Resonant optical studies of GaAs/AlGaAs Multiple Quantum Well based Bragg Structures at excited states
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.21
Resonant optical studies of GaAs/AlGaAs Multiple Quantum Well based Bragg Structures at excited states Nikesh Maharjan1, Vladimir Chaldyshev2, and Mim Lal Nakarmi1 1
Department of Physics, Brooklyn College and the Graduate Center of the City University of New York, Brooklyn, NY 11210, USA
2
Ioffe Institute, 26 Polyteckhnicheskaya, St. Petersburg 194021, Russia
ABSTRACT
Optical reflectance (OR) and electro-reflectance (ER) spectroscopies were employed to study the resonant optical properties of GaAs/AlGaAs multiple quantum wells based Resonant Bragg Structures (RBS) at excited states. The RBS samples have 60 periods of GaAs/AlGaAs quantum well/barrier grown on semi-insulating GaAs substrates by molecular beam epitaxy with slightly different thicknesses of well/barrier. We observed enhanced OR and ER features when exciton energy coincides with the energy of the Bragg reflection peak, a double resonance condition. Bragg peak can significantly be tuned by changing the angle of incidence of the light. Exciton energies can be tuned by changing the temperature, external electric field and the thickness of the quantum wells. By tuning the Bragg peak for double resonance in the RBS samples of different thicknesses, we observed the electro-reflectance features related to the transitions of x(e2-hh2), x(e2-hh1), x(e2-lh1), x(e2-hh3) and x(e1-hh3) excitons along with the sharp features of x(e1-hh1) and x(e1-lh1) ground state exciton transitions from the ER experiments. The excitonic transitions x(e2-hh1), x(e2-lh1) and x(e2hh3) which are prohibited at zero electric field, were also observed due to the increased overlap of the electron and hole wave functions caused by the electric field; built-in electric field or applied DC bias.
INTRODUCTION An optical medium can be fabricated with a periodic modulation of index of refraction using different materials in order to obtain resonant optical reflectance and
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transmittance features. Dielectric photonic crystal is a famous example of such structure [1]. One-dimensional such structure called distributed Bragg reflector is used in Vertical cavity surface emitting laser [2]. Periodic thin layered structure of semiconductors of different bandgaps forming multiple quantum wells (MQWs) is similar to the periodic modulation of refractive index for Bragg diffraction of electromagnetic waves. Bragg resonance depends on the periodicity of the well/barrier in the MQW system. This kind of resonant optical media consisting of semiconductor MQWs was first proposed by Ivchenko et al., called Resonant Bragg Structure (RBS) [3]. Semiconductor quantum wells (QWs) have intrinsic electronic states such as excitons. Energy states in the quantum well depend on the thickness of
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