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An investigation of medium-range ordering and the orientational proximity effect in amorphous germanium by high resolution transmission electron microscopy
250
Journal of Non-Crystalline Solids 106 (1988) 250 255 North-Holland. Amsterdam S
AN INVESTIGATION OF MEDIUM-RANGE ORDERING AND THE ORIENTATIONAL PROXIMITY EFFECT IN AMORPHOUS GERMANIUM BY HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY
P.H. Gaskell and A. Saeed University of Cambridge, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K.
Detection of microcrystallites in an amorphous film of silicon by High Resolution Electron Microscopy (HREM) has been reported recently. Microcrystallites were said to be orientated parallel to the lattice planes of a substrate of crystalline silicon and this so-called Orientational Proximity Effect, was found to operate within about 80 nm of the interface. In an attempt to assess the generality of this effect, we have investigated the structure of amorphous germanium evaporated onto particles of c-Ge deposited immediately beforehand. HREM at 500 keV and 200 keV was used to study the mediumrange structure of the amorphous film in the region of the interface. Bright field real-space images, and optical microdiffraction patterns recorded from them, were used to estimate the extent of medium-range order. Lattice fringe features, similar to those observed previously by a number of workers, were seen in very thin regions of the specimens, indicating a degree of medium-range ordering. We have found no evidence of preferred orientation, however. 1.
INTRODUCTION
ited an orientational correspondence with the diffraction
As part of an extensive series of investigations on the
pattern of the crystalline substrate. The term "Orientational
structure of amorphous semiconductors and glasses,
Proximity Effect" was coined to describe the phenomenon.
Ourmazd, Bean and Phillips 1, Phillips, Bean, Wilson and
The term and the experiments described above have sub-
Ourmazd 2 and Saito 3 have examined several specimens of
sequently figured in a defence of the thesis that many amor-
amorphous silicon and germanium by high resolution elec-
phous solids consist of sub-microcrystallites 3,4.
tron microscopy. The most influential experiments 2 were
In an attempt to examine the generality of the Orientat-
carried out on thin cross-sections of a composite specimen,
ional Proximity Effect, the structure of a-Ge near the
consisting of a-Si deposited in UHV on a { 100} face of a
crystal-amorphous phase boundary has been studied by high
crystalline Si wafer, the surface having been cleaned by Ar
resolution electron microscopy. The experiments described
ion sputtering followed by annealing at 800C to remove
here differ from those of Phillips et al 1,2 in several respects.
surface damage. Thin cross sections were prepared by Ar
Firstly, of course, we are dealing with a different material;
ion-milling the surfaces of a specimen cooled to 100K.
Ge rather than Si. Secondly, as described below, the spec-
Examination using H R E M revealed the presence of
imens were evaporated in two stages: an amorphous film
ordered regions that were claimed to be 3 nm "submicro-
was deposited at room temperature onto a discontinuous
crystallites".
The degree of ordering decreased with
crystalline film previously produced on a hot substrate.
distance from the amorphous-crystalline interface but was
Thus, any orientational proximity effect can, in principle, be
detectable within about 80 nm of the interface. The evid-
studied as a function of the nature of the interfacial facet:
ence for ordering rested on the visual appearance of "lattice"
since the OPE might be expected to depend to some extent on
fringes in the real-space image, and on patterns formed by
whether low or high index planes intersect the surface of the
diffraction of a fine laser beam from regions of the micro-
crystal. It is also interesting to speculate that any OPE might
graph equivalent to 5 nm on the scale of the specimen. The
be strongly affected by interactions at corners where two or
optical microdiffractograms were judged to originate from
three planes meet. Further, any OPE would be expected to
Diffraction
be affected when two crystals lie within about 80 nm of each
spots were small, consistent with crystallites of diameter
other. A third difference with the experiments of Phillips et
microcrystallites for the following reasons.
3nm, some of the patterns showed a degree of rotational
al is that the deposition was not conducted in UHV condit-
symmetry and many of the microdiffraction patterns exhib-
ions. However, the specimens remained in a vacuum better
0022 3093/88/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.H. GaskelL A. Saeed / lm'estigation o[medium - range ordering
251
than 5 x 10 -4 Pa, so that the presence of a thick native oxide
about 2 nm. In the region of the crystalline islands, the total
film is excluded The experiments described below could
thickness increased to about 7 nm.
thus throw some light on the sensitivity of any OPE to thin
High resolution micrographs were obtained at 500kV
layers of contamination. Finally, thin films, suitable for
using the Cambridge University High Resolution Trans-
HREM are formed directly. Any possible artefacts created
mission Electron Microscope 6 and a JEM 200 CX with a
by the ion-beam milling process, necessary to obtain cross-
high resolution polepiece (Jefferson et a17). The microscope
sections, are eliminated.
was carefully aligned: for the 500 kV instrument, the final beam alignment required that the quality of the image be unchanged as the beam was tilted through equal and opposite
2. EXPERIMENTAL Two specimens were prepared by the vacuum deposition
angles about the X and Y tilt axes8, 9. Through-focal series
of germanium; each involving two successive evaporations
were recorded, without an objective aperture, micrographs
on the same freshly cleaved NaC1 substrate.
being spaced at defocus intervals of 10 nm, the first at a
For a first
specimen, evaporation was carried out with a substrate
small overfocus. Optical diffractograms were recorded and
temperature, Ts = 500C and a pressure of about 5x10 -4 Pa.
micrographs nearest optimum defocus examined in detail.
The substrate was then allowed to cool to 20C under
Thin areas of the first specimen were examined using the
vacuum. A second evaporation was carried out at 3 x 10 -4
Cambridge HREM. Figure 1 shows one of the through-focal
Pa. The total time, from the deposition of the first crystall-
series. Unfortunately, the later micrographs in this series
ine film, was about 75 minutes. A second set of specimens
were lost as the film etched in the beam. Figure 1 corresp-
was prepared with Ts = 460C and 4 x l 0 "5 Pa; then with Ts =
onds to an overfocus of about 30 nm. The optical diffract-
20C and 10 -4 Pa. Before and after deposition of the first
ogram shows that the largest interplanar spacing of Ge, i.e.,
layer, the chamber was filled with argon to reduce the
d l l l = 0.327 nm, lies within the range of spatial frequencies
residual oxygen content. The interval between the stages of
"admitted" by the first passband of the contrast transfer
the evaporation was thereby reduced to about 60 minutes.
function of the microscope: the first zero of the CTF is
Films were floated off in distilled water, onto holey carbon grids.
Selected area diffraction patterns from the
calculated to be 0.29 nm. The information limit, however, lies far beyond, as shown by the presence of {311} and
Ge islands indicated a very high degree of crystallinity and,
{400} spacings.
little preferred orientation. The composition and thickness
0.163 nm half spacing of the { 111 } planes are seen.
of the second set of specimens were examined by Electron
In addition, fringes associated with the
Three regions, the smallest about 10 x 10 nm, were
energy loss spectroscopy (EELS) on a scanning transmission
examined by optical microdiffraction. Each region included
electron microscope (Vacuum Generators HB501). Spectra
part of the periphery of a large crystallite. One is enlarged
were recorded at various points, with the film covering a
in figure 2. Using an unmagnified laser beam, about 80 diff-
hole in the carbon support in each case. There was evidence
ractograms were obtained from each region, from areas as
for some oxidation. Most probably this occurred after
small as 3-4 nm diameter on the scale of the specimen.
deposition, and the oxide thus coats the specimen surface and
Micrographs relating to the second specimen are shown
not the amorphous / crystal interface. Further measure-
in figures 3-5. Figure 3 is an annular dark field image that
ments are in progress to check this. Energy loss spectra in
clearly shows the dendritic nature of the amorphous film.
the range 0-50 eV were obtained from amorphous regions
Similar growth patterns have been observed previously in a-
of the specimen and estimates of the thickness, t, obtained using the relation t = Lp P(1)/P(0), where P(1) and P(0) are
Ge films evaporated onto NaCI in a clean ambient 10-12. At
the intensities of the first plasmon and zero-loss peaks, and Lp is the plasmon mean free path 5. Amorphous regions of
age value of 2 nm: values as low as 0.7 nm were measured. Several through-focal series were obtained using the JEM
the film were uniform in thickness with a mean value of
200CX, examples are shown in figures 4 and 5.
the edges of the dendrites, the film is thinner than the aver-
252
P.H. Gaskell, A. Saeed / In~,estigation of medium - range ordering
Figure 1. Islands of crystalline Ge embedded in a-Ge. Arrows mark 0.17, 0.16, 0.14 nm fringes associated with {311}, 1/2{111} and {400} planes. Boxes show areas scanned by optical diffractometry with a fine laser beam. Inserts show optical diffractograms. (Right) With a wide beam illuminating both crystalline and amorphous regions,
showing spots due to { 111} planes from c-Ge and the "white" spectrum from a-Ge extending beyond with no "zeros" in the CTF. (Left) With a "3 n m " beam illuminating a crystalline region: note the diffraction spots and the signal/noise ratio. showed none of the features noted by Phillips et al: that is,
Again, almost all of the first peak of the structure factor of Ge lies within the first passband of the microscope.
sharp diffraction spots with a recognisable symmetry and orientational relationship to the diffraction spots of c-Ge. This result was disappointing, and unexpected, since
3.
DISCUSSION Optical microdiffractograms obtained from the cryst-
signs of apparently ordered structures are present at several points in the micrograph and recognisable diffraction patterns might be expected from these regions. A possible
alline islands shown in figure 1 demonstrate that clearly distinguishable diffraction patterns can be obtained from
explanation for these apparently contradictory results may
areas corresponding to 3-4 nm.
These also give some
lie in the insensitivity of optical microdiffraction for order-
indication of the signal to noise ratio (see insert, Fig 1). Optical microdiffractograms obtained from the amorphous
ed regions smaller than 3 nm in diameter. An amorphous region in which "{ 111 }" planes can be seen over a distance of about 1.5 nm (such as that shown in figure 2) would
regions, contiguous with the crystalline islands, however,
P.H. Gaskell, A. Saeed / l n e , estigation of medium - range ordering
253
Figure 3. STEM annular dark-field image showing the dendritic nature of the amorphous film. The crystalline islands are the more regular features, Figure 2 Enlargement of upper right box of Fig. 1 showing an ultra-thin amorphous region with several ordered "{ 111}" planes. These bear no orientational correlation to the large crystallite.
amorphous regions of these micrographs but there is no real evidence, from the many micrographs examined, that they occur preferentially near the amorphous-crystal boundary.
produce spots with more than twice the diameter of those
Nor are there any indications of an orientational relationship
shown in figure 1 from 3-4 nm diameter crystalline regions
between the {111 } planes of the ordered amorphous regions
but with only one-eighth of the intensity. It seems unlikely,
and the lattice planes of the crystal. In short, our results give
then, that distinct spots would emerge from the background
no support for the Orientational Proximity Effect.
of noise generated by the amorphous region. Similar con-
The most persuasive signs of structural order visible in
clusions emerged from the earlier work of Gaskell and
the real-space images also have relatively low contrast as
Mistryl3: optical microdiffraction patterns recorded from real-space images of particulate a-SiO2 only showed dis-
they originate from thin parts of the specimen. The reasons
cernable symmetry for the most obviously ordered partic-
proportional to its volume. The surrounding amorphous
for this are clear: an ordered region contributes a "signal"
les. In other cases, order, or more correctly, pattern, could
material also contributes to the real-space image but in this
be detected more readily in the real-space images.
case, the contribution represents noise. A rough rule of
Examination of the second specimen was therefore lim-
thumb (Krivanek, Gaskell and Howie 14) is that a micro-
ited to a visual inspection of the micrographs (figures 4, 5).
crystallite of Ge, orientated so that the {111 } planes are
Regions of order are visible in several parts of the
diffracting, will be visible if the foil thickness does not
254
P.H. Gaskell, A. Saeed / hwestigation of medium - range ordering
Figure 4. Three members of a through-focal series showing parallel fringes but with no orientational correlation to the { 111 } planes of the larger crystallite above.
exceed about three times the crystallite diameter. Microcrystallites of about 1.5 nm diameter may be observable in a foil of thickness 4 nm but with difficulty in thicker regions. Ordered regions can be seen in the ultra-thin region of the specimen to the right of the large hole visible in figure 2. These observations are in accord with the results of Smith et a115 who found that specimens lnm thick showed a lower level of contrast but appeared to contain more structuredependent information than specimens with a nominal thickness of 10nm. Considerable structure can be seen in figure 5 where the average thickness is about 2 nm and probably less near the edge of the film. 4. CONCLUSION Micrographs reported here were obtained under conditions in which the first peak of the structure factor of germanium was included within the first passband of the CTF of the microscope. The micrographs from thin regions of both
Inserts show power spectra - the scale bar corresponds to Ge { 111 } planes, indicating adequate information transfer.
specimens show distinct signs of structure- especially the second specimen (figure 5) prepared in a clean ambient with an average thickness of 2 nm. Although there is evidence for a degree of order in the micrographs, there is no support for the orientational proximity effect. The reasons may lie with the differences between these experiments and those of Phillips et al, mentioned in the introduction. Clearly, there could also be some differences of technique: in particular, the optical microdiffraction technique could almost certainly be improved. But, any Orientational Proximity Effect should have been detectable in the real-space images - and it was not. These results suggest either that the OPE is specific to interfaces between crystalline and amorphous silicon only, or that the interfacial area parallel to the direction of observation should be large, or that absence of the effect can be attributed to the crucial role of a detail of the deposition process. Greatest suspicion falls on the differences in oxidation state of the
P.H. Gaskell. A. Saeed / lntrestigation of m e d i , m - range ordering
255
Figure 5. Clear, extensive, ordered areas are seen in members of this through-focal series obtained from a thin
portion of an amorphous Ge film. The average thickness is estimated to be 2 nm.
crystal surface before deposition of the amorphous phase
4. 5.
and differences due to ion-beam thinning used to produce cross-sections by Phillips et al 1,2. Further experiments are needed to establish whether the OPE is crucially dependent
6.
on such fine details or whether it really is a much more general phenomenon.
7.
ACKNOWLEDGEMENTS P.H.G. acknowledges the generous financial support of
8.
Pilkington PLC. We also thank Dr. D.A. Jefferson and Mr. R.A. Camps, for provision of microscope facilities and
9.
assistance with their use. Thanks are also due to Dr. J. Yuan
10.
for help with collecting the EELS data. Partial funding from The British Vacuum Council (A.S.) is also gratefully
11.
acknowledged.
12.
1.
13. 14.
2. 3.
A. Ourmazd, J.C. Bean and J.C. Phillips, Phys.Rev. Letts. 55 (1985) 1599. J.C. Phillips, J.C. Bean, B.A. Wilson and A. Ourmazd, Nature 325 (1987) 121. Y. Saito, J. Phys. Soc. Japan 53 (1984) 4230.
15.
J.C. Phillips, Solid State Commun. 60 (1986) 299. D.C. Joy, in: Introduction to Analytical Electron Microscopy, eds. J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum, New York, 1979) p. 230. V.E. Cosslett, R.A. Camps, W.O. Saxton, D.J. Smith, W.C. Nixon, H. Ahmed, C.J.D. Catto, J.R.A. Cleaver, K.C.A. Smith, A.E. Timbs, P.W. Turner and P.M. Ross, Nature 281 (1979) 49. D.A. Jefferson, R.D. Brydson, A. Harriman, G.R. Millward, J.M. Thomas and K. Tsuno, Nature 323 (1986) 428. D.J. Smith, R.A. Camps, L.A. Freeman, R. Hill, W.C. Nixon and K.C.A. Smith, J. Microscopy 130 (1983) 127. D.J. Smith, W.O. Saxton, M.A. O'Keefe, G.J. Wood and W.M. Stobbs, Ultramicroscopy 11 (1983) 263. K.L. Chopra, A.C. Rastogi and D.K. Pandya, Phil. Mag. 30 (1974) 935. A. Barna, P.B, Barna, G. Radnoczi, H. Sugawara and P. Thomas, Thin Solid Films 48 (1978) 163. A.B. Mistry, PhD Thesis (University of Cambridge) (1978) p.51. P.H. Gaskell and A.B, Mistry, Phil. Mag.39 (1978) 245 O.L. Krivanek, P.H. Gaskell and A. Howie, Nature 262 (1976) 454. D.J. Smith, W.M. Stobbs and W.O. Saxton, Phil. Mag. B 43 (1981) 907.
Journal of Non-Crystalline Solids 106 (1988) 250 255 North-Holland. Amsterdam S
AN INVESTIGATION OF MEDIUM-RANGE ORDERING AND THE ORIENTATIONAL PROXIMITY EFFECT IN AMORPHOUS GERMANIUM BY HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY
P.H. Gaskell and A. Saeed University of Cambridge, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K.
Detection of microcrystallites in an amorphous film of silicon by High Resolution Electron Microscopy (HREM) has been reported recently. Microcrystallites were said to be orientated parallel to the lattice planes of a substrate of crystalline silicon and this so-called Orientational Proximity Effect, was found to operate within about 80 nm of the interface. In an attempt to assess the generality of this effect, we have investigated the structure of amorphous germanium evaporated onto particles of c-Ge deposited immediately beforehand. HREM at 500 keV and 200 keV was used to study the mediumrange structure of the amorphous film in the region of the interface. Bright field real-space images, and optical microdiffraction patterns recorded from them, were used to estimate the extent of medium-range order. Lattice fringe features, similar to those observed previously by a number of workers, were seen in very thin regions of the specimens, indicating a degree of medium-range ordering. We have found no evidence of preferred orientation, however. 1.
INTRODUCTION
ited an orientational correspondence with the diffraction
As part of an extensive series of investigations on the
pattern of the crystalline substrate. The term "Orientational
structure of amorphous semiconductors and glasses,
Proximity Effect" was coined to describe the phenomenon.
Ourmazd, Bean and Phillips 1, Phillips, Bean, Wilson and
The term and the experiments described above have sub-
Ourmazd 2 and Saito 3 have examined several specimens of
sequently figured in a defence of the thesis that many amor-
amorphous silicon and germanium by high resolution elec-
phous solids consist of sub-microcrystallites 3,4.
tron microscopy. The most influential experiments 2 were
In an attempt to examine the generality of the Orientat-
carried out on thin cross-sections of a composite specimen,
ional Proximity Effect, the structure of a-Ge near the
consisting of a-Si deposited in UHV on a { 100} face of a
crystal-amorphous phase boundary has been studied by high
crystalline Si wafer, the surface having been cleaned by Ar
resolution electron microscopy. The experiments described
ion sputtering followed by annealing at 800C to remove
here differ from those of Phillips et al 1,2 in several respects.
surface damage. Thin cross sections were prepared by Ar
Firstly, of course, we are dealing with a different material;
ion-milling the surfaces of a specimen cooled to 100K.
Ge rather than Si. Secondly, as described below, the spec-
Examination using H R E M revealed the presence of
imens were evaporated in two stages: an amorphous film
ordered regions that were claimed to be 3 nm "submicro-
was deposited at room temperature onto a discontinuous
crystallites".
The degree of ordering decreased with
crystalline film previously produced on a hot substrate.
distance from the amorphous-crystalline interface but was
Thus, any orientational proximity effect can, in principle, be
detectable within about 80 nm of the interface. The evid-
studied as a function of the nature of the interfacial facet:
ence for ordering rested on the visual appearance of "lattice"
since the OPE might be expected to depend to some extent on
fringes in the real-space image, and on patterns formed by
whether low or high index planes intersect the surface of the
diffraction of a fine laser beam from regions of the micro-
crystal. It is also interesting to speculate that any OPE might
graph equivalent to 5 nm on the scale of the specimen. The
be strongly affected by interactions at corners where two or
optical microdiffractograms were judged to originate from
three planes meet. Further, any OPE would be expected to
Diffraction
be affected when two crystals lie within about 80 nm of each
spots were small, consistent with crystallites of diameter
other. A third difference with the experiments of Phillips et
microcrystallites for the following reasons.
3nm, some of the patterns showed a degree of rotational
al is that the deposition was not conducted in UHV condit-
symmetry and many of the microdiffraction patterns exhib-
ions. However, the specimens remained in a vacuum better
0022 3093/88/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.H. GaskelL A. Saeed / lm'estigation o[medium - range ordering
251
than 5 x 10 -4 Pa, so that the presence of a thick native oxide
about 2 nm. In the region of the crystalline islands, the total
film is excluded The experiments described below could
thickness increased to about 7 nm.
thus throw some light on the sensitivity of any OPE to thin
High resolution micrographs were obtained at 500kV
layers of contamination. Finally, thin films, suitable for
using the Cambridge University High Resolution Trans-
HREM are formed directly. Any possible artefacts created
mission Electron Microscope 6 and a JEM 200 CX with a
by the ion-beam milling process, necessary to obtain cross-
high resolution polepiece (Jefferson et a17). The microscope
sections, are eliminated.
was carefully aligned: for the 500 kV instrument, the final beam alignment required that the quality of the image be unchanged as the beam was tilted through equal and opposite
2. EXPERIMENTAL Two specimens were prepared by the vacuum deposition
angles about the X and Y tilt axes8, 9. Through-focal series
of germanium; each involving two successive evaporations
were recorded, without an objective aperture, micrographs
on the same freshly cleaved NaC1 substrate.
being spaced at defocus intervals of 10 nm, the first at a
For a first
specimen, evaporation was carried out with a substrate
small overfocus. Optical diffractograms were recorded and
temperature, Ts = 500C and a pressure of about 5x10 -4 Pa.
micrographs nearest optimum defocus examined in detail.
The substrate was then allowed to cool to 20C under
Thin areas of the first specimen were examined using the
vacuum. A second evaporation was carried out at 3 x 10 -4
Cambridge HREM. Figure 1 shows one of the through-focal
Pa. The total time, from the deposition of the first crystall-
series. Unfortunately, the later micrographs in this series
ine film, was about 75 minutes. A second set of specimens
were lost as the film etched in the beam. Figure 1 corresp-
was prepared with Ts = 460C and 4 x l 0 "5 Pa; then with Ts =
onds to an overfocus of about 30 nm. The optical diffract-
20C and 10 -4 Pa. Before and after deposition of the first
ogram shows that the largest interplanar spacing of Ge, i.e.,
layer, the chamber was filled with argon to reduce the
d l l l = 0.327 nm, lies within the range of spatial frequencies
residual oxygen content. The interval between the stages of
"admitted" by the first passband of the contrast transfer
the evaporation was thereby reduced to about 60 minutes.
function of the microscope: the first zero of the CTF is
Films were floated off in distilled water, onto holey carbon grids.
Selected area diffraction patterns from the
calculated to be 0.29 nm. The information limit, however, lies far beyond, as shown by the presence of {311} and
Ge islands indicated a very high degree of crystallinity and,
{400} spacings.
little preferred orientation. The composition and thickness
0.163 nm half spacing of the { 111 } planes are seen.
of the second set of specimens were examined by Electron
In addition, fringes associated with the
Three regions, the smallest about 10 x 10 nm, were
energy loss spectroscopy (EELS) on a scanning transmission
examined by optical microdiffraction. Each region included
electron microscope (Vacuum Generators HB501). Spectra
part of the periphery of a large crystallite. One is enlarged
were recorded at various points, with the film covering a
in figure 2. Using an unmagnified laser beam, about 80 diff-
hole in the carbon support in each case. There was evidence
ractograms were obtained from each region, from areas as
for some oxidation. Most probably this occurred after
small as 3-4 nm diameter on the scale of the specimen.
deposition, and the oxide thus coats the specimen surface and
Micrographs relating to the second specimen are shown
not the amorphous / crystal interface. Further measure-
in figures 3-5. Figure 3 is an annular dark field image that
ments are in progress to check this. Energy loss spectra in
clearly shows the dendritic nature of the amorphous film.
the range 0-50 eV were obtained from amorphous regions
Similar growth patterns have been observed previously in a-
of the specimen and estimates of the thickness, t, obtained using the relation t = Lp P(1)/P(0), where P(1) and P(0) are
Ge films evaporated onto NaCI in a clean ambient 10-12. At
the intensities of the first plasmon and zero-loss peaks, and Lp is the plasmon mean free path 5. Amorphous regions of
age value of 2 nm: values as low as 0.7 nm were measured. Several through-focal series were obtained using the JEM
the film were uniform in thickness with a mean value of
200CX, examples are shown in figures 4 and 5.
the edges of the dendrites, the film is thinner than the aver-
252
P.H. Gaskell, A. Saeed / In~,estigation of medium - range ordering
Figure 1. Islands of crystalline Ge embedded in a-Ge. Arrows mark 0.17, 0.16, 0.14 nm fringes associated with {311}, 1/2{111} and {400} planes. Boxes show areas scanned by optical diffractometry with a fine laser beam. Inserts show optical diffractograms. (Right) With a wide beam illuminating both crystalline and amorphous regions,
showing spots due to { 111} planes from c-Ge and the "white" spectrum from a-Ge extending beyond with no "zeros" in the CTF. (Left) With a "3 n m " beam illuminating a crystalline region: note the diffraction spots and the signal/noise ratio. showed none of the features noted by Phillips et al: that is,
Again, almost all of the first peak of the structure factor of Ge lies within the first passband of the microscope.
sharp diffraction spots with a recognisable symmetry and orientational relationship to the diffraction spots of c-Ge. This result was disappointing, and unexpected, since
3.
DISCUSSION Optical microdiffractograms obtained from the cryst-
signs of apparently ordered structures are present at several points in the micrograph and recognisable diffraction patterns might be expected from these regions. A possible
alline islands shown in figure 1 demonstrate that clearly distinguishable diffraction patterns can be obtained from
explanation for these apparently contradictory results may
areas corresponding to 3-4 nm.
These also give some
lie in the insensitivity of optical microdiffraction for order-
indication of the signal to noise ratio (see insert, Fig 1). Optical microdiffractograms obtained from the amorphous
ed regions smaller than 3 nm in diameter. An amorphous region in which "{ 111 }" planes can be seen over a distance of about 1.5 nm (such as that shown in figure 2) would
regions, contiguous with the crystalline islands, however,
P.H. Gaskell, A. Saeed / l n e , estigation of medium - range ordering
253
Figure 3. STEM annular dark-field image showing the dendritic nature of the amorphous film. The crystalline islands are the more regular features, Figure 2 Enlargement of upper right box of Fig. 1 showing an ultra-thin amorphous region with several ordered "{ 111}" planes. These bear no orientational correlation to the large crystallite.
amorphous regions of these micrographs but there is no real evidence, from the many micrographs examined, that they occur preferentially near the amorphous-crystal boundary.
produce spots with more than twice the diameter of those
Nor are there any indications of an orientational relationship
shown in figure 1 from 3-4 nm diameter crystalline regions
between the {111 } planes of the ordered amorphous regions
but with only one-eighth of the intensity. It seems unlikely,
and the lattice planes of the crystal. In short, our results give
then, that distinct spots would emerge from the background
no support for the Orientational Proximity Effect.
of noise generated by the amorphous region. Similar con-
The most persuasive signs of structural order visible in
clusions emerged from the earlier work of Gaskell and
the real-space images also have relatively low contrast as
Mistryl3: optical microdiffraction patterns recorded from real-space images of particulate a-SiO2 only showed dis-
they originate from thin parts of the specimen. The reasons
cernable symmetry for the most obviously ordered partic-
proportional to its volume. The surrounding amorphous
for this are clear: an ordered region contributes a "signal"
les. In other cases, order, or more correctly, pattern, could
material also contributes to the real-space image but in this
be detected more readily in the real-space images.
case, the contribution represents noise. A rough rule of
Examination of the second specimen was therefore lim-
thumb (Krivanek, Gaskell and Howie 14) is that a micro-
ited to a visual inspection of the micrographs (figures 4, 5).
crystallite of Ge, orientated so that the {111 } planes are
Regions of order are visible in several parts of the
diffracting, will be visible if the foil thickness does not
254
P.H. Gaskell, A. Saeed / hwestigation of medium - range ordering
Figure 4. Three members of a through-focal series showing parallel fringes but with no orientational correlation to the { 111 } planes of the larger crystallite above.
exceed about three times the crystallite diameter. Microcrystallites of about 1.5 nm diameter may be observable in a foil of thickness 4 nm but with difficulty in thicker regions. Ordered regions can be seen in the ultra-thin region of the specimen to the right of the large hole visible in figure 2. These observations are in accord with the results of Smith et a115 who found that specimens lnm thick showed a lower level of contrast but appeared to contain more structuredependent information than specimens with a nominal thickness of 10nm. Considerable structure can be seen in figure 5 where the average thickness is about 2 nm and probably less near the edge of the film. 4. CONCLUSION Micrographs reported here were obtained under conditions in which the first peak of the structure factor of germanium was included within the first passband of the CTF of the microscope. The micrographs from thin regions of both
Inserts show power spectra - the scale bar corresponds to Ge { 111 } planes, indicating adequate information transfer.
specimens show distinct signs of structure- especially the second specimen (figure 5) prepared in a clean ambient with an average thickness of 2 nm. Although there is evidence for a degree of order in the micrographs, there is no support for the orientational proximity effect. The reasons may lie with the differences between these experiments and those of Phillips et al, mentioned in the introduction. Clearly, there could also be some differences of technique: in particular, the optical microdiffraction technique could almost certainly be improved. But, any Orientational Proximity Effect should have been detectable in the real-space images - and it was not. These results suggest either that the OPE is specific to interfaces between crystalline and amorphous silicon only, or that the interfacial area parallel to the direction of observation should be large, or that absence of the effect can be attributed to the crucial role of a detail of the deposition process. Greatest suspicion falls on the differences in oxidation state of the
P.H. Gaskell. A. Saeed / lntrestigation of m e d i , m - range ordering
255
Figure 5. Clear, extensive, ordered areas are seen in members of this through-focal series obtained from a thin
portion of an amorphous Ge film. The average thickness is estimated to be 2 nm.
crystal surface before deposition of the amorphous phase
4. 5.
and differences due to ion-beam thinning used to produce cross-sections by Phillips et al 1,2. Further experiments are needed to establish whether the OPE is crucially dependent
6.
on such fine details or whether it really is a much more general phenomenon.
7.
ACKNOWLEDGEMENTS P.H.G. acknowledges the generous financial support of
8.
Pilkington PLC. We also thank Dr. D.A. Jefferson and Mr. R.A. Camps, for provision of microscope facilities and
9.
assistance with their use. Thanks are also due to Dr. J. Yuan
10.
for help with collecting the EELS data. Partial funding from The British Vacuum Council (A.S.) is also gratefully
11.
acknowledged.
12.
1.
13. 14.
2. 3.
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