U.S. patent application number 15/416134 was filed with the patent office on 2017-05-11 for lateral electrochemical etching of iii-nitride materials for microfabrication.
This patent application is currently assigned to YALE UNIVERSITY. The applicant listed for this patent is YALE UNIVERSITY. Invention is credited to JUNG HAN.
Application Number | 20170133826 15/416134 |
Document ID | / |
Family ID | 49778116 |
Filed Date | 2017-05-11 |
United States Patent
Application |
20170133826 |
Kind Code |
A1 |
HAN; JUNG |
May 11, 2017 |
LATERAL ELECTROCHEMICAL ETCHING OF III-NITRIDE MATERIALS FOR
MICROFABRICATION
Abstract
Conductivity-selective lateral etching of III-nitride materials
is described. Methods and structures for making vertical cavity
surface emitting lasers with distributed Bragg reflectors via
electrochemical etching are described. Layer-selective, lateral
electrochemical etching of multi-layer stacks is employed to form
semiconductor/air DBR structures adjacent active multiple quantum
well regions of the lasers. The electrochemical etching techniques
are suitable for high-volume production of lasers and other
III-nitride devices, such as lasers, HEMT transistors, power
transistors, MEMs structures, and LEDs.
Inventors: |
HAN; JUNG; (WOODBRIDGE,
CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
YALE UNIVERSITY |
NEW HAVEN |
CT |
US |
|
|
Assignee: |
YALE UNIVERSITY
NEW HAVEN
CT
|
Family ID: |
49778116 |
Appl. No.: |
15/416134 |
Filed: |
January 26, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13923248 |
Jun 20, 2013 |
9583353 |
|
|
15416134 |
|
|
|
|
61665617 |
Jun 28, 2012 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/306 20130101;
H01S 5/343 20130101; H01L 21/30635 20130101; H01L 2924/0002
20130101; H01S 5/34333 20130101; H01S 5/18341 20130101; H01L 33/105
20130101; H01L 21/30612 20130101; H01S 5/187 20130101; H01L
2924/0002 20130101; H01L 33/0075 20130101; H01L 2924/00 20130101;
H01S 5/18369 20130101; H01S 5/18363 20130101; B82Y 20/00
20130101 |
International
Class: |
H01S 5/343 20060101
H01S005/343; H01S 5/183 20060101 H01S005/183; H01S 5/187 20060101
H01S005/187 |
Claims
1. A III-nitride DBR device comprising: a multi-layer structure
having first and second layers formed of III-nitride material,
wherein a conductivity of the first layers is different from a
conductivity of the second layers; a MQW structure formed adjacent
the multi-layer structure, wherein the MQW structure comprises an
active region of the device; vias formed into the multi-layer
structure proximal to the MQW structure; and regions adjacent the
vias in which portions of the second layers have been completely
removed to form at least two first layers separated by one or more
layers of air.
2. The DBR device of claim 1, wherein the MQW structure forms an
active region of a laser.
3. The DBR device of claim 1, wherein the first layers and second
layers comprise GaN.
4. The DBR device of claim 3, wherein the second layers comprise
high n-type conductivity material.
5. The DBR device of claim 1, wherein the thicknesses of the first
layers are substantially the same.
6. The DBR device of claim 1, wherein the thicknesses of the first
layers correspond to approximately one-quarter of a selected
emission wavelength for the laser.
7. The DBR device of claim 1, wherein the removed portions of the
second layers and remaining first layers form a structure having
periodic contrast of optical refractive index.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
application Ser. No. 61/665,617 titled "III-Nitride Vertical Cavity
Surface Emitting Lasers (VCSEL) with GaN/air DBR by Electrochemical
Etching," filed on Jun. 28, 2012, which is incorporated herein by
reference in its entirety.
BACKGROUND
[0002] Technical Field
[0003] The technology relates to methods and structures for
performing etching of sacrificial layers of III-nitride material.
The etching techniques may be used for micro- and nano-fabrication
of integrated devices, such as vertical cavity surface emitting
lasers (VCSELs) based on III-nitride semiconductor material. The
VCSELs may include air/semiconductor, distributed Bragg reflector
(DBR) structures formed adjacent to the VCSEL using lateral,
electrochemical etching techniques.
[0004] Discussion of the Related Art
[0005] The etching of semiconductor materials is an important
technique that is used in microfabrication processes. Various kinds
of etching recipes have been developed for many materials used in
semiconductor manufacturing. For example, Si and certain oxides may
be routinely etched using dry (e.g., reactive-ion etching) or wet
chemical etching techniques that yield desired etch rates and etch
morphologies. III-nitride materials have recently emerged as
attractive materials for semiconductor manufacturing, however these
materials can be chemically inert to standard wet etchants.
[0006] Some attractive applications for III-nitride materials
include micro-photonic devices, such as LEDs and lasers. Some
existing methods of making III-nitride VCSELs involve forming
structures that comprise alternating layers of AlGaN/GaN or
AlInN/GaN. However, these structures are difficult to produce and
do not exhibit high refractive index contrast between the
alternating layers. Accordingly, to obtain a suitable reflectivity
for a laser cavity, the number of layers must be increased (e.g.,
to about 40) making the overall cavity thick. Additionally, it is
difficult to match the cavity mode with an active layer emission
wavelength. Another approach to making a VCSEL with a DBR structure
is to form dielectric DBRs at two ends of the cavity using layer
lift-off techniques. However, this approach is complex to implement
and suffers from low yield during manufacture.
[0007] As InGaN light emitting diodes (LEDs) gradually approach
technological maturity in performance for blue/green emissions,
microcavity based LEDs (resonant-cavity LED, RCLED) and laser
diodes (vertical-cavity surfaceemitting laser, VCSEL) become
appealing alternatives that may offer advantages in enhancing
radiative recombination rates, improving beam directionality, and
possibly reducing the cost in manufacturing due to their planar
configuration.
[0008] The encasing of the optical active region into a cavity of a
few wavelengths may be done using distributed Bragg reflectors
(DBRs) with minimum absorption and high reflectivity. For
nitride-based emitters, top p-side DBRs may be prepared by
depositing dielectric, quarter-wavelength stacks as a last step of
device fabrication. The bottom (n-side) DBRs may be implemented
using either dielectric stacks or epitaxially grown (Ga,Al,In)N/GaN
periodic heterostructures. However, challenges still exist in both
approaches toward ultimate manufacturability. The dielectric
approach can require complicated thin-film lift-off and wafer
bonding in order to expose the n-type GaN. For epitaxial DBRs, 20
to 40 pairs of heterostructures are needed for good peak
reflectance (above 95%) due to the small contrast of refractive
indices. Such a thick DBR structure causes a narrow stopband
(<50 nm), creates issues in stress management on the device, and
reduces the benefit of the Purcell effect.
[0009] Semiconductor/air structures have been pursued using a
photo-assisted electrochemical (PEC) etch where minority-holes are
photo-generated and confined in narrower-bandgap sacrificial layers
to facilitate selective etching. Also, selective chemical etching
has been identified for the AlInN/GaN system. The membrane
structures prepared by the two techniques generally suffer from
etched surface roughness that contributes to scattering losses in
the optical devices. The maximum reflectance in the blue/green
range from GaN/air DBRs, prepared by either PEC or selective wet
etching, has not exceeded 75%.
SUMMARY
[0010] The described technology relates to methods and structures
for lateral electrochemical etching of III-nitride materials to
produce optically smooth surfaces, and for controllably forming
porous III-nitride material. The etching processes are compatible
with InGaN based light emitters and integrated devices. The etching
techniques may be used for manufacturing various microstructures
and microdevices. For example, the etching techniques may be used
for making vertical cavity surface emitting lasers (VCSELs) that
include distributed Bragg reflector (DBR) structures. According to
some embodiments, the VCSELs and DBR structures comprise
III-nitride semiconductor materials that may be deposited in
multiple layers on a substrate, and the formed DBR structure
comprises a limited number of alternating III-nitride/air layers.
In various embodiments, the electrochemical etching of a DBR
structure laterally removes alternating layers in a multilayer
structure, and leaves optically smooth air/semiconductor
interfaces. The electrochemical etching may be highly selective to
the conductivity of the materials. The conductivity (and resulting
etching properties) may be controlled by doping the materials
during their deposition. By modulating the doping of epitaxial
layers, highly-doped n-type layers may be laterally etched over
large distances (>10 microns) in confined regions (<500 nm
layer thickness sandwiched between non-etched layers) at high etch
rates (>5 microns/min). The etching techniques may be used to
fabricate other III-nitride devices, e.g., LEDs, high electron
mobility transistors, high-power tranistors, and MEMs devices
[0011] The electrochemical etching may be controlled to form
optically smooth surfaces or nanoporous structures wherein the
pores may have selected properties. The etching may be controlled
to produce a desired etching morphology by controlling etching
parameters that include one or more of: etchant solution,
electrical bias between the sample to be etched and the etchant
solution, and dopant concentration of the material to be
etched.
[0012] According to some embodiments, a method for laterally
etching III-nitride material comprises depositing a first layer of
III-nitride material having a first conductivity on a substrate,
and depositing a second layer of material over the first layer. The
method may further include forming a via in the second layer to
expose a surface area of the first layer, and electrochemically and
laterally etching at least a portion of the first layer using a
hydrofluoric-based etchant. The portion of the first layer etched
may undercut the second layer.
[0013] In some embodiments, a method for etching III-nitride
material may comprise depositing a first layer of III-nitride
material having a first conductivity on a substrate, and depositing
a second layer of material adjacent the first layer. The method may
further include electrochemically etching at least a portion of the
first layer using a hydrofluoric-based etchant. The etched portion
of the first layer may be a component of an LED device.
[0014] In some embodiments, a method for forming a distributed
Bragg reflector (DBR) laser comprising III-nitride material
comprises depositing a first multi-layer structure on a substrate,
wherein the first multi-layer structure comprises first and second
layers. The first layers may have a conductivity different than
that of the second layers. The method may further include
depositing a multiple quantum well (MQW) active structure adjacent
the first multi-layer structure, and forming vias into the first
multi-layer structure so as to provide access for an etchant to the
second layers. The method for forming a distributed Bragg reflector
(DBR) laser may further comprise laterally and electrochemically
etching the second layers, so as to remove at least a portion of
the second layers and form a DBR structure adjacent the MQW region.
The DBR structure may comprise at least two first layers separated
by one or more layers of air.
[0015] Structures related to the methods are also contemplated. In
some implementations, a III-nitride DBR device manufactured
according to the disclosed techniques may comprise a multi-layer
structure having first and second layers formed of III-nitride
material, wherein a conductivity of the first layers is different
from a conductivity of the second layers. The device may further
comprise a MQW structure formed adjacent the multi-layer structure,
wherein the MQW structure comprises an active region of the device,
and vias formed into the multi-layer structure proximal to the MQW
structure. The DBR device may further include regions adjacent the
vias in which portions of the second layers have been completely
removed to form at least two first layers separated by one or more
layers of air.
[0016] The foregoing and other aspects, embodiments, and features
of the present teachings can be more fully understood from the
following description in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The skilled artisan will understand that the figures,
described herein, are for illustration purposes only. It is to be
understood that in some instances various aspects of the
embodiments may be shown exaggerated or enlarged to facilitate an
understanding of the embodiments. In the drawings, like reference
characters generally refer to like features, functionally similar
and/or structurally similar elements throughout the various
figures. The drawings are not necessarily to scale, emphasis
instead being placed upon illustrating the principles of the
teachings. Where the drawings relate to microfabrication of
integrated devices, only one device may be shown of a large
plurality of devices that may be fabricated in parallel. The
drawings are not intended to limit the scope of the present
teachings in any way.
[0018] FIG. 1 depicts a III-nitride DBR laser with a MQW active
region, according to some embodiments;
[0019] FIGS. 2A-2H depict methods for fabricating structures that
include laterally-etched III-nitride materials, according to some
embodiments;
[0020] FIG. 3A-3B are SEM micrographs of nanoporous GaN formed by
electrochemical etching in HF-based etchant, according to some
embodiments;
[0021] FIG. 4 depicts anodic current-voltage relationship for n-GaN
etching in HF; according to some embodiments;
[0022] FIGS. 5A-5B are SEM micrographs showing a change in pore
density as a function of etching time;
[0023] FIGS. 6A-6B are SEM micrographs showing synchronized pore
diameter oscillation;
[0024] FIG. 7 depicts results of etching phases for varying dopant
densities as a function of applied etching bias; according to some
embodiments;
[0025] FIGS. 8A-8D are SEM micrographs illustrating different pore
morphologies as a function of dopant density;
[0026] FIG. 9 plots pore diameter and wall thickness as a function
of dopant density, according to some embodiments;
[0027] FIG. 10 depicts results of etching phases for varying HF
concentration as a function of applied etching bias; according to
some embodiments;
[0028] FIG. 11 plots pore diameter and wall thickness as a function
of HF concentration, according to some embodiments;
[0029] FIGS. 12A-12B are representations of possible pore
morphologies based on the ratio of space charge region thickness
and pore separation;
[0030] FIG. 13A depicts an embodiment of an air/semiconductor DBR
structure, according to some embodiments;
[0031] FIGS. 13B-13C show SEM micrographs of air/GaN layers of a
DBR structure fabricated using electrochemical etching, according
to some embodiments;
[0032] FIG. 13D shows a DIC image of a DBR structure after
electrochemical etching, where the formation of air gaps between
layers results in a circular pattern;
[0033] FIG. 14 shows measured and simulated reflectance spectra for
a DBR structure; according to some embodiments;
[0034] FIG. 15 shows emission spectra of MQWs samples (1) without
DBRs, (2) with bottom DBRs, and (3) with bottom DBRs and silver
capping layer, where significant linewidth narrowing can be seen
for samples with DBRs;
[0035] FIG. 16 illustrates FWHM linewidths from experiment and from
simulated spectra of emitters in a cavity, where agreement between
experiments and simulation of high reflectance bottom DBRs
indicates membrane DBRs can be used to improve spectral purity via
optical cavity modes;
[0036] FIG. 17A shows a DIC image of the measurement spots on an EC
etched sample with MQWs; and
[0037] FIG. 17B illustrates the corresponding emission peak and
Raman shift at the measured spot locations.
[0038] The features and advantages of the embodiments will become
more apparent from the detailed description set forth below when
taken in conjunction with the drawings.
DETAILED DESCRIPTION
[0039] Because III-nitride materials can be chemically inert to wet
etchants, microfabrication of integrated optical or integrated
electronic devices based on these materials poses manufacturing
challenges. Although some etching techniques (e.g., dry
reactive-ion etching or photochemical etching) have been developed
to etch these materials, these processes can be costly and/or
difficult to implement. Described herein are methods that may be
used to readily etch III-nitride materials at high etch rates
(e.g., at rates up to 10 microns/minute) and with uniform and
controllable surface properties and etch morphologies.
[0040] The etching processes are based on electrochemical etching
of III-nitride materials that have been selectively doped to tune
the etching properties of the materials. The merits of
conductivity-based wet etching include, but are not limited to (1)
high scalability to large areas--the etching does not require UV
illumination and therefore does not suffer from problems related to
uniformity of illumination, (2) improved manufacturability--the
electrochemical etch rate can be several orders of magnitude faster
than chemical wet etch, (3) controllable etching morphology--the
etching morphology may be controlled by controlling the doping
profile(s) in the material(s) to be etched. For example, modulation
of dopant can define complex structures in a single material.
[0041] The etching techniques may be used for various
microfabrication applications, e.g., the manufacture of
microstructures that include III-nitride materials. Examples of
microstructures include microcavity lasers, DBR lasers, microcavity
LEDs, resonant cavity LEDs, enhanced emission LEDs, transistors,
and MEMs devices. In some embodiments, the electrochemical etching
may be used in the preparation of highly reflective distributed
Bragg reflectors (DBRs) from III-nitride materials. These DBR
structures may provide a useful building block for integrated
photonics, such as the manufacture of DBR lasers, microcavity
structures, and enhanced emission structures. Other devices
fabricated from III-nitrides may comprise detectors and emitters in
the blue/green region of the optical spectrum, as well as
transistors that may be used in high power and/or high electron
mobility devices. For integrated optical applications, an
integrated DBR structure may be used to provide tailored
reflectance or transmission bands for integrated photonic
structures.
[0042] Research on porous semiconductors has drawn much attention,
since the discovery of intense luminescence from porous silicon.
The creation of nanopores through electrochemical (EC) anodization
transforms conventional semiconductors into three-dimensional
meshed networks or foams that are inherently single crystalline.
Applications of porous III-V semiconductors in microelectronics,
optics, sensing, and light harvesting have been demonstrated.
Extension of the porosification study to wide-bandgap GaN is
especially appealing due to its demonstrated importance in light
emitting diodes and high power electronics. GaN is chemically inert
with no available wet etching process at room temperature.
Accordingly, an electrochemical etching process for III-nitride
materials will have attractive technological implications.
[0043] GaN epilayers are typically prepared with a high density of
dislocations. Porous GaN can conceivably influence and block the
propagation of dislocations. Additionally, it has been shown that
the surface states of GaN tend to have a much slower recombination
velocity than those from conventional As- and P-based III-V
compounds, making it possible to consider the usage of porous GaN
as an active medium.
[0044] By way of explanation, methods for making integrated DBR
structures for integrated photonic devices are described below to
exemplify how the etching techniques may be used to form advanced
integrated optical structures. However, the invention is not
limited to the formation of DBR structures for VCSELs only. The
etching techniques may be applied to other microfabrication
processes involving III-nitride materials. For example, the etching
techniques may be used to form LEDs, transistors, cantilevers, or
microelectromechanical structures based on III-nitride materials.
In some embodiments, the etching techniques may be used to form
thin membranes. In some implementations, the etching techniques may
be used to release membranes or devices from a substrate.
[0045] FIG. 1 shows one example of an integrated photonic DBR laser
102 that includes a highly reflective DBR structure 110 that may be
fabricated using electrochemical etching of sacrificial III-nitride
layers. The highly reflective DBR structure 110 may form a
wavelength-selective mirror at a first end of a cavity of the DBR
laser 102. The active or gain region of the laser may comprise a
multiple quantum well (MQW) structure 140. The MQW 140 may be
sandwiched between a layer of n-type semiconductor material 130,
which may inject electrons into the MQW region, and a p-type
semiconductor material 150, which may inject holes into the MQW
region. The holes and electrons may recombine in the MQW region to
produce photons. There may be a first contact pad 135 formed on the
n-type semiconductor material 130 and a second contact pad 155
formed on the p-type semiconductor material 150.
[0046] In some embodiments, a second end of the DBR laser cavity
may comprise a dielectric DBR structure 160 that functions as a
wavelength-selective mirror. In other embodiments, a dielectric DBR
structure may not be used, and a reflective or semi-reflective
layer of material (e.g., a thin film of metal such as silver,
chrome, or gold) may be deposited at the second end of the laser
cavity. The reflectivity of the dielectric DBR 160 or deposited
material may be less that the reflectivity of the highly reflective
DBR 110, such that the majority of light from the laser emits from
the second end of the cavity along the optical axis 170, as
depicted in the drawing.
[0047] In the depicted embodiment, the highly reflective DBR
structure 110 comprises an air/semiconductor multilayer stack. The
semiconductor may be a III-nitride semiconductor (e.g., GaN, InGaN,
InAlGaN). According to various embodiments, the layers of
semiconductor 112 are separated by layers of air 114. According to
some embodiments, the layers of air may be continuous and uniform
where the sacrificial layers have been removed, e.g., there are no
remnants of the sacrificial layer protruding into the air layers.
In some embodiments, the regions shown as air layers may comprise
highly porous sacrificial material, e.g., the regions may contain a
sparse density of filaments remaining from the sacrificial layer.
The structure may further include vias 120, which may be formed for
etching purposes, as described further below.
[0048] FIGS. 2A-2G outline methods that may be used to fabricate a
structure like that shown in FIG. 1. For brevity, not all
fabrication steps are shown in the drawings. Steps omitted as not
being necessary for understanding the methods will be readily
deduced by those skilled in the art of microfabrication. According
to some embodiments, a method for fabricating an integrated
photonic device (e.g., a DBR laser) with a highly reflective DBR
structure may comprise preparing or obtaining a substrate 205 with
a multi-layer stack 210 of III-nitride semiconductor material
formed on the substrate. The substrate may be any suitable
substrate, e.g., a semiconductor, a glass, crystalline material, a
ceramic, a polymer. In some embodiments, the substrate 205 may be
formed from sapphire.
[0049] The stack 210 may comprise alternating layers of III-nitride
material, wherein first layers 211 have a different electrical
conductivity than second layers 213. The III-nitride materials may
comprise (Ga,Al,In)N materials where the constituents may be
present in any III-nitride combination. In some embodiments, the
stack 210 may comprise (Ga,Al,In)N/GaN periodic heterostructures.
In some implementations, any one layer may comprise
Al.sub.xIn.sub.yGa.sub.1-x-y N where x and y may range between 0
and 1 in a manner such that 0.ltoreq.1-x-y.ltoreq.1. The layers may
be deposited by one or a combination of deposition techniques,
e.g., chemical vapor deposition (CVD), plasma enhanced CVD (PECVD),
metal organic CVD (MOCVD), hydride vapor phase epitaxy (HVPE),
molecular beam epitaxy (MBE) or atomic layer deposition (ALD),
according to some embodiments. The first and second layers may
alternate in the stack, as depicted, according to some embodiments.
In other embodiments, the first and second layers may be arranged
in other orders within the stack, or there may be only two layers
present. In some embodiments, additional layers may be deposited
that may comprise materials other than III-nitrides.
[0050] The thicknesses of the layers 211, 213 may be selected to
satisfy optical design parameters for a DBR structure, according to
some embodiments. For example, the thicknesses of the first layers
211 may be approximately one-quarter and/or three-quarter of a
selected emission wavelength from the laser. In some embodiments,
all first layers may have approximately the same thickness. In
other embodiments, at least one of the first layers 215 may have a
thickness different than other first layers. The thicknesses of the
layers 211, 213 may be between approximately 50 nm and
approximately 500 nm.
[0051] Some of the layers, e.g., second layers 213, may be
designated as sacrificial layers. These sacrificial layers may be
removed in whole, or in part, during an etching step. The
conductivity, or dopant concentration, of the sacrificial layers
may be selected such that these layers will etch at a significantly
higher rate than the first layers 211. In some embodiments, the
dopant concentration of the sacrificial layers may be selected such
that these layers etch so as to provide a particular
morphology.
[0052] During deposition, a dopant may be added to yield a selected
conductivity of the layers. For example, the second layers 213 may
be doped with an n-type dopant (e.g., Si) at a dopant concentration
between approximately 10.sup.17 to approximately 10.sup.20
cm.sup.-3. The inventors have found that electrochemical etching of
n-type III-nitride material in HF-based etchants can occur at high
etch rates (e.g., greater than 10 microns/minute) for materials
doped in this range. Additionally, the etched surface can exhibit
high smoothness that may be beneficial for photonic devices, or the
etched material may be made porous, depending on the selected
etching conditions. The first layers 211 may be lightly doped as
n-type material, in some embodiments, or may be undoped in other
embodiments. Modulation of the doping in the multi-layer stack can
define the etchant profiles and morphologies.
[0053] According to some embodiments, an additional layer (a ground
layer, not shown) may be deposited proximal to the multi-layer
stack 210. The ground layer may comprise a conductive layer formed
from doped III-nitride material or other conductive material. The
ground layer may provide uniform current spreading for an
electrochemical etching process. For example, a potential bias or
reference potential may be applied to the ground layer during
electrochemical etching, and the ground layer may aid in uniformly
spreading the electric field across the region to be etched.
[0054] With reference to FIG. 2B, active layers 220 for a laser or
LED may be formed adjacent the multi-layer stack 210. In some
embodiments, the active layers 220 may form a multiple quantum well
(MQW) structure or a superlattice (SL). The active layers may
comprise multiple layers of semiconductor materials, e.g., layers
of III-nitride materials. Additional layers of other materials,
e.g., insulators, metals, polymers, may be included with the active
layers in some implementations.
[0055] The active layers 220 may be patterned using lithography and
etching techniques, as depicted in FIGS. 2C-2D. A resist may be
applied uniformly over the active layers 220, and subsequently
patterned, e.g., via photolithography, to provide a resist mask
225. The exposed regions of the active layers 220 may be etched
away using an anisotropic dry etching process, e.g., reactive ion
etching, to form an active MQW region 228 of a device.
[0056] The resist mask 225 may be stripped from the substrate, and
another resist mask formed to define vias 230 in the multi-layer
stack 210, as depicted in FIG. 2E. The vias may be proximal the
active region 228, and may be formed using a dry anisotropic etch
process. There may be multiple vias around the active region, as
depicted in FIG. 1 for example, and the vias may or may not be
circular in cross section. The vias 230 along their sidewalls may
expose surface regions of the first and second layers 211, 213. The
resist may be stripped from the substrate to yield the structure
shown in FIG. 2E.
[0057] After formation of the vias, the substrate and multi-layer
stack 210 may be subjected to electrochemical etching in an
HF-based etchant. The etching may take place in an etchant bath,
wherein a potential bias is applied between the etchant solution
and the multi-layer stack. Arrangements for etching may include
those described in U.S. patent application Ser. No. 13/559,199
filed Jul. 26, 2012, which is incorporated herein by reference in
its entirety. The applied bias may be between approximately 5V and
approximately 60V, according to some embodiments. In some
embodiments, the applied bias may be controllably varied during the
etching steps, for example as described in "Nanopores in GaN by
electrochemical anodization in hydrofluoric acid: formation and
mechanism," by D. Chen et al., J. App. Phys., 112 (2012) p. 064303,
which is incorporated herein by reference in its entirety.
[0058] According to some embodiments, the HF-based etchant may be
substantially non-aqueous. The inventors have found that an aqueous
solution of HF can contribute to a swelling, deformation, or
increased stress of the remaining, unetched layers. In some
implementations, an HF/ethanol etchant solution may be used, in
which any water content is minimized in the etchant to less than
about 10%.
[0059] In some implementations, the HF may be aqueous, e.g.,
.about.50% in water, which is then diluted with ethanol and/or
glycerol. In some cases, the volume ratio of ethanol to HF
(.about.50%) may be between approximately 0.5:1 and approximately
9:1. In some embodiments, the water content of the etchant may be
reduced by adding ethanol and/or glycerol to between 5% and
15%.
[0060] The inventors have also found that adding glycerol to the
HF/ethanol etchant can improve lateral etching rates and smoothness
of the etched surfaces. In some embodiments, the etchant comprises
ethanol/glycerol added to HF acid. According to some embodiments,
the etchant may comprise .about.10% HF, .about.10% water,
.about.32% ethanol, and .about.48% glycerol, where the percentages
refer to volume. In some embodiments, these percentages may be
varied by up to .+-.5% from the values listed. In other
embodiments, these percentages may be varied by .+-.5% or more from
the values listed. The etching may be carried out at room
temperature, in some embodiments. In some implementations, the
etching may be carried out between approximately 10.degree. C. and
approximately 40.degree. C. The bias applied during the
electrochemical etching may be between approximately 10V and
approximately 30V, in some implementations. The dopant
concentration may be between approximately 1.times.10.sup.19
cm.sup.-3 and approximately 5.times.10.sup.19 cm.sup.-3, in some
cases. In some embodiments, the dopant concentration may be between
approximately 3.times.10.sup.18 cm.sup.-3 and approximately
2.times.10.sup.19 cm.sup.-3. With these etchant parameters, lateral
etch rates of between about 2 microns/min and about 10
microns/minute may be achieved. According to some embodiments,
etching rates in this range can be achieved in closed-space
configurations (approximately 200 nm layer separation). The etching
may yield surface smoothness of approximately 5 nm RMS over an
etched area of 25 square microns.
[0061] If porous GaN is desired, other etching parameters may be
used. Parameters for forming porous GaN are discussed below in
connection with Example 1. Porous GaN may be used for the
manufacture of III-nitride LEDs. In some embodiments, a DBR
structure may be formed using highly porous GaN intermediate layers
in the DBR stack. The highly porous layers may provide significant
refractive index contrast and also provide support for uniform
spacing of the unetched DBR layers.
[0062] As a result of electrochemical etching and removal of the
second layers 213, a DBR structure 280 as depicted in FIG. 2F may
be formed. The lateral etching may remove portions of the second
layers to form continuous layers of air between layers of
semiconductor. With the layer thicknesses chosen to be
approximately one-quarter of a target emission wavelength, the DBR
structure can provide high reflectivity (e.g., >70%) over a wide
wavelength band (e.g., .about.150 nm) with six or fewer
semiconductor layers. According to some embodiments, a peak
reflectance of more than 98% may be achieved with as few as four
semiconductor layers.
[0063] FIG. 2G depicts another embodiment of a DBR structure 282
formed adjacent an active MQW region. In this embodiment, the
sacrificial layers are not completely removed beneath the MQW
structure. A pillar 290 may remain to provide uniform spacing of
the first, unetched layers. The pillar may be sub-wavelength in
cross-sectional dimension in some embodiments, and may or may not
be circular in cross section. In some embodiments, the pillar 290
may have a cross-section that is approximately as wide as the MQW
active region. In some embodiments where a pillar 290 is formed,
the via 232 may comprise an annular ring surrounding the MQW
region. The discs of the first layer material formed by etching may
be supported entirely by the pillar structure.
[0064] The inventors have found that stresses in the unetched
layers 211 can lead to warping, buckling, or out-of-plane
deformations when the sacrificial layers 213 are removed. A
structure shown in FIG. 2G may mitigate out-of-plane deformations.
In other embodiments, one or more stress-compensating layers 295
may be deposited proximal the etched regions, as depicted in FIG.
2H. The stress compensating layer 295 or layers may impart a stress
opposite to that of the unetched first layers 211 to the etched
region, and thereby compensate for the stresses of the first layers
211. In some implementations, stress compensating layers may be
included as multiple thin layers in the multi-layer stack 210,
e.g., each adjacent the a respective first layer.
[0065] A DBR laser cavity may be completed by capping the MQW
structure with a dielectric DBR structure (as depicted in FIG. 1)
or a reflective material, e.g., a thin metallic film such as a
layer of silver. The dielectric DBR or reflective material may be
deposited after the active layers 220, and may be patterned before
or at the same time as the patterning of the MQW region. According
to some embodiments, a second mirror for the cavity may be formed
using techniques developed for the manufacture of LEDs.
[0066] Electrochemical etching techniques may also be employed
during the fabrication of LED devices. For example, an
air/semiconductor DBR structure as described above may be formed
adjacent a planar III-nitride LED to reflect emission from a
substrate to a direction away from the substrate, e.g., towards a
viewer. In some implementations, air/semiconductor DBR structures
may be formed adjacent an LED to make resonant cavity and/or
microcavity LED emitters. The DBR structures may enhance emission
from the LEDs. Additionally or alternatively, a portion of the LED
device may be made porous using electrochemical etching.
Porosification of the LED may enhance photon emission from the
devices active region. Selection and control of pore morphology, as
described in the following example, may improve LED emissions.
EXAMPLES
Example 1. Controlled Porosification of GaN
[0067] This example describes the use of hydrofluoric acid (HF) as
a versatile electrolyte for preparing porous GaN with a wide range
of morphology from curved to highly parallel pores. The interplay
among different electro-chemical processes may be elucidated using
cyclic voltammetry. Under suitable conditions, a record-high rate
of porosification (up to 150 .mu.m/min) is observed. The influence
of electrolyte concentration, sample conductivity (doping), and
anodization potential is investigated. The rich variety of pore
morphology can be explained by a depletion-region model where the
available current flow pathways in GaN, determined by the ratio of
inter-pore spacing and the width of depletion region, define the
directions of pore propagation and branching.
[0068] Electrochemical porosification experiments were conducted in
a two-electrode cell at room temperature with n-type GaN as the
anode and a platinum wire as the counter electrode (cathode).
Ga-polar Si doped GaN of 2 .mu.m thickness was grown on c-plane
sapphire by metal-organic chemical vapor deposition (MOCVD) with a
doping range of 10.sup.17-10.sup.19 cm.sup.-3 and defect density
range 10.sup.8-10.sup.9 cm.sup.-2. Underneath the Si-doped layer,
an undoped GaN layer of 500 nm was used as an etch stop. An n-GaN
layer (5.times.10.sup.18 cm.sup.-3) with a thickness of 500 nm was
grown prior to the etch stop and contacted with conductive tape to
ensure uniform distribution of the anodization bias across the
sample (.about.1.times.1 cm.sup.2). The electrolytes were prepared
by adding ethanol to HF (49%) with a volume ratio from 0.5:1 to
9:1. The anodization process was carried out in a potentiostatic
(constant voltage) mode controlled by a Keithley 2400 source meter,
while etching current is recorded under room light without UV
illumination. After anodization, samples were rinsed with deionized
(DI) water and dried in N.sub.2. Scanning electron microscopy
(Hitachi SU-70) was used to study the pore morphology.
Representative cross-section and plan-view images of GaN are shown
in FIGS. 3A-3B. In the cyclic voltammetry experiment, the source
meter is programmed to provide a constant sweep rate of 6 V/s. The
scale bars for these SEM images is 1 .mu.m. For the sample shown in
FIG. 3A, the bias was 8V. For the sample shown in FIG. 3B, the bias
was 18V.
[0069] The morphology of porous textures and patterns may be
determined by the highly inhomogeneous etching process, which
consists of successive steps including carrier transport in the
space-charge (SC) layer, oxidation at the semiconductor surface,
transport of ions in the oxide layer (OL), field-enhanced
dissolution of oxide at the oxide-electrolyte interface, and ionic
transport within the interfacial double layer (DL). The local
electric field within each layer plays a role in regulating or
facilitating the aforementioned process. The richness in porous
morphology is derived from (1) the curvature-driven electrochemical
etching due to the concentration of electric field near surface
undulations/defects, with a statistical nature not dissimilar to
nucleation, and (2) the competing dynamics in the growth and
dissolution of intermediate oxide, resulting in different
distributions of electric field in SC, OL, and DL layers, which can
alter the rate-limiting steps in the EC etching and the final
morphology. To gain insight into the underlying redox process over
a wide range of anodization potential, cyclic voltammetry (or
voltammogram) was performed.
[0070] FIG. 4 shows a typical cyclic current-voltage (CV)
characteristic for n-type GaN in aqueous HF
(N.sub.D=8.times.10.sup.18 cm.sup.-3, [HF]=25%). In the forward
direction, three regions with different CV slopes are
distinguished, reflecting differences in the underlying
rate-limiting steps. Region I corresponds to low bias,
pre-breakdown conditions. Leakage current across the reverse-biased
electrolyte/n-GaN junction is responsible for the etching and the
formation of surface pits (see FIG. 5A). Surface pits formed under
region I do not propagate into GaN layers to become nanopores due
to a lack of a consistent supply of holes from thermalization only.
This region is subsequently denoted as "no etching."
[0071] The stochastic nature of such an etching mechanism is
manifested by a linear increase of pit density with etching time,
as seen in the SEM images of FIGS. 5A-5B. The n-type dopant density
for this sample was 1.2.times.10.sup.19 cm.sup.-3, and the sample
was etched in 25% HF for 10 min (FIG. 5A) and 25 min (FIG. 5B). The
corresponding surface pore densities were found to be
9.5.times.10.sup.9 cm.sup.-2 (FIG. 5A) and 2.0.times.10.sup.10
cm.sup.-2 (FIG. 5B).
[0072] In region II, the electric field is sufficient to induce
local breakdown in reverse biased GaN, providing holes for the
redox reaction and etching. Porous GaN can be formed consistently
in this region, and the process is rate-limited by carrier
transport (SC). The appearance of multiple current peaks in this
region implies the emergence of a multi-step redox reaction
involving intermediate oxidation states. At the onset of pore
formation, (structural) defect-assisted etching prevails such that
the pore density remains unchanged during etching and comparable to
the dislocation density. Increasing the anodization bias beyond the
first oxidation peak (.about.10 V), an additional dissolution
process emerges that causes the etching front to propagate very
rapidly. An etch rate as high as 2.5 .mu.m/s has been obtained
under a bias of 20V (N.sub.D=8.times.10.sup.18 cm.sup.-3,
[HF]=25%). This etch rate is one to two orders of magnitude faster
than previously reported results. When the static anodic potential
is between the first and second oxidation peaks, synchronized
oscillations in pore diameter with a spatial coherence over tens of
microns were observed, as shown in FIG. 6. Such a phenomenon has
been observed in other III-V system and has been explained by a
"current burst" model. According to this model, the self-induced
oscillation is attributed to the formation and dissolution of an
intermediate OL, causing a dynamic modulation of resistance,
current, and consequently the pore diameter.
[0073] In region III, the dissolution of oxide occurs homogeneously
across all GaN/electrolyte interfaces, and electropolishing rather
than porosification is observed. The decrease of current with an
increasing bias is due to the depletion of ions near the interface
and the corresponding increase in the thickness of the diffusive
layer (also known as Guoy layer). The redox process in region III
may then be limited by ion diffusion (DL). The asymmetry in the
up-sweep and down-sweep traces (i.e., the absence of reduction
peaks) suggests that the reverse electron transfer rate is
negligible, unlike the conventional cases with metallic anodes, and
indicates the porosification process of GaN is irreversible.
[0074] A wide variety of pore morphologies can be created in GaN by
electrochemical etching with HF. The effects of doping level and
electrolyte concentration are discussed below. N-GaN with four
different doping levels (3.times.10.sup.18 cm.sup.-3,
5.times.10.sup.18 cm.sup.-3, 8.times.10.sup.18 cm.sup.-3, and
1.2.times.10.sup.19 cm.sup.-3) were anodized in 25% HF with an
applied bias ranging from 5V to 50 V. FIG. 7 summarizes the phase
diagram of the observed morphology, consisting of mainly no
etching, nanoporous etching, and electro-polishing regions. (These
three regions are delineated by solid lines as a visual guide.)
[0075] Within the nanoporous region, the doping level has a
profound effect on the pore morphology, as shown in FIGS. 8A-8D.
These SEM images show cross-sectional and plan-view (inset) images
of samples having different doping levels. The doping levels were:
sample A, N.sub.D=3.times.10.sup.18 cm.sup.-3, sample B
N.sub.D=5.times.10.sup.18 cm.sup.-3, sample C
N.sub.D=8.times.10.sup.18 cm.sup.-3, and sample D
N.sub.D=1.2.times.10.sup.19 cm.sup.-3. All samples were etched in
25% HF at 20 V.
[0076] Under same etching conditions, an increase in doping,
corresponding to an upward move in FIG. 7, results in (i) an
increase in pore density, (ii) a decrease in pore diameter, and
(iii) the pores being from horizontal/tree-branch-like to highly
vertically aligned. The trends of (i) and (ii) are summarized
quantitatively in FIG. 9. The dopant used for the results of FIG. 7
and FIG. 9 was silicon. The samples were etched in 25% HF.
[0077] Keeping the doping level constant, we have investigated the
effects of electrolyte concentration using HF:ethanol mixtures.
Again, three regions are mapped out in FIG. 10 in a manner similar
to the doping-bias study in FIG. 7. Additionally, a dashed line is
used to delineate the observed first oxidation peak extracted from
voltammetry. Two notable trends were observed as the HF
concentration increases: (1) a decrease in the breakdown potential
and the first oxidation potential, and (2) the delayed onset for
electropolishing to occur or the widening of the window for porous
etching. As the HF concentration increases (to no more than 35%),
the ion concentration in Helmholtz and diffusive layers increases,
the effective capacitance increases, and the voltage drop in the
electrolyte decreases, giving rise to an apparent reduction in the
breakdown potential (the boundary between no-etching and porous
regions in FIG. 10) and the first oxidation peak. Under a higher
anodization bias, on the other hand, a higher HF concentration
increases the oxide dissolution rate, making it less likely for
oxide-induced, homogeneous electropolishing to take place,
effectively broadening the window of the porous region.
[0078] The graph shown in FIG. 11 plots the geometric parameters
(pore diameter and wall thickness) at various HF concentrations,
which shows that high HF concentration produces smaller pores with
thicker sidewalls. A high HF concentration can be used to fine tune
the structure of the pores, while a low HF concentration is more
effective in electropolishing GaN surfaces. Electropolishing of GaN
to produce optically smooth surfaces may be between HF
concentrations of about 5% and 15%, in some embodiments. For the
data of FIG. 11, the n-GaN was doped at
N.sub.D.about.8.times.10.sup.18 cm.sup.-3 and etched at 20 V.
[0079] Several models have been developed for the variations in
pore morphology in Si and other III-V semiconductor materials. The
depletion layer model treats porous media as a reverse-biased
Schottky diode and explains the morphology by the curvature-induced
concentration of the electric field. Lehmann and Gosele proposed a
quantum wire model to explain the difficulties in hole transport
for micropores (diameters from below 2 nm to 20 nm). The rates of
carrier transport in Si and mass transport in the electrolyte are
thought to be responsible for the observed fractal morphology of
macropores in the diffusion limited model. Additionally, as has
been briefly described in FIG. 6, the oscillation in pore diameters
has been attributed to current burst.
[0080] Based on the above findings, ion diffusion does not appear
to be a rate limiting step once the HF electrolyte exceeds a
certain concentration. The porous morphology in the majority of
region II depends on the availability of minority carriers (holes)
created by impact ionization. The trajectory of the pore can be
considered as a mapping of the electric field/carrier flow
contours. The possible current flowing pathway is determined by the
inter-pore spacing d.sub.w, and the thickness of space charge
region d.sub.sc. The measured d.sub.w and calculated d.sub.sc are
listed in Table I for the four samples of different doping levels.
Correlating Table I with the observed morphologies (FIGS. 8A-8D), a
consistent model emerges where the pore morphology is determined by
the presence and pinch-off of the conduction paths in-between
pores, quantifiable in terms of a ratio d.sub.w/d.sub.sc.
TABLE-US-00001 TABLE I Comparison of space charge region thickness
and pore separation in samples A-D, etched in 25% HF at 20 V.
Sample.sup.a N.sub.D (.times.10.sup.18 cm.sup.-3) d.sub.w
(nm).sup.b d.sub.sc (nm).sup.c d.sub.w:d.sub.sc A 3 175 81.5 2.15 B
5 95 63.1 1.51 C 8 58 50.1 1.16 D 12 25 40.7 0.61 .sup.aCorresponds
to samples in FIG. 5 and FIG. 6. .sup.bd.sub.w is deduced from the
diameter and wall thickness of pores. .sup.cd.sub.sc = {square root
over (2.epsilon..epsilon..sub.oU.sub.sc/(qN.sub.D))}, where
.epsilon. is the dielectric constant, .epsilon..sub.o is the
permittivity of GaN, U.sub.sc is the drop across the depletion
region (deduced from reference electrode), q is the charge of an
electron, and N.sub.D is the donor density.
[0081] The modeling of conduction paths in the three-dimensional
porous structure is a complicated case. Here, a simplified planar
model is illustrated in FIGS. 12A-12B to give the trend. When
d.sub.w>2d.sub.sc (FIG. 12A), there are sufficient current
pathways between pores; reverse breakdown takes place in between
pores and at the tips, causing horizontal widening, branching of
the pores in addition to vertical downward propagation as observed
in FIG. 8A. Conversely, when d.sub.w<d.sub.sc, as sketched in
FIG. 12B, the space between pores is completely depleted due to
coalescence of the space charge layers surrounding two adjacent
pores. Carriers can only be supplied from underneath the pore tip,
forcing the pores to propagate vertically as observed in FIG. 8D.
Samples B and C represent anodization under an intermediate
condition with a mixed character of vertical and inclined pores
(FIGS. 8B and 8C). It is worth noting that the critical
d.sub.w/d.sub.sc ratio presented in this planar model is subject to
modification when largely curved interface is involved, in which
case this model should serve only as a qualitative guide. Except in
the cases of low concentrations of electrolyte and semiconductor
doping, the observed nanoporous morphology can be interpreted and
rationally controlled by the depletion model d.sub.sc= {square root
over (2.di-elect cons..di-elect cons..sub.oU.sub.sc/(qN.sub.D))}
where more sophisticated porosity profiles can be engineered
through real-time modulation of anodization bias (U.sub.sc) and/or
the employment of doping control (N.sub.D).
[0082] This example demonstrates the use of HF as a versatile and
effective electrolyte in etching or porosifying n-type GaN. Rate
limiting steps for this electrochemical process were investigated
and identified by cyclic voltammetry. A record-high rate of
porosification (>100 .mu.m/min) was observed. A detailed mapping
of the etching and porosity phase-diagrams was conducted that
includes parameters such as doping level, electrolyte
concentration, and anodization bias. The morphology of the
nanoporous region can be understood quantitatively by a
depletion-layer model. These findings can enable the rational
control of porous morphology towards the fabrication of GaN
structures having the desired optical, mechanical, and/or
electrical properties.
Example 2. Fabrication of a MQW/Air-Semiconductor DBR Structure
[0083] This example describes in further detail the fabrication of
a membrane-based GaN/air-gap DBR for blue/green light emitting
devices. The formation of membrane DBRs and microcavities may
employ electrochemical etching described above in which selective
lateral etching of layers is achieved by adjusting the conductivity
of layers rather than chemical composition. This can relieve
greatly the burden in creating epitaxial DBRs. The lateral-etching
process is found to be compatible with InGaN based light
emitters.
[0084] The emission of InGaN multiple quantum wells (MQWs) was
modified successively with the formation of a DBR underneath the
MQW and the capping of the MQW with a silver top mirror.
Micro-reflectance measurements of a fabricated DBR structure shows
over 98% peak reflectance and a wide stopband (over 150 nm) with
only four pairs of GaN/air-gap layers. Micro-photoluminescence
spectra of InGaN multiple quantum wells (MQWs) on DBRs show reduced
linewidth and improved emission efficiency. After capping the MQWs
on DBRs with a silver reflective layer, a significant linewidth
narrowing indicates the modification of spontaneous emission due to
the presence of a planar microcavity. The reduction of PL linewidth
from 20 to 8 nm agrees well with the theoretical prediction of the
distribution of cavity modes, indicating that the GaN/air gap DBRs
can be a viable building block for III-nitride photonics.
[0085] In this example, the DBR structures were grown on sapphire
by metalorganic chemical vapor deposition (MOCVD) using a standard
two-step growth process described in F. Bernardini et al., Phys.
Rev. B, 56 (1997) R10024, which is incorporated herein by reference
in its entirety. To facilitate lateral etching, windows were opened
by inductively coupled plasma reactive-ion etching (ICP-RIE, Oxford
Plasmalab 100) with a Ni mask to form vias in DBR stack and expose
sidewalls.
[0086] The sample was then subjected to an electrochemical etch
that was carried out in a potentiostatic (constant voltage) mode
without UV illumination. The electrolyte was prepared by adding
ethanol/glycerol to equal part of hydrofluoric acid (HF, 49%). It
was found that replacing .about.2/3 of ethanol with glycerol could
reduce the roughness of the etched surfaces. The formation and
quality of the membrane were examined by scanning electron
microscopy (SEM, Hitachi SU-70), atomic force microscopy (AFM,
Veeco MultiMode), and differential interference contrast (DIC)
microscopy (Nikon Optiphot). As the membrane DBRs were formed
laterally with a finite spatial extent, .mu.-reflectance,
.mu.-photoluminescence (.mu.-PL, excitation wavelength=405 nm), and
.mu.-Raman (HORIBA Jobin Yvon, confocal LabRAMVR Raman 300)
measurements were carried out with a spatial resolution of 4, 2,
and 4 .mu.m in diameter, respectively. FIGS. 13A and 13B provide
conceptual sketches of the formation of a GaN membrane DBR
structure using the EC etching. Four pairs of undoped
(u-GaN)/n-type GaN (n-GaN, 1.2.times.10.sup.19 cm.sup.-3) were
grown where the n-GaN region becomes the air gap during subsequent
EC etching. The thickness of the undoped (GaN membrane) layer was
either 1/4.lamda. or 3/4.lamda. (where .lamda..about.500 nm), while
the thickness of n-GaN was kept at 1/4.lamda. for mechanical
stability. The use of a 3/4.lamda. membrane layer has the effect of
reducing the width of the stopband from 320 nm to 140 nm based on
modeling using transfer matrix method. Underneath the DBR layers,
an undoped GaN layer of 500 nm was used as a spacer and an etch
stop. An n-GaN layer (5.times.10.sup.18 cm.sup.-3) with a thickness
of 500 nm was grown prior to the etch stop (shaded in light grey in
FIG. 13A to ensure uniform distribution of the anodization bias
across the sample (.about.1.times.1 cm.sup.2). This lower n-GaN
layer is referred to as the "ground plane" or "ground layer."
[0087] The optical quality of the finished membrane DBRs is
affected by both the pattering dry etching and the EC etch. The
roughness of the ICP-etched sidewall was found to be correlated
with striations formed on the GaN membrane surface. The chlorine
based ICP-RIE process needed to be adjusted to minimize corrugation
of the via sidewall surfaces. The parameters of lateral HF EC
etching have been explored in D. Chen et al., J. App. Phys., 112
(2012) p. 064303. In general, a lower anodization voltage (<20
V) results in smoother surface and a reduced etching rate. We also
note that the quality of EC etching in such a close-spaced
configuration (0.2 .mu.m) depends on the effective transport of
electrolyte to the etching surface. The partial substitution of
ethanol with glycerol was found useful in further improving the
membrane smoothness. FIGS. 13B-13C show cross-sectional SEM images,
and a plan-view DIC picture (FIG. 13D) with a pattern that
correlates with the schematic of FIG. 13A. The sharp transition
between GaN membrane and air is clearly shown in FIGS. 13B-13C
demonstrates that good quality DBR can be maintained over a long
distance. The surface roughness can be maintained under 5 nm (root
mean square, RMS value) over an area of 5.times.5 .mu.m.sup.2 at a
bias of approximately 12 V. The lateral etch rate at this bias was
estimated to be 5 .mu.m/min. The DBR regions were formed around the
windows over a length of 10 to 30 .mu.m. Substantial color contrast
can be seen in FIG. 13D. The outer, lighter circle defines the
range where DBR structure was formed laterally. The inner (and
somewhat non-uniform) circle is a result of unintentional
porosification of the ground-plane at roughly half the rate.
[0088] The result from .mu.-reflectance measurement is shown in
FIG. 14 (darker trace). Absolute reflectance value was obtained
from calibration with a standard silver mirror (Thorlabs
PF03-03-P0123) over 340-800 nm and the accuracy is further checked
with a sapphire wafer. With four pairs of GaN/airgap layers, the
peak reflectance exceeds 98% at .lamda.=503 nm, and the width of
the stopband is around 150 nm, which is three to five times wider
than reported epitaxial DBR mirrors. The measured reflectance
spectrum is also compared to a theoretical model (FIG. 14, lighter
trace) that is calculated using the transfer matrix method, taking
into account the effect of oblique incident/collection angle (half
angular range .about.10.degree.). Absorption of GaN is included
through a wavelength-dependent, complex index of refraction. A
scattering loss from surface roughness of 4 nm RMS is assumed. The
discrepancy between the measured reflectance spectrum and the
simulation is likely due to the fluctuation of the of air-gap
thickness in the membrane-based DBRs.
[0089] Toward the fabrication of InGaN microcavity devices, an
active region was prepared consisting of 30 pairs of
In.sub.0.06Ga.sub.0.94N (2 nm)/GaN (2 nm) superlattices (SLs) and
10 periods of In.sub.0.17Ga.sub.0.83N (3 nm)/GaN (8 nm) MQWs, on
top of four periods of undoped/n-type GaN designed to form
3/4.lamda. DBR for 460 nm emission. The sample was similarly
patterned by ICP etching and EC etched to form the membranes of the
DBR structure. The MQWs remained intact after the EC etching. The
.mu.-PL spectra (spot diameter 2 .mu.m) of as-grown (black, broad
trace) and with/without membrane DBRs are shown in FIG. 15. A
significant linewidth narrowing effect of the MQWs with DBR can be
seen. The spectra of as grown sample (black) and after EC etching
(middle, gray trace) are shown in FIG. 15. Emission from the
as-grown MQW exhibits typical fringes due to the interference
effect of reflections at GaN/sapphire and GaN/air interfaces. The
full-width-half-maximum (FWHM) of the spontaneous emission is
around 20 nm and in good agreement with the reported data. After
the EC etch, the .mu.-PL spectrum (middle, gray trace) shows a
reduced linewdth (17 nm), a five to ten times increase in
intensity, and the disappearance of interference fringes as the
effective cavity width was reduced to 0.38 .mu.m estimated by
summing up the thicknesses of InGaN MQWs, InGaN SLs and the
penetration depth into the DBR. We note that the linewidth
reduction of less than 20% is due to the fact that the upper
"mirror" has rather limited reflectivity (18%). To improve the
quality of the micro-cavity, 50 nm of silver (with a measured
reflectance at 460 nm=70%) was deposited on a piece of EC etched
sample using e-beam evaporation. Normalized .mu.-PL spectrum in
FIG. 15 (narrow, gray trace) shows a further reduction in linewidth
to less than 8 nm.
[0090] To ascertain the optical quality of the bottom GaN/air
membrane DBRs, we modeled the linewidth of the spontaneous emission
by fitting the experimental linewidths with two different top
mirrors (R1=18% and 70%), using the reflectivity of the bottom
mirror (R2) as the fitting parameter. For each combination of R1
and R2, the predicted linewidth is obtained semi-empirically by
convoluting the theoretical density of cavity modes with the
measured spectrum of as-grown InGaN MQW sample assuming
thick-cavity effect is negligible. The calculated linewidths versus
R1 (top mirror) for three separate values of R2 (10%, 50%, and 95%)
are shown in FIG. 16, together with the experimental data.
Qualitatively, a low-reflectivity top mirror (left part of FIG. 16)
will diminish the cavity effect, and the quality of the bottom
mirror (R2) is best revealed with a highly reflective top mirror
(R1, lower right part of FIG. 16). Preparation of a dielectric top
mirror is currently in progress. The good agreement between the
experimental and simulated FWHM with highly reflective bottom DBR
indicates that the high reflectance membrane DBR can be used to
control the modes of spontaneous emission.
[0091] To ascertain the effect of underlying DBR formation on the
optical emission, .mu.-PL measurement was performed on a sample
prior to the silver coating, from a region unaffected by the EC
etching (spot 1 in FIG. 17A), to an area where the membrane DBRs
are formed underneath (spot 2), then to the center of the etched
pattern that can be considered freely suspended (spot 3). The
measurement continued along a separate branch (spots 4 and 5) for a
consistency check. FIG. 17B plots the peak energy of .mu.-PL
determined at each point. A systematic blue shift amounting to
35-40 meV was observed. Since the distances among all spots
measured were within 100 .mu.m, the inhomogeneity in composition or
thickness due to growth is tentatively ruled out. The systematic
emission peak shift can be a result of the reduced piezoelectric
effect as InGaN/GaN MQWs undergo relaxation during the formation of
membrane DBR. To determine the change of strain, .mu.-Raman was
carried out on the exact spots and summarized as triangles in FIG.
17B. A shift of the E.sub.2.sup.H from 569.6 cm.sup.-1 (as grown)
to 567.7 cm.sup.-1 (after EC etch) corresponds to the relief of a
compressive strain of 0.104% as a result of EC lateral etching, in
good agreement with the reported data of residual strain for GaN on
sapphire. The corresponding change in piezoelectric field can be
calculated with known parameters and the total electric field can
be calculated. Solving the Schrodinger equation for electrons and
heavy holes, the effective band gap of strain relaxed MQWs was
found to be about 34 meV larger than the strained ones. The good
agreement between experimental and theoretical results suggests
that the piezoelectric effect is the dominant factor responsible
for the observed emission shift. It is noteworthy that the emission
peak of the silver coated sample exhibits a further blue-shift in
FIG. 15. Possible causes contributing to the blue-shift include
coupling between the MQW emission and surface plasmons (SPs) and
wavelength variation across sample (<2 nm).
[0092] This example demonstrates an embodiment of a method that may
be used to produce highly reflective short-wavelength membrane DBRs
(e.g., air/semiconductor DBRs) that may be used to fabricate
innovative high performance GaN-based optical devices.
[0093] The technology described herein may be embodied as a method,
of which at least one example has been provided. The acts performed
as part of the method may be ordered in any suitable way.
Accordingly, embodiments may be constructed in which acts are
performed in an order different than illustrated, which may include
performing some acts simultaneously, even though shown as
sequential acts in illustrative embodiments. Additionally, a method
may include more acts than those illustrated, in some embodiments,
and fewer acts than those illustrated in other embodiments.
[0094] Having thus described at least one illustrative embodiment
of the invention, various alterations, modifications, and
improvements will readily occur to those skilled in the art. Such
alterations, modifications, and improvements are intended to be
within the spirit and scope of the invention. Accordingly, the
foregoing description is by way of example only and is not intended
as limiting. The invention is limited only as defined in the
following claims and the equivalents thereto.
* * * * *