Synthesis of hybrid nanowires comprising uniaxial and coaxial InGaN/GaN MQWs with a nano-cap
We propose a novel hybrid nanostructure which comprises both uniaxial and coaxial multi-quantum wells (MQWs) on nanowires topped with an InGaN nano-cap. The growth process included both top-down and bottom-up approaches followed by the intentional growth of an InGaN nano-cap to offer larger active area. The In composition was optimized to absorb light at green and blue wavelengths by the uniaxial and coaxial quantum wells respectively. Extensive structural and optical characterizations were carried out. Field emission scanning electron microscopy (FE-SEM) revealed a high density of nanowires. High resolution transmission electron microscopy (HR-TEM) images displayed 5 pairs of uniaxial multi-quantum wells, 6 pairs of coaxial multi-quantum wells and the existence of the intended nano-caps. A photoluminescence (PL) spectrum was recorded for the grown structure at room temperature. The resultant emission spectrum comprised two distinct peaks resulting from each of the multi-quantum well assemblies and emission from the nano-cap. Cathodoluminescence (CL) mapping data revealed discrete bright field images of the InGaN nano-caps and uniaxial multi-quantum well structures. Emission peaks for the nano-caps and both of the multi-quantum well structures were observed in the CL point spectrum which in turn corroborated the PL measurement. During an energy-dispersive X-ray (EDX) study, a high composition of In was found in the nano-cap area along with the distinct presence of both types of multi-quantum wells. In addition, to investigate the opto-electronic device applicability of the grown MQW structure, the photocurrent was measured at various light intensities. The photocurrent density was observed to increase linearly with the increasing light power density. Also the photocurrent density was found to be higher for the hybrid structure than a single uniaxial or coaxial assembly.