We present an approach to the molecular beam epitaxy of high-$Q$ planar GaAs-based microcavities in which the AlGaAs high-index layers of the distributed Bragg reflectors are replaced by short-period GaAs/AlAs superlattices (digital alloys) engineered to provide the same effective Al content. This design enables a significant reduction of interface roughness, precise control of both the $\lambda /4$ optical thickness and the effective Al content, suppression of the propagation of certain structural defects, and efficient tuning of the intrinsic absorption at the polariton emission wavelength through optimization of the superlattice parameters. Using this approach, we have grown a microcavity with a low polariton lasing threshold of $P_{\mathrm{th}}\approx 570\pm 90\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ and a high experimental quality factor of $Q_{\mathrm{expt}}\approx (5.4\pm 0.1)\times {10}^4$. This value exceeds by a factor of 2 the theoretical estimate obtained within a model in which the digital alloy is replaced by a ternary AlGaAs alloy with the same effective Al content. We demonstrate that accurate modeling of the stop-band characteristics and the $Q$ factor requires incorporation of the modified electronic density of states in the superlattice, including quantum-confinement and excitonic effects.

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• Crystal growth
• Exciton polariton
• Bragg structures
• Heterostructures
• III-V semiconductors
• Layered semiconductors
• Microcavity & microdisk lasers
• Optical microcavities
• Polariton condensate
• Quantum wells
• Superlattices
• Atomic force microscopy
• Molecular beam epitaxy
• Optical microscopy
• Photoluminescence
• Reflection high-energy electron diffraction
• Reflectivity
• Transfer matrix method