TEMPERATURE PROGRAMMED DESORPTION STUDY OF GAAS(100)-C(4X4) AND AS4 EXPOSED (2X4) SURFACES

GaAs(100)-c(4 x 4) and As4 exposed (2 x 4) surfaces are studied using temperature programmed desorption (TPD) and reflection high energy ele ctron diffraction (RHEED). The As sensitive factor for a quadrupole ma ss spectrometer is precisely calibrated and the As coverage is determi ned from As2 desorption spectra. For the c(4 x 4) surface, there are t wo steps in the As desorption process. Coverage analysis indicates tha t the c(4 x 4) structure is formed at an As coverage, theta(As), of 1. 28 monolayer (ML) and an additional As adsorption is possible up to a theta(As), of 1.61 ML without disturbing the reconstruction. At higher theta(As), a repulsive interaction between adsorbed As dimers is sugg ested. On the As4 exposed (2 x 4) surfaces, the chemisorbed As desorbs at 430 and 500-degrees-C as As2. Based on RHEED observations, As2 pea king at 500-degrees-C is assigned to desorption due to transition from a (2 x 4)-gamma to a (2 x 4) surface structure. The structures of the se surfaces are discussed based on our TPD results and previously prop osed structure models.