Semiconductor light receiving element

Abstract

A semiconductor light receiving element comprises: a semiconductor substrate; a high concentration layer of a first conductivity type formed on the semiconductor substrate; a low concentration layer of a first conductivity type formed on and in contact with the high concentration layer of the first conductivity type; a low concentration layer of a second conductivity type configured to form a PN junction interface together with the low concentration layer of the first conductivity type; and a high concentration layer of a second conductivity type formed on and in contact with the low concentration layer of the second conductivity type. The carrier concentration of the low concentration layer is less than 1×10 16 / cm 3 The carrier concentration of the high concentration layer is 1×10 17 / cm 3 At least one of the low-concentration layers includes an absorption layer having a band gap for absorbing incident light.

Description

Technical Field

[0001] The present disclosure relates to a semiconductor light receiving element. Background Art

[0002] A semiconductor light receiving element for high-speed optical communication may include a PIN-type photodiode, in which an undoped semiconductor layer is sandwiched between p-type and n-type semiconductor layers, and when a bias voltage is applied in a use state, the semiconductor layer is depleted and serves as an absorption layer for optical signals from an optical fiber. In some cases, in order to achieve light receiving sensitivity and high-speed responsiveness, a configuration in which the absorption layer is formed by a p-type doped semiconductor layer and an undoped semiconductor layer can be used. In other cases, in order to achieve a high-speed response to high light input, even at a low voltage, the semiconductor light receiving element may include an absorption layer located between an n-layer and a p-layer, and the absorption layer may be formed by a layer containing n-type impurities and a layer containing p-type impurities, the concentration of the n-type impurities gradually decreases from the junction surface with the n-layer inward, and the concentration of the p-type impurities gradually decreases from the junction surface with the p-layer inward.

[0003] In some cases, in order to improve the frequency response speed of the semiconductor light receiving element, it is effective to reduce the element capacitance. The element capacitance is composed of PN junction capacitance and parasitic capacitance, and the PN junction capacitance can be reduced by thickening the depletion layer. For example, the undoped absorption layer can be thickened. However, even when the undoped absorption layer is thickened, unless the entire undoped absorption layer can be depleted under the bias voltage applied under the use conditions, the actual depletion layer becomes thinner and the desired capacitance reduction effect cannot be obtained. Under a constant bias voltage, as the carrier concentration becomes lower, the actual depletion layer becomes thicker. Therefore, when the undoped absorption layer is an intrinsic semiconductor layer with no carriers at all, it is easy to deplete all the undoped absorption layers even at low voltages.

[0004] However, in many cases, the undoped absorption layer actually contains a low concentration of carriers. The undoped absorption layer is formed by epitaxial growth, for example, using an MOCVD method or an MBE method. During epitaxial growth, the absorption layer is intentionally not doped with p-type or n-type carriers to form an undoped absorption layer. However, the absorption layer actually contains a non-zero background level of carriers, and no intrinsic semiconductor layer is truly formed. The concentration of unintentionally included carriers (carriers included at the background level) depends on the growth apparatus and growth conditions, but is, for example, about 2×10 for n-type 15 / cm 3 Therefore, when a practical undoped absorption layer is used, the area that can be depleted is limited depending on the carrier concentration contained at a constant voltage. In other words, simply thickening the undoped absorption layer may not be sufficient to ensure sufficient thickness of the depletion layer under actual use conditions.

[0005] Even if an undoped absorption layer is used as described above, there is actually a low concentration of carriers, so the area that can be depleted is limited. When the applied bias voltage increases, the area that can be depleted increases, and even if the carrier concentration is the same, the capacitance can be reduced. However, from the perspective of power consumption, the bias voltage that can be applied to the semiconductor light receiving element is limited, and it is impossible to obtain a sufficient bias voltage. Therefore, in actual use, it is desired that the semiconductor light receiving element has a large area that can be depleted with a lower voltage. Summary of the invention

[0006] In some embodiments, a semiconductor light receiving element has a large area that can be depleted, low capacitance, and excellent high-speed responsiveness.

[0007] In some embodiments, the semiconductor light receiving element includes: a semiconductor substrate; a high concentration layer of a first conductivity type formed on the semiconductor substrate; a low concentration layer of a first conductivity type formed on and in contact with the high concentration layer of the first conductivity type; a low concentration layer of a second conductivity type configured to form a PN junction interface with the low concentration layer of the first conductivity type; and a high concentration layer of a second conductivity type formed on and in contact with the low concentration layer of the second conductivity type, wherein: the low concentration layer of the first conductivity type and the low concentration layer of the second conductivity type each have a thickness less than 1×10 16 / cm 3 The first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer, the second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer, and the first conductivity type high concentration layer and the second conductivity type high concentration layer each have 1×10 17 / cm 3 or higher carrier concentration, and at least one of the first conductivity type low concentration layer or the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light.

[0008] In some embodiments, the semiconductor light receiving element has low capacitance, and thus excellent high-speed responsiveness can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1A is a cross-sectional view of a semiconductor light receiving element according to the first exemplary embodiment.

[0010] Figure 1B is an energy band diagram of the semiconductor light receiving element according to the first exemplary embodiment.

[0011] Figure 2is a graph showing the electric field intensity of the semiconductor light receiving element according to the first exemplary embodiment.

[0012] Figure 3A is a cross-sectional view of a semiconductor light receiving element according to a comparative exemplary embodiment.

[0013] Figure 3B is an energy band diagram of a semiconductor light receiving element according to a comparative exemplary embodiment.

[0014] Figure 4 is a graph showing the electric field intensity of a semiconductor light receiving element according to a comparative exemplary embodiment.

[0015] Figure 5A is a cross-sectional view of a semiconductor light receiving element according to a second exemplary embodiment.

[0016] Figure 5B is an energy band diagram of the semiconductor light receiving element according to the second exemplary embodiment.

[0017] Figure 6 is a graph showing the electric field intensity of the semiconductor light receiving element according to the second exemplary embodiment.

[0018] Fig. 7A is a cross-sectional view of a semiconductor light receiving element according to a third exemplary embodiment.

[0019] Figure 7B is an energy band diagram of the semiconductor light receiving element according to the third exemplary embodiment.

[0020] Figure 8 is a graph showing the electric field intensity of the semiconductor light receiving element according to the third exemplary embodiment.

[0021] Fig.9A is a cross-sectional view of a semiconductor light receiving element according to a fourth exemplary embodiment.

[0022] Fig. 9B is an energy band diagram of a semiconductor light receiving element according to the fourth exemplary embodiment.

[0023] Fig.10 is a graph showing the electric field intensity of the semiconductor light receiving element according to the fourth exemplary embodiment.

[0024] Fig.11A is a cross-sectional view of a semiconductor light receiving element according to a modified example of the fourth exemplary embodiment.

[0025] Fig. 11B is an energy band diagram of a semiconductor light receiving element according to a modified example of the fourth exemplary embodiment.

[0026] Fig.12 is a graph showing the electric field intensity of a semiconductor light receiving element according to a modified example of the fourth exemplary embodiment. DETAILED DESCRIPTION

[0027] Some embodiments are described in detail below with reference to the accompanying drawings. In the accompanying drawings, the same components are represented by the same reference numerals and have the same or equivalent functions, and their repeated descriptions are omitted. The size of the figure is not necessarily consistent with the magnification.

[0028] Figure 1A 1 is a cross-sectional view of a semiconductor light receiving element 10 according to a first exemplary embodiment. The semiconductor light receiving element 10 may be a top illuminated semiconductor light receiving element capable of receiving an optical signal having a wavelength between 840 nanometers (nm) and 950 nm, and the optical signal is transmitted, for example, through a multimode optical fiber. The semiconductor light receiving element 10 includes a semiconductor light receiving element 10 doped with a concentration of 5×10 18 / cm 3 The silicon (Si) has an n-type InP contact layer 12 with a thickness of 1 micrometer (μm), an n-type low-concentration indium gallium arsenide (InGaAs) absorption layer 13 with a thickness of Wn, and a doping layer with a concentration of 2×10 15 / cm 3 The p-type low-concentration InGaS absorption layer 14 is doped with beryllium (Be) at a concentration of 5×10 19 / cm 3 The p-type InGaS contact layer 16 is formed of beryllium with a thickness of 0.1 μm. Here, the n-type low-concentration InGaAs absorption layer 13 may be formed as undoped in the multilayer growth, but may also be formed as a layer with a carrier density of 2×10 15 / cm 3 n-type semiconductor layer. Intentional doping of Si and / or other elements in multilayer growth can be used to obtain a desired carrier concentration. In addition, the n-type low-concentration InGaS absorption layer 13, the p-type low-concentration InGaS absorption layer 14, and the p-type InGaS contact layer 16 can be layers with a band gap capable of absorbing incident light. In the first exemplary embodiment, Wn=0.9μm, Wp=0.9μm. The n-type InP contact layer 12 may not be limited to InP. Similarly, the p-type InGaAs contact layer 16 may also be a semiconductor layer made of another material.

[0029] The n-type low-concentration InGaS absorption layer 13, the p-type low-concentration InGaS absorption layer 14, and the p-type InGaS contact layer 16 can be etched into a cylindrical shape to form a light receiving mesa. In addition, at the top of the light receiving mesa, the annular p-type electrode 17 can be electrically connected to the p-type InGaAs contact layer 16. In addition, the n-type electrode 18 can be electrically connected to the n-type indium phosphide contact layer 12. The surface other than some parts of the p-type electrode 17 and the n-type electrode 18 can be coated with a silicon nitride film 19. The n-type InP contact layer 12, the n-type low-concentration InGaS absorption layer 13, the p-type low-concentration InGaS absorption layer 14, and the p-type InGaS contact layer 16 can be lattice matched with the InP substrate 11. The silicon nitride film 19 can be used as an anti-reflection film and can have a reflectivity of 1% or less for the wavelength of incident light inside the annular p-type electrode 17.

[0030] In the semiconductor light receiving element 10, when a reverse bias voltage is applied between the p-type electrode 17 and the n-type electrode 18, the n-type low-concentration InGaS absorption layer 13 and the p-type low-concentration InGaS absorption layer 14 can be depleted to serve as an absorption layer for signal light with a wavelength between 840nm and 950nm that has entered the annular interior of the p-type electrode 17. The p-type InGaAs contact layer 16 may also have a band gap capable of absorbing incident light, but due to high concentration doping, it may not be depleted at the use voltage and may not function as an absorption layer. This article describes the relationship between carrier concentration and depletion.

[0031] Figure 1B An energy band diagram of the semiconductor light receiving element 10 when a reverse bias voltage is applied between the p-type electrode 17 and the n-type electrode 18 is shown.

[0032] Figure 2 is a graph showing the electric field intensity when a reverse bias voltage of 2 volts (V) is applied between the p-type electrode 17 and the n-type electrode 18 . Figure 2 The horizontal axis represents the layer thickness when the PN junction interface is set to 0. Figure 2 In the equation, the + (positive) direction represents the n-type electrode 18 side of the PN junction interface, and the - (negative) direction represents the p-type electrode 17 side of the PN junction interface. In addition, the vertical axis represents the electric field strength. The interface between the n-type low-concentration InGaS absorption layer 13 and the p-type low-concentration InGaS absorption layer 14 can be a PN junction, at which the electric field strength can be maximum, and the electric field strength can decrease as the thickness becomes farther away from the PN junction interface. When the electric field strength at the PN junction interface is represented by E0 (V / m), the electric field strength En (V / m) in the n-type low-concentration InGaAs absorption layer 13 can be represented by Equation 4.

[0033] [Equation 4]

[0034] En(V / m)=E0-(q / εn)·Nn·Dn

[0035] In Equation 4, q is the elementary charge (=1.6×10 19 (C)), εn is the dielectric constant of the n-type low-concentration InGaAs absorption layer 13, which is the dielectric constant of InGaAs in this case (=1.23×10 -10 (F / m)). Nn is the carrier density of the n-type low-concentration InGaAs absorption layer 13, which is 2×10 15 / cm 3 Dn is the distance from the PN junction interface. When the distance from the PN junction interface where the electric field intensity En becomes zero is represented by Dn0, E0 can be represented by Equation 5.

[0036] [Equation 5]

[0037] E0=(q / εn)·Nn·Dn0

[0038] Similarly, the electric field intensity Ep (V / m) in the p-type low-concentration InGaAs absorption layer 14 is expressed by Equation 6.

[0039] [Equation 6]

[0040] Ep(V / m)=E0-(q / εp)·Np·Dp

[0041] εp is the dielectric constant of the p-type low-concentration InGaAs absorption layer 14, which in this case is the dielectric constant of InGaAs (=1.23×10 -10 (F / m)). In addition, Np is the carrier density of the p-type low-concentration InGaAs absorption layer 14, which is 2×10 15 / cm 3 , Dp is the distance from the PN junction interface. When the distance from the PN junction interface where the electric field intensity Ep becomes zero is represented by Dp0, E0 is represented by Equation 7.

[0042] [Equation 7]

[0043] E0=(q / εp)·Np·Dp0

[0044] When the reverse bias voltage is represented by Vr, and the built-in potential at the interface between the n-type low-concentration InGaS absorption layer 13 and the p-type low-concentration InGaS absorption layer 14 is represented by Vb, the sum of the value obtained by integrating the electric field intensity En from the PN junction interface as Dn0 and the value obtained by integrating the electric field intensity Ep from the PN junction interface as Dp0 is (Vr+Vb)(V), so the following equation 8 is established.

[0045] [Equation 8]

[0046] E0·Dp0+E0·Dn0=2·(Vr+Vb)

[0047] From Equation 5, Equation 7, and Equation 8, Equation 1, Equation 2, and Equation 3 are derived.

[0048] [Equation 1]

[0049]

[0050] [Equation 2]

[0051] Rp=1+(εn / εp)·(Np / Nn)

[0052] [Equation 3]

[0053] Dn0=(Rp-1)·Dp0

[0054] An optical module for optical communication (e.g., an optical transceiver) can be driven by a single 3.3V power supply. Due to the influence of the voltage drop in the internal circuit of the optical module (e.g., transimpedance amplifier circuit), the voltage applied to the semiconductor light receiving element 10 included in the optical module can be a minimum of about 2V. In addition, the built-in potential is determined by the semiconductor material, and in this case, due to the PN junction between the InGaAs layers, the built-in potential can be 0.54V. Here, when Vr=2V and Vb=0.54V are substituted into Equation 1, Dp0=0.988μm is obtained. In addition, from Equation 2 and Equation 3, Dn0=0.988μm is obtained. In other words, it can be seen that the n-type low-concentration InGaAs absorption layer 13 and the p-type low-concentration InGaAs absorption layer 14 each have 2×10 15 / cm 3 In the case of a carrier concentration of , when a voltage of 2V is applied, the n-type low-concentration InGaAs absorption layer 13 and the p-type low-concentration InGaAs absorption layer 14 can be depleted to a width of 0.988μm. In the first exemplary embodiment, because Wn=0.9μm, Wp=0.9μm, the entire area of ​​the n-type low-concentration InGaS absorption layer 13 and the p-type low-concentration InGaS absorption layer 14 is depleted. In other words, the width (thickness) depleted by the semiconductor light receiving element 10 can be 1.8μm.

[0055] Therefore, the absorption layer can be formed of a low-concentration p-type semiconductor layer and a low-concentration n-type semiconductor layer. Here, low concentration means a concentration at a level that allows sufficient depletion when a voltage of several volts is applied, which can be used in practical applications. Specifically, less than 1×10 16 / cm 3The p-type InGaAs contact layer 16 contains a carrier concentration of 5×10 19 / cm 3 The Be of low concentration p-type semiconductor layer and low concentration n-type semiconductor layer is used to form an absorption layer, and thus it is not depleted at several volts and does not actually function as an absorption layer. By using an absorption layer formed of a low concentration p-type semiconductor layer and a low concentration n-type semiconductor layer, when a certain bias voltage is applied, the depletion layer area can be expanded. As a result, a semiconductor light receiving element is provided that achieves reduced capacitance and has excellent high-speed responsiveness.

[0056] Figure 3A : is a cross-sectional view for explaining a semiconductor light receiving element 30 according to a comparative exemplary embodiment. The difference from the semiconductor light receiving element 10 according to the first exemplary embodiment is the configuration of the absorption layer, and the other configurations are the same. The absorption layer of the semiconductor light receiving element 30 may be formed of an n-type low-concentration InGaAs absorption layer 33 having a thickness Wnc. As with the n-type low-concentration InGaAs absorption layer 13 in the first exemplary embodiment, the n-type semiconductor layer has a thickness of 2×10 15 / cm 3 The background level contains an n-type carrier concentration of . In addition, Wnc may be 1.8 μm. In other words, the absorption layer may have the same concentration and the same thickness as the semiconductor light receiving element 10. The only difference may be that all the absorption layers are n-type. Figure 3B is an energy band diagram of the semiconductor light receiving element 30 when a reverse bias voltage is applied.

[0057] Figure 4 The electric field intensity when a reverse bias voltage of 2 V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 30 is shown in the comparative exemplary embodiment. Figure 2 As shown, on the horizontal axis, the PN junction interface is set to 0. The interface between the n-type low-concentration InGaAs absorption layer 33 and the p-type InGaAs contact layer 16 can be a PN junction. The electric field intensity in the n-type low-concentration InGaAs absorption layer 33 can be maximum at the PN junction interface, and can decrease as the thickness becomes farther away from the PN junction interface. When the electric field intensity at the PN junction interface is represented by E0 (V / m), the electric field intensity En (V / m) in the n-type low-concentration InGaAs absorption layer 33 can be represented by Equation 4. Similarly, when the distance from the PN junction interface where the electric field intensity En becomes zero is represented by Dn0, E0 can be represented by Equation 5.

[0058] When the reverse bias voltage is represented by Vr and the built-in potential at the interface between the n-type InGaS absorption layer 33 and the p-type InGaS contact layer 16 is represented by Vb, the value obtained by integrating the electric field strength E from the PN junction interface to Dn0 is (Vr+Vb)(V), so Dn0 can be expressed by Equation 9.

[0059] [Equation 9]

[0060]

[0061] When Vr=2V and Vb=0.54V are substituted into Equation 9, Dn0=1.397μm is obtained. In other words, although the n-type low concentration InGaAs absorption layer 33 has a thickness of 1.8μm, when 2V is actually applied, the n-type low concentration InGaAs absorption layer 33 is depleted by only 1.397μm.

[0062] In the semiconductor light receiving element 10 according to the first exemplary embodiment, the n-type low-concentration InGaS absorption layer 13 and the p-type low-concentration InGaS absorption layer 14 form an absorption layer with a thickness of 1.8 μm. In addition, when 2V is applied, the entire absorption layer is depleted. In contrast, in the semiconductor light receiving element 30 according to the comparative exemplary embodiment, the absorption layer is made of only n-type low-concentration InGaAs and has a thickness of 1.8 μm. However, when 2V is applied, only a thickness of 1.397 μm is depleted. Therefore, compared with the semiconductor light receiving element 30, the semiconductor light receiving element 10 according to the first exemplary embodiment is a semiconductor light receiving element having a small capacitance and excellent high-speed responsiveness. In addition, for the semiconductor light receiving element 10 according to the first exemplary embodiment, the voltage for obtaining the same depletion region is smaller than that of the semiconductor light receiving element 30 according to the comparative exemplary embodiment. Therefore, the semiconductor light receiving element 10 according to the first exemplary embodiment is a semiconductor light receiving element with which the absolute value of the voltage required to obtain the same capacitance (ie, the same level of high-speed responsiveness) is small and excellent in low power consumption. This characteristic is obtained by the combination of a p-type low-concentration absorption layer and an n-type low-concentration absorption layer, each of which has a thickness of less than 1×10 16 / cm 3 The carrier concentration of , using the carrier concentration, the absorption layer can be depleted.

[0063] In some cases, the semiconductor light receiving element may be composed of a layer containing an n-type impurity and a layer containing a p-type impurity, the concentration of the n-type impurity gradually decreases from the junction surface with the n-layer inward, and the concentration of the p-type impurity gradually decreases from the junction surface with the p-layer inward. At the interface between the n-type impurity layer and the p-type impurity layer, the concentrations of the n-type impurity and the p-type impurity are 0. With this structure, both the p-type InGaS absorption layer and the n-type InGaS absorption layer have a structure in which the junction surface with the p-type layer or the n-type layer (which may be in contact with the p-type InGaS absorption layer or the n-type InGaS absorption layer) has a 1×10 16 / cm 3 or higher, and the concentration gradually decreases from the junction surface to the inside. Therefore, not all of the n-type impurity layer and the p-type impurity layer are depleted, only the carrier concentration is less than 1×10 16 / cm 3 The region is depleted. Other cases involve the generation of an internal electric field by a concentration gradient in each impurity layer and the prevention of a reduction in carrier movement speed even at low voltage. Therefore, it is not assumed that the carrier concentration of the entire impurity layer is set to less than 1×10 16 / cm 3 In addition, in the n-type impurity layer, even if the concentration is 1×10 16 / cm 3 The carrier concentration also gradually changes around. Therefore, the electric field is not properly applied to the actual depletion region, and the interface between the actual depletion layer and the non-depletion region is blurred. As a result, a sufficient carrier movement speed cannot be obtained, which is disadvantageous in terms of high-speed responsiveness.

[0064] In contrast, in the semiconductor light receiving element 10 according to the first exemplary embodiment, the n-type low-concentration InGaS absorption layer 13 is in contact with the n-type InP contact layer 12 and is doped to a high concentration equal to or greater than 10 times the concentration of the n-type low-concentration InGaS absorption layer 13, which is 5×10 18 / cm 3 Therefore, the electric field is appropriately applied to the n-type low-concentration InGaAs absorption layer 13, resulting in the semiconductor light receiving element being able to respond at high speed with excellent carrier splitting performance. In order to sufficiently apply the electric field to the depletion layer region, the carrier concentration of the layer in contact with the low-concentration layer is preferably 1×10 17 / cm 3 or higher.

[0065] Figure 5A is a cross-sectional view of a semiconductor light receiving element 50 according to the second exemplary embodiment. Figure 5B1 shows an energy band diagram when a reverse bias voltage is applied to the semiconductor light receiving element 50. The difference from the semiconductor light receiving element 10 described in the first exemplary embodiment is the difference in concentration and thickness between the n-type low-concentration InGaS absorption layers 13 and 53 and the p-type low-concentration InGaS absorption layers 14 and 54. In the second exemplary embodiment, the n-type low-concentration InGaAs absorption layer 53 has a 3×10 15 / cm 3 The p-type low-concentration InGaAs absorption layer 54 has a 2×10 15 / cm 3 In other words, the n-type low-concentration InGaAs absorption layer 53 can be formed thinner than the p-type low-concentration InGaAs absorption layer 54, and the thickness of the absorption layer as a whole can be 1.5 μm. The other configurations are the same as the semiconductor light receiving element 10.

[0066] Figure 6 is a graph showing the electric field intensity when a reverse bias voltage of 2 V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 50. Figure 2 As shown, on the horizontal axis, the PN junction interface is set to 0. The area that can be depleted (i.e., the distance Dn0 and Dp0 from the PN junction interface where the electric field strength is 0) can be determined based on Equations 1, 2, and 3. When Vr=2V and Vb=0.54V are substituted into Equations 1, 2, and 3, Dp0=1.082μm and Dn0=0.721μm are obtained. In other words, the p-type low-concentration InGaAs absorption layer 54 with a thickness of 0.9μm and the n-type low-concentration InGaAs absorption layer 53 with a thickness of 0.6μm can both be depleted. In addition, when the carrier concentrations of the n-type low-concentration InGaS absorption layer 53 and the p-type low-concentration InGaS absorption layer 54 have the above-mentioned values, the maximum depletion layer thickness is Dp0+Dn0=1.803μm. This shows that although the thickness of the absorption layer can be 1.5μm in the second exemplary embodiment, the absorption layer can be completely depleted even when the thickness is 1.803μm.

[0067] Meanwhile, in the comparative exemplary embodiment, for example, when the concentration of the n-type low-concentration InGaAs absorption layer 33 is set to 3×10 15 / cm 3When , which is the same concentration as that of the n-type low-concentration InGaAs absorption layer 53 in the second exemplary embodiment, the width of the maximum depletion region becomes 1.14 μm. Therefore, even with the same carrier concentration, the width of the maximum depletion region can be widened by the combination of the p-type low-concentration absorption layer and the n-type low-concentration absorption layer as described in the second exemplary embodiment. In addition, it can be seen from Equation 1, Equation 2, and Equation 3 that when the p-type low-concentration absorption layer and the n-type low-concentration absorption layer have the same carrier concentration, the maximum depletion region can be obtained at a specific bias voltage.

[0068] By light absorption, a pair of electrons and a hole are generated. Then, the hole moves to the p-type InGaAs contact layer 16 side, and the electron moves to the n-type InP contact layer 12 side. Light is absorbed in the entire absorption layer, and the hole generated on the side close to the n-type InP contact layer 12 needs to move a long distance compared to the hole generated on the side close to the p-type InGaAs contact layer 16. Compared with electrons, holes are heavy, so it takes time to move. In high-frequency driving, holes can be allowed to move quickly from the depletion layer region to increase high-speed responsiveness. In the second exemplary embodiment, when the absorption layer is regarded as a whole, the PN junction interface where the electric field intensity is maximized is closer to the n side. Therefore, a stronger electric field is applied to the holes generated in the n-type low-concentration InGaAs absorption layer 53 that need to move a longer distance. Therefore, these holes are accelerated by the strong electric field and are therefore allowed to move faster to the p-type InGaAs contact layer 16 side. As a result, a semiconductor light receiving element with superior high-speed responsiveness can be provided.

[0069] Fig. 7A is a cross-sectional view of a semiconductor light receiving element 60 according to the third exemplary embodiment. Figure 7B 1 shows an energy band diagram when a reverse bias voltage is applied to the semiconductor light receiving element 60. The difference from the semiconductor light receiving element 10 described in the first exemplary embodiment is the difference in concentration and thickness between the n-type low-concentration InGaS absorption layers 13 and 63 and the p-type low-concentration InGaS absorption layers 14 and 64. In the third exemplary embodiment, the n-type low-concentration InGaAs absorption layer 63 has a 2×10 15 / cm 3 The p-type low-concentration InGaAs absorption layer 64 has a concentration of 3×10 15 / cm 3In other words, one of the n-type low-concentration InGaAs absorption layer 63 and the p-type low-concentration InGaAs absorption layer 64 closer to the light entry surface (the p-type low-concentration InGaAs absorption layer 64) can be formed thinner than the other (the n-type low-concentration InGaAs absorption layer 63), and the thickness of the entire absorption layer can be 1.5 μm. The other configurations are the same as the semiconductor light receiving element 10.

[0070] Figure 8 is a graph showing the electric field intensity when a reverse bias voltage of 2 V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 60. Figure 2 As shown, on the horizontal axis, the PN junction interface is set to 0. The area that can be depleted (i.e., the distance Dn0 and Dp0 from the PN junction interface where the electric field strength is 0) can be determined based on Equations 1, 2, and 3. When Vr=2V and Vb=0.54V are substituted into Equations 1, 2, and 3, Dp0=0.721μm and Dn0=1.082μm are obtained. In other words, the p-type low-concentration InGaAs absorption layer 64 with a thickness of 0.6μm and the n-type low-concentration InGaAs absorption layer 63 with a thickness of 0.9μm can both be depleted. In addition, when the carrier concentrations of the n-type low-concentration InGaS absorption layer 63 and the p-type low-concentration InGaS absorption layer 64 have the above-mentioned values, the maximum depletion layer thickness is Dp0+Dn0=1.803μm. This indicates that although the thickness of the absorption layer may be 1.5 μm in the third exemplary embodiment, the absorption layer may be completely depleted even when the thickness is 1.803 μm.

[0071] Meanwhile, in the comparative exemplary embodiment, for example, when the concentration of the n-type low-concentration InGaAs absorption layer 33 is set to 2×10 15 / cm 3 When the concentration is the same as that of the n-type low-concentration InGaAs absorption layer 63 in the third exemplary embodiment, the width of the maximum depletion region becomes 1.14 μm. Therefore, even with the same carrier concentration, the width of the maximum depletion region can be widened by the combination of the p-type low-concentration absorption layer and the n-type low-concentration absorption layer as described in the third exemplary embodiment.

[0072] The semiconductor light receiving element 60 can be a top-illuminated optical semiconductor light receiving element, and the received light signal can enter the p-type low-concentration InGaAs absorption layer 64 through the p-type InGaAs contact layer 16. The light signal can reach the n-type low-concentration InGaS absorption layer 63 while being absorbed by the p-type low-concentration InGaS absorption layer 64, and can also be absorbed into the n-type low-concentration InGaS absorption layer 63. Therefore, the amount of light absorbed in the p-type low-concentration InGaS absorption layer 64 can be greater than the amount of light absorbed in the n-type low-concentration InGaS absorption layer 63. Therefore, in the region of the p-type low-concentration InGaAs absorption layer 64, the number of holes and electron pairs generated by light absorption can also be greater. In order to allow the differentially generated large number of carriers to move at high speed, it is preferred to apply a strong electric field. In the third exemplary embodiment, a higher electric field strength can be applied to the p-type low-concentration InGaAs absorption layer 64 with a larger amount of light absorption, and the carriers are allowed to move effectively.

[0073] As described in the second exemplary embodiment, in some cases, it may be better to increase the electric field strength of the n-type low-concentration InGaAs absorption layer 63. For example, in the case of a top-irradiated type, the semiconductor light receiving element 60 according to the third exemplary embodiment may be preferred so as to allow a larger amount of carriers generated on the p-side to move. On the contrary, in the case of a back-illuminated semiconductor light receiving element, it may be that the n-type low-concentration InGaAs absorption layer 63 absorbs a larger amount of light, and the semiconductor light receiving element 50 according to the second embodiment may be better in high-speed responsiveness. In addition, even in a top-irradiated semiconductor light receiving element, which of the extractability of carriers generated in large quantities on the p-side and the extractability of holes on the n-side affects the high-speed responsiveness as a whole depends on the incident light intensity, the applied voltage, and, for example, the thickness and concentration of the absorption layer. Therefore, as part of the overall design, it is preferred to determine the thickness and concentration of the p-type low-concentration absorption layer and the n-type low-concentration absorption layer.

[0074] Fig.9A is a cross-sectional view of a semiconductor light receiving element 70 according to the fourth exemplary embodiment. Figure 7B 1 shows an energy band diagram when a reverse bias voltage is applied to the semiconductor light receiving element 70. The difference from the semiconductor light receiving element 10 described in the first exemplary embodiment is that the n-type low-concentration InGaAs absorption layer 13 is composed of a carrier concentration of 1×10 15 / cm 3 The n-type low-concentration InP wide bandgap layer 73 is replaced, and the p-type low-concentration InGaAs absorption layer 74 has a thickness of 0.8 μm. The p-type low-concentration InGaAs absorption layer 74 has the same carrier concentration of 2×10 15 / cm 3 . The carriers of the n-type low-concentration InP wide bandgap layer 73 may be intentionally doped or contained in the background. The n-type low-concentration InP wide bandgap layer 73 has a thickness of 1.5 μm. In other words, the absorption layer is only the p-type low-concentration InGaAs absorption layer 74, and has a thickness of 0.9 μm. The other configurations are the same as the semiconductor light receiving element 10.

[0075] The n-type low-concentration InP wide bandgap layer 73 has an InP bandgap wavelength shorter than the wavelength of 840 nm to 950 nm of the incident light and therefore does not act as an absorption layer. However, the carrier concentration may be less than 1×10 16 / cm 3 , so it may be a depletion layer. Among the carriers generated in the p-type low-concentration InGaAs absorption layer 74, only electrons are allowed to drift at high speed by the internal electric field. In this way, the structure reduces the element capacitance while suppressing the increase in carrier movement time. In order to increase the high-speed responsiveness of the semiconductor light receiving element, in addition to the reduction in capacitance, it is also important how quickly the carriers can leave the absorption layer. From the latter point of view, this structure may be better. As long as the layer serves as a wide bandgap of the wavelength of the incident light, the injection described in this article may not be limited to the InP layer, and for example, an InAlAs layer or an InAlGaAs layer can be used.

[0076] Fig.10 is a graph showing the electric field intensity when a reverse bias voltage of 2 V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 70. Figure 2 As shown, on the horizontal axis, the PN junction interface is set to 0. The area that can be depleted (i.e., the distance Dn0 and Dp0 from the PN junction interface where the electric field strength becomes 0) can be determined based on Equation 1, Equation 2, and Equation 3. In the fourth exemplary embodiment, the PN junction interface can be an interface between an InGaAs layer and an InP layer, so Vb can be 0.59V. In addition, the dielectric constant of InP can be 1.11×10 -10(F / m). When Vr=2V and Vb=0.59V are substituted into Equation 1, Equation 2, and Equation 3, Dp0=0.844μm and Dn0=1.517μm are obtained. In other words, the p-type low-concentration InGaAs absorption layer 74 with a thickness of 0.8μm and the n-type low-concentration InP wide bandgap layer 73 with a thickness of 1.5μm can both be depleted. In other words, the semiconductor light receiving element 70 has a small capacitance, and when 2V is applied, the thickness of the depletion layer is as large as 2.3μm, and due to the n-type low-concentration InP wide bandgap layer 73, the semiconductor light receiving element 70 is an element capable of high-speed response with excellent carrier separation performance. In the fourth exemplary embodiment, the n-type low-concentration layer can be a wide bandgap layer. However, even when the p-type low-concentration layer is an InP wide bandgap layer and the n-type low-concentration layer is an n-type InGaAs absorption layer, contrary to the situation in the fourth embodiment, a wide depletion layer can be obtained.

[0077] As described in the fourth exemplary embodiment, it is not always necessary to make the low-concentration layer only of a material having a band gap capable of absorbing incident light, and by using 16 / cm 3 The configuration in which the p-type low-concentration layer with a carrier concentration of 0.1% and the n-type low-concentration layer form a PN interface can expand the depletion layer.

[0078] A modified exemplary embodiment of the semiconductor light receiving element 70 according to the fourth exemplary embodiment is described. Fig.11A is a cross-sectional view of a semiconductor light receiving element 90 according to a modified example. Fig. 11B An energy band diagram when a reverse bias voltage is applied to the semiconductor light receiving element 90 is shown. Fig.12 1 is a graph showing the electric field intensity when a reverse bias voltage of 2 V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 90. The difference from the semiconductor light receiving element 70 described in the fourth exemplary embodiment is that a p-type high-concentration InGaS absorption layer 99 may be inserted between the p-type low-concentration InGaS absorption layer 74 and the p-type InGaS contact layer 16. The p-type high-concentration InGaAs absorption layer 99 may be doped with a concentration of 1×10 18 / cm 3 The p-type high-concentration InGaS absorption layer 99 may have a thickness of 1×10 16 / cm 3or higher carrier concentration, and therefore will not be depleted at a voltage of about several volts. Therefore, the effect of reducing the capacitance by providing this p-type high-concentration InGaS absorption layer 99 is not obtained. However, the p-type high-concentration InGaAs absorption layer 99 has a band gap for absorbing incident light, and therefore effectively increases the light receiving sensitivity. The actual depletion layer of the semiconductor light receiving element 90 may be the same as the depletion layer of the semiconductor light receiving element 70 described in the fourth exemplary embodiment. The p-type high-concentration InGaAs absorption layer 99 may have a concentration gradient. However, it should be noted that even the region with the lowest concentration will not be depleted in concentration when a bias voltage applied under the conditions of use is applied. Specifically, 1×10 17 / cm 3 In addition, as in the first exemplary embodiment, an n-type low-concentration InGaS absorption layer may be inserted between the p-type low-concentration InGaS absorption layer 74 and the n-type low-concentration InP wide bandgap layer 73 .

[0079] Generating a to-be-depleted layer having a thickness derived from Equation 1, Equation 2, and Equation 3 may be better in terms of low capacitance and high-speed responsiveness, but the low-concentration layer may be formed to be about 10% thicker than the thickness of the layer to be depleted obtained from Equation 1, Equation 2, and Equation 3. In this case, there is a layer of undepleted p-type low-concentration layer near the interface with the p-type high-concentration layer, or there is a layer of n-type low-concentration layer near the interface with the n-type high-concentration layer. However, these layers are not thick, so there is no significant effect on the degradation of carrier separation performance, and it is at a practical level.

[0080] When the structures described in the above exemplary embodiments are combined with each other, the effects of some embodiments described herein can be obtained. In other words, as long as at least one of the n-type low-concentration layer or the p-type low-concentration layer includes an absorption layer that absorbs incident light, the n-type low-concentration layer and the p-type low-concentration layer can be formed by two layers: an absorption layer and a wide bandgap layer, or only by one of them. In addition, the n-type low-concentration layer and the p-type low-concentration layer can have the same thickness, the n-type low-concentration layer can be thicker than the p-type low-concentration layer, or the n-type low-concentration layer can be thinner than the p-type low-concentration layer. In addition, a top-irradiated semiconductor light receiving element capable of receiving a light signal with a wavelength of 840 nanometers to 950 nanometers has been described. However, a top-irradiated semiconductor light receiving element or a back-irradiated semiconductor light receiving element capable of receiving a light signal of another wavelength band (for example, a 1.3 μm band or a 1.55 μm band) can be used. In addition, the optical fiber of the input light signal can be a single-mode optical fiber. In addition, in the above embodiment, a structure in which the n-type is closer to the InP substrate side has been described. However, some implementations described herein are not limited thereto, and similar effects can be obtained even when a structure with opposite polarity, ie, p-type, can be located closer to the InP substrate side. In addition, a conductive semiconductor substrate can be used instead of a semi-insulating InP substrate.

[0081] The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the implementation to the precise form disclosed. Modifications and variations may be made based on the foregoing disclosure, or may be obtained from the practice of the implementation. In addition, any implementation described herein may be combined, unless the foregoing disclosure explicitly provides reasons why one or more implementations cannot be combined.

[0082] Even though specific combinations of features are referenced in the claims and / or disclosed in the specification, these combinations are not intended to limit the disclosure of the various embodiments. In fact, many of these features can be combined in ways that are not specifically described in the claims and / or disclosed in the specification. Although each dependent claim listed below may be directly dependent on only one claim, the disclosure of the various embodiments includes each dependent claim and each other claim in the claim group. As used herein, a phrase referring to "at least one" in a list of items refers to any combination of those items, including single members. For example, "at least one of a, b, or c" is intended to cover a, b, c, ab, ac, bc, and abc, as well as any combination of multiples of the same item.

[0083] Unless explicitly described, the elements, actions or instructions used herein should not be interpreted as critical or necessary. In addition, as used herein, the article "one" is intended to include one or more items and can be used interchangeably with "one or more". In addition, as used herein, the article "the" is intended to include one or more items associated with the article "the" and can be used interchangeably with "the one or more". In addition, as used herein, the term "group" is intended to include one or more items (e.g., a combination of related items, unrelated items, or related and unrelated items), and can be used interchangeably with "one or more". When only one item is intended, the phrase "only one" or similar language is used. In addition, as used herein, the term "having" etc. is intended to be an open term. In addition, the phrase "based on" is intended to mean "based at least in part", unless otherwise explicitly stated. In addition, as used herein, the term "or" is intended to be included when used in series, and can be used interchangeably with "and / or", unless otherwise explicitly stated (e.g., if used in combination with "either" or "only one of them"). Additionally, for ease of description, spatially relative terms, such as "lower," "below," "upper," "above," etc., may be used herein to describe the relationship of one element or feature to another element or feature as depicted in the figures. Spatially relative terms are intended to encompass different orientations of the device, apparatus, and / or element in use or operation in addition to the orientation depicted in the figures. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein should be interpreted accordingly.

[0084] Cross-references to related applications

[0085] This application claims priority from Japanese application No. JP2021-078578 filed on May 6, 2021, which claims priority from Japanese application No. JP2021-010676 filed on January 26, 2021, the contents of which are expressly incorporated herein by reference.

Claims

1. A semiconductor light receiving element, comprising: Semiconductor substrate; A high-concentration layer of a first conductivity type is formed on a semiconductor substrate; a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer; a second conductivity type low concentration layer configured to form a PN junction interface together with the first conductivity type low concentration layer; and a high-concentration layer of the second conductivity type, formed on and in contact with the low-concentration layer of the second conductivity type, wherein: The first conductivity type low concentration layer and the second conductivity type low concentration layer each have a thickness of less than 1×10 16 / cm 3 The carrier concentration, The first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer, The second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer, The first conductivity type high concentration layer and the second conductivity type high concentration layer each have a 1×10 17 / cm 3 or higher carrier concentration, and at least one of the first conductivity type low concentration layer or the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light, When a voltage is applied to the first conductivity type low concentration layer and the second conductivity type low concentration layer, the entire area of ​​the first conductivity type low concentration layer and the second conductivity type low concentration layer is depleted, wherein the thickness of the first conductivity type low concentration layer is a thickness Dn0 or less, and the thickness Dn0 is defined by Equation 1, Equation 2, and Equation 3, and The thickness of the second conductive type low-concentration layer is Dp0 or less, and the thickness Dp0 is defined by Equation 1, Equation 2, and Equation 3: [Equation 1] [Equation 2] R p = 1 + (εn / εp)·(Np / Nn) [Equation 3] Dn0=(Rp-1)·Dp0 in: Vr: Bias voltage applied from the outside Vb: built-in potential q: elementary charge εp: Dielectric constant of the low-concentration layer of the second conductivity type εn: Dielectric constant of the low-concentration layer of the first conductivity type Np: Carrier density of the second conductivity type low concentration layer Nn: Carrier density of the low-concentration layer of the first conductivity type.

2. The semiconductor light receiving element according to claim 1, wherein One of the first conductivity type low-concentration layer and the second conductivity type low-concentration layer, which is closer to the surface into which light enters, is thinner than the other of the first conductivity type low-concentration layer and the second conductivity type low-concentration layer.

3. A semiconductor light receiving element, comprising: Semiconductor substrate; A high-concentration layer of a first conductivity type is formed on a semiconductor substrate; a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer; a second conductivity type low concentration layer configured to form a PN junction interface together with the first conductivity type low concentration layer; and a high-concentration layer of the second conductivity type, formed on and in contact with the low-concentration layer of the second conductivity type, wherein: The first conductivity type low concentration layer and the second conductivity type low concentration layer each have a thickness of less than 1×10 16 / cm 3 The carrier concentration, The first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer, The second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer, The first conductivity type high concentration layer and the second conductivity type high concentration layer each have a 1×10 17 / cm 3 or higher carrier concentration, and at least one of the first conductivity type low concentration layer or the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light, wherein the high concentration layer of the first conductivity type is a contact layer of the first conductivity type, The first conductivity type low concentration layer is a first conductivity type low concentration InGaAs absorption layer having a band gap for absorbing incident light, The second conductivity type low concentration layer is a second conductivity type low concentration InGaAs absorption layer having a band gap for absorbing incident light, The second conductivity type high concentration layer is a second conductivity type contact layer, and When a voltage is applied to the first conductivity type low concentration layer and the second conductivity type low concentration layer, the entire regions of the first conductivity type low concentration layer and the second conductivity type low concentration layer are depleted.

4. A semiconductor light receiving element, comprising: Semiconductor substrate; A high-concentration layer of a first conductivity type is formed on a semiconductor substrate; a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer; a second conductivity type low concentration layer configured to form a PN junction interface together with the first conductivity type low concentration layer; and a high-concentration layer of the second conductivity type, formed on and in contact with the low-concentration layer of the second conductivity type, wherein: The first conductivity type low concentration layer and the second conductivity type low concentration layer each have a thickness of less than 1×10 16 / cm 3 The carrier concentration, The first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer, The second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer, The first conductivity type high concentration layer and the second conductivity type high concentration layer each have a 1×10 17 / cm 3 or higher carrier concentration, and at least one of the first conductivity type low concentration layer or the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light, wherein the high concentration layer of the first conductivity type is a contact layer of the first conductivity type, The first conductivity type low concentration layer is a first conductivity type low concentration wide band gap layer having a band gap that does not absorb incident light, The second conductivity type low concentration layer is a second conductivity type low concentration InGaAs absorption layer having a band gap for absorbing incident light, The second conductivity type high concentration layer is a second conductivity type contact layer, and When a voltage is applied to the first conductivity type low concentration layer and the second conductivity type low concentration layer, the entire regions of the first conductivity type low concentration layer and the second conductivity type low concentration layer are depleted.

5. The semiconductor light receiving element according to claim 4, wherein The first conductivity type low-concentration wide bandgap layer is made of one of InP, InAlAs and InAlGaAs.

6. A semiconductor light receiving element, comprising: Semiconductor substrate; A high-concentration layer of a first conductivity type is formed on a semiconductor substrate; a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer; a second conductivity type low concentration layer configured to form a PN junction interface together with the first conductivity type low concentration layer; and a high-concentration layer of the second conductivity type, formed on and in contact with the low-concentration layer of the second conductivity type, wherein: The first conductivity type low concentration layer and the second conductivity type low concentration layer each have a thickness of less than 1×10 16 / cm 3 The carrier concentration, The first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer, The second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer, The first conductivity type high concentration layer and the second conductivity type high concentration layer each have a 1×10 17 / cm 3 or higher carrier concentration, and at least one of the first conductivity type low concentration layer or the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light, wherein the first conductivity type is n-type, and the second conductivity type is p-type, and When a voltage is applied to the first conductivity type low concentration layer and the second conductivity type low concentration layer, the entire regions of the first conductivity type low concentration layer and the second conductivity type low concentration layer are depleted. 7 . The semiconductor light receiving element according to claim 6 , wherein the low-concentration layer of the first conductivity type is thinner than the low-concentration layer of the second conductivity type.

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