We studied the effect of different deposition conditions on the properties of multiple InN orientations grown by plasma-assisted molecular beam epitaxy (PA-MBE).  We found that the optimized growth conditions for In-face, N-face, m-plane, and a-plane oriented InN films generally include an In-wetting layer on the InN surface during growth (III/V ratio ≥ 1).  Optimized InN growth temperatures for the different orientations, however, were somewhat different.  N-face InN had the lowest thermal decomposition and was deposited at the highest growth temperatures (up to 620°C) compared to the other InN orientations.  In-face InN had optimal deposition temperatures somewhat lower than N-face InN (~ 470°C), with complete thermal decomposition around 500°C.  Finally, both the non-polar m- and a-plane InN orientations were grown at about 400°C and showed no growth beyond 430°C.

The origin of bulk electron concentrations in In-face InN was studied by considering the effects of both unintentionally incorporated impurities and threading dislocation densities on the electron transport properties of InN.  The concentrations of the unintentionally incorporated impurities oxygen and hydrogen scaled with the bulk electron concentrations while TDs had no observed effect on the electron concentrations.  We conclude that impurities were the significant source of electrons and TDs acted only as scattering centers limiting the mobility of electrons in InN.  Using the optimized growth conditions, unintentionally doped In-face InN films have been realized with record room temperature as-measured electron mobilities as high as 2900 cm²/Vs (3750 cm²/Vs bulk electron mobility as determined by 2-layer Hall analysis) and free electron concentrations as low as 2.3 x 10¹⁷ cm^-3 (6.6 x 10¹⁶ cm^-3 bulk concentration by 2-layer Hall).

The optical properties of the different InN orientations were measured by reflectance, transmission, and photoluminescence (PL).  The measured band gap values for the different InN orientations were in good agreement and ranged from 0.65 eV to 0.70 eV.  The PL peak wavelength was red-shifted by ~70meV with respect to the band gap energy for m- and a-plane orientations, and only ~25meV for the c-plane orientations.  This disparity is most likely due to increased band tail states from stacking faults and increased impurity incorporation for the non-polar samples due to the lower growth temperatures.

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