JPH08264895A - Semiconductor device - Google Patents

Abstract

PURPOSE: To obtain large change of an absorption coefficient by applying a small electric field by making deeper the quantum well at the inner quantum well layer side of a quantum well formed at a valence band than other quantum wells and locating the energy level of the lowest hole nearly at the center of the quantum well. CONSTITUTION: One quantum well for an electron is formed by a quantum well layer 12 at a conduction band and two quantum wells for a hole are formed by quantum well layers 12 and 16 at a valence band, where the quantum well at the side of the quantum well layer 16 out of the quantum wells formed at the valence band is deeper than other quantum wells and the energy level of the lowest hole is located nearly at the center of the quantum well. Also, a barrier due to a quantum barrier layer 14 is formed between the quantum well layer 12 and the quantum well layer 16 at the valence band. While no electric field is applied, the peak of the wave function of an electron should be located at the center of the quantum well layer 12 and that of the wave function of a hole should be located at the quantum well layer 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device, and more particularly to a semiconductor device having a quantum well structure.

[0002]

2. Description of the Related Art Quantum well structures have been studied under application to laser diodes, optical modulators and the like. A conventional type I quantum well structure will be described with reference to FIGS. 18 is a band diagram of a type I quantum well structure, FIG. 19 is a graph showing wave functions of electrons and holes when an electric field is applied to the type I quantum well structure, and FIG. 20 is a type I quantum well structure. FIG. 21 is a graph showing changes in transition wavelength when an electric field is applied, and FIG. 21 is a graph showing overlap of wave functions when an electric field is applied to a type I quantum well structure. The type I is one of band structures formed when bonding different kinds of semiconductors, and a forbidden band of a semiconductor having a small forbidden band width is included in a forbidden band of a semiconductor having a large forbidden band width. The state of being.

The type I quantum well layer is formed of a material having a narrow bandgap and a quantum well layer 12 having a wide bandgap, and quantum barrier layers 10 and 14 for confining carriers in the quantum well layer 12.
It is composed of In the state where no electric field is applied to the type I quantum well, the wave functions of electrons and holes are located in the center of the well layer 12. When an electric field is applied, the wave functions of electrons and holes shift according to the electric field. For example, when a negative electric field is applied to the structure shown in FIG. 18, the wave function of electrons shifts to the right and the wave function of holes shifts to the left (FIG. 19).

Therefore, in the type I quantum well, the overlap between the electron wave function and the hole wave function becomes maximum when no electric field is applied, and becomes smaller as the electric field is applied (FIG. 21). As a result, when an electric field is applied to the type I quantum well structure, the optical transition wavelength is on the long wavelength side,
That is, it shifts to the red side (FIG. 20). Next, a conventional coupled quantum well structure will be described with reference to FIGS.

FIG. 22 is a band diagram of a coupled quantum well structure, FIG. 23 is a graph showing wave functions of electrons and holes when an electric field is applied to the coupled quantum well structure, and FIG. 24 is a coupled quantum well. FIG. 25 is a graph showing changes in transition wavelength when an electric field is applied to the well structure, and FIG. 25 is a graph showing overlap of wave functions when an electric field is applied to the coupled quantum well structure.

As shown in FIG. 22, the coupled quantum well structure has a quantum barrier layer 14 in which two type I quantum wells are thin.
It has a structure separated by. Quantum barrier layer 14
The two quantum well layers 12 and 16 separated by are coupled by tunneling of electrons and holes through the quantum barrier layer 14 between the quantum well layers 12 and 16. When an electric field is applied to the coupled quantum well structure, electrons and holes move to different quantum well layers. For example, when a negative electric field is applied to the structure shown in FIG. 22, the electron wave function shifts to the quantum well layer 16 and the hole wave function shifts to the quantum well layer 12 (FIG. 23).

Therefore, like the type I quantum well, the optical transition wavelength shifts to the long wavelength side, that is, red (FIG. 2).
4). In addition, since the change in the wave function overlap is larger than that in the type I quantum well structure, a larger energy shift can be obtained than in the type I quantum well structure (FIG. 25). In addition to the above quantum well structure, a graded asymmetric quantum well structure has been proposed.

In an ordinary graded quantum well structure, the peak positions of the wave function of electrons and holes when no electric field is applied do not match. The quantum well is pre-biased depending on the positions and shapes of the electron wave function and the hole wave function. Therefore, in the graded quantum well structure, a peak of the oscillator strength occurs when an electric field that cancels this bias is applied.

[0009]

However, in the conventional semiconductor device having the type I quantum well structure described above,
Since the peak of the wave function overlap occurs when the electric field is not applied, there are problems that the wave function overlaps and the absorption oscillator strength decreases due to the application of the electric field.

Since the amount of energy shift is proportional to the fourth power of the width of the quantum well layer, it is desirable to increase the width of the quantum well layer in order to increase the amount of energy shift. If the thickness is increased, the peaks of the wave function of the electron and the hole are separated when a high electric field is applied, and there are problems that the wave function overlaps, the oscillator strength, and the absorption peak are reduced.

In a conventional semiconductor device having a coupled quantum well structure, when an electric field is applied, the wave functions of electrons and holes move in different quantum well directions, so that the wave functions overlap and vibrate. There was a problem that the child strength decreased. In addition, in a semiconductor device having a graded quantum well structure, the peaks of the wavefunctions of electrons and holes can be separated even when no electric field is applied, but the amount of displacement thereof is limited to the distance of the barrier layer. there were.

The object of the present invention is that the wave functions of electrons and holes are separated in the state where no electric field is applied, and the overlap of wave functions increases with the application of an electric field, resulting in large changes in oscillator strength and absorption. It is to provide a semiconductor device that can be obtained.

[0013]

The above object is to form a first semiconductor layer and a level of the conduction band lower end on the first semiconductor layer, the conduction band lower end level being lower than the conduction band lower end level. A second semiconductor layer which is lower than the first valence band and is higher than the valence band upper level of the first semiconductor layer and which is a quantum well for electrons and holes; Formed on the semiconductor layer, the lower level of the conduction band is higher than the lower level of the conduction band of the second semiconductor layer, and the upper level of the valence band is higher than the valence band of the second semiconductor layer. Of the third semiconductor layer, which is higher than the level of the third semiconductor layer and becomes a quantum well for holes, and the level of the bottom of the conduction band is formed on the third semiconductor layer and the bottom of the conduction band of the third semiconductor layer. The level of the valence band upper end is lower than the level of the valence band upper end of the third semiconductor layer. It is achieved by a semiconductor device characterized by having a conductor layer.

In the above semiconductor device, the level of the bottom of the conduction band between the second semiconductor layer and the third semiconductor layer is the level of the bottom of the conduction band of the second semiconductor layer. The level higher and higher than or equal to or lower than the conduction band lower end level of the third semiconductor layer, and the valence band upper end level is higher than the second band.
It is desirable to further include a fifth semiconductor layer having a level lower than the valence band upper end level of the semiconductor layer and the valence band upper end level of the third semiconductor layer.

In the above semiconductor device, it is desirable that the second semiconductor layer be thicker than the third semiconductor layer. An optical modulator using the above semiconductor device, characterized in that incident light is modulated by an electric field applied between the first semiconductor layer and the fourth semiconductor layer. Also achieved by.

Further, in the above semiconductor device, the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer,
It is desirable to be composed of an InGaAsP-based semiconductor material. In the above semiconductor device, the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer is an InAlAsP-based semiconductor material. It is desirable to be composed by.

Further, in the above semiconductor device, the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer,
It is desirable to be composed of an InGaAlP-based semiconductor material. In the above semiconductor device, the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer is an InSbAsP-based semiconductor material. It is desirable to be composed by.

[0018]

According to the present invention, the first semiconductor layer is formed on the first semiconductor layer, and the lower level of the conduction band is lower than the lower level of the conduction band of the first semiconductor layer. A second semiconductor layer whose level at the top of the band is higher than the level at the top of the valence band of the first semiconductor layer and serves as a quantum well for electrons and holes;
On the semiconductor layer, the lower level of the conduction band is higher than the lower level of the conduction band of the second semiconductor layer, and the upper level of the valence band is higher than the upper level of the valence band of the second semiconductor layer. Higher than the second semiconductor layer and forming a quantum well with respect to holes, and a level at the bottom of the conduction band formed on the third semiconductor layer and a level at the bottom of the conduction band of the third semiconductor layer. Since the quantum well structure is constituted by the fourth semiconductor layer which is substantially equal to or lower than the valence band upper end level of the third semiconductor layer, the quantum well structure is formed by applying a slight electric field. A large change in absorption coefficient can be obtained.

Further, since modulation is possible by changing the strength of the oscillator, it is possible to construct a modulator having a negative α parameter. Moreover, since the nonlinear χ ² parameter can be adjusted without applying a bias due to the asymmetric quantum well structure, a large nonlinear optical effect can be obtained.

Further, between the second semiconductor layer and the third semiconductor layer, the level of the bottom of the conduction band is higher than the level of the bottom of the conduction band of the second semiconductor layer and the level of the third semiconductor layer. The level at the top of the valence band is approximately equal to or lower than the level at the bottom of the conduction band,
The above effect can be obtained also in a semiconductor device further including a fifth semiconductor layer having a level lower than the valence band upper end level of the second semiconductor layer and a valence band upper end level of the third semiconductor layer.

In the above semiconductor device, if the thickness of the second semiconductor layer is larger than the width of the third semiconductor layer,
Absorption by light holes can be suppressed to a small level. Further, if the optical modulator is configured by the above semiconductor device, a high modulation effect can be obtained with a low applied electric field. In addition, InG is used for the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer.
An aAsP-based semiconductor material can be applied.

In addition, the first semiconductor layer, the second semiconductor layer,
An InAlAsP-based semiconductor material can be applied to the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer. In addition, the first semiconductor layer, the second semiconductor layer, the third
An InGaAlP-based semiconductor material can be applied to the semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer.

In addition, the first semiconductor layer, the second semiconductor layer,
An InSbAsP-based semiconductor material can be used for the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer.

[0024]

EXAMPLE A structure of a semiconductor device according to a first example of the present invention will be described with reference to FIGS. FIG. 1 is a sectional view and a band diagram showing the structure of a semiconductor device according to this embodiment,
FIG. 3 is a diagram showing a change of a wave function by applying an electric field in the semiconductor device according to the present embodiment, FIG. 3 is a diagram showing a change of a transition wavelength by applying an electric field in the semiconductor device according to the present embodiment, and FIG. 4 is a semiconductor according to the present embodiment. It is a figure which shows the change of the overlap of a wave function by the application of the electric field in an apparatus.

The semiconductor device according to the present embodiment is formed by stacking a quantum barrier layer 10, a quantum well layer 12, a barrier layer 14, a quantum well layer 16 and a barrier layer 18 (FIG. 1).
(A)). The quantum well layer 12 forms one quantum well for electrons in the conduction band, and the quantum well layers 12 and 16 form two quantum wells for holes in the valence band. Here, among the quantum wells formed in the valence band, the quantum well on the side of the quantum well layer 16 is deeper than the other quantum well, and the lowest hole energy level is located almost at the center of the quantum well. Further, a barrier is formed by the quantum barrier layer 14 between the quantum well layer 12 and the quantum well layer 16 in the valence band (FIG. 1 (b)).

The structure of the semiconductor device according to this embodiment is an intermediate structure between the type I quantum well structure and the type II quantum well structure, that is, it is a type 1.5 quantum well structure. In order to form such a quantum well structure, for example, a Ga _x In _{1 -x} As _y P _1-y- based material shown in Table 1 is used.
It can be realized by sequentially stacking on an nP substrate.

[0027]

[Table 1] Next, the effect of applying an electric field to such a quantum well structure will be described. In the quantum well structure shown in FIG.
In the state where no electric field is applied, the peak of the electron wave function is located in the center of the quantum well layer 12, and the peak of the hole wave function is located in the quantum well layer 16.

When a positive electric field is applied in this state, electrons move to the left side of the quantum well layer 12 and holes move to the quantum well layer 16.
To the right of (Fig. 2). As the electrons and holes move in this way, the transition wavelength largely shifts to the red side (FIG. 3), and the carrier overlap is reduced (FIG. 4). When a negative electric field is applied, the electrons move to the right side of the quantum well layer 12. On the other hand, the holes move to the left side of the quantum well layer 16 and the overlap of carrier wave functions increases (FIGS. 2 and 4). This causes the transition energy to shift slightly to the blue side (Fig. 3).

When the applied electric field is increased, the wave function of the heavy hole having the lowest energy penetrates through the quantum barrier layer 14 and is located near the center of the quantum well layer 12.
This makes it possible to form a sharply rising oscillator strength peak on the low electric field side (FIG. 4). When the electric field is further increased, the hole wave function moves to the left side of the quantum well layer 12 and the overlap of the wave function decreases (FIG. 4).
This causes a red-side shift at the lowest energy transition (e ₀ -hh ₀ transition) (Fig. 3).

As described above, when the semiconductor device is formed by using the quantum well structure formed according to this embodiment, a very large change in absorption can be obtained by slightly changing the applied electric field. Therefore, such a quantum well has a desirable structure as an electron absorption modulator. For example, in the case of a modulator having a background loss of -3 dB and an attenuation ratio of 12 dB, it is necessary for the absorption coefficient to change about four times, but as shown in FIG. It is possible to obtain

Since the change of the absorption coefficient is caused by the change of the oscillator strength rather than the shift of the transition wavelength to the red side, the light of the wavelength on the blue side of the absorption band edge should be used as the input light to be modulated. You can As a result, the refractive index decreases as the absorption coefficient increases, and the α parameter becomes a negative value. When the α parameter is negative, it is possible to improve the transmission performance by a pulse of a long distance and a high bit rate more than that of an optical fiber.

Although the transition from the energy level of the second-highest hole to the energy level of electrons is not usually observed, the absorption and the oscillator strength proportional to the overlap of the wave functions are asymmetrical in structure. Therefore, it becomes large (Figs. 3 and 4). Therefore, if the wavelength separation of the two absorption peaks is small, the absorption effect of the second lowest transition obscures the modulation effect of the transition at low energy. Therefore, it is necessary to pay attention to this point when forming the quantum well structure according to the present embodiment.

Next, the structure of the semiconductor device according to the second embodiment of the present invention will be described with reference to FIGS. 5 is a cross-sectional view and a band diagram showing the structure of the semiconductor device according to the present embodiment, FIG. 6 is a diagram showing changes in the wave function due to the application of an electric field in the semiconductor device according to the present embodiment, and FIG. 8 is a diagram showing a change in transition wavelength due to application of an electric field in FIG. 8, FIG. 8 is a diagram showing a change in overlap of wave functions due to application of an electric field in the semiconductor device according to this embodiment, and FIG. 9 is a diagram showing an electric field in the semiconductor device according to this embodiment. FIG. 10 is a diagram showing a change in band structure when applied, and FIG. 10 is a diagram for explaining the operation of the semiconductor device according to the present embodiment.

The semiconductor device according to the present embodiment is formed by stacking a quantum barrier layer 10, a quantum well layer 12, a quantum well layer 16 and a barrier layer 18 (FIG. 5A). The structure of the semiconductor device according to the present embodiment is almost the same as that of the semiconductor device according to the first embodiment, but is characterized in that the quantum barrier layer 14 is not formed. That is, one quantum well for electrons due to the quantum well layer 12 is formed in the conduction band, and quantum wells 12 and 16 are merged with a step in the valence band to form a quantum well for holes. here,
The quantum well layer 16 is deeper than the quantum well layer 12 with respect to holes, and the lowest hole energy level is the quantum well layer 16.
It is located almost in the center (Fig. 5 (b)).

In order to form such a quantum well structure, for example, it is realized by sequentially laminating the Ga _x In _{1 -x} As _y P _{1 -y} type materials shown in Table 2 on an InP substrate. You can

[0036]

[Table 2] Next, the effect of applying an electric field to such a quantum well structure will be described. In the quantum well structure shown in FIG. 5,
In the state where no electric field is applied, the peak of the electron wave function is located in the center of the quantum well layer 12, and the peak of the hole wave function is located in the quantum well layer 16. It should be noted that compared with the first embodiment shown in FIG. 2, the tail of the hole wave function is widened, but it is sufficiently separated from the electron wave function.

When a positive electric field is applied in this state, the band structure of the quantum well is tilted as shown in FIG. 9 (a). At the same time, the electrons move to the left side of the quantum well layer 12 and the holes move to the right side of the quantum well layer 16 (FIG. 6). Due to the movement of electrons and holes, the transition wavelength largely shifts to the red side (FIG. 7), and the overlap of carrier wave functions is reduced (FIG. 8).

When a negative electric field is applied, the band structure of the quantum well tilts as shown in FIG. 9B, and the electrons move to the right side of the quantum well layer 12. On the other hand, the holes move to the left side of the quantum well layer 16 and the overlap of carrier wave functions increases (FIGS. 6 and 8). This causes the transition energy to shift slightly to the blue side (Fig. 7). As the applied electric field is increased, the wave function of the heavy hole having the lowest energy comes to be located near the center of the quantum well 12. This makes it possible to form a sharply rising oscillator strength peak on the low electric field side (FIG. 8).

When the electric field is further increased, the Hall wavefunction moves to the left side of the quantum well 12 and the overlap of the wavefunctions decreases (FIG. 8). This causes a red-side shift at the lowest energy transition (e ₀ -hh ₀ transition) (FIG. 7). Next, the operation of the semiconductor device according to the present embodiment will be described using the absorption spectrum.

FIG. 10A shows the absorption spectrum of the e ₀ -hh ₀ transition in the state where no electric field is applied, and FIG. 10C shows the absorption spectrum of the e ₀ -hh ₁ transition in the state where no electric field is applied. Shows. FIG. 10E shows an absorption spectrum in the case where both the e ₀ -hh ₀ transition and the e ₀ -hh ₁ transition are taken into consideration in the state where no electric field is applied. FIG.
(B) is e in the state where an electric field of −50 kV / cm is applied
The absorption spectrum of the _0- hh ₀ transition is -5 in FIG.
Shows the absorption spectrum of e ₀ -hh ₁ transition while applying an electric field of 0 kV / cm. -50 in FIG.
The absorption spectrum is shown in the case where both the e ₀ -hh ₀ transition and the e ₀ -hh ₁ transition are taken into consideration in the state where an electric field of kV / cm is applied.

Comparing FIG. 10 (a) and FIG. 10 (b), it is found that the absorption increases due to the increase of the oscillator strength due to the application of the electric field, and the e ₀ -hh ₀ transition is slightly shifted to the blue side. I understand. Comparing FIG. 10C and FIG. 10D,
It can be seen that in the e ₀ -hh ₁ transition when an electric field is applied, the transition wavelength shifts to the red side and the absorption decreases.

Therefore, considering both of these transitions,
In the state where no electric field is applied, the modulation signal is on the shorter wavelength side than the e ₀ -hh ₀ transition, but on the longer wavelength side than the e ₀ -hh ₁ transition. In this wavelength band, the absorption spectrum of the e ₀ -hh ₀ transition remains, but the oscillator strength is small and the absorption is not large when no electric field is applied (FIG. 10).
(E)).

When an electric field is applied in this state, the oscillator strength of the e ₀ -hh ₀ transition increases with the absorption of the modulation signal wavelength. Furthermore, e ₀ -hh increased absorption in the modulation wavelength due ₁ shift to the red side of the transition was observed, the absorption increases from alpha ₀ before the electric field applied to α ₀ + Δα (FIG 10 (f)). Thus, a large increase in absorption can be obtained by applying an electric field.

Next, the influence of the width of the quantum well will be shown.
11 to 14 are characteristic diagrams when the quantum well is configured by the structure shown in Table 3. The difference from the quantum well structure shown in FIGS. 5 to 8 is that the width of the quantum well layer 12 is increased to 9 nm.

[0045]

[Table 3] As shown, thickening the quantum well layer 12 sharpens the peak of the oscillator strength of the e ₀ -hh ₀ transition.
Further, since the wave function of the electrons is distributed more widely, it is possible to reduce the overlap of the wave function without applying the electric field.

When a negative electric field is applied, the wave motion of electrons and holes
The shift of the function becomes large and the oscillator strength
The sharpness of the peak of the degree and the large shift of the transition wavelength
I hear On the other hand, if the width of the quantum well layer 12 becomes thick, e₀
-Hh₀Transition energy and e₀-Hh₁Energy of transition
The difference between the₀−
hh ₁Unexpected absorption from the transition becomes unacceptable. Well
In addition, when forming a thick quantum well with a material with lattice strain
In addition, there is a problem that the manufacturing process is difficult.

Regarding the width relationship between the quantum well layer 12 and the quantum well layer 16, it is desirable that the width of the quantum well layer 12 be larger than the width of the quantum well layer 16. This is because absorption by light holes can be suppressed to a small level. Next, the dipole moment unique to the quantum well structure according to this example will be described. The dipole moment enhances the nonlinear effect of the material.

In the conventional type I quantum well, dipole moment does not exist because the wave functions of electrons and holes are symmetric. Further, in the conventional type II quantum well, an offset is provided in the wave function of the electron and the hole, but the dipole moment is canceled out because of the symmetry between the adjacent wells. On the other hand, in the quantum well structure according to the present example, the wave functions of electrons and holes do not match as shown in FIG. 15 in the state where no external electric field is applied. With such an asymmetrical offset, a dipole moment is induced in the material and causes an interaction with the electric field or light.

When the dipole moment interacts with an electric field or light, a χ ² nonlinear effect occurs. Therefore, an application using nonlinear frequency conversion can be constructed. In addition, in order to perform the nonlinear frequency conversion, in addition to using a material that enhances the nonlinear effect, it is necessary to perform phase matching between the pump and the probe beam. As described above, the quantum well structure formed according to the present embodiment, like the quantum well structure according to the first embodiment where the quantum barrier layer 14 is formed, has a very large change in absorption by slightly changing the applied electric field. Can be obtained.

Therefore, such a quantum well has a desirable structure as an electron absorption modulator. Since the change of the absorption coefficient is caused by the change of the oscillator strength rather than the shift of the transition wavelength to the red side, the light of the wavelength on the blue side of the absorption band edge can be used as the input light to be modulated. As a result, the refractive index decreases as the absorption coefficient increases, and the α parameter becomes a negative value. When the α parameter is negative, it is possible to improve the transmission performance by a pulse of a long distance and a high bit rate more than that of an optical fiber.

Further, in the semiconductor device according to the present embodiment, the above effect can be obtained without forming the thin quantum barrier layer 14.
Therefore, the quantum well structure according to the present invention does not largely depend on the quantum barrier layer 14 in its characteristics. In this respect, there is an advantage over the conventional coupled quantum well structure in which the quantum barrier layer greatly affects the characteristics. Further, in order to form the thin quantum barrier layer 14, precise control is required in the manufacturing process, but in the quantum well structure according to this embodiment, the thin quantum barrier layer 1 is formed.
4, the manufacturing process can be greatly simplified.

Further, in the semiconductor device according to the present embodiment, a dipole moment is induced in the material and a χ ² non-linear effect occurs, so that an application using non-linear frequency conversion can be constructed. In the semiconductor device according to the first and second embodiments, the escape time of electrons and holes can be controlled independently. In order to shorten the carrier escape time, it is necessary to avoid the problem of saturation and the frequency response of the modulator due to the screening effect of the electric field. However, the carrier escape time must be longer than the exciton lifetime in order to reduce the sharpness of the absorption edge and to avoid the exciton spreading effect which reduces the modulator damping ratio. For holes, the barrier layer is simply confined by applying an electric field, but for electrons, a barrier layer similar to the well layer on the hole side forms a barrier. Therefore, the electrons have an effective barrier region that reduces the escape time as compared with the escape time of holes. The faster electron escape time is InGaA where the valence band offset is generally larger than the conduction band offset and the electron mass is larger than the hole mass.
It is very advantageous for sP-based materials.

Although the quantum well structure is formed of the InGaAsP-based material in the above-mentioned embodiment, the above effect can be obtained by forming the band structure shown in FIG. 1B or 5B. It is good and is not limited to this. For example, the same results as above can be obtained by growing an InAlAsP-based material, an InGaAlP-based material, an InSbAsP-based material, or the like on an InP substrate.

Next, the structure of the semiconductor device according to the third embodiment of the present invention will be described with reference to FIG. FIG. 16 is a diagram showing the structure of a semiconductor device according to the third embodiment of the present invention. In the semiconductor device according to the present embodiment, the optical modulator using the quantum well according to the first embodiment or the second embodiment is a DFB.
(Distributed Feedback Type) Distributed feedback is formed integrally with a laser diode.

On the N-type InP substrate 20, an N-type cladding layer 22, an undoped isolation / confinement heterostructure layer 24, the quantum well 26 described in the first or second embodiment, and an undoped layer. Isolation confinement heterostructure layer 28
, P-type cladding layer 30, and P-type contact layer 32
And are stacked and deposited to form an optical modulator. The optical modulator configured in this way includes the DFB laser 3
4 are connected.

It is possible to confine light in the direction perpendicular to the waveguide type optical modulator by changing the refractive index using various materials. However, in order to guide the electron absorption modulation by doping each layer. It enables an application of applying an electric field. In addition, since the quantum well described in the first or second embodiment has an asymmetric structure, P-type and N-type doping is performed so that a strong electric field can be applied to increase absorption. .

The laminate thus formed has a width of 1
It is patterned in a stripe shape with a thickness of up to 3 μm. A semi-insulating InP buried heterostructure layer 36 is formed between the patterned stripe-shaped active quantum materials. Further, a polyimide film 38 is formed to reduce the device capacitance because modulation at a high frequency is possible.

Further, the InP substrate 20 is thin so that it can be easily cleaved and an N-type contact can be easily formed. Next, the operation of the modulator will be described. As the input signal to be modulated, for example, 1.31 μm TE polarized light is used. When used as an optical waveguide type modulator, use TE polarized light as an input signal in order to cause a large interaction with heavy holes and a small interaction with light holes. Is desirable.

If the wavelength of the input signal is shorter than the e ₀ -hh ₀ transition wavelength in the absence of an applied electric field, unwanted absorption will occur in the background of the input signal. But,
If the absorption amount is relatively small, the oscillator strength is 0 kV / c
It becomes relatively small under an applied electric field of m. When a negative electric field is applied, e ₀ -hh ₀ transition is slightly shifted to the blue side (FIG. 7). However, as shown in FIG. 8, the shift to the blue side is caused by a sharp increase in the oscillator strength. Such a change in the oscillator strength is extremely rapid as compared with a normal type I quantum well when an electric field of 50 kV / cm is applied.

A rapid increase in oscillator strength increases absorption in the material, and a small electric field is needed to obtain a rapid increase in oscillator strength, which means that the material is used under a low electric field. It shows that you can do it. Low bias operation is desirable in the design of optical modulators because it can be adapted to low voltage, high speed electronic devices.

In the modulator according to the present embodiment, the oscillator strength increases with the applied electric field, so the absorption peak of the material is
It becomes larger than that of the conventional type I quantum well in which the oscillator strength decreases and the absorption peak decreases with the application of an electric field.
The quantum well structure having a large absorption peak enables the design with a short length and a small capacitance, so that a high-speed optical modulator can be constructed.

E₀-Hh₀Using the energy level of the transition
If e₀-Hh₀Of the background from the transition
E not only for absorption₀-Hh₁Back from transition
Ground absorption also occurs. Normal type I quantum well
Then e₀-Hh₁The transition has a small overlap of wave functions
It does not happen for the sake of safety. However, in the asymmetric structure, e ₀
-Hh₁A transition occurs, and e₀-Hh
₀It has a larger oscillator strength than the transition.

These facts show that a negative α parameter can be obtained when the optical modulator is operated in a state where the difference between the transition wavelength and the signal wavelength is very small. As described above, according to the present embodiment, the optical modulator can be configured by using the quantum well structure according to the first embodiment or the second embodiment.

Although the DFB laser diode and the optical modulator are integrally formed in the above embodiment, the optical modulator may be formed as a single unit. In addition, although the quantum well structure of only one cycle is shown in the first and second embodiments, a multi-cycle quantum well may be configured as shown in FIG.

[0065]

As described above, according to the present invention, the first semiconductor layer is formed on the first semiconductor layer, and the lower level of the conduction band is lower than the lower level of the conduction band of the first semiconductor layer. Second level which is lower than the upper level of the valence band of the first semiconductor layer and is higher than the upper level of the valence band of the first semiconductor layer to form a quantum well for electrons and holes.
And the second semiconductor layer, the lower level of the conduction band is higher than the lower level of the conduction band of the second semiconductor layer, and the upper level of the valence band is the second semiconductor layer. Of the third semiconductor layer, which is formed on the third semiconductor layer and a third semiconductor layer which is higher than the upper level of the valence band of and which serves as a quantum well for holes. A quantum well structure is constituted by a fourth semiconductor layer which is almost equal to or lower than the lower level of the conduction band and whose upper level of the valence band is lower than the upper level of the valence band of the third semiconductor layer. Therefore, a large change in absorption coefficient can be obtained by applying a slight electric field.

Further, since modulation is possible by changing the oscillator strength, it is possible to construct a modulator having a negative α parameter. Moreover, since the nonlinear χ ² parameter can be adjusted without applying a bias due to the asymmetric quantum well structure, a large nonlinear optical effect can be obtained.

In addition, between the second semiconductor layer and the third semiconductor layer, the level at the bottom of the conduction band is higher than the level at the bottom of the conduction band of the second semiconductor layer and the level of the third semiconductor layer. The level at the top of the valence band is approximately equal to or lower than the level at the bottom of the conduction band,
The above effect can be obtained also in a semiconductor device further including a fifth semiconductor layer having a level lower than the valence band upper end level of the second semiconductor layer and a valence band upper end level of the third semiconductor layer.

In the above semiconductor device, if the thickness of the second semiconductor layer is larger than the width of the third semiconductor layer,
Absorption by light holes can be suppressed to a small level. Further, if the optical modulator is configured by the above semiconductor device, a high modulation effect can be obtained with a low applied electric field. In addition, InG is used for the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer.
An aAsP-based semiconductor material can be applied.

Further, the first semiconductor layer, the second semiconductor layer,
An InAlAsP-based semiconductor material can be applied to the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer. In addition, the first semiconductor layer, the second semiconductor layer, the third
An InGaAlP-based semiconductor material can be applied to the semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer.

In addition, the first semiconductor layer, the second semiconductor layer,
An InSbAsP-based semiconductor material can be used for the third semiconductor layer, the fourth semiconductor layer, or the fifth semiconductor layer.

[Brief description of drawings]

FIG. 1 is a sectional view and a band diagram showing a structure of a semiconductor device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing changes in wave function due to application of an electric field in the semiconductor device according to the first example of the present invention.

FIG. 3 is a diagram showing changes in transition wavelength due to application of an electric field in the semiconductor device according to the first example of the present invention.

FIG. 4 is a diagram showing a change in overlap of wave functions due to application of an electric field in the semiconductor device according to the first example of the present invention.

FIG. 5 is a sectional view and a band diagram showing a structure of a semiconductor device according to a second embodiment of the present invention.

FIG. 6 is a diagram showing changes in wave function due to application of an electric field in a semiconductor device according to a second example of the present invention.

FIG. 7 is a diagram showing changes in transition wavelength due to application of an electric field in a semiconductor device according to a second example of the present invention.

FIG. 8 is a diagram showing changes in overlap of wave functions due to application of an electric field in a semiconductor device according to a second example of the present invention.

FIG. 9 is a diagram showing a change in band structure when an electric field is applied to the semiconductor device according to the second embodiment of the present invention.

FIG. 10 is a diagram for explaining the operation of the semiconductor device according to the second embodiment of the present invention.

FIG. 11 is a band diagram showing a structure of a semiconductor device according to a modification of the second embodiment.

FIG. 12 is a diagram showing changes in wave function due to application of an electric field in a semiconductor device according to a modification of the second embodiment.

FIG. 13 is a diagram showing a change in transition wavelength due to application of an electric field in a semiconductor device according to a modification of the second embodiment.

FIG. 14 is a diagram showing a change in overlap of wave functions due to application of an electric field in a semiconductor device according to a modification of the second embodiment.

FIG. 15 is a diagram illustrating a dipole moment in the semiconductor device according to the second example of the present invention.

FIG. 16 is a schematic diagram showing the structure of a semiconductor device according to a third embodiment of the present invention.

FIG. 17 is a diagram showing a modification of the quantum well in the semiconductor device according to the third embodiment of the present invention.

FIG. 18 is a band diagram showing a conventional type I quantum well structure.

FIG. 19 is a diagram showing changes in wave function due to application of an electric field in a conventional type I quantum well structure.

FIG. 20 is a diagram showing changes in transition wavelength due to application of an electric field in a conventional type I quantum well structure.

FIG. 21 is a diagram showing a change in overlap of wave functions due to application of an electric field in a conventional type I quantum well structure.

FIG. 22 is a band diagram showing a conventional coupled quantum well structure.

FIG. 23 is a diagram showing a change in wave function due to application of an electric field in a conventional coupled quantum well structure.

FIG. 24 is a diagram showing changes in transition wavelength due to application of an electric field in a conventional coupled quantum well structure.

FIG. 25 is a diagram showing a change in overlap of wave functions due to application of an electric field in a conventional coupled quantum well structure.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Quantum barrier layer 12 ... Quantum well layer 14 ... Quantum barrier layer 16 ... Quantum well layer 18 ... Quantum barrier layer 20 ... InP substrate 22 ... Clad layer 24 ... Separation and confinement heterostructure layer 26 ... Quantum well 28 ... Separation and confinement heterostructure Layer 30 ... Clad layer 32 ... Contact layer 34 ... DFB laser 36 ... InP buried heterostructure layer 38 ... Polyimide film

Claims (8)

[Claims] 1. A first semiconductor layer and a valence band upper end formed on the first semiconductor layer, wherein a conduction band lower end level is lower than a conduction band lower end level of the first semiconductor layer. A second semiconductor layer whose level is higher than the level of the upper end of the valence band of the first semiconductor layer and serves as a quantum well for electrons and holes, and is formed on the second semiconductor layer, The lower level of the conduction band is higher than the lower level of the conduction band of the second semiconductor layer, the upper level of the valence band is higher than the upper level of the valence band of the second semiconductor layer, and holes A third semiconductor layer to be a quantum well, and the level of the bottom of the conduction band is substantially equal to the level of the bottom of the conduction band of the third semiconductor layer formed on the third semiconductor layer. A fourth semiconductor layer having a low valence band top level lower than a valence band top level of the third semiconductor layer. Semiconductor device to collect. 2. The semiconductor device according to claim 1, wherein between the second semiconductor layer and the third semiconductor layer, the level of the conduction band lower end is the conduction band lower end of the second semiconductor layer. Of the valence band of the second semiconductor layer and the level of the top of the valence band of the second semiconductor layer is higher than or equal to or lower than the level of the bottom of the conduction band of the third semiconductor layer. A semiconductor device further comprising a fifth semiconductor layer lower than the level of the upper end of the valence band of the third semiconductor layer. 3. The semiconductor device according to claim 1, wherein the second semiconductor layer is thicker than the third semiconductor layer. 4. An optical modulator using the semiconductor device according to claim 1, wherein the light is incident by an electric field applied between the first semiconductor layer and the fourth semiconductor layer. A semiconductor device characterized by modulating light that is emitted. 5. The semiconductor device according to claim 1, wherein the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the semiconductor layer. The fifth semiconductor layer is composed of an InGaAsP-based semiconductor material, and is a semiconductor device. 6. The semiconductor device according to claim 1, wherein the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the semiconductor layer. The fifth semiconductor layer is composed of an InAlAsP-based semiconductor material, and is a semiconductor device. 7. The semiconductor device according to claim 1, wherein the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the semiconductor layer. The fifth semiconductor layer is composed of an InGaAlP-based semiconductor material, and is a semiconductor device. 8. The semiconductor device according to claim 1, wherein the first semiconductor layer, the second semiconductor layer, the third semiconductor layer, the fourth semiconductor layer, or the semiconductor layer. The fifth semiconductor layer is composed of an InSbAsP-based semiconductor material, and is a semiconductor device.