JP4557111B2 - Quantum dot semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser serving as a light source for optical communication, optical interconnection, and the like, and more particularly to a semiconductor laser using quantum dots as an active layer.
[0002]
[Prior art]
A quantum dot structure in which electrons are confined three-dimensionally has a density of states having a delta function-like discrete level. Therefore, when electrons (or holes) are injected into the quantum dots, the electrons (or holes) are concentrated at discrete level energy. As a result, the emission spectrum from the quantum dots has a very narrow energy spread and a high intensity. When this quantum dot structure is applied to the active layer of a semiconductor laser, it is expected that the threshold of the semiconductor laser is reduced, temperature characteristics are improved, and the modulation band is expanded.
[0003]
In order to operate a quantum dot laser with these excellent characteristics, it is necessary to form a quantum dot structure with high density and high uniformity without deterioration of crystallinity.
[0004]
Conventionally, the quantum dot structure is formed by processing such as lithography and dry etching. In this case, since processing damage is introduced into the crystal, the efficiency of light emission is significantly reduced.
[0005]
On the other hand, a self-forming method (or self-organization method) that forms a quantum dot structure only by crystal growth has been proposed. For example, by growing lattice mismatched InGaAs on a GaAs substrate, InGaAs grows in an island shape when the critical film thickness is exceeded, and this island-shaped crystal has a size of several tens of nanometers suitable for a quantum dot structure. (D. Leonard et al., Applied Physics Letters, 63, 3202 (1993)). Thereafter, this self-forming method was established as a method for producing quantum dots, and its application to semiconductor lasers has been actively studied.
[0006]
[Problems to be solved by the invention]
Among the characteristics of quantum dot lasers, the modulation band was examined (D. Klotzkin et al., IEEE Photonics Technology Letters, 9, 1301 (1997)), but the band was 4.5 GHz at room temperature. . This is equivalent to or less than that of a quantum well laser and indicates that the high-speed modulation of the quantum dot laser is significantly limited.
[0007]
As a limiting factor of such high-speed modulation, there is a slow capture time of about 30 ps of carriers from the barrier layer surrounding the quantum dots to the quantum dots. This is because the energy levels of the quantum dots are discrete, so that phonon relaxation used for trapping carriers in the conventional quantum well is difficult to occur. Furthermore, since it is considered that the relaxation between the quantum levels in the quantum dots takes about 5 to 10 ps, the laser modulation band cannot be extended to 10 GHz.
[0008]
The present invention has been made in view of this point, and an object of the present invention is to promote the relaxation of carriers to quantum dots and enable high-speed modulation of quantum dot lasers.
[0009]
[Means for Solving the Problems]
The present invention comprises a quantum dot structure that constitutes an active layer and a quantum well structure for carrier injection, and is between the first level and the second level that are any two quantum levels of the quantum well structure. A semiconductor laser characterized in that an energy difference is equal to an energy difference between a first level having a lower energy than the second level and any quantum level of the quantum dot structure having a lower energy than the first level. About.
[0011]
In addition, the present invention includes a quantum dot structure constituting an active layer and a quantum well structure for carrier injection, and between the first level and the second level, which are any two quantum levels of the quantum well structure. This energy difference is equal to the energy difference between the first level of energy lower than the second level and the continuous level of the quantum dot structure of energy lower than the first level.
[0012]
Further, the present invention comprises a quantum dot structure constituting an active layer, a quantum well structure for carrier injection, and a tunnel barrier layer sandwiched between them, and any quantum level of the quantum well structure And a continuous level energy of the quantum dot structure.
[0013]
Since the continuous level has an energy width, the energy difference (ΔE _CON ) between the quantum level and the continuous level is the difference between the energy of the quantum level and the maximum energy of the continuous level (ΔE _MAX ). shall have a difference (Delta] E _MIN) to the energy range (ΔE _MAX ~ΔE _MIN) of the minimum energy of the energy and the continuous level of the quantum level from.
[0014]
Therefore, the energy difference (ΔE1) between two quantum levels is equal to the energy difference (ΔE _CON ) between the quantum level and the continuous level. ΔE1 is within this energy range (ΔE _{MAX to} ΔE _MIN ). It means that there is.
[0015]
The energy of a certain quantum level is equal to the energy of a continuous level means that the energy of a certain quantum level is within the range from the minimum value to the maximum value of the energy of the continuous level.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described.
[0017]
First, the working principle will be described with reference to an embodiment of the present invention.
[0018]
FIG. 1A is a sectional view of a laser structure according to an embodiment of the present invention. In the figure, an n-type AlGaAs cladding layer 2, an undoped GaAs optical confinement layer 3 and an undoped GaAs barrier layer 4 are formed on an n-type GaAs substrate 1, and an InGaAs quantum well structure 5 for carrier injection is provided thereon. Yes. An InAs quantum dot 6 is directly formed thereon, and an undoped GaAs barrier layer 7, an undoped GaAs optical confinement layer 8, and a p-type AlGaAs cladding layer 9 are formed. A p-type AlGaAs cap layer 10 is formed thereon.
[0019]
In this structure, as shown in the energy band diagram of FIG. 1B, the first quantum level 11 (base level) (referred to as “E1 _QW ”) and the second of the InGaAs quantum well 5 for carrier injection. The energy difference (E2 _QW −E1 _QW ) from the quantum level 12 (excited level) (E2 _QW ) of the first quantum level 11 (E1 _QW ) of the quantum well and the InAs quantum dot 6 The layer thickness and composition of the quantum well 5 are adjusted so as to be equal to the energy difference (E1 _QW −E1 _QD ) from the first quantum level 13 (referred to as “E1 _QD ”). In this case, the electrons in the first quantum level 11 (E1 _QW ) of the quantum well _undergo a relaxation transition to the first quantum level 13 (E1 _QD ) of the quantum dot. _QW− E1 _QD ) is given to another electron in the first level 11 (E1 _QW ) of the quantum well, and this electron is excited and transitioned to the second quantum level 12 (E2 _QW ) of the quantum well. That is, the Auger transition process is caused by the condition of E1 _QW -E1 _QD = E2 _QW -E1 _QW . Since the second quantum level 12 of the quantum well has a higher degree of degeneracy than the first quantum level 11, the number of states is large. Therefore, since there are a lot of vacancies in the state of the second quantum level, the first electron is more likely to undergo excitation transition. This promotes the relaxation of electrons to the quantum dots and provides a ps order electron relaxation rate.
[0020]
In order to cause light emission from the quantum dot, it is necessary to inject holes into the quantum dot. However, since the mass of holes is larger than that of electrons, many holes remain in the quantum dots having a low potential. Therefore, when the carrier is modulated at a high speed, electrons are actually injected into the dot at a high speed, and as a result, the optical output of the laser is modulated at a high speed. Depending on the injection speed of the hole, the modulation speed of the laser is limited. Not.
[0021]
A laser structure sectional view of another embodiment is shown in FIG. As shown in the figure, an n-type AlGaAs cladding layer 2, an undoped GaAs optical confinement layer 3, and an undoped GaAs barrier layer 4 are formed on an n-type GaAs substrate 1, and an InGaAs quantum well structure 5 for carrier injection is formed thereon. An AlAs tunnel barrier layer 14 is provided. An InAs quantum dot 6 is formed thereon, and an undoped GaAs barrier layer 7, an undoped GaAs optical confinement layer 8, and a p-type AlGaAs cladding layer 9 are formed. A p-type AlGaAs cap layer 10 is formed thereon.
[0022]
In this structure, as shown in the energy band diagram of FIG. 2B, the quantum level 15 of the InGaAs quantum well 5 (referred to as “E _QW ”, here, the ground level) and the quantum level of the InAs quantum dot 6. The layer thickness and composition of the quantum well are adjusted so that the energy of 16 (referred to as “E _QD ”) is equal. As a result, electrons in the quantum level 15 (E _QW ) of the quantum well are tunnel-injected to the quantum level 16 (E _QD ) of the quantum dot through the tunnel barrier layer 14. This tunnel injection occurs at a ps order rate.
[0023]
As described above, high-speed modulation exceeding 10 GHz of the quantum dot laser becomes possible by injecting electrons into the quantum dots at high speed using Auger transition or tunnel injection.
[0024]
In addition, the quantum levels of InAs quantum dots on GaAs are known to have continuous levels on the high energy side in addition to discrete quantum levels (Y. Toda et al., Physical Review Letters, 82, 4114 (1999)). This is considered to be caused by the fact that the magnitude of distortion in the quantum dots continuously changes toward the substrate side. When trapping carriers in the quantum dots, carriers may be injected into the continuous level of the quantum dots. This continuous level has the advantage that the energy has a spread, so that it is not necessary to strictly adjust the layer thickness or composition of the quantum well structure in order to determine the quantum level energy of the quantum well.
[0025]
EXAMPLES Next, an Example is given and this invention is demonstrated in detail, referring drawings.
[0026]
Example 1
FIG. 3A is a structural sectional view of the semiconductor laser according to the first embodiment of the present invention.
[0027]
An n-type AlGaAs cladding layer 18 (Al composition 0.3, thickness 3 μm, carrier concentration 1 × 10 ¹⁸ cm ⁻³ ) on an n-type GaAs substrate 17 using a molecular beam epitaxy (MBE) apparatus, An undoped GaAs optical confinement layer 19 (thickness 0.15 μm) and an undoped GaAs barrier layer 20 (thickness 20 nm) are grown. Subsequently, an undoped InGaAs quantum well 21 (In composition 0.19) for carrier injection is grown to a thickness of 9.0 nm. Since this thickness does not exceed the critical thickness of InGaAs of this composition, InGaAs grows in layers. When InAs is supplied on this by the thickness of 4 atomic layers, InAs exceeds the critical strain thickness, island-like growth occurs, and quantum dots 22 are formed. The size of the dot is 30 nm in diameter and the thickness is 8 nm. On top of this, an undoped GaAs barrier layer 23 (thickness 20 nm), an undoped GaAs optical confinement layer 24 (thickness 0.15 μm), a p-type AlGaAs cladding layer 25 (Al composition 0.3, thickness 2 μm, carrier concentration 5 ×) 10 ¹⁷ cm ⁻³ ) and a p-type AlGaAs cap layer 26 (Al composition 0.15, thickness 0.5 μm, carrier concentration 5 × 10 ¹⁸ cm ⁻³ ) are sequentially grown using an MBE apparatus.
[0028]
The carrier injection layer 21 has a quantum well structure sandwiched between GaAs barrier layers 20 and 23, and the energy difference between the first quantum level 27 (base level) and the second quantum level 28 is 79 meV. Yes (see energy band diagram in FIG. 3B). On the other hand, the energy difference between the fourth quantum level 29 of the InAs quantum dots 22 and the first quantum level 27 of the carrier injection layer is 79 meV, which is the energy between the quantum levels of the carrier injection layer. be equivalent to. Therefore, another electron in which the electrons in the first quantum level 27 of the carrier injection layer 21 are in the same level is excited to the second quantum level 28, and the fourth quantum level 29 of the quantum dot is excited. ease. Since this Auger transition process occurs at a high speed of the order of 1 ps, carrier injection into dots can be performed at several ps. As a result, the laser can be modulated at a high speed of 10 GHz or higher.
[0029]
In this embodiment, the substrate is GaAs, but this may be InP, and the cladding layer, optical confinement layer, and barrier layer may be InAlGAs or InGaAsP. In this case, since the emission wavelength from the InAs quantum dots exceeds 1.3 microns, the light source laser for optical communication with a long wavelength is obtained.
[0030]
(Example 2)
FIG. 4A is a structural sectional view of a semiconductor laser according to a second embodiment of the present invention.
[0031]
Using an MBE apparatus, an n-type AlGaAs cladding layer 31 (Al composition 0.75, thickness 3 μm, carrier concentration 1 × 10 ¹⁸ cm ⁻³ ), undoped AlGaAs optical confinement layer 32 (Al composition) is formed on an n-type GaAs substrate 30. 0.5, thickness 0.15 μm) and an undoped AlGaAs barrier layer 33 is grown. Subsequently, an undoped InGaAs quantum well 34 (In composition 0.2) for carrier injection is grown to a thickness of 4.9 nm. Since this thickness does not exceed the critical thickness of InGaAs, InGaAs grows in layers. When InAs is supplied on this by the thickness of 4 atomic layers, InAs exceeds the critical strain thickness, island-like growth occurs, and quantum dots 35 are formed. The size of the dot is 30 nm in diameter and the thickness is 8 nm. On top of this, an undoped AlGaAs barrier layer 36, an undoped AlGaAs optical confinement layer 37 (Al composition 0.5, thickness 0.15 μm), a p-type AlGaAs cladding layer 38 (Al composition 0.75, thickness 2 μm, carrier concentration 5) × 10 ¹⁷ cm ⁻³ ) and p-type AlGaAs cap layer 39 (Al composition 0.15, thickness 0.5 μm, carrier concentration 5 × 10 ¹⁸ cm ⁻³ ) are grown sequentially with an MBE apparatus.
[0032]
The carrier injection layer 34 of this embodiment has a quantum well structure sandwiched between AlGaAs barrier layers 33 and 36, and an energy difference between the first quantum level 40 (base level) and the second quantum level 41. Is 234 meV (see energy band diagram in FIG. 4B). This energy difference is equal to the energy difference from the first quantum level 42 (base level) among a plurality of quantum levels of the quantum dots, so that electrons from the carrier injection layer 34 are transferred to the higher order levels of the quantum dots. It is directly injected into the ground level 42 without passing through the position. Thereby, the relaxation time (about 5-10 ps) when relaxing between each level in a quantum dot is not required. Therefore, the laser can be modulated at a higher speed than in the case of injection at a higher level.
[0033]
(Example 3)
FIG. 5A is a structural sectional view of a semiconductor laser according to a third embodiment of the present invention.
[0034]
Using an MBE apparatus, an n-type AlGaAs cladding layer 44 (Al composition 0.3, thickness 3 μm, carrier concentration 1 × 10 ¹⁸ cm ⁻³ ), undoped GaAs optical confinement layer 45 (thickness) is formed on an n-type GaAs substrate 43. 0.15 μm), an undoped GaAs barrier layer 46 (thickness 20 nm) is grown. Subsequently, undoped InGaAs 47 (In composition 0.4) for carrier injection is grown to a thickness of 3.6 nm. Since this thickness does not exceed the critical thickness of InGaAs, InGaAs grows in layers. On this, an AlAs tunnel barrier layer 48 is grown by 2 nm. Subsequently, when InAs is supplied by the thickness of four atomic layers, InAs exceeds the critical strain thickness, island-like growth occurs, and quantum dots 49 are formed. The size of the dot is 30 nm in diameter and the thickness is 8 nm. On top of this, an undoped GaAs barrier layer 50 (thickness 20 nm), an undoped GaAs optical confinement layer 51 (thickness 0.15 μm), a p-type AlGaAs cladding layer 52 (Al composition 0.3, thickness 2 μm, carrier concentration 5 ×). 10 ¹⁷ cm ⁻³ ) and a p-type AlGaAs cap layer 53 (Al composition 0.15, thickness 0.5 μm, carrier concentration 5 × 10 ¹⁸ cm ⁻³ ) are sequentially grown using an MBE growth apparatus.
[0035]
The carrier injection layer 47 of this embodiment has a quantum well structure, and the quantum level 54 (base level) and the energy of the quantum level 55 of the quantum dot coincide with each other (energy band diagram of FIG. 5B). reference). Accordingly, electrons in the carrier injection layer 47 tunnel through the AlAs tunnel barrier layer and enter the quantum level of the quantum dot. Since this tunneling process takes about 1 ps, high-speed carrier injection into the quantum dots is performed. Accordingly, when laser carrier modulation is performed, high-speed modulation of 10 GHz or more is possible.
[0036]
Example 4
FIG. 6A is a structural sectional view of a semiconductor laser according to a fourth embodiment of the present invention.
[0037]
An n-type AlGaAs cladding layer 57 (Al composition 0.3, thickness 3 μm, carrier concentration 1 × 10 ¹⁸ cm ⁻³ ), undoped GaAs optical confinement layer 58 (thickness) is formed on an n-type GaAs substrate 56 using an MBE apparatus. 0.15 μm), an undoped GaAs barrier layer 59 (thickness 20 nm) is grown. Subsequently, undoped InGaAs60 (In composition 0.2) for carrier injection is grown to a thickness of 5 nm. Since this thickness does not exceed the critical thickness of InGaAs, InGaAs grows in layers. On this, an AlAs tunnel barrier layer 61 is grown by 2 nm. Subsequently, when InAs is supplied by the thickness of four atomic layers, InAs exceeds the critical strain thickness, island-like growth occurs, and quantum dots 62 are formed. The size of the dot is 30 nm in diameter and the thickness is 8 nm. On top of this, an undoped GaAs barrier layer 63 (thickness 20 nm), an undoped GaAs optical confinement layer 64 (thickness 0.15 μm), a p-type AlGaAs cladding layer 65 (Al composition 0.3, thickness 2 μm, carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) and a p-type AlGaAs cap layer 66 (Al composition 0.15, thickness 0.5 μm, carrier concentration 5 × 10 ¹⁸ cm ⁻³ ) are sequentially grown using an MBE growth apparatus.
[0038]
The carrier injection layer 60 of this embodiment has a quantum well structure, and its quantum level 67 (base level) is equal to the continuous level 68 on the high energy side of the quantum level in the quantum dot. Set. Accordingly, electrons in the carrier injection layer 60 tunnel through the AlAs tunnel barrier layer and enter the continuous level of the quantum dots. Since this tunneling process takes about 1 ps, high-speed carrier injection into the quantum dots is performed. Accordingly, when laser carrier modulation is performed, high-speed modulation of 10 GHz or more is possible. Further, when the quantum dots are injected into the continuous level 68 as in this embodiment, the continuous level has an energy width of about 100 meV, so that the quantum level 67 of the quantum well structure that constitutes the carrier injection layer. There is an advantage that it is not necessary to precisely control and match the quantum level of the quantum dot.
[0039]
【The invention's effect】
In the structure of a semiconductor laser using a GaAs substrate, the quantum well structure constituting the carrier injection layer is provided next to the InAs quantum dot structure constituting the active region. Carriers can be efficiently injected into the dots by an Auger transition process with another carrier in the carrier injection layer. In addition, according to the configuration of the present invention in which the tunnel barrier layer is provided between the carrier injection layer and the quantum dots, carriers can be efficiently injected from the carrier injection layer into the quantum dots through the tunnel process. By these injection methods, it is possible to increase the direct modulation of the laser at a high speed exceeding 10 GHz, and this laser can be used as a light source for high-speed optical communication.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the principle of an Auger transition process in a quantum dot semiconductor laser of the present invention.
FIG. 2 is a diagram for explaining the principle of a tunnel injection process in the quantum dot semiconductor laser of the present invention.
FIGS. 3A and 3B are a cross-sectional view and an energy band diagram of a semiconductor laser for explaining a first embodiment of the present invention. FIGS.
FIGS. 4A and 4B are a sectional view and an energy band diagram of a semiconductor laser for explaining a second embodiment of the present invention. FIGS.
FIGS. 5A and 5B are a sectional view and an energy band diagram of a semiconductor laser for explaining a third embodiment of the present invention. FIGS.
FIGS. 6A and 6B are a structural cross-sectional view and an energy band diagram of a semiconductor laser for explaining a fourth embodiment of the present invention. FIGS.
[Explanation of symbols]
1 n-type GaAs substrate 2 n-type AlGaAs cladding layer 3 undoped GaAs optical confinement layer 4 undoped GaAs barrier layer 5 InGaAs quantum well 6 for carrier injection InAs quantum dot 7 undoped GaAs barrier layer 8 undoped GaAs optical confinement layer 9 p-type AlGaAs cladding layer 10 p-type AlGaAs cap layer 11 first quantum level of InGaAs quantum well 12 second quantum level of InGaAs quantum well 13 first quantum level of InAs quantum dot 14 AlAs tunnel barrier layer 15 quantum of InGaAs quantum well Level 16 InAs quantum dot quantum level 17 n-type GaAs substrate 18 n-type AlGaAs cladding layer 19 undoped GaAs optical confinement layer 20 undoped GaAs barrier layer 21 InGaAs quantum well 22 for carrier injection InA s quantum dot 23 undoped GaAs barrier layer 24 undoped GaAs optical confinement layer 25 p-type AlGaAs cladding layer 26 p-type AlGaAs cap layer 27 first quantum level 28 of InGaAs quantum well second quantum level 29 of InGaAs quantum well 29 InAs 4th quantum level 30 of quantum dot n-type GaAs substrate 31 n-type AlGaAs cladding layer 32 undoped AlGaAs optical confinement layer 33 undoped AlGaAs barrier layer 34 InGaAs quantum well 35 for carrier injection InAs quantum dot 36 undoped AlGaAs barrier layer 37 undoped AlGaAs Optical confinement layer 38 p-type AlGaAs cladding layer 39 p-type AlGaAs cap layer 40 InGaAs quantum well first quantum level 41 InGaAs quantum well second quantum level 4 2 First quantum level of InAs quantum dots 43 n-type GaAs substrate 44 n-type AlGaAs cladding layer 45 undoped GaAs optical confinement layer 46 undoped GaAs barrier layer 47 InGaAs quantum well for carrier injection 48 AlAs tunnel barrier layer 49 InAs quantum dot 50 Undoped GaAs barrier layer 51 Undoped GaAs optical confinement layer 52 p-type AlGaAs cladding layer 53 p-type AlGaAs cap layer 54 InGaAs quantum well quantum level 55 InAs quantum dot quantum level 56 n-type GaAs substrate 57 n-type AlGaAs cladding layer 58 Undoped GaAs optical confinement layer 59 Undoped GaAs barrier layer 60 InGaAs quantum well 61 for carrier injection AlAs tunnel barrier layer 62 InAs quantum dot 63 Undoped GaAs barrier layer 6 4 Undoped GaAs optical confinement layer 65 p-type AlGaAs cladding layer 66 p-type AlGaAs cap layer 67 InGaAs quantum well quantum level 68 InAs quantum dot continuous level

Claims (3)

  An energy difference between a first level and a second level, which are any two quantum levels of the quantum well structure, comprising a quantum dot structure constituting an active layer and a quantum well structure for carrier injection, A semiconductor laser characterized by being equal in energy difference between a first level having a lower energy than the second level and any quantum level of the quantum dot structure having a lower energy than the first level.   An energy difference between a first level and a second level, which are any two quantum levels of the quantum well structure, comprising a quantum dot structure constituting an active layer and a quantum well structure for carrier injection, A semiconductor laser characterized by being equal to an energy difference between a first level having a lower energy than the second level and a continuous level of the quantum dot structure having a lower energy than the first level.   A quantum dot structure forming an active layer; a quantum well structure for carrier injection; and a tunnel barrier layer sandwiched between the quantum dot structure and the quantum well structure. A semiconductor laser characterized in that the energy of the continuous level of the dot structure is equal.

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