Quantum effects in multilayer Si-Ge nanoheterostructures
The lateral photoconductivity spectra and photofield electron emission were used to investigate multilayer Ge/Si heterostructures with Ge quantum dots. Earlier we have revealed a close connection between elastic strain in Ge quantum dots originating due to the lattice mismatch during the epitaxial g...
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Інститут хімії поверхні ім. О.О. Чуйка НАН України
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| Cite this: | Quantum effects in multilayer Si-Ge nanoheterostructures / Yu.N. Kozyrev, M.Yu. Rubezhanska, A.V. Sushyі, S.V. Kondratenko, O.V. Vakulenko, A.A. Dadykin, A.G. Naumovets // Поверхность. — 2008. — Вип. 14. — С. 176-185. — Бібліогр.: 25 назв. — англ. |
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Kozyrev, Yu.N. Rubezhanska, M.Yu. Sushyі, A.V. Kondratenko, S.V. Vakulenko, O.V. Dadykin, A.A. Naumovets, A.G. 2019-02-12T14:29:05Z 2019-02-12T14:29:05Z 2008 Quantum effects in multilayer Si-Ge nanoheterostructures / Yu.N. Kozyrev, M.Yu. Rubezhanska, A.V. Sushyі, S.V. Kondratenko, O.V. Vakulenko, A.A. Dadykin, A.G. Naumovets // Поверхность. — 2008. — Вип. 14. — С. 176-185. — Бібліогр.: 25 назв. — англ. 2617-5975 https://nasplib.isofts.kiev.ua/handle/123456789/146920 538.94 The lateral photoconductivity spectra and photofield electron emission were used to investigate multilayer Ge/Si heterostructures with Ge quantum dots. Earlier we have revealed a close connection between elastic strain in Ge quantum dots originating due to the lattice mismatch during the epitaxial growth and additional energy level forming in the strained Si-Ge heterojunction region. Thus, it appeared to be possible observing intraband transitions in Ge quantum dots that are absent in two-dimensional Si-Ge heterostructures using such simple and informative methods. While an influence of the number of Ge quantum dot layers on lateral photoconductivity spectra is not essential, the photofield electron emission characteristics showed considerable shift to middle infrared area, as the number of Ge quantum dot layers increased. It was revealed that size and composition parameters of Ge quantum dots correspond to energy levels in the valence band of the latter with the energy distance between them about 0.32 and 0.34 eV with a high accuracy. The results of our investigation make it possible to expect their possible application in new nano- and optoelectronic devices. Методами фотопольової емісії та повздовжньої фотопровідності було досліджено багатошарові наногетероструктури з квантовими точками германію. Попередні дослідження показали тісний зв'язок пружних напружень, що виникають при епітаксійному рості матеріалів з різними постійними граток, з виникненням додаткових рівнів в зоні такого гетеропереходу. Таким чином, відносно простими але інформативними методами спостерігалися міжзонні переходи, які відсутні в двовимірних гетероструктурах Si-Ge. Якщо вплив кількості шарів на фотопольову емісію несуттєвий, то при дослідженні фотопровідності виявлено прямий зв'язок зсуву чутливості повздовжньої фотопровідності в середній інфрачервоний діапазон від кількості шарів з квантовими точками германію. Показано, що розмірні характеристики та мольний склад квантових точок відповідають енергетичним рівням з енергетичною відстанню між ними приблизно 0,32 та 0,34 еВ з дуже високою точністю. Результати досліджень дозволяють сподіватися на їх використання в перспективних приладах нано- та оптоелектроніки. The research was implemented within the bilateral ÖAD Project UA No 2007/05 and supported by the program of fundamental research of the National Academy of Sciences of Ukraine “Nanostructured systems, nanomaterials, nanotechnologies” through the Project No 9/07 and by the Ministry of Education and Science of Ukraine through Project No M/139-07. en Інститут хімії поверхні ім. О.О. Чуйка НАН України Поверхность Физико-химия поверхностных явлений Quantum effects in multilayer Si-Ge nanoheterostructures Квантові ефекти в багатошарових Si-Ge наногетероструктурах Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Quantum effects in multilayer Si-Ge nanoheterostructures |
| spellingShingle |
Quantum effects in multilayer Si-Ge nanoheterostructures Kozyrev, Yu.N. Rubezhanska, M.Yu. Sushyі, A.V. Kondratenko, S.V. Vakulenko, O.V. Dadykin, A.A. Naumovets, A.G. Физико-химия поверхностных явлений |
| title_short |
Quantum effects in multilayer Si-Ge nanoheterostructures |
| title_full |
Quantum effects in multilayer Si-Ge nanoheterostructures |
| title_fullStr |
Quantum effects in multilayer Si-Ge nanoheterostructures |
| title_full_unstemmed |
Quantum effects in multilayer Si-Ge nanoheterostructures |
| title_sort |
quantum effects in multilayer si-ge nanoheterostructures |
| author |
Kozyrev, Yu.N. Rubezhanska, M.Yu. Sushyі, A.V. Kondratenko, S.V. Vakulenko, O.V. Dadykin, A.A. Naumovets, A.G. |
| author_facet |
Kozyrev, Yu.N. Rubezhanska, M.Yu. Sushyі, A.V. Kondratenko, S.V. Vakulenko, O.V. Dadykin, A.A. Naumovets, A.G. |
| topic |
Физико-химия поверхностных явлений |
| topic_facet |
Физико-химия поверхностных явлений |
| publishDate |
2008 |
| language |
English |
| container_title |
Поверхность |
| publisher |
Інститут хімії поверхні ім. О.О. Чуйка НАН України |
| format |
Article |
| title_alt |
Квантові ефекти в багатошарових Si-Ge наногетероструктурах |
| description |
The lateral photoconductivity spectra and photofield electron emission were used to investigate multilayer Ge/Si heterostructures with Ge quantum dots. Earlier we have revealed a close connection between elastic strain in Ge quantum dots originating due to the lattice mismatch during the epitaxial growth and additional energy level forming in the strained Si-Ge heterojunction region. Thus, it appeared to be possible observing intraband transitions in Ge quantum dots that are absent in two-dimensional Si-Ge heterostructures using such simple and informative methods. While an influence of the number of Ge quantum dot layers on lateral photoconductivity spectra is not essential, the photofield electron emission characteristics showed considerable shift to middle infrared area, as the number of Ge quantum dot layers increased. It was revealed that size and composition parameters of Ge quantum dots correspond to energy levels in the valence band of the latter with the energy distance between them about 0.32 and 0.34 eV with a high accuracy. The results of our investigation make it possible to expect their possible application in new nano- and optoelectronic devices.
Методами фотопольової емісії та повздовжньої фотопровідності було досліджено багатошарові наногетероструктури з квантовими точками германію. Попередні дослідження показали тісний зв'язок пружних напружень, що виникають при епітаксійному рості матеріалів з різними постійними граток, з виникненням додаткових рівнів в зоні такого гетеропереходу. Таким чином, відносно простими але інформативними методами спостерігалися міжзонні переходи, які відсутні в двовимірних гетероструктурах Si-Ge. Якщо вплив кількості шарів на фотопольову емісію несуттєвий, то при дослідженні фотопровідності виявлено прямий зв'язок зсуву чутливості повздовжньої фотопровідності в середній інфрачервоний діапазон від кількості шарів з квантовими точками германію. Показано, що розмірні характеристики та мольний склад квантових точок відповідають енергетичним рівням з енергетичною відстанню між ними приблизно 0,32 та 0,34 еВ з дуже високою точністю. Результати досліджень дозволяють сподіватися на їх використання в перспективних приладах нано- та оптоелектроніки.
|
| issn |
2617-5975 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/146920 |
| citation_txt |
Quantum effects in multilayer Si-Ge nanoheterostructures / Yu.N. Kozyrev, M.Yu. Rubezhanska, A.V. Sushyі, S.V. Kondratenko, O.V. Vakulenko, A.A. Dadykin, A.G. Naumovets // Поверхность. — 2008. — Вип. 14. — С. 176-185. — Бібліогр.: 25 назв. — англ. |
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2025-11-27T04:31:07Z |
| last_indexed |
2025-11-27T04:31:07Z |
| _version_ |
1850800349399482368 |
| fulltext |
Химия, физика и технология поверхности. 2008. Вып. 14. С. 176 – 185
176
УДК 538.94
QUANTUM EFFECTS IN MULTILAYER
Si-Ge NANOHETEROSTRUCTURES
Yu.N. Kozyrev1, M.Yu. Rubezhanska1, A.V. Sushyі1,
S.V. Kondratenko2, O.V. Vakulenko2, A.A. Dadykin3, A.G. Naumovets3
1О.О. Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine
General Naumov Str. 17, 03164 Kyiv-164
2Kiev National Taras Shevchenko University, Physics Department,
Acad. Glushkov Str. 2, 03022 Kyiv, Ukraine
3Institute of Physics, National Academy of Sciences of Ukraine
Prospect Nauki 46, 03028 Kyiv, Ukraine
The lateral photoconductivity spectra and photofield electron emission were used to
investigate multilayer Ge/Si heterostructures with Ge quantum dots. Earlier we have revealed
a close connection between elastic strain in Ge quantum dots originating due to the lattice
mismatch during the epitaxial growth and additional energy level forming in the strained Si-Ge
heterojunction region. Thus, it appeared to be possible observing intraband transitions in Ge
quantum dots that are absent in two-dimensional Si-Ge heterostructures using such simple and
informative methods. While an influence of the number of Ge quantum dot layers on lateral
photoconductivity spectra is not essential, the photofield electron emission characteristics
showed considerable shift to middle infrared area, as the number of Ge quantum dot layers
increased. It was revealed that size and composition parameters of Ge quantum dots
correspond to energy levels in the valence band of the latter with the energy distance between
them about 0.32 and 0.34 eV with a high accuracy. The results of our investigation make it
possible to expect their possible application in new nano- and optoelectronic devices.
Introduction
With the progress in nanotechnologies new possibilities were opened for easy creation
of nanosize objects, in particular Ge-Si heterostructures with Ge quantum dots, which attract
much attention of researchers all over the world as systems that allow introduction of band gap
engineering into silicon technology. This opens promising ways for the development of new
Si-based high-frequency and optoelectronic devices, for instance photoemission detectors for
infrared (IR) range that have been already elaborated [1 – 3]. Due to their geometry, ensembles
of nanosized Ge quantum dots make possible the realisation of high electric fields ~ 107 V/cm
near the dot apex at moderate voltages ~103–104 V, which causes an intense field electron
emission.
The field emission from semiconductor cathodes was widely investigated by several
research groups beginning from 1960’s [4 – 8]. To our knowledge, the first field emission
measurements from Ge nanoclusters grown on Si were reported in 2000 by Tondare et al. [9].
The first works on this system recorded no peculiarities in the current-voltage (I-V) curves of
the field emission. We also investigated such systems [10, 11] and revealed interesting
features – reproducible peaks of current in the I-V-curves of the field emission from Ge
nanocluster structures on Si(100). The number of the peaks was found to depend on the cluster
size. We connected the peaks with the effects of energy quantization in the Ge quantum dots.
Moreover, the field emission current showed a considerable photosensitivity in the wavelength
range from 0.4 to 10 mm. For the photodiodes that were used, it was found that the field
emission current increased at room temperature by a factor of 5 to 3 under the irradiation by
177
light with the wavelength of 2 and 10 mm, respectively [12]. Besides that, lateral
photoconductivity spectra from the same structures that we investigated independently revealed
photocurrent in the mid-infrared range caused by optical transition from localized states in the
valence band of QDs [13, 14].
It should also be noted that considering photoelectric and electronic properties of Ge-Si
heterostructures with Ge quantum dots one has to take into account the morphology of such
systems. They are generally referred to the second type heterostructures, in which holes are
localized in the quantum dots, while electrons are supposed to be free in the conduction band.
A large number of experimental works were devoted to investigation of the energy-band
structure in a quantum dot. It is supposed that the positions of energy levels in a quantum dot
are determined by its size, shape and composition, as well as the value of inhomogeneous
strain inside it caused by the lattice mismatch. Morphological investigations may make
possible the optimization of epitaxial conditions and growth mode of the heterostructure
formation [15]. Another important parameter that essentially influences optical and
photoelectric properties of Ge/Si heterostructures is the value of the valence band offset. The
average valence band offset of Si/Si1-xGex heterojunction was estimated as 0.54×x (eV), where x
is Ge concentration. This value describes rather accurately the properties of heterojunctions of
different compositions. The significant valence band offset between silicon and Si1-xGex alloys
determines a wide range of spectral sensitivity variation of photodetectors with the SiGe
quantum dots [16].
We present here our experimental results on lateral photoconductivity and photofield
electron emission from multilayer Ge/Si heterostructures with Ge quantum dots. They seem to
reveal quantum size effects which mirror the nature of electron transport in such structures.
Thus one of the main purposes of the present work was to search for a correlation between the
quantum regularities of the photofield electron emission and the lateral photoconductivity in
Ge/Si heterostructures with Ge quantum dots.
Experimental methods
The molecular beam epitaxy (MBE) technique (“Katun’-B” set-up, produced in
Novosibirsk, Russia) was used to prepare multilayer Ge-Si(100) nanocluster arrays with the
islands of various sizes and surface density. The (100) oriented wafers of n-Si with 7.5 and 20
Ohm×cm resistivity and diameter of 76 mm were used as substrates. In order to prepare
multilayer quantum dot systems with regular nanoisland distribution over the substrate surface,
we had earlier proposed to use a system of Si1-xGex intermediate layers with a sub-critical
thickness [11]. The Ge mole fraction x was gradually increased from layer to layer grown at
gradually decreasing substrate temperature started from Ts=500 оС. The growth process, in
particular the moment of the 2D®3D transition in the Stranski-Krastanov growth regime, was
controlled via RHEED (reflection high energy electron diffraction). To study the surface
morphology, atomic force microscopy (AFM) measurements were carried out using an MFP-
3D AFM from Asylum Research with a closed loop scanner. Standard Si cantilevers with tips
having a half opening angle of 10° were employed as probes. Fig. 1 represents an AFM image
of the top nanoisland layer of the investigated samples. It shows that the nanoclusters are
shaped as tetrahedral pyramids with the base about 40 nm and height about 2 nm. The average
cluster density was about 1010 сm–2. The growth of each Si intermediate layer was continued
until a high-contrast Si(100)2´1 RHEED pattern was produced typical of clean Si. Thus, the
multilayer Ge-Si(100) nanocluster arrays with three, five and eight layers of SiGe quantum
dots alternated by Si layers of the thickness about 3.5 nm were grown at the temperature
Ts=500 оС. The Ge fraction in the investigated structures was estimated by secondary ion mass-
spectroscopy (SIMS) technique. This technique was applied to monitor sequentially the Ge
fraction within the Si/Ge layers with QDs in the growth direction with a step of 1 nm. Taking
178
into account the value of QD surface density and the average size of QDs, the average Ge
fraction in the QD ensemble was estimated at the range from 75 to 85 %.
Fig. 1. A 1´1 mm2 AFM image of the QDs grown at 500 оС. The average height and base
diameter of dots are about 2 nm and 40 nm, respectively.
The measurements of the field and photofield electron emission were carried out in a
diode cell, in which the distance between the electrodes was 300 mm (fig. 2, a). In order to
visualize the spatial emission distribution, a ZnS cathodoluminescent screen was deposited
onto a glass plate having a SnO2 conducting layer which served as the anode. The details of the
experiment were presented in [12].
а b
Fig. 2. (a) Diode cell for measurements of the field and photofield electron emission.
(b) Sample for photoconductivity measurements.
To investigate lateral photoconductivity of SiGe quantum dot multilayer structures, two
Au ohmic contacts 1 mm in diameter and placed 10 mm apart were fused into the surface with
the SiGe QD layers [14] (fig. 2, b). As a result, the contacts were brought to each QD layer and
to Si substrate. The conduction current-voltage characteristics of all the investigated structures
were linear in the voltage range from U = –50 V to +50 V at the temperatures 77 – 290 К.
Spectral dependences of lateral photoconductivity were measured using infrared spectrometer
in the hn range from 0.3 to 1.1 eV at U = 5 V. The photocurrent was measured using a current
amplifier and the standard lock-in detection technique. The measured spectral dependences
were brought to a constant quantum quantity using a nonselective pyroelectric receiver.
179
Results and discussion
Field and photofield electron emission
The investigated structures showed a stable field emission at T=290 К and a
macroscopic electric field ~ 105 V/cm. It should be kept in mind that that the local electric
fields, which render the surface potential barrier sufficiently transparent to ensure the field
emission, are much higher (~107 V/cm). It is difficult to give their accurate evaluation, since
they are determined by details of geometrical and compositional heterogeneities on the
nanometer scale [17]. Curve 1 in fig. 3 is an I-V characteristic of photofield emission from a
heterostructure with SiGe quantum dots of the height ≈ 2 nm. The sample was irradiated with a
10 Wt light halogen lamp placed at 20 cm from it. The dark field emission current measured
without irradiation was several orders smaller (fig. 3, curve 2). Neither field emission nor
photofield emission current was recorded for the Si samples without quantum dots in the
investigated voltage range 2.0 – 4.3 kV. It is thus supposed that the observed photo-induced
current is produced by emission of electrons from the quantum dots.
2,0 2,5 3,0 3,5 4,0
0
5
10
15
20
25
1
2A
ve
ra
ge
c
ur
re
nt
d
en
si
ty
, m
A
/c
m
2
U, kV
Fig. 3. (1) The current-voltage curve of photofield emission from a Si/Ge heterostructure with
8 layers of quantum dots of the height ~3 nm. (2) The dark field emission current-
voltage for the same heterostructure.
The particular sample whose I-V curves are given in fig. 3 showed practically no dark
current in the voltage range studied. However, in our previous works11 we presented results
recorded for other Si/Ge quantum dot heterostructures which did generate rather intense field
emission currents in the dark (~10–7 to 10–5 A at the anode voltages of ~102 to 104 V and the
total sample area of ~10–1 cm2). It was found [11] that the structures with the quantum dots ≈ 3
to 5 nm high had the field emission current-voltage curves with a few distinct maxima (peaks).
The numbers of the peaks in the field emission I-V curve increases with QD size, so that
eventually no clear current peaks can be observed for the QDs higher than ≈ 10 nm. This effect
was attributed to resonant electron tunneling through quantized energy levels in the dots.
We suppose that the current peak in the I-V curve of photofield emission (fig. 3, cur-
ve 1) may also be connected with energy quantization, in particular the presence of discrete
energy levels in the valence band of the quantum dots. In the absence of external electric field,
Si/Ge heterojunction is referred to the second type heterostructures, in which a potential well is
formed for one type of carriers. Namely, the valence band of Ge nanoislands is a potential well
for holes causing a localization of states. Possible energy-band diagrams for Si/SiGe
heterojunction in the absence and presence of electric field are suggested in fig. 4, a and
fig. 4, b, respectively. It should be stressed that the potential well near the surface is in any
event highly asymmetric. Besides, the shape and transparence of the barrier Si/QD/vacuum
180
depends drastically on the strength of applied electric field. That is why the considered
Si/QD/vacuum “heterostructure” can neither be unambiguously classified as a first type nor as
a second type heterojunction. We have thus to treat it as a special type of heterojunction whose
properties depend upon the value of the applied electric field and require a separate approach.
In particular, the shape and width of the potential well for holes in the valence band of
SiGe nanoislands changes essentially when the electric field is applied to the surface. As a
consequence, the discrete energy values En change too. At some value of the applied electric
field, the energy position of a quantization level may coincide with the top of the Si valence
band, so that resonance tunneling of electrons can become probable from this band into
vacuum via the energy quantization level in the SiGe quantum dot (fig. 4, b). As the electric
field is increased further, the potential well becomes shallower and resonance tunneling can
proceed via the next energy level. The observation of a current peak for the structure with the
QDs about 2 nm high may indicate that at least one localized energy state exists in the valence
band of SiGe nanoisland in the absence of electric field.
a)
Si SiGe vacuum
e-
Si
SiGe
- eFx
vacuum
a b
Fig. 4. An energy-band diagram of Si/SiGe heterojunction without (a) and in the presence of
electric field F (b).
In our previous works [11], we considered another potential diagram for the field
emission from Si/Ge dots corresponding to the case of a strong downward band bending and
resonance tunneling through the discrete energy levels that can arise in the QD conduction
band. It should be admitted that the real potential configuration near the QD surface still
remains undetermined due to its complex dependence on a number of poorly evaluated factors,
such as exact QD geometry, composition and strain distribution, field penetration, the nature
and properties of surface states.
The essential increase of field emission current under the irradiation observed for the
structures with Ge quantum dots may be caused by optical transitions in the quantum dots
involving energy quantization levels. In low-dimensional Si/Ge heterostructures with Ge
quantum dots, there exists a possibility of electronic transitions in near infrared range from
energy quantization levels in the valence band of Ge quantum dots into the conduction band of
Ge quantum dots and conduction band of Si surrounding [18].
An Estimation of the energy level positions in the valence band of Ge nanoislands
The number of energy levels in the valence band of Ge nanoislands depends on Ge
nanocluster size, shape, elastic strain value and the corresponding potential well depth. When
treating the QD approximately as a box with a finite confinement potential, with a thickness Lz
and lateral dimension Lx = Ly, the energy level nml nx my lzE E E E= + + and the wave
181
function nml nx my lzy = y y y are three-dimensional. The corresponding energy values nxE ,
myE , lzE and functions nxy , myy , lzy are the solutions of one-dimensional equations. If
one considers the simplest model of a rectangular finite potential well, the position of energy
levels is determined by solving the transcendental equation [19]:
arcsin nK
KL n
G
æ ö= p - ç ÷
è ø , (1)
where *2 /n h nK m E= h , h/2 0
*VmG h= , 1,2,3...n = , *
hm is the effective mass of holes
in the potential well with the depth 0V , and L is the width of the potential well. It should be
noted that this estimation is approximate, since the real shape of Ge nanoislands, as well as
their inhomogeneous composition are not considered.
The depth of potential barriers is taken equal to value of valence band offset. It is
generally assumed that the valence band offsets for heavy and light hole states of the strained
Ge/Si heterojunction depend linearly on the Ge mole fraction and residual elastic strain values
exx and ezz. Uniaxial strains entail lifting of the valence band degeneracy in the SiGe
nanoislands. As a result, light and heavy hole states of the valence band are shifted in opposite
directions, and the shifts can be determined using the relation for the average valence band
offset av
vED in the following way [20]:
( )
2/1
2
0
2
00 4
9
2
1
4
1
6
1
úû
ù
êë
é +D+D++D-=D EEEELH ddd , (2)
EEHH d
2
1
3
1
0 -D=D , (3)
where the value Ed for elastic strain in the direction [001] can be given as:
( )xxzzbE eed -= 2001 . (4)
The values of spin-orbit splitting of the valence band 0D , deformation potential b for SiGe
nanoislands are determined using a linear interpolation of the data for clean Si and Ge.
According to our experimental results and estimations in 21, residual elastic strain value
in the investigated SiGe quantum dot multilayer structures amounts to exx »0.026¼0.027. The
best correspondence of theoretical calculations and experimental results concerning the
dependence of the valence band offset of a compressively strained Ge on Si gives value
av
VED = 0.54 eV for average value and hh
VED = 0.74 eV value for heavy hole discontinuity.16
The estimations showed that the valence band offsets for the strained interface Si0.2Ge0.8/Si
come to av
vED = 0.43 eV and hhED = 0.59 eV.
In particular, for a Ge quantum box with the width Lz = 2 nm, lateral dimensions
Lx = Ly = 40 nm, and *
00.34hhm m= × , one has E111 = 0.132 еV, E121 = E211 = 0.135 еV, E112 =
0.472 еV, E212 = 0.475 eV.
The proposed model is a simplified one allowing only a rough estimate of the number
of the localized states in the valence band of the QDs. It is evident that taking into account the
real shapes of the nanoislands, as well as inhomogeneous strain values and Ge fraction
182
distribution inside the clusters will give more precise positions of the energy levels in the
valence band.
Spectral dependences of lateral photoconductivity
Spectral dependences of lateral photocurrent of Ge/Si heterostructure with quantum
dots were measured at normal incidence and side illumination at 77 K (fig.5). The photocurrent
is caused by intraband transition between localized states in the valence band of nanoislands.
0,4 0,6 0,8 1,0
0
10
20
30
40
50
0,30 0,35 0,40
0,00
0,05
0,10
0,15
0,20
0,25
2
1
x10
Ph
ot
oc
ur
re
nt
,
ar
b.
un
.
hn,eV
2
hn , eV
1
Fig. 5. Spectral dependence of lateral photoconductivity of Ge/Si heterostructure with Ge
quantum dots measured at side excitation (1) and normal incidence (2) at 77 K. The
inset shows in details the low-energy part of the spectra.
In the case of side excitation, when unpolarized light spreads along the basis of Ge
nanoislands, there exists a component of the vector E oscillating along the growth direction (z-
component), in which the confinement is the most pronounced. As it is known, intraband
transitions in potential wells may be caused only by z-component of the vector E.
The optical absorption coefficient for the transition from level i to f in a QDs is
proportional to the matrix element:
2
i fa µ y yp , (4)
where p is the momentum operator for corresponding transition. The integral defining the
transitions between the levels has the form:
*
mn i f
k
P i dxdydz
x
ò ò ò
¶
= - y y
¶
h , (5)
where , ,kx x y z= . Polarization selection rules will be specified by nonzero terms of this
integral. Only transitions with 1nD = , i. e. between the first and second levels are dominant
[22]. The transitions 111 112E E® , 121 211 122 212, ,E E E E® and 221 222E E® are allowed
with polarization along the growth axis.
Selection rules can change if a potential well has a finite depth and effective masses of
carriers in the well and barrier layer are different. As a consequence, electron transitions under
the irradiation of light polarized in the lateral structure direction become possible (under the
183
action of x- and y-components of the vector E). As regards GaAs/AlGaAs quantum wells, the
absorption coefficient for the light polarized along quantum dimensional layers is several
orders smaller than for the light with z-polarization [23]. However, selection rules are modified
in quantum dots making it possible to observe intraband transitions at one band also at normal
incidence of exciting irradiation. As a result, the photocurrent value in our photoconductivity
experiments depended essentially on the way of irradiation of the heterostructure. The
photocurrent in the range from 0.3 to 1.0 еV at normal incidence of exciting irradiation was
much smaller (fig. 5, сurves 2).
In the case of side excitation in the spectral range from 0.3 to 0.37 еV, two peaks of
current were observed at 0.32 еV and 0.34 еV and the photocurrent increased monotonically as
the quantum energy increased in the range hv >0.38 еV. The peaks of current observed in the
spectral range from 0.3 to 0.37 еV can be attributed to hole transitions between the levels in the
valence band of nanoislands. It can be seen that these values, by order of magnitude, fall within
the range of excitation energies which follow from the level positions estimated above for SiGe
quantum dots. The photoresponse in the range hv > 0.38 еV may be caused by hole transitions
from the ground state of the valence band of SiGe nanoislands to two-dimensional continuum
states of the valence band of a wetting layer or intermediate layers of Si surrounding
[17, 24, 25]. Nonequilibrium carriers excited by such transitions may contribute to the
observed lateral photocurrent, as far as no potential barriers exist for electron transport in the
lateral direction. However, it is impossible to distinguish a contribution of bound-to-continuum
transitions to the states existing in a wetting layer or Si intermediate layers in the conditions of
given experiment.
Сonclusions
The current density J of field emission depends on the product of supply function S(E)
and the transmission factor of the potential barrier T(E). The observation of peak in I-V curves
of photofield emission indicates to existing of quantized energy levels in QDs that causes the
dependence of barrier transmission of Si/QD/vacuum barrier on applied voltage value. It may
reflect resonant tunneling of electrons via energy levels in QD potential well. The investigation
of lateral photoconductivity in Si/Ge heterostructures with SiGe nanoislands revealed the
presence of localized states in the valence band of Ge nanoislands with the energy distance
between them about 0.32 and 0.34 eV. Moreover, the lateral photocurrent is determined by
features of transport of nonequilibrium carriers, therefore the supply function properties.
Intraband transitions between the localized states of the valence band of Ge nanoislands, in our
opinion, are responsible for the observed photosensitivity of lateral photoconductivity and
photofield emission in Si/Ge heterostructures with Ge quantum dots in the midinfrared range.
Acknowledgements
The research was implemented within the bilateral ÖAD Project UA № 2007/05 and
supported by the program of fundamental research of the National Academy of Sciences of
Ukraine “Nanostructured systems, nanomaterials, nanotechnologies” through the Project
№ 9/07 and by the Ministry of Education and Science of Ukraine through Project № M/139-07.
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Квантові ефекти в багатошарових
Si-Ge наногетероструктурах
Ю.М. Козирев1, М.Ю. Рубежанська1, А.В. Суший1,
С.В. Кондратенко2, О.В. Вакуленко2, A.A. Дадикін3, А.Г. Наумовець3
1Інститут хімії поверхні ім. О.О. Чуйка Національної академії наук України
вул. Генерала Наумова 17, 03164 Київ
2Київський національний університет імені Тараса Шевченка,
фізичний факультет,
вул. Академіка Глушкова 2, 03022 Київ
3Інститут фізики Національної академії наук України
Проспект Науки 46, 03028 Київ
Методами фотопольової емісії та повздовжньої фотопровідності було
досліджено багатошарові наногетероструктури з квантовими точками германію.
Попередні дослідження показали тісний зв'язок пружних напружень, що виникають при
епітаксійному рості матеріалів з різними постійними граток, з виникненням
додаткових рівнів в зоні такого гетеропереходу. Таким чином, відносно простими але
інформативними методами спостерігалися міжзонні переходи, які відсутні в
двовимірних гетероструктурах Si-Ge. Якщо вплив кількості шарів на фотопольову
емісію несуттєвий, то при дослідженні фотопровідності виявлено прямий зв'язок зсуву
чутливості повздовжньої фотопровідності в середній інфрачервоний діапазон від
кількості шарів з квантовими точками германію. Показано, що розмірні
характеристики та мольний склад квантових точок відповідають енергетичним рівням
з енергетичною відстанню між ними приблизно 0,32 та 0,34 еВ з дуже високою
точністю. Результати досліджень дозволяють сподіватися на їх використання в
перспективних приладах нано- та оптоелектроніки.
Introduction
Results and discussion
|