U.S. patent application number 14/807504 was filed with the patent office on 2015-12-24 for two-dimensional photonic crystal laser and method of producing the same.
The applicant listed for this patent is KYOTO UNIVERSITY, ROHM CO., LTD.. Invention is credited to Wataru KUNISHI, Yoshikatsu MIURA, Eiji MIYAI, Kazuya NAGASE, Susumu NODA, Dai OHNISHI, Takui SAKAGUCHI.
Application Number | 20150372452 14/807504 |
Document ID | / |
Family ID | 45526667 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150372452 |
Kind Code |
A1 |
NODA; Susumu ; et
al. |
December 24, 2015 |
TWO-DIMENSIONAL PHOTONIC CRYSTAL LASER AND METHOD OF PRODUCING THE
SAME
Abstract
A two-dimensional photonic crystal laser according to the
present invention includes a two-dimensional photonic crystal layer
15 having a base body made of Al.sub..alpha.Ga.sub.1-.alpha.As
(0<.alpha.<1) or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1) with modified refractive
index areas (air holes) 151 periodically arranged therein and an
epitaxial growth layer 16 created on the two-dimensional photonic
crystal layer 15 by an epitaxial method. Since
Al.sub..alpha.Ga.sub.1-.alpha.As and
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P are
solid even at high temperatures, the air holes 151 will not be
deformed in the process of creating the epitaxial growth layer 16,
so that the performance of the two-dimensional photonic crystal
layer 15 as a resonator can be maintained at high levels.
Inventors: |
NODA; Susumu; (Kyoto-shi,
JP) ; SAKAGUCHI; Takui; (Kyoto-shi, JP) ;
NAGASE; Kazuya; (Kawanishi-shi, JP) ; KUNISHI;
Wataru; (Kyoto-shi, JP) ; MIYAI; Eiji;
(Kyoto-shi, JP) ; MIURA; Yoshikatsu; (Kyoto-shi,
JP) ; OHNISHI; Dai; (Otsu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KYOTO UNIVERSITY
ROHM CO., LTD. |
Kyoto-shi
Kyoto-shi |
|
JP
JP |
|
|
Family ID: |
45526667 |
Appl. No.: |
14/807504 |
Filed: |
July 23, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13192852 |
Jul 28, 2011 |
9130348 |
|
|
14807504 |
|
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Current U.S.
Class: |
438/32 |
Current CPC
Class: |
H01S 5/18316 20130101;
H01S 5/3013 20130101; H01S 5/1231 20130101; H01S 5/18319 20130101;
H01S 5/187 20130101; H01S 5/105 20130101; H01S 2301/17
20130101 |
International
Class: |
H01S 5/10 20060101
H01S005/10; H01S 5/187 20060101 H01S005/187; H01S 5/12 20060101
H01S005/12; H01S 5/30 20060101 H01S005/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 30, 2010 |
JP |
2010-171933 |
Jul 30, 2010 |
JP |
2010-171934 |
Jul 30, 2010 |
JP |
2010-171935 |
Claims
1. A method of producing a two-dimensional photonic crystal laser,
comprising: a) a base-body layer creation process for creating a
base-body layer having a same crystal structure as
Al.sub.xGa.sub.1-xAs (0.4<=x<1); b) an air-hole formation
process for periodically forming air holes in the base-body layer,
each of the air holes having a maximum width d in planer shape and
a depth h, where d satisfies d<=200 nm and a depth-to-width
ratio h/d satisfies 1.3<=h/d<=5; and c) an epitaxial-layer
creation process for creating a layer made of the aforementioned
Al.sub.xGa.sub.1-xAs on the base-body layer and the air holes by an
epitaxial method.
2. The method of producing a two-dimensional photonic crystal laser
according to claim 1, wherein the base-body layer is a layer
created by epitaxially growing Al.sub..alpha.Ga.sub.1-.alpha.As
(0<.alpha.<1) or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1).
3. The method of producing a two-dimensional photonic crystal laser
according to claim 1, wherein the base-body layer has a multi-layer
structure having a plurality of layers.
4. The method of producing a two-dimensional photonic crystal laser
according to claim 1, wherein a process for forming a
crystal-growth inhibiting film for inhibiting a crystal growth of
the aforementioned Al.sub.xGa.sub.1-xAs on at least a portion of an
inner surface of the air holes is included between the air-hole
formation process and the epitaxial-layer creation process.
5. The method of producing a two-dimensional photonic crystal laser
according to claim 4, wherein the crystal-growth inhibiting film is
made of a material selected from a group of SiO.sub.2,
Si.sub.3N.sub.4, ZnO and ZrO.sub.2.
6. The method of producing a two-dimensional photonic crystal laser
according to claim 1, wherein the air hole has an elliptic planer
shape with a major diameter being directed in a growth direction of
Al.sub.xGa.sub.1-xAs within a plane parallel to the base-body
layer.
7. The method of producing a two-dimensional photonic crystal laser
according to claim 1, wherein a planer shape of the air hole is a
polygon with a groove-like projected portion extending outward from
each vertex of the polygon.
8. A method of producing a two-dimensional photonic crystal,
comprising: a) a base-body layer creation process for creating a
base-body layer having a same crystal structure as
Al.sub.xGa.sub.1-xAs (0<x<=0.8); b) an air-hole formation
process for periodically forming air holes in the base-body layer,
each of the air holes having a maximum width d in planer shape and
a depth h, where d satisfies d<=200 nm and a depth-to-width
ratio h/d satisfies 0.1<=h/d<=1.2; c) a modified refractive
index area formation process for forming, by an epitaxial method,
modified refractive index areas made of the aforementioned
Al.sub.xGa.sub.1-xAs in the air holes; and d) an epitaxial-layer
creation process for creating, by the aforementioned epitaxial
method, a layer made of Al.sub.yGa.sub.1-yAs (0<=y<=1) on the
base-body layer having the modified refractive index areas formed
therein.
9. The method of producing a two-dimensional photonic crystal laser
according to claim 8, wherein x equals y.
10. The method of producing a two-dimensional photonic crystal
laser according to claim 9, wherein the modified refractive index
area formation process and the epitaxial-layer creation process are
simultaneously performed.
11. The method of producing a two-dimensional photonic crystal
laser according to claim 8, wherein a process for forming a
crystal-growth inhibiting film for inhibiting an epitaxial growth
of the aforementioned Al.sub.xGa.sub.1-xAs on the base-body layer
is included between the base-body layer creation process and the
modified refractive index area formation process.
12. The method of producing a two-dimensional photonic crystal
laser according to claim 11, wherein the crystal-growth inhibiting
film is made of a material selected from a group of SiO.sub.2,
Si.sub.3N.sub.4, ZnO and ZrO.sub.2.
13. The method of producing a two-dimensional photonic crystal
laser according to claim 8, wherein the base-body layer is a layer
created by epitaxially growing Al.sub..alpha.Ga.sub.1-.alpha.As
(0<.alpha.<1) or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1).
14. The method of producing a two-dimensional photonic crystal
laser according to claim 8, wherein the base-body layer has a
multi-layer structure having a plurality of layers.
15. A method of producing a two-dimensional photonic crystal laser,
comprising: a) a modified refractive index area formation process
for periodically forming columnar modified refractive index areas
on an epitaxial-growth substrate layer having a same crystal
structure as Al.sub.xGa.sub.1-xAs (0<x<=0.65), the modified
refractive index areas being made of a material whose refractive
index differs from that of the aforementioned Al.sub.xGa.sub.1-xAs;
b) a base-body creation process for creating, by an epitaxial
method, a base body made of the aforementioned Al.sub.xGa.sub.1-xAs
in a space between the modified refractive index areas; and c) an
epitaxial-layer creation process for creating, by the
aforementioned epitaxial method, a layer made of
Al.sub.yGa.sub.1-yAs (0<=y<=1) on a layer in which the
modified refractive index areas and the base body have been
formed.
16. The method of producing a two-dimensional photonic crystal
laser according to claim 15, wherein the modified refractive index
areas are formed by etching the epitaxial-growth substrate layer in
such a manner that a portion thereof in a thickness direction and
the modified refractive index areas are left.
17. The method of producing a two-dimensional photonic crystal
laser according to claim 15, wherein x equals y.
18. The method of producing a two-dimensional photonic crystal
laser according to claim 17, wherein the base-body creation process
and the epitaxial-layer creation process are simultaneously
performed.
19. The method of producing a two-dimensional photonic crystal
laser according to claim 15, wherein a process for forming a
crystal-growth inhibiting film for inhibiting an epitaxial growth
of the aforementioned Al.sub.xGa.sub.1-xAs on a top face of the
modified refractive index areas is included between the modified
refractive index area formation process and the base-body layer
creation process.
20. The method of producing a two-dimensional photonic crystal
laser according to claim 19, wherein the crystal-growth inhibiting
film is made of a material selected from a group of SiO.sub.2,
Si.sub.3N.sub.4, ZnO and ZrO.sub.2.
21. The method of producing a two-dimensional photonic crystal
laser according to claim 15, wherein the modified refractive index
areas are made of a material selected from a group of SiO.sub.2,
Si.sub.3N.sub.4, ZnO and ZrO.sub.2.
22. The method of producing a two-dimensional photonic crystal
laser according to claim 15, wherein the epitaxial-growth substrate
layer is a layer created by epitaxially growing
Al.sub..alpha.Ga.sub.1-.alpha.As (0<=.alpha.<=1) or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<=1, 0<=.gamma.<=1).
Description
[0001] The present application is a divisional application of U.S.
patent application Ser. No. 13/192,852, filed Jul. 28, 2011, which
claims foreign priority to each of JP 2010-171933, JP 2010-171934
and JP 2010-171935, all filed on Jul. 30, 2010. The disclosures of
each of the above applications are hereby incorporated by reference
in their entireties.
TECHNICAL FIELD
[0002] The present invention relates to a two-dimensional photonic
crystal laser having a structure suitable for a production process
using an epitaxial method, as well as a method of producing such a
laser.
BACKGROUND ART
[0003] In recent years, new types of lasers using a two-dimensional
photonic crystal have been developed. A two-dimensional photonic
crystal consists of a plate-shaped dielectric base body with a
periodic structure of refractive index formed therein. Typically,
this device is created by providing the base body with a periodic
arrangement of areas whose refractive index differs from that of
the base body. (This area is hereinafter called the "modified
refractive index area.") This periodic structure causes a Bragg
diffraction within the crystal and creates an energy band gap for
the energy of light. There are two types of two-dimensional
photonic crystal lasers: one type utilizes a band-gap effect to
make a point defect function as a resonator, and the other type
utilizes a standing wave at a band edge where the group velocity of
light becomes zero. Each of these devices causes laser oscillation
by amplifying light of a predetermined wavelength.
[0004] The latter type of the two-dimensional photonic crystal
laser utilizing a standing wave has a layered structure in which a
layer having a two-dimensional photonic crystal structure (this
layer is hereinafter called the "two-dimensional photonic crystal
layer") is stacked on an active layer either directly or via
another layer. These layers are sandwiched by other layers, such as
a cladding layer for injecting electric charges into the active
layer, a contact layer to be in contact with an external element,
and a spacer layer for connecting these layers.
[0005] Patent Document 1 discloses a method of creating a
two-dimensional photonic crystal laser by a process including the
following steps: a structure including a cladding layer, spacer
layer and other layers is prepared; this structure is stacked on a
two-dimensional photonic crystal layer consisting of a base body
with air holes (modified refractive index areas) periodically
arranged therein, with the spacer layer being in contact with the
two-dimensional photonic crystal layer; and the two-dimensional
photonic crystal layer and the spacer layer are fused together by
heat (thermal fusion bonding). In one example disclosed in Patent
Document 1, both the base body of the two-dimensional photonic
crystal layer and the spacer layer are made of n-type InP, and the
heating temperature is 620.degree. C. The layer stacked on the
two-dimensional photonic crystal layer is hereinafter referred to
as the "upper layer."
[0006] Patent Document 2 discloses a method of creating an upper
layer by epitaxially growing AlGaN directly on a two-dimensional
photonic crystal layer consisting of a base body made of GaN with
air holes periodically formed therein.
[0007] The methods described in Patent Document 2 can be broadly
classified into the following three types: (i) A method in which a
two-dimensional photonic crystal layer consisting of a base body
with air holes as the modified refractive index areas periodically
arranged therein is created, and then an upper layer is formed
without filling the air holes; (ii) a method in which a layer
including a base body with air holes periodically arranged therein
is created, and then an upper layer is formed while filling the air
holes to form modified refractive index areas; and (iii) a method
in which modified refractive index areas in the form of columns are
formed on a substrate, and then the spaces around them are filled
by epitaxial growth to continuously form the base body and the
upper layer.
[0008] In the case of the methods (ii) and (iii), the modified
refractive index areas are made of a material other than air (more
specifically, the same material as that of the upper layer). Such a
structure has a lower light-confining effect as compared to the
structure using air holes as the modified refractive index areas.
However, the former structure is advantageous in that a single-mode
laser oscillation can be more easily generated over a large
area.
BACKGROUND ART DOCUMENT
Patent Document
[0009] Patent Document 1: JP-A 2000-332351
[0010] Patent Document 2: WO-A1 2006/062084
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0011] In the method described in Patent Document 1, the electric
resistance at the interface between the two-dimensional photonic
crystal layer and the upper layer increases due to an interface
state at the fused faces of these layers. This increases the
operating voltage and impedes the continuous oscillation of the
laser. Furthermore, the thermal fusion bonding process may deform
air holes and thereby deteriorate the performance of the
two-dimensional photonic crystal layer as a resonator.
[0012] The methods described in Patent Document 2 are free from the
problem of the increase in the electric resistance at the interface
between the two-dimensional photonic crystal layer and the upper
layer. However, in epitaxially growing the upper layer, the
two-dimensional photonic crystal layer needs to be heated to
600.degree. C. This urges atomic migrations and disturbs the atomic
crystal structure in the two-dimensional photonic crystal layer,
which results in a deformation of the holes. Such atomic migrations
also disturbs the atomic arrays (or crystal structure) at the
surface so that the upper layer epitaxially growing on the surface
inherits disturbances in the atomic crystal structures.
[0013] Further, the method (i) has the problem that the material of
the upper layer intrudes deeply into the air holes during the
epitaxial growth of the upper layer, disfiguring the whole shape of
the air holes. In the case of the methods (ii) and (iii), on the
contrary, it is difficult to completely fill the air holes or the
spaces between the columnar modified refractive index areas with
the material of the upper layer, so that voids are likely to be
formed. In any of these cases, the periodic structure of the
refractive index becomes imperfect, which lowers the performance of
the two-dimensional photonic crystal layer as a resonator and hence
deteriorates the laser characteristics of the two-dimensional
photonic crystal laser.
[0014] One of the problem solved by the present invention is,
therefore, to provide a two-dimensional photonic crystal laser in
which the two-dimensional photonic crystal layer is robust against
high-temperature and adequately produces a sound upper epitaxial
layer. Another problem solved by the present invention is to
provide a method of producing a two-dimensional photonic crystal
laser in which the epitaxial growth on the two-dimensional photonic
crystal layer can be adequately controlled so that: in the case of
producing the upper layer on a two-dimensional photonic crystal
including holes, as in the above method (i), the upper layer
material is prevented from intruding to the bottom of the holes,
minimizing the disfigure of holes; in the case of producing a
two-dimensional photonic crystal with modified refractive index
areas made of the same material as the upper layer, as in the above
method (ii), the holes in the base-body layer can be filled with
the material of the upper layer with less voids; and in the case of
producing a two-dimensional photonic crystal with a base-body layer
made of the same material as the upper layer, as in the above
method (iii), the base-body layer can be made with less voids.
Means for Solving the Problems
(1) Means for Solving the First Problem
[0015] A two-dimensional photonic crystal laser according to the
first aspect of the present invention aimed at solving the
aforementioned first problem includes:
[0016] a two-dimensional photonic crystal layer having a base-body
layer made of Al.sub..alpha.Ga.sub.1-.alpha.As (0<.alpha.<1)
or (Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1) with modified refractive
index areas periodically arranged therein; and
[0017] an epitaxial growth layer created on the two-dimensional
photonic crystal layer by an epitaxial method.
[0018] It should be noted that the terms "upper" and "lower" used
in the present application merely indicate the positional
relationship between layers and should not be interpreted as
defining the direction of the layers with respect to the gravity
during the manufacturing process or the direction of the completed
two-dimensional photonic crystal laser with respect to the
gravity.
[0019] As described before, in order to create an upper layer on
the two-dimensional photonic crystal layer by an epitaxial method,
it is necessary to heat the two-dimensional photonic crystal layer
to as high as approximately 600.degree. C. The material used in the
first aspect of the present invention, i.e.
Al.sub..alpha.Ga.sub.1-.alpha.As or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P, is
solid even at such high temperatures and atomic migrations hardly
occur. Therefore, in the case of epitaxially growing an upper layer
on a base-body layer having a hole or holes (as in the case of
two-dimensional photonic crystal with air holes), the air hole or
air holes hardly deform in the epitaxially growing process. Also in
the case of forming a two-dimensional photonic crystal with the
modified refractive index areas made of the material of the upper
layer, the holes in the base-body layer hardly deform. In any case,
the performance of the two-dimensional photonic crystal layer as a
resonator can be maintained at high levels. Further, since the
atomic crystal structure of the base-body layer is less disturbed
due to atomic migrations at such high temperature, the upper layer
epitaxially growing on the base-body layer bears less disturbances
in the atomic crystal structures in the first aspect of the present
invention.
[0020] As the material for the epitaxial growth layer,
Al.sub.xGa.sub.1-xAs (0<x<1) is desirable. The gas-diffusion
length of Al.sub.xGa.sub.1-xAs varies according to the content of
Al, and its growth characteristic correspondingly changes.
Therefore, it is possible to maintain the performance of the
two-dimensional photonic crystal layer as a resonator at high
levels by optimizing the value of x according to the structure of
the lower layer, i.e. the photonic crystal layer.
[0021] The epitaxial growth layer can be used, without any change,
as a p-type or n-type cladding layer in a light-emitting diode
(LED). However, it is also possible to use the epitaxial growth
layer as a regrowth interface layer for separately forming a
cladding layer by epitaxial growth. When the parameter x is changed
according to the structure of the photonic crystal layer in the
aforementioned manner, if the epitaxial growth layer is used as the
cladding layer without any change, the composition of the cladding
layer will also change according to the structure of the photonic
crystal layer. The introduction of the regrowth interface layer
allows the creation of a cladding layer independently of the
structure of the photonic crystal layer. Therefore, the
two-dimensional photonic crystal laser can be produced with higher
degrees of freedom in its structure.
(2) Method for Solving the Second Problem
[0022] The second aspect of the present invention achieved to solve
the second problem described above has the following three modes.
Any of the three modes correspond to the two-dimensional photonic
crystal laser as the above-described first aspect wherein the base
body is made of Al.sub..alpha.Ga.sub.1-.alpha.As and the value of x
is limited to a specific range.
[0023] (2-1) First Mode of the Method of Producing Two-Dimensional
Photonic Crystal Laser According to the Second Aspect of the
Present Invention
[0024] The first mode of the method of producing a two-dimensional
photonic crystal laser according to the present invention
includes:
[0025] a) a base-body layer creation process for creating a
base-body layer having a same crystal structure as
Al.sub.xGa.sub.1-xAs (0.4<=x<1);
[0026] b) an air-hole formation process for periodically forming
air holes in the base-body layer, each of the air holes having a
maximum width d in planer shape and a depth h, where d satisfies
d<=200 nm and a depth-to-width ratio h/d satisfies
1.3<=h/d<=5; and
[0027] c) an epitaxial-layer creation process for creating a layer
made of the aforementioned Al.sub.xGa.sub.1-xAs on the base-body
layer and the air holes by an epitaxial method.
[0028] The "maximum width" in the present application is defined as
the length of the longest line segment that can be included in the
planer shape of the air hole (i.e. the sectional shape of the air
hole parallel to the surface of the base-body layer). For example,
when the planer shape of the air hole is circular, the diameter of
the circle corresponds to the maximum width. For an elliptic hole,
the major diameter of the ellipse corresponds to the maximum width.
For a triangular hole, the longest side of the triangle equals the
maximum width.
[0029] In the first-mode method, the vertical sectional shape (the
sectional shape vertical to the surface of the base-body layer) and
planer shape of the air holes in the two-dimensional photonic
crystal layer before epitaxial growth are determined based on the
growth characteristics of Al.sub.xGa.sub.1-xAs so as to make the
shape of the air holes after the regrowth as close to the desired
shape as possible and thereby maintain the performance of the
crystal layer as a photonic crystal at high levels. Specifically,
the ratio of the depth h to the maximum width d of the air hole,
h/d, (which is hereinafter referred to as the "aspect ratio") is
set to be 1.3 or greater. This setting is aimed at giving the air
holes an adequate depth so that a large quantity of the material
used for creating the epitaxial layer (upper layer) will not enter
to the bottom and fill the air holes in the epitaxial-layer
creation process. Setting the parameter x to be a large value equal
to or greater than 0.4 is also aimed at preventing the material of
the upper layer from entering the air holes. This is based on the
fact that the gas-diffusion length of the materials belonging to
the Al.sub.xGa.sub.1-xAs group decreases as the value of x
increases. The values of h/d and x are appropriately adjusted
within the aforementioned ranges so that the shape of the air hole
after the regrowth becomes as close to the desired shape as
possible. In this manner, a two-dimensional photonic crystal laser
with high laser characteristics can be created without
deteriorating the performance of the two-dimensional photonic
crystal layer as a resonator.
[0030] If the depth h is too large or the maximum width d is too
small, the two-dimensional periodic structure may become
insufficient. This situation can be avoided by setting an upper
limit of the aspect ratio h/d, which is 5 in the first-mode
method.
[0031] In the first-mode method, a process for forming a
crystal-growth inhibiting film for inhibiting an epitaxial growth
of Al.sub.xGa.sub.1-xAs on at least a portion of the inner surface
of the air holes may be added between the air-hole formation
process and the epitaxial-layer creation process. This film will
more assuredly inhibit the formation of the crystal of
Al.sub.xGa.sub.1-xAs inside the air holes. Examples of the
materials available for the crystal-growth inhibiting film include
silicon dioxide (SiO.sub.2), silicon nitride (Si.sub.3N.sub.4),
zinc oxide (ZnO) and zirconium dioxide (ZrO.sub.2).
[0032] When a film is formed by crystal growth, the film
anisotropically grows at different rates in the in-plane direction
(i.e. the direction parallel to the surface of the substrate)
depending on the direction of the flow of the material gas used in
the production process or other factors. Therefore, if the layer
above the base-body layer and the air holes is formed by an
epitaxial method, the air holes will change their shapes depending
on the difference in the growth rate. Given this problem, in the
first-mode method, it is desirable to investigate the difference in
the growth rate by a preliminary experiment or other the like and
design the planer shapes of the air holes before the creation of
the upper layer (epitaxial layer) in accordance with that
difference. By this method, the planer shape of the air holes after
the creation of the epitaxial layer can be made to be close to the
desired shape.
[0033] (2-2) Second Mode of the Method of Producing Two-Dimensional
Photonic Crystal Laser according to the Second Aspect of the
Present Invention
[0034] The second mode of the method of producing a two-dimensional
photonic crystal laser according to the present invention
includes:
[0035] a) a base-body layer creation process for creating a
base-body layer having a same crystal structure as
Al.sub.xGa.sub.1-xAs (0<x<=0.8);
[0036] b) an air-hole formation process for periodically forming
air holes in the base-body layer, each of the air holes having a
maximum width d in planer shape and a depth h, where d satisfies
d<=200 nm and a depth-to-width ratio h/d satisfies
0.1<=h/d<=1.2;
[0037] c) a modified refractive index area formation process for
forming, by an epitaxial method, modified refractive index areas
made of the aforementioned Al.sub.xGa.sub.1-xAs in the air holes;
and [0038] d) an epitaxial-layer creation process for creating, by
the aforementioned epitaxial method, a layer made of
Al.sub.yGa.sub.1-yAs (0<=y<=1) on the base-body layer having
the modified refractive index areas formed therein.
[0039] In the second-mode method, the aspect ratio h/d, which
satisfies 0.1<=h/d<=1.2, is smaller than the values used in
the first-mode method. Furthermore, the parameter x has a
relatively small value of 0.8 or less. These settings are aimed at
making the air holes easy to be filled with the material of the
modified refractive index areas, i.e. Al.sub.xGa.sub.1-xAs. Filling
the air holes with this material effectively suppresses the
formation of voids in the modified refractive index areas and
thereby prevents deterioration in the performance of the
two-dimensional photonic crystal layer as a resonator. As a result,
a two-dimensional photonic crystal laser with high laser
characteristics is obtained.
[0040] (2-3) Third Mode of the Method of Producing Two-Dimensional
Photonic Crystal Laser according to the Second Aspect of the
Present Invention
[0041] The third mode of the method of producing a two-dimensional
photonic crystal laser according to the present invention
includes:
[0042] a) a modified refractive index area formation process for
periodically forming columnar modified refractive index areas on an
epitaxial-growth substrate layer having a same crystal structure as
Al.sub.xGa.sub.1-xAs (0<x<=0.65), the modified refractive
index areas being made of a material whose refractive index differs
from that of the aforementioned Al.sub.xGa.sub.1-xAs;
[0043] b) a base-body creation process for creating, by an
epitaxial method, a base body made of the aforementioned
Al.sub.xGa.sub.1-xAs in a space between the modified refractive
index areas; and [0044] c) an epitaxial-layer creation process for
creating, by the aforementioned epitaxial method, a layer made of
Al.sub.yGa.sub.1-yAs (0<=y<=1) on a layer in which the
modified refractive index areas and the base body have been
formed.
[0045] In the third-mode method, columnar modified refractive index
areas are initially formed, after which a base body is formed by
filling the spaces around the modified refractive index areas with
Al.sub.xGa.sub.1-xAs. The base body formed by filling the spaces
with Al.sub.xGa.sub.1-xAs is less likely to allow the formation of
voids than the modified refractive index areas formed by filling
air holes with Al.sub.xGa.sub.1-xAs, and hence will not cause
deterioration in the performance of the two-dimensional photonic
crystal layer as a resonator. As a result, a two-dimensional
photonic crystal laser with high laser characteristics is obtained.
The material of the epitaxial-growth substrate layer may be the
same as or different from the material used for forming the base
body or the modified refractive index areas.
Effects of the Invention
[0046] In the two-dimensional photonic crystal laser according to
the first aspect of the present invention, a material selected from
the group of Al.sub..alpha.Ga.sub.1-.alpha.As or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P, which
is solid even at high temperatures, is used for the base body.
Therefore, the air holes will not be deformed in the process of
epitaxially growing the upper layer, so that the performance of the
two-dimensional photonic crystal layer as a resonator can be
maintained at high levels.
[0047] In the two-dimensional photonic crystal laser according to
the present invention, when the epitaxial growth layer serving as
the upper layer is made of Al.sub.xGa.sub.1-xAs, the performance of
the two-dimensional photonic crystal layer as a resonator can be
maintained at even higher levels. Furthermore, in the
two-dimensional photonic crystal laser according to the present
invention, when the epitaxial growth layer is used as a regrowth
interface layer for creating a p-type or n-type cladding layer by
epitaxial growth, the two-dimensional photonic crystal laser can be
produced with higher degrees of freedom in its structure.
[0048] In the first mode of the method of producing a
two-dimensional photonic crystal laser according to the second
aspect of the present invention, the entry of the material of the
epitaxial layer into the air holes of the two-dimensional photonic
crystal layer is prevented by giving a high aspect ratio to the air
holes before epitaxial growth and using Al.sub.xGa.sub.1-xAs
(0.4<=x<1) which barely diffuses to the bottom of the holes
in the process of epitaxial growth. As a result, the shape of the
air hole after the regrowth becomes close to the desired shape,
whereby the performance of the crystal layer as a photonic crystal
is maintained at high levels. In this manner, a two-dimensional
photonic crystal laser with high laser characteristics can be
created without deteriorating the performance of the
two-dimensional photonic crystal layer as a resonator.
[0049] In the second and third modes of the method of producing a
two-dimensional photonic crystal laser according to the second
aspect of the present invention, as described previously, the
inside of the air holes (second mode) or the spaces between the
modified refractive index areas (third mode) can be easily filled
with Al.sub.xGa.sub.1-xAs. Therefore, it is possible to create the
two-dimensional photonic crystal layer with no void formed in the
air holes or in the spaces between the modified refractive index
areas, whereby the performance of the two-dimensional photonic
crystal as a resonator is prevented from deterioration. As a
result, a two-dimensional photonic crystal laser with high laser
characteristics is obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1 is a perspective view showing the first embodiment
(Embodiment 1) of the two-dimensional photonic crystal laser
according to the present invention.
[0051] FIG. 2 is a top view showing one example of the structure of
the two-dimensional photonic crystal layer.
[0052] FIGS. 3A-3E are vertical sectional views showing a method of
producing the two-dimensional photonic crystal laser of Embodiment
1.
[0053] FIG. 4 is a vertical sectional view showing the second
embodiment (Embodiment 2) of the two-dimensional photonic crystal
laser according to the present invention.
[0054] FIGS. 5A-5E are vertical sectional views showing a method of
producing the two-dimensional photonic crystal laser of Embodiment
2.
[0055] FIG. 6 is a vertical sectional view showing the third
embodiment (Embodiment 3) of the two-dimensional photonic crystal
laser according to the present invention.
[0056] FIGS. 7A and 7B are diagrams for illustrating the depth h
and the maximum width d of an air hole.
[0057] FIGS. 8A-8C are vertical sectional views showing a method of
producing the two-dimensional photonic crystal laser of Embodiment
3.
[0058] FIG. 9 is a vertical sectional view showing the fourth
embodiment (Embodiment 4) of the two-dimensional photonic crystal
laser according to the present invention.
[0059] FIGS. 10A-10C are vertical sectional views showing a method
of producing the two-dimensional photonic crystal laser of
Embodiment 4.
[0060] FIG. 11 is a vertical sectional view showing the fifth
embodiment (Embodiment 5) of the two-dimensional photonic crystal
laser according to the present invention.
[0061] FIGS. 12A-12D are vertical sectional views showing a method
of producing the two-dimensional photonic crystal laser of
Embodiment 5.
[0062] FIG. 13 is an electron microscopic image showing a vertical
sectional shape of air holes formed in a two-dimensional photonic
crystal layer created by regrowing the upper layer by a
conventional epitaxial method.
[0063] FIGS. 14A-14C are images each of which shows a change in the
vertical sectional shape of air holes before and after the
formation of an epitaxial layer.
[0064] FIG. 15 is a graph showing a change in the effect of
interference inside the air holes formed in a two-dimensional
photonic crystal layer.
[0065] FIGS. 16A-16E are images and diagrams showing a change in
the horizontal sectional shape of air holes having a circular
planer shape before and after the formation of an epitaxial
layer.
[0066] FIGS. 17A-17D are images each of which shows a change in the
horizontal sectional shape of air holes having a
equilateral-triangular planer shape before and after the formation
of an epitaxial layer.
[0067] FIGS. 18A and 18B are diagrams each of which shows the
horizontal sectional shape of an air hole before regrowth, each air
hole being specifically designed so that it will have a circular or
equilateral-triangular horizontal sectional shape after the
regrowth.
[0068] FIGS. 19A-19D are diagrams showing one embodiment of the
first mode of the method of producing a two-dimensional photonic
crystal laser according to the present invention.
[0069] FIGS. 20A-20D are diagrams showing a variation of the first
mode of the method of producing a two-dimensional photonic crystal
laser.
[0070] FIG. 21 is a top view showing the structure of a
two-dimensional photonic crystal layer formed by the second or
third mode of the method of producing a two-dimensional photonic
crystal laser according to the present invention.
[0071] FIGS. 22A-22D are vertical sectional views showing one
embodiment (Embodiment 8) of the second mode of the method of
producing a two-dimensional photonic crystal laser according to the
present invention.
[0072] FIGS. 23A and 23B are microscopic images each of which shows
one example of the air holes filled with the material of the
modified refractive index areas by a method of Embodiment 8.
[0073] FIGS. 24A-24E are vertical sectional views showing one
embodiment (Embodiment 9) of the second mode of the method of
producing a two-dimensional photonic crystal laser.
[0074] FIGS. 25A-25E are vertical sectional views showing one
embodiment (Embodiment 10) of the second mode of the method of
producing a two-dimensional photonic crystal laser.
[0075] FIGS. 26A-26F are vertical sectional views showing one
embodiment (Embodiment 11) of the third mode of the method of
producing a two-dimensional photonic crystal laser according to the
present invention.
[0076] FIG. 27A is an electron microscopic image of the modified
refractive index areas 851 taken in one production step (FIG. 26C)
of Embodiment 11, and FIG. 27B is an electron microscopic image of
a vertical section of the two-dimensional photonic crystal laser
produced in Embodiment 11.
[0077] FIGS. 28A-28D are vertical sectional views showing one
embodiment (Embodiment 12) of the third mode of the method of
producing a two-dimensional photonic crystal laser.
[0078] FIGS. 29A-29E are vertical sectional views showing one
embodiment (Embodiment 13) of the third mode of the method of
producing a two-dimensional photonic crystal laser.
BEST MODES FOR CARRYING OUT THE INVENTION
[0079] Embodiments of the two-dimensional photonic crystal laser
and the first through third modes of the method of producing a
two-dimensional photonic crystal laser according to the present
invention are hereinafter described by means of FIGS. 1-29E. Among
the following embodiments, Embodiments 1-5 are examples of the
two-dimensional photonic crystal laser according to the present
invention, Embodiments 6 and 7 are examples of the first mode of
the method of producing a two-dimensional photonic crystal laser,
Embodiments 8-10 are examples of the second mode of the method of
producing a two-dimensional photonic crystal laser, and Embodiments
11-13 are examples of the third mode of the method of producing a
two-dimensional photonic crystal laser.
Embodiment 1
[0080] As shown in FIG. 1, the two-dimensional photonic crystal
laser 10 of Embodiment 1 includes a substrate 11, on which a first
cladding layer 12, an active layer 13, a carrier-blocking layer 14,
a two-dimensional photonic crystal layer 15, a second cladding
layer (epitaxial growth layer) 16 and a contact layer 17 are
stacked in this order. A lower electrode 18 is provided under the
substrate 11, while an upper electrode 19 is provided on the
contact layer 17.
[0081] As shown in FIG. 2, the two-dimensional photonic crystal
layer 15 consists of a plate-shaped base-body layer 152 in which
air holes 151 having a specific planer shape, such as a circle or
triangle, are periodically formed. In the present embodiment,
Al.sub.0.1Ga.sub.0.9As is used as the material of the base-body
layer 152. This is because Al.sub.0.1Ga.sub.0.9As is solid even at
high temperatures and can prevent deformation of the air holes 151
even if the temperature is increased to create the second cladding
layer 16 by an epitaxial method as described later.
[0082] It should be noted that the material of the base-body layer
152 is not limited to the one used in the present embodiment; any
material selected from the group of
Al.sub..alpha.Ga.sub.1-.alpha.As (0<.alpha.<1) or
(Al.sub..beta.Ga.sub.1-.beta.).gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1) may be used. The former
group is suitable for producing a laser oscillation having a
wavelength within the near-infrared region. The latter group is
suitable for producing a laser oscillation having a wavelength
within the red region.
[0083] For the second cladding layer 16, a material that can form a
layer on the two-dimensional photonic crystal layer 15 by an
epitaxial method is used. The material used for the second cladding
layer 16 in the present embodiment is p-type
Al.sub.0.65Ga.sub.0.35As, which includes a triply-charged positive
(Al.sub.0.65Ga.sub.0.35) site doped with a minor amount of
doubly-charged positive impurity. It should be noted that the
material of the second cladding layer 16 is not limited to the one
used in the present embodiment; any material categorized as p-type
Al.sub.xGa.sub.1-xAs (0.4<=x<1) can be suitably used. As the
value of x increases, the gas-diffusion length of
Al.sub.xGa.sub.1-xAs decreases, making this material more difficult
to enter the air holes 151. Accordingly, it is possible to prevent
the formation of an unnecessary crystal of p-type
Al.sub.0.65Ga.sub.0.35As inside the air holes 151.
[0084] For the layers other than the two-dimensional photonic
crystal layer 15 and the second cladding layer 16, the following
materials are used in the present embodiment: For the substrate 11
and the first cladding layer 12, n-type GaAs and n-type
Al.sub.0.65Ga.sub.0.35As are used, respectively. These materials
can be obtained by doping the Ga site of GaAs or
(Al.sub.0.65Ga.sub.0.35) site of Al.sub.0.65Ga.sub.0.35As with a
minor amount of quadruply-charged positive impurity. The active
layer 13 is made of InGaAs/GaAs multiple quantum wells. The
carrier-blocking layer 14 is made of Al.sub.0.4Ga.sub.0.6As. The
contact layer 17 is made of p-type GaAs. It should be noted that
these layers may also be made of materials other than the
aforementioned ones.
[0085] Opposite to the present embodiment, it is also possible to
use p-type materials for the substrate 11 and the first cladding
layer 12 and n-type materials for the second cladding layer 16 and
the contact layer 17.
[0086] One example of the method of producing the two-dimensional
photonic crystal laser 10 of the present embodiment is hereinafter
described by means of FIGS. 3A-3E. It should be noted that, unlike
the previously described first and second modes of the method of
producing a two-dimensional photonic crystal laser, there is no
specific limitation on the aspect ratio of the air holes in the
present embodiment. First, the first cladding layer 12, the active
layer 13 and the carrier-blocking layer 14 are created in this
order on the substrate 11 by epitaxially growing each layer by a
gas-phase process (FIG. 3A). Next, the base-body layer 152 is
created by epitaxially growing it on the carrier-blocking layer 14
by a gas-phase process (FIG. 3B). Subsequently, a resist 21 is
applied to the top surface of the base-body layer 152, and a
pattern corresponding to the arrangement of air holes 151 is formed
in the resist 21 by electron beam lithography, after which the air
holes 151 are formed in the base-body layer 152 by etching. As a
result, the two-dimensional photonic crystal layer 15 is obtained
(FIG. 3C).
[0087] After that, the resist 21 is removed, and the second
cladding layer 16 is created by epitaxially growing it on the
two-dimensional photonic crystal layer 15 by a gas-phase process
(FIG. 3D). During the process of epitaxially growing the second
cladding layer 16, the two-dimensional photonic crystal layer 15 is
heated to approximately 600.degree. C. If the base-body layer 152
is made of GaAs with no Al contained therein and heated to such a
high temperature, atomic migration may occur, causing the
disfiguring of the air holes 151. In the present embodiment, the
use of Al.sub.0.1Ga.sub.0.9As as the material of the base-body
layer 152 enables the air holes 151 to maintain their shape even at
such a high temperature.
[0088] After the second cladding layer 16 is created, the contact
layer 17 is epitaxially grown on the second cladding layer 16 by a
gas-phase process. Then, the lower and upper electrodes 18 and 19
are respectively created under the substrate 11 and on the contact
layer 17 by vapor deposition to obtain the two-dimensional photonic
crystal laser 10 of the present embodiment (FIG. 3E).
Embodiment 2
[0089] Embodiment 2 of the two-dimensional photonic crystal laser
according to the present invention is hereinafter described by
means of FIG. 4. In the two-dimensional photonic crystal laser 10A
of the present embodiment, a two-dimensional photonic crystal layer
15A, which will be described later, is used in place of the
two-dimensional photonic crystal layer 15 used in Embodiment 1. The
other elements are the same as those of the two-dimensional
photonic crystal laser 10 of Embodiment 1.
[0090] The two-dimensional photonic crystal layer 15A has a
base-body layer 152A having a double-layer structure including a
first base-body layer 1521A made of Al.sub.0.65Ga.sub.0.35As
(.alpha.=0.65) on which a second base-body layer 1522A made of
Al.sub.0.1Ga.sub.0.9As (.alpha.=0.1) is formed. The second
base-body layer 1522A is thinner than the first one 1521A. Air
holes 151A having the same shape and space intervals as in
Embodiment 1 are formed in the base-body layer 152A. The second
base-body layer 1522A is characterized in that it has an Al content
lower than that of the first base-body layer 1521A and therefore is
less likely to be oxidized in the production process described
later.
[0091] A method of producing the two-dimensional photonic crystal
laser 10A of the present embodiment is hereinafter described by
means of FIGS. 5A-5E. It should be noted that the present
embodiment also imposes no specific limitation on the aspect ratio
of the air holes. First, similarly to the method of Embodiment 1,
the first cladding layer 12, the active layer 13 and the
carrier-blocking layer 14 are created in this order on the
substrate 11 by epitaxially growing each layer by a gas-phase
process (FIG. 5A). Next, the first base-body layer 1521A is created
by epitaxially growing it on the carrier-blocking layer 14 by a
gas-phase process (FIG. 5B). Subsequently, the second base-body
layer 1522A is created by epitaxially growing it on the first
base-body layer 1521A by a gas-phase process (FIG. 5C). All of
these processes are performed in the same chamber while changing
the type of the material gas.
[0092] Next, similarly to Embodiment 1, air holes 151A are formed
in the base-body layer 152A by electron beam lithography and
etching (FIG. 5D). This process use techniques different from
epitaxy and hence needs to be performed in a chamber different from
the chamber used for the previous processes. Subsequently, the
chamber is replaced with the previously used one, and the second
cladding layer 16 is created by epitaxially growing it on the
second base-body layer 1522A by a gas-phase process (FIG. 5E).
After that, the contact layer 17, the lower electrode 18 and the
upper electrode 19 are created by a method similar to Embodiment 1
to obtain the two-dimensional photonic crystal laser 10A of the
present embodiment.
[0093] As just described, the present method requires changing the
chamber before and after the process of forming the air holes 151A.
In the process of changing the chamber, the surface of the
base-body layer may be oxidized. In the present embodiment, this
oxidation of the surface of the base-body layer is suppressed by
the second base-body layer 1522A made of a material that is more
resistant to oxidation than the material of the first base-body
layer 1521A.
Embodiment 3
[0094] Embodiment 3 of the two-dimensional photonic crystal laser
according to the present invention is hereinafter described by
means of FIG. 6. The two-dimensional photonic crystal laser 10B of
the present embodiment is created by the first mode of the method
of producing a two-dimensional photonic crystal laser.
[0095] The two-dimensional photonic crystal laser 10B is a
variation of the device described in Embodiment 1 and additionally
includes a regrowth interface layer 31 made of Al.sub.xGa.sub.1-xAs
(0.4<=x<1) located between the two-dimensional photonic
crystal layer 15 and the second cladding layer 16. The maximum
width d in planer shape of the air hole 151 is equal to or smaller
than 200 nm. The ratio of the depth h to the maximum width d of the
air hole 151 (the aspect ratio h/d) satisfies 1.3<=h/d<=5.
The maximum width d corresponds to the length of the longest line
segment that can be included in the planer shape of the air hole
151 (see FIGS. 7A and 7B). For example, when the air hole 151 has a
circular planer shape, its diameter corresponds to the maximum
width. For an air hole having an equilateral-triangular planer
shape, its one-side length corresponds to the maximum width. For a
non-equilateral triangle, the length of the longest side of the
triangle equals the maximum width.
[0096] In the present embodiment, the material gas for creating the
regrowth interface layer 31 is prevented from easily entering the
air hole 151 by giving a relatively high value to the Al content x
of the regrowth interface layer 31 and setting the aspect ratio h/d
to be equal to or higher than 1.3. Therefore, a crystal of the
material used for the regrowth interface layer 31 will be barely
formed in the air holes 151. The reason for setting an upper limit
of the aspect ratio h/d, which is 5 in the present case, is because
the two-dimensional periodic structure of the air holes 151 may
become insufficient if h is too large or d is too small.
[0097] A method of producing the two-dimensional photonic crystal
laser 10B of the present embodiment is hereinafter described by
means of FIGS. 8A-8C. Other examples of the first mode of the
method of producing a two-dimensional photonic crystal laser will
be described in more detail in Embodiments 6 and 7.
[0098] First, the first cladding layer 12, the active layer 13, the
carrier-blocking layer 14 and the two-dimensional photonic crystal
layer 15 are created on the substrate 11 by a method similar to
Embodiment 1 (FIG. 8A). Next, the regrowth interface layer 31 is
created by epitaxially growing it on the two-dimensional photonic
crystal layer 15 by a gas-phase process to close the upper side of
the air holes 151 (FIG. 8B). In this process, as already explained,
the material of the regrowth interface layer 31 is prevented from
entering the air holes 151. Furthermore, the aforementioned choice
of the material for the base-body layer 152 prevents the occurrence
of atomic migration in this process. Subsequently, the second
cladding layer 16 is created by epitaxially growing it on the
regrowth interface layer 31 by a gas-phase process (FIG. 8C). After
that, the contact layer 17, the lower electrode 18 and the upper
electrode 19 are created by a method similar to Embodiment 1 to
obtain the two-dimensional photonic crystal laser 10B of the
present embodiment.
Embodiment 4
[0099] Embodiment 4 of the two-dimensional photonic crystal laser
according to the present invention is hereinafter described by
means of FIG. 9. The two-dimensional photonic crystal laser 10C of
the present embodiment is created by the second mode of the method
of producing a two-dimensional photonic crystal laser.
[0100] In the two-dimensional photonic crystal laser 10C, a
regrowth interface layer 31A made of Al.sub.xGa.sub.1-xAs
(0<x<=0.8) is provided in place of the regrowth interface
layer 31 used in Embodiment 3. Furthermore, in place of the air
holes 151, modified refractive index members 32 made of the same
material as that of the regrowth interface layer 31A are
periodically arranged in the two-dimensional photonic crystal layer
15C. The maximum width d in planer shape of the modified refractive
index member 32 is equal to or smaller than 200 nm. The aspect
ratio h/d is set to satisfy 0.1<=h/d<=1.2. The definitions of
the maximum width d and the aspect ratio h/d are the same as those
of the air hole 151.
[0101] In the present embodiment, the Al content x of the regrowth
interface layer 31A is set to a relatively low value. Furthermore,
the aspect ratio h/d is set to be equal to or lower than 1.2. These
settings are aimed at helping the material gas for the regrowth
interface layer 31 to enter the air holes 151 in the process of
creating the regrowth interface layer 31A. The reason for setting
the lower limit of the aspect ratio h/d, which is 0.1 in the
present case, is because the two-dimensional periodic structure of
the air holes 151 may become insufficient if h is too small or d is
too large.
[0102] A method of producing the two-dimensional photonic crystal
laser 10C of the present embodiment is hereinafter described by
means of FIGS. 10A-10C. Other examples of the second mode of the
method of producing a two-dimensional photonic crystal laser will
be described in more detail in Embodiments 8-10.
[0103] First, the first cladding layer 12, the active layer 13, the
carrier-blocking layer 14 and the two-dimensional photonic crystal
layer 15 are created on the substrate 11 by a method similar to
Embodiment 1 (FIG. 10A). At this stage, the two-dimensional
photonic crystal layer 15 still has air holes 151, similar to the
one created in Embodiment 1; the modified refractive index members
32 are not yet formed therein. Next, the regrowth interface layer
31A and the modified refractive index members 32 are simultaneously
created on the two-dimensional photonic crystal layer 15 and in the
air holes 151, respectively, by epitaxially growing them by a
gas-phase process (FIG. 10B). In this process, the modified
refractive index members 32 can be created without any voids since,
as explained previously, the material gas can easily enter the air
holes 151. Furthermore, the aforementioned choice of the material
for the base-body layer 152 prevents the occurrence of atomic
migration in this process. Subsequently, the second cladding layer
16 is created by epitaxially growing it on the regrowth interface
layer 31A by a gas-phase process (FIG. 10C). After that, the
contact layer 17, the lower electrode 18 and the upper electrode 19
are created by a method similar to Embodiment 1 to obtain the
two-dimensional photonic crystal laser 10C of the present
embodiment.
Embodiment 5
[0104] Embodiment 5 of the two-dimensional photonic crystal laser
according to the present invention is hereinafter described by
means of FIG. 11. In the two-dimensional photonic crystal laser 10D
of the present embodiment, the two-dimensional photonic crystal
layer 15D has a structure composed of columnar modified refractive
index members 32A periodically arranged on the carrier-blocking
layer 14 and a base body 152B filling the spaces around the
modified refractive index members 32A. In the present embodiment,
the material of the modified refractive index members 32A is not
limited to Al.sub.xGa.sub.1-xAs (0<x<=0.8) but may be another
kind of semiconductor or dielectric material. Additionally, a
regrowth interface layer 31B made of the same material as that of
the base body 152B is formed on the two-dimensional photonic
crystal layer 15D. The other elements are the same as those of
Embodiment 1.
[0105] A method of producing the two-dimensional photonic crystal
laser 10D of the present embodiment is hereinafter described by
means of FIGS. 12A-12D. First, the first cladding layer 12, the
active layer 13 and the carrier-blocking layer 14 are created on
the substrate 11 by a method similar to Embodiment 1 (FIG. 12A).
Next, a precursor layer 33 for the modified refractive index areas,
which is made of a material for the modified refractive index
members 32A, is created by epitaxially growing it on the
carrier-blocking layer 14 by a gas-phase process (FIG. 12A).
Subsequently, the precursor layer 33 is removed from the top
surface to a middle depth by electron beam lithography and etching,
leaving a group of periodically arranged columnar areas. As a
result, columnar modified refractive index members 32A are formed
on a spacer layer 33 formed by the remaining bottom portion of the
precursor layer 33 (FIG. 12B). Next, the base body 152B is created
by epitaxially growing it on the spacer layer 33A by a gas-phase
process. The creation of the crystal by epitaxial growth is
continued even after the base body 152B has reached the level of
the top faces of the modified refractive index members 32A. In this
manner, the regrowth layer 31B is formed above the modified
refractive index members 32A and the base body 152B (FIG. 12C).
Subsequently, the second cladding layer 16 is created by
epitaxially growing it on the regrowth interface layer 31B by a
gas-phase process (FIG. 12D). After that, the contact layer 17, the
lower electrode 18 and the upper electrode 19 are created by a
method similar to Embodiment 1 to obtain the two-dimensional
photonic crystal laser 10D of the present embodiment.
[0106] The two-dimensional photonic crystal laser according to the
present invention is not limited to Embodiments 1-5. For example,
the base-body layer may be composed of two or more layers made of
different materials. Furthermore, one of those layers may be made
of GaAs which contains no Al. Even this configuration can more
effectively suppress the influence of atomic migration than the
configuration in which the base-body layer is entirely made of
GaAs.
Embodiment 6
[0107] Embodiments of the first mode of the method of producing a
two-dimensional photonic crystal are hereinafter described. In the
following embodiments, basically, the previously described
two-dimensional photonic crystal laser as illustrated in FIGS. 1
and 2 are created. To create such a type of two-dimensional
photonic crystal laser, it is necessary to form the second cladding
layer 16 immediately above the air holes 151 of the two-dimensional
photonic crystal layer 15. In recent years, the idea of using an
epitaxial method of forming a layer on the two-dimensional photonic
crystal layer 15 has been proposed. However, using a conventional
method to implement this idea causes the problem that the air holes
151 become partially filled during the regrowth process and
deformed, as shown in FIG. 13. The first mode of the method of
producing a two-dimensional photonic crystal laser is characterized
in that, in the process of forming the epitaxial layer (second
cladding layer) 16 on the two-dimensional photonic crystal layer
15, the shape of the air holes 151 of the two-dimensional photonic
crystal layer 15 before the regrowth is designed taking into
account the characteristics of the material of the epitaxial layer
16 (this material is hereinafter referred to as the "regrowth
material") so as to make the shape of the air holes after the
regrowth as close to the desired shape as possible and thereby
maintain the performance of the crystal layer 15 as a photonic
crystal at high levels.
[0108] Initially, one example of the first mode of the method of
producing a two-dimensional photonic crystal laser is described
with reference to experimental data. In the following description,
a material selected from the group of
Al.sub..alpha.Ga.sub.1-.alpha.As (0<.alpha.<1) or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1) is used for the base-body
layer 152. This is because, in the process of forming the epitaxial
layer 16 on the two-dimensional photonic crystal layer 15 after a
photonic crystal structure has been formed, the substrate must be
heated to approximately 600.degree. C., and if the substrate is
made of GaAs or a similar material, the air holes 151 may be
disfigured due to atomic migration during the heating process.
Meanwhile, a material selected from the Al.sub.xGa.sub.1-xAs group
is use as the regrowth material. Materials belonging to the
Al.sub.xGa.sub.1-xAs group have the characteristic that its
gas-diffusion length of decreases as the value of x increases,
making this material less likely to enter the air holes 151.
Accordingly, Al.sub.xGa.sub.1-xAs can be suitably used as the
material for the epitaxial layer 16 to be formed on the
two-dimensional photonic crystal layer 15.
[0109] The experiments described below were carried out under the
conditions that the air holes 151 had a circular planer shape and
the value of x was 0.65.
[0110] [Experiment on Vertical Sectional Shape]
[0111] FIGS. 14A-14C show experimental data on the vertical
sectional shape of the air holes 151 before regrowth and that of
the air holes 151B after the regrowth. The definitions of the
maximum width d and the depth h of the air holes are as already
explained in Embodiment 3 (see FIGS. 7A and 7B).
[0112] A comparison between FIGS. 14A and 14B demonstrates that,
for approximately the same value of d, the vertical sectional shape
of the air holes 151 before the regrowth can be maintained to a
greater extent by increasing the aspect ratio h/d. On the other
hand, a comparison between FIGS. 14B and 14C demonstrates that, for
approximately the same value of h/d, the vertical sectional shape
of the air holes 151 before the regrowth can be maintained to a
greater extent by increasing the maximum width d. These results
suggest that it is possible to make the vertical sectional shape of
the air holes 151B after the regrowth closer to the shape of the
air holes 151 before the formation of the epitaxial layer 16 by
appropriately determining one or both of the maximum width d and
the aspect ratio h/d of the air holes for the Al content rate x of
the regrowth material.
[0113] It has been experimentally confirmed that the parameters x,
d and h/d should preferably be set within the ranges of
0.4<=x<1, d<=200 nm and 1.3<=h/d, respectively. It is
basically unnecessary to specify the upper limit of the aspect
ratio h/d. However, in the present embodiment, an upper limit of 5
is given to the aspect ratio h/d. This is because the
two-dimensional periodic structure of the air holes 151 may become
insufficient if h is too large or d is too small.
[0114] Inside the air hole 151, a diffracted light from the active
layer 13 and a diffracted light from the second cladding layer 16
constructively or destructively interfere with each other. The
interference condition depends on the material of the base-body
layer 152, the depth h of the air hole 151 and the vertical
sectional shape of the air hole 151. For example, a constructive
interference occurs when the base-body layer 152 is made of
Al.sub.0.1Ga.sub.0.9As and the air hole 151 has a rectangular
vertical sectional shape with a depth h=120 nm. If the depth h is
further increased, the interference will gradually change from the
constructive state toward the destructive one.
[0115] In the regrowth method of the present embodiment, as shown
in FIGS. 14A-14C, a bullet-like vertical sectional shape pointing
toward the second cladding layer (epitaxial layer) 16 is obtained.
FIG. 15 shows how the effect of the interference occurring in the
air hole changes depending on the vertical sectional shape of the
air hole. When the vertical sectional shape has an upward-tapering
conical area 154, the diffracted light from the second cladding
layer 16 becomes weaker and the interference becomes less
effective, as shown in the graph of FIG. 15.
[0116] The result shown in FIG. 15 demonstrates that the effect of
the interference can be reduced by changing the depth h.sub.1 of
the conical area 154 and the depth h.sub.2 of the rectangular
portion 155. Since the depth h.sub.1 of the conical area 154 and
the depth h.sub.2 of the rectangular portion 155 can be controlled
through the parameters d, h and x, it is possible to prevent
destructive interference of the diffracted light from the active
layer 13 and the diffracted light from the second cladding layer 16
by appropriately adjusting the parameters d, h and x.
[0117] [Experiment on Planer Shape (Horizontal Sectional
Shape)]
[0118] Experimental data on the planer shape of the air holes 151
before the regrowth and that of the air holes 151B after the
regrowth are shown in FIGS. 16A-17D.
[0119] FIGS. 16A-16E show the result of an experiment on the growth
of the epitaxial layer 16 in the in-plane direction (i.e. the
direction perpendicular to the stacking direction) on the
two-dimensional photonic crystal layer 15. More specifically, FIG.
16A is an electron microscopic image of air holes 151 before the
formation of the epitaxial layer 16. FIGS. 16B-16D are electron
microscopic images taken when Al.sub.0.65Ga.sub.0.35As was
epitaxially regrown to a thickness of 40 nm on the two-dimensional
photonic crystal layer 15, where FIG. 16B shows an image taken from
above, FIG. 16C shows a vertical section at a plane perpendicular
to an orientation-flat (001) face, and FIG. 16D shows a vertical
section at a plane parallel to the orientation-flat (001) face.
[0120] In the example of FIGS. 16A-16E, Al.sub.xGa.sub.1-xAs more
easily grows in the direction parallel to the orientation-flat
(001) face. Therefore, while the epitaxial layer 16 is being
formed, the circular air hole 151 will gradually change to an
elliptic hole with its minor diameter extending parallel to the
orientation-flat (001) face. Taking this nature into account, the
rate of growth in each direction is determined for each value of
the Al content rate x of the regrowth material, for example, by a
preliminary experiment. Based on the result of this experiment, the
planer shape of the air hole 151 before the regrowth can be
designed so that the air hole 151B after the regrowth will have the
desired planer shape. For example, when x=0.65, the ratio between
the growth rate a in the direction parallel to the orientation-flat
(001) face to the growth rate b in the direction perpendicular to
the orientation-flat (001) face is b/a=1.3. In this case, the
desired planer shape will be eventually obtained by giving the air
hole 151 before the regrowth an elliptic planer shape having the
major diameter extending parallel to the orientation-flat (001)
face and the minor diameter extending perpendicular to the same
face, with the ratio of the major diameter a to the minor diameter
b being a/b=1.3 (FIG. 16E).
[0121] When the air hole 151 has a polygonal planer shape, such as
a triangle, the crystal grows inward from each side of the polygon.
FIGS. 17A-17D show experimental data for various air holes 151 each
having an equilateral-triangular planer shape. As in these cases,
when the two neighboring growth faces make an angle equal to or
smaller than 90.degree., the crystal grows while gradually filling
each vertex of the polygon. Therefore, in the case of FIG. 17A, the
planer shape of the air holes 151B after the regrowth has become
nearly circular. Taking this into account, in the examples of FIGS.
17B-17D, a groove-like projected portion 153 is formed at each
vertex where the two growth faces meet so that the air holes 151B
after the regrowth will have a triangular planer shape.
[0122] FIGS. 18A and 18B show the results of experiments for
circular and equilateral-triangular planer shapes. The experiment
for the circular shape has demonstrated that the planer shape after
the regrowth will be a circle when the air hole before the regrowth
has an elliptic planer shape, with the major diameter extending
parallel to the growth face and the minor diameter extending
perpendicular to the same face, and the ratio of the major diameter
a to the minor diameter b satisfying 1<a/b<=1.5 (FIG. 18A).
The experiment for the equilateral-triangular shape has
demonstrated that the planer shape after the regrowth will be an
equilateral triangle when the air hole before the regrowth has a
equilateral-triangular planer shape having a projected portion at
each vertex, with the length a of the line segment from the center
of gravity of the triangle to the end of the projected portion and
the length b of a perpendicular from the center of gravity to each
side of the triangle satisfying 2<a/b<=3 (FIG. 18B). In this
manner, it is possible to make the planer shape of the air hole
after the regrowth close to the desired shape by appropriately
changing the shape of the air hole before the regrowth with respect
to the direction of the growth face as well as the Al content rate
x of the regrowth material.
Embodiment 7
[0123] One embodiment of the method of producing a two-dimensional
photonic crystal laser according to the present invention is
hereinafter described by means of FIGS. 19A-19D.
[0124] First, an n-type Al.sub.0.65Ga.sub.0.35As layer (n-type
cladding layer) 42, an InGaAs/GaAs layer (active layer) 43, an
Al.sub.0.4Ga.sub.0.6As layer (carrier-blocking layer) 44 and an
Al.sub.0.1Ga.sub.0.9As layer 45 are epitaxially grown in this order
on a GaAs substrate 41 (FIG. 19A). Next, a group of air holes 451
with a predetermined periodic structure is created in the
Al.sub.0.1Ga.sub.0.9As layer 45 by etching in such a manner that
the maximum width d of the air hole satisfies d<=200 nm and the
ratio of the depth h to the maximum width d satisfies
1.3<=h/d<=5 (FIG. 19B). As a result, a two-dimensional
photonic crystal layer 45A is obtained. Subsequently, a p-type
Al.sub.0.65Ga.sub.0.35As layer (p-type cladding layer) 46 is
created by epitaxially growing it on the two-dimensional photonic
crystal layer 45A (i.e. the Al.sub.0.1Ga.sub.0.9As layer 45 with
the air holes 451 formed therein), after which a p-type GaAs layer
(contact layer) 47 is formed on the p-type Al.sub.0.65Ga.sub.0.35As
layer 46 (FIG. 19C). After that, a lower electrode (window-shaped
electrode) 48 and an upper electrode 49 are formed under the
substrate 41 and on the p-type GaAs layer 47, respectively (FIG.
19D). As a result, a two-dimensional photonic crystal laser with
high laser characteristics is obtained. It should be noted that the
base body of the two-dimensional photonic crystal layer 45A may be
made of a material selected from the group of
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1).
[0125] Although the planer shape of the air holes 451 before the
regrowth is not specified in the above method, it is preferable to
appropriately design their planer shape based on the growth face of
the p-type Al.sub.0.65Ga.sub.0.35As layer 46 during the epitaxial
growth of this layer, as shown in FIGS. 16A-18B, so as to improve
the performance of the two-dimensional photonic crystal layer 45A
and thereby achieve high laser characteristics.
[0126] In the method shown in FIGS. 19A-19D, a process for forming
a growth-inhibiting film made of a material that can inhibit the
epitaxial growth of the materials of the Al.sub.xGa.sub.1-xAs group
may be added between the processes of FIGS. 19B and 19C. Examples
of the growth-inhibiting materials include SiO.sub.2,
Si.sub.3N.sub.4, ZnO and ZrO.sub.2. This variation is hereinafter
described by means of FIGS. 20A-20D.
[0127] After, the air holes 451 have been formed in the
Al.sub.0.1Ga.sub.0.9As layer 45A (FIG. 20A), an SiO.sub.2 film 50
is formed on this layer (FIG. 20B). Next, the SiO.sub.2 film 50 is
removed by dry etching (FIG. 20C). Since the etching rate on the
surface 452 of the Al.sub.0.1Ga.sub.0.9As layer 45A is higher than
in the air holes 451, the SiO.sub.2 film 50 remains only in the air
holes 451. The SiO.sub.2 film 50 remaining in the air holes 451
functions as a growth-inhibiting film for inhibiting the epitaxial
growth of Al.sub.xGa.sub.1-xAs. Accordingly, the formation of the
crystal in the air holes 451 during the epitaxial growth of the
p-type cladding layer 46 is more effectively prevented.
[0128] In the case of forming the growth-inhibiting film in the air
holes 451, it is desirable to design the vertical and/or horizontal
sectional shape of the air holes 451 before the regrowth by the
previously described method, although the laser characteristics can
be considerably improved by merely forming the growth-inhibiting
film in the air holes 451.
[0129] Embodiments 6 and 7 are mere examples of the first mode of
the method of producing a two-dimensional photonic crystal laser,
and any change, modification or addition may be appropriately made
within the spirit of the present invention. For example, in the
aforementioned embodiments, the base-body layer had a one-layer
structure made of Al.sub.0.1Ga.sub.0.9As. This can be changed to a
multi-layer structure having a plurality of
Al.sub..alpha.Ga.sub.1-xAs layers with different values of a. This
structure may be further modified by replacing a portion of the
layers with a GaAs layer or another kind of semiconductor layer.
Similarly, when a material of the
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P group
is used for the base-body layer, it is possible to adopt a
multi-layer structure having a plurality of layers with different
values of .beta. and .gamma., and to further modify this structure
by replacing a portion of the layers with a GaAs layer or another
kind of semiconductor layer.
Embodiment 8
[0130] Embodiments 8-10 are examples of the second mode of the
method of producing a two-dimensional photonic crystal laser. The
basic structure of the two-dimensional photonic crystal laser
created by the second-mode method is the same as shown in FIG. 1.
An important difference is that, in place of the air holes,
modified refractive index members made of a material different from
that of the base body are used as the modified refractive index
areas 151C (FIG. 21). The structure using the modified refractive
index members as the modified refractive index areas has a lower
light-confining effect as compared to the structure using air
holes. However, the former structure is advantageous in that a
single-mode laser oscillation can be more easily generated over a
large area.
[0131] The production method according to Embodiment 8 is
hereinafter described by means of FIGS. 22A-22D. First, a first
cladding layer 62 made of n-type Al.sub.0.4Ga.sub.0.6As, an active
layer 63 made of InGaAs/GaAs multiple quantum wells, a
carrier-blocking layer 64 made of Al.sub.0.4Ga.sub.0.6As, and a
base-body layer 652A made of Al.sub.0.1Ga.sub.0.9As are created in
this order on a substrate 61 by an epitaxial method (FIG. 22A).
Next, a large number of air holes 651 having a circular planer
shape with the diameter (maximum width) d being 110 nm, the depth h
being 120 nm, and hence the aspect ratio h/d being 1.09, are
periodically formed at predetermined space intervals in the
base-body layer 652A by electron beam lithography and etching (FIG.
22B). Subsequently, a crystal of p-type Al.sub.0.4Ga.sub.0.6As is
epitaxially grown in the air holes 651A and on the base-body layer
652A (FIG. 22C). In this process, modified refractive index areas
651 made of p-type Al.sub.0.4Ga.sub.0.6As are formed in the air
holes 651A, whereby a two-dimensional photonic crystal layer 65
composed of the base-body 652 and the modified refractive index
areas 651 is created. Simultaneously, a second cladding layer 66
made of p-type Al.sub.0.4Ga.sub.0.6As is created on the
two-dimensional photonic crystal layer 65. Subsequently, a lower
electrode (window-shaped electrode) 68 is formed under the
substrate 61, while a contact layer 67 and an upper electrode 69
are formed in this order on the second cladding layer 66 (FIG.
22D). Thus, the two-dimensional photonic crystal laser is
completed.
[0132] The materials of the layers are not limited to the
aforementioned ones. For example, Al.sub.xGa.sub.1-xAs
(0<x<=0.8) may be used as the material for the modified
refractive index areas 651 and the second cladding layer 66. This
material has the characteristic that, as the content rate of Al
decreases, the molecules of the material gas used in the creation
process more easily diffuse, thereby helping the material to enter
the air holes 651. As the material for the base-body layer 652A,
Al.sub..alpha.Ga.sub.1-.alpha.As (0<.alpha.<1, where
.alpha..noteq.x) or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<1, 0<.gamma.<1) having the same crystal
structure as that of the material of the modified refractive index
areas 651 may be used.
[0133] The planer shape of the air hole 651A (and the modified
refractive index area 651 created by filling the air hole 651A with
the modified refractive index member) is not limited to a circle;
there are various choices, such as an ellipse or triangle. The
maximum width d and the aspect ratio h/d are not limited to the
aforementioned values; a sufficient amount of gas molecules will
reach the bottom of the air hole 651A as long as these parameters
satisfy the conditions of d<=200 nm and 0.1<=h/d<=1.2. The
definitions of the maximum width d and the depth h of the air holes
are as already explained in Embodiment 3 (see FIGS. 7A and 7B).
[0134] FIGS. 23A and 23B are electron microscopic images showing
the results of an experiment in which a crystal of p-type
Al.sub.0.4Ga.sub.0.6As was epitaxially grown in the air holes 651A
by the method of Embodiment 8. In this experiment, the crystal
growth was discontinued when the crystal of p-type
Al.sub.0.4Ga.sub.0.6As reached the thickness of 50 nm. Two types of
air holes 651A were created in this experiment; the first type
measured d=130 nm, h=60 nm and h/d=0.46 (FIG. 23A), while the
second type measured d=130 nm, h=80 nm, and h/d=0.63 (FIG. 23B).
The obtained electron microscopic images demonstrate that the air
holes 651A were filled without any voids after the crystal
growth.
Embodiment 9
[0135] Another embodiment of the second mode of the method of
producing a two-dimensional photonic crystal laser is hereinafter
described by means of FIGS. 24A-24E. In the present embodiment, the
modified refractive index areas and the second cladding layer are
respectively created by separate processes. First, the first
cladding layer 62, the active layer 63, the carrier-blocking layer
64 and the base-body layer 652A are formed in this order on the
substrate 61 (FIG. 24A), using the same method and materials as
used in Embodiment 1, after which the air holes 651A are formed in
the base-body layer 652A (FIG. 24B). Next, modified refractive
index areas 651B are formed in the air holes 651A by growing a
crystal of Al.sub.yGa.sub.1-yAs (0<=y<=1) in the air holes
651A by an epitaxial method until the air holes 651A are entirely
filled (FIG. 24C). In this process, a buffer layer 653 made of
Al.sub.0.4Ga.sub.0.6As is also created on the base-body layer 652A.
Subsequently, the second cladding layer 66A made of p-type
Al.sub.0.4Ga.sub.0.6As is created on the buffer layer 653 by an
epitaxial method (FIG. 24D). After that, similarly to Embodiment 8,
the lower electrode 68, the contact layer 67 and the upper
electrode 69 are formed (FIG. 24E) to complete the two-dimensional
photonic crystal laser.
Embodiment 10
[0136] Another embodiment of the second mode of the method of
producing a two-dimensional photonic crystal laser is hereinafter
described by means of FIGS. 25A-25E. First, the first cladding
layer 62, the active layer 63, the carrier-blocking layer 64 and
the base-body layer 652A are formed in this order on the substrate
61, using the same method and materials as used in Embodiment 8,
after which a crystal-growth inhibiting film 71 made of SiO.sub.2
is formed on the base-body layer 652A (FIG. 25A). SiO.sub.2 is a
material capable of inhibiting the epitaxial growth of
Al.sub.xGa.sub.1-xAs (x=0.4 in the present embodiment) used for
forming the modified refractive index areas. It is also possible to
use Si.sub.3N.sub.4, ZnO or ZrO.sub.2 as the material for the
crystal-growth inhibiting film 71. Next, a pattern of a resist 72
for masking the areas other than the modified refractive index
areas is formed by electron beam lithography, and an etching
process using an etchant capable of removing the crystal-growth
inhibiting film 71 is carried out. After that, another etching
process using a different etchant is performed for the base-body
layer 652A. As a result, air holes 651C which penetrate through the
crystal-growth inhibiting film 71 to the base-body layer 652A are
created (FIG. 25B). Subsequently, after the resist 72 is removed,
the modified refractive index areas 651D made of
Al.sub.xGa.sub.1-xAs is created by forming a crystal of
Al.sub.xGa.sub.1-xAs in the air holes 651C by an epitaxial method
(FIG. 25C). In this process, the epitaxial growth of
Al.sub.xGa.sub.1-xAs does not occur on the portions of the top
surface of the base-body layer 652A where no air holes 651C exist,
because the crystal-growth inhibiting film 71 still remains on
those portions. In this manner, the crystal of Al.sub.xGa.sub.1-xAs
is prevented from horizontally extending from the top surface of
the base-body layer 652 and closing the air holes 651C, so that no
void will be formed inside the modified refractive index areas
651D. Next, the crystal-growth inhibiting film 71 is removed, and
the second cladding layer 66B made of p-type Al.sub.0.4Ga.sub.0.6As
is created on the base-body layer 652A and the modified refractive
index areas 651D by an epitaxial method (FIG. 25D). After that,
similarly to Embodiment 8, the lower electrode 68, the contact
layer 67 and the upper electrode 69 are formed (FIG. 25E) to
complete the two-dimensional photonic crystal laser.
Embodiment 11
[0137] Embodiments of the third mode of the method of producing a
two-dimensional photonic crystal laser according to the present
invention are hereinafter described. Initially, one of the
embodiments is described by means of FIGS. 26A-26F. a first
cladding layer 82 made of n-type Al.sub.0.65Ga.sub.0.35As, an
active layer 83 made of InGaAs/GaAs multiple quantum wells, and a
carrier-blocking layer 84 made of Al.sub.0.4Ga.sub.0.6As are formed
in this order on a substrate 81 by an epitaxial method (FIG. 26A).
Next, a precursor layer 851A for modified refractive index areas,
which layer is made of SiO.sub.2, is formed on the carrier-blocking
layer 84 by sputtering (FIG. 26B). Subsequently, the precursor
layer 851A is partially removed by electron beam lithography and
etching, leaving a number of periodically arranged columnar areas.
In this manner, columnar modified refractive index areas 851 made
of SiO.sub.2 are formed on the carrier-blocking layer 84 (FIG.
26C). After that, a base body 852 is formed in the spaces between
the modified refractive index areas 851 by epitaxially growing a
crystal of p-type Al.sub.0.65Ga.sub.0.35As on the portions of the
carrier-blocking layer 84 from which the precursor layer 851A has
been removed (FIG. 26D). In this process, the aforementioned
portions of the carrier-blocking layer 84 serve as the substrate
(i.e. the epitaxial-growth substrate layer). Thus, a
two-dimensional photonic crystal layer 85 having the base body 852
filling the spaces between the modified refractive index areas 851
is created. Since the modified refractive index areas 851 are made
of SiO.sub.2, the p-type Al.sub.0.65Ga.sub.0.35As crystal will not
grow on the modified refractive index areas 851 until the base body
852 grows to the same level as the modified refractive index areas
851. After the base body 852 has grown to the same level as the
modified refractive index areas 851, the growth of the p-type
Al.sub.0.65Ga.sub.0.35As crystal is further continued. Then, the
p-type Al.sub.0.65Ga.sub.0.35As crystal begins to grow not only in
the vertical direction but also in the horizontal direction. As a
result, a second cladding layer 86 made of p-type
Al.sub.0.65Ga.sub.0.35As is formed over the two-dimensional
photonic crystal layer 85, including the top faces of the modified
refractive index areas 851 (FIG. 26E). After that, similarly to
Embodiment 8, a lower electrode 88 is formed under the substrate
81, while a contact layer 87 and an upper electrode 89 are formed
in this order on the second cladding layer 86 (FIG. 26F). Thus, the
two-dimensional photonic crystal laser is completed.
[0138] In the present embodiment, there is no specific limitation
on the size (aspect ratio) of the modified refractive index area
851. Furthermore, the materials of the layers are not limited to
the aforementioned ones. For example, Si.sub.3N.sub.4, ZnO or
ZrO.sub.2 may be used as the material for the modified refractive
index areas 851 (and the precursor layer 851A for modified
refractive index areas). Al.sub.xGa.sub.1-xAs (0<x<=0.65) may
be used as the material for the base body 852 and the second
cladding layer 86. The shape of the modified refractive index area
851 is not limited to a column; there are various choices, such as
an elliptical column or triangular prism.
[0139] FIG. 27A is an electron microscopic image of the precursor
layer 851A for modified refractive index areas, which image was
taken immediately after the etching of that layer (corresponding to
FIG. 26C). This image demonstrates that a large number of modified
refractive index areas 851 in the form of triangular prisms were
formed. FIG. 27B is an electron microscopic image showing a
vertical section of the two-dimensional photonic crystal laser
created in the present embodiment, which image was taken after the
completion of the laser. This image demonstrates that the base body
852 was solidly formed (without any void) around the modified
refractive index areas 851.
[0140] In the case of creating the base body 852 using a material
different from that of the second cladding layer 86, the material
supplied to the surface of the modifier refractive index areas 851
and the base body 852 is changed to a material for the second
cladding layer 86 after the base body 852 has grown to the same
level as the modified refractive index areas 851 (FIG. 26D).
Embodiment 12
[0141] Another embodiment of the third mode of the method of
producing a two-dimensional photonic crystal laser is hereinafter
described by means of FIGS. 28A-28D. In the present embodiment, the
precursor layer 851B for modified refractive index areas is created
by an epitaxial method using a material having the same crystal
structure as the base body 852, such as
Al.sub..alpha.Ga.sub.1-.alpha.As (0<=.alpha.<=1) or
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P
(0<=.beta.<=1, 0<=.gamma.<=1) (FIG. 28A). After the
precursor layer 851B is formed, the precursor layer 851B is
partially removed from the top surface to a middle level by
electron beam lithography and etching, leaving a group of
cyclically arranged columnar areas. As a result, columnar modified
refractive index areas 851C are formed on a spacer layer 853 which
consists of the remaining bottom portion of the precursor layer
851B (FIG. 28B). Subsequently, using the spacer layer 853 and the
modified refractive index areas 851C as the epitaxial-growth
substrate layer, the base body 852 and the second cladding layer 86
are created by epitaxially growing a crystal of p-type
Al.sub.0.65Ga.sub.0.35As on the epitaxial-growth substrate layer
(FIG. 28C). After that, similarly to Embodiment 11, the lower
electrode 88, the contact layer 87 and the upper electrode 89 are
formed (FIG. 26D) to complete the two-dimensional photonic crystal
laser.
Embodiment 13
[0142] Another embodiment of the third mode of the method of
producing a two-dimensional photonic crystal laser is hereinafter
described by means of FIGS. 29A-29E. First, the first cladding
layer 82, the active layer 83, the carrier-blocking layer 84 and
the precursor layer 851B for modified refractive index areas are
formed in this order on the substrate 81 by an epitaxial method
using the same method and materials as used in Embodiment 12. Next,
a crystal-growth inhibiting film 91 made of SiO.sub.2 is formed on
the precursor layer 851B (FIG. 29A). SiO.sub.2 is a material
capable of inhibiting the epitaxial growth of Al.sub.xGa.sub.1-xAs
(x=0.65 in the present embodiment) used for forming the base body.
It is also possible to use Si.sub.3N.sub.4, ZnO or ZrO.sub.2 as the
material for the crystal-growth inhibiting film 91. Subsequently, a
pattern of a resist 92 for masking the areas corresponding to the
modified refractive index areas is formed by electron beam
lithography, and an etching process using an etchant capable of
removing the crystal-growth inhibiting film 91 is carried out.
After that, another etching process using a different etchant is
performed for the precursor layer 851B. As a result, modified
refractive index areas 851C with the crystal-growth inhibiting film
91 on their top faces are formed (FIG. 29B). Subsequently, the base
body 852 is created by epitaxially growing a crystal of p-type
Al.sub.0.65Ga.sub.0.35As on the space layer 853, which is formed by
the remaining lower portions of the precursor layer 851B and serves
as the epitaxial growth substrate layer (FIG. 29C). In this
process, the crystal-growth inhibiting film 91 on the top faces of
the modified refractive index areas 851C prevents the p-type
Al.sub.0.65Ga.sub.0.35As crystal from horizontally extending from
the top faces of the modified refractive index areas 851C and
closing the spaces between the modified refractive index areas
851C, so that no void will be formed inside the base body 852.
Next, the crystal-growth inhibiting film 91 is removed, and the
second cladding layer 86 made of p-type Al.sub.0.65Ga.sub.0.35As is
created on the modified refractive index areas 851C and the
base-body layer 852 by an epitaxial method (FIG. 29D). After that,
similarly to Embodiment 11, the lower electrode 88, the contact
layer 87 and the upper electrode 89 are formed (FIG. 29E) to
complete the two-dimensional photonic crystal laser.
[0143] In the present embodiment, after the base body 852 is formed
(FIG. 29C), it is possible to create the second cladding layer 86
without removing the crystal-growth inhibiting film 91. In this
case, after the crystal of the second cladding layer 86 has grown
to a level higher than the crystal-growth inhibiting film 91, the
crystal grows not only in the vertical direction but also in the
horizontal direction. As a result, the upper surface of the
crystal-growth inhibiting film 91 will be covered with the second
cladding layer 86. It is also possible to use different materials
for the base body 852 and the second cladding layer 86.
[0144] The previously described embodiments of the second and third
modes are mere examples, and any change, modification or addition
may be appropriately made within the spirit of the present
invention. For example, in the aforementioned embodiments, the
base-body layer had a one-layer structure made of
Al.sub.0.1Ga.sub.0.9As. This can be changed to a multi-layer
structure having a plurality of Al.sub..alpha.Ga.sub.1-.alpha.As
layers with different values of a. This structure may be further
modified by replacing a portion of the layers with a GaAs layer or
another kind of semiconductor layer. Similarly, when a material
selected from the
(Al.sub..beta.Ga.sub.1-.beta.).sub..gamma.In.sub.1-.gamma.P group
is used for the base-body layer, it is possible to adopt a
multi-layer structure having a plurality of layers with different
values of .beta. and .gamma., and to further modify this structure
by replacing a portion of the layers with a GaAs layer or another
kind of semiconductor layer.
EXPLANATION OF NUMERALS
[0145] 10, 10A, 10B, 10C, 10D . . . Two-Dimensional Photonic
Crystal Laser [0146] 11, 41, 61, 81 . . . Substrate [0147] 12, 62,
82 . . . First Cladding Layer [0148] 13, 43, 63, 83 . . . Active
Layer [0149] 14, 44, 64, 84 . . . Carrier-Blocking Layer [0150] 15,
15A, 15C, 15D, 45, 45A, 65, 85 . . . Two-Dimensional Photonic
Crystal Layer [0151] 151, 151A, 151B, 451, 651A, 651C . . . Air
Hole [0152] 151C . . . Modified Refractive Index Area Consisting of
Modified Refractive Index Member [0153] 152, 152A, 652A . . .
Base-Body Layer [0154] 1521A . . . First Base-Body Layer [0155]
1522A . . . Second Base-Body Layer [0156] 152B, 652, 852 . . . Base
Body [0157] 16, 66, 66A, 66B, 86 . . . Second Cladding Layer
(Epitaxial Growth Layer) [0158] 17, 67, 87 . . . Contact Layer
[0159] 18, 47, 68, 88 . . . Lower Electrode [0160] 19, 48, 69, 89 .
. . Upper Electrode [0161] 21 . . . Resist [0162] 31, 31A, 31B . .
. Regrowth Interface Layer [0163] 32, 32A . . . Modified Refractive
Index Member [0164] 33, 851A, 851B . . . Precursor Layer for
Modified Refractive Index Areas [0165] 33A, 853 . . . Spacer Layer
[0166] 42 . . . n-type Cladding Layer [0167] 452 . . . Surface of
Al.sub.0.1Ga.sub.0.9As [0168] 46 . . . p-type Cladding Layer [0169]
50 . . . SiO.sub.2 Film [0170] 651B, 651D, 851, 851C . . . Modified
Refractive Index Areas [0171] 653 . . . Buffer Layer [0172] 71, 91
. . . Crystal-Growth Inhibiting Film [0173] 72, 92 . . . Resist
* * * * *