The traditional elemental semiconductors Si and Ge from group IV of the periodic table cannot address all emerging optoelectronic applications because the bandgap is not correct for the wavelength of interest. In addition to having the appropriate composition, the materials must be used in high quality single crystal form. Group IV semiconducting properties can be obtained by combining elements from group III with group V or group II with group VI. Accordingly, binary compounds such as GaAs (III-V) and CdTe (II-VI) have been used to provide the desired bandgap or lattice constant for specific devices.
Still, these binary compounds cannot provide the optoelectronic parameters to satisfy all device applications. Binary compounds can be combined to produce pseudo-binary (ternary and quaternary) compounds such as InSb and GaSb. The lattice constant and bandgap of the resulting material will have parameters that lie between the values of the two binary compounds.
The desired properties can be tailored by selecting the appropriate ratio of the compounds (ie,
where x = the composition mole % GaSb). In some cases, epitaxial thin film ternary and quaternary compounds of desired composition can be deposited on single crystal elemental or binary compound substrates. This capability is limited due to lattice mismatch problems.
Accordingly, there is a vital need for ternary and quaternary crystals in bulk substrate form. Unfortunately, these materials have a non-congruent melting point and cannot be successfully grown by traditional melt growth techniques because the composition of the crystal will vary continuously. An examination of the typical pseudo-binary phase diagram shows the separation between the liquidus and solidus line for all composition except for the binary components.
The THM can solve the composition problem and produce almost all binary, ternary and quaternary compounds in high quality bulk single crystal form. It is only necessary to prepare quenched polycrystalline feed having the average target composition.
The diagram shows the energy gap and corresponding cut-on wavelength as well as the lattice dimensions and melting points for the most popular II-V and II-VI compound semiconductors.