Atomic force microscopy study of self-organized Ge islands grown on Si(100) by low pressure chemical vapor deposition

Article abstract of DOI:10.1063/1.118191

In this letter, we present an atomic-force-microscopy investigation of the Stranski-Krastanov growth of Ge on Si(100). Upon increasing the base width of the islands, two morphology transitions are found. The first transition occurs at a base width of ～50-60 nm and marks the evolution from few-monolayer-thick terraces to square-base pyramidal islands. In the second transition, which takes place when the base width exceeds ～300 nm, the island shape changes from square base pyramids to tetragonal truncated pyramids. Both transitions are brought about by the need for the system to minimize the elastic energy.

Full text of DOI:10.1063/1.118191

Atomic force microscopy study of self-organized Ge islands grown on Si(100) by low
pressure chemical vapor deposition
G. Capellini, L. Di Gaspare, F. Evangelisti, and E. Palange
Citation: Applied Physics Letters 70, 493 (1997); doi: 10.1063/1.118191
View online: http://dx.doi.org/10.1063/1.118191
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Atomic force microscopy study of self-organized Ge islands grown
on Si(100) by low pressure chemical vapor deposition
G. Capellini,a) L. Di Gaspare, and F. Evangelisti
`
`
Dipartimento di Fisica ‘‘E. Amaldi’’ and Unita INFM, Universita di Roma TRE, V. Vasca Navale 84,
00146 Roma, Italy
E. Palange
`
Dipartimento di Fisica, Universita di Roma ‘‘La Sapienza’’, P.le A. Moro 2, 00185 Roma, Italy
͑Received 10 July 1996; accepted for publication 20 November 1996͒
In this letter, we present an atomic-force-microscopy investigation of the Stranski–Krastanov
growth of Ge on Si͑100͒. Upon increasing the base width of the islands, two morphology transitions
are found. The first transition occurs at a base width of ϳ50–60 nm and marks the evolution from
few-monolayer-thick terraces to square-base pyramidal islands. In the second transition, which takes
place when the base width exceeds ϳ300 nm, the island shape changes from square base pyramids
to tetragonal truncated pyramids. Both transitions are brought about by the need for the system to
minimize the elastic energy. © 1997 American Institute of Physics. ͓S0003-6951͑97͒02104-9͔
Recently the physical properties of quantum confined
semiconductor heterostructures have attracted wide attention
both from the experimental and theoretical point of view.1
The main reason is the demonstrated capability of tailoring
the optical and electronic properties of such heterostructures
by controlling the growth at the atomic scale. In the case of
the fourth group semiconductors, we deal with indirect band-
gap materials exhibiting poor luminescence efficiency. It
would be attractive, therefore, to enhance the carrier radia-
tive recombination efficiency by means of quantum confine-
ment effects, in order to realize Si-based optoelectronic de-
vices. In particular, quantum dots ͑QDs͒ are expected to give
a significant increase of the quantum efficiency.2
In the literature, a variety of recipes for realizing QDs
has been reported.3–7 Among them, the self-organization of
islands seems to be one of the most promising ways to pro-
duce zero dimensional quantum confined systems. A pos-
sible way to achieve self-organization is to take advantage of
the spontaneous formation of clusters under the Stranski–
Krastanov ͑SK͒ growth conditions. In this way, it is possible
to produce QDs of good quality with a narrow distribution of
sizes and with an increase of the optical quantum efficiency.8
These promising experimental findings require detailed
knowledge of the SK growth dynamics.
In this letter, we report an atomic force microscopy
͑AFM͒ study of the evolution of SK islands in Ge grown by
UHV low pressure chemical vapor deposition ͑LPCVD͒ on
Si͑100͒. Atomic force microscopy measurements were per-
formed in contact mode with a Park Scientific Instruments
CP microscope equipped with a high aspect ratio conical
͑80° sidewall angle͒ Ultralever™ tips with 5 nm minimum
radius.9 We have done standard tests and scan calibrations on
freshly cleaved mica in order to obtain atomic imaging and
to avoid tip imaging misleading information.
The Si͑100͒ substrates used in this study were treated
with an ex situ chemical etch in and immediately loaded into
a UHV growth chamber, whose basic pressure was in the
low 10Ϫ10 Torr range, and then heated up to 950 °C in H2
atmosphere. As determined by x-ray photoemission spectros-
copy, reflection high-energy electron diffraction, and AFM
the surfaces were clean ͑C and O contamination below
0.1%͒, 2ϫ1 reconstructed and flat ͑RMS roughness ϳ1 ML͒.
Moreover, photoluminescence measurements showed that ra-
diative dislocations were absent.
The Ge films were deposited by LPCVD from high pu-
rity germane without carrier gas at pressures, Pdep , in the
range 0.5–2 mTorr. The samples considered in the present
investigation grew according to the Stranski–Krastanov
mode. Under our experimental conditions, this growth mode
was observed for deposition temperatures, Tdep , in the range
400–650 °C. For TdepϽ400 °C, a layer-by-layer Frank–Van
der Merwe growth mode occurred, while for TdepϾ650°C
the samples grew according to the Volmer–Weber mode.
In Fig. 1, we show AFM topographies of samples grown
at Tdepϭ600 °C and Pdepϭ1 mTorr for four different depo-
sition times. We see in Fig. 1͑a͒ that 0.5 min deposition time
is enough to develop on top of the wetting layer a large
number of small islands characterized by a narrow distribu-
tion of sizes. Longer deposition times ͓Figs. 1͑b͒–͑d͔͒ bring
about an increase of the average island size, a spreading of
the size distribution, and a slight decrease of the number of
the islands in agreement with recent theoretical
predictions.10–12 These observations suggest that the new
material deposited contributes to enlarge the existing islands.
In such conditions, the deposition rate was estimated to cor-
respond to a Ge effective thickness of 3 nm per min.
A detailed analysis of the size distributions and shapes
shows that the island evolution follows a well defined and
regular pattern. Concerning the size and shape evolution, this
can be illustrated with reference to Fig. 2, where the island
height, h, as a function of the square root of the base area,
i.e., the base width b, is plotted for different growth times.
We see ͓points with bϽϳ50 nm and negligible h in Figs.
2͑a͒ and 2͑b͔͒ that at the initial stage, the island heights are
very small as compared to the corresponding bases. Accord-
ingly, the islands should be considered, more properly, as
few-monolayer-thick terraces. These terraces expand later-
ally without an appreciable vertical growth until they reach a
critical value bc of 50–60 nm for the base width.
a͒
Electronic mail: Capellini@roma1.infn.it
At the critical width bc , a vertical growth begins until
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FIG. 1. AFM topographies of four samples grown for different deposition times: ͑a͒ 0.5, ͑b͒ 1, ͑c͒ 3, and ͑d͒ 4 min. Note the variation of the vertical scale
from ͑a͒ to ͑d͒.
the terraces become islands with an aspect ratio h/bϳ0.25.
These islands now have the shape of square-base pyramids
with ͕311͖ facets. The direct estimation of the facet orienta-
tion was performed by calculating the gradient in each point
of the 256ϫ256 mesh used to acquire AFM topographies.
From this analysis, we found that the facet angle increases
from a value close to zero for bϽbc to ␽ϳ25 deg for b
Ͼbc . This value is very close to that between the ͕311͖ and
the ͕100͖ planes, i.e., 25.2 deg. It is worth noting that the
aspect ratio of an ideal square pyramid with ͕311͖ facets is
0.24, in good agreement with our experimental finding of
h/bϭϳ0.25. It is worth noticing that the ͕311͖ facets were
evaluated13 to exhibit the minimum surface energy for the
growth of Ge on Si͑100͒. The base edges of the islands are
¯
parallel to the ͓011͔ and ͓011͔ directions. These orientations
were determined by referring to the cleaving directions of the
Si͑100͒ substrates. Possible artifacts on the AFM images due
to the raster scan direction were ruled out by changing the
scanning direction relative to the sample orientation.
It has been pointed out14,15 that the island formation re-
sults from the possibility to relax laterally for the lattice
planes that are compressed in a coherently grown planar
film. In this way, the elastic energy is reduced. Therefore, the
above experimental observations show that bcϭ50–60 nm
represents the critical width for the planar growth on the
wetting layer under the growing conditions used in the
present work. For bϾbc , the new Ge atoms impinging on
the terrace find energetically more favorable to form succes-
sive layers at whose periphery the lateral lattice compression
can relax than to expand the bottom terrace itself. The evo-
FIG. 2. Island height h vs base-width b for the samples shown in Fig. 1. The
continuous line shows the h/bϭ0.24 linear dependence, i.e., the growth ac-
cording to a fixed 0.24 aspect ratio.
lution we are observing is in good agreement with the theo-
retical predictions of Tersoff and Tromp,10 who found that,
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494 Appl. Phys. Lett., Vol. 70, No. 4, 27 January 1997 Capellini et al.
On: Wed, 24 Dec 2014 16:45:18

exhibit a shape like that reported in Fig. 3͑b͒. A triangular
growing front, 20 nm thick, and oriented on the ͑311͒ direc-
tion, is clearly visible on one side of the pyramidal island.
This observation fits nicely within the growth cycle model
recently proposed,16 which the relaxation of the strain in the
islands occurs by the insertion of dislocations that nucleate at
the edge of the islands themselves, becoming a preferential
site for the attachment of Ge. In particular, every time a
dislocation is created, the island grows by ϳ20 nm in the
direction of the strain relief. We conclude, therefore, that the
growing fronts we have observed are originated by the inser-
tion of dislocations. Moreover, the growth of the front occurs
in a ledge-by-ledge mode from the base upwards in order to
minimize the interaction between the surface steps.15 After
the insertion of a certain number of dislocations, the typical
shape of the islands becomes the truncated pyramid reported
in Fig. 3͑a͒ with ͕311͖ lateral facets and a ͑100͒ top facet.
The appearance of the ͑100͒ top facet confirms the insertion
of dislocations.17 We point out that the critical height at
which the morphological transition occurs is in good agree-
ment with the critical value of 50 nm for the maximum
height of dislocation free SK islands found by Eaglesham
and Cerullo.14 Under these conditions, the subsequent lateral
increase of the islands is much faster than the vertical one, so
that h remains nearly constant. Moreover, the lateral facets
orientation remains ͕311͖.
In conclusion, we have analyzed the evolution of the SK
Ge islanding on Si͑100͒. We demonstrated that two morphol-
ogy transitions occurred upon increasing the base width of
the islands. The first transition marks the evolution from
few-monolayer-thick terraces to square-base pyramidal is-
lands. In the second transition the island shape changes from
square-base pyramids to tetragonal truncated pyramids. Both
transitions are brought about by the need for the system to
minimize the elastic energy.
FIG. 3. ͑a͒ View of a typical island with the shape of tetragonal truncated
pyramid with rectangular base ͑hϭ60 nm and bϭ320 nm͒. ͑b͒ View of a
square-base pyramidal island ͑hϭ55 nm and bϭ300 nm͒ on which a dislo-
cation has been just inserted, forming a new growing ledge. The vertical
magnification, shading and azimuth are the same for both images. The scan
direction is aligned in the ͓011͔ sample direction.
as the islands grow, they become triangular in cross section.
Following their model, we find that, in our case, the critical
width for the development of pyramidal islands is bc
ϳ40 nm.
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For bϾbc , the islands increase in size without varying
their shape, keeping fixed the aspect ratio at h/bϳ0.25. This
is illustrated in Figs. 2͑b͒–2͑d͒ where a linear relationship
between h and b is evident for values of b in the range
70–300 nm. Furthermore, the island distribution is quite nar-
row with ⌬b/bmeanϳ0.3, where ⌬b is the standard deviation
of the Gaussian distribution of the base widths and bmean
their mean value. These data show unambiguously that the
island growth proceeds in the ledge-by-ledge mode as re-
cently suggested.15
The increase of the island width beyond ϳ300 nm brings
about a variation in the shape and a change of the growth
mode. Referring to Figs. 2͑c͒ and 2͑d͒, we see that the island
height tends to a limiting value hcϳ55 nm. The island
shape becomes that of truncated pyramids with rectangular
base, as shown in Fig. 3͑a͒. Concomitantly, there is an in-
crease in the width of their size distribution, becoming
⌬b/bmeanϳ0.5. Thus, a second morphology transition occurs
at bϳ300 nm. We believe that this lateral growth is origi-
nated by the insertion of dislocations in order to relax the
stress accumulated in the islands. Indeed, we have observed
that at the beginning of the lateral growth, i.e., when h and b
are close to the values at the transition point, the islands
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