Ultrathin ALD-Al2O3 layers for Ge(001) gate stacks: Local composition evolution and dielectric properties

Correlations among physical and electrical properties of atomic layer deposited (ALD)-Al2O3 on H2O-prepulsed Ge(100) have been investigated to evaluate Al2O3 as an ultrathin interface passivation layer for higher-k/Al2O3/Ge gate stacks. In situ XPS in the ALD environment provides insights into the local composition evolution during the initial stages of ALD, evidencing (a) an incubation regime that may limit the minimum achievable capacitance equivalent thickness (CET) of these gate stacks, and (b) residual hydroxyl incorporation in the film consistent with the observed dielectric constant ∼7.2. Thickness scaling of the CET is consistent with a nearly abrupt interface as measured by synchrotron radiation photoemission spectroscopy (SRPES). SRPES studies also reveal that forming gas anneal provides passivation through monolayer-level formation of stoichiometric GeO2, suggesting a complex chemical interaction involving residual -OH groups in the as-grown ALD-Al2O3. Valence and conduction band offsets of prepulsed ALD-Al2O3 with respect to Ge are calculated to be 3.3 ± 0.1 and 2.6 ± 0.3 eV, indicating that these layers offer an effective barrier to hole and electron injection.