Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to semiconduc-tor lasers of the double
heterostructure type.
Typically double heterostructure lasers are made from IlI-V
type materials. Examples of such materials are -the ternary GaAlAs/GaAs
and the quaternary GaInAsP/InP systems~ By appropriately -tailoring the
III-V material composition, desired bandgap diFferences between adjacent
layers of the double heterostructllre can be obtained and crystalline
integrity of the device is secured.
Typically a double heterostructure laser has a substrate9 a
first confining layer epitaxially grown on the substra-te, an active layer
epitaxially grown on the first confining layer, a second confining layer
epitaxially grown on the active layer and a capping layer epitaxially
grown on the second conFining layer. Opposed facets of the device are
rendered very smooth by cleaving in order to deFine the ends of a resonant
cavity within the active layer. The lateral limits of the resonant cavity
can be defined by a number of techniques but one of the most popular is
the provision of a narrow stripe contact on the top surFace of the laser.
A broad area contact is deposited on the bottom, substrate side of the
device~
Typically the substrate and first conFining layer are n-type
materials and the second confining and capping layers are p-type
materials. The active layer can be n- or p-type but is of lower bandgap
and higher refractive index than the conFining layers. In operation,
current flows through the double heterostructure from the top stripe
contact to the bottom contact. Carriers are injected into the active
layer at the Forward biased pn junction within the double heterostructure
and end up in an excited energy state. A recombination process occurs
39~
during which photors are emitted within an active region determined by the
diffusion length of the injected carriers. ~lithin the resonant cavity
between the two mirror facets stimulated emission occurs; in other words,
the device lases. By appropriately conFining current ancl gain within the
laser~ it can be made to emit very intense light from a very narrow region
in ~he active layer the light ideally being of low order mode since this
has advantages in reducing fiber optic -transmission losses.
In addition to the stripe contact it is known to introduce a
highly conductive region into the heterostructure by providing a narrow
p-type diffusion from the top surface of the device to a point just short
of the active layer.
One of the problems with performing a p-type diffusion into a
p-type material is that it is very difficult to detect the diffusion front
using a scanning electron microscope. If the diffusion front is too far
from the active layer then it is found that the lasing region of the
active layer is far wider than is optimum for achieving low order modes
and intense output. On the other hand if the p type diffusion extends
into or past the active layer then the required threshold current is
higher and mode problems occur. Another disadvantage of the narrow
diffusion being a p-type diffusion within a p-type second confining layer
is that charge carriers are encouraged to stay within the diffusion only
by the dif`ference in resistivity of the diffused region compared to the
resistivity of the parts of the second confining layer which flank it.
A double heterostructure laser structure is now proposed
which makes process control easier and which shows stronger charge carrier
confinement than known stripe diffusion double heterostructure lasers.
7~
According -to one aspec-t of the invention there is provided a
semiconductor laser comprising a substrate~ a first confining layer, an
active layer, a second conFining layer, a blocking layer9 and a capp1ng
layer~ the laser further having a top metal contact layer contacting the
capping layer and a bottom metal contact l~yer contacting the substrate,
and a narrow difFusion extending frorn a narrow stripe region of the top
contact layer through the capping layer, the diffusion having a diffusion
front partly within the second confininy layer and within 0~5 microns of
the active layer, the second confining layer being no greater than 0.5
microns in thickness and being of the sarne conductivity type as said
diffusion but of opposite conductivity type to the blocking layer.
The device can have an n-type substrate and first confining
layer a p- or n-type active layer, a p-type second confining layer and an
n-type blocking layer. The diffusion is preferably a p-type zinc or
cadmium diffusion. The wafer can be epitaxially produced on a III-V
substrate by liquid phase epitaxy. The laser can be fabricated using any
suitable III-V system, for example the GaAlAs or the GaInAsP systems. The
operation of diffusing into the blocking layer can be clearly detected
using a scanning electron microscope which after suitable chemical etching
?n can distinguish between the diffuse p-type region and outlying parts of
the n-type blocking layer. The narrow diffusion region serves to funnel
current ir,to a narrow striped region. The pn junction between the second
confining layer and the n-type blocking layer laterally outside the stripe
is reverse biased in use and so accentuates that confinement.
An embodiment of the invention will now be described by way
of example with reference to the accompanying drawing which is a sectional
view of a laser according to the invention.
The laser il'lustra-ted in the Figure has in vertically
ascending order the following layers:-
an n-type InP substrate 10 abou-t 75 microns thick;
an n~type InP tin doped first confining layer 12 about 3 to 5
microns thick;
an n- or p-type Ga1 xInxAs1 yPy tin or zinc doped active
layer 14 about 0.1 to 0.3 microns thick;
a p-type InP zinc or cadmium doped second confining 'layer 16
of thickness 0.5 microns or less;
an n type InP tin doped blocking 'layer 18 of a thickness
about 1 to 1.3 microns; and
an n-type GaO 47InO 53As capping layer 20 of thickness 0.5 microns.
Extending into the top of the chip is a narrow p-type
diffusion 22 which extends from the top surface through the capping and
blocking layers to merge with the p-type second confining layer. ~ecause
it is so merged, the diffusion front 24 must inevitably be separated from
the active layer 14 by less than 0.5 microns. A bottom metal contact
layer 30 extends over the lower surface of the entire substrate and a top
metal contact layer 32 extends substantially over the whole of the
difFused region of the capping layer 20~ The top contact layer 32 is
separated from the capping layer 20 outside the diffused region by an
SiO2 layer 3~.
In operation a voltage is applied across the device in a
direction so as to reverse bias the pn junction 33 between the second
confining layer 16 ancl the blocking layer 18.
As previously indicated, at the active layer 1~ electrons
are injected from the n-side to the p-side at one of the active layer
junctions with -the confining layers, these electrons then being in an
exci-ted eneryy sta-te within the conduction ban~. A recombination process
occurs on the p-side within an active region width determined by the
diffusion length of the injected carriers. When the device is pumped by
current directed across the active layer l~, electrons are excited to a
higher energy level to achieve population invasion and so emit photons of
the same wavelength, direction and phase as the stimulating photons. The
stimulated emission process, or lasing, occurs very quickly. Although not
shown in the Figure, mirrors are needed to define the lasing re~ion or
resonant cavity to obtain the stimulated ernission characteristic oF laser
behaviour. Such mirrors are produced hy natural cleavage of the wafer
following processing.
The function of the narrow diFfuse region 22 is to funnel
current through a narrow region of the active layer 14 in order to obtain
intense, localized laser output. By maintaining the diffusion front 24
within 0.5 microns of the active layer 14, the laser output can be limited
to low order modesO As shown in the Figure the diffusion front of the
p-type diffused region is far more easily distinguishable within the
n-type blocking layer 18 than in the p-type second confining layer 16. As
previously mentioned since the sPcond confining layer used in conventional
stripe diffusion heterostructure lasers have been p-type it has proved
difficult to detect the diffusion front due to lack of contrast hetween
the diffusion and the second confining layer. By splitting the
conventional second confining layer into a relatively thin p-type second
confining layer 16 and a relatively wide n-type blocking layer 18 the
bandgap, dopant level and refractive index differences between the second
confining layer and the active layer required for lasing can be maintained
and yet the diffusion front 24 can be accurately detected except where it
is within the p-type second confining layerO The deep di-ffusion 22
funnels current to a very narrow active region by virtue of its lower
resistivity. However that funnelling is accentuated by the presence of a
reverse biased pn junction between the second confining layer 16 and ~he
blocking layer 18 in the regions flanking the diffused region 22.
The device shown in the Figure is fabricated as follows~
Firstly the double heterostructure 127 1~, 16, the blocking layer 18 and
the capping layer 20 are epitaxially ~rown on a InP substrate 10 using,
for example, the liquid phase epitaxy or the organometallic pyrolysis
techniques which are well known in the art. A 1000 ~ 1500 A thick layer
3~ of SiO2 is then chemically vapour deposited onto the top
surface of the wafer and is photodefined and etched to provide a stripe
window 40 of width W1 typically 2 5 ~m. A ~inc or cadmium
diffusion is then performed through the window 40 to produce the p-type
diffusion 22. The d-iffusion is carefully monitored as the diffusion front
spreads through the n-type blocking layer. Using the scanning electron
microscope hoth the diffusion front and the pn junction between the
blocking layer and the second confining layer can be easily seen. Since
the layer 16 is less than 0,5 ~m thick, then as soon as the diffusion
front broaches the junction 33, it must inevitably be within 0.5 microns
of the active layer. The sectional view shown in the Figure is not to
scale. In fact, in practice9 side diffusion s is approximately equal to
diffusion depth d.
If narrower current confinement is desired then the p-type
second confining layer 16 can be made even thinner for example down to 0.2
microns and the same method can be used for monitoring the progression of
the diffusion front to within 0.2 microns of the active layer. Clearly,
better current confinement can be obtained lF the second con~ining layer
is less than 0.~ ~ms. However less process tolerances are permi-tted if a
layer of, for example, 0.2 ~m is cleposited.
Once the diFFusion is complete a top (Cr/Au) and bottom
metal (AuGe) contacts 32, 30 are vapour deposited onto the top and bo-ttom
surfaces respectively of the wafer which is then cleaved into individual
chips. The cleaving process leaves facets which are planar, mirror smooth
and parallel to one another in order to de~ine opposecl surFaces of -the
laser resonant cavities.
As previously indicated -the semiconductor laser can be
produced using other III-V systems, for example~ the GaAlAs ternary
system.
