Photovoltaics with Piezoelectric Core−Shell Nanowires

Abstract

We report on a theoretical discovery of a generic piezoelectric field in strained core-shell compound semiconductor nanowires. We show, using both an analytical model and numerical simulations based on fully electroelastically coupled continuum elasticity theory, that lattice-mismatch-induced strain in an epitaxial core-shell nanowire gives rise to an internal electric field along the axis of the nanowire. This piezoelectric field results predominantly from atomic layer displacements along the nanowire axis within both the core and shell materials and can appear in both zinc blende and wurtzite crystalline core-shell nanowires. The effect can be employed to separate photon-generated electron-hole pairs in the core-shell nanowires and thus offers a new device concept for solar energy conversion.

Strain in crystalline core-shell nanowires

“…Several comparisons have been given with numerical treatments using either a valence force field model [2,3,6,17,26,28], or a finite element implementation of continuum elasticity [26]. Even if commercial packages now exist which will give the same results as these numerical treatments, the analytical treatment remains faster, and it favors a more comprehensive understanding.…”

“…This is the case of NWs with the wurtzite structure grown along the c-axis, as well as NWs with the zinc-blende structure grown along the 1 1 1 axis. The relevant strain components entering the longitudinal polarization are [3] ε zz and (ε rr + ε θθ ) in the first case, and (2ε zz − ε xx − ε yy ) in the second case. In addition, confinement effects should be taken into account if the NW radius is small enough, and the confining potential is modified by these two mechanisms.…”

“…Again, the polarization in the core is along the axis, determined by −e 14 (ε xx + ε yy − 2ε zz )/ √ 3 where e 14 is the unique coefficient of the piezoelectric tensor (the indices refer to the cubic axes). A complex lanscape however emerges in the shell from the presence of in-plane and axial shear strains, and of additional terms in the piezoelectric tensor written in the trigonal axes [3,26,27].…”

“…In turn, the built-in strain can be used as a further adjustable parameter: strain engineering can be used to lower the degeneracy in the valence band and select the type of holes with a larger spin for spintronics applications (for instance a larger spin-carrier coupling in diluted magnetic semiconductors) [1], or a smaller longitudinal mass to achieve a better mobility in transport properties [2]. The strain can also be designed to induce a built-in piezoelectric field, resulting in a faster separation of the electron-hole pairs in photovoltaic applications [3]. Finally, strain is an important parameter when engineering Si-Ge NWs to obtain direct bandgap configurations and efficient emission of light [4].…”

Residual strain and piezoelectric effects in passivated GaAs/AlGaAs core-shell nanowires

“…In solar cells, they can boost electron-hole pair separation, reducing recombination losses. PZ effects are particularly attractive for III-V nanowire solar cell structures [5,6] because nanowires preferentially grow along the polar <111> direction and are easily strained by growing a shell around them [7][8][9]. The shell also acts as protective layer for the core surface.…”