Complex Formation between Magnesocene (MgCp2) and NH3:  Implications for p-Type Doping of Group III Nitrides and the Mg Memory Effect

Magnesocene (biscyclopentadienylmagnesium) is a common precursor used for the p-type doping of GaN and other group III nitride materials. Unfortunately, difficulties remain with predictably controlling the incorporation of Mg during metal organic chemical vapor deposition (MOCVD) film growth, which often exhibits poorly understood “memory effects.” Although the formation of a reaction product between magnesocene and ammonia has been previously speculated, one has never been experimentally isolated or identified. We have spectroscopically observed and identified, for the first time, the adducts formed between magnesocene and ammonia. Density functional theory (DFT) quantum chemistry calculations have also been performed on the system to determine the structures and energetics of the reaction products. It was found that ammonia can form condensable Lewis acid−base complexes with magnesocene in both 1:1 and 2:1 ratios (i.e., NH3−MgCp2 and (NH3)2−MgCp2) via nucleophilic attack of NH3 at the positively charged Mg center of MgCp2. Adduct formation is reversible, and the 1:1 and 2:1 products can be converted to one another by controlling the NH3 partial pressure. The formation and condensation of both adducts at room temperature is the probable parasitic source that leads to many of the observed Mg incorporation difficulties during the p-type doping of group III nitride materials.