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.
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.
True blue lasers with wavelengths of ~450 nm are of great interest for full color laser projection. These kind of
applications usually require high output power and, in particular, an excellent wall plug efficiency within a wide
temperature range. In this paper we therefore present experimental and theoretical investigations of the temperature
behavior of 60mW InGaN lasers in a range of -10 °C to 100 °C.
The laser parameters threshold current density, slope efficiency and operating voltage describe the wall plug efficiency
of the device. The slope efficiency does not show any significant temperature dependence which is due to an almost
temperature independent injection efficiency in the temperature range that is of interest for most commercial
applications. In contrast, the laser threshold current density increases with temperature and we determine a characteristic
temperature T0 of about 141K for our devices emitting at 445nm. This increasing threshold current density can be
explained by lower gain of the quantum wells at higher temperature. Furthermore, Auger recombination influences the
threshold as verified by simulations. The second electro-optical parameter is the electrical voltage, which is dominated
by electrical barriers. The voltage decreases with increasing temperature and compensates the increasing threshold
current resulting in a nearly constant high wall plug efficiency of 13% between -10°C and 100°C.
The measurement of the bias and temperature dependent photoluminescence, photocurrent and their decay times allows
to deduce important physical properties such as barrier height, electron-hole overlap and the magnitude of the
piezoelectric field in InGaN quantum wells. However the analysis of these experiments demands for a detailed physical
model based on a realistic device structure which is able to predict the measured quantities. In this work a selfconsistent
model is presented based on a realistic description of the alloy and doping profile of a green InGaN single
quantum well light emitting diode. The model succeeds in the quantitative prediction of the quantum confined Stark
shift and the associated change in the electron-hole overlap measured via the change in the bimolecular decay rate using
literature parameters for the piezoelectric constants. The blue shift of the emission under forward current conditions can
be attributed to the carrier induced screening of the piezoelectric charges as predicted by the model. The photocurrent is
calculated via thermionic tunneling through the barriers using a WKB-approximation and the calculated potential profile
for the tunneling barrier. From the fact that the bias and temperature dependence of the experimentally observed
photocurrent cannot be described by the thermionic tunneling model even though the theoretical potential profile fits
excellent to the luminescence data, we conclude that the carrier escape is dominated by a different mechanism such as
defect- or phonon-assisted tunneling.
The internal quantum efficiency as a function of the internal electric field was studied in InGaN/GaN based quantumwell
heterostructures. Most striking, we find the IQE to be independent of the electron hole overlap for a standard green-emitting
single quantum-well LED structure. In standard c-plane grown InGaN quantum wells, internal piezo-fields are
responsible for a reduced overlap of electron and hole wavefunction. Minimization of these fields, for example by
growth on non-polar m- and a-planes, is generally considered a key to improve the performance of nitride-based light
emitting devices. In our experiment, we manipulate the overlap by applying different bias voltages to the standard c-plane
grown sample, thus superimposing a voltage induced band-bending to the internal fields. In contrast to the IQE
measurement, the dependence of carrier lifetime and wavelength shift on bias voltage could be explained solely by the
internal piezo-fields according to the quantum confined Stark effect. Measurements were performed using temperature
and bias dependent resonant photoluminescence, measuring luminescence and photocurrent simultaneously.
Furthermore, the doping profile in the immediate vicinity of the QWs was found to be a key parameter that strongly
influences the IQE measurement. A doping induced intrinsic hole reservoir inside the QWs is suggested to enhance the
radiative exciton recombination rate and thus to improve saturation of photoluminescence efficiency.