SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

2.9. DOPING IN POLAR MATERIALS 67

C

a

Figure 2.24: The wurtzite crystal structure unit cell. In the ideal wurtzite structure theclattice

constant is related to thealattice constant by the relationc=2

√

2
3 a. However, when the cation-
anion bond lengths cause a deviation from this relationship a net spontaneous polarization is
created.

Polar Charge at Heterointerfaces

If there is a net movement of one sublattice against each other, a polarization field is set up.
This results in a positive and negative polar charge. Under most conditions the polar charge on
the free surfaces is neutralized by charges present in the atmosphere. This causes depolarization
of the material. If, however, a heterostructure is synthesized and the two materials forming the
structure have different values for the polarization, there is a net polar charge (and polarization)
at the interface as shown in figure 2.30. In semiconductors this polar charge can cause a built-in
electric field

E=

P



(2.9.1)

The interface chargePA−PBand the built-in interface field (see figure 2.26) can be exploited in
device design since for most applications this fixed polar charge can act as dopant (see figure 2.27
and figure 2.28).