JPH07254750A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To prevent deterioration of laser characteristics and temperature characteristics, by hanging the carrier concentration of a P-type buried layer in a current blocking layer which comes directly into contact with the side surface of an active layer, in the inside region and the outside region. CONSTITUTION:Clad layers 21 and 25 are formed below and above an active layer 23, respectively. Current blocking layers 26, 27 are formed on both sides of the active layer 23. A first buried layer 26 of the current blocking region which comes directly into contact with the side surface of the active layer 23 exhibits P-type conductivity. The carrier concentration of at least a part of the outside region 26b distant from the active layer 23 of the first buried layer 26 is set higher than the carrier concentration of the inside region 26a. Thereby the diffusion of P-type impurities into the active layer 23 can be restrained, and the built-in potential to an adjacent N-type buried layer 27 can be made large, so that the leakage current is reduced, the threshold value is lowered, and excellent temperature characteristics can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser used for optical communication devices and information processing, and more particularly to a semiconductor laser having a structure in which the side surface of a mesa stripe including an active layer is filled with a semiconductor layer for current confinement. Regarding

[0002]

2. Description of the Related Art Recently, as a light source for information processing and optical communication,
A semiconductor laser device using an InP-based material has been developed, and higher performance is desired. In particular, there are strong demands for low threshold characteristics, high temperature stability with small output fluctuations with respect to temperature, and high reliability, and research and development have been actively conducted. In order to realize a semiconductor laser device that oscillates at a low threshold value, a buried laser structure in which the width of the active layer is narrowed and both sides thereof are filled with a current constriction structure is used.

FIG. 11 is a sectional view showing a conventional semiconductor laser formed on a p-type semiconductor substrate by MOCVD. Such lasers are, for example, ELECTRONI
CSLETTERS Vol. 28 No. 19 p1
844. This laser is manufactured by the following procedure.

First, a p-type InP substrate 1, a p-type InP buffer layer 2, an InGaAsP active layer 3 having an emission wavelength of 1.3 μm, an n-type InP clad layer 4, and an n-type InGaAs etching dummy layer (not shown) are sequentially formed by MOCVD. To grow. Next, a SiO ₂ mask is formed in a stripe shape in the <011> direction, and then a mesa stripe having a height of about 3 μm is formed by chemical etching. Then S
The p-type InP burying layer 5, the n-type InP current blocking layer 6, and the p-type InP current blocking layer 7 are selectively grown sequentially by the MOCVD method while leaving the iO ₂ mask. Next, after removing the SiO ₂ mask and the InGaAs etching dummy layer, the n-type InP clad layer 8 and the n-type InGaAs contact layer 9 are grown on the entire surface by MOCVD. Finally, the electrodes 10 and 11 are formed and the substrate is polished, and cleaved into individual semiconductor laser chips to complete a semiconductor laser.

In this semiconductor laser, p on both sides of the active layer is used.
Due to the current block structure of the npn thyristor structure, current is efficiently injected into the active layer. Further, in this laser, in order to obtain good device characteristics with a small leak current,
The side surface of the active layer is a p-type I having a higher electric resistance than n-type InP.
It is important that it is covered with nP and that the n-type InP clad layer and the n-type InP block layer are separated.

The manner in which such a structure can be realized on the side surface of the active layer will be described with reference to FIG. 12 in FIG.
Is Si used for mesa etching and buried selective growth.
It is an O ₂ mask. First, the p-type InP buried layer 5
Of about 1 μm (FIG. 12A), the (111) B plane 13 and the (221) B plane 1 having slow growth rates are formed on the side surface of the mesa.
4 is formed. Next, the n-type InP current blocking layer 6 is grown to such an extent that the (221) B plane 14 is not completely filled (FIG. 12 (b)), and finally the p-type InP current blocking layer 7 is grown (FIG. 12 (c)). ). When grown in this manner, the second n-type InP layer 6 does not grow on the (111) B plane 13 formed in the first p-type InP layer 5, so the n-type InP current blocking layer 6 is formed. And n-type InP clad layer 8 can be separated without connecting.

As described above, the n-type InP current blocking layer and the n-type InP current blocking layer and
A buried semiconductor laser that is not connected to the InP clad layer can be manufactured, and an element having excellent characteristics with a small leak current can be obtained. On the other hand, however, the (111) B plane has a very low crystal growth rate on this plane, which causes a problem that flat burying does not proceed on both sides of the mesa (ELECTRONICS LETTERS).
Vol. 28 No. 19 p1844).

Further, when growing the n-type InP current blocking layer 6 of FIG. 12B, a (22) layer having a length of about 0.5 μm is formed.
1) When the B side 14 is completely filled, (11
1) Since the crystal grows on the B-face 13 as well, the n-type InP current blocking layer 6 and the n-type InP cladding layer 8 cannot be separated from each other, and the device characteristics are significantly deteriorated. Therefore, in the structure as shown in FIG. 11, it is necessary to make the thickness of the n-type InP current blocking layer 6 relatively small. However, in an embedded semiconductor laser having a normal pnpn current block structure, if the thickness of the current block layer is small, the leakage current to the buried layer becomes large at the time of high current injection or high temperature. Therefore, with the element having the structure shown in FIG. 11, it is difficult to perform high output operation and high temperature operation.

FIG. 13 shows an n-type InP current blocking layer 6 and n.
FIG. 11 is a cross-sectional view showing a conventional semiconductor laser in which separation from the type InP clad layer 8 is performed by another method. In this laser, p-type impurities are diffused from the p-type InP buried layer 5 and the p-type InP current blocking layer 7 to the portion 15 of the n-type InP current blocking layer 6 to change the conductivity type to p-type. Is obtained. In this case, the higher the p-type impurity concentration of the buried layer 5 and the block layer 7, the more easily diffusion occurs, and the desired structure is easily obtained. However, p in InP
The type impurities are easily diffused, and if the p-type carrier concentration is too high, the p-type impurities diffuse into the active layer, which deteriorates the characteristics of the laser. This problem can be avoided by lowering the carrier concentration of the p-type buried layer 5 (first buried layer) that is in direct contact with the active layer. In this case, however, the breakdown voltage of the reverse bias buried region composed of the buried layers 6 and 7 is reduced. The temperature decreases, the leak current in this region increases, the threshold value increases, and the temperature characteristic deteriorates.

[0010]

One of the objects of the present invention is to control the carrier concentration of the "p-type first buried layer" in contact with the side surface of the active layer in the buried laser of InP-based material. Is to solve the problem.
That is, if the carrier concentration of the first burying layer is too high, impurities are diffused into the active layer and the laser characteristics are deteriorated and the laser reliability is deteriorated. If it is too low, the threshold value is increased and the temperature characteristics are deteriorated. Is to solve the problem.

Another object of the present invention is to perform S growth in embedded growth by MOCVD method capable of high performance laser production.
Solves the problem that the (111) B plane is easily crystallized directly under the iO ₂ mask and the filling cannot be performed well, and it has low threshold, good temperature characteristics, high reliability, and is reproducible by MOCVD. To provide a simple semiconductor laser.

Still another object of the present invention is to provide a semiconductor laser in which the thickness of the n-type InP current blocking layer is increased so that a sufficiently high output operation and high temperature operation can be performed.

[0013]

The first aspect of the present invention is as follows.
In a semiconductor laser in which cladding layers are formed above and below an active layer and current blocking regions are formed on both sides of the active layer, a first buried layer in the current blocking region that is in direct contact with a side surface of the active layer is a p-type layer. The conductivity type, the first
The carrier concentration of at least a part of the outer region of the buried layer remote from the active layer is higher than the carrier concentration of an inner region of the first buried layer that is in direct contact with the active layer.

Preferably, the cladding layer contains InP, and the width of the active layer is defined by forming a mesa stripe. Further, the carrier concentration of the at least a part of the outer region and the carrier concentration of the inner region of the first buried layer are about 7 × 10 ^{17 to} 3 × 10, respectively.
¹⁸ cm ⁻³ and about 1 × 10 ¹⁷ to 4 × 10 ¹⁷ cm ⁻³ .
Further, the formation temperature of the current block region is higher than the formation temperature of the active layer. Further, the mesa stripe and the current block region are composed of a layer formed by a metal organic chemical vapor deposition method.

Here, the impurity concentration is about 110 ¹⁷ to 4 × 1.
0 ¹⁷ cm ^-3 and about 7 × 10 ^{17 to} 3 × 10 ¹⁸ cm ^-3 are Secondary Ion Mass Spec.
It is a value measured by rometry (SIMS), and includes an error of factor 3 as an absolute value.
However, the relative error between the high density and the low density is 20% or less.

The essence of the second aspect of the present invention is to devise the side shape of the mesa stripe before embedding and growing the n-type InP.
The purpose is to increase the thickness of the current blocking layer. A second aspect of the present invention is that the cladding layer is made of InP.
In a semiconductor laser in which the width of the active layer is defined by forming a mesa stripe, the side surface of the mesa stripe is a smooth curve, and in a region of approximately half of the side close to the top of the mesa stripe. The radius of curvature at each point on the curve is 10 μm or more, and the angle between the tangent at each point on the curve in this region and the normal to the substrate is 1
It is characterized by being 0 degrees or less.

Here, the region approximately half the height of the mesa stripe from the top refers to a region in the range of 40 to 60% of the height of the mesa stripe. Further, it is important that the shape of the side surface of the mesa is a continuous smooth curve or straight line. For example, when a crystal plane is formed and there is a non-smooth point, a high-order crystal plane with a very small growth rate that appears in the process of embedded growth is not formed.

Preferably, the depth of the mesa stripe is about 3 μm, the upper half of the mesa stripe is substantially perpendicular to the substrate, and the radius of curvature of the lower half is about 1.5.
The width is such that the width of the mesa stripe gradually increases in μm, so that the shape is smooth. The active layer is preferably located as close as possible to the top of the mesa stripe, and the distance from the top to the active layer is 0.3 μm.
It should be below.

[0019]

According to the first aspect of the present invention, the concentration of p-type impurities in the inner region of the p-type first buried layer in contact with the active layer is low. Therefore, diffusion of p-type impurities into the active layer can be suppressed, and accumulation of p-type impurities in the active layer and generation of a deep level can be suppressed. As a result, it is possible to prevent the characteristics of the laser from deteriorating and the characteristics of the laser from changing with time due to the movement of impurities in the active layer, so that the reliability of the laser can be prevented from degrading. Further, since the concentration of the p-type impurity is high in the outer region of the p-type first buried layer away from the active layer, a large built-in potential can be obtained between the p-type first buried layer and the adjacent n-type conductivity type buried layer, and the current confinement can be achieved. It is possible to reduce the leak current in the reverse junction buried layer, reduce the threshold value, and obtain good temperature characteristics.

Since the p-type impurity concentration changes significantly above and below the boundary of approximately 5 × 10 ¹⁷ cm ^-3 ,
In the first buried layer, the impurity concentration in the inner region may be 4 × 10 ¹⁷ cm ⁻³ or less, which is sufficiently lower than this,
The impurity concentration in the outer region on the opposite side is sufficiently higher than this 7
If × 10 ¹⁷ cm ^-3 or more concentration, characteristics of the laser is further improved.

When the embedded growth is performed by using the overhang of the SiO ₂ mask by the MOCVD method, the (111) B plane is formed near the upper end of the mesa structure when the p-type buried layer near the active layer is formed. However, once this surface is formed, growth on this surface is unlikely to occur, and good buried growth does not proceed. In this regard, as a result of research by the present inventor, in the buried growth on the lower surface of the mask, the condition that the (111) B plane is likely to be formed is when the Zn concentration is approximately 7 × 10 ¹⁷ cm ⁻³ or more and below that. In the case of the concentration, it was confirmed that the (111) B plane was hardly formed and the embedding proceeded normally. Therefore, when the p-type impurity concentration in the inner region of the first burying layer is set to about 4 × 10 ¹⁷ cm ⁻³ or less as in the semiconductor laser of the present invention, normal burying becomes possible when forming the current blocking layer. As a result, a p-type layer having a sufficient thickness can be formed on the side surface of the active layer. Moreover, since the p-type impurity concentration in the outer region of the first buried layer is set to 1 × 10 ¹⁷ cm ⁻³ or more, the built-in potential with the n-type cladding layer can be made sufficiently large and the current at the contact portion between the two layers can be increased. Leakage can be reduced and a low threshold laser can be realized.

According to the first aspect of the present invention, when the p-type first buried layer is formed, a region having a low concentration of p-type impurities is first formed, and the side surface of the active layer has a sufficient thickness. p
Type InP. Therefore, when the outer region having a high impurity concentration of the p-type first buried layer is formed, even if the (111) B plane is formed and the growth rate is lowered due to the buried growth, the side surface of the active layer is not formed. Since the InP layer having a sufficient thickness has already been formed, no problem occurs.

On the contrary, the concentration of the p-type impurity in the outer region is set to about 7.
With × 10 ¹⁷ cm ^-3 or more, (111) to form a B side, it is possible to form a predetermined structure by using a specific crystal plane. Especially when forming a laser on a p-type substrate, (111) B when forming the p-type first buried layer.
When the surface is formed, the n-type current blocking layer is hardly formed on the surface, so that the n-type current block and the n-type cladding layer can be easily separated by only crystal growth. . Therefore, the threshold value of the laser can be lowered and the temperature characteristics can be improved. of the p-type first buried layer,
In when the concentration of p-type impurities is 3 × 10 ¹⁸ cm ⁻³ or more
The density of deep levels in P rises, crystal defects increase rapidly, and the laser characteristics deteriorate rapidly.

When the buried region is formed, the formation temperature thereof is made higher than the temperature at which the active layer is formed, whereby diffusion of impurities is promoted, the position of the regrowth interface and the homojunction are shifted, and the threshold value of the laser is increased. Can be lowered.
In particular, when crystal growth is carried out by the MOCVD method, the active layer of the laser is 620 in terms of forming a hetero interface with high quality and accurately controlling the composition of InGaAsP.
It is desirable to grow at ~ 670 ° C. On the other hand, by growing the embedded region at a growth temperature of 670 to 770 ° C., abnormal growth of the embedded region and deterioration of the surface can be prevented, and a low threshold value can be realized. Further, when the growth temperature is raised, the concentration of p-type impurities causes (111)
It becomes possible to control the presence or absence of B-side formation with better controllability. Therefore, the embedded structure can be controlled more accurately and the characteristics of the laser can be improved.

According to the second aspect of the present invention, since the side surface of the mesa stripe is substantially perpendicular to the substrate within a certain range from the SiO ₂ mask used in the manufacturing process, the p-type InP layer, which is a first layer, is formed. The length of the (221) B plane that appears when the first burying layer is grown can be made larger than before.
Along with this, the thickness of the second n-type InP current blocking layer can be made twice as thick as the conventional one, and a semiconductor laser capable of high-power operation and high-temperature operation can be obtained. In addition, since the allowable range of the thickness of the n-type InP current blocking layer that can prevent the n-type InP current blocking layer and the n-type InP cladding layer from being connected to each other is large, the manufacturing yield can be increased as compared with the conventional case.

[0026]

The present invention will be described below with reference to the illustrated embodiments.

FIG. 1 is a sectional view showing a schematic structure of a semiconductor laser according to an embodiment of the present invention. Carrier concentration 10 ¹⁸
cm ^-3 n-type InP consisting of Si-added n-type InP substrate
On the clad layer 21, Si with a carrier concentration of 10 ¹⁷ cm ^-3
An n-type GaInAsP optical guide layer 22 having a thickness of 0.1 μm, a GaInAsP quantum well barrier layer 23 a having a thickness of 10 nm and 3.5 nm, and a GaInAsP well layer (bandgap 1.42 μm, strain amount 0.5%) 23 b. 9 and 10 layers of the active layer 23 are repeatedly stacked, and a carrier concentration of 10 ¹⁶ cm ^{−3 and} a Si-added GaI layer having a thickness of 0.1 μm.
nAsP optical guide layer 24, p-type InP cladding layer 25 with a carrier concentration of 10 ¹⁸ cm ⁻³ and Zn addition of 0.5 μm in thickness
Are sequentially stacked. This layer is formed at a growth temperature of 620 ° C. A stripe-shaped mesa structure having a width of about 1.5 μm is formed above the n-type InP clad layer 21.

Both sides of the mesa stripe are filled with a p-type InP first burying layer 26 and an n-type InP current blocking layer 27. The first buried layer 26 is located on the side of the active layer, has a thickness of 1 μm, and has a Zn-containing inner region 26a having a carrier concentration of 3 × 10 ¹⁷ cm ⁻³ , and is located on a side far from the active layer and has a thickness of 0.5 μm. Zn with carrier concentration of 1 × 10 ¹⁸ cm ^-3
And an additional outer region 26b. The block layer 27 has a thickness of 1 μm and is made of Si-doped n-type InP having a carrier concentration of 10 ¹⁸ cm ⁻³ . These layers 26, 27 are formed at a growth temperature of 670 ° C.

As shown in FIG. 2, the region 26a of the p-type InP layer 26 grows at the bottom of the SiO ₂ mask 34 and the upper end of the mesa stripe 33 without forming a singular surface during the buried growth to form an active layer. Completely covers both sides of. On the other hand, since the Zn concentration is high in the outer region 26b of the p-type InP layer 26, (111) is formed on the upper end of the mesa stripe 33.
The B surface 35 is formed. With the formation of (111) B side,
A surface 36 having a slow growth rate is also formed on the side surface of the mesa stripe 33, and the first buried layer 26 is formed on the side surface of the mesa stripe.
The outer region 26b has a smaller thickness.

On the p-type InP clad layer 25 and the n-type InP current blocking layer 27, the carrier concentration is 10 ¹⁸ cm ^−3.
Zn-added p-type InP layer 28 having a thickness of 0.5 μm and a carrier concentration of 5 × 10 ¹⁸ cm ⁻³ and Zn-added p-type Ga _0.47
The In _0.53 As contact layer 29 is formed. GaIn
An Au-Zn / Au p-side metal electrode 31 is provided on the As contact layer 28, and Au-Ge / N is provided on the n-type InP substrate 21.
The i / Au n-side metal electrode 32 is deposited. In this semiconductor laser, the cavity length is 200 μm, and a high-reflection coating having a reflectance of 70% on one end face and a reflectance of 90% on the other end face is applied by a multilayer film of Si and SiO ₂ .

The oscillation wavelength of this semiconductor laser is 1.32 μm.
m, the threshold current is 3 mA, and the temperature variation of the slope efficiency η is η (75 ° C) / η (25 ° C) to 0.8, which is a large value for a laser with such a low threshold current. It was Without dividing the p-type first buried layer 26 into two regions, an inner region 26a and an outer region 26b having different Zn concentrations,
Η (75 ° C) / with a constant low carrier concentration
It became (eta) (25 degreeC) -0.6. Further, in the case where the carrier concentration is high, the p-type first burying layer 26 has (11
1) Forming the B surface, η (75 ° C) / η (25 ° C) to 0.
It became 65.

Particularly, when the thickness of the GaInAsP light guide layer 24 is 0.3 μm, the p-type InP cladding layer 25 is not formed, and the diffraction grating is formed on the GaInAsP light guide layer 24, the effect of the present invention is remarkable. Met. That is,
When the Zn concentration of the first buried layer 26 was divided into the inner region 26a and the outer region 26b, there was no significant difference in the laser characteristics regardless of the presence or absence of the p-type InP cladding layer 25.

However, Zn in the p-type first buried layer 26
When the concentrations are all increased, the n-type GaInAsP light guide layer 22, the active layer 23, and the G layer are formed by forming the (111) B plane.
A p-type conduction type InP layer having a sufficient thickness was not formed on the side surface of the aInAsP optical guide layer 24, normal buried growth did not proceed, and only a laser with extremely poor characteristics could be produced. Conversely, when the p-type InP clad layer 25 is formed, the influence of the (111) B plane being formed and the growth rate being lowered on the side surface of the mesa stripe was mitigated. The most suitable thickness of the p-type InP clad layer 25 is about 0.3 to 1.2 μm.

The leakage current in the buried layers 26 and 27 is
The growth temperature of the buried layer is from 620 ° C. to 670-770.
As the temperature was raised to 0 ° C., it was remarkably decreased from about 2 mA to 0.5 mA. This is because the diffusion rate of atoms on the crystal surface rises rapidly as the growth temperature rises, and as a result, it becomes possible to bury it flat and prevent crystal defects and electric field concentration due to unevenness at the bonding interface, which is active. It is considered that this is because the breakdown voltage of the buried layer around the layer increased.

In the laser of the present embodiment, the crystal growth temperature of the active layer is low, and the crystal growth is performed under the condition that the temperature dependence of the composition of InGaAsP is weak, so that the composition control is accurate.
Variation of laser oscillation wavelength is 1.32 ± 0.01 μm
Could be controlled. With the quantum well laser having a structure similar to that of this example, the variation could be suppressed to about 50% or less as compared with the case where the active layer was grown at 670 ° C.

GaInAsP light guide layer 22, GaIn
The composition of the AsP quantum well barrier layer 23a and the GaInAsP light guide layer 24 had the threshold value decreased from 5 mA to 1.9 mA as the band gap was decreased from 1.03 μm to 1.23 μm. When the band gap was further reduced, the threshold value rose sharply. It is considered that this is because carriers can be uniformly injected into the quantum well as the bandgap of the barrier layer is made smaller, and when the thickness is smaller than 1.23 μm, the overflow of carriers from the quantum well becomes remarkable.

FIG. 3 is a sectional view showing a semiconductor laser according to another embodiment of the present invention. In the semiconductor laser of the present embodiment, Zn-doped p with a carrier concentration of 4 × 10 ¹⁸ cm ⁻³ is used.
On the p-type InP substrate 41, a Zn-doped p-type InP clad layer 4 having a thickness of 2.5 μm and a carrier concentration of 10 ¹⁸ cm ^−3.
2. GaInAsP (bandgap 1.13 μm) optical guide layer 43 having a thickness of 0.1 μm, GaInAsP (bandgap 1.13 μm) quantum well barrier layers 44 a and GaInAsP (bandgap 1.
4 μm, lattice mismatch of 0.6%) Quantum well layer 44b
The active layer 4 is formed by repeating 5 and 6 layers respectively.
4. Si-added GaInAsP (bandgap 1.13μ, thickness 0.2 μm, carrier concentration 5 × 10 ¹⁷ cm ⁻³ )
m) An optical guide layer 45 and a Si-doped n-type InP cladding layer 46 having a carrier concentration of 2 × 10 ¹⁸ cm ⁻³ are sequentially formed. The width above the upper portion of the p-type InP clad layer 42 is about 1.
A 5 μm stripe mesa structure is formed.

Both sides of the mesa stripe are filled with a first buried layer 47 of p-type InP layer having a thickness of 1 μm, and the carrier concentration of this layer is a region far from the active layer from 3 × 10 ¹⁷ cm ⁻³ near the active layer. It changes intermittently up to 1 × 10 ¹⁸ cm ^-3 . Here, the region where the carrier concentration is 4 × 10 ¹⁷ cm ⁻³ or less has a thickness of 0.6 μm or more, and the carrier concentration is 7 × 10 7.
A region of ¹⁷ cm ⁻³ or more is formed with a thickness of 0.2 μm or more.
Furthermore, S having a thickness of 0.8 μm and a carrier concentration of 10 ¹⁸ cm ⁻³
The periphery of the mesa stripe is filled with an i-doped n-type current blocking layer 48 and a Zn-doped p-type InP layer 49 having a thickness of 1 μm and a carrier concentration of 10 ¹⁸ cm ⁻³ .

In this embodiment, the p-type InP first buried layer 4 is used.
Since the carrier concentration of No. 7 is as low as 3 × 10 ¹⁷ cm ^-3 inside, 0.4 μm p-type conductivity type InP having a sufficient thickness is formed on the side surface of the active layer. Then, the carrier concentration rises to 1 × 10 ¹⁸ cm ^{−3 in the} outer region far from the active layer, so that the (111) B plane 55 is formed near the upper end of the mesa structure. Further, a surface (S surface) 56 having a relatively low growth rate is formed along the side surface of the mesa stripe. Furthermore, in a region away from the mesa stripe, a crystal plane (F
Surface 57 is formed. Therefore, the current blocking layer 48 is hardly laminated on the (111) B plane 55 and the S plane 56, but is laminated on the F plane 57. Further, since the p-type InP layer 49 is buried, the p-type InP layer 47 and the p-type I
Due to the effect of diffusion of impurities from the nP layer 49, the current block layer 48 is embedded so as to be surrounded by the p-type region.

[0040] n-type InP on the clad layer 46 and the p-type InP layer 49, n-type InP layer 50 and the carrier concentration of the Si addition of carrier concentration ^{10 18 cm -3 5 × 10 18} cm -3,
0.5 μm thick Si-added n-type Ga _0.47 In _0.53 As
The contact layer 51 is formed. An Au-Ge / Ni / Au n-side metal electrode 52 is deposited on the GaInAs contact layer 51. Au-Z is formed on the p-type InP substrate 41.
A p-side metal electrode 53 of n / Au is deposited. In this semiconductor laser, the cavity length is 200 μm, and a high-reflection coating having a reflectance of 70% on one end face and a reflectance of 90% on the other end face is applied by a multilayer film of Si and SiO ₂ .

The oscillation wavelength of this semiconductor laser is 1.3 μm, the threshold current is 2.3 mA, and the temperature variation of the slope efficiency η is η (75 ° C.) / Η (25 ° C.) to 0.7. It showed the same excellent characteristics as the laser formed above, and showed excellent characteristics as a laser having a small threshold current formed on a p substrate.

Zn of the entire p-type InP first buried layer 47
When the concentration was lowered, as shown in FIG. 4, no (111) B plane was formed near the upper end of the mesa structure, and no S plane was formed along the side surface of the mesa stripe. Therefore, the n-type current blocking layer 48 grows to the lower end of the SiO ₂ mask (similar to the mask 34 shown in FIGS. 2 and 5), and when the n-type InP clad layer 50 is formed, the two n-type layers are connected. have done. In addition, the built-in potential of the p-type first buried layer 47 and the n-type cladding layer 50 was also reduced. For this reason, the current blocking layer 48 did not function effectively, the leak current was large, and the temperature characteristics of the laser were poor.

When the Zn concentration of the entire first buried layer 47 is increased, the GaInAsP light guide layer 43, the active layer 44, and the Ga layer are formed by forming the (111) B plane as shown in FIG.
A p-type conductive InP layer having a sufficient thickness was not formed on the side surface of the InAsP light guide layer 45, and normal buried growth did not proceed. In addition, the influence of Zn diffusion from the p-type buried layer in the immediate vicinity of the active layer to the active layer was large, and only a laser with poor temperature characteristics could be produced.

However, as shown in FIG. 6, when the thickness of the n-type InP clad layer 46 is increased, the active layer is separated from the upper end of the mesa stripe, so that the (111) B surface is formed and the growth on the side surface of the mesa stripe. Even if the speed is reduced, it is formed on the side surface of the active layer 44. The thickness of the p-type layer in the immediate vicinity of the active layer is increased, and the deterioration of the laser characteristics is reduced.
C.). Eta. (25.degree. C.) to about 0.6. The thickness of the n-type InP clad layer 46 is 0.3 to 0.
The time of about 7 μm was the most suitable. When it was too thin, the temperature characteristics were poor, and when it was too thick, the depth of mesa etching and the amount of side etching from the side surface of the active layer became unstable, resulting in poor reproducibility.

When formed on an n-type substrate, it is desirable that the mesa etching depth and the thickness of the buried layer match with an error of about 0.5 μm. However, as shown in FIG. 3, p
When it is formed on the substrate of the mold, the depth of the mesa etching is deeper than the sum of the thickness of the p-type InP first buried layer 47 and the thickness of the current block layer 48, and the surface of the current block layer 48 is If embedded in the p-type InP layer 49 and not in contact with the n-type InP clad layer 46, p-type InP
The thickness of the layer 49 is approximately the same as the width of the mesa or the p-type InP layer 4
If the angle θ formed between the n-type InP clad layer 50 and the n-type InP clad layer 50 was 45 degrees or less, the characteristics did not change significantly. That is, it is called reproducibility to make the depth of mesa etching as shallow as possible within the range of the condition that the surface of the current block layer 48 is embedded in the p-type InP layer 49 and does not contact the n-type InP cladding layer 46. In terms of advantages. The reproducibility was the best when the depth of the mesa etching was approximately equal to the sum of the thickness of the p-type InP layer 47 near the active layer and the thickness of the current block layer 48.

FIG. 7 shows p according to another embodiment of the present invention.
It is sectional drawing which shows schematic structure of the embedded semiconductor laser using a board | substrate. Here, InGaA using an InP substrate is used.
The description will be made by taking an sP-based semiconductor laser in the 1.3-micron band as an example, but the present invention can be similarly implemented in other material systems and other wavelength bands.

On the p-type InP substrate 61, a p-type InP buffer layer 62, an InGaAsP active layer 63, and an n-type InP are formed.
Clad layer 64, p-type InP buried layer 65, n-type In
P current blocking layer 66, p-type InP current blocking layer 6
7, the n-type InP clad layer 68 and the n-type InGaAs contact layer 69 are formed. An n-side electrode 70 and a p-side electrode 71 are provided on the contact layer 69 and the substrate 61, respectively.

Next, with reference to FIG. 8, the manufacturing process of the semiconductor laser having the above structure and the details of each part will be described.

First, as shown in FIG. 8A, p-type InP is used.
On the substrate 61, a p-type InP buffer layer 62 (p = 1 × 10 ¹⁸ cm ⁻³ , thickness 1 μm) and InGa are formed by MOCVD method.
AsP active layer 63, n-type InP clad layer 64 (n = 1
× 10 ¹⁸ cm ^-3 , thickness 0.2 μm), n-type InGaAs
The P etching dummy layer 73 is crystal-grown.

Next, as shown in FIG. 8B, <011
SiO ₂ stripe mask 7 with a width of 4.2 μm in the> direction
After forming No. 2, chemical etching is performed to form a mesa stripe having a height of 3 μm as shown in FIG. At this time, what is different from the conventional example shown in FIG. 11 is that the shape of the side surface of the mesa stripe is almost perpendicular to the substrate within a range of about 1.5 μm from the SiO ₂ mask, and the width of the mesa stripe is lower in the area below the side surface. It is a smooth shape that gradually expands. The radius of smooth curve is about 1.5.
It is about μm. At this time, an etchant in which bromine, hydrobromic acid and water were mixed was used as the chemical etchant, and etching was performed while the wafer was stationary in the etchant. Further, in order to obtain the mesa shape as shown in FIG. 8C, it is desirable to set the band gap wavelength of the InGaAsP etching dummy layer 73 to about 1.1 to 1.2 μm.

Next, as shown in FIG. 8D, SiO ₂
With the mask 72 left, the p-type I is formed by the MOCVD method.
nP buried layer 65 (p = 1 × 10 ¹⁸ cm ⁻³ , thickness 0.
5 μm), n-type InP current blocking layer 66 (n = 7 × 1)
0 ¹⁷ cm ^-3, 1.5μm thickness), p-type InP current blocking layer ^{67 (p = 1 × 10 18} cm -3, thickness 1.0 .mu.m) the crystal growth. At this time, although the (111) B plane and the (221) B plane appear on the side surface of the mesa stripe when one p-type InP buried layer is grown, as in FIG. 12A,
The length of the (221) B-plane cross section is about 1 μm, which is about twice as long as that of the conventional structure. Therefore, even if the thickness of the n-type InP current blocking layer 66 is sufficiently thick as 1.5 μm, the (221) B plane on the side surface of the mesa stripe does not disappear.

If the entire shape of the side surface of the mesa stripe is close to a right angle with respect to the substrate to some extent (221) B.
No face is formed. Therefore, it is important to set the local radius of the smooth curved region below the portion perpendicular to the substrate on the side surface of the mesa stripe to at least 1 μm or more. The most preferred shape is such that about half the height of the mesa stripe is perpendicular to the substrate and the other half is a smooth curve. Further, the p-type InP region between the active layer and the n-type InP current blocking layer serves as a current leak path, but it is necessary to make the width of this leak path as narrow as possible. Therefore, since it is important that the active layer be as close to the SiO ₂ mask as possible on the mesa stripe, the thickness of the n-type InP cladding layer 64 was reduced to 0.2 μm.

Next, as shown in FIG. 8 (e), SiO ₂
After removing the mask 72 and the InGaAsP etching dummy layer 73, the n-type InP clad layer 68 (n = 1 × 10 ¹⁸ cm ⁻³ , thickness 1.5 μm) is formed by MOCVD.
n-type InGaAs contact 9 (n = 1 × 10 ¹⁹ c
m ⁻³ , thickness 0.5 μm). Finally, Figure 8
As shown in (f), the n-side electrode 70 and the p-side electrode 71 are formed, the substrate is polished, and cleaved into individual semiconductor lasers, whereby a semiconductor laser as shown in FIG. 7 can be manufactured.

The semiconductor laser thus manufactured is
Since the thickness of the n-type InP current block layer can be made twice as thick as that of the conventional structure, the leakage current flowing through the current block layer is small even at the time of high current injection or high temperature operation. It becomes possible to operate. In the actually manufactured semiconductor laser, an optical output of 20 mA or more was obtained even at 100 ° C. with a double-sided cleaved element having a cavity length of 300 μm.

By the way, the InGaAsP active layer 6 shown in FIG.
3 is a multi-quantum well structure with seven wells having a structure as schematically shown in FIG.
(Thickness 20 nm), non-doped barrier layer 82 (thickness 10 n
m), non-doped strained well layer 83 (thickness 8 nm, compressive strain 1%), n-type n-side optical confinement layer 84 (n = 5 × 10 ¹⁷
cm ⁻³ , thickness 80 nm). At this time, the band gap wavelengths of the optical confinement layer and the barrier layer are 1.
The oscillation wavelength of the laser is 1.
It was set to 3 μm.

FIG. 10 is a perspective view showing a semiconductor laser according to still another embodiment of the present invention. This embodiment is a distributed feedback semiconductor laser, and p-type InP is used in the first crystal growth.
Only the buffer layer 62 and the InGaAsP active layer 63 are crystal-grown, and the interference exposure method is used to form the diffraction grating 75 thereon. The shape of the side surface of the mesa stripe is such that the width of the mesa stripe is widest immediately below the SiO ₂ mask and decreases slightly as it moves away from the mask.

The angle formed between the tangential direction of the side surface of the upper half region of the mesa stripe and the normal direction of the substrate is set within 10 degrees. If this angle becomes too large, the (221) B plane will not be formed during the embedded growth. As described above, the mesa stripe width is reduced near the SiO ₂ mask as it moves away from the mask because the p-type InP of the first buried layer grows on the side surface of the active layer immediately below the SiO ₂ mask. This is to make it easier. The other parts are exactly the same as the embodiment of FIG. However, in the third crystal growth, it is necessary to satisfy the condition that the diffraction grating is appropriately deformed by the storage of the diffraction grating and the thermal etching. For this purpose, for example, the growth start temperature is preferably 550 ° C. to 620 ° C., and the time until the growth starts after the temperature is raised to 400 ° C. or higher is 30.
It is desirable that the time is from 3 seconds to 30 minutes. Also, PH ₃
The pressure is preferably 0.02 to 20 Torr. The active layer 63 has a multiple quantum well structure as in the embodiment of FIG. 7, and the diffraction grating is formed in the n-side optical guide layer.
The present invention is also suitable for a distributed feedback semiconductor laser having a structure in which the diffraction grating is above the active layer with respect to the substrate.

In the above embodiment, Planer Bur
Although the laser having the ied Heterostructure structure has been described, the present invention is applicable to lasers having other structures, integrated lasers, surface emitting lasers, integrated devices such as modulators, as long as it is a device for performing current constriction by using a reverse junction. Application is also possible.

In the above embodiment, the laser having the active layer of the multiple quantum well structure having the wavelength of 1.3 μm is described, but the wavelength may be any wavelength. Also, InAlAs, InGaAs is formed on at least a part of the reverse junction buried layer.
_{P, In x Ga y Al 1} -xy P (0 ≦ x, y ≦ 1) or may be is used material other than InP of this combination and the like. A layer containing InP and Al, that is, InAlA
The s layer or the InAlGaP layer or the like is embedded in the buried layers 5, 6, 7
When provided in any of the above, 50 to 20 layers containing Al
A thin film having a thickness of about 0 Å and a structure including an InP layer having a thickness of 50 to 2000 angstroms between layers containing Al facilitates selective growth and is expected to improve characteristics. Further, it is desirable that the number of layers containing Al is 1 to 10 and that the total thickness is 1000 angstroms or less in order to improve the characteristics. The width of the SiO ₂ mask 34 is preferably 4 μm or less.

The structure of the active layer does not matter. The device containing the device, the buried layer on the GaAs substrate _{In x Ga y Al 1-xy} P (0 ≦ x, y ≦ 1) and comprising the present invention is InP cladding layer formed on the GaAs substrate or the Si substrate It is also applicable to. Although Zn is used as the p-type impurity, the same effect as that of Zn can be obtained when another group II element such as Cd is used as the p-type impurity. That is, by setting a low carrier concentration in the vicinity of the active layer 26a in the buried layer 26 in contact with the active layer and a high carrier concentration in the outer side 26b away from the active layer, it is possible to suppress diffusion of p-type impurities into the active layer, and The embedded shape can be obtained. In this case, the high impurity concentration refers to the impurity concentration at which the efficiency of taking in impurities from the raw material changes drastically, and in the case of Cd as well as Zn, it is about 5%.
It becomes × 10 ¹⁷ cm ^-3 . Further, as the n-type impurity, Si
Although a group IV element such as Sn or a group VI element such as S or Se may be used.

[0061]

According to the first aspect of the present invention, diffusion of p-type impurities into the active layer is prevented, and the side surface of the active layer does not adversely affect the active layer. With the embedding layer of the mold, you can smoothly embed it in a sufficient thickness,
It is possible to provide a semiconductor laser which has a low threshold value, good temperature characteristics, high reliability, and can be produced with high reproducibility by the MOCVD method. This effect can be applied to a high-power semiconductor laser, and the improvement of device characteristics can be realized.

According to the second aspect of the present invention, in the buried type semiconductor laser by MOCVD method using the p substrate, it appears on the side surface of the mesa stripe during the buried growth (2
21) The length of the B surface can be made longer than in the conventional case. Therefore, the thickness of the n-type InP current blocking layer can be increased, and a semiconductor laser suitable for high power operation and high temperature operation can be obtained.

[Brief description of drawings]

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

FIG. 2 is a cross-sectional view illustrating a process of filling a side surface of an active layer of the semiconductor laser shown in FIG. 1 with a p-type buried layer.

FIG. 3 is a sectional view showing a semiconductor laser according to another embodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a laser structure in which the side surface of the active layer of the semiconductor laser shown in FIG. 3 is filled with a low-concentration p-type buried layer.

5 is a cross-sectional view illustrating a process when the side surface of the active layer of the semiconductor laser illustrated in FIG. 3 is filled with a high-concentration p-type buried layer.

6 is a cross-sectional view for explaining the effect on the thickness of an n-type cladding layer when the side surface of the active layer of the semiconductor laser shown in FIG. 3 is filled with a high-concentration p-type buried layer.

FIG. 7 is a sectional view showing a semiconductor laser according to still another embodiment of the present invention.

8A to 8C are cross-sectional views sequentially showing manufacturing steps of the semiconductor laser shown in FIG.

9 is a schematic diagram showing a band structure of multiple quantum wells in the active layer of the semiconductor laser shown in FIG.

FIG. 10 is a sectional view showing a semiconductor laser according to still another embodiment of the present invention.

FIG. 11 is a cross-sectional view showing a conventional semiconductor laser.

12 is a cross-sectional view showing a buried growth process on the side surface of the mesa stripe of the semiconductor laser shown in FIG.

FIG. 13 is a sectional view showing another conventional semiconductor laser.

[Explanation of symbols]

21 ... n-type InP substrate-side clad, 22 ... n-type GaIn
AsP light guide layer, 23 ... Active layer, 23a ... Quantum well barrier layer, 23b ... Quantum well well layer, 24 ... Light guide layer, 2
5 ... p-type InP clad layer, 26 ... p-type first buried layer InP layer, 26a ... inner region of the first buried layer, 26b
... outer region of the first buried layer, 27 ... n-type InP current blocking layer, 28 ... p-type InP layer, 29 ... Ga _0.47 In
_0.53 As contact layer, 31 ... P-side electrode, 32 ... N-side electrode, 34 ... SiO ₂ mask, 33 ... Mesa stripe, 3
5 ... (111) B surface, 36 ... Slow growth rate surface, 41 ...
p-type InP substrate, 42 ... p-type InP clad layer, 43 ...
GaInAsP optical guide layer, 44 ... Active layer, 44a ... Quantum well barrier layer, 44b ... Quantum well well layer, 45 ... GaI
nAsP optical guide layer, 46 ... n-type InP clad layer, 4
7 ... p-type InP active layer immediate vicinity layer, 48 ... n-type InP current blocking layer, 49 ... p-type InP layer, 50 ... n-type InP layer,
51 ... N-type GaInAs contact layer, 52 ... N-side electrode, 53 ... P-side electrode, 55 ... (111) B surface, 56 ... S
Surface, 57 ... F surface, 61 ... p-type InP substrate, 62 ... p-type I
nP buffer layer, 63 ... InGaAsP active layer, 64 ...
n-type InP clad layer, 65 ... p-type InP buried layer,
66 ... n-type InP current blocking layer, 67 ... p-type InP current blocking layer, 68 ... n-type InP cladding layer, 69 ... n
Type InGaAsP contact layer, 70 ... N side electrode, 71
... p-side electrode, 72 ... SiO ₂ mask, 73 ... n-type InG
aAsP etching dummy layer, 81 ... P-side light guide layer,
82 ... Barrier layer, 83 ... Well layer, 84 ... N side light guide layer.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hideto Furuyama 1 Komukai Toshiba-cho, Kouki-ku, Kawasaki-shi, Kanagawa Toshiba Research & Development Center (72) Inventor Yoshihiro Kokubun Komukai, Ko-ku, Kawasaki-shi, Kanagawa Toshiba Town No. 1 Inside Toshiba Research and Development Center

Claims (6)

[Claims] 1. A semiconductor laser in which a cladding layer is formed above and below an active layer, and a current block region is formed on both sides of the active layer, wherein a first buried region of the current block region is in direct contact with a side surface of the active layer. The layer exhibits a p-type conductivity, and the carrier concentration of at least a part of the outer region of the first buried layer remote from the active layer is the carrier of the inner region of the first buried layer that is in direct contact with the active layer. A semiconductor laser characterized by being higher than the concentration. 2. The semiconductor laser according to claim 1, wherein the cladding layer contains InP, and the width of the active layer is defined by forming a mesa stripe. 3. The carrier concentration of the at least a part of the outer region and the carrier concentration of the inner region of the first buried layer are about 7 × 10 ^{17 to} 3 × 10 ¹⁸ cm ⁻³ and about 1 × 10 ^{17 respectively.} The semiconductor laser according to claim 1, which has a density of about 4 × 10 ¹⁷ cm ^-3 . 4. The semiconductor laser according to claim 1, wherein the formation temperature of the current block region is higher than the formation temperature of the active layer. 5. The semiconductor laser according to claim 1, wherein the mesa stripe and the current blocking region are layers formed by a metal organic chemical vapor deposition method. 6. The shape of the side surface of the mesa stripe is a smooth curve, and the radius of curvature at each point on the curve in an approximately half region near the top of the mesa stripe is 10 μm or more and this region 3. The semiconductor laser according to claim 2, wherein an angle formed by a tangent line at each point on the curve and the normal line of the substrate is 10 degrees or less.