JPS6216035B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor optical device including a current distribution semiconductor injection laser that spatially controls injection current, a selective amplification element, and a modulation element.

Recently, optical communication has almost entered the practical stage due to improvements in the longevity of transmitting sources such as semiconductor injection lasers and light emitting diodes, and reduction in transmission loss in focusing fibers. Furthermore, their integration is progressing rapidly. Furthermore, various structures have been proposed to obtain efficient stimulated radiation. In semiconductor injection lasers, injected electrons, holes, or both,
Stimulated radiation is more efficient if the generated light is spatially confined to the same area. The currently common structure is
The area where light is confined is made of a material with a higher refractive index than the materials on both sides, and the forbidden band width is narrower in the area where electrons and holes are confined.
It improves the efficiency of stimulated radiation by improving the confinement of electrons and holes, increasing the density of injected carriers relative to the injected current, increasing recombination efficiency, and improving the confinement of light due to the difference in refractive index. . An example is shown in FIG.

Substrate 11 is n-type GaAs and layer 12 is n-
Ga _1-x Al _x As, active layer 13 is GaAs, layer 14 is p-
Ga _1-x Al _x As, layer 15 is p-GaAs, and 16 and 17 are metal electrodes. Reference numeral 18 indicates one end face constituting the optical resonator. The active layer 13 is the stimulated emission region of light. In this case, Ga _1-x Al _x As is GaAs
A single crystal containing AlAs in a ratio of 1-x:x is shown, but x is usually 0.2 or more. layers 11, 12,
14 and 15 are regions close to the stimulated radiation region. The more AlAs is added, the wider the forbidden band and the lower the refractive index. When x=0.3
Ga ₀ . ₇ Al ₀ . ₃ As has a difference in refractive index of about 5% compared to GaAs, which improves light confinement, resulting in extremely high stimulated radiation efficiency. Of course, structures have also been developed that increase the light confinement area compared to the carrier confinement area. Even in that case, it is desirable that the area where the carrier is confined is located in a place where the intensity of light is strong.
The region where light, electrons, and holes are confined is called the active layer. Since both sides of this layer are sandwiched between materials with different forbidden widths, this is usually called a double heterostructure (DH structure). This structure is of course also possible with other materials, for example, the active layer can be
If Ga _1-x In _x As _1-y P _y is formed and InP is grown on both sides, this becomes a DH structure. However, x,
y represents the component ratio, and by changing x and y, the forbidden band changes and the emission wavelength changes. With this material, a thickness of about 1 μm to 1.3 μm is possible, which can be matched to the low loss region of optical transmission glass fibers. Other active layers
A DH structure is possible with various materials, such as GaAsSb on both sides and AlGaAsSb. For ternary and quaternary materials, if you change the components,
In addition to being able to change the emission wavelength, it is also possible to adjust the lattice spacing and eliminate lattice distortion by adjusting the components. In the structure of this semiconductor injection laser, a Fabry-Perot resonator is formed using the deep surface of the substrate as a reflector, so it is difficult to obtain light with a single wavelength, and only laser light containing many wavelengths is used. In many cases, this is not possible, causing various problems in optical communications. Particularly when the injected current becomes large in order to obtain a large output, light with wavelengths determined by a large number of axial modes is emitted. To solve these problems, distributed feedback semiconductors create a periodic structure of refractive index in the direction of light propagation in the resonator of a semiconductor injection laser (the place where electrons, holes, or light are confined) and utilize the bragged reflection. Injection lasers have been realized. An example of this is shown in FIG. For example, the substrate 11 is made of n-GaAs, and the layer 12 is made of GaAs or GaAlAs.
is n-Ga _1-x Al _x As, active layer 13 is p-GaAs or GaAs, layer 24 is p-Ga _1-y Al _y As, layer 25 is p-Ga 1-x Al x As, and layer 15 is p-Ga _1-x Al _x As. p-GaAs, 16,1
7 is a metal electrode. However, x>y.
A periodic structure is embedded between layers 24 and 25. In this case, the layer of p-GaAs 13 is a stimulated emission region in which electrons and holes are trapped, where they are recombined and light is emitted. However, layer 24 has a low content of AlAs, and layer 13 and layer 2
The refractive index difference between layer 25 and layer 12 is small.
Due to the high content of AlAs, light confinement is performed in both the active layer 13 and the layer 24. Due to the periodic structure between the layers 24 and 25, only a specific wavelength is selected, and oscillation of a single wavelength occurs.
Previous forms of this structure lacked layer 24 and active layer 23.
A periodic structure was directly formed in the layer shown in Fig. 2, but this resulted in defects in the active layer 23 and severe deterioration of the diode, making it impossible to obtain a diode that could operate continuously at room temperature. For the first time, a device that can operate continuously at room temperature has been obtained. In a structure with such a periodic refractive index, the region in which electrons and holes are confined and the region in which light is confined in the direction perpendicular to the junction surface do not match, and the carrier confinement region occurs where the light intensity becomes weak. This is undesirable because it reduces the efficiency of stimulated radiation. Furthermore, in this structure, the current flows almost uniformly in the horizontal direction on the junction surface within the semiconductor laser, so the current flowing through the nodes of the standing wave of light hardly contributes to light emission, leading to a decrease in efficiency. This may cause oscillation in other axis modes, and the maximum value of the oscillation output per mode will be suppressed.

So far, we have talked about the length direction in which the standing wave of light stands, but in the direction perpendicular to this, there is a striped structure in which current flows only in the shaded area in Figure 3, and there are various striped structures. The oscillation mode in the horizontal direction is controlled by considering the following. However, as shown in the figure, the current flows almost uniformly over the entire stripe width, so the oscillation state is unstable, and the current-optical characteristics may be distorted, or if a step current is passed. , its state changes over time, and its operating state changes as the directivity changes or as the temperature rises or the injection current level rises.

It is an object of the present invention to provide a current distribution type semiconductor optical device that overcomes these disadvantages, has a relatively simple structure, and is suitable for integration.

The present invention will be described below with reference to the drawings.
This will be explained in detail by taking a No. 5 semiconductor injection laser as an example.

Normally, when a semiconductor injection laser emits laser light, the resistance at the diode junction decreases, making it easier for current to flow. For example, in a GaAs-GaAlAsDH laser, although it depends on the thickness of the active layer, the resistance of the junction becomes 1/10 when the laser starts emitting light even with a thickness of 0.5 μm.
decreases below. The decrease in resistance is more pronounced as the thickness of the active layer becomes thinner. However, since the laser is emitted by forward injection, the resistance at the junction is small.In fact, the series resistance due to the resistance of the substrate and the contact resistance between the metal used as the electrode and the semiconductor part is large. The resistance change is small compared to that, and in many cases, in the current range where laser emission occurs, the current distribution is almost determined by the distribution of series resistance outside the junction. If these series resistances are made very small to be equal to or smaller than the resistance of the junction, the resistance will be low in areas where the light is strong and a large amount of current will flow, and the resistance will hardly decrease in areas where the light is weak, so the current will flow. A current distribution is generated corresponding to the intensity distribution of the standing wave of light, such that the current distribution decreases. For example, FIG. 4 is a simple diagram for explanation. layer 4
4 is an active layer portion for guiding light, and layer 41 and layer 4
3 indicates the low refractive index portion sandwiching it, and as shown in FIG. 1, the layer 41 is the layers 11 and 12,
Layer 43 corresponds to layers 14 and 15. An arrow 44 indicates that current flows in the direction of the arrow, with a large amount of current flowing in the area indicated by the arrow, and a wavy line 45 indicates a standing wave of light. The direction in which the standing wave stands is the direction in which the light travels. As shown in the figure, when a laser is first emitted and a standing wave 45 of light is created in the resonator, the electric field strength is strong at the antinode part 46 of the standing wave, and the electric field strength is weak at the node part 47. If the laser diode has a series resistance that is very small compared to the resistance of the junction, the current will naturally concentrate where the electric field strength is strong, and the current flowing through the weak electric field strength will decrease, causing the current to decrease. Utilization efficiency increases. Since the current is concentrated at the antinode of the light, the gain for light is large in that part, and the gain for light is small at the nodes, and the gain for light becomes periodic, so light with a wavelength that does not match this period is Instead of being amplified, only the light with a wavelength that matches the period of the standing wave is amplified, resulting in laser emission with a single wavelength, and the efficiency of stimulated radiation is also extremely high. Naturally, the luminous output obtained in single mode is also increased. However, in a normal laser diode, as mentioned above, due to the presence of a large series resistance, the current is rather determined by the series resistance, and in a laser diode with a normal structure, the current flows uniformly through the active layer. Even if the existing wave of one wavelength is generated, the current distribution is uniform, so the gain left at the nodes of the standing wave described above can be used to emit laser light of various other axial mode wavelengths. It happens. That is, high-intensity light emission of a single wavelength is suppressed. Therefore, in order to solve this problem, by giving a spatial period to the resistor in advance, creating a current distribution, and giving a period to the gain, it is possible to create a semiconductor with a large limit on the output that can be obtained in a single wavelength oscillation and in a single mode. An injection laser is obtained. When a current is passed with a predetermined spatial period, the oscillation mode becomes stable, the luminous efficiency is high, temperature dependence and current injection level dependence are extremely small, and stable laser light can be obtained. Examples thereof are shown in FIGS. 5a, b, c, and d. The basic structure is the same as that shown in Figure 1, but with a spatial distribution of resistance. However, the spatial distribution of resistance must not extend to the layer that traps light, electrons, and holes (active layer: stimulated light emission region). This is because during manufacturing, when techniques such as ion implantation, ion milling, chemical etching, or selective diffusion are used to create the period, defects are likely to occur, so it is necessary to prevent them before they reach the active layer. In this case, the luminous efficiency deteriorates, making it impossible to manufacture a semiconductor injection laser with good characteristics. That is, it must be limited to only the area close to the stimulated radiation area. for example,
Taking the material GaAs (1.43 eV) as an example, its oscillation wavelength is about λ ₀ = 8500 Å in free space, so the refractive index is approximately n = 3.6, and GaAs
Within this range, the basic period should be Λ=1180 mÅ. m is an integer value, m=1, 2, 3, 4. Currently, due to manufacturing technology, m = 3,
It forms a periodic structure with an interval of about Λ = 3500 Å. Of course, at present, experiments with Λ=1200Å have also been successfully conducted. However, this period varies depending on the material used, and InSb (forbidden band width 0.18eV (300〓))
If a semiconductor laser is made of InAs (bandwidth forbidden: 0.35 eV (300〓)), the oscillation wavelength will be longer, so it can be manufactured with a smaller value of m. (The smaller m is, the better the Bragg reflection efficiency is for objects with a refractive index distribution, and the higher the current utilization efficiency is for objects with a current distribution type.) In the case of InSb , 300〓,
Λ=about 8100mÅ. (m: integer) If this is the case, a periodic structure can be easily created with m=1. Furthermore, ternary systems such as PbSnTe and HgCdTe can also be produced with long wavelengths. For example, x=0.2 for Pb _1-x Sn _x Te
, the wavelength in free space is about 10 μm, and the refractive index is slightly larger than 4, so the period is approximately Λ = (1.1 m) μm (m: integer),
The periodic structure can be manufactured very easily. Ternary systems (in addition to the above, InGaP, GaPSb, GaAlSb, etc.) and four-element systems
Element system (InGaAsP, GaAlSbAs, AlGaInAs, etc.)
Then, by changing the ratio of the components, the period changes. In particular, in the case of long-wavelength materials such as PbSnTe and HgCdTe, when the current flows uniformly, the distance between the node and the antinode becomes long. It is rare for the current distribution shape of the present invention to increase the efficiency of current utilization, since it is rare for the current distribution shape of the present invention to increase the efficiency by spreading to the antinodes of the standing waves.

In FIG. 5a, hydrogen molecules or the like are implanted into the upper two layers to periodically form high-resistance layers. A black striped area 56 represents the ion-implanted area, and 11, 12, 13, 14, and 15 are the same as in FIG. When ion implantation is performed, defects are created in the region, but they can be recovered considerably by heat treatment at a temperature of only 200 to 400 degrees Celsius, which is much lower than the growth temperature, for example, 800 degrees Celsius. Furthermore, since the region where the defect occurs is a high-resistance layer, almost no current flows, and since the ion implantation has not reached the active layer, there is no light, so the semiconductor implanted laser is susceptible to deterioration. has almost no effect. or,
For ion implantation, it is possible to use not only hydrogen, but also sulfur, silicon, beryllium, cadmium, etc. depending on the situation. (Even a small defect can become a large defect if current flows or strong light is present, causing the semiconductor injection laser to deteriorate.)
In addition, for example, Zn or the like may be selectively diffused periodically.

The optical resonator end face of a semiconductor injection laser is the antinode of the standing wave of light, so m: (integer value) has an odd period (a period that is an odd multiple of the half wavelength). In the case of , when the peak is cut in the middle of the high resistance region, the antinode of the standing wave coincides with the region where a large amount of current flows, and the gain of light with a wavelength that matches the predetermined period is high. Become. However, when m is an even number, the antinode of the standing wave deviates from the middle position of the 1/4 wavelength high resistance region, so care must be taken in the position of the cleavage plane.

However, if the position of the resonator end face deviates from the position corresponding to the antinode of the standing wave, no current will flow at the antinode of the standing wave. In extreme cases, a large amount of current may flow at the nodes of the standing wave. In this case, the efficiency becomes worse than when the current flows uniformly over the entire surface, the threshold current increases, and characteristics such as light emission mode deteriorate. Therefore, the position of the resonator end face is extremely important, and the position of the antinode of the standing wave must match the cleavage surface. However, matching includes not only exact matching but also almost matching. The same applies to the following structures. Figure 5b is the 3rd layer day 14
We perform ion implantation only for this reason. Growth may be stopped at the third layer, ion implantation may be performed, and growth may be performed thereon, or ion implantation may be performed after growth up to the fourth layer 15. In addition to ion implantation, growth may be stopped at the third layer and selective diffusion may be performed. The depth at which ions penetrate into the material is approximately determined by the acceleration energy when they are implanted, and the ions are scattered in an approximately Gaussian distribution in the depth direction centered on the average depth. Of course, the depth will vary depending on the ions being implanted and the material being implanted. When light atomic ions are implanted with high energy, they can be implanted into deep places.

FIG. 5c shows that only the top layer 15 is periodically removed by ion milling plasma etching or chemical etching, and then the third layer 14 is etched periodically.
A resistive contact is made by depositing metal on top of the exposed surface. However, what is used here is that the third layer 14 is p-Ga _{1-x Al x As and the fourth layer 15 is p-Ga 1-x} Al _x As.
Since it is p-GaAs, there is a difference of approximately two orders of magnitude in the contact resistance between the two, and the portion in contact with the third layer 14 has a high resistance and no current flows. 4th layer 1
Where it is in contact with 5, the resistance is low and current flows. In this way, periodic high resistance regions are created due to the contact resistance difference between the materials, and a periodic current is caused to flow. Similar things can be done with other materials. On the contrary, layer 1
If a material with low contact resistance is used at point 4, the current distribution will be opposite to the above.

In FIG. 5d, the third layer p-Ga _1-x Al _x As 14 is given periodicity. p-GaAs15 is p-
Ga _1-x Al _x As the resistance is lower than that of As14, the third layer14
Where it appears, the resistance is high and it becomes difficult for current to flow. This structure is similar to Fig. 2, but
24 in the figure has a small AlAs content (y < x) and is used as an optical waveguide, but in Fig. 5 d, the AlAs content is large (x≒0.3), so it cannot be used as an optical waveguide. First, there is no blur reflection of light due to this, and by periodically arranging high-resistance regions and low-resistance regions using the periodic structure, it is possible to control the magnitude of the current and obtain a periodic gain distribution.

I will briefly explain the manufacturing method using the following materials (GaAs, GaAlAs).

Crystal growth is carried out by, for example, using a liquid phase epitaxial method to grow a first crystal on a Te-doped (10 ¹⁸ - 10 ¹⁹ cm ^-3 ) GaAs substrate.
Layer 5 × 10 ¹⁷ cm ^-3 With addition of _{Te, Ga 0.7 Al 0.3 As} (3
~5 μm thick), second layer, GaAs (0.1 μm ~ 0.5 μm) layer with no intentional additions. The third layer is a Ga _0.7 Al 0.3 As layer ( _{~1 μm) to which Ge is added to approximately 5×10 17 cm -3.The fourth layer is a Ga 0.7} ^{Al 0.3} _{As layer} (~1 μm) to which Ge is added to approximately 10 ¹⁸ cm ^-3 .
Grow a GaAs layer (0.5 μm to 1 μm). Only in FIG. 5D, in order to insert a periodic structure in the middle, it is necessary to stop the growth once up to the third layer, create the periodic structure, and then grow again. You can create a periodic structure after all the others have grown.

A photoresist (e.g.
AZ1350) and rotate at 6000 rpm to make a film thickness of about 3000 Å. Bake this at 125℃ for 2.5 minutes to make it stick. After that, a single-frequency Ar ⁺ laser (for example, 4579 Å) is split into two beams of equal intensity, and when they are passed through a 10 μm wide slit and made to interfere on the photoresist at an angle of about 40 degrees, interference occurs. The period of the stripes is approximately 3600 Å. Expose for 12 seconds at approximately 2.5 mW/cm ² . By soaking it in undiluted AZ developer for 1 to 2 minutes,
Periodic stripes are formed. In this way, a photoresist mask is created. Depending on the purpose, conditions such as the thickness of the photoresist can be changed, and the spacing between stripes can also be easily changed. The subsequent steps differ depending on each structure, and in the case of FIGS. 5a and 5b, ion implantation and selective diffusion are performed using this as a mask.

FIG. 5c, this can be done by ion milling or chemical etching. All that remains is to attach the electrodes.

Above, we have mainly talked about semiconductor injection lasers with a double heterostructure, but this structure can also be applied to semiconductor injection lasers with a single heterostructure with only one heterojunction and a homojunction structure with no heterojunction. can.

Although the laser has been described above as a semiconductor injection laser, it can also be used as an amplification element, modulation element, parametric amplifier, etc. in an optical integrated circuit or during optical transmission by passing a current below a threshold value. In this case, since only the wavelength determined by the periodic structure is amplified, it can function as a selective amplification element or a modulation element for a selective wavelength, and can be used as various semiconductor optical devices on an optical waveguide. Needless to say, the present invention includes other applicable methods as long as they do not deviate from the spirit of the present invention.

[Brief explanation of the drawing]

1, 2, and 3 are examples of conventional semiconductor optical devices, FIG. 4 is a diagram of a standing wave of light in a resonator, and FIGS. This is an example with periodic distribution.

Claims (1)

[Claims] 1. On at least a semiconductor substrate of a first conductivity type, a first semiconductor layer of a first conductivity type, an active layer of a first conductivity type or a second conductivity type of a conductivity type opposite to the first conductivity type. ,
Only the second semiconductor layer is a semiconductor layer in which second and third semiconductor layers of the second conductivity type are stacked substantially in parallel, and the forbidden band width of the first and second semiconductor layers is larger than the forbidden band width of the active layer; In a semiconductor optical device, regions of a first conductivity type of opposite conductivity type are arranged in the second and third semiconductor layers at a period of an integral multiple of a half wavelength of light in the direction of propagation of light. and a pair of optical resonator end faces perpendicular to the light propagation direction are regions of the first conductivity type of the opposite conductivity type in only the second semiconductor layer arranged at the period or in the second and third semiconductor layers. 1. A semiconductor optical device characterized in that the semiconductor optical device is formed approximately at the center of a semiconductor layer having a second conductivity type intermediate between . 2. At least a first
a first conductivity type semiconductor layer, a first conductivity type or a first conductivity type semiconductor layer;
An active layer of a second conductivity type opposite to the conductivity type, and second and third semiconductor layers of the second conductivity type are stacked almost in parallel, and the forbidden band width of the first and second semiconductor layers is equal to that of the active layer. In a semiconductor optical device in which the third semiconductor layer is a semiconductor layer larger than the forbidden band width and is removed in the light propagation direction at a period that is an integral multiple of the half wavelength of light, the third semiconductor layer is perpendicular to the semiconductor layer and perpendicular to the light propagation direction. A semiconductor optical device characterized in that a pair of optical resonator end faces are formed substantially in the center of a remaining region of the third semiconductor layer removed at the period. 3 At least a first
a first conductivity type semiconductor layer, a first conductivity type or a first conductivity type semiconductor layer;
an active layer of a second conductivity type opposite the conductivity type;
The second and third semiconductor layers of the second conductivity type are stacked almost in parallel, and the forbidden band width of the first and second semiconductor layers is larger than that of the active layer. In a semiconductor optical device that has a concave-convex structure with a part thickened in the light propagation direction at a period of an integral multiple of the half wavelength of the light, a pair of optical resonator end faces perpendicular to the semiconductor layer and perpendicular to the light propagation direction are used. is formed substantially at the center of the periodic convex portion of the second semiconductor layer.