JPH02238432A - Optical modulating device - Google Patents

Abstract

PURPOSE:To heighten the electric field strength applied to an optical modulation layer and to improve the S/N ratio of a modulation pulse by interposing a first undoped joint layer and n-type auxiliary layer whose thickness is thin between the optical modulation layer and a second joint layer of an opposite conductive type. CONSTITUTION:An auxiliary layer 4 consisting of a first semiconductor is formed on an optical modulation layer 2 of a superlattice, and a first substantially undoped joint layer 3 consisting of a first n-type semiconductor is formed on this auxiliary layer 4. On this first joint layer 3, a second joint layer 5 consisting of a first semiconductor of an opposite conductive type is formed. On a substrate 1 and on a second joint layer 5, electrodes 6, 7 are formed, and a modulated light incident means 9 and a modulated light emitting means 10 are provided so as to come into contact opposingly with an optical path 8 for passing through the optical modulation layer 2. In such a way, the electric field strength applied to the optical modulation layer 2 is heightened and a variation of an optical absorption factor against a variation of an external electric field is enlarged enough, and the S/N ratio of a modulation pulse can be improved.

Description

[Detailed Description of the Invention] [Summary] Controlling the absorption edge wavelength using the quantum Stark effect,
Turns the incident light on and off by changing the absorption rate for the incident light
Regarding the improvement of a light modulation device that performs F modulation, by increasing the electric field intensity applied to the light modulation layer, the change in light absorption rate in response to a change in the external electric field is sufficiently increased, and the S/N ratio of the modulation pulse is improved. The object of the present invention is to provide an optical modulation device in which a thin film made of a second semiconductor having a predetermined forbidden band width and a thin film made of a second semiconductor having a predetermined forbidden band width are formed on a substrate made of a first semiconductor of one conductivity type. A light modulation layer having a superlattice structure in which thin films made of a third semiconductor having a smaller forbidden band width than the second semiconductor are formed alternately is formed, and an auxiliary film made of the first semiconductor is formed on this light modulation layer. A layer is formed on this auxiliary layer, and a first bonding layer made of the first semiconductor of one conductivity type and practically undoped is formed on this auxiliary layer, and a first bonding layer of the opposite conductivity type is formed on this first bonding layer. A second bonding layer made of the first semiconductor is formed, and electrodes are formed on the substrate and the second bonding layer, and electrodes are formed on the substrate and the second bonding layer, and electrodes are formed on the substrate and the second bonding layer, and electrodes are formed on the substrate and on the second bonding layer. This is achieved by a light modulation device in which a modulated light input means and a modulated light output means are provided adjacent to each other. [Industrial Application Field] The present invention is an improvement of a light modulation device that controls the absorption edge wavelength using the quantum Stark effect and changes the absorption rate of incident light to perform ON/OFF modulation of incident light. In particular, it relates to improvements in increasing the electric field strength applied to the light modulation layer to sufficiently increase the change in absorption rate. [Prior art] See Figure 3. Figure 3 shows the configuration of a light modulation device according to the prior art. In the figure, 1 is an n-type InP substrate, and 11 is an n-type I
A buffer layer made of nP, 2 a light modulation layer made of a superlattice, 3 a first bonding layer made of n-type InP, and 5 a second bonding layer made of p-type InP. , 12 is a contact layer made of p-type InGaAs, 6 and 7 are electrodes, 9 is a modulated light input means for inputting the modulated light along the optical path 8, and 10 is the modulated light input means. This is the injection means. As an example, in the case of manufacturing a light modulation device that absorbs light in the 1.5μ band, the light modulation layer 2 is composed of an InGaAs P layer and an InP layer each having a composition such that the emission wavelength is 1.6u. It consists of a superlattice body with 10 layers stacked alternately in each layer. A light change made of a superlattice! ll
When an electric field is applied to N2, the absorption edge wavelength of the light modulation layer 2 shifts due to the quantum Stark effect, and the absorption rate of the incident light changes, resulting in ON/OFF modulation. See Figure 4 By the way, if even a small electric field is applied to the superlattice, the absorption edge suddenly begins to blur, and light of wavelengths that were previously transmitted begins to be absorbed. Therefore, it is necessary to prevent any electric field from being applied to the superlattice at zero bias. Therefore, a first bonding layer 3 made of n-type 1nP is formed between the photovariable tlil layer 2 made of a superlattice and a second bonding layer 5 made of p-type InP, as shown in FIG. 4(b).
As shown in FIG. 2, the end of the depletion layer disappears into the first bonding layer 3 when O bias is applied, so that no diffusion potential is applied to the light modulation layer 2. When a reverse bias voltage is applied to the optical modulator and its value is gradually increased, the edge of the depletion layer in the first bonding layer 3 extends toward the substrate l, and when it reaches the substrate l, the optical modulation begins. An electric field starts to be applied to the layer 2, and when the reverse bias voltage is further increased, a desired electric field is applied to the light modulating layer 2 as shown in FIG. 4(C), and the absorption edge wavelength is shifted to the long wavelength side due to the quantum Stark effect. , the absorption rate for the incident light changes, and ON-OFF adjustment is performed. [Problems to be Solved by the Invention] When applying an electric field to the light modulation layer 2 and changing the absorption rate of incident light by shifting the absorption edge wavelength by the quantum Stark effect to perform light modulation, the light modulation layer 2 changes to The stronger the applied electric field, the more the absorption edge wavelength is shifted, and the S/N ratio of the modulated pulse is improved. However, when applying a reverse bias voltage to the optical modulator to increase the electric field applied to the optical modulating layer 2, the electric field applied to the pn junction between the first junction layer 3 and the second junction layer 5 also increases. Of course it will be expensive. If the impurity concentration of the first junction layer 3 is lowered in an attempt to lower the electric field applied to this pn junction, the O
In order to keep the end of the depletion layer within the first junction layer 3 during bias, the first junction 713 must be formed thick. As a result, the area where the electric field is applied becomes wider, and the ratio of change in the electric field in the optical modulation layer 2 to changes in the external electric field becomes smaller, resulting in poor modulation performance. In other words, in order to make a larger change in the absorption rate with respect to changes in the external electric field, the first bonding layer 3 should be formed thinner, and the
Since it is necessary to keep the edge of the depletion layer therein during biasing, the electric field strength applied to the pn junction inevitably increases.

As a result, when the reverse bias voltage is increased, the light modulating layer 2
The disadvantage is that the pn junction breaks down before a sufficiently high electric field strength is applied. The purpose of the present invention is to eliminate this drawback, and by increasing the electric field intensity applied to the light modulation layer, the change in light absorption rate with respect to the change in the external electric field is sufficiently increased.
An object of the present invention is to provide an optical modulation device that improves the S/N ratio of modulated pulses. An auxiliary layer (4) made of a first semiconductor is formed on this light modulation layer (2). A substantially non-doped first bonding layer (3) made of the first semiconductor of one conductivity type is formed in the first bonding layer (3).
), a second bonding layer (5) made of the first semiconductor of opposite conductivity type is formed on the substrate (1) and the second bonding layer (5). Electrode (6)/(7)
is formed, and a modulated light input means (9) and a modulated light output means (10) are provided in opposition to the optical path (8) passing through the light modulation layer (2). This is accomplished by a device. [Means for solving the problem] The above object is to provide a substrate (
1) A thin film (21) made of a second semiconductor having a predetermined forbidden band width and a thin film (22) made of a third semiconductor having a smaller bandgap width than the second semiconductor are alternately laminated on top. Refer to FIG. 5 In the optical modulator according to the present invention, the impurity concentrations of the thin auxiliary layer 4 and the second bonding layer 5 are respectively IXI
If it is about O"Cll-", the electric field strength at 0 bias will be uniformly distributed only in the non-doped first bonding layer 3, as shown in FIG. 5(b). Note that the area of this square corresponds to the diffusion potential. Compared to conventional optical modulators designed to have the same depletion layer thickness at 0 bias, or conversely, the maximum electric field strength at the pn junction can be made equal. Therefore, it is possible to apply a stronger electric field to the light modulation layer 2 than in a light modulation device with a conventional structure, or to increase the ratio of change in the electric field in the light modulation layer 2 to changes in the external electric field. can. [Embodiment] An optical modulation device according to an embodiment of the present invention will be described below with reference to the drawings. Refer to Figure 1. N-type InP with an impurity concentration of about 5X10"cm-' (
100) Si-doped n-type 1nP buffer layer 11 having the same impurity concentration as this is formed on the substrate 1 by 0.2
A light modulation layer 2 made of an undoped superlattice is then formed. As an example! ．． 5 When forming a light modulation layer that absorbs n-band light, an I nC;aAs P layer 2 having a composition that gives an emission wavelength of 1.6n is used.
The InP layer 22 and the InP layer 22 are alternately laminated by 10 layers each with a thickness of 100 layers. Note that when the light modulation layer 2 is grown using the metal organic chemical vapor deposition method (MOCVD method), the background concentration is IXIOISel.
It becomes an n-type of about 1-'. Then, Si
The n-type 1nP auxiliary layer 4 doped to about cm-3 is 0.0
It is formed as thin as In, and undoped InP is formed on top of it.
A first bonding layer 3 made of the following is formed to a thickness of approximately 0.1 nm.
The background concentration of the first bonding layer 3 is 'o'c
It is about m-s. Then, Zn was
A second bonding layer 5 made of p-type 1nP doped to the extent of an arch
is formed to a thickness of about 1n, and p-type 1nC;aAs is formed on it to a thickness of about 1n.
A contact layer 12 is formed to have a thickness of approximately 0.2 μm. T i / P t / on the surface of n-type 1nP substrate l
A positive electrode 6 made of a triple layer of Au is formed, and a p-type 1 nG
A negative layer 11ffi7 made of an AuGe/Au double layer is formed on a contact layer 12 made of aAs, and a modulated light input means 9 and a modulated light output means 10 are provided in opposition to the optical path 8 passing through the light modulation J12. It goes without saying that when the optical path is provided in a direction perpendicular to the electrodes 6 and 7, measures such as providing an opening in the electrodes 6 and 7 that transmits light are necessary. Refer to FIG. 2 Also, as shown in FIG. 2, the light modulator is mesa-etched to form a trapezoidal laminate, and an Fe-doped InP layer 13 is formed around the trapezoidal laminate. It can also be used as According to the depletion approximation calculation with reference to FIG.
It remains within the type 1nP auxiliary layer 4. δ V At this time, the electric field strength 6x applied to the pn junction is 1.3
It becomes XIO' V/C11. It is useful for calculations,
The bandgap of rnP was set to 1.35ev, and the dielectric constant was set to 12.4. Now, when a reverse bias voltage of 12.1V is applied to the optical modulator 1, the maximum electric field strength at the pn junction is 5. O XIO' v/cm, and the electric field strength applied to the light modulation layer 2 is 3.5 XIO5 v/cm.

Refer back to FIG. 4 On the other hand, in the light modulation device of the conventional structure shown in FIG.
The thickness of the first bonding layer 3 made of 1nP type is 0.114, and the impurity concentration is 1.4XIO"CI1-3 of n-type.
Then, the end of the depletion layer at 0 bias comes to the end of the first bonding layer 3 made of n-type InP on the side that is in contact with the light modulation layer 2.

The maximum electric field strength at the pn junction at this time is 2.2
xto' v/cm. In order to apply an electric field of 3.5 x 10' v/car to the light modulation layer 2 as above, the reverse bias voltage may be 1.21 V as above, but the maximum of the pn junction at that time is Electric field strength is 5.8
XIO' v/cm. That is, when the electric field strength applied to the light modulation layer 2 is 3.5 XIO'V/CI, the electric field strength applied to the pn junction is 5.8 XIO'V/CI1 in the conventional structure. However, in the present invention, this decreases to 5×10'v/cm. if,
If the design is made so that the electric field strength applied to the pn junction is the same, the film thickness from the pn junction to the light modulation layer 2 can be made thinner. Note that the impurity concentration of the n-type 1nP buffer layer 11 and the second bonding layer 5 made of P-type InP is 5XIO".
If the CI is 1-3 or more, the shape of the electric field distribution will be much the same as the shape shown in FIG.

In addition, in the embodiment, a case where an optical modulator is constructed using an InP-based semiconductor is described, but even when constructed using a GaAs-based semiconductor, the principle is exactly the same as the case of an InP-based semiconductor, with the only difference being the physical constants. It is. [Effects of the Invention] As explained above, in the light modulation device according to the present invention, the non-doped first bonding layer and the extremely thin thickness are provided between the light modulation layer and the second bonding layer of the opposite conductivity type. By interposing an auxiliary layer of one conductivity type, the electric field intensity applied to the pn junction can be lowered by changing the electric field distribution shape, so the electric field applied to the optical modulation layer can be reduced without causing breakdown of the pn junction. Since the intensity can be increased to sufficiently increase the change in light absorption rate, it is possible to significantly improve the S/N ratio of the modulated pulse.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a light modulation device according to an embodiment of the present invention. Figure 2 is a cross-sectional view of the stripe type optical modulator. FIG. 3 is a block diagram of a conventional optical modulation device. FIG. 4 is an electric field strength distribution diagram of a conventional optical modulation device. FIG. 5 is an electric field strength distribution diagram of the optical modulation device according to the present invention. l...Substrate, 2...Photochangeable TJ4 layer, 3...First bonding layer, - Auxiliary layer, - Second bonding layer, - Positive electrode, - Negative electrode, - Optical path, - Modulated Light input means, - Modulated light output means, - Buffer layer, - Contact layer, - Thin film made of the second semiconductor, - Thin film made of the third semiconductor.

Claims (1)

[Claims] A thin film (21) made of a second semiconductor having a predetermined forbidden band width on a substrate (1) made of a first semiconductor of one conductivity type.
and a thin film (22) made of a third semiconductor having a smaller band gap than that of the second semiconductor are alternately stacked to form a light modulation layer (2) having a superlattice structure, 2) An auxiliary layer (4) made of the first semiconductor on top
) is formed on the auxiliary layer (4), and the first layer of one conductivity type is formed on the auxiliary layer (4).
A substantially non-doped first bonding layer (3) made of a semiconductor of 5) is formed, electrodes (6) and (7) are formed on the substrate (1) and the second bonding layer (5), and an optical path passing through the light modulation layer (2). A light modulation device characterized in that a modulated light input means (9) and a modulated light output means (10) are provided in opposition to (8).