Molybdenum thin films have garnered significant attention in diverse technological applications owing to their outstanding characteristics. The high melting point and stability of molybdenum ensure that it remains structurally intact under the harsh operating conditions of solar cells . This stability is essential for long-term reliability and performance. The low resistivity of Mo thin films makes them desirable for integrated circuits, where they contribute to the efficient flow of electrical current . Furthermore, their optical properties make them well suited for a use as a protective coating in energy storage and electronic devices . Mo films deposited on substrates under suitable conditions lead to improvements in functionality and address the needs of various cutting-edge industries .

For the deposition of Mo thin films, various techniques such as chemical vapor deposition, physical vapor deposition (RF sputtering and DC sputtering) , and electron beam evaporation have been reported in the literature. RF sputtering is the predominant technique for thin film deposition because of its benefits regarding layer adhesion, uniformity, composition, and deposition rate compared to other methods . In the deposition of molybdenum films, RF sputtering allows for precise control of film thickness, shape, and stoichiometry, making it a key method for preparing films with specific characteristics .

Numerous approaches exist to improve the performance of thin films, including ion implantation techniques that enable precise alteration of material characteristics. Ion implantation is one of the most attractive techniques because it introduces considerable changes in the surface morphology and composition of the films . The uses of implanted Mo thin films cover a broad range of applications including microelectronics and optoelectronics. One major use area is in microelectromechanical systems, where these films are used in parts such as microsensors, actuators, and microfluidic devices .

In this regard, the effects of different ion beams on the characteristics of Mo thin films have been explored . Ahmed et al. investigated the effect of helium ion irradiation on the structural and electrical properties of Mo thin films. They noted that α-particles create defects that reduce charge carrier mobility, and the hardness increased from low to high ion fluence. Hoffman et al. investigated the effects of argon ion bombardment on the characteristics of Mo thin films. Films with reduced porosity and larger grains demonstrated increased reflectance and decreased resistivity, consistent with electron mean free path analysis results. Navin et al. investigated self-organized pattern generation in Mo thin films with a low-energy argon ion beam (1 keV) across different ion fluences (1016–1018 ions·cm−2). Thornton et al. examined a transition from tensile to compressive stress in argon-ion-implanted Mo thin films as the sputtering gas pressure decreased. Sun et al. also analyzed the properties of argon-ion-implanted Mo thin films deposited via ion beam sputtering, varying deposition parameters such as accelerating voltage, incidence angle, and chamber pressure. Films deposited at near-normal incidence exhibited compressive stress and a nearly linear increase with the accelerating voltage. At grazing incidence, the observed stress is either minimal or slightly tensile and is mostly unaffected by the accelerating voltage. Tripathi et al. examined the temperature-dependent surface alterations in Mo films induced by He+ ion irradiation within the 773–1073 K range as a prospective substitute for tungsten in plasma-facing components of fusion devices. Klaver et al. investigated the impact of irradiation with low-energy helium ions on the physical properties of molybdenum thin films. Ono et al. studied the degradation of the optical characteristics of single- and polycrystalline Mo mirrors for plasma diagnostics when treated with low-energy He+ ion irradiation at ambient temperature and 400 °C. Takamura et al. examined the effects of He plasma irradiation on Mo thin films. The temperature range for nanostructure growth was within a temperature range of 800 to 1050 K, under incident helium ion energies of 50 to 100 eV. Nitrogen gas offers a fascinating opportunity for ion implantation in Mo thin films because of its high reactivity . The incorporation of nitrogen ions alters the characteristics of Mo thin films, potentially improving their performance in a wide range of applications . Kim et al. examined the impact of a 3 × 1017 N2+·cm−2 ion fluence on the structural characteristics, surface morphology, and thermal stability of Mo thin films. The internal stress of these films transitioned from strongly compressive to weakly tensile after annealing. Lee et al. efficiently incorporated nitrogen ions into epitaxial Mo films, forming a buried superconducting layer. They deposited atomically flat epitaxial Mo(110) films on Al2O3(0001) substrates. Carreri et al. investigated high-temperature (800–1200 °C) plasma-based nitrogen ion implantation on molybdenum thin films. They also examined the phase development and tribological alterations caused by ion implantation. Furthermore, Nakano et al. investigated the deterioration of optical characteristics in polycrystalline Mo mirrors exposed to irradiation with helium or deuterium ions. With increasing fluence and energy of the ions, a greater extent of deterioration was observed in helium-irradiated specimens than in deuterium-irradiated specimens. Mändl et al. examined the impact of nitrogen ion implantation on Mo, focusing on nitride phase formation and nitrogen diffusion behavior within a temperature range of 330 to 580 °C. They observed the formation of a new cubic Mo2N phase. In addition, they also examined the impact of high ion fluence and temperature on nitrogen implantation in molybdenum with supplementary heating within the temperature range of 500 to 750 °C.

It is well documented that ion beam implantation, including helium, nitrogen, and argon ions, is a versatile technique for tuning the properties of molybdenum thin films. Researchers examined the effects of ion fluence, implantation temperature, and ion energy on the characteristics of Mo thin films. Despite these advances, a noticeable gap exists in the systematic exploration of nitrogen implantation with variations in the thickness of Mo films. To address this gap, this study employs an extensive approach, implementing low-energy nitrogen ion implantation with a systematic variation in film thickness and investigating the effects of these factors on the characteristics of molybdenum thin films. This study promotes the development of next-generation Mo thin films with precisely tuned characteristics. The findings of this research contribute to the optimization and development of Mo thin films for various applications in photovoltaics, sensors, optoelectronics, and electronic devices.

In the present work, Mo thin films with varying thickness of 150, 200, 250, and 300 nm were deposited on Si(100) substrates using radio frequency (RF) sputtering in an argon environment at ambient temperature. Some films with different thicknesses were implanted with 1 × 1017 N2+·cm−2 at 30 keV using a current density of 4 µA·cm−2. This study illustrates the effects of nitrogen ion implantation and film thickness on the structural, optical, and electrical properties of thin molybdenum films.

In this study, molybdenum thin films of varying thickness were deposited at room temperature on Si(100) substrates via RF sputtering using a pure 2″ diameter Mo target (99.99% purity) in Ar gas atmosphere with a flow rate of 10 sccm. The plasma was obtained by setting the RF power to 100 W, while careful minimizing the reflected power. Before deposition, the target surface was pre-sputtered for 15 min to remove any surface contamination. The silicon substrates were meticulously cleaned by washing them with distilled water and isopropyl alcohol and rinsing them with acetone. The vacuum chamber was evacuated to a base pressure of 2.0 × 10−3 mbar, with a working pressure during deposition of approximately 1.2 × 10−2 mbar; deposition times varied from 7 to 12 min, resulting in films with thicknesses from 150 to 300 nm, as measured by spectroscopic ellipsometry. After deposition, some films with different thicknesses were implanted with 1 × 1017 N2+·cm−2 at 30 keV using a current density of 4 µA·cm−2.

The structural properties of the deposited Mo thin films were investigated using a GXRD Bruker AXS GmbH D8 Advance X-ray diffractometer in grazing incidence geometry, employing Cu Kα radiation with a wavelength of 1.5405 Å. Measurements were conducted with a fixed incident angle of 0.5°, and the X-ray tube was operated at 40 kV and 40 mA. The surface morphology was analyzed using a Bruker Multimode-8 atomic force microscopy (AFM). The optical characteristics of the molybdenum thin films were analyzed using a SENresearch 4.0 spectroscopic ellipsometer across a wide spectral range of 200–800 nm. The electrical properties of the films were evaluated using a Keithley 4200 A-SCS parametric analyzer equipped with two probes. All these analytical instruments are available at the Ion Beam Centre, Kurukshetra University, Kurukshetra, ensuring comprehensive and accurate characterization of the properties of Mo thin films.

Figure 1: (A) Nitrogen ion trajectories, (B) distribution of ions, (C) distribution of losses due to ionization, and (D) vacancy distribution in a Mo thin film.

Figure 2: GXRD patterns of as-deposited and N2+-implanted Mo thin films. (A) 150 nm, (B) 200 nm, (C) 250 nm, and (D) 300 nm.

where k is a constant (k = 0.94) , λ is the X-ray wavelength (λ = 1.5406 Å), θ is the Bragg angle, and β is the FWHM.

Additionally, the microstrain (ε) developed in the thin films due to lattice distortions or mismatch was calculated using Wilson’s equation :

The dislocation density (δ) gives more information about the number of defects in the films; it was calculated from the relation :

where D is the average crystallite size.

The interplanar spacing (d) and lattice constant (a) were calculated using the relations :

Table 1: Structural parameters of as-deposited and implanted Mo thin films.

Figure 3: GXRD pattern of the (111) diffraction peak of as-deposited and N2+-implanted Mo thin films (a, a′) 150 nm, (b, b′) 200 nm, (c, c′) 250 nm, and (d, d′) 300 nm.

Figure 4: Stress in (A) as-deposited and (B) N2+-implanted Mo thin films as a function of the thickness.

Table 2: Lattice constants and texture coefficients of as-deposited and implanted Mo thin films.

The texture coefficients of as-deposited and N2+-implanted Mo thin films demonstrate a strong orientation preference in the (111) direction. The TC111 values of these films are significantly larger than those for other orientations. Also, the texture coefficient for the (200) plane, though less than that for the (111) plane, is larger than those for the (220) and (311) planes. These observations reveal that the plane (111) is the dominant crystalline orientation in both as-deposited and nitrogen-implanted Mo thin films. The (111) plane in face-centered cubic structures, such as Mo, typically has the lowest surface energy . Therefore, atoms tend to arrange themselves to minimize the overall energy, leading to a predominant (111) orientation. This tendency is even more pronounced for as-deposited films, where the absence of any external influence would have allowed for the natural minimization of surface energy. Nitrogen ion implantation could change the crystalline orientation through defects and stress introduced in the Mo thin films. The residual stress due to the implantation may also be an influential factor regarding the preferred orientation. If the applied stress promotes the (111) orientation, this plane will be more pronounced in the resultant thin film .

Additionally, the texture coefficients increase with the thickness for both as-deposited and implanted Mo thin films. Thicker films (i.e., 250 and 300 nm) generally exhibit stronger orientation preferences because of the increased volume for developing certain planes. In contrast, thinner films (i.e., 150 and 200 nm) exhibit a specific orientation preference owing to the restricted volume, which restricts the formation of certain planes . Furthermore, the interaction between the substrate and the thin film can generate stress and strain, affecting the growth of specific planes. If the substrate promotes the (111) orientation through lattice compatibility, the resultant Mo thin film exhibits a pronounced (111) orientation.

Crystallite size and texture coefficients of these Mo thin films significantly increase with the thickness of Mo thin films. Moreover, a significant reduction has been observed in both these parameters after nitrogen implantation compared to equivalently thick as-deposited films. The increase in crystallite size with greater film thickness directly signifies enhanced crystallinity, observable by the formation of larger crystallites in thicker films. Additionally, the texture coefficient rises, signifying that crystallites attain a more uniform orientation in the film.

Figure 5: 2D and 3D AFM images of Mo thin films: as-deposited (a) 150 nm, (b) 200 nm, (c) 250 nm, and (d) 300 nm, and implanted (a′) 150 nm, (b′) 200 nm, (c′) 250 nm, and (d′) 300 nm. The fast Fourier transform images are insets in the 2D AFM micrographs.

Table 3: Values of RMS surface roughness and particle size of as-deposited and implanted Mo thin films.

This increase in roughness and particle size is related to structural alterations due to the increasing film thickness. At the lowest thickness, a greater number of particles are found to be small . As the thickness of Mo thin films increases, smaller particles coalesce, which increases the surface roughness. The increase in roughness and particle size of implanted films results from structural modifications caused by N2+ implantation. AFM analysis reveals that N2+ implantation and thickness of films significantly influence the surface morphology of Mo thin films.