Inhomogeneous free-electron distribution in InN nanowires: Photoluminescence excitation experiments

Abstract

Photoluminescence excitation ͑PLE͒ spectra have been measured for a set of self-assembled InN nanowires ͑NWs͒ and a high-crystalline quality InN layer grown by molecular-beam epitaxy. The PLE experimental lineshapes have been reproduced by a self-consistent calculation of the absorption in a cylindrical InN NW. The differences in the PLE spectra can be accounted for the inhomogeneous electron distribution within the NWs caused by a bulk donor concentration ͑N D + ͒ and a two-dimensional density of ionized surface states ͑N ss + ͒.For NW radii larger than 30 nm, N D + and N ss + modify the absorption edge and the lineshape, respectively, and can be determined from the comparison with the experimental data.

Probing the electrical transport properties of intrinsic InN nanowires

“…15,32 In this derivation, the carrier distribution is assumed to be uniform across the radial direction of nanowires, due to the absence of surface electron accumulation, 45 which is in contrast to the n-type degenerate InN nanowires where high density of electrons are accumulated on the nanowire surfaces. 8,10,12,13 The derived free carrier concentration increases significantly with decreasing the nanowire radius following n 0 / r À2:3 6 0:2 . Such radius dependent free carrier concentration is also observed in InAs nanowires.…”

“…7,8 This significantly reduced electron mobility, compared to the theoretical prediction, has been attributed to the presence of extremely high free carrier concentration and large dislocation density, 6,9 as well as the surface electron accumulation. 8,[10][11][12][13] Recently, dislocation-free InN nanowires with the absence of surface electron accumulation and extremely low free carrier concentration (on the order of 10 15 cm À3 , or less) have been achieved by the drastically improved molecular beam epitaxial growth process, wherein an in situ deposited In seeding layer was utilized prior to the growth initiation, to promote the nucleation and formation of InN nanowires. 14,15 There is, therefore, an urgent need to develop a fundamental understanding of the charge carrier transport properties and their dependence on the nanowire dimensions in such intrinsic InN nanowires.…”

“…The free carrier concentration is in the range of 10 13 to 10 15 cm À3 (depending on the nanowire radius), compared to the commonly reported values of 10 17 to 10 18 cm À3 , or even higher, in nominally undoped InN thin films 6,31 and tapered nanowires. 8,12,13 It is further derived that the charge traps distribute exponentially just below the conduction band edge, with a characteristic energy $65 meV.…”

Understanding the role of Si doping on surface charge and optical properties: Photoluminescence study of intrinsic and Si-doped InN nanowires

“…[16][17][18] To date, however, the practical device applications of InN-based materials have been severely limited by the presence of extremely large residual electron density and the uncontrolled surface charge properties, as well as the difficulty in achieving p-type conductivity. 8,[19][20][21][22][23][24][25][26][27][28][29][30] For example, in general, the currently reported nominally undoped InN is n-type degenerate, with the residual electron densities in the range of ∼ 1 × 10 18 cm −3 , or higher. 8,11,25,[31][32][33][34] Moreover, it has been generally observed that there exists a very high electron concentration (∼ 1 × 10 13−14 cm −2 ) at both the polar and nonpolar grown surfaces of InN films, 19,35 and the Fermi-level (E F ) is pinned deep into the conduction band at the surfaces; 19,20,29,30 similar electron accumulation profile has also been measured at the lateral nonpolar grown surfaces of [0001]-oriented wurtzite InN nanowires.…”

“…7,9,40 Meanwhile, significant efforts have also been made to study the optical properties of InN nanowires. 8,11,21,24,28,33,45 The PL spectra of nominally undoped InN nanowires typically exhibit a very large inhomogeneous broadening, a peak energy considerably larger than the bandgap of InN, as well as an anomalous dependence on temperature. Due to these strong n-type characteristics of nominally undoped InN nanowires, the optical properties of intrinsic InN nanowires, and the dependence on the charge carrier concentrations and surface charge properties had remained not clear.…”