The results of a reflectance-difference spectroscopy study of GaAs grown on ͑100͒ GaAs substrates by low-temperature molecular-beam epitaxy ͑LT-GaAs͒ are presented. In-plane optical anisotropy resonances which come from the linear electro-optic effect produced by the surface electric field are observed. The RDS line shape of the resonances clearly shows that the depletion region of LT-GaAs is indeed extremely narrow ͑Ӷ200 Å͒. The surface potential is obtained from the RDS resonance amplitude without the knowledge of space-charge density. The change of the surface potential with post-growth annealing temperatures reflects a complicated movement of the Fermi level in LT-GaAs. The Fermi level still moves for samples annealed at above 600°C, instead of being pinned to the As precipitates. This behavior can be explained by the dynamic properties of defects in the annealing process. ͓S0163-1829͑97͒51712-7͔GaAs grown by low-temperature molecular-beam epitaxy ͑LT-GaAs͒ has recently attracted much interest due to its unique properties and potential electronic and optoelectronic applications. [1][2][3][4][5] This material is crystalline, with a high content of excess arsenic, 4 which agglomerates into small metallic clusters after post-growth annealing. 5,6 With the formation of arsenic precipitates the electrical and optical properties of LT-GaAs change significantly. [3][4][5][6] For example, annealing at temperatures above 600°C usually results in LT-GaAs with extremely high resistivity. However, the mechanism of such high resistivity is still in debate. Some believe that it was the result of depletion of carriers by the Schottky barrier at the surface of the metallic As precipitates, 5,6 while others suggested a compensation model of defects similar to the conventional semi-insulating ͑SI͒ GaAs. 4 The change of carrier and defect densities inevitably changes the Fermi level, and the determination of which is therefore important in the study of defect-state evolution in post-growth annealing.Photoreflectance ͑PR͒ is extensively used to study the surface electric field and the related Fermi level of semiconductors. However, so far no Franz-Keldysh Oscillation ͑FKO͒ produced by the electric field inside LT-GaAs was ever detected except for samples annealed at above 700°C. 7,8 We believe that this is due to the highly nonuniform electric field and the extremely narrow surface depletion region which are the results of high density of defects and traps in LT-GaAs samples. 4 As a result, the Fermi level of LT-GaAs could only be determined indirectly by PR measurements of the electric field in the adjacent normal GaAs region near a LT-GaAs/ GaAs interface. 9 Here we report a direct measurement of the surface potential of LT-GaAs by reflectance-difference spectroscopy ͑RDS͒. Recent RDS studies of doped GaAs showed strong anisotropy resonances at the critical points E 1 and E 1 ϩ⌬ 1 due to the electro-optic ͑LEO͒ effect generated by the surface electric field. [10][11][12] In this paper we present our RDS study of a series of L...