Surface chemistry of oxygen, carbon and hydrogen on indium tin oxide with implications on work function modification
Dr. John Chaney,
University of Louisville, Kentucky
Wednesday, February 13, 2002 2:15pm,
Ben Bandy Conference Center, Center for Applied Energy Research
Indium tin oxide (ITO) continues to be developed as an important material for opto-electronics allocations. Much research has been focused on the work function (F) modification of ITO. Increasing FITO to higher values allows for more efficient hole injection in light emitting diodes. However, the full exploitation of ITO requires that several surface chemical issues be addressed, especially the susceptibility of ITO toward contamination.
We have explored fundamental chemical changes of ITO with regard to hydrogen, oxygen and carbon, and we have evaluated the impact of these chemical changes on the physical properties of ITO, namely the work function. ITO was exposed in situ to molecular hydrogen (H2), hot-filament activated oxygen (O2*), and hot-filament activated deuterium (D2*). Surface chemical changes have strong effects on FITO as seen by the Kelvin probe technique. Exposure of clean ITO to O2* can increase F to ~5.6 eV, but the increase is short lived. The decrease in F over time can be related to the uptake of carbon impurities in ultra high vacuum as monitored by Auger. Vibrational analysis of ITO reveals significant hydrocarbon impurities. Chemical reduction produces a metallic surface and dehydrogenates the adsorbed hydrocarbons. Re-oxidation of metallic ITO and oxidation of clean ITO both temporarily remove adventitious carbon from the surface, but oxidized ITO adsorbs an even larger quantity of carbon over time.
It was seen that ITO has a strong inherent affinity for surface carbon contamination. It was also observed that ex-situ cleaning of ITO may have little effect or an adverse effect in reducing surface carbon contamination. While C contamination may be the major technological limitation of ITO, relatively little research has addressed this important interaction.