APL Mater. 8, 031115 (2020); https://doi.org/10.1063/1.5142491 8, 031115
© 2020 Author(s).
Dislocations as channels for the
fabrication of sub-surface porous GaN by
electrochemical etching
Cite as: APL Mater. 8, 031115 (2020); https://doi.org/10.1063/1.5142491
Submitted: 13 December 2019 . Accepted: 05 March 2020 . Published Online: 25 March 2020
Fabien C.-P. Massabuau , Peter H. Griffin , Helen P. Springbett, Yingjun Liu , R. Vasant Kumar,
Tongtong Zhu , and Rachel A. Oliver 
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APL Materials ARTICLE scitation.org/journal/apm
Dislocations as channels for the fabrication
of sub-surface porous GaN by electrochemical
etching
Cite as: APL Mater. 8, 031115 (2020); doi: 10.1063/1.5142491
Submitted: 13 December 2019 • Accepted: 5 March 2020 •
Published Online: 25 March 2020
Fabien C.-P. Massabuau, Peter H. Griffin, Helen P. Springbett, Yingjun Liu, R. Vasant Kumar,
Tongtong Zhu,a) and Rachel A. Olivera)
AFFILIATIONS
Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road,
Cambridge CB3 0FS, United Kingdom
a)Authors to whom correspondence should be addressed: tongtong.zhu@porotech.co.uk and rao28@cam.ac.uk
ABSTRACT
Porosification of nitride semiconductors provides a new paradigm for advanced engineering of the properties of optoelectronic materials.
Electrochemical etching creates porosity in doped layers while leaving undoped layers undamaged, allowing the realization of complex three-
dimensional porous nanostructures, potentially offering a wide range of functionalities, such as in-distributed Bragg reflectors. Porous/non-
porous multilayers can be formed by etching the whole, as-grown wafers uniformly in one simple process, without any additional processing
steps. The etch penetrates from the top down through the undoped layers, leaving them almost untouched. Here, atomic-resolution electron
microscopy is used to show that the etchant accesses the doped layers via nanometer-scale channels that form at dislocation cores and trans-
port the etchant and etch products to and from the doped layer, respectively. Results on AlGaN and non-polar GaN multilayers indicate that
the same mechanism is operating, suggesting that this approach may be applicable in a range of materials.
© 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5142491., s
Gallium nitride and its alloys are enabling an ever-increasing
range of applications across electronics and optoelectronics. Since
the development of the first commercial nitride light emitting diodes
(LEDs) in the mid-1990s, the potential applications of this family
of materials have burgeoned to include laser diodes,1 single pho-
ton sources,2 solar cells,3 power electronic devices,4 and integrated
photonic circuits.5 In all of these devices, the key method by which
the properties of the nitride materials have been systematically engi-
neered is by changing their composition, principally by altering the
proportion of the different group III metals (indium, gallium, alu-
minum, and more recently boron) forming the binary compound
with nitrogen.
Recently, an alternative method of engineering the materi-
als properties of nitride semiconductors has emerged: by intro-
ducing porosity into monolithic nitride layers, it is possible to
achieve very significant changes in a wide range of properties
including electrical6 and thermal7 conductivity, refractive index,8
birefringence,9 mechanical properties,10 strain relief,11 chemical
activity,12 and piezo-electricity.13 Porous GaN layers are already
enabling enhanced device performance in LEDs14,15 and laser diodes
(LDs)16 and the development of novel composites of GaN with other
materials.17 The porosification of other nitride alloys has allowed the
formation of novel quantum structures.18 Introducing porosity to
nitride materials provides a new degree of freedom, allowing hith-
erto unanticipated physical phenomena and device concepts to be
explored.
Porosification of GaN can be achieved by an electrochemi-
cal etching process in which highly n-doped GaN immersed in
an acid-based electrolyte is subjected to a positive anodic bias.19
This results in oxidation of GaN by holes at the surface inver-
sion layer, resulting in material decomposition and pore formation.
This mechanism requires the n-doped GaN surface to be directly
exposed to the electrolyte. However, for many potential applications
of porous GaN, a multilayer of alternating porous and non-porous
APL Mater. 8, 031115 (2020); doi: 10.1063/1.5142491 8, 031115-1
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materials is required. A key example here is a distributed Bragg
reflector (DBR) in which the refractive index contrast between layers
results in reflection at the interfaces between the layers. High reflec-
tivity occurs when the quarter-wavelength of the light incident on
the DBR matches the layer thickness. For porous nitride DBRs, this
typically implies the formation of a stack of several pairs of layers,
consisting of a non-porous layer and a porous layer, each a few 10s
of nanometers thick.
Such a structure can be formed by growing a simple (non-
porous) GaN structure, consisting of alternating layers of undoped
GaN (which will remain non-porous) and highly n-doped GaN
(which will be porosified). In order to access the sub-surface doped
layers and achieve porosity by electrochemical etching, it is typical
to first cover the GaN surface with SiO2 and then pattern this SiO2
by opening up windows of uncovered GaN. SiO2 is used as an etch
mask in the etching of deep trenches into GaN. This exposes the
sidewalls of sub-surface, highly n-doped GaN layers. Electrochem-
ical porosification then proceeds laterally into the masked regions.
This approach has been very successful for the formation of DBRs
over small regions, of lateral extent 10s–100s of micrometers.20
For example, Huang et al.21 showed lateral etching over a range
of about 80 μm, and Hsu et al.22 showed a greater etch penetra-
tion of 155 μm. Lateral etching has been used to create optically
pumped microcavities8 and electrically injected laser structures.16
However, the requirement to etch deep trenches at regular inter-
vals adds significant complexity and cost to the cheap and simple
electrochemical etching process and is limiting in terms of device
design.23
For porous GaN to be integrated into existing device process-
ing paradigms, it would be preferable to develop a simple process
that allows uniform porosification of sub-surface layers across whole
industrial-size wafers, without the requirement for surface coating
and trench etching. We recently demonstrated14 such a wafer-scale
porosification approach for the formation of DBRs, and the same
approach is relevant to a broad range of other sub-surface multilayer
structures. We showed that etching can occur through an undoped
surface GaN layer and also through the multiple undoped layers in
the DBR stack, porosifying the doped layers and leaving the undoped
GaN almost unaltered. We suggested that etching was occurring via
intrinsic structural defects, potentially threading dislocations, which
are ubiquitously present in the heteroepitaxial nitride materials that
dominate the nitride device market. Here, we address the role of
threading dislocations in the electrochemical etching process and
reveal the details of the porosification mechanism.
For this study, structures consisting of alternating layers of
undoped GaN and doped GaN were deposited by metal-organic
vapor phase epitaxy (the standard industrial technique) in a 6 × 2 in.
Thomas Swan close-coupled showerhead reactor on c-plane sap-
phire substrates using trimethylgallium and ammonia as precursors,
hydrogen as a carrier gas, and silane for n-type doping. After the
growth of a 4 μm thick c-plane GaN buffer layer with a nominal dis-
location density of ∼3 × 108 cm−2, 500 nm n-doped GaN (silicon
doping concentration ∼ 1 × 1018 cm−3) was first grown to enable
the uniform distribution of etching current across the entire area
of the wafer. Thereafter, pairs of alternating highly doped n+-GaN,
with a nominal silicon doping concentration of ∼1 × 1019 cm−3, and
undoped GaN layers, with an electron concentration of <1017 cm−3,
were grown.
Porosification of the sub-surface doped GaN layers was
achieved using a simple one-step electrochemical etching process, as
detailed elsewhere.14 In this process, the GaN sample is immersed in
0.25M oxalic acid with the as-grown undoped GaN surface exposed
and a DC potential of 6 V applied to the sample with respect to an
inert platinum counter electrode. No SiO2 or other protective layer
was deposited on the GaN surface, and no trenches were etched. The
electrochemical porosification thus proceeds vertically from the top
surface, but as we have previously shown,14 the top surface is left
unaffected by the etching process, within the accuracy achieved by
atomic force microscopy measurements.
To examine how the porosity evolves with etching time, we
used a sample consisting of 12 periods of alternating highly doped
and undoped GaN, with the top (surface) layer being undoped.
Pieces of the sample were etched for time periods of increasing
duration and then cleaved and examined in the scanning elec-
tron microscope (SEM) using a Nova NanoSEM with the through
lens secondary electron detector and an acceleration voltage of
5 kV. After a short etching period (500 s), porosity is only observed
in patches of the uppermost doped layer [Fig. 1(a)]. After 1000 s
FIG. 1. Cross-sectional SEM images of a DBR sample after
various etch times, showing how the etch proceeds in depth
as time passes. Scale bars are 1 μm.
APL Mater. 8, 031115 (2020); doi: 10.1063/1.5142491 8, 031115-2
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[Fig. 1(b)], porosity is observed throughout the top layer and the
second layer has also mostly been porosified. After 2500 s, poros-
ity is observed throughout the top four layers and about half of
the fifth is porous [Fig. 1(c)]. In this case, the full 12 pair struc-
ture was porosified after 6000 s. The etch proceeds from the top
surface down, but the undoped layers between the porous layers
remain essentially undamaged within the resolution of the SEM
observation. Hence, it is vital to address how the etchant is accessing
each doped layer after etching of the previous layer completes and
indeed how the etchant accesses the first doped layer through the
undoped cap.
In a previous study, we inferred that the etch accesses the
sub-surface layers via channels at dislocations,14 postulating that
these defects—usually seen to be a weakness of heteroepitaxial GaN
materials—can, in fact, be used to great advantage in engineering the
sub-surface porous morphology. Here, we use plan view high angle
annular dark field imaging in the scanning transmission electron
microscope (HAADF-STEM) to observe the structure of threading
dislocation cores in a porous DBR sample and an unetched mate-
rial from the same wafer (Fig. 2). The HAADF-STEM imaging was
performed on an aberration-corrected FEI Titan operated at 300 kV.
The sample was prepared in the plan-view geometry by mechanical
polishing, followed by Ar+ ion milling at 5 kV until a hole was vis-
ible, and final cleaning at 0.1–1 kV. Dislocations in the sample that
has not been porosified exhibit a filled core structure with a range
of atomic arrangements, consistent with those previously reported
in the literature.24 Figure 2(a) shows an example of an edge disloca-
tion with the usual core structure consisting of a five-membered and
a seven-membered ring. This is in strong contrast with the porosi-
fied sample, where the dislocations we observed had a hollow core
[Fig. 2(b)]. In Figs. 2(a) and 2(b), a Burgers circuit has been drawn
around the suggested position of the dislocation. The Burgers cir-
cuit drawn around the ∼5–10 nm diameter hole in Fig. 2(b) con-
firms the existence of an edge- or mixed-type dislocation at the hole
location.
Lower magnification HAADF-STEM images of a single porous
layer in a porosified sample [Fig. 2(c)] reveal the structure of
the porous layer around the holes at the dislocation locations. In
Fig. 2(c), the contrast relates to the thickness of the sample (given
that all of the material is GaN) and the holes at dislocation locations
are seen as dark spots (one of these is ringed in white). Radiating out
from these dark spots, we see a finger-like structure, with the pores
(dark gray contrast) alternating with bright unetched walls. Domains
are observed, which are separated by unetched walls wherever the
finger-like pores emanating from different dislocations would other-
wise meet. A subset of these walls are marked in yellow in Fig. 2(c).
These data suggest that porosification occurs through dislocations,
where a dislocation forms a pipe that connects the surface of the
sample to a subsurface layer, which is subsequently etched via that
pipe.
Very occasionally, we observe domains of pores that have an
unetched dislocation core at their center (Fig. 3). However, in this
case, we observe a dark region to one side of the dislocation core,
indicating that the material is thinner in this region. In this case, we
suggest that the pipe connecting the dislocation core to the surface
terminates at the porous layer under observation and that the dark
recession relates to the pipe position. These rare structures give us
the opportunity to assess the strain field around the dislocation and
FIG. 2. High resolution HAADF-STEM image of a dislocation seen along the⟨0001⟩ zone axis (i.e., seen end-on) in the (a) non-porous sample and (b) porous
sample. (c) Lower magnification HAADF-STEM image of a porous layer in the
porosified sample. Here, contrast is thickness-related (i.e., dark means pore). A
hole at the location of a dislocation is marked in white, while a subset of the walls
between domains emanating from separate dislocations are marked in yellow.
how it relates to the position of the pipe. The STEM data in Fig. 3 are
overlaid with a geometrical phase analysis map,25 revealing a com-
pressively strained region on one side of the dislocation core and a
tensile strained region on the other. The dark region corresponding
to the pipe location is located on the same side of the dislocation
core as the compressively strained region. Silicon dopants have a
tendency to segregate to the compressively strained regions adjacent
to dislocation cores.26 Hence, these observations are consistent with
outdiffusion of silicon from the doped to the undoped layers in the
vicinity of dislocations. The compressively strained region adjacent
to the dislocation core would then have an enhanced conductivity,
allowing electrochemical etching to take place. Such silicon segre-
gation would be expected to occur in the very near vicinity of the
dislocation core,26 consistent with our observation of the formation
of pipes that remove the core structure. Other unintentional dopant
APL Mater. 8, 031115 (2020); doi: 10.1063/1.5142491 8, 031115-3
© Author(s) 2020
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FIG. 3. High resolution HAADF-STEM image of a (dissociated mixed-type) dislo-
cation seen along the ⟨0001⟩ zone axis (i.e., seen end-on) in the porous sample.
This dislocation was not etched completely, and a recession from the etch channel
could be seen in darker contrast (indicated by a circle). The recession is not cen-
tered on the unetched dislocation core but is instead shifted on the compressive
region part of the core, as indicated by the geometrical phase analysis overlay.
Inset: lower magnification HAADF-STEM image of the same region.
incorporation (e.g., oxygen) could also lead to a conductivity in the
near core region, facilitating etching.
It is worth briefly commenting on why the dislocation in Fig. 3
might have an intact core below the etched layer. We suggest that
this occurs when etching down different nanopipes occur at slightly
different rates, and hence, in the next doped layer below the one pic-
tured in Fig. 3, the material close to this dislocation could have been
etched via other, faster etching nanopipes before this etch pathway
penetrated below the layer in this figure. Hence, etching down this
pathway would terminate, leaving the dislocation core intact.
Based on the TEM data [e.g., Fig. 2(c)], we estimate that the
volume fraction of the material removed from the porous layers is
approximately 45%. Etch times vary between samples, but the etch-
ing of a single layer can take as little as 120 s. Assuming that nitrogen
is evolved from the overall etching reaction as N2 molecules, then for
a dislocation density of ca. 3× 108 cm−2 and the formation of pipes at
the dislocation core of 5 nm radius, this implies a flux of N2 down the
pipe of roughly 0.03 mol m−2 s−1. Assuming that nitrogen is trans-
ported via diffusion through the solvent (water) contained in the
pipe, we calculate (see the supplementary material) that this rate of
mass transport would require a supersaturation of nitrogen in water
of around a factor of two in comparison to the equilibrium solubility
at atmospheric pressure. Such a super-saturation is not unreason-
able, given that the formation of gaseous nitrogen would require
the nucleation of gas bubbles, and the critical radius for nucleation
might require bubbles to be formed which are larger than the pore
size.
Dislocations in GaN are generally considered to be a nui-
sance, deteriorating the properties of devices. Here, however, we
have shown that they can be usefully exploited as pathways for
electrochemical etching of sub-surface layers, providing a pipeline
for the flow of the etchant through the undoped material and leav-
ing the surrounding defect-free material undamaged. This approach
has been shown to be viable not only for the polar GaN studied
here but also for non-polar GaN,14 and our STEM studies (see the
supplementary material, Fig. 1) show similar pore morphologies
for the non-polar case. Focused ion beam/SEM studies of AlGaN
DBRs23 show a similar morphology to the porous layer, suggest-
ing that the same mechanism operates in other alloys (see the sup-
plementary material, Fig. 2). This flexibility between the different
crystal orientations and compositions suggests that defect assisted
electrochemical etching may also be viable in other heteroepitaxial
semiconductor materials.
See the supplementary material for details of calculations on the
rate of mass transport down the pipes at dislocation cores and for
images of porous layers formed via dislocation related mechanisms
in non-polar GaN and c-plane AlGaN.
This work was supported, in part, by the UK Engineering
and Physical Sciences Research Council (EPSRC) under Grant Nos.
EP/M011682/1 and EP/M010589/1 and the EPSRC Impact Acceler-
ation Account Follow-On Fund of the University of Cambridge.
DATA AVAILABILITY
The data that support the findings of this study are available
within the article and its supplementary material.
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