Title: Semiconductor laser with a lattice structure
Abstract: A semiconductor laser (22) with a semiconductor substrate (11), a laser layer (13) arranged on the semiconductor substrate, a waveguide ridge (15) arranged at a distance from the laser layer, and a strip-shaped lattice structure (23) arranged in parallel to the laser layer is disclosed. The lattice structure (23) includes two structural regions (24, 25) which are arranged on both sides of the waveguide ridge (15) and are formed at a distance from the laser layer (13) above the laser layer (13). A process for the production of such a semiconductor laser is also disclosed.
Patent Number: 6,846,689 Issued on 01/25/2005 to Forchel, et al.
| Inventors:
|
Forchel; Alfred (Abtsleitenweg 25a, D-97082 Wuerzburg, DE);
Kamp; Martin (Marienberg, D-97082 Wuerzburg, DE)
|
| Appl. No.:
|
677903 |
| Filed:
|
October 2, 2003 |
Foreign Application Priority Data
| Aug 31, 1998[DE] | 298 15 522 |
| Current U.S. Class: |
438/31; 372/49.01; 438/32; 438/39 |
| Intern'l Class: |
H01L 021/00 |
| Field of Search: |
438/31-32,39
372/49
|
References Cited [Referenced By]
Other References
High Performance laterally gain coupled InGaAs / AlGaAs DFB lasers, Kamp et
al., May 1998.
CW Performance of an InGaAs-GaAs-AlGaAs Laterally-Coupled Distributed
Feedback (LC-DFB) Ridge Laser Diode, Martin et al., Mar. 1995.
"Low-threshold high-quantum-efficiency laterally gain-coupled InGaAs/AlGaAs
distributed feedback lasers" , Kamp et al., Jan. 25, 1999.
"Low threshold II-VI laser diodes with transversal and longitudinal
single-mode emission", Legge et al., 2000.
"1.3-MUM wavelength INP laterally coupled distributed feedback ridge
laser", Chen et al., 1997.
|
Primary Examiner: Fourson; Goerge
Assistant Examiner: Pham; Thanh V
Attorney, Agent or Firm: McGlew and Tuttle, P.C.
Parent Case Text
RELATED APPLICATIONS
This is a Divisional Application of application Ser. No. 09/296,059 filed
Apr. 21, 1999 now U.S. Pat. No. 6,671,306, and the entire disclosure of
this prior application is considered to be part of the disclosure of the
accompanying application and is hereby incorporated by reference therein.
Claims
What is claimed is:
1. A process for production of a semiconductor laser based on a
semiconductor substrate with a laser layer arranged on said semiconductor
substrate and a strip-shaped lattice/grating structure, the process
comprising the steps of:
producing a complete semiconductor laser structure in an epitaxial process;
forming a waveguide ridge by subjecting said semiconductor laser structure
to a material removal process to form carrier surfaces arranged on both
sides of a waveguide ridge; and
applying the lattice/grating structure to said carrier surfaces.
2. The process according to claim 1, wherein before applying the
lattice/grating structure to said carrier surfaces an insulating layer is
formed on the carrier surfaces.
3. The process according to claim 1, wherein said step of applying the
lattice/grating structure includes applying a metallic lattice/grating
structure with a lithographic process, said lithographic process being
employed followed by metallization of said lithographic structure.
4. The process according to claim 3, wherein a plurality of semiconductor
lasers are produced in a composite wafer in according to the steps
comprising:
producing a semiconductor laser wafer by application of an epitaxial
structure to a semiconductor substrate;
forming the ridge-like waveguide structure comprising waveguide ridges
extending in parallel to one another on said surface of said semiconductor
laser wafer;
dividing said semiconductor laser wafer into individual semiconductor laser
chip units;
forming one or more lattice/grating structures on said semiconductor laser
chip units.
Description
FIELD OF THE INVENTION
The present invention relates to a semiconductor laser with a semiconductor
substrate, a laser layer arranged on the semiconductor substrate, a
waveguide ridge arranged at a distance from the laser layer, and a
strip-shaped lattice structure arranged in parallel to the laser layer.
The present invention further relates to a process for the production of
such a semiconductor laser.
BACKGROUND OF THE INVENTION
Known semiconductor lasers of the type defined in the introduction, also
referred to in the art as so-called DFB (distributed feedback) laser
diodes, have a lattice structure which extends through the laser layer and
which facilitates the construction of a monomode laser diode in which, in
contrast to multi-mode laser diodes, laser radiation with one specified
laser mode is emitted and other modes are suppressed by the lattice
structure. The production of the DFB laser diodes constructed in the known
manner proves extremely costly, in particular due to the production and
test process employed and the high reject quota associated therewith. For
the production of the known DFB laser diodes on the basis of a wafer on a
semiconductor substrate base, epitaxy is used to form the structure of the
semiconductor wafer on the semiconductor substrate. For the formation of
the lattice structure in the laser layer, when approximately half the
layer height of the epitaxial structure has been reached the epitaxial
growth is interrupted and the lattice structure is introduced in a
lithographic--and removal process. Then the epitaxial growth is continued.
The interruption of the epitaxy in the formation of the laser layer and
the following overgrowth of the lattice structure introduced into the
half-layer induces defects in the laser layer which disadvantageously
affect the properties of the laser layers and possibly manifest in a
higher current consumption or a reduced life of the laser diodes.
As a result of the mutual influences between the laser layer and the
lattice structure formed in the laser layer in terms of the amplification
properties of the semiconductor laser wafer, the properties of a
semiconductor laser wafer produced in the described way cannot be
predetermined in an exact manner. As the properties of the semiconductor
laser wafer cannot be determined until after the conclusion of the
epitaxial growth and the complete formation of the laser layer in the test
operation, the amplification spectrum of the semiconductor laser wafer
also cannot be determined until after the formation of the lattice
structure in the laser layer, with the result that the lattice structure
cannot be accurately adapted to the amplification spectrum of the laser
layers and consequently the known DFB laser diodes also cannot be produced
in a precise manner in accordance with predefined specifications relating
to the desired laser mode or the desired wavelength. Rather, the structure
of the known DFB laser diodes described in the foregoing requires a
production process in which different lattice structures must be formed in
the laser layer of a semiconductor laser wafer in order that, by checking
the laser diodes separated from the semiconductor laser wafer, precisely
those laser diodes which emit the desired laser mode with the desired
wavelength can be retrospectively determined. It is thus apparent that the
structural design of the known DFB laser diodes necessitates the
production of a plurality of laser diodes in order that the laser diodes
suitable for the intended application, i.e. those laser diodes which emit
a laser radiation with the desired wavelength, can be selected from this
plurality of laser diodes by testing of their laser properties.
SUMMARY AND OBJECTS OF THE INVENTION
The primary object of the present invention is to propose a laser diode
with a structure which facilitates a simple and reproducible manufacture
of laser diodes with a defined wavelength. It is also an object of the
present invention to propose a DFB laser diode with improved power output.
A further object of the present invention is to propose a process
particularly suitable for the production of a DFB laser diode according to
the invention.
According to the invention, a semiconductor laser is provided with a
semiconductor substrate. A laser layer is arranged on the semiconductor
substrate. A waveguide ridge is arranged at a distance from the laser
layer, and a strip-shaped lattice or grating structure is arranged in
parallel to the laser layer. The lattice structure includes two structural
regions which are arranged on both sides of the waveguide ridge and are
formed at a distance from the laser layer above the laser layer.
An embodiment of the invention provides a DFB laser diode with a lattice
structure produced following the conclusion of the epitaxial growth of the
laser layer for completion of the semiconductor laser wafer and following
the formation of the waveguide ridge. By virtue of this structurally
required, subsequent production of the lattice structure it is possible to
determine the individual amplification spectrum of the laser layer and
semiconductor laser wafer before the production of the lattice structure
in order then, by selective predefinitions of the parameters of the
lattice structure, to be able to subsequently produce the desired laser
profile in an exact manner and thus to be able to reproducibly manufacture
DFB laser diodes with a precisely defined wavelength or laser mode.
The structural design according to the invention also facilitates an
undisturbed, continuous formation of the laser layer in the epitaxial
process so that unnecessary defects, which can impair the power output
characteristic of the laser layer or the DFB laser diode, do not arise at
all. The arrangement of the lattice structure at a distance from the
active laser layer also prevents the subsequent impairment of the laser
layer. The lattice structure modulates periodically the losses and the
refractive power for light propagating through the laser. In this way the
DFB laser diode according to the invention facilitates a complex coupling
of the laser radiation with the lattice structure with lateral modulation
of the real--and imaginary parts of the refractive index. Laser diodes
according to the invention therefore have a high degree of insensitivity
to back-reflections, which enables them to be used without an optical
isolator, for example in applications for glass fiber transmission.
To permit the precisest possible setting of the distance or relative
position between the lattice structure and the active laser layer of the
DFB laser diode in the production of the DFB laser diode, the lattice
structure can be arranged on a barrier layer arranged in parallel to the
laser layer.
If a metal, for example chromium, aluminum, scandium, titanium, vanadium,
chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium,
zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver,
cadmium, tin, hafnium, tantalum, tungsten, rhenium, osmium, iridium,
platinum, gold, thallium, lead, bismuth, lanthanum, cerium, praseodymium,
neodymium, promethium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, lutetium and alloys
thereof is used to form the lattice structure, the advantageous effects
described in the foregoing can be achieved to a particularly comprehensive
extent. Irrespectively of the material selected to construct the lattice
structure, the lattice structure can also be formed by material removal,
thus not only by material application.
It proves particularly advantageous for the structural regions of the
lattice structure to be arranged adjacent to sides of the waveguide ridge
and for the width of the waveguide ridge to be dimensioned such that base
points of the sides are located in the peripheral region of the radiation
emitted from the active zone of the laser layer. This ensures that the
amplification power of the laser is influenced to the least extent
possible by the lattice structure and in particular ensures effective
coupling between the laser radiation and the lattice structure.
In order to improve the electrical injection the metallic lattice structure
is placed on a thin insulator layer (e.g. native or artificial oxide). The
insulator layer should be chosen in such a way that the refractive index
is closer to that of the semiconductor and that the insulation is obtained
a small thickness (typically a few nanometers). This layer is also used to
suppress a potential penetration of the grating material into the
semiconductor material of the laser layers and therefore serves as a
barrier layer too.
For the optimization of the electrical injection surface and the effects of
the lattice structure, it also proves advantageous for the sides of the
waveguide ridge to be arranged substantially at right angles to the plane
in which the lattice structure extends, within the accuracy attainable by
the manufacturing process.
In a process according to the invention, on the basis of a semiconductor
substrate a complete semiconductor laser structure is produced in an
epitaxial process with the subsequent formation of a waveguide ridge by
subjecting the semiconductor laser structure to a material removal process
to form carrier surfaces arranged on both sides of the waveguide ridge and
subsequent application of a lattice structure to the carrier surfaces.
Irrespectively of the material selected to form the lattice structure, the
lattice structure can also be produced by material removal, thus not only
by material application.
The processes according to the invention described in the foregoing thus
facilitate the production of functional laser diodes in a first process
phase, thereby facilitating the precise checking and determination of the
electrical and optical properties, thus for example determination of the
individual amplification spectrum of the semiconductor wafer used for the
laser fabrication. Only thereafter in a second process phase, by the
formation of lattice structures alongside the waveguide ridge with defined
parameters, are the originally multi-mode laser diodes converted into
monomode DFB laser diodes with properties in each case defined as a
function of the parameters of the lattice structures.
In the event that the lattice structure is produced by the application of a
lattice structure to the carrier surfaces, the use of a lithographic
process, in particular the use of an electron beam lithographic process
with subsequent metallization of the lithographic structure, proves
particularly advantageous.
A variant of the production process which is particularly advantageous from
the economic standpoint is possible if, for the production of a plurality
of DFB laser diodes with different properties, a semiconductor laser wafer
is firstly produced by applying an epitaxial structure to a semiconductor
substrate, whereupon the waveguide ridges associated with the individual
laser diodes are produced in the composite wafer by forming a strip-shaped
waveguide structure which is arranged on the surface of the semiconductor
laser wafer and which comprises waveguide ridges extending in parallel to
one another and interlying carrier surfaces. Only thereafter is the
semiconductor laser wafer divided up into separate semiconductor laser
chip units, whereby the properties associated with the individual laser
diodes are then precisely defined by the application or implantation of a
lattice structure with corresponding structural parameters on the surface
of a selected number of the laser diodes.
It is thus possible for the laser diodes which have been produced in the
composite wafer and are already provided with the waveguide ridge to be
used as basic laser diodes or "unfinished" laser diodes with defined
electrical and optical properties whereupon, from this reservoir of
identically formed basic laser diodes, the required number of laser diodes
can then be selected and, by the application or implantation of defined
lattice structures, the desired number of monomode DFB laser diodes with
precisely defined optical and electrical properties can be produced
substantially without rejects.
In the following the construction of an embodiment of a DFB laser diode
according to the invention and a possible process for the production
thereof will be explained in detail making reference to the drawing.
The various features of novelty which characterize the invention are
pointed out with particularity in the claims annexed to and forming a part
of this disclosure. For a better understanding of the invention, its
operating advantages and specific objects attained by its uses, reference
is made to the accompanying drawings and descriptive matter in which
preferred embodiments of the invention are illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1a is a view illustrating a stage in the production of a DFB laser
diode with a lateral lattice structure;
FIG. 1b is a view illustrating a different stage in the production of a DFB
laser diode with a lateral lattice structure;
FIG. 1c is a view illustrating a different stage in the production of a DFB
laser diode with a lateral lattice structure;
FIG. 2 is a scanning electron microscope image of a plan view of the
lattice structure arranged on both sides of a waveguide;
FIG. 3 is a simplified perspective view of a laser diode illustrating the
active zone of a laser layer;
FIG. 4 is a diagram illustrating a possible amplification spectrum of the
laser diode shown in FIG. 3;
FIG. 5 is a graphic representation of the dependence between the
amplification spectrum or wavelength of the radiation emitted from the
laser diode and the lattice constant of the lattice structure;
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings in particular, FIG. 1a is a simplified
perspective view of a semiconductor laser or basic laser diode 10
comprising a semiconductor substrate 11 and an epitaxial structure 12
grown thereon. Part of the epitaxial structure 12 is formed by a laser
layer 13 based on a buffer and contact layer 31 and covered at the top by
a covering layer 14.
The basic laser diode 10 shown in FIG. 1a is of cuboid formation with a
flat diode surface 16. Commencing from the basic laser diode 10
illustrated in FIG. 1a, the embodiment of a DFB laser diode 22 according
to the invention shown in FIG. 1c is produced in two essential process
phases; as a transitional stage following the implementation of a first
process phase, FIG. 1b shows a waveguide diode 17 in which the diode
surface 16 has been subjected to a material removal process, such as for
example a dry etching process, in order to obtain the illustrated stepped
surface formation with a waveguide ridge 15 extending in the longitudinal
direction of the waveguide diode 17. The aforementioned material removal
process gives rise to surfaces which are formed on both sides of sides 18,
19 of the waveguide ridge 15 and which will be referred to in the
following as carrier surfaces 20 and 21, the carrier surfaces being
covered by a thin insulating layer 26.
Commencing from the waveguide diode 17 illustrated in FIG. 1b, the
embodiment of a DFB laser diode 22 shown in FIG. 1c is produced by forming
a metallic lattice structure 23 with two structural regions 24 and 25 in
each case arranged in the carrier surfaces 20 and 21 respectively by
subjecting the carrier surfaces 20 and 21 to an electron beam lithographic
process and a following metallization process not described in detail
here. This second process phase results in the DFB laser diode 22
illustrated in FIG. 1c with the metallic lattice structure 23 arranged in
the carrier surfaces 20 and 21 above the laser layer 13. To be able to
precisely define the position of the structural regions 24 and 25 of the
metallic lattice structure 23 arranged on both sides of the waveguide
ridge 15 in the epitaxial structure 12 relative to the laser layer 13, the
insulating layer 26, for example in the form of an etch-stop layer, is
provided in the epitaxial structure above the laser layer 13, which
insulating layer 26 limits the depth, in the epitaxial structure 12, of a
lithographic structure produced using an etching process and thereby
defines the position of the metallic lattice structure 23 relative to the
laser layer 13.
As shown by the electron microscope image of a plan view of the metallic
lattice structure 23 schematically illustrated in FIG. 1c, the structural
regions 24 and 25 constructed from lattice ridges 27, here arranged
equidistantly from one another, extend up to the sides 18 and 19 of the
waveguide 15. The characteristic of the metallic lattice structure 23 is
determined by the distance between the lattice ridges 27 or the lattice
constant d, the geometric configuration of the lattice ridges 27, and the
metal used for the metallic lattice structure 23.
FIG. 3 is a qualitative illustration of the active zone 28 of the laser
layer 13 in the form of an intensity distribution depicted in the outlet
cross-section of the laser diode 22. It will be apparent that, since the
individual lattice ridges 27 are connected as directly as possible to the
sides 18, 19, in particular in the region of base points 29, 30 of the
sides 18, 19 a coupling is advantageously achieved between the metallic
lattice structure 23 and the laser radiation in its peripheral zone.
FIG. 4 clarifies the filter effect obtained by means of the metallic
lattice structure 23 in that, as can be seen from FIG. 4, subsidiary modes
of the laser radiation emitted from the active zone 28 are effectively
suppressed and substantially only the emission of one laser mode with a
precisely defined wavelength is permitted.
FIG. 5 illustrates the effects of changes in the lattice constant d (FIG.
1c) upon the wavelength. It will be apparent from FIG. 5 that by changing
the lattice constant d it is possible to achieve a highly accurate fine
adjustment of the wavelength so that, commencing from a predetermined
individual amplification spectrum of a basic laser diode 10 illustrated by
way of example in FIG. 1a, by purposively selecting the parameters of the
lattice structure, thus for example here the lattice constant, a highly
accurate setting of the wavelength can be obtained for the intended
application of the laser diode in question. For example, by means of the
choice of metal for the metallic lattice structure 23, the complex
coupling between the laser radiation and the lattice structure, thus for
example the absolute value and the relative ratio of the real--and
imaginary parts of the refractive index--and thus the component of
refractive index coupling and absorption coupling--can be set within wide
limits. The so-called "duty factor" of the lattice structure, thus the
ratio of the lattice ridge width to the lattice constant, also constitutes
a variable which can be optimized.
While specific embodiments of the invention have been shown and described
in detail to illustrate the application of the principles of the
invention, it will be understood that the invention may be embodied
otherwise without departing from such principles.
*
