No abstract

The influence of dislocations on electron transport properties of undoped InN thin films grown by molecular-beam epitaxy on AlN͑0001͒ pseudosubstrates is reported. The microstructure and the electron transport in InN͑0001͒ films of varying thickness were analyzed by transmission electron microscopy and variable temperature Hall-effect measurements. It was found that crystal defects have strong effects on the electron concentration and mobility of the carriers in the films. In particular, the combined analysis of microscopy and Hall data showed a direct dependence between free carrier and dislocation densities in InN. It was demonstrated that threading dislocations are active suppliers of the electrons and an exponential decay of their density with the thickness implies the corresponding decay in the carrier density. The analysis of the electron transport yields also a temperature-independent carrier concentration, which indicates degenerate donor levels in the narrow band-gap InN material. The relative insensitivity of the mobility with respect to the temperature suggests that a temperature-independent dislocation strain field scattering dominates over ionized impurity/defect and phonon scattering causing the increase of the mobility with rising layer thickness due to the reducing dislocation density. Room temperature mobilities in excess of 1500 cm 2 V −1 s −1 were obtained for ϳ800 nm thick InN layers with the dislocation densities of ϳ3 ϫ 10 9 cm −2 .

Articles you may be interested inImpact of varying buffer thickness generated strain and threading dislocations on the formation of plasma assisted MBE grown ultra-thin AlGaN/GaN heterostructure on silicon AIP Advances 5, 057149 (2015); 10.1063/1.4921757 Improved electron mobility in InSb epilayers and quantum wells on off-axis Ge (001) substratesThe strain-relaxation phenomena and the formation of a dislocation network in 2H-InN epilayers during molecular beam epitaxy are reported. Plastic and elastic strain relaxations were studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. Characterization of the surface properties has been performed using atomic force microscopy and photoelectron spectroscopy. In the framework of the growth model the following stages of the strain relief have been proposed: plastic relaxation of strain by the introduction of geometric misfit dislocations, elastic strain relief during island growth, formation of threading dislocations induced by the coalescence of the islands, and relaxation of elastic strain by the introduction of secondary misfit dislocations. The model emphasizes the determining role of the coalescence process in the formation of a dislocation network in heteroepitaxially grown 2H-InN. Edge-type threading dislocations and dislocations of mixed character have been found to be dominating defects in the wurtzite InN layers. It has been shown that the threading dislocation density decreases exponentially during the film growth due to recombination and, hence, annihilation of dislocations, reaching ϳ10 9 cm −2 for ϳ2200 nm thick InN films.

A model for the influence of different contributions to the high electron concentration in dependence on the film thickness of state-of-the-art InN layers grown by molecular-beam epitaxy is proposed. Surface accumulation has a crucial influence for InN layers <300nm and superimposes the background concentration. For air-exposed InN, it can be assigned to a surface near doping by oxygen. For InN layers in the micron range the density of dislocations is the major doping mechanism. Finally, point defects such as vacancies and impurities have minor influence and would dominate the free electron concentration only for InN >10μm.