JPH0251269B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Technical Field of the Invention The present invention relates to semiconductor light emitting devices, particularly InP/InGaAsP
The present invention also relates to a semiconductor light emitting device that has a structure in which an active layer and the like are buried in striped grooves using an InGaAs semiconductor, and whose linearity, threshold current, etc. are greatly improved, especially at high temperatures.

(b) Technical background Optical communication systems, which use light as the medium for information signals, continue to make remarkable progress as a major pillar of the information society. Semiconductor light-emitting devices are the most important components in systems that use light as a medium, such as optical communications. , improvements in various characteristics such as improved quantum efficiency, improved linearity of current-light output characteristics, and increased output have contributed to the advancement of systems.

(c) Prior Art and Problems As a result of the research and development described above, many structures for semiconductor light emitting devices have already been provided. Among them, indium-phosphorus/indium-gallium-
VSB (V-grooved Substrate Buried double) using arsenic/phosphorus (InP/InGaAsP) compound semiconductor
heterostructure) lasers are known.

FIG. 1 is a sectional view showing an example of an InP/InGaAsP VSB laser. In manufacturing the VSB laser of this conventional example, an n-type InP single crystal (100)
A p-type InP layer 2 is grown on a substrate 1 with the surface facing up, and a striped groove extending in the <011> direction is formed using a hydrochloric acid (HCl)-based etching solution to a depth that penetrates the p-type InP layer 2. to form. This stripe groove is
It has a V-shaped cross section that exposes the (111)B plane of the InP single crystal.

In this stripe groove, an n-type InP confinement layer 3,
A heterojunction structure consisting of an InGaAsP active layer 4 and a p-type InP confinement layer 5 is grown by a liquid phase epitaxial growth method or the like. At this time, the n-type InP layer 3a and the InGaAsP layer 4a are grown at the same time as the confinement layer 3 and the active layer 4, respectively, and the p-type InP confinement layer 5 is grown until it fills the stripe trench and extends outside the trench. Further, a p-type InGaAsP layer 6 is continuously formed.
grow. Further, 7 is a p-side electrode, and 8 is an n-side electrode. The optical cavity of the VSB laser in this conventional example is the first
It is a Fabry-Perot type in which the two cleavage planes parallel to the cross section shown in the figure serve as reflecting mirror surfaces, and laser oscillation occurs when gain and loss are balanced in the vicinity of the wavelength that has the maximum gain within the resonator. . This threshold current decreases as the resonator length L becomes shorter.

In addition, the differential quantum efficiency η of a semiconductor laser is generally expressed as follows, where the cavity length is L [cm], the reflectance of the cavity end face is R, and the loss coefficient within the cavity is α [cm ^-1 ]. Using η ₀ determined by the semiconductor substrate, it is expressed as η=η ₀ 1/Lln1/R/α+1/Lln1/R, and by shortening the resonator length L, the influence of loss can be relatively reduced. , and the differential quantum efficiency η can be increased.

For these reasons, the conventional InP/InGaAsP system
In the VSB laser, the cavity length L is approximately 200 μm. Figure 2 shows the resonator length L≒200μm.
It is a diagram showing an example of the current-light output characteristics of an InP/InGaAsP-VSB laser, and shows the light output [mw] per end facet at temperatures of 25°C and 70°C.

Examining these characteristic curves, we can see that even at a temperature of 25°C, the differential quantum efficiency η increases as the current increases.
decreases, the characteristic curve curves and deviates from linearity. Furthermore, it is known that at a temperature of 70° C., the threshold current increases and the differential quantum efficiency decreases significantly.

Such characteristics are mainly controlled by reactive current, that is, leakage current that does not flow through the active layer 4. The current paths of the VSB laser shown in FIG.
and is the above-mentioned reactive current.

Current flows from p-type InP confinement layer 5 to p-type InP layer 2
→Flows through the path of n-type InP substrate 1, and the current value is p
It is governed by the voltage-current characteristics of the pn junction formed by the InP type InP layer 2 and the n-type InP substrate 1.

Next, outside the stripe groove, (a) the p-type regions of the InGaAsP layer 6 and the InP confinement layer 5, and (b) the InP layer 3.
It has a structure in which the n-type region of a, (c) the p-type region of the InP layer 2, and (d) the n-type region of the InP substrate 1 are stacked.
A current corresponding to the above current is blocked by the np reverse junction formed by the n-type InP layer 3a and the p-type InP layer 2. However, due to (a) to (d) above, a pnpn junction is formed between the two electrodes, and when the current flows, it operates as a thyristor using this as the gate current, and a current corresponding to the anode current is generated. It may turn on. For this thyristor operation, the impurity concentration of the p-type InP layer 2, which was conventionally about 5 x 10 ¹⁷ cm ^-3 , is increased to about 2 x 10 ¹⁸ cm ^-3 , and electrons in this layer are increased. Improvements have been made by shortening the lifetime of the device to prevent it from reaching turn-on, at least at room temperature. In the characteristic example shown in Figure 2, in the case of 25°C, the increase in leakage current that is controlled by the forward voltage-current characteristics of the pn junction affects the temperature, and in the case of 70°C, the temperature of the forward current of the pn junction It is considered that the current due to the thyristor operation is added to the increase in current based on the characteristics. The deterioration of characteristics due to this temperature rise is
For example, in InP/InGaAsP lasers, GaAs/
This is especially noticeable compared to AlGaAs lasers.

Non-linearity of optical output, increase in threshold current due to temperature rise, deterioration of differential quantum efficiency, etc. as mentioned in the example above are important problems for light sources for optical communications, etc., and are important issues for the progress of optical communications systems. Improvements are requested.

(d) Purpose of the Invention The present invention relates to a semiconductor laser in which an active layer, etc. is embedded in a stripe groove using InP/InGaAsP or InGaAs-based semiconductor material, and improves the above-mentioned characteristics by suppressing the influence of leakage current. The purpose is to improve.

(e) Structure of the Invention The object of the present invention is to provide a first semiconductor layer of a second conductivity type formed on an indium-phosphorus compound semiconductor substrate of a first conductivity type, and a semiconductor layer that penetrates the first semiconductor layer. a striped groove having a V-shaped cross section reaching the substrate; a second semiconductor layer of a first conductivity type formed in contact with the substrate and the first semiconductor layer inside the groove; a third semiconductor layer made of an indium-gallium-arsenic, phosphorous compound, or an indium-gallium-arsenic compound, which is in contact with the second semiconductor layer, is a single layer, and has a narrower bandgap than the second semiconductor layer; a fourth semiconductor layer that embeds the third semiconductor layer and extends onto the first semiconductor layer outside the trench; and a fourth semiconductor layer that extends onto the first semiconductor layer outside the trench of the fourth semiconductor layer. A resonator for light guided in the stripe direction is provided in a semiconductor substrate having a fifth semiconductor layer of the first conductivity type located between the stretched portion and the first semiconductor layer below the stretched portion. , and the length of the resonator in the waveguide direction is 300 μm or more and 400 μm or less.

Figure 3 shows an example of the correlation between the cavity length L, threshold current Ith, and threshold current density Jth for an InP/InGaAsP VSB laser, and Figure 4 shows an optical output of 5 mW at a temperature of 70°C. 2 is a diagram showing an example of the correlation between the current required for the resonator length L and the resonator length L. FIG.

The threshold current Ith usually increases as the resonator length L increases. However, the threshold current density Jth is large when the resonator length L is short, decreases as the resonator length L becomes longer, and then becomes approximately constant. This is because when the resonator length L is short, the gain coefficient g=α+1/Lln1/R required to generate oscillation becomes large, and the current density threshold Jth needs to be large, and the resonator length L is long. This is because the effect of the second term in the above equation is relatively reduced. This threshold current density Jth
The magnitude of L means the level of voltage applied to the semiconductor substrate, and by selecting a resonator length L with a small threshold current density, the applied voltage can be lowered. If the applied voltage is reduced, the voltage between the p-type InP layer 2 and the n-type InP substrate 1, which dominates the current explained earlier, will decrease.
The voltage applied to the pn junction decreases, and the leakage current decreases in accordance with the forward characteristics of the pn junction.

As mentioned earlier, the threshold current density Jth is, for example, in the example shown in Fig. 3, when the resonator length = 350 μm.
Above a certain level, it becomes almost constant. In this way, if the amplitude length L is increased within a range in which there is no effect of reducing the threshold current density Jth, the threshold current Ith will only be increased needlessly without the effect of reducing the voltage. This situation is also clear from the example shown in FIG. 4; for example, the current required to obtain an optical output of 5 mW at a temperature of 70° C. decreases as the resonator length L increases, and then begins to increase. In this illustrated example, the resonator length L≒
The current value is minimum at 350 μm, and the bottom of the curve is in the range where the resonator length L is about 300 μm to 400 μm.

Lasers with a trench-embedded structure made of InP/InGaAsP semiconductor materials exhibit similar characteristics with respect to the cavity length L even if their configurations are different, and by setting the cavity length L between 300 μm and 400 μm, the quantum efficiency can be improved. can be maximized, and as will be exemplified later, the objective of the present invention is to improve the characteristics.

(f) Examples of the invention The present invention will be explained in more detail below using examples.

As an example of the present invention, a VSB laser having the cross-sectional structure shown in FIG. 1 was manufactured as follows.

The n-type InP substrate 1 of this embodiment has an impurity concentration of, for example, 2×10 ¹⁸ cm ^-3 , and the p-type InP layer 2 has an impurity concentration of, for example, 2
×10 ¹⁸ cm ^-3 , thickness 1.5 μm, n-type InP confinement layer 3
For example, the impurity concentration is 5×10 ¹⁷ cm ^-3 , the InGaAsP active layer 4 has a luminescence wavelength of 1.3 μm, the thickness on the center line is 0.15 μm, and the p-type InP confinement layer 5 has an impurity concentration of 5×10 ¹⁷ cm ⁻³ , for example. For example, the p-type InGaAsP layer 6 has an impurity concentration of 1×10 ¹⁹ cm ⁻³ .

The resonator of this embodiment has a length L of about 350 μm, and both end faces are cleavage planes. The current-light output characteristics of this example are illustrated in FIG. In the figure, the solid line indicates this embodiment, and the broken line indicates the conventional example with the resonator length L≈200 μm shown in FIG. 2 earlier. The temperature is 25℃
The light output [mw] per end face is displayed for the two cases of temperature and 70°C.

First, at a temperature of 25.degree. C., the threshold current in this embodiment is increased compared to the case where L≈200 .mu.m, as described above. However, in this embodiment, the differential quantum efficiency increases and the linearity is improved, and the characteristic curves intersect, and at a current above this value, the quantum efficiency of this embodiment becomes larger than that of the conventional example.

The effect of the present invention is remarkable at a temperature of 70°C. In other words, both the threshold current and differential quantum efficiency have been improved; for example, the current required to obtain an optical output of 5 mW is significantly reduced from approximately 127 mA when L≒200 μm to approximately 80 mA when L≒350 μm. ing. Temperature characteristics are a major problem.
This improvement in temperature characteristics is particularly effective for InP/InGaAsP lasers, but the synergistic effect of reducing leakage current, which in turn reduces heat generation, and reduces the temperature rise of the semiconductor substrate is significant. It is having an effect.

Although the above explanation is directed to a VSB laser in which the cross-sectional shape of the stripe groove is V-shaped, the present invention is applicable to an InP laser that forms an active layer, etc. in any stripe groove, regardless of the cross-sectional shape of the groove, crystal orientation, etc. /
It can be applied to InGaAsP lasers. Further, the present invention can be similarly applied to a laser whose active layer is made of InGaAs.

(g) Effect of the invention As explained above, according to the present invention, InP/
Regarding InGaAsP or InGaAs-based trench-filling lasers, the threshold current at high temperatures, which was a major problem with conventional InP/InGaAsP-based lasers,
Deterioration in differential quantum efficiency, ie linearity, etc., is significantly improved, and linearity is also improved even at room temperature.

As a result, optical communication systems can adapt to wider environmental conditions than before, improve reliability, or increase the optical output of the light source and extend the relay interval, contributing to the advancement of optical communication systems. can.

Furthermore, the degree of freedom regarding thermal conditions is expanded for arraying semiconductor lasers or integrating them with other electronic elements, thereby further promoting the development of the system.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing an example of a VSB laser, Figure 2 is a diagram showing a conventional example of current-light output characteristics, and Figure 3 is a diagram showing a conventional example of the current-light output characteristics.
The figure shows an example of the correlation between the threshold current and its density and the resonator length. Figure 4 shows an example of the correlation between the drive current and the resonator length. Figure 5 shows the current-light output characteristics. FIG. 3 is a diagram showing a comparison between an embodiment of the invention and a conventional example. In the figure, 1 is an n-type InP substrate, 2 is a p-type InP substrate
layers, 3 is n-type InP confinement layer, 4 is InGaAsP active layer, 5 is p-type InP confinement layer, 6 is p-type InGaAsP
7 indicates a p-side electrode, and 8 indicates an n-side electrode.

Claims (1)

[Scope of Claims] 1: a first semiconductor layer of a second conductivity type formed on an indium-phosphorus compound semiconductor substrate of a first conductivity type; and a semiconductor layer that penetrates the first semiconductor layer to reach the substrate. a striped groove having a V-shaped cross section; a second semiconductor layer of a first conductivity type formed in contact with the substrate and the first semiconductor layer inside the groove; and a second semiconductor layer of a first conductivity type formed in contact with the second semiconductor layer. , and a third semiconductor layer made of an indium-gallium-arsenic-phosphorus compound or an indium-gallium-arsenic compound that is a single layer and has a narrower band gap than the second semiconductor layer; and the third semiconductor layer. a fourth semiconductor layer that embeds and extends onto the first semiconductor layer outside the trench; a portion of the fourth semiconductor layer that extends onto the first semiconductor layer outside the trench; A resonator for light guided in the stripe direction is provided in a semiconductor substrate having a fifth semiconductor layer of a first conductivity type located between a lower first semiconductor layer, and a resonator for light guided in the stripe direction; The length in the waveguide direction is
A semiconductor light emitting device characterized by having a diameter of 300 μm or more and 400 μm or less.