CHY8428: Which semiconductor properties can be obtained from measurements of the absorption coefficient: Energy and Materials Assignment, NU, UK

1. Which semiconductor properties can be obtained from measurements of the absorption coefficient?
2. Estimate the bandgap of Si and GaAs using Figure 1. Compare the obtained values with values estimated in figure 2. What is the smallest value for the energy of a direct optical transition for Si (Figure 1).
3. Neglecting any reflection losses, calculate the depth at which 95% of a 500 nm light beam has been absorbed in CuInSe2 and in Si (use Figure 2) given that absorptivity (α) is related to distance (d) by:

   α ≈(1/d)ln(I0/I)
   

Figure 1. Energy band structure of Si and GaAs.

Figure 2. Absorption spectra of various semiconductors.

• Identify the processes numbered in Figure 1 below.
  
  Figure 1. Processes that determine the efficiency of a solar cell.
• Below are the characteristics for three record efficiency devices (X, Y, Z)
  

Cell X

Cell Y