TiO2-based Ultraviolet Photodetectors by Ultrasonic Spray Pyrolysis Technique

Abstract

The research mainly investigates on the TiO2-based ultraviolet (UV) photodetectors (PDs) by using ultrasonic spray pyrolysis technique Among TiO2 thin film deposition methods ultrasonic spray pyrolysis deposition (USPD) which is a non-vacuum and low cost approach is used to fabricate the titanium dioxide active layer Furthermore this technique is applied to two different structured UV PDs including metal-semiconductor-metal (MSM) and metal-insulator-semiconductor-insulator-metal (MISIM) structures In order to know crystal phases crystal types surface roughness grain sizes chemical composition oxygen vacancies refractive index and thin film thickness of the oxide layer the (1) X-ray diffraction (XRD) (2) Microscopes Raman spectrometer (3) Atomic Force Microscopy (AFM) (4) Scanning electron microscope (SEM) (5) Electron Spectroscopy for Chemical Analysis (ESCA) (6) Photoluminescence spectrometer (PL) (7) Ellipsometry (8) Transmission electron microscopy (TEM) and (9) Deep-level transient spectroscopy are adopted in this research After the material analysis of the TiO2 film the USPD technique is used to grow the TiO2 active layer of the MSM UV PDs and the Al2O3 layer for the passivation and insulator of the MISIM UV PDs and the PECVD is applied to grow the SiO2 for insulator of the MISIM UV PDs To investigate the characteristics of the different finger spacing widths in the MSM UV PDs four masks of different finger spacing widths are designed From the I-V characteristics of the MSM UV PDs it was found that the smaller the finger spacing width is the larger the current is This phenomenon is attributed to the stronger electric field when the finger spacing width gets smaller To find out the optimized annealing temperature the TiO2 with 400oC and 600oC annealing are used to be active layers for the MSM UV PDs The results show that the TiO2 MSM UV PD with 600oC annealing has better performance because the higher annealing temperature repaired the surface defects Among the material analyses and electrical characteristics the 600oC annealing temperature is the better for fabricating TiO2 MSM UV PDs in this thesis To enhance the performance of the MSM UV PDs the Al2O3 film on the MSM UV PD as a passivation layer The dark current of the passivated MSM UV PD is suppressed 10 times lower than that of the MSM UV PD and this increases the photo-to-dark current ratio the UV-to-visible rejection ratio and decreases the noise current Among these electrical analyses the Al2O3 passivated MSM UV PD effectively improve the performances Adding an insulator layer between the metal electrode and active layer in MSM structure is MISIM structure 20 nm thickness of the Al2O3 and SiO2 grown by USPD&PECVD are used as the insulator layers in the MISIM structures The photo-to-dark current ratio the UV-to-visible rejection ratio and the detectivity of the Al2O3 MISIM UV PD are better than those of the SiO2 MISIM UV PD However the dark current and the dark current-to temperature sensitivity of the SiO2 MISIM UV PD are better than those of the Al2O3 MISIM UV PD Among all the Al2O3 MISIM UV PD is more suitable for UV PDs In summaries the Al2O3 MISIM and the Al2O3 passivated MSM fabricated by the USPD technique improve the performance of the MSM UV PDs The USPD technique makes good quality of titanium dioxide in a cost-effective way It’s very promising technique to be used in the future industry

Date of Award	2015 Jul 27
Original language	English
Supervisor	Wei-Chou Hsu (Supervisor)