  We realize a two-dimensional electron system (2DES) in ZnO by simply
depositing pure aluminum on its surface in ultra-high vacuum, and characterize
its electronic structure using angle-resolved photoemission spectroscopy. The
aluminum oxidizes into alumina by creating oxygen vacancies that dope the bulk
conduction band of ZnO and confine the electrons near its surface. The electron
density of the 2DES is up to two orders of magnitude higher than those obtained
in ZnO heterostructures. The 2DES shows two $s$-type subbands, that we compare
to the $d$-like 2DESs in titanates, with clear signatures of many-body
interactions that we analyze through a self-consistent extraction of the system
self-energy and a modeling as a coupling of a 2D Fermi liquid with a Debye
distribution of phonons.
