Learning goals

src/lecture_6.md

@@ -151,8 +151,9 @@ Extended BZ (n-th band within n-th BZ):
 
     After this lecture you will be able to:
 
-    - describe how an insulating or conducting nature of a material is related to the material's band structure.
-    - examine 1D and 2D band structures and argue if you expect the corresponding material to be an insulator/semiconductor or a conductor.
+    - describe how an insulating or conducting nature of a material is related to the material's electronic band structure.
+    - examine 1D and 2D electronic band structures and argue if you expect the corresponding material to be an insulator/semiconductor or a conductor.
+    - describe how the light absorption spectrum of a material may be understood through its band structure.
     
 ## Band structure
 
@@ -321,7 +322,7 @@ Here $E'_G\approx 0.9eV$ and $E_G\approx 0.8 eV$. The two steps visible steps ar
 
 ![](figures/direct_indirect.svg)
 
-The band structure has two band gaps: *direct*, the band gap at $k=0$, $E'_G$ and *indirect* gap $E_G$ at any $k$. In Ge $E_G > E'_G$, and therefore it is an *indirect band gap semiconductor*. Silicon also has an indirect band gap. Direct band gap materials are for example GaAs and InAs.
+The band structure has two band gaps: *direct*, the band gap at $k=0$, $E'_G$ and *indirect* gap $E_G$ at any $k$. In Ge $E_G < E'_G$, and therefore it is an *indirect band gap semiconductor*. Silicon also has an indirect band gap. Direct band gap materials are for example GaAs and InAs.
 
 Photons carry very little momentum and a very high energy since $E = c \hbar k$ and $c$ is large. Therefore to excite electrons at $E_G$, despite a lower photon energy is sufficient, there is not enough momentum. Then an extra phonon is required. Phonons may have a very large momentum at room temperature, and a very low energy since atomic mass is much higher than electron mass.