< Back to previous page

Project

Energy landscaping of thin-film image sensor devices

Title: Energy landscaping of thin-film image sensor devices Supervisor: Paul Heremans Brief description of the proposed dissertation research: The purpose of this PhD is to obtain full description of the electronic band structure (including band alignment) of the thin-film layer stacks for photodiodes. These thin-film materials are deposited from either solution process (polymers, metal-oxides, or quantum dots) or by evaporation (organic small molecules, metal-oxides). The energy band alignment of a full photodiode stack consists of a minimum of 5 different materials has strong influence on the key performance metrics. Therefore, the primary goals of this PhD are to determine the energy structures of the utilized thin-film semiconductors and the interfacial effects between layers. A collaboration between the material characterization and thin-film fabrication groups will ensure the in-depth characterization of thin-films using photoemission spectroscopy (X-ray and ultraviolet photoemission spectroscopy (XPS & UPS)) and thin-film photodiode fabrication, respectively. This study will bring an in-depth understanding of thin-film photodiode energy structures and their impact on the performance of photodiode, which will ease the optimization of photodiode for high-quality imagers. Detailed proposal: Having the advantages of being less scattered by the particles smaller than wavelength, infrared (IR) imaging made its way to the primary choice for applications such as surveillance, missile guiding systems, machine vision, industrial inspection, satellite imaging, and spectroscopy [1–3]. In recent years, biometric sensors for facial recognition and light detection and ranging systems (LiDAR) for depth sensing based on IR imaging have become standard features for consumer smartphones. At the same time, new applications like augmented and virtual reality (AR-VR), autonomous cars will continue to emerge under the IR imaging spotlight [3–6]. Silicon has been the ultimate choice of material for the state-of-the-art visible and near-infrared (300 – 1000 nm) imagers. Beyond silicon spectral range epitaxially grown semiconductors like InGaAs and HgCdTe are good absorbers of IR light, however the integration to CMOS read-out IC with flip-chip architecture is detrimental to achieve high-resolution imagers [2, 6]. This is where thin-film solution processed semiconducting materials like organic small molecules and polymers, quantum-dots (QD), and perovskites come to play with clear advantages like monolithic CMOS integration, spectral tunability, large-area room temperature fabrication, processibility on flexible substrates [2, 7]. Undoubtedly, the photodiode is the most important part of an imager as it is responsible for the detection of light, thus understanding the photodiode characteristics well is one of the primary steps to realize high-quality imager. Unlike silicon and InGaAs based photodiodes where the device characteristics are well understood and materials are extensively studied [8-9], the thin-film photodiodes (TFPD) are relatively unexplored in terms of device characteristics and material perspectives. A thin-film photodiode generally consists of a minimum 5 different materials where the interfacial effects and energy band alignments are particularly important to understand the device characteristics as the transport properties depend on the conduction and valence band discontinuities that accommodate the difference in bandgap between the materials, better known as, the conduction and valence band offsets [10]. Thus, tuning the energy offsets between layers can have advantageous or detrimental impacts on the device performance metrics which need to be comprehensively studied to improve the TFPD performance. Therefore, the primary aim of this PhD will be to investigate the energy structure of the used soft semiconductors and establish a correlation between performance metrics and energetics aiming to optimize the TFPD. To determine the energetics of the TFPD materials and devices, the underlying measurement framework for reliable and reproducible energetics measurement using state-of-the-art photoemission techniques like XPS and UPS for these thin-film semiconductors is a challenging task. These techniques are highly surface sensitive and together with the thin-films’ sensitivity to the ambient conditions making the ex-situ measurement strongly dependent on appropriate surface cleaning before collecting any meaningful data [11]. Thus, developing proper measurement frameworks for the utilized thin-film semiconductors is one of my first goals as a PhD student. Contrarily, anticipating device characteristics based on energetics of thin-film stacks may not be sufficient to understand all the performance metrics of a TFPD considering the inherent energy disorder of soft semiconductors like organics [12], however such a full energy structure of photodiode stack will already be a great achievement in terms of understanding material and interface from a fundamental device physical perspective. Since QD and organics show remarkable variations in material properties based on the type of ligand and chemical groups attached respectively, a selection of appropriate TFPD materials and further optimization of devices are the big challenges to overcome in coming years [14]. Selecting materials based on energetics will ideally allow me to choose the appropriate materials in the first place and afterwards device optimization can be eased with manipulating the energy band alignment. In my predoctoral research project at KU Leuven and imec, I’ve already started with the energetics measurement of thin films using the photoemission techniques XPS and UPS where the energies of valence band minima (VBM)/highest occupied molecular orbital (HOMO) for inorganic and organic materials respectively can be extracted from the photoemission spectra. Further analysis of VBM spectra, Fermi edge and bandgap of the materials provides the opportunity to also extract conduction band maxima (CBM), ionization energy, work function, electron affinity to determine the complete energy structure of a thin-film material. During my predoctoral period, I stared with an organic photodiode (OPD) stack where I determined a complete energy structure of the stack to develop the correlation between the energetics and the key device performance metrics like dark current, external quantum efficiency. The plan for the first year PhD is to compare the energy structures of different OPD stacks to deduce a correlation between performance metrics and energetics. In the following years, I’ll analyze the energetics of quantum dots depending upon different ligand exchange and process flow to optimize the quantum dot photodiode (QDPD). Properly functioning IR imager based on QDPD is already realized with monolithic CMOS read-out IC integration, however, further improve the QDPD photodiode is required for better quality and reliable imagers [3]. With very little information on the QD energetics available in the literature, this is quite challenging since the first step to improve the QDPD performance is to understand the characteristics of QD thin-films and their interfaces. In addition, optical and electrical properties of quantum dots can be very different depending upon the size and ligand which arises further ambiguity to choose proper materials and respective optimization schemes in achieving better performing photodiodes. Energy landscaping of quantum dots can provide fundamental understanding of electronic and optical transport parameters to understand the correlation between the device performance and electronic band structure. Such a study will add a new dimension to thin-film quantum dot knowledge of energetics and will ease the QDPD optimization process. References: [1] Rieger C. J. and Frank G Carpenter 1959 Light Scattering by Commercial Sugar Solutions Journal of Research of National Bureau of Standards-A. Physics and Chemistry 63A [2] P. E. Malinowski et al., 'Thin-film quantum dot photodiode for monolithic infrared image sensors', Sensors, vol. 17, no. 12, pp. 2867, 2017. [3] E. Georgitzikis , Doctoral dissertation, Infrared Sensitive Thin-Film Photodetectors for Integration on Top of CMOS, KU Leuven [4] R. S. Ghiass, O. Arandjelović, A. Bendada and X. Maldague, 'Infrared face recognition: A comprehensive review of methodologies and databases', Pattern Recognit., vol. 47, no. 9, pp. 2807-2824, Sep. 2014 [5] Fei Liu and Stefan Seipel, 'Infrared-visible image registration for augmented reality-based thermographic building diagnostics', Visualization in Engineering, vol. 3, no. 1, pp. 16, 2015 [6] Livache, C., Martinez, B., Goubet, N. et al. A colloidal quantum dot infrared photodetector and its use for intraband detection. Nat Commun 10, 2125 (2019) [7] P. E. Malinowski et al., “Miniaturization of NIR/SWIR image sensors enabled by thin-film photodiode monolithic integration,” in Optical Architectures for Displays and Sensing in Augmented, Virtual, and Mixed Reality (AR, VR, MR) II, Mar. 2021, p. 31. [8] K. Yamada et al., “High-performance silicon photonics technology for telecommunications applications,” Science and Technology of Advanced Materials, vol. 15, no. 2, p. 024603, Apr. 2014, doi: 10.1088/1468-6996/15/2/024603. [9] D. Yin, T. He, Q. Han, Q. Lü, Y. Zhang, and X. Yang, “High-responsivity 40 Gbit/s InGaAs/InP PIN photodetectors integrated on silicon-on-insulator waveguide circuits,” J. Semicond., vol. 37, no. 11, p. 114006, Nov. 2016, doi: 10.1088/1674-4926/37/11/114006. [10] M. Peressi, N. Binggeli, and A. Baldereschi, “Band engineering at interfaces: theory and numerical experiments,” J. Phys. D: Appl. Phys., vol. 31, no. 11, pp. 1273–1299, Jun. 1998. [11] S. Olthof, “The Impact of UV Photoelectron Spectroscopy on the Field of Organic Optoelectronics—A Retrospective,” Adv. Optical Mater., vol. 9, no. 14, p. 2100227, Jul. 2021. [12] P. Li, G. Ingram, J.-J. Lee, Y. Zhao, and Z.-H. Lu, “Energy disorder and energy level alignment between host and dopant in organic semiconductors,” Commun Phys, vol. 2, no. 1, p. 2, Dec. 2019. [13] D. M. Kroupa et al., “Tuning colloidal quantum dot band edge positions through solution-phase surface chemistry modification,” Nat Commun, vol. 8, no. 1, p. 15257, Aug. 2017.

Date:15 Oct 2021 →  Today
Keywords:Photodiode, Thin-film, Quantum dots, Organics, Thin-film imagers, Material characterization, X-ray photoelectron spectroscopy, Ultraviolet photoelectron spectroscopy, Energy band alignment
Disciplines:Semiconductor devices, nanoelectronics and technology, Photodetectors, optical sensors and solar cells, Nanoscale characterisation, Spectroscopic methods
Project type:PhD project