Leed, aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Abstract

Epitaxially grown GaAs(001), (111) and (1̄1̄1̄) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH 3 atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10 14 atoms cm −2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1̄1̄1̄) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10 14 atoms cm −2. The maximum photoemission from all surfaces is in the order of 50μA Im −1 for white light ( T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10 14 atoms cm −2 at 260dgC and one of 2.7 × 10 14 atoms cm −2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10 14 atoms cm −2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1̄1̄1̄) crystal shows a stability region at 260°C with a coverage of 3.8 × 10 14 atoms cm −2 with ordering of the Cs atoms in a GaAs(1̄1̄1̄) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10 14 at cm −2 with the formation of a GaAs(1̄1̄1̄) (√21 × √21)Cs structure. Possible models of the GaAs(1̄1̄1̄) (4 × 4)Cs, GaAs(1̄1̄1̄)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.