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STM combined with field-emission microscope
Ultramicroscopy 42-44 (1992) 1574-1579 North-Holland
Thermofield tip formation in U H V / S T M combined with field-emission rriicroscope A.O. Golubok, S.A. M a s a l o v a n d N . A . T a r a s o v Institute for Analytical Instrumentation, 26 Ogorodnikova, St. Petersburg 198103, Russia Received 12 August 1991
One of the essential STM experimental stages is tip formation and refinement. For this aim the UHV/STM (P = 10 - 7 10 -8 Pa) combined with a field-emission microscope has been designed. Refinement and formation of the tungsten tip surface were made at strong electrical field (F = 10 6 - 1 0 7 g / c m ) and high temperature (T = 1500-2500 K). The compact electron gun mounted not far from the tip microscanner was used for tip heating. The series of field-emission tungsten-tipsurface images and Fowler-Nordheim characteristics obtained at the different thermofield treatment stages are shown and discussed. The field-emission W-tip images and Fowler-Nordheim characteristic examination allowed us to estimate the changes of tip geometry and refinement level in the process of thermofield treatment.
1. Introduction Tip preparation is one of the important stages of the STM experiment. First the spatial resolution A is due to the size and electronic properties of the tip [1]:
A ~ [( R + d l / v / ~ ] '/2,
(1)
where R is the tip radius, d the width of the tunneling barrier, and ~p the work function. Also, the tip shape may define the structure of the S T M image. For example, in ref. [2] the authors have seen a n o m a l o u s images that can be acc o u n t e d for by simultaneous tunneling from double tips. It is clear that the tip shape will play an essential role w h e n the tip and the surface corrugation have similar sizes. In the second place, the tunneling current is a convolution of both the tip electronic properties and the sample surface characteristics J~
E IMts
[ 2[f(et) -f(es+eV)]pt(et)
of the tip, and p~(e) the density of states of the sample. Thus, it is clear that stable tips with electronic properties known in advance are necessary. The aim of this work is to develop a tip formation and examination m e t h o d compatible with ultrahigh-vacuum ( U H V ) STM. F r o m our point of view field-ion microscopy ( F I M ) a n d / o r field-electron microscopy (FEM), where the object u n d e r investigation is the tip surface placed in a high electric field [3,4], are the most convenient m e t h o d s for this purpose. In ref. [5] results of a tip study using F I M compatible with U H V / S T M have b e e n shown. We have tried to apply for tip formation and examination the F E M compatible with U H V / S T M . In contrast to FIM, the F E M does not need a buffer gas or low temperatures and works at lower potential differences applied between the tip and the screen; and, therefore, it may be m o r e easily c o m b i n e d with U H V / S T M .
t,s
Xps(Es + e V ) S ( e t - e s ) ,
(2)
where Mts is the tunneling matrix element, f ( e ) is the Fermi function, pt(E) the density of states
2. UHV/STM-FEM design Fig. la shows the total view of the analytical unit of the S T M / F E M combination. T h e piezo-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A.O. Golubok et al. / Thermofield tip formation in UHV / STM
ceramic tube microscanner (1) of the tip is connected to the metal base (2), which contains the quartz plate (3). The base (2), through the piezoceramic tubes (4), is connected to the more heavy base (5). The quartz holder of the sample (6) is placed on the quartz plate (3). The fluorescence screen (7) is used for field-emission observation. The tungsten foil ring with radius of ~ 2 mm, thickness of ~ 0.05 mm and width of ~ 0.5 mm is used as the electron gun (8). The tip is placed in the center of the ring. The STM is suspended with the help of metallic springs, contained in rubber bumpers, in the vacuum chamber at a pressure P = 10-7-10 -8 Pa. For coarse approach of the sample to the tip asymmetric voltage impulses (with different time front) are applied to the piezoceramic tubes (4). As a result they are bent to the left side or to the right side (fig. la),and the quartz plate (3) is irregularly moved relative to the base (5). Varying the shape and the frequency of the impulses applied to the piezoceramic tubes (4), it is possible to change the sample holder velocity and the single step length. Fig. lb shows the histogram of the step-length distribution for the single-step-motion regime of the sample holder. The histogram was measured using STM in the following way. The tip position z] has been recalled after the tunneling current capture. Then the tip is moved back from the sample surface (the tip motion range along the z~ direction is L = 103 nm), and after that the asymmetric single-voltage impulse has been applied to the tubes (4). As a result the sample holder (6)
1575
has made a single step in the direction of the tip. After that, the microscanner (1) has moved the tip again in the direction of the sample. When the tunneling current has been captured the new tip position z 2 is recalled. We took the value z l - z 2 as a single step length. Varying the shape of the applied impulse, we could change the direction of the sample holder motion. Repeating the above-described procedure many times it was possible to measure the mean value and dispersion of the sample holder step in both positive and negative z-direction with good enough statistics. It is seen from figs. lb and lc that the value of the mean step is z ( + ) = 3 7 . 7 +30.2 nm and z ( - ) = 47.4 + 41.6 nm; that is quite large enough for capturing the tunneling current without tipsample contact. In the single-step regime the sample holder velocity was v = 102 n m / s . When the distance between the tip and sample was large enough, ~ 103 nm, we increased the frequency of the applied impulses and, consequently, the sample holder velocity up to v ~ 10 6 nm/s.
On the extreme left sample-holder position the tip-sample distance is ~ 10 mm. This is sufficient for obtaining a field-emission image of the tip apex on the screen (7). In this case a positive potential V = 500-10000 V is applied to the tip apex relative to the screen. In order to clean the surface the tip apex is heated with the help of the electron gun (8). The power of the electron gun was R ~ 3 0 W. The voltage, about 500 V, is applied between the tip and W heater. Radiated 50
r
a 311
-100
0
100
-100
0
100
Fig. 1. Total view of the analytical unit of the S T M / F E M combination (a) and experimental step-length distribution, for two directions of motion: (b) z( + ) direction, (c) z( - ) direction.
1576
A.O. Golubok et al. / Thermofield tip formation in U H V / STM
by the gun the thermofield electrons are focused on the apex of the tip. It was possible to heat the tip apex using short impulses (0.1-5 s) up to about 3000 K without destruction of the piezoceramic polarization. The temperature was measured by an optic pyrometer.
3. R e s u l t s
sponding to curve 1 consisted of a set of disordered spots on the screen. This image was not quite stable. In figs. 3a, 3b, 3c and 3d the fieldemission images correspond to curves 2, 3, 4 and 8 of fig. 2. Note that the images corresponding to curves 5, 6, 7 and 8 are identical. As it is well known the slope of the F / N curves is caused by the work function value ¢
and discussion
d[lg(I/U2)]
Examination of the tip surface was made using both field-emission images on the screen (7) and measurement of the F o w l e r - N o r d h e i m ( F / N ) dependencies [4]:
J( F, ~o) =A( F, q~)F 2 ×exp(-
B( F,
~) q)3/2 ) F v(F, q~)
(3)
where F(F, ~) is the emission-current density, F the electric field, ~o the work function, v(F, q~) the Nordheim renormalisation function, and A(F, q~) and B(F, q~) are slowly changing functions. Fig. 2 shows the set of F / N curves which have been obtained after some annealing stages of the tip. Each stage is characterized by the time of heating, 1 min. The temperature of heating was T---2000 K. The field-emission image corre-
-22
mr-
- -0"296q~3/2s( F' g~)/3-"
d[1/U]
(4) where I is the field-emission current, U the t i p screen potential difference, s(F, q~) the Nordheim renormalisation function, and /3 the field factor ( F =/3U). As is seen from fig. 2a, the work function becomes constant after 4 - 5 heating stages, that is the argument of the surface refinement. The corresponding field-emission image is shown in fig. 3d. It is known [6] as the field-emission image of the W surface with {110} crystal orientation and work function q~{ll0} = 4.5 eV. So, having the set of F / N curves and the set of field-emission images it is possible to get information about the power of the tip surface refinement, crystal orientation and work function. In order to define the values of the radius and the work function of the tip surface we used the F / N expression (3). Substituting I(F, ¢, r ) = J(F, ~p)Trr2, F(U, k, r) = U/kr and approximating the Nordheim function in the form of v(F, q~) = 0.965 - 0.739(3.79F 1/2/¢) one can obtain 7 r × 1 . 5 3 7 × 1 0 10¢ (3r U ) 2
-25
I=
--
0.931q~4(kr) 2 + × exp
[
6.821q)2Ukr + 12.489U 2
-0.659q~3/2kr U
7.25 1 + ¢,/2 ] '
(5)
-28 -- 8
-sl
, 0
I 10
,,
I 20
, 30
io4m Fig. 2. Set of experimental Fowler-Nordheim curves for some annealing stages of the tip.
H e r e r is the tip radius in cm, q~ the work function in eV, U the t i p - s c r e e n potential difference in V, I the field-emission current in A, and k the field coefficient (/3 = 1/kr). The leastsquares method and a X2 criterion were used for the definition of r, q~ and k as parameters. Fig. 4 shows a comparison of the experimental data (curve 8 of fig. 2) with the F / N theory; the
1577
A.O. Golubok et al. / Thermofield tip formation in UHV / STM
e x p e r i m e n t a l d a t a w e r e r e c o n s t r u c t e d using o t h e r c o o r d i n a t e s , which a r e m o r e useful for a c o m p a r i son with the theory. F o r t h e c l e a n surface (curve 8) the following p a r a m e t e r v a l u e s have b e e n o b t a i n e d at the 90% c o n f i d e n c e level: r = 130 + 14 nm, q~ = 4.53 + 0.67 eV, k = 4.9 + 1.7, a n d F = V / k r = 0.047 + 0.017 V / n m . It is seen t h a t t h e p r o p o s e d p r o c e d u r e of the e x p e r i m e n t a l d a t a p r o c e s s i n g gives quite reas o n a b l e values for t h e tip r a d i u s a n d w o r k function. It is i n t e r e s t i n g to n o t e t h a t using this app r o a c h it is possible to see the d y n a m i c s of the tip r a d i u s a n d w o r k function c h a n g e d u r i n g the therm o f i e l d t r e a t m e n t p r o c e s s (fig. 5). A f t e r h e a t i n g up to 2000 K the W tip surface has b e e n r e f i n e d a n d s m o o t h e d a n d as a result t h e tip r a d i u s has b e e n i n c r e a s e d . But f r o m t h e
40
0,4
I 2.7
L 2.9
I
I 5,1
I
I 33
I |.5
k7
Fig. 4. Comparison of the least-squares method fitting with data of fig. 2, curve 8.
Fig. 3. Field-emission images of the tip for various annealing stages: (a) corresponds to curve 2, (b) to curve 3, (c) to curve 4 and (d) to curve 8 of fig. 2.
1578
A.O. Golubok et al. / Thermofield tip formation in UHV / STM
S
t]t]
4' v 9-
Tso
t ]
D ~ 2 r ~ (2 × l O ' ) / x ) [ e x p ( y ) / r r ] 1/2 = 5.9 nm
'°° I
so[ t o0 ,
t 2
4
50
T
,5
t it [
8
c
t 0
6
W e o b t a i n e d a value of the m i c r o t i p a r e a of about
t -29
~, a
~
,~ 4
. . . . 6
8
(.
o tt
-33
tt~i 2
4
6
8
2
4
6
8
Fig. 5. Dependencies of the best-fit parameters (see eq. (5)) on heating stages: (a) work function ~, (b) tip radius r, (c) field coefficient k, (d) example of electrical-field behavior, which was calculated at V = 2 kV. p o i n t o f view of the S T M s p a c e r e s o l u t i o n it will be very useful to have the possibility for c r e a t i o n of a m i c r o t i p on the tip surface. In o r d e r to form m i c r o t i p s we u s e d t h e r m o f i e l d t r e a t m e n t of the tip surface [6,7]. T h e W - t i p was h e a t e d up to 2000 K in t h e p r e s e n c e of a strong electrical field (the p o t e n t i a l d i f f e r e n c e b e t w e e n tip a n d s c r e e n was up to 10 kV). A s a result a m i c r o t i p o r i e n t e d along c e r t a i n crystal d i r e c t i o n s can o c c u r owing to surface diffusion o f the W a t o m s in t h e strong electrical field. T h e s e m i c r o t i p s are frozen in t h e p r e s e n c e of the strong electrical field w h e n the h e a t i n g is switched off. It is k n o w n [7] that a m i c r o t i p p l a c e d on t h e s m o o t h tip surface gives the b e n d p e c u l i a r i t y on the F / N d e p e n d e n c e . Fig. 6a shows this kind of F / N curve which was m e a s u r e d a f t e r t h e t h e r m o f i e l d t r e a t m e n t of t h e tip surface. T h e i n c r e a s e o f t h e t u n n e l c u r r e n t with r e g a r d to the a p p l i e d field is s m a l l e r t h a n the n o r m a l F / N behavior. T h e d e v i a t i o n from the n o r m a l F / N b e h a v i o r m a y b e d u e to t h e p r e s e n c e of s p a c e c h a r g e n e a r t h e microtip. It is k n o w n [7,8] t h a t the influence of t h e space c h a r g e occurs w h e n t h e e m i s s i o n - c u r r e n t density J > 10 6 A / c m < So, using the c o o r d i n a t e s o f the p e c u l i a r p o i n t B in fig. 6a we m a y define the m i c r o t i p size.
\
-3r~ _ . 2
3
104/V
Fig. 6. F / N characteristics, which was measured after thermofield treatment at T = 2500 K, V= 2.5 kV and t _>10 rain (a); and consistent field-emission image of the tip, which was obtained at V = 8 kV (b).
A.O. Golubok et aL / Thermofield tip formation in UHV / STM at x = 1 0 4 / U = 2 a n d y = l n ( I / U z) = - 3 2 . 1 5 . Practically, this single microtip u n d e r our experim e n t a l c o n d i t i o n s c a n n o t be observed o n the screen due to the insufficient brightness. A f t e r increase of the a p p l i e d voltage a n u m b e r of microtips a p p e a r e d o n the screen (fig. 6b).
4. Conclusion T h e p r o p o s e d m e t h o d of the U H V / S T M a n d F E M c o m b i n a t i o n allows o n e to c l e a n a n d form the tip surface, to define the crystal tip surface o r i e n t a t i o n , a n d to o b t a i n q u a n t i t a t i v e estimates for the tip radius a n d work function. This experim e n t a l m e t h o d seems to be fruitful essentially for local spectroscopic surface investigation.
1579
References [1] W. Sacks, S. Gauthier, S. Rousset, J. Klein and M. Esrick, Phys. Rev. B 36 (1987) 961. [2] S. Park, J. Nogami and C.F. Quate, Phys. Rev. B 36 (1987) 2863. [3] E.W. Muller and T.T. Tsong, in: Field Ion Microscopy, Principles and Applications (Elsevier, New York, 1969). [4] A. Motions, in: Field, Thermionic and Secondary Electron Emission Spectroscopy (Plenum, New York, 1984). [5] Th. Michely, K.H. Besocke and M. Teske, J. Microscopy 152 (1988) 77. [6] J.A. Vlasov, O.L. Golubev and V.N. Shreds, J. Physique C 49 (1988) C6-131. [7] Vu Thien Binh and J. Marian, Surf. Sci. 202 (1988) L539. [8] W.P. Dyke and W.W. Dolan, Adv. Electron. Electron Phys. 8 (1956) 89.
Thermofield tip formation in U H V / S T M combined with field-emission rriicroscope A.O. Golubok, S.A. M a s a l o v a n d N . A . T a r a s o v Institute for Analytical Instrumentation, 26 Ogorodnikova, St. Petersburg 198103, Russia Received 12 August 1991
One of the essential STM experimental stages is tip formation and refinement. For this aim the UHV/STM (P = 10 - 7 10 -8 Pa) combined with a field-emission microscope has been designed. Refinement and formation of the tungsten tip surface were made at strong electrical field (F = 10 6 - 1 0 7 g / c m ) and high temperature (T = 1500-2500 K). The compact electron gun mounted not far from the tip microscanner was used for tip heating. The series of field-emission tungsten-tipsurface images and Fowler-Nordheim characteristics obtained at the different thermofield treatment stages are shown and discussed. The field-emission W-tip images and Fowler-Nordheim characteristic examination allowed us to estimate the changes of tip geometry and refinement level in the process of thermofield treatment.
1. Introduction Tip preparation is one of the important stages of the STM experiment. First the spatial resolution A is due to the size and electronic properties of the tip [1]:
A ~ [( R + d l / v / ~ ] '/2,
(1)
where R is the tip radius, d the width of the tunneling barrier, and ~p the work function. Also, the tip shape may define the structure of the S T M image. For example, in ref. [2] the authors have seen a n o m a l o u s images that can be acc o u n t e d for by simultaneous tunneling from double tips. It is clear that the tip shape will play an essential role w h e n the tip and the surface corrugation have similar sizes. In the second place, the tunneling current is a convolution of both the tip electronic properties and the sample surface characteristics J~
E IMts
[ 2[f(et) -f(es+eV)]pt(et)
of the tip, and p~(e) the density of states of the sample. Thus, it is clear that stable tips with electronic properties known in advance are necessary. The aim of this work is to develop a tip formation and examination m e t h o d compatible with ultrahigh-vacuum ( U H V ) STM. F r o m our point of view field-ion microscopy ( F I M ) a n d / o r field-electron microscopy (FEM), where the object u n d e r investigation is the tip surface placed in a high electric field [3,4], are the most convenient m e t h o d s for this purpose. In ref. [5] results of a tip study using F I M compatible with U H V / S T M have b e e n shown. We have tried to apply for tip formation and examination the F E M compatible with U H V / S T M . In contrast to FIM, the F E M does not need a buffer gas or low temperatures and works at lower potential differences applied between the tip and the screen; and, therefore, it may be m o r e easily c o m b i n e d with U H V / S T M .
t,s
Xps(Es + e V ) S ( e t - e s ) ,
(2)
where Mts is the tunneling matrix element, f ( e ) is the Fermi function, pt(E) the density of states
2. UHV/STM-FEM design Fig. la shows the total view of the analytical unit of the S T M / F E M combination. T h e piezo-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A.O. Golubok et al. / Thermofield tip formation in UHV / STM
ceramic tube microscanner (1) of the tip is connected to the metal base (2), which contains the quartz plate (3). The base (2), through the piezoceramic tubes (4), is connected to the more heavy base (5). The quartz holder of the sample (6) is placed on the quartz plate (3). The fluorescence screen (7) is used for field-emission observation. The tungsten foil ring with radius of ~ 2 mm, thickness of ~ 0.05 mm and width of ~ 0.5 mm is used as the electron gun (8). The tip is placed in the center of the ring. The STM is suspended with the help of metallic springs, contained in rubber bumpers, in the vacuum chamber at a pressure P = 10-7-10 -8 Pa. For coarse approach of the sample to the tip asymmetric voltage impulses (with different time front) are applied to the piezoceramic tubes (4). As a result they are bent to the left side or to the right side (fig. la),and the quartz plate (3) is irregularly moved relative to the base (5). Varying the shape and the frequency of the impulses applied to the piezoceramic tubes (4), it is possible to change the sample holder velocity and the single step length. Fig. lb shows the histogram of the step-length distribution for the single-step-motion regime of the sample holder. The histogram was measured using STM in the following way. The tip position z] has been recalled after the tunneling current capture. Then the tip is moved back from the sample surface (the tip motion range along the z~ direction is L = 103 nm), and after that the asymmetric single-voltage impulse has been applied to the tubes (4). As a result the sample holder (6)
1575
has made a single step in the direction of the tip. After that, the microscanner (1) has moved the tip again in the direction of the sample. When the tunneling current has been captured the new tip position z 2 is recalled. We took the value z l - z 2 as a single step length. Varying the shape of the applied impulse, we could change the direction of the sample holder motion. Repeating the above-described procedure many times it was possible to measure the mean value and dispersion of the sample holder step in both positive and negative z-direction with good enough statistics. It is seen from figs. lb and lc that the value of the mean step is z ( + ) = 3 7 . 7 +30.2 nm and z ( - ) = 47.4 + 41.6 nm; that is quite large enough for capturing the tunneling current without tipsample contact. In the single-step regime the sample holder velocity was v = 102 n m / s . When the distance between the tip and sample was large enough, ~ 103 nm, we increased the frequency of the applied impulses and, consequently, the sample holder velocity up to v ~ 10 6 nm/s.
On the extreme left sample-holder position the tip-sample distance is ~ 10 mm. This is sufficient for obtaining a field-emission image of the tip apex on the screen (7). In this case a positive potential V = 500-10000 V is applied to the tip apex relative to the screen. In order to clean the surface the tip apex is heated with the help of the electron gun (8). The power of the electron gun was R ~ 3 0 W. The voltage, about 500 V, is applied between the tip and W heater. Radiated 50
r
a 311
-100
0
100
-100
0
100
Fig. 1. Total view of the analytical unit of the S T M / F E M combination (a) and experimental step-length distribution, for two directions of motion: (b) z( + ) direction, (c) z( - ) direction.
1576
A.O. Golubok et al. / Thermofield tip formation in U H V / STM
by the gun the thermofield electrons are focused on the apex of the tip. It was possible to heat the tip apex using short impulses (0.1-5 s) up to about 3000 K without destruction of the piezoceramic polarization. The temperature was measured by an optic pyrometer.
3. R e s u l t s
sponding to curve 1 consisted of a set of disordered spots on the screen. This image was not quite stable. In figs. 3a, 3b, 3c and 3d the fieldemission images correspond to curves 2, 3, 4 and 8 of fig. 2. Note that the images corresponding to curves 5, 6, 7 and 8 are identical. As it is well known the slope of the F / N curves is caused by the work function value ¢
and discussion
d[lg(I/U2)]
Examination of the tip surface was made using both field-emission images on the screen (7) and measurement of the F o w l e r - N o r d h e i m ( F / N ) dependencies [4]:
J( F, ~o) =A( F, q~)F 2 ×exp(-
B( F,
~) q)3/2 ) F v(F, q~)
(3)
where F(F, ~) is the emission-current density, F the electric field, ~o the work function, v(F, q~) the Nordheim renormalisation function, and A(F, q~) and B(F, q~) are slowly changing functions. Fig. 2 shows the set of F / N curves which have been obtained after some annealing stages of the tip. Each stage is characterized by the time of heating, 1 min. The temperature of heating was T---2000 K. The field-emission image corre-
-22
mr-
- -0"296q~3/2s( F' g~)/3-"
d[1/U]
(4) where I is the field-emission current, U the t i p screen potential difference, s(F, q~) the Nordheim renormalisation function, and /3 the field factor ( F =/3U). As is seen from fig. 2a, the work function becomes constant after 4 - 5 heating stages, that is the argument of the surface refinement. The corresponding field-emission image is shown in fig. 3d. It is known [6] as the field-emission image of the W surface with {110} crystal orientation and work function q~{ll0} = 4.5 eV. So, having the set of F / N curves and the set of field-emission images it is possible to get information about the power of the tip surface refinement, crystal orientation and work function. In order to define the values of the radius and the work function of the tip surface we used the F / N expression (3). Substituting I(F, ¢, r ) = J(F, ~p)Trr2, F(U, k, r) = U/kr and approximating the Nordheim function in the form of v(F, q~) = 0.965 - 0.739(3.79F 1/2/¢) one can obtain 7 r × 1 . 5 3 7 × 1 0 10¢ (3r U ) 2
-25
I=
--
0.931q~4(kr) 2 + × exp
[
6.821q)2Ukr + 12.489U 2
-0.659q~3/2kr U
7.25 1 + ¢,/2 ] '
(5)
-28 -- 8
-sl
, 0
I 10
,,
I 20
, 30
io4m Fig. 2. Set of experimental Fowler-Nordheim curves for some annealing stages of the tip.
H e r e r is the tip radius in cm, q~ the work function in eV, U the t i p - s c r e e n potential difference in V, I the field-emission current in A, and k the field coefficient (/3 = 1/kr). The leastsquares method and a X2 criterion were used for the definition of r, q~ and k as parameters. Fig. 4 shows a comparison of the experimental data (curve 8 of fig. 2) with the F / N theory; the
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A.O. Golubok et al. / Thermofield tip formation in UHV / STM
e x p e r i m e n t a l d a t a w e r e r e c o n s t r u c t e d using o t h e r c o o r d i n a t e s , which a r e m o r e useful for a c o m p a r i son with the theory. F o r t h e c l e a n surface (curve 8) the following p a r a m e t e r v a l u e s have b e e n o b t a i n e d at the 90% c o n f i d e n c e level: r = 130 + 14 nm, q~ = 4.53 + 0.67 eV, k = 4.9 + 1.7, a n d F = V / k r = 0.047 + 0.017 V / n m . It is seen t h a t t h e p r o p o s e d p r o c e d u r e of the e x p e r i m e n t a l d a t a p r o c e s s i n g gives quite reas o n a b l e values for t h e tip r a d i u s a n d w o r k function. It is i n t e r e s t i n g to n o t e t h a t using this app r o a c h it is possible to see the d y n a m i c s of the tip r a d i u s a n d w o r k function c h a n g e d u r i n g the therm o f i e l d t r e a t m e n t p r o c e s s (fig. 5). A f t e r h e a t i n g up to 2000 K the W tip surface has b e e n r e f i n e d a n d s m o o t h e d a n d as a result t h e tip r a d i u s has b e e n i n c r e a s e d . But f r o m t h e
40
0,4
I 2.7
L 2.9
I
I 5,1
I
I 33
I |.5
k7
Fig. 4. Comparison of the least-squares method fitting with data of fig. 2, curve 8.
Fig. 3. Field-emission images of the tip for various annealing stages: (a) corresponds to curve 2, (b) to curve 3, (c) to curve 4 and (d) to curve 8 of fig. 2.
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A.O. Golubok et al. / Thermofield tip formation in UHV / STM
S
t]t]
4' v 9-
Tso
t ]
D ~ 2 r ~ (2 × l O ' ) / x ) [ e x p ( y ) / r r ] 1/2 = 5.9 nm
'°° I
so[ t o0 ,
t 2
4
50
T
,5
t it [
8
c
t 0
6
W e o b t a i n e d a value of the m i c r o t i p a r e a of about
t -29
~, a
~
,~ 4
. . . . 6
8
(.
o tt
-33
tt~i 2
4
6
8
2
4
6
8
Fig. 5. Dependencies of the best-fit parameters (see eq. (5)) on heating stages: (a) work function ~, (b) tip radius r, (c) field coefficient k, (d) example of electrical-field behavior, which was calculated at V = 2 kV. p o i n t o f view of the S T M s p a c e r e s o l u t i o n it will be very useful to have the possibility for c r e a t i o n of a m i c r o t i p on the tip surface. In o r d e r to form m i c r o t i p s we u s e d t h e r m o f i e l d t r e a t m e n t of the tip surface [6,7]. T h e W - t i p was h e a t e d up to 2000 K in t h e p r e s e n c e of a strong electrical field (the p o t e n t i a l d i f f e r e n c e b e t w e e n tip a n d s c r e e n was up to 10 kV). A s a result a m i c r o t i p o r i e n t e d along c e r t a i n crystal d i r e c t i o n s can o c c u r owing to surface diffusion o f the W a t o m s in t h e strong electrical field. T h e s e m i c r o t i p s are frozen in t h e p r e s e n c e of the strong electrical field w h e n the h e a t i n g is switched off. It is k n o w n [7] that a m i c r o t i p p l a c e d on t h e s m o o t h tip surface gives the b e n d p e c u l i a r i t y on the F / N d e p e n d e n c e . Fig. 6a shows this kind of F / N curve which was m e a s u r e d a f t e r t h e t h e r m o f i e l d t r e a t m e n t of t h e tip surface. T h e i n c r e a s e o f t h e t u n n e l c u r r e n t with r e g a r d to the a p p l i e d field is s m a l l e r t h a n the n o r m a l F / N behavior. T h e d e v i a t i o n from the n o r m a l F / N b e h a v i o r m a y b e d u e to t h e p r e s e n c e of s p a c e c h a r g e n e a r t h e microtip. It is k n o w n [7,8] t h a t the influence of t h e space c h a r g e occurs w h e n t h e e m i s s i o n - c u r r e n t density J > 10 6 A / c m < So, using the c o o r d i n a t e s o f the p e c u l i a r p o i n t B in fig. 6a we m a y define the m i c r o t i p size.
\
-3r~ _ . 2
3
104/V
Fig. 6. F / N characteristics, which was measured after thermofield treatment at T = 2500 K, V= 2.5 kV and t _>10 rain (a); and consistent field-emission image of the tip, which was obtained at V = 8 kV (b).
A.O. Golubok et aL / Thermofield tip formation in UHV / STM at x = 1 0 4 / U = 2 a n d y = l n ( I / U z) = - 3 2 . 1 5 . Practically, this single microtip u n d e r our experim e n t a l c o n d i t i o n s c a n n o t be observed o n the screen due to the insufficient brightness. A f t e r increase of the a p p l i e d voltage a n u m b e r of microtips a p p e a r e d o n the screen (fig. 6b).
4. Conclusion T h e p r o p o s e d m e t h o d of the U H V / S T M a n d F E M c o m b i n a t i o n allows o n e to c l e a n a n d form the tip surface, to define the crystal tip surface o r i e n t a t i o n , a n d to o b t a i n q u a n t i t a t i v e estimates for the tip radius a n d work function. This experim e n t a l m e t h o d seems to be fruitful essentially for local spectroscopic surface investigation.
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References [1] W. Sacks, S. Gauthier, S. Rousset, J. Klein and M. Esrick, Phys. Rev. B 36 (1987) 961. [2] S. Park, J. Nogami and C.F. Quate, Phys. Rev. B 36 (1987) 2863. [3] E.W. Muller and T.T. Tsong, in: Field Ion Microscopy, Principles and Applications (Elsevier, New York, 1969). [4] A. Motions, in: Field, Thermionic and Secondary Electron Emission Spectroscopy (Plenum, New York, 1984). [5] Th. Michely, K.H. Besocke and M. Teske, J. Microscopy 152 (1988) 77. [6] J.A. Vlasov, O.L. Golubev and V.N. Shreds, J. Physique C 49 (1988) C6-131. [7] Vu Thien Binh and J. Marian, Surf. Sci. 202 (1988) L539. [8] W.P. Dyke and W.W. Dolan, Adv. Electron. Electron Phys. 8 (1956) 89.