| Molecular Beam Epitaxy (MBE) is a versatile thin film growth technique with the ability to control the growth with atomic layer accuracy. Novel devices with complex structures can be prepared by MBE to meet specific needs. Intersubband (ISB) devices, such as quantum cascade lasers (QCLs) and quantum well infrared photodetectors (QWIPs) operating in the mid-infrared range, have potential advantages over conventional interband devices due to the increased ability to engineer the devices, as well as their intrinsic faster relaxation time and higher quantum efficiencies.; The shortest wavelength that can be obtained is limited by the band offset in the conduction band of the heterostructures currently used for these devices. Semiconductors of the II-VI family have shown potential in fabricating this kind of devices due to their widely adjustable bandgaps. In particular, the ZnxCd1-xSe/Znx'Cdy'Mg1-x'-y' Se QW structure is a II-VI semiconductor system recently investigated in our lab, whose emission can cover the entire visible wavelength range by simply changing the QW width. A modulation technique, contactless electroreflectance (CER), has been used to determine the conduction band offset (CBO) of the system. For the limit composition of this lattice-matched system, Zn 0.5Cd0.5Se/Zn0.13Mg0.87Se, the CBO has been estimated to be as large as 1.12 eV.; In this work, the growth of these materials is optimized, and methods to study the ISB transitions in the material system, including theoretical prediction by the envelope function approximation, CBO determination and ISB transition energy estimation by CER, and ISB absorption measurement by Fourier transform infrared (FTIR) spectroscopy, are investigated systematically. A series of high quality ZnxCd1-xSe/Znx'Cd y'Mg1-x'-y'Se single QW and multiple QW structures with different QW thicknesses and composition (and thus, strain) have been grown on InP substrates by MBE. The samples were characterized by X-ray diffraction (XRD), Photoluminescence (PL), time-resolved PL, CER and FTIR measurements, etc. All the data indicate excellent material properties which meet the requirements of device design and demonstrated the potential of this II-VI material system for ISB device applications. |
