- Email: [email protected]
Depth profiles of Fe and Cr in InP after annealing
Nuclear Instruments and Methods 209/210 (1983) 671-675 North-Holland Publishing Company
671
D E P T H PROFILES OF Fe A N D Cr IN lnP AFTER ANNEALING M. G A U N E A U , H. L ' H A R I D O N , A. R U P E R T and M. S A L V I L A B / I C M / M P A , CNET, 22301 Lannion, France
The migration of the compensating dopants, iron or chromium, either in undoped Fe or Cr implanted indium phosphide, or in Se implanted semi-insulating InP substrates has been studied using secondary ion mass spectrometry. Annealing at temperatures ranging from 550 to 750°C results in profiles divided into three regions: the near-surface zone where metals pile-up, the zone in front of the projected range (R p), around 0.8 R p, where Fe and Cr atoms are trapped by residual damage, and a region around R p + A R p where a well defined peak, with maximum concentration reaching 3-4 × 1018 cm-3 is observed. Results are interpreted according to the calculations of L.A. Christel et al. on stoichiometric disturbances in ion implanted compound semiconductors. But in the case of Se-implanted semi-insulating substrates, another mechanism must be involved.
1. Introduction I n d i u m p h o s p h i d e is b e c o m i n g of technological interest for o p t o e l e c t r o n i c and microwave devices. Device f a b r i c a t i o n requires semi-insulating substrates a n d one goal of p r e s e n t - d a y integrated circuit technology is to m a k e clear the b e h a v i o r of these substrates d u r i n g thermal processes. This p a p e r is c o n c e r n e d with iron a n d c h r o m i u m which are c o m m o n l y used as c o m p e n s a t i n g impurities in InP. T h e r m a l r e d i s t r i b u t i o n of c h r o m i u m in G a A s has received c o n s i d e r a b l e a t t e n t i o n over the past few years [1-5], b u t very few studies concern iron, or c h r o m i u m , in InP. The p u r p o s e of this p a p e r is to describe the b e h a v i o r of iron a n d c h r o m i u m , especially iron, in u n d o p e d , or semi-insulating, i m p l a n t e d substrates. T h e profiles, o b t a i n e d b y s e c o n d a r y ion mass s p e c t r o m e t r y (SIMS) will be tentatively e x p l a i n e d according to recent results on s t o i c h i o m e t r i c d i s t u r b a n c e s in ion i m p l a n t e d comp o u n d semiconductors.
2. Experimental I n P substrates used in this study were of (100) o r i e n t a t i o n a n d grown b y the liquid e n c a p s u l a t i o n C z o c h r a l s k y technique at C N E T l a b o r a t o r i e s or p u r c h a s e d from S u m i t o m o . Two types of samples were p r e p a r e d : 1) n - t y p e samples ( u n i n t e n t i o n a l l y d o p e d , - 2 × 1016 e cm 3), i m p l a n t e d at 800 keV to fluences of 1013 and 1014 Fe or Cr a t o m s cm 2; 0167-5087/83/0000-0000/$03.00
in this case, i m p l a n t a t i o n was p e r f o r m e d at r o o m t e m p e r a t u r e except for one C r / I n P s a m p l e which was i m p l a n t e d at 77 K. 2) Semi-insulating samples ( F e doped), i m p l a n t e d at 400 keV to fluences of 1014 a n d 5 × l014 Se atoms cm-2; these i m p l a n t a tions were carried out at 200°C to avoid a m o r p h i zation. D u r i n g i m p l a n t a t i o n , the substrates were tilted 7 ° from the (100) axis to prevent c h a n n e l i n g effects. A f t e r i m p l a n t a t i o n , the samples were annealed: 1) at 750°C in a 0.4% P H 3 + H 2 ambient; 2) at 700 or 550°C u n d e r Si3N 4 encapsulation. C o n t r o l specimens (not annealed) were always kept for comparison. The d e p t h profiles were recorded by S I M S using an oxygen p r i m a r y b e a m and m o n i t o r i n g 56Fe+ or Cr +. Two ion m i c r o a n a l y z e r s were used: the A E I I M 20 a p p a r a t u s , and the C A M E C A I M S - 3 F ion microanalyzer. C a l i b r a t i o n of atomic concentrations was established using the u n a n n e a l e d d e p t h profiles as s t a n d a r d s . To correlate d e p t h d o p a n t profiles and i m p l a n t a t i o n residual damage, R u t h e r f o r d b a c k s c a t t e r i n g analyses were carried out using 2 MeV 4He+ ion channeling. The c h a n n e l i n g m e a s u r e m e n t s were p e r f o r m e d in the (100) direction with the d e t e c t o r oriented at 170 ° to the incident b e a m axis. Residual d a m a g e d i s t r i b u t i o n was d r a w n using the stopp i n g power d e t e r m i n e d with a 5 × 1015 Bi cm 2 i m p l a n t e d silicium s t a n d a r d a n d assuming bulk d e n s i t y for the a m o r p h o u s InP.
© 1983 N o r t h - H o l l a n d
Vl. SEMICONDUCTORS
672
M. Gauneau et al. / Depth profiles of Fe and Cr in InP
191
3. Results
3.1. Crystalline quality At first, crystalline quality was investigated by channeling measurements. As stated in previous studies [6,7] we know that: 1) At r o o m temperature, a 800 keV 1014 Cr cm 2 implantation amorphizes the implanted layer up to a depth of 8000 A, while after a 800 keV 1013 Cr cm -2 implantation, only a buried d a m a g e d layer appears between 3000 and 5000 A. 2) At 77 K, the implantation of 1014 Cr cm -2 into InP leads to a thicker a m o r p h o u s layer with a sharp c r y s t a l l i n e - a m o r p h o u s interface. 3) At r o o m temperature a fluence of 4 × 1012 Se cm 2 is sufficient to amorphize the substrate; when the implanted fluence is increased the a m o r p h o u s layer broadens both towards the surface and towards the bulk. However, at 200°C, InP is not amorphized even for a l014 Se cm z fluence. 4) After a 750°C anneal in a PH 3 atmosphere, a 1014 Cr cm -2 implanted layer is not fully regrown, a heavily d a m a g e d zone centered at 0.8 R r (-3500 A) remains. The same result is observed for a 1014 S e c m 2 implanted layer, but in this case, the d a m a g e d zone lies closer to the surface. On the contrary, a low-dose light-element implanted sample (1013 Cr cm 2) completely recovers the original crystallinity. Due to the small mass difference between Fe and Cr, iron is expected to behave similarly to chromium.
3.2. Iron profiles in non-doped implanted InP substrates Fig. 1 depicts the kinetics of the iron migration in 800 keV 10 t4 Fe cm -2 implanted substrates which have been annealed under Si3N 4 encapsulation, at 550 and 700°C, for times ranging from 10 rain to 60 min. The projected range, Rp, of the as-implanted profile is 5600 ,~ and the standard deviation, ARn, is 2100 A. After sixty minutes at 550°C, iron atoms begin to move towards a depth greater than Rp" a shoulder is observed on the corresponding profile. After a 700°C anneal for 10 min, a depleted zone is largely formed in the 0.8Rp region, but Fe atoms are gettered and form a first peak. For longer annealing times, the iron concentration is more and more depleted at a depth
56Fe+/InP 1014atoms cm -2 800 keY --
u nonneoLed AnneaLed z~ 60 min 5 5 0 ° C o 10 rain 700 °C D 30 rain, 700 ° C -4- 60 rain 7 0 0 ° C
Rp
ld 8
03i
o~
I
<
g 17
c
8 unonneoLed
,,,o
oa
o
1016
lO
l
o
,
,
,
'5o'oo'
' 1oooo'
%oo'
'
Angstroms
Fig. 1. Iron depth profiles, measured by SIMS, for a 800 keV 1014 cm 2 Fe-implant into InP: unannealed (solid curve); annealed at 550°C for 60 min (triangles); annealed at 700°C for 10 min (circles); annealed at 700°C for 30 rain (squares); annealed at 700°C for 60 min (solid curve, diamonds).
equal to R p, creating a large dip separating the first peak at 0.8 Rp and a second one, at about R p-4-Z~Rp, in which the iron concentration is higher than at Rp in the as-implanted profile. It is worth noticing that this second p e a k is formed of two contributions, one at 7000 A and the other at 8000 8500 A. A diffusion tail appears towards the bulk. The Fe concentrations at the bottom of the dip and at the shoulder of the tail are similar, about 1 × 1017 cm 3, which is rather close to the admitted limit solubility for Fe in InP at 700°C [8]. The diffusion coefficient, estimated from the variation of the diffusion tail versus time is found to be 8 × 10 -12 cm -2 s 1 which is in good agreement with the results of Holmes et al [9]. In the case of a sample implanted at a lower dose (1013 Fe cm 2) and annealed at 700°C for 10
M. Gauneau et al. / Depth profiles of Fe and Cr in lnP
min, the redistribution features are not so clear. but basically the same, as has been previously described [7].
3.3. Chromium profiles in non-doped implanted InP substrates Fig. 2 describes the chromium profiles in 800 l014 cm 2 implanted substrates, unannealed or annealed at 700 and 750°C for 10 rain. After anneal the profiles are still divided into three regions: (1) the near surface zone where Cr atom accumulation takes place; (ii) the 0 . 8 R p z o n e where c h r o m i u m is gettered and forms a first peak; (iii) the R p + A R p zone where a second, double peak is observed. The migration of Cr atoms in the 0.8Rp zone depends strongly on temperature since the peak concentration decreases from 6 x 1017 cm 3 at 700°C for 10 min, to 5 × 1016 c m 3 at 750°C keV
10 9 52Cr+/InP, 1014atoms cm2,800 keV~ Rp - - unannealed ' I o#!~. anneaLed i o 10 rnin 700°C J -'- 10 min. 750°C [
for l0 min. The same fast decrease of concentration was observed for iron between 10 and 30 min at 700°C (fig. 1), but comparison with Fe profiles shows that the 0 . 8 R p z o n e is more deeply depleted for chromium. In particular, the concentration at the b o t t o m of the dip, a t R p , is 2 x 1016 cm 3, and diffusion towards the bulk is not so evident for chromium. The same general behavior is observed for Cr atoms in a sample implanted at 77 K to 1014 c m - 2 and annealed at 750°C [7]. Nevertheless, in this case, two main differences are noticed: 1) Cr accumulation a t 0.SRp is more important and a peak concentration of 8 x 1017 cm 3 is registered; 2) the Rp + ARp peak is essentially formed with one accumulation at 7500 A.
3.4. Iron profiles in semi-insulating Se-implanted substrates: Semi-insulating, Fe-doped substrates were implanted with selenium ions at 400 keV to fluences 9
o E
g -
Rp÷ARp ' (O) // ~
I',/
Fe profiles
in semi-insulating Se- implanted tnP { Q ) Se, 1014Se cm-2 , Qs i replanted ( b) Fe ,10I& Se cm-2*lSmin Qt 7S0=C (C)Fe S101/~S~ cm-2.1S min at 7S0"C
<
o
o ~
Rp
t I ,"~, i~ ,~
? E
?
673
o i o o
: ,
(b)
i!,
i017
l~ l ' , ? t \ /
g (..)
ld7
o
o o
6 o o
\
o
lJ 5 10 0
5000
10000 Angstroms
15000
Fig. 2. Chromium depth profiles for a 800 keV 1014 cm 2 Cr-implant into InP: as-implanted (solid curve); annealed at 700°C for 10 min (circles); annealed at 750°C for 10 min (solid curve squares).
o5
i microns
Fig. 3. Depth profiles in semi-insulating Se-implanted substrates: (a) as-implanted Se profile; (b) Fe profile, in a 1014 Se cm 2 sample, annealed at 750°C for 15 min; (c) Fe profile, in a 5X 1014Se cm 2 sample, annealed at 750°C for 15 min. VI. S E M I C O N D U C T O R S
674
M. Gauneau et al. / Depth profiles of Fe and Cr in l n P
of 1014 and 5 × l014 cm -2. During a 750°C anneal, Fe redistribution takes place as shown in fig. 3. In that case, a rather different feature is observed. The 1014 Se cm 2 sample shows a surface pile-up, a peak at 5000 A and a large depleted zone going to 1.5 /~m. The 5 × 1014 Se cm -2 sample exhibits about the same behavior except the fact that two peaks are recorded: the first, at about 2700 A, is preceded by a small dip located at 2000 ,~; the second, with a peak density equal to 3 × 1017 cm 3, is located at 5600 A.
4. Discussion The surface pile-up is attributed to the dopant out diffusion. Here, Cr and Fe show a similar behavior in InP as chromium in GaAs [3,4,10]. The 0.8Rp peak is generally attributed to gettering by implant damage. That behavior is observed when fluences higher than the critical amorphizing dose are reached, and when a buried amorphized layer remains after annealing [5]. This is the case for 1014 Fe cm -2, or 1014 Cr c m 2, implanted samples described here. More difficulties are encountered to explain implanted species accumulation within zones at a depth greater than Rp. A R p + A R p peak has often been reported [see 5,10,11], and many mechanisms have been invoked to explain it. In this study, the same well defined R v + A R p peak is located in a epitaxially regrown layer and is observed whether the implanted layer is amorphized (i.e. Fe or Cr fluences greater than 5 × 1013 cm -2 in InP), or not (fluence equal to 1013 cm 2). So, amorphization of the implanted layer does not seem to have a major effect on the redistribution process of Fe or Cr, in InP or GaAs, at depths greater than _Rp. The crystal nonstoichiometry resulting from the implantation, as described by Christel and Gibbons [12] can be inferred here to explain the Fe and Cr redistribution in InP. After implantation, the crystal stoichiometry is destroyed in the implanted layer and two regions are distinguished: 1) the shallower one where the P vacancies create a net indium excess; 2) the deeper one where the P recoiled atoms exceed the In recoiled atoms and where a net P excess exists. Between the two layers, the stoichiometry is preserved in a narrow zone. These In and P excess densities depend on
the dopant/substrate combinations and on the fluences. As an example, fig. 3 in ref. [12] demonstrates that, in a 10 ~5 S e c m -2 InP implanted wafer, the crystal is stoichiometric at about R p, and that the net indium or phosphorus excess density is about 1019 cm -3. In our case, we estimate that the stoichiometric zone is also near R p (about 5600 ,~ for Fe or Cr implanted into InP) while the P excess density should be slightly reduced for a 10 ~5 cm 2 fluence or ten times lower for a 1014 cm -2 fluence. Figs. 1 and 2 show that, where the crystal stoichiometry is preserved, near R v' Fe or Cr concentrations are depleted down to the solubility limits (1 X 1017 Fe cm -3, or 2 × 10 ~6 Cr cm -3 in InP). Metal atoms are piled up in the region with a P excess, probably as F e - P or C r - P pairs, or as metal phosphide precipitates, which increase the apparent solubility to a value close to the excess P concentration (about 1018 cm 3 for an implantation of 1014 cm-2). Then, from this quasi-infinite source, iron atoms diffuse towards the bulk (fig. 1). It has been noticed that the Rp + ARp peak was composed of two contributions: one at 7000 ,~, the other at 8000-8500 ,~. The third peak, at about 8300 A, is observed for iron (fig. 1) as well as for chromium (fig. 2). It does not occur: 1) in InP implanted to 1013 Fe cm 2; 2) in InP implanted to 1014 Cr cm 2 at 77 K; 3) in GaAs implanted to 1014 Cr cm -2 [5,7]. This third gettering peak is deeper than expected in the implanted layer. A secondary gettering process is needed to explain this behavior. A similar behavior is observed in semi-insulating Se-implanted substrates after a 750°C anneal (fig. 3). The higher peak located at 5000 A for a 1014 c m 2 dose, or at 5600 A for a 5 × 1014 cm 2 dose is deeper than Rp + ARp: in the case of 400 keV Se-implanted substrates the projected range of selenium is equal to 1500 A, and R p + / ~ R p is equal to about 2400 ,~ [13]. In fig. 3, only the first peak of the 5 × 1014 cm 2 profile could be attributed to phosphorus gettering. It is worth noticing that this first peak is preceded by a dip at a depth equal to 2000 ,~. Two remarks can be made about this deeper gettering process: 1) the Fe accumulation and so the peak concentration do not depend on the Se implanted fluence between l01-~ and 5 × 1014 cm-2; 2) this deep peak seems to move towards the bulk as the fluence increases: from 3200 A for1013 cm 2 t o 5 6 0 0 A f o r 5 × 10 T4cm 2
M. Gauneau et al. / Depth profiles of Fe and Cr in l n P
As a possible explanation it could be advanced that this deep gettering process is due to defects created during implantation at room temperature or at 200°C. Such defects nucleation at low temperature has been observed in gallium arsenide [11,14] and it is well known than InP is more sensitive to thermal processing than GaAs.
5. Conclusion
This work concerns iron and chromium after 700-750°C anneal redistributions in implanted InP substrates. The post anneal depth profiles are divided into three regions: 1) the near-surface zone where metals pile up as is generally observed in III-V semiconductors; 2) t h e 0.8Rp zone where the implanted species remain trapped in the residual damage peak; 3) the R p - I - A R p depth region where gettering processes take place. In this third region two types of gettering processes seems to be involved: one due to phosphorus excess according to the results of Christel and Gibbons [12] concerning atom displacements after implantation into InP, the other due to point defects created during implantation. To be well established the thermal processing of InP needs much more experimental work. In particular, transmission electron microscopy and low temperature annealing must be undertaken to specify the size and the nature of the defects at depths greater than Rp.
675
References [1] B. Deveaud and P.N. Favennec, Solid State Commun. 24 (1979) 473. [2] A.M. Huber, G. Morillot, N.T. Linh, P.N. Favennec, B. Deveaud and B. Toulouse, Appl. Phys. Lett. 34 (1979) 858. [3] P.K. Vasudev, R.G. Wilson and C.A. Evans, Appl.Phys. Lett. 36 (1980) 837. [4] V. Eu, M. Feng, W.B. Henderson, H.B. Kim, and J.M. Whelan, Appl. Phys. Lett. 37 (1980) 473. [5] P.K. Vasudev, R.G. Wilson and C.A. Evans, Appl. Phys. Lett. 37 (1980) 308. [6] P. Auvray, A. Guivarc'h, H. L'Haridon, G. Pelous, M. Salvi and P. Henoc, J. Appl. Phys. 53 (1982) 6202. [7] M. Gauneau, H. L'Haridon, A. Rupert and M. Salvi, J. Appl. Phys. 53 (1982) 6823. [8] H. Mordoc, J.T. Andrews and S.B. Hyder, Electron. Lett. 14 (1978) 715. [9] D.E. Holmes, R.G. Wilson and P.W. Yu, J. Appl. Phys. 52 (1981) 3396. [10] C.A. Evans, V.R. Deline, T.W. Sigmon, and A. Lidow, Appl. Phys. Lett. 35 (1979) 291. [11] T.J. Magee, H. Kawayoshi, A.D. Ormond, L.A. Christel, J.F. Gibbons, C,G. Hopkins, C.A. Evans, and D.S. Day, Appl. Phys. Lett. 39 (1981) 906. [12] L.A. Christel and J.F. Gibbons, J. Appl. Phys. 52 (1981) 5050.
[13] M. Gauneau, A. Rupert and P.N. Favennec, Secondary Ion Mass Spectrometry, SIMS Ill, eds., A. Benninghoven, J. Giber, J. Laszlo, M. Riedel and H.W. Werner, Springer Series in Chemical Physics (Springer, Berlin, (1981) p. 342. [14] T.J. Magee, K.S. Lee, R. Ormond, C.A. Evans, R.J. Blattrier and C. Hopkins, Appl. Phys. Lett. 37 (1980) 635.
VI. SEMICONDUCTORS
671
D E P T H PROFILES OF Fe A N D Cr IN lnP AFTER ANNEALING M. G A U N E A U , H. L ' H A R I D O N , A. R U P E R T and M. S A L V I L A B / I C M / M P A , CNET, 22301 Lannion, France
The migration of the compensating dopants, iron or chromium, either in undoped Fe or Cr implanted indium phosphide, or in Se implanted semi-insulating InP substrates has been studied using secondary ion mass spectrometry. Annealing at temperatures ranging from 550 to 750°C results in profiles divided into three regions: the near-surface zone where metals pile-up, the zone in front of the projected range (R p), around 0.8 R p, where Fe and Cr atoms are trapped by residual damage, and a region around R p + A R p where a well defined peak, with maximum concentration reaching 3-4 × 1018 cm-3 is observed. Results are interpreted according to the calculations of L.A. Christel et al. on stoichiometric disturbances in ion implanted compound semiconductors. But in the case of Se-implanted semi-insulating substrates, another mechanism must be involved.
1. Introduction I n d i u m p h o s p h i d e is b e c o m i n g of technological interest for o p t o e l e c t r o n i c and microwave devices. Device f a b r i c a t i o n requires semi-insulating substrates a n d one goal of p r e s e n t - d a y integrated circuit technology is to m a k e clear the b e h a v i o r of these substrates d u r i n g thermal processes. This p a p e r is c o n c e r n e d with iron a n d c h r o m i u m which are c o m m o n l y used as c o m p e n s a t i n g impurities in InP. T h e r m a l r e d i s t r i b u t i o n of c h r o m i u m in G a A s has received c o n s i d e r a b l e a t t e n t i o n over the past few years [1-5], b u t very few studies concern iron, or c h r o m i u m , in InP. The p u r p o s e of this p a p e r is to describe the b e h a v i o r of iron a n d c h r o m i u m , especially iron, in u n d o p e d , or semi-insulating, i m p l a n t e d substrates. T h e profiles, o b t a i n e d b y s e c o n d a r y ion mass s p e c t r o m e t r y (SIMS) will be tentatively e x p l a i n e d according to recent results on s t o i c h i o m e t r i c d i s t u r b a n c e s in ion i m p l a n t e d comp o u n d semiconductors.
2. Experimental I n P substrates used in this study were of (100) o r i e n t a t i o n a n d grown b y the liquid e n c a p s u l a t i o n C z o c h r a l s k y technique at C N E T l a b o r a t o r i e s or p u r c h a s e d from S u m i t o m o . Two types of samples were p r e p a r e d : 1) n - t y p e samples ( u n i n t e n t i o n a l l y d o p e d , - 2 × 1016 e cm 3), i m p l a n t e d at 800 keV to fluences of 1013 and 1014 Fe or Cr a t o m s cm 2; 0167-5087/83/0000-0000/$03.00
in this case, i m p l a n t a t i o n was p e r f o r m e d at r o o m t e m p e r a t u r e except for one C r / I n P s a m p l e which was i m p l a n t e d at 77 K. 2) Semi-insulating samples ( F e doped), i m p l a n t e d at 400 keV to fluences of 1014 a n d 5 × l014 Se atoms cm-2; these i m p l a n t a tions were carried out at 200°C to avoid a m o r p h i zation. D u r i n g i m p l a n t a t i o n , the substrates were tilted 7 ° from the (100) axis to prevent c h a n n e l i n g effects. A f t e r i m p l a n t a t i o n , the samples were annealed: 1) at 750°C in a 0.4% P H 3 + H 2 ambient; 2) at 700 or 550°C u n d e r Si3N 4 encapsulation. C o n t r o l specimens (not annealed) were always kept for comparison. The d e p t h profiles were recorded by S I M S using an oxygen p r i m a r y b e a m and m o n i t o r i n g 56Fe+ or Cr +. Two ion m i c r o a n a l y z e r s were used: the A E I I M 20 a p p a r a t u s , and the C A M E C A I M S - 3 F ion microanalyzer. C a l i b r a t i o n of atomic concentrations was established using the u n a n n e a l e d d e p t h profiles as s t a n d a r d s . To correlate d e p t h d o p a n t profiles and i m p l a n t a t i o n residual damage, R u t h e r f o r d b a c k s c a t t e r i n g analyses were carried out using 2 MeV 4He+ ion channeling. The c h a n n e l i n g m e a s u r e m e n t s were p e r f o r m e d in the (100) direction with the d e t e c t o r oriented at 170 ° to the incident b e a m axis. Residual d a m a g e d i s t r i b u t i o n was d r a w n using the stopp i n g power d e t e r m i n e d with a 5 × 1015 Bi cm 2 i m p l a n t e d silicium s t a n d a r d a n d assuming bulk d e n s i t y for the a m o r p h o u s InP.
© 1983 N o r t h - H o l l a n d
Vl. SEMICONDUCTORS
672
M. Gauneau et al. / Depth profiles of Fe and Cr in InP
191
3. Results
3.1. Crystalline quality At first, crystalline quality was investigated by channeling measurements. As stated in previous studies [6,7] we know that: 1) At r o o m temperature, a 800 keV 1014 Cr cm 2 implantation amorphizes the implanted layer up to a depth of 8000 A, while after a 800 keV 1013 Cr cm -2 implantation, only a buried d a m a g e d layer appears between 3000 and 5000 A. 2) At 77 K, the implantation of 1014 Cr cm -2 into InP leads to a thicker a m o r p h o u s layer with a sharp c r y s t a l l i n e - a m o r p h o u s interface. 3) At r o o m temperature a fluence of 4 × 1012 Se cm 2 is sufficient to amorphize the substrate; when the implanted fluence is increased the a m o r p h o u s layer broadens both towards the surface and towards the bulk. However, at 200°C, InP is not amorphized even for a l014 Se cm z fluence. 4) After a 750°C anneal in a PH 3 atmosphere, a 1014 Cr cm -2 implanted layer is not fully regrown, a heavily d a m a g e d zone centered at 0.8 R r (-3500 A) remains. The same result is observed for a 1014 S e c m 2 implanted layer, but in this case, the d a m a g e d zone lies closer to the surface. On the contrary, a low-dose light-element implanted sample (1013 Cr cm 2) completely recovers the original crystallinity. Due to the small mass difference between Fe and Cr, iron is expected to behave similarly to chromium.
3.2. Iron profiles in non-doped implanted InP substrates Fig. 1 depicts the kinetics of the iron migration in 800 keV 10 t4 Fe cm -2 implanted substrates which have been annealed under Si3N 4 encapsulation, at 550 and 700°C, for times ranging from 10 rain to 60 min. The projected range, Rp, of the as-implanted profile is 5600 ,~ and the standard deviation, ARn, is 2100 A. After sixty minutes at 550°C, iron atoms begin to move towards a depth greater than Rp" a shoulder is observed on the corresponding profile. After a 700°C anneal for 10 min, a depleted zone is largely formed in the 0.8Rp region, but Fe atoms are gettered and form a first peak. For longer annealing times, the iron concentration is more and more depleted at a depth
56Fe+/InP 1014atoms cm -2 800 keY --
u nonneoLed AnneaLed z~ 60 min 5 5 0 ° C o 10 rain 700 °C D 30 rain, 700 ° C -4- 60 rain 7 0 0 ° C
Rp
ld 8
03i
o~
I
<
g 17
c
8 unonneoLed
,,,o
oa
o
1016
lO
l
o
,
,
,
'5o'oo'
' 1oooo'
%oo'
'
Angstroms
Fig. 1. Iron depth profiles, measured by SIMS, for a 800 keV 1014 cm 2 Fe-implant into InP: unannealed (solid curve); annealed at 550°C for 60 min (triangles); annealed at 700°C for 10 min (circles); annealed at 700°C for 30 rain (squares); annealed at 700°C for 60 min (solid curve, diamonds).
equal to R p, creating a large dip separating the first peak at 0.8 Rp and a second one, at about R p-4-Z~Rp, in which the iron concentration is higher than at Rp in the as-implanted profile. It is worth noticing that this second p e a k is formed of two contributions, one at 7000 A and the other at 8000 8500 A. A diffusion tail appears towards the bulk. The Fe concentrations at the bottom of the dip and at the shoulder of the tail are similar, about 1 × 1017 cm 3, which is rather close to the admitted limit solubility for Fe in InP at 700°C [8]. The diffusion coefficient, estimated from the variation of the diffusion tail versus time is found to be 8 × 10 -12 cm -2 s 1 which is in good agreement with the results of Holmes et al [9]. In the case of a sample implanted at a lower dose (1013 Fe cm 2) and annealed at 700°C for 10
M. Gauneau et al. / Depth profiles of Fe and Cr in lnP
min, the redistribution features are not so clear. but basically the same, as has been previously described [7].
3.3. Chromium profiles in non-doped implanted InP substrates Fig. 2 describes the chromium profiles in 800 l014 cm 2 implanted substrates, unannealed or annealed at 700 and 750°C for 10 rain. After anneal the profiles are still divided into three regions: (1) the near surface zone where Cr atom accumulation takes place; (ii) the 0 . 8 R p z o n e where c h r o m i u m is gettered and forms a first peak; (iii) the R p + A R p zone where a second, double peak is observed. The migration of Cr atoms in the 0.8Rp zone depends strongly on temperature since the peak concentration decreases from 6 x 1017 cm 3 at 700°C for 10 min, to 5 × 1016 c m 3 at 750°C keV
10 9 52Cr+/InP, 1014atoms cm2,800 keV~ Rp - - unannealed ' I o#!~. anneaLed i o 10 rnin 700°C J -'- 10 min. 750°C [
for l0 min. The same fast decrease of concentration was observed for iron between 10 and 30 min at 700°C (fig. 1), but comparison with Fe profiles shows that the 0 . 8 R p z o n e is more deeply depleted for chromium. In particular, the concentration at the b o t t o m of the dip, a t R p , is 2 x 1016 cm 3, and diffusion towards the bulk is not so evident for chromium. The same general behavior is observed for Cr atoms in a sample implanted at 77 K to 1014 c m - 2 and annealed at 750°C [7]. Nevertheless, in this case, two main differences are noticed: 1) Cr accumulation a t 0.SRp is more important and a peak concentration of 8 x 1017 cm 3 is registered; 2) the Rp + ARp peak is essentially formed with one accumulation at 7500 A.
3.4. Iron profiles in semi-insulating Se-implanted substrates: Semi-insulating, Fe-doped substrates were implanted with selenium ions at 400 keV to fluences 9
o E
g -
Rp÷ARp ' (O) // ~
I',/
Fe profiles
in semi-insulating Se- implanted tnP { Q ) Se, 1014Se cm-2 , Qs i replanted ( b) Fe ,10I& Se cm-2*lSmin Qt 7S0=C (C)Fe S101/~S~ cm-2.1S min at 7S0"C
<
o
o ~
Rp
t I ,"~, i~ ,~
? E
?
673
o i o o
: ,
(b)
i!,
i017
l~ l ' , ? t \ /
g (..)
ld7
o
o o
6 o o
\
o
lJ 5 10 0
5000
10000 Angstroms
15000
Fig. 2. Chromium depth profiles for a 800 keV 1014 cm 2 Cr-implant into InP: as-implanted (solid curve); annealed at 700°C for 10 min (circles); annealed at 750°C for 10 min (solid curve squares).
o5
i microns
Fig. 3. Depth profiles in semi-insulating Se-implanted substrates: (a) as-implanted Se profile; (b) Fe profile, in a 1014 Se cm 2 sample, annealed at 750°C for 15 min; (c) Fe profile, in a 5X 1014Se cm 2 sample, annealed at 750°C for 15 min. VI. S E M I C O N D U C T O R S
674
M. Gauneau et al. / Depth profiles of Fe and Cr in l n P
of 1014 and 5 × l014 cm -2. During a 750°C anneal, Fe redistribution takes place as shown in fig. 3. In that case, a rather different feature is observed. The 1014 Se cm 2 sample shows a surface pile-up, a peak at 5000 A and a large depleted zone going to 1.5 /~m. The 5 × 1014 Se cm -2 sample exhibits about the same behavior except the fact that two peaks are recorded: the first, at about 2700 A, is preceded by a small dip located at 2000 ,~; the second, with a peak density equal to 3 × 1017 cm 3, is located at 5600 A.
4. Discussion The surface pile-up is attributed to the dopant out diffusion. Here, Cr and Fe show a similar behavior in InP as chromium in GaAs [3,4,10]. The 0.8Rp peak is generally attributed to gettering by implant damage. That behavior is observed when fluences higher than the critical amorphizing dose are reached, and when a buried amorphized layer remains after annealing [5]. This is the case for 1014 Fe cm -2, or 1014 Cr c m 2, implanted samples described here. More difficulties are encountered to explain implanted species accumulation within zones at a depth greater than Rp. A R p + A R p peak has often been reported [see 5,10,11], and many mechanisms have been invoked to explain it. In this study, the same well defined R v + A R p peak is located in a epitaxially regrown layer and is observed whether the implanted layer is amorphized (i.e. Fe or Cr fluences greater than 5 × 1013 cm -2 in InP), or not (fluence equal to 1013 cm 2). So, amorphization of the implanted layer does not seem to have a major effect on the redistribution process of Fe or Cr, in InP or GaAs, at depths greater than _Rp. The crystal nonstoichiometry resulting from the implantation, as described by Christel and Gibbons [12] can be inferred here to explain the Fe and Cr redistribution in InP. After implantation, the crystal stoichiometry is destroyed in the implanted layer and two regions are distinguished: 1) the shallower one where the P vacancies create a net indium excess; 2) the deeper one where the P recoiled atoms exceed the In recoiled atoms and where a net P excess exists. Between the two layers, the stoichiometry is preserved in a narrow zone. These In and P excess densities depend on
the dopant/substrate combinations and on the fluences. As an example, fig. 3 in ref. [12] demonstrates that, in a 10 ~5 S e c m -2 InP implanted wafer, the crystal is stoichiometric at about R p, and that the net indium or phosphorus excess density is about 1019 cm -3. In our case, we estimate that the stoichiometric zone is also near R p (about 5600 ,~ for Fe or Cr implanted into InP) while the P excess density should be slightly reduced for a 10 ~5 cm 2 fluence or ten times lower for a 1014 cm -2 fluence. Figs. 1 and 2 show that, where the crystal stoichiometry is preserved, near R v' Fe or Cr concentrations are depleted down to the solubility limits (1 X 1017 Fe cm -3, or 2 × 10 ~6 Cr cm -3 in InP). Metal atoms are piled up in the region with a P excess, probably as F e - P or C r - P pairs, or as metal phosphide precipitates, which increase the apparent solubility to a value close to the excess P concentration (about 1018 cm 3 for an implantation of 1014 cm-2). Then, from this quasi-infinite source, iron atoms diffuse towards the bulk (fig. 1). It has been noticed that the Rp + ARp peak was composed of two contributions: one at 7000 ,~, the other at 8000-8500 ,~. The third peak, at about 8300 A, is observed for iron (fig. 1) as well as for chromium (fig. 2). It does not occur: 1) in InP implanted to 1013 Fe cm 2; 2) in InP implanted to 1014 Cr cm 2 at 77 K; 3) in GaAs implanted to 1014 Cr cm -2 [5,7]. This third gettering peak is deeper than expected in the implanted layer. A secondary gettering process is needed to explain this behavior. A similar behavior is observed in semi-insulating Se-implanted substrates after a 750°C anneal (fig. 3). The higher peak located at 5000 A for a 1014 c m 2 dose, or at 5600 A for a 5 × 1014 cm 2 dose is deeper than Rp + ARp: in the case of 400 keV Se-implanted substrates the projected range of selenium is equal to 1500 A, and R p + / ~ R p is equal to about 2400 ,~ [13]. In fig. 3, only the first peak of the 5 × 1014 cm 2 profile could be attributed to phosphorus gettering. It is worth noticing that this first peak is preceded by a dip at a depth equal to 2000 ,~. Two remarks can be made about this deeper gettering process: 1) the Fe accumulation and so the peak concentration do not depend on the Se implanted fluence between l01-~ and 5 × 1014 cm-2; 2) this deep peak seems to move towards the bulk as the fluence increases: from 3200 A for1013 cm 2 t o 5 6 0 0 A f o r 5 × 10 T4cm 2
M. Gauneau et al. / Depth profiles of Fe and Cr in l n P
As a possible explanation it could be advanced that this deep gettering process is due to defects created during implantation at room temperature or at 200°C. Such defects nucleation at low temperature has been observed in gallium arsenide [11,14] and it is well known than InP is more sensitive to thermal processing than GaAs.
5. Conclusion
This work concerns iron and chromium after 700-750°C anneal redistributions in implanted InP substrates. The post anneal depth profiles are divided into three regions: 1) the near-surface zone where metals pile up as is generally observed in III-V semiconductors; 2) t h e 0.8Rp zone where the implanted species remain trapped in the residual damage peak; 3) the R p - I - A R p depth region where gettering processes take place. In this third region two types of gettering processes seems to be involved: one due to phosphorus excess according to the results of Christel and Gibbons [12] concerning atom displacements after implantation into InP, the other due to point defects created during implantation. To be well established the thermal processing of InP needs much more experimental work. In particular, transmission electron microscopy and low temperature annealing must be undertaken to specify the size and the nature of the defects at depths greater than Rp.
675
References [1] B. Deveaud and P.N. Favennec, Solid State Commun. 24 (1979) 473. [2] A.M. Huber, G. Morillot, N.T. Linh, P.N. Favennec, B. Deveaud and B. Toulouse, Appl. Phys. Lett. 34 (1979) 858. [3] P.K. Vasudev, R.G. Wilson and C.A. Evans, Appl.Phys. Lett. 36 (1980) 837. [4] V. Eu, M. Feng, W.B. Henderson, H.B. Kim, and J.M. Whelan, Appl. Phys. Lett. 37 (1980) 473. [5] P.K. Vasudev, R.G. Wilson and C.A. Evans, Appl. Phys. Lett. 37 (1980) 308. [6] P. Auvray, A. Guivarc'h, H. L'Haridon, G. Pelous, M. Salvi and P. Henoc, J. Appl. Phys. 53 (1982) 6202. [7] M. Gauneau, H. L'Haridon, A. Rupert and M. Salvi, J. Appl. Phys. 53 (1982) 6823. [8] H. Mordoc, J.T. Andrews and S.B. Hyder, Electron. Lett. 14 (1978) 715. [9] D.E. Holmes, R.G. Wilson and P.W. Yu, J. Appl. Phys. 52 (1981) 3396. [10] C.A. Evans, V.R. Deline, T.W. Sigmon, and A. Lidow, Appl. Phys. Lett. 35 (1979) 291. [11] T.J. Magee, H. Kawayoshi, A.D. Ormond, L.A. Christel, J.F. Gibbons, C,G. Hopkins, C.A. Evans, and D.S. Day, Appl. Phys. Lett. 39 (1981) 906. [12] L.A. Christel and J.F. Gibbons, J. Appl. Phys. 52 (1981) 5050.
[13] M. Gauneau, A. Rupert and P.N. Favennec, Secondary Ion Mass Spectrometry, SIMS Ill, eds., A. Benninghoven, J. Giber, J. Laszlo, M. Riedel and H.W. Werner, Springer Series in Chemical Physics (Springer, Berlin, (1981) p. 342. [14] T.J. Magee, K.S. Lee, R. Ormond, C.A. Evans, R.J. Blattrier and C. Hopkins, Appl. Phys. Lett. 37 (1980) 635.
VI. SEMICONDUCTORS