Luminous Efficiency of Ordered Arrays of GaN Nanowires with Subwavelength Diameters

The linear optical response of a bulk material is fully described by the wavelength-dependent complex refractive index n(λ). However, if the material is structured on a scale comparable to or smaller than the wavelength of the light it interacts with, the optical response is additionally affected or even governed by the geometry and dimensions of the nanostructure. The interaction of light with the material can thus be tailored such as to provide functionalities not attainable with planar devices. In particular, both light absorption and extraction can be enhanced well beyond the limits of homogeneous materials.

Top: Exemplary bird’s eye view scanning electron micrograph of an ordered GaN nanowire array array. Bottom: Experimental luminous efficiency (solid squares) and computed extraction efficiency (solid circles and line) of a nanowire in a periodic array as a function of its diameter d*. The inset shows the emission pattern of this nanowire.

In this work, we investigate the spontaneous emission from ordered arrays of GaN nanowires with well-defined diameters, representing the building blocks for light-emitting as well as light-detecting devices integrated on Si substrates. The luminous efficiency of these arrays is observed to decrease by more than an order of magnitude when the nanowire diameter is increased from 120 to 240 nm. A detailed analysis of both steady-state and transient photoluminescence data reveals that this quenching is not caused by a corresponding decrease of the internal quantum efficiency. Hence, we examine the coupling of light into and out of the nanowire arrays by appropriate numerical simulations. While the change in absorbance is minor in the investigated diameter range, the extraction efficiency for thin nanowires is enhanced by an order of magnitude as compared to thick nanowires. This phenomenon primarily originates in the efficient coupling of the spontaneous emission to free space for sub-wavelength diameter nanowires. Additionally, our results show that light, which after extraction from a nanowire propagates laterally, may be diffracted at the periodic array and redirected into free space, thus further enhancing the extraction efficiency for certain nanowire diameters.

1 Author C. Hauswald , I. Giuntoni , T. Flissikowski , T. Gotschke , R. Calarco , H. T. Grahn , L. Geelhaar , O. Brandt

Luminous efficiency of ordered arrays of GaN nanowires with sub-wavelength diameters

Source ACS Photonics , 4 , 52 ( 2017 )
DOI : 10.1021/acsphotonics.6b00551 | 2823 Cite : Bibtex RIS
C. Hauswald, I. Giuntoni, T. Flissikowski, T. Gotschke, R. Calarco, H. T. Grahn, L. Geelhaar, and O. Brandt