JPH0653537A - Semiconductor light receiving element - Google Patents

Abstract

PURPOSE:To quicken response by forming a diffraction lattice type lens, comprising a plurality of concentric grooves with the gap (d) between adjacent grooves being represented by specific conditions, on the surface of a substrate on the inside of a window. CONSTITUTION:A diffraction lattice type leans 11 comprising a plurality of concentric grooves having the same depth is formed on the rear surface of an InP substrate 1. An antireflection film 10 is formed on the surface of the diffraction lattice lens 11 whereas an n-type ohmic electrode 9 is formed on the surface of the substrate on the outside of the diffraction lattice leans 11. An n-type InP buffer layer 2, an n-type InGaAs light absorption layer 3, and an n-type InP cap layer 4 are formed sequentially on the surface of the InP substrate 1. The diffraction lattice type lens 11 is obtained by making a plurality of grooves 11a, 11b, 11c, 11d concentrically at an interval d=lambda/2n.sintheta (theta= tan-1(r/1)) and a depth lambda/4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an optical semiconductor device, especially In
The present invention relates to a light receiving element using a compound semiconductor substrate such as P single crystal, and relates to a technique effectively used for a PIN photodiode for signal reception in optical communication, for example.

[0002]

2. Description of the Related Art Compound semiconductor photodetectors have been put into practical use as high-sensitivity photodetectors for optical communication and optical information processing.
Avalanche type photodiodes and PIN type photodiodes are known. Among them, the avalanche type photodiode has a high internal current gain due to avalanche multiplication of electrons or holes in a high electric field by being biased in the vicinity of the breakdown voltage, and has been put to practical use as a high sensitivity element. On the other hand, since the PIN photodiode can be operated at a low voltage, it can share a power source with other electronic circuits, and has a great advantage that a device having a single power source voltage can be configured. Therefore, for example, it is widely used as a photoelectric conversion element in a transmission / reception module that constitutes a branch point (node) or the like of a local area network using a double ring type optical fiber. In particular, in a cable television distribution network using an optical fiber for distributing digitized moving image information, a light receiving element capable of supporting a high speed of about 2 Gbit / sec is required for transmission of a large amount of image information in the future. Become.

Conventionally, a PIN for signal reception in optical communication
As the type photodiode, for example, there is a back illuminated type photodiode as shown in FIG. That is, the n-type InP buffer layer 2 and the n-type InGaA are formed on the surface (the lower surface in the figure) of the InP substrate 1 by the hydride vapor phase epitaxial growth method or the like.
s The light absorption layer 3 and the n-type InP cap layer 4 are sequentially grown. Then, Zn is formed on a part of the InP cap layer 4 by
By diffusing a p-type impurity such as
The p-type diffusion region 6 is formed so as to reach the interface with the light absorption layer 3. Further, a protective film 7 made of a silicon nitride film is formed on the surface of the cap layer 4, and an ohmic electrode 8 is formed on the surface of the p-type diffusion region 6. On the other hand, an n-type electrode 9 is formed on the back surface (upper surface in the figure) of the InP substrate 1, a light entrance opening 9a is formed in the center thereof, and a film thickness is formed inside the opening 9a to minimize the reflectance. The antireflection film 10 made of controlled silicon nitride is formed.

In the light receiving element having the above structure, a reverse voltage is applied between the electrodes 8 and 9, that is, a negative voltage is applied to the electrode 8 and a positive voltage is applied to the electrode 9. In such a reverse bias state, the depletion layer 30 at the boundary of the p-type diffusion region 6 spreads mainly in the InGaAs light absorption layer 3. In this state, the light incident from the light receiving portion is not absorbed by the InP substrate 1 and the buffer layer 2 but reaches the light absorption layer 3 and is absorbed, and an electron-hole pair is generated. The electron-hole pair generated in the depletion layer 30 is accelerated by the electric field in the depletion layer 30 and reaches the electrodes 8 and 9, and is observed as a photocurrent in an external circuit. by the way,
In the PIN photodiode, two main factors that limit the response speed are the CR time constant due to the junction capacitance and parasitic resistance parasitic on the light receiving portion, and the transit time of the electron-hole pair generated by light absorption. There is one. Regarding the limitation by the CR time constant, the speed can be increased by reducing the area of the light receiving portion (p-type diffusion layer 6) to reduce the junction capacitance or by reducing the contact resistance of the electrode. However,
If the size of the p-type diffusion region 6 is reduced to reduce the area, optical coupling with an optical fiber becomes difficult. Therefore, a technique has been proposed in which a lens as indicated by a chain line A is integrally formed on the back surface of the substrate 1 so that incident light is focused on the light absorption layer 3.

[0005]

However, in order to integrally form a lens on the back surface of the substrate, a highly controlled etching technique is required, which complicates the process, and if the lens is processed only by etching. There are drawbacks that the precision of the lens shape is low and the curvature of the lens changes, and the condensing position shifts due to decentering. Therefore, it has been found that the p-type diffusion region 6 cannot be made so small in the conventional lens-integrated photodiode, and there is a limit to speeding up by reducing the pn junction capacitance. The present invention has been made in view of the above problems, and an object thereof is to realize a semiconductor light receiving element having a fast response speed without using an advanced etching technique.

[0006]

SUMMARY OF THE INVENTION According to the present invention, a light absorption layer formed on a semiconductor substrate via a buffer layer and formed of a semiconductor layer of a first conductivity type has a forbidden band more than the light absorption layer. A cap layer made of a semiconductor layer of the first conductivity type having a large width is formed, a diffusion region of the second conductivity type is formed in a part of the cap layer, and an electrode is formed on the surface of the diffusion region. And a window portion through which light can be incident is formed on the back surface of the semiconductor substrate, the wavelength of the received light is λ, and the semiconductor substrate is a substrate surface inside the window portion. When the refractive index of the crystal is n, the thickness of the semiconductor substrate is 1, and the distance from the center of the window is r, the distance d between adjacent grooves is expressed by the following equation: d = λ / (2n · sin θ), θ = Tan- ¹ (r / l) and a plurality of concentric circles with a depth of λ / 4 A diffraction grating type lens composed of grooves is formed.

[0007]

According to the above means, the incident light can be condensed on the light absorption layer by the same principle as the diffraction grating by the diffraction grating type lens formed inside the window on the back surface of the semiconductor substrate. In comparison with the one in which a normal convex lens is integrally formed on the back surface of the substrate, a larger aperture and a shorter focus can be realized, and there is no change in curvature due to processing variations or deviation of the focusing position (focal point) due to eccentricity. It becomes easy to optically couple with p and the area of the p-type diffusion region can be reduced, whereby a semiconductor light receiving element with a fast response speed in which the pn junction capacitance is reduced can be obtained. Further, since the diffraction grating lens for condensing light on the back surface of the semiconductor substrate is composed of a plurality of concentric circular grooves having a constant depth (λ / 4), it is necessary to form a convex lens or Fresnel lens on the substrate. Since it is not necessary to form a curved surface or an inclined surface as described above, it can be easily formed by selective etching by a normal photolithography technique, and a semiconductor light receiving element having a lens structure can be manufactured without complicating the process. become able to. The depth of the groove is set to λ / 4 because the depth of the groove where the first-order intensity distribution is the strongest in this type of diffraction grating is λ / 4.
It is selected because it is an odd multiple of 1 and it facilitates processing in the peripheral portion where the lattice spacing is narrow.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a PIN photodiode using InP as a substrate will be described below with reference to the drawings. FIG. 1 shows an embodiment of a PIN photodiode according to the present invention. In the figure, the same reference numerals as those shown in FIG. 3 indicate the same or corresponding portions. In this embodiment, a diffraction grating lens 11 having a plurality of concentric circular grooves having a constant depth is formed on the back surface (upper surface in the figure) of the InP substrate 1, and the silicon nitride is formed on the surface of the diffraction grating lens 11. The antireflection film 10 made of
An n-type ohmic electrode 9 is formed on the substrate surface outside the diffraction grating lens 11. Then, an n-type InP buffer layer 2, an n-type InGaAs light absorption layer 3, and an n-type InP cap layer 4 are sequentially formed on the surface (lower surface in the figure) of the InP substrate 1.

In the diffraction grating lens, the distance d between adjacent grooves is expressed by the following equations d = λ / (2n · sin θ) and θ = tan- ¹ (r / l), and the depth is λ / A plurality of grooves 11a, 11
It is obtained by forming b, 11c and 11d so as to form concentric circles. In this case, the size of the outer diameter of the lens may be determined first, and the distance between the grooves satisfying the above formula may be calculated in order from the outer side to the inner side, or the size of the flat portion in the center of the lens may be calculated. Alternatively, the distance between the grooves satisfying the above formula may be calculated in order from the inner side to the outer side and then calculated.

Further, Zn is formed on a part of the cap layer 4.
The p-type diffusion region 6 is provided so that the p-type impurity as described above is diffused and reaches the interface with the n-type InGaAs light absorption layer 3. Further, a surface protective film 7 made of silicon nitride is formed on the surface of the cap layer 4, and an opening is formed in a portion of the surface protective film corresponding to the p-type diffusion region 6, and the inside of the opening is formed. 2 through a p-type InGaAs contact layer 16 having a bandgap smaller than that of the cap layer 4, as shown in an enlarged view in FIG. 2, a laminated thin film structure of a group IIB metal such as Ti and Zn serving as p-type impurities ( Ti layer / Zn
Layer) p-type ohmic electrode 8a and Ti / Pt
The second p-type electrode 8b having the / Au structure is formed by lamination.

In the PIN type photodiode of this embodiment, even if the incident light is irradiated from the upper side of the device to the whole, the depletion layer 30 at the interface of the light absorption layer 3 is formed by the diffraction grating type lens 11. In addition, since the central portion of the diffraction grating lens 11 is flat, the curvature of the focal point (focal point) is deviated due to a change in curvature or decentering due to processing variations as in a normal convex lens, and the Fresnel There is no axial misalignment due to mask misalignment in the lens. In addition, the size of the central flat part is changed according to the way the light spreads due to the difference in the incident conditions (distance from the optical fiber, core diameter, numerical aperture) from the optical fiber (for example, in the case of a straight incident, the central flat part is changed). The area is enlarged, and the central flat area is narrowed when entering a widening eye.)
It can be dealt with. This facilitates optical coupling with the optical fiber. As a result, it is possible to reduce the area of the p-type diffusion region, reduce the pn junction capacitance,
The cutoff frequency of the element, which is determined by the CR time constant, increases, and the response of the element improves.

In this embodiment, the p-type electrode is a second p-type electrode having a Ti / Pt / Au structure on the p-type ohmic electrode 8a having a laminated structure (Ti layer / Zn layer) containing a Group IIB metal.
Since the type electrodes 8b are laminated, an alloying layer such as an ohmic electrode made of a conventional Au alloy is not required, so that heat treatment at a high temperature for a long time is not required, and a diffraction grating lens due to thermal strain. It is possible to prevent the protective film (antireflection film 10) from peeling or cracking particularly in the peripheral portion of 11, and improve the thermal stability of electrical contact. Furthermore, since a low contact resistance is obtained, the series resistance becomes small, the cutoff frequency of the element determined by the CR time constant becomes high, and the response speed of the element becomes fast.

Next, the manufacturing process of the PIN photodiode of the above embodiment will be described. The n-type InP buffer layer 2, the n-type InGaAs light absorption layer 3, and the n-type InP cap layer 4 on the InP substrate 1 are formed by sequentially epitaxially growing by the MOVPE method (metal organic chemical vapor deposition method). The p-type diffusion region 6 is an n-type I.
After epitaxially growing a p-type InGaAs layer on the nP cap layer 4, a silicon nitride film is formed on the p-type InGaAs layer and a circular diffusion window is opened in a portion corresponding to the light receiving region, and Zn (zinc) is used as a diffusion mask. N-type InGaAs
It is formed by selective diffusion until reaching the interface with the light absorption layer 3. After removing the diffusion mask, the contact layer 16 is p-type InGaAs only on the portion above the p-type diffusion region 6.
It is formed by etching so that the layer remains.

The surface protective film 7 on the surface of the cap layer 4 is formed of a silicon nitride film by plasma CVD and then 500
The film is heat-treated at a temperature of not more than 0 ° C. (usually 400 to 450 ° C.) to make the film uniform and dense. Then, an opening is formed in a portion of the surface protective film 7 corresponding to the p-type diffusion region 6. The p-type electrodes 8a and 8b inside this opening are
First, a Ti layer of about 200 Å and a Zn layer of about 10 are formed on the p-type InGaAs contact layer 16 so as to cover the opening.
A thickness of 0Å and a Ti layer of about 200Å on it, P
The t-layer is formed to a thickness of about 1000Å. Then 45
Heat treatment is performed at a temperature of about 0 ° C. in a non-oxidizing atmosphere for several tens of seconds to solid-phase diffuse Zn into the p-type InGaAs contact layer 16 to reduce the electrical contact resistance of the Ti layer to form the ohmic electrode 8a. Then, a Ti layer is deposited on the ohmic electrode 8a to a thickness of about 500Å, a Pt layer is deposited to a thickness of about 1000Å, and an Au layer is deposited to a thickness of about 3000Å, and then patterned to form a second p-type electrode 8b.

In the diffraction grating lens 11 on the back surface of the substrate, the distance d between the grooves is d = λ / on the back surface of the mirror-polished substrate.
After forming a plurality of annular silicon oxide films satisfying (2n · sin θ) by concentric circles by lithography technology, the back surface of the substrate is selectively etched to a depth of λ / 4 by vapor phase etching using this as an etching mask. To form. After forming the plurality of ring-shaped grooves, the silicon oxide film serving as the etching mask is removed, and then the n-type electrode 9 made of a metal such as aluminum is formed on the back surface of the substrate. After opening the light incident window portion 9a corresponding to the lens 11, an antireflection film 10 made of silicon nitride having a film thickness controlled so that the reflectance is minimized is formed inside thereof.

In the above embodiment, the method of forming the photoelectric conversion portion (p-type diffusion region, p-type electrode) on the front surface side of the substrate has been described for convenience of explanation. It is possible to first form either the diffraction grating lens or the photoelectric conversion portion on the substrate surface side. Further, in addition to the p-type electrodes (8a, 8b) of the embodiment, the present invention can be applied to a so-called flip chip type light receiving element in which an electrode portion corresponding to the n-type electrode 9 is provided on the substrate surface. Furthermore, although the present invention has been described in the embodiment as applied to a PIN photodiode, it can also be applied to an avalanche photodiode and other semiconductor light receiving elements.

[0017]

As described above, according to the present invention, a light-absorbing layer made of a semiconductor layer of the first conductivity type formed on a semiconductor substrate with a buffer layer interposed between the light-absorbing layer and the light-absorbing layer is more forbidden than the light-absorbing layer. A cap layer made of a semiconductor layer of the first conductivity type having a large band width is formed, and a diffusion region of the second conductivity type is formed in a part of the cap layer, and a surface of the diffusion region is formed. In a light receiving element in which an electrode is formed and a window through which light can enter is formed on the back surface of the semiconductor substrate, a substrate surface inside the window has a wavelength of received light of λ, a semiconductor When the refractive index of the substrate crystal is n, the thickness of the semiconductor substrate is l, and the distance from the center of the window is r, the distance d between adjacent grooves is expressed by the following equation: d = λ / (2n · sin θ), A concentric circular compound with a depth of λ / 4, expressed as θ = tan- ¹ (r / l). Since a diffraction grating type lens consisting of a number of grooves is formed, the incident light is guided to the light absorbing layer by the same principle as the diffraction grating by the diffraction grating type lens formed inside the window on the back surface of the semiconductor substrate. Can be focused. Therefore, compared to a normal convex lens integrally formed on the back surface of the substrate, it is possible to realize a larger aperture and a shorter focus, and there is no change in curvature due to processing variations or deviation of the focus position (focus) due to eccentricity. The optical coupling with the optical fiber is facilitated, and the area of the p-type diffusion region can be reduced, which makes it possible to obtain a semiconductor light receiving element having a fast response speed with a reduced pn junction capacitance.

Further, since the diffraction grating lens for collecting light on the back surface of the semiconductor substrate is composed of a plurality of concentric circular grooves having a constant depth (λ / 4), a convex lens or Fresnel lens is formed on the substrate. Since it is not necessary to form curved surfaces or inclined surfaces as in the case of manufacturing, it can be easily formed by selective etching by ordinary photolithography technology, and a semiconductor light receiving element having a lens structure can be manufactured without complicating the process. There is an effect that can be done.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of an embodiment in which the present invention is applied to a PIN photodiode using InP as a substrate.

FIG. 2 is an enlarged cross-sectional view showing details of a main part (photoelectric conversion part) of the PIN photodiode of the above embodiment.

FIG. 3 is a sectional view showing an example of a conventional PIN photodiode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 substrate 3 light absorption layer 4 cap layer 6 p type diffusion region (light receiving part) 7 protective film (silicon nitride film) 8a, 8b, 9 electrode 9a window part 10 antireflection film 11 diffraction grating lens 11a to 11d concentric circles groove

Claims (1)

[Claims] 1. A first conductivity type having a forbidden band width larger than that of the light absorption layer, which is formed on a semiconductor substrate via a buffer layer and is composed of a semiconductor layer of the first conductivity type. And a second conductive type diffusion region is formed on a part of the cap layer, and an electrode is formed on the surface of the diffusion region.
In a semiconductor light receiving element in which a circular window through which light can be incident is formed on the back surface of the semiconductor substrate, the wavelength of the received light is λ, and the semiconductor substrate crystal is refracted on the substrate surface inside the window. When the ratio is n, the thickness of the semiconductor substrate is l, and the distance from the center of the window is r, the distance d between adjacent grooves is given by the following equation: d = λ / (2n · sin θ), θ = tan- A semiconductor light receiving element, characterized in that a diffraction grating type lens formed by a plurality of concentric circular grooves having a depth of λ / 4, which is represented by ¹ (r / l), is formed.