Evaluation of Ge Oxidation State in Ge Nanoparticles Formed in Thin SiO2 Layer by Negative-Ion Implantation and Successive Two-Stage Annealing

Low-voltage electroluminescence (EL) at 390 nm was obtained in a MIS structure at applied voltage of 15-30 V from 3 at.% Ge-implanted 50-nm SiO2 layer samples after the successive two-stage annealing. From the optical emission, Ge-related oxygen deficiency centers (Ge-ODCs) are speculated to exist at a shallow depth. To clarify the creation depth of Ge-ODCs and roles of nanoparticles (NPs), Ge-oxidation states in the SiO2 layer and Ge NP were measured by two methods, i.e., X-ray photoelectron spectrometry (XPS) and 3-dimensional atom probe (3D-AP) after annealing: (1) in nitrogen gas flow at 700℃ for 1h and (2) in air flow at 700℃ for 1h. In XPS with Ar-etching, a significant increase of Ge-O bonds was shown at 15 - 30 nm in depth. Ge-Ge bonds decreased in the whole depth region. In 3D-AP, relatively large four Ge NPs were detected in the Ge-implanted SiO2 layer. The shallowest NP was spherical with at diameter of 5 nm and had a Ge-core at 2 nm in diameter and Ge-oxide shell of with 1 - 2 in thickness. So Ge-O bonds were surely created between core and shell. The role of Ge NP is to ensure forming G-ODCs in NP and interrupting progress of Ge oxidation to the deeper side.