Buffer Layer Engineering for High ( $\geq 10^{\mathrm {13}}$ cm $^{\mathrm {-2}}$ ) 2-DEG Density in ZnO-Based Heterostructures

In this paper, we report on the prospect of achieving very high values (≥ 10
¹³
cm
^-2
) of 2-D electron gas (2-DEG) density in ZnO-based heterostructures through buffer layer engineering. A physics-based analytical model is developed and utilized to demonstrate up to 25× higher 2-DEG values in MgZnO/CdZnO as compared with that in the MgZnO/ZnO heterostructure at lower Mg composition of 0.10 in barrier layer. It is shown that a lower spontaneous polarization in buffer layer due to more electronegative Cd and higher lattice constant of CdZnO, which introduces tensile piezoelectric strain in the barrier layer, favorably add up, and increase polarization difference at barrier-buffer interface, which eventually enhances 2-DEG density. This paper demonstrates new opportunities to effectively utilize buffer layer properties to significantly improve the 2-DEG sheet density (~4 × 10
¹³
cm
^-2
) in ZnO heterostructures.