Two-dimensional electron gas in Zn polar ZnMgO∕ZnO heterostructures grown by radical source molecular beam epitaxy

Abstract

A two-dimensional electron gas was observed in Zn polar $\mathrm{Zn}\mathrm{Mg}\mathrm{O}∕\mathrm{Zn}\mathrm{O}$ (ZnMgO on ZnO) heterostructures grown by radical source molecular beam epitaxy. The electron mobility of the $\mathrm{Zn}\mathrm{Mg}\mathrm{O}∕\mathrm{Zn}\mathrm{O}$ heterostructures dramatically increased with increasing Mg composition and the electron mobility $(\mu \sim 250{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s})$ at RT reached a value more than twice that of an undoped ZnO layer $(\mu \sim 100{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s})$ . The carrier concentration in turn reached values as high as $\sim 1\times {10}^{13}{\mathrm{cm}}^{-2}$ and remained nearly constant regardless of Mg composition. Strong confinement of electrons at the $\mathrm{Zn}\mathrm{Mg}\mathrm{O}∕\mathrm{Zn}\mathrm{O}$ interface was confirmed by $C\text{-}V$ measurements with a concentration of over $4\times {10}^{19}{\mathrm{cm}}^{-3}$ . Temperature-dependent Hall measurements of $\mathrm{Zn}\mathrm{Mg}\mathrm{O}∕\mathrm{Zn}\mathrm{O}$ heterostructures also exhibited properties associated with well defined heterostructures. The Hall mobility increased monotonically with decreasing temperature, reaching a value of $2750{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ at $4\mathrm{K}$ . Zn polar “ZnMgO on ZnO” structures are easy to adapt to a top-gate device. These results open new possibilities for high electron mobility transistors based upon ZnO-based materials.