Spectroscopy of buried states in black phosphorous with surface doping

Electrostatic gating or alkali metal evaporation can be successfully employed to tune the interface of layered black phosphorus (BP) from the semiconductor to a 2D Dirac semimetal. Although Angle Resolved Photoelectron Spectroscopy (ARPES) experiments have captured the collapse of the band gap in the inversion layer, the quantitative estimate of the band structure evolution has been hindered by the short escape depth and matrix elements of the probed photoelectrons. Here, we precisely monitor the evolution of electronic states by time-resolved ARPES at photon energy of 6.3 eV. The probing depth of laser based ARPES is long enough to observe the buried electronic states originating from the valence band maximum. Our data show that the band gap has the maximal value of 0.32 eV in the pristine sample and it shrinks down monotonically by increasing the carrier concentration in the topmost layer. Most interestingly, the band velocity of valence band increases by a factor two along the armchair direction, overcoming the value reported in graphene on silicon carbide (SiC). This control of band structure via external gating is of interest for the design of optoelectronic devices.