Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers
Optical and structural properties of undoped ZnSe epilayers with thickness ranging from 0.5 to 2 mm grown by molecular beam epitaxy on GaAs (001) substrates have been investigated by depth resolved optical and X-ray methods. It was found that the epilayers with thicknesses above some value (>1 μm...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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| Zitieren: | Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers / G.N. Semenova, E.F. Venger, N.O. Korsunska, V.P. Klad’ko, L.V. Borkovska, M.P. Semtsiv, M.B. Sharibaev, V.I. Kushnirenko, Yu.G. Sadofyev // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 2. — С. 133-137. — Бібліогр.: 13 назв. — англ. |
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irk-123456789-1211822017-06-14T03:06:50Z Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers Semenova, G.N. Venger, E.F. Korsunska, N.O. Klad’ko, V.P. Borkovska, L.V. Semtsiv, M.P. Sharibaev, M.B. Kushnirenko, V.I. Sadofyev, Yu.G. Optical and structural properties of undoped ZnSe epilayers with thickness ranging from 0.5 to 2 mm grown by molecular beam epitaxy on GaAs (001) substrates have been investigated by depth resolved optical and X-ray methods. It was found that the epilayers with thicknesses above some value (>1 μm) contain three regions of different structural and optical quality. It is shown that two of these regions (near top surface and near interface ones) contain higher defect density. The nature of luminescence line at 446.1nm (4.2 K) is discussed. It was found that the radiation enhanced defect reactions occurred in the top surface region of epilayer. 2002 Article Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers / G.N. Semenova, E.F. Venger, N.O. Korsunska, V.P. Klad’ko, L.V. Borkovska, M.P. Semtsiv, M.B. Sharibaev, V.I. Kushnirenko, Yu.G. Sadofyev // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 2. — С. 133-137. — Бібліогр.: 13 назв. — англ. 1560-8034 PACS: 66.30.Jt, 66.30.Qa, 61.72.Vv, 61.72.Ff, 61.72.Yx http://dspace.nbuv.gov.ua/handle/123456789/121182 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Optical and structural properties of undoped ZnSe epilayers with thickness ranging from 0.5 to 2 mm grown by molecular beam epitaxy on GaAs (001) substrates have been investigated by depth resolved optical and X-ray methods. It was found that the epilayers with thicknesses above some value (>1 μm) contain three regions of different structural and optical quality. It is shown that two of these regions (near top surface and near interface ones) contain higher defect density. The nature of luminescence line at 446.1nm (4.2 K) is discussed. It was found that the radiation enhanced defect reactions occurred in the top surface region of epilayer. |
| format |
Article |
| author |
Semenova, G.N. Venger, E.F. Korsunska, N.O. Klad’ko, V.P. Borkovska, L.V. Semtsiv, M.P. Sharibaev, M.B. Kushnirenko, V.I. Sadofyev, Yu.G. |
| spellingShingle |
Semenova, G.N. Venger, E.F. Korsunska, N.O. Klad’ko, V.P. Borkovska, L.V. Semtsiv, M.P. Sharibaev, M.B. Kushnirenko, V.I. Sadofyev, Yu.G. Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Semenova, G.N. Venger, E.F. Korsunska, N.O. Klad’ko, V.P. Borkovska, L.V. Semtsiv, M.P. Sharibaev, M.B. Kushnirenko, V.I. Sadofyev, Yu.G. |
| author_sort |
Semenova, G.N. |
| title |
Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers |
| title_short |
Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers |
| title_full |
Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers |
| title_fullStr |
Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers |
| title_full_unstemmed |
Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers |
| title_sort |
defects and radiation-enhanced defect reactions in znse/(001)gaas mbe layers |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2002 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121182 |
| citation_txt |
Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers / G.N. Semenova, E.F. Venger, N.O. Korsunska, V.P. Klad’ko, L.V. Borkovska, M.P. Semtsiv, M.B. Sharibaev, V.I. Kushnirenko, Yu.G. Sadofyev // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 2. — С. 133-137. — Бібліогр.: 13 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-07-08T19:21:16Z |
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2025-07-08T19:21:16Z |
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1837107752423391232 |
| fulltext |
133© 2002, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2002. V. 5, N 2. P. 133-137.
PACS: 66.30.Jt, 66.30.Qa, 61.72.Vv, 61.72.Ff, 61.72.Yx
Defects and radiation-enhanced defect reactions
in ZnSe/(001)GaAs MBE layers
G.N. Semenova, E.F. Venger, N.O. Korsunska, V.P. Klad�ko, L.V. Borkovska,
M.P. Semtsiv, M.B. Sharibaev, V.I. Kushnirenko, Yu.G. Sadofyev1)
Institute of Semiconductor Physics, NAS of Ukraine, 45 prospect Nauky, 03028 Kyiv, Ukraine,
Phone: (044) 265-96-45; fax: (044) 265-83-44; e-mail: bork@lumin.semicond.kiev.ua
1)P.N. Lebedev Physical Institute RAS, 117927 Moscow, Russia
Abstract. Optical and structural properties of undoped ZnSe epilayers with thickness ranging
from 0.5 to 2 µm grown by molecular beam epitaxy on GaAs (001) substrates have been
investigated by depth resolved optical and X-ray methods. It was found that the epilayers
with thicknesses above some value (>1 µm) contain three regions of different structural and
optical quality. It is shown that two of these regions (near top surface and near interface ones)
contain higher defect density. The nature of luminescence line at 446.1nm (4.2 K) is discussed.
It was found that the radiation enhanced defect reactions occurred in the top surface region of
epilayer.
Keywords: MBE, ZnSe, photoluminescence, degradation.
Paper received 07.05.02; revised manuscript received 03.06.02; accepted for publication 25.06.02.
1. Introduction
In recent years, ZnSe-based II-VI heterostructures have
been intensively investigated as the promising candidates
for light-emitting devices in the blue-green spectral range.
However, their application is still limited by degrada-
tion processes [1]. It is known that degradation rate de-
pends on the density of dislocations generated on ZnSe/
GaAs heterointerface. So, the buffer layer of thickness
h<hC (hC - critical thickness for strain relaxation by mis-
fit dislocations which is 0.15-0.2 µm for ZnSe/GaAs) is
used as a rule. From the other hand, it was shown [2] that
decrease of the total thickness of ZnSe layers separated
the GaAs substrate and the active layers results in the
drop of thermal stability of heterostructures. Thus, in-
vestigation of ZnSe layer characteristics in dependence
on their thickness is to be interesting.
In present work dependence of the structural and lu-
minescence characteristics of ZnSe epilayers both on their
thickness and as a function of depth has been investi-
gated. As it will be shown the top region of thick (1.3÷2
µm) epilayers contains the higher concentration of the
extended defects and plays a noticeable role in radiation
enhanced defect reaction.
2. Experiment and samples treatment
Five series of undoped ZnSe layers of different thickness
(0.5-2 µm) were grown by molecular beam epitaxy (MBE)
on semiinsulating Cr-doped oriented 3° off (001) towards
[110] GaAs substrates in a CATYN� machine equipped
with conventional effusion cells for Zn and Se. The re-
sidual pressure in the chamber was ~8⋅10�11 Torr. For
deoxidation the GaAs substrates were heated up to the
temperature of about 580 0C without or with the use of an
As beam. Before the deposition of ZnSe epilayer the GaAs
surface was treated in Zn flux during 100 s at pressure
~4⋅10�7 Torr for prevention the chemical reaction of Se
and the excess Ga on the GaAs surface. Reflection high-
energy electron diffraction, RHEED, was applied to con-
trol the surface during deoxidation and deposition proc-
esses. ZnSe epilayers were grown at the temperatures of
260�340 0C and Se/Zn beam pressure ratios were 1.2�
1.5. The growth rate was ~0.6 µm/hour. The free carrier
concentration, n, and mobility, µ, were obtained from
Hall effect measurements at 300 K. Table 1 shows some
technological conditions of the growth process and pa-
rameters of the samples.
134 SQO, 5(2), 2002
G.N. Semenova et al.: Defects and radiation-enhanced defect reactions...
X-ray diffraction methods and X-ray topography as
well as exciton and impurity luminescence spectroscop-
ies were used to control the epilayer properties. Photolu-
minescence (PL) of ZnSe was excited by 365 nm line of
the 200 W mercury lamp. PL spectra were recorded in
the temperature range 4.2-77 K using the grating
spectrometer MDR-23. Depth uniformity of ZnSe
epilayers (ELs) was investigated using step etching. For
simulation of the degradation processes the samples were
subjected to the illumination by UV light of 500 W mer-
cury lamp.
3. Experimental results and discussions
The (004) rocking curves for ZnSe/GaAs epilayers of dif-
ferent thickness grown under approximately the same
conditions (Tg=300 0C, Se/Zn beam pressure ratio ~1.2)
are shown in Fig.1. The angle between the ZnSe and
GaAs (004) reflections increases with the increase of layer
thickness. Using the angle value, the ratio of the out-of
plane lattice parameter a⊥ to in-plane lattice parameter
a|| a⊥/a||<1 was obtained, and a small residual biaxial
tensile strain component was evaluated (ε≤2⋅10-3) for all
the series of the samples. It is known that at 300 K the
mismatch ~ 0.27 % between ZnSe and GaAs (aZnSe >
aGaAs) should result in the in-plane biaxial compressive
strain. Since all investigated samples were grown of thick-
ness h>hC a partial strain relaxation is expected to occur
during the growth processes [3, 4]. Therefore, small ten-
sile strains observed at 300 K are the thermally induced
strains stemming from different thermal expansion coef-
ficients of the ZnSe epilayer and the GaAs substrate [3].
The (004) rocking curves full width at half maximum
(FWHM) changed nonmonotonically with the increase
of layer thickness (Fig.1). The narrowest curve was ob-
tained from the 1.3 µm thick films that demonstrates a
crystalline quality of such epilayers.
An additional information about the epilayer quality
was obtained from PL data. The PL spectra of the inves-
tigated ZnSe ELs at 4.2 K are shown in Fig. 2. The spec-
tra of thick samples (curves 2, 3) in the near band edge
region consist of the narrow bands at λm1=442 nm,
λm2=443.5 nm, λm3=446.8 nm and λm4=476.5 nm. The
position of first peak corresponds to free-exciton transi-
tion, IFX. On the more large scale it consist of two peaks
(main peak and shoulder at shorter wavelengths). This
double peak can be ascribed to valence band splitting on
light-hole, lh, and heavy-hole, hh, branches due to strain
[3]. Position of the second PL peak corresponds to neu-
tral donor-bound transition, I2 (D0, X). The I2 (D0, X)
peak is attributed usually to GaZn [5,6].
The band at 446.8 nm labeled as IV
0 was attributed to
extended defects [7-9] as well as the band Y0 at 476.5 nm
[7]. On the short wavelength side of IV
0 peak the shoulder
or the peak at 446 nm (IX) is often observed in our sam-
ples also (see Fig. 4). Spectral position of last peak is
close to the positions of two-electron satellite (2EL) of
elpmaS snoitidnocygolonhceT mliflaixatipE
oN
nZ/eS
oitar Tg,
oC
ecafruS
noitcurtsnocer
,ssenkcihT
µm
n K003 mc, 3-
ytiliboM µ K003 ,
mc 2 V/ ⋅s
eSronZ
dehcir
ecafrus
05 2.1 003 c )2x2( 0.2 * * nZ
11 2.1 003 c )2x2( 3.1 * * nZ
43 2.1 003 c )2x2( 5.0 8⋅ 01 71 022 nZ
53 2.1 062 )1x2( 5.0 3.1 ⋅ 01 81 091 eS
63 5.1 043 c )2x2( 6.0 1.2 ⋅ 01 71 052 nZ
Table 1. Technology conditions and initial parameters of ZnSe epilayers.
* high resistivity
G.N. Semenova et al.: Defects and radiation-enhanced defect reactions...
135SQO, 5(2), 2002
(D0, X) transitions and neutral acceptor bound exciton
(connected with As or its complexes) [3]. Its possible na-
ture will be considered later. In the spectral region be-
tween IV
0 and Y0
bands the weak luminescence of differ-
ent DA-pairs, namely DAP-1 (456.5 nm), Q-DAPs (460-
461 nm) and DAP-2 (462 nm), are observed [10].
In the 500-700 nm spectral region the PL bands con-
nected with deep level defects are observed in all the sam-
ples. For the thick layers weak PL bands were observed
at 500 nm and 560-580 nm (DAP) [10] while PL spectra
of the thin samples showed a broad peak at 620 nm. The
latter one is due to donor-acceptor pairs VZn-D, where D
is the group III element [10].
The low value of ratio of donor bound exciton peak
intensity to that of free-exciton ξ≈1 (ξ =I(D0,X)/I (FX)) in
our thick epilayers confirms their high optical quality. It
should be noted that intensity of IV
0 and Y0
relatively to
IFX increases when epilayer thickness increases from 1.3
to 2 µm (Fig. 2, curve 2, 3). The increase of IV
0 intensity
relatively to IFX and I2 with epilayer thickness shows the
deterioration of epilayer when its thickness is above some
optimal value. This is in agreement with nonmonotonical
dependence of FWHM of X-ray rocking curve on h.
The PL spectra of thinner samples (No 34-36, see
Table) show the peak at 443.5 nm only (Fig. 1, curve 1).
The FWHM of these peaks varies from 7 meV (sample
No 36) to 11 meV (sample No 34). Optical reflection
spectra contained the excitonic peculiarities for the sam-
ple No 36 only. The absence of the exciton peculiarities
in the optical reflection spectra and large FWHM of PL
peak in samples No 34 and 35 testify to its band-to-band
recombination nature that is in agreement with high car-
rier density obtained from Hall measurements. Presence
of the I2 peak in samples No 50, 11 and 36 indicates
diffusion of Ga from the substrate to ZnSe epilayer [5].
We think that high carrier concentration in samples N34,
35 is connected with GaZn also because the highest free
electron concentration is observed in the sample ¹35,
which is grown on the Ga-riched substrate. So, PL meas-
urements show the higher quality (lower impurity con-
centration) of thick samples comparing to thin ones that
is in agreement with results obtained in [11].
PL spectra transformation at step etching revealed
considerable depth inhomogeneity of 2 µm thick sample
that was characterized by high value of FWHM of rock-
ing curve. Fig. 3 represents 4.2 K PL spectra before (curve
1) and after (curves 2, 3) step etching of sample. After 0.2
mm etching an intensity of IV
0 line decreases essentially.
At the same time the ratio I2/IFX changes insignificantly.
Etching of the 1.5 µm layer leads to subsequent decrease
of IV
0/IFX and to increase of I2/IFX ratios (inset in Fig.3).
It is known that IV
0 intensity drops with the increase of
impurity concentration [12], that may be the reason of
IV
0 intensity drop after etching. But insignificant change
of I2/IFX ratio after 0.2 µm layer etching testifies to that
IV
0 decrease is due to not the change of Ga concentration
but to the decrease of extended defect concentration. So,
we can conclude that the top region of thick epilayers
contains higher extended defect concentration.
-800 -600 -400 -200 0 200
3
2
1
4'53"
6'20"
7'32"
X
-r
a
y
in
te
n
s
it
y
(a
.
u
.)
θ (a rcsec )
Fig. 1. (004) X-ray diffraction curves of ZnSe epilayers of differ-
ent thicknesses: 0.5 (curve 1), 1.3 (2) and 2.0 µm (curve 3).
440 450 460 470 480
3
2
1
x 8
x 8
x 8
I
X
2.602.702.80
E n erg y (eV )
Y
0
I
V
0
I
2
I
F X
D A P
P
L
in
te
n
s
it
y
(a
rb
.
u
n
.)
W a v e leng th (n m )
Fig. 2. Typical 4.2 K photoluminescence spectra from ZnSe
epilayers of different thicknesses: 0.6 (curve 1), 1.3 (curve 2)
and 2.0 µm (curve 3) λexc=365.0 nm.
136 SQO, 5(2), 2002
G.N. Semenova et al.: Defects and radiation-enhanced defect reactions...
The X-ray measurements fulfilled on 2 µm thick ZnSe
epilayer under step etching confirmed the depth inhomo-
geneity of the ZnSe epilayers. The X-ray double-crystal
topography of the samples gives an additional informa-
tion about this inhomogenity. The topograms were ob-
tained from (311) asymmetric reflection before and after
etching. From these experimental results we point out
the existence of two regions with higher defect density:
thin near top surface region and near interface one. The
latter has the thickness ~ 0.5 µm and contains high misfit
dislocation and point defect density. PL spectra confirm
this assertion. Increase of the I2 peak intensity in com-
parison with IFX one after 1.5 µm layer etching is in agree-
ment with supposed donor nature (GaZn) of this peak (dif-
fusion from GaAs substrate). It should be noted that in
the thin samples dislocations are present in the whole
ZnSe layer volume.
Let us consider now the possible nature of IX band. It
is essential that the high ratio (~0.3) of IX/I2 intensities
can be observed in our samples (Fig. 4). This fact testifies
against the two-electron satellite of (D0, X) nature of IX.
To get an additional information about the IX band we
investigated the temperature dependence of PL spectra
of the thick samples. As figure 4 shows the temperature
dependence of IX intensity does not correlate with the
same dependencies for I2 and IFX. This fact does not al-
low us to suppose that IX is the I2 satellite or acceptor
bound exciton. This is confirmed by the absence of corre-
0 1 2
440 442 444 446 448 450
3
2
1
I
V
0
/I
F X
I
2
/I
F X
2 . 7 52 . 8 0
E n erg y (eV )
I
V
0
I
X
I
2
I
F X
P
L
in
te
n
s
it
y
(a
rb
.
u
n
.)
W av e leng th (n m )
d (µm )
Fig. 3. 4.2 K PL spectra of thick 2.0 µm ZnSe epilayer:
unetched (curve 1), etched down to 1.8 µm (curve 2), etched
down to 1.3 µm (curve 3), λexc=365.0 nm. The inset shows the
I2
Ga
and IV
0 peak intensities normalized by height of IFX line as a
function of depth (d) in the ZnSe epilayer.
440 442 444 446 448 450 452
x 7 4 5 K
x 7
2 0 K
x 3 .5
1 0 K
x 2
6 K
2.80 2.75
E n erg y (eV )
x 1
E
g
I
V
0
I
X
I
2I
F X
T = 4 .2 K
P
L
in
te
n
s
it
y
(a
rb
.
u
n
.)
W a v e leng th (nm )
Fig. 5. 77 K PL spectra of 0.6 µm ZnSe epilayer before (a) and
after (b) UV irradiation λexc=365 nm.
Fig. 4. Temperature transformation of the excitonic photolumi-
nescence of 1.3 µm ZnSe epilayer, λexc=365 nm.
500 600 700
x 1 0
I
6
2.80 2.75 2.70 2.50 2.25 2.00 1.75
440 450 460
x 2
I
FX
(A 0,X )
(D ,A )
}
E n erg y (eV )
2
1
P
L
in
te
n
s
it
y
(
a
rb
.
u
n
.)
W a v e leng th (nm )
G.N. Semenova et al.: Defects and radiation-enhanced defect reactions...
137SQO, 5(2), 2002
lation between the temperature dependence of IX and IFX
band maximum positions: IX band position does not
change practically up to T = 45-50 K whereas FX posi-
tion shifts toward the low energies. Such temperature
dependencies of IX intensity and peak position in con-
junction with its small FWHM are typical for PL lines
connected with dislocations in II-VI compounds [13].
For elucidation of the top imperfect layer contribu-
tion in degradation processes we investigated the influ-
ence of UV-radiation on its PL characteristics. Fig. 5
shows the 77K photoluminescence spectra from ZnSe
epilayers before (curve 1) and after UV-irradiation (curve
2) (sample N36). The near band edge emission at this
temperature shows the combination of band IFX and shoul-
der at 447 nm. The last PL band was identified as exciton
bound to neutral acceptor (A0,X) caused by VZn [10]. In
addition a weak structural donor-to-acceptor (D,A) band
connected with Ga (450-480 nm) and the one connected
with VZn-GaZn (620 nm - IA) are observed. After UV irra-
diation within 3 hours at room temperature (D,A) band
disappears completely (Fig. 5, curve 2). The intensity of
the (A0,X) band as well as the IA band decreases as the
time of treatment increases. This fact gives the evidence
that UV light enhanced point defect reactions including
GaZn and VZn. take place in the investigated layers.
Because the UV light is absorbed in the near surface re-
gion (~ 0.1 µm) the observed point defect reactions take
place in the near top part of epilayer.
4. Conclusion
In conclusion, we have found that increase of epilayer
thickness up to 1.3 µm results in the increase of its struc-
tural quality and decrease of impurity concentration.
However, further increase of epilayer thickness leads to
epilayer deterioration. Besides, we have shown that the
thick samples are depth inhomogeneous and consist of
three region with different extended defect and impurity
concentration: (i) near the interface region with high den-
sity of misfit dislocations and point defects; (ii) the re-
gion with low extended defect and impurity concentra-
tion and (iii) near the top surface region with higher ex-
tended defect concentration. Experiment on step etching
and temperature dependence of PL spectra allow us to
conclude that IX line (λ=446 nm) is connected with ex-
tended defects (dislocations).
Acknowledgments
It is a pleasure to acknowledge Dr I.A. Mazarchuk
for sample etching.
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