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United States Patent |
5,012,486
|
Luryi
,   et al.
|
April 30, 1991
|
Vertical cavity semiconductor laser with lattice-mismatched mirror stack
Abstract
In a vertical semiconductor laser, the top mirror is composed of
alternating layers of lattice-mismatched semiconductors. Quantum
reflections and other charge transport barriers for majority carriers at
the interface, and hence electrical resistance and power dissipation, are
reduced by choosing the lattice-mismatched semiconductor materials in such
a manner as to align their band edges for majority carriers. On the other
hand, the semiconductor materials are selected to supply relatively large
refractive index differences, and hence relatively large optical
reflections, at their interfaces. The lattice-mismatching may also produce
vertical thread dislocations through the stack, which increase the
electrical conductivity.
Inventors:
|
Luryi; Sergey (Bridgewater, NJ);
Xie; Ya-Hong (Flemington, NJ)
|
Assignee:
|
AT&T Bell Laboratories (Murray Hill, NJ)
|
Appl. No.:
|
506413 |
Filed:
|
April 6, 1990 |
Current U.S. Class: |
372/45; 372/99 |
Intern'l Class: |
H01S 003/19 |
Field of Search: |
372/45,43
11/99
|
References Cited [Referenced By]
U.S. Patent Documents
4943970 | Jul., 1990 | Bradley | 372/45.
|
4949351 | Aug., 1990 | Imanaka | 372/45.
|
Foreign Patent Documents |
0081887 | May., 1985 | JP | 372/45.
|
0081888 | May., 1985 | JP | 372/45.
|
0094689 | Apr., 1989 | JP | 372/45.
|
Primary Examiner: Epps; Georgia
Attorney, Agent or Firm: Caplan; D. I.
Claims
We claim:
1. A semiconductor laser structure comprising an active semiconductor layer
upon which a cladding layer and a mirror stack have been grown, the mirror
stack comprising a plurality of semiconductor layers at least one of which
is lattice-mismatched with respect to another contiguous thereto.
2. A laser structure in accordance with claim 1 in which every other layer
in the mirror stack has a same first lattice constant and all the
remaining layers in the stack have a same second lattice constant
different from the first.
3. The laser structure of claim 2 in which the difference between the first
and second lattice constants amounts to at least 0.30 percent.
4. The laser structure of claim 2 in which the difference between the first
and second lattice constants amounts to at least about 0.20 percent.
5. The laser structure of claim 1 in which the lattice mismatch amounts to
at least 0.30 percent.
6. The laser structure of claim 1 in which the lattice mismatch amounts to
at least 0.20 percent.
7. The laser structure of claim 1 in which the active layer is essentially
GaAs and in which the mirror-stack comprises a plurality of alternating
layers of essentially AlAs and In.sub.z Ga.sub.1-z P, where z is
approximately equal to 0.55.
8. The laser structure of claim 1 further including an electrode attached
to the mirror stack, and in which the active layer is essentially InGaAs
which generates light having a vacuum wavelength of about 1.55 .mu.m in
response to electrical current supplied to the electrode, and in which the
mirror-stack comprises a plurality of alternating layers of essentially
InP and Al.sub.y In.sub.1-y As, where y is approximately equal to 0.82.
9. The structure of claim 8 in which y is approximately equal to 0.8.
10. An optical system including the structure of claim 1 and means for
utilizing optical radiation which can be emitted by the structure.
11. An optical system including the structure of claim 2 and means for
utilizing optical radiation emitted by the structure.
12. An optical system including the structure of claim 3 and means for
utilizing optical radiation emitted by the structure.
13. An optical system including the structure of claim 4 and means for
utilizing optical radiation emitted by the structure.
14. An optical system including the structure of claim 5 and means for
utilizing optical radiation emitted by the structure.
15. An optical system including the structure of claim 6 and means for
utilizing optical radiation emitted by the structure.
16. An optical system including the structure of claim 7 and means for
utilizing optical radiation emitted by the structure.
17. An optical system including the structure of claim 8 and means for
utilizing optical radiation emitted by the structure.
18. An optical system including the structure of claim 9 and means for
utilizing optical radiation emitted by the structure.
Description
TECHNICAL FIELD
This invention relates to optical systems comprising semiconductor lasers
and more particularly to those systems comprising lasers which have
vertical cavities for the emission of light (optical radiation) through a
major ("horizontal") surface of the semiconductor.
BACKGROUND OF THE INVENTION
In optical systems of prior art, the structure of one useful form of
semiconductor lasers is a vertical cavity (or simply "vertical") laser. In
a vertical laser, there is an active region in a semiconductor body
(substrate) which includes a planar pn junction. Typically the plane of
this pn junction is parallel to a major surface of a semiconductor
substrate body, the major surface of the substrate being considered
arbitrarily to be horizontal. In a vertical laser, light is emitted from
the top or the bottom (major) surface, or both, of the semiconductor body,
a vertical optical cavity being created therein by virtue of reflecting
optical mirror(s) located on the top or bottom surface thereof, or both.
The structure of a vertical laser can be made circularly symmetric.
Therefore, a vertical laser can have the advantage of relatively low
astigmatism. Also, because a vertical laser can be made with a relatively
large aperture, it can have the further advantage of a low divergence of
the emerging beam as compared with other laser, such as "edge" lasers in
which light is emitted from a side edge of the semiconductor body.
A vertical laser typically is built as a double heterostructure (two
junctions between chemically dissimilar materials), for example, by
successive epitaxial growth of the following semiconductor layers in
spatial sequence upon a semiconductor substrate: the bottom mirror, a
bottom optical cladding region, the active region, a top cladding region,
and the top mirror. Typically, in a vertical laser each mirror(s) is
formed by a quarter-wavelength stack, such as a mirror stack formed by
alternating layers of two semiconductor materials with differing chemical
compositions and hence differing refractive indices, which thus form a
semiconductor superlattice. The choice of the semiconductor materials for
the mirror stack is made so as to result in large differences in these
refractive indices, in order to maximize optical reflectivity and hence
minimize the number of periods in the superlattice, and thus minimize
undesirable vertical electrical resistance and unwanted power dissipation.
In an optically pumped semiconductor laser, optical radiation of
wavelength(s) shorter than that (those) to be emitted by the laser is
directed upon the laser to create an electronic population inversion. In a
typical electrically pumped (driven) vertical cavity semiconductor laser,
electrical current is passed between a top electrode formed on the top
major surface of the top mirror and a bottom electrode formed on the
bottom major surface of the semiconductor substrate. Many such vertical
lasers can be built on a single such substrate, as by trench or other
isolation, in such a way that the intensity of light--e.g., ON vs.
OFF--emitted by each laser can be controlled by an electrical signal
independently of all other lasers on the substrate. Thus, vertical lasers
appear especially attractive for use in practical applications where more
than one independently controllable source of light is desired on a single
substrate. Alternatively, many separate lasers can be mass produced from
the single substrate, as by masking and etching apart the individual
lasers.
In prior art, the semiconductor substrates that have been used for double
heterostructure vertical lasers have been mostly gallium arsenide or
indium phosphide. It has been believed to be necessary to build such
lasers with very nearly lattice matching of the double heterostructure,
including the mirror(s), in order to achieve the high quality (low defect
density) epitaxial growth needed for the desirably low optical absorption
and high quantum efficiency of light emission. Consequently, the choice of
materials for the mirror stack has been limited, in order at the same time
to preserve the large difference in refractive index between contiguous
layers in the stack. In turn, this limited choice of semiconductor
materials usually results in undesirably large conduction and valence band
edge discontinuities, whereby undesirably high electrical resistance is
exhibited by the mirror stack, owing to the resulting high quantum
reflection coefficients and other charge transport barriers ("effective
barriers") for both electrons and holes at the interfaces of contiguous
layers in the mirror stack. In turn, this high resistance results in
undesirably high power loss (dissipation) in the laser.
It would therefore be desirable to have a vertical laser which mitigates
the problem of high electrical resistance and power dissipation.
SUMMARY OF THE INVENTION
The top mirror of a semiconductor vertical laser structure is made of
lattice-mismatched materials having band edge discontinuities that are
relatively small--that is, less than about 50 meV, i.e., the equivalent of
about 2 kT (k=Boltzmann's constant, T=room temperature)--for the majority
carriers in the stack. To achieve this end, the lattice mismatch is
typically in the range of about 0.20 or 0.30 percent to 3.0 percent or
more. In this way, electrical resistance and power dissipation can be
significantly reduced, and at the same time the structure can have
suitably large refractive index differences between contiguous layers in
the top mirror stack. At the same time also, the lattice defects
(dislocation lines) caused by the lattice mismatch may supply useful
vertical conduction paths for electrical current and thus may further
reduce the electrical resistance.
BRIEF DESCRIPTION OF THE DRAWING(S)
This invention together with its features and characteristics may be better
understood from the following detailed description when read in
conjunction with the drawing in which the FIGURE is a side view in cross
section of a semiconductor laser structure, together with means for
utilizing its optical output, in accordance with a specific embodiment of
the invention. Only for the sake of clarity, the FIGURE is not drawn to
any scale.
DETAILED DESCRIPTION
As shown in the FIGURE, an optical system comprises a vertical laser
structure 100, together with means 200 for utilizing optical output beam
20 emitted by the laser 100. Illustratively, the vertical laser structure
100 comprises an n.sup.+ type indium phosphide (InP) substrate body 10, a
bottom n.sup.+ type mirror stack 11, a bottom n.sup.+ type InP cladding
layer 12, a lattice-matched p type indium gallium arsenide (In.sub.x
Ga.sub.1-x As) active layer 13 (wavelength, .lambda.=1.55 .mu.m), a top
p.sup.+ type InP cladding layer 14, and a top p.sup.+ type mirror stack 15
(described in greater detail below). The structure also includes a bottom
electrode 9 having an aperture 8 located on the body 10 and a top
electrode 16 located on the top surface of the mirror stack 15. By
"lattice-matched" InGaAs is meant In.sub.x Ga.sub.1-x As with x=0.53. The
top cladding layer 14, the active region 13, the bottom cladding layer 12,
the bottom mirror stack 11, and the substrate 10 are mutually lattice
matched. However, the top mirror stack is not lattice matched: it contains
at least one layer which is not lattice matched, whereby the top mirror
stack is threaded with desirable dislocations.
The bottom n.sup.+ type mirror stack 11 typically comprises about 20
periods or more of alternating quarter wavelength thick layers of n.sup.+
type InP and lattice-matched n.sup.+ type InGaAs (each period having one
quarter wavelength thick layer of n.sup.+ type InP and one quarter
wavelength thick layer of lattice-matched n.sup.+ type InGaAs).
The top mirror stack 15 illustratively comprises alternating quarter
wavelength thick layers of p.sup.+ type InP and lattice-mismatched p.sup.+
type aluminum indium arsenide, to wit, Al.sub.y In.sub.1-y As, with y
approximately equal to 0.82. In this way, the discontinuities of valence
band edges at interfaces of contiguous lattice-mismatched layers are less
than about 2 kT, in order to reduce the effective barrier to majority
carriers (holes) at the interfaces and hence to reduce electrical
resistance otherwise increased by the band-edge discontinuities. The
lattice mismatch in this case amounts to about 2.2 percent.
During laser operation, a current source 7 drives the laser structure 100,
whereby the output beam 20 of optical radiation is emitted by the laser
structure 100 and is incident upon utilization means 200, which may
include such elements as an optical detector, an optical lens, an optical
fiber, or a combination thereof.
In an illustrative example, the InP substrate 10 and the bottom
lattice-matched mirror stack 11 are heavily doped with donor impurities,
such as sulfur or tin, to a concentration of approximately 1E18/cm.sup.3
--i.e., 1.times.10.sup.18 per cubic centimeter. On the top surface of the
optical cladding layer 12 is located the active region 13 having a
thickness of about 0.5 .mu.m. The doping level in the bottom cladding
layer 12 is made suitable for that of a laser, typically approximately
1E18/cm.sup.3.
The top cladding layer 14 illustratively has a thickness and an impurity
doping level suitable for a laser, for example, a doping level of about
5E18/cm.sup.3, and may be graded. The doping level in the Al.sub.y
In.sub.1-y As layers in the top mirror stack 15 is illustratively about
5E18/cm.sup.3 ; that of the InP layers therein is also about
5E18/cm.sup.3. The top electrode 16 is made of a material, such as gold or
silver, which acts as an ohmic contact, and also acts as a mirror to
enhance the reflection by the top mirror stack 15, if need be. The bottom
aperture 8 in the bottom mirror 9 enables exit of light from the laser
100.
Instead of lattice-matched InGaAs, the active layer 13 can be made of
alternating ("multiple") quantum well layers of InP and lattice-matched
InGaAs. In such a case the thickness of each of the quantum well InP
layers is approximately 5 to 50 nm, and the thickness of each of the
quantum well InGaAs layers is in the approximate range of 1 to 20 nm.
Typically, there are from 1 to 20 spatial periods in such an active layer
formed by multiple quantum well layers.
Fabrication of the layers 11, 12, 13, 14, and 15 can be accomplished by
such known methods as metal organic vapor phase epitaxy (MOVPE), also
known as metal organic chemical vapor deposition (MOCVD), or by molecular
beam epitaxy (MBE), or by hydride vapor phase epitaxy (HVPE). If needed,
added annealing may be used to reduce the band edge discontinuity in the
top mirror stack 15 further. The ohmic contact layer 9 can be fabricated
by such known techniques as evaporation followed by photolithography to
achieve a desirable annular shape, for example. The metal mirror contact
16 can be fabricated by such known techniques as evaporation of the mirror
metal, followed by masking and lift-off or etching as known in the art.
It should be understood that many lasers can be fabricated on a single
substrate by first forming all the semiconductor and metal layers all the
way across the surfaces of the InP body 10, then forming by etching the
individual metal mirror contacts 16 (one for each laser), thereafter
etching the apertures 8 in the ohmic contacts 9, and then further masking
and etching (or cleaving) apart the individual lasers. Alternatively, many
lasers can remain integrated in an array on the same body 10 and at the
same time can be mutually isolated by trench or mesa isolation techniques,
as known in the art.
Although the invention has been described in detail with reference to a
specific embodiment, various modifications can be made without departing
from the scope of the invention. For example, in order to increase the
energy (decrease the wavelength) per photon of the emitted light, the
active region can contain phosphorus in an amount to form lattice-matched
quaternary InGaAsP. As a further option, the active layer 13 could be GaAs
in combination with a top mirror composed of alternating layers of AlAs
and In.sub.z Ga.sub.1-z P with z approximately equal to 0.55,
corresponding to a lattice mismatch of about 0.36 percent.
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