Defects in Transparent Conductive beta-Ga2O3 Single Crystals
EMSL Project ID
39397
Abstract
In this proposal we motivate and describe a series of ion beam experiments and density functional theory calculations to be conducted at EMSL, which will answer fundamental questions about the origin of the conductivity in single crystals of the transparent conductive oxide (TCO) beta-Ga2O3. TCOs like Ga2O3, In2O3, SnO2, ZnO, and their alloys, are of key importance as transparent conductors and gas sensing materials. Yet, despite the myriad technological applications, the origin of the ubiquitous n-type conductivity in these materials is hotly debated. P-type doping of oxides, which would allow new technologies such as transparent light-emitters and optical sensors, remains an elusive goal. If any progress is to be made, then a detailed understanding of the factors affecting the always-observed n-type conductivity is necessary. Compared to other TCOs, relatively little is known experimentally or theoretically about Ga2O3. Based on an inverse correlation between oxygen pressure during growth, and by analogy to the other TCOs, oxygen vacancies are commonly assumed to be responsible for the n-type conduction. However, there is no experimental evidence for non-stoichiometry in Ga2O3, let alone its correlation with conductivity. A recent paper [Vill08] shows the conductivity can be intentionally controlled over three orders of magnitude by silicon doping on the order of typical Si impurity levels in Ga2O3 source materials. This suggests Si impurities, being the major dopant in commercially available source materials, could be responsible for the observed n-type conductivity. This leaves the actual role of oxygen vacancies, if any, unclear. To date, while the DFT results on defects in the other TCOs are evolving at their second and third generations, no such study exists for Ga2O3. This is despite the fact that large single crystals can be grown [Vill04], and that beta-Ga2O3 exhibits the largest band gap (~4.8 eV) of the TCOs, making it unique for applications where near-UV transparency might be required. We propose to use DFT as implemented in VASP to calculate the formation energies of various defects, which directly translates to their abundance in the material and their effect on the electronic structure. We also propose to use Rutherford backscattering (RBS) and other ion beam techniques to systematically study how various defects, impurities, and crystal disorder correlate with conductivity.
Project Details
Project type
Exploratory Research
Start Date
2010-03-22
End Date
2011-03-27
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Lovejoy TC, R Chen, EN Yitamben, V Shutthanandan, SM Heald, EG Villora, K Shimamura, ST Dunham, FS Ohuchi, and MA Olmstead. 2012. "Incorporation, valence state, and electronic structure of Mn and Cr in bulk single crystal B-Ga2O3." Journal of Applied Physics.