Luminous Efficiency of Axial In_<i>x</i>Ga_1–<i>x</i>N/GaN Nanowire Heterostructures: Interplay of Polarization and Surface Potentials

Using continuum elasticity theory and an eight-band <b>k</b>·<b>p</b> formalism, we study the electronic properties of GaN nanowires with axial In_<i>x</i>Ga_1–<i>x</i>N insertions. The three-dimensional strain distribution in these insertions and the resulting distribution of the polarization fields are fully taken into account. In addition, we consider the presence of a surface potential originating from Fermi level pinning at the sidewall surfaces of the nanowires. Our simulations reveal an in-plane spatial separation of electrons and holes in the case of weak piezoelectric potentials, which correspond to an In content and layer thickness required for emission in the blue and violet spectral range. These results explain the quenching of the photoluminescence intensity experimentally observed for short emission wavelengths. We devise and discuss strategies to overcome this problem

Strain Engineering of Nanowire Multi-Quantum Well Demonstrated by Raman Spectroscopy

An analysis of the strain in an axial nanowire superlattice shows that the dominating strain state can be defined arbitrarily between unstrained and maximum mismatch strain by choosing the segment height ratios. We give experimental evidence for a successful strain design in series of GaN nanowire ensembles with axial In_<i>x</i>Ga_1–<i>x</i>N quantum wells. We vary the barrier thickness and determine the strain state of the quantum wells by Raman spectroscopy. A detailed calculation of the strain distribution and LO phonon frequency shift shows that a uniform in-plane lattice constant in the nanowire segments satisfactorily describes the resonant Raman spectra, although in reality the three-dimensional strain profile at the periphery of the quantum wells is complex. Our strain analysis is applicable beyond the In_<i>x</i>Ga_1–<i>x</i>N/GaN system under study, and we derive universal rules for strain engineering in nanowire heterostructures