Monolayer P-type doping in silicon using gallium and boron coordinate complexes

Conformal and controlled semiconductor doping is needed for applications in next generation nanoscale devices. Molecular monolayer doping (MLD) in silicon is a novel technique based on the formation of a self-assembled monolayer of dopant molecules. A single monolayer effectively controls the dopant concentration that can incorporate into the silicon lattice following thermal annealing. As a result, MLD is capable of forming ultra-shallow junctions with high atomic accuracy, and minimum defects in silicon. In addition to our recent work in n-type doping using literature-reported phosphorous systems, we have demonstrated, for the first time, the application of gallium p-type doping using the MLD approach. Tris(2,4-pentanedionato)gallium (III) was synthesized, and used to dope HF-etch silicon wafers. Secondary ion mass spectroscopy (SIMS) results confirmed gallium doping with a 0.3µm junction depth in silicon at a surface concentration of 10¹⁹ atoms/cm³. Boron doping, on the other hand, currently uses an expensive allylboronic acid pinacol ester complex. Using insight into the mechanism of action between the dopant and hydrogen-terminated silicon surface, we have demonstrated doping equivalent to, or exceeding, the pinacol ester using significantly cheaper boron alkoxide systems. Trimethoxyborate, for example, was used to form a shallower 0.2mm junction with a surface concentration of about 10¹⁸ atoms/cm³. In order to optimize and control the junction depth, a series of gallium and boron dopant complexes were synthesized and characterized. Ligands, with a range of electron withdrawing and donating character, were selected to gain further understanding of the nature of bond formation between the dopant functional group(s) and the silicon surface. A facile preparation of a p-type dopant complex containing unsaturated ligand functionality capable of free-radical mediated bonding will be presented. Future plans to move p- and n-type doping out of the cleanroom environment will be discussed.