A. Wierzbicka

Influence of substrate nitridation temperature on epitaxial alignment of GaN nanowires to Si(111) substrate

An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.

Growth by molecular beam epitaxy and properties of inclined GaN nanowires on Si(001) substrate

The growth mode and structural and optical properties of novel type of inclined GaN nanowires (NWs) grown by plasma-assisted MBE on Si(001) substrate were investigated. We show that due to a specific nucleation mechanism the NWs grow epitaxially on the Si substrate without any Si(x)N(y) interlayer, first in the form of zinc-blende islands and then as double wurtzite GaN nanorods with Ga-polarity. X-ray measurements show that orientation of these nanowires is epitaxially linked to the symmetry of the substrate so that [0001] axis of w-GaN nanowire is directed along the [111]Si axis. This is different from commonly observed behavior of self-induced GaN NWs that are N-polar and grow perpendicularly to the surface of nitridized silicon substrate independently on its orientation. The inclined NWs exhibit bright luminescence of bulk donor-bound excitons (D(0)X) at 3.472 eV and exciton-related peak at 3.46 eV having a long lifetime (0.7 ns at 4 K) and observable up to 50 K.

High-resolution X-ray diffraction analysis of strain distribution in GaN nanowires on Si(111) substrate

In this work, the influence of micro- and macro-deformation profiles in GaN nanowires (NWs) on the angular intensity distribution of X-ray diffraction are studied theoretically. The calculations are performed by using kinematical theory of X-ray diffraction and assuming the deformation decays exponentially from the NW/substrate interface. Theoretical modeling of X-ray scattering from NWs with different deformation profiles are carried out. We show that the shape of the (002) 2θ/ω X-ray diffraction profile (XDP) is defined by initial deformation at the NWs bottom and its relaxation depth given by the decay depth of the exponential deformation profile. Also, we demonstrate that macro-deformation leads to XDP shift, whereas micro-deformations are the cause of XDPs asymmetry and its symmetrical broadening. A good correlation between calculated and experimental XDP from self-assembled GaN NWs on Si(111) substrate was achieved by taking into account all parameters of micro- and macro-deformation profiles.

Spatially resolved x-ray diffraction study of GaSb layers grown laterally on SiO2-masked GaAs substrates

In this work spatially resolved x-ray diffraction (SRXRD) is used to analyze strain in GaSb layers grown by epitaxial lateral overgrowth (ELO) on SiO2-masked (001) GaAs substrates. We show that this heteroepitaxial structure contains local mosaicity in the wing area that cannot be detected by selective etching. While the standard x-ray diffraction measurements only suggest the presence of grain structure of the ELO layer, SRXRD allows examining the microscopic strain distribution in the sample. In particular, size of microblocks and their relative misorientation are determined.

An influence of the local strain on cathodoluminescence of GaN/AlxGa1−xN nanowire structures

Near-band-edge excitonic emission shift is investigated as a measure of the local strain in GaN nanowires with single AlxGa1−xN sections of various Al contents (x = 0.0, 0.22, 0.49, 1.0). Complementary data obtained by spatially and spectrally resolved cathodoluminescence spectroscopy and imaging of individual nanowires at low temperature, high resolution X-ray diffraction, and transmission electron microscopy are used to determine the correspondence between the cathodoluminescence emission energy and the strain in the GaN core of the nanowire surrounded by the AlxGa1−xN shell formed during the growth of AlxGa1−xN sections by catalyst-free plasma-assisted molecular beam epitaxy. In majority of nanowires, the blue-shift of GaN cathodoluminescence follows the evolution expected for the GaN core under uniaxial compressive strain along the axis of the structure.