Surface and interface properties of ZrO_2/GaAs, SiO_2/GaAs and GaP/GaAs hetero structures investigated by surface photovoltage spectroscopy

摘要

The surface and interface properties of III-V semiconductor heterostructures are of extreme importance for increasing the efficiency of multijunction solar cells. However, presence of surface and interface states, deteriorate the performance of the devices made from these structures. Therefore, in this work, surface and bulk states of n(+)-GaAs substrate, grown by vertical gradient freeze (VGF) method, and GaP/GaAs grown using metal organic vapour phase epitaxy are identified from the surface photovoltage amplitude and phase spectra. The surface states are due to complexes formed by silicon and arsenic while the bulk sates are mainly due to boron acceptor states that are commonly observed in VGF grown n(+)-GaAs substrate due to encapsulation of molten flux by B2O5 during growth. The surface passivation effects, by high (ZrO2) and low (SiO2) k dielectrics, on the optical properties of n(+)-GaAs remains similar in nature as confirmed from the SPV amplitude and phase spectra. Further, the pre-cleaning of the substrate by chemical treatment followed by dielectric deposition show improved surface passivation as confirmed from SPV and X-ray reflectivity. Interestingly, similar surface passivation results are also observed in GaP/GaAs heterostructures. After identification of surface states and their isolation, carrier redistribution processes occurring at surface, interface and substrate are distinguished using wavelength dependent SPV magnitude and phase spectra of GaP/GaAs. Subsequently, corresponding band alignment diagram along with band bending at the interface for this hetero structure is constructed. Identification of surface and interface states of semiconductor heterostructures by SPV, surface passivation with suitable materials and construction of the band diagram would be useful for increasing the efficiency of advanced multi junction photovoltaic devices.