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Microscopy of surfaces and applications to molecular beam epitaxy
Ultramicroscopy 17 (1985) 185-192 North-Holland, Amsterdam
185
MICROSCOPY OF SURFACES AND APPLICATIONS TO MOLECULAR BEAM EPITAXY P.M. PETROFF, C.H. CHEN and D.J. WERDER A T&T Bell Laboratories, Murray Hill, New Jersey 07974, USA. Received 13 June 1985
A scanning transmission electron microscope has been specifically designed for studies of reconstructed surfaces and epitaxial growth processes. Incorporation of a molecular beam epitaxy system, Auger spectrometer and ion gun along with ultra-high vacuum make this instrument ideally suited for this type of investigation. An analysis of the reconstructed GaAs surface is also given to demonstrate the instrument capabilities.
1. Introduction The relatively new applications of transmission electron microscopy (TEM) to the field of surface analysis and surface reconstruction provide a novel approach to understanding semiconductors and metal surfaces [1-5]. Indeed TEM provides both direct, real-space imaging of the surface and reciprocal-space information via the transmission electron diffraction pattern (TED). As opposed to low-energy electron diffraction (LEED), interpretation of TED patterns for the purpose of modeling the reconstructed surfaces is straightforward in the framework of the kinematical diffraction theory which is applicable to the thin surface layers [6,7]. The most recent applications of surface TEM have been carried out on modified transmission electron microscopes that provide a UHV environment around the sample [8,9]. This experimental approach is limiting for the following reasons: (a) often the sample preparation prior to observation of the reconstructed surface requires a procedure for surface cleaning, e.g. ion bombardment and annealing, (b) a high-sensitivity analysis of the chemical species remaining on the surface after a surface cleaning treatment or a deposition is often needed, (c) the possibility of observing the interactions of various atomic species with the reconstructed surface demands exposing the surface to evaporation sources in an UHV environment. The
following paper describes a microscope which meets the above experimental requirements. A detailed description of the instrument is given. Selected example are given to illustrate the potential applications of this new experimental approach to the study of surfaces and the initial stages of the epitaxial process.
2. Experimental 2.1. The microscope basic functions The instrument is based on a 100 keV scanning transmission electron microscope column (Vacuum Generators HB501A) which has been modified to provide an UHV environment and equipped with a high-brightness electron field emission gun. The additional components which have been added to the basic electron-optical column are shown in fig. 1. These consist of a middle chamber equipped with an Auger electron spectrometer and an upper chamber comprising an argon ion gun and a cluster of 4 effusion cells for molecular beam epitaxy (MBE). The essential functions of the various chambers are given in fig. 1 and the ray diagram for the various modes of operation shown in fig. 2. Structural analysis is carried out on a 3 mm disk sample located inside the objective lens pole-piece (position I in fig. 2A) with a point-to-point resolu-
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
186
P.M. Petroff et al. / Microscopy of surfaces and appfications to MBE
VACUUM
RESOLUTION
SIGNALS
5 10-5 Torr TO 3 ! O- I 0 Torr
2ooX
BRIGHT AND DARK FIELD STEM DIFFRACTION SEM
5 10-5 Torr TO 3 I0 -10 Torr
FUNCTIONS
-
SURFACE CLEANINGAND MBE DEPOSITION
'f////////J
"a3
DARK FIELD STEM -DIFFRACTION
l
I p,-
~ooi
SEM AUGER
si
SURFACE.STRUCTUREANALYSIS AUGER ANALYSIS m
ml
HIGH RESOLUTION, STRUCTURALANALYSIS
I 0-8 Torr TO 3 10 -I0 Torr
ELECTRONOPTICS: LENSES IOL. CL2. CLI) SCANNING COILS
5 I0-11 Torr
FIELD EMISSION GUN 5 - 100 keV
Fig. 1. Schematicgiving the various instrument componentswith their main function and important characteristics.
tion of --- 4-5 ,~. Chemical and structural analyses are performed on 3 mm samples or larger samples (up to 1 cm 2) located ,.~ 1 cm above the objective lens pole-piece (position 2 in fig. 2C). Scanning transmission electron microscopy (STEM) and T E D operations are also possible along with scanning reflection electron microscopy (SREM) and reflection high-energy electron d i f f r a c t i o n ( R H E E D ) for samples located in position 2. The resolution on thin film samples at the position 2 level is equal to the probe size -- 50 ,~. In position 3, the samples (3 mm diameter disks or -- 1 cm 2 samples) are located -- 50 cm above the objective lens pole-piece. Surface preparation and epitaxial deposition are carried out in position 3 (fig. 2D). A focused ion gun allows surface cleaning by ion bombardment as well as thinning of a pre-thinned sample using Ar ÷ ions (500 eV to 5 keV). In this position the sample can be annealed at temperatures up to 1000°C under UHV conditions.
2.2. T h e v a c u u m system
The U H V environment is maintained by a corqbination of cold trap and diffusion pump for the upper chambers of the microscope and ion pump for the electron optics column. The electron gun section is also pumped by an ion pump. The electron gun section is separated from the remainder of the instrument by a 200 # m diameter differential pumping aperture. This allows operating the microscope with a partial gas pressure of up to 10 -5 Torr in the upper chambers of the instrument. Thus MBE deposition or ion thinning of the sample can be performed while imaging the sample in position 3 with a resolution of -300-500 A. The instrument base pressure in the MBE chamber part is 3 × 10-1° Torr with the 4 effusion cells loaded with Ga, Al, As and Si. The sample transfer system which is essential for moving the samples between the various posi-
P.M. Petroff et al. / Microscopy of surfaces and applications to MBE
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P.M. Petroff et al. / Microscopy of surfaces and applications to MBE
188
tions in the instrument is based on a plunger tool actuated from outside the microscope. Two rotary-motion feedthroughs provide this plunger with sample holder grabbing functions as well as a vertical and rotational motion. An air-lock system allows the introduction of a fresh sample into the UHV environment in less than 5 rain.
13158V 1359tV ~] I~ 13g' " q I I ~1341 "
Eo= IOOKoV 1,= 05/IA AI~GliA,
2.3. The A uger electron spectrometer (AES) l[
The AES is based on a hemispherical electrostatic analyzer. The sample located in position 2 is imaged vi.a the secondary electron detector and the area of interest is chosen for AES analysis. For this analysis the sample is inclined at more than 45 ° with respect to the incident electron beam. An einzel lens between the sample and the AES is used for collecting and focusing the Auger electrons on the entrance slit of the spectrometer. A channeltron tube detects the Auger electrons and provides signal versus electron energy plot via an analogue signal-processing chain. The spatial resolution in the Auger mode is considerably worse than in the imaging mode because of the sample-e- beam geometry when collecting the Auger signal. This configuration emphasizes the contribution of elastically backscattered electrons to the Auger signal. The resolution in this mode is -~ 2000 A for bulk samples. Nevertheless the Auger detection system is suffi-
1500
I
1400
As I
|
1000
1,00
/
1100
1000
p
levl
Fig. 4. Auger electron spectrum for an Al epitaxial ( > 100 ,~) film deposited on the (100)GaAs c ( 4 x 4 ) surface at room temperature. Note some As but no Ga is detected on the AI surface.
ciently sensitive to carry out Auger analysis of thin film (100 ,~ Au) surfaces. Two examples of the signal-to-noise achieved with this instrument are given for a (100)GaAs c(4x 4) reconstructed surface (fig. 3) and for an AI epitaxial film (fig. 4) deposited by MBE onto a (100)GaAs c(4 X 4) surface. Both samples were prepared inside the microscope. The signal-to-noise ratio in this energy range is large enough to avoid the need for differentiation of the Auger signal. 2.4. The MBE deposition system
/
As I
6IAs (1001
Ee = I00 keY I o = 05/AA
\
"
1300
I 1200
I 1100
I 1000
000
t ItVl
Fig. 3. Auger electron spectrum for GaAs(100) reconstructed surface. Primary electron beam energy (100 keV) and beam current 0.5 #A.
As shown in fig. 2D, the MBE deposition system allows for epitaxial deposition while the sample is imaged in the STEM or SEM mode. In this system, 4 effusion cells are mounted on a honeycomb cryopanel which is cooled to liquid nitrogen temperature during operation of the sources. The temperature regulation of each MBE cell is stabilized to the preset level by a Eurotherm temperature controller. Each cell is equipped with a shutter actuated by a rotary feedthrough. Fig. 5 shows the 4 boron nitride cells inside the honeycomb cryopanel. The entire MBE system is modular and adapted to the microscope through an 8" OD Conflat flange. To prevent contamination of the electron-optics column and Auger analysis chamber, a liquid-nitrogen-cooled cyroshield is used
P.M. Petroff et aL / Microscopy of surfaces and applications to MBE
189
Fig. 5. The modular MBE system contains a honeycomb cyroshield (A) into which 4 boron nitride cells (B) are actuated through a system of rotary feedthroughs.
(fig. 2D). This system is efficient in preventing contamination. Partial pressures of the various species in the MBE section of the microscope are monitored through a vacuum ionization gauge or a quadrupole mass analyzer. Typical base' pressure with the MBE cells cold ( < 200°C) are in the 2 to 3 × 10 -1° Torr range. The remaining detected species in the system are mostly hydrogen, water vapor, nitrogen and methane.
3. Applications to MBE-grown epitaxial surfaces The imaging and analysis of GaAs surface grown in situ by MBE deposition on a (100)GaAs substrate is used as an example of the capabilities of this instrument.
A (100)GaAs semi-insulating substrate was cleaned and polished using the standard diluted (2%) bromine methanol solution followed by a final etch in a H2SO4, H 2 0 2 , H 2 0 (5 : 1 : 1) solution prior to introduction in the microscope. A GaAs layer = 1200 ,~ thick was deposited at a substrate temperature Ts = 580°C after sublimination of the GaAs oxide at T~ = 630°C. The sample was then cooled under an As flux to 400°C and to room temperature without the As flux. After lowering the sample in position 2, Auger analysis was carried out (fig. 3). In addition to Ga and As, the Auger spectrum indicates a small amount of oxygen present on the surface; however, no carbon or other elements could be detected. Imaging of the surface was carried out by tilting the sample almost parallel to the electron beam. The angle of
190
P.M. Petroff et al. / Microscopy of surfaces and appfications to MBE
incidence of the e - beam on the surface, -- 10-2 rad, was estimated from the sample size and its image detected using the dark field annular detector or the secondary electron detector. With the electron beam incident along a (110)
direction, the R H E E D pattem of the surface (fig 6B) shows the superlattice spots characteristic of the c(4 × 4) or c(8 × 2) reconstructed surface. The SEM imaging shows a rough surface with terrace sizes in the range 0.5 to 10/Lm. This imaging mode is very sensitive to the surface topography, and monoatomic changes in surface topography are easily detected by secondary reflection electron microscopy (SREM). The surface topography of this MBE-grown GaAs is similar to that grown'in a conventional MBE system. As reported earlier [4], the MBE-grown GaAs surface is not completely flat and terraces with height ranging from -- 5 A to 50 A are prevalent when growing under As-rich conditions. The length of the superlattice streaks in the R H E E D pattern reflects the presence of the terraces and indicates a mosaic-type surface with small deviations of the terraces from the (100) orientation. The roughness of the MBE-grown GaAs surface is somewhat surprising in view of earlier results which indicated that the interfaces between GaAs and GaAIAs epitaxial layers are extremely abrupt and flat [11]. Since the surface examined here was cooled down to room temperature, there remains the possibility that the surface topography changes as it is cooled from the growth temperature. In fact, rapid changes in the R H E E D spot intensities and shapes caused by changing the impinging Ga flux (by closing or opening the Ga shutter) have been reported [11]. These have been rationalized by a surface smoothing via step motions. Thus a large difference in the surface topography could exist between the surface at low and high temperatures. Step motion and step bunching upon cooling of the sample are needed to explain the rough GaAs surface observed at room temperature.
4. Conclusions
Fig. 6. (A) Secondary reflection electron micrograph (SREM) of the surface of (100)GaAs epitaxial film produced by MBE. The arrow is parallel to a [011] direction and indicates the direction of the incidem electron beam (E0 =.100 keV). The angle of incidenceto the surfaceis = 5 × 10 -2 rd. (B) RHEED pattern of the surface showing a c(4x4) or c(2 × 8) reconstructed surface. The central beam is in the lower-leftpart of the pattern.
A new type of electron microscope combining U H V technology with chemical analysis of surface layers and molecular beam epitaxy has been described. Some of the performances of this instrument have been demonstrated with a surface analysis of an MBE-grown GaAs epitaxial film.
P.M. Petroff et aL / Microscopy of surfaces and applications to MBE
References [1] K. Yagi, T. Takayanagi, K. Kobayaski, N. Osakabe, Y. Taniskiro and G. Honjo, Surface Sci. 86 (1979) 174. [2] K. Takayanagi, Ultramicroscopy 8 (1982) 145. [3] K. Takayanagi, in: Proc. 10th Intern. Congr. on Electron Microscopy, Hamberg, 1981, p. 43. [4] P.M. Petroff, in: Electron Microscopy of Materials, Mater. Res. Soc. Symp., Vol. 31, Eds. W. Krakow, D.A. Smith and L.W. Hobbs (North-Holland, New York, 1984) p. 117. [5] P.M. Petroff and R.J. Wilson, Phys. Rev. Letters 51 (1983) 199.
191
[6] J.C.H. Spence, Ultramicroscopy 11 (1983) 117. [7] E.G. McRae and P.M. Petroff, Surface Sci. 147 (1984) 385. [8] R.J. Wilson and P.M. Petroff, Rev. Sci. Instr. 54 (1983) 1534. [9] K. Takayanagi, K. ~ragi, K. Kobayashi and G. Honjo, J. Phys. E l l (1978) 441. [10] P.M. Petroff, A.C. Gossard, W. Wiegmann and A. Savage, J. Crystal Growth 44 (1978) 5. [11] J.M. Van Hove and P.I. Cohen, J. Vacuum Sci. Technoi. 20 (1982) 726.
185
MICROSCOPY OF SURFACES AND APPLICATIONS TO MOLECULAR BEAM EPITAXY P.M. PETROFF, C.H. CHEN and D.J. WERDER A T&T Bell Laboratories, Murray Hill, New Jersey 07974, USA. Received 13 June 1985
A scanning transmission electron microscope has been specifically designed for studies of reconstructed surfaces and epitaxial growth processes. Incorporation of a molecular beam epitaxy system, Auger spectrometer and ion gun along with ultra-high vacuum make this instrument ideally suited for this type of investigation. An analysis of the reconstructed GaAs surface is also given to demonstrate the instrument capabilities.
1. Introduction The relatively new applications of transmission electron microscopy (TEM) to the field of surface analysis and surface reconstruction provide a novel approach to understanding semiconductors and metal surfaces [1-5]. Indeed TEM provides both direct, real-space imaging of the surface and reciprocal-space information via the transmission electron diffraction pattern (TED). As opposed to low-energy electron diffraction (LEED), interpretation of TED patterns for the purpose of modeling the reconstructed surfaces is straightforward in the framework of the kinematical diffraction theory which is applicable to the thin surface layers [6,7]. The most recent applications of surface TEM have been carried out on modified transmission electron microscopes that provide a UHV environment around the sample [8,9]. This experimental approach is limiting for the following reasons: (a) often the sample preparation prior to observation of the reconstructed surface requires a procedure for surface cleaning, e.g. ion bombardment and annealing, (b) a high-sensitivity analysis of the chemical species remaining on the surface after a surface cleaning treatment or a deposition is often needed, (c) the possibility of observing the interactions of various atomic species with the reconstructed surface demands exposing the surface to evaporation sources in an UHV environment. The
following paper describes a microscope which meets the above experimental requirements. A detailed description of the instrument is given. Selected example are given to illustrate the potential applications of this new experimental approach to the study of surfaces and the initial stages of the epitaxial process.
2. Experimental 2.1. The microscope basic functions The instrument is based on a 100 keV scanning transmission electron microscope column (Vacuum Generators HB501A) which has been modified to provide an UHV environment and equipped with a high-brightness electron field emission gun. The additional components which have been added to the basic electron-optical column are shown in fig. 1. These consist of a middle chamber equipped with an Auger electron spectrometer and an upper chamber comprising an argon ion gun and a cluster of 4 effusion cells for molecular beam epitaxy (MBE). The essential functions of the various chambers are given in fig. 1 and the ray diagram for the various modes of operation shown in fig. 2. Structural analysis is carried out on a 3 mm disk sample located inside the objective lens pole-piece (position I in fig. 2A) with a point-to-point resolu-
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
186
P.M. Petroff et al. / Microscopy of surfaces and appfications to MBE
VACUUM
RESOLUTION
SIGNALS
5 10-5 Torr TO 3 ! O- I 0 Torr
2ooX
BRIGHT AND DARK FIELD STEM DIFFRACTION SEM
5 10-5 Torr TO 3 I0 -10 Torr
FUNCTIONS
-
SURFACE CLEANINGAND MBE DEPOSITION
'f////////J
"a3
DARK FIELD STEM -DIFFRACTION
l
I p,-
~ooi
SEM AUGER
si
SURFACE.STRUCTUREANALYSIS AUGER ANALYSIS m
ml
HIGH RESOLUTION, STRUCTURALANALYSIS
I 0-8 Torr TO 3 10 -I0 Torr
ELECTRONOPTICS: LENSES IOL. CL2. CLI) SCANNING COILS
5 I0-11 Torr
FIELD EMISSION GUN 5 - 100 keV
Fig. 1. Schematicgiving the various instrument componentswith their main function and important characteristics.
tion of --- 4-5 ,~. Chemical and structural analyses are performed on 3 mm samples or larger samples (up to 1 cm 2) located ,.~ 1 cm above the objective lens pole-piece (position 2 in fig. 2C). Scanning transmission electron microscopy (STEM) and T E D operations are also possible along with scanning reflection electron microscopy (SREM) and reflection high-energy electron d i f f r a c t i o n ( R H E E D ) for samples located in position 2. The resolution on thin film samples at the position 2 level is equal to the probe size -- 50 ,~. In position 3, the samples (3 mm diameter disks or -- 1 cm 2 samples) are located -- 50 cm above the objective lens pole-piece. Surface preparation and epitaxial deposition are carried out in position 3 (fig. 2D). A focused ion gun allows surface cleaning by ion bombardment as well as thinning of a pre-thinned sample using Ar ÷ ions (500 eV to 5 keV). In this position the sample can be annealed at temperatures up to 1000°C under UHV conditions.
2.2. T h e v a c u u m system
The U H V environment is maintained by a corqbination of cold trap and diffusion pump for the upper chambers of the microscope and ion pump for the electron optics column. The electron gun section is also pumped by an ion pump. The electron gun section is separated from the remainder of the instrument by a 200 # m diameter differential pumping aperture. This allows operating the microscope with a partial gas pressure of up to 10 -5 Torr in the upper chambers of the instrument. Thus MBE deposition or ion thinning of the sample can be performed while imaging the sample in position 3 with a resolution of -300-500 A. The instrument base pressure in the MBE chamber part is 3 × 10-1° Torr with the 4 effusion cells loaded with Ga, Al, As and Si. The sample transfer system which is essential for moving the samples between the various posi-
P.M. Petroff et al. / Microscopy of surfaces and applications to MBE
i
~%i o i,,.i ~ uMl'-
187
,--m
°
i.
i i-"n~T°T
]
0
0
o~:~ ~:w J ~o
,~o
~ ~
zu
.
0!. o ~_
e~
o~
~z w ~ o ~ w
z o
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g a
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w ~ v
~w
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iv
i
P.M. Petroff et al. / Microscopy of surfaces and applications to MBE
188
tions in the instrument is based on a plunger tool actuated from outside the microscope. Two rotary-motion feedthroughs provide this plunger with sample holder grabbing functions as well as a vertical and rotational motion. An air-lock system allows the introduction of a fresh sample into the UHV environment in less than 5 rain.
13158V 1359tV ~] I~ 13g' " q I I ~1341 "
Eo= IOOKoV 1,= 05/IA AI~GliA,
2.3. The A uger electron spectrometer (AES) l[
The AES is based on a hemispherical electrostatic analyzer. The sample located in position 2 is imaged vi.a the secondary electron detector and the area of interest is chosen for AES analysis. For this analysis the sample is inclined at more than 45 ° with respect to the incident electron beam. An einzel lens between the sample and the AES is used for collecting and focusing the Auger electrons on the entrance slit of the spectrometer. A channeltron tube detects the Auger electrons and provides signal versus electron energy plot via an analogue signal-processing chain. The spatial resolution in the Auger mode is considerably worse than in the imaging mode because of the sample-e- beam geometry when collecting the Auger signal. This configuration emphasizes the contribution of elastically backscattered electrons to the Auger signal. The resolution in this mode is -~ 2000 A for bulk samples. Nevertheless the Auger detection system is suffi-
1500
I
1400
As I
|
1000
1,00
/
1100
1000
p
levl
Fig. 4. Auger electron spectrum for an Al epitaxial ( > 100 ,~) film deposited on the (100)GaAs c ( 4 x 4 ) surface at room temperature. Note some As but no Ga is detected on the AI surface.
ciently sensitive to carry out Auger analysis of thin film (100 ,~ Au) surfaces. Two examples of the signal-to-noise achieved with this instrument are given for a (100)GaAs c(4x 4) reconstructed surface (fig. 3) and for an AI epitaxial film (fig. 4) deposited by MBE onto a (100)GaAs c(4 X 4) surface. Both samples were prepared inside the microscope. The signal-to-noise ratio in this energy range is large enough to avoid the need for differentiation of the Auger signal. 2.4. The MBE deposition system
/
As I
6IAs (1001
Ee = I00 keY I o = 05/AA
\
"
1300
I 1200
I 1100
I 1000
000
t ItVl
Fig. 3. Auger electron spectrum for GaAs(100) reconstructed surface. Primary electron beam energy (100 keV) and beam current 0.5 #A.
As shown in fig. 2D, the MBE deposition system allows for epitaxial deposition while the sample is imaged in the STEM or SEM mode. In this system, 4 effusion cells are mounted on a honeycomb cryopanel which is cooled to liquid nitrogen temperature during operation of the sources. The temperature regulation of each MBE cell is stabilized to the preset level by a Eurotherm temperature controller. Each cell is equipped with a shutter actuated by a rotary feedthrough. Fig. 5 shows the 4 boron nitride cells inside the honeycomb cryopanel. The entire MBE system is modular and adapted to the microscope through an 8" OD Conflat flange. To prevent contamination of the electron-optics column and Auger analysis chamber, a liquid-nitrogen-cooled cyroshield is used
P.M. Petroff et aL / Microscopy of surfaces and applications to MBE
189
Fig. 5. The modular MBE system contains a honeycomb cyroshield (A) into which 4 boron nitride cells (B) are actuated through a system of rotary feedthroughs.
(fig. 2D). This system is efficient in preventing contamination. Partial pressures of the various species in the MBE section of the microscope are monitored through a vacuum ionization gauge or a quadrupole mass analyzer. Typical base' pressure with the MBE cells cold ( < 200°C) are in the 2 to 3 × 10 -1° Torr range. The remaining detected species in the system are mostly hydrogen, water vapor, nitrogen and methane.
3. Applications to MBE-grown epitaxial surfaces The imaging and analysis of GaAs surface grown in situ by MBE deposition on a (100)GaAs substrate is used as an example of the capabilities of this instrument.
A (100)GaAs semi-insulating substrate was cleaned and polished using the standard diluted (2%) bromine methanol solution followed by a final etch in a H2SO4, H 2 0 2 , H 2 0 (5 : 1 : 1) solution prior to introduction in the microscope. A GaAs layer = 1200 ,~ thick was deposited at a substrate temperature Ts = 580°C after sublimination of the GaAs oxide at T~ = 630°C. The sample was then cooled under an As flux to 400°C and to room temperature without the As flux. After lowering the sample in position 2, Auger analysis was carried out (fig. 3). In addition to Ga and As, the Auger spectrum indicates a small amount of oxygen present on the surface; however, no carbon or other elements could be detected. Imaging of the surface was carried out by tilting the sample almost parallel to the electron beam. The angle of
190
P.M. Petroff et al. / Microscopy of surfaces and appfications to MBE
incidence of the e - beam on the surface, -- 10-2 rad, was estimated from the sample size and its image detected using the dark field annular detector or the secondary electron detector. With the electron beam incident along a (110)
direction, the R H E E D pattem of the surface (fig 6B) shows the superlattice spots characteristic of the c(4 × 4) or c(8 × 2) reconstructed surface. The SEM imaging shows a rough surface with terrace sizes in the range 0.5 to 10/Lm. This imaging mode is very sensitive to the surface topography, and monoatomic changes in surface topography are easily detected by secondary reflection electron microscopy (SREM). The surface topography of this MBE-grown GaAs is similar to that grown'in a conventional MBE system. As reported earlier [4], the MBE-grown GaAs surface is not completely flat and terraces with height ranging from -- 5 A to 50 A are prevalent when growing under As-rich conditions. The length of the superlattice streaks in the R H E E D pattern reflects the presence of the terraces and indicates a mosaic-type surface with small deviations of the terraces from the (100) orientation. The roughness of the MBE-grown GaAs surface is somewhat surprising in view of earlier results which indicated that the interfaces between GaAs and GaAIAs epitaxial layers are extremely abrupt and flat [11]. Since the surface examined here was cooled down to room temperature, there remains the possibility that the surface topography changes as it is cooled from the growth temperature. In fact, rapid changes in the R H E E D spot intensities and shapes caused by changing the impinging Ga flux (by closing or opening the Ga shutter) have been reported [11]. These have been rationalized by a surface smoothing via step motions. Thus a large difference in the surface topography could exist between the surface at low and high temperatures. Step motion and step bunching upon cooling of the sample are needed to explain the rough GaAs surface observed at room temperature.
4. Conclusions
Fig. 6. (A) Secondary reflection electron micrograph (SREM) of the surface of (100)GaAs epitaxial film produced by MBE. The arrow is parallel to a [011] direction and indicates the direction of the incidem electron beam (E0 =.100 keV). The angle of incidenceto the surfaceis = 5 × 10 -2 rd. (B) RHEED pattern of the surface showing a c(4x4) or c(2 × 8) reconstructed surface. The central beam is in the lower-leftpart of the pattern.
A new type of electron microscope combining U H V technology with chemical analysis of surface layers and molecular beam epitaxy has been described. Some of the performances of this instrument have been demonstrated with a surface analysis of an MBE-grown GaAs epitaxial film.
P.M. Petroff et aL / Microscopy of surfaces and applications to MBE
References [1] K. Yagi, T. Takayanagi, K. Kobayaski, N. Osakabe, Y. Taniskiro and G. Honjo, Surface Sci. 86 (1979) 174. [2] K. Takayanagi, Ultramicroscopy 8 (1982) 145. [3] K. Takayanagi, in: Proc. 10th Intern. Congr. on Electron Microscopy, Hamberg, 1981, p. 43. [4] P.M. Petroff, in: Electron Microscopy of Materials, Mater. Res. Soc. Symp., Vol. 31, Eds. W. Krakow, D.A. Smith and L.W. Hobbs (North-Holland, New York, 1984) p. 117. [5] P.M. Petroff and R.J. Wilson, Phys. Rev. Letters 51 (1983) 199.
191
[6] J.C.H. Spence, Ultramicroscopy 11 (1983) 117. [7] E.G. McRae and P.M. Petroff, Surface Sci. 147 (1984) 385. [8] R.J. Wilson and P.M. Petroff, Rev. Sci. Instr. 54 (1983) 1534. [9] K. Takayanagi, K. ~ragi, K. Kobayashi and G. Honjo, J. Phys. E l l (1978) 441. [10] P.M. Petroff, A.C. Gossard, W. Wiegmann and A. Savage, J. Crystal Growth 44 (1978) 5. [11] J.M. Van Hove and P.I. Cohen, J. Vacuum Sci. Technoi. 20 (1982) 726.