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Self-organized quantum dot formation by ion sputtering
ELSEVIER
Microelectronic Engineering 53 (2000) 245-248 www.elsevier.nl/locate/mee
S e l f - o r g a n i z e d q u a n t u m dot f o r m a t i o n b y ion sputtering S. Facsko, T. Dekorsy, C. Trappe, and H. Kurz Institute for Semiconductor Electronics, RWTH Aachen, Sommerfeldstr. 24, 52074 Aachen, Germany
Key requirements for the fabrication of semiconductor quantum dots (QD) are their size, size distribution and density. Beyond conventional lithography methods self-organization processes are promising for the realization of QDs. We present a new route for the generation of quantum dots, which in contrast to epitaxial growth is a subtractive self-organized and self-ordered method. It is based on a surface instability induced by low energy and normal incidence ion bombardment of semiconductor surfaces. Uniform crystalline islands with dimensions as small as 15 nm and densities up to 1011cm-2 are formed by a cooperative process on the surface of semiconductors during continuous ion sputtering.
1.
~TRODUCTION
Semiconductor quantum dots are expected to play an important role in future electronic and optoelectronic applications, like low threshold lasers [1], single electron transistors, or lateral resonant tunneling diodes. Self-organization methods for the production of nanostructures have been successfully used in the fabrication of small and defect-free quantum dots. Spontaneous growth of quantum dots by the Stranski-Krastanow (SK) mode was performed at large in the past. Their potential in optoelectronics and nanoelectronics are severely hampered by a rather large spread in size distribution through thermal fluctuations and the complexity of the MBE-process involved. Recently, we discovered a new self-organization process during the erosion of semiconductor surfaces by ion sputtering for the generation of QDs [2]. Under normal incident and low energy ion bombardment a regular dot pattern appears on initially flat GaSb (100) and InSb (100) surfaces. The uniformity and ordering of the dots results from a surface instability induced by the curvature dependence of the sputtering yield [3]. Surface morphologies induced by ion bombardment have been a subject of intense investigations since years [4,5]. In general the roughness of surfaces increases during the erosion by ion sputtering. It has been found that under certain sputter conditions regular ripple patterns appear under bombard0167-9317/00/$ - see front matter PII: S 0 1 6 7 - 9 3 1 7 ( 0 0 ) 0 0 3 0 7 - 5
ment with non-normal incident ions, which have been observed on semiconductor [6,7], metal [8,9] and insulator surfaces [10]. Their direction is either parallel or perpendicular to the direction of the ion beam, depending on the angle of incidence. In contrast to these investigations we use normal incident ions. Instead of coherent ripple formation we observe a regular dot pattern with a characteristic length in the nanometer regime. With regard to future applications it is important to note that a once generated dot pattern can be transferred into the material enabling the formation of quantum dots with a three dimensional electronic confinement.
2.
T H E O R E T I C A L DESCRIPTION
The evolution of the surface height and the dot formation mechanism during ion sputtering can be described by a stochastic growth equation adapted for the erosion by ion sputtering [3,11,12]. The interplay of roughening and smoothing processes governing the formation of periodic structures can be best understood in terms of the spatial frequency spectrum of the surface height h(q,t), with q the spatial wavevector on the surface [7]:
O h ( q , t ) 2 / Ot= vq 2 h(q,t)l 2 - D q 4lh(q,t)l 2 ,
© 2000 Elsevier Science B.V. All rights reserved.
246
S. Facsko et al. / Microelectronic Engineering 53 (2000) 245-248
with v the negative surface tension, and D a positive constant related to the surface diffusivity. The surface instability induced by the curvature dependence of the sputtering yield is expressed as a negative surface tension term v in the erosion equation and describes the tendency of the sputtering process to maximize the surface. The competition between the negative surface tension and the surface diffusion D generates structures with a characteristic wavelength
ranged in a hexagonal superstructure with a spatial correlation extending over more than ten periods. The size distribution obtained from the SEM imagc and a Gaussian fit with FWHM of 7 nm is displayed in Fig. 2. In the steady state regime, explained below, the relative size distribution narrows significantly indicating a new regime of self-organized dot formation with a nearly perfect hexagonal order.
/~ o~ x / - D - ~ . Besides thermal diffusion additional smoothing mechanisms may be important, i.e. ioninduced diffusion and viscous flow on amorphous surfaces [6,7]. In the case of ripple evolution the curvature dependence of the erosion velocity is anisotropic and the spatial frequency is given by the maximum of the negative surface tension defining the ripple orientation. At normal incident ion bombardment there is no orientation defined by the sputtering process. The most stable solution for this conditions is the formation of a hexagonal arrangement of dots as observed in our experiments•
3.
EXPERIMENTAL DETAILS
The regular dot structures are produced in a mass spectrometer and separately in a commercial ion etching system. The dots are produced on GaSb (100) wafers and on heterostructures with molecular beam epitaxy (MBE) grown GaSb layers on different substrates (AISb, GaAs). Ar ÷ ions with energies of 100 eV to 2 keV are directed in normal incidence onto the surface with a typical ion fluence of 1 x 1016 c m -2 s -t. The experiments are performed at two temperatures of 60°C and -100°C by cooling the samples with water or liquid nitrogen. During the dot formation the sample temperature is held constant. The dot pattern evolution is observed by analyzing the GaSb surfaces for consecutive ion exposure times and measuring the densities and dot diameters by scanning electron microscopy (SEM).
Fig. 1. SEM-image of a self-organized nanodot structure on a GaSb (100) surface induced by ion bombardment with 420 eV Ar ÷ ions. 50
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.
.
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.
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.
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.
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Dot diameter[nm] Fig. 2. Size distribution of the dots in Fig. 1.
4.
EXPERIMENTAL RESULTS
Fig. 1 shows a SEM image of GaSb dots on GaAs substrate produced by ion sputtering with 450 eV Ar ÷ ions. The dots have an average diameter of 26 nm and a dot density of 6 x 10 ~° cm 2. They are ar-
A key requirement for the application of quantum dots is a high crystal quality. Fig. 3 shows the crosssectional transmission electron microscope image (TEM) of a patterned GaSb surface. The dots are sinusoidal shaped with side wall angles of 60 ° to 70 °
S. Facsko et al. / Microelectronic Engineering 53 (2000) 245-248
and have a high aspect ratio as compared to SK quantum dots. The GaSb crystal structure and the same crystal orientation as the GaSb substrate are observed within the dots covered by an amorphous boundary of approx. 2 nm induced by the low energy ions.
Fig. 3. The cross sectional transmission electron microscope image of a dot pattern on GaSb reveals the high crystallinity inside the dots. The quantum dot formation process is schematically sketched in Fig. 4. The regular nanoscaled dot pattern is formed after some seconds of the ion bombardment. A characteristic wavelength defined by the sputtering and diffusive processes is amplified out from a white noise spectrum corresponding to random surface corrugations. The dot diameter and amplitude increases with exposure time, whereas the dot density remains constant. After an exposure time of 400 s a steady-state morphology is formed, characterized by a close-packed hexagonal arrangement. This pattern is transferred during the
continuous sputtering into the material. In MBE grown multilayer samples the dot structure can be transferred down to the interface generating insulated dots of GaSb on the underlying layer. The controlled stopping of the sputter process is monitored by the mass spectrometric analysis of the sputtered atoms. In the case of a GaSb/AISb sample the appearance of AI in the mass spectrum defines the interface between the GaSb and the AISb Layer. Further sputtering transfers the regular pattern into the substrate allowing the nanostructuring of different materials for which the formation process would be difficult to be initiated. The characteristic wavelength of the superstructure depends on the sputtering conditions (ion energy, ion flux, and substrate temperature), as well as on the properties of the bombarded material (density and penetration depth of the ions). A great advantage for the quantum dot formation by ion sputtering is the separate control over the dot density via the process parameters and over the dot diameter via sputtering time. A careful parametric study leads to the conclusion that for the dot formation on GaSb surfaces the dominant smoothing term is ion-induced diffusion. This result is strongly supported by the observation that the same dot density is achieved at 60°C and 100°C substrate temperature. The uncovered GaSb quantum dots on a AISb substrate show a weak and broad photoluminescence (PL) spectrum with a peak position at 1.1 eV and a FWHM of 300 meV, which is blue shifted relative to the GaSb bulk PL [3]. This shift is attributed to quantum confinement in the dots. The broad spectrum and the low luminescence efficiency is due to the ion induced surface defects, which is visible in the TEM image (Fig. 3) as an amorphous boundary. In order to obtain higher efficiency and narrower
Ar* ions r
GaSb AISb
Dotpattern~, ;; ~,~' ,~ GaSb AISb
GaSb i' ~,; ; quantumdots,. ; ; AlSb
Fig. 4. Principle of quantum dot formation by transferring the self-organized dot pattern in a MBE grown GaSb layer down to the underlying layer
247
248
S. Facsko et al. I Microelectronic Engineering 53 (2000) 245-248
luminescence spectra the surface defects must be removed by appropriate annealing and covering of the dots.
REFERENCES
1.
5.
CONCLUSIONS
In conclusion, we have presented a new selforganization and self-ordering route for the fabrication of uniform semiconductor dots with high densities. The surface instability induced by the curvature dependence of the sputtering yield in competition with diffusive processes leads to the build up of structures with a characteristic spatial period. Under normal incident ion bombardment regular dot structures arranged in a hexagonal pattern appear on GaSb (100) surfaces. The universality of the formation mechanism suggests, that nanoisland can be produced on other III-V and group IV semiconductor materials. The method of QD formation by ion sputtering promises a controlled and cost effective fabrication of crystalline dots. To apply this technique for the fabricating of quantum devices, however, massive further work on narrowing the size distribution, removal of the amorphous cover layer, and possible transfer to other material classes is requested.
ACKNOWLEDGMENTS
The authors would like to thank A. Vogt and H. L. Hartnagel for the epitaxial grown GaSb samples, D. Meertens and M. Feuerbacher for the TEM measurements and C. Moormann and C. Zancke for the SEM images.
Y. Arakawa and H. Sakaki, Appl. Phys. Lett 40, 939 (1982). 2. S. Facsko et. al., Science 285, 1551 (1999). 3. R.M. Bradley and J. M. E. Harper, J. Vac. Sci. Technol. A 6, 2390 (1988). 4. G. Carter, B. Navinsek, and L. Whitton, in Sputtering by Particle Bombardment III, Vol. 64 of Topics in Applied Physics, edited by R. Behrisch and K. Wittmaack (Springer Verlag, 1991). 5. O. Auciello, J. Vac. Sci. Technol., 19 (4), 841 (1981). 6. E. Chason et al., Phys. Rev. Lett. 72, 3040 (1994). 7. G. Carter and V. Vishnyakov, Phys. Rev. B 54, 17647 (1996). 8. S. Rusponi, C. Boragno, and U. Valbusa, Phys. Rev. Lett. 78, 2795 (1997). 9. S. Rusponi, G. Constantini, C. Boragno, and U. Valbusa, Phys. Rev. Lett. 81, 4184 (1998). 10. T.M. Mayer, E. Chason, and A.J. Howard, J. Appl. Phys. 76, 1633 (1994). 11. R. Cuerno and A.-L. Barab~si, Phys. Rev. Lett. 74, 4746 (1995). 12. R. Cuerno et al., Phys. Rev. Lett. 75, 4464 (1995).
Microelectronic Engineering 53 (2000) 245-248 www.elsevier.nl/locate/mee
S e l f - o r g a n i z e d q u a n t u m dot f o r m a t i o n b y ion sputtering S. Facsko, T. Dekorsy, C. Trappe, and H. Kurz Institute for Semiconductor Electronics, RWTH Aachen, Sommerfeldstr. 24, 52074 Aachen, Germany
Key requirements for the fabrication of semiconductor quantum dots (QD) are their size, size distribution and density. Beyond conventional lithography methods self-organization processes are promising for the realization of QDs. We present a new route for the generation of quantum dots, which in contrast to epitaxial growth is a subtractive self-organized and self-ordered method. It is based on a surface instability induced by low energy and normal incidence ion bombardment of semiconductor surfaces. Uniform crystalline islands with dimensions as small as 15 nm and densities up to 1011cm-2 are formed by a cooperative process on the surface of semiconductors during continuous ion sputtering.
1.
~TRODUCTION
Semiconductor quantum dots are expected to play an important role in future electronic and optoelectronic applications, like low threshold lasers [1], single electron transistors, or lateral resonant tunneling diodes. Self-organization methods for the production of nanostructures have been successfully used in the fabrication of small and defect-free quantum dots. Spontaneous growth of quantum dots by the Stranski-Krastanow (SK) mode was performed at large in the past. Their potential in optoelectronics and nanoelectronics are severely hampered by a rather large spread in size distribution through thermal fluctuations and the complexity of the MBE-process involved. Recently, we discovered a new self-organization process during the erosion of semiconductor surfaces by ion sputtering for the generation of QDs [2]. Under normal incident and low energy ion bombardment a regular dot pattern appears on initially flat GaSb (100) and InSb (100) surfaces. The uniformity and ordering of the dots results from a surface instability induced by the curvature dependence of the sputtering yield [3]. Surface morphologies induced by ion bombardment have been a subject of intense investigations since years [4,5]. In general the roughness of surfaces increases during the erosion by ion sputtering. It has been found that under certain sputter conditions regular ripple patterns appear under bombard0167-9317/00/$ - see front matter PII: S 0 1 6 7 - 9 3 1 7 ( 0 0 ) 0 0 3 0 7 - 5
ment with non-normal incident ions, which have been observed on semiconductor [6,7], metal [8,9] and insulator surfaces [10]. Their direction is either parallel or perpendicular to the direction of the ion beam, depending on the angle of incidence. In contrast to these investigations we use normal incident ions. Instead of coherent ripple formation we observe a regular dot pattern with a characteristic length in the nanometer regime. With regard to future applications it is important to note that a once generated dot pattern can be transferred into the material enabling the formation of quantum dots with a three dimensional electronic confinement.
2.
T H E O R E T I C A L DESCRIPTION
The evolution of the surface height and the dot formation mechanism during ion sputtering can be described by a stochastic growth equation adapted for the erosion by ion sputtering [3,11,12]. The interplay of roughening and smoothing processes governing the formation of periodic structures can be best understood in terms of the spatial frequency spectrum of the surface height h(q,t), with q the spatial wavevector on the surface [7]:
O h ( q , t ) 2 / Ot= vq 2 h(q,t)l 2 - D q 4lh(q,t)l 2 ,
© 2000 Elsevier Science B.V. All rights reserved.
246
S. Facsko et al. / Microelectronic Engineering 53 (2000) 245-248
with v the negative surface tension, and D a positive constant related to the surface diffusivity. The surface instability induced by the curvature dependence of the sputtering yield is expressed as a negative surface tension term v in the erosion equation and describes the tendency of the sputtering process to maximize the surface. The competition between the negative surface tension and the surface diffusion D generates structures with a characteristic wavelength
ranged in a hexagonal superstructure with a spatial correlation extending over more than ten periods. The size distribution obtained from the SEM imagc and a Gaussian fit with FWHM of 7 nm is displayed in Fig. 2. In the steady state regime, explained below, the relative size distribution narrows significantly indicating a new regime of self-organized dot formation with a nearly perfect hexagonal order.
/~ o~ x / - D - ~ . Besides thermal diffusion additional smoothing mechanisms may be important, i.e. ioninduced diffusion and viscous flow on amorphous surfaces [6,7]. In the case of ripple evolution the curvature dependence of the erosion velocity is anisotropic and the spatial frequency is given by the maximum of the negative surface tension defining the ripple orientation. At normal incident ion bombardment there is no orientation defined by the sputtering process. The most stable solution for this conditions is the formation of a hexagonal arrangement of dots as observed in our experiments•
3.
EXPERIMENTAL DETAILS
The regular dot structures are produced in a mass spectrometer and separately in a commercial ion etching system. The dots are produced on GaSb (100) wafers and on heterostructures with molecular beam epitaxy (MBE) grown GaSb layers on different substrates (AISb, GaAs). Ar ÷ ions with energies of 100 eV to 2 keV are directed in normal incidence onto the surface with a typical ion fluence of 1 x 1016 c m -2 s -t. The experiments are performed at two temperatures of 60°C and -100°C by cooling the samples with water or liquid nitrogen. During the dot formation the sample temperature is held constant. The dot pattern evolution is observed by analyzing the GaSb surfaces for consecutive ion exposure times and measuring the densities and dot diameters by scanning electron microscopy (SEM).
Fig. 1. SEM-image of a self-organized nanodot structure on a GaSb (100) surface induced by ion bombardment with 420 eV Ar ÷ ions. 50
.
.
.
.
.
.
.
.
.
.
.
.
.
¢/1
40d z
.
,
3o-
~
~o-
o
•
10
...... ~ =
15
,
)
2O
,
)
25
•
i
30
.
i~-,> i
35
. . . . .) . .
40
.
)
45
Dot diameter[nm] Fig. 2. Size distribution of the dots in Fig. 1.
4.
EXPERIMENTAL RESULTS
Fig. 1 shows a SEM image of GaSb dots on GaAs substrate produced by ion sputtering with 450 eV Ar ÷ ions. The dots have an average diameter of 26 nm and a dot density of 6 x 10 ~° cm 2. They are ar-
A key requirement for the application of quantum dots is a high crystal quality. Fig. 3 shows the crosssectional transmission electron microscope image (TEM) of a patterned GaSb surface. The dots are sinusoidal shaped with side wall angles of 60 ° to 70 °
S. Facsko et al. / Microelectronic Engineering 53 (2000) 245-248
and have a high aspect ratio as compared to SK quantum dots. The GaSb crystal structure and the same crystal orientation as the GaSb substrate are observed within the dots covered by an amorphous boundary of approx. 2 nm induced by the low energy ions.
Fig. 3. The cross sectional transmission electron microscope image of a dot pattern on GaSb reveals the high crystallinity inside the dots. The quantum dot formation process is schematically sketched in Fig. 4. The regular nanoscaled dot pattern is formed after some seconds of the ion bombardment. A characteristic wavelength defined by the sputtering and diffusive processes is amplified out from a white noise spectrum corresponding to random surface corrugations. The dot diameter and amplitude increases with exposure time, whereas the dot density remains constant. After an exposure time of 400 s a steady-state morphology is formed, characterized by a close-packed hexagonal arrangement. This pattern is transferred during the
continuous sputtering into the material. In MBE grown multilayer samples the dot structure can be transferred down to the interface generating insulated dots of GaSb on the underlying layer. The controlled stopping of the sputter process is monitored by the mass spectrometric analysis of the sputtered atoms. In the case of a GaSb/AISb sample the appearance of AI in the mass spectrum defines the interface between the GaSb and the AISb Layer. Further sputtering transfers the regular pattern into the substrate allowing the nanostructuring of different materials for which the formation process would be difficult to be initiated. The characteristic wavelength of the superstructure depends on the sputtering conditions (ion energy, ion flux, and substrate temperature), as well as on the properties of the bombarded material (density and penetration depth of the ions). A great advantage for the quantum dot formation by ion sputtering is the separate control over the dot density via the process parameters and over the dot diameter via sputtering time. A careful parametric study leads to the conclusion that for the dot formation on GaSb surfaces the dominant smoothing term is ion-induced diffusion. This result is strongly supported by the observation that the same dot density is achieved at 60°C and 100°C substrate temperature. The uncovered GaSb quantum dots on a AISb substrate show a weak and broad photoluminescence (PL) spectrum with a peak position at 1.1 eV and a FWHM of 300 meV, which is blue shifted relative to the GaSb bulk PL [3]. This shift is attributed to quantum confinement in the dots. The broad spectrum and the low luminescence efficiency is due to the ion induced surface defects, which is visible in the TEM image (Fig. 3) as an amorphous boundary. In order to obtain higher efficiency and narrower
Ar* ions r
GaSb AISb
Dotpattern~, ;; ~,~' ,~ GaSb AISb
GaSb i' ~,; ; quantumdots,. ; ; AlSb
Fig. 4. Principle of quantum dot formation by transferring the self-organized dot pattern in a MBE grown GaSb layer down to the underlying layer
247
248
S. Facsko et al. I Microelectronic Engineering 53 (2000) 245-248
luminescence spectra the surface defects must be removed by appropriate annealing and covering of the dots.
REFERENCES
1.
5.
CONCLUSIONS
In conclusion, we have presented a new selforganization and self-ordering route for the fabrication of uniform semiconductor dots with high densities. The surface instability induced by the curvature dependence of the sputtering yield in competition with diffusive processes leads to the build up of structures with a characteristic spatial period. Under normal incident ion bombardment regular dot structures arranged in a hexagonal pattern appear on GaSb (100) surfaces. The universality of the formation mechanism suggests, that nanoisland can be produced on other III-V and group IV semiconductor materials. The method of QD formation by ion sputtering promises a controlled and cost effective fabrication of crystalline dots. To apply this technique for the fabricating of quantum devices, however, massive further work on narrowing the size distribution, removal of the amorphous cover layer, and possible transfer to other material classes is requested.
ACKNOWLEDGMENTS
The authors would like to thank A. Vogt and H. L. Hartnagel for the epitaxial grown GaSb samples, D. Meertens and M. Feuerbacher for the TEM measurements and C. Moormann and C. Zancke for the SEM images.
Y. Arakawa and H. Sakaki, Appl. Phys. Lett 40, 939 (1982). 2. S. Facsko et. al., Science 285, 1551 (1999). 3. R.M. Bradley and J. M. E. Harper, J. Vac. Sci. Technol. A 6, 2390 (1988). 4. G. Carter, B. Navinsek, and L. Whitton, in Sputtering by Particle Bombardment III, Vol. 64 of Topics in Applied Physics, edited by R. Behrisch and K. Wittmaack (Springer Verlag, 1991). 5. O. Auciello, J. Vac. Sci. Technol., 19 (4), 841 (1981). 6. E. Chason et al., Phys. Rev. Lett. 72, 3040 (1994). 7. G. Carter and V. Vishnyakov, Phys. Rev. B 54, 17647 (1996). 8. S. Rusponi, C. Boragno, and U. Valbusa, Phys. Rev. Lett. 78, 2795 (1997). 9. S. Rusponi, G. Constantini, C. Boragno, and U. Valbusa, Phys. Rev. Lett. 81, 4184 (1998). 10. T.M. Mayer, E. Chason, and A.J. Howard, J. Appl. Phys. 76, 1633 (1994). 11. R. Cuerno and A.-L. Barab~si, Phys. Rev. Lett. 74, 4746 (1995). 12. R. Cuerno et al., Phys. Rev. Lett. 75, 4464 (1995).