Two-temperature synthesis of ZnGeP2

Article abstract of DOI:10.1134/S0020168507100020

We describe a modified two-temperature process for reproducible high-volume (up to 500 g) synthesis of the nonlinear-optical semiconductor ZnGeP ₂, which enables the preparation of nominally stoichiometric material. The major reaction intermediates in the two-temperature ZnGeP ₂ synthesis are ZnP₂, Zn₃P₂, GeP, and Ge. Using x-ray diffraction, we refined the interplanar spacings in the tetragonal structure of ZnGeP₂ and fcc structure of GeP and indexed peaks missing in the PDF cards 33-1471 (ZnGeP₂) and 21-353 (GeP).

Full text of DOI:10.1134/S0020168507100020

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 10, pp. 1040–1045. © Pleiades Publishing, Inc., 2007.
Original Russian Text © G.A. Verozubova, A.I. Gribenyukov, Yu.P. Mironov, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 10, pp. 1164–1169.
Two-Temperature Synthesis of ZnGeP
G. A. Verozubova , A. I. Gribenyukov , and Yu. P. Mironov
2
a
a
b
a
Institute of Monitoring of Climatic and Ecological Systems, Siberian Division, Russian Academy of Sciences,
Akademicheskii pr. 10/3, Tomsk, 634055 Russia
b
Institute of Strength Physics and Materials Science, Siberian Division, Russian Academy of Sciences,
Akademicheskii pr. 2/1, Tomsk, 634021 Russia
e-mail: verozubova@mail.tomsknet.ru
Received June 14, 2006; in final form, February 18, 2007
Abstract—We describe a modified two-temperature process for reproducible high-volume (up to 500 g) syn-
thesis of the nonlinear-optical semiconductor ZnGeP , which enables the preparation of nominally stoichiomet-
2
ric material. The major reaction intermediates in the two-temperature ZnGeP synthesis are ZnP , Zn P , GeP,
2
2
3 2
and Ge. Using x-ray diffraction, we refined the interplanar spacings in the tetragonal structure of ZnGeP and
2
fcc structure of GeP and indexed peaks missing in the PDF cards 33-1471 (ZnGeP ) and 21-353 (GeP).
2
DOI: 10.1134/S0020168507100020
INTRODUCTION
application of ZnGeP still depends on developing a
2
safe, reproducible process for the high-volume synthe-
sis of this compound. In this paper, we present the
results of our studies aimed at resolving this problem.
ZnGeP , a compound semiconductor with the chal-
2
copyrite structure, offers a wide transmission window
in the mid-IR region, a large nonlinear optical coeffi-
cient, and birefringence sufficient for phase matching
in wide spectral ranges, which makes it an attractive
material for high-efficiency mid-IR frequency convert-
ers [1] and terahertz lasers [2]. Transition-metal-doped
EXPERIMENTAL
Two-temperature syntheses of ZnGeP were per-
2
formed in a two-zone horizontal furnace tilted at an
angle of 7°. The temperature profile in the furnace was
determined with an accuracy of ±2°C, and the temper-
ZnGeP is believed to have considerable potential for
2
use in spintronics [3].
A serious impediment to many potential applica- ature was maintained with a stability of ±0.1°C. Reac-
tions of ZnGeP is that material of high optical quality tors were fabricated from fused silica tubes. The posi-
2
is difficult to prepare. This compound has a relatively tion of the zinc and germanium in the reactor corre-
high melting point, 1027°ë, with a relatively high satu- sponded to the hot zone of the furnace. Phosphorus was
rated vapor pressure over its melt. According to separated from the metals by a quartz glass insert, and
Buchler and Wernick [4], the total vapor pressure over its position corresponded to the cold zone of the fur-
5
ZnGeP at its melting point is 3.5 × 10 Pa, and the nace. To produce a vapor phase over the melt of the syn-
2
thesized compound, to a stoichiometric amount of
phosphorus was added an excess corresponding to the
vapor phase consists predominantly of phosphorus.
Elemental synthesis of ZnGeP continues to be a chal-
2
equilibrium vapor pressure over molten ZnGeP at the
lenge. In particular, using a single-temperature process
2
homogenization temperature. The reactor was typically
Schunemann and Pollack [5] were able to synthesize no
9
70 mm in length, and its diameter was varied from 10
more than 25 g of ZnGeP . Moreover, in their prepara-
2
to 45 mm, depending on the charge weight. The starting
tions ampules often exploded because of the high phos-
phorus pressure at elevated temperatures. Conventional
two-temperature synthesis, widely used in the technol-
ogy of binary semiconductors, cannot be used to pre-
mixture was sealed in the reactor under a vacuum of
–
4
~
10 Pa. After synthesis, the reactor was opened, and
the residual phosphorus was burnt.
pare ZnGeP because, at a fixed temperature of the cold
To gain insight into the chemical processes in the
2
zone, volatile binary zinc phosphides condense in the reaction zone during the two-temperature synthesis of
gradient region (between the hot and cold zones), and ZnGeP , we used control samples (≤5 g) obtained by
2
stoichiometric ZnGeP cannot be obtained. For this rea- quenching the reactor in water from hot-zone tempera-
2
son, use is made of a modified two-temperature pro- tures of 850, 900, 950, 1010, and 1050°ë at a cold-zone
cess, in which the temperature of the cold zone is raised temperature of 510°ë. We carried out two series of
after the reaction [6, 7]. However, even in the modified experiments. In one of them, the reactor was withdrawn
process the reactor often explodes. Thus, practical from the furnace just after the desired temperature of
1
040

TWO-TEMPERATURE SYNTHESIS OF ZnGeP2
1041
Table 1. Compounds and elements identified by XRD in samples obtained by quenching in different steps of two-tempera-
ture ZnGeP synthesis
2
Compounds and elements present
Sample no.
t, °C
50
Total phosphorus, at %
>
10–15 mol %
<10–15 mol %
8
1
2
3
4
5
6
7
Zn, Ge
ꢀ1
ꢀ11
ꢀ21
ꢀ30
ꢀ46
ꢀ50
ꢀ50
900
950
Ge, Zn3P2
ZnP ,GeP, Zn
2
Ge, GeP, Zn3P2
Ge, GeP, ZnP2
Ge, GeP, Zn P , ZnP
2
Zn, ZnP2
1010
900
Zn, Zn3P2
ZnGeP2
3
2
1010
1050
ZnGeP2
ZnGeP2
Ge, ZnP , GeP, Zn P
3 2
2
–
Note: Samples 1–4 were prepared in the first series of experiments (quenching), and samples 5–7, in the second series (1-h hold before
quenching).
the hot zone had been reached. In the other series, the zone (and, accordingly, the phosphorus pressure) and
sample was held at temperature for 1 h before quench- the synthesis time. The temperature of the hot zone was
ing.
1010°ë, and that of the cold zone was varied from 480
to 520°ë, which corresponds to phosphorus vapor pres-
sures from 0.4 to 1.05 MPa. The weight of the starting
mixture was 300 to 500 g.
The phase composition of the powder samples thus
prepared was determined by x-ray diffraction (XRD)
on a DRON-4 diffractometer (CuK or CÓK radiation,
α
α
2
θ = 12°–165°, step-scan mode with a step size of
0
.05°) at room temperature. The powders were pressed
RESULTS AND DISCUSSION
with Zapon into disks. XRD data were analyzed using
the RENEX program (Institute of Strength Physics and
Materials Science, Siberian Division, Russian Acad-
emy of Sciences), designed for refining the intensity,
angular position, and full width at half maximum
Diffusional/convective Zn transport in the vapor
phase. In preliminary two-temperature syntheses of
ZnGeP , the temperature of the cold zone was 450 to
2
4
80°ë, and the corresponding phosphorus vapor pres-
(FWHM) of diffraction peaks. The angular position and
sure was 0.17 to 0.45 MPa. The hot zone was main-
tained at 1030°ë; the corresponding saturated vapor
pressure over pure zinc is 0.4 MPa. Based on Raoult’s
law, the Zn pressure was assumed to be approximately
a factor of 2 lower because of the mixing of zinc with
germanium. Such conditions ensured zinc vapor trans-
port toward the cold zone, resulting in deposition of
zinc phosphides (ZnP and Zn P ) on the reactor wall
midway between the cold and hot zones. The phos-
phides clogged up the reactor, preventing phosphorus
vapor transport to the hot zone. Owing to the presence
of free phosphorus, subsequent increase in the temper-
ature of the cold zone caused a sharp rise in pressure
and, as a consequence, explosion.
FWHM of individual peaks were determined with an
uncertainty within 0.02°. In phase analysis, we used
JCPDS Powder Diffraction File data [8] for the com-
pounds and constituent components of the ternary sys-
tem Zn–Ge–P: Ge, Zn, P, GeP, GeP , GeP , ZnP , Zn P ,
3
5
2
3 2
ZnP , and ZnGeP . We took into account that some of
4
2
the compounds in this system might be present in sev-
eral polymorphs and that the presence of phosphorus, a
low-Z element, reduced the reflectance of the phase.
The ratio of the integrated intensity of diffraction peaks
from the sample to that given in the PDF was used to
estimate the volume fraction of the phase in the sample.
In this way, the phases present were classed into major
2
3 2
(>10–15%) and secondary (<10–15%) phases. The
content of combined phosphorus was evaluated as the
difference between the sample weights before and after
the experiment.
Analysis of equations of diffusional/convective zinc
vapor transport in phosphorus vapor [9] indicates that,
to prevent zinc transport in the vapor phase during reac-
To optimize the temperature of the cold zone (and, tion, one must markedly raise the phosphorus pressure
accordingly, the phosphorus pressure in the reactor) and reduce the zinc pressure, which can be achieved by
and evaluate the phosphorus vapor consumption in the properly raising the cold-zone temperature and lower-
reaction, we performed a series of experiments in ing the hot-zone temperature. Zinc diffusion may, how-
which ZnGeP synthesis was interrupted. In those ever, occur before the reaction. One must then more
2
experiments, we varied the temperature of the cold rapidly heat the reactor to temperatures at which phos-
INORGANIC MATERIALS Vol. 43 No. 10 2007

1
042
VEROZUBOVA et al.
1
1
100
000
tm
Homogenization
Synthesis
9
8
7
6
5
4
3
2
00
00
00
00
00
00
00
00
Cooling
Hot zone
Cold zone
0
3
6
9
12
15
75
Time, h
78
Fig. 1. Temperature–time program for ZnGeP synthesis (470-g starting mixture).
2
Fig. 2. 470-g ZnGeP ingot prepared by the proposed process.
2
phorus vapor actively reacts with the zinc–germanium to 1010°C, the increase in the phosphorus content of the
melt in the hot zone.
sample is accompanied by a reduction in Zn P concen-
3
2
The temperature at which phosphorus vapor begins tration, because ZnP reacts with phosphorus vapor to
2
to react with zinc–germanium melts and chemical pro- form ZnGeP , which becomes a major phase, together
2
cesses in the Zn–Ge–P system during the two-tempera-
with Ge and GeP. At phosphorus concentrations below
30 at %, ZnGeP was not detected. During holding at
00°ë for 1 h (sample 5), most of the phosphorus was
combined in the hot zone; the phosphorus content of
sample 5 was ꢀ46 at %. XRD examination, however,
revealed only a small amount of ZnGeP . The major
phases in that sample were Ge, GeP, Zn P , and ZnP .
After holding at 1010°ë for 1 h (sample 6), the predom-
inant phase was the ternary compound ZnGeP , while
Ge, ZnP , GeP, and Zn P were present in small
amounts. Raising the temperature of the hot zone to
050°ë and homogenizing the material for 1 h, we
obtained nominally stoichiometric ZnGeP , phase-pure
ture synthesis of ZnGeP were determined in this work
2
2
since such data are not available in the literature.
9
Chemical transformations during two-tempera-
ture ZnGeP synthesis. Table 1 lists the phase compo-
2
sitions of samples obtained by quenching in different
2
steps of two-temperature ZnGeP synthesis. Active
2
3
2
2
reaction between phosphorus vapor and Zn–Se melts
(the liquidus temperature of an equiatomic mixture of
2
Zn and Ge is 740°C) in the hot zone begins at ꢀ900°ë.
The phosphorus content of the sample quenched from
this temperature (sample 2) was ꢀ11%. This reaction
leads primarily to the formation of Zn P . Increasing
2
3 2
1
3
2
the temperature of the hot zone to 950°C raises the
phosphorus concentration in the melt owing to the reac-
tion between P and Ge, which increases the GeP con-
2
by XRD (sample 7). Thus, our XRD data indicate that
the reaction intermediates in the synthesis of the ternary
centration. As the temperature of the hot zone is raised compound ZnGeP are Zn P , ZnP , GeP, and Ge. The
2 3 2 2
INORGANIC MATERIALS Vol. 43 No. 10 2007

TWO-TEMPERATURE SYNTHESIS OF ZnGeP2
Table 2. XRD data for ZnGeP (tetragonal structure)
1043
2
d , Å
I, %
JCPDS, 33-1471
100
50
d , Å
±∆d, Å
h k l
FWHM (2θ), deg
I, %
hkl
hkl
this work
3
2
2
1
1
1
1
1
1
1
1
1
1
.126
.726
.667
.927
.901
.812
.642
.616
.578
.364
.335
.251
.238
–
3.135
0.003
1 1 2
2 0 0
0 0 4
2 2 0
2 0 4
3 0 1
3 1 2
1 1 6
2 2 4
4 0 0
0 0 8
3 3 2
3 1 6
4 2 0
4 0 4
2 0 8
4 2 4
2 2 8
5 1 2
3 3 6
1 1 10
4 4 0
4 0 8
5 3 2
5 1 6
1 310
4 4 4
4 2 8
6 2 0
6 0 4
2 0 12
5 3 6
3 3 10
6 2 4
6 3 1
2 2 12
4 4 8
0.18
0.16
0.16
0.17
0.16
–
100
9
2.734
0.002
50
75
100
25
90
75
50
50
75
50
75
–
2.6785
1.9326
1.9123
–
0.002
3
0.001
34
41
~0
33
14
4
0.001
–
1.6446
1.6204
1.5666
1.3662
1.3382
1.2523
1.2414
1.2219
1.2169
1.2019
1.1115
1.1002
1.0507
1.0444
1.0319
0.9661
0.9561
0.9232
0.9188
0.9102
0.9087
0.90245
0.86406
0.86228
0.84826
0.82978
0.82343
0.82229
0.81235
0.81003
0.78336
0.0008
0.0008
0.0007
0.0005
0.0005
0.0004
0.0004
0.0004
0.0004
0.0004
0.0003
0.0003
0.0003
0.0003
0.0002
0.0002
0.0002
0.0002
0.0002
0.0002
0.0002
0.00014
0.00012
0.00011
0.00010
0.00009
0.00008
0.00008
0.00007
0.00007
0.00004
0.15
0.16
0.15
0.14
0.13
0.14
0.13
0.14
0.12
0.15
0.12
0.17
0.17
0.14
0.17
0.13
0.16
0.16
0.17
0.17
0.15
0.16
0.17
0.17
0.17
0.20
0.20
0.23
0.20
0.24
0.32
6
3
7
12
1
1
1
1
1
1
1
1
0
0
0
0
0
.218
.199
.111
.098
.050
.043
.030
.965
.955
.923
.919
.910
–
25
25
25
75
75
50
50
25
50
50
50
50
–
2
1
14
10
7
5
2
5
6
6
6
5
1
–
–
2
0
0
0
.864
–
50
–
7
5
.848
–
25
–
5
7
.824
–
25
–
5
2
–
–
1
0
.811
–
50
–
1
13
INORGANIC MATERIALS Vol. 43 No. 10 2007

1
044
VEROZUBOVA et al.
Table 3. XRD data for GeP (cubic structure)
d , Å
h k l
I, %
d , Å
±∆d, Å
h k l
I, %
hkl
hkl
JCPDS, 21-353
this work
3
2
1
1
1
1
1
.16
1 1 1
2 0 0
2 2 0
3 1 1
2 2 2
–
100
10
50
25
6
3.168
2.743
1.9388
1.6534
–
0.003
1 1 1
2 0 0
2 2 0
3 1 1
100
5
.734
.933
.647
.577
.379
.252
0.002
0.0009
0.0005
85
30
–
10
16
1.3715
1.2583
0.0003
0.0002
0.0002
0.0001
4 0 0
3 3 1
4 2 0
4 2 2
5
3 3 1
15
3
1
.2275
1
1
1
.121
.114
.050
–
10
16
6
1.1202
–
13
–
4 2 2
5 1 1
1.05561
0.00009
0.00007
0.00005
0.00003
0.00002
5 1 1, 3 3 3
4 4 0
4
0
0
0
0
.96951
.92740
.86769
.83662
7
5 3 1
12
8
6 2 0
5 3 3
4
process involves two steps: the formation of zinc and reaction of phosphorus vapor with a mixture of con-
germanium phosphides and then the formation of the densed zinc and germanium), heating of the cold
ternary compound.
(empty) zone to the hot-zone temperature or above,
melt homogenization, melt solidification, and cooling
of the reactor.
Effect of phosphorus pressure on the diffu-
sional/convective Zn transport in the vapor phase.
Experimental data obtained at various phosphorus pres-
Figure 1 shows the temperature–time programs for
sures in the vapor phase indicate that, at pressures from the cold and hot zones in the process under consider-
0
.4 to 0.6 MPa, the material deposited in the gradient ation, and Fig. 2 is a photograph of a 470-g ZnGeP2
region consisted of elemental zinc and zinc phosphides. ingot prepared by this process.
At higher phosphorus pressures, only zinc phosphides
Lattice parameters and XRD intensities of
were deposited. Increasing the temperature of the cold
zone reduces zinc losses in the reaction zone, which
become insignificant (≤1–2 g) in comparison with the
total weight of the starting mixture at 520°ë (the calcu-
lated vapor pressure over red phosphorus is 1.05 MPa).
This small amount is easy to sublime and transfer to the
reaction zone by raising the temperature of the cold
zone. Experimental data obtained at a cold-zone tem-
perature of 520°ë and various total weights of the start-
ing mixture indicate that, at total weights of the starting
mixture of 300, 400, and 500 g, all of the phosphorus is
ZnGeP and GeP. Detailed analysis of XRD patterns
2
at high diffraction angles made it possible to accurately
determine the interplanar spacings d in the crystal lat-
hkl
tices of the ZnGeP and GeP compounds and to index
2
XRD peaks not included in the PDF cards 33-1471
(
ZnGeP ) and 21-353 (GeP). In particular, card 33-
2
1
6
4
472 gives no data for the 420, 444, 428, 604, 536, 624,
31, and 448 reflections, and card 21-353, for the 420,
40, 531, 620, and 533 reflections. The parameters of
the tetragonal chalcopyrite cell of ZnGeP were deter-
2
combined in 50, 90, and 120 min, respectively. Our data mined to be a = 5.4650 ± 0.0002 Å and c = 10.7088 ±
make it possible to obtain reliable estimates of the 0.0005 Å. The parameter of the fcc lattice of GeP was
nonisothermal holding time needed for the cold zone to determined to be a = 5.4861 ± 0.0006 Å.
contain no free phosphorus in order to avoid pressures
exceeding the strength of the reactor.
The d , FWHM, and XRD intensity data for
hkl
ZnGeP and GeP are presented in tables 2 and 3, respec-
2
Schedule of two-temperature ZnGeP synthesis. tively.
2
Based on our experimental data, we developed a repro-
ducible two-temperature process for high-volume
CONCLUSIONS
We investigated chemical transformations in differ-
ZnGeP synthesis.
2
The process involves several steps: heating of the
reactor to the synthesis conditions, holding at the syn- ent steps of two-temperature ZnGeP synthesis. It is
2
thesis conditions (here, synthesis means the chemical shown that active reaction between phosphorus vapor
INORGANIC MATERIALS Vol. 43 No. 10 2007

TWO-TEMPERATURE SYNTHESIS OF ZnGeP2
1045
and zinc–germanium melts begins at a hot-zone tem-
perature of ꢀ900°ë. The synthesis of the ternary com-
pound involves two steps: the formation of zinc and
germanium phosphides and then the formation of
ZnGeê . The major reaction intermediates in ZnGeê
manium Phosphides, Appl. Phys. Lett., 2003, vol. 83,
no. 5, pp. 848–850.
3. Sato, K., Medvedkin, G.A., Ishibashi, T., and Hirose, K.,
Room Temperature Ferromagnetism in the Novel Mag-
netic Semiconductor Based on the Chalcopyrite Com-
pounds, J. Cryst. Growth, 2002, vols. 237–239,
pp. 1363–1369.
2
2
synthesis are ZnP , Zn P , GeP, and Ge. Raising the
2
3 2
phosphorus vapor pressure to 1.0–1.1 MPa and reduc-
ing the hot-zone temperature to below the melting point
of the ternary compound (to 1010°ë) suppresses the
diffusion of the second volatile component (Zn) in the
vapor phase and makes it possible to obtain nominally
4
. Buchler, E. and Wernick, J.H., Concerning Growth of
Single Crystals II–IV–V2 Diamond-like Compounds
ZnSiP , CdSiP , ZnGeP , and CdSnP and Standard
2
2
2
2
Enthalpies of Formation for ZnSiP2 and CdSiP2,
J. Cryst. Growth, 1971, vol. 8, pp. 324–332.
stoichiometric ZnGeP . Based on the present data, we
2
5
6
. Schunemann, P.G. and Pollack, T.M., Ultralow Gradient
HGF-Grown ZnGeP and CdGeAs and Their Optical
developed a modified two-temperature process for
2
2
reproducible high-volume (up to 500 g) ZnGeP syn-
2
Properties, MRS Bull., 1998, no. 7, pp. 23–27.
thesis.
. Andreev, A.Yu., Voevodin, V.G., Vyatkin, A.P., and
We refined most of the interplanar spacings dhkl in
Zuev, V.V., Nonlinear-Optical II–IV–V Crystals for IR
2
the tetragonal structure of ZnGeP and fcc structure of
2
Laser Radiation Conversion, Elementnaya baza optiko-
elektronnykh priborov (Materials and Components of
Optoelectronic Devices), Zuev, V.E. and Kabanov, M.V.,
Eds., Tomsk: Rasko, 1992.
GeP and indexed peaks missing in the PDF cards 33-
1
471 and 21-353.
7
. Ginsar, V.E., Gribenyukov, A.I., Zuev, V.V., and Kry-
gin, V.V., USSR Inventor’s Certificate no. 1641037,
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1
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2
. Wei Shi and Yujie, J. Ding, Continuously Tunable and
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INORGANIC MATERIALS Vol. 43 No. 10 2007