PA-MBE Growth and Characterization of Nitride Semiconductors, from InGaN Thin-films to GaN and AlN Self-assembled Nanowires

In this thesis the Plasma-Assisted Molecular Beam Epitaxy (PA-MBE) growth and characteristics of different families of III-N semiconductors are discussed. The two main issues preventing realization of III-N full potential are addressed to be the difficulty in obtaining high-In-composition InGaN crystals, and the existence of huge densities of defects in heteroepitaxially-grown III-N semiconductors. The prophecy of this thesis is to remedy these two issues. As an effective solution to obtain high-In-composition InGaN crystals, the growth dynamics of Ga(In)N semiconductors by PA-MBE at low temperatures (T = 450 °C) is investigated. The presence of droplets at the growth surface is found to strongly affect the adatom incorporation dynamics, hence making the growth rate a decreasing function of the metal flux impinging on the surface. This phenomenon is explained via a model taking into account the droplet effects on the incorporation of metal adatoms into the crystal. A relevant role is played by the vapor-liquid-solid (VLS) growth mode that takes place under the droplets due to nitrogen molecules directly impinging on the droplets. An inverse relation between the surface covered by metal droplets and Indium incorporation into the crystal is observed. As soon as metal droplets generate on the surface, a drastic reduction of In incorporation occurs and it continues by increasing the surface coverage by metal droplets. Raman spectroscopy as a fast and simple alternative method for XRD and PL, with ability of local measurement, can be used to measure the InGaN composition. In this study, by filling the gap of data in literature for the x values (Indium percentage) between 30%-65%, it is found that in contrary to the linear dependence of A1 (LO) with In fraction reported in the literature, in the central zone of InGaN it follows a polynomial relationship with a bowing toward higher frequencies in the middle region. At the end of this part, it is shown that by precisely controlling of III/V ratio close to unity, there is a very narrow window where it is possible to obtain high-quality, composition controlled, droplet free, and flat In0.5Ga0.5N single crystal on Silicon substrates. Regarding to the common problem with existence of huge densities of defects in heteroepitaxially-grown III-N semiconductors, growing nanowires (NWs) instead of thin-films, seems a promising solution as they can release the epitaxial strain through a lateral plastic relaxation. So in the next part of this thesis, GaN and AlN NWs growth on TiNx sputtered surface are investigated, GaN NW ensembles are synthesized by MBE on in situ prepared TiNX films. NW nucleation is observed for TiNx films deviating from the stoichiometry (x=0.94) and results in an ensemble free of coalescence. The photoluminescence spectrum of the NWs is dominated by the donor-bound exciton with a linewidth below 2 meV. The donor-bound exciton lifetime exceeds 500 ps, which is significantly longer than the typical lifetime reported for NWs on Si. Benefiting from TiN high thermal stability, high-temperature growth of self-assembled AlN NWs on TiN film is studied. Nearly free of coalescence AlN NWs are grown on the TiN film at 1180 °C. Raman spectroscopy and X-ray diffraction confirm the absence of degradation of the TiN film during the high temperature growth. The AlN NWs are single-crystalline and free of extended defects. The cathodoluminescence (CL) of the AlN NWs is dominated by the band-edge emission at 10 K. The TiN film shows metallic behavior, which can be used as a back contact or a mirror.