Study On TiO₂ Based Metal-semiconductor-metal Ultraviolet Photodetectors

Ultraviolet (UV) detectors have attracted intense in recent years because of military and civil value. Recently, many wide-band-gap semiconductors with good optical properties such as doped GaN,ZnO,SiC and diamond have been used for UV detectors. UV photodetectors based on these wide bandgap semiconductors are widely reported, but few concerns with TiO₂. Titanium dioxide is a wide bandgap (anatase 3.2 eV, rutile 3.0 eV) semiconductor, and it has been researched in many aspects such as photocatalysis,solar cell and gas sensors, due to its outstanding physical and chemistry properties, especially optical properties. The applications based on the property that TiO₂ transform UV irradiation to chemical or electrical energy. The property reminds us fabricating UV photodetectors with the wide bandgap semiconductor-TiO₂.In this paper, meatel-semicongductor-metal (MSM) UV photodetector based on nanocrystalline TiO₂ thin films with Schottky contact was demonstrated firstly.TiO₂ thin films were prepared by tetrabutyl titanate (Ti (OC4H9)4), which was generally used for fabricating TiO₂ thin films or TiO₂ fine particles through sol-gel method. The surface of substrates used for the deposition of the TiO₂ films was oxidized, and the thickness of the SiO₂ layers was chosen to be high enough (1μm) in order to ensure good isolation of the TiO₂ layers from silicon substrate. The samples were then annealed for 2 h at 650℃in a muffle furnace, leading to polycrystalline oxide film structures. The characteristics were studied through X-ray diffraction (XRD), atomic force microscope (AFM), UV-Vis spectrum, scanning electron microscope (SEM), X-ray photoelectron spectrometer (XPS). The XPS result show that the valence of Ti atom is +4.The XRD result show that the polycrystalline oxide film is pure anatase, and the average crystallite size of TiO₂ was 38.02 nm. The morphology of the films is smooth and dense as indicated by the AFM and SEM photography. Compare with the absorbance edge relative to the bandgap of anatase, obvious blue shift is shown in the UV-Vis spectrum, which is caused by nanometer effect.In order to adapt actual application, doped TiO₂ were prepared to adjust the absorbance spectrum.Metal-semiconductor contacts play an important part in semiconductor devices, and a lot of efforts have been made to prepare the metal/TiO₂ contacts and study their properties. The fabrication of Schottky diodes from TiO₂ is an important and interesting area where much work has been done recently. Al/TiO₂/Al devices were fabricated firstly, and the liner IV characteristic means that the contact between Al and TiO₂ was ohmic. Radio frequency (RF) magnetron sputtering technique was used in fabricating Cr/TiO₂, Au/TiO₂ and Ni/TiO₂ Schottky barrier diodes with Al ohmic electrode. The effective barrier heightΦb and ideal factor n were calculated from thermionic emission model. The height of Schottky barriers increases with the value of work function indicating that metal/TiO₂ junction was dependent on contacting metals, metals with high work function are tend to form Schottky contact, and those with low work function are tend to form ohmic contact.Interdigitated Au/TiO₂/Au circular structure was designed and fabricated. Au film was deposited by RF magnetron sputtering. The planar interdigitated electrodes were prepared by standard photolithography and lift-off technique. At 5 V applied bias, it was found that dark current of the fabricated devices was only 1.9 nA, and high responsivity of 539 A/W was found when irradiated by the UV light (λ=260 nm) which was much higher than most of other wide band gap photovoltaic photodectors with MSM structure. The high responsivity may owe not only to the decrease of the height of Schottky barrier under UV light but also to the neutral semiconductor between the two back to back Schottky barriers. The trapping states introduced by surface defects reduce the height of Schoottky barrier under UV light irradiation, and the reduction causes more carriers getting across the barrier and then enlarges the current. Moreover, the wide gap between the electrodes induced long neutral region. The neutral region works in the photoconductive mode as photosensitive resistance. The reduction of Schoottky barrier and the neutral region introduces high gain, which is responsible for the high photoresponsivity. Assuming all the photons are absorbed by the semiconductor, we calculate the total gain and find that it is 1896.The devices show a slow time response with a rise time of 85.03ms and a decay time of 1.793s. We deduced the slow time response was generated by defect traps which widely distributed in nanocrysal, acting as recombination centers in the neutral region. The response time will be shorter if the crystal grows better and the finger space becomes smaller. The improvement of TiO₂ films in crystalline quality will reduce the recombination induced by the defects and then reduce the response time. Smaller finger space may reduce the neutral region, even making all the areas between the electrodes to become exhausted region, where the photogenerated carriers will be separated by the build-in field quicklyNi/TiO₂/Ni UV photodetectors were fabricated and tested. At 5V bias, the dark current was 1.5nA, and high responsivity of 8974 A/W was found. The rise time was 3.75ms and the decay time was1.413s. Compare with Au/TiO₂/Au devices, the performance of Ni/TiO₂/Ni UV photodetectors was much better. We deduced that maybe caused by the higher Scottky barrier of Ni/TiO₂ junctiong.Cr/TiO₂/Cr and Au/ RxTi1-xO₂/Au devices were also were fabricated and tested. The results show that Cr/TiO₂/Cr devices and most of Au/ RxTi1-xO₂/Au devices were not sensitive to UV light and a few of Au/ RxTi1-xO₂/Au devices were sensitive to UV light slightly. The mechanics of these devices are not clear and much work still needs to be done.