18 CHAPTER 1. STRUCTURAL PROPERTIES OF SEMICONDUCTORS
VacancySelf
interstitialImpurity
interstitialSubstitutionalPOINT DEFECTS- Effect is localized
to a few atomic sites
TYPICAL POINT DEFECTS IN CRYSTALS
Figure 1.13: A schematic showing some important point defects in a crystal.ii) New effective substrate: High quality substrates are only available for Si, GaAs and InP
(sapphire, SiC and quartz substrates are also available and used for some applications). Since
most semiconductors are not lattice-matched to these substrates a solution that has emerged is
to grow a thick overlayer on a mismatched substrate. If the conditions are right, dislocations are
generated and eventually the overlayer forms its own substrate. This process allows a tremen-
dous flexibility in semiconductor technology. Not only can it, in principle, resolve the substrate
availability problem, it also allows the possibility of growing GaAs on Si, CdTe on GaAs, GaN
on SiC etc. Thus different semiconductor technologies can be integrated on the same wafer.
In figure 1.15 we show a TEM image of an InP/InAs double-barrier resonant tunneling device
(DBRT). The InP barriers are 5 nm wide, enclosing a 15 nm InAs quantum dot. The InP is
coherently strained, with no dislocations created at the interfaces. The sharpness of the interfaces
was determined to be 1-3 lattice spacings.
Coherent and Incoherent Structures
Consider situation shown schematically in figure 1.16 where an overlayer with lattice constant
aLis grown on a substrate with lattice constantaS. The strain between the two materials is
defined as
=
aS−aL
aL(1.3.1)
If the lattice constant of the overlayer is maintained to beaL, it is easy to see that after every 1/
bonds between the overlayer and the substrate, either a bond is missing or an extra bond appears
as shown in figure 1.16b. In fact, there would be a row of missing or extra bonds since we have
a 2-dimensional plane. These defects are the dislocations discussed earlier.
An alternative to the incoherent case is shown in figure 1.16c. Here all the atoms at the in-
terface of the substrate and the overlayer are properly bonded by adjusting the in-plane lattice