Correlative Nanoscale Luminescence and Elemental Mapping in InGaN/(Al)GaN Dot‐in‐a‐wire Heterostructures

Abstract

Ternary InGaN compounds show great promise for light‐emitting diode (LED) applications because of bandgap energies (0.7 – 3.4 eV) that can be tailored to have emission wavelengths spanning the entire visible spectrum. Complex III‐N device heterostructures have been incorporated into GaN nanowires (NWs) recently, but exhibit emission linewidths that are broader than expected for their corresponding planar counterparts, as measured with photoluminescence (PL) spectroscopy [1]. Nanoscale elemental mapping has provided evidence of alloy non‐uniformity in NWs as a likely cause [2]. It is thus critical to understand how the structural and optical properties interplay within individual NW structures, using combined spectroscopic methods that can resolve different localized signals at the nanoscale with analytical scanning transmission electron microscopy (STEM). Multiple InGaN/(Al)GaN quantum dot (QD) embedded nanowire heterostructures (NWHs), grown catalyst‐free on Si(111) substrates by molecular beam epitaxy, were characterized by STEM. To investigate the inhomogeneous broadening observed in PL from an ensemble of NWs [1], nanometer‐resolution STEM‐cathodoluminescence (CL) spectral imaging on single NWs was performed at 150 K using a system as described in [3]. Individual NWs examined show diverse optical responses, but most NWs exhibit multiple sharp emission peaks (25 – 50 nm at FWHM) centered between 500 – 625 nm in the yellow‐green wavelengths (Fig. 1i) from the active region (Fig. 1b–d), identified using the annular dark‐field (ADF) signal collected concurrently. This is consistent with the PL, indicating that the broad emission originates from within single NWs. Subsequent aberration‐corrected STEM‐HAADF images on the same NWs were acquired to evaluate their structural properties, such as the size and morphology of the 10 QDs within the NWH (Fig. 1f). Additionally, electron energy‐loss spectroscopy (EELS) spectrum imaging (SI), together with multiple linear least‐squares fitting, was used to extract the In‐distribution to quantify the In‐composition projected through thickness (Fig. 1g,h) [2]. Apparent spatial‐spectral correlation can be made between shifts in the CL emission wavelength to the relative In‐content between successive QDs from the STEM‐EELS (Fig. 1h, regions are color‐coded to the corresponding emission wavelength based on the legend in Fig. 1i inset). The luminescence intensity within NWs is related to the presence of a GaN or AlGaN shell surrounding the InGaN/GaN NWHs, formed due to sidewall incorporation during the growth of the subsequent GaN barrier and p ‐AlGaN electron‐blocking layer (EBL), respectively. Both can enhance the in‐plane confinement of carriers, hence reducing non‐radiative surface recombination. Therefore, the utilization of larger bandgap AlGaN as barriers were also investigated for their expected enhancement in carrier confinement [4]. The InGaN/AlGaN NWHs exhibit a nested core‐shell structure made up of an Al‐rich AlGaN shell surrounding the InGaN QDs along axial and radial directions (Fig. 2g). STEM‐CL spectral imaging shows a progressive red‐shifting of the emission peaks along the growth direction (Fig. 2b–d,f,h). Spatial localization of individual spectral features suggests superior three‐dimensional carrier confinement, which can be assigned to specific QDs as resolved in the bright‐field (BF) image (Fig. 2a). Lastly, the observed spatial asymmetry in the luminescence intensity distribution, which is affected by charge carrier diffusion and drift in the presence of spontaneous and piezoelectric polarization fields in the InGaN/(Al)GaN NWHs, will also be addressed [5].