Amorphous Gadolinium Aluminate as a Dielectric and Sulfur for Indium Phosphide Passivation

The passivation of n-type InP (100) using sulfur in combination with a gadolinium aluminate (GAO) dielectric layer has been studied. Photoluminescence, minority-carrier lifetime, and capacitance-voltage measurements indicate that a (NH4)(2)S vapor passivation step prior to atomic layer deposition of the oxide effectively lowers the interface state density. Surface and interface chemistry were studied by synchrotron radiation photoemission spectroscopy (SRPES). The effect of ex situ surface passivation after native oxide removal in HCl solution was examined. It was observed that surface reoxidation occurred during (NH4)(2)S vapor exposure, leading to the formation of In-x(HPO4)(y). S was present on the surface as a sulfide in both surface and subsurface sites. After atomic layer deposition of GAO, sulfates were detected in addition to In-x(HPO4)(y), which was confirmed by near-edge X-ray absorption fine structure analysis. The S in the stack was quantified using reference-free grazing incidence X-ray fluorescence analysis. X-ray absorption spectroscopy showed that Gd was oxidized and present in the 3+ oxidation state, most likely as a phosphate close to the InP interface and possibly mixed with sulfates. Energy-dependent SRPES measurements of Al 2p and Gd 4d core levels, complemented by transmission electron microscopy, further suggest that the dielectric layer was segregated. Valence band measurements confirm the effective passivation of InP, indicating unpinning of the surface Fermi level.