Effects of stoichiometry on electrical, optical, and structural properties of indium nitride

A series of indium nitride (InN) epilayers with different excess indium (In) concentration are grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates. The increasing excess In concentration of the epilayers correlates with an increasing free-electron concentration and a decreasing electron mobility. Photoluminescence (PL) illustrates a 0.77-0.84 eV transition for all samples with a redshift in the peak energy with increasing In concentration (for the highest free-electron concentration of 4× 1021 cm-3). This suggests that the ∼0.8 eV PL transition is not consistent with the band-edge transition in InN. Moreover, an additional PL transition at 0.75 eV along with the In clusters observed in transmission electron microscopy analysis are found only in the 29% excess In sample. This implies a relationship between the new PL transition and the presence of In clusters. Finally, secondary-ion mass spectrometry is used to verify that the contamination, especially hydrogen (H) and oxygen (O) impurities, has no influence on the redshift of the ∼0.8 eV PL peaks and the existence of the additional 0.75 eV peak in the sample containing In clusters.