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

Journal of Chemistry, Vol. 44 (6), P. 762 - 765, 2006<br /> <br /> <br /> Electrodeposition of Co thin Film onto n-Si(111)/Au<br /> substrate<br /> Received 27 May 2005<br /> Mai Thanh Tung1, chu van thuan2, Nguyen Hoang Nghi3<br /> 1<br /> Faculty of Chemical Technology, Hanoi University of Technology<br /> 2<br /> International Training Institute for Material Science (ITIMS)<br /> 3<br /> Lab. of Amophours Material and Nanocrystalline- Hanoi University of Technology<br /> <br /> SUMMARY<br /> Electrodeposition kinetics of Co thin film were investigated and magnetic coercivity of the<br /> deposited films were studied using potentiostatic technique and magnetic hyteresis loop<br /> measurements. Results showed that the nucleation mechanism of the electrodeposition process<br /> changed from instantaneous mode to progressive mode when potential was changed from –0.7 V<br /> to –0.8 V. The magnetic coercivity Hc increases with deposition time and the film with Hc < 15 Oe<br /> can be obtained with deposition time t < 5 s.<br /> <br /> I - INTRODUCTION between kinetics and morphology, magnetic<br /> properties are still not well understood. In this<br /> Magnetic thin films are intensively studied study, we will show results on electrodeposition<br /> during the last decades due to their various kinetics and magnetic properties of the<br /> applications in electronics, sensor and actuator deposition process of Co on to n-Si(111)/Au.<br /> technology ect. [1 - 5]. Usually, the materials<br /> for those applications are soft and hard magnetic II - EXPERIMENTAL<br /> thin films. Recently, new type of films with the<br /> so-called Giant Magnetoresistance (GMR) or Deposition of Co was performed from a<br /> Giant Magnetoimpedance (GMI) effects have electrolyte containing 0.5 M CoSO4, H3BO3 0.2<br /> been focused due to their very high sensitivity to M, pH = 4 and deposition at room temperature.<br /> magnetic field. Among the methods for Potentiostatic experiments were carried out at<br /> fabrication of the magnetic thin films, potentials E = -0.7 V and E = -0.8 V. All<br /> electrodeposition technique is one of the most potentials are referred to the Calomel<br /> widely used process [1 - 5]. In the previous Hg/Hg2Cl2/NaCl electrode. All experiments<br /> works, we have shown that the multilayer Co/Cu were performed either on phosphorous-doped n-<br /> with GMR effect can be electrodeposited from a type silicon (111) wafers (Goodfellows, UK)<br /> single bath by potential pulse method [2, 3]. The coated by sputtered 10nm Au with a resistivity<br /> magnetic properties of the electrodeposited of 7.5 cm. Prior to each experiment the Si/Au<br /> layer are clearly influenced by the kinetics of samples were sequentially cleaned<br /> the process (nucleation and growth), particularly ultrasonically for 10 minutes in ethanol and<br /> the depostion of Co, which is the magnetic water. The substrates were mounted onto a<br /> component of the multilayer [4]. However, the Teflon holder and exhibited an active surface<br /> kinetics of the Co deposition and the relation area of 0.28 cm2. The potentiostatic experiments<br /> <br /> 762<br /> were carried out using potentiostat Autolab films were measured using vibrating sample<br /> (Ecochemie, the Netherland). Using the magnetometer (VSM). The nominal thicknesses<br /> potentiostatic curves, the nucleation of electrodeposited films were estimated by<br /> mechanisms are analysed by the model calculating the deposition charge of the<br /> proposed by Scharifker and Staikov [6, 7]: potentiostatic curves (Fig. 1). The procedure for<br /> 2<br /> calculation of the nominal thickness were<br /> 2<br /> 2 t described elsewhere [2].<br /> i t 2.3367<br /> t max<br /> = 1.2254 max 1 e<br /> imax prog<br /> t III- RESULTS AND DISCUSSION<br /> (progressive nucleation) (1)<br /> In the previous study, it has been shown that<br /> 2<br /> 2 t Co deposits onto the Si/Au substrate at potential<br /> i t max 1.2564<br /> t max<br /> = 1.5942 1 e E < -0.65 V [2, 3]. In order to investigate<br /> imax inst<br /> t kinetics of the deposition, potentiostatic method<br /> combined with SEM measurements at potentials<br /> (instantaneous nucleation) (2)<br /> E = -0.7 V and E = -0.8 V were carried out. The<br /> where the progressive nucleation represents the obtained chronoamperometric curves is shown<br /> mechanism, in which the nucleation occurs in Fig. 1a and the curves in reduced coordinates<br /> preferentially on freshly formed nuclei, leading (i/imax)2 vs. (t/tmax) are plotted in Fig. 1b.<br /> to the formation of course clusters. Meanwhile, In order to analyse nucleation mechanism,<br /> the instantaneous nucleation takes place the curves in the reduced coordinates are<br /> preferentially on the substrate and the well compared to the equations (1) and (2) and the<br /> distribution of nuclei on the surface can be mechanism is decided by that fact that the<br /> obtained as a result. The difference between (1) curves are well fitted to (1) or (2). Fig 1b shows<br /> and (2) is the coefficient standing in front of the measured curve (denoted as black and white<br /> (t/tmax) due to the difference in nucleation circles) and fitted curve (continuous and dashed<br /> mechanism. In these equations the coordinates curves) following equations (1) and (2). As can<br /> of the current transient maximum imax and tmax be seen, the curve at –0.7 V is well fitted to<br /> are coupled by: equation (1), indicating that the primary Co<br /> (i 2<br /> t<br /> max max prog ) = 0.2598( zFc ) D<br /> 2<br /> (3) nucleation on n-Si(111)/Au follows a<br /> progressive mechanism at –0.7 V. Meanwhile<br /> (i 2<br /> t<br /> max max inst ) = 0.1629( zFc ) D<br /> 2<br /> (4) the curve at –0.8V is well fitted to equation (2),<br /> showing that the nucleation mechnism of Co<br /> 1/ 2<br /> deposition at E = -0.8 V is instantaneous one. It<br /> (i 2 3<br /> t ) = 0.2898( zF )<br /> 2 c3 (5) should be pointed that the instantaneous mode is<br /> 8 Vm ( AN 0 )<br /> max max prog 2<br /> the desirable mechanism for the nucleation<br /> 1/ 2 since this mechanism facilitates the 2D growth<br /> (i 2 2<br /> t<br /> max max inst ) = 0.065( zF )<br /> 2 c3<br /> 8 Vm N 02<br /> (6) mode of the deposited layer.<br /> Additional SEM measurements confirm this<br /> observation (Fig. 2). As can be seen in the SEM<br /> where c is the metal ion concentration in the image, electrodeposited Co clusters at E = -0.7<br /> electrolyte, D is the diffusion coefficient, Vm is V are coarse and low density. This behaviour is<br /> the molar volume of the metal, N0 is the characteristic for a progressive nucleation<br /> nucleation site density and A represents the mechanism and is in agreement with the current<br /> nucleation frequency per nucleation site. transient. On the other hand, the nuclei size is<br /> The metal nuclei were recorded using lower and density is remarkably higher at<br /> Scanning Electron Microscope (SEM) (JMS deposition potential E = -0.8 V, indicating that<br /> 5410-Jeol). Magnetic hyteresis loop and the corresponding nucleation mechanism is<br /> magnetic coervivity Hc of the electrodeposited instantaneous.<br /> 763<br />  Figure 1: Chronoamperometric curves for Co deposition at E = -0.7 V and E = -0.8 V<br /> in (a) original form and (b) reduced form (i/imax)2 vs. (t/tmax)<br /> <br /> (a) -0.7V (b) -0.8V<br /> <br /> <br /> <br /> <br /> Figure 2: SEM image of the Co nuclei after 1s deposition at (a) –0.7 V, (b) –0.8 V<br /> <br /> <br /> 0.3 90 80<br /> t=10s, d=33nm<br /> t=15s, d=52nm 80<br /> 0.2<br /> t=20s, d=78nm 70 70<br /> <br /> 60<br /> Thickness (d) /nm<br /> <br /> <br /> <br /> <br /> Coercivity (HC)/ Oe<br /> <br /> <br /> <br /> <br /> 0.1<br /> B (Wb/m )<br /> <br /> <br /> <br /> <br /> thickness 60<br /> 2<br /> <br /> <br /> <br /> <br /> 50<br /> 0.0 40<br /> -Hc Hc<br /> 50<br /> 30<br /> -0.1<br /> 20<br /> Hc 40<br /> 10<br /> -0.2<br /> 0<br /> 30<br /> -0.3 0 5 10 15 20<br /> -1000 -800 -600 -400 -200 0 200 400 600 800 1000<br /> Time /s<br /> H (Oe)<br /> Figure 3: Magnetic hyteresis loops of Co film Figure 4: Dependence of the magnetic<br /> after deposition time of 10s, 15s, 20s coercivity Hc and film thickness on deposition<br /> time<br /> 764<br />  Fig. 3 shows the hyteresis loops of Co films Acknowledgements: Financial support of this<br /> deposited at –0.8 V for different time 10, 20 and work by VLIR-HUT fund (project VLIR-<br /> 50s, corresponding to the nominal thicknesses HUT/IUC/PJ10) and Basic research Fund<br /> of 51, 78 and 162 nm, respectively. The (project 81.18.05) are also gratefully<br /> hyteresis loops show arelatively abrupt acknowledged.<br /> magnetization reversal, which is similar in all<br /> samples. It should also be mentioned the REFERENCES<br /> coercivity Hc is dependence on thickness of the<br /> deposition layer (Fig. 3). Fig. 4 summarizes the 1. M. N. Baibich, J. M. Broto, A. Fert, Nguyen<br /> dependence of coercivity Hc on deposition time<br /> Van Dau, R. F Petroff, P. Eitenne, G.<br /> and corresponding deposited film thickness.<br /> Results show that Hc increases with deposition Creuzet, A. Freiderich and J. Chazelas.<br /> time and the films with Hc < 15 Oe can only be Phys. Rev. Lett., 61, P. 2472 - 2479 (1988).<br /> obtained with deposition time t < 5s. It should 2. Mai Thanh Tung, Nguyen Hoang Nghi,<br /> also be mentioned that Co layers with low Hc is J.W. Schultze. 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