Thermal Atomic Layer Deposition of Polycrystalline Gallium Nitride

We report the successful preparation of polycrystalline gallium nitride (poly-GaN) layers by thermal atomic layer deposition (ALD) at low temperatures (375–425 °C) from trimethylgallium (TMG) and ammonia (NH3) precursors. The growth per cycle (GPC) is found to be strongly dependent on the NH3 pulse duration and the NH3 partial pressure. The pressure dependence makes the ALD atypical. We propose that the ALD involves (i) the reversible formation of the hitherto-unreported TMG:NH3 surface adduct, resulting from NH3 physisorbing on a TMG surface site and (ii) the irreversible conversion of neighboring surface adducts to Ga–NH2–Ga linkages. The pressure dependence arises from the presumed reversible nature of the adduct formation on the surface, equivalent to the known reversible nature of its formation in the gas phase in metal organic chemical vapor deposition reactions. Using in situ spectroscopic ellipsometry (SE), the GPC monitored as a function of several ALD parameters is as high as 0.1 nm/cycle at 60 ...