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Fast gain and phase recovery of semiconductor optical amplifiers based on submonolayer quantum dots
Submonolayer quantum dots as active medium in opto-electronic devices promise to combine the high density of states of quantum wells with the fast recovery dynamics of self-assembled quantum dots. We investigate the gain and phase recovery dynamics of a semiconductor optical amplifier based on InAs submonolayer quantum dots in the regime of linear operation by one- and two-color heterodyne pump-probe spectroscopy. We find an as fast recovery dynamics as for quantum dot-in-a-well structures, reaching 2 ps at moderate injection currents. The effective quantum well embedding the submonolayer quantum dots acts as a fast and efficient carrier reservoir.
Ultrafast Nanoimaging of Carrier Funneling in Composition-Graded Semiconductor Nanowires
Recent advances in bandgap engineering of low-dimensional semiconductors have enabled high-efficiency carrier transport in miniaturized electronic and optoelectronic devices. The physical properties and functionalities of these materials are governed by complex carrier dynamics coupled with multiple transport mechanisms in tailored band structures. Here, we report ultrafast nanoimaging of carrier funneling and recombination in composition-grade CdSxSe1-x nanowires using pump-probe near-field nanoscopy. Leveraging the high resolution of this technique in both space and time, we resolve nanoscale local carrier dynamics along composition-graded nanowires, revealing the local variation of composition-dependent carrier mobilities and lifetimes that significantly differ from their uniform composition counterparts. Furthermore, we demonstrate a length-dependent behavior wherein shorter nanowires exhibit enhanced funneling effects, accelerating carrier transport by up to 33%. Our findings provide direct visualization of nanoscale carrier transport while supporting an effective approach for investigating complex carrier interactions in inhomogeneous semiconductor nanostructures, with implications for optimizing next-generation optoelectronic devices.
Interface modulated currents in periodically proton exchanged Mg doped lithium niobate
Conductivity in Mg doped lithium niobate (Mg:LN) plays a key role in the reduction of photorefraction and is therefore widely exploited in optical devices. However, charge transport through Mg:LN and across interfaces such as electrodes also yields potential electronic applications in devices with switchable conductivity states. Furthermore, the introduction of proton exchanged (PE) phases in Mg:LN enhances ionic conductivity, thus providing tailorability of conduction mechanisms and functionality dependent on sample composition. To facilitate the construction and design of such multifunctional electronic devices based on periodically PE Mg:LN or similar ferroelectric semiconductors, fundamental understanding of charge transport in these materials, as well as the impact of internal and external interfaces, is essential. In order to gain insight into polarization and interface dependent conductivity due to band bending, UV illumination, and chemical reactivity, wedge shaped samples consisting of polar oriented Mg:LN and PE phases were investigated using conductive atomic force microscopy. In Mg:LN, three conductivity states (on/off/transient) were observed under UV illumination, controllable by the polarity of the sample and the externally applied electric field. Measurements of currents originating from electrochemical reactions at the metal electrode–PE phase interfaces demonstrate a memresistive and rectifying capability of the PE phase. Furthermore, internal interfaces such as domain walls and Mg:LN–PE phase boundaries were found to play a major role in the accumulation of charge carriers due to polarization gradients, which can lead to increased currents. The insight gained from these findings yield the potential for multifunctional applications such as switchable UV sensitive micro- and nanoelectronic devices and bistable memristors.
Coupled optical and electrical study of thin-film InGaAs photodetector integrated with surface InP Mie resonators
High-index dielectric and semiconductor nanostructures with characteristics of low absorption loss and artificially controlled scattering properties have grasped an increasing attention for improving the performance of thin-film photovoltaic devices. In this work, combined optical and electrical simulations were performed for thin-film InP/In{sub 0.53}Ga{sub 0.47}As/InP hetero-junction photodetector with periodically arranged InP nano-cylinders in the in-coupling configuration. It is found that the carefully designed InP nano-cylinders possess strongly substrate-coupled Mie resonances and can effectively couple incident light into the guided mode, both of which significantly increase optical absorption. Further study from the electrical aspects shows that enhancement of external quantum efficiency is as high as 82% and 83% in the configurations with the optimized nano-cylinders and the optimized period, respectively. Moreover, we demonstrate that the integration of InP nano-cylinders does not degrade the electrical performance, since the surface recombination is effectively suppressed by separating the absorber layer where carriers generate and the air/semiconductor interface. The comprehensive modeling including optical and electrical perspectives provides a more practical description for device performance than the optical-only simulation and is expected to advance the design of thin-film absorber layer based optoelectronic devices for fast response and high efficiency.
Revealing Surface Modifications of Potassium-Fluoride-Treated Cu(In,Ga)Se 2 : A Study of Material Structure, Chemistry, and Photovoltaic Performance
The effects of alkali post-deposition treatments and device properties for polycrystalline thin film Cu(In,Ga)Se2 have been investigated. It is reported that these surface treatments lead to differences in interface chemistry and device properties. The behavior of defects in the space charge region as a function of different growth parameters is investigated by correlative analytical microscopy. The latter combines electron microscopy based imaging, Kelvin probe force microscopy, and atom probe tomography. Alkali treatments lead to copper depletion and consequent sharpening of the compositional profiles, and the measured electric potential differences of exposed Cu(In1-x,Gax)Se2 surfaces. Measurable differences in resistivity and potential have also been observed, which are expected to relate to the improved open-circuit voltage, fill-factor, and device efficiency. This study frames one perspective as to why post-deposition alkaline treatments lead to copper depletion, a mildly n-type semiconductor interface, and higher efficiency for a Cu(In,Ga)Se2 thin-film photovoltaic device.
Modeling Electron Injection at Semiconductor–Molecule Interfaces using First-Principles Dynamics Simulation: Effects of Nonadiabatic Coupling, Self-energy, and Surface Models
Excited electron transfer across semiconductor–molecule heterogeneous interfaces is central to various future electronic and optoelectronic devices. At the same time, first-principles modeling of such dynamical processes remains as a great challenge in theoretical chemistry and condensed matter physics for developing better understanding at the molecular scale. Excited electron transfer from a molecule to semiconductor surface is a particularly difficult case to model accurately, because the initial state of such an electron injection process often lies deep within the dense manifold of the conduction band states in the semiconductor. Nonadiabatic couplings and energy level alignments at such interfaces as well as the finite size error of the surface model all play important roles in numerical modeling of electron injection via first-principles theory. Here, using representative interfaces between a well-defined hydrogen-terminated Si(111) surface and series of covalently adsorbed conjugated molecules, we investigate the extent to which these theoretical and numerical considerations influence the description of electron injection at the semiconductor–molecule interface.
Improving the performance of organic thin film transistors formed on a vacuum flash-evaporated acrylate insulator
A systematic investigation has been undertaken, in which thin polymer buffer layers with different ester content have been spin-coated onto a flash-evaporated, cross-linked diacrylate gate-insulator to form bottom-gate, top-contact organic thin-film transistors. The highest device mobilities, ∼0.65 cm{sup 2}/V s and ∼1.00 cm{sup 2}/V s for pentacene and dinaphtho[2,3-b:2′,3′-f]-thieno[3,2-b]thiophene (DNTT), respectively, were only observed for a combination of large-grain (∼1–2 μm) semiconductor morphology coupled with a non-polar dielectric surface. No correlation was found between semiconductor grain size and dielectric surface chemistry. The threshold voltage of pentacene devices shifted from −10 V to −25 V with decreasing surface ester content, but remained close to 0 V for DNTT.
Electrooptical properties of hybrid liquid crystalline systems containing CdSe quantum dots
In this paper, we present electrooptical properties of hybrid liquid crystalline systems, which contained CdSe quantum dots (QDs). We have shown by experiments of degenerated two-wave mixing and transverse conductivity measurements that liquid crystal cells filled with nematic and doped with semiconductor nanoparticles exhibit photorefractive effect associated with photoconductivity appearing in the system. We also present the mathematical model, which explains the relationship between the photoconductivity of the layer on which the QDs reside and the generation of holographic gratings. Our research may help to develop better understanding of processes observed in such systems and create more efficient materials for holographic data storage.
A Novel Voltage Balancing Control With dv/dt Reduction for 10-kV SiC MOSFET-Based Medium Voltage Modular Multilevel Converter
Using high voltage (HV) silicon carbide (SiC) power semiconductors in a modular multilevel converter (MMC) is promising because it results in fewer submodules and lower switching loss compared to conventional Si based solutions. The nearest level pulsewidth modulation (NL-PWM) is commonly used in the MMC for medium voltage applications. However, with the NL-PWM and existing voltage balancing control, there are many submodules that switch their modes in a control cycle, resulting in a high dv/dt during the deadtime of the power semiconductor, which could be multiple times of the dv/dt of the single device. This poses great challenges on the noise immunity and insulation design in the MMC using HV SiC devices, which have very fast switching speed. A novel voltage balancing control, which ensures only two submodules switch their modes in a control cycle, is proposed in this article, limiting the maximum dv/dt to the dv/dt of a single power semiconductor and also maintaining the voltage balance performance. Furthermore, the proposed voltage balancing control is experimentally validated in a 10-kV SiC mosfet based MMC with four submodules per arm.
Synthetic Pathways to gamma-Graphyne and Related Allotropes of Carbon
Graphynes, two-dimensional carbon lattices combining sp 1 and sp 2 hybridized atoms, were predicted theoretically more than three decades ago, but few structures have been realized to date. These carbons are believed to possess remarkable mechanical and electronic properties, including high charge carrier mobilities comparable to those in graphene (10 4 to 10 5 cm 2 V -1 s -1 ). Unlike graphene, certain graphynes are predicted to be intrinsic semiconductors. Among these intriguing structures, γ-graphyne stands out as the structurally simplest and most symmetric sp 1 /sp 2 lattice. γ-Graphyne was first theorized in 1987. In contrast with graphene, γ-graphyne will be a semiconductor with a small band gap suitable for fabrication of electronic devices. This solves one of the fundamental problems of carbon-based electronics, the necessity for inducing a band gap in graphene. γ-Graphyne has the potential to form the basis for the next generation of carbon-based electronics operating at speeds unattainable by traditional silicon chips. Unlike silicon, γ-graphyne is a direct band gap semiconductor, and it will feature exceptional strength comparable to that of other 2D carbon allotropes. Such combination of properties may enable a new generation of highly efficient, ultra-light and flexible solar cells. Despite being a potentially “magical” material, γ-graphyne remained synthetically elusive for over three decades. The primary goals of this project were: (1) Synthesis of bulk γ-graphyne phases through solution-phase 2D polymerizations; (2) Experimental exploration of the physical and chemical properties of γ-graphyne; and (3) Mechanistic and theoretical studies of the novel chemical transformations developed in Goal 1. Common pyrolytic and vapor-deposition methodologies used for the synthesis of graphitic allotropes are unsuitable for graphyne and other sp 1 -contaning structures, as acetylenes readily convert to graphene and amorphous carbon at high temperatures. In contrast, this proposal is based on solution-based 2D polymerization. The major advantages of this approach over the traditional high temperature techniques are the potential to adjust the structure of the material with atomic precision, and the possibility of using structurally complex and relatively fragile repeat units. The outcomes of this research can revolutionize carbon nanotechnology, expanding the field’s structural toolbox beyond primarily graphitic and benzenoid structures. Understanding the chemistry of sp 1 carbon allotropes can lead to entirely new classes of structures with unique properties, including graphyne ribbons, nanotubes, quantum dots, and heterostructures with other 2D materials. Furthermore, the development of reliable and robust synthetic pathways towards periodic covalent molecular sheets with atomically precise structures shall have a profound impact on chemistry and materials science.
Interactions between nonfullerene acceptors lead to unstable ternary organic photovoltaic cells
For organic photovoltaic (OPV) devices to achieve consistent performance and long operational lifetimes, organic semiconductors must be processed with precise control over their purity, composition, and structure. This is particularly important for high volume solar cell manufacturing where control of materials quality has a direct impact on yield and cost. Ternary-blend OPVs containing two acceptor–donor–acceptor (A–D–A)-type nonfullerene acceptors (NFAs) and a donor have proven to be an effective strategy to improve solar spectral coverage and reduce energy losses beyond that of binary-blend OPVs. Here, we show that the purity of such a ternary is compromised during blending to form a homogeneously mixed bulk heterojunction thin film. We find that the impurities originate from end-capping C=C/C=C exchange reactions of A–D–A-type NFAs, and that their presence influences both device reproducibility and long-term reliability. The end-capping exchange results in generation of up to four impurity constituents with strong dipolar character that interfere with the photoinduced charge transfer process, leading to reduced charge generation efficiency, morphological instabilities, and an increased vulnerability to photodegradation. As a consequence, the OPV efficiency falls to less than 65% of its initial value within 265 h when exposed to up to 10 suns intensity illumination. We propose potential molecular design strategies critical to enhancing the reproducibility as well as reliability of ternary OPVs by avoiding end-capping reactions.
Optical Properties of Tensilely Strained Ge Nanomembranes
Group-IV semiconductors, which provide the leading materials platform of micro- electronics, are generally unsuitable for light emitting device applications because of their indirect- bandgap nature. This property currently limits the large-scale integration of electronic and photonic functionalities on Si chips. The introduction of tensile strain in Ge, which has the effect of lowering the direct conduction-band minimum relative to the indirect valleys, is a promising approach to address this challenge. Here we review recent work focused on the basic science and technology of mechanically stressed Ge nanomembranes, i.e., single-crystal sheets with thicknesses of a few tens of nanometers, which can sustain particularly large strain levels before the onset of plastic deformation. These nanomaterials have been employed to demonstrate large strain-enhanced photoluminescence, population inversion under optical pumping, and the formation of direct-bandgap Ge. Furthermore, Si-based photonic-crystal cavities have been developed that can be combined with these Ge nanomembranes without limiting their mechanical flexibility. These results highlight the potential of strained Ge as a CMOS-compatible laser material, and more in general the promise of nanomembrane strain engineering for novel device technologies.
Effect of different sulphur precursors on morphology and band-gap on the formation of Cu{sub 2}ZnSnS{sub 4} (CZTS) particles with microwave irradiation
Cu{sub 2}ZnSnS{sub 4} (CZTS) is a promising semiconductor material for ecological cost effective thin film Photovoltaic (PV) devices. As it contains earth abundant and non-toxic elements, it has the advantages over commercially available CIGS and CdTe thin film PV devices. In the present work, the pure phase Cu{sub 2}ZnSnS{sub 4} particles were successfully synthesised with microwave irradiation. The morphology and phase study was carried out for the samples prepared with two different sulphur precursors viz. thiourea and thioacetamide (TAA). CZTS particles with thiourea as sulphur precursor are more crystalline than CZTS particles with TAA. The band gap of 1.654 eV and 1.713 eV were calculated for the samples prepared with thiourea and TAA respectively.
Epitaxial PbGeSe thin films and their photoluminescence in the mid-wave infrared
PbSe is a narrow bandgap IV–VI compound semiconductor with application in mid-wave infrared optoelectronics, thermoelectrics, and quantum devices. Alkaline-earth or rare-earth elements such as Sr and Eu can substitute Pb to widen the bandgap of PbSe in heterostructure devices, but they come with challenges such as deteriorating optical and electronic properties, even in dilute concentrations due to their dissimilar atomic nature. We substitute Pb instead with column IV Ge and assess the potential of rocksalt phase PbGeSe as a wider bandgap semiconductor in thin films grown by molecular beam epitaxy on GaAs substrates. Low sticking of GeSe adatoms requires synthesis temperatures below 260 °C to incorporate Ge, but this yields poor structural and compositional uniformity as determined by x-ray diffraction. Consequently, as-grown films in the range Pb0.94Ge0.06Se–Pb0.83Ge0.17Se (6%–17% Ge) show much less bandgap widening in photoluminescence than prior work on bulk crystals using absorption. We observe that post-growth rapid thermal annealing at temperatures of 375–450 °C improves the crystal quality and recovers bandgap widening. Rapid interdiffusion of Ge during annealing, however, remains a challenge in harnessing such PbGeSe materials for compositionally sharp heterostructures. Annealed 17% Ge films emit light at 3–3.1 μm with a minimal shift in wavelength vs temperature. These samples are wider in bandgap than PbSe films by 55 meV at room temperature, and the widening increases to 160 meV at 80 K, thanks to sharply different dependence of bandgap on temperature in PbSe vs PbGeSe.
Advanced Electronic Structure Calculations For Nanoelectronics Using Finite Element Bases and Effective Mass Theory.
This paper describes our work over the past few years to use tools from quantum chemistry to describe electronic structure of nanoelectronic devices. These devices, dubbed "artificial atoms", comprise a few electrons, con ned by semiconductor heterostructures, impurities, and patterned electrodes, and are of intense interest due to potential applications in quantum information processing, quantum sensing, and extreme-scale classical logic. We detail two approaches we have employed: nite-element and Gaussian basis sets, exploring the interesting complications that arise when techniques that were intended to apply to atomic systems are instead used for artificial, solid-state devices.
Toward non-Si electronics: From remote epitaxy to layer splitting of 2D materials for mixed dimensional heterostructures
The current electronics has been mainly dominated by Si-based devices due to their mature processing system and exceptional cost-effectiveness. However, next generation electronics needs novel functionalities that cannot be realized by Si because of intrinsic limitation of Si. Accordingly, demand for non-Si electronics has been getting substantially high. Unfortunately, current methodology requires extremely high cost for non-Si materials, which impedes the progress in developing the non-Si based electronics. Here, I will discuss about our group’s efforts to address this issue. Our team recently conceived a new crystalline growth, termed as “remote epitaxy”, which can copy/paste crystalline information from substrates remotely through graphene, thus generating single-crystalline films on graphene. As interfacial binding energy is attenuated by inserting graphene at interface, the single-crystalline films can be easily exfoliated from the slippery graphene surface. Also, the graphene-coated substrates can be, in principle, reused infinitely to produce single-crystalline films. Thus, the remote epitaxy can produce non-Si semiconductor films with unprecedented cost efficiency while allowing additional flexible device functionality required for current ubiquitous electronics. Next, I will discuss about a layer splitting technique which can be a potential solution to overcome the problem in obtaining large-scale and monolayer 2D materials. A 2D material-based heterostructure has been intensively studied because of its unique device functionalities and novel physics. However, it is extremely challenging to secure large-scale and monolayer 2D materials because of following issues: 1) poor scalability for laboratory fabrication processes of 2D heterostructures and 2) lack of well-defined control parameters for kinetics of 2D materials and predictable number of layers of 2D materials. To resolve this issue, we conceived a new approach called “layer-resolved splitting” which obtains multiple monolayer from multilayer 2D materials by controlling interfacial toughness contrast. As this method is versatile and universal, we can, in principle, apply to all 2D materials. We succeeded in having large-scale, monolayer 2D materials through our approach and, thereby 2D heterostructures were demonstrated for functional devices. Lastly, I would like to discuss opportunities of mixed-dimensional heterostructure demonstrated by remote epitaxy and layer-resolved splitting. As they produce freestanding 3D bulk films and 2D atomic layers, a new type of 3D/2D heterostructures can be realized where a new physics and new device architecture are revealed. Therefore, I believe that a new opportunity will be discovered through the mixed-dimensional heterostructures.
Hexagonal boron nitride: Epitaxial growth and device applications
As a newest family member of the III-nitrides, BN is considered amongst the remaining frontiers in wide energy bandgap semiconductors with potentials for technologically significant applications in deep UV (DUV) optoelectronics, solid-state neutron detectors, electron emitters, single photon emitters, switching/memory devices, and super-capacitors. It was shown that it is possible to produce h-BN epilayers with high hexagonal phase purity, UV transparency, and film stoichiometry by employing nitrogen-rich growth conditions. The quasi-2D nature of h-BN supports unusually strong optical transitions near the band edge and a large exciton binding energy on the order of 0.7 eV. Since the isotope of B-10 has a large capture cross-section for thermal neutrons, h-BN is an ideal material for the fabrication of solid-state neutron detectors for special nuclear materials detection, well and geothermal logging, and medical imaging applications. Freestanding B-10 enriched h-BN (h- 10 BN) epilayers with varying thicknesses up to 200 μm have been successfully synthesized by metal organic chemical vapor deposition (MOCVD) as of this writing. By utilizing the conductivity anisotropy nature of h-BN, 1 cm 2 lateral detectors fabricated from 100 μm thick h-10BN epilayers have demonstrated a detection efficiency of 59% for thermal neutrons, which is the highest on record among all solid-state neutron detectors as of today. It was noted that high growth temperatures, long growth times and the use of sapphire substrate tend to incorporate oxygen related impurities into h- 10 BN epilayers, which strongly impacted the carrier mobility-lifetime (μτ) products and charge collection efficiencies of h- 10 BN neutron detectors. As the h-BN material technology further develops, improved carrier mobilities and μτ products will allow the fabrication of h-BN devices with enhanced performance.