Abstract
We report the catalyst free growth of wurtzite InN nanorods (NRs) and microislands on bare Si (111) by plasma-assisted molecular beam epitaxy at various temperatures. The morphological evolution from NRs to three dimensional (3D) islands as a function of growth temperature is investigated. A combination of tapered, non-tapered, and pyramidal InN NRs are observed at 490 °C, whereas the InN evolves to faceted microislands with an increase in growth temperature to 540 °C and further developed to indented and smooth hemispherical structures at extremely high temperatures (630 °C). The evolution from NRs to microislands with increase in growth temperature is attributed to the lowering of the surface free energy of the growing crystals with disproportionate growth velocities along different growth fronts. The preferential adsorption of In atoms on the (0001) c-plane and (10-10) m-plane promotes the growth of NRs at relatively low growth temperature and 3D microislands at higher temperatures. The growth rate imbalance along different planes facilitates the development of facets on 3D microislands. A strong correlation between the morphological and structural properties of the 3D films is established. XRD studies reveal that the NRs and the faceted microislands are crystalline, whereas the hemispherical microislands grown at extremely high growth temperature contain In adlayers. Finally, photoluminescent emissions were observed at ∼0.75 eV from the InN NRs.
Highlights
Among group III nitride wide band-gap semiconductors, indium nitride (InN) has attracted attention due to its unique properties such as low effective mass, high carrier mobility, relatively high absorption coefficient and narrow band gap energy (0.7–0.9 eV) [1, 2]
We report the initial stages of InN NRs growth, the morphological evolution from NRs to microislands at high growth temperatures >490 °C, as well as the detailed properties, geometry and crystal structure
All the InN films were grown at similar N/In beam equivalent pressure (BEP) ratio (≈300) but at different growth time and substrate temperatures (Tg) which were measured by a thermocouple
Summary
Among group III nitride wide band-gap semiconductors, indium nitride (InN) has attracted attention due to its unique properties such as low effective mass, high carrier mobility, relatively high absorption coefficient and narrow band gap energy (0.7–0.9 eV) [1, 2]. InN holds enormous potential in various device applications including lasers, high-speed field-effect transistors [3] and nanogenerators [4]. Despite this huge potential, the growth of InN has been very challenging due to its low decomposition temperature, the high equilibrium vapour pressure of nitrogen over indium and the shortage of lattice-matched substrates. NRs have a number of advances including large surface-to-volume ratio, high density of electronic states, diameter-dependent band gap, enhanced optical absorption and dislocation-free structures on foreign substrates [7, 8].
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