Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range
The possibility to create uncooled photodetector (PD) in the region close to l = 10 μm being based on p(Pb₁₋xSnxSe)-n(CdSe) heterojunction has been conceptually and practically confirmed. Design and technology of uncooled thin-film PD based on Pb Sn Se₁₋x p -n(CdSe) heterojunction in which broad-...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2014
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| Cite this: | Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range / Ya.I. Lepikh, I.A. Ivanchenko, L.M. Budiyanskaya // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 408-411. — Бібліогр.: 10 назв. — англ. |
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irk-123456789-1184292017-05-31T03:08:01Z Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range Lepikh, Ya.I. Ivanchenko, I.A. Budiyanskaya, L.M. The possibility to create uncooled photodetector (PD) in the region close to l = 10 μm being based on p(Pb₁₋xSnxSe)-n(CdSe) heterojunction has been conceptually and practically confirmed. Design and technology of uncooled thin-film PD based on Pb Sn Se₁₋x p -n(CdSe) heterojunction in which broad-band CdSe layer is located on the illuminated surface and plays the role of the optical filter with respect to the lower layer of ternary compound. The PD spectral characteristics at room temperature have been researched, which confirms the photoactivity of both heterojunction layers. The mechanism of current flow in the PD structure based on the above heterojunction and the mechanism of the PD samples sensitivity at room temperature in the far infrared spectrum, the determining factor of which is the amount of wide-gap semiconductors where space charge-limited current appears, have been investigated. The uncooled PD detectability typical for polycrystalline structures 106 …107 сm*Hz¹/²/W has been discovered. 2014 Article Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range / Ya.I. Lepikh, I.A. Ivanchenko, L.M. Budiyanskaya // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 408-411. — Бібліогр.: 10 назв. — англ. 1560-8034 PACS 07.57.Kp, 73.40.-c http://dspace.nbuv.gov.ua/handle/123456789/118429 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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English |
| description |
The possibility to create uncooled photodetector (PD) in the region close to
l = 10 μm being based on p(Pb₁₋xSnxSe)-n(CdSe) heterojunction has been conceptually
and practically confirmed. Design and technology of uncooled thin-film PD based on
Pb Sn Se₁₋x p -n(CdSe) heterojunction in which broad-band CdSe layer is located on
the illuminated surface and plays the role of the optical filter with respect to the lower
layer of ternary compound. The PD spectral characteristics at room temperature have
been researched, which confirms the photoactivity of both heterojunction layers. The
mechanism of current flow in the PD structure based on the above heterojunction and
the mechanism of the PD samples sensitivity at room temperature in the far infrared
spectrum, the determining factor of which is the amount of wide-gap semiconductors
where space charge-limited current appears, have been investigated. The uncooled PD
detectability typical for polycrystalline structures 106
…107 сm*Hz¹/²/W has been
discovered. |
| format |
Article |
| author |
Lepikh, Ya.I. Ivanchenko, I.A. Budiyanskaya, L.M. |
| spellingShingle |
Lepikh, Ya.I. Ivanchenko, I.A. Budiyanskaya, L.M. Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Lepikh, Ya.I. Ivanchenko, I.A. Budiyanskaya, L.M. |
| author_sort |
Lepikh, Ya.I. |
| title |
Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range |
| title_short |
Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range |
| title_full |
Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range |
| title_fullStr |
Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range |
| title_full_unstemmed |
Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range |
| title_sort |
uncooled р(pb₁₋xsnxse)-n(cdse) heterostructure-based photodetector for the far infrared spectral range |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2014 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118429 |
| citation_txt |
Uncooled р(Pb₁₋xSnxSe)-n(CdSe) heterostructure-based photodetector for the far infrared spectral range / Ya.I. Lepikh, I.A. Ivanchenko, L.M. Budiyanskaya // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 408-411. — Бібліогр.: 10 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT lepikhyai uncooledrpb1xsnxsencdseheterostructurebasedphotodetectorforthefarinfraredspectralrange AT ivanchenkoia uncooledrpb1xsnxsencdseheterostructurebasedphotodetectorforthefarinfraredspectralrange AT budiyanskayalm uncooledrpb1xsnxsencdseheterostructurebasedphotodetectorforthefarinfraredspectralrange |
| first_indexed |
2025-07-08T13:58:15Z |
| last_indexed |
2025-07-08T13:58:15Z |
| _version_ |
1837087427517218816 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 408-411.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
408
PACS 07.57.Kp, 73.40.-c
Uncooled р(Pb1–xSnxSe)n(CdSe) heterostructure-based
photodetector for the far infrared spectral range
Ya.I. Lepikh, I.A. Ivanchenko, L.M. Budiyanskaya
I.I. Mechnikov Odessa National University,
2, Dvoryanskaya str., 65082 Odessa, Ukraine
Phone/fax: +38(048) 723-34-61, e-mail: ndl_lepikh@onu.edu.ua
Abstract. The possibility to create uncooled photodetector (PD) in the region close to
= 10 μm being based on p(Pb1xSnxSe)n(CdSe) heterojunction has been conceptually
and practically confirmed. Design and technology of uncooled thin-film PD based on
SeSnPb xx1p n(CdSe) heterojunction in which broad-band CdSe layer is located on
the illuminated surface and plays the role of the optical filter with respect to the lower
layer of ternary compound. The PD spectral characteristics at room temperature have
been researched, which confirms the photoactivity of both heterojunction layers. The
mechanism of current flow in the PD structure based on the above heterojunction and
the mechanism of the PD samples sensitivity at room temperature in the far infrared
spectrum, the determining factor of which is the amount of wide-gap semiconductors
where space charge-limited current appears, have been investigated. The uncooled PD
detectability typical for polycrystalline structures 106…107 сmHz1/2/W has been
discovered.
Keywords: infrared photodetector, heterostructures, heterojunction, energy band
diagram, detectability.
Manuscript received 14.01.14; revised version received 23.07.14; accepted for
publication 29.10.14; published online 10.11.14.
1. Introduction
In the far infrared (IR) spectral range ( 10 μm), the
highest sensitivity is shown by extrinsic silicon and
germanium photodetectors (PD) and also intrinsic PD
based on HgCdTe and PbSnTe narrow-gap solid
solutions (alloys) cooled to liquid helium and liquid
nitrogen temperatures with the detectability D* ≈
(3…5)∙1010 сmHz1/2/W [1]. Since application of cooled
photodetectors is complicated and not always desirable,
for example, as indicator sensors, the uncooled
photodetector creation in this range of the spectrum is
quite topical.
Another intrinsic narrow-gap semiconductor is
SeSnPb xx1 . Electrical properties of SeSnPb xx1 alloy
showed the dependence of this phenomenon on the
temperature resistance [2]. Within the temperature range
25 to 180 K, the resistance changes linearly for alloy
with x ≈ 0.25. When temperature exceeds the value
180 K, there takes place a transition from the linear
dependence to the nonlinear one, and this temperature is
close to the temperature of band inversion ТВ . Тhe
inversion temperature does not depend on the carrier
concentration in the alloy, but changes significantly with
the change in alloy composition.
Eg dependence of Pb1–xSnxSe alloy on composition
and temperature suggested by Strauss [3] has the
following form
xTEg 0.89104.50.13eV)( 4 . (1)
According to (1), band inversion is observed at the
temperatures 77, 195, 300 K and x = 0.19, 0.25, 0.30,
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 408-411.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
409
Table.
Parameter Pb1–xSnxSe CdSe
band-gap Eg, eV 0.12 1.67 [6]
electron affinity χ, eV 4.85 (calcul.) 4.95 [6]
work function φ, eV 4.97 (experim.)
lattice type cubic [6] hexagonal [7]
respectively. Thus, creation of intrinsic uncooled PD for
λ = 10 μm is possible. Heterojunction based on
SeSnPb xx1 with x = 0.05 showed the maximum
photosensitivity near λ ~ 10 μm at the temperature of
liquid nitrogen [4].
The proposed IR photodetecting structure is based
on SeSnPb xx1p −n(CdSe) thin-film heterojunction.
This heterojunction belongs to the anisotype ones
with energy band profiles that can be described within
the Anderson model that ignores the states at the
boundary [5].
To construct the energy band diagram of
SeSnPb xx1p -n(CdSe) heterojunction shown in Fig. 1,
the constituent semiconductor parameters listed in Table
were used. In Fig. 1, the Pb1–xSnxSe parameters are
denoted by the index 1, and CdSe – by the index 2.
For Pb1–xSnxSe ternary compound Eg = 0.12 eV
was adopted. Pb1–xSnxSe electron affinity is obtained by
calculation based on the work function φ = 4.97 eV
measured using contact potential difference method and
the assumptions that the energy of the valence band top
Ev is close to the Fermi level energy EF in narrow-gap
semiconductor [5]
eV85.4 gE .
EF
Ec
Ev
Ev
Ec
n(CdSe)p(Pb1-xSnxSe)
ΔEv
ΔEc
φ1
χ1
χ2 φ2
VD
Eg1
Eg2
Fig. 1. р(Pb1–xSnxSe)n(CdSe) heterojunction energy band
diagram.
In accordance with the type system proposed in [5],
the expected band diagram of p(Pb1xSnxSe)n(CdSe)
heterojunction refers to the type II, in which the narrow-
gap semiconductor electron affinity is lower than in the
wide-gap ones. An example of the type II heterojunction
and an analog of the proposed one with significant
difference in the crystal structure is p(ZnTe)-n(CdSe)
heterojunction formed also by semiconductors with
hexagonal and cubic crystal lattice [6].
According to Table, the energy bands are broken at
the boundary of the heterojunction
eV10.021 cE ,
eV65.12211 ggv EEE ,
and the contact potential difference
eV02.021 DV .
In the heterojunction on each side of the interface,
the depleted layers that make up the space-charge region
were formed. Their thickness is determined using the
concentration of majority charge carriers and the barrier
height for them. At the layers boundary in this
heterojunction type, composed with semiconductors with
large Eg, there is a significant difference between the
crystal lattice type, the surface states of acceptor type
that determines the presence of the barrier in CdSe are
formed. Since the barrier height for electrons is much
lower than that for holes, and the donor concentration in
CdSe is much lower than the acceptor concentration in
the layer of narrow-gap Pb1–xSnxSe, then a depleted
layer is almost entirely located in CdSe. Thus, the
determining factor in the mechanism of current flow in
the structure is the amount of wide-gap semiconductors,
in which the space charge-limited current appears
(SCLC).
The absorption of infrared radiation falling onto the
heterojunction of the CdSe side is as follows.
The infrared radiation passes through a layer of
wide-gap CdSe, which plays the role of the optical
window, and excites an electron-hole pairs in the lower
layer of narrow-gap Pb1–xSnxSe.
The PD construction based on p(Pb1xSnxSe)
n(CdSe) heterojunction is shown in Fig. 2. There is
CdSe (Eg = 1.7 eV) wide-gap layer located at the
illuminated surface and it serves as the optical narrow-
band filter with respect to the lower layer of
SeSnPb xx1 (Eg = 0.12 eV) semiconductor. There are
two ohmic contacts attached to both layers. The sample
has a square shape formed by the intersection of the
narrow- and wide-gap layers with an active area of
221 cm10...10 . The dark resistance of the CdSe high-
resistance layer is 105...106 Ohm, and that one of
SeSnPb xx1 low-resistance layer is 10...103 Ohm. The
optimum layers thickness was 1...3 μm for CdSe and
1 μm for Pb1–xSnxSe.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 408-411.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
410
1
2
3
4
3
2
1
4
Fig. 2. PD sample construction: 1 – dielectric substrate, 2 –
PbSnSe layer, 3 – CdSe layer, 4 – ohmic contacts.
Technological difficulties in manufacturing PD
experimental samples are related to the need to obtain
film layers of the Pb1–xSnxSe solid solution of certain
stoichiometric composition, the initial components of
which, at relatively close to the melting point are very
different in temperature and evaporation rate.
SeSnPb xx1 films, obtained by thermal evaporation in
vacuum of component compositions from three different
sources, and the powder mixture components from one
source have shown considerable heterogeneity in
composition. The best results are achieved when using
evaporation of the Pb1–xSnxSe synthesized poly-
crystalline ingots with subsequent heat treatment of the
films [7]. CdSe films production in vacuum flow sheet,
similar to most A2B6 compounds, is described in [8].
The main proof of the heterojunction and
photoactivity operation of both layers are the PD spectral
characteristics (Fig. 3) measured across
SeSnPb xx1p −n(CdSe) structure and along CdSe top
layer. The main peaks near 0.8 and 10.6 μm correspond
to intrinsic absorption in CdSe and Pb1–xSnxSe layers,
respectively. The rise of the sensitivity around 3.0 μm
seems to be related with absorption in PbSe binary
compound, and around 8.0 μm it is connected with
absorption in the ternary Pb1–xSnxSe alloy with deviation
in the stoichiometric composition.
Investigation of the current flow mechanism in the
photodetector structure was carried out using the
current-voltage characteristics (CVC) of the samples
measured within the temperature range °C90...40
(Figs 5 and 6).
The presence of ohmic and blocking contacts in the
structure of p(Pb1xSnxSe)n(CdSe) causes the effect of
straightening. Straightening the direct and reverse CVC
branches shown in double logarithmic coordinates
indicates exponential dependence of current on the
voltage I ~ U n, where n is the angle of the slope of the
nUI lglg plot. The direct CVC branch (Fig. 5)
straightens at room and high temperatures with a slope
close to 1, i.e. I ~ U 2, and CVC is linear. At lower
temperatures, CVC is divided into two sections with
different slopes. The initial part (at low voltages) is close
to linear, while in the second section I ~ U 2. A similar
distinction between linear and quadratic parts occurs in
the reverse CVC branch in the entire temperature range
(Fig. 5). Straightening the I (U) dependence in both
branches and the presence of two straightened sections –
linear and quadratic – are typical for the SCLC
mechanism in the film structure [9].
Measurement of the PD threshold parameters in the
far-infrared spectrum was carried out using a CO2 laser
LG-15, emitting at λ = 10.6 μm with a power up to
35 W. Laser radiation was regulated by the frequency fm
with the mechanical modulator and the power Fl – with
the grid attenuator.
Fig. 3. р(Pb1-xSnxSe)-n(CdSe) heterojunction photodetectors
spectral characteristics at room temperature: ● across the
structure, ▲ along CdSe upper layer.
Fig. 4. CVC direct branches at various temperatures.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 408-411.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
411
Fig. 5. CVC reverse branches at various temperatures.
Proceeding from the necessity of exposure to the
PD entire surface, its area SPD is smaller than cross-
sectional area of the laser beam Sl. And the luminous
flux F illuminating PD can be expressed as
l
l
PD F
S
S
F , (2)
where Fl is the luminous flux at a certain laser
attenuator.
The threshold flux Fp in a single band of an
amplifier Δfa is defined as [10]
ans
p
fUU
F
F
1
, (3)
where Us – signal voltage, Un – voltage noise.
With regard to (2), the working formula to
calculate Fp has the form
21HzW
1
ansl
lPD
p
fUUS
FS
F
. (4)
The specific threshold flux Fp referred to PD unit
area
21HzcmW
1
ansl
PDl
p
fUUS
SF
F .
The specific detectability D* as a parameter
characterizing PD regardless of the sensing element area
and of the amplifier channel properties is reverse to Fp
WHzcm
1 21
pF
D . (5)
For the sample series characterized by SPD =
221 cm10...10 , Us and Un = V10 4 , with fm = 50 Hz
and fa = 1 MHz, calculated from equations (4) and (5)
are values Fp = 2/175 HzW10...10 and, respectively,
D* = 106...107 cm∙Hz1/2/W. The obtained values of the
threshold parameters are satisfactory for polycrystalline
structures.
Conclusions
1. Design of the thin-film uncooled PD based on
p(Pb1xSnxSe)n(CdSe) heterojunction has been
developed.
2. The spectral characteristics of PD at room
temperature containing the main peaks in the CdSe
and Pb1xSnxSe areas of intrinsic photoconductivity
have been researched.
3. The mechanism of the samples sensitivity in the far
infrared spectral region corresponding to the SCLC
mechanism has been researched.
4. Photodetector detectability reaches D* =
106...107 cm∙Hz1/2/W, which is typical for
polycrystalline structures.
References
1. F.F. Sizov, Photoelectronics for Vision Systems in
“Invisible” Spectral Ranges. Akademperiodika,
Kyiv, 2008 (in Russian).
2. G.F. Hoff, J.R. Dixon // Solid State Communs.
10(5), p. 433-437 (1972).
3. A.J. Strauss // Phys. Appl. 157(3), p. 608-611 (1967).
4. S.P. Chaschin, T.L. Safian, N.S. Baryshev, I.S.
Averyanov, N.P. Markina, Photosensitive p-n
heterojunctions in Pb1-xSnxSe-PbS system // Fizika
tekhnika poluprovodnikov, 6(5), p. 969 (1972), in
Russian.
5. B.L. Sharma, R.K. Purokhit, Semiconductor
Heterojunctions. Sov. Radio, Moscow, 1979 (in
Russian).
6. A. Millns, D. Feucht, Heterojunctions and Metal
Semiconductor Junctions. Mir, Moscow, 1975 (in
Russian).
7. N.M. Bondar, V.M. Zheludkov, I.A. Ivanchenko,
I.Ya. Shapiro, Uncooled photodetectors for the far
infra-red range based on Pb1-xSnxSe. Abstracts of
works of the 1-st Soviet conference on the
atmosphere optics, Tomsk, June 23-25, 1986,
Part II.
8. Y.F. Waksman, L.M. Budiyanskaya, I.A. Ivan-
chenko, Colour-detecting photodetector based on
thin film heterojunction p(Cu2O)-n(CdS) with
adjustable spectral sensitivity coordinate //
Photonics, 12, p. 76-79 (2003).
9. M.I. Elinson, G.V. Stepanov, P.I. Perlov,
V.I. Pokalyakin, Compilation of Works. Problems
of Film Electronics. Sov. Radio, Moscow, 1966 (in
Russian).
10. G.G. Ishanin, E.D. Pankov, A.L. Andreev,
G.V. Polschikov, Radiation Sources and Detectors.
Politekhnika, St.-Petersburg, 1991 (in Russian).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 408-411.
PACS 07.57.Kp, 73.40.-c
Uncooled р(Pb1–xSnxSe)(n(CdSe) heterostructure-based photodetector for the far infrared spectral range
Ya.I. Lepikh, I.A. Ivanchenko, L.M. Budiyanskaya
I.I. Mechnikov Odessa National University,
2, Dvoryanskaya str., 65082 Odessa, Ukraine
Phone/fax: +38(048) 723-34-61, e-mail: ndl_lepikh@onu.edu.ua
Abstract. The possibility to create uncooled photodetector (PD) in the region close to ( = 10 μm being based on p(Pb1(xSnxSe)(n(CdSe) heterojunction has been conceptually and practically confirmed. Design and technology of uncooled thin-film PD based on
(
)
Se
Sn
Pb
x
x
1
-
p
(n(CdSe) heterojunction in which broad-band CdSe layer is located on the illuminated surface and plays the role of the optical filter with respect to the lower layer of ternary compound. The PD spectral characteristics at room temperature have been researched, which confirms the photoactivity of both heterojunction layers. The mechanism of current flow in the PD structure based on the above heterojunction and the mechanism of the PD samples sensitivity at room temperature in the far infrared spectrum, the determining factor of which is the amount of wide-gap semiconductors where space charge-limited current appears, have been investigated. The uncooled PD detectability typical for polycrystalline structures 106…107 сm(Hz1/2/W has been discovered.
Keywords: infrared photodetector, heterostructures, heterojunction, energy band diagram, detectability.
Manuscript received 14.01.14; revised version received 23.07.14; accepted for publication 29.10.14; published online 10.11.14.
1. Introduction
In the far infrared (IR) spectral range (( ( 10 μm), the highest sensitivity is shown by extrinsic silicon and germanium photodetectors (PD) and also intrinsic PD based on HgCdTe and PbSnTe narrow-gap solid solutions (alloys) cooled to liquid helium and liquid nitrogen temperatures with the detectability D* ≈ (3…5)∙1010 сm(Hz1/2/W [1]. Since application of cooled photodetectors is complicated and not always desirable, for example, as indicator sensors, the uncooled photodetector creation in this range of the spectrum is quite topical.
Another intrinsic narrow-gap semiconductor is
Se
Sn
Pb
x
x
1
-
. Electrical properties of
Se
Sn
Pb
x
x
1
-
alloy showed the dependence of this phenomenon on the temperature resistance [2]. Within the temperature range 25 to 180 K, the resistance changes linearly for alloy with x ≈ 0.25. When temperature exceeds the value 180 K, there takes place a transition from the linear dependence to the nonlinear one, and this temperature is close to the temperature of band inversion ТВ . Тhe inversion temperature does not depend on the carrier concentration in the alloy, but changes significantly with the change in alloy composition.
Eg dependence of Pb1–xSnxSe alloy on composition and temperature suggested by Strauss [3] has the following form
x
T
E
g
0.89
10
4.5
0.13
eV)
(
4
-
×
+
=
-
.
(1)
According to (1), band inversion is observed at the temperatures 77, 195, 300 K and x = 0.19, 0.25, 0.30,
Table.
Parameter
Pb1–xSnxSe
CdSe
band-gap Eg, eV
0.12
1.67 [6]
electron affinity χ, eV
4.85 (calcul.)
4.95 [6]
work function φ, eV
4.97 (experim.)
lattice type
cubic
[6]
hexagonal [7]
respectively. Thus, creation of intrinsic uncooled PD for λ = 10 μm is possible. Heterojunction based on
Se
Sn
Pb
x
x
1
-
with x = 0.05 showed the maximum photosensitivity near λ ~ 10 μm at the temperature of liquid nitrogen [4].
The proposed IR photodetecting structure is based on
(
)
Se
Sn
Pb
x
x
1
-
p
−n(CdSe) thin-film heterojunction.
This heterojunction belongs to the anisotype ones with energy band profiles that can be described within the Anderson model that ignores the states at the boundary [5].
To construct the energy band diagram of
(
)
Se
Sn
Pb
x
x
1
-
p
-n(CdSe) heterojunction shown in Fig. 1, the constituent semiconductor parameters listed in Table were used. In Fig. 1, the Pb1–xSnxSe parameters are denoted by the index 1, and CdSe – by the index 2.
For Pb1–xSnxSe ternary compound Eg = 0.12 eV was adopted. Pb1–xSnxSe electron affinity is obtained by calculation based on the work function φ = 4.97 eV measured using contact potential difference method and the assumptions that the energy of the valence band top Ev is close to the Fermi level energy EF in narrow-gap semiconductor [5]
eV
85
.
4
=
-
j
=
c
g
E
.
E
F
E
c
E
v
E
v
E
c
n(CdSe)
p(Pb
1-x
Sn
x
Se)
ΔE
v
ΔE
c
φ
1
χ
1
χ
2
φ
2
V
D
E
g1
E
g2
Fig. 1. р(Pb1–xSnxSe)(n(CdSe) heterojunction energy band diagram.
In accordance with the type system proposed in [5], the expected band diagram of p(Pb1(xSnxSe)(n(CdSe) heterojunction refers to the type II, in which the narrow-gap semiconductor electron affinity is lower than in the wide-gap ones. An example of the type II heterojunction and an analog of the proposed one with significant difference in the crystal structure is p(ZnTe)-n(CdSe) heterojunction formed also by semiconductors with hexagonal and cubic crystal lattice [6].
According to Table, the energy bands are broken at the boundary of the heterojunction
eV
10
.
0
2
1
=
c
-
c
=
D
c
E
,
(
)
(
)
eV
65
.
1
2
2
1
1
=
+
c
-
+
c
=
D
g
g
v
E
E
E
,
and the contact potential difference
eV
02
.
0
2
1
=
j
-
j
=
D
V
.
In the heterojunction on each side of the interface, the depleted layers that make up the space-charge region were formed. Their thickness is determined using the concentration of majority charge carriers and the barrier height for them. At the layers boundary in this heterojunction type, composed with semiconductors with large Eg, there is a significant difference between the crystal lattice type, the surface states of acceptor type that determines the presence of the barrier in CdSe are formed. Since the barrier height for electrons is much lower than that for holes, and the donor concentration in CdSe is much lower than the acceptor concentration in the layer of narrow-gap Pb1–xSnxSe, then a depleted layer is almost entirely located in CdSe. Thus, the determining factor in the mechanism of current flow in the structure is the amount of wide-gap semiconductors, in which the space charge-limited current appears (SCLC).
The absorption of infrared radiation falling onto the heterojunction of the CdSe side is as follows.
The infrared radiation passes through a layer of wide-gap CdSe, which plays the role of the optical window, and excites an electron-hole pairs in the lower layer of narrow-gap Pb1–xSnxSe.
The PD construction based on p(Pb1(xSnxSe)(n(CdSe) heterojunction is shown in Fig. 2. There is CdSe (Eg = 1.7 eV) wide-gap layer located at the illuminated surface and it serves as the optical narrow-band filter with respect to the lower layer of
Se
Sn
Pb
x
x
1
-
(Eg = 0.12 eV) semiconductor. There are two ohmic contacts attached to both layers. The sample has a square shape formed by the intersection of the narrow- and wide-gap layers with an active area of
2
2
1
cm
10
...
10
-
-
. The dark resistance of the CdSe high-resistance layer is 105...106 Ohm, and that one of
Se
Sn
Pb
x
x
1
-
low-resistance layer is 10...103 Ohm. The optimum layers thickness was 1...3 μm for CdSe and 1 μm for Pb1–xSnxSe.
1
2
3
4
3
2
1
4
Fig. 2. PD sample construction: 1 – dielectric substrate, 2 – PbSnSe layer, 3 – CdSe layer, 4 – ohmic contacts.
Technological difficulties in manufacturing PD experimental samples are related to the need to obtain film layers of the Pb1–xSnxSe solid solution of certain stoichiometric composition, the initial components of which, at relatively close to the melting point are very different in temperature and evaporation rate.
Se
Sn
Pb
x
x
1
-
films, obtained by thermal evaporation in vacuum of component compositions from three different sources, and the powder mixture components from one source have shown considerable heterogeneity in composition. The best results are achieved when using evaporation of the Pb1–xSnxSe synthesized poly-crystalline ingots with subsequent heat treatment of the films [7]. CdSe films production in vacuum flow sheet, similar to most A2B6 compounds, is described in [8].
The main proof of the heterojunction and photoactivity operation of both layers are the PD spectral characteristics (Fig. 3) measured across
(
)
Se
Sn
Pb
x
x
1
-
p
−n(CdSe) structure and along CdSe top layer. The main peaks near 0.8 and 10.6 μm correspond to intrinsic absorption in CdSe and Pb1–xSnxSe layers, respectively. The rise of the sensitivity around 3.0 μm seems to be related with absorption in PbSe binary compound, and around 8.0 μm it is connected with absorption in the ternary Pb1–xSnxSe alloy with deviation in the stoichiometric composition.
Investigation of the current flow mechanism in the photodetector structure was carried out using the current-voltage characteristics (CVC) of the samples measured within the temperature range
°C
90
...
40
-
(Figs 5 and 6).
The presence of ohmic and blocking contacts in the structure of p(Pb1(xSnxSe)(n(CdSe) causes the effect of straightening. Straightening the direct and reverse CVC branches shown in double logarithmic coordinates indicates exponential dependence of current on the voltage I ~ U n, where n is the angle of the slope of the
(
)
n
U
I
lg
lg
plot. The direct CVC branch (Fig. 5) straightens at room and high temperatures with a slope close to 1, i.e. I ~ U 2, and CVC is linear. At lower temperatures, CVC is divided into two sections with different slopes. The initial part (at low voltages) is close to linear, while in the second section I ~ U 2. A similar distinction between linear and quadratic parts occurs in the reverse CVC branch in the entire temperature range (Fig. 5). Straightening the I (U) dependence in both branches and the presence of two straightened sections – linear and quadratic – are typical for the SCLC mechanism in the film structure [9].
Measurement of the PD threshold parameters in the far-infrared spectrum was carried out using a CO2 laser LG-15, emitting at λ = 10.6 μm with a power up to 35 W. Laser radiation was regulated by the frequency fm with the mechanical modulator and the power Fl – with the grid attenuator.
Fig. 3. р(Pb1-xSnxSe)-n(CdSe) heterojunction photodetectors spectral characteristics at room temperature: ● ( across the structure, ▲ ( along CdSe upper layer.
Fig. 4. CVC direct branches at various temperatures.
Fig. 5. CVC reverse branches at various temperatures.
Proceeding from the necessity of exposure to the PD entire surface, its area SPD is smaller than cross-sectional area of the laser beam Sl. And the luminous flux F illuminating PD can be expressed as
l
l
PD
F
S
S
F
=
,
(2)
where Fl is the luminous flux at a certain laser attenuator.
The threshold flux Fp in a single band of an amplifier Δfa is defined as [10]
a
n
s
p
f
U
U
F
F
D
×
=
1
,
(3)
where Us – signal voltage, Un – voltage noise.
With regard to (2), the working formula to calculate Fp has the form
[
]
2
1
Hz
W
1
a
n
s
l
l
PD
p
f
U
U
S
F
S
F
D
×
=
.
(4)
The specific threshold flux Fp referred to PD unit area
[
]
2
1
Hz
cm
W
1
×
D
×
=
*
a
n
s
l
PD
l
p
f
U
U
S
S
F
F
.
The specific detectability D* as a parameter characterizing PD regardless of the sensing element area and of the amplifier channel properties is reverse to Fp
[
]
W
Hz
cm
1
2
1
×
=
*
*
p
F
D
.
(5)
For the sample series characterized by SPD =
2
2
1
cm
10
...
10
-
-
, Us and Un =
V
10
4
-
, with fm = 50 Hz and fa = 1 MHz, calculated from equations (4) and (5) are values Fp =
2
/
1
7
5
Hz
W
10
...
10
-
-
and, respectively, D* = 106...107 cm∙Hz1/2/W. The obtained values of the threshold parameters are satisfactory for polycrystalline structures.
Conclusions
1. Design of the thin-film uncooled PD based on p(Pb1(xSnxSe)(n(CdSe) heterojunction has been developed.
2. The spectral characteristics of PD at room temperature containing the main peaks in the CdSe and Pb1(xSnxSe areas of intrinsic photoconductivity have been researched.
3. The mechanism of the samples sensitivity in the far infrared spectral region corresponding to the SCLC mechanism has been researched.
4.
Photodetector detectability reaches D* = 106...107 cm∙Hz1/2/W, which is typical for polycrystalline structures.
References
1.
F.F. Sizov, Photoelectronics for Vision Systems in “Invisible” Spectral Ranges. Akademperiodika, Kyiv, 2008 (in Russian).
2.
G.F. Hoff, J.R. Dixon // Solid State Communs. 10(5), p. 433-437 (1972).
3.
A.J. Strauss // Phys. Appl. 157(3), p. 608-611 (1967).
4.
S.P. Chaschin, T.L. Safian, N.S. Baryshev, I.S. Averyanov, N.P. Markina, Photosensitive p-n heterojunctions in Pb1-xSnxSe-PbS system // Fizika tekhnika poluprovodnikov, 6(5), p. 969 (1972), in Russian.
5.
B.L. Sharma, R.K. Purokhit, Semiconductor Heterojunctions. Sov. Radio, Moscow, 1979 (in Russian).
6.
A. Millns, D. Feucht, Heterojunctions and Metal Semiconductor Junctions. Mir, Moscow, 1975 (in Russian).
7.
N.M. Bondar, V.M. Zheludkov, I.A. Ivanchenko, I.Ya. Shapiro, Uncooled photodetectors for the far infra-red range based on Pb1-xSnxSe. Abstracts of works of the 1-st Soviet conference on the atmosphere optics, Tomsk, June 23-25, 1986, Part II.
8.
Y.F. Waksman, L.M. Budiyanskaya, I.A. Ivan-chenko, Colour-detecting photodetector based on thin film heterojunction p(Cu2O)-n(CdS) with adjustable spectral sensitivity coordinate // Photonics, 12, p. 76-79 (2003).
9.
M.I. Elinson, G.V. Stepanov, P.I. Perlov, V.I. Pokalyakin, Compilation of Works. Problems of Film Electronics. Sov. Radio, Moscow, 1966 (in Russian).
10.
G.G. Ishanin, E.D. Pankov, A.L. Andreev, G.V. Polschikov, Radiation Sources and Detectors. Politekhnika, St.-Petersburg, 1991 (in Russian).
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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