Electrodeposition of Co thin Film onto n-Si(111)/Au substrate

762 Journal of Chemistry, Vol. 44 (6), P. 762 - 765, 2006 Electrodeposition of Co thin Film onto n-Si(111)/Au substrate Received 27 May 2005 Mai Thanh Tung1, chu van thuan2, Nguyen Hoang Nghi3 1Faculty of Chemical Technology, Hanoi University of Technology 2International Training Institute for Material Science (ITIMS) 3Lab. of Amophours Material and Nanocrystalline- Hanoi University of Technology SUMMARY Electrodeposition kinetics of Co thin film were investigated and magnetic coercivity of the deposited films were studied using potentiostatic technique and magnetic hyteresis loop measurements. Results showed that the nucleation mechanism of the electrodeposition process changed from instantaneous mode to progressive mode when potential was changed from –0.7 V to –0.8 V. The magnetic coercivity Hc increases with deposition time and the film with Hc < 15 Oe can be obtained with deposition time t < 5 s. I - INTRODUCTION Magnetic thin films are intensively studied during the last decades due to their various applications in electronics, sensor and actuator technology ect. [1 - 5]. Usually, the materials for those applications are soft and hard magnetic thin films. Recently, new type of films with the so-called Giant Magnetoresistance (GMR) or Giant Magnetoimpedance (GMI) effects have been focused due to their very high sensitivity to magnetic field. Among the methods for fabrication of the magnetic thin films, electrodeposition technique is one of the most widely used process [1 - 5]. In the previous works, we have shown that the multilayer Co/Cu with GMR effect can be electrodeposited from a single bath by potential pulse method [2, 3]. The magnetic properties of the electrodeposited layer are clearly influenced by the kinetics of the process (nucleation and growth), particularly the depostion of Co, which is the magnetic component of the multilayer [4]. However, the kinetics of the Co deposition and the relation between kinetics and morphology, magnetic properties are still not well understood. In this study, we will show results on electrodeposition kinetics and magnetic properties of the deposition process of Co on to n-Si(111)/Au. II - EXPERIMENTAL Deposition of Co was performed from a electrolyte containing 0.5 M CoSO4, H3BO3 0.2 M, pH = 4 and deposition at room temperature. Potentiostatic experiments were carried out at potentials E = -0.7 V and E = -0.8 V. All potentials are referred to the Calomel Hg/Hg2Cl2/NaCl electrode. All experiments were performed either on phosphorous-doped n- type silicon (111) wafers (Goodfellows, UK) coated by sputtered 10nm Au with a resistivity of 7.5
cm. Prior to each experiment the Si/Au samples were sequentially cleaned ultrasonically for 10 minutes in ethanol and water. The substrates were mounted onto a Teflon holder and exhibited an active surface area of 0.28 cm2. The potentiostatic experiments 763 were carried out using potentiostat Autolab (Ecochemie, the Netherland). Using the potentiostatic curves, the nucleation mechanisms are analysed by the model proposed by Scharifker and Staikov [6, 7]: 22 3367.2 max 2 progmax maxe12254.1             =    t t t t i i (progressive nucleation) (1) 2 2564.1 max 2 instmax maxe15942.1           =    t t t t i i (instantaneous nucleation) (2) where the progressive nucleation represents the mechanism, in which the nucleation occurs preferentially on freshly formed nuclei, leading to the formation of course clusters. Meanwhile, the instantaneous nucleation takes place preferentially on the substrate and the well distribution of nuclei on the surface can be obtained as a result. The difference between (1) and (2) is the coefficient standing in front of (t/tmax) due to the difference in nucleation mechanism. In these equations the coordinates of the current transient maximum imax and tmax are coupled by: ( ) ( ) DzFcti 2progmax2max 2598.0= (3) ( ) ( ) DzFcti 2instmax2max 1629.0= (4) ( ) ( ) ( ) 2/1 2 0m 3 2 prog 3 max 2 max 8 2898.0   = ANV czFti (5) ( ) ( ) 2/1 2 0m 3 2 inst 2 max 2 max 8 065.0   = NV czFti (6) where c is the metal ion concentration in the electrolyte, D is the diffusion coefficient, Vm is the molar volume of the metal, N0 is the nucleation site density and A represents the nucleation frequency per nucleation site. The metal nuclei were recorded using Scanning Electron Microscope (SEM) (JMS 5410-Jeol). Magnetic hyteresis loop and magnetic coervivity Hc of the electrodeposited films were measured using vibrating sample magnetometer (VSM). The nominal thicknesses of electrodeposited films were estimated by calculating the deposition charge of the potentiostatic curves (Fig. 1). The procedure for calculation of the nominal thickness were described elsewhere [2]. III- RESULTS AND DISCUSSION In the previous study, it has been shown that Co deposits onto the Si/Au substrate at potential E < -0.65 V [2, 3]. In order to investigate kinetics of the deposition, potentiostatic method combined with SEM measurements at potentials E = -0.7 V and E = -0.8 V were carried out. The obtained chronoamperometric curves is shown in Fig. 1a and the curves in reduced coordinates (i/imax) 2 vs. (t/tmax) are plotted in Fig. 1b. In order to analyse nucleation mechanism, the curves in the reduced coordinates are compared to the equations (1) and (2) and the mechanism is decided by that fact that the curves are well fitted to (1) or (2). Fig 1b shows the measured curve (denoted as black and white circles) and fitted curve (continuous and dashed curves) following equations (1) and (2). As can be seen, the curve at –0.7 V is well fitted to equation (1), indicating that the primary Co nucleation on n-Si(111)/Au follows a progressive mechanism at –0.7 V. Meanwhile the curve at –0.8V is well fitted to equation (2), showing that the nucleation mechnism of Co deposition at E = -0.8 V is instantaneous one. It should be pointed that the instantaneous mode is the desirable mechanism for the nucleation since this mechanism facilitates the 2D growth mode of the deposited layer. Additional SEM measurements confirm this observation (Fig. 2). As can be seen in the SEM image, electrodeposited Co clusters at E = -0.7 V are coarse and low density. This behaviour is characteristic for a progressive nucleation mechanism and is in agreement with the current transient. On the other hand, the nuclei size is lower and density is remarkably higher at deposition potential E = -0.8 V, indicating that the corresponding nucleation mechanism is instantaneous. 764 Figure 1: Chronoamperometric curves for Co deposition at E = -0.7 V and E = -0.8 V in (a) original form and (b) reduced form (i/imax) 2 vs. (t/tmax) (a) -0.7V (b) -0.8V Figure 2: SEM image of the Co nuclei after 1s deposition at (a) –0.7 V, (b) –0.8 V -1000 -800 -600 -400 -200 0 200 400 600 800 1000 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 t=10s, d=33nm t=20s, d=78nm t=15s, d=52nm Hc-Hc H (Oe) B (W b/ m 2 ) 0 5 10 15 20 0 10 20 30 40 50 60 70 80 90 Hc thickness Coercivity (H C )/O eTh ic kn es s (d )/ nm Time /s 30 40 50 60 70 80 Figure 3: Magnetic hyteresis loops of Co film after deposition time of 10s, 15s, 20s Figure 4: Dependence of the magnetic coercivity Hc and film thickness on deposition time 765 Fig. 3 shows the hyteresis loops of Co films deposited at –0.8 V for different time 10, 20 and 50s, corresponding to the nominal thicknesses of 51, 78 and 162 nm, respectively. The hyteresis loops show arelatively abrupt magnetization reversal, which is similar in all samples. It should also be mentioned the coercivity Hc is dependence on thickness of the deposition layer (Fig. 3). Fig. 4 summarizes the dependence of coercivity Hc on deposition time and corresponding deposited film thickness. Results show that Hc increases with deposition time and the films with Hc < 15 Oe can only be obtained with deposition time t < 5s. It should also be mentioned that Co layers with low Hc is desirable for the purpose of electrodeposition of multilayer Co/Cu with GMR effect. Thus, it can be concluded from the obtained results that the optimal parameters for deposition of multilayer should be E = -0.8 V and t < 5 s. IV- CONCLUSIONS The electrodeposition kinetics of Co thin film were investigated by potentiostatic measurements combined with SEM observation. 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