A. O. Ajagunna

InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy

Single crystalline and single phase In(x)Ga(1-x)N nanopillars were grown spontaneously on (111) silicon substrate by plasma assisted molecular beam epitaxy. The surface morphology, structural quality, and optoelectronic properties of InGaN nanopillars were analyzed using scanning electron microscopy (SEM), energy dispersive x-ray (EDXA) analysis, high resolution x-ray diffraction (HR-XRD), and both room and low temperature photoluminescence spectra. The EDXA results showed that these nanopillars were composed of InGaN and the amount of indium incorporation in In(x)Ga(1-x)N NPs could be controlled by changing the growth temperature. The room temperature and low temperature PL spectra revealed that the emission wavelength could be tuned from a blue to green luminescent region depending on the growth temperature. The wavelength tuning was attributed to a higher amount of In incorporation at a lower growth temperature which was consistent with the EDXA and HR-XRD results.

Epitaxial growth, electrical and optical properties of a-plane InN on r-plane sapphire

The heteroepitaxy of a-plane (112¯0) InN films on r-plane (11¯02) sapphire substrates, by nitrogen radio frequency plasma-assisted molecular beam epitaxy, has been investigated and compared to that of c-plane (0001) InN. The epitaxial growth of a-plane InN proceeded through the nucleation, growth, and coalescence of three-dimensional islands, resulting in surface roughness that increased monotonically with epilayer thickness. The full width at half maximum of (112¯0) x-ray diffraction rocking curves decreased significantly with increasing InN thickness, characteristic of structural improvement, and it reached the value of 24 arcmin for a 1 μm thick film. Hall-effect measurements exhibited a similar dependence of electron concentration and mobility on thickness for both the a- and c-plane InN films. The analysis of the Hall-effect measurements, by considering the contribution of two conducting layers, indicates a similar accumulation of low mobility electrons with Ns>1014 cm−2 at the films’ surface/interfa...

Growth mechanism and microstructure of low defect density InN (0001) In-face thin films on Si (111) substrates

Transmission electron microscopy has been employed to analyze the direct nucleation and growth, by plasma-assisted molecular beam epitaxy, of high quality InN (0001) In-face thin films on (111) Si substrates. Critical steps of the heteroepitaxial growth process are InN nucleation at low substrate temperature under excessively high N-flux conditions and subsequent growth of the main InN epilayer at the optimum conditions, namely, substrate temperature 400–450 °C and In/N flux ratio close to 1. InN nucleation occurs in the form of a very high density of three dimensional (3D) islands, which coalesce very fast into a low surface roughness InN film. The reduced reactivity of Si at low temperature and its fast coverage by InN limit the amount of unintentional Si nitridation by the excessively high nitrogen flux and good bonding/adhesion of the InN film directly on the Si substrate is achieved. The subsequent overgrowth of the main InN epilayer, in a layer-by-layer growth mode that enhances the lateral growth o...

Optical characterization of free electron concentration in heteroepitaxial InN layers using Fourier transform infrared spectroscopy and a 2 × 2 transfer-matrix algebra

Fourier Transform Infrared (FTIR) reflectance spectroscopy has been implemented as a non-destructive, non-invasive, tool for the optical characterization of a set of c-plane InN single heteroepitaxial layers spanning a wide range of thicknesses (30–2000 nm). The c-plane (0001) InN epilayers were grown by plasma-assisted molecular beam epitaxy (PAMBE) on GaN(0001) buffer layers which had been grown on Al2O3(0001) substrates. It is shown that for arbitrary multilayers with homogeneous anisotropic layers having their principal axes coincident with the laboratory coordinates, a 2 × 2 matrix algebra based on a general transfer-matrix method (GTMM) is adequate to interpret their optical response. Analysis of optical reflectance in the far and mid infrared spectral range has been found capable to discriminate between the bulk, the surface and interface contributions of free carriers in the InN epilayers revealing the existence of electron accumulation layers with carrier concentrations in mid 1019 cm−3 at both t...

Self-annihilation of inversion domains by high energy defects in III-Nitrides

Low-defect density InN films were grown on Si(111) by molecular beam epitaxy over an ∼1 μm thick GaN/AlN buffer/nucleation layer. Electron microscopy observations revealed the presence of inverse polarity domains propagating across the GaN layer and terminating at the sharp GaN/InN (0001¯) interface, whereas no inversion domains were detected in InN. The systematic annihilation of GaN inversion domains at the GaN/InN interface is explained in terms of indium incorporation on the Ga-terminated inversion domains forming a metal bonded In-Ga bilayer, a structural instability known as the basal inversion domain boundary, during the initial stages of InN growth on GaN.

Angular-dependent Raman study of a- and s-plane InN

Angular-dependent polarized Raman spectroscopy was utilized to study nonpolar a-plane ( 11¯20) and semipolar s-plane ( 101¯1) InN epilayers. The intensity dependence of the Raman peaks assigned to the vibrational modes A1(TO), E1(TO), and E2h on the angle ψ that corresponds to rotation around the growth axis, is very well reproduced by using expressions taking into account the corresponding Raman tensors and the experimental geometry, providing thus a reliable technique towards assessing the sample quality. The s- and a-plane InN epilayers grown on nitridated r-plane sapphire (Al2O3) exhibit good crystalline quality as deduced from the excellent fitting of the experimental angle-dependent peak intensities to the theoretical expressions as well as from the small width of the Raman peaks. On the contrary, in the case of the s-plane epilayer grown on non-nitridated r-plane sapphire, fitting of the angular dependence is much worse and can be modeled only by considering the presence of two structural modificat...