Semiconducting III-V Compounds
Where does that leave us with optics - what kind of light can we get out of these compound semiconductors? The next figure will provide the answer for this question. How about GaP? It has an indirect band gap but is still used for making LED 's just believe it? Yes - there are tricks to get an indirect semiconductor to emit light.
For some materials they work, for others they don't. How it is done is beyond our ken at present; things like "excitons" one of the many quasiparticles of solid state quantum theory "quantum wires" or "quantum dots" will be invoked. This link moves you on to a suitable module of a graduate course if you are curious.
Does the last column imply that we can also have mixed cases? Yes - but only for thin films. Is it technically important if we have wurtzite or sphalerite; how about the lattice constants?
Yes and yes - this is even extremely important. Nothing, of course - provided that the ternary phase diagram provides for such a compound. Now we have a large research program: Find out what can be done for all kinds of combinations of III's , V's , x 's and y 's. Lattice constant : As long as the lattice type doesn't change, the lattice constant most likely will just smoothly change from one extreme value - e.
GaAS - to the other - e. Band gap : If we have no choice but guessing, your best guess would be exactly as above: The band gap probably changes from one extreme value to the other one - somehow. Direct - indirect band gap : That's where guessing ends - except that for very small or very large x 's we probably get whatever the pure material will have. Index of refraction : Well - we will have a smooth change, most likely.
Lattice constant vs. Band gap and refractive index vs. So it is pretty much as expected. Pretty smooth changes of lattice constants and index of refraction with composition, but not strictly linear with composition. Notice that the lattice constant hardly changes going from GaAs to AlAs.
This will have tremendous technical consequences. The band gap case shows two curves - direct and indirect band gap for all compositions. In contrast, the nitrides are found as wurtzite structures e. As with the group IV elements the lattice parameters are highly temperature dependent; however, additional variation arises from any deviation from absolute stoichiometry.
Zinc atoms are shown in green small , sulfur atoms shown in red large , and the dashed lines show the unit cell. The homogeneity of structures of alloys for a wide range of solid solutions to be formed between III-V compounds in almost any combination. While quaternary alloys of the type III x -III 1-x -V y -V 1-y allow for the growth of materials with similar lattice parameters, but a broad range of band gaps. A very important ternary alloy, especially in optoelectronic applications, is Al x -Ga 1-x -As and its lattice parameter a is directly related to the composition x.
Not all of the III-V compounds have well characterized high-pressure phases. Thus, AlP undergoes a zinc blende to rock salt transformation at high pressure above kbar, while AlSb and GaAs form orthorhombic distorted rock salt structures above 77 and kbar, respectively. Indium arsenide InAs undergoes two-phase transformations. The structures of the II-VI compound semiconductors are less predictable than those of the III-V compounds above , and while zinc blende structure exists for almost all of the compounds there is a stronger tendency towards the hexagonal wurtzite form.
In several cases the zinc blende structure is observed under ambient conditions, but may be converted to the wurtzite form upon heating. In general the wurtzite form predominates with the smaller anions e. One exception is mercury sulfide HgS that is the archetype for the trigonal cinnabar phase. Mercury selenide HgSe and mercury telluride HgTe convert to the mercury sulfide archetype structure, cinnabar, at high pressure.
Sodium ions are shown in purple and chloride ions are shown in red. Copper atoms are shown in blue, iron atoms are shown in green and sulfur atoms are shown in yellow. The dashed lines show the unit cell.
Lattice parameters for tetragonal cell. One of the advantages of the copper indium chalcogenide compounds is the formation of solid solutions alloys of the formula CuInE2-xE'x, where the composition variable x varies from 0 to 2.
3 - Silicon, III-V Compound Semiconductor, Glass and Metal Category
As would be expected from a consideration of the relative ionic radii of the chalcogenides the lattice parameters of the CuInS2-xSex alloy should increase with increased selenium content. In fact, we plot in Fig. In Fig. The agreement between the calculated and experimental data is excellent. The important optical transition energies observed at energies higher than E 0 are labeled E 1 and E 2.
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Since the carrier effective mass is strongly connected with the carrier mobility, it is known to be one of the most important device parameters. For conventional semiconductors, the values of the effective mass are known to decrease with decreaseing bandgap energy Fig. This behavior is rather unusual, and in fact is opposite to what is seen in conventional semiconductors.
The deformation potentials of the electronic states at the Brillouin zone centers of semiconductors play an important role in many physical phenomena. The lattice mobilities of holes are also strongly affected by these potentials.
Due to the large scatter in the experimental binary endpoint values, it is very difficult to establish any evolution of the deformation potentials with composition. Until more precise data become available, we suggest employing the linear interpolation expressions in order to estimate the parameter values of these poorly explored properties.
These two methods are found to provide almost the same interpolated values. The optical constants in the interband transition regions of semiconductors depend fundamentally on the electronic energy band structure of the semiconductors. The refractive indices and absorption coefficients of semiconductors are the basis of many important applications of semiconductors, such as light-emitting diodes, laser diodes and photodetectors. We plot in Fig. The three major features of the spectra seen in Fig.
It is found that the E 0 and E 1 structures move to higher energies with increasing x , while the E 2 structure does not do so to any perceptible degree. We can see that the MDF calculation enables us to calculate the optical spectra for optional compositions of alloy semiconductors with good accuracy.
Structures of Element and Compound Semiconductors - Chemistry LibreTexts
There are various carrier scattering mechanisms in semiconductors, as schematically shown in Fig. We plot in Figs. The solid lines in Figs. Martienssen Ed. Watts, A. Willoughby: J. Vergara, R. Monroy, F. Calle, E. Krieger, H. Sigg, N.
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