Molecular beam epitaxy growth and characterization of wide bandgap zinc-magnesium-selenium semiconductor materials and heterostructures for intersubband devices

This thesis describes the molecular beam epitaxy (MBE) growth and characterization of the ZnxMg(1-x)Se based semiconductor material system grown on InP substrates for quantum cascade lasers (QCLs) and other intersubband devices for room temperature operation and application in ultra fast optical communication.; Wide band gap II-VI semiconductor materials containing Mg, such as Zn xMg1-xSe, are of considerable interest for the fabrication of optoelectronic devices because their band gaps range from 2.75 to 3.7 eV, which allows us to reach higher bandgap values than those available from currently used II-VI materials. A series of high crystalline quality zincblende Zn xMg1-xSe alloys was grown lattice matched to InP (001) substrates by molecular beam epitaxy. Since we have used III-V InP substrates to grow this material, a lattice matched III-V InGaAs buffer layer, Zn irradiation and low temperature ZnCdSe interfacial layer was grown to improve the interface quality between III-V and II-VI material. The use of InP as a substrate allows us to optimize the material quality of the high Mg content compositions, since near lattice matching to this substrate is achieved with as much as ∼87% Mg concentration. This lattice-matched alloy has a bandgap of 3.6 eV at 77K. The crystalline quality was assessed by X-ray diffraction techniques. The band gap energies of the alloys were determined using photoluminescence measurements and were plotted as a function of Mg concentration. A linear dependence between the band gap and the Mg concentration was observed for the entire range. From the extrapolation of our experimental data, the bandgap of zincblende MgSe was determined to be 3.74 eV.; Using ZnxMg1-xSe as a barrier layer we have grown several Znx'Cd(1-x') Se/ZnxMg(1-x)Se single quantum well (QW) structures. These samples exhibited emission that ranges form the near-UV, throughout the visible range of the spectrum. The dependency of the QW emission with QW layer thickness was modeled using a finite barrier model. Finally, using a modulated spectroscopic technique known as contactless electroreflectance, a very large conduction band offset for this QW structure, of ∼1.12 eV, was estimated. This result makes this a very promising material for applications in the fabrication of intersubband devices such as quantum cascade lasers operating at room temperature or at short wavelengths such as 1.55 mum, as needed for ultra fast optical communications and other intersubband devices in the short wavelength region.