Pinar Dogan

Direct experimental determination of the spontaneous polarization of GaN

We present a universal approach for determining the spontaneous polarization Psp of a wurtzite semiconductor from the emission energies of excitons bound to the different types of stacking faults in these crystals. Employing micro-photoluminescence and cathodoluminescence spectroscopy, we observe emission lines from the intrinsic and extrinsic stacking faults in strain-free GaN micro-crystals. By treating the polarization sheet charges associated with these stacking faults as a plate capacitor, Psp can be obtained from the observed transition energies with no additional assumptions. Self-consistent Poisson-Schroedinger calculations, aided by the microscopic electrostatic potential computed using density-functional theory, lead to nearly identical values for Psp. Our recommended value for Psp of GaN is -0.022+/-0.007 C/m^{2}.

Luminescence associated with stacking faults in GaN

Basal-plane stacking faults are an important class of optically active structural defects in wurtzite semiconductors. The local deviation from the 2H stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound state in the gap of the host crystal, resulting in the localization of excitons. Due to the two-dimensional nature of these planar defects, stacking faults act as quantum wells, giving rise to radiative transitions of excitons with characteristic energies. Luminescence spectroscopy is thus capable of detecting even a single stacking fault in an otherwise perfect wurtzite crystal. This review draws a comprehensive picture of the luminescence properties related to stacking faults in GaN. The emission energies associated with different types of stacking faults as well as factors that can shift these energies are discussed. In this context, the importance of the quantum-confined Stark effect in these zinc-blende/wurtzite heterostructures, which results from the spontaneous polarization of wurtzite GaN, is underlined. This discussion is extended to zinc-blende segments in a wurtzite matrix. Furthermore, other factors affecting the emission energy and linewidth of stacking fault-related peaks as well as results obtained at room temperature are addressed. The considerations presented in this article should also be transferable to other wurtzite semiconductors.

Nucleation, Growth, and Bundling of GaN Nanowires in Molecular Beam Epitaxy: Disentangling the Origin of Nanowire Coalescence

We investigate the nucleation, growth, and coalescence of spontaneously formed GaN nanowires in molecular beam epitaxy combining the statistical analysis of scanning electron micrographs with Monte Carlo growth models. We find that (i) the nanowire density is limited by the shadowing of the substrate from the impinging fluxes by already existing nanowires, (ii) shortly after the nucleation stage, nanowire radial growth becomes negligible, and (iii) coalescence is caused by bundling of nanowires. The latter phenomenon is driven by the gain of surface energy at the expense of the elastic energy of bending and becomes energetically favorable once the nanowires exceed a certain critical length.

Nature of excitons bound to inversion domain boundaries: Origin of the 3.45-eV luminescence lines in spontaneously formed GaN nanowires on Si(111)

We investigate the 3.45-eV luminescence band of spontaneously formed GaN nanowires on Si(111) by photoluminescence and cathodoluminescence spectroscopy. This band is found to be particularly prominent for samples synthesized at comparatively low temperatures. At the same time, these samples exhibit a peculiar morphology, namely, isolated long nanowires are interspersed within a dense matrix of short ones. Cathodoluminescence intensity maps reveal the 3.45-eV band to originate primarily from the long nanowires. Transmission electron microscopy shows that these long nanowires are either Ga polar and are joined by an inversion domain boundary with their short N-polar neighbors, or exhibit a Ga-polar core surrounded by a N-polar shell with a tubular inversion domain boundary at the core/shell interface. For samples grown at high temperatures, which exhibit a uniform nanowire morphology, the 3.45-eV band is also found to originate from particular nanowires in the ensemble and thus presumably from inversion domain boundaries stemming from the coexistence of N- and Ga-polar nanowires. For several of the investigated samples, the 3.45-eV band splits into a doublet. We demonstrate that the higher-energy component of this doublet arises from the recombination of two-dimensional excitons free to move in the plane of the inversion domain boundary. In contrast, the lower-energy component of the doublet originates from excitons localized in the plane of the inversion domain boundary. We propose that this in-plane localization is due to shallow donors in the vicinity of the inversion domain boundaries.

Atomic-scale behavior of adatoms in axial and radial growth of GaN nanowires: Molecular dynamics simulations

The effect of the flux ratio of N atoms to Ga atoms on the radial/axial growth of GaN nanowires during molecular-beam epitaxy has been investigated by molecular dynamics simulations. By studying adatoms on a surface during the growth of GaN nanowires, we explore the underlying mechanisms and find that both a preferable adsorption surface for N and Ga adatoms and the desorption of N adatoms play a key role for such axial/radial growth. These observations on the atomic scale are crucial for understanding the self-induced growth of GaN nanowires in general as well as for achieving their desired morphology under different growth conditions.

The hydride vapor phase epitaxy of GaN on silicon covered by nanostructures

GaN several tens of μm thick has been deposited on a silicon substrate using a two-step hydride vapor phase epitaxy (HVPE) process. The substrates were covered by AlN layers and GaN nanostructures grown by plasma-assisted molecular-beam epitaxy. During the first low-temperature (low-T) HVPE step, stacking faults (SF) form, which show distinct luminescence lines and stripe-like features in the cathodoluminescence images of the cross-section of the layers. These cathodoluminescence features provide an insight into the growth process. During a second high-temperature (high-T) step, the SFs disappear, and the luminescence of this part of the GaN layer is dominated by the donor-bound exciton. For templates consisting of both a thin AlN buffer and GaN nanostructures, the incorporation of silicon into the GaN grown by HVPE is not observed. Moreover, the growth mode of the (high-T) HVPE step depends on the specific structure of the AlN/GaN template, where in the first case, epitaxy is dominated by the formation of slowly growing facets, while in the second case, epitaxy proceeds directly along the c-axis. For templates without GaN nanostructures, cathodoluminescence spectra excited close to the Si/GaN interface show a broadening toward higher energies, indicating the incorporation of silicon at a high dopant level.

Influence of nanowire template morphology on the coalescence overgrowth of GaN nanowires on Si by molecular beam epitaxy

GaN nanowires are grown on Si(111) as templates for pendeoepitaxial coalescence overgrowth under different V/III ratios by molecular beam epitaxy. The degree of coalescence in the nanowire template increases with decreasing V/III ratio or doping with Mg. The morphology of the GaN nanowire template strongly influences that of the pendeoepitaxial layer after coalescence as well as its optical quality.