Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals
The paper elucidates the peculiarities of photoplasticization effect (PPE) occurring in narrow-gap crystals of mercury chalcogenides illuminated by white light during the process of uniaxial deformation. As found, the irradiation has an influence on the plastic deformation in narrow-gap CdxHg₁₋xTe c...
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Інститут металофізики ім. Г.В. Курдюмова НАН України
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nasplib_isofts_kiev_ua-123456789-1459462025-02-09T09:44:41Z Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals Фотопластический эффект в узкощелевых кристаллах халькогенидов ртути Фотопластичний ефект у вузькощілинних кристалах халькогенідів меркурію Koman, B.P. Balitskii, O.O. Leonov, D.S. Физика прочности и пластичности The paper elucidates the peculiarities of photoplasticization effect (PPE) occurring in narrow-gap crystals of mercury chalcogenides illuminated by white light during the process of uniaxial deformation. As found, the irradiation has an influence on the plastic deformation in narrow-gap CdxHg₁₋xTe crystals. Negative photoplasticization effect (NPPE) described here is concerned with reducing of plastic stress fluidity under white-light irradiation during plastic deformation of crystals at a constant rate. As found, in contrary to wide-bandgap crystals of the II–VI groups exhibiting positive PPE, NPPE observed in CdxHg₁₋xTe occurs without internal photoeffect. A model explaining the nature of NPPE relies on a decreasing of positive charge in a native oxide layer at the surface of crystal. Charge decreasing reduces potential barrier for exiting on surface of dislocations generated by near-surface sources during dynamical loading. As a result, the level of fluidity stress in the deformed crystal is reduced. В работе рассмотрены особенности фотопластического эффекта (ФПЭ), наблюдаемого в узкощелевых (Eg ≈ 0,2 эВ) кристаллах халькогенидов ртути при облучении белым светом в процессе их одноосной деформации. Установлено, что облучение влияет на пластическую деформацию узкощелевых кристаллов твёрдых растворов CdxHg₁₋xTe (х — молярный состав). Исследованный в работе отрицательный фотопластический эффект (ОФПЭ) связан с уменьшением при облучении белым светом напряжения пластического течения в условиях пластической деформации кристалла с постоянной скоростью нагружения. Установлено, что, в отличие от широкозонных кристаллов соединений II–VI групп, проявляющих положительный ФПЭ, в кристаллах узкощелевых твёрдых растворов CdxHg₁₋xTe ОФПЭ наблюдается в отсутствие внутреннего фотоэффекта. Модель, объясняющая природу ОФПЭ, основывается на установленном факте уменьшения при освещении кристалла положительного заряда в оксидном слое приповерхностной области кристалла. Этот процесс понижает потенциальный барьер для выхода на поверхность дислокаций, порождаемых приповерхностными источниками в процессе динамического нагружения. Следствием является уменьшение напряжения пластического течения кристалла. У роботі розглянуто особливості фотопластичного ефекту (ФПЕ), що виникає у вузькощілинних кристалах (Eg ≈ 0,2 еВ) халькогенідів ртуті, освітлених білим світлом, у процесі їх одновісної деформації. Встановлено, що опромінення впливає на пластичну деформацію вузькощілинних кристалів твердих розчинів CdxHg₁₋xTe (х — молярний склад). Досліджений в роботі від’ємний фотопластичний ефект (ВФПE) пов’язаний зі зменшенням напруги пластичної плинности при опроміненні білим світлом в умовах одновісної пластичної деформації кристалу за постійної швидкости навантаження. Виявлено, що, на відміну від широкозонних кристалів сполук II–VI груп, які виявляють позитивний ФПЕ, в кристалах твердих розчинів CdxHg₁₋xTe ВФПE спостерігається за відсутности внутрішнього фотоефекту. Модель, що пояснює природу ВФПE, ґрунтується на встановленому факті зменшення при освітленні кристалу позитивного заряду в оксидному шарі приповерхневої области кристалу. Зменшення заряду понижує потенціяльний бар’єр для виходу на поверхню дислокацій, що породжуються приповерхневими джерелами в процесі динамічного навантаження. Наслідком є зменшення напруги плинности деформації кристалу. 2018 Article Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals / B.P. Koman, O.O. Balitskii, D.S. Leonov // Металлофизика и новейшие технологии. — 2018. — Т. 40, № 4. — С. 529-540. — Бібліогр.: 28 назв. — англ. 1024-1809 PACS: 61.72.Lk, 61.80.Ba, 61.82.Fk, 62.20.fq, 81.40.Lm, 81.40.Wx, 83.60.Np DOI: https://doi.org/10.15407/mfint.40.04.0529 https://nasplib.isofts.kiev.ua/handle/123456789/145946 en Металлофизика и новейшие технологии application/pdf Інститут металофізики ім. Г.В. Курдюмова НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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English |
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Физика прочности и пластичности Физика прочности и пластичности |
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Физика прочности и пластичности Физика прочности и пластичности Koman, B.P. Balitskii, O.O. Leonov, D.S. Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals Металлофизика и новейшие технологии |
| description |
The paper elucidates the peculiarities of photoplasticization effect (PPE) occurring in narrow-gap crystals of mercury chalcogenides illuminated by white light during the process of uniaxial deformation. As found, the irradiation has an influence on the plastic deformation in narrow-gap CdxHg₁₋xTe crystals. Negative photoplasticization effect (NPPE) described here is concerned with reducing of plastic stress fluidity under white-light irradiation during plastic deformation of crystals at a constant rate. As found, in contrary to wide-bandgap crystals of the II–VI groups exhibiting positive PPE, NPPE observed in CdxHg₁₋xTe occurs without internal photoeffect. A model explaining the nature of NPPE relies on a decreasing of positive charge in a native oxide layer at the surface of crystal. Charge decreasing reduces potential barrier for exiting on surface of dislocations generated by near-surface sources during dynamical loading. As a result, the level of fluidity stress in the deformed crystal is reduced. |
| format |
Article |
| author |
Koman, B.P. Balitskii, O.O. Leonov, D.S. |
| author_facet |
Koman, B.P. Balitskii, O.O. Leonov, D.S. |
| author_sort |
Koman, B.P. |
| title |
Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals |
| title_short |
Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals |
| title_full |
Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals |
| title_fullStr |
Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals |
| title_full_unstemmed |
Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals |
| title_sort |
photoplastic effect in narrow-gap mercury chalcogenide crystals |
| publisher |
Інститут металофізики ім. Г.В. Курдюмова НАН України |
| publishDate |
2018 |
| topic_facet |
Физика прочности и пластичности |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/145946 |
| citation_txt |
Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals / B.P. Koman, O.O. Balitskii, D.S. Leonov // Металлофизика и новейшие технологии. — 2018. — Т. 40, № 4. — С. 529-540. — Бібліогр.: 28 назв. — англ. |
| series |
Металлофизика и новейшие технологии |
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| fulltext |
529
PACS numbers: 61.72.Lk, 61.80.Ba, 61.82.Fk, 62.20.fq, 81.40.Lm, 81.40.Wx, 83.60.Np
Photoplastic Effect in Narrow-Gap Mercury Chalcogenide Crystals
B. P. Koman, O. O. Balitskii, and D. S. Leonov*
Ivan Franko National University of Lviv,
Faculty of Electronics and Computer Technologies,
1 Universytetska Str.,
UA-79000 Lviv, Ukraine
*Technical Centre, N.A.S. of Ukraine,
13 Pokrovs’ka Str.,
04070 Kyiv, Ukraine
The paper elucidates the peculiarities of photoplasticization effect (PPE) oc-
curring in narrow-gap crystals of mercury chalcogenides illuminated by
white light during the process of uniaxial deformation. As found, the irradi-
ation has an influence on the plastic deformation in narrow-gap CdxHg1 xTe
crystals. Negative photoplasticization effect (NPPE) described here is con-
cerned with reducing of plastic stress fluidity under white-light irradiation
during plastic deformation of crystals at a constant rate. As found, in con-
trary to wide-bandgap crystals of the II–VI groups exhibiting positive PPE,
NPPE observed in CdxHg1 xTe occurs without internal photoeffect. A model
explaining the nature of NPPE relies on a decreasing of positive charge in a
native oxide layer at the surface of crystal. Charge decreasing reduces poten-
tial barrier for exiting on surface of dislocations generated by near-surface
sources during dynamical loading. As a result, the level of fluidity stress in
the deformed crystal is reduced.
Key words: photoplasticization effect, narrow-gap semiconductor, mercury
chalcogenides, dislocations, deformation.
У роботі розглянуто особливості фотопластичного ефекту (ФПЕ), що ви-
никає у вузькощілинних кристалах (Eg 0,2 еВ) халькогенідів ртуті, осві-
тлених білим світлом, у процесі їх одновісної деформації. Встановлено,
Corresponding authors: Bohdan Petrovych Koman
E-mail: sonce_28@ukr.net
Citation: B. P. Koman, O. O. Balitskii, and D. S. Leonov, Photoplastic Effect in
Narrow-Gap Mercury Chalcogenide Crystals, Metallofiz. Noveishie Tekhnol., 40,
No. 4: 529–540 (2018), DOI: 10.15407/mfint.40.04.0529.
Ìåòàëëîôèç. íîâåéøèå òåõíîë. / Metallofiz. Noveishie Tekhnol.
2018, т. 40, № 4, сс. 529–540 / DOI: 10.15407/mfint.40.04.0529
Îòòèñêè äîñòóïíû íåïîñðåäñòâåííî îò èçäàòåëÿ
Ôîòîêîïèðîâàíèå ðàçðåøåíî òîëüêî
â ñîîòâåòñòâèè ñ ëèöåíçèåé
2018 ÈÌÔ (Èíñòèòóò ìåòàëëîôèçèêè
èì. Ã. Â. Êóðäþìîâà ÍÀÍ Óêðàèíû)
Íàïå÷àòàíî â Óêðàèíå.
https://doi.org/10.15407/mfint.40.04.0529
https://doi.org/10.15407/mfint.40.04.0529
530 B. P. KOMAN, O. O. BALITSKII, and D. S. LEONOV
що опромінення впливає на пластичну деформацію вузькощілинних кри-
сталів твердих розчинів CdxHg1 xTe (х — молярний склад). Досліджений в
роботі від’ємний фотопластичний ефект (ВФПE) пов’язаний зі зменшен-
ням напруги пластичної плинности при опроміненні білим світлом в умо-
вах одновісної пластичної деформації кристалу за постійної швидкости
навантаження. Виявлено, що, на відміну від широкозонних кристалів
сполук II–VI груп, які виявляють позитивний ФПЕ, в кристалах твердих
розчинів CdxHg1 xTe ВФПE спостерігається за відсутности внутрішнього
фотоефекту. Модель, що пояснює природу ВФПE, ґрунтується на встано-
вленому факті зменшення при освітленні кристалу позитивного заряду в
оксидному шарі приповерхневої области кристалу. Зменшення заряду
понижує потенціяльний бар’єр для виходу на поверхню дислокацій, що
породжуються приповерхневими джерелами в процесі динамічного нава-
нтаження. Наслідком є зменшення напруги плинности деформації крис-
талу.
Ключові слова: фотопластичний ефект, вузькощілинні напівпровідники,
халькогеніди ртуті, дислокації, деформація.
В работе рассмотрены особенности фотопластического эффекта (ФПЭ),
наблюдаемого в узкощелевых (Eg 0,2 эВ) кристаллах халькогенидов
ртути при облучении белым светом в процессе их одноосной деформации.
Установлено, что облучение влияет на пластическую деформацию узко-
щелевых кристаллов твёрдых растворов CdxHg1 xTe (х — молярный со-
став). Исследованный в работе отрицательный фотопластический эффект
(ОФПЭ) связан с уменьшением при облучении белым светом напряжения
пластического течения в условиях пластической деформации кристалла с
постоянной скоростью нагружения. Установлено, что, в отличие от ши-
рокозонных кристаллов соединений II–VI групп, проявляющих положи-
тельный ФПЭ, в кристаллах узкощелевых твёрдых растворов CdxHg1 xTe
ОФПЭ наблюдается в отсутствие внутреннего фотоэффекта. Модель, объ-
ясняющая природу ОФПЭ, основывается на установленном факте умень-
шения при освещении кристалла положительного заряда в оксидном слое
приповерхностной области кристалла. Этот процесс понижает потенци-
альный барьер для выхода на поверхность дислокаций, порождаемых
приповерхностными источниками в процессе динамического нагруже-
ния. Следствием является уменьшение напряжения пластического тече-
ния кристалла.
Ключевые слова: фотопластический эффект, узкощелевые полупровод-
ники, халькогениды ртути, дислокации, деформация.
(Received March 21, 2018)
1. INTRODUCTION
As recently described in Refs. [1–3], variation of electron-subsystem
states in semiconducting materials essentially influences dislocation
movement conditions. In particular, irradiation induces changes of
PHOTOPLASTIC EFFECT IN NARROW-GAP MERCURY CHALCOGENIDE CRYSTALS 531
point defect and dislocations charges in a crystal and affects the inter-
action between mobile dislocations and effective stoppers. A striking
example of the external factors influencing a plastic deformation
stress is the photoplasticization effect (PPE). It changes fluidity stress
in the crystal excited by intrinsic absorption under dynamical loading
within the plastic deformation region. PPE is much less studied as
compared to magnetoplastic effect where atomic mechanisms have al-
ready been drawn [4–7] mainly due to practical implementation of the
effect.
Nadeau discovered PPE in alkali halide crystals [8]. In more details,
its nature was studied in [9], followed by the quantitative mathematical
model with estimated values of plasticity shifts [10]. PPE in wide-
bandgap II–VI-groups compounds was described in [11]. It was found
that irradiation of CdS under plastic deformation with visible light
leads to hardening and plastic deformation shifts towards increased
values of shear stress. Generalization of experimental results on PPE in
above-mentioned compounds was done by Osip’yan and Petrenko [12].
An important fact for understanding the nature of this effect was eluci-
dation of the role of nonequilibrium carriers generated by an internal
photoeffect in the processes of changing the charge state of point de-
fects and mobile dislocations. It was found that light exciting РPE sig-
nificantly increases the linear charge density of dislocations [12]. Name-
ly, there is a correlation between the changes of deforming tension and
dislocation charge density. Therefore, the dislocation mobility decrease
can be natured from its charge rise. It was established that the charge of
mobile dislocations is generated by electrons collected by trap levels [3].
As the number of electrons at these levels grows, the dislocation charge
increases and dislocation mobility reduced. Irradiation of a crystal with
bandgap matched light stimulates number of traps filled with electrons,
leading to an increase in the dislocations charge. Alternative view of the
PPE nature in wide-bandgap semiconductor crystals is based on the
changes of movement kinetics under irradiation and multiplication of
the double bends number during the plastic deformation of a crystal [1].
Since PPE is a typical phenomenon for wide-bandgap crystal of A
2B6
family (for example, CdTe exhibits positive and negative PPE) [14, 15],
it is interesting to examine manifestation of this effect in narrow-band
crystals belonging to ternary CdxHg1 xTe semiconductors (x 0.16–
0.22) with Eg 0.1 eV. We are aiming the present work to study the fea-
tures of PPE in mixed CdxHg1 xTe crystals and to establish the physical
nature of this phenomenon.
2. MATERIALS AND METHODS
Single crystal samples of CdxHg1 xTe solid solution (x 0.16–0.22)
were grown by Bridgman and solid-state recrystallization methods.
532 B. P. KOMAN, O. O. BALITSKII, and D. S. LEONOV
Own oxide was formed on the surface of a crystal sealed in an oxygen
filled ampoule during thermal annealing at 200 C for 24 hours in order
to master under-gate insulator. Deformation of the samples (2 2 6
mm3) was performed on a Regel–Dubov relaxometer along the longer
crystal edge at a constant rate. The rate of deformation was chosen in
the range 2 10 6–9∙10 5
s
1. Irradiation of the sample in the process of
deformation was carried out by using of two different methods: i)
white light from an incandescent lamp (P 20 W, flux 80 lm) or ii) con-
tinuous CO2 laser operating at 10.6 m. For the generation of light
pulses, mechanical shutter camera was used, producing light pulses
with 1 to 100 s periodicity. Experimental measurements of voltage–
capacitance characteristics (VCCs) were conducted at the frequency of
10 kHz test signal with linear voltage sweep rate of 10 2
Hz.
3. RESULTS AND DISCUSSION
Figure 1 illustrates a typical stress–strain diagram with the photosof-
tening (PS) effect for Cd0.2Hg0.8Te crystal. The white-light irradiated
crystal under uniaxial deformation possesses initially a step-like
change of fluidity tension. Then, the radiation induced rapid decrease
is observed (i.e. NPPE) and the crystal continues to deform with the
previous coefficient of deformation hardening. As the light is turned
off, deformation restores at the initial value. This phenomenon occurs
repeatedly until the sample is destroyed. The described effect was ob-
served with the absence of photoconductivity in the temperature range
of 290–430 K.
The systematic study of NPPE stress curves indicates that steady
stress fluidity is not achieved immediately after light is switched
Fig. 1. Negative photoplasticization effect in Cd0,2Hg0,8Te crystal ( —light on,
—light off); V 5.85 10 3
mm/min, T 300 K. The white light from an in-
candescent lamp was used for irradiation.
PHOTOPLASTIC EFFECT IN NARROW-GAP MERCURY CHALCOGENIDE CRYSTALS 533
on/off, but there is a delay namely a relaxation time. Thus, a memory
effect is observed while previous level of deforming stress is reached.
Such behaviour can be natured from two types of processes. In one
hand, characteristic times for electronic subsystem to reach the equi-
librium are sufficiently long and substantially dependent on tempera-
ture. In the other hand, light induced tension growth can be connected
with the restructuring of the dislocation structure. The latter depends
on the area of the total plastic deformation accumulated in the sample.
The relaxation time growth at the higher degree of deformation can be
explained by increasing of dislocation density. In addition, changes in
a point defects system are accompanied by variations in conditions of
their movement and reproduction. In this case, mercury vacancies in
Cd0.2Hg0.8Te serve as efficient centres for interaction with mobile dis-
locations [16].
Typical experimental dependencies of PS on deformation level for
Cd0.2Hg0.8Te crystals with different degree of the stoichiometry devia-
tion are shown in Fig. 2.
All the samples possess a common feature: PS value decreases with
the plastic deformation accumulated. However, crystals with different
deviations from stoichiometry show different decrease tendencies. In
particular, sample with electronic conductivity (n 1015
cm
3) mani-
fests a linear behaviour. For the sample with hole conductivity (p
8 1016
cm
3), such a dependence consists of two sections. Primarily,
the value of PS drops sharply down to 50%, and then changes slightly
reaching a saturation at large values strain. The smallest changes for
PS are observed in the sample with intrinsic conductivity (p n): the
value of the effect reaches as little as 20%. As discussed above,
CdxHg1 xTe crystals are characterized by a memory effect to the previ-
ous deforming stress. In order to draw a physical model of the effect,
one must gather information about the lifetimes of the centres respon-
sible for the observed PPE. For this purpose, we have studied the effect
Fig. 2. Relative PS in Cd0.2Hg0.8Te crystals as a function of strain degree: 1—
p 8 1016
cm
3, 2—n 1015
cm
3, 3 — p n.
534 B. P. KOMAN, O. O. BALITSKII, and D. S. LEONOV
of pulsed irradiation on the kinetics of the PS (Fig. 3).
It is evident that the PS characteristic can by divided into three con-
secutive stages. The first corresponds to the rapid growth (up to 5 s)
while at the second (up to 33 s) the growth rate becomes slightly slow-
er. The final stage corresponds to a saturation of the PS effect. Typi-
cally, during performed experiments a steady state is reached after ap-
proximately 80 s of irradiation. That can be explained as follows. Ini-
tially, a certain amount of PS softening centres is photogenerated in a
crystal, and then their concentration exponentially decreases after
light is turned off. Each centre corresponds to the exact type of de-
forming stress. However, total stress is a combined result of interac-
tion of mobile dislocations with conservative and non-conservative
centres in irradiated crystal and not following the concentration of
photosensitive centres. The rate of such changes depends on the de-
formation and plastic properties of the sample. In this regard, stress
decreases were postponed after switching the light off. Due to suffi-
Fig. 3. The PS dependence on irradiation pulse duration.
Fig. 4. PS effects after different rates of loading for Cd0.2Hg0.8Te crystals: 1—
2.34 10 2
mm/min, 2—5.85 10 3
mm/min, 3—2.93 10 3
mm/min, 4—
1.46 10 3
mm/min.
PHOTOPLASTIC EFFECT IN NARROW-GAP MERCURY CHALCOGENIDE CRYSTALS 535
ciently longer centre lifetimes, the concentration of centres during
certain period of time is not matching with achieved deforming stress.
Decrease of mechanical stress is terminated when the stress values and
the centres’ concentration are matched. Further increase of the de-
forming stress is due to the subsequent decrease of the centres’ concen-
tration. Growth of the deformation rate is accompanied by more rapid
achievement of consistency between the deforming stress and the cen-
tres’ concentration. Therefore, the time of the aftereffect decreases
and the value of PPE increases.
According to the technique suggested in [17], one should combine
curves related to different strain rates (Fig. 4). Combined dependences
(Fig. 5) will be further used to elucidate PPE parameters.
That dependence is linearized in semi-logarithmic scale in Fig. 5, b.
The centres lifetime deducted is about 6 seconds. Investigation of tem-
perature dependencies of centres lifetime responsible for PPE in the
existence region (300–400 K) allowed estimating the activation energy
of thermal destruction, which is equal to 0.34 eV. It was found that the
highest value of lifetime for PPE centres has been observed for the
samples highly manifested PS effect. Therefore, we assume that the
magnitude of the PS effect is proportional to the lifetime of the centres
responsible for PPE. In other words, the PPE in narrow-gap
CdxHg1 xTe crystals is not natured by interaction of charged disloca-
tions with photosensitive centres as for the case of binary wide-
bandgap crystals of II–VI-groups’ family.
It should be noted that the manifestation of PPE under visible light
excitation is not expected in the narrow-band materials ( 0.1 eV). The
attempts to draw the appropriate model based on the generation of
non-equilibrium carriers’ theory as was done for wide gap have not yet
been successful. Let us now focus on some aspects describing unusual
nature of this effect in ternary CdxHg1 xTe crystals and compare them
Fig. 5. The plot determines the lifetime of the centres responsible for PPE in
Cd0.2Hg0.8Te crystals.
536 B. P. KOMAN, O. O. BALITSKII, and D. S. LEONOV
with those of wide-bandgap II–VI-groups crystals. In particular, i)
NPPE is observed in narrow-bandgap crystal CdxHg1 xTe excited by
white light ( 0.35–2.5 m) being beyond the absorption range. Thus,
the internal photoeffect responsible for the positive PPE in A
2B6
com-
pounds is not manifested. Some samples manifest the effect under la-
ser irradiation with the wavelength of 10.6 m (CO2 laser) at room
temperature. ii) PPE is observed in crystals with stable own oxide lay-
er. iii) Relaxation times are rather large establishing the permanent
stress deformation after switching the light on and off in NPPE exper-
iment. At sufficiently low values for Peierls barriers for mobile dislo-
cations (0.14–0.41еV) in CdxHg1 xTe crystals [16], one should expect
the influence of oxide layer on the dislocation-related processes at the
near-surface layer during the entire time of dynamical loading of crys-
tals. Primarily, we observe the inception of dislocations with subse-
quent transformation of dislocation structure as described by kinetics
of deformation processes [18].
Key factors in the development of physical processes in near-surface
layer should are properties of the oxide layer (adhesion, band gap, op-
tical characteristics etc.). Therefore, under the influence of external
factors such as incident light, observed effects are different: positive
PPE in CdTe or NPPE in CdxHg1 xTe. In contrary to binary compounds,
ternary CdxHg1 xTe own oxide has significantly larger number of com-
ponents drastically affecting physics and chemistry of surface. An ex-
ample of such influence is a long-time relaxation of photoconductivity
observed in CdxHg1 xTe crystals, which is characterized by time up to
several minutes [19]. In particular, studied objects are characterized
by uneven distribution of Hg, Te, Cd and O and the own oxide growth
orientation directed towards the crystal bulk. The chemical composi-
tion is a mixture of CdO, CdTeO3, TeO2, and HgTeO3 [20]. Oxide layer
has a relatively high density of positive embedded charge (6 1011–
1 1012
cm
2), fast surface states with the density of 2.6 1011
cm
2
and
slow states with the density of 1 1011
cm
2. The near-surface layer of
the crystal is a classic insulator-semiconductor junction with positive
embedded charge and the expected electric field of 102–104
V/sm. To
analyse the charge distribution in the light excited intrinsic oxide–
semiconductor system in the PPE experiments, we studied the VCC of
MOS-structures based on n-Cd0.25Hg0.75Te (Fig. 6).
VСC plots of MOS-structures are presented in Fig. 7. It was estab-
lished that initial VCC is shifted by value of VCB 2.5 V in respect to
ideal characteristic. Such a shift is natured by an existence of effective
positive charge in the undergate insulator. Stimulated by irradiation,
a significant offset of voltage planar zones towards positive values ap-
peared. Moreover, a number of excited electrons changes the surface
condition undergate from enriched to inverse. Reverse offset of volt-
age planar zones occurs when the structure is placed in a dark area.
PHOTOPLASTIC EFFECT IN NARROW-GAP MERCURY CHALCOGENIDE CRYSTALS 537
Time needed for the full recovery to initial VCC at T 300 K is 30–
150 s.
The most important result of studies establishing the nature of PPE
is a radiation induced reduction of effective positive charge in the in-
sulator relaxing after switching off the light with a time constant of
30–60 s. Increasing of the exposure time is accompanied by higher
relaxation times of nonequilibrium charge. Let us briefly analyse the
optical properties of typical oxides on the surface of the CdxHg1 xTe
crystals. CdO, according to [21], possesses 70–80% of transparency at
500–800 nm wavelengths. Paratellurite TeO2 has much broader range
of transparency: 80% within 0.36–5.0 m window [22]. Hence, multi-
component native oxide layer on the surface of n-CdxHg1 xTe structure
is transparent in almost entire spectrum range of incandescent lamp
down to the insulator–semiconductor interface. Assuming the average
band gap of the oxide layer to be about 3.4 eV and band gap of the con-
tacting semiconductor 0.2 eV, the photoinduced charge in the oxide
can be explained by external photoemission of electrons from the va-
Fig. 6. Sketch of MOS structure based on n-CdHgTe with semi-transparent
gate: 1—gate electrode, 2—semi-transparent gate, 3—undergate insulator,
4—Cd0.25Hg0.75Te.
Fig. 7. VCCs for initial (1) and irradiated for 80 s by visible light (2) CdHgTe
MOS structures.
538 B. P. KOMAN, O. O. BALITSKII, and D. S. LEONOV
lence band of the semiconductor oxide into the conduction band of ox-
ide followed by their capturing at traps in the oxide (Fig. 8), which are
localized near the oxide–semiconductor interface.
Thus, the neutralization of fixed charge in oxide is realized. Proba-
ble mechanism of the electrons injection is the tunnelling of carriers
through the interface. The energy of visible light ( 0.6 µm) is close to
the value of the oxide–semiconductor energy barrier. When the struc-
ture is placed in dark, conditions for thermal generation of electrons
captured at traps and their further recombination in the semiconduc-
tor exist. Generally, excitation of studied crystals with visible light is
accompanied by a decrease in the effective positive charge at the insu-
lator–semiconductor interface.
Such a behaviour of the charge results in reduced mechanical stress
on the interface, and reduced barrier for generated dislocations exit on
the surface of the crystal in the process of deformation. These factors
stimulate the lowering of the fluidity stress level in the deformed crys-
tal with simultaneous irradiation (NPPE). Schematic representation of
such processes is shown in Fig. 9.
As known, interface interaction is natured from physical, chemical,
thermodynamical and mechanical incompatibilities between interac-
tion materials [23]. The parameters characterizing above-mentioned
interaction are interface energy and tension m and m, respectively.
The model elaborated [24] combines equations of nonequilibrium
thermodynamics and surface physics describing mechanical stresses
and charges at the interface. After applying such calculations, as was
done in [25], the values for CdxHg1 xTe–own oxide system are as follow:
m 0.893 J/m2, m 0.739 N/m. Those parameters are as much as five
time lower than that for Si–SiO2 [26]. Therefore, the interface interac-
tion in CdxHg1 xTe–own oxide system is rather weak and can effective-
Fig. 8. Sketch of energy diagram of the ‘own oxide–n-CdxHg1 xTe (x 0.24)
crystal’ interface, illustrating the reduction of positive embedded charge in
the insulator excited by white light.
PHOTOPLASTIC EFFECT IN NARROW-GAP MERCURY CHALCOGENIDE CRYSTALS 539
ly be tuned by irradiation. As we mentioned, the mechanism of such
tuning lies in the decrease of embedded insulator charge. Note, the
presence of own oxide film on the CdHgTe surface eliminates the pos-
sibilities of periodic structures formation under combined effect of
light pressure and mechanical stresses as was illustrated in [27].
CdHgTe own oxide is impenetrable by white light making processes of
self-organization under macroscopic plastic deformation impossible
[28].
4. CONCLUSIONS
1. Negative PPE is discovered in CdxHg1 xTe crystals manifested in re-
duced fluidity stress in these crystals excited with white light during
dynamical loading at a constant rate at the stage of plastic fluidity.
2. VCCs of MOS structures based on n-CdxHg1 xTe indicate that white-
light excitation effectively decreases the positive charge in the insula-
tor by means of injecting electrons from the valence band to the trap in
semiconductor oxide, spatially localized near the oxide–semiconductor
interface. Possible mechanism of the injection is electrons tunnelling
through the interface.
3. NPPE in narrow-bandgap CdxHg1 xTe crystals is natured from the
reduction in mechanical stress under the influence of light due to the
decrease of the embedded positive charge at own oxide–semiconductor
boundary. It is accompanied by the lowering of deformation barrier
for the exit of generated dislocations on the surface, resulting in re-
duced level of fluidity stress in the deformed crystal under irradia-
Fig. 9. Model representing the processes in uniaxially deformed CdxHg1 xTe
crystal (the implementation of negative PPE): 1—deformed single crystal,
2—dislocation slipping planes and directions of their movement, 3—oxide
layer on the surface of a crystal, 4—positive charge embedded in the insulator
on the intrinsic oxide–semiconductor interface, 5—intrinsic oxide–single
crystal boundary.
540 B. P. KOMAN, O. O. BALITSKII, and D. S. LEONOV
tion.
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