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Submitted on 1 Jan 1975
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CONCLUDING REMARKS
M. Cardona, J. Jerphagnon
To cite this version:
M. Cardona, J. Jerphagnon. CONCLUDING REMARKS. Journal de Physique Colloques, 1975, 36 (C3), pp.C3-189-C3-191. �10.1051/jphyscol:1975335�. �jpa-00216305�
JOURNAL DE PHYSIQUE Colloque C3, supplGment au no 9, Tome 36, Seprembre 1975, page C3-189
CONCLUDING REMARKS
M. CARDONA
Max Planck Institut fur Festkorperforschung, Stuttgart, FRG and
J. JERPHAGNON
Centre National d'Etudes des TCl~communications, Bagneux, France
These remarks grew out of a panel discussion held a t the end of the Conference and moderated by the authors. It was felt that comments ought to be made by the participants on the recent progress as well as on the prospects in the field of ternaries. The moderators attempted to come to some consensus as to the value of research in ternary semiconductors from the point of view of technical device applications and basic solid state science. We shall try here to jot down a few notes concerning this very spirited discussion including occasionnally some thoughts which occurred to us afterwards.
Technical device applications. - There is no doubt that one of the main reasons for the large increase in the activity on ternary compounds during the past few years is the possible use of the specific semiconducting and nonlinear optical properties of these materials for practical purposes. It is time to question how significant have been the achievements in that respect. D o we still think that ternaries offer high promises ?
The consensus seemed to be general that, at present, the three most likely fields of applications were light emitting diodes (LED) and lasers, solar cells and light detectors, and nonlinear optical elements.
The large variety of materials allowing more flexibility, for instance in selecting a compound with an appropriate band gap, was in favor of the applications as LED, especially if the materials were available off the shelf. This is unfortunately not the case. It was also hoped that the difficulties encountered for changing the type (p or n) of large band gap binaries could be, in some way, avoided. One has to face the fact that the problem appears to be even more complex in ternaries.
Yet most of the ternaries are p type while the binaries are n type, thus allowing heterojunctions. It was felt that ternaries, complicated from the structural point of view, should have a very hard time competing with the rather advanced technology of 111-V alloys.
According to the opinion of some participants they might however be useful as LED for the blue-violet
region. No consensus could be reached on this matter although agreement was general that the LED appli- cations of ternaries were, at best, rather limited.
Similar comments apply to lasers. It was nevertheless gratifying to see that laser action had been obtained in a large number of chalcopyrite materials.
The prospects seem to be brighter for applications as light detectors and, in particular, as solar cells.
Concerning solar cells it was pointed out that competi- tion with silicon, with which large solar eficiencies had been reached, would be rather tough. The Bell group counteracted the argument along two lines.
First the broad band spectral response is in favor of CuInSe,-CdS heterojunctions. More important, silicon, in spite of the low cost of the element, is too expensive in high purity single crystal form to allow profitable coverage of large areas. With ternaries one should stay away from gallium compounds because of the cost of the element but indium is quite inexpensive.
There are indications that heterojunctions could be made by thin film techniques and would consequently be much cheaper than silicon cells. Since large sums are being spent in research and development to lower the cost of silicon solar cells, it is worth having a much less costly try at I-111-VIZ-CdS heterojunctions.
Nonlinear optical elements seemed to be the natural application for the ternary chalcopyrites. In addition to their large transparency range, which can be selected according to composition, they have large nonlinear susceptibilities and natural birefringence.
It has to be noted that ternaries are not as exciting for guided waves as for bulk interactions since in the former case birefringence is not needed to fulfil1 phase matching conditions. Investigations and measure- ments have been performed on a number of materials but the bottleneck is the crystal preparation techno- logy. At the Conference success has been reported for AgGaSe, and AgGaS, which are available as large crystals of spectaculary high optical quality and trans- parency. Only two usable compounds are available so far, a rather poor achievement indeed. However growing crystals of suitable optical quality for technical
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975335
C3-190 M. CARDONA AND J. JERPHAGNON devices require a lot of time and effort, as demons-
trated by the case of nonlinear optical materials which are currently part of parametric sources. The most immediate concern seems to be the use of AgGaSe, and AgGaS, as parametric oscillators in the near and infrared region although they are in competition with CdSe. Another prospect is using ternaries as frequency mixers for submillimeter sources.
Basic solid state science. - The authors tried to moderate the discussion towards reaching a consensus as to whether the work being done on ternaries, with their large variety estimated by Nitsche to be of the order of 30 000, was simply stamps collecting, or our knowledge of basic semiconductors physics was being enriched by it.
Here the consensus was quite positive. It was felt that something basically new had been brought up in many areas, such as structural stability, preparation and crystal growth, defects chemistry, electronic pro- perties, lattice dynamics and electron-phonon inter- actions. There are still important questions calling for further investigations. Noticeable is the fact that very little attention has been paid so far to certain aspects of ternaries not to mention that the study of ternaries has not yet benefited of all the recent experimental techniques.
To be more explicit, a particularly intriguing ques- tion concerning matters of structural stability is the difficulty of synthetizing anion-substituted tetrahedral crystals. U p to now only two of them (AIzCO and ZnzNF) have been prepared. In the discussion this was attributed, possibly equivalently, to the large size of the anions and also to the fact that nearly all the valence electrons, responsible for the bonding, were concentrated around the anions. Under these condi- tions anionic substitutions may not be possible because of lower energy of the segregated phases.
The question still remains of what can be done under hydrostatic pressure or, epitaxially, in thin film form.
Since there are materials (Ge-Sn alloys) which can be prepared in amorphous form while they cannot be obtained as crystals, what about the anionically substituted tetrahedral ternaries ? On the whole the absence at the Conference of work on amorphous ternaries is quite surprising, especially in view of the large recent interest in the corresponding binary materials.
Ternaries with average four valence electrons per atom crystallize in various structures such as dis- torted zinc blende (chalcopyrite), distorted wurtzite, rocksalt and more complicated structures as well.
They thus constitute a fertile ground for testing our ideas about the relationships between ionicity, bonding, and crystal structure. Most likely, distorted zinc- blende and wurtzite modifications of a given material differ little in free energy. Can an unstable phase be stabilized by epitaxial growth as a very thin film ? Quite clearly ternaries present a number of interesting
problems in the realm of crystal structure and lattice stability which provide for good physics and not merely stamps collecting. Among them are those referring to the parameters (the cla ratio and the anion displacement) characterizing the distortion of chalcopyrite from pure tetrahedral bonding. The theoretical understanding of these quantities, their pressure and temperature dependence, is still not very satisfactory and cannot be improved by a mere extension of the work on binaries. Of interest are also various questions like the distribution of cationic charges but we would like to emphasize that there should be more systematic study of simple tetrahedral defect structure, particularly with respect to the influence of lone pairs. There has lately been some atten- tion being paid to the properties and characteristics of materials such as selenium, tellurium, GaSe, GeSe, GazS,, GeSe,, etc ... One may consider them as tetra- hedral defect structures (0-VIZ, 111,-0-VI,, IVz-0-VIZ, 111,-0-VI,, IV-0-VIZ respectively) and they could therefore be a link between tetrahedral and various other structures, allowing for instance the generaliza- tion of the bond ionicity picture which has been rather successful1 in explaining several features of tetrahedral compounds.
Another open question of considerable interest which may help us to understand the behaviour of binary materials is the fact that large band gap chal- copyrites are mostly only p-type. In contrast most large band gap 11-V1 compounds are only n-type.
As already pointed out this may turn to be a blessing for the preparation of p-n heterojunctions. A simple explanation has been proposed by Shay but additional work is clearly called for especially concerning the Cu-111-VI, compounds (according to the results published by the du Pont de Nemours people, CuAlS, can be prepared both p and n type).
The spin-orbit splitting of many I-TIT-VIZ are nega- tive while only two tetrahedral binaries are known with this property (ZnO, CuCl). Stamps collecting indeed, but collecting of very rare stamps. With few exceptions these negative splittings are understood as due to an admixture of d-wavefunctions of the group I element (Cu, Ag). By the way it is worth noting that no gold tetrahedral binary nor ternary seems to have been observed. Is that a question of size of the gold atom ? From the spin-orbit splittings hybridiza- tion coefficients can be derived and used to interpret other hybrjdization quantities such as energy gaps and their pressure and temperature coefficients, oscilla- tors strengths, effective g-factors, etc ... The beautiful work of Sermage et al. accounts for the anomalous temperature dependence of the band gaps of the I-111-VI, compounds as due to a change in hybridiza- tion with temperature. It remains to be answered why this hybridization takes place and what exactly are its consequences (to go further than the mere statement this is due to d-wavefunctions in explaining any unusual electronic property). Actually the effect of d-hybridiza-
CONCLUDING REMARKS C3-19 1
tion in the I-V11 compounds is so strong that the jump between 11-V1 and I-V11 is too large for an orderly inter- pretation of various properties by extrapolation. The lattice dynamics of the I-VII, for instance, is highly anharmonic and a large number of phases are present with only small differences in free energy. The 1-111-VIZ represent an intermediate case and are therefore a valuable link in the systematics of semiconductors.
The Conference brings forth the fact that materials preparation, in particular growth of large and high quality single crystals, is very much of a problem.
Consequently the crystals are largely in the hands of the chemists and, most often, the physical experiments which are performed are somewhat unsophisticated.
One sees a lack of contributions making use of highly advanced experimental techniques such as magneto- optics, soft X-ray emission and absorption, synchro- tron radiation (absorption, reflection, photoemission in the far UV), UV and X-rays photoelectron spec- troscopy (ESCA), and neutron diffraction. Even Raman scattering, about which we heard a very spirited discussion between the Edinburgh and the Max Planck workers, is conducted in the long wavelength. non- resonant, range.
One of the moderators (MC) was particularly gra- tified to see how useful, because of its simplicity, the electrolyte method of electroreflectance has become for exploratory studies, as demonstrated by Shay in his initial work on band gap in chalcopyrites. In spite of the availability of this technique, it is disturb- ing to see assignments to direct allowed -or forbidden transitions being performed on the basis of the func- tional dependence of the edges of transmission, through thin samples, in the small absorption coefficient range.
Finally we would like to comment on Raman scattering with particular attention to resonant Raman scattering (RRS). For simple binary tetrahedral mate- rials only one L 0 and one TO phonons (taking into account the degeneracy) belonging to the same r
representation are involved in first order scattering.
In ternaries and taking again into account degeneracy the number of optical phonons is 15 among which 13 are Raman active. A natural theoretical problem is the interpretation of the relative strengths of the corresponding Raman lines, both at long wavelengths and when approaching the various energy gaps (RRS).
This problem, which can be tackled at different levels of sophistication, requires essentially the solution of the interactions of the various energy gaps with the various phonons. It is thus a rather complete and splendid exercice in electron-phonon interaction. The eigenvectors of the r phonons have been calculated by Bettini while the gaps can be obtained from pseudo- potential theory (except, of course, for the more chal- lenging effect of the d-electrons in the I-111-VI, compounds). One just has to sit down and do it. The strength of the T, mode in the I-111-VIZ as compared with the 11-IV-V, is as yet uninterpreted. Are again the d-electrons responsible for that ? The mode does not even correspond to a r mode of zinc-blende ! In this respect we regret the absence of the Soviet participants ; the paper by Polygalov et al. would have surely contributed to clarify this point.
In conclusion it seems to be the general opinion that there are still challenging questions to be answered and fruitfull work to be done in the field of ternaries.
Thanking you all for your cooperation, we hope t o see you again at the third international meeting.
