Growth and Characterization of III-Nitride Quantum Dots and their Application to Emitters

Abstract

GaN and its alloys with InN and AlN are III-V semiconductor compounds having the wurtzite crystal structure and a direct energy bandgap which is suitable for optoelectronic applications, such as light emitting diodes and laser diodes. The relationship between the direct energy gap and the lattice constant (a) of wurtzite (Al,Ga,In)N is shown in Fig. 15.1. The recent discovery that InN has a bandgap of ∼0.7 eV (Lu et al. 2001, Walukiewicz et al. 2004, Monemar et al. 2005) indicates that the Al-Ga-In-N system encompasses a broad spectral region ranging from the deep UV to infra red (200 nm – 1770 nm). Another advantage of the nitride semiconductors over other wide bandgap semiconductor is the strong chemical bond which makes the material very stable and resistant to degradation under conditions of high electric current injection and intense light illumination. Quantum dots (QDs) can be used as the active region of opto-electronic devices, such as LEDs, lasers, modulators, and detectors. The fabrication methods of QDs can be classified into two major categories: the post-growth method and the selfassemble method. The post-growth lateral patterning of 2D quantum wells, as an indirect method, suffers from insufficient lateral resolution and interface damage caused by the patterning process. Self-organization of QDs on crystal surface is a more promising method to fabricate quantum dots. These techniques require large lattice mismatch between the growing layer and the substrate to achieve elastic relaxation. Specifically, elastic strain relaxation on facet edges and island interaction via the strained substrate are the driving forces for the self-organization of ordered arrays of uniform, strained islands on crystal surfaces. Stranski-Krastanov (SK) growth (with wetting layer, WL) and Volmer-Weber (VW) growth (without WL) in highly lattice-mismatched material systems like InAs-(Al)GaAs, GaSb-GaAs or InAs-Si are used to fabricate self-assembled 3D quantum dots. Both molecular beam epitaxy (MBE) and metal organic vapor-phase epitaxy (MOVPE) are currently used to grow QDs using the above mechanisms. GaN QDs grown on an AlN buffer follow the classical pattern of the StranskiKrastanov mode (the lattice mismatch between GaN and AlN is ∼2.5%), where the formation of a wetting layer is followed by the formation of GaN QDs (Xu