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A mechanism of misfit dislocation reaction for GaInAs strained layers grown onto off-axis GaAs substrates
Journal of Crystal Growth U2 (1991) 359—367 North-Holland
359
A mechanism of misfit dislocation reaction for GaInAs strained layers grown onto off-axis GaAs substrates P. Kightley,
P.J. Goodhew
Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, UK
R.R. Bradley and P.D. Augustus Plessev Research Caswell, Ltd., Caswell Towcester, Northants, NNI 2 8EQ, UK Received 10 May 1990; manuscript received in final form 15 February 1991
The structure of misfit dislocations at the interface between compressively strained epitaxial In
0 12Ga088As and GaAs has been studied by TEM. Two dislocation line directions, separated by 2° in both the nominal [1101 and [110] directions, are observed for growth on (001) substrates tilted — 2° off toward (010). Only [110] and [110] glide directions are observed for growth on nominally on-axis substrates. It is shown that the misfit dislocations are constrained to lie in the interface plane often generating long segments of edge dislocation where the misfit dislocations are forced to converge by the low angle offset. The edge segments curve Out of the interface plane into the buffer layer.
1. Introduction The presence of misft dislocations alloy interfaces is well catalogued [1,2], there is little quantitative information effect upon device performance. From luminescence studies, Fitzgerald et al
at Ill—V although on their cathodo[3] have
proposed a dependence of the electrical upon dislocation type, with curved edge activity ~aGaAs (110) dislocations, having Burgers vectors within the (001) plane, displaying greater electrical activity than those where the Burgers vector is inclined at 600 to the dislocation line. In this communication we propose a mechanism for the partial dependence of the formation of these curved edge dislocations upon substrate orientation. The formation of the edge interfacial defects is thus, in part, dependent not only upon mismatch energy and the density of fixed dislocation sources but also upon the substrate misorientation. The predominant mixed 60 0 misfit dislocations revealed in this study are initially sourced from the existing grown-in dislocations of the substrate [4]. No in0022-0248/91/503.50 © 1991
—
crease in threading dislocation density is noted for any of the samples studied. The misfit dislocations are primarily located within the first strained layer to substrate (or lattice matched buffer layer) interface. The threading dislocation density was measured by planar view from material well above this first interface for2 all It important was alwaysnot estior samples. better. It is to mated io~cm dislocation densities at the mimeasureas threading tial interface because during TEM observation dislocations can be annealed out, being attracted under their image force toward the free surface. This gives rise to segments displaying pendellosung contrast that can be wrongly interpreted as the existing threading dislocations that formed the misfit array. Reaction centres that may appear to be a product of the Hagen—Strunk mechanism [5,61are attributed to normal reaction nodes associated with the crossing of dislocations with identical Burgers vector and not part of a multiplication mechanism. Our observations indicate that each individual misfit dislocation is formed by a single slip operation; i.e. that prior to extensive
Elsevier Science Publishers B.V (North-Holland)
360
P. Kightlev et al.
/
Mechanism of misfit dislocation reaction for GalnAs SLs
reaction, the 60° misfit dislocations all form long straight lines, 2. Experimental Analysis of four samples is presented in this paper. Sample A is a 10 period strained layer supenlattice (SLS)of In 0 12Ga085As/GaAs with layers 90 and 100 A thick.~respectively. Samples B and C are 0.5 p.m thick single layers of Ga088 In0 12As grown in the same fabrication run. Sampie D has 100 periods of Ga0881n012As/GaAs with thicknesses of 95 and 100 A, respectively, Specimens A, B and D were grown on semi-in-
sulating (SI) (001) GaAs substrates tilted by 2° toward (010). Sample C was grown using an onaxis (±0.5°) (001) SI substrate. A 0.5 ~rm undoped GaAs buffer layer was grown between the strained layer and the substrate for all of the samples. The MOVPE system was operated at atmospheric pressure using trimethylgallium, tnmethylindium and a liquid arsine source. The growth temperature was always 720°C. TEM [110] cross-section and [001] plan view analysis has been used throughout this study. Bright field, dark field and weak beam imaging techniques were used at accelerating voltages of 100 or 120 keV.
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Fig. 1. (001) plan vIe~from sample A. Note the intersecting misfit dislocation lines, shown as AA’ and BB’.
P. Kightlev et al.
/
Mechanism of misfit dislocation reaction for GalnAs SI..s
3. Results and discussion
361
20
A
B
Laue back reflection radiography from the substrates of samples A, B and D confirmed the substrate misorientation to be 2° toward (010) from (001). No misorientation could be detected for sample C. The substrate misorientation is cxpected to lead to the_formation of surface steps
with both [110] and [110] ledge directions. Fig. 1 shows a (001) TEM plan view prepared from sample A. Careful inspection of the misfit dislocation structure reveals that in each of the approximate [110] and [110] directions there are two 60° misfit dislocation line directions. Their separation is described by the angles 20 and 20’. These are measured from fig. 1 as 1.8 and 2.2 respectively. The angles result in the dislocations converging to create crossing points; this is illustrated by dislocation lines AA’ and BB’ on the micrograph and is constructed schematically in fig. 2. Cross-sectional TEM images show the dislocations to be located approximately at the SLS to buffer layer interface. Superposition of the dcctron diffraction pattern onto the dislocation image
[iTo
20’
B’
A’
Fig. 2. Schematic of the intersecting dislocation lines. —
(fig 3) shows that the [110] and [110] directions bisect the angles between the dislocation lines with each dislocation displaced either 0 or 0’ from the true [110] or [110] directions respectively. Using the distances A, B and C indicated in fig. 3, measured at a fixed distance either side of the
micrograph, allowed the calculation of the values
Fig. 3. Superposition of the electron diffraction pattern and the dislocation image. Measurements from these micrographs confirmed neither dislocation set to be along the true [110].
362
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Mechanism of misfit dislocation reaction for GalnAs SLi
of 1.25°±0.5° for both 0 and 0’ illustrating the deviation of the overall line from the true <110) for all the 60° misfit dislocations.
reasons. Among these are the presence of kinks on the dislocation line, the dissociation of the dislocation or the interaction of point defects with the
Ideally 60°dislocations should lie in directions
dislocation. Figs. 4a and 4b show (001) micro-
most densely packed with atoms, i.e. they should be straight, lying in (110) directions on their {111} slip planes. Local lengths of dislocation line could deviate from an exact (110) for a number of
graphs from samples B and C. Observation of the underlying directions of the glide dislocation shows that the misfit dislocations for the on-axis (001) substrate lie exactly along (110) directions,
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I ~im Fig. 4. (a) (001) plan view of sample B, growth onto a substrate tilted — 2° from (001) to (010). Note the underlying line directions of the glide dislocations where the MD intersect similar to sample A, fig. 1. (b) (001) plan view of sample C, growth onto an exactly oriented (001) substrate. Here none of the glide dislocations converge in the manner described for samples A and B.
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Mechanism of misfit dislocation reaction for GalnAs SLs
whereas for the 2° offcut substrate once again offset angles (20 and 20’) of 2° are seen. This comparison shows that the deviation of the misfit dislocation line direction is directly influenced by the choice of substrate orientation. To be most efficient at misfit relief, the misfit dislocation is required to remain in the first InGaAs/GaAs interface. For an on-axis wafer the
dislocation glide plane intersects the interface with an exact (110) direction and the dislocation lines adopt this overall line direction appearing straight in a TEM image. When using vicinal wafers, the glide plane intersects the interface with a direction slightly off from (110), thus for the dislocation to remain in the interface plane, it has to be displaced on its glide plane adopting a line direction other than exactly (110). A simple explanation of how this could occur is through the gliding thread ing dislocation (which is forming a MD segment
in the manner described by Matthews et al. [7])
363
_________ _________ ____________
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_________
_____________________________ _________
interacting with the surface steps. Fig. 5a shows a threading dislocation prior to incidence upon a surface step riser (or ledge). For a wafer 2° off axis from (001) to (010), the ledges will occur approximately every 60 A in both the [110] and [110] directions. The ledge probably consists of an exposed part of a (111) plane with a [110] ledge
Fig. 5. (a) A laterally gliding dislocation, trailing a length of
direction. The misfit dislocation segment contains
dislocations with different threading dislocation line directions.
a random distribution of kinks and has a [110] line direction overall. The threading segment has a [011] line direction. The dislocation does not glide through the ledge, but leaves a kink there. For the example shown, a portion of the threading dislocation, with an [011] line direction, remains at the ledge joining the two misfit segments. This is illustrated in fig. Sb. The overall line of the dislocation will appear offset at each of these ledges. As the kinks formed are probably only single height steps, it is not suprising that the dislocation displacement at the ledges cannot be imaged even by weak beam microscopy. The net effect in the TEM image is of a straight dislocation line that is offset from the true [110] by 0°. It is clear that dislocations that have their misfit segment along [110] and their threading dislocation segment along [011] will at some point intersect dislocations that have their misfit segment likewise along [110] but their threading segment along [101], being sep-
arated by the angle of 20 shown in fig. 1. This is illustrated in fig. Sc. The magnitude of 0 is determined by the facet plane of the surface step, the line direction of the threading portion of the misfit dislocation, the offcut angle and direction of the substrate and the efficiency of the steps in creating the displacement. Our model requires that the dislocations lie within the strained layer to substrate interface. The offset displacement angle from [110] or [110] does not depend upon the Burgers vector of the dislocation; in fig. Sb the threading end of the dislocation could adopt either [011] or [101] line direction adopting either 600 or screw character for any of the possible ~aGaAs (110) 600 dislocations, being also independent of a or /3 character. So far we have only concerned ourselves with the more common 60° dislocations. Analysis of the
_________
MD, prior to incidence upon a surface step riser (ledge). Note that the threading segment is in a screw orientation. (b) The dislocation structure after encountering the ledge. A portion of screw dislocation remains at the ledge. (c) The intersection of
364
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Mechanism of misfit dislocation reaction for GalnAs SLs
dislocation structure of this interface shows there are also dislocations that have ~aGaAs [110] or ~UGaAs [110] Burgers vectors, i.e. vectors within the (001) plane. These dislocations are often formed as a reaction product of the 60 disloca-
Prior to any dislocation reaction, the misfit dislocation segments studied can all be determined to have been of the 60° type. Individual substrate threading dislocations may form misfit dislocations with a terminating { 111 } plane in
tions at, and close to, the induced crossing points of the dislocations caused by the substrate misorientation. In figs. 6a to 6d, we show part of a dislocation analysis from one of these wafers. At
either substrate or epilayer, dependent upon the
the intersection of many of these dislocations long edge dislocation segments (labelled E in fig. 6b)
ment should be formed with a line direction such that the terminating plane at the dislocation core
are formed, with the product dislocation approximately tracing the path of the initially formed
will be within the substrate (although a small number of non-misfit relieving dislocations have
60°dislocations. For all the original glide dislocations g b 0 in either fig. 6a or 6c. When g b 0
been reported by Dixon and Goodhew [8]). One well documented reaction [2,3,9] of these disloca-
the dislocations show strong residual contrast where u — f110}; when the line direction changes
tions can lead to the formation of a Lomer prod-
°
=
line direction chosen by the laterally gliding dislocation. For the relief of the compressive stress generated for this growth system, the misfit seg-
=
uct dislocation, i.e.
to {100} at intersection points (i.e. labelled I in fig. 6a) both g b 0 and g (b X u) 0 simulta=
——
=
—
——
neously and the dislocation contrast is seen to
2~j0h11 + ~aGaAs[1O1] ~aGaAs [110]. (1) Crossing of dislocations with identical Burgers
cancel to zero.
vectors (but not chemical type) very often leads to
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2 Fig. 6. Part of the dislocation analysis of sample A. The 60° MD are out of contrast for gb = 0 simultaneously with g(b >< u) = 0 when they adopt <100) line directions at the points of intersection (see text). Note also the lengths of edge dislocation formed at the intersections of many MDs.
P. Kightley et a!.
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Mechanism of misfit dislocation reaction for GaInAs SLs
the annihilation of the crossing node. Possible reactions that are not observed are the formation of large Burgers vector a0~~(OOl) or 2aGaAs (112) dislocations. Our observations show that reactions such as that described by eq. (1) can occur where the dislocations intersect orthogonally or where they intersect at 20 or 20’. Short segments of edge dislocation can be formed at the orthogonal intersection of dislocations, whereas much longer seetions of edge dislocation (up to 40 p.m) can be formed where the dislocations intersect at the smaller angles. Reaction to annihilate the crossing nodes has, so far, only been identified for approximately orthogonal intersections. For a sample displaying very long dislocation lines intersecting at small angles, each dislocation will cross many other MD. It is thus very likely to intersect other dislocations of the “correct” type to form lengths of edge MD. The lengths of edge MD are intersected by the orthogonal MD which cut them into shorter segments. The formation of these edge dislocations requires the 60° disloca-
365
tions to glide out of the interface so they can react. For a low angle intersection, the crossing point will be in the interface plane. As the distance away from this crossing point increases, the separation of the two 60° dislocations also increases, and thus the distance they are required to glide out of this interface plane to be able to meet also increases. When an orthogortal dislocation intersects these dislocations, then often they are constrained to be linked to the originating interface. The completion of the reaction may thus require a climb motion, perhaps made possible by the mobility of interstitials at the growth temperature. For an angle of intersection of 2° and an edge dislocation length of 40 p.m, the 60°dislocations appear to react until they reach a separation of 700 A. It is likely that misfit segments that are formed parallel to each other will also glide out of this interface to form edge dislocations, providing they are close enough. These dislocation reactions have been reported by Chang et al. [9] for growth onto on-axis substrates and their ob—
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Fig. 7. [1101 cross-section of sample D. The plan view of fig. 8 is taken from the area marked PV.
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366
P. Kightley ci at.
/
Mechanism of misfit dislocation reaction for GalnAs SLs
servations are supported by this work where these reactions can be identified, although in lower
edge segments, initially, form only below the interface, within the GaAs buffer layer.
numbers. It is of course difficult to distinguish between reactions of this type and those of very low angle intersections for nominally on-orientation wafers. As the mismatch energy is increased, the line density of edge dislocation rises. A cross-section of sample D is shown in fig. 7. The specimen is tilted toward (111) to enable the dislocation arrangement at the interface to be viewed. Plan view analysis was performed at different levels throughout this sample by carefully etching away portions of the epilayer. Fig. 8 shows part of a dislocation analysis from an area just below the multilayer stack, indicated PV in fig. 7. These dislocations are almost all edge in nature and penetrate into the buffer layer by up to 0.45 p.m. Similar analysis from the area just above the interface shows them to be absent and therefore these
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4. Conclusions Laterally gliding dislocations on a vicinal substrate form kinks in the resultant misfit dislocations probably at surface steps. The effect of these kinks is to rotate the dislocation line within the plane of the interface. Resultant MDs lie along directions at a small angle ±0to the exact <110). Where the nearly parallel dislocations cross they react to form edge dislocations. Because each dislocation crosses many other dislocations there are more opportunities for the formation of edge segments than exist for on-axis substrates. The formation of the edge dislocation requires that the 60° MDs glide out of the heterointerface. This behaviour is observed and, for the 10 period strained layer superlattice studied here, it is esti-
.
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Fig. 8. Plan view dislocation analysis showing the curved edge dislocations formed in the buffer layer directly below the interface.
P. Kightley et al.
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Mechanism of misfit dislocation reaction for GalnAs SLs
mated that dislocations have undergone this reaction up to a separation of 700 A. It is probable that this distance is mismatch energy and growth temperature dependent.
Acknowledgements The authors gratefully acknowledge the encouragement and support of Dr. R.H. Wallis and thank Dr. D.J. Dunstan (Surrey University) for valuable discussions. Mrs. G.B. Davies is also acknowledged for her expert technical assistance.
References [1] A. Okamoto, H. Toyoshima and K. Ohata, Japan. J. AppI. Phys. 26 (1987) 539.
367
[2] Y. Ashizawa, S. Akbar. W.J. Schaff, L.F. Eastman, E.A. Fitzgerald and D.G. Ast. J. AppI. Phys. 64 (1988) 4065. [3] J.M. E.A. Woodall, Fitzgerald,J. D.G. P.D. Kirchner, Appl. Ast, Phys. 63 (1988) 693.GD. Pettit and [4] G.H. Olsen and M. Ettenberg, in: Crystal Growth: Theory and Techniques, Ed. C.H.L. Goodman (Plenum, London, 1978) p. 23. [5] E.A. Fitzgerald, G.P. Watson, RE. Proano, D.G. Ast, P.D.
Kirchner, G,D. Pettit and J.M. Woodall, J. AppI. Phys. 65 (1989) 2220. [6] W. Hagen and H. Strunk. Appl. Phys. 17 (1978) 85. [7[ J.W. Matthews, A.E. Blakeslee and S. Mader, Thin Solid Films 33 (1976) 253. [8] RI-I. Dixon and P.J. Goodhew, J. Appl. Phys. 68 (1990) 3163.
[9] K.H. Chang, P.K. Bhattacharya and R. Gibala, J. AppI. Phys. 66 (1989) 2993.
359
A mechanism of misfit dislocation reaction for GaInAs strained layers grown onto off-axis GaAs substrates P. Kightley,
P.J. Goodhew
Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, UK
R.R. Bradley and P.D. Augustus Plessev Research Caswell, Ltd., Caswell Towcester, Northants, NNI 2 8EQ, UK Received 10 May 1990; manuscript received in final form 15 February 1991
The structure of misfit dislocations at the interface between compressively strained epitaxial In
0 12Ga088As and GaAs has been studied by TEM. Two dislocation line directions, separated by 2° in both the nominal [1101 and [110] directions, are observed for growth on (001) substrates tilted — 2° off toward (010). Only [110] and [110] glide directions are observed for growth on nominally on-axis substrates. It is shown that the misfit dislocations are constrained to lie in the interface plane often generating long segments of edge dislocation where the misfit dislocations are forced to converge by the low angle offset. The edge segments curve Out of the interface plane into the buffer layer.
1. Introduction The presence of misft dislocations alloy interfaces is well catalogued [1,2], there is little quantitative information effect upon device performance. From luminescence studies, Fitzgerald et al
at Ill—V although on their cathodo[3] have
proposed a dependence of the electrical upon dislocation type, with curved edge activity ~aGaAs (110) dislocations, having Burgers vectors within the (001) plane, displaying greater electrical activity than those where the Burgers vector is inclined at 600 to the dislocation line. In this communication we propose a mechanism for the partial dependence of the formation of these curved edge dislocations upon substrate orientation. The formation of the edge interfacial defects is thus, in part, dependent not only upon mismatch energy and the density of fixed dislocation sources but also upon the substrate misorientation. The predominant mixed 60 0 misfit dislocations revealed in this study are initially sourced from the existing grown-in dislocations of the substrate [4]. No in0022-0248/91/503.50 © 1991
—
crease in threading dislocation density is noted for any of the samples studied. The misfit dislocations are primarily located within the first strained layer to substrate (or lattice matched buffer layer) interface. The threading dislocation density was measured by planar view from material well above this first interface for2 all It important was alwaysnot estior samples. better. It is to mated io~cm dislocation densities at the mimeasureas threading tial interface because during TEM observation dislocations can be annealed out, being attracted under their image force toward the free surface. This gives rise to segments displaying pendellosung contrast that can be wrongly interpreted as the existing threading dislocations that formed the misfit array. Reaction centres that may appear to be a product of the Hagen—Strunk mechanism [5,61are attributed to normal reaction nodes associated with the crossing of dislocations with identical Burgers vector and not part of a multiplication mechanism. Our observations indicate that each individual misfit dislocation is formed by a single slip operation; i.e. that prior to extensive
Elsevier Science Publishers B.V (North-Holland)
360
P. Kightlev et al.
/
Mechanism of misfit dislocation reaction for GalnAs SLs
reaction, the 60° misfit dislocations all form long straight lines, 2. Experimental Analysis of four samples is presented in this paper. Sample A is a 10 period strained layer supenlattice (SLS)of In 0 12Ga085As/GaAs with layers 90 and 100 A thick.~respectively. Samples B and C are 0.5 p.m thick single layers of Ga088 In0 12As grown in the same fabrication run. Sampie D has 100 periods of Ga0881n012As/GaAs with thicknesses of 95 and 100 A, respectively, Specimens A, B and D were grown on semi-in-
sulating (SI) (001) GaAs substrates tilted by 2° toward (010). Sample C was grown using an onaxis (±0.5°) (001) SI substrate. A 0.5 ~rm undoped GaAs buffer layer was grown between the strained layer and the substrate for all of the samples. The MOVPE system was operated at atmospheric pressure using trimethylgallium, tnmethylindium and a liquid arsine source. The growth temperature was always 720°C. TEM [110] cross-section and [001] plan view analysis has been used throughout this study. Bright field, dark field and weak beam imaging techniques were used at accelerating voltages of 100 or 120 keV.
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Fig. 1. (001) plan vIe~from sample A. Note the intersecting misfit dislocation lines, shown as AA’ and BB’.
P. Kightlev et al.
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Mechanism of misfit dislocation reaction for GalnAs SI..s
3. Results and discussion
361
20
A
B
Laue back reflection radiography from the substrates of samples A, B and D confirmed the substrate misorientation to be 2° toward (010) from (001). No misorientation could be detected for sample C. The substrate misorientation is cxpected to lead to the_formation of surface steps
with both [110] and [110] ledge directions. Fig. 1 shows a (001) TEM plan view prepared from sample A. Careful inspection of the misfit dislocation structure reveals that in each of the approximate [110] and [110] directions there are two 60° misfit dislocation line directions. Their separation is described by the angles 20 and 20’. These are measured from fig. 1 as 1.8 and 2.2 respectively. The angles result in the dislocations converging to create crossing points; this is illustrated by dislocation lines AA’ and BB’ on the micrograph and is constructed schematically in fig. 2. Cross-sectional TEM images show the dislocations to be located approximately at the SLS to buffer layer interface. Superposition of the dcctron diffraction pattern onto the dislocation image
[iTo
20’
B’
A’
Fig. 2. Schematic of the intersecting dislocation lines. —
(fig 3) shows that the [110] and [110] directions bisect the angles between the dislocation lines with each dislocation displaced either 0 or 0’ from the true [110] or [110] directions respectively. Using the distances A, B and C indicated in fig. 3, measured at a fixed distance either side of the
micrograph, allowed the calculation of the values
Fig. 3. Superposition of the electron diffraction pattern and the dislocation image. Measurements from these micrographs confirmed neither dislocation set to be along the true [110].
362
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Mechanism of misfit dislocation reaction for GalnAs SLi
of 1.25°±0.5° for both 0 and 0’ illustrating the deviation of the overall line from the true <110) for all the 60° misfit dislocations.
reasons. Among these are the presence of kinks on the dislocation line, the dissociation of the dislocation or the interaction of point defects with the
Ideally 60°dislocations should lie in directions
dislocation. Figs. 4a and 4b show (001) micro-
most densely packed with atoms, i.e. they should be straight, lying in (110) directions on their {111} slip planes. Local lengths of dislocation line could deviate from an exact (110) for a number of
graphs from samples B and C. Observation of the underlying directions of the glide dislocation shows that the misfit dislocations for the on-axis (001) substrate lie exactly along (110) directions,
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I ~im Fig. 4. (a) (001) plan view of sample B, growth onto a substrate tilted — 2° from (001) to (010). Note the underlying line directions of the glide dislocations where the MD intersect similar to sample A, fig. 1. (b) (001) plan view of sample C, growth onto an exactly oriented (001) substrate. Here none of the glide dislocations converge in the manner described for samples A and B.
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Mechanism of misfit dislocation reaction for GalnAs SLs
whereas for the 2° offcut substrate once again offset angles (20 and 20’) of 2° are seen. This comparison shows that the deviation of the misfit dislocation line direction is directly influenced by the choice of substrate orientation. To be most efficient at misfit relief, the misfit dislocation is required to remain in the first InGaAs/GaAs interface. For an on-axis wafer the
dislocation glide plane intersects the interface with an exact (110) direction and the dislocation lines adopt this overall line direction appearing straight in a TEM image. When using vicinal wafers, the glide plane intersects the interface with a direction slightly off from (110), thus for the dislocation to remain in the interface plane, it has to be displaced on its glide plane adopting a line direction other than exactly (110). A simple explanation of how this could occur is through the gliding thread ing dislocation (which is forming a MD segment
in the manner described by Matthews et al. [7])
363
_________ _________ ____________
b _______
________
_____________ ________
[011] [11O]~
c -______
_________
_____________________________ _________
interacting with the surface steps. Fig. 5a shows a threading dislocation prior to incidence upon a surface step riser (or ledge). For a wafer 2° off axis from (001) to (010), the ledges will occur approximately every 60 A in both the [110] and [110] directions. The ledge probably consists of an exposed part of a (111) plane with a [110] ledge
Fig. 5. (a) A laterally gliding dislocation, trailing a length of
direction. The misfit dislocation segment contains
dislocations with different threading dislocation line directions.
a random distribution of kinks and has a [110] line direction overall. The threading segment has a [011] line direction. The dislocation does not glide through the ledge, but leaves a kink there. For the example shown, a portion of the threading dislocation, with an [011] line direction, remains at the ledge joining the two misfit segments. This is illustrated in fig. Sb. The overall line of the dislocation will appear offset at each of these ledges. As the kinks formed are probably only single height steps, it is not suprising that the dislocation displacement at the ledges cannot be imaged even by weak beam microscopy. The net effect in the TEM image is of a straight dislocation line that is offset from the true [110] by 0°. It is clear that dislocations that have their misfit segment along [110] and their threading dislocation segment along [011] will at some point intersect dislocations that have their misfit segment likewise along [110] but their threading segment along [101], being sep-
arated by the angle of 20 shown in fig. 1. This is illustrated in fig. Sc. The magnitude of 0 is determined by the facet plane of the surface step, the line direction of the threading portion of the misfit dislocation, the offcut angle and direction of the substrate and the efficiency of the steps in creating the displacement. Our model requires that the dislocations lie within the strained layer to substrate interface. The offset displacement angle from [110] or [110] does not depend upon the Burgers vector of the dislocation; in fig. Sb the threading end of the dislocation could adopt either [011] or [101] line direction adopting either 600 or screw character for any of the possible ~aGaAs (110) 600 dislocations, being also independent of a or /3 character. So far we have only concerned ourselves with the more common 60° dislocations. Analysis of the
_________
MD, prior to incidence upon a surface step riser (ledge). Note that the threading segment is in a screw orientation. (b) The dislocation structure after encountering the ledge. A portion of screw dislocation remains at the ledge. (c) The intersection of
364
P. Kightley et al.
/
Mechanism of misfit dislocation reaction for GalnAs SLs
dislocation structure of this interface shows there are also dislocations that have ~aGaAs [110] or ~UGaAs [110] Burgers vectors, i.e. vectors within the (001) plane. These dislocations are often formed as a reaction product of the 60 disloca-
Prior to any dislocation reaction, the misfit dislocation segments studied can all be determined to have been of the 60° type. Individual substrate threading dislocations may form misfit dislocations with a terminating { 111 } plane in
tions at, and close to, the induced crossing points of the dislocations caused by the substrate misorientation. In figs. 6a to 6d, we show part of a dislocation analysis from one of these wafers. At
either substrate or epilayer, dependent upon the
the intersection of many of these dislocations long edge dislocation segments (labelled E in fig. 6b)
ment should be formed with a line direction such that the terminating plane at the dislocation core
are formed, with the product dislocation approximately tracing the path of the initially formed
will be within the substrate (although a small number of non-misfit relieving dislocations have
60°dislocations. For all the original glide dislocations g b 0 in either fig. 6a or 6c. When g b 0
been reported by Dixon and Goodhew [8]). One well documented reaction [2,3,9] of these disloca-
the dislocations show strong residual contrast where u — f110}; when the line direction changes
tions can lead to the formation of a Lomer prod-
°
=
line direction chosen by the laterally gliding dislocation. For the relief of the compressive stress generated for this growth system, the misfit seg-
=
uct dislocation, i.e.
to {100} at intersection points (i.e. labelled I in fig. 6a) both g b 0 and g (b X u) 0 simulta=
——
=
—
——
neously and the dislocation contrast is seen to
2~j0h11 + ~aGaAs[1O1] ~aGaAs [110]. (1) Crossing of dislocations with identical Burgers
cancel to zero.
vectors (but not chemical type) very often leads to
~
g~2o
I
~
‘
~
......—I
.
~
.
I
____________
~di1 H ‘~tU1:I11 ___ ‘.
‘~
“
L
2 Fig. 6. Part of the dislocation analysis of sample A. The 60° MD are out of contrast for gb = 0 simultaneously with g(b >< u) = 0 when they adopt <100) line directions at the points of intersection (see text). Note also the lengths of edge dislocation formed at the intersections of many MDs.
P. Kightley et a!.
/
Mechanism of misfit dislocation reaction for GaInAs SLs
the annihilation of the crossing node. Possible reactions that are not observed are the formation of large Burgers vector a0~~(OOl) or 2aGaAs (112) dislocations. Our observations show that reactions such as that described by eq. (1) can occur where the dislocations intersect orthogonally or where they intersect at 20 or 20’. Short segments of edge dislocation can be formed at the orthogonal intersection of dislocations, whereas much longer seetions of edge dislocation (up to 40 p.m) can be formed where the dislocations intersect at the smaller angles. Reaction to annihilate the crossing nodes has, so far, only been identified for approximately orthogonal intersections. For a sample displaying very long dislocation lines intersecting at small angles, each dislocation will cross many other MD. It is thus very likely to intersect other dislocations of the “correct” type to form lengths of edge MD. The lengths of edge MD are intersected by the orthogonal MD which cut them into shorter segments. The formation of these edge dislocations requires the 60° disloca-
365
tions to glide out of the interface so they can react. For a low angle intersection, the crossing point will be in the interface plane. As the distance away from this crossing point increases, the separation of the two 60° dislocations also increases, and thus the distance they are required to glide out of this interface plane to be able to meet also increases. When an orthogortal dislocation intersects these dislocations, then often they are constrained to be linked to the originating interface. The completion of the reaction may thus require a climb motion, perhaps made possible by the mobility of interstitials at the growth temperature. For an angle of intersection of 2° and an edge dislocation length of 40 p.m, the 60°dislocations appear to react until they reach a separation of 700 A. It is likely that misfit segments that are formed parallel to each other will also glide out of this interface to form edge dislocations, providing they are close enough. These dislocation reactions have been reported by Chang et al. [9] for growth onto on-axis substrates and their ob—
~c,.
—
I
©ii
—a.
Fig. 7. [1101 cross-section of sample D. The plan view of fig. 8 is taken from the area marked PV.
pv
366
P. Kightley ci at.
/
Mechanism of misfit dislocation reaction for GalnAs SLs
servations are supported by this work where these reactions can be identified, although in lower
edge segments, initially, form only below the interface, within the GaAs buffer layer.
numbers. It is of course difficult to distinguish between reactions of this type and those of very low angle intersections for nominally on-orientation wafers. As the mismatch energy is increased, the line density of edge dislocation rises. A cross-section of sample D is shown in fig. 7. The specimen is tilted toward (111) to enable the dislocation arrangement at the interface to be viewed. Plan view analysis was performed at different levels throughout this sample by carefully etching away portions of the epilayer. Fig. 8 shows part of a dislocation analysis from an area just below the multilayer stack, indicated PV in fig. 7. These dislocations are almost all edge in nature and penetrate into the buffer layer by up to 0.45 p.m. Similar analysis from the area just above the interface shows them to be absent and therefore these
g22o
g
2 20
r.
~
-
.
.
4. Conclusions Laterally gliding dislocations on a vicinal substrate form kinks in the resultant misfit dislocations probably at surface steps. The effect of these kinks is to rotate the dislocation line within the plane of the interface. Resultant MDs lie along directions at a small angle ±0to the exact <110). Where the nearly parallel dislocations cross they react to form edge dislocations. Because each dislocation crosses many other dislocations there are more opportunities for the formation of edge segments than exist for on-axis substrates. The formation of the edge dislocation requires that the 60° MDs glide out of the heterointerface. This behaviour is observed and, for the 10 period strained layer superlattice studied here, it is esti-
.
~,.A~!i’
~.
gip4o
40
2
~ifli
Fig. 8. Plan view dislocation analysis showing the curved edge dislocations formed in the buffer layer directly below the interface.
P. Kightley et al.
/
Mechanism of misfit dislocation reaction for GalnAs SLs
mated that dislocations have undergone this reaction up to a separation of 700 A. It is probable that this distance is mismatch energy and growth temperature dependent.
Acknowledgements The authors gratefully acknowledge the encouragement and support of Dr. R.H. Wallis and thank Dr. D.J. Dunstan (Surrey University) for valuable discussions. Mrs. G.B. Davies is also acknowledged for her expert technical assistance.
References [1] A. Okamoto, H. Toyoshima and K. Ohata, Japan. J. AppI. Phys. 26 (1987) 539.
367
[2] Y. Ashizawa, S. Akbar. W.J. Schaff, L.F. Eastman, E.A. Fitzgerald and D.G. Ast. J. AppI. Phys. 64 (1988) 4065. [3] J.M. E.A. Woodall, Fitzgerald,J. D.G. P.D. Kirchner, Appl. Ast, Phys. 63 (1988) 693.GD. Pettit and [4] G.H. Olsen and M. Ettenberg, in: Crystal Growth: Theory and Techniques, Ed. C.H.L. Goodman (Plenum, London, 1978) p. 23. [5] E.A. Fitzgerald, G.P. Watson, RE. Proano, D.G. Ast, P.D.
Kirchner, G,D. Pettit and J.M. Woodall, J. AppI. Phys. 65 (1989) 2220. [6] W. Hagen and H. Strunk. Appl. Phys. 17 (1978) 85. [7[ J.W. Matthews, A.E. Blakeslee and S. Mader, Thin Solid Films 33 (1976) 253. [8] RI-I. Dixon and P.J. Goodhew, J. Appl. Phys. 68 (1990) 3163.
[9] K.H. Chang, P.K. Bhattacharya and R. Gibala, J. AppI. Phys. 66 (1989) 2993.