SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
A.V.DVURECHENSKII, A.I.YAKIMOV, N.P.STEPINA,
V.V.KIRIENKO, P.L.NOVIKOV
Institute of Semiconductor Physics, Siberian Branch of Russian Academy
of Science, Novosibirsk
Abstract. The electronic and optical phenomena, as well as possible device-oriented application in Ge/Si and Ge/SiO2 nanodots that have been synthesized by molecular-beam growth are the scope of present article. We will
focus on the fundamental aspects and device applications of the small size
dots which electronic states resemble certainly those an atom even at room
temperature.
Keywords: quantum dots, germanium, silicon, field effect transistor, infrared photo
detector, memory device.
1. Introduction
A new branch of semiconductor physics that studies the behavior of electrons confined in precisely tailored manmade potential emerged during last
20 years. This field has developed from the obvious progress in technology
that now allows for the routine fabrication of nanometer-scale solid state
structures that contain small number of conduction electrons (<100) in geometries of size comparable to their de Broglie wavelength. Usually they
are called “quantum dots” (QDs), referring to their quantum confinement in
all three spatial dimensions1. The three-dimensional confinement leads to
formation of a discrete carrier energy spectrum, resembling that of an atom.
Next to this purely quantum effect, an important element of artificial atom
filling is Coulomb blockade. An extra electron can only be added to the dot
if enough energy is provided to overcome the Coulomb repulse on between
______
 To whom correspondence should be addressed. Anatoly V. Dvurechenskii. Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Science, Lavrentiev Prospect13, 630090 Novosibirsk, Russia.
1
Book Title Information
© 2005 Springer. Printed in the Netherlands.
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SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
the electrons. Studying the QD systems manifests in striking ways the interplay of the discreteness of charge carried by a single electron and of quantum effects2,3.
2. Quantum dot field-effect transistor
In a single quantum dot weakly coupled by tunneling barriers to two leads,
the interplay of single-electron charging effects and resonant tunneling
through quantized states leads to conductance oscillations as the electrochemical potential of the dot is tuned. This phenomenon underlies working
of nano-scale single-electron transistors. The behavior of double dot systems is found to be mainly affected by electrostatic coupling between the
two dots inside the artificial molecule. The next step is the large arrays of
QDs in close proximity, allowing Coulomb interaction and tunneling between them. The behavior of a multi-dot structure is more complicated for
several reasons: (i) the QDs are inevitably not sufficiently identical in size
that can cause smearing of their atomic-like properties; (ii) in contrast to a
single dot, the interaction of the dots in an ensemble can be significant; (iii)
transport through the system may be dominated by thermally assisted hopping between the dots rather than by resonant tunneling between source and
drain electrodes.
Little work has been done on the Ge/Si quantum-dot field-effect transistors (QDFETs), which use the quantum transport through discrete energy
states in zero-dimensional systems4. To date, most work in the field of
QDFETs has concentrated on InAs/GaAs SAQDs and on Si-based quantum
dots defined by very sophisticated patterning techniques, such as electronbeam litography in combination with anisotropic etching and selective oxidation or by tunable gates3.
The promising approach to reveal the phenomena of charge transport
through the array of QDs based on a few technological successes. First, the
drain current leakage should be reduced by using the superficial Si layer
thickness in a silicon-on-insulator (SOI) structure. Second, the average QDs
size should be in nanoscaled range to provide the stronger carrier confinement in the dots and the larger energy level separation resulting in a more
clear resolution of the current peaks at high temperatures. In order to raise
the operation temperature of QDFETs up to 300 K, the size of QDs has to
be smaller than 10 nm. This requirement considerably restricts the possibility of using the lithographic processes for fabrication ultrasmall QDs. Third,
inhomogeneous broadening due to dot size variations and long-range random Coulomb potentials may be reduced by decreasing the QDFET and by
using the gate recess configuration.
Si of 47 nm thick in SOI structure has been used in experiments. The
Ge self-assembled dots were grown with the density 4 × 1011 cm-2 and sub-
SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
3
sequently capped by Si layer. The average in-plane diameter and height of
the Ge dots are 10 and 1 nm, respectively. To supply holes on the dots, a
boron delta-doping (6 × 1012 cm-2) Si layer inserted above the Ge layer was
grown (Fig.1). The channel was patterned by photolithography to form a Si
island of 4-μm length and 1-μm width, etched down to the underlying SiO2.
Source and drain electrodes were made using Al evaporation and annealing
at 450 C in a N2 atmosphere. A plasma-enhanced chemical-vapor deposition
silicon dioxide of 60 nm thicknesses was deposited as the gate insulator
and, finally, a Al gate of 4 μm width and 1 μm length was formed. The
amount of oxide charge, estimated from the admittance measurements, was
about 31010 cm-2. Figure 1 shows an atomic force microscopy picture of
the transistor. Several samples with designed channel widths W ranging
from 2 to 1 um are fabricated. The sidewall depletion width is determined to
be 0.9 um from measurements of drain current versus W at zero gate voltage. Assuming a uniform density of 4000 dots per um2 these different gate
areas of the samples contain number of active dots from 400 to 4000. The
hole concentration in the boron -doping Si layer is sufficient to fill, after
spatial transfer, all hole bound states in the Ge islands and to populate twodimensional states in the Ge wetting layer. As a result, the channel conductance at zero gate voltage is found to show the non-activated behavior and
depend only slightly on temperature. The drain current (Id) as a function of
the gate voltage (Vg) was measured at different temperatures with the drain
voltage fixed at 5 mV. Figure 2 shows the typical Id-Vg characteristics of the
1 um gate QD transistor. When a positive bias is applied to the gate the
channel is depleted and current flow between the source and drain contacts
is suppressed. Above the threshold voltage Vth  4 V the deep hole states in
the dots come into resonance with the Fermi energy and the current starts to
oscillate (Fig. 3).
At room temperature, the current bump is clearly observable around 6
V. As the temperature decreases, four well-pronounced equidistant peaks
with a gate voltage separation Vg  1.1 V appear after onset of the conductance. The number and relative position of the peaks are well reproducible at different cold cycles and in different samples of similar sizes. Four
equidistant current peaks is suggested to be due to tunneling through fourfold-degenerate excited state in the Ge QDs.
One may estimates position of two additional peaks corresponding to
transport through the twofold-degenerate hole ground state. Due to confinement and Coulomb effect, the energy difference between loading the
second hole into the ground state and the first hole into the excited state of
the used Ge SAQDs is approximately 200 meV. The filling of the ground
state is expected to be at Vg 16 V, but, in this region, large leakage current
through the gate insulator prevents measurements of Id–Vg characteristics.
For larger size of QDs the ground states were observed5.
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SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
The charging energy (Ec) of the dots can be determined by using Ec
=eVg, where the gate modulation coefficient  relates the gate voltage to
the hole energy inside the dots. This coefficient can be calculated from the
temperature dependence of the full width at half maximum (FWHM) of the
current peaks, which, for a single dot showing Coulomb blockade oscillations, should be broadening with T as 3.5 kBT /(e) (Ref. [6]). On the basis
of this estimation the charging energy is 43±3 meV.
Figure 4 shows the temperature dependence of the current maxima. A
clear thermally enhanced transport through the dots with the activation energy Ea =21 ± 3 meV is evident. Possible mechanisms of charge transport
may be 1) thermal activation of holes from the dots over the barriers or 2)
hole tunneling between neighboring dots7. In a latter case, the activation energy is typical disorder energy in the system, which comes from dispersion
of the dot sizes and potential fluctuations caused by random distribution of
the charged dots and interface states. Because the experimental value Ea is
the same for all peaks (i.e., it does not depend on the effective barrier
height), the conduction mechanism is attributed to the nearest-neighbor
hopping of holes between the dots. With scanning the gate voltage, the
Fermi level moves across the zero-dimensional density of states. The maximum current occurs when the given hole level in the dots is half-filled because this maximizes the product of possible initial and final states for tunneling process and avoids increasing the energy of the system due to appearance of extra charge in a final dot.
The disorder energy, Ed, in ensemble of the dots can be found from
residual full width at half maximum (FWHM) of current peaks. The value
of Ed was found to be 19±3 meV, which is consistent with the experimental
value of activation energy observed.
3. Quantum dots infrared photodetectors
The potential advantages of the quantum dots infrared photodetectors
(QDIPs) for near infrared operation as compared with two-dimensional systems are (i) increased sensitivity to normally incident radiation as a result of
breaking of the polarization selection rules, so eliminating the need for reflectors, gratings or optocouplers, (ii) expected large photoelectric gain associated with a reduced capture probability of photo excited carriers due to
suppression of electron-phonon scattering, (iii) small thermal generation
rate, resulted from zero-dimensional character of the electronic spectrum,
that renders a much improved signal-to-noise ratio, (iv) possibility of the
narrow-band detection due to discreteness of the density of states. Furthermore, since the spatial extent of the electron or carrier wavefunction in QDs
is in the order of their size (about 10 nm) or more, the dipole matrix element
SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
5
for the intersubband transitions can be large, which is not the case for natural deep impurities in semiconductors.
To provide a high performance of QDIPs, the photosensitive region of
detectors should consist of a dense array of QDs. In this way more advantageous and hence more relevant for application in QDIPs are self-assembled
Ge/Si QDs, in which the surface density of the dots can be achieved as high
as 1011–1012 cm-2 by choosing appropriate growth conditions.
The concept of QDIP using intersubband transitions was proposed and
analyzed theoretically8-11. Figure 5 shows Ge/Si QDIP with vertical photocurrent consisting of a p+–p–n+ silicon structure with a planar array of Ge
quantum dots embedded in the undoped or lightly doped with boron p-Si
region. The structures p+–p–n+ and n+–p–n+ were used for mid-and nearinfrared12-18.
The integration of Si/Ge heterostructures on a Si chip and their compatibility with Si based electronic circuitry presents a high potential for designing low-cost optoelectronic modules, operating at the 1.3 µm and 1.55
µm telecommunication wavelength (bound-to-continuum interband transition from Ge to Si). A fundamental feature of staggered QDs Ge/Si is the
spatial separation of electrons and holes resulting in formation of spatially
indirect excitons.
An example of near infrared Ge/Si p–i–n photodetector operating at
the 1.3 µm and 1.55 µm telecommunication wavelengths is shown in Figure
6. To increase the interaction length between the light and the QDs layers
and to provide the intrachip interconnections, a vertical stacking of 36 layers of coherent Ge nanoislands was inserted into a waveguide obtained with
a SOI structure. The sample was processed into ridge waveguide. Devices
with lengths going from L = 0.1 to 5 mm were fabricated18. The roomtemperature quantum efficiency of a 4 mm device versus reverse bias Ub is
shown in Figure 7. The light is coupled through the edge of the detector.
The maximum external quantum efficiency achieved is 16% for λ=1.55 µm
and 21% for λ=1.3 µm at L>3 mm and Ub >3 V.
To the best of our knowledge, the first observation of midinfrared photoconductivity in Ge QDs has been reported in 1999 [Ref. 15]. The QDIP
under investigation was a p+–p–n+ silicon diode embedded with a single
layer of pyramidal Ge SAQDs. The average size of the dot base length is 15
nm, the height is .1.5 nm. As the heterostructure was growth at low temperature (Ge layer was grown at 300 C and covered with Si at 500 C), the segregation and interdiffusion effects are negligible and Ge islands contain no
silicon atoms. The large areal density of Ge QDs (3×1011 cm-3) realized the
high absorption coefficient.
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SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
4. Ge nanodots on SiO2 for non-volatile memory device
A nanocrystal in a dielectric matrix has attracted much attention as a
promising candidate for a charging node in a single-electron memory device
(SEMD). An advantage of the nano-floating gate memory over the continuous floating gate is its improved endurance due to preventing lateral charge
movement. Faster writing/erasing time, lower operating voltage and longer
retention time have been demonstrated in memory device based on Si islands embedded in SiO2 (Ref. [19]). Recently it was shown that Ge-based
SEMD has the superior properties over Si-based SEMD in terms of the
writing /erasing time and the operating voltage. Since then different methods were used for fabrication Ge nanodots in dielectric matrix, such as ion
beam synthesis20, oxidation and reduction of Ge/Si islands21, rapid thermal
annealing of cosputtered22 and chemical-vapor-deposition layers, and
pulsed-laser deposition23. However practically all of these techniques create
the random distribution of nanodots inside the dielectric, that yields the ensemble-averaged information only. To suppress the tunneling distance fluctuation and allow carrying out the detail analysis of tunneling-out mechanism it is necessary to form in-plane distribution of nanodots. Moreover,
when used nanodots for charge storage devices, the general requirements
that can be placed upon the control are the size of nanodots, theirs density
and homogeneity in growth plane, that is the problem for most of abovementioned growth methods.
In work24 pulsed low-energy (100-200 eV) ion-beam-induced nucleation was found to stimulate Ge nanocrystals formation on relatively thick
films of SiO2 (about 100 nm) prepared by thermal oxidation of Si. It was
shown that the ion-beam action causes an increase in nanocrystal density
and size homogeneity. The idea of using the ion-beam action is that each
ion impact into the SiO2 surface (a) produces a vacancy depression with a
surface steps being nucleation sites for nanocrystals growth; (b) generates
adatoms assisting in nuclei growth; c) stimulates surface adatom diffusion.
Ion energy should be low enough to avoid introducing defects into SiO2
film, but high enough as compared to that of thermal energy of deposited
atoms. The regime of pulsed ion-beam irradiation allows synchronous nucleation of new phase, favorable for highly homogeneous nanocrystal size
distribution.
The pulsed low-energy (200 eV) ion-beam-induced nucleation during
Ge deposition on SiO2 was used to study the formation of Ge nanocrystals
arrays on thin films of dielectric which are usually tunnel layers in memory
devices. A 4.5 nm thick SiO2 film were growth by thermal oxidation on
(111) p-type silicon substrates at 850 C (tunnel insulator).
After dioxide formation the wafers were washed, dried-up and inserted
into the ultrahigh-vacuum (UHV) chamber. About 1 nm-thick SiO2 layer
SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
7
was removed from the top surface in situ using a Si flux at 820 C before Ge
deposition. Molecular beam deposition (MBD) was carried out in an UHV
chamber of molecular beam epitaxy setup equipped with effusion cell (boron nitride crucible) for Ge. The system of ionization and acceleration of
Ge+ ions provided the degree of ionization of Ge molecular beam from
0.1% to 0.5%. A pulse accelerating voltage supply unit generated ioncurrent pulses with duration of 0.5-1 s and ion energy of 200 eV. The angle
of incidence of the molecular and ion beams on the substrate was 54 degree
out of the surface normal. The analytical assembly unit of the chamber included reflection high energy (20 keV) electron diffraction (RHEED) unit.
The substrate temperature of Ge deposition was varied from 250 to 400 C.
In all experiments 20 monolayers (ML) as an effective Ge layer thickness
were intended for deposition. The rate of Ge deposition was varied between
0.08 - 0.19 ML/s. Two different regimes of ion stimulation were studied. In
the first case (regime 1) three ML of Ge were deposited without ion-beam
action. Pulsed ion-beam actions were applied in series at the effective Ge
layer thickness of 3 ML, 4 ML and 5 ML. In the second case ion implantation was starting simultaneously with Ge deposition in series at the effective
thickness of 1ML, 2 ML and 3ML (regime 2). Ge nanocrystal density and
size distribution were studied with high resolution electron microscopy
(HREM) of plan-view and cross-sectional geometries. To prevent Ge oxidation some samples were capped by thin Si layer (~5 nm) in the same UHV
chamber. The chemical composition of the uncapped samples and tunnel insulator thickness variation were studied with Electron Spectroscopy for
Chemical Analysis (ESCA) technique by transferring samples via ambient
atmosphere to ESCA spectrometer. The Al source with K line 1486.6 eV
was used for exciting X-ray photoelectron spectra with the energy resolution of 0.7 eV and the probing depth of from 1 to 4 nm in dependence of
photoelectrons energy.
To find the influence of ion beam action on thickness of tunneling isolator during Ge deposition the intensity of Si 2p line was analyzed. The
thickness of tunneling isolator was evaluated from the intensity ratio of the
oxide component Si 2p (103.0 eV) to substrate component Si 2p (98.7 eV).
The significant decrease of tunneling isolator thickness (from 4.2 to 2.4 nm)
is observed, when ion irradiates uncapped SiO2 surface (regime 2). When
ion action takes place on surface capped by Ge (the regime 1) the remains
tunneling isolator keeping thickness. In both cases there are no any changes
in stoichiometry of SiO2 layers. As a measure of germanium content the line
intensity of Ge 2p x-ray photoelectron may be taken to represent information with probing depth about 0.89 nm (the inelastic mean free path of
photoelectrons). The Ge 2p states are clearly resolved with a spin-orbit
splitting of 31.1 eV. Both Ge 2p1/2 and Ge 2p3/2 lines show pronounced superposition of two components with energy shift of 2.3 eV. These two com-
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SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
ponents have peak positions at 1217.7 eV and at 1220.0 eV of binding energies and are related with germanium in pure and partially oxidized states,
correspondingly. The origin of GeOx fraction is oxidation during transferring of samples to ESCA spectrometer via ambient atmosphere.
The HREM image shows homogeneous Ge nanocrystals of 7 nm average size on SiO2 surface (Fig.8). An additional cross-section study (Fig.9)
allows concluding on the ball-like shape of Ge nanocrystals. The density of
nanocrystals increases with top value up to 1012 cm-2 as the substrate
temperature goes down (Fig.10).
The strong Ge desorption was observed during deposition as substrate
temperature increases. The pulsed ion beam nucleation in regime 1 was
found to gives rise to suppressing of Ge desorption. This effect is clearly
demonstrated with ESCA measurements of Ge peaks intensity and HREM
studies of nanocrystal’s size distribution (Fig.11). At temperature of substrate 300 C the pulsed ion beam action in regime 1 allows to increase the
total amount of Ge deposited on SiO2 by a factor 1.7 as compared with
common MBD. But even in this case the effect of desorption still dominates. From 20 effective Ge ML intended for deposition just 2 ML remains
on the SiO2 surface.
The growth procedure was optimized with respect to uniform dot-size
distribution around 6 nm and a dot density about 1012 cm-2 Main parameters
determining these regimes are the deposition temperature and Ge flux.
To clarify the mechanism of Ge nanocrystals formation and the role of
ion irradiation we have carried out the kinetic Monte Carlo (MC) simulation. The kinetics of nanocrystsls formation was calculated using "lattice
gas" model25, which includes Ge atoms deposition on SiO2 surface, their
surface diffusion, desorption, precipitation, and ion-beam action. The sites
occupied by Ge atoms and SiO2 were restricted to faced-centered cubic
(FCC) lattice, which is well suited to describe precipitation in isotropic
amorphous matrices (e.g. SiO2). The lattice includes 12812832 sites with
cylindric boundary conditions in the lateral plane. The number and types of
nearest neighbors to a Ge atom determine its interaction energy with other
Ge atoms and SiO2 matrix.
The energy per one Ge-Ge bond in FCC lattice was taken equal to 0.2
eV, which corresponds to an activation surface diffusion energy 0.8 eV at
free Ge surface obtained from molecular dynamics calculation26. For Ge in
SiO2 environment the interaction energy per one neighbor site occupied by
SiO2 was chosen as much as 0.076 eV, which provides the reproduction of
the experimental dependence of Ge amount remaining in the surface upon
the Ge amount deposited.
The simulated nanocrystals were found to have isotropic shape. This
result was not expected in advance, since the growth proceeds under
nonequilibrium conditions. An irradiation effect was described by the con-
SIGE NANODOTS IN ELECTRO-OPTICAL SOI DEVICES
9
cept of collisional mixing, i.e. the displacements of atoms. In the simulation
process each Ge atom has a probability P to be displaced by a distance R
during one MC step. Ion-beam action results in the smaller nanocrystals
size and the higher density due to precipitation and nucleation of new nanocrystals by atoms knocked out from initial ones to the SiO2 surface.
In conclusion we have shown that pulsed low-energy ion beam nucleation
during Ge deposition on SiO2 films allows to suppress Ge desorption , increase the density of Ge nanocrystals, decrease the average nanocrystal size
and size dispersion. The predeposition of Ge before pulsed ion beam action
was found to allow keeping thickness of thin SiO2 film (tunnel isolator in
memory device) practically unchangeable. The C-V curves of the structures
with control insulator over Ge nanocrystals show the hysteresis with
memory window up to 7 V indicating the charge storage effect in Ge nanocrystals.
Acknowledgment. This work was supported by the Russian Foundation for Basic Research (Grant No.06-02-08077).
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Captions.
Fig. 1. Atomic force microscopy image and schematic cross section of the transistor channel.
The source, drain, and the gate are labeled by S, D, and G, respectively.
Fig.2. Gate voltage dependence of drain current at various temperatures.
Fig.3. Top of the valence band of the transistor for positive gate bias. The SOI substrate is
not shown. The holes reside in the Ge dots. When the Fermi level is aligned with the quantum levels in the Ge dots at a certain gate voltage, holes will flow through that quantum level
in the plane of Ge SAQDs.
Fig.4. Temperature dependence of the current maxima at different gate voltages. Inset: Temperature dependence of the average FWHM of four current maxima with a linear .t to the data. To obtain the FWHM of each peak, the observed current oscillations were decomposed
into four Gaussians.
Fig.5. Schematic of the Ge/Si photodetector structure.
Fig.6. Schematic layout of waveguide near-infrared photodetector on a silicon-on-insulator
substrate.
Fig.7. Quantum efficiency of a 4 mm waveguide photodetector as a function of reverse bias.
Fig.8. The HREM image (plane view) of Ge nanocrystals on SiO2.
Fig.9. Cross-section HREM image of Ge nanocrystals on SiO2.
Fig. 10. Density of Ge nanocrystals on SiO2 versus the substrate temperature.
Fig.11. Ge nanocrystal’s size distributions: 1 – Ge deposition with pulsed ion-beam action
(regime 1); 2 – no ion-beam action during Ge deposition.