Measurements at conjugate points on the ground near L = 4 of the power spectra of magnetic‐field fluctuations in the frequency range 0.5 to 20 mHz are used as a means of estimating daily values for the relativistic‐electron radial‐diffusion coefficient DLL for two periods in December 1971 and January 1972. The values deduced for L−10 DLL show a strong variation with magnetic activity, as measured by the Fredricksburg magnetic index KFR. The radial‐diffusion coefficient typically increases by a factor of ∼10 for a unit increase in KFR. When KFR ≲ 2, it is generally found that DLL ≲ 2 × 10−9 L10 day−1 for equatorially mirroring electrons having a first invariant M = 750 Mev/gauss; a value of DLL ∼4 × 10−7 L10 day−1 is deduced for one day on which the mean KFR was 4.5. The quantity L−10 DLL theoretically depends on energy and L as (L/M)(s−2)/2 for relativistic particles, where s is the logarithmic slope of the power‐law spectrum of magnetic fluctuations observed on the ground. For the time period analyzed, s typically had values between 1 and 3.

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First-principles molecular-dynamics calculations have been used to calculate the formation energy of the lowest-energy As interstitial configuration relative to the formation energies of As antisites and Ga vacancies in As-rich GaAs, and to identify and study the properties of energetically favorable complexes containing one As antisite and one As interstitial. It is suggested that the electronic and optical properties of the antisiteinterstitial complexes match the properties of the defects responsible for the dominant donor band in some samples grown around 350°C.

Results of first-principles calculations are reported for the adsorption of As 2 molecules on the stable surface reconstructions of the GaAs (001) surface, including adsorption paths and barriers for strongly bound sites. It is shown that a novel chemisorption state acts together with an intermediate physisorbed plateau in the total energy to hold the As 2 molecules near the surface and funnel them into strongly bonding sites during epitaxial growth, and that this state can explain the transition from the b2͑2 3 4͒ to the c͑4 3 4͒ reconstruction under low-temperature, very arsenic-rich conditions. [S0031-9007 (99)09336-9] PACS numbers: 82.65.My, 68.45.Da, 81.10.AjA wide variety of electronic and optoelectronic devices are based on multilayered structures composed of III-V semiconductors, such as GaAs. Understanding the fundamental kinetic processes of epitaxial growth is an important step toward optimizing the characteristics of these semiconductor layers by altering the growth conditions. Because of its technological importance as the most commonly used surface for epitaxial growth of GaAs, the GaAs (001) surface has been extensively studied. However, although the relative energies of various static reconstructions for this surface, with different surface stoichiometries, have been thoroughly studied by firstprinciples calculations [1,2], gallium adatom diffusion is the only kinetic process that has been thoroughly investigated by such methods, including reaction paths and energy barriers as well as the binding energies for stable and metastable adatom sites [3]. In particular, almost nothing is known about the microscopic reaction paths, reaction barriers, and kinetics of the important processes involving incoming arsenic, such as adsorption, diffusion, and incorporation into strongly bonded sites on the surface. Experimental work, from the earliest studies of the interaction of gallium and arsenic beams with the GaAs surface [4-6] to recent scanning tunneling microscopy studies of island shapes and size distributions immediately following a growth interruption [7], has shown that the kinetics of arsenic adsorption and attachment play an important role in GaAs growth. Although both As 2 and As 4 sources are used for epitaxial growth of GaAs, it is simpler to understand the growth processes when As 2 is used, since the GaAs (001) surface reconstructions produced in standard epitaxial growth are terminated with arsenic dimers, so the As 2 molecule does not necessarily have to break up in order to become incorporated into the growing surface. For both As 2 and As 4 sources, in order to reproduce growth without assuming unphysically large As:Ga flux ratios, the growth models used in kinetic Monte Carlo simulations commonly assume that arsenic molecules are initially adsorbed into a "molecular precursor" state [5][6][7][8].However, no microscopic description of this precursor state has been given, and no microscopic model has been proposed for the incorporation of arsenic molecules from the precursor state into the ...