Surface transfer doping of hydrogen-terminated diamond by C ₆₀F ₄₈: Energy level scheme and doping efficiency

Surface sensitive C1s core level photoelectron spectroscopy was used to examine the electronic properties of C ₆₀F ₄₈ molecules on the C(100):H surface. An upward band bending of 0.74 eV in response to surface transfer doping by fluorofullerene molecules is measured. Two distinct molecular charge states of C ₆₀F ₄₈ are identified and their relative concentration determined as a function of coverage. One corresponds to ionized molecules that participate in surface charge transfer and the other to neutral molecules that do not. The position of the lowest unoccupied molecular orbital of neutral C ₆₀F ₄₈ which is the relevant acceptor level for transfer doping lies initially 0.6 eV below the valence band maximum and shifts upwards in the course of transfer doping by up to 0.43 eV due to a doping induced surface dipole. This upward shift in conjunction with the band bending determines the occupation of the acceptor level and limits the ultimately achievable hole concentration with C ₆₀F ₄₈ as a surface acceptor to values close to 10 ¹³ cm ^-2 as reported in the literature.