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        Maria C. Tamargo
        Professor at the City College and the Graduate Center of CUNY
        B.S., University of Puerto Rico; M.S., Ph.D., 
The Johns Hopkins University

      There are several semiconductor materials projects within our molecular beam epitaxy (MBE) program at City College-CUNY. They include growth and characterization of new wide bandgap II-VI alloys, nano-structures for visible light emitters, and in-situ patterning by selected area epitaxy (SAE). Our facilities consist of a two chamber MBE growth system with connecting UHV modules and a metal deposition chamber. Characterization instruments include a photoluminescence set-up, a Hall effect measurement apparatus, single and double crystal X-ray diffractometers, a Nomarski interference microscope, an electrochemical capacitance-voltage (ECV) profiler and  C-V and IV probes. 

      In the past several years, we have investigated the growth and properties of a family of wide bandgap II-VI alloys, ZnCdMgSe, grown lattice-matched to InP substrates (see figure1). By optimizing the growth conditions we have reduced defect densities to the levels comparable to those of other II-VI materials grown on GaAs. These new alloys and heterostructures possess properties (band structure, lattice constants, band offsets, doping properties) that are attractive and offer some advantages for the design of improved visible semiconductor lasers and light emitting diodes (LED's). These devices are of interest for optical recording, displays and communications applications.  We have made LED’s with electroluminescence  in the red, green and blue (R-G-B) region of the visible range (see figure 2).  These LED’s have identical structures except for the thickness or composition of the QW active layer.  We have also made photopumped lasers in the R-G-B (see figure 3). The possibility of fabricating VCSEL’s  is being explored by developing distributed Bragg reflector (DBR) structures from these materials.

      Bipolar doping of wide bandgap II-VI compounds  is another area of research we pursue.  In collaboration with Prof. G. Neumark and her group at Columbia University, we are exploring non-equilibrium doping techniques, such as d-doping, to enhance the activation of acceptors in ZnSe and other wide bandgap semiconductors (see figure 4).

     New alloys including BeSe and BeTe semiconductors are also being pursued.  Be-chalcogenides are expected to impart hardness to the materials due to their strong covalent bonding character. We have recently observed a new (2 ´ 1) Be-terminated reconstruction on the GaAs (100) surface which provides a excellent surface for II-VI/III-V heteroepitaxy.  

      In-situ patterning enables fabrication of devices within the growth environment and thus can greatly improve the processing yields of device fabrication. We have recently demonstrated the technique of selected area epitaxy (SAE) during MBE growth to obtain lateral patterning of CdTe layers for IR detector arrays. More recently, these techniques are being applied to the ZnCdSe/ZnCdMgSe materials to integrate R-G-B emitters on a single substrate.