Search query: Direct bandgap semiconductor

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abstract: Highly stable antimonene, as the cousin of phosphorene from group VA, has opened up exciting realms in the two dimensional (2D) materials family. However, pristine antimonene is an indirect band gap semiconductor, which greatly restricts its applications for optoelectronics devices. Identifying suitable materials, both responsive to incident photons and efficient for carrier transfer, is urgently needed for ultrathin devices. Herein, by means of first principles computations we found that it is rather feasible to realize a new class of 2D materials with a direct bandgap and high carrier mobility, namely antimonene oxides with different content of oxygen. Moreover, these tunable direct bandgaps cover a wide range from 0 to 2.28 eV, which are crucial for solar cell and photodetector applications. Especially, the antimonene oxide (18Sb 18O) is a 2D topological insulator with a sizable global bandgap of 177 meV, which has a nontrivial Z2 topological invariant in the bulk and the topological states on the edge. Our findings not only introduce new vitality into 2D group VA materials family and enrich available candidate materials in this field but also highlight the potential of these 2D semiconductors as appealing ultrathin materials for future flexible electronics and optoelectronics devices.
author_names: Shengli Zhang

Wenhan Zhou

Yandong Ma

Jianping Ji

Bo Cai

Shengyuan A Yang

Zhen Zhu

Zhongfang Chen

Haibo Zeng
title: Antimonene Oxides: Emerging Tunable Direct Bandgap Semiconductor and Novel Topological Insulator.
venue: Nano letters
year: 2017

abstract: Solar water splitting via multi junction semiconductor photoelectrochemical cells provides direct conversion of solar energy to stored chemical energy as hydrogen bonds. Economical hydrogen production demands high conversion efficiency to reduce balance of systems costs. For sufficient photovoltage, water splitting efficiency is proportional to the device photocurrent, which can be tuned by judicious selection and integration of optimal semiconductor bandgaps. Here, we demonstrate highly efficient, immersed water splitting electrodes enabled by inverted metamorphic epitaxy and a transparent graded buffer that allows the bandgap of each junction to be independently varied. Voltage losses at the electrolyte interface are reduced by 0.55 V over traditional, uniformly p doped photocathodes by using a buried p n junction. Advanced on sun benchmarking, spectrally corrected and validated with incident photon to current efficiency, yields over 16% solar to hydrogen efficiency with GaInP/GaInAs tandem absorbers, representing a 60% improvement over the classical, high efficiency tandem III V device. Solar water splitting efficiency can be enhanced by careful bandgap selection in multi junction semiconductor structures. Young et al. demonstrate a route that allows independent bandgap tuning of each junction in an immersed water splitting device, enabling a solar to hydrogen efficiency of over 16%
author_names: James L Young

Myles A Steiner

Henning Doscher

John A Turner

Todd G Deutsch
title: Direct solar to hydrogen conversion via inverted metamorphic multi junction semiconductor architectures
year: 2017

abstract: Methylammonium lead iodide perovskites are considered direct bandgap semiconductors. Here we show that in fact they present a weakly indirect bandgap 60 meV below the direct bandgap transition. This is a consequence of spin orbit coupling resulting in Rashba splitting of the conduction band. The indirect nature of the bandgap explains the apparent contradiction of strong absorption and long charge carrier lifetime. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure induced reduction in local electric field around the Pb atom. The nature of the bandgap becomes increasingly more direct, resulting in five times faster charge carrier recombination, and a doubling of the radiative efficiency. At hydrostatic pressures above 325 MPa, MAPI undergoes a reversible phase transition resulting in a purely direct bandgap semiconductor. The pressure induced changes suggest epitaxial and synthetic routes to higher efficiency optoelectronic devices.
author_names: Tianyi Wang

Benjamin Daiber

Jarvist Moore Frost

Sander A Mann

Erik C Garnett

Aron Walsh

Bruno Ehrler
title: Indirect to direct bandgap transition in methylammonium lead halide perovskite
year: 2016

abstract: Recently rediscovered black phosphorus is a layered semiconductor with promising electronic and photonic properties. Dynamic control of its bandgap can allow for the exploration of new physical phenomena. However, theoretical investigations and photoemission spectroscopy experiments indicate that in its few layer form, an exceedingly large electric field in the order of several volts per nanometre is required to effectively tune its bandgap, making the direct electrical control unfeasible. Here we reveal the unique thickness dependent bandgap tuning properties in intrinsic black phosphorus, arising from the strong interlayer electronic state coupling. Furthermore, leveraging a 10 nm thick black phosphorus, we continuously tune its bandgap from ~300 to below 50 meV, using a moderate displacement field up to 1.1 V nm 1. Such dynamic tuning of bandgap may not only extend the operational wavelength range of tunable black phosphorus photonic devices, but also pave the way for the investigation of electrically tunable topological insulators and semimetals.
author_names: Bingchen Deng

Vy Tran

Yujun Xie

Hao Jiang

Cheng Li

Qiushi Guo

Xiaomu Wang

He Tian

Steven J Koester

Han Wang

Judy J Cha

Qiangfei Xia

Li Yang

Fengnian Xia
title: Efficient electrical control of thin film black phosphorus bandgap
venue: Nature communications
year: 2017

abstract: The possibility of tailoring physical properties by changing the number of layers in van der Waals crystals is one of the driving forces behind the emergence of two dimensional materials. One example is bulk MoS2, which changes from an indirect gap semiconductor to a direct bandgap semiconductor in the monolayer form. Here, we show a much bigger tuning range with a complete switching from a metal to a semiconductor in atomically thin PtSe2 as its thickness is reduced. Crystals with a thickness of ~13 nm show metallic behavior with a contact resistance as low as 70 O*um. As they are thinned down to 2.5 nm and below, we observe semiconducting behavior. In such thin crystals, we demonstrate ambipolar transport with a bandgap smaller than 2.2 eV and an on/off ratio of ~105. Our results demonstrate that PtSe2 possesses an unusual behavior among 2D materials, enabling novel applications in nano and optoelectronics.The electronic band structure of van der Waals crystals is strongly sensitive to the number of layers. Here, the authors observe a thickness dependent metal to semiconductor transition in layered PtSe2 by means of electrical transport measurements.
author_names: Alberto Ciarrocchi

Ahmet Avsar

Dmitry Ovchinnikov

Andras Kis
title: Thickness modulated metal to semiconductor transformation in a transition metal dichalcogenide
venue: Nature Communications
year: 2018

abstract: Few layer Tellurium, an elementary semiconductor, succeeds most of striking physical properties that black phosphorus (BP) offers and could be feasibly synthesized by simple solution based methods. It is comprised of non covalently bound parallel Te chains, among which covalent like feature appears. This feature is, we believe, another demonstration of the previously found covalent like quasi bonding (CLQB) where wavefunction hybridization does occur. The strength of this inter chain CLQB is comparable with that of intra chain covalent bonding, leading to closed stability of several Te allotropes. It also introduces a tunable bandgap varying from nearly direct 0.31 eV (bulk) to indirect 1.17 eV (2L) and four (two) complex, highly anisotropic and layer dependent hole (electron) pockets in the first Brillouin zone. It also exhibits an extraordinarily high hole mobility ~10$^5$ cm$^2$/Vs) and strong optical absorption along the non covalently bound direction, nearly isotropic and layer dependent optical properties, large ideal strength over 20% better environmental stability than BP and unusual crossover of force constants for interlayer shear and breathing modes. All these results manifest that the few layer Te is an extraordinary high mobility, high optical absorption, intrinsic anisotropy, low cost fabrication, tunable bandgap, better environmental stability and nearly direct bandgap semiconductor. This "one dimension like" few layer Te, together with other geometrically similar layered materials, may promote the emergence of a new family of layered materials.
author_names: Jingsi Qiao

Yuhao Pan

Feng Yang

Cong Wang

Yang Chai

Wei Ji
title: Few layer Tellurium: one dimensional like layered elementary semiconductor with striking physical properties
year: 2017

abstract: Since the establishment of perovskite solar cells (PSCs) there has been an intense search for alternative materials to replace lead and improve their stability toward moisture and light. As single metal perovskite structures have yielded unsatisfactory performances, an alternative is the use of double perovskites that incorporate a combination of metals. To this day, only a handful of these compounds have been synthesized, but most of them have indirect bandgaps and/or do not have bandgaps energies well suited for photovoltaic applications. Here we report the synthesis and characterization of a unique mixed metal <111> oriented layered perovskite, Cs4CuSb2Cl12 (1) that incorporates Cu2+ and Sb3+ into layers that are three octahedra thick (n 3) In addition to being made of abundant and nontoxic elements, we show that this material behaves as a semiconductor with a direct bandgap of 1.0 eV and its conductivity is 1 order of magnitude greater than that of MAPbI3 (MA methylammonium) Furthermore, 1 has high photo and thermal stability and is tolerant to humidity. We conclude that 1 is a promising material for photovoltaic applications and represents a new type of layered perovskite structure that incorporates metals in 2+ and 3+ oxidation states, thus significantly widening the possible combinations of metals to replace lead in PSCs.
author_names: Brenda Cabral Vargas

Estrella Ramos

Enrique Perez-Gutierrez

Juan Carlos Alonso

Diego Solis-Ibarra
title: A Direct Bandgap Copper Antimony Halide Perovskite.
venue: Journal of the American Chemical Society
year: 2017

abstract: The development of non equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. Here, we describe the fabrication of uniform diameter, direct bandgap Ge1 xSnx alloy nanowires, with a Sn incorporation up to 9.2 at. far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom up growth paradigm using noble metal and metal alloy catalysts. Metal alloy catalysts permitted a greater inclusion of Sn in Ge nanowires compared with conventional Au catalysts, when used during vapour liquid solid growth. The addition of an annealing step close to the Ge Sn eutectic temperature (230 degC) during cool down, further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple phase boundary during growth.
author_names: Subhajit Biswas

Jessica Doherty

Dzianis Saladukha

Quentin M Ramasse

Dipanwita Majumdar

Moneesh Upmanyu

Achintya Singha

Tomasz J Ochalski

Michael A Morris

Justin D Holmes
title: Non equilibrium induction of tin in germanium: towards direct bandgap Ge1 xSnx nanowires
venue: Nature communications
year: 2016

abstract: Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN) based superconductors with the wide bandgap family of semiconductors silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN) We apply molecular beam epitaxy to grow an AlGaN/GaN quantum well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high mobility, two dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor an electronic gain element to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.
author_names: Rusen Yan

Guru Khalsa

Suresh Vishwanath

Yimo Han

John Wright

Sergei Rouvimov

D Scott Katzer

Neeraj Nepal

Brian P Downey

David A Muller

Huili Grace Xing

David J Meyer

Debdeep Jena
title: GaN/NbN epitaxial semiconductor/superconductor heterostructures
venue: Nature
year: 2018

abstract: The CH3NH3PbI3 (methylammonium lead triiodide) perovskite semiconductor system has been viewed as a blockbuster research material during the last five years. Because of its complicated architecture, several of its technological, physical and geometrical issues have been examined many times. Yet this has not assisted in overcoming a number of problems in the field nor in enabling the material to be marketed. For instance, these studies have not clarified the nature and type of hydrogen bonding and other noncovalent interactions involved; the origin of hysteresis; the actual role of the methylammonium cation; the nature of polarity associated with the tetragonal geometry; the unusual origin of various frontier orbital contributions to the conduction band minimum; the underlying phenomena of spin orbit coupling that causes significant bandgap reduction; and the nature of direct to indirect bandgap transition features. Arising from many recent reports, it is now a common belief that the I**H N interaction formed between the inorganic framework and the ammonium group of CH3NH3+ is the only hydrogen bonded interaction responsible for all temperature dependent geometrical polymorphs of the system, including the most stable one that persists at low temperatures, and the significance of all other noncovalent interactions has been overlooked. This study focussed only on the low temperature orthorhombic polymorph of CH3NH3PbI3 and CD3ND3PbI3, where D refers deuterium. Together with QTAIM, DORI and RDG based charge density analyses, the results of density functional theory calculations with PBE with and without van der Waals corrections demonstrate that the prevailing view of hydrogen bonding in CH3NH3PbI3 is misleading as it does not alone determine the a b+a tilting pattern of the PbI64 octahedra. This study suggests that it is not only the I**H/D N, but also the I**H/D C hydrogen/deuterium bonding and other noncovalent interactions (viz. tetrel pnictogen and lump hole bonding interactions) that are ubiquitous in the orthorhombic CH3NH3PbI3/CD3ND3PbI3 perovskite geometry. Their interplay determines the overall geometry of the polymorph, and are therefore responsible in part for the emergence of the functional optical properties of this material. This study also suggests that these interactions should not be regarded as the sole determinants of octahedral tilting since lattice dynamics is known to play a critical role as well, a common feature in many inorganic perovskites both in the presence and the absence of the encaged cation, as in CsPbI3/WO3 perovskites, for example.
author_names: Pradeep R Varadwaj

Arpita Varadwaj

Helder M Marques

Koichi Yamashita
title: Significance of hydrogen bonding and other noncovalent interactions in determining octahedral tilting in the CH3NH3PbI3 hybrid organic inorganic halide perovskite solar cell semiconductor
venue: Scientific Reports
year: 2019