JP2005142499A - Semiconductor light-receiving element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a structure for improving a wavelength selection ratio by suppressing the effect of noise carrier diffusion for a semiconductor light-receiving element capable of selectively receiving long-wavelength optical signals out of a mixture of short-wavelength and long-wavelength lights. <P>SOLUTION: A semi-insulating layer is formed between the NCE layer and the contact layer of the second conductivity type in this semiconductor light-receiving element, wherein the NCE layer selectively absorbs short-wavelength lights only and converts a generated noise carrier into a long wavelength light not to be absorbed by the light-receiving layer for the extinction of the noise carrier. The NCE layer and the light-receiving layer are electrically isolated from each other in this structure for the prevention of noise carrier diffusion. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a semiconductor light receiving element, and more particularly to a semiconductor light receiving element for optical communication.

In an optical access system typified by a PON system among optical communication systems, optical signals having wavelengths of 1.3 μm band, 1.5 μm band, and the like travel through a single optical fiber. Therefore, in order not to cause a signal recognition error, the light receiving element is required to be able to selectively receive each wavelength.

In order to selectively receive short wavelength (eg, 1.3 μm band) light without receiving long wavelength (eg, 1.5 μm band) light, the band gap wavelength (λg) of the light receiving layer is set to long wavelength light and short wavelength light. It can be easily realized by setting the interval between (for example, λg = 1.4 μm).

It is relatively difficult to selectively receive only long-wavelength (eg, 1.5 μm band) light without receiving short-wavelength (eg, 1.3 μm band) light. There is Japanese Patent Application No. 2002-270371.

In the light receiving element described in the publication, short wavelength (noise) light is absorbed by the first and second semiconductor (InGaAsP) layers in the optical filter layer, and generated noise carriers are diffused in the third semiconductor (InGaAs) layer, Light emission is recombined in the InGaAs layer. Since this InGaAs layer has a band gap wavelength longer than that of the light receiving layer, the recombination light is not absorbed by the light receiving layer, and noise carriers can be effectively eliminated. Further, since the long wavelength (signal) light reaches the light receiving layer with almost no absorption in the optical filter layer, the long wavelength light can be selectively received. Hereinafter, the optical filter layer having the semiconductor multilayer structure is referred to as an NCE (Noise Carrier Extension) layer.

FIG. 1 shows a schematic cross-sectional view of a prototype sample. FIG. 1A shows a light receiving element in which an optical filter layer is constituted only by an InGaAsP layer 17. FIG. 1B shows a light receiving element (described in the publication) in which an optical filter layer is configured by the NCE layer 2. Both optical filter layer thicknesses were 4.3 μm, and the light receiving layer 5 thickness was also 2.5 μm. Thereby, the short wavelength (noise) light transmittance of the optical filter layer and the long wavelength (signal) light absorption rate of the light receiving layer 5 are substantially the same.

FIG. 2 shows the result of comparing the frequency dependence of the wavelength selection ratio. From this result, it can be seen that the wavelength selective ratio of the light receiving element incorporating the NCE layer 2 in FIG. 2B is improved by about 5 dB. In the light receiving element incorporating the NCE layer 2, a wavelength selection ratio of about −13 dB in the low frequency range and about −20 dB in the high frequency range of 20 MHz or higher was obtained. Incidentally, the ratio of the light-receiving sensitivity of short-wavelength light to the light-receiving sensitivity of long-wavelength light is called a wavelength selection ratio and is expressed in dB. Equation 1 is a formula for calculating the wavelength selection ratio. In the case of a long wavelength light selective light receiving element, the smaller this value, the better the wavelength selection ratio.

The noise caused by the short wavelength light is considered to be mainly due to (1) light transmitted through the optical filter layer, (2) recombination light within the optical filter layer, and (3) diffuse noise carrier. In (1), short wavelength (noise) light is not completely absorbed in the optical filter layer and reaches the light receiving layer, thereby degrading the wavelength selection ratio. In (2), light emission is recombined by the InGaAsP layer in the optical filter layer, and the wavelength selective ratio is deteriorated by the recombination light being absorbed by the light receiving layer. (3) deteriorates the wavelength selection ratio by diffusing noise carriers generated in the optical filter layer to the light receiving layer.

When the evaluation result of FIG. 2 is analyzed, it can be seen that it is composed of a low frequency component 21 and a high frequency component 22. The low frequency component 21 is considered to be caused by the diffusion noise carrier (3) having a slow response speed. The high frequency component 22 is considered to be caused by the light (1) and (2) having a high response speed.

The light receiving element with a built-in NCE layer described in the above publication can reduce the recombination light in the optical filter layer (2) and the diffusion noise carrier (3) due to its structure. Therefore, as shown in the evaluation results of FIG. 2, the wavelength selection ratio can be improved by about 5 dB in both the low frequency range and the high frequency range.
Japanese Patent Application No. 2002-270371

Although the wavelength selection ratio is improved in the light receiving element with a built-in NCE layer, the diffusion noise carrier in (3) is not completely extinguished. Noise is also generated. In order to further improve the wavelength selection ratio, it is essential to reduce the above (1) and (3). In particular, the disappearance of the diffusion noise carrier in (3) is the key to the improvement of the wavelength selection ratio.

For the reduction of the light transmitted through the optical filter layer (1), the NCE layer thickness is increased, the band gap wavelength of the InGaAsP layer in the NCE layer is shifted to the long wave side, and the absorptance of short wavelength light is increased. Can be considered. However, the total epitaxial thickness of the prototype (total thickness epitaxially grown on the InP substrate 1: thickness obtained by adding the NCE layer 2, the InP second contact layer 4, the light receiving layer 5, and the InP first contact layer 6) is about 10 μm. Therefore, it is almost impossible to increase the thickness of the NCE layer. In addition, the method of adjusting the band gap wavelength also has a problem that it cannot be expected to greatly improve the wavelength selection ratio in consideration of the transmittance of long wavelength (signal) light.

For reducing the diffusion noise carrier in (3), the thickness of the InP second contact layer 4 is increased, the n-type carrier concentration of the InP second contact layer 4 is increased, and the InGaAs layer in the NCE layer It is conceivable to increase the valence band gap of hindering the diffusion of noise carriers. However, as described above, it is difficult to increase the layer thickness. Further, since the carrier concentration of the prototype InP second contact layer is set to a relatively high value of 1.7E + 18 cm ⁻³ , there is a problem that it is difficult to desire a carrier concentration higher than this.

In order to solve the above problem, the semiconductor light receiving element according to the first invention of the present application is characterized in that an insulating layer or a semi-insulating layer is formed between the NCE layer and the InP second contact layer.

The semiconductor light-receiving device according to the second invention of the present application uses an epi-wafer (epitaxially grown wafer) having an optical filter layer such as the NCE layer or the InGaAsP layer described in the above publication, removes the light-receiving layer other than the light-receiving region, The light is reflected so that the light passes through the optical filter layer twice or more. Further, the light receiving region and the optical filter layer primary passage position are separated by 10 μm or more.

The semiconductor light-receiving element according to the third invention of the present application is characterized in that in the semiconductor light-receiving element according to the second invention, the optical filter layer between the first pass position of the optical filter layer and the light-receiving region is removed. To do.

The semiconductor light-receiving element according to the fourth invention of the present application is characterized in that a pn junction is formed at or near the light reflection position of the semiconductor light-receiving element according to the second invention.

In the semiconductor light receiving element according to the first invention of the present application, the NCE layer and the light receiving layer are made electrically independent by forming an insulating layer or a semi-insulating layer between the NCE layer and the InP second contact layer. Therefore, noise carriers generated in the NCE layer do not diffuse and enter the light receiving layer. As a result, it is possible to suppress degradation of the wavelength selection ratio in the low frequency range due to the diffuse noise carrier.

In the semiconductor light receiving element according to the second invention of the present application, the total thickness of the apparent optical filter layer is gained by reflecting the light inside the element and setting the light to pass through the optical filter layer twice or more. The wavelength selection ratio can be improved by reducing the light transmitted through the optical filter of (1). Furthermore, since the light receiving region and the first pass position of the optical filter layer are separated, the diffusion noise carrier (3) cannot reach the light receiving region, and the wavelength selection ratio in the low frequency region can be improved. Even when an InGaAsP layer is used as the optical filter layer instead of the NCE layer described above, even if the generated noise carriers are recombined with light emission, the light receiving region and the optical filter layer primary passage position are separated, so that the light receiving region is separated. The solid angle that enters is reduced, and the wavelength selection ratio is improved almost without being affected by the light emission recombination light of (2).

In the semiconductor light receiving element according to the third invention of the present application, by removing the optical filter layer between the portion where the light passes through the optical filter layer and the light receiving layer, noise carriers generated inside the optical filter layer are removed from the optical filter layer. It is possible to prevent the light from diffusing into the light receiving portion. As a result, it is possible to prevent deterioration of the wavelength selection ratio in the low frequency range due to the diffuse noise carrier, and further improve the wavelength selection ratio.

In the semiconductor light receiving element according to the fourth aspect of the present invention, by forming a pn junction at or near the light reflection position, it becomes possible to trap and eliminate the diffuse noise carriers generated in the optical filter layer. Thereby, an improvement in the wavelength selection ratio can be expected as compared with the first invention.

Examples of the present application are shown below.

An embodiment of the first invention of the present application will be described below. FIG. 3 is a cross-sectional view of the light receiving element of this embodiment, and illustrates a back-illuminated light receiving element.

In this light receiving element, an NCE layer 2, an Fe-doped InP semi-insulating layer 3, an n + type InP second contact layer 4, an n − type InGaAs light receiving layer 5 and an n − type InP layer 7 are sequentially formed on an InP substrate 1. The manufactured epi-wafer is processed. A p + -type InP first contact layer 6 is formed by diffusing Zn or the like only in a region where the pn junction of the n − -type InP layer 7 is to be formed. Here, the n − -type InP layer 7 may be a p + -type junction epi wafer. The NCE layer 2 includes an InGaAsP first noise light absorption layer 9, an InGaAsP second noise light absorption layer 10, an InGaAs noise carrier extinction layer 11, and an InP diffusion carrier barrier layer 12, and has a multilayer structure as shown in FIG. ing. The n − type InP layer 7 other than the light receiving region 8 is removed with a hydrochloric acid etching solution or the like, and the light receiving layer 5 is removed with a sulfuric acid etching solution or the like.

The band gap wavelengths were 1.39 μm for the first noise light absorption layer 9, 1.43 μm for the second noise light absorption layer 10, 1.83 μm for the noise carrier extinction layer 11, and 1.65 μm for the light receiving layer 5.

The p electrode 13 is formed on the first contact layer 6, and the n electrode 14 is formed on the second contact layer 4. The top surface of the first contact layer 6 other than the p electrode 13, the top surface and end surface of the n − -type InP layer 7, the end surface of the light receiving layer 5, and the top surface of the second contact layer 4 other than the n electrode 14 are insulated as a passivation film 15 such as a silicon nitride film. A film is formed. An antireflection film 16 for 1.55 μm of signal light is formed on the surface opposite to the epitaxial growth surface of the InP substrate 1 (back surface of the light receiving element). A silicon nitride film having a refractive index of about 1.9 is used for the antireflection film 16.

When 1.55 μm light as signal light is incident from the back surface of the light receiving element of this embodiment, the light is transmitted through the NCE layer 2 without being absorbed and reaches the light receiving layer 5 and is absorbed. On the other hand, when 1.31 μm light as noise light is incident, it is absorbed in the NCE layer 2 and generates noise carriers. Since this noise carrier has a higher probability of occurrence as it is closer to the back side of the element, a potential gradient is formed toward the light receiving layer 5 even under no bias. As a result, although noise carriers are diffused, most of them disappear due to recombination of light emission in the noise carrier extinction layer 11 in the NCE layer 2. However, the noise carriers that could not be eliminated diffuse along the potential gradient and try to enter the light receiving layer 5, but are electrically blocked by the semi-insulating layer 3 and therefore cannot enter the light receiving layer 5. Become. Therefore, it is possible to reduce the deterioration of the wavelength selection ratio in the low frequency range due to the diffuse noise carrier. Incidentally, when an Fe-doped semi-insulating substrate is used for the InP substrate 1, the absorption loss in the substrate can be reduced, and thus high light receiving sensitivity of signal light can be ensured. Although this embodiment is effective when the element size cannot be increased, crystal defects such as morphology when the semi-insulating layer is epitaxially grown may be a problem.

An embodiment of the second invention of the present application will be described below. FIG. 4 is a cross-sectional view of the light receiving element of this embodiment, and illustrates an end face incident type light receiving element. The optical filter layer is exemplified by the NCE layer.

This light receiving element is manufactured by processing an epi-wafer in which an NCE layer 2, an n + type InP second contact layer 4, an n − type InGaAs light receiving layer 5 and an n − type InP layer 7 are sequentially formed on an InP substrate 1. Is done. A p + -type InP first contact layer 6 is formed by diffusing Zn or the like only in a region where the pn junction of the n − -type InP layer 7 is to be formed. Here, the n − -type InP layer 7 may be a p + -type junction epi wafer. The NCE layer 2 is the same as in the first embodiment. The mesa surface 19 is formed by etching the back surface of the InP substrate 1 with a bromine-based etchant or the like. Here, the mesa surface 19 only needs to form a forward taper regardless of the type of etching solution and the type of formation method (dry etching, processing with a dicer, etc.). Further, the n − type InP layer 7 other than the light receiving region 8 is removed with a hydrochloric acid-based etching solution, and the light receiving layer 5 is removed with a sulfuric acid-based etching solution. Again, the type of etching solution and the formation method are not limited, and the upper surface of the second contact layer 4 or the upper surface of the NCE layer 2 may be exposed. The p electrode 13 is formed on the first contact layer 6, and the n electrode 14 is formed on the second contact layer 4. An insulating film such as a silicon nitride film is formed as a passivation film 15 on the upper surface of the first contact layer 6 other than the p electrode 13, the upper surface and the end surface of the n − -type InP layer 7, and the end surface of the light receiving layer 5. Further, an insulating film such as a silicon nitride film (a silicon oxide film, polyimide, SOG, or the like may be used on the upper surface of the second contact layer 4 other than the n-electrode 14 and the back surface of the InP substrate 1, and the back surface of the InP substrate 1 and the second contact. On the upper surface of the layer 4, insulating films having different types and different film thicknesses may be formed, and function as the reflective film 18. On the mesa surface 19 corresponding to the light incident surface, an antireflection film 16 for signal light of 1.55 μm is formed. An insulation film (silicon oxide film or the like) such as a silicon nitride film is formed on the antireflection film 16. The light receiving element is completed by chipping from the apex of the mesa surface 19 with a dicer or a cleavage machine.

When light is incident in parallel to the bottom surface of the light receiving element of this embodiment, 1.55 μm light as signal light is refracted by the mesa surface 19 and enters the inside of the light receiving element. In the case of this light receiving element, the taper angle of the mesa surface 19 is about 54 °. Therefore, when the refractive index of InP with respect to 1.55 μm light is about 3.17, the refraction angle with respect to the mesa surface is 10.7 °. That is, the light travels upward at an angle of 25.3 ° with respect to the bottom surface of the light receiving element. The light traveling inside the element passes through the NCE layer 2 and reaches the interface between the second contact layer 4 and the reflective film 18. The incident angle of light to this interface is 64.7 °. Assuming that the refractive index of the reflective film 18 is about 1.9, total reflection occurs when the thickness of the reflective film 18 is 3618 mm or more. The totally reflected light passes through the NCE layer 2 again, is totally reflected at the interface between the InP substrate 1 and the reflective film 18 on the back surface of the element, and travels upward again. The light transmitted through the NCE layer 2 three times is absorbed by the light receiving layer 5. On the other hand, 1.31 μm light, which is noise light, goes through a similar process, but is almost completely absorbed while passing through the NCE layer 2 three times. As described above, it is possible to suppress deterioration of the wavelength selection ratio due to light transmitted through the optical filter layer.

Further, in this embodiment, when the total thickness of the light receiving element is 150 μm, the light receiving region 8 and the optical filter layer primary passage position 24 are extremely long as about 600 μm. Since a large amount of noise carriers due to 1.31 μm light are generated in the NCE layer 2 in the vicinity of the optical filter layer primary passage position 24, it is difficult to think that it will diffuse and enter if it is more than 600 μm away from the light receiving region. Therefore, in this embodiment, since the influence of the diffuse noise carrier can be suppressed, the wavelength selection ratio in the low frequency region can be improved.

In the case of the present embodiment, since the light path becomes long, if an n + InP substrate is used for the InP substrate 1, the absorption loss in the substrate becomes large. Therefore, it is better to use a Fe-doped InP semi-insulating substrate. When a Fe-doped semi-insulating substrate is used, the total thickness of the light receiving element is 150 μm, the NCE layer is 4 μm, and the light receiving layer is 2.5 μm, the calculated light receiving sensitivity of 1.55 μm light is 0.89 A / W, wavelength The selectivity is -41.4dB, which is a very good value.

By the way, in this embodiment, the end face incident type using the mesa surface as the light receiving surface is exemplified. However, the light enters the optical filter layer several times in the front side incident type, the back side incident type, and other end face incident types as shown in FIG. The same result can be obtained if the light-receiving position and the optical filter layer primary passage position are separated by 10 μm or more. In this example, the same result can be obtained even when the epi-wafer of Example 1 is used. In the case of the present embodiment, the element size increases depending on the light incident method, so it is necessary to consider mounting restrictions.

An embodiment of the third invention of the present application will be described below. FIG. 6 is a cross-sectional view of the light receiving element of this embodiment, and illustrates an end face incident type light receiving element.

This light receiving element is removed by etching the n + -type InP second contact layer 4 and the NCE layer 2 in the optical filter layer removal region 23 of the light receiving element exemplified in the second embodiment with a bromine-based etchant or the like. ing. Here, the optical filter layer removal region 23 is an NCE between the light receiving region 8 and the optical filter layer primary passage position 24 regardless of the type of etching solution and the type of removal method (dry etching, dicer processing, etc.). The layer 2 may be removed.

In the case of the present embodiment, it is possible to prevent diffusion noise carriers from diffusing in the NCE layer 2 and entering the light receiving layer. When a Fe-doped semi-insulating substrate is used for the InP substrate 1, it is possible to electrically separate the generation source of the diffuse noise carrier from the light receiving region. Therefore, the influence of the diffuse noise carrier can be completely ignored, and the wavelength selection ratio deterioration in the low frequency region is eliminated. However, since the processing cost for removing the optical filter layer is higher than that of the second invention, an increase in cost is inevitable.

An embodiment of the fourth invention of the present application will be described below. FIG. 7 is a cross-sectional view of the light receiving element of this embodiment, and illustrates an end face incident type light receiving element.

In the light receiving element of FIG. 7A, Zn or the like is diffused into the n + -type InP second contact layer 4 in the reflection region 25 of the light-receiving element exemplified in the second embodiment to form a p + -type, and the pn junction 26 is formed. Forming. This pn junction 26 may be formed in the NCE layer 2. Further, even if a pn junction 26 is formed in the vicinity of the reflection region 25 as shown in FIG.

In the case of the present embodiment, by forming the pn junction 26 at or near the light reflection position, it is possible to trap and extinguish diffusion noise carriers that diffuse in the second contact layer 4 and enter the light receiving layer. It becomes possible. Therefore, since the influence by the diffusion noise carrier can be reduced, it is possible to suppress deterioration of the wavelength selection ratio in the low frequency range. However, the cost increase for the diffusion process is inevitable.

In Examples 2, 3, and 4, the NCE layer has been specialized. However, even if an InGaAsP layer or the like is used as the optical filter layer, the generated noise carrier hardly reaches the light receiving region. Even if the light emission is recombined, the solid angle becomes small due to the distance to the light receiving region, and the light caused by noise entering the light receiving region becomes weak. As a result, even when an InGaAsP layer is employed as the optical filter layer, a wavelength selection ratio that is not significantly different from that of the NCE layer can be obtained.

Although there are several applications similar in structure to Examples 2, 3, and 4, in the present application, an effective optical filter layer thickness is obtained by reflecting light inside the device. The feature is that the next passing position and the light receiving area are separated from each other to prevent the generated noise carrier from reaching the light receiving area, and there is no application whose main purpose is this. These characteristics can be predicted for the first time by the analysis of the noise component of FIG. 2 from the experiments and trial manufactures of the present inventor, and it can be said that the inventive idea of the present application is high.

Cross-sectional schematic diagram of a prototype sample of a long wavelength light selective light receiving element (background technology) Measured frequency dependence of wavelength selection ratio (background technology) Cross-sectional view of a back-illuminated light receiving element according to an embodiment of the first invention (Example 1) Sectional view of an edge-incident type light receiving element according to an embodiment of the second invention (Example 2) Application example of the second invention (Example 2) Sectional view of an end-face incident type light receiving element according to an embodiment of the third invention (Example 3) Sectional view of an end face incident type light receiving element according to an embodiment of the fourth invention (Embodiment 4)

Explanation of symbols

1: InP substrate 2: NCE layer 3: Fe-doped InP semi-insulating layer 4: n + type InP second contact layer 5: n− type InGaAs light receiving layer 6: p + type InP first contact layer 7: n− type InP Layer 8: Light receiving region 9: InGaAsP first noise light absorption layer 10: InGaAsP second noise light absorption layer 11: InGaAs noise carrier extinction layer 12: InP diffusion noise carrier barrier layer 13: p electrode 14: n electrode 15: passivation film 16: Antireflection film 17: InGaAsP optical filter layer 18: Reflective film 19: Mesa surface 20: Light (signal light, noise light)
21: Low frequency component 22: High frequency component 23: Optical filter layer removal region 24: Optical filter layer primary passage position 25: Reflection region 26: pn junction

Claims (5)

In the case where the first light (short wavelength) and the second light (long wavelength) are incident, an optical filter layer and a light receiving layer are formed in the traveling direction of the light, and the band gap wavelength is set in the optical filter layer. A first semiconductor layer located between the wavelength of the first light and the wavelength of the second light; a second semiconductor layer having a band gap wavelength longer than that of the first semiconductor layer; and a band gap greater than the wavelength of the second light. A third semiconductor layer having a longer wavelength, and the light receiving layer is formed of a fourth semiconductor layer having a shorter band gap wavelength than the third semiconductor layer, and is insulated between the optical filter layer and the light receiving layer. Alternatively, a semi-insulating layer is formed to electrically separate the optical filter layer and the light receiving layer, the first light is absorbed by the optical filter layer, and light is recombined in the third semiconductor layer to receive the light. The light is not received by the layer and cannot be converted The noise carrier is prevented from diffusing and entering the light receiving layer by the insulating or semi-insulating layer, and only the second light transmitted through the optical filter layer is selectively received by the light receiving layer. Semiconductor light receiving element. When the first light (short wavelength) and the second light (long wavelength) are incident, in the semiconductor light-receiving element having an optical filter layer that absorbs the first light, the light filter reflects the light inside the element. The apparent thickness of the optical filter layer is increased by passing through the layer a plurality of times, and the primary passing position of the optical filter layer is separated from the light receiving region, so that noise carriers generated in the optical filter layer are diffused. A semiconductor light receiving element that prevents the light from reaching the light receiving region. 3. The semiconductor light receiving device according to claim 2, wherein noise carriers propagating in the optical filter layer are prevented by removing the optical filter layer between a primary passage position of the optical filter layer and the light receiving region. element. 3. The semiconductor light receiving element according to claim 2, wherein a noise carrier is eliminated at the pn junction by forming a pn junction at or near the light reflection position. The optical filter layer having a semiconductor multilayer structure according to claim 1, wherein noise carriers generated by absorbing the first light are converted into light having a wavelength that is not absorbed by the light receiving layer and disappears in the optical filter layer. The semiconductor light receiving element according to claim 2, 3, or 4.

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