Robert A. R. Leute

Photoluminescence of highly excited AlN: Biexcitons and exciton-exciton scattering

Low-temperature photoluminescence spectra of nominally undoped high quality AlN layers on SiC and Al2O3 substrates are reported. Under high excitation conditions, we observe several bands that increase superlinearly with the excitation density. Based on temperature and excitation level dependences recorded on different samples, we identify a band 36 meV below the free A-exciton transition as due to exciton-exciton scattering (P2 band) and a second band down-shifted from the A-exciton transition by 27 meV as due to biexciton recombination. The combined data yield an exciton binding energy of 48 meV.

Determination of axial and lateral exciton diffusion length in GaN by electron energy dependent cathodoluminescence

We demonstrate the application of low-temperature cathodoluminescence (CL) with high lateral, depth, and spectral resolution to determine both the lateral (i.e., perpendicular to the incident primary electron beam) and axial (i.e., parallel to the electron beam) diffusion length of excitons in semiconductor materials. The lateral diffusion length in GaN is investigated by the decrease of the GaN-related luminescence signal when approaching an interface to Ga(In)N based quantum well stripes. The axial diffusion length in GaN is evaluated from a comparison of the results of depth-resolved CL spectroscopy (DRCLS) measurements with predictions from Monte Carlo simulations on the size and shape of the excitation volume. The lateral diffusion length was found to be (95 ± 40) nm for nominally undoped GaN, and the axial exciton diffusion length was determined to be (150 ± 25) nm. The application of the DRCLS method is also presented on a semipolar (112¯2) sample, resulting in a value of (70 ± 10) nm in p-type GaN.

LEDs on HVPE grown GaN substrates: Influence of macroscopic surface features

We demonstrate the strong influence of GaN substrate surface morphology on optical properties and performance of light emitting devices grown on freestanding GaN. As-grown freestanding HVPE GaN substrates show excellent AFM RMS and XRD FWHM values over the whole area, but distinctive features were observed on the surface, such as macro-pits, hillocks and facets extending over several millimeters. Electroluminescence measurements reveal a strong correlation of the performance and peak emission wavelength of LEDs with each of these observed surface features. This results in multiple peaks and non-uniform optical output power for LEDs on as-grown freestanding GaN substrates. Removal of these surface features by chemical mechanical polishing results in highly uniform peak wavelength and improved output power over the whole wafer area.

Ga(In)N Photonic Crystal Light Emitters with Semipolar Quantum Wells

We present directional photonic crystal light emitters produced as periodic semipolar GaInN quantum wells, grown by selective area metal organic vapour phase epitaxy. The emitted angle-dependent modal structure for sub-micrometer stripes and embedded photonic crystal structures is analyzed experimentally in detail, and the introduction of an Al0.12Ga0.88N cladding layer is investigated. We provide a complete simulation based on the finite-difference time-domain method, which allows to identify all leaky modes as well as their spectral and angular dependence.

GaN laser structure with semipolar quantum wells and embedded nanostripes

Using nanoimprint lithography, we fabricate GaN nano-structures with semipolar quantum wells on 2-inch c-oriented substrates and embed them within a planar waveguide to create a separate confinement heterostructure. Electroluminescence, transmission electron microscopy (TEM) and spatially resolved cathodoluminescence inside a scanning TEM (STEM-CL) is applied.