The immobilization of DNA on passivated n-type InAs (100) surfaces has been
studied using X-ray and ultraviolet photoelectron spectroscopy. The benefits
of sulfur passivation using ammonium sulfide solution
((NH4)2S) for DNA immobilization were examined. The
XPS/UPS data carried out on non-functionalized and functionalized surfaces
demonstrate that the DNA probes reacted with the sulfur-passivated InAs
surface. The XPS data in combination with fluorescently-tagged DNA indicate
that the sulfur passivation process leads to a higher and more uniform
attachment of DNA over the surface compared to non-sulfur-passivated InAs
surfaces. The XPS data obtained immediately after sulfur passivation clearly
observes In-S bonding, with little or no As-S. In addition, the XPS spectra
of As 3d core-levels immediately after sulfur passivation shows that there
is a negligible amount of As-Ox, but the peak become considerable
after exposure to the aqueous DNA probe solution. The increase in
As-Ox is likely due to the presence of non-sulfur bonded As
atoms present on the surface. The presence of sulfur on the surface does
lead to the high areal density of attached ssDNA. This system forms the
basis of a DNA sensing system. While chemically passivating the surface
against oxidation and facilitating probe attachment, the changes in Fermi
level position were also monitored by UPS. UPS spectra show that the Fermi
level of a clean InAs surface is located ~0.6 eV above the valence band
maximum. The changes in electronic states induced by sulfur passivation and
the pinning of EF are discussed.