JPH0656458B2 - Light control element - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical control element for performing modulation, switch operation, and the like.

(Prior Art and its Problems) Due to the concept of an optical transmission network that handles high-speed / broadband signals such as image information in recent years, there has been an increasing demand for a high-speed / broadband optical transmission system. The semiconductor laser (LD) used as a light source in the current optical transmission system has advantages such as small size, high efficiency, and direct modulation capability. Direct modulation gradually becomes difficult due to approaching the upper limit and fluctuations in wavelength during direct modulation. As one method for solving this problem, it is considered to use a combination of an LD driven by direct current and an ultra-high speed external modulator, and therefore, an ultra-high speed external modulator which is compact and can be driven at a low voltage is required. . As a means for realizing such an ultra-high-speed external modulator, an electro-optical effect without carrier movement,
It is considered to use the Franz-Keldish effect or the like.

In the modulator using the electro-optic effect, Ti on LiNbO ₃ substrate
A branching interference type or directional coupler type traveling wave electrode structure using a diffusion waveguide has obtained a modulation band close to 20 GHz, but in order to obtain a practical modulation voltage of about 4 V or less, 1
It is difficult to reduce the size because the element length of 0 mm or more is required.
For such a modulator, see "Applied Physics Letters," Vol.
Volume 3, CM Gee (CMGee) published on pages 998 to 1001 in 1983.
Papers by others and the journal "IE Journal of Quantum Electronics (IEEE Jour
nal of Quantm Electronics) ", Volume 19, 1983,
RC Alphanes (RCAlfern, published on pages 1339 to 1341)
ess) in a paper by others.

On the other hand, in the one using the Franz Keldysh effect, the magazine "Applied Physics Letters (Applied R
hysics Letters) ", Volume 28, 1976, pp. 544-546.
GE Stillman et al. Reported that extinction ratio is ~ 20dB and rise time is 1nsec or less for a device of 1mm or less. Generally, the modulation voltage is high and the voltage of high speed signal (generally about 2V or less). Considering, it is difficult to actually obtain ultra-high speed modulation operation.

On the other hand, “Conoerence on Lasers ann'84 (Conference on Lasers and Electro-Optics) held in the United States in June 1984.
d Electro-Optics '84) ”, a paper by TH Wood et al., which uses the electric field absorption effect in a GaAlAs / GaAs multiple quantum well structure different from the usual Franz-Keldysh effect. 100pse
It has been reported that rising and falling responses of c or less were obtained. For the structure and operating principle of this modulator, refer to the magazine "Applied Phgsics Lez".
"Tters)", Vol. 44, pp. 16-18, 1984, TH Wood, et al. (abbreviated as "Reference 1" below), and this modulation is also based on that paper. The structure and operating principle of the vessel will be described.

FIG. 3 shows the structure of a conventionally known electro-absorption optical modulator. n ⁺ -GaAs n ⁺ -GaAs buffer layer 2 on the substrate 1,
n ⁺ -AlGaAs etch stop layer 3 n ⁺ -AlGaAs / GaAs superlattice contact layer 4, i-AlGaAs / GaAs superlattice buffer group 5, i-AlGaAs / GaAs multiple quantum well layer 6, i-AlGaAs / Ga
The As superlattice buffer layer 7, p ⁺ -AlGaAs / GaAs superlattice contact layer 8 and p ⁺ -AlGaAs contact layer 9 are grown by the molecular beam epitaxial (MBE) method. The epitaxial layer side is etched up to the n ⁺ -GaAs substrate 1 in a cylindrical shape,
It forms a mesa pin diode. In this optical modulator, signal light passes vertically through the epitaxial layer, so p ⁺ -AlG
An ohmic electrode 10 having a hole for transmitting light is formed in the center of the aAs contact layer 9. On the other hand, on the side of the n ⁺ -GaAs substrate 9 as well, in order to eliminate the absorption of light by the substrate, the ohmic electrode 11 is formed in the lower part of the substrate by removing the portion corresponding to the light path up to the n ⁺ -GaAs buffer layer 2 by etching. Modulated light
20a enters the modulator in a beam form from the epitaxial layer side, and modulated light 20b is extracted from the substrate side.

Next, the operating principle of this modulator will be described. Since the structure shown in FIG. 3 is a pin diode structure, when a reverse bias is applied between the electrodes 10 and 11, an electric field is applied to the high resistance i layer portion. FIG. 4 is a diagram for explaining an energy band state of one quantum well in a multiple quantum well (MQW) layer depending on the presence or absence of an electric field. In the quantum well structure, electrons and holes are confined in the GaAs well, and their energy levels are discretized to form levels Eo and Eo '. ((A)) Energy of this level,
Although the number depends on the band gap difference between the well and the barrier, the well layer thickness, etc., here, for simplicity, it is assumed that one level is formed for each of the electron and hole. MQ like this
When an electric field is applied vertically to the W layer, the entire potential tilts according to the electric field strength, and the energy of the quantized level changes due to the seepage of the wave function, and the apparent band gap becomes small (b). That is, the bandgap can be controlled by the electric field in the direction perpendicular to the MQW layer, and a modulator using this phenomenon is possible for light near the bandgap wavelength. This phenomenon is due to bulk Franz
Very similar to the Keldysh effect, but with M
By adopting the QW structure, the efficiency of modulation can be greatly improved. FIG. 5 shows the wavelength characteristic of the light transmittance of the modulator having the structure shown in FIG. 3 reported in the literature 1). Due to the MQW structure, excitons can exist stably even at room temperature, and exciton resonance absorption peaks corresponding to heavy holes (HH) and light holes (LH) clearly appear. It can be seen that the peak shifts to the low energy side with the application of an electric field, indicating that this effect can be used for a modulator for light of a fixed wavelength. Similar to the Franz-Keldish effect, this effect enables an ultra-high-speed response, and the rise and fall of 100 psoc or less is actually obtained by reducing the CR time constant of the electric system to apply the electric field. Moreover, the optical path length is on the order of μm, and a very small modulator can be obtained.

However, the electric field absorption effect of this MQW structure is higher in modulation efficiency than the bulk Franz-Keldysh effect, but it is still insufficient for lowering the voltage for a true high-speed response. As can be seen from FIG. 5, even if the wavelength of the modulated light is selected to be 853 mm (photon energy 1.454 eV), the modulation degree obtained by applying a voltage of 8 V is only -50%.

(Object of the Invention) An object of the present invention is to eliminate the above-mentioned problems, and to provide a small-sized light control element capable of performing light modulation with a low voltage, an ultrahigh speed, and a high degree of modulation.

(Structure of the Invention) The present invention has multiple quantum wells sandwiching a first semiconductor layer having a thickness of about de Broglie wavelength and a second semiconductor layer having a wider bandgap than the first semiconductor layer in the layer thickness direction. A multi-quantum well structure, means for propagating light in each layer of the multi-quantum well structure horizontally or across the multi-quantum well structure, and means for applying an electric field to a light propagating portion of the multi-quantum well structure. And the direction of the electric field applied by the means for applying the electric field is inclined with respect to each layer of the multiple quantum well structure.

(Principle of the Invention) Of the effects of applying an electric field to the MQW, the case where an electric field is applied vertically to the MQW layer has already been described. It is known that when a horizontal electric field is applied to the MQW layer, an effect due to a completely different mechanism appears. In this regard, the magazine "Applied Physics Letters (Applied
Phgsics Letters) Vol. 42, pp. 864-866, 1983, published by DS Chemla et al. (Abbreviated as Reference 2 below). The invention is MQW
Since the modulation efficiency is improved by using the electric fields in the vertical and horizontal directions in combination with the layer, the effect of the electric field in the horizontal direction on the MQW layer will be described first.

Excitons exist in a plane similar to the two-dimensional hydrogen atom in the plane horizontal to the MQW layer. Therefore, the clonal interaction works between holes and electrons that form excitons, similar to ordinary hydrogen atoms. The electric field in the direction horizontal to the MQW layer has no effect on the energy band structure of the MQW, but it significantly deforms the Coulomb potential shape in the exciton. When the energy due to the electric field becomes larger than the ionization energy of the excitons, the excitons disappear and naturally the corresponding resonance absorption peak also disappears. Sixth
The figure shows the absorption coefficient spectrum corresponding to the light transmitted perpendicularly to the MQW due to the electric field in the direction horizontal to the MQW reported in Reference 2). Again, the heavy hole (H
Resonance absorption peaks corresponding to H) and light holes (LH) appear, and it can be seen that the peak shifts to the low energy side due to the application of the electric field, and the absorption coefficient itself also decreases.

Considering the above and the effect of the electric field in the direction perpendicular to the MQW described above, the resonance absorption peak of exciton shifts to the low photon energy side due to the electric field in both directions, and the absorption coefficient You can see that the size itself is getting smaller. Therefore, the modulation efficiency can be additively and synergistically increased by using the effect of the electric field in the horizontal and vertical directions together with the MQW. It is effective to apply an electric field obliquely to the MQW layer in order to use the effect of the electric field in the horizontal and vertical directions together in the MQW layer. For example, if an electric field is applied to the MQW layer at an angle of 45 °, the electric field components in the directions horizontal and vertical to the MQW will be become. For simplicity, it is assumed that the electric fields in the horizontal and vertical directions to the MQW contribute to the optical modulation degree to the same degree, but even if the electric fields in both directions are merely additive, the oblique electric field applies the electric field only in the horizontal or vertical direction to the MQW. Modulation efficiency compared to Can be doubled. Therefore, the voltage required to obtain the same degree of modulation can be reduced, and a compact, high extinction ratio, low voltage modulator can be realized.

Hereinafter, the present invention will be described in detail with reference to Examples.

(Embodiment) FIG. 1 shows a first embodiment of the light control element according to the present invention. First, the manufacturing method of this embodiment will be described with reference to FIG. Here, the case of using an AlGaAs / GaAs-based semiconductor material will be described, but it goes without saying that the present invention is not limited to this material.

On the (100) n ⁺ -GaAs substrate 1, the n ⁺ -AlGaAs buffer layer 2 (S
i-doped, thickness-1 μm) i-AlGaAs cladding layer 30 (thickness-0.6 μm), i-multiple quantum well layer 6, i-AlGaAs buffer layer 31 (thickness-0.6 μm), i-AlGaAs cap layer 32 (thickness- 0.5 μm) molecular beam epitaxial MBE
Grow by law. i-Multiple quantum well layer 6 has a thickness of up to 100
It consists of 10 repetitions of Å GaAs well and AlGaAs barrier, and the total layer thickness is ~ 0.21μm. The Al molar ratio of AlGaAs is 0.
Is 3. Through the SiO ₂ mask 33 having _two parallel gaps close to the wafer thus manufactured
Expand the number of Zn. At this time, the diffusion front is set to i-AlGaA
It is controlled to stop in the s clad layer 31 or the i-multiple quantum well layer 6. In the p ⁺ − region due to diffusion, the refractive index is lowered by the plasma effect, and further, the i − multiple quantum well layer 6
In the inner diffusion region, the multiple quantum well structure is collapsed and AlGaAs having an intermediate composition between the well and the barrier is formed, so that the refractive index is further lowered. Therefore, the p ⁺ -diffusion region 34 of the i-multiple quantum well layer 6 is
An intermediate portion 36 between a and 34b serves as a channel optical waveguide. Finally, the ohmic electrodes 35a, 35b, 36 are formed on the p ⁺ -diffusion regions 34a, 34b and the n ⁺ -GaAs substrate 1, respectively, and the end faces for entering and exiting the light are manufactured by micro cleaving or dry etching, and the end faces are not reflected. Apply a coat. Light waveguide length is several to several
It was set to 10 μm.

Next, the operation of this embodiment will be described. Light in the vicinity of the exciton resonance absorption wavelength in the i-multiple quantum well structure 6 is guided to the channel optical waveguide 36. Electrodes, 35a or 35b and 3
When a reverse bias voltage is applied between 6 and p ⁺ -diffusion regions 34a, 3
A depletion layer is formed in the front part of 4b, and an electric field works in the depletion layer. Here, the channel optical waveguide 36 is located near the edges of the p ⁺ -diffusion regions 34a and 34b, and an electric field inclined from the epitaxial layer thickness direction is applied to the guided light. Therefore, as described above, the efficiency of the electric field in the horizontal and vertical directions can be used together in each layer of the multiple quantum well structure, and the modulation operation can be performed at a low voltage. Moreover, since the electric field strength at the edge portion of the diffusion front is generally higher than that at the central portion of the diffusion front, it is possible to further reduce the voltage. As a voltage application method, one of the P-side electrodes 35a and 35b may be used, or both may be short-circuited and used.

FIG. 2 shows a second embodiment of the present invention. In the present embodiment, an example in which light is propagated across the multiple quantum well structure without using the multiple quantum well structure as an optical waveguide is shown. First, the manufacturing method of this embodiment will be described with reference to FIG. Here, the case of using the AlGaAs / GaAs material will be described. On (100) n ⁺ -GaAs substrate 1, n ⁺ -GaAs buffer layer 2, n ⁺ -AlGaAs etch stop layer 3, n ⁺ -AlGaAs
First, the buffer layer 41 is grown. A growth method other than the MBE method may be used for the growth. Next, a hole 41 having a slanted wall shape is formed in the i-AlGaAs buffer layer by chemical etching.
form a. The i-AlGaAs buffer layer 42, i-multiple quantum well layer 6, i-
The AlGaAs buffer layer 43 and the i-GaAs contact layer 44 are M
It grows by the BE method. At this time, in the MBE method, since the growth rate has no anisotropy, each layer is grown according to the shape of the hole 41a. Next, a Schottky electrode 10a made of Ti, Pt, Au or the like is formed on the i-GaAs contact layer 44, and is used as a mask to form a cylindrical n
⁺ -Mesa etching is performed up to GaAs substrate 1. Finally, a hole for light transmission is formed in the center of the Schottky electrode 10a, an ohmic electrode 11 is formed on the substrate side, and the substrate of the light transmission part is n ⁺ -AlGaA.
s The etch stop layer 3 is removed halfway.

Next, the operation of this embodiment will be described. In this embodiment, the modulated light 20a is vertically incident on the substrate and is extracted as the modulated light 20b. The wavelength of the modulated light is set near the exciton resonance absorption wavelength in the i-multiple quantum well layer 6. When a reverse bias is applied between the Schottky electrode 10a and the electrode 11, an electric field is applied between the Schottky electrode 10a and the n ⁺ -AlGaAs contact layer 40, but the electric field is increased because the i-multiple quantum well layer 6 is inclined to the substrate 1. It is applied with an inclination with respect to the multiple quantum well layer. Therefore, as described above, horizontal to each layer of the multiple quantum well structure,
The effect of the electric field in the vertical direction can be used together, and the operating voltage becomes possible. A pin structure may be used as the electric field applying means.

Although the AlGaAs / GaAs system is considered as the material in the above embodiments, a multi-quantum well structure such as InGaAsP / InP or InAs-GaSb system may be used. Although only the optical modulation operation is described in the embodiment, an optical bistable device (magazine “Applied Physics Letters” has a similar structure.
"Tters)", Vol. 45, 1984, pp. 13 to 15) and the like can be realized, and at that time, the low power consumption of the operation can be realized by applying the present invention.

(Effects of the Invention) As described above, according to the present invention, it is possible to obtain a light control element which is small in size, capable of performing light modulation with a low voltage at an ultrahigh speed and a high modulation degree.

[Brief description of drawings]

1 and 2 show the first and second light control elements according to the present invention.
FIG. 3, FIG. 4, FIG. 5, FIG. 6 and FIG. 6 are views for explaining the structure and operation of a conventionally known light control element. In the figure, 1, 2, 3, 4, 5, 6, 7, 8, 9, 30, 31, 32, 34a, 3
4b, 40, 41, 42, 43, 44 are semiconductors, 10, 10a, 11, 35a, 35b are electrodes, 33 is SiO ₂ , 20a, 20b is light, 41a is a hole, and 36 is a channel waveguide.

Claims (1)

[Claims] 1. A multiple quantum well structure having multiple quantum wells sandwiching a first semiconductor layer having a thickness of about de Broglie wavelength and a second semiconductor layer having a wider band gap than the first semiconductor layer in a layer thickness direction. A means for propagating light horizontally in each layer of the multi-quantum well structure or across the multi-quantum well structure, and means for applying an electric field to the light propagating portion of the multi-quantum well structure, A light control element, wherein the direction of the electric field applied by the means for applying the electric field is inclined with respect to each layer of the multi-quantum well structure.