U.S. patent application number 09/245503 was filed with the patent office on 2001-11-22 for inxalygazn optical emitters fabricated via substrate removal.
Invention is credited to COMAN, CARRIE CARTER, KISH, FRED A. JR., KRAMES, MICHAEL R., MARTIN, PAUL S..
Application Number | 20010042866 09/245503 |
Document ID | / |
Family ID | 22926939 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010042866 |
Kind Code |
A1 |
COMAN, CARRIE CARTER ; et
al. |
November 22, 2001 |
INXALYGAZN OPTICAL EMITTERS FABRICATED VIA SUBSTRATE REMOVAL
Abstract
Devices and techniques for fabricating InAlGaN light-emitting
devices are described that result from the removal of
light-emitting layers from the sapphire growth substrate. In
several embodiments, techniques for fabricating a vertical InAlGaN
light-emitting diode structure that result in improved performance
and or cost-effectiveness are described. Furthermore, metal
bonding, substrate liftoff, and a novel RIE device separation
technique are employed to efficiently produce vertical GaN LEDs on
a substrate chosen for its thermal conductivity and ease of
fabrication.
Inventors: |
COMAN, CARRIE CARTER; (SAN
JOSE, CA) ; KISH, FRED A. JR.; (SAN JOSE, CA)
; KRAMES, MICHAEL R.; (MT. VIEW, CA) ; MARTIN,
PAUL S.; (PLEASANTON, CA) |
Correspondence
Address: |
SKJERVEN MORRILL MACPHERSON LLP
25 METRO DRIVE
SUITE 700
SAN JOSE
CA
95110
US
|
Family ID: |
22926939 |
Appl. No.: |
09/245503 |
Filed: |
February 5, 1999 |
Current U.S.
Class: |
257/103 ; 257/99;
438/22 |
Current CPC
Class: |
H01L 33/40 20130101;
H01S 5/0421 20130101; H01S 5/0216 20130101; H01S 5/18341 20130101;
H01S 5/32341 20130101; H01L 33/32 20130101 |
Class at
Publication: |
257/103 ; 257/99;
438/22 |
International
Class: |
H01L 033/00; H01L
021/00 |
Claims
We claim:
1. An InAlGaN light-emitting device comprising: a host substrate;
an AlInGaN light-emitting structure, including device layers of a
first and second polarity, proximate to a top side of the host
substrate; a first device contact to a top side of the AlInGaN
light-emitting structure; a wafer bonding layer, interposing the
host substrate and the AlInGaN structure; and a second device
contact, positioned within the wafer bonding layer, electrically
connected to a bottom side of the AlInGaN light-emitting
structure.
2. A device, as defined in claim 1, wherein the second device
contact contains at least 50% silver.
3. A device, as defined in claim 1, wherein the second device
contact contains at least 50% aluminum.
4. A device, as defined in claim 1, wherein the host substrate is
selected from a group that includes metals and semiconductors.
5. A device, as defined in claim 4, wherein the host substrate is
selected from a group that includes silicon, germanium, glass,
copper, and gallium arsenide.
6. A device, as defined in claim 4, wherein the host substrate is a
semiconductor, further comprising a first substrate ohmic contact
positioned on the top side of the host substrate.
7. A device, as defined in claim 6, further comprising a second
substrate ohmic contact that is electrically connected to a bottom
side of the host substrate.
8. A device, as defined in claim 1, further comprising a pair of
polished mirrors positioned on two opposing side faces of the
InAlGaN light-emitting structure forming an edge emitting
laser.
9. A device, as defined in claim 1, further comprising: a first
dielectric Bragg reflector mirror, positioned on the top side of
the InAlGaN light-emitting structure; and a second dielectric Bragg
reflector mirror, positioned within the wafer bonding layer,
adjacent to the bottom side of the InAlGaN light emitting
structure.
10. A method for fabricating a vertical conducting AlInGaN
light-emitting device comprising the steps of: growing an AlInGaN
light-emitting structure that has device layers of a first and a
second polarity on a growth substrate; depositing a first ohmic
metal layer onto an exposed side of the InAlGaN light-emitting
structure; depositing a second ohmic metal layer onto a host
substrate; and wafer bonding the first and second ohmic metal
layers to form a first electrical contact within the wafer bond
interface.
11. A method, as defined in claim 10, wherein the first ohmic metal
layer is selected from a group that includes silver, nickel,
aluminum, gold, and cobalt.
12. A method, as defined in claim 10, further comprising the steps
of: removing the growth substrate; and fabricating a second
electrical contact to a newly exposed side of InAlGaN
light-emitting structure.
13. A method, as defined in claim 12, further comprising the step
of etching mesas through the AlInGaN light-emitting structure
corresponding to a desired device size.
14. A method, as defined in claim 13, further comprising the step
of singulating the host substrate.
15. A method, as defined in claim 10, wherein the step of growing
an InAlGaN light-emitting structure comprises the step of growing
an AlInGaN film having a thickness greater than 50 microns on the
growth substrate.
16. A method, as defined in claim 10, wherein the host substrate is
selected from a group that includes metals and semiconductors.
Description
FIELD OF INVENTION
[0001] The present invention relates generally to the field of
semiconductor optical emission devices, more particularly to a
method for fabricating highly efficient and cost effective InAlGaN
devices.
BACKGROUND
[0002] Sapphire has proven to be the preferred substrate for
growing high efficiency InAlGaN light emitting devices because of
its stability in the high temperature ammonia atmosphere of the
epitaxial growth process. However, sapphire is an electrical
insulator with poor thermal conductivity resulting in unusual and
inefficient device designs. A typical LED structure grown on
sapphire has two top side electrical contacts and a semitransparent
metal layer to spread current over the p-contact. This contrasts
with the standard vertical structure for current flow in LEDs grown
on conducting substrates such as GaAs or GaP in which an electrical
contact is on the top side of the semiconductor devcie and one is
on the bottom. The two top side contacts on the sapphire based LED
reduce the usable light emitting area of the device.
[0003] Furthermore, the low conductivity of the p-type InAlGaN
layer results in the need for a semitransparent metal layer to
spread current over the p-type semiconducting layer. The index of
refraction of the sapphire (n.about.1.7) is also lower than that of
the InAlGaN layers (n.about.2.2-2.6) grown upon it. Consequently,
this mismatch in index of refraction (with the Sapphire being
lower) results in waveguiding of the light between the absorbing
semitransparent p-side current-spreading metallization and the
sapphire. This results in absorption of 10-70% of the light
generated in commercial InAlGaN device by the semitransparent metal
layer.
[0004] Wafer bonding can be divided into two basic categories:
direct wafer bonding, and metallic wafer bonding. In direct wafer
bonding, the two wafers are fused together via mass transport at
the bonding interface. Direct wafer bonding can be performed
between any combination of semiconductor, oxide, and dielectric
materials. It is usually done at high temperature (>400 C) and
under uniaxial pressure. One suitable direct wafer bonding
technique is described by Kish, et al, in U.S. Pat. No. 5,502,316.
In metallic wafer bonding, a metallic layer is deposited between
the two bonding substrates to cause them to adhere. This metallic
layer may serve as an ohmic contact to either the active device,
the substrate or both. One example of metallic bonding is flip-chip
bonding, a technique used in the micro- and optoelectronics
industry to attach a device upside down onto a substrate. Since
flip-chip bonding is used to improve the heat sinking of a device,
removal of the substrate depends upon the device structure and
conventionally the only requirements of the metallic bonding layer
are that it be electrically conductive and mechanically robust.
[0005] A vertical cavity optoelectronic structure is defined to
consist of an active region that is formed by light emitting layer
interposing confining layers that may be doped, un-doped, or
contain a p-n junction. The structure also contains at least one
reflective mirror that forms a Fabry-Perot cavity in the direction
normal to the light emitting layers. Fabricating a vertical cavity
optoelectronic structure in the
GaN/In.sub.xAl.sub.yGa.sub.zN/Al.sub.xGa.sub.1-xN (where x+y+z=0.5)
material systems poses challenges that set it apart from other
III-V material systems. It is difficult to grow
In.sub.xAl.sub.yGa.sub.zN structures with high optical quality.
Current spreading is a major concern for In.sub.xAl.sub.yGa.sub.zN
devices. Lateral current spreading in the p-type material is 30
times less than that in the n-type material. Furthermore, the low
thermal conductivity of the substrates adds complexity to the
device design, since the devices should be mounted p-side down for
optimal heat sinking.
[0006] One vertical cavity optoelectronic structure, e.g. a
vertical cavity surface emitting laser (VCSEL), requires high
quality mirrors, e.g. 99.5% reflectivity. One method to achieve
high quality mirrors is through semiconductor growth techniques. To
reach the high reflectivity required of distributed Bragg
reflectors (DBRs) suitable for VCSELs (>99%), there are serious
material issues for the growth of semiconductor
In.sub.xAl.sub.yGa.sub.zN DBRs, including cracking and dopant
incorporation. These mirrors require many periods/layers of
alternating indium aluminum gallium nitride compositions
(In.sub.xAl.sub.yGa.sub.zN/In.sub.x,Al.sub.y,Ga.sub.z,N).
Dielectric DBRs (D-DBR), in contrast to semiconductor DBRs, are
relatively straightforward to make with reflectivities in excess of
99% in the spectral range spanned by the In.sub.xAl.sub.yGa.sub.zN
system. These mirrors are typically deposited by evaporation or
sputter techniques, but MBE (molecular beam epitaxal) and MOCVD
(metal-organic chemical vapor deposition) can also be employed.
However, only one side of the active region can be accessed for
D-DBR deposition unless the host substrate is removed. Producing an
In.sub.xAl.sub.yGa.sub.zN vertical cavity optoelectronic structure
would be significantly easier if it was possible to bond and/or
deposit D-DBRs on both sides of a In.sub.xAl.sub.yGa.sub.z- N
active region.
[0007] In "Low threshold, wafer fused long wavelength vertical
cavity lasers", Applied Physics Letters, Vol. 64, No. 12, 1994,
pp1463-1465, Dudley, et al. taught direct wafer bonding of
AlAs/GaAs semiconductor DBRs to one side of a vertical cavity
structure while in "Room-Temperature Continuous-Wave Operation of
1.430 .mu.m Vertical-Cavity Lasers", IEEE Photnoics Technology
Letters, Vol. 7, Nol. 11, November 1995, Babic, et al. taught
direct wafer bonded semiconductor DBRs to both sides of an InGaAsP
VCSEL to use the large refractive index variations between
AlAs/GaAs. As will be described, wafer bonding D-DBRs to
In.sub.xAl.sub.yGa.sub.zN is significantly more complicated than
semiconductor to semiconductor wafer bonding, and was not known
previously in the art.
[0008] In "Dielectrically-Bonded Long Wavelength Vertical Cavity
Laser on GaAs Substrates Using Strain-Compensated Multiple Quantum
Wells:, IEEE Photonics Technology Letters, Vol. 5, No. 12, December
1994, Chua et al. disclosed AlAs/GaAs semiconductor DBRs attached
to an InGaAsP laser by means of a spin-on glass layer. Spin-on
glass is not a suitable material for bonding in a VCSEL between the
active layers and the DBR because it is difficult to control the
precise thickness of spin on glass, and hence the critical layer
control needed for a VCSEL cavity is lost. Furthermore, the
properties of the spin-on glass may be inhomogeneous, causing
scattering and other losses in the cavity.
[0009] Optical mirror growth of Al.sub.xGa.sub.1-xN/GaN pairs of
semiconductor DBR mirrors with reflectivities adequate for VCSELs,
e.g. >99%, is difficult. Theoretical calculations of
reflectivity suggest that to achieve the required high
reflectivity, a high index contrast is required that can only be
provided by increasing the Al composition of the low-index
Al.sub.xGa.sub.1-xN layer and/or by including more layer periods
(material properties taken from Ambacher et al., MRS Internet
Journal of Nitride Semicoductor Research, 2(22)1997). Either of
these approaches introduces serious challenges. If current will be
conducted through the DBR layers, it is important that the DBRs be
conductive. To be sufficiently conductive, the Al.sub.xGa.sub.1-xN
layer must be adequately doped. Dopant incorporation is
insufficient unless the Al composition is reduced to below 50% for
Si (n-type) doping and to below 17% for Mg (p-type) doping.
However, the number of layer periods needed to achieve sufficient
reflectivity using lower Al composition layers requires a large
total thickness of Al.sub.xGa.sub.1-xN material, increasing the
risk of epitaxial layer cracking and reducing compositional
control. Indeed, the Al.sub..30Ga.sub..70N/GaN stack of FIG. 1 is
already 2.5 .mu.m thick and is far from sufficiently reflective for
a VCSEL. Thus, a high reflectivity DBR based on this layer pair
requires a total thickness significantly greater than 2.5 .mu.m and
would be difficult to grow reliably given the mismatch between AlN
and GaN growth temperatures. Even though the cracking is not as
great of an issue if the layers are undoped, compositional control
and the AlN/GaN growth temperatures still pose great challenges to
growing high reflectivity DBRs. Hence, even in applications where
the DBRs do not have to conduct current, mirror stacks with
reflectivities >99% in the In.sub.xAl.sub.yGa.sub.zN material
system have not been demonstrated. For this reason,
dielectric-based DBR mirrors are preferred.
[0010] Semiconductor devices are manufactured many thousands to
tens of thousands at a time on wafers. The wafers must be diced
into individual die prior to packaging. If sapphire is used as the
growth substrate one must thin and dice the sapphire substrate. The
hardness and hexagonal crystal structure of sapphire make the
dicing operation difficult and expensive.
SUMMARY OF THE INVENTION
[0011] In this invention, devices and techniques for fabricating
InAlGaN light-emitting devices are described that result from the
removal of light-emitting layers from the sapphire growth
substrate. In several embodiments, techniques for fabricating a
vertical InAlGaN light-emitting diode structure that result in
improved performance and or cost-effectiveness are described.
Furthermore, metal bonding, substrate liftoff, and a novel RIE
device separation technique are employed to efficiently produce
vertical GaN LEDs or vertical cavity surface emitting lasers
(VCSELs) on a substrate chosen for its thermal conductivity and
ease of fabrication.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates a preferred embodiment of an InAlGaN
light-emitting device with a bonding layer comprised of ohmic
contact layers to the InAlGaN heterostructure and adhesion layers
to the host substrate.
[0013] FIG. 2 illustrates a preferred embodiment of an InAlGaN
light-emitting device with a bonding layer comprised of ohmic
contact layers to the InAlGaN heterostructure and also ohmic
contact layers to an electrically conducting host substrate.
[0014] FIG. 3 illustrates a preferred embodiment of an InAlGaN
light-emitting device with opposing distributed Bragg reflector
(DBR) mirror stacks on either side of the light emitting layers to
form vertical cavity device. The bonding layer is comprised of
ohmic contact layers to the InAlGaN heterostructure and also ohmic
contact layers to an electrically conducting host substrate.
[0015] FIGS. 4A-D illustrate a preferred method for dicing InAlGaN
light-emitting devices.
[0016] In FIG. 4A, InAlGaN layers grown on a sapphire substrate are
coated with ohmic contact and bonding layers.
[0017] In FIG. 4B, a host substrate is bonded to the InAlGaN layers
prior to removal of the sapphire substrate.
[0018] In FIG. 4C, the InAlGaN devices are defined by mesa etching
through the InAlGaN device.
[0019] In FIG. 4D, Devices are finally singulated by dicing the
host substrate.
DETAILED DESCRIPTION OF THE DRAWINGS
[0020] This invention is concerned with building vertically
conducting InAlGaN light emitting devices defined as devices in
which the ohmic contacts to the InAlGaN device layers are on
opposite sides, top & bottom, of the InAlGaN device layers.
[0021] One preferred structure according to the present invention
is shown in FIG. 1. Initially, an InAlGaN light emitting device 20
is grown on a sacrificial growth substrate 30 such as sapphire. The
structure is grown with the p-type layer 20a exposed. A reflective
ohmic contact 18 is deposited on top of the p-type InAlGaN layers
20a. The InAlGaN structure is then bonded to a host substrate 12 by
means of bonding layers 16 interposing the InAlGaN light emitting
layers 20 and the host substrate 12. The bonding layer 16 materials
are chosen to provide a strong mechanical bond and to be
electrically conductive. In general, the bonding layer includes a
plurality of layers, the first bonding layer 16a that are deposited
on the InAlGaN device layers and the second bonding layers 16b that
are deposited on the host substrate. The bonding layers 16 are
deposited by any number of means known in the prior art, such as
electron-beam evaporation, sputtering, and electroplating. After
bonding, the sacrificial sapphire growth substrate 30 is removed
via one of many substrate removal techniques as known in the prior
art such as laser melting, mechanical polishing, and chemical
etching of sacrificial layers. Then the InAlGaN layers are
patterned, etched, and contacted to provide for an electrical
injection light emitting device. The bonding layer serves as a low
resistivity current spreading layer, an ohmic contact to the
p-InAlGaN layers, and an adhesion layer to the host substrate.
[0022] Another preferred embodiment is shown in FIG. 2. As in FIG.
1, InAlGaN light-emitting device layers are grown atop a
sacrificial substrate 30 and a reflective ohmic contact 18 is
deposited on top of the exposed p-type layer 20a. Now, the InAlGaN
structure 20+18 is bonded to a host substrate 12 that is
electrically conductive via bonding layers 16. This substrate may
be a semiconductor, dielectric, or metal. In the case of a
semiconductor substrate, the bonding layer must be adjacent or
comprised of ohmic contact layers to the substrate 24a, and a
second ohmic contact is applied to the side of the substrate
opposing the bonded interface 24b. After attaching the host
substrate, the sacrificial growth substrate is removed and an
n-type ohmic contact 22 is provided to the n-InAlGaN layers. As a
result, a vertically conductive InAlGaN light-emitting device is
achieved. This device exhibits excellent current spreading due to
the low resistitivity of the semiconductor or metal host substrate
resulting in low forward voltage and high electrical to optical
conversion efficiency. In addition, because there is only a single
ohmic contact on the top of the device and none of the active
region of the device is removed during the fabrication of the
second ohmic contact to the device, more than 75% of the available
active region is preserved for unblocked light emission compared to
less than 40% in commercially available devices.
[0023] Another preferred embodiment is shown in FIG. 3. In this
case, a DBR mirror stack 26a is deposited to the p-InAlGaN layer
20a in addition to the p-side ohmic contacts 18. The mirror stack
can consist of one or more of the following materials: dielectric,
semiconductor and metal. The structure is bonded to a host
substrate 12 via bonding layers 16 which provides adhesion to the
host substrate 12 and electrical contact to the p-side ohmic 18
contact metals. The bonding layer 16 material and thickness should
be chosen to avoid compromising the DBR mirror stack reflectivity
during the attachment of the host substrate. After removal of the
sacrificial growth substrate 30, a second DBR mirror stack 26b is
deposited on the InAlGaN vertical cavity optoelectronic structure
on the side opposing the first mirror stack 26a. The optional
second mirror stack 26b is patterned and etched to provide areas
for n-type ohmic contacts 22. For a vertical cavity surface
emitting laser, the mirrors must have very high reflectivity
>99%. For an resonant cavity LED, the reflectivity requirement
of the mirror(s) is relaxed (>60%). The first and second
substrate ohmic contacts 24a, 24b provide for a vertically
conductive device.
[0024] A preferred method for fabricating InAlGaN light-emitting
devices is shown in FIG. 4. FIG. 4a shows InAlGaN light emitting
layers 20a and 20b grown on a growth substrate 30 with a reflective
ohmic silver contact 18 deposited on top of the p-type InAlGaN
layer. Silver is preferred for the p-type ohmic contact because of
its high reflectivity to the wavelengths of light typically emitted
from an InAlGaN light-emitting device and for its low contact
resistance to p-type InAlGaN. Alternatively, for devices in which
the InAlGaN layers are grown with the n-type layer furthest from
the sapphire growth substrate, aluminum is an excellent choice for
an ohmic metal since it also has high reflectivity in the visible
wavelength region of light typically emitted by InAlGaN devices and
also makes an excellent ohmic contact to n-type InAlGaN. Above the
device structure is shown a low resistivity host substrate 12
provided with first 24a and second 24b ohmic contacts to facilitate
vertical conduction. A bonding layer 16a may be deposited on top of
the first substrate ohmic contact. A second bonding layer 16 is
optionally deposited on top of the p-side ohmic contact 18 to
facilitate a mechanically strong metallic wafer bond in a later
step. In FIG. 4b, the host substrate is shown wafer bonded to the
InAlGaN layers via the bonding layers. In FIG. 4c, the growth
substrate 30 has been removed and ohmic contact 22 to the n-InAlGaN
layers is provided. Then, mesas 32 are etched through the InAlGaN
layers to define individual device active areas. In FIG. 4d, the
host substrate has been diced to singulate individual InAlGaN light
emitting devices. Silicon is preferred for the host substrate
because it is easy to thin and saw into very small chips, and can
have low electrical resistivity and high thermal conductivity
compared to other common substrates. This method allows simple
dicing of the InAlGaN devices and avoids the problems associating
with dicing sapphire. It is also possible to etch mesas prior to
attaching the host substrate, rather than after removal of the
growth substrate.
* * * * *