Strain-Induced Band Gap Engineering in Selectively Grown GaN–(Al,Ga)N Core–Shell Nanowire Heterostructures

Abstract

We demonstrate the selective area growth of GaN−(Al,Ga)N core−shell nanowire heterostructures directly on Si(111). Photoluminescence spectroscopy on as-grown nanowires reveals a strong blueshift of the GaN band gap from 3.40 to 3.64 eV at room temperature. Raman measurements relate this shift to compressive strain within the GaN core. On the nanoscale, cathodoluminescence spectroscopy and scanning transmission electron microscopy prove the homogeneity of strain-related luminescence along the nanowire axis and the absence of significant fluctuations within the shell, respectively. A comparison of the experimental findings with numerical simulations indicates the absence of a significant defect-related strain relaxation for all investigated structures, with a maximum compressive strain of −3.4% for a shell thickness of 50 nm. The accurate control of the nanowire dimensions, namely, core diameter, shell thickness, and nanowire period, via selective area growth allows a specific manipulation of the resulting strain within individual nanowires on the same sample. This, in turn, enables a spatially resolved adjustment of the GaN band gap with an energy range of 240 meV in a one-step growth process.

Surface passivation and self-regulated shell growth in selective area-grown GaN–(Al,Ga)N core–shell nanowires

“…1e), compressive strain within the GaN core which leads to a potential shift and a change of the energetically highest valence band from A-exciton to C-exciton. 4,9,10 In contrast, the energetic shift of the YL emission is 99 meV with respect to the bare GaN and is, thus, less pronounced than the band gap shift. Since the donor band for the DAP recombination of YL is similar to the conduction band of GaN (at room temperature), 28 the smaller strain-induced shift implies a discrepancy in the deformation potentials of the unknown deep acceptor band and the GaN valence bands.…”

“…4 A main characteristic for different (Al,Ga)N shell thicknesses is the energetic variation of the PL peak centers of the GaN core, which has been assigned to compressive strain within the GaN core along its growth direction. 4,[8][9][10][11] A phenomenon which has not been addressed so far is the loss of PL intensity of the NWs grown on Si once coated with the (Al,Ga)N shell. In particular, a decrease of the PL intensity of core-shell NWs with t shell = 30′ (blue curve) by two orders of magnitude with respect to the bare GaN NWs (green curve) can be identified.…”

“…A detailed description of GaN-(Al,Ga)N coreshell NW SAG can be found in a previous publication. 4 The measurement setups and the corresponding parameters for μ-PL, CL and STEM are equivalent to those in ref. 4.…”

“…4 The measurement setups and the corresponding parameters for μ-PL, CL and STEM are equivalent to those in ref. 4. TEM, applied for the shell nucleation study, has been performed with a FEI Tecnai transmission electron microscope at an acceleration voltage of 200 kV.…”

“…[1][2][3] Moreover, in a recent publication, we have demonstrated the controlled strain-induced tuning of the GaN NW band gap toward higher energies with the help of an (Al,Ga)N shell. 4 However, in particular for coreshell structures with comparably thick shells (>30 nm), where strain effects play a major role, the nucleation mechanism of the shell and defects within the heterostructures are scarcely investigated. Hence, the nanoscale optical and structural properties of heteroepitaxial selectively grown GaN-(Al,Ga)N core-shell NWs are presented in this work.…”