CdSe With Mixed Zincblende and Wurtzite Phases Grown on Lattice-Matched InAs Substrates Using Molecular Beam Epitaxy

The II–VI compound semiconductor CdSe, which has both zincblende (ZB) and wurtzite (WZ) phases with bandgap energies of 1.67 and 1.74 eV, respectively, is an ideal candidate material for the top cell in a tandem solar cell with a Si bottom cell to achieve higher power conversion efficiency. In this work, molecular beam epitaxy growth of CdSe thin films on lattice-matched InAs(100) substrates reveals a single-phase ZB structure with high crystallinity and a low defect density. In contrast, all CdSe layers grown on InAs(111)B substrates have mixed ZB and WZ phases in coexistence, as confirmed by high-resolution X-ray diffraction, transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL efficiencies of the CdSe layers grown on (111)B substrates are substantially lower than those grown on (100) substrates. This result is attributed to defects at the boundaries between the two different phases. Postgrowth annealing of CdSe layers grown on InAs(111)B at 250–450 °C converts most of the CdSe ZB material into the WZ phase, as evidenced by improved PL efficiency and TEM images. Density functional theory simulations confirm that the formation energy difference between the ZB and WZ phases for CdSe is very small compared with other conventional compound semiconductors, which is in good agreement with the experimental observations.