What is a Metal Oxide Semiconductor?

Jump to: Structure-Property Relationship | Examples | Band gap | Defects | Impurities and Doping | Applications

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Metal oxide semiconductors consist of positive metallic ions connected to negative oxygen ions via ionic bonds. Typically in solid form, they are non-stoichiometric compounds, which means their chemical formula cannot be exactly represented by a ratio of small natural numbers. Ionic bonds are not as strong as covalent bonds, so metal oxide semiconductors have more intrinsic defects than elemental semiconductor materials such as silicon, leading to the non-stoichiometry.

Structure-Property Relationship

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Each metal oxide semiconductor has different crystalline structures, defect types, and energy band levels. Depending on the desired properties of the resulting material, these structural features can be tailored via doping, particle size and more. The movement of electrons and holes in semiconductor nanomaterials are affected by size and geometry of said materials.

The s electronic shell of metal oxides is typically filled which gives them excellent thermal and chemical stability. The d electronic shells vary in their degree of filling depending on the metal which gives rise to diverse optical and electronic properties. As the metal varies, so does its valency, with many metals having multiple oxidation states, resulting in different oxygen-to-metal ratios leading to a variety of electronic structures across the material range.

Metal oxide semiconductor properties include a large range of band gaps from copper (II) oxide’s low 1.35 eV band gap to tin (IV) oxide’s large 3.8 eV band gap. Metal oxides also have high dielectric constants, otherwise known as permittivity, desirable for energy storage and electronic applications. Other properties include fast electronic transitions, electrochromic behavior and enhanced electrical conductivity.

Examples of Metal Oxide Semiconductors

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Metal Oxides

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Metal Oxide Semiconductor Band gap

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Band theory in the context of metal oxide semiconductors describes the range of energy levels within the materials and helps explain their semi-conducting properties. The valence band is the energy level that holds the outermost electrons. The conduction band contains energy levels that do not have electrons in them at room temperature. The band gap is the difference in energy between the valence and conduction band. This difference is the minimum energy that electrons need to move from the valence band to the conduction band.

Once electrons transfer from valence band to conduction band, they will be able to move freely, which means the formation of current. External heat or a photon with energy higher than band gap energy could excite electrons so that they can leave the valence band and transit into the conduction band, leaving orbital holes in the valence band.

For metal oxide semiconductors the conduction band minimum (CBM) mainly consist of the metal (M) ns orbitals and valence band maximum (VBM) consists mainly of the oxygen (O) 2p orbitals. These orbitals do experience hybridization with some orbitals of the opposite element. The conduction band varies significantly depending on the metal's electronic structure and orbital filling. However, this also effects the valence band as the strength of hybridization between the oxygen p orbitals and metal orbitals varies.

The interactions between the metal and oxygen orbitals results in a big difference between the transport of the different charge carriers. Metal ns orbitals forming the CBM are highly dispersive, resulting in a small effective mass for electrons. While the oxygen 2p orbitals forming the VBM are more localized, leading to a larger effective mass for holes. As a result, electrons typically exhibit much higher mobility than holes in these materials.

Charge Carrier Mobility

Charge carrier mobility is inversely proportional to the carrier effective mass (m*) where τ is the free carrier scattering time:

μ = eτ/m*

Therefore, the smaller electron effective mass in metal oxide indicates the better electron transport in comparison to hole transport. This means that most metal oxide semiconductors are n-type.

Band Gap Energy

For metal oxide semiconductors the band gap is generally wide (E_g = >3 eV):

The wide-band gap means these materials can withstand large electric fields and have high breakdown voltages. Electronic devices based on metal oxide semiconductors can be operated at high temperatures and power as a result of these properties. The wide band gap also provides high optical transparency in the visible region of the electromagnetic spectrum, which is a desired characteristic in transparent electronics.

Defects

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There are a range of defect types that influence the properties of metal oxide semiconductors including, point defects, line defects, plane defects, and volume defects.

Point Defects

Point defects are zero-dimensional defects that cause imperfections in the crystal lattice of a material that are localized to a single atomic site or a small group of adjacent sites and can be mainly divided into three kinds:

In metal oxide semiconductors there are six classic point defects:

Line Defects

Line defects are one-dimensional and arise due to the misalignment of atoms in a crystal lattice. There are two types of line defects: edge dislocations and screw dislocation.

Plane Defects

Planar defects are two-dimensional and include the formation of planes or boundaries that separate the particle into regions with the same crystal structure but different orientations. There are three types of planar defects including stacking faults, grain boundaries and twin boundaries.

Volume Defects

Volume defects are three-dimensional and include changes in crystal structure to include two or more chemical species in one or more crystal sites. As a result, voids and different disordered states appear in the crystal lattice.

Impurities and Doping

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The impurity level in metal oxide semiconductors significantly affects their carrier concentration, microstructure, and electronic structure. While impurities can sometimes reduce desirable properties, they can also be deliberately introduced to enhance or tune material characteristics. This intentional introduction of impurities to improve material properties is known as doping.

Doping can introduce extra energy levels within the band gap either close to the valence band or conduction band depending on the type of dopant:

• Electron donor or n-type doping brings in energy levels near the conduction band
• Electron acceptor or p-type doping brings in energy levels near the valence band

Doped semiconductors have higher carrier concentration compared to their intrinsic state because both electrons in n-type semiconductors and holes in p-type semiconductors contribute to conductivity significantly. Doping directly impacts charge carrier mobility and can lead to conductivities that are comparable to metals.

Doping Example

The most famous example of a doped metal oxide is indium tin oxide (ITO), widely used as a transparent electrode in optoelectronic applications. Tin-doped indium oxide typically consists of approximately 74% indium, 8% tin, and 18% oxygen by weight. Compared to undoped indium oxide, ITO exhibits a significantly lower resistivity.

This behavior is characteristic of n-type doping, where new energy levels form near the conduction band. Crucially, this is due to the valency difference between the ions: indium normally exists as In³⁺, while tin substitutes as Sn⁴⁺. When a Sn⁴⁺ ion replaces an In ion in the lattice, it introduces an extra electron into the structure, increasing the number of free charge carriers.

The presence of extra electrons affects the band structure and Fermi level of the n-type semiconductor (the energy level at which the probability of an electron being present is 50%). The Fermi level is shifted closer to the conduction band. This is because the extra electrons populate the energy states near the bottom of the conduction band, reducing the energy gap.

To maintain charge balance, this substitution also leads to the formation of oxygen vacancies, which themselves act as electron donors. The combination of extra electrons from higher-valency Sn ions and oxygen vacancies significantly increases the carrier concentration, lowers the activation energy for electrical conduction, and enhances the overall conductivity of the material.

Applications

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