16 CHAPTER 1. STRUCTURAL PROPERTIES OF SEMICONDUCTORS
D lAlAs (perfect crystal)GaAs (perfect crystal)GaAsGaAs2.1 ML AlAs(a)(b)Figure 1.11: (a) A schematic picture of the interfaces between materials with similar lattice
constants such as GaAs/AlAs. No loss of crystalline lattice and long range order is suffered
in such interfaces. The interface is characterized by islands of heightΔand lateral extentλ.
(b) High resolution cross-sectional TEM image along with schematic diagram of (411A) GaAs
with a very thin (2.1 monolayer) AlAs layer in the middle. A small amount of roughness can
be observed at the interface. TEM image courtesy of S. Shimomura and S. Hiyamizu of Osaka
University.
energy,kdis a dimensionless parameter with values ranging from 1 to 10 in semiconductors, and
T, the crystal growth temperature. Defect formation energy is in the range of an eV for most
semiconductors.
Dislocations
In contrast to point defects, line defects (called dislocations) involve a large number of atomic
sites that can be connected by a line. Dislocations are produced if, for example, an extra half
plane of atoms are inserted (or taken out) of the crystal as shown in figure 1.14. Such dislocations
are called edge dislocations. Dislocations can also be created if there is a slip in the crystal so
that part of the crystal bonds are broken and reconnected with atoms after the slip. In the nitride
technology where alternate substrates are used, dislocation densities can be quite large.