Experimental Demonstration of Nonlinear Photoconductive Gain in N-Doped $β$-Ga$_2$O$_3$ Devices

Technical Summary: Experimental Demonstration of Nonlinear Photoconductive Gain in N-Doped β-Ga₂O₃ Devices

Problem Statement
While β-Ga₂O₃ is a promising ultra-wide-bandgap (UWBG) semiconductor for high-voltage optoelectronics due to its large bandgap (~4.8 eV) and high theoretical critical electric field, current photodetectors face significant limitations. Most existing devices rely on band-to-band excitation in the deep-ultraviolet (DUV) spectral range. This necessitates bulky, difficult-to-integrate light sources with complex optical delivery and packaging constraints. Furthermore, reported β-Ga₂O₃ photoconductive devices typically operate in a linear regime where photocurrent scales one-to-one with absorbed optical power. This linear relationship limits system efficiency and power gain, requiring high-energy optical excitation to achieve appreciable current levels. There is a need to explore nonlinear, high-gain regimes in β-Ga₂O₃ that can be triggered by lower-energy visible light, similar to regimes well-established in GaAs-based photoconductive switches (PCSS).

Methodology
The authors fabricated vertical photoconductive devices using a 5.6-µm-thick nitrogen-doped (~2 × 10¹⁶ cm⁻³) semi-insulating β-Ga₂O₃ epitaxial layer grown via halide vapor phase epitaxy (HVPE) on a highly conductive Sn-doped substrate. The devices featured Ti/Au contacts (20 nm/100 nm), with a uniform bottom electrode and an interdigitated top electrode to ensure uniform field distribution and optical access.

Experimental characterization was conducted at room temperature using a probe station immersed in Fluorinert dielectric fluid to prevent air breakdown. The devices were illuminated by a 3-W, 445-nm continuous-wave (CW) diode laser (sub-bandgap excitation). Electrical measurements were performed using a high-voltage source-measure unit (Keithley 2657A) to record dark and photocurrent I-V characteristics and temporal responses.

To support experimental findings, two-dimensional drift-diffusion TCAD simulations were performed using Silvaco ATLAS. The simulations modeled the specific device geometry, including the n-type epilayer with residual Si doping and deep nitrogen acceptor traps (modeled at 1.9 × 10¹⁶ cm⁻³) to replicate semi-insulating behavior. Impact-ionization generation was modeled using the Selberherr model to analyze carrier multiplication mechanisms.

Key Results

• Nonlinear Transition: Under 445-nm CW illumination, the devices exhibited a distinct transition from a linear photoconductive regime to a nonlinear high-gain regime. This transition occurred at a threshold electric field of approximately 0.67 MV/cm (corresponding to bias voltages between 325 V and 350 V).
• Gain Enhancement: Beyond the threshold, the photocurrent increased dramatically. For Device 1, the current rose from ~20 µA in the linear regime to ~476 µA at 435 V, representing an enhancement factor of approximately 23. Device 2 showed a similar ~21-fold enhancement, reaching ~450 µA at 445 V. Peak photocurrents approached 475 µA.
• Temporal Dynamics: At low bias (300 V), the photocurrent response was small and slowly varying. At higher biases (400–425 V), the response showed a pronounced transient peak followed by a decay to a steady state, indicating trap-mediated carrier dynamics and enhanced carrier generation at high fields.
• Simulation Insights: TCAD simulations revealed that at biases above 400 V, the electric field near the contact regions exceeded 1.5 MV/cm. This high-field localization triggered a rapid increase in impact-ionization generation rates (reaching 10¹⁶–10¹⁸ cm⁻³·s⁻¹), confirming that avalanche-assisted carrier multiplication contributes significantly to the observed gain.

Significance and Claims
The paper claims to present the first experimental demonstration of nonlinear, field-tunable photoconductive gain in β-Ga₂O₃ devices under sub-bandgap visible-light excitation. The authors assert that by leveraging deep-level trap states activated under visible illumination, combined with high-field impact ionization, it is possible to achieve a current multiplication factor of ~20 without deep-ultraviolet sources.

The significance of this work lies in demonstrating a viable pathway toward compact, high-field photoconductive detection in β-Ga₂O₃ that operates without the packaging and integration constraints of DUV sources. The results suggest that defect-assisted transport and avalanche multiplication can be harnessed to create field-tunable gain photodetectors suitable for harsh environment sensing and optoelectronic applications.