  We have studied the efficacy of (NH4)2Sx surface passivation on the (311)A
GaAs surface. We report XPS studies of simultaneously-grown (311)A and (100)
heterostructures showing that the (NH4)2Sx solution removes surface oxide and
sulfidizes both surfaces. Passivation is often characterized using
photoluminescence measurements, we show that while (NH4)2Sx treatment gives a
40 - 60 x increase in photoluminescence intensity for the (100) surface, an
increase of only 2 - 3 x is obtained for the (311)A surface. A corresponding
lack of reproducible improvement in the gate hysteresis of (311)A
heterostructure transistor devices made with the passivation treatment
performed immediately prior to gate deposition is also found. We discuss
possible reasons why sulfur passivation is ineffective for (311)A GaAs, and
propose alternative strategies for passivation of this surface.
