and applied physics letters 36 1980 698 the exponential increase of rate with temperature below 500 u00b0 c is typical of a surface process under kinetic control and the apparent activation energy of 38 k cal mo is tyupical of such processes d w shaw 34 mechanisms in vapor epitaxy of semiconductors 34 crystal growth theory and techniques vol i ed c h l goodman plenum press london 1974 a further argument for control of the etch by surface kinetics at the lower temperatures is seen in the temperature dependence of 100 morphologies for h 2 carrier gas the etch pits at 550 u00b0 c and lower have distinct crystallographic orientation and faceting typical of etch rates limited by the surface kinetics for the 700 u00b0 c to 750 u00b0 c range where etching is mass transport limited the rate is controlled by processes away from the substrate surface the insensitivity to crystal surface chemistry reduces the etch pit formation however at these temperatures a degree of thermal pitting occurs regardless of increased ph 3 concentrations for the 575 u00b0 c to 600 u00b0 c regime the etch pits are seen to be shallow and less defined by crystallographic facets the absence of strong crystallographic dependence weakens any argument that this regime is dominated solely by surface kinetic processes also there are some specimens etched in this regime which showed absence of etch pits the edges of samples which correspond to the outer edge of the polished wafer typcially are free of pits the outer surface of the wafer has been polishd and etched more than the central region and as a result the surface is slightly curved so it is no longer parallel to 100 the use of substrates oriented 3 u00b0.