Along the last two centuries, the story of semiconductor materials ranged from a mix of disbelief and frustration to one of the most successful technological achievements ever seen. Such a progress comprised the development of materials and models that, allied to the knowledge provided by spectroscopic techniques, resulted in the (nowadays) omnipresent electronic gadgets. Within this context, optically-based methods were of special importance since, amongst others, they presented details about the electronic states and energy bandgap E_gapof semiconductors which, ultimately, decided about their application in devices. Stimulated by these aspects, this work investigated the semiconductors silicon, germanium, and gallium-arsenide in the crystalline (bulk and powder) and amorphous (film) forms. The detailed analysis of the experimental results indicates that accurate E_gapvalues can be obtained by fitting a sigmoid (Boltzmann) function to their corresponding optical absorption spectra. The method is straightforward and, contrary to the traditional approaches to determine E_gap, it is exempt from errors due to experimental spectra acquisition and data processing. Additionally, it complies with the requirements of direct, indirect, and amorphous bandgap semiconductors, and it is able to probe the (dis)order of the material as well. In view of these characteristics, a new−unified methodology based on the fitting of the absorption spectrum with a Boltzmann function is being proposed to efficiently determine the optical bandgap of semiconductor materials.

In this work the relationship between the characteristic energy of the Urbach edge $E_0$ and the parameter $B^{\frac{1}{2}}$ of the Tauc's representation of the absorption coefficient of $a$-SiN- and $a$-GeN-based alloys is presented and discussed. No correspondence has been experimentally found between $B^{\frac{1}{2}}$ and the topological disorder induced by small impurity concentrations in the network (less than a few at.%), which provokes a broadening of the Urbach tail. In the alloy regime, nevertheless, $E_0$ and $B^{\frac{1}{2}}$ present a linear correspondence. This fact is discussed in terms of the structural changes induced by atoms of different atomic coordination, i.e., on the base of the dominant bonding character (which changes from purely covalent to partially ionic) and the electronic states at the top of the valence band, as the nitrogen content is increased. The effects of hydrogen, carbon, and silicon in the $a$-Si and $a$-Ge networks are also discussed in terms of the Tauc slope $B^{\frac{1}{2}}$ parameter.

The known properties of nitrogen as an impurity in, and as an alloy element of, the germanium network are reviewed in this article. Amorphous and crystalline germanium–nitrogen alloys are interesting materials with potential applications for protective coatings and window layers for solar conversion devices. They may also act as effective diffusion masks for III-V electronic devices. The existing data are compared with similar properties of other group IV nitrides, in particular with silicon nitride. To a certain extent, the general picture mirrors the one found in Si–N systems, as expected from the similar valence structure of both elemental semiconductors. However, important differences appear in the deposition methods and alloy composition, the optical properties of as grown films, and the electrical behavior of nitrogen-doped amorphous layers. Structural studies are reviewed, including band structure calculations and the energies of nitrogen-related defects, which are compared with experimental data. Many important aspects of the electronic structure of Ge–N alloys are not yet completely understood and deserve a more careful investigation, in particular the structure of defects associated with N inclusion. The N doping of the $a$ -Ge:H network appears to be very effective, the activation energy of the most effectively doped samples becoming around 120 meV. This is not the case with N-doped $a$ -Si:H, the reasons for the difference remaining an open question. The lack of data on stoichiometric $\mathrm{\beta -}{\mathrm{Ge}}_{\mathrm{3}}{\mathrm{N}}_{\mathrm{4}}$ prevents any reasonable assessment on the possible uses of the alloy in electronic and ceramic applications.

Green light emission at room temperature was achieved from nonhydrogenated amorphous silicon–nitrogen (a-SiN) thin films. The films were deposited by cosputtering a silicon target covered with metallic erbium platelets in an Ar+N2 atmosphere. According to the deposition conditions, the nitrogen concentration [N] reached ∼40 at. % rendering an optical gap of approximately 3.5 eV while the Er concentration [Er] was estimated to be ∼10 at. % in the present films. The high [Er] associated to the optical band gap allows the direct excitation of Er3+ ions. This optical excitation is more efficient at low temperatures as a consequence of the reduction in nonradiative processes, and when exciting the samples with the 488.0 nm line of an Ar+ laser which is in resonance with the F47/2Er3+ energy level. In addition to light emission at ∼520 and ∼545 nm, transitions in the infrared energy region could be easily verified in as-deposited samples.

Titanium-dioxide (TiO₂) is a low-cost, chemically inert material that became the basis of many modern applications ranging from, for example, cosmetics to photovoltaics. TiO₂ exists in three different crystal phases − Rutile, Anatase and, less commonly, Brookite − and, in most of the cases, the presence or relative amount of these phases are essential to decide the TiO₂ final application and its related efficiency. Traditionally, X-ray diffraction has been chosen to study TiO₂ and provides both the phases identification and the Rutile-to-Anatase ratio. Similar information can be achieved from Raman scattering spectroscopy that, additionally, is versatile and involves rather simple instrumentation. Motivated by these aspects this work took into account various TiO₂ Rutile+Anatase powder mixtures and their corresponding Raman spectra. Essentially, the method described here was based upon the fact that the Rutile and Anatase crystal phases have distinctive phonon features, and therefore, the composition of the TiO₂ mixtures can be readily assessed from their Raman spectra. The experimental results clearly demonstrate the suitability of Raman spectroscopy in estimating the concentration of Rutile or Anatase in TiO₂ and is expected to influence the study of TiO₂-related thin films, interfaces, systems with reduced dimensions, and devices like photocatalytic and solar cells.