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The homoepitaxy of GaAs on (631)-oriented substrates has been studied as a function of the growth temperature. We observed the spontaneous formation of a high density of large scale features on the surface. The hilly like features are elongated towards the ½À5; 9; 3 direction. When the growth temperature was varied from 490 to 580 C the hillocks length exponentially increases from 1.8 to 4.3 mm, their height linearly increases from 35 to 50 nm, and the density exponentially decreases from 2:8  10 6 to 3  10 5 /cm 2 . The hillocks formation is discussed in terms of adatoms diffusion anisotropy, sticking properties at step edges, and Ehrlich-Schwoebel diffusion barriers.The molecular beam epitaxial (MBE) growth of III-V compound semiconductors on high index GaAs substrates has attracted much interest due to the strong optical anisotropies and piezoelectric effects present in these crystal planes. 1,2) Another important reason to study the growth on high index substrates is the possibility of obtaining self organized phenomena to synthesize low dimensional structures. 3) Some of the high index surfaces where the MBE growth of GaAs has been studied are: (11n), (122), (133), (012), and ð2; 5; 11Þ. 4-7) However, to our knowledge, there is no report on the MBE growth of GaAs on (631) oriented substrates. The (631) plane is interesting because facets of this orientation naturally appear during the MBE growth of III-V semiconductors under different circumstances. For example; (631)A facets are formed during the GaAs MBE growth on GaAs(100) substrates patterned with squareshaped mesa structures with sides oriented along h010i directions. 8) The (631) facets also appear during the self assembling of InAs quantum dots on GaAs(100) substrates. 9-11) These experimental results suggest that the (631) plane should have interesting surface diffusion properties, which motivate MBE growth studies on this plane. In this letter we report the MBE growth of GaAs on GaAs(631)A substrates as function of growth temperature.For the study we employed GaAs(631)A undoped wafers, some crystallographic directions relevant to the discussion are indicated in the schematic drawing of a wafer in the inset of Fig. 1. The wafers were degreased and etched, and then they were loaded in a Riber C21 MBE system. Before the GaAs growth the surface oxide was desorbed in the MBE chamber by annealing the substrates at 580 C under arsenic overpressure. The growths were carried out employing a constant As 4 beam-equivalent pressure of 1  10 À5 Torr, and a GaAs growth rate of 1 ML/s. Different samples were prepared with the substrate temperature varied in the range of 490 -580 C, all the samples were grown to a thickness of $500 nm. The samples surface was in-situ monitored by reflection high-energy electron diffraction (RHEED). Immediately after the growths the samples surface was analyzed by atomic force microscopy (AFM) in air.First of all we will present some basic aspects of the (631) plane structure. Figure 1 shows a ball-and-stick model of the unreco...

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