Back-incident light-receiving element

Abstract

An n-type contact layer (2), a p-type lightly doped light absorption layer (3), an n-type lightly doped light absorption layer (4), and a window layer (5) are laminated in this order on a substrate (1). A p-type region (7) is formed in a part of the window layer (5) and the n-type lightly doped light absorption layer (4).

Description

Technical Field

[0001] The present disclosure relates to a back-illuminated light-receiving element used for optical fiber communications and the like. Background Art

[0002] Photodiodes are semiconductor light-receiving elements used in optical fiber communications and the like. Representative structures of photodiodes include mesa-type and planar structures. In the mesa-type structure, the size of the light-receiving portion that can receive light is determined by etching away the peripheral portion of the light-receiving portion previously formed by the epitaxial stacking structure. In the planar structure, the light-receiving portion is formed by locally converting a layer called a window layer to a p-type or n-type. Since the pin junction of the light-receiving portion is not exposed on the surface or side of the element, the planar structure has a small leakage current and generally has an advantage in reliability.

[0003] As mentioned above, in a planar structure, it's necessary to convert a portion of the epitaxial layer to p-type or n-type. However, in compound semiconductor processes, localized p-type conversion is easier than localized n-type conversion. Furthermore, because it offers greater controllability, a pin junction is often employed, where the substrate side of the epitaxial layer is n-type, while a region formed locally on the surface side is p-type.

[0004] Generally, because the window layer needs to transmit incident light and protect the device surface, a material with a larger band gap than the light-absorbing layer can be used. Consequently, a difference in band gap occurs at the interface between the window layer and the light-absorbing layer. The p-type region is often formed below the interface between the window layer and the light-absorbing layer to prevent carrier movement from being impeded at the interface between the window layer and the light-absorbing layer. In this case, the light-absorbing layer of the photodiode is often lightly doped with n-type to stabilize the shape of the p-type region (see, for example, Patent Document 1).

[0005] Furthermore, the structures of semiconductor light-receiving elements are broadly classified into surface-incident light-receiving elements, where light enters from the surface side of the epitaxial layer; back-incident light-receiving elements, where light enters from the back side of the substrate; and end-incident light-receiving elements, where light enters from the side. Back-incident light-receiving elements are particularly commonly used in high-speed communication applications because they easily achieve both high-speed operation and a large light-receiving diameter. Back-incident light-receiving elements absorb a large amount of light from the side of the light-absorbing layer closest to the substrate in the direction of light incidence, generating a large number of carriers in this area.

[0006] Patent Document 1: Japanese Patent No. 4956944

[0007] In a conventional back-illuminated light-receiving element in which the light-absorbing layer is doped to an n-type, the electric field intensity on the side of the light-absorbing layer closest to the substrate decreases. However, since a large amount of light is absorbed in this region of low electric field intensity, a large number of carriers are generated. Especially when the input light intensity is high, the generated carriers are retained, resulting in a space charge effect in which the electric field acts in a direction that cancels the electric field applied to the light-absorbing layer. Therefore, there is also the problem of carrier immobility and deterioration of high-speed responsiveness. Although the degradation of high-speed responsiveness can be suppressed by increasing the electric field by increasing the externally applied voltage, there are limitations on the applied voltage. Summary of the Invention

[0008] The present disclosure has been made to solve the above-mentioned problems, and an object thereof is to provide a back-illuminated light-receiving element having high-speed response while being able to increase input light intensity without increasing applied voltage.

[0009] The back-incident light-receiving element disclosed herein is characterized in that it comprises: a substrate; an n-type contact layer, a p-type lightly doped light-absorbing layer, an n-type lightly doped light-absorbing layer, and a window layer stacked in sequence on the substrate; and a p-type region formed in the window layer and a portion of the n-type lightly doped light-absorbing layer.

[0010] Another back-incident light-receiving element disclosed herein is characterized in that it comprises: a substrate; an n-type contact layer, a first n-type lightly doped light absorption layer, a p-type doped light absorption layer, a second n-type lightly doped light absorption layer and a window layer stacked in sequence on the above-mentioned substrate; and a p-type region formed in the above-mentioned window layer and a portion of the above-mentioned second n-type lightly doped light absorption layer.

[0011] In this disclosure, in a back-illuminated light-receiving element with an n-type light-absorbing layer, the substrate side of the light-absorbing layer is converted to p-type. This increases the electric field strength on the substrate side of the light-absorbing layer, thereby suppressing the space charge effect. As a result, high-speed responsiveness can be improved at input light intensities without increasing the applied voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a cross-sectional view showing the semiconductor light-receiving element according to the first embodiment.

[0013] Figure 2 It is a cross-sectional view showing a first modification of the semiconductor light-receiving element according to the first embodiment.

[0014] Figure 3 It is a cross-sectional view showing a second modification of the semiconductor light-receiving element according to the first embodiment.

[0015] Figure 4It is a cross-sectional view showing a semiconductor light-receiving element according to a comparative example.

[0016] Figure 5 Graphs showing the electric field intensity distribution of the depleted portion when voltage is applied to the semiconductor light-receiving elements of Embodiment 1 and the comparative example.

[0017] Figure 6 It is a cross-sectional view showing a third modification of the semiconductor light-receiving element according to the first embodiment.

[0018] Figure 7 It is a cross-sectional view showing a semiconductor light-receiving element according to the third embodiment.

[0019] Figure 8 Graphs showing the electric field intensity distribution of the depleted portion when voltage is applied to the semiconductor light-receiving elements of the third embodiment and the comparative example.

[0020] Figure 9 It is a cross-sectional view showing a semiconductor light-receiving element according to the fourth embodiment.

[0021] Figure 10 Graphs showing the electric field intensity distribution of the depleted portion when voltage is applied to the semiconductor light-receiving elements of the fourth embodiment and the comparative example.

[0022] Figure 11 It is a cross-sectional view showing a semiconductor light-receiving element according to the fifth embodiment. DETAILED DESCRIPTION

[0023] The back-illuminated light-receiving element according to the embodiment will be described with reference to the accompanying drawings. The same reference numerals are used for the same or corresponding components, and redundant description may be omitted.

[0024] Implementation method 1.

[0025] Figure 1 This is a cross-sectional view showing a semiconductor light-receiving element according to Embodiment 1. On an InP substrate 1, an n-type contact layer 2, a p-type lightly doped light-absorbing layer 3, an n-type lightly doped light-absorbing layer 4, a window layer 5, and a p-type contact layer 6 are sequentially stacked. Here, lightly doped means 5×10 17 cm －3 Therefore, the impurity concentration of the p-type lightly doped light absorbing layer 3 and the n-type lightly doped light absorbing layer 4 is 5×10 17 cm －3 the following.

[0026] The p-type region 7 is formed continuously from the surface side of the epitaxial layer to the window layer 5 and a portion of the n-type lightly doped light absorbing layer 4, functioning as a light-receiving portion. The p-type electrode metal 11 is formed to be conductive with at least the p-type contact layer 6, functioning as an anode. This planar structure, in which the p-type region 7 is partially formed from the rear, offers superior reliability compared to a mesa structure.

[0027] The material of the n-type contact layer 2 is InGaAs, InP, InGaAsP, AlInAs, AlGaInAs or a combination thereof. The material of the p-type lightly doped light absorption layer 3 and the n-type lightly doped light absorption layer 4 is a material that generates carriers when light is incident, that is, a material with a small band gap relative to the incident light, such as InGaAs, InGaAsP, InGaAsSb, or a combination thereof. The material of the window layer 5 is a material that transmits incident light, such as InP, InGaAsP, AlInAs, AlGaInAs or a combination thereof. The material of the p-type contact layer 6 is InGaAs, InP, InGaAsP, AlInAs, AlGaInAs or a combination thereof. In order to alleviate band discontinuity, a band discontinuity relaxation layer composed of InGaAsP, AlGaInAs, etc. can be provided between each epitaxial layer or between the p-type electrode metal 11 and the epitaxial layer. As long as the characteristics required for operation as a semiconductor light-receiving element can be obtained, any material can be used for each layer, and the above-mentioned materials are not limited to the scope.

[0028] As p-type dopants that impart conductivity to III-V semiconductor crystals, Group II atoms such as Be, Mg, Zn, and Cd can be used. As n-type dopants, Group IV atoms such as S, Se, and Te can be used. As amphoteric impurities that act as dopants of either conductivity type in semiconductor crystals, Group IV atoms such as C, Si, Ge, and Sn can be used.

[0029] Figure 2 This is a cross-sectional view showing a modified example 1 of the semiconductor light-receiving element according to the first embodiment. The side and top surfaces of the epitaxial layer are covered with a passivation film 8. The material of the passivation film 8 is SiO2, SiN, SiON, or a combination thereof. The portion of the back surface of the substrate that does not obstruct the incidence of light is covered with a back metal 9. The InP substrate 1 is an n-type, and the back metal 9 functions as a cathode. An anti-reflection film 10 is formed on the portion of the back surface of the substrate where light is incident. The p-type contact layer 6 is annular when viewed from the top surface, but may also be circular.

[0030] Figure 3 This is a cross-sectional view showing a second modification of the semiconductor light-receiving element according to Embodiment 1. The InP substrate 1 of the second modification is semi-insulating. A cathode metal 12 is formed on the surface of the epitaxial layer in contact with the n-type contact layer 2 and functions as a cathode.

[0031] Next, the manufacturing method of the semiconductor light-receiving element involved in Embodiment 1 is described. First, an epitaxial layer is grown on an InP substrate 1 using a growth method such as liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), particularly metal organic vapor phase epitaxy (MO-VPE), or molecular beam epitaxy (MBE).

[0032] Next, using conventional photolithography techniques, with a mask opened only for the desired portion, a p-type dopant such as Zn is vapor-phase diffused or solid-phase diffused to form the p-type region 7. Next, using conventional photolithography techniques, with a mask opened only for the desired portion, a metal material such as Ti, Pt, or Au is deposited by electron beam evaporation or sputtering, and the metal material is removed from the unnecessary portions to form the p-type electrode metal 11 and cathode metal 12. Alternatively, the p-type electrode metal 11 and cathode metal 12 can be formed by depositing the metal material on the entire surface, then etching away the unnecessary portions using conventional photolithography techniques while leaving the mask open only for the desired portion.

[0033] After forming an insulating film by plasma-enhanced chemical vapor deposition (PE-CVD) or sputtering, the passivation film 8 is formed by etching unnecessary portions of the insulating film using conventional photolithography while leaving only desired portions masked.

[0034] Next, the InP substrate 1 is inverted and fixed to another supporting substrate or the like. Using conventional photolithography techniques, a metal material such as Ti, Pt, Ni, or Au is formed into a film by electron beam evaporation or sputtering, with a mask opened only for the desired portion. The metal material in the unnecessary portion is then removed, thereby forming the back metal 9. Alternatively, the back metal 9 can be formed by forming a film of the metal material on the entire surface and then etching the unnecessary portion of the metal material using conventional photolithography techniques, with a mask left open only for the desired portion.

[0035] Next, the InP substrate 1 is inverted and fixed to another supporting substrate, etc. After forming an insulating film by PE-CVD or sputtering, the insulating film of the unnecessary part is etched using conventional photolithography technology while leaving a mask only on the desired part, thereby forming an anti-reflection film 10.

[0036] The effects of this embodiment will be described in comparison with a comparative example. Figure 4 1 is a cross-sectional view showing a semiconductor light-receiving element according to a comparative example. In the comparative example, the p-type lightly doped light-absorbing layer 3 below the light-absorbing layer is absent, and the light-absorbing layer is formed only by the n-type lightly doped light-absorbing layer 4 . Figure 5 This figure shows the distribution of electric field intensity in the depleted portion when voltage is applied to the semiconductor light-receiving elements of Embodiment 1 and the Comparative Example. In the Comparative Example, the electric field intensity in the region near the InP substrate 1 is reduced. On the other hand, in Embodiment 1, the electric field intensity in the region near the InP substrate 1 is higher than in the Comparative Example. Therefore, even when strong light is incident and a large number of carriers are generated in the region near the substrate in the light-absorbing layer, space charge effects are less likely to occur, preventing degradation of frequency characteristics.

[0037] As described above, in this embodiment, in a back-illuminated light-receiving element in which the light-absorbing layer is doped to n-type, the substrate side of the light-absorbing layer is converted to p-type. This increases the electric field intensity on the substrate side of the light-absorbing layer, thereby suppressing the space charge effect. As a result, high-speed responsiveness at input light intensity can be improved without increasing the applied voltage.

[0038] However, if the impurity concentration of the p-type lightly doped light absorbing layer 3 is increased, the applied voltage required for depletion will increase. Therefore, from the perspective of practical use, the impurity concentration of the p-type lightly doped light absorbing layer 3 is preferably 1×10 17 cm －3 the following.

[0039] Furthermore, since light is absorbed by the p-type lightly doped light absorbing layer 3 and the n-type lightly doped light absorbing layer 4, each layer needs a certain thickness. To improve sensitivity and obtain the effects of this embodiment, the n-type lightly doped light absorbing layer 4 is preferably 0.3 μm or thicker.

[0040] Figure 6This is a cross-sectional view showing a third variation of the semiconductor light-receiving element according to Embodiment 1. A back lens 17 is formed on the back side of the InP substrate 1, on the opposite side from the surface on which the epitaxial layer is formed. The back lens 17 can be formed by various methods, such as isotropic wet etching or dry etching with the center of the lens covered with a mask, dry etching while varying the etching mask diameter, or etching with the photoresist shaped into a lens. The back lens 17 focuses incident light onto the p-type region 7. Therefore, the apparent light-receiving diameter can be enlarged. Therefore, this structure can be used in high-speed communication elements that achieve a lower capacity by reducing the size of the p-type region 7. On the other hand, although the light density increases in a portion due to the focusing of light, making the space charge effect more likely to occur, the structure of Embodiment 1 can achieve a higher effect of suppressing the space charge effect. Furthermore, in the following embodiments, a higher effect of suppressing the space charge effect can also be achieved in the structure in which the back lens 17 is formed.

[0041] Implementation method 2.

[0042] In Embodiment 1, the p-type lightly doped light absorbing layer 3 and the n-type lightly doped light absorbing layer 4 are formed from the same semiconductor material having a bandgap for absorbing incident light. However, in this embodiment, the bandgap energy of the p-type lightly doped light absorbing layer 3 is greater than that of the n-type lightly doped light absorbing layer 4. This reduces the absorption coefficient of the p-type lightly doped light absorbing layer 3 and the amount of light absorbed, thereby suppressing carrier concentration on the side of the light absorbing layer closer to the substrate. Consequently, compared to the structure of Embodiment 1, the space charge effect can be suppressed. The remaining structure and effects are the same as those of Embodiment 1.

[0043] Implementation method 3.

[0044] Figure 7 This is a cross-sectional view of a semiconductor light-receiving element according to Embodiment 3. In this embodiment, the region near the InP substrate 1 in the p-type lightly doped light-absorbing layer 3 of Embodiment 1 is replaced with a p-type doped light-absorbing layer 14 having a high impurity concentration. An n-type lightly doped light-absorbing layer 13 is formed between the p-type doped light-absorbing layer 14 and the n-type contact layer 2. The impurity concentration of the n-type lightly doped light-absorbing layers 4 and 13 is 5×10 17 cm －3 The impurity concentration of the p-type doped light absorbing layer 14 is 5×10 17 cm －3 above.

[0045] Figure 8This graph shows the electric field intensity distribution of the depleted portion when voltage is applied to the semiconductor light-receiving elements of Embodiment 3 and the comparative example. As in Embodiment 1, the electric field intensity in the region near the InP substrate 1 is higher than in the comparative example. Therefore, even when strong light is incident and a large number of carriers are generated in the region near the substrate in the light-absorbing layer, space charge effects are less likely to occur, preventing degradation of frequency characteristics.

[0046] By forming an n-type lightly doped light absorbing layer 13 at the interface on the back side of the light absorbing layer, the electric field intensity near the interface is reduced. However, light incident from the back side of the substrate is not completely absorbed at the interface on the back side of the light absorbing layer. Instead, it is gradually absorbed as it moves from the interface toward the inner side of the light absorbing layer. In this embodiment, by using a p-type doped light absorbing layer 14 with a high impurity concentration, the electric field intensity can be increased within a certain thickness range on the back side of the light absorbing layer. This prevents degradation of frequency characteristics.

[0047] Implementation method 4.

[0048] Figure 9 This is a cross-sectional view of a semiconductor light-receiving element according to Embodiment 4. An n-type lightly-doped electron transit layer 15 is inserted between the n-type contact layer 2 and the p-type lightly-doped light-absorbing layer 3. An n-type doped layer 16 is inserted between the n-type lightly-doped electron transit layer 15 and the p-type lightly-doped light-absorbing layer 3. Although the n-type lightly-doped electron transit layer 15 and the n-type doped layer 16 are continuous, a band discontinuity mitigation layer may be inserted between them.

[0049] The n-type lightly doped electron transport layer 15 and the n-type doped layer 16 are made of a material that does not absorb incident light, for example, InP, InGaAsP, AlInAs, AlGaInAs, or a combination thereof.

[0050] Here, when light is absorbed in the light absorption layer, electrons and holes are generated. Holes move to the anode side where a negative voltage is applied, and electrons move to the cathode side where a positive voltage is applied. Of the carriers generated in the light absorption layer, only the electrons that move to the substrate side move to the n-type lightly doped electron transport layer 15. Since a small electric field needs to be applied to the n-type lightly doped electron transport layer 15, the impurity concentration of the n-type lightly doped electron transport layer 15 is reduced to 1×10 16 cm －3 the following.

[0051] The n-type doped layer 16 adjusts the electric field applied to the n-type lightly doped electron transport layer 15. The impurity concentration of the n-type doped layer 16 is higher than that of the n-type lightly doped electron transport layer 15 and lower than that of the n-type contact layer 2. In order to reduce the resistance when contacting with the cathode electrode, the impurity concentration of the n-type contact layer 2 is as high as 1×10 18 cm －3 above.

[0052] With the recent increase in information communication volume, photodiodes are required to improve their high-speed responsiveness. To achieve this, it is necessary to strike a balance between reducing capacitance and shortening carrier transit time. To achieve this, in this embodiment, a non-light-absorbing n-type lightly doped electron transport layer 15 is inserted beneath the light-absorbing layer.

[0053] When the n-type lightly doped electron transport layer 15 is inserted, the electron movement time becomes longer due to the increase in the movement distance of the electrons, but the hole movement time remains unchanged. Since the movement speed of holes is several times slower than that of electrons, the hole movement time determines the overall high-speed responsiveness. Since only electrons move in the n-type lightly doped electron transport layer 15, the actual carrier transit time does not increase. Therefore, even if the n-type lightly doped electron transport layer 15 is inserted, it will not affect the overall high-speed responsiveness. On the other hand, since the depletion layer width is expanded by an amount corresponding to the inserted n-type lightly doped electron transport layer 15, low capacity can be achieved. Therefore, both low capacity and shortened carrier transit time can be taken into account.

[0054] Furthermore, the lower the electric field strength in the n-type lightly doped electron transport layer 15, the faster the electrons move. Since the higher the electron speed, the thicker the n-type lightly doped electron transport layer 15 is, the lower the capacitance is, which is beneficial for high-speed operation. The addition of the n-type doped layer 16 can reduce the electric field strength in the n-type lightly doped electron transport layer 15.

[0055] Figure 10 This graph shows the electric field intensity distribution of the depleted portion when voltage is applied to the semiconductor light-receiving elements of Embodiment 4 and the comparative example. As in Embodiment 1, the electric field intensity in the region near the InP substrate 1 is higher than in the comparative example. Therefore, even when strong light is incident and a large number of carriers are generated in the region near the substrate in the light-absorbing layer, space charge effects are less likely to occur, preventing degradation of frequency characteristics.

[0056] Furthermore, although the electric field strength of the n-type lightly doped electron transport layer 15 is reduced, it is known that the drift velocity of electrons is abnormally high in regions with low electric field strength.

[0057] Implementation method 5.

[0058] Figure 11 This is a cross-sectional view of a semiconductor light-receiving element according to Embodiment 5. When viewed from above, a trench 18 is formed outside the p-type region 7 by etching at least once, extending from the window layer 5 to the lower layer of the p-type lightly doped light-absorbing layer 3. The trench 18 is filled with a buried semiconductor layer 19. The buried semiconductor layer 19 is composed, for example, of InP. Since the formation of the trench 18 enables a lower capacitance, high-speed responsiveness is improved. However, leakage current may flow through the interface between the buried semiconductor layer 19 and the etched side portion.

[0059] For this reason, the impurity concentration in the outer peripheral portion 3a of the p-type lightly doped light absorbing layer 3 in contact with the trench 18 is lower than the impurity concentration in the central portion of the p-type lightly doped light absorbing layer 3. As a result, while the electric field intensity in the central portion of the light-receiving portion, where a large number of carriers are generated by incident light, is increased, the electric field intensity in the outer peripheral portion of the p-type lightly doped light absorbing layer 3 in contact with the trench 18 can be reduced, thereby reducing leakage current. The remaining structure and effects are the same as those of the first embodiment.

[0060] Description of Reference Numerals

[0061] 1...InP substrate (substrate); 2...n-type contact layer; 3...p-type lightly doped light absorption layer; 3a...peripheral portion; 4...n-type lightly doped light absorption layer (second n-type lightly doped light absorption layer); 5...window layer; 7...p-type region; 13...n-type lightly doped light absorption layer (first n-type lightly doped light absorption layer); 14...p-type doped light absorption layer; 15...n-type lightly doped electron transport layer (n-type electron transport layer); 16...n-type doped layer; 18...trench; 19...semiconductor buried layer.

Claims

1. A back-illuminated light-receiving element, characterized in that: have: substrate; an n-type contact layer, a p-type lightly doped light absorbing layer, an n-type lightly doped light absorbing layer, and a window layer sequentially stacked on the substrate; and A p-type region is formed in the window layer and a portion of the n-type lightly doped light absorbing layer.

2. The back-illuminated light-receiving element according to claim 1, wherein: The impurity concentration of the p-type lightly doped light absorbing layer and the n-type lightly doped light absorbing layer is 5×10 17 cm －3 the following.

3. The back-illuminated light-receiving element according to claim 2, wherein: The impurity concentration of the p-type lightly doped light absorbing layer is 1×10 17 cm －3 the following.

4. The back-illuminated light-receiving element according to any one of claims 1 to 3, wherein: The thickness of the n-type lightly doped light absorbing layer is greater than 0.3 μm.

5. The back-illuminated light-receiving element according to any one of claims 1 to 4, wherein: The p-type lightly doped light absorbing layer and the n-type lightly doped light absorbing layer are formed of the same semiconductor material having a band gap for absorbing incident light.

6. The back-illuminated light-receiving element according to any one of claims 1 to 4, wherein: The p-type lightly doped light absorbing layer and the n-type lightly doped light absorbing layer are formed of a semiconductor material having a band gap for absorbing incident light. The band gap energy of the p-type lightly doped light absorbing layer is greater than the band gap energy of the n-type lightly doped light absorbing layer.

7. The back-illuminated light-receiving element according to any one of claims 1 to 6, wherein: Also features: An n-type electron transport layer is inserted between the n-type contact layer and the p-type lightly doped light absorbing layer; and The n-type doping layer is inserted between the n-type electron transport layer and the p-type lightly doped light absorption layer, and has an impurity concentration higher than that of the n-type electron transport layer.

8. The back-illuminated light-receiving element according to any one of claims 1 to 7, wherein: In a plan view, a trench is formed outside the p-type region from the window layer to the lower layer of the p-type lightly doped light absorbing layer. An impurity concentration of an outer peripheral portion of the p-type lightly doped light absorbing layer contacting the trench is lower than an impurity concentration of a central portion of the p-type lightly doped light absorbing layer.

9. A back-illuminated light-receiving element, characterized in that: have: substrate; an n-type contact layer, a first n-type lightly doped light absorbing layer, a p-type doped light absorbing layer, a second n-type lightly doped light absorbing layer, and a window layer sequentially stacked on the substrate; and A p-type region is formed in the window layer and a portion of the second n-type lightly doped light absorbing layer.

10. The back-illuminated light-receiving element according to claim 9, wherein: The impurity concentration of the first n-type lightly doped light absorbing layer and the second n-type lightly doped light absorbing layer is 5×10 17 cm －3 the following.

11. The back-illuminated light-receiving element according to claim 9 or 10, wherein: The first n-type lightly doped light absorbing layer, the p-type doped light absorbing layer, and the second n-type lightly doped light absorbing layer are formed of the same semiconductor material having a band gap for absorbing incident light.

Images