In the fields of microelectronics and optoelectronics, the optical properties of semiconductor thin film materials play a decisive role in their practical application in various devices. The transparency of a material is closely linked to its band structure, electronic behavior, and microscopic morphology. This paper selects silicon-based materials, metals, and metal compounds as research subjects to systematically study the patterns of transparency variation and the underlying physical mechanisms.
I. Transparent Semiconductor Thin Films Materials and Their Operating Principles
✅ Oxides and Nitrides: Unique Advantages of Wide Bandgap Materials
Silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) are representative transparent materials. SiO₂ has a bandgap as high as 9 eV, significantly higher than the photon energy range of visible light (1.6–3.1 eV), meaning visible light is almost not absorbed, thereby exhibiting high transparency. Si₃N₄ has a bandgap of about 5 eV, slightly lower than that of SiO₂, but still above the energy level of visible light. In the 400–700 nm wavelength range, its transmittance exceeds 90%. Tantalum oxide (TaO) has a bandgap of about 4.4 eV and shows good transmittance to visible light but absorbs near-ultraviolet light (wavelength < 280 nm). The transparency of such materials originates from their wide band structures, where photon energy is insufficient to induce electronic transitions, resulting in only weak lattice vibration absorption.
✅ Properties of Amorphous Silicon
Amorphous silicon (a-Si) has a bandgap of about 1.7 eV, wider than that of crystalline silicon (1.1 eV), but it can still absorb some visible light (such as blue and green light). Its light transmission properties are closely related to film thickness: when the film thickness is less than 50 nm, red light transmittance can reach 40%, thus it is often used in tandem solar cells to achieve selective light transmission.
II. Opaque Semiconductor Thin Films Materials and Their Mechanisms
✅ Silicon-Based Semiconductors: Relationship Between Bandgap and Light Absorption
Both crystalline and polycrystalline silicon have a bandgap of 1.1 eV, corresponding to an absorption edge wavelength of about 1127 nm. High-energy photons in visible light (such as blue light at 2.75 eV) can be strongly absorbed, making the material opaque in the visible range. Even when the thickness is reduced to 10 nm, transmittance remains below 30%, mainly limited by the high absorption coefficient of direct bandgap transitions.
✅ Metal Materials: Shielding Effect of Free Electrons
Metals such as copper, aluminum, and titanium contain a large number of free electrons, and their plasma frequencies are in the ultraviolet range (>10¹⁵ Hz). When visible light is incident, the photon energy is lower than the plasma frequency, causing collective oscillations of electrons, which in turn results in high reflection (>90%) and strong absorption. For example, a 100 nm thick aluminum film has a visible light reflectivity of up to 95%, with a transmittance of less than 0.1%.
Knowledge Expansion:
Presence of Free Electrons: Metals like copper, aluminum, and titanium have large numbers of freely moving electrons, which are not tightly bound to atoms as in insulators but can move freely throughout the metal crystal structure. This is a key reason for the good electrical conductivity of metals and is essential for understanding their optical properties.
Plasma Frequency: The free electrons exhibit plasma-like behavior and have a characteristic frequency known as the plasma frequency. For metals, this frequency lies in the ultraviolet range (above 10¹⁵ Hz), meaning effective “resonance” with external electromagnetic fields only occurs when their frequency reaches or exceeds this value.
Interaction Between Visible Light and Electrons: The frequency of visible light is below the plasma frequency of metals. When visible light hits a metal surface, the photon energy is insufficient to induce electron transitions to higher energy levels, but it does cause collective electron oscillations. This oscillation can be imagined as a large group of electrons moving together under the push of photons, like a school of fish disturbed in a pond.
High Reflection and Strong Absorption: During collective oscillations, electrons interact with surrounding metal ions, converting absorbed photon energy into heat—this is the absorption. Simultaneously, oscillating electrons re-emit electromagnetic waves, which appears as reflection. Due to this mechanism, metals have very high reflectivity to visible light (over 90%) and extremely low transmittance (e.g., visible light transmittance of less than 0.1% for 100 nm thick aluminum film), making most metals appear opaque with a shiny metallic luster.
✅ Metal Compounds: Correlation Between Conductivity and Light Absorption
Titanium nitride (TiN) has metallic-like conductivity and its optical properties resemble those of metals. In the visible light region, TiN has a large imaginary part of the complex refractive index (k > 2), leading to strong light absorption. Experimental data show that a 50 nm thick TiN film has a transmittance of only 0.5% at a wavelength of 550 nm and is currently mainly used in photothermal conversion devices.
III. Key Factors in Regulating the Transparency of Semiconductor Thin Films
✅ Band Engineering and Doping Techniques
Elemental doping can be used to adjust the bandgap of materials. For example, doping SiO₂ with fluorine can widen the bandgap to 9.5 eV, further reducing ultraviolet absorption; doping tantalum oxide (Ta₂O₅) with nitrogen to form TaON can lower the bandgap to 2.4 eV, allowing partial transmission of visible light.
✅ Microstructure Design
Porous structures can reduce the effective refractive index of materials and minimize interface reflection losses. For example, a porous SiO₂ film (500 nm thick) can achieve 99.6% transmittance, approaching the theoretical limit. Nanocrystalline/amorphous composite structures (such as nc-Si/a-SiO₂) can utilize quantum confinement effects to regulate the light absorption edge.
✅ Ultra-thin Fabrication Techniques
When the thickness of metal films is less than 10 nm, the mean free path of electrons becomes limited, leading to anomalous light transmission. For example, a 5 nm thick gold film can achieve 15% transmittance in the green light band, which can be used in the fabrication of transparent electrodes.
IV. Research Conclusions
The transparency of a material fundamentally depends on its electronic band structure and carrier concentration. Wide bandgap insulators (such as SiO₂, Si₃N₄) and some oxides (Ta₂O₅) are transparent due to the lack of low-energy light absorption pathways; narrow bandgap semiconductors (such as Si) and metals are opaque due to interband absorption or free electron response. In the future, precise control of band structures and optimization of micro/nano structures are expected to further expand the application scope of transparent materials and promote the development of flexible optoelectronic device technologies.
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