Surface Chemistry and Interface Formation during the Atomic Layer Deposition of Alumina from Trimethylaluminum and Water on Indium Phosphide

The surface chemistry and the interface formation during the initial stages of the atomic layer deposition (ALD) of Al 2 O 3 from trimethylaluminum (TMA) and H 2 O on InP(100) were studied by synchrotron radiation photoemission spectroscopy and scanning tunneling microscopy. The effect of the ex situ surface cleaning by either H 2 SO 4 or (NH 4 ) 2 S was examined. It is shown that the native oxide on the InP surface consisted mainly of indium hydrogen phosphates with a P enrichment at the interface with InP. After a (NH 4 ) 2 S treatment, S was present on the surface as a sulfide in both surface and subsurface sites. Exposure to TMA led to the formation of a thin AlPO 4 layer, irrespective of the surface cleaning. The surface Fermi level of p-type InP was found to be pinned close to midgap after H 2 SO 4 cleaning and moved only slightly further toward the conduction band edge upon TMA exposure, indicating that the AlPO 4 /InP interface was rather defective. (NH 4 ) 2 S passivation led to a Fermi level position of p-type InP close to the conduction band edge. Hence, the InP surface was weakly inverted, which can be attributed to surface doping by S donors. TMA exposure was found to remove surface S, which was accompanied by a shift of the Fermi level to midgap, consistent with the removal of (part of) the S donors in combination with a defective AlPO 4 /InP interface. Further TMA/H 2 O ALD did not lead to any detectable changes of the AlPO 4 /InP interface and suggested simple overgrowth with Al 2 O 3 .