A strong dependence of sputtering power on c‐axis oriented aluminium nitride on Si (111): A structural and electrical study

Abstract

Growing and controlling the c‐axis orientation of the aluminium nitride (AlN) thin film on unheated Si (111) substrates using reactive magnetron sputtering are challenging. Sputtering parameters such as nitrogen concentration and sputtering power were effectively tuned to grow the c‐axis oriented AlN thin film. The results show that a low concentration of (25%) N₂ is enough for forming AlN at a reduced flow rate, whereas a higher flow rate requires a higher concentration of N₂. Low concentration with a low flow rate is preferred to grow AlN at low temperature and power. The poor crystallinity of AlN with (100) orientation was improved by varying the power from 75 to 175 W. The X‐ray diffraction results confirmed the improvisation of crystallinity of grown AlN films and indicated the strong dependence of preferred c‐axis orientation on sputtering power. Also, the dependence of sputtering power on microstrain and stress was analysed. The surface morphology study by field emission scanning electron microscopy and topography measured by an atomic force microscope shows a dependence on sputtering power. The high c‐axis orientation was observed at 175 W with low surface roughness, low leakage current density (2 × 10⁻⁹ A/cm²) and low dielectric constant (6.8).

INTRODUCTION

Wurtzite aluminium nitride (AlN) thin films have gained much interest as a piezoelectric material in microelectromechanical system (MEMS) devices because of their unique properties and the advancement in related technologies in the last decade [1]. It has a moderate piezoelectric constant (d₃₃) of 5.2 pC/N, wide band gap (6.2 eV), high electrical resistivity (10¹³ Ω-cm), high breakdown voltage (2–5 MV/cm), and low dielectric constant of 8.3 [1–4]. These balanced properties with complementary metal-oxide semiconductor (CMOS) compatibility make it an excellent choice over other superior piezoelectric materials for MEMS applications such as lead zirconate titanate. Sensors, microwatt energy harvesters, and radio frequency (RF) filters such as surface acoustic wave and bulk acoustic wave devices are the major applications of these MEMS devices [5]. Aluminium nitride thin films can be grown using different deposition techniques such as MBE, metal organic chemical vapor deposition and sputtering [6–9]. However, the CMOS microfabrication processing temperature limit is around 450° C. Therefore, DC sputtering is most suited, wherein the deposition can be at substrate temperatures as low as room temperature [10]. The DC power source is cost-effective and has a higher deposition rate; the only issue with this is arcing (high discharge current) in case of reactive sputtering of metal targets for depositing the insulating thin film. Arcing is caused by the presence of reactive gas (N₂) essential for compound thin film (AlN) formation, which also reacts at the surface of the metal target and forms an insulating layer on the surface, known as a poisoned target. This thin insulting layer will break down with arcing, that is, high discharge current, which results in the melting of the target in small areas less than 50 μm. These fine melts may incorporate into a sputtered thin film, resulting in the formation of defects. However, the DC power supply's arc detection and rapid switching off the power supply prevent these issues. The radio frequency power source will not suffer from these arcing issues since it discharges accumulated charges in the positive cycle, but it has some limitations such as power loss, less thin film deposition rate, higher cost and complexity in scaling-up [11]. The piezoelectric property is the highest in highly c-axis oriented AlN because of the high electromechanical coupling coefficient in this polar axis. It has been demonstrated by Yarar et al. that the piezoelectric coefficient (d_{33, f}) is strongly dependent on film thickness, and the drastic increment observed from 2.30 to 5 pm/V when the thickness is increased from 30 to 500 nm, above which it slightly increases up to 5.57 pm/V at 2 μm [12]. So, it is preferable to deposit the film thickness of 500 nm to 1 μm, which is required to achieve a piezo response for sensor and energy harvester applications. Many research groups have grown c-axis oriented AlN on sapphire, silicon, and flexible polymer substrates. However, obtaining c-axis oriented films on silicon substrates is essential considering the micromachining and CMOS integration requirements. Hence, we have chosen low resistive Si (111) substrates because of good lattice matching with c-axis AlN, and low resistance will double up as a bottom electrode [13].

Highly c-axis AlN on Si (111) substrates can be obtained by controlling the sputtering parameters. The peak width (full width at half-maximum (FWHM)) of the peak corresponding to the (002) plane will provide the quality of c-axis orientation—the lower the FWHM, the better the crystallinity. Also, estimating the residual stress is essential, which decides the device performance [14, 15]. In literature, the sputtering parameters such as power, pressure, and nitrogen concentration are primarily optimised for obtaining c-axis AlN [16]. However, there are few reports on a systematic approach to improving the poor crystallinity to highly c-axis orientation, which is crucial to optimising the quality of AlN thin films. In this study, we have shown that the improvisation of poor crystallinity (no peak) to highly c-axis orientation is possible by switching the correct parameters. Initially, the nitrogen concentration is varied, an important factor controlling the reaction between aluminium and nitrogen, by keeping low power (75 W), which is essential for getting smooth films. Further, the sputtering power is varied to study its effectiveness in controlling the quality of the films. Crystallinity and preferred orientation were analysed by high-resolution X-ray diffraction (HRXRD), and the quality was estimated by peak width (FWHM), and crystallite size, microstrain, and stress were calculated using Scherrer's formula. The power dependence on the surface morphology was studied by field emission scanning electron microscopy (FE-SEM), and the topography was measured using an atomic force microscope (AFM).

EXPERIMENTAL DETAILS

Aluminium nitride thin films with varied thicknesses (200–1000 nm) were grown on RCA-cleaned Si (111) substrates in a DC-magnetron sputtering system (Excel instruments, India). A high-purity aluminium (Al) target (99.999%) with a diameter of 50.8 mm was fixed on a magnetron sputtering gun. The substrate-to-target distance of 130 mm was fixed for initial optimisation. The base pressure below 6 × 10⁻⁷ mbar is obtained before each deposition, with the help of a turbo molecular pump (TMP). After the evacuation, ultra-high pure argon gas of purity 99.999% was introduced at 20–40 sccm using a mass flow controller to pre-sputter the Al target for approximately 10 min before each film growth for target cleaning purposes. The pre-decided sputtering pressure was controlled by adjusting the gate valve throttling between the deposition chamber and TMP. After pre-sputtering, N₂ gas was introduced to perform reactive sputtering to grow AlN. The pre-decided Ar/N₂ ratio was maintained for a complete deposition duration of 2 h. Other variables for the experiments of N₂ concentration with the total gas flow rate and power are given in Tables 1 and 2, respectively.

TABLE 1 N₂ concentration and total gas flow rate variation at the DC power of 75 W.

Sample name	N-1	N-2	N-3	N-4
N2 concentration	25%	40%	50%	25%
Total gas flow rate (sccm)	40	20
Sputtering pressure (×10−3 mbar)	8.2 +/− 0.1	6.3 +/− 0.1

TABLE 2 DC power variation at 20 sccm of the total gas flow rate with 25% of N₂ concentration and pressure 6.3 × 10⁻³ mbar.

Sample name	P-1	P-2	P-3	P-4	P-5
Power (W)	75	100	125	150	175
Deposition rate (nm/min)	1.7	2.5	5.8	6.7	8.3

The thickness of the sputtered AlN films was measured with a stylus surface profilometer (Dektak, Bruker). These thickness values were used to calculate the sputter rate. The HRXRD (Smart Lab, Rigaku) was utilised to investigate the crystalline property of AlN thin films, which uses a single crystal monochromator (Ge) suitable for single crystal substrates [17]. Scherer's formulas were utilised to measure the lattice parameters and microstrain. The formula for finding the wurtzite hexagonal crystal lattice constants ‘a’ and ‘c’ is as follows. [18, 19]. 1 $$\frac{1}{{d}_{(hkl)}^{2}}=\frac{4}{3}\left(\frac{{h}^{2}+hk+{k}^{2}}{{a}^{2}}\right)+\frac{{l}^{2}}{{c}^{2}}$$ where d _(hkl) is the inter-planar distance, which can be determined by X-ray diffraction (XRD) characterisation using the X-ray of wavelength λ with an incident angle with relation derived from Bragg's law: 2 $$\lambda =2{d}_{(hkl)}\,\mathit{sin}{\theta }_{(hkl)}$$

Further, the equation for lattice constants can be simplified as follows. 3 $$a=\frac{\lambda }{\sqrt{3}\mathit{sin}{\theta }_{(100)}}$$ 4 $$c=\frac{\lambda }{\mathit{sin}{\theta }_{(002)}}$$

Any strain developed because of the variation in crystal lattice constant results in crystal distortion and crystal asymmetry, affecting the piezoelectricity. Crystallite size, microstrain, and stress are essential characteristics of piezoelectric thin films, resulting in the broadening of the XRD peak, and they can be analysed using the Debye–Scherrer method. Average crystallite size D is inversely proportional to the peak broadening or FWHM of the peak referred to as β. 5 $$D=\frac{k\lambda }{\beta \,\mathit{cos}\,\theta }$$

The k-value is constant and depends mainly on the crystal shape. Microstrain is the root mean square of the variations in the lattice parameters across the thin film caused by the distributed interplanar distance, stacking fault, dislocations, lattice mismatch etc. The microstrain is directly dependent on the peak broadening and follows the decreasing trend with the increased X-ray incidence angle, and crystallite size must be consistent over a long range; the relation below is used for calculation. 6 $$\langle \varepsilon \rangle =\frac{\beta \,\mathrm{cot}\,\theta }{4}$$ whereas the stress (σ) can be calculated using the equation below. 7 $$\sigma =Y\ast \langle \varepsilon \rangle $$ where Y stands for Young's Modulus of the material. Dislocation density (δ) is calculated using the formula below. 8 $$\delta =\frac{1}{{D}^{2}}$$

These structural properties are crucial for device fabrication. The surface morphology of thin films was analysed using a FE-SEM and the cross-section using a scanning electron microscope (EVO MA 18, Carl Zeiss), and surface roughness was measured using an AFM (Flex-Axiom, Nanosurf).

The electrical characterisation, such as leakage current density by current–voltage (I-V) characterisation from −10 V to +10 V using a source metre (Keithley 2636B) and capacitance-voltage measurements was done using an impedance analyser (Wayne Kerr 6500-B). These characterisations were performed after fabricating the metal-insulator-semiconductor (MIS) structures. The 100 nm thick Al electrode with a diameter of 0.5 mm is the metal contact deposited by thermal evaporation on AlN/Si (111); Al was deposited on the backside of the Si substrates to use as a back contact. C–V characterisation of the films was measured at different frequencies, 1 kHz, 5 kHz, 10 kHz, 50 kHz, 100 kHz, 500 kHz, and 1 MHz, by varying DC voltage bias from −10 to +10 V along with a 250 mV ac signal. The dielectric constant was calculated using these capacitance values.

RESULTS AND DISCUSSION

Structural properties

The crystalline structure of AlN thin film grown as a function of N₂ concentration and total gas flow rate is investigated by HR-XRD. Obtained XRD patterns of the films deposited according to the sputtering parameters listed in Table 1 are presented in Figure 1. The patterns confirm the only silicon (111) substrate peak at 28.71° with high intensity and a small peak around the (101) peak position of AlN for the films deposited with a constant 40 sccm of flow rate and 25% of N_2, indicating poor crystallinity of the grown films. Upon increasing the concentration of N₂ to 40% and 50% in the total gas mixture with the same flow rate of 40 sccm, only a peak at 2theta position 33.28° confirming the formation of crystalline AlN with (100) is observed. These peaks match well with the standard JCPDS file (# 96-900-8861). Further increasing the nitrogen concentration may not help in the formation of AlN at low power without any substrate heating. The total mass is reduced as the mass of the nitrogen is lower than that of argon, and the momentum transfer to the target is decreased as the nitrogen concentration increases [20, 21]. These results indicate that the poor crystallinity of the AlN (100) plane and no preferential (002) plane may be because of the low kinetic energy of adatoms. The kinetic energy of the adatoms can be increased by lowering the sputtering pressure and reducing the total gas flow rate [22]. So, AlN films were grown at a total of 20 sccm with 25% of N_2, wherein the sputtering pressure reduced to 6.3 × 10⁻³ mbar from 8 × 10⁻³ mbar resulted in only the (100) plane of AlN with poor crystallinity—further lowering the flow rate results in arcing and no stable plasma. It is also observed that 25% of N₂ concentration was not helping to form crystalline AlN at 40 sccm of the total gas flow rate with a high sputter pressure of 8.2 × 10⁻³ mbar, whereas the same concentration is forming AlN at a lowered flow rate of total gas of 20 sccm with a pressure 6.3 × 10⁻³ mbar. Mirpuri et al. reported no crystalline AlN at 25% N₂ with 8.1 × 10⁻³ mbar pressure, even with an RF power of 400 W [23]. The decreased sputtering pressure at 20 sccm increases the kinetic energy of adatoms than at 40 sccm and the formation of AlN with (100) orientation. However, these changes in nitrogen concentration and total gas flow rate are not helping in the formation of c-axis oriented AlN. This may be due to the low kinetic energy of adatoms at 75 W; the formation of AlN (002) may require some critical power, and at that power, N₂ concentration may affect the quality of crystallinity [23]. Increasing the sputtering power will be the right choice to increase adatoms' kinetic energy, resulting in improved film crystalline quality. From the literature, we observed that a higher concentration of N₂ (>50%) requires high power (>300 W) to form AlN [24]. Therefore, a low flow rate and low concentration will be beneficial to grow at low power (<200 W) [16].

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This is essential to lower the roughness of the thin film, so it is advantageous to grow at 20 sccm with 25% of N₂ at increased power [25]. Accordingly, the sputtering power was varied from 75 to 175 W to check the effect on the plane of orientation. The measured thickness values were used to calculate the rate of deposition. It is essential to measure the rate of deposition for varying sputtering conditions. This helps to grow films with constant thickness at comparable parameters to avoid thickness-related effects [26]. It can be considered that the effect of power is the main contributor to the properties of AlN thin films since this study focuses on the improvisation of crystallinity from the poor crystallinity of AlN. The experimental condition for power variation is given in Table 2. It is observed that there is an increase in the deposition rate as the sputtering power increases. The enhancement in the ionisation of gas would give a high density of Ar⁺, obviously improving the deposition rate [25]. Figure 2 shows the X-ray diffraction patterns of AlN thin films at different sputtering powers between 75 and 175 Win 6.3 × 10⁻³ mbar of sputtering pressure and 5:15 of N₂: Ar atmosphere. Major peaks at around 33.28° and 36.03° are observed, confirming the hexagonal wurtzite structured AlN thin films with (100) and (002) planes, respectively. The observed peaks match well with the standard JCPDS file (# 96-900-8861).

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The XRD results showed that the AlN grown with low powers (75–100 W) is poor crystalline in nature. The wurtzite hexagonal structure can be confirmed by the low intensity peaks corresponding to (100) and (101) for the films grown at 75 W. For the films grown at 100 W, a broad peak corresponding to (002) planes also starts emerging with these low intensity peaks. The low intensity of all these peaks might be because of the low kinetic energy of the imping atoms, which may lead to no long-range AlN crystallite. Also, low thickness contributes to low intensity. Whereas a further increase in power to 125 and 150 W results in a crystalline AlN with a wurtzite hexagonal structure with peaks of (100) (002) and with increasing peak intensity and a small (101) peak. When the power increased to 175 W, the films grew with only a preferential oriented (002) plane. At these sputtering conditions, the kinetic energy of impinging atoms has increased, which will break the weakly bonded (100) plane, and only the (002) plane with a strong bond will be constructed, which is preferential for the piezoelectric property [24, 27, 28]. Also, as sputtering power increased, (002) peak values of AlN films slightly shifted towards a higher 2θ angle; this can be attributed to compressive stress on the film [28].

The effect of sputtering power on lattice constant a and c of the AlN thin film is shown in Figure 3a. Lattice constant ‘a’ is calculated utilising (100) peak observed only for samples 75–150 W and is in the range of 3.112–3.117 Å, which agrees well with the value (3.112 Å) calculated using density functional theory (DFT, results are not discussed here) The lattice constant ‘c’ determined using (002) also agrees with the calculated value of c = 4.984 Å. Sputtering power influences lattice constants, and ‘c’ decreased from 4.989 to 4.967 when power increased from 100 to 175 W. Which is directly dependent on the shift in the diffraction angle due to the microstrain of the crystallite [29]. Crystalline quality can be analysed by the peak width and intensity of the preferential (002) plane of AlN thin films. Figure 3b shows the effect of sputtering power on the peak intensity and peak width of AlN thin films. Peak intensity increases when power is increased from 100 to 175 W. Aluminium nitride grown at 100 W shows a very low intensity of 55 cps and a peak width of 0.52°. The films grown at 125 and 150 W show a peak width around 0.18°, whereas at 175 W, it is 0.23° with a high intensity of 3980 cps. The increased intensity and low FWHM of the peak suggest good crystallinity, and this may be due to increased thickness at higher sputtering power [30]. The shift in peak position (2 theta) and peak width (FWHM) is attributed to the crystallite size and microstrain or lattice strain caused due to the growth condition. The average crystallite size is inversely proportional to both FWHM and 2theta position, whereas the microstrain is directly proportional, and these can be calculated using Scherrer’s formula [29].

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Figure 4a shows the effect of sputtering power on the average crystallite size and microstrain corresponding to (002) plane or c-axis oriented crystallites. Crystallite size and microstrain were found to be 21.58 nm, and 5.91 × 10⁻³ at 100 W. Large crystallite size (>42 nm) and low microstrain (<2.9 × 10⁻³) were found at 125 and 150 W of sputtering power, and at 175 W of sputtering power it is found to be 38 nm and 3.1 × 10⁻³_, respectively. Microstrain originated by lattice mismatch between the Si (111) and the AlN thin films. Aluminium nitride deposition parameters control molecules' kinetic energy, which results in deformed growth [31]. The calculated microstrain is still in the good range of the highly c-axis oriented AlN thin film.

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Figure 4b shows the average stress and the dislocation density of AlN thin films grown at different powers. The compressive stresses and dislocation density of the films slowly decreased from 1.96 to 1.02 GPa and 2.52 × 10¹⁵ lines per m² to 0.70 × 10¹⁵ lines per m², respectively, when sputtering power increased from 100 to 175 W. These values and the trend are similar to the reported work [28]. This can be attributed to defects such as interstitials, Al or N vacancies, and dislocation density generated by the high-energy atom bombardment [35]. As the sputtering power increases, more Al atoms sputtered from the target with more kinetic energy. So, these highly energetic particles transfer more momentum to the deposited thin film. This will help adatoms rearrange themselves to form strong c-axis oriented planes, and on some occasions, some adatoms are knocked out of the growing film. This leads to the formation of defects and stress in the thin film growth process [28]. The structural properties of other reported c-axis AlN thin films on the silicon substrate are given in Table 3.

TABLE 3 Structural property comparison of aluminium nitride (AlN) grown at 175 W with other reported c-axis oriented AlN thin films on the silicon substrate.

Sputtering source – power [Reference]	Peak position 2θ (002)	FWHM	Crystallite size	Micro strain	Dislocation density	Stress
(°)	(°)	(nm)	(×10−3)	(×1015 lines/m2)	(GPa)
RF- 200 W [32]	36.12	0.118	71.00	∼1.58	∼0.20	∼0.52
DC- 160 W [33]	36.02	0.295	28.40	∼3.96	∼1.25	∼1.31
DC- 1000 W [29]	36.05	∼0.276	31.50	3.71	∼1.01	∼1.23
RF- 200 W [34]	35.70	0.535	15.90	∼7.25	∼3.96	∼2.40
DC- 175 W [This work]	36.12	0.231	37.68	3.10	0.70	1.02

Surface morphology and topography

The effect of sputtering power on the surface morphology of the AlN on silicon (111) substrate is analysed using FE-SEM images, and Figure 5 shows the non-porous surface and dense and homogeneous grain distribution. The effect of sputtering power on the surface morphology of the AlN on the silicon (111) substrate is analysed using FE-SEM images, and Figure 5 shows the non-porous surface and dense and homogeneous grain distribution. As the DC power increases to 125–175 W, grain size is increased and found to be approximately 20 nm at 125 W, 80–100 nm at 150 nm and 150–200 nm at 175 W, as estimated with reference to the scale bar in the image.

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A low power of 125 W resulted in a low deposition rate (5.7 nm/min) because of less yield in sputtering and low adatom mobility because of the low adatom kinetic energy, so the small spherical grains were observed. The increased power of 150 W resulted in a slightly increased deposition rate (6.7 nm/min) and adatom mobility, resulting in larger pyramid cone-shaped grains. With an increased power of 175 W, the deposition rate was (8.8 nm/min) and larger grains of cubic and pyramid shape were observed. This agrees well with an earlier study [36]. The cross-sectional image depicted in Figure 5d confirms the columnar growth pattern for highly c-axis orientated AlN grown at 175 W. Figure 6 shows the surface topography of AlN thin films grown at different sputtering powers. Roughness values (R_q or rms) were observed to be 1.38 nm, 8.74 nm, and 3.22 nm for 125 W, 150 W, and 175 W, respectively. Generally, sputtering power results in an increased surface roughness due to high kinetic energy and nucleation rate [36]. It is also possible that increased adatom energy would assist in coarsening the grains and obstruct the formation of (002) orientation [37].

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When it surpasses the kinetic energy required for a weak (100) plane at high power, it helps grow only (002) orientation. Aluminium nitride with c-axis orientation has good piezoelectric response and CMOS compatibility; these are preferred for microwatt energy harvesters and surface acoustic wave devices with low surface roughness. Roughness more than wavelength will obstruct the waves in surface acoustic wave devices [38, 39].

Electrical properties

The current-voltage (I- V) characteristics of the AlN layer grown at 175 W are shown in Figure 7a using the fabricated MIS (schematic is shown in the inset of Figure 7a). The leakage current density at 2 V of samples grown at different sputtering power from 75 W, 100 W, 125 W, 150 and 175 W is 5.0 × 10⁻⁴A/cm², 3.2 × 10⁻⁸ A/cm², 1.7 × 10⁻⁹ A/cm², 8.7 × 10⁻⁹ A/cm² and 2.1 × 10⁻⁹ A/cm²_, respectively. The leakage current density in the polycrystalline insulating films occurs mainly through grain boundaries. The increased sputtering power resulted in an increase in crystallite size, which means a smaller number of grain boundaries, resulting in a lower leakage current. Typical C–V characteristics at different frequencies (1 kHz to 1 MHz) are shown in Figure 7b of the MIS structure consisting of AlN grown at 175 W.

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All samples show the nature of a standard metal-oxide-semiconductor (MOS) capacitor with the n-type Si (111) substrate, which has an accumulation (from +10 V to +0.5 V), depletion and inversion regions. The increased frequency resulted in the decrease of capacitance in the accumulation region (i.e., capacitance of insulating AlN layer). Using this value at the accumulation region, we can calculate the relative dielectric constant of AlN by the formula given by Abdallah et al. [40]. The dielectric constant of AlN at a frequency of 1 MHz for samples grown at 75, 100, 125, 150 and 175 W is 1.7, 4.6, 7.2, 16.7 and 6.8, respectively, and are shown in Figure 7c, which is in comparison to earlier reported values [41, 42].

CONCLUSION

In this study, AlN thin films with highly c-axis orientation were grown on unheated Si (111) substrates by varying N₂ concentration and sputtering power. XRD measurement confirms the w-hexagonal AlN thin films and c-axis orientation strongly depend on sputtering power. The low concentration of (25%) N₂ is enough to form w-hexagonal AlN at a reduced flow rate, whereas a higher flow rate requires a higher concentration of N₂. Changing N₂ concentration at low power does not help to grow c-axis oriented AlN. Improving the crystallinity from poor to highly c-axis orientation strongly depends on high sputtering power (175 W). The crystalline quality was analysed in terms of FWHM of (002) XRD peak, and other structural parameters such as lattice constant, crystallite size, micro-strain and stress on the films were calculated using XRD peaks. The results confirm that the improvisation of AlN crystallinity is strongly dependent on sputtering power. Furthermore, the surface morphology and surface roughness were measured using field emission scanning electron microscopy and AFM, respectively. These results show dependence on sputtering power and switching of highly c-axis orientation. Highly c-axis oriented AlN, non-porous, thin film with low surface roughness, low leakage current density (2 × 10⁻⁹ A/cm²) and low dielectric constant (6.8) is preferred for piezoelectric-based microwatt energy harvesters and surface acoustic wave device applications.

AUTHOR CONTRIBUTIONS

Sandeep Sanjeeva: Formal analysis; Investigation; Methodology; Visualisation; Writing – original draft. Jyothilakshmi Rudresh: Data curation; Investigation; Methodology; Validation. K. B. Vinayakumar: Validation; Visualisation; Writing – review & editing. K. K. Nagaraja: Conceptualisation; Funding acquisition; Resources; Supervision; Visualisation; Writing – review & editing.

ACKNOWLEDGEMENTS

This research was funded by the Department of Science and Technology, Government of India, under grant SRG/2019/002225 and Manipal Academy of Higher Education for the Seed Money Grant for Faculty (ID: 00000693, October 2022). Sanjeeva would like to acknowledge the financial support of the Ministry of Tribal Affairs-Government of India given through the National Fellowship for ST Students, Award Number: 202122-NFST-KAR-03286.

CONFLICT OF INTEREST STATEMENT

The authors have no competing interests to declare that are relevant to the content of this article.

PERMISSION TO REPRODUCE MATERIALS FROM OTHER SOURCES

None.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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