Van der Waals Heterojunctions for Emerging Device Applications
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Low-dimensional systems are an exciting platform for exploring new physics and realizing novel devices. The intriguing features, such as the existence of strongly bound multiparticle complexes and thickness-dependent band structures, enable us to utilize them to overcome many challenges faced by bulk materials and conceive new technologies. Since the isolation of graphene, the class of two-dimensional materials has grown tremendously. The array of materials one can choose from for implementing an idea is vast. Nevertheless, understanding the underlying physics is essential for utilizing these properties for real-life applications. Here, we explore the optical, electrical, and optoelectrical characteristics of heterostructures based on 2D layered systems. The strongly bound excitonic complexes hosted by monolayer transition metal dichalcogenide semiconductors (TMDC) are an excellent platform for probing many-body physics. The strong luminescence and a plethora of exciting properties make them a good candidate for applications such as single photon emitters and light-emitting diodes. In the first work, we explore new ways to tune the emission from these particles without compromising their luminescence. Using a high-quality graphene/hBN/WS2/hBN/Au vertical heterojunction, we demonstrate for the first time an out-of-plane electric field-driven change in the sign of the Stark shift from blue to red for four different excitonic species, namely, the neutral exciton, the charged exciton (trion), the charged biexciton, and the defect-bound exciton. We also find that the encapsulating environment of the monolayer TMDC plays a vital role in wave function spreading and hence in determining the magnitude of the blue Stark shift. We also provide a theoretical framework to understand the underlying physics better. The findings have important implications in probing many-body interaction in the two dimensions and developing layered semiconductor-based tunable optoelectronic devices. A significant advantage of the 2D material system is its robustness against lattice mismatch between the successive layers and the ability to extract exciting characteristics from the resultant system. The final system's behavior greatly depends on how the energy bands of the individual materials line up and can result in drastically different properties. In the second work, we demonstrate how an additional ultra-thin barrier layer modifies the properties of a black phosphorus (BP)/SnSe2 tunnel diode. While the system without the barrier layer showed a linear relationship between current and voltage, the additional barrier layer modified it to a highly nonlinear relation and exhibited negative differential resistance (NDR). Moreover, the tunnel diodes exhibited highly repeatable, ultra-clean, and gate tunable NDR characteristics with a signature of intrinsic oscillation and a large peak-to-valley current ratio (PVCR) of 3.6 at 300 K (4.6 at 7 K), making them suitable for practical applications. We then show that the thermodynamic stability of the van der Waals (vdW) tunnel diode circuit can be tuned from astability to bistability by altering the constraint by choosing a voltage or a current bias, respectively. After exploring the dynamics of the device, we assess its viability for designing systems with real-life applications. In the astable mode under voltage bias, we demonstrate a compact, voltage-controlled oscillator without needing an external tank circuit. In the bistable mode under current bias, we demonstrate a highly scalable, single element, a one-bit memory cell promising for dense random access memory applications in memory-intensive computation architectures. In the third work, we explore the usage of vdW materials for generating a cryptographically secure true random number generator. Such generators rely on external entropy sources for their indeterminism. Physical processes governed by the laws of quantum mechanics are excellent sources of entropy available in nature. However, extracting enough entropy from such systems for generating truly random sequences is challenging while maintaining the feasibility of the extraction procedure for real-world applications. Here, we design a compact and an all-electronic vdW heterostructure-based device capable of detecting discrete charge fluctuations for extracting entropy from physical processes and use it for the generation of independent and identically distributed (IID) true random sequences. Using the proposed scheme, we extract a record high value (> 0.98 bits/bit) of min-entropy. We demonstrate an entropy generation rate tunable over multiple orders of magnitude and show the persistence of the underlying physical process for temperatures ranging from cryogenic to ambient conditions. We verify the random nature of the generated sequences using tests such as the NIST SP 800-90B standard and other statistical measures and verify the suitability of our random sequence for cryptographic applications using the NIST SP 800-22 standard. The generated random sequences are then used to implement various randomized algorithms in real life without preconditioning steps. We then investigate how knowledge of the dynamics of optically generated carriers, ability to sense discrete charge fluctuation, and transport of carriers across vdW heterostructure can be combined to design a comprehensive system to detect single photons. Single-photon detectors (SPDs) are crucial in applications ranging from space and biological imaging to quantum communication and information processing. The SPDs operating at room temperature are particularly interesting to broader application spaces as the energy overhead introduced by cryogenic cooling can be avoided. Although silicon-based single photon avalanche diodes (SPADs) are well matured and operate at room temperature, the bandgap limitation restricts their operation at telecommunication wavelength (1550 nm) and beyond. On the other hand, InGaAs-based SPADs are sensitive to 1550 nm photons but suffer from relatively lower efficiency, high dark count rate, afterpulsing probability, and pose hazards to the environment from the fabrication process. By coupling a low bandgap (~350 meV) absorber (black phosphorus) to a sensitive van der Waals probe capable of detecting discrete electron fluctuation, we demonstrate a room-temperature single-photon detector. While the device is capable of covering up to a wavelength of ~3.5 um, we optimize the device for operation at 1550 nm and demonstrate an overall quantum efficiency of 21.4% (estimated as 42.8% for polarized light) and a minimum dark count of ~720 Hz at room temperature.