Compensation of hole conductivity in CdTe crystals doped with Cr
We present the results of optical and electrophysical investigations of CdTe:Cr crystals. A model explaining a considerable shift of the fundamental absorbtion edge in the crystals into the long-wave region is proposed. It is found that the doping of cadmium telluride crystals with Cr impurity le...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2007
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| Series: | Semiconductor Physics Quantum Electronics & Optoelectronics |
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| Cite this: | Compensation of hole conductivity in CdTe crystals doped with Cr / E.S. Nikonyuk, Z.I. Zakharuk, M.I. Kuchma, M.O. Kovalets, A.I. Rarenko, I.M. Yuriychuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 3. — С. 77-79. — Бібліогр.: 8 назв. — англ. |
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irk-123456789-1181282017-05-29T03:05:10Z Compensation of hole conductivity in CdTe crystals doped with Cr Nikonyuk, E.S. Zakharuk, Z.I. Kuchma, M.I. Kovalets, M.O. Rarenko, A.I. Yuriychuk, I.M. We present the results of optical and electrophysical investigations of CdTe:Cr crystals. A model explaining a considerable shift of the fundamental absorbtion edge in the crystals into the long-wave region is proposed. It is found that the doping of cadmium telluride crystals with Cr impurity leads to the introduction of deep donors with Еv + 0.19…0.32 eV 2007 Article Compensation of hole conductivity in CdTe crystals doped with Cr / E.S. Nikonyuk, Z.I. Zakharuk, M.I. Kuchma, M.O. Kovalets, A.I. Rarenko, I.M. Yuriychuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 3. — С. 77-79. — Бібліогр.: 8 назв. — англ. 1560-8034 PACS 61.72.Vv, 71.55.-i http://dspace.nbuv.gov.ua/handle/123456789/118128 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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We present the results of optical and electrophysical investigations of CdTe:Cr
crystals. A model explaining a considerable shift of the fundamental absorbtion edge in
the crystals into the long-wave region is proposed. It is found that the doping of cadmium
telluride crystals with Cr impurity leads to the introduction of deep donors with
Еv + 0.19…0.32 eV |
| format |
Article |
| author |
Nikonyuk, E.S. Zakharuk, Z.I. Kuchma, M.I. Kovalets, M.O. Rarenko, A.I. Yuriychuk, I.M. |
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Nikonyuk, E.S. Zakharuk, Z.I. Kuchma, M.I. Kovalets, M.O. Rarenko, A.I. Yuriychuk, I.M. Compensation of hole conductivity in CdTe crystals doped with Cr Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Nikonyuk, E.S. Zakharuk, Z.I. Kuchma, M.I. Kovalets, M.O. Rarenko, A.I. Yuriychuk, I.M. |
| author_sort |
Nikonyuk, E.S. |
| title |
Compensation of hole conductivity in CdTe crystals doped with Cr |
| title_short |
Compensation of hole conductivity in CdTe crystals doped with Cr |
| title_full |
Compensation of hole conductivity in CdTe crystals doped with Cr |
| title_fullStr |
Compensation of hole conductivity in CdTe crystals doped with Cr |
| title_full_unstemmed |
Compensation of hole conductivity in CdTe crystals doped with Cr |
| title_sort |
compensation of hole conductivity in cdte crystals doped with cr |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2007 |
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http://dspace.nbuv.gov.ua/handle/123456789/118128 |
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Compensation of hole conductivity in CdTe crystals doped with Cr / E.S. Nikonyuk, Z.I. Zakharuk, M.I. Kuchma, M.O. Kovalets, A.I. Rarenko, I.M. Yuriychuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 3. — С. 77-79. — Бібліогр.: 8 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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AT nikonyukes compensationofholeconductivityincdtecrystalsdopedwithcr AT zakharukzi compensationofholeconductivityincdtecrystalsdopedwithcr AT kuchmami compensationofholeconductivityincdtecrystalsdopedwithcr AT kovaletsmo compensationofholeconductivityincdtecrystalsdopedwithcr AT rarenkoai compensationofholeconductivityincdtecrystalsdopedwithcr AT yuriychukim compensationofholeconductivityincdtecrystalsdopedwithcr |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 3. P. 77-79.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
77
PACS 61.72.Vv, 71.55.-i
Compensation of hole conductivity
in CdTe crystals doped with Cr
E.S. Nikonyuk1, Z.I. Zakharuk2, M.I. Kuchma1, M.O. Kovalets1,
A.I. Rarenko2, I.M. Yuriychuk2
1 National University of Water Management and Conservation
11, Soborna str., 35011 Rivne, Ukraine; phone: (0362)230420, e-mail: semirivne@mail.ru
2 Chernivtsi National University
2, Kotsyubynsky str., 58012 Chernivtsi, Ukraine
Phone: (0372)584875; e-mail: microel@chnu.cv.ua
Abstract. We present the results of optical and electrophysical investigations of CdTe:Cr
crystals. A model explaining a considerable shift of the fundamental absorbtion edge in
the crystals into the long-wave region is proposed. It is found that the doping of cadmium
telluride crystals with Cr impurity leads to the introduction of deep donors with
Еv + 0.19…0.32 eV.
Keywords: CdTe, chromium, doping, mobility, Hall effect, electrical conductivity,
optical transmission.
Manuscript received 16.07.07; accepted for publication 27.09.07; published online 31.10.07.
1. Introduction
Cadmium telluride crystals doped with Cr impurity have
recently attracted considerable interest as a promising
material for high-performance lasers in the middle
infrared range [1] and as a semimagnetic semiconductor
in spintronics [2]. At the same time, there are few works
devoted to the study of optical and electrophysical
properties of CdTe:Cr [3-7], and the problem of the
mechanism of the introduction of Cr impurity into the
CdTe lattice remains to be solved. On the base of earlier
studies [4], it was considered that the doping of CdTe
with Cr does not involve the introduction of electrically
active centers. This conclusion is refuted by the results
in [5], since the reliably observed semiinsulating state of
CdTe-Cr crystals cannot be attributable only to the
presence of uncontrolled impurities. In addition, it
should be taken into account that Cr ions can be in
different charge states [5, 6], as well as that this impurity
can form clusters in CdTe crystals [5].
An isolated chromium atom in the cadmium
telluride lattice should have the electronic configuration
3d4s2 (state Cr2+), which means the absence of impurity
electrical activity [4]. At the same time, the magnetic
studies of CdTe:Cr crystals [5] suppose the existence of
the Cr+ state in the case where the impurity is a part of a
cluster. This provides the possibility to realize a
quasichemical reaction of the donor type:
Cr+→Cr2+ + e−. Since there is no reason to expect that
these donors are shallow, the donor nature of an impurity
can be established only by studying a series of doped
and undoped crystals under similar technological
conditions.
CdTe and CdTe:Cr single crystals were grown
from a charge, prepared from stoichiometric weights of
Cd and Te additionally purified by the zone melting.
Chromium at the concentration С0 = 1018…1019 сm−3
was loaded into a quartz ampoule together with Cd and
Te. Following a long synthesis of the charge, single
crystals were grown by the Bridgman method (the
temperature gradient at the crystallization front was
10…15 K/сm, and the growth rate was 2 mm/h).
The temperature behaviors of the electrical
conductivity (σ) and the Hall constant (RH) were studied
by standard methods on the rectangle-shaped samples in
the temperature range 77…400 K. Samples for
measurements were cut of different parts of doped
ingots. The position of a specimen in the ingot was
characterized by a reduced coordinate g = x/L, where L
is the ingot length, and x is the distance from the
beginning of the ingot. Two pairs of probe contacts
manufactured of the following alloys were used: Cu+In
– for high-resistance specimens of the р-type;
Au+Cu+In, with a preliminary spark treatment of contact
pads – for high-resistance specimens of the p-type;
Cu+(In+Sn) – for low-resistance specimens of the n-
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 3. P. 77-79.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
78
type. The mobility of carriers was calculated by the
formula µ = RHσ, i.e. the Hall factor was assumed equal
to unity. The ionization energies of acceptors or donors
were found from the temperature dependences of RH in
the framework of the model of compensated centers.
For optical measurements, the 1-mm-thick polished
wafers were made. The measurements were performed at
room temperature on the universal spectral computer
system including an MDR-12 monochromator.
The study of the optical transmission at Т = 300 K
showed that the doping with chromium causes a
noticeable displacement of the fundamental absorption
edge into a long-wave region as compared with pure
CdTe depending on the impurity concentration [∆ =
(0.15…0.25) eV in different samples, Fig. 1]. A similar
displacement was observed in [3] at Т = 4 K.
The temperature dependences of the Hall
coefficient of seven samples with different impurity
concentrations С0 in a melt are represented in Fig. 2.
Only one sample (7) has electron conductivity, the rest –
hole conductivity. In the samples with hole conductivity,
the carrier concentration p lies at 300 K in the interval
(2.1011…1014) сm−3 which is several orders less than that
in the undoped crystals, where р = (1015…1016) сm−3.
The energy of ionization centers (εА), which determine
р-conductivity (let’s formally call them acceptors), is
equal to (0.19...0.32) eV and monotonously increases
with decrease in the carrier concentration. The Fermi
level lies well above all the energy levels of acceptors
(for example, for sample 6 with εА = 0.32 eV, the Fermi
level lies at 300 K at Еv + 0.6 eV), which testifies to a
very high degree of compensation of acceptors: k =
[A−]/[A] = (0.99…0.999). It is worth noting the increase
of k with the ionization energy of acceptors, which is not
typical of undoped р-CdTe crystals with casual deep
acceptors.
The above regularities can be understood on the
assumption that the experimentally observed levels at
Ev = (0.19…0.32) eV are caused not by heavily
compensated acceptors, but by weakly compensated
very deep donors (DD) related to the chromium impurity
as a part of a cluster. The donor action of Cr ions is
provided by the quasichemical reaction Cr+→Cr2+ + e−
with the subsequent capture of an electron by acceptors
А1 (Ev + 0.05 eV) and А2 (Ev + 0.13 eV). As long as the
binding energy of an electron must depend on the cluster
structure (the number of impurity ions and intrinsic
defects, mutual arrangement, etc), one should expect a
wide energy spectrum of deep levels (Fig. 3). In a
specific doped specimen, the “acceptor” thermal
transition (T) of an electron from the v-band to the
lowest compensated level takes place. The character of
compensation is determined by the ratio arising under
these conditions between the chromium impurity
concentration in clusters of different configurations,
acceptors A1 and A2, and shallow donors D1
(Еc−0.01 eV) always present in CdTe crystals.
Fig. 1. Optical transmission spectra of CdTe:Cr crystals: 1 –
CdTe; 2 – С0 = 1018 cm−3 (g = 0.25); 3 – С0 = 6.1018 cm−3
(g = 0.3); 4 – С0 = 1019 cm−3 (g = 0.3); 5 – С0 = 1019 cm−3
(g = 0.8).
Let us give the qualitative estimates which will
confirm the possibility to realize the above-described
situation. The total concentrations of acceptors А1 and А2
can be estimated at the level of ~1017 сm−3. As long as
none of the samples exhibits the signs of at least the
beginning of the area of full ionization of the centers at
high temperatures, one can expect (for examples, for
samples 1 and 2) that this area is realized at рsat ≥
1016…1017 сm−3. This gives, with regard for the degree
of compensation of cluster centers, the value within 1017-
1018 сm−3. The concentration of the dissolved chromium
impurity can be of the same or somewhat larger value,
taking into account its high solubility [5] and the fact
that the coefficient of segregation of this impurity in
cadmium telluride is a little less than unity [7].
Fig. 2. Temperature dependences of the Hall coefficient in
CdTe-Cr crystals prepared from different parts of doped
samples: 1, 2 – С0 = 1018 cm−3 (g = 0.3, 0.76); 3 – С0 =
6.1018 cm−3 (g = 0.8); 4, 5 – С0 = 8.1018 cm−3 (g = 0.06, 0.74);
6, 7 – С0 = 1019 cm−3 (g = 0.29, 0.81).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 3. P. 77-79.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
79
Fig. 3. Energy diagram of CdTe-Cr crystals.
The mobility of carriers in р-type samples (Fig. 4)
monotonously increases with decrease in the
temperature. On the one hand, this testifies to the
absence of drift barriers for carriers, that is the absence
of a potential relief caused by the inhomogeneous spatial
distribution of electrically active centers. On the other
hand, the concentration of these centers (NI) (calculated
from the µ = f (T) dependence) does not exceed
5.1017 сm−3, which is in agreement with (NI) = 2([А1] +
+[А2]) ≈ 2.1017 сm−3 (perhaps, except sample 2). At the
same time, the mobility of carriers in the only one
sample of the n-type quickly drops with decrease in the
temperature, which attests to the existence of drift
barriers (εbar ≈ 0.06 eV) caused by the presence of
overlapped space charge areas [8]. In this case, the
impurity clusters in the context of carrier scattering
processes cannot be considered, obviously, as point
formations any longer. In this case, the binding energy
of an electron must be essentially different from that in
the р-type samples. If the above suggestion is correct,
one can state that the binding energy should be reduced
with a complication of the cluster structure, i.e. the
donor level approaches the с-band.
The proposed model allows us to explain the
presence of a powerful optical absorption band (α ≥
102 сm−1) with a long-wave edge at hν = (1.35-1.45) eV
which is observed at 4 К in the samples with С0 = (1018-
1019) сm−3 [3]. This band is due to the optical (О)
transitions of electrons from the uncompensated (filled
with electrons) deep levels to the с-band. Assuming the
equality of thermal and optical ionization energies, the
sum of the energies of optical and thermal transitions
should be somewhat greater than the band gap. Really,
we have (1.35…1.45) eV + (0.32…0.19) eV > Eg =
1.60 eV. In addition, the shifts of the absorption band
edge ∆ = (0.15…0.25) eV at 300 K (Fig. 1) are in
agreement with the activation energies of thermal
“acceptor” transitions εА = (0.19-0.32) eV (taking into
account the thermal reduction of the band gap
=300
gЕ 1.49 eV).
Fig. 4. Temperature dependences of the mobility of carriers in
CdTe-Cr crystals (the numbering of samples as in Fig. 1).
The doping of cadmium telluride crystals with the
Cr impurity is accompanied by the introduction of deep
donors, whose levels lie in the lower half of the
forbidden band gap (Еv + 0.19…0.32 eV). We suppose
that the donor action reveals Cr impurity ions as a part of
impurity clusters according to the quasichemical reaction
Cr+ ⇔ Cr2+ + e− in the forward direction. The opposite
direction of the reaction is realized as the thermal
“acceptor” transitions that determine the temperature
dependence of the concentration of holes.
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vedi, S.W. Kutcher, C.C. Wang, Observation of
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Mater. 31(7), p. 806-810 (2002).
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magnetization in Cr-doped CdTe crystals // Appl.
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