JPS6320393B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a buried semiconductor laser capable of high optical output and continuous operation at high temperatures by minimizing leakage current flowing outside the active layer.

Embedded semiconductor lasers are suitable as light sources for optical fiber communication because they exhibit a low oscillation threshold, require a small drive current, and emit an isotropic beam, which is advantageous for coupling with optical fibers. The authors are
55-123261, he invented a structure in which a Pn semiconductor layer is formed on the side surface of a mesa stripe containing an active layer to reduce leakage current, which is a problem in buried semiconductor lasers, and developed InGaAsP with an InP substrate. Minimum lasing threshold of 8.5 as an embedded semiconductor laser
mA, and a maximum continuous operating temperature of 100°C was obtained. However, in the embedded semiconductor laser with this structure,
The turn-on voltage of the PnPn structure at the periphery of the mesa is not high enough to completely prevent leakage current, and as the forward bias voltage applied to the semiconductor laser increases, the leakage current flowing through the PnPn layer increases, resulting in high injection There was a tendency for the optical output to saturate when the current was applied or for the oscillation threshold to rapidly increase in the high temperature region.

Therefore, an object of the present invention is to prevent the leakage current of a buried semiconductor laser, especially the buried semiconductor laser having the structure described in Japanese Patent Application No. 55-123261, to obtain a high optical output and to be able to operate continuously at a high temperature. The object of the present invention is to provide an embedded semiconductor laser with a new possible structure.

According to the present invention, an n-type first cladding layer (gap band width Eg ⁿ¹ ), an n-type second cladding layer (gap band width Eg ² , Eg ⁿ² >Eg ^{n1 ), and an active layer (gap band width Eg n1} ) are formed on an n-type semiconductor substrate. A multilayer film substrate including at least a layer structure in which a forbidden band width Eg ^a , Eg ^a <Eg ⁿ² ) and a P-type cladding layer (Eg ^p1 , Eg ^p1 > Eg ^a ) is laminated in this order is formed into an n-type first cladding layer. The band-like mesa structure containing the active layer is formed ^by mesa etching at a ^depth _reaching . _g > E ⁿ¹ _g ), an n-type current confinement layer (forbidden band width E ^c _g , E ^c _g > E ⁿ¹ _g ) formed except for the upper part of the mesa of the band-shaped mesa structure, and continuously formed over the entire surface. P-type buried layers (gap band width E ^E _g , E ^E _g >E ⁿ¹ _g ) are successively stacked in the order of 3.
A buried type semiconductor laser or the like is obtained, which is characterized in that it is buried in a multilayer film including at least two layers.

Next, embodiments of the present invention will be described using the drawings.

FIG. 1 shows an InGaAsP buried semiconductor laser using n-type InP as a substrate, as described in Japanese Patent Application No. 55-123261. This semiconductor laser is an n-type with a (001) plane.
n-type InP buffer layer 2 (thickness 3μ) on InP substrate 1
m), InGaAsP active layer 3 (emission wavelength 1.3 μm, thickness
After laminating a P-type InP layer 4 (0.5 μm thick), a mesa stripe 30 with a height of 2 μm and a width of 2 μm is formed in parallel to the <110> direction, and then a P-type InP layer 4 (thickness 0.5 μm) is formed on the mesa side surface.
An InP type current blocking layer 5 and an n type InP current confinement layer 6 are formed, and the whole is further covered with a p type InP buried layer 7 and an n type InP cap layer 8, and a Zn layer is formed on the top of the mesa.
It has a structure in which a selective diffusion region 9 is formed. When a bias voltage is applied between the P-type electrode 20 and the N-type electrode 21 of this semiconductor laser, with the P-type electrode being positive, the mesa part is forward biased to the Pn junction, so current flows into the active layer, which is a Pn junction. is injected, but
Since the parts other than the mesa part are PnPn junctions or nPnPn junctions, almost no current flows. Therefore, current is effectively concentrated only in the active layer, resulting in a low oscillation threshold of about 10 to 20 mA. However, 20~30mW
In order to obtain a high optical output, the injection current becomes large and the voltage applied to the lower part of the selective diffusion region 9 also becomes high. The current flowing through 5 also increases. This current acts as a gate current of the PnPn junction on the side of the mesa, that is, an InP thyristor, and serves to lower the turn-on voltage of this PnPn junction. Therefore, as the applied voltage increases, the leakage current flowing through the mesa side increases and the proportion of current injected into the active layer decreases, so the saturation tendency of the optical output gradually becomes stronger. The PnPn junction on the side of the mesa is turned
When turned on, the light output drops rapidly. This situation is shown in FIG. This leakage current also affects the temperature characteristics of the element. That is, since the oscillation threshold increases with temperature, the voltage applied to the element increases at high temperatures, which ultimately increases the leakage current flowing through the PnPn junction on the side surface of the mesa. Therefore, the increase in leakage current is added to the increase in the oscillation threshold of the active layer itself, and the temperature characteristics of the element deteriorate rapidly in the high temperature region. Therefore, the maximum continuous operating temperature of the InGaAsP buried semiconductor laser having the structure shown in Figure 1 is at most 80°C.
The temperature was about ~100℃.

In order to improve the optical output and continuous operation upper limit temperature of the semiconductor laser shown in FIG. 1, it is important to minimize the increase in leakage current on the mesa side surface.
For this purpose, the turn-on voltage of the PnPn junction on the side of the mesa is high, and even if a fairly large gate current is injected, it is sufficient that it does not decrease significantly.

FIG. 3 shows an embodiment of the invention. The difference from the conventional example shown in Fig. 1 is that the n-type InP substrate 1 and the
InP type buffer layer 2 (film thickness 0.6 μm in Figure 3)
between the n-type InGaAsP first cladding layer 10
(emission wavelength 1.3 μm, thickness 1 μm),
The n-type InGaAsP first cladding layer is exposed in the flat part by mesa etching, and the n-type InP cap layer 8 is not used.

In the buried semiconductor laser having the structure shown in FIG. 3, the structure of the mesa portion is PInP/InGaAsP (active layer)/nInP/nInGaAsP/nInP (substrate).
The Pn junction is inside the InGaAsP active layer 3, and
InGaAsP active layer 3 and nInGaAsP first cladding layer 3
0, since the nInP buffer layer 2 with a thickness of 0.6 μm is present, there is almost no injection carrier leaking from the InGaAsP active layer 3 to the nInP buffer layer 2. Also, the mesa side part is P type InP/n type InP/P type
It is a PnPn junction with a structure of InP/n-type InGaAsP/n-type InP (substrate). The PnPn junction in this structure is based on the InP homojunction in the structure shown in Figure 1.
Compared to the PnPn junction, the turn-on voltage can be maintained higher, especially in the presence of gate current. This can be thought of as follows.

In the PnPn structure on the side of the mesa in FIGS. 1 and 3, the P-type InP buried layer 7 side is the anode, and the n-type
The InP buffer layer 2 or n-type InGaAsP first cladding layer 10 side is used as the cathode, and the gate current is P.
It can be considered as a thyristor injected into the InP current blocking layer 5. At this time, the P-type current blocking layer 5 into which the gate current is injected is sandwiched.
Considering the nPn junction, in the case of Figure 1 it is an InP homojunction transistor, whereas in the case of Figure 2 it is a transistor with the smallest emitter bandgap, so The efficiency with which electrons are injected from the emitter of the n-type first cladding layer 10 to the base region of the P-type InP current blocking layer 5 is reduced. Therefore, the current gain α of the nPn junction transistor becomes small, so even if a gate current exists, the current transported from the n-type InGaAsP first cladding layer 10 to the n-type InP current confinement layer 6 is small. Therefore, the turn of the entire PnPn junction is
The on-voltage can be maintained higher than in the PnPn junction shown in FIG. 1 even in the presence of gate current.

From the above, in the buried semiconductor laser having the structure of the embodiment of the present invention shown in FIG. 3, even if the applied voltage is high, the PnPn junction on the mesa side surface is difficult to turn on, and the leakage current flowing through this region is extremely low. small. Therefore, there is no need to restrict the current path on the P side, and there is no need to form a selective diffusion region as shown in FIG. Therefore, the manufacturing process can be simplified and the ohmic resistance can also be reduced. The oscillation threshold of this device at room temperature is 20 mA, and the optical output increases linearly to about 50 mW, with no tendency to saturation.
In addition, even if the oscillation threshold increases in the high temperature region, the leakage current is small, so the temperature characteristics of the element do not deteriorate rapidly, and the characteristic temperature T expressed by Ith∝exp(T/T ₀ ) ₀ is 70k from room temperature to about 100℃
This value was obtained, and CW operation was possible up to 100°C or higher.

In the above embodiment, the final layer is a P-type InP layer, but in order to further reduce the ohmic resistance, a P-type InP layer is used.
An InGaAsP electrode forming layer may be added. or,
In the above implementation, an InGaAsP layer with a composition equivalent to an emission wavelength of 1.3 μm was used as the n-type InGaAsP first cladding layer, but this was changed to a composition equivalent to 1.1 μm, and the n-type InP buffer layer 3 was adjusted to an appropriate thickness. By setting the light emitted in the active layer to n-type InGaAsP
It is also possible to have a structure in which the waveguide is coupled to the first cladding layer. Further, although an InP substrate is used in this embodiment, it is also possible to use a GaAs substrate or to fabricate it from an AlGaAs-based semiconductor material.

Finally, to summarize the features of the present invention, by introducing a low bandgap layer that can maintain a high on-on voltage of the PnPn layer on the side surface of the mesa in a buried semiconductor laser, the linearity of the optical output is improved and the It is possible to generate output, and the temperature characteristics of the device have been improved, allowing continuous operation up to high temperatures of 100°C or higher.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a conventional buried type semiconductor laser. FIG. 2 is a graph showing the injection current vs. optical output characteristic of a conventional buried semiconductor laser. FIG. 3 is a sectional view showing an embodiment of a buried semiconductor laser according to the present invention. In the figure, 1... n-type InP substrate, 2... n-type InP buffer layer, 3... InGaAsP active layer, 4... P-type
InP cladding layer, 5... P-type InP current blocking layer,
6...N-type InP current confinement layer, 7...P-type InP
Buried layer, 8... n-type InP cap layer, 9...
Zn selective diffusion region, 10... n-type InGaAsP 1st
Cladding layer, 20...P side metal electrode, 21...n
Side metal electrode, 30...Mesa stripe.

Claims (1)

[Claims] 1. On an n-type semiconductor substrate, an n-type first cladding layer (forbidden band width E n1 _g ) and an n-type second cladding layer (forbidden band width E ⁿ¹ g ) are formed.
E ⁿ² _g , E ⁿ² _g > E ⁿ¹ _g ), active layer (gap E ^a _g , E ^a _g <
E ⁿ² _g ) and a P-type cladding layer (E ^p1 _g , E ^p1 _g >E ^a _g ) are laminated in this order. The band-shaped mesa structure including the active layer formed by mesa etching in the band-shaped mesa structure has a P-type current blocking layer (with forbidden band widths E ^B _g , E ^B _g >E ⁿ¹ _g ), an n-type current confinement layer (forbidden band width E ^c _g , E ^c _g >E ⁿ¹ _g ) formed except for the upper part of the mesa of the band-shaped mesa structure, and continuously formed over the entire surface. Embedded type embedded with a multilayer film including at least three P-type embedded layers (gap band width E ^E _g , E ^E _g >E ⁿ¹ _g ) successively stacked in this order. semiconductor laser. 2. The n-type semiconductor substrate according to claim 1 is an n-type InP substrate having a (001) plane, the n-type first cladding layer is an n-type InGaAsP layer, and the active layer is an InGaAsP layer. The n-type second cladding layer and the n-type current confinement layer are n-type InP layers, and the P-type cladding layer, the P-type buried layer, and the P-type current blocking layer are P-type InP layers, and the band-like mesa structure is is formed substantially parallel to the <110> direction.