JP4072107B2 - Semiconductor optical modulator and laser with optical modulator - Google Patents

Description

  The present invention relates to an optical waveguide type semiconductor optical modulator and a laser with an optical modulator, which are used for optical communication in a wavelength band of 1.3 to 1.5 μm, for example, using a glass fiber as a transmission medium.

In recent years, with the development of an advanced information society, optical communication networks using optical fibers have been developed around the world, and many communications centered on telephones have been made by optical communication.
In order to realize optical communication using an optical fiber, it is necessary to convert an electrical signal into an optical signal using a certain conversion circuit. A device that performs such electro-optical conversion is an optical modulator.

  There are two types of optical modulators that perform electro-optical conversion: direct modulation and external modulation. The direct modulation method uses a semiconductor laser (LD) to change the modulation signal as it is and to change the intensity of the light source. On the other hand, the external modulation method is a method of applying modulation from the outside to the output light from the semiconductor laser that is operating in direct current (CW).

   Direct modulation using a semiconductor laser has the advantage that the configuration is simple and the size can be reduced. For this reason, direct modulation using a semiconductor laser has been widely used. However, in this modulation method, when the frequency becomes several GHz or higher, the transmission speed can be limited due to the chirping phenomenon of the semiconductor laser. Chirping means that during high-speed modulation (several GHz or more) of a semiconductor laser, the refractive index of the active layer varies with the time variation of the carrier, and the resonance wavelength of the laser resonator changes. It is a phenomenon that fluctuates (wavelength fluctuation, relaxation oscillation).

On the other hand, the external modulator modulates the stable light from the semiconductor laser operating in direct current by the electro-optic effect, etc., so that there is no problem of chirping and enables high-speed transmission over a long distance. .
In optical communication, signal light having a wavelength band of 1.3 to 1.5 μm is used, and a silica glass fiber is mainly used as a transmission medium. In such optical communication, for example, a semiconductor optical modulator made of a compound semiconductor (crystal) such as InGaAlAs or InGaAsP is used (see Patent Document 1).

  In recent years, semiconductor multi-quantum well structures and superlattice structures have emerged as a result of the development of extremely thin compound semiconductor thin films such as molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE). Yes. With these structures, it is possible to significantly improve the characteristics of the optical element, such as an increase in modulation speed, as compared with conventional bulk semiconductors. Among these, the electroabsorption effect in which a multiple quantum well structure is adopted for the light absorption layer and an electric field is applied thereto to change the absorption coefficient, the improvement of the characteristics is very remarkable as compared with the bulk semiconductor.

  By using these characteristics, an element capable of being driven at a higher speed and at a lower voltage is realized as the semiconductor optical modulator described above. This semiconductor optical modulator modulates the input signal light (light source) by applying a reverse bias electric signal to the light absorption layer having a multiple quantum well structure and shifting the wavelength of the absorption edge of the exciton, Electric signals are converted into optical signals.

The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
JP-A-8-86987 Fabio Bernardini, Vincenzo Fiorentini, and David Vanderbilt "Spontaneous Polarization and piezoelectric constants of III-V nitrides", Phys. Rev. B, 56 (1997) R10024. K. Osamura, K. Nakajima and Y. Murakami, "Fundamental absorption edge GaN, Inn and their alloys" Solid State Comm., 11 (1972) 617 N.Puychevrier and M.Menoret, "Synthesis of III-V semiconductor nitrides by reactive cathodic sputtering" Thin Solid Films, 36 (1976) 141 TLTansley and CPFo1ey, "Optical band gap of indium nitride" J. App1.Phys., 59 (1986) 3241. T. Matsuoka, H. Tanaka, T. Sasaki and A. Katsui, "Wide-Gap Semiconductor (In, Ga) N", International Symposium on GaAs and Related Compounds, (Karuizawa, Japan, 1989); in Inst. Phys. Conf.Ser., 106.pp.141-146 T. Matsuoka, "Phase Separation in Wurtzite In1-X-YGaXAlYN", MRS Internet J. Nitride Semicond. Res. 3, 54 (1998)

However, the conventional semiconductor optical modulator has the following problems.
First, an element using an InGaAsP-based material has a problem that it is easily affected by the operating temperature because the band discontinuity in the conduction band in the band structure is small. For this reason, at present, an element using an InGaAsP-based material having a zinc blende type crystal structure has a problem that a characteristic temperature, which is one of practically important characteristics, is low. InGaAsP-based materials have a large Auger effect because there is no energy gap between heavy holes and light holes at the Γ point of the valence band, that is, spin split-off energy Δsp. As a result, the element using the InGaAsP material has poor temperature characteristics.

  Further, in the conventional semiconductor optical modulator described above, it is required to increase the wavelength shift amount, but in order to increase the shift amount, it is necessary to apply a large voltage as the above-described reverse bias electric signal. It becomes. This is because at present, it is almost impossible to make the light absorption layer in an ideal state without carriers. For example, impurities may enter through diffusion from p-type semiconductor layers and n-type semiconductor layers above and below the light absorption layer. Thus, in reality, carriers exist in the light absorption layer, and the built-in voltage generated by the pn junction cannot completely deplete the light absorption layer. As a result, a part of the electric signal applied to the light absorption layer is consumed for depletion of the light absorption layer. This portion consumed for depletion is required as a reverse bias electric signal.

Therefore, first, the conventional semiconductor optical modulator is required to withstand a high reverse bias. However, it is not easy to grow a high-purity crystal having a large withstand voltage against a reverse voltage.
In addition, since an ultra-high speed electronic circuit that operates the optical modulator cannot generate a large voltage, the operating voltage of the optical modulator needs to be as low as possible. For this reason, the metal electrode provided in the semiconductor optical modulator needs to be in an ohmic junction state with the compound semiconductor layer constituting the semiconductor optical modulator.

  This means that the semiconductor layer in contact with the metal electrode is in a high impurity concentration state, and the active region made of the above-described high-purity crystal is sandwiched between the high impurity concentration layers. In an external modulator for optical communication, among materials that can be selected as layers sandwiching a quantum well structure, InP has the maximum band gap energy, and this band gap energy is 1.42 eV. With such a configuration, if a large reverse bias is applied in order to increase the degree of modulation, the layers around the quantum well may cause dielectric breakdown.

  Fabrication of such a structure in which a high-purity crystal layer and a crystal layer with a high impurity concentration are stacked requires an expensive crystal growth apparatus, and the process for growing the crystal layer is complicated and not easy. . Furthermore, many steps are required for the element manufacturing process following the crystal growth. As a result, the above-described conventional semiconductor optical modulator requires a great deal of cost for device fabrication, and the yield of device fabrication is low, making it an expensive device.

  Conventionally, it is necessary to simultaneously apply a DC reverse bias and a high-frequency electrical signal to the semiconductor optical modulator. For this reason, it is necessary to use an electronic circuit component called a bias T. For example, when the semiconductor optical modulator is applied to a communication speed of 10 Gbit / sec, the output band required for the electronic circuit component is DC to 60 GHz, which is very expensive. In addition, it is difficult to obtain electronic circuit components having good characteristics even when the output band is DC.

Here, a general configuration and operation of a conventional optical modulator will be described in more detail.
Conventional optical modulators have a crystal structure in which a lower clad layer, an impurity-free multiple quantum well layer, an upper clad layer, and a p-type contact layer are sequentially laminated on a substrate. The multiple quantum well layer functions as an absorption layer that absorbs light, and the upper and lower cladding layers constitute an optical waveguide. A p-electrode is formed on the contact layer, and an n-electrode is formed below the substrate.

  The light incident from the light incident end face at one end of the optical waveguide is ON / OFF-modulated and propagates from the light emitting end face opposite to the light incident end face while propagating through the optical waveguide. Fig. 14 shows the dependence of the light absorption characteristics on the wavelength λ of the incident light when no voltage is applied between the p electrode and the n electrode (OFF) and when a reverse bias is applied (ON). In FIG. 14, the wavelength λ of light incident on this optical modulator is shown as the operating wavelength λs.

  When no voltage is applied, the light absorption coefficient at the operating wavelength λs is sufficiently small, and incident light is emitted as it is from the light exit surface. If a sufficient reverse bias is applied to this state, the absorption characteristics of the light absorption layer shift to the longer wavelength side, so that light absorption increases and the light is not emitted from the emission end. In this way, by turning on / off the reverse bias voltage, the light propagating through the optical waveguide is also turned off / on, and an electric signal can be converted into an optical signal.

  The semiconductor layer structure constituting the optical modulator is formed by crystal growth, but p and n impurities in the cladding layers above and below the light absorption layer diffuse during the growth process, and the diffused impurities enter the light absorption region. ing. For this reason, the light absorption region is not completely depleted by the built-in voltage generated at the pn junction. Therefore, a sufficient electric field is not generated in the multiple quantum well layer (light absorption layer) that controls the quenching operation. Even if light absorption occurs once, the charge generated by light absorption remains, and more No light absorption occurs.

  In other words, light absorption hardly occurs in the conventional optical modulator. On the other hand, when a reverse bias voltage is applied from the outside, the multiple quantum well layer is depleted, light absorption increases as the reverse bias voltage applied increases, and quenching increases. The explanation for this situation is the voltage dependence of the extinction ratio in Fig. 15. Therefore, for optimum operation, it is necessary to make it completely depleted by applying a reverse bias voltage from the outside up to the point of the bias point shown in Fig. 15, and an additional circuit such as a bias T for this purpose. Is essential for optimal operation.

  In addition, when there is an SCH layer that forms a separate confinement heterostructure between the cladding layer and the multiple quantum well structure, even if a reverse bias voltage is applied from the outside, the upper part sandwiching the light absorption layer The lower SCH layer is in a state where a weak electric field that is finally depleted is applied. For this reason, in the optical modulator having the SCH structure, carriers generated by absorption in the light absorption layer or the like are extracted only by a weak electric field, and the traveling speed of the carriers is low. Furthermore, the hetero barrier present between the SCH layer and the clad layer hinders carrier travel, and increases the carrier accumulation.

  Therefore, when modulating high-intensity light, the amount of carriers generated by light absorption is larger than the amount of carriers that can be extracted, resulting in high-speed operation due to the non-uniformity of the electric field caused by the accumulated carriers and the electric field shielding effect. Cannot be obtained. In order to suppress such carrier accumulation, there is a technique of forming a SCH layer with a semiconductor layer having a graded composition or a staircase composition, and performing carrier extraction as smoothly as possible. However, with such a configuration, crystal growth becomes extremely complicated, and an extremely high-precision growth technique is required, which makes it difficult to manufacture the device.

  Further, in the conventional device, the upper and lower SCH layers are not less than the diffusion length so that even if the p and n impurities in the upper and lower cladding layers diffuse, the multiple quantum well layer that is the modulation layer does not reach at a high concentration. To reduce the modulation function. For this reason, in the modulation operation state by the modulation voltage, the modulation voltage is applied to the whole of the upper and lower SCH layers and the multiple quantum well layer, but the multiple quantum well layer governs the light modulation characteristics as an effective voltage component. Only a part of this modulation voltage is applied. The components divided by the upper and lower SCH layers are reactive voltage components that do not directly contribute to the modulation (quenching) characteristics.

Further, although it is possible to operate in a state where a reverse bias voltage is not applied from the outside, a driving voltage having an extremely large amplitude corresponding to the bias is required. Furthermore, in this case, until the multiple quantum well layer is completely depleted, the electric field intensity is not linear with respect to the voltage, but is nonlinear. Therefore, the transmitted light intensity amplitude waveform is extremely nonlinear, reflecting the extinction characteristic. It becomes distorted.
As described above, the conventional semiconductor optical modulator has various problems because it is easily affected by the operating temperature and it is necessary to apply a reverse bias having a higher voltage than the outside.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a semiconductor optical modulator that is not easily affected by the operating temperature. It is another object of the present invention to provide a semiconductor optical modulator that does not require voltage application with a high reverse bias. For example, in the built-in voltage generated by the formation of a pn junction due to the increase in residual impurities in the multiple quantum well layer as described above or the increase of impurities due to diffusion of p and n impurities in the upper and lower cladding layers, the multiple quantum well layer Improve the situation where the device structure (including the SCH layer including the SCH layer) is not completely depleted, and can operate optimally without applying a reverse bias voltage from the outside, and modulate high intensity light. However, an object of the present invention is to provide a semiconductor optical device that can operate at high speed without accumulating carriers, has a small modulation voltage, and has a low nonlinearity with respect to a driving voltage.

A semiconductor optical modulator according to the present invention includes a first conductivity type lower cladding layer, a light absorption layer having a quantum well structure formed on the lower cladding layer, the quantum well structure and a barrier layer, And an upper cladding layer of the second conductivity type formed on the light absorption layer, wherein the quantum well layer is In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦ 1, 0 ≦ X + Y ≦ 1). And the barrier layer is made of In _{1-X′-Y ′} Ga _{X ′} Al _{Y ′} N (0 ≦ X ′, Y ′ ≦ 1, 0 ≦ X ′ + Y ′ ≦ 1), The lower cladding layer, the light absorption layer, and the upper cladding layer constitute an optical waveguide having a light incident end, and the quantum well and the barrier layer have a composition range without phase separation other than an immiscible region of InGaAlN. and, the light absorbing layer is polarized in a non-bias state is generated, the polarization the light absorbing layer It is obtained as is the sum of the spontaneous polarization and piezoelectric polarization in the same direction that Oite occurred.
As a result of this configuration, the light absorption layer is made of a material having a small Auger effect. In addition, an electric field due to natural polarization is generated in the light absorption layer.

In the semiconductor modulator, the light absorption layer has a multiple quantum well structure.
In the semiconductor optical modulator, the lower cladding layer may be formed on a predetermined substrate.

In the semiconductor optical modulator, the quantum well layer and the barrier layer may have different lattice constants. For example, the quantum well layer has a larger lattice constant than the barrier layer, or the quantum well layer is The lattice constant may be smaller than that of the barrier layer.
In the semiconductor optical modulator, the quantum well layer may be composed of InN crystal, and the barrier layer may be composed of GaN crystal.

The laser with an optical modulator of the present invention is a laser with an optical modulator composed of a waveguide type semiconductor laser and a semiconductor optical modulator integrated on the same substrate, and the semiconductor optical modulator is a substrate. A first-conductivity-type lower cladding layer formed thereon, a quantum well structure light-absorbing layer formed on the lower clad layer and composed of a quantum well layer and a barrier layer; and on the light-absorbing layer An upper cladding layer of the second conductivity type formed, and the quantum well layer is made of In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦ 1, 0 ≦ X + Y ≦ 1), The barrier layer is made of In _{1-X′-Y ′} Ga _{X ′} Al _{Y ′} N (0 ≦ X ′, Y ′ ≦ 1, 0 ≦ X ′ + Y ′ ≦ 1), and the lower cladding layer, optical layer The absorption layer and the upper cladding layer constitute an optical waveguide having a light incident end, and the quantum well and the barrier layer are Are the non-non-miscible region of phase separation without the composition range of InGaAlN, the light absorbing layer is polarization occurs under no bias state, the polarization of the same direction are generated in the light absorbing layer The sum of natural polarization and piezo polarization .
As a result of this configuration, the light absorption layer is made of a material having a small Auger effect. In addition, an electric field due to natural polarization is generated in the light absorption layer.

In the laser with an optical modulator, the quantum well layer and the barrier layer may have different lattice constants. For example, the quantum well layer has a larger lattice constant than the barrier layer, or the quantum well layer is What is necessary is just to have a lattice constant smaller than a barrier layer.
In the laser with an optical modulator, the quantum well layer can be composed of an InN crystal, and the barrier layer can be composed of a GaN crystal.

As described above, according to the present invention, in the quantum well structure constituting the light absorption layer, the quantum well layer is composed of In _{1 -XY} Ga _X Al _Y N (0 ≦ X, Y ≦ 1, 0 ≦ X + Y ≦ 1), and the barrier layer is composed of In _{1-X′-Y ′} Ga _{X ′} Al _{Y ′} N (0 ≦ X ′, Y ′ ≦ 1, 0 ≦ X ′ + Y ′ ≦ 1). Therefore, the light absorption layer is made of a material having a small Auger effect, and is hardly affected by the use temperature.

  In addition, spontaneous polarization always occurs in the quantum well layer acting as a light absorption layer. As described above, according to the present invention, even when no bias voltage is applied, an electric field is generated in the light absorption layer, and the band of the quantum well layer constituting the light absorption layer is inclined. I tried to be. As a result, in the present invention, the band gap energy difference between electrons and holes constituting the exciton can be increased and absorbed by applying a signal voltage to further smooth the slope of the quantum well band. The wavelength to be changed can be changed. As a result, according to the present invention, it is possible to perform modulation, for example, by an operation by a forward bias.

  In the semiconductor optical device of the present invention, even if there is an increase in impurities in the multiple quantum well layer due to diffusion of residual impurities or p and n impurities in the upper and lower cladding layers in the multiple quantum well layer, The built-in voltage generated by the formation of the pn junction completely depletes the multiple quantum well layer, and an optimum operation is possible without applying a reverse bias voltage from the outside. The same applies to the case where the SCH layer is provided, and the SCH layer is depleted even if the impurity in the SCH layer increases.

  For this reason, an additional circuit such as a bias T for applying a reverse bias voltage from the outside becomes unnecessary, and further, a power supply device becomes unnecessary. Further, since the low modulation voltage operation is realized, the demand for the driver for driving the modulator is reduced, and the cost and power consumption of the driver can be reduced. As a result, the transmitter can be significantly reduced in size and cost.

  Also, the lattice constant of the barrier layer is made larger than the lattice constant of the quantum well layer to distort the quantum well layer and generate piezo polarization in addition to the above-described natural polarization, so that no bias voltage is applied. However, it is also possible to incline the band of the quantum well layer that constitutes the light absorption layer by setting an electric field in the light absorption layer. However, when the directions of natural polarization and piezoelectric polarization, such as tensile strain, are different, the compositions of the quantum well layer and the barrier layer are combined so that the magnitude of the natural polarization is different from the magnitude of the piezoelectric polarization.

  Further, it is not necessary to apply a large DC reverse bias used in the case of the conventional semiconductor optical modulator to the semiconductor optical modulator and the laser with the semiconductor optical modulator of the present invention. Depending on the configuration of the present invention, no DC bias need be applied. This eliminates the need for high quality crystals that can withstand reverse bias. This increases the margin of the crystal growth process. As a result, the yield of device fabrication is improved, and a cheaper device can be provided. In addition, since it is not necessary to apply a large reverse bias, the lifetime of the element is extended. Therefore, if a communication system is configured using the element of the present invention, the reliability is improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a first embodiment of the present invention.
First, a first embodiment of the present invention will be described.
FIG. 1A is a schematic cross-sectional view schematically showing a configuration example of a semiconductor optical modulator according to an embodiment of the present invention. FIG. 1B is a perspective view schematically showing the configuration of the semiconductor optical modulator. In the following description, compound semiconductor materials such as GaN, AlGaN, AlN, and InN are single crystals unless otherwise specified.

In this semiconductor optical modulator, first, a buffer layer 102 made of GaN having a thickness of 20 nm is formed on a nitrided surface of a single crystal substrate 101 made of (0001) plane sapphire (Al ₂ O ₃ crystal) having a thickness of 330 μm. , A 4 μm thick electrode layer 103 made of p-type GaN doped with Mg, and a 0.5 μm thick clad layer 104 made of p-type Al _0.1 Ga _0.9 N doped with Mg. Note that the single crystal substrate 101 is not limited to sapphire, and may be made of a crystal such as silicon carbide (SiC), zinc oxide (ZnO), or lithium gallate (LiGaO ₂ ).

A light absorption layer 105 having a multiple quantum well structure is provided on the cladding layer 104. The light absorption layer 105 has a stacked structure of a barrier layer 151 made of non-doped GaN, which is an InGaAlN-based material, and a quantum well layer 152 made of non-doped InN. The quantum well layer 152 is composed of In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦ 1, 0 ≦ X + Y ≦ 1), and the barrier layer 151 is composed of In _{1-X′-Y ′} Ga. _{X ′} Al _{Y ′} N (0 ≦ X ′, Y ′ ≦ 1, 0 ≦ X ′ + Y ′ ≦ 1) may be used. The combination of these compositions will be described in detail later. The light absorption layer 105 only needs to have a quantum well structure, and may have a single quantum well structure.

  The film thickness of the quantum well layer 152 is, for example, a film thickness at which the quantum effect is obtained and the spread of the electron wave function is not more than. The barrier layer 151 only needs to have a thickness such that the wave function does not substantially overlap with the quantum well layer 152. Here, the lattice constant of the material constituting the quantum well layer 152 is larger than the lattice constant of the material constituting the barrier layer 151. In this embodiment mode, a stacked structure of five barrier layers 151 and three quantum well layers 152 is employed. The number of stacks may be set as appropriate depending on the state of light absorption, and may be other stacks.

On the light absorption layer 105, a 0.5 μm thick clad layer 106 made of n-type Al _0.1 Ga _0.9 N doped with Si is formed.
Therefore, in the semiconductor optical modulator shown in FIG. 1, the clad layer 104, the light absorption layer 105, and the clad layer 106 constitute an optical waveguide having a light incident end and a light exit end.

In the semiconductor optical modulator of FIG. 1, a contact layer 107 made of n-type GaN doped with Si and having a thickness of 0.1 μm and an n-type metal electrode 108 are formed. Although not shown, the n-type metal electrode 108 has a laminated structure of a 50 nm thick aluminum layer and a 200 nm thick gold layer that are in direct contact with the contact layer 107.
The electrode layer 103 includes an exposed region formed by partially etching each of the layers above, and a p-type metal electrode 109 is provided in the exposed region. Although not shown, the p-type metal electrode 109 has a laminated structure of a nickel layer with a thickness of 50 nm that is in direct contact with the exposed region and a gold layer with a thickness of 200 nm formed thereon.

In the present embodiment, as described above, the semiconductor optical modulator is configured using an InGaAlN-based material that is a wurtzite crystal. As a result, the light absorption layer 105 is made of a material having a small Auger effect and is hardly affected by the use temperature.
In the above embodiment, InN and GaN are used for the light absorption layer 105 as an InGaAlN-based material, and a multiple quantum well structure is formed using these. These wurtzite crystals have a polarization effect that does not exist in zinc blende crystals.

  Due to this polarization effect, the light absorption layer 105 is polarized without bias. Thus, even if carriers exist in the light absorption layer 105, a state where the light absorption layer 105 is depleted is obtained. As described above, in the semiconductor optical modulator according to the present embodiment, the light absorption layer 105 is made of an InGaAlN-based material, and in addition, polarization is generated without bias. The light can be modulated by applying a forward voltage without consumption.

  Thus, according to the present invention, since the light absorption region is depleted by the built-in voltage generated by the formation of the polarization electric field and the pn junction, the optimum operation can be performed without applying a reverse bias voltage from the outside. It is possible to obtain high-speed operation without accumulating carriers even for modulation of high-intensity light.

Hereinafter, the polarization effect will be described in more detail.
When a multi-quantum well structure is formed by laminating a quantum well layer and a barrier layer made of an InGaAlN-based material that is a wurtzite crystal, natural polarization provided in the crystal itself always occurs. In addition, in this embodiment, since the lattice constant of the quantum well layer is larger than the lattice constant of the barrier layer, piezoelectric polarization due to crystal strain (compression strain) due to this lattice mismatch also exists.

  In the light absorption layer 105 having the above-described configuration, the natural polarization and the piezo polarization due to compressive strain are in the same direction. Therefore, the multi-quantum well structure constituting the light absorption layer 105 includes the natural polarization and the piezo polarization. An electric field is generated by polarization. The band structure in the layer structure centering on the light absorption layer 105 shown in FIGS. 1A and 1B having such a structure is as shown in FIGS. 2A and 2B. become. 2A shows a state where no forward voltage is applied (no bias state), and FIG. 2B shows a state where a forward voltage is applied. This is a case where the p-type metal electrode 109 is an anode and the n-type metal electrode 108 is a cathode. In FIGS. 2A and 2B, the electrons constituting the excitons are indicated by white circles, and the holes constituting the excitons are indicated by black circles.

  As shown in FIG. 2A, in the multiple quantum well structure (light absorption layer 105), electrons and holes constituting excitons in the quantum well layer 152 are spatially separated. In the case of this embodiment, the distance (band gap energy difference) between these electrons and holes is changed by applying a forward voltage. When a forward voltage is applied, as shown in FIG. 2B, the band inclination in the quantum well layer 152 decreases, and the distance between electrons and holes constituting excitons becomes longer (band gap energy). The difference between

  As a result, in the state where the forward voltage is applied, the absorption edge wavelength of the exciton becomes shorter than in the state where no voltage is applied. Therefore, in this embodiment, when the wavelength of light incident on the light incident end of the semiconductor optical modulator is shorter than the absorption edge wavelength when no bias is applied and longer than the absorption edge wavelength when forward bias is applied, The signal is transmitted through the modulator only when a bias is applied.

  As described above, in the semiconductor optical modulator according to the present embodiment, the wavelength of light absorbed in the light absorption layer 105 is changed by applying a forward voltage. Therefore, according to the semiconductor light modulation in this embodiment, the light incident on the light absorption layer 105 can be modulated and output by applying a forward voltage. In addition, since the semiconductor optical modulator in the present embodiment is an element that can be operated by applying a forward voltage, unlike a conventional element, a crystal with a high reverse breakdown voltage is not required. This simplifies the crystal growth apparatus for manufacturing the device and simplifies the crystal growth process. In addition, the production yield of elements can be improved. As a result, according to the present embodiment, a semiconductor optical modulator that can be easily manufactured can be provided.

  By the way, in a multiple quantum well layer composed of a compound semiconductor, TE polarized light is absorbed by both excitons composed of heavy holes and electrons and excitons composed of light holes and electrons. Only absorbed by excitons consisting of light holes and electrons. For this reason, in order to modulate the light regardless of the polarization of the light, the spectra in both excitons must match.

  FIG. 3A shows a band structure of a wurtzite crystal in which a tensile strain is applied to the quantum well layer having the quantum well structure. At the Γ point, the light hole band energy level can be designed to be much lower than the heavy hole band energy level. Further, as shown in FIG. 3B, the energy bands of both holes can be matched.

  Because of the polarization effect in the crystal, the degree of freedom in design of the band structure in the wurtzite type crystal described above is much greater than that in the zincblende type crystal. Therefore, it is very easy to match the spectrum of both excitons as described above under no electric field. For these reasons, according to the optical modulator in the present embodiment using a wurtzite crystal, modulation without polarization dependence is possible.

  However, the direction of piezo polarization due to tensile strain may be the same as or different from the direction of natural polarization of the wurtzite crystal. This is because the direction of natural polarization depends on the polarity of the crystal. Therefore, depending on the combination of materials, the magnitude of the natural polarization and the magnitude of the piezo polarization may be equal, canceling the effect of polarization, and no electric field may be generated. In such a case, in order to obtain the effect of polarization, it is necessary to combine materials so that the magnitude of piezoelectric polarization due to tensile strain differs from the magnitude of natural polarization.

  4A and 4B show the stacking order of Ga atoms and N atoms in a single crystal obtained by crystal growth of GaN in the c-axis direction. This stacking order corresponds to the polarity. As shown in FIG. 4A, a state in which three N atoms are bonded under the Ga atom when viewed from the surface of the GaN single crystal is called Ga polarity or + c polarity. On the other hand, as shown in FIG. 4B, the state in which three Ga atoms are bonded under the N atom when viewed from the surface of the GaN single crystal is called N polarity or -c polarity. This polarity is independent of whether the atoms constituting the surface of the GaN single crystal are Ga or N atoms.

  For example, adatoms are also present on the surface of a GaN single crystal. In that case, even if it is Ga polarity, the case where N atom exists in the surface is also considered. Even in such a case, the stacking order of atoms in consideration of the number of bonds can be seen in the internal structure of the crystal. Whatever phenomenon occurs on the surface and what atoms are adsorbed, the internal structure does not change and the polarity in the crystal is uniquely determined.

  When GaN is considered as a dielectric, the direction of the polarization electric field is determined by the polarity. Polarization determined by the polarity is called natural polarization. The relationship between polarity and natural polarization is as shown in FIGS. 4 (a) and 4 (b). When the grown crystal is Ga-polar, the direction of the electric field due to natural polarization is from the inside of the crystal to the surface. When the polarity is N, the direction of the electric field due to natural polarization is directed from the surface to the inside of the crystal.

  The piezo polarization is as follows. A case where a GaN single crystal layer is formed by crystal growth on a sapphire substrate whose main surface is a (0001) plane (c-plane) will be described. Crystal growth is generally performed at a temperature higher than room temperature, and the device is used at room temperature. The lattice constant along the a-axis is larger for GaN than for sapphire. In addition, the thermal expansion coefficient along the a-axis is larger for sapphire than for GaN.

  From this, at room temperature, the GaN grown on the sapphire substrate is in a state where stress is generated as shown in the schematic cross section of FIG. This stress acts in a direction parallel to the interface between the sapphire substrate and GaN, and acts on GaN as a compressive stress. The compressive stress in this case is a biaxial strain.

  Therefore, the GaN crystal lattice formed on the sapphire substrate contracts in a direction parallel to the interface and extends in a direction perpendicular to the interface, as shown in FIG. As a result, as shown in FIG. 4C, an electric field due to piezoelectric polarization is generated in a direction perpendicular to the interface, that is, in a direction parallel to the c-axis. The direction of this piezo electric field depends on the direction of stress. In the case of compressive stress, the direction of the piezo electric field is directed to the sapphire substrate side (FIG. 4C). In the case of tensile stress, the direction is opposite to that in FIG. 4C, and the direction is from the sapphire substrate side to the GaN layer side.

The natural polarization charges are 0.081, 0.029, and 0.032 C / m ² for AlN, GaN, and InN, respectively. These values are much larger than the perovskite structure crystals that are ferroelectrics. Further, piezoelectric polarization charge P _piezo is expressed by the following equation.
P _piezo (C / m ² ) = e ₃₃ (c−c ₀ ) / c ₀ + e ₃₁ (a−a ₀ ) / a ₀ (1)
In the above equation (1), c ₀ is a lattice constant when there is no strain along the c axis, and c is a lattice constant when there is a strain along the c axis. Further, a ₀ is a lattice constant when there is no distortion along the a-axis, and a is a lattice constant when there is a distortion along the a-axis.

Further, e ₃₃ and e ₃₁ are piezoelectric coefficients and have values shown in Table 1 below. The values shown in Table 1 are about one third of the perovskite crystal.

For example, the estimated values of crystal deformation and piezoelectric polarization in GaN are shown. Here, it is assumed that the volume of the crystal is constant. When a GaN single crystal is stretched 10% in the c-axis direction, the a-axis shrinks 4.7%. Piezoelectric polarization charge P _piezo at this time is _{"P piezo = 0.73 × 0.1 + 0.49} × 0.047 = 0.096 " (see Non-Patent Document 1).

  To summarize the above description, first, if the light absorption layer is polarized in a non-biased state, light modulation can be performed without applying a reverse bias having a higher voltage than the outside. A state in which polarization occurs without bias can be obtained, for example, if spontaneous polarization occurs in the light absorption layer. In this state, for example, when the quantum well layer is made of InGaN and the barrier layer is made of InGaAlN, a state in which natural polarization occurs in the light absorption layer can be obtained. InGaN and InGaAlN can have the same lattice constant.

  In addition, a state in which polarization is generated without bias occurs when natural polarization and piezo polarization are generated in the light absorption layer, and the sum thereof is not zero. A state in which piezo polarization is generated in addition to natural polarization in the light absorption layer can be obtained if the lattice constant of the quantum well layer and the lattice constant of the barrier layer are different. However, if the natural polarization and the piezo polarization have the same magnitude and different directions, they cancel each other, so that no polarization is generated in the light absorption layer without bias. Except for this state, even if piezo polarization occurs, the light absorption layer is polarized without bias. For example, when the quantum well layer is made of InN and the barrier layer is made of GaN, in the light absorption layer, in addition to the natural polarization in the direction of the present embodiment, the piezoelectric polarization in the same direction is generated. Is obtained.

Next, an InGaAlN-based compound semiconductor constituting the semiconductor optical modulator in the present embodiment will be described.
Conventionally, there has been no idea to apply an InGaAlN material to a semiconductor optical modulator that uses a wavelength of 1.3 μm to 1.58 μm, which is an optical communication wavelength region. This is because the material with the smallest band gap (Eg) in this material system is InN, and this Eg was 1.9 to 2.1 eV.

  In the era when Eg was measured, since InN was to be formed by a method such as reactive sputtering, only polycrystalline InN could be formed, and the above-mentioned band gap measurement results were for polycrystalline InN. (See Non-Patent Document 2, Non-Patent Document 3, and Non-Patent Document 4).

  Among the compounds AlN, GaN, and InN that constitute InGaAlN, the equilibrium vapor pressure of nitrogen on the solid phase of InN is five orders of magnitude higher than others (see Non-Patent Document 5). With InN having such characteristics, it is almost impossible to form high-quality crystals by a method such as reactive sputtering in which the pressure in a container (for example, a vacuum chamber) in which a film is formed cannot be increased. In the above-mentioned era, since an InN film was formed on a glass substrate, lattice information for single crystal growth could not be obtained, and the single crystal could not be grown.

In response to the above, in 1988, the inventors performed single crystal growth of InN on a single crystal sapphire substrate using a metal organic vapor phase epitaxy method (hereinafter referred to as MOVPE) in which nitrogen pressure is applied. Tried and succeeded in growing the single crystal for the first time in the world. In 1989, the results were presented at an academic conference.
However, the single crystal growth at this time has insufficient crystallinity, and the optical characteristics of InN have not been obtained. After this, the inventors succeeded in the growth of high quality InN single crystal that can measure optical properties for the first time in the world in 2001 by various technical improvements (Takashi Matsuoka, Masashi Nakao, Hiroshi Okamoto, Hiroshi Harima, Eiji Kurimoto, Ebara. Megumi, “InN Bandgap Energy”, Proceedings of the 49th Joint Conference on Applied Physics, p.392 (29p-ZM-1) (2002). 8th, the conference will be held from March 27th to 30th, and the presentation date will be March 29th.

  The first technical improvement for the high-quality InN single crystal growth is that GaN having a lattice constant between sapphire and InN is formed on a sapphire substrate, and InN is formed on the GaN layer. It is a growing point. In this case, the GaN layer is required to have high crystallinity. Therefore, the GaN growth conditions were optimized for the following points.

1. 1. Cleaning the surface of the sapphire substrate in a growth furnace 2. Nitridation of sapphire surface with ammonia 3. Growth of low temperature grown GaN as a buffer layer to alleviate lattice mismatch between sapphire and GaN 4. High-temperature annealing for achieving single crystallization of the buffer layer High quality GaN growth at high temperature

  The second technical improvement is that the growth conditions of InN were examined on GaN grown under the conditions optimized as described above. Items examined were growth temperature, growth rate, ratio of Group V material to In material, gas flow rate in the growth furnace, and the like. As a result of studying each of these items, the growth of InN that can measure optical properties has finally been achieved for the first time. At present, it is possible to directly form an InN layer on a sapphire substrate without using a GaN layer.

Here, the characteristics of the formed InN film are shown in FIGS. FIG. 5 is a characteristic diagram showing the relationship between the square of absorption and the photon energy in the formed film. As shown in FIG. 5, in the formed InN film, a clear absorption edge is detected from optical absorption. Since this relationship is almost linear, InN is a direct transition type, and Eg can be estimated to be 0.8 eV.
FIG. 6 shows photoluminescence measured at room temperature of the InN film formed under the above conditions, and it can be seen that light is emitted in the vicinity of a wavelength of 1.59 μm (0.8 eV). These absorption and photoluminescence results shown in FIGS. 5 and 6 indicate that the Eg of the InN film formed under the above conditions is 0.8 eV. From these results, it can be estimated that Eg of InN is in the vicinity of 0.8 eV.

  For the first time, the Eg of single-crystal InN was found to be 0.8 eV, which is less than half of the conventional measurement results, through various technical improvements and long-term efforts described above. FIG. 7 is a characteristic diagram showing the relationship between Eg of InGaAlN and the a-axis lattice constant a. In FIG. 7, the broken line is the old data, and the solid line is the result reflecting the data obtained by the inventor (Eg = 0.8 eV).

  Therefore, the composition of each of the barrier layer 151 and the quantum well layer 152 constituting the light absorption layer 105 has a higher band gap energy in the barrier layer 151 in the triangle composed of the three black dots shown in FIG. What is necessary is just to set suitably so that it may become a composition. For example, the quantum well layer 152 may be made of non-doped GaN, and the barrier layer 151 may be made of non-doped AlN. However, as described above, it is necessary to consider the combination of materials and the process of crystal growth so that the sum of piezoelectric polarization and natural polarization does not become zero.

  Next, a method for manufacturing the semiconductor optical modulator in the present embodiment shown in FIGS. 1A and 1B will be described. First, a metal organic vapor phase growth apparatus having a vertical growth furnace is used for crystal growth for forming each crystal layer. Also, ammonia is used as the nitrogen raw material. As the carrier gas, nitrogen gas is used for the growth of the quantum well structure, and hydrogen gas is used for the growth of the other crystal layers. The growth pressure is normal pressure.

  First, the substrate temperature is set to 1050 °, the surface of the single crystal substrate 101 made of sapphire is nitrided in an ammonia atmosphere, and an extremely thin nitride layer is formed on the surface of the single crystal substrate 101. The nitride layer is a very thin layer and is not shown in the figure. Thereafter, the substrate temperature is set to 550 ° C., and the buffer layer 102 in which GaN is grown is formed on the nitrogen layer of the single crystal substrate 101. In the growth of GaN, triethylgallium (TEG) having a relatively low gallium vapor pressure is used as a gallium raw material. Subsequently, the single crystal substrate 101 is annealed at 1050 ° C. for 9 minutes, and the buffer layer 102 is single crystallized.

Next, the temperature of the single crystal substrate is set to 1020 ° C., p-type GaN doped with Mg and p-type Al _0.1 Ga _0.9 N doped with Mg are sequentially grown to form the electrode layer 103 and the cladding layer 104. To do. Trimethylaluminum (TMA) is used as the aluminum material in the vapor phase growth, and trimethylgallium (TMG) having a relatively high vapor pressure is used as the gallium material. Further, methylcaptan / biscyclopenta / dienylmagnesium (MeCP ₂ Mg) is used as a raw material for adding Mg. This raw material is liquid, and the reproducibility of the added concentration of Mg is better than that of biscyclopentadienylmagnesium (Cp ₂ Mg), which is a commonly used solid raw material.

  Next, GaN crystal growth and InN crystal growth are alternately performed on the cladding layer 104 to form a laminated structure (multi-quantum well structure) of the barrier layer 151 and the quantum well layer 152. In the crystal growth of InN used as the quantum well layer 152, since the nitrogen equilibrium vapor pressure on the solid phase of InN is high, the growth temperature (substrate temperature) is set to 575 ° C., for example. As the In raw material for the crystal growth, trimethylindium (TMI) may be used.

  In InN crystal growth, the ratio of ammonia to TMI, that is, V / III, is set to 660000 in order to prevent precipitation of metal In and grow high quality InN. The carrier gas and bubbling gas are both nitrogen. This is because decomposition of ammonia is suppressed when hydrogen is used as a carrier gas (see Non-Patent Document 5).

  As an example of the relationship between growth conditions and crystallinity in InN, the ratio V / III (ammonia / indium) of the supply amount of ammonia, which is a nitrogen raw material, to the growth furnace and the supply amount of trimethylindium (TMI), which is an indium raw material, is The X-ray diffraction spectra of InN grown by changing are shown in FIGS. FIG. 8A, FIG. 8B, and FIG. 8C show the ω-2θ scan spectrum. As shown in FIGS. 8A and 8B, in the film formed under the condition that V / III is 160000 or less and V / III is 320,000, a signal from metallic indium is observed. On the other hand, as shown in FIG. 8C, in the film formed under the condition where V / III is 660000, it can be seen that the peak of metallic indium disappears and no metallic indium is contained.

As described above, when a high quality InN crystal is formed and GaN serving as the barrier layer 151 is grown, InN is etched and the crystallinity is deteriorated. Nitrogen is used, and the substrate temperature is 575 ° C., which is similar to the growth of InN.
In this way, after the GaN barrier layer 151 and the InN well layer 152 were sequentially stacked in a predetermined number of layers to form the light absorption layer 105, the substrate temperature was raised to 1020 ° C., and Si was added. N-type GaN doped with n-type Al _0.1 Ga _0.9 N and Si is sequentially grown to form a cladding layer 106 and a contact layer 107.

In order to add Si to each layer, silane (SiH ₄ ) gas having a concentration of 1 ppm diluted with hydrogen is used. The same raw material was used for addition of Si in other steps.
After forming the cladding layer 106 and the contact layer 107, annealing is performed in a nitrogen atmosphere at 700 ° C. for 30 minutes in order to activate Mg added to these layers.

  Next, using an electron beam evaporation apparatus, a 50 nm-thick aluminum layer and a 200 nm-thick gold layer are sequentially deposited to form a metal layer to be the n-type metal electrode 108. Thereafter, a novolac positive photoresist pattern is formed on the metal layer by a known photolithography technique, and the lower layer is etched using this pattern as a mask to form an exposed region in the electrode layer 103. In this etching, for example, the etching of the crystal layer may be performed by reactive ion etching using chlorine gas.

  Etching to form the exposed region is performed, the pattern of the photoresist is removed, the single crystal substrate 101 and the structure formed thereon are cleaned, nickel having a thickness of 50 nm is formed on the exposed region of the electrode layer 103, 200 nm gold is sequentially deposited and laminated, and these are patterned to form a p-type metal electrode 109.

  As described above, after forming up to the p-type metal electrode 109, the back surface of the single crystal substrate is polished, the single crystal substrate 101 is thinned to a thickness of 80 μm, and the back surface is finished to a mirror surface. If the back surface is not finished to a mirror surface, the single crystal substrate 101 may be deformed or cracked when the single crystal substrate 101 is peeled from the polishing table, so that the back surface is finished to a mirror surface. Thereafter, these are washed, and a predetermined portion is cleaved, whereby the semiconductor optical modulator shown in FIGS. 1A and 1B having an element length of 1 mm, for example, is obtained.

  Here, the band structure in the above-described multiple quantum well structure (light absorption layer 105) and the polarity of the crystal which is important in the polarization effect will be described. The GaN of the barrier layer 151 constituting the light absorption layer 105 is in the order of nitrogen atom, gallium atom, nitrogen atom, gallium atom from the direction of the single crystal substrate 101, that is, a group V atom, a group III atom, a group V atom, Grows in order of group III atoms. This stacking order of atoms is generally called + (plus) polarity.

  In a crystal growth method such as MOVPE or MBE, when a high-quality crystal is obtained, growth with the above polarity is dominant. At the time of starting crystal growth, even if-(minus) polarity and + polarity in the opposite direction to + polarity are mixed, the growth condition of + polarity is dominant under the growth conditions that give a good quality crystal, and to some extent When the film thickness is increased, all become + polarity. When the crystal is grown with + polarity, the band structure of the multiple quantum well structure is as shown in FIG. Note that by selecting a substrate, each crystal layer can be grown with -polarity. In this case, the gradient of the band structure is opposite to the state shown in FIG. When each crystal is configured with a polarity, the band structure has a downward slope, unlike FIG. In this case, when a reverse voltage is applied, the slope of the band decreases.

  Even when a substrate similar to that of the semiconductor optical modulator of the present embodiment is used, each crystal layer can be configured with a negative polarity by the following. For example, first, the surface of the sapphire c-plane substrate is nitrided by exposure to an ammonia atmosphere at a high temperature. On the sapphire substrate subjected to the nitriding treatment, first, a buffer layer made of GaN is grown at a low temperature to a predetermined film thickness, and the growth is interrupted for a predetermined time. During this growth interruption, the already formed buffer layer is annealed at a high temperature of 1000 ° C. or higher. Subsequently, GaN is grown at the same high temperature of 1000 ° C. or higher, so that a single crystal layer of -polar GaN can be formed.

  This is a technique known as a two-stage growth method. The -polar GaN single crystal layer thus formed has a rocking curve width obtained by X-ray diffraction of 40 seconds or less. The + polar GaN single crystal has a rocking curve width of 100 seconds or more. Compared with this, it can be seen that the above-described -polar GaN single crystal layer has a good crystallinity.

  Next, the operating characteristics of the semiconductor optical modulator in the present embodiment described above will be described. Hereinafter, the n-type metal electrode 108 in FIGS. 1A and 1B is operated as a cathode and the p-type electrode 109 is operated as an anode. That is, it is operated as a state in which a voltage is applied in the forward direction of the diode. By applying the voltage in this way, the band inclination of the quantum well layer 152 made of InN shown in FIG. 2A can be reduced, flattened, and further reversed. Note that in this embodiment mode, operation is performed without current injection.

  FIG. 9 is a characteristic diagram showing the characteristics of the semiconductor optical modulator according to the present embodiment, and shows the relationship between the modulation applied voltage and the transmitted light intensity. As shown in FIG. 9, the semiconductor optical modulator operates with a forward bias. The wavelength of the light source used for investigating the characteristics shown in FIG. 9 is 1.53 μm, and the input light intensity into the device is 10 μW.

In the light absorption layer 105, the thickness of the quantum well layer 152 and the In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦, constituting the quantum well layer 152 and the barrier layer 151 are considered in consideration of the polarization effect. A wavelength that can be modulated can be selected by appropriately designing a combination of compositions of 0 ≦ X + Y ≦ 1).
Note that phase separation is considered in the combination of In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦, 0 ≦ X + Y ≦ 1) compositions used for the light absorption layer. As shown in FIG. 10, in the crystal growth of an InGaAlN-based material, an immiscible region exists, and phase separation occurs under the condition of the immiscible region, so that the crystal cannot grow. FIG. 10 is a characteristic diagram showing the results of obtaining the non-mixing region of InGaAlN from experiments and calculations (see Non-Patent Document 6).

In FIG. 10, the InGaAlN immiscible region when grown at 600 ° C. is indicated by a one-dot chain line, the InGaAlN immiscible region when grown at 800 ° C. is indicated by a solid line, and the InGaAlN immiscible when grown at 1000 ° C. Regions are indicated by broken lines, and regions inside each line are immiscible regions.
Therefore, if the composition is determined in consideration of the conditions of the mixing region shown in FIG. 10, a multiple quantum well structure using an InGaAlN-based material can be realized.

  Since InN and GaN constituting the light absorption layer 105 in this embodiment shown in FIGS. 1A and 1B are binary compounds, phase separation as described above occurs in principle. Therefore, the conditions are easier to manufacture. In addition, the barrier layer 151 and the well layer 152 are in a state in which the lattice constants of the crystals are mismatched. This can be achieved by determining the composition of InGaAlN in consideration of the characteristics of the InGaAlN-based material shown in FIG. .

  By the way, the semiconductor optical modulator of the present embodiment shown in FIGS. 1A and 1B can be monolithically integrated with a semiconductor laser to realize a laser with an optical modulator. For example, as shown in FIG. 11, an n-type GaN layer 1002 is formed on an n-type semiconductor substrate 1001, and a waveguide-type semiconductor laser 1003 is formed thereon, as shown in FIGS. When the semiconductor optical modulator 1004 having the layer configuration shown in FIG. 5 is integrated with the optical axes overlapped, an external modulation type optical modulator can be configured. The semiconductor laser 1003 is made of a normal nitride semiconductor material.

  As an electrode, a common n-type electrode may be provided on the n-type GaN layer 1002, and the p-type electrode may be provided separately for the semiconductor laser 1003 and the semiconductor optical modulator 1004. According to this configuration, the cathode is shared, the circuit configuration is facilitated, and light modulation at a higher frequency is possible. In FIG. 11, a space is provided between the semiconductor laser 1003 and the semiconductor optical modulator 1004. However, the present invention is not limited to this, and these may be disposed in close contact with a known structure such as a butt joint. May be. Further, the semiconductor laser 1003 and the semiconductor optical modulator 1004 may be optically coupled by other methods.

In addition, various impurities are introduced into each layer constituting the semiconductor laser 1003, but these have almost no adverse effect on the semiconductor optical modulator 1004.
As described above, since the semiconductor optical modulator of this embodiment is operated by the forward bias, a high-purity crystal layer that can withstand a high reverse bias is not required, and therefore the presence of impurities is a problem. Don't be.

Although the forward bias structure has been described above, the reverse bias structure may be operated by applying a reverse bias. For example, the electrode layer 103 shown in FIGS. 1A and 1B is made of n-type GaN, the cladding layer 104 is made of n-type Al _0.1 Ga _0.9 N, and the cladding layer 106 is made of p-type. consist of al _0.1 Ga _0.9 n, constitute a contact layer 107 from the p-type GaN, is formed on the contact layer 107 form a p-type metal electrode, the electrode layer 103 to form an n-type metal electrodes, n-type It is also possible to operate the metal electrode as an anode and the p-type metal electrode as a cathode.

  In the above embodiment, the metal electrode is directly formed on the semiconductor layer such as the contact layer 107. For example, an insulating layer is formed instead of the contact layer 107, and the metal electrode is provided thereon. Also good. By providing an increase in insulation in this way, carrier injection into the light absorption layer 105 can be prevented more reliably.

  Further, the quantum well layer constituting the light absorption layer may have a lattice constant smaller than that of the barrier layer, and the magnitude of the natural polarization of the quantum well layer and the piezo polarization generated in the quantum well layer may be different. . In this case, the reverse bias can be applied most efficiently if the directions of the natural polarization and the piezo polarization are matched and the matched direction is matched with the reverse bias direction. When the directions of the natural polarization and the piezo polarization are different from each other, it is only necessary that the sum of both polarizations exists and this direction matches the reverse bias direction.

[Embodiment 2]
Next, another embodiment of the present invention will be described.
FIG. 12 is a schematic perspective view schematically showing another configuration example of the semiconductor optical modulator according to the embodiment of the present invention.
In this semiconductor optical modulator, first, a light absorption region formed in a ridge shape is provided on the surface of a substrate 1101 made of n-type GaN having a thickness of 300 μm. The light absorption region includes a lower clad layer 1102 made of Si doped n-type Al _0.1 Ga _0.9 N with a thickness of about 1.5 μm and a lower SCH (separate confinement) made of non-doped InGaAlN with a thickness of 0.5 μm. (Hetero) layer 1103, light absorption layer 1104 having a multiple quantum well structure, upper SCH layer 1105 having a thickness of 0.5 μm made of non-doped InGaAlN, and a film made of p-type Al _0.1 Ga _0.9 N doped with Mg An upper cladding layer 1106 having a thickness of 1.5 μm and a contact layer 1107 having a thickness of 0.1 μm made of p-type GaN doped with Mg are formed.

The light absorption layer 1104 includes a quantum well layer composed of In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦ 1, 0 ≦ X + Y ≦ 1), In _{1-X′-Y ′} Ga _{X ′} It is a multiple quantum well in which barrier layers composed of Al _{Y ′} N (0 ≦ X ′, Y ′ ≦ 1, 0 ≦ X ′ + Y ′ ≦ 1) are alternately stacked.
The light absorption region in which these layers are stacked to form a ridge structure is filled with, for example, a polyimide layer 1110 on both sides, and the upper surface of the contact layer 1107 and the upper surface of the polyimide layer 1110 are formed on substantially the same plane. Yes.

On the contact layer 1107, a p-type electrode 1108 having a laminated structure of a nickel layer with a thickness of 50 nm and a gold layer with a thickness of 200 nm formed thereon is formed. On the other hand, an n-type electrode 1109 having a laminated structure of a 50 nm-thick aluminum layer and a 200 nm-thick gold layer is formed on the back surface of the substrate 1101.
As described above, in the semiconductor optical modulator shown in FIG. 12, the cladding layers provided above and below the light absorption layer 1104 are divided into the lower cladding layer 1102, the lower SCH layer 1103, the upper SCH layer 1105, and the upper cladding layer. 1106 and the SCH structure.

  Here, the SCH structure will be described. In the optical modulator as described above, it is necessary to confine incident light in the light absorption layer 1104. When the thickness of the light absorption layer is approximately one-tenth or more of the wavelength of light, light can be confined by using the double heterostructure. On the other hand, when the light absorption layer has a thin quantum well structure, the light spreads greatly in the cladding layer, and the degree of light confinement in the active layer is reduced.

  Here, when a second cladding layer having a refractive index intermediate between these is inserted between the cladding layer and the light absorption layer, the electric field distribution of the light confined in the light absorption layer is greater than that of the double heterostructure. Will be much higher. The waveguide structure using the second cladding layer is the SCH structure described above, and in the semiconductor optical modulator of FIG. 11, the lower SCH layer 1103 and the upper SCH layer 1105 correspond to the second cladding layer. .

In the present invention, since the light absorption layer has a quantum well structure, the light confinement effect in the light absorption layer becomes higher when the SCH structure is used like the semiconductor optical modulator of FIG. Absorption efficiency can be increased.
Note that the above-described cladding layer and the second cladding layer used in the above-described SCH structure are configured to have a larger band gap energy than the active region (light absorption region).

  In the description of FIGS. 2A and 2B, the case where the forward bias is applied is shown. However, when the reverse bias is applied, the state shown in FIG. The band slope increases, and the energy between electrons and holes in the quantum well 152 becomes smaller than when no bias is applied. As a result, the wavelength of the absorption edge becomes longer. That is, the wavelength of light incident on the light incident end of the semiconductor optical modulator shown in FIGS. 1A and 1B is longer than the absorption edge wavelength when no bias is applied, and the absorption edge when a reverse bias is applied. When the wavelength is shorter than the wavelength, the light is transmitted through the modulator only when there is no bias.

  As described above, also in the second embodiment, the light absorption region is depleted by the built-in voltage generated by the formation of the polarization electric field and the pn junction, so that no reverse bias voltage is applied from the outside. Optimal operation is possible, and high-speed operation can be obtained without accumulating carriers even for modulation of high-intensity light. In addition, according to the present embodiment, the upper and lower SCH layers and the light absorption layer (multiple quantum well layer) are depleted by the polarization electric field, so that the upper and lower SCH layers are the minimum necessary for optical confinement. It is possible to make a thin layer.

  As a result, according to the present embodiment, the reactive voltage component due to the SCH layer can be reduced by thinning, and the modulation voltage from the outside contributes to optical modulation very effectively, and low voltage driving is realized. . Furthermore, according to the present embodiment, it is possible to increase the number of multiple quantum wells of the light absorption layer corresponding to the thinning of the upper and lower SCH layers. For this reason, according to the present embodiment, the extinction characteristic can be changed more steeply with respect to the voltage, and an even lower voltage operation can be realized. Furthermore, it becomes possible to provide a semiconductor optical device having a small nonlinearity with respect to the drive voltage.

1A and 1B are a schematic cross-sectional view (a) and a perspective view (b) illustrating a configuration example of a semiconductor optical modulator according to a first embodiment of the present invention. FIG. 2 is a band diagram showing a band structure in a partial configuration of the semiconductor optical modulator shown in FIG. 1. It is explanatory drawing which shows the band structure of the wurtzite type crystal which gave the tensile strain to the quantum well layer of a quantum well structure. Explanatory diagrams (a) and (b) showing the stacking order of Ga atoms and N atoms of single crystal GaN grown in the c-axis direction, and the state in which stress is generated in GaN grown on the sapphire substrate. FIG. 2 is a schematic cross-sectional view (c) and an explanatory view (d) showing the expansion and contraction of a GaN crystal lattice formed on a sapphire substrate. It is. It is a characteristic view showing the relationship between the square of absorption and photon energy in the formed InN film. It is a characteristic view which shows the photoluminescence measured at room temperature of the InN film | membrane. It is a characteristic view which shows the relationship between Eg of InGaAlN and the lattice constant a of the a axis. FIG. 6 is a characteristic diagram showing an X-ray diffraction spectrum of InN grown under a predetermined condition of a ratio V / III (ammonia / indium) between a supply amount of ammonia to a growth furnace and a supply amount of trimethylindium (TMI) as an indium raw material. is there. It is a characteristic view which shows the relationship between the modulation applied voltage of the semiconductor optical modulator and transmitted light intensity in this Embodiment. FIG. 6 is a characteristic diagram showing a state of a mixed region and a non-mixed region in crystal growth of an InGaAlN-based material. It is a perspective view which shows the structural example of the laser with an optical modulator. It is a typical perspective view showing roughly other examples of composition of a semiconductor optical modulator in an embodiment of the invention. It is a band figure which shows the band structure example in the partial structure of the semiconductor optical modulator in embodiment of this invention. FIG. 6 is a characteristic diagram showing the dependence of the light absorption characteristics on the wavelength λ of incident light when no voltage is applied and when a reverse bias is applied. It is a characteristic view which shows the voltage dependence of an extinction ratio.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... Single crystal substrate, 102 ... Buffer layer, 103 ... Electrode layer, 104 ... Cladding layer, 105 ... Light absorption layer, 106 ... Cladding layer, 107 ... Contact layer, 108 ... n-type metal electrode, 109 ... p-type metal electrode 151 ... barrier layers, 152 ... quantum well layers.

Claims (13)

A lower cladding layer of a first conductivity type;
A light absorption layer having a quantum well structure formed on the lower cladding layer and composed of a quantum well layer and a barrier layer;
An upper cladding layer of a second conductivity type formed on the light absorption layer;
Comprising at least
The quantum well layer is composed of In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦ 1, 0 ≦ X + Y ≦ 1),
The barrier layer is made of In _{1-X′-Y ′} Ga _{X ′} Al _{Y ′} N (0 ≦ X ′, Y ′ ≦ 1, 0 ≦ X ′ + Y ′ ≦ 1),
The lower cladding layer, the light absorption layer, and the upper cladding layer constitute an optical waveguide having a light incident end,
The quantum well and the barrier layer have a composition range without phase separation other than an immiscible region of InGaAlN,
The light absorption layer is polarized in an unbiased state,
The semiconductor optical modulator according to claim 1, wherein the polarization is a sum of natural polarization and piezo polarization in the same direction generated in the light absorption layer. The semiconductor optical modulator according to claim 1.
The semiconductor light modulator, wherein the light absorption layer has a multiple quantum well structure. The semiconductor optical modulator according to claim 1 or 2,
2. The semiconductor optical modulator according to claim 1, wherein the lower cladding layer is formed on a predetermined substrate. The semiconductor optical modulator according to any one of claims 1 to 3,
The semiconductor optical modulator, wherein the quantum well layer and the barrier layer have different lattice constants. The semiconductor optical modulator according to claim 4.
The semiconductor optical modulator, wherein the quantum well layer has a larger lattice constant than the barrier layer. The semiconductor optical modulator according to claim 4.
The semiconductor optical modulator, wherein the quantum well layer has a lattice constant smaller than that of the barrier layer. The semiconductor optical modulator according to any one of claims 1 to 3,
The quantum well layer is composed of an InN crystal,
The semiconductor optical modulator according to claim 1, wherein the barrier layer is made of a GaN crystal. A laser with an optical modulator composed of a waveguide type semiconductor laser and a semiconductor optical modulator integrated on the same substrate,
The semiconductor optical modulator is:
A first conductivity type lower cladding layer formed on a substrate, a quantum well structure light absorption layer formed on the lower cladding layer and comprising a quantum well layer and a barrier layer, and on the light absorption layer The quantum well layer is made of In _1-XY Ga _X Al _Y N (0 ≦ X, Y ≦ 1, 0 ≦ X + Y ≦ 1). The barrier layer is made of In _{1-X′-Y ′} Ga _{X ′} Al _{Y ′} N (0 ≦ X ′, Y ′ ≦ 1, 0 ≦ X ′ + Y ′ ≦ 1),
The lower cladding layer, the light absorption layer, and the upper cladding layer constitute an optical waveguide having a light incident end ,
The quantum well and the barrier layer have a composition range without phase separation other than an immiscible region of InGaAlN,
The light absorption layer is polarized in an unbiased state,
2. The semiconductor optical modulator according to claim 1, wherein the polarization is a sum of natural polarization and piezo polarization in the same direction generated in the light absorption layer. The laser with an optical modulator according to claim 8,
The laser with an optical modulator, wherein the optical modulator has a multiple quantum well structure. The laser with an optical modulator according to claim 8 or 9,
The laser with an optical modulator, wherein the quantum well layer and the barrier layer have different lattice constants. The laser with an optical modulator according to claim 10,
The laser with an optical modulator, wherein the quantum well layer has a larger lattice constant than the barrier layer. The laser with an optical modulator according to claim 10,
The laser with an optical modulator, wherein the quantum well layer has a lattice constant smaller than that of the barrier layer. The laser with an optical modulator according to claim 8 or 9,
The quantum well layer is composed of an InN crystal,
The laser with an optical modulator, wherein the barrier layer is made of a GaN crystal.

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