T. Baron

Laser diodes based on beryllium-chalcogenides

Beryllium chalcogenides have a much higher degree of covalency than other II–VI compounds. Be containing ZnSe based mixed crystals show a significant lattice hardening effect. In addition, they introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures due to their band gaps, lattice constants, and doping behavior. Therefore, these compounds seem to be very interesting materials for short wavelength laser diodes. Here, we report on the first fabrication of laser diodes based on the wide band gap II–VI semiconductor compound BeMgZnSe. The laser diodes emit at a wavelength of 507 nm under pulsed current injection at 77 K, with a threshold current of 80 mA, corresponding to 240 A/cm2.

Novel beryllium containing II-VI compounds : basic properties and potential applications

Abstract An additional, yet untried approach to further improve the reliability of ZnSe-based devices is to use beryllium containing II–VI compounds. BeS, BeSe and BeTe are characterized by a considerable amount of covalent bonding and a high bond energy. This distinguishes these materials from the conventional ionic wide gap II–VI semiconductors like ZnSe, ZnTe or CdTe. Recently, thin film structures using Be-compounds have been fabricated and characterized. It became clear that — besides the application aspects — these materials are also very interesting from a more fundamental point of view. Using Be-containing II–VI compounds, ionic and covalent lattice matched II–VI materials can be combined in quantum well structures. The type II band alignment of BeTe and ZnSe gives additional freedom in the band gap engineering, and it is possible to grow lattice matched quaternaries of low polarity onto silicon. Here, basic properties of Be containing II–VI compounds will be described, and the potential of these novel materials will be discussed.

Reduction of the extended defect density in molecular beam epitaxy grown ZnSe based II-VI heterostructures by the use of a BeTe buffer layer

The reduction of extended defects in ZnSe based II-VI heterostructures grown by molecular beam epitaxy on (001) GaAs is reported, using BeTe buffer layers as a novel approach. After defect selective chemical etching three different types of etch pits could be observed by optical microscopy. By the application of a thin BeTe buffer layer the density of paired Frank type stacking faults could be strongly reduced to values below 103 cm−2. The role of Se in the background pressure for the defect nucleation at the II-VI/GaAs interface is significant. It has been found that BeTe can form a smooth interface to GaAs and ZnSe which is reflected in high resolution x-ray diffraction data.

Optical creation of a metastable two-dimensional electron gas in a ZnSe/BeTe quantum structure

A microstructure has been designed in which an electron gas is created optically with variable density. ZnSe and BeTe quantum wells are separated by a BeZnMgSe alloy with band gap 3 eV. When electron-hole pairs are created by ultraviolet light in the barriers, electrons accumulate in the ZnSe quantum well, and holes in the BeTe quantum well. This is because the BeTe valence band lies 900 meV higher than that of ZnSe. Reflectivity and luminescence spectra of the ZnSe well show the destabilization of excitons by an electron gas and formation of trions. The metastable charge-separated state has a very long lifetime (2 s at 2 K).

Nitrogen doping in molecular-beam epitaxy growth of II–VI semiconductors

Abstract We report on the p-doping properties of the II–VI related compounds ZnTe, CdTe, BeTe and MgTe. Following a comparative study of nitrogen doping in these materials, we perceive the formation of nitride compounds as responsible for the trend observed in N-doped telluride materials. We have also investigated a possible enhanced intermixing process in CdZnMgTe/CdZnTe and ZnSe/BeTe superlattices due to nitrogen incorporation.