  Properties of semiconductors are largely defined by crystal imperfections
including native defects. Van der Waals (vdW) semiconductors, a newly emerged
class of materials, are no exception: defects exist even in the purest
materials and strongly affect their electrical, optical, magnetic, catalytic
and sensing properties. However, unlike conventional semiconductors where
energy levels of defects are well documented, they are experimentally unknown
in even the best studied vdW semiconductors, impeding the understanding and
utilization of these materials. Here, we directly evaluate deep levels and
their chemical trends in the bandgap of MoS2, WS2 and their alloys by transient
spectroscopic study. One of the deep levels is found to follow the conduction
band minimum of each host, attributed to the native sulfur vacancy. A
switchable, DX center - like deep level has also been identified, whose energy
lines up instead on a fixed level across different hosts, explaining a
persistent photoconductivity above 400K.
