Yu. M. Sirenko

Piezoelectric effect on optical properties of pseudomorphically strained wurtzite GaN quantum wells

The presence of internal strain in wurtzite quantum-well (QW) structures may lead to the generation of large polarization fields. These piezoelectric fields cause a spatial separation of the electrons and holes inside the QW to screen the internal fields. A self-consistent calculation of optical gain and the corresponding differential gain is presented in pseudomorphically strained GaN quantum wells as a function of carrier density. Based on the local exchange-correlation potential, electron and hole band structures are obtained by coupling Poissons equation with an effective-mass Schrodinger equation in the conduction band and an envelope-function (or k/spl middot/p) Hamiltonian in the valence band. Our calculations show that self-consistent calculations including the piezoelectric effects are essential for accurate description of strained wurtzite QW structures.

Acoustic phonon quantization in buried waveguides and resonators

Starting from a classical Hamiltonian for nonhomogeneous elastic media, a procedure is developed for acoustic phonon quantization in resonators as well as linear and planar waveguides. The formalism is illustrated in an example of acoustic phonon modes in a buried cylindrical waveguide. The deformation potential Hamiltonian for electron - acoustic phonon interaction is also obtained.

Strain effects on valence band structure in würtzite GaN quantum wells

The effect of strain on valence band spectra in both bulk and in pseudomorphic GaN quantum wells is studied theoretically using recent experimental results for deformation potential constants. Wave functions and dispersion curves for A‐, B‐ and C‐hole subbands are obtained from linear combination of bulk analytical solutions for the 3×3 wurtzite block‐Hamiltonian. A detailed analysis is presented for the dependence of hole spectrum on quantum well width, depth, and strain due to lattice mismatch.

Strain effects on optical gain in wurtzite GaN

Strain effects on optical gain in hexagonal bulk GaN are calculated and explained in terms of the change in the effective hexagonal crystal field component. Qualitatively, even unstrained wurtzite structures correspond to cubic crystals with a proper biaxial stress applied. Such biaxial stress results in effective tensile deformation along the c axis ([111] direction in cubic crystals) and compressive strain in the perpendicular plane. Therefore, the light mode with a polarization vector parallel to the c axis is suppressed, while the mode with a perpendicular polarization is enhanced in wurtzite structures. Thus, compared to cubic structures with similar material parameters, a strong optical anisotropy of wurtzites results in enhanced gain for certain light polarizations, which make wurtzite structures superior for lower-threshold lasing. These qualitative arguments are illustrated by numerical calculations of optical gain in biaxially strained wurtzite GaN, based on a 6×6 envelope-function Hamiltonian.

Valence band parameters of wurtzite materials

Abstract We deduced the valence band parameters of several wurtzite materials (ZnS, CdS, CdSe, and GaN) by matching the results of existing full-band calculations of the energy spectrum with analytical expressions of the envelope-function formalism. The calculated A-, B-, and C-type hole dispersion relations show strongly anisotropic characteristics and anti-crossing features in spectrum due to band mixing effects. We demonstrated that for all materials considered except CdSe, the spherical cubic approximation for six Luttinger-like parameters holds with good accuracy, so that the anisotropy arises mainly owing to the crystal field splitting term. Thus, the top valence band may be described with (in addition to crystal field and spin-orbit splitting energies) only two Luttinger-like parameters, γ 1 and γ 2 .

Anisotropic hole scattering in hexagonal GaN

We calculate the hole scattering rate owing to the interaction with polar optical phonons and ionized impurities in bulk wurtzite GaN. The valence band states of this hexagonal material are obtained from a matrix Hamiltonian. The calculated scattering rate shows strong orientational dependence with respect to the crystal c-axis. Analysis shows that both anisotropy in the dispersion relations and the Bloch overlap factors are important for a proper description of hole scattering.

Carrier capture in quantum well embedded quantum wire structures

We propose a novel quantum wire (QWR) laser structure with improved carrier capture characteristics, where the carriers are injected into a quantum well (QWL) and subsequently recombine within an embedded QWR. The corresponding electron capture rates via polar optical phonon scattering are calculated for this system. An oscillatory behavior of the electron capture rate is observed as a function of the QWR thickness at the temperatures considered (77 K and 300 K). The amplitude of these oscillations also increases as the QWR width decreases. Our calculations show that the electron capture rate in the QWL embedded QWR structure can be improved by more than 30% when compared to single QWLs. Therefore, the proposed QWR laser design provides improved modulation bandwidth and optical gain over conventional QWL and QWR structures.

Carrier capture in cylindrical quantum wires

We present quantum mechanical calculations of electron capture rates in cylindrical quantum wires via polar‐optical phonon scattering. The capture rate dependence on quantum wire radius and lattice temperature is investigated. An oscillatory behavior of the electron capture rate is observed as a function of the quantum wire radius at the temperatures considered in this study (20–300 K). However, the amplitude of these oscillations decreases significantly at large wire radii and high lattice temperatures.