Emerging Semiconductor Material Research Articles

Investigation of the nano and micromechanical performance of β-Ga2O3 epitaxial films on sapphire using nanoindentation

Beta-phase gallium oxide (β-Ga2O3) has attracted considerable attention because of its excellent properties as one of the emerging semiconductor materials. However, not only the fabrication of high quality β-Ga2O3 thin films on heterogeneous substrates is challenging, but there is also a lack of research pertaining to their mechanical properties. In this work, the mechanical properties of β-Ga2O3 epitaxial films of varying thicknesses on c-plane sapphire substrates fabricated via plasma enhanced chemical vapor deposition were investigated through nanoindentation and nano-scratch experiments. The β-Ga2O3 hetero-epitaxial thin films exhibit preferential orientation growth along the (−201) plane when deposited on the c-plane sapphire substrate. The elastic modulus, hardness, and fracture toughness of β-Ga2O3 films grown on sapphire substrates are 249.1 ± 14.1 GPa, 18.7 ± 0.9 GPa, and 1.822 MPa m1/2, respectively. The elastic-plastic behavior of β-Ga2O3 thin films with varying thicknesses is consistent. The critical loads for the elastic-plastic and plastic-brittle transitions are 9.4 ± 1.8 mN and 50.7 ± 2.7 mN respectively, while the scratch depths for the same are 91.8 ± 7.8 nm and 216.4 ± 10.1 nm, respectively. The critical adhesion and adhesive energy for the desquamation of the β-Ga2O3 film from the substrate are dependent on the thickness of the film. These results establish an experimental basis for understanding the mechanical behavior in the preparation and application of β-Ga2O3 thin films grown on sapphire substrates.

Correlation between structural, morphological, and electrical transport properties of nano-dimensional Tellurium thin film by low energy ion beam irradiation

Tellurium (Te), an emerging semiconductor material, exhibits intriguing physical properties in low-dimensional forms. This study investigates the structural, morphological, and transport properties of low-dimensional Te thin film deposited by physical vapor deposition after 400 keV Kr2+ ion irradiation with varying fluences of 1 × 1013, 5 × 1013, and1 × 1014 ions-cm−2. The X-ray diffraction (XRD) pattern reveals the hexagonal phase of pristine Te. As ion fluence increases, intensity of the XRD peaks decreases and the full-width half maxima (FWHM) increases, indicating a reduction in crystallinity. The intensity of the Raman peak decreases upon irradiation, resulting in a red shift in the spectra of the irradiated sample. The evolution of morphology at different fluences has been examined with atomic force microscopy (AFM) measurements. The results indicate the segregation of grains and an increase in root mean square (RMS) roughness from 2.59 pm to 5.11 pm with ion fluence up to 5 × 1013 ions-cm−2. At the highest fluence, i.e., 1 × 1014 ions-cm−2, there is an agglomeration of grains, and hence a decrease in RMS roughness to 3.92 pm is observed. The DC resistivity measurement indicates the semiconducting nature of all films. Additionally, this measurement signifies the rapid decrease in activation energy at both band-to-band conduction and nearest neighbour hopping (NNH) conduction. Following irradiation, Hall measurement demonstrates a decrease in the concentration of charge carriers and an increase in resistivity due to scattering originating from grain boundaries. However, when the fluence is increased to 1 × 1014 ions-cm−2, there is a reduction in resistivity and an increase in carrier concentration with enhanced Hall mobility in comparison to pristine Te. Therefore, this study showcases that low-energy ion irradiation has a significant impact on the modification of the structure, morphology, and electrical transport properties of semiconducting Te thin film.

Understanding the synthesis mechanism, chemical structures and optical properties of aromatic carbon nitride

The chemical structures and optical properties of aromatic carbon nitride and intermediates have been systematically studied; hereby the detailed synthesis mechanism of aromatic carbon nitride is explicitly proposed.

Defects in lead halide perovskite light-emitting diodes under electric field: from behavior to passivation strategies.

Lead halide perovskites (LHPs) are emerging semiconductor materials for light-emitting diodes (LEDs) owing to their unique structure and superior optoelectronic properties. However, defects that initiate degradation of LHPs through external stimuli and prompt internal ion migration at the interfaces remain a significant challenge. The electric field (EF), which is a fundamental driving force in LED operation, complicates the role of these defects in the physical and chemical properties of LHPs. A deeper understanding of EF-induced defect behavior is crucial for optimizing the LED performance. In this review, the origins and characterization of defects are explored, indicating the influence of EF-induced defect dynamics on LED performance and stability. A comprehensive overview of recent defect passivation approaches for LHP bulk films and nanocrystals (NCs) is also provided. Given the ubiquity of EF, a summary of the EF-induced defect behavior can enhance the performance of perovskite LEDs and related optoelectronic devices.

Crystal orientation engineering in two-dimensional perovskites for improved photoelectric properties and applications

Two-dimensional perovskite is considered an emerging semiconductor material because of its structural diversity and stability, making it useful in solar cells, light-emitting diodes, and lasers. However, compared with conventional three-dimensional perovskite materials, issues such as lower charge transport efficiency due to organic cation spacers could hinder their widespread use. Therefore, vertically oriented two-dimensional perovskites have been extensively studied because they can provide direct carrier transport channels, improve trap density, and reduce lattice stress. Here, the article summarizes two-dimensional vertically oriented perovskite from orientation methods, characterization methods, properties, and applications. Finally, the article discusses the challenges and expectations of developing two-dimensional oriented perovskites.

Tuneable Schottky contact of MoSi2N4/TaS2 van der Waals heterostructure

The two-dimensional MoSi2N4 monolayer is an emerging semiconductor material that offers considerable promise due to its ultra-thin profile, tuneable mechanical properties, excellent optoelectronic properties and exceptional environmental stability. The van der Waals (vdW) heterostructure formed by stacking such two-dimensional monolayers has demonstrated superior performance across various domains. In this study, a vdW heterostructure combining the two-dimensional MoSi2N4 and TaS2 monolayers is examined using first-principles density functional theory. In its ground state, this van der Waals heterostructure establishes an ohmic contact with an exceptionally low potential barrier height. By modulating the vdW heterostructure with an applied electric field of -0.1 V/Å and under vertical stress, we discovered that MoSi2N4 and TaS2 can transition from an ohmic contact to a p-type Schottky with an ultra-low Schottky barrier height (SBH). Our observations may give valuable insights for designing reconfigurable, tuneable Schottky nano-devices with enhanced electronic and optical properties based on MoSi2N4/TaS2.

Printable organic light-emitting diodes for next-generation visible light communications: a review

Visible light communication (VLC) is an emerging technology employing light-emitting diodes (LEDs) to provide illumination and wireless data transmission simultaneously. Harnessing cost-efficient printable organic LEDs (OLEDs) as environmentally friendly transmitters in VLC systems is extremely attractive for future applications in spectroscopy, the internet of things, sensing, and optical ranging in general. Here, we summarize the latest research progress on emerging semiconductor materials for LED sources in VLC, and highlight that OLEDs based on nontoxic and cost-efficient organic semiconductors have great opportunities for optical communication. We further examine efforts to achieve high-performance white OLEDs for general lighting, and, in particular, focus on the research status and opportunities for OLED-based VLC. Different solution-processable fabrication and printing strategies to develop high-performance OLEDs are also discussed. Finally, an outlook on future challenges and potential prospects of the next-generation organic VLC is provided.

A thienyl‐benzodithiophene‐based two‐dimensional conjugated covalent organic framework for fast photothermal conversion

AbstractTwo‐dimensional conjugated covalent organic frameworks (2D c‐COFs) are emerging semiconductor materials for optoelectronic and photothermal applications. In particular, the highly tailorable, semiconducting thiophene‐based 2D c‐COFs have attracted considerable interest due to their nontrivial physicochemical properties such as photo‐activity, broad optical absorption, tunable electronic structures, and so forth. Herein, we demonstrate a novel, crystalline 2D c‐COF based on thienyl‐functionalized benzodithiophene (BDT) and biphenyl (BP) via the Schiff‐base polycondensation reaction. The resultant BDT‐BP‐COF exhibits a broad optical absorption up to ca. 600 nm and decent π‐conjugation along the 2D polymer skeleton, as revealed by the optical absorption and theoretical calculations. The favorable π‐conjugation and the abundant electron‐rich thiophene units confer excellent photo‐activity to BDT‐BP‐COF towards the usage of solar energy. As a proof‐of‐concept application, we explore BDT‐BP‐COF in photothermal conversion, in which it shows a fast surface‐temperature increase upon light irradiation for seconds.

Mechanical stability parameters of chalcogenides and pnictides based optoelectronic materials

A study of experimental data reveals that the bulk modulus of chalcogenides and pnictides based chalcopyrites (AIIBIVC2 V and AI BIIIC2 VI) can be explained by a simple scaling rule that rely only on the crystal ionicity, ionic charge product, and the melting temperature. PVV theory of crystal ionicity, temperature dependence of elasticity and product of ionic charge theory are taken into account for the study. Based on this result, a simple microhardnessbulk modulus relation is applied to evaluate the microhardness of the complex compounds; which correspond well with the experimental data and other published results. The proposed findings support in the modeling of emerging semiconductor materials and even understanding of their mechanical properties for optoelectronics, photovoltaic, electromagnetic (EM) screening, and spintronic applications. PACS: 62.20.-x; 62.20.Qp

(Digital Presentation) MOCVD Growth of Ga2O3, Algao and Heterostructures

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability. It has a band gap of 4.5-4.9 eV, much wider than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic β-phase Ga2O3 represents the thermodynamically stable crystal among the known five known phases (α, β, γ, d, ε/κ). The breakdown field of β-Ga2O3 is predicted to be 6-8 MV/cm, which is much larger than that of the 4H-SiC and GaN. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. Single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Metalorganic chemical vapor deposition (MOCVD) growth technique was demonstrated to produce high quality β-Ga2O3 thin films and its ternary (AlxGa1-x)2O3 alloys. Control of background and n-type doping in β-Ga2O3 will be discussed. Record carrier mobilities of 194 cm2/V·s at room temperature and ~10,000 cm2/V·s at low temperature were measured for MOCVD β-Ga2O3 thin films. Growth and fundamental understanding of β-(AlxGa1-x)2O3 are still limited. The incorporation of Al in beta-phase Ga2O3 has not been well understood, although it was predicted up to 60% of Al composition could be incorporated into β-Ga2O3. MOCVD growth of β-AlGaO targeting for Al composition of > 40% will be discussed. N-type doping capability as a function of Al composition in (AlxGa1-x)2O3 is another important fundamental question. Charge carrier transport properties in (AlxGa1-x)2O3 will be discussed. In addition, MOCVD growth of different phases of Ga2O3 and AlGaO will be presented. Acknowledgement: The authors acknowledge the funding support from the Air Force Office of Scientific Research FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir) and the National Science Foundation (1810041, 2019753, 1755479).

Deriving the absorption coefficients of lattice mismatched InGaAs using genetic algorithm

Lattice-mismatched InGaAs has appeared to be emerging semiconductor materials for sensors and photovoltaic applications. The absorption coefficients of the materials are crucial in designing high-performance semiconductor devices. Nevertheless, the absorption coefficient of lattice-mismatched InGaAs were not comprehensively studied to cater for the 2000–3000 nm applications. This study aims to determine the absorption coefficients of lattice-mismatched In0.73Ga0.27As and In0.83Ga0.17As semiconductor materials through photocurrent measurement which enables the absorption tail information to be extracted. In addition, this work demonstrates the incorporation of an innovative artificial intelligence-based method in solving the absorption coefficient of lattice-mismatched InGaAs, considering the detailed information of the structure design and material parameters. By selecting the best gene for the next iteration, the utilization of Genetic Algorithm has significantly reduced the number of iterations from a maximum of 10 000 to 300. Validation of the algorithm was conducted, showing a good agreement of absorption coefficient result compared to the published work on In0.72Ga0.28As. The absorption coefficient of In0.83Ga0.17As with an extended cutoff wavelength near 2.6 μm is newly reported in this paper. In addition, the extrapolation of the obtained absorption results demonstrates energy gaps of 0.475 eV for In0.73Ga0.27As and 0.55 eV for In0.83Ga0.17As, which are compatible with the reported bandgaps of these materials. The extracted absorption coefficient information can be used in the design of semiconductor devices for emerging technologies such as focal plane array, short wave infrared sensing and thermophotovoltaic.

Empirical predictions for bulk and shear moduli of zinc-blende structured binary solids

An empirical relationship has been found to evaluate the bulk modulus (B) and shear modulus (G) with the help of two experimental physical quantities (the melting temperature and the crystal ionicity), which are drawn from various pieces of literature. The PVV theory of crystal ionicity and temperature dependence of elasticity are taken into account for the study. The evaluated bulk modulus and shear modulus values are found to be in excellent conformity with experimental and reported values of other researchers. The current research supports the modelling of emerging semiconductor materials and even understanding of their mechanical properties for photovoltaic, spintronics, LiB systems, FLB phenomenon, and superhard material applications.

Charge-Carrier Transport in Quasi-2D Ruddlesden-Popper Perovskite Solar Cells.

In recent years, 2D Ruddlesden-Popper (2DRP) perovskite materials have been explored as emerging semiconductor materials in solar cells owing to their excellent stability and structural diversity. Although 2DRP perovskites have achieved photovoltaic efficiencies exceeding 19%, their widespread use is hindered by their inferior charge-carrier transport properties in the presence of diverse organic spacer cations, compared to that of traditional 3D perovskites. Hence, a systematic understanding of the carrier transport mechanism in 2D perovskites is critical for the development of high-performance 2D perovskite solar cells (PSCs). Here, the recent advances in the carrier behavior of 2DRP PSCs are summarized, and guidelines for successfully enhancing carrier transport are provided. First, the composition and crystal structure of 2DRP perovskite materials that affect carrier transport are discussed. Then, the features of 2DRP perovskite films (phase separation, grain orientation, crystallinity kinetics, etc.), which are closely related to carrier transport, are evaluated. Next, the principal direction of carrier transport guiding the selection of the transport layer is revealed. Finally, an outlook is proposed and strategies for enhancing carrier transport in high-performance PSCs are rationalized.

Silicon-integrated monocrystalline oxide–nitride heterostructures for deep-ultraviolet optoelectronics

New opportunities for high-performance CMOS-compatible optoelectronic devices have accelerated the interest in vertically configured device topologies that enable next-generation photonic technologies. Lately, TiN has been identified as a promising refractory metal–ceramic for the hybrid integration of emerging semiconductor materials on a variety of substrates, including Si, MgO, and sapphire. Among these, Si is the least expensive and most commonly used element and substrate material in the semiconductor device industry. Following these examples, a hybrid oxide–nitride–Si stack is proposed and thoroughly investigated herein for its potential use in DUV optoelectronic device applications. The stack comprises β-Ga2O3 thin films grown heteroepitaxially on TiN/Si platforms, wherein the TiN interlayers were heteroepitaxially grown on bulk (100)-oriented Si and act as lattice-mismatched templates and bottom device electrodes. Albeit the relatively large lattice mismatch between Si and TiN, a low in-plane rotation of 3 ∘ revealed that the TiN layers continued to grow as a bulk crystal, paving the way for heteroepitaxial β-Ga2O3 thin films being grown without exhibiting amorphous and metastable phases. DUV photodetectors based on this optoelectronic heterostructure exhibited average peak spectral responsivity and external quantum efficiency levels as high as 249 A/W and 1.23 × 105%, respectively, in the ultraviolet-C regime at an illuminating power density of around 12 µW/cm2.

Hybrid Thin-Film Materials Combinations for Complementary Integration Circuit Implementation

Beyond conventional silicon, emerging semiconductor materials have been actively investigated for the development of integrated circuits (ICs). Considerable effort has been put into implementing complementary circuits using non-silicon emerging materials, such as organic semiconductors, carbon nanotubes, metal oxides, transition metal dichalcogenides, and perovskites. Whereas shortcomings of each candidate semiconductor limit the development of complementary ICs, an approach of hybrid materials is considered as a new solution to the complementary integration process. This article revisits recent advances in hybrid-material combination-based complementary circuits. This review summarizes the strong and weak points of the respective candidates, focusing on their complementary circuit integrations. We also discuss the opportunities and challenges presented by the prospect of hybrid integration.

Low Temperature Producing Copper‐Doped Gallium Oxide as Hole Transport Layers of Perovskite Solar Cells Enhanced by Impurity Levels

In inverted perovskite solar cells (PSCs), metal oxides become kind of promising hole transport layers for their facile synthesis and low cost. For conventional hole transport materials, the valence band match between metal oxides and perovskite layers is usually necessary for the hole extraction process. Ga2O3 is an emerging semiconductor material with ultrawide bandgap, but a significant energy level mismatch exists at Ga2O3/perovskite interfaces. In this work, Cu‐doped Ga2O3 (Ga2O3:Cu) nanocrystals are synthesized by the hydrothermal method and used as the hole transport material of inverted PSCs for the first time. It is found that Cu dopants can substantially improve the performance of Ga2O3 layers, and the efficiency of PSCs is increased from 7.6% to 19.5%. This improvement can be attributed to the additional hole transport channels from impurity levels of Cu dopants, which exactly match with the valence band of perovskite layers. As a consequence, Ga2O3:Cu layers can effectively extract holes and inhibit the recombination in perovskite layers. This work also provides an alternative route for the design of hole transport materials.

(Invited) Nanoscale Investigation of Extended Defects in Wide Bandgap Semiconductors By Exploiting Phonon-Resonant Near-Field Interaction

The use of wide-bandgap semiconductor materials like SiC and GaN can dramatically improve the performance of electronic devices at high powers and temperatures. However, the propensity of extended defects in these materials does challenge their implementation in commercial electronic and optical applications. Spectroscopic and microscopic tools for identifying and characterizing these defects typically offer either spectroscopic or microscopic information exclusively and/or may be destructive. Here, we show how extended defects within 4H-SiC manifest in the nanoscale infrared phonon response probed by scattering-type scanning near-field optical microscopy (s-SNOM), a nondestructive method capable of simultaneously collecting topographic and spectroscopic information with frequency-independent nanoscale spatial resolution (≈20 nm).We correlate the s-SNOM response of various defects in 4H-SiC with UV-photoluminescence, secondary electron and electron channeling contrast imaging, and transmission electron microscopy. We identify evidence of step-bunching, recombination-induced stacking faults, and threading screw dislocations, and also demonstrate the interaction of surface phonon polaritons with extended defects. Phonon-enhanced infrared nanospectroscopy and spatial mapping via s-SNOM thus offer significant insights into extended defects within emerging semiconductor materials and devices. It thus serves as an important diagnostic tool to help advance material growth efforts for electronic, photonic, phononic, and quantum optical applications.[1] B. Hauer, C. E. Marvinney, et al. Advanced Functional Materials 30, 1907357 (2020).

Emerging light-emitting diodes for next-generation data communications

The continuing development of consumer electronics, mobile communications and advanced computing technologies has led to a rapid growth in data traffic, creating challenges for the communications industry. Light-emitting diode (LED)-based communication links are of potential use in both free space and optical interconnect applications, and LEDs based on emerging semiconductor materials, which can offer tunable optoelectronics properties and solution-processable manufacturing, are of particular interest in the development of next-generation data communications. Here we review the development of emerging LED materials—organic semiconductors, colloidal quantum dots and metal halide perovskites—for use in optical communications. We examine efforts to improve the modulation performance and device efficiency of these LEDs, and consider potential applications in on-chip interconnects and light fidelity (Li-Fi). We also explore the challenges that exist in developing practical high-speed LED-based data communication systems. This Review examines the development of emerging semiconductor materials—organic semiconductors, colloidal quantum dots and metal halide perovskites—for light-emitting diodes, considering efforts to improve modulation performance and device efficiency, as well as potential applications in on-chip interconnects and light fidelity (Li-Fi).

Solar-TiO2 immobilized photocatalytic reactors performance assessment in the degradation of methyl orange dye in aqueous solution

Heterogeneous photocatalysis with titanium dioxide (TiO2) nanoparticles (NPs) has been demonstrated an emerging semiconductor material and widely been used in the treatment of wastewater due to is its high photocatalytic activity. In this study, TiO2 NPs with and without hydrolysis were synthesized by sol–gel and heat treatment methods. About 67 times higher surface area was obtained for TiO2 NPs synthesized with hydrolysis as compared to without hydrolysis. The X-ray diffraction showed an increased amount of anatase phase of TiO2 NPs with hydrolysis whereas the rutile phase of TiO2 NPs was obtained without hydrolysis. Immobilized-TiO2 NPs borosilicate glass (BG), cement coated borosilicate glass (CCBG), and steel wire mesh (SWM) reactors were constructed and the photocatalytic performance of TiO2 NPs was investigated to the degradation of methyl orange (MO) dye under solar irradiation. The photocatalytic removal of 10 mg/L MO dye was 97.8%, 88.6%, and 21.5%, respectively, at the same dose of immobilized-TiO2 on BG, CCBG, and SWM surfaces at pH 6.2 with 5 h of contact time. The kinetic study showed a pseudo-first-order reaction model fitted the best for MO dye removal. The increase of dye concentration decreases the performance of BG reactor, whereas, the addition of oxidant H2O2 increases the performance of BG reactor. The lowest TiO2 NPs loss from the study was observed for BG reactor (10.78%). This study indicates that the solar-TiO2 immobilized borosilicate glass reactor has high potential in the treatment of toxic compounds in textile wastewater.

Fine-control-valve of halide perovskite single crystal quality for high performance X-ray detection

Halide perovskite single crystals (HPSCs) provide a unique platform to study the optoelectronic properties of such emerging semiconductor materials, while the temperature induced crystal growth method often has an increased solute integration speed and/or unavoidable solute consumption, resulting in a soaring or slumping crystal growth rate of HPSCs. Here, we developed a universal and facile solvent-volatilization-limited-growth (SVG) strategy to finely control the crystal growth rate by the fine-control-valve for high quality crystal grown through solution processes. The grown HPSCs by SVG method exhibited a record low trap density of 2.8 × 108 cm−3 and a high charge carrier mobility-lifetime product (μτ product) of 0.021 cm2/V, indicating the excellent crystal quality. The crystal surface defects were further passivated by oxygen suppliers as Lewis base, which led to a reduction of surface leakage current by two times when using for low dose rate X-ray detection. Such HPSC X-ray detector displayed a high sensitivity of 1274 µC/(Gyair cm2) with a lowest detectable dose rate of 0.56 μGyair/s under 120 keV hard X-ray. Further applications including alloy composition analysis and metal flaw detection by HPSC detectors were also demonstrated, which not only shows the bright future for product quality inspection and non-destructive materials analysis, but also paves the way for growing high quality single crystals and fabricating polycrystalline films.