Gallium nitride (GaN), as a third-generation wide bandgap semiconductor material, has excellent characteristics such as a large bandgap width (3.4eV), high critical breakdown electric field, high electron saturation drift velocity, and strong radiation resistance. It has shown broad application prospects in the field of ultraviolet detection. Especially in cutting-edge fields such as ultraviolet astronomy, environmental monitoring, and space communication, GaN photodetector, like avalanche photodiodes (APDs), can achieve high sensitivity detection at the single photon level. Regardless of the device structure, the quality of epitaxial materials - including substrate selection, buffer layer design, doping distribution, defect density control, etc. - directly determines the core performance of APD, such as dark current, gain, breakdown voltage, and reliability.

1. Epitaxial Fundamentals of GaN-Based APDs

1.1 Substrate Selection

Bulk GaN substrates are scarce and expensive, so most GaN epitaxy is performed on heterogeneous substrates. A comparison of common substrates and their key parameters is presented below:

The main challenges with heterogeneous substrates are large lattice and thermal mismatches, which must be addressed through buffer layer technology to release stress and filter dislocations. For example, growing GaN on silicon substrates typically employs an AlN nucleation layer and a graded (Al,Ga)N buffer layer; on sapphire substrates, a low-temperature GaN or AlN nucleation layer is often used.

1.2 Buffer Layer Design

The buffer layer serves to block the extension of substrate defects into the epitaxial layer and to manage thermal stress. Hamdaoui et al. (2024) reported a high-quality GaN-on-Si APD structure using an approximately 1.5μm thick (Al,Ga)N superlattice buffer combined with a high-temperature AlN nucleation layer, achieving a threading dislocation density of about 5×10⁸cm⁻². For sapphire substrates, a low-temperature GaN buffer followed by a high-temperature GaN layer further reduces dislocation density. Proper buffer layer design can also adjust wafer bowing and improve uniformity on large-diameter substrates.

1.3 Doping Control and Defect Density

N-type doping control in GaN epitaxy typically uses silane (SiH₄), with doping concentrations ranging from 5×10¹⁵cm⁻³ (lightly doped drift layer) to 5×10¹⁸cm⁻³ (n⁺ contact layer). P-type doping employs bis(cyclopentadienyl)magnesium (Cp₂Mg), but due to the high activation energy of Mg in GaN (approximately 170meV), high-temperature annealing is required for activation, and the effective doping concentration is usually much lower than the stoichiometric concentration. Defect density, especially threading dislocations, exacerbates reverse leakage current, reduces breakdown voltage, and leads to premature device breakdown. Therefore, reducing defect density is the core objective of epitaxial optimization.

2. GaN Epitaxial Structure Design for Ultraviolet Avalanche Photodiodes

2.1 GaN Photodetector p-i-n Structure Epitaxy

The most fundamental structure for GaN photodetector is the p-i-n structure. The typical epitaxial layer sequence is: on a substrate, grow successively an n⁺-GaN contact layer, a lightly doped or unintentionally doped i-GaN layer (serving as both absorption and multiplication region), and a p-GaN layer. Lei et al. reported a sapphire-based p-i-n structure APD epitaxy operating in back-illuminated mode, where UV light enters from the sapphire side and the i-GaN layer acts as the main absorption and multiplication region. Notably, the hole impact ionization coefficient in GaN is larger than that of electrons, so backside illumination (hole injection into the multiplication region) reduces excess noise. Related studies show that an n-i-p structure on free-standing GaN substrates further reduces dark current density to 1.5×10⁻⁵A/cm² and achieves a gain as high as 10⁵.

For AlGaN APDs requiring solar-blind characteristics, the epitaxy incorporates a high-Al-composition AlGaN absorption layer (e.g., Al₀.₄Ga₀.₆N) together with a low-Al-composition AlGaN multiplication layer (e.g., Al₀.₂Ga₀.₈N). However, due to the lack of native homoepitaxial substrates, AlGaN epitaxy must be grown on sapphire or AlN substrates, resulting in high defect density. By inserting a step-graded n-type layer (n-GaN → n-Al₀.₀₂Ga₀.₉₈N), the dark current density can be reduced from 6.5×10⁻⁵A/cm² to below 1×10⁻⁷A/cm².

2.2 GaN Separate Absorption and Multiplication (SAM) Structure Epitaxy

The SAM structure achieves electric field engineering by inserting a charge layer between the absorption layer and the multiplication layer, concentrating a high electric field in the multiplication region and a low electric field in the absorption region, thereby reducing noise and increasing gain. A typical AlGaN SAM structure APD epitaxy includes: p-AlGaN contact layer / AlGaN multiplication layer / AlGaN charge layer / AlGaN absorption layer / n-AlGaN layer / substrate. The multiplication layer uses a low Al composition (e.g., Al₀.₂Ga₀.₈N) to achieve a high hole impact ionization coefficient; the absorption layer uses a high Al composition (e.g., Al₀.₄Ga₀.₆N) for solar-blind characteristics; the charge layer precisely controls the doping concentration and thickness to tailor the electric field distribution between the two layers. A structure reported in the literature employs a dual-multiplication-layer design with high/low Al composition, lowering the breakdown voltage from 116.3V to 96.9V. Moreover, inserting a one-dimensional photonic crystal filter layer on the backside of the SAM structure further enhances solar-blind detection capability.

2.3 Polarization-Enhanced Epitaxy Design

The strong spontaneous and piezoelectric polarization effects in III-nitride heterojunctions can be harnessed to enhance the built-in electric field, thereby reducing the operating voltage of APDs. This approach is known as polarization engineering. Researchers have proposed replacing the p-GaN layer in a p-i-n APD with p-In₀.₀₅Ga₀.₉₅N; the polarization charge between InGaN and GaN enables the multiplication region to reach the critical electric field at a lower bias, reducing the breakdown voltage by more than 26V. Further, introducing a highly polar Sc₀.₀₅Ga₀.₉₅N layer into the SAM structure yields an internal electric field of 2.8 MV/cm and increases the gain by 60% to 7.2×10⁴. Such polarization engineering provides new approaches for developing low-power, high-sensitivity GaN photodetectors.

3. Commercial GaN Photodetector Epitaxy Specifications

Based on the above epitaxial design principles, PWG offers various commercial epitaxy GaN photodetector wafers with customizable structures, covering different substrates and application requirements. Two typical product structures are presented below, corresponding to sapphire-based back-illuminated APDs and silicon-based low-cost UV detection, respectively.

1) Sapphire-based Back-illuminated p-i-n Structure

2) Silicon-based p-i-n Structure

Note: Specific thickness and doping values are customizable.

The above commercial epitaxy products fully consider the differentiated requirements of various application scenarios regarding epitaxial layer thickness, doping concentration, and substrate type (including sapphire, SiC, silicon, and GaN). Users can select suitable standard products or request custom epitaxy(p-i‑n structure, SAM structure, etc.) according to their device structure.

References:

1. Lei, Q., Li, L., Lu, W., Tao, J., Ling, R., Zhang, L., ... & Gao, F. (2025). Preparation and research progress of GaN-based avalanche photodetectors. Microstructures, 5(4), N-A.

2. Hamdaoui, Y., Michler, S., Bidaud, A., Ziouche, K., & Medjdoub, F. (2024). 1200-V fully vertical GaN-on-silicon pin diodes with avalanche capability and high on-state current above 10 A. IEEE Transactions on Electron Devices, 72(1), 338-343.