Effect of growth conditions on the morphology and structural perfection of vapor-grown PbI2 crystals

Article abstract of DOI:10.1023/A:1014787220190

Data are presented on the morphology and structural perfection of PbI ₂ crystals grown from the vapor phase in a closed system. By varying growth conditions, platelike, ribbon, needle, twinned, and dendritic crystals were prepared, as well as combinations and intergrowths of these habits.

Full text of DOI:10.1023/A:1014787220190

Inorganic Materials, Vol. 38, No. 3, 2002, pp. 288–291. Translated from Neorganicheskie Materialy, Vol. 38, No. 3, 2002, pp. 362–365.
Original Russian Text Copyright © 2002 by Kurilo, Rybak.
Effect of Growth Conditions on the Morphology
and Structural Perfection of Vapor-Grown PbI2 Crystals
I. V. Kurilo and O. V. Rybak
Lviv State Polytechnical University, ul. St. Bandery 12, Lviv, 79013 Ukraine
Received July 17, 2001
Abstract—Data are presented on the morphology and structural perfection of PbI2 crystals grown from the
vapor phase in a closed system. By varying growth conditions, platelike, ribbon, needle, twinned, and dendritic
crystals were prepared, as well as combinations and intergrowths of these habits.
INTRODUCTION
RESULTS AND DISCUSSION
The crystal habit is known to be governed by the
thermal conditions of growth, partial pressures of the
constituent components (stoichiometry of the growing
crystal), and contamination level [1]. Depending on the
relative importance of these factors, crystal may take a
wide variety of habits, from ribbons oriented along a
certain crystallographic direction to dendrites contain-
ing precipitates and polycrystalline inclusions.
In a number of cases, the growth habit is governed
by the crystal structure, but typically a key role in deter-
mining the morphology of crystals is played by the
growth conditions—temperature, impurities, supersat-
uration, and deviations from stoichiometry.
In most cases, under the optimal growth conditions,
we obtained platelike or ribbon crystals. The growth
habit was responsive to the thermal conditions, which
determine the stoichiometry of the growing crystal.
Typically, the platelets and ribbons had the same orien-
tation. It is believed that, in vapor growth, platelike
morphology is due to the lateral growth of needle crys-
tals, acting as growth leaders. Platelike crystals may
result not only from the vapor–liquid–solid mechanism
but also from layer-by-layer growth by the vapor–solid
mechanism.
The introduction of excess iodine with pI = 2.2 kPa
2
(PbI2 flow of 15.4 × 10–5 mol/(m2 s)) favors platelike
The purpose of this work was to assess the effect of
growth conditions on the crystal habit of PbI2 grown
from the vapor phase in the presence of excess iodine.
morphology. At pI = 4.0 kPa (PbI2 flow of 9.93 ×
2
10−5 mol/(m2 s)), we observe growth of ribbons 10 mm
in length and 1–2 mm in width. The source temperature
in those runs was 770 K, and the deposition tempera-
ture was 650 K. Perfect ribbon crystals were also
EXPERIMENTAL
The growth conditions of PbI2 crystals were opti-
mized using thermodynamic analysis of the equilib-
rium vapor-phase composition in the Pb–I2 system and
theoretical and experimental studies of mass transport:
obtained at pI = 8.5 kPa (PbI2 flow of 4.54 ×
2
10−5 mol/(m2 s)) and the above source and deposition
temperatures. The presence of impurities leads to a
wide variety of growth habits, from isometric to acicu-
source temperature, 750–800 K;
deposition temperature, 630–660 K;
iodine overpressure, 4–10 kPa;
growth time, 2–4 h.
lar. At Tsource = 820 K and pI = 8.5 kPa (PbI2 flow of
18.45 × 10–5 mol/(m2 s)), we obtained imperfect crys-
tals and polycrystalline material.
2
Figure 1 illustrates the morphology of PbI2 crystals
grown from the vapor phase in the form of ribbons, nee-
dles, and their intergrowths. The following types of
morphology can be distinguished: long ribbons and
their intergrowths (often twins), elongated crystals with
sharp terminations, bent ribbons, complex habits based
on ribbons and their intergrowths, and misshapen plate-
lets (Fig. 1b).At high supersaturations, we observe den-
dritic morphology (Fig. 1c). Figure 1d shows a mis-
shapen crystal with a twin band (dark area).
Crystals were grown in a two-zone furnace. The
growth charge synthesized from a high-purity elemen-
tal mixture was sealed in a silica tube under vacuum.
After holding for 2–4 h at different combinations of
source and deposition temperatures, the tube was
cooled to room temperature, and the crystals were with-
drawn.
The structural perfection of the crystals was exam-
ined by optical and scanning electron microscopy
(SEM) techniques and x-ray diffraction (Laue photo-
graphs).
On the surface of regular ribbon crystals, we some-
times observe striations tilted to the crystal axis
0020-1685/02/3803-0288$27.00 © 2002 MAIK “Nauka/Interperiodica”

EFFECT OF GROWTH CONDITIONS ON THE MORPHOLOGY
289
(b)
(a)
(c)
(d)
(e)
(f)
Fig. 1. (a–e, g) Optical and (f, h–j) SEM micrographs illustrating the morphology of PbI crystals; magnifications: (a–e, g) ×50–200,
2
(f, h) ×2000, (i, j) ×1000.
(Fig. 1e). Such striations can also be met on misshapen irregular triangles and hexagons (Figs. 1g–1j). Figure 1i
crystals.
shows vicinal hillocks with a stepped structure. In
Fig. 1j, one can see details of a hexagonal pattern
formed by a set of parallel steps. On sufficiently perfect
surfaces, we observe triangular patterns corresponding
to stacking faults.
The surface of most regular crystals is perfect; stri-
ations are discernible only in SEM micrographs taken
at large magnifications. Other morphological features
are illustrated in Figs. 1f–1j. Figure 1f shows an accu-
mulation of growth spirals, which lead to the formation
The peripheral parts of crystals grown at different
of so-called kinematic density waves at growth steps temperatures are characterized by vicinal developments
[2]. The appearance of such waves is commonly attrib- in the form of terraces, some with striations. We
uted to the particular character of the diffusion field observed intergrowth of three terraces into a single
near step sources and kinematic wave fronts. On the aggregate. At long growth times, such terraces disap-
surface of less perfect crystals, we observe growth pat- pear and, as a result of the tapering of other platelets, a
terns and vicinal developments in the form of regular or monolithic structure is formed. The elongation of cer-
INORGANIC MATERIALS Vol. 38 No. 3 2002

290
KURILO, RYBAK
(g)
(h)
(i)
(j)
Fig. 1. (Contd.)
tain faces indicates that the growth rate is anisotropic, of CdTe; that is, inclusions are capable of generating
which is due to the anisotropy in the effective surface deformation twins.
energy. The direction normal to the plane of Fig. 1 cor-
Twinning is also favored by sharp changes in growth
responds to the highest surface energy, while the in-
conditions, which may cause a polymorphic transfor-
plane directions correspond to the lowest surface
mation. On the surface of the crystals rapidly cooled
energy, as evidenced by the higher rate of lateral
after growth, we observe twinned structures. To avoid
growth, especially in the direction of the long axis.
twinning, slow growth rates (low thermal gradients)
and high purity of the starting materials are required.
On inhomogeneous surfaces, we observe bundles of
The amount of twinning was found to rise with
increasing PbI2 vapor pressure. Crystallization at high
supersaturations gives rise to the formation of cyclic
twins. Note that some of the twins extend throughout
the crystal and are easy to reveal by the brightness con-
trast under an optical microscope. The highest supersat-
urations result in dendritic structures (Fig. 1c).
slip bands characteristic of plastically deformed crys-
tals. The slip bands run along the easy slip directions
and correspond to the outcrops of slip planes. Occa-
sionally, such slip bands alternate with twin bound-
aries. The latter can be distinguished by the character-
istic tapering at the end of the twin boundary. The
development of slip bands is related to the steep tem-
perature gradient during growth.
It is of interest to note that, in the range of a steep
temperature gradient, we observe the formation of both
single crystals and polycrystalline material uniform in
grain size. By varying the thermal conditions, we are
able to control, to a certain extent, the grain size of
polycrystals. This finding is of technological interest in
preparing materials with controlled properties.
The observed twinning may be due to the significant
supersaturation during growth, thermal stress, and sec-
ond-phase inclusions. In the last case, high mechanical
stresses may arise because of the lattice and elastic
modulus mismatches between the crystal and inclusion.
Such stresses may give rise to twinning, as observed in
some of the crystals. Analogous twins around inclu-
sions were revealed in II–VI compounds, in particular,
in CdTe [3], which is characterized by a low energy of
stacking faults. The components of the elastic stress
tensor around a Te inclusion, calculated in the Mott–
Thus, our results demonstrate that, depending on the
supersaturation during growth, needle and platelike
crystals, twinned platelets, and dendritic twins may be
obtained.
The high structural perfection of the crystals
Nabarro model [4], are close to the theoretical strength selected for physical measurements is evidenced by the
INORGANIC MATERIALS Vol. 38 No. 3 2002

EFFECT OF GROWTH CONDITIONS ON THE MORPHOLOGY
291
2. Obrazovanie kristallov (Crystal Formation), vol. 3 of
Sovremennaya kristallografiya (Modern Crystallogra-
phy), Chernov, A.A. et al., Eds., Moscow: Nauka, 1980.
high intensity and small width of excitonic emissions.
The presence of excess iodine in the growth system
ensures a low intensity, if any, of the defect-related pho-
toluminescence (PL) bands at 620 and 700 nm, which
are commonly assigned to Pb and I vacancies, respec-
tively [5].
3. Kurilo, I.V. and Kuchma, V.I., Twinning in CdTe and
CdxHg1 – xTe Crystals, Izv. Akad. Nauk SSSR, Neorg.
Mater., 1982, vol. 18, no. 4, pp. 569–572.
Low-temperature PL studies of PbI2 crystals grown
under different conditions showed that, in the PL spec-
tra of some crystals, the lowest-energy exciton state has
a complex structure, which is commonly attributed to
polytypism, inherent in layered crystals. Some of the
crystals grown under nonoptimal conditions were
found to consist of 2H- and 4H-polytypes (mixed-poly-
type crystals [6, 7]).
4. Kurilo, I.V., Rudyi, I.O., Virst, I.S., and Vlasenko, O.I.,
Effect of Inclusions of the Mechanical Properties of
CdTe, HgTe, and CdxHg1 – xTe Crystals, Ukr. Fiz. Zh.
(Ukr. Ed.), 2002, vol. 47, no. 1, pp. 30–42.
5. Blazhkiv, V.S., Gamernyk, R.V., Grigorovich, V.M.,
et al., Defect System in PbI2 Crystals, Ukr. Fiz. Zh.
(Russ. Ed.), 1992, vol. 37, no. 3, pp. 425–428.
6. Brodin, M.S., Blonskii, I.V., Nitsovich, I.V., and Nitsov-
ich, V.V., Dinamicheskie effekty v mnogokomponentnom
gaze kvazichastits (Dynamic Effects in a Multicompo-
nent gas of Quasiparticles), Kiev: Naukova Dumka,
1992.
CONCLUSION
The habit, surface morphology, and structural per-
fection of PbI2 crystals were studied as a function of
growth conditions.
7. Blonskii, I.V., Nabytovych, Y., Loon, Yu.O., and
Rybak, O.V., Peculiarities of Manifestation of Different
Forms of Structure Disordering in the Exciton Spectra of
the PbI2, Phys. Status Solidi A, 1999, vol. 174, no. 2,
pp. 353–360.
REFERENCES
1. Stroitelev, S.A., Kristallograficheskii aspekt tekhnologii
poluprovodnikov (Crystallographic Aspects of Semicon-
ductor Technology), Novosibirsk: Nauka, 1976.
INORGANIC MATERIALS Vol. 38 No. 3 2002