Mass and energy spectra of Mo ions in field evaporation

apphed surface s o e n c e ELSEVIER

Applied Surface Scmnce 76/77 (1994) 73-79

Mass and energy spectra of Mo ions in field evaporation Jong-bor

Wang,

Tten T

Tsong

*

Instttute of Physws, Academta Smtca, Nankang, Talpez, Tatwan, ROC

(Recewed 2 August 1993, accepted for pubhcatlon 10 September 1993)

Abstract The field-evaporation behavior of Mo is studied with a high-resolution pulsed-laser ume-of-fl~ght atom probe The ion energy distribution of Mo 2+ shows a double peak structure and the ion energy distribution of Mo ~+ has a broad single energy peak To explain the formation of the low energy peak of the M o 2+ I o n s and the abnormally small critical energy deficit of the Mo 3+ ions, we propose a two-step field-evaporation mechanism for molybdenum in which two Mo atoms line up with the field before they are field-ionized Also at low field strengths (3 7 to 4 2 V / , ~ ) and high temperature (about 600 K), triply charged Mo dlmers can be detected

1. Introduction F i e l d e v a p o r a t i o n , t h e d e s o r p t l o n o f surface a t o m s by high e l e c t r i c field, can o c c u r f r o m the surface of a s h a r p tip w h e n a high v o l t a g e is a p p l i e d to t h e tip It has b e e n u s e d for c l e a n i n g a n d s h a p i n g the tip to its e n d f o r m T h e i m p o r t a n c e of it is s e e n in t h e a t o m - p r o b e e x p e r i m e n t s a n d in t h e m a n i p u l a t i o n of a t o m s at t h e surface [1,2] Several m o d e l s have b e e n p r o p o s e d in t h e p a s t to e x p l a i n field e v a p o r a t i o n i n c l u d i n g o n e - d i mensional and three-dimensional desorptlon m e c h a n i s m s [1-4] H o w e v e r , the f u n d a m e n t a l asp e c t o f ~t is by no m e a n s well u n d e r s t o o d W e r e p o r t h e r e a study o f field e v a p o r a t i o n using a high-resolution pulsed-laser TOF atom-probe field ion m i c r o s c o p e U s i n g t h e s a m e t e c h n i q u e ~t was d e m o n s t r a t e d e a r h e r t h a t in the l o w - t e m p e r a t u r e high-field c o n d i t i o n t h e r e was no e v i d e n c e

* Corresponding author Fax + 886 2 7899601

of p o s t field i o n i z a t i o n o f singly c h a r g e d ions in m o s t m e t a l s [1,5] In this study, we focus o u r a t t e n t i o n at t h e f i e l d - e v a p o r a t i o n b e h a v i o r of M o tip b e c a u s e we have f o u n d s o m e novel f e a t u r e s in the s p e c t r a in an e a r l i e r study [5]

2. Experimental T h e h i g h - r e s o l u t i o n p u l s e d - l a s e r time-of-flight A P we u s e d has a m a s s a n d e n e r g y r e s o l u t i o n o f o n e p a r t in 50 000 a n d has b e e n d e s c r i b e d e a r l i e r [6,7] M o l y b d e n u m was f i e l d - e v a p o r a t e d from t h e e m i t t e r tip by p u l s e d - l a s e r h e a t i n g a n d mass- a n d e n e r g y - a n a l y z e d by a time-of-flight mass spect r o m e t e r T h e d a t a w e r e s e p a r a t e d into two g r o u p s a c c o r d i n g to the e x p e r i m e n t a l fields O n e g r o u p of d a t a w e r e c o l l e c t e d at fields 4 8 to 4 2 V / A , a n o t h e r g r o u p o f d a t a w e r e c o l l e c t e d at fields 4 2 to 3 8 V / , ~ All d a t a w e r e c o l l e c t e d u n d e r U H V with a b a s e p r e s s u r e o f a b o u t 2 × 10-10 T o r r W i t h a careful c o n t r o l o f the expert-

0169-4332/94/$07 00 © 1994 Elsevier Scmnce B V All rights reserved SSDI 0169-4332(93)E0279-U

J b Wang, T T Tsong /Apphed Surface Science 76/77 (1994) 73-79

74

mental conditions, we have achieved 0 2 eV accuracy m the Mo 2+, Mo 3+ ion energy dlstnbuUon measurements, which allows us to get good appearance energies, or the critical ton energy deficxts, and peak w~dths from the spectra We have also consxdered the relaUve abundances of various ions of dxfferent charge states Since they depend on the field and temperature, we have determined the t~p temperature from the lowering of the evaporation field The field strength is

determined from the onset field of field xomzatlon of He gas which is 4 4 V / , ~

3. Results We have first measured the critical energy deficit of He ~ons desorbed from the Mo tip The value of it xs 19 8 eV which gives us the average work function of the fltght tube used in our

90

M o 2+ 98

72 95

92

0 O4

96

54 0

IO0

97

94

36

I

I

.KI

E

18

7

272700

275500

278290

281090

283880

286680

Fhght Trme (t u ) Fig 1 T h e flight t~me d~strlbut~on o f M o 2+ ~ons at 8200 V A t i m e u m t ~s 156 25 ps N u m b e r s above p e a k s are mass n u m b e r s o f isotopes

120

98

Mo3+

96 95

o 92 "5

94

#_ "6

96

72

97

48

IO0

(D

E Z

24

4,.. 222500

224780

227060

~ .... 229340

1~.,~ 231620

,

,!~ o,

_

233900

Flight Time (t u ) Fig 2 T h e flight t i m e d l s t n b u U o n of M o ~+ ions at 8200 V A t i m e unit Is 156 25 ps

J b Wang, T T Tsong/Apphed Surface Sctence 76/77 (1994) 73-79

75

270 2+

>

Me

216

03 O

__>_

g_ "5

108

E z

54

<~

3 8eV

34 8 20 2

32

o

64

96

128

160

Energy Deficit (eV) Fig 3 The energy distribution of Mo 2+ ions after combining distributions of all the seven isotopes

temperature exceeded about 350 K Some of the results of field-evaporated M o 2+ ~ ons are shown m Fig 1 and those of Mo 3+ ions are shown in Fig 2 Both of them show clearly seven ~sotope peaks After combining all seven ~sotopes together, we got better energy distributions of Mo 2+ and Mo 3+ ions, as shown m Fig 3 and m Fig 4 A surpnsmg double peak structure appeared m the energy dlstrlbutxon of Mo 2+ The first peak

experiments to be 4 8 +_ 0 1 eV The system constants used in this calculation have been described earher [10] For the high-field-low-temperature behavior, the pulsed-laser-sumulated field desorptmn of Mo startedo at the DC evaporation field of about 4 8 V / A , with a very low laser power, and ocontinued until the field dropped to about 4 2 V / A , with the laser power increased only shghtly In no cases, however, the surface

240

Ug + >

g

192

o

o

144 ---?

<-- 7 8eV

96 AE 3+, = 42 6 e V E z

48

0

32

64

96

128

160

Energy Dehclt (eV) Fig 4 The energy distribution of Mo 3+ ions after combining distributions of all the seven isotopes

76

J b Wang, T T Tsong /Apphed Surface Sctenee 76 / 77 (1994) 73-79

Table 1 Experimental data

Mo 2+ 1st Mo 2+ 2nd Mo 3+

AE c (eV)

AEmax (eV)

FWHM (eV)

X~ (~,)

202+02 348+_08 192+02

244_+02 444+02 280+02

38+04 141+04 78+_04

21

?

80

4

64

o~"

h a s a crttlcal i o n e n e r g y d e f i c i t o f 2 0 2 _+ 0 2 e V and aFWHMof38_+04 eV The second peak h a s a c r m c a l i o n e n e r g y d e f i c t t o f 3 4 8 _+ 0 8 e V and aFWHMofabout 141_+04 eV The peak posmons o f t h e 1st a n d 2 n d p e a k s h a v e t h e e n e r g y d e f l c t t s o f 2 4 4_+ 0 2 a n d 4 4 4_+ 0 2 e V ,

Mo3+

~..

48 1st

peak

~

~ ~

o Mo 2~

32 :3

~~ ~-

.... :_~21\ 2nd

16

peak

Mo 2+

0

49

I

I

I

48

47

46

1

45

I

I

I

44

43

42

41

Fteld Strength (V/A)

Fig 5 The relative abundances of Mo ~+ ions and Mo 2+ ions in the first and second peaks depend on the apphed field We use a reverse &rection for the field strength scale for the reason that high field data were collected first As the tip dulled gradually, low field data were collected

240 [ 2

192

MO 2+

o

co 144 #_

"6

96

z

/ 200000

250000

300000

Fhght T i m e

350000

400000

450000

(t u )

Fig 6 A fl~ght rime s p e c t r u m o b t a i n e d in the field r a n g e f r o m 4 15 to 3 80 V / A . which contains Mo ~+, Mo 2+, Mo~ + and Mo 2+ ion species

J b Wang, T T Tsong /Apphed Surface Sctence 76/77 (1994) 73-79 10

/

of the ion mass lines as it A well accepted and experimentally well established equation for metals is [1,10,11]

2nd peak

\

8

MO2+

Mo23+ /

AE~'+ =~Q+ 6 o ~J

\ 4

~22L

I

4 10

400

3 90

3 80

LI,-ngo-Q,

(1)

t=l

Mo22+

0 4 2O

77

3 70

Fletd Strength (V/A)

Fig 7 The relatwe abundances of Mo Ions obtained m the field range 415 to 380 V/,~

respectively A very surprising result of Mo 3+ is that it shows a high energy peak with a critical ion energy deficit of 19 2 _+ 0 2 eV, which is even lower than that for the 1st peak of Mo 2÷ The F W H M of 7 8 + 0 4 eV is about twice larger than the F W H M of the 1st peak of Mo 2÷ Table 1 summarizes these results The relative abundances of Mo 3+ ions and Mo 2+ Ions In the first and second peaks depend on the apphed field This dependence is shown In Fig 5 When the applied field drops below 4 2 V / , ~ , the N o 3÷ peak disappears rapidly and Mo 2÷ peaks, especially the first peak, dominate Also M o 3+ i o n s start to appear Fig 6 shows a spectrum oobtained in the field range from 4 2 to 3 8 V / A , Fig 7 shows their relative abundances as function of the field

4. Discussion 4 1 C n n c a l ton energy defictt

Critical ion energy deficit is the energy loss of the most energetic ions with respect to the full energy of the acceleration voltage, neV, where n is the charge state of the ion species In our case, we take the 2% peak height at the leading edge

where A E g + is the critical energy deficit of the atomic Ions, ~(~ IS the binding energy of the surface atoms, I, is the tth lomzatlon energy, 4) is the average work funcnon of the flight tube, and Q is the thermal energy This equation is for the field evaporation of atomic ions which is the case in most low-temperature field desorptlon experiments of metals A general equation for cluster Ions is given by [12]

aEc"+..= . Q +

LI,--Eb(m)--ngo-- Q,

(2)

t=l

where

aEc%

is the critical energy deficit of cluster ions, and Eb(m) is the total binding energy of the cluster of m atoms It IS not easy to use Eq (2) for our data analysis since the parameters such as Eb(m) and the ionization energies of clusters I, are mostly unknown Fortunately, only Eq (1) is needed for most of our data analysis A recent experimental measurement finds that the binding energy term In Eq (1) agrees with the cohesive energy available in the literature for many metals [10] Even though an early experiment on Rh came to a slightly different conclusion [13] From Eq (1), the critical energy deficit of Mo 2+ is calculated to be 20 2 eV and that of Mo 3+ is 42 6 eV by using a cohesive energy of 6 8 eV (the work function is 4 8 eV and the thermal energy is 0 2 eV) A summary of the results from Eq (1) is listed in Table 2 Compared to our experimental data, the M o 2+ 1st peak shows exactly the same crmcal energy deficit as that predicted by Eq (1) It means that these M o 2+ i o n s are formed right above the "surface" with a critical distance of 2 1 and an ionization zone width of about 0 4 * at

78

J b Wang, T T Tsong / A p p h e d Surface Sctence 76 / 77 (1994) 73-79

Table 2 Calculated A E c from Eq (1)

4 3 Posszble mechamsms m fteld evaporation of Mo

A E c (eV)

Cohesive (eV)

X~ in F = 4 7 V / A

Mo 2+

202

68

21A

Mo ~+

426

68

31A

a field o f 4 7 V / A T h e critical e n e r g y deficit of M o 3+, however, IS 2 3 4 e V s m a l l e r t h a n p r e d i c t e d Is th~s a new p h e n o m e n o n 9 W e have c h e c k e d a b o u t all p u b l i s h e d d a t a a n d f o u n d t h a t t h e r e w e r e s o m e cases t h a t a high e n e r g y tall d o e s exist F o r e x a m p l e , u n d e r i n t e n s e laser lrrad l a t l o n on Ir e m i t t e r , t h e Ir 2+ ions m a y have excess e n e r g i e s as large as 340 eV, a l m o s t hund r e d t i m e s t h e p h o t o n energy, 3 68 e V [5] H o w ever, t h e n u m b e r o f ions in t h e tall ~s quite small In o u r case, t h e M o ~+ ions w e r e f o r m e d in very low laser p o w e r d e n s i t y T h e e m i t t e r t e m p e r a t u r e d o e s not e x c e e d 350 K A l s o , most of the ions d e t e c t e d have e n e r g i e s shifted m the h i g h - e n e r g y r e g i o n T h e r e f o r e , this is s o m e t h i n g which has not yet b e e n f o u n d b e f o r e

4 2 The double peak structure of Mo 2+ T h e M o 2+ Ions show clearly a d o u b l e p e a k s t r u c t u r e in the e n e r g y & s t n b u t l o n T h e first a n d s e c o n d p e a k s a r e s e p a r a t e d by 16 2 e V a c c o r d i n g to t h e i r critical ion e n e r g y deficits T h e d o u b l e p e a k s t r u c t u r e has b e e n f o u n d m a c l u s t e r field d e s o r p t l o n case like H e R h 2+ [8] In this case, t h e R h a t o m leaves t h e surface as a h e h d e ion, t h e n the H e a t o m t u n n e l s o u t a n d leaves R h 2+ T h e R h 2+ ions so p r o d u c e d f o r m t h e s e c o n d p e a k But o u r e x p e r i m e n t s w e r e p e r f o r m e d in U H V c o n d i t i o n at t h e limit of t h e v a c u u m g a u g e W e d~d n o t d e t e c t m o l y b d e n u m h e h d e ions e i t h e r A l s o the r a t i o of M o 2+ 1st a n d 2nd p e a k s d e p e n d s on t h e fields In high field, t h e r e a r e m o r e s e c o n d p e a k ions t h a n first p e a k ions A s t h e field is l o w e r e d , t h e n u m b e r o f ions in the 1st p e a k increases, w h e r e a s t h a t in the 2nd p e a k dec r e a s e s T h e r e f o r e , this is s o m e t h i n g d i f f e r e n t from t h a t o f H e R h 2+

F o r all the m e t a l s we have s t u d i e d so far, t h e critical ion e n e r g y deficits m e a s u r e d a g r e e exc e e d i n g l y well with E q (1) N e i t h e r d o the ion e n e r g y d i s t r i b u t i o n s show any d o u b l e p e a k struct u r e M o s e e m s to be a p e c u l i a r case w h e r e the g e n e r a l l y a c c e p t e d p i c t u r e o f field e v a p o r a t i o n , l e field e v a p o r a t i o n o c c u r r i n g from kink sites o n e a t o m at a time, d o e s n o t a p p l y H e r e we p r o p o s e a m e c h a n i s m to e x p l a i n the o b s e r v e d ion energy distributions In field e v a p o r a t i o n of Mo, we very o f t e n find t h a t a kink site a t o m is a c t i v a t e d to a site n e a r t h e e d g e o f the surface layer above, b e f o r e it is fielde v a p o r a t e d This p r e l i m i n a r y o b s e r v a t i o n l e a d s us to p r o p o s e a t w o - s t e p f i e l d - e v a p o r a t i o n m e c h a nism as i l l u s t r a t e d in Fig 8 First, a kink site a t o m is a c t i v a t e d to t h e e d g e site of the surface layer above, as shown for a t o m 1 in F i g 8a S e c o n d , a n o t h e r kink site a t o m ( a t o m 2) is activ a t e d to t h e apex site above, p u s h i n g a t o m 1 f u r t h e r away to form an a t o m i c chain as illust r a t e d in F i g 8b A t t h a t s a m e instance, electronic t r a n s i t i o n s occur, a t o m 1 is d o u b l y i o n i z e d a n d a t o m 2 is triply i o n i z e d T h e s e two ions t h e n a c c e l e r a t e away from t h e surface by t h e a p p l i e d field A l t h o u g h we artificially resolve t h e fielde v a p o r a t i o n act into two s e p a r a t e steps, in reality

(a)

(b)

Fig 8 A proposed two-step field evaporation mecbamsm for molybdenum

J b Wang, T T Tsong /Apphed Surface Sctence 76/77 (1994) 73-79 they occur successwely wtthln a few atomic vtbrat l o n a l p e r i o d s , a n d , thus, a r e d i f f i c u l t to o b s e r v e s e p a r a t e l y N o w let us e x p l a m w h y w e p r o p o s e thts s e e m i n g l y v e r y c o m p h c a t e d d t m e r - e v a p o r a t t o n m e c h a n i s m W e will c o n s i d e r t h e c r i t i c a l ion e n e r g y d e f i c i t s o f t h e M o 3+ a n d M o 2÷ t o n s so f o r m e d I n this c a l c u l a t i o n w e u s e t h e f a c t t h a t o t h e a t o m i c r a d m s o f t h e M o a t o m is ~ 1 4 A, t h e r e f o r e , r 2 = 2 8 ,~ T h e poSltlOn o f t h e i m a g e p l a n e ts m o r e u n c e r t a i n , b u t tt is r e a s o n a b l e to a s s u m e t h a t tt p a s s e s t h r o u g h t h e n u c l e u s p o s l t t o n o f t h e k i n k site a t o m T h e r e f o r e , r I = 2 8 -A W h e n M o 2÷ a n d M o 3+ a r e f o r m e d , t h e r e p u l s t v e f o r c e b e t w e e n t h e m will e v e n t u a l l y t r a n s f o r m m t o k i n e t i c e n e r g y w h i c h is s h a r e d e q u a l l y b e t w e e n t h e t w o tons I n o t h e r w o r d s , a k i n e t i c e n e r g y o f (144×2×3)/(2×28) eV=154 e V is g a m e d for e a c h t o n T h u s , t h e crttlcal ton e n e r g y d e f l c t t s of these ion spectes are AE2c + = n F ( r 1 + r2) - 15 4 e V =(2×45×56-154)

eV=35eV

AE3c + = n F r j - 15 4 e V =(3×45×28-154)

eV=

22 4 e V

I n thts c a l c u l a t i o n , w e h a v e u s e d t h e a v e r a g e o field, 4 5 V / A , w h e r e t h e h t g h f i e l d d a t a w e r e collected These crmcal ion energy deficits agree w e l l w i t h t h e e x p e r i m e n t a l d a t a , as c a n b e expected from such a crude esttmatlon We are a w a r e t h a t t h e p r o p o s e d m e c h a n i s m ts s o m e w h a t o u t o f t h e o r d m a r y , b u t so is t h e f i e l d - e v a p o r a tton behavior of molybdenum Before a better e x p l a n a t t o n c a n b e g w e n , this m e c h a m s m s e e m s to b e a r e a s o n a b l e c h o i c e s m c e it e x p l a i n s b o t h

79

the critical ion energy deflctts we have measured for t h e M o 3+ i o n s a n d t h e M o 2÷ i o n s in t h e 2 n d peak

5. References [1] T T Tsong, Atom-Probe Field Ion Microscopy (Cambridge Umverslty Press, Cambndge, 1990) [2] N M Mlskovsky, C M Wel and T T Tsong, Phys Rev Lett 69 (1992) 2427 [3] A R Waugh, E D Boyes and M J Southon, Surf Scl 61 (1976) 109 [4] E W Muller, Phys Rev 102 (1956) 618, R Gomer and L W Swanson, J Chem Phys 38 (1963) 1613 [5] T T Tsong, Surf So 177 (1986) 596 [6] T T Tsong, S B McLane and T J Kmkus, Rev Scl Instrum 53 (1982) 1442 [7] T T Tsong, Yung Liou and Jlang Lm, J gac Sc! Technol A 7 (1989) 1758 [8] T T Tsong, Phys Rev Lett 55 (1985)2826 [9] G L Kellogg, Phys Rev B 24 (1981) 1848 [10] Jmng Lm, Chun-wu Wu and T T Tsong, Phys Rev B 43 (1991) 11595 [11] T T Tsong, W A Schmldt and O Frank, Surf So 65 (1977) 109, N Ernst, G Bozdesh and J H Block, Int J Mass Spectrom Ion Phys 28 (1978) 33 [12] T T Tsong, Phys Rev B 30 (1984) 4946 [13] N Ernst, Surf Sca 87 (1979) 469 This experiment was done by heating the Rh tip from 100 to 600 K Smce it 1s ~mposstble to separate the temperature effect from the field effect, thmr conclumon that the critical ion energy defiots of Rh + and Rh 2+ are field dependent is speculatwe at best The present work as well as earher works by Tsong (Ref [5]) and by Lm et al (Ref [10]) do not support Ernst's result An earher attempt by A R Waugh and M J Southon (J Phys D 9 (1976) 1017) to measure energy deficits of metal ions suffers from poor resolution Thmr result neither agrees wRh ours nor with Eq (1)