Note: Descriptions are shown in the official language in which they were submitted.
~ P~ul A. Kirby - 2
f'' (~evision~
~ ~7~Zc3
Fie].d oE the I~ven~.ion
__. ___ __
This inve~ticn relates to heterostructure sem.iconductor
wavegu.ide structures.
Back~round of the Invèntion
____ __ .
S . Heterostructure rib waveguide i.njection lasers such as that
described by T. P. Lee et al. in a paper entitled 'AlGaAs
Double Heterostructure Rib WavecJuide Injection Lasers'
appearing in the XEEE Journal of Quantum El2ctronics Spec~al
lssue Vol. 11 No. 7 pp ~32 - 43S (July 1975~ are known.
L0 disadvantage o that structure is howeve.~ that its method of
construction involves halting the e~i~axy at an interrnediate
sta~e, removing the device from the epitax~ furnace, and masking
and etching the device before returning it to the furnace for the
. completi.on of the epitaxy. This interruption of the epitaxy
is very lLable to produce a poor quality interface between
' the later grown matexial and the earlier grown material. "
: Problems are encountered with the later grown material fail,ing
` to nucleate properly un].ess there is less than about 0.8% AlAs ,in
;, the materia]. upon whi.ch tlle later qrown materlal is to be
'20 deposited. Th.is is attributed to effects of ex.idation of the ,,
- 's~rface while it is removed from the furnace. .'
One'example of a known buried heterostructure injec~ion
laser is that describecl by T. Tsukada in a paper en-titled
5Ga,~s Ga~ As Buried Heterostructure Injection Lasers'
appearirlg i.n the Journal of Applied Physics Vol. 45 pp 4899-49~6
: , (Nov. 1974). ~owever that structure is like the one descri~ed'
in the IEEE Journal ref~rre'~ to previously, in that its manu-
factur~ involves halting the epitaxy at an i,ntermedi.ate stage t
r~moving the device from the ep.itaxy ~urnace, and masking and
,30 , etching th~ device ~efore returning it to the furnace fc~r com-
pletion of the epitaxy~ ,'
--2--
, .
~6~76~
S ary o:E the Inventi.on
Acco~ling to the present invention there is provided a method of
heterostruc-turc semiconductor waveguide manufacture wherein a first COII-
tinuous layer o:~ semiconductive material is grown by liquid phase epitaxy
upon a substTate surface having one or more grooves extending therein, the
thickness of the layer and the conditions of growth being such that a
groove is formecl in the exposed surface of the first layer overlying the
groove in the substrate, wherein semi.conductive material of higher refrac-
tive index is gro~n by liquid phase pitaxy in said groove in the exposed
surface of the first layer and wherein the epitaxially deposited higher
refractive index material is covered by the growth by liquid phase epitaxy
of a further layer of semiconductive material which further layer has a
refractive index less than that of said higher refractive index material.
The first epitaxially deposited layer may be of opposite conduc-
tivity type to each succeeding epitaxially deposited layer, and the higher
reractive index material may consist of or include a region forming the
active material of an injection laser or light amplifier.
According to the present invention there is also provided a semi-
conductor light emitting device comprising: a substrate having a surface
with a first groove provided.therein; a first layer of low refractive index
material of one conductivity type provided on the surface, said first layer
having a surface wi~h a second groove provided therein overlying the first
groove; an active layer of higher refractive index material provided in ~he
second groove; a second layer of low refractive index material of the other
conductivity type provided over said active layer and said first layer; and
an electrode layer provided over said second layer.
Description of the Invention
., ~ .
T~e invention relies upon the tendency for grooves in a substrate
~ ~ to become preferentially filled when l.ayers are grown on ~he subs~rate by
:.~ 30 liquid phase epitaxy~ Thus if a groove of depth _ is provided in the
surface of a substrate on which material is grown by liquid phase epit~xy,
the depth a of the groove in the surface of grown material approximately
.. :, .
~' ~
~7~ 9
follows the relationship a = aO exp-t/c, where t i9 the thickness of the
grown material covering a p:Lane part of the substrate, cmd c is a decay
constant dependent upon the melt composition and the conditions oE the
growth. ~le decay constant has been found to be particularly small for
growth on a substrate whose surface i9 closely aligned w:Lth one of the low
index
3a-
Paul A. Kirby - 2
,~t~Z9 ( Revision~
p~anes, such as a {100~ plan~ in GaAlAs. In this i.ns-tance
however, the decay constant is si~nificantly increased when the
--. ancJle bet~een the substrate surface and the {100} plane is f
increased to greater than 1/2 Thus, Eor instance, it is
Eound -that when a suhstrate surace, that is tilted by
betweenl ~2~nd 3 about a<100> direction from a {100~ plane,
is provided with a 51,m width Vee-shaped g:roove extending
in a direction at right angles to the ti.lt axis, the groove
profile in the surface of the grown material is reduced
to 1.5~1m under typical growth conditions when the depth o
the grown material in plane regions near the groove has a
-thi.ckness of about 2.5~m. Thus any epitaxially deposited layer
has a region of increased thickness that follows the track o
the groove in the surface of the underlying substrate. ~y
lS arranging that the material bounding this layer is of lower
refractive index than the layer, this ].ayer is provided with
the optical guiding properties of a 'rib' waveguide.
For certain applications use may also be made of the
fact that in liquid phase epitaxy the equilibrium saturation
temperature of a melt in contact with a solid surface is a
function o:~ the curvature of tne interface between them. In
particular the equilibrium saturation -temperature is higher
where the solid is convex and lower where it is concave, and,
for surfaces having a curvature in one direction only, sati6fies
the equationo-
Tr = T~ y L 1 r~l)
where Tr is the equilibrium temperature in a region where
the surface has a radius of curvature r,
T~ is the equilibrium temperature in a region where the
surface is planar,
y ls the free surEace energy at the liquid solid interface,
ancl
F'
Paul ~. Kirby - 2
tRevis.ion)
~t7
L is the latent heat o~ Eusi.c-n per Ul):it vol~lme.
Thereore by choosing to perform a liquid phase epitaxy und~r
condi-t.iolls in which the melt is sli.ghtly undersa-turated wi.th
respec~ -to a plane surface, the epitaxial growth can be
S confined to a strip along the bottom of the groove where the
solid surface is concave. By ar:ran~ing for the growth of the g
strip to be on lower refractive index material, and by arranging
for the strip to be covered by the growth of a layer of lower
refractive index material the s*rip is provided with the optical
guidi.ng properties of a 'clad' wav`eguide.
This selectivity of deposition arising from curvature
~~~ differences may also be used to make complex integrated optics
st.ructures incorporating active elements, lasers or light ampli--
fiers, wi.th passive waveguides. For this purpose the profile
of the groove or grooves is arranged to be different in different
parts o~ the substra-te so that an epitaxy can be performed which
will result in the deposition of the active material required for 1`
the lasers or light amplifiers to occur in one groove but not t
another, or to occur in a po.rtion of a groove, but not over the
~20 whole of its l.ength. .
There follows a description of -the methods of manufacture
of constructions of ~aAlAs heterostructure laser embodying the
invention in preferred forms. The description refers to the
accom~anying drawings in which:-
Brie~ Description o the_Drawings
I
~ igure 1 is a transverse section through a buried 'rib'
injection laser a~cording to the inven-tion.
Figure 2 depicts a transverse section through an isolated
~ stripe heterostructure injection laser, according to the ~.
lnvention, and
- S
' ,
,
Paul A. Kirby 2
(Revision)
~t7~
Figure 3 depicts a transverse section through a
buried heterostructure injection laser according to the
invention.
Descript,ion of the Preferred Embodiment
S Referriny to Figure 1 a substrate 10 has a Vee-shaped groove
11 e~tending substantially in a <110> direction along t.he
substrate surface. This surface exten'ds in a {100} plane, but
- is tilted b~ between 1/2 and 3 about an axis substantially
perpendicu],ar to the groove direction. The substrate is placed
.
10 - in a mul-ti-well liquid phase epitaxy graphite boat (not shown)
provided with melts for growing three layers 12, 13 and 14 on
the substrate. The boat is placed in an epitaxy furnace and
the three layers are grown withou-t interruption. Layers 12 and
14 are constructed of GaAlAs of opposite conductivity type. ~;
Normally the substrate is n-type, and hence layer 12 is
n-type and layer 14 p-type. Layer 13 is grown in lower band
gap higher refractive index material than the other two layers, ~
and contains either a reduced AlAs content or substantially no ~,
AlAs. Layers 12 and 14 are of opposite polarity type in order
to form a p~n junction near ~he surface of or within layer 13, ,
which is the ac-tive layer, and which may be o~ either conductivity
type, but is usually grown p-type. Typically the doping levels
used for these layers lie in the region of 5 x 1017 carriers cm 3-
Typically layer 13 is about 0.2 ~m thick, and in order to 3
provide adequate lateral optical guidance, is required to have
its centre about 0.02~m thicker than its edges. The amount of
thickening can readily be controlled by adjusting the thickness
of the underlying layer 12 because the depth of the groove in ita , ~-
surface varies substantially exponentially with thickness. It
is also possible to control the exponential decay constant by
varying the growth conditions, in particular -to speed up the
,.
r
Paul ~. Kirby - 2
~7~ ( Revision)
"
growth rate so as to reduce the rate of (lecay w:ith depth.
After the growth of the three ep.itaxial layers, the st.ructure
is prov,ided with an insulator layer 16. This i9 ca:refully masked
fnr etching a channel throu~h i~ which will accurately register
with the underl.ying buried ri.b. Then, a~ter the etchi.ng, a ,'
metal elect.rode layer 17 is prov.ided over the insulator layer 16 ~.;
to make a stripe electrical contact with the exposed portio.n of
layer 14.
The structure of figure 1 provides more lateral optical
guidance tllan a conventional stripe double heterostructure laser
employing p:Lane layers because the pr.imary lateral optical guidarlce
." ~
in the onventional stripe laser is that provided by gain guiding.
The basic structure of figure 1 is progressively modified
~'~ in the structure now to be described with reference to f.igures
2 and 3.
The only modification required to make the structure of ',~
f.i.gu~e 2 involves making a slight change to the melt conditions
employed in the growth of the active material 13'. The supe.r-
saturation of the melt is reduced so that, while the melt is
still super saturated with respect to plane surfaces and concave ¦.
suxfaces, i.t is unsaturated with respect to the concave surfaces 1-
that occur at the si.des of the groove in layer 12. The result is
that layer 13' is interrupted by two reyions 20 where no active I '
materlal is deposited. In between these two regions 20 there
is a stripe o active material 21 encircled by the lower refract.ive
index material of layers 12 and 14. The optical guiding provided
by this stripe 21 is thus not that of a 'rib' structure, but
instead that ~f a 'core surrounded by lower.refractive index
claddingl structure.
In a particular exa~lple of this ~ype of structure the ,.
~...................................................................... i
stripe 2~ was found to be approximately 17~m wide. The stripe
'. ' Il
. ~ I
Paul ~. Kirby - 2
~Revision)
was appr~:~in~ate:ly 0,28~m thiclc alony its cent:r~ lire ~rlc' amooth~y
tapered to nothin~ at the sides. In the Elanking reylons 23
of layer 1~' o~tsicle the regions 20 the thickrless of tlle active
material was found to increase again to a maximum of about 0.15~m.
In this st.ructure there will be substant:ially no current
~low across the pn junction in -the regions 20 because here the
junction is bounc3ed on both sides by the higher band-gap
materia]. of layers 12 and 14, whereas in ~the strlpe 21 it is
bounded on at least one side by the lower band-gap act,:ive material.
A stripe contact top electrode is however still necessary in order
to reduce the current flow across the pn junction in the regions
23 outside the strips 20.
In the str~lcture of E.igure 3 the modification of the
figure 2 construction is taken a stage fur-ther with the melt
~~' -- 15 cond.itions used for growing the active material 13" arranged f
so that the melt is unsaturated with respect to a plane, and f
. supersaturated only with respect to the concave surface at the
bottom of the groove in the surface of layer 12. As a result the
growth of the active material is confined to the stripe 31.
In a particular example of this type of structure the
stripe 31 was found to be about 9~m w.ide and about 0.15~m thi.ck
along its centre line, the thickness smoothly tapering to nothing
at the sides.
In this structure there is suhstant.ially no current flow
across the pn junction outside the region of the stripe because
outside this region the junction is bounded on both s.ides by
the h,igher band-gap material of layers 12 and '14 whereas inside
the region it is bounded on at least one side by the lower band-gap
material 13". As a resu1t there is no need' to use a stripe
electr.ical contact, and hence the steps of depositing the insulating
layer, masking it, and etchin~ it~ may be omit~ed~
.~ : . . . . : - ,. ~
Paul K. Kir~ly ~ 2
(Revision)
~ 3~
It is to be ullclerstooc1 that the invention is not limitec1
to the construc1;on oE lasers ancl light an1F)Lifiers, bnt i~ appll~
cable also to ~he construction of inteqratecl circuit structures.
For example a directional coupler is Eormecl by ùsing the invention
to form waveguiding on a substrate provided with two grooves,
which, over a portion of their length, exltend side ~y side in
close proxim:ity. If a pn junction is ~or1ned in the region of
the waveguides the structure can be arranged to be an active
device in which the coupling between the guides is capable of
l~ bein-~ varied electrically. This is achievable by reverse biasing
the junction to establish a depletion region in which the re- ~-
fractive index is changed in part as a result of the extraction
`- o~ free carriers and in part by the electro-optic light effect
produ~ed by the field extending in the depletion region. More
complex integrated optics structures may be produced by having i
diferent profiles o groove so that liquid phase epitaxy can be
used to deposite~ active material in one portion of the groove
s-tructure but not another. This is then followed by an additional
li~uid phase epitaxy preceding the deposition of the lower
refractive index material. This additional expi~y is used to
deposit passive material along the entire length o the groove
structure. In this way there is produced a waveguide structure
of passive material under which there are locali%ed regions of
active material. The passive material has a larger band-gap
than the active material and hence current across the pn junction
~s substantially conined to the active regions which thereby
provi~e optical gain for the structure by skimulated emission.
It will be appreciated ~hat the control of the saturation
o~ the melt from which the higher refractive index material is
grown is a particularly critical matter when structures of the
- type~ illustrated in figures 2 and 3 are being made. In
_ 9 _
Paul ~. Kirby - 2
~l~evis.ion)
In particl1lar account has to be tak~n of the rapid in:itial growth
-that is l:i.ablt? to occur when a melt first comes :into conl:act '
with the surface upon which ex~taxi~ growth is required. Good
contIol oE this rap:icl initlal g.rowth phenor~e1lon is ach.ievable
by use of an epitaxy furnace with a controllable vextical temper~
a-ture gradient. The appropri.ate vertical. temperature gradier1t
for a par-ti.cular material and structu~e will depend upon
cooli.ng rate.
In the laser structures of figure l and 2 t.he current flow
across the pn junctior1 is substantially confined to the requireA
region by use of a stripe contac-t. Current flow across the
junction outside -the required region is effectively a leakage ~1;
current. Such leakage can be reduced or eliminated by mesaing
away the junctions in -the leakage regions or by converting
lS these regions to semi-insulating material for instance by t
proton bombardment. If either of these expedients are adopted
it i.s not necessary Eor the insulating layer ~o be provided
which defines the stripe contact.
It is to be understood that the foregoing description of
speciic examples of this invention is made by way of example
only and is not to be considered as a limitation on its scope. '
RA~:sq
October 26, 1976
--- .
- 10 --
- , . ' , . . ::, ~. : , .
