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>
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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 ont
~ ~ et6
S ~prkparks par transport chimique avec de l'iode,
en prenant cornme mattriaux de depart des aIliages respectifs 6laborks soit par croissance en solution, 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-&treun 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&meseparation
de champ cristallin (0,ll eV) ii travers tout le systkme.
Abstract. - The CuAl I - ~ G ~ alloys
, S ~ 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 composition 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 comparing 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 transport 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 solidification). 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 temperature-zones furnace of 9000C (powder end) and
600 OC (growth end) for 1-4 days of each run. Typically, 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 yellowgreen of CuGaS, via blue of the intermediates. However, 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 tcmperature 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 character. -- 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 analysis, 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
0.02 in X for the CuAI, -,Ga,S, alloy system.
is
+
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 chemically 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 composition, 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 rcference 131, is just on the point of the intersection of the
broken line on to the X = 1 line. Therefore, our chemialloys are consically transported CuAI, -,Ga,S,
dered to be off-stoichiometric ones having some
Cu-vacancies (and perhaps AI and/or Ga 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
I1111
Monochromtor
Polar~zer
(1121
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
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 measuring 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 ALLOYS
~ ~ S ~ 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].
22
2.L
26
28
3.0
3.2
31
36
38
Photon energy leV)
FIG. 3. - Phase-shift difference spectra of the C ~ A I I - ~ G ~ X S Z 4.
alloys at 110 K.
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 separation 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 dependence~of the encrgy gaps deduced from the l , peaks
were plotted in figure 4, which indicate some nonlinear dependences upon composition.
In conclusion, the CuA1, -,Ga,S, alloy crystals of
the whole range were prepared by the chemical trans-
Con,position depcndences of the energy gaps deduced
from the energies of A and B peaks of figure 3.
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 crystal density as compared with that of the starting alloys.
This may arise fiom the transporting ~lnbalancebetween
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. Hamakawa 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
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N., KURO,H. and MIYAUCH~,
T., Japan. J.
Aypl. Yhys. 14 (1975) 299.
l21 YAMAMOTO,
N.and MIYAUCHI,
T., Japan. 3 . APP[.P h ~ s 13
.
[S] S O N O ~ ~ UH.,
R ANAKMORI,
,
T. and Mry~ucrrr,T., Appf. Phys.
Lert. 22 (1973) 532.
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 ' M
j l ,. , 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.