Direct band structure measurement of a buried two-dimensional electron gas

Buried two dimensional electron gasses (2DEGs) have recently attracted considerable attention as a testing ground for both fundamental physics and quantum computation applications. Such 2DEGs can be created by phosphorus delta (\delta) doping of silicon, a technique in which a dense and narrow dopant profile is buried beneath the Si surface. Phosphorous \delta-doping is a particularly attractive platform for fabricating scalable spin quantum bit architectures, compatible with current semiconductor technology. The band structure of the \delta-layers that underpin these devices has been studied intensely using different theoretical methods, but it has hitherto not been possible to directly compare these predictions with experimental data. Here we report the first measurement of the electronic band structure of a \delta-doped layer below the Si(001) surface by angle resolved photoemission spectroscopy (ARPES). Our measurements confirm the layer to be metallic and give direct access to the Fermi level position. Surprisingly, the direct observation of the states is possible despite them being buried far below the surface. Using this experimental approach, buried states in a wide range of other material systems, including metallic oxide interfaces, could become accessible to direct spectroscopic investigations.