Generation of substrate-free III--V nanodisks from user-defined
multilayer nanopillar arrays for integration on SiPAPER • OPEN
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Generation of substrate-free III–V nanodisks from user-defined
multilayer nanopillar arrays for integration on Si
S Naureen, N Shahid1, A Dev and S Anand
School of Information and Communication Technology, KTH-Royal
Institute of Technology, Electrum 229, Kista SE-16440, Sweden
E-mail:
[email protected]
Received 23 February 2013, in final form 28 March 2013 Published 30
April 2013 Online at stacks.iop.org/Nano/24/225301
Abstract High material quality InP-based multilayer nanopillar (NP)
arrays are fabricated using a combination of self-assembly of
silica particles for mask generation and dry etching. In
particular, the NP arrays are made from user-defined epitaxial
multilayer stacks with specific materials and layer thicknesses. An
additional degree of flexibility in the structures is obtained by
changing the lateral diameters of the NP multilayer stacks.
Pre-defined NP arrays made from InGaAsP/InP and InGaAs/InP NPs are
then used to generate substrate-free nanodisks of a chosen material
from the stack by selective etching. A soft-stamping method is
demonstrated to transfer the generated nanodisks with arbitrary
densities onto Si. The transferred nanodisks retain their smooth
surface morphologies and their designed geometrical dimensions.
Both InP and InGaAsP nanodisks display excellent photoluminescence
properties, with line-widths comparable to unprocessed reference
epitaxial layers of similar composition. The multilayer NP arrays
are potentially attractive for broad-band absorption in
third-generation solar cells. The high optical quality,
substrate-free InP and InGaAsP nanodisks on Si offer a new path to
explore alternative ways to integrate III–V on Si by bonding
nanodisks to Si. The method also has the advantage of re-usable
III–V substrates for subsequent layer growth.
S Online supplementary data available from
stacks.iop.org/Nano/24/225301/mmedia
(Some figures may appear in colour only in the online
journal)
1. Introduction
The development of fabrication technologies for semiconduc- tor
nanostructures, although initially driven by the demand for device
miniaturization [1], has opened up new possibilities for realizing
ultra-light, smart, multifunctional devices [2, 3], in addition to
exploring the rich physics of confined systems. Recent research
efforts have demonstrated applications
Content from this work may be used under the terms of the Creative
Commons Attribution 3.0 licence. Any further
distribution of this work must maintain attribution to the
author(s) and the title of the work, journal citation and DOI. 1
Present address: Department of Electronic Materials Engineering,
Research School of Physics and Engineering, The Australian National
University, Canberra ACT 0200, Australia.
of III–V nanostructures in a variety of electronic and photonic
applications, featuring better performance and/or new functions
[4–8]. In addition, heterogeneous integration of III–V compound
semiconductors on Si has been extensively studied to combine the
advantageous properties of III–Vs, such as their direct bandgaps
and high carrier mobilities, with the low cost and mature
processing advantage of Si technology [2, 5, 9–11]. In addition,
GaAs and InP-based multi-junction solar cells offer the highest
efficiency [12, 13] and can be integrated to design self-powered
devices. However, direct integration of several relevant III–V
materials (e.g. InP, GaAs etc) on Si by heteroepitaxy remains a
major challenge due to the large lattice mismatch between the III
and V materials and Si [14–17]. On the other hand, low-temperature
wafer bonding is not subject to these lattice
10957-4484/13/225301+08$33.00 c© 2013 IOP Publishing Ltd Printed in
the UK & the USA
Nanotechnology 24 (2013) 225301 S Naureen et al
Figure 1. Schematic demonstration of various steps for (a) nanodisk
fabrication using dry etching and wet etching of the multilayered
structure; (b) stamping of nanodisks on a Si substrate using
PDMS.
matching conditions and several integrated devices have been
demonstrated [17–19]. However, this approach often involves
removing the III–V substrate after bonding and the electronic
quality of the III–V/Si interface has to be improved for device
applications that require intimate contact at the interface [18,
19]. Several new methods are currently being developed for
large-scale integration without the restriction of substrate
compatibility, such as epitaxial lift-off [20, 21], contact
printing, and solution transfer [2, 11, 22, 23]. In this context,
the present work demonstrates a method to generate and transfer
high optical quality InP and InGaAsP nanodisks from user-defined
InP-based multilayer nanopillar (NP) arrays. Using the designed
material layers, one can obtain nanodisks of a given material by
selectively etching the others. The physical dimensions of the
nanodisk can also be tailored—its thickness is defined by growth
and the lateral size is determined by the NP diameter. The
nanodisks, thus generated, can be transferred to Si for
integration. This method is equally valid for multi-layer NPs
obtained by top-down or bottom-up approaches, each having its own
advantages. Such multilayer NP arrays are also attractive
candidates for broad-band absorption in solar cells. With advances
in NP growth, it may be possible to control the doping profiles,
crystalline quality and material composition to obtain tailored
multilayered NPs [24, 25]. Alternatively, we can benefit from
state-of-the-art epitaxial growth of III–V thin films to fabricate
user-defined material stacks and then adapt a top-down approach for
InP-based NP arrays [26] to generate multilayer NPs. In this case,
with appropriate patterning it is also possible to obtain
micro-pillars. Finally, depending on the transfer procedure, III–V
disks with lateral dimensions ranging from nanometers to
micrometers can be assembled on the substrate of choice.
In this paper, we demonstrate a simple method to fabricate InP,
InGaAsP and InGaAs nanodisks with user- defined dimensions, and
transfer them onto Si or other desired substrates with arbitrary
densities. The fabrication of InGaAsP/InP and InGaAs/InP multilayer
NP arrays is based on colloidal lithography for mask generation and
dry etching. The InGaAsP/InP and InGaAs/InP multilayer structures
were grown by metal organic vapor phase epitaxy (MOVPE). From a
given multilayer NP array, nanodisks of the selected material (e.g.
InP) are obtained by the selective etched removal of the
other material (e.g. InGaAsP). We also show good control over the
diameter of the nanodisks by resizing of the mask particles and/or
by post-fabrication nano-sculpting [27]. The nanodisks are
transferred to Si substrates manually using a polydimethylsiloxane
(PDMS) stamp. Scanning electron microscopy (SEM) and atomic force
microscopy (AFM) investigations of stamped nanodisks show smooth
surface morphologies and confirm that the physical dimensions of
the nanodisks are consistent with the grown multilayer and NP
dimensions. Photoluminescence (PL) measurements performed at both
room temperature and low temperature (77 K) shows strong
luminescence and narrow line-widths, very similar to that of
high-quality epitaxial layers. Strong PL signals were observed even
from isolated disks.
2. Experimental details
2.1. Fabrication of multilayer nanopillar arrays
The fabrication method for nanodisks is schematically shown in
figure 1(a). We used a two-step process. In the first step,
nanopillar arrays were fabricated by vertically etching InGaAsP/InP
and InGaAs/InP multilayer planar structures. A colloidal
lithography technique applied to pattern etch-masks, followed by
inductively coupled plasma reactive ion etching (ICP-RIE) was used
to fabricate ordered arrays of vertically aligned nanopillars with
alternating layers of InGaAsP/InP or InGaAs/InP. In the second
step, wet-chemical etching was used to selectively remove one
material layer, forming disk-shaped nanostructures of the other
material. The initial multilayer structures (on InP substrate) were
grown by MOVPE, and all the epitaxial layers were nominally
undoped. The details of the nanopillar fabrication process are
reported elsewhere [26, 27]. Briefly, the as-grown samples were
first cleaned by standard organic solvents and then treated with O2
plasma to enhance the wettability of the surface. An aqueous
suspension of colloidal SiO2 particles, used as an etch-mask, was
then spin coated on the substrate. The size of the SiO2 particles
also defines the diameter of the nanopillars, and hence the
diameter of the nanodisk. For this work, SiO2 particles of 500 nm
diameter were first coated on the substrate and then isotropically
etched using RIE to reduce the size. RIE operating at 15 mT with a
CHF3 flow of 25 sccm offers
2
Nanotechnology 24 (2013) 225301 S Naureen et al
a 20 nm min−1 etch rate, and thus allows trimming of the silica
particle diameter. By shrinking the particle size, while
maintaining their original position, the spacing between the
adjacent particles is increased. This also helps to reduce the
well-known lag effect observed during etching of InP using closely
packed particles as etch-masks. SiO2 particles with diameters in
the range 150 nm–3 µm can be used to obtain nanodisks of different
diameters, according to requirement. Finally, ICP-RIE (Oxford
Plasma lab System 100) with a Cl2/H2/CH4 chemistry was used to
fabricate InP nanopillars by etching the exposed substrate. The NPs
were organically cleaned and treated with 50% HF to remove the
remaining silica mask particles. Finally, the samples were rinsed
in DI water and dried under N2 flow.
2.2. Selective wet-chemical etching
In the second step, selective wet-chemical etching was used to
fabricate InP, InGaAsP and InGaAs disks from the multilayer NP
arrays. For the fabrication of InGaAsP and InGaAs nanodisks, InP
layers in the pillar arrays were selectively etched with HCl:H2O in
a 2:1 ratio for 5 min. On the other hand, to generate InP
nanodisks, InGaAs or InGaAsP layers (depending on the vertical
structure) were selectively removed either by using an etching
solution of H3PO4:H2O2:H2O or H2O:H2SO4:H2O2. Both the etchants
work well. However,results with H3PO4:H2O2:H2O, used in a 1.1:1
ratio, having an etch rate of ∼1 nm s−1, were found to be more
satisfactory and equally good for selective etching of both InGaAs
and InGaAsP. All the samples were rinsed in DI water and dried at
room temperature inside a fume hood. Additional treatment in 50% HF
solution for 1 min removes residues and native oxide layers from
the disks.
We note here that, after selective etching, most of the nanodisks
settle on the substrate due to their large size. During etching,
the samples were left undisturbed (no agitation). After etching,
the sample was carefully taken out of the etch solution without
tilting and transferred to a separate beaker containing DI water.
The sample was left in DI water for about 2 min, carefully lifted
out and left to dry in a fume hood. The same procedure was followed
for subsequent HF treatment of the nanodisks after stamping. These
steps were sufficient for the present purposes. However, it is
likely that some nanodisks are lost during transfer from the
solutions. In the present work, it was not possible to quantify the
yield, which requires further investigations mapping the surface
density of the nanopillars and the remnant nanodisks. We foresee
the need to develop a robust and reproducible procedure for
applications that require close to 100% yield or a pre-designed
arrangement (as defined by the nanopillars) of the nanodisks.
2.3. Stamping method for nanodisks transfer onto Si
The stamping procedure uses a PDMS stamp, shown schematically in
figure 1. For the preparation of the PDMS stamp, Sylgard 184 from
Dow Corning with a composition of base and curing agent in a 10:1
ratio was used. The two parts were thoroughly mixed in a small
beaker and the mixture
was put into a desiccator under vacuum for about 30 min for
de-gassing. Then the mixture was poured on a Si wafer to the
desired thickness (∼2 mm in this case). Care was taken to avoid any
bubble formation. The PDMS was then cured at 100 C for one
hour.
We comment here that our main purpose was to demonstrate the
feasibility of transferring nanodisks with a reasonable yield. We
used a very simple approach, that is, by gently pressing the PDMS
stamp manually to both collect and transfer the nanodisks. Although
this manual procedure works rather well, the yield of transfer
could be different from run to run. In general, not all the
collected disks are transferred in one stamping step. This, we have
used positively to control the average spatial density of the
stamped nanodisks (as shown in figure 4 and in supporting
information, S3 available at stacks.iop.org/Nano/24/225301/
mmedia). However, the ideal controlling parameters, such as force,
temperature, and contact time, involved in the stamping process
will be necessary for the best reproducibility and yield. In
particular, such control can enable faithful transfer of nanodisks
in specific arrangements, if so desired.
2.4. Characterization methods
The morphology and physical dimensions of the stamped nanodisks
were investigated by scanning electron microscopy (SmartSEM V5.00)
and atomic force microscopy in tapping mode. Photoluminescence
measurements were made using a HORIBA JOBIN YVON HR800 Raman and
micro PL system using an InGaAs detector. The samples were excited
by a 514.5 nm Ar laser, with an excitation density of 0.5 mW
cm−2.
3. Results and discussions
The process flow for the fabrication of InP-based nanodisks and
their transfer onto Si is schematically shown in figure 1. In the
first step, nanopillar arrays were fabricated by vertically etching
InGaAsP/InP and InGaAs/InP multilayered planar structures using a
previously optimized inductively coupled plasma reactive ion
etching (ICP-RIE) process [26]. The multilayer NP and nanodisk
fabrication are validated on different multilayered vertical
structures. A schematic of a representative vertical structure,
used for both InGaAsP and InP nanodisk fabrication, is shown in
figure 2(a). The structure contains five InGaAsP layers, of
different thickness, 20, 50, 100 and 200 nm; these layers are
separated by 100 nm thick InP (barrier) layers. Figure 2(b) shows
an SEM image of a nanopillar array fabricated from this structure.
Highly ordered vertical nanopillar arrays with circular
cross-sections are clearly seen. The fabrication details are
similar to those previously developed for NP arrays in InP and
InP/InGaAsP QW structures [26]. The typical etch selectivity is
>10:1 over the colloidal silica mask. The ICP-RIE parameters
were adjusted to obtain a uniform NP height of ∼800 nm to expose
most of the grown multilayers. The magnified image (figure 2(c))
shows clear material contrast along the length of the pillar,
delineating the different layers. Five 100 nm
Figure 2. (a) Schematic of a multilayered structure used for InP
and InGaAsP nanodisk fabrication; (b) 12 tilted cross-section SEM
image showing ordered arrays of pillars fabricated using ICP-RIE.
(c) Magnified cross-sectional SEM image of the nanopillars showing
material contrast from alternating layers.
InP segments and one 100 nm, one 50 nm and two 20 nm InGaAsP
segments are accessible for subsequent substrate- free nanodisk
generation. The CH4/H2/Cl2 chemistry used to etch the NPs is
advantageous since a wide range of III–V materials can be etched
with smooth surface profiles and it is not very material selective.
Therefore, it gives the facility to etch hetero-structures with
minimal exposure of the hetero-interfaces. The SEM images (figures
2(b) and (c)) show no detectable material-dependent lateral
etching. The pillar diameter at the top closely follows that of the
SiO2 etch-mask, but is wider at the base due to slow but continuous
erosion of silica particles with etch time [28]. The evolution of
the pillar shape from cylindrical to conical as a function of etch
time is shown in section S1 in the supporting information
(available at stacks.iop.org/Nano/ 24/225301/mmedia). However, with
a more resistant mask material (e.g. metal particles) taper-free
pillars are possible. Alternatively, after pillar fabrication,
controlled sculpting using a sulfur-oleylamine solution provides
NPs with uniform lateral dimensions [27]. The supporting
information discusses these results in detail (S2 available at
stacks.iop.org/Nano/24/ 225301/mmedia).
Figure 3 shows cross-sectional SEM images of the nanopillar arrays
after different durations of material-selective etching,
demonstrating the different intermediate steps in the nanodisk
fabrication. Nanopillars with partially etched InGaAsP layers after
30 s and 1 min of selective etching are shown in figures 3(a) and
(b), respectively. Clearly, the InP layer shows very good etch
resistance in H3PO4:H2O2:H2O, whereas the InGaAsP layer is removed
rapidly. In contrast, the HCl:H2O solution rapidly etches only the
InP layer (figure 3(c)), without any detectable erosion of the
InGaAsP
layers. Similar etching selectivity of HCl:H2O solution was
observed in the case of an InGaAs/InP vertical structure, thus
allowing fabrication of InGaAs nanodisks using the same etchant.
Keeping the samples in the etching solution for a slightly longer
time ensured complete removal of the sacrificial layers. A
comparison of the etching behavior of InP and InGaAsP layers in
H3PO4:H2O2:H2O and HCl:H2O solution is shown in figure 3(d). The
red arrows indicate the etching behavior of the InGaAsP layers in
H3PO4:H2O2:H2O (left) and HCl:H2O (right) solutions, whereas the
white arrows indicate the same for the InP layers.
Figures 4(a) and (b) show the SEM images of the fabricated InGaAsP
and InP nanodisks, respectively, taken after cleaning and stamping
on Si substrate. The high density of nanodisks on the Si substrate
clearly indicates the very high yield of the stamping process.
Control over the nanodisk density can be achieved either by varying
the number of layers in the initial multilayer structures, and thus
altering the number of disks per NP, or by modifying the density of
the pillars in the arrays by controlling the density of the
colloidal silica particles. Further control of the density of
nanodisks on Si can be attained during the stamping process
(supporting information S3 available at stacks.iop.org/Nano/
24/225301/mmedia). The observed variation in the diameters of the
nanodisks (inset of figures 4(a) and (b)) is consistent with the
tapered shape of the NPs.
AFM measurements were performed to determine the thickness and
surface morphology of the stamped nanodisks. Figures 5(a) and (b)
show the AFM images of InGaAsP and InP nanodisks, respectively. The
thickness of InGaAsP and InP nanodisks analyzed at different parts
of the sample are shown in figures 5(c) and (d), respectively. As
expected
Nanotechnology 24 (2013) 225301 S Naureen et al
Figure 3. (a) Partially etched InGaAsP layers after 30 s, (b)
partially etched InGaAsP layers after 1 min, (c) partially etched
InP layers after 1 min. (d) Comparison of etching behavior of an
InGaAsP layer in H3PO4:H2O2:H2O (left) and an InP layer in HCl:H2O
solution (right).
Figure 4. (a) SEM image showing highly dense InGaAsP nanodisks
stamped on a Si substrate; inset shows a circular cross-section of
the disks. (b) SEM image of InP disks showing a large distribution
in their diameters.
from the choice of the vertical structure, the InGaAsP nanodisk
(figure 5(c)) sample shows nanodisks with three different
thicknesses of 20, 50 and 100 nm, whereas InP nanodisks (figure
5(d)) are close to 100 nm thick in all parts of the sample. This is
a clear indication of the very high material selectivity of both of
the etchants. The slight thickness deviations from the designed
values (figure 2(a)) are predominantly due to similar deviations in
the grown multilayer structure. The surfaces of individual disks at
several parts of the sample were carefully investigated by AFM and
the measured roughness (rms) was found to be less than 1 nm. Such a
smooth surface morphology further confirms very good material
selectivity of the chemical etchants. It is also evident that the
stamping process is equally efficient for both thin (20 nm, in this
case) and thick (100 nm) nanodisks. Furthermore, nanostructures
with different shapes arising from conical NPs are obtained,
as discussed in section S4 of the supporting information (figures
S4-1 and S4-2 available at stacks.iop.org/Nano/24/ 225301/mmedia).
The nanodisk fabrication method was also validated by preparing 20
nm thick InGaAs and 100 nm InP nanodisks from an InGaAs/InP
multilayered structure (section S5 in the supporting information
available at stacks.iop.org/ Nano/24/225301/mmedia). The results
clearly demonstrate the versatility of this process to prepare
substrate-free high-quality InP-based nanodisks. Although the work
focuses on InP-based materials, the general principles are
applicable to other lattice matched semiconductor materials with
appropriate NP fabrication and material selective etchants.
Chemical treatment, in many cases, has been observed to influence
the optical quality of InP-based materials [29] by introducing
additional surface states. Such mechanisms increase the surface
recombination rate and significantly reduce their PL yield. In
addition, PL line-widths might
Nanotechnology 24 (2013) 225301 S Naureen et al
Figure 5. (a) AFM image of InGaAsP nanodisks (magnified view in the
inset). (b) InP nanodisks on a Si substrate. (c) Sectional analysis
performed on (a) showing the thickness distribution of InGaAsP
nanodisks. (d) Sectional analysis performed on (b) showing the
identical thickness of ∼100 nm as expected from the multilayered
structure.
also increase. Depending on the chemicals and the nature of their
interaction with the semiconductor, the extent of such effects can
be different [29]. In order to investigate the optical quality of
the fabricated nanodisks, PL measurements were performed both at
room temperature (RT) and 77 K, and the results were compared with
the PL spectra recorded from nanopillar arrays (reference
sample).
The PL spectra of InGaAsP nanodisks are presented in figures 6(a)
and (b). Room-temperature PL spectra (figure 6(a)) recorded at two
different density regions (number density of the scattered
nanodisks) show a strong luminescence band centered at 1.02 eV,
corresponding to bulk InGaAsP. The line-width of the PL band is
about 60 meV. The PL line-widths and peak positions for the disks
and the reference sample are comparable (inset of figure 6(a)). The
small blue-shift observed for the NPs is attributed to the
contribution from the 20 nm QW layers. It is reasonable to assume
that absorption of the 514 nm excitation light (Ar laser), with a
corresponding light penetration depth in InP of ∼90 nm, occurs
mostly in the upper part of the NPs. Thus, one expects the PL to
depend on the vertical structure, with the bulk-like 100 and 50 nm
InGaAsP layers being dominant. However, due to the relatively lower
band offsets between InP/InGaAsP, carriers can be thermally
re-emitted from the bulk-like GaInAsP layers to the InP barrier,
which are then transferred to the 20 nm QW layers. Thus, the PL
contribution from the 20 nm QW layers will blue-shift the PL peaks
for the NPs compared to the bulk nanodisks, as observed
(∼10 meV) in the inset of figure 6(a). At lower temperatures, the
carrier redistribution by thermal re-emission from the bulk GaInAsP
layers is suppressed. Thus, one would expect a negligible
contribution from those 20 nm QWs situated at ∼450 nm (and lower)
from the NP top. Consistent with this, as seen in figure 6(b), the
PL peak positions of the NPs and the GaInAsP nanodisks show no
shift. The above observations are supported by PL investigations of
nanopillar arrays fabricated from two different vertical
structures, and the results are discussed in section S6 (and the
figures therein) of the supporting information (available at
stacks.iop.org/Nano/ 24/225301/mmedia). In structure I (figure
S6(a1)) bulk-like GaInAsP layers are in the upper part of NPs,
while in structure II (figure S6(a2)) the 20 nm QWs are at the top.
In the case of structure II, the PL will have a significant
contribution from the 20 nm InGaAsP QWs, and thus will be
blue-shifted compared to structure I. This is confirmed by PL
results reported in figures S6(b) and (c). As expected, at RT, due
to redistribution of carriers in the nanopillars, the PL spectra
(figure S6(b)) are broader in both the structures. The shoulders
are also visible, indicated by arrows in figure S6(b). However, at
77 K, as expected, the line-widths are narrow and comparable
(figure S6(c)).
Figure 6(b) shows the normalized low-temperature (77 K) PL spectra
of InGaAsP nanodisks and the reference sample. Both the peak
position and the line-width of the nanodisks correspond very well
with those of the NPs. These results are also consistent with the
arguments presented above. Both the
Nanotechnology 24 (2013) 225301 S Naureen et al
Figure 6. (a) Room-temperature PL spectra of InGaAsP nanodisks
taken at different density regions; inset shows normalized PL
spectra of nanodisks (blue line) and the reference sample (black
line); (b) normalized low-temperature (77 K) PL spectra of InGaAsP
nanodisks (blue line) and the reference sample (black line); inset
shows the comparison of the intensity at different density regions.
(c) Room-temperature PL spectra of InP nanodisks and the reference
sample (green line, 50 times magnified); inset shows normalized
spectra; (d) low-temperature (77 K) PL spectra of InP
nanodisks.
RT and 77 K PL data indicate that the dominant contribution comes
from bulk-like nanodisks. The PL measurements indicate that
bulk-like nanodisks are better suited—both for their superior
optical properties and known bulk properties. µ-PL measurements
(spot size ∼ 2 µm) over regions containing a few nanodisks (three
to five nanodisks) are shown together with PL from high-density
regions in the inset of figure 6(b). As shown, PL could be observed
even from regions of extremely low density, suggesting the good
optical quality of individual bulk nanodisks.
Comparison of the PL intensity before and after chemical etching
could reveal more about the influence of chemical treatment on the
luminescence quality of the bulk nanodisks. However, such a
comparison is not meaningful since the geometry of the structures
is very different, affecting the light coupling. Furthermore,
efficient carrier transfer from InP barriers to lower bandgap
InGaAsP layers occurs in the NPs. The latter has two
consequences—the PL from InGaAsP will be much stronger, whereas
that from InP will be very weak.
PL investigations of InP nanodisks, as expected, also show
bulk-like luminescence properties. The room- temperature PL spectra
of InP nanodisks and the correspond- ing reference sample are
demonstrated in figure 6(c). InP nanodisks show strong luminescence
at room temperature whereas the luminescence from the reference
sample is very weak due to the reasons explained earlier. The
low-temperature PL spectra from InP nanodisks shows
(figure 6(d)) a narrow line-width of ∼18 meV, which corresponds
very well with our investigation of single InP nanopillars [26].
All the observations and comparisons made above clearly indicate
that the as-prepared nanodisks possess a very good optical quality,
even without any surface treatment. By developing low-temperature
bonding procedures [18, 19], it will be possible to integrate the
III–V GaInAsP or InP nanodisks on Si for device applications.
4. Conclusion
In summary, we have demonstrated a simple and cost-effective method
to fabricate nanodisks of InP, InGaAsP and InGaAs from epitaxially
grown InP-based multilayer structures. The fabrication method
involves a low-damage ICP-RIE process, followed by
material-selective wet-chemical etching. The demonstrated methods
offer flexibility for the precise control of the diameter and
thickness of the nanodisks, while maintaining good
photoluminescence properties. Transfer of the generated nanodisks
to Si using a soft stamp, with variable spatial coverage, is
demonstrated. Although only planar stamps were used in this work,
the results suggest that transfer printing of nanodisks in selected
areas will be possible using appropriately patterned stamps. The
excellent material selectivity of the wet-chemical etching also
ensures smooth surfaces, with less than 1 nm rms roughness. The
InGaAsP and InP nanodisks on Si exhibit high
photoluminescence
7
Nanotechnology 24 (2013) 225301 S Naureen et al
even at room temperature. The measured PL line-widths of different
nanodisks is comparable to the corresponding reference layers,
indicating excellent fabrication quality. The multilayer NP arrays
are potentially attractive for broad-band absorption in
third-generation solar cells. High optical quality, substrate-free
InP and InGaAsP nanodisks on Si offer a new path to explore
alternative ways to integrate III–V materials on Si by bonding
nanodisks to Si. The method also has the advantage of re-usable
III–V substrates for subsequent layer growth.
Acknowledgments
The work was performed within the Linne Center for Advanced Optics
and Photonics (Grant number: 349-2007- 8664) funded by the Swedish
Research Council. Partial supports from the EU Network of
Excellence ‘Nanophotonics for Energy Efficiency’ (Grant number:
248855) and from ‘Nanordsun’ (Grant number: 10048) funded by the
Nordic Innovation Centre are also acknowledged. SN and NS
acknowledge the Higher Education Commission of Pakistan for
partially supporting their PhD studies (scholarship). The authors
thank M Hammar and J Berggren for MOVPE growth, and A Berrier for
useful discussions.
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Introduction
Selective wet-chemical etching
Characterization methods