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Glasses and amorphous materials

Which correctly compares the properties of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon?

  1. AIts disordered atomic arrangement means it absorbs almost no visible light, so it is used for transparent electrodes in place of ITO
  2. BIt absorbs visible light more strongly than crystalline silicon, so films under 1 µm suffice, but its electron mobility is 2 to 3 orders lower
  3. CIts electron mobility is higher than that of crystalline silicon, so it is widely used in place of it for transistors in high-speed microprocessors
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Answer: B. It absorbs visible light more strongly than crystalline silicon, so films under 1 µm suffice, but its electron mobility is 2 to 3 orders lower

Crystalline silicon is an indirect-gap semiconductor that absorbs the longer wavelengths of visible light weakly, so solar cells usually use substrates 100 µm thick or more. In a-Si:H there is no long-range periodicity, so the constraint of conserving momentum in a transition is relaxed and its visible absorption coefficient is larger than that of the crystal, letting a film 1 µm thick or less absorb the light. On the other hand, carriers are trapped in band-tail states created by the disordered network, so the electron mobility is about 1–10 cm²/(V·s), 2 to 3 orders of magnitude lower than the roughly 1400 cm²/(V·s) of crystalline silicon. It is therefore unsuited to high-speed logic circuits, but taking advantage of large-area deposition at about 300 °C, it has been used for pixel TFTs in liquid crystal displays and for thin-film solar cells. With a band gap of about 1.7 eV it absorbs visible light strongly and cannot be a transparent electrode.

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More Glasses and amorphous materials questions

  1. Which statement about the atomic arrangement of an oxide glass such as soda-lime glass is correct?
  2. Which set correctly lists network-forming oxides, which form glass on their own and build the network skeleton of oxide glasses?
  3. How do the structure and properties of SiO₂ glass change when Na₂O is added?
  4. In a sodium silicate glass of 30 mol% Na₂O-70 mol% SiO₂, assuming each Na₂O creates two non-bridging oxygens, which is closest to the number of non-bridging oxygens per SiO₄ tetrahedron (NBO/T)?
  5. The viscosity η (Pa·s) of a soda-lime glass is described by the VFT equation log₁₀η = A + B/(T − T₀) with A = −2.5, B = 4300 K and T₀ = 520 K. Which is closest to the temperature at which the viscosity reaches 10¹² Pa·s (a guide to the annealing point, close to the glass transition temperature)?