XRD and TEM Characterization of Compound Semiconductor Solid Solutions:
Sn(S,Se) and (Pb,Cd)S
B.R. Jarabek., D.G. Grier., D.L. Simonson,
Center for Main
Department
North Dakota
Fargo, ND
D.J. Seidler, P. Boudjouk
Group Chemistry
of Chemistry
State University
58 105-55 16
and G.J. McCarthy
L.P. Keller
MVA Inc.
5500 Oakbrook Pkwy., Suite 200
Norcross, GA 30093
Abstract
X-ray powder diffraction (XRD) is an essential component of research into use of organometallic
precursors as a route to low temperature (<500 “C) synthesis of bulk compound semiconductors.
This paper illustrates the use of XRD to study solid solution behavior in the systems SnS-SnSe
and PbS-CdS. XRD characterization demonstrated that phase pure intermediate members of the
Sn(S,Se) solid solution prepared by this low temperature route showed the typical dependence of
unit cell parameters on composition. PbS-CdS exhibits essentially no solid solution behavior at
low temperatures according to established phase diagrams. Characterization of organometallic
precursor thermolysis products with intermediate compositions initially suggested that a
metastable solid solution with perhaps 30% of CdS in PbS could be prepared at 150°C. More
detailed XRD and high resolution transmission electron microscopy (HRTEM) characterization
indicated that the CdS was in fact present as a separate phase with such small crystallites (2-5 nm)
that peak broadening made it essentially undetectable in concentrations below 30%. The actual
amount of solid solution was 2% or less of CdS in PbS.
Introduction
Organometallic precursors have been found to provide a facile route to low temperature
synthesis of bulk compound semiconductor metal chalcogenide solid solutions’. The primary
advantage of producing solid solutions by organometallic precursor decomposition is intimate
mixing of the inorganic constituents at the molecular level. This mixing occurs in the melt phase,
during heating of mixtures of the organometallic precursor end-member molecules, prior to
decomposition of these precursor molecules and subsequent crystallization of the inorganic end
products, The organometallic compounds decompose at relatively low temperatures typically less
than 300°C. Synthesis of solid solutions by conventional means, i.e. from the elements or binary
end member compounds, would require temperatures of 500-lOOO”C, long heating times,
intermediate grindings, and possibly explosive conditions.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
Copyright 0 JCPDS-International
Centre for Diffraction Data 1997
This document was presented at the Denver X-ray
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XRD characterization, including the use of calculated X-ray powder diffraction patterns2, is an
essential component of these studies. Here we describe results from studies of two binary
chalcogenide systems, one in which XRD characterization was routine, and one in which the
characterization was more challenging.
The first binary system was that of the chalcogen anion-substituted solid solution, Sn(S,Se), in
which complete solid solution between the isostructural (GeS structure) SnS and SnSe
components is known3. Phase pure intermediate members of the Sn(S,Se) solid solution prepared
at 450°C showed the typical dependence of unit cell parameters on composition.
The equilibrium phase diagram for PbS-CdS4 indicates essentially no solid solution between
the cubic (Fm3m) rocksalt structure PbS and the hexagonal (P63mc) wiirtzite structure CdS end
members at temperatures below 500 “C. However, some solid solution of CdS in PbS is known to
occur under equilibrium conditions at higher temperatures (maximum of ca. 40 mol% at the
eutectic temperature of 1052°C)4. In addition, complete solid solution with metastable retention
of the rocksalt structure at ambient conditions has been reported for high pressure syntheses’. The
organometallic precursor route was explored as a possible low temperature (<5OO”C), ambient
pressure synthesis of metastable (Pb,Cd)S. Examination of routine powder diffraction patterns of
products of syntheses of intermediate compositions suggested at first that a metastable solid
solution with perhaps 30% of CdS in PbS could be prepared at 150°C. However, the rocksalt unit
cell parameter did not show the substantial decrease that substitution of the smaller Cd for Pb
would produce. We describe here the additional characterization by XRD and TEM that led to the
conclusion that up to about 30%, the CdS was actually present in thermolysis products as a
crystalline phase, but with such small crystallite sizes (2-5 nm) that peak broadening made it
essentially impossible to detect in the X-ray diffraction patterns.
Experimental
To study the low temperature synthesis of SnS-SnSe solid solutions, thermolysis of mixtures
of (Bn$nS)s and (Bn$nSe)j (where Bn = benzyl, CHzCsHs) was carried out in a programmable
tube furnace under nitrogen flow. The mixtures were heated to, and held for 2 h at, 450 “C before
cooling to room temperature. With the PbS-CdS system, mixtures of Pb(SBn)z and Cd(SBn);l
were heated for 12 h at 150 “C. The compositions studies were 2, 5, 10, 15, 20, 30, 35, 40, 45,
50, 60, 70, 75 and 90 mol% CdS. Three replicate runs were made for the 5%, 20%, 35% and
50% test compositions.
X-ray powder diffraction (XRD) patterns were recorded from ethanol slurry-mounted samples
on zero background quartz slides using a Philips automated vertical diffractometer with CuKa
radiation. The diffractometer setup included a theta-compensating
variable divergence slit, a
graphite diffracted beam monochrometer, and a scintillation detector. Data reduction was
performed on personal computers using MD1 Jade software. Unit cell parameters were determined by the least squares method, also using MD1 Jade software. NIST SRM 640b silicon was used
as the internal standard for the SnS-SnSe system. For the PbS-CdS system, NIST SRM 660 LaB6
was used as the internal standard for cell parameter and crystallite size determinations.
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Calculated powder patterns were used in characterizing all decomposition products, including
the intermediate members of the SnS-SnSe and PbS-CdS systems, using the POWD code.6
Indexing and subsequent unit cell refinements of the orthorhombic patterns were aided by
calculated pattern intensities, which allowed unique indexing of many peaks that would otherwise
have been multiply indexed. Intermediate Sn(S,Se) patterns were calculated assuming random
substitution in the chalcogenide site, using literature values’ for atomic parameters and
interpolated unit cell dimensions. Intensity variations in a potential PbS-CdS rocksalt structure
solid solution phase were also modeled with POWD, estimating all thermal parameters as
Bi,,=l .O. Neutral atom scattering factors were used in all cases, with corrections for anomalous
dispersion for all atoms heavier than Li. The Philips diffractometer was modeled using Cauchy
peak profiles and a dit?i-acted beam monochromator setting.
For the transmission electron microscope (TEM) analyses, an aliquot of each sample was
ground in acetone, and a drop of the suspension was placed on a holey carbon thin film that was
supported by a copper TEM grid. High-resolution TEM (HRTEM) images, selected-area electron
diffraction (SAED) patterns, and chemical analyses of the materials were obtained using a JEOL
2010 (200 KeV) TEM equipped with a NORAN thin window energy-dispersive X-ray (EDX)
spectrometer. The high resolution TEM images were recorded at magnifications of 400kX to
8OOkX.
Results and Discussion
SnS-SnSe System
Sn(S,Se) formed in the expected continuous solid solution series, with random chalcogen
substitution at the anion site. The products of the 450 “C thermolysis were crystalline phase pure,
with no peak splitting to suggest phase separation, and no significant elevation in diffuse background scattering to suggest a significant amorphous contribution (Figure 1). In many cases,
upon exposure to humid laboratory conditions during specimen preparation, minor oxidation of
the platy crystallites (with large surface areas) occurred, producing trace to minor tin oxide. Unit
cell volume varied linearly with sulfur content in the SnS-SnSe solid solution (Figure 2). The
diffraction patterns also showed peak broadening of the 101 reflection of the intermediate member
Sn(Seo.&s) with FWHM=0.267(40 nm). This peak broadening is greater than both the SnSe
endmember with FWHM=0.210 (55 nm) and the SnS endmember with FWHM=O. 190 (80 m-n).
PbS-CdS System
Initial examination of diffraction patterns of thermolysis products of intermediate
compositions in the PbS-CdS system seemed to suggest that extensive solid solubility of CdS in
PbS had occurred, i.e., the diffraction patterns appeared to show crystalline phase pure FCC
materials, with broad peaks but without peak splitting, which would suggest phase separation, and
with no prominent elevations in background scattering, which would suggest amorphous phase(s)
(Figure 3). These results seemed to hold true throughout the range of CdS addition until
approximately 60% CdS, at which point minor broad hexagonal CdS peaks typical of the end
member were observed (Figure 4). However, careml, re-examination of background regions
containing these peaks in more PbS-rich mixtures revealed barely observable broad hexagonal
CdS “peaks” to approximately 30% CdS, but not below.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
26
28
30
32
34
36
3s
2-Theta
Figure 1.
Diffraction patterns of SnS, Sn(So.s Seo.s), and SnSe, showing typical solid solution
behavior.
210
195
190
J
40
60
80
Composition, x, in Sn(S l-x, Se X)
Figure 2.
Unit cell variation in Sn(S1, Se,). For the linear regression, R* = 0.991.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
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Figure 3.
40
2-Theta
Diffraction patterns from PbS, CdS, and an 80% PbS-20% CdS run. The cubic phase
is the only apparent crystalline phase in the product, suggesting incorporation of CdS
into the PbS structure.
PbSlCdS
PbS
2-Theta
Diffraction patterns from PbS, CdS, and a 40% PbS-60% CdS run. With 60% CdS
addition, increases in background from broad hexagonal peaks of the CdS end member were
noted, indicating that at least some portion of the CdS was now crystallizing in a second,
hexagonal phase.
Figure 4.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
Despite the initial implication that solid solutions had formed in samples containing up to 30%
CdS, little variation in unit cell parameters was observed across the range of input stoichiometries
compared to the end member PbS (Figure 5). The large peak shifts predicted by Vegard’s law for
solid solution and observed in a previous high pressure study’ are absent in the experimental data
of Figure 3. These results suggested that very little (<2%) solid solution had actually formed.
Unfortunately, this interpretation leaves most of the CdS input to the system unaccounted for.
The thermolysis apparatus included a liquid nitrogen trap to capture volatile byproducts. Bn$
was the only byproduct. No cadmium was detected by XRF in the byproducts, which indicated
that all of the input cadmium had been retained in the thermolysis product. Two hypotheses were
suggested by these observations: (1) Cd was present in the crystalline FCC phase observed
(without causing a resultant change in unit cell parameters), or (2) Cd was present in a separate
phase that was not observable by XRD.
In order to test these hypotheses, more detailed investigations of the diffraction data were
performed. Clearly, since the peak positions and unit cells remained constant throughout the range
of compositions, the diffraction peak positional data were not helpful. Instead, the relative peak
intensities were used to obtain another line of evidence in support of a separate cadmium sulfide
phase. Site substitution of Cd (atomic no. 48) for Pb (atomic no. 82) in (Pb,Cd)S (hypothesis (l)),
would lead to variation in average scattering factor on the metal site, which should affect
diffraction peak intensities of certain reflections. The three peaks from the calculated diffraction
patterns shown in Figure 6 predict, a significant decrease in relative intensity of the (111) peak
relative to either the (200) or (220). These ratios, then, should decrease with addition of CdS, if
major solid solution were occurring. Figure 7 is a plot of the ratios of the (111) to (220)
reflections for all of the runs made. There is significant scatter of the data, but the peak height
ratio of most run products remained between 0.8 and 0.9, supporting hypothesis (2), and did not
decrease toward 0.6 as would occur if Cd were substituting for Pb in (Pb,Cd)S. Similar results
were obtained for the (111) / (200) ratio.
Based on all of these data, it was concluded that at most only 2% CdS had substituted in the
rocksalt PbS structure. The remaining CdS was present as a separate phase that was below the
detection limit of our instrument for compositions to 30% of CdS in the CdS-PbS binary. As the
weight fraction was increased past 30% the hexagonal phase of CdS became apparent. A
crystallite size estimate of 5-7 nm was obtained using the Scherrer equation on the diffraction
pattern of pure CdS.
TEM analysis was then performed to learn more about the CdS phase. HRTEM and SAED
showed 20-100 nm crystallites of rocksalt phase intermixed with nanocrystalline (2-5 nm) CdS
(Figures 8 and 9). These crystallite sizes of the rocksalt structure phase were consistent with the
20-40 nm crystallite size estimates obtained from XRD using the Scherrer equation. An important
observation for analytical XRD in general was that although the CdS phase was nearly “X-ray
amorphous” in the mixtures, the HRTEM results demonstrated that it is actually quite crystalline.
The very small crystallite sizes led to extreme peak broadening in XRD making it impossible to
distinguish the CdS phase under 30 mole% CdS and requiring great magnification of the
background to see the nanocrystalline CdS phase in scans between 30 and 60 mole%.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
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Figure 5:
Measured unit cell parameters of the FCC phases in all run products as a function of
nominal composition, and the cell parameter variation in solid solutions quenched from
high pressure syntheses.’
3
2-Theta
Figure 6:
Calculated (Pb,Cd)S FCC phase solid solution intensities for the (11 l), (200) and
(220) reflections. Only the (111) varies substantially with substitution of Cd for Pb.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
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Figure 7:
system.
Observed and calculated ratio of the (111) to the (220) reflection in the PbS-CdS
I
’
PbS
50nm
Figure 8:
TEM image from a run products at an intermediate composition.
with much smaller 2-5 nm crystallites of hexagonal CdS.
Copyright 0 JCPDS-International
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Cubic PbS crystals
Figure 9. HRTEM image showing a 20 nm PbS crystallite and randomly oriented 2-5 nm CdS
crystallites.
EDX analysis of the larger, rocksalt structure crystallites indicated that no more than 2 mol%
Cd could be substituting for Pb in this phase. It is also possible that this small amount of Cd
observed by EDS under TEM was present as a thin coating of CdS on larger, pure PbS crystallites. The presence of a small amount of Cd in solid substitution for Pb in the rocksalt structure
phase, rather than as a coating, is consistent with the XRD result shown in Figure 5.
Conclusions
The low temperature organometallic precursor synthesis produced complete solid solution
between SnS and SnSe, and the XRD characterization was routine. Characterization by XRD and
TEM of possible solid solution between PbS and CdS indicated that, for the synthesis conditions
used, at most only 2% of CdS was miscible in rocksalt structure PbS. TEM analysis indicated that
the CdS in other intermediate compositions consisted of 3 to 5 nm crystallites. XRD could not
detect the CdS phase(s) in the products of experiments with up to 30% CdS in PbS-CdS due to
the resulting peak broadening. This study illustrates the need for XRD analysts to be alert to
nanocrystalline phases which may go unnoticed in mixtures with much better crystallized phases.
Acknowledgments
Financial support for this research was provided by the National Science Foundation
Grant No. OSR 9452892.
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through
References
1. Boudjouk, P., Seidler D. J., Grier, D., McCarthy G. (1996) “Benzyl-Substituted Tin
Chalcogenides. Efficient Single-Source Precursors for Tin Sulfide, Tin Selenide, and
Sn(S,Ser,),”
J. Chem. Mater. 8, 1189-l 196.
2. McCarthy, G. J., Martin, K.J., Holzer, J.M., Grier, D.G., Syvinski, W.M., Nodland, D.W.
(1992). “Calculated Patterns in X-Ray Powder DifTraction Analysis,” Adv. X-Ray Anal. 35,
17-23.
3. Albers, W.; Haas, C.; Ober, H.; Schodder, G. R.; Wasscher, J.D. (1962) “Preparation and
Properties of Mixed Crystals Sn So-X)Se.,” J. Phys. Chem. Solids 23, 215-220.
4. Leute, V., Schmidt, R. 2. (1991). “The Quasiternary System (Cdx Pbr_x)(Sr_LTel-L),” Phys.
Chem. 172, 81-103.
5. Sood, A. K., Wu, K., Zemel, J. N. (1978). “Metastable Pbi_,Cd, S Epitaxial Films 1. Growth
and Physical Properties,” Thin Solid Films 48, 73-86.
6. Smith, K. L. and Smith, D.K. (1993). “Micro-POWD, Ver. 2.3,” Materials Data Inc.,
Liver-more, CA, U. S. A.
7. Wiedemeier, H., Georg, H., von Schnering, H.G. (1978). “Refinement of the structures of
GeS, GeSe, SnS and SnSe,” Z. Krist. 148, 295-303.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997