Enhanced Electrical Properties of Copper Nitride Films Deposited via High Power Impulse Magnetron Sputtering

High Power Impulse Magnetron Sputtering (HiPIMS) has generated a great deal of interest by offering significant advantages such as high target ionization rate, high plasma density, and the smooth surface of the sputtered films. This study discusses the deposition of copper nitride thin films via HiPIMS at different deposition pressures and then examines the impact of the deposition pressure on the structural and electrical properties of Cu3N films. At low deposition pressure, Cu-rich Cu3N films were obtained, which results in the n-type semiconductor behavior of the films. When the deposition pressure is increased to above 15 mtorr, Cu3N phase forms, leading to a change in the conductivity type of the film from n-type to p-type. According to our analysis, the Cu3N film deposited at 15 mtorr shows p-type conduction with the lowest resistivity of 0.024 Ω·cm and the highest carrier concentration of 1.43 × 1020 cm−3. Furthermore, compared to the properties of Cu3N films deposited via conventional direct current magnetron sputtering (DCMS), the films deposited via HiPIMS show better conductivity due to the higher ionization rate of HiPIMS. These results enhance the potential of Cu3N films’ use in smart futuristic devices such as photodetection, photovoltaic absorbers, lithium-ion batteries, etc.


Introduction
Transition metal nitrides (TMNs) have been proved to be significant materials with a myriad of uses that hugely impact our daily lives. TMNs are highly advantageous in that they offer good hardness [1] and high temperature stability [2], resulting in them being highly recommended as a coating to protect mechanical tools [3]. Interestingly, many of these materials possess rock-salt structures [4], due to which they can either be metallic (like TiN [5]) or semiconductors (like ScN [6], YN [7] etc.). Among various TMNs with semiconductor properties, researchers in a quest for new kinds of materials possessing remarkable optoelectronic performance have put their key focus on Copper Nitride (Cu 3 N). Cu 3 N in crystalline form has an unambiguous cubic anti-ReO 3 structure (a = 0.38 nm and α = β = γ = 90 • ) [8]. Optical band gap is temperature-dependent, lying within the range of 1.2 eV and 1.9 eV [9], and presenting less reflectivity [10] and high transparency in the IR region [11]. At room temperature, Cu 3 N with a lattice constant lower than 38 Å possesses huge electrical resistivity [10]. Thanks to abundant resource availability, cheap manufacturing costs, non-toxicity, low deposition temperature and also being highly adaptable to several substrates [12,13], Cu 3 N can be applied in numerous The composition of Cu3N thin films produced via HiPIMS on silicon substrates at various deposition pressures is shown in Figure 2. Cu content reduces with increasing deposition pressure, while N content rises in the films. When the deposition pressure rises from 5 mtorr to 15 mtorr, Cu and N content suddenly changes. This is due to the fact The composition of Cu 3 N thin films produced via HiPIMS on silicon substrates at various deposition pressures is shown in Figure 2. Cu content reduces with increasing deposition pressure, while N content rises in the films. When the deposition pressure rises from 5 mtorr to 15 mtorr, Cu and N content suddenly changes. This is due to the fact that when the deposition pressure initially increases, more nitrogen molecules or atoms in the chamber react with copper atoms (ions), therefore the N content in the thin film increases. As the deposition pressure exceeds 15 mtorr, the variation in Cu and N content tends to moderate. Presumably, the composition of Cu 3 N film has reached saturation. Likewise, the deposition rate of the Cu 3 N thin film linearly declines with the increase in the deposition pressure. That is mainly due to the fact that as the amount of nitrogen and argon within the chamber rises, additional collisions among the particles will shorten their mean free path, which affects the deposition rate [21]. The composition of Cu3N thin films produced via HiPIMS on silicon substrates at various deposition pressures is shown in Figure 2. Cu content reduces with increasing deposition pressure, while N content rises in the films. When the deposition pressure rises from 5 mtorr to 15 mtorr, Cu and N content suddenly changes. This is due to the fact that when the deposition pressure initially increases, more nitrogen molecules or atoms in the chamber react with copper atoms (ions), therefore the N content in the thin film increases. As the deposition pressure exceeds 15 mtorr, the variation in Cu and N content tends to moderate. Presumably, the composition of Cu3N film has reached saturation. Likewise, the deposition rate of the Cu3N thin film linearly declines with the increase in the deposition pressure. That is mainly due to the fact that as the amount of nitrogen and argon within the chamber rises, additional collisions among the particles will shorten their mean free path, which affects the deposition rate [21].  The X-ray diffractograms of Cu 3 N thin films ( Figure 3) deposited on glass substrates at different deposition pressures mainly show diffraction peaks of the crystal planes of (100), (110), (111), (200), (210), and (220). The peaks corresponding to Cu 3 N phase are associated to JCPDS: 86-2283. The two most intense XRD peaks of Cu 3 N thin films are (111) and (100) related to copper and nitrogen content; usually considered as copper-rich and nitrogen-rich thin films, respectively [14]. Absence of impurity peaks in XRD results (like Cu) indicates the successful Cu nitration that helps in depositing Cu 3 N thin film with a high degree of purity. A diffraction peak of the Cu 3 N thin film deposited at a deposition pressure of 5 mtorr appears between the Cu (111) peak (JCPDF: 70-3038) and the Cu 3 N (111) peak (JCPDF: 86-2283). With the further increase in the deposition pressure, the diffraction peak (111) continues to move to lower Bragg angles until the deposition pressure exceeds 15 mtorr, which is completely matched with Cu 3 N, implying the development of pure Cu 3 N thin films. Although the (100) diffraction peak that produces the N-rich (100) plane does not appear as a result of insufficient nitrogen. Furthermore, there is a strong dominance of the (111) plane over the (100) plane in Cu 3 N at 10 mtorr, although in actuality, a small amount of the Cu 3 N (100) plane still exists, which is difficult to analyze by XRD. Additionally, the Cu 3 N (111) peak intensity declines with the rise in the deposition pressure. This may be the result of the decline in the copper content of the film (as observed in EPMA). It can be seen from some reports [37,39,40] that when the Cu content decrease, it will make it difficult to generate a Cu-rich (111) peak, thus resulting in a decrease in the intensity of the (111) peak.
although in actuality, a small amount of the Cu3N (100) plane still exists, which is difficult to analyze by XRD. Additionally, the Cu3N (111) peak intensity declines with the rise in the deposition pressure. This may be the result of the decline in the copper content of the film (as observed in EPMA). It can be seen from some reports [37,39,40] that when the Cu content decrease, it will make it difficult to generate a Cu-rich (111) peak, thus resulting in a decrease in the intensity of the (111) peak.  The microstructure of the Cu 3 N nanocrystal on glass substrates was determined through HRTEM. The TEM sample was prepared through focused ion beam (FIB) for structural analysis. The HRTEM images of the cross-section of Cu 3 N thin films deposited at different deposition pressure were analyzed using Gatan Digital Micrograph software ( Figure 4). After inverse Fourier transformation, the interplanar d-spacing value of the Cu 3 N thin-film lattice was calculated. Under low deposition pressure (10 mtorr) (Figure 4a), the film appears partially amorphous, with the d-spacing value as 2.114 ± 0.04 Å of the (111) plane. As the deposition pressure increased to 15 mtorr and 20 mtorr (Figure 4b,c) the d-spacing values of the (111) plane rise to 2.254 ± 0.04 Å and 2.255 ± 0.04 Å, respectively. Unlike the (111) plane, the d-spacing of the (100) plane in Figure 4b,c have similar value as 3.868 ± 0.04 Å and 3.854 ± 0.04 Å, respectively. From the XRD pattern ( Figure 3), it is clear to see that when the deposition pressure rises from 10 mtorr to 20 mtorr, the Cu 3 N (111) diffraction peak progressively shifts to a lower angle, which means that the lattice constant gradually increases. This further confirms that the value of d-spacing increases with the increasing deposition pressure.
Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 12 mtorr, the Cu3N (111) diffraction peak progressively shifts to a lower angle, which means that the lattice constant gradually increases. This further confirms that the value of d-spacing increases with the increasing deposition pressure. The surface roughness of the deposited thin film plays an essential role in determining the film's quality. This characteristic of Cu3N thin films on glass substrates (over an area of 1 µm × 1 µ m) prepared by varying the deposition pressure was analyzed using atomic force microscopy (AFM, Figure 5). A significant reduction in the surface roughness of the Cu3N films with the increasing deposition pressure was observed. The value of the lowest surface roughness is Ra = 0.78 nm obtained at 25 mtorr. This corroborates that the smoothness of the films was improved with the increase in deposition pressure. When the deposition pressure increases, more Ar + ions strike the copper target resulting in the high energy of the ion bombardment on the substrate due to the high peak power density of HiPIMS, which results in a close arrangement of the incident atoms in Cu3N The surface roughness of the deposited thin film plays an essential role in determining the film's quality. This characteristic of Cu 3 N thin films on glass substrates (over an area of 1 µm × 1 µm) prepared by varying the deposition pressure was analyzed using atomic force microscopy (AFM, Figure 5). A significant reduction in the surface roughness of the Cu 3 N films with the increasing deposition pressure was observed. The value of the lowest surface roughness is Ra = 0.78 nm obtained at 25 mtorr. This corroborates that the Nanomaterials 2022, 12, 2814 6 of 11 smoothness of the films was improved with the increase in deposition pressure. When the deposition pressure increases, more Ar + ions strike the copper target resulting in the high energy of the ion bombardment on the substrate due to the high peak power density of HiPIMS, which results in a close arrangement of the incident atoms in Cu 3 N films. In this way, the thin film with high deposition pressure achieved a smooth surface. In addition, the surface quality of the Cu 3 N film prepared via DCMS at 15 mtorr was also analyzed by AFM (as shown Figure S2). Its surface roughness (1 µm × 1 µm) Ra value of about 1.96 nm is higher than the roughness of the film deposited via HiPIMS (with Ra value of about 1.06 nm) (Figure 5c). During deposition using HiPIMS, the power density is much higher than that in the DCMS deposition process, as shown in (Figure 1c). As a result, more kinetic energy can be transferred from the bombarding Ar + ions to the target atoms, which have Cu ions combined with nitrogen to the substrate, and result in a high-density film with a smooth surface. Therefore, the thin films via DCMS possessing low ionization have relatively rough surface. The optical properties of Cu3N thin films deposited via HiPIMS on glass substrates were analyzed by UV-Vis spectroscopy (Figure 6a,b). The Cu3N thin films show high absorbance in the UV range (300-400 nm). As the wavelength increases, it starts to decrease gradually in the visible range from 400 nm to 700 nm. Beyond the visible region, it shows a low absorbance in the near IR range (>700 nm). Correspondingly, the Cu3N thin films show high transmittance in the IR range (700-900 nm), and as the wavelength decreases, it starts to decline gradually in the visible range from 700 nm to 450 nm wavelength. Below this region, it shows low transmittance in the near UV range (<400 nm). This behavior seems to be very common in the deposited Cu3N thin films. The deposition pressure drastically affects both the absorption coefficient and the transmittance of Cu3N films. When the deposition pressure changes, a significant variation in the absorption coefficient and the transmittance with the wavelength can be observed. Upon increasing the deposition pressure from 5 mtorr to 15 mtorr, there is an enormous decrease in the absorption coefficient and an impressive increase in transmittance. This drastic change is attributable to the significant decline in the Cu content in the films. Beyond 15 mtorr, the reduction in absorption coefficient and the transmittance seems to be weakened as a result of little variation in the film's composition. The optical bandgap (Eg) of the Cu3N can be calculated using the absorption coefficient. The Tauc method assumes that the energy-dependent absorption coefficient α can be expressed by the following equation: The optical properties of Cu 3 N thin films deposited via HiPIMS on glass substrates were analyzed by UV-Vis spectroscopy (Figure 6a,b). The Cu 3 N thin films show high absorbance in the UV range (300-400 nm). As the wavelength increases, it starts to decrease gradually in the visible range from 400 nm to 700 nm. Beyond the visible region, it shows a low absorbance in the near IR range (>700 nm). Correspondingly, the Cu 3 N thin films show high transmittance in the IR range (700-900 nm), and as the wavelength decreases, it starts to decline gradually in the visible range from 700 nm to 450 nm wavelength.
Below this region, it shows low transmittance in the near UV range (<400 nm). This behavior seems to be very common in the deposited Cu 3 N thin films. The deposition pressure drastically affects both the absorption coefficient and the transmittance of Cu 3 N films. When the deposition pressure changes, a significant variation in the absorption coefficient and the transmittance with the wavelength can be observed. Upon increasing the deposition pressure from 5 mtorr to 15 mtorr, there is an enormous decrease in the absorption coefficient and an impressive increase in transmittance. This drastic change is attributable to the significant decline in the Cu content in the films. Beyond 15 mtorr, the reduction in absorption coefficient and the transmittance seems to be weakened as a result of little variation in the film's composition. The optical bandgap (E g ) of the Cu 3 N Nanomaterials 2022, 12, 2814 7 of 11 can be calculated using the absorption coefficient. The Tauc method assumes that the energy-dependent absorption coefficient α can be expressed by the following equation: where α is the absorption coefficient, h is the Planck constant, ν is the photon frequency, E g is the bandgap, and A is a constant. The power of the x-factor depends on the nature of the electron transition, i.e., equal to 1/2 or 2 for the direct and indirect transition bandgap, respectively (Cu 3 N has the indirect transition bandgap, where x-factor is 2). Tauc plot of Cu 3 N representing the SI fitting as shown in Figure S3. Here, it is observed that with the rise in deposition pressure from 15 mtorr to 25 mtorr, the optical bandgap of p-type Cu 3 N also increases (Figure 6c). Hall measurement was used to examine the electrical properties of the films deposited on glass substrates at various deposition pressure (Figure 7), while conductivity type was confirmed through hot probe measurement. Generally, the electrical properties depend upon the composition, phase structure and vacancy defects amount. By increasing the deposition pressure, the amount of nitrogen increases inside the reaction chamber. In low deposition pressure, the amount of nitrogen is inadequate to form Cu3N phase, which results in Cu-rich n-type Cu3Nx film. At 5 mtorr and 10 mtorr, the carrier concentration of n-type Cu3N films is found to be higher than 2.25 × 10 22 cm −3 and 1.05 × 10 22 cm −3 , respectively. Their n-type conductivity occurs from the excessive electrons within Cu-rich films with low resistivity of 8.67 × 10 −5 Ω·cm and 1.42 × 10 −3 Ω·cm, respectively. Meanwhile, the mobility decreases from 3.2 to 0.42 cm 2 V −1 s −1 with the increase in deposition pressure from 5 mtorr to 10 mtorr. Upon further increasing the deposition pressure, the N2 turns to be sufficient to form p-type Cu3N film by producing Cu + vacancies. This conversion of the electrical conductivity type was observed by hot probe measurement at the deposition pressure of 15 mtorr, where the film's resistivity and carrier concentration are 0.024 Ω·cm and 1.43 × 10 20 cm −3 , respectively. With increasing the deposition pressure, the resistivity increases and the carrier concentration decreases gradually. At the same time, the mobility increases from 1.79 to 7.54 cm 2 V −1 s −1 . After the deposition pressure exceeds 20 mtorr, the resistivity becomes very high to the point where it exceeds the measuring limit of the machine. This variation in the behavior of film's electrical performance is consistent with the structural change of the films as observed from XRD data. The diffraction peak intensity becomes relatively weak as the deposition pressure increases to 20 mtorr and 25 mtorr, which is due to the change in the composition of Cu3N films. In addition, the electrical properties of Cu3N films deposited via HiPIMS were Hall measurement was used to examine the electrical properties of the films deposited on glass substrates at various deposition pressure (Figure 7), while conductivity type was confirmed through hot probe measurement. Generally, the electrical properties depend upon the composition, phase structure and vacancy defects amount. By increasing the deposition pressure, the amount of nitrogen increases inside the reaction chamber. In low deposition pressure, the amount of nitrogen is inadequate to form Cu 3 N phase, which results in Cu-rich n-type Cu 3 N x film. At 5 mtorr and 10 mtorr, the carrier concentration of n-type Cu 3 N films is found to be higher than 2.25 × 10 22 cm −3 and 1.05 × 10 22 cm −3 , respectively. Their n-type conductivity occurs from the excessive electrons within Cu-rich films with low resistivity of 8.67 × 10 −5 Ω·cm and 1.42 × 10 −3 Ω·cm, respectively. Meanwhile, the mobility decreases from 3.2 to 0.42 cm 2 V −1 s −1 with the increase in deposition pressure from 5 mtorr to 10 mtorr. Upon further increasing the deposition pressure, the N 2 turns to be sufficient to form p-type Cu 3 N film by producing Cu + vacancies. This conversion of the electrical conductivity type was observed by hot probe measurement at the deposition pressure of 15 mtorr, where the film's resistivity and carrier concentration are 0.024 Ω·cm and 1.43 × 10 20 cm −3 , respectively. With increasing the deposition pressure, the resistivity increases and the carrier concentration decreases gradually. At the same time, the mobility increases from 1.79 to 7.54 cm 2 V −1 s −1 . After the deposition pressure exceeds 20 mtorr, the resistivity becomes very high to the point where it exceeds the measuring limit of the machine. This variation in the behavior of film's electrical performance is consistent with the structural change of the films as observed from XRD data. The diffraction peak intensity becomes relatively weak as the deposition pressure increases to 20 mtorr and 25 mtorr, which is due to the change in the composition of Cu 3 N films. In addition, the electrical properties of Cu 3 N films deposited via HiPIMS were compared with those produced by other conventional methods, such as DCMS and RF sputtering (Table 1). Since p-type Cu 3 N film deposited via HiPIMS at 15 mtorr presents better electrical properties, as it possesses the lowest resistivity resulting from the high ionization rate offered by HiPIMS, we used the same parameters to deposit Cu 3 N film through DCMS. The results show that p-type Cu 3 N can be achieved, but its resistivity is above 18.82 Ω·cm (Table 1), which is excessive compared to that of the film deposited via HiPIMS.  The comparative investigation of the chemical bonding state in Cu3N thin films was also performed by X-ray photoelectron spectroscopy (XPS) to confirm the change in Cu 2+ content within Cu3N thin films fabricated via both DCMS and HiPIMS technology on silicon substrates (Figure 8). The photoelectron emission from the Cu-2p spectra of Cu3N thin films was analyzed. The fitting curve of Cu-2p spectra was compared in Figure 8a,b. The Cu 2+ /Cu + ratio of the film deposited via HiPIMS (0.62) is higher than that of the film  The comparative investigation of the chemical bonding state in Cu 3 N thin films was also performed by X-ray photoelectron spectroscopy (XPS) to confirm the change in Cu 2+ content within Cu 3 N thin films fabricated via both DCMS and HiPIMS technology on silicon substrates (Figure 8). The photoelectron emission from the Cu-2p spectra of Cu 3 N thin films was analyzed. The fitting curve of Cu-2p spectra was compared in Figure 8a,b. The Cu 2+ /Cu + ratio of the film deposited via HiPIMS (0.62) is higher than that of the film deposited via DCMS (0.41), which indicates that a relatively high proportion of Cu 2+ ions exist in the Cu 3 N thin films deposited via HiPIMS.

Discussion
Based on the conductivity type mechanism of NiO thin films, holes get generated by replacing two Ni 2+ cations with two Ni 3+ cations, which result in Ni 2+ vacancies [48,49]. This theory suits the best as proof to describe the conductivity type transition mechanism of Cu3N (as shown in Figure S4). The above findings confirm that the replacement of Cu + ions by Cu 2+ ions are more evident in HiPIMS than that in DCMS, which results in more Cu + vacancies during the deposition using HiPIMS process. It could contribute to higher p-type carrier concentration in the films deposited via HiPIMS technology, which helps in reducing the electrical resistivity of the films as compared to DCMS. These results clearly show the potential of Cu3N films deposited through HiPIMS technology to be applied in future optoelectronic devices. This technique helps in obtaining a flatter interface when the multilayer films are coated, which can improve the device performance [50].

Conclusions
Cu3N thin films were deposited via HiPIMS technology at different deposition pressures, while peak power density and the mixture of argon and nitrogen reactive gas remained constant. At lower deposition pressure, the existence of Cu-rich phase enhances the film's n-type conductivity. Upon increasing the deposition pressure, the deposition rate and surface roughness decrease while nitrogen content in the film increases with the reduction in the copper content in the film. At the higher deposition pressures, the existence of a Cu3N phase is confirmed to increase the film's transmittance and its optical bandgap, while also enhancing the resistivity of the Cu3N film. In addition, the study compares the chemical bonding state, quality, and electrical properties of Cu3N deposited through HiPIMS and DCMS methods. Beneficial due to its higher ionization rate and peak power density, HiPIMS offers lower roughness of the films and a higher Cu 2+ /Cu + ratio, which results in more Cu + vacancies. This indicates that HiPIMS technology is an ideal technology to achieve better electrical properties of Cu3N films in comparison to the transitional DCMS methods.

Supplementary Materials:
The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: The instantaneous voltage and current during the (a) HiPIMS and (b) DCMS process are represented by CH1 and CH2, respectively.; Figure S2: AFM images showing the surface (1 × 1 µ m 2 ) of Cu3N films deposited by DCMS at 15 mtorr.; Figure S3: Tauc plot of Cu3N representing the SI fitting.; Figure S4: The p-type conductivity mechanism of Cu3N.

Discussion
Based on the conductivity type mechanism of NiO thin films, holes get generated by replacing two Ni 2+ cations with two Ni 3+ cations, which result in Ni 2+ vacancies [48,49]. This theory suits the best as proof to describe the conductivity type transition mechanism of Cu 3 N (as shown in Figure S4). The above findings confirm that the replacement of Cu + ions by Cu 2+ ions are more evident in HiPIMS than that in DCMS, which results in more Cu + vacancies during the deposition using HiPIMS process. It could contribute to higher p-type carrier concentration in the films deposited via HiPIMS technology, which helps in reducing the electrical resistivity of the films as compared to DCMS. These results clearly show the potential of Cu 3 N films deposited through HiPIMS technology to be applied in future optoelectronic devices. This technique helps in obtaining a flatter interface when the multilayer films are coated, which can improve the device performance [50].

Conclusions
Cu 3 N thin films were deposited via HiPIMS technology at different deposition pressures, while peak power density and the mixture of argon and nitrogen reactive gas remained constant. At lower deposition pressure, the existence of Cu-rich phase enhances the film's n-type conductivity. Upon increasing the deposition pressure, the deposition rate and surface roughness decrease while nitrogen content in the film increases with the reduction in the copper content in the film. At the higher deposition pressures, the existence of a Cu 3 N phase is confirmed to increase the film's transmittance and its optical bandgap, while also enhancing the resistivity of the Cu 3 N film. In addition, the study compares the chemical bonding state, quality, and electrical properties of Cu 3 N deposited through HiPIMS and DCMS methods. Beneficial due to its higher ionization rate and peak power density, HiPIMS offers lower roughness of the films and a higher Cu 2+ /Cu + ratio, which results in more Cu + vacancies. This indicates that HiPIMS technology is an ideal technology to achieve better electrical properties of Cu 3 N films in comparison to the transitional DCMS methods.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano12162814/s1, Figure S1: The instantaneous voltage and current during the (a) HiPIMS and (b) DCMS process are represented by CH1 and CH2, respectively.; Figure S2: AFM images showing the surface (1 × 1 µm 2 ) of Cu 3 N films deposited by DCMS at 15 mtorr.; Figure S3: Tauc plot of Cu 3 N representing the SI fitting.; Figure S4: The p-type conductivity mechanism of Cu 3 N.