Arquitecturas basadas en micro- y nanohilos de Ga₂O₃ con aplicaciones en fotónica

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 23-10-2020Transparent Conductive Oxides (TCOs) are materials which combine a very low absorption in the visible range of the electromagnetic spectrum with a moderate electrical conductivity. Among them, gallium oxide ( -Ga2O3) is one of the widest band gap semiconductors (almost 5 eV). Lately, this material is experiencing an exponential increase in its research interest, mainly due to two reasons: the better performance and lower cost of some high power electronic devices of Ga2O3 compared to the semiconductor giants SiC and AlN and the recent development of high quantum efficiency Ga2O3 deep-ultraviolet (deep-UV) photodetectors...Los óxidos conductores transparentes (TCOs, por sus siglas en inglés) son materiales que combinan una baja absorción en el rango visible del espectro electromagnético con una conductividad eléctrica moderada. Perteneciente a la familia de los TCOs, el óxido de galio ( -Ga2O3) tiene uno de los mayores band gaps de todos los semiconductores (casi 5 eV). Últimamente, el interés científico sobre este material está aumentando exponencialmente por dos principales motivos: el desarrollo de dispositivos de Ga2O3 de alta potencia con mejores capacidades y menor coste que los semiconductores comerciales de este sector (SiC y AlN) y el desarrollo de fotodetectores ultravioleta con gran eficiencia cuánticaFac. de Ciencias FísicasTRUEunpu

Influence of Li doping on the morphology and luminescence of Ga_2O_3 microrods grown by a vapor-solid method

Gallium oxide microrods have been grown by an evaporation-deposition method by using a precursor containing lithium in order to check the influence of such dopant on the morphology and physical properties of the obtained ß-Ga_2O_3 structures. SEM studies show that the morphology is modified with respect to undoped gallium oxide, promoting the growth of micropyramids transversal to the microwire axis. Raman analysis reveals good crystal quality and an additional Raman peak centred at around 270 cm^(-1), characteristic of these samples and not present in undoped monoclinic gallium oxide. The presence of the Li^(+) ions also influences the luminescence emission by inducing a red-shift of the characteristic UV-blue defect band of gallium oxide. In addition, an intense sharp peak centred around 717 nm observed both by cathodoluminescence (CL) and photoluminescence (PL) is also attributed to the presence of these ions. The Li related luminescence features have also been investigated by PL excitation (PLE) spectra and by the temperature dependence of the luminescence

Shape engineering driven by selective growth of SnO2 on doped Ga2O3 nanowires

Tailoring the shape of complex nanostructures requires control of the growth process. In this work, we report on the selective growth of nanostructured tin oxide on gallium oxide nanowires leading to the formation of SnO2/Ga2O3 complex nanostructures. Ga2O3 nanowires decorated with either crossing SnO2 nanowires or SnO2 particles have been obtained in a single step treatment, by thermal evaporation. The reason for this dual behavior is related to the growth direction of trunk Ga2O3 nanowires. Ga2O3 nanowires grown along the [001] direction favor the formation of crossing SnO2 nanowires. Alternatively, SnO2 forms rhombohedral particles on [110] Ga2O3 nanowires leading to skewer-like structures. These complex oxide structures were grown by a catalyst-free vapor-solid process. When pure Ga and tin oxide were used as source materials and compacted powders of Ga2O3 acted as substrates, [110] Ga2O3 nanowires grow preferentially. High-resolution transmission electron microscopy analysis reveals epitaxial relationship lattice matching between the Ga2O3 axis and SnO2 particles, forming skewer-like structures. The addition of chromium oxide to the source materials modifies the growth direction of the trunk Ga2O3 nanowires, growing along the [001], with crossing SnO2 wires. The SnO2/Ga2O3 junctions does not meet the lattice matching condition, forming a grain boundary. The electronic and optical properties have been studied by XPS and CL with high spatial resolution, enabling us to get both local chemical and electronic information of the surface in both type of structures. The results will allow tuning optical and electronic properties of oxide complex nanostructures locally as a function of the orientation. In particular, we report a dependence of the visible CL emission of SnO2 on its particular shape. Orange emission dominates in SnO2/Ga2O3 crossing wires while green-blue emission is observed in SnO2 particles attached to Ga2O3 trunks. The results show that the Ga2O3-SnO2 system appears to be a benchmark for shape engineering to get architectures involving nanowires via the control of the growth direction of the nanowires

Modal analysis of β-Ga₂O₃:Cr widely tunable luminescent optical microcavities

Optical microcavities are key elements in many photonic devices, and those based on distributed Bragg reflectors (DBRs) enhance dramatically the end reflectivity, allowing for higher quality factors and finesse values. Besides, they allow for wide wavelength tunability, needed for nano-and microscale light sources to be used as photonic building blocks in the micro- and nanoscale. Understanding the complete behavior of light within the cavity is essential to obtaining an optimized design of properties and optical tunability. In this work, focused ion-beam fabrication of high refractive-index contrast DBR-based optical cavities within Ga₂O₃:Cr microwires grown and doped by the vapor-solid mechanism is reported. Room-temperature microphotoluminescence spectra show strong modulations from about 650 nm up to beyond 800 nm due to the microcavity resonance modes. Selectivity of the peak wavelength is achieved for two different cavities, demonstrating the tunability of this kind of optical system. Analysis of the confined modes is carried out by an analytical approximation and by finite-difference-time-domain simulations. A good agreement is obtained between the reflectivity values of the DBRs calculated from the experimental resonance spectra, and those obtained by finite-difference-time-domain simulations. Experimental reflectivities up to 70% are observed in the studied wavelength range and cavities, and simulations demonstrate that reflectivities up to about 90% could be reached. Therefore, Ga₂O₃:Cr high-reflectivity optical microcavities are shown as good candidates for single-material-based, widely tunable light emitters for micro- and nanodevices

Direct observation of tunnelled intergrowth in SnO2/Ga2O3 complex nanowires

beta-Ga_2O_3 intergrowths have been revealed in the SnO_2 rutile structure when SnO_2/Ga_2O_3 complex nanostructures are grown by thermal evaporation with a catalyst-free basis method. The structure is formed by a Ga_2O_3 nanowire trunk, around which a rutile SnO_2 particle is formed with [001] aligned to the [010] Ga_2O_3 trunk axis. Inside the SnO_2 particle, beta-Ga_2O_3 units occur separated periodically by hexagonal tunnels in the (210) rutile plane. Orange (620 nm) optical emission from tin oxide, with a narrow linewidth indicating localised electronic states, may be associated with this beta-Ga_2O_3 intergrowth

Growth, catalysis and faceting of α\alpha-Ga2_2O3_3 and α\alpha-(Inx_xGa1−x_{1-x})2_2O3_3 on mm-plane α\alpha-Al2_2O3_3 by molecular beam epitaxy

The growth of $\alpha$-Ga$_2$O$_3$ and $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ on $m$-plane $\alpha$-Al$_2$O$_3$(10$\bar{1}$0) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure $\alpha$-Ga$_2$O$_3$(10$\bar{1}$0) and $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$(10$\bar{1}$0) thin films are realized. The presence of In on the $\alpha$-Ga$_2$O$_3$ growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio ($R_{\text{O}}$), In incorporates into $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ up to $x \leq 0.08$. Upon a critical thickness, $\beta$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ nucleates and subsequently heteroepitaxially grows on top of $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ facets. Metal-rich MOCATAXY growth conditions, where $\alpha$-Ga$_2$O$_3$ would not conventionally stabilize, lead to single-crystalline $\alpha$-Ga$_2$O$_3$ with negligible In incorporation and improved surface morphology. Higher $T_{\text{G}}$ further results in single-crystalline $\alpha$-Ga$_2$O$_3$ with well-defined terraces and step edges at their surfaces. For $R_{\text{O}} \leq 0.53$, In acts as a surfactant on the $\alpha$-Ga$_2$O$_3$ growth surface by favoring step edges, while for $R_{\text{O}} \geq 0.8$, In incorporates and leads to a-plane $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ faceting and the subsequent ($\bar{2}$01) $\beta$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ growth on top. Thin film analysis by STEM reveals highly crystalline $\alpha$-Ga$_2$O$_3$ layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline $\alpha$-Ga$_2$O$_3$ on $\alpha$-Al$_2$O$_3$(10$\bar{1}$0)

Exciting and confining light in Cr doped gallium oxide

On one hand, interest on the tunability of the optical microcavities has increased in the last few years due to the need for selective nano-and microscale light sources to be used as photonic building blocks in several applications. On the other, transparent conductive oxide (TCO) beta-Ga_2O_3 is attracting attention in the optoelectronics area due to its ultra wide band gap and high breakdown field. However, at the micro- and nanoscale there are still some challenges to face up, namely the control and tuning of the optical and electrical properties of this oxide. In this work, Cr doped Ga_2O_3 elongated microwires are grown using the vapor-solid (VS) mechanism. Focused Ion Beam (FIB) etching forms Distributed Bragg Reflector (DBR)-based resonant microcavities. Room temperature microphotoluminescence (mu-PL) spectra show strong modulations in the red-NIR range on five cavities with different lengths. Selectivity of the peak wavelengths is obtained, proving the tunability of this kind of optical systems. The confined modes are analyzed experimentally, analytically and via finite difference time domain (FDTD) simulations. Experimental reflectivities up to 78% are observed

Correlative analysis on InGaN/GaN nanowires: structural and optical properties of self-assembled short-period superlattices

: The influence of self-assembled short-period superlattices (SPSLs) on the structural and optical properties of InGaN/GaN nanowires (NWs) grown by PAMBE on Si (111) was investigated by STEM, EDXS, µ-PL analysis and k·p simulations. STEM analysis on single NWs indicates that in most of the studied nanostructures, SPSLs self-assemble during growth. The SPSLs display short-range ordering of In-rich and In-poor InxGa1-xN regions with a period of 2-3 nm that are covered by a GaN shell and that transition to a more homogenous InxGa1-xN core. Polarization- and temperature-resolved PL analysis performed on the same NWs shows that they exhibit a strong parallel polarized red-yellow emission and a predominantly perpendicular polarized blue emission, which are ascribed to different In-rich regions in the nanostructures. The correlation between STEM, µ-PL and k·p simulations provides better understanding of the rich optical emission of complex III-N nanostructures and how they are impacted by structural properties, yielding the significant impact of strain on self-assembly and spectral emission

Temperature-Dependent Anisotropic Refractive Index in β-Ga2O3: Application in Interferometric Thermometers

An accurate knowledge of the optical properties of β-Ga2O3 is key to developing the full potential of this oxide for photonics applications. In particular, the dependence of these properties on temperature is still being studied. Optical micro- and nanocavities are promising for a wide range of applications. They can be created within microwires and nanowires via distributed Bragg reflectors (DBR), i.e., periodic patterns of the refractive index in dielectric materials, acting as tunable mirrors. In this work, the effect of temperature on the anisotropic refractive index of β-Ga2O3 n(λ,T) was analyzed with ellipsometry in a bulk crystal, and temperature-dependent dispersion relations were obtained, with them being fitted to Sellmeier formalism in the visible range. Micro-photoluminescence (μ-PL) spectroscopy of microcavities that developed within Cr-doped β-Ga2O3 nanowires shows the characteristic thermal shift of red–infrared Fabry–Perot optical resonances when excited with different laser powers. The origin of this shift is mainly related to the variation in the temperature of the refractive index. A comparison of these two experimental results was performed by finite-difference time-domain (FDTD) simulations, considering the exact morphology of the wires and the temperature-dependent, anisotropic refractive index. The shifts caused by temperature variations observed by μ-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(λ,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.This work was supported by MICINN projects (RTI2018-097195-B-I00, RTI2018-096918-B-C41, PID2021-122562NB-I00 and PID2021-123190OB-I00/AEI/10.13039/501100011033/FEDER, UE). The authors acknowledge the financial support of the excellence research network RED2018-102609-T by MINECO. The authors acknowledge the support from the Air Force Office of Scientific Research under Award No. FA8655-20-1-7013 (Program Manager: Ali Sayir). M.A.-O. acknowledges financial support from MICINN (FPU contract No. FPU15/01982) and thanks the Central Research Development Fund (CRDF) of the University of Bremen for funding (ZF04/2021). J.S.M. and J.J. were supported by the Air Force Office of Scientific Research under award number FA9550–21–1–0507, monitored by Dr. Ali Sayir. Any opinions, finding, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the United States Air Force