E. Calleja

High-performance GaN p-n junction photodetectors for solar ultraviolet applications

The fabrication and characterization of ultraviolet photodetectors based on GaN p-n junctions is reported. The devices are grown by metalorganic vapour phase epitaxy on basal-plane sapphire substrates. These detectors are visible-blind with a sharp wavelength cut-off at 360 nm. The photocurrent is linear with incident power from up to , with a responsivity of at 360 nm. The device time response is dominated by the effective resistance-capacitance time constant, and a 105 ns response is estimated for very low load resistances. A comparison with the response of GaN photoconductor detectors is also presented. The application of these high-performance photodetectors for solar ultraviolet monitoring is described.

Self-Regulated Radius of Spontaneously Formed GaN Nanowires in Molecular Beam Epitaxy

We investigate the axial and radial growth of GaN nanowires upon a variation of the Ga flux during molecular beam epitaxial growth. An increase in the Ga flux promotes radial growth without affecting the axial growth rate. In contrast, a decrease in the Ga flux reduces the axial growth rate without any change in the radius. These results are explained by a kinetic growth model that accounts for both the diffusion of Ga adatoms along the side facets toward the nanowire tip and the finite amount of active N available for the growth. The model explains the formation of a new equilibrium nanowire radius after increasing the Ga flux and provides an explanation for two well-known but so far not understood experimental facts: the necessity of effectively N-rich conditions for the spontaneous growth of GaN nanowires and the increase in nanowire radius with increasing III/V flux ratio.

Annealing effects on the crystal structure of GaInNAs quantum wells with large In and N content grown by molecular beam epitaxy

The impact of rapid thermal annealing on the optical emission of GaInNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy with high In and N content is shown to be highly dependent on the crystal structure of the QWs, as determined by transmission electron microscopy. Due to the presence of higher concentrations of nonradiative recombination centers, the annealing temperature required to obtain maximum photoluminescence emission is higher for the QW with strong structural modulation of the upper interface [at the onset of three-dimensional (3D) growth], intermediate for the two-dimensional (2D) grown QW with compositional fluctuations, and lower for the homogeneous 2D grown QW. Moreover, the transition from homogeneous 2D growth, to 2D growth with compositional fluctuations, and finally 3D growth, leads to progressively deeper carrier localization states below the conduction-band edge. Increasing annealing temperatures gradually shifts the localization states closer to the conduction-band edge, pr...

Wet etching of GaN grown by molecular beam epitaxy on Si(111)

A wet etching method for GaN and AlxGa1-xN, based on aqueous solutions of KOH, is presented. A detailed analysis of the etching rate dependence with temperature and concentration is described. This etching has been used for the fabrication of high optical quality pyramidal nanostructures in wurtzite N-face GaN grown on AlN-buffered Si(111) substrates by molecular beam epitaxy. These nanostructures have been studied by high-resolution transmission and scanning electron microscopy and their optical quality has been analysed by low-temperature photoluminescence (PL) measurements. The pyramids are parallel to the basal plane and limited by {111} planes and its presence improves the overall PL response of the sample. The relationship between the polarity of GaN and the characteristics of the AlN buffer has also been analysed.

Formation mechanisms of GaN nanowires grown by selective area growth homoepitaxy.

This work provides experimental evidence and theoretical explanations regarding the formation mechanisms of GaN nanowires grown by selective area growth on GaN-on-sapphire templates. The first growth stage, driven by selective area growth kinetics, consists of initial nucleation (along the nanohole inner periphery), coalescence onset and full coalescence, producing a single nanocrystal within each nanohole. In the second growth stage, driven by free-surface-energy minimization, the formed nanocrystal undergoes morphological evolution, exhibiting initial cylindrical-like shape, intermediate dodecagonal shape and a final, thermodynamically stable hexagonal shape. From this point on, the nanowire vertical growth proceeds while keeping the stable hexagonal form.

AlGaN ultraviolet photodetectors grown by molecular beam epitaxy on Si(111) substrates

Abstract The performance of AlGaN metal–semiconductor–metal (MSM) photodetectors grown on Si(111) is presented in this article. It is shown that the growth of an adequate AlN buffer layer is critical to achieve visible-blind devices, and that its role as an effective electrical insulator of the conductive substrate was found to be more efficient for N-excess AlN growth. The increase of Al content produced a transition from photoconductor to MSM photodiode behaviour, as determined from the detector responsivity, temporal response, and UV/visible contrast. The effect of the contact metal on photoconductive gain and UV/visible contrast was also studied.

Radiative defects in GaN nanocolumns: Correlation with growth conditions and sample morphology

Low-temperature photoluminescence is studied in detail in GaN nanocolumns (NCs) grown by plasma-assisted molecular beam epitaxy under various conditions (substrate temperature and impinging Ga/N flux ratio). The relative intensities of the different emission lines, in particular those related to structural defects, appear to be correlated with the growth conditions, and clearly linked to the NC sample morphology. We demonstrate, in particular, that all lines comprised between 3.10 and 3.42 eV rapidly lose intensity when the growth conditions are such that the NC coalescence is reduced. The well-known line around 3.45 eV, characteristic of GaN NC samples, shows, however, a behavior that is exactly the opposite of the other lines, namely, for growth conditions leading to reduced NC coalescence, this line tends to become more prominent, thus proving to be intrinsic to individual GaN NCs.

AlGaN Nanocolumns and AlGaN/GaN/AlGaN Nanostructures Grown by Molecular Beam Epitaxy

This work reports on the characterization of hexagonal, single crystal AlGaN nanocolumns with diameters in the range of 30 to 100 nm grown by molecular beam epitaxy on Si(111) substrates. The change of the flux ratio between the Al and the total III-element controls the alloy composition. The Al composition trend versus the Al flux is consistent both with the E 2 phonon energy values measured by inelastic light scattering and the luminescence emission peaks position. High quality low dimensional AlGaN/GaN/AlGaN heterostructures with five GaN quantum discs, 2 and 4 nm thick, embedded into the AlGaN columns, were designed in order to study the quantum confinement effects.

Structural and optical characterization of thick InGaN layers and InGaN/GaN MQW grown by molecular beam epitaxy

Abstract Thick In x Ga 1− x N (0.20 x 2 O 3 templates. The strain and In-content is estimated from high-resolution X-ray diffraction, showing that the bulk samples are not fully relaxed. A bowing parameter of 3.6 eV is obtained from absorption measurements of In x Ga 1− x N layers. Strong In-dependent excitonic localization is observed in these bulk layers, leading to an increase in the absorption band edge with the In content. Regarding the MQWs structures, high-resolution transmission electron microscopy reveals an increase in the interface roughness for high In content. The dominant PL emission of the MQWs shows a red-shift when increasing the well thickness for a given In-content, due to internal piezoelectric field. The excitonic localization is studied and compared between thick layers and MQWs structures.

Growth of GaN layers on SiC/Si(111) substrate by molecular beam epitaxy

High quality GaN layers were grown by molecular beam epitaxy (MBE), with and without AlN buffer, on SiC/Si(111) substrates obtained by C implantation into Si(111) at 500 °C to avoid amorphization. Thermal annealing at 1150 °C for 6 h produced crystalline SiC layers embedded into the Si(111). The SiC top stoichiometry is controlled by reactive ion etching (RIE), after what, all SiC layers show a fairly flat (4 nm) and specular surface. Photoluminscence spectra reveal that all GaN layers are under tensile biaxial strain of thermal origin. GaN layers grown on stoichiometric SiC have lower mosaicity, but also less photoluminescence efficiency and tensile strain, as compared to GaN on bare non-stoichiometric SiC. This, most likely, relates to changes in microcrystals size and grain boundaries density, that depend strongly on the GaN nucleation process. Despite the partial polycrystalline nature of the SiC/Si(111), the grown GaN quality is as high or even better than that from GaN grown on Si(111).