Alloys

Alloys are mixtures of at least two elements in a solid solution. In other words, they maintain an overall underlying crystal structure, but the atoms on that structure are mixed at the atomic scale level and the occupancy of each lattice site by different elements is essentially random in contrast to binary compounds, where it is fully ordered. Of course, different degrees of ordering may occur. In semiconductor alloys, one can either mix the cations or anions on their respective sub-lattices or both. For example, Ga_1-xIn_xN or GaAs_1-xN_x. They provide an important means to gradually tune the properties between those of the two end compounds. According to the number of elements being alloyed, the alloys are categorized as binary, ternary, quaternary and pentanary alloy. For example, Ga_1-xIn_xN should be considered a binary alloy even though it has three components and is therefore sometimes called a ternary semiconductor. In contrast to the here studied class of heterovalent ternary compounds, it would be a homovalent ternary. For specific compositions ordering of the cations could occur and form well defined crystallographic compounds.

In current technology the use of alloys is seen in heterojunction bipolar transistors, laser diodes, and numerous other devices. Laser diodes used in fiber-optic communications, for example, are required to operate at a high modulation rate and at very specific wavelengths where the absorption coefficient in the fiber is unusually low. Thus, it is critical to these devices to know how to control the energy gap of the semiconductor effectively. Alloying is also necessary for improvement of laser efficiencies as well.

Group III nitrides GaN,InN and AlN which are being used particularly for the light-emitting and laser diodes operating in green and blue spectral regions, has stimulated the study of their alloys Al_xGa_1−xN, In_xGa_1−xN and In_xAl_1−xN. The first motivation comes from their large and direct band gaps, 3.5 eV for the GaN, 1.89 eV, and 0.9 eV for the InN, and 6.28 eV for the AlN in the wurtzite structure, which would allow to cover an exceptionally large spectrum [1].

In the family of II-IV-N₂ materials, we can in principle alloy the various group IV elements (Si,Ge,Sn), the various group-II elements(Be,Mg,Cd,Zn,Hg) and for each one a continuous change in composition is possible. This rapidly explodes the number of potential alloys because all pair combinatins should be considered and provides potentially unprecedented control or fine tuning of the properties. At present only very limited information is available on the II-IV-N₂ alloys. They are organized by pair of elements considered in the alloy.

The bandgap energy varies with the different composition of an alloy and the variations of the lattice constant directly relate with the band gap energy. The lattice parameter usually obey Vegard’s Law, which states that the lattice constant of an alloy crystal, a₀(A_1-xB_x), is an average of the lattice constants of the pure compounds, a₀(A) and a₀(B), weighted by the composition: a₀(A_1-xB_x) = (1-x)a₀(A) + xa₀(B). In other words, the lattice constant changes linearly with composition of the alloy from that of one compound or elemental constituent to that of the other. Most mechanical properties also shift linearly with alloy composition. Because these properties are determined by substantial volumes of the semiconductor crystal, local composition fluctuations are averaged out and do not affect the property overall. Any property of a material that is determined by an ensemble average over a significant volume of the material will generally scale linearly in the alloy with composition. [2]

Most ternary alloys have a parabolic compositional dependence with the band gaps. The magnitude of the parabolic factor is known as the bowing 'b'. The experimental values of the bowing factor exhibit significant scatter for each alloys.

Because of the need for single phase and single crystal structure, method of fabrication of the alloy plays major role in alloy industry. There are two common options for making high quality single crystal. To grow them as bulk or to form them as thin “epitaxial” single crystals on a large single-crystal substrate.

Currently, several important theoretical calculation has been made and optimization of each characteristic are being investigated in order to enhance the efficiency of alloy materials.

References.

[1]. Z Dridi, B Bouhafs and P Ruterana,Semi. Sci. Tech Vol 18, 9, 8 Aug 2003.

[2]. Rockett. A,The material science of semiconductors,Nov 30, 2007, ISBN0387256539

[3]. Narang, P at al. (2014), Bandgap Tunability in Zn(Sn,Ge)N₂ Semiconductor Alloys. Adv. Mater., 26: 1235–1241.

[4]. S. J. Pearton at al,J. Appl. Phys., 2047(2002).