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Dopant
Dopant
Dopant
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A dopant (also called a doping agent) is a small amount of a substance added to a material to alter its physical properties, such as electrical or optical properties. The amount of dopant is typically very low compared to the material being doped.

When doped into crystalline substances, the dopant's atoms get incorporated into the crystal lattice of the substance. The crystalline materials are frequently either crystals of a semiconductor such as silicon and germanium for use in solid-state electronics, or transparent crystals for use in the production of various laser types; however, in some cases of the latter, noncrystalline substances such as glass can also be doped with impurities.

In solid-state electronics using the proper types and amounts of dopants in semiconductors is what produces the p-type semiconductors and n-type semiconductors that are essential for making transistors and diodes.

Transparent crystals

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Lasing media

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The procedure of doping tiny amounts of the metals chromium (Cr), neodymium (Nd), erbium (Er), thulium (Tm), ytterbium (Yb), and a few others, into transparent crystals, ceramics, or glasses is used to produce the active medium for solid-state lasers. It is in the electrons of the dopant atoms that a population inversion can be produced, and this population inversion is essential for the stimulated emission of photons in the operation of all lasers.

In the case of the natural ruby, what has occurred is that a tiny amount of chromium dopant has been naturally distributed through a crystal of aluminium oxide (corundum). This chromium both gives a ruby its red color, and also enables a ruby to undergo a population inversion and act as a laser. The aluminium and oxygen atoms in the transparent crystal of aluminium oxide served simply to support the chromium atoms in a good spatial distribution, and otherwise, they do not have anything to do with the laser action.

In other cases, such as in the neodymium YAG laser, the crystal is synthetically made and does not occur in nature. The human-made yttrium aluminium garnet crystal contains millions of yttrium atoms in it, and due to its physical size, chemical valence, etc., it works well to take the place of a small minority of yttrium atoms in its lattice, and to replace them with atoms from the rare-earth series of elements, such as neodymium. Then, these dopant atoms actually carry out the lasing process in the crystal. The rest of the atoms in the crystal consist of yttrium, aluminium, and oxygen atoms, but just as above, these other three elements function to simply support the neodymium atoms. In addition, the rare-earth element erbium can readily be used as the dopant rather than neodymium, giving a different wavelength of its output.

In many optically-transparent hosts, such active centers may keep their excitation for a time on the order of milliseconds, and relax with stimulated emission, providing the laser action. The amount of dopant is usually measured in atomic percent. Usually the relative atomic percent is assumed in the calculations, taking into account that the dopant ion can substitute in only part of a site in a crystalline lattice. The doping can be also used to change the refraction index in optical fibers, especially in the double-clad fibers. The optical dopants are characterized with lifetime of excitation and the effective absorption and emission cross-sections, which are main parameters of an active dopant. Usually, the concentration of optical dopant is of order of few percent or even lower. At large density of excitation, the cooperative quenching (cross-relaxation) reduces the efficiency of the laser action.

Examples

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The medical field has some use for erbium-doped laser crystals for the laser scalpels that are used in laser surgery. Europium, neodymium, and other rare-earth elements are used to dope glasses for lasers. Holmium-doped and neodymium yttrium aluminium garnets (YAGs) are used as the active laser medium in some laser scalpels.[1]

Phosphors and scintillators

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In context of phosphors and scintillators, dopants are better known as activators, and are used to enhance the luminescence process.[2]

Semiconductors

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Main article: Doping (semiconductor)

The addition of a dopant to a semiconductor, known as doping, has the effect of shifting the Fermi levels within the material.[citation needed] This results in a material with predominantly negative (n-type) or positive (p-type) charge carriers depending on the dopant variety. Pure semiconductors that have been altered by the presence of dopants are known as extrinsic semiconductors (see intrinsic semiconductor). Dopants are introduced into semiconductors in a variety of techniques: solid sources, gases, spin on liquid, and ion implanting. See ion implantation, surface diffusion, and solid sources footnote.

Others

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The color of some gemstones is caused by dopants. For example, ruby and sapphire are both aluminium oxide, the former getting its red color from chromium atoms, and the latter doped with any of several elements, giving a variety of colors.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Dopant

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from Grokipedia
A dopant is an impurity atom or intentionally introduced into a pure host material, most commonly a such as or , to alter its electrical conductivity and other properties by creating either excess electrons or holes as charge carriers. This process, known as doping, enables the control and enhancement of conductance in semiconductors by introducing excess charge carriers, forming the foundation of modern . Dopants are classified into two primary types based on their effect on the semiconductor's band structure: n-type dopants, which are typically group V elements like , , or that donate extra electrons to the conduction band, and p-type dopants, which are group III elements such as , aluminum, or that create "holes" in the valence band by accepting electrons. In n-type semiconductors, electrons become the majority charge carriers, while in p-type, holes dominate, allowing for the creation of p-n junctions essential for devices like diodes and transistors. The concentration of dopants, often ranging from 10^13 to 10^18 atoms per cubic centimeter, precisely controls the material's resistivity and enables tailored performance in applications. The significance of dopants lies in their role in enabling semiconductor devices that power , , and technologies, with doping techniques having evolved from early methods in the mid-20th century to advanced for nanoscale precision. By modulating the bandgap and carrier mobility, dopants not only enhance conductivity but also influence , such as in light-emitting diodes (LEDs) and solar cells, where they improve and wavelength tuning. Beyond semiconductors, dopants are employed in optical materials, dielectrics, and superconductors to achieve desired properties such as lasing or enhanced critical temperature. Ongoing research focuses on novel dopants, including rare earth elements, to address challenges in high-speed and .

Fundamentals

Definition and Purpose

A dopant is a trace impurity element or compound intentionally introduced into a pure host material, such as a semiconductor or crystal lattice, at concentrations typically below 1% to modify its electrical, optical, thermal, or mechanical properties without significantly altering the host's overall structure. These impurities, often atoms from group III, V, or rare-earth elements, are added in controlled amounts—usually on the order of parts per million to parts per billion—to tailor the material's functionality for specific technological needs. The primary purpose of doping is to engineer desirable behaviors in the host material, such as generating charge carriers (free electrons or holes) in semiconductors, inducing color centers for optical absorption, enhancing efficiency, or fine-tuning the bandgap for light emission or detection. Dopants achieve this by occupying either substitutional sites (replacing host atoms in the lattice) or sites (fitting between lattice positions), which introduces localized states that interact with the host's electronic . For instance, in semiconductors, acts as a donor to increase , while in optical crystals like yttrium aluminum garnet, ions enable action through radiative transitions. In terms of general effects, doping shifts the —the energy at which the probability of finding an is 50%—and introduces donor or acceptor energy levels within the bandgap of the host material. For n-type doping, donor levels lie just below the conduction band, facilitating the release of s into it and elevating the ; conversely, p-type doping places acceptor levels above the valence band, promoting holes and lowering the . In energy band diagrams, these modifications appear as shallow impurity levels splitting off from the band edges, enabling controlled carrier concentrations that transform insulators or poor conductors into highly functional materials. The concept of doping was first recognized in early 20th-century research, building on studies of impurity effects in materials like and during the and . A pivotal milestone occurred in 1947 at Bell Laboratories, where the invention of the relied on intentionally doped to achieve amplification, marking the dawn of modern technology.

Types and Classification

Dopants are classified primarily by their chemical elements, which determine their valence electron contribution and interaction with the host lattice. Elements from Group III of the periodic table, such as and aluminum, act as acceptors in silicon-based semiconductors by providing three s, creating hole-accepting sites that enhance p-type conductivity. Similarly, Group V elements like and serve as donors, contributing five s to introduce excess electrons for n-type doping. Transition metals, including and iron, are employed for their ability to create deep energy levels that influence recombination processes and in the host material. Rare-earth elements, such as and , are notable for inducing effects through intra-ionic transitions, making them suitable for optical applications. Classification by valence further distinguishes dopants based on their energy level positions relative to the host's band edges. Shallow dopants, like in , have ionization energies close to the conduction band (approximately 0.045 eV), allowing easy and effective carrier generation at . In contrast, deep dopants, such as gold in , position their levels near the midgap (around 0.54 eV), acting as efficient recombination centers or traps rather than primary carriers. Isovalent dopants maintain the same valence as the host atoms, exemplified by in , where they introduce lattice strain without net charge compensation, influencing band structure indirectly. Aliovalent dopants, differing in valence (e.g., in ), require charge compensation mechanisms like vacancy formation to maintain lattice neutrality. Dopants can also be categorized by their functional behavior in the host material. Electrical dopants primarily modify conductivity by introducing free carriers, as seen with group IV-V impurities in elemental semiconductors. Optical dopants alter absorption and emission spectra, often through rare-earth ions that enable specific interactions. Magnetic dopants, typically transition metals, incorporate localized spins to enable quantum effects like spin-dependent transport. A critical aspect of dopant classification involves their limits and segregation coefficients, which govern incorporation stability during processing. The limit represents the maximum concentration before occurs; for in , this is approximately 3 × 10^{20} cm^{-3} at typical annealing temperatures, beyond which inactive precipitates form, reducing effective doping. Segregation coefficients, defined as the ratio of dopant concentration in the solid to that in the liquid phase (k = C_s / C_l), quantify partitioning during ; for in , k ≈ 0.3, leading to uneven distribution and potential pile-up at interfaces. These parameters explain why high concentrations often result in for many dopants, limiting achievable carrier densities.

Doping Processes

Common Techniques

is a thermal process used to introduce dopants into materials, where dopant atoms spread from regions of high concentration to low concentration, primarily through vacancy or mechanisms in the lattice. This method involves heating the material in an atmosphere containing dopant sources, such as gases or doped oxides, allowing atoms to incorporate into the surface and diffuse inward. The process is governed qualitatively by , which describe the of dopants; Fick's states that the diffusion JJ is proportional to the negative gradient of concentration CC, expressed as J=−D∇C,J = -D \nabla C, where DD is the diffusion dependent on temperature and material properties. Ion implantation introduces dopants by accelerating ions to high energies, typically in the range of 10-500 keV, and directing them into the subsurface of the target material to embed them at controlled depths. This physical process creates a precise dopant profile but induces lattice damage from ion collisions, necessitating a subsequent annealing step at elevated temperatures (often 500-1300°C) to repair the and electrically activate the dopants by placing them on substitutional lattice sites. Other techniques include (CVD), which enables epitaxial doping by simultaneously depositing the host material and incorporating dopants from vapor-phase precursors, achieving uniform layers with controlled thickness. (MBE) provides precise control over layer thickness and doping at the atomic scale under , using directed beams of atoms or molecules for sequential deposition. Neutron transmutation doping offers exceptional uniformity, particularly for in , through the nuclear reaction 30Si(n,γ)31Si→31P+β−^{30}\mathrm{Si}(n,\gamma)^{31}\mathrm{Si} \rightarrow ^{31}\mathrm{P} + \beta^-, where thermal neutrons convert stable silicon isotopes into dopants during irradiation in a reactor. Diffusion excels in achieving uniform bulk doping due to its equilibrium-driven nature, while is preferred for shallow junctions with sharp profiles, though it requires additional annealing to mitigate damage. was commercialized in the , revolutionizing precise doping in .

Concentration and Distribution

Dopant concentration is typically expressed in units of atoms per cubic centimeter (cm⁻³) or atomic percent (at%), with the former being more common in contexts due to its direct relation to carrier density. In semiconductors like , typical doping levels range from 10¹⁴ to 10²⁰ cm⁻³, spanning lightly doped regimes for devices requiring low conductivity to heavily doped contacts. These concentrations are controlled to achieve precise electrical properties, with atomic percent providing a host-lattice-normalized measure (e.g., 10¹⁸ cm⁻³ equates to roughly 0.002 at% in , given its atomic density of ~5 × 10²² cm⁻³). The of dopants is modeled using specific profiles that reflect the introduction method, enabling prediction and control of concentration gradients. For , the as-implanted profile approximates a Gaussian distribution, given by: C(x)=Q2πΔRpexp⁡[−(x−Rp)22ΔRp2]C(x) = \frac{Q}{\sqrt{2\pi} \Delta R_p} \exp\left[ -\frac{(x - R_p)^2}{2 \Delta R_p^2} \right]
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