Note: Descriptions are shown in the official language in which they were submitted.
105~043
This invention relates to optoelectronic devices
and the control of light propagation therein, particularly to
at least reduce the emission of photons from, or into, other than
the desired areas of a device.
In optoelectronic devices, such as light emitting
diodes (LEDs), 1asers, modulators and detectors, there is often
a need to accurately define the active region of these devices,
that is the region at which light emission or light absorption
takes place. For example, in heterostructure GaAs/GaAlAs devices~ ~
10 a fraction of the light which is either generated directly in the -
active layer (LED's or lasers) or coupled into the active region
(modulators or detectors) will escape from the active or guiding -
layer since the confinement will not be perfect. Such unguided
light may exit through the GaAlAs confin7ng layers adjacent to the
guiding layer and trigger undesired optical response of subsequent
optical elements.
The present invention provides a way of at least
reducing the effects of imperfect confinement by providing a
barrier, or barriers to the unguided light in the confining layers.
This invention will be readily understood by the following
descriptlon of certain embodiments, by way of example, in
conjunction with the accn~panying drawings, in which:-
Figure 1 is a diagrammatic cross-section through
a device illustrating the basic concept of the inventioni
Figure 2 is a curve illustrating the light
transmission for different parts of the device in Figure l;
Figure 3 is a curve illustrating the ratios of
light transmission through bombarded and nonbombarded regions of ~ ;
a device as in Figure 1, for different waveleng~hs,
3V Figure 4 is a diagrammatic cross-section through
an integrated LED - modulator device incorporating one form of
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the invention;
Figure 5 illustrates the improvement in modulator
extinction ratios, using the invention;
Figures 6 to 9 illustrate steps in the production
of an LED emitter - modulator structure, incorporating one form
of the invention, Figures 8 and 9 being cross-sections on the
lines VIII-VIII and IX-IX of Figures 6 and 7 respective7y;
Figure 10 is a diagrammatic cross-section of the
finished structure, as on the line X-X of Figure 7;
Figure 11 is a diagrammatic cross-section of a
finished structure incorporating another form of the invention;
Figure 12 is a diagrammatic cross-section of a
structure, similar to that of Figure 11, but incorporating the
present invention in a further form;
Figures 13 and 14 illustrate diagrammatic cross-
sections through two devices which have well-defined active or
guiding layers into which optically absorbing barriers are
introduced by proton bombardment or crystal growth techniques
respectively.
The present invention provides a barrier, or
barriers which are incorporated into the confining layer, or
layers of double heterostructure devices to prevent unguided
light in those confining layers from exiting through the side
facets. This provides a variety of advantages for LED's, lasers,
modulators, and detectors. One advantage is that the only light
which exits from these devices comes from the active or guiding
layer. This will essentially eliminate undesired optical responses
generated by stray light. A second advantage is that the geometry
of the active region is well-defined so that emitting areas can
be made comparable in size to the cores of optical fibers which
might be attached to the end faces. A third advantage occurs in
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electroabsorption or phase modulators in which the light which
escapes into and propages along the confining layers will reduce
the modulation depth. For example, integrated LED emitter-
modulator structures can achieve up to 20dB extinction ratios
via the process of electroabsorption. However these high
extinction ratios are only achieved by limiting the area of the ~--
detector so that only light which exits from the active, or
guiding layer is recorded by the detector. If the light which
propagates outside the active, or guiding layer is also detected,
10 then the extinction ratio is significantly reduced.
In a typical double heterostructure device, the }guiding or active layer consists of Gal yAlyAs material with
y'O.l, such a layer has an optical absorption edge in the range
8~0-870 nm and will guide photons with wavelengths longer than
the aborption edge value. It is proposed that one way of over-
coming light spill of these photons into and out of the Gal xAlxAs
confining layers is by introducing optically absorbing regions
into the confining layers by the method of proton bombardment.
Thus photons from the guiding layer must be absorbed in a -
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~;~20 confining layer of completely different material (typically
GaO 7Alo 3As) where the band edge is near 680 nm. This is a
completely different situation to that in which proton bombardment
of GaAs provides absorption for wavelengths close to the GaAs
absorption edge. The basic validity of the above proposal has
been established using the device illustrated in Figure 1. A 5~m
thick GaO 7Al o 3As layer was first grown on an n-GaAs substrate.
Approximately one half of the area of this layer was bombarded
at 390 keY, 3 x 1015 cm 2, the other half was shielded from the
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beam. The crystal was then glued to a glass slide with a trans-
3~ parent photoresist and the whole of the n-type substrate was -
removed by using a selective etch (H202~NH40H, pH = 8.70). After
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an etch time of about 60 minutes, only the 5 ~m thick layer
remained. In Figure 1 the glass slide is indicated at 10, the
photoresist layer at 11 and the 5 ~m thick GaO 7Alo 3As layer is
indicated at 12. The proton bombarded area is indicated at 13.
Light from a monochromatic source was then passed through both
the bombarded and nonbombarded regions of the crystal, as
indicated by arrors X, and detected by a cooled photo-multipler.
Figure 2 illustrates a typical variation in the
transmitted light variation across a crystal for a fixed wave-
length of 750 nm. The undulations are due to surface roughnessof the etched surface but the location of the boundary 14 between
bombarded and nonbombarded regions is easily identified. The
two intensities of light are indicated on Figure 1, and as an
average on Figure 2, as Tb and To for bombarded and nonbombarded
regions respectively.
Figure 3 illustrates how the ratio of light
transmission through the unbombarded and bombarded regions of a
crystal (i.e. Io/~b) varies as a function of wavelength. As
will be seen, the ratio varies from about 1.2 for wavelengths of
2~ 7Z5 nm t~ about 0.45 for wavelengths of 900 nm.
As an example of a device employing the invention,
an integrated LED - modulator structure is illustrated in Figure
4. The structure illùstrated is a double heterostructure
comprising a GaAs substrate 20, a first Gal xAlxAs (x ~ 0.3)
confining layer 21, an active GaAs layer 22, a second Gal xAlxAs
(x ~ 0.3) confining layer 23 and an optional capping layer 24.
A masking layer 25 is formed on the capping layer 24 and proton
bombardment forms regions of high optical absorption in the second
confining layer 23 (and in the capping layer 24 although this is
incidental). The conductivity type of the layers can vary provided
there is the correct relationship. Thus the substrate is n-type,
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the first confining layer and the active layer are n-type while
the second confining layer (and capping layer) are p-type. If
the substrate is p-type, the first confining layer and active
layer are also p-type and the second confining layer (and capping
layer) are n-type.
Hole 26 is then etched through the substrate 20.
A suitable etch is as referred to previously, for removal of the
substrate in the preparation of the device of Figure l. Conven-
iently the etch is selective for GaAs, stopping at the first
confining layer 21, the bottoms of the hole 26 being at the
boundary between substrate 20 and confining layer 21. A further
proton bombardment is carried out from the substrate side of the
structure to form a region 27 of high optical absorption at the
bottom of the hole as well as a reg;on 30 along the periphery of
the hole. The LED emitter section is at 28 and is energized by
an appropriate potential or bias applied to the capping layer 24
in the emitter section and to the contact on the substrate 20.
The modulator section 29 modulates the light emission from the
active layer 22 in the emitter section, again by suitable potentials
applied to the capping layer 24 and substrate 20.
The photons labelled B and C pass into the
confining layers 21 and 23. Photons B will be absorbed by the
proton damaged regions 26 and 27. The photons C will be absorbed
to some extent by the proton damage at the periphery of the hole
26, indicated at 30. Complete abosrption in the substrate can be
assured if the n-type active layer 22 contains a small amount of
Al which will shift the photon energy to values beyond the
absorption edge of substrate 20. A detector is indicated at 31.
The possible improvement gains are illustrated by the
curves in Figure 5. The curves illustrate extinction ratio versus
effective detector width at the modulator exit face. The highest
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extinction ratios are obtained when the effective detector width
is narrower than the thickness of the guiding layer 22. When the
effective width is greater than the guiding layer 22, so that
light from the conf;ning layers 21 and 23 is also included, the
extinction ratio is reduced by 8-10 dB. By preventing the
propagation of light rays through the confining layers of the
modulator, the size of the effective detector width is not so
critical. A wider effective detector width can be used and still
obtain high extinction ratios. The extinction ratio will be
improved by 8-lOdB relative to the same wider effective detector
width without photon absorption. Curves 33 and 34 illustrate
extinction ratio versus effective detector width for two convent-
ional modulator devices operated at negative biases of 24 volts
and 18 volts, respectively. Curves 33a and 34a illustrate the
improvements achieved by introducing optical absorption into the
confining layers by proton bombardment.
Figures 6 to 9 illustrate two steps in producing
a high-speed high extinction ratio LED emitter-modulator structure,
and Figure 10 is a cross-section through the structure - on the
line X-X of Figure 7. The structure ;llustrated is a double
heterostructure, as in Figure 4, with a substrate 35, first
confining layer 36, active or guiding layer 37, a second confining
layer 38 and optional capping layer 39. The conductivity type of
substrate 35 and layers 36, 37, 38 and 39 as previously described
in rélation to Figure 4. High speed operation is obtained by
limiting the junction capacity with a first proton bombardment.
A metal stripe 40 is produced on the capping layer 39 and the
structure bombarded. The bombardment alters the layers 39, 38
and 37, also part of the layer 36, as seen in Figure 8. Narrow
3~ gaps 42 are then etched into the metal stripe and a second proton
bombardment which alters only layers 39 and 38 forms electrical
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isolation between sections and ensures optical absorption in the
top confining layer 38. The structure is then as in Figure 9.
Finally holes 43 are etched through the substrate 35 to the
first confining layer 36 and a third proton bombardment is
performed into these holes from the substrate side, to form regions
44 which provide optical isolat;ons (absorption) in the lower, or
first, confining layer. The mask layer for etching the holes 43
and masking from the third bombardment is indicated at 45. A
certain amount of bombardment damage also occurs on the sides of
the holes 43 at 46. For both second and third bombardments the
proton beam energy is accurately controlled to ensure that protons
penetrate only to a minimum extent into the active or guiding
layer 37. In Figure 10, the emitter section is the central one
third portion between the bombarded regions 42. The structure
illustrated in Figure 10 has one LED emitter section 46 with
modulator sections 47 positioned on either side. These devices
are most conveniently made by fabricating a large number of
sections on a common substrate and then dividing along the dashed
line 48 (Figure 10).
ln relation to Figures 6 to 10, if high modulator -
speed is not critical, the first proton bombardment can be
eliminated. The only bombardment required is that which creates
regions 42, (now extending in stripes all across the crystal)
to provide electrical (and optical) isolation between sections
~; and prevent propagation of leakage photons in the second confining
layer 38. It is also possible to provide an alternative optical
isolation structure for the first confining layer. In Figure 11
a photon absorbing region is formed by initial profiling of a
substrate. As illustrated in Figure 11 a substrate 50 is masked
and etched on one sur~ace to form upstanding ribs or ridges 51.
The first confining layer 5~ is then formed followed by formation
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of the active or guiding layer 53. The thickness of layer 52 can
be controlled for careful crystal growth such that the gap between
the top surface of the ribs 51 and active layer 53 is small. The
second confining layer 54 is formed followed by capping layer 55.
A masking layer 56 is formed on the capping layer 55 and isolation
regions 57 are formed by proton bombardment through layers 54
and 55 down to the upper surface of the active or guiding layer
53. The proton bombardment regions 57 prevent propagation of
photons B along the upper or second confining layer 54 into the
modulator section 60, from the emitter section 61, while the ribs
51 absorb the photons B propagating in the lower or first
confining layer 52. The photons C are absorbed in the substrate.
To ensure substantially complete absorption of unwanted protons
the active layer 53 contains some Al, having the for~ n-Gal yA1yAs -
with y - 0.1. The photons A will be the only light to emit from
the emitter 61 and propagate through the modulator 60.
As an alternative to the proton bombarded regions
57 which were used in Figure 11 to provide optical absorpt;on,
some devices can effectively utilize profiling plus crystal growth
techniques to provide the required optical absorption in both
confining layers. This is illustrated in Figure 12 using the same
referencesas in Figure 11 where applicable. In this case, up-
standing ridges 51 provide optical absorption in the ~irst
confining layer while inverted ridges 62 provide optical absorption
in the second confining layer. In such an arrangement ~he capping
layer 55 is not optional and would have properties the same as
substrate 50, that is, be essentially GaAs (with little or no Al
content). This would ensure absorption of photons B provided
the guiding or active layer 53 is of GaAlAs, for example
30 GaO gAlo lAs. ;~ -
In addition to the integrated emitter-modulator
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devices discussed previously, the inventions are applicable to
many discrete optoelectronic devices such as LED's, laser, -
modulators and detectors. Diagrammatic cross-sections are shown
in Figures 13 and 14 for the cases of optical barriers introduced
by the techniques of proton bombardment and crystal growth,
respectively. In each case a well-defined active or guiding layer
is defined which provides those advantages discussed previously.
In these devices, the structure comprises a substrate 63 with a
p- or n-type active layer 64 and confining layers 65 and 66 on
either side o~ the active layer. A capping layer 67 is on top
of confining layer 66. Substrate and first confining layer are
typically n-type while second confining layer and capping layer
are of p-type. Precise definition of the active layer is provided
at the exit facets at 68 by proton bombardment in Figure 13 ~;
and at 69 by crystal growth in Figure l4.
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