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ELECTRICAL CHARACTERISTICS OF Au
SCHOTTKY CONTACTS EVAPORATED ON
PULSED ELECTRON BEAM ANNEALED N-TYPE
(100) SILICON
M. Doghmane, Damien Barbier, A. Laugier
To cite this version:
Colloque C5, suppl6ment au nO1O, Tome 44, octobre 1983 page C5-297
E L E C T R I C A L C H A R A C T E R I S T I C S OF A u SCHOTTKY CONTACTS EVAPORATED ON PULSED ELECTRON BEAM ANNEALED N-TYPE (100) S I L I C O N
M.S. Doghmane, D. B a r b i e r and A. L a u g i e r
Laboratoire de Physique de Za Matidre (LA 3 5 8 ) , Institzrt NationaZ des Sciences AppZiquBes de Lyon, 20, Avenue Albert Einstein,
69621 ViliZZeurbanne Ceder, France RESUME
Des diodes Schottky ont k t 6 rkalisees sur du silicium de t y p e N dont la s u r f a c e a subi un recuit par bombardement Clectronique pulse (P.E.B.A.) Au-delh d'un seuil situe aux environs d e 1.0 3/cm2, l e recuit P.E.B.A. provoque une f o r t e ddtCrioration des caract6ristiques Clectriques d e s diodes, une diminution d e l a barriere d e potentiel a u contact Au-Si(N) e t une augmentation rapide du f a c t e u r d'idkalite. Des mesures d e c a p a c i t b montrent qu'une f o r t e concentration d e donneurs est introduite par le recuit P.E.B.A. Leur concentration superficielle, pouvant dkpasser
1 6
10 e t leur profil o n t
6t6
dkterminhs par des oxydations anodiques successives. En outre, une am6lioration sensible des caractdristigues des diodes est obtenue lorsque l e t r a i t e m e n t P.E.B.A. e s t suivi d'un recuit thermique iiochrone sous hydrogkne durant 30 minutes jusqula 6OO0C.ABSTRACT
Gold Schottky contacts were deposited on Pulsed Electron Beam Annealed (P.E.B.A.) virgin N-type (100) silicon. Above a I 3/cm2 fluence threshold a strong degradation of t h e Schottky diodes I(V) characteristics has been observed with a drop of t h e barrier height and a f a s t increase of t h e ideality factor. Moreover, capacitance measurements combined with oxide
16
stripping have shown t h a t donor centers in excess of 10 a r e generated in t h e P.E.B.A. induced regrowth layer. Isochronal thermal annealing under
H2
atmosphere was performed on-
P.E.BiA. processed samples before depositing t h e Gold contacts. Improvement of t h e diode characteristics have been obtained when increasing t h e annealing t e m p e r a t u r e up t o 600°C.
I. INTRODUCTION
Among other f a s t liquid phase epitaxy processes Pulsed Electron Beam Annealing (P.E.B.A.) has been successfully used t o a c t i v a t e implanted dopants in silicon /1,2/. However, pulsed laser or electron beam annealing is generally accompanied by electrically a c t i v e defects generation in t h e regrowth layer 131. In a previous work DLTS measurements have shown t h a t various majority carrier traps a r e induced by P.E.B.A. depending o n t h e electron beam parameters
/4/.
In this work, t h e degradation of Au/Si Schottky diodes made on P.E.B.A. processed (100) N-type silicon has been studied a s
a
function of t h e electron beam parameters (Energy deposition profile-fluence). The e f f e c t of a post-P.E.B.A. isochronal t h e r m a l t r e a t m e n t under H2 atmosphere was also investigated in order t o study t h e induced defects annealing behaviour.11. EXPERIMENT
Czochralski grown (100) N-type silicon wafers 3 t o IOQ .cm in resistivity w e r e f i r s t HF rinsed and then annealed with 50 ns duration electron beam pulses under cryogenic vacuum using a SPIRE 300 pulsed electron beam processor. This machine is described in ref.2. I t allows for polykinetic electron beam parameters adjustment by mean of t h e field emission diode perveance which is controlled by t h e r/do ratio, r being t h e cathode radius and do t h e cathode-anode distance. In this work, t w o values of t h e r/do ratio w e r e selected resulting in a r a t h e r penetrating electron depth-dose profile for pulse n O 1 (r/d = 7.3, mean electron energy =
0
C5-298 JOURNAL DE PHYSIQUE
1 5 keV) and in a sharper electron depth-dose profile for pulse n02 (rido = 6.5, mean electron energy = 12 keV). For each electron pulses t h e beam fluence has been varied from 0.8 t o 1.5 JlcmL. A f t e r P.E.B.A. Al back contacts w e r e evaporated and annealed f o r 1 5 mn a t 150°C. Finally, t h e Au c o n t a c t was evaporated on t h e P.E.B.A. processed area. Reference diodes made o n non-P.E.B.A. processed silicon exhibit good and reproducible rectifier characteristics.
IIL.
RESULTSThe I(V) characteristics of Schottky diodes made on pulse n O 1 processed silicon a r e
-
plotted on Fig.1 a s a function of t h e beam fluence. The rectifier e f f e c t desappears a t 1.1 J / c m L and t h e ideality f a c t o r n which was 1.16 f o r t h e reference diode reache values a s high a s 2.7 atonly 1.2 3/cmL a s shown o n Fig.2. O n t h e s a m e figure is also plotted t h e values of n versus fluence for diodes made o n pulse n 0 2 processed silicon. We notice t h e s a m e threshold around
-
1 J / c m \ h a t e v e r t h e t y p e of electron beam pulse but t h e ideality f a c t o r remains slightly lower for pulse n02 compared t o pulse no I .Fig.1
-
I(V) characteristics of Au/Si-N Schottky diodes made o n silicon processed .with electron beam pulse no 1 a t variable fluence.indicating t h e s a m e trend a s f o r t h e ideality f a c t o r evolution. C(V) measurements have been c a r r i e d o u t at 1
MHz
and Fig.4 shows t h e f a s t increase of t h e measured capacity a t -0.5 V above2
1 J l c m
.
This result is coherent with t h e degradation threshold deduced from I(V) curves of Fig.1.E (J/cm2)
Fig.3
-
Evolution of t h e barrier height Vbn versus fluenceFig.4
-
Evolution of t h e diode c a p a c i t y a t -0.5 V a s a function of t h e beam fluence.Oxide stripping of t h e P.E.B.A. processed silicon layer was performed in order t o investigate t h e diode characteristics evolution a s a function of depth. Au c o n t a c t s w e r e deposited at e a c h s t e p a f t e r removal of a 1 5 nm thick oxide layer. A t only 90 nm beneath t h e i n i t i t a l s u r f a c e t h e I(V) curve of a Schottky diode m a d e on silicon which has been processed with
2
C5-300 JOURNAL DE PHYSIQUE
f a c t o r recovers a value of 1.25 and t h e barrier height rises from 0.57 V t o 0.7 V. Fig.5 shows t h e decrease of t h e capacitance measured at -1 V a f t e r e a c h 1 5 nm silicon removal. One can observe t h a t t h e reference diode capacitance i s recovered a t about 90nm. A good-fit of t h e depth-capacitance curve of Fig.5 has been carried out using a n exponential d e f e c t concentration profile 151. This leads t o donor c e n t e r s surface concentration in excess of 1 0 l ~ c m - ~ . In addition capacitance measurements a f t e r oxide stripping of silicon samples processed with pulse n02
-
at1.2 ~ l c m ~ indicates t h a t t h e donor profile extends deeper than 90 nm. This result c a n be related t o t h e differences in t h e P.E.B.A. induced thermal e f f e c t s between t h e t w o types of electron beam pulse a s will b e discussed in t h e next section.
Moreover, t a b l e I shows t h e evolution of t h e diode characteristics when P.E.B.A. i s followed by
a
3 0 mn thermal t r e a t m e n t under H2 atmosphere. A1 and Au c o n t a c t s w e r e deposited a f t e r thermal annealing. Considerable improvement of t h e diode characteristics is observed when t h e annealing temperature is increased from 300°C t o 600°C. But even a t this t e m p e r a t u r e t h e typical parameters of a reference diode a r e not completely recovered.Fig.5
-
Diode capacitance at -1 V a s a function of t h e removed silicon layer thickness.CAPACITY(PF)
150
SiN-Au - 1 V : 1 MHz
50
Table 1
-
Evolution of t h e diode parameters a f t e r a 30 mn thermal t r e a t m e n t under H2 atmosphere at variable temperature.2
For a beam fluence lower t h a n 1 J / c m P.E.B.A. produces only a small change in t h e Au/Si-N Schottky diode e l e c t r i c a l parameters. However, above this threshold a f a s t degradation o c c u r s with t h e t w o types of electron beam pulses used in this work. T h e sharp increase of t h e c a p a c i t a n c e measured a t -1 V indicates t h a t donor c e n t e r s a r e induced by P.E.B.A. in high concentration in t h e silicon regrowth layer. They exhibit a s t e e p profile which is m o r e penetrating in t h e c a s e of pulse n 0 2 processed samples. Computer simulation have shown t h a t f o r
-
Z
fluences higher than 1 J / c m t h e silicon s u r f a c e layer turns i n t o melt e i t h e r for pulse n O 1 o r f o r
-
pulse n02 /6/. However, a t 1.2 J/cmL, t h e P.E.B.A. induced fully molten depth is 0.6 p m f o r pulse n 0 2 while i t is only 0.1 y m f o r pulse nO1. So we think t h a t t h e most damaging d e f e c t s a r e g e n e r a t e d in t h e P.E.B.A. induced molten layer. Because of t h e improvement obtained with t h e post-P.E.B.A. thermal t r e a t m e n t up t o 600°C we suggest t h a t quenched-in point d e f e c t s a r e mostly responsible f o r t h e diodes degradation.Moreover, P a t e l et al. have shown t h a t dislocations could be annealed only at t e m p e r a t u r e s higher than 700°C /7/. So, t h e residual diode degradation a f t e r P.E.B.A. plus 600°C t h e r m a l t r e a t m e n t under Hz atmosphere could be r e l a t e d e i t h e r t o residual point d e f e c t s n o t annealed below 600°C o r more probably t o dislocations. These have been recently observed by m e a n of e l e c t r o n microscopy in t h e P.E.B.A. induced melting layer /8/.
V. CONCLUSION
Gold Schottky diodes made on P.E.B.A.
,.
processed N-type silicon exhibit f a s t degradation above a 1 J / c m L fluence threshold. T h e highly damage layer coincides with t h e P.E.B.A. induced molten layer where donor c e n t e r s a r e likely t o b e generated during t h e freezing process. However, a post-P.E.B.A. t h e r m a l t r e a t m e n t a t 600°C during 30 mn in t h e presence of molecular hydrogen is not sufficient t o recover t h e typical p a r a m e t e r s of a Au/Si-N r e f e r e n c e diode.REFERENCES
/ I / GREENWALD A.C., KIRKPATRICK A.R., LITTLE R.G., MINNUCCI J.A., J. Appl. Phys. 50,
2 (1979) 783.
/2/ BARBIER D., LAUGIER A., CACHARD A., J. d e Physique, Col.C5, Suppl.n012,
9
(1982) 411./3/ KIMMERLING L.C., BENTON J.L., Proceedings of t h e MRS Symposium : Laser and Electron Beam Processing of Materials, Academic Press (1980) 385.
/4/ BARBIER D., KECHOUANE M., CHANTRE A., LAUGIER A., Proceedings of t h e MRS Symposium : Laser-Solid interactions and transient t h e r m a l processing of Materials, Elsevier (1982) (to b e published)
/5/ PONPON J.P., BUTTUNG E., SIFFERT P., Rev.Phys.Appl.17 (1982) 687.
/ 6 / CHEMISKY G., BARBIER D., LAUGIER A., (see this symposium)
