In this letter, we discuss the interfacet diffusion of group-III species fr
om {111} B facets to the (100) plane in planar selective area epitaxy. In g
eneral, this leads to enhanced vertical growth at the edges of the (100) su
rface. From such edge profiles, several groups have extracted adatom "diffu
sion lengths" of similar to 1 mu m. This is a factor of 100 larger than rep
orted diffusion lengths obtained by reflective high energy electron diffrac
tion, scanning tunneling microscopy, or growth-rate analysis. We show that
these values are severely overestimated and that edge profiles only give in
formation on the propagation velocity of macrosteps. (C) 1999 American Inst
itute of Physics. [S0003-6951(99)04115-7].