| MBE GROUP
DEPARTMENT OF CHEMISTRY
The City College of New York
138th Street and Convent Avenue, NY, NY 10031
Maria C. Tamargo
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 LEDs with electroluminescence
in the red, green and blue (R-G-B) region of the visible range (see figure 2). These
LEDs 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 VCSELs 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.