THE INTERACTION OF HYDROGEN WITH GaAs SURFACES

Aspects of the interaction of hydrogen with MBE grown GaAs surfaces have been studied in situ with synchrotron radiation excited photoemission core level spectroscopy. We show how this characterization technique is a sensitive probe of the surface chemical composition. Using this technique, the GaAs(100) and (111) surface phase diagram at room temperature has been obtained as well as aspects of the AlAs surface. The measurements showed that for GaAs(100) the surface reconstructions proceed through a series of centered [c(4x4), c(2x8), c(8x2)] and primitive structures [(1x6) ,(4x6)] and for the (111) surface (2x2), (√ 3x √ 3)R(30°), (√ l9x √ l9)R(23.4°) as the surface As to Ga ratio is decreased. The study of the clean surfaces has been extended with an investigation of the interaction with atomic and molecular hydrogen. This is of interest both because hydrogen is a common growth ambient and because there is some indication that MBE growth in the presence of hydrogen improves crystal electrical quality. In all cases, the experiments showed a saturation hydrogen coverage was achieved at an exposure of 106L of hydrogen when a cracking filament is used. Core level spectroscopy shows that the surface composition is modified by the presence of hydrogen and we have deduced a mechanism whereby the hydrogen forms a surface gallium hydride which sublimes. The hydrogen converts the clean surface reconstructions to a distinct semiconducting reconstruction with a specific composition independent of the starting reconstruction. These results indicate that hydrogen could modify the MBE growth surface thereby effect impurity incorporation

Storage of electrons in shallow donor excited states of GaP:Te

Tellurium donors in GaP have been ionized by phonon-assisted tunneling in the electric field of pulsed far-infrared laser radiation. In response to the laser pulse a photoconductive signal has been detected with a fast component that follows in time the laser pulse and a slow component that rises after the irradiation has ceased and finally exponentially decays with a strongly temperature-dependent time constant varying from several microseconds to milliseconds. It is shown that this temporal structure of the signal is due to a storage of carriers in the valley-orbit split 1s(E) shallow donor state. Observation of far-infrared to mid-infrared up-conversion demonstrates that the final step of cascade recombination is achieved by radiative transitions

Diffraction efficiencies of holographic transmission gratings in the region 80–1300 eV

Experimental measurements of diffraction efficiencies for holographic transmission gratings have been performed. The transmitted intensity distribution is characterized by an overall efficiency of about 10% into first order and a strong resonance enhancement, up to 20% efficiency, around the region of anomalous dispersion. The intensity distribution is well described by a grating model which predicts the overall efficiency, as well as the detailed behavior of the grating around the regions of anomalous dispersion. The model can be used to predict the efficiencies for an arbitrary grating material and thickness, and thus aid in the determination of grating structure for a specific experimental application