InN and In-rich InGaN Nanocolumns by Molecular Beam Epitaxy

Abstract

The focus of this thesis is on InN and InGaN semiconductor-nanowires. The aim is to gain a deeper understanding of the electronic properties of InN nanowires and the growth mechanisms of InN and InGaN nanowires by molecular beam epitaxy (MBE) on silicon. The chemical, structural and electrical properties of InN nanowires were analyzed. The combination of optical and electrical measurements resulted in a consistent picture of electron concentration and distribution. The cores of the wires showed a for InN low electron concentration in the range of 3.5x10^(17) cm^(-3) to 2x10^(18) cm^(-3) and high electron mobility of about 2400-4800 cm^2 /(Vs). This is comparable to layers with the lowest carrier concentration reported in literature. An electron accumulation was found at the interface of an oxide outer shell and the InN core. The electron surface density was about 1 to 2x10^(13) cm^(-2) and the corresponding volume mobility is considerably lowered with about 600 cm^2 /(Vs) compared to the core. Furthermore, In-rich InGaN nanowires were successfully synthesized. The Ga:In ratio of the wires was significantly lower than the ratio of the impinging fluxes. The distributions of In and Ga on the samples were explained by a diffusion growth model assuming a diffusion length which is considerably larger than the next-neighbor-distance of the wires for In and close to zero for Ga. In addition, InGaN markers were introduced into InN nanowires to monitor their growth rates which were found to be nearly equal for all wires on a sample and stepwise constant in time. The changes in growth rates were assigned to nucleation of new nanowires. Furthermore, first steps towards position- and size-controlled growth of single InN nanowires on silicon were presented using a mask approach.