The research projects in Professor Holloway's group may be divided into the following categories:
Wide Bandgap Compound Semiconductors for Band Edge Devices
Wide Bandgap Compound Semiconductors for State-to-State Devices
Research is currently being conducted into ohmic contacts to gallium nitride. The problem of most interest is contacts to p-type materials, since this requires a metal with too large a work function (7.5 eV). Because no metal has a work function this large, several metal systems which exhibit interfacial reactions have been investigated.
Our group is a world leader in the understanding of interfacial reactions between metals and semiconductors, and how such reaction may lead to low resistance contacts. In the case of p-GaN, interfacial reactions with pure metals have only led to high resistance contacts. Currently we are working with transparent, thin oxide/metal contacts to both reduce the total contact resistance as well as allow emission of light from LEDs or diode lasers.
The above image shows Gallium nitride light emitting diodes giving either blue or white light. The blue comes directly from an indium gallium nitride quantum well. The white comes from a quantum well emitting in the ultraviolet/blue region, with conversion of UV to white light by a phosphor, similar to the phosphors in a fluorescent light. Semiconductor LEDs will revolutionize lighting in our appliances, car and home over the next few years.
Ohmic Contacts to SiC
Silicon carbide devices have a large potential to be used as logic elements and amplifiers at temperatures up to 600oC. However, the ohmic contacts to the devices must be stable for long times at these elevated temperatures. We are determining the role of interfacial reactions between contact metals and SiC substrates to yield low resistance ohmic contacts. In addition, further interfacial reactions during use at high temperatures must either reduce the specific contact resistance, or at least not cause it to increase. Appropriate layered metal thin film structures, deposition conditions and geometrical configurations to achieve these objectives are being studied.
The image to the left depict one
application of silicon carbide high temperature microelctronics for control
of electric power.
High voltage, fast thyristors are being developed for these applications.
Zinc Oxide Contacts to CuInSe2 Thin Film Solar Cells
ZnO contacts are used on the top surface of CuInSe2 solar cells to allow light to be adsorbed at the junction to generate power, but at the same time conduct the current to the attached devices. A compromise is necessary since low resistivity ZnO requires higher carrier concentrations, but these carriers adsorb more light and reduce the solar cell efficiency. Both intrinsic and doped ZnO thin films are being sputter deposited onto substrates with thicknesses between 1 and 100 nm. The film morphology and composition are correlated with both deposition conditions and electrical properties to understand how to achieve better conductivity in the thin films. The objective is to improve the microstructure of the ZnO thin films such that the resistivity is lower, even for a lower carrier concentration, resulting in more efficient solar cells.
Atomic force microscope images of a smooth (left; RMS roughness = 0.3 nm) and rough (right; RMS roughness = 0.9 nm) thin film of zinc oxide are shown.
Wide Bandgap Compound Semiconductors for State-to-State Devices (Phosphors)
Electroluminescent Phosphors
In several research projects, we are depositing thin films of zinc sulfide doped with either transition or rare earth elements. This thin film phosphor is sandwiched between dielectric layers and electrodes deposited on both sides. When a field of about 1 MV/cm is applied across the phosphor, visible or infrared light may be emitted. The wavelength of the light varies with the dopant element. The intensity and efficiency of emission may be varied by annealing and co-doping with other elements. Several research projects are active in this area. In particular, we are studying doping with terbium to achieve brighter, more efficient emission of green light for full color electroluminescent flat panel displays. We are also studying the doping of ZnS with other rare earth elements for enhanced emission of infrared wavelengths in the range from 0.8 to 2 micrometers. Infrared emission can be used for thermometry and communication purposes. Finally we are studying the electroluminescence of nanopowder phosphors to determine if quantum confinement of the excited electron state may be used to enhance the radiative efficiency of the phosphor.
To the left is an active matrix
electroluminescent full color display intended to be used in head mounted
displays because of its
small size. The phosphors being developed will allow brighter, more power
efficient displays.
