Biomedical subjects

Z Liliental-Weber

Publications and source records attributed to Z Liliental-Weber.

8 recordsLinked to original sources

PubMed2004

Atomic structure of defects in GaN:Mg grown with Ga polarity.

The atomic structure of characteristic defects (Mg-rich hexagonal pyramids and truncated pyramids) in GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities within the defects have inside walls covered by GaN of reverse polarity. We propose that lateral overgrowth of the cavities restores matrix polarity on the defect base. From matrix to defect, exchange of Ga and N sublattices leads to a 0.6+/-0.2 A displacement of Ga sublattices. We observe a [1100]/3 shift from matrix AB stacking to BC stacking for the entire pyramid. Electron energy loss spectroscopy detected changes in N edge and presence of oxygen on the defect walls. Our results explain commonly observed decrease of acceptor concentration in heavily doped GaN:Mg.

Journal Article↗

PubMed2004

Screw dislocations in GaN grown by different methods.

A study of screw dislocations in hydride-vapor-phase-epitaxy (HVPE) template and molecular-beam-epitaxy (MBE) overlayers was performed using transmission electron microscopy (TEM) in plan view and in cross section. It was observed that screw dislocations in the HVPE layers were decorated by small voids arranged along the screw axis. However, no voids were observed along screw dislocations in MBE overlayers. This was true both for MBE samples grown under Ga-lean and Ga-rich conditions. Dislocation core structures have been studied in these samples in the plan-view configuration. These experiments were supported by image simulation using the most recent models. A direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high-resolution images was applied. It was shown that the core structures of screw dislocations in the studied materials were filled. The filed dislocation cores in an MBE samples were stoichiometric. However, in HVPE materials, single atomic columns show substantial differences in intensities and might indicate the possibility of higher Ga concentration in the core than in the matrix. A much lower intensity of the atomic column at the tip of the void was observed. This might suggest presence of lighter elements, such as oxygen, responsible for their formation.

Gallium↗

PubMed2000

Derivation of growth mechanism of nano-defects in GaN from TEM data

Transmission electron microscopy (TEM) has been used to describe 'hollow' defects, e.g. nanotubes and pinholes (empty inside) formed in GaN hetero- and homoepitaxial layers. These defects are believed to be formed due to the presence of impurities and dopants. These defects appear to be even more deleterious than dislocations for the application of GaN in devices because their formation interrupts two-dimensional growth of quantum wells, and therefore will influence electrical and optical properties of the material. Experimental observations show that both these defects start from V-shaped indentations delineated by (1011) planes, and their density increases with higher concentrations of impurities or dopants. These 'hollow' defects are often found to be attached to dislocations, but the frequently observed termination of these defects inside the crystal eliminates dislocations as the source of these defects. Theoretical predictions also do not suggest that dislocations would have large-diameter empty cores like these of nanotubes and pinholes.

Journal Article↗

PubMed1996