Recent developments in the MOVPE growth of ZnSe-based compounds and heterostructures

This work briefly reviews the state of the art of p-type doping metalorganic vapor phase epitaxy (MOVPE) grown ZnSe-based compounds and heterostructures. In particular, the origin of unintentional hydrogen (H) incorporation during the MOVPE of these wide band-gap II-VI compounds is discussed. H in ZnSe effectively passivates intentional nitrogen (N) acceptors, during the MOVPE fabrication of blue-light emitting diodes and laser diodes. It is shown that H enters in MOVPE-grown ZnSe as result of specific surface reactions between alkyls, whose efficiency depends on the chemical nature of the VI-group precursor. The existence of a marked trade-off between the proclivity of most common Se alkyls to incorporate H and their thermal stability is pointed out. The use of a novel class of VI-group alkyl precursors of the form R2X2 [where X=Se, S and R is an ethyl (Et) or methyl (Me) radical] will be then proposed as a convenient solution of the above problem. These alkyls allow a strong reduction of H incorporation in ZnSe-based materials, whilst retaining the low temperatures required by the MOVPE growth of device quality wide band-gap II-VI compounds. Dimethyldiselenide (Me2Se2) and diethyldisulphide (Et2S2) allow the pyrolytic MOVPE growth of Zn(S)Se-based heterostructures below 400 °C, i.e. ca. 150°C lower than what necessary for the usual dimethylselenide and diethylsulphide alkyls. Mass spectrometry fragmentation experiments performed on the alkyl molecular ions allowed us to investigate the alkyl relative bond strengths and suggest likely decomposition paths. The reduced thermal stability of these alkyls is then attributed to a weakening of the X-C bonds in the R2X2 molecule induced by the stronger X-X bond. Secondary ion mass spectrometry (SIMS) analysis shows that as-grown ZnSe epilayers have [H] values around (1-3)x10^17 cm-3, i.e. amongst the lowest ever reported for MOVPE-grown layers. SIMS elemental depth profiling measurements performed on the ZnSe samples show unintentional halogens (Cl and I) which contaminate the epilayers at the 10^14-10^15 cm^-3 level; these impurities originate from residual halides in the Me2Se2 batch. The functional validation of the new S and Se alkyls is completed by the structural, morphological and optical characterization of Zn(S)Se-based heterostructures. The results of double-crystal X-ray diffraction (XRD) measurements and scanning electron microscopy observations performed on ZnSe, ZnS and ZnSSe epilayers grown on (100)GaAs will be presented. The epilayer structural properties, determined by a statistical XRD theory under the kinematical approximation, compare well with what reported for molecular beam epitaxy grown ZnSe. 10K cathodoluminescence (CL) measurements performed on ZnSe and ZnS samples are also presented. CL spectra of as-grown ZnSe epilayers appear of good quality, with pronounced band-edge emissions and reduced deep level contributions.