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Temperature dependence of the silicon field evaporation voltage
Surface Science 124 (1982) L55-L59 North-Holland Publishing Company
L55
SURFACE SCIENCE LETTERS TEMPERATURE DEPENDENCE OF THE SILICON FIELD EVAPORATION VOLTAGE * G.L. K E L L O G G Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
Received 22 November 1982
The silicon field evaporation voltage has been measured as a function of temperature in ultra-hi~,h vacuum for the first time. A linear decrease in evaporation voltage by nearly a factor of two is observed as the temperature is increased from 50 to 530 K. The anomalous field evaporation (random, cluster desorption) previously observed at low temperatures is found to be replaced by uniform, layer-by-layer field evaporation at temperatures above 150 K. The temperature dependence is discussed in relation to previous measurements carried out in hydrogen, to recent theoretical predictions, and to the mechanism of pulsed laser stimulated field evaporation of silicon.
F i e l d e v a p o r a t i o n is the high field r e m o v a l of surface a t o m s as positive ions [1,2]. Because of its i m p o r t a n c e in field ion m i c r o s c o p y [1] and a t o m - p r o b e m a s s s p e c t r o s c o p y [2], the process of field e v a p o r a t i o n has been extensively s t u d i e d b o t h e x p e r i m e n t a l l y a n d theoretically [1,2]. Very few of these studies, however, have involved semiconductors, in p a r t i c u l a r silicon. M e l m e d a n d Stein [3], who o b t a i n e d the first o r d e r e d field ion images of silicon, found that field e v a p o r a t i o n of silicon in the presence of h y d r o g e n was u n i f o r m a n d p r o c e e d e d l a y e r - b y - l a y e r as in metals. Later, Sakurai et al. [4] f o u n d that the low t e m p e r a t u r e field e v a p o r a t i o n of silicon in v a c u u m was a n o m a l o u s , i.e., the e v a p o r a t i o n process was s p o r a d i c with surface a t o m s d e s o r b i n g p r i m a r i l y as clusters. This result i n d i c a t e d that clean, o r d e r e d silicon surfaces could not be p r e p a r e d b y l o w - t e m p e r a t u r e field e v a p o r a t i o n . A n o t h e r surprising result was r e p o r t e d b y S a k a t a a n d Block [5], who f o u n d that the field e v a p o r a t i o n voltage of silicon in h y d r o g e n increased with increasing temperature, j u s t the o p p o s i t e of the expected behavior. W i t h increasing activity in the a p p l i c a t i o n of high field techniques to studies of silicon surfaces [5-9], it is i m p o r t a n t to o b t a i n m o r e i n f o r m a t i o n on the field e v a p o r a t i o n of silicon a n d a t t e m p t to clarify the a b o v e results. * This work performed at Sandia National Laboratories supported by the US Department of Energy under contract No. DE-AC04-76DP00789. 0039-6028/83/0000-0000/$03.00
© 1983 N o r t h - H o l l a n d
1.56
G.L. Kellogg / Temperature dependence of Si field et:aporation voltage
In the study reported here, we have examined the temperature dependence of the silicon field evaporation voltage in vacuum. The purpose of the study was threefold: (1) to determine the temperature range of the so-called anomalous field evaporation, (2) to ascertain that in vacuum the evaporation voltage decreases with increasing temperature, and (3) to test recent theoretical predictions [10] of the temperature dependence of evaporation fields. An additional motivation for the study relates to the mechanism involved in pulsed-laser-assisted field evaporation [Ill of silicon. The pulsed-laser technique, which employs a short duration laser pulse and a DC voltage to stimulate field evaporation, has been shown to be particularly suited to the study of silicon [8,11]. Thus an understanding of the underlying process is desirable. In the case of metals the desorption mechanism is almost certainly a thermallystimulated process [12]. However, because the laser photon energy is typically greater than the silicon band gap, photoconductivity effects may dominate in silicon. The magnitude of the decrease in evaporation voltage with temperature, in comparison with previous measurements on metal samples [12], should give an indication of the relative importance of thermal effects. The experimental arrangement used for the measurements consisted of a silicon tip mounted on a Pt wire loop opposite to a curved channel plate assembly. Field evaporation was initiated by applying a high DC voltage to the tip and was detected by observing the appearance of ions on the phosphor screen behind the channel plate assembly. The temperature of the tip was varied from its base value of 50 K by passing a DC current through the Pt support loop. The voltage drop across the end portion of the loop was monitored by small diameter potential leads, and resistivity measurements were used to determine the temperature. The entire temperature controller assembly was isolated from ground so that the temperature could be varied while simultaneously applying the high voltage required for field evaporation [121. Prior to conducting the vacuum field evaporation measurements, the tip was first annealed in vacuum for 1 h at - 600 K. Surface oxides were then removed by field evaporation in hydrogen. The hydrogen gas was subsequently pumped out and vacuum field evaporation measurements were begun at a pressure of 5.0 × 10 ~0 Torr. At low temperatures, the sporadic field evaporation reported previously [4] was observed, and the probability of tip failure was quite high. Above 150 K, however, the field evaporation proceeded quite normally with the collapsing (111) planes clearly visible. In fig. 1, a multilayer desorption image of silicon at room temperature is shown. Although the integrated image does not show the ring structure seen in real time, it is clearly more uniform than the "blotchy" pattern observed at low temperatures [4]. That the process is non-random is indicated by the dark region corresponding to the (111) plane (the tip orientation can be seen in the field ion image, shown in fig. lb). The likelihood of tip failure was also found to be significantly reduced at temperatures above 150 K.
G.L, Kellogg / Temperaturedependenceof Si field evaporation voltage
L57
Fig. 1. (a) A multi-layer vacuum field desorption image of silicon at 300 K. The anomalous field evaporation observed at low temperatures does not occur above 150 K. (b) A low temperature field ion image of the same tip in hydrogen showing the location of the (111) plane.
The evaporation voltage required to produce the onset of field evaporation was measured in steps of approximately 30 K up to a temperature of 530 K. Since the rate of evaporation increased dramatically with increasing applied voltage, the onset voltage was very well defined. Except for the base temperature of - 50 K, measurements below 150 K were not taken because of the high probability of tip failure at low temperatures and the inaccuracy of the temperature calibration in this range. Fig. 2 shows the results. A linear decrease of the evaporation voltage over the entire temperature range is seen. The monotonic decrease in evaporation voltage with increasing temperature is the expected result. The previous, unexpected result of Sakata and Block [5] can thus be attributed to the presence of surface hydrogen, as they originally suggested. It is well known that even low partial pressures of hydrogen significantly reduce the evaporation field of silicon [4]. As the sample temperature is increased, surface hydrogen is thermally desorbed causing an increase in the evaporation field. The linear dependence of the evaporation voltage, V, on the temperature, T, does not agree with recent theoretical predictions by Chibane and Forbes [10]. Their model, which assumes a parabolic surface atom bonding well, predicts a linear behavior for a plot of V- ~ v e r s u s T 1/2. The discrepancy probably occurs because of their choice of atomic potential. The parabolic approximation may be valid at low temperatures (good agreement is found for field evaporation of metals below 150 K), but at high temperatures a more realistic atomic potential is required. The experimental data presented here should help in defining the nature of that potential.
G.L. Kellogg / Temperature dependence of Si field evaporation t'oltage
1.58 6.0
~
5.0
> z 0
4.0
I
I
I
I
T Silicon
~ 3.0 u.i 2.0
0
L
L
J
I
I
100
200
300
400
500
600
TEMPERATURE {K )
Fig. 2. A plot of the silicon evaporation voltage versus temperature, the voltage scale can be converted to field strengths using (V/Vo)£ o, where V is the evaporation voltage, V0 the evaporation voltage at 50 K and /;0 the evaporation field at 50 K (usually assumed to be - 2.2 V/~,).
The size of the decrease observed for silicon is comparable to that previously observed for metals [12]. An increase in temperature of 170 K produces a decrease in evaporation voltage of 1.0 kV, a decrease even larger than that observed for W and Mo over the same temperature range. Since thermal processes are responsible for pulsed-laser-assisted field evaporation in metals, this is an indication that they are also important in silicon. A further indication that the mechanism in silicon is not due to photoconductivity effects comes from previous measurements of the charge state distribution of field evaporated silicon ions [13]. As the power of the applied laser pulses is increased and the applied voltage is decreased to produce a constant evaporation rate, the charge state changes from Si 2+ to Si + . The shifting charge states indicate that the field strength is changing as the applied voltage is reduced [13]. If photoconductivity effects were entirely responsible, the field strength at the time of desorption, and hence the charge state, would not change (laser induced photoconduction would increase the voltage to produce the field strength necessary for field evaporation). Thus, we conclude that the mechanism is due to thermal, rather than photoconductivity, processes. In summary, the temperature dependence of the silicon evaporation voltage has been measured. The evaporation voltage is found to decrease with increasing temperature, in agreement with the accepted picture of field evaporation (i.e., a thermally activated desorption over a field-reduced surface potential). Previous results, where the evaporation voltage was found to increase with increasing temperature, now can be explained unambiguously by the thermal desorption of surface hydrogen, whose presence lowers the silicon evaporation
G.L. Kellogg / Temperature dependence of Si field evaporation voltage
[.59
v o l t a g e . T h e l i n e a r d e p e n d e n c e of the d e c r e a s e in e v a p o r a t i o n v o l t a g e d o e s n o t a g r e e w i t h r e c e n t t h e o r e t i c a l p r e d i c t i o n s , m o s t likely d u e to the a s s u m p t i o n of a p a r a b o l i c s u r f a c e a t o m p o t e n t i a l , w h i c h is n o t valid at h i g h e r t e m p e r a t u r e s . T h e a n o m a l o u s field e v a p o r a t i o n p r e v i o u s l y o b s e r v e d in v a c u u m is f o u n d n o t to o c c u r at t e m p e r a t u r e s a b o v e 150 K. T h i s i n d i c a t e s that the p r e p a r a t i o n o f clean, o r d e r e d silicon s u r f a c e s by v a c u u m field e v a p o r a t i o n c a n b e a c c o m p l i s h e d by e m p l o y i n g t e m p e r a t u r e s a b o v e this value, F i n a l l y , the m a g n i t u d e of the d e c r e a s e a l o n g w i t h p r e v i o u s m e a s u r e m e n t s of c h a r g e state d i s t r i b u t i o n s suggest t h a t p u l s e d - l a s e r - s t i m u l a t e d field e v a p o r a t i o n of silicon is a t h e r m a l l y s t i m u l a t e d process. T h e a u t h o r wishes to a c k n o w l e d g e the technical a s s i s t a n c e of L . M . K a r k i e w i c z in this i n v e s t i g a t i o n .
References [1] See, for example, E.W. Mialler and T.T. Tsong, Field Ion Microscopy, Principles and Applications (Elsevier, New York, 1969). [2t See, for example, E.W. Mtiller and T.T. Tsong, in: Progress in Surface Science, Ed. S.G. Davison (Pergamon, New York, 1973) Vol. 4, Part 1. [3] A.J. Melmed and R.J. Stein, Surface Sci. 49 (1975) 645. [4] T. Sakurai, R.J. Culbertson and A.J. Melmed, Surface Sci. 78 (1978) L221. [5] T. Sakata and J.H. Block, in: Proc. 28th Intern. Field Emission Symp., Portland, OR, 1981, Eds. L. Swanson and A. Bell, p. 143. [6] T. Sakurai, E.W. MOiler, R.J. Culbertson and A.J. Melmed, Phys. Rev. Letters 39 (1977) 578. [7] G.L. Kellogg, Appl. Surface Sci. 11/12 (1982) 186. [8] G.D.W. Smith, C.R.M. Grovenor, K.M. Delargy, T.J. Godfrey and A.R. McCabe, in: Proc. 29th Intern. Field Emission Symp., Eds. H. Nord6den and H-O. Andr+n (Almqvist and Wiksell, Stockholm, 1982). [9] G.L. Kellogg, J. Vacuum Sci. Technol. in press. [10] K. Chibane and R.G. Forbes, Surface Sci. 122 (1982) 191. [11] G.L. Kellogg and T.T. Tsong, J. Appl. Phys. 51 (1980) 1184. [12] G.L. Kellogg, J. Appl. Phys. 52 (1981) 5320. [13] G.L. Kellogg, Surface Sci. 120 (1982) 319.
L55
SURFACE SCIENCE LETTERS TEMPERATURE DEPENDENCE OF THE SILICON FIELD EVAPORATION VOLTAGE * G.L. K E L L O G G Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
Received 22 November 1982
The silicon field evaporation voltage has been measured as a function of temperature in ultra-hi~,h vacuum for the first time. A linear decrease in evaporation voltage by nearly a factor of two is observed as the temperature is increased from 50 to 530 K. The anomalous field evaporation (random, cluster desorption) previously observed at low temperatures is found to be replaced by uniform, layer-by-layer field evaporation at temperatures above 150 K. The temperature dependence is discussed in relation to previous measurements carried out in hydrogen, to recent theoretical predictions, and to the mechanism of pulsed laser stimulated field evaporation of silicon.
F i e l d e v a p o r a t i o n is the high field r e m o v a l of surface a t o m s as positive ions [1,2]. Because of its i m p o r t a n c e in field ion m i c r o s c o p y [1] and a t o m - p r o b e m a s s s p e c t r o s c o p y [2], the process of field e v a p o r a t i o n has been extensively s t u d i e d b o t h e x p e r i m e n t a l l y a n d theoretically [1,2]. Very few of these studies, however, have involved semiconductors, in p a r t i c u l a r silicon. M e l m e d a n d Stein [3], who o b t a i n e d the first o r d e r e d field ion images of silicon, found that field e v a p o r a t i o n of silicon in the presence of h y d r o g e n was u n i f o r m a n d p r o c e e d e d l a y e r - b y - l a y e r as in metals. Later, Sakurai et al. [4] f o u n d that the low t e m p e r a t u r e field e v a p o r a t i o n of silicon in v a c u u m was a n o m a l o u s , i.e., the e v a p o r a t i o n process was s p o r a d i c with surface a t o m s d e s o r b i n g p r i m a r i l y as clusters. This result i n d i c a t e d that clean, o r d e r e d silicon surfaces could not be p r e p a r e d b y l o w - t e m p e r a t u r e field e v a p o r a t i o n . A n o t h e r surprising result was r e p o r t e d b y S a k a t a a n d Block [5], who f o u n d that the field e v a p o r a t i o n voltage of silicon in h y d r o g e n increased with increasing temperature, j u s t the o p p o s i t e of the expected behavior. W i t h increasing activity in the a p p l i c a t i o n of high field techniques to studies of silicon surfaces [5-9], it is i m p o r t a n t to o b t a i n m o r e i n f o r m a t i o n on the field e v a p o r a t i o n of silicon a n d a t t e m p t to clarify the a b o v e results. * This work performed at Sandia National Laboratories supported by the US Department of Energy under contract No. DE-AC04-76DP00789. 0039-6028/83/0000-0000/$03.00
© 1983 N o r t h - H o l l a n d
1.56
G.L. Kellogg / Temperature dependence of Si field et:aporation voltage
In the study reported here, we have examined the temperature dependence of the silicon field evaporation voltage in vacuum. The purpose of the study was threefold: (1) to determine the temperature range of the so-called anomalous field evaporation, (2) to ascertain that in vacuum the evaporation voltage decreases with increasing temperature, and (3) to test recent theoretical predictions [10] of the temperature dependence of evaporation fields. An additional motivation for the study relates to the mechanism involved in pulsed-laser-assisted field evaporation [Ill of silicon. The pulsed-laser technique, which employs a short duration laser pulse and a DC voltage to stimulate field evaporation, has been shown to be particularly suited to the study of silicon [8,11]. Thus an understanding of the underlying process is desirable. In the case of metals the desorption mechanism is almost certainly a thermallystimulated process [12]. However, because the laser photon energy is typically greater than the silicon band gap, photoconductivity effects may dominate in silicon. The magnitude of the decrease in evaporation voltage with temperature, in comparison with previous measurements on metal samples [12], should give an indication of the relative importance of thermal effects. The experimental arrangement used for the measurements consisted of a silicon tip mounted on a Pt wire loop opposite to a curved channel plate assembly. Field evaporation was initiated by applying a high DC voltage to the tip and was detected by observing the appearance of ions on the phosphor screen behind the channel plate assembly. The temperature of the tip was varied from its base value of 50 K by passing a DC current through the Pt support loop. The voltage drop across the end portion of the loop was monitored by small diameter potential leads, and resistivity measurements were used to determine the temperature. The entire temperature controller assembly was isolated from ground so that the temperature could be varied while simultaneously applying the high voltage required for field evaporation [121. Prior to conducting the vacuum field evaporation measurements, the tip was first annealed in vacuum for 1 h at - 600 K. Surface oxides were then removed by field evaporation in hydrogen. The hydrogen gas was subsequently pumped out and vacuum field evaporation measurements were begun at a pressure of 5.0 × 10 ~0 Torr. At low temperatures, the sporadic field evaporation reported previously [4] was observed, and the probability of tip failure was quite high. Above 150 K, however, the field evaporation proceeded quite normally with the collapsing (111) planes clearly visible. In fig. 1, a multilayer desorption image of silicon at room temperature is shown. Although the integrated image does not show the ring structure seen in real time, it is clearly more uniform than the "blotchy" pattern observed at low temperatures [4]. That the process is non-random is indicated by the dark region corresponding to the (111) plane (the tip orientation can be seen in the field ion image, shown in fig. lb). The likelihood of tip failure was also found to be significantly reduced at temperatures above 150 K.
G.L, Kellogg / Temperaturedependenceof Si field evaporation voltage
L57
Fig. 1. (a) A multi-layer vacuum field desorption image of silicon at 300 K. The anomalous field evaporation observed at low temperatures does not occur above 150 K. (b) A low temperature field ion image of the same tip in hydrogen showing the location of the (111) plane.
The evaporation voltage required to produce the onset of field evaporation was measured in steps of approximately 30 K up to a temperature of 530 K. Since the rate of evaporation increased dramatically with increasing applied voltage, the onset voltage was very well defined. Except for the base temperature of - 50 K, measurements below 150 K were not taken because of the high probability of tip failure at low temperatures and the inaccuracy of the temperature calibration in this range. Fig. 2 shows the results. A linear decrease of the evaporation voltage over the entire temperature range is seen. The monotonic decrease in evaporation voltage with increasing temperature is the expected result. The previous, unexpected result of Sakata and Block [5] can thus be attributed to the presence of surface hydrogen, as they originally suggested. It is well known that even low partial pressures of hydrogen significantly reduce the evaporation field of silicon [4]. As the sample temperature is increased, surface hydrogen is thermally desorbed causing an increase in the evaporation field. The linear dependence of the evaporation voltage, V, on the temperature, T, does not agree with recent theoretical predictions by Chibane and Forbes [10]. Their model, which assumes a parabolic surface atom bonding well, predicts a linear behavior for a plot of V- ~ v e r s u s T 1/2. The discrepancy probably occurs because of their choice of atomic potential. The parabolic approximation may be valid at low temperatures (good agreement is found for field evaporation of metals below 150 K), but at high temperatures a more realistic atomic potential is required. The experimental data presented here should help in defining the nature of that potential.
G.L. Kellogg / Temperature dependence of Si field evaporation t'oltage
1.58 6.0
~
5.0
> z 0
4.0
I
I
I
I
T Silicon
~ 3.0 u.i 2.0
0
L
L
J
I
I
100
200
300
400
500
600
TEMPERATURE {K )
Fig. 2. A plot of the silicon evaporation voltage versus temperature, the voltage scale can be converted to field strengths using (V/Vo)£ o, where V is the evaporation voltage, V0 the evaporation voltage at 50 K and /;0 the evaporation field at 50 K (usually assumed to be - 2.2 V/~,).
The size of the decrease observed for silicon is comparable to that previously observed for metals [12]. An increase in temperature of 170 K produces a decrease in evaporation voltage of 1.0 kV, a decrease even larger than that observed for W and Mo over the same temperature range. Since thermal processes are responsible for pulsed-laser-assisted field evaporation in metals, this is an indication that they are also important in silicon. A further indication that the mechanism in silicon is not due to photoconductivity effects comes from previous measurements of the charge state distribution of field evaporated silicon ions [13]. As the power of the applied laser pulses is increased and the applied voltage is decreased to produce a constant evaporation rate, the charge state changes from Si 2+ to Si + . The shifting charge states indicate that the field strength is changing as the applied voltage is reduced [13]. If photoconductivity effects were entirely responsible, the field strength at the time of desorption, and hence the charge state, would not change (laser induced photoconduction would increase the voltage to produce the field strength necessary for field evaporation). Thus, we conclude that the mechanism is due to thermal, rather than photoconductivity, processes. In summary, the temperature dependence of the silicon evaporation voltage has been measured. The evaporation voltage is found to decrease with increasing temperature, in agreement with the accepted picture of field evaporation (i.e., a thermally activated desorption over a field-reduced surface potential). Previous results, where the evaporation voltage was found to increase with increasing temperature, now can be explained unambiguously by the thermal desorption of surface hydrogen, whose presence lowers the silicon evaporation
G.L. Kellogg / Temperature dependence of Si field evaporation voltage
[.59
v o l t a g e . T h e l i n e a r d e p e n d e n c e of the d e c r e a s e in e v a p o r a t i o n v o l t a g e d o e s n o t a g r e e w i t h r e c e n t t h e o r e t i c a l p r e d i c t i o n s , m o s t likely d u e to the a s s u m p t i o n of a p a r a b o l i c s u r f a c e a t o m p o t e n t i a l , w h i c h is n o t valid at h i g h e r t e m p e r a t u r e s . T h e a n o m a l o u s field e v a p o r a t i o n p r e v i o u s l y o b s e r v e d in v a c u u m is f o u n d n o t to o c c u r at t e m p e r a t u r e s a b o v e 150 K. T h i s i n d i c a t e s that the p r e p a r a t i o n o f clean, o r d e r e d silicon s u r f a c e s by v a c u u m field e v a p o r a t i o n c a n b e a c c o m p l i s h e d by e m p l o y i n g t e m p e r a t u r e s a b o v e this value, F i n a l l y , the m a g n i t u d e of the d e c r e a s e a l o n g w i t h p r e v i o u s m e a s u r e m e n t s of c h a r g e state d i s t r i b u t i o n s suggest t h a t p u l s e d - l a s e r - s t i m u l a t e d field e v a p o r a t i o n of silicon is a t h e r m a l l y s t i m u l a t e d process. T h e a u t h o r wishes to a c k n o w l e d g e the technical a s s i s t a n c e of L . M . K a r k i e w i c z in this i n v e s t i g a t i o n .
References [1] See, for example, E.W. Mialler and T.T. Tsong, Field Ion Microscopy, Principles and Applications (Elsevier, New York, 1969). [2t See, for example, E.W. Mtiller and T.T. Tsong, in: Progress in Surface Science, Ed. S.G. Davison (Pergamon, New York, 1973) Vol. 4, Part 1. [3] A.J. Melmed and R.J. Stein, Surface Sci. 49 (1975) 645. [4] T. Sakurai, R.J. Culbertson and A.J. Melmed, Surface Sci. 78 (1978) L221. [5] T. Sakata and J.H. Block, in: Proc. 28th Intern. Field Emission Symp., Portland, OR, 1981, Eds. L. Swanson and A. Bell, p. 143. [6] T. Sakurai, E.W. MOiler, R.J. Culbertson and A.J. Melmed, Phys. Rev. Letters 39 (1977) 578. [7] G.L. Kellogg, Appl. Surface Sci. 11/12 (1982) 186. [8] G.D.W. Smith, C.R.M. Grovenor, K.M. Delargy, T.J. Godfrey and A.R. McCabe, in: Proc. 29th Intern. Field Emission Symp., Eds. H. Nord6den and H-O. Andr+n (Almqvist and Wiksell, Stockholm, 1982). [9] G.L. Kellogg, J. Vacuum Sci. Technol. in press. [10] K. Chibane and R.G. Forbes, Surface Sci. 122 (1982) 191. [11] G.L. Kellogg and T.T. Tsong, J. Appl. Phys. 51 (1980) 1184. [12] G.L. Kellogg, J. Appl. Phys. 52 (1981) 5320. [13] G.L. Kellogg, Surface Sci. 120 (1982) 319.