Crystal growth and properties of PbI2 doped with Fe and Ni

Article abstract of DOI:10.1007/s10789-005-0271-1

A procedure is described for doping PbI₂ single crystals with Fe and Ni during vapor-phase growth in a closed system in the presence of excess iodine. The rate of mass transport in the system and the doping level of the crystals are shown to be governed by the dopant content in the source material and the source temperature. The effect of Fe and Ni doping on the low-temperature (5 K) exciton photoluminescence spectrum of PbI₂ is discussed.

Full text of DOI:10.1007/s10789-005-0271-1

Inorganic Materials, Vol. 41, No. 10, 2005, pp. 1124–1127. Translated from Neorganicheskie Materialy, Vol. 41, No. 10, 2005, pp. 1272–1276.
Original Russian Text Copyright © 2005 by Rybak, Lun’, Bordun, Omelyan.
Crystal Growth and Properties
of PbI2 Doped with Fe and Ni
O. V. Rybak, Yu. O. Lun’, I. M. Bordun, and M. F. Omelyan
Lviv State Polytechnic University, ul. St. Bandery 12, Lviv, 79013 Ukraine
e-mail: orybak@polynet.lviv.ua
Received December 1, 2004; in final form, March 25, 2005
Abstract—A procedure is described for doping PbI2 single crystals with Fe and Ni during vapor-phase growth
in a closed system in the presence of excess iodine. The rate of mass transport in the system and the doping
level of the crystals are shown to be governed by the dopant content in the source material and the source tem-
perature. The effect of Fe and Ni doping on the low-temperature (5 K) exciton photoluminescence spectrum of
PbI2 is discussed.
INTRODUCTION
After synthesis and purification, FeI2 and NiI2 were
stored in evacuated ampules.
PbI2 layered crystals are of scientific interest for
gaining greater insight into electronic processes in
highly anisotropic systems and have considerable
potential for use in nonlinear optical devices and x-ray
and gamma detectors [1–3]. Doping of PbI2 single crys-
tals with 3d transition metals is of interest for the devel-
opment of new materials—quasi-two-dimensional
diluted magnetic semiconductors [4].
For mass transport studies, 2 g of a PbI2–MI2 alloy
(0.001–2 at % M) or mixture of PbI2 and MI2 (5–30 at %
M) and a capillary containing iodine were sealed in an
ampule under vacuum. Next, the capillary was opened,
and the ampule was mounted in a gradient furnace so
that the charge was situated in the low-temperature
zone and melted. After homogenization for 2 h, the
thermal conditions were set to initiate crystal growth,
which was conducted for 2–4 h. After cooling and
opening the ampule, the grown single crystals were
withdrawn, and the residual charge (alloy) was
weighed.
The dopant concentration in the source material and
crystals was determined by x-ray microanalysis. The
structural perfection of the crystals was examined on
optical and scanning electron microscopes.
Low-temperature PL spectra were measured near
the main excitonic transition in PbI2 crystals. Excitation
was provided by the 442.1-nm cw He–Cd laser line.
The spectral slit width was 0.02 nm. Thermal control
during measurements near 5 K was accomplished with
a well-proven UTREKS system.
The vapor-phase growth of PbI2 crystals in a closed
system in the presence of excess iodine was described
in [5–7]. The vapor-phase growth of PbI2 crystals
doped with Fe and Ni has not yet been investigated in
sufficient detail.
The purpose of this work was to study the effect of
transition-metal dopants on the rate of mass transport in
crystal growth of PbI2, the quality of the resultant crys-
tals, and their photoluminescence (PL) spectra.
EXPERIMENTAL
Given that the introduction of elemental dopants
(M = Fe or Ni) adds much complexity to processes in
the PbI2–I2 system, the dopants were introduced in the
form of MI2 or PbI2–MI2 alloys.
RESULTS AND DISCUSSION
FeI2 and NiI2 were synthesized from stoichiometric
high-purity (≤10–5 at % impurities) elemental mixtures,
which were sealed in silica ampules under vacuum. The
ampules were mounted in a two-zone furnace and held
for 3–5 h at a source temperature of 1000–1200 K and
deposition temperature of 700–750 K. For further puri-
fication, the synthesized iodides were transferred to
precleaned silica ampules, which were sealed off under
vacuum and placed in a lower gradient (8–10 K/cm)
The doping level of vapor-grown crystals depends
primarily on the dopant content of the source material
and the temperatures of the source and deposition
zones. In addition, at considerable dopant (MI2) con-
centrations in the system, the doping level of the crys-
tals is also influenced by the relationship between the
rates of PbI2 and MI2 transport. The effect of process
parameters on the rate of PbI2 transport in the absence
furnace, in which the source temperature was 880– of impurities was studied experimentally and theoreti-
900 K, and the deposition temperature was 700–750 K. cally in [6]. In this work, mass transport was studied
0020-1685/05/4110-1124 © 2005 Pleiades Publishing, Inc.

CRYSTAL GROWTH AND PROPERTIES OF PbI2 DOPED WITH Fe AND Ni
1125
Table 1. Doping of PbI2 with Fe (FeI2)
Run no. at % Fe (alloy) Flux ×105, mol/(m2 s)
Results
1
2
3
4
0.5
1
4.41
3.47
2.94
2.29
PbI2 single crystals 10 × 1 mm2 in area containing ~0.03 at % Fe
Crystals 3 × 1 mm2 in area containing 0.1 at % Fe
Single crystals 2 × 1 mm2 in area containing ~0.2 at % Fe
3
5
No PbI2 crystals; deposition of polycrystalline material
in the 630- and 690-K zones
5
10
1.5
Deposition of polycrystalline material in the 690-K zone
Note: At a source temperature of 1120 K, we obtained small PbI single crystals containing ~0.3 at % Fe from an alloy containing 6 at % Fe.
2
Table 2. Doping of PbI2 with Ni (NiI2)
Run no. at % Ni (alloy)
Flux ×105, mol/(m2 s)
Results
1
0.5
4.32
PbI2 single crystals 5 × 1 mm2 in area containing
~0.02 to 0.03 at % Ni
2
3
4
1
3
6
3.56
3.04
2.54
Crystals 3 × 1 mm2 in area containing 0.2 at % Ni
Crystals 2 × 1 mm2 in area containing 0.3–0.4 at % Ni
No single crystals; deposition of polycrystalline material
in the 630- and 760-K zones
5
10
1.72
No single crystals; deposition of polycrystalline material
in the 760-K zone
Note: At a source temperature of 1120 K, we obtained small single crystals containing ~0.7 at % Ni from an alloy containing 7.2 at % Ni.
under the optimal conditions for the crystal growth of crystals was an order of magnitude lower than that of
undoped PbI2: source temperature of 770 K, deposition the source.
temperature of 650 K, iodine overpressure of 8.5 kPa
(PbI2 flux of 4.5 × 10–5 mol/(m2 s)), and growth time of
2–4 h. The rate of FeI2 and NiI2 transport was found to
be an order of magnitude lower than that of PbI2 tran-
sport.
The introduction of 0.5 to 5 at % dopant (Fe, Ni)
reduced the PbI2 deposition flux by a factor of 1.5–2,
obviously because of the increase in MI2 concentration
in the vapor phase. The dopant content of the crystals
was an order of magnitude lower than that of the alloy.
With increasing MI2 concentration in the source mate-
rial, the crystal size decreased, and the deposition zone
broadened (toward higher temperatures) owing to the
deposition of polycrystalline material. As the dopant
content of the source material was raised from 5 to
10 at %, the rate of mass transport dropped by a factor
of 2, and another, well-defined deposition zone (670–
770 K) appeared. In the 630- to 660-K zone, polycrys-
talline material containing ~0.1 at % dopant was depos-
ited. Increasing the source temperature to 1020–1090 K
increased the rate of mass transport and led to the
growth of small crystals containing 0.5–2 at % dopant
in the 640- to 660-K zone.
The Fe and Ni contents of the source material were
varied from 0.001 to 30 at %. The rate of PbI2 transport
and the doping level of the grown crystals were deter-
mined as functions of the dopant concentration in the
source under the optimal conditions for the crystal
growth of undoped PbI2 and also as functions of the
source temperature at a fixed dopant content of the
source material.
At dopant contents of the source from 0.001 to
0.1 at %, the rate of mass transport was on the same
order as in the undoped system. The dopant concentra-
tion in the crystals was found to be proportional to that
in the source material.
At dopant contents from 10 to 30 at % and a source
temperature of 1270 K, no crystal growth occurred:
mass transport increased markedly, and polycrystalline
material containing 3–5 at % dopant was deposited.
X-ray microanalysis data indicated that the dopant
profile in the crystals grown from alloys containing
Our results on mass transport and crystal quality at
dopant contents from 0.1 to 30 at % are summarized in
Tables 1 and 2. At dopant contents of the starting alloy
from 0.1 to 0.5 at %, the reduction in the rate of PbI2
transport was within 5%, but the dopant content of the 1−5 at % dopant was nonuniform.
INORGANIC MATERIALS Vol. 41 No. 10 2005

1126
RYBAK et al.
doping on PbI2 transport could only be studied with
495
PbI2–MI2 alloys as evaporation sources.
(a)
Understanding the processes involved in the doping
of PbI2 crystals with transition-metal ions requires
knowledge of the vapor composition in the PbI2–MI2
system.
The study of the exciton spectra of doped semicon-
ductors is of special interest because the shape of exci-
ton luminescence peaks is very sensitive to various lat-
tice defects, which allows one to gain detailed informa-
tion about microscopic processes in crystals. The low-
temperature (5 K) exciton PL spectra of the vapor-
grown PbI2 single crystals doped with Fe or Ni to
0.001–0.01 at % during growth indicate that doping has
the following effects on the structural and optical prop-
erties of the crystals:
494.8
(b)
1. Doping leads to predominant growth of 4H-PbI2
crystals (different PbI2 polytypes can be identified
readily be decomposing exciton resonances [8]).
2. Doping during vapor growth gives rise to one-
dimensional structural disordering: formation of stack-
ing faults in the 4H polytype.
(c)
494.5
3. Increasing the doping level leads to a nonlinear
broadening and weakening of the exciton peak.
Typical low-temperature (5 K) PL spectra of Ni-
doped PbI2 crystals are shown in Fig. 1. Increasing the
Ni content from 0.001 to 0.01 at % shifts the exciton
peak to shorter wavelengths by 0.5 nm and reduces its
intensity. Moreover, the full width at half maximum
(FWHM) of the exciton peak increases by almost a fac-
tor of 2, from 26 to 46 meV.
400
450
500
550
600
Wavelength, nm
Fig. 1. 5-K PL spectra of PbI crystals doped with (a) 0.001,
2
(b) 0.005, and (c) 0.01 at % Ni.
Table 3 lists the spectral parameters of the exciton
luminescence in doped PbI2 crystals. It can be seen that
the parameters are monotonic, nonlinear functions of
dopant content. The concentration coefficient of peak
position is dE/dN = 2.5 meV for both Fe and Ni,
whereas the concentration coefficient of broadening,
d(WHM)/dN, for Ni doping is almost twice that for Fe
In the case of separate PbI2 and MI2 sources, MI2
was found to react with PbI2 in the source zone. The
reaction had a significant effect on the rate of mass
transport and impeded control over the doping process.
For this reason, the doping process and the effect of doping.
Table 3. Parameters of the exciton luminescence in doped PbI2 crystals
Dopant
Ni
at %
λ, nm
FWHM, meV Relative intensity dE/dN, meV d(FWHM)/dN, meV
0.001
0.005
0.01
495
26
1
2.5
20
494.8
494.5
495.2
494.7
494.1
40.5
46
0.71
0.44
1
Fe
0.001
0.005
0.01
37
2.5
10.5
42
0.71
0.55
47.5
Note: λ is the peak position, dE/dN is the concentration coefficient of peak position, and d(FWHM)/dN is the concentration coefficient
of broadening.
INORGANIC MATERIALS Vol. 41 No. 10 2005

CRYSTAL GROWTH AND PROPERTIES OF PbI2 DOPED WITH Fe AND Ni
1127
To understand the origin of the nonlinear variation
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INORGANIC MATERIALS Vol. 41 No. 10 2005