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

Abstract

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 ∼ 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 ∼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.

Propagating Polaritons in III-Nitride Slab Waveguides

“…The critical points were fitted using the approach introduced by Aspnes. 29,30 The comparison between low-temperature PL and PR spectra ( Fig. 3(a)) reveals a Stokes shift of 13 meV, which is similar to that previously reported for equivalent GaN/AlGaN MQW samples.…”

“…3(a)) reveals a Stokes shift of 13 meV, which is similar to that previously reported for equivalent GaN/AlGaN MQW samples. 30 Note that since the PL and PR measurements were conducted with the sample in different cryostats, there is an uncertainty of ∼500 µm -comparable to the PR spot size-on the relative sample location probed in both experiments.…”

Homoepitaxial nonpolar (10-10) ZnO/ZnMgO monolithic microcavities: Towards reduced photonic disorder

“…Still, an open issue remains concerning the exploitation of such improved photonic figures of merit in strongly-coupled microcavities, especially in terms of the active region to be used. Several open possibilities exist, including the use of ZnCdO QWs or pure ZnCdO layers with low Cd content, to limit the associated inhomogeneous broadening, 20 as well as the use of ZnMgO/ZnMgO-based DBRs and ZnO as active region, 32 the problem being in this last situation the reduction of the DBR stopband. 19 One might think to combine the advantages of both solutions and try to mitigate, thereby, their respective disadvantages.…”

“…In extreme cases, where the disorder is comparable with the system Rabi splitting, it can even lead to the loss of the strong coupling. 20 In this respect, current inorganic wide-bandgap microcavities resemble more the first polariton lasers based on CdTe 6,23 rather than the current GaAs-based microcavities. [10][11][12][13] In wurtzite materials as GaN and ZnO the quantum confined Stark effect 24,25 (QCSE) exacerbates the excitonic disorder due to quantum wells thickness fluctuations in polar quantum wells.…”

“…Compared with the most mature GaAs-based microcavities, [10][11][12][13] whose optical quality has been continuously improved, [14][15][16][17] wide-bandgap microcavities still exhibit a large degree of disorder: this disorder is most often associated to the polariton photonic component due to in-plane fluctuations of the cavity or DBRs thicknesses; 18,19 still, polariton disorder can also display an excitonic component, 20 particularly if quantum wells (QWs) are used as active medium. The polariton disorder can largely influence the polariton condensation/lasing process, 21,22 irrespective of its actual origin.…”