RESUMEN
In this work, a systematic photoluminescence (PL) study on three series of gallium oxide/aluminum gallium oxide films and bulk single crystals is performed including comparing doping, epitaxial substrates, and aluminum concentration. It is observed that blue/green emission intensity strongly correlates with extended structural defects rather than the point defects frequently assumed. Bulk crystals or Si-doped films homoepitaxially grown on (010) ß-Ga2O3 yield an intense dominant UV emission, while samples with extended structural defects, such as gallium oxide films grown on either (-201) ß-Ga2O3 or sapphire, as well as thick aluminum gallium oxide films grown on either (010) ß-Ga2O3 or sapphire, all show a very broad PL spectrum with intense dominant blue/green emission. PL differences between samples and the possible causes of these differences are analyzed. This work expands previous reports that have so far attributed blue and green emissions to point defects and shows that in the case of thin films, extended defects might have a prominent role in emission properties.
RESUMEN
Three-dimensional (3D) semimetals have been predicted and demonstrated to have a wide variety of interesting properties associated with their linear energy dispersion. In analogy to two-dimensional (2D) Dirac semimetals, such as graphene, Cd3As2 has shown ultrahigh mobility and large Fermi velocity and has been hypothesized to support plasmons at terahertz frequencies. In this work, we experimentally demonstrate synthesis of high-quality large-area Cd3As2 thin films through thermal evaporation as well as the experimental realization of plasmonic structures consisting of periodic arrays of Cd3As2 stripes. These arrays exhibit sharp resonances at terahertz frequencies with associated quality factors ( Q) as high as â¼3.7 (at 0.82 THz). Such spectrally narrow resonances can be understood on the basis of a long momentum scattering time, which in our films can approach â¼1 ps at room temperature. Moreover, we demonstrate an ultrafast tunable response through excitation of photoinduced carriers in optical pump/terahertz probe experiments. Our results evidence that the intrinsic 3D nature of Cd3As2 might provide for a very robust platform for terahertz plasmonic applications. Moreover, the long momentum scattering time as well as large kinetic inductance in Cd3As2 also holds enormous potential for the redesign of passive elements such as inductors and hence can have a profound impact in the field of RF integrated circuits.
RESUMEN
Ultra-fast thermal annealing of semiconductor materials using a laser can be revolutionary for short processing times and low manufacturing costs. Here we investigate Cu-In-Se thin films as precursors for CuInSe2 semiconductor absorber layers via laser annealing. The reaction mechanism of laser annealed metal stacks is revealed by measuring ex situ X-ray diffractograms, Raman spectra and composition. It is shown that the formation of CuInSe2 occurs via the formation of Cu x Se/In x Se y binary phases as in conventional annealing routes, despite the entirely different annealing time scale. Pre-alloying the Cu and In metals prior to laser annealing significantly enhances the selenisation reaction rate. Laser annealing for six seconds approaches a near phase-pure material, which exhibits similar crystalline quality to the reference material annealed for ninety minutes in a tube furnace. The estimated quasi Fermi level splitting deficit for the laser annealed material is only 60 meV lower than the reference sample, which implies a high optoelectronic quality.
RESUMEN
Despite numerous studies on three-dimensional topological insulators (3D TIs), the controlled growth of high quality (bulk-insulating and high mobility) TIs remains a challenging subject. This study investigates the role of growth methods on the synthesis of single crystal stoichiometric BiSbTeSe2 (BSTS). Three types of BSTS samples are prepared using three different methods, namely melting growth (MG), Bridgman growth (BG) and two-step melting-Bridgman growth (MBG). Our results show that the crystal quality of the BSTS depend strongly on the growth method. Crystal structure and composition analyses suggest a better homogeneity and highly-ordered crystal structure in BSTS grown by MBG method. This correlates well to sample electrical transport properties, where a substantial improvement in surface mobility is observed in MBG BSTS devices. The enhancement in crystal quality and mobility allow the observation of well-developed quantum Hall effect at low magnetic field.
RESUMEN
The performance of polycrystalline CdTe photovoltaic thin films is expected to depend on the grain boundary density and corresponding grain size of the film microstructure. However, the electrical performance of grain boundaries within these films is not well understood, and can be beneficial, harmful, or neutral in terms of film performance. Electron backscatter diffraction has been used to characterize the grain size, grain boundary structure, and crystallographic texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment in order to correlate performance with microstructure. Weak fiber textures were observed in the as-deposited films, with (111) textures present at lower deposition pressures and (110) textures observed at higher deposition pressures. The CdCl2-treated samples exhibited significant grain recrystallization with a high fraction of twin boundaries. Good correlation of solar cell efficiency was observed with twin-corrected grain size while poor correlation was found if the twin boundaries were considered as grain boundaries in the grain size determination. This implies that the twin boundaries are neutral with respect to recombination and carrier transport.
RESUMEN
Wave guidance is an important aspect of light trapping in thin film photovoltaics making it important to properly model the effects of loss on the field profiles. This paper derives the full-field solution for electromagnetic wave propagation in a symmetric dielectric slab with finite absorption. The functional form of the eigenvalue equation is identical to the lossless case except the propagation constants take on complex values. Additional loss-guidance and anti-guidance modes appear in the lossy model which do not normally exist in the analogous lossless case. An approximate solution for the longitudinal attenuation coefficient αz is derived from geometric optics and shows excellent agreement with the exact value. Lossy mode propagation is then explored in the context of photovoltaics by modeling a thin film solar cell made of amorphous silicon.
RESUMEN
We present a concept for improving the efficiency of thin-film solar cells via scattering from dielectric particles. The particles are embedded directly within the semiconductor absorber material with sizes on the order of one wavelength. Importantly, this geometry is fully compatible with the use of an anti-reflective coating (ARC) to maximize light capture. The concept is demonstrated through finite-difference time domain (FDTD) simulations of spherical SiO(2) particles embedded within a 1.0 microm layer of crystalline silicon (c-Si) utilizing a 75 nm ARC of Si(3)N(4). Several geometries are presented, with gains in absorbed photon flux occurring in the red end of the spectrum where silicon absorption is weak. The total integrated absorption of incident photon flux across the visible AM-1.5 spectrum is on the order of 5-10% greater than the same geometry without any dielectric scatterers.
