Christopher Kölper

Luminescence and efficiency optimization of InGaN/GaN core-shell nanowire LEDs by numerical modelling

We present a computational study on the anisotropic luminescence and the efficiency of a core-shell type nanowire LED based on GaN with InGaN active quantum wells. The physical simulator used for analyzing this device integrates a multidimensional drift-diffusion transport solver and a k · p Schr¨odinger problem solver for quantization effects and luminescence. The solution of both problems is coupled to achieve self-consistency. Using this solver we investigate the effect of dimensions, design of quantum wells, and current injection on the efficiency and luminescence of the core-shell nanowire LED. The anisotropy of the luminescence and re-absorption is analyzed with respect to the external efficiency of the LED. From the results we derive strategies for design optimization.

All-InGaN Phosphorless White Light Emitting Diodes: An Efficiency Estimation

In this theoretical study we investigate the efficiency potential of monolithic white light emitting diodes (LEDs) that are free of wavelength-converting phosphors and are based solely on the InGaN material system. For that purpose we develop a numerical model that handles multiple active layers of different emission wavelength and takes photon reabsorption and -emission as well as internal non-radiative and optical losses into account. It is applied both to thin film structures as well as novel nanorod LEDs featuring disc-like active layers. In both cases, the active layers may either consist of multiple thin quantum wells or a single thick, bulk-like InGaN layer.

Optical properties of individual GaN nanorods for light emitting diodes: influence of geometry, materials, and facets

We present a systematic analysis of the optical properties of GaN nanorods (NRs) for the application in Light Emitting Diodes (LEDs). Our focus is on NR emitters incorporating active layers in the form of quantum-disc or core-shell geometries. We concentrate on the properties of individual NRs, neglecting any coupling with neighbouring NRs or ensemble effects. The distribution of power among guided and radiative modes as well as Purcell enhancement is discussed in detail in the context of different NR geometries, materials and the presence of interfaces.

Fabrication of hole pattern for position-controlled MOVPE-grown GaN nanorods with highly precise nanoimprint technology

Nano Imprint Lithography (NIL) is a promising technology that combines low costs with high throughput for fabrication of sub 100 nm scale features. One of the first application areas in which NIL is used is manufacturing of various types of LEDs. The wafers used for producing LEDs are typically III/V semiconductor materials grown with epitaxial processes. These types of substrates suffer from growth defects like hexagonal spikes, vpits, waferbowing, atomic steps and surface corrugations on a scale of few 10 μm or even large islands of irregularities. The mentioned irregularities are particularly disturbing when NIL based processes are utilized to create patterns onto the wafer surface. The nanopatterns created by NIL can be applied to control metal organic vapour phase epitaxy (MOVPE) growth of GaN nanorods. This paper will show that NIL is an excellent technology to produce nanopatterned GaN substrates highly suitable to grow defect free arrays of positioncontrolled nanorods for ultrahigh brightness LED applications.