J. Pal

Enhancement of efficiency of InGaN-based light emitting diodes through strain and piezoelectric field management

We report calculations of the strain dependence of the piezoelectric field within InGaN multi-quantum wells light emitting diodes. Such fields are well known to be a strong limiting factor of the device performance. By taking into account the nonlinear piezoelectric coefficients, which in particular cases predict opposite trends compared to the commonly used linear coefficients, a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. We propose that such reduction of the field can be obtained by including a metamorphic InGaN layer below the multiple quantum well active region.

Non-linear piezoelectricity in zinc blende GaAs and InAs semiconductors

We investigate the strain dependence of piezoelectric effect, both linear and non linear, in zincblende GaAs and InAs semiconductors. We expanded the polarization in terms of the ionic and dipole charges, internal displacement and the exploited the ab-initio Density Functional Theory (DFT) to evaluate the dependence of all quantities on the strain tensor. By this detailed study of the non linear piezoelectric effect, we report that even third order effects are significant.

Elastic and vibrational properties of group IV semiconductors in empirical potential modelling

We have developed an interatomic potential that with a single set of parameters is able to accurately describe at the same time the elastic, vibrational and thermodynamics properties of semiconductors. The simultaneous inclusion of radial and angular forces of the interacting atom pairs (short range) together with the influence of the broken crystal symmetry when the atomic arrangement is out of equilibrium (long range) results in correct predictions of all of the phonon dispersion spectrum and mode-Grüneisen parameters of silicon and germanium. The long range interactions are taken into account up to the second nearest neighbours, to correctly influence the elastic and vibrational properties, and therefore represent only a marginal computational cost compared to the full treatment of other proposed potentials.Results of molecular dynamics simulations are compared with those of ab initio calculations, showing that when our proposed potential is used to perform the initial stages of the structural relaxation, a significant reduction of the computational time needed during the geometry optimization of density functional theory simulations is observed.

A review of non linear piezoelectricity in semiconductors

The piezoelectric effect in polar semiconductor has seen increased interest in recent years because of the prospect of exploiting semiconducting behavior and piezoelectric response, i.e. generating electric fields in response to pressure, in novel optoelectronic devices with applications as pressure sensors and energy harvesting. In this paper we review the basic concepts and recent findings related to the novel concept of non-linear piezoelectricity, which can be exploited in composite nanostructured materials to increase the piezoelectric response compared to bulk materials. Applications to light emitting diodes and nanowires will also be discussed. We will show how the non-linear theory of piezoelectricity can in some cases lead to opposite predictions compared to the classic linear theory.

Beyond ZnO nanowires for piezotronics and nanogenerators

In the past decade ZnO nanowires have been the key enabling material for demonstrating novel electronics components in the field of piezotronics and in the first realization of a nanogenerator. What are the materials that will be crucial in demonstrating even more novel devices in future years? We propose the use of both core shell nanowires and graphene as key enablers of new functionalities.

Ripples, phonons and bandgap in strained graphene

Using a novel interatomic force field, called MMP, we study the morphology of Graphene layers under a variety of strain conditions. We report that strain induced ripples possess the “right” kind of elastic deformation that is necessary in order to produce appreciable bandgap opening, which we calculate using Tight Binding, even for low enough strain that can be accessed through realistic means. At the same time the vibrational properties, calculated from analytic derivatives of the MMP force field and used within the dynamics matrix method, can be easily linked to strain obtained from Molecular Dynamics, opening the way for accurate modelling of Raman data. We also show that our models have allowed us to realize in practice novel devices based on our predictions.

Non linear piezoelectricity in zincblende GaAs and InAs semiconductors

We investigate the strain dependence of piezoelectric effect, both linear and non linear, in zincblende GaAs and InAs semiconductors. We expanded the polarization in terms of the ionic and dipole charges, internal displacement and the exploited the ab-initio Density Functional Theory (DFT) to evaluate the dependence of all quantities on the strain tensor. By this detailed study of the non linear piezoelectric effect, we report that even third order effects are significant.

Strain dependence of internal displacement and effective charge in wurtzite III-N semiconductors

The elastic and dielectric properties of binary III-N wurtzite semiconductors have been investigated as a function of strain. Using an ab initio density functional theory (DFT), we concentrate on the internal displacement (u) and Born effective charge (Z*) and show that our model provides a unique non linear dependence of the III-N material properties as a function of strain.

Investigating the effect of non linear piezoelectricity on the excitonic properties of III-N semiconductor quantum dots

We investigate the effects of linear and non linear piezoelectricity in wurtzite III-N semiconductors and their influence on the electronic properties of low dimensional quantum dots. By studying the dependence of the biexciton on structural and geometrical parameters of the nanostructure, we show second order to be important particularly when the strain in the nanostructure is reduced

Erratum: Second-order piezoelectricity in wurtzite III-N semiconductors [Phys. Rev. B 84, 085211 (2011)]

We note two typographical errors in our recent paper. First, in Table III, the Lw/Lb ratio in the first row should be 3/5, not 3/50. Second, we gave an incorrect sign for some of the parameters listed in Table IV. The parameters of e311, e333, and e133 in Table IV should read as given here. Because the correct signs were used to calculate the fields in the original work, these corrections do not affect our conclusions.