M.J. Hernández

On the true optical properties of zinc nitride

Refractive index (n) and extinction coefficient (k) of Zn3N2 layers deposited by radio-frequency magnetron sputtering at temperatures (Ts) between 298 and 523 K were determined by spectroscopic ellipsometry. Results showed strong variations of the apparent optical constants with Ts and time attributed to surface effects. Resonant Rutherford backscattering and spectroscopic ellipsometry confirmed the formation of a ZnO surface layer provoked by the ambient exposure. Samples grown at low Ts presented the lowest surface roughness and exhibited 2.0 < n < 2.8 and 0.6 < k < 1.0 in the 1.5–4.5 eV energy range. The extracted n and k values accurately reproduced the reflectance properties.

Investigation of surface plasmon resonance in Au nanoparticles deposited on ZnO:Al thin films

Spectroscopic ellipsometry in external reflection (ER) and total internal reflection (TIR) modes is used to characterize surface plasmon resonance in Au nanoparticles (AuNPs) deposited on Al-doped ZnO films via surface thiolation. ER ellipsometry exhibits high sensitivity to the alkanethiol layer as well as to localized surface plasmons at energies around 2.3 eV. TIR ellipsometry reveals resonances at higher energies (2.9–3.35 eV), which are dependent on the environment used: air or deionized water. Coupling between charge dipoles inside the AZO layer and surface plasmons may account for the existence of those resonances.

Shallow buried SiNx layers

High dose nitrogen implantations have been performed at an energy of 30 keV. After high temperature annealing, 1200 °C, a buried layer composed mostly of silicon nitride is formed leaving an overlayer with a high fraction of crystalline silicon. The lattice constant of the overlayer and the region below the SiNx are reduced in 0.13% and 0.089%, respectively. The substitutional N seems to be responsible for this reduction.

Fabrication and Characterization of Multiband Solar Cells Based on Highly Mismatched Alloys

Multiband solar cells are one type of third generation photovoltaic devices in which an increase of the power conversion efficiency is achieved through the absorption of low energy photons while preserving a large band gap that determines the open circuit voltage. The ability to absorb photons from different parts of the solar spectrum originates from the presence of an intermediate energy band located within the band gap of the material. This intermediate band, acting as a stepping stone allows the absorption of low energy photons to transfer electrons from the valence band to the conduction band by a sequential two photons absorption process. It has been demonstrated that highly mismatched alloys offer a potential to be used as a model material system for practical realization of multiband solar cells. Dilute nitride GaAs1-xNx highly mismatched alloy with low mole fraction of N is a prototypical multiband semiconductor with a well-defined intermediate band. Currently, we are using chemical beam epitaxy to synthesize dilute nitride highly mismatched alloys. The materials are characterized by a variety of structural and optical methods to optimize their properties for multiband photovoltaic devices.

Effect of the deposition temperature on the properties of Zn 3 N 2 layers grown by rf magnetron sputtering

In this work, polycrystalline zinc nitride films were prepared on Si and glass substrates by rf magnetron sputtering in N2 ambient using Zn as a target. Substrate temperature (Ts) was different for each deposition process ranging from 298 K to 523 K. Optical transmission experiments showed that the optical band gap of the resultant layers decreased as Ts increased. X-ray diffraction scans presented a pattern of oriented crystals along the (100) direction at low Ts in contrast to the highly disoriented patterns obtained at high Ts. Hall measurements were carried out and exhibited n-type character for all samples, with mobilities ranging between 10 and 30 cm2/Vs, and a resistivity reduction as substrate temperature increases. By scanning electron microscopy, it was possible to study the grain structure forming the surface of zinc nitride. The size of the grains became larger as Ts increased, which accounts for the reduction of the electrical resistivity.