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Fabrication of quantum dots on the InSb (110) surface
Superlattices and Microstructures, Vol. 11, No. 4, 1992
461
FABRICATION OF QUANTUM DOTS ON THE InS b ( i i 0 )
SURFACE
Yong L l a n g Laboratory for Research in the Structure of Matter University of Pennsylvania, Philadelphia, PA 19104 and William E. Packard and John D. Dow Department of Physics and Astronomy Arizona State University, Tempe, AZ 85287 (Received 11 March 1992)
Using a scanning tunneling microscope, quantum dots with diameters of order 25 A have been fabricated on the cleaved InSb(llO) surface in UHV. Both In and Sb atoms were clearly resolved on the cleaved surface.
S c a n n i n g t u n n e l i n g m i c r o s c o p e s (STM's) p r e s e n t the opportunity not only to image a semiconductor surface with unprecedented resolution but also to controllably alter the surface on the Angstrom scale, as a result of the tip-surface interaction. B e c k e t et a l . demonstrated that single atoms could be m a n i p u l a t e d on t h e G e ( l l l ) s u r f a c e with a s i n g l e voltage pulse [I]. A number of authors [2] have "machined" various surfaces on the nanometer scale, either by crashing the STM tip into the surface, by applying voltage pulses, or by local heating -- with experiments performed in air, under liquids, or in ultra-hlgh vacuum. In this paper we present some of the first [3] STM images of the InSb(ll0) surface under ultra-hlgh vacuum and we show how quantum dots can be controllably engraved in the surface. We e m p l o y e d a P a c h y d e r m - 4 s c u l p t e d STM [ 4 ] , which is machined almost in its e n t i r e t y from a single block of stainless steel and hence has unusual vibrational stability. Wlth this STM, we obtained the images of Fig. 1 and Fig. 2 by scanning the In~(ll0) surface. This surface was cleaved in 10 " " Tort vacuum,, and the images were taken with a tunneling current of =120 pA. Fig. i was taken with a negative sample bias of - 0 . 2 V, and so is sensitive to the electronically occupied, Sb-llke states of the valence band. The p r o m i n e n t f e a t u r e s o f t h e imag e a r e Sb a t o m s , s p a c e d b~ t h e accepted lattice c o n s t a n t s ( a s s u m i n g a 29 ° R i g i d R o t a t i o n Model [5] of the surface relaxation). The number o f v i s i b l e d e f e c t s i ~ t h i s r e g i o n o f t h e s u r f a c e i s a b o u t one p e r =10 a t o m s . F i g . 2 shows InSb b e f o r e a n d a f t e r a q u a n t u m dot hole is fabricated on i t s ( 1 1 0 ) s u r f a c e . Positive sample bias of +0.3 V produced the
0 7 4 9 - 6 0 3 6 / 9 2 / 0 4 0 4 6 1 + 03 $ 0 2 . 0 0 / 0
images of Fig. 2a and 2b, which are sensitive to electronically empty, In-derived states. Negative bias of -0.3 V, yielded Fig. 2c. y p o s i t i o n i n g t h e STM t i p o v e r a s p o t for = 10Ls we f o r m e d q u a n t u m d o t s , s u c h a s t h o s e o f F i g s . 2b a nd 2 c . The p o s i t i o n s o f t h e d o t s were easily controllable. The p e r f e c t InSb(llO)
F i g . 1. STM i ma ge o f I n S b ( l l O ) w i t h a t o m i c resolution. The s u r f a c e was c l e a v e d in u l t r a - h i g h vacuum (10 - = ` tort range) a nd imaged at - 0 . 2 V s a m p l e b i a s w i t h 120 pA tunneling current. With negative bias the Sb a t o m s a r e p r o m i n e n t a n d t h e I n a t o m s a r e almost invisible. The d a t a were F o u r i e r filtered.
© 1992 Academic Press Limited
462
Superlattices and Microstructures, Vol. 11, No. 4, 1992
F i g . 2. STH image of InSb(ll0) with a sample bias of (a) +0.3 V, (b) +0.3 V, and (c) -0.3 V; the tunnel current was -120 pA. Individual In atoms are clearly resolved in (a) and (b); Sb atoms are resolved in (c). A quantum dot is visible in (b) and (c). The data were Fourier filtered.
s u r f a c e was more s u s c e p t i b l e to disruption by t h e STM t i p t h a n o t h e r s u r f a c e s we have s t u d i e d [2,6], and s o p r e s e r v a t i o n of the perfect surface required rapid scanning, while the f o r m a t i o n o f quantum d o t s was a l m o s t e f f o r t l e s s . The d i a m e t e r o f t h e d o t i n F i g . 2 i s a b o u t ~ 25 A as can be deduced by recognizing that the Sb atoms o f F i g . 2b e x h i b i t a r e c t a n g u l a r s u r f a c e unit cell of 6.5 A x 4.6 A. Close inspection of Figs. 2b and 2c reveals images of individual Sb and In atoms. The quantum d o t is a small surface void, with a depth of one or two atoms, 2 A to 4 A, as determined by scanning in the dot. The size of
t h e v o i d s can be i n c r e a s e d by h o l d i n g t h e t i p f i x e d f o r a l o n g e r time o r by s c a n n i n g o v e r a very small area. These r e s u l t s d e m o n s t r a t e t h a t A n g s t r o m - s c a l e STM l i t h o g r a p h y o f I n S b ( l l 0 ) s u r f a c e s w i l l be f e a s i b l e , and that it might be p o s s i b l e , i f the electronic structures o f t h e quantum d o t s a r e f a v o r a b i e , to f a b r i c a t e A n g s t r o m - s c a l e memory bite onthis surface. Acknowledsment -- We a r e g r a t e f u l t o t h e U.S. Army R e s e a r c h O f f i c e f o r t h e i r s u p p o r t o f t h i s work ( C o n t r a c t No. DAAL03*87-K-0112). We have also benefited from s t i m u l a t i n g d i a c u e e l o n e o f
Superlattices and Microstructures, Vol. 11, No. 4, 1992 STM design with R. Jaklevlc, W. Kaiser, and S.L. Tang, and we thank B. Swartzentruber for a copy of his STM software. Much of this research was performed when all three authors were at the University of Notre Dame. We thank Prof. M. A. Bowen for being acting editor for this manuscript.
REFERENCES [1] R. $. Becker, J. A. Golovchenko, & B. S. Swartzentruber, Nature 325, 419 (1987). [2] W. E. Packard, Y. Liang, N. Dal, J. D. Dow, R. Nlcolaldes, R. C. Jaklevic, & W. J. Kaiser, J. Microscopy 152, 715 (1988) & references therein.
463 [3] see Y. Lian8, W.-M: Hu, W. E. Packard, & J. D. Dow, Bull. Am. Phys. Soc. 35, 227 (1990). L. J . Whitman, J . k. Strosclo, R. A. Dragoset, Bull. Am. Phys. Soc. 3-5, 226 (1990) have i n d e p e n d e n t l y r e p o r t e d images o f the InSb(110) surface recently. [4] Pachyderm S c i e n t i f i c Industries technlcal r e p o r t TR1000-01. [5] S. Y. Tong, A. R. L u b i n s k y , B. J . M r s t i k , & M. A. Van Hove, Phys. Rev. B 1__77, 3303 ( 1 9 7 8 ) ; D. J . Chadl, Phys. Rev. B 18, 1800 (1978); 1_99, 2074 ( 1 9 7 9 ) ; V. V. F r o e l l c h , M. E. L a p e y r e , J. D. Dow, & R. E. A l l e n , Superlatt. M i c r o s t r u c t . 2, 87-89 ( 1 9 8 5 ) ; R. V. K a s o w s k l , M.-H. T s a i , & J . D. Dow, J . Vac. S c l . T e c h n o l . B 5, 953-5 ( 1 9 8 7 ) . [6] W. E. P a c k a r d , N. Dal, J . D. Dow, R. C. Jaklevic, W. J . Kaiser, b S. L. Tang, J . Vac. S c i . T e c h n o l . A 8, 3512 ( 1 9 9 0 ) .
461
FABRICATION OF QUANTUM DOTS ON THE InS b ( i i 0 )
SURFACE
Yong L l a n g Laboratory for Research in the Structure of Matter University of Pennsylvania, Philadelphia, PA 19104 and William E. Packard and John D. Dow Department of Physics and Astronomy Arizona State University, Tempe, AZ 85287 (Received 11 March 1992)
Using a scanning tunneling microscope, quantum dots with diameters of order 25 A have been fabricated on the cleaved InSb(llO) surface in UHV. Both In and Sb atoms were clearly resolved on the cleaved surface.
S c a n n i n g t u n n e l i n g m i c r o s c o p e s (STM's) p r e s e n t the opportunity not only to image a semiconductor surface with unprecedented resolution but also to controllably alter the surface on the Angstrom scale, as a result of the tip-surface interaction. B e c k e t et a l . demonstrated that single atoms could be m a n i p u l a t e d on t h e G e ( l l l ) s u r f a c e with a s i n g l e voltage pulse [I]. A number of authors [2] have "machined" various surfaces on the nanometer scale, either by crashing the STM tip into the surface, by applying voltage pulses, or by local heating -- with experiments performed in air, under liquids, or in ultra-hlgh vacuum. In this paper we present some of the first [3] STM images of the InSb(ll0) surface under ultra-hlgh vacuum and we show how quantum dots can be controllably engraved in the surface. We e m p l o y e d a P a c h y d e r m - 4 s c u l p t e d STM [ 4 ] , which is machined almost in its e n t i r e t y from a single block of stainless steel and hence has unusual vibrational stability. Wlth this STM, we obtained the images of Fig. 1 and Fig. 2 by scanning the In~(ll0) surface. This surface was cleaved in 10 " " Tort vacuum,, and the images were taken with a tunneling current of =120 pA. Fig. i was taken with a negative sample bias of - 0 . 2 V, and so is sensitive to the electronically occupied, Sb-llke states of the valence band. The p r o m i n e n t f e a t u r e s o f t h e imag e a r e Sb a t o m s , s p a c e d b~ t h e accepted lattice c o n s t a n t s ( a s s u m i n g a 29 ° R i g i d R o t a t i o n Model [5] of the surface relaxation). The number o f v i s i b l e d e f e c t s i ~ t h i s r e g i o n o f t h e s u r f a c e i s a b o u t one p e r =10 a t o m s . F i g . 2 shows InSb b e f o r e a n d a f t e r a q u a n t u m dot hole is fabricated on i t s ( 1 1 0 ) s u r f a c e . Positive sample bias of +0.3 V produced the
0 7 4 9 - 6 0 3 6 / 9 2 / 0 4 0 4 6 1 + 03 $ 0 2 . 0 0 / 0
images of Fig. 2a and 2b, which are sensitive to electronically empty, In-derived states. Negative bias of -0.3 V, yielded Fig. 2c. y p o s i t i o n i n g t h e STM t i p o v e r a s p o t for = 10Ls we f o r m e d q u a n t u m d o t s , s u c h a s t h o s e o f F i g s . 2b a nd 2 c . The p o s i t i o n s o f t h e d o t s were easily controllable. The p e r f e c t InSb(llO)
F i g . 1. STM i ma ge o f I n S b ( l l O ) w i t h a t o m i c resolution. The s u r f a c e was c l e a v e d in u l t r a - h i g h vacuum (10 - = ` tort range) a nd imaged at - 0 . 2 V s a m p l e b i a s w i t h 120 pA tunneling current. With negative bias the Sb a t o m s a r e p r o m i n e n t a n d t h e I n a t o m s a r e almost invisible. The d a t a were F o u r i e r filtered.
© 1992 Academic Press Limited
462
Superlattices and Microstructures, Vol. 11, No. 4, 1992
F i g . 2. STH image of InSb(ll0) with a sample bias of (a) +0.3 V, (b) +0.3 V, and (c) -0.3 V; the tunnel current was -120 pA. Individual In atoms are clearly resolved in (a) and (b); Sb atoms are resolved in (c). A quantum dot is visible in (b) and (c). The data were Fourier filtered.
s u r f a c e was more s u s c e p t i b l e to disruption by t h e STM t i p t h a n o t h e r s u r f a c e s we have s t u d i e d [2,6], and s o p r e s e r v a t i o n of the perfect surface required rapid scanning, while the f o r m a t i o n o f quantum d o t s was a l m o s t e f f o r t l e s s . The d i a m e t e r o f t h e d o t i n F i g . 2 i s a b o u t ~ 25 A as can be deduced by recognizing that the Sb atoms o f F i g . 2b e x h i b i t a r e c t a n g u l a r s u r f a c e unit cell of 6.5 A x 4.6 A. Close inspection of Figs. 2b and 2c reveals images of individual Sb and In atoms. The quantum d o t is a small surface void, with a depth of one or two atoms, 2 A to 4 A, as determined by scanning in the dot. The size of
t h e v o i d s can be i n c r e a s e d by h o l d i n g t h e t i p f i x e d f o r a l o n g e r time o r by s c a n n i n g o v e r a very small area. These r e s u l t s d e m o n s t r a t e t h a t A n g s t r o m - s c a l e STM l i t h o g r a p h y o f I n S b ( l l 0 ) s u r f a c e s w i l l be f e a s i b l e , and that it might be p o s s i b l e , i f the electronic structures o f t h e quantum d o t s a r e f a v o r a b i e , to f a b r i c a t e A n g s t r o m - s c a l e memory bite onthis surface. Acknowledsment -- We a r e g r a t e f u l t o t h e U.S. Army R e s e a r c h O f f i c e f o r t h e i r s u p p o r t o f t h i s work ( C o n t r a c t No. DAAL03*87-K-0112). We have also benefited from s t i m u l a t i n g d i a c u e e l o n e o f
Superlattices and Microstructures, Vol. 11, No. 4, 1992 STM design with R. Jaklevlc, W. Kaiser, and S.L. Tang, and we thank B. Swartzentruber for a copy of his STM software. Much of this research was performed when all three authors were at the University of Notre Dame. We thank Prof. M. A. Bowen for being acting editor for this manuscript.
REFERENCES [1] R. $. Becker, J. A. Golovchenko, & B. S. Swartzentruber, Nature 325, 419 (1987). [2] W. E. Packard, Y. Liang, N. Dal, J. D. Dow, R. Nlcolaldes, R. C. Jaklevic, & W. J. Kaiser, J. Microscopy 152, 715 (1988) & references therein.
463 [3] see Y. Lian8, W.-M: Hu, W. E. Packard, & J. D. Dow, Bull. Am. Phys. Soc. 35, 227 (1990). L. J . Whitman, J . k. Strosclo, R. A. Dragoset, Bull. Am. Phys. Soc. 3-5, 226 (1990) have i n d e p e n d e n t l y r e p o r t e d images o f the InSb(110) surface recently. [4] Pachyderm S c i e n t i f i c Industries technlcal r e p o r t TR1000-01. [5] S. Y. Tong, A. R. L u b i n s k y , B. J . M r s t i k , & M. A. Van Hove, Phys. Rev. B 1__77, 3303 ( 1 9 7 8 ) ; D. J . Chadl, Phys. Rev. B 18, 1800 (1978); 1_99, 2074 ( 1 9 7 9 ) ; V. V. F r o e l l c h , M. E. L a p e y r e , J. D. Dow, & R. E. A l l e n , Superlatt. M i c r o s t r u c t . 2, 87-89 ( 1 9 8 5 ) ; R. V. K a s o w s k l , M.-H. T s a i , & J . D. Dow, J . Vac. S c l . T e c h n o l . B 5, 953-5 ( 1 9 8 7 ) . [6] W. E. P a c k a r d , N. Dal, J . D. Dow, R. C. Jaklevic, W. J . Kaiser, b S. L. Tang, J . Vac. S c i . T e c h n o l . A 8, 3512 ( 1 9 9 0 ) .