OPTOELECTRONIC PROPERTIES OF PULSED LASER DEPOSITED TiO2 & Nb:TiO2 THIN FILMS ON Si SUBSTRATE

Abstract

From last few decades, transition metal oxides and their alloys have been drawn great attention due to their versatile and remarkable structural, optical and electronic properties. Depending on their crystal structure and most importantly, the bonding between the metal cation and the oxygen atoms of the transition metal oxides exhibit a wide range of device applications including optoelectronics. Among all the oxide-based semiconductors, Titanium dioxide (TiO2) and Nb-doped TiO2 (Nb:TiO2) has emerged as a promising n-type wide band gap (~3.2 eV for anatase TiO2) semiconductor for optoelectronic applications due to its good photo-stability and high photo-conversion efficiency. Apart from these, TiO2 and Nb:TiO2 are non-toxic, abundant on earth, cost effective, and manufacturing technology of TiO2 is well established in the industries. Owing to the wide band gap ~3.2 eV, their applications are restricted only to the ultraviolet (UV) region. However, its application can be extended to UV-Visible region of the solar spectrum by forming a heterojunction with p-type narrow band gap semiconductors like Si. In this kind of heterojunction, TiO2 or Nb:TiO2 absorbs the photons of the UV wavelength region while Si absorbs those of the visible region. Thus, this type of heterostructure has a great potential to work as a solar cell and UV-Visible photodetector. In the conventional Si-based photodetectors/solar cells, the reflection from the illuminating surface must be minimum. Hence, to suppress the reflection of solar irradiance, texturing of the Si surface is preferentially implemented. More improvement in the antireflection property of the device could be achieved by conformal deposition of another layer with a wide band gap material like TiO2 and Nb:TiO2. Moreover, a heterojunction of TiO2 or Nb:TiO2 with textured p-type Si not only creates solar-blind structure but also photo-generates a large number of charge carriers due to the deep penetration of photons inside the device. Indeed, this kind of heterostructure is found suitable for UV-Visible photodetector, antireflection coating, and solar cells applications, which has a broader range of commercial and military applications. In this thesis the optoelectronic properties of anatase TiO2 and Nb:TiO2 thin film grown on two kind of p-Si substrates (p-Si) viz. pristine p-Si and pyramidally textured p-Si is presented systematically. The fabrication of semiconducting thin films of TiO2 and Nb:TiO2 on glass substrates are successfully demonstrated by pulsed laser deposition (PLD) technique. The Abstract ii structural, morphological, optical and electrical properties of the grown thin films are investigated and correlated comprehensively. The electrical properties of TiO2 are improved significantly by extrinsic doping with Nb, which substitute into the host Ti sites and provide an extra electron to the conduction band of the TiO2 thin film. After tuning the deposition parameters such as doping concentration and oxygen ambient pressure during growth, the lowest resistivity of 1.1×10-2 Ω-cm of the Nb:TiO2 thin film is achieved with moderate optical transmittance over 55% in visible and near-infrared (NIR) region. Further, anatase TiO2 thin film of different thicknesses (39-201 nm) is grown on pristine p- Si substrates to explore the influence of the TiO2 film thickness on the antireflection, solar cell efficiency and photodetection properties of the n-TiO2/p-Si heterojunction structure. It has been shown that the thickness of the TiO2 layer plays a crucial role on the photovoltaic performance of the n-TiO2/p-Si heterostructure. The photocurrent of the n-TiO2/p-Si heterostructure is found to decrease with an increment of the thickness of the TiO2 thin film. This is attributed to the increment of sheet resistance with increase of TiO2 film thickness. It is observed that the optical reflectivity of TiO2 thin film of the particular optimized thickness (55 nm) deposited on p-Si could be lowest. TiO2(55 nm)/p-Si heterostructure exhibits the minimum reflectivity ~4.67% at the wavelength of 576 nm. With the inclusion of Ag nanoislands layer sandwiched between Si and TiO2, the inflection point of minima, as well as the antireflection region, consistently gets redshifted from 576 nm to 732 nm due to the increment of average Ag metal nano-particle size from 15.9 nm to 32.5 nm, respectively. Results are explained in the light of localized surface plasmon resonance effects of Ag nanoislands in the multilayer structure. To improve the photoresponse of the optimized n-TiO2 (55 nm)/p-Si heterojunction diode, TiO2 layer (55 nm) is deposited on chemically etched pyramidal textured p-Si substrate. After tuning of chemical etching parameters, the minimum total reflectance of ~7.6% is achieved for textured-Si substrate in the wavelength range of 300-900 nm, which further drops down to 6.5% when the TiO2 layer is deposited on the top surface of the textured Si substrate. TiO2(55 nm)/Textured-Si heterostructure exhibits a better response to solar irradiance as well as to the UV light with responsivity 1.86 times and 2.0 times higher, respectively, at -2V bias. Abstract iii The responsivity values of the best photodiode, i.e., TiO2 (55 nm)/Textured-Si under the illumination of both UV and Solar irradiance is found to be ~0.22 A/W and ~0.13 A/W at -2V bias, respectively. One of the reasons for lower photoresponse is identified as the high resistivity of TiO2. Hence, to improve the conductivity, Nb doping is introduced in the TiO2 thin film and deposited on p-Si substrates. Consequently, the UV-Visible photoresponse and the electrical properties of the n-Nb:TiO2/p-Si heterojunctions is investigated. A comparative study on the UVVisible light detection properties of the optimized Nb:TiO2 thin films for two different concentrations of Nb doping (3.1 and 4.2 at.%) with the undoped TiO2 is explored. Under the illuminated condition, n-Nb:TiO2/p-Si heterojunction having higher Nb doping concentration (4.2 at.%) in Nb:TiO2 film exhibits better photoresponse. Further, the role of the film thickness of Nb:TiO2 (120 nm, 198 nm and 322 nm) on UV-Visible light-driven photoresponse and electrical properties of n-Nb:TiO2/p-Si heterojunctions is investigated systematically on two types of substrates, viz. pristine and pyramidal textured p-Si. The n-Nb:TiO2/p-Si heterojunction diode based on textured-Si substrates exhibits better photo-responsivity to solar irradiance as well as to the UV light. The best responsivity is achieved for the Nb:TiO2 (198 nm)/Textured-Si heterojunction photodiode with responsivity value (at -3V bias) 1.22 A/W and 1.85 A/W under solar simulated light and UV light, respectively. Moreover, both the heterojunction photodiodes exhibit fast photoresponse to UV as well as solar simulated light (rise time ~28 ms and fall time ~30 ms). These results are explained in terms of change in their optical and electrical properties. The present finding will be certainly important for fabricating high-speed optoelectronic devices based on reverse-biased n-Nb:TiO2/p-Si heterojunctions.