Matthias Sabathil

On the importance of radiative and Auger losses in GaN-based quantum wells

Fully microscopic many-body models are used to study the importance of radiative and Auger carrier losses in InGaN∕GaN quantum wells. Auger losses are found to be negligible in contrast to recent speculations on their importance for the experimentally observed efficiency droop. Good agreement with experimentally measured threshold losses is demonstrated. The results show no significant dependence on details of the well alloy profile.

Identification of nnp and npp Auger recombination as significant contributor to the efficiency droop in (GaIn)N quantum wells by visualization of hot carriers in photoluminescence

We report the direct observation of hot carriers generated by Auger recombination via photoluminescence spectroscopy on tailored (AlGaIn)N multiple quantum well (QW) structures containing alternating green and ultra-violet (UV) emitting (GaIn)N QWs. Optically pumping solely the green QWs using a blue emitting high power laser diode, carrier densities similar to electrical light-emitting diode (LED) operation were achieved, circumventing possible leakage and injection effects. This way, luminescence from the UV QWs could be observed for excitation where the emission from the green QWs showed significant droop, giving direct evidence for Auger generated hot electrons and holes being injected into the UV QWs. An examination of the quantitative relation between the intensity of the UV luminescence and the amount of charge carriers lost due to drooping of the QWs supports the conclusion that Auger processes contribute significantly to the droop phenomenon in (AlGaIn)N based light-emitting diodes.

Influence of indium content and temperature on Auger-like recombination in InGaN quantum wells grown on (111) silicon substrates

High-efficiency InGaN-based light-emitting diodes have been grown on (111) silicon substrates and investigated with regard to efficiency and carrier lifetime as a function of current density. Using a single quantum well active layer ensures a well-defined active volume which enables the precise determination of the recombination coefficients in the ABC rate model for different emission wavelengths and junction temperatures. Good agreement of the resulting C values with calculated Auger coefficients is found both with respect to absolute value as well as their dependence on bandgap energy and temperature.

Experimental Determination of the Dominant Type of Auger Recombination in InGaN Quantum Wells

We investigate theoretically the influence of type and density of background carriers in the active region on the quantum efficiency of InGaN-based light emitters using an extension of the ABC rate model. A method to determine experimentally whether a certain type of Auger recombination is relevant in InGaN quantum wells is derived from these considerations. Using this approach, we show that the physical process which is the dominant cause for the efficiency droop is superlinear in the electron density and can thus be assigned to nnp-Auger recombination.

Gain of blue and cyan InGaN laser diodes

Experimental gain spectra of 450 and 490 nm laser diodes on c-plane GaN are analyzed by detailed comparison with the results of a fully microscopic theory. The gain calculation shows the importance of electron LO-phonon coupling. The whole spectral gain shape, not only the low energy tail, is strongly influenced by the LO-phonon contribution. The inhomogeneous broadening parameter increases by a factor of about two for the cyan laser diode in comparison with the blue laser structure. This indicates an increase in alloy and thickness fluctuations for the longer wavelength material.

Self-consistent modeling of resonant PL in InGaN SQW LED-structure

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.

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.

Temperature dependence of blue InGaN lasers

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.

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.

Transport and capture properties of Auger-generated high-energy carriers in (AlInGa)N quantum well structures

Recent photoluminescence experiments presented by M. Binder et al. [Appl. Phys. Lett. 103, 071108 (2013)] demonstrated the visualization of high-energy carriers generated by Auger recombination in (AlInGa)N multi quantum wells. Two fundamental limitations were deduced which reduce the detection efficiency of Auger processes contributing to the reduction in internal quantum efficiency: the transfer probability of these hot electrons and holes in a detection well and the asymmetry in type of Auger recombination. We investigate the transport and capture properties of these high-energy carriers regarding polarization fields, the transfer distance to the generating well, and the number of detection wells. All three factors are shown to have a noticeable impact on the detection of these hot particles. Furthermore, the investigations support the finding that electron-electron-hole exceeds electron-hole-hole Auger recombination if the densities of both carrier types are similar. Overall, the results add to the evidence that Auger processes play an important role in the reduction of efficiency in (AlInGa)N based LEDs.