0000000000239943
AUTHOR
José M. Llorens
Deformation profile in GaN quantum dots: Medium-energy ion scattering experiments and theoretical calculations
Medium energy ion scattering (MEIS) has been used to measure at the scale of the monolayer the deformation profile of self-organized GaN quantum dots grown on AlN by molecular-beam epitaxy. The effect of capping the GaN dots by a thin layer of AlN has also been studied. It is shown that GaN dots are partially relaxed in every situation. Capping them with AlN has little effect on the basal plane, as expected, but strongly modifies the strain of the upper part of dots. The experimental results are compared with theoretical calculations, allowing one to conclude that GaN quantum dots experience a nonbiaxial strain, which drastically decreases when going from the basal plane up to the apex of t…
Energy levels of a quantum ring in a lateral electric field
Abstract The electronic states of a semiconductor quantum ring (QR) under an applied lateral electric field are theoretically investigated and compared with those of a quantum disk of the same size. The eigenstates and eigenvalues of the Hamiltonian are obtained from a direct matrix diagonalization scheme. Numerical calculations are performed for a hard-wall confinement potential and the electronic states are obtained as a function of the electric field and the ratio r2/r1, where r2 (r1) is the outer (inner) radius of the ring. The effects of decreasing symmetry and mixing on the energy levels and wave functions due to the applied electric field are also studied. The direct optical absorpti…
Resonant Raman scattering in self-assembledGaN∕AlNquantum dots
Self-assembled $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ quantum dots have been investigated by means of Raman scattering. A resonant enhancement of the Raman peaks has been observed when the excitation is tuned above the GaN band-gap energy. The polar mode nature, either quasiconfined or interfacial, has been assigned after comparing with the polar optical modes of spheroidal dots calculated within the framework of the anisotropic dielectric continuum model. The built-in strain of the GaN dots induced a substantial blueshift of the nonpolar ${E}_{2H}$ Raman mode frequency. A theoretical model that analyzes the three-dimensional strain distribution in the quantum dots has been employed …
Raman study and theoretical calculations of strain in GaN quantum dot multilayers
Changes in strain and phonon mode energy in stacks of self-assembled GaN quantum dots embedded in AlN have been studied by means of Raman spectroscopy as a function of the number of periods. The ${E}_{2H}$ phonon modes related to the quantum dots and AlN spacers are clearly resolved, and their energies allow monitoring the state of strain of the dots and AlN spacers simultaneously. The evolution of the measured phonon frequencies and the associated strains are discussed in comparison with theoretical calculations of the inhomogeneous strain distribution in a system of coherent misfitting inclusions.
Structural properties of GaN quantum dots
The strain state and the deformation profile of GaN quantum dots embedded in AlN have been measured by high resolution electron microscopy, medium energy ion scattering and grazing incidence X-ray diffraction. The results are compared with theoretical calculations, allowing one to conclude that GaN quantum dots experience a non biaxial strain which drastically decreases when going from the basal plane up to the apex of the dots. We also demonstrate that AlN is distorted in the surroundings of the dots, which provides the driving force for vertical correlation of GaN dots when the AlN spacer between successive planes is thin enough.
Raman study of strain in GaN/AlN quantum dot multilayered structures
Raman spectroscopy has been used to investigate self-assembled stacks of GaN/AlN quantum dots with increasing number of periods. The E2H phonon modes associated to GaN and AlN are clearly resolved with visible excitation, and their energies allow the simultaneous monitoring of the dot and barrier strain states. The compression of the quantum dots is evidenced by a shift of the E2H phonon mode of circa 29 cm–1 to higher energies with respect to its relaxed value. The strain of the AlN spacer is found to be correlated to that of the dot, with an increase in its tensile component for the samples with fewer periods and a partial relaxation for samples over 50 periods. Additionally, resonant eff…
Vibrational modes and strain in GaN/AlN quantum dot stacks: dependence on spacer thickness
We have investigated the influence of spacer thickness on the vibrational and strain characteristics of GaN/AlN quantum dot multilayers (QD). The Raman shift corresponding to the E2h vibrational mode related to the QDs has been analyzed for AlN thicknesses ranging from 4.4 nm to 13 nm, while the amount of GaN deposited in each layer remained constant from sample to sample. It is shown that there is a rapid blue shift of the GaN vibrational mode with spacer thickness when its value is smaller than 7 nm while it remains almost constant for thicker spacers. A rapid increase of the Raman line-width in the thicker samples is also observed. The experimental behavior is discussed in comparison wit…
Electronic structure of a quantum ring in a lateral electric field
The electronic states of novel semiconductor quantum rings (QR's) under applied lateral electric fields are theoretically investigated for different values of the ratio ${r}_{2}{/r}_{1},$ where ${r}_{2}$ ${(r}_{1})$ is the outer (inner) radius of the ring. The eigenstates and eigenvalues of the Hamiltonian are obtained from a direct matrix diagonalization scheme. Numerical calculations are performed for a hard-wall confinement potential and the electronic states are obtained as a function of the electric field and the ratio ${r}_{2}{/r}_{1}.$ An anomalous behavior in the energy vs. electric-field fan plot due to the break of symmetry is predicted. Analytical expressions for the energy level…