JPH0449792B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to semiconductor laser technology, and particularly to a semiconductor laser device with a low threshold current.

[Technical background of the invention and its problems]

Digital Audio Disk (DAD),
2. Description of the Related Art As semiconductor lasers are increasingly being used as optical disk devices such as video disks, document files, etc. and as light sources for optical communications, techniques for mass production of semiconductor lasers are becoming necessary. Conventionally, the liquid phase epitaxial growth method (LPE method) using the sliding boat method has been used as a thin film multilayer heterojunction crystal manufacturing technology for semiconductor lasers.
With the LPE method, there is a limit to how large the wafer area can be.
For this reason, crystal growth techniques such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), which have excellent uniformity and control over large areas, are attracting attention.

A semiconductor laser as shown in FIG. 1 is an example of a built-in waveguide laser that takes advantage of the characteristics of the MOCVD method. In the figure, 1 is an N-GaAs substrate, and 2 is an N-GaAs substrate.
A GaAlAs cladding layer, 3 a GaAlAs active layer, 4 a P-GaAlAs cladding layer, 5 an N-GaAs current blocking layer, and 7 and 8 metal electrodes. In a laser with such a structure, when looking at a cross section perpendicular to the electrode surface, there is only a simple PN junction in the striped part where the current blocking layer is missing, whereas PNPN junctions are formed on both sides of the striped part. has been done. Therefore, when a forward voltage is applied, a reverse bias is applied to one of the PNPN junctions, and almost no current flows through the PNPN junction, and current only flows through the stripe portion. It will flow.

Note that the laser with the above structure requires the first crystal growth from the substrate 1 to the current blocking layer 5, and the first crystal growth from the substrate 1 to the current blocking layer 5.
It is formed by a two-step crystal growth process called a second crystal growth in which a part of the substrate is etched into a stripe shape and then a covering layer 6 and a contact layer 7 are formed. Here, the growth to the cladding layer 4 at the start of the second crystal growth is growth on the GaAlAs surface whose surface is once exposed to the air.
For this reason, it is difficult to grow using the conventional LPE method.
It was only through the MOCVD method, which allows easy growth on GaAlAs surfaces, that it became possible to manufacture it with good controllability.

By the way, the laser with the structure shown in Fig. 1 has a current confinement structure called an internal stripe structure, but its current confinement effect is not as sufficient as that of a laser with a so-called buried structure (BH laser). It's hard to say. That is, while the threshold current of a typical BH laser is 20 [mA] or less, the threshold current of the laser having the structure shown in FIG. 1 is at least about 41 [mA]. This difference in threshold current between the two is due to the following reason.

(1) With a BH laser, the active layer in the areas where current is not injected is removed, and the active layer left in stripes is uniformly excited. In contrast, in the laser shown in FIG. 1, the active layer exists in a planar shape, and the transverse mode oozes out to the active layer, which is relatively weakly excited. Therefore, the internal loss of the laser cavity is BH
Larger than laser.

(2) In the case of a BH laser, a high-resistance buried layer or a PNPN junction buried layer is formed on both sides of the striped active region that forms the light emitting region.
The structure is such that almost no current flows outside the active region. On the other hand, in the laser shown in FIG. 1, the current narrowed by the striped groove also spreads laterally through the P-type cladding layer and flows through the active layer on both sides of the striped groove. For this reason,
A reactive current is generated, leading to an increase in the threshold current.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser device in which the reactive current flowing outside the striped light emitting region can be minimized in an internal stripe structure and the threshold value can be lowered.

[Summary of the invention]

The present invention provides an N-type semiconductor substrate, a first N-type semiconductor layer formed on the N-type semiconductor substrate and constituting a first cladding layer, an active layer, and a first cladding layer constituting a second cladding layer. a double heterojunction structure made of a P-type semiconductor layer; and a second current blocking layer formed on the first P-type semiconductor layer and having a striped groove extending to the first P-type semiconductor layer. and the second N-type semiconductor layer including the groove portion.
a second P-type semiconductor layer formed on the N-type semiconductor layer, and the PN junction in the region corresponding to the striped groove in the double heterojunction structure is inside the active layer or with the active layer. first or second
The first N-type semiconductor layer is formed at the junction with the cladding layer, and the portion of the first N-type semiconductor layer that is in contact with the active layer and excluding the region corresponding to the striped groove is P.
The present invention provides a semiconductor laser device characterized in that the active layer corresponding to the region inverted to P type is also inverted to P type.

[Effects of the Invention] According to the present invention, in the regions on both sides of the striped groove where a part of the N-type cladding layer is inverted to P-type,
A PN junction is formed in the N-type cladding layer. On the other hand, the region below the striped groove is formed inside the active layer or on either side of the clad layer with which the active layer is in contact. The built-in current barrier associated with a PN junction is determined primarily by the bandgap of the semiconductor constituting the PN junction, particularly the bandgap of the narrower of the P-type and N-type semiconductor layers. Furthermore, the magnitude of the current that flows when a forward current is applied to the PN junction depends not only on the carrier concentration of the semiconductor layer but also on the magnitude of the built-in potential barrier. On the other hand, the bandgap of the semiconductor layer forming the cladding layer is usually set to be about 100 to 200 [meV] larger than the bandgap of the active layer. Therefore, the built-in potential barrier of a PN junction formed in the cladding layer is 100 to 200 times larger than that of a PN junction formed in the active layer or between the active layer and the cladding layer.
[meV] It becomes something big.

Therefore, if the same forward voltage is applied to both PN junctions, the density of the current flowing through the PN junction in the cladding layer will be less than that of the PN junction in the active layer or between the active layer and the cladding layer. It becomes smaller by more than one order of magnitude. Therefore, the proportion of current flowing through the PN junctions under the striped grooves becomes extremely large compared to the case where the PN junctions on both sides of the striped grooves are not formed in the N-type cladding layer.
That is, the current confinement effect is much more effective than in the structure shown in FIG. 1, and the laser oscillation threshold current can be significantly reduced.

In addition, in order to obtain sufficient effects as described above, it is necessary to
The junction needs to be formed sufficiently far away from the active layer. This measure is the diffusion length of electrons, which are minority carriers in the P-type layer, injected from the N-type cladding layer, and the larger the diffusion length, the greater the effect. When the carrier concentration was 10 ¹⁸ [cm ^-3 ] in the P-type Ga _0.55 Al _0.45 As layer, the diffusion length of minority carriers was about 0.5 [μm].

Directly below the striped groove, a PN junction is formed between the active layer and the cladding layer, and on both sides of the striped groove, a PN junction is formed in the N-type cladding layer.
As a method of forming the active layer at a position about the diffusion length of a fractional carrier, it is possible to take advantage of the fact that P-type impurities doped into semiconductors easily diffuse at high temperatures and also easily diffuse into the gas phase. . For example, in the case of Zn doped as a P-type impurity in a Ga _0.55 Al _0.45 As layer, it diffuses through the solid phase several micrometers per hour at a high temperature of 750 degrees Celsius, and furthermore, it diffuses into the gas phase from the crystal surface. It evaporates inside. Using this effect, a desired structure can be obtained extremely easily. That is, after the N-type current blocking layer is grown through the first growth, a part of the N-type current blocking layer is etched away in a stripe shape, and then subjected to high temperature treatment in a vapor phase. In this case, since the P-type cladding layer is exposed to the gas phase surface in the region directly below the striped groove, the P-type impurity doped in the P-type cladding layer evaporates, causing the P-type impurity in the P-type cladding layer to evaporate. concentration decreases. Therefore, P in the P-type cladding layer
The type impurity diffuses into the active layer and even into the N-type clad layer, but since the P-type impurity concentration in the P-type clad layer decreases, an appropriate relationship between the P-type impurity concentration and the N-type impurity concentration in the N-type clad layer is established. By doing so, it is possible to suppress the N-type cladding layer from being inverted to the P-type conductivity type. On the other hand, in the regions on both sides of the striped groove, N is in contact with the P-type cladding layer.
Due to the presence of the N-type current blocking layer, the diffusion of P-type impurities in the P-type cladding layer is significantly suppressed compared to the case where the N-type current blocking layer is not present. Therefore, the amount of decrease in the P-type impurity concentration in the P-type cladding layer is small. Therefore, the P-type impurity can be sufficiently diffused into the N-type cladding layer.
By making the P-type impurity concentration in the P-type cladding layer a certain amount higher than the N-type impurity concentration in the N-type cladding layer, the side of the N-type cladding layer in contact with the active layer is made to have a P-type impurity concentration.
It becomes possible to invert the type conductive layer.

Furthermore, in the present invention, since the current confinement structure is formed in a self-aligned manner on both sides of the active layer, even if the impurity concentration of the P-type cladding layer is increased, it is possible to suppress the increase in reactive current flowing outside the stripe portion. can. Therefore, it is possible to increase the maximum temperature at which CW operation is possible and the optical output saturation level, making it extremely effective as a light source for optical disk writing.

Furthermore, there are other effects as follows. The current blocking layer functions to confine light emitted from the active layer due to the difference in refractive index with surrounding layers. Therefore, since it is necessary to arrange the current blocking layer near the active layer, the P-type cladding layer provided between the active layers of the current blocking layer is formed thin. However, if this layer is thin, it will form in areas other than the striped grooves.
Current breakthrough occurs in this P-type cladding layer in the PNPN structure. In the present invention, the P-type cladding layer is extended into the N-type cladding layer to substantially increase the thickness of the P-type cladding layer. Of course, since the N-type cladding layer is not limited in thickness, it can be made sufficiently thick. In this way, it is possible to provide a highly reliable semiconductor laser device in which current penetration is less likely to occur due to the PNPN structure in which each layer has a sufficient thickness.

[Embodiment of the Invention] FIG. 2 is a sectional view showing a schematic structure of a semiconductor laser according to an embodiment of the invention. 11 in the figure is N
-GaAs substrate, 12 is N-Ga _0.55 Al _0.45 As cladding layer, 13 is Ga _0.85 Al _0.15 As active layer, 14 is P-
A Ga _0.55 Al _0.45 As cladding layer, 15 an N-GaAs current blocking layer, 16 a P-GaAs covering layer (contact layer), 17 and 18 metal electrode layers, and 19 a P-type inversion layer, respectively.

The laser having the above structure is realized by the steps shown in FIGS. 3a to 3d. First, as shown in FIG. 3a, an N-GaAs substrate 11 (Si doped 2
×10 ¹⁸ cm ^-3 ) with a thickness of 2 [μm] of N-Ga _0.55
Al _0.45 As cladding layer 12, (Se doped 2×10 ¹⁷ cm
^-3 ), undoped ga _0.85 Al _0.15 As with a thickness of 0.1 [μm]
Active layer 13, P-Ga _0.55 Al _0.45 As with a thickness of 2 [μm]
A cladding layer 14 (Zn doped, 4×10 ¹⁸ cm ^-3 ) and a 2 μm thick N-GaAs current blocking layer 15 (Se doped, 5×10 ¹⁸ cm ^-3 ) were grown in sequence. The MOCVD method was used for this first crystal growth, and the growth conditions were: substrate temperature 750 [℃], V/=20, carrier gas (H ₂ ) flow rate ~10 [/min], and the raw material was trimethyl gallium (TMG: (CH) ₃ Ga), trimethylaluminum (TMA: (CH ₃ ) ₃ Al), aluminium (AsH ₃ ), p-dopant: diethylzinc (DEZ:
(C ₂ H ₅ ) ₂ Zn), n-dopant: hydrogen selenide (H ₂ Se), and the growth rate was 0.25 [μm/min]. Note that the first crystal growth does not necessarily result in M-
Although it is not necessary to use the CVD method, using the MO-CVD method, which allows crystal growth with good uniformity over a large area, is advantageous over the LPE method when considering mass production.

Next, as shown in FIG. 3b, a photoresist 21 is applied on the current blocking layer 15, a striped window with a width [μm] is formed in the resist 21, and the current blocking layer 15 is selectively etched using this as a mask. death,
Striped grooves 22 were formed. Next, after removing the resist 21 and performing surface cleaning treatment,
Prior to the second crystal growth, 750° C.
High temperature heat treatment was performed at [°C] for 90 minutes. As a result, the P-type impurity in the P-type cladding layer 14 is diffused into the N-type cladding layer 12, and as shown in FIG.
The portion in contact with is inverted to P-type, and a P-type inversion layer 19 is formed. Here, the reason why the P-type inversion layer 19 is not formed in the region below the striped groove is that the P-type impurity at the level of the P-type cladding layer 14 diffuses into the gas phase below the striped groove. After this, a second crystal growth was performed using the MOCVD method. That is, as shown in Figure 3c, the entire surface is coated with a thickness of 2 [μm].
N-Ga _0.55 Al _0.45 As coating layer 16 (Se doped 1×
10 ¹⁸ cm ^-3 ) and 8 [μm] thick N-GaAs contact layer 17 (Se doped 1×10 ¹⁸ ) was successively grown.

After this, a Cu-Ar electrode layer 18 is formed on the contact layer 17 by a normal electrode attaching process, and a Cu-Ar electrode layer 18 is applied to the substrate 11.
An Au--Ge electrode 19 was deposited on the lower surface to obtain the structure shown in FIG. 2. The thus obtained sample was cleaved into a Fabry-Perot laser with a cavity length of 250 [μm]. The characteristics of the device were as follows:
It was extremely low at 30 [mA]. This value is extremely low compared to the structure shown in FIG. 1, and it can be seen that the present invention has a remarkable effect of reducing reactive current.

Note that the present invention is not limited to the embodiments described above. For example, the step of inverting a portion of the N-type cladding layer to P-type is not limited to the embodiment and can be modified as appropriate. One example is a method in which the N-type current blocking layer is doped with a P-type impurity at the first crystal growth stage within a range that does not exceed the N-type impurity concentration. In this case, in the regions on both sides of the striped groove, the P-type impurity in the N-type current blocking layer suppresses the diffusion of P-type impurity from the P-type cladding layer, so that the P-type impurity is transferred from the P-type cladding layer to the N-type cladding layer in the heat treatment process. Impurity diffusion can be performed more effectively. Furthermore, as another example, there is a method in which the region of the N-type cladding layer in contact with the active layer is sufficiently doped with P-type impurities in advance, and a P-type inversion layer is formed at the time of the first crystal growth. be. In this case, during the heat treatment process prior to the second crystal growth, the P-type impurity evaporates into the gas phase through the striped grooves, so the P-type impurity concentration under the striped grooves decreases, and the P-type The conductivity type of the N-type cladding layer doped with impurities is also reversed to the original N-type. It goes without saying that the same effects as in the embodiments described above can be obtained even with the two methods described above.

Further, N-GaAlAs may be used instead of N-GaAs as the N-type current blocking layer, and a two-layer or multilayer structure including an N-type layer may be used. Further, the double heterojunction structure including the active layer is not limited to a symmetrical three-layer structure, but may be an asymmetric structure or a multilayer structure having three or more layers. In addition, as a P-type impurity, Zn
Of course, any impurity species that can be implanted by thermal diffusion at high temperatures can be applied to the present invention as a reverse process of implantation by diffusion. Further, the constituent material is not limited to GaAlAs, and compound semiconductor materials such as InGaAsP and GaAlInP may be used. Furthermore, as a crystal growth method, it is also possible to use the MBE method instead of the MO-CVD method. In addition, various modifications can be made without departing from the gist of the present invention.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a schematic structure of a conventional semiconductor laser, FIG. 2 is a sectional view showing a schematic structure of a semiconductor laser according to an embodiment of the present invention, and FIGS. 3 a to d
FIG. 2 is a cross-sectional view showing the manufacturing process of the laser according to the embodiment. 11...N-GaAs substrate, 12...N-Ga _0.55
Al _0.45 As clad layer, 13...Undoped Ga _0.85
Al _0.15 As active layer, 14...P-Ga _0.55 Al _0.45 As cladding layer, 15... N-GaAs current blocking layer, 16
...P-Ga _0.55 Al _0.45 As coating layer (contact layer),
17, 18... Metal electrode layer, 19... P-type inversion layer, 22... Striped groove.

Claims (1)

[Claims] 1. An N-type semiconductor substrate, and a first N-type semiconductor substrate formed on the N-type semiconductor substrate and constituting a first cladding layer.
a double heterojunction structure formed on the first P-type semiconductor layer and comprising a first P-type semiconductor layer constituting a type semiconductor layer, an active layer, and a second cladding layer; A second N constituting a current blocking layer having a striped groove extending to the semiconductor layer.
a second P-type semiconductor layer that includes the groove and is formed on the second N-type semiconductor layer; A PN junction is formed inside the active layer or at a junction between the active layer and the first or second cladding layer, and is in contact with the active layer of the first N-type semiconductor layer and corresponds to the striped groove. A semiconductor laser device characterized in that a portion of the active layer other than the region is inverted to P type, and the active layer corresponding to the region inverted to P type is also inverted to P type. 2 P-type impurities in the portion of the first N-type semiconductor layer that is inverted to P-type and P-type impurities in the first P-type semiconductor layer.
2. The semiconductor laser device according to claim 1, wherein the type impurities are the same. 3. The semiconductor laser device according to claim 2, wherein the P-type impurity is made of Zn.