Tube Furnace Preparation of MoS₂ Coating: Technology for S101/SiO₂ Substrate

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Tube Furnace Preparation of MoS₂ Coating: Technology for S101/SiO₂ Substrate

As a typical two-dimensional transition metal dichalcogenide, molybdenum disulfide (MoS₂) has excellent electrical on-off ratio, chemical and thermal stability, offering broad prospects in optoelectronic devices, lithium-ion batteries and field-effect transistors. Compared with mechanical exfoliation, the tube furnace chemical vapor deposition (CVD) method ensures large-area uniform MoS₂ coatings with controllable layers and strong silicon-based process compatibility, becoming the mainstream for high-quality MoS₂ coatings on S101/SiO₂ substrates. This paper analyzes the core principle, operation process, key parameter optimization and performance characterization of this tube furnace CVD process, providing references for R&D and industrial application.

I. Core Principle of the Process

The tube furnace preparation of MoS₂ coating relies on high-temperature CVD gas-phase reaction between molybdenum and sulfur sources. Generated MoS₂ active species deposit and crystallize on S101/SiO₂ substrates to form continuous uniform coatings. The substrate’s surface SiO₂ layer, with good insulation, flatness and MoS₂ compatibility, regulates nucleation density and crystallization quality, facilitating high-performance two-dimensional coatings—an advantage critical for optimizing the tube furnace CVD process.

Typically, molybdenum trioxide (MoO₃) and sulfur powder (S) serve as sources, with high-purity Ar or N₂ as carrier gas. Trace H₂ or O₂ may be added for slight etching to repair defects and terminate single-layer growth, enabling controllable double/multi-layer coatings. The core reaction is:

2MoO₃ + 7S → 2MoS₂ + 3SO₂↑

II. Detailed Process Operation Flow

2.1 Substrate Pretreatment

Surface cleanliness of S101/SiO₂ substrates directly affects coating adhesion and crystallization. Multi-step cleaning removes oil, impurities and hydroxyl groups:

1. Ultrasonically clean the substrate in acetone, absolute ethanol and deionized water (10-15 minutes each) to remove organic pollutants and particles;

2. After cleaning, blow dry the surface moisture of the substrate with high-purity nitrogen (purity ≥99.999%) to avoid residual water stains causing coating defects;

3. Optional step: Place the dried substrate in a tube furnace and perform thermal annealing at 300-400℃ for 30 minutes to further remove residual water vapor and adsorbed impurities on the surface.

2.2 Reaction System Construction

A dual/three-zone tube furnace builds the reaction system, requiring precise control of component placement and ratio:

1. Sulfur source placement: Put sulfur powder into a quartz boat and place it in the low-temperature zone upstream of the gas flow in the tube furnace, which is heated and evaporated by thermal radiation from the molybdenum source zone;

2. Molybdenum source placement: Put MoO₃ powder (purity ≥99.9%) into another quartz boat, and place it together with the pretreated S101/SiO₂ substrate in the high-temperature zone downstream of the gas flow. The substrate can be buckled upside down above the molybdenum source or placed horizontally near the downstream to ensure uniform coverage of gas-phase species;

3. Source distance: Control MoO₃-sulfur horizontal spacing at 25-40cm. Excessively close spacing causes premature sulfur evaporation; overly far leads to insufficient sulfur and incomplete vulcanization;

4. Vacuum and carrier gas replacement: After closing the tube furnace chamber, evacuate to 10⁻³Pa level, and flush the chamber with high-purity carrier gas 3-5 times to completely eliminate air impurities and avoid oxidation reactions affecting coating quality.

2.3 Temperature Control and Reaction Process

Segmented heating enables controllable coating growth, with typical parameters as follows:

1. Preheating: Heat at 5-30℃/min to 150-350℃ (molybdenum source/substrate zones), hold 60-180 minutes to purify the substrate and preheat sources;

2. Pre-evaporation and pre-nucleation stage: Adjust the temperature of the molybdenum source zone to 600-800℃, the temperature of the substrate zone to 600-750℃, control the temperature difference between the two zones at -150~150℃, and keep it warm for 10-40 minutes to make MoO₃ evaporate initially to form gaseous MoO₃₋ₓ species and pre-nucleate on the substrate surface;

3. Main growth: Heat sulfur zone to 130-220℃, molybdenum source zone to 800-900℃, substrate zone to 700-850℃, hold 10-60 minutes. Adjust carrier gas flow to 10-80sccm to carry vapors for MoS₂ deposition. For double layers, add 1-10sccm H₂/O₂ for etching to terminate the first layer before re-deposition;

4. Cooling stage: After the reaction, turn off the heat source, keep the carrier gas continuously flowing, and let the tube furnace cool down to room temperature naturally to avoid coating cracking or peeling caused by rapid cooling.

III. Key Parameter Optimization Points

3.1 Temperature Parameters

Temperature is core for regulating MoS₂ coating layers and crystallization. The first substrate preset temperature (650-750℃) determines nucleation density, yielding uniform single layers with 5-25 minutes of holding. The second preset temperature (750-900℃, higher than the first) with 10-35 minutes of holding (longer than the first) enables second-layer growth. Molybdenum source-substrate temperature difference is critical: negative difference (molybdenum source < substrate) reduces nucleation density (suitable for single layers); positive difference increases thickness (facilitating multi-layer/rod-like structures).

3.2 Carrier Gas and Source Material Ratio

Optimal carrier gas flow is 50-100sccm: too low causes insufficient sulfur and incomplete vulcanization; too high boosts nucleation density and reduces coating size. Control sulfur-MoO₃ mass ratio at 20:1~250:1—excess sulfur avoids MoO₁₋ₓSₓ impurities and improves purity.

3.3 Substrate Selection and Treatment

The thickness of the SiO₂ layer of the S101/SiO₂ substrate is recommended to be controlled at about 280nm. Too thick will easily lead to decreased coating adhesion, and too thin will affect insulation performance. The substrate after ultrasonic cleaning must be thoroughly dried to avoid residual moisture causing coating defects at high temperature.

IV. Coating Performance Characterization Methods

1. Raman spectroscopy: Determine layers and crystallization via characteristic peak spacing. Single-layer MoS₂ has ~20cm⁻¹ spacing between E₁₂g (~384cm⁻¹) and A₁g (~404cm⁻¹) peaks; double/multi-layers exceed 24cm⁻¹ (e.g., 24.5cm⁻¹ for 5 layers);

2. Atomic Force Microscopy (AFM): Observe the surface morphology and thickness of the coating. High-quality coatings have triangular or hexagonal island-like structures on the surface, no obvious particle agglomeration, and uniform and controllable thickness;

3. Photoluminescence (PL) spectroscopy: Analyze the optical properties of the coating. The free exciton and bound exciton characteristic peaks of single-layer MoS₂ are around 1.98eV and 1.84eV respectively, and the defect density can be judged by the peak intensity;

4. X-ray Photoelectron Spectroscopy (XPS): Detect the elemental composition and bonding state of the coating. No additional shoulder peaks indicate no impurity phase, and the atomic ratio of Mo to S close to 1:2 is optimal.

V. Process Advantages and Application Scenarios

Tube furnace CVD on S101/SiO₂ substrates achieves large-area, layer-controllable high-quality MoS₂ coatings with excellent electrical/optical properties, overcoming mechanical exfoliation’s low yield and small size. This optimized combination suits back-gate transistors, photodetectors and photocatalytic electrodes. Particularly for next-generation semiconductors, it mitigates silicon-based short-channel effects, supporting Moore’s Law extension and laying the groundwork for industrialization.

VI. Conclusion

Precise control of temperature difference, carrier gas flow and source ratio is key for tube furnace MoS₂ coating on S101/SiO₂ substrates. Segmented heating and slight etching enable controllable single/multi-layer growth. Future efforts will optimize parameters, reduce defects, enhance device compatibility and promote industrial application in optoelectronics and spintronics.

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