JPH01302791A - Buried structure semiconductor laser - Google Patents

Abstract

PURPOSE:To reduce leak current to almost zero, and realize oscillation of high efficiency by arranging a band gap barrier layer between a current flowing part and a current blocking layer, which barrier layer has always a forbidden bandwidth larger than that of an active layer and that of a clad layer. CONSTITUTION:A mesa type current flowing part, in which an active layer 11 is sandwiched by a P-type clad layer 12 and an N-type clad layer 13, is filled with a current blocking layer 14 composed of a semiconductor layer having a deep impurity level. A current blocking layer 15, in which forbidden bandwidth of the parts in contact with the region 11 and the clad layer 12 is always larger than that of region 11 and that of the clad layer 12, is formed. Even in the case where the difference of the forbidden bandwidth at the interface of the part 15 is small, the interface sufficiently operates as a barrier, if only the difference is room temperature energy 0.025eV or more. As a result, leak current component is remarkably reduced. Thereby the leak current becomes almost zero, and oscillation of high efficiency is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor laser used in optical communications and optical information processing.

(Prior art and its problems) Semiconductor lasers are one of the key devices used in optical communications and optical information processing, and are expected to have high-speed modulation characteristics and oscillate with high efficiency. However, it has been difficult for conventional semiconductor lasers to satisfy both of the above two characteristics. Hereinafter, the problems of conventionally manufactured semiconductor lasers will be described first, and then the remaining problems of the second conventional technique, which has been attempted to overcome these problems, will be described.

A typical semiconductor laser manufactured conventionally is a double-groove planar buried structure semiconductor laser (DoubleChan).
nel Planar Buried Heteros
Structure La5er Diode: Abbreviation DC-
PBHLD), Journal of Lightwave Technology, Volume LT-1, March 1983 Issue, 1
Details are given on pages 95-202. This semiconductor laser has a groove portion of pnpn+f! as shown in FIG. 2(a). The current is blocked by the junction potential of the pn junction formed at the buried interface. In the current blocking structure using the junction potential of a p-n junction, which is typified by this structure, voltage is directly applied to the junction, so the voltage applied to the junction is high and the current flowing across the junction tends to be large, and the p-n junction is 10 pF. There is a problem in that high-speed modulation of nearly 10 Gb/s is difficult because of the capacitance above.

Next, in order to improve these problems, a semiconductor laser was developed in which both sides of the active region are buried with semi-insulating semiconductor layers containing impurities that create deep levels, as shown in Figure 2(b). There is.

A fabrication example of this semiconductor laser is described in Electronics Letters (Electron, Lett, Vol. 22 (19
(1986, p. 1214). This conventional second
In this semiconductor laser, since the p layer and n layer are interposed through a high-resistance buried layer, the capacitance is low and the high-speed modulation characteristics are significantly improved compared to the conventional first semiconductor laser. However, this conventional element has a problem in that when the voltage applied to the element is increased, current flows through the buried layer and the oscillation efficiency decreases.

An object of the present invention is to reduce the current in the buried layer to almost zero, which is the problem with the conventional art, and to provide a semiconductor laser that has excellent high-speed modulation characteristics and oscillation efficiency.

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

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

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

As shown in FIG. 1, the semiconductor laser of the present invention has a mesa-shaped current-carrying portion in which an active region 11 is sandwiched between a p-type rad layer 12 and an n-type rad layer 13, and a deep impurity level. The forbidden band width of the current blocking layer 14 made of a semiconductor layer having a semiconductor layer and at least the part in contact with the active region 11 and the p-type rad layer 12 is smaller than the forbidden band width of the active region 11 and the p-type rad layer 12, respectively. The feature is that a large current blocking layer portion 15 is always provided in the portion. 16 is a substrate, and 17 is an electrode. On the other hand, in the conventional first semiconductor laser, as shown in FIG. 2(a), the active layer 11, p
Shaped Rad Layer 12) It has a basic structure in which a mesa-shaped current carrying part made of an n-shaped rad layer is buried with a p-type buried layer 21. At this time, since the applied voltage is almost directly applied to the p-type buried layer 21 and the n-type rad layer 13, the leakage current flowing outside the active region is a non-negligible amount of current 23 that crosses the p-n junction. become. In particular, when the n-type buried layer 22 is present, the layer structure of the p-type buried layer 21 and the n-type rad layer 13 may amplify leakage current due to transistor action. For this reason, an attempt has been made to provide a current 24 flowing through the p-type buried layer 21 to the outside of the double trench to reduce the voltage applied to the pn junction. However, there has been a conventional problem that the current 24 flowing to the outside of the double groove increases as a leakage current. Furthermore, since the pn junction capacitance of the buried layer is large, the element capacitance cannot be reduced. .

In the second conventional example, a structure is formed in which a current blocking layer 14 having a deep impurity level is embedded as shown in FIG. 2(b). As a result, the problem of junction capacitance is significantly improved since the voltage is applied sequentially across the current blocking layers. However, in the second conventional example, the forbidden band width 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 a deep electron-trapping impurity level is used as the current blocking layer. I was worried. In this case, although the semiconductor layer used as the current blocking layer alone has extremely high resistance,
In the laser structure as shown in Fig. 2(b), the total current is 105
The problem of front and rear leakage current 24 remained. The cause of this leakage current 25 is that holes in the p-type rad layer 12 are drawn toward the negative charges of electrons captured by impurities in the vicinity of the current blocking layer 14 in contact with the p-type rad layer 12. The region where the electron concentration is high as well as the hole concentration is p
This is because it occurs on both sides of the shaped rad layer. That is, this is due to the fact that holes are not captured by impurities.

In the present invention, in order to eliminate the cause of this leakage current, p
The present device has a layer structure that prevents high-concentration holes in the shaped rad layer 12 and active region 11 from flowing into the current blocking layer and prevents electrons from flowing in from the n-shaped rad layer 13 and the active region 11. It was introduced in That is, in the present invention, the bandgap barrier layer 15 is used as a current blocking layer region whose forbidden band width is always larger than the forbidden band width of the P-shaped rad layer 12 and the active region 11, at least in the vicinity of the region where holes may flow. introduced. In addition to this, in order to reduce the inflow of electrons from the n-type cladding layer, a thin bandgap barrier layer with a forbidden band width larger than that of the n-type cladding layer is also applied to the region in contact with the n-type cladding layer. 15, the leakage current can be further reduced. Even if the difference in forbidden fluorescence 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 will function as a sufficient barrier, so the leakage current component 24 will be extremely reduced.

Furthermore, when a hole-trapping type semiconductor having a deep impurity level is used for the current blocking layer 14, the bandgap barrier layer 15 with a larger forbidden band width is similarly placed near the region where uncaptured electrons may flow. All you have to do is set it up. That is, if a gap barrier layer is provided in a portion where the current blocking layer contacts the active region and the n-type rad layer, leakage current can be reduced compared to the conventional method. Furthermore, in order to make the leakage current low range more effective, the band gap barrier layer 15
may be provided over the entire surface of the current carrying part including the p-type cladding layer.

Next, the operation of the second aspect of the present invention will be explained. InGaAsP/I is a typical buried structure semiconductor laser.
When introducing the bandgap barrier layer 15 of the present invention into an nP-based semiconductor laser, InGaP can be considered as a material with a larger forbidden band than InP for the cladding layer. However, since the lattice constant of InGaP decreases from that of InP as the Ga content increases, dislocations occur during film growth, making it difficult to grow a thick film. The present invention attempts to alleviate the above-mentioned problems by forming the band gap barrier layer structure into a multilayer film structure. That is, by forming a thin film of InGaP with a wider forbidden band width and smaller lattice constant than InP on the current-carrying part side, and then creating a multilayer structure by combining layers with large and small lattice constants, dislocations can be generated. can be avoided. That is, in the case of a multilayer structure, there is a factor that absorbs strain due to lattice mismatch between adjacent layers, so that film growth with fewer 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 in FIG. In this embodiment, first, an n-type InP substrate 16 is
InP type cladding layer 13, wavelength 1.3pm composition InGa
An AsP active layer 11 and a p-type InP cladding layer 12 are sequentially grown and etched to a depth of about 2.5 pm according to a normal etching process.
Grooves with a width of 10 μm were formed on both sides of an inverted mesa-shaped current carrying portion with an active region width of 1.5 μm. Next, as a band gap 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 carrying part. The InGaP film was grown to a thickness within an acceptable range under lattice constant mismatch. In this example, In has a forbidden band width of 1.7 eV. , 5Gao, sP layer was grown and the film thickness was 0.05 pm. A vapor phase growth method was used as the growth method. This growth method has a characteristic that the growth rate differs depending on the crystal plane, so growth can be selectively performed on the side surfaces of the groove. In this example, the crystal plane exposed in the p-type rad layer 12 portion of the inverted mesa-shaped current carrying part is the A-plane, which has a high growth rate, while the surface of the element, etc., is the (001) plane, which has an extremely slow growth rate. Therefore, selective growth of InGaP onto the mesa side surface was easily performed.

Thereafter, similarly to the second conventional example, InP doped with iron (Fe) having a deep electron-trapping impurity level was buried all over the trench to produce a planar buried type semiconductor laser.

As a result 1, in this example, as shown by the solid line 31 in Figure 3, the ratio of leakage current to the total current (leakage current ratio) is approximately 2% when the current density in the active region is around 2KA/cmz. was shown by computer simulation. This leakage current ratio is approximately 30%, which is the estimated leakage current ratio of the first conventional element shown by the broken line 41 in FIG. This is a significant decrease compared to the value of about 8%, and experiments have confirmed that the leakage current is small.

In this example, the InGaP layer 15 and the InP current blocking layer were doped with electron-trapping type Fe impurities, and the trap concentration was set to 5 x 10110l5 in both cases. Also, this trap concentration is set to 2 × 1015 cm−
When changing from 3 to 2×10 16 cm −3 , the leakage current ratio was similarly small. Note that the wide 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, it is possible to oscillate with higher efficiency than in the prior art and to perform high-speed modulation.

(Example 2) FIG. 4 is a schematic sectional view of an element showing a second example. The basic structure is the same as that of Example 1, but the semiconductor layer of the current blocking layer 44 is of the hole trap type instead of the electron trap type. Furthermore, the p-type and n-type conductivity types of the p-type substrate and the cladding layer are switched from those in the first embodiment. In this example, Ti was doped as a hole-trapping deep impurity. At this time, (I'llI
A thin film of InGaP layer was grown on the wall 5, and as in Example 1, good current blocking properties were obtained. Furthermore, it was confirmed that the leakage current is low when Co or Cr is doped instead of Ti, and even when undoped.

Note that in Examples 1 and 2 above, the band gap barrier layer 1
5 is formed on the side wall portion of the current blocking layer 44, but the effects of the invention can be sufficiently obtained if they are provided only on the side wall portions in contact with the p-type rad layer and the active layer, and the n-type rad layer and the active layer, respectively.

(Example 3) FIG. 5 is a schematic cross-sectional view of an element showing a third example. The basic structure is the same as that of Example 1, but the difference is that the band gap barrier layer 15 is grown on the entire surface of the groove on both sides of the mesa. The growth method used was metal organic chemical vapor deposition (MOCVD). In this growth method, the dependence of the growth rate on the crystal plane is small, so growth can be performed over the entire surface of the groove. In this example, the entire groove surface of the reverse mesa-shaped current carrying part is made of InGa
The growth of P was easy. After that, similar to the second conventional example, iron (Fe) having a deep electron-trapping impurity level is
A planar buried type semiconductor laser was fabricated by filling the entire groove with InP doped with .

As a result, in this example, as shown by the solid line 32 in FIG. 3, the ratio of the leakage current to the total current (leakage current ratio) is less than 1% when the current density of the active region is around 2KA/cmz. Computer simulations showed that it is zero. This leakage current ratio is approximately 30%, which is the estimated leakage current ratio of the first conventional element shown by the broken line 41 in FIG. This is a significant decrease compared to the value of about 8%, and experiments have confirmed that the leakage current is small. In this example, the InGaP layer 15 and the InP current blocking layer are doped with electron-trapping Fe impurities, and the trap concentration is 5×10 15 c.
It was set as m-3. Also, this trap concentration is 2×
When changing from 10110l5 to 2 x 1016cm-3, the leakage current ratio was similarly small. Note that the bandgap 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, it is possible to oscillate with higher efficiency than in the prior art and to perform high-speed modulation.

(Example 4) FIG. 6 is a schematic sectional view of an element showing a fourth example. The basic structure is the same as that of Example 3, but the semiconductor layer of the current blocking layer 44 is of the hole trap type instead of the electron trap type. Furthermore, the p-type and n-type conductivity types of the p-type substrate and the cladding layer are switched from those in the first embodiment. In this example, Ti was doped as a hole-trapping deep impurity. At this time, InG is formed on the sidewalls of the n-cladding layer 13 and the active region 11.
A thin film of the aP layer was grown, and as in Example 1, good current blocking properties were obtained. 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 this example is the same as that of Example 3 (FIG. 5), but the difference is that the bandgap barrier layer 15 is composed of a multilayer structure 71, which is a distinctive point. InGa as the material of the band gap barrier layer 15
A P layer and an InP layer were used. Although the InGaP layer has a lattice mismatch with the InP layer, for example, in the case of Ino, 6Gao, and 4P compositions, the wide gap layer can be repeatedly stacked without dislocations 5 as long as the thickness is up to about 0.015 μm. In this example, metal organic vapor phase epitaxy is used as the growth method, and In. , 6Gao, 4P layer and InP layer are 0.0
Ten layers of 111 m each were grown alternately. In this example,
Thereafter, a p-type, low concentration InP layer was used as the current blocking layer 14 and buried in the trench to produce 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 no deterioration of the oscillation threshold current value due to dislocation or the like was observed. Furthermore, it goes without saying that a semi-insulating InP layer may be used as the current blocking layer 14 in this embodiment. In this embodiment, an example of an n-type InP substrate was shown, but the multilayer structure barrier layer 15 was also effective in reducing the leakage current in the low range in the case of a p-type InP substrate. Above Example 1~
5, the active region contains I having a composition that oscillates at a wavelength of 1.3 pm.
Although nGaAsP was used, it is clear that the composition is not limited to this.

Further, in the above embodiments, an example of selective film formation by vapor phase epitaxy was shown, but the growth method is not limited to vapor phase epitaxy, and other growth methods such as gas source molecular beam epitaxial growth may be used.

Furthermore, in the above embodiment, InGaAsP on the InP substrate
/InP material system was used, but Al on GaAs substrate
GaAs/GaAs material system and AIGaInP/GaIn
The same applies to P material systems.

(Effects of the Invention) The semiconductor laser of the present invention can reduce leakage current to almost zero compared to conventional elements, and therefore oscillates with high efficiency, and is also capable of high-speed modulation due to lower capacitance, which was an improvement over conventional elements. , suitable for light source elements for optical communication and optical information processing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing the device structure of the first invention of the embedded semiconductor laser of the present invention, FIG. 2(a) is a sectional view showing the schematic structure of the first conventional device, FIG. 2(b) is a cross-sectional view showing the schematic structure of the second conventional device, and FIG. 3 shows the amount of leakage current and active region current density of the device of the present invention, the first conventional device, and the second conventional device. 4 is a schematic cross-sectional view showing the element of the second embodiment (14 structures,
FIG. 5 is a schematic cross-sectional view of the device of the third embodiment, and FIG. 6 is the device diagram of the fourth embodiment.
FIG. 7 is a schematic sectional view of an element according to the second embodiment of the invention. 11... Active region, 12... P-type rad layer, 13
... n-shaped rad layer, 14 ... current blocking layer, 15.
... Bandgap barrier layer, 16... N-type substrate, 1
7... Electrode, 21... P-type buried layer, 22... N-type buried layer 23, 24... Leakage current path of first conventional element, 25... Second conventional element Element leakage current path, 31...
・Leakage current/current density relationship of the element of Example 1 of the present invention, 32...Leakage current/current density relationship of the element of Example 3 of the present invention 41...Leakage of the first conventional element Current/current density relationship 4
2... Leakage current/current density relationship of second conventional element, 4
3...p-type substrate, 44...hole-trapping type current blocking layer 71...multilayer structure.

Claims (2)

[Claims] (1) The upper and lower sides of the active region are connected to the first conductivity type and the second conductivity type, respectively.
In a buried structure semiconductor laser having a structure in which a mesa-shaped current carrying part is sandwiched between conductive type cladding layers and a current blocking layer made of a semiconductor is buried on both sides of the current carrying part, the active region and at least one conductive type are buried. A band gap barrier layer having a forbidden band width that is always larger than the forbidden band width of the active region and the cladding layer is provided between the current carrying part and the current blocking layer at a position in contact with the cladding layer. A buried structure semiconductor laser. (2) A buried structure semiconductor laser characterized in that the bandgap barrier layer according to claim 1 has a multilayer structure in which films having different lattice constants are laminated.