M.P.F. de Godoy

Strain and localization effects in InGaAs(N) quantum wells: Tuning the magnetic response

We investigated effects of localization and strain on the optical and magneto-optical properties of diluted nitrogen III–V quantum wells theoretically and experimentally. High-resolution x-ray diffraction, photoluminescence (PL), and magneto-PL measurements under high magnetic fields up to 15 T were performed at low temperatures. Bir-Pikus Hamiltonian formalism was used to study the influence of strain, confinement, and localization effects. The circularly polarized magneto-PL was interpreted considering localization aspects in the valence band ground state. An anomalous behavior of the electron-hole pair magnetic shift was observed at low magnetic fields, ascribed to the increase in the exciton reduced mass due to the negative effective mass of the valence band ground state.

Insulating substrates for cubic GaN-based HFETs

The growth of cubic group III-nitrides is a direct way to eliminate polarization effects, which inherently limits the fabrication of normally off heterojunction field-effect transistors (HFETs) in the GaN technology. However, for the achievement of electronic devices with cubic nitrides an important precondition is the availability of a high-resistive substrate or GaN buffer layer with zinc-blende crystal structure. We investigated the applicability of carbonized high resistance Si (001)-substrates and thick conductive free-standing 3C-SiC (100) substrates with an Ar^+-ion-damaged surface layer for this purpose and studied the use of carbon-doped GaN buffer layers for electrical insulation. We found that Ar-implantation of 3C-SiC is an appropriate alternative to fabricate insulation layer for cubic GaN (c-GaN) growth and that C-doped GaN buffers introduce non-linear I-V characteristics. The structural properties of c-GaN on Ar-implanted 3C-SiC are comparable to GaN on untreated 3C-SiC whereas on carbonized Si substrates an increase of dislocation density and surface roughness is observed.

Atmosphere-Dependent Photoconductivity of ZnO in the Urbach Tail

Photoconductivity is a fundamental and highly applicable phenomenon for semiconductor oxide-based devices, and the presence of defects plays a significant role in this mechanism. Here, we present an investigation based on different atmospheres and light excitation (above and below bandgap) dependences of zinc oxide thin film grown by spray-pyrolysis. As-grown ZnO presents a representative Urbach tail associated to the presence of localized levels in the bandgap. Photoconductivity response and decay times are investigated for air and inert atmospheres as well as under vacuum conditions with significant features due to light excitation conditions. The observed characteristics are explained based on oxygen photodesorption when excitation is above bandgap while this process is suppressed when excitation is below bandgap.

Magnetic and structural properties of Fe-implanted cubic GaN

In this article, we report on structural and magnetic properties of cubic GaN epitaxial layers implanted with Fe ions and subsequently subjected to thermal annealing. The epitaxial quality of the layers was studied by X-ray diffraction rocking curves (ω-scans) and Raman spectroscopy. The results show that the implantation damages the crystal structure producing an expansion of the lattice parameter in the implanted region. These damages are partially removed by the thermal treatment. Room temperature ferromagnetism is observed for the sample implanted with a dose of 1.2 × 1016 cm−2, while samples implanted with 2.4 × 1016 cm−2 show a coexistence of ferromagnetism and paramagnetism due to disperse Fe3+. Thermal annealing changes these magnetic properties. For the low dose sample, the ferromagnetism is converted into paramagnetism while for the high dose we observed an enhancement of the ferromagnetic contribution characterized by a superparamagnetism behavior attributed to Fe-based particles.

Electron and hole scattering in short-period InGaAs/InP superlattices

The combination of photoluminescence and magneto-transport measurements is used to study the single-particle relaxation time and the transport scattering time in short-period InGaAs/InP superlattices. Both the single-particle relaxation times of the electrons and of the holes were obtained in the same samples and were shown to be determined by the remote-impurity scattering. The transport scattering time for electrons was found to be dominated by the interface-roughness scattering with lateral length Λ=10 nm and height Δ = 0.13 nm. We also discuss the importance of multiple-scattering effects for small well widths and of alloy scattering for large well widths.