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We report an extensive study of the first stages of the buildup by molecular-beam epitaxy of III-V-compound semiconductor heterojunctions (AlAs, InAs, and CraAs with each other, and GaAs/GaP and InAs/InP). Surface-sensitive techniques have been applied in situ, first to yield information on the geometry and crystal structure of monolayer-thick overlayers of a III-V compound grown upon another III-V substrate. For atomically well-defined heterointerfaces, the buildup of the electron states at the interface has then been analyzed. In these cases, with respect to the vacuum level, the substrate band structure remains stationary, and the overlayer surfaceand bulkderived electron states appear at the position they have in the bulk overlayer material, within a +0. 1 eV accuracy limit. The band offsets that we deduce are in good agreement with the electron-af5nity rule and with recent experimental and theoretical data.UILDUP OF III-V-COMPOUND SEMICONDUCTOR HETEROJUNCTIONS: 1773 buffer layers at =1 ML/s. Growth temperatures lie around 600'C, except for In-containing structures where it is reduced to 450'C in order to avoid In desorption.Arsenic pressures are tuned during growth in order to obtain the "As-stabilized" regime except for In-containing layers where the maximum pressure ensuring an "Instabilized" growth is used. The final thickness of the buffer layers is large enough to avoid effects of the initial contamination and to accommodate the eventual lattice mismatch, thus yielding unstrained substrates.After cooling down, most often under As pressure down to 300 C and then under vacuum down to room temperature (see Sec. III C), the structures are transferred to the analysis section and probed by LEED, AES, XPS, UPS, and EELS. All electronic spectra are obtained with the CLAM 100 analyzer. AES is performed with a primary electron energy of 2 keV and a modulation of 2.5 V and EELS with 120 eV and 1 V, respectively, XPS with

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