Advances in Gallium-Based Metal Monochalcogenide Photodetectors: From Fundamental Physics to Heterojunction Engineering

Abstract

Two-dimensional (2D) metal monochalcogenides (MMCs) have emerged as a pivotal class of semiconductors, offering unique optoelectronic properties that bridge the gaps left by graphene and transition metal dichalcogenides. This review provides a comprehensive analysis of gallium-based MMCs, specifically GaS and GaSe, detailing their evolution from fundamental material physics to advanced device applications. We begin by elucidating the core sensing mechanisms-photoconductive, photogating, and photovoltaic effects-that govern light-matter interactions in 2D systems, followed by a rigorous definition of the figures-of-merit essential for evaluating photodetector performance. The article systematically surveys contemporary synthesis strategies, contrasting top-down exfoliation methods with bottom-up growth techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and pulsed laser deposition (PLD). A significant focus is placed on heterojunction engineering, highlighting how the integration of Ga-based MMCs into van der Waals heterostructures and 2D/3D interfaces can enhance carrier mobility, prolong photocarrier lifetimes, and enable self-powered operation. By synthesizing the current state of research, this work underscores the transformative potential of gallium-based MMCs in the development of next-generation, high-sensitivity multispectral photodetectors.

Introduction

The isolation of graphene in 2004 opened a new era in 2D materials science, demonstrating exceptional carrier mobility and thermal conductivity. However, graphene lacks a natural bandgap, limiting its use in optoelectronics. This led to the exploration of 2D semiconductors like transition metal dichalcogenides (TMDCs) and, more recently, metal monochalcogenides (MMCs) such as GaS, GaSe, InSe, and GeSe.

MMCs exhibit unique electronic properties, including layer-dependent direct-to-indirect bandgap transitions, high surface-to-volume ratios, and tunable band edges, making them highly sensitive to light and ideal for multispectral photodetection (UV, visible, NIR).

Gallium-based MMCs are particularly important:

• GaSe: stable p-type 2D semiconductor, enabling efficient p–n junctions without doping.

• GaS: wide bandgap (~3 eV), solar-blind, ideal for UV sensing.

Advances include CVD and MBE growth, heterojunction engineering with n-type oxides or TMDCs, and flexible/wearable photodetectors, though ambient stability remains a challenge.

Photodetection Mechanisms

Three primary mechanisms govern 2D photodetector operation:

1. Photoconductive Effect

   • Light increases semiconductor conductivity by generating electron-hole pairs.

   • Gain depends on the ratio of carrier lifetime to transit time; higher gain reduces response speed.

2. Photogating Effect

   • A variant of photoconduction where defect-trapped carriers act as local gates.

   • Leads to high gain but slower response due to trap-mediated processes.

3. Photovoltaic Effect

   • Built-in electric fields at junctions separate photogenerated carriers without external bias.

   • Photodiodes have low gain (≤1) but fast response; ideal for high-speed applications.

Figures-of-Merit in Photodetectors

Performance parameters include:

• Dark current (I_d): low values are preferable.

• Illumination current (I_λ) and photocurrent (I_ph): higher values indicate better sensitivity.

• Photo-to-dark current ratio (PDCR): measures signal-to-noise.

• Photoresponsivity (R_λ): current per unit incident power; higher is better.

• Noise Equivalent Power (NEP): minimum detectable power.

• Specific Detectivity (D*): ability to detect weak signals; higher indicates better performance.

• External Quantum Efficiency (EQE): fraction of photons converted to carriers.

• Response time (τ): rise and decay times, with fast and slow components.

• Gain (G): number of carriers generated per photon.

• Linear Dynamic Range (LDR): input power range producing a linear response.

Synthesis and Assembly of MMCs

Reliable production of MMCs is crucial for electronics and optoelectronics. Methods are categorized as:

1. Top-Down Approaches

• Utilize weak van der Waals forces to separate layers.

• Techniques: mechanical exfoliation, chemical exfoliation, and liquid-phase intercalation.

2. Bottom-Up Approaches (briefly mentioned)

• Techniques include vapor deposition, solvothermal synthesis, pulsed laser deposition.

• Allow controlled layer thickness and morphology for device integration.

Conclusion

The emergence of two-dimensional Gallium-based metal monochalcogenides (MMCs) has established a new frontier in next-generation optoelectronics, effectively bridging the performance and bandgap limitations inherent to graphene and transition metal dichalcogenides (TMDCs). This review has systematically highlighted the unique intrinsic properties of GaSe and GaS, including their distinctive direct-to-indirect thickness-dependent bandgap transitions, exceptionally low carrier effective masses, and inherent p-type or solar-blind characteristics. By leveraging photoconductive, photogating, and photovoltaic sensing mechanisms, devices based on these materials have demonstrated remarkable optoelectronic figures-of-merit, achieving ultra-high specific detectivities (exceeding ?10?^14 Jones) and massive external quantum efficiencies. A defining advantage of Ga-based MMCs lies in their ultrafast interfacial charge dynamics and unprecedented stacking tunability. As elucidated through recent computational models and experimental transient absorption spectroscopy, the weak non-adiabatic coupling in these systems enables a substantial suppression of electron-hole recombination. Consequently, properly engineered van der Waals heterostructures—such as GaSe/GaTe, GaSe/Ga?O?, and GaSe/Si—can prolong photocarrier lifetimes by orders of magnitude compared to TMDC-only architectures. This interfacial mastery facilitates the realization of Type-II band alignments that drive highly efficient, self-powered photodetection at zero bias. However, the transition of Ga-based MMC photodetectors from laboratory-scale prototypes to commercial optoelectronic integration faces critical engineering challenges. While significant progress has been made in top-down exfoliation and bottom-up synthesis methods (CVD, PVD, and PLD), achieving wafer-scale, single-crystalline uniformity with precise layer control remains a primary hurdle. Furthermore, mitigating surface defect densities and ensuring the long-term ambient stability of these atomically thin layers will require the development of robust defect-passivation and encapsulation strategies. Moving forward, the synergistic combination of ab initio computational predictions (DFT and NAMD) with advanced time-resolved spectroscopy will be essential to rationally design optimal stacking registries and heterojunction interfaces. Future research paradigms should prioritize the integration of these high-gain, self-driven MMC sensors into flexible substrates, multispectral imaging arrays, and low-power logic circuits. Ultimately, Gallium-based metal monochalcogenides possess the transformative potential to redefine the operational limits of high-sensitivity, broadband, and wearable optoelectronic systems.

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Copyright © 2026 Sahin Sorifi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.