Valence band structure in boron-zinc oxide films characterized by secondary electron emission

Abstract

Properties of the valence band structure in boron-zinc oxide (BZO) films were investigated using the secondary electron emission due to the Auger neutralization of helium ions, with respect to the application of BZO films to the development of solar cells, in which the conductivity of the BZO films plays a critical role in improving cell performance. The characteristic energy ɛ0 corresponding to the peak density of states in the valence band showed that BZO film prepared with a 3000 SCCM B2H6 gas flow rate (SCCM denotes cubic centimeters per minute at standard temperature and pressure) had a shallow characteristic energy ɛ0 = 5 eV, whereas film without boron doping had a deep characteristic energy ɛ0 = 8.2 eV, suggesting that a high concentration of boron impurity in BZO films might enhance the transition of electrons and holes through the bandgap from the valence to the conduction band in zinc oxide crystals, thereby improving the conductivity of the film. The measurement method developed here demonstrates that the secondary electron emission is very useful in the determination of the band structure in various synthetic films.