JPH0758403A - High-resistance buried semiconductor laser - Google Patents

Abstract

PURPOSE:To enable high speed modulation in a semiconductor laser, by reducing leakage current passing through areas other than an active layer to reduce threshold current, and reducing capacitance intrinsic to the semiconductor laser. CONSTITUTION:An active layer 3 is formed on a substrate 1 of a first conductivity type in a semiconductor laser of mesa stripe structure. The side of the active layer 3 is coated with a low-concentration InP thin film layer 6 of a second conductivity type using a MOVPE method or gas source MBE method. In addition the active layer is coated or filled with high-purity InAlAs 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high resistance semiconductor laser used for optical communication and the like.

[0002]

2. Description of the Related Art Optical fiber communication technology has made remarkable progress in recent years, and large-capacity optical communication systems such as 400 Mb / s and 1.6 Gb / s have been introduced not only in Japan but also through foreign countries through submarine cables. ing. A key device indispensable for this optical communication technology is a semiconductor laser which is a light source for optical communication. With the long distance and large capacity of the system, semiconductor lasers are required to have higher output and higher speed.

Currently used semiconductor lasers generally have a thyristor structure in which the current block region is pnpn. However, in this structure, when the current is increased, when the thyristor is turned on or when the gain of the transistor forming the thyristor is large, a non-negligible current may flow and the optical output may be saturated. In addition, since the thyristor has parasitic capacitance, R
Since the C time constant is large, it is difficult to realize high-speed modulation.

On the other hand, a semiconductor laser using a high-resistance semiconductor layer in the current block region has a small leak current, which enables a low threshold operation, and a small parasitic capacitance, which enables a high-speed modulation. Recently, research and development have been actively conducted. A conventional example of a high-resistance embedded semiconductor laser is shown in FIG.
Shown in. In FIG. 3, a double hetero structure including an n-type InP clad layer 2, an InGaAsP active layer 3, a p-type InP clad layer 4, and a p-type InGaAsP contact layer 5 is formed in a mesa shape on an n-type InP substrate 1. A semi-insulating (hereinafter abbreviated as SI) InP layer 8 is buried, the n-side electrode 11 is on the substrate 1 side, and the p-side electrode 1 is on the contact layer 5 side.
0 is formed.

[0005]

The conventional high-resistance semiconductor laser as shown in FIG. 3 described above has the following problems. The SI semiconductor layer used for the current blocking layer is
It is not an insulator through which no current flows. For example, in the case of InP doped with iron (Fe), which is the most common, I
Fe forming a deep level in the forbidden band of nP functions as an electron trap. That is, when alternating current is applied by sandwiching Fe-doped InP with p-type and n-type InP,
The holes injected from the n-type InP into the Fe-doped InP recombine with the electrons already captured by the Fe acceptor, and this becomes a recombination current. Therefore, in the structure as shown in FIG. 3, there is a leakage current flowing from the p-type InP clad layer 4 to the SI-InP layer 8, a sufficient current blocking effect cannot be obtained, and an increase in threshold current and a decrease in efficiency are caused. This may cause saturation of light output.

On the other hand, when a hole trap such as titanium (Ti) or cobalt (Co) is used as a deep impurity, it functions as a hole trap, and therefore the n-type InP substrate 1 and the n-type I are used.
A current flows from the nP clad layer 2 to the SI-InP layer 8.

Unless these problems are solved, the characteristics of the high-resistance embedded semiconductor laser, such as lower threshold, high efficiency and high speed modulation, cannot be fully utilized.

An object of the present invention is to provide a high resistance embedded semiconductor laser which solves the above problems.

[0009]

A high resistance buried semiconductor laser according to a first aspect of the present invention has a double hetero structure including a semiconductor active layer and a second conductivity type semiconductor clad layer formed on a first conductivity type semiconductor substrate. , A stripe-shaped mesa, and undoped InAlAs high resistance semiconductor layers are formed on both sides of the mesa. The second
According to another aspect of the present invention, in the high resistance embedded semiconductor laser, a double hetero structure including a semiconductor active layer and a second conductivity type semiconductor clad layer formed on a first conductivity type semiconductor substrate is formed in a stripe mesa. Low concentration on both sides of the second
The conductive type semiconductor layer and the undoped InAlAs high resistance semiconductor layer are formed in this order.

[0010]

In the semiconductor laser of the first invention, undoped high-purity InAlAs is used as the high-resistance buried semiconductor. According to our experimental results, high-purity InAlAs grown by molecular beam epitaxy (MBE) has a very high resistivity of about 10 ⁸ Ωcm. The mechanism by which InAlAs has high resistance is not clear, but carriers are trapped at a deep level formed by doping an impurity such as generally known Fe-doped InP, and thus have high resistance. I'm not. That is, it has high resistance to both electrons and holes. Therefore, in the high-resistance embedded semiconductor laser using InAlAs as the high-resistance embedded semiconductor, there is little leakage current due to recombination current, and a semiconductor laser with low threshold, high efficiency and high speed modulation can be obtained. Further, since it is not necessary to dope the impurities, the problem of contamination of the crystal growth apparatus by the impurities does not occur.

In the first invention, InA containing Al is used.
Since 1As is used as a buried layer, the deterioration of crystallinity at the crystal interface becomes a problem, and leakage current occurs at the interface. Therefore, in the second invention, a semiconductor thin film layer having a low concentration second conductivity type different from the substrate having the first conductivity type is inserted between the InAlAs burying layer used in the first invention and the mesa. As a result, deterioration of crystallinity, which is a problem in the first invention, is avoided. By lowering the concentration of the buried second-conductivity-type semiconductor layer to increase the resistance, the second-conductivity-type cladding layer of the mesa and the first-conductivity-type cladding layer are passed through the second-conductivity-type semiconductor layer on the side surface of the mesa. It is possible to reduce the leakage current flowing to the first conductivity type substrate 1. Further, the effect of using InAlAs for the high-resistance embedded semiconductor layer is similar to that of the first invention, and the low oscillation threshold,
High luminous efficiency and high speed modulation are possible.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a high resistance embedded semiconductor laser of the present invention will be described in detail with reference to the drawings.

FIG. 1A is a sectional view showing the structure of the semiconductor laser described in the first invention. This example
This is the case where the n-type semiconductor substrate 1 is used.

First, the double hetero structure was crystal-grown by the metal organic chemical vapor deposition (MOVPE) method. The grown crystal is Si-doped n-type I on the n-type semiconductor substrate 1.
nP clad layer 2 (carrier concentration 4 × 10 ¹⁸ cm ⁻³ , layer thickness 0.4 μm), undoped InGaAsP active layer 3
(Layer thickness 0.2 μm), Zn-doped p-type InP clad layer 4 (carrier concentration 5 × 10 ¹⁷ cm ⁻³ , layer thickness 1 μm), Z
It is a double heterostructure crystal composed of an n-doped p-type InGaAsP contact layer 5 (carrier concentration 1 × 10 ¹⁹ cm ⁻³ , layer thickness 0.2 μm).

Next, a surface SiO ₂ film is deposited and processed into a stripe having a width of 1.5 μm by a photolithography technique, and an n-type etching solution of bromine-methanol is used.
Mesa etching was performed until the type InP substrate 1 was reached.
Then, using the gas source MBE method, the SiO ₂ stripe is used as a mask in the atomic hydrogen and the undoped InAl is used.
The As layer 7 was embedded and grown. When crystal growth is performed in atomic hydrogen, crystal growth does not occur on the dielectric mask, and InGa
The side wall of the mesa including the AsP active layer 3 is covered with a crystal, and the mesa is embedded when the crystal growth is further continued. The SiO ₂ stripe was removed, and Ti / Pt / A was used as the p-side electrode 10.
After sputtering u and heat-treating at 430 ° C. for 5 minutes, the n-type substrate 1 is polished so that the total thickness is about 100 μm, AuGeNi is vapor-deposited as the n-side electrode 11, and 410 ° C.
And heat treated for 5 minutes.

FIG. 1B shows the case where the p-type semiconductor substrate 20 is used. In this case, since the n-side ohmic electrode can be taken directly from the InP clad layer, a contact layer is not necessary.

FIG. 2A is a sectional view showing the structure of the semiconductor laser described in the second invention. In this example, n
This is a case where a mold semiconductor substrate is used. For the double hetero structure, the MOVPE method was used, and a crystal structure similar to that of the example of FIG. The mesa etching method is also shown in FIG.
Was carried out in the same manner as in the above example. Next, the Zn-doped p ⁻ type InP burying layer 6 (carrier concentration 1
× 10 ¹⁷ cm ⁻³ , layer thickness 500 angstrom) was grown. In the MOVPE method, crystal growth does not occur on the dielectric mask and the side wall of the mesa including the InGaAsP active layer 3 is covered. Further, similarly to the embodiment of FIG. 1A, the undoped InAlAs embedded layer 7 was grown by using the gas source MBE method with the SiO ₂ stripe as a mask in atomic hydrogen. Subsequent electrode steps and heat treatment steps were performed in the same manner as in the example of FIG.

FIG. 2B shows the case where the p-type semiconductor substrate 20 is used. In this case, the thin film semiconductor layer inserted between the mesa and InAlAs is an n ⁻ layer.

After the above process, the wafer was cleaved to have a cavity length of 300 μm, mounted on a Si heat sink and assembled, and the device characteristics were evaluated. When the current-light output characteristics of the cut out device were measured, the threshold current was 10 mA on average, the efficiency was 0.23 W / A on average, and the maximum light output was 30 mW on average. This is because a leak current was generated in the high resistance buried portion, and the recombination current disappeared because InAlAs was used for the high resistance buried semiconductor layer. Also, the element capacity is about 4p
When F was small and the small signal response characteristics were measured, good characteristics in a modulation frequency band of 12 GHz were obtained.

In each of the above embodiments, the growth of InAlAs is carried out by the gas source MBE method, but the MOVPE method or the like may be applied. Further, as a constituent material of the laser, I
Although nGaAsP / InP is explained, AlGaAs / Ga
The same effect can be obtained by using a double hetero structure composed of a mixed crystal of other compounds such as As and AlGaInP / GaInP.

[0021]

According to the present invention, it is possible to obtain a semiconductor laser which has no leakage current, has a low threshold value and high efficiency, has a small parasitic capacitance, and can be modulated at high speed.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a structure of a high-resistance embedded semiconductor laser according to a first invention, in which an n-type semiconductor substrate and a p-type semiconductor substrate are used, respectively.

FIG. 2 is a cross-sectional view showing the structure of a high-resistance embedded semiconductor laser according to a second invention, which uses an n-type semiconductor substrate and a p-type semiconductor substrate, respectively.

FIG. 3 is a cross-sectional view showing the structure of a conventional high-resistance embedded semiconductor laser.

[Explanation of symbols]

1 n-type substrate 2 n-type clad layer 3 active layer 4 p-type clad layer 5 p-type contact layer 6 p ^- type buried layer 7 InAlAs high resistance buried semiconductor layer 8 semi-insulating semiconductor buried layer 10 p-side electrode 11 n-side electrode 20 p-type substrate 21 n ^- type buried layer

Claims (2)

[Claims] 1. A double hetero structure, which is formed on a semiconductor substrate of the first conductivity type and comprises a semiconductor active layer and a semiconductor clad layer of the second conductivity type, is formed in a stripe mesa, and undoped InAlAs is formed on both sides of the mesa. A high-resistance embedded semiconductor laser, characterized in that a high-resistance semiconductor layer is formed. 2. A double hetero structure, which is formed on a semiconductor substrate of the first conductivity type and comprises a semiconductor active layer and a semiconductor clad layer of the second conductivity type, is formed in a stripe-shaped mesa and has a low concentration on both sides of the mesa. A high-resistance embedded semiconductor laser comprising a second conductivity type semiconductor layer and an undoped InAlAs high-resistance semiconductor layer formed in this order.