Electromechanically Coupled III-N Quantum Dots.

We exploit the three-dimensional (3D) character of the strain field created around InGaN islands formed within the multilayer structures spaced by a less than 1-nm-thick GaN layer for the creation of spatially correlated electronically coupled quantum dots (QDs). The laterally inhomogeneous vertical out-diffusion of In atoms during growth interruption is the basic mechanism for the formation of InGaN islands within as-deposited 2D layers. An anisotropic 3D strain field created in the first layer is sufficient to justify the vertical correlation of the islands formed in the upper layers spaced by a sufficiently thin GaN layer. When the thickness of a GaN spacer exceeds 1 nm, QDs from different layers under the same growth conditions emit independently and in the same wavelength range. When extremely thin (less than 1 nm), a GaN spacer is formed solely by applying short GI, and a double wavelength emission in the blue and green spectral ranges evidences the electromechanical coupling. With k→·p→ calculations including electromechanical fields, we model the optoelectronic properties of a structure with three InGaN lens-shaped QDs embedded in a GaN matrix, with three different configurations of In content. The profiles of the band structures are strongly dependent on the In content arrangement, and the quantum-confined Stark effect is significantly reduced in a structure with an increasing gradient of In content from the top to the bottom QD. This configuration exhibits carrier tunneling through the QDs, an increase of wave functions overlap, and evidence emerges of three distinct peaks in the spectral range.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

AlGaN HEMT Structures Grown on Miscut Si(111) Wafers.

A complex study was performed on a set of AlGaN/GaN high-electron-mobility transistor structures grown by metalorganic vapor phase epitaxy on miscut Si(111) wafers with a highly resistive epitaxial Si layer to investigate the influence of substrate miscut on their properties. The results showed that wafer misorientation had an influence on the strain evolution during the growth and surface morphology, and could have a strong impact on the mobility of 2D electron gas, with a weak optimum at 0.5° miscut angle. A numerical analysis revealed that the interface roughness was a main parameter responsible for the variation in electron mobility.

Scattering Analysis of AlGaN/AlN/GaN Heterostructures with Fe-Doped GaN Buffer.

The results of the study of the influence of Fe segregation into the unintentionally doped GaN channel layer in AlGaN/AlN/GaN heterostructures with Fe-doped GaN buffer layer on the electrical properties of two-dimensional electron gas are presented. A set of several samples was grown by metal-organic vapor-phase epitaxy and characterized by the van der Pauw method. The dependence of concentration and mobility of the two-dimensional electron gas on the channel layer thickness was analyzed theoretically by self-consistent solving of 1D Poisson and Schrödinger equations and scattering rate calculations within the momentum relaxation time approximation. It was found that both concentration and mobility decreases were responsible for the increase in the sheet resistance in the structures with a thinner channel layer, with a drop in mobility being not only due to ionized impurity scattering, but also due to a combined effect of weakening of screening, lower carrier energy and change in form-factors on scattering by interface roughness, dislocations and polar optical phonons.