UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption and dissociation. A nitride passivation layer can be formed on the GaAs surface at 100 K by simultaneous exposure to ammonia and uv photons in a UHV environment. The nitride layer consists of a mixture of Ga and As nitrides. While the dominating GaN surface species is thermally stable, AsN desorbs below 800 K. Surface NH2 is identified as an intermediate. The implication of this study for selective area passivation and GaN growth is discussed.

The use of trimethylgallium (TMGa) as the gallium source during epitaxial growth of GaAs often leads to high levels of carbon incorporation. Using temperature programmed desorption, high-resolution electron energy loss spectroscopy (HREELS), and static secondary ion mass spectroscopy (SSIMS), we have identified a likely carbon incorporation pathway initiated by methyl group dehydrogenation. Methyl group dehydrogenation is evidenced by a small amount of hydrogen evolution around 430 °C. Extended TMGa exposures in this temperature regime yield substantial coverages of methylene (CH2) adsorbate that is detected by HREELS and SSIMS. The CH2 adsorbate undergoes further reaction at higher temperature, yielding acetylene (C2H2), H2, and CH3 radicals as desorption products at ∼550 °C. All of these products can be attributed to a mechanism involving CH2 dehydrogenation, hydrogenation, and recombination reactions. The rate of CH3 dehydrogenation is consistent with carbon doping levels typically obtained by metalorganic molecular beam epitaxy (MOMBE) and related techniques. High temperature exposure to arsine (AsH3) consumes the CH2 adsorbate, apparently by hydrogenating them back to CH3 groups that then desorb. This observation explains why carbon doping is lower during atomic layer epitaxy as compared to MOMBE.

Monolayer nitridation of Ga-rich GaAs(100) is achieved at 100 K by simultaneous exposure to ammonia and 6.4 eV photons in a vacuum environment. This process is a result of nonthermal photodissociation of adsorbed ammonia. Surface NH2 is identified as an important intermediate in nitridation.

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UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption, with a mean translational temperature of <Etrans/2k≳=300 K, independent of photon energy and isotope substitution. However, the photodesorption cross section depends strongly on isotope substitution: σNH3/σND3=4.1 at hν=6.4 eV. This isotope effect is too large to be accounted for by the mass difference in the leaving particles (NH3 vs ND3), but can be successfully explained in terms of an isotope effect in the internal N–H(D) coordinates. We take this as evidence for uv-driven photodesorption from electronically quenched, but vibrationally hot ground state ammonia.