Molecular beam epitaxy growth and characterization of GaN, AlN and AlGaN/GaN heterostructures

Doctoral thesis, 2005

The III-nitride materials, consisting of Al, Ga, In and N, have several physical properties that make them attractive for semiconductor devices in electronic and optic applications. A wide range of direct energy bandgaps (0.7 eV - 6.1 eV) can be obtained. By utilizing this energy span light emitting diodes of all colors and lasers with blue and ultra-violet light have been made. In electronic applications the good electron mobility and the high breakdown field of GaN and AlN make it possible to design high speed and high power diodes and transistors. AlGaN/GaN heterostructure field effect transistors have exhibited very high power densities at microwave frequencies. The III-nitride materials are grown by epitaxial techniques. Sapphire is the most commonly used substrate, even though the lattice mismatch is large (13% to AlN). During the initial growth the difference in physical properties causes strain and as the layer relaxes dislocations are formed. The lattice disorder may propagate through the consecutive layers and thereby effecting device performance. Hence the understanding of initial growth steps are decisive for the growth of high quality layers. In this thesis GaN and AlN layers and AlGaN/GaN heterostructures have been grown on sapphire by molecular beam epitaxy. The material transition from sapphire to a III-nitride was investigated in detail. To facilitate growth the sapphire surface was pre-treated by annealing, degreasing, gallium cleaning and nitridation. The annealing provided a smooth surface with atomic steps, suitable for epitaxial growth. During the nitridation a surface atom periodicity close to AlN was formed (mismatch <2.5 %). Hence the sapphire was transformed to give better conditions for the initial growth of a thin AlN nucleation layer. The thickness and growth conditions of the nucleation layer are decisive to the quality of the epitaxial layers. Our results show that the best GaN and AlN layers are obtained for approximatelly 5 nm thick AlN nucleation layers grown at nitrogen rich conditions. The crystalline quality expressed as the full width half maximum (FWHM) of X-ray diffraction rocking curves. FWHM values less than 100 arcsec have been obtained for both GaN(0002) and AlN(0002). The surface morphology was characterized by atomic force microscopy and the roughness, calculated as the root-mean-square value of the surface height, was less than 1 nm for the GaN and AlN layers. AlGaN/GaN heterostructures and the two-dimensional electron gas (2DEG) formed at the interface were investigated. The 2DEG sheet density was studied with respect to AlGaN layer thickness Al-content and background doping. Our experimental and simulated results showed that the density primarily is determined by the Al-content and that the background doping can be used to explain very high sheet densities. To understand the mobility in the 2DEG system various scattering mechanisms were investigated by calculations. These showed that at temperatures below 50 K the ionized impurity, line dislocation and interface roughness scattering limited the mobility. At room temperature the optical phonon scattering was predominant. Also, for samples having a rough interface between the AlGaN top layer and the GaN channel, the roughness scattering can significantly effect the mobility at high temperatures.