Current research trends in semiconductor materials for microelectronics and optoelectronics

Due to their superior material properties, wide bandgap (WBG) semiconductors enable the application of power electronics at higher temperature operation, higher frequencies, and higher efficiencies compared to silicon (Si). However, the commonly-used aluminum wire bonding as topside interconnection technology prevents WBG semiconductors from reaching their full potential, due to inherent parasitic inductances, large size, heat dissipation, and reliability issues of wire bonding technology. Therefore, this article presents a comprehensive review of topside interconnection technologies of WBG semiconductor power devices and modules. First, the challenges and driving factors for the interconnection of WBG semiconductor dies are discussed. Second, for each widely commercially used WBG semiconductor, i.e., silicon carbide and gallium nitride, technical details and innovative features of state-of-the-art interconnection techniques in packages are reviewed. Then, the majority of existing topside interconnection materials for WBG semiconductors are categorized and compared, followed by a discussion of their advantages, challenges, and failure modes. Based on this elaborate discussion, potential future directions of the interconnection technology development are given. It is concluded that the superior performance of WBG semiconductors can be obtained by combining novel materials with innovative designs for the topside interconnections.

As discussed in the accompanying articles in this issue ofMRS Bulletin, the optical properties of rare-earth (RE) elements have led to many important photonic applications, including solid-state lasers, components for telecommunications (optical-fiber amplifiers, fiber lasers), optical storage devices, and displays. In most of these applications, the host materials for the RE elements are various forms of oxide and nonoxide glasses. The emission can occur at visible or infrared (IR) wavelengths, depending on the electronic transitions of the selected RE element and the excitation mechanism. Until recently, the study of semiconductors doped with RE elements such as Pr and Er has concentrated primarily on the lowest excited state as an optically active transition. The presence of transitions at IR wavelengths (1.3 and 1.54 μm) that are coincident with minima in the optical dispersion and the loss of silica-based glass fibers utilized in telecommunications, combined with the prospect of integration with semiconductor device technology, has sparked considerable interest.The status and prospects of obtaining stimulated emission in Si:Er are reviewed by Gregorkiewicz and Langer in this issue and by Coffa et al. in a previousMRS Bulletinissue. While great progress is being made in enhancing the emission intensity of Er-doped Si, it still experiences significant loss in luminescence efficiency at room temperature, as compared with low temperatures. This thermal quenching was shown by Favennec et al. to de crease with the bandgap energy of the semiconductor. Hence wide-bandgap semiconductors (WBGSs) are attractive candidates for investigation as hosts for RE doping.

Semiconductors in the shape of nanostructures, including quantum dots (0-dimension) and nanoplatelets (2-dimension) demonstrate unique electronic and optoelectronic properties because of the quantum mechanism related to excitons confined in low dimensions. Such confinement can provide intriguing properties like high capability of photo-activated carriers, band gap tunability, splendid photoluminescence (PL) yield, narrow emission bandwidth, high absorption efficiency of light, fast fluorescence lifetime, etc. These properties attract great attention due to their high potential in applications like solar cells, light-emitting diodes (LEDs), lasers, photodetectors, catalysts, etc. Novel semiconductors including MoS2, MXene and perovskite have received increasing research interest in recent years. Unlike traditional semiconductor materials like Si, Ge, II-VI and III-V materials, the novel ones usually have low-dimensional form natively, which have emerged as very promising candidates for advanced electronic and optoelectronic devices. Among the various novel semiconductor materials, this thesis focuses on two types of them, which are MXene Sc2CO2 and cesium lead halide perovskite CsPbBr3. The two materials have some features in common. Both of them are direct band gap materials, which makes them available for optical applications. And they can be synthesized directly into two-dimensional structure, which o ers the opportunity to learn the influence of one-dimensional confinement. To extend the study into another dimension, the quantum dots of CsPbBr3 are also studied. The first part of the thesis focuses on the study of Sc2COx which is a type of semi-conductive MXene. The material is synthesized for the first time by using magnetron sputtering. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are performed to determine the composition of the sample. The band gap of Sc2COx sample is determined by ultraviolet-visible (UV-VIS) range absorption and photoluminescence (PL) spectrum. The Tauc plot indicates that Sc2COx is a direct band gap material. The high sensitivity of scandium to oxygen makes the sample contain both scandium carbide and scandium oxide. Thermal annealing introduces more oxidation into the sample. The structure of MXene will be broken when the annealing temperature is higher than 600C. The second part of the thesis focuses on the study of perovskite CsPbBr3 in forms of nanoplatelets and quantum dots. For CsPbBr3 nanoplatelets, the effect of thickness of cesium lead halide perovskite CsPbBr3 nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an 8-band k · p model which is based on effective-mass envelope function theory with exciton binding energy consideration. We first reported the CsPbBr3 nanoplatelets' band structure and optical gain with exciton effect. As the thickness of NPLs decreases, their band gap increases and the band mixing is more obvious which influences the transition matrix element (TME). The optical gain of CsPbBr3 nanoplatelets is calculated by taking into account TME, Fermi factor, injected carrier density and thickness. A blue shift of peak position in optical gain can be observed as the thickness of NPLs decreases. For any given NPL with a certain thickness, there is a slight blue shift in the peak position of optical gain as the carrier density increases because of band filling effect. The maximum optical gain of thinner NPLs needs higher carrier density to reach saturation. To obtain high differential gain, the carrier density in thick NPLs has to maintain in a small range. Experimental work is carried out and the results agree with our theoretical results very well. For quantum dot, the size effect on the electronic structure and optical properties of cubic CsPbBr3 perovskite quantum dots are investigated by using an 8-band k · p model which is also based on effective-mass envelope function theory with exciton binding energy consideration. Quantum dots with smaller sizes have larger band gaps due to quantum confinement. The transition matrix element (TME) is also influenced by the size. The optical gain of such material is calculated by considering TME, Fermi factor, injected carrier density, quantum dot size, and dephasing rate. A higher density of the injected carrier is needed for smaller quantum dots to get a positive optical gain. The peak of optical gain has a blue shift as the size of a quantum dot decreases. When the carrier density increases for a quantum dot with a certain size, a blue shift in emission peak position can be observed due to the band filling effect. Smaller quantum dots have higher differential optical gain at carrier densities within a certain range. In a word, two direct band gap semiconductor materials, Sc2COx and CsPbBr3 are studied via theoretical and experimental approaches. Their band-structure and optical properties are presented, followed by the analysis of their potential in applications.

Advancing photocatalytic performance for enhanced visible-light-driven H2 evolution and Cr (VI) reduction of g-C3N4 through defect engineering via electron beam irradiation

Transition metal dichalcogenides (TMDs) are of great promise for various nonlinear optical (NLO) applications due to their unique electronic and optoelectronic properties, such as tunable optical bandgap, strong spin–orbit coupling, and exciton effects. However, the desired NLO performances of regular 2H‐TMDs are usually restricted by their limited absorption at atomic thickness. With this regard, a structurally novel spiral MoTe2 (s‐MoTe2) nanopyramids is reported with unique and superior NLO response, enabled by their broken inversion symmetry, weak interlayer coupling, exciton resonance, and strong light–matter interaction from the edge‐rich 3R‐like quasi‐multilayer structure. The excellent NLO response over a wide spectral range from the near‐infrared to visible region is demonstrated, where second‐ and third‐order NLO responses have been simultaneously observed. Moreover, the second‐order nonlinear susceptibility of s‐MoTe2 is estimated to be around 1–2 order(s) of magnitude larger than those of most reported TMDs. The demonstration of a superior NLO response in such s‐MoTe2 not only paves a new way for designing the best NLO TMD structures, but also greatly prompts their practical applications in micro–nano NLO devices on chips in future.

Phosphorene, mono/few‐layered black phosphorous with advantages of tunable energy bandgaps and strong light–matter interaction, is fabricated by electrochemical intercalation with large area (≈3 µm) and controllable thickness (mainly four layers). Thanks to the direct gap and resonant absorption of four‐layer phosphorene at the telecommunication band, all‐optical thresholding and optical modulation are demonstrated for optical communications by using few‐layer phosphorene‐decorated microfibers. This device is experimentally verified as an efficient noise suppressor that can enhance the signal‐to‐noise ratio and reshape the deteriorated signal pulse, and also as an optical modulator that can switch the signal on/off by pumping light. The findings, as the first prototypic device of all‐optical thresholding and optical modulation, might facilitate the development of phosphorene‐based optical communication technologies.

A brief biography of Professor Lester F. Eastman is given highlighting his professional achievements. Professor Eastman is one of the world leaders in the physics and technology of compound semiconductor materials and devices. He has invented, fabricated, and investigated many novel semiconductor materials and devices, and his work on so many occasions has resulted in breakthroughs enabling important practical applications. The best solid-state device groups have followed his ideas with great success, and his former students and associates work in leadership positions in hundreds of leading companies, research laboratories and Universities in the United States and around the globe.

Ultraviolet optoelectronic devices based on AlGaN-SiC platform: Towards monolithic photonics integration system

The synthesis and characterization of wide bandgap II–VI (~3.6 eV) semiconductor materials has been the major area of research in the field of nanotechnology since last 10 years. The nano-sized wide bandgap semiconductor crystallites have unique chemical, physical, and electronic properties, which support their potential application in opto-electronics, data storage devices, quantum dot lasers, nanosensors, nanophosphors, biological markers, and efficient photocatalytic applications [1–6]. When the size of semiconductor nanoparticles is reduced below a critical diameter (<100 nm), the spatial confinement of the charge carrier causes them to behave quantum mechanically. As a result, the bands split into discrete electronic states in the valence and conduction bands, and nanoparticles behave more like a giant atom. Due to the quantum confinement effect [7,8], semiconductor nanomaterials also exhibit large surface area to volume ratio. The high surface-to-volume ratio of nanoclusters leads to high surface reactivity and enhance the chemical reaction dynamics and photon absorption efficiency. In case of photocatalytic processes, the electron–hole pairs generated on the surface of semiconductor materials by visible or ultraviolet (UV) photon undergo redox reactions with molecules adsorbed onto the surface, thereby breaking them into smaller fragments. The process finds its applications for the mineralization of toxic organic compounds, hazardous inorganic constituents, and bacteria disinfection. An ideal photocatalyst should be stable, inexpensive, nontoxic, and highly photoactive. Doping of pure semiconductor nanomaterials with rare earths and/or transition metals creates quasistable energy states within the bandgap, thereby affecting the electronic structure, optical, and transition probabilities. In case of doped semiconductor nanocrystals, impurity ion occupies the host atom lattice site and behaves as trap site for electron and holes. The electrons are excited from the conduction band by absorbing the energy equal or greater than their bandgap energy. Subsequent relaxation of these photoexcited electrons to some surface states or levels is followed by radiative and nonradiative decay processes. In case of photocatalysis, increased electron trapping due to higher defect sites leads to enhancement in the photocatalytic efficiency. This increase in photocatalytic efficiency is possible provided the electron– hole pair recombination rate is lower than the rate of electron transfer to adsorbed molecules. Therefore, the photocatalytic and luminescent properties of wide bandgap semiconductor nanomaterials are strongly dependent upon the type of dopant ions. Moreover, photophysical and photochemical behavior correlation and their dependence on doping concentration and nanostructure morphology will be very beneficial for making the phosphor and photocatalytic calibration curves, which will serve as the backbone for the future applications of these smart materials. CONTENTS

Recent advances in wide band-gap (WBG) semiconductor materials, such as silicon carbide (SiC) and gallium nitride (GaN) are enabling a new generation of power semiconductor devices that far exceed the performance of silicon-based devices. Past ARPA-E programs (ADEPT, Solar ADEPT, and SWITCHES) have enabled innovations throughout the power electronics value chain, especially in the area of WBG semiconductors. The two recently launched programs by ARPA-E (CIRCUITS and PNDIODES) continue to investigate the use of WBG semiconductors in power electronics. From materials and devices to modules and circuits to application-ready systems integration, ARPA-E projects have demonstrated the potential of WBG semiconductors to lower the cost of high-efficiency power electronics to enable broad adoption in energy applications.

Two-dimensional (2D) semiconductor materials with a large surface area, tunable band gap, and high catalytic activity are emerging as a new class of photoelectrocatalysts. However, the 2D photoelectrocatalysts usually suffer from lower photoelectrochemical efficiency because of the challenges in effectively utilizing the 2D photoelectrocatalysts in three-dimensional electrodes with a reserved surface area and 2D electronic properties. Here, we report novel nanoporous heterojunctions composed of 3D nanoporous reduced graphene oxide (rGO) as visible light absorber and 2D monolayer MoS2 as electron transfer bridge and hydrogen evolution reaction (HER) electrocatalyst for photoelectrochemical hydrogen production. With a designed band alignment between monolayer MoS2 and rGO and a large surface and contact interface, the photoelectrocatalyst shows an outstanding photoenhanced HER activity with a high solar-to-hydrogen conversion efficiency.

Chapter 16 - Oxyhalides-based photocatalysts: the case of bismuth oxyhalides

Deep‐level defects in semiconductor materials usually induce carrier trapping and non‐radiative recombination. However, defects caused by unintentional contaminants in antimony selenide (Sb2Se3) solar cells have rarely been reported. Herein, the correlation between defect properties and unintentional impurities in the Sb2Se3 absorber is investigated, which is prepared by injection vapor deposition with Sb2Se3 source purities ranging from 99.9% to 99.9999%. The analysis of deep‐level transient spectra reveals that an increase in impurity concentration does not result in new defect types. Nevertheless, the higher impurity level causes an increase in both the defect density and the capture cross section of the original defect. To address this challenge, defect engineering is developed to regulate the growth of the Sb2Se3 absorber. This strategy completely suppresses deepest defect states E3, a mixture of intrinsic defect (SbSe) and impurity Si related defect. As a result, the carrier lifetime increases significantly from 0.37 to 3.4 ps. This enables to fabricate Sb2Se3 solar cells with a power conversion efficiency of 10.41%. This work uncovers the characteristics of unintentional contaminant‐related defects, and provides a strategy for fabricating highly efficient Sb2Se3 solar cells with low‐purity source materials.

The study of the magnetism and optical properties of semiconductor materials by defect engineering has attracted much attention because of their potential uses in spintronic and optoelectronic devices. In this paper, first-principle calculations discloses that cationic vacancy formation energy of the doped wurtzite materials can be sharply decreased due to alkali metal dopants and shows that their magnetic properties strongly depend on defect and doping concentration. This effect can be ascribed to the volume change induced by foreign elements doped into the host system and atomic population's difference. The symmetric deformation induced by biaxial strain can further regulate this behavior. Our results suggest that the formation of cationic vacancy can be tailored by strain engineering and dopants incorporation.

Fundamentals and engineering of defects