Effect of Substrate and Annealing on Electrical, Magnetic and Morphological Properties of Ni Thin
Films
Md. Kamrul Hasan and Md. Johurul Islam*
Department of Applied Physics and Electronic Engineering University of Rajshahi
Rajshahi, Bangladesh
[email protected], [email protected]*
Abstract—Nickel thin films have been prepared by using electron beam evaporation technique at a pressure of 4105 mbar on silicon and glass substrates. Some of as-deposited films have been annealed at 573 K for 1.5 hours in open air. Electrical and morphological properties of both as-deposited and annealed films are investigated. The magnetic properties of as-deposited Ni films are studied by VSM at room temperature. The resistivity of Ni films on silicon substrate is higher than resistivity of Ni films found on glass substrate. T.C.R. of Ni films is found to be positive indicating metallic nature of the samples. Coercivity of Ni films increases with the increase of film thickness. The coercivity of 80 nm as-deposited Ni film on glass substrate is 9 Oe. The rms value of surface roughness of 150 nm as-deposited Ni films on glass substrate is 12 nm. It becomes 7 nm after annealing. On the other hand the coercivity of 90 nm and 160 nm as-deposited Ni films on silicon substrate is 50 Oe and 85 Oe respectively. The rms value of surface roughness of 120 nm as-deposited Ni film on Si substrate is 16 nm. It becomes 3 nm after annealing.
Index Terms—Magnetic Materials, Coercivity, Surface Roughness, Electrical Properties.
I. INTRODUCTION
The spin of electrons in ferromagnetic materials has been ignored for electronics application for many years. But ferromagnetic materials like Nickel in the form of thin films have received much attention over decades due to their potential technological applications like magnetic sensors, ultra-high-density magnetic storage, spin polarized transistors etc. [1-6]. Material scientists need to understand the relation between structures and magnetic properties of ferromagnetic films in order to improve production methods and material quality. The influence of thickness, substrate, magnetic field and deposition parameters on magnetic properties of Nickel film is being studied worldwide [7-10]. Not only single layer but also multilayer Ni film is important for research. Haque et al. reported a two phase spin reversal [11] and dissimilar hysteresis loop [12] in multilayer Ni films. Not only magnetic properties but also electrical properties of thin film are influenced by thickness [13-14], temperature [15], substrate etc. Morphological property such as surface roughness is also gaining much attention as it is directly related to optical scattering and resistivity due to grain boundary and thickness
[16]. Zhao et al. [17] and Prosen et al. [18] reported that surface roughness has effect on magnetic properties.
Comparative study of electrical, magnetic and morphological properties of Ni thin films on different substrates have been investigated and reported in this paper.
II. EXPERIMENTAL DETAILS
Thin films of nickel have been deposited on silicon and glass substrates using E-beam evaporation technique. Before deposition glass substrates were cleaned with acetone and silicon substrates were cleaned with H2O2 (30% solution). Pure Nickel (purity 99.99%) was deposited onto glass and silicon substrates in vacuum by Edwards E306 vacuum coating unit at a base pressure of 4105 mbar. The deposition rate was kept 2 nms1. Films with various thicknesses have been prepared and investigated. The thickness of the films is measured by Fizeau fringes [19] method. After deposition, some of as-deposited samples have been annealed at 573 K for 1.5 hours at atmospheric pressure in a Carbolite heavy duty oven. The resistivity of both as-deposited and annealed films has been measured by van der Pauw [20] method. The temperature coefficient of resistivity (T.C.R) was calculated from resistivity data. The magnetic behavior (hysteresis loop) of the films was studied by using a VSM at room temperature. The surface morphology of the films was observed by AFM [21]. The surface roughness of the films was calculated from AFM image.
III. RESULT AND DISCUSSION
The electrical resistivity of the films was measured by Van der Pauw method. Variation of resistivity with respect to temperature and thickness is studied in the temperature range 313 K – 483 K. The resistivity (ρ) versus temperature (T) curve for Ni film on glass substrate having thickness 70 nm, 90 nm and 120 nm is shown in Fig.1. It is seen that the resistivity of both as-deposited (Fig.1a) and annealed (Fig.1b) films increases with temperature.
It is also seen from the graph that the resistivity decreases with increasing film thickness which affirms the Fuchs and Sondheimer [22-23] theory of size effect. The resistivity has a
linear relationship with temperature in the observed range of temperature.
Fig. 1. Resistivity (ρ) vs. Temperature (T) curve for (a) as-deposited and (b) annealed Ni films on glass substrate.
It is also observed that the the value of resistivity of as- deposited Ni films are higher than the resistivity of annealed Ni films. Recrystallization of Ni films occurs at high temperature.
Defect density greatly becomes reduced during annealing.
Therefore, the resistivity of annealed Ni films is lower than the resistivity of as-deposited films.
Fig.2 shows the temperature dependent resistivity of (a) as- deposited and (b) annealed Ni films on silicon substrate having thicknesses 50 nm, 120 nm and 180 nm. It is observed from the Figure that the resistivity of Ni films increases with increasing temperature but resistivity decreases with increasing film thickness [22-23]. The resistivity of Ni films decreases after annealing.
The resistivity of Ni films on Si substrate is found to be higher than that of Ni films on glass substrate. Nickel forms NiSi at the Ni/Si interface [24-25]. It is known that the resistivity of NiSi is higher than that of Ni metal. Therefore, the overall resistivity of Ni films on Si subatrate is higher that the resistivity of Ni fims on glass substrate[25].
Temperature coefficient of resistivity (T.C.R) versus temperature graph of (a) as-deposited and (b) annealed Ni films on glass substrates is shown in Fig.3. From the Figure it is observed that the T.C.R is always positive for both the as- deposited and annealed films. This is evident that the samples are metallic in nature. It is also observed from the Figure that the T.C.R. curve is not linear over the entire temperature range.
This is due to the fluctuation of temperature during the measurement.
Fig. 2.Resi
stivity (ρ) vs. Temperature (T) curve for (a) as-deposited and (b) annealed Ni films on silicon substrate.
Fig. 3. T.C.R variation with respect to Temperature; (a) as-deposited, (b) annealed Ni films on glass substrate.
Magnetization process of Ni films on both the glass and silicon substrates was measured at room temperature. It is known that the magnetization process of ferromagnetic materials is affected by substrates. To study the effect of substrates on the magnetization process, Ni films of different thicknesses are deposited on both the Si and glass substrates.
Fig.4 shows the hysteresis loop of 80 nm as-deposited Ni film on glass substrate. The coercivity and squareness of the film is 9 Oe and 0.17 respectively.
Fig. 4. Hysteresis loop (M-H curve) of 80 nm as-deposited Ni film on glass substrate.
The Hysteresis loop of 90 nm and 160 nm Ni films on silicon substrate is shown in Fig.5. It is observed from Fig.5 (a) and Fig.5 (b) that the coercivity, squareness and remanent magnetization increases with film thickness. This complies with the result reported by Haque et al. [7]. The coercivity of 90 nm and 160 nm as-deposited Ni films are 50 Oe and 85 Oe respectively. The squareness of 90 nm Ni film is 0.08 but it is increased to 0.2 for 160 nm film.
Fig. 5. Hysteresis loop of (a) 90 nm and (b) 160 nm as-deposited Ni film on silicon substrate.
It is seen from Fig.4 and Fig.5 (a) that the coercive field of Ni film on glass substrate is lower than the coercivity of Ni films on Si substrate. This may happen due to better short- range order of the films deposited on the crystalline substrates [26].
The relationship between coercivity and roughness for a relatively thick film is complicated, rather than a simple monotonic relationship [17]. In general, films cannot be grown perfectly. Roughness appearing in the film plays significant role in modulating the magnetic properties. For in-plane magnetization, local magnetizing fields at the surface reduce its anisotropy. The surface roughness of ferromagnetic thin film depends on both annealing and substrates. In order to understand the effect of surface roughness on coercivity, the roughness was studied by an AFM.
Fig.6 shows the AFM images of 150 nm as-deposited and annealed Ni film on glass substrate. The rms value of surface roughness of as-deposited film is 12 nm which becomes 7 nm after annealing.
Fig. 6. AFM image of 150 nm (a) as-deposited and (b) annealed (at 573K for 1.5 hours) Ni films on glass substrate.
To observe the effect of substrate on the surface roughness, Ni thin film of 120 nm is deposited on Si substrate. The AFM image of the film is shown in Fig.7. The rms value of surface roughness of as-deposited Ni film on Si substrate is 16 nm which becomes 3 nm after annealing. It is seen that the surface roughness of Ni film on glass substrate changes 42 % after annealing. On the other hand the change is 81 % for Ni films on Si substrate. It is known that the surface roughness decreases after annealing [27]. Nickel forms Nickel silicide (NiSi) at relatively low temperature but it becomes Nickel disilicide (NiSi2) at high temperature [25]. For glass substrate the interfacial reaction is zero. It is observed from the above discussion that the change in surface roughness of Ni films on Si substrate is much higher than the change in surface
roughness found in Ni films on glass substrate. This may be due to the formation of Nickel silicides or disilicides at the interface of Ni/Si.
Fig. 7. AFM image of 120 nm (a) as-deposited and (b) annealed (at 573K for 1.5 hours) Ni film on Si substrate.
IV. CONCLUSION
In this study, we have observed an increase in the resistivity of as-deposited and annealed Ni films on glass and Si substrates with increasing temperature. It is also observed that the coercivity of Ni films on glass and Si substrates increases with increasing film thickness. Morphological studies of as- deposited and annealed Ni films on glass and Si substrates show that the surface roughness of Ni film is reduced after annealing.
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