SnO2 is a prototype material for chemical gas sensing. The analysis clearly showed that best I-V responses were obtained when one of the barrier height is 0 eV, which is consistent with the observed experimental results. Theoretical analysis using the Simmons Simmons, JAP, 1963 models were carried out to identify the best barrier parameters to achieve desired I-V response of a MIM structure. As per the theory, if one of the work function of the metals and the electron affinity of the insulator are close (i.e., the barrier height at the interface is close to zero), it results in a device with high asymmetry and if a suitable insulator thickness is chosen yields a very low turn-on voltage. Using a trapezoidal barrier model and the experimental results, an empirical theory relating the work function values of the metal and the electron affinity of the insulator layer is proposed. The other MIM structures based on Al 2O 3 or Y 2O 3 or HfO 2 exhibited linear or symmetric I-V curves and hence are not suitable for rectification. Among the devices tested, MIM systems based on Nb-Nb 2O 5-Pt and Nb-TiO 2-Pt gave excellent diode characteristics such as low turn-on voltage (0.02 V), high asymmetry value (defined as |I forward/I reverse|) of 4 to 10 at a bias voltage of 0.5 V and high non-linearity. I-V response of the devices were measured using a custom built I-V test stand assembled for point-contact configuration. The ultra-thin film oxides were characterized using x-ray reflectivity, atomic force microscopy and x-ray photoelectron spectroscopy. Al 2O 3, Y 2O 3 and HfO 2 were deposited via atomic layer deposition. Nb 2O 5 and TiO 2 were deposited via anodization and pulsed layer deposition, respectively. ![]() TiO 2, Nb 2O 5, Al 2O 3, Y 2O 3 and HfO 2 are the insulating oxides investigated in this study to fabricate MIM structures in point-contact configuration. In this work, MIM diodes with different insulator layers were fabricated and characterized with the focus on rectification characteristics at low frequencty. As tunneling probability decays exponentially with insulator thickness, an ultra-thin (1-5 nm) high quality (pin-hole free) insulator layer is required to achieve tunneling in MIM diodes. Tunneling is an ultra-fast process and hence allows ∼THz frequency operation. Metal-Insulator-Metal (MIM) diodes are actively considered for high frequency (THz) rectification when operated in the fast tunneling transport regime. Thus, we succeeded in the synthesis of transparent and metallic Nb 12O 29 films on glass substrates. In addition, this film showed relatively high transparency with the transmittance of ~70 % in the visible. We employed synchrotron-radiation photoemission spectroscopy (SR-PES) to gain direct evidence for the metallic nature of Nb 12O 29, consequently revealing the metallic Fermi edge. The HRTEM images strongly support that this film is composed of a Nb 12O 29 based on their rectangle block structures and the corresponding diffraction patterns. Structural properties characterized by grazing incidence x-ray diffraction (GIXRD) and high-resolution transmittance electron microscope (HRTEM) indicate that these films can be assigned to be Nb 12O 29. Sputtering of Nb metal under a critical oxygen flow ratio of 6.5%, followed by annealing at 1000☌ in vacuum, resulted in the transparent and conductive thin film. ![]() Highly-conductive Nb 12O 29 thin films were fabricated on glass substrates using dc magnetron sputtering. Further investigations of new class of TCOs on glass substrates have been more and more required in light of cost issues and improvement of various optoelectronic devices. In the last decade, the industrial demands for transparent conductive oxides (TCOs) have been growing to provide a wide range of novel devices, flat panel displays, light-emitting devices, and solar cells.
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