JP3044604B2 - Semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly, to a semiconductor laser having a low threshold value at a high temperature in a 1.3 μm band, excellent in temperature characteristics, and requiring no cooling mechanism.

[0002]

2. Description of the Related Art A 1.3 .mu.m band semiconductor laser for subscriber optical communication is required to be capable of operating in a high temperature range. One of the causes of the deterioration of the characteristics of semiconductor lasers at high temperatures is that the carriers (especially electrons) injected into the active layer become high-energy states due to heat, exceed the height of the barrier layer, overflow the active layer, and emit light. Is reduced.

In a semiconductor laser having a quantum well layer, in order to suppress the overflow of electrons from the quantum well layer, a quantum well having a large energy discontinuity in the conduction band between the quantum well layer and the barrier layer is used. The structure is considered valid. A quantum well structure using a mixed crystal containing Al for the barrier layer can have a large conduction band discontinuity, so that high temperature characteristics are expected. Zah et al.
"IEJ Journal of Quantum Electronics (IEEE J. Quantum Electron.)"
Vol. 30, No. 2, pp. 511-523 (1994) reports that good temperature characteristics were obtained in a semiconductor laser using AlGaInAs as a barrier layer.

Further, it has been reported that the threshold current density can be reduced by changing the valence band structure by forming the quantum well layer into a strained quantum well structure. Especially 1.3
It has been reported that in semiconductor lasers in the μm band, the threshold is lowered by using a strained InAsP quantum well layer. Ohashi et al., “Electronicsletters (Electronicslet
Vol. 31, No. 7, p. 556 (1995), reports a low threshold value and high efficiency characteristics of a strained quantum well semiconductor laser formed of an InAsP strained quantum well layer and an InGaAsP barrier layer. .

[0005]

However, in the semiconductor laser reported by Zar et al., An AlG
Since the aInAs layer is used, it is difficult to lower the threshold. In general, when the quantum well layer is made of a material containing Al, the non-radiative recombination lifetime of the quantum well becomes short, so that it is difficult to lower the threshold.

On the other hand, in the semiconductor laser reported by Ohashi et al., Since the barrier layer is composed of an InGaAsP layer, the carrier overflow at a high temperature is large and its characteristic temperature is as small as 60 to 70K.

An object of the present invention is to provide a semiconductor laser having a low threshold value and a high characteristic temperature.

[0008]

Means for Solving the Problems The present inventor has made various studies in order to achieve the above object, and as a result, completed the present invention.

According to a first aspect of the present invention, a first optical waveguide layer having a first conductivity and a second optical waveguide layer having a conductivity different from the first conductivity are stacked on an InP substrate. And a semiconductor laser having an active layer region sandwiched between the two optical waveguide layers, wherein the active layer region has at least one quantum well layer made of InAsP having compressive strain. The well layer is sandwiched between barrier layers made of InAlGaAs, and the barrier layer is combined with the strained quantum well layer.
A semiconductor laser having a composition for forming a type I superlattice .

A second invention relates to the semiconductor laser according to the first invention, wherein the composition of the barrier layer made of InAlGaAs is such that the barrier layer lattice-matches with InP.

According to a third aspect of the present invention, the composition of the barrier layer made of InAlGaAs is such that the barrier layer has a tensile strain with respect to the InP substrate and compensates for at least a part of the compressive strain of the quantum well layer. The present invention relates to a semiconductor laser according to a first aspect of the present invention having the following composition.

A fourth invention relates to the semiconductor laser according to the first invention, wherein a spacer layer made of InP is provided between the quantum well layer and the barrier layer, and the thickness of the spacer layer is 1 to 10 molecular layers. .

In the first invention, the use of an InAsP strained quantum well layer energetically separates heavy holes and light holes and reduces the effective mass of in-plane holes, thereby reducing the threshold voltage. At the same time, the barrier layer is made of InA.
By using lGaAs, the conduction band discontinuity with InAsP is increased, and the overflow of electrons to the cladding layer is reduced.

In the second invention, InAlGaAs
Since the barrier layer has a composition lattice-matched to InP, crystal growth is easy, and a high-quality active layer region can be formed.

In a third aspect of the present invention, InAlGaAs
By imparting tensile strain to the barrier layer, at least part of the compressive strain of the strained quantum well layer is compensated and reduced, so that the structural stability of the strained quantum well layer is increased and reliability at high temperature operation is improved. And the degree of freedom of design parameters such as the amount of strain and the thickness of the strained layer when the active layer region is formed of a multiple quantum well layer.

In the fourth aspect of the present invention, by introducing a spacer layer composed of 1 to 10 molecular layers of InP between the strained quantum well layer and the barrier layer, an interval between the quantum well layer and the barrier layer is ensured. Thereby, it is possible to suppress capture of carriers at non-radiative recombination centers such as lattice defects and impurities contained in the mixed crystal containing Al, and to prevent a decrease in luminous efficiency in the quantum well layer. Become. Further, when the thickness of the InP spacer layer is increased, it becomes difficult to inject holes into the InAsP layer. Therefore, the thickness is desirably 10 molecular layers or less.

[0017]

FIG. 1 is a sectional view showing a basic structure of an embodiment of a semiconductor laser according to the present invention. This semiconductor laser is a ridge waveguide type laser. An active layer region 12 is laminated on an n-type optical waveguide layer 11, and a p-type optical waveguide layer 13 is laminated thereon. Further, a high-concentration p-type doped contact layer 14 is laminated on the p-type optical waveguide layer 13. The present semiconductor laser has two trenches 19 for realizing current confinement, and an insulator film 15 is formed to prevent current from flowing except for the ridge portion 18 sandwiched between the two trenches 19. Have been. Also on the outside p
-Type contact metal film 16 and n-type contact metal film 17
Are formed respectively. When a voltage is applied between the two metal films by the voltage power supply 10, a current flows only in the ridge portion 18 and light is emitted in the active layer region 12. The generated light is guided in the ridge stripe direction by the upper and lower optical waveguide layers 11 and 13. The end of the stripe is a cleavage plane, and a part of the light is reflected to form an optical resonator as a whole.
In this way, laser oscillation can be obtained at a threshold current density or higher.

An important part of the present invention is the configuration of the active layer region 12 in the above basic structure, which will be specifically described below with reference to a detailed structural diagram. FIG. 2 shows a layer structure of the semiconductor laser according to the first embodiment of the present invention.
First, the same n-type InP layer 21 is laminated on the n-type InP substrate 20, and these two serve as the n-type optical waveguide layer 11. A lower InAlAs layer 22 and InAlGa
As barrier layers 23 and strained InAsP quantum well layers 24 are alternately stacked, and an upper InAlAs layer 25 is stacked thereon, thereby forming the active layer region 12 as a whole. Lower I
The nAlAs layer 22 and the upper InAlAs layer 25 are for improving light confinement in the active layer region. In
The composition of the AlGaAs barrier layer 23 needs to be selected so that, in combination with the strained InAsP quantum well layer 24, a type I superlattice in which electrons and holes are both confined in the quantum well layer.

FIG. 4 shows the strained InAsP quantum well layer 24 forming the active layer region 12 of the semiconductor laser of the present invention and InA.
This shows the composition dependence of the band structure of the lGaAs barrier layer 23. The right half of the figure shows the band structure of InAsP, and the left half shows the band structure of InAlGaAs lattice-matched to InP. The upper line in each band structure diagram represents the conduction band, and the lower line represents the valence band. First, the composition of the strained InAsP quantum well layer serving as the quantum well layer is determined to be As from the requirement that the oscillation wavelength of the semiconductor laser is around 1.3 μm.
An appropriate composition is 0.3 or more. InAlGaAs
As the barrier layer, for example, As of a strained InAsP quantum well layer
Assuming that the composition is 0.45, a type I superlattice is formed with an Al composition of 0.5 or more from FIG.

In the above description, the case where the InAlGaAs barrier layer 23 is lattice-matched to InP has been described, but the InAlGaAs barrier layer 23 may also be strained. In particular, an appropriate amount of tensile strain is InAlGaAs.
When applied to the s-barrier layer 23, the strain is balanced as a whole in the active layer region 12, so that the active layer region 12 is structurally stabilized. Even in this case, what is important is that the InAlGaAs barrier layer 23
Is selected so that, in combination with the strained InAsP quantum well layer 24, a type I superlattice in which both electrons and holes are confined in the quantum well layer.

FIG. 3 is a band structure diagram of the semiconductor laser of the present invention thus formed. First, n-type InP
The lower InAlAs layer 22 stacked on the layer 21 raises both the conduction band and the valence band upward (toward the vacuum level). Subsequently, the InAlGaAs barrier layer 23 and the strain I
A quantum well structure is formed by alternately stacking the nAsP quantum well layers 24. A feature of the present invention is that, as shown in FIG. 3, the confinement energy of electrons is larger than the confinement energy of holes, so that overflow from the quantum well layer can be suppressed.

Second Embodiment FIG. 5 shows a layer structure of a semiconductor laser according to a second embodiment of the present invention. Most is the same as FIG. 2 except that an InP spacer layer 51 is interposed between the strained InAsP quantum well layer 24 and the InAlGaAs barrier layer 23. By inserting the InP spacer layer 51,
An interval between the quantum well layer and the barrier layer is ensured, thereby
It is possible to suppress trapping of carriers at non-radiative recombination centers such as lattice defects and impurities contained in the nAlGaAs barrier layer 23, and to prevent a reduction in luminous efficiency in the quantum well layer. If the layer thickness of the InP spacer layer is too large, it becomes difficult to inject holes into the strained InAsP quantum well layer 24. Therefore, the layer thickness is desirably 10 molecular layers or less.

[0023]

EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

Example 1 A semiconductor laser having the layer structure shown in FIG. 2 was manufactured by using a gas source molecular beam epitaxy method. First, on the Sn-doped (3 × 10 ¹⁸ cm ⁻³ ) InP substrate 20,
0 nm Si-doped (1 × 10 ¹⁸ cm ⁻³ ) InP layer 21
Were laminated. Next, a thickness of 50 nm lattice-matched to InP
Undoped In _0.53 Al _0.47 As layer 22 is laminated, followed by 10.5 nm thick In _0.53 (Al _0.6 Ga _0.4 ).
_0.47 As barrier layer 23 and 6 nm thick strained InAs _0.45 P
Four layers of _0.55 quantum well layers 24 are stacked, and undoped In _0.53 Al having a thickness of 50 nm lattice-matched to InP is stacked thereon.
_The active layer region 12 was formed by laminating a _0.47 As layer 25.
A 2000 nm thick Be-doped layer (1 × 10 ¹⁸ c
m- ³ ) An InP layer was laminated to form a p-type optical waveguide layer 13.
Finally, as a p-type contact layer, Be-doped (1 × 10 ¹⁹⁾
cm ^-3 ) InGaAs contact layer 14 is 50 nm thick
Was laminated.

In this embodiment, the strained InAsP quantum well layer 2
4 and the conduction band discontinuity of the InAlGaAs barrier layer 23 are 200 meV, and a large electron confinement effect is expected.

For the ridge stripe laser, first, trenches 19 were formed on both sides of the ridge stripe by lithography and chemical etching as shown in FIG. A 200 nm thick Si is applied to the region other than the top surface of the ridge stripe.
An O ₂ insulator film 15 was formed so that current could be injected only into the ridge stripe portion. Then, p of Ti / Au
A contact metal film 16 and an n-type contact metal film 17 of Ti / Au were formed and alloyed. The threshold current density of this semiconductor laser at room temperature was 1.3 KA / cm ² . In addition, the characteristic temperature up to 85 ° C. was about 100 K, and good characteristics were obtained.

Embodiment 2 This embodiment is an example in which tensile strain is applied to the InAlGaAs barrier layer. The layer structure is the same as that described in the first embodiment except for the active layer region.
The composition and thickness of the As barrier layer are different. In this embodiment,
The composition of the InAlGaAs barrier layer was changed to In _0.375 (Al _0.6 G
a _0.4 ) _0.625 As, and a tensile strain of about 1.08% was introduced into the barrier layer. The strained InAs _0.45 P _0.55 quantum well layer has a compressive strain of about 1.43%, a thickness of 6 nm, and a strained InAlGaAs.
When the thickness of the s barrier layer is 8 nm, the strain in the active layer region is almost compensated. As a result, the structure of the strained superlattice can be stabilized, and the reliability of the semiconductor laser during high-temperature operation can be improved.

Embodiment 3 An example in which the InP spacer layer 51 shown in FIG. 5 is inserted will be described. The layer structure is the strain compensation InAsP / InA described above.
Five molecular layers of an InP spacer layer 51 were inserted between each quantum well layer 24 and the barrier layer 23 in the 1GaAs quantum well structure. Thereby, the photoluminescence intensity from the active layer region was increased about 5 times, and the optical characteristics were improved. The threshold current density at room temperature of the semiconductor laser thus manufactured was 1.1 KA / cm ² , and the characteristic temperature up to 85 ° C. was about 116 K. Improvements in properties were seen.

Although the present invention has been described with reference to the ridge stripe semiconductor laser, similar effects can be obtained with other laser structures such as a buried semiconductor laser. Further, as a growth method, a method other than the gas source molecular beam epitaxy method, for example, a vapor phase growth method such as a metalorganic vapor phase epitaxy method may be used. Further, the light confinement structure in the active layer region is I
A GRIN-SCH structure in which the composition of nAlGaAs is changed may be used. In order to reduce the p-type contact resistance, InGaAsP is provided between the p-type InP layer (p-type optical waveguide layer) 13 and the p-type InGaAs contact layer 14.
The effect of the present invention can be obtained even if such a layer is inserted. Similarly,
A stepped layer of InAlGaAs or the like may be provided between the n-type InP layer 21 and the lower InAlAs layer 22.

[0030]

As is apparent from the above description, according to the present invention, high efficiency laser characteristics with a small threshold current can be realized even at a high temperature of about 85.degree. The reason is that the use of the strained InAsP layer as the quantum well layer enables the threshold value to be reduced and the efficiency to be improved, and the InAlGaAs layer has a type I quantum well structure in combination with the strained InAsP layer. This is because by selecting the Al composition, the overflow of electrons is suppressed, and the characteristic deterioration at high temperatures is suppressed.

The reliability of the laser is improved by using the tensile strained InAlGaAs layer, and the insertion of the InP spacer layer is effective in lowering the threshold value of the laser.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a basic structure of an embodiment of a semiconductor laser of the present invention.

FIG. 2 is a layer structure diagram of a first embodiment of the semiconductor laser of the present invention.

FIG. 3 is a band structure diagram of the first embodiment of the semiconductor laser of the present invention. .

FIG. 4 is a diagram showing the composition dependence of the band structure of a strained InAsP quantum well layer and an InAlGaAs barrier layer forming an active layer region of the semiconductor laser of the present invention.

FIG. 5 is a layer structure diagram of a second embodiment of the semiconductor laser of the present invention.

[Explanation of symbols]

 Reference Signs List 10 voltage power supply 11 n-type optical waveguide layer 12 active layer region 13 p-type optical waveguide layer 14 high-concentration p-type doped contact layer 15 insulator film 16 p-type contact metal film 17 n-type contact metal film 18 ridge portion 19 trench 20 n Type InP substrate 21 n-type InP layer 22 lower InAlAs layer 23 InAlGaAs barrier layer 24 strained InAsP quantum well layer 25 upper InAlAs layer 51 InP spacer layer

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-2-130988 (JP, A) JP-A-7-135370 (JP, A) JP-A-5-267773 (JP, A) JP-A-5-267773 7054 (JP, A) IEEE. J. Quantum. Electron 30 [2] (1994) p. 524-532 IEEE. J. Quantum. Electron 30 [2] (1994) p. 511-523 Phys. Rev .. B 53 [4] (1996) p. 1990-1996 (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 JICST file (JOIS)

Claims (4)

(57) [Claims] A first optical waveguide layer having a first conductivity, a second optical waveguide layer having a conductivity different from the first conductivity, laminated on the InP substrate; In a semiconductor laser having an active layer region sandwiched between two optical waveguide layers,
In wherein the active layer region has at least one compressive strain.
A strained quantum well layer sandwiched between barrier layers made of InAlGaAs ;
The barrier layer is tied in combination with the strained quantum well layer.
A semiconductor laser having a composition for forming a type I superlattice . 2. The semiconductor laser according to claim 1, wherein the composition of the barrier layer made of InAlGaAs is such that the barrier layer lattice-matches with InP. 3. The composition of the barrier layer made of InAlGaAs is such that the barrier layer has a tensile strain with respect to the InP substrate and compensates for at least a part of the compressive strain of the quantum well layer. The semiconductor laser according to claim 1. 4. The semiconductor device according to claim 1, wherein I is between said quantum well layer and said barrier layer.
a spacer layer made of nP, wherein the thickness of the spacer layer is 1 to
2. The semiconductor laser according to claim 1, which has 10 molecular layers.