Suppression of cross-hatched polariton disorder in GaAs/AlAs microcavities by strain compensation

Physical Review B, 2012

We report on polariton states bound to defects in planar GaAs/AlAs microcavities grown by molecular beam epitaxy. The defect types relevant for the spatial polariton dynamics in these structures are cross-hatch misfit dislocations, and point-like defects extended over several micrometers. We attribute the latter defects to Ga droplets emitted occasionally by the Ga cell during the growth. These defects, also known as oval defects, result in a dome-like local modulation of surface, which is translated into the cavity structure and leads to a lateral modulation of the cavity polariton energy of up to 15 meV. The resulting spatially localized potential landscape for the in-plane polariton motion creates a series of bound states. These states were characterized by spectrally resolved transmission imaging in real and reciprocal space, and reveal the spatial potential created by the defects. Interestingly, the defect states exhibit long lifetimes in the 10 ps range, which we attribute to a spatially smooth confinement potential.

Physical Review B, 2008

Applied Physics B, 2011

The well-distinguished lower polariton branches (LPBs) and upper polariton branches (UPBs) are characteristics of strong coupling in semiconductor microcavities (MCs). In practice, however, the UPBs are often broadening especially in wide-bandgap material MCs. We present in detail the possible physical mechanisms for the broadening of UPBs for different designs of MCs by numerical simulations based on GaAs, GaN and ZnO materials. The calculated results show that the UPBs of the GaN-and ZnObased MCs will become indistinct when the thickness of optical cavity is larger than λ and 0.25λ, respectively, mainly attributed to the larger product of the absorption coefficient and the active layer thickness. In wide-bandgap materials, it would be relatively easier to observe the UPB in the case of negative exciton-cavity mode detuning due to the excitonlike UPB and lower absorption of scattering states. In addition, the inhomogeneous broadening would be an important factor causing the invisible UPB in wide-bandgap semiconductor MCs. We demonstrate that in multiple quantum well embedded ZnO-based MCs, the UPB could be well defined due to the large 2D exciton binding energy and the small product of absorption coefficient and active layer thickness. These results show that the UPBs can be properly defined in wide-bandgap semiconductor MCs by appropriate design of the MC structures.

MRS Proceedings, 2008

We present an overview of our work concerning the fabrication of GaN-based microcavities grown on silicon substrates dedicated to the observation of the strong light-matter coupling regime. In the view of recent promising results in the field, prospects regarding the improvement of heterostructures in order to observe room temperature polariton lasing from a GaN-based microcavity grown on a silicon substrate will be discussed.

Physical Review B, 2012

We present a correlative study of structural and optical properties of natural defects in planar semiconductor microcavities grown by molecular beam epitaxy, which are showing a localized polariton spectrum as reported in Zajac et al., Phys. Rev. B 85, 165309 (2012). The three-dimensional spatial structure of the defects was studied using combined focussed ion beam (FIB) and scanning electron microscopy (SEM). We find that the defects originate from a local increase of a GaAs layer thickness. Modulation heights of up to 140 nm for oval defects and 90 nm for round defects are found, while the lateral extension is about 2µm for oval and 4µm for round defects. The GaAs thickness increase is attributed to Ga droplets deposited during growth due to Ga cell spitting. Following the droplet deposition, the thickness modulation expands laterally while reducing its height, yielding oval to round mounds of the interfaces and the surface. With increasing growth temperature, the ellipticity of the mounds is decreasing and their size is increasing. This suggests that the expansion is related to the surface mobility of Ga, which with increasing temperature is increasing and reducing its anisotropy between the [110] and [110] crystallographic directions. Comprehensive data consisting of surface profiles of defects measured using differential interference contrast (DIC) microscopy, volume information obtained using FIB/SEM, and characterization of the resulting confined polariton spectrum are presented.

physica status solidi (b), 2007

We present both experimental and theoretical results which outline our development of the molecular beam epitaxy of GaN microcavities on (111) silicon. In particular we show that although in this material system the strong-light matter coupling regime can be observed at 300 K even with relatively low quality factor structures (Q = 60) in reflectivity measurements, it is necessary to increase the Q-factor by at least a factor of two to observe strong coupling in the emission. For an optimized microcavity structure (Q = 160), polaritonic emission is observed at 300 K, with the origin of the broadened luminescence features confirmed by co-incident reflectivity measurements.

The polariton condensation phase diagram is compared in GaN and ZnO microcavities grown on mesa-patterned silicon substrate. Owing to a common platform, these microcavities share similar photonic properties with large quality factors and low photonic disorder, which makes it possible to determine the optimal spot diameter and to realize a thorough phase diagram study. Both systems have been investigated under the same experimental conditions. The experimental results and the subsequent analysis reveal clearly that longitudinal optical phonons have no influence in the thermodynamic region of the condensation phase diagram, while they allow a strong (slight) decrease of the polariton lasing threshold in the trade-off zone (kinetic region). Phase diagrams are compared with numerical simulations using Boltzmann equations, and are in satisfactory agreement. A lower polariton lasing threshold has been measured at low temperature in the ZnO microcavity, as is expected due to a larger Rabi splitting. This study highlights polariton relaxation mechanisms and their importance in polariton lasing.

Physical Review B, 2011

We present a comprehensive study of the anisotropic optical properties of nonpolar GaN/AlGaN multiple quantum wells intentionally designed to act as an active region of a planar microcavity operating in the strongcoupling regime. The strain induced by the underlying AlGaN-based Bragg reflector leads to a redistribution of exciton oscillator strength as revealed by photoluminescence and reflectivity measurements. Complementary k • p calculations show an excellent agreement with experiments and emphasize the opportunity to tune the nature of the light-matter coupling in a microcavity by means of strain engineering. Finally, the validity of the developed model is proven by angle-resolved photoluminescence studies carried out on the complete microcavity structure. The recorded eigenmode spectra reveal the coexistence of the weak-and the strong-coupling regime along the two orthogonal polarization planes.

Applied Physics Letters, 2013

The strong light-matter coupling regime and lasing in a GaN microcavity fabricated by incorporating a high optical quality GaN membrane inside an all-dielectric mirror cavity is demonstrated at room temperature. A nonlinear increase of the emission and line narrowing marks the onset of polariton lasing regime with significantly reduced threshold compared with previous reports for bulk GaN microcavity. This combination of low lasing thresholds and ease of fabrication allows incorporation of quantum wells and electrical contacts into the active region, paving the way for electrically driven room temperature polariton laser devices.

Journal of Applied Physics, 2010

Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on ͑111͒ Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al 0.2 Ga 0.8 N / AlN Bragg mirror followed by a / 2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2 m thick GaN layer is grown, and progressively thinned to a final thickness of . Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations.