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Gallium phosphide
Gallium phosphide
Gallium phosphide
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Gallium phosphide
GaP ingots (impure)
GaP wafer (electronic device quality)
Names
IUPAC name
Gallium phosphide
Other names
Gallium(III) phosphide
gallanylidynephosphane
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.031.858 Edit this at Wikidata
RTECS number
  • LW9675000
UNII
  • InChI=1S/Ga.P checkY
    Key: HZXMRANICFIONG-UHFFFAOYSA-N checkY
  • InChI=1/Ga.P/rGaP/c1-2
    Key: HZXMRANICFIONG-ZZOGKRAHAQ
  • [Ga]#P
  • [Ga+3].[P-3]
Properties
GaP
Molar mass 100.697 g/mol[1]
Appearance pale orange solid
Odor odorless
Density 4.138 g/cm3[1]
Melting point 1,457 °C (2,655 °F; 1,730 K)[1]
insoluble
Band gap 2.24 eV (indirect, 300 K)[2]
Electron mobility 300 cm2/(V·s) (300 K)[2]
−13.8×10−6 cgs[2]
Thermal conductivity 0.752 W/(cm·K) (300 K)[1]
2.964 (10 µm), 3.209 (775 nm), 3.590 (500 nm), 5.05 (354 nm)[3]
Structure
Zinc blende
T2d-F-43m
a = 544.95 pm[4]
Tetrahedral
Thermochemistry
−88.0 kJ/mol[5]
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
3
1
0
Flash point 110 °C (230 °F; 383 K)
Related compounds
Other anions
 Gallium nitride
Gallium arsenide
Gallium antimonide
Other cations
 Aluminium phosphide
Indium phosphide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Gallium phosphide (GaP), a phosphide of gallium, is a compound semiconductor material with an indirect band gap of 2.24 eV at room temperature. Impure polycrystalline material has the appearance of pale orange or grayish pieces. Undoped single crystals are orange, but strongly doped wafers appear darker due to free carrier absorption. It is odorless and insoluble in water.

GaP has a microhardness of 9450 N/mm2, a Debye temperature of 446 K (173 °C), and a thermal expansion coefficient of 5.3 ×10−6 K−1 at room temperature.[4] Sulfur, silicon or tellurium are used as dopants to produce n-type semiconductors. Zinc is used as a dopant for the p-type semiconductor.

Gallium phosphide has applications in optical systems.[6][7][8] Its static dielectric constant is 11.1 at room temperature.[2] Its refractive index varies between ~3.2 and 5.0 across the visible range, which is higher than in most other semiconducting materials.[3] In its transparent range, its index is higher than almost any other transparent material, including gemstones such as diamond, or non-oxide lenses such as zinc sulfide.

Light-emitting diodes

[edit]

Gallium phosphide has been used in the manufacture of low-cost red, orange, and green light-emitting diodes (LEDs) with low to medium brightness since the 1960s. It is used standalone or together with gallium arsenide phosphide.

Pure GaP LEDs emit green light at a wavelength of 555 nm. Nitrogen-doped GaP emits yellow-green (565 nm) light, zinc oxide doped GaP emits red (700 nm).

Gallium phosphide is transparent for yellow and red light, therefore GaAsP-on-GaP LEDs are more efficient than GaAsP-on-GaAs.

Crystal growth

[edit]

At temperatures above ~900 °C, gallium phosphide dissociates and the phosphorus escapes as a gas. In crystal growth from a 1500 °C melt (for LED wafers), this must be prevented by holding the phosphorus in with a blanket of molten boric oxide in inert gas pressure of 10–100 atmospheres. The process is called liquid encapsulated Czochralski (LEC) growth, an elaboration of the Czochralski process used for silicon wafers.

References

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Cited sources

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Gallium phosphide

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from Grokipedia
Gallium phosphide (GaP) is a III-V compound with the GaP, characterized by a zinc blende and a of 5.45 . It exhibits an indirect bandgap of approximately 2.26 eV at , rendering it transparent in the visible to spectral range from about 0.54 μm to 11 μm, with a averaging 3.1 at 1.4 μm and higher values up to 3.6 at 500 nm. High-purity GaP appears as a transparent, orange, glassy material with a of 4.14 g/cm³, a of around 1470 °C under phosphorus pressure, and notable mechanical properties including a Knoop of 850 kg/mm² and flexural of 100 MPa. Synthesized through methods such as liquid encapsulated Czochralski (LEC) growth from gallium-rich melts or vapor-phase reactions involving precursors like trimethylgallium and , GaP is typically doped n-type with or to achieve carrier concentrations of 10¹⁷–10¹⁸ cm⁻³. As a wide-bandgap , GaP has been pivotal in since the 1960s, particularly for low-cost light-emitting diodes (LEDs) emitting red, orange, and light, including as a substrate for AlGaInP-based devices producing orange-yellow- emission. Despite its indirect bandgap, GaP exhibits visible through mechanisms such as donor-acceptor recombination, with LED emission around 555 nm, while its high stability and chemical resistance support applications in , solar cells, and biological imaging via thin films or nanoparticles. In advanced , GaP's low extinction coefficient (near zero beyond 560 nm) and high dispersion facilitate its use in metasurfaces for visible light manipulation, such as transmission gratings achieving up to 50% efficiency and metalenses with focal lengths around 2.4 mm at 450 nm. Additionally, its transparency and durability make it suitable for visible and mid- to long-wave (MWIR/LWIR) components, including lenses for CO₂ lasers and imagers, often paired with materials like ZnSe in achromatic systems.

Properties

Physical properties

Gallium phosphide has the GaP and a of 100.697 g/mol. It appears as a pale orange to yellow crystalline solid and is odorless. The of gallium phosphide is 4.138 g/cm³ at . Its point is 1,457 °C, though it may decompose before melting under certain conditions, such as during high-temperature processing. Gallium phosphide crystallizes in the zincblende structure, which is cubic, with a of 5.4505 at 300 . This structure contributes to its stability as a material. Key thermal properties include a thermal conductivity of approximately 0.81 /cm· and a coefficient of of 4.65 × 10⁻⁶ /, making it suitable for applications requiring moderate heat dissipation. Mechanically, gallium phosphide is a brittle solid with a Mohs of 5. It has a Knoop of 850 kg/mm². GaP is insoluble in and most organic solvents but can be etched by hot concentrated acids or bases.

Electronic and optical properties

Gallium phosphide (GaP) is a III-V compound characterized by an indirect bandgap, which influences its efficiency in optoelectronic applications by requiring assistance for momentum conservation in radiative transitions. The indirect bandgap occurs at the X-point in the with an energy of 2.26 eV at 300 K, while the direct bandgap at the Γ-point is approximately 2.78 eV, making radiative recombination less favorable compared to direct-bandgap materials like GaAs. In undoped GaP, electron mobility is approximately 100 cm²/V·s, and hole mobility is around 50 cm²/V·s, reflecting the material's moderate carrier properties suitable for low-power devices. The static constant is 11.1, and the high-frequency constant is 9.11, contributing to its use in capacitors and waveguides. The is about 3.4 at 500 nm, enabling strong light confinement in photonic structures. The lies near 550 nm, corresponding to green light, beyond which GaP exhibits high transparency extending from the into the mid-infrared region (up to ~11 μm), ideal for optical windows and lenses. Doping modulates conductivity: n-type doping with or introduces donor levels, increasing carrier concentrations up to 10¹⁸ cm⁻³ and enhancing n-type conductivity, while p-type doping with creates acceptor levels, boosting concentrations and p-type behavior. Phonon properties play a key role in GaP's light emission efficiency due to the indirect bandgap; the longitudinal optical (LO) energy is approximately 45 meV, facilitating phonon-assisted recombination processes that can limit quantum yields in undoped material.

Synthesis and production

Gallium phosphide (GaP) was first synthesized in 1926 by . Early vapor transport techniques were developed in the mid-20th century to produce higher-purity material for applications. One common laboratory method involves the direct combination of metal and vapor. In this process, is heated to its molten state and exposed to vapor generated from elemental , typically at temperatures between 900 and 1,100 °C under an inert atmosphere such as carrier gas to prevent oxidation. The reaction proceeds according to the Ga + (1/4)P₄ → GaP, but requires an excess of to maintain equilibrium and avoid of the GaP product due to volatility. An alternative route utilizes the reduction of gallium oxide (Ga₂O₃) precursors. Here, Ga₂O₃ is reduced with vapor or (PH₃) at approximately 1,000 °C in a flow system employing a stream to facilitate the reaction and remove byproducts like . This method allows for the conversion of oxide impurities in sources into GaP, yielding polycrystalline material suitable for further processing. Recent colloidal methods, such as heating acetylacetonate with phosphorus sources in high-boiling solvents like , enable the synthesis of size-tunable GaP quantum dots with quantum yields up to 40% as of 2023, suitable for optoelectronic devices such as color converters in LEDs.

vapor-phase approaches

The reaction of chlorides, such as GaCl or GaCl₃, with (PH₃) is used in halide vapor phase epitaxy (HVPE). The precursors are introduced into a where the chloride vapor reacts with PH₃ at elevated temperatures, typically in the range of 600–1,000 °C, to deposit GaP. This route offers control over stoichiometry through gas flow ratios and is valued for its scalability in producing high-purity material. High-pressure synthesis employs sealed vessels to counteract phosphorus loss and achieve precise stoichiometric balance. and are loaded into a within a high-pressure chamber pressurized with (e.g., ) to 36–50 bars, heated to 1,000–1,500 °C to drive the direct reaction while minimizing evaporation. This technique ensures complete reaction and is particularly useful for initial polycrystalline formation under controlled conditions. For polycrystalline GaP, typical yields exceed 90% under optimized conditions, with purity levels controlled by vacuum baking of precursors and inert atmospheres to limit impurities from oxygen (forming oxides) or carbon (from reactors). Silicon and other trace elements remain the primary contaminants, often below 50 ppm when using high-vacuum setups.

Crystal growth techniques

Gallium phosphide (GaP) single crystals are essential for devices due to their wide bandgap and lattice-matching properties, but growing high-quality crystals is complicated by the material's high and volatility. Several techniques have been developed to produce defect-minimized and epitaxial layers, focusing on controlling loss and structural imperfections. These methods typically start from polycrystalline GaP synthesized via chemical routes and aim for oriented single crystals suitable for substrates and . The liquid encapsulated Czochralski (LEC) method involves pulling single crystals from a Ga-P melt encapsulated in molten (B₂O₃) to suppress evaporation. The process occurs at approximately 1,500 °C, with the rotated at 10–20 rpm to ensure uniform growth and minimize thermal stresses. This technique produces cylindrical up to 2 inches in diameter, though contamination from and encapsulant can introduce oxygen impurities, typically at levels of 10¹⁷–10¹⁹ atoms/cm³. In the vertical Bridgman technique, occurs in sealed or ampoules containing polycrystalline GaP, with the assembly lowered through a of 20–50 °C/cm. Internal heating and pressurization to 50 bars with address the high (minimum 36 bars at the 1,467 °C ), enabling congruent without significant phosphorus loss. Crystals grown this way achieve lengths of 10–15 mm and diameters of 8–10 mm, exhibiting good translucency and cleavage, though crucibles yield higher-quality single crystals compared to silica. Vapor phase (VPE), often implemented as VPE, deposits GaP layers on substrates using precursors like gallium chloride (GaCl) and (PH₃) in a carrier gas at 800–1,000 °C. This open-tube or horizontal reactor method allows precise control over growth, producing epitaxial layers 1–100 µm thick with reduced defects compared to bulk methods, particularly when using a two-step process to promote single-domain orientation on mismatched substrates like . Metalorganic chemical vapor deposition (MOCVD) employs organometallic precursors such as trimethylgallium (TMGa) and (PH₃) in a low-pressure reactor, enabling low-temperature growth (typically 700–800 °C) and high-purity layers with defect densities below 10⁴ cm⁻² under optimized conditions like pulsed . This technique excels in producing thin films on or GaP substrates, minimizing antiphase boundaries and dislocations through off-cut substrates and nucleation layers. A primary challenge in GaP crystal growth is the high , which prevents and leads to non-stoichiometric compositions, twinning, and dislocations (densities up to 10⁶–10⁸ cm⁻² in unoptimized growth). Encapsulation with B₂O₃, high-pressure ampoules, and controlled temperature gradients mitigate these issues, while doping (e.g., with or ) and precise gradient control further reduce defects, though residual imperfections can slightly alter the bandgap by introducing shallow levels. For nanoscale structures, GaP nanowires are grown via of a GaP-gallium mixture in a at 800–1,000 °C under flow, yielding wires 20–50 nm in and hundreds of micrometers long through an oxide-assisted vapor-liquid-solid mechanism. These defect-containing structures (with twins and stacking faults) enable studies of one-dimensional quantum effects but require careful purification to avoid metallic impurities.

Applications

Light-emitting diodes

Gallium phosphide (GaP) has been employed as both a substrate and active layer material in light-emitting diodes (LEDs) since the , enabling emission in the (around 700 nm) and (around 555 nm) regions of the . Its indirect bandgap of approximately 2.26 eV allows for visible light emission when radiative recombination is facilitated through impurities. Electroluminescence in GaP LEDs is achieved via p-n junction structures, where doping strategies tailor the emission color and efficiency. For green emission, n-type doping with (S) or (Te) is combined with isoelectronic (N) traps, which localize excitons and overcome the indirect bandgap limitation to enable efficient recombination at . This doping, pioneered at Bell Laboratories in the mid-1960s, resulted in initial external quantum efficiencies of about 0.5–1%, with peak emission near 565 nm for yellow-green light. Red emission, peaking around 700 nm, relies on p-type zinc (Zn) doping paired with oxygen (O) to form donor-acceptor pairs that promote radiative transitions. These diodes typically operate at a forward voltage of about 2.2 V and exhibit operational lifetimes exceeding 10,000 hours under standard conditions. The historical development of GaP LEDs accelerated in the early 1960s through efforts at Bell Laboratories, where researchers like R. A. Logan demonstrated efficient green electroluminescence via nitrogen doping in solution-grown p-n junctions. Similarly, red emission was advanced by M. Gershenzon and colleagues using Zn-O complexes, achieving early efficiencies around 0.1%. Commercial production of GaP-based red and green LEDs emerged by the late 1960s, following initial breakthroughs in visible-spectrum devices. Brightness improvements, driven by optimized nitrogen doping at Bell Labs, elevated output to medium levels, typically up to 10 millicandela (mcd), making them suitable for indicators and displays. GaP LEDs offered advantages such as low production costs due to straightforward synthesis and reliable in low-power applications, with the indirect bandgap challenges mitigated by impurity-induced radiative paths. However, their efficiencies remained lower than those of direct-bandgap materials like gallium arsenide phosphide (GaAsP), limiting adoption for high-brightness needs. By the , GaP LEDs were largely phased out in favor of higher-efficiency (InGaN) devices for demanding illumination roles.

Substrates and other uses

Gallium phosphide (GaP) serves as a substrate for growing AlGaAs and InGaP layers in red and yellow light-emitting diodes (LEDs) and lasers, enabling high-quality epitaxial structures despite lattice mismatch with these alloys. Commercial GaP wafers are produced in diameters up to 50 mm (2 inches), supporting fabrication for optoelectronic devices. Defect densities in these substrates are reduced through techniques like epitaxial lateral overgrowth, which filters threading dislocations during growth on patterned templates, improving material quality for device performance. In photonic devices, GaP enables the fabrication of optical metasurfaces that manipulate for applications such as holograms and lenses, leveraging its high refractive index and low optical loss in the . These metasurfaces achieve precise phase control over wavefronts, allowing compact, flat with efficiencies suitable for integrated systems. Additionally, GaP-based high Q-factor nanocavities exceed Q > 1,700, supporting enhanced light-matter interactions in structures for visible wavelengths. GaP is employed as a buffer layer in III-V/Si multi-junction solar cells, particularly in GaAsP/Si tandems, where its wide bandgap facilitates epitaxial integration on while minimizing defects. Research on these tandems has demonstrated efficiencies exceeding 23%. In other , GaP is employed in high-temperature diodes and switches, benefiting from its thermal stability and wide bandgap that maintain performance under elevated conditions. It also supports anodic bonding processes for integrating GaP with other materials in atomic clocks, enabling robust, hermetic seals for precision timing devices. Emerging applications include GaP nanowires for quantum dots, which provide near-transform-limited photon emission for quantum optics and single-photon sources. Additionally, GaP serves as a phosphor material in displays, contributing to color conversion layers in LEDs for enhanced visible emission. The market for GaP is primarily driven by demand for LED substrates in optoelectronics, with annual production on the order of tons to meet growing needs in lighting and displays.

References

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