Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PHASE-LOCKED SEMI CONDUCTOR LASER ARRAY AND
-
A MET~IOD OF MAKING SAME
The invention relates to a monolithic array of
semiconductor lasers whose indlvidual optical fields
interact with one another to produce a single, coherent
output light beam.
BACKGROUND OF T~IE INVENTION
A semiconductor injection laser typically
comprises a body of semiconductor material having a thin
active region between cladding regions of opposite conduc-
tivity type. To increase the output power from such a
laser, a guide layer having a refractive index which is
intermediate between that of the active and cladding
layers is interposed between one of the cladding regions
and the active region. Light generated in the active
layer propagates in both the active and guide layers
thereby forming a larger beam at the emitting facet of the
body. A thin active layer restricts oscillation in the
transverse direc-tion, the direction perpendicular to the
2~ plane of the layers, to the fundamental optical mode. In
the lateral direction, the direction in the plane of the
layers and perpendicular to a line between the laser
facets, a similar restriction does not exist and oscil-
lation typically occurs simultaneously in several dif-
ferent optical modes.
It has been found useful to introduce lateralvariations into the laser structure which produce an
optical waveguide which restricts the oscillation to the
fundamental optical mode in the lateral direction. A
channelled substrate laser formed by liquid phase epitaxy
over a single channel in a substrate has an optical
waveguide formed by lateral variations in the layer
thicknesses and the close proximity of the absorbing
substrate at the sides of the emitting region over the
channels. The lateral flow of electrical current is not,
however, automatically restricted to the emitting region
over the channel but rather tends to flow to the substrate
at the sides of the channel. To inhibit this lateral
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current flow, back-biased P-N junctions are typically formed
in the substrate or overlying layers at the sides of the
channels. Botez in U.S. patent No. 4,347,486 has dis-
closed a laser having a pair of channels in the surface of a
substrate with a mesa therebetween. The layers overlying
this channelled surface have laterally varying thicknesses
because of the tendency for faster liquid-phase epitaxy -
growth over concave as opposed to flat or convex surfaces.
This structure restricts the current flow to the region over
the mesa and, because of the laterally varying layer
thicknesses, produces an optical waveguide which restrlcts
the oscillation to the fundamental lateral mode over the
mesa up to an output power in excess of 40 milliwatts.
To increase the output power in the coherent light
beam beyond the capability of an individual laser,
monolithic arrays of spaced-apart laser devices have been
fabricated where the modes of oscillation of the individual
lasers are coupled to one another to form a single phase-
locked coupled oscillator. Such lasers include a striped-
oxide defined array having planar layers over a planar
~; substrate which operates only in a pulsed mode and an array
of mesa waveguide lasers where the emitting regions are over
mesas on the substrate surface. This array appears to
operate in a phase-locked mode in pulsed operation but is
only partially phase-locked in continuous wave operation.
Botez in U.S. Patent No. 4,385,389 has disclosed a phase-
locked array comprising a plurality of spaced-apart lasing
elements of the type disclosed in U.S. Patent No. ~,347,485,
which can be operated cw in a fundamental lateral mode. In
this array coupling between the modes of oscillation of the
different elements of the array can occur over comparatively
long distances because the individual devices have high
lateral radiation leakage. However, the large inter-element
spacing of the Botez array, required by the use of pairs of
channels and the curvature of the layers, is undesirable
since it increases the number of lobes in the far-filed
-; pattern.
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Thus it would be desirable to have a phase-locked laser
array having the minlmum spacing between the emitting
elements and which operates in a single narrow beam peaked
at o.
SUM~ARY OF THE INVENTION
A phase-locked laser array includes a substrate
having a plurality of substantially parallel channels in a
surface thereof. A first cladding layer, a cavlty region,
comprising a guide layer and an active layer, and a second
cladding layer se~uentially overlie the surface of the
substrate and the channels. A broad area electrical
contact overlies the second claddlng layer over the
channels.
The individual oscillators over the separate
channels are coupled to one another by the overlap of
their evanescent fields primarily in the guide layer. The
waveguides formed over the separate channels suppress the
oscillation of higher order lateral modes over a wide
range of output powers. The broad area contact provides a
sufficiently uniform current distribution across the
channels without the requirement for current confinement
to the regions over the channels and without significantly
increasing the threshold current.
The invention also includes a method Gf fabri-
cating a laser array having planar active and guide layersover a channelled substrate which includes the steps of
forming on the surface of the substrate a mesa having a
plurality of corrugations in the surface thereof, sequen-
tially depositing by liquid~phase expitaxy over the
channels the first cladding layer, the active region, and
the second cladding layer; and forming a broad electrical
contact. The meltback of the convex portions of the
corrugations during the initial stages of the deposition
forms channels with lands therebetween and delays the
growth of the layers over the channels thereby allowing
the formation of the planar layers over the channelled
surface.
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BRIEF DESCRIPTION OF THE DRAWING
FIGURE 1 is a prospective view of a flrst
emoodiment of the phase-locked array of the invention.
FIGURES 2 and 3 are cross-sectional views oE
second and third embodiments of the phase-]ocked array of
the invention.
FIGUR~S 4-6 are cross-sectional views of the
substrate at different steps of the formatlon of the mesa
- with the channels in a surface thereof.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In the FIGU~ES the same elements in the dif-
ferent embodiments of the invention have the same identi-
fication.
In FIGURE 1 the laser array 10 comprises a body
12 of single crystalline semiconductor material having
spaced, parallel end facets 14 which are reflecting to
light at the laser wavelength with at least one of the end
facets 14 being partially transparent so that light may be
emitted therefrom. The body 12 also includes spaced,
substantially parallel side surfaces 16 which extend
between and are p~rpendicular to the end facets 14.
The body 12 includes a substrate 18 having
spaced, parallel first and second major surfaces 20 and
22, respectively, which extend between and are perpen~
dicular to both the end facets 14 and the side surfaces
16. In the first major surface 20 is a mesa 23 having a
surface 24. A plurality of spaced, substantially paral-
lel, vee-shaped channels 26 extends a distance into the
mesa 23 from the surface 24 between the facets 14. A
first cladding layer 28 overlies the surfaces 20 and 24 of
the substrate and the mesa, respectively, and fills the
channels 26. A cavity region 30, comprising a guide layer
32, overlying the first cladding layer 28 and an active
layer 34 overlying the guide layer 32, overlies the first
~ladding layer 28. A second cladding layer 36 overlies
the cavity region 30 and a capping layer 38 overlies the
second cladding layer 36. An electrically insulating
layer 40, having an opening 42 extending therethrough over
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the channels 26, overlies the capping layer 38. A broad
area electrical contact 44 overlies the cappi.ng layer 38
in the region of the opening 42 and the electrically
insulating layer 40. A substrate electrical contact 46
overlies the second major surface 22 of the substrate 18.
In FIGURE 2 the laser array 100 differs from the
array 10 of FIGURE 1 in that the channels 102 extend a
distance into a substantially flat major surface 104 of
the substrate 18. The channels 102 differ from the
channels 26 of the array lO in that the channel bottoms
are flat.
In FIGURE 3 the laser array 200 differs from the
array 10 in that vee-shaped channels 202 extend a distance
- into a substantially flat major surface 204 of the sub-
strate 18. The array 200 differs from the arrays 10 and
100 in that the cladding layers 28 and 36, the guide layer
: 32, and the active layer 34 have laterally varying thick-
nesses with the guide layer 32 and the active layer 34
being thickest over the channels 202.
The laser array of the i.nvention may be formed
of materials such as GaAs and AlGaAs which have the
requisite refractive index differences. Alternatively,
other Group III or ~ elements such as InP, Ga and As may
be used. The alloys used for the particular layers of the
array should be such that the refractive index of the
active layer 34 is greater than the refractive index of
the guide layer 32 which in turn is greater than the
refractive index of the cladding layers 28 and 36.
The substrate 18 and the first cladding layer 28
are of one conductivity type and the second cladding layer
36 and the capping layer 38 are of the opposite conduc-
tivity type. In the cavity region 30 the positions of the
guide layer 32 and the active layer 34 are inter-
changeable. The guide layer 32 is preferably positioned
between the first cladding layer 28 and the active layer
34 and, in this case, has the same conductivity as the
first cladding layer 28. In some applications the guide
layer 32 may be positioned between the active layer 34 and
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the second cladding layer 36 in which case the guide layer
32 has the same conductivity type as the second cladding
layer 36.
The substrate 18 is preferably composed of
N-type GaAs having a first major surface 20 which is
parallel to the (100) crystallographic plane. The sub-
strate may be misoriented from this orientation but
preferably a {100} plane is used. The channels are pref-
erably uniformly spaced and are typically between about
1.5 and 2.5 micrometers (~m) deep, having a width at the
surface 20 between about 3.5 and 4.5 ~m with a typical
center-to-center spacing between the channels of between
about 4 and 6 ~m. Larger center-to-center spacings are
also useful in which case the other dimensions change
accordingly. The channels are typically vee-shaped with
planar surface lands therebetween. Alternatively the
channels may have another shape such as the flat bottom
channels illustrated in FIGURE 2.
The first cladding layer 28 is typically com-
posed of N-type AlrGal rAs where r is between about 0.20
and 0.45 and preferably between about 0.25 and 0.35. This
; layer is comparatively thin over the lands between the
channels, being between about 0.1 and 0.4 ~m and typically
about 0.25 ~m thick and preferably fills the channels,
producing a planar layer surface of the first cladding
layer 28 upon which the succeeding layers are deposited.
Alternatively the deposition of the first cladding layer
28 is controlled so that the channels are not filled
thereby producing a curved surface of the first cladding
layer 28.
The guide layer 32 is typically composed of
N-type AlxGal xAs where x is between about 0.15 and 0.30
and preferably between about 0.18 and 0.25. The layer is
typically planar and is between about 0.3 and 0.6 ~m thick
and preferably about 0.4 ~m thick. If the layer is non-
planar, as illustrated in FIGU~E 3, it is typically
between about 0.3 and 0.6 ~m thick over the channels and
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between about 0.1 and 0.4 ~m thick over the lands between
the channels.
The active layer 34 is typically composed of
AlyGal yAs where y is between about 0.0 and 0.15, and
preferably between about 0.03 and 0.12 and is preferably
undoped. If the active layer 34 is planar, it is typi-
cally between about 0.05 and 0.12 ~m thick. If it is
formed on a curved surface, it is typically between about
0.05 and 0.12 ~m thick over the channels and thinner but
non-zero over the planar lands between the channels.
The second cladding layer 36 is typically
composed of P type AlzGal zAs where z is between about
0.25 and 0.45, and preferably between about 0.28 and 0.35.
This layer is typically between about 0.18 and 1.5 ~m
thick.
The capping layer 38 is typically composed of
P-type GaAs and is used to facilitate ohmic electrical
contact between the underlying semiconductor material and
the overlying metal contact. This layer is typically
between about 0.5 and 1.0 ~m thick.
The electrically insulating layer 40 is pre-
erably composed of silicon dioxide which is deposited on
the capping layer 38 by pyrolytic decomposition of a
silicon-containing gas, such as silane, in oxygen or water
vapor. An opening 40 is formed through the electrically
insulating layer down to the capping layer 38 over the
channels 26 using standard photolithographic masking
techniques and etching processes. The broad area elec-
trical contact 44 is then deposited over the capping layer
38 where it is exposed to the opening 42. The broad area
electrical contact 44 is preferably composed of sequen-
tially evaporated titanium, platinum, and gold. A sub-
strate electrical contact 46 is deposited on the major
surface 22 of the substrate 18 by sequential evaporation
and sintering of tin and silver followed by a plated
nickel layer and a layer of gold.
The emitting end facet 14 of the array is
typically coated with a layer of A12O3 or a similar
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material having a thickness of about one-half wave at the
lasing wavelength as disc'osed by Ladany et al in U.S.
Patent No. 4,178,564. The opposed end facet 14 is coated
; with a reflector consisting of an electrical insulator such
as SiO2 coated with a layer of gold as disclosed by Caplan
et al in U.S. Patent No. 3,701,047~ Alternatively the
mirror may be a multi-layer reflector such as that disclosed
by Ettenberg in U~S. Patent No. 4,092,659.
The laser array of the invention may be fabricated
using well-known photolithographic masking techniques and
etching processes to form the channels followed by standard
liquid-phase epitaxy techniques such as those disclosed by
Lockwood et al in U.S. Patent No. 3,753,801 and by Botez in
U.S. Patent No. 4,347,486 to deposit the layers over the
substrate surface containing the channels. The etching
processes to form the channels may include selective
chemical etching of a surface having a particular
crystallographic orientation or ion etching. These
techniques are well known in the art.
Alternatively the layers may be formed by first
forming a series of adjacent vee-shaped channels thereby
forming a corrugated surface as shown in FIGURES 4 and 5.
In FIGURE 4 a GaAs substrate 300 has *ormed on a major
surface 302 thereof, which is preferably the (100)
crystallographic plane, a plurality of stripes 304 composed
of an etch-resistant material such as SiO2. The stripes
are formed using standard photolithographic masking
techniques and etching processes and are preferably oriented
along a (011) crystallographic direction on the (100)
oriented surface. A preferential etch is then applied to
the exposed surface of the substrate to form the vee-shaped
channels as shown in FIGURE 5. Channels 402 having a vee
shape are formed by underetching the stripes 304 to the
point where only a small portion of the original surface
remains to support the stripes. The surface 302 outside
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the region of the stripes is also removed forming a new
surface 404, leaviny a mesa with a plurality of corruga-
tions in the surface thereof.
The substra-te having the corrugated surface is
then inserted into a liquid-phase epitaxy apparatus such
as that disclosed by Lockwood et al and brought into
contact with the solution from which the first layer is to
be deposited on the channels 402 and the surface 404.
The sequence of events which occur at the onset
of the liquid-phase epitaxy deposition process depends
critically on the characteristics of the solution and the
shape of the substrate surface contacted by the solution.
In the simplest case, a saturated solution of the elements
- to be deposited and an element which is a solvent for the
substrate material is brought into contact with a planar
surface. At this point neither deposition nor meltback of
the substrate occurs since the substrate and solution are
in equilibrium. The combination of the substrate and
solution is then cooled causing the solution to become
super-saturated and deposition occurs.
However, if the surface is not planar but has a
locally varying radius of curvature, then the degree of
saturation of the contacting solution also locally varies.
If the solution is just saturated for a planar surface, it
will be super-saturated over concave portions of the
surface, as viewed from the direction of the solution, and
; under-saturated ovex convex portions of the surface. Over
the curved portion of the surface two effects can then
occur. First, deposition occurs on the concave portions
of the surface over which the solution is super-saturated
and, second, melting of the substrate occurs on the convex
portions of the surface over which the solution is under-
saturated.
Applyiny these principles to the corrugated
surface of the substrate 300, as shown in FIGURE 6, the
convex portions of the corrugations, that is, the tips of
the projections which form the channel, undergo meltback
forming the planar lands 502 between the concave portions
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of the corrugations, now ~orming the channels. This
meltback locally delays the growth over the newly formed
lands 502. The growth proceeds in the channels 402 so that
after an interval of time the surface of the deposited layer
over the channels 402 and the lands 502 becomes planar. The
growth then proceeds uniformly over the entire planar
surface.
We have found that an AlGaAs cladding layer can be
deposited on the corrugated surface to provide planar layers
over channels by carefully controlling the temperature of
the solution, the degree of super-cooling used, and the
cooling rate. In particular, we have found that an initial
growth temperature, typically the temperature at which the
solution is equilibrated, between 700C and 800C, and
preferably about 760C should be used. The range of
temperatures through which the solution and substrate are
cooled to a lower temperature is between about 2 and 10C
and preferably between 4 and 5UC. The cooling rate is
between about 0.5 and 5C and preferably about 1C per
minute. In this temperature range the rate of growth is
significantly slower than for the range of growth
temperatures between 850aC and 950C which are typically
used. In addition, the meltback of the convex surfaces can
be controlled and fill-in of the channels to form a planar
;~ 25 surface occurs more readily. At higher temperatures the
thermal decomposition rate of the corrugated surface due to
arsenic loss is much higher.
The steps of the novel method for fabricating a
phased array of closely spaced lasers are to form a plura-
; 30 lity of adjacent corrugations on the surface of a substrate
with the axis of the corrugations extending between the end
facets. Typically, a solution containing the elements to be
deposited is equilibrated at a first temperature in contact
with a source wafer as disclosed by Lockwood et al in U.S.
35 Patent No. 3,7~1,825. Preferably, the solution and
substrate are then separately cooled through a range of
temperatures to crease a super-saturated growth condition
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for concave and planar portions of the surface and a
slightly under-saturated grow-th condition for convex
portions of the surface. The solution and substrate are
then contacted by sliding a wafer into contact with the
solution thereby causing a partial melt-back of the convex
portions, or tips, of the corrugations to form the lands
between the concave portions of the corrugations, the
channels~ Deposition of the first cladding layer begins
and continues until the planar sur~ace of the deposited
layer is formed over the convex and concave portions of
the original corrugations. The remaining semiconductor
layers of the laser array are then deposited on this
planar surface using standard liquid-phase epitaxy
techniques.
In the operation of the laser array under
forward bias, electrical current flows into the semi-
conductor material through the broad-area electrical
contact which extends laterally over all the active
channels. Lasing action occurs over each channel in the
active region and propagation of the lasing light beam
occurs in both the active and guide layers over each
- channel. Surprisingly, we have found that the emission
from the individual oscillators is in the fundamental
lateral mode without the use of any lateral conductivity
variations to confine the electrical current to the
portions of the active layer over the channels, as is
typically required for single oscillators of this type.
It appears that the combination of uniform current flow
from the broad-area contact coupled with the close prox-
imity of the absorbing substrate to the active and guidelayers over the lands between the channels is sufficient
to allow only the fundamental lateral mode to oscillate.
The close proximity of the individual oscillators to one
another in this structure permits this to happen without
incurring an excessive penalty in increased threshold
current.
Coupliny between the oscillators over adjacent
channels occurs through the overlap of their evanescent
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optical fields. This coupling can occur with a 0 phase
shift between the oscillators which, for coupled oscil-
lators each oscillating in the fundamental lateral mode,
can produce a single output beam normal to the facet.
Alternatively, the coupling can occur with a 1~0 phase
shift between the oscillators which, for oscillators
oscillating in the fundamental mode, can produce a pair of
symmetrical output lobes in the output beam which are
spaced apart by an angle of about 5-10 depending upon the
lateral separation of the oscillators.
The invention is illustrated by the following
Example but it is not intended to be limited to the
details described therein.
EXAMPLE
Nine element phase-locked laser arrays were
fabricated using the method of the invention. A mask
having sets of stripes which included 2 ~m openings
between 3 ~m stripes of sio2 were formed on a (100)
surface of an N-type GaAs substrate wafer. The long
dimensions of the stripes were oriented along the [011]
crystallographic direction of the substrate. The sub-
strate was then etched to form the sets of channels in an
etchant solution comprising 1 H2SO4:8 H2O2:8 H2O. Under-
etching of the sio2 stripes produced approximately
triangular-shaped channels 2.2 ~m deep with a 5 ~m
center-to-center spacing. The material outside the region
of the channels was also removed so that the triangular-
shaped portions of the substrate material projected from
the resulting etched surface of the remainder of the
substrate for each set.
The substrate was then inserted into a multi-bin
boat of the type described by Lockwood et al which con-
tained in one bin a solution formed by combining 3 grams
of Ga, 25 milligrams (mg) of GaAs, 1.9 mg of A1 and 200 mg
of Sn. The solution was equilibrated against a GaAs
source wafer at 760~C. The substrate and the growth
solution for the first cladding layer were then separately
cooled about 4-5C from an initial temperature of 760C at
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a rate of about 1C per minute. The substrate and solu-
tion were then brought into contact for deposition of the
first cladding layer. The triangular-shaped projections
were initially melted back by about 1.3 Ilm leaving 0.9 ~m
deep triangularly shaped channels with planar lands
therebetween. Deposition of the layers was then carried
out resulting in the followlng sequence of layers: an
N-type Alo 3Ga0 7As layer 0.25 ~m thick over the lands; an
N-type Alo 22Ga0 78As guide layer 0.4 ~m thick; an
Alo 07Ga0 93As active layer 0.06 ~m thick; a P-type
Alo 35Ga0 65As second cladding layer 0.8 ~m thick and a
P-type GaAs capping layer 0.3 ~m thick. An sio2 insu-
lating layer about 0.1 ~m thick was deposited on the
capping layer and 50 ~m wide openings for the broad area
contacts were formed over the channels using standard
photolithographic and etching techniques. Ti, Pt, and Au
were then deposited over the oxide and the capping layer
by vacuum evaporation. The substrate electrical contact
was formed by vacuum deposition of Ag and Sn followed by a
sintering step. This surface was then plated with Ni and
coated with Au.
The wafer was then cleaved to form slivers. One
facet of the slivers was coated with about 0.27 ~m of
Al2O3 and the second facet was coated with a six-layer
dielectric stack reflector. Individual dice from the
slivers were then mounted for test.
The devices were tested in a pulse mode using
100 nsec pulses at a 1 kHz rate and at cw. Different
devices exhibited threshold currents between ~50 and 400
ma with peak pulsed output powers up to 400 mw and cw
output powers up to 80 mw. A number of the devices tested
exhibited a two~lobe far field pattern consistent with
180 phase-shift operation. others exhibited a single
lobe characteristic of 0 phase-shift operation in pulsed
mode operation. The quality of these far field patterns
improved with increasing cw output power indicating that
the coupling between emitting elements is increasing with
increasing drive level.
