TABLE OF CONTENTS An ideal crystalline solid has a periodic structure that is based on the chemical properties of its constituent atoms (see Chapter 1). However, real crystals are not perfect. They always have imperfections such as extra/missing atoms or impurities, which are called defects.
The periodicity characterizes the crystals as we learned in previous Chapters. For example, the periodic potential of the lattice modulates the wave function, and we can establish relationships between the energy and wave vector using the Bloch theorem as shown by the Kronig-Penney model (Chapter 4). The existence of defects perturbs the potential of the lattice and this modifies the band diagram in the crystals.
While many properties of crystalline systems depend upon the periodic lattice arrangement, many additional properties can be manipulated by adding defects or dopants to the crystal. These properties enable us to fabricate various devices in the modern world of semiconductor technology. On the other hand, unintentionally introduced defects can also have a profound impact on the properties of materials or on the performance of these devices. Therefore it is a challenging goal to have precise control of defects in crystals.
The defects can determine the color of the crystal, its electric conductivity, and they can also introduce modifications in the lattice vibrations. For example, Silicon becomes p-type with Boron doping. Al 2O 3 has red color as a ruby when a small amount of Cr 3+ substitutes Al 3+ but Al 2O 3 has...
