Threading Dislocation Densities in GaAs Grown on Reduced Area Si Substrates
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THREADING DISLOCATION DENSITIES IN GaAs GROWN ON REDUCED AREA Si SUBSTRATES J. KNALL, L.T. ROMANO, D.K. BIEGELSEN, AND R.D. BRINGANS Xerox Palo Alto Research Center, Palo Alto, CA 94304
ABSTRACT We have studied the possibilities of reducing the threading dislocation (TD) density in GaAs on Si using free side wall growth on patterned Si substrates and/or using a soft ZnSe interlayer in combination with post growth annealing procedures. TD densities were accurately determined using large area plan view TEM and were found to be unaffected by proximity to free side walls andby the ZnSe interlayer. Post growth heat treatments led to a factor of -2 reduction in TD density and to bunching of dislocations throughout the thickness of the film. INTRODUCTION GaAs on Si technology is still limited by a high density of threading dislocations (TDs), which is attributed to the 4% lattice mismatch and the large difference in thermal expansion between GaAs and Si. The lattice mismatch introduces a high density of misfit dislocations which can terminate either at the film surface or at the sample edge. Fitzgerald found that for systems with smaller lattice mismatch ( type lying in the surface plane. We observed that both in annealed and unannealed films, only - 20 % of the dislocations have a Burgers vector parallel to the surface plane. From our stereo images it was found that unentangled dislocations often had line directions either along < 110 > or < 112 > directions. Close to mesa edges many dislocations were found to follow the < 112 > direction that pointed away from the edge when going from substrate to film surface. This could be related to the asymmetric stress that prevails near the edges. The dislocation density is much higher close to the substrate than at the surface of a 2 lim film as can be seen in the TEM cross section of an as grown film in Fig. 5. This is due to a large number of dislocation reactions between threading segments that leads to annihilation and/or superposition of threading segments. DISCUSSION The result that proximity to free side walls does not strongly affect the TD density can be interpreted in two ways. Either (i) the films are dislocation free at high temperature and TDs move in during cool down because of thermal expansion mismatch or (ii) the threading segments are always present and have low mobility due to interactions at the interface. The fact that the threading dislocations were unentangled throughout the top part of the unannealed films indicates that these dislocations have not moved far during cool down from the growth temperature. This leads us to suggest that they are present already during growth. It is probable
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Figure 4. TEM plan view of an as grown GaAs film on a 10 pM Si mesa (near a [0011 zone). In the left panel the diffraction vector g was [220], and in the right g was [040]. In the micrographs the label "a" indicates dislocations with Burgers vector b=± %a[110] or b= ± %a[110], the label "b" indicates b= ± %a[1011 or
b= ± %a[1O1],"cl"indicates b= ± %a[110],and"c2"in
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