JPH0697574A - Semiconductor laser system - Google Patents

Abstract

PURPOSE:To provide a semiconductor laser having excellent characteristics capable of sufficiently augmenting the surge breakdown voltage due to the photoallowance level of an active layer. CONSTITUTION:Within the semiconductor laser system comprising a semiconductor laser and a diode monolithically formed on the same substrate to be parallel connected, the semiconductor laser composed of an InGaP active layer 24 formed of a double heterojunction held by an InGaAlP clad layers 23 and 25. Furthermore, the diode is formed of a pn junction of the n-InGaAlP clad layer 23 and a p-InAlP layer 33 while the threshold voltage of the diode is made higher than that of the semiconductor laser as well as the on-resistance of the diode is made lower than that of the semiconductor laser.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser using a compound semiconductor material, and more particularly to a semiconductor laser device having a protection function against surge voltage.

[0002]

2. Description of the Related Art In recent years, short wavelength lasers have been developed for application to high density optical disk systems, high speed laser printers, bar code readers and the like. Among these, In having an oscillation wavelength in the 0.6 μm band (red)
GaAlP-based semiconductor lasers are attracting attention as promising short-wavelength semiconductor lasers.

In order to make full use of the characteristics of the InGaAlP semiconductor laser and put it to practical use, it is required that its various characteristics and reliability are as good as those of the conventional GaAlAs semiconductor laser. Regarding the long-term reliability of the InGaAlP semiconductor laser, practical levels of 10,000 hours or longer at a level of 3 to 5 mW and 2000 hours or longer at a level of 10 to 20 mW have been obtained. However, the surge withstand capability, which represents the breakdown level for instantaneous voltage application, is GaAl.
It is significantly lower than the As laser, which is a big problem when mounting the laser.

It has been clarified that the mechanism of destruction of the InGaAlP semiconductor laser by the surge is the same as that of the GaAlAs semiconductor laser. That is, a pulse current flows when a voltage is applied, and the level of light generated at that time exceeds the light-allowable level of the active layer, causing irreversible destruction. In the case of the InGaAlP semiconductor laser, the light acceptance level of this active layer is small. In addition, since the optical tolerance level improvement measures used in GaAlAs semiconductor lasers such as forming a dielectric film on the end face portion or thinning the active layer are not effective, laser breakdown due to surge has become a very serious problem. It was

[0005]

As described above, the In
The GaAlP semiconductor laser has no countermeasure against surge at all, and there is a problem that the surge tolerance is remarkably lower than that of the GaAlAs semiconductor laser due to the low light allowable level of the active layer.

The present invention has been made in consideration of the above circumstances, and it is an object of the present invention to sufficiently increase the surge resistance due to the light permissible level of the active layer.
Another object is to provide a semiconductor laser device having excellent characteristics.

[0007]

The essence of the present invention is InG
By connecting a bypass diode in parallel with a semiconductor laser with low surge resistance such as aAlP series, and applying a high voltage to this diode, a current equal to or higher than that of the semiconductor laser is made to flow, so that the surge resistance can be maintained without changing the element characteristics of the semiconductor laser. Is to raise.

That is, the present invention is a semiconductor laser device constituted by connecting a semiconductor laser and a diode having a pn junction in parallel. The rising voltage of the diode is higher than the rising voltage of the semiconductor laser and the diode is turned on. The resistance is smaller than the on-resistance of the semiconductor laser.

More specifically, the rising voltage of the diode is higher than the rising voltage of the semiconductor laser, the diode does not turn on during normal driving of the semiconductor laser, and the rising voltage of the diode is lower than the voltage at which the semiconductor laser is destroyed. The diode is turned on to bypass the current flowing through the semiconductor laser when the voltage nears the breakdown voltage of the semiconductor laser.

[0010]

According to the present invention, since the rising voltage of the diode is higher than that of the semiconductor laser, the diode does not operate at the normal operating voltage of the semiconductor laser, and the semiconductor laser operates in the same manner as in the case without the diode. . However, the diode operates when a voltage exceeding the normal operating voltage of the semiconductor laser is applied, whereby an excessive current of the semiconductor laser can be leaked. Furthermore, since the on-resistance of the diode is lower than the on-resistance of the semiconductor laser, more current flows through the diode above the voltage at which the diode rises. This makes it possible to suppress irreversible destruction of the semiconductor laser.

[0011]

The details of the present invention will be described below with reference to the illustrated embodiments.

FIG. 1 is a side view showing a schematic structure of a semiconductor laser device according to the first embodiment of the present invention. In this embodiment, the bypass diode is formed separately from the laser section and incorporated at the time of mounting. In the figure, 11 is a heat sink portion, 12 is a semiconductor laser, 13 is a bypass diode, and the specific structures of the semiconductor laser 12 and the bypass diode 13 are the same as in the second embodiment described later.

FIG. 2 is a view showing the schematic arrangement of a semiconductor laser device according to the second embodiment of the present invention. In this embodiment, the bypass diode is formed monolithically. In the figure, 21 is an n-GaAs substrate, and this substrate 21
The n-GaAs buffer layer 22, n-In _0.5 (G
a _0.3 Al _0.7 ) _0.5 P cladding layer 23 (Si-doped,
3-5 × 10 ¹⁷ cm ^-3 ), InGaP active layer 24 (undoped), p-In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P cladding layer 25 (Zn-doped, 3-5 × 10 ¹⁷ cm ^-3 ),
p-In _0.5 Ga _0.5 P etching stop layer 26 (Zn-doped, 3 to 5 × 10 ¹⁷ cm ⁻³ ), and p-In _0.5 (G
a _0.3 Al _0.7 ) _0.5 P cladding layer 27 (Zn-doped,
A double heterojunction structure composed of ^{3 to} 5 × 10 ¹⁷ cm ⁻³ ) is formed. On the clad layer 27, p-In
_0.5 Ga _0.5 P cap layer 28 (Zn-doped, 1 × 10
¹⁸ cm ^-3 ) are formed.

Double-heterojunction layers and cap layer 2
8 has a lattice constant equal to that of the GaAs substrate 21, and the band gap energies of the clad layers 23 and 25 are equal to those of the active layer 2.
The composition of In, Ga and Al is determined so as to be larger than that of No. 4. A p-GaAs contact layer 30 (Zn-doped, 5 × 10 ¹⁸ cm ⁻³ ) is formed on the cap layer 28 and the etching stop layer 26. And
A metal electrode 31 is deposited on the upper surface of the contact layer 30, and a metal electrode 32 is deposited on the lower surface of the substrate 21.

The current constriction in this laser is 27,3.
0 p-InGaAlP / p-GaAs heterojunction,
27, 28, 30 p-InGaAlP / p-InGa
This is done by utilizing the difference in voltage drop in the P / p-GaAs heterojunction (p-I having an intermediate bandgap).
If the nGaP layer is sandwiched, the barrier becomes low, and the voltage drop can be reduced), and p-InGaP is formed only in the ridge portion.
The cap layer 28 is provided for current confinement. The optical waveguide is a clad layer 25 formed on a stripe-shaped mesa,
27. The thickness of the clad layer 23 is 1.0 μ
m, the thickness of the active layer 24 is 0.04 μm, the cladding layer 2
5, 27 have a thickness of 0.7 μm in the ridge portion, the thickness of the etching stop layer 26 is 0.01 μm, and the width of the bottom of the mesa is 5 μm.
And The buffer layer 22 is for improving the quality of the InGaAlP-based crystal formed on GaAs.

The laser structure up to this point is the same as that of the conventional device, but in this embodiment, a bypass diode is additionally formed. That is, the etching stop layer 26,
Part of the p-clad layer 25, the active layer 24 and the n-clad layer 23 is removed by etching, and p-In _0.5 A
An l _0.5 P layer 33 and a p-In _0.5 Ga _0.5 P layer 34 are formed. Then, the p-InAlP layer 33 and the n-clad layer 23 form a diode having a pn junction. The current confinement in this diode is performed by utilizing the difference in voltage drop between the p-InAlP / p-GaAs heterojunction and the p-InAlP / p-InGaP / p-GaAs heterojunction, as in the case of the laser. .

In order to obtain a laser having a sufficiently high surge resistance, it is necessary to set the material and composition of the bypass diode within a predetermined range, and the above-mentioned values are examples thereof. The optimization of this bypass diode will be described below.

First, the relationship between surge and light destruction will be described. FIG. 3A shows the relationship between the current flowing through the laser and the light output and the light allowable level. As shown in FIG. 3B, the surge occurs because a pulse current flows through the laser when a voltage is applied by electrostatic discharge, and the level of laser light generated at that time exceeds the light allowable level of the active layer. . At this time, if a means for leaking the current only when an abnormal voltage is applied, such as electrostatic discharge, is provided so as not to exceed the light allowable level of the active layer, surge breakdown does not occur. The bypass diode is effective for surge prevention based on this principle, but in order to prevent leakage from occurring during normal operation and to turn it on only when an abnormal voltage is applied, the bypass diode is used as described above. Structural design including the materials and composition of is important.

The present inventors have paid attention to the relationship between the band gap of the pn junction diode and the position of the conduction band edge from the vacuum level and the rising voltage of the diode. Figure 4
The positional relationship from the vacuum level of the two types of p-type and n-type semiconductor layers is shown in FIG. In the figure, the p-type and n-type Fermi levels are shown. The difference Vj between these is approximately the rising voltage of the diode as shown in FIG. 5, that is, the voltage at which the band becomes flat and the recombination current starts to flow. . Thus, a combination in which the conduction band is located as high as possible for the n-type and the valence band is located as low as possible for the p-type is in the direction of increasing the rise voltage, and the band gap itself is large. And the rising voltage also increases.

FIG. 6 shows the band positions from the vacuum level of the main Al composition used in the InGaAlP laser.
In a normal laser, n-type InGaP (x =
0), and InGaAlP (x = 0.
7) is used, and the current-voltage characteristic is as shown by A in FIG. As shown in FIG. 6, InGaA as n-type
1P (x = 0.7), p-type InAlP (x = 1.
0), the rising voltage rises by about 0.5 V more than that of the laser section, and the current-voltage characteristic thereof does not leak in the normal operating range of the laser as shown by B in FIG. The current can be set to be bypassed only when an abnormal voltage that exceeds the voltage is applied.

In order to avoid a current in the bypass diode when a high voltage is applied, it is important to make the resistance component of the bypass diode smaller than that of the laser section. Therefore, the diode has carrier concentration of p, n
Both are required to be 1 × 10 ¹⁷ cm ⁻³ or more, and the p-type layer having a high resistivity is desirably 1 × 10 ¹⁸ cm ⁻³ or more. In addition, using an inclined substrate as the substrate is effective from the viewpoint of improving the carrier concentration.

In this embodiment, the p-type layer is thinned to 0.5 μm, and the junction area of the bypass diode is 300.
× 300 μm ² , which is 10 times larger than that of a semiconductor laser. By reducing the resistance of the bypass diode in this way, 100 mA is applied when a high voltage is applied.
A large current that exceeds the current flows. The resulting temperature change due to the Peltier effect was also effective as a surge countermeasure.

Here, the bypass diode needs to have a rising voltage higher than that of the laser so that it is turned on when an abnormal voltage is applied to the semiconductor laser, and the ON resistance is lower than that of the laser in order to protect the laser. is necessary. Specifically, it is desirable that the current intersection of the diode and the laser is in the range of 50 to 90% of the optical breakdown level.
The reason for the upper limit of 90% is that the current-light output characteristics become steep during laser oscillation, so a margin of about 10% is required. The reason for the lower limit of 50% is that if it is lower than this, the reactive current to the bypass diode in the normal operation increases and the light output becomes unstable due to its heat generation.

As described above, the bypass diode is connected to the semiconductor laser, the rising voltage of the diode is made higher than that of the semiconductor laser, and the on-resistance of the diode is made smaller than that of the laser. For voltage application exceeding the normal operating voltage of
An excessive current of the semiconductor laser can be leaked to the diode, and irreversible destruction of the semiconductor laser can be suppressed. In the examples of FIGS. 1 and 2, the laser oscillated with a threshold value of 50 mA when the resonator length was 400 μm. At this time, there were no problems in temperature characteristics and reliability characteristics. Further, even when comparing the surge characteristics due to discharge from the capacitor, no surge deterioration was observed in the measurement range.

The present invention is not limited to the above embodiments. In the embodiment, the bypass diode is mounted or formed monolithically on the same substrate as the semiconductor laser, but the diode may be installed as an external device. Also, as the bypass diode, p
Besides the n-junction, a MIS junction or a Schottky junction type diode may be used. Furthermore, if a bypass diode having a wider gap than that of the material system forming the laser, such as SiC, ZnS, AnSe, or ZnCdTeSe, is used, the above-described suitable bypass diode can be easily obtained. Further, as the target laser, in addition to the InGaAlP-based laser, GaAlAs-based laser or InGaA-based laser is used.
It can be applied to s series. In addition, various modifications can be made without departing from the scope of the present invention.

[0026]

As described above in detail, according to the present invention, I
By connecting a bypass diode in parallel with a semiconductor laser having a low surge resistance such as nGaAlP series, the rising voltage of this diode is made higher than that of the laser, and the on-resistance of the diode is made smaller than that of the laser, thereby making the active layer It is possible to realize a semiconductor laser device in which the surge withstand level due to the light permissible level can be sufficiently increased and the characteristics are also excellent.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a semiconductor laser device according to a first embodiment.

FIG. 2 is a diagram showing a schematic configuration of a semiconductor laser device according to a second embodiment.

FIG. 3 is a diagram showing a relationship between a surge applied voltage and a current-optical output characteristic.

FIG. 4 is a diagram showing a positional relationship from a vacuum level of p-type and n-type semiconductor layers.

FIG. 5 is a diagram showing a current-voltage characteristic of a pn diode.

FIG. 6 is a diagram showing the Al composition dependence of the band position from the vacuum level of InGaAlP.

FIG. 7 is a diagram showing current-voltage characteristics of a semiconductor laser and a bypass diode.

[Explanation of symbols]

 11 ... Heat sink part, 12 ... Semiconductor laser, 13 ... Bypass diode, 21 ... N-GaAs substrate, 22 ... N-GaAs buffer layer, 23 ... n-InGaAlP clad layer, 24 ... InGaP active layer, 25, 27 ... P- InGaAlP clad layer, 26 ... p-InGaP etching stop layer, 28 ... p-InGaP cap layer, 30 ... p-GaAs contact layer, 31, 32 ... Metal electrode, 33 ... p-InAlP layer, 34 ... p-InGaP layer.

Claims (1)

[Claims] 1. A semiconductor laser and a diode having a pn junction are connected in parallel, the rising voltage of the diode is higher than the rising voltage of the semiconductor laser, and the ON resistance of the diode is higher than the ON resistance of the semiconductor laser. A semiconductor laser device characterized by being small.