Epitaxial scandium nitride thin films and heterostructures for thermoelectric, electronic, and artificial synaptic device applications

Abstract

Nitrides are a technologically important class of materials consisting of well-known III(A)-nitrides, transition metal and rare-earth nitrides. While conventional wurtzite nitride semiconductors, such as GaN, InN, AlGaN, have significantly impacted lighting and high-frequency-highpower electronics, they are inadequate for effectively addressing contemporary challenges such as energy conversion, neuromorphic computing, quantum computing, and data security. In the quest for new materials with novel functionalities enabling it to meet the current requirements, this thesis explores a relatively less explored III(B)-transition metal nitride, scandium nitride (ScN). Unlike conventional wurtzite III(A)- nitrides exhibiting direct optical bandgbap, ScN has a cubic rocksalt structure and an indirect bandgap. With some resemblance (composition, doping, etc.) and with some contrast (structure, bandgap, mobility, etc.) to GaN, ScN provides high hope for various current scenario applications. This thesis delves into the exploration of ScN films for applications in thermoelectrics, electronics, and artificial optoelectronic synapses. The thesis comprises of six chapters, and a brief summary on each is presented below. In Chapter 1, the significance, evolution, and characteristics of the emerging III(B)-nitride semiconductor, ScN, are presented. Additionally, a concise introduction to thermoelectricity, Schottky diodes, and artificial synapses is provided, elucidating their fundamental concepts and illustrating how ScN showing how ScN seamlessly fits well into each area. As ScN exhibit degenerate semiconducting properties and a low thermal conductivity among ambient-stable III-nitrides, it is a potential candidate for thermoelectric application. Chapter 2 explores the thermoelectric performance of MBE deposited thin film. In part-A of this chapter, MBE deposited epitaxial single-crystalline ScN thin film on MgO substrates is shown to have a high Seebeck co-efficient ( -175 µV/K at 950K) and high thermoelectric power factor (∼2.3×10−3 W/m.K2 ). MBE growth and electronic transport properties are investigated in detail in this part. Previously, theoretical calculations have shown that scandium and nitrogen vacancies in ScN lead to asymmetric density of state peaks near the Fermi energy that should be useful for improving the Seebeck coefficient. In part B, we have introduced such defects in ScN by lithium-ion implantation and investigate the thermoelectric performance. The defects increase the Seebeck coefficient and reduce the thermal conductivity to half of the thermal conductivity in pristine ScN. A significant rectification of electrical current in the metal/ScN interface is necessary to realize the ScN-based Schottky diode and thermionic emission in metal/ScN superlattices. Smooth and compact ScN thin films with low carrier concentration are essential to achieve significant current rectification. In part A of this chapter, the energy barrier for various growth modes in ScN thin film deposition with DC reactive magnetron sputtering and its impact on film morphology are investigated. It is found that a high substrate temperature (∼800 °C) is required to obtain smooth and compact ScN films. In part B, the electron concentration in ScN is reduced with magnesium (hole) doping. Electrical characterization of indium (In), silver (Ag), and gold (Au) contacts on electron-compensated n-ScN reveals that the Indium forms an Ohmic contact, while Ag and Au form Schottky contact with significant current rectification. The barrier heights are measured to be 0.55 eV for Ag/ScN and 0.53 eV for Au/ScN diodes. The von Neumann bottleneck in conventional computing systems can be overcome by implementing biological neuron-like hardware having both processing and memory in the same unit. In chapter 4, we show that the negative photoresponse in ScN and positive photoresponse in Mg-doped ScN can be used as inhibitory and excitatory synapses, respectively, and the persistence in photo-response decay can be exploited as memory. With these devices, we emulate some of the basic neural functionalities, like the transition from short-term memory (STM) to long-term memory (LTM), learning and forgetting curves, frequencydependent paired pulse facilitation (PPF) and paired pulse depression (PPD), dynamic filtering, Hebbian learning, and logic gate operations. In chapter 5, we try to understand a drastic decrease in electrical conductivity with increasing Mg compensation in ScN with the quasiclassical Anderson transition theory proposed by Efros and Shklovskii. The resistivity of intentionally-undoped ScN increases by nine orders on adding about 3% Mg due to the long-range potential fluctuation created by the inhomogeneous distribution of the impurities. The carrier droplets trapped in the potential wells must tunnel unless activated to percolation level. This room-temperature metal-insulator transition in heavily n-doped ScN on high compensation with Mg is accompanied by peculiar phenomena like persistent photoconductivity and increasing mobility with increasing temperature. Since the crystallinity of ScN is maintained across the transition, ScN can now be used in epitaxial single-crystalline devices with a wide range of resistivity The final chapter summarizes the comprehensive findings of the thesis. MBE deposition of ScN films and deliberately inducing defects to enhance the thermoelectric performance adds a new materials engineering aspect to ScN. The demonstration of significant current rectification in Ag/ScN and Au/ScN Schottky diodes marks a pivotal development. The innovative application of ScN as an artificial optoelectronic synapse introduces a promising avenue for practical implementations. The study of the quasi-classical Anderson transition in ScN with hole doping not only advances the understanding of ScN’s electronic compensation behaviour but also contributes to the broader comprehension of the electronic nature of narrow bandgap semiconductors. Altogether, this thesis delves into the versatility of ScN, showcasing promising advancements that set the stage for future developments and applications in the realm of materials science.