As-grown Material Research Articles

Few-layer Bi2O2Se: a promising candidate for high-performance near-room-temperature thermoelectric applications

Advancements in high-temperature thermoelectric (TE) materials have been substantial, yet identifying promising near-room-temperature candidates for efficient power generation from low-grade waste heat or TE cooling applications has become critical but proven exceedingly challenging. Bismuth oxyselenide (Bi2O2Se) emerges as an ideal candidate for near-room-temperature energy harvesting due to its low thermal conductivity, high carrier mobility and remarkable air-stability. In this study, the TE properties of few-layer Bi2O2Se over a wide temperature range (20-380 K) are investigated, where a charge transport mechanism transitioning from polar optical phonon to piezoelectric scattering at 140 K is observed. Moreover, the Seebeck coefficient (S) increases with temperature up to 280 K then stabilizes at∼-200μV K-1through 380 K. Bi2O2Se demonstrates high mobility (450 cm2V-1s-1) within the optimum power factor (PF) window, despite itsT-1.25dependence. The high mobility compensates the minor reduction in carrier densityn2Dhence contributes to maintain a robust electrical conductivity∼3 × 104S m-1. This results in a remarkable PF of 860μW m-1K-2at 280 K without the necessity for gating (Vg= 0 V), reflecting the innate performance of the as-grown material. These results underscore the considerable promise of Bi2O2Se for room temperature TE applications.

Synthesis of High-Quality Bulk Single-Crystal Black Phosphorus by the Circulating Vapor Growth Approach.

Black phosphorus (BP), a promising two-dimensional (2D) layered semiconductor material, has gained enormous attention due to its impressive properties over the past several years. Although plenty of methods have been developed to synthesize high-quality BP, most of the currently available BP materials still suffer from unsatisfactory crystallization, purity, and stability in air, hindering their practical application. A facile approach to synthesizing ultrahigh-quality single-crystal BP is of significance to shed light on the nature of 2D semiconductor materials and their massive application. In this work, we present the facile and efficient circulating vapor growth approach to growing bulk single-crystal BP. The as-grown BP material features high crystallinity and ultrahigh purity (higher than 99.999 at %), exceeding those of all the previously reported and some commercially available BP crystals. It also maintains excellent stability in air and water after 15 consecutive days of test. Moreover, the as-synthesized BP material features good thermal stability, oxidation resistance, and excellent electrical properties, as well. This study provides a new approach for the fabrication of ultrahigh-quality BP material and thus promotes its application.

Room-Temperature Growth of Square-Millimeter Single-Crystalline Two-Dimensional Metal Halides on Silicon.

Silicon is the cornerstone of electronics and photonics. In this context, almost all integrated devices derived from two-dimensional (2D) materials stay rooted in silicon technology. However, as the growth substrate, silicon has long been thought to be a hindrance for growing 2D materials through bottom-up methods that require high growth temperatures, and thus, indirect routes are usually considered instead. Although promising growth of large-area 2D materials on silicon has been demonstrated, the direct growth of single-crystalline materials using low-thermal-budget synthesis methods remains challenging. Here, we report the room-temperature growth of millimeter-scale single-crystal 2D metal halides on silicon substrates with a hydroxyl-terminated surface. Theoretical calculations reveal that the activation energy for surface diffusion can be reduced by an order of magnitude by terminating the surface with hydroxyl groups, from which on-silicon growth is greatly facilitated at room temperature and enables a 4-order-of-magnitude increase in area. The high quality and uniformity of the resulting single crystals are further evidenced. The optoelectronic devices employing the as-grown materials show an ultralow dark current of 10-13 A and a high detectivity of 1013 Jones, thereby corroborating a weak-light detection ability. These results would point to a rich space of surface modulation that can be used to surmount current limitations and demonstrate a promising strategy for growing 2D materials directly on silicon at room temperature to produce large single crystals.

Growth and characterization of physical properties of photovoltaic (K,Ba)(Ni,Nb)O3 single crystals

The substitution of fossil energy for green-energy sources is a world demand. Photovoltaic devices are a well-known safety solution to convert solar energy into electricity. Ferroelectric materials are one of the most promising candidates for substituting Si-based compounds in solar-energy conversion devices due to their relatively low cost and bulk photovoltaic effect instead of the classical p-n junction. In this work, (K,Ba)(Nb,Ni)O3 single crystal was grown by vertical Bridgman-Stockbarger method, and structural, chemical, and optical properties of the as-grown material were characterized. Growth parameters were chosen based on the thermal properties of the precursor oxide and on theoretical models.

Thermal stability, work function and Fermi level analysis of 2D multi-layered hexagonal boron nitride films

Thermal stability and Fermi level analysis of commercial multilayer (ML) and as-grown 10 nm thick hexagonal boron nitride (hBN) films on Cu foil by atmospheric chemical vapor deposition (APCVD) by heating in N2 and air at temperatures ≤900 °C are studied. For all samples, progressive surface oxidation was observed correlating with higher hBN nucleation density. Thermal stability was measured in air from 200 °C to 400 °C (relative humidity ∼65%) where commercial ML and APCVD material degraded; BN peaks begin to be replaced by BO peaks at temperatures >300 °C. It is shown that hBN thermal stability is sensitive to Tgrowth and Cu foil surface preparation methods such as electropolishing (EP). We evaluate as-grown material for different Tgrowth conditions (1000 °C, 1030 °C, 1050 °C and 1060 °C) by ultraviolet photoelectron spectroscopy (UPS) to locate and understand subsequent Fermi level and work function variation, confirming our material is in general n-type, which becomes increasingly so as thermal stability regresses. To this effect, we observe a work function increase of 0.45 eV as BO increases with in-situ anneal temperature. An average electron affinity of 2.13 eV and 1.7 eV is determined for commercial hBN and APCVD material, respectively. Further work is required by material scientists to optimize the material properties of CVD hBN.

Synthesis, experimental and theoretical studies of morpholinium bromide single crystal for NLO application

Due to the electronic applications, non-linear optical (NLO) materials are receiving research attention. A non-linear optical material, morpholinium bromide (MBr) single crystal with dimensions of 14 × 9 × 3 mm3 was grown successfully by slow evaporation solution growth technique. The synthesized crystal belongs to monoclinic crystal with space group P21/c, which was confirmed through single crystal XRD (SXRD) analysis. Further, powder XRD analysis was used to record several dispersion peaks and the presence of dispersion peak positions for the as-grown material is correlated with the scattering peak state. The optimised molecular structure, HOMO-LUMO, NBO, MEP and Mulliken atomic charges of MBr crystal were theoretically investigated using B3LYP with a 6–311 G ++(d, p) basis set. FTIR studies revelas the functional components present in the grown material. The optical band gap was found to be 3.3 eV with a transmittance cut-off wavelength of 362 nm. TG-DTA analysis was done to understand the as-grown MBr crystal's temperature withstanding capability by assessing the weight loss associated with the corresponding decomposition peak. The potential of the crystal to stand up to laser power is investigated using laser damage threshold (LDT) studies. The Z-scan study is implemented to the polished MBr crystal to assess its nonlinear refractive index (n2), absorption coefficient (β) and susceptibility χ(3) for its usefulness in optoelectronic applications.

Evaluation of electron lifetime for Te inclusions free CdZnTe

Te secondary phase defects are known as major obstacle for improving charge carriers transport properties of CdTe compounds material. The enhanced detector performance was reported for Te inclusions free CZT detector implemented through two-step annealing (i.e., first in Cd and second in Te). We applied the time-of-flight (TOF) technique to measure the electron lifetime in as-grown and two-step annealed CZT material. TOF has several advantages over Hecht equation fitting such as independence of the electric field distribution, direct measurement of carrier lifetime, and high accuracy for high mobility-lifetime product CZT material. The detector performance improved drastically for the two-step annealed CZT detector except which containing inordinate number and size of prismatic punching defects. The average electron lifetime increased more than 10 times after successful two-step annealing.

Efficient Activation and High Mobility of Ion-Implanted Silicon for Next-Generation GaN Devices

Selective area doping via ion implantation is crucial to the implementation of most modern devices and the provision of reasonable device design latitude for optimization. Herein, we report highly effective silicon ion implant activation in GaN via Symmetrical Multicycle Rapid Thermal Annealing (SMRTA) at peak temperatures of 1450 to 1530 °C, producing a mobility of up to 137 cm2/Vs at 300K with a 57% activation efficiency for a 300 nm thick 1 × 1019 cm−3 box implant profile. Doping activation efficiency and mobility improved alongside peak annealing temperature, while the deleterious degradation of the as-grown material electrical properties was only evident at the highest temperatures. This demonstrates efficient dopant activation while simultaneously maintaining low levels of unintentional doping and thus a high blocking voltage potential of the drift layers for high-voltage, high-power devices. Furthermore, efficient activation with high mobility has been achieved with GaN on sapphire, which is known for having relatively high defect densities but also for offering significant commercial potential due to the availability of cheap, large-area, and robust substrates for devices.

Improved performance of InGaAs/AlGaAs quantum well lasers on silicon using InAlAs trapping layers.

InGaAs/AlGaAs multiple quantum well lasers grown on silicon (001) by molecular beam epitaxy have been demonstrated. By inserting InAlAs trapping layers into AlGaAs cladding layers, misfit dislocations easily located in the active region can be effectively transferred out of the active region. For comparison, the same laser structure without the InAlAs trapping layers was also grown. All these as-grown materials were fabricated into Fabry-Perot lasers with the same cavity size of 20 × 1000 µm2. The laser with trapping layers achieved a 2.7-fold reduction in threshold current density under pulsed operation (5 µs-pulsed width, 1%-duty cycle) compared to the counterpart, and further realized a room-temperature continuous-wave lasing with a threshold current of 537 mA which corresponds to a threshold current density of 2.7 kA/cm2. When the injection current reached 1000 mA, the single-facet maximum output power and slope efficiency were 45.3 mW and 0.143 W/A, respectively. This work demonstrates significantly improved performances of InGaAs/AlGaAs quantum well lasers monolithically grown on silicon, providing a feasible solution to optimize the InGaAs quantum well structure.

UV LEDs and Their Applications

The purpose of this research is to design and develop a semiconductor light source emitting ultraviolet (UV) light. The final device will then be integrated into a biomedical application to assist clinicians in patient diagnosis. A light emitting diode (LED) is the specific UV source of interest, which has been the subject of major research and development over the past 20 years, with applications spanning surface disinfection to biomedical imaging. To realise the UV LED, the semiconductor structure must first be ‘grown’ which is a research area in and of itself. This project focuses specifically on taking the as-grown material of the LED and forming individual functioning devices, characterisation and then further optimisation of the growth. Challenges currently being faced include the physical constraints (electrical and optical properties) of certain materials, and extracting as much UV light as possible from the device. With 2 years left in the project, these challenges will hopefully be overcome to produce a high efficiency UV LED, integrated into a biophotonics diagnostic tool.

Nano-LED driven phase change evolution of layered chalcogenides for Raman spectroscopy investigations

We present a device driving testing platform based on vertically integrated nano light emitting diodes (nano-LEDs). The nano-LEDs with a peak wavelength emission centered at ∼ 445 nm were arranged in arrays and conditioned using a laser-micro-annealing process to individually tune their intensity. They were coupled with freestanding monocrystalline Ge1Sb2Te4 nano-membranes with three different thicknesses (∼40, ∼60 and ∼ 90 nm) with the aim of initializing ultrafast switching processes and of observing phase changed states simultaneously by Raman spectroscopy. Raman spectroscopy studies reveal that the optical pulses emitted from the nano-LEDs induce substantial, local changes in the nano-membranes’ states of the Ge1Sb2Te4 layered material. Beside the crystalline state in non-exposed areas (as-grown material), amorphous and different intermediate states were identified in exposed areas as island-like structures with diameters ranging from ∼ 300 nm up to ∼ 1.5 µm. The latter confirms the nano-LEDs’ emission role in both near- and far-field regimes, depending on the distance between nano-LED and nano-membrane, for driving i.e. inducing the phase change process. The results presented demonstrate the suitability and potential of the vertically integrated nano-LEDs as the key components for a testing platform/for electro-optical convertors driving phase change processes in optically active media. They could also play an important role in the development of future, e.g., non-volatile data storage as well as in optical and neuromorphic computing architectures based on transmistor devices.

A complete solid solution between GeO2 and SiO2 with the α-quartz structure: Additional X-ray diffraction data from Ge1−xSixO2 flux-grown crystals

Millimeter-sized crystals of formula Ge1−xSixO2, i.e. GeO2 materials partially substituted by SiO2, were obtained for the first time both in the middle and the Si-rich parts of the GeO2–SiO2 composition domain (0.5 ​≤ ​x ​≤ ​0.93) by spontaneous nucleation using the high-temperature flux method. The X-ray crystal diffraction (XRD) data at room temperature confirm the non-centrosymmetric α-quartz-type structure and the low level of defects for these as-grown materials. The silicon content in the Ge1−xSixO2 crystals was determined by interpolation from the inter-tetrahedral bridging angle. The complete mutual solubility is confirmed between α-SiO2 and α-GeO2. The deviation from the regularity of the TO4 tetrahedra (T ​= ​Si, Ge) decreases linearly when Ge is gradually replaced by Si.

Design and synthesis of highly efficient nitrogen-doped carbon nano-onions for asymmetric supercapacitors

The preparation of highly graphitic carbon materials encourages the synthesis of sustainable energy storage for fast charging power applications. In this consideration, at present work, porous nitrogen-doped carbon nano-onions (N-CNO) were prepared by a different strategic, simple, low-cost, one-step pyrolysis technique. The fabricated material was characterized by various physical and electroanalytical measurements. The D, G, and 2D bands from Raman and pyridinic-N, pyrrolic-N, graphitic-N, and pyridinic-N-O from the X-ray photoelectron spectroscopy confirmed the graphitic nature of the N-CNO material. The results of specific capacitance for the as-grown materials containing N-CNO at a current density of 1 A/g have achieved a specific capacitance of 234 F g− 1. Moreover, the retention of N-CNO after 5000 long charge/discharge was 98%. The utilizing voltages of the synthesized asymmetric devices were expanded to 1.6 V in 2 M KOH solution, providing a notable specific capacitance of 79 F g−1 and higher energy densities of 27.2 Wh kg−1 at 1 A g−1. Mainly, even at higher current densities (10 A/g) this device still holds higher power densities (7.5 kW kg−1) while keeping 12 Wh kg−1 energy densities. Nevertheless, the well-being asymmetric device explores notable electrochemical cycling stabilities and capacitance retention of 98% over 5000 at 8 A g−1. These values evident that N–CNO will have been utilized as a surrogate electrode source and a potential candidate in SCs with higher specifical energy/power densities.

Composition and phase engineering of metal chalcogenides and phosphorous chalcogenides.

Two-dimensional (2D) materials with multiphase, multielement crystals such as transition metal chalcogenides (TMCs) (based on V, Cr, Mn, Fe, Cd, Pt and Pd) and transition metal phosphorous chalcogenides (TMPCs) offer a unique platform to explore novel physical phenomena. However, the synthesis of a single-phase/single-composition crystal of these 2D materials via chemical vapour deposition is still challenging. Here we unravel a competitive-chemical-reaction-based growth mechanism to manipulate the nucleation and growth rate. Based on the growth mechanism, 67 types of TMCs and TMPCs with a defined phase, controllable structure and tunable component can be realized. The ferromagnetism and superconductivity in FeXy can be tuned by the y value, such as superconductivity observed in FeX and ferromagnetism in FeS2 monolayers, demonstrating the high quality of as-grown 2D materials. This work paves the way for the multidisciplinary exploration of 2D TMPCs and TMCs with unique properties.

Engineering Highly Fluorescent and Colloidally Stable Blue-Emitting CsPbBr3 Nanoplatelets Using Polysalt/PbBr2 Ligands

Two-dimensional (2D) colloidal perovskite CsPbBr3 nanoplatelets (NPLs) have recently attracted attention as promising blue-emitting materials because of their large exciton binding energy and precisely controlled thickness. However, synthesizing NPLs that are homogeneous and highly fluorescent while exhibiting long-term stability has been challenging, which has limited the successful integration of such materials in optoelectronic devices (e.g., light-emitting devices). Herein, we introduce a strategy relying on surface ligand-engineering, using PbBr2-complexed polymers as coating ligands that impart morphological and colloidal stability while enhancing the photoluminescence quantum yield of perovskite NPLs. The polymers are salt-rich compounds that present several ammonium (or imidazolium) bromide anchoring groups and alkyl chains with different lengths as solubilizing blocks. The presence of quaternary ammonium salts facilitates the dissolution of PbBr2 in organic solutions, forming polysalt-[PbxBry]2x−y complexes. These complexes strongly interact with the NPLs and eliminate surface defects, compared to the as-grown materials, and improve their structural and morphological stability. This strategy has yielded high-quality blue-emitting NPLs that maintain color purity (with narrow profiles at ∼460 nm) with photoluminescence quantum yield (PLQY) up to 80%. Additionally, significantly enhanced long-term stability of the nanoplatelets under several challenging conditions, including UV irradiation and highly diluted conditions, has been achieved.

Mg-doping and free-hole properties of hot-wall MOCVD GaN

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range (2.45×1018 cm−3 up to 1.10×1020 cm−3) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance–voltage (C–V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 Ω cm and 8.4×1017 cm−3, respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

Growth and Strain Modulation of GeSn Alloys for Photonic and Electronic Applications.

GeSn materials have attracted considerable attention for their tunable band structures and high carrier mobilities, which serve well for future photonic and electronic applications. This research presents a novel method to incorporate Sn content as high as 18% into GeSn layers grown at 285–320 °C by using SnCl4 and GeH4 precursors. A series of characterizations were performed to study the material quality, strain, surface roughness, and optical properties of GeSn layers. The Sn content could be calculated using lattice mismatch parameters provided by X-ray analysis. The strain in GeSn layers was modulated from fully strained to partially strained by etching Ge buffer into Ge/GeSn heterostructures . In this study, two categories of samples were prepared when the Ge buffer was either laterally etched onto Si wafers, or vertically etched Ge/GeSnOI wafers which bonded to the oxide. In the latter case, the Ge buffer was initially etched step-by-step for the strain relaxation study. Meanwhile, the Ge/GeSn heterostructure in the first group of samples was patterned into the form of micro-disks. The Ge buffer was selectively etched by using a CF4/O2 gas mixture using a plasma etch tool. Fully or partially relaxed GeSn micro-disks showed photoluminescence (PL) at room temperature. PL results showed that red-shift was clearly observed from the GeSn micro-disk structure, indicating that the compressive strain in the as-grown GeSn material was partially released. Our results pave the path for the growth of high quality GeSn layers with high Sn content, in addition to methods for modulating the strain for lasing and detection of short-wavelength infrared at room temperature.

Investigation into the InAs/GaAs quantum dot material epitaxially grown on silicon for O band lasers

InAs/GaAs quantum dot (QD) lasers were grown on silicon substrates using a thin Ge buffer and three-step growth method in the molecular beam epitaxy (MBE) system. In addition, strained superlattices were used to prevent threading dislocations from propagating to the active region of the laser. The as-grown material quality was characterized by the transmission electron microscope, scanning electron microscope, X-ray diffraction, atomic force microscope, and photoluminescence spectroscopy. The results show that a high-quality GaAs buffer with few dislocations was obtained by the growth scheme we developed. A broad-area edge-emitting laser was also fabricated. The O-band laser exhibited a threshold current density of 540 A/cm2 at room temperature under continuous wave conditions. This work demonstrates the potential of large-scale and low-cost manufacturing of the O-band InAs/GaAs quantum dot lasers on silicon substrates.

Unexpected atmosphere effect on the iron oxide growth by the Laser Floating Zone method

The control of the oxidation state represents an important challenge for materials engineering. It is fairly known that iron oxide can be present in several oxidation states each one with unique physical and chemical properties. In the present work the effects of the different oxygen partial pressure on the growth of Fe3O4 fibers, using hematite (Fe2O3) as a precursor material was studied. The fibers were grown by the Laser Floating Zone (LFZ) method at a constant pulling rate (100 mm/h) under different atmospheres (air, vacuum, argon and nitrogen). The structure and morphology of the as-grown fibers were studied by X-ray diffraction, SEM/EDS/EBSD and optical microscopy. The fibers show a core/shell structure, where Fe3O4 is the core and Fe2O3 the shell, putting in evidence that the partial pressure of oxygen is a crucial parameter to control the thickness and grains morphology of the shell. Low oxygen partial pressure increases the width of the Fe2O3 insulating phase shell. The electrical conductivity, of the samples grown under an atmosphere with low oxygen partial pressure, is improved due to the morphology of the grains in the shell. However, no significant changes in the magnetization measurements were detected. From the results, we can conclude that applying an atmosphere during the LFZ growth allows tunning the transport properties envisaging electrical applications of the as-grown material.

Computational synthesis of 2D materials grown by chemical vapor deposition

The exotic properties of 2D materials made them ideal candidates for applications in quantum computing, flexible electronics, and energy technologies. A major barrier to their adaptation for industrial applications is their controllable and reproducible growth at a large scale. A significant effort has been devoted to the chemical vapor deposition (CVD) growth of wafer-scale highly crystalline monolayer materials through exhaustive trial-and-error experimentations. However, major challenges remain as the final morphology and growth quality of the 2D materials may significantly change upon subtle variation in growth conditions. Here, we introduced a multiscale/multiphysics model based on coupling continuum fluid mechanics and phase-field models for CVD growth of 2D materials. It connects the macroscale experimentally controllable parameters, such as inlet velocity and temperature, and mesoscale growth parameters such as surface diffusion and deposition rates, to morphology of the as-grown 2D materials. We considered WSe2 as our model material and established a relationship between the macroscale growth parameters and the growth coverage. Our model can guide the CVD growth of monolayer materials and paves the way to their synthesis-by-design.Graphic abstract