Tunneling from an ultrathin quantum well in a strong electrostatic field: A comparison of different methods

A case study of field emission from a zero-range quantum well has been carried out. The various strong-field approximations, both time-dependent and time-independent, are discussed and compared with exact numerical results. The exponential decay law is shown to be a good approximation on the average, but it cannot account for short-time ripples in the ionisation probability.