Focused Ion Beam Technology for Optoelectronic Devices
J.P. Reithmaier, L. Bach, A. Forchel
Technische Physik, University of Würzburg, Germany
Abstract. High-resolution proximity free lithography was developed using InP as anorganic resist for ion beam
exposure. InP is very sensitive on ion beam irradiation and show a highly nonlinear dose dependence with a contrast
function comparable to organic electron beam resists. In combination with implantation induced quantum well
intermixing this new lithographic technique based on focused ion beams is used to realize high performance nano
patterned optoelectronic devices like complex coupled distributed feedback (DFB) and distributed Bragg reflector
(DBR) lasers.
Ion beam enhanced wet chemical etching, which
was demonstrated first in the AlGaAs/GaAs material
system,3-5 needs several orders of magnitude lower
doses and strongly reduces the damage problem by
selectively removing the highly damaged area. This
technology could be successfully transferred to InP
which allow the implementation of FIB based nano
patterning techniques to the fabrication of telecommunication related optoelectronic devices.6-9
INTRODUCTION
High-resolution lithographic techniques gained
more and more importance for the realization of
advanced microelectronic and optoelectronic devices
due to reduced structure dimensions and the demand
on size control in the nanometer range. The control of
structure dimensions below 0.1 µm by optical
lithography is quite sophisticated. Improved resolution
is possible by high-resolution e-beam writing systems.
Both techniques are based on exposure of organic
resists and need additional mask and structure transfer
steps to form semiconductor nanostructures. This
pattern transfer process causes additional deviations
and has to be corrected. Important factors are depth of
focus for optical lithography that limits the exposure of
multi-level surfaces, and the proximity effect for ebeam exposure caused by backscattered electrons.
In this paper the newly developed nano fabrication
technique based on FIB exposure of InP as inorganic
resist material is reviewed. Examples will be
discussed, where this technique was applied for the
fabrication of optical feedback gratings to realize
single mode emitting lasers and monolithically
integrated devices.
Also with focused ion beams (FIB) high-resolution
exposure is possible in the range well below 100 nm.
Due to the large penetration depth and the low
probability to create secondary electrons, proximity
effects can be neglected. In contrast to the other
lithographic techniques, FIB can also applied for
maskless patterning. Different effects can be used, like
implantation induced quantum well intermixing1 or
direct sputtering of material.2 Unfortunately, sputtering
needs relatively high ion doses and causes significant
crystal damage, which may significantly degrade
device performances.
MASKLESS PATTERNING
For implantation, an FIB system from Eiko Corp. is
used with a beam diameter (FWHM) of about 30 nm.10
For maskless patterning, 100 keV Ga+ ions were
directly implanted into InP and the damaged regions
were removed by a 10% HF acid in an ultrasonic bath
at 80 °C. In Fig. 1, the dose dependence of the etch
depth is plotted which show a resist like strongly nonlinear characteristic. Below an ion dose of 1.5×1013
cm-3 etching is completely stopped while above 2×1013
cm-3 the etch depth saturates to about the ion
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quantum wells are within this depth, the channeled
ions can induce thermal intermixing effects in spatial
alignment to the etched patterns. In Fig. 3, the
implantation induced thermal intermixing effect is
illustrated which creates a local band gap shift.13
penetration depth (not taking into account channeling
effects).
FIGURE 3. Schematic illustration of implantation induced
thermal intermixing effect in semiconductor quantum films.
Starting from an untreated quantum film (left) the transition
energy is shifted to higher energies due to a narrowing of the
well caused by material interdiffusion.
FIGURE 1. Dose dependence of etch depth in InP after 100
keV Ga implantation and 10 min etching in 10% HF.
Because of the high contrast function the etching
stops at a fixed ion dose level. This can be seen in a
secondary electron microscopy (SEM) image of the
cross section of such a line in Fig. 2. The etch front is
highlighted by a dashed line which coincides quite
nicely with the dose profile calculated by Monte Carlo
simulations. The feature size at the surface is 25 nm
and broadens to about 40-50 nm due to scattering
effects. Because the dose is quite low, only 125 ions
are involved within an area of 25×25 nm2 and
statistical deviations are already significant. Due to the
high sensitivity of InP on ion beam implantation, the
writing speed can be as fast as e-beam exposure of
organic resists.
GRATING FABRICATION
Both effects can be used separately or
simultaneously to fabricate gratings for optical
feedback in single mode emitting devices like
distributed feedback (DFB) or distributed Bragg
reflector (DBR) lasers. If only the material contrast is
used, an index coupled grating is built while by using
the intermixing effect, a gain coupled grating is
formed.14 Best results in terms of wavelength control
and device performance are achieved by combining
both effects in so-called complex coupled gratings.9
+
Ga
FIGURE 4. Schematic illustration of the definition of
laterally complex coupled gratings by FIB technology. The
optical feedback is based on the overlap with the evanescent
part of the propagating optical mode.
FIGURE 2. SEM cross-section of wet chemically etched
single implanted line. The dashed line highlights the edge of
the groove.
In addition to the creation of a damage network
near the surface, which is removed by the above
discussed wet chemical etching process, an essential
part of the ions penetrate much deeper into the
semiconductor due to channeling in crystal
direction.11,12 In (100) InP the penetration depth for
100 keV Ga ions is in the range of about 300 nm. If
In Fig. 4, the grating fabrication technique is
illustrated. First, a conventional ridge waveguide
(RWG) laser is processed by optical lithography and
wet chemical etching. The etch depth is controlled by a
GaInAsP etch stop layer. Second, the grating geometry
is defined by direct Ga implantation without any mask
process. The highly damaged parts lateral to the ridge
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process technology. The contact layers from gain and
DBR sections are separated and only the gain section
is pumped.
are removed by HF. The ridge itself is still protected
by the previous etch mask. Afterwards, the sample is
annealed by a rapid thermal annealing step (typical
parameters: 700 °C, 60 s) to intermix the underlying
quantum wells and to form a self-aligned bandgap
shifted absorption grating to the index surface grating.
gain section
grating
section
In Fig. 5, an SEM image of an FIB defined grating
is shown after the wet chemical etch process. Due to
the proximity free exposure, the grating is well defined
near to the ridge. Shadowing of the slightly underetched ridge causes a well controlled lateral
displacement of the grating of about 100 nm. The
rectangular unpatterned area in Fig. 5 was formed by
beam shadowing of the top under-etched contact layer,
which is broken away after the process.
FIGURE 6. Schematic structure of a DBR laser with gain
and passive grating sections.
Despite the simple fabrication technology, the
device performance is very high.9 In Fig. 7, the light
output characteristic of such a device with a total
device length of 600 µm is shown. The light output
was detected from the cleaved backside of the device.
The device has a very low threshold current of 8 mA
and very high differential efficiency of 0.374 W/A.
The inset shows the emission spectrum with a high
side mode suppression ratio (SMSR) of 48 dB. The
wavelength is precisely defined by the grating period
and the fabrication yield is nearly 100%. Lifetime
measurements show no degradation up to 10000 h.7
FIGURE 5. Top-view of a lateral grating after the wet
chemical etch process. The unpatterned area at the end of the
ridge is caused by beam shadowing of an under-etched part
of the top contact layer (not visible any more).
The grating depth is controlled by a second
GaInAsP etch stop layer, the groove width by the
exposed ion dose. The cross section of a grating line is
nearly rectangular with a width and depth of about 80
nm. The grating period is about 240 nm. The bandgap
shift caused by the intermixing is in the order of 40
meV.
DEVICE EXAMPLES
The definition of 1st order gratings by conventional
techniques for DFB or DBR lasers is still sophisticated
because an additional overgrowth process is needed
and the wavelength control is difficult. With this new
approach the device processing can be strongly
simplified and the fabrication yield improved. In the
following two examples will be briefly discussed.
FIGURE 7. Light output characteristic (solid line) and
SMSR values (open dots) for a DBR laser with 300 µm
grating and 300 µm gain sections. The inset shows the
emission spectrum at a drive current of 70 mA.
For wavelength division multiplexing (WDM)
systems, light sources with different emission wavelengths are necessary. By using the newly developed
grating fabrication technique by FIB, the integration of
DFB lasers with different grating periods is strongly
simplified and the total fabrication yield is quite high.
In Fig. 6, the schematic structure of a DBR laser
with an active gain section and a passive DBR section
is shown. Except for the grating definition by FIB, all
process steps are identical to a conventional RWG
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In Fig. 8, a realized device design is shown which
integrates four DFB lasers emitting at different
wavelengths.8 The light of all 4 lasers is coupled into a
single output ridge waveguide which simplifies fiber
coupling.
ACKNOWLEDGMENTS
The supply of epitaxial laser structures by Alcatel
Corp. Res. Center, Opto+, and the financial support by
the European Community (LTR project NANOLASE,
IST project BigBand) and the State of Bavaria is
gratefully acknowledged.
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