Single-emitter and collective effects in light emission from ordered arrays of InGaN-GaN nanowires

It is widely known that the size of a light emitting devices (LED) has a profound effect on their emissive properties. In particular, wire-shaped nanostructures have been proposed for increasing the external quantum... [ view full abstract ]

It is widely known that the size of a light emitting devices (LED) has a profound effect on their emissive properties. In particular, wire-shaped nanostructures have been proposed for increasing the external quantum efficiency of light emitters and for controlling the direction of their emission.  Yet achieving such benefits is not straightforward because of intricate interactions between the light source and the realized geometry of a nanowire.  Also, the constraints of nanofabrication can limit the separation between adjacent devices when ordered arrays of emissive nanowires are required.  This paper describes an investigation of the roles of single nano-device emission and collective effects on the emissive properties of ordered arrays InGaN-GaN light emitting nanowires.

InGaN-GaN nano-LEDs were fabricated in two ways: (a) arrays of 200-450 nm diameter nanowires were etched from pre-grown InGaN-GaN epitaxial layers containing a single InGaN quantum well (QW); or (b) from GaN homo-epitaxial layers and then over-grown with a single InGaN-GaN QW by metalorganic vapour phase epitaxy. The angle dependence of resonantly excited room temperature photoluminescence (PL) was measured using a goniometer system.

The angle dependence of the PL from a hexagonal array (600 nm pitch) of short (850 nm high) core-shell nano-LEDs (Fig.1) shows traditional diffraction behaviour.  However, increasing the height of the core-shell nanowires causes a dramatic change in the angular distribution of the radiation (Fig.2).

Measurements of free-space wave number (k_o) versus lateral wave-vector (k_||) of the type of diffraction feature seen in Fig. 2 revealed their origin is the formation of the partial bandgaps of a two-dimensional photonic crystal fabricated on a high index substrate, due to only weak coupling of light propagating laterally in an array of tall (4000 nm high) nanowires to the underlying substrate.  Maps of k_o versus k_|| of other emissive nanowire array structures revealed that features characteristic of the emission of light from single nanowires are typically superimposed on diffraction patterns of the types shown in Figs. 1 and 2.    The impact of these nano-photonic emission effects and on the properties of nano-LED devices, including ring-shaped nanostructures, and light trapping in the substrate will be discussed.