ABSTRACT
To make a radiation detector, we first need to grow a crystal of the particular semiconductor material. To do this we need to understand a little of crystal structure.
The term “crystal” is derived from the Greek word κρύσταλλος (krystallos), meaning “clear ice,” and was originally used to refer to materials which looked similar to ice (such as “rock
crystal” the colorless form of quartz)—because ice was the most obvious manifestation of crystal structure in ancient times. The structure of a crystal is usually defined in terms of lattice points, which mark the positions of the atoms forming the basic unit cell of the crystal. Cullity [1] defines a lattice point as “an array of points in space so arranged that each point has (statistically) identical surroundings.” The word “statistically” is introduced to allow for solid solutions, where fractional atoms would otherwise be required. The defining property of a crystal is thus its inherent symmetry, by which we mean that under certain operations the crystal remains unchanged. For example, rotating the crystal 180 degrees about a certain axis may result in an atomic configuration that looks identical to the original configuration. The crystal is then said to have a twofold rotational symmetry about this axis. In addition to rotational symmetries like this, a crystal may have symmetries in the form of mirror planes and translational symmetries.
