Topological defects and low-dimensional nitride layers

Abstract A large research effort is going on with wurtzite nitride semiconductors, due to their large direct bandgap which provides a possibility for optoelectronic devices from red to the ultraviolet range [1]. The bandgaps of InxGa1_xN alloys cover a wide spectral range, from red (InN: l.89eV) to UV (GaN: 3.SeV), and that of the AlGaN goes to deep UV(AlN: 6.2eV). As expected, the most difficult part is InGaN, due mainly to the large mismatch (10%) between GaN and InN. In this case, ordering [2], phase separation [3], and growth instabilities have been reported [4]. For AlN and GaN, the misfit is smaller (ce2.5%) and one would expect more stable growth. The growth of quantum wells (QWs) appears to follow the 2D growth mode and that of high-quality quantum dots (QDs) has been achieved [5]. However, it was in this system that ordering along the c-axis was reported in AlGaN by Korakakis et al. [6], where the Ga and Al atoms separately occupy two simple hexagonal sublattices of the wurtzite system, giving rise to the lower symmetry P3ml space group. Active areas in visible GaN-based electroluminescent diodes (ELDs) and laser diodes (LDs) consist mainly of InGaN/GaN multiple quantum wells. Results have been published concerning the phase separation which was mainly supposed to give rise to the formation of layers composed of InN and GaN patches. This was first reported in polycrystalline InGaN films that had been annealed at temperatures below 700 °C [7]. More recently, it was shown that a ternary layer grown by molecular beam epitaxy (MBE) with In fraction larger than 30% contains regions of pure In, but atomic In concentrations up to 80% could be incorporated without phase separation in GaN/InGaN/GaN heterostructures [3]. This was tentatively explained by the fact that the two compounds have a large mismatch of lattice parameters (-10% ). In fact, the growth of InGaN layers may lead to the formation of the ternary alloy InxGa1_xN, InN and GaN, In, and finally in some cases to a ternary ordered alloy. Depending on the growth conditions one may obtain a combination of the various cases, along with the characteristic crystallographic defects.