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Comments on “red intensity oscillations during MBE of GaAs”
L444
Surface Science 108 (1981) L444-L446 North-Holland Publishing Company
SURFACE SCIENCE LETTERS COMMENTS ON “RED INTENSITY OSCILLATIONS
DURING MBE OF GaAs”
J.J. HARRIS and B.A. JOYCE PhilipsResearch Laboratories, Redhill, Surrey, UK
P.J. DOBSON Departrnent of Physics, Imperial College of Science and Technology, Prince Consort Road, London, UK Received 30 March 1981
In his letter [l], Wood makes a number of speculations on the origin of the .oscillating RHEED patterns reported in our previous publication [2]. However, some of his remarks lead us to conclude that the contents of ref. [2] were not as clearly expressed as we might have wished; we would therefore like to clarify a few points raised by Wood, as well as to comment on his various proposed interpretations of the effect. The first difficulty arises over the cause of the static changes in RHEED pattern with increasing coverage of Sn or other species [ 1,3]. As we pointed out in ref. [2], there is no generally accepted model for the configuration and bonding of even the basic reconstructed (lOO)GaAs surface, and it thus seems a little premature to enter into a detailed discussion of the origin of modifications to such a surface. However, four points might usefully be made: (a) Our proposal of a strain field surrounding each chemisorbed Sn atom was intended primarily to account for the increased diffuseness of the RHEED pattern, since we are not in a position to explain the changes in structure. We would emphasise that it requires a variation in surface symmetry to cause a modification to the spacing of the RHEED streaks, and such modifications are not indicative of the range of coherent surface order, whereas the length and diffuseness of the streaks are [4]. (b) The unequal spacing of the reconstruction streaks shown in tig. 1 of ref. [ 1] is simply an intermediate stage in the gradual transition of the (2 X 3) structure into the (1 X 2); the extra lines in the 4-fold reconstructed azimuth of the (2 X 4) surface appear to drift towards the mid-point between the bulk lines as the Sn coverage increases, eventually merging together and finally disappearing completely. 0039-6028/81/0000-0000/$02.50
0 North-Holland
J.J. Harris et al. /RED intensity oscillationsduring MBE of GaAs
I.445
Thus even the 3-fold reconstruction may be simply a transitory stage in this process. (c) Comparison of the changes described above with those occurring when the substrate temperature is varied may not be valid, since in general the latter changes are instantaneous, rather than gradual, and probably correspond to abrupt changes in surface structure [ 51. (d) The suggestion was made in ref. [l] that the changes in RHEED pattern arise from changes in the range of surface order, or from the presence of adjacent domains of different reconstructions. Although these ideas are not inconsistent with the tentative model proposed in ref. [2] (the strained area around each Sn atom reduces the surface ordering, and may introduce a domain whose structure differs from the surrounding unstrained GaAs), nevertheless as we mentioned above, changes in the range of surface order can only account for changes in the sharpness and length of the RHEED streaks; furthermore, it seems probable that adjacent domains of different symmetry will give rise to superimposed RHEED patterns, rather than to modifications of the type observed. Turning now to the oscillation of the RHEED patterns, it is clear from fig. 1 of ref. [2] (though admittedly, not from the text), that the oscillation results from the apparent motion of the intensity distribution along the rods, and not from an inphase variation of the brightness of the whole pattern. (It is interesting to note that the initial movement of the bright point on the rods appears to be towards the shadow edge, and not away, as suggested in ref. [l] ,) The intensity variation along the RHEED streaks arises from a complex superposition of the scattered intensity distributions from the surface atoms, and it is far from clear how Wood’s suggestion of transient changes in the degree of surface ordering can influence anything more than the sharpness of the pattern. Considering the effect of linear steps in the surface, a simple Ewald sphere construction shows that these will introduce extra diffraction features normal to the step direction [6], and thus his alternative explanation also seems inconsistent with ‘observation. Furthermore, since a step edge does not give rise to transmission diffraction spots, the suggestion that the oscillations are due to changes in step height is clearly untenable, We do not completely discount a topographical interpretation of the effect; however, Gilmer [7] has predicted periodic surface roughening under certain transient conditions during crystal growth, and although it is not obvious how such a phenomenon can account for the RHEED oscillations, we are currently looking for evidence of this effect as an alternative explanation. In this context, the report in ref. [l] of weak oscillations on nominally undoped layers, while apparently supporting a model of this latter type, is not necessarily inconsistent with a surface impurity model, since recent SIMS measurements confirm that several common substrate impurities, notably Cr [8] and Fe [9], are surfaceaccumulated during substrate cleaning and are subsequently carried forward on the growing epilayer surface in a similar way to Sn. Finally, although we are not as convinced as Wood that oscillating RHEED
L446
J.J. Harris et al. /RED
intensity oscillatiotw during MBE of GaAs
patterns are “a two-dimensional manifestation of step propagation crystal growth mechanisms” (mainly because it is not clear exactly what is meant by that phrase), we agree wholeheartedly with his conclusion that “further work is needed to clarify the exact mechanism”, and we look forward to the publication of any contributions he can make in this direction.
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
C.E.C. Wood, Surface Sci. 108 (1981) 441. J.J. Harris, B.A. Joyce and P.J. Dobson, Surface Sci. 103 (1981) L90. A.Y. Cho and M. Panish, Appl. Phys. Letters 43 (1972) 5118. P.K. Larsen, J.H. Neave and B.A. Joyce, J. Phys. C (Solid State Phys.) 14 (1981) 167. J.H. Neave and B.A. Joyce, J. Crystal Growth 44 (1978) 387. F. Hottier, J.B. Theeten, A. Masson and J.L. Domange, Surface Sci. 65 (1977) 563. G.H. Gilmer, J. Crystal Growth 49 (1980) 465. H. Morkoc, C. Hopkins, CA. Evans and A.Y. Cho, J. Appl. Phys. 51 (1980) 5986. D.W. Covington, J. Comas and P.W. Yu, Appl. Phys. Letters 37 (1980) 1094.
Surface Science 108 (1981) L444-L446 North-Holland Publishing Company
SURFACE SCIENCE LETTERS COMMENTS ON “RED INTENSITY OSCILLATIONS
DURING MBE OF GaAs”
J.J. HARRIS and B.A. JOYCE PhilipsResearch Laboratories, Redhill, Surrey, UK
P.J. DOBSON Departrnent of Physics, Imperial College of Science and Technology, Prince Consort Road, London, UK Received 30 March 1981
In his letter [l], Wood makes a number of speculations on the origin of the .oscillating RHEED patterns reported in our previous publication [2]. However, some of his remarks lead us to conclude that the contents of ref. [2] were not as clearly expressed as we might have wished; we would therefore like to clarify a few points raised by Wood, as well as to comment on his various proposed interpretations of the effect. The first difficulty arises over the cause of the static changes in RHEED pattern with increasing coverage of Sn or other species [ 1,3]. As we pointed out in ref. [2], there is no generally accepted model for the configuration and bonding of even the basic reconstructed (lOO)GaAs surface, and it thus seems a little premature to enter into a detailed discussion of the origin of modifications to such a surface. However, four points might usefully be made: (a) Our proposal of a strain field surrounding each chemisorbed Sn atom was intended primarily to account for the increased diffuseness of the RHEED pattern, since we are not in a position to explain the changes in structure. We would emphasise that it requires a variation in surface symmetry to cause a modification to the spacing of the RHEED streaks, and such modifications are not indicative of the range of coherent surface order, whereas the length and diffuseness of the streaks are [4]. (b) The unequal spacing of the reconstruction streaks shown in tig. 1 of ref. [ 1] is simply an intermediate stage in the gradual transition of the (2 X 3) structure into the (1 X 2); the extra lines in the 4-fold reconstructed azimuth of the (2 X 4) surface appear to drift towards the mid-point between the bulk lines as the Sn coverage increases, eventually merging together and finally disappearing completely. 0039-6028/81/0000-0000/$02.50
0 North-Holland
J.J. Harris et al. /RED intensity oscillationsduring MBE of GaAs
I.445
Thus even the 3-fold reconstruction may be simply a transitory stage in this process. (c) Comparison of the changes described above with those occurring when the substrate temperature is varied may not be valid, since in general the latter changes are instantaneous, rather than gradual, and probably correspond to abrupt changes in surface structure [ 51. (d) The suggestion was made in ref. [l] that the changes in RHEED pattern arise from changes in the range of surface order, or from the presence of adjacent domains of different reconstructions. Although these ideas are not inconsistent with the tentative model proposed in ref. [2] (the strained area around each Sn atom reduces the surface ordering, and may introduce a domain whose structure differs from the surrounding unstrained GaAs), nevertheless as we mentioned above, changes in the range of surface order can only account for changes in the sharpness and length of the RHEED streaks; furthermore, it seems probable that adjacent domains of different symmetry will give rise to superimposed RHEED patterns, rather than to modifications of the type observed. Turning now to the oscillation of the RHEED patterns, it is clear from fig. 1 of ref. [2] (though admittedly, not from the text), that the oscillation results from the apparent motion of the intensity distribution along the rods, and not from an inphase variation of the brightness of the whole pattern. (It is interesting to note that the initial movement of the bright point on the rods appears to be towards the shadow edge, and not away, as suggested in ref. [l] ,) The intensity variation along the RHEED streaks arises from a complex superposition of the scattered intensity distributions from the surface atoms, and it is far from clear how Wood’s suggestion of transient changes in the degree of surface ordering can influence anything more than the sharpness of the pattern. Considering the effect of linear steps in the surface, a simple Ewald sphere construction shows that these will introduce extra diffraction features normal to the step direction [6], and thus his alternative explanation also seems inconsistent with ‘observation. Furthermore, since a step edge does not give rise to transmission diffraction spots, the suggestion that the oscillations are due to changes in step height is clearly untenable, We do not completely discount a topographical interpretation of the effect; however, Gilmer [7] has predicted periodic surface roughening under certain transient conditions during crystal growth, and although it is not obvious how such a phenomenon can account for the RHEED oscillations, we are currently looking for evidence of this effect as an alternative explanation. In this context, the report in ref. [l] of weak oscillations on nominally undoped layers, while apparently supporting a model of this latter type, is not necessarily inconsistent with a surface impurity model, since recent SIMS measurements confirm that several common substrate impurities, notably Cr [8] and Fe [9], are surfaceaccumulated during substrate cleaning and are subsequently carried forward on the growing epilayer surface in a similar way to Sn. Finally, although we are not as convinced as Wood that oscillating RHEED
L446
J.J. Harris et al. /RED
intensity oscillatiotw during MBE of GaAs
patterns are “a two-dimensional manifestation of step propagation crystal growth mechanisms” (mainly because it is not clear exactly what is meant by that phrase), we agree wholeheartedly with his conclusion that “further work is needed to clarify the exact mechanism”, and we look forward to the publication of any contributions he can make in this direction.
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
C.E.C. Wood, Surface Sci. 108 (1981) 441. J.J. Harris, B.A. Joyce and P.J. Dobson, Surface Sci. 103 (1981) L90. A.Y. Cho and M. Panish, Appl. Phys. Letters 43 (1972) 5118. P.K. Larsen, J.H. Neave and B.A. Joyce, J. Phys. C (Solid State Phys.) 14 (1981) 167. J.H. Neave and B.A. Joyce, J. Crystal Growth 44 (1978) 387. F. Hottier, J.B. Theeten, A. Masson and J.L. Domange, Surface Sci. 65 (1977) 563. G.H. Gilmer, J. Crystal Growth 49 (1980) 465. H. Morkoc, C. Hopkins, CA. Evans and A.Y. Cho, J. Appl. Phys. 51 (1980) 5986. D.W. Covington, J. Comas and P.W. Yu, Appl. Phys. Letters 37 (1980) 1094.