Abstract

Indium nitride (InN) represents a significant III-V semiconductor compound with the chemical formula InN and molar mass of 128.83 g/mol. This black crystalline solid crystallizes in the wurtzite structure with lattice parameters a = 354.5 pm and c = 570.3 pm. The compound exhibits a direct bandgap of approximately 0.65 eV at 300 K, making it a narrow-gap semiconductor with exceptional electron mobility of 3200 cm²/(V·s). Indium nitride demonstrates thermal conductivity of 45 W/(m·K) and refractive index of 2.9. Primary applications include high-speed electronic devices, solar cells, and optoelectronic components, particularly when alloyed with gallium nitride to form InGaN systems spanning bandgaps from infrared to ultraviolet wavelengths.

Introduction

Indium nitride constitutes an inorganic compound classified among the III-V semiconductors, characterized by the combination of indium from group 13 and nitrogen from group 15 of the periodic table. The material gained significant scientific attention following the correction of its bandgap value from the previously accepted 1.97 eV to approximately 0.7 eV, fundamentally altering understanding of its electronic properties. This revision positioned indium nitride as the semiconductor with the smallest bandgap among the III-nitride family, enabling applications across a broader spectral range than previously possible. The compound's exceptional electron transport properties and thermal characteristics make it particularly valuable for high-frequency electronic devices and efficient photovoltaic systems.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Indium nitride adopts the wurtzite crystal structure with space group C_6v⁴-P6₃mc, featuring tetrahedral coordination geometry around both indium and nitrogen atoms. Each indium atom bonds to four nitrogen neighbors at bond distances of approximately 214 pm, while each nitrogen atom coordinates with four indium atoms in a complementary tetrahedral arrangement. The hexagonal unit cell parameters measure a = 354.5 pm and c = 570.3 pm, with a c/a ratio of 1.61, slightly deviating from the ideal wurtzite value of 1.633.

The electronic structure derives from the interaction between indium's 5s²5p¹ valence electrons and nitrogen's 2s²2p³ configuration. Molecular orbital theory indicates strong sp³ hybridization, resulting in four equivalent bonding orbitals directed toward the corners of a tetrahedron. The conduction band minimum occurs at the Γ-point of the Brillouin zone, characteristic of direct bandgap semiconductors. Density functional calculations reveal significant charge transfer from indium to nitrogen atoms, with calculated Born effective charges indicating substantial ionic character to the predominantly covalent bonding.

Chemical Bonding and Intermolecular Forces

The In-N bond exhibits mixed ionic-covalent character with approximately 47% ionic contribution based on Pauling electronegativity differences. X-ray photoelectron spectroscopy measurements indicate binding energies of 443.5 eV for In 3d_5/2 and 396.2 eV for N 1s core levels. The bond dissociation energy measures approximately 2.8 eV, slightly lower than that of gallium nitride (3.2 eV) but higher than most II-VI semiconductor compounds.

In the solid state, primary intermolecular interactions include dipole-dipole forces between polarized In-N bonds and van der Waals forces between adjacent layers. The compound demonstrates significant polarity with spontaneous polarization estimated at -0.042 C/m² along the c-axis. The static dielectric constant measures 15.3, while the high-frequency dielectric constant reaches 8.4, reflecting substantial electronic polarization capability.

Physical Properties

Phase Behavior and Thermodynamic Properties

Indium nitride appears as a black polycrystalline powder with density of 6.81 g/cm³ at 298 K. The compound melts at approximately 1100°C with decomposition, precluding observation of a true liquid phase under atmospheric conditions. High-pressure studies indicate possible phase transitions to rocksalt structure above 12 GPa, though these transformations exhibit significant hysteresis.

Standard enthalpy of formation measures -32.1 kJ/mol, with Gibbs free energy of formation at 298 K calculated as -26.4 kJ/mol. The Debye temperature derived from specific heat measurements equals 660 K, significantly lower than that of gallium nitride (1100 K) due to the greater atomic mass of indium. Thermal expansion coefficients measure 3.5 × 10^-6 K^-1 along the a-axis and 2.8 × 10^-6 K^-1 along the c-axis, demonstrating moderate anisotropy.

Spectroscopic Characteristics

Fourier transform infrared spectroscopy reveals Reststrahlen band features between 450-590 cm^-1, with longitudinal optical phonon frequency at 586 cm^-1 and transverse optical phonon frequency at 447 cm^-1. Raman spectroscopy demonstrates characteristic modes including E₂^high at 488 cm^-1, A₁(LO) at 583 cm^-1, and E₁(LO) at 561 cm^-1.

Photoluminescence spectroscopy shows near-band-edge emission at 0.69 eV (1800 nm) at low temperatures, shifting to 0.65 eV (1900 nm) at room temperature due to bandgap narrowing effects. Ultraviolet photoelectron spectroscopy measurements place the valence band maximum 1.5 eV below the Fermi level in unintentionally doped n-type material. Electron energy loss spectroscopy reveals plasmon peaks at 12.5 eV and 20.3 eV, corresponding to volume and surface plasmons respectively.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Indium nitride undergoes hydrolysis in aqueous environments according to the reaction: InN + 3H₂O → In(OH)₃ + NH₃. The reaction proceeds with activation energy of 68 kJ/mol and follows first-order kinetics with respect to InN surface area. Oxidation occurs above 400°C in air or oxygen atmospheres, forming indium(III) oxide: 4InN + 3O₂ → 2In₂O₃ + 2N₂.

The compound demonstrates relative stability in dry atmospheres up to 600°C, with decomposition kinetics following the contracting sphere model. Etching rates in common acids measure 5 nm/min in HCl (1M) and 2 nm/min in H₂SO₄ (1M) at 25°C, while alkaline solutions exhibit negligible etching below pH 10. Plasma etching using chlorine-based chemistries proceeds at rates up to 200 nm/min at 200°C substrate temperature.

Acid-Base and Redox Properties

Indium nitride behaves as a Lewis base through nitrogen lone pair donation, forming adducts with Lewis acids including boron trifluoride and aluminum trichloride. The compound exhibits negligible solubility in aqueous acids and bases, though surface oxidation occurs under both conditions. Standard reduction potential for the InN/In couple estimates at -0.45 V versus standard hydrogen electrode, indicating moderate thermodynamic stability against reduction.

Electrochemical impedance spectroscopy reveals n-type semiconductor behavior with flatband potential of -0.32 V vs. SCE in pH 7 buffer solution. The space charge layer capacitance follows Mott-Schottky behavior with donor density typically ranging from 10¹⁸ to 10²⁰ cm^-3 in unintentionally doped material. Surface states density at the electrolyte interface measures approximately 10¹³ cm^-2eV^-1, influencing charge transfer kinetics.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Metalorganic chemical vapor deposition represents the predominant method for indium nitride thin film growth, utilizing trimethylindium (TMIn) or triethylindium (TEIn) as indium precursors with ammonia as nitrogen source. Typical growth conditions involve temperatures between 500-600°C, V/III ratios of 10,000-50,000, and reactor pressures of 50-200 Torr. Growth rates typically range from 0.1-1.0 μm/h, with higher temperatures favoring decomposition over deposition.

Molecular beam epitaxy enables growth at lower temperatures (400-500°C) using elemental indium and nitrogen from plasma sources. This technique produces films with superior crystalline quality and lower background carrier concentrations, typically around 5×10¹⁷ cm^-3. Radio-frequency nitrogen plasma sources operating at 200-500 W provide active nitrogen species, with growth rates limited to 0.05-0.2 μm/h by nitrogen incorporation kinetics.

Industrial Production Methods

Industrial production employs modified MOCVD reactors with capacity for multiple 4-inch or 6-inch wafers per growth run. Precursor utilization efficiency reaches 30-40% for indium sources through reactor design optimization and precursor recycling systems. Ammonia consumption remains substantial due to the high V/III ratios required, with typical consumption of 500-1000 g per wafer.

Bulk crystal growth presents significant challenges due to the high equilibrium nitrogen pressure over InN, estimated at 20-50 kbar at 1000 K. High-pressure solution growth techniques employing nitrogen pressures up to 20 kbar and temperatures around 1500 K produce small crystallites up to 1 mm in dimension. Hydride vapor phase epitaxy offers alternative approaches with growth rates exceeding 10 μm/h, though crystal quality requires further improvement for device applications.

Analytical Methods and Characterization

Identification and Quantification

X-ray diffraction provides definitive identification through comparison with reference patterns (JCPDS 02-1450), with characteristic reflections at 31.3° (100), 32.9° (002), and 36.1° (101) using Cu Kα radiation. Energy-dispersive X-ray spectroscopy permits quantitative elemental analysis with detection limits of 0.5 atomic percent for indium and 1.0 atomic percent for nitrogen. Rutherford backscattering spectrometry achieves superior accuracy for composition determination, with uncertainties below 2% for both elements.

Secondary ion mass spectrometry enables depth profiling with detection limits below 10¹⁶ cm^-3 for common impurities including oxygen, carbon, and hydrogen. Hall effect measurements determine electrical properties with typical accuracy of 5% for carrier concentration and 10% for mobility values. Temperature-dependent Hall measurements distinguish conduction mechanisms and quantify impurity activation energies.

Purity Assessment and Quality Control

High-quality indium nitride films exhibit background electron concentrations below 1×10¹⁸ cm^-3 and room-temperature mobilities exceeding 2000 cm²/(V·s). X-ray diffraction rocking curve full width at half maximum values below 200 arcseconds indicate good crystalline quality for heteroepitaxial layers. Photoluminescence full width at half maximum below 30 meV at 10 K signifies minimal impurity and defect contributions.

Transmission electron microscopy reveals threading dislocation densities typically between 10⁹-10¹⁰ cm^-2 for layers grown on sapphire substrates, while growth on native substrates reduces this to below 10⁷ cm^-2. Atomic force microscopy surface roughness measurements below 1 nm RMS over 5×5 μm areas indicate smooth growth surfaces suitable for device fabrication.

Applications and Uses

Industrial and Commercial Applications

Indium nitride serves primarily as a component in indium gallium nitride (InGaN) heterostructures for high-electron-mobility transistors operating at microwave and millimeter-wave frequencies. Devices demonstrate cutoff frequencies exceeding 200 GHz and maximum oscillation frequencies above 300 GHz, enabling applications in radar systems and high-speed communications. The small effective electron mass of 0.055 m₀ contributes to high electron saturation velocities approaching 4×10⁷ cm/s.

InGaN-based solar cells utilizing indium nitride's narrow bandgap theoretically achieve conversion efficiencies beyond 50% under concentrated sunlight through spectrum splitting approaches. Current experimental devices demonstrate 3-5% efficiency for single-junction cells, limited primarily by material quality and doping challenges. Thermophotovoltaic systems employing InN converters target efficiency improvements through better matching to infrared emitters.

Research Applications and Emerging Uses

Research focuses on InN-based heterojunctions for hot-carrier solar cells exploiting the material's large phonon energy and slow carrier cooling rates. Time-resolved spectroscopy measurements indicate hot carrier lifetimes exceeding 10 ps, substantially longer than conventional semiconductors. Superconducting properties observed below 4 K in heavily doped material stimulate investigations into nitride-based superconducting devices and quantum computing applications.

Nanostructured indium nitride including nanowires and quantum dots enables novel optoelectronic devices through quantum confinement effects. Nanowire arrays demonstrate bandgap widening to 1.2 eV for diameters below 10 nm, extending the accessible spectral range. Plasmonic applications utilize the compound's negative dielectric constant above 12.5 eV for ultraviolet metamaterials and subwavelength imaging systems.

Historical Development and Discovery

Initial synthesis of indium nitride occurred in the 1960s through ammonia reaction with indium metal or compounds, though material quality limited characterization. Early optical measurements incorrectly indicated a bandgap of 1.9-2.0 eV, persisting in literature until the early 2000s. Improved epitaxial growth techniques during the 1990s enabled production of higher quality material, leading to the landmark recognition around 2002 that the true bandgap measured approximately 0.7 eV.

This revision emerged from concerted efforts across multiple research groups employing advanced characterization techniques including photoluminescence, optical absorption, and electron energy loss spectroscopy. The discovery fundamentally altered understanding of III-nitride semiconductor properties and stimulated renewed research interest. Subsequent investigations established the exceptional electron transport properties and narrow bandgap characteristics that distinguish indium nitride from other nitride semiconductors.

Conclusion

Indium nitride represents a unique III-V semiconductor with the smallest bandgap among nitride compounds, exhibiting exceptional electron transport properties and interesting fundamental physics. The material's narrow bandgap enables optoelectronic applications across the infrared spectrum, while its high electron mobility suits high-frequency electronic devices. Significant challenges remain in material synthesis, particularly regarding p-type doping and heteroepitaxial growth with low defect densities.

Future research directions include development of native substrates, understanding and controlling point defects, and exploiting the material's superconducting properties at low temperatures. Alloying with gallium and aluminum nitrides continues to expand the accessible property range for specialized applications. Advances in growth techniques and fundamental understanding promise to realize the full potential of this remarkable semiconductor material.