JPH0697586A - Multiple quantum well semiconductor laser - Google Patents

Abstract

PURPOSE:To increase the gain of a quantum well as much as possible and to provide a multiple quantum well layer, which has a low threshold value and is superior in temperature characteristics. CONSTITUTION:An active layer of a multiple quantum well structure and clad layers 111 and 116, which are formed in such a way as to insert at least said active layer between them, are formed on a compound semiconductor substrate 110. Quantum well layers 113 of the active layer are respectively formed into an Inx(Ga1-yAly)1-xP layer and barrier layers 114 of this active layer are respectively formed into an Inn(Ga1-mAlm)1-nP layer (provided that, y<m and m=0.5 to 0.7). The sum total of the thicknesses of the above quantum well layers 113 is formed into a thickness of 15[nm] to 40[nm].

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device used as a light source for optical information processing, optical measurement, etc.
The present invention relates to a semiconductor laser device using an nGaAlP-based material.

[0002]

2. Description of the Related Art Recently, I having an oscillation wavelength in the 0.6 μm band
A red semiconductor laser using an nGaAlP-based material has been commercialized, and is expected as a light source for a high-density optical disk device, a laser beam printer light source, a bar code reader, optical measurement, and the like. At present, in order to increase the optical information processing speed and the optical recording density, the semiconductor laser of this system is required to have high output and short wavelength. In this case, it is effective to use a quantum well structure in which a well layer is sandwiched by barrier layers as an active layer and increase the gain by the quantum effect.

Conventionally, in the case of producing a laser in the 630 nm band, a manufacturing process as shown in FIG. 11, for example, is used by metal organic chemical vapor deposition (MOCVD). Figure 1
Briefly explaining with reference to No. 1, first, n-GaA
An n-In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P clad layer having a thickness of about 1.0 μm is formed on the s (100) substrate 110.
111 and In _0.5 (Ga of about 50 nm thick)
_0.5 Al _0.5 ) _0.5 P optical guide layer 112, and
In _0.5 Ga _0.5 P well layer with a thickness of about 3 nm 11
3 and a thickness of about 4 nm In _0.5 (Ga _0.5 Al _0.5 )
_0.5 P barrier layer 114 and a quantum well structure active layer (for example, 4 wells), and a thickness of about 50 nm
In _0.5 (Ga _0.5 Al _0.5 ) _0.5 P optical guide layer 1
15 and p-In having a thickness of about 0.2 μm
_0.5 (Ga _0.3 Al _0.7 ) _0.5 P Clad layer 11
6 and p-In _0.5 Ga _{0.5 with} a thickness of about 5 nm
P etching stop layer 117 and p-In _0.5 (Ga _0.3 Al _0.7 ) _0.5 having a thickness of about 0.8 μm
P clad layer 118, p-In _0.5 Ga _0.5 P conduction facilitating layer 119 having a thickness of about 50 nm, and n-GaAs current constriction layer 1 having a thickness of about 1 μm
22 and a p-GaAs contact layer 123 having a thickness of about 1 μm are formed respectively. And finally,
AuGe / Au 124 is formed as an electrode on the substrate side, and AuZn / Au is formed as an electrode on the opposite side to the substrate.
Form 125.

In such a conventional multiple quantum well laser,
Since InGaP is used for the well layer, it is necessary to reduce the well width to about 3 nm in order to create a laser in the 633 nm band. However, when the well becomes thin to this extent, the wave function of the electron existing in the well overlaps with the wave function of the electron in the adjacent well. At this time, the quantum level formed in the quantum well is further subbanded, and its width is widened. Also, the energy difference between the base level sub-band and the second level sub-band decreases. As the energy difference between the subbands decreases, the gain of the quantum well decreases, and the effect of using the quantum well in the active layer cannot be obtained. Further, if the well layer is thin, the light confinement efficiency is lowered and the threshold value is increased. Therefore, in such a conventional multi-quantum well laser, almost no improvement in operating characteristics can be obtained by using the multi-quantum well as the active layer.

[0005]

As described above, the well layer is
The well width must be set to 3 nm in order to obtain the oscillation wavelength of 630 nm by nGaP. In this case, the coupling between the quantum wells becomes strong, the quantum level subbands in the quantum wells are wide, and the energy difference between the quantum levels is small. Therefore, the gain of the quantum wells becomes small, and the quantum wells are formed in the active layer. The effect used will not be obtained so much. As a result,
The characteristics of the quantum well laser are the same as those of a normal DH laser, there is a problem that the oscillation threshold is high and the temperature characteristics are not so good.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a structure capable of increasing the gain of a quantum well of a multiple quantum well laser as much as possible, having a threshold value lower than that of a conventional one, and a temperature. It is to provide a multi-quantum well laser having excellent characteristics.

[0007]

In order to achieve the above object, a multiple quantum well semiconductor laser device of the present invention includes an active layer having a multiple quantum well structure and at least the active layer sandwiched on a compound semiconductor substrate. And a cladding layer formed as described above, wherein the quantum well layer of the active layer is In _x (Ga _{1 -y}
Al _y ) _1-x P and the barrier layer of the active layer is In _n
(Ga _1-m Al _m ) _1-n P (provided that y <m, m = 0.
5 to 0.7). The total thickness of the quantum well layers is 15 [nm] to 40 [nm].

Further, the thickness of each barrier layer in the multiple quantum well structure is 4 nm to 6 nm, and the thickness of each quantum well layer is 3.
It is 5 nm to 9 nm.

Further, the lattice constant in the vertical direction of each quantum well layer in the multiple quantum well structure is 0.
It is 5% to 2% smaller.

Further, the vertical lattice constant of each barrier layer in the multiple quantum well structure is 0.5 than the lattice constant of the substrate.
% To 2% larger.

That is, the gist of the present invention is to form an InGaAlP-based multiple quantum well laser having guide layers at both ends of the active layer, and the total well layer thickness in the multiple quantum well structure forming the active layer is The number of wells is determined to be 15 to 40 nm, the thickness of the well layer is set to 3.5 nm to 9 nm, and the thickness of the barrier layer is set to 4 nm to 6 nm. [Negative lattice mismatch (Δa / a = (lattice constant of growth layer-substrate lattice constant) / substrate lattice constant) −0.5% to −2%]
The purpose of this is to manufacture a multi-quantum well laser with a low threshold and excellent temperature characteristics.

[0012]

[Function] The active layer is InGaP and the light guide layer is In
_0.5 (Ga _1-x Al _x ) _0.5 P (x = 0.5), and the cladding layer is In _0.5 (Ga _1-x Al _x ) _0.5 P (x = 0.
7), SCH (Separate Confin)
In the element heterostructure) structure, the threshold value depends on the active layer thickness as shown in FIG. When the active layer is thin, the threshold increases because the volume of the light emitting layer increases as the active layer becomes thicker, but the threshold decreases as a whole due to the increase in light confinement efficiency, so the threshold as a whole. The value decreases.

However, as the thickness of the active layer further increases, the light confinement efficiency saturates to 1. Therefore, the increase of the threshold value due to the increase of the volume of the light emitting layer becomes dominant, and the threshold value increases as a whole. Go. As a result, as shown in FIG.
The threshold is minimal at ~ 40 nm. Experiments show that this characteristic is the same even when the active layer is a multiple quantum well, and the lowest threshold value is obtained when the total well layer thickness is 15 to 40 nm. .

Further, the relationship between the quantum level of the multiple quantum well and the well width Lω is as shown in FIG. The gain of the quantum well decreases as the quantum levels become subbands and the energy difference between the quantum levels decreases. From the figure, by setting the well width to 3.5 nm to 9 nm, the energy difference between the quantum levels can be made relatively large. Moreover, since the coupling between the quantum wells is reduced by making the barrier layer thick, the energy width of each subband quantum level can be reduced.

However, if the barrier layer is made too thick, holes will not be uniformly injected into each quantum well on the valence band side. According to experiments, the barrier layer has a thickness of 4n.
It can be seen that m to 6 nm is preferable. Therefore, in order to increase the quantum well gain and shorten the wavelength of the multiple quantum well laser, the well layer thickness and the barrier layer thickness may be selected as described above.

The band structure of the valence band can be changed by introducing a negative lattice mismatch to each quantum well layer of the active layer. That is, the bands of light holes and heavy holes that have degenerated at the apex of the valence band are separated, the band of holes lighter than the heavy holes comes to the apex, and the light holes at the valence band are Become dominant. As a result, the densities of states in the conduction band and the valence band are substantially symmetrical, so that population inversion is likely to occur and laser oscillation is likely to occur, and the threshold value can be lowered. According to the experiment, the negative lattice mismatch amount is -0.5% ~
It was found that the threshold value becomes low when the value is -2%.

By forming a positive lattice mismatch opposite to that of each quantum well layer in each barrier layer of the active layer, it is possible to prevent the occurrence of defects in the multiple quantum well structure.

[0018]

The details of the present invention will be described with reference to the illustrated embodiments. FIG. 1 shows a sectional view of a first embodiment of the present invention. First, as shown in FIG. 3A, an n-GaAs (100) substrate 110 tilted by 15 ° in the [011] direction was formed on the n-GaAs (100) substrate 110 by a metal organic chemical vapor deposition (MOCVD) method so as to have a thickness of 1.0 μm. -In _0.5 (Ga _0.3 Al
_0.7 ) _0.5 P clad layer 111 and thickness 50
nm of In _0.5 (Ga _0.5 Al _0.5 ) _0.5 P optical guide layer 112 and a thickness of 6 nm of In _0.5 Ga
_0.5 P well layer 113 and 4 nm thick In _0.5 (Ga
_0.5 Al _0.5 ) _0.5 P barrier layer 114 and an active layer having a multiple quantum well structure with four wells and a thickness of 50 n
m of _{In 0.5 (Ga 0.5 Al 0. 5} ) 0.5 P optical guide layer 115, and a thickness of 0.2μm p-In _0.5
(Ga _0.3 Al _0.7 ) _0.5 P clad layer 116, p-In _0.5 Ga _0.5 P etching stop layer 117 having a thickness of 5 nm, and a thickness of 0.8 μm.
p-In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P clad layer 118 and p-In _0.5 (G having a thickness of 50 nm)
a _0.5 P accelerating electricity conduction layer 119 and thickness 150
nm of n-an In _0.5 the (Ga _0.3 Al _{0.7) 0.5} P cap layer 120 are formed, respectively.

After that, as shown in FIG. 3B, stripe-shaped SiO whose stripe direction is the [011] direction.
₂ After forming the mask 121, etching is performed until the etching stop layer is exposed to form a ridge stripe having a stripe direction of [011] and a width of 5 μm.

Then, as shown in FIG.
_An n-GaAs current confinement layer 122 is selectively grown so as not to grow on ₂ 121. After that, as shown in FIG. 3D, SiO ₂ 121 and n-In
_0.5 (Ga _0.3 Al _0.7 ) _0.5 P cap layer 120
Under the same conditions after removing the
The aAs contact layer 123 is regrown.

Finally, AuGe / A is used as an electrode on the substrate side.
u 124 as AuZn / A as an electrode on the side opposite to the substrate.
u 125 are each formed. The resonator length is 4
It is set to 00 μm. The composition profile of the active layer is as shown in FIG.

In addition to this, the number of wells was set to 2 to 8 by the same process as above. As a result, the dependency of the oscillation threshold on the number of wells was obtained as shown in FIG. A low threshold value was obtained when the number of wells was 4 to 6. In the multiple quantum well laser, the light confinement efficiency is determined by the well layer 11 rather than the number of wells.
Since it is determined by the total thickness of the quantum well layers 3, the result of FIG. 3 shows that the threshold value can be lowered when the total thickness of the quantum well layers is around 15 nm to 40 nm.

Such a dependency of the threshold value on the number of wells was obtained even when the well width was set to 4 nm or 8 nm. When the well width is 6 nm and the number of wells is 5, the oscillation wavelength is 652.
nm, threshold is 30mA, 5mW up to 90 ° C
The optical output of was obtained. In this embodiment, the substrate is made of (100)
From the substrate is inclined by 15 ° in the [011] direction. This is because when multiple quantum wells are formed on the substrate,
The interface flatness is good and a steep hetero interface is obtained.
This is because the compositional uniformity of each layer in the multiple quantum well is improved and the non-issuing centers are reduced, so that the oscillation characteristics of the multiple quantum well laser are improved. This result shows that the substrate tilt angle is 5 to
If it is 40 °, almost the same effect can be obtained. Similar effects can be obtained even when the substrate tilt angle is in the [01-1] direction. Also,
Even if a normal n-GaAs (100) just substrate is used, the operating characteristics are improved as compared with the conventional case by making the structure of the multiple quantum well laser as described above.

A second embodiment of the present invention is shown in FIG. The laser forming process is the same as the process of the first embodiment shown in FIG. FIG. 4 shows the composition profile in this example. As shown in the figure, the well layer has a thickness of 4 nm of In _0.5 Ga _0.5 P 313 and the number of wells is 6
As a multi-quantum well laser, the width of the In _0.5 (Ga _0.5 Al _0.5 ) _0.5 P barrier layer 314 was changed from 2 nm to 8 nm.

As a result, as shown in FIG. 5, the barrier width dependence of the threshold value was obtained. A low threshold was obtained when the barrier width was 4 to 6 nm. Especially when the barrier width is set to 5 nm,
It oscillated at an oscillation wavelength of 633 nm and a threshold value of 55 mA.
A light output of 3 mW was obtained up to 75 ° C.

A third embodiment of the present invention is shown in FIG. Also in this example, a laser was produced by the same steps as those shown in FIG. However, the well layer is made of 8 nm InGaP
413, the number of wells is 4, and In _0.5 (Ga _0.5 Al
_0.5 ) _0.5 P The barrier layer 414 was set to have a width of 4 nm, and the lattice mismatch of the well layer was changed from Δa / a = 0 to −2.0% to prepare a multiple quantum well laser.

As a result, FIG. 7 was obtained as the oscillation wavelength dependence of the maximum continuous oscillation temperature at which an optical output of 3 mW was obtained. In the figure, ● indicates the well width of 8 nm and Δa / a =
In the case of 0 to -2.0%, ○ indicates Δa / a
= 0 and the well width is changed from 4 nm to 8 nm. The maximum continuous oscillation temperature is improved by setting Δa / a <0. When Δa / a ≦ −0.5, the maximum continuous oscillation temperature is improved by about 30 ° C. as compared with Δa / a = 0 regardless of the lattice mismatch amount. Well width 8 nm, Δa / a
= -2.0% and the number of wells is 4, the oscillation wavelength is 6
It oscillated at 33 nm and a threshold value of 55 mA, and an optical output of 3 mW was obtained even at 80 ° C. operation. In this embodiment, the amount of lattice mismatch is set to -2.0%. However, when the amount of lattice mismatch is more than this, the quantum well structure as the active layer causes lattice relaxation, and the operating characteristics of the laser. Is likely to deteriorate.

A fourth embodiment of the present invention is shown in FIG. Also in this embodiment, the forming process is the same as the previous embodiments. The composition profile is shown in FIG.
FIG. 10B shows the relationship between the oscillation wavelength, the well width, and the lattice mismatch when the well layer 513 is InGaP, and the well layer 51.
3 shows the relationship between the oscillation wavelength, the well width and the Al composition x when In _0.5 (Ga _1-x -Al _x ) _0.5 P is set to 3. Since the oscillation wavelength changes when the well width is changed while keeping the lattice mismatch amount constant, the well width and the lattice mismatch amount are set so that the oscillation wavelength becomes 633 nm based on FIG. Changed. The width of the In _0.5 (Ga _0.5 Al _0.5 ) _0.5 P barrier layer 514 was 4 nm. The number of wells was determined for each well width so that the total well layer thickness was about 24 nm.

FIG. 9 shows the relationship between the threshold value and the well width of the multiple quantum well thus obtained. From here, when the well width is about 6 nm, the lowest threshold value is 45 mA.
A light output of 3 mW was obtained even at 0 ° C. The characteristic temperature with respect to the threshold value at this time was 85K. This characteristic is excellent as never before for a 633 nm band laser.

From the figure, the well width is set to 3.5 nm to 9 nm.
If it is set to nm, the threshold value can be set to 65 mA or less. Such a relationship between the threshold value and the well width is almost the same when Al is added as shown in FIG. 7B instead of giving the negative lattice mismatch to the well layer and the oscillation wavelength of 633 nm is aimed at. A similar relationship between threshold and well width was obtained.
The well layer was made of In _0.5 (G) of 6 nm with Al composition x = 0.08.
a _1−x −Al _x ) _0.5 P, the oscillation threshold is 52
It was mA, and a light output of 3 mW was obtained even at 85 ° C.

A fifth embodiment of the present invention is shown in FIG. In this embodiment, the barrier layer and the light guide layer in the third embodiment are made of In (Ga _0.4 Al _0.6 ) P (Δa / a = +
2%) to give a lattice mismatch in the direction opposite to the well layer. That is, the well layer is formed of 8 nm InGaP (Δa / a
= -2%) 613, the barrier layer is made of In (Ga _0.4 A) of 4 nm.
l _0.6 ) P (Δa / a = + 2%) 614, and the optical guide layer is 50 nm thick In (Ga _0.4 Al _0.6 ) P (Δa / a = +
2%) 612 and 615, and the number of wells was 4.

By doing so, the amount of lattice mismatch in the multiple quantum well active layer is averaged and reduced to 63.
The life time of the optical output 3 mW operation at 50 ° C. with the oscillation wavelength of 3 nm was improved by about 2 times as compared with the third embodiment, from 4000 hours to 8000 hours. The other characteristics were almost the same as the results of the third embodiment. It is considered that the strain in the multi-quantum well active layer is reduced and the device reliability is improved by making the barrier layer have a lattice mismatch in the direction opposite to the well layer.

[0033]

As described above, in the multiple quantum well laser, the gain of the quantum well is maximized by setting the well layer thickness to 3.5 nm to 9 nm, and the total well layer thickness is set to 15 to 40 nm. By doing so, the threshold is minimized. As a result, the threshold value is lower than that of the conventional multiple quantum well laser, and the temperature characteristics and the reliability of long-term operation are improved.

Further, at the wavelength of 630 nm band, since In _0.5 Ga _0.5 P was conventionally used for the well layer, the well width had to be 3 nm.
GaP (Δa / a <0) or In _0.5
By using (Ga _1-x Al _x ) _0.5 P, the well width can be set within the above range, the threshold value is lowered, and the temperature characteristics and the reliability of long-term operation are significantly improved as compared with the conventional one. . In particular, InGaP (−2.0% ≦ Δa /
a ≦ −0.5%), the band structure of the valence band changes, so that the threshold value can be further reduced and the temperature characteristics can be improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing a manufacturing process of a multiple quantum well laser according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a composition profile of the first example of the present invention.

FIG. 3 is a graph showing the number of wells dependence of the threshold value of the multiple quantum well laser obtained according to the first embodiment of the present invention.

FIG. 4 is a diagram showing a composition profile of a second example of the present invention.

FIG. 5 is a diagram showing the barrier width dependence of the threshold value of the multi-quantum well laser obtained according to the second embodiment of the present invention.

FIG. 6 is a diagram showing a composition profile of a third example of the present invention.

FIG. 7 is a diagram showing the relationship between the maximum continuous oscillation temperature and the oscillation wavelength of the multiple quantum well laser obtained according to the third embodiment of the present invention.

FIG. 8 is a diagram showing a composition profile (a) and a relationship (b) between a lattice mismatch amount (or Al composition), a well width, and an oscillation wavelength according to a fourth embodiment of the present invention.

FIG. 9 is a diagram showing the relationship between the threshold and the well width of the multiple quantum well laser obtained according to the fourth embodiment of the present invention.

FIG. 10 is a diagram showing a composition profile of a fifth example of the present invention.

FIG. 11 is a diagram showing a cross-sectional device structure (a) and a composition profile (b) of a conventional quantum well laser, respectively.

FIG. 12 is a diagram showing the dependence of the threshold of the SCH structure laser on the active layer thickness.

FIG. 13 is a diagram showing a relationship between a quantum level and a well width of a multiple quantum well laser.

[Explanation of symbols]

10 ... n-GaAs (100) substrate 111 ... n-In _0.5 (Ga _0.3 A
l _0.7 ) _0.5 P clad layer 112, 115 ... In _0.5 (Ga _0.5 Al _0.5 ).
_0.5 P optical guide layer 113 ... In _0.5 Ga _0.5 P well layer 114 ... In _0.5 (Ga _0.5 Al _0.5 ).
_0.5 P barrier layer 116 ... p-In _0.5 (Ga _0.3 A
l _0.7 ) _0.5 P cladding layer 117 ... p-In _0.5 Ga _0.5 P etching stop layer 118 ... p-In _0.5 (Ga _0.3 A)
1 _0.7 ) _0.5 P clad layer 119 ... p-In _0.5 Ga _0.5 P current facilitation layer 120 ... n-In _0.5 (Ga _0.3 A)
1 _0.7 ) _0.5 P cap layer 121 ... SiO ₂ mask 122 ... n-GaAs current confinement layer 123 ... p-GaAs contact layer 124 ... AuGe / Au 125 ... AuZn / Au 313 ... In _0.5 Ga _0.5 P (4 nm) well layer 314 … In _0.5 (Ga _0.5 Al _0.5 )
_0.5 P (thickness 2 nm to 8 nm) barrier layer 413 ... In _0.5 Ga _0.5 P (thickness 8 n
m) Well layer 414 ... In _0.5 (Ga _0.5 Al _0.5 ).
_0.5 P (thickness 4 nm) barrier layer 513 ... In _0.5 Ga _0.5 P, In
_0.5 (Ga _1-x Al _x ) _0.5 P Well layer 514 ... In _0.5 (Ga _0.5 Al _0.5 ).
_0.5 P (thickness 4 nm) barrier layer 612, 615 ... In (Ga _0.4 Al _0.6 ) P photo guide layer, (Δa / a = + 2%, 50 nm) 613 ... InGaP well layer, (Δa / a
= -2%, 8 nm) 614 ... In (Ga _0.4 Al _0.6 ) P barrier layer, (Δa / a = + 2%, 4 nm)

Claims (4)

[Claims] 1. A multiple quantum well semiconductor laser device having an active layer having a multiple quantum well structure on a compound semiconductor substrate, and a cladding layer formed so as to sandwich at least the active layer. The quantum well layer is In _x (Ga _1-y Al _y ).
_1-x P, the barrier layer of the active layer is In _n (Ga _1-m Al _m ) _1-n
P (however, y <m, m = 0.5 to 0.7), and the total thickness of the quantum well layers is 15 [nm] to 40.
[Nm] is a multiple quantum well semiconductor laser device. 2. The barrier layer in the multiple quantum well structure has a thickness of 4 nm to 6 nm, and each quantum well layer has a thickness of 3.5 n.
The multi-quantum well semiconductor laser device according to claim 1, characterized in that the thickness is m to 9 nm. 3. The lattice constant in the vertical direction of each quantum well layer in the multiple quantum well structure is 0.5% of the lattice constant of the substrate.
2. The multiple quantum well semiconductor laser device according to claim 1, wherein the multiple quantum well semiconductor laser device is smaller than .about.2%. 4. The lattice constant in the vertical direction of each barrier layer in the multi-quantum well structure is 0.5% to 2 than the lattice constant of the substrate.
%. The multiple quantum well semiconductor laser device according to claim 1, wherein the multiple quantum well semiconductor laser device is large.