Optical Properties of Stacked Ge/Si Quantum Dots with Different Spacer
Thickness Grown by Chemical Vapor Deposition
Wen-Yen Chen, Wen-Hao Chang, An-Tai Chou, and Tzu-Min Hsu*
Department of Physics, National Central University, Chung-li, 32054,
Taiwan, Republic of China
Pan-Shiu Chen, Zingway Pei and Li-Shyue Lai
Electronic Research and Service Organization (ERSO), ITRI, Hsinchu,
Taiwan, Republic of China
Photoluminescence spectroscopy has been used to study the optical properties of
multiple stacked Ge/Si quantum dots (QDs) with different thickness of Si spacers
inserted between the Ge dot layers. According to the emission energy of the stacked
Ge/Si QDs, we found that thinner spacer will lead to significant material intermixing.
Such intermixing degrades the interface sharpness and the hole confinement depth of
the dots. The thermal activation energy of PL intensity quenching for different spacer
thicknesses also confirms this finding. We point out that thinner spacer is in fact
detrimental to the emission properties of the stacked Ge/Si QDs. To obtain better
luminescence efficiency at room temperature, the influence of the material
intermixing on stacked Ge/Si QDs should be minimized.
PACS: 71.55.Cn
Keyword: material intermixing, strain relaxation, QDs, quantum dot, germanium.
E-mail: tmhsu@phy.ncu.edu.tw
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1. Introduction
SiGe has become a promising material system for fabricating high-integration and
low-cost optoelectronic devices that could be operated at the wavelength for optical
communication [1]-[3]. In recent years, the development in growing high-quality
self-assembled Ge/Si quantum dots (QDs) further makes this aim feasible [4]. Because the Si
is an indirect material, radiative recombination has to involve phonon-assisted processes. In
Ge/Si QD system, it is a common belief that phononless luminescence may be significantly
enhanced due to the localized exciton at Ge/Si interface [5], [6]. Since the the Ge/Si band
alignment is type II, strain relaxation of the dots and interface sharpness will significantly
affects the QD emission properties. For multiple stacked Ge/Si QDs, the thickness of Si
spacer layer becomes even crucial. The strain relaxation can be achieved not only by
transferring strain of the dots into the spacer, but also by material intermixing. [7], [8]. Both
effects affect the Ge/Si interface properties as well as the confinement potential. Therefore,
spacer thickness is of great importance in both the formation of stacked Ge/Si QDs and their
optical properties. In this work, we use photoluminescence (PL) spectroscopy to study the
optical properties of multiple stacked Ge/Si QDs with different spacer thicknesses. The
effects of strain transfer and material intermixing are discussed. Finally, we point out some
important factors that are relevant to the room temperature luminescence efficiency of the
Ge/Si QD stacks.
2. Experimental Details
The samples were grown on Si(001) substrates by a hot-wall multi-wafers ultra-high
vacuum chemical vapor deposition (UHV-CVD) system under a base pressure of 5 10 9
2
torr. We use pure SiH4 and 5 % He-diluted GeH4 as the precursor gas. Before loading into the
growth chamber, the Si substrate was dipped in an HF solution to remove the native oxide
and form an H-passivation layer. After depositing a 100-nm Si buffer layer on the substrate,
tenfold stacks of Ge/Si bilayers were then deposited at 600 C. The nominal Ge amount is
12.8 equivalent-monolayer (eq-ML, 1 eq-ML= 6.271014 atoms/cm2). Finally, the layer
structures were completed by depositing a 100-nm Si cap layer. A series of samples with
different spacer thicknesses (ds) were grown and the nominal ds are 8, 12, 18, 24, and 48 nm,
respectively. The structural properties of these samples have been characterized by double
crystal X-ray diffraction combined with cross-section transmission electron microscopy
(TEM) using a JEOL-2010F microscope operated at 200 kV. The photoluminescence (PL)
spectroscopy was performed by using a 532-nm solid-state laser as an excitation source,
under an excitation power of about 200 mW. The PL spectra were then analyzed by a 0.5-m
monochomator in conjunction with a liquid-nitrogen-cooled Ge detector using the standard
lock-in technique.
3. Results and Discussions
The structural properties of the investigated samples were first characterized by TEM.
Figure 1 shows the cross-section TEM micrographs for the tenfold-stacked Ge/Si QDs with
various spacer thicknesses. For QD stacks with ds= 18 nm, vertical correlation is apparent.
The lateral dot size increases slightly from the lowest to the topmost layers, indicating that
strain field transferring from the underlying Ge dots into the Si spacer is reduced [9]. As the
ds is increased up to 48 nm, the vertical correlation of the QD stacks disappeared. Besides, the
average dot size is found to be smaller, and the size variation becomes larger. When the ds is
reduced down to 8 nm, significant increase in dot size can be seen. In addition, since the
accumulated strain in Si spacers is too large, dislocations are formed along the QD edges.
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According to the TEM study, two important effects concurrently occur when the spacer
thickness is varied. First, when the spacer thickness is reduced, more strain are transferred
from the dots into the spacer, as can be inferred from the increasing vertical correlation. On
the other hand, the decreasing spacer thickness also leads to significant material intermixing.
For the same amount of Ge deposition, larger dot size indicates more Si contents were
incorporated into the QDs, indicating more pronounced material intermixing occurred at the
Ge/Si interface.
The normalized PL spectra for the investigated samples measured at room temperature
(RT, T= 300 K) are shown in Fig. 2(a). All of these spectra consist of three main features: the
laser sharp line at 1.167 eV, the transverse-optical (TO) phonon replica of the Si band edge
emission near 1.1 eV and the Ge/Si QD emission band in the range of 0.7-1.0 eV. The QD
peak energy (EQD) as a function of ds are shown in Fig. 2(b). It is found that the EQD shifts
from 0.79 to 0.85 eV as the ds is reduced from 48 to 8 nm. The observed energy shift with
respect to the spacer thickness reveals an important trend. In fact, the strain transfer and
material intermixing are two counteracting effects on the QD emission energy, which are
schematically depicted in Fig. 3. The strain transfer effect on the band alignment for a “pure”
Ge QDs embedded in “pure” Si matrix is shown in Fig. 3 (a). Since the deformation potential
of the Si band is about three times larger than that of the Ge Γ band, the strain transfer
effect tends to lower the QD emission energy [7]. On the other hand, as shown in Fig. 3(b),
the material intermixing tends to increase the Si content inside the dots, which not only
shallows the potential depth of the QDs, but also softens the dot/spacer interface [8]. This
means the material intermixing will increase the emission energy of QDs. Although it is
difficult to quantitatively specify both effects, the observed energy shift in Fig. 2 reveals that
the material intermixing may be dominated over the strain transfer effect when the spacer
thickness is reduced. Consequently, due to the predominant intermixing, reducing the spacer
thickness degrades both the interface sharpness and the hole confinement of the QDs.
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The intermixing effect also reflects influences on the PL quenching behavior. Figure 4
shows the temperature dependencies of the integrated QD PL intensity for the samples with
12-nm and 24-nm spacer layers. The PL intensity starts to decrease at T~ 120 K for ds = 12
nm and at ~200K for ds = 24 nm. Moreover, the RT PL intensity for ds= 24 nm is still rather
high, which only drops to about 1/3 of the intensity at T= 100 K. The thermal activation
energy, EA, can be obtained from the Arrhenius plot of the PL intensity. In the inset of Fig. 3,
the activation energy, EA, as a function of ds, are shown. The deduced EA increases from 85
meV for ds= 8 nm, to 183 meV for ds> 24 nm. Since the electron-hole recombination process
in Ge/Si QDs is spatially indirect, (i.e., electron in the spacer and hole in the dots,) the
obtained EA would correspond to the energy for the hole escaping from the QDs to the barrier.
The dependence of the EA on ds also consists with the material intermixing effects. With the
decreasing spacer thickness, intermixing effects become more pronounced, which tends to
shallow the hole confinement in the dots and hence reduce the thermal activation energy. It is
worth to point out that thinner spacer is in fact detrimental to the RT emission properties of
multiple stacked Ge/Si QDs. This means that, whenever one intends to design a room
temperature light emitter based on stacked Ge/Si QDs, optimal spacer thickness with minimal
material intermixing is very important.
4. Conclusions
The optical properties of multiple stacked Ge/Si QDs with different spacer thickness
are studied. We found that the material intermixing plays a crucial role when the spacer
thickness is reduced. Although reducing the thickness may provide better confinement for the
electron in Si conduction band due to the strain transfer effect, the material intermixing tend
to counteracts this effects, decreasing hole confinement in the dots. We also found that the
activation energy for the PL intensity quenching increases with the increasing ds. This further
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confirms the intermixing effect. Finally, we point out that thinner spacer is in fact detrimental
to the emission properties of the stacked Ge/Si QDs. To obtain better luminescence efficiency
at room temperature, the influence of the material intermixing on in stacked Ge/Si QDs has to
be minimized.
Acknowledgments
This works is supported in part by the Electronic Research and Service Organization
(ERSO) of ITRI within the Novel-Nanoelectronics project under Contract No. 03911019 and
the National Science Council of the Republic of China under Grant No. NSC-
91-2112-M-008-037.
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Figure Captions
FIG. 1 Cross-section TEM micrographs for the tenfold stacked Ge/Si QDs with ds = (a) 18
nm, (b) 48 nm and (c) 8 nm.
FIG. 2 (a) The normalized PL spectra for a series of samples with different spacer
thickness measured at T= 300 K. (b) The PL peak energy (EQD) of the QD emission
as a function of spacer thickness (ds).
FIG 3 Schematic diagram for the effects of strain transfer (a) and material intermixing (b)
on the emission energy of the Ge/Si QDs.
FIG. 4 The temperature dependencies of the integrated QD PL intensity for the samples
with 12- and 24-nm spacer layers. The inset shows the fitted activation energy EA as
a function of ds.
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FIG. 1/4 Chen et al.
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FIG. 2/4 Chen et al.
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FIG. 3/4 Chen et al.
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FIG. 4/4 Chen et al.
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