Gaseous Source Epitaxy Technologies For Wide Bandgap II-VI Semiconductors

Substantial progress in recent years by the solid source molecular beam epitaxy (MBE) community has demonstrated the feasibility of the (Zn,Mg)(S,Se) material system for realizing laser diodes operating in the green to blue spectral region. For the Zn-chalcogenide quaternary, all of the constituent elements have very high vapor pressures, and require the regulation of thermal effusion ovens operating at low temperatures (-100-300"C), thus creating a difficulty in reproducibility of the 11-VI alloy composition and emission wavelength. Gaseous source epitaxy technologies [metalorganic (MOMBE) and gas source molecular beam epitaxy (GSMBE)] address these difficulties by employing mass flow controllers to regulate the flux of hydride and metalorganic gas sources. The ultrahigh vacuum environment of MOMBWGSMBE enables implementation of a nitrogen plasma source, which is necessary for p-type doping by the incorporation of nitrogen acceptors into the 11-VI material system. In this paper, the application of the gaseous source epitaxial growth approaches to the fabrication of wide bandgap 11-VI materials and heterostructures will be summarized. In addition, the use of photo-assisted epitaxy, which enables in situ modification of surface chemical reactions, has provided a significant growth rate enhancement, as well as a growth rate retardation, and is dependent on the species present at the growth front. (An electron beam incident to the surface has also been found to simulate the photo-assisted epitaxy effect by creating a significant growth rate enhancement.) To expand the range of lattice constants available for the heteroepitaxy of ZnSe-based heterostructures, the epitaxial growth of ZnSe onto novel 111-V epitaxial layers containing (In,Ga)P is also under intense investigation; the initial characterization of this new IIVyIII-V heterostructure will be described. Utilizing gas source molecular beam epitaxy, ZnSe has been grown using a hydride compound for the source of the high vapor pressure anion species. Nand p-type doping has been investigated using a nitrogen plasma cell for acceptor species of nitrogen and a solid ZnC12 source for donor species of chlorine, respectively. The use of hydride compounds, however, raises the issue of hydrogen incorporation and the possibility that hydrogen may electrically passivate donors or acceptors. High quality ZnSe:Cl has been grown with atomic c1 concentrations approaching lo2' as indicated by secondary ion mass spectrometry (SIMS). At incorporation levels greater than lo2' ~ m ~ , an appreciable decrease in the growth rate has been observed. The sharp transition to a negligible growth rate is atmbuted to the Occurrence of a surface chemical reaction originating from C1 and H which are present in the GSMBE environment. For C1 concentrations as high as 4 ~ 1 0 ' ~ the films exhibited high crystalline quality, as indicated by photoluminescence originating from a single intense donor-bound excitonic transition. In the case of ZnSe:Cl, H was present in the ZnSe layers, but did not appear to adversely affect the electrical properties of the n-type films. Electrical characterization of the ZnSe:Cl layers using capacitance-voltage (C-V) profiling and Hall effect measurements indicated values for the electron concentration which were in relatively close agreement with those obtained by SIMS. The hydrogen incorporation in ZnSe doped p-type with nitrogen is markedly different