Metastable interstitials in CdSe and CdS crystals
An "anomalous" defect drift in external electric field, namely, transport of acceptorlike centres from the anode to the cathode, has been observed in CdS:Cu, CdS:Ag and nominally undoped CdSe crystals at 350-700 K. The effect is accounted for by transformation of acceptors into donors unde...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2003
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| Цитувати: | Metastable interstitials in CdSe and CdS crystals / L.V. Borkovska, B.M. Bulakh, L.Yu. Khomenkova, N.O. Korsunska, I.V. Markevich // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2003. — Т. 6, № 4. — С. 437-440. — Бібліогр.: 14 назв. — англ. |
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irk-123456789-1180822017-05-29T03:04:35Z Metastable interstitials in CdSe and CdS crystals Borkovska, L.V. Bulakh, B.M. Khomenkova, L.Yu. Korsunska, N.O. Markevich, I.V. An "anomalous" defect drift in external electric field, namely, transport of acceptorlike centres from the anode to the cathode, has been observed in CdS:Cu, CdS:Ag and nominally undoped CdSe crystals at 350-700 K. The effect is accounted for by transformation of acceptors into donors under heating. The donors are metastable centres that do not display themselves in the equilibrium state and can be revealed only by drift in electric field. The acceptors are shown to be substitutional impurity atoms, acceptor-to-donor transformation occurring due to transition of these atoms from lattice sites to interstitials. Under cooling reverse donor-to-acceptor transition takes place. 2003 Article Metastable interstitials in CdSe and CdS crystals / L.V. Borkovska, B.M. Bulakh, L.Yu. Khomenkova, N.O. Korsunska, I.V. Markevich // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2003. — Т. 6, № 4. — С. 437-440. — Бібліогр.: 14 назв. — англ. 1560-8034 PACS: 61.72 Ji; 66.30 Jt; 66.30 Qa http://dspace.nbuv.gov.ua/handle/123456789/118082 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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An "anomalous" defect drift in external electric field, namely, transport of acceptorlike centres from the anode to the cathode, has been observed in CdS:Cu, CdS:Ag and nominally undoped CdSe crystals at 350-700 K. The effect is accounted for by transformation of acceptors into donors under heating. The donors are metastable centres that do not display themselves in the equilibrium state and can be revealed only by drift in electric field. The acceptors are shown to be substitutional impurity atoms, acceptor-to-donor transformation occurring due to transition of these atoms from lattice sites to interstitials. Under cooling reverse donor-to-acceptor transition takes place. |
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Article |
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Borkovska, L.V. Bulakh, B.M. Khomenkova, L.Yu. Korsunska, N.O. Markevich, I.V. |
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Borkovska, L.V. Bulakh, B.M. Khomenkova, L.Yu. Korsunska, N.O. Markevich, I.V. Metastable interstitials in CdSe and CdS crystals Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Borkovska, L.V. Bulakh, B.M. Khomenkova, L.Yu. Korsunska, N.O. Markevich, I.V. |
| author_sort |
Borkovska, L.V. |
| title |
Metastable interstitials in CdSe and CdS crystals |
| title_short |
Metastable interstitials in CdSe and CdS crystals |
| title_full |
Metastable interstitials in CdSe and CdS crystals |
| title_fullStr |
Metastable interstitials in CdSe and CdS crystals |
| title_full_unstemmed |
Metastable interstitials in CdSe and CdS crystals |
| title_sort |
metastable interstitials in cdse and cds crystals |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2003 |
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http://dspace.nbuv.gov.ua/handle/123456789/118082 |
| citation_txt |
Metastable interstitials in CdSe and CdS crystals / L.V. Borkovska, B.M. Bulakh, L.Yu. Khomenkova, N.O. Korsunska, I.V. Markevich // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2003. — Т. 6, № 4. — С. 437-440. — Бібліогр.: 14 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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2025-07-08T13:20:06Z |
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2025-07-08T13:20:06Z |
| _version_ |
1837085026381987840 |
| fulltext |
437© 2003, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2003. V. 6, N 4. P. 437-440.
PACS: 61.72 Ji; 66.30 Jt; 66.30 Qa
Metastable interstitials in CdSe and CdS crystals
L.V. Borkovska, B.M. Bulakh, L.Yu. Khomenkova, N.O. Korsunska, I.V. Markevich
V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine,
45, pr. Nauky, 03028 Kyiv, Ukraine, E-mail: khomen@lumin.semicond.kiev.ua,
Fax: +380 (44) 2658342; Phone: +380 (44) 2657234
Abstract. An �anomalous� defect drift in external electric field, namely, transport of acceptor-
like centres from the anode to the cathode, has been observed in CdS:Cu, CdS:Ag and nomi-
nally undoped CdSe crystals at 350�700 K. The effect is accounted for by transformation of
acceptors into donors under heating. The donors are metastable centres that do not display
themselves in the equilibrium state and can be revealed only by drift in electric field. The
acceptors are shown to be substitutional impurity atoms, acceptor-to-donor transformation
occurring due to transition of these atoms from lattice sites to interstitials. Under cooling
reverse donor-to-acceptor transition takes place.
Keywords: metastable centres, defect drift in electric field.
Paper received 25.09.03; accepted for publication 11.12.03.
1. Introduction
One of the main factors of semiconductor material deg-
radation is transformation of the so-called metastable de-
fects (MDs) under various external influences [1�3]. Such
processes also pose a number of fundamental problems
essential to physics of defects in semiconductors. That is
why MDs are investigated intensively.
It was shown that MDs can be both complex [1,2] and
point [3, 4] defects. The former rearrange due to their
recharge after photocarrier capture and following long-
range diffusion of components [1,2], while the later form
different centres because of short-range shift occurring
in the region of one configuration cage [1,4]. Complex
MD�s were found earlier in CdS and CdSe crystals and
were proved to be responsible for photo-enhanced defect
reactions [1,2]. The present investigations have shown
that, in these compounds, point MD�s, namely, metastable
interstitials can also be present and display themselves in
such phenomenon, as an �anomalous� defect drift under
electric field.
Drift of lattice defects under electric field is the well-
known effect. When external electric field is applied to
semiconductor at a fit temperature, redistribution of
charged mobile defects along the sample occurs, donors
being collected near the cathode and acceptors being
accumulated near the anode [5]. In n-type semiconduc-
tor, this redistribution must result in the increase of sam-
ple conductivity near the cathode and its decrease near
the anode. Such �normal� effect was observed, in par-
ticular, in nominally undoped and doped with Li CdS
crystals, where mobile shallow donors Cdi and Lii were
shown to be present [6]. Drift of these donors in external
electric field caused the increase of conductivity and pho-
tosensitivity in the near-cathode crystal region [7]. It has
been found, however, that sometimes an �anomalous�
effect, namely, the accumulation of acceptors near the
cathode can take place. This effect that has been observed
in nominally undoped CdSe, as well as in doped with
copper and silver CdS crystals, is described and investi-
gated in this work.
2. Experimental procedure
Bulk and platelet CdS crystals doped with Cu or Ag and
nominally undoped CdSe platelets were investigated. The
crystals were highly-resistive: ρ > 105 Ohm⋅cm for CdSe
and ρ > 109 Ohm⋅cm for CdS. All the samples were of n-
type, which is usual for CdS and CdSe. Ohmic In or Cd
electrodes were applied to the crystals as shown in Fig.1.
In the initial state, i.e. before action of electric field,
dark current (DC), as well as photocurrent (PC) and pho-
toluminescence (PL) spectra were measured at 300 or
77 K between electrodes 1, 2 and 3, 4 (Fig.1, a). Then
electrodes 1, 2 and 3, 4 were closed (Fig.1, b), the sample
was heated to Td = 350�700 K and direct electric field
Ed = 50�100 V/cm was applied to it. After a time interval
∆t, the sample was quickly cooled to room temperature,
438
SQO, 6(4), 2003
L.V. Borkovska et al.: Metastable interstitials in CdSe and CdS crystals ...
various sample regions between electrodes 1, 3 and 2, 4
showed that with ∆t increase the region with quenched IR
band and PC extrinsic maximum spread along the sam-
ple to the cathode. The effect was reversible: when the
sample was heated again and electric field of opposite
direction was applied to it, the strengthening of IR band
and PC extrinsic maximum near the new cathode and
their quenching near the new anode occurred.
Similar changes of PL and PC spectra took place also
in CdS:Ag crystals, where in the initial state orange PL
band λm = 0.61 µm dominated in PL spectra and PC ex-
trinsic maximum peaked at about 0.6 µm was observed
(Fig. 4 a, b).
The dark currents in investigated CdS:Cu and CdS:Ag
crystals both before and after action of electric field were
too small to be measured exactly.
1
1
a
b
±Ed
2
2
3
3
4
4
Fig. 1. Appearance of the sample mounted for measurements of
DC and PC spectra (a) and for drift carrying out (b).
0.5
10
�6
10
10
�4
�5
0.6 0.7 0.8 0.9
3
2
Wavelength, µm
P
h
o
to
cu
rr
en
t,
A
1
a
0.8
100
50
0
1.0 1.2 1.4 1.81.6
2
1
3
Wavelength, µm
P
L
i
n
te
n
si
ty
,
a
rb
.
u
n
.
b
Fig. 2. PC (a) and PL (b) spectra of CdSe crystal at 300 K (a)
and 77 K (b) measured between the electrodes 1,2 before (1)
and after (2,3) the action of electric field, when the electrodes
1,2 were the cathode (2) and the anode (3) (Ed = 100 V/cm,
Td = 400K, ∆td = 1 min). The values of dark current are shown as
dashed lines.
the electric field was switched off, electrodes 1, 2 and 3, 4
were opened, and above-mentioned characteristics were
measured again. To control DC and PC in any sample
region, additional indium electrodes were applied be-
tween 1,3 and 2,4 ones.
3. Experimental reasults
CdSe. The influence of electric field on CdSe crystal
characteristics was found to take place already at
Td = 350�370 K. After application of Ed = 50�100 V/cm
during ∆t = 30�40 s at this temperature, DC and PC val-
ues increased at the anode and decreased at the cathode
(Fig. 2, a). Simultaneously, the only present in PL spec-
trum band at λm = 0.93 µm strengthened near the anode
and quenched near the cathode (Fig. 2, b). With ∆t in-
crease, the high-conductivity region spread to the cath-
ode, and at last only thin low-conductivity strip was ob-
served near this electrode, while the rest crystal became
highly conductive. If then electric field of opposite direc-
tion was applied to the sample at the same Td, the low-
conductivity region created near the new cathode. This
process could be repeated many times, and the results
were reproduced.
CdS. In the initial state in PL spectra of CdS:Cu crys-
tals an infrared band at λm = 1.0 µm was the most inten-
sive and in PC spectra a strong extrinsic maximum peaked
approximately at 0.75 µm was present (Fig. 3, a,b).
Application of electric field at Td = 650�700 K dur-
ing ∆t = 3�5 min resulted in sharp drop of IR-band in-
tensity near the anode and its rise near the cathode, while
the intensity of the other PL band did not change notice-
ably (Fig. 3, a). Simultaneously, PC extrinsic maximum
value decreased near the anode and increased near the
cathode (Fig. 3, b). The measurements of PL and PC in
L.V. Borkovska et al.: Metastable interstitials in CdSe and CdS crystals ...
439SQO, 6(4), 2003
In the initial state, CdS:Cu and CdS:Ag crystals had
reddish-brown and bright brown colour, respectively.
Described above changes in PL and PC spectra were
accompanied by the change of crystal colour: after switch-
ing electric field on at first, a thin bright yellow (like undo-
ped CdS) strip appeared near the anode. Then, this strip
broadened with ∆t increase, and at last only a thin inten-
sively coloured region was observed near the cathode.
Under electric field of opposite direction, the crystal ac-
quired reddish-brown or bright brown colour again, and
then bright yellow strip appeared near the new anode.
Induced by electric field characteristic changes kept
for many months at 300 K both for CdS and CdSe crys-
tals.
Wavelength, µm
P
L
i
n
te
n
si
ty
,
a
rb
.
u
n
.
0.7 0.8 0.9 1.0 1.1 1.2 1.3
0
50
100
2
3
1
a
Wavelength, µm
P
h
o
to
cu
rr
en
t,
A
0.5 0.6 0.7 0.8
10
�5
10
�7
10
�9
3
b
2
1
Fig. 3. PL (a) and PC (b) spectra of CdS:Cu crystal at 77K
measured between electrodes 1,2 before (1) and after (2,3) the
action of electric field, when the electrodes 1,2 were the cath-
ode (2) and the anode (3) (Ed = 70V/cm, Td = 650K, ∆td = 5min.)
Wavelength, µ m
P
L
i
n
te
n
si
ty
,
a
rb
.
u
n
.
0.6 0.7 0.8
0
50
100
3
3
2
1
a
Wavelength, µ m
P
h
o
to
cu
rr
en
t,
A
0.5 0.6 0.7
10
�9
10
�8
10
�7
10
�6
3
b
2
1
Fig. 4. PL (a) and PC (b) spectra of CdS:Ag crystal at 77 K
measured between the electrodes 1,2 before (1) and after (2,3)
action of electric field, when the electrodes 1,2 were the catho-
de (2) and the anode (3) (Ed = 90 V/cm, Td = 700 K, ∆td = 10 min).
4. Discussion
Above results show that, in investigated crystals, trans-
port of acceptors, i.e. negatively charged particles, to
the cathode takes place during drift process. Really, re-
distribution of DC along the sample in CdSe crystals in-
dicates directly that the change of acceptor density with
respect to donor one occurs under electric field. The in-
crease of DC in the major part of the sample and its sharp
decrease near the cathode testifies that this change is due
to extraction of acceptors from the anode-side region and
their accumulation at the cathode. This process is not
accompanied by the rise of any PL band intensity, so,
one can think that the acceptors are nonradiative recom-
440
SQO, 6(4), 2003
L.V. Borkovska et al.: Metastable interstitials in CdSe and CdS crystals ...
bination centres. The increase of such centres density must
lead to the drop of PC and PL intensity [4], which is ob-
served indeed (Fig. 2, a,b).
In CdS crystals electric field induces considerable
changes in IR (for CdS:Cu) and orange (for CdS:Ag) PL
band intensities, while the intensities of other bands re-
main almost unchanged. This is the evidence that ob-
served PL spectrum transformations result from changes
of densities of radiative centres responsible for λm =
= 1.0 µm and λm = 0.61 µm bands [8]. These bands are
known to result from recombination of free electrons on
CuCd and AgCd acceptors respectively, and observed in
PC spectra extrinsic maxima were shown to be due to
photoionization of electrons from these acceptors to c-
band [8, 9]. Since Cucd and Agcd acceptors are «sensi-
tizing» recombination centres [8,9], the values of the in-
trinsic PC maxima correlate with their densities too. Thus,
during drift process, the densities of CuCd and AgCd cen-
tres decrease in the anode-side region and increase near
the cathode.
It is known that Cu and Ag incorporate in CdS and
diffuse inside the crystal lattice interstitially as donors
Cui
+ and Agi
+ with following reactions Cui+VCd→CuCd
and Agi+VCd→AgCd superimposed on the diffusion proc-
esses, diffusivity of interstitials being much more than
that of substitutes [10,11]. Acceptors CuCd and AgCd are
well known radiative recombination centres [8,12], while
donors Cui and Agi do not display themselves in the equi-
librium state [10,13]. Obtained results testify that in pre-
viously doped crystals reverse reactions CuCd→Cui+VCd
and AgCd→Agi+VCd are intensive enough at 650�700K.
Because of these processes, the impurity atoms acting at
300 K as acceptors, at elevated temperatures drift in elec-
tric field as donors and become acceptors again under
cooling. So, the impurities are extracted from the anode-
side sample region and accumulated near the cathode.
The change of crystal colour after the action of electric
field confirms this conclusion.
Transformation process similar to described above
takes place, obviously, in CdSe crystals. Although these
crystals were nominally undoped, none of native defects
could play the role of the centre under consideration.
Really, a distinctive feature of drifting defects in CdSe is
their high diffusivity. The only native defect that is mo-
bile enough at 350�370K is shallow donor Cdi [6, 7, 12],
but it by no means can transform to acceptor. Thus, de-
fects responsible for anomalous drift in CdSe crystals must
be some residual impurity atoms, in all probability, in
cadmium sublattice, because Cd substitutes are much
more mobile than Se ones [5,12]. Therefore, elements of
the first Group should be considered. The impurity, how-
ever, is neither Cu or Ag, nor Li because: i) diffusion of
Cu or Ag in CdSe at 350�370 K is too slow to explain
observed effect [13,14]; ii) CuCd and AgCd acceptors in
CdSe are radiative centres [8, 9,12] and should be ob-
served in PL spectra; iii) although Lii has high mobility
(its drift is observed already at 250 K [6,7]), LiCd→Lii
transition was not found in CdS and CdSe crystals up to
700 K [13]. Since, in CdSe platelets growth, H2Se va-
pour was present, one can suppose that the impurity un-
der consideration is hydrogen. This impurity is known to
have the highest mobility in semiconductors [5,12] and
to form metastable centres in some of them [4].
In conclusion, an �anomalous� defect drift in electric
field, namely, transport of acceptors from the anode to
the cathode was observed in CdS:Cu, CdS:Ag and nomi-
nally undoped CdSe crystals at 350�700K. The effect
was accounted for by transformation of acceptors into
donors under heating. Transformation process was con-
cluded to consist in replacement of substitutional impu-
rity atoms from Cd sites to interstitial positions. In CdS
crystals, impurities responsible for anomalous drift have
been shown to be Cu and Ag. In CdSe platelets the impu-
rity under consideration is supposed to be hydrogen.
Acknowledgments
This work was supported by National Academy of Sci-
ences of Ukraine, and one author (L.Yu.K.) were sup-
ported by Grant of the President of Ukraine for young
scientists.
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