Abstract
We have systematically characterized oxygen (O) and carbon (C) impurities in undoped gallium aluminum arsenide ( Alx Ga1-x As) 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≤0.63 the increase rate of O concentration, [O], with x is nearly proportional to the flow rate of trimethylaluminum (TMA), indicating that the oxygen is contained in the TMA molecules. In contrast, [O] and [C] increase superlinearly with x for x≥0.83, and particularly, [O] exceeds 1018 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 O2 or H2O molecules. The mechanism of the superlinear increase is discussed in terms of statistic consideration of the –Al– arrangement on the Alx Ga1-x As surface. Hall measurements show that the hole concentration markedly decreases for x>0.63 and the Alx Ga1-x As 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.
Published Version
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