Self-assembled columnar AlGaN/GaN nanocavities, with an active region of GaN quantum disks embedded in an AlGaN nanocolumn and cladded by top and bottom AlN/GaN Bragg mirrors, were grown. The nanocavity has no cracks or extended defects, due to the relaxation at the Si interface and to the nanocolumn free-surface to volume ratio. The emission from the active region matched the peak reflectivity by tuning the Al content and the GaN disks thickness. Quantum confinement effects that depend on both the disk thickness and the inhomogeneous strain distribution within the disks are clearly observed.
Polarization-sensitive photodetectors for the ultraviolet spectral range based on M-plane GaN films grown on LiAlO 2 substrates have been fabricated and characterized. These detectors exploit the dichroic properties of strained, M-plane GaN films. For a 400-nm-thick film, a maximum contrast of 7.25 between the detection of light polarized perpendicular and parallel to the c-axis is reached at 363 nm. Considerations for the detector design show that thin strained M-plane GaN films will enhance the polarization-sensitive bandwidth, while the maximum contrast can be obtained for relaxed thick films under weak signal detection conditions.
A theoretical study of the effect of inhomogeneous strain in piezoelectric materials is presented. It is shown that for deformation potentials opposite to strain-induced piezoelectric potentials, electrons and holes can drift in opposite directions, both in the in-plane and growth directions. The resultant potential will dramatically affect the confinement of carriers in mesoscopic quantum structures. In this case, states spatially separated in the in-plane direction can be formed, giving rise to unusual optical properties. The proposed model allows for the interpretation of experimental results obtained for ͑Al, Ga͒N / GaN nanocolumnar structures, namely, the dependence of the luminescence intensity on the well thickness, an unusual spectral line broadening, and long lifetime contributions. This model aims to provide design rules to enhance the radiative efficiency in this type of structure.
Carrier confinement effects in nanocolumnar Al x Ga 1−x N / GaN multiple quantum disks have been studied by photoluminescence, as a function of the Al content and quantum disk thickness. Experimental emission energies are compared to theoretical calculations based on a one-dimensional Schrödinger-Poisson solver, including spontaneous and piezoelectric polarizations, surface potentials, and strain. An inhomogeneous biaxial ͑in-plane͒ strain distribution within the GaN quantum disks, pseudomorphically grown on strain-free Al x Ga 1−x N nanocolumns, results from a reduction of the accumulated elastic energy at the disk free surface ͑GaN-air boundary͒. This strain reduction annihilates partially the piezoelectric field, giving rise to a specific carrier confinement mechanism ͑strain confinement͒, that depends on the disk thickness. This strain confinement mechanism is the origin of the luminescence quenching in very thin GaN quantum disks, as well as the main source of the emission linewidth broadening.
Influence of threading dislocations on GaN-based metal-semiconductor-metal ultraviolet photodetectors Appl. Phys. Lett. 98, 011108 (2011);The spectral response of metal-semiconductor-metal ͑MSM͒ and Schottky barrier photodiodes have been studied in the near-and vacuum ultraviolet ͑VUV͒. Devices were fabricated on micro-epitaxial lateral overgrowth GaN layers, which presented dislocation densities as low as 7ϫ10 7 cm Ϫ2 . Experimental results indicate that the surface properties become critical for the optical response at short wavelengths. Schottky barrier photodiodes showed a lower VUV sensitivity than MSMs as a result of the radiation absorption in the semitransparent Au top-layer. However, Schottky photodiodes yielded a better time stability operating in photovoltaic mode. For photon energies above 10.5 eV, the quantum efficiency of the MSM photodiodes was enhanced as a consequence of the different nature of light-semiconductor interactions, which provoke a decrease of the absorption coefficient and the generation of multiple electron-hole pairs for each impinging photon. The ionization energy for GaN has been also estimated.
The effect of strain induced by electric field in AlGaN/GaN high-electron-mobility transistors is investigated by theoretical calculations based on the minimization of the electric enthalpy functional. Results of the proposed model show that the converse piezoelectric effect increases (decreases) the stored elastic energy at positive gate voltage under biaxial tensile (compressive) strain, whereas it decreases (increases) at negative gate voltage. Hence, strain relaxation of piezoelectric origin is only expected in the on-state operation. In contrast, the degradation in the off-state operation could be identified with the effect of the electrostatic force generated by the increase in the stored electrostatic energy.
The authors demonstrate a photodetection configuration where the responsivity in the ultraviolet spectral region is limited to a few nanometers, representing high-quality-factor, narrow-band detection together with polarization sensitivity. Both features are obtained by utilizing a polarization-sensitive photodectector in combination with a polarization filter made from two identical M-plane GaN films on γ-LiAlO2 (100) substrate. The optical band gap of these films depends on the direction of the in-plane polarization vector of the incident light beam with respect to the c axis. Electronic-band-structure calculations show that the naturally present anisotropic in-plane strain in these films is the crucial parameter to achieve both a high responsivity and a high polarization contrast.
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