Direct Energy, 2018a

106 6.3 Semiconductors and Energy Level Diagrams
around 1000 ◦ C. Some boron will diuse in and replace silicon atoms.
Remove the excess boron. The same procedure can be used to dope with
other donors or acceptors. What is the most dangerous part of the process?
Etching the oxide o the silicon because hydrouoric acid HF, a dangerous
acid, is used [69].
Sometimes it is possible to grow one layer of a semiconductor material
on top of a layer of a dierent type of material. A stack of dierent
semiconductors on top of eachother is called a heterostructure. Not all
materials can be made into heterostructures. GaAs and AlAs have almost
the same atomic spacings, so heterostructures of these materials can be
formed. The spacing between atoms, also called lattice constant, in AlAs is
0.546 nm, and the spacing between atoms in GaAs is 0.545 nm [9]. If the
atomic spacing in the two materials is too dierent, mechanical strain in
the resulting material will pull it apart. Even moderate mechanical strain
can negatively impact optical properties of a device because defects may
be introduced at the interface between the materials. These defects can
introduce additional energy levels which can trap charge carriers.
6.3.2 Energy Levels in Isolated Atoms and in Semiconductors
In a solar cell, light shining on a semiconductor causes electrons to ow
which allows the device to convert light to electricity. How much energy
does it take to cause an electron in a semiconductor to ow? To answer
this question, we will look at energy levels of:
• An isolated Al atom at T =0K
• An isolated P atom at T =0K
• Isolated Al atom and P atoms at T>0 K
• An AlP crystal at T =0K
• An AlP crystal at T>0 K
Aluminum has an electron conguration of 1s 2 2s 2 2p 6 3s 2 3p 1 . It has 13 total
electrons, and it has 3 valence electrons. More specically, it has two valence
electrons in the 3s subshell and one in the 3p subshell. Phosphorous
has an electron conguration of 1s 2 2s 2 2p 6 3s 2 3p 3 , so it has 5 valence electrons.
Ideas in this section apply to materials regardless of whether they
are crystalline, amorphous, or polycrystalline.