Note: Descriptions are shown in the official language in which they were submitted.
-- 1 --
I~ETHOD OE` PREE'EI~ENTTALLY ETCHI~IG OPTICALLY FLAT
MI R~OR FACETS I~ InGaAsP/InP HETEROSTKUCTURES
Technical Field
This invention relates to a method of chemical
etching an optically flat facet on a preferred
crystallographic plane of a multilayer InGaAsP/InP device.
Back~round of the Invention
In general, an optoelectronic device such as a
laser is fabricated along a preferred crystallographic
direction. Mirror facets for such a device are formed on a
plane perpendicular to the preferred direction and
sidewalls of the device are formed on planes parallel to
that direction. Also~ it is desirable for at least the
mirror facets to exhibit the characteristic of optical
flatness.
Optically flat mirror facets are created by
manual cleaving or by etching. Though manual cleaving does
produce high quality mirror facets, this technique has a
low yield.
Etching methods encompass both wet and dry
chemical etching. Wet chemical etching techniques
gen~r~lly cause mask undercutting thereby not producing the
desired flatnessr Examples of wet chemical etching
techniques are given in the following references: K. Iga
et al., "GaInAsP/InP DH Lasers with a Chemically Etched
Facet," IEEE Journal of Quantum Electronics, QE-16, p. 1044
(1980), (a solution o HCl: CH3COOH: d202 = (1:2:1));
P. D. Wright et al., "InGaAsP Double Heterostructure Lasers
tl = 1-3~m~ with Etched Reflectors," Applied Physics
Letters, Vol. 36, p. 518 (1980), (a solution of Br: CH30H);
and S. Arai et al., "New 1.6 ~m Wavelength GaInAsP/InP
Buried Heterostructure Lasers," Electronics Letters,
Vol. 16, p. 349 (1980), (a sequential process of ~r: CH30H
followed by 4HCl-H20).
~'
-- 2 --
Dry chemical etching techniques include reactive
ion etchingl reactive-ion beam etching and plasma etching.
For separate descrip-tions of each of the above, see R. E.
Howard et al~ eactive-Ion Etching of III-V Compoundsr"
Topical Meetin~ on Integrated and Guided Wave Optics Digest
(IEEE: New York 1980) WA-2; M.A. Bosch et al., "Reactive-
Ion Beam Etching of InP with C12," Applied Physics
Letters, Vol~ 38, p. 264 (1980); and Ro H. Burton et al.,
"Plasma Separation of InGaAsP/InP Light-Emittiny Diodes,"
Ap~lied Physics Letters, Vol. 37, p. 411 (1980~.
Reactive ion etching avoids some of the problems of
the wet chemical etching methods and is useful in making
grooves in a heterostructure system. This type of etching
is effectively a single step process which results in
facets which are approximately planar but "overcut". That
is, the facets which form the groove slope toward each
other from the top of the groove to the bottom. Although
these facets are reproducible, they lack the optically
flat mirror quality necessary for certain applications.
Similarly, the other dry etching techniques create facets
satisfactory for use as waveguide sidewalls and die separ-
ations but lack the optically flat mirror quality necessary
for optoelectronic and integrated optics devices.
Summary_of the Invention
According to the invention there is provided a
method for etching a multilayer se~iconductor heterostruc-
ture body having alternating layers of InGaAsP and InP,
characterized by etching a given surface of the semicon-
ductor body to expose a first crystallographic surface
through the alternating layers of the semiconductor body,
selectively etching the exposed surface to expose InGaAsP
portions of a preferred crystallographic plane substan-
tially perpendicular to the given surface~ and etching the
exposed InP layers with HCl to expose the InP portions
of the preferred crystallographic plane to comple~e an
~1 t;~9
- 2a -
optically flat mirror facet of the semiconductor
heterostructure body.
Other aspects of this invention are claimed in our
copending Canadian patent application Serial No. 405,781
filed on June 23, 1982 of which the present application
is a division, and in other divisions thereof.
Brief Description of the Drawings
FIG. 1 shows a portion of a multilayer semiconductor
heterostructure body having a stripe-mask thereon;
6~i
FIGS. 2 and 3 show structural changes of the
semiconductor body in FIG. 1 after successive steps in a
first exemplary etching method embodying the invention;
FIG~. 4, 5, 6 and 7 show structural changes of
the semiconductor ~ody in FIG. 1 after each of four
sequential steps in a second exemplary etching method
embodying the invention;
FIGS~ 8, 9, 10 and 11 illustrate structural
changes of the semiconductor body in FIG. 1 after each of
four sequential steps in a third exemplary etching method
embodyiny the invention;
FXG. 12 shows a portion of a multilayer
semiconductor heterostructure body having a stripe-mask
thereon in a direc~ion different from that in FIG. l; and
FIGSo 13, 14 and 15 show structural changes of
the semiconductor body of FIG~ 12 after each of three
successive steps in a fourth exemplary etching method
embodyin~ the invention.
Detailed Description
_
Optoelectronic and integrated optics devices are
grown in certain desirable crystallographic directions.
For III-V semiconductor heterostructure lasers and the like
composed of InGaAsP/InP on a (100) substrate, the desirable
direction for the laser axis is <011>. Hence, it is
necessary to create optically flat, mirror quality facets
on the (oll) crystallograpnic plane, because this plane is
perpendicular (vertical) to the <011> direction and the
(100) plane of the heterostructure heterostructure
substrate.
EIG. 1 shows a multilayer InGaAsP/InP
heterostructure body having mask 1 on the (100)
crystallographic plane. Also included in FIG. 1, as well
as all remaining figures, is a set of basis lattice vectors
indicating the three-dimensional orientation of the
semiconductor body.
Tne semiconductor heterostructure of FIG. 1
comprises mask layer 1, p+-type cap layer 2, p-type upper
6~
clad~ing cladding layer 3, n-type or undoped active
layer 4, n-ty~e lower cladding layer 5, and n-type
substrate 6. The conductivity type for each layer can be
reversed so that-each p-layer becomes an n-layer and each
n-layer becomes a p-layer. For the example described
herein, cap layer 2 is approximately 3000-5000 angstroms
thick, cladding layers 3 and 5 are a?proximately 1.5-2~m
thick, active layer 4 is approximately 1000-3000 angstroms
thickr and substrate 6 is approximately 75-100 ~m thick.
Semiconductor materials for the heterostructure
are chosen from the group of lII-V compounds. In
uarticular, a binary III-V compound, InP, is employed for
cladding layers 3 and 5 and for substrate 6. A quaternary
III-V compound, Inl_yGayAsxPl_x, is utili~ed for cap
layer 2 and active layer 4, wherein the alloy composition
ratios x and y are chosen to produce a particular
wavelength or energy bandgap and lattice constant for the
heterostruct~re. For a description of techniques for
choosing x and y, see R. Moon et al, "Bandgap and Lattice
Constant of GaInAsP as a Function of Alloy Composition", J.
Electron. Materials, Vol. 3, p. 635 (1974). In the
description which follows, exemplary composition ratios,
x = 0.52 and y = 0.22, are selected to produce a wavelength
of 1.3 ~m 10.95eV). It is important to note that the
2S inventive method is equally applicable when these ratios
are varied to produce wavelengths in the range of 1.1 ~m to
1.7 ~m. For concentration ratios producing wavelengths
above 1.5 ~m, it is necessary to grow a quaternary
antimeltback layer between layers 3 and 4 during liquid
phase e~itaxial growth of the heterostructure. The
presence of such an antimeltback layer requires the
inventive method to be modified only slightly, in terms of
etching exposure times, to provide acceptable results.
A mask layer is deposited on the (100) plane of
the semiconductor body by any suitable deposition process
such as chemical vapor deposition or the like. An
exemplary mask layer is chemically composed of silicon-
nitride. Mask 1 is formed by ~hotolithography and dryetching of the silicon nitride to l~ave edges which are
substantially smooth. Striped regions in mask 1 leave
surface areas such as surface 10 completely exposed, as
opposed to being covered by mask 1. The stripe in mask 1
is aligned with the <011> direction of the semiconductor
heterostructure ~ody. Although this type of stripe mask
produces a groove in the semiconductor body, other masks
such as the one shown in FIGo 12 can be utilized to produce
a single wall, i.e., for effectively slicing away an
unmasked portion of the semiconductor body.
FIG. 2 illustrates the structural changes in the
semiconductor body of FIG. 1 after processing that body
with a wet chemical etchant. A wet chemical etchant
suitable for creating the structural change shown in FIG. 2
in a single step is HCl Hl~03 = (1 ), where 1< <5 and,
preferably, is equal to 3~ The etching process is
anisotropic and is substantially self-stop~ing when the
(01l) plane is reached in each of the various layers of the
semiconductor body. This plane is perpendicular to the
(100) plane. The proportion of HCl and HN03 is critical to
ensuring that no step discontinuities appear at the
interface of the heterojunction and surface 20 exposed by
the HCl:HN03 etchant. By experimentation, it has been
found that, for less HN03 than an a.mount dictated by an
opti~um proportion, ~uaternary layer 4 is etched more
slowly than binary layer 3. This gives surface 20 the
appearance of being stepped outward toward the etched
groove such that layers 4 and 5 protrude into the groove
be~ond the ex~osed edge of layer 3. If the amount of HN03
exceeds the optimurn proportion, the opposite result appears
because quaternary layer 4 etches rriore quickly than
layer 3. So, surface 20 appears to be stepped inward from
the etched groove and layer 3 protrudes into the groove
beyond tlle exposed edges of layers 4 and 5. Optimization
of the value of ~ermits the etchant to react with both
the binary layers (layers 3, 5, and 6) and quaternary
layers (layers 2 and 4) at approximately the same rate.
Hence, surface 20, which is exposed by this optimized
etchant, is substantially planar through at least layers 2,
3, and 4.
In practice, optimization is performed by using a
small sample of the semiconductor body to be etched. The
sa~ple is then subjected to the etchant while the value of
~ is adjusted until the optimum value is found~ Certain
factors influence the selec~ion of a value ~or a such as
the alloy composition ratios x and y, the thickness of each
semiconductor layer in the heterostructure, the age and
strength or diluteness of the etchan~ component chemicals,
and tlle temperature of the etchant.
Assuming that the value of ~ is optimized for the
wet chemical etchant, HCl: a~O3, the exposed
crystallographic surface 20 is substantially perpendicular
to the (100) plane.
In one exa~ple from experimental practice, the
semiconductor heterostructure body defined above is
immersed and agitated in a chemical bath of ~C1:3HNO3 for
approximately 30 seconds at 22 degrees Centigrade. After
this immersion, the etching process is halted by rinsing
the ~C1:3HN03 from the se~iconductor body with deionized
water and surface 20 is exposed. However, surface 20 has a
rouyhened appearance exhibiting irregular characteristics
such as high spots and striations generally along the ~100>
direction, and a polishing step is necessary to remove
these irregularities from exposed crystallographic
surface 20.
3U FIG. 3 illustrates the structural changes which
aupear after t`ne semiconductor body of FIG. 2 is polished
with a chemical etchant. In this instance, polishing
entails contacting exposed surface 20 (FIG. 2) with HCl for
a time sufficient to expose a preferred crystallographic
plane of the semiconductor body. HCl is both material
selective and orientationally preferential (anisotropic) as
an etchant. As before, the semiconductor body of FIG. 2 is
immersed in a bath of HCl and agitated. The polishing
proces~ is halted by rinsing the etched semiconductor body
in deionized water. In one example, concentrated HCl is
utilized in the bath at 22 degrees Centigrade with an
immersion or etching time of approxiamtely 3 seconds. For
more dilute concentrations of HCl, the etching time must be
adjusted and increased accordingly.
For the example shown in FIG. 3, the
crystallographic plane preferentially exposed by the HCl
etchant is (011) plane, denoted as surface 21, which is
perpendicular to the (loO) plane. Surface 21 is an
op~ically flat mirror facet. Althouyh HCl preferentially
exposes the (011) crystallographic plane of only the InP
layers, i.e., layers 3 and 5, and does not etch the
quaternary layers, layers 2 and ~, the amount of etching
(polishin~) is so small as not to impair the substantially
coplanar relationship of the groove walls. If desired, the
value of a can be selected to cause a slight undercutting
of the layers 2 and 4 during the first etching step,
whereupon the polishing of the layers 3 and 5 moves the
side walls thereof into coplanar relationship with the side
walls o layers 2 and 4. Generally, however, such fine
adjustment is not necessary.
At the lower pOrtion of a trough or groove in
layers 5 and 6 created by the etching process,
crystallographic plane (111) denoted as surface 22) is also
ex~osed as a polished facet. Surface 22 is generically
reerred to as a (111)~ crystallographic plane which
includes planes (111, (111), (111), and (111). The suffix
30 'B' means that the particular plane includes only
phosphorous atoms which are chemically reactive and,
therefore, capable of being removed by a chemical etchantO
Similarly, a (lll~A crystallographic plane, which will be
discussed balow, includes planes (111), (111), ~111), and
(111). The suffix 'A' means that the particular plane
includes only indium atoms which appear to be substantially
inert and resist removal by chemical etching.
FIGS. 4, 5, 6, and 7 sho~ structural changes
which appear after the semiconductor heterostruc~ure body
of E`IG. 1 is sub~ected to the etchants in a sequential
etching process. The method shown in FIGS. 4 through 7 is
called sequential etching ~ecause each layer of the
multilayer structure directly under exposed surface 10
(FIG. 1) is etched away in sequence. That isr the portion
of cap layer 2 directly under surface 10 is etched a-~ay
with a wet or dry chemical etchant to expose surface 12 on
cladding layer 3. Preferably~ each etchant used is
material selective, i.e., an etchant which attacks either
layers 2 and 4 or layers 3 and 5, but not both. An
advantage of this is that it provides greater control of
the process. Eor example, variations in the per~ormance of
one etchin~ step has little or no affect uyon the
performance of the next step.
Several wet chemical etchants have been shown to
be effective for selectively etching quaternary layers such
as layers 2 and 4O Examples of several selective etchants
include: a solution of H2SO4:H2O2:H2O = (10:1:1) as
descriDed in R. J. Nelson et al., "~igh-Output Power in
InGaAsP/InP (~ = 1.3 ~m~ Strip-Buried Heterostructure
Lasers," Applied Physics Letters, Vol. 36, p. 35~ (1980);
or AB etchant, wherein the A solution is (4O.OID1.
H2O + 0.3g.Ag NO3 t 40.0ml. HF) and the B solution is
(40.0g~ CrO3 + 40.0ml. H2O) and A:B=(l:l) as described in
G. ~. ~lsen et al., "Universal Stain/Etchant for Interfaces
in III-V Compounds," Journal of Applied Physics, Vol. 45,
No. 11, p. 5112 (1974); or a solution of KO~I:K3Fe(CN)6:H2O.
Etching time for the quaternary layers varies according to
thickness oE the quaternary layer r temperature, and alloy
composition ratios, x and y, for the quaternary layers.
For a 3000 arlgstroms ~hickness of layer 2 (~ = 1.3~m) and a
tempel-ature of 22 degrees Centigrade, the following
approximate &tching times produce the results shown in
FIGS. 4 and 6: AB etchant for approximately 15 seconds, and
KOH:K3E`e(CN~6: H2O etch for approximately 8 seconds. This
~tchiny step is halted by rinsing the etched semiconductor
body in deionized water.
FIGo S shows the structural change of the
semiconductor body in FIG. 4 after etching in an InP
selective etch. ~or this etching step, HCl is a suitable
etchant to cut away the portion of layer 3 under surface 12
(E`IG. 4~ thereby exposing surface 13 on quaternary
layer 4. This etchant stops reacting automatically at
surface 13. For an InP layer thickness of approximately
1.5 ~m, an exemplary etching time period for concentrated
HCl is approximately 45 seconds to produce the results
shown both in FIG. 5 and FIG. 7. After this etching step
as shown in FIG 5, it is irnportant to note that the
etched, exposed walls of layer 3 exhibit crys~allographic
smoothness.
FIG. 6 illustrates the structural change apparent
in the semiconductor heterostructure body, after the body
shown in FIGo 5 is contacted with a wet or dry chemical
etchant to selectively etch quaternary layer 4 directly
under surface 13 for a time period sufficient to expose
surface 14 on layer 5. Also, crystallographic surface 20
is exposed at a preselected ~e.g., perpendicular) slope to
the surface containing mask 1 and surface 10 (EIG~ 1). The
etching procedure and the etchants employed at this step
have been described above in relation to FIG~ 4~
FIG. 7 shows the completion of all structural
changes caused by the sequential etching process. Again,
ar, InP selective etchant, HCl, is contacted with exposed
surfaces of the semiconductor body create an optically flat
mirror facet at surface 21. In particular, surface 14 and
crystallographic surface 20 are brought into contact, via
immersion and agitation as described above~ with a solution
of HCl for a time period sufficient to expose a preferred
crystallographic plane as the optically flat mirror facet.
Yor this example, the etching time period in a bath of
concentrated HCl required to produce exyosure of the (011)
cr~stallogra~hic plane at surface 21 is approximately 20
-- 10 --
!
seconds. As stated above, surface 22 is also exposed
through the InP material comprising layer 5 and
substrate 6. surface 22 is a (lll)B crystallographic
plane~
Although, as aforedescribed, either wet or dry
etching can be used for the layers 2 and 4, dry etching i5
generally better because it avoids any undercutting
problems associated with wet etching.
FIGS. 8, 9, 10, and 11 show anotller process for
etching the semiconductor heterostructure of FIG. 1
In FIG. 8, the semiconductor body is shown to
have a groove directly under surface 10 (FIG. 1), i.e., the
unmasked stripe region between adjacent sections of mask 1.
~his groove is created by either the wet or dry chemical
etchants described in relation to FIG. 2. FIG. 8 shows the
result using a dry etching process, the walls of the
grooves thus slightly converging towards one another as is
characteristic of generally known dry etching procedures.
Also, no undercutting of the mask 1 occurs.
FIG. 9 illustrates the structural changes to
layers 3 and 5 and to substrate 6 after the semiconductor
body of FIG. 8 is immersed in an InP selective polishing
etchant such as HCl. The etched surfaces of layers 3 and 5
and substrate 6 are depicted as being crystallographically
smooth. Immersion and agitation in this step are required
for only a short time period, for example, 3 seconds. This
immersion is followed by a rinse in deionized water to halt
tlle etchin~ process.
The results shown in FIG. 9 are exaggerated for
clarity of presentation. Layers 4 and 5 protrude only
slightly into the groove because of the inclination of the
sidewalls of the groove. If it is necessary to remove this
slight protrusion, the remaining steys shown in FIGS. 10
and 11 are available to create a perfectly flat surface.
FIG. 10 shows structural changes to quaternary
layers 2 and 4 after yrocessing the semiconductor body of
FIG. 9 with a quaternary material selective etch such as
the wet or dry cllemical etchants described above in
relation to FIG. 4. For the wet chemical etci~ants
described above, the etching times to complete this step at
room temperatur~ are shorter, by a factor of approximately
one-quarter to one-third, than the etching times given in
relation to the step s~lown in FIG. 4. It is critical that
the etching be controlled to align the exposed surfaces of
layers 2 and 4 on substantially the same plane.
A final polishing step for the semiconductor body
of GIG. 10 is shown in FIG. 11. After this step,
crystallographic plane (01I) is preferentially exposed by
HCl at surface 21. surface 21 is an optically flat mirror
facet because of the alignment of layers 3, 4, and 5. For
concentrated HCl, this polishing etch step is necessary for
only a short time period such as 3 seconds.
It is also possible to omit the first HCl etching
step (the result ~eing shown in FIG. 9) and proceed
directly to the layers 2 and 4 etching step (FIG. 10). The
final polishing step (FIG. 11) corrects any roughness left
by omission of t~le FIG. 9 step. Also, the step illustrated
in FIG. 10, of sliyhtly etching back the layer 4 prior to
the final pclishing step, can be done in the aforedescribed
process illustrated by FIGS. 1-3. That is, such layer 4
etching step can be performed to remove any ledge of layer
4 protruding into the groove after the first etching step,
as well as for slightly recessing the layer 4 edge to
insure its coplanar relationship with layers 3 and 5 after
the final polishing step.
The semiconductor body shown in FIG. 12 is at an
orientation disulaced 9o degrees from that sho~n in FIG. 1.
Mirror facets are created on crystallographic plane (011)
in this orientation and are useful in producing ring lasers
and sidewalls parallel to the laser axis for
heterostructure lasers. The (100) surface of layer 2 has
mask 1 partially disposed thereon with stripes in the <011>
direction. Layer 7 is quaternary layer similar in
com~osition and thickness to layers ~ and 4.
- 12-
After the semiconductor body of FIG. 12 has
reacted with a chemical etchant such as a Br:CH30~l solution
(1~, for 30 seconds) or HCl:HN03 = ~ ) (as described
earlier), for 30 secondsr crystallographic surface 30
appears as shown in FIG. 13. surface 30 is an oblique
surface cutting through each layer of the semiconductor
heterostructure near a (lll)A crystallographic plane which
is inert as mentioned above in relation to FIG. 3.
FIG. 14 shows the structural changes which result
in quaternary layers 4 and 7 after the semiconductor
heterostructure of FIG. 13 is contacted by a selective
quaternary etchant such as one described above in relation
to FIG. 4. It is important that the thickness and
composition of layers 4 and 7 be chosen appropriately so
that the selective quaternary etchant causes surfaces 31 to
be substantially coplanar. For the exemplary se~iconductor
body and layer thicknesses described above, etching time
periods approximately seven or eight times longer than
those defined for the quaternary layer selective wet
chemical etchants described in relation to FIG. 4 above.
The results of a final polish etching step on the
semiconductor heterostructure body of FIG~ 14 are shown in
FIG. 15. The polish etchant is HCl which is material
selective (InP) and orientationally preferential
(crystallographic plane (011)). Because of the hollow
cavities left by the removal of quaternary layers 4 and 7,
the HCl is able to etch layers 3 and 5 from underneath
through the (111)B plane and exposing, thereby,
crystallo~raphic plane (011) at surface 32 on layers 3 and
5. By proper experimentation with layer and composition
thickness and etching times, it is possible to have
surfaces 31 and 32 of layers 3 and 5, and 2, 4, and 7 ,
respectively, coplanar in crystallographic plane (011) as
an optically flat mirror facet. For concentrated HCl, the
etching time period is determined to be approximately 20
seconds.
