Date of Graduation

7-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Microelectronics-Photonics

Advisor

Omar Manasreh

Committee Member

Colin Heyes

Second Committee Member

Zhong Chen

Third Committee Member

Morgan E. Ware

Fourth Committee Member

Rick Wise

Keywords

Applied physics, Colloidal quantum dots, Graphene, Light sensors, Nanomaterial, Photodetectors, Photonics

Abstract

Photodetectors are devices that capture light signals and convert them into electrical signals. High performance photodetectors are in demand in a variety of applications, such as optical communication, security, and environmental monitoring. Among many appealing nanomaterials for novel photodetection devices, graphene and semiconductor colloidal nanocrystals are promising candidates because of their desirable and unique properties compared to conventional materials.

Photodetector devices based on different types of nanostructured materials including graphene and colloidal nanocrystals were investigated. First, graphene layers were mechanically exfoliated and characterized for device fabrication. Self-powered few layers graphene phototransistors were studied. At zero drain voltage bias and room temperature, the devices operated and achieved high responsivity and detectivity on the order of 2.7 × 104 A/W and 5 × 1012 cm·Hz0.5/W, respectively. A ratio of 29 between the photocurrent and the dark current was obtained. The device showed an excellent tunable photo response as a function of the applied back-gate voltage. Second, colloidal PbSe nanocrystals or quantum dots were synthesized via a hot injection method and activated by ligand exchange process to be applied in near infrared photoconductors. Interdigital electrode photodetectors based on PbSe nanocrystals on glass substrate were investigated to examine the functionality of the synthesized nanocrystals. The devices showed a perfect response for the visible to near-infrared region. Then, the PbSe nanocrystals and the mechanically exfoliated graphene layers were combined to build near-infrared photodetectors based on hybrid graphene-colloidal PbSe quantum dots. The device showed a remarkable responsivity of 1265 A/W and detectivity of 3.4 ×1010 cm·Hz0.5/W. Single layer graphene was grown via chemical vapor deposition (CVD) and transferred to a light p-type Si/SiO2 substrates. Single layer CVD-grown graphene-based phototransistor was fabricated and studied. Photogating effect was found to be established via the Si/SiO2 interface and the light doping Si in the substrate. Due to the photogating effect, the conductivity of the single layer graphene in the channel was enhanced, and the device achieved high responsivity and detectivity at very low or zero drain bias voltages and negative back gate voltages. For device optimization, devices with different channel length were fabricated and characterized. Responsivities of 9.6 × 102 A/W and 2.6 × 103 A/W were recorded for devices with 15 µm and 5 µm channel lengths, respectively.

Finally, colloidal CdSe nanocrystals and silver nanoparticles were grown using chemical approaches, and the CdSe nanocrystals were functionalized by changing their ligand. Colloidal quantum dot photodetectors enhanced by plasmonic silver nanoparticles were fabricated and examined. The device consisted of interdigital gold electrodes on a glass substrate. The silver nanocrystals were deposited underneath the CdSe nanocrystals layer in the device channel. The device performance was compared with a CdSe nanocrystals based control device, and an enhancement in the device photo response was obtained.

Available for download on Saturday, September 10, 2022

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