The recent debate over the bandgap energy of InN has been based on measurements of the wurtzite phase of the material. Several attempts have been made to grow the metastable zincblende phase on cubic substrates, but the tendency of the material to revert to the wurtzite phase has prevented a bandgap being assigned to the cubic phase. [1,2] Measurement of the zincblende bandgap of InN would shed light on the current bandgap energy debate as typically the zincblende phase of III-V semiconductors has a slightly narrower bandgap energy than the wurtzite phase. For example α-GaN (wurtzite) and β-GaN (zincblende) have bandgaps of 3.4 and 3.2 eV respectively, while α-AlN and β-AlN have gaps of 6.2 and 5.1 eV respectively.

Indium nitride thin films have been grown on GaN buffer layers atop sapphire substrates by plasma assisted molecular beam epitaxy. Two films were investigated using high resolution transmission electron microscopy and both were observed to be majority wurtzite, although isolated zincblende grains were also identified within both films. Quantifying the amount of zincblende phase within each film proved difficult due to the small sample sizes investigated within the high resolution scans. However, it was clear that one film had a significantly higher cubic content and corresponded to n-type carrier concentrations and Hall mobility of 3 × 10¹⁹ cm^-3 and 200 cm²/Vs, respectively, as opposed to 1 × 10¹⁹ cm^-3 and 500 cm²/Vs for the lower cubic content. Photoluminescence studies revealed strong emission near 0.7 eV from both films, but the higher cubic content PL exhibited a low energy shoulder at 0.63 eV. The emission at the low energy shoulder persisted at comparable intensities to the 0.7 eV peak at temperatures from 10 to 300 K.

[1] A. Tabata, A. P. Lima, L. K. Teles et al. Appl. Phys. Lett. 74, 362 (1999)

[2] V. Cimalla, J. Pezoldt, G Ecke et al. Appl. Phys. Lett. 83, 3468 (2003)

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