Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures
Analyzed in this paper is the influence of various modifications of substrate surface in n-CdTe single crystals (laser, thermal and photothermal) on electric and photoelectric properties of diode structures based on them.
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2009
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| Назва видання: | Semiconductor Physics Quantum Electronics & Optoelectronics |
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| Цитувати: | Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures / V.P. Makhniy, N.V. Skrypnyk, Yu.N. Boyko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2009. — Т. 12, № 2. — С. 143-146. — Бібліогр.: 12 назв. — англ. |
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nasplib_isofts_kiev_ua-123456789-1186912025-06-03T16:26:27Z Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures Makhniy, V.P. Skrypnyk, N.V. Boyko, Yu.N. Analyzed in this paper is the influence of various modifications of substrate surface in n-CdTe single crystals (laser, thermal and photothermal) on electric and photoelectric properties of diode structures based on them. 2009 Article Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures / V.P. Makhniy, N.V. Skrypnyk, Yu.N. Boyko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2009. — Т. 12, № 2. — С. 143-146. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS 73.40.Cg, Gk, Lq https://nasplib.isofts.kiev.ua/handle/123456789/118691 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Analyzed in this paper is the influence of various modifications of substrate
surface in n-CdTe single crystals (laser, thermal and photothermal) on electric and
photoelectric properties of diode structures based on them. |
| format |
Article |
| author |
Makhniy, V.P. Skrypnyk, N.V. Boyko, Yu.N. |
| spellingShingle |
Makhniy, V.P. Skrypnyk, N.V. Boyko, Yu.N. Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Makhniy, V.P. Skrypnyk, N.V. Boyko, Yu.N. |
| author_sort |
Makhniy, V.P. |
| title |
Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures |
| title_short |
Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures |
| title_full |
Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures |
| title_fullStr |
Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures |
| title_full_unstemmed |
Perspective of surface modification of CdTe single crystal substrate for creation of photosensitive barrier structures |
| title_sort |
perspective of surface modification of cdte single crystal substrate for creation of photosensitive barrier structures |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2009 |
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https://nasplib.isofts.kiev.ua/handle/123456789/118691 |
| citation_txt |
Perspective of surface modification of CdTe single crystal substrate
for creation of photosensitive barrier structures / V.P. Makhniy, N.V. Skrypnyk, Yu.N. Boyko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2009. — Т. 12, № 2. — С. 143-146. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT makhniyvp perspectiveofsurfacemodificationofcdtesinglecrystalsubstrateforcreationofphotosensitivebarrierstructures AT skrypnyknv perspectiveofsurfacemodificationofcdtesinglecrystalsubstrateforcreationofphotosensitivebarrierstructures AT boykoyun perspectiveofsurfacemodificationofcdtesinglecrystalsubstrateforcreationofphotosensitivebarrierstructures |
| first_indexed |
2025-12-01T09:24:21Z |
| last_indexed |
2025-12-01T09:24:21Z |
| _version_ |
1850297353667346432 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2009. V. 12, N 2. P. 143-146.
© 2009, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
143
PACS 73.40.Cg, Gk, Lq
Perspective of surface modification of CdTe single crystal substrate
for creation of photosensitive barrier structures
V.P. Makhniy, N.V. Skrypnyk, Yu.N. Boyko
Yuri Fedkovych Chernivtsi National University, 2, Kotsyubynsky, Chernivtsi 58012, Ukraine
Phone: +38 (037)-2244221, e-mail: Skrup@meta.ua
Abstract. Analyzed in this paper is the influence of various modifications of substrate
surface in n-CdTe single crystals (laser, thermal and photothermal) on electric and
photoelectric properties of diode structures based on them.
Keywords: surface-barrier diode, modificated surface, current-voltage characteristic.
Manuscript received 19.11.08; accepted for publication 18.03.09; published online 20.03.09.
1. Introduction
Cadmium telluride continues to remain one of the most
perspective materials for creation of various devices in
semiconductor electronics, which is caused by the
number of their physical-and-technical parameters. The
relatively wide bandgap provides work of devices at
high temperatures and is optimum for converting sun
energy into the electric one [1]. The high density and
atomic number in combination with high radiation
hardness promote effective and stable work of various
types of CdTe-detectors in a wide range of ionizing
radiation energies.
It is noteworthy that the basis for most of above
devices is rectifying structures, the separate class of
which is represented by surface-barrier diodes (SBDs). It
is caused by a number of their advantages in comparison
with other types of diode structures. The first advantage
consists in relatively simple technology providing
creation of single- and multi-element active structures
with arbitrary topology on mono- or polycrystalline
substrates in a unique cycle. Second, anomalous low
temperatures for deposition of barrier contacts do not
change the parameters of base substrates. Third,
presence of strong electric surface field causes more
effective separation of photocarriers born by high energy
photons. And finally, SBDs can possess lower values of
the serial resistance as compared with that of homo- and
heterojunctions, since one of semiconductor ranges is
eliminated in the first case.
Despite the above advantages of SBDs, the main
problems in their production are to obtain a maximal
height of the potential barrier and provide minimal speed
of surface recombination. Application of the known
technological methods does not result in successful
solution of these tasks, which requires new approaches
and principles.
One of possible ways to solve the above problems
lies in additional special treatment (modification) of
semiconductor substrate surface before creation of the
rectifying contact. In this work, influence of various
types of annealing (thermal, photothermal and laser) on
electric and photoelectric properties of SBDs based on n-
CdTe is analyzed.
2. Technology and methodology
As initial substrates, we used the plates with typical
dimensions 441 mm, which were cut out from the
bulk single-crystals of CdTe with the specific resistance
10 to 20 Ohm·cm at 300 K. Crystals were obtained using
the Bridgmen method growing from melt, non-doped
additionally with any impurities and had electronic
conductivity. Plates passed stage-by-stage mechanical
and chemical polishing in solution of composition
K2Cr2O7:H2O:HNO3 = 4:20:10, and also careful cleaning
in deionized water and final drying. The prepared
substrates had mirror surfaces, in one of which the
conductive indium contacts were infused. Before
creation of the rectifying contact made of semi-
transparant gold layer deposited in vacuum, some
substrates were treated in specific ways described below.
First group of substrates was processed in the water
suspensions of salts of alkaline metals and, in what
follows, marked as CdTe:AS [8]. Substrates from second
and third groups were treated by laser annealing
(CdTe:L) and thermal annealing in air, i.e., in molecular
oxygen (CdTe:O2) [9]. It is necessary to note that
photothermal oxidation of plates (annealing in atomic
oxygen) results in formation of new compound CdO [9],
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2009. V. 12, N 2. P. 143-146.
© 2009, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
144
e/2kT
e/kT
10-5
10-6
10-7
10-8
10-9
1
If, А
0 V, V0.2 0.4 0.6 0.8
2 3
Fig. 2. Initial parts of forward branches of I-V characteristics
for the diodes based on CdTe (1), CdTe:АS (2) and
CdTe:О2 (3) substrates at 300 К.
and, consequently, in creation of the heterojunction
CdO-CdTe. As the latter cannot be related to the class of
SBDs, they will not be considered in this work. Note
also that annealing of CdTe substrates in the same
conditions but in molecular oxygen results in formation
of quantum size surface [10]. The fourth group of
samples consists of chemically etched substrates that
were not treated in addition and marked as CdTe. SBDs
based on them served as the standard ones when
comparing their characteristics with those of diode
structures in other groups.
3. Results and discussion
Researches showed that SBDs based on CdTe:AS and
CdTe:O2 substrates have a considerably higher height of
the potential barrier than that in standard ones. Unlike
the above mentioned treatments, the laser annealing
causes some decrease in the value φ0. It is illustrated in
Fig. 1, where the direct branches of I-V characteristic for
objects under study are shown in the range of their
linearity. Let us pay attention to the insignificant
difference of slopes in linear parts of I-V characteristic
for diode structures in all the groups. It testifies to the
close values of the serial resistance R0 that is the sum of
resistances of contacts and the base of diode. On the
other hand, it is also indicative of negligibly low
contribution to the value R0 from surface layers modified
by different methods. Further analysis of investigated
electric properties showed that the change of surface
modification method considerably influences not only on
the value φ0 but also on the character of flowing physical
processes. First of all, it can be seen from the
dependences of forward and backward currents, which
are considered in detail below. As obvious from Fig. 2,
the initial parts of I-V characteristics of SBDs based on
CdTe, CdTe:AS and CdTe:O2 substrates at eV > 3kT are
described by the known expression [1, 5-7, 11]
)/(exp0 nkTeVII , (1)
I, mA
15
10
5
0
0.5 1.0 V, V1.5
4 1 2 320
Fig. 1. Forward branches of I-V characteristics of surface-
barrier diodes based on CdTe (1), CdTe:АS (2), CdTe:О2 (3)
and CdTe:L (4) substrates at 300 К.
where I0 is the cut-off current for V = 0, n – coefficient
of ideality. The latter can be easily determined from the
linear part of I-V characteristics, built in semi-
logarithmic coordinates. For the diodes Au-CdTe n = 1,
which indicates the over-barrier character of the forward
current origin and is characteristic for SBDs created on
reasonably doped substrates. Beside, it means that an
intermediate layer between metal and semiconductor is
tunnel transparent, and the height of barrier does not
exceed the half of the bandgap for the substrate base.
Unlike the contacts of Au-CdTe, the latter condition is
not valid for SBDs based on CdTe:AS and CdTe:O, as
the value φ0 for them is noticeably higher than
Eg /2 ≈ 0.75 eV (Fig. 1). For these structures at low
voltages in accord with [5, 11], the recombination
current becomes dominating in the space charge region
(SCR) which is described by the formula (1) at n = 2. As
it is evident from Fig. 1, the initial parts of direct
branches in I-V characteristics of SBDs with φ0 > Еg /2
well follow the expression (1), and the energy
dependence slope is 1.6 еV and agrees with Eg of
cadmium telluride at 0 К [2].
Note that deviation of experimental points from the
expected dependences in Fig. 1 is caused by a voltage
drop on the serial resistance R0 in this diode structure.
Calculating the resistance results in straightening the
Іf (V) dependences in semi-logarithmic coordinates. In
this case, the slope of straight lines for all SBDs in Fig. 1
is the same and equals е/кТ. It is indicative of the over-
barrier character of the forward current at a large bias
and does not contradict to the known mechanisms of
current transfer in SBDs [1, 4-6, 11].
The considered above over-barrier current and
generation-recombination cut-off one І0 at 300 К do not
exceed 10-10 A (Fig. 1), аnd, moreover, in obedience to
the theory depend on reverse bias not stronger than V
[1, 5, 11]. On the other hand, there observed in
experiment are more sharp dependences Іr(V), which
indicates other current transfer mechanisms. It is
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2009. V. 12, N 2. P. 143-146.
© 2009, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
145
Ir, А
0 ( 0 – eV)-1/2, eV-1/2
10-6
10-7
10-5
10-8
0.2 0.4 0.6 0.8
3 2
10-4
1
Fig. 3. Comparison to (2) experimental reverse branches of
I-V characteristics for the diodes based on CdTe (1),
СdTe:AS (2) and CdTe:О2 (3) substrates at 300 К.
reasonable to assume that for diodes based on wide-
bandgap semiconductors (including the surface-barrier
ones), tunnelling of charge carriers can be both interband
and with participation of local levels [11]. This
assumption is confirmed by Fig. 3, from which it is
evident that the reverse I-V characteristics are described
by the known expression for a tunnel current through a
sharp barrier [5, 11].
eV
b
aI
0
exp . (2)
Here, а and b are the coefficients defined by the
parameters of a substrate and diode structure. The sign
“–“ under the root indicates the reverse bias, and the
voltage should be put with the negative sign. Different
slopes of initial parts in reverse I-V characteristics for
the objects under study testifies to the difference just of
the diode parameters, and not of the substrate ones, since
the latter are identical for all of them. Note that the
coefficient b depends on the height and width of barrier,
energy depth of local centres in SCR, their distribution
and other reasons [11], and in this relation needs
separate more detailed consideration. Also, the same
concerns explanation of various dependences in Fig. 3 at
large biases, in the range of which, except for the factors
marked above, avalanch processes can take part yet
[6, 11].
Unlike to the considered above SBD current-
voltage characteristics, the ones based on laser annealled
substrates have quite another character. As seen from
Fig. 4, they follow the law
I V m, (3)
where the exponent takes values from the interval 1 to 6
in dependence on the sign and magnitude of the applied
bias voltage. The analysis shows that dependence of the
type (3) is characteristic for currents limited by the space
charge. In accord with the theory of mono-polar
injection, the initial part of I-V characteristics should be
described by the Ohm law (m = 1), and at higher V
should be changed with the quadratic one named as
trapless. Instead, in our case the exponent in this part is
larger and equals to 3, which does not coincide with the
expected one m = 2.
Similar dependences were observed, in particular,
in thin CdS crystals, which was explained by the authors
[12] accounting for the presence of a large number of
traps with the shallow energy depths. Note that in our
case it is fully probable, as after the laser annealing in
this modified layer there can be a large number of
defects, authentication of which needs additional
researches. Further growth of the forward current in the
part with m = 6 corresponds to the complete filling of
traps, which is again changed with a slower growth
region, namely, by the expected trapless quadratic law.
The reverse branch of I-V characteristic (curve 2 in
Fig. 4) in a wide range of changing voltages follows the
Ohm law, and at high V values is changed with the
sharper one. It can be caused by many mechanisms, the
most probable of which are tunneling with participation
of local levels and impact ionization.
Researches of light characteristics showed that
dependences for all the diodes are qualitatively similar.
So, in particular, the current of short circuit Isc is the
linear function of the illumination level L when the latter
changes within the limits of more than four orders. The
open circuit voltage Voc lgL at low illumination levels
and follows to the saturation at the larger ones. The
absolute values Voc depends on the type of diode and L,
and their maximal values are summarized in Table.
It is noteworthy that the effective photosensitive
area of the samples was ~2·10-2 cm2, and photoelectric
parameters were measured at the maximal illumination
level provided with a 100-W tungsten filament lamp.
Note also the substantial improvement of basic electric
and photoelectric parameters (φ0, Voc and Isc) of SBDs
with the modified surface as compared with the standard
barrier structure.
I, А
V, V
10-8
10-6
10-4
10-2
10-2 10-1 100
m=1
m=3
m=2
m=6
1
2
Fig. 4. Forward (1) and reverse (2) I-V characteristics of diodes
Au-CdTe:L at 300 К.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2009. V. 12, N 2. P. 143-146.
© 2009, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
146
S, rel.un.
1 ћ, eV
1
1.0
0.5
0
2 3 4
2 34
Fig. 5. Photosensitivity spectra of surface-barrier base
substrates CdTe (1), CdTe:AS (2), CdTe:О2 (3) and CdTe:L (4)
at 300 K.
Table. Parameters and types of the diode structures.
Diode type Au–
CdTe
Au–
CdTe:AS
Au–
CdTe:O2
Au–
CdTe:L
0, eV 0.7±0.1 1.2±0.1 1.35±0.1 0.55±0.05
R0,Ohm 30±2 30±2 35±2 30±2
Voc, V 0.4±0.05 0.75±0.05 0.9±0.05 0.45±0.05
P
ar
am
et
er
Isc, mA 0.1±0.02 0.2±0.02 0.3±0.02 0.4±0.02
At the end, let us analyze the influence of
modification on the concentration of surface defects Ns
that usually form the undesirable channels of
recombination in the impurity range. As the effective
length of absorption for radiation with the quantum
energy ħω > Еg in cadmium telluride le ≤ 10-5 cm, non-
equilibrium electron-hole pairs are practically generated
in a subsurface layer. In this relation, in the first
approximation it is possible to conceive that the
efficiency of edge luminescence and photosensitivity in
the area of fundamental absorption are inversely
proportional to the concentration Ns. As it is seen from
Fig. 5, the highest photosensitivity in the high-energy
spectral range inherent to SBDs based on CdTe:O2
substrates, the surface of which has a quantum-size
structure [10]. These substrates provide also the highest
efficiency of edge luminescence that can reach 10 % at
300 К. The least short-wave photosensitivity is observed
for the contacts of Au-CdTe:L, which is well explained
by a considerable number of surface defects appearing
after the laser annealing. It is confirmed by the results of
measuring I-V characteristics in these diodes, and also
complete absence of mentioned above edge
luminescence at 300 К.
4. Сonclusions
Thus, the obtained results testify to perspectives of
modification of n-CdTe substrate surface to control
electric and photoelectric properties of surface-barrier
structures based on them. In this case, every method has
its advantages and deficiencies as compared to each
other both in technological aspect and in achieving the
maximal values of physical-and-technical parameters of
photodiodes. In this relation, the following investigations
should be aimed at not only development of new
technologies for surface modification but also at
searching the optimum combinations of treatments
considered in this work in application to cadmium
telluride substrates.
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