Enhanced quantum‐confined Stark effect in ZnSe/ZnMgSSe asymmetric coupled quantum wells for blue‐ultraviolet optical modulators

Abstract

We report on the enhanced quantum-confined Stark effects (QCSE) in ZnSe/ZnMgSSe asymmetric coupled quantum wells (ACQWs), and its application for blue-ultraviolet optical modulator. A new electronresonant-type of ACQW structure: [ZnSe(6ML)/ZnMgSSe(6ML)/ZnSe(12ML)] is designed and fabricated by molecular beam epitaxy (MBE). This new structure device exhibits a strong coupling of the electron envelope wavefunctions and a large Stark shift of ∆E ∼ 28 meV at room temperature. This field enhanced Stark shift is found to be superior to the usual heavy-hole-resonant type ACQW (∆E ∼ 21 meV), which is consistent with the theoretical simulation based on the effective mass approximation.1 Introduction A quantum confined Stark effect (QCSE) of excitonic transition is attracting for optical functional devices such as high speed light modulators, switches or self electro-optic effect devices (SEEDs) [1,2]. The exciton binding energy in theII-VI widegap semiconductors is much larger than that of III-V compounds, and therefore, these widegap compounds have a large potential for QCSE devices in new short wavelength region. Furthermore, asymmetric-coupled quantum wells (ACQWs) of II-VI widegap compounds have a big advantage on the large Stark shift under low electric field [3].In this paper, enhanced Stark shifts in ZnSe/ZnMgSSe ACQWs are proved experimentally, and the physical mechanism is examined by theoretical analysis. The MQW optical modulators used in this study are p-i-n structure diodes including 50 periods ACQWs for the active layers, grown by molecular beam epitaxy (MBE) on n + -GaAs(100) substrates. Excitonic transition energies and Stark shifts were determined by electroreflectance (ER) and electroabsorption (EA) experiments. The theoretical analysis of ACQWs under high electric fields are based on the finite element method in the framework of the effective mass approximation [4,5].

High efficiency blue‐violet optical modulators of ZnSe/ZnMgSSe asymmetric coupled quantum wells

“…Maximum modulation depths are 40 % and 51 % for (a) and (b) at the exciton resonance. These values are fairly large compared with the modulation depth (26 %) of the heavy-hole resonant ACQWs [4]. The 6-2-12 ML ACQW with thin barrier has especially large modulation depth, and it is due to that thermal dissociation of exciton is suppressed under high electric field.…”

“…Furthermore, asymmetriccoupled quantum wells (ACQWs) of II-VI widegap compounds have a big advantage on the large Stark shift under low electric field [3]. In our previous study, electron-resonant type asymmetric-coupled quantum wells (ACQWs) of ZnSe/ZnMgSSe have revealed enhanced Stark shift without saturation under high electric field [4].In this paper we present a high efficiency blue-violet optical modulator consists of ZnSe/ZnMgSSe asymmetric-coupled quantum wells (ACQWs) with modulation efficiency exceeding 50% at room temperature. The new device designed with an electron-resonant type ACQW has revealed a large Stark shift of ∆E ∼ 34 meV at room temperature in low electric field condition of 480 kV/cm and very stable device operation.…”

“…In these optical devices, high electric-field induced excitonic Stark effect plays essential role, where large Stark shift without exciton dissociation must be realized. We have made systematic study on the development of new short wavelength optical modulator using II-VI widegap compounds based quantum wells because of its large exciton binding energy (> 20 meV) [3,4]. Furthermore, asymmetriccoupled quantum wells (ACQWs) of II-VI widegap compounds have a big advantage on the large Stark shift under low electric field [3].…”