Thin film electronics and optoelectronics based on layered and traditional materials

Date of Award

Fall 10-1-2021

Document Type


Degree Name

Doctor of Philosophy (PhD)


Electrical Engineering (ENAS)

First Advisor

Xia, Fengnian


Over the past few decades, transistors and integrate circuits experienced almost constant rapid development following Moore’s law, where the number of transistors in dense integrated circuits doubles in around every two years was predicted. The performance of integrated circuits has therefore experienced orders of magnitude of increase and shaped many aspects of our modern life. This rapid development is driven by the scaling of planar CMOS technology, which is also facing increasing challenges as the transistors size decreases. The classic Dennard scaling eventually ended at 130nm technology. Innovative transistor designs are introduced to enable further scaling of the transistors, by incorporating new structures and materials to the transistor design. Due to the advantages of low leakage current and parasitic capacitance, ultra-thin body devices based on thin film semiconductor became a promising candidate for further scaling. After the discovery of graphene, which is a material formed by a single layer of carbon atoms, a whole group of 2-dimensional (2D) material consists of single or few layer atoms were explored. Due to their 2D nature and layers are bonded by Van der Waals force, 2D materials showed promising properties like reduced dangling bonds, ultra clean interfaces, etc. Besides these common properties, each 2D material also has its unique properties. In this thesis, we explored the synthesis, device application and physical properties of 2D materials. We also explored device applications of traditional thin film material such as Si film, since the more mature processing based on traditional material allows for fabrication of more complex MEMS structures. We first explored some emerging layered materials. Known as a layered material and most stable most stable allotrope of phosphorus, black phosphorus (BP)was rediscovered as a layered semiconductor in 2014. BP has high hole mobility and a sizable bandgap, widely tunable bandgap by number of layers or external electric field. This makes it an interesting candidate for electronic and optoelectronic devices. However, previously high quality few-layer and thin-film BP are produced primarily by mechanical exfoliation, limiting their potential in future applications. We show the synthesis of highly crystalline thin-film BP on 5-milimeter sapphire substrates by conversion from red to black phosphorus at 700 degrees Celsius and 1.5 Gigapascals. The synthesized ~50-nm thick BP thin films are poly-crystalline with a crystal domain size ranging from 40 to 70 m long, as indicated by Raman mapping and infrared extinction spectroscopy. At room temperature, field-effect mobility of the synthesized BP thin film is found to be around 160 cm2V-1s-1 along armchair direction, and reaches up to about 200 cm2V-1s-1 at around 90 K. Moreover, red phosphorus (RP) covered by exfoliated hexagonal boron nitride (hBN) before conversion shows atomically sharp hBN/BP interface and perfectly layered BP after the conversion. Our demonstration represents a critical step towards the future realization of large scale, high quality BP devices and circuits. BP’s electronic properties made it a promising two-dimensional material for high-frequency electronic devices. Further, for metal-oxide-semiconductor field-effect transistors (MOSFETs) operating at high frequencies, they must have a top gate of submicron length instead of the commonly used global back gate. However, without the global back gate to electrostatically induce doping in BP, top-gated submicron BP MOSFETs have not reached its full potential mainly due to large contact resistances. Here we further explored BP’s applications in electronic devices. We show top-gated submicron BP MOSFETs with local contact bias electrodes to induce doping in the contact region. This resulted in reduced contact resistance and, in turn, improvement in current capacity and peak transconductance, if compared with top-gated BP transistors without any back-gating scheme. In turn, these improvements resulted in a forward current-gain cutoff frequency of 37 GHz and a maximum frequency of oscillation of 22 GHz at room temperature, the highest reported for BP MOSFETs up to date. Our BP RF transistors are based on hBN/BP/hBN highly heterostructures, and the device are often operating at high lateral electrical field. Our fabrication and testing of high quality 2D heterostructures based on hBN driven by high bias voltage showed electron-phonon coupling at work in saturation velocity. We further studied this process in the heterostructure of hBN/graphene/hBN, as this heterostructure offers high mobility in graphene and high saturation velocity limited by hBN optical phonon scattering. Further, as hBN is a hyperbolic polar dielectric, the coupling between graphene electrons and hBN hyperbolic phonon opens a cooling pathway where hot carrier cooling rate is expected to be much higher than conventional carrier cooling. Although studies suggest that excitation of hBN hyperbolic phonon can exist, direct optical measurement of such effect has yet to be achieved. Here we showed the evidence for electrically generate hyperbolic phonons based on hBN/graphene/hBN heterostructure by driving carriers into velocity saturation. On the other hand, thin film devices intrinsically have small volume, leading to reduced heat capacity and thermal time constant. Therefore, thin film devices are also suitable for temperature sensing since a short response time can be expected. Here we showed high speed bolometers based on MSM structure fabricated on Si thin film in SOI. Microbolometers are a type of uncooled sensor for electromagnetic radiation in mid-infrared and terahertz range, and they have become one of the dominating technologies for uncooled IR imaging. Due to the high demand, high performance bolometers with high sensitivity have been demonstrated. However, commercial devices still have room for improvement, such as TCR, difficulty for scaling down, relatively long thermal time constant, etc. Our demonstrated bolometers based on SOI MSM structures addressed these issues and showed high sensitivity, small pitch, and very low thermal time constant. In summary, this thesis focused on the demonstration of electronic and optoelectronic devices based on emerging layered and traditional thin film materials. The large-scale synthesis, device applications which leverage fundamental properties, and their physical properties have been explored. We demonstrated millimeter wave operation of BP RF devices, and bolometers with low response time. These demonstrations highlight the unique properties of thin film materials and shows the promising applications for future generation of electronic and optoelectronic devices.

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