JP2018189780A - Compound semiconductor based light modulation element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-speed semiconductor light modulation element formed on a compound semiconductor substrate, having a layer structure in which p-type layers can be laminated in a highly precisely controlled manner.SOLUTION: A semiconductor light modulation element formed on a crystal substrate of III-V group compound semiconductor includes on the surface of the crystal substrate at least a first n-type clad layer, a non-doped core layer, a p-type clad layer, and a second n-type clad layer formed thereon. A V group element of at least one of the first or the second n-type clad layer includes P (phosphorus), a V group element of the p-type clad layer includes only As(arsenic). and C(carbon) is used for dopant of the p-type clad layer.SELECTED DRAWING: Figure 3

Description

  The present invention relates to the field of optical communication, and more particularly to a compound semiconductor-based light modulation element that functions as a high-speed light modulator.

  In the optical communication field, in addition to an increase in optical communication capacity, there is a demand for miniaturization and low power consumption of optical devices used there. Mach-Zehnder type (MZ) optical modulators are listed as important element devices that control their characteristics, and many researches and developments have been made. Conventionally, optical modulators using lithium niobate (LN) or the like have been developed, but in recent years, compound semiconductor materials such as InP have been developed in order to achieve further reduction in size and power consumption. The optical modulator used is attracting attention.

  The characteristics of the compound semiconductor optical modulator will be briefly described below. A compound semiconductor is a semiconductor composed of a compound of two or more elements, and in order to operate as an optical modulator, an interaction between an electric field of a modulated electric signal applied to a semiconductor junction and propagating light is used. In general, a semiconductor junction has a pin structure in which a core layer for confining light is a non-doped layer (i-type layer) to which no impurity (dopant) is added, and the i-type layer is sandwiched between p-type and n-type cladding layers. By applying a reverse bias voltage in which a modulated electric signal is superimposed on this pin structure, light modulation is realized by changing the phase of light based on a strong interaction between electricity and light.

  Hereinafter, a device structure using an InP (indium phosphorus) -based material, which is one of III-V group compound semiconductors, will be described.

  In InP-based materials, zinc (Zn) or beryllium (Be) is usually used as a p-type dopant for forming a p-type semiconductor, and silicon (Si) or selenium (selenium) is used as an n-type dopant for forming an n-type semiconductor. Se) and sulfur (S) are used. Since the p-type semiconductor formed in this way has a material electrical resistivity and optical absorptance that are higher by an order of magnitude than those of an n-type semiconductor, it has major problems in speeding up the modulation operation and reducing optical loss. ing. In addition, since the contact area between the p-type semiconductor and the electrode is smaller than that of the n-type, the contact resistance increases, resulting in band degradation.

  In order to solve these problems, there has been proposed an optical modulator having an npin type structure in which the p-type semiconductor is replaced with an n-type semiconductor and a thin film p-type layer functioning as a current blocking layer is inserted (for example, the following patent document). 1). Since the npin type structure does not use a p-type cladding having a large optical loss, a relatively long waveguide can be used. In addition, since there is a degree of freedom that the thickness of the depletion layer can be designed optimally, it is easy to satisfy the reduction in drive voltage and electrical speed / light speed matching at the same time, which is advantageous for increasing the response speed of the optical modulator. is there.

  Since the npin structure uses a thin film p-type layer as a current blocking layer when a voltage is applied, its electrical characteristics greatly depend on the characteristics of the thin film p-type layer. The thin film p-type layer is less affected by optical loss, and the layer thickness is determined within a range in which the electric withstand voltage can be secured, and is usually 50 to 100 nm.

Japanese Patent No. 4047785 Japanese Patent No. 5497678

  However, in actual device fabrication, it is difficult to stack the thin film p-type layer with high accuracy. The main reason is that the p-type dopant Zn, which is most frequently used in the above-mentioned InP-based materials, has a large thermal diffusion coefficient, and heat treatment (epitaxial growth temperature: about 600 degrees) is added to the fabrication process after crystal deposition. This is because Zn diffuses into other layers starting from the p-type layer. As a result, for example, a part of the non-doped layer adjacent to the p-type layer becomes p-type, so that not only a desired depletion region (electric capacity) cannot be obtained but also an increase in optical loss may occur. Further, when the p-type layer is a thin film, the impurity concentration of the p-type layer after diffusion is lowered, making it difficult to secure a desired breakdown voltage, and a large variation in electrical characteristics within the wafer surface. There is concern about becoming.

  In addition, although there is Be having a smaller diffusion coefficient than Zn as a p-type dopant, Be is concerned about the memory effect of the dopant into the growth furnace in the process of metal organic chemical vapor deposition (MOCVD). The problem remains. In the molecular beam epitaxy (MBE), Be is used, but high temperature annealing treatment (crystal regrowth by MOCVD whose growth temperature is 150 ° C. higher than MBE) is added in the manufacturing process of the modulator. Like Zn, diffusion is also a concern with Be.

  Further, as a p-type dopant of the InP-based material, a group IV element C (carbon) is also known. FIG. 1 is one example of a layer structure of a conventional C-doped InP optical device, in which a C-doped p + type InGaAs contact layer 6 is provided on a Zn-doped p-type InP cladding layer 5. The structure of the optical device includes a semi-insulating InP substrate 1, a Si-doped n-type InGaAs contact layer 2, a (Si-doped) n-type InP cladding layer 3, and a non-doped core / cladding layer 4 in order from the lower layer of FIG.

  FIG. 2 shows another example of the layer structure of a conventional C-doped InP optical device. On the same optical device structure as in FIG. 1, a C-doped p-type InAlAs cladding layer 7, a Zn-doped p-type InP. A clad layer 5 and a Zn-doped p + type InGaAs contact layer 8 are formed.

  However, since these conventional C-doped p-type layers are used together with a Zn-doped P-based compound semiconductor (p-type), they have the above-mentioned problems of Zn doping. It was difficult to control the interface state between the compound crystals with high accuracy, and on the contrary, the electrical characteristics could be deteriorated. As a result, there are many process problems beyond the advantage of a small diffusion coefficient, which is an original feature, and it has not been applied to practical devices. Of course, for the MZ modulator constituting the IQ optical modulator, Zn is mainly used as the dopant of the p-type layer.

  From the above, Zn is still used as the p-type dopant of the currently produced compound semiconductor optical modulator, and the same problem exists in the optical modulator having the npin structure.

The present invention has been made in view of such problems, and its object is to
In a semiconductor light modulation device formed on a compound semiconductor substrate, a high-speed semiconductor light modulation device is realized as a layer structure that can be stacked by controlling a p-type layer with high accuracy.

In order to achieve such an object, the compound semiconductor light modulation device of the present invention is characterized by having the following configuration.
(Structure 1 of the invention)
A semiconductor light modulation element formed on a crystal substrate of a III-V compound semiconductor,
At least a first n-type cladding layer, a non-doped core layer, a p-type cladding layer, and a second n-type cladding layer are formed on the surface of the crystal substrate,
At least one group V element of the first or second n-type cladding layer includes P (phosphorus),
The group V element of the p-type cladding layer is composed only of As (arsenic),
A semiconductor light modulation device using C (carbon) as a dopant of the p-type cladding layer.
(Configuration 2)
2. The semiconductor light modulation device according to Configuration 1, wherein the crystal composition of the p-type cladding layer is InAlAs.
(Structure 3 of the invention)
The non-doped core layer and the p-type clad layer are sandwiched between the first and second n-type clad layers, the non-doped core layer is i-type, and the n − toward the substrate surface of the crystal substrate. 3. The semiconductor light modulation element according to Configuration 1 or 2, wherein the layers are stacked in the order of i-pn or n-pn.
(Configuration 4)
4. The semiconductor light modulation device according to claim 1, wherein the crystal substrate is InP, and InP is contained in at least one of the first or second n-type cladding layer.
(Structure 5 of the invention)
The semiconductor optical modulator according to any one of the configurations 1 to 4 of the invention constitutes a phase modulator formed in an arm optical waveguide of an optical circuit of a Mach-Zehnder interference type. Zehnder type optical modulator.

  According to the present invention described above, in a semiconductor light modulation device formed on a compound semiconductor crystal substrate, a p-type layer can be stacked with high precision, and a high-speed semiconductor light modulation device can be realized. It becomes possible to do.

It is a figure which shows the layer structure example 1 of the conventional C dope InP type | system | group optical device. It is a figure which shows the layer structure example 2 of the conventional C dope InP type | system | group optical device. It is sectional drawing of the semiconductor light modulation element produced as Example 1 of this invention. It is sectional drawing of the semiconductor light modulation element produced as Example 2 of this invention. FIG. 6 is a schematic plan view of a Mach-Zehnder optical modulator that is Embodiment 3 of the present invention. It is a figure which shows the structure outline | summary of the optical modulator cross section in VI-VI in FIG. It is a figure which shows the structure outline | summary of the optical modulator cross section in VII-VII in FIG. It is a figure which shows the structure outline | summary of the optical modulator cross section in VIII-VIII in FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

The present invention has one of the main characteristics in that C (carbon, carbon) is used as a p-type dopant in a III-V compound semiconductor light modulation device. There are three points to consider when using C as a p-type dopant.
(1) C is a dopant having the smallest diffusion coefficient among p-type dopants such as Zn and Be.

For this reason, it is possible to suppress dopant diffusion due to heat treatment after doping.
(2) C is known as a dopant having the highest activation rate among p-type dopants such as Zn and Be.

That is, since the absolute number of impurity atoms doped to obtain a certain carrier concentration can be reduced, the optical material absorption per unit length can be reduced. This is a very advantageous point in a semiconductor light modulation device in which the waveguide length is longer than that of a laser or the like and optical loss reduction is required.
(3) In order to make a group III-V compound semiconductor p-type using C as a dopant, it is necessary to have a composition that does not contain phosphorus (P) at the group V site of the crystal.

  This is because C is a group IV element and acts on an III-V compound semiconductor as an amphoteric dopant which can be p-type or n-type depending on the combination of compound elements. Therefore, in order to make C function as a p-type dopant, it is a compound semiconductor such as InGaAs, AlGaAs, and InAlAs that does not contain P in the V group, for example, the V group is composed only of arsenic (As) (arsenic compound). There is a need. This is because it is generally known that a compound crystal containing P (P-based compound) becomes n-type by doping C instead of p-type.

  These crystals are often used mainly in devices on a GaAs substrate, and as described in the conventional example of FIGS. 1 and 2, the application of C as a p-type dopant to the device on an InP substrate is limited. . As described above, as an application example of C doping to an InP optical device, for example, as a high concentration p-type contact layer, a composition crystal smaller than the band gap of a core such as InGaAs is doped with C. (FIG. 1), with a C-doped p-type InAlAs layer sandwiched between a non-doped core layer and a Zn-doped p-type InP cladding, etc., to prevent Zn diffusion into the non-doped layer (FIG. 2) Is mentioned.

  However, since these conventional C-doped p-type layers are used together with a Zn-doped P-based compound semiconductor (p-type), they have the above-mentioned problems of Zn doping. It was difficult to control the interface state between the system compound crystals with high accuracy, and on the contrary, the electrical characteristics could be deteriorated. As a result, as described above, there are more problems in the process than the advantage that the diffusion coefficient, which is the original feature, is small, and it has not been applied to a practical device. Of course, for the MZ modulator constituting the IQ optical modulator, Zn is mainly used as the dopant of the p-type layer.

  On the other hand, in the optical modulator according to the present invention, since the p-type semiconductor only has to function as a current block, it is not necessary to deposit in combination with the Zn-doped P-based compound semiconductor as described above. In other words, there are few process problems, high-precision control is possible as compared with Zn doping, and optical loss and electrical characteristics are expected to be improved.

(Example 1)
FIG. 3 shows a cross-sectional view of a semiconductor light modulation device manufactured as Example 1 of the present invention.
The semiconductor optical modulation device of Example 1 in FIG. 3 has a nipn type semiconductor layer structure on a semi-insulating InP substrate 31. Specifically, from the upper layer, the semiconductor layers are an n-type contact layer 37, an n-type cladding layer 35, a non-doped core / clad layer 34 (i-type), a thin film p-type cladding layer 36 serving as a current barrier, and an n-type cladding layer 33. The n-type contact layer 32 is stacked on the InP substrate 31 in this order.

For example, the n-type contact layers 32 and 37 are made of Si-doped InGaAs having a carrier concentration of 1E + 19 cm ⁻³ , and the n-type cladding layers 33 and 35 are made of Si-doped InP having a carrier concentration of 1E + 18 cm ⁻³ . The carrier concentration of the p-type cladding layer 36 is 5E + 17 to 1E + 18 cm ⁻³ InAlAs in view of the light absorption coefficient and the potential energy that becomes an electron barrier. In order to use C as a p-type dopant in the InAlAsp type cladding layer 36, CBr ₄ was used as a raw material.

  The thickness of the C-doped InAlAsp type cladding layer 36 is determined within a range in which a sufficient electric withstand voltage is ensured in the wafer surface and the optical / RF loss is sufficiently small. Specifically, in consideration of doping concentration and layer thickness variation (for example, ± 10%) in the wafer surface, it is desirable that the thickness be at least 50 nm, and a thickness at which loss is negligible (for example, 200 nm) is desirable. In Example 1, the thickness that can sufficiently secure the withstand voltage was set to 100 nm.

  Crystal growth was deposited on the semi-insulating InP (100) substrate 31 by MBE. The band gap wavelength of the core layer 34 is determined within a range where the electro-optic effect is effectively applied with high efficiency at the operating light wavelength and the light absorption is not a problem. For example, in the case of the 1.55 micron band, the light emission wavelength of the core layer 34 is set to about 1.4 microns. The core layer 34 is preferably formed of a multiple quantum well structure of InGaAlAs / InAlAs from the viewpoint of high-efficiency modulation, but the usefulness of the present invention is not lost even if it is formed of a multiple structure such as InGaAsP / InP or InGaAsP / InGaAsP. Is clear.

  Further, the composition of the contact layer and the cladding layer other than the p-type layer 36 is not limited to the above, and there is no problem if, for example, an InGaAsP composition is used. As the p-type layer 36, there is no problem even if InGaAlAs or the like having a band gap larger than InP is used because the V group is composed only of As in addition to InAlAs. The composition crystal is basically lattice-matched with InP, but there is no problem even if the composition is changed from the lattice-matching condition within a range not reaching the critical film thickness. The crystal growth method is not limited to MBE, and the usefulness of the present invention is not lost even when MOCVD is used, for example.

(Example 2)
FIG. 4 shows a cross-sectional view of a semiconductor light modulation device manufactured as Example 2 of the present invention.

  In Example 2 of FIG. 4, the same layers as those in Example 1 of FIG. 3 are given the same numbers. However, unlike Example 1, the C-doped p-type cladding layer 36 is formed on the non-doped core / cladding layer 34. The npin-type layer structure deposited in (1) is adopted. Such a layer structure does not change the usefulness of the present invention. In this case, it is necessary to pay attention to the fact that the power supply polarity when the reverse bias voltage is applied is opposite to that of the nipn type, and that the waveguide stripe direction of the modulator is 90 degrees different from the case of the nipn type in terms of modulation efficiency. Yes (see Patent Document 2).

(Example 3: Mach-Zehnder type optical modulator)
FIG. 5 is a schematic plan view of a Mach-Zehnder optical modulator that is Embodiment 3 of the present invention.

  The Mach-Zehnder optical modulator according to the third embodiment bifurcates an optical signal input from the optical waveguide 53 into upper and lower arm optical waveguides, and optical phases are generated by the phase modulators 52 provided in the respective arm optical waveguides. This is a Mach-Zehnder interferometer-type optical circuit configuration that outputs light after being modulated and combined. Each phase modulator 52 is constituted by the semiconductor optical modulation elements of the first and second embodiments. FIG. 5 also shows an electrode 51 for applying a bias voltage.

  As a manufacturing process of the optical modulator shown in FIG. 5, a layer structure corresponding to the semiconductor optical modulator shown in FIG. 3 or 4 is first formed on a substrate. Thereafter, the upper n-type cladding layer 35 and the n-type contact layer 37 in a region not contributing to modulation are removed by dry etching and wet etching for the purpose of electrical isolation between elements. Further, from the viewpoint of reducing optical loss, the removed portion is further backfilled with semi-insulating InP (see also FIGS. 7 and 8 described later).

Thereafter, as shown in FIG. 5, an MZ interferometer waveguide pattern made of SiO ₂ formed in a direction equivalent to the [011] plane direction is formed, and a high mesa optical waveguide 53 is formed by dry etching. For example, a ridge-shaped waveguide other than the high mesa shape may be used for the optical waveguide 53. Subsequently, in order to apply a bias voltage to the lower n-type cladding, dry / wet etching is further performed to expose a part of the lower n-type contact layer, and an electrode 51 for applying a bias voltage is formed thereon. Thereafter, a high frequency electrode pattern is formed by a gold plating method to obtain a Mach-Zehnder type optical modulator.

  FIG. 6 is a diagram showing a structural outline of a cross section of the optical modulator taken along line VI-VI in FIG. 5 of the third embodiment. The cross-sections of the two arm optical waveguides of the Mach-Zehnder optical modulator of FIG. 5 are shown with the same structure. The cross-sectional structure (61 to 67) of each arm corresponds to the cross-sectional structure of the semiconductor optical modulation element of FIG.

  FIG. 7 is a diagram showing a structural outline of a cross section of a modulation element taken along the line VII-VII in FIG. 5 of the third embodiment. Since the phase modulator 52 shown in FIG. 5 is not provided, the electrode 68 shown in FIG. 6 is not provided. Instead of the upper n-type cladding layer 65 and the contact layer 67 (Fe-doped), a semi-insulating InP cladding layer 75 is used. The main structure of the two covered optical waveguides and the structural outline of the cross section of the bias voltage application electrode 51 are shown.

  FIG. 8 is a diagram showing a structural outline of a cross section of a modulation element in VIII-VIII in FIG. 5 of the third embodiment. A cross section of one optical waveguide similar to that of FIG. 7 but after combining is shown.

  The high frequency electrode of the phase modulator 52 is preferably a traveling wave type (distributed constant type) from the viewpoint of speeding up, but the usefulness of the present invention is not lost even if, for example, a lumped constant type electrode is used.

  According to the present invention described above, in a semiconductor light modulation device formed on a compound semiconductor substrate, a p-type layer can be controlled and stacked with high accuracy, and a high-speed compound semiconductor light modulation device can be realized. It becomes.

1, 31, 61 Semi-insulating InP substrate 2, 32, 37, 62, 67 Si-doped n-type InGaAs contact layers 3, 33, 35, 63, 65 (Si-doped) n-type InP cladding layers 4, 34, 64 Non-doped Core-clad layer 5 Zn-doped p-type InP clad layer 6 C-doped p + -type InGaAs contact layer 7 C-doped p-type InAlAs clad layer 8 Zn-doped p + -type InGaAs contact layer 36, 66 C-doped p-type clad layer 51 Bias voltage application Electrode 52 Phase modulator 53 Optical waveguide 68 Electrode 75 (Fe doped) Semi-insulating InP clad layer

Claims (5)

A semiconductor light modulation element formed on a crystal substrate of a III-V compound semiconductor,
At least a first n-type cladding layer, a non-doped core layer, a p-type cladding layer, and a second n-type cladding layer are formed on the surface of the crystal substrate,
At least one group V element of the first or second n-type cladding layer includes P (phosphorus),
The group V element of the p-type cladding layer is composed only of As (arsenic),
A semiconductor light modulation device using C (carbon) as a dopant of the p-type cladding layer. 2. The semiconductor light modulation device according to claim 1, wherein the crystal composition of the p-type cladding layer is InAlAs. The non-doped core layer and the p-type clad layer are sandwiched between the first and second n-type clad layers, the non-doped core layer is i-type, and the n − toward the substrate surface of the crystal substrate. 3. The semiconductor light modulation device according to claim 1, wherein the semiconductor light modulation devices are stacked in the order of i-p-n or n-p-i-n. 4. The semiconductor light modulation device according to claim 1, wherein the crystal substrate is InP, and at least one of the first or second n-type cladding layer includes InP. 5.   5. The Mach-Zehnder according to claim 1, wherein the semiconductor optical modulation element constitutes a phase modulator formed in an arm optical waveguide of a Mach-Zehnder interference type optical circuit. Type optical modulator.

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