Optoelectronic Semiconductor Research Articles

Native point defects in 2D transition metal dichalcogenides: A perspective bridging intrinsic physical properties and device applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) hold immense promise as ultrathin-body semiconductors for cutting-edge electronics and optoelectronics. In particular, their sustained charge mobility even at atomic-level thickness as well as their absence of surface dangling bonds, versatile band structures, and silicon-compatibility integration make them a prime candidate for device applications in both academic and industrial domains. Despite such high expectations, group-VI TMDs reportedly exhibit a range of enigmatic properties, such as substantial contact resistance, Fermi level pinning, and limited unipolar charge transport, which are all rooted in their inherent defects. In other words, intrinsic physical properties resulting from their native defects extend their influence beyond the material level. Bridging point-defect-induced material properties and their behavior at the device level, this Perspective sheds light on the significance of crystalline defects beyond a rather simple defect–property relationship. As a distinctive approach, we briefly review the well-established defect model of conventional III–V semiconductors and further apply it to the emergent defect behaviors of 2D TMDs such as their defect-induced gap states. Within the main discussion, we survey a range of behaviors caused by the most prevalent intrinsic defect, namely, vacancies, within 2D TMDs, and their implications for electronic and optoelectronic properties when employed at the device level. This review presents an in-depth summary of complexities in material properties as well as device characteristics arising from intrinsic point defects and provides a solid foundation for the cross-links among native defects and material/device properties.

Low-Cost Nonfused-Ring Electron Acceptors Enabled by Noncovalent Conformational Locks.

ConspectusSince the first bilayer-structured organic solar cells (OSCs) in 1986, fullerenes and their derivatives have dominated the landscape for two decades due to their unique properties. In recent years, the breakthrough in nonfullerene acceptors (NFAs) was mainly attributed to the development of fused-ring electron acceptors (FREAs), whose photovoltaic performance surpassed that of fullerene derivatives. Through the unremitting efforts of the whole community, the power conversion efficiencies (PCEs) have surpassed 19% in FREA-based OSCs. However, FREAs generally suffered from complex synthetic approaches and high product costs, which hindered large-scale production. Therefore, many researchers are seeking a new type of NFA to achieve cost-effective, highly efficient OSCs.In collaboration with Marks and Facchetti in 2012, Huang et al. (Huang, H. J. Am. Chem. Soc. 2012, 134, 10966-10973, 10.1021/ja303401s) proposed the concept of "noncovalent conformational locks" (NoCLs). In the following years, our group has been focusing on the theoretical and experimental exploration of NoCLs, revealing their fundamental nature, formulating a simple descriptor for quantifying their strength, and employing this approach to achieve high-performance organic/polymeric semiconductors for optoelectronics, such as OSCs, thin-film transistors, room-temperature phosphorescence, and photodetectors. The NoCLs strategy has been proven to be a simple and effective approach for enhancing molecular rigidity and planarity, thus improving the charge transport mobilities of organic/polymeric semiconductors, attributed to reduced reorganization energy and suppressed nonradiative decay.In 2018, Chen et al. (Li, S. Adv. Mater. 2018, 30, 1705208, 10.1002/adma.201705208) reported the first example of nonfused-ring electron acceptors (NFREAs) with intramolecular noncovalent F···H interactions. The NoCLs strategy is essential in NFREAs, as it simplifies the conjugated structures while maintaining high coplanarity comparable to that of FREAs. Due to their simple structures and concise synthesis routes, NFREAs show great potential for achieving cost-effective and highly efficient OSCs. In this Account, we provide an overview of our efforts in developing NFREAs with the NoCLs strategy. We begin with a discussion on the distinct features of NFREAs compared with FREAs, and the structural simplification from FREAs to NFREAs to completely NFREAs. Next, we examine several selected typical examples of NFREAs with remarkable photovoltaic performance, aiming to provide an in-depth exploration of the molecular design principle and structure-property-performance relationships. Then, we discuss how to achieve a balance among efficiency, stability, and cost through a two-in-one strategy of polymerized NFREAs (PNFREAs). Finally, we offer our views on the current challenges and future prospects of NFREAs. We hope this Account will trigger intensive research interest in this field, thus propelling OSCs into a new stage.

Multiphoton photocurrent in wide bandgap semiconductors for nonlinear optoelectronics: Comparison of GaP, GaN/InGaN, and SiC

Wide bandgap semiconductors are ideally suited for nonlinear optoelectronics. Because their bandgaps are larger than 2 eV, simultaneous absorption of two or more near-infrared photons is necessary to excite the electrons from the valence to the conduction band. Understanding of the processes that affect multiphoton absorption is important in the design and fabrication of optoelectronic devices. Here, we present an overview of the photocurrent response in photodetectors made from GaP, GaN, InGaN, and SiC when they are excited by photons at 1.2 eV. Recent measurements have shown that sub-bandgap absorptions contribute to photocurrent in GaP, and, thus, it is not a good material for nonlinear optoelectronics. Similarly, the response of GaN is affected by long-lived trapped charges. Photocurrents in InGaN and SiC are predominantly from three- and four-photon absorption, respectively. Moreover, these materials can withstand excitation intensities higher than 1011 W cm−2, making them appropriate platforms for nonlinear optoelectronics.

Toward Smart, Flexible, and Omnidirectional Self-Powered Photodetection by an All-Solution-Processed In2O3/Pbl2 Heterojunction.

Amorphous In2O3 film is emerging as a promising oxide semiconductor for next-generation electronics and optoelectronics owing to high mobility and wide band gap. However, the persistent photocurrent phenomenon and high carrier concentration in amorphous In2O3 film are challenging the photodetection performances, resulting in a long response time and low Ilight/Idark ratio. In this work, the In2O3/PbI2 heterojunction is constructed by an all-solution synthesis process to inhibit the persistent photocurrent phenomenon and large dark current. Benefiting from the built-in electric field at the heterojunction interface, the In2O3/PbI2 heterojunction photodetector exhibits excellent self-powered photodetection performances with an ultralow dark current of 10-12 A, a high Ilight/Idark ratio of 104, and fast response times of 0.6/0.6 ms. Furthermore, the entire solution synthesis process and amorphous characteristics enable the fabrication of an In2O3/PbI2 heterojunction photodetector on arbitrary substrates to realize specific functions. When configured onto the polyimide substrate, the In2O3/PbI2 heterojunction photodetector shows excellent mechanical flexibility, bending endurance, and photoresponse stability. When implanted onto the transparent substrate, the In2O3/PbI2 heterojunction photodetector exhibits an outstanding omnidirectional self-powdered photodetection performance and imaging capability. All results pave the way for an all-solution-processed amorphous In2O3 film in advanced high-performance photodetectors.

Recent developments in perovskite materials, fabrication techniques, band gap engineering, and the stability of perovskite solar cells

Organic-inorganic hybrid metal halide perovskite solar cells (PSC) represent a novel class of optoelectronic semiconductors that have garnered significant attention from photovoltaic researchers globally. This is due to their continually improving efficiency, straightforward solution processing methods, use of lightweight and cost-effective materials, and other notable features. The advantageous properties of perovskite materials, such as superior charge transport, tunable band gap, and distinctive electronic structure, contribute to their appeal. Over the past 6 to 7 years, diverse device architectures and fabrication techniques for PSC have emerged, achieving an impressive power conversion efficiency (PCE) of 25.7%. This review article primarily focuses on recent advancements in PSC fabrication techniques, synthesis, device architecture, charge transport mechanisms, band gap engineering, and stability. Additionally, it provides a summary of recently reported materials used in various layers of PSC, including the perovskite layer, as well as electron and hole transport layers. Lastly, the article outlines the challenges faced in the development of PSC, offers recommendations, and suggests potential directions for future research to guide the field forward.

Cu-doped lithium oxide films with high mobility and bandgap prepared by pulsed direct-current sputtering

In this study, Cu-doped lithium carbonate (Cu:Li2O2) films are prepared by pulsed direct-current (DC) magnetron sputter using a lithium carbonate and copper oxide mixed target. The oxygen flow rate is varied during the sputtering process to systematically investigate its effect on structural, electrical and optical properties of the films. The experimental results show that all the Cu:Li2O2 films are mainly amorphous as confirmed by X-ray diffraction. Cu incorporation is able to reduce the aggregated clusters of the lithium oxides. At the oxygen flow rate of 20 sccm the film with a Cu content of ∼0.7 at.% has a low p-type resistivity of 0.82 Ω-cm, hole concentration of 1.1 × 1017 cm−3 and the highest mobility of 73 cm2V−1s−1, which is at least twice higher than those of other p-type metal oxides prepared at similarly low temperatures. The film also has a high optical band gap of 2.87 eV. Moreover, the Cu:Li2O2/ZnO heterojunction demonstrating a clear diode rectification supports the p-type nature of Cu:Li2O2 films. The pulsed DC Cu:Li2O2 films with these remarkable optoelectronic properties demonstrate great potential to be adopted as transparent oxide semiconductors in high-performance optoelectronics.

A REVIEW OF EPOXY-NANOCOMPOSITE PROPERTIES

Epoxy resins have been the subject of many studies as a consequence of their extensive usage in recent years. The brittleness and low resistance to propagation and crack initiation of epoxy resins are well-recognized characteristics. Therefore, in recent years, experts have concentrated on increasing epoxy resin's fracture resistance. Adding inorganic nanoparticles like titanium dioxide (TiO2) TiO2, silica (SiO2), carbon black, alumina (Al2O3), and others to the polymer matrix is one of the most investigated techniques in polymer science. Despite having a modest nanofiller content, the resulting nanocomposites may enhance their thermal, mechanical, rheological, electrical, and optical characteristics. These nanocomposites are an alternative to metal-based materials. They have great promise as multifunctional materials in a range of applications, including optoelectronic devices, semiconductor devices, civil engineering, automotive, and aerospace. To show potential future directions and market prospects for polymer nanocomposites reinforced with TiO2 nanoparticles, current results, and trends have been examined and highlighted. In addition, the current review surveys many studies that highlighted using nanoparticles as reinforcement, their different structure, the interface, and the geometry and structure of the resulting nano-materials reinforced resin.

Cesium lead bromide perovskite nanocrystals synthesized via supersaturated recrystallization at room temperature: comparison of one-step and two-step processes.

Over more than a decade, lead halide perovskites (LHPs) have been popular as a next-generation semiconductor for optoelectronics. Later, all-inorganic CsPbX3 (X = Cl, Br, and I) nanocrystals (NCs) were synthesized via supersaturated recrystallization (SR) at room temperature (RT). However, compared to the hot injection (HI) method, the formation mechanism of NCs via SR-RT has not been well studied. Hence, this study will contribute to elucidating SR-RT based on the LaMer model and Hansen solubility parameter. Herein, we also demonstrate the entropy-driven mixing between two dissimilar polar-nonpolar (DMF-toluene) solvents. Next, we find that, in a poor solvent (toluene ≫ DMF in volume), ∼60 nm sized CsPbBr3 NCs were synthesized in one step, whereas in a marginal solvent (toluene ≈ DMF), ∼3.5 nm sized NCs were synthesized in two steps, indicating the importance of solvent polarity, specifically the 'solubility parameter'. In addition, in the presence of a CuBr2 additive, high-quality cubic NCs (with ∼3.8 nm and ∼21.4 nm edge sizes) were synthesized. Hence, through this study, we present a 'solubility parameter-based nanocrystal-size control model' for SR-RT processes.

Achievements and prospects: 25 years of SPQEO journal

The Ukrainian journal Semiconductor Physics, Quantum Electronics and Optoelectronics (SPQEO) was launched in 1998 artificially combining three main areas of scientific activity inherent to the V. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, namely semiconductor physics, quantum electronics and optoelectronics. After a decade this artificial base turned into some kind of organic symbiosis, namely: (i) the main optoelectronic systems are based on semiconductor devices, for example, fiber-optic networks; (ii) semiconductor lasers dominate in a huge number of applications in quantum electronics; (iii) semiconductor physics proposes new types of LEDs with extremely high efficiency, and so on. This article is dedicated to the 25 th anniversary of SPQEO. According to the Google Scholar Citation statistics, above 1680 articles cited more than 7350 times in total were published in the journal from 1998 to 2023. The statistics of references of journal articles and the scientific areas of the most cited articles are presented.

Molecular Design of Multifunctional Integrated Polymer Semiconductors with Intrinsic Stretchability, High Mobility, and Intense Luminescence.

Multifunctional semiconductors integrating unique optical, electrical, mechanical, and chemical characteristics are critical to advanced and emerging manufacturing technologies. However, due to the trade-off challenges in design principles, fabrication difficulty, defects in existing materials, etc., realizing multiple functions through multistage manufacturing is quite taxing. Here, an effective molecular design strategy is established to prepare a class of multifunctional integrated polymer semiconductors. The pyridal[1,2,3]triazole-thiophene co-structured tetrapolymers with full-backbone coplanarity and considerable inter/intramolecular noncovalent interactions facilitate short-range order and excellent (re)organization capability of polymer chains, providing stress-dissipation sites in the film state. The regioregular multicomponent conjugated backbones contribute to dense packing, excellent crystallinity, high crack onset strain over 100%, efficient carrier transport with mobilities exceeding 1cm2 V-1 s-1 , and controllable near-infrared luminescence. Furthermore, a homologous blending strategy is proposed to further enhance the color-tunable luminescent properties of polymers while effectively retaining mechanical and electrical properties. The blended system exhibits excellent field-effect mobility (µ) and quantum yield (Φ), reaching a record Φ · µ of 0.43cm2 V-1 s-1 . Overall, the proposed strategy facilitates a rational design of regioregular semicrystalline intrinsically stretchable polymers with high mobility and color-tunable intense luminescence, providing unique possibilities for the development of multifunctional integrated semiconductors in organic optoelectronics.

New Applications of Thermomigration in Semiconductor Technologies and Optoelectronics Developments (Review)

New Applications of Thermomigration in Semiconductor Technologies and Optoelectronics Developments (Review)

Synthesis and investigation of the structure, thermal, optical, and electrical properties of poly (indol-thiazolo-thiadiazole) derivatives at room temperature

This research work is intended for the novel synthesis and study of structural, thermal, optical, electrical, and dielectric properties of semiconductor poly (2-amino-5-(1H-indolyl)-5H-thiazolo[4,3-b]-1,3,4-thiadiazole) (PAITTD) and its N-substituted indole derivatives with a side chain containing chlorobenzoyl (PACBITTD) and bromobenzene sulphonyl (PABBSITTD). The characterization techniques such as H1NMR, FTIR, SEM, and TGA/DSC ensured the structural and thermal properties of the as-synthesized polymers. In addition, the molecular weight of the PAITTD, PACBITTID, and PABBSITTD was calculated via H1NMR technique. The optical band gap as obtained from the absorbance study was found to be in the range of 1.5 eV to 2.58 Ev. Moreover, the ground-state geometries and the absorption wavelengths for the studied compounds were investigated by DFT and TD-DFT, where the theoretical results show coincidence with these of the experimental where the calculated (λmax) values are approximately near to the experimental (λmax) values for the PAITTD, PACBITTID, and PABBSITTD. The impedance ac conductivity and dielectric properties of the PAITTD, PACBITTID, and PABBSITTD were performed in the frequency ranges from 20 Hz to 10 MHz at room temperature, where their values were 3.0 × 10−7, 8.7 × 10−7, and 2.6 × 10−6, respectively. The Nyquist plot confirmed the presence of both grain and grain boundary effects for conducting PAITTD, PACBITTID, and PABBSITTD, and Double relaxation behavior was also observed. The variation of ac conductivity with frequency obeys the modification of Jonscher’s universal power law (JUPL). Based on the optical and electrical properties, synthetic PAITTD, PACBITTID, and PABBSITTD are promising materials in the design of optoelectronic semiconductor devices.

Self-supervised deep learning for tracking degradation of perovskite light-emitting diodes with multispectral imaging

Emerging functional materials such as halide perovskites are intrinsically unstable, causing long-term instability in optoelectronic devices made from these materials. This leads to difficulty in capturing useful information on device degradation through time-consuming optical characterization in their operating environments. Despite these challenges, understanding the degradation mechanism is crucial for advancing the technology towards commercialization. Here we present a self-supervised machine learning model that utilizes a multi-channel correlation and blind denoising to recover images without high-quality references, enabling fast and low-dose measurements. We perform operando luminescence mapping of various emerging optoelectronic semiconductors, including organic and halide perovskite photovoltaic and light-emitting devices. By tracking the spatially resolved degradation in electroluminescence of mixed-halide perovskite blue-light-emitting diodes, we discovered that lateral ion migration (perpendicular to the external electric field) during device operation triggers the formation of chloride-rich defective regions that emit poorly—a mechanism that would not be resolvable with conventional imaging approaches.

Luminescent Behavior of Sb3+-Activated Luminescent Metal Halide.

Metal halide perovskites have unparalleled optoelectronic properties and broad application potential and are expected to become the next epoch-making optoelectronic semiconductors. Although remarkable achievements have been achieved with lead halide perovskites, the toxicity of lead inhibits the development of such materials. Recently, Sb3+-activated luminescent metal halide perovskite materials with low toxicity, high efficiency, broadband, large Stokes shift, and emission wavelengths covering the entire visible and near-infrared regions have been considered one of the most likely luminescent materials to replace lead halide perovskites. This review reviews the synthesis, luminescence mechanism, structure, and luminescence properties of the compounds. The basic luminescence properties of Sb3+-activated luminescent metal halide perovskites and their applications in WLED, electroluminescence LED, temperature sensing, optical anti-counterfeiting, and X-ray scintillators are introduced. Finally, the development prospects and challenges of Sb3+-activated luminescent metal halide perovskites are discussed.

Lead‐Free Halide Perovskite Materials and Optoelectronic Devices: Progress and Prospective

AbstractHalide perovskites, in the form of thin films and colloidal nanocrystals, have recently taken semiconductor optoelectronics research by storm, and have emerged as promising candidates for high‐performance solar cells, light‐emitting diodes (LEDs), lasers, photodetectors, and radiation detectors. The impressive optical and optoelectronic properties, along with the rapid increase in efficiencies of solar cells and LEDs, have greatly attracted researchers across many disciplines. However, most advances made so far in terms of preparation (colloidal nanocrystals and thin films), and the devices with highest efficiencies are based on Pb‐based halide perovskites, which have raised concerns over their commercialization due to the toxicity of Pb. This has triggered the search for lower‐toxicity Pb‐free halide perovskites and has led to significant progress in the last few years. In this roadmap review, researchers of different expertise have joined together to summarize the latest progress, outstanding challenges, and future directions of Pb‐free halide perovskite thin films and nanocrystals, regarding their synthesis, optical spectroscopy, and optoelectronic devices, to guide the researchers currently working in this area as well as those that will join the field in the future.

Defect Emission and Its Dipole Orientation in Layered Ternary Znln2 S4 Semiconductor.

Defect engineering is promising to tailor the physical properties of 2D semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors are not clarified. Here, the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2 S4 is thoroughly studied via versatile spectroscopic tools in combination with theoretical calculations. It is demonstrated that the Zn-In antistructural defects induce the formation of a series of donor and acceptor energy levels and sulfur vacancies induce donor energy levels, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair in Znln2 S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef ). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2 S4 is directly revealed by back focal plane imagining, where it presents obviously in-plane dipole orientation within a dozen-layer thickness of Znln2 S4 . These unique features of defects in Znln2 S4 including extrinsic absorption, rich recombination paths, gate tunability, and in-plane dipole orientation are definitely a benefit to the advanced orientation-functional optoelectronic applications.

Enhanced Photoluminescence and Prolonged Carrier Lifetime through Laser Radiation Hardening and Self-Healing in Aged MAPbBr3 Perovskites Encapsulated in NiO Nanotubes.

Organic-inorganic perovskites hold great promise as optoelectronic semiconductors for pure color light emitting and photovoltaic devices. However, challenges persist regarding their photostability and chemical stability, which limit their extensive applications. This paper investigates the laser radiation hardening and self-healing-induced properties of aged MAPbBr3 perovskites encapsulated in NiO nanotubes (MAPbBr3@NiO) using photoluminescence (PL) and fluorescence lifetime imaging (FLIM). After deliberately subjecting the MAPbBr3@ NiO to atmospheric conditions for two years, the sample remains remarkably stable. It exhibits no changes in PL wavelength during UV laser irradiation and self-healing. Furthermore, exposure to UV light at 375 nm enhances the PL of the self-healed MAPbBr3@NiO. FLIM analysis sheds light on the mechanism behind photodegradation, self-healing, and PL enhancement. The results indicate the involvement of many carrier-trapping states with low lifetime events and an increase in peak lifetime after self-healing. The formation of trapping states at the perovskite/nanotube interface is discussed and tested. This study provides new insights into the dynamics of photo-carriers during photodegradation and self-healing in organic-inorganic perovskites.

(Electronics and Photonics Division Award) Epitaxy of Group-IV Semiconductors for Nanoelectronics and Optoelectronics

Epitaxy stands for the deposition of a perfectly ordered layer on a monocrystalline substrate, which is typically, for group-IV semiconductors, bulk Si or Silicon-On-Insulator (SOI). If the layer is chemically different from the substrate and lattice parameters are not the same, epitaxy can be of two types. If the atomic columns of the substrate extend into the layer with the same in-plane lattice parameter, the layer will be pseudomorphic. The built-in strain will be compressive if the layer has a larger lattice parameter than that of the substrate or tensile if it is lower. If the layer thickness exceeds the so-called critical thickness for plastic relaxation, it will relax, with the formation of misfit dislocations to accomodate the lattice parameter mimatch.Epitaxy of group-IV semiconductors is key for numerous Si-based nanoelectronics and optoelectronics devices. Although they crystallize in the same diamond structure, C, Si, Ge and Sn bandgaps (5.47, 1.12, 0.66 and -0.41 eV), lattice parameters (3.57, 5.43, 5.66 and 6.49 Å) and fusion temperatures (> 4000°C, 1414°C, 938°C and 232°C) are very different. This can be an advantage (bandgap and mobility engineering in devices) or a difficulty (when growing stacks). Finally, although Si and Ge are miscible in all proportions, solid solubilities of C in Si and Sn in Ge are exceedingly low (less than 10-4 % and 0.8%, respectively). Low growth temperatures and high growth rates will then be mandatory in order to have high quality alloys.In that presentation, I will show how epitaxy can be put to good use in order to boost the properties of devices. Layers or stacks were grown on 200 mm and 300 mm diameter wafers in industrial Reduced-Pressure – Chemical Vapour Deposition tools with temperatures in the 300°C-900°C range and pressures in the 20-100 Torr range, typically. Precursors were mono-methysilane (for C), silane, dichlorosilane or disilane (for Si), germane or digermane (for Ge) and tin tetrachloride (for Sn). Finally, diborane and phosphine were the P-type and N-type dopant precursors, respectively.The electrical properties of p-type Metal Oxyde Semiconductor – Field Effect Transistors (MOSFETs) can be improved by the insertion of a thin pseudomorphic SiGe layer beneath the gate of transistors. The compressive strain in it will boost by a factor of ~2 the hole mobility in pMOSFETs. The use of SiGe:B in the sources and drains regions on each side of such devices will help in minimizing the contact resistivity and increasing the compressive strain in the channel. Ramping-up, at high temperatures, the Ge concentration in microns thick layers grown on Si and capping them with constant composition layers will result in SiGe virtual substrates which are almost fully relaxed (e.g. with a lattice parameter close to that of bulk SiGe), slightly defective (Threading Dislocations Density (TDD) around 105 cm-2) and with surface cross-hatches. Growing thin tensily-strained Si layers on top and transferring them onto oxydized Si will result in strained SOI substrates, with an increase by a factor of two of the electron mobility in n-type MOSFETs built on top. Using low contact resistance tensily-strained SiP sources and drains will further boost nMOSFET performances.Microns-thick Ge layers grown directly on Si thanks to a low temperature/high temperature approach can be used as templates for III-V epitaxy. The surface root mean square roughness will be less than 1 nm and the TDD around 107 cm-2 with some short duration thermal cycling. Ge can also be grown selectively in trenches at the end of Si waveguides and near infra-red photo-detectors processed. Microns-thick SiGe layers (capped with Si) and pure Ge layers (capped with SiGe) can otherwise be used to fabricate mid and long Infra-Red waveguides. Mid-IR supercontinuum light was generated when sending ultrashort light pulses in such waveguides. Finally, germanium can be transformed into a direct bandgap semiconductor thanks to high amounts of tensile strain or some alloying with more than 8% of tin. Optically pumped lasing at room temperature was recently achieved in bonded or pillared micro-disks made of thick, plastically relaxed GeSn 16-17% layers.Finally, the feasability of selectively etching SiGe versus Si can be used to fabricate stacked nanosheet MOSFETs, with superior ION/IOFF tradeoffs for a given footprint. The core structures of such devices are epitaxial Si/SiGe superlattices, with the alternance of typically ten nm thick Si and SiGe layers. SiGe layers are later on selectively etched. The resulting voids are conformally filled with gate dielectrics and metals. We recently succeeded in stacking seven Si nanosheets, with a record nMOSFET saturation current of 2.8mA/µm for a 1V drain voltage. Figure 1

Ultraviolet photodetectors based on ZnO nanowires with SiO2/ZnO multilayers

Ultraviolet photodetectors (UV PDs) have been a frequently studied focus of optoelectronic semiconductor devices because of their large range of applications. As one of the most famous materials in third-generation semiconductors, ZnO-based UV PDs have received a great deal of attention in various research areas. Due to the carrier transport channel and higher exciton bond energy, ZnO nanowires (NWs) have better photoelectron sensitivity under UV light than ZnO films. Here, well-organized ZnO NWs were fabricated on ZnO-seeded substrates via a hydrothermal method. Multilayers of double SiO2/ZnO(S/Z) layers were designed on the NWs, and the Schottky barrier was introduced by the close contact of the ZnO NWs and gold electrodes to improve the performance of ZnO UV PDs. Characteristics of the dark current, photocurrent, rise time, decay time, photo-to-current ratio (S), and responsivity values (Rs) of the PDs were studied. Furthermore, multilayer UV PDs were achieved on both Si and quartz substrates. S of the ZnO S/Z-based UV PDs fabricated on a silica substrate can reach 55.57, which is better than the values of ZnO UV PDs reported previously. Compared with silica-based UV PDs with S/Z layers, quartz-based UV PDs with S/Z layers show a higher performance of Rs, which reach 12.62 A/W. ZnO NWs with multilayers and the Schottky contact between ZnO NWs and Au are promising candidates in high-sensitivity UV PDs.

Prospectives for AlN electronics and optoelectronics and the important role of alternative synthesis

Future applications for emerging AlN semiconductor electronics and optoelectronics are facilitated by emerging doping technologies enabled by low temperature, non-equilibrium epitaxy. Defect and impurity compensation can be reduced by controlling the surface chemistry with reducing compensating vacancy concentrations being a key driver for lower temperature growth. Contrary to common understanding, low temperature, metal-rich vacuum processes are shown to have higher diffusion lengths than high temperature nitrogen-rich methods. This feature can be utilized to inhibit silicon-DX center formation without compromises in crystal quality. First principles calculations identify the valence split-off band as the dominant hole band contributing to impurity band formation (as opposed to the heavy and light hole bands in other nitrides). This anomalous band structure causes an impurity band to form at dopant concentrations similar to GaN even though AlN has a deeper isolated acceptor energy and results in hole mobilities that are substantially higher than possible in GaN. AlN hole concentrations of ∼4.4 × 1018 cm−3 and 0.045 Ω cm resistivity and electron concentrations of ∼6 × 1018 cm−3 and ∼0.02 Ω cm resistivity are shown and offer substantial promise for future generations of AlN bipolar electronic and optical devices.