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PREPARATION OF CuAl1-xGaxS2 ALLOYS AND MEASUREMENT OF PHASE-SHIFT DIFFERENCE
UPON REFLECTION
N. Yamamoto, T. Miyauchi
To cite this version:
N. Yamamoto, T. Miyauchi. PREPARATION OF CuAl1-xGaxS2 ALLOYS AND MEASUREMENT OF PHASE-SHIFT DIFFERENCE UPON REFLECTION. Journal de Physique Colloques, 1975, 36 (C3), pp.C3-155-C3-157. �10.1051/jphyscol:1975328�. �jpa-00216298�
JOURNAL DE PHYSIQUE Colloque C3, suppl6ment au no 9, Tome 36, Septembre 1975, page C3-155
PREPARATION OF CuA1,-,Ga,S, ALLOYS AND MEASUREMENT OF PHASESHIFT DIFFERENCE UPON REFLECTION
N. YAMAMOTO and T. MIYAUCHI College of Engineering, University of Osaka Prefecture
Mozu, Sakai, Osaka 591, Japan
Rbumti. - Les alliages C U A I I - ~ G ~ ~ S ~ ont et6 prkparks par transport chimique avec de l'iode, en prenant cornme mattriaux de depart des aIliages respectifs 6laborks soit par croissance en solu- tion, soit partir du bain fondu.
La mesure de la densite de ces alliages ainsi que de leurs matkriaux de depart, indique que la densite des alliages de depart est en accord avec celle calculQ A partir de la cellule Clementaire, mais que la densite des alliages transportes chimiquement est lkgkrement plus faible que celle des alliages de depart. Ceci peut suggerer que les alliages transport& chimiquement comporteraient quelques vacances de Cu (et peut-&tre un e x d s de AI et/ou de Ga), provenant d'un deskquilibre au cours du transport entre Cu2S et A12S3 et/ou Ga2S3, plut8t que d'une Ikgkre dtviation de la composition x au cours de la croissance.
Une mesure directe du dephasage A la reflection est effectuk afin de determiner la variation du gap et de la separation de la bande de valence en fonction de la composition. Le resultat montre une dkpendance non lintaire du gap en fonction de la composition, et sensiblement la m&me separation de champ cristallin (0,ll eV) ii travers tout le systkme.
Abstract. - The CuAl I - ~ G ~ , S ~ alloys of the whole range are prepared by the chemical transport method using iodine as transport agent, where are used as the starting materials the respective alloys initially grown by the solution growth method and the melt growth method.
Measurements of the crystal density on these alloys as well as their starting materials indicate that the densities of the starting alloys agree well with those of the calculated value from their cell dimensions but that the densities of the chemically transported alloys are slightly smaller than those of the starting alloys. This may suggest that the chemically transported alloys should have some Cu-vacancies (and perhaps AI and/or Ga excess) due to the transporting unbalance between Cu2S and Al2S> and/or GazS,, rather than the slight deviation in the alloy composition X , during the growth process.
Direct measurement of the phase-shift difference upon reflection is also made to value the compo- sition dependences of the energy gaps and the valence band splittings. The result shows a nonlinear character of the energy gap vs. composition curves and almost the same crystal field splitting (0.1 1 eV) throughout the system.
1. Introduction. - The CuA1,-, Ga,S2 alloys have the direct energy gap from 3.48 eV (CuAlS,) t o 2.46 eV (CuGaS,), and therefore, are the possible candidates for the LED materials of ultra-violet t o green spectral range. In the previous paper [I], it was reported that these alloys of the whole range could be grown by the solution growth method using in a s solvent. This report describes the preparation of the same alloys by the chemical transport method using iodine as transport agent and also describes the slight off-stoichiometric character of the yielded alloys which is deduced from the decrease in density compar- ing with that of the starting alloys.
Direct measurement of the phase-shift difference upon reflection [2] a t low temperature is also made t o value the conlposition dependences of the energy gaps and the valence band splittings in the alloy system.
2. Alloy preparation. - The CuA1,-, Ga,S2 alloys of the whole range were grown by the chemical trans- port method using iodine a s transport agent. Starting materials were the respective alloys which were initially grown by the solution growth method using in a s solvent [l] or by the melt growth method (direct soli- dification). Usually, 500 mg of the powdered materials and 90 m g of iodine were sealed in a evacuated silica ampoule, and the ampoule was heated in a two tempe- rature-zones furnace of 9000C (powder end) and 600 OC (growth end) for 1-4 days of each run. Typi- cally, thin platelet crystals u p t o 5 mm X 5 mm X 0.2 m m in dimensions were grown and which colours changed from colourless transparent of CuAlS, t o yellow- green of CuGaS, via blue of the intermediates. Howe- ver, for example in the case of CuAIS,, blue o r dark blue coloured varieties a n d yellow o r brown coloured varieties were also yielded a t slightly higher and
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975328
C3-156 N. YAMAMOTO A N D T. MIYAUCHI
lower tempcrature zonc, respectively, than the tcmpe- rature zone where the colourless transparent crystals were yielded. This tendency to yield many colour varieties was observed in the whole alloys examined, and some discussions on CuGaS, wcre already reported by the authors [3]. For thc purpose of the density and the reflection measurerncnts, the most transparent varietics other than the dark or thc yellow varieties were selected.
3. Density measurement and off-stoichiometric cha- racter. -- In the case of the CuA1,-, Ga,S, alloys system, the accurate determination of the alloy composition, X , is impossible from the measurement of the lattice constants alone due to the very small difference (0.4 X) in the lattice constants between CuAIS, and CuGaS,. The X-ray spectroscopic analy- sis, which was adopted in the case of the CuGa, -,In,S, alloy system [4], is also expected to be insensitive to detect Al. However, the difference in density between CuAIS, and CuGaS, is sufficiently large (25 %) enough to determine the alloy composition. To measure the crystal density, was adopted thc floatation method using Clerici's heavy fluid [5], in which method the accuracy' of determining thc composition is + 0.02 in X for the CuAI, -,Ga,S, alloy system.
FIG. 1. -Composition dependences of alloy densities of the solution grown, the melt grown and the chemically transported crystals at 300 K. The solid line is the calculated density for the alloys having ideal chalcopyrite structure and the same lattice constants a s those of the chcmically transported alloys.
Figurc 1 shows the results of the alloy densities of the solution grown crystals, the melt grown crystals and the chemically transported crystals. The solid line is the calculated density curve for the alloy crystals having ideal chalcopyrite structure and the same lattice constants as those measured for the chemically
transported crystals. The experimental plots for the solution grown alloys and the melt grown alloys scatter well along the solid line, and this indicates that both of these starting materials have almost the same compositions as those of the initial charge of the constituent elemcnts. However, the plots for the che- mically transported alloys, which scatter along the broken line, are systematically smaller than those of the starting materials. This decrease in density of the chcmically transported alloys should be attributed to, either (I) the small deviation in the alloy composi- tion, X , or (2) the slight off-stoichiometric deviation in which some Cu-vacancies might be produced so that the crystal density was decreased. The fact that even the density of CuAIS, ( X = 0) is smaller than that of thc starting material suggests the validity of the latter reason. Besides, the plot for the red coloured CuGaS,, which we could assign as to be Ga,S,-rich (and hence having Cu-vacancies) CuGaS, in rcfe- rence 131, is just on the point of the intersection of the broken line on to the X = 1 line. Therefore, our chemi- cally transported CuAI, -,Ga,S, alloys are consi- dered to be off-stoichiometric ones having some Cu-vacancies (and perhaps AI and/or G a excess) which lead to the decrease in density. The result by Donohue et al. [6], i. e., the colourless transparent CuAIS, is slightly AI-rich (and/or Cu-deficient), also agrees with our result.
4. Direct measurement of phase-shift difference upon reflection. - Anisotropic oscillator strength of the direct excitons of these materials gives rise to the large phase-shift difference upon reflection between the ordinary and the extraordinary light components.
The modulation measurement of this phase-shift was first made by Bettini [7] and the direct measurement without using any modulation technique was reported by the authors [2]. The same method was adopted to the CuAI, -,Ga,S, alloy system at l l0 K.
Sample Monochromtor I1111 Polar~zer
(1121
Photomult~plier FIG. 2. Schematic diagram of the measuring apparatus for
the phase-shift difference upon reflection.
Figure 2 shows the schematic diagram of the mea- suring apparatus. Well-collimated monochromatic light was irradiated to the (1 12) surface of the crystal through a prism polarizer whose axis was set to make 450 from the [l 111 direction of the crystal. The reflected light was measured by a photomultiplier behind another prism polarizer (analyser) which was
PREPARATION O F C U A I I - ~ G ~ ~ S ~ ALLOYS AND MEASUREMENT O F PHASE-SHIFT DIFFERENCE C3-157
set to make - 450 from the [ I l l ] direction of the crystal. Therefore, the light intensity measured by the photomultiplier, I,, is, to the first approximation, proportional to the square of the phase-shift difference upon reflection [2].
2 2 2.L 2 6 2 8 3.0 3.2 3 1 36 3 8 Photon energy leV)
FIG. 3. - Phase-shift difference spectra of the C ~ A I I - ~ G ~ X S Z 4.
Con,position depcndences of the energy gaps deduced alloys at 110 K.
from the energies of A and B peaks of figure 3.
Figure 3 shows the I, spectra of the CuAI, -,Ga,S, alloys ( X = 0, 0.25, 0.50, 0.75 and 1) at l l0 K, where the I , spectrum of the CuGaS, ( X = 1) is, however, the result for the stoichiometric yellow coloured CuGaS, since the red coloured CuGaS, did not show any spectrum [3]. Each I, curve has two peaks, A and B, which values of energy correspond to those of the direct cxcitons associating to the uppermost valence bands. For the two terminate crystals, CuAIS, and CuGaS2, thc values of A and B agree well with those by Bettini [7] and by Shay et al. [g]. The sepa- ration of A-B splitting, which should be attributed to the crystal field splitting 181, was about 0.1 1 eV throughout the alloy system. The composition depen- d e n c e ~ of the encrgy gaps deduced from the l , peaks were plotted in figure 4, which indicate some non- linear dependences upon composition.
In conclusion, the CuA1, -,Ga,S, alloy crystals of the whole range were prepared by the chemical trans-
port method. However, in the present stage, these alloys are considered to be slightly off-stoichiometric ones having some Cu-vacancies (and perhaps A1 and/or Ga excess) which results in the decrease in crys- tal density as compared with that of the starting alloys.
This may arise fiom the transporting ~lnbalance between Cu2S and A12S3 (andlor Ga,S,) during the growth process.
The direct measurement of the phase-shift difference upon reflection was also made and the result shows that the composition dependences of the two energy gaps are nonlinear and the crystal iield splitting is almost the same throughout the system.
The authors would like to thank Prof. Y. Hama- kawa and Dr. T. Nishino of Osaka university and Dr. H. Sonomura of this university for valuable discussions. They also thank Messrs. H. Kubo and T. Fujii for their helpful assistances.
References
[l] YAMAMOTO, N., KURO, H. and MIYAUCH~, T., Japan. J . [S] S O N O ~ ~ U R A , H., NAKMORI, T. and Mry~ucrrr, T., Appf. Phys.
Aypl. Yhys. 14 (1 975) 299. Lert. 22 (1973) 532.
l21 YAMAMOTO, N. and MIYAUCHI, T., Japan. 3 . APP[. P h ~ s . 13 161 DONOHUE, P. C., BIEKLEIN, J. D., HANLON, J. E. and JAK-
(1974) 1919. RETT, H. S., 3 . Electrochern. Soc. 121 (1974) 829.
L31 YAMAMOTO, N., TOHGE, N. and MIYAUCHI, T.9 Japan. J . A P P ~ . [7] B ~ ~ r l l ' j l , M . , Solid Slate Conlmun. 13 (1973) 599, Phys. 14 (1975) 192.
[ g ] SHAY, J. L. and TALL, B., SurJ Sci. 37 (1973) 748.
[4] YAMAMOTO, N. and MIYAUCHI, T., Jupun. J. Appl. Phys. 11 (1972) 1383.
