JPH0878789A - Semiconductor laser device - Google Patents

Abstract

PURPOSE: To provide a semiconductor laser device in which a large capacitance generated between a current block layer and a second clad can be lowered without damaging excellent current block structure and an output is hard to be lowered at the time of high-frequency operation. CONSTITUTION: In a semiconductor laser device in which an active layer 14, a second clad layer 16, a current block layer 17 having an opening part from which the predetermined places of the second clad layer 16 are exposed, an easily electrifying layer 18 to the opening part and a second conductivity type contact layer covering the current block layer 17 of the easily electrifying layer 18 are successively formed, the current block layer 17 is formed with a small band gap and reverse conductivity type relative the second clad layer 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to optical communication, optical information processing,
Alternatively, the present invention relates to a semiconductor laser device used as a light source for optical measurement or the like, and particularly to a semiconductor laser device using an InGaAlP-based material.

[0002]

2. Description of the Related Art Recently, I having an oscillation wavelength in the 0.6 μm band
Red semiconductor lasers using nGaAlP materials have been commercialized, and high-density optical disk devices, laser beam printers,
It is expected as a light source for bar code readers and optical measurement. At present, in order to increase the optical information processing speed and increase the optical recording density, the semiconductor laser device of this system is required to have high output. In addition, in order to reduce noise when reading from an optical disc, 400M
An operation in which a high frequency of Hz or higher is superimposed is necessary. But,
In high frequency operation, the optical output of the semiconductor laser device sharply decreases as the frequency increases. This decrease in light output is due to the electrostatic capacitance existing in parallel in the easily energized layer of the semiconductor laser device. As this capacitance is reduced, the decrease in optical output during high frequency operation can be suppressed. Practically, the optical output of the semiconductor laser device needs to be more than half that of DC operation. Conventionally, the capacitance value is large, and the operating frequency is 200 MHz or less, and the optical output of DC operation is half. It was The device structure of the conventional semiconductor laser will be described in detail below with reference to sectional views.

Conventionally, as shown in FIG. 3A, n-G
current blocking layer 24 (n = 10 ¹⁸ c
m ⁻³ ) and the contact layer 19 made of p-GaAs and p
-In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P second
By inserting it between the cladding layers 16, the current is cut off at this portion, and p-In _0.5 (Ga _0.3 Al _0.7 )
_The current is made to flow only in the ridge stripe portion 25 made of _0.5 P. This is because the current-carrying layer 18 made of In _0.5 Ga _0.5 P formed on the upper surface of the ridge stripe portion 25 is used as the contact layer 19 and the ridge stripe portion 2.
This is to bring ohmic contact between the five. This is S
It is called a BR (Selectively burried ridge waveguide) structure. AuZn / Au is formed on the contact layer 19.
An electrode 21 composed of is formed. Second clad layer 1
An etching stop layer 26 made of p-In _0.5 Ga _0.5 P is formed on the upper surface of the second cladding layer 1.
6 below, the optical guide layer 15 made of In _0.5 (Ga _0.5 Al _0.5 ) _0.5 P, the MQW active layer 14, and the In _0.5 (Ga
_0.5 Al _0.5 ) _0.5 P and the optical guide layer 13 and n-I
The first cladding layer 12 made of n _0.5 (Ga _0.3 Al _0.7 ) _0.5 P, the substrate 11 made of n-GaAs, and AuGe /
Electrodes 20 made of Au are sequentially formed.

In this structure, as shown in FIG.
The light generated in the active layer 14 is absorbed by the current blocking layer 24 to generate electrons and holes. Of these, holes flow into the cladding layer 16 side by the electric field applied to the junction between the current blocking layer 24 and the cladding layer 16. As a result, since the electrons have just been injected into the current blocking layer 24, a forward bias is applied to the junction between the contact layer 19 and the current blocking layer 24, causing a transistor operation to start a current flow. Here, when the net donor concentration of the current blocking layer 24 is high and the layer thickness is large, the contact layer 19
The holes that have flowed into the current blocking layer 24 from the above cause recombination in the current blocking layer 24 with the electrons injected by photoexcitation, which are majority carriers, and beyond this, at the junction between the contact layer 19 and the current blocking layer 24. Forward bias is no longer applied. Therefore, although a small amount of current flows at the beginning, no current finally flows. The current can be blocked on this principle. At this time, the current blocking layer 2
In order to set the layer thickness of No. 4 to about 0.5 μm to 1.0 μm, the net donor concentration had to be 4 × 10 ¹⁷ cm ⁻³ or more.

However, when the net donor concentration of the current blocking layer 24 is set to 4 × 10 ¹⁷ cm ^-3 or more, the depletion of the junction between the current blocking layer 24 and the second cladding layer 16 which is reverse biased during operation. The spread of the layer width becomes smaller.
For this reason, in the conventional semiconductor laser device, the value of the above-mentioned capacitance becomes large, and it is not possible to cope with the operating frequency of 400 MHz, and the optical output is reduced to less than half.
In addition, in order to reduce the capacitance, the current blocking layer 24
Decreasing the net donor concentration of the current blocking layer 24
Since the holes that have flowed into the second cladding layer 16 do not disappear sufficiently due to recombination and flow into the second cladding layer 16, the function of the current block described above is lost. This problem is caused by IS (Inner strike
This is also serious in a gain-guided semiconductor laser having a pe) structure.

[0006]

As described above, in the conventional semiconductor laser using the etching stop layer, it is necessary to set the net donor concentration and the acceptor concentration high in the current blocking layer and the second cladding layer, respectively. Because of this, the electrostatic capacity at this joint increases,
The high frequency characteristics were not practical.

The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a semiconductor laser with improved high frequency characteristics by reducing the electrostatic capacitance originally possessed by the current block structure.

[0008]

In order to achieve the above object, in the present invention, an active layer, a second conductivity type second clad layer, and a second clad layer are formed on a first conductivity type first clad layer. A current blocking layer having an opening exposing a predetermined portion of the layer, a second conductive type conductive easy layer in the opening, and a conductive layer of the second conductive type covering the easy conductive layer and the current block layer are sequentially formed. In the semiconductor laser device described above, the current blocking layer is formed with a conductivity type opposite to that of the second cladding layer and having a small bandgap.

[0009]

As shown in FIG. 1 (a), the second cladding layer 16
On top, a current blocking layer 17 made of n ^-- GaAs and p
^A forward bias voltage is applied so that the voltage on the contact layer 19 side is higher than that on the cladding layer 16 by using the current block structure formed by the contact layer 19 made of ⁺ -GaAs. At this time, a reverse bias voltage is applied to the junction surface between the current blocking layer 17 and the cladding layer 16, and an operating voltage is applied between the contact layer 19 and the first cladding layer 12, whereby the current blocking layer 17 and the cladding layer 1 are
The depletion layer at the junction with 6 spreads to the current block layer 17 side. As a result, it is possible to suppress the capacitance generated at the joint surface between the current blocking layer 17 and the cladding layer 16 to be small.
In the conventional current block structure, the net donor concentration could not be reduced in order to reduce the capacitance, but in the structure shown in the same figure, the energy of FIG.
As shown in the band diagram, the holes are blocked by the hetero barrier generated by the current blocking layer 17 and the cladding layer 16, so that the current can be blocked even if the net donor concentration of the current blocking layer 17 is reduced. Here, for example, if the net donor concentration of the current blocking layer 17 is set to 4 × 10 ¹⁶ cm ⁻³ or less, a semiconductor laser having a cavity length of 800 μm can be used for direct current operation even in high-speed modulation operation of 400 MHz or more. You can get half the light output. The relationship between the net donor concentration and the net acceptor concentration in the pn junction at this time is shown by the hatched line in FIG.

[0010]

FIG. 1 shows the first embodiment of the present invention.
This is a cross-sectional view of a semiconductor laser device having an SBR structure. Metal organic CVD system
m, hereinafter referred to as MOCVD method), for example, n-In _0.5 (Ga _0.3 Al _0.7 ) _{0.5 having} a thickness of 1.0 μm is formed on a substrate 11 made of n-GaAs (crystal direction is [100]).
C cladding layer 12 made of P, In _{0.5 having} a thickness of 50 nm
An optical guide layer 13 made of (Ga _0.5 Al _0.5 ) _0.5 P,
6.5 nm In _0.62 Ga _0.38 P well layer and 4 nm In
MQW active layer 14 having a two-layer structure of _0.5 (Ga _0.5 Al _0.5 ) _0.5 P barrier layer, In _0.5 (Ga _0.5
Al _0.5 ) _0.5 P optical guide layer 15, width 5 μm
Processed to have a striped ridge structure of
The thickness of the ridge is 1.0 μm, and the thickness outside the ridge is 0.2 μm. P-In _0.5 (Ga _0.3 Al _0.7 ).
_The cladding layer 16 made of _0.5 P (for example, p = 6 × 10
¹⁷ cm ⁻³ ), and then a current blocking layer 17 (for example, n = 4 × 10 ¹⁶ cm ⁻³ ) of 0.8 μm made of n ⁻ -GaAs is formed outside the ridge of the cladding layer 16.
And 50 nm p-In on the ridge portion of the cladding layer 16.
_An easy conduction layer 18 made of _0.5 Ga _0.5 P is formed, and p ⁺ -G is further formed on the current blocking layer 17 and the easy conduction layer 18.
A contact layer 19 made of aAs was formed, and finally AuGe / Au20 was formed as an electrode on the substrate side, and AuZn / Au21 was formed as an electrode on the opposite side to the substrate. The semiconductor laser device thus obtained has a cavity length of 800 μm.
(In all the embodiments described below, the resonator length is 800
.mu.m), and for a high frequency operation signal of about 700 MHz or more, more than half of the output during DC operation was obtained. In addition,
It is difficult to do the MOCVD method to dope the current blocking layer 17 made of GaAs with such a low concentration that the net donor concentration is about 4 × 10 ¹⁶ cm ⁻³ . MOC
When a crystal containing Al is undoped grown by the VD method, oxygen is easily incorporated into the crystal and the donor concentration is 1 × 10.
A low-concentration n-type crystal of ¹⁵ to 1 × 10 ¹⁶ cm ⁻³ can be easily obtained. Therefore, instead of GaAs, AlGaAs grown undoped as the current blocking layer 17 or
It is also very effective to use InGaAlP. For example, with undoped growth of Al _0.1 Ga _0.9 As, a low concentration n-type layer having a donor concentration of about 3 × 10 ¹⁵ cm ⁻³ was obtained. Further, the semiconductor laser device using this undoped Al _0.1 Ga _0.9 As current blocking layer also provided good frequency characteristics as in the first embodiment. Further, in the present structure, when the current blocking layer 17 is heavily doped (for example, n ≧ 6 × 10 ¹⁷ cm ⁻³ ) as in the conventional structure, the frequency characteristic is not improved, but the second cladding layer is different from the conventional structure. 1
The current leaking to the ridge side of No. 6 is suppressed, and it is very effective in reducing the threshold current of laser oscillation.

A second embodiment of the present invention is shown in FIG. 50 nm of In _0.5 (Ga _0.5)
After forming the optical guide layer 15 made of Al _0.5 ) _0.5P , 1.0 μm of p-In _0.5 (Ga _0.3 Al _0.7 )
The second cladding layer 16 made of _0.5 P, and the current blocking layer 17 made of 1.0 μm n-GaAs having a stripe-shaped opening with a width of 5 μm (for example, n = 1 × 10 ¹⁶ c
m ^-3 ), p-In _0.5 (Ga _0.3 Al exposed at the opening)
_0.7 ) _0.5 P 50 nm p-In on the clad layer 16
_{After the} easy-current-carrying layer 18 made of _0.5 Ga _0.5 P is sequentially formed, the contact layer 19 made of p-GaAs is formed. Finally, similarly to the first embodiment, the electrode 20 made of AuGe / Au and the electrode 21 made of AuZu / Au21 are formed. This is the case where the present invention is used in a gain-guided semiconductor laser device having an IS structure, and the same effect as that of the first embodiment can be obtained. That is, the donor density of the current blocking layer 17 is 4 × 10 ¹⁶ c due to the current blocking effect of the current blocking layer 17 and the second cladding layer 16.
Even if it is less than m ⁻³ , the current will flow into the easy-current-carrying layer 18. As a result, the frequency characteristic is greatly improved as in the first embodiment. Further, similar to the first embodiment, AlGaAs and InGaA are used as the low concentration n-type current blocking layer.
It is effective to use 1P. Further, even if the donor concentration of the current blocking layer 17 is set to about 1 × 10 ¹⁸ cm ⁻³ as in the conventional example, the frequency characteristic is not improved, but the leak current due to holes passing through the current blocking layer 17 can be suppressed. The threshold current of laser oscillation can be reduced.

[0012]

According to the present invention, it is possible to reduce a large capacitance, which is a problem in the conventional semiconductor laser device, without damaging a good current block structure, and as a result, it is possible to output even at a high frequency operation. It is possible to provide a semiconductor laser device that does not easily deteriorate.

[Brief description of drawings]

FIG. 1 is a cross sectional view of an element structure of a semiconductor laser device (SBR structure) showing the first embodiment of the present invention and its energy band diagram.

FIG. 2 is a sectional view of an element structure of a semiconductor laser device (IS structure) showing a second embodiment of the present invention.

FIG. 3 is a sectional view of an element structure of a conventional semiconductor laser device (SBR structure) and its energy band diagram.

FIG. 4 is a characteristic diagram of acceptor concentration with respect to donor concentration for high-speed modulation operation of 400 MHz or higher.

[Explanation of symbols]

 11 substrate 12 first clad layer 13 light guide layer 14 MQW active layer 15 light guide layer 16 second clad layer 17 current block layer 18 easy-to-carry layer 19 contact layer 20 electrode 21 electrode 24 current block layer 25 ridge stripe portion 26 etching stop layer

Claims (4)

[Claims] 1. A current block layer having an active layer, a second clad layer of a second conductivity type, and an opening exposing a predetermined portion of the second clad layer on a first clad layer of the first conductivity type,
In a semiconductor laser device in which a second conductive type easily conductive layer and a second conductive type contact layer covering the current conductive layer and the current conductive layer are sequentially formed in the opening, the current block layer may be: A semiconductor laser device characterized in that the second clad layer is formed to have a small band gap and an opposite conductivity type. 2. The semiconductor laser device according to claim 1, wherein the second conductivity type second cladding layer has a ridge portion. 3. The semiconductor laser device according to claim 1, wherein the current blocking layer is made of AlGaAs and InGaAlP. 4. The impurity concentration of the current blocking layer is 4 ×
The semiconductor laser device according to claim 3, wherein the semiconductor laser device has a ^{diameter of} 10 ¹⁶ cm ^-3 or less.