Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties
The technique of thermal vacuum deposition of Ge onto GaAs substrates has been used for obtaining nanocrystalline Ge films. Nanocrystalline character of the films is confirmed by atomic force microscopy of their surface and by the data of Raman light scattering. The most probable size of the nano...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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irk-123456789-1184852017-05-31T03:07:56Z Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties Borblik, V.L. Korchevoi, A.A. Nikolenko, A.S. Strelchuk, V.V. Fonkich, A.M. Shwarts, Yu.M. Shwarts, M.M. The technique of thermal vacuum deposition of Ge onto GaAs substrates has been used for obtaining nanocrystalline Ge films. Nanocrystalline character of the films is confirmed by atomic force microscopy of their surface and by the data of Raman light scattering. The most probable size of the nanocrystallites forming the films decreases monotonically with decreasing their thickness. Electro conductivity of such the films proves to be high enough (1-10 Ohm·cm at room temperature) and has a character of variable range hopping conduction of Mott’s type. The hops, presumably, take place through the localized states connected with the grain boundaries. 2014 Article Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties / V.L. Borblik, A.A. Korchevoi, A.S. Nikolenko, V.V. Strelchuk, A.M. Fonkich, Yu.M. Shwarts, M.M. Shwarts // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 3. — С. 237-242. — Бібліогр.: 10 назв. — англ. 1560-8034 PACS 73.63.-b, 81.07.-b http://dspace.nbuv.gov.ua/handle/123456789/118485 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The technique of thermal vacuum deposition of Ge onto GaAs substrates has
been used for obtaining nanocrystalline Ge films. Nanocrystalline character of the films
is confirmed by atomic force microscopy of their surface and by the data of Raman light
scattering. The most probable size of the nanocrystallites forming the films decreases
monotonically with decreasing their thickness. Electro conductivity of such the films
proves to be high enough (1-10 Ohm·cm at room temperature) and has a character of
variable range hopping conduction of Mott’s type. The hops, presumably, take place
through the localized states connected with the grain boundaries. |
| format |
Article |
| author |
Borblik, V.L. Korchevoi, A.A. Nikolenko, A.S. Strelchuk, V.V. Fonkich, A.M. Shwarts, Yu.M. Shwarts, M.M. |
| spellingShingle |
Borblik, V.L. Korchevoi, A.A. Nikolenko, A.S. Strelchuk, V.V. Fonkich, A.M. Shwarts, Yu.M. Shwarts, M.M. Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Borblik, V.L. Korchevoi, A.A. Nikolenko, A.S. Strelchuk, V.V. Fonkich, A.M. Shwarts, Yu.M. Shwarts, M.M. |
| author_sort |
Borblik, V.L. |
| title |
Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties |
| title_short |
Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties |
| title_full |
Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties |
| title_fullStr |
Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties |
| title_full_unstemmed |
Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties |
| title_sort |
nanocrystalline ge films created by thermal vacuum deposition on gaas substrates: structural and electric properties |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2014 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118485 |
| citation_txt |
Nanocrystalline Ge films created by thermal vacuum deposition on
GaAs substrates: structural and electric properties / V.L. Borblik, A.A. Korchevoi, A.S. Nikolenko, V.V. Strelchuk, A.M. Fonkich, Yu.M. Shwarts, M.M. Shwarts // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 3. — С. 237-242. — Бібліогр.: 10 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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| first_indexed |
2025-07-08T14:05:26Z |
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2025-07-08T14:05:26Z |
| _version_ |
1837087887325134848 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 237-242.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
237
PACS 73.63.-b, 81.07.-b
Nanocrystalline Ge films created by thermal vacuum deposition on
GaAs substrates: structural and electric properties
V.L. Borblik, A.A. Korchevoi, A.S. Nikolenko, V.V. Strelchuk, A.M. Fonkich,
Yu.M. Shwarts, M.M. Shwarts
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
Corresponding author e-mail: borblik@isp.kiev.ua
Abstract. The technique of thermal vacuum deposition of Ge onto GaAs substrates has
been used for obtaining nanocrystalline Ge films. Nanocrystalline character of the films
is confirmed by atomic force microscopy of their surface and by the data of Raman light
scattering. The most probable size of the nanocrystallites forming the films decreases
monotonically with decreasing their thickness. Electro conductivity of such the films
proves to be high enough (1-10 Ohm·cm at room temperature) and has a character of
variable range hopping conduction of Mott’s type. The hops, presumably, take place
through the localized states connected with the grain boundaries.
Keywords: thermal vacuum deposition, germanium, gallium arsenide, nanocrystalline
films, hopping conduction.
Manuscript received 07.02.14; revised version received 25.06.14; accepted for
publication 16.09.14; published online 30.09.14.
1. Introduction
The first works concerning with germanium epitaxy on
gallium arsenide substrate (in 60-ies) were carried out by
means of thermal deposition in vacuum and had not
satisfied researchers in many respects because they had
failed to get the films with electric properties close to
those of bulk Ge.
Since then the variety of more complete technique
have been developed (molecular beam epitaxy,
chemical-vapor deposition, high-vacuum magnetron
sputtering et al.) that have resulted in refining such kind
of films up to so-called “device quality” which is
characterized by high carrier mobility and large enough
lifetimes of non-equilibrium carriers, i. e. by high
crystalline perfection (see, for example, [1]).
At the same time a new science direction has
appeared (nanophysics) in the framework of which non-
perfect crystallinity, more exactly – nanocrystallinity, is
just desirable property. Wide investigation of
nanocrystalline (or nanostructured) objects is connected
with significant changes of all properties of the materials
(mechanical, electrical, optical) in nanostructured state
[2, 3]. This circumstance allows returning to simple and
cheap technology of thermal vacuum deposition.
Nanostructured state of Ge film is attractive,
uppermost, in connection with the possibility to obtain a
luminescence in visible range of the spectrum. It is
important also that these films should have a high
enough electro conduction that will allow obtaining the
active light emitting devices on their base.
In the course of present investigations, the different
regimes of Ge deposition on GaAs substrate have been
studied and those of them which result in nanocrystalline
films have been found. Then structural and electric
properties of the films obtained have been studied.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 237-242.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
238
2. Experimental
The crucible made of tantalum-graphite which was
heated by alternate current up to temperature of 1400-
1600 °C and derived thermal evaporation of Ge, was
located in vacuum camera with pressure of
Torr10)52( 6 . Temperature of the (100)GaAs
substrate with resistivity of 108 Ohm·cm could be
supported in the range of 200-550 °C by the special
heater.
The film thickness was measured by means of
micro profilometer “Alpha-Step 100” with accuracy of
4 nm and by micro interferometer МII–4.
Measurements of micro Raman light scattering
were carried out in reflection geometry at room
temperature by means of triple Raman spectrometer T-
64000 Horiba Jobin-Yvon equipped with cooled CCD
detector. For the sample excitation, light from Ar-Kr
ionic laser with wavelength of 488 nm are focused to the
sample by objective 50×/0.75 to the spot of ~1 µm size
under the radiation power near 1 mW.
Investigations of the film surface were carried out
by atomic force microscope NanoScope IIIa Dimention
3000 (Digital Instruments, США) in the regime of
periodic contact. The serial silicon probes NSG-11 with
nominal tip curvature radius of 10 nm (NT-MDT,
Russia) were used.
For measuring conductive properties of the films
obtained, planar contacts with the spacing of 1 mm were
deposited on them.
3. Results and discussion
3.1. Technological conditions for nanocrystallinity
of the films
Fig. 1 represents the data of Raman light scattering for
three films: #2 and #3 – deposited at temperature of 500
and 350 °С, respectively, on the substrates annealed at
500 °С for 20 min, and #4 – deposited at temperature of
300 °С on non-annealed substrate.
250 300
#4 - = 300.1 cm-1, = 6.5 cm-1
#2 - = 301.2 cm-1, = 5.3 cm-1
a-Ge
c-Ge
#3
#2
#4
R
am
an
in
te
n
si
ty
,
ar
b.
un
.
Raman shift, cm-1
Fig. 1. Spectra of Raman light scattering for the samples #2, #3
and #4; dashed curve represents spectrum of bulk Ge.
At low deposition temperature (350 °С), Ge film
turns out amorphous as it follows from style of the curve
for sample #3 in the form of diffuse peak. But at
deposition temperature of 500 °С (sample #2) the sharp
peak appears near the optic phonon frequency of
1cm300 which corresponds to crystalline Ge. However
this peak is broadened in comparison with that in the
bulk material (dashed curve in Fig. 1) due to
polycrystalline (grain) structure of the obtained film and
is asymmetrical because of the effect of quantum
confinement for phonons in the nano-dimensional
crystallites. Also the peak maximum undergoes the shift
to lower frequencies. And the film deposited at 300 °С
on non-annealed substrate (sample #4) turns out
amorphous as well (though with some portion of
crystalline phase).
3.2 Structure of the films surfaces
Results of the atomic force microscopy (AFM) are
represented in Fig. 2 for a number of the films (obtained
by Ge deposition at temperature of 380 °С on the
substrates annealed previously at 400 °С for 8 min)
which have different thicknesses – 450, 150 and 75 nm.
Fig. 3 represents analogous data for even more thin film
with thickness of 15 nm obtained under somewhat
another conditions – deposition at 400 °C on the non-
annealed substrate.
All of them have a grain, polycrystalline, structure.
But characteristic grain sizes turn out different
depending on the film thickness. Distribution of the
effective lateral grain sizes is presented on the
corresponding histograms in Figs 2 and 3, and
distribution of the vertical scales of the surface relief is
presented in Fig. 4 (along with the results of their
processing as the gauss curves).
As it follows from the data represented, decrease in
the film thickness (from 450 to 15 nm) results in
decreasing all characteristic scales of their surface relief.
So the most probable lateral grain size decreases from
40-60 nm through 35-40 nm (then 30-35 nm) down to
24-26 nm. The most probable value of the relief height
decreases from 19.26 nm through 11.68 nm (then
7.98 nm) down to 3.97 nm, and the root-mean-square
height deviation from the average value (roughness) –
from 5.73 nm through 3.3 nm (then 2.60 nm) down to
1.18 nm. It is obvious that gain of the grain size with
increase in the film thickness is connected directly with
increase in the time spent for their growth that results in
enlargement of the crystallites.
It is interesting that qualitatively similar
dependence between film thickness and grain size has
been observed in the case of hydrogenised germanium
films on glass [4]. As was noted in Ref. [4] the values
obtained by means of AFM prove to be somewhat
overrated because of the finite tip radius; therefore the
lateral scale of the film nanostructurization can turn out
even smaller in fact.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 237-242.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
239
20 40 60 80 100 120
0
5
10
15
20
25
C
o
un
t
Grain diameter, nm
# 22
450 nm
a
20 30 40 50 60 70
0
5
10
15
20
C
o
un
t
Grain diameter, nm
# 20
150 nm
b
15 20 25 30 35 40 45 50 55
0
5
10
15
20
25
C
o
u
n
t
Grain diameter, nm
# 12
75 nm
c
Fig. 2. Images of the film surface in atomic force microscope and histograms of the lateral grain sizes for Ge films with the
thickness of 450 (a), 150 (b) and 75 nm (c) on the GaAs substrates.
Above obtained dependences of the characteristic
scales of the surface relief on the film thickness are
summarized in Fig. 5. It is worthwhile to pay attention to
practically complete identity of the thickness
dependences of the characteristic scales for lower
boundary of the lateral sizes of the grains (a, bottom)
and their characteristic height (b).
In all the cases a ratio of the double characteristic
height to the characteristic lateral size is less than 1, i.e.
the grains (at least, in the near-surface layer) are oblate
in direction perpendicular to the surface, and their
oblateness increases with decrease in the film thickness.
In Fig. 6 this fact is demonstrated for the thinnest film
(15 nm) where the height of each element of the film
surface relief is plotted versus its effective lateral size. It
should be noted that authors of paper [5] also took notice
an oblateness of the grains on the surface of the films
consisting of Ge nanocrystals embedded in SiO2 matrix.
3.3. Conductive properties of the films
All the obtained films had linear current-voltage
characteristics up to applied voltage of 10 V in the
temperature range of 77-300 K.
Fig. 7 represents Arrenius curves for the current I at
the fixed voltage U = 1 V for two such the films: #13
(the film with thickness of 150 nm deposited at
temperature of 400 °С on the substrate annealed
previously at 450 °С for 5 min) and #16 (the film with
the same thickness of 150 nm deposited at temperature
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 237-242.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
240
10 15 20 25 30 35 40 45 50
0
20
40
60
80
100
120
C
o
u
n
t
Grain diameter, nm
# 23
15 nm
Fig. 3. The same as in Fig. 2 for the Ge film with the thickness of 15 nm on the GaAs substrate.
0 5 10 15 20 25 30 35 40
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
# 23 - 15 nm
x
C
3.96598 ±0.00295
w 2.37491 ±0.00759
A 1.91411 ±0.00711
# 22
450 nm
# 12 - 75 nm
x
C
7.97735 ±0.00974
w 5.21231 ±0.02472
A 4.27062 ±0.02338
# 20 - 150 nm
x
C
11.68154 ±0.00843
w 6.59268 ±0.0227
A 4.9867 ±0.02053
# 23
15 nm
# 22 - 450 nm
x
C
19.25845 ±0.01525
w 11.45986 ±0.04234
A 8.30859 ±0.03753# 20
150 nm
C
o
u
n
t,
%
Height, nm
# 12
75 nm
0 100 200 300 400 500
0
10
20
30
40
50
60
C
h
ar
ac
te
ri
st
ic
s
ca
le
s,
n
m
Thickness, nm
a
b
c
Fig. 4. Distribution of the height probabilities for the surface
relief of the films shown in Figs 2 and 3 along with results of
the data approximation by the gauss curves (by means of
program Origin).
Fig. 5. Dependences of the characteristic lateral non-
homogeneity scale of the film surface (a), the characteristic
scale of the relief height (b) and the root-mean-square
deviation of the height from average value (roughness) (c)
on the film thickness.
of 380 °C on the substrate annealed previously at 400 °C
for 6 min). These curves determine temperature
dependences of the film electro conductivity σ which (as
it is seen from the figure) is characterized by the variable
activation energy. These dependences are straightened
well being plotted as a function of 4/11 T as it is
demonstrated by Fig. 8. It follows from this that the film
conductivity is realized by means of variable range
hopping conduction of Mott’s type when
4/1
0 /ln TT . Here aEkg
T
F
0
const
where k is the
Boltzmann constant, a is the localization radius of the
states through which the hops take place, FEg is the
density of these states on the Fermi level. Values of
parameter T0 determined from the line slope for two
samples differ noticeably from each other.
0 5 10 15 20 25 30 35 40 45 50 55
0
1
2
3
4
5
6
H
ei
g
h
t,
n
m
Effective lateral diameter, nm
Fig. 6. Dependence between the heights of the surface relief
elements and their effective lateral sizes for the film with the
thickness 15 nm (the sample #23).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 237-242.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
241
It is worthwhile to look for a reason for significant
difference in the conductive properties of these two films
in their morphology. Shown in Figs 9 and 10 are the data
of the AFM of their surfaces.
It follows from these figures that the film #13
consists of much larger crystallites than the film #16. It
is obvious also that the distances between crystallites
will be more substantial in the film #13 than in the film
#16 that can explain its significantly smaller
conductivity.
From comparison of the characteristic temperatures
T0, it follows that the film #13 has larger density of
localized states g(EF) than the film #16 (if the
localization radii may be considered as equal
approximately). This fact can be connected with the
larger grains in the film #13 possessing by
correspondingly larger the external surface on which, by
hypothesis, the localized states connected with dangling
bonds are located.
The same type of conductivity (hopping, with the
variable activation energy) is known also for another
nanostructured objects – for example, for the films
containing Ge nanoclusters in SiO2 matrix [6] (in Ref.
[6], conductivity was measured across the films).
0.002 0.004 0.006 0.008 0.010 0.012 0.014
1E-6
1E-5
1E-4
Ge/GaAs #16
substrate annealing - 400oC (6 min), deposition - 380oC
I,
A
(
U
=
1
V
)
T -1, K -1
Ge/GaAs #13
substrate annealing - 450oC (5 min), deposition - 400oC
0.24 0.26 0.28 0.30 0.32 0.34
1E-6
1E-5
1E-4
Ge/GaAs #13
T0 = 5.63x105 K
Ge/GaAs #16
T0 = 7.46x106 K
I,
A
(
U
=
1
V
)
T -1/4, K -1/4
Fig. 7. Arrenius curves for the current along the film at
fixed voltage of 1 V for the samples #13 and #16.
Fig. 8. Temperature dependences of the current along the
film at fixed voltage of 1 V for the samples #13 and #16 as a
function of (1/T)1/4.
60 80 100 120 140 160 180 200 220
0
10
20
30
Grain diameter, nm
C
o
u
n
t
# 13
150 nm
a
20 25 30 35 40 45 50 55 60
0
10
20
30
C
o
u
n
t
Grain diameter, nm
# 16
150 nm
b
Fig. 9. Images of the film surface in atomic force microscope and histograms of the lateral grain sizes for the samples #13 (a)
and #16 (b).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 237-242.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
242
Authors of this paper have observed decrease in T0 with
the growth of Ge content in the films, i.e. with increase
in either sizes or number of conductive clusters that
results in increasing FEg .
Note that in the case when Ge in SiO2 matrix has
been transformed into nanocrystalline inclusions with
sizes of 4-9 nm [7] (as a result of following thermo-
treatment), the same authors have observed the
temperature dependence of the film conductivity in the
form of 2/1/1ln T . In spite of similarity of this
dependence with that for variable range hopping
conduction under coulomb gap [8], this mechanism is
not considered as appropriate to given situation.
The temperature dependence in the form of
4/1
0 /ln TT has been observed also in the paper [9]
for the conductivity along the films of similar type (Ge
nanoparticles embedded in SiO2 matrix) and for the
conductivity across the films which consist of Ge
nanocrystals on silicon or silica substrates grown at
room temperature [10]. In all similar cases, the variable
range hopping conduction of Mott’s type is explained by
the existence of the localized states connected with the
grain boundaries.
4. Conclusions
Thus the high enough temperature of the GaAs substrate
under deposition of Ge film (above 380-400 °С) is the
condition of obtaining its nanocrystalline structure. The
thinner the film, the smaller are the crystallites forming
it; this allows us to choose purposefully their desirable
size (at the average). In view of smallness of the
distances between the adjacent nanograins in the films,
their conductivity proves to be high enough (1-
10 Ohm·cm at room temperature) that will allow
creation of active light emitting devices on the base of
such the films.
References
1. S.-H. Tang, E. Y. Chang, M. Hudait, J.-S. Maa,
C.-W. Liu, G.-L. Luo, H.-D. Trinh, and Y.-H. Su,
High quality Ge thin film grown by ultrahigh
vacuum chemical vapor deposition on GaAs
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(2011).
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3. R.A. Andrievskii and A.V. Ragulya, Nano-
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3751 (1996).
7. M. Fujii, O. Mamezaki, S. Hayashi, and K.
Yamamoto, Current transport properties of SiO2
films containing Ge nanocrystals // J. Appl. Phys.
83(3), p. 1507-1512 (1998).
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Properties of Doped Semiconductors. Nauka,
Moscow, 1979.
9. I. Stavarache, A.-M. Lepadatu, A.V. Maraloiu, V.S.
Teodorescu, and M.L. Ciurea, Structure and
electrical transport in films of Ge nanoparticles
embedded in SiO2 matrix // J. Nanopart. Res.
14(7), Article: 930(1-9) (2012).
10. S. Banerjee, Non-ohmic hopping conduction in Ge
nanocrystalline film // Physica E, 15(3), p. 164-168
(2002).
0 10 20 30 40 50 60 70
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
# 16
# 13
C
o
u
n
t,
%
Height, nm
Fig. 10. Distribution of the height probabilities for the surface
relief of the samples #13 and #16.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 237-242.
PACS 73.63.-b, 81.07.-b
Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties
V.L. Borblik(, A.A. Korchevoi, A.S. Nikolenko, V.V. Strelchuk, A.M. Fonkich,
Yu.M. Shwarts, M.M. Shwarts
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
(Corresponding author e-mail: borblik@isp.kiev.ua
Abstract. The technique of thermal vacuum deposition of Ge onto GaAs substrates has been used for obtaining nanocrystalline Ge films. Nanocrystalline character of the films is confirmed by atomic force microscopy of their surface and by the data of Raman light scattering. The most probable size of the nanocrystallites forming the films decreases monotonically with decreasing their thickness. Electro conductivity of such the films proves to be high enough (1-10 Ohm·cm at room temperature) and has a character of variable range hopping conduction of Mott’s type. The hops, presumably, take place through the localized states connected with the grain boundaries.
Keywords: thermal vacuum deposition, germanium, gallium arsenide, nanocrystalline films, hopping conduction.
Manuscript received 07.02.14; revised version received 25.06.14; accepted for publication 16.09.14; published online 30.09.14.
1. Introduction
The first works concerning with germanium epitaxy on gallium arsenide substrate (in 60-ies) were carried out by means of thermal deposition in vacuum and had not satisfied researchers in many respects because they had failed to get the films with electric properties close to those of bulk Ge.
Since then the variety of more complete technique have been developed (molecular beam epitaxy, chemical-vapor deposition, high-vacuum magnetron sputtering et al.) that have resulted in refining such kind of films up to so-called “device quality” which is characterized by high carrier mobility and large enough lifetimes of non-equilibrium carriers, i. e. by high crystalline perfection (see, for example, [1]).
At the same time a new science direction has appeared (nanophysics) in the framework of which non-perfect crystallinity, more exactly – nanocrystallinity, is just desirable property. Wide investigation of nanocrystalline (or nanostructured) objects is connected with significant changes of all properties of the materials (mechanical, electrical, optical) in nanostructured state [2, 3]. This circumstance allows returning to simple and cheap technology of thermal vacuum deposition.
Nanostructured state of Ge film is attractive, uppermost, in connection with the possibility to obtain a luminescence in visible range of the spectrum. It is important also that these films should have a high enough electro conduction that will allow obtaining the active light emitting devices on their base.
In the course of present investigations, the different regimes of Ge deposition on GaAs substrate have been studied and those of them which result in nanocrystalline films have been found. Then structural and electric properties of the films obtained have been studied.
2. Experimental
The crucible made of tantalum-graphite which was heated by alternate current up to temperature of (1400-1600 °C and derived thermal evaporation of Ge, was located in vacuum camera with pressure of
Torr
10
)
5
2
(
6
-
×
-
. Temperature of the (100)GaAs substrate with resistivity of 108 Ohm·cm could be supported in the range of 200-550 °C by the special heater.
The film thickness was measured by means of micro profilometer “Alpha-Step 100” with accuracy of 4 nm and by micro interferometer МII–4.
Measurements of micro Raman light scattering were carried out in reflection geometry at room temperature by means of triple Raman spectrometer T-64000 Horiba Jobin-Yvon equipped with cooled CCD detector. For the sample excitation, light from Ar-Kr ionic laser with wavelength of 488 nm are focused to the sample by objective 50×/0.75 to the spot of ~1 µm size under the radiation power near 1 mW.
Investigations of the film surface were carried out by atomic force microscope NanoScope IIIa Dimention 3000 (Digital Instruments, США) in the regime of periodic contact. The serial silicon probes NSG-11 with nominal tip curvature radius of 10 nm (NT-MDT, Russia) were used.
For measuring conductive properties of the films obtained, planar contacts with the spacing of 1 mm were deposited on them.
3. Results and discussion
3.1. Technological conditions for nanocrystallinity
of the films
Fig. 1 represents the data of Raman light scattering for three films: #2 and #3 – deposited at temperature of 500 and 350 °С, respectively, on the substrates annealed at 500 °С for 20 min, and #4 – deposited at temperature of 300 °С on non-annealed substrate.
250
300
#4 -
w
= 300.1 cm
-1
,
G
= 6.5 cm
-1
#2 -
w
= 301.2 cm
-1
,
G
= 5.3 cm
-1
a-Ge
c-Ge
#3
#2
#4
Raman intensity, arb.un.
Raman shift, cm
-1
Fig. 1. Spectra of Raman light scattering for the samples #2, #3 and #4; dashed curve represents spectrum of bulk Ge.
At low deposition temperature (350 °С), Ge film turns out amorphous as it follows from style of the curve for sample #3 in the form of diffuse peak. But at deposition temperature of 500 °С (sample #2) the sharp peak appears near the optic phonon frequency of
1
cm
300
-
which corresponds to crystalline Ge. However this peak is broadened in comparison with that in the bulk material (dashed curve in Fig. 1) due to polycrystalline (grain) structure of the obtained film and is asymmetrical because of the effect of quantum confinement for phonons in the nano-dimensional crystallites. Also the peak maximum undergoes the shift to lower frequencies. And the film deposited at 300 °С on non-annealed substrate (sample #4) turns out amorphous as well (though with some portion of crystalline phase).
3.2 Structure of the films surfaces
Results of the atomic force microscopy (AFM) are represented in Fig. 2 for a number of the films (obtained by Ge deposition at temperature of 380 °С on the substrates annealed previously at 400 °С for 8 min) which have different thicknesses – 450, 150 and 75 nm. Fig. 3 represents analogous data for even more thin film with thickness of 15 nm obtained under somewhat another conditions – deposition at 400 °C on the non-annealed substrate.
All of them have a grain, polycrystalline, structure. But characteristic grain sizes turn out different depending on the film thickness. Distribution of the effective lateral grain sizes is presented on the corresponding histograms in Figs 2 and 3, and distribution of the vertical scales of the surface relief is presented in Fig. 4 (along with the results of their processing as the gauss curves).
As it follows from the data represented, decrease in the film thickness (from 450 to 15 nm) results in decreasing all characteristic scales of their surface relief. So the most probable lateral grain size decreases from 40-60 nm through 35-40 nm (then 30-35 nm) down to 24-26 nm. The most probable value of the relief height decreases from 19.26 nm through 11.68 nm (then 7.98 nm) down to 3.97 nm, and the root-mean-square height deviation from the average value (roughness) – from 5.73 nm through 3.3 nm (then 2.60 nm) down to 1.18 nm. It is obvious that gain of the grain size with increase in the film thickness is connected directly with increase in the time spent for their growth that results in enlargement of the crystallites.
It is interesting that qualitatively similar dependence between film thickness and grain size has been observed in the case of hydrogenised germanium films on glass [4]. As was noted in Ref. [4] the values obtained by means of AFM prove to be somewhat overrated because of the finite tip radius; therefore the lateral scale of the film nanostructurization can turn out even smaller in fact.
Above obtained dependences of the characteristic scales of the surface relief on the film thickness are summarized in Fig. 5. It is worthwhile to pay attention to practically complete identity of the thickness dependences of the characteristic scales for lower boundary of the lateral sizes of the grains (a, bottom) and their characteristic height (b).
0
10
20
30
40
50
60
70
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
# 16
# 13
Count, %
Height, nm
In all the cases a ratio of the double characteristic height to the characteristic lateral size is less than 1, i.e. the grains (at least, in the near-surface layer) are oblate in direction perpendicular to the surface, and their oblateness increases with decrease in the film thickness. In Fig. 6 this fact is demonstrated for the thinnest film (15 nm) where the height of each element of the film surface relief is plotted versus its effective lateral size. It should be noted that authors of paper [5] also took notice an oblateness of the grains on the surface of the films consisting of Ge nanocrystals embedded in SiO2 matrix.
3.3. Conductive properties of the films
All the obtained films had linear current-voltage characteristics up to applied voltage of 10 V in the temperature range of 77-300 K.
Fig. 7 represents Arrenius curves for the current I at the fixed voltage U = 1 V for two such the films: #13 (the film with thickness of 150 nm deposited at temperature of 400 °С on the substrate annealed previously at 450 °С for 5 min) and #16 (the film with the same thickness of 150 nm deposited at temperature of 380 °C on the substrate annealed previously at 400 °C for 6 min). These curves determine temperature dependences of the film electro conductivity σ which (as it is seen from the figure) is characterized by the variable activation energy. These dependences are straightened well being plotted as a function of
(
)
4
/
1
1
T
as it is demonstrated by Fig. 8. It follows from this that the film conductivity is realized by means of variable range hopping conduction of Mott’s type when
(
)
4
/
1
0
/
ln
T
T
µ
s
. Here
(
)
a
E
kg
T
F
0
const
=
where k is the Boltzmann constant, a is the localization radius of the states through which the hops take place,
(
)
F
E
g
is the density of these states on the Fermi level. Values of parameter T0 determined from the line slope for two samples differ noticeably from each other.
0.002
0.004
0.006
0.008
0.010
0.012
0.014
1E-6
1E-5
1E-4
Ge/GaAs #16
substrate annealing - 400
o
C (6 min), deposition - 380
o
C
I, A (U = 1 V)
T
-1
, K
-1
Ge/GaAs #13
substrate annealing - 450
o
C (5 min), deposition - 400
o
C
0
5
10
15
20
25
30
35
40
45
50
55
0
1
2
3
4
5
6
Height, nm
Effective lateral diameter, nm
Fig. 6. Dependence between the heights of the surface relief elements and their effective lateral sizes for the film with the thickness 15 nm (the sample #23).
It is worthwhile to look for a reason for significant difference in the conductive properties of these two films in their morphology. Shown in Figs 9 and 10 are the data of the AFM of their surfaces.
It follows from these figures that the film #13 consists of much larger crystallites than the film #16. It is obvious also that the distances between crystallites will be more substantial in the film #13 than in the film #16 that can explain its significantly smaller conductivity.
0.24
0.26
0.28
0.30
0.32
0.34
1E-6
1E-5
1E-4
Ge/GaAs #13
T
0
= 5.63x10
5
K
Ge/GaAs #16
T
0
= 7.46x10
6
K
I, A (U = 1 V)
T
-1/4
, K
-1/4
From comparison of the characteristic temperatures T0, it follows that the film #13 has larger density of localized states g(EF) than the film #16 (if the localization radii may be considered as equal approximately). This fact can be connected with the larger grains in the film #13 possessing by correspondingly larger the external surface on which, by hypothesis, the localized states connected with dangling bonds are located.
The same type of conductivity (hopping, with the variable activation energy) is known also for another nanostructured objects – for example, for the films containing Ge nanoclusters in SiO2 matrix [6] (in Ref. [6], conductivity was measured across the films). Authors of this paper have observed decrease in T0 with the growth of Ge content in the films, i.e. with increase in either sizes or number of conductive clusters that results in increasing
(
)
F
E
g
.
Note that in the case when Ge in SiO2 matrix has been transformed into nanocrystalline inclusions with sizes of 4-9 nm [7] (as a result of following thermo-treatment), the same authors have observed the temperature dependence of the film conductivity in the form of
(
)
2
/
1
/
1
ln
T
µ
s
. In spite of similarity of this dependence with that for variable range hopping conduction under coulomb gap [8], this mechanism is not considered as appropriate to given situation.
The temperature dependence in the form of
(
)
4
/
1
0
/
ln
T
T
µ
s
has been observed also in the paper [9] for the conductivity along the films of similar type (Ge nanoparticles embedded in SiO2 matrix) and for the conductivity across the films which consist of Ge nanocrystals on silicon or silica substrates grown at room temperature [10]. In all similar cases, the variable range hopping conduction of Mott’s type is explained by the existence of the localized states connected with the grain boundaries.
4. Conclusions
Thus the high enough temperature of the GaAs substrate under deposition of Ge film (above 380-400 °С) is the condition of obtaining its nanocrystalline structure. The
60
80
100
120
140
160
180
200
220
0
10
20
30
Grain diameter, nm
Count
# 13
150 nm
a
20
25
30
35
40
45
50
55
60
0
10
20
30
Count
Grain diameter, nm
# 16
150 nm
b
thinner the film, the smaller are the crystallites forming it; this allows us to choose purposefully their desirable size (at the average). In view of smallness of the distances between the adjacent nanograins in the films, their conductivity proves to be high enough (1-10 Ohm·cm at room temperature) that will allow creation of active light emitting devices on the base of such the films.
References
1.
S.-H. Tang, E. Y. Chang, M. Hudait, J.-S. Maa,
C.-W. Liu, G.-L. Luo, H.-D. Trinh, and Y.-H. Su, High quality Ge thin film grown by ultrahigh vacuum chemical vapor deposition on GaAs substrate // Appl. Phys. Lett. 98(16), 161905(1-3) (2011).
2.
H. Gleiter, Nanostructured materials: basic concepts and microstructure // Acta Materialia, 48(1), p. 1-29 (2000).
3.
R.A. Andrievskii and A.V. Ragulya, Nanostructured Materials. Асаdemia, Moscow, 2005.
4.
W.B. Jordan, E.D. Carlson, T.R. Johnson, and S. Wagner // Mat. Res. Soc. Symp. Proc. 762, paper A6.5 (2003).
5.
I. Stavarache, A.-M. Lepadatu, T. Stoica, and M.L. Ciurea, Annealing temperature effect on structure and electrical properties of films formed of Ge nanoparticles in SiO2 // Appl. Surf. Sci. 285, Part B, p. 175-179 (2013).
6.
M. Fujii, Y. Inoue, S. Hayashi, and K. Yamamoto, Hopping conduction in SiO2 films containing C, Si, and Ge clusters // Appl. Phys. Lett. 68(26), p. 3749-3751 (1996).
7.
M. Fujii, O. Mamezaki, S. Hayashi, and K. Yamamoto, Current transport properties of SiO2 films containing Ge nanocrystals // J. Appl. Phys. 83(3), p. 1507-1512 (1998).
8.
B.I. Shklovskii and A.L. Efros, The Electronic Properties of Doped Semiconductors. Nauka, Moscow, 1979.
9.
I. Stavarache, A.-M. Lepadatu, A.V. Maraloiu, V.S. Teodorescu, and M.L. Ciurea, Structure and electrical transport in films of Ge nanoparticles embedded in SiO2 matrix // J. Nanopart. Res. 14(7), Article: 930(1-9) (2012).
10.
S. Banerjee, Non-ohmic hopping conduction in Ge nanocrystalline film // Physica E, 15(3), p. 164-168 (2002).
�
Fig. 10. Distribution of the height probabilities for the surface relief of the samples #13 and #16.
��
��
�
Fig. 7. Arrenius curves for the current along the film at fixed voltage of 1 V for the samples #13 and #16.�
Fig. 8. Temperature dependences of the current along the film at fixed voltage of 1 V for the samples #13 and #16 as a function of (1/T)1/4.�
�
�
Fig. 9. Images of the film surface in atomic force microscope and histograms of the lateral grain sizes for the samples #13 (a) and #16 (b).
�
Fig. 3. The same as in Fig. 2 for the Ge film with the thickness of 15 nm on the GaAs substrate.
��
��
�
Fig. 4. Distribution of the height probabilities for the surface relief of the films shown in Figs 2 and 3 along with results of the data approximation by the gauss curves (by means of program Origin).�
Fig. 5. Dependences of the characteristic lateral non-homogeneity scale of the film surface (a), the characteristic scale of the relief height (b) and the root-mean-square deviation of the height from average value (roughness) (c) on the film thickness.�
�
�
Fig. 2. Images of the film surface in atomic force microscope and histograms of the lateral grain sizes for Ge films with the thickness of 450 (a), 150 (b) and 75 nm (c) on the GaAs substrates.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
237
10
15
20
25
30
35
40
45
50
0
20
40
60
80
100
120
Count
Grain diameter, nm
# 23
15 nm
0
5
10
15
20
25
30
35
40
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
# 23 - 15 nm
x
C
3.96598
±
0.00295
w
2.37491
±
0.00759
A
1.91411
±
0.00711
# 22
450 nm
#
12 - 75 nm
x
C
7.97735
±
0.00974
w
5.21231
±
0.02472
A
4.27062
±
0.02338
# 20 - 150 nm
x
C
11.68154
±
0.00843
w
6.59268
±
0.0227
A
4.9867
±
0.02053
# 23
15 nm
# 22 - 450 nm
x
C
19.25845
±
0.01525
w
11.45986
±
0.04234
A
8.30859
±
0.03753
# 20
150 nm
Count, %
Height, nm
# 12
75 nm
0
100
200
300
400
500
0
10
20
30
40
50
60
Characteristic scales, nm
Thickness, nm
a
b
c
20
40
60
80
100
120
0
5
10
15
20
25
Count
Grain diameter, nm
# 22
450 nm
a
20
30
40
50
60
70
0
5
10
15
20
Count
Grain diameter, nm
# 20
150
nm
b
15
20
25
30
35
40
45
50
55
0
5
10
15
20
25
Count
Grain diameter, nm
# 12
75 nm
c
_1466426106.unknown
_1466426330.unknown
_1473601853.unknown
_1473601589.unknown
_1466426176.unknown
_1466426310.unknown
_1466425987.unknown
_1466426102.unknown
_1466425144.unknown
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