Optimizing 4H‑SiC SACM Avalanche Photodiodes for Superior Ultraviolet Sensitivity

Introduction

Silicon carbide (SiC) and aluminum gallium nitride (AlGaN) are prominent wide‑bandgap semiconductors, offering high critical electric fields, superior radiation tolerance, and excellent thermal conductivity—key attributes for ultraviolet (UV) detection [1–3]. While AlGaN photodetectors can tune their cut‑off wavelengths from 365 to 200 nm, the growth challenges of Al‑rich AlGaN often result in higher dark currents compared to SiC devices [4]. Consequently, SiC photodetectors have attracted considerable research interest.

To date, 4H‑SiC UV detectors include Schottky barrier diodes, metal–semiconductor–metal (MSM) photodiodes, p‑i‑n photodiodes, and avalanche photodiodes (APDs) [5–9]. APDs, with their high avalanche gain, low dark current, and minimal noise, are especially attractive for applications such as fire detection, quantum communication, and missile warning [10–12]. However, SiC’s low absorption coefficient poses a challenge: thin multiplication layers hinder quantum efficiency until the SACM structure was introduced. In SACM APDs, a dedicated absorption layer captures UV photons while a separate multiplication layer amplifies the signal via impact ionization, with the charge‑control layer terminating the avalanche [13–15]. This configuration reduces noise by allowing only the high‑ionization‑rate carriers to reach the multiplication region [16].

Large‑area SACM APDs promise higher detectivity, but increased device area also elevates surface and bulk leakage currents. Thus, device design must balance active area against leakage, demanding high‑quality epitaxial layers and meticulous fabrication. Recent advances, such as variable‑temperature photoresist reflow for smooth beveled mesas, have achieved multiplication gains exceeding 10^6 with dark currents around 0.2 nA/cm^2 [18]. Nonetheless, the influence of structural design on carrier transport and detectivity remains underexplored. This work addresses that gap by systematically evaluating how layer thicknesses, doping concentrations, and mesa bevel angles affect the optoelectronic performance of large‑area 4H‑SiC SACM UV APDs.

Research Methods and Physics Models

Figure 1a illustrates the cross‑section of a standard 4H‑SiC SACM APD. The stack consists of a p^+ substrate, a 3‑µm p^+ ohmic contact layer (N_a = 1×10^19 cm^−3), a 0.5‑µm n^− multiplication layer (N_d = 1×10^15 cm^−3), a 0.2‑µm n‑type charge‑control layer (N_d = 5×10^18 cm^−3), a 0.5‑µm n^− absorption layer (N_d = 1×10^15 cm^−3), and a 0.3‑µm n^+ ohmic contact (N_d = 1×10^19 cm^−3). A positive bevel angle (θ = 8°) mitigates edge breakdown [22,23], and the device diameter is 800 µm. Cathode and anode contacts are treated as ideal ohmic in the simulations.

Simulations using APSYS solve Poisson’s equation, carrier continuity, and drift‑diffusion equations, incorporating impact ionization and Zener tunneling. Shockley‑Read‑Hall lifetimes are set to 1 µs [24]. Impact‑ionization coefficients for 4H‑SiC follow Chynoweth relations: α_n = 1.98×10^6 exp[−(9.46×10^6/E)^1.42] cm^−1 (1) and β_p = 4.38×10^6 exp[−(1.14×10^7/E)^1.06] cm^−1 (2). The absorption coefficient for 4H‑SiC as a function of wavelength λ (nm) is described by equation (3) [26].

Figure 1b depicts the energy band diagram under reverse bias, illustrating how photo‑generated carriers traverse from the absorption layer through the charge‑control layer into the multiplication region. The electric field in the multiplication layer, the band alignment at the charge‑control/absorption interface, and the n‑type ohmic contact influence carrier transport and thus photocurrent.

a Schematic cross‑section (not drawn to scale), b energy band diagram under reverse bias, c calculated current–voltage characteristics and multiplication gain, d calculated spectral response at 10 V reverse bias. Insets show experimental data.

Results and Discussions

Effect of the n‑Type Ohmic Contact Layer

To isolate the influence of the n‑type ohmic contact layer, we fabricated reference, L1–L4 (varying thickness), and A1–A4 (varying doping) devices. Table 1 lists their structures.

a Breakdown voltage, b vertical electric field distribution at −160 V, c photocurrent–voltage at 280 nm, d spectral response at −100 V. Insets show key results.

Breakdown voltage is essentially unchanged across thickness variations (≈161.6 V) because the charge‑control layer confines the depletion region. However, thicker contacts increase non‑radiative recombination, reducing photocurrent and responsivity. Doping variations likewise have negligible effect on breakdown voltage and photocurrent, underscoring that thickness is the critical parameter for the ohmic contact layer. Maintaining a thin, heavily doped (≈1×10^19 cm^−3) contact layer (~0.2 µm) optimizes responsivity while keeping dark current low.

Effect of the Absorption Layer

Devices M1–M4 (varying thickness) and B1–B4 (varying doping) were analyzed. Table 2 summarizes their designs.

a Breakdown voltage, b electric field at −160 V, c photocurrent–voltage at 280 nm, d spectral response, e carrier profiles in the multiplication layer. Insets show key data.

Increasing absorption‑layer thickness lowers photocurrent and responsivity due to enhanced non‑radiative recombination. In contrast, raising the absorption‑layer doping reduces energy barriers at the charge‑control interface, boosting hole injection and thus responsivity (device B4 shows ~0.11 A/W at 280 nm). Therefore, the absorption layer should be thin (~0.5 µm) and lightly doped (~1×10^15 cm^−3) to balance absorption depth and carrier transport.

Effect of the Charge‑Control Layer

Devices N1–N4 (varying thickness) and C1–C4 (varying doping) were fabricated. Table 3 lists their parameters.

a Breakdown voltage, b electric field at −160 V, c photocurrent–voltage at 280 nm, d spectral response at −100 V. Insets highlight key trends.

Thicker charge‑control layers slightly reduce photocurrent by increasing recombination, while doping reductions below 2×10^18 cm^−3 expand the depletion region, lowering the electric field and raising breakdown voltage to ~315 V (device C1). This depletion extension also diminishes the band‑barrier at the multiplication/charge‑control interface, enhancing hole injection and thus responsivity. Optimal design balances a moderate doping (~1×10^18 cm^−3) and thickness to maintain high gain without excessive dark current.

Effect of the Multiplication Layer

Devices P1–P4 (varying thickness) and D1–D4 (varying doping) are detailed in Table 4.

a Breakdown voltage, b electric field at −160 V, c photocurrent–voltage at 280 nm, d spectral response at −100 V. Insets illustrate key observations.

Increasing the multiplication‑layer thickness from 0.3 to 0.7 µm raises the breakdown voltage from 110 to 210 V and slightly improves responsivity, as a wider depletion region facilitates impact ionization. Conversely, higher doping (>10^18 cm^−3) reduces breakdown voltage and increases dark current by enhancing space‑charge generation; thus, intrinsic doping (~1×10^15 cm^−3) is preferred to preserve low dark current while maintaining high gain.

Effect of the Beveled Mesa Angle

Six devices (E1–E5) with bevel angles from 5° to 15° were analyzed (Table 5).

a Dark current–voltage, b photocurrent–voltage at 280 nm, c spectral response at −100 V. Insets highlight trends.

As the bevel angle increases, the edge electric field rises, leading to premature breakdown and higher dark current. The lateral field distribution (Figure 13a) shows a gradual decrease toward the mesa edge, and smaller angles shift the breakdown away from the surface. While a smaller bevel (≈10–20°) suppresses leakage, it also reduces the active area and responsivity. Thus, a compromise angle between 10° and 20° is recommended based on the epitaxial quality and surface conditions.

Conclusions

Our comprehensive simulation study demonstrates how each structural parameter governs the performance of 4H‑SiC SACM APDs:

• High‑doping (≈1×10^19 cm^−3) but thin (≈0.2 µm) n‑type ohmic contacts keep breakdown voltage stable while maximizing responsivity.
• Lightly doped (≈1×10^15 cm^−3) absorption layers, kept thin (~0.5 µm), optimize carrier generation and transport.
• Charge‑control layers with doping around 1×10^18 cm^−3 effectively confine the electric field; reducing doping below this expands the depletion region, raising breakdown voltage and improving responsivity.
• Intrinsic‑doped multiplication layers (~1×10^15 cm^−3) minimize dark current, while a thickness of 0.5–0.7 µm balances high gain and acceptable breakdown voltage.
• Beveled mesa angles between 10° and 20° provide the best trade‑off between leakage suppression and active area.

These insights furnish a clear design roadmap for fabricating cost‑effective, high‑performance 4H‑SiC SACM UV APDs.

Availability of Data and Materials

The data and analyses presented here are available from the corresponding authors upon reasonable request.