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Indium phosphide
Indium phosphide
Indium phosphide
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Indium phosphide
Names
Other names
Indium(III) phosphide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.040.856 Edit this at Wikidata
UNII
  • InChI=1S/In.P checkY
    Key: GPXJNWSHGFTCBW-UHFFFAOYSA-N checkY
  • InChI=1/In.P/rInP/c1-2
    Key: GPXJNWSHGFTCBW-HIYQQWJCAF
  • [In+3].[P-3]
  • [In]#P
Properties
InP
Molar mass 145.792 g/mol
Appearance black cubic crystals[1]
Density 4.81 g/cm3, solid[1]
Melting point 1,062 °C (1,944 °F; 1,335 K)[1]
Solubility slightly soluble in acids
Band gap 1.344 eV (300 K; direct)
Electron mobility 5400 cm2/(V·s) (300 K)
Thermal conductivity 0.68 W/(cm·K) (300 K)
3.1 (infrared);
3.55 (632.8 nm)[2]
Structure
Zinc blende
a = 5.8687 Å [3]
Tetrahedral
Thermochemistry[4]
45.4 J/(mol·K)
59.8 J/(mol·K)
−88.7 kJ/mol
−77.0 kJ/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Toxic, hydrolysis to phosphine
Safety data sheet (SDS) External MSDS
Related compounds
Other anions
 Indium nitride
Indium arsenide
Indium antimonide
Other cations
 Aluminium phosphide
Gallium phosphide
Related compounds
 Indium gallium phosphide
Aluminium gallium indium phosphide
Gallium indium arsenide antimonide 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 ?)

Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic ("zincblende") crystal structure, identical to that of GaAs and most of the III-V semiconductors.

Manufacturing

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Indium phosphide nanocrystalline surface obtained by electrochemical etching and viewed under scanning electron microscope. Artificially colored in image post-processing.

Indium phosphide can be prepared from the reaction of white phosphorus and indium iodide at 400 °C.,[5] also by direct combination of the purified elements at high temperature and pressure, or by thermal decomposition of a mixture of a trialkyl indium compound and phosphine.[6]

Applications

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The application fields of InP splits up into three main areas. It is used as the basis for optoelectronic components,[7] high-speed electronics,[8] and photovoltaics[9]

High-speed optoelectronics

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InP is used as a substrate for epitaxial optoelectronic devices based other semiconductors, such as indium gallium arsenide. The devices include pseudomorphic heterojunction bipolar transistors that could operate at 604 GHz.[10]

InP itself has a direct bandgap, making it useful for optoelectronics devices like laser diodes and photonic integrated circuits for the optical telecommunications industry, to enable wavelength-division multiplexing applications.[11] It is used in high-power and high-frequency electronics because of its superior electron velocity with respect to the more common semiconductors silicon and gallium arsenide.

Optical Communications

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InP is used in lasers, sensitive photodetectors and modulators in the wavelength window typically used for telecommunications, i.e., 1550 nm wavelengths, as it is a direct bandgap III-V compound semiconductor material. The wavelength between about 1510 nm and 1600 nm has the lowest attenuation available on optical fibre (about 0.2 dB/km).[12] Further, O-band and C-band wavelengths supported by InP facilitate single-mode operation, reducing effects of intermodal dispersion.

Photovoltaics and optical sensing

[edit]

InP can be used in photonic integrated circuits that can generate, amplify, control and detect laser light.[13]

Optical sensing applications of InP include

  • Air pollution control by real-time detection of gases (CO, CO2, NOX [or NO + NO2], etc.).
  • Quick verification of traces of toxic substances in gases and liquids, including tap water, or surface contaminations.
  • Spectroscopy for non-destructive control of product, such as food. Researchers of Eindhoven University of Technology and MantiSpectra have already demonstrated the application of an integrated near-infrared spectral sensor for milk.[14] In addition, it has been proven that this technology can also be applied to plastics and illicit drugs.[15]

References

[edit]

Cited sources

[edit]
[edit]
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Indium phosphide

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Indium phosphide (InP) is a binary III–V compound with the chemical formula InP, consisting of and in a 1:1 stoichiometric ratio. It adopts a zincblende with a of 5.869 Å and exhibits a direct bandgap of 1.34 eV at 300 K, enabling efficient light emission and absorption. Key physical properties include a of 4.81 g/cm³, a of 1062 °C, high electron mobility of up to 5400 cm²/·s, and hole mobility of up to 200 cm²/·s at . These attributes, combined with its superior electron velocity compared to , position InP as a critical material for advanced electronic and photonic technologies. InP's optoelectronic properties make it indispensable in high-speed and high-frequency devices, including laser diodes, light-emitting diodes (LEDs), photodetectors, and modulators essential for fiber-optic telecommunications. It serves as the substrate for photonic integrated circuits (PICs) that enable compact, high-performance systems for data centers, including high-speed interconnects for AI-driven applications, networks, and free-space optical communications. Advances in waveguide fabrication, particularly achieving more vertical sidewall angles, have reduced propagation losses in InP membrane waveguides, thereby improving the performance of photonic integrated circuits. Additionally, InP-based high-electron-mobility transistors (HEMTs) support terahertz applications and . In , InP solar cells offer high radiation resistance, making them ideal for missions where they achieve efficiencies up to 19.1% under AM0 conditions. Beyond electronics, InP finds use in quantum dot synthesis for displays and bioimaging, leveraging its tunable bandgap through nanostructuring. InP quantum dots are also explored for emerging applications such as photonic synapses in neuromorphic computing, though they exhibit traditionally lower quantum efficiency and stability compared to CdSe quantum dots; these can be partially mitigated through fluorination via ligand exchange, which adds processing steps, while long-term stability and large-array integration remain unproven. Its production typically involves methods like liquid-encapsulated Czochralski (LEC) growth for single crystals, ensuring low defect densities for device-grade material. Despite its benefits, handling InP requires caution due to its toxicity and potential carcinogenicity from phosphine release upon hydrolysis. Ongoing research focuses on improving yield and scalability to meet demands in emerging technologies like and .

Introduction

Chemical composition and structure

Indium phosphide (InP) is a binary III-V compound composed of (In), an element from of the periodic table, and (P), from group 15. The InP reflects its stoichiometric 1:1 atomic ratio, where each indium atom pairs with one phosphorus atom to form the stable compound. This composition endows InP with semiconducting properties that arise from the partial ionic and covalent bonding between the electropositive indium and electronegative phosphorus. In the solid state, the In-P bond length measures approximately 2.54 Å, contributing to the material's structural integrity and electronic characteristics. At room temperature and ambient pressure, InP crystallizes in the zincblende structure, a face-centered cubic lattice analogous to diamond but with alternating indium and phosphorus atoms. In this arrangement, each indium atom is tetrahedrally coordinated to four phosphorus atoms via sp³ hybridized orbitals, and each phosphorus atom is similarly coordinated to four indium atoms, resulting in a highly symmetric, isotropic network of covalent bonds. This tetrahedral coordination is characteristic of many III-V compounds and underpins their favorable optoelectronic behavior. Like other III-V semiconductors such as (GaAs), InP shares the and possesses a comparable of 5.87 , enabling epitaxial growth and lattice matching in heterostructures, though InP's direct bandgap is slightly narrower at 1.34 eV compared to GaAs's 1.42 eV. InP exhibits polymorphic forms beyond the zincblende phase; under , it undergoes a to the rocksalt structure at approximately 9 GPa, accompanied by changes in coordination and density. This transition highlights the material's structural adaptability, though the zincblende form remains the thermodynamically stable polymorph under standard conditions.

Historical development

Significant interest in InP as a emerged in the 1950s, when Heinrich Welker at Laboratories developed methods for producing single crystals using the horizontal Bridgman technique and identified its promising semiconducting characteristics, including rectification behavior in point-contact devices. Welker's work laid the foundation for InP's application in , highlighting its potential over earlier materials like due to superior electrical properties. During the 1960s and 1970s, research focused on InP's high , exceeding 4,000 cm²/V·s, which positioned it as a for high-speed devices; early efforts included prototypes at institutions like RCA, where patents explored InP junctions for amplification. The saw a pivotal shift toward commercialization, with the adoption of metalorganic chemical vapor deposition (MOCVD) for growing high-quality epitaxial layers, enabling scalable production driven by the rising demand for fiber-optic components operating at 1.55 μm wavelengths. In the 1990s, InP facilitated key integrations in , particularly in laser diodes with multiple structures for improved efficiency and wavelength stability in telecom systems. By the 2000s, advancements in InP-based bipolar transistors (HBTs) achieved cutoff frequencies over 300 GHz, supporting terahertz applications and high-data-rate circuits.

Physical and chemical properties

Crystal structure and lattice parameters

Indium phosphide (InP) adopts the zincblende crystal structure, a face-centered cubic lattice characteristic of many III-V semiconductors, with the space group F4‾3mF\overline{4}3m (No. 216). In this structure, indium and phosphorus atoms occupy alternating tetrahedral sites, forming a three-dimensional network stabilized by covalent bonding with partial ionic character. The lattice parameter aa is 5.869 Å at 300 K, reflecting the average In-P bond length of approximately 2.54 Å. The of InP is isotropic due to its cubic , with a linear of 4.6×10−64.6 \times 10^{-6} K−1^{-1} over typical operating temperatures. This low value indicates dimensional stability under moderate thermal cycling, which is advantageous for device fabrication. Native point defects play a critical role in the material's properties, including vacancies (VInV_{\mathrm{In}}) and antisites (PInP_{\mathrm{In}}), which can act as compensating centers or traps. Formation energies for these defects range from 2-3 eV in neutral or low-charge states, varying with the and growth conditions such as In- or P-rich environments. For instance, PInP_{\mathrm{In}} defects exhibit lower formation energies under In-rich conditions, promoting their prevalence in undoped crystals. Doping strategies leverage shallow impurities to control carrier type and concentration while preserving lattice integrity. N-type doping is typically achieved with group VI (, S) or group IV (, Si) donors, which substitute on the In site and introduce levels approximately 0.005 eV below the conduction band minimum, enabling nearly complete ionization at . P-type doping employs group II acceptors like (Zn), substituting on the In site with an acceptor level about 0.03 eV above the valence band maximum, though higher concentrations can lead to challenges during growth. X-ray diffraction (XRD) serves as a primary tool for verifying the zincblende phase and assessing lattice quality, with powder patterns displaying characteristic Bragg peaks. Prominent reflections include the (220) plane at approximately 43.5° 2θ and the (400) plane at approximately 63.3° 2θ using Cu Kα radiation (λ = 1.5406 Å), corresponding to interplanar spacings derived from the lattice constant. These peaks allow quantification of strain, orientation, and defect-induced broadening in epitaxial or polycrystalline samples.

Thermal and mechanical properties

Indium phosphide (InP) exhibits a of 1060 °C when maintained under sufficient overpressure to prevent decomposition into indium and vapor. Without this overpressure, InP undergoes incongruent decomposition above approximately 600 °C in conditions, resulting in loss and the formation of indium-rich phases on the surface. The density of InP at is 4.81 g/cm³. Its is 0.31 J/g·K, while the thermal conductivity is 68 W/m·K along the <100> direction at 300 K. These thermal properties contribute to InP's suitability for high-temperature device processing, where controlled atmospheres are essential to maintain . The thermal expansion of InP is isotropic due to its cubic , with a linear of 4.6×10−64.6 \times 10^{-6} K−1^{-1}. Mechanically, InP is characterized by a of approximately 61 GPa along the direction and a of 0.36. The material's hardness is around 600 HV, reflecting its moderate typical of III-V semiconductors, which influences its and behavior during fabrication. Overall, these properties enable InP's use in robust optoelectronic structures, balancing thermal management with mechanical integrity under operational stresses.

Electronic and optical properties

Indium phosphide (InP) is a direct bandgap III-V with a room-temperature bandgap of 1.34 eV, corresponding to a of approximately 925 nm, which enables efficient radiative recombination for optoelectronic applications. The bandgap exhibits temperature dependence described by the Varshni equation, Eg(T)=1.421−4.9×10−4T2T+327E_g(T) = 1.421 - \frac{4.9 \times 10^{-4} T^2}{T + 327} eV, where TT is in , reflecting electron-phonon interactions that reduce the gap with increasing temperature. in undoped InP reaches 5400 cm²/V·s at 300 K, significantly higher than hole mobility of up to 200 cm²/V·s, due to the lighter effective mass of electrons (me∗=0.07m0m_e^* = 0.07 m_0) compared to holes (mh∗=0.45m0m_h^* = 0.45 m_0), facilitating high-speed charge transport in n-type devices. The relative dielectric constant is 12.4, influencing capacitance and screening effects in heterostructures. Optically, InP interacts strongly with near-infrared light near its bandgap, exhibiting a of 3.17 at 1.55 μm, which supports confinement in photonic integrated circuits. The absorption coefficient rises sharply to approximately 10410^4 cm⁻¹ just above the bandgap, enabling efficient photon absorption for photodetection while maintaining transparency at longer wavelengths. in InP peaks at around 920 nm at , slightly blue-shifted from the bandgap due to excitonic effects, with an binding energy of about 4 meV that promotes bound electron-hole pairs but allows thermal dissociation at ambient conditions. These properties collectively underpin InP's role in balancing electronic speed and optical efficiency.

Chemical properties

Indium phosphide is chemically stable under dry conditions but hydrolyzes in the presence of moisture, releasing toxic (PH₃) gas. It is insoluble in but slightly soluble in acids, where dissolution may also produce phosphine.

Synthesis and production

Laboratory synthesis methods

Laboratory synthesis of indium phosphide (InP) in research settings typically employs techniques that prioritize high purity, precise control over crystal morphology, and small-scale production suitable for device prototyping and fundamental studies. These methods allow researchers to tailor the material's properties, such as defect density and doping, under controlled conditions. One foundational approach is the direct combination of elemental and , often conducted in sealed ampoules under low pressure to produce polycrystalline InP. High-purity is heated to 945–1055 °C while red is maintained at 412–520 °C in a two-zone furnace, enabling the vapor-phase reaction to form stoichiometric InP ingots with carrier concentrations as low as 3.16 × 10¹⁵ cm⁻³ and mobilities exceeding 38,000 cm²/V·s at temperatures. This method yields high-purity material suitable for substrate preparation, though it requires careful evacuation to 5 × 10⁻⁷ to minimize impurities. Hydride vapor phase epitaxy (HVPE) is widely used for growing epitaxial InP layers on substrates, offering rapid deposition rates in laboratory reactors. The process involves the in-situ generation of indium chloride (InCl) from metal and HCl, which reacts with (PH₃) at approximately 610 °C to deposit InP via the reaction InCl + (1/4)PH₃ → InP + HCl + (3/4)H₂, typically at a V/III ratio of 10. Growth rates of 1–10 μm/h are achieved, enabling thick layers up to 21 μm in 2.5 hours with smooth morphology and radiative properties comparable to bulk InP. This technique is valued for its simplicity and ability to produce low-defect films for optoelectronic research. Molecular beam epitaxy (MBE) provides atomic-layer precision for InP thin films in environments, essential for heterostructure development. Elemental is evaporated from a Knudsen cell alongside red , which dissociates from P₄ to primarily P₂ at high temperatures, directed onto a heated substrate at 480–530 °C to form high-quality epitaxial layers with mobilities over 3700 cm²/V·s. Pre-growth substrate baking above 450 °C removes oxides, ensuring smooth interfaces. Stoichiometry control during MBE growth is critical to prevent indium droplet formation and achieve uniform zincblende structure. A phosphorus-rich (P₂) maintains the low-temperature (2×4) , monitored in-situ by reflection high-energy (RHEED), which shows weak intermediate streaks indicative of excess phosphorus; shifting to higher temperatures or reduced P transitions to the cation-rich high-temperature (2×4) phase, risking In excess and droplets if not balanced. patterns allow real-time adjustment of beam for optimal V/III ratios. For nanoscale applications, colloidal synthesis produces InP quantum dots or nanoparticles with tunable optoelectronic properties. Indium chloride (InCl₃) serves as the indium precursor, combined with phosphorus sources in trioctylphosphine (TOP) or similar coordinating solvents, heated to 200–300 °C in a hot-injection setup to yield particles sized 2–10 nm, often with additives for monodispersity. This solution-phase method enables size control via reaction time and ligands like , resulting in tetrahedral or spherical morphologies suitable for displays and sensors.

Industrial manufacturing processes

The industrial production of phosphide (InP) primarily involves synthesizing high-purity polycrystalline material as a precursor for single-crystal . High-purity , recovered mainly from byproducts and further purified via zone to achieve 7N (99.99999%) grade, is reacted with vapor under , typically around 30 atm, using the horizontal gradient freeze (HGF) technique. This process yields stoichiometric polycrystalline InP ingots weighing up to 1.25 kg with diameters of about 45 mm, serving as feedstock for subsequent crystal . Single-crystal InP is predominantly grown via the liquid encapsulated Czochralski (LEC) method, where the polycrystalline material is melted at approximately 1065 °C in a high-pressure vessel and encapsulated with a layer of molten (B₂O₃) to minimize volatilization. The melt is then slowly pulled and rotated to form cylindrical ingots with diameters of 2–4 inches (50–100 mm), suitable for slicing; these crystals typically exhibit etch pit densities (indicating levels) of 10⁴–10⁵ cm⁻² due to stresses during growth. An advanced alternative to LEC is the vertical gradient freeze (VGF) technique, which solidifies the melt directionally in a by gradually lowering the temperature gradient, resulting in significantly lower densities below 10³ cm⁻² and improved uniformity. VGF is widely adopted for commercial production, including by , enabling 4-inch (100 mm) ingots with average etch pit densities as low as 3 cm⁻² when combined with vertical Bridgman refinements; this method supports scalable output for high-performance substrates while reducing defects that could impair device yield. Further purification of InP occurs through zone refining of the precursor materials or the polycrystalline ingot itself, effectively segregating impurities such as to levels below 0.1 ppm, which is critical for achieving 6N (99.9999%) overall purity required in applications. Global production remains limited, with the supply chain heavily reliant on indium mining and refining dominated by , which accounts for 70% of worldwide output as of 2024. In August 2024, an indium phosphide wafer fabrication facility resumed operations in , supporting efforts to diversify domestic production. Substrate costs for bulk InP wafers are approximately $500 per kg, reflecting the material's specialized processing and raw material constraints. The market for InP substrates is dominated by several key companies, including Sumitomo Electric Industries, AXT, Inc., Freiberger Compound Materials GmbH, and Coherent Corp. (formerly II-VI Incorporated). These firms lead in the production of high-quality InP wafers through advanced techniques such as VGF and LEC, collectively holding a significant portion of the market share as of 2024, though market leadership can fluctuate over time due to technological advancements, strategic acquisitions, and evolving demands in telecommunications and photonics.

Applications

Optoelectronic devices

Indium phosphide (InP) plays a central role in optoelectronic devices, particularly those operating in the near-infrared spectrum, owing to its lattice matching with ternary and alloys like InGaAs and InGaAsP, which enable bandgap engineering for emission and detection at 1.3–1.55 μm. These wavelengths align with the low-loss windows of optical fibers, making InP-based structures essential for high-speed light-emitting and detecting components. The material's high in InGaAs channels, typically over 10,000 cm²/V·s at 300 K, supports ultrafast carrier transport, facilitating device bandwidths exceeding tens of GHz. Light-emitting diodes (LEDs) based on InP substrates utilize InGaAsP/InP multiple quantum wells to achieve emission in the 1.3–1.55 μm range. These structures leverage quantum confinement to enhance radiative recombination efficiency, with internal quantum efficiencies reaching approximately 80% in optimized InP-based materials. External quantum efficiencies are lower due to but can exceed 6% in microcavity designs that improve light extraction, enabling output powers of several milliwatts at moderate drive currents. Such LEDs serve as compact sources for short-haul optical links and testing in photonic systems. Laser diodes, including distributed feedback (DFB) types, are fabricated on InP using InGaAsP active regions for single-mode operation at telecom wavelengths. DFB lasers incorporate a for wavelength selection, achieving threshold currents as low as 10–15 mA in 250 μm cavity devices grown by metalorganic vapor phase epitaxy. Their spectral linewidth is typically less than 10 MHz, ensuring phase stability for coherent applications, with initial developments for CD/DVD systems extended to high-bit-rate telecom transmitters operating up to 40 Gb/s. Slope efficiencies around 0.2 W/A and side-mode suppression ratios over 40 dB characterize these devices. Photodetectors, such as PIN diodes, exploit InP/InGaAs heterostructures for high-responsivity detection at 1.55 μm. These devices feature an undoped InGaAs absorption layer sandwiched between p- and n-doped InP layers, yielding responsivities of approximately 0.9 A/W, corresponding to quantum efficiencies near 70%. Bandwidths surpass 40 GHz in waveguide-integrated designs, supporting error-free operation at data rates beyond 100 Gb/s with low dark currents under reverse bias. The high saturation power, often >10 mW, makes them suitable for analog RF links and direct detection receivers. Heterojunction bipolar transistors (HBTs) based on InP/InGaAs lattices achieve exceptional high-frequency performance, with cutoff frequencies (f_T) exceeding 600 GHz in scaled emitter designs using InAlAs/InGaAs heterostructures. A record f_T/f_max of 0.71/1.05 THz was reported in 2021 for double-heterojunction variants with 175 nm emitters, enabling terahertz amplifiers and mixers. Power handling capabilities surpass 10 W/mm in multifinger layouts, with power-added efficiencies up to 35% at 170 GHz, positioning InP HBTs for mm-wave power amplification in and systems. InP quantum dots are emerging in applications such as photonic synapses for neuromorphic computing, where they enable light-induced synaptic behaviors mimicking biological processes. However, compared to CdSe quantum dots, InP quantum dots traditionally exhibit lower photoluminescence quantum yields and external quantum efficiencies, although strategies like fluorination and ligand exchange can mitigate these issues to some extent but may not fully close the performance gap. Additionally, ligand exchange introduces extra process steps, and challenges with long-term stability and large-array integration remain unproven. Integration of these components into monolithic microwave integrated circuits (MMICs) on InP platforms allows for compact optoelectronic systems, such as combined laser and semiconductor optical amplifier (SOA) arrays. These circuits monolithically combine DFB lasers with SOAs for power boosting, achieving output powers over 20 mW across tunable ranges spanning 40 nm while maintaining linewidths below 100 kHz. Such integration reduces packaging complexity and enables photonic integrated circuits for transceivers.

Telecommunications and photonics

Indium phosphide (InP) is pivotal in and , enabling high-speed, low-loss optical and transmission in optic networks. Its ability to support active and passive components at the 1550 nm telecom window, combined with integration capabilities, makes it ideal for dense (DWDM) systems and photonic integrated circuits (PICs) that handle terabit-per-second data rates. InP-based devices offer superior performance in amplification, wavelength selection, modulation, and detection compared to alternatives, due to their direct bandgap properties that facilitate efficient light-matter interactions. InP-based optical amplifiers (SOAs) function as compact, integrable alternatives to traditional erbium-doped amplifiers, providing high gain at 1550 nm for signal boosting in long-haul transmission. These devices achieve small-signal gains exceeding 30 dB, enabling efficient amplification of multi-wavelength signals with low noise figures and polarization-independent operation, which is essential for maintaining in dense optical networks. For (WDM), tunable lasers fabricated on InP platforms allow precise channel selection across the C-band, supporting channel spacings as fine as 50 GHz and accommodating over 100 channels per fiber. These lasers, often based on sampled grating (SG-DBR) or Vernier-effect designs, exhibit wide tuning ranges (>40 nm) and low , facilitating flexible bandwidth allocation in reconfigurable optical add-drop multiplexers (ROADMs) for dynamic network traffic management. Photonic integrated circuits (PICs) leveraging indium phosphide-on-insulator (InP-OI) substrates integrate multiple functions on a single chip, including arrayed waveguide gratings (AWGs) for demultiplexing WDM signals with insertion losses below 5 dB. This platform reduces footprint and power consumption while enabling heterogeneous integration with , supporting scalable transceivers for data centers and metro networks, particularly high-speed interconnects in AI-driven data centers where InP-based lasers and modulators facilitate photonic AI accelerators and aggregate data rates exceeding 40 TBps; for instance, 16-channel AWGs on InP membranes demonstrate crosstalk below -25 dB and compact sizes under 1 cm². In InP membrane waveguides, which are essential to high-performance PICs, sidewall angles closer to 90° (vertical) reduce propagation loss compared to more tilted angles. Tilted sidewalls increase loss due to stronger interaction of the optical mode with the sidewalls, leading to higher scattering. An optimized single-step reactive ion etching (RIE) process achieving an 88° sidewall angle resulted in a record-low propagation loss of 2.5 dB/cm, improved from previous values around 3.3 dB/cm with 84–85° angles. Electro-optic Mach-Zehnder modulators (MZMs) utilizing InP quantum wells provide high-speed modulation for coherent and direct-detection systems, achieving electrical bandwidths greater than 100 GHz with low drive voltages (Vπ < 2 V). These modulators exploit the quantum-confined Stark effect in strained InGaAsP/InP multiple quantum wells for efficient phase shifts, enabling error-free transmission at 200 Gbaud PAM-4 formats with minimal chirp and high extinction ratios (>10 dB). In the context of and emerging networks, InP avalanche photodiodes (APDs) integrated into photonic mm-wave links support ultra-high bit rates exceeding 400 Gbps through optical upconversion and direct detection. Waveguide-coupled InP APD arrays with multiplication layers exhibit 3-dB bandwidths up to 30 GHz and high linearity (OIP3 > 20 dBm), enabling low-noise reception in fiber-wireless hybrid systems for backhaul and fronthaul applications.

Photovoltaics and sensors

Indium phosphide (InP) is integral to multi-junction solar cells, serving as a substrate or tunnel junction material in lattice-matched structures such as GaInP/InGaAs on InP or GaInP/GaAs configurations adapted for high-performance photovoltaics. In these designs, InP enables low-resistance interconnects between subcells, facilitating efficient current flow and spectrum splitting for enhanced overall conversion. For instance, InP-based tunnel junctions in triple-junction cells lattice-matched to InP have demonstrated high transparency and peak current densities exceeding 100 A/cm², supporting applications in concentrator photovoltaics. Such multi-junction cells, incorporating InP components, have achieved efficiencies over 40% under AM1.5 illumination, with efficiencies up to 46% under concentration for four-junction cells lattice-matched to InP, as demonstrated in 2014. In space applications, InP solar cells excel due to their exceptional radiation tolerance, outperforming and GaAs/Ge counterparts in high-radiation environments like satellite orbits. Under 1 MeV electron irradiation, InP cells exhibit minimal degradation, retaining approximately 75% of initial efficiency after a fluence of 10^{15} e/cm², attributed to defect annealing and robust minority carrier diffusion lengths. This radiation hardness makes InP ideal for long-duration missions, where cumulative damage from protons and electrons is a primary concern. For thermophotovoltaics, InP serves as a substrate for InGaAsP cells tuned to bandgaps around 0.6–0.75 eV, matching the spectral output of emitters at approximately 1500 K for efficient recovery in . These systems convert from sources like exhaust into , with InP-based devices demonstrating external quantum efficiencies above 70% in the target range. InP-based materials are widely employed in optical sensors, particularly infrared detectors for systems operating at 1.55 μm. Avalanche photodiodes (APDs) on InP substrates, such as AlInAsSb/InGaAs designs, offer low excess noise factors (k ≈ 0.012) and dark currents below 55 μA/cm², enabling high-sensitivity detection. These detectors achieve (NEP) values around 10^{-12} W/√Hz, suitable for ranging and in automotive and . Chemical sensing applications leverage surface-functionalized InP nanowires for selective gas detection, including (NO₂) at parts-per-million levels. Self-powered InP arrays exhibit an 84% response to 1 ppm NO₂, with limits of detection down to sub-ppb concentrations, due to enhanced surface-to-volume ratios and modulation. Indium phosphide nanomembranes further extend this capability, offering tunable dynamic ranges from 0.2 ppb to 10 ppm through thickness control, ideal for wearable and portable environmental sensors.

Safety and environmental aspects

Health hazards and handling

Indium phosphide (InP) poses significant health risks primarily through of its dust or particles, leading to respiratory such as and pulmonary inflammation. Indium compounds, including InP, are classified by the International Agency for Research on Cancer (IARC) as probably carcinogenic to humans (Group 2A), based on sufficient evidence of carcinogenicity in experimental animals, including increased incidences of lung neoplasms in rats and mice exposed to InP aerosols. Acute exposure to InP can cause skin and eye damage due to the potential release of gas upon contact with moisture, which is highly toxic and can lead to severe or burns. Inhalation of InP particles at high concentrations may result in immediate respiratory distress, including coughing and . Chronic exposure to InP dust is associated with serious lung conditions, notably indium lung disease, characterized by , , and . The first cases of indium lung disease were reported in in 2003 among workers handling indium-tin oxide particles, with subsequent cases linked to InP and other indium compounds showing progressive lung and impaired lung function. Long-term studies in animals demonstrate persistent lung tissue accumulation of with slow clearance, exacerbating fibrotic changes. Safe handling of InP requires strict adherence to protective measures to minimize exposure. Operations involving InP powders or particles should be conducted in well-ventilated fume hoods or enclosed systems to prevent airborne dispersion. (PPE) including or gloves, safety goggles, and N95 or higher-rated respirators with particulate filters is essential to protect against , eye, and hazards. Avoid conditions that could lead to release, such as contact with water or acids, and ensure proper storage in sealed, dry containers to prevent from gas evolution. Contaminated clothing should be removed and laundered separately, with thorough handwashing after handling. Regulatory limits for occupational exposure to indium and its compounds, including InP, are established to mitigate health risks. The (OSHA) sets a (PEL) of 0.1 mg/m³ as an 8-hour time-weighted average for (as In). Under the European Union's REACH regulation, InP is registered, with specific requirements for nanomaterial forms including detailed safety data submission and risk assessments for handlers.

Environmental impact and recycling

Indium phosphide (InP) production contributes to resource scarcity due to the limited global supply of , estimated at 990 metric tons in 2023 and 1,080 metric tons in 2024, primarily as a of . Approximately 95% of refined originates from processing, making its availability dependent on market dynamics and leading to supply constraints. This dependency has caused significant price volatility, with prices peaking at around $1,000 per kilogram in before declining to approximately $400 per kilogram by 2010. The synthesis of InP generates environmental waste, including phosphine (PH₃) emissions of about 0.2 grams per square meter of wafer produced, a toxic gas that acts as a greenhouse gas and poses ecological risks if not properly managed. End-of-life optoelectronic devices incorporating InP, such as lasers and photodetectors, contribute to e-waste streams containing 10–100 milligrams of InP per unit, exacerbating the accumulation of critical material residues in landfills. Recycling efforts focus on hydrometallurgical methods, such as leaching InP wafers with (HCl), which achieves over 90% recovery of through processes like solvent extraction following acid dissolution. Pilot-scale operations, including those explored by companies like since the mid-2010s, demonstrate the feasibility of recovering high-purity from semiconductor scrap, reducing reliance on primary . Life-cycle assessments of InP production reveal a of approximately 50 of CO₂ equivalent per of InP, driven largely by energy-intensive epitaxial growth and precursor synthesis, alongside water usage of around 100 liters per . These impacts highlight the need for optimized to mitigate the material's environmental burden compared to silicon-based alternatives. Sustainability initiatives include the European Union's of 2023, which entered into force in May 2024 and mandates that at least 25% of annual consumption of strategic raw materials like be sourced from by 2030, establishing quotas to enhance circularity in InP supply chains. Research into alternatives, such as (GaAs) substrates or InP-on-GaAs hybrids, aims to substitute InP in applications like , potentially lowering demand and associated ecological pressures.

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