Alteration of structural and optical properties in quaternary Al0.1In0.1Ga0.8N films using ultraviolet assisted photo-electrochemical etching route

Effects of ultraviolet-assisted electrochemical etching current densities on structural and optical characteristics of porous quaternary AlInGaN alloys

We report the room-temperature anisotropic dielectric functions (DFs), refractive indices, and absorption coefficients as well as birefringence and dichroism of wurtzite GaN in the spectral range between 0.6 eV and 6 eV. They have been determined by combined spectroscopic ellipsometry, optical retardation, and transmission measurements on a series of m- and c-plane bulk substrates prepared from crystals grown by hydride vapor phase epitaxy. The accuracy of the derived DFs is estimated by investigation of the role of mosaicity-related crystal imperfections, self-consistency test based on a Kramers-Kronig analysis, and examination of the influence of kind of overlayer. We also briefly discuss optical properties of a highly defective near-surface layer of GaN crystals introduced by their mechanical polishing.

Anisotropic optical constants, birefringence, and dichroism of wurtzite GaN between 0.6 eV and 6 eV

The third-generation wide bandgap semiconductor GaN currently occupies a hot spot in the fields of high-power electronics and optoelectronics. Fully exploring its optical and optoelectronic characteristics is of great significance. Here, we provide a systematic study on the temperature-dependent dielectric functions of GaN grown by metal-organic chemical vapor deposition in the spectral range of 0.73-5.90 eV via spectroscopic ellipsometry experiments and first-principles calculations. Ellipsometric measurements identify two typical absorption peaks that originate from the excitonic and phonon-assisted indirect absorption process, respectively. To explore the underlying physics, we perform first-principles calculations using the independent-particle approximation, model Bethe-Salpeter equation (mBSE), and phonon-assisted indirect absorption process (Inabs). In comparison with ellipsometric measurements, the mBSE calculation determines the absorption peak contributed by the many-body excitonic effect, while the Inabs calculation successfully predicts the second absorption peak. When heating the crystal, it observes the redshift and weakening of absorption peaks, intrinsically due to the nontrivial electron-phonon interaction as lattice vibration strengthens. While doping GaN with Fe or Si elements, the introduced free carriers modify the electronic interband transition. As the temperature increases, more free carriers are excited, and the temperature influence on the absorption peak is more significant than that of the undoped one. This work fully explores the physical origins of the temperature and doping effect on UV-Vis dielectric functions of GaN, aiming to promote its application in the fields of high-power electronic devices.

Ellipsometric and first-principles study on temperature-dependent UV–Vis dielectric functions of GaN

We carried out reflectance and spectroscopic ellipsometry measurements of high-purity c -plane epitaxial films of wurtzite GaN in a temperature range 5-840 K. Analysis of the data reveals three main contributions to the dielectric function related to optical transitions involving discrete exciton states, excitonic continuum and exciton-phonon complexes, respectively. The observed contributing mechanisms show different temperature dependence. This behavior is related to specific interactions between optical excitations and thermal LO phonons. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

F. Bärwolf

Papers

Excitons and exciton‐phonon coupling in the optical response of GaN