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Epi-ready InP wafers by oxide passivation
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n this article we have investigated the potential of two epi-ready oxidation procedures t h r o u g h the q u a l i t y o f thermally cleaned lnP (100) surfaces and that of G a I n A s and InP layers s u b s e q u e n t l y g r o w n by MBE. G a l n A s / l n P structures were fabricated using a multichamber integrated UHV system combining a Riber 2300 MBE reactor and a VSW surface analysis chamber with high resolution XPS facilities. The epitaxial InP layers have been grown in a n o t h e r Riber 2300 reactor by gas source MBE ( G S M B E ) . This study has been performed on semi-insulating iron doped 2" lnP wafers from Crismatech InPact, formerly M6taux Sp6ciaux.
N a t i v e oxides To grow protective oxide layers, a variety of wet chemical treatments have been tried but they led to unacceptably high deoxidation temperatures. In contrast, excellent results have been o b t a i n e d with dry oxidation processes. Two procedures have been compared, i) thermal oxidation (TO) and ii) U V / o z o n e oxidation (UVO) ~ . Such treatments have already b e e n used to p r e p a r e GaAs and silicon wafers. The critical step in the thermal oxidation process is the last deoxidation chemical treatment since carbonic impurities have to be minimized. This step is less critical for UV oxidation since this latter technique is used both to remove hydrocarbon contamination and to form a protective oxide layer. The thickness and composition of TO and UVO films were i n v e s t i g a t e d using XPS. The estimated thickness for both oxides was around 7-10 A and composition of
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Epi ready InP Wafers By Oxide
as observed for wafers protected by a natural oxide.
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efficiency' o f the two substrate preparation procedures was evaluated by investigating the quality of lattice matched G a l n A s layers grown on the substrates. Special attention was paid to the density of oval defects related to the substrate preparation. Oval defects without core particulates (called ~x) and oriented in the (110) direction are generally related to remaining oxygen or carbon contaminations. XPS measurements on epi-ready substrates have shown that raising the annealing temperature to 560-565'~C, just below the (2x4) -, (4x2) surface phase transition, allows the elimination of any remaining oxygen surface atom (Figure 1). In parallel, Nomarski phase contrast observations reveal that such treatment minimizes the density of oval detects. In fact, an optimized annealing procedure had to be defined in o r d e r to find the best compromise between annealing temperature and annealing duration, without degrading the substrate m o r p h o l o g y . The optimized procedure illustrated in Figure 2 allows complete elimination of the oval defects.
From Crismatech InPact At the InP Conference in Cardiff, Tony Overs presented collaborative work demonstrating the availability and advantages of epi-ready InP substrates for MBE applications. The availability of etched, ready to use wafers, with no chemical pretreatment by the grower, can be crucial for the fabrication of reproducible high quality epitaxial layers. about InP0. 9 04.s. Thermal oxides were found to be homogeneous with a composition of InPO4. In cont r a s t , UV o x i d e s a r e inhomogeneous in depth. A surface composition of lnP0.3sO~.s was f o u n d whereas the interface comp o s i t i o n was c l o s e to lnPO4. No carbon contamination was detected on wafers introduced in the XPS chamber immediately after the oxidation procedure. In contrast, large quantities of contamination localized on top of the oxide area were de~ected on wafers stored in air atmosphere. For comparison, a natural oxide layer built up on an oxide-free surface after several months storage has a composition of a b o u t InP0.9OV4.8 with some carbon localized at the interface and incorporated within the oxide during growth. The high oxygen c o n c e n t r a t i o n is indicative of some oxygen atoms belonging to OH or C-O contamination species.
appearance of the two fold periodicity R H E E D pattern in the (110) azimuth, with very narrow streaks. Thicker dry oxides and wet chemical oxides give worse surface morphologies (spotty R H E E D patterns). The chemical composition of the clean arsenic stabilized surfaces, prepared at 510-530°C, was analyzed using in situ XPS. It was found that no carbon contamination is present at the surface whereas some oxygen is still detected. For comparison, naked surfaces deoxidized using HF or H2SOa-based solutions, have the (2x4) surface reconstruction appearing at 465-470°C. However both oxygen and carbon residual contaminations still remain on the
In situ clean
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Both TO and UVO oxides with thicknesses less than 12 A desorb, under As flux, at 510°C, leaving a flat InP surface, as revealed by the ~+ #~ ~= i ~~~ , r ~
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Figure 1: X P S survey scans ~or two lnP substrates aJter arsenic stabilization at 540°C: a) Surface preparation by chemical cleaning b) Surface preparation by Epi-Ready oxidation
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U V O p r e p a r a t i o n s . The electronic quality o f epilayer-substrate interfaces was investigated by measuring the cartier concentration around InP/InP interfaces. Figure 4 shows cartier profiles for a chemically cleaned surface and for an epi-ready substrate. It is observed that there is a decrease in the surface carrier concentration for epi-ready samples which demonstrates the higher quality of the surface preparation.
The effect of oxidation and sublimation was studied by probing the contamination concentration near the substrate-epitaxial interface using SIIVIS and C-V carrier profiling techniques. Figure 3 shows the SIMS oxygen profiles at the GaInAs/InP interface for two different substrate preparations. The first sample (A) was p r e p a r e d using the optimized procedure described above and did not have any oval defects in the epitaxial layer. The second sample (B) was deoxidized at lower temperature (520°C) and was characterized by a high density of oval defects ( > lOScm-~). The interfacial oxygen concentration for sample B is one order of magnitude higher than sample A.
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microns Figure 3." SIMS oxygen profilesfor 2 UVO Epi-Ready substrates preparations a) optimized deoxidation procedure b) deoxidation at 520°C
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Figure ~l. Comparison of C- V carrierprofilesfor chemically clean wafers (1) and UVO Epi-Ready substrates (2)
Authors
between the deoxidation conditions (temperature and heating time), the interface oxygen concentration and the density o f oval defects. The absolute number of oxygen atoms was determined by XPS on surfaces prepared similarly. For samples o f type B, a surface oxygen concentration o f the order of 0.6 monolayer (1 M L = 5.8 10 TM at cm -2) was found, fairly well in agreement with the SIMS estimation, considering a m e a s u r e d interface width of 400 A and a maximum concentration of 9.1019 at cm "3. The SIMS signals were calibrated from oxygen implants measured in the same experimental conditions. The oxygen surface concentration on samples of type B is below the XPS detection limit (0.1 ML) in agreement with the SIMS results (1.4 10 ~9 at cm-3). Note that no difference in the SIMS carbon profiles at the interface was found between epi-ready TO and
D. Gallet, M. Gendry, G. Hollinger, L E E C de Lyon, Ecully, France. M. Gauneau, H. L'Haridon, D. Lecrosnier, CNET, Lannion, France. A. Overs, G. Jacob, B. Boudard, Crismatech InPact, Pombliere St. Marcel, 73600, Moutiers France. Tel: [33] (0)79 24 2780. Fax: [33] (0) 79 24 4517.
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n this article we have investigated the potential of two epi-ready oxidation procedures t h r o u g h the q u a l i t y o f thermally cleaned lnP (100) surfaces and that of G a I n A s and InP layers s u b s e q u e n t l y g r o w n by MBE. G a l n A s / l n P structures were fabricated using a multichamber integrated UHV system combining a Riber 2300 MBE reactor and a VSW surface analysis chamber with high resolution XPS facilities. The epitaxial InP layers have been grown in a n o t h e r Riber 2300 reactor by gas source MBE ( G S M B E ) . This study has been performed on semi-insulating iron doped 2" lnP wafers from Crismatech InPact, formerly M6taux Sp6ciaux.
N a t i v e oxides To grow protective oxide layers, a variety of wet chemical treatments have been tried but they led to unacceptably high deoxidation temperatures. In contrast, excellent results have been o b t a i n e d with dry oxidation processes. Two procedures have been compared, i) thermal oxidation (TO) and ii) U V / o z o n e oxidation (UVO) ~ . Such treatments have already b e e n used to p r e p a r e GaAs and silicon wafers. The critical step in the thermal oxidation process is the last deoxidation chemical treatment since carbonic impurities have to be minimized. This step is less critical for UV oxidation since this latter technique is used both to remove hydrocarbon contamination and to form a protective oxide layer. The thickness and composition of TO and UVO films were i n v e s t i g a t e d using XPS. The estimated thickness for both oxides was around 7-10 A and composition of
::::::::::::::::.
:::::
======================================= :~" : ~ i : : ~ : "
~':~:~"
:"
,.
":'".
" "::
i:~:~: :: :::
Epi ready InP Wafers By Oxide
as observed for wafers protected by a natural oxide.
m
Epi growth
Passivation
efficiency' o f the two substrate preparation procedures was evaluated by investigating the quality of lattice matched G a l n A s layers grown on the substrates. Special attention was paid to the density of oval defects related to the substrate preparation. Oval defects without core particulates (called ~x) and oriented in the (110) direction are generally related to remaining oxygen or carbon contaminations. XPS measurements on epi-ready substrates have shown that raising the annealing temperature to 560-565'~C, just below the (2x4) -, (4x2) surface phase transition, allows the elimination of any remaining oxygen surface atom (Figure 1). In parallel, Nomarski phase contrast observations reveal that such treatment minimizes the density of oval detects. In fact, an optimized annealing procedure had to be defined in o r d e r to find the best compromise between annealing temperature and annealing duration, without degrading the substrate m o r p h o l o g y . The optimized procedure illustrated in Figure 2 allows complete elimination of the oval defects.
From Crismatech InPact At the InP Conference in Cardiff, Tony Overs presented collaborative work demonstrating the availability and advantages of epi-ready InP substrates for MBE applications. The availability of etched, ready to use wafers, with no chemical pretreatment by the grower, can be crucial for the fabrication of reproducible high quality epitaxial layers. about InP0. 9 04.s. Thermal oxides were found to be homogeneous with a composition of InPO4. In cont r a s t , UV o x i d e s a r e inhomogeneous in depth. A surface composition of lnP0.3sO~.s was f o u n d whereas the interface comp o s i t i o n was c l o s e to lnPO4. No carbon contamination was detected on wafers introduced in the XPS chamber immediately after the oxidation procedure. In contrast, large quantities of contamination localized on top of the oxide area were de~ected on wafers stored in air atmosphere. For comparison, a natural oxide layer built up on an oxide-free surface after several months storage has a composition of a b o u t InP0.9OV4.8 with some carbon localized at the interface and incorporated within the oxide during growth. The high oxygen c o n c e n t r a t i o n is indicative of some oxygen atoms belonging to OH or C-O contamination species.
appearance of the two fold periodicity R H E E D pattern in the (110) azimuth, with very narrow streaks. Thicker dry oxides and wet chemical oxides give worse surface morphologies (spotty R H E E D patterns). The chemical composition of the clean arsenic stabilized surfaces, prepared at 510-530°C, was analyzed using in situ XPS. It was found that no carbon contamination is present at the surface whereas some oxygen is still detected. For comparison, naked surfaces deoxidized using HF or H2SOa-based solutions, have the (2x4) surface reconstruction appearing at 465-470°C. However both oxygen and carbon residual contaminations still remain on the
In situ clean
~ooo
Both TO and UVO oxides with thicknesses less than 12 A desorb, under As flux, at 510°C, leaving a flat InP surface, as revealed by the ~+ #~ ~= i ~~~ , r ~
:~i~?~i~:~:':::~:~a ~ ' ~ ' ~ , .•~:...~..':::-':~
I
®
3%,
60o
@
i
o
binding energy(eV)
Figure 1: X P S survey scans ~or two lnP substrates aJter arsenic stabilization at 540°C: a) Surface preparation by chemical cleaning b) Surface preparation by Epi-Ready oxidation
....... "~#*~::~:~='~. .~: ~*~i~~,. "~!~*~~ ~i~!~ ' • .:::*:.:,::::.=:::{~i
'
Tl
- 560"C 520"C
~ t(rr~n)
TIME Figure 2: Optimizedprocedure.[br deoxidation of Epi-Ready lnP (100) 10
ib
t0
® IQ
I0
~O
i
U V O p r e p a r a t i o n s . The electronic quality o f epilayer-substrate interfaces was investigated by measuring the cartier concentration around InP/InP interfaces. Figure 4 shows cartier profiles for a chemically cleaned surface and for an epi-ready substrate. It is observed that there is a decrease in the surface carrier concentration for epi-ready samples which demonstrates the higher quality of the surface preparation.
The effect of oxidation and sublimation was studied by probing the contamination concentration near the substrate-epitaxial interface using SIIVIS and C-V carrier profiling techniques. Figure 3 shows the SIMS oxygen profiles at the GaInAs/InP interface for two different substrate preparations. The first sample (A) was p r e p a r e d using the optimized procedure described above and did not have any oval defects in the epitaxial layer. The second sample (B) was deoxidized at lower temperature (520°C) and was characterized by a high density of oval defects ( > lOScm-~). The interfacial oxygen concentration for sample B is one order of magnitude higher than sample A.
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Correlation T h e r e is a g o o d c o r r e l a t i o n
15 I0
,'1 , I, I', t , 1 r[ , I, I , I ~
microns Figure 3." SIMS oxygen profilesfor 2 UVO Epi-Ready substrates preparations a) optimized deoxidation procedure b) deoxidation at 520°C
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Figure ~l. Comparison of C- V carrierprofilesfor chemically clean wafers (1) and UVO Epi-Ready substrates (2)
Authors
between the deoxidation conditions (temperature and heating time), the interface oxygen concentration and the density o f oval defects. The absolute number of oxygen atoms was determined by XPS on surfaces prepared similarly. For samples o f type B, a surface oxygen concentration o f the order of 0.6 monolayer (1 M L = 5.8 10 TM at cm -2) was found, fairly well in agreement with the SIMS estimation, considering a m e a s u r e d interface width of 400 A and a maximum concentration of 9.1019 at cm "3. The SIMS signals were calibrated from oxygen implants measured in the same experimental conditions. The oxygen surface concentration on samples of type B is below the XPS detection limit (0.1 ML) in agreement with the SIMS results (1.4 10 ~9 at cm-3). Note that no difference in the SIMS carbon profiles at the interface was found between epi-ready TO and
D. Gallet, M. Gendry, G. Hollinger, L E E C de Lyon, Ecully, France. M. Gauneau, H. L'Haridon, D. Lecrosnier, CNET, Lannion, France. A. Overs, G. Jacob, B. Boudard, Crismatech InPact, Pombliere St. Marcel, 73600, Moutiers France. Tel: [33] (0)79 24 2780. Fax: [33] (0) 79 24 4517.
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