Structured 2D Electron Gas and MXene-Based Optoelectronics

As the branch of technology concerned with the combined use of electronics and light, optoelectronics affects many aspects of modern life, including enabling long distance communication and data transfer through data centers around the globe. Specifically, high speed and broad bandwidth are crucial for data centers that support applications such as artificial intelligence, autonomous vehicles, Internet of Things, and other Internet demands. Photonic integrated circuits (PICs) are the technology that is crucial in meeting these demands. Furthermore, in many of these optoelectronic and photonic circuits, high responsivity light detection devices are required, resulting in the need for new materials that can interact with light more efficiently when implemented in such devices. Two dimensional (2D) materials can meet these needs, being candidates for substitution of conventional materials in platforms such as photonic integrated circuits, and in many applications, such as sensing, mapping, detection, and light manipulation. Herein, we investigate the use of two types of structured 2D materials in various devices. We study the propagation of plasma waves in low dimensional electron systems forming at the interface of two dissimilar semiconductors, for electrical detection of an optical signal. First, we analyze the effects of structured 2D electron gas in a planar structure for detection of THz excitation. We analyze and construct a channel consisting of a two-dimensional electron gas meta-surface for coupling the excitation photons to the free carriers at the meta-surface. This structure allows theelectrical measurement of plasma wave propagation across the channel at room temperature at a speed much faster than the drift of electrons in the channel of a conventional field effect transistor (FET), while eliminating the need for external photodetectors by producing a voltage difference across the meta-structure. These devices have the potential to be used as room-temperature (RT) detectors of THz radiation, since the resonant frequency of the plasmons in the channel lies within this range. We also dig deep into the physics of this behavior, by the analogy of this structure with shallow water-based systems and solving the hydrodynamic equations. Additionally, we study the low dimensional electron gas systems, fortuitously formed at the heterojunction of core-shell nanowires (CSNW), showing their role in plasmonic enhancement of nanowire optical cavities. These systems enhance the light confinement and quality factor of the cavity while allowing resonant modes to exist in deep sub-wavelength regions which otherwise could not be achieved. Moreover, the ability to modify the concentration of the electron in the electron gas allows dynamic tuning of the resonant peaks. We have compared the figures of merit of these enhanced nanowire cavities with those of noble metals and graphene showing that the low dimensional electron system pseudo mirror performs as well as the others in enhancing of the quality factors. These results indicate that the 2DEG can enhance mode confinement, allowing us to engineer nanostructured cavities with prescribed absorption and emission spectra. Next, we implement structured patterns of a group of 2D materials, called MXenes, as contact materials for optoelectronic components. We have implemented MXenes in a family of photodetectors, MXene-Semiconductor-MXene (MX-S-MX) through a simple fabrication process. These detectors offer higher responsivity, quantum efficiency, and detectivity compared to conventional detectors made with gold contacts, while lowering the cost of manufacturing and maintaining high response speeds. These photodetectors that outperform conventional ones using Ti/Au can be fabricated through a three-step process using only tabletop equipment. This ambient condition process is promising for integration into microelectronics, photonic integrated circuits, and silicon photonics technologies and proves that MXenes can efficiently function as electrode materials in place of metals like gold. One of the shortcomings of Metal-Semiconductor-Metal (MSM) photodetectors is the relatively large dark current and fall time. In the next step of this study, we fabricated a similar device on a delta doped AlGaAs/i-AlGaAs (intrinsic) substrate, that is formed on a layered structure comprised of a GaAs/AlGaAs heterostructure, where this layer isolates and eliminates the photocarriers generated deep in the device. This fact, in addition to using low temperature grown GaAs in the wafers, leads to extremely low dark currents and very fast detectors. These devices are fabricated using a simple 3-step fabrication process that reduces the cost of manufacturing while improving the dark current performance by a factor of 100, faster response by a factor of more than 200 times, and more than 4 times higher responsivity and quantum efficiency. Finally, we incorporate the patterns of MXene in the form of a wire grid in designing THz polarizers on transparent quartz. These devices show comparable extinction ratios to conventional metallic wire grid polarizers made of gold or tungsten with up to 32 dB contrast between two perpendicular polarizations when optimized, at a fraction of thickness of the films. MXene used in these devices is solution-processed and can be readily deposited into various substrates such as flexible ones. This method can be applied in the future to other MXenes, resulting in a variety of novel THz photonic devices. Additionally, we propose a double layer wire grid structure that allows us to tune the operation wavelength dynamically, by using a tunable carrier concentration 2DEG as the second layer of the grid.