibited an enhanced mobility of 3 cm²/V-s from a value of 1.25 cm²/V-s for as-deposited NiO_x/SiO₂ TFT, subthreshold swing of 0.65 V/decade, on/off current ratio of 6.5 × 10⁴ and threshold voltage of −12.2 V. The concentration of defect gap states as a result of light absorption processes explains the enhanced performance of laser-irradiated NiO_x. Additionally, laser irradiation results in complex thermal and photo thermal changes, thus resulting in an enhanced electrical performance of the p-type NiO_x/SiO₂ TFT structure.

1. Introduction

Metal oxide semiconductors have been widely used for applications in thin-film transistors (TFTs), gas sensors and carrier transporting layers in solar cells due to their defect engineering capabilities [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 can accentuate next-generation Complementary Metal Oxide Semiconductor (CMOS) circuits [2,3]. Few p-type semiconductors such as the compounds Tin Oxide (SnO), Copper Oxide (Cu_xO) and Nickel Oxide (NiO_x) were reported to have a reasonable electrical performance as p-type channels. High-performance n-type equivalent p-type oxides still remain challenging due to the deep valence band maximum (~6.5 eV) [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 is to determine the extent of tuning of the behavior of p-type TFTs.

NiO_x is a particularly interesting and promising material due to its wide range of applicability vis-à-vis rectifying diodes [5], p-type Metal Insulator Semiconductor (MIS) structures [6], tin whisker growth mitigation by NiO sublayers [7], etc. The stoichiometric NiO is a Mott insulator with a conductivity of 10⁻¹³ S/cm, while nonstoichiometric NiO_x is a wide-band-gap p-type semiconductor [8]. Several methods have been used for growing NiO films, including sputtering [9,10,11,12], e-beam evaporation [13,14], the chemical and plasma-enhanced chemical vapor method [15], sol-gel [16], pulsed laser deposition [17] and spray pyrolysis [18,19]. Sputtering, which is a type of physical vapor deposition technique, is preferred due to its industrial scalability. Indeed, the sputter-deposited NiO_x films often exhibit a varied electrical conductivity [20,21]. Defect engineering is not well-established in O-rich NiO films [22]. Some recent studies used working gas and thermal annealing as viable techniques for interstitial/defect changes. However, the dynamics of interstitials/vacancies created due to laser irradiation are little discussed regarding non-stoichiometric NiO films [23].

In continuation of the previous work in [23], laser irradiation was also employed to study its effects in modifying the 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 a continuous wave laser with a wavelength of 355 nm to analyze the temporal behavior of absorption annealing. In [34], the postdeposition annealing of tin oxide (SnO₂) thin films by ultra-short laser pulses resulted in a change in the refractive index and conductivity of the films. A significant modification in the stoichiometry, desorption of dopant atoms and adsorption of hydrogen atoms from the atmosphere were also observed. The 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 the 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 a 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 the aluminum oxide Al₂O₃ gate dielectric significantly improved the electrical performance of NiO_x TFT compared to the SiO₂-based dielectric. The hole mobility was reportedly enhanced to 4.4 cm²/Vs. Similarly, ink-jet printed p-type NiO_x TFTs annealed at 280 °C in [39] gave the best electrical performance with a field-effect mobility of 0.78 cm²/Vs, SS of 1.68 V/dec, on/off current ratio (I_on/I_off) of 5.3 × 10⁴ with a 50 nm Al₂O₃ insulator layer. The authors in [40] successfully achieved p-type NiO TFTs with a mobility of 6.0 cm²/Vs and on/off current ratio of 10⁷ at a temperature of 250 °C. Hence, from our previous works [5,6,7,23,29], we propose a noncontact laser irradiation on sputter-deposited NiO_x/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 NiO_x, threshold voltage, on/off ratio and subthreshold swing of the NiO_x/SiO₂ TFT device is also studied extensively.

2. Device Fabrication Details

Nickel oxide films were deposited on SiO₂/Si substrate by reactive sputtering of 99.99% pure Ni target in an 80:20 oxygen–argon gas mixture at 300 °C. The cleaning procedure for the substrate is as follows: wash in cleaning solution (Micro-90), thoroughly rinse with DI water and conduct an ultra-sonication bath in methanol and ethanol for about 20 min. Simultaneously, the surfaces are dry-blown with nitrogen. The thickness of the films was found to be about 100 nm using spectroscopic ellipsometry studies [29]. Figure 1 depicts the layered structure of the NiO_x-based TFT. The gate metal (sputter-deposited Ni film of a thickness of 50 nm is deposited on the Si substrate (0.5 mm)). Then, an SiO₂ layer of thickness 200 nm is sputter-deposited by step-projection masking. Now, the NiO film of thickness 100 nm is deposited by reactive sputtering of Ni in 20% oxygen atmosphere. Post-deposition, the NiO layer is subjected to an Nd:YAG laser irradiation laser with a fourth harmonic wavelength λ = 266 nm and optimized laser fluence (smoothening of the film due to localized melting) of 150 mJ/cm² for different numbers of pulses (Figure 2a–d). At laser fluences slightly above and below the optimum laser fluence (i.e., 175 mJ/cm² and 125 mJ/cm², respectively), a marginal discontinuity (assumed to be a microhole) in the film can be observed at the center of the laser spot. This may arise due to a slight nonuniformity in the thickness of the sputter-deposited NiO film. The laser fluence is kept below a threshold (200 mJ/cm²) for any observable physical damage/ablation. Finally, the source and drain metal (Al film of thickness 50 nm) are deposited by RF sputtering. The mobility was obtained using a Hall effect measurement setup at room temperature with a magnetic field of 2500 G [29]. The polycrystalline structure of the as-deposited NiO_x is verified on a Rigaku X-ray diffractometer with Cu K_a radiation (λ = 0.154 nm) and a Ni filter.

3. Results and Discussion

Figure 3a depicts the relationship between the mobility and the number of laser pulses. The mobility is affected by several scattering mechanisms, including lattice vibrations, ionized impurities, grain boundaries, interface surface roughness, lattice strain and other structural defects, velocity saturation and electron trapping [41]. The field-effect mobility, which is the most widely used parameter to evaluate the TFT performance, gradually increases from 1.25 cm²/V-s to 3 cm²/V-s as the number of laser pulses increases in steps of 100 pulses to 500 pulses, after which, from 600 pulses to 1000 pulses, the hole concentration ceases to increase. This is now reflected in the mobility, decreasing to about 2.5 cm²/V-s and finally reaching a value of 2.4 cm²/V-s.

Similarly, 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. 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, the value of SS decreases from 3.8 to 0.65 V/decade (Figure 3a). After 500 pulses up to 1000 pulses, the SS starts to increase to 2 V/decade, which suggests that the NiO layer is now slowly reduced to an amorphous film from a polycrystalline film [29]. As a result, this change degrades the dielectric/semiconductor interface quality. Hence, from Figure 3a, it is evident that 500 laser pulses is the point where both the mobility and SS are tuned for an optimum performance of NiO TFT.

Figure 3b represents the relationship between the on/off current ratio and the number of laser pulses. The on/off current ratio I_on/I_off is extracted from the transfer characteristic curve. If the number of laser pulses increases from 0 to 500, then the ratio value constantly increases to 6.5 × 10⁴. For 600 to 1000 laser pulses, the formation of Ni-rich NiO [29] results in a relatively lower mobility, thus reducing the on/off current ratio from 6.5 × 10⁴ to 5.5 × 10⁴.

Figure 3b also establishes the relationship between the threshold voltage and the number of laser pulses. The threshold voltage ($V_{TH}$) is the measure of how well the channel is formed near the dielectric layer/active layer interface in TFTs. Depending on the values of V_TH 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, the value of the threshold voltage decreases to −12.5 V. This is attributed to the increase in the carrier concentration of NiO. For 600 to 1000 laser pulses, as the carrier concentration stabilizes, the V_TH also stabilizes and reaches a value of −11.5 V.

Table 1. Comparison of reported works on p-type NiO TFTs with the present work.

Ref.	Published Year	Oxide Layer	Mobility (cm2/V-s)	Subthreshold Swing (V/decade)	On/off Current Ratio	Threshold Voltage (V)
[42]	2013	NiO	5.20	3.91	2.2 × 103	-
[43]	2015	NiO	0.05	2.6	103	8.6
[44]	2016	Sn:NiOx	0.97	0.24	106	1.44
[45]	2017	Cu:NiOx	1.53	0.13	3 × 104	0.45
This work	2021	NiOx	3.00	0.65	6.5 × 104	−12.5

presents a comparison of the electrical parameters reported in this work with previously reported NiO TFTs.

Furthermore, the associated (I_DS)^½ and I_GS as a function of the V_GS are shown in Figure 4a–c. It was found that the saturation drain-source current and the gate leakage current were −40 μA and −8 nA, respectively, when the p-type NiO TFTs operated at a V_DS of −10 V and a V_GS of −9 V. The associated on-to-off current ratio was 6.5 × 10⁴. The associated threshold voltage and subthreshold swing were −8 V and 0.56 V/decade, respectively.

The output characteristics of the p-type NiO-based TFT are divided into two main operating regions depending on the value of V_DS: linear and saturation regions. In the linear region, V_GS controls the channel resistance, thus accumulating charges uniformly in the channel. In the saturation region, for a constant V_GS, I_DS is constant, which suggests a lack of channel region due to a depletion of charges in the accumulation layer. For the 500 pulses laser-irradiated NiO_x/SiO₂ TFT, the output characteristics are shown in Figure 5.

At a constant V_GS of 0 V, when the voltage V_DS decreases from 0 V to −20 V, the value of the current I_DS decreases from 0 µA to −1.6 µA. At a constant V_GS of −3 V, when the voltage V_DS decreases from 0 V to −20 V, the value of the current I_DS decreases from 0 µA to −3 µA. At a constant V_GS of −6 V, when the voltage V_DS decreases from 0 V to −7.5 V, the value of the current I_DS decreases from 0 µA to −15 µA and remains constant up to −20 V. Again, at a constant V_GS of −9 V, when voltage V_DS decreases from 0 V to −10 V, the value of the current I_DS decreases from 0 µA to −40 µA and remains constant up to −20 V.

4. Conclusions

In this work, we studied the effects of laser irradiation in enhancing the electrical performance of p-type NiO_x TFT with SiO₂ as a high-k dielectric layer. The mobility increased from 1.25 cm²/V-s for the as-deposited NiO_x thin film to 3 cm²/V-s for the 500 laser pulse-irradiated sample. Similar enhancements were observed for the laser-irradiated sample in terms of the threshold voltage, subthreshold swing and the on/off current ratio. The plausible mechanism responsible for the reported phenomenon was that the excess energy the charge carriers possess, combined with the very high light intensity, resulted in a complex defect/interstitial tuning (Ni/O defect formations), thus enhancing the electrical performance of the TFT.