Fabrication of nanometer-scale Si field emitters using self-assembled Ge nanomasks

Article abstract of DOI:10.1149/1.3267512

Large-area, nanometer-scale Si field emitters have been fabricated by selective chemical etching of self-assembled Ge islands on Si. Taking advantage of the relatively low etching rate, uniform Ge islands act as virtual nanomasks for the underlying Si substrate. During selective chemical etching, Ge nanomasks shrink into small Ge-core islands, which determine the apex sharpness of the resulting Si pyramidal tips. The results demonstrate that Si pyramidal tips exhibited improved antireflective and electron field emission characteristics compared to as-grown Ge islands. The high field enhancement factor can be attributed to high tip density, nanoscale apex, and well-controlled spacing between Si pyramidal tips. This work offers a low cost alternative for designing and fabricating high efficiency Si-based field emitters or nanodevices.

Full text of DOI:10.1149/1.3267512

H174
Journal of The Electrochemical Society, 157 ͑2͒ H174-H177 ͑2010͒
0013-4651/2009/157͑2͒/H174/4/$28.00 © The Electrochemical Society
Fabrication of Nanometer-Scale Si Field Emitters
Using Self-Assembled Ge Nanomasks
,z
*
Sheng-Wei Lee, Bo-Lun Wu, and Hung-Tai Chang
Institute of Materials Science and Engineering, National Central University, Jhong-Li 32001, Taiwan
Large-area, nanometer-scale Si field emitters have been fabricated by selective chemical etching of self-assembled Ge islands on
Si. Taking advantage of the relatively low etching rate, uniform Ge islands act as virtual nanomasks for the underlying Si substrate.
During selective chemical etching, Ge nanomasks shrink into small Ge-core islands, which determine the apex sharpness of the
resulting Si pyramidal tips. The results demonstrate that Si pyramidal tips exhibited improved antireflective and electron field
emission characteristics compared to as-grown Ge islands. The high field enhancement factor can be attributed to high tip density,
nanoscale apex, and well-controlled spacing between Si pyramidal tips. This work offers a low cost alternative for designing and
fabricating high efficiency Si-based field emitters or nanodevices.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3267512͔ All rights reserved.
Manuscript submitted May 19, 2009; revised manuscript received November 2, 2009. Published December 9, 2009.
monolayers ͑eq-ML, 1 eq-ML = 6.27 ϫ 1014 Ge atom/cm2͒. The Si
wafers were dipped in a 10% HF solution to achieve the hydrogen-
passivated surface, and then transferred into a UHV/CVD system. A
50 nm thick Si buffer layer was first grown to cover the wafer sur-
face. After depositing the Si buffer, a Ge layer of 12 eq-ML was
then deposited to form the self-assembled Ge islands via the
Stranski-Krastanow ͑SK͒ mode.17 Finally, the selective wet etching
process was performed by dipping the as-grown samples in 50%
aqueous tetramethylammonium hydroxide ͑TMAH͒ solutions at
50°C for various periods of time.
The surface topographies of the etched samples were analyzed ex
situ by the atomic force microscopy ͑AFM͒ in a tapping mode. The
transmission electron microscopy ͑TEM͒ was carried out with a
JEOL 2100 TEM operating at 200 kV to reveal more detailed infor-
mation about the microstructures of the etched samples. We also
measured the total hemispherical reflectance spectra in air on a spec-
trophotometer with an integrating sphere ͑Hitachi U-4100,
350–850 nm͒ to characterize the surface morphologies after etching.
The electron field emission properties were measured under a pres-
sure of 1 ϫ 10−7 Torr in a scanning-probe field emission system. A
platinum sphere with a diameter of 1 mm was used as an anode to
collect electrons from the samples. The measurement distances be-
tween the anode and the emitting surface were fixed at 30 ␮m. The
measurements were repeated several times to obtain the stable and
reproducible current-density–electric-field characteristics.
In recent years, intensive research efforts have been devoted to
the design and fabrication of Si-based field emitters in pursuit of a
new generation of flat panel displays and vacuum microelectronic
devices.1-3 Sharp field emitters are highly desirable because an elec-
tron emission strongly depends on the electric field, and the electric
field scales with emitter sharpness.4 Processes typically use wet
chemical etching with an oxide pattern mask to control the shape of
the Si tips or pyramids, but in micrometer sizes.5,6 Several ap-
proaches have been used to fabricate nanometer-scale Si tips or
pyramids. An electron-beam lithography combined with etching can
define features down to sub-10 nm,7 but the cost is high and the
throughput is low. A nanoimprint lithography is a possible choice,
but it requires a reliable mold which has a low fabrication cost and
a long lifetime.8 A nanosphere lithography, which exploits a self-
organized, order, and close-packed sphere array as a deposition or
etching mask, is an effective method for the fabrication of periodic
nanostructures.9 Nevertheless, it usually requires a combination of
other techniques, such as reactive ion etching10 or metal deposition11
to realize the fabrication of the designed structures.
With a moderate lattice mismatch ͑4.2%͒, Ge/Si has emerged as
a model system for the fabrication and investigation of nanometer-
scale heteroepitaxy.12-14 Recently, Tondare et al. reported the feasi-
bility of using the self-assembled Ge islands on Si͑111͒-7 ϫ 7 as
field emitters.15 In the previous study, we also demonstrated the
improved field emission properties in Si-capped Ge islands.16 This
work, taking advantage of the relatively low etching rate and uni-
form size of self-assembled Ge islands, proposes a low cost method
to fabricate nanoscale Si field emitters over a large area. The self-
assembled Ge islands act as virtual nanomasks for the underlying Si
substrate during selective chemical etching. The resulting Si pyra-
midal tips have an average height and base width of 47 and 95 nm,
respectively. The density of Si pyramidal tips is as high as 8
ϫ 109 cm−2. Furthermore, these Si pyramidal tips exhibit improved
antireflective and field emission characteristics. The fabrication pro-
cess is compatible with the existing Si/SiGe-based device technol-
ogy.
Results and Discussion
For the as-grown sample, Fig. 1a and b shows the well-known
bimodal islands, pyramids with shallow ͕105͖ facets and domes with
multiple steeper facets, which are commonly observed in high tem-
perature depositions.18 The island density is about 9 ϫ 109 cm−2
and roughly 88% of the Ge islands are highly uniform domes. The
Ge domes have an average diameter of 72 nm. Figure 1b-f illustrates
the topographic evolution of the Ge islands during the selective
chemical etching. When immersed in a 50% TMAH solution for
20 s, the morphology of the Ge islands remains almost unchanged
͑not shown here͒. The self-assembly of Ge islands on Si is known to
develop in the SK growth mode, in which heteroepitaxial growth
starts with the formation of a two-dimensional wetting layer fol-
lowed by the nucleation of three-dimensional islands.19 Therefore, a
wetting layer of several monolayers is thought to exist on the Si
surface. Such a thin wetting layer may act as a protective layer for
the underlying Si substrate during the initial etching process. How-
ever, after etching for 30 s, the wetting layer surrounding the Ge
islands started to be attacked by the etchant ͑see arrows in Fig. 1c͒.
With 40 s immersion in 50% TMAH solution, the surface heights of
the Ge islands increased significantly ͑Fig. 1d͒. The average diam-
eter of the etched nanostructures increased slightly to 86 nm. As
marked by a dotted line, it is evident that the apexes of etched
structures exhibit a similar geometry to the as-grown Ge domes. At
Experimental
p-Type ͑001͒-oriented Si wafers ͑10–25 ⍀ cm, 150 mm diam-
eter͒ were used in the present study. All Ge-island samples were
grown at 600°C in a multiwafer ultrahigh vacuum chemical vapor
deposition ͑UHV/CVD͒ system under a base pressure of 5
ϫ 10−9 Torr. Pure SiH4 and GeH4 were used as precursors. Both
gas flow rates were kept at 100 sccm. The growth rates for Si and
Ge are 0.03 nm/s and 0.8 eq-ML/s, respectively. Note that the
amount of Ge deposition is expressed in the unit of equivalent-
*
Electrochemical Society Active Member.
z E-mail: swlee@ncu.edu.tw
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Journal of The Electrochemical Society, 157 ͑2͒ H174-H177 ͑2010͒
H175
Figure 1. ͑Color online͒ AFM images
͑1 ϫ 1 ␮m͒ of ͑a͒ as-grown Ge islands
and three-dimensional view of Ge-island
samples after etching for ͑b͒ 0, ͑c͒ 30, ͑d͒
40, ͑e͒ 50, and ͑f͒ 60 s, respectively.
Pyramid-like and domelike Ge islands in
͑a͒ and ͑b͒ are marked as P and D, respec-
tively.
this stage, the monodisperse Ge islands act as nanomasks to prevent
the underlying Si substrate from being etched. In the meantime,
anisotropic etching proceeds quickly on the unmasked Si regions
located in between the islands. After etching for 50 s, the etched
nanostructures became sharper. It was speculated that the upper Ge
islands were removed, leaving the Si pyramid-shaped nanostructures
͑pyramidal tips͒ on the surface, as shown in Fig. 1e. By the AFM
cross-sectional analysis, the pyramidal tips are primarily bounded by
value as the Si pyramid tips formed ͑50 s in this case͒. The lower
reflectance of the Si pyramid tips can be attributed to the highly
roughened surface. However, after etching the sample for 60 s, the
reflectance increases significantly. This result is consistent with the
AFM observation that Si pyramidal tips disappear or smooth out
with an overetching time. Recently, many techniques including
laser-chemical20 and metal-assisted chemical etching21 have been
reported to fabricate “black silicon” with an ultralow reflectance.
Compared with black silicon, the reflectance of the Si pyramidal tips
is still too high for photovoltaic applications. Nevertheless, this sur-
face nanoroughening process could be an approach applied to other
Si-based nanodevices, such as metal-oxide-Si tunneling diodes22 or
light-emitting diodes.23
The cross-sectional transmission electron microscopy ͑XTEM͒
images in Fig. 3 show the representative microstructures of the as-
grown and etched samples. As shown in Fig. 3a, a typical multifac-
eted Ge island with a very thin wetting layer was observed in the
as-grown sample. After etching for 40 s, the wetting layer was com-
pletely destroyed and anisotropic etching of the Si substrate oc-
curred, resulting in a mushroomlike shape in Fig. 3b. The mush-
roomlike structure consists of an upper Ge island and an underlying
sigma ͑⌺͒-shaped Si pedestal. The ⌺-shaped Si pedestal was pro-
duced by the anisotropic etching of Si sidewalls.24 With further etch-
ing for 45 s, as seen in Fig. 3c, the heights of mushroomlike struc-
tures continue to increase. Meanwhile, the upper Ge islands
apparently shrink. As reported previously, Ge islands usually suffer
from serious Si–Ge intermixing during island growth. The Ge-rich
area in the Ge islands exists as a core of the islands encapsulated by
the intermixed phase.25 Therefore, the outer parts of Ge islands are
easily etched out during the process, leaving the smaller Ge-rich
cores of islands on Si. As seen in the XTEM image, the dimension
of Ge-core islands is an important parameter for determining the
apex sharpness of the resulting structures. After etching the sample
for 50 s, the Ge-core islands were removed, leaving the Si pyrami-
dal tips on the Si substrate. As shown in Fig. 3d, the diameter of the
apex of Si pyramidal tips is less than 20 nm. Eventually, after fur-
ther etching for 60 s ͑Fig. 3e͒, the Si pyramids tended to smooth out,
leading to an increase in reflectance. We also find that Ge-core is-
lands are highly resistant to a TMAH etchant. Figure 4 gives an
example of a freestanding Ge-core island, which was left on Si after
2 min of etching. It also indicates that Ge-core islands were not
eroded away but lifted off from the Si pyramidal tips. In the present
study, we use the AFM tips with a typical tip radius of less than
7 nm and a conical angle better than 25°, which is much smaller
than the apex angle of the Si pyramidal tips. Therefore, tip-sample
͕111͖ facets. The tip density is estimated to be about
8
ϫ 109 cm−2, very close to that of the as-grown Ge domes. These Si
pyramidal tips have an average height and base diameter of 47 and
95 nm, respectively. The Si pyramidal tips are expected to exhibit
improved field emission characteristics because they have a much
higher aspect ratio compared to self-assembled Ge nanodots.16
However, after further etching for 60 s, the Si pyramidal tips tended
to shrink or even disappear, as shown in Fig. 1f.
Optical reflectance is a sensitive nondestructive method to exam-
ine the etched surface morphology. Figure 2 shows the optical re-
flection spectra with wavelengths from 300 to 850 nm for the as-
grown and etched samples. The inset of Fig. 2 is a plot showing the
variation in reflectance at 400 nm as a function of etching time. The
reflectance decreased with the etching time and reached a minimum
Figure 2. ͑Color online͒ Optical reflection spectra with wavelengths from
300 to 850 nm for the as-grown and etched samples measured at an incident
angle of 5°. The inset shows the variation in reflectance at 400 nm as a
function of etching time.
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H176
Journal of The Electrochemical Society, 157 ͑2͒ H174-H177 ͑2010͒
Figure 4. XTEM image of a Ge-core island remaining after 2 min immer-
sion in a 50% TMAH solution.
in Fig. 5. First, the thin wetting layer was removed ͑Fig. 5a and b͒.
Subsequently, Ge islands act as nanomasks to prevent the underlying
Si substrate from being etched, resulting in a ⌺-shaped Si pedestal
͑Fig. 5c͒. With a longer etching time, the height of the ⌺-shaped Si
pedestal continues to increase ͑Fig. 5d͒. Meanwhile, Ge islands
shrink into Ge-rich cores, thus reducing the lateral size of the upper
Si pedestal. Underetching is a well-known problem in the Si etching
technology.26,27 This is not observed experimentally in this work,
possibly because Ge nanomasks shrink at the same time. Neverthe-
less, we speculate that, with further etching, underetching of the
upper Si pedestal may occur and shrink the dimension of the upper
Si pedestal because Ge-rich cores do not shrink anymore. Eventu-
ally, Ge-core islands were lifted off and Si pyramidal tips form on
the Si substrate ͑Fig. 5e͒. We expect that the pyramid structures can
be further optimized. The size uniformity of Ge nanomasks is an
important parameter for the fabrication of pyramid structures. Post-
growth annealing can be performed to obtain more uniform Ge is-
lands as nanomasks28 and thus yield more uniform Si pyramidal
tips. In addition, the aspect ratios of the resulting structures in the
present study are limited to the formation of Si pyramids. Recently,
Figure 3. ͑Color online͒ XTEM images of ͑a͒ as-grown Ge islands on Si
after etching for ͑b͒ 40, ͑c͒ 45, ͑d͒ 50, and ͑e͒ 60 s, respectively. Note that
WL in ͑a͒ refers to the wetting layer and ͑c͒ and ͑d͒ were taken along ͗010͘
directions.
convolution effects can be largely neglected. Here, the inconsistency
between AFM and XTEM images mainly occurs in those etched
samples with a ⌺-shaped Si pedestal, where AFM tips cannot access
the re-entrant surface and thus display a distorted facet in imaging.
Nevertheless, for the Si pyramidal tips, the AFM results, including
pyramidal shape, facet, and base diameter, are very consistent with
those obtained from XTEM observations because the ⌺-shaped ped-
estal has disappeared.
Figure 5. Schematic depiction of the formation of Si pyramidal tips with Ge
nanomasks: ͑a͒ as-grown Ge islands, ͑b͒ etching on wetting layer, ͑c͒ aniso-
tropic etching to produce a mushroomlike structure, ͑d͒ Ge nanomask shrink,
and ͑e͒ underetching and final structure with Si pyramidal tips.
Based on these experimental observations, the formation proce-
dures for the Si pyramidal tips with Ge nanomasks are summarized
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Journal of The Electrochemical Society, 157 ͑2͒ H174-H177 ͑2010͒
Conclusion
H177
In summary, large-area, nanometer-scale Si field emitters have
been successfully fabricated using self-assembled Ge islands as na-
nomasks. During the etching process, Ge nanomasks shrink into
small Ge-core islands that determine the apex sharpness of the Si
pyramidal tips. These Si pyramidal tips have an average height and
base width of 47 and 95 nm, respectively. The results also demon-
strate that Si pyramidal tips exhibited improved antireflective and
field emission properties compared to as-grown Ge islands. The high
field enhancement factor can be attributed to high tip density, nano-
scale apex, and well-controlled spacing between Si pyramidal tips.
This process promises to be applicable for fabricating future high
efficiency Si-based field emitters.
Acknowledgment
The research was supported by the National Nano Device Labo-
ratories and the National Science Council of Taiwan under contract
no. NSC 97-2221-E-008-091-MY3 and no. NSC 98-2623-E-008-
007-NU.
Figure 6. ͑Color online͒ Plots of field emission current density J ͑mA/cm2͒
against local applied field E ͑V/␮m͒ curves for Si pyramidal tips and as-
grown Ge islands. The inset presents the emission current data plotted with
the FN relationship for the Si pyramidal tips.
National Central University assisted in meeting the publication costs of
this article.
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where J is the current density, E is the applied electric field,
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pyramidal tips. The ln͑J/E2͒ − 1/E plot gives a straight line at a
high field, suggesting an FN tunneling process. The field enhance-
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higher than that of as-grown Ge islands, which is 1043. The high ␤
value of Si pyramidal tips can be attributed to high tip density ͑8
ϫ 109 cm−2͒, nanoscale apex ͑less than 20 nm͒, and well-controlled
spacing between Si pyramidal tips so that the antenna effect is mini-
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