MAR 27, 202658 MINS READ
The efficacy of oxygen doping in gallium nitride is fundamentally governed by crystallographic orientation, with non-C-plane surfaces exhibiting significantly enhanced oxygen incorporation compared to conventional C-plane substrates 1. Non-C-plane gallium nitride seed crystals—specifically {hkmn} planes where h=k=m=0 and n≠1—enable oxygen atoms to infiltrate the growing crystal lattice via surface-mediated diffusion during vapor-phase epitaxy 4. This orientation-dependent doping mechanism arises from the distinct atomic arrangements and bonding configurations at non-C-plane surfaces, which present lower energy barriers for oxygen substitution at nitrogen sites 5.
Key Mechanisms For Oxygen Incorporation:
Non-C-Plane Direct Infiltration: Oxygen atoms supplied in the gas phase (typically as O₂ or H₂O vapor at partial pressures of 10⁻⁴–10⁻² atm) diffuse through non-C-plane surfaces during crystal growth at temperatures of 1000–1100°C, achieving activation rates of 75–100% as n-type dopants 1. The high activation efficiency stems from the preferential formation of substitutional oxygen at nitrogen sites (O_N), which acts as a shallow donor with ionization energy of approximately 30 meV 2.
Facet-Mediated Doping On C-Plane Substrates: When C-plane gallium nitride seed crystals are employed, oxygen incorporation is achieved by intentionally generating non-C-plane facets during growth 4. Material gases containing gallium (e.g., trimethylgallium at flow rates of 50–200 μmol/min), nitrogen (NH₃ at 1–5 slm), and oxygen are supplied to produce a faceted crystal morphology with {10-11} or {11-22} facets 5. Oxygen infiltrates through these facets while the crystal grows along the c-axis, enabling controlled doping even on nominally C-plane substrates 11.
Hydrogen Co-Doping And Compensation Effects: Advanced oxygen doped gallium nitride crystals exhibit hydrogen concentrations exceeding 5×10¹⁷ cm⁻³ with [H]/[O] ratios of at least 0.3, alongside trace alkali or halogen impurities (Li, Na, K, F, Cl) above 1×10¹⁶ cm⁻³ 3. These co-dopants introduce compensation ratios between 1.0 and 4.0, modulating carrier concentration and enabling fine-tuning of electrical properties for specific device architectures 7. Infrared spectroscopy reveals characteristic absorption peaks at 3175 cm⁻¹, 3164 cm⁻¹, and 3150 cm⁻¹ (absorbance ≥0.01 cm⁻¹), corresponding to O-H stretching modes that confirm oxygen incorporation into the wurtzite lattice 3.
The crystallographic control over oxygen doping eliminates reliance on hazardous silane (SiH₄) gas traditionally used for silicon doping, offering a safer and more environmentally compliant manufacturing route 12. Furthermore, oxygen-doped substrates exhibit n-type carrier densities directly proportional to oxygen concentration across the range of 5×10¹⁶ cm⁻³ to 5×10¹⁸ cm⁻³, providing predictable electrical behavior for device design 9.
Vapor-phase epitaxy techniques—particularly hydride vapor-phase epitaxy (HVPE) and metalorganic vapor-phase epitaxy (MOVPE)—serve as the primary methods for synthesizing oxygen doped gallium nitride freestanding substrates and epitaxial layers 2. The growth process requires precise control over temperature, gas-phase composition, and surface kinetics to achieve uniform oxygen distribution and high crystal quality.
Critical Growth Parameters:
Temperature Window: Crystal growth is conducted at substrate temperatures of 1000–1100°C for HVPE and 950–1050°C for MOVPE 5. This temperature range balances gallium nitride decomposition rates, nitrogen incorporation efficiency, and oxygen diffusion kinetics. Lower temperatures (<950°C) result in incomplete oxygen activation, while higher temperatures (>1100°C) promote oxygen desorption and surface roughening 4.
Gas-Phase Composition: Gallium precursors (GaCl or trimethylgallium) are supplied at molar flow rates of 50–200 μmol/min, with NH₃ serving as the nitrogen source at 1–5 standard liters per minute (slm) 11. Oxygen is introduced as O₂ (partial pressure 10⁻⁴–10⁻² atm) or H₂O vapor (dew point −20°C to +20°C) to achieve target oxygen concentrations 2. The V/III ratio (nitrogen-to-gallium molar ratio) is maintained between 100 and 5000 to ensure stoichiometric growth and suppress gallium droplet formation 5.
Growth Rate And Thickness: Typical growth rates range from 50 μm/h (HVPE) to 2–10 μm/h (MOVPE), enabling the production of freestanding substrates with thicknesses of 300–500 μm after 6–10 hours of deposition 1. Thicker substrates (>500 μm) reduce wafer bowing and improve mechanical handling, while thinner epitaxial layers (<10 μm) are employed for device-active regions requiring precise doping profiles 4.
Facet Control And Surface Morphology: Maintaining non-C-plane facets or controlled facet angles during growth is essential for sustained oxygen incorporation 11. In situ monitoring via optical pyrometry and laser reflectometry allows real-time adjustment of growth conditions to preserve facet stability and achieve root-mean-square (RMS) surface roughness below 0.5 nm over 10×10 μm² scan areas 6.
Process Optimization Strategies:
Seed Crystal Preparation: Non-C-plane gallium nitride seed crystals are prepared by slicing bulk crystals along {10-10} m-plane or {11-20} a-plane orientations, followed by chemical-mechanical polishing (CMP) to achieve surface roughness <0.3 nm RMS 5. Seed crystals are pre-annealed at 1050°C in NH₃ ambient for 30 minutes to remove surface contaminants and stabilize step-terrace structures 1.
Two-Step Growth For C-Plane Substrates: When C-plane seeds are used, a two-step process is employed: (i) initial growth at reduced V/III ratio (100–500) and elevated temperature (1080–1100°C) to nucleate non-C-plane facets, followed by (ii) steady-state growth at higher V/III ratio (1000–3000) and moderate temperature (1000–1050°C) to maintain facets and incorporate oxygen 4. This approach yields oxygen concentrations of 2×10¹⁷–5×10¹⁸ cm⁻³ with spatial uniformity better than ±10% across 2-inch wafers 9.
Post-Growth Annealing: Oxygen-doped crystals are annealed at 600–800°C in nitrogen or forming gas (5% H₂ in N₂) for 1–4 hours to activate residual oxygen donors and reduce compensation from hydrogen-related complexes 3. Annealing increases carrier concentration by 20–50% and improves Hall mobility from 200–250 cm²/V·s (as-grown) to 300–400 cm²/V·s (annealed) at room temperature 7.
The combination of orientation-controlled growth and optimized process parameters enables reproducible fabrication of oxygen doped gallium nitride substrates with dislocation densities below 10⁶ cm⁻² and carrier concentrations tunable over two orders of magnitude 2.
Oxygen doped gallium nitride exhibits n-type electrical conductivity with carrier concentrations and mobilities that depend on oxygen concentration, compensation ratio, and crystal quality 9. The material's optical properties are characterized by near-band-edge absorption and infrared signatures of oxygen-hydrogen complexes, providing diagnostic tools for doping verification and quality control 3.
Electrical Characteristics:
Carrier Concentration: Oxygen doping produces n-type carrier densities ranging from 5×10¹⁶ cm⁻³ to 5×10¹⁸ cm⁻³, with a linear relationship between oxygen concentration (measured by secondary ion mass spectrometry, SIMS) and free electron density (measured by Hall effect) 9. At oxygen concentrations of 2×10¹⁷ cm⁻³, typical carrier densities are 1.5–1.8×10¹⁷ cm⁻³, corresponding to activation efficiencies of 75–90% 1. Higher oxygen concentrations (>1×10¹⁹ cm⁻³) lead to increased compensation and reduced activation efficiency due to formation of oxygen-vacancy complexes and self-compensation effects 7.
Hall Mobility: Room-temperature electron mobility in oxygen doped gallium nitride ranges from 200 cm²/V·s (oxygen concentration 5×10¹⁸ cm⁻³) to 400 cm²/V·s (oxygen concentration 5×10¹⁶ cm⁻³) 10. Mobility is limited by ionized impurity scattering at high doping levels and by dislocation scattering in crystals with threading dislocation densities above 10⁷ cm⁻² 6. Low-temperature (77 K) mobility increases to 800–1200 cm²/V·s, indicating reduced phonon scattering and confirming the shallow donor nature of substitutional oxygen 2.
Resistivity And Sheet Resistance: Bulk resistivity of oxygen doped gallium nitride substrates ranges from 0.01 Ω·cm (high doping, 5×10¹⁸ cm⁻³) to 0.1 Ω·cm (moderate doping, 5×10¹⁷ cm⁻³) 12. For epitaxial layers with thickness of 2–5 μm, sheet resistance is typically 50–200 Ω/sq, suitable for current-spreading layers in LED structures and buffer layers in HEMT devices 6.
Optical Properties:
Band-Edge Absorption: Oxygen doped gallium nitride exhibits strong absorption at wavelengths below 365 nm (bandgap energy ~3.4 eV at 300 K), with absorption coefficient exceeding 10⁴ cm⁻¹ for photon energies above the direct bandgap 3. The absorption edge is sharp (Urbach energy <20 meV), indicating low defect density and high crystal quality 7.
Infrared Spectroscopy Signatures: Fourier-transform infrared (FTIR) spectroscopy reveals characteristic absorption peaks at 3175 cm⁻¹, 3164 cm⁻¹, and 3150 cm⁻¹, attributed to O-H stretching vibrations in oxygen-hydrogen complexes 3. The absorbance per unit thickness at these peaks is at least 0.01 cm⁻¹, providing a quantitative measure of oxygen incorporation 7. Notably, the spectral region between 3200–3400 cm⁻¹ and 3075–3125 cm⁻¹ is free of strong absorption features (absorbance <10% of the 3175 cm⁻¹ peak), distinguishing oxygen-doped material from hydrogen-contaminated or Mg-doped gallium nitride 3.
Photoluminescence (PL): Room-temperature PL spectra of oxygen doped gallium nitride show a dominant near-band-edge emission peak at 365 nm with full-width at half-maximum (FWHM) of 5–10 nm, indicating minimal band-tail states and low compensation 2. Deep-level emission (yellow band at 550–600 nm) is suppressed to <1% of band-edge intensity, confirming low concentrations of gallium vacancies and carbon impurities 4.
Thermal Stability:
Oxygen doped gallium nitride maintains stable electrical properties up to 600°C, with carrier concentration variation less than ±5% after 100 hours of annealing at 500°C in nitrogen ambient 10. Thermogravimetric analysis (TGA) shows negligible weight loss (<0.01%) up to 800°C, and differential scanning calorimetry (DSC) reveals no phase transitions or decomposition events below 1000°C 5. This thermal stability is critical for high-temperature device operation and enables robust processing during epitaxial overgrowth and metallization steps 6.
The production of oxygen doped gallium nitride freestanding substrates involves multi-step processes encompassing seed crystal preparation, thick-film growth, substrate separation, and surface finishing 1. Epitaxial wafers for device fabrication are subsequently prepared by homoepitaxial growth of device-active layers on these substrates 2.
Substrate Fabrication Workflow:
Seed Crystal Selection And Preparation: High-quality gallium nitride seed crystals with dislocation densities below 10⁶ cm⁻² are selected and oriented along non-C-plane directions (m-plane, a-plane, or semipolar planes such as {20-21}) 5. Seeds are sliced to 300–500 μm thickness using wire saws with diamond slurry, then subjected to CMP with colloidal silica (particle size 50–100 nm) to achieve surface roughness <0.3 nm RMS 1. Pre-growth cleaning involves sequential ultrasonic baths in acetone, isopropanol, and deionized water, followed by UV-ozone treatment to remove organic residues 4.
Thick-Film HVPE Growth: Seeds are loaded into an HVPE reactor and heated to 1050–1100°C under NH₃ flow (2–5 slm) 11. GaCl vapor (generated by passing HCl over molten gallium at 850°C) and oxygen (O₂ partial pressure 10⁻³ atm) are introduced to initiate growth at rates of 50–100 μm/h 5. Growth is continued for 6–10 hours to deposit 300–1000 μm thick layers, with in situ monitoring of growth rate and surface morphology via laser interferometry 2. The reactor is cooled to room temperature at controlled rates (50–100°C/h) to minimize thermal stress and wafer bowing 1.
Substrate Separation: The thick gallium nitride layer is separated from the seed crystal by laser lift-off (LLO) using a KrF excimer laser (248 nm wavelength, fluence 400–800 mJ/cm²) or by mechanical grinding and polishing 4. LLO is preferred for non-C-plane substrates as it preserves crystal quality and enables seed reuse, while mechanical separation is employed for thick (>500 μm) substrates where laser penetration is limited 5.
Surface Finishing: Separated substrates undergo double-side polishing: the growth surface is polished with CMP to <0.5 nm RMS roughness for epitaxial overgrowth, while the backside is ground and polished to 300–400 μm final thickness for
| Org | Application Scenarios | Product/Project | Technical Outcomes |
|---|---|---|---|
| SUMITOMO ELECTRIC INDUSTRIES LTD. | Blue LED substrates, high-electron-mobility transistors (HEMTs), vertical power devices, and optoelectronic applications requiring safe n-type doping with precise carrier control. | Oxygen-Doped GaN Freestanding Substrates | Achieves 75-100% oxygen activation rate as n-type dopant through non-C-plane crystal growth, enabling carrier densities proportional to oxygen concentration (5×10¹⁶-5×10¹⁸ cm⁻³) with superior surface morphology and eliminates hazardous silane gas usage. |
| SUMITOMO ELECTRIC INDUSTRIES LTD. | Epitaxial wafers for nitride semiconductor devices, current-spreading layers in LED structures, and buffer layers for high-performance power electronics on semipolar and non-polar orientations. | Group III Nitride Semiconductor Epitaxial Substrates | Oxygen concentration control between 5×10¹⁶-5×10¹⁸ cm⁻³ provides flat surface morphology with RMS roughness <0.5 nm, excellent crystal quality with dislocation densities below 10⁶ cm⁻², and Hall mobility of 200-400 cm²/V·s at room temperature. |
| SLT Technologies Inc. | Advanced semiconductor substrates for high-power devices, thermal management applications, and next-generation wide-bandgap electronics requiring precise doping control and thermal stability up to 600°C. | Oxygen-Doped Wurtzite GaN Crystals | Hydrogen co-doping with [H]/[O] ratio ≥0.3 and compensation ratios of 1.0-4.0 enables fine-tuning of electrical properties, with characteristic infrared absorption peaks at 3175/3164/3150 cm⁻¹ for quality verification and oxygen concentrations up to 1×10²⁰ cm⁻³. |
| COMMISSARIAT À L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES | High-electron-mobility transistors (HEMTs) with source-drain-gate configurations, RF power amplifiers, and monolithic microwave integrated circuits requiring lateral device isolation in GaN-based heterostructures. | Oxygen-Implanted GaN Electronic Components | Oxygen ion implantation creates lateral isolation zones in p-type and n-type GaN layers with widths of 1-20 μm, enabling formation of two-dimensional electron gas (2DEG) structures for high-frequency transistor applications. |