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Ge concentration dependence of Sb surface segregation during SiGe MBE
Journal of Crystal Growth 201/202 (1999) 560}563
Ge concentration dependence of Sb surface segregation during SiGe MBE Kiyokazu Nakagawa*, Nobuyuki Sugii, Shinya Yamaguchi, Masanobu Miyao Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan
Abstract Surface segregation of antimony atoms during Si Ge molecular beam epitaxy (MBE) was studied by using the \V V delta-doping technique. The segregation of Sb increases with increasing growth temperature and increasing Ge concentration of Si Ge . This result is well explained by a two-state model in which the energy barrier to Sb surface \V V segregation decreases with increasing Ge concentration of Si Ge . 1999 Published by Elsevier Science B.V. All \V V rights reserved. PACS: 64.75.#g; 81.15.Hi Keywords: Surface segregation of Sb; SiGe molecular beam epitaxy
1. Introduction Si/SiGe heterostructures o!er great advantages because of their higher carrier mobilities than those in conventional Si devices [1]. To realize high-performance devices with ultrahigh electron mobilities, however, controlled modulation doping is needed. We have found that the dopant segregation phenomenon for Sb in a Si Ge layer increases \V V with increasing Ge concentration (x), and Sb doping is di$cult to control in Si Ge . \V V In this paper, we present experimental results concerning the surface segregation of Sb atoms on various Si Ge surfaces. These results enable us \V V
* Corresponding author. Tel.: #81-423-23-1111; fax: #81423-27-7748; e-mail: [email protected].
to clarify the mechanisms of Sb incorporation in the Si Ge layer. \V V
2. Experimental procedure The Si MBE apparatus used is a Vacuum Generator's V80 system with a base pressure of about 5;10\ Torr. This system consists of an e-gun evaporator for Si deposition and e!usion cells for Ge deposition and Sb doping. First, 0.1 ML Sb atoms were deposited on cleaned silicon surfaces at 6003C. Undoped Si Ge layers with various \V V thicknesses and various Ge concentrations (x) were then overgrown on the Sb-deposited surfaces at various substrate temperatures. The surface concentration of the Sb atoms segregated on the Si Ge overgrown surfaces was measured by \V V using X-ray photoelectron spectroscopy (XPS).
0022-0248/99/$ } see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 8 9 - X
K. Nakagawa et al. / Journal of Crystal Growth 201/202 (1999) 560}563
561
Since the escape depth of Sb(3d) photoelectrons is less than about 1 nm, the XPS signals observed mainly come from Sb atoms segregated on the Si Ge epitaxial surface and the contribution \V V from Sb dopant atoms incorporated in the Si Ge is negligible. \V V 3. Experimental results The Sb atoms deposited on the Si surfaces are incorporated partly in the Si Ge layers grown \V V on top of the initial surfaces. And the residual Sb atoms segregate on the MBE overgrown Si Ge \V V surfaces. The segregation length is de"ned as the Si Ge deposition thickness at which the Sb sur\V V face concentration becomes 1/e of the initial surface concentration of Sb (e is the base of natural logarithms). Fig. 1 shows the dependence of the Sb segregation length on temperature. Here the lengths increase steeply with increasing temperature from about 2 nm at 1003C and reach about 1000 nm at 5003C. To study the in#uence of Ge concentration of the Si Ge layer on the Sb \V V segregation, the segregation length at the growth temperature of 3003C was estimated using samples with various Ge concentrations. As shown in Fig. 2,
Fig. 1. Sb surface segregation length as a function of growth temperature.
Fig. 2. Surface segregation length as a function of Ge concentration (x) of Si Ge . \V V
the increase in Ge concentration enhances segregation. We also studied the in#uence of threading dislocations and strain on Sb segregation in Si Ge . \V V These dislocations are introduced when the thickness of the Si Ge layer grown on an Si substrate \V V exceeds the critical thickness in order to release the strain energy caused by the lattice mismatch between Si and Si Ge . While measuring seg\V V regated Sb concentration in the growing Si Ge \V V layer on Si, the threading dislocations were introduced in the Si Ge layer. To clarify the in#uence \V V of these dislocations and strain on surface segregation, we also prepared relaxed surfaces without threading dislocations as follows. An SiGe bu!er layer consisting of a 2 lm stacked layer (combining a 1 lm constant-Ge-content (x) layer with a 1 lm graded-Ge-content (0)y)x) layer below it) was grown on Si(1 0 0) substrates. Transmission electron microscopy (TEM) observation showed that mis"t dislocations are con"ned inside the graded layer; no threading dislocations are observed inside the SiGe top layer, which is nearly strain-free. The segregation lengths on these relaxed layers were the same as those described in Figs. 1 and 2, con"rming that the changes of both threading dislocation density and strain do not cause any di!erence in surface segregation phenomena.
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K. Nakagawa et al. / Journal of Crystal Growth 201/202 (1999) 560}563
4. Discussion
In the model, the Sb concentration in each layer is deduced using the following equations:
The segregation phenomenon can be explained by a two-energy-state model [2,3]. Fig. 3 shows the potential energy diagram for Sb in the Si Ge \V V layer and Sb segregation paths of Si(1 0 0) substrates. The energy of the surface state is lower than that of the subsurface state by an amount equal to E , and the Sb atoms jump over the potential barrier of E and segregate to the surface. In this case, the jumping probability r of Sb from the subsurface to the surface can be written as r "l exp(!E /k¹),
dn (t) "!d r n (t)#d r n (t), dt
(3)
dn (t) "d r n (t)!d r n (t), dt
(4)
where n (t) is the Sb occupant ratio of the surface layer, n (t) is that of the subsurface layer, and d is the number of segregation paths; that is, d is 2 on the Si(1 0 0) surface, as shown in Fig. 3. The "rst
(1)
and the jumping probability r of the reverse pro cess from surface to subsurface can be written as r "l exp(!(E #E )k¹),
(2)
where l is the frequency of attempts to jump over the potential barriers and k is the Boltzmann constant.
Fig. 3. Sb segregation paths of Si(1 0 0) substrates shown as cross-section and schematic illustration of an energy diagram of the two-energy-state model.
Fig. 4. Result of a simulation based on the two-state model. Both decrease of E and increase of E cause the increase of segregation length. As shown in Fig. 2, the temperature dependence becomes lower as the Ge concentration increases. This result is well reproduced by the model in which the increase in the segregation length is due to decrease in E (a) and not due to increase in E (b).
K. Nakagawa et al. / Journal of Crystal Growth 201/202 (1999) 560}563
terms of Eqs. (3) and (4) represent the rates of Sb jumping from the surface to the subsurface, and the second terms represent those from the subsurface to the surface. As the growth temperature increases, the reverse probability (r ) as well as the forward probability (r ) also increase and the system becomes a kind of themal equilibrium state. In this high-temperature region, the ratio of n /n can be expressed only using a Boltzmann factor, exp(E /k¹). This ratio de creases with temperature, that is, the surface segregation phenomenon is reduced with temperature. As a result, the segregation phenomenon reaches its maximum when the reverse process starts to become signi"cant under high-temperature growth conditions, as can be seen in Fig. 1. E mainly determines the segregation behavior in low-temperature regions and E mainly that in high-temper ature regions. Segregation lengths with various "tting parameters are schematically shown in Fig. 4. As shown experimentally in Fig. 1, the segregation length increases with increasing Ge concentration, and the temperature dependencies of the segregation length shift toward lower temperature regions. These shifts are well explained by the decrease of the potential barrier E and not by the increase of the energy di!erence E . In Fig. 4, we only give general trends of segregation phenomena. In order to determine the values of E and E exactly, detailed ex periments are now underway.
563
5. Summary We investigated the dependence of Sb surface segregation in an SiGe layer on both the substrate temperature and the Ge concentration of the Si Ge layer. We obtained the following results. \V V E Segregation length is larger in an SiGe layer with higher Ge concentration. E Enhanced surface segregation caused by Ge atoms is attributed to the reduced energy barrier to Sb segregation. E Threading dislocations and strain (introduced in Si Ge by lattice mismatch between the Si \V V substrate and the Si Ge layer) cause no \V V observable in#uence on Sb surface segregation.
Acknowledgements We thank our co-worker Takeshi Ikezu for his expert technical assistance with the MBE.
References [1] N. Sugii, K. Nakagawa, Y. Kimura, M. Miyao, Jpn. J. Appl. Phys. 37 (1998) 1308. [2] K. Nakagawa, Y. Kimura, M. Miyao, J. Crystal Growth 175/176 (1997) 481. [3] J.J. Harris, D.E. Ashenford, C.T. Foxon, P.J. Dobson, B.A. Joyce, Appl. Phys. A 33 (1984) 87.
Ge concentration dependence of Sb surface segregation during SiGe MBE Kiyokazu Nakagawa*, Nobuyuki Sugii, Shinya Yamaguchi, Masanobu Miyao Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan
Abstract Surface segregation of antimony atoms during Si Ge molecular beam epitaxy (MBE) was studied by using the \V V delta-doping technique. The segregation of Sb increases with increasing growth temperature and increasing Ge concentration of Si Ge . This result is well explained by a two-state model in which the energy barrier to Sb surface \V V segregation decreases with increasing Ge concentration of Si Ge . 1999 Published by Elsevier Science B.V. All \V V rights reserved. PACS: 64.75.#g; 81.15.Hi Keywords: Surface segregation of Sb; SiGe molecular beam epitaxy
1. Introduction Si/SiGe heterostructures o!er great advantages because of their higher carrier mobilities than those in conventional Si devices [1]. To realize high-performance devices with ultrahigh electron mobilities, however, controlled modulation doping is needed. We have found that the dopant segregation phenomenon for Sb in a Si Ge layer increases \V V with increasing Ge concentration (x), and Sb doping is di$cult to control in Si Ge . \V V In this paper, we present experimental results concerning the surface segregation of Sb atoms on various Si Ge surfaces. These results enable us \V V
* Corresponding author. Tel.: #81-423-23-1111; fax: #81423-27-7748; e-mail: [email protected].
to clarify the mechanisms of Sb incorporation in the Si Ge layer. \V V
2. Experimental procedure The Si MBE apparatus used is a Vacuum Generator's V80 system with a base pressure of about 5;10\ Torr. This system consists of an e-gun evaporator for Si deposition and e!usion cells for Ge deposition and Sb doping. First, 0.1 ML Sb atoms were deposited on cleaned silicon surfaces at 6003C. Undoped Si Ge layers with various \V V thicknesses and various Ge concentrations (x) were then overgrown on the Sb-deposited surfaces at various substrate temperatures. The surface concentration of the Sb atoms segregated on the Si Ge overgrown surfaces was measured by \V V using X-ray photoelectron spectroscopy (XPS).
0022-0248/99/$ } see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 8 9 - X
K. Nakagawa et al. / Journal of Crystal Growth 201/202 (1999) 560}563
561
Since the escape depth of Sb(3d) photoelectrons is less than about 1 nm, the XPS signals observed mainly come from Sb atoms segregated on the Si Ge epitaxial surface and the contribution \V V from Sb dopant atoms incorporated in the Si Ge is negligible. \V V 3. Experimental results The Sb atoms deposited on the Si surfaces are incorporated partly in the Si Ge layers grown \V V on top of the initial surfaces. And the residual Sb atoms segregate on the MBE overgrown Si Ge \V V surfaces. The segregation length is de"ned as the Si Ge deposition thickness at which the Sb sur\V V face concentration becomes 1/e of the initial surface concentration of Sb (e is the base of natural logarithms). Fig. 1 shows the dependence of the Sb segregation length on temperature. Here the lengths increase steeply with increasing temperature from about 2 nm at 1003C and reach about 1000 nm at 5003C. To study the in#uence of Ge concentration of the Si Ge layer on the Sb \V V segregation, the segregation length at the growth temperature of 3003C was estimated using samples with various Ge concentrations. As shown in Fig. 2,
Fig. 1. Sb surface segregation length as a function of growth temperature.
Fig. 2. Surface segregation length as a function of Ge concentration (x) of Si Ge . \V V
the increase in Ge concentration enhances segregation. We also studied the in#uence of threading dislocations and strain on Sb segregation in Si Ge . \V V These dislocations are introduced when the thickness of the Si Ge layer grown on an Si substrate \V V exceeds the critical thickness in order to release the strain energy caused by the lattice mismatch between Si and Si Ge . While measuring seg\V V regated Sb concentration in the growing Si Ge \V V layer on Si, the threading dislocations were introduced in the Si Ge layer. To clarify the in#uence \V V of these dislocations and strain on surface segregation, we also prepared relaxed surfaces without threading dislocations as follows. An SiGe bu!er layer consisting of a 2 lm stacked layer (combining a 1 lm constant-Ge-content (x) layer with a 1 lm graded-Ge-content (0)y)x) layer below it) was grown on Si(1 0 0) substrates. Transmission electron microscopy (TEM) observation showed that mis"t dislocations are con"ned inside the graded layer; no threading dislocations are observed inside the SiGe top layer, which is nearly strain-free. The segregation lengths on these relaxed layers were the same as those described in Figs. 1 and 2, con"rming that the changes of both threading dislocation density and strain do not cause any di!erence in surface segregation phenomena.
562
K. Nakagawa et al. / Journal of Crystal Growth 201/202 (1999) 560}563
4. Discussion
In the model, the Sb concentration in each layer is deduced using the following equations:
The segregation phenomenon can be explained by a two-energy-state model [2,3]. Fig. 3 shows the potential energy diagram for Sb in the Si Ge \V V layer and Sb segregation paths of Si(1 0 0) substrates. The energy of the surface state is lower than that of the subsurface state by an amount equal to E , and the Sb atoms jump over the potential barrier of E and segregate to the surface. In this case, the jumping probability r of Sb from the subsurface to the surface can be written as r "l exp(!E /k¹),
dn (t) "!d r n (t)#d r n (t), dt
(3)
dn (t) "d r n (t)!d r n (t), dt
(4)
where n (t) is the Sb occupant ratio of the surface layer, n (t) is that of the subsurface layer, and d is the number of segregation paths; that is, d is 2 on the Si(1 0 0) surface, as shown in Fig. 3. The "rst
(1)
and the jumping probability r of the reverse pro cess from surface to subsurface can be written as r "l exp(!(E #E )k¹),
(2)
where l is the frequency of attempts to jump over the potential barriers and k is the Boltzmann constant.
Fig. 3. Sb segregation paths of Si(1 0 0) substrates shown as cross-section and schematic illustration of an energy diagram of the two-energy-state model.
Fig. 4. Result of a simulation based on the two-state model. Both decrease of E and increase of E cause the increase of segregation length. As shown in Fig. 2, the temperature dependence becomes lower as the Ge concentration increases. This result is well reproduced by the model in which the increase in the segregation length is due to decrease in E (a) and not due to increase in E (b).
K. Nakagawa et al. / Journal of Crystal Growth 201/202 (1999) 560}563
terms of Eqs. (3) and (4) represent the rates of Sb jumping from the surface to the subsurface, and the second terms represent those from the subsurface to the surface. As the growth temperature increases, the reverse probability (r ) as well as the forward probability (r ) also increase and the system becomes a kind of themal equilibrium state. In this high-temperature region, the ratio of n /n can be expressed only using a Boltzmann factor, exp(E /k¹). This ratio de creases with temperature, that is, the surface segregation phenomenon is reduced with temperature. As a result, the segregation phenomenon reaches its maximum when the reverse process starts to become signi"cant under high-temperature growth conditions, as can be seen in Fig. 1. E mainly determines the segregation behavior in low-temperature regions and E mainly that in high-temper ature regions. Segregation lengths with various "tting parameters are schematically shown in Fig. 4. As shown experimentally in Fig. 1, the segregation length increases with increasing Ge concentration, and the temperature dependencies of the segregation length shift toward lower temperature regions. These shifts are well explained by the decrease of the potential barrier E and not by the increase of the energy di!erence E . In Fig. 4, we only give general trends of segregation phenomena. In order to determine the values of E and E exactly, detailed ex periments are now underway.
563
5. Summary We investigated the dependence of Sb surface segregation in an SiGe layer on both the substrate temperature and the Ge concentration of the Si Ge layer. We obtained the following results. \V V E Segregation length is larger in an SiGe layer with higher Ge concentration. E Enhanced surface segregation caused by Ge atoms is attributed to the reduced energy barrier to Sb segregation. E Threading dislocations and strain (introduced in Si Ge by lattice mismatch between the Si \V V substrate and the Si Ge layer) cause no \V V observable in#uence on Sb surface segregation.
Acknowledgements We thank our co-worker Takeshi Ikezu for his expert technical assistance with the MBE.
References [1] N. Sugii, K. Nakagawa, Y. Kimura, M. Miyao, Jpn. J. Appl. Phys. 37 (1998) 1308. [2] K. Nakagawa, Y. Kimura, M. Miyao, J. Crystal Growth 175/176 (1997) 481. [3] J.J. Harris, D.E. Ashenford, C.T. Foxon, P.J. Dobson, B.A. Joyce, Appl. Phys. A 33 (1984) 87.