J. Malindretos

Tracking defect-induced ferromagnetism in GaN:Gd

We report on the magnetic properties of GaN:Gd layers grown by molecular beam epitaxy (MBE). A poor reproducibility with respect to the magnetic properties is found in these samples. Our results show strong indications that defects with a concentration of the order of 10^19 cm^-3 might play an important role for the magnetic properties. Positron annihilation spectroscopy does not support the suggested connection between the ferromagnetism and the Ga vacancy in GaN:Gd. Oxygen co-doping of GaN:Gd promotes ferromagnetism at room temperature and points to a role of oxygen for mediating ferromagnetic interactions in Gd doped GaN.

Phase Separation in Single InxGa1–xN Nanowires Revealed through a Hard X-ray Synchrotron Nanoprobe

In this work, we report on the composition, short- and long-range structural order of single molecular beam epitaxy grown In(x)Ga(1-x)N nanowires using a hard X-ray synchrotron nanoprobe. Nano-X-ray fluorescence mapping reveals an axial and radial heterogeneous elemental distribution in the single wires with Ga accumulation at their bottom and outer regions. Polarization-dependent nano-X-ray absorption near edge structure demonstrates that despite the elemental modulation, the tetrahedral order around the Ga atoms remains along the nanowires. Nano-X-ray diffraction mapping on single nanowires shows the existence of at least three different phases at their bottom: an In-poor shell and two In-rich phases. The alloy homogenizes toward the top of the wires, where a single In-rich phase is observed. No signatures of In-metallic precipitates are observed in the diffraction spectra. The In-content along the single nanowires estimated from X-ray fluorescence and diffraction data are in good agreement. A rough picture of these phenomena is briefly presented. We anticipate that this methodology will contribute to a greater understanding of the underlying growth concepts not only of nanowires but also of many nanostructures in materials science.

X-ray absorption in GaGdN: A study of local structure

In this study, we report on the incorporation of dilute Gd amounts into GaN films grown by molecular beam epitaxy. A combination of x-ray fluorescence with x-ray absorption spectroscopic techniques enabled us to examine not only the distribution of rare earth atoms in the GaN matrix but also the short-range structural order. Our results show Gd atoms in a trivalent state with tetrahedral coordination, thus substituting Ga in the wurtzite GaN structure.

Optical Emission of Individual GaN Nanocolumns Analyzed with High Spatial Resolution

Selective area growth has been applied to fabricate a homogeneous array of GaN nanocolumns (NC) with high crystal quality. The structural and optical properties of single NCs have been investigated at the nanometer-scale by transmission electron microscopy (TEM) and highly spatially resolved cathodoluminescence (CL) spectroscopy performed in a scanning transmission electron microscope (STEM) at liquid helium temperatures. TEM cross-section analysis reveals excellent structural properties of the GaN NCs. Sporadically, isolated basal plane stacking faults (BSF) can be found resulting in a remarkably low BSF density in the almost entire NC ensemble. Both, defect-free NCs and NCs with few BSFs have been investigated. The low defect density within the NCs allows the characterization of individual BSFs, which is of high interest for studying their optical properties. Direct nanometer-scale correlation of the CL and STEM data clearly exhibits a spatial correlation of the emission at 360.6 nm (3.438 eV) with the location of basal plane stacking faults of type I1.

Intra-atomic photoluminescence at 1.41 eV of substitutional Mn in GaMnN of high optical quality

We report on a characteristic photoluminescence feature of the substitutional Mn in high quality GaMnN layers. The lattice site was identified using atom localization by channeling enhanced microanalysis with a transmission electron microscope. It shows that 96.5%±5.0% of the Mn atoms are incorporated on the substitutional Ga site. In photoluminescence a feature appears at 1.41 eV with a phonon sideband related to the GaN matrix. The temperature evolution is characteristic of an intra-atomic transition and it is assigned to the internal transition E5→T52 of the Mn3+ ion. The assignment is supported by absorption experiments. The persistence of the clear PL signal up to about 1% Mn concentration is proposed to be a fingerprint of high quality diluted GaMnN.

Spontaneous core–shell elemental distribution in In-rich InxGa1−xN nanowires grown by molecular beam epitaxy

The elemental distribution of self-organized In-rich In(x)Ga1-xN nanowires grown by plasma-assisted molecular beam epitaxy has been investigated using three different techniques with spatial resolution on the nanoscale. Two-dimensional images and elemental profiles of single nanowires obtained by x-ray fluorescence and energy-dispersive x-ray spectroscopy, respectively, have revealed a radial gradient in the alloy composition of each individual nanowire. The spectral selectivity of resonant Raman scattering has been used to enhance the signal from very small volumes with different elemental composition within single nanowires. The combination of the three techniques has provided sufficient sensitivity and spatial resolution to prove the spontaneous formation of a core–shell nanowire and to quantify the thicknesses and alloy compositions of the core and shell regions. A theoretical model based on continuum elastic theory has been used to estimate the strain fields present in such inhomogeneous nanowires. These results suggest new strategies for achieving high quality nonpolar heterostructures.

Structure and Elemental Distribution of (Ga,Mn)N Nanowires

(Ga,Mn)N nanowires were grown by plasma-assisted molecular beam epitaxy on p-type Si(111) substrates. Chemical composition and elemental distribution of single nanowires were analyzed by energy dispersive X-ray spectroscopy revealing an inhomogeneous Mn distribution decreasing from the surface of the nanowires toward the inner core region. The average Mn concentration within the nanowires is found to be below 1%. High-resolution transmission electron microscopy shows the presence of planar defects perpendicular to the growth direction in undoped and Mn-doped GaN nanowires. The density of planar defects dramatically increases under Mn supply.

Synchrotron nanoimaging of single In-rich InGaN nanowires

This work reports on the elemental distribution and local structure of single InxGa1–xN nanowires (NWs) grown by molecular beam epitaxy on Si (111) substrates using X-ray fluorescence nanoprobe. Ga and In maps reveal an inhomogeneous elemental distribution along the NWs, with a higher Ga concentration at the bottom of the NW. Scanning electron microscopy images show that the inhomogeneous axial distribution is not correlated with a X-ray beam induced damage, and therefore, should be an intrinsic characteristic of the NWs arising from the growth process. Spatially resolved X-ray absorption near edge structure spectroscopy data acquired around the In K-edge show that the tetrahedral structure is preserved around the absorbing In-atoms all along the NW, and suggests that the compositional modulation could be affecting its long-range order.

Polarity dependent strongly inhomogeneous In-incorporation in GaN nanocolumns.

In this work, GaN/InGaN/GaN nanocolumns (NCs) have been grown by molecular beam epitaxy. Selective area growth (SAG) and self-organized growth (SOG) were performed simultaneously in patterned and unpatterned regions of the same substrate, respectively. The resulting structures show different tip morphologies and structural properties due to the different polarity along the growth direction, namely Ga-polar with r-plane faceted tips for the SAG NCs and N-polar with c-plane top facet for the SOG ones. When growing Ga-polar GaN/InGaN NCs, no indium is incorporated at a substrate temperature of [Formula: see text]°C. Rather, indium incorporation takes place under the same growth conditions on the N-polar NCs. The In-incorporation is investigated by means of nano x-ray fluorescence and diffraction, high-angle annular dark-field scanning transmission electron microscopy and high-resolution transmission electron microscopy.

Method to Predict and Optimize Charge Sensitivity of Ungated AlGaN/GaN HEMT-Based Ion Sensor Without Use of Reference Electrode

In this paper, we report on a methodology for theoretical prediction and optimization of charge sensitivity for ungated AlGaN/GaN high electron mobility transistor-based ion sensors operated in the reference electrode free configuration. We have performed numerical simulations of device sensitivity, specifically the change in channel electron concentration with the change in surface potential, for different Al mole fractions and AlGaN thicknesses. These results can be used for device optimization, signal analysis, and sensor calibration purposes. To validate the model, six ungated AlGaN/GaN transistor-based devices of different Al mole fractions and AlGaN thicknesses were fabricated. These devices were exposed to KOH solutions with different pH values, and the voltage change at the gate area was indirectly measured as a function of ionic concentration. The gain in conductivity across the measured range of pH values was experimentally extracted for each device and closely matched the sensitivity predicted by simulation.