JP2605911B2 - Optical modulator and photodetector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical modulator and a photodetector used in a future ultra-high-speed optical communication system or the like.

(Prior Art and its Problems) With the development of optical communication systems in recent years, there has been an increasing demand for optical modulators that can operate at an ultra-high speed and operate at a low voltage and that can be easily miniaturized and integrated. As an example that satisfies these requirements, there is a prototype of "20 GHz optical modulator using InGaAlAs / InAlAs MQW structure" manufactured by Wakita et al. (C-474 Spring Meeting of the Institute of Electronics, Information and Communication Engineers, 1989). This is an absorption-type modulator that utilizes the exciton peak shift generated by an electric field when a reverse bias voltage is applied to a semiconductor PIN structure.
An n-InAlAs clad layer, an i-MQW guide layer, and a p-InAlAs clad layer were formed on an n-InP substrate by MBE. The modulation frequency band Δf of such a modulator is substantially determined by the capacitance C of the element and is expressed by Δf = 1 / (πCR). The capacitance C of the element is the junction capacitance C at the pn junction, the wiring capacitance C _i , and the pad capacitance at the bonding pad.
_{Expressed as} the sum of C _p . In the case of the above modulator, since ultra-high-speed modulation is aimed at, the lower portion of the pad portion is filled with polyimide to reduce the capacitance, and as a result, the device capacitance is as low as about 0.2 pF. However, the essential junction capacitance C _j on the modulator even in this case is less than half of the total, the remainder being inherently unnecessary wiring capacitance and pad capacitance caused by inter-wiring electrode and the n-InP substrate. Also, the element length of this modulator is about 100 μm, and considering the characteristics of the modulator, it is difficult to further reduce the junction capacitance further, and furthermore, using a conductive substrate such as an n-InP substrate. Therefore, it is also difficult to further reduce the wiring capacitance and the pad capacitance. Therefore, in the optical modulator having the conventional structure, the modulation band is at most 20 to 25 GHz, and it is difficult to apply the optical modulator to a future ultra-high-speed optical modulator (band ≧ 50 GHz).

Further, as an example of an element in which a modulator and a semiconductor laser (LD) as a light source are integrated, there is an optical modulator / DFB laser integrated light source prototyped by Hida et al. (100C'89 Technical Digest 20PDB-5). This is a device in which a DFB-LD and a modulator utilizing light absorption by the Franz-Keldysh effect are integrated on an n-InP substrate, and both sides of the LD and the optical waveguide of the modulator are embedded with high-resistance InP. It is. Since the conductive substrate is used similarly to the above-described conventional example, the capacitance at the pad portion is large, and only the element capacitance of 0.55 pF and the modulation band of about 10 GHz can be obtained. As described above, the conventional optical modulator has a problem to be solved regarding the modulation speed.

An object of the present invention is to provide a wide-band optical modulator and a photodetector capable of performing ultra-high-speed modulation by reducing the element capacitance.

(Means for Solving the Problems) In the present invention, (1) a first conductivity type clad layer formed on a high resistance semiconductor substrate, and a light formed on the first conductivity type clad layer. A mesa stripe including an absorption layer and a second conductivity type clad layer thereon, and a high resistance block layer formed on the first conductivity type clad layer so as to fill both side surfaces of the mesa stripe. Means for increasing the resistance of the first conductivity type cladding layer below the high resistance block layer on one side of the mesa stripe, and a part of the high resistance block layer on the other side of the mesa stripe. Means for converting to the first conductivity type from the surface to the interface between the high-resistance block layer and the first conductivity type clad layer, and on the mesa stripe and the high-resistance block layer on one side. First electrode, A structure of a semiconductor optical modulator comprising a second electrode on the conductive high-resistance block layer on the other side; and (2) formed on a high-resistance semiconductor substrate. A first conductivity type clad layer, a mesa stripe including an active layer formed on the first conductivity type clad layer and a second conductivity type clad layer thereon, and the first conductivity type clad layer. A high resistance block layer formed on the layer so as to fill both side surfaces of the mesa stripe, wherein the active layer is divided into two parts with respect to the traveling direction of the mesa stripe and made of semiconductors having different band gaps from each other. Means for increasing the resistance of the first conductivity type cladding layer under the high resistance block layer on one side of the mesa stripe, and a part of the high resistance block layer on the other side of the mesa stripe. High from the high resistance layer surface Means for converting to the first conductivity type up to the interface between the anti-blocking layer and the first conductivity type clad layer, the height of the mesa stripe on the active layer having a narrow bandgap and the height of one side. The first electrode on the resistance block layer,
A second electrode electrically insulated from the first electrode on the mesa stripe on the active layer having a wide bandgap and on the high-resistance block layer on one side; And (3) a first conductivity type formed on the high-resistance semiconductor substrate, wherein the third electrode is provided on the conductive high-resistance block layer. A cladding layer, a mesa stripe including a light absorption layer formed on the first conductivity type cladding layer and a second conductivity type cladding layer thereon, and a Means for increasing the resistance of the first conductivity type cladding layer under the high resistance block layer on one side of the mesa stripe, comprising a high resistance block layer formed so as to fill both side surfaces of the front mesa stripe. , The other immediate high resistance block of the mesa stripe Means for converting a part of the backing layer to the first conductivity type from the surface of the high resistance layer to the interface between the high resistance block layer and the cladding layer of the first conductivity type. A first electrode is provided on the high resistance block layer on the side of the first side, and a second electrode is provided on the conductive high resistance block layer on the other side, and the band gap wavelength of the light absorption layer is provided. The above-mentioned problem is solved by using a photodetector structure characterized in that the wavelength is longer than the wavelength of the incident light.

(Function) The present invention uses a high-resistance substrate and further embeds an optical waveguide having a PIN structure with a high-resistance layer, thereby minimizing the capacitance of a portion unrelated to the actual operation of the modulator and the photodetector. The overall capacity is reduced, and the modulator and the photodetector can have a wider band.

Generally, the capacitance C can be expressed by C = ε _{s 0} S / d. Here, ε _s is a relative dielectric constant, ε ₀ is a dielectric constant of vacuum, S is an electrode area (or pn junction area), and d is a distance between electrodes (or a depletion layer thickness). As described in the section of the conventional example, the capacitance C _i of the entire device is a junction capacitance C _j , a wiring capacitance C _i , and a pad capacitance.
The C _p, is represented by _{C t = C j + C i} + C p. Since the junction capacitance C _j affects the static characteristics of the modulator, it was designed so as not to degrade, the waveguide width 2 [mu] m, the waveguide length 100 [mu] m, the junction capacitance C _j and the depletion layer 0.3μm is about 74fF Become. It is desirable that the remaining wiring capacitance C _i and pad capacitance C _p be reduced for widening the bandwidth of the modulator. According to the present invention, by using the high-resistance substrate and the high-resistance layer buried structure, the distance d between the electrodes can be about 100 μm, and a conventional conductive substrate is used. About 1/10 of the structure (d = 2 to 3 μm, ε _s ３3) buried with a dielectric material (d = 2 to 3 μm.ε _s). Approx. 1/30 compared to ~ 12)
To this extent, the wiring capacitance C _i and the pad capacitance C _p can be reduced. As a result, the capacitance C _t of the entire device is almost equal to the junction capacitance C _j.
And a wider band of the modulator can be achieved.

Also, formed on a high-resistance substrate and embedded with a high-resistance layer
An optical waveguide having a PIN structure is similar in many respects to a semiconductor laser in terms of structure, and it is easy to apply an optical modulator and a semiconductor laser to an integrated device with a similar configuration, and it is possible to realize an ultra-high speed as an integrated device. It is.

Further, in the structure of the optical modulator according to the present invention, the composition of the light absorption layer is set to a composition having a band gap wavelength longer than the wavelength of the light source, and the photocurrent due to the light absorbed by the light absorption layer is independently formed in the PIN structure. P-side electrode, n
By detecting from the side electrode, it can be used as a waveguide type photodetector. Also in this case, as described above, the capacity of the element can be greatly reduced, so that an ultra wide band photodetector can be obtained.

(Embodiment) FIG. 1 is a perspective view showing a first embodiment of the optical modulator according to the present invention. As a material system, a waveguide having a DH structure using an InGaAsP / InP system will be described. However, the material and structure are not limited to these, and an InGaAs / InAlAs system and a GaAs / Al
A GaAs-based material or an MQW structure may be used.

In the first embodiment shown in FIG.
An n ⁺ -InP cladding layer 2 is laminated on a substrate 1, and an i-InGaAsP light absorbing layer 3, a p-InP cladding layer 4, and ap ⁺ -In
A mesa stripe in which GaAs cap layers 5 are sequentially laminated is further laminated. Both sides of the mesa stripe are buried with the high resistance block layer 6 and the upper surface of the web is flattened. On one side of the mesa stripe, proton injection is performed from the upper surface of the high resistance block layer to the high resistance InP substrate 1. N ⁺ -InP cladding layer 2 in region 7
Has increased resistance. On the other side of the mesa stripe, ions for converting the high-resistance block layer to n-type, for example, Si ions, are implanted from the upper surface of the high-resistance block layer to the n ⁺ -InP cladding layer 2. A p-side electrode 9 is formed on the mesa stripe and on the proton-implanted region 7 (part of the high-resistance block layer 6) into which protons have been implanted.
Is formed, and an n-side electrode 10 is formed in an ion-implanted region 7 (a part of the high-resistance block layer 6) formed into an n-type by ion implantation. Here, the thickness of each layer is n ⁺ −
InP clad layer 2 is about 1 μm, i-InGaAsP light absorbing layer 3
But 0.3 [mu] m, p-InP cladding layer 4 is 1μm approximately, p ⁺ -In
The GaAs cap layer is about 0.2 μm. Also, i-InGaA
The band gap wavelength of the sP light absorbing layer 3 is set to 1.475 μm. The width of the mesa stripe is 2 μm.

The manufacturing method of this embodiment shown in FIG. 1 will be briefly described. First, an n ⁺ -InP cladding layer 2, an i-InGaAsP light absorbing layer 3, a p-InP cladding layer 4, and a p ⁺
-InGaAs cap layer 5 is sequentially grown by MOVPE method or the like. After that, a stripe-shaped SiO ₂ film serving as an etching mask when forming the mesa stripe is formed on the p ⁺ -InGaAs cap layer 5 by a usual photolithography method. P ⁺ -InGaAs cap layer 5 in the portion other than the mesa stripe with the stripe-shaped SiO ₂ mask, p-an In
The P cladding layer 4 and the i-InGaAsP light absorbing layer 3 are removed by etching. Then, using the stripe-shaped SiO ₂ as it is as a mask for selective growth, the sides of the mesa stripe
It is selectively buried in a high-resistance block layer 6 made of Fe-doped high-resistance InP. After removing the stripe-shaped SiO ₂ mask, a photoresist film or metal film that will serve as a mask for proton implantation will be formed on the entire surface in the future, and the window will be formed only in the portion corresponding to the proton implantation region by ordinary photolithography. After the opening, protons are injected until reaching the high-resistance substrate 1 to form a proton injection region 7. After removing the mask for proton implantation, a photoresist film or metal film serving as a mask is formed on the entire surface when ions for converting the high-resistance block layer to the n-type are formed. After windowing is performed only on the portion corresponding to the implantation region, ions for converting the high-resistance layer to n-type, for example, Si ions are converted into n ⁺ -InP.
The ion implantation is performed until the cladding layer 2 is reached, thereby forming an ion implantation region 7. After removing the mask for ion implantation, p ⁺ -InGaAs
A p-side electrode 9 is formed on the mesa stripe exposing the cap layer 5 and on the high-resistance block layer 6 on the side where the resistance of the n ⁺ -InP cladding layer 2 is increased by proton implantation. Then, an n-side electrode 10 is formed on the high resistance block layer which has been converted to the n-type by ion implantation on the opposite side.
After forming the electrodes, the substrate is polished to a thickness of about 100 μm, and the end face of the modulator is formed by cleavage. The element length of the modulator is 100 μm. The area of the p-side electrode is 2 μm × 100 μm in the stripe portion and 100 μm × 100 in the pad portion.
μm.

Next, the operation of the optical modulator according to the first embodiment will be described. First, the static characteristics will be described. The wavelength of the incident light is 1.55 μm for optical communication. When no reverse bias voltage is applied between the p-side electrode 9 and the n-side electrode 10, the incident light is output as it is as the outgoing light. The propagation loss at this time is
Approximately 1.5 dB when the element length is 100 μm and the wavelength difference between the incident light and the band gap of the light absorbing layer 3 is 75 nm (that is, when the band gap wavelength of the i-InGaAsP light absorbing layer 3 is 1.475 μm).
And a small value. When a reverse bias voltage is applied between the p-side electrode 9 and the n-side electrode 10 and an electric field is applied to the i-InGaAsP light absorbing layer 3, incident light propagates through the i-InGaAsP light absorbing layer 3 due to the Franz-Keldysh effect. The light is absorbed inside and no emitted light is output. At this time, the extinction ratio is 10 dB or more at a voltage of 3 V, and good characteristics can be obtained. Next, the modulation characteristics will be described. As described in the operation section, the bandwidth of the modulator is substantially determined by the capacitance C of the element using the electric field effect, and Δf = 1 / (π
CR). In the case of the embodiment, the non-dielectric constant of the semiconductor is 1
When calculated as 2.5, the junction capacitance C _j is 74 fF and the wiring capacitance C _i
And the pad capacitance C _p is 12FF, the capacity of the entire element 86fF
It is. Therefore, by adopting the modulator structure using the high-resistance substrate according to the present invention, the value of the element capacitance for determining the modulation speed can be reduced from a fraction to 1/10 or less of the conventional one, and the modulation band is 74 GHz. Is obtained, and a modulator capable of performing ultra-high-speed modulation is obtained.

FIG. 2 is a view showing an embodiment of an element integrated in a semiconductor laser and a modulator according to the present invention. FIG. 2A is a perspective view of the element, and FIG. FIG. 3 is a cross-sectional view of the device, cut along a plane passing through line A ′ and perpendicular to the substrate 1.

In the integrated device of the present embodiment shown in FIG. 2, an n ⁺ -InP cladding layer 2 is laminated on a high-resistance InP substrate 1, and n ⁺ -
On the InP cladding layer 2, a diffraction grating 22 parallel to the <10> direction is formed only in one of the two regions that divides the element into two. The period of the diffraction grating 22 is 2400. An n-InGaAsP light guide layer 21 having a band gap wavelength of 1.2 μm is formed on the n ⁺ -InP cladding layer 2 having a partial diffraction grating.
i-InGaAsP active layer 24, p-InP cladding layer 4, p ⁺ -InGa
The mesa stripe in which the As cap layers 5 are sequentially laminated is <
110> direction. i-InGaAsP active layer
Numeral 24 is a semiconductor having different band gaps in the direction along the mesa stripe, and the upper part of the diffraction grating 22 is an emission layer 24A made of InGaAsP having a wavelength composition of 1.55 μm and emitting light by current injection. The portion on which is not formed is made of InGaAsP having a wavelength composition of 1.475 μm and functions as the light absorption layer 24B of the modulator. Here, the thickness of each layer is n
⁺ -InP cladding layer 2 is 1.5 μm, n-InGaAsP light is guiding layer 21 is 0.1 μm, i-InGaAsP light absorbing layer 24B and i-InG
aAsP light emitting layer 24A is 0.3 μm, p-InP cladding layer 4 is 1 μm
The m, p ⁺ -InGaAs gap layer is 0.2 μm. The mesa stripe width is 2 μm. Both sides of the mesa stripe are buried with the Fe-doped high resistance InP block layer 6, and the upper surface of the wafer is flattened. On one side of the mesa stripe, from the upper surface of the high-resistance block layer to the high-resistance InP substrate, both the DFB laser portion and the optical modulator portion are subjected to proton injection and the n ⁺ -InP cladding layer 2 in the proton-injected region 7. Has increased resistance. On the other side of the mesa stripe, ions that convert the high-resistance block layer to n-type from the upper surface of the high-resistance block layer to the n ⁺ -InP cladding layer 2, for example, Si ions, are emitted from the DFB laser unit, The vessel is also injected. The p-side electrode 29 of the optical modulator is formed on a part of the mesa stripe having the light absorbing layer 24B and on the high resistance block layer into which protons are injected.
The p-side electrode 39 on the B laser side is formed on a part of a mesa stripe having the light emitting layer 24B and a part of a high resistance block layer into which protons have been injected. The n-side electrode, which is a common ground electrode, is n-
It is formed on a high-resistance block layer converted into a mold.
An anti-reflection coating film 23 is formed on the end face on the modulator side. The total length of the element is 410 μm,
FB laser section 300μm, optical modulator section 100μm, depth 1.2μ for electrical separation between DFB laser section and optical modulator section
The groove length of m is 10 μm. The area of the p-side electrode 29 in the modulator section is 100 μm × 2 μm in the stripe section, 10 μm × 20 μm in the wiring section, and 100 μm × 100 μm in the pad section.

Next, the operation of the integrated optical modulator of this embodiment will be described. When a forward current is applied between the electrodes 39 and 10, the laser oscillates at a wavelength of 1.55 μm, and the light output from the modulator side end surface passes through the light absorption layer 24B which is optically connected to the light emitting layer 24A. can get. Modulator section p-side electrode 29 and n-side common electrode 10
When a reverse bias voltage is applied during the period, the light is absorbed and modulated by the light Franz-Keldysh effect propagating in the light absorption layer 24B. The operation of the modulator is the same as that of the embodiment shown in FIG. 1 and has already been described, so that the description is omitted here. However, the modulation band of the modulator is also 50 GHz or more, and the An integrated optical modulator with integrated devices can be used as a light source capable of ultra-high-speed modulation.

FIG. 2 shows the case where the DFB laser and the first embodiment shown in FIG. 1 are integrated using an InP-based material, but the material, structure and manufacturing method of the integrated optical modulator are shown. Is not limited to this embodiment.

FIG. 3 is a diagram showing an embodiment of the photodetector according to the present invention.

In this embodiment, the structure and manufacturing method are the same as those of the embodiment of the optical modulator shown in FIG. 1 except that the light absorbing layer 33 is i-InGaAs lattice-matched with InP. A detailed description of the structure and the manufacturing method will be omitted.

In the photodetector shown in FIG. 3, the band gap of the i-InGaAs light absorption layer is 1.55 μm for incident light.
Since the incident light is 1.67 μm, which is longer than the wavelength of the incident light, the incident light is efficiently absorbed in the light absorbing layer 3, and the photocurrent due to the absorbed light is detected from the p-side electrode and the n-side electrode. The element shown in FIG. 3 functions as a waveguide type photodetector. Also in this case, if the element length and the thickness of the i-InGaAs light absorbing layer 33 are substantially the same as those in the first embodiment, the capacitance of the element can be set to 0.1 pF or less, and the ultra-wide band light can be obtained by the present invention. A detector is obtained.

(Effects of the Invention) As described in detail above, according to the present invention, an optical modulator capable of ultrahigh-speed modulation and a photodetector related thereto can be obtained, which contributes to the realization of a future ultrahigh-speed optical communication system. It is big to do.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an optical modulator of the present invention, and FIG. 2 is a diagram showing an embodiment of an integrated optical modulator in which a semiconductor laser and an optical modulator according to the present invention are integrated. FIG. 3 is a diagram showing an embodiment of the photodetector according to the present invention. In the figure, 1 is a high resistance InP substrate, 2 is an n ⁺ -InP cladding layer, 3 is an i-InGaAsP light absorbing layer, 4 is a p-InP cladding layer, 5 is a p ⁺ -InGaAs cap layer, and 6 is a high resistance block. Layer, 7 is a proton implantation region, 8 is an ion implantation region, 9 is a p-side electrode, 10 is an n-side electrode, 21 is a light guide layer, 22 is a diffraction grating, 23 is a non-reflective coating film, 24 is an active layer, 24A Is a light emitting layer, 24B is a light absorbing layer, 29 is a modulator side p-side electrode, 39 is a DFB
The laser unit p-side electrode 33 is an i-InGaAs light absorbing layer.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomoji Terakado 53-13-1 Shiba, Minato-ku, Tokyo NEC Corporation (56) References JP-A-1-185612 (JP, A) JP-A-Hei 1-239973 (JP, A)

Claims (3)

(57) [Claims] A first conductive type clad layer formed on a high resistance semiconductor substrate; a light absorbing layer formed on the first conductive type clad layer; and a second conductive type clad thereon. A high resistance block layer formed on the cladding layer of the first conductivity type so as to fill both side surfaces of the mesa stripe, and a high resistance block layer on one side of the mesa stripe. Means for increasing the resistance of the first conductivity type cladding layer below the first conductivity type cladding layer and a part of the high resistance block layer on the other side of the mesa stripe from the high resistance layer surface to the high resistance block layer and the first conductivity type cladding. Means for converting to the first conductivity type up to the interface of the layers, wherein a first electrode is provided on the mesa stripe and on the high resistance block layer on one side, and on the other side. Conductive high resistance block Optical modulator characterized by comprising a second electrode on. 2. A cladding layer of a first conductivity type formed on a high resistance semiconductor substrate, an active layer formed on the cladding layer of the first conductivity type, and a cladding layer of a second conductivity type thereon. And a high-resistance block layer formed on the first conductivity type cladding layer so as to fill both side surfaces of the mesa stripe, and the active layer is arranged in a traveling direction of the mesa stripe. Means for increasing the resistance of the first-conductivity-type cladding layer under the high-resistance block layer on one side of the mesa stripe; Means for converting a part of the high resistance block layer on the other side to the first conductivity type from the surface of the high resistance layer to the interface between the high resistance block layer and the first conductivity type clad layer, Narrow bandgap The first electrode on the mesa stripe on the active layer and on the high resistance block layer on one side, on the mesa stripe on the active layer with a wide bandgap and on the high resistance block layer on one side. A second electrode electrically insulated from the first electrode, characterized in that it has a third electrode on the other side of the conductive high resistance block layer on the other side. Light modulator. 3. A cladding layer of a first conductivity type formed on a high resistance semiconductor substrate, a light absorbing layer formed on the cladding layer of the first conductivity type, and a cladding of a second conductivity type thereon. A high resistance block layer formed on the cladding layer of the first conductivity type so as to fill both side surfaces of the mesa stripe, and a high resistance block layer on one side of the mesa stripe. Means for increasing the resistance of the first conductivity type cladding layer below the first conductivity type cladding layer and a part of the high resistance block layer on the other side of the mesa stripe from the high resistance layer surface to the high resistance block layer and the first conductivity type cladding. Means for converting to the first conductivity type up to the interface of the layers, wherein a first electrode is provided on the mesa stripe and on the high resistance block layer on one side, and on the other side. Conductive high resistance block Photodetector and comprising a second electrode, the band gap wavelength of the light absorbing layer is equal to or longer than the wavelength of the incident light on the.