Despite of remarkable improvements in crystal quality during the last years, as-grown InN still exhibits a strong n-type tendency and considerably high defect concentrations [1]. Concerning native defects, recent calculations [2] predict In-vacancies to be the most abundant defect type in n-type InN and possess the tendency for forming complexes with other intrinsic defects or impurities [3], which raises the question of their impact on the materials properties. In this contribution we present results obtained with positron annihilation spectroscopy in MBE-grown Si-doped InN with electron concentrations from 1 x 10¹⁸ cm^-3 to 6.6 x 10²⁰ cm^-3. Temperature dependent Doppler Broadening measurements show In-vacancies in concentrations from 2 x 10¹⁶ cm^-3 to 7 x 10¹⁷ cm^-3 which increase with increasing electron concentration in the material. These values are remarkably higher than what can be estimated from theoretically predicted formation energies of In-vacancies in InN based on experimentally determined Fermi levels in the investigated samples [4]. Additionally, a strong inhomogeneity of the defect distribution with an increasing vacancy concentration towards the layer/substrate-interface can be observed. These results suggest that the formation energy of In-vacancies is locally reduced in the vicinity of extended defects, which are especially abundant in the interface area - possibly through the formation of defect complexes. From the observed threshold for the formation of In-vacancies, which is seen by a comparison with earlier results in He-irradiated InN [5], it can be deduced that in spite of their still relatively low absolute concentrations In-vacancies play an important role in reducing the electron mobility in the material.

[1] B. Monemar et al., Phys. Stat. Sol. B 244, 1759-1768 (2007)

[2] C. Stampfl et al., Phys. Rev. B 61, R7846-R7849 (2000)

[3] X. M. Duan et al., Phys. Rev. B 79, 035207 (2009)

[4] P. D. C. King et al., Phys. Rev. B 77, 045316 (2008)

[5] F. Tuomisto et al., Phys. Rev. B 75, 193201 (2007)

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