JPH0530254B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical modulator that can be driven at low voltage to obtain a high extinction ratio and that can perform high-speed modulation.

(Prior art) An optical modulator that utilizes changes in the energy level of electrons or holes in a quantum well caused by the application of an electric field was proposed by Dr. Yamanishi and published in the Japanese Journal of Applied Physics. Applied Physics) magazine
Reported in Volume 22, page L22, 1983. This is done by shifting the optical absorption spectrum of the quantum well layer as a whole toward longer wavelengths by applying an electric field.
This makes use of the increase in the light absorption coefficient in the long wavelength region that results from this.

(Problems to be Solved by the Invention) In the conventional optical modulator described above, when the optical absorption spectrum shifts to the long wavelength side due to an electric field, the wave functions of electrons and holes are spatially separated from each other at the same time, and the optical This has the disadvantage that the absorption probability becomes small, and therefore the absorption coefficient near the optical absorption edge shifted to the long wavelength side becomes small. Furthermore, the driving voltage required to obtain a high extinction ratio is still high in practice.

(Means for Solving the Problems) In order to solve the above-mentioned problems, an optical modulator provided by the present invention is made up of at least three conductor layers laminated in sequence, and the thickness of these layers is A multiple quantum well formed by alternately stacking quantum well layers whose sum is smaller than the mean free path of an electron and barrier layers made of semiconductor layers having a larger forbidden band width than any of the semiconductor layers constituting the quantum well layers. structure, and has means for applying an electric field in the stacking direction of the multi-quantum well structure, at least one semiconductor which is not adjacent to the barrier layer among the plurality of semiconductor layers constituting the quantum well layer. The layer is a narrow bandgap layer having a narrower bandgap width than a semiconductor layer in contact with this semiconductor layer, and the bandgap width of a portion of the quantum well layer excluding this narrow bandgap layer changes monotonically with respect to the stacking direction. There is something special about what you do.

(Function) Hereinafter, the function of the present invention will be explained with reference to the drawings. For ease of understanding, the following description focuses on one quantum well structure (consisting of one quantum well layer and two barrier layers on both sides thereof) included in the multiple quantum well structure. However, exactly the same effect occurs with other quantum well structures.

Figures 1a and 1b show the schematic band structure of the quantum well structure according to the present invention and electron FIG. 3 is a diagram showing wave functions of and holes. Figures 2a and b show the schematic band structure of a conventional rectangular quantum well structure when no electric field is applied (Figure a), and when an electric field is applied in the stacking direction (Figure b), and the electron and positive It is a figure which shows the wave function of a hole.

As a result of analysis using the ordinary effective mass approximation that is widely used to analyze the electronic state in quantum wells, it was found that in the quantum well structure of the present invention, the long wavelength of the optical absorption edge can be adjusted by applying an electric field in the stacking direction. It is easy to convert
It was shown that the amount of wavelength shift when the same electric field is applied is larger than that of the conventional rectangular quantum well structure.
FIG. 4 is a diagram showing the amount of shift of the optical absorption edge when an electric field is applied in the stacking direction (in this figure, reference numeral 41 indicates the characteristic line of the quantum well structure of the present invention, and reference numeral 42 indicates the conventional ) shows the characteristic line of a rectangular quantum well structure. From FIG. 4, it can be seen that the quantum well structure of the present invention induces a large optical absorption edge change at a small electric field intensity compared to the conventional rectangular quantum well structure. However, in this calculation example, the quantum well layer thickness was set to 100 Å. Further, in the quantum well structure of the present invention, the narrow bandgap layer thickness in the quantum well layer was 10 Å. The direction of the electric field in Figure 4 is the first
This is the direction shown in Figure b and Figure 2b.

Figure 5 shows the electric field dependence of the square of the overlap integral |Mcv| ² , which is approximately proportional to the light absorption coefficient. Here, Mcv is the electron wave function Ψ _e 〈Z〉,
When the wave function of a hole is Ψ _h 〈Z〉, it is given by Mcv=∫ ^∞ _-∞ dZΨ _e 〈Z〉Ψ _h 〈Z〉 (In Fig. 5, the symbol 51 indicates the characteristics of the quantum well structure of the present invention. 52 indicates a characteristic line of a conventional rectangular quantum well structure). From Figures 4 and 5, it can be seen that for the electric field strength that lengthens the wavelength of the optical absorption edge by the same amount, the square of the overlap integral | Mcv
| The value of ² is higher in the quantum well structure of the present invention.
It can be seen that this is larger than that in the conventional rectangular quantum well structure. However, in this calculation example, the quantum well layer thickness was 100 Å, and in the quantum well structure of the present invention, the narrow bandgap layer thickness in the quantum well layer was 10 Å. Further, the direction of the electric field in FIG. 5 is the direction shown in FIG. 1b and FIG. 2b.

FIG. 6 is a diagram schematically showing the change in light absorption coefficient due to an electric field for the quantum well structure of the present invention.

Note that the quantum well layer consists of only one layer, and is simply
The quantum well structure of the present invention has a larger optical absorption coefficient than one in which the forbidden band width simply changes monotonically. Characteristic line 43 in Figure 4 and characteristic line 5 in Figure 5
3, each quantum well layer consists of only one layer,
The amount of shift of the optical absorption edge and the square of the overlap integral when an electric field is simply applied in the stacking direction of a quantum well structure whose forbidden band width changes monotonically | Mcv
| Shows the electric field dependence of the value of ² . Figures 3a and 3b show the schematic band structure and wave functions of electrons and holes in this quantum well structure when no electric field is applied (Figure a) and when an electric field is applied (Figure b). FIG. In this calculation example, the quantum well layer thickness is
It was set to 100Å. From FIGS. 4 and 5, it is clear that in a quantum well structure in which the quantum well layer consists of only one layer and its forbidden band width simply changes monotonically, the amount of shift of the light absorption edge due to the electric field is the same as in the quantum well structure of the present invention. However, it can be seen that the value of the square of the overlap integral |Mcv| ² is reduced by applying an electric field to a greater extent than in the quantum well structure of the present invention, which has the disadvantage that the probability of light absorption is reduced. The quantum well structure of the present invention avoids the above-mentioned drawbacks by providing a narrow bandgap layer therein. In other words, since the narrow bandgap in the quantum well layer plays the role of capturing the electron wave function, the broadening of the electron wave function is suppressed even when an electric field is applied, and therefore the square of the overlap integral |Mcv | The value of ² is difficult to become small. In addition, since this narrow bandgap layer only has to play the role of slightly trapping the wave function of electrons, it is possible to have multiple layers instead of a single layer, and if the narrow bandgap layer is located near the center, it does not necessarily correspond to the quantum well layer. It doesn't have to be in the center.

Finally, the basic operation of the optical modulator of the present invention will be summarized. First, the energy corresponding to the frequency of light incident on the optical modulator is set to be slightly lower than the optical absorption edge when no electric field is applied.
The optical modulator is left in a state that allows light to pass through it. Next, by applying an electric field (the direction of the electric field is the same as in Figure 1b), the optical absorption edge is shifted to the lower energy side.
It absorbs light and makes it impossible for the optical modulator to transmit the light.

Through the above series of operations, the optical modulator having a quantum well structure of the present invention has a function of effectively blocking incident light by applying an electric field by low voltage driving.

(Example) Next, examples of the present invention will be described.

FIG. 7a shows a first embodiment of the present invention in a perspective view, and FIG. 7b shows an enlarged sectional view of a multiple quantum well structure in this embodiment. This example was manufactured using the molecular beam epitaxy (MBE) method. In manufacturing this example, first, a 1.0 μm thick Si-doped n
A type GaAs buffer layer 72 and a Si-doped n-type Al _0.4 Ga _0.6 As cladding layer 73 having a thickness of 2.0 μm are laminated.
Next, a non-doped Al _x Ga _1-x As layer 74 with a thickness of 45 Å was formed in which the Al composition ratio x was continuously changed from 0 to 0.075.
1, a non-doped GaAs layer 742 with a thickness of 10 Å,
Non-doped Al _x Ga _1-x As layer 743 with a thickness of 45 Å with the Al composition ratio x continuously changed from 0.075 to 0.15
A quantum well layer 74 having a total thickness of 100 Å and a non-doped Al _0.4 Ga _0.6 As barrier layer 75 having a thickness of 80 Å are alternately stacked in this order.
Layer periodically. On top of that is a Be-doped p-type Al _0.4 Ga _0.6 As cladding layer 76 with a thickness of 2.0 μm, and a cladding layer 76 with a thickness of 0.5 μm.
A Be-doped p-type GaAs contact layer 77 was grown to fabricate a multilayer structure. The partial cross-sectional view of the multiple quantum well structure in FIG. 7b is a part of the circle indicated by the reference numeral in FIG. 7a. Next, this was made into a size of about 5 x 5 mm, the upper and lower GaAs layers were removed in a circular shape by selective etching, and electrodes 78 were formed on the remaining GaAs layers to complete the fabrication of this example.

Light was incident perpendicularly to this circular portion where the GaAs layer was removed, and a voltage was applied between the electrodes to examine the voltage dependence of the transmitted light spectrum. When the voltage was applied in the positive direction, that is, when the n-side electrode was grounded and a negative voltage was applied to the p-side electrode, the light absorption edge became longer wavelength. When a voltage from +2 V to 0 V was applied, the amount of change in the optical absorption edge was from -6 meV to 0 meV, and the optical absorption rate near the optical absorption edge hardly changed.
And the light modulation properties were good.

Next, a second embodiment of the present invention will be described.
FIG. 8a is a perspective view of this embodiment, and FIG. 8b is an enlarged sectional view of the multiple quantum well structure in this embodiment. In manufacturing this example, first, a film with a thickness of 2.0 μm was deposited on an S-doped n-type InP substrate 81 using a vapor phase growth method.
An S-doped n-type InP buffer layer 82 is laminated,
Next, a non-doped In _0.75 Ga _0.25 AS _x P _1-x with a thickness of 50 Å was prepared by changing the As composition ratio x continuously from 0.50 to 0.55.
layer 831 and 20 Å thick undoped In _0.75 Ga _0.25
As _0.50 P _0.50 layer 832 and As composition ratio x from 0.55 to 0.60
A non-doped film with a thickness of 50 Å that was continuously varied up to
Three layers, In _0.75 Ga _0.25 As _x P _1-x layer 833, are laminated in this order to form a quantum well layer 83 with a total thickness of 120 Å and a non-doped InP barrier layer 8 with a thickness of 60 Å.
4 was laminated alternately for 6 cycles, and on top of that, a 2.5 μm thick
Z _o doped p-type InP cladding layer 85, 0.5 μm thick
A substrate on which a Z _o doped p-type InGaAsP contact layer 86 was laminated was manufactured. The partial sectional view of the multiple quantum well structure shown in FIG. 8b is a part of the circle indicated by the symbol I in FIG. 8a. Next, electrodes 87 are formed on both sides of the substrate, and after a SiO ₂ film is deposited on the top surface of the substrate by CVD, a 1.5 μm wide stripe of SiO 2 is left and other SiO ₂ is deposited using a normal photolithography method. ₂ was removed, and then the portion to which SiO ₂ was not attached was removed by etching up to the n-type InP buffer layer 82, and the remaining SiO ₂ was removed to form a waveguide structure. completed.

When light was incident on this waveguide structure and the dependence of the transmission spectrum of the guided light on the applied voltage was measured, the optical absorption edges were 1295 nm, 1305 nm, and 1305 nm, respectively, when the applied voltage was +1V, 0V, and -1V. It was 1314nm. The waveguide length is 200μm, and the wavelength is 1300nm.
When the above voltage was modulated and applied, about 1% of the original laser light intensity was extracted when +1V was applied, and 70% when -1V was applied. The extinction ratio at this time was approximately 18 dB, which was a very high extinction ratio. When the intensity of light was modulated by modulating this applied voltage, the highest frequency that could be modulated was approximately 3 GHg, and this was determined by the parasitic capacitance between the electrodes.
This was not due to the element structure.

Two examples have been described above, but
The present invention describes a method for changing the forbidden band width in a quantum well,
The material system, semiconductor growth method, etc. are not limited to the above-mentioned embodiments.

(Effects of the Invention) As described above, the optical modulator according to the present invention can obtain a high extinction ratio at low voltage and can perform high-speed modulation.

[Brief explanation of the drawing]

FIGS. 1a and 1b are schematic diagrams showing a band structure and wave functions of electrons and holes for explaining the principle of the present invention. FIG. 2 is a schematic diagram showing the band structure and wave functions of electrons and holes of the conventional quantum well structure shown in FIGS. 2a and 2b. Figures 3a and 3b are schematic diagrams showing the band structure and wave functions of electrons and holes of a quantum well structure in which the quantum well layer consists of only one layer and its forbidden band width simply changes monotonically. be. FIG. 4 is a diagram showing the amount of change in the optical absorption edge due to the electric field in the quantum well structure. Figure 5 is the square of the overlap integral due to the electric field in a quantum well structure |
FIG. 2 is a diagram showing changes in the value of Mcv| ² . FIG. 6 is a schematic diagram showing how the light absorption coefficient changes depending on the electric field in the quantum well structure of the present invention. Fig. 7a is a perspective view showing the first embodiment of the present invention, Fig. 7b
is a sectional view showing a portion of a multiple quantum well structure in the example. FIG. 8a is a perspective view showing a second embodiment of the present invention, and FIG. 8b is a sectional view showing a portion of the multiple quantum well structure in the embodiment of FIG. 8a. 11, 21, 31...quantum well layer, 12, 22,
32... Barrier layer, 13... Narrow forbidden band width, 41... Characteristic line showing electric field dependence of optical absorption edge shift in quantum well structure of the present invention, 42... Electric field dependence of optical absorption edge shift in conventional rectangular quantum well structure Characteristic line showing the electric field dependence of the optical absorption edge shift in a quantum well structure in which the quantum well layer consists of only one layer and the forbidden band width thereof simply changes monotonically, 51... Characteristic line showing the electric field dependence of the square of the overlap integral |Mcv| ² in the quantum well structure of the present invention, 52...Characteristic showing the electric field dependence of the square of the overlap integral |Mcv| ² in the conventional rectangular quantum well structure Line, 53...The square of the overlap integral in a quantum well structure in which the quantum well layer consists of only one layer and its forbidden band width simply changes monotonically |
Characteristic line showing electric field dependence of Mcv｜ ² , 71...n type
GaAs substrate, 72...n-type GaAs buffer layer, 7
3...n-type Al _0.4 Ga _0.6 As cladding layer, 74... non-doped quantum well layer, 75... non-doped Al _0.4 Ga _0.6 As
Barrier layer, 76...p-type Al _0.4 Ga _0.6 As cladding layer, 7
7...p-type GaAs contact layer, 78...electrode, 74
1...Non-doped Al _x Ga _1-x As layer (0x0.075),
742... Non-doped GaAs layer, 743... Non-doped Al _x Ga _1-x As layer (0.075x0.15), 81... N-type
InP substrate, 82... n-type InP buffer layer, 83...
Non-doped quantum well layer, 84... Non-doped InP barrier layer, 85... P-type InP cladding layer, 86... P-type
InGaAsP contact layer, 87...electrode, 831...
Non-doped In _0.75 Ga _0.25 As _x P _1-x layer (0.50x
0.55), 832...Non-doped In _0.75 Ga _0.25 As _0.50 P _0.50
Layer, 833...Non-doped In _0.75 Ga _0.25 As _x P _1-x layer (0.55x0.60).

Claims (1)

[Claims] 1. A quantum well layer consisting of at least three semiconductor layers stacked one after another, where the sum of the layer thicknesses of these layers is smaller than the mean free path of an electron, and a forbidden band larger than any of the semiconductor layers constituting this quantum well layer. In an optical modulator, the optical modulator includes a multiple quantum well structure formed by alternately laminating barrier layers made of semiconductor layers having a width, and has means for applying an electric field in the lamination direction of the multiple quantum well structure. Among the plurality of semiconductor layers that constitute the semiconductor layer, at least one semiconductor layer that is not adjacent to the barrier layer is a narrow bandgap layer that has a narrower bandgap than the semiconductor layer that is in contact with this semiconductor layer; An optical modulator characterized in that the forbidden band width of a portion excluding the narrow band gap layer changes monotonically in the lamination direction.