Scanning transmission electron microscopy study of III-V nitrides

A study of the electronic properties of the III-V nitrides and the characterization of nitride-based heterostructures, interfaces and quantum wells using scanning transmission electron microscopy (STEM) are presented. Examination of the epitaxially grown GaN/Al_0.25Ga_0.75N heterostructure shows that, in contrast to expectations, the interface between GaN buffer and Al_0.25Ga_0.75N barrier is not atomically sharp, but diffuse. High spatial resolution electron energy loss spectroscopy measurements and annular dark field imaging indicate that these interfaces can be up to 20 Å wide. The effects of the presence of the diffuse interface between the GaN and Al_xGa_{1− x}N layers on formation of the quasi-two-dimensional electron gas at the heterointerface are studied. Electronic energy levels and the distribution of these highly localized electrons at the interface are calculated for different interface widths and for various physical parameters of the structure.; The long-range and atomic level uniformities of the GaN quantum wells grown in an AlN matrix are characterized using electron energy loss spectroscopy and annular dark field imaging. The effects of the incident electron beam broadening inside the specimen on STEM measurements are discussed and mechanisms to minimize them are suggested. For quantitative correlation, the measured intensity of the nitrogen K-edge is compared with the propagating beam intensity obtained from multislice calculations. Possible effects of strain in the structure on its electronic states and energy-loss spectra are predicted.; The electron-beam-induced damage of the wurtzite InN in STEM is studied and knock-on type damage with ejection of nitrogen atoms from a sample is observed. From comparison of the measured integrated intensity of the nitrogen K-edge and indium M_4,5-edge with a calculated mass-loss model the vacancy-enhanced displacement energy for nitrogen atoms in InN is obtained. Investigations of the electronic structure of the wurtzite InN are carried out and excellent agreement between measured spectra of the nitrogen K-edge and the indium M_4,5 -edge and corresponding nitrogen 2p and the indium 5p partial density of states (DOS) of the conduction band is obtained. The calculations of DOS are performed using density functional theory (DFT). DFT calculations of nitrogen 2p partial DOS of the conduction band in strained and relaxed wurtzite InN combined with multiple scattering x-ray absorption near edge structure calculations of the corresponding nitrogen K-edge spectra suggest that a strong modification of the electronic structure should be expected in epitaxially grown multilayer structures when a significant mismatch in lattice constants between layers is present.