I. Introduction
Ge has been extensively studied to replace Si due to its higher electron and hole mobility [1], [2], which can enhance metal oxide semiconductor field effect transistor (MOSFET) speed without any physical scaling. Before the development of high-k oxide, the hygroscopic property of GeO2 impeded the implementation of Ge MOSFET since SiO2 has a better stability as native oxide layer, grown on Si [3]. Nevertheless, after metal/high-k gate stack was introduced, the formation of appropriate thickness of GeOx or GeO2 between high-k and Ge substrate was reported to have low interface state density () [4]. Growing a few Ångström GeO2/GeOx intentionally is, therefore, necessary to obtain good interface quality for high-k/Ge gate stacks [5]. The thickness of GeO2/GeOx layer will impact the overall equivalent oxide thickness (EOT) because of its low dielectric constant compared to high-k layer [6]. Therefore, the tradeoff between EOT and is an inevitable issue. Besides, interface layer treatment of Ge and controlling its thickness are also critical to further the scaling EOT below 1 nm [7]. High-k dielectrics like HfO2 and ZrO2 have already been integrated into CMOS technology. It is reported that HfO2/Ge gate stack shows a larger capacitance-voltage (CV) hysteresis than Al2O3 [8], and Al2O3 can block electron injection from substrate effectively by large conduction band offset related to Ge substrate [8]. Moreover, ZrO2 showed lower leakage current than HfO2 with similar dielectric constant [9]. Therefore, a bilayer high-k stack (ZrO2/Al2O3) was used in this study. The IL quality (interface state density, ), EOT, and XPS analysis of IL atomic composition of these stacks were reported in a previous work [10] and it is summarized in the Table I. EOT, , , and Interfacial Layer Type for Three Samples
Sample name | EOT (nm) | (V) | (cm/eV) at −0.15 eV | Interfacial Layer |
---|---|---|---|---|
Ge/SPAO/Al2O3/ZrO2 | 0.98 | 0.015 | GeOx | |
Ge/Al2O3/SPAO/ZrO2 | 1.29 | −0.228 | GeOx | |
Ge/Al2O3/ZrO2/SPAO | 1.13 | −0.362 | GeO2 |
†The data were reported in previous work [10].