JP2850985B2 - Semiconductor waveguide type photo detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor waveguide type photodetector having a pin structure in a semiconductor optical waveguide layer.

[0002]

2. Description of the Related Art A layer made of a material having a high light absorption and thus a high refractive index is used for a photoelectric conversion layer and a photoelectric conversion layer of a light receiving element by utilizing the property of a substance that the refractive index increases as the light absorption increases. 2. Description of the Related Art Conventionally, there has been a waveguide type light receiving element characterized by having both functions of a core layer of a waveguide.

In such a waveguide type light receiving element, since the light incident direction is perpendicular to the traveling direction (pn junction direction) of the photoexcited carriers, the element length in the light incident direction which is a factor of photoelectric conversion efficiency is reduced. The element thickness, which is a factor of high speed, can be set independently, and therefore, it is suitable as a high-speed and high-efficiency light receiving element.

Another advantage is that monolithic integration with a semiconductor laser, a waveguide type optical switch or the like is easy due to the similarity of the structure.

In a conventional general waveguide type light receiving element, when a signal light wavelength is 1.3 μm as an example, as shown in FIG. 7, a p electrode disposed on an n ⁺ -InP substrate 20 is used. Layer 2
P ⁺ -InP as 1 and 0.2 μm thick n-In _0.72 Ga _0.28 As _0.6 P as the low carrier concentration layer 22
_0.4 and n ⁺ -InP as the n-electrode layer 23 and has a stripe shape of 2 μm in width and 30 μm in length (JE Bowers, CA Burru, CA).
s, Electronics Letters, 22
No. 905, 1986). N-In as layer 22
Only _0.72 Ga _0.28 As _0.6 P _0.4 is a material capable of absorbing a signal light having a wavelength of 1.3 μm, and this waveguide type light receiving element is designed to be able to receive the signal light 24 having a wavelength of 1.3 μm.

Here, as a waveguide structure, n-In _0.72 Ga _0.28 As _0.6 P _0.4 having a high refractive index is composed of a core layer,
P ⁺ -InP and n ⁺ -InP having low refractive indices constitute an upper cladding layer and a lower cladding layer, respectively. As shown in FIG. Wave light can be confined in the vicinity of the core layer.

This light receiving element has a p ⁺
-InP and n ⁺ -InP, which is the n-electrode layer 23, apply a reverse bias voltage to apply In _0.72 Ga _0.28 As _0.6 P
_{0.4 A} depletion layer is formed in the low carrier concentration layer 22, and photoelectric conversion is performed using a high electric field applied to the depletion layer.

In FIG. 7, a signal light 24 enters from an end face of a light receiving element, and is photoelectrically converted by the core layer 22 while guiding the signal light 24 through the waveguide along the core layer 22. However, not all of the signal light emitted from the optical fiber can be guided in the waveguide. That is, of the light emitted from the optical fiber, the coupling efficiency represented by the spot size W _OF of the light field distribution emitted from the optical fiber and the spot size W _WG of the guided light field distribution unique to each waveguide structure, Only the ratio of η = 2 / (W _OF / W _WG + W _WG / W _OF ) is guided inside the waveguide type light receiving element and photoelectrically converted, and the other parts are radiated out of the waveguide and photoelectrically converted. I can't get it.

As shown in FIG. 9, the value of the coupling efficiency η is W _OF
= W maximum value of 1 when the _WG, difference between W _OF and W _WG is reduced the greater.

By the way, the spot size of the distribution of the emitted light field from the optical fiber is substantially equal to the core diameter of the optical fiber.
It is about 10 μm, which is 1 μm or more even when a spherical fiber is used. On the other hand, in the conventional waveguide type light receiving element, the refractive index difference between the core layer and the cladding layer is 0.4 μm.
Since it is large, the guided light is confined in the vicinity of the core layer, and the spot size is about 0.2 μm, which is almost equal to the thickness of the core layer of the waveguide.

[0011]

Due to such a difference in spot size from that of the optical fiber, the conventional waveguide type light receiving element cannot sufficiently guide the signal light into the light receiving element, so that the waveguide type light receiving element cannot be used. It has been difficult to realize the high-speed and high-efficiency properties inherent in the light-receiving element.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a waveguide type light receiving element for photoelectrically converting signal light from an optical fiber with high efficiency.

[0013]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a semiconductor substrate, a first semiconductor layer disposed on the semiconductor substrate, and a first semiconductor layer on the first semiconductor layer. A second semiconductor layer having a bandgap energy smaller than that of the first semiconductor layer, being a non-doped layer and having a thickness of 0.15 μm or less, and being disposed on the second semiconductor layer. And a third semiconductor layer having a larger band gap energy than the second semiconductor layer,
A pn junction is provided near the boundary between the second and third semiconductor layers or in the third semiconductor layer.

Here, part or all of the second and third semiconductor layers can be formed in a stripe shape having a length of 20 μm or less and a width of 5 μm or less.

The first, second and third semiconductor layers are formed by I
lattice-matched to _{nP In 1-x Ga x As} y P 1-y (0 ≦
x ≦ 1,0 ≦ y ≦ 1) based semiconductor and without, the first semiconductor layer _{In 1-x Ga x As y} P 1-y (x <0.42, y
<0.9), the second semiconductor layer is formed of In _1-x Ga _x As.
_{y P 1-y (x ≧} 0.42, y ≧ 0.9), said third semiconductor layer _{In 1-x Ga x As y} P 1-y (x <0.42,
y <0.9).

[0016]

FIG. 1 shows the relationship between the core layer of the waveguide type light receiving element and the spot size of the waveguide light field distribution obtained by calculation. The spot size is minimum when the core layer is 0.2 μm, and increases sharply as the core layer becomes smaller than this value at about 0.15 μm. At this time, the materials of the core layer and the cladding layer are the same as those of the first embodiment described later, but the calculation result is hardly changed for other combinations of the core layer and the cladding layer.

According to the present invention, the confinement of the guided light in the core layer is reduced by setting the thickness of the core layer of the waveguide type light receiving element to 0.15 μm or less based on the relationship shown in FIG. Thus, the configuration is such that the spot size of the guided light field distribution of the waveguide type light receiving element is close to the spot size of the emitted light field distribution from the optical fiber. There is an advantage that the signal light from the optical fiber can be guided into the element with high efficiency as compared with a waveguide type light receiving element having a thickness of 0.2 μm or more.

Further, according to the present invention, the width of the element is 5 μm.
The following is a characteristic feature that the signal light is confined within the width of the element and guided, and as in the conventional case where the element width is 5 μm or more, the signal light is remarkably guided in the element width direction. This is different from a side-incident type light receiving element having no wave mode.

Furthermore, the device structure of the present invention is different from a semiconductor laser in that the length of the device is set to 20 μm or less in order to enable high-speed operation of 50 GHz or more by reducing the capacity. .

FIG. 2 shows the cut-off frequency estimated based on the capacitance calculated from the length of the element. It can be seen that the operation at 50 GHz or more requires an element length of 20 μm or less.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 3 is a view for explaining a first embodiment of the present invention.

In FIG. 3, 1 is a carrier concentration of 1 × 10
¹⁸ cm ^-3 of n ⁺ -InP substrate, 2 is a thickness of 2μm, n ⁺ -In _1-x of the carrier concentration of _{^{1 × 10 18 cm -3 Ga x}} As y
_{P 1-y (x = 0.37} , y = 0.8) cladding layer, 3 is a non-doped layer having a thickness of 0.1 [mu] m, n-type carrier concentration of 1 × 10 ¹⁶ cm ^-3 or less of an In _{1- x Ga x As y P 1-} y
(X = 0.42, y = 0.9) core layer, 4 has a thickness of 2μ
m, p ⁺ -In _1-x G with a carrier concentration of 1 × 10 ¹⁸ cm ^-3
_{a x As y P 1-y} (x = 0.37, y = 0.8) is clad layer, disposed on the substrate 1 a layer 2, 3 and 4 in this order. _{In 1-x Ga x As y} P 1-y (x = 0.3
7, y = 0.8) and _{In 1-x Ga x As y} P 1-y
(X = 0.42, y = 0.9) is a semiconductor lattice-matched to InP.

Here, from the diode structure,
The substrate 1 and the cladding layer 2 are n-electrode layers, the core layer 3 is a low carrier concentration layer, and the cladding layer 4 is a p-electrode layer. Of these, only the core layer 3 can absorb light having a wavelength of 1.55 μm. Layer.

The light receiving element having the structure shown in FIG. 3 is manufactured as follows. First, after the layers 2, 3 and 4 are epitaxially grown on the substrate 1, the layers 2, 3 and 4 are processed by etching into stripes having a length of 20 μm and a width of 2 μm, and P ⁺ -In _{1 -x} Ga _x As _y P _1-y (x = 0.3
7, y = 0.8) p-type ohmic electrode, InP
An n-type ohmic electrode is deposited on the back surface of the substrate 1 to form a light receiving element.

The wavelength of 1.55 μ emitted from the optical fiber
The signal light of m is applied to the end face of the element, and a part thereof is guided in the element.

FIG. 4 shows the guided light field distribution in the device calculated by the Maxwell equation, corresponding to each layer in FIG. The spot size representing the degree of spread of the guided light field distribution is about 1 μm in the structure of this embodiment, and the spot size is 0.4 μm in the case of the conventional example (FIG. 8) in which the thickness of the core layer is 0.4 μm. It is much larger than that. Actually, the light receiving efficiency of the waveguide type light receiving element having the structure of this embodiment is about 60% by the experiment using the spherical fiber, and the light receiving efficiency of the waveguide type light receiving element having the core layer as thick as 0.4 μm is about 30%. % Was significantly improved.

(Embodiment 2) FIG. 5 is a view for explaining a second embodiment of the present invention.

In FIG. 5, reference numeral 5 denotes a carrier concentration of 1 × 10
¹⁸ cm ^-3 n ⁺ -InP substrate. 6 and 7 are In
_{1-x Ga x As y P} 1-y (x = 0.37, y = 0.8)
The cladding layer 6 has a thickness of 2 μm, an n-type carrier concentration of 1 × 10 ¹⁸ cm ⁻³ , and the cladding layer 7 has a thickness of 0.1 μm.
A 2 μm non-doped layer having an n-type carrier concentration of 1 ×
It is 10 ¹⁶ cm ^-3 or less. 8 is a non-doped layer having a thickness of 0.1 [mu] m, n-type carrier concentration of 1 × 10 ¹⁶ cm ^-3 or less of _{n-In 1-x Ga x} As y P 1-y (x = 0.42, y
= 0.9) core layer. 9 and 10 are In _1-x Ga
_{x As y P 1-y (} x = 0.37, y = 0.8) is a cladding layer. The cladding layer 9 is a non-doped layer having a thickness of 0.2 μm and has an n-type carrier concentration of 1 × 10 ¹⁶ cm ⁻³ or less. The cladding layer 10 has a thickness of 2 μm and a p-type carrier concentration of 1.
× 10 ¹⁸ cm ^-3 .

Here, from the diode structure,
The substrate 5 and the cladding layer 6 are an n-electrode layer, a cladding layer 7,
The core layer 8 and the cladding layer 9 are low carrier concentration layers, and the cladding layer 10 is a p-electrode layer.
And 9, only the core layer 8 is a photoelectric conversion layer capable of absorbing light having a wavelength of 1.55 μm.

The structure of the second embodiment is different from that of the first embodiment.
Since the depletion layer is thick, there is also an effect that the junction capacitance can be reduced.

FIG. 6 shows the guided light field distribution in the device calculated from the refractive index distribution by the Maxwell equation, corresponding to each layer in FIG. The spot size representing the degree of spread of the guided light field distribution is about 1 μm in the structure of this embodiment.
m, and the light receiving efficiency was greatly improved as in Example 1.

In the present embodiment, the thickness of the cladding layer 9 is the same as that of the cladding layer 7, but the same effect can be obtained even if the cladding layer 9 is made thinner than the cladding layer 7. In this structure, since the distance between the photoelectric conversion layer 8 and the p-electrode layer 10 is short, the time required for the holes generated in the photoelectric conversion layer 8 to reach the p-electrode layer 10 is short, and therefore, the light receiving speed is higher. Suitable for devices.

[0034] above, in the embodiment of the present invention, an In _1-x Ga as a material of the cladding layer and the core layer _x As _y P
_{1-y (0 ≦ x ≦} , 0 ≦ y ≦ 1) has shown an example of using the system _{semiconductor, In 1-xy Al x Ga} y As (0 ≦ x ≦ 1,0
The same effect can be expected by using ≦ y ≦ 1).

Alternatively, a similar effect can be expected even if a part or all of the cladding layer and the core layer are made of a material that does not lattice match with the InP substrate.

Further, in the embodiment of the present invention, an example in which the signal light wavelength is 1.55 μm is shown. However, the present invention can be applied to signal light having a wavelength other than 1.55 μm by appropriately selecting the material. A waveguide type light receiving element having the same effect as the example can be realized.

Further, a GaAs substrate and Al _1-x Ga
_x As (0 ≦ x ≦ 1), GaSb substrate and Al _1-x Ga _{x
As y Sb 1-y (0} ≦ x ≦ 1,0 ≦ y ≦ 1) can also be a semiconductor layer.

Furthermore, by applying the structure of the present invention to a semiconductor optical modulator, it is possible to realize a semiconductor optical modulator having good coupling with an optical fiber.

[0039]

As described above, according to the present invention,
By reducing the thickness of the core layer of the waveguide type light receiving element to 0.15 μm or less, it is possible to guide the signal light from the optical fiber into the element with high efficiency. There is an advantage that a light receiving element can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a thickness of a core layer of a waveguide type light receiving element and a spot size of a guided light field distribution.

FIG. 2 is a diagram showing a relationship between a length of an element and a cutoff frequency.

FIG. 3 is a schematic view showing Example 1 of the waveguide type light receiving element of the present invention.

FIG. 4 is a distribution diagram showing a refractive index distribution and a guided light field distribution in the waveguide type light receiving element of Example 1 shown in FIG.

FIG. 5 is a schematic view showing Example 2 of the waveguide type light receiving element of the present invention.

FIG. 6 is a distribution diagram showing a refractive index distribution and a guided light field distribution in the waveguide type light receiving element of Example 2 shown in FIG.

FIG. 7 is a schematic view of a conventional waveguide light receiving element.

FIG. 8 is a distribution diagram showing a refractive index distribution and a guided light field distribution in a conventional waveguide type light receiving element.

FIG. 9 is a diagram showing a relationship between a spot size _WWG of a waveguide light field distribution unique to a waveguide type light receiving element structure and a coupling efficiency of signal light from an optical fiber.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 clad layer 3 core layer 4 clad layer 5 substrate 6 clad layer 7 clad layer 8 core layer 9 clad layer 10 clad layer 20 substrate 21 p electrode layer 22 low carrier concentration layer 23 n electrode layer 24 signal light

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Junichi Yoshida 1-6-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-2-59625 (JP, A) JP-A-Hei 1-150374 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) H01L 31/10

Claims (3)

(57) [Claims] 1. A semiconductor substrate, a first semiconductor layer disposed on the semiconductor substrate, and a bandgap energy smaller than the first semiconductor layer disposed on the first semiconductor layer. A non-doped layer having a thickness of 0.1
A second semiconductor layer having a thickness of 5 μm or less, and a third semiconductor layer disposed on the second semiconductor layer and having a bandgap energy larger than that of the second semiconductor layer; A pn junction provided near the boundary of the semiconductor layer or in the third semiconductor layer. 2. The semiconductor waveguide light receiving device according to claim 1, wherein a part or all of the second and third semiconductor layers are formed in a stripe shape having a length of 20 μm or less and a width of 5 μm or less. Semiconductor waveguide type light receiving element. In the semiconductor waveguide type light receiving element according to 3. A process according to claim 1 or 2, wherein the first, second and third semiconductor layers lattice-matched to InP and _{In 1-x Ga x As y} P 1- _y
(0 ≦ x ≦ 1,0 ≦ y ≦ 1) based semiconductor and without, the first semiconductor layer _{In 1-x Ga x As y} P 1-y (x <0.
42, y <0.9), and the second semiconductor layer is formed of In _1-x G
_{a x As y P 1-y} (x ≧ 0.42, y ≧ 0.9), the third semiconductor layer _{In 1-x Ga x As y} P 1-y (x <
0.42, y <0.9).