JP3829153B2 - Optical semiconductor device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in an optical semiconductor device using a laminated body formed by multilayering layers in which self-assembled quantum dots are generated.
[0002]
At present, the self-assembled quantum dot layer is used as an active layer of a semiconductor laser, for example, and is an effective means for lasing with a low threshold current, but the size of the quantum dots is not uniform. Since the point is regarded as a problem, the present invention discloses a means for solving the problem.
[0003]
[Prior art]
For example, semiconductor lasers have been steadily improved in performance over the past 20 years or so, and the driving force is the active layer used as the light emitting part of the semiconductor laser. The structure of is advanced.
[0004]
In the early 1980s, a bulk material having a thickness of 100 nm or more was used as the active layer, but since the mid 1980s, a quantum well structure composed of a thin film having a thickness of 10 nm was used. , Began to use the quantum effect.
[0005]
In the 1990s, a strained quantum well structure in which strain is intentionally introduced into a quantum well is realized, and the laser oscillation threshold current is changed from 100 [mA] to 1 [mA depending on these technological advances. ], Ie, about two orders of magnitude.
[0006]
In recent years, in order to achieve further lower threshold current, attempts to construct the active layer with semiconductor quantum dots are becoming popular, and such development and research has become possible. The reason is that a new crystal growth technique for self-forming a formed crystal has been discovered, and a practical quantum dot can be grown relatively easily.
[0007]
Quantum dots are used to confine carriers in a three-dimensionally narrow region and fully quantize the energy. By utilizing this property for the active layer of a semiconductor laser, it does not contribute to laser oscillation. Since excess carriers can be reduced and optical gain can be generated efficiently, as a result, a low threshold current can be achieved.
[0008]
As a technique for creating quantum dots, a method using a step on a slightly inclined substrate, a method using a lithography technique and an etching technique, a method using growth on a shaped substrate (stepped substrate), and the like are known.
[0009]
However, the quantum dots created by the above methods are
(1) Poor crystal quality (2) Large size and sufficient quantum effect cannot be obtained (3) There are drawbacks such as low density. Therefore, to realize a high performance semiconductor laser as expected could not.
[0010]
However, according to the quantum dot formation method based on self-organization, it has become possible to produce a quantum dot that is sufficiently practical as an active layer of a semiconductor laser.
[0011]
FIG. 7 is a cut-away side view of an essential part showing an optical semiconductor device at a process point for qualitatively explaining self-organization of quantum dots.
[0012]
Refer to FIG. 7A. 7- (1)
A wet layer (wetting layer) 2 made of a semiconductor that is lattice-mismatched with the semiconductor is formed on the semiconductor substrate 1 by applying a MOCVD (Metalical Chemical Vapor Deposition) method or an MBE (Molecular Beampitity) method.
[0013]
If the wetting layer 2 is about one molecular layer or two molecular layers, epitaxial growth occurs two-dimensionally as shown.
[0014]
Refer to FIG. 7B. 7- (2)
Since the wetting layer 2 is distorted, strain energy is accumulated in the layer. Here, when a thicker crystal is deposited, the strain energy increases, and as shown in FIG. A shaped island 3 is generated.
[0015]
The cluster-like islands 3 are extremely small and have sufficient characteristics as quantum dots.
[0016]
Attempts have been made to improve the performance of semiconductor lasers by using, as an active layer, a quantum dot layer containing many self-assembled quantum dots as described above.
[0017]
FIG. 8 is a cutaway side view of a main part showing a semiconductor laser using a quantum dot layer as an active layer.
[0018]
In the figure, 11 is an n-type semiconductor substrate, 12 is an n-side cladding layer, 13 is a light confinement layer, 14 is a quantum dot active layer, 15 is a light confinement layer, 16 is a p-side cladding layer, 17 is a cap layer, Reference numeral 18 denotes a p-side electrode, and 19 denotes an n-side electrode.
[0019]
FIG. 9 is an enlarged side view of the principal part of the quantum dot active layer in the semiconductor laser described with reference to FIG. 8, and the same reference numerals as those used in FIG. It shall have meaning.
[0020]
In the figure, reference numerals 14A to 14D denote first to fourth quantum dot layers for constituting the quantum dot active layer 14, and 21A to 21C denote first to third interlayer semiconductor layers. , Q1 to Q4 indicate the first to fourth quantum dots, respectively.
[0021]
In order to cause laser oscillation in a semiconductor laser, it is necessary to have an optical gain necessary for it, and in order to achieve the purpose, the quantum dot active layer 14 has a plurality of as shown in the figure. The quantum dot layers are stacked.
[0022]
By the way, as is clear from FIG. 9, when the quantum dot layer 14A and the like are stacked,
(1) The quantum dots Q1... Are arranged in the vertical direction. (2) The quantum dots Q1... Are larger as they are included in the layer on the surface side of the stacked quantum dot layers. It has been known.
[0023]
There is no particular problem with respect to the point where the quantum dots listed as (1) are arranged in the vertical direction, but the point where the size of the quantum dots mentioned in (2) changes is a problem with semiconductor lasers.
[0024]
That is, the difference in the size of each quantum dot means that the light emission energy in each quantum dot is different, so that the contribution of the optical gain to the resonator optical mode is reduced. In order to achieve the purpose of reducing the threshold current of laser oscillation, it is preferable that the quantum dots have the same size.
[0025]
[Problems to be solved by the invention]
In the present invention, when a layer in which self-assembled quantum dots are generated is formed in multiple layers, the size is uniform not only within the layer but also between quantum dots of different layers. To.
[0026]
[Means for Solving the Problems]
The present inventor considered whether the included quantum dots increase as the number of stacked layers increases when the self-assembled quantum dot layers are stacked.
[0027]
Here, consider a case where a material having a lattice constant a _e larger than a _sub is crystal-grown on a semiconductor substrate having a lattice constant a _sub . The same argument can be made for a material having a smaller lattice constant than a _sub .
[0028]
FIG. 1 and FIG. 2 are fragmentary cutaway side views showing a self-assembled quantum dot layer in a process essential point for explaining an experiment conducted in realizing the optical semiconductor device of the present invention.
[0029]
1A and 1B show a state in which a quantum dot layer is further grown, wherein 21 is a semiconductor substrate, 22 is a first layer wetting layer, 23 is a first layer quantum dot, 24 Is the interlayer semiconductor layer, a ₁ is the lattice constant at the apex of the first layer quantum dot 23, a ₂ is the lattice constant at the surface of the first wetting layer 22, and a ₃ is the first layer quantum. shows the surface in the lattice constant of the interlayer semiconductor layer 24 located on the apex of the dots 23, a ₄ is the in lattice constant on the surface of the interlayer semiconductor layer 24 overlying the first layer wetting layer 22, respectively Yes.
[0030]
Note that in the quantum dot layer shown in FIG. 1A, the lattice constant a _{2 at} the surface of the wetting layer 22 and the lattice constant a _{1 at} the apex of the quantum dot 23 are different. There must be.
[0031]
That is, the top of the quantum dot 23 is considerably free from strain and has a large lattice constant close to the original lattice constant of the growth material, but the wetting layer 22 is distorted and the surface is in the same plane as the substrate 21. The lattice constant is small.
[0032]
Here, the lattice constant a _{1 at} the vertex of the quantum dot 23 is
a _e > a ₁ > a _sub
The lattice constant a _{2 on} the surface of the first wetting layer 22 is
a ₂ = a _sub
It is.
[0033]
FIG. 1B shows a state in which an interlayer semiconductor layer 24 made of the same crystal as the substrate 21 is grown on the entire surface of the quantum dot layer described with reference to FIG. The lattice constant a ₄ on the surface of the interlayer semiconductor layer 24 located above is equal to the lattice constant a _sub of the substrate 21, but the lattice constant a on the surface of the interlayer semiconductor layer 24 located on the vertex of the quantum dot 23. _{Although 3} is slightly smaller, the lattice constant is larger than the lattice constant a _{sub of the} substrate 21.
[0034]
Here, the lattice constant a _{3 on} the surface of the interlayer semiconductor layer 24 on the top of the quantum dot 23 is
a ₁ > a ₃ > a _sub
The lattice constant a _{4 on} the surface of the interlayer semiconductor layer 24 on the first wetting layer 22 is
a ₄ = a ₂ = a _sub
It is.
[0035]
FIGS. 2A and 2B illustrate a process of growing the second quantum dot layer after forming the first quantum dot layer as described with reference to FIG. Reference numeral 25 denotes a second layer wetting layer, 25A denotes a growth nucleus, and 26 denotes a second layer quantum dot.
[0036]
By the way, as described with reference to FIG. 1, the lattice constant a _{3 on} the surface of the interlayer semiconductor layer 24 on the top of the quantum dot 23 is slightly larger than the lattice constant a ₁ on the top of the quantum dot 23. Although smaller, when compared with the lattice constant a _sub of the substrate 21, that is, when compared with the lattice constant a _{4 on} the surface of the interlayer semiconductor layer 24 on the first wetting layer 22, the lattice constant is large. And
a _e −a ₃ > a _e −a ₄
It is.
[0037]
As a result, the growth nucleus 25A, which is generally InAs, is generated in the portion of the interlayer semiconductor layer 24 on the top of the quantum dot 23 where the strain energy is small, and once the growth nucleus 25A is generated. Then, the second-layer quantum dots 26 are grown there, and this is the reason why the quantum dots 23, 26,.
[0038]
The second layer quantum dot 26 has a difference in lattice constant with the lattice constant a _{3 on} the surface of the interlayer semiconductor layer 24 on the base, that is, on the vertex of the quantum dot 23, so that the first layer quantum dot Smaller than 23,
a _e −a ₃ <a _e −a _sub
Therefore, since the strain energy is also small, a larger quantum dot 26 is formed.
[0039]
Therefore, when the process as described above is repeated, a larger quantum dot is generated as the quantum dot layer stacks upward.
[0040]
In order to solve this problem and to align the size of the quantum dots, it is necessary to reduce the lattice constant of the interlayer semiconductor layer grown on the entire surface after forming a certain quantum dot layer. If the lattice constant is a _c ,
a _c <a _sub
What should I do?
[0041]
FIG. 3 and FIG. 4 are side sectional views showing the main part of the self-assembled quantum dot layer at the process points for explaining the principle of the present invention.
[0042]
FIGS. 3A and 3B show a state in which the quantum dot layer is grown one layer and then the interlayer semiconductor layer is grown, where 31 is the semiconductor substrate, 32 is the first wetting layer, and 33 is First layer quantum dots, 34 is an interlayer semiconductor layer, 34A is a growth nucleus, a ₅ is a lattice constant of the interlayer semiconductor layer 34 on the top of the quantum dot 33, and a ₆ is on the first wetting layer 32 The lattice constants on the surface of the interlayer semiconductor layer 34 in FIG. 3 and 4, the lattice constant at the apex of the first layer quantum dot 33 is a ₁ , and the first layer wetting layer 32 is the same as described with reference to FIGS. 1 and 2. The lattice constant is assumed to be a ₂ .
[0043]
In the present invention, when the lattice constant of the material constituting the interlayer semiconductor layer 34 is a _c , as described above,
a _c <a _sub
When the material is used, the lattice constant a _{5 on} the surface of the interlayer semiconductor layer 34 on the top of the quantum dot 33 is
a ₃ > a ₅ > a _sub
, And the thus, inter semiconductor layer 34 by appropriately selecting the size of the lattice constant a _c of the material constituting the surface in the lattice constant of the interlayer semiconductor layer 34 located on the vertices of the quantum dots 33 a ₅ can be made close to the lattice constant a _sub of the substrate 31, and the increase in size in the quantum dots can be made small enough to be ignored.
[0044]
In this case, the lattice constant a _{6 on} the surface of the interlayer semiconductor layer 34 on the first wetting layer 32 is
a ₆ = a _sub
It becomes.
[0045]
FIG. 4 illustrates the second quantum dot layer formed in the same manner as the first quantum dot layer after the first quantum dot layer described with reference to FIGS. 3A and 3B is formed. The step of growing a layer is shown. 35 indicates a second-layer wetting layer, 36 indicates a second-layer quantum dot, and L indicates the size of the quantum dot.
[0046]
As shown in the figure, the size L of the first layer quantum dots 33 and the second layer quantum dots 36 are almost the same, and this state can be realized even if the quantum dot layer is further multilayered. it can.
[0047]
As described above, in the optical semiconductor device of the present invention, the optical semiconductor device has a stacked body in which a plurality of quantum dot layers in which self-assembled quantum dots are formed are stacked on a substrate, and a _sub : substrate (for example, semiconductor substrate 31). : lattice constant of the reference Figure 3), a _c: interlayer semiconductor layer of the quantum dot layers (e.g. interlayer semiconductor layer 34: lattice constant of the reference Figure 3), a _e: lattice constant of the semiconductor material (semiconductor forming the wetting layer A _c <a _sub <a _e or a _e <a _sub <a _c .
[0048]
When the layers in which self-assembled quantum dots are generated are formed in multiple layers by adopting the above means, the size is not limited to the inside of the layers but also between the quantum dots of different layers. Therefore, it is preferable to construct a semiconductor laser that oscillates with a low threshold current using the quantum dot layer as an active layer.
[0049]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 5 and FIG. 6 are fragmentary cutaway side views showing a self-assembled quantum dot layer at a process point for explaining a process for manufacturing an optical semiconductor device according to an embodiment of the present invention. . Hereinafter, description will be given with reference to the drawings.
[0050]
Refer to FIG. 5A. 5- (1)
By applying the MOCVD method, the wetting layer 42 and the first layer quantum dots 43 are formed on the substrate 41.
[0051]
Here, main data regarding each semiconductor part is exemplified as follows.
(1) About substrate 41 Material: GaAs
Lattice constant a _sub = 5.6533 [Å]
(2) Wetting layer 42
Material: InAs
Lattice constant a _e = 6.058 [Å]
(3) About the first layer quantum dots 43 Material: InAs
Vertex lattice constant a ₁ : a _sub <a ₁ <a _e
[0052]
Refer to FIG. 5B. 5- (2)
By applying the MOCVD method, the interlayer semiconductor layer 44 is formed on the first quantum dot layer.
[0053]
Examples of main data regarding the interlayer semiconductor layer 44 are as follows.
Material: GaAs _0.7 P _0.3
Lattice constant a _c : 5.593 [Å]
Lattice constant a _{5 at} the surface of the portion on the first layer quantum dot 43
: Slightly larger than a _sub Lattice constant a _{6 on} the surface of the portion on the first wetting layer 42
: ≒ a _sub
Therefore, a _sub <a ₆ <a ₅ .
[0054]
Refer to FIG. 6 (A) 6- (1)
By applying the MOCVD method, when the source gas for forming the wetting layer is sprayed on the interlayer semiconductor layer 44, the growth nucleus 44A is generated spontaneously.
[0055]
Refer to FIG. 6 (B) 6- (2)
Subsequently, a film is formed on the interlayer semiconductor layer 44 by applying the MOCVD method, and the wetting layer 45 and the second-layer quantum dots 46 are formed.
[0056]
Here, main data regarding each semiconductor part is exemplified as follows.
(1) About the wetting layer 45 Material: InAs
Lattice constant a _e = 6.058 [Å]
Thickness: 1 molecular layer to 2 molecular layer (2) About the second layer quantum dot 46 Material: InAs
Vertex lattice constant a ₁ : a _sub <a ₁ <a _e
[0057]
As shown in the figure, the first layer quantum dots 43 and the second layer quantum dots 46 are almost the same size, and this state can be realized without change even if the quantum dot layer is further multilayered. .
[0058]
The present invention is not limited to the above embodiment, and many other modifications can be realized.
[0059]
For example, in the above-described embodiment, the multilayer body in which a plurality of quantum dot layers in which self-assembled quantum dots are formed is stacked on a substrate is used as an active layer in a semiconductor laser. The laminate can be used as an active layer in a semiconductor optical modulator or a semiconductor optical switch.
[0060]
In the above embodiment, GaAs is used for the substrate, InAs is used for the quantum dots, and GaAsP is used for the interlayer semiconductor layer between the quantum dots.
(1) InGaAs is used for quantum dots in the material system,
(2) Use GaAs for the substrate, InGaAs or InAs for the quantum dots, and InGaP for the interlayer semiconductor layer between the quantum dots.
(3) Use GaAs for the substrate, InGaAs or InAs for the quantum dots, and InGaAsP for the interlayer semiconductor layer between the quantum dots.
(4) Use GaAs for the substrate, InGaAs or InAs for the quantum dots, and AlAsP for the interlayer semiconductor layer between the quantum dots.
(5) InP can be used for the substrate, InPAs or InAs for the quantum dots, and InGaP, GaAsP, InGaAsP, or AlAsP can be used for the interlayer semiconductor layer between the quantum dots.
[0061]
【The invention's effect】
In the optical semiconductor device according to the present invention, a plurality of quantum dot layers in which self-assembled quantum dots are formed has a laminate formed on a substrate, a _sub : lattice constant of the substrate, a _c : Lattice constant of the interlayer semiconductor layer between the quantum dot layers, a _e : lattice constant of the semiconductor material (lattice constant of the semiconductor material itself constituting the wetting layer) a _c <a _sub <a _e or a _e < characterized in that it is a a _sub <a _c.
[0062]
When the layers in which self-assembled quantum dots are generated are formed in multiple layers by adopting the above configuration, the size is not limited to the inside of the layer, but also between the quantum dots of different layers. Therefore, it is preferable to construct a semiconductor laser that oscillates with a low threshold current using the quantum dot layer as an active layer.
[Brief description of the drawings]
FIG. 1 is a cutaway side view of a principal part showing a self-assembled quantum dot layer in a process essential point for explaining an experiment conducted when realizing an optical semiconductor device of the present invention.
FIG. 2 is a cut-away side view of a main part showing a self-assembled quantum dot layer in a process key point for explaining an experiment conducted when realizing an optical semiconductor device of the present invention.
FIG. 3 is a cut-away side view of a principal part showing a self-assembled quantum dot layer at a process point for explaining the principle of the present invention.
FIG. 4 is a cut-away side view of a principal part showing a self-assembled quantum dot layer at a process point for explaining the principle of the present invention.
FIG. 5 is a cut-away side view of a main part showing a self-assembled quantum dot layer at a process point for explaining a process for manufacturing an optical semiconductor device according to an embodiment of the present invention.
FIG. 6 is a cutaway side view of a principal part showing a self-assembled quantum dot layer at a process point for explaining a process for manufacturing an optical semiconductor device according to an embodiment of the present invention.
FIG. 7 is a cut-away side view of an essential part showing an optical semiconductor device at a process point for qualitatively explaining self-organization of quantum dots.
FIG. 8 is a cutaway side view of a main part showing a semiconductor laser using a quantum dot layer as an active layer.
FIG. 9 is an enlarged side view of a principal part showing an enlarged quantum dot active layer in the semiconductor laser described with reference to FIG. 8;
[Explanation of symbols]
31 semiconductor substrate 32 first layer wetting layer 33 first layer quantum dot 34 interlayer semiconductor layer 34A growth nucleus a _sub lattice constant a _e lattice constant a _c lattice constant a _{1 to} a ₆ lattice constant

Claims (1)

A plurality of quantum dot layers in which self-assembled quantum dots are formed have a laminate in which the substrate is laminated on the substrate,
a _sub : lattice constant of the substrate a _c : lattice constant of the interlayer semiconductor layer between the quantum dot layers a _e : lattice constant of the semiconductor material itself constituting the wetting layer a _c <a _sub <a _e or a _e <A _sub < _ac
An optical semiconductor device characterized by the above.

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