Optoelectronic Properties and Applications of 3-D Hybrid a-Si:H/ZnO Nanowire Structures

Abstract

This Ph.D. dissertation presents the study and development of optoelectronic properties of 3-D hybrid zinc oxide nanowire (ZnO NW) and hydrogenated amorphous silicon (a-Si:H) core-shell structures and a demonstration of their application in infrared photodiodes. In addition, the demonstration of infrared photoconductors using the 3-D a-Si:H/ZnO NW structure, which are then integrated onto conventional back channel etched (BCE) a-Si:H thin film transistors (TFTs) for potential large-area sensor applications is also presented.

A hybrid 3-D core-shell structure formed using ZnO NW cores and conformally coated a-Si:H thin film shell was found to be infrared sensitive for wavelengths up to 2.5 µm wavelength. Scattering and multiple reflection enabled by the 3-D morphology were found to enhance the effective thickness of the a-Si:H shell by ~3 orders of magnitude. With the enhanced effective thickness, defects within the a-Si:H material were associated with enabling infrared absorption, achieving up to 73% infrared absorption at 2.3 µm infrared wavelength for 500 nm coated a-Si:H film on 7 µm long ZnO NWs having a nanowire density of 4.3 × 107 NW/cm2. Comprehensive materials and device characteristics were studied to show a defect mediated infrared absorption process in the 3-D a-Si:H/ZnO NW material system.

3-D infrared photoconductors were fabricated afterwards at a temperature of ≤ 150°C using the infrared sensitive 3-D a-Si:H/ZnO NW hybrid material system. An intentional ‘NW gap’ was created between the edge of the NW array and the contacts of the infrared photoconductors to minimize parasitic conduction from conductive and connected NWs thereby reducing the dark current of the 3-D photoconductor. An ON/OFF ratio of 3.2 × 102 was achieved for 1 µm thick a-Si:H shell coating on 2.7 µm long ZnO NWs with a nanowire density of 3.9 × 108 NW/cm2 using 1.55 µm LED illumination.

As an alternative infrared photodetector, 3-D infrared photodiodes were also fabricated using similar process conditions. Dark current as low as 1.6 × 10-9 A/cm2 was achieved for a diode with NW length vs a-Si:H thickness ratio of 1.5× and NW density of 6.1 × 107 NW/cm2 giving an infrared signal to noise ratio of 2.5 × 102 with 1.55 µm LED irradiation. The factors that influence the dark currents were studied and several optimizations were implemented. The top contact was optimized by replacing aluminum doped zinc oxide (AZO) top conducting oxide (TCO) with thinned-down, conductive gallium indium zinc oxide (GIZO) and p+ doped a-Si:H to minimize the window layer absorption and enhance the its infrared transmission.

3-D infrared photoconductors were also integrated onto a-Si:H BCE TFT at a process temperature ≤ 150°C. The process development and the effects of both the structure and the integration process flow were evaluated. A 3× signal to noise ratio due to infrared irradiation using a heat lamp was obtained for the integrated device with a photoconductor section that contains only 20% 3-D a-Si:H/ZnO NW structure.