Thickness Dependent Structural and Electrical Properties of Magnetron Sputtered Nanostructured CrN Thin Films

In the present work, we have investigated the structural and electrical properties of CrN thin films for a thickness t in the 30-220 nm range, grown on Si (100) substrates. The CrN/Si (100) films exhibits a structural transition from hexagonal phase (β-Cr2N) to cubic phase (CrN) with the increasing in film thickness, the change in structural transition is attributed to the decrease of film-substrate interfacial strain. From electrical resistivity measurements, the thickness of 150 nm CrN/Si (100) film shown the metal-semiconductor phase transition at around 250 K with energy band gap (Eg) 81 meV in semiconducting region, whereas the thickness of 30, 110 and 220 nm CrN/Si(100) films were shown only semiconducting behaviour for whole temperature range of 50-400 K. On the other hand, a clear grain size was increased in CrN/Si films with increasing thickness and its influence on transport properties was also seen. The possibility of phase transition and occurrence of semiconducting behaviour in the CrN films were analysed.


Introduction
Over the past decades, CrN has received a great technological attention due to its remarkable mechanical properties such as high hardness, high wear resistance, and high temperature oxidation resistance [1][2][3] .Besides the technological importance, CrN also shows very interesting novel properties like magnetic, optical, and electronic properties.It is well known that bulk CrN is paramagnetic behaviour (PM) at room temperature with a B1 NaCl crystal structure.At the Neel temperature in a range of 273-283 K it undergoes a first order phase transition to antiferromagnetic (AFM) with an orthorhombic Pnma Crystal structure 4,5 .However, many authors studied the structural, electrical, mechanical and magnetic properties of CrN thin films grown on different substrates, such as Si (100), MgO (001) and Al 2 O 3 (0001) etc. [6][7][8][9][10][11] .Recently, Ney et al. 12 reported that the CrN thin films deposited on MgO (001) substrate and Al 2 O 3 (0001) substrate showed the paramagnetic behaviour at low temperature and ferromagnetic behaviour above room temperature, respectively.Filippetti et al. 13 concluded that CrN was a metal in its PM state but a weak metal in the antiferromagnetic state.Constantin et al. 14 observed that electronic, magnetic and structural phase transitions might be correlated in CrN/MgO film deposited by molecular-beam epitaxy method.Some experimental studies revealed different results on the electron transport properties of CrN material.Therefore, it is still open issue in this material how the structural and electronic phase transitions are correlated 15 .In this study, CrN thin films grown on Si (100) substrates at different thickness ranges, 30-220 nm, by reactive dc magnetron sputtering and systematically examined the structural and electrical properties.

Experimental Details
The different thicknesses CrN thin films were grown onto Si (100) substrates using dc reactive magnetron sputtering at 350 o C temperature.High purity (99.99%)Cr metal target of 50 mm diameter and 3 mm thickness was used.Before sputtering deposition, the chamber was evacuated to a base pressure of the order of 2×10 -6 torr and no post annealing was performed after deposition.Working pressure was maintained at a constant value of 10 m Torr.The target and substrate distance was kept constant approximately 5 cm.The deposition was carried out for 5, 10, 15 and 20 min to get the different thicknesses (30, 90, 150, 220 nm) and power was kept constant at 90 W for all depositions.The chamber environment was 70%Ar+ 30%N 2 during deposition.The crystallographic orientations of the deposited films were studied using a Bruker advanced diffractrometer of Cu Kα (1.54Å) radiations in θ-2θ geometry at a scan speed of 1 • / min.The deposited films were subjected to morphological characterization using Zeiss Evo18 Scanning electron microscope (SEM) and atomic force microscopy (AFM).The resistivity verses temperatures (ρ-T) measurements were performed by four-probe resistivity method using a cryohead with helium compressor interfaced with Keithley instruments over temperature range from 50 K to 400 K.

Results and Discussion
Figure 1 shows the XRD patterns of CrN films with different thicknesses grown on Si (100) substrates.The CrN/ Si (100) films with thicknesses t = 30 nm was found to be hexagonal (β-Cr 2 N) crystal structure with preferred (002) orientation along with small orientation peaks ( 200) and (311), and further increasing thickness (t =150 and 220nm) the films were exhibited structural phase transition from hexagonal β-Cr 2 N with (002) orientation to (111) orientation of cubic CrN.The film with an intermediate thickness of 90 nm was found to have mixed phase of hexagonal β-Cr 2 N (002) and cubic CrN phase with fundamental peak (111) and small intensity of ( 200) and (311) peaks were observed.Many authors were reported in the literature, the CrN films exhibits structural phase transition from NaCl cubic crystal structure at room temperature to orthorhombic structure at Neel temperature 16 .In this present study, the CrN/Si films exhibited structural transition due to the variation of thickness.Nevertheless, we have noticed the mixed CrN phase for the 90 nm film due to structural transition from the lower thickness of hexagonal β-Cr 2 N phase to higher thickness of cubic CrN phase.This structural transition could be due to the lattice misfit of CrN phase w.r.t to substrate.In order to investigate the micro structural properties, we studied the scanning electron microscope (SEM) and atomic force microscopy (AFM) analysis for all deposited CrN/Si (100) films.Figure 2(a)-(d) and Figure 3(a)-(d) depicts the scanning electron microscope (SEM) and atomic force microscope (AFM) microstructures of various thicknesses (t = 30, 90, 150 and 220 nm) of CrN/Si (100) films, respectively.The SEM and AFM images show that the CrN thickness effectively changes the microstructure of the films.All deposited films were homogeneous and crack-free.Both SEM and AFM microstructures replicate an increase in grain size with increasing of CrN thickness and all the films are spherical grains in nature.The values of average grain size (for both SEM and AFM) and root mean square (RMS) surface roughness were given in Table 1.The grain size and RMS roughness of the films were observed to increase with the increasing CrN thickness.Figure 4 (a) shows electrical resistivity as a function of temperature ρ (T) measured between 50 K to 400 K of all CrN/Si (100) films with different thicknesses.Thickness of 150 nm CrN/Si film was polycrystalline cubic structure in nature conformed from XRD results.In the ρ (T) curve, the t =90 nm CrN/Si film resistivity (ρ) slightly decreasing with increasing T between 50 and 250 K, showing semiconducting behaviour with dρ/dT < 0. At 250 K, ρ begins to increase sharply and reaches 284 K, it indicates the metallic behvior with dρ/dT > 0. Further increasing the  temperature from 284 K to 400 K, the ρ decreasing sharply showed the semiconducting behaviour, this could be attributed to the presence of a band gap or carrier localization due to grain boundaries or N-vacancies 17,18 .The room temperature resistivity is 36.6Ω-cm, this is within the wide range of previously reported values, 3× 10 -4 to 600 Ω-cm, obtained from polycrystalline CrN thin films 19 .The resistivity shows the discontinuity at around 250-284 K (marked in curve), as shown in Figure 4(b), which is associated with a structural phase transition.This discontinuity could be occurred due to the grain boundary changes in the film and as well as changing in the shape and volume of the unit cell during phase transition 20 .Most polycrystalline CrN films exhibits the phase transitions due to distort the individual grains in different directions and the associated volume reduction would provide sufficient space within the microstructure for grain boundary slide.To study the electrical transport mechanism of this film, we have fitted the curve above the Neel temperature (T N ) of CrN in the semiconducting (284-400 K) region, as shown in Figure 5.

By using an activation law
we have calculated the slope of the curve equal to E g /2K and corresponding E g value 80 meV with ρ 0 =4.59 × 10 -3 Ω cm, the room temperature resistivity ρ 300 K = 36.6Ω-cm.The E g value is consistent with these previously reported values and range from 48 to 81 meV 14 .
Further, the thickness of t= 30, 1 50 and 220 nm CrN films deposited on Si (100) substrates shows the resistivity decreases monotonically with increasing temperature range 50-400 K, and shows no discontinuity at around 280 K, suggesting no phase transition.This is in contrast to some earlier reported authors 4,21 .Here, XRD results shows that t = 30 nm CrN film is a hexagonal crystal structure and not shown any structural phase transition in ρ (T) curve, this could be due to the lack of appropriate N-vacancy concentration in this thickness for the phase transition.When thickness reaches t = 150 nm, cubic phase, shown

Conclusions
In the summery, we have successfully deposited CrN thin films for a thickness t in the 30-220 nm range onto Si (100) substrate and investigated the structural and electrical properties.Deposited CrN/Si (100) films have showed structural transition from hexagonal phase (β-Cr 2 N) to cubic phase (CrN) with the increasing in film thickness.From ρ (T) measurements, the thickness of 150 nm CrN/Si (100) film shown the metal-semiconductor phase transition at around 250 K with energy band gap (E g ) 81 meV in semiconducting region.Further, AFM microstructures shown CrN/Si films grain size increases with increasing of thickness.range hopping (VRH) conduction mechanism with ρ α T -1/4 22,23 .Mott VRH describes well the temperature dependence of ρ for approximately 50-230 K, for all thicknesses t = 30, 150 and 220 nm of CrN films.This suggests a hopping like conduction mechanism is fitted for our semiconducting CrN thin films in the region of 50-230 K.When the film thickness is decreased, the relative impact of the strain generated at the film-substrate interface becomes increasingly important and result in the change of electrical properties of these films.On the other hand, AFM micrographs indicated that the thicker films have larger grain size, the larger grain size results in a lower density of grain boundaries, which behaves as traps for free carriers and barriers for carrier transport in the film 24 .Hence, an increase in grain size can cause a decrease in grain boundary scattering, which leads to decrease in resistivity.

a
Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro Gyeongsan-si Gyeongsangbuk-do, 3854, Republic of Korea b Department of Physics, Madanapalle Institute of Technology & Science, Madanaplle, Andhra Pradesh-517325, India Thickness Dependent Structural and Electrical Properties of Magnetron Sputtered Nanostructured CrN Thin Films

Table 1 .
Various parameters of different thicknesses of CrN/Si(100) films temperature and show no first order phase transition.The temperature variations of resistivity are not the simple activation type one.The data are fitted using a Mott variable-