Research

We are investigating optoelectronic materials for the next generation of infrared detectors, solar cells, and thin film electronics. Our emphasis is on the study of compound semiconductors (II-VI and III-V) and oxides. Our primary expertise and experimental capabilities are

  • Molecular Beam Epitaxy (Riber 32 MBE system for II-VI compounds)
  • Pulsed Laser Deposition (Neocera PLD system for ZnO and ferroelectric oxides)
  • Simulation and Modeling of Optoelectronic Devices (Comsol, Sentaurus, Custom)
  • Materials and Device Fabrication and Testing (EMAL, LNF, MIBL)

 

Infrared Detectors

HgCdTe nBn singe element detectors.

Infrared focal plane arrays (IR FPAs) are of high importance for a variety of defense, scientific, and commercial applications. HgCdTe is currently the premier material for high performance infrared detection applications. This narrow-bandgap II-VI compound semiconductor alloy is extremely challenging to grow and process. Though this material has been studied for decades, the ability to achieve and control the growth of high quality epitaxial HgCdTe material for FPA applications has only occured recently. Molecular beam epitaxy (MBE) has enabled the growth and control over sophisticated multilayer HgCdTe structures needed for detectors demanding high-performance, multi-spectral detection, and high operating temperature.

We are currently working with EPIR Technologies to develop advanced HgCdTe IR detector structures, detector fabrication technologies, and integrated optical elements.

Solar Cell Materials and Device Concepts

Depiction of solar energy conversion in an intermediate band solar cell utilizing band to band, valence band to intermediate band, and intermediate band to conduction band optical transitions.

The growing importance of identifying renewable, clean energy sources has spurred increasing interest in photovoltaics. The viability of photovoltaics will depend on the solar cell cost, availability and hazards associated with solar cell materials/resources, and distributed overall cost of energy generation. The viability of photovoltaics may be improved with significant breakthroughs in solar cell efficiency, or through new methods to significantly reduce solar cell cost. We are currently investigating both new materials and device structures for high-efficiency, low-cost solar cells consisting of non-hazardous materials. Our current work includes the study of intermediate band solar cells based on oxygen doping and nanostructures in II-VI materials. Our group is a part of the Center for Solar and Thermal Energy Conversion (CSTEC), an Energy Frontier Research Center (EFRC) supported by the US Department of Energy (DOE).

Zinc Oxide and Related II-VI Oxide Materials and Devices

Id-Vds characteristics of ZnO TFT.

Zinc oxide is a wide-bandgap II-VI compound semiconductor (3.4 eV bandgap) that shows tremendous potential for optoelectronic and electronic devices due to its native substrate availability, strong excitonic binding energy (~60 meV), polar properties, large breakdown field, and rad hard capabilities. These characteristics show potential to compete with III-N compound semiconductors. ZnO may be alloyed with Mg or Cd to tune the bandgap energy for a wide range of visible and ultraviolet (UV) wavelengths. ZnO is also a transparent semiconductor that may be synthesized at low temperature at low cost, lending itself to display electronics and transparent conductors for displays, solar cells, and other photonic devices.

We are researching the epitaxial growth and thin film deposition of ZnO, and the application of these materials to photodetectors, light emitters, and thin film transistors. We have demonstrated the epitaxial growth of single-crystal ZnO with quality similar to bulk ZnO, ZnO/MgZnO quantum wells with excellent optical properties, and ferroelectric/ZnO heterojunctions demonstrating charge control through polarization switching. We have also demonstrated ZnO thin film transistors integrating high-k gate dielectrics with performance among the highest reported values.

Ferroelectric Thin Films and Device Applications

Piezo force microscopy image (2µm x 2µm) of a BaTiO3 thin film.

Ferroelectric materials exhibit a spontaneous polarization, which may be modulated upon application of an electric field. Properties associated with these materials are large dielectric constants, large electro-optic coefficients, and the ability to switch the polarizaton direction upon application of an electric field. Integrating these characteristics with semiconductors are of high interest for electronic and optoelectronic devices. Applications of interest include

  • Alternative gate dielectrics for field-effect transistors
  • Non-volatile memories
  • Novel charge-control devices
  • Integrated optics
  • Reconfigurable microwave circuits

We are currently investigating the electronic and optical properties of ferroelectric thin films deposited by pulsed-laser deposition (PLD). One area of particular focus is the integration of ferroelectrics on semiconductors for optoelectronic device integration and multi-functional devices. In addition, we are investigating ferroelectric thin film devices for reconfigurable microwave circuits, in collaboration with Professor Amir Mortazawi.