Optoelectronic Semiconductor Devices Principals and Characteristics

Table 1.: Recombination coefficients for several semiconductors. 
[6]
Material Band-gap 
type B
rec
 in cm
3
/s 
Si Indirect 1.79·10
-15
Ge Indirect 5.25·10
-15
GaP Indirect 5.37·10
-14
InSb Direct 4.58·10
-11
InAs Direct 8.5·10
-11
GaSb Direct 2.39·10
-10
GaAs Direct 7.21·10
-10
Each elemental and binary semiconductor has a specific band-gap at a given temperature. If the choice of 
semiconductors were restricted to elemental and binary semiconductors (Binary semiconductors - 
compounds made from the elements of two different groups of the periodic table (For example: III and V, 
II and IV, etc.)) only, then the available wavelengths would be rather limited. 
But as we know, light is emitted by solid solutions of semiconductors as well. By mixing two or more 
binary semiconductors, we can create ternary or quaternary crystalline solid solutions. The energy band-
gap, refractive index, and lattice constant of such solid solutions can be adjusted by varying the 
composition of the contributing materials and the growth conditions of the solutions. 
2.2 SEMICONDUCTOR MATERIALS FOR DIODE LASERS 
The successful fabrication of a diode laser relies very heavily upon the properties of the materials 
involved. There is very limited set of semiconductors that possess all of the necessary properties to make 
a good laser. For the desired DH (double heterostructure) at least two compatible materials must be 
found, one for the cladding layers and another for the active region. In more complex geometries, three or 
four different band-gaps may be required within the same structure. 
The fundamental requirement for these different materials is that they have the same crystal structure and 
nearly the same lattice constant (Lattice Constant - a length that denotes the size of the unit cell in a 
crystall lattice. With respect to the cubic crystal, this is the length of the side of the unit cell. However, a 
simple definition of the term is difficult, and the lattice constant must be considered with the geometry of 
the structure in each case.), so that single-crystal, defect-free films of one can be epitaxially grown on the 
other. 
We need to understand how to select materials that meet these fundamental boundary conditions.