InGaN alloys and heterostructures

The III-nitride semiconductor material system - (InAlGa)N - is of highest interest for optoelectronic applications due to its direct bandgap, tunable from the ultraviolet to the infrared spectral range. The most well-known are white light-emitting diodes, which are presently revolutionizing the general lighting market, and 405 nm laser diodes, the key elements of the Blu-ray Disc[TM] technology. Furthermore, sophisticated devices like electrically-driven verticalcavity surface-emitting lasers, which are microcavity based lasers, have been demonstrated recently, paving the way for electrically-driven low threshold room temperature polariton lasers. In 1996 Imamoglu and coworkers predicted the possibility of achieving such a low threshold coherent light source from a polariton condensate in a suitable microcavity. The eigenmodes of such a microcavity are exciton-polaritons, admixed particles resulting from the strong coupling between a photonic mode and an excitonic resonance. Thanks to their very light effective mass at the center of the Brillouin zone (105 times lighter than that of a free electron) and efficient relaxation mechanism a phase transition toward an exciton-polariton condensate might occur at elevated temperatures (observed up to 340 K in GaN-based microcavities). The goal of the present study is to provide a detailed theoretical analysis of the main emission features of electrically-driven polariton lasers based on planar GaN microcavities with embedded InGaN/GaN multiple quantum wells and to further derive the stringent requirements necessary for their experimental implementation. The complete polariton phase diagram is established for two experimentally relevant pumping geometries. Furthermore, the steady-state, the high-speed current modulation response, the relative intensity noise, and the Schawlow- Townes linewidth of those two geometries are derived. The resulting general expressions can be applied to any inorganic semiconductor polariton laser diodes. Then the building blocks of such microcavities are experimentally analyzed separately, i.e., the bottom III-nitride based distributed Bragg reflector, the active medium, and the top dielectric Bragg reflector. An innovative optical characterization method allows to study the effect of the substrate on lattice-matched InAlN/GaN Bragg reflectors. The best optical quality is obtained for such Bragg reflectors when grown on high quality free-standing GaN substrates. Furthermore, particular attention is paid to the excitonic localization via simulations and optical characterizations. For this purpose, a photoreflectance setup allowing the determination of the Stokes shift in the InGaN alloys grown on free-standing GaN substrates either as thick layers or as heterostructures (quantum wells) was carefully designed to operate from cryogenic to room temperature. The stacking of several In0.1Ga0.9N/GaN quantum wells results in a detrimental increase in the Stokes shift and thus the absorption linewidth, which is incompatible with the requirements of the strong coupling regime. Various possible reasons such as inhomogeneous built-in field distribution among the quantum wells and their partial strain relaxation are then identified, hence explaining why it is presently not possible to achieve the strong coupling regime with such an active region embedded in a semi-hybrid microcavity. Alternative solutions for the microcavity design to achieve the strong coupling regime with the InGaN alloy are discussed, supported by results obtained from transfer matrix simulations. The inhomogeneous absorption linewidth of thick InGaN layers is deduced from transmission measurements and for low In content layers (x < 12%) it is successfully reproduced by a model based on a stochastic distribution of indium atoms at the atomic scale. It is finally proposed that gain dilution in green laser diodes should not be much higher than in blue laser diodes provided that the In content within the QWs is homogeneous and abrupt interfaces are present.

Generic picture of the emission properties of III-nitride polariton laser diodes: Steady state and current modulation response

Raphaël Butté, Munise Nevin Cobet, Marlene Glauser, Nicolas Grandjean, Jacques Levrat, Georg Rossbach

The main emission characteristics of electrically driven polariton lasers based on planar GaN microcavities with embedded InGaN quantum wells are studied theoretically. The polariton emission dependence on pump current density is first modeled using a set of semiclassical Boltzmann equations for the exciton polaritons that are coupled to the rate equation describing the electron-hole plasma population. Two experimentally relevant pumping geometries are considered, namely the direct injection of electrons and holes into the strongly coupled microcavity region and intracavity optical pumping via an embedded light-emitting diode. The theoretical framework allows the determination of the minimum threshold current density J(thr),(min) as a function of lattice temperature and exciton-cavity photon detuning for the two pumping schemes. A J(thr, min) value of 5 and 6 A cm(-2) is derived for the direct injection scheme and for the intracavity optical pumping one, respectively, at room temperature at the optimum detuning. Then an approximate quasianalytical model is introduced to derive solutions for both the steady-state and high-speed current modulation. This analysis makes it possible to show that the exciton population, which acts as a reservoir for the stimulated relaxation process, gets clamped once the condensation threshold is crossed, a behavior analogous to what happens in conventional laser diodes with the carrier density above threshold. Finally, the modulation transfer function is calculated for both pumping geometries and the corresponding cutoff frequency is determined.

Amer Physical Soc2012

Investigation of indium-rich InGaN alloys and kinetic growth regime of GaN

Nils Asmus Kristian Kaufmann

Nowadays, light emitting diodes (LEDs) and laser diodes (LDs) are part of our daily life. More and more devices incorporate InGaN-based optoelectronic devices. In fact, since the first demonstration of a candela-class InGaN-based LED in the beginning of the nineties, those LEDs have quickly become popular. The efficiency of blue InGaN LEDs is nowadays very high and when coated with a yellow phosphor, they can efficiently emit white light. Such white LEDs are more and more used for different lighting applications, from home lighting to car lighting. Another application of blue InGaN emitters that is found in many households is the Blu-ray disc, based on a 405 nm LD. The great success of the III-nitrides in the blue range created a strong interest to extend the wavelength range into the green, as efficient green emitters are missing. However, their performance is still low compared to their blue counterparts. The goal of this dissertation is to investigate issues that limit the performance of green InGaN LEDs and LDs. To achieve efficient green emission, the growth temperature needs to be reduced to incorporate more indium into the active region. In this context, the present study is divided into two parts. In the first one, the growth regime of GaN as a function of the temperature is investigated and the physical origin of the surface features is discussed. When lowering the temperature, the so called Ehrlich-Schwöbel barrier, a barrier at the step-edges, causes a different attachment probability for the adatoms arriving from the different sides. This asymmetry can lead to the appearance of three different kinetic surface morphologies: hillocks, fingers, or step-bunching. After presenting the theory, the three different regimes are demonstrated for GaN, and the growth parameters that allow to control the morphology are investigated. Indeed, the different morphologies usually reported for the different growth techniques can be attributed to the different growth parameters commonly used. In the end of the first part, the influence of the underlaying morphology on the properties of the active region of an LED is discussed. In the second part, the impact of the thermal budget on the properties of InGaN quantum wells (QWs) is discussed. Active regions with a high In content can degrade easily when the p-type layer is grown on top, due to being exposed to a too high thermal budget. This is especially critical for green LDs which require thick cladding layers, i.e. a long growth time. The origin of the degradation and the parameters influencing it are discussed first: when an In-rich QW is annealed at a too high temperature, its emission intensity is reduced and dark spots appear on PL maps. The annealing temperature causing this degradation depends on the indium content, the QW thickness and the number of QWs. It is shown that these dark spots exhibit a peculiar emission spectra, with a characteristic emission at 2.75 eV. This emission is tentatively attributed to nitrogen vacancies that are created by the formation of metallic indium clusters in the active region. Then the focus is laid on the beneficial effects that can take place when an In-rich active region is exposed to a moderate thermal budget. Apart from reducing the linewidth of the emission, a strong improvement in photoluminescence intensity can be achieved. This improvement increases for high indium content QWs, i.e. when the growth temperature is low. In fact, this improvement is more pronounced for molecular beam epitaxy grown structures whose active regions are grown at a much lower temperatures than their metal organic vapor phase epitaxy counterparts. This is tentatively ascribed to the reduction of point defects by thermal annealing. As a consequence, a moderate annealing can be beneficial for both to reduce the non-radiative recombinations and the linewidth. It is finally proposed to perform a short annealing step of high indium content QWs before growing the p-type (cladding) layer at low temperature.

EPFL2013

InGaN/GaN quantum wells for polariton laser diodes: Role of inhomogeneous broadening

Raphaël Butté, Jean-François Carlin, Eric Feltin, Marlene Glauser, Nicolas Grandjean, Christian Mounir, Georg Rossbach

Contrary to the case of III-nitride based visible light-emitting diodes for which the inhomogeneous linewidth broadening characteristic of InGaN-based multiple quantum well (MQW) heterostructures does not appear as a detrimental parameter, such a broadening issue can prevent a microcavity (MC) system entering into the strong light-matter coupling regime (SCR). The impact of excitonic disorder in low indium content (x similar to 0.1) InxGa1-xN/GaN MQW active regions is therefore investigated for the subsequent realization of polariton laser diodes by considering both simulations and optical characterizations. It allows deriving the requirements for such MQWs in terms of absorption, emission linewidth, and Stokes shift. Systematic absorption-like and photoluminescence (PL) spectroscopy experiments are performed on single and multiple In0.1Ga0.9N/GaN quantum wells (QWs). Micro-PL mappings reveal a low temperature PL linewidth of similar to 30 meV, compatible with SCR requirements, for single QWs for which the microscopic origin responsible for this broadening is qualitatively discussed. When stacking several InGaN/GaN QWs, a departure from such a narrow linewidth value and an increase in the Stokes shift are observed. Various possible reasons for this degradation such as inhomogeneous built-in field distribution among the QWs are then identified. An alternative solution for the MC design to achieve the SCR with the InGaN alloy is briefly discussed. (C) 2014 AIP Publishing LLC.

Amer Inst Physics2014

Investigation of indium-rich InGaN alloys and kinetic growth regime of GaN

Nils Asmus Kristian Kaufmann

EPFL2013