Tantalum/ Nitrogen and n-type WO3 semiconductor/FTO structures as a cathode for the future of nano devices

INTRODUCTION

Unspecified nanoelectronic devices such as OFET (Organic filed effect transistors), CMOS (Complementary metal oxide semiconductor) transistors, and OLED (organic light emission diode) devices need an electrode amidst the high-grade connection point at in especially carrier’s flux tube or channel [1-3]. The other devices, here, the cathode deposited on fluorine-doped tin oxide (FTO) coated glass substrate by PVD (Physical vapor deposition) method, needs more trap centers as well as a cathode for electrochromic (EC) and nanodevices [4-8].

It is completely apprehended that the nitrogen ion implantation can improve the surface properties of metals by converting their structures and compositions in the surface coatings. Because of this understanding, the nitrogen ion implantation has been investigated for many years, and several nitrogen-metal systems have been studied in classification to clarify the composition ion mechanisms of multiple metal nitrides and their influences on surface properties. The impressions of nitrogen ion implantation (NII) on nanostructural, electro-optical properties of various materials have been investigated [9-11]. The degree of surface modification by NII depends on the ion energy, current density, irradiation time and the substrate temperature. Metal nitride coatings have different physical characteristics like melting points and exclusive electrical stability. In this regard, ion implantation is one of the prospective ways for changing chemical and physical properties in the near-surface part of metals by the formation of nitride phases [12–14].

Tantalum nitride (Ta/N) has many attractive characteristics, for instance, high melting point, hardness, chemical stability and thermal conductivity and low electrical resistivity, which make it possible to use as diffusion barriers for suitable cabling classifications in silicon-based integrated circuits (IC) [13-16].

Moreover, TaN can also be manipulated in a spacious variety of applications such as corrosion-resistant materials and high-speed thermal printing head as well as thin film resistors. While Tantalum nitride deposition procedure can be tuned to achieve a wide range of electrical properties variable the film stoichiometry [17-21], as one of the barriers for copper back-end-of-line processing, high resistivity and narrow process window continue to limit the usability of these films as thin film resistors in Silicon IC manufacturing. Belii et al [18] reported Ta₂Nand TaN formation by ion-implantation at a dose of 1×10¹⁷ions/cm², while at higher dose of 5 × 10¹⁷ion/cm², only FCC (Face Cubic Center) TaN was identified. They also found that, the electrical resistivity of the implanted layer increases with increasing ion-dose. Wang et al. [19] studied the structure and the composition of nitrogen implanted tantalum at 1-l0 keV with a dose of 5 × 10¹⁷ ion/cm². Zhou et al. [20] have reported the ion-dose dependence of the structure of the nitrogen implanted tantalum in the range of 5×10¹⁶ to 5×10¹⁷ (80 keV energy). Yadav et al. [22] reported the formation of Ta₂N and TaN_o.8 at all doses and TaN at higher doses (5×10¹⁷ ion/cm² to1×10¹⁸ ion/cm²).

Considering tantalum samples, a moderate bombarding dose (1×10 ¹⁷ -1×10¹⁸ ions/cm²) of medium-energy heavy-ions would be enough for amorphization without producing substantial topographical changes.

In the present work, the surface topography was analyzed with an Atomic Force Microscope (AFM). AFM images show nitrogen ion bombardment of tantalum surfaces induced topographical changes at normal incidence and standard doses. The structure phases of TaN and WO3 have analyzed with X-ray diffraction (XRD) technique. The energy gap of WO₃, determined with UV- visible tool, was also found. The obtained results indicate that Ta/N with more trap centers can be identified as a possible element of the subsequent ECD and nanodevices [23-27].

MATERIALS AND METHODS

The method of ion bombardment was made on samples of tantalum with sizes of 1cm×1cm and 0.58 mm thicknesses by the ion implantation equipment in the Plasma Physics Research Center (PPRC), and at the Science and Research Branch. The Ion bombardment procedure was implemented by nitrogen ions (99.999%) with the energy of 30 keV and doses of (1-10)×10 ¹⁷ ion/cm² at ambient temperature, while the angle between the implanted ions and surface of samples was 90°. Prior to ion implantation, the sample surfaces were finished in a manner that resulted in a glossy coating using diamond paste. They were then ultrasonically cleaned in alcohol and acetone. The next phase was to dry them in an oven with a 100 ⁰C temperature.

The extracted ions (without mass selection) were accelerated to the maximum energy of 30 keV. Ion beam energy and current densities were kept fixed for all the samples. The samples were implanted at doses of 1×10¹⁷ to 10×10¹⁷ ions/cm². It was expected that the ion beam cross-section uniformly cover all the sample area. The implantation parameters have been listed in Table 1. Before starting the ion implantation, the sample was at room temperature. However, this temperature altered during the implantation due to the heat transfer which came from ion bombardment to the samples.

Structural characters and the composition of the samples of tantalum were investigated with various methods. The nano- microstructure of the samples before and after ion implantation and WO₃ crystallite phases were characterized by X-ray diffraction (XRD) analysis. For this purpose, we used the STOE model STADI MP system using Cu k_α radiation (wavelength = 1.5405 Å ) with a tungsten filament at 40 kV, 40 mA and step size of 0.04 and shown in Fig. 1 and Fig. 2.

The implantation-induced modification of surface roughness and surface topography of WO₃ were studied employing atomic force microscopy (AFM). The facility was an AFM (SPM Auto Probe CP, Park Scientific Instruments, USA) in contact mode with a low-stress Tantalum nitride tip of less than 20 nm radiuses and a tip opening of 18.AFM analysis in contact mode. Resistivity values of tantalum sample were measured at room temperature using four-point probe technique.

Parallel to Ta/ N experiments, for constructing FTO/WO₃/Ag, Ta/WO₃/Ag electrochromic (EC) arrangement is deposited toward fluorine-doped tin oxide (FTO) coated glass substrate by physical vapor deposition (PVD) method. Current (Å), deposition time (second) and deposition rate (Å/Sec) of PVD is shown in Fig. 3. After passing 40 min of deposition time, the highest deposition rate (0.53 Å/Sec) and 60 min of deposition time, the maximum amount of electrical current (170 A) could be determined.

In this method, first the WO₃ nanoparticles powder with an average rate of 0.4 Ås^-1 are deposited in vacuum on FTO substrate for 80 minutes, and then the Ag nanoparticles powder is deposited on prior layer for 1 minute. In the beginning, WO₃ electrochromic layer with an average of 0.4 Ås^-1 is deposited on prior layers for 5 minutes. Finally, the layer of Ag nanoparticles is deposited for 1 minute with the rate of 0.1 Ås^-1. The fabricated electrochromic devices (ECDs) post annelid to investigate the effects of temperature on energy gap of WO₃. For this purpose, the temperatures of 100, 200 and 500 °C are selected, and ECDs annelid for two minutes in vacuum. The maximum of current was about 3.5 mA in oxidation state for this sample. Furthermore, the change of transmittance for this sample was upgraded to 40.30% at continuous switching steps. This arrangement (WO₃/Ag/WO₃/Ag) can be used in ECDs with its excellent properties.

RESULTS AND DISCUSSION

Phases, crystalline structure

Fig. 1 illustrates XRD results of both un-implanted and implanted samples of tantalum as a function of implantation dose. The spectra are limited to the specific region from 24 to 82˚for 2θ with four peaks concerning Ta (110),Ta (200),Ta (211) and Ta (200) at different angles of 2θ=38.66, 55.76, 69.82 and 82.50. Nevertheless, ions of high energy, those with doses of 3×10¹⁷ ions/cm² and 3×10¹⁷ ions/cm² show the TaN_0.43 diffraction peak location in the second and third samples. Aforementioned can be explained by the fact that nitrogen ions take interstitial positions in Tantalum to form Tantalum nitride. According to SRIM simulation, in low ion energy, electron stopping is dominant; nitrogen ions are not able to remove host atoms. In fact, the ions of nitrogen interact with electrons of the host atom in order to break the bonds between Tantalums.

It can be seen that all peaks have changed positions, while a few of them have broadened. Since constant parameters like temperature, ion density and energy have been applied in all experiments; the peaks’ presence depends on the production of conjunctions between Nitrogen and Tantalum due to the increase in the ion dose. The XRD patterns of samples before and after ion implantation have been listed in Table 2. Besides, two dominated peaks of WO₃ powder determined by using x- Powder software are also shown in Fig. 4.

Fig. 1 indicates that peak intensity and substrate broadness for implanted samples have been modified. This increase or decrease in peak intensity can be explained according to the implantation of nitrogen ions on tantalum substrate and the formation of a TaN new phase. It seems that during the process of the formation of phase structures in tantalum, nitrogen atoms get the interstitial positions. This does not result in the creation of sharp boundaries between the nitride and metallic phase structures.

Moreover, as it can be seen in Fig. 1, the diffraction peaks of the substrate are shifted at lower angles (2θ=37.53, 54.44, 68.14 and 80.59) that belong to the diffraction line of the TaN phase (refer to standard card No: #71- 0265). The broadening and shift of the peaks are both connected to the phase concerning TaN_0.43 (110) expanded austenite, which was per second produced by the super saturation of nitrogen, as well as the stress which was caused by nitrogen’s remaining in the solid solution of the lattice. Furthermore, another three additional peaks are observed by increasing nitrogen does from 1×10¹⁷ ions/cm² to 3×10¹⁷ ions/cm² which relate to the nitride phase. This fact confirmed the start of the nitride forming phase where the structures are amorphous in low dose. Also, increasing the dose further to 5×10¹⁷ ions/cm², the intensity of TaN_0.43 (201) and TaN_0.43 (300) diffraction peaks increases. This is while for doses 7×10¹⁷ and 10×10¹⁷ ions/cm², the diffraction peaks of TaN_0.43 with conversion to amorphous structures, are reduced. In addition, decreasing and increasing roughness in the dose of 1×10¹⁷ ions/cm² might happen by an increase in the values of surface dispersion of tantalum atoms on the sample’s surface. Moreover, by increasing the ion’s dose, the intensity of TaN_0.43 diffraction peaks changes. This phenomenon can be the result of structural defects of the surface sample caused by ion bombardment. Thus, variation in surface roughness results in a non-uniform corrosion of the samples. Noteworthy to mention is that when low density is considered, there are no changes in tantalum structure. Nevertheless, the angles have reduced, thus the space between the two plans increased, and nitrogen atoms have been placed among tantalum atoms. The rise in peak intensity is connected to the increase in nitrogen atoms as well as the likelihood of the existence of a bond between nitrogen and tantalum atoms.

Surface morphology, and roughness

The Surface roughness of the samples will undergo a substantial change after bombardment. As stated before, AFM is used for the investigation of surface topography as well as the roughness of Ta/N (Fig. 5) and WO₃ (Fig. 6) sample surface. As an example of DM- SPM software for measuring surface roughness is shown in Fig. 7 and the measurement data are invested in Tables 3 and 4. Fig. 8 also reveals roughness variation curve. Fig. 8 exhibits that by increasing the implantation dose, the topography and the size of grains on the samples change.

The surface roughness, as a statistical property of a surface, is described with DMSPM software image and data (See Fig. 7). There are many various parameters: The surface root-mean-squared roughness in nm² (S_a), Mean in nm (S_m), two orientation roughness values (S_y, S_z) which show y-z (the Peak-Valley Height) plane are also measured and Root Mean Square (S_q).These are calculated from the data according to the following formulas (1-3) [28, 29]:

 (1)

 (2)

 (3)

Table (3) summarizes the roughness parameters of WO3 nanoparticles obtained from DM- SPM software.

By comparing the values of WO₃ sample roughness parameters obtained from DMSPM software (data are given in Table (3)), and Ta/N with exposing to 10×10¹⁷ nitrogen ions per cm², show different surface topography.

Moreover, as can be seen from Table 4, by increasing the ion dose to 3×10¹⁷ and 10×10¹⁷ ions/cm², the surface roughness of the Tantalum is significantly raised. It seems that by entering nitrogen ions into the surface, the etching process has occurred.

On the other hand, surface diffusion processes, due to ion bombardment, increase the surface roughness at higher doses (10×10¹⁷ ions/cm²). Also, at the highest implantation dose (10×10¹⁷ ions/cm²), some grooving development effect dominates the diffusion and roughness enhances. Fig. 5 shows the root mean square (RMS) roughness and average roughness un-implanted and some of the implanted samples. It is clear from Fig. 8, that with increasing the dose from 1×10¹⁷ ions/cm² - 10×10¹⁷ ions/cm² with variations in surface roughness, the grains size formed on the surface is also changed. The result shows that the surface roughness increases with nitrogen ion doses. The AFM images show that there is an increase in the RMS roughness of the sample’s surface with an increasing dose. The surface diffusion mechanism, due to a greater number of ions in the implantation process, is responsible for this change.

Electrical resistivity

The square resistance (R) of all films was also measured using the four-point-probe method. Electrical resistivity (ρ) was obtained from the sheet resistance and film thickness from the following relation: ρ = R_S t Where ρ is resistivity (Ω.cm), and t is thickness from SEM (cm) [30- 40].

Fig. 9 shows the results of electrical resistivity measurements at room temperature that there is an improvement in the electrical resistivity of the sample’s surface with a growing dose. According to our laboratory results, a robust relationship between the resistivity and the RMS roughness can be perceived. According to Fig. 8 with increasing the surface roughness the resistivity values improvements. The nitrogen ion implantation causes the structural and compositional variations in the modified surface layer, and that is why the electrical resistivity can be developed. Table 4, shows the extent of improvement in resistivity with the increase of ion doses.

Indeed, the resistivity values for the tantalum acquired using ion implantation technique have been in a range of 182-272 mΩ.cm. It can be seen that the electrical resistivity for the tantalum which doses 10×10¹⁷ ion/cm², had a high value of 272 mΩcm. Similar behavior of the sheet resistance of 40 keV nitrogen implanted tantalum film on glass for doses ranging from 5 ×10^l6 to 3 × 1 0¹⁷ions/cm² has been reported elsewhere [30]. Fig. 9 shows the I-V curves obtained for unplanted and implanted samples. It can be recognized that linear I-V curves are achieved. The values of average resistance for all samples are presented in Table 4 (last column). The results show that average resistance increases with nitrogen ion dose.

This can be attributed to the transition of metal phase to nitride metal phase by nitrogen ion implantation method and increasing of nitrogen ion dose. Additionally, the energy gap of WO₃ samples with 200 and 300 nm in size, determined with UV- visible (Fig. 10) reveals that as annealing temperature increases, the energy gap reduced. It means energy gap of WO₃ at higher temperature, around 800- 900^oC could be so small and cannot be as n-type semiconductor, while Ta/N can stand even high temperature

CONCLUSION

XRD technique investigated tantalum nitride produced by nitrogen ion implantation in Tantalum with different fluxes Diversifying from 1×10¹⁷ into 1×10¹⁸ ions/cm², applied to obtain the structural characteristics, while the AFM technique was employed to concern the morphology of the surfaces of the samples. We can see from the above commentary that there is a structural correlation between different tantalum nitrides. XRD analysis confirms the successful establishment of tantalum nitride phase which is promoted by increasing ion implantation dose. Nitrogen ion implantations of the samples with different fluxes lead to get a functionalized surface with much more trap centers and as an exemplary cathode. The shift and broadening of the peak are associated with, produced by super nitrogen saturation and related stress caused by the nitrogen remaining in solid solution in the lattice. For ion-implanted sample at 1×10¹⁷ ions/cm2, new phases are not perceived. At a higher dose of (3-5)×l0¹⁷ ions/cm2 many additional peaks are observed are due to FCC TaN_0.43, Ta₃N₅ and bcc tantalum phases respectively.

The results show that TaN_0.43 phase of Tantalum nitride formed with nitrogen ion implantation, and intensity of this peak increases with nitrogen doses. Furthermore enhancing nitrogen flux increased surface roughness and average resistance of samples. The electrical resistivity of tantalum of implanted samples especially after increasing ion dose increased. This cathode with more trap centers and possible surface dangling bonds can be advised for the future of ECD and nanoelectronic devices.

CONFLICT OF INTERESTS

The authors declare that there is no conflict of interests regarding the publication of this paper.