THE SINGLE-SOURCE PRECURSOR CONCEPT - A CASE-STUDY OF GALLIUM-ARSENIDE

The viability of the GaAs single-source chemical beam epitaxy (CBE) precursors [Me2Ga(mu-AsiPr2)]3 (1), [Me2Ga(mu-AstBu2)], (2), [Et2Ga(mu-AstBu2)]2 (3) and Ga(AstBu2)3 (4) has been explored. Precursor 1 (and analogous trimeric molecules) fail to yield satisfactory thin films of GaAs because of facile loss of tetraalkyldiarsine. Both 2 and 3 afford epitaxial p-type films of GaAs at > 500-degrees-C and 10(-5) Torr pressure. However, thin films grown from 3 exhibit anomalous Hall mobilities due to Ga island formation. The best thin films of GaAs were grown from the homoleptic precursor 4 at 480-degrees-C and 10(-5) Torr pressure. These thin films are n-type, of low carbon content, and exhibit an exciton peak in the 4.2 K photoluminescence spectra. Pyrolysis studies (10(-2) Torr pressure) have been carried out on 1-3. These studies confirm that the diarsine As2((i)Pr)4 is readily eliminated from 1. For both 2 and 3, the (t)Bu ligands begin to react at approximately 330-degrees-C with formation of both isobutane and isobutene. Methane elimination from 2 commences at 445-degrees-C. However, the Et groups of 3 begin to react at a significantly lower temperature (350-degrees-C), thus explaining the formation of Ga islands. Temperature programmed desorption (TPD) studies at 2 x 10(-10) Torr pressure yield similar results and, in addition, indicate that in the case of 4 the sequence of reactions is (t)Bu ligand desorption, beta-hydride elimination from (t)Bu2As moieties, followed by H-2 desorption and Ga-As bond cleavage. The potential use of single-source precursors for nanosize particle formation, access to metastable or unusual phases, selective deposition, and growth modification is also discussed.