Effects of aluminum incorporation on band alignment at the SiO2/HfO2 interface

A high-k HfO2/SiO2 gate stack is taking the place of SiO2 as a gate dielectric in a modern field effect transistor. The use of HfO2 requires employing a metal gate electrode that limits available options for controlling transistor threshold voltage by modulating the electrode work function. An alternative approach is to modify the overall band alignment in the gate stack. To guide the experiment in finding a way of engineering the stack composition in order to control the band offset, we investigate theoretically the effects of Al incorporation in the high-k HfO2/SiO2 gate stack (effectively, this creates a pseudobinary alloy Al2O3-HfO2). It is shown that O vacancies are stabilized in the vicinity of substitutional Al in the SiO2 interfacial layer. We find that Al interstitial atoms pair up and form a stable Al-vacancy complex in the vicinity of the HfO2/SiO2 interface. Based on a previously introduced model, which suggests that the oxygen depleted interface provides much less effective screening of the interfacial dipole, we show that Al incorporation in the SiO2 layer may shift significantly the band alignment between hafnia and silica, increasing the band offset by up to 1.8 eV.