JPH03284891A - Optical semiconductor element - Google Patents

Abstract

PURPOSE:To perform integration of modulators allowing superhigh-frequency modulation with low voltage by constituting an optical absorbing amount of MQW of wavelength having exciton wavelength and an optical waveguide layer of MQW of wavelength, whose exciton wavelength is longer than the above. CONSTITUTION:For instance, a GaInAs waveguide layer 3 of composition 1.3mum is provided on an InP substrate 1, and a grating 2 is formed in the laser part at the interface both of them. In a modulator part, an MQW layer 23 consisting of a GaInAs layer having the thickness 70Angstrom is provided, for instance, on the ten-layered waveguide layer 3. Each layer generates a level corresponding to exciton wavelength of 1.53mum. And, the thickness of a non-doped InP host layer 24 between the respective MQW layers is made 100Angstrom . On the other hand, in the laser part, the GaInAs layer consisting of MQW 23' shall have its constitution the same with the MQW layer 23 of the modulator part and the number of layers shall be equally ten-layered while the thickness of each layer shall be thickened to 120Angstrom . Thereby, the exciton wavelength of each layer becomes 1.55m. A p-InP clad layer 5, a cap layer 12 of p<+>-GaInAsP and a positive electrode 13 are provided on the upper part of both MQW construction.

Description

[Detailed Description of the Invention] [Summary] The present invention relates to an optical semiconductor device in which a semiconductor laser for optical communication and an optical modulator are integrally formed. With the aim of creating a structure that can be formed integrally, the optical waveguide layer of the laser and the optical absorption layer of the modulator, which are optically coupled, are formed of semiconductor layers with the same composition, and the thicknesses of the constituent layers are different. By doing so, the exciton wavelength on the laser side is made longer than that on the modulator side.

[Industrial application field]

The present invention relates to a semiconductor laser for optical communication, and more particularly to an optical semiconductor device in which a semiconductor laser and an optical modulator are integrally formed.

In optical communication using a semiconductor laser as a signal light source, conventionally, the laser is intermittently oscillated using an excitation current corresponding to a signal waveform, and then transmitted through an optical fiber. In such an internal modulation method, when ultra-high-speed modulation of nearly 1 OGHz is performed, relaxation oscillation occurs, and fluctuations in the oscillation wavelength called chirping are unavoidable. This makes it difficult to extend the transmission distance.

As a countermeasure, attempts have been made to employ an external modulation method in which a laser is used as a DC light source that continuously oscillates light of a constant wavelength, and an optical signal is obtained by an optical modulator coupled to the laser. As an optical semiconductor element used in this case, one in which a DC laser light source and an external modulator are combined into one chip is known.

[Prior Art and Problems to be Solved by the Invention] An example of a device in which a DC laser light source and an external modulator are combined into one chip is shown in FIG.

The cross-sectional structure of this layer is schematically shown, with the region on the left being a laser and the region on the right being an optical modulator.

Both are formed on a common InP substrate 1, and in the laser part, the interface with the waveguide layer 3 thereon is a diffraction grating, and D
It has been given the function of FB relay. Further, 4 is an active layer, and 5 is a cladding layer, and the intensity of the oscillated light is distributed around the active layer as indicated by a thick solid line 9 in the figure.

- On the power end modulator side, an absorption layer 6 is formed on top of the waveguide layer 3 and etching stop layer 15 in order to align the height with the active layer of the laser, and the scattering layer is the same as the waveguide layer. Optically coupled. In other areas, 5 is the cladding layer, 12
is the cap layer, 13 is the positive electrode, 14 is the negative electrode, 15 is A
This is an R coating film, and the high resistance isolation layer 7 is provided to electrically isolate the two cladding layers.

When an integrated element with such a structure is formed based on an optimal design, an optical output of 15 mW or more can be obtained, a cutoff frequency of 10 GHz, and a wavelength chirp of 0. Although it is possible to achieve a characteristic of less than IA, the operating voltage requires approximately 3V. It is extremely difficult with current technology to integrate a drive circuit that can output a voltage of 3 V at a frequency of about 10 GHz, and a modulator that can apply intensity modulation to signal light with a lower control voltage is desired. It will be done.

A modulator that meets these demands is a multiple quantum well (M
A modulator using QW) has been proposed.

As shown schematically in Figure 3, this is the cross-sectional structure.
The light absorption layer has an MQW structure.

In this layer, l is an n-InP substrate, 5 is a p-InP cladding layer, 12 is an I)-GalnAsP cap layer, 13 is a positive electrode, 14 is a negative electrode, and a non-doped Ga1nAsP layer 21 (1 , 6 μm composition) and non-doped InP layers 22 are alternately stacked to form an MQW layer. As mentioned above, it is common practice to express the composition of a semiconductor layer by length, and the band gap value, which changes depending on the composition, is expressed by an equivalent wavelength of light.

The optical modulator used in the device shown in Figure 5 utilizes the effective change in the bandgap of the light absorption layer by applying a reverse bias voltage, whereas the MQW modulator uses the change in the conduction band and value. The level difference created in the electron band well is changed by applying a reverse bias voltage. The fluctuation of the absorption edge wavelength in exciton absorption has a larger fluctuation rate with respect to the applied voltage than when changing the band gap, so even if there is a limit to the output voltage value of the drive circuit, Frequency modulation becomes possible.

Forming such an MQW modulator alone requires the M.O.
It's not that difficult if you use VPE technology. However, it is extremely difficult to integrate and form the laser as a DC light source. In the example in Figure 5, the optical modulator part is MQ
In order to form the W structure, the portions above the leading wave layer 5 or the etching stop layer 15 must be formed separately for the laser section and the modulator section. It is almost impossible to optically couple the absorbing layer 6 of .

An object of the present invention is to provide a semiconductor laser that integrates a modulator capable of ultra-high frequency modulation at a lower voltage, and such a semiconductor laser and optical modulator can be integrated without complicated manufacturing processes. An object of the present invention is to provide an optical semiconductor device.

[Means to solve the problem]

In order to achieve the above object, the optical semiconductor device of the present invention includes a semiconductor laser and an optical modulator that are integrally formed, and an optical waveguide layer of the semiconductor laser and an optical absorption layer of the optical modulator that are optically coupled. An optical semiconductor device, wherein the light absorption layer is made of a first MQW whose exciton wavelength is a first wavelength, and the optical waveguide layer is composed of a first MQW whose exciton wavelength is a second wavelength longer than the first wavelength. It is configured by a certain second MQW.

In order for the optical semiconductor device to satisfy the above-mentioned conditions related to the exciton wavelength, both the MQWs are composed of a group of semiconductor layers having the same composition, and the thickness of the well layer among the semiconductor layers constituting the second MQW is The thickness is greater than the thickness of the well layer constituting the first MQW.

[For production]

First, the energy band structure of MQW and the situation in which exciton absorption occurs will be briefly explained.

When a second semiconductor layer (well layer) with a narrower bandgap exists in a host crystal (barrier layer) with a bandgap value of . A depression is formed in the conduction band in the second semiconductor layer, but if the thickness of the diffused layer is small, a subband is generated in this depression, and electrons exist at this level. The thicker the layer, the closer the subband is to the bottom of the conduction band.

This is called a quantum well, and similar levels occur in the valence band, and the recombination energy of electron/hole pairs is equal to the energy difference E between the two levels. It becomes 1. An MQW is one in which semiconductor layers with different band gaps are alternately stacked in this way to create a plurality of quantum wells.

When a bias voltage is applied across the quantum well layer, the band structure changes as shown in Figure 4 (bl), and in the well region, the recombination energy of electron/hole pairs decreases from the above Ec+ and becomes EGI. becomes.

Corresponding to the light transmission characteristics, when no bias voltage is applied, the wavelength of the absorption edge is λ corresponding to Ec+
1, but by applying a bias voltage, it changes to this wavelength or E. , changes to λ2 corresponding to . That is, it shifts to the long wavelength side. Light absorption due to recombination between quantum well levels as mentioned above is exciton absorption, and its absorption edge wavelength is called exciton wavelength. Exciton absorption occurs when the band gap is changed by applying a bias voltage. In comparison, it shows a more acute wavelength shift.

The optical semiconductor device of the present invention has a structure whose cross-sectional schematic diagram is shown in FIG. As clearly shown in the figure, the active layer of the laser in this device has an MQW structure. M in the laser section
In QW, the thickness of the quantum well layer is larger than the thickness of the MQW constituent layers of the modulator section. That is, when comparing the energy gaps of the levels generated in the quantum wells, the energy gap of the modulator section is larger than that of the laser section.

This means that the oscillation wavelength of the laser is longer than the exciton wavelength of the modulator when no bias is applied, and the laser oscillation light is output to the outside without being absorbed by the MQW layer, which is the absorption layer of the modulator. It means. However, the difference between the laser oscillation wavelength and the absorption edge wavelength of the modulator when no bias is applied is 0.
．． When a reverse bias voltage is applied to the modulator, the exciton wavelength shifts to a longer wavelength side than the oscillation wavelength of the laser, so the laser output light is absorbed by the modulator and transmitted to the outside. is not output. That is, it is possible to modulate the DC output light of the laser by applying a signal voltage to the modulator.

As already mentioned, the drive voltage of the MQW modulator is not required to be any voltage value when driving a modulator with a normal structure, so ultra-high frequency modulation by an integrated drive circuit is possible, and the integrated drive circuit of the present invention The type optical semiconductor device can be formed by a known processing method, as explained in the Examples section.

〔Example〕

FIG. 1 shows an optical semiconductor device according to an embodiment of the present invention in which a semiconductor laser operating at a wavelength of 1.55 μm and an optical modulator are integrated. Ga1n with a composition of 1.3 μm on an InP substrate
Similar to the integrated device shown in FIG. 5, an As waveguide layer 3 is provided and the interface between the two forms a diffraction grating in the laser section.

In the modulator section, a non-doped InP layer 24 is used as a matrix,
An MQW structure consisting of a 70 nm thick Ga InAs layer 23 is provided on the waveguide layer. The composition of the GaInAs layer produces a level corresponding to an exciton wavelength of 1.53 μm at the thickness. The figure shows only two scattered layers, but in the example there are 10 layers, and this MQ
The thickness of the matrix layer 24 between the W layers is 100 mm. On the top of the MQW structure, the same p-InP cladding layer 5, p
” -GaInAsP cap layer 12, positive electrode 1
3 is provided, and 16 is an AR coating film.

On the other hand, in the laser section, the GaInAs layer 23 constituting the MQW
′ is 3′ on top of the 2nd MQW layer in the modulator section, and the number of layers is also the same <10 layers, but the thickness of each layer is 120. As a result, The exciton wavelength of the scattered layer is 1.55 μm.

On top of this MQW structure, a p-InP cladding layer 5, p
It is the same as the device shown in FIG. 5 in that a cap layer 12 of -GaInAsP and a positive electrode 13 are provided, and also in that a negative electrode 14 is provided in common with the modulator section. In the above structure, the InP layer 24, which is the host crystal of the MQW structure, has a 1.15 μm and 5 μm composition InAsP.
It may be.

In this way, an MQW structure with the same composition but only partially changed thickness can be formed by repeating normal epitaxial growth and selective etching, but the growth rate in selective growth depends on the growth area. It is also possible to form it using gender.

In the selective growth of a band-shaped region, the growth rate increases when the width of the aperture is narrow, and decreases when the width of the aperture is wide.

The narrow band gap layer constituting the MQW will be shaped into a band shape later by mesa etching, so it is not necessary to form it to cover the entire surface of the laser section and modulator section (it is not necessary to form it in the same shape in the image section). Therefore, in the epitaxial growth of the scattered layer, the growth substrate is selectively masked with a film such as 5iOz, and the width of the band-shaped opening is varied to perform selective growth.
It can be grown thickly in the laser part and thinly in the modulator part.

A perspective view of the optical semiconductor device of the above embodiment is shown in FIG. 2, where IO is a DFB laser portion and 11 is an optical modulator portion.

〔Effect of the invention〕

Since the optical semiconductor device of the present invention has a modulator portion that is an MQW modulator, it can be driven at a lower voltage than a modulator with a normal structure, and ultra-high frequency modulation that reduces the output voltage of the drive circuit is possible. . Furthermore, since the active layer of the laser portion of the optical semiconductor device of the present invention has an MQW structure, it is easy to form a structure integrated with the modulator.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing the structure of an embodiment of the present invention. FIG. 2 is a perspective view of an embodiment of the present invention. FIG. 3 is a schematic cross-sectional view showing the structure of an MQW modulator. Figure 5 is a schematic cross-sectional diagram showing a conventional integrated optical signal generator, in which I is an InP substrate, 2 is a diffraction grating, 3 is a waveguide layer, and 4 is a guide layer. wave layer, 5 is a cladding layer, 6 is an absorption layer, 7 is a high resistance separation layer, 9 is an optical distribution curve, 10 is a DFB laser, 11 is an optical modulator, 12 is a cap layer, 13 is a positive electrode, 14 is a negative electrode 15 is an etching stop layer, 16 is an AR coating film, 21, 23.23' are MQW layers, and 22, 24 are host crystals. 'Schematic cross-sectional diagram showing the structure of an MQW modulator Figure 3 (a) (b) Diagram showing the energy level of MQW Figure 4 Schematic cross-sectional diagram showing the structure of an embodiment of the present invention Figure 1 Implementation of the present invention Fig. 2 is a perspective view of an example. Coating film Fig. 5 is a schematic diagram showing a conventional integrated optical signal generator.

Claims (2)

[Claims] (1) A semiconductor laser and an optical modulator are integrally formed, and the optical waveguide layer of the semiconductor laser and the optical absorption layer of the optical modulator are composed of multiple quantum wells having a multiple structure of well layers and barrier layers, and an optical semiconductor element in which the two are optically coupled, the light absorption layer having a first layer having an exciton wavelength of the first wavelength.
The optical waveguide layer includes a second wavelength whose exciton wavelength is longer than the first wavelength.
An optical semiconductor device comprising multiple quantum wells. (2) The optical semiconductor device according to claim 1, wherein the second multiple quantum well is constituted by a group of semiconductor layers having the same composition as the first multiple quantum well; An optical semiconductor device characterized in that the thickness of the well layer is greater than the thickness of the well layer constituting the first multiple quantum well.