Temperature-Dependent Photoluminescence Study on CdZnS/ZnS/MgZnS Separate-Confinement Heterostructures

Abstract

CdZnS/ZnS/MgZnS separate-confinement heterostructures (SCHs) were grown on (100)-oriented GaAs substrates by low-pressure metalorganic chemical vapor deposition. Optical properties of SCHs were investigated by means of temperature-dependent photoluminescence (PL) spectroscopy in the temperature range of 4 to 300 K. In order to improve the crystalline quality of SCHs, a ZnS buffer layer was inserted prior to the growth of SCH structure on substrates. For a Cd 0.2 Zn 0.8 S/ZnS/Mg 0.25 Zn 0.75 S SCH structure with a ZnS buffer layer, which has the appropriate thickness of 100 nm, the decrease in PL intensity from 4 to 300 K was about two orders of magnitude. In comparison with the decrease observed for MQW structure, thermal quenching of the PL intensity was suppressed by about two orders of magnitude. It is expected that the suppression was attributed to the enhancement of carrier confinement by the insertion of a MgZnS cladding layer.Introduction II±VI semiconductors such as ZnS and ZnSe have intensively attracted attention both for optical devices in the ultraviolet (UV) spectral range and for basic research. Since ZnS has a large bandgap energy of 3.73 eV and an exciton binding energy of 36 meV, it is especially expected that the optical devices due to the radiative recombination of excitons or biexcitons can be realized in ZnS-based systems at RT. In fact, the excitonic stimulated emission was observed for different structures by various groups [1 to 3]. We also reported that the stimulated emission connected with the recombination of biexcitons was observed up to about 200 K [4]. In order to realize the biexcitonic stimulated emission at RT, it is important to enhance the carrier confinement. In ZnSe-based laser diodes (LD), Itoh et al. reported that a carrier overflow from an active layer was suppressed by the insertion of a Mg-compound cladding layer [7]. Therefore it is expected that the use of a MgZnS cladding layer leads to an enhancement of the carrier confinement in the quantum wells.In this study, we report the optical properties of CdZnS/ZnS multiple-quantum wells (MQWs) and CdZnS/ZnS/MgZnS separate confinement heterostructures (SCHs) by means of temperature-dependent photoluminescence (PL) measurements. We also report the improvement of crystalline quality in MgZnS mixed films by the insertion of a ZnS buffer layer. Furthermore, we demonstrate the enhancement of the luminescence intensity at RT from SCH structures grown on ZnS buffer layers.

Effect of Induced Strain on Valence Band in CdxZn1-xS Wells of CdxZn1-xS/ZnS/BeyZn1-yS Separate-Confinement Heterostructures

“…To increase light and carrier confinements in Cd x Zn 1Àx S/ ZnS quantum wells (QWs), Cd x Zn 1Àx S/ZnS/Mg y Zn 1Ày S separate-confinement heterostructures (SCHs) were grown. 1,2) In this system, a Cd x Zn 1Àx S/ZnS multiple quantum well (MQW) is sandwiched between two Mg y -Zn 1Ày S cladding layers where the band gap energy of MgS [4.45 eV at room temperature (RT) 3) ] is larger than that of ZnS (3.68 eV at RT 4) ). In ZnSe-based SCHs, 5) the use of the cladding layers led to an increase in carrier confinement; thus, SCHs are useful for designing the light-emitting devices.…”

Effect of Induced Strain Due to Lattice Mismatch between MgyZn1-yS Cladding Layers and CdxZn1-xS/ZnS Quantum Wells on Effective Band Gap Energy in CdxZn1-xS/ZnS/MgyZn1-yS Separate-Confinement Heterostructures

“…Recently, Cd x Zn 1Àx S/ZnS/Mg y Zn 1Ày S separate-confinement heterostructures (SCHs) have been grown using metalorganic chemical vapor deposition (MOCVD) and their optical properties have been reported. [1][2][3] In SCH structures, a cladding layer is added on either side of the quantum well (QW) layer. In general, the cladding layers in the SCH structures have a lower reflective index than the QW layers, and thus the light is effectively confined.…”

Reduction in Induced Strain in CdxZn1-xS Well Layers Using ZnS1-ySey Barrier Layers and Inhibition of Induced Strain due to Lattice Mismatch between MgzZn1-zS Cladding Layers and CdxZn1-xS/ZnS1-ySey Multiple Quantum Wells by Adjusting Mg Content

“…In fact, the band lineups and conduction and valence band offsets in ZnSe-containing alloy-material-based MQWs, such as ZnSe/ZnS x Se 1Àx QWs, 25) are easily estimated, compared with those in Cd x Zn 1Àx S/Mg y Zn 1Ày S MQWs. In addition, the growth temperature of ZnSe/ZnS x Se 1Àx QWs 25) is lower than that of Mg y Zn 1Ày S layers, [26][27][28] and thus, the ZnS x Se 1Àx layers can be grown at lower temperatures than Mg y Zn 1Ày S layers. It is expected that the inter diffusion between the well and barrier layers can be reduced, and that the flatness of the interface between the well and barrier layers in Cd x Zn 1Àx S/ ZnS 1Ày Se y MQWs is better than that between the well and barrier layers in Cd x Zn 1Àx S/Mg y Zn 1Ày S MQWs.…”

“…In previous studies, the Mg y Zn 1Ày S epitaxial layers and Cd x Zn 1Àx S/ZnS/ Mg y Zn 1Ày S SCHs were grown, and their optical properties investigated. 21,[26][27][28] effective for the improvement of carrier confinement efficiency in Cd x Zn 1Àx S/ZnS 1Ày Se y MQWs.…”