Molecular Beam Epitaxy Growth of Indium Nitride and Indium Gallium Nitride Materials for Photovoltaic Applications
The objective of the proposed research is to establish the technology for material growth by molecular beam epitaxy (MBE) and fabrication of indium gallium nitride/gallium nitride (InxGa1-xN/GaN) heterojunction solar cells. InxGa1-xN solar cell have the potential to span 90% of the solar spectrum, however there has been no success with high indium (In) incorporation and only limited success with low In incorporation InxGa1-xN. Therefore, this present work focuses on 15 - 30% In incorporation leading to a bandgap value of 2.3 - 2.8 eV. This work will exploit the revision of the indium nitride (InN) bandgap value of 0.68 eV, which expands the range of the optical emission of nitride-based devices from ultraviolet to near infrared regions, by developing transparent InxGa1-xN solar cells outside the visible spectrum. Photovoltaic devices with a bandgap greater than 2.0 eV are attractive because over half the available power in the solar spectrum is above the photon energy of 2.0 eV. The ability of InxGa1-xN materials to optimally span the solar spectrum offers a tantalizing solution for high-efficiency photovoltaics.
Using the metal modulated epitaxy (MME) technique in a new, ultra-clean refurbished MBE system, an innovative growth regime is established where In and Ga phase separation is diminished by increasing the growth rate for InxGa1-xN. The MME technique modulates the metal shutters with a fixed duty cycle while maintaining a constant nitrogen flux and proves effective for improving crystal quality and p-type doping. We demonstrate the ability to repeatedly grow high hole concentration Mg-doped GaN films using the MME technique. The highest hole concentration obtained is equal to 4.26 e19 cm-3, resistivity of 0.5 ?-cm, and mobility of 0.28 cm2/V-s. We have achieved hole concentrations significantly higher than recorded in the literature, proving that our growth parameters and the MME technique is feasible, repeatable, and beneficial. The high hole concentration p-GaN is used as the emitter in our InxGa1-xN solar cell devices.
Advisor:Graham, Samuel; Shen, Shyh-Chiang; Rohatgi, Ajeet; Ferguson, Ian; Doolittle, W. Alan
School:Georgia Institute of Technology
School Location:USA - Georgia
Source Type:Master's Thesis
Keywords:electrical and computer engineering
Date of Publication:03/12/2009