JP2780275B2 - Embedded semiconductor laser - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor laser used for optical communication and optical information processing.

(Prior art and its problems) A semiconductor laser is one of key devices used for optical communication and optical information processing, and is expected to have high-speed modulation characteristics and oscillate with high efficiency. However, it has been difficult for a conventional semiconductor laser to satisfy both of the above two characteristics. Hereinafter, the problems of the conventionally manufactured semiconductor laser will be described first, and then the remaining problems of the second prior art which has been attempted to improve these problems will be described.

Conventionally manufactured typical semiconductor lasers include 2
Double Channel Pla
nar Buried Heterostructure Laser Diode: DC-PBH
LD), which is described in detail in Journal of Lightwave Technology, LT-1, Vol. 1983, March 1983, pp. 195-202. This semiconductor laser has a pnpn structure in a groove portion as shown in FIG.
The current is blocked by the junction potential of the junction. In a current blocking structure based on the junction potential of a pn junction represented by this structure, since a voltage is directly applied to the junction, the voltage applied to the junction is high and the current flowing across the junction is likely to be large, and the pn junction is 10 pF There is a problem that high-speed modulation near 10 Gb / s is difficult due to the above-mentioned capacitance.

Next, in order to solve these problems, a semiconductor laser in which both sides of an active region are buried with a semi-insulating semiconductor layer containing an impurity for forming a deep level as shown in FIG. 2B has been developed. I have.

An example of manufacturing this semiconductor laser is described in Electronics Letters (Electron. Lett., Vol. 22, 1986, page 1214).
Since the p-layer and the n-layer are interposed through the high-resistance buried layer, the capacity and the high-speed modulation characteristic are significantly improved compared to the conventional first semiconductor laser. However, this conventional device has a problem that when the voltage applied to the device is increased, a current flows through the buried layer and the oscillation efficiency is reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor laser in which the current in the buried layer, which is a conventional problem described above, is reduced to almost zero, and high-speed modulation characteristics and oscillation efficiency are excellent.

(Means for Solving the Problems) The present invention provides a mesa-shaped current-carrying part in which the upper and lower sides of an active region are sandwiched by cladding layers of a first conductivity type and a second conductivity-type, respectively. In a buried structure semiconductor laser having a structure in which both sides are buried with a current blocking layer made of a semiconductor having a deep impurity level, a forbidden band width is provided at a position in contact with the active region and the cladding layer of at least one conductivity type. A band gap barrier layer made of a semiconductor that is always larger than the forbidden band width of the active region and the cladding layer is provided between the current conducting portion and the current blocking layer.

A second invention is characterized in that the bandgap barrier layer has a multi-layer structure in which films having different lattice constants are stacked.

In a third aspect, the band gap barrier layer is made of InGaP.
Characterized in that it is a layer.

(Operation and Principle of the Invention) The operation and principle of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the semiconductor laser according to the present invention includes a mesa-shaped current-carrying portion in which an active region 11 is sandwiched between a p-type cladding layer 12 and an n-type cladding layer 13 by a semiconductor having a deep impurity level. The band gap buried in the current blocking layer 14 and at least in contact with the active region 11 and the p-type cladding layer 12 is always smaller in each part than the band gap of the active region 11 and the p-type cladding layer 12. The feature is that a large current blocking layer portion 15 is provided. 16 is a substrate and 17 is an electrode. On the other hand, in the conventional first semiconductor laser, the second
As shown in FIG. 1A, the element has a basic structure in which a mesa-shaped current-carrying portion including an active layer 11, a p-type cladding layer 12, and an n-type cladding layer is buried with a p-type buried layer 21. At this time, the leakage current flowing outside the active region is almost directly applied to the p-type buried layer 21 and the n-type cladding layer 13, so that the current 23 crossing the pn junction is not negligible. Especially n-type buried layer
When the p-type buried layer 21 and the n-type cladding layer 13 are present,
There is a fear that the layer structure of this may leak due to the action of the npn transistor and amplify the current. For this reason, the current 24 flowing in the p-type buried layer 21 to the outside of the double groove is provided to reduce the voltage application to the pn junction. However, there has been a problem that the current 24 flowing outside the double groove leaks and increases as a current. Also, the pn junction capacitance of the buried layer was large, so that the device capacitance could not be reduced.

In the second conventional example, a structure in which a current blocking layer 14 having a deep impurity level is buried as shown in FIG. 2B is formed. As a result, since the voltage is sequentially applied to the current blocking layer, the problem relating to the junction capacitance is greatly improved. However, in the second conventional example, the band gap of the current blocking layer is equal to that of the p-type cladding layer or the n-type cladding layer, and a semi-insulating semiconductor having an electron trapping type deep impurity level is used as the current blocking layer. I was In this case, although the semiconductor layer used alone as the current blocking layer has an extremely high resistance, the leakage current 24 is about 10% of the total current in the laser structure as shown in FIG. 2B. The problem remained. The cause of the leakage current 25 is that the holes in the p-type cladding layer 12 are attracted to the negative charges of the electrons trapped by the impurities in the vicinity of the current blocking layer 14 in contact with the p-type cladding layer 12. This is because regions having both a high hole concentration and a high electron concentration are formed on both sides of the p-type cladding layer. That is, this is because holes are not captured by impurities.

In the present invention, in order to remove the cause of the leakage current,
High concentration holes in the p-type cladding layer 12 and the active region 11 are prevented from flowing into the current blocking layer and the n-type cladding layer
A layer structure for preventing electrons from flowing from the active region 13 and the active region 11 was introduced into this device. That is, in the present invention, the band gap barrier layer 15 is used as a current blocking layer region in which the forbidden band width is always larger than the forbidden band width of the p-type cladding layer 12 or the active region 11 at least in the vicinity of the region where holes may flow. Introduced. In addition, in order to reduce the inflow of electrons from the n-type cladding layer, also in the region in contact with the n-type cladding layer,
By providing the thin-film bandgap barrier layer 15 whose forbidden band width is larger than the forbidden band width of the n-type cladding layer, the leakage current can be further reduced. Even if the difference in the forbidden band width at the interface of the bandgap barrier layer 15 is small, if the temperature energy at room temperature is about 0.025 eV or more, it works as a sufficient barrier,
The leakage current component 24 is extremely reduced.

Also, when a semiconductor having a deep impurity level of a hole trapping type is used for the current blocking layer 14, a band gap barrier layer 15 having a larger forbidden band width is similarly provided in the vicinity of a region where electrons not trapped may flow. Should be provided. That is, if a bandgap barrier layer is provided at a portion where the current blocking layer contacts the active region and the n-type cladding layer, the leakage current can be reduced as compared with the conventional case. Further, in order to make the leakage current low region more effective, the band gap barrier layer 15 may be provided on the entire surface of the current carrying portion including the p-type cladding layer.

Next, the operation of the second invention of the present invention will be described. When trying to introduce the band gap barrier layer 15 of the present invention into an InGaAsP / InP-based semiconductor laser which is a typical buried structure semiconductor laser, InGaP is considered as a material having a larger forbidden band than InP of the cladding layer. However, the lattice constant of InGaP decreases from the lattice constant of InP as the Ga component increases, so that there is a problem that dislocations occur in the film growth and it is difficult to grow a thick film. The present invention is intended to alleviate the above-mentioned problems by making the band gap barrier layer structure a multilayer film structure. In other words, InP
By forming a thin film of an InGaP layer having a wider bandgap and a lattice constant smaller than that of InP, and subsequently combining a layer having a large lattice constant and a layer having a small lattice constant to form a multilayer structure, dislocations can be avoided. In other words, in the case of a multilayer structure, there is a factor that absorbs lattice mismatch distortion between adjacent layers.
Film growth with few dislocations can be achieved.

Hereinafter, the present invention will be described in more detail with reference to examples.

Embodiment 1 The schematic structure of the first embodiment of the present invention is the same as that of FIG. In this embodiment, first, an n-type I
nP cladding layer 13, wavelength 1.3 μm composition InGaAsP active layer 11, p
An InP cladding layer 12 is sequentially grown, and a groove having a depth of about 2.5 μm and a width of 10 μm is formed in accordance with a normal etching process.
It was formed on both sides of a μm inverted mesa current conducting portion. Next, as a bandgap barrier layer 15, an InGaP layer having a forbidden band width larger than the forbidden band width of InP of 1.35 eV was grown on the side wall of the mesa-shaped current conducting portion. The thickness of InGaP grew to an acceptable range under lattice mismatch. In this embodiment, an In _0.6 Ga _0.4 P layer having a forbidden band width of 1.7 eV is grown to a thickness of 0.05 μm.
m. As a growth method, a vapor phase growth method was used. In this growth method, since the growth rate varies depending on the crystal plane, the growth can be selectively performed on the side surface of the groove. In this embodiment, the crystal face exposed on the p-type cladding layer 12 of the reverse-mesa-shaped current-carrying portion is the A-plane having a high growth rate, while the element surface is the (001) plane having a very low growth rate. InGaP was easily grown selectively on the mesa side. Thereafter, similarly to the second conventional example, an electron trapping type InP doped with iron (Fe) having a deep impurity level is buried in the entire surface of the groove to produce a planar buried semiconductor laser.

As a result, in this embodiment, as shown by the solid line 31 in FIG. 3, the ratio of the leakage current to the total current (leakage current ratio) is about 2% when the current density in the active region is around ² KA / cm ^2. This was shown by computer simulation. The leakage current ratio is approximately 30%, which is the estimated value of the leakage current ratio of the first conventional element shown by the broken line 41 in FIG. 3, or the estimated value of the leakage current ratio of the second conventional element, shown by the dashed line 42 in FIG. It is much smaller than about 8%, and it was confirmed in experiments that the leakage current was small. In this embodiment, the InGaP layer 5 and the InP current blocking layer have an electron trapping type.
Doped with Fe impurities, the trap concentration of which is 5 ×
It was set to 10 ¹⁵ cm ^-3 . In addition, this trap concentration is set to 2 × 10
^The leakage current ratio was similarly small when changing from ¹⁵ cm ⁻³ to 2 × 10 ¹⁶ cm ⁻³ . The wide gap current blocking layer may be doped with an electron trapping type impurity other than Fe.
It may be undoped. As a result, in this embodiment, oscillation was performed with higher efficiency than in the prior art, and high-speed modulation was possible.

Example 2 FIG. 4 is a schematic sectional view of an element showing a second example.
Although the basic structure is the same as that of the first embodiment, the semiconductor layer of the current blocking layer 44 is of a hole trapping type instead of an electron trapping type. Further, the p-type substrate and the p-type and n-type conductivity types of the cladding layer are replaced with those in the first embodiment. In this embodiment, Ti is doped as a hole trapping type deep impurity. At this time, a thin film of an InGaP layer was grown on the side walls of the n-cladding layer 13 and the active region 11, and good current blocking characteristics were obtained as in Example 1. Also, it was confirmed that leakage current was small even when Co or Cr was doped instead of Ti, or even when undoped.

In Examples 1 and 2, the band gap barrier layer was used.
Although 15 is formed on the side wall portion of the current blocking layer 44, the effect of the present invention can be sufficiently obtained if it is provided only on the side wall portion in contact with the p-type cladding layer and the active layer and the n-type cladding layer and the active layer.

Example 3 FIG. 5 is a schematic sectional view of an element showing a third example.
The basic structure is the same as that of the first embodiment, except that the band gap barrier layer 15 is grown on the entire surface of the groove on both sides of the mesa. As a growth method, a metal organic chemical vapor deposition (MOCVD) method was used. In this growth method, the growth rate depends on the crystal plane is small, so that the entire surface of the groove can be grown. In this example, InGaP could be easily grown on the entire surface of the groove of the reverse-mesa-shaped current conducting portion. Thereafter, similarly to the second conventional example, an electron trapping type InP doped with iron (Fe) having a deep impurity level is buried in the entire surface of the groove to produce a planar buried semiconductor laser.

As a result, in this embodiment, as shown by the solid line 32 in FIG. 3, the ratio of the leakage current to the total current (leakage current ratio) is smaller than 1% when the current density of the active region is around ² KA / cm ^2. Computer simulation showed that it was almost zero. The leakage current ratio is approximately 30%, which is the estimated value of the leakage current ratio of the first conventional element shown by the broken line 41 in FIG. 3, or the estimated value of the leakage current ratio of the second conventional element, shown by the dashed line 42 in FIG. It is much smaller than about 8%, and it was confirmed in experiments that the leakage current was small. In this example, the InGaP layer 15 and the InP current blocking layer were doped with an electron trapping type Fe impurity, and the trap concentration was set to 5 × 10 ¹⁵ cm ⁻³ . When the trap concentration was changed from 2 × 10 ¹⁵ cm ⁻³ to 2 × 10 ¹⁶ cm ^{−3, the} leakage current ratio was similarly small. The band gap current blocking layer portion may be doped with an electron trapping type impurity other than Fe or may be undoped. As a result, in this embodiment, oscillation oscillated with higher efficiency than that of the conventional one and high-speed modulation was made possible. .

Example 4 FIG. 6 is a schematic sectional view of an element showing a fourth example.
Although the basic structure is the same as that of the third embodiment, the semiconductor layer of the current blocking layer 44 is of a hole trap type instead of an electron trap type. Further, the p-type substrate and the p-type and n-type conductivity types of the cladding layer are replaced with those in the first embodiment. In this embodiment, Ti is doped as a hole trapping type deep impurity. At this time, a thin film of an InGaP layer was grown on the side walls of the n-cladding layer 13 and the active region 11, and good current blocking characteristics were obtained as in Example 1. It was also confirmed that the leakage current was small when at least one of Co and Cr was doped instead of Ti, and even when undoped.

Embodiment 5 FIG. 7 is a schematic sectional view of an element showing an embodiment of the second invention. The basic structure of the present embodiment is the same as that of Embodiment 3 (fifth embodiment).
This is the same as FIG. 2 except that the bandgap barrier layer 15 has a multi-layer structure 71. InGaP layer and InP as band gap barrier layer 15 material
Layers were used. Although the InGaP layer has lattice mismatch with the InP layer, for example, in the case of In _0.6 Ga _0.4 P composition, the wide gap layer is about 0.01.
If the film thickness is up to 5 μm, it is possible to repeatedly laminate without dislocation. In this embodiment, metalorganic vapor phase epitaxy is used as a growth method, and the In _0.6 Ga _0.4 P layer and the InP layer
Ten μm layers were grown alternately. In this embodiment, a p-type low concentration InP layer is used as a current blocking layer 14 in the trench to fabricate a semiconductor laser having a flat buried structure. As a result,
The leakage current of the semiconductor laser of this example was sufficiently small near the oscillation threshold current value, and good device characteristics were obtained in which the oscillation threshold current value did not deteriorate due to dislocation or the like. In addition, a semi-insulating InP layer may of course be used for the current blocking layer 14 of this embodiment. In this embodiment, the example of the n-type InP substrate is shown. However, also in the case of the p-type InP substrate, the multi-layered structure barrier layer 15 simultaneously leaks and is effective in a low current region. In Examples 1 to 5, InGaAsP having a composition oscillating at a wavelength of 1.3 μm was used in the active region, but it is clear that the composition is not limited to this composition.

Further, in the above embodiment, an example of selective film formation by a vapor phase growth method has been described, but the growth method is not limited to the vapor phase growth method, and another growth method such as gas source molecular beam epitaxial growth may be used.

Further, in the above embodiment, the InGaAsP / InP material system on the InP substrate was used.
The same applies to lGaInP / GaInP material systems.

(Effect of the Invention) The semiconductor laser of the present invention can make the current almost zero as compared with the conventional device, oscillates with high efficiency, and can perform high-speed modulation by reducing the capacity, which was an object of improvement of the conventional device. ,
Suitable for light source devices for optical communication and optical information processing.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an element structure of a buried semiconductor laser according to the first invention of the present invention, FIG. 2 (a) is a sectional view showing a schematic structure of a first conventional element, and FIG. FIG. 3 is a cross-sectional view showing a schematic structure of a second conventional element, and FIG. 3 shows a computer simulation result of a relationship between a leakage current amount and an active region current density of the element of the present invention, the first conventional element, and the second conventional element. FIG. 4 is a schematic sectional view showing the element structure of the second embodiment, and FIG.
FIG. 6 is a schematic sectional view of the element of the third embodiment, FIG. 6 is a schematic sectional view of the element of the fourth embodiment, and FIG. 7 is a schematic sectional view of the element of the embodiment of the second invention. 11 active region, 12 p-type cladding layer, 13 n-type cladding layer, 14 current blocking layer, 15 band gap barrier layer, 16 n-type substrate, 17 electrode 21 ... p-type buried layer, 22 ... n-type buried layer 23, 24 ... leakage current path of the first conventional element, 25 ... leakage current path of the second conventional element, 31 ... Leakage current / current density relationship of the device according to the first embodiment of the present invention, 32: leakage current / current density relationship of the device of the third embodiment of the present invention 41 ... Leakage current / current density of the first conventional device Relationship 42: leakage current / current density relationship of the second conventional element 43: p-type substrate 44: hole trapping type current blocking layer 71: multi-layer structure.

Claims (3)

(57) [Claims] A semiconductor having a mesa-shaped current-carrying portion having upper and lower sides of an active region sandwiched by cladding layers of a first conductivity type and a second conductivity type, respectively, and a deep impurity level on both sides of the current-carrying portion. In a buried structure semiconductor laser having a structure buried with a current blocking layer, the forbidden band has a forbidden band width at a position in contact with the active region and the cladding layer of at least one conductivity type. A buried semiconductor laser, wherein a bandgap barrier layer made of a semiconductor that is always larger than a width is provided between the current conducting portion and the current blocking layer. 2. The buried semiconductor laser according to claim 1, wherein said band gap barrier layer has a multi-layer structure in which films having different lattice constants are stacked. 3. A buried semiconductor laser according to claim 1, wherein said band gap barrier layer is an InGaP layer.