Optoelectronic Device Structures Research Articles

The Role of Interface Band Alignment in Epitaxial SrTiO3/GaAs Heterojunctions.

Correlated oxides are known to have remarkable properties, with a range of electronic, magnetic, optoelectronic, and photonic functionalities. A key ingredient in realizing these properties into practical technology is the effective and scalable integration of oxides with conventional semiconductors. Unlocking the full spectrum of functionality requires atomically abrupt oxide-semiconductor interfaces and intimate knowledge of their potential landscape and charge transport. In this study, we investigated the electrical properties of epitaxial SrTiO3/GaAs heterostructures by examining the band alignment and transport behavior at the interface. We employ X-ray photoelectron spectroscopy (XPS) to measure the barriers for electrons and holes across the interface and, through them, explain the transport behavior for junctions with n- and p-type GaAs. We further show qualitative evidence of the strong photoresponse of these structures, illustrating the potential of these structures in optoelectronic devices. These results establish the fundamental groundwork for utilizing these interfaces toward new devices and define their design space.

Optical detection of passive thermal lattice deformation of monolayer WS2 in van der Waals heterostructures of WS2/h-BN

Van der Waals heterostructures (vdWHs) of the constituent two-dimensional (2D) layered materials are quickly emerging as functional structures for diverse novel electronic and optoelectronic devices. To further explore the device applications of the vdWHs, an open question, which is whether the lattice of atomic thin active layer follows the deformation of its neighboring thick layers in a vdWH, needs to be clarified. Herein, the thermal deformation of WS2 monolayers in vdWHs with and without hexagonal boron nitride (h-BN) layers on SiO2/Si substrates is investigated with combined optical spectroscopies of photoluminescence (PL) and Raman scattering. With a developed strategy extracting the pure strain-induced effects from the temperature-dependent PL and Raman scattering spectroscopic results, the relative deformations of the WS2 monolayers and the h-BN layers with respect to the substrate have been identified. Both the Raman and PL results consistently reveal that the lattice variation of the WS2 monolayers passively match the deformation of the h-BN layers, and the WS2 monolayer on the SiO2/Si substrate exhibits the largest strain. These findings have not only demonstrated the passive behavior of the thermal strain of 2D semiconductor monolayers in vdWHs heterostructures, but also paved a way to tailor the thermal strain via designing and constructing vdWHs.

Current-induced nonreciprocal transmittance in graphene-embedded photonic multilayers comprising the Octonacci sequence

In this work, we have studied the nonreciprocal properties of quasi-periodic Octonacci multilayer incorporating current-induced drifted graphene monolayers at the interfaces. We have analyzed the transmittance of the structure and compared it with that of regular periodic photonic crystals. Our investigation has revealed that the transmission of light is significantly improved through configurations comprising dielectric layers and graphene monolayers. The proposed structures exhibit nonreciprocal optical responses across a broad range of frequencies. The coupling of surface plasmon polaritons in the quasi-periodic structure exceeds that of periodic structures. We have examined nonreciprocal behaviors versus the drift velocity of electrons and the Fermi energy of graphene layers. It has been demonstrated that the reciprocity of the system can be adjusted by manipulating the Fermi energy and the drift velocity of the graphene layers. Moreover, the direction of nonreciprocity is dictated by the sign of the drift velocity. Nonreciprocal behavior is absent under zero DC bias and can be tuned up to 4.5 THz with bias. These distinctive properties suggest potential applications of these structures in optoelectronic devices.

Direct CVD-growth and optoelectronic characterizations of monolayer ribbon-like MoS2 flakes

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) materials provide a perspective development for optoelectronic, catalytic and quantum devices. However, relative approach to modify corresponding physical performance via explicit synthetic control of edge structure and dimensionality is still an enormous challenge. Here, we elucidate the successful fabrication of high-yield monolayer ribbon-like MoS2 crystals via a catalyst-free chemical vapor deposition (CVD) strategy. These ribbon-like MoS2 flakes exhibit a monolayer feature with tunable width in the range of 0.6–4.2 µm and a changed optical/electronic performance depending on the flake width. Furthermore, the proposed growth mechanism of such unique structures is analyzed at the basis of the change of Mo/S atom ratio. Our result emphasizes the potential to enable the successful preparation of special quasi one-dimensional (1D) TMDC structures for next-generation micro-nano optoelectronic devices.

Optical ridge waveguides in Nd3+‐doped fluorophosphate glasses fabricated by carbon ion implantation and femtosecond laser ablation

AbstractIn this work, the ridge waveguide was formed by the femtosecond laser ablation with a central wavelength of 0.808 μm and a speed of 50 μm/s on the planar Nd3+‐doped fluorophosphate glass waveguide that was fabricated by the C3+ ion implantation with an energy of 6.0 MeV and a fluence of 1.0×1012 C3+s/mm2. The mechanism of the ion‐implanted waveguide was discussed by the Stopping and Range of Ions in Matter 2013. The cross‐section morphology of the ridge waveguide was captured by an optical microscope. Its mode characteristics were analyzed by the end‐face coupling method. The ridge Nd3+‐doped fluorophosphate glass waveguides have potential as fundamental structures for many optoelectronic devices including waveguide amplifiers and lasers.

Graphene Oxide: Key to Efficient Charge Extraction and Suppression of Polaronic Transport in Hybrids with Poly (3-hexylthiophene) Nanoparticles.

Nanoparticles (NPs) of conjugated polymers in intimate contact with sheets of graphene oxide (GO) constitute a promising class of water-dispersible nanohybrid materials of increased interest for the design of sustainable and improved optoelectronic thin-film devices, revealing properties exclusively pre-established upon their liquid-phase synthesis. In this context, we report for the first time the preparation of a P3HTNPs-GO nanohybrid employing a miniemulsion synthesis approach, whereby GO sheets dispersed in the aqueous phase serve as a surfactant. We show that this process uniquely favors a quinoid-like conformation of the P3HT chains of the resulting NPs well located onto individual GO sheets. The accompanied change in the electronic behavior of these P3HTNPs, consistently confirmed by the photoluminescence and Raman response of the hybrid in the liquid and solid states, respectively, as well as by the properties of the surface potential of isolated individual P3HTNPs-GO nano-objects, facilitates unprecedented charge transfer interactions between the two constituents. While the electrochemical performance of nanohybrid films is featured by fast charge transfer processes, compared to those taking place in pure P3HTNPs films, the loss of electrochromic effects in P3HTNPs-GO films additionally indicates the unusual suppression of polaronic charge transport processes typically encountered in P3HT. Thus, the established interface interactions in the P3HTNPs-GO hybrid enable a direct and highly efficient charge extraction channel via GO sheets. These findings are of relevance for the sustainable design of novel high-performance optoelectronic device structures based on water-dispersible conjugated polymer nanoparticles.

Two-dimensional metal halide perovskites and their heterostructures: from synthesis to applications

Abstract Size- and shape-dependent unique properties of the metal halide perovskite nanocrystals make them promising building blocks for constructing various electronic and optoelectronic devices. These unique properties together with their easy colloidal synthesis render them efficient nanoscale functional components for multiple applications ranging from light emission devices to energy conversion and storage devices. Recently, two-dimensional (2D) metal halide perovskites in the form of nanosheets (NSs) or nanoplatelets (NPls) are being intensively studied due to their promising 2D geometry which is more compatible with the conventional electronic and optoelectronic device structures where film-like components are usually employed. In particular, 2D perovskites exhibit unique thickness-dependent properties due to the strong quantum confinement effect, while enabling the bandgap tuning in a wide spectral range. In this review the synthesis procedures of 2D perovskite nanostructures will be summarized, while the application-related properties together with the corresponding applications will be extensively discussed. In addition, perovskite nanocrystals/2D material heterostructures will be reviewed in detail. Finally, the wide application range of the 2D perovskite-based structures developed to date, including pure perovskites and their heterostructures, will be presented while the improved synergetic properties of the multifunctional materials will be discussed in a comprehensive way.

Fabrication of GaN-air channels for embedded photonic structures

Refractive index engineering is a key element in the design of optoelectronic device structures. In this work we present a method for the fabrication of buried hollow channels, periodically arranged inside GaN structures. The method is based on selective area doping and electrochemical etching. Channels width is defined by the selective area ion implantation mask geometry while the channel vertical dimension depends on the implantation energy and annealing conditions. In this work we used implantation with Si 100 keV and dose 2.6·1015 cm−2 into GaN. Next, a 100 nm GaN layer was grown by plasma-assisted molecular beam epitaxy (PAMBE). After annealing at 1070 °C for 5 min, selectively doped regions were removed in the process of electrochemical etching (ECE), forming periodically arranged empty volumes. Annealing resulted in partial strain removal as observed by X-Ray diffraction. Surface roughness was evaluated at every stage of the process: after implantation, after regrowth and after annealing, and it is promising for the integration of the proposed technology with device fabrication and processing. The flexibility in the mask design for selective ion implantation and a high refractive index contrast between GaN and air, make the proposed technology attractive for the fabrication of buried photonic structures such as diffraction gratings.

Shape‐Controlled Solution‐Epitaxial Perovskite Micro‐Crystal Lasers Rivaling Vapor Deposited Ones

AbstractEpitaxial growth methods usually need dedicated equipment, high energy consumption to maintain pure vacuum conditions and evaporation of source materials, and elevated substrate temperatures. Solution epitaxial growth requires nothing of that but is rarely used because the achieved microstructures are of low quality, not homogeneous, and finally exhibit worse performances in devices. Here, an antisolvent‐vapor‐assisted‐crystallization of metal‐halide‐perovskites as a method overcoming these disadvantages is demonstrated. The methylammonium lead tribromide exhibits van‐der‐Waals type of epitaxial growth on mica substrates, resulting in micro‐crystallites whose shape can be controlled to be either triangular micro‐prism or micro‐cuboid. These micro‐crystallites act as optical resonators supporting various optical modes and lasing is achieved under optical excitation with low thresholds and record high environmental stability. Selecting suitable resonators from a large variety of sizes allows control of mode spacing and finally mono‐mode operation, considered to be an important feature of semiconductor laser devices. The achieved results are essentially competitive to those obtained by vapor phase epitaxial microstructures, highlighting that epitaxy of high‐quality optoelectronic device structures is feasible by minimum technological efforts and energy consumption, which are of increasing importance considering issues such as global warming and the current energy crisis.

Effect of Electron – Phonon Interaction on the Magneto-optical Absorption in Monolayer Phosphorene

In this work, we investigate the magneto-optical absorption in a phosphorene monolayer subjected to a perpendicular static magnetic field. The magneto-optical absorption power is calculated using the projection operator technique taking account of the effect of electron – optical phonon interaction. Numerical results showed that in the case of absence of any interaction, the higher the resonant frequency, the smaller absorbed power was. If the electron - phonon interaction exists, there appear the cyclotron-phonon resonant peaks which show the electron transitions between Landau levels by absorbing a photon accompanied by absorbing/emitting an optical phonon. The obtained results are important for further studies and applications of the phosphorene based structures in optoelectronic devices.

Growth and Optoelectronic Properties of Large-Scale Bilayer WS2 Ribbons with Unusual Shapes via Chemical Vapor Deposition

Atomically thin transition metal dichalcogenides (TMDCs) have attracted worldwide interest for their prospective potential in electronic and optoelectronic fields. However, TMDCs tend to grow into two-dimensional (2D) flakes rather than one-dimensional (1D) micro/nanoribbons due to their intrinsic layered structure. Here, large-scale bilayer WS2 ribbons with unusual shapes have been achieved through the space-confined chemical vapor deposition method via utilizing WO3 and S powders as the precursors. The microstructures and optical and electronic properties of as-grown bilayer WS2 ribbons have been systematically researched. Moreover, the possible growth mechanism of such special structures is revealed by the growth-time-dependent morphological evolution of the WS2 crystals. Our results emphasize the prospects for the successful fabrication of novel 1D TMDC structures for high-performance optoelectronic devices at nanoscale, including transistors, photocatalysts, photovoltaic cells, and spintronic devices.

Nanoscale Charge Density and Dynamics in Graphene Oxide.

Graphene oxide (GO) is widely used as a component in thin film optoelectronic device structures for practical reasons because its electronic and optical properties can be controlled. Progress critically depends on elucidating the nanoscale electronic structure of GO. However, direct experimental access is challenging because of its disordered and nonconductive character. Here, we quantitatively mapped the nanoscopic charge distribution and charge dynamics of an individual GO sheet by using Kelvin probe force microscopy (KPFM). Charge domains are identified, presenting important charge interactions below distances of 20 nm. Charge dynamics with very long relaxation times of at least several hours and a logarithmic decay of the time correlation function are in excellent agreement with Monte Carlo simulations, revealing an universal hopping transport mechanism best described by Efros–Shklovskii’s law.

Light Management With Grating Structures in Optoelectronic Devices.

Ordered and patterned micro/nanostructure arrays have emerged as powerful platforms for optoelectronic devices due to their unique ordered-dependent optical properties. Among various structures, grating structure is widely applied because of its simple fabrication process, easy adjusting of size and morph, and efficient light trapping. Herein, we summarized recent developments of light management with grating structures in optoelectronic devices. Typical mechanisms about the grating structures in optoelectronic devices have been reviewed. Moreover, the applications of grating structures in various optoelectronic devices have been presented. Meanwhile, the remaining bottlenecks and perspectives for future development have been discussed.

Integration of Colloidal Quantum Dots with Photonic Structures for Optoelectronic and Optical Devices.

Colloidal quantum dot (QD), a solution‐processable nanoscale optoelectronic building block with well‐controlled light absorption and emission properties, has emerged as a promising material system capable of interacting with various photonic structures. Integrated QD/photonic structures have been successfully realized in many optical and optoelectronic devices, enabling enhanced performance and/or new functionalities. In this review, the recent advances in this research area are summarized. In particular, the use of four typical photonic structures, namely, diffraction gratings, resonance cavities, plasmonic structures, and photonic crystals, in modulating the light absorption (e.g., for solar cells and photodetectors) or light emission (e.g., for color converters, lasers, and light emitting diodes) properties of QD‐based devices is discussed. A brief overview of QD‐based passive devices for on‐chip photonic circuit integration is also presented to provide a holistic view on future opportunities for QD/photonic structure‐integrated optoelectronic systems.

(Invited) Technology of Gallium Nitride Crystal Growth - Challenges and Perspectives

It is well known that gallium nitride (GaN) is the basic material for fabricating optoelectronic devices as Light Emitting Diodes (LEDs) and Laser Diodes (LDs). GaN is also an attractive choice for electronic high-frequency and high-power devices. This is due to its wide band gap, high electron velocity, high breakdown field strength and high thermal conductivity. Most of the mentioned GaN-based devices are built on foreign substrates (sapphire, silicon, or silicon carbide). For many years, there was no mass production of native substrates. This happened because GaN crystal growth is quite a challenging process. The compound melts congruently at high temperature and extremely high nitrogen pressure (>6 GPa). Thus, today, it is impossible to grow GaN from the melt. The crystals should be obtained by other techniques requiring lower pressure and temperature. Crystallization from gas phase, solution, or any combination thereof must be included. For more than three decades, crystallization of bulk GaN has remained a big challenge for the nitride community. Work on developing bulk GaN has mainly been motivated by the LD market. All commercially available LD epi-structures are built on GaN wafers. Today, however, not only the LDs, but also the electronic industry and the demand for vertical high power transistors and diodes seem to be the driving force for crystallizing bulk GaN. Thus, GaN substrates of relatively high quality have recently become available. Today, GaN is crystallized by three main methods: i/ sodium flux (Na-flux); ii/ halide vapor phase epitaxy (HVPE); and iii/ basic or acidic ammonothermal growth. Perspectives of all the mentioned above technologies will be presented in this lecture. Special attention will be paid to the basic ammonothermal method and HVPE. It will be shown that growth from the vapor phase needs a new philosophy. The crystallization processes on foreign seeds (sapphire and gallium arsenide) reached their maximum possibilities. Despite many tricks during crystal growth runs the structural quality as well as yield of the obtained GaN crystals and wafers are very low and cannot be improved. The only way is GaN-on-GaN crystallization. Recently, significant progress has been presented in the basic ammonothermal method. High structural quality crystals of lateral size larger than 2 inches and thickness exceeding 6 mm were obtained in one crystal growth process. Use of ammonothermally grown GaN as seed for HVPE will be presented. All challenges and difficulties to grow bulk GaN from the vapor phase will be demonstrated. Scenarios for the further development of bulk GaN crystallization will be shown. Important parameters of GaN substrates for epitaxy of optoelectronic and electronic device structures will be discussed. A new way of better controlling the electrical properties of GaN crystals and wafers will be proposed.

Back-contact structures for optoelectronic devices: Applications and perspectives

Interdigitated back contact (IBC) structures formed by integrating two functional materials of opposite polarity on the rear side of device have attracted a significant attention for optoelectronic device design. Currently, this type of IBC design has been widely developed with a variety of applications in the photovoltaic (PV) community and associated fields. In terms of PV devices, the IBC designs can eliminate the need for a front electrode, endowing a potential in high power conversion efficiency. Moreover, by opening the front surface on IBC structures provides a platform for further designs, which enables us to realize a series of derived applications. This review summarizes the recent progress in IBC based PV devices, including c-Si solar cells (SCs), perovskite SCs, and perovskite/c-Si tandem SCs. Furthermore, the focus herein is focused on the latest advances in the specific applications in the studies of photon recycling, and in-situ / in-operando measurements of photoelectric properties. Finally, a perspective into the remaining challenges and future opportunities to accelerate the practical applications of IBC devices is highlighted. • A systematic review including physical mechanism and design principles is summarized. • A large number of applications for optoelectronic devices are covered, which is of great interest to not only the fundamental researchers but also to the photovoltaic industry. • The content presented in this review includes many of the latest reports and sheds new light on the device designs and characterization.

Perovskite Solar Cells: Surface Ligand Management Aided by a Secondary Amine Enables Increased Synthesis Yield of CsPbI3 Perovskite Quantum Dots and High Photovoltaic Performance (Adv. Mater. 32/2020)

Emerging all-inorganic CsPbI3 perovskite quantum dots offer colloidal synthesis and processing using industrially friendly solvents. This is desirable for solar-cell applications and will catalyze new structures for optoelectronic devices. In article number 2000449, Jianyu Yuan, Wanli Ma, and co-workers report the development of a new surface ligand management strategy, which improves on the state-of-the-art efficiency of CsPbI3 quantum-dot solar cells.

Controllable Growth of High-Quality Inorganic Perovskite Microplate Arrays for Functional Optoelectronics.

Inorganic perovskite single crystals have emerged as promising vapor-phase processable structures for optoelectronic devices. However, because of material lattice mismatch and uncontrolled nucleation, vapor-phase methods have been restricted to random distribution of single crystals that are difficult to perform for integrated device arrays. Herein, an effective strategy to control the vapor-phase growth of high-quality cesium lead bromide perovskite (CsPbBr3 ) microplate arrays with uniform morphology as well as controlled location and size is reported. By introducing perovskite seeds on substrates, intractable lattice mismatches and random nucleation barriers are surpassed, and the epitaxial growth of perovskite crystals is accurately controlled. It is further demonstrated that CsPbBr3 microplate arrays can be monolithically integrated on substrates for the fabrication of high-performance lasers and photodetectors. This strategy provides a facile approach to fabricate high-quality CsPbBr3 microplates with controllable size and location, which offers new opportunities for the scalable production of integrated optoelectronic devices.

Realization of high-quality InGaAs/GaAs quantum dot growth on Ge substrate and improvement of optical property through ex-situ ion implantation

Epitaxial growth of III-V heterostructures on non-native substrates such as Silicon (Si) or Germanium (Ge) is one of the promising research topics for the last two decades. The interface between polar III-V semiconductors and non-polar substrates (Si or Ge) plays a crucial role in monolithic integration. However, there is an anticipation of epitaxial GaAs growth on Ge substrate because of the lower mismatch of lattice constants and thermal expansion coefficients between them. Therefore, the high-quality growth of III-V semiconductor heterostructures on Ge substrates would overcome the impediment to Si-photonics, where the monolithic integration of optoelectronic device structures can be done using a Ge graded layer on Si. Here, we have explored the epitaxial growth of multi-layer InGaAs/GaAs quantum dot heterostructures on Ge substrates and compared the optical and structural properties with the QDs grown on GaAs substrate. The optical properties of all samples are investigated with the help of photoluminescence (PL) and time-resolved photoluminescence (TRPL), whereas the morphology of the QDs is observed through cross-sectional transmission electron microscopy (XTEM) images. An enhancement in the optical characteristics (PL peak wavelength, activation energy, carrier lifetime) is found for the QDs grown on Ge substrate with the super-lattice buffer (SLB) layer. The minimization of defects and dislocations in the heterostructure is also realized for the structure with the SLB layer. Furthermore, a two-fold enhancement in PL intensity and 24 meV increment in activation energy is achieved through ex-situ H− ion-implantation, which approached the values obtained for the QD heterostructure grown on GaAs substrate.

Improving External Quantum Efficiency by Subwavelength Nano Multi-Layered Structures for Optoelectronic Devices

Based on thin film optics (TFO) and finite element method (FEM), we have theoretically investigated improving external quantum efficiency (EQE) by anti-reflection (AR) films constructed from subwavelength nano multi-layers (NML) of low and high index materials, where the low and high index materials are MgF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , respectively. This kind of NML dielectric structures have the advantages of low-cost and flexible. Three kinds of substrates have been studied here, which are glass, polyethylene naphthalate (PEN), and polyethylene terephthalate (PET), respectively. TFO theory has been used to obtain the transmittance and reflectance, which agrees well with FEM results. For NML structure, TFO is more efficient than FEM in terms of calculation time and accuracy. The average AR effects (AAREs) are about 3.69%, 3.46% and 3.07% on glass substrate, about 6.15%, 5.56% and 5.02% on PEN substrate, and about 5.06%, 4.62% and 4.17% on PET substrate, for AR bands (ARBs) 350~800 nm, 350~1100 nm and 350~1500 nm, respectively. The results also reflect that wider AR bandwidth needs more NML layers. In practice, this kind of AR films can be widely applicable to enhance the EQE of the optoelectronic devices (OEDs).