Extraction of band offsets in Strained Si/Strained Si1-yGey on relaxed Si1-xGex dual-channel enhanced mobility structures

A dual-channel structure consisting of a thin (~3 nm) strained Si layer on strained Si1-yGey on relaxed Si1-xGex (Fig. 1) has been shown to offer large enhancements in both electron and hole mobilities in a single epitaxial layer stack [1,2]. Mobility enhancements as large as 1.8x and 10x have been measured in nand p-MOSFETs respectively [3]. It is essential to know the band structure in order to accurately model these devices. In particular the valence band offset, ∆EV, between the strained Si and strained Si1-yGey layers (Fig. 1) determines the degree of hole confinement in the Si1-yGey layer. The positions of the conduction band edge in the strained Si and the valence band edge in the strained Si1-yGey significantly impact the effective bandgap of the structure and can be used to tune the threshold voltage of both nand pMOSFETs for use with a single workfunction metal gate [4]. Theoretical predications of these band parameters are uncertain by ±100 meV [5,6]. In this work, for the first time, we extract these critical band parameters over a wide range of Ge composition and strain, using a combination of experiment and modeling. It is also shown that the appropriate density of states for the strained layers must be used in order to accurately model the capacitance-voltage (C-V) behavior. Epitaxial layers were grown in an Applied Materials “Epi-Centura” system. The layer structure is illustrated in Fig. 1. High frequency C-V was measured and compared to simulations taking into account quantum corrections using the density gradient model in Dessis [7]. The valence band offset, ∆EV was extracted from the analysis. As the bias is swept negative, hole accumulation occurs first in the strained Si1-yGey layer. A plateau forms in the C-V curve, which is very sensitive to ∆EV (Fig. 2). The effective valence band density of states NV decreases with strain and increasing Ge content [8]. NV values calculated by the nonlocal empirical pseudopotential method from full-band Monte Carlo simulations [9] were used in the Dessis C-V simulations. Fig. 3 illustrates the improved C-V fitting obtained when the appropriate values are used for NV in the strained Si and strained SiGe, and the impact on extracted oxide thickness and ∆EV. Fig. 4 compares values of extracted ∆EV for a more well studied structure, strained SiGe on relaxed Si. Using the full-band strained NV values we obtain a slope of 101 meV/10% Ge for ∆EV, and a corresponding value of 87 meV/10% Ge when the relaxed Si value is used for NV. For strained Si on strained Si1yGey, ∆EV increases with both y and strain (Fig. 5). The sensitivity of Vt and subthreshold slope to both ∆EV and NV demonstrate the importance of using appropriate values for accurate modeling of these structures (Fig. 6). In summary, MOS C-V analysis has been used to extract critical band parameters for strained Si/strained SiGe dual-channel structures. These parameters are essential for modeling such high mobility structures. References: [1] Leitz et al, Appl. Phys. Lett. 2001. [2] Jung et al, EDL, 2003. [3] Lee et al, IEDM, 2003. [4] S. Yu et al, submitted to EDL. [5] People, J. Quant. Electronics, 1986. [6] Schaffler, Semicond Sci. Technol. , 1997. [7] Ancona et al, Phys. Rev.B, 1989. [8] Manku et al, J. Appl. Phys., 1991. [9] Rieger et al, Phys. Rev.B, 1993. [10] Lang et al, Appl. hys. Lett. 1985. P [11] King et al, Trans. Electron Devices, 1989.