UV-enhanced photodetector with nanocrystalline-TiO2 thin film via CMOS compatible process

This research presents nanocrystal titaniumdioxide (nanocrystal-TiO₂) film deposition technique with CMOS compatible process [1] to extend the optical response bandwidth of silicon based photodetecting devices toward ultraviolet range [2]. The thin films were initially deposited as Titanium Nitride (TiN) using DC magnetron reactive sputtering system. It was then annealed under nitrogen atmosphere at 800°C. After analyzing crystal structures and surface morphology with X-ray diffraction and FE-SEM systems, it was found that the deposited thin films showed the crystal phase of TiO₂ oriented along (200) plane of Rutile crystal structure with 50 nm grain size and increasing with film thickness. Using electroreflectance (ER) spectroscopy measurement [3], the bandgap of nanocrystal-TiO₂ was revealed to be 3.16 eV. PN-heterojunction photodiodes were fabricated with Nanocrystal-TiO₂/SiO₂/p-Si structures. Interdigitated aluminum structures were used as electrodes. By varying the thickness of nanocrystal-TiO₂ film, i.e. 30, 60, and 90 nm, the devices could response further into the UV range. The absorption edge wavelength investigated by photoresponse measurement was at 275 nm and shifting to the shorter wavelength as a function of the nanocrystal-TiO₂ grain size due to quantum confinement phenomenon [4]. The nanocrystal-TiO₂/SiO₂/p-Si photodetector had dark current = 5.31nA (2V), photocurrent = 0.9 uA, rise time = 58 us, fall time = 47 us at 30 nm thickness of TiO₂.