Characterization of oxygen and carbon in undoped AlGaAs grown by organometallic vapor-phase epitaxy

We have systematically characterized oxygen (O) and carbon (C) impurities in undoped gallium aluminum arsenide (AlxGa1-xAs) epitaxial layers grown by organometallic vapor-phase epitaxy (OMVPE). The concentrations of O and C impurities are evaluated by secondary-ion mass spectroscopy. For x less than or equal to 0.63 the increase rate of O concentration, [O], with x is nearly proportional to the flow rate of trimethylaluminum (TR/IA), indicating that the oxygen is contained in the TMA molecules. In contrast, [O] and [C] increase superlinearly with x for x greater than or equal to 0.83 and particularly, [O] exceeds 10(18) cm(-3). The vibration mode of Al-O bonds is observed at 900-1050 cm(-1) for the x=0.83 sample using Fourier-transformation infrared measurements. The superlinear increase of [O] with x is attributed to the increased adsorption of residual O-2 or H2O molecules. The mechanism of the superlinear increase is discussed in terms of statistic consideration of the -Al- arrangement on the AlxGa1-xAs surface. Hall measurements show that the hole concentration markedly decreases for x >0.63 and the AlxGa1-xAs layer becomes semi-insulating, which was attributed to hole compensation by the O-related deep hole traps. The hole mobility also decreases in the same x range.