CRITERIA FOR THE DESIGN OF MONOALKYLPHOSPHINE PRECURSORS FOR INP METALORGANIC VAPOR-PHASE EPITAXY

We have studied the reaction between n-butylphosphine and sec-butylphosphine with trimethylindium in the dihydrogen atmosphere of an InP metalorganic chemical vapour deposition (MOCVD) reactor using Fourier transform infrared (FTIR) spectroscopy, since these precursors are structurally related to tertiarybutyl phosphine, an established InP MOCVD precursor. It is found that at room temperature under conditions designed to amplify precursor interactions an undesirable pre-reaction occurs between sec-butylphosphine and TMIn resulting in a clear, viscous, involatile liquid. For n-butylphosphine a similar reaction is observed but to a much lesser extent than that for the case of the sec-butylphosphine. We have further characterised the structure of the involatile product formed in the case of sec-butylphosphine using FTIR spectroscopy and have assigned this structure to that of a polymer of the type (Me-In-P-sBu)(n). We have made a comparison of these findings with data previously reported from our laboratories for the precursors tertiarybutylphosphine, cyclohexylphosphine, cyclopentylphosphine, benzylphosphine and cyanoethylphosphine. With the exception of tertiarybutylphosphine all of the precursors studied give rise to an undesirable pre-reaction with TMIn resulting in an involatile product. This is believed to be a quite general phenomenon for this system with the polymer having the generic structure (Me-In-P-R)(n). The uniqueness of tertiarybutylphosphine has been examined in the light of these data using a series of proposed reaction steps and simple molecular modelling and it is suggested that the key to the lack of pre-reaction in the case of tertiarybutlyphosphine lies in a combination of its base strength and structure (steric effects) in relation to the other phosphines studied.