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.
EPFL2014
