Visible band-gap ZnCdO heterostructures grown by molecular beam epitaxy

Abstract

Single-phase ZnCdO alloys with a band gap extending from the violet to yellow spectral range are fabricated by molecular beam epitaxy using extremely low growth temperatures in conjunction with O-rich growth conditions. The Cd concentration can be systematically adjusted via the Cd∕Zn beam pressure ratio. Despite growth temperatures as low as 150°C, layer-by-layer growth is accomplished allowing for the preparation of ZnCdO∕ZnO quantum well structures. Both epilayers and quantum wells exhibit strong band-gap-related emission at room temperature in the whole composition range.

Ab initiodescription of heterostructural alloys: Thermodynamic and structural properties ofMgxZn1−xOandSchleife1, Eisenacher2, Rödl3 et al. 2010Phys. Rev. B

“…Experimental studies of Cd x Zn 1−x O show an ambivalent picture: While two groups report very low thermodynamic solubility limits 23 of only x ≈ 0.07 or phase separation at even lower Cd concentrations 24 , another group observed Cd concentrations up to x = 0.32 in samples produced by means of highly non-equilibrium MBE 12 . Unfortunately, they have not tried for higher concentrations.…”

“…A thermodynamical miscibility only exists at very high temperatures, i.e., where the mixing entropy is sufficiently large; at lower temperatures there is a tendency for a phase separation of the alloys 17 . However, isovalent but heterostructural II-VI alloys seem to be stable under certain conditions: X x Zn 1−x O alloy films occur in wz structure for x < 0.55 (X = Mg) [18][19][20] or x < 0.32 (X = Cd, grown under non-equilibrium conditions) 12,13 . Depending on the composition x the film preparation may give rise to a non-uniformity of pseudobinary thin films as observed experimentally for Mg x Zn 1−x O 21 .…”

“…On the other hand, metal-organic chemical vapor deposition (MOCVD) techniques produced alloys up to x = 0.7 13 . Molecular beam epitaxy (MBE) leads to epilayers with a structure close to the wz one without an indication for a phase separation up to a Cd concentration of x = 0.32 12 .…”

“…Mixed Mg x Zn 1−x O crystals allow to tune the energy gap from about 3.4 eV of wurtzite (wz) ZnO towards the ultraviolet spectral region with at least 4.4 eV in Mg x Zn 1−x O [9][10][11] . Conversely, pseudobinary Cd x Zn 1−x O alloys tend to close the gap and are, therefore, suitable candidates for optoelectronic devices in the visible spectral range 12 . Moreover, quantum-well and other heterostructures based on the combinations Mg x Zn 1−x O/ZnO and ZnO/Cd x Zn 1−x O allow an additional tailoring of electronic and optical properties by means of quantum-confinement effects 6,12,13 .…”

Computer simulation of coherent BaZrO3/MgO interfaces

“…Given that the difference is less than 0.02 ¡, this is not expected to have a significant impact on the results obtained for the interface. Also, as is typical for GGA, the band gaps are underestimated; at 2.4 and 4.7 eV for BaZrO 3 and MgO, respectively, they are roughly half the typical experimentally measured values of 4.0 23) and 7.7 eV, 24) respectively. Including a component of the exact exchange energy calculated from Hartree-Fock theory in the form of a hybrid potential would no doubt provide more accurate values for the band gaps, but such a computationally expensive method was deemed too time-consuming in this case because of the comparatively large system required to model a heterogeneous interface.…”