Growth of Silicon-Germanium Alloys
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- Keywords
- 2 – Effect of the Electric Field in Dopant Segregation
- 3. Experimental Procedures
- 3.1. Obtention of Polycrystalline Si:Ge Alloys
- 4. Alloys Metalographic Analysis
- 4.1. Regional Chemical Analysis
- 4.2. Spot Chemical Analysis
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Growth of Silicon-Germanium Alloys Octaviano, E.S.* , Andreeta, J.P. and Alves, L.M. Grupo de Crescimento de Cristais Instituto de Física de São Carlos Universidade de São Paulo C.P. 369, 13560-970 – São Carlos – SP – Brazil *Permanent Address Academia da Força Aérea Divisão de Ensino Campo Fontenelle – Pirassununga – 13630-000 - SP – Brazil ABSTRACT The Silicon-Germanium alloys (Si80Ge20) are used in several applications, monocrystallyne as well as polycrystalline, in advanced electronic and optoelectronic devices, and mainly for radioisotope thermoelectric generators (RTG) for energy conversion by thermoelectrical effects. In this work the Si:Ge alloys are grown by the Czochralski technique under applied electric field, and the alloy obtained are analyzed by chemical etching and electronic microscopy (EDX). Keywords: crystal growth, electric field, segregation, Silicon, Germanium 1. Introduction The Silicon-Germanium (Si:Ge) alloys have very extensive technological applications (1-4), like, for example, in the generation of electrical energy for satellites, nuclear batteries for a wide range of applications , RTG calls, radioisotope thermoelectrical generators, in electronics and, also, for applied research in several areas. The main problem found is that the growth of these alloys, be it in the monocrystalline for or in the polycrystalline ( both have applications ) is very hard, due to the high degree of Ge segregation. Many know process for the growth always come up against this same problem. 2 – Effect of the Electric Field in Dopant Segregation The effective segregation coefficient is an indicator of the quantity of dopant that is incorporated in a crystal during the growth process. The most usual description of the effective segregation coefficient is based on the theory of Burton, Prim and Slichter [5] (the BPS theory), where only the normal crystal parameter dependence is considered. In some others' work [6-11] it has been observed that the effective segregation coefficient is also a function of the applied electric field in the crystal growth process or of the electric field that appears in the crystal-melt interface, due to the great thermal gradient, as in the single crystal fibers growth of oxide materials [12-16]. One of the reasons for which this electric field is applied in crystal growth is the possibility of obtaining single domain ferroelectric crystals directly from the experiment [17-18]. In some cases the influence of electric field in dopant incorporation is explained by a growth rate change due to the Peltier effect and constitutional supercooling [6,8,19]. In another case [12-16,20] the phenomena that explain the dopant incorporation rate changes are related to electrochemical processes and ion eletromigration in the boundary layer, and Seebeck effect, with a change in the velocity growth. All these effects normally modify the crystal growth rate due to the dopant concentration profile changes that again provoke new modifications in the dopant distribution. The first studies were done by Angus et al. [21], Pfann and Wagner [22], Hay and Scala [23] and Verhoeven [24]. The result of these works led to a modification in expression for the effective segregation coefficient obtained by BPS [5], and a new expression to keff. However, such new expression, according to the authors themselves, was only good for a qualitative evaluation of the phenomenon, due to the fact that it is impossible to determine the terms involved in real growth process. So it became a limit to this kind of analysis. Tiller and Uda [12,13] and Uda et al [14-16], developed an excellent formulation for the variation of the effective segregation coefficient and of the growth velocity due to a electric field in the interface of growth, mainly applied in the single crystal fiber growth in oxide materials through laser-heated pedestal growth (LHPG) and micro-pulling (µ-PD) techniques. There’s still in Garendet [25] an expression for the keff due to changes in growth velocity, mainly applied in the case of semiconductors materials, where the electric field that is applied causes a cooling in the interface through Peltier effect. In previous works [26,27] we built a model in order to explain the changes that took place in the keff, for the case of oxide materials (e.g. LiNbO3), as well as in semiconductor materials, grown through the Czochralski’s method with an electric field applied. The equation for the keff was found:   (1)
where is the crystal growth rate modification that must be obtained as a laboratory parameter, f is the growth velocity and ko is the equilibrium segregation coefficient. The simplest equation to represent as a function of the electric current density, we could write as follows (this case occurs when an increase in the current density causes a decrease in keff): = aJ (2) where a is a constant to be experimentally determined and J is the applied electrical current density. Combining Equations 1 and 2 we obtain  (3)
For [J] = [mA cm-2] and [] = [cm s-1], dimensional analysis requires that [a] = [cm3 mA-1 s-1]. There is still the possibility (depending on the polarization of the crucible-melt system) that an increase in the current density causes a decrease in the rejection of the dopant in the solid phase, and consequently an increase in keff. We can express this variation as inversely proportional to the current density, i.e. = b/J (4) where b is the constant to be experimentally determined. Combining eq. 1 and 4 we obtain  (5)
In this case the dimensional analysis requires that [b] = [mA cm-1 s-1]. 3. Experimental Procedures The Si:Ge alloys are grown through Czochralski’s method, using a Kokusai DP-1300 A system of growing for semiconductors. The limitations of the equipment make the obtained crystals with reduced dimensions and relatively high concentration of impurities. These limitations exist due to the physical characteristics of the equipment: reduced limitation of the hot zone of the chamber of growing, inexistence of vacuum systems and of purification of atmosphere and of automatic diameter’s control. The electric field’s applications in the growing of silicon and germanium wasn’t possible up to then due to the impossibility of getting an electric contact with the crucible , and any attempt of contact through fused stage with some metallic conductors was also impossible, due to the fact that the fused silicon chemically attacked and dissolved these contacts. We used then a contact with a silicon electrode , where said silicon acted as a second seed, being introduced through an auxiliary opening in the edge of a alumina rod. This electrode is connected to a DC source and the crystals is linked to another polarity of the source through the cold finger and a mobile contact. This system is used when one wants to apply an electric field during the growing process. In this process the electrode is introduced in the fused stage next to the crucible’s edge.
We wish to emphasize that to obtention of polycrystalline Si:Ge alloys is of great importance in the Si:1.549 Ge composition (corresponding a atomic percentage of 80% of Si and 20% of Ge). Therefore, one should use the same procedure for the obtention of monocrystalline Si:Ge , but in the germination we don’t take the necessary care for obtention of a monocrystal. Therefore, we must either use a polycrystalline seed, and/or initiate the growing at a high speed. In this experiment we obtained a polycrystal of approximately 17mm by 10mm of diameter which was cut in three parts and analyzed here. 4. Alloys Metalographic Analysis In this section we describe the metalographic analysis of the crystal grown through Czochralski's pulling technique with electric field applied and the chemical analysis through Energy's Dispersive X-Ray (EDX).  Fig. 4.1 - Crystal's micrography - Image obtained through electrons backscattering - Increase 50 X - Area 1- beginning. With Scanning Electronic Microscopy and the EDX chemical analysis, it was possible to detect differences in the phases , differences in the chemical compositions, and the solute concentration over all the samples, particularity those grown without a proper applied electric field. However, in these samples where the electric field was used , a decrease in the germanium segregation was observed as shown in the figures. Typical m  icrography of the regions where the chemical analysis were made, with a contrast enhancement to differentiate the slightly different chemical compositions phases are shown in figures 4.1 to 4.4. F  IG. 4.2 - Crystal's micrography - Image obtained through electrons backscattering -Increase 50 X - Area 2 -middle. Fig. 4.3 - Crystal's micrography - Image obtained through electrons backscattering - Increase 50 X - Area 3- end. 4.1. Regional Chemical Analysis A regional chemical analysis was initially made taking as leig as possible detect through the electronic microscope which was of 4.5 mm x 4.5 mm, to find a average composition obtained. The results found in these three areas analyzed are shown in table 1. Table 1 Regional Chemical Analysis in Si:Ge
The chemical analysis results shown in Table 1 show a good homogeneity in the whole crystal in relation to previous techniques . The micrographies of these areas were made, using detection of electrons backscattering through material (fig. 4.4) whith the advantages of differentiating the chemical phases that posses different densities. The lighter areas are shown through denser material which certainly correspond to germanium and the darker ones correspond to silicon. The percentage for each one of these elements can only be measured in each of these micro-areas, through a spot chemical analysis. 4.2. Spot Chemical Analysis We observed in each one of the three analyzed areas basically three different micro-areas : a light area a grey one and a dark one, as shown in figure 4.4. The Table 2 shows a spot chemical analysis in each of these micro-areas. Table 2 – Spot Chemical Analysis of the Si:Ge - Initial Area
Table 3 – Spot Chemical Analysis of the Si:Ge - Central Area
Table 4 – Spot Chemical Analysis of the Si:Ge - Final Area
It's provide then the existence of a larger germanium percentage according to the tonality of grey , tending from black to white . the darker area contain less germanium than the grey ones and these less than the light area, and for the silicon the relation is inverse.  FIG. 4.4 - Typical micrography of the micro-area where the chemical analysis were made, with a contrast highlighting, to differentiate the chemical composition stages that are slightly different. 5. Discussion We could see that the idea of using the electric field to avoid the germanium segregation through Peltier Effect was good and the results obtained so far were promising. However, it is necessary to alter the electrode's geometry for the effect to be homogeneous over the whole material. The effect of the electric field over the growth favoured the alloy's homogenization as show in figures 4.1 to 4.4, because the alloys obtained through ECZ show a decrease of the germanium segregation in all the material. Therefore, it showed a better distribution of said germanium in relation to those obtained through the same Czochralski's technique with-out applied electric field (CZ), or through simple techniques of fusion. The alteration in the dopants incorporation in semiconductors through crystal's growth techniques with applied electric field mainly due to the Peltier Effect. The efficiency in the homogenization through electric field application can be attributed to the following factors:
This method of crystal growth (ECZ) is an alternative to the making of thermoelements, cheaper than the Hot-Pressing technique, whose equipment is scarce in Brazil and presents competitive results. We can still get a better homogeneity with this technique, adjusting the crystal growth's parameters, such as : pulling velocity, rotation rate and the electric field value. 6. Acknowledgments We acknowledge partial financial support from the Conselho Nacional de Desenvolvimento Científico e Tecnógico (CNPq) and the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). 7. References 1 - Kürten, M. and Schilz, J. - J. Crystal Growth 139 (1994) 1. 2 - Kadokura, K. and Takano, Y. - J. Crystal Growth 171 (1997) 56. 3 - Abrosimov, N.V. et all. - J. Crystal Growth 174 (1997) 182. 4 - Abrosimov, N.V. et all. - J. Crystal Growth 166 (1996) 657. 5 - Burton, J. A., Prim, R.C. and Slicther, W. P. - The J. Chem. Phys., 21 n° 11(1953) 1987. 6 - Lawrence, D. J. and Eastman, L.F. - J. Crystal Growth 30 (1975) 267. 7 - Rauber, A. - Mat. Res. Bull., 11 (1976) 497. 8 - Daniele, J.J. - App. Phys. Letters, 27 (1975) 373. 9 - Raüber, A. and Feisst, A. - J. Crystal Growth 63 (1983) 337. 10 - Raüber, A. - Chemistry and Physics of LiNbO3 - Current Topics in Materials Science, North-Holland, 1978. 11 - Reiche, P. et al. - J. Crystal Growth 108 (1991) 759. 12 - Uda, S. and Tiller, W. A. - J. Crystal Growth 121 (1992) 93. 13 - Uda, S. and Tiller, W. A. - J. Crystal Growth 126 (1993) 396. 14 - Uda, S. et al. - J. Crystal Growth 179 (1997) 567. 15 - Uda, S. et al. - J. Crystal Growth 182 (1997) 403. 16 - Uda, S. et al. - J. Crystal Growth 155 (1995) 229 17 - Nassau, K. , Levinstein, H.J. and Loiacono, G. M. - J. Phys. Chem. Solids, 27 (1966) 989. 18 - Octaviano, E. S. and Andreeta, J.P. - Revista de Física Aplicada e Instrumentação, vol. 3, 1(1988) 30. 19 - Corre, S. et al. – J. Crystal Growth 180(1997)604 20 – Uda, S and Tiller, W. A - J. Crystal Growth 121(1992)155 21 - Angus, J. et al. - Physical Chemistry of Process Metallurgy, Part II, Interscience, N. York, (1961) 833. 22 - Pfann, W.G. and Wagner, R. S. - Trans AIME, 224 (1962) 1139.
24 - Verhoeven, J. D. - Trans AIME, 223 (1965) 1156. 25 - Garandet, J.P. – J. Crystal Growth 131(1993)431 26 - Andreeta, J.P. , Octaviano, E.S. and Gallo, N.J.H. - Journal of Mat. Science 28 (1993)65. 27 - Octaviano, E. S – Ph. D Thesis, Instituto de Física e Química de São Carlos, USP,1991. Yüklə 49,16 Kb. Dostları ilə paylaş:
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