Ultraviolet Photodetectors Based On Zr_xTi_1-xO₂Solid Solution

Sensor, as the interface between system monitoring and research object, plays animportant role in obtaining information. Ultraviolet detector as an important branch oftraditional sensors, has attracted a great deal of interests in civil and military applications.Due to the smaller volume, higher efficiency, lower power dissipation and cost, wide bandgap semiconductors based Ultraviolet detectors have become the mainstream. Studies onthe substrate materials of UV detector were mainly concentrated on GaN and SiC duringthe past years, and extended to some metal oxide, such as ZnO, TiO2in recent years.However, the binary wide-band gap materials have their own limitations, the responsivitystill needs to be improved and the detection range could be extended further. To date, a lotof attention has been paid to the complex oxide materials, such as ZnxMg1-xO, Zn2GeO4and Zr_xTi_1-xO₂due to their improved physical and chemical properties and the tunable bandgap.Zr_xTi_1-xO₂exhibits similar material advantages as pure TiO2and offers a way ofobtaining titania with higher surface area and better thermal stability. In addition, the bandgap of Zr_xTi_1-xO₂can be tuned from3.2eV (TiO2) to7.8eV (ZrO2) by varying the Zrcomposition. Therefore, it has much potential superiority in adjusting its UV absorptionedge and fabricating solar blind UV detector.In this letter, Zr_xTi_1-xO₂nano films were prepared by a sol-gel method and characterizedby means of XRD, SEM, XPS, AFM and UV-visible absorption spectra. The XRD patternsshow that Zr_xTi_1-xO₂solid solution was formed when a few Zr was doped; SEM imagesshow that there was serious crack appear on pure TiO2sample and ZrO2sample surface,and the combination of TiO2and ZrO2would improve the surface smoothness of the film; the UV-visible absorption spectra of Zr_xTi_1-xO₂films shows that the doping of Zr increasesthe UV absorption edge and the band gap of Zr_xTi_1-xO₂; The ratio between Zr and Ti on thefilm surface was semi-quantitatively estimated by XPS spectra, it is found that Zr waseasier react with oxygen due to its lower electronegativity.Based on Zr_xTi_1-xO₂thin films, MSM structure UV electrodes were designed, and the I-Vcharacteristics and the responsivity of the devices were measured. For the Zr_xTi_1-xO₂detectors with lower Zr doping, higher photoresponse was obtained than that of TiO2,which can be explained by the introduction of Zr atom. It is well known that theelectronegativity of zirconium atom is weaker than that of titanium atom. As theisoelectronic impurities of Ti, Zr in the lattice could be treated as hole traps, and thusfurther results in the reduction of Schottky barrier. That is, higher responsivity is obtained.However with further increasing of Zr, the response peaks shift to shorter wavelengthdirection and the photoresponses drop significantly. This is because that the introduction ofZr could not only increase the resistance of the neutral region between electrodes, but alsodecrease the absorption coefficient of Zr_xTi_1-xO₂films. However, the responsivity could beimproved by optimizing the film quality and the distance between electrodes. So thefabricating of high response solar blind UV detector is quite possible.High responsivity metal-semiconductor-metal Zr_xTi_1-xO₂ultraviolet photodetectors withNi, Pt and Au electrodes were identically fabricated to explore the influence of metalelectrodes on device performance. It has been confirmed that when Ni is deposited on topof the Zr_xTi_1-xO₂film, the work function of Ni will decrease. It may be because that theelectron would transfer from Ni to the substrate, which will form a dipole layer between Niand Zr_xTi_1-xO₂, and result in the decrease of SBH, therefore high responsivity was obtained.For the Pt electrodes, it produces the highest Schottky barrier and low Schottky barriercapacitance, so fast recover time was obtained. In addition, different device structures werealso studied to explore the influence on the performance of ultraviolet detectors. Thesandwich structure UV detector fabricated by adding an additional Zr_xTi_1-xO₂lay on theoriginal MSM UV detector, achieves higher responsivity and long-term stability because ofthe three-dimensional structure of back-to-back Schottky junction and the protective effect of metal electrode from corrosion by air, water vapor.The carrier transport mechanism and its gain mechanism for MSM devices were alsoanalyzed in detail. For the significant gain of Zr_xTi_1-xO₂based detector with Au electrodes(967), the traditional theory was difficult to explain such a high gain. Therefore,phototransistor model was firstly proposed in this paper.The Zr_xTi_1-xO₂nanowire array was prepared via a low-temperature hydrothermal methodand characterized by means of XRD, SEM, and XPS. One-dimensional Zr_xTi_1-xO₂nanowire could decrease the recombination probability of photogenerated electron-hole andlargely increase the photoresponse owing to the fact that the photogenerated holes may betrapped in the surface states of Zr_xTi_1-xO₂nanowire caused by the adsorbed O2. In addition,well-aligned Zr_xTi_1-xO₂NWS arrays have been demonstrated with fast electron transportand straight conduction pathways, which are beneficial in obtaining a faster response time.The diameter and length of the nanowire were20nm and4μm, respectively. For thegrowth mechanism of nanowires, growth unit model and over-saturation growth modelwere proposed to analyze the growth rate and growth mechanism. In addition, thecontrolled growth of the nanowire diameter was also analyzed in detail.Based on the Zr_xTi_1-xO₂nanowire array, Schottky photodiode UV detectors with Agelectrodes were fabricated. Back-side illumination was adopted to increase the lightabsorption effectively. In addition, the Zr_xTi_1-xO₂/Metal Schottky contact characteristicswas researched in detail and the barrier height value was calculated, this Schottky diodedetector with simple fabrication process, low cost and superior performance wouldprovides a new way in fabricating UV imaging arrays. The Schottky barrier height of thedevice was analyzed by theoretical calculations, the thermionic emission theory and theXPS spectra analysis, it is found that the work-function of Ag increased considerably due tothe oxygen adsorbed on Ag, which could greatly increased the Schottky barrier heightbetween Ag and Zr_xTi_1-xO₂. In addition, the Schottky photodiode UV detector also obtainedinternal gain, which may be induced by the reduction of the Schottky barrier at Ag/TiO2interface under illumination.