Adsorption and electrophysical studies of the sensitivity and selectivity of the surface of the InSb-CdTe system with respect to toxic gases

Article abstract of DOI:10.1134/S0036024408050233

The piezoquartz microweighing and probe compensation methods were used to study the adsorption properties with respect to NO₂ and changes in the electrical conductivity of solid solutions and the binary components of the InSb-CdTe system under the influence of NO₂ and NO₂ + SO₂ and NO₂ + CO mixtures at various temperatures and relative gas contents. Solid solutions were prepared by isothermal diffusion and certified using the results of X-ray, thermographic, IR spectroscopic, and electrophysical measurements. The (InSb)_0.03(CdTe)_0.97 component of the system was found to possess high selective sensitivity (adsorption and electronic) already at room temperature. This component was recommended for creating a sensor for NO₂ microimpurities.

Full text of DOI:10.1134/S0036024408050233

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 5, pp. 830–834. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © I.A. Kirovskaya, E.V. Mironova, E.I. Bykova, O.T. Timoshenko, T.N. Filatova, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 5,
pp. 949–953.
PHYSICAL CHEMISTRY
OF SURFACE PHENOMENA
Adsorption and Electrophysical Studies of the Sensitivity
and Selectivity of the Surface of the InSb–CdTe System
with Respect to Toxic Gases
I. A. Kirovskaya, E. V. Mironova, E. I. Bykova, O. T. Timoshenko, and T. N. Filatova
Omsk State Technical University, Omsk, Russia
e-mail: phiscem@omgtu.ru
Received May 2, 2007
Abstract—The piezoquartz microweighing and probe compensation methods were used to study the adsorp-
tion properties with respect to NO and changes in the electrical conductivity of solid solutions and the binary
2
components of the InSb–CdTe system under the influence of NO and NO + SO and NO + CO mixtures at
2
2
2
2
various temperatures and relative gas contents. Solid solutions were prepared by isothermal diffusion and cer-
tified using the results of X-ray, thermographic, IR spectroscopic, and electrophysical measurements. The
(
InSb)0.03(CdTe)0.97 component of the system was found to possess high selective sensitivity (adsorption and
electronic) already at room temperature. This component was recommended for creating a sensor for NO2
microimpurities.
DOI: 10.1134/S0036024408050233
INTRODUCTION
physicochemical properties of this material and sub-
strate [2]. Solid solution powders were obtained by iso-
thermal diffusion of the binary components in evacu-
ated sealed quartz ampules at temperatures above the
melting point of the low-melting component (InSb) [3].
The composition of solid solutions obtained depended
on the mutual solubility of the binary components (up
to 6 mol % InSb in CdTe and up to 5 mol % CdTe in
InSb).
The most important problem of the chemistry of
semiconductors is the preparation of new systems on
the basis of known binary compounds. These systems
should retain the properties of the initial binary com-
pounds and exhibit new, possibly predictable, charac-
teristics. Such systems are semiconducting solid solu-
tions, whose typical representatives are solid solutions
based on InSb and CdTe considered in this work. The
unique properties of InSb and CdTe (electric, photo-
and piezoelectric, and optical) and their capabilities
already used in several technologies and semiconduct-
The thickness of films was determined by interfer-
ometry, from changes in piezoquartz resonator fre-
quency [4], and using the equation
ing catalysis allow us to regard (InSb) (CdTe)
solid
2
x
1 – x
D = msinβ/(4πl ρ),
solutions as promising adsorbents, catalysts, and mate-
rials for modern technology, sensor electronics in the
first place. The effectiveness of use in any of these areas
depends on the degree to which the physicochemical
state of the surface (including chemical composition,
structure, and acid–base, adsorption, and electronic
where m is the sample weight, ρ is the sample density,
l is the distance from the vaporizer to the substrate, and
β is the vaporization angle. The D value was 0.25–
0
.35 µm.
The structure of films was studied by X-ray diffrac-
properties and their selective changes under the action tion. X-ray diffraction along with thermographic anal-
of various media) has been studied. The present work is ysis and forbidden band width and electrical conductiv-
concerned with exactly this aspect of studies of the sur- ity measurements was also used to certify the solid
face of InSb–CdTe system components.
solutions obtained [5].
The adsorbates were prepared following standard
procedures [2, 6]; that is, carbon(II) oxide, by the
decomposition of formic acid in the presence of con-
centrated sulfuric acid; nitrogen(IV) oxide, by the
action of concentrated nitric acid on copper shavings;
and sulfur(IV) oxide, by the action of concentrated sul-
furic acid on sodium sulfite.
EXPERIMENTAL
The samples studied, InSb, CdTe, and solid solution
films, were prepared by discrete thermal deposition in a
vacuum (Tcond = 298 K, p = 1.33 × 10 Pa) onto elec-
trode surfaces of piezoquartz resonators followed by
annealing in vapor of the initial material [1]. The con-
–
3
Adsorption was studied by piezoquartz micro-
–
11
2
ditions of annealing were determined on the basis of the weighing (sensitivity 1.23 × 10 g/(cm Hz)) over the
8
30

ADSORPTION AND ELECTROPHYSICAL STUDIES
831
a, Å
a, Å
6
.49
(
a)
6
6
6
.60
.52
.44
(a)
6
.48
6.47
d , Å
hkl
(
b)
6
∆
.46
E, eV
3
.80
0
.23
3
3
.75
.70
(b)
0
.21
3
ρr, g/Òm
(
c)
5
.80
0.19
0.17
5
5
.76
.72
0
.15
0
1
2
3
4
5
6
0
1
2
3
4
5
InSb, mol %
CdTe, mol %
Fig. 1. Concentration dependences of (a) lattice parameter
(
‡), (b) interplanar distance (d ), and (c) X-ray density (ρ )
Fig. 2. Dependences of (a) lattice parameter and (b) forbid-
den band width on the composition of the InSb–CdTe sys-
tem in the presence of excess InSb in solid solutions.
hkl
r
of InSb–CdTe solid solutions in the region of InSb solubil-
ity in CdTe.
temperature and pressure ranges 253–358 K and 4– these dependences were comparatively complex in
3 Pa. Adsorbent films were deposited onto electrode character (Fig. 2). It was, however, shown and
1
surfaces of piezoquartz resonators having the shape of explained in [3, 8, 9] that the dependences of lattice
an AT cut lens. Resonator intrinsic frequencies were of parameters and other characteristics of semiconducting
7
–8 MHz [4].
substitution solid solutions on composition could be
nonlinear.
The same samples were used to simultaneously study
changes in electrical conductivity and, accordingly, sur-
It is also noteworthy that the X-ray patterns do not
face charging under the influence of adsorbed gases with contain additional lines corresponding to unreacted
the use of the probe compensation method [4].
binary components, and the main lines are not smeared,
which is evidence of solid solution synthesis comple-
tion. According to the positions and intensity distribu-
tion of the main X-ray pattern lines, all system compo-
RESULTS AND DISCUSSION
nents (InSb, CdTe, and (InSb) (CdTe)1 – x) have
x
The Formation of Substitution Solid Solutions
in the InSb–CdTe System
sphalerite cubic structures.
The endothermic peaks in the thermograms of InSb,
CdTe, and their solid solutions caused by melting and
sample oxidation shift predominantly toward higher tem-
The X-ray diffraction, thermographic, IR spectro-
scopic, and electrophysical data show that substitution
solid solutions are formed in the InSb–CdTe system [7].
peratures in the series InSb
(InSb) (CdTe)
x 1 – x
According to the X-ray data, X-ray pattern lines CdTe, which is indirect evidence of the formation of
were shifted with respect to the binary component substitution solid solutions. The same follows from a
lines, but the number of lines remained unchanged. The smooth increase in electrical conductivity σ as the con-
dependences of the lattice parameter (‡), interplanar tent of InSb grows (Fig. 3).
distances (d ), and X-ray density (ρ ) on the composi-
hkl
r
The forbidden band widths found from the IR data
tion of solid solutions obtained in the region of InSb
solubility in CdTe were close to linear (a small devia-
tion from linearity was only observed for the
(
from the hν intercept of the tangent to the absorption
edge [5]) pass a minimum at 3 mol % CdTe as the com-
position changes (Fig. 2). That is, like ∆E = f(c II VI ),
A B
(
InSb)0.03(CdTe)0.97 component) (Fig. 1). For solid solu-
the a = f(c II VI ) dependence is nonlinear. The appear-
tions obtained in the region of CdTe solubility in InSb,
A B
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 5 2008

8
32
KIROVSKAYA et al.
σ × 10 , Ω cm–1
8
–1
α × 10 , mol/m
4
2
8
1
4
6
4
2
1
0
2
1
6
0
1
2
3
4
5
InSb, mol %
2
50
275
300
325
350
T, ä
Fig. 3. Dependence of specific conductivity on the compo-
sition of the InSb–CdTe system in the presence of excess
CdTe in solid solutions.
Fig. 4. Isobars of the adsorption of NO on (1) CdTe and
2
(2) (InSb)0.03 (CdTe)0.97 solid solution at p = 8 Pa.
n
α × 10 , mol/m2
4
2
1
4
2
1
2
8
4
0
α × 10 , mol/m
2
1
1
2
8
4
0
3
5
7
9
11
13
p, P‡
10
30
50
70
t, min
Fig. 5. Equilibrium isotherms of the adsorption of NO on
Fig. 6. Kinetic isotherms of the adsorption of NO on
2
0.03 n
and (2) 328 K.
2
(
1) CdTe and (2) (InSb)
(CdTe)0.97 solid solution at
(InSb)
(CdTe)0.97 solid solution at p = 8 Pa and (1) 303
0
.03
3
03 K.
ance of the minimum is in all probability related to the tures. This is substantiated by the results of thermody-
accumulation of defects during the interaction of the namic and kinetic analyses. The heats of adsorption cal-
binary components accompanied by the formation of culated by the Clausius–Clapeyron equation for
donor-acceptor complexes [3, 10].
descending isobar portions α = f(T) and the semiempir-
p
ical equation suggested by one of these authors [4] at
various T and α over the whole temperature range stud-
ied are of 6.1–13.4 kJ/mol, which corresponds to chem-
ical adsorption on diamond-like semiconductors [2].
The mean activation energy values calculated by the
Roginskii equation [4] valid for our systems are 43–
Such a behavior of σ is in conformity with the
scheme of the component conductivity mechanism sug-
III
V
II VI
gested in [3] for A B –A Ç systems.
The Adsorption Properties
of the InSb–CdTe System Components
8
6 kJ/mol at various surface coverages (adsorption val-
ues α).
The adsorption properties of the InSb–CdTe system
components were studied most thoroughly with respect
to nitrogen dioxide because of its highest activity
among the gases selected.
The observed decrease in the heat and increase in
the activation energy of adsorption as surface coverage
grows and linear kinetic isotherms in the logarithmic
The adsorption values of NO on all the system coordinates are indicative of surface inhomogeneity
2
–4
−3
2
components vary from α × 10 to α × 10 mol/m . and the presence of active centers of different strengths
The typical experimental dependences of adsorption in different energy states. The same follows from the
shown in Figs. 4–6 are similar for all the components. acid–base properties of our adsorbents determined
They are evidence of physical adsorption below 273 K using IR spectroscopy, isoelectric state pH measure-
and chemical activation adsorption at higher tempera- ments, and mechanochemical and conductometric titra-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 5 2008

ADSORPTION AND ELECTROPHYSICAL STUDIES
833
σ × 10 , Ω cm–1
7
–1
∆
3
2
1
1
8
2
6
10
9
7
80
70
1
2
1
5
6
5
0
0
8
3
3
7
12
17
gNO2, µg
1
0
1
2
3
4
5
InSb, mol %
Fig. 7. Dependences of changes in the specific conductivity
of a thin (InSb)0.95 (CdTe)0.05 solid solution film on the
Fig. 8. Dependences of (1) adsorption value, (2) activation
energy of adsorption, and (3) heat of adsorption on the com-
content of NO in the sample (g
) at the carrier gas vol-
NO2
2
ume flow rate 8 ml/min and temperatures of (1) 20, (2) 50,
and (3) 80°ë.
position of the InSb–CdTe system at p = 8 Pa and T = (1, 3)
n
300 and (2) 300–358 K.
tion data [11]. These studies allowed us to identify at sition diagrams can be suggested as a material for a
least four types of acid centers. As on the other dia- sensor for NO microimpurities. We studied changes
2
mond-like semiconductors, coordination unsaturated in the electrical conductivity of this solid solution and
atoms with vacancy defects around them (Lewis cen- the other system components under the action of NO2
–
ters) and adsorbed ç é molecules and éç groups and, to determine selectivity, mixtures of NO with the
2
2
(
Broensted centers) can be responsible for them.
other gases.
The acid–base properties of the surface of the adsor-
bents and the electronic structure of NO molecules led
2
The Influence of the Adsorption of Nitrogen Dioxide
on the Electrical Conductivity
us to suggest a donor–acceptor mechanism of the
adsorption of NO (by analogy with the adsorption of
2
of InSb–CdTe System Components
ëé [12]), predominantly with the participation of sur-
2
face A atoms (with more pronounced metallic proper-
ties) and B atom vacancies, adsorbate molecules (NO2)
playing the role of donors.
Our studies showed that indium antimonide was vir-
tually insensitive to NO at room temperature. An
2
increase in the content of cadmium telluride in the
The electron donor function of NO molecules is sub-
2
InSb–CdTe system increases sensitivity to NO , which
2
stantiated by an increase in electrical conductivity (σ)
reaches a maximum for the (InSb)0.95 (CdTe)0.05 solid
under the conditions of the adsorption of NO (Fig. 7).
2
solution (Fig. 9). Heating increases the sensitivity to
A comparison of binary and quaternary (solid solu- NO of all the system components, most noticeably that
2
tion) components of the InSb–CdTe system and adsor- of (InSb)0.95 (CdTe)0.05 (Fig. 7). The volume flow rate
bents with respect to NO shows them to be similar. (Vvol) of the carrier gas (argon) also influences sensitiv-
2
Solid solutions, however, exhibit special features. Sim- ity (NO
signal value).
2
ilarity follows from similar shapes of the α = f(T),
p
A more optimal value for the (InSb)0.95(CdTe)0.05
solid solution was Vvol = 8 ml/min (Fig. 9).
α = f(p), and α = f(t) experimental dependences, sim-
T
T
ilar adsorption values and thermodynamic and kinetic
characteristics, the same nature of active centers and
Since the (InSb)0.95 (CdTe)0.05 solid solution was
mechanisms and adsorption interaction characteristics. recommended as a material for a sensor for NO micro-
2
The special feature of quaternary adsorbents is the impurities, it was important to estimate the time of the
presence of extrema in adsorption characteristic–com- restoration of the initial value of its specific conductiv-
position dependences (Fig. 8).
According to these
InSb)0.03(CdTe)0.97 solid solution has the highest
ity (relaxation time) after contact with the gas to be
determined (NO ) and its selectivity with respect to this
dependences,
the
2
gas in the presence of the other gases (CO and SO2).
(
The relaxation time in argon was 2–7 min depending on
adsorption activity. For the same solution, the depen-
dence of the lattice constant on system composition
deviates from the linear Vegard law (Fig. 1), which,
according to [3], can be related to the largest number of
defects and coordination unsaturation of surface atoms.
the concentration of NO and temperature and notice-
2
ably decreased as the temperature increased.
The (InSb)0.95 (CdTe)0.05 solid solution exhibits the
highest selectivity with respect to NO at room temper-
2
The (InSb)0.03 (CdTe)0.97 solid solution as the com- ature. The presence of CO and SO (in an amount four
2
ponent of the system most active with respect to NO2 times larger than that of NO ) virtually did not influence
2
found with the use of adsorption characteristic–compo- the signal of NO . The signal of NO insignificantly
2
2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 5 2008

8
34
KIROVSKAYA et al.
σ × 10 , Ω cm–1
7
–1
InSb−CdTe system described above [13, 14]. This
allowed us to study the influence of not only binary
∆
1
0
8
6
4
2
III
V
II VI
(
A B and A B ) but also elemental (A and B) con-
stituents on adsorption and electronic properties. These
results should be considered separately.
2
1
REFERENCES
1
2
. Thin Films of Indium Antimonide (Shtiintsa, Chisinau,
989), p. 162 [in Russian].
. I. A. Kirovskaya, Surface Properties of Diamond-Like
Semiconductors: Adsorption of Gases (Irkutsk. Gos.
Univ., Irkutsk, 1984) [in Russian].
1
3
. I. A. Kirovskaya, Surface Properties of Diamond-Like
Semiconductors: Solid Solutions (Tomsk. Gos. Univ.,
Tomsk, 1984) [in Russian].
. I. A. Kirovskaya, Adsorptson Processes (Irkutsk. Gos.
Univ., Irkutsk, 1995) [in Russian].
. I. A. Kirovskaya, O. P. Azarova, E. G. Shubenkova, and
O. N. Dubina, Neorg. Mater. 38 (2), 667 (2002) [Inorg.
Mater. 38 (2), 91 (2002)].
0
1
2
3
4
5
CdTe, mol %
2
4
6
8
10
4
5
Vvol, ml/min
Fig. 9. Dependences of changes in the specific conductivity
of (1) thin InSb–CdTe films on composition at an NO con-
2
tent of 10 µg in the sample, 20°ë, and carrier gas volume
flow rate 8 ml/min and (2) a thin (InSb)
(CdTe)0.05 solid
6. F. M. Rapoport and A. A. Il’inskaya, Laboratory Meth-
ods for Preparation of Pure Gases (Goskhimizdat, Mos-
cow, 1963) [in Russian].
0
.95
solution film on carrier gas volume flow rate.
7
. I. A. Kirovskaya and E. V. Mironova, Dokl. Akad. Nauk
VSh RF, No. 1 (8), 1 (2007).
. I. A. Kirovskaya, Zh. Fiz. Khim. 59 (1), 194 (1985).
. V. A. Brodovoi, N. G. Vyalyi, L. M. Knorozok, and
I. Ya. Markiv, Neorg. Mater. 33 (3), 303 (1997) [Inorg.
Mater. 33 (3), 248 (1997)].
decreased as the temperature increased to 80°ë, and the
material became slightly sensitive toward CO and SO2.
8
9
To summarize, the InSb–CdTe solid solutions
obtained in the form of powders and films were certi-
fied using the X-ray diffraction, thermographic, IR
spectroscopic, and electrophysical methods. The
adsorption and electronic (specific conductivity
changes) properties of the solid solutions and binary
components (InSb and CdTe) brought in contact with
1
1
0. L. A. Skorobogatova and E. N. Khabarov, Fiz. Polupro-
vodn., No. 2, 401 (1974).
1. I. A. Kirovskaya and E. V. Mironova, Zh. Fiz. Khim. 79
(
4), 755 (2005) [Russ. J. Phys. Chem. 79 (4), 649
Né , Né + SO , and NO + CO were studied. The
2
2
2
2
(2005)].
adsorbent ((InSb)0 (CdTe)0.05 solid solution) with the
.95
12. I. A. Kirovskaya, Surface Phenomena (Omsk. Gos.
highest surface (adsorption and electronic) sensitivity
Tekhn. Univ., Omsk, 2001) [in Russian].
and selectivity with respect to NO already at room
2
1
3. I. A. Kirovskaya, E. G. Shubenkova, L. V. Novgorod-
tseva, et al., Sovremennye Problemy Nauki Obrazo-
vaniya, No. 2, 49 (2006).
temperature was found. This allowed us to recommend
it as a material for the corresponding sensor.
A similar study was performed for the InSb–CdS 14. I. A. Kirovskaya and O. T. Timoshenko, Dokl. Akad.
and InP–CdS systems in comparison with the
Nauk VSh RF, No. 1, 69 (2006).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 5 2008