Development of an epitaxial lift-off technology for II-VI nanostructures using ZnMgSSe alloys

An epitaxial lift-off technique for removing wide bandgap II–VI heterostructures from GaAs substrates has previously been demonstrated using lattice-matched MgS as the sacrificial layer. However, using MgS as an etch release layer prevents its use as a wide bandgap barrier in the rest of the structure. Here, we describe the use of the etch-resistant alloy Zn.2Mg.8S.64Se.36 which we have developed as a replacement for MgS. We demonstrate that this alloy can be grown by MBE together with MgS in heterostructures and used as a barrier for ZnSe. A ZnSe quantum well with Zn.2Mg.8S.64Se.36 barriers shows no decrease in photoluminescence intensity after the etching process but shows a shift in emission wavelength associated with the changing strain state. & 2008 Elsevier Ltd. All rights reserved. For many applications of epitaxial semiconductor layers, it is desirable to remove the substrate on which they have been deposited. An ability to transfer the layer to different substrates generates the possibility of new device structures and measurements which would not be possible otherwise. One method of accomplishing this is by means of an epitaxial lift-off technique, which was first demonstrated in GaAs/AlAs structures by Yablonovitch et al. [1]. Until recently, a similar lift-off technology has not been available in II–VI semiconductors. However, Mg chalcogenides are extremely soluble in dilute acids, and we have demonstrated that ZnSe/ZnCdSe quantum wells deposited on an MgS layer grown in the zinc blende structure can be removed without damage from the GaAs substrate [2]. In these structures, the MgS is used as a sacrificial layer due to the difference in etch rates of MgS and ZnSe in HCl of approximately 10:1. Our aim therefore has been to develop a technology which will allow II–VI multilayers containing either quantum wells or quantum dots to be removed from the substrate and transferred to materials of different functionality. Recently, we have used this technique to transfer ZnSe/ZnCdSe quantum wells onto Bragg dielectric mirror stacks. This has allowed us the potential of combining the benefits of commercially available dielectric mirrors with MBE grown active regions in hybrid devices and has enabled us to observe exciton–photon coupling in the quantum well [3,4]. In addition to its use as a sacrificial layer in these devices, we have developed other used for ZB MgS. This compound is a very ll rights reserved. : +441314513473. useful wide bandgap material which has a bandgap of 5 eV that can be grown epitaxially on GaAs in conjunction with ZnSe-based alloys which we have demonstrated forms an excellent barrier material for both ZnSe quantumwells [5,6] and for CdSe quantum wells and dots [7,8]. However, it is not possible to use MgS in a structure both as a large bandgap barrier and as a sacrificial layer for epitaxial lift-off. We have therefore examined other MgS-rich, wide bandgap ZnMgSSe alloys to determine those compositions which are acid resistant. Recently, we were able to demonstrate that the quaternary alloy Zn.2Mg.8S.64Se.36 has many desirable characteristics. In addition to having a bandgap much larger than ZnSe, it has a smaller strain than MgS when grown epitaxially on GaAs. In our preliminary study, interfaces with ZnSe appeared smooth, and this alloy demonstrated excellent resistance to acid attack [9]. Here, we demonstrate the further development of the lift-off technology by firstly comparing the quantum confinement of Q and MgS and secondly using both materials together in structures and comparing the photoluminescence (PL) of these layers before and after lift-off. One aim of this work is to use this material as a barrier with CdSe quantum dots in structures designed for lift-off. However, due to the broad emission spectrum obtained from CdSe quantum dots, it is not possible to determine any changes in emission attributable to the lift-off procedure. Therefore, in this study, we have used samples containing 4nm ZnSe quantum wells. Wells of this thickness have been grown routinely with MgS barriers and we are therefore able to compare directly with this material. Two samples denoted as A and B were grown by molecular beam epitaxy (MBE) for this study. Sample growth was carried out at 240 1C using the standard growth method as described