JPH03120775A - Embedded structure semiconductor and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a low-threshold current, a high efficiency, and a high-frequency characteristic by providing a mesa protection layer and a spacer layer consisting of two kinds of semiconductor layers consisting of an n-type InP which is in contact with an activation layer and a P-type InP which is in contact with this n-type InP. CONSTITUTION:An activation layer 40 is sandwiched by an n-type InP clad layer 38 and a P-type InP clad layer 34 on a P<+>-InP substrate 35 from the upper and lower directions. An electrode layer 37 consisting of an n<+>-InGaAsP is provided on the n-type InP clad layer 38 to achieve an improved contact with an n-type electrode 31. A mesa protection layer 39 consisting of a P-type InP is formed only at a part from an area closer to a boundary 41 between the electrode layer 37 and the n-type InP clad layer 38 to the substrate. A mesa part 42 with the mesa protection layer is sandwiched at both sides by a current block layer 33 in semi-insulation crystal which is the Fe-doped InP, thus reducing leak current passing through the mesa protection layer, restricting diffusion of Fe into the activation layer, and achieving a stable operation over a long period of time.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a buried structure semiconductor laser, particularly a buried structure semiconductor laser having a structure in which rnP containing Fe is embedded on both sides of a mesa stripe region. This relates to reduction and improvement of luminous efficiency.

[Prior art] InP-based long-wavelength semiconductor lasers, which have already been put into practical use as light sources for optical communications, are capable of high-speed modulation, which is extremely important in realizing large-capacity optical transmission. .

In order to reduce the oscillation threshold current and stabilize the single mode, the structure of this semiconductor laser for optical communication is generally as follows.
An embedded heterostructure is adopted.

In this structure, the active layer is formed into a stripe with a width of about 2 microns, and both sides of the active layer are sandwiched between current blocking layers.

For example, in the case of an n-type substrate, the current blocking layer is a P layer on the substrate.
InP type and n type InP are sequentially laminated to form a pnpn thyristor structure.

However, in this structure, a parasitic capacitance exists in the reverse bias application part of the pn junction that constitutes the current blocking layer.
For this reason, there is a problem in that the modulation depth decreases during high-speed modulation.

In order to solve this problem, attempts have been made in the past to use Fe-doped InP semiconductor crystals as the current blocking layer. FIG. 4 shows a cross-sectional view of a typical Fe-embedded structure semiconductor laser. This structure includes an n+ type InP substrate 4, an n type InP cladding layer 3, an InGaAsP active layer 8. P-type InP cladding layer 7 and P″″TnGaA
sP electrode layers 6 are sequentially formed and sandwiched between Fe-doped semi-insulating InP current blocking layers on both sides of the active layer, and further provided with an n-type electrode 5 and a p-type electrode 1. A semiconductor laser having this structure has a high band characteristic in which the modulated light intensity is reduced by 3 dB and the cutoff frequency is 10 GHz or more.

However, this Fe-embedded semiconductor laser has the following two problems.

1) Generally, Fe in rnP-based semiconductor crystals is
Because it diffuses relatively easily and forms deep levels,
Becomes a non-radiative recombination center. As a result, in the structure shown in FIG. 4, for example, Fe diffuses into the active layer, leading to a decrease in luminous efficiency when operated over a long period of time.

Further, Fe piles up on the side surface of the active layer, which causes an increase in threshold current and a decrease in reliability.

2) In the Fe-embedded structure semiconductor laser, the thickness of the current blocking layer is increased to 2 to 3 microns in order to reduce the element capacitance. For this reason, if the stripe width of the mesa portion including the active layer is as narrow as 2 microns, the resistance of the mesa portion, and therefore the Re time constant, increases, impairing high frequency characteristics. Furthermore, the voltage applied to the current blocking layer increases, leading to an increase in leakage current due to double injection, making it difficult to reduce the threshold current and perform high output operation. This problem becomes more pronounced as the stripe width of the P-type InP cladding layer becomes narrower.

In order to solve the above problem l), basically Fe
The doped InP layer and the active layer may be separated by a third semiconductor layer to prevent diffusion of Fe into the active layer or pile-up of Fe on the sides of the active layer. From this point of view,
The following two methods have been proposed.

j) A mesa protective layer 9 made of InP is formed on the side surface of the mesa portion including the active layer 3, thereby separating the active layer 8 and the Fe-doped InP current blocking layer 2 (Fig. 5: Japanese Patent Publication No. 14-77979). (see publication).

if) forming semiconductor spacer layers on both sides of the active layer to separate the active layer from the Fe-doped InP layer;

FIG. 6 shows an example of a semiconductor laser having such a structure, in which an n-InP buffer layer 1 is formed on an n-InP substrate 11.
2. Active layer 13 consisting of rno, 59Gao4+ASo, t+Po, +. P-InP cladding layer 14. P
"-1no5+Gao, 478S contact layer 15 and high resistance InP layer 16 are formed, and further active layer 13
InP spacer layers 17 are formed on both sides. 18
is 5in2 membrane.

19 and 20 are electrodes (Japanese Patent Application Laid-open No. 63-1
28786).

On the other hand, in order to solve the above-mentioned problem 2), the stripe width of the mesa portion may be increased to 4 to 5 microns to lower the resistance. At this time, only the active layer is selectively etched to a width of 2 microns, and the etched portion is closed by crystal growth or mass transport. After that, embedding is performed using InP containing Fe [Problems to be Solved by the Invention] However, in the embedding structure formed by the methods i) and if) described above, there are several problems listed below. Ta.

i) When forming a mesa protective layer, leakage current through the mesa protective layer must be suppressed as much as possible.

For this purpose, it is necessary to make the mesa protective layer thinner. When the mesa protective layer is formed by liquid phase growth, the mesa protective layer 9 becomes thick as shown in FIG. In contrast, in the organometallic vapor phase epitaxy method, a thin mesa protective layer 9 can be formed on the side surfaces of the mesa portion, as shown in FIG.

However, in this growth method, the mesa protective layer 9 is formed on the entire side surface of the mesa portion, and the upper electrode and the substrate are connected. Therefore, the mesa protective layer becomes a current leak bus, leading to an increase in threshold current and a decrease in luminous efficiency.

it) When the spacer layer is P-type InP, the P-type dopant diffuses into the active layer or piles up at the buried interface, leading to a decrease in luminous efficiency and an increase in threshold current.

Ni) For the reason of if) above, the spacer layer located beside the active layer is preferably n-type InP.

However, n-type InP has a lower resistance than p-type InP, so if the stripe width is made wider to solve the above problem 2), the width of the spacer layer that fills the selectively etched portion becomes large. This results in an increase in leakage current.

The present invention solves the above-mentioned conventional drawbacks and provides low threshold current,
The purpose of the present invention is to provide a buried structure semiconductor laser with high efficiency and high high frequency characteristics.

[Means for Solving the Problems 1] The semiconductor laser of the present invention is formed in a stripe shape, and includes a mesa portion including at least an active layer and two cladding layers sandwiching the active layer and having different conductivity types, and both sides of the mesa portion. In a buried structure semiconductor laser including a semi-insulating InP current blocking layer containing Fe embedded therein, the mesa portion includes an inverted mesa-shaped portion made of a (111) A plane located above the active layer, and a mesa protective layer made of P-type InP provided on the side surface of the part, an n-type InP sandwiched between the two cladding layers on the same plane as the active layer and in contact with the active layer, and a P-type InP in contact with the active layer. It is characterized by having a spacer layer composed of two types of semiconductor layers made of InP type.

The method of the present invention includes a substrate having a first conductivity type, a cladding layer having the same conductivity type, an active layer, and a first buried structure semiconductor laser. 4) a step of sequentially laminating a cladding layer of a first conductivity type and an electrode layer of a second conductivity type; a step of providing a cladding layer and a second-shaped mask; forming a mesa portion by processing the electrode layer into an inverted mesa shape, the second conductivity type cladding layer, the active layer, and the first conductivity type cladding layer into a mesa shape; selecting the active layer; defining the width of the active layer through the active layer;
, forming a mesa protective layer on the sides of the two cladding layers and the spacer layer, and forming a current blocking layer on both sides of the mesa portion. shall be.

[Function] In order to improve the efficiency and lower the threshold current of the Fe-buried structure semiconductor laser, it is effective to separate the Fe-doped InP current blocking layer and the active layer by a third semiconductor layer. For forming the mesa protective layer, it is desirable to use a metal organic vapor phase epitaxy method, which allows the mesa protective layer to be made thinner on the side surfaces of the mesa portion. However, conventionally, when a mesa protective layer of InP semiconductor was formed on the side surface of a mesa portion including an active layer by the same growth method, the upper electrode and the substrate were connected by the mesa protective layer.

In general, in metal organic vapor phase epitaxy, the growth rate of InP on the (111)A plane is slow. Therefore, by forming part of the mesa portion into an inverted mesa shape consisting of the (111)A plane, it is possible to suppress the growth of InP crystals on the side surfaces of the inverted mesa shape. In addition, when filling both sides of a mesa stripe region by metalorganic vapor phase epitaxy, an eaves are formed on the top of the mesa portion using a mask provided for selective filling growth (K, Nakai et al., Jour et al.
nal of Crystal Growth 93 (
1988) 248-253). Thereby, it is possible to reduce the raw material concentration on the side surface of the mesa portion directly under the eaves during crystal growth, and to reduce the growth rate. Furthermore, (1
11) The growth of InP on the side surfaces of the inverted mesa shape can be suppressed by utilizing both the low growth rate on the A-plane and the reduction in raw material concentration due to the eaves of the mask.

2 FIG. 1 shows a mesa structure for Fe embedding according to the present invention. For example, on the n-type InP substrate 26,
n-type InP cladding layer 25. InGaAsP active layer,
P-type InP cladding layer 23. InGaAsP electrode layer 2
A mesa structure consisting of 2 is formed. InGaAs on top
The P electrode layer 22 is a P-type InP cladding layer 23. active layer 24
．． It has an inverted mesa shape opposite to that of the n-type InP cladding layer 25, and its side surfaces are (111)A planes. 21 is 5i02
The end of the mask has an eave-like shape that protrudes beyond the electrode layer. In this way, by forming the side surface of the mesa portion directly under the mask into an inverted mesa shape consisting of the (111) A plane, the growth of InP crystal on the side surface of the inverted mesa portion can be significantly suppressed. Such a mesa structure is I
By providing an electrode layer made of nGaAsP, it can be manufactured with good reproducibility.

Taking advantage of these points, the electrode layer has an inverted mesa shape consisting of the (111)A plane, and the mesa protective layer on the side surface of the electrode layer is made extremely thin, or by not forming a mesa protective layer on the side surface of the electrode layer. Up to now, there has been no known Fe-embedded semiconductor laser having a structure in which the upper electrode and the mesa protective layer are disconnected.

In addition, the mesa protective layer is generally made of P-type InP, but in order to prevent the diffusion of P-type dopants from P-type InP to the active layer, a spacer layer made of an n-type InP layer is provided between the cladding layers. Structured semiconductor lasers are also hitherto unknown.

On the other hand, a method of separating the Fe-doped InP current blocking layer and the active layer by forming only a spacer layer between the cladding layers has also been proposed.

In this case, the spacer layer is preferably made of n-type InP, but as the spacer layer becomes wider, the leakage current increases, so a part of the spacer layer is replaced with p-type InP, and only the sides of the active layer are made of n-type InP. There is a need. However, there has never been a buried structure semiconductor laser that has an n-type InP layer on the side of the active layer and a p-type InP layer on the side of the n-type InP layer, and a spacer layer composed of these two types of semiconductor layers between the cladding layers. .

Furthermore, in order to reduce leakage current, it is necessary to make the width of the n-type InP spacer layer as narrow as possible. For that purpose, P
As the type dopant, it is desirable to use Mn, which diffuses slower than the commonly used Zn, and therefore is less likely to cause pile-up beside the active layer through the n-type InP spacer layer. However, there has been no buried structure semiconductor laser including a spacer layer containing Mn as a P-type dopant, and furthermore, a mesa protective layer containing Mn as a P-type dopant.

The buried structure semiconductor laser according to the present invention has an electrode layer (
111) It has an inverted mesa shape consisting of the A-plane, and has a mesa protective layer that is extremely thin or not formed only at the electrode layer core, and a spacer layer whose side surface of the active layer is at least n-type InP between the cladding layers. threshold current,
High efficiency and high high frequency characteristics can be achieved.

[Example] Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a cross-sectional view of an Fe-embedded structure semiconductor laser which is an embodiment of the present invention.

5 The active layer 40 is made of InG corresponding to an emission wavelength of 1.30 μm.
It is an aAsP semiconductor crystal. The active layer 4o is made of InP
On the substrate 35, it is sandwiched between an n-type InP cladding layer 38 and a p-type InP cladding layer 34 from above and below. n-type I
An electrode layer 37 made of n"-InGaAsP is provided on the nP cladding layer 38 so as to make good contact with the n-type electrode 31. In the mesa portion including the active layer 4o, the electrode layer 37 and n-type InP cladding layer 3
A mesa protective layer 39 made of P-type InP is formed only in a portion extending from the vicinity of the boundary 41 of 8 to the substrate. The mesa portion 42 having the mesa protective layer is made of Fe-doped In
Due to the current blocking layer 33 of semi-insulating crystal made of P,
It's pinched on both sides. 5iz21e! on the upper surface of the current blocking layer 33 so that the injected current flows selectively to the mesa portion 42 including the active layer 40. 32 are provided. The P-type electrode 36 is formed on the entire surface of the P-type substrate 35, and the n-type electrode 31 is formed on the entire surface of the upper surface of the device.

Next, FIG. 3 shows a cross-sectional view of a product formed at each stage of the manufacturing process of this embodiment. First, a 6 (100) plane P-type InP substrate 35 (carrier concentration 2x1
018 cm'-3), a P-type InP buffer layer 34 with Zn as a dopant (carrier concentration 1 x 1018 cm
-”, thickness 1 μml, non-doped InGaAsP active layer 40 (thickness 0.1 μm) corresponding to emission wavelength 1.30 μm.
m), n-type InP cladding layer 38 with S as a dopant
(Carrier concentration 5x10"cm", thickness 1.5 μm
), n''-InGaAsP electrode layer 37 with S as a dopant (carrier concentration 2xlO'8c+N', thickness 0
．． 5 μm) are sequentially stacked (FIG. 3(a)). Next, only the side surface of the electrode layer 37 has an inverted mesa shape consisting of the (111)A plane, and the stripe width gradually becomes wider from near the boundary 41 between the electrode layer 37 and the n-type InP cladding layer 38 to the substrate 35. A mesa structure having a width of 3.5 μm in the active layer 40 is formed by etching using a 5 in 2 mask 45 (FIG. 3(b)). An example of a method for manufacturing such a mesa structure will be described. First, 5i0 on the electrode layer 37
2 masks 45 are formed. Next, dry etching is performed so that the 5in2 mask 45 has a predetermined shape. At this time, the electrode layer 37 is etched so that a constant thickness remains not only at the bottom of the mask but also over the entire surface. Next, the remaining thickness of the electrode layer is completely removed by wet etching using 1 (2S 04) as an etchant. At this time, the lower electrode part of the mask 45 is side-etched, and the mask 45 is shaped like an eaves. Furthermore, by dry etching, the n-type InP cladding layer 38,
The active layer 40 and the P-type InP buffer layer 34 are removed leaving the lower part of the mask. However, a portion of the buffer layer 34 may be left on the substrate. When wet etching is then performed using promethanol, the electrode layer 37 has an inverted mesa shape with side surfaces having (111)A planes, and the portions below the InP cladding layer 38 have a mesa shape. Further, both sides of the active layer 40 are selectively etched using H2SO4 to make the width of the active layer 40 1.5 .mu.m (FIG. 3(C)). Next, by low-pressure MOVPE, a part of the selectively etched portion was etched with undoped n-type InP (carrier concentration 1xl).
O16cm-') is blocked by the spacer layer 44 (FIG. 3(d)), followed by a spacer layer 43 of P-type InP doped with Mn (carrier concentration 1xlO"c).
n+-3) are sequentially formed (FIG. 3(e)). At this time, as shown in the figure, the side surface of the electrode layer 37 is (111)
At the entrance surface, the mesa protective layer 39 is hardly formed, and a mesa protective layer with a thickness of about 0.1 μm is formed only in the curved part from near the boundary between the electrode layer 37 and the n-type InP cladding layer 38 to the substrate. (Figure 3(e)). Subsequently, Fe-doped InP current blocking layers 33 are grown on both sides of the mesa portion (FIG. 3(f)). Finally, 5j02 membrane 32,
Then, electrodes 31 and 36 were formed and cut into individual laser chips to obtain a desired Fe-buried structure laser as shown in FIG.

The characteristics of the fabricated semiconductor laser at room temperature are: oscillation threshold current 71IA % external differential efficiency 0.25mW/mA
, the maximum output is 30 mW, the capacitance of the entire element is about 1 pF, and the high frequency characteristics are excellent at 10 GHz or more. Further, in this example, there is almost no decrease in efficiency due to an increase in the injection current.

Furthermore, it was possible to obtain semiconductor lasers with uniform characteristics at a high yield compared to the number of wafers.

9 [Effects of the Invention] In the buried structure semiconductor laser according to the present invention, the mesa portion including the active layer and formed in a stripe shape has (
111) An electrode layer having an inverted mesa shape consisting of an incident surface, and a P-type In doped with Mn that is extremely thin or not formed only on the side surfaces of the mesa portion.
A mesa protective layer made of P and a spacer layer beside the active layer that is at least n-type InP between the cladding layer,
The mesa part composed of these is I containing Fe.
Embedded by nP.

By adopting such a configuration, leakage current through the mesa protective layer was reduced. Further, the diffusion of Fe into the active layer was suppressed, and there was no Fe pile-up on the side surfaces of the active layer. Furthermore, by optimizing the spacer layer structure, it is possible to suppress the diffusion of P-type dopants from the mesa protective layer to the active layer, and to increase the stripe width, thereby reducing the resistance in the mesa part. We were able to.

As a result of 0 or more, reduction of threshold current, high output operation, and
Stable operation over long periods of time is now possible.

FIG. 8 is a cross-sectional view of a conventional Fe-embedded structure semiconductor laser having a mesa protective layer formed by metal organic vapor phase epitaxy.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a mesa structure for embedding Fe according to the present invention, FIG. 2 is a cross-sectional view of a semiconductor laser with an Fe-embedded structure according to an embodiment of the present invention, and FIG. 3 is a diagram showing each of the manufacturing steps according to the present invention. Figure 4 is a cross-sectional view of a conventional Fe-embedded structure semiconductor laser, and Figure 5 is a cross-sectional view of a conventional F semiconductor laser with a mesa protective layer on the mesa side.
Fig. 6 is a cross-sectional view of a conventional Fe-embedded structure semiconductor laser having a spacer layer beside the active layer, and Fig. 7 is a cross-sectional view of a conventional Fe-embedded structure semiconductor laser having a mesa protective layer formed by liquid phase growth. 1... P-type electrode, 2... Semi-insulating InP current blocking layer, 3... N-type TnP cladding layer, 4... N+-type InP substrate, 5 ... n-type electrode, 6-P''1nGaAsP electrode layer, 7... P-type InP cladding layer 8 ... = InGaAsP active layer, 9... mesa protective layer, 1l-n-InP substrate, 12... n-InPn 9 Fufu 14...p-InP cladding layer, 15.=P”-1no5sGa. , 47AS contact layer, 16... High resistance InP layer, 17... InP spacer layer, 1B... 5i02 film, 19.20... Electrode, 21... Mask, 22=-1nGa 8sP to Pole layer, 23... P-type TnP cladding layer, 24-1nGa 8sP hemilayer, 26-n-type InP substrate, 31... n-type electrode, 32... 5i02 film, 33... semi-insulating InP current blocking layer, 34...
P-type ■RoP cladding layer, 35...P1 type I-port P substrate, 36...P-type electrode, 37.=n"1nGa8sP pole layer, 38...n-type InP cladding layer, 39 ...Mesa protective layer, 40--InGa 8sP polar layer, 41... Boundary between electrode layer and n-type InP cladding layer, 4
2... Mesa portion having a mesa protective layer, 43... P-type InP spacer layer, 3 44... N-type 1-hole P spacer layer, 45... 5i02 mask.

Claims (1)

[Scope of Claims] 1) A mesa portion formed in a stripe shape and including at least an active layer and two cladding layers sandwiching the active layer and having different conductivity types, and Fe embedded on both sides of the mesa portion.
In the buried structure semiconductor laser including a semi-insulating InP current blocking layer containing
1) An inverted mesa-shaped part made of the A-plane, a mesa protective layer made of P-type InP provided on the side surface of the mesa part, and sandwiched between the two cladding layers on the same plane as the active layer, n-type InP in contact with the layer and the n-type In
A buried structure semiconductor laser characterized by having a spacer layer composed of two types of semiconductor layers made of P-type InP in contact with P. 2) The buried structure semiconductor laser according to claim 1, wherein the inverted mesa-shaped portion is an electrode layer having a composition of InGaAsP. 3) The buried structure semiconductor laser according to claim 1 or 2, wherein the mesa protective layer and the spacer layer made of P-type InP are each made of InP containing Mn. 4) A buffer layer and an active layer having the same conductivity type on a substrate having the first conductivity type, a cladding layer of the second conductivity type having a conductivity type different from the first conductivity type, and an electrode of the second conductivity type. a step of sequentially laminating layers, a step of providing a mask of a predetermined shape on the electrode layer, and a step of etching the laminated layer through the mask to form the electrode layer into an inverted mesa shape, and forming the second conductivity type cladding layer. , processing the active layer and the buffer layer of the first conductivity type into a mesa shape to form a mesa portion; selectively etching the active layer to define its width; forming a spacer layer having a second conductivity type in contact with the active layer on both sides; forming a mesa protective layer on the sides of the two cladding layers and the spacer layer; and applying an electric current on both sides of the mesa. A method for manufacturing a buried structure semiconductor laser, the method comprising the step of forming a blocking layer.