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AIAs heterostructures
I
elf-diffusion studies have been proved to be extremely important for the understanding of the diffusion mechanisms of b o t h native and impurity atoms in semiconductors. Yet despite the technological importance of GaAs, self-diffusion coefficients of Ga and As in this material have not yet been reliably established. The early work involved the diffusion of radioactive isotopes but there were n u m e r o u s difficulties associated with the short half-lives of the Ga and As isotope sources and the very slow self-diffusion rates. These limitations have made the conventional techniques used in radiotracer experiments difficult to apply and so little data is available. Consequently it was necessary to examine other methods that may indirectly measure self-diffusion coefficients. Interdiffusion studies involve the exchange of different atoms of the same group across an interface, say GaAs-A1As, and although this is a somewhat different process, the mechanisms involved are believed to be essentially the same. Moreover, theoretical calculations predict that the rates at which these two mechanisms should proceed can be related. Recent developments in the growth of alternating epitaxial layers and their characterisation as well as the discovery of the impurity induced disordering p h e n o m e n o n have renewed the interest in self-diffusion studies of IIIV compounds. The most investigated systems are (A1,Ga) As based heterostructures. This is due to the small lattice mismatch between GaAs and the (AI,Ga)As ternery alloy. This allows the investigation of strain-free
S
A Dynamic SIMS Study of Interdiffusion in GaAs/AIAs Heterostructures Growth and characterisation of alternating epilayers have renewed interest in self-diffusion studies of III-V compounds. A collaborative project between CSMA Manchester and the University of Nottingham has investigated the extent of interdiffusiOn as a function of depth across GaAs-AIAs interfaces in MBF epilayers. SIMS and TEM have given direct evidence of a depth dependency and the enhancement of interdiffusion near the surface is believed to be caused by the diffusion of Group III vacancies from the surface.
. ~
~
85 A GaAs - - ~ , . . ~
0.5 p,m GaAs:
Capping Layer
£ BufforY/
~ Layer7/
0.2 p,m AlAS
0.2 l~m AlAS
Figure 1: Schematic of Heterostructure
10 t
|
Ga-A1 interdiffusion across the material interfaces, and there are a wide choice of available techniques which can be employed. The simplicity of some of these techniques and there being no restrictions in annealing times or temperature for interdiffusion studies present a real advantage over the self-diffusion m e t h o d s where diffusion time is limited by isotope halflives. The main techniques currently used include d y n a m i c secondary ion mass spectrometry, SIMS, and sputter-Auger profiling techniques along with X-ray diffraction, Raman spectroscopy, photoluminescence and cathodoluminescence. More direct techniques such as transmission electron microscopy, TEM, and angle-lapping are also used where interdiffusion can be visually estimated.
THIS STUDY The aim of this study was to investigate the extent of interdiffusion as a function of depth that had occurred across GaAs-A1As interfaces of the MBE grown structure (Figure 1) due to annealing. The dynamic SIMS technique was used to produce depth profiles before and after treatment. TEM micrographs were also recorded from the same samples.
Ga + 106"1
EXPERIMENTAL
_ + E~a 2 . . . . AI +
104
'
'°'I ,l
\
," . . . . . . . . . . . . . . . . . . . . .
; "
ii
i} ,:'.. i1
i'. ',' '.!
• I, ,i
i " ";
. . . . ,~., ,
'.:'
,oo
', !. O0
4,4 10
~
t~!
.'
.... 95
". . . . . . . . . . . . . . . .
15
i 20
D e p l h / i~m
Figure 2: Dynamic SIMS Profile of Unannealed Structure
'I'" 2~
The analysis was carried out using a VG SIMSLAB which is a quadrupole based ultra high v a c u u m instrument. A focussed primary beam of mass filtered high energy oxygen ions was raster scanned over a well defined region u p o n the sample causing sputtering of secondary ions. Electropositive species were extracted from the analysis region to be energy analysed and mass filtered. Monitoring several
FEAT U RE
annealing due to out-diffusion into adjacent GaAs layers. It is quite clear from the profiles that the extent of interdiffusion was depth related with the outmost AlAs and GaAs layers significantly more interdiffused. The TEM micrograph recorded from this annealed sample (Figure 5) supports the conclusion that extensive intermixing of the outer layers had occurred, with the inner layers affected to a lesser extent, remaining more intact. Both techniques also suggest that the intermixing process occurred more so in the direction of the sample surface.
masses sequentially by rapid switching of the quadrupole as the sample surface was eroded away by the ion bombardment produced real-time profiles of composition versus depth. Profile quality was optimised by incorporating optical and electronic gating of the secondary ion signal such that only ions were detected from a small central area (~16%) of the crater bottom. OBSERVATIONS The profile of the as-grown structure (Figure 2) clearly indicates the different regions within the structure. The two 85A GaAs sandwich layers were easily resolved, illustrating that good depth resolution was maintained throughout the analysis. This was imperative for the effect of the annealing with depth to be observed. Comparison of the Ga+ and Ga+2 profiles show the improved dynamic range attainable by monitoring diatomic species. Although generally lower in intensity than atomic ions the diatomic ion current is proportional to the square of the atom density. The features are also sharper than for the monatomic ion profile hence showing better depth resolution. A TEM micrograph from this as-grown sample (Figure 3) gave clear, welldefined layer information. In this image the surface is at bottom left and the multilayer structure shown in Figure 1 is clearly visible proceeding to the top right. Figure 4 is the depth profile obtained from the annealed structure. Clearly significant interdiffusion had occurred. The outermost GaAs sandwich layer was extensively disordered whilst the innermost sandwich was to a lesser extent, still being partially resolved. The two AlAs layers were broader after
Figure 3: TEM of unannealed structure
107 • Ga + 106 "
Ga~ . . . . + A~ 2
105 104 "
...... • /~ , ..... •
103.
,
~
~
i;:
\I- ~. . . . . . . . . . . . l~
,.,,,
102 °
%
101 °
t
1
I 00 00
U
05
r
10
~
15
2
0
Depth !~ m
Figure 4: Dynamic SIMS Profile of Annealed Stnlcture
Figure 5: TEM of annealed structure
25
CONCLUSIONS To date the evidence for the depth dependency of the interdiffusion has relied mainly in the interpretation of photoluminescence results. This short study has provided direct evidence of this effect. The enhancement of interdiffusion near the surface is believed to be caused by the diffusion of Group III vacancies from the surface. The use of dynamic SIMS for this type of analysis requires good depth resolution to be maintained whilst using fast etch rates to produce short analysis times. Flat, smooth samples are required since, at best, depth resolution will only reflect the flatness of the crater bottom produced. Uniform sputtering is essential and this requires very stable primary ion beam current and density along with good choice of the optical and electronic gated area. • S Hibbert, Centre for Surface & Materials Analysis, UMIST, Manchester, M60 1QD, UK. N Baba-Ali & I Harrison. Dept Electrical & Electronic Engineering, University of Nottingham, University Park, Nottingham, who supplied the samples and TEM micrographs.
elf-diffusion studies have been proved to be extremely important for the understanding of the diffusion mechanisms of b o t h native and impurity atoms in semiconductors. Yet despite the technological importance of GaAs, self-diffusion coefficients of Ga and As in this material have not yet been reliably established. The early work involved the diffusion of radioactive isotopes but there were n u m e r o u s difficulties associated with the short half-lives of the Ga and As isotope sources and the very slow self-diffusion rates. These limitations have made the conventional techniques used in radiotracer experiments difficult to apply and so little data is available. Consequently it was necessary to examine other methods that may indirectly measure self-diffusion coefficients. Interdiffusion studies involve the exchange of different atoms of the same group across an interface, say GaAs-A1As, and although this is a somewhat different process, the mechanisms involved are believed to be essentially the same. Moreover, theoretical calculations predict that the rates at which these two mechanisms should proceed can be related. Recent developments in the growth of alternating epitaxial layers and their characterisation as well as the discovery of the impurity induced disordering p h e n o m e n o n have renewed the interest in self-diffusion studies of IIIV compounds. The most investigated systems are (A1,Ga) As based heterostructures. This is due to the small lattice mismatch between GaAs and the (AI,Ga)As ternery alloy. This allows the investigation of strain-free
S
A Dynamic SIMS Study of Interdiffusion in GaAs/AIAs Heterostructures Growth and characterisation of alternating epilayers have renewed interest in self-diffusion studies of III-V compounds. A collaborative project between CSMA Manchester and the University of Nottingham has investigated the extent of interdiffusiOn as a function of depth across GaAs-AIAs interfaces in MBF epilayers. SIMS and TEM have given direct evidence of a depth dependency and the enhancement of interdiffusion near the surface is believed to be caused by the diffusion of Group III vacancies from the surface.
. ~
~
85 A GaAs - - ~ , . . ~
0.5 p,m GaAs:
Capping Layer
£ BufforY/
~ Layer7/
0.2 p,m AlAS
0.2 l~m AlAS
Figure 1: Schematic of Heterostructure
10 t
|
Ga-A1 interdiffusion across the material interfaces, and there are a wide choice of available techniques which can be employed. The simplicity of some of these techniques and there being no restrictions in annealing times or temperature for interdiffusion studies present a real advantage over the self-diffusion m e t h o d s where diffusion time is limited by isotope halflives. The main techniques currently used include d y n a m i c secondary ion mass spectrometry, SIMS, and sputter-Auger profiling techniques along with X-ray diffraction, Raman spectroscopy, photoluminescence and cathodoluminescence. More direct techniques such as transmission electron microscopy, TEM, and angle-lapping are also used where interdiffusion can be visually estimated.
THIS STUDY The aim of this study was to investigate the extent of interdiffusion as a function of depth that had occurred across GaAs-A1As interfaces of the MBE grown structure (Figure 1) due to annealing. The dynamic SIMS technique was used to produce depth profiles before and after treatment. TEM micrographs were also recorded from the same samples.
Ga + 106"1
EXPERIMENTAL
_ + E~a 2 . . . . AI +
104
'
'°'I ,l
\
," . . . . . . . . . . . . . . . . . . . . .
; "
ii
i} ,:'.. i1
i'. ',' '.!
• I, ,i
i " ";
. . . . ,~., ,
'.:'
,oo
', !. O0
4,4 10
~
t~!
.'
.... 95
". . . . . . . . . . . . . . . .
15
i 20
D e p l h / i~m
Figure 2: Dynamic SIMS Profile of Unannealed Structure
'I'" 2~
The analysis was carried out using a VG SIMSLAB which is a quadrupole based ultra high v a c u u m instrument. A focussed primary beam of mass filtered high energy oxygen ions was raster scanned over a well defined region u p o n the sample causing sputtering of secondary ions. Electropositive species were extracted from the analysis region to be energy analysed and mass filtered. Monitoring several
FEAT U RE
annealing due to out-diffusion into adjacent GaAs layers. It is quite clear from the profiles that the extent of interdiffusion was depth related with the outmost AlAs and GaAs layers significantly more interdiffused. The TEM micrograph recorded from this annealed sample (Figure 5) supports the conclusion that extensive intermixing of the outer layers had occurred, with the inner layers affected to a lesser extent, remaining more intact. Both techniques also suggest that the intermixing process occurred more so in the direction of the sample surface.
masses sequentially by rapid switching of the quadrupole as the sample surface was eroded away by the ion bombardment produced real-time profiles of composition versus depth. Profile quality was optimised by incorporating optical and electronic gating of the secondary ion signal such that only ions were detected from a small central area (~16%) of the crater bottom. OBSERVATIONS The profile of the as-grown structure (Figure 2) clearly indicates the different regions within the structure. The two 85A GaAs sandwich layers were easily resolved, illustrating that good depth resolution was maintained throughout the analysis. This was imperative for the effect of the annealing with depth to be observed. Comparison of the Ga+ and Ga+2 profiles show the improved dynamic range attainable by monitoring diatomic species. Although generally lower in intensity than atomic ions the diatomic ion current is proportional to the square of the atom density. The features are also sharper than for the monatomic ion profile hence showing better depth resolution. A TEM micrograph from this as-grown sample (Figure 3) gave clear, welldefined layer information. In this image the surface is at bottom left and the multilayer structure shown in Figure 1 is clearly visible proceeding to the top right. Figure 4 is the depth profile obtained from the annealed structure. Clearly significant interdiffusion had occurred. The outermost GaAs sandwich layer was extensively disordered whilst the innermost sandwich was to a lesser extent, still being partially resolved. The two AlAs layers were broader after
Figure 3: TEM of unannealed structure
107 • Ga + 106 "
Ga~ . . . . + A~ 2
105 104 "
...... • /~ , ..... •
103.
,
~
~
i;:
\I- ~. . . . . . . . . . . . l~
,.,,,
102 °
%
101 °
t
1
I 00 00
U
05
r
10
~
15
2
0
Depth !~ m
Figure 4: Dynamic SIMS Profile of Annealed Stnlcture
Figure 5: TEM of annealed structure
25
CONCLUSIONS To date the evidence for the depth dependency of the interdiffusion has relied mainly in the interpretation of photoluminescence results. This short study has provided direct evidence of this effect. The enhancement of interdiffusion near the surface is believed to be caused by the diffusion of Group III vacancies from the surface. The use of dynamic SIMS for this type of analysis requires good depth resolution to be maintained whilst using fast etch rates to produce short analysis times. Flat, smooth samples are required since, at best, depth resolution will only reflect the flatness of the crater bottom produced. Uniform sputtering is essential and this requires very stable primary ion beam current and density along with good choice of the optical and electronic gated area. • S Hibbert, Centre for Surface & Materials Analysis, UMIST, Manchester, M60 1QD, UK. N Baba-Ali & I Harrison. Dept Electrical & Electronic Engineering, University of Nottingham, University Park, Nottingham, who supplied the samples and TEM micrographs.