High-speed and high-gain graphene photovoltaic phototransistor gated by a van der Waals heterojunction

Technical Summary: High-speed and High-gain Graphene Photovoltaic Phototransistor Gated by a van der Waals Heterojunction

Problem Statement
Two-dimensional (2D) material-based phototransistors offer a unique integration of optical sensing, signal amplification, and logic operation. However, they fundamentally suffer from an inherent trade-off between gain and speed. In conventional phototransistors, high responsivity is achieved through the photogating (PG) effect, where photogenerated carriers are trapped, extending their lifetime ($\tau_{life}$) and allowing the opposite carrier type to recirculate multiple times. This results in a photoconductive gain $G = \tau_{life}/\tau_{tr}$ (where $\tau_{tr}$ is the transit time). While this mechanism yields ultrahigh responsivity, it typically incurs a severe penalty in response speed, often resulting in response times in the millisecond to second range. Conversely, devices relying solely on the photovoltaic (PV) effect or intrinsic carrier transport (like pure graphene) offer high speed but suffer from low responsivity (typically bounded by $R_0 = \lambda/1240$ A/W) due to the absence of gain. Previous attempts to introduce built-in electric fields to improve speed while maintaining gain have struggled to achieve sub-microsecond response times without compromising gain, often due to prolonged charge interactions within the device channel.

Methodology
To overcome this limitation, the authors demonstrate an all-2D material photovoltaic phototransistor (PVPT) architecture that decouples the light-harvesting/generation region from the carrier transport channel. The device consists of a graphene (Gr) transistor channel gated by a MoS$_2$/PtSe$_2$ van der Waals heterojunction, separated by a hexagonal boron nitride (hBN) insulating layer.

• Device Fabrication: The constituent 2D materials (MoS$_2$, PtSe$_2$, hBN, and Gr) were mechanically exfoliated and deterministically transferred using PDMS stamps onto a SiO$_2$/Si substrate. Source and drain electrodes (Au/Cr) were fabricated via standard photolithography and electron-beam evaporation.
• Operating Principle: The design utilizes an "interfacial gating" mechanism. The MoS$_2$/PtSe$_2$ heterojunction acts as the photovoltaic unit, generating an ultrafast PV response and separating electron-hole pairs via a built-in electric field. The accumulated charges in the heterojunction (specifically holes in PtSe$_2$ or electrons in MoS$_2$, depending on stacking) electrostatically gate the underlying graphene channel through the hBN barrier. This separates the photogating process from the transport channel, suppressing prolonged charge interactions.
• Characterization: The study employed transfer characteristic measurements, Raman spectroscopy, atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM) to verify material quality and band alignment. Photodetection performance was evaluated using transient photocurrent measurements under modulated laser illumination (532 nm and broadband 405–1550 nm) using choppers, TTL, and acousto-optic modulators (AOM) to assess response speeds.

Key Contributions and Results
The study reports a PVPT that simultaneously achieves ultrahigh gain and sub-microsecond response speed, effectively breaking the classical gain-speed trade-off.

1. Performance Metrics:

   • Gain and Responsivity: The device exhibits a photoconductive gain of up to $10^8$, corresponding to a maximum responsivity of $1.14 \times 10^5$ A/W at 532 nm under weak illumination ($P_{in} = 0.016$ mW/cm$^2$).
   • Response Speed: The response time is governed by the carrier transit time in the graphene channel rather than trap lifetimes. Using an AOM for modulation, the rise and fall times were extracted as 440 ns. The authors conservatively estimate the actual response time to be below 550 ns, limited only by the instrumentation's sampling interval (0.55 $\mu$s).
   • Detectivity: The device achieves a specific detectivity ($D^*$) exceeding $10^{11}$ Jones (maximum $9.7 \times 10^{11}$ Jones) with a noise equivalent power (NEP) of 0.82 fW/Hz$^{1/2}$.
   • Broadband Operation: Leveraging the optical properties of the MoS$_2$/PtSe$_2$ heterojunction, the device demonstrates broadband photodetection from the visible to the near-infrared (405–1550 nm), maintaining high responsivity ($\sim 10^4$ A/W) and detectivity across this spectrum.

2. Mechanism Verification:

   • Interfacial Gating vs. Trapping: The authors argue that the ultrahigh gain is not dominated by conventional charge trapping (which would imply a gain $< 10^3$ given the fast response). Instead, the gain originates from the interfacial gating effect where photogenerated carriers accumulate in the heterojunction and modulate the graphene channel. This is supported by temperature-dependent measurements showing stable photocurrent from 10–300 K and gate-sweep-rate independence, suggesting minimal contribution from deep traps.
   • Band Alignment: KPFM and energy band diagrams confirm a type-I or type-II alignment at the MoS$_2$/PtSe$_2$ interface with a built-in electric field oriented from MoS$_2$ to PtSe$_2$. This facilitates efficient separation of electron-hole pairs, with holes accumulating in PtSe$_2$ to induce n-doping in the graphene channel (for $V_G > V_{Dirac}$).

3. Design Flexibility:

   • The authors demonstrated that reversing the stacking order to PtSe$_2$/MoS$_2$ allows for electron accumulation in the MoS$_2$ layer, enabling control over the sign of the photocurrent while maintaining comparable performance.

Significance
The paper establishes a new paradigm for high-performance 2D phototransistors by harnessing the synergy between photovoltaic and photogating effects. By utilizing an interfacial gating architecture with a MoS$_2$/PtSe$_2$ heterojunction, the device successfully overcomes the classical gain-speed trade-off that has long limited phototransistor design. The results demonstrate that high gain can be attained without compromising speed, as the response time is dictated by the ultrafast carrier transit in graphene rather than slow trap dynamics. The authors posit that this architecture, offering ultrahigh sensitivity, high speed, and broadband response, paves a promising way for next-generation optical communication, sensing, and imaging systems. The versatility of the photovoltaic heterojunction design further suggests potential for extending the spectral range in diverse optoelectronic applications.