Band-gap engineering

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Band-gap engineering is the process of controlling or altering the band gap of a material. This is typically done to semiconductors by controlling the composition of alloys, constructing layered materials with alternating compositions, or by inducing strain either epitaxially or topologically. A band gap is the range in a solid where no electron state can exist. The band gap of insulators is much larger than in semiconductors. Conductors or metals have a much smaller or nonexistent band gap than semiconductors since the valence and conduction bands overlap. Controlling the band gap allows for the creation of desirable electrical properties.

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Molecular-beam epitaxy (MBE)

Molecular-beam epitaxy is a technique used to construct thin epitaxial films of materials ranging from oxides to semiconductors to metals. Different beams of atoms and molecules in an ultra-high vacuum environment are shot onto a nearly atomically clean crystal, creating a layering effect. This is a type of thin-film deposition. Semiconductors are the most commonly used material due to their use in electronics. Technologies such as quantum well devices, super-lattices, and lasers are possible with MBE. Epitaxial films are useful due to their ability to be produced with electrical properties different from those of the substrate, either higher purity, or fewer defects or with a different concentration of electrically active impurities as desired. [1] Varying the composition of the material alters the band gap due to bonding of different atoms with differing energy level gaps.

Strain-induced band-gap engineering

Semiconducting materials are able to be altered with strain-inducing from tunable sizes and shapes due to quantum confinement effects. A larger tunable bandgap range is possible due to the high elastic limit of semiconducting nanostructures (Guerra, [2] and Guerra and Vezenov [3] ). Strain is the ratio of extension to original length, and can be used on the nanoscale. [4] [5]

Thulin and Guerra (2008) [6] theoretically quantified a strain-inducing method that they used to engineer the material properties of anatase titania. They studied its electronic band structure over a range of biaxial strain by utilizing both the density functional theory within the generalized gradient approximation (GGA) and quasiparticle theory calculations within the GW approximation. They found that the strain-modified material is suitable for use as a high efficiency photoanode in a photoelectrochemical cell. They tracked the changes to the band gap and the charge carrier effective masses versus the total pressure associated with the strained lattice. Both the GGA and the GW approximation predict a linear relationship between the change in band gap and the total pressure, but they found that the GGA underestimates the slope by more than 57% with respect to the GW approximation result of 0.0685 eV/GPa.


ZnO nanowires

ZnO Nanowires are used in nanogenerators, nanowire field effect transistors, piezo-electric diodes, and chemical sensors. Several studies have been conducted on the effect of strain on different physical properties. Sb-doped ZnO nanowires experience variation in resistance when exposed to strain. Bending strain can induce an increase in electrical conductance. Strain can also induce change of transport properties and band-gap variation. By correlating these two effects under experimentation the variation of transport properties as a function of band-gap can be generated. Electrical measurements are obtained using scanning tunnelling microscope-transmission electron microscope probing system. [4]

Energy band-gap engineering of graphene nanoribbons

When lithographically generated graphene ribbons are laterally confined in charge it creates an energy gap near the charge neutrality point. The narrower the ribbons result in larger energy gap openings based on temperature dependent conductance. A narrow ribbon is considered a quasi one dimensional system in which an energy band gap opening is expected. Single sheets of graphene are mechanically extracted from bulk graphite crystals onto a silicon substrate and are contacted with Cr/Au metal electrodes. Hydrogen silsesquioxane is spun onto the samples to form an etch mask and then oxygen plasma is used to etch away the unprotected graphene. [7]

Related Research Articles

A semiconductor is a material, which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. Its conducting properties may be altered in useful ways by introducing impurities ("doping") into the crystal structure. When two differently doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers, which include electrons, ions, and electron holes, at these junctions is the basis of diodes, transistors, and most modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the so-called "metalloid staircase" on the periodic table. After silicon, gallium arsenide is the second-most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.

<span class="mw-page-title-main">Band gap</span> Energy range in a solid where no electron states can exist

In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy required to promote a valence electron bound to an atom to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current. It is closely related to the HOMO/LUMO gap in chemistry. If the valence band is completely full and the conduction band is completely empty, then electrons cannot move within the solid because there are no available states. If the electrons are not free to move within the crystal lattice, then there is no generated current due to no net charge carrier mobility. However, if some electrons transfer from the valence band to the conduction band, then current can flow. Therefore, the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have very small band gaps or none, because the valence and conduction bands overlap to form a continuous band.

<span class="mw-page-title-main">Epitaxy</span> Crystal growth process relative to the substrate

Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epitaxial film or epitaxial layer. The relative orientation(s) of the epitaxial layer to the seed layer is defined in terms of the orientation of the crystal lattice of each material. For most epitaxial growths, the new layer is usually crystalline and each crystallographic domain of the overlayer must have a well-defined orientation relative to the substrate crystal structure. Epitaxy can involve single-crystal structures, although grain-to-grain epitaxy has been observed in granular films. For most technological applications, single domain epitaxy, which is the growth of an overlayer crystal with one well-defined orientation with respect to the substrate crystal, is preferred. Epitaxy can also play an important role while growing superlattice structures.

<span class="mw-page-title-main">Quantum well</span> Concept in quantum mechanics

A quantum well is a potential well with only discrete energy values.

A "photoelectrochemical cell" is one of two distinct classes of device. The first produces electrical energy similarly to a dye-sensitized photovoltaic cell, which meets the standard definition of a photovoltaic cell. The second is a photoelectrolytic cell, that is, a device which uses light incident on a photosensitizer, semiconductor, or aqueous metal immersed in an electrolytic solution to directly cause a chemical reaction, for example to produce hydrogen via the electrolysis of water.

Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction. Whereas traditional electronics are based on control of charge carriers, practical magnetic semiconductors would also allow control of quantum spin state. This would theoretically provide near-total spin polarization, which is an important property for spintronics applications, e.g. spin transistors.

<span class="mw-page-title-main">Indium antimonide</span> Chemical compound

Indium antimonide (InSb) is a crystalline compound made from the elements indium (In) and antimony (Sb). It is a narrow-gap semiconductor material from the III-V group used in infrared detectors, including thermal imaging cameras, FLIR systems, infrared homing missile guidance systems, and in infrared astronomy. The indium antimonide detectors are sensitive between 1–5 μm wavelengths.

Indium gallium arsenide (InGaAs) is a ternary alloy of indium arsenide (InAs) and gallium arsenide (GaAs). Indium and gallium are elements of the periodic table while arsenic is a element. Alloys made of these chemical groups are referred to as "III-V" compounds. InGaAs has properties intermediate between those of GaAs and InAs. InGaAs is a room-temperature semiconductor with applications in electronics and photonics.

<span class="mw-page-title-main">Bismuth telluride</span> Chemical compound

Bismuth telluride (Bi2Te3) is a gray powder that is a compound of bismuth and tellurium also known as bismuth(III) telluride. It is a semiconductor, which, when alloyed with antimony or selenium, is an efficient thermoelectric material for refrigeration or portable power generation. Bi2Te3 is a topological insulator, and thus exhibits thickness-dependent physical properties.

Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons.

Strain engineering refers to a general strategy employed in semiconductor manufacturing to enhance device performance. Performance benefits are achieved by modulating strain, as one example, in the transistor channel, which enhances electron mobility and thereby conductivity through the channel. Another example are semiconductor photocatalysts strain-engineered for more effective use of sunlight.

<span class="mw-page-title-main">Transparent conducting film</span> Optically transparent and electrically conductive material

Transparent conducting films (TCFs) are thin films of optically transparent and electrically conductive material. They are an important component in a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics. While indium tin oxide (ITO) is the most widely used, alternatives include wider-spectrum transparent conductive oxides (TCOs), conductive polymers, metal grids and random metallic networks, carbon nanotubes (CNT), graphene, nanowire meshes and ultra thin metal films.

Photoelectrochemistry is a subfield of study within physical chemistry concerned with the interaction of light with electrochemical systems. It is an active domain of investigation. One of the pioneers of this field of electrochemistry was the German electrochemist Heinz Gerischer. The interest in this domain is high in the context of development of renewable energy conversion and storage technology.

<span class="mw-page-title-main">Silicene</span> Two-dimensional allotrope of silicon

Silicene is a two-dimensional allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene. Contrary to graphene, silicene is not flat, but has a periodically buckled topology; the coupling between layers in silicene is much stronger than in multilayered graphene; and the oxidized form of silicene, 2D silica, has a very different chemical structure from graphene oxide.

<span class="mw-page-title-main">Core–shell semiconductor nanocrystal</span>

Core–shell semiconducting nanocrystals (CSSNCs) are a class of materials which have properties intermediate between those of small, individual molecules and those of bulk, crystalline semiconductors. They are unique because of their easily modular properties, which are a result of their size. These nanocrystals are composed of a quantum dot semiconducting core material and a shell of a distinct semiconducting material. The core and the shell are typically composed of type II–VI, IV–VI, and III–V semiconductors, with configurations such as CdS/ZnS, CdSe/ZnS, CdSe/CdS, and InAs/CdSe Organically passivated quantum dots have low fluorescence quantum yield due to surface related trap states. CSSNCs address this problem because the shell increases quantum yield by passivating the surface trap states. In addition, the shell provides protection against environmental changes, photo-oxidative degradation, and provides another route for modularity. Precise control of the size, shape, and composition of both the core and the shell enable the emission wavelength to be tuned over a wider range of wavelengths than with either individual semiconductor. These materials have found applications in biological systems and optics.

<span class="mw-page-title-main">Transition metal dichalcogenide monolayers</span> Thin semiconductors

Transition-metal dichalcogenide (TMD or TMDC) monolayers are atomically thin semiconductors of the type MX2, with M a transition-metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. They are part of the large family of so-called 2D materials, named so to emphasize their extraordinary thinness. For example, a MoS2 monolayer is only 6.5 Å thick. The key feature of these materials is the interaction of large atoms in the 2D structure as compared with first-row transition-metal dichalcogenides, e.g., WTe2 exhibits anomalous giant magnetoresistance and superconductivity.

<span class="mw-page-title-main">I-III-VI semiconductors</span>

I-III-VI2 semiconductors are solid semiconducting materials that contain three or more chemical elements belonging to groups I, III and VI (IUPAC groups 1/11, 13 and 16) of the periodic table. They usually involve two metals and one chalcogen. Some of these materials have a direct bandgap, Eg, of approximately 1.5 eV, which makes them efficient absorbers of sunlight and thus potential solar cell materials. A fourth element is often added to a I-III-VI2 material to tune the bandgap for maximum solar cell efficiency. A representative example is copper indium gallium selenide (CuInxGa(1–x)Se2, Eg = 1.7–1.0 eV for x = 0–1), which is used in copper indium gallium selenide solar cells.

<span class="mw-page-title-main">Gallium nitride nanotube</span>

Gallium nitride nanotubes (GaNNTs) are nanotubes of gallium nitride. They can be grown by chemical vapour deposition.

Zinc cadmium phosphide arsenide (Zn-Cd-P-As) is a quaternary system of group II (IUPAC group 12) and group V (IUPAC group 15) elements. Many of the inorganic compounds in the system are II-V semiconductor materials. The quaternary system of II3V2 compounds, (Zn1−xCdx)3(P1−yAsy)2, has been shown to allow solid solution continuously over the whole compositional range. This material system and its subsets have applications in electronics, optoelectronics, including photovoltaics, and thermoelectrics.

References

  1. ↑ Arthur, John R. (2002). "Molecular beam epitaxy". Surface Science. Elsevier BV. 500 (1–3): 189–217. Bibcode:2002SurSc.500..189A. doi:10.1016/s0039-6028(01)01525-4. ISSN   0039-6028.
  2. ↑ U.S. Pat. No. 7,485,799, "Stress-induced bandgap-shifted semiconductor photoelectrolytic/photocatalytic/photovoltaic surface and method for making same," John M. Guerra, Priority date May 7, 2002. Assigned to Nanoptek Corporation.
  3. ↑ NASA Contract No. NAS2-03114 with Nanoptek Corporation, "Stress-induced bandgap-shifted titania photocatalyst for hydrogen generation," J. Guerra and D. Vezenov, 2002.
  4. 1 2 Shao, Rui-wen; Zheng, Kun; Wei, Bin; Zhang, Yue-fei; Li, Yu-jie; et al. (2014). "Bandgap engineering and manipulating electronic and optical properties of ZnO nanowires by uniaxial strain". Nanoscale. Royal Society of Chemistry (RSC). 6 (9): 4936–4941. Bibcode:2014Nanos...6.4936S. doi:10.1039/c4nr00059e. ISSN   2040-3364. PMID   24676099.
  5. ↑ "Stress & Strain." PhysicsNetcouk RSS. Accessed December 4, 2014. http://physicsnet.co.uk/a-level-physics-as-a2/materials/stress-strain/.
  6. ↑ Thulin, Lukas; Guerra, John (May 14, 2008). "Calculations of strain-modified anatase TiO 2 band structures". Physical Review B. 77 (19): 195112. doi:10.1103/PhysRevB.77.195112. ISSN   1098-0121.
  7. ↑ Han, Melinda Y.; Özyilmaz, Barbaros; Zhang, Yuanbo; Kim, Philip (May 16, 2007). "Energy Band-Gap Engineering of Graphene Nanoribbons". Physical Review Letters. 98 (20): 206805. arXiv: cond-mat/0702511 . Bibcode:2007PhRvL..98t6805H. doi:10.1103/physrevlett.98.206805. ISSN   0031-9007. PMID   17677729. S2CID   6309177.
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