Study On Ultraviolet Photodetectors Based On TiO₂

Ultraviolet photo-detectors (UVPDs) have attracted intense in recent years because of military and civil value. The UVPDs based on Si or Ge have been utilized, especially for Si UVPDs which have been commercialized because of the mature technology. Recently, UVPDs is developed rapidly along with the development of the technology of material. In recent years, many wide-band-gap semiconductors with good optical properties such as doped GaN, ZnO, SiC and diamond have been used for UVPDs.Nano TiO₂ film has many unique characteristic of nanomaterials in common, for example, surface effect, small size effect, quantum size effect. At the same time, nano TiO₂ film has broad application prospects in many areas such as photocatalyst, UV-ray absorber, gas sensor and photovoltaic cell because of its good performance in weatherability, resistance to chemical corrosion, anti-UV ability, transparency and optical properties. However, it is very few reported about UVPDs based on nano TiO₂ film in spite of its good optical properties. In this paper, metal-semiconductor-metal (MSM) UVPDs based on nanocrystalline TiO₂ thin films with Schottky contact was demonstrated.The feature of the nano TiO₂ film should be studied before the UVPD was fabricated. TiO₂ thin films were prepared through sol-gel method. 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 XRD result shows the diffraction peak of the TiO₂ films prepared by sol-gel method is at 25.3°which indicates that the film is anatase. The UV-Vis spectrum shows the absorption edge of TiO₂ films is about 320nm which is blue shifted obviously compare with the absorption edge of bulk anatase. The blue shift is caused by quantum size effect. The morphology of the films is smooth and dense as indicated by the AFM and SEM photography. The XPS result shows that a large number of oxygen vacancies exist at the surface of the film.The absorption edge of material is adjustable by changing its bandgap. Then Ti_xZr_1-xO₂ solid solution was prepared by sol-gel method in order to fabricate UVPDs whose absorption range is adjustable. The XRD result shows the diffraction peak of the complex was at 24.95°which was offset compare with the diffraction peak of the anatase TiO₂, while the diffraction peak of ZrO₂ was not observed. Therefore, the Ti_xZr_1-xO₂ solid solution was formed. The SEM result shows the surface of the film was dense, the particle size was uniform, the cracks and holes were not obvious which mean good surface properties. The different Ti_xZr_1-xO₂ solid solution films were prepared by changing x which presented the ratio of Ti and Zr atoms. UV-Vis spectrums of solid solution films with different x were tested and compared with UV-Vis spectrums of pure TiO₂ and ZrO₂. The comparing result shows that smaller the value of x was, more obvious the blue shift was, and bigger the bandgap of the solid solution film was. At the same time, the bandgap of the solid solution film-x characteristic was studied. The bandgap-x curve shows there was a linear relationship between bandgap and x, in other words, the values of bandgap become smaller as x increase. That means the bandgap can be adjusted through changing the ratio of the Ti and Zr atoms which lays a good foundation for fabricating wide absorption range UVPD in the future.The TiO₂ films prepared by normal method are polycrystalline which exist a large number of defects and grain boundaries at the surface. Therefore, in order to overcome the above shortcomings, the TiO₂ films were prepared by self-assembly technique on quartz glass substrate. The XRD result shows that the diffraction peak of the TiO₂ films prepared by self-assembly is at 25.3°which indicates that the film is pure anatase. The UV-Vis spectrum shows the absorption edge of TiO₂ films is about 320nm which is similar with the absorption edge of TiO₂ films prepared by sol-gel. The SEM shows the surface of the film was uniform and dense, and the average crystallite size of TiO₂ was about 10-25 nm.Then MSM UVPDs with Au electrode were fabricated based on three different TiO₂ films mentioned above respectively. The optical-electrical properties of three types UVPDs were below:a. Under 5 V applied bias, the dark current of the UVPD based on TiO₂ film prepared by sol-gel was only 4.42 nA. Such a small dark current was caused by restricting electrons of TiO₂ in the area of two back-to-back Schottky barriers. Under irradiation of 250 nm UV light, the photo current could reach to 5.67μA. The ratio of photocurrent-to-darkcurrent was about three orders of magnitude. The photo response wasn't obvious until the wavelength of incident light was shorter than 350nm which were agreed with absorption edge. The peak responsivity of 447 A/W was found under irradiation of 260 nm UV light. The reason of high responsivity was that the hole traps at the semiconductor-metal interface captured photo-generated holes which made the average lifetime of hole longer and larger gain. At the same time, because of the existence of holes captured at the interface, negative mirror charges were generated in the semiconductor side which resulted in the local electric field enhancement and an average Schottky barrier lowering, as a result, more electrons would cross the barrier. The rise time of the device was 2.82 ms and the decay time was 14.51 ms.b. Under 5 V applied bias, the dark current of the UVPD based on Ti0.8Zr0.2O₂ film was 2.72 nA. Under irradiation of 250 nm UV light, the photo current was 77.9μA. The photo response began to be obvious under irradiation of 340 nm which means the absorption edge of device was adjusted. The peak responsivity of 756 A/W was found under irradiation of 260 nm UV light. The photo response decreased rapidly when the wavelength of incident light was shorter than 260nm. The reason is that the wavelength was too short to penetrate and the surface recombination was easier, photogenerated carriers were combined before they crossed depletion layer. The rise time of the device was 3.178 ms and the decay time was12.4 ms.c. Under 5 V applied bias, the dark current of the UVPD based on TiO₂ film prepared by self-assembly was 5.95 nA. Under irradiation of 250 nm UV light, the photo current was 21.9μA. The peak responsivity of 73.65 A/W was found under irradiation of 260 nm UV light. The rise time of the device was 3.664 s and the decay time was 4.183 s. Compared with the other two devices, the disparities of UVPD based on TiO₂ film prepared by self-assembly were obvious. The self-assembly method was not qualified, there were many areas for improvement, for example, control of experimental conditions, film quality, component selection. As a result, the photo-electronic characteristics of the device were not as good as the other two devices mentioned above. However, we can anticipate that the performance of the device would be promoted if the self-assembly method was improved and film with better quality was prepared.In order to understand the property of TiO₂-metal Schottky contact barrier and find an appropriate metal as electrode, MSM UVPDs based on nanocrystalline TiO₂ thin films with Au, Ni Schottky contacts were fabricated.Dozens of devices with Au and Ni electrodes were fabricated. The Schottky barrier height (SBH) and ideality factor n were evaluated from I-V characteristics of each device by thermionic emission theory. The values of SBH and n obey a Gaussian distribution. Therefore, using the Gaussian function, we can evaluate SBH value of 0.838±0.001 eV and an average n value of 2.09±0.24 for the Ni/TiO₂/Ni devices. The respective values for the Au/TiO₂/Au devices are 0.841±0.002 ev and 1.72±0.24. At the same time, there is a linear relationship between SBH and ideality factors, in other words, the values of SBH become smaller as ideality increase. Thus, the homogeneous barrier heightФ_hom can be determined by evaluating the values ofФ_b and n of many devices, fitting the curve ofФ_b versus n and extrapolating to n=1.01. Extrapolating theФ/n curve to n=1.01, we can getΦ_bi^Ni=0.839 eV for Ni/TiO₂/Ni devices andΦ_bi^Au=0.842 eV for Au/TiO₂/Au devices. The result thatΦ_bi^Niis lower thanΦ_bi^Au shows, under irradiation of specific UV light, the photo-generated carriers at the interface of TiO₂/Ni are much easier to transit across the barrier than those at the interface of TiO₂/Au. As a consequence, better photoelectric properties for Ni/TiO₂/Ni ultraviolet detectors are expected, which agree the with experimental data. As a result, Ni is more appropriate as electrode.In order to make UVPDs practical, using the feature of generating photo-current under irradiation of UV light, a UV radiation intensity tester was fabricated. The basic theory is: according to the relationship that the voltage of load resistance varies with the UV radiation intensity, we test the voltage of load resistance in order to get the radiation intensity indirectly, and using the digital processing system of peripheral circuit to output the result on the LCD. The result shows that the voltage of load resistance increased as the radiation intensity was higher, and the relationship of the two values was nonlinear. The tester we made can work well under both high and low radiation intensity which means the adaptability of the tester is wide.