Exceptional Lateral Photovoltaic Response in MoS₂/GaAs Heterojunctions for Ultra‑Sensitive Position Detection

Abstract

We report a record lateral photovoltaic effect (LPE) in molybdenum disulfide (MoS₂) thin films deposited on n‑type GaAs via DC magnetron sputtering. The MoS₂/n‑GaAs heterojunctions display a linear photovoltage that rises to 208.2 mV when the laser spot is near the electrodes, corresponding to a sensitivity of 416.4 mV mm⁻¹. This value far exceeds sensitivities reported for MoS₂/Si and other MoS₂‑based devices. In contrast, MoS₂/p‑GaAs junctions exhibit a much weaker LPE (7.3 mV). Energy‑band alignment analysis attributes the superior performance to a strong built‑in field (E_bi = 1.07 V) that forms a high‑conductivity inversion layer in the n‑GaAs, enabling efficient lateral carrier diffusion. The optimal MoS₂ thickness is ≈30 nm, balancing carrier diffusion and recombination. These findings demonstrate that MoS₂/GaAs heterostructures are promising for high‑performance position‑sensitive detectors.

Background

Molybdenum disulfide (MoS₂) is a prototypical two‑dimensional semiconductor that offers a tunable bandgap, high on/off ratios (>10⁸) in transistors, and photodetectors with high responsivity [1–5]. Unlike gapless graphene, MoS₂’s direct bandgap (~1.8 eV for monolayers, decreasing with layer number) makes it ideal for optoelectronic devices [6]. Recent work has integrated MoS₂ with III–V semiconductors such as GaAs, Si, and GaN to exploit complementary properties [9–13]. GaAs, with a 1.42 eV direct gap and electron mobility ~8000 cm² V⁻¹ s⁻¹, has enabled MoS₂/GaAs solar cells (PCE > 9 %) [9] and self‑driven photodetectors with detectivity 3.5 × 10¹³ Jones [10]. However, MoS₂/GaAs lateral photovoltaic detectors have been largely unexplored. The LPE, arising from lateral diffusion and recombination of photo‑generated carriers in an inversion layer, yields a photovoltage that varies linearly with laser spot position and is highly valuable for position‑sensitive detection (PSD) in robotics, biomedical sensing, and process control [14–18].

Methods

MoS₂ powders (99 % purity) were cold‑pressed (20 MPa) to form a 60 mm × 4.5 mm target and sputtered onto (100) GaAs substrates (n‑, p‑, and intrinsic) at 400 °C under 1 Pa, 10 W. Film thicknesses (≈10–90 nm) were tuned by deposition time and confirmed by cross‑sectional SEM. In pads (300 µm × 0.5 mm) served as electrodes. Characterization included Raman spectroscopy (488 nm), AFM, XPS (Kratos Axis ULTRA), UV‑Vis transmission (Shimadzu UV‑3150), UPS (He‑I 21.22 eV), and electrical measurements (Keithley 2000/2400). The LPE was probed with a 650 nm laser spot (≈0.1 mm) scanned between two In contacts (~1 mm apart). Sensitivity S = LPV_max/L.

Results and Discussion

Structural and spectroscopic validation—Raman peaks at 406.7 cm⁻¹ (A₁g) and 378.9 cm⁻¹ (E¹²g) confirm crystalline MoS₂; A₁g/E¹²g intensity ratio ≈2.1 aligns with literature [19]. AFM shows dense cone‑like grains (≈76 nm) with RMS roughness 1.7 nm, enhancing light absorption.

Electrical behavior—MoS₂/n‑GaAs heterojunctions exhibit rectifying I–V with a ratio of ~520 at ±1 V (Fig. 3a). Transverse I–V curves reveal a slight nonlinearity, indicating an inversion layer in the n‑GaAs (Fig. 3b). In contrast, MoS₂ on intrinsic GaAs shows linear I–V, confirming ohmic In/MoS₂ contacts.

Lateral photovoltaic effect—The LPE curves for MoS₂/n‑GaAs are linear with respect to laser spot position, fitted by the diffusion model LPV = K₀[exp(−|L−x|/d)−exp(−|L+x|/d)]. The maximum LPV is 208.2 mV, while MoS₂/p‑GaAs shows only 7.3 mV, confirming carrier‑type dependence. Sensitivity peaks at 416.4 mV mm⁻¹ for a 30 nm MoS₂ film, decreasing for thicker films due to increased recombination, and rising for thinner films due to reduced built‑in field (Fig. 5).

Energy‑band analysis—UPS yields a MoS₂ work function of 5.24 eV and a valence‑band offset of 0.51 eV, indicating p‑type behavior. Combined with GaAs parameters (E_g = 1.42 eV, E_F = 4.17 eV for n‑GaAs), the interface forms a p–n junction with E_bi = 1.07 V pointing from GaAs to MoS₂, creating a high‑conductivity inversion layer (Fig. 7a–b). In the p‑GaAs case, the weak E_bi (0.08 V) yields a lower carrier density in the inversion layer and a correspondingly weaker LPE (Fig. 7d–e).

Optimizing the MoS₂ thickness balances carrier diffusion against recombination, yielding the maximum LPE at ~30 nm.

Conclusions

DC magnetron‑sputtered MoS₂ films on n‑GaAs generate an exceptional LPE with 416.4 mV mm⁻¹ sensitivity and linear voltage response to laser position. The strong built‑in field and inversion layer underpin this performance, while the p‑GaAs counterpart shows markedly lower sensitivity (7.3 mV mm⁻¹). A 30 nm MoS₂ thickness optimizes the trade‑off between carrier diffusion and recombination. These insights affirm MoS₂/GaAs heterojunctions as promising platforms for high‑performance position‑sensitive detectors.

Abbreviations

ΔE

Distance between E_V and E_F

d_MoS2

Thickness of the MoS₂ film

E_bi

Built‑in field

E_C

Conduction band level

E_F

Fermi energy level

E_g

Band‑gap energy

E_V

Valence band level

I–V

Current–voltage

LPE

Lateral photovoltaic effect

LPV

Lateral photovoltage

LPV_max

Maximum lateral photovoltage

MoS2

Molybdenum disulfide

PSD

Position‑sensitive detector

UPS

Ultraviolet photoelectron spectroscopy

W

Work function