JP3610545B2 - Waveguide type photodetector with semiconductor layer for non-alloy contact - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a waveguide photodetector having a mushroom structure and a semiconductor layer for non-alloy contact having excellent high frequency characteristics.
[0002]
[Prior art]
Recent optical communication systems are making rapid progress. In particular, there is a great demand for speeding up, and a communication speed of 10 Gbps is currently in practical use, but a communication speed of 40 Gbps or more is required as a system in the near future.
The main components of the optical communication system are a light source, an optical transmission line, and a photodetector. Recently, waveguide-type photodetectors having excellent high-frequency characteristics have attracted attention. However, for further speedup, capacity reduction is necessary.
[0003]
As shown in FIG. 6, a conventional waveguide type photodetector generally has a buffer layer 92 and a waveguide layer on an n-type InP substrate 91 (hereinafter, “semiconductor substrate” is abbreviated as “substrate”). 93, followed by formation of the light absorption layer 94, a waveguide layer 95 and a p-type InP layer 96 are sequentially laminated, and finally an electrode contact layer 97 doped with a high concentration of p-type dopant is disposed. have. In the case of a waveguide type photodetector for a wavelength of 1.5 μm, which is often used for optical communication, the composition of the light absorption layer 94 is InGaAs.
This semiconductor stacked structure S is formed by MOCVD (Metal Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy), CBE (Chemical Beam Epitaxy), and CBE (Chemical Beam Epitaxy). Created using
[0004]
The semiconductor multilayer structure S is processed by photolithography into a mesa structure Me (table-shaped multilayer structure with a flat top and a steep slope around the top) having a width of about 6 μm and a length of about 10 μm (FIG. 6 (a) conceptually). In order to reduce the capacitance of the waveguide type photodetector, a structure in which the width of the light absorption layer 94 located at the center of the mesa structure Me is reduced to about 2 μm, that is, a so-called mushroom structure Mu (the middle layer portion of the mesa structure Me). It is effective to form a constricted structure (conceptually shown in FIG. 6 (b)). In order to make the light absorption layer 94 thin, wet etching is used. In the semiconductor stacked structure S described above, since the light absorption layer 94 and the electrode contact layer 97 are both semiconductors having the same composition, InGaAs, the light absorption layer 94. It is difficult to create a mushroom structure Mu by selectively thinning only (X in FIG. 6B).
[0005]
Therefore, in order to realize the mushroom structure Mu, it is conceivable to use InGaAsP having a relatively low etching rate for the electrode contact layer 97 and InGaAsP having a relatively high etching rate for the light absorption layer 94.
[0006]
[Problems to be solved by the invention]
However, when InGaAsP is doped with p-type dopants such as Be (beryllium) or Zn (zinc), the concentration of holes that can be induced in InGaAsP is smaller than that of InGaAs. More specifically, the hole concentration that can be achieved in InGaAsP decreases as the PL wavelength (photoluminescence wavelength) becomes shorter. This tendency does not depend on the growth method as described above. That is, if InGaAsP is used, the etching rate can be reduced, but the hole concentration (= carrier concentration) decreases.
Therefore, when InGaAsP is used for the electrode contact layer 97, a mushroom structure Mu with a narrowed central portion can be formed, but there is a problem that the electrode contact layer 97 having a low electric resistance cannot be formed.
[0007]
The present invention provides a waveguide-type photodetector having a mushroom structure and a semiconductor layer for non-alloy contact having a low electric resistance and a high carrier concentration in order to further increase the speed of optical communication. Objective.
[0008]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention provides a waveguide-type photodetector comprising a semiconductor substrate and a plurality of semiconductor layers formed on the semiconductor substrate, and (a) the waveguide-type photodetector. The electrode contact layer for forming an electrode is a p-type In _1-x Ga _x As _1-y P _y layer for non-alloy contact, and (a) the electrode contact layer And (c) the electrode contact layer has tensile strain with respect to the semiconductor substrate made of InP, and the p-type In _1-x Ga _x As _1. it _-y P _y layer is set to the composition is x> y, and, (d) the waveguide photodetector that has a mushroom structure, the characteristics and the for non-alloy contact Waveguide-type photodetector with a semiconductor layer Proposed. According to this, it is possible to provide a waveguide type photodetector for high-speed optical communication, in which the electrical resistance of the electrode contact layer on which the electrode is installed is small and the high frequency characteristics are excellent.
The present invention (Claim 2) is characterized in that, in the configuration of Claim 1, ( d ' ) the waveguide type photodetector has a mushroom structure in which the light absorption layer is thinned. .
Further, according to the present invention (Claim 3), in the configuration of Claim 1 or Claim 2, the x is set to a value in which the tensile strain value is in a range of -0.5 to -1.0%. It is characterized by.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a conceptual perspective view showing an example of a waveguide type photodetector provided with a semiconductor layer for non-alloy contact according to the present invention. Here, (a) shows a mesa structure and (b) shows a mushroom structure. FIG. 2 is a diagram showing the relationship between the composition of InGaAsP carbon-doped with CBr ₄ and the carrier concentration. (A) is a graph which shows the relationship between Ga composition x (PL wavelength) and carrier concentration, (b) is a figure which shows the semiconductor layer used for the test.
[0010]
FIG. 3 is a graph showing the relationship between the tensile strain (Ga composition x) of the 1.4Q layer and the hole concentration. (A) is a graph which shows the relationship between tensile strain (Ga composition x) and hole concentration, (b) is a figure which shows the semiconductor layer used for the test. FIG. 4 is a diagram showing the relationship between the composition of InGaAsP and the etching rate. (A) is a graph which shows the etching rate of a 1.4Q layer, a 1.5Q layer, and an InGaAs layer, (b) is a figure which shows the semiconductor layer used for the test.
FIG. 5 is a conceptual cross-sectional view showing an example of a waveguide-type photodetector provided with a semiconductor layer for non-alloy contact according to the present invention.
[0011]
Description of Configuration and Manufacturing Method of Waveguide Type Photodetector As shown in the example of FIG. 1, the waveguide type photodetector D provided with the semiconductor layer for non-alloy contact according to the present invention includes at least a substrate 1 and a buffer. It has a mesa structure Me (semiconductor multilayer structure S) in which the layer 2, the waveguide layer 3, the light absorption layer 4, the waveguide layer 5, the p-type InP layer 6, and the electrode contact layer 7 are sequentially stacked. Further, the waveguide type photodetector D has a mushroom structure Mu in which a central portion (a portion of the light absorption layer 4) of the mesa structure Me is constricted (FIG. 1B). The size of the mesa structure Me is, for example, about 6 μm in width and about 10 μm in length, and varies depending on the application and required function. The thickness of each layer also differs depending on the application and function.
[0012]
In order to create the mushroom structure Mu, first, a semiconductor thin film structure S in which predetermined semiconductor layers are sequentially stacked on the substrate 1 by using a semiconductor thin film growth method such as the MOCVD method, the MBE method, or the CBE method described above. Need to produce. Of the above growth methods, the CBE method has the advantage that p-type high concentration doping is possible.
[0013]
Each of the semiconductor layers is made of a semiconductor material made of a III-V group. Specifically, the substrate 1 is n-type InP, the buffer layer 2 is n-type InP, the light absorption layer 4 is InGaAs, and the electrode contact layer 7 is carbon-doped and p-type having tensile strain. InGaAsP is used.
[0014]
As a Group III element raw material, an organic metal having three methyl groups bonded thereto can be used. For example, TMI (trimethylindium) or the like is used as the In material, and TMG (trimethylgallium) or the like is used as the Ga material.
Further, as the raw material for the group V element, for example, arsine (AsH ₃ ), phosphine (PH ₃ ), or the like is used.
[0015]
As a carbon raw material for carbon doping, CBr ₄ (carbon tetrabromide) or the like is used. For example, when a target semiconductor layer is grown by the CBE method, carbon is doped simultaneously with the growth of the semiconductor layer by adding a predetermined amount of CBr ₄ to the source gas.
[0016]
Next, the completed semiconductor stacked structure S is processed into a mesa structure Me having a predetermined width and length by photolithography. Since the etching is performed in the vertical direction, reactive ion etching (RIE) or the like is used.
[0017]
The mesa structure Me is wet-etched from the side surface using, for example, a mixed solution of H ₂ SO ₄ : H ₂ O ₂ : H ₂ O to form a mushroom structure Mu. It is the portion of the light absorption layer 4 that is selectively etched.
[0018]
Finally, although not shown in FIG. 1, both sides of this mushroom structure Mu are protected by being embedded with a polymer resin such as polyimide, and an electrode Po is attached to the electrode contact layer 7 to form a waveguide type photodetector D. .
[0019]
That is, the waveguide type photodetector D according to the present invention is:
1) Production process of semiconductor multilayer structure S 2) Mesa structure Me production process by photolithography and etching 3) Mushroom structure Mu production process by wet etching 4) Embedding process with resin 5) Production by manufacturing method comprising electrode installation process, etc. Is done.
[0020]
This waveguide type photodetector D has a structure in which light is incident from the side and guided to the light absorption layer 4 through the waveguide layers 3 and 5, so that it can be installed on the same plane as an optical fiber or an electronic device. is there. In addition, since the light absorption layer 4 is sandwiched between InPs (buffer layer 2 and p-type InP layer 6) having a large band gap, high quantum efficiency and high-speed response can be easily obtained. Furthermore, since the electrical resistance value of the electrode contact layer 7 is small, the electrode Po can be connected with high quality, and in addition, since it has the mushroom structure Mu, the capacity can be reduced and the high frequency characteristics are excellent. High-speed response is possible.
[0021]
Relationship between InGaAsP Composition and Carrier Concentration In the waveguide type photodetector D according to the present invention, it is an important point how to maintain a high carrier concentration in the electrode contact layer 7 made of InGaAsP. This is because it is impossible to make an electrically low-resistance contact with the electrode Po unless the carrier concentration is increased.
[0022]
FIG. 2A shows a semiconductor stacked structure comprising 1) an n-type InP substrate 21, 2) an InP buffer layer 22, and 3) a carbon-doped InGaAsP layer 23 using a CBE method capable of p-type high concentration doping. Several types of S (FIG. 2 (b)) that differ only in the composition of the uppermost layer were prepared and tested, and the results were summarized. The thickness of the uppermost layer is 3000 A (Angstrom).
[0023]
In FIG. 2A, the vertical axis represents carrier concentration (hole or electron concentration), the horizontal axis represents the PL wavelength of InGaAsP, and the upper side of the horizontal axis represents In _1-x Ga _x As _1-y P _y . The Ga composition x of each is shown.
The PL wavelength indicates the wavelength of photoluminescence which is secondary light emitted when InGaAsP receives light. This PL wavelength varies depending on the composition of InGaAsP. The uppermost InGaAsP layer 23 is lattice-matched to the n-type InP substrate 21.
[0024]
From FIG. 2A, when the Ga composition x = 0.47, the carrier concentration (hole concentration) is as high as 7 × 10 ¹⁹ cm ⁻³ . As the numerical value of the Ga composition x becomes smaller (as the PL wavelength becomes smaller), the carrier concentration also decreases. When the Ga composition x becomes smaller and the PL wavelength becomes 1.4 μm, the carrier concentration becomes 5 × 10 ¹⁸ cm. ^-3 .
When the Ga composition x is further reduced and the PL wavelength is shortened, the carrier concentration is minimized and the p-type InGaAsP is inverted into an n-type semiconductor at a place where the PL wavelength is 1.3 μm.
[0025]
Thus, carbon (C) in InGaAsP (and InGaAs) functions as an amphoteric impurity, and can be an acceptor or a donor depending on the partner bonded in the crystal. Further, when the P composition y is fixed, if the Ga composition x is increased, the number of acceptors is increased. As a result, the hole concentration is increased and the carrier concentration is also increased.
[0026]
Further, the influence of the Ga composition x and the P composition y on the characteristics of the carbon-doped InGaAsP semiconductor is that the PL wavelength is 1.5 μm and the p-type when x = 0.47 and y = 0. The PL wavelength when x = 0.33 and y = 0.28 is 1.4 μm and is p-type, and the PL wavelength when x = 0.28 and y = 0.39 is 1. Since it is 3 μm and is n-type (FIG. 2A), p-type InGaAsP is obtained by carbon doping because the composition of In _1-x Ga _x As _1-y P _y is x> y This is the case.
[0027]
Relationship between tensile strain of semiconductor layer and hole concentration 1.4 Relationship between tensile strain (Ga composition x) of the Q layer 33 and hole concentration is shown in FIG. In the figure, the Ga composition x on the horizontal axis on the upper side is calculated based on the lattice constant determined by X-ray diffraction.
The 1.4Q layer 33 refers to a carbon-doped InGaAsP layer having a PL wavelength of 1.4 μm. The same applies to the other 1.4Q layers 45 and 1.5Q layers 47 described below.
[0028]
FIG. 3A shows a semiconductor stacked structure S (1) composed of 1) n-type InP substrate 31, 2) InP buffer layer 32, 3) 1.4Q layer 33 by CBE method capable of p-type high concentration doping. FIG. 3 (b)) shows a summary of the results obtained by preparing and testing several types that differ only in the uppermost Ga composition x.
Thereby, in addition to the relationship between the tensile strain of the 1.4Q layer 33 and the hole concentration (= carrier concentration), the relationship between the Ga composition x and the hole concentration is also known.
Note that the thickness of the uppermost 1.4Q layer 33 is 3000 A, and the PL wavelength is the same. Further, the P composition y is 0.28 as shown in FIG.
[0029]
FIG. 3A shows that the hole concentration increases as the Ga composition x increases. Specifically, the hole concentration when the value of the Ga composition x is 0.33 (that is, the tensile strain is 0 in the case of lattice matching) is about 5 × 10 ¹⁸ cm ⁻³ , but x = 0.5 ( That is, at a tensile strain of 1%), the hole concentration increases to about 1.5 × 10 ¹⁹ cm ⁻³ by about three times. This hole concentration is not less than 1 × 10 ¹⁹ cm ^−3, which is a carrier concentration (= hole concentration) required for non-alloy contact, and an electrically low-resistance contact with the electrode Po can be realized.
If the Ga composition x exceeds 0.5, the tensile strain becomes too large (−1%), and crystal defects (transition) are likely to occur, which is not preferable.
[0030]
Therefore, the value of tensile strain is preferably in the range of −0.5% where the hole concentration is 1 × 10 ¹⁹ cm ⁻³ or more to −1.0% where the risk of crystal defects is high, and the Ga of the InGaAsP layer is high. The value of the composition x is preferably changed within the range of the tensile strain value.
Crystal defects are more likely to occur when the semiconductor layer is thicker, but the thickness of the electrode contact layers 7 and 61 in which InGaAsP is used is as thin as 3000 A or less. There is no.
[0031]
Relationship between P composition y of InGaAsP and etching rate An important point of the waveguide photodetector D according to the present invention is how to form the mushroom structure Mu in the mesa structure Me.
4 shows 1) n-type InP substrate 41, 2) n-type InP buffer layer 42, 3) InGaAs layer 43 having a thickness of 2000A, 4) InP layer 44 having a thickness of 5000A, and 5) 1.4Q layer 45 having a thickness. 6) InP layer 46 having a thickness of 5000A, 7) 1.5Q layer 47 (InGaAsP layer with a PL wavelength of 1.5 μm) having a thickness of 2000A, and 8) Finally, an InP layer 48 having a thickness of 5000A is sequentially stacked. The etching rate of the mesa structure Me (FIG. 4B) grown in this manner was examined. The 1.4Q layer 45 and the 1.5Q layer 47 are both carbon-doped and have tensile strain.
[0032]
4A, the etching rate of the InGaAs layer 43 containing no P is 0.25 μm / min, whereas the 1.5Q layer 47 containing P is 0.14 μm / min. The etching rate of the including 1.4Q layer 45 is 0.1 μm / min.
Incidentally, the P composition y of the 1.4Q layer is 0.27, and the P composition y of the 1.5Q layer is 0.2.
[0033]
Thus, the etching rate of the 1.4Q layer 45 containing P is reduced to half or less as compared with the InGaAs layer 43 containing no P. In addition, it has been confirmed for each InGaAsP composition that the etching rate depends only on the P composition y regardless of the presence or absence of carbon doping. That is, as the value of P composition y increases, the etching rate also decreases.
[0034]
【Example】
Next, the present invention will be described in more detail by way of examples.
[0035]
6 shows a waveguide-type photodetector D provided with a semiconductor layer for non-alloy contact shown in FIG. This is created by the following procedure.
[0036]
First, using CBE method or the like, 1) n-type InP substrate 51, 2) n-type InP buffer layer 52 (4000A), 3) n-type 1.3Q layer 53 (5000A), 4) n-type InP layer 54 ( 200) 5) n-type 1.3Q layer 55 (5000A), 6) non-doped InGaAs layer 56 (20A), 7) p-type InGaAs light absorption layer 57 (1000A), 8) p-type 1.3Q layer 58 (9000A) ), 9) p-type InP layer 59 (5000 A), 10) p-type 1.3Q layer 60 (2000 A), and 11) as the uppermost layer A semiconductor multilayer structure S composed of electrode contact layer 61 (2000 A) was created (FIG. 5). ).
The electrode contact layer 61 is a carbon-doped 1.4Q layer (p-type InGaAsP layer with a PL wavelength of 1.4 μm) having tensile strain. The numbers in parentheses indicate the thickness of each layer.
[0037]
Next, a mask was formed by photolithography, and the mesa structure Me was processed by etching vertically by the RIE based on the mask. The mesa structure Me has a width of 6 μm and a length of 10 μm.
[0038]
The mesa structure Me is selectively wet-etched from the side surface using a mixed solution of H ₂ SO ₄ : H ₂ O ₂ : H ₂ O so that the width of the p-type InGaAs light absorption layer 57 becomes 2 μm, and the mushroom structure Mu Produced.
The electrode contact layer 61 had a width of about 4.5 μm, and good selective etching could be performed.
[0039]
Thereafter, both sides of the mushroom structure Mu are embedded with polyimide with little light loss, and finally, TiPtAu (titanium / platinum / gold) is deposited on the uppermost electrode contact layer 61 to form an electrode Po (non-alloy electrode). . The electric resistance of the electrode contact layer 61 was reduced to about ¼ compared to that having no distortion in the lattice.
[0040]
As a result, the frequency band of the waveguide-type photodetector D is greatly improved from 50 GHz to 70 GHz, and the response speed is excellent, resulting in an element capable of high-speed optical communication.
[0041]
【The invention's effect】
As described above, the waveguide type photodetector D provided with the semiconductor layer for non-alloy contact according to the present invention has a high carrier concentration (= hole concentration) in the electrode contact layer, so that it is electrically The resistance is low, and the electrode Po can be connected with high quality.
In addition, since the mushroom structure Mu is provided, the capacity can be reduced, the high frequency characteristics (high-speed response) are excellent, and high-speed optical communication can be supported.
[0042]
Although the present invention has been described mainly with respect to the 1.4Q layer (composition with a PL wavelength of 1.4 μm), it goes without saying that the present invention can also be applied to InGaAsP with other compositions.
[Brief description of the drawings]
FIG. 1 is a conceptual perspective view showing an example of a waveguide-type photodetector provided with a semiconductor layer for non-alloy contact according to the present invention. (A) shows a mesa structure and (b) shows a mushroom structure.
FIG. 2 is a diagram showing the relationship between the composition of InGaAsP carbon-doped with CBr ₄ and the carrier concentration. (A) is a graph which shows the relationship between Ga composition x (PL wavelength) and carrier concentration, (b) is a figure which shows the semiconductor layer used for the test.
FIG. 3 is a diagram showing a relationship between tensile strain (Ga composition x) of a 1.4Q layer and hole concentration. (A) is a graph which shows the relationship between tensile strain (Ga composition x) and hole concentration, (b) is a figure which shows the semiconductor layer used for the test.
FIG. 4 is a diagram showing the relationship between the composition of InGaAsP and the etching rate. (A) is a graph which shows the etching rate of a 1.4Q layer, a 1.5Q layer, and an InGaAs layer, (b) is a figure which shows the semiconductor layer used for the test.
FIG. 5 is a conceptual cross-sectional view showing an example of a waveguide-type photodetector provided with a semiconductor layer for non-alloy contact according to the present invention.
FIG. 6 is a conceptual perspective view showing a conventional waveguide type photodetector. (A) shows a mesa structure and (b) shows a mushroom structure.
[Explanation of symbols]
D; Waveguide type detector Me; Mesa structure Mu; Mushroom structure Po; Electrode S; Semiconductor laminated structure x; Ga composition y; P composition 1, 21, 31, 41, 51, 91; Substrate (semiconductor substrate, n Type InP substrate)
2, 22, 32, 42, 52, 92; buffer layers 3, 5, 93, 95; waveguide layers 4, 57, 94; light absorption layer 6 96; p-type InP layers 44, 46, 48; InP layer 54 N-type InP layers 7, 61 and 97; electrode contact layer 23; InGaAsP layer 43; InGaAs layer 56; non-doped InGaAs layers 53 and 55; n-type 1.3Q layers 58 and 60; p-type 1.3Q layers 33 and 45 1.4Q layer 47; 1.5Q layer

Claims (3)

A waveguide detector comprising a semiconductor substrate and a plurality of semiconductor layers formed on the semiconductor substrate,
(A) The light absorption layer of the waveguide type photodetector is an InGaAs layer (indium, gallium, arsenic), and the electrode contact layer for electrode formation is p-type In _1-x Ga _x As for non-alloy contact. _1-y P _y layer to consist of (indium gallium arsenide phosphide),
(A) The electrode contact layer is doped with C (carbon) as a dopant;
(C) The electrode contact layer has a tensile strain with respect to the semiconductor substrate made of InP (indium / phosphorus), and is a p-type In _1-x Ga _x As _1-y P that is the electrode contact layer. _{the y} layer is set to a composition where x> y,
(D) the waveguide type photodetector has a mushroom structure;
A waveguide type photodetector comprising a semiconductor layer for non-alloy contact characterized by the above. (D ’) The waveguide type photodetector has a mushroom structure in which the light absorption layer is thin;
A waveguide-type photodetector comprising the semiconductor layer for non-alloy contact according to claim 1. (U ’) X is set to a value in which the tensile strain value falls within a range of -0.5 to -1.0%,
The waveguide type photodetector provided with the semiconductor layer for non-alloy contacts according to claim 1 or 2.

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