Excitons in asymmetric quantum wells

Abstract: Resonance dielectric response of excitons is studied for the high-quality GaAs/InGaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances with various profiles. A standard approach for the phenomenological analysis of the profiles is generalized by introducing of different phase shifts for the light waves reflected from the QWs at different exciton resonances. Perfect agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to obtain reliable parameters of the exciton resonances including the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of Schr\"{o}dinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation during the heterostructure growth is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.

• The paper demonstrates that exciton reflectance spectra in asymmetric GaAs/InGaAs QWs are modulated by indium segregation and potential profile asymmetry.
• It utilizes numerical solutions of the Schrödinger equation with Coulomb interactions to compute exciton energies, wave functions, and radiative decay rates.
• The study presents a generalized phenomenological model that predicts multiple exciton resonances and informs the design of advanced optoelectronic devices.

Excitons in Asymmetric Quantum Wells

Introduction

The paper "Excitons in asymmetric quantum wells" investigates the dielectric response of excitons in GaAs/InGaAs heterostructures with wide, asymmetric quantum wells (QWs). The research focuses on comprehensive microscale modeling of exciton states through Schrödinger equation solutions, accounting for indium segregation. The impact of QW asymmetry on optical reflectance spectra is studied, offering insights into the quantum-confined exciton states observed.

Experimental Methodology

The study examines reflectance spectra for several InGaAs/GaAs heterostructures grown via molecular beam epitaxy (MBE). Two noteworthy structures, S1 and S2, were analyzed: S1 incorporates a nominally square QW, while S2 features a triangle-like QW with asymmetric profile walls. Spectroscopic measurements were conducted using femtosecond Ti:Sapphire lasers and halogen lamps, focusing on small beam areas under cryogenic conditions to achieve high spectral resolution.

Phenomenological Model Generalization

Standard theories of exciton reflectance were enhanced to encompass asymmetric QWs with multiple exciton resonances. Interference patterns stemming from light waves interacting with the surface and QWs were evaluated. The paper introduces phase shifts associated with resonant exciton states, vital for understanding the modulatory effects of asymmetric potential profiles on light reflectance.

Microscopic Modeling and Numerical Results

The eigenproblem for heavy-hole excitons in QWs was addressed using a stationary Schrödinger equation with Coulomb interactions, adopting finite difference methods for harmonic potential computation. Parameters including exciton energies, wave functions, radiative decay rates, and phase shifts were calculated, validating experimental observations. The segregation effect played a crucial role in modifying potential profiles—segregation lengths and indium concentrations were critical metrics in achieving modeling accuracy.

Implications and Future Directions

This research underscores the importance of QW structure asymmetries in affecting exciton states and achievable optical properties. The generalized phenomenological model, combined with precise numerical analyses, offers enhanced predictive capabilities in designing heterostructures with tailored optoelectronic properties. Potential applications include semiconductor devices with superior electron spin-orbit coupling, enhanced optical nonlinearities, and efficient terahertz interactions.

Conclusion

Findings reiterate the sensitivity of exciton properties to QW asymmetry and advocacy for using exciton resonances to infer precise QW potential profiles. The comprehensive approach combining experimental spectroscopy, phenomenological modeling, and numerical simulations establishes a robust framework for the analysis and design of complex heterostructures in future research endeavors.

These insights contribute significantly to the field of quantum well physics, fostering technological advancements in semiconductor optoelectronics. The methodologies and conclusions detailed in the study pave the way for further exploration and refinement in exciton-optics integration within high-quality heterostructures.