Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x
We consider the growth technology and investigations of indium antimonide doped concurrently with acceptor (Сd) and donor (Те) impurities taken in equiatomic ratio. The optimal modes of single crystal synthesis and crystallization are determined. It is shown that, when doping the indium antimonide,...
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Venger, E.F. Knorozok, L.M. Melnichuk, L.Yu. Melnichuk, O.V. 2017-06-14T10:57:59Z 2017-06-14T10:57:59Z 2006 Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x / E.F. Venger, L.M. Knorozok, L.Yu. Melnichuk, O.V. Melnichuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 2. — С. 80-86. — Бібліогр.: 11 назв. — англ. 1560-8034 PACS 72.80.Ey, 78.30.Fs https://nasplib.isofts.kiev.ua/handle/123456789/121438 We consider the growth technology and investigations of indium antimonide doped concurrently with acceptor (Сd) and donor (Те) impurities taken in equiatomic ratio. The optimal modes of single crystal synthesis and crystallization are determined. It is shown that, when doping the indium antimonide, its lattice parameter changes considerably. This leads to deformation of the electron energy spectrum, changes the bandgap and charge carrier effective mass and affects the optical and electrical properties of indium antimonide samples. As a result, such material becomes suitable for fabrication of IR photodetectors. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x Article published earlier |
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Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x |
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Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x Venger, E.F. Knorozok, L.M. Melnichuk, L.Yu. Melnichuk, O.V. |
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Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x |
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Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x |
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Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x |
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Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x |
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synthesis and properties of semiconductor solid solutions (insb)₁₋x(cdte)x |
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Venger, E.F. Knorozok, L.M. Melnichuk, L.Yu. Melnichuk, O.V. |
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Venger, E.F. Knorozok, L.M. Melnichuk, L.Yu. Melnichuk, O.V. |
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2006 |
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English |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Article |
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We consider the growth technology and investigations of indium antimonide doped concurrently with acceptor (Сd) and donor (Те) impurities taken in equiatomic ratio. The optimal modes of single crystal synthesis and crystallization are determined. It is shown that, when doping the indium antimonide, its lattice parameter changes considerably. This leads to deformation of the electron energy spectrum, changes the bandgap and charge carrier effective mass and affects the optical and electrical properties of indium antimonide samples. As a result, such material becomes suitable for fabrication of IR photodetectors.
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1560-8034 |
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Synthesis and properties of semiconductor solid solutions (inSb)₁₋x(CdTe)x / E.F. Venger, L.M. Knorozok, L.Yu. Melnichuk, O.V. Melnichuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 2. — С. 80-86. — Бібліогр.: 11 назв. — англ. |
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2025-11-25T21:20:35Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 80-86.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
80
PACS 72.80.Ey, 78.30.Fs
Synthesis and properties of semiconductor solid solutions
(ІnSb)1−х(СdТе)x
E.F. Venger1, L.M. Knorozok2, L.Yu. Melnichuk2, O.V. Melnichuk2
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
2Mykola Gogol Nizhyn State University, 2, Kropyv’yanskogo str., 16600 Nizhyn, Ukraine
Abstract. We consider the growth technology and investigations of indium antimonide
doped concurrently with acceptor (Сd) and donor (Те) impurities taken in equiatomic ratio.
The optimal modes of single crystal synthesis and crystallization are determined. It is
shown that, when doping the indium antimonide, its lattice parameter changes
considerably. This leads to deformation of the electron energy spectrum, changes the
bandgap and charge carrier effective mass and affects the optical and electrical properties
of indium antimonide samples. As a result, such material becomes suitable for fabrication
of IR photodetectors.
Keywords: indium antimonide, lattice parameter, effective mass, electron energy
spectrum.
Manuscript received 04.03.06; accepted for publication 29.03.06.
1. Introduction
Further progress of advanced semiconductor electronics
requires development of novel materials with preset
properties and parameters, as well as both theoretical and
experimental investigations of them. At present, a special
emphasis is put upon studies of disordered systems. To
simulate them, it is convenient to use heavily doped
crystalline semiconductor binary compounds (e.g.,
АIIIВV), among which solid solutions (SS) hold a
significant place. By studying them, it is possible to
follow modification of the physical properties of the
above compounds due to imperfections of the crystal
lattice occurring at gradual replacement of anions or
cations in it. Such studies also enhance substantially the
possibilities to develop semiconductor devices on the
basis of the abovementioned materials.
When the atomic (ionic) radii of dopants and host
material are different, then a considerable local strain
appears around the sites where an atom is replaced by
another one. At high dopant concentration, this may lead
to a considerable change of the lattice parameter of
doped material, and this effect, in its turn, will result in
modification of the electron energy spectrum. It was
shown in [1] that in this case relative changes of the
bandgap and effective mass of charge carriers may be
order of magnitude bigger than relative change of material
volume caused by strains. Such big relative changes of the
bandgap and effective mass of charge carriers are
particular characteristics of narrow-band semiconductors.
A typical representative of such materials is indium
antimonide doped concurrently with acceptor (Cd) and
donor (Те) impurities taken in the equiatomic ratio. In this
case, the atomic and ionic radii of the dopants differ
essentially from those of the replaced atoms of the host
material (indium antimonide).
In our previous works [2, 3], it was shown that, along
with applied aspects, an important role in investigation of
narrow-gap semiconductors with moderate and high degree
of doping with specially chosen impurities belongs to
theoretical and experimental studies how the properties of
the host material depend on crystal lattice deformation,
electron-deformation interaction and interaction between
impurities, complexes formed by impurity atoms and those
of host material, structural defects, etc.
The objective of this work is development of
technology of synthesis of SS (InSb)1−x(CdTe)x,
obtaining an information concerning the effect of its
composition on the band structure parameters, and
investigation of its optical and electrical properties in the
course of concurrent doping the SS with the
corresponding impurities.
2. Growth procedure and investigation of SS
(InSb)1−x(CdTe)x
Synthesis of SS (InSb)1−x(CdTe)x was carried out in a
closed quartz container, at a controlled pressure of volatile
component vapors. The starting materials were In, Sb, Сd
and Те, with uncontrolled impurities of no more than
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 80-86.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
81
10−4 wt. %. The reagents were weighed with an analytical
balance (within the error limits 5×10−4 g). Additional
amounts of the volatile components (Sb and Сd) that were
required to ensure an equilibrium pressure of their vapors
in the crystallization zone were calculated according to the
theory of ideal gases [4]. The container was pre-evacuated
down to a pressure of 10−1…10−3 Pa.
Before loading with substances, the ampoules were
annealed at the temperatures 970…1020 °С for 8 to 10 h.
After this, they were etched in a mixture of nitric and
hydrofluoric acids for 20 to 40 min, and then several
(3…5) times were washed off with distilled water.
The dopants were introduced immediately into the
starting weight of substances. It was determined in the
course of technological experiments that the reaction
between the SS starting components proceeds up to an
end. The distribution of impurities is more uniform when
synthesis occurs at the temperatures 800…830 °С, i.e.,
much over the InSb melting point (525 °С). Depending on
the impurity concentration, time of synthesis was from 20
up to 100 h. To promote the process and ensure uniform
distribution of impurities, a vibratory agitation of the melt
was used sometimes.
We tested the horizontally- and vertically-oriented
crystallization techniques to obtain (InSb)1−x(CdTe)x
crystals from melt. Based on our experimental studies of
SS (InSb)1−x(CdTe)x, we gave preference to the
horizontally-oriented crystallization. The following two
versions of this technique were realized.
(i) Motion of the melt crystallization front was made
automatically by decreasing the temperature in the furnace
using a specially designed programmable facility. The
starting temperature in the reactor was set within the range
from 800 to 900 °С. After the synthesis process came to an
end, the temperature was reduced manually (with the rate
70 to 100 °С/h) down to 540 °С, and then automatically
(with the rate 3…5 °С/h) until the complete melt
crystallization. When the temperature reached
470…480 °С, its further decrease was much faster (40 to
50 °С/h). Our technological experiments showed that this
version is more appropriate for SS with rather low CdTe
content (up to 0.5…1.0 mol. % СdТе in ІnSb).
(ii) According to another version, motion of the
melt crystallization front was made by motion of the
furnace with a preset velocity (0.5…3.0 mm/h) relatively
to the stationary container with the melt. An abrupt
temperature gradient in the crystallization zone was made
using an additional spiral heater.
Fig. 1 shows the scheme of the set-up. A multizone
(3…4 coils) resistance furnace 1 was fixed on a movable
platform 2 that was moved along the rails 3 with a dc motor
4 via a special driving gear. This gear involved a reducer 5
and screw gear (worm gear) 6. A screw gear lead, reduction
coefficient and motor rotation rate were chosen in such a
way to ensure the possibility for velocity variation of
furnace horizontal motion between 0.5 and 10 mm/h.
Application of dc motor made it possible to vary smoothly
the velocity of furnace motion over a preset range.
Fig. 1. Scheme of the set-up for production of SS crystals
using horizontally-oriented crystallization.
An ampoule 10 of a particular shape with weights of
the starting elements was moved in a quartz tube 7 that
was fixed on the same axis as the furnace. The reactor
temperature was monitored with chromel-alumel
thermocouples 8 that were introduced through fireclay
dummy plugs 9. Automatic recording of thermocouple
readings was made using PC. The temperature of the
resistance furnace coils was adjusted and kept with special
electronic devices and autotransformers. To obtain big
temperature gradients (50 to 70 °С/cm) in the
crystallization zone, the latter (right) coil of the furnace
was made as a spiral (coil diameter of 5…6 mm) of
nichrome wire (diameter 1…1.2 mm). The set-up made it
possible to keep temperature in the furnace within 1.5 °С.
When synthesizing SS, the temperature in the
furnace first was increased (with the rate 70…80 °С/h)
up to 550…600 °С, and then was kept at this level during
6 to 7 h, to ensure the diffusion of volatile components
(Сd and Sb) into the melt. Then the temperature in the
synthesis zone was increased up to 850–900 °С, to ensure
an intense interdiffusion of the components and required
degree of SS crystal homogeneity. The time of synthesis
depended on the content of СdТе in SS; it was 10 (48) h at
0.1 (5) mol. % of СdТе in ІnSb.
The temperature distribution in the furnace when
SS crystal growing using the horizontally-oriented
crystallization technique is shown in Fig. 2. The
temperature in the crystallization zone was 3 to 5 °С over
that of crystallization. Then the mechanism of resistance
furnace motion was started and the crystallization process
proper began. Using this technique, we obtained SS
crystals (mass up to 100 g, length up to 11 cm) with CdTe
content up to 5 mol. %.
Fig. 2. Temperature distribution in the furnace.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 80-86.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
82
Table. The concentration and Hall mobility of electrons in the
(InSb)1−x(CdTe)x crystals at the temperature 300 K.
# of the
batch
samples
СdТе
content
in SS
(mol. %)
Electron con-
centrat. (cm-3)
Electron mobility
(cm2V−1s−1)
1
2
3
4
5
6
7
8
9
0
0.0025
0.005
0.0075
0.01
0.02
0.03
0.04
0.05
(7.2±4.6)×1016
(3.5±1.2)×1018
(8.4±7.3)×1018
(6.4±1.9)×10 18
(2.8±0.8)×1018
(7.2±1.6)×1017
(6.6±2.1)×1017
(7.7 ±3.5)×1017
(1.1±0.7)×1019
(2.3±1.1)×104
(6.2±3.8)×103
(8.2±7.1)×103
(3.2±2.4)×103
(3.5±1.8)×103
(6.6±5.4)×103
(6.4±5.2)×103
(6.0±4.3)×103
(8.7±6.3)×102
When applying the above technique, one should
give particular attention to choice of the magnitudes of
the temperature gradient and velocity of furnace motion
relatively to the stationary reactor. It is precisely these
parameters that affect strongly the form of crystallization
front. At a tiny temperature gradient and high velocity of
crystallization front motion, the front becomes bent in the
melt [4]. In this case, the cooling through the side surface
occurs. This leads to spontaneous production of a great
number of crystalline nuclei is a result of the polycry-
stalline growth. The optimal conditions for obtaining
single or polycrystal samples were determined experi-
mentally and were as follows: the temperature gradient of
50…70 °С/cm and velocity of crystallization front motion
υ = 0.1…5 mm/h (it depends strongly on x – the content of
СdТе in SS: the bigger is х, the lower should be υ).
The above crystallization conditions ensure
uniformity in the cross-sections of the grown crystals.
However, some variations of composition along the
sample length still remain. So it is necessary to monitor
the chemical composition and degrees of homogeneity and
uniformity of the ingots. Such monitoring was realized
using the X-ray spectrum analysis, X-ray diffraction
analysis and fine-structural investigations (see [1]). The
results obtained evidence that chemical composition of the
middle part of the ingots is close to that of the starting
weight of elements. The impurity distribution is practically
uniform in the ingot middle part with a length of about
70 % of the total ingot length.
Our investigations of the Hall concentration and
mobility of electrons showed that they vary non-
monotonically with chemical composition of SS at 2–
3 mol. % of СdТе (Table). In the middle part of the ingots
(2/3 length), the departure of the concentration from the
mean value did not exceed 15%, and the crystals were
sufficiently uniform as concerned the Hall concentration of
free charge carriers (electrons).
Fig. 3 demonstrates how the screw dislocation
density in SS depends on the crystal composition and
growth conditions. It was shown that, under the optimal
crystallization conditions, the density D of screw
dislocations in a purposely undoped InSb material was
(3…6)×104 cm−2; however, after addition of СdТе, it grew
abruptly up to ~5×106 cm−2 at the impurity concentration
of 1.5 to 2.5 mol. %. This evidences that the growth of SS
crystal proceeds according to the layer-by-layer and
dislocation mechanisms.
When studying the SS optical properties at room
temperature, we used those crystals whose parameters
are summarized in Table. They were flat plates, with the
diameter from 10 up to 15 mm and thickness of 2 to
3 mm; their surface finish class was no less than 14. The
experimental reflection spectra of SS (taken in the 200 to
4000 cm−1 frequency range) were registered using a
spectrophotometer SPECORD-M80 with a reference
mirror (for experimental procedure see [5]). The reflection
coefficient values were determined within 1…2 %. The
resolving ability in the above frequency range was 1 cm−1.
3. Results of investigations of the optical and electrical
properties of SS (InSb)1−x(CdTe)x and discussion
The results of investigations of electrical and magnetic
properties obtained in [1, 3, 6] indicate a complex
character of interaction of Cd and Те impurity atoms
between themselves, as well as with the InSb lattice.
This leads to a nonmonotonous dependence of the lattice
parameter and both concentration and mobility of charge
carriers on SS chemical composition, as well as to
appearance of peaks in the temperature dependence of
the Hall constant, etc. There exist rather detailed
discussions of investigations of optical and electrical
properties of semiconductors performed using the
nondestructive techniques of IR reflection and
transmission spectroscopies [5, 7]. However, as to SS
(InSb)1−x(CdTe)x crystals, these problems practically
were not considered.
Fig. 4 shows the experimental reflection coefficient
as a function of the energy of incident photons for all the
SS compositions. All the curves (numbers of which
correspond to those in Table) have clearly pronounced
typical minima that correspond to resonance absorption
by free charge carriers. One can see that, as the photon
energy increases, the R(ν) curve approaches 0.36 (for all
the SS compositions), thus deriving for high-frequency
permittivity the value ∞ε = 16.
Fig. 3. Density of screw dislocations as a function of the
CdTe content in SS.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 80-86.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
83
Fig. 4. The reflection coefficient of (InSb)1−x(CdTe)x as a
function of the energy of incident photons at various values of the
CdTe content in SS x: 1 – 0.005; 2 – 0.0075; 3 – 0.01; 4 – 0.02; 5 –
0.03.
According to [1, 7], one can use the dependence
of the position and depth of reflection coefficient
minimum in heavily doped samples on the
concentration, mobility and effective mass of charge
carriers for determination of any of the above three
quantities, if the other two are known. The
concentration and mobility of free charge carriers
(electrons) were determined from the Hall effect. The
charge carrier effective mass ∗m at the Fermi level was
determined from the position of the reflection
coefficient minimum. In our case, the error of ∗m and
∞ε determination did not exceed 10…15 %. The
obtained dependence of the effective mass ∗m at the
Fermi level on the SS chemical composition is shown
in Fig. 5.
By applying the Kane dispersion law, one can
present the relationship between ∗m and electron
concentration n0 as [8]:
( )3/2
0
2
0
0
/1
/
nf
mm
mm
=⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
− ∗
∗
, (1)
where m0 is the free electron mass. Shown in Fig. 6a are
the experimental ( )3/2
0nf curves for various values of
CdTe content х, while Fig. 6b shows the dependence of
the InSb bandgap on the SS composition. For given x, the
∗m values were determined from the concentration
dependences of the reflection coefficient spectra. One can
see from Fig. 6a that the experimental points fall on a
straight line. This fact evidences that the Kane theory is
applicable to all the SS compositions studied.
We determined the optical bandgap value opt
gE for
every SS composition from the lengths cut off by the
straight lines on the ordinate axis (Fig. 6a). The
dependences of the effective mass and optical bandgap on
SS composition are complex and nonmonotonous
(Fig. 6b). The minimal values, т*/т0 = 0.8⋅10−2 and
opt
gE = 0.07 eV, are observed for х = 0.02…0.03.
Fig. 5. The effective mass of charge carriers at the Fermi
level as a function of the CdTe content in SS.
We concluded from the studies performed that the
character of dependences of SS bandgap and effective
mass on the SS composition (x) is determined
unambiguously by the change of the crystal lattice
parameter а(х). As а(х) increases (decreases), then
( )xmm 0/∗ and opt
gE decrease (increase). It was shown in
[1] that the а(х) dependence is determined by complex
character of interaction of dopant atoms between
themselves, as well as with the InSb lattice. For SS
(InSb)1−x(CdTe)x the bandgap is much narrower than in
pure indium antimonide. This fact makes SS more
promising for fabrication of IR photodetectors.
Fig. 6. а) ( )3/2
0
2
0
0
/1
/
nf
mm
mm
=⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
− ∗
∗
at the various values of the
CdTe content in SS х: 1 – 0.0025; 2 – 0.005; 3 – 0.0075; 4 –
0.01; 5 – 0.02; 6 – 0.03; b) InSb bandgap as a function of CdTe
content in SS.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 80-86.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
84
Fig. 7. The Fermi level (relative to the bottom of the
conduction band) as a function of CdTe content in SS.
The т* value can be estimated using the following
expression [1]:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
∗
p
g
E
E
mm
111
0
, (2)
where gE is the direct gap between the edges of allowed
bands, wave functions of which are of different
symmetries (for example, s- and p-symmetry). It was
found experimentally that Ep ≈ 20 eV for all crystals with
diamond- and sphalerite-type lattices [8]. The obtained т*
values are given in Fig. 5. One can see that the ( )xfm =∗
curve follows, on the whole, that of m0(х).
According to [9], the dependence of the effective
mass on the energy may be presented as
( ) ( )opt/21 gEEmEm += ∗∗ . (3)
Knowing the effective mass value at the Fermi level,
one can estimate from Eq. (3) the energy Е = EF
corresponding to it. Using the т*(х) curves (Fig. 5) and
opt
gE value, one can determine the dependence of the
Fermi level position (relatively to the conduction band
edge) in the SS composition (Fig. 7).
Shown in Fig. 8 are the absorption coefficient vs
photon energy curves, ( )ωhk , for four crystals with
various contents of Сd and Те. There are two growing
portions of absorption curves (at high and low energies)
that characterize interband transitions and absorption by
free charge carriers, respectively. Using (at ωh < 0.3 eV)
the known relation pAk λ= between k and wavelength λ
of incident light (the power р depends on the charge
carrier scattering mechanism), one can obtain information
on the predominant scattering mechanism for each alloy.
It follows from Fig. 9 (where the ( )λk curves are
plotted on a log-log scale) that p = 2, 1.94 and 3.5 for x =
0.0025, 0.05 and 0.03, respectively. The р values that are
close to 2 are the characteristics of strongly degenerated
materials. For these SS, earlier it was shown that the
electron mobility does not depend on temperature over the
wide temperature range (100 to 700 K). The values p =
= 3…3.5 are observed, if charge carriers are scattered by
impurity ions. This conclusion was also reached from the
analysis of the temperature dependence of the charge
carrier mobility in 100…400 K temperature range:
αμ T≈ , with α close to 1.5. Thus, the conclusions
concerning the mechanisms of electron scattering in alloys
of various compositions (that were obtained from an
analysis of the temperature dependences of charge carrier
mobility) find their confirmation in the investigation of
( )λk dependence in the region of free charge carrier
absorption.
At higher energies, an analysis of the shape of low-
energy edge of intrinsic absorption is complicated for
several reasons. The most essential of them are as follows.
When the charge carriers are strongly degenerated, then
absorption of light by free charge carriers is superimposed
on that near the low-energy edge; besides, the impurity
states take part in optical transitions. The absorption by
free charge carriers can be approximated from its shape in
the low-energy region and then subtracted from the total
absorption. After this is made, one can see that all the
Fig. 8. Absorption coefficient spectral curves ( )ωhk for
various values of the CdTe content in SS х: 1 – 0.0025; 2 –
0.05; 3 – 0.01; 4 – 0.03.
Fig. 9. Log-log scale dependence of free charge carrier
absorption coefficient k on the light wavelength λ for
various values of CdTe content in SS х: 1 – 0.0025; 2 –
0.05; 3 – 0.01; 4 – 0.03.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 80-86.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
85
studied SS samples demonstrate exponential dependence
of the optical absorption edge on photon energy (Fig. 10)
(when this energy is below the bandgap). The
characteristic energy Е in the expression ( )Ekk /exp0 ωh=
for the absorption coefficient is the same (about 27 meV)
for all SS, i.e., it corresponds to the energy value kT for
300 К.
It is known [10] that the criterion for heavy doping
can be applied to semiconductors, when the mean distance
between the impurity atoms is much less than the Bohr
radius of electron on a donor: Na3 >> 1. Due to
fluctuations of the concentration of charged impurities, the
density of electron states in such materials does not vanish
at the boundaries of allowed bands. It decreases in
accordance with the exponential law when going deeper
into the forbidden band [10]. The investigations of
galvanomagnetic properties of a number of narrow-gap
semiconductors, in particular, strongly compensated,
showed that there is a deep potential relief of the
conduction band bottom due to impurity aggregations.
The depth of the most probable potential wells, γ, is 40 to
50 meV. Such wells have bound electron states. In this
case, ( ) 22/
12
0
2 ≈
−
srmhγ , and electron tunneling through
the barrier seems unlikely. So, such potential barrier may
not lead to optical reduction of bandgap.
At the same time, we observed absorption of
photons with energy deficiency: ωh−=Δ opt
gE > 0. In
[11], it was shown that this is related to the presence of
smaller-scale fluctuations of the charged impurity
concentration. They result in shallow deformations of
potential barriers against the background of deep potential
wells. Due to a big difference between the effective mass
values for electrons and holes in InSb, the electrons will
not feel a shallower potential relief, while the holes
(having a bigger effective mass) can have bound states in
the shallow potential wells. In that case, spatial
distribution of the valence band energy levels follows the
small-scale potential relief, while the conduction band
structure remains unchanged. As a result, the tail of
density of states (that is responsible for absorption of
photons whose energy is below the bandgap) appears at
the valence (and not conduction) band. So, the absorption
curve at ωh < opt
gE may be determined by electron
transitions from the valence band tail to the conduction
band bottom (distorted with the presence of impurities).
Now let us consider the curves of edge absorption in
SS samples when ωh < opt
gE . In this case, because the
electron gas is strongly degenerated, the optical transitions
from the valence band tail are to the levels lying deeply in
the conduction band (near the Fermi level). At the
temperature 300 K, the characteristic energy that enters
into the exponential law for absorption edge is determined
by the Fermi distribution of empty levels in the
conduction band and is about 27 meV, i.e., the kT value at
300 К.
4. Conclusions
Our technological, physical-and-chemical and physical
investigations demonstrated that the most promising
manufacturing technique for perfect (InSb)1−x(CdTe)x
crystals (х < 0.05) is horizontal-oriented crystallization of
melt with controlled pressure of volatile components. It
was found that synthesis and melt crystallization should
be combined in a single technological process. This makes
it possible to decrease both the departure of SS chemical
composition from the stoichiometric one and
contamination with uncontrolled impurities. The required
degrees of sample homogeneity and uniformity are
obtained at high (800…830 °С) temperatures and long-
term (up to 100 h) synthesis. Single-crystalline growth is
ensured only by application of low (0.5…3.0 mm/h)
velocities of crystallization front motion and rather high
(50…70 K/cm) values of the crystallization temperature
gradient.
A correlation was found between the dependences of
the crystal lattice parameter, effective masses of charge
carriers and optical bandgap on the chemical composition
of SS crystals. A complicated character of these
dependences is due to the features of interaction of the
introduced impurity atoms between themselves and with
the InSb lattice.
The principal parameters of SS band structure
(optical bandgap and charge carrier effective mass at both
the Fermi level and conduction band bottom) were
determined for the samples with different contents of Cd
and Те. It was shown that opt
gE = 0.07 eV, when the
composition х = 0.02, so the corresponding alloys are
promising for fabrication of IR detectors.
Fig. 10. Semilog scale spectral dependence of the
absorption coefficient k for various values of CdTe content
in SS х: 1 – 0.0025; 2 – 0.05; 4 – 0.03.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 80-86.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
86
An analysis of the intrinsic absorption edge in SS
samples was performed. It was shown that, at small values
of the absorption coefficient k, the light absorption is due
to transitions between the tails of density of valence band
states and the conduction band bottom. An assumption as
to the possibility of formation in SS of neutral complexes
that involve impurity atoms and those of the host lattice
was confirmed. The free electron scattering mechanisms
in SS of different compositions were determined.
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