【作者】 解艳茹；

【导师】 梅良模； 陈延学；

【作者基本信息】 山东大学 ， 凝聚态物理， 2014， 博士

【摘要】 紫外探测技术是如今研究开发的重要课题之一,在军事、通讯、空间科学、环境监测、生物细胞癌变监测等领域有着十分广泛的应用。随着宽禁带半导体材料与器件技术的快速发展,基于宽禁带半导体的紫外探测器有望取代传统的硅基紫外探测器,具有更广泛的应用前景和良好的发展潜力。宽禁带半导体材料具有较大的禁带宽度,与传统硅基紫外探测器相比,宽禁带半导体基紫外探测器可以避免使用结构复杂、价格昂贵的滤光器,实现“可见光盲区”紫外探测器,因此其在通讯、航空航天等领域具有很大的吸引力,得到人们的普遍关注和广泛研究。近年来,人们相继研究了多种半导体材料体系的紫外探测器,例如基于金刚石、GaN、SiC、ZnO等材料的探测器,但这些材料体系大多存在制备工艺困难、稳定性差等局限性,不利于大规模的应用。此外,同一种材料体系,由于制备的工艺不同,获得的光电特性差异很大。因此寻找更为理想的制备紫外探测器的半导体材料和结构具有十分重要的意义。TiO2作为一种宽禁带半导体材料,具有优良的物理化学稳定性、光电特性优良且价格低廉,在光催化、气敏器件以及太阳能电池等光电领域有着重要的应用前景。TiO2体材料的禁带宽度为3.0-3.2eV,主要吸收紫外光波段,对可见光几乎不吸收,紫外光区/可见光区响应对比度较高,因此具有高紫外探测灵敏度和可见光盲特性。此外,TiO2薄膜材料制备的工艺简单,为大批量生产低成本、高稳定性紫外探测器提供了有利条件。相对于金刚石基、GaN基、SiC基、ZnO基等较为成熟的紫外探测器,基于TiO2的紫外光电探测器的研究工作还比较少。目前为止,TiO2基紫外探测器的研究大多集中在锐钛矿相的TiO2。对于金红石相TiO2基光电探测器的制备及其性能还知之甚少,需要进行系统深入的研究。要制备高性能的紫外探测器,生长制备高质量的TiO2是至关重要的。金红石TiO2薄膜的单晶外延生长对于其物性研究及材料体系的其他应用也有着十分重要的意义。目前,金红石相TiO2单晶外延薄膜的生长主要使用的衬底是蓝宝石衬底,由于晶格失配较大,制备的薄膜存在大量的氧缺陷。本论文中,我们利用脉冲激光沉积方法,在具有良好晶格匹配性的单晶MgF2(110)衬底上外延生长出高质量的金红石Ti02薄膜。MgF2本身是一种十分重要的光学窗口材料,其折射率很低,从紫外到红外波长都具有非常好的透光性,被广泛应用于一些光学元件中。为制备紫外探测器器件,我们利用光刻技术在制备好的金红石Ti02外延薄膜刻制叉指电极,再利用溅射仪在图样上溅射一层Ni做为金属电极,完成面内结构的Ni-TiO2-Ni紫外探测器。光电探测结果显示器件对于波长大于370nm的光几乎没有响应(因低能入射光子不足以激发二氧化钛的能级跃迁),在300nm处产生了光电响应极大值,约为4.85A/W。对于波长小于260nnm的紫外光,探测器的光电响应明显下降。这是因为当入射光波长很短时,材料的吸收系数较大,光子穿透的深度减小,表面的复合速率变大,即光生载流子在还没有穿透耗尽层的时候就复合掉了,表现出响应的急剧减小。除了Ti02外延薄膜的探测器外,近期人们尝试将纳米晶Ti02薄膜制作成为光化学电池结构,并获得了性能十分优良的紫外探测器。和纳米晶Ti02薄膜相比,一维有序材料,如纳米棒、纳米管、纳米线阵列,具有很大的界面面积并可以提供载流子定向传输的快速通道。如果以Ti02的有序纳米材料代替传统的薄膜材料,这将有可能进一步提升Ti02的紫外探测性能。水热法制备纳米二氧化钛具有成本低廉、工艺简单、生长周期短等优点受到越来越多的关注,水热法在高压条件下低温即可实现Ti02的结晶。本论文通过水热法成功生长了Ti02纳米结构的样品,并对其制备的光电探测器的性能进行研究。利用水热法我们在FTO衬底上生长了金红石Ti02纳米棒阵列,对样品的结构、形貌及光学性质进行了表征,并利用所得样品制备了光化学电池结构的自供电紫外探测器。首先我们使用去离子水做为电解液,取代传统的碘离子电解液。获得的器件表现出和基于碘离子电解液的探测器可比拟的光电响应性能。而水对环境无污染,解决了碘离子液体容易腐蚀金属电极、长期稳定性差等问题。Ti02纳米棒阵列样品制备的器件在1.25mW/cm2紫外光照射下(λ=365nm),短路电流(ISc)和开路电压(Voc)分别是4.67μA和0.408V。器件光谱响应范围是在310nm到420nm之间,表明器件很适合UV-A波段的探测。无外加电压的情况下,器件响应度的极大值是0.025A/W出现在波长350nm处。对于辐射的强度是1.25mW/cm2的紫外光(λ=365nm),器件光电流的上升响应时间是0.15s,下降响应时间是0.05s。为了进一步增大TiO2的比表面积,进而获得响应度更高的紫外探测器,我们对TiO2纳米棒阵列样品作了改进,制备了纳米树结构的样品。TiO2纳米树紫外探测器最大的光电响应度是0.22A/W,在352nm处,IPCE是77%。与纳米棒阵列样品相比,TiO2纳米树基紫外探测器的Jsc和Voc都明显增加。性能最好的器件在2mW/cm2紫外光(λ=365nm)下Jsc和Voc分别是373μA/cm2和0.44V,光伏特性有明显的改善。器件性能的提高归结于以下几点：(1)与TiO2纳米棒阵列相比,TiO2纳米树的比表面积和粗糙度都更大,TiO2/电解液接触面积大,能产生更多的光生电子-空穴对；(2)单晶纳米棒是光电子的良好通道,光电子能很快的传导到FTO衬底；(3)纳米树杈的存在一定程度上增加了样品对光的散射,使光子的吸收效率提高。为了解决液体电解液容易泄露的问题,我们利用基于液晶的固态电解液代替传统的液体电解液,制备了新型准固态的TiO2基自供电紫外探测器。器件的光电响应比液态电解液制备的探测器响应度更好,同时解决了液体容易流失、长期稳定性差等问题。器件的工作的波长范围310-420nm,适合探测UV-A波段的光子。器件的响应度很高,最大值是0.09A/W,在383nm的位置。对于辐射的强度是2.5mW/cm2的紫外光(λ=365nm),器件光电流的上升响应时间和下降响应时间均小于0.3s。此外,我们还利用常见的有机p型半导体spiro-MeOTAD作为空穴传导层材料制备了自供电的全固态无机-有机异质结光电探测器。器件有良好的波长选择性,光谱响应曲线的峰值位于400到420nm,半峰宽(FWHM)仅有26nm,非常适合蓝光区域的探测。器件在不加外偏压的情况下可持续稳定工作,且响应速度快。强度75μW/cm2蓝光(λ=410nm)照射下,器件上升响应时间是0.12s,下降响应时间是0.06s。更多还原

【Abstract】 Ultraviolet (UV) photodetectors are becoming increasingly important for their applications in many fields, such as military use, UV communication, environmental monitoring, space research, biological cells canceration monitoring, and so on. As the grown technology developes rapidly, wide band gap semiconductor will replace Si for the fabrication of photodetectors and has enormous potential and broader prospects.Compared with photodetectors based on Si, devices based on wide band gap semiconductors can avoid expensive optical filter and realize visible-blind detection. Wide band gap based photodetectors, which have been used in the fields such as radar, communication and aviation, have attracted lots of attention. In recentyears, wide bandgap semiconductors, such as diamond, GaN, SiC and ZnO, have been studied for wide spreading usage in photodetection, especially in ultraviolet region. However, these materials are either difficult to preparare or have bad stability. Moreover, results from different research groups are sometimes contradictory with each other; therefore, looking for more suitable semiconductor to make photodetectors is crucial.Wide band gap semiconductor TiO2has many advantages, such as good physical and chemical stability, high reactivity, low-cost and good photoelectric characteristics. TiO2has already received enormous attention for photocatalyst, gas sensers and solar cell applications. TiO2has a band gap of3.0-3.2eV and nearly does not absorb visiblelight, which makes TiO2-based photodetectors show high sensitivity to UV/visible. What is more, the growth of TiO2films is simple, which is beneficial to low-cost and large-scale UV photodetectors manufacture.Compared with UV photodetectors based on diamond, GaN, SiC and ZnO, TiO2based UV photodetectors were only reported by several groups. And the reported TiO2UV photodetectors are mostly based on anatase phase. Research on rutile TiO2based UV photodetectors is strongly desired.Immobilization of TiO2as a thin film was a postulate for high performance photodetectors applications. The epitaxial growth of high-quality rutile TiO2films is important for studying its photoelectric property. Untill now, only a fewstudies have dealt with single crystalline, epitaxial rutile TiO2films, most of which deal with Al2O3substrate. In our work, high-quality epitaxial rutile TiO2(110) films have been deposited on MgF2(110) substrate by pulsed laser deposition. Since MgF2has a high optical transmittance in a wide range from UV to near-infrared, it is an important optical material and widely used as window materials in optical elements. During growth, the substrate temperature was maintained at500℃under controlled oxygen partial pressure. Then, photolithography was used to prepare interdigitated electrode. UV photodetector with Ni/TiO2/Ni structure was designed and fabricated to test the UV photoresponse performance. The maximum of the response is4.85A/W under5V bias, which occurred at300nm. As the wavelength became longer than370nm, the device showed no response. As the wavelength became shorter than260nm, the photocurrent descended gradually.Recently, photoelectrochemical cell-based UV photodetectors made by nano-structured TiO2have shown promising potential due to its low-cost, simple fabrication process and fast response. One-dimensional well-aligned nanostrctured semiconductors such as nanorods, nanowires and nanotubes have attracted extensive research interest due to their high surface-to-volume ratio and good electron conductivity. In our work, rutile TiO2nanostructures were grown directly on fluorine-doped tin oxide (FTO) glass by a low temperature hydrothermal method. The crystal structure, surface morphology and optical transmittance of the TiO2nanorod arrays were characterized. Then, self-powered UV detectors based on TiO2nanorod arrays were fabricated and studied. High quality deionized water was injected intothe device as an electrolyte instead of I-/I3-redox couples electrolyte. Water may be the most safe, stable and environmentally friendly electrolyte. Excellent UV-light detection selectivity in a spectral range between310and420nm is observed, which indicates that the device can be used as photodetector for UV-A range(320～400nm) application. Under the illumination of1.25mW/cm2UV light (λ=365nm), the UV detector shows an excellent photovoltaic performance, yielding a short-circuit current of4.67μA and an open-circuit voltage of0.408V. Under the same illumination, the rise time and the decay time of the UV detector are approximately0.15and0.05s.In order to overcome the drawback of a low specific surface area of TiO2thin film and bare nanorod arrays, TiO2nano-branched arrays with a larger surface roughness and surface area were used as an active photoanode for efficient photodetectors. The highest peak responsivityof TiO2nano-branched arrays is approximately0.22A/W at352nm. The corresponding IPCE is higher than77%, which is much higher than TiO2nanorod arrays detectors and commercial silicon detectors at this wavelength. The Jsc and Voc are obviously increased for the TiO2nano-branched arrays based UV photodetectors. The TiO2nano-branched arrays outperform the bare TiO2nanorod arrays film mainly for three reasons:(1) The specific surface area and roughness of TiO2nano-branched arrays are markedly enlarged, leading to enlarged TiO2/electrolyte contact area and flourishing electron-hole pairs;(2) The good electron conductivity is remained in the vertical nanorod trunk, which enables fast transport of photo-generated electrons from the TiO2nanobranches to the collecting FTO substrates through the single-crystalline TiO2nanorods; and (3) The branches can fill the gaps between the nanorods and improve the light harvesting efficiency.A new type of quasi-solid-state UV photodetector was fabricated using liquid crystal (LC)-embedded electrolyte with light-trapping scheme. The photodetecting performance of the device improved significantly, which overcomes leakage of organic solvents and bad stability. Excellent UV-light detection selectivity in a spectral range between310and420nm is observed, which indicates that the device is very suitable for UV-A range application. The maximum responsivity of the spectrumis about0.09A/W, located at the wavelength of383nm. Exposed to365nm UV-light on/off switching irradiation with light intensities of2.5mW/cm2, the photocurrent was observed to be excellent reproducible with fast response (both decay and rise times are less than0.3s).P-type organic semiconductorspiro-MeOTAD was also used as the light absorber and hole transport layer. Solid-state photodetector based on an inorganic/organic heterojunction of TiO2nanorod/spiro-MeOTAD is constructed. Excellent detection selectivity in a spectral range between400and420nm with a full width at half maximum (FWHM) of only26nm is observed, indicating that the device is very suitable for blue range application. The TNAs/spiro-MeOTAD heteroj unction shows reproducible photoresponse with fast respons. Exposed to410nm UV-light on/off switching irradiation with light intensities of75μW/cm2,the rise time and the decay time of the detector are approximately0.12and0.06s respectively.更多还原