- Email: [email protected]
FIM and atom-probe studies of defects in doped tungsten lamp wires
385
Abstracts of 25th International Field Emission Symposium
We now b e l i e v e that p h o t o c o n d u c t i v i t y is m a i n l y r e s p o n s i b l e for this effect. The p o t e n t i a l drop across an oxide layer depends on the layer thickness, the tip temperature, and the light intensity. It may amount to a few kV. This quantity can be d e r i v e d quite a c c u r a t e l y from I-V curves. In a d d i t i o n the elect r o n - h o l e r e c o m b i n a t i o n rate, w h i c h again depends on both the tip temperature and the oxide layer thickness, and which may amount to several seconds, can be m e a s u r e d from the r e l a x a t i o n time of the FI enhancement. By c h a n g i n g the oxide layer thickness the effect of band b e n d i n g on the e l e c t r o n - h o l e rec o m b i n a t i o n rate can be investigated. It is i m p o r t a n t to note that the effect of b a n d b e n d i n g is exactly the same as that of field p e n e t r a t i o n , and can t h e r e f o r e be c o n t r o l l e d by a p p l i e d field. Our study should y i e l d significant i n f o r m a t i o n on physics of semic o n d u c t o r s and i n s u l a t o r surfaces. (3) We also found that field e m i s s i o n from a Si tip d e v i a t e s s i g n i f i c a n t l y from the F o w l e r - N o r d h e i m plot by light illumination. Such effect p e r s i s t s even w h e n a m o n o c h r o m a t i c light of 12,000 (% 1 eV < Egap) is used. The effect is e x p l a i n e d in terms of field p e n e t r a t i o n into the near surface layers. This work was DMR-76-11418.
supported
with a s u b s t a n t i a l l y lower cobalt content than in the primary y' precipitates. (2) I n v e s t i g a t i o n of the initial stages of d e c o m p o s i t i o n of the solutiont r e a t e d alloy. C o m p u t e r m e t h o d s have been d e v i s e d to follow the l o c a l i z e d comp o s i t i o n f l u c t u a t i o n s o c c u r r i n g during the e a r l y stages of y' formation. Fluctuations in the c o n c e n t r a t i o n s of any nulaber of elements can be f o l l o w e d simultaneously. (3) D e t e r m i n a t i o n of the c o m p o s i t i o n of the p r i m a r y MC carbide phase. All the major c a r b i d e - f o r m i n g elements (Ti, Zr, Ta, Nb, Cr and W) were found to be present in carbide p a r t i c l e s of o v e r a l l comp o s i t i o n MC0. SN i t r o g e n and boron were also p r e s e n t , 8 £ h e latter b e i n g e n r i c h e d at the c a r b i d e / m a t r i x interface. Work is also in p r o g r e s s to investigate the effects of p r o l o n g e d exposure, under skress, at e l e v a t e d t e m p e r a t u r e s on the m i c r o s t r u c t u r e and p r o p e r t i e s of superalloys. Highly a n i s o t r o p i c ¥' particle shapes nave been o b s e r v e d by FIM in a crept specimen of NIMONIC 90. Similar w o r k is b e i n g u n d e r t a k e n on IN939. i. P. A. Beaven, M. K. Miller, G. D. W. Smith, 24th International Field Emission Symposium, Oxford, England, 1977.
by NSF Grant
i. T. T. Tsong, Yee S. Ng, A. J. Melmed, to be p u b l i s h e d in Surface Sci.
FIM AND A T O M - P R O B E STUDIES OF DEFECTS IN DOPED T U N G S T E N LAMP W I R E S P.
A.
Beaven,
Miller, P R O G R E S S W I T H F I M AND A T O M - P R O B E I N V E S T I G A T I O N S OF N I C K E L - B A S E D SUPERALLOYS K. M. Delargy, P. A. Beaven, Miller, G. D. W. Smith
M.
K. M.
Delargy,
P. R. Williams,
M.
G. D. W.
K.
Smith
university of Oxford, Department of Metallurgy a n d S c i e n c e of M a t e r i a l s , P a r k s R o a d , O x f o r d OXI 3PH, E n g l a n d
K.
university of O x f o r d , D e p a r t m e n t of Metallurgy and Science of Materials, P a r k s R o a d , O x f o r d OX! 3PH, E n g l a n d
F u r t h e r i n v e s t i g a t i o n s of the micros t r u c t u r e and fine-scale c o m p o s i t i o n of n i c k e l - b a s e d s u p e r a l l o y s have been carried out u s i n g TEM, FIM, T.O.F. A t o m Probe and Imaging A t o m - P r o b e techniques. These studies follow the initial work on the d e t e r m i n a t i o n of y and y' phase c o m p o s i t i o n in alloy IN939, r e p o r t e d last year. New results to be p r e s e n t e d c o n c e r n ing alloy IN939 include the following: (i) D e t e r m i n a t i o n of the c o m p o s i t i o n of the u l t r a - f i n e secondary p r e c i p i t a t e s formed during the low t e m p e r a t u r e (700 ° C) final aging treatment. The phase p r e s e n t has been i d e n t i f i e d as ¥', but
P r e v i o u s FIM o b s e r v a t i o n s of defects in a l u m i n u m - p o t a s s i u m - s i l i c o n ('AKS') doped tungsten lamp wire I'2 have been s u b s t a n t i a l l y extended. The a c c u m u l a t e d e v i d e n c e shows that a large n u m b e r of o b s e r v a t i o n s in the e a r l i e r F i M literature of t u n g s t e n c o n c e r n i n g the "crossover effect", " d i s s o c i a t e d d i s l o c a t i o n loops", "horseshoe d i s l o c a t i o n s " etc. can be a s c r i b e d to the imaging of small defects i n t r o d u c e d into the t u n g s t e n wire during the doping process. These defects fall into two main classes: small voids (believed s t a b i l i z e d by the p r e s e n c e of potassium) and small solid particles, of h i t h e r t o unknown composition. Each of these classes of defect has been i n v e s t i g a t e d u s i n g the T.O.F. AtomProbe. In the case of the small voids, attempts at chemical analysis have been u n s u c c e s s f u l , owing to i o n - o p t i c a l aberration effects. In some cases, no substrate ions at all w e r e c o l l e c t e d from
386
Abstracts of 25th International Field Emission Symposium
the r e g i o n of the void, d e s p i t e the p r e s ence of a large image gas ion c u r r e n t at the detector, thus d e m o n s t r a t i n g a substantial trajectory difference between the image gas ions and the s u b s t r a t e ions in the r e g i o n of the void. M i c r o a n a l y s i s of solid p a r t i c l e s was m o r e successful. Particles located both in g r a i n b o u n d a r y and m a t r i x r e g i o n s h a v e b e e n analyzed. The o v e r a l l c o m p o s i t i o n s o b t a i n e d w e r e in the range W50 to W30, w h i c h c o r r e s p o n d s a p p r o x i m a t e l y to that e x p e c t e d for the 8 - t u n g s t e n phase. T r a c e s of d o p a n t e l e m e n t s w e r e found in solid s o l u t i o n in these p a r t i c l e s , s u p p o r t i n g e a r l i e r s u g g e s t i o n s that the 8 - t u n g s t e n phase may be of i m p o r t a n c e in r e t a i n i n g the trace e l e m e n t s w i t h i n the m i c r o s t r u c t u r e d u r i n g the w i r e m a n u f a c t u r i n g processes. 3 1.
2.
3.
T. J. Godfrey, R. J. Lewis, D. A. Smith, G. D. W. Smith, 21st International Field E m i s s i o n Symposium, Marseille, 1974. T. J. Godfrey, R. J. Lewis, D. A. Smith, G. D. W. Smith, J. Less Common M e t a l s 44, 319 (1976). H. L. Spier, P h i l l i p s Res. R e p o r t s Suppl. 1962, No. 3.
E. W. P l u m m e r Physics Department, University Pennsylvania, Philadelphia, Pennsylvania
of
R e c e n t d e v e l o p m e n t s in a n g l e - r e s o l v e d p h o t o e l e c t r o n s p e c t r o s c o p y u s i n g synchrotron r a d i a t i o n w i l l be d i s c u s s e d . Experiments w h i c h s h e d light on the n a t u r e of surfaces and s u r f a c e r e a c t i o n s w i l l be emphasized. The c o m b i n a t i o n of prolonged light and a n g l e - r e s o l v e d c o l l e c t i o n s allows the e x p e r i m e n t a l i s t to apply simple s y m m e t r y rules to d e t e r m i n e the s y m m e t r y of each e l e c t r o n i c state at the surface. This w i l l be i l l u s t r a t e d w i t h e x a m p l e s of c l e a n i n t r i n s i c s u r f a c e states and ads o r b a t e e n e r g y levels. In contrast, these same s y m m e t r y rules can be u s e d w i t h m o l e c u l a r s~ates of k n o w n s y m m e t r y to d e t e r m i n e the b o n d i n g c o n f i g u r a t i o n . The c o n t i n u o u s range of p h o t o n e n e r g i e s a v a i l a b l e u s i n g s y n c h r o t r o n r a d i a t i o n allows one to look at r e s o n a n c e s in the c o n t i n u u m , for d e f r a c t i o n e f f e c t s and to d e t e r m i n e the p h o t o - i o n i z a t i o n c r o s s section. H o w this i n f o r m a t i o n can be used to d e t e r m i n e the s p a t i a l n a t u r e of the e l e c t r o n i c states at the s u r f a c e as w e l l as the b o n d i n g c o n f i g u r a t i o n of ads o r b a t e s w i l l be i l l u s t r a t e d .
ROUND-ROBIN ATOM-PROBE EXPERIMENT: PRELIMINARY RESULTS A. J. Melmed, Brenner ~
J. J. Carroll,
S. S.
surface science Division, National B u r e a u of S t a n d a r d s , Washington, D.C. 20234; ~U. S. S t e e l R e s e a r c h Laboratory, Monroeville, Pennsylvania 15146
F o l l o w i n g d i s c u s s i o n s at the 24th IFES in Oxford, we i n i t i a t e d a roundr o b i n e x p e r i m e n t i n t e n d e d to p r o v i d e i n f o r m a t i o n about the r e l a t i v e and "absolute" a c c u r a c i e s of a t o m - p r o b e instruments. A t e r n a r y M o - b a s e d alloy was chosen as the s p e c i m e n m a t e r i a l , and s p e c i m e n s w e r e sent to all of the p a r t i c i p a t i n g l a b o r a t o r i e s for analysis. D e t a i l s of the s p e c i m e n s e l e c t i o n and p r e p a r a t i o n t e c h n i q u e s w i l l be described, e x p e r i m e n t a l p r o b l e m s w i l l be d i s c u s s e d , and a p r e l i m i n a r y a n a l y s i s of r e s u l t s w i l l be p r e s e n t e d .
EMISSION
SPECTROSCOPIES
ANGLE-RESOLVED PHOTO-EMISSION S T U D I E S OF S U R F A C E S U S I N G SYNCHROTRON RADIATION ( D i s t i n g u i s h e d G u e s t Lecture)
T O T A L E N E R G Y D I S T R I B U T I O N S OF F I E L D - E M I T T E D E L E C T R O N S AT HIGH CURRENT DENSITY A.
E. Bell, Oregon Oreton
L. W.
Graduate 97005
Swanson Center,
Beaverton,
M e a s u r e m e n t s of the total e n e r g y dist r i b u t i o n s (TED) have b e e n c a r r i e d out at h i g h - c u r r e n t d e n s i t y J = 106 to 108 A / c m 2 for a h i g h - w o r k f u n c t i o n (built-up (100)W, ~ = 4.5 eV) and l o w - w o r k function (Zr/(100)W, ~ = 2.5 eV) field emitter. At h i g h v a l u e s of J, both e m i t t e r s give v a l u e s of the full w i d t h at half m a x i m u m (FWHM) of the T E D w h i c h e x c e e d the values b a s e d on F o w l e r - N o r d h e i m t h e o r y by a few eV. This d e v i a t i o n of the F W H M v a l u e s i n c r e a s e s w i t h J, is i n d e p e n d e n t of temp e r a t u r e and d e c r e a s e s w i t h e m i t t e r radius for a s p e c i f i e d b e a m a c c e p t a n c e angle. Studies of these a n o m a l o u s values of F W H M v a l u e s e x t e n d from 84 to 1975 ~. The b r o a d e n i n g of the e n e r g y d i s t r i b u t i o n o c c u r s on b o t h sides of the Fermi p e a k and r e s u l t s in a F W H M of up to 3.5 eV for the 2.5 eV w o r k f u n c t i o n field e m i t t e r c o m p a r e d to an a p p r o x i m a t e l y 0.S eV c a l c u l a t e d value. S i m i l a r F W ~ M values w e r e o b s e r v e d for the 4.5 eV w o r k f u n c t i o n emitter. The a n g u l a r i n t e n s i t y
Abstracts of 25th International Field Emission Symposium
We now b e l i e v e that p h o t o c o n d u c t i v i t y is m a i n l y r e s p o n s i b l e for this effect. The p o t e n t i a l drop across an oxide layer depends on the layer thickness, the tip temperature, and the light intensity. It may amount to a few kV. This quantity can be d e r i v e d quite a c c u r a t e l y from I-V curves. In a d d i t i o n the elect r o n - h o l e r e c o m b i n a t i o n rate, w h i c h again depends on both the tip temperature and the oxide layer thickness, and which may amount to several seconds, can be m e a s u r e d from the r e l a x a t i o n time of the FI enhancement. By c h a n g i n g the oxide layer thickness the effect of band b e n d i n g on the e l e c t r o n - h o l e rec o m b i n a t i o n rate can be investigated. It is i m p o r t a n t to note that the effect of b a n d b e n d i n g is exactly the same as that of field p e n e t r a t i o n , and can t h e r e f o r e be c o n t r o l l e d by a p p l i e d field. Our study should y i e l d significant i n f o r m a t i o n on physics of semic o n d u c t o r s and i n s u l a t o r surfaces. (3) We also found that field e m i s s i o n from a Si tip d e v i a t e s s i g n i f i c a n t l y from the F o w l e r - N o r d h e i m plot by light illumination. Such effect p e r s i s t s even w h e n a m o n o c h r o m a t i c light of 12,000 (% 1 eV < Egap) is used. The effect is e x p l a i n e d in terms of field p e n e t r a t i o n into the near surface layers. This work was DMR-76-11418.
supported
with a s u b s t a n t i a l l y lower cobalt content than in the primary y' precipitates. (2) I n v e s t i g a t i o n of the initial stages of d e c o m p o s i t i o n of the solutiont r e a t e d alloy. C o m p u t e r m e t h o d s have been d e v i s e d to follow the l o c a l i z e d comp o s i t i o n f l u c t u a t i o n s o c c u r r i n g during the e a r l y stages of y' formation. Fluctuations in the c o n c e n t r a t i o n s of any nulaber of elements can be f o l l o w e d simultaneously. (3) D e t e r m i n a t i o n of the c o m p o s i t i o n of the p r i m a r y MC carbide phase. All the major c a r b i d e - f o r m i n g elements (Ti, Zr, Ta, Nb, Cr and W) were found to be present in carbide p a r t i c l e s of o v e r a l l comp o s i t i o n MC0. SN i t r o g e n and boron were also p r e s e n t , 8 £ h e latter b e i n g e n r i c h e d at the c a r b i d e / m a t r i x interface. Work is also in p r o g r e s s to investigate the effects of p r o l o n g e d exposure, under skress, at e l e v a t e d t e m p e r a t u r e s on the m i c r o s t r u c t u r e and p r o p e r t i e s of superalloys. Highly a n i s o t r o p i c ¥' particle shapes nave been o b s e r v e d by FIM in a crept specimen of NIMONIC 90. Similar w o r k is b e i n g u n d e r t a k e n on IN939. i. P. A. Beaven, M. K. Miller, G. D. W. Smith, 24th International Field Emission Symposium, Oxford, England, 1977.
by NSF Grant
i. T. T. Tsong, Yee S. Ng, A. J. Melmed, to be p u b l i s h e d in Surface Sci.
FIM AND A T O M - P R O B E STUDIES OF DEFECTS IN DOPED T U N G S T E N LAMP W I R E S P.
A.
Beaven,
Miller, P R O G R E S S W I T H F I M AND A T O M - P R O B E I N V E S T I G A T I O N S OF N I C K E L - B A S E D SUPERALLOYS K. M. Delargy, P. A. Beaven, Miller, G. D. W. Smith
M.
K. M.
Delargy,
P. R. Williams,
M.
G. D. W.
K.
Smith
university of Oxford, Department of Metallurgy a n d S c i e n c e of M a t e r i a l s , P a r k s R o a d , O x f o r d OXI 3PH, E n g l a n d
K.
university of O x f o r d , D e p a r t m e n t of Metallurgy and Science of Materials, P a r k s R o a d , O x f o r d OX! 3PH, E n g l a n d
F u r t h e r i n v e s t i g a t i o n s of the micros t r u c t u r e and fine-scale c o m p o s i t i o n of n i c k e l - b a s e d s u p e r a l l o y s have been carried out u s i n g TEM, FIM, T.O.F. A t o m Probe and Imaging A t o m - P r o b e techniques. These studies follow the initial work on the d e t e r m i n a t i o n of y and y' phase c o m p o s i t i o n in alloy IN939, r e p o r t e d last year. New results to be p r e s e n t e d c o n c e r n ing alloy IN939 include the following: (i) D e t e r m i n a t i o n of the c o m p o s i t i o n of the u l t r a - f i n e secondary p r e c i p i t a t e s formed during the low t e m p e r a t u r e (700 ° C) final aging treatment. The phase p r e s e n t has been i d e n t i f i e d as ¥', but
P r e v i o u s FIM o b s e r v a t i o n s of defects in a l u m i n u m - p o t a s s i u m - s i l i c o n ('AKS') doped tungsten lamp wire I'2 have been s u b s t a n t i a l l y extended. The a c c u m u l a t e d e v i d e n c e shows that a large n u m b e r of o b s e r v a t i o n s in the e a r l i e r F i M literature of t u n g s t e n c o n c e r n i n g the "crossover effect", " d i s s o c i a t e d d i s l o c a t i o n loops", "horseshoe d i s l o c a t i o n s " etc. can be a s c r i b e d to the imaging of small defects i n t r o d u c e d into the t u n g s t e n wire during the doping process. These defects fall into two main classes: small voids (believed s t a b i l i z e d by the p r e s e n c e of potassium) and small solid particles, of h i t h e r t o unknown composition. Each of these classes of defect has been i n v e s t i g a t e d u s i n g the T.O.F. AtomProbe. In the case of the small voids, attempts at chemical analysis have been u n s u c c e s s f u l , owing to i o n - o p t i c a l aberration effects. In some cases, no substrate ions at all w e r e c o l l e c t e d from
386
Abstracts of 25th International Field Emission Symposium
the r e g i o n of the void, d e s p i t e the p r e s ence of a large image gas ion c u r r e n t at the detector, thus d e m o n s t r a t i n g a substantial trajectory difference between the image gas ions and the s u b s t r a t e ions in the r e g i o n of the void. M i c r o a n a l y s i s of solid p a r t i c l e s was m o r e successful. Particles located both in g r a i n b o u n d a r y and m a t r i x r e g i o n s h a v e b e e n analyzed. The o v e r a l l c o m p o s i t i o n s o b t a i n e d w e r e in the range W50 to W30, w h i c h c o r r e s p o n d s a p p r o x i m a t e l y to that e x p e c t e d for the 8 - t u n g s t e n phase. T r a c e s of d o p a n t e l e m e n t s w e r e found in solid s o l u t i o n in these p a r t i c l e s , s u p p o r t i n g e a r l i e r s u g g e s t i o n s that the 8 - t u n g s t e n phase may be of i m p o r t a n c e in r e t a i n i n g the trace e l e m e n t s w i t h i n the m i c r o s t r u c t u r e d u r i n g the w i r e m a n u f a c t u r i n g processes. 3 1.
2.
3.
T. J. Godfrey, R. J. Lewis, D. A. Smith, G. D. W. Smith, 21st International Field E m i s s i o n Symposium, Marseille, 1974. T. J. Godfrey, R. J. Lewis, D. A. Smith, G. D. W. Smith, J. Less Common M e t a l s 44, 319 (1976). H. L. Spier, P h i l l i p s Res. R e p o r t s Suppl. 1962, No. 3.
E. W. P l u m m e r Physics Department, University Pennsylvania, Philadelphia, Pennsylvania
of
R e c e n t d e v e l o p m e n t s in a n g l e - r e s o l v e d p h o t o e l e c t r o n s p e c t r o s c o p y u s i n g synchrotron r a d i a t i o n w i l l be d i s c u s s e d . Experiments w h i c h s h e d light on the n a t u r e of surfaces and s u r f a c e r e a c t i o n s w i l l be emphasized. The c o m b i n a t i o n of prolonged light and a n g l e - r e s o l v e d c o l l e c t i o n s allows the e x p e r i m e n t a l i s t to apply simple s y m m e t r y rules to d e t e r m i n e the s y m m e t r y of each e l e c t r o n i c state at the surface. This w i l l be i l l u s t r a t e d w i t h e x a m p l e s of c l e a n i n t r i n s i c s u r f a c e states and ads o r b a t e e n e r g y levels. In contrast, these same s y m m e t r y rules can be u s e d w i t h m o l e c u l a r s~ates of k n o w n s y m m e t r y to d e t e r m i n e the b o n d i n g c o n f i g u r a t i o n . The c o n t i n u o u s range of p h o t o n e n e r g i e s a v a i l a b l e u s i n g s y n c h r o t r o n r a d i a t i o n allows one to look at r e s o n a n c e s in the c o n t i n u u m , for d e f r a c t i o n e f f e c t s and to d e t e r m i n e the p h o t o - i o n i z a t i o n c r o s s section. H o w this i n f o r m a t i o n can be used to d e t e r m i n e the s p a t i a l n a t u r e of the e l e c t r o n i c states at the s u r f a c e as w e l l as the b o n d i n g c o n f i g u r a t i o n of ads o r b a t e s w i l l be i l l u s t r a t e d .
ROUND-ROBIN ATOM-PROBE EXPERIMENT: PRELIMINARY RESULTS A. J. Melmed, Brenner ~
J. J. Carroll,
S. S.
surface science Division, National B u r e a u of S t a n d a r d s , Washington, D.C. 20234; ~U. S. S t e e l R e s e a r c h Laboratory, Monroeville, Pennsylvania 15146
F o l l o w i n g d i s c u s s i o n s at the 24th IFES in Oxford, we i n i t i a t e d a roundr o b i n e x p e r i m e n t i n t e n d e d to p r o v i d e i n f o r m a t i o n about the r e l a t i v e and "absolute" a c c u r a c i e s of a t o m - p r o b e instruments. A t e r n a r y M o - b a s e d alloy was chosen as the s p e c i m e n m a t e r i a l , and s p e c i m e n s w e r e sent to all of the p a r t i c i p a t i n g l a b o r a t o r i e s for analysis. D e t a i l s of the s p e c i m e n s e l e c t i o n and p r e p a r a t i o n t e c h n i q u e s w i l l be described, e x p e r i m e n t a l p r o b l e m s w i l l be d i s c u s s e d , and a p r e l i m i n a r y a n a l y s i s of r e s u l t s w i l l be p r e s e n t e d .
EMISSION
SPECTROSCOPIES
ANGLE-RESOLVED PHOTO-EMISSION S T U D I E S OF S U R F A C E S U S I N G SYNCHROTRON RADIATION ( D i s t i n g u i s h e d G u e s t Lecture)
T O T A L E N E R G Y D I S T R I B U T I O N S OF F I E L D - E M I T T E D E L E C T R O N S AT HIGH CURRENT DENSITY A.
E. Bell, Oregon Oreton
L. W.
Graduate 97005
Swanson Center,
Beaverton,
M e a s u r e m e n t s of the total e n e r g y dist r i b u t i o n s (TED) have b e e n c a r r i e d out at h i g h - c u r r e n t d e n s i t y J = 106 to 108 A / c m 2 for a h i g h - w o r k f u n c t i o n (built-up (100)W, ~ = 4.5 eV) and l o w - w o r k function (Zr/(100)W, ~ = 2.5 eV) field emitter. At h i g h v a l u e s of J, both e m i t t e r s give v a l u e s of the full w i d t h at half m a x i m u m (FWHM) of the T E D w h i c h e x c e e d the values b a s e d on F o w l e r - N o r d h e i m t h e o r y by a few eV. This d e v i a t i o n of the F W H M v a l u e s i n c r e a s e s w i t h J, is i n d e p e n d e n t of temp e r a t u r e and d e c r e a s e s w i t h e m i t t e r radius for a s p e c i f i e d b e a m a c c e p t a n c e angle. Studies of these a n o m a l o u s values of F W H M v a l u e s e x t e n d from 84 to 1975 ~. The b r o a d e n i n g of the e n e r g y d i s t r i b u t i o n o c c u r s on b o t h sides of the Fermi p e a k and r e s u l t s in a F W H M of up to 3.5 eV for the 2.5 eV w o r k f u n c t i o n field e m i t t e r c o m p a r e d to an a p p r o x i m a t e l y 0.S eV c a l c u l a t e d value. S i m i l a r F W ~ M values w e r e o b s e r v e d for the 4.5 eV w o r k f u n c t i o n emitter. The a n g u l a r i n t e n s i t y