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Abstract

To our knowledge this paper presents the first study on the persistent photoconductivity behavior of hydrogen-terminated type-IIa diamond in the presence of a surface conductivity, particularly in exploringthe effect of temperature. Photoconductivity measurements were performed in vacuum over a range of sample temperatures, and with a variety of photo excitation sources of varying wavelength in addition to the transport measurements on the van der Pauw devices. From the determination of the hole sheet density, the position of the Fermi energy level, with respect to the valence band maximum was determined to be between -0.18 eV and -0.22 eV. The trap states that are responsible for photo-excitation in the diamond band gap is within about 2.4 eV of the valence band maximum. It is found that there was no significant difference in the levels of excited photocurrent for devices with optically exposed and shielded metal contacts, confirming that the photo-effects observed arise in the diamond. Our interpretation suggests an evolution from a slowly decaying process dominated by the photoexcitation and spatial separation of electrons in the near-surface regime at high temperature to a faster decay process dominated by charge trapping by boron acceptors in the bulk at low temperature. Temperature dependent transport measurements showed that our samples became more resistive, with a concurrent decrease in measured hole sheet density, as the temperature was reduced, consistent with carrier freeze out at low temperatures due to a degree of surface disorder that has been reported extensively for all but the highest quality hydrogenated diamond surfaces.