JPH05102513A - Semiconductor phtodetector - Google Patents

Abstract

PURPOSE:To obtain a PIN photo diode capable of quick response to incident light which is subjected to high speed modulation, by forming a light reflecting layer between a light absorption layer and a buffer layer, so as to cover the outside of a diffusion region. CONSTITUTION:A light reflecting layer 12 composed of a semiconductor multifilm layer is formed between a light absorption layer 3 and a buffer layer 14, so as to cover the outside of a diffusion layer 6. A reflecting mirror structure composed of a dielectric film 16 and a metal film 8b is formed on the surface of a substrate. A lens structure is formed in a body in an incidence window part 9a on the substrate surface through which window a light enters. A contact layer 15 whose forbidden bandwidth is smaller than that of a cap layer is formed between an electrode 8a of a photo detection element and the surface of a diffusion layer 6. Thereby the light is restrained from entering the light absorption layer 3, and electron-hole pairs can be prevented from being generated in a region distant from a depletion layer 30, so that the fall of a photoelectric waveform becomes sharp and a PIN photodiode capable of quick response can be obtained.

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 In a large capacity optical communication system such as an optical CATV system, an ultra-high speed and highly sensitive photodetector is indispensable. To meet this demand, a low loss wavelength band of 1.3 .mu.m of a silica optical fiber is used. A semiconductor light receiving element called a PIN photodiode has been proposed as an element for efficiently converting light of ˜1.55 μm into an electric signal. As a PIN photodiode for signal reception in optical communication,
For example, there is a back-illuminated type as shown in FIG.

That is, an n-type InP buffer layer 2, an n-type InGaAs light absorption layer 3 and an n-type InP cap layer 4 are grown in this order on the surface (the lower surface in the figure) of the InP substrate 1. Then, a p-type impurity layer such as Zn is diffused in a part of the InP cap layer 4 to form a p-type diffusion layer 6 that reaches the interface with the n-type InGaAs 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 8a is formed on the surface of the p-type diffusion layer 6. On the other hand, an n-side 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 so that the reflectance is minimized. The antireflection film 10 made of controlled silicon nitride is formed. A p-side electrode 8b made of Au or the like is formed on the surface side of the substrate so as to be bonded to the surface of the ohmic electrode 8a.

In the light receiving element having the above structure, a reverse voltage, that is, a negative voltage is applied to the electrode 8b and a positive voltage is applied to the electrode 9 between the electrodes 8b and 9. In such a reverse bias state, the depletion layer 30 at the boundary of the p-type diffusion layer 6 spreads mainly in the InGaAs light absorption layer 3. In this state, the light having a predetermined wavelength that has entered 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. Of these, the electron-hole pairs generated in the depletion layer 30 are accelerated by the electric field in the depletion layer, reach the electrodes 8b and 9, and are observed as a photocurrent in an external circuit. On the other hand, the electron-hole pairs generated in the light absorption layer 3 outside the depletion layer 30 reach the depletion layer by diffusion and contribute to the photocurrent.

[0005]

By the way, in the PIN photodiode having the above-mentioned structure, the main factor limiting the response speed of the diode is the junction capacitance parasitic on the light receiving portion and the CR time constant due to the parasitic resistance. There are two transit times of electron-hole pairs generated by light absorption. Regarding the limitation due to the CR time constant, the area of the light receiving portion (p-type diffusion layer 6) is reduced to reduce the junction capacitance, the contact resistance of the electrode is reduced, and the protective film 7 formed on the substrate surface is reduced.
And an electrode 8b on the MIS (metal-insulating film-
Higher speed can be achieved by reducing the capacitance of the (semiconductor) structure.

However, the limitation of speeding up due to the traveling time of the electron-hole pair generated by light absorption can be dealt with by reducing the traveling distance of the electron-hole pair, but in order to reduce the traveling distance, the It is necessary to reduce the thickness of the absorption layer 3 or the diameter of the p-type diffusion layer 6. However, when the thickness of the light absorbing layer 3 is reduced, the thickness of the depletion layer 30 is also reduced, and conversely the junction capacitance is increased. Therefore, to increase the response speed, the thickness of the light absorbing layer 3 is too small. It cannot be thinned.

On the other hand, if the diameter of the p-type diffusion layer 6 is reduced, the ratio of electron-hole pairs generated outside the depletion layer 30 increases. Therefore, the holes generated in the region away from the depletion layer 30 reach the depletion layer by diffusion and contribute to the photocurrent, but the diffusion speed is slow, so that the time until it contributes to the photocurrent is delayed. As a result, when a pulsed optical signal as shown in FIG. 5 is incident, the trailing edge of the photocurrent waveform is delayed as shown by the solid line B in FIG. I found out that there is. The present invention has been made focusing on the above problems,
It is an object of the invention to provide a PIN photodiode capable of responding at high speed to incident light which is modulated at high speed and with high intensity.

[0008]

SUMMARY OF THE INVENTION According to the present invention, a forbidden band is provided on a light absorption layer formed of a semiconductor layer of a first conductivity type formed on a semiconductor substrate with a buffer layer interposed therebetween. A cap layer composed 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 light receiving element in which a window through which light can enter is formed on the back surface of the semiconductor substrate, and the outside of the diffusion region is covered between the light absorption layer and the buffer layer. A light reflection layer made of a semiconductor multilayer film is formed. Further, a reflecting mirror structure including a dielectric film and a metal film is formed on the front surface side of the substrate. Further, a lens structure is integrally formed on the entrance window portion on the back surface of the substrate where light is incident. Further, a contact layer made of a second conductive type semiconductor having a band gap smaller than that of the cap layer is formed between the electrode of the light receiving element and the surface of the diffusion region.

[0009]

According to the above means, the light reflection layer suppresses the incidence of light on the light absorption layer outside the depletion layer as the light receiving portion, and the electron-hole pair is generated in the region away from the depletion layer. Since it can be prevented, the fall of the photocurrent waveform becomes sharp,
A PIN photodiode capable of high-speed response can be obtained. In addition, since a reflecting mirror structure composed of a dielectric film and a metal film is formed on the substrate surface side, the amount of light that reaches the substrate surface without being absorbed even if it enters and passes through the depletion layer is incident. The light that is not absorbed is reflected by the surface of the substrate and passes through the depletion layer again, but a part of the light that is still not absorbed reaches the light reflection layer and is reflected. As compared with the conventional device, the amount of light absorbed is significantly increased, and the sensitivity of the device is improved.

Further, since the lens structure is integrally formed in the entrance window portion on the back surface of the substrate where the light is incident, the light absorption layer outside the depletion layer can be formed without increasing the area of the depletion layer as the light receiving portion. It is possible to prevent the delay of the trailing edge of the photocurrent waveform by reducing the ratio of the electron-hole pairs generated in 1) and further improve the response of the device. Further, since the contact layer made of the second conductive type semiconductor having a band gap smaller than that of the cap layer is formed between the electrode of the light receiving element and the surface of the diffusion region, the ohmic contact is directly formed on the cap layer. It is possible to form an electrode having a smaller contact resistance than that of the conventional element in which the electrodes are in contact, the cutoff frequency of the element determined by the CR time constant can be increased, and the response of the element can be improved.

[0011]

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. 3 shows an embodiment of a PIN photodiode according to the present invention. In the figure, the same reference numerals as those shown in FIG. 4 indicate the same or corresponding portions.
In this embodiment, the lens portion 11 is formed on the back surface (the upper surface in the figure) of the InP substrate 1, the antireflection film 10 made of silicon nitride is formed on the surface of the lens portion 11, and the substrate surface outside the lens portion 11 is formed. The ohmic electrode 9 is formed. Then, on the surface (the lower surface in the figure) of the InP substrate 1,
Light reflection consisting of an n-type InP buffer layer 2 and a semiconductor multilayer film in which a large number of semiconductor layers having different refractive indexes are laminated (for example, a structure in which 20 pairs of InP (film thickness 50Å) / InGaAsP (film thickness 30Å) are laminated) Layer 12 and n-type InP second
The cap layer 13 is sequentially formed.

Further, in the center of the light reflection layer 12, there is provided a window portion 12a which is made to be a mixed crystal by selectively diffusing sulfur with the silicon nitride film formed by the plasma CVD method as a mask after the formation of the second cap layer 13. Has been formed. Further, on the lower surface of the second cap layer 13, an n-type InP intermediate buffer layer 14, an n-type InGaAs light absorption layer 3,
The n-type InP first cap layer 4 is sequentially formed.
A p-type diffusion layer 6 is provided in a part of the first cap layer 4 so that a p-type impurity such as Zn 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 first cap layer 4, and an opening is formed in a portion of the surface protective film corresponding to the p-type diffusion layer 6, so that the p inside the opening is formed. An ohmic electrode 8a of Au / Zn structure is formed on the periphery of the type diffusion layer 6 via a ring-shaped contact layer 15 made of p-type InGaAs having a band gap smaller than that of the first cap layer 4. At the same time, on the surface side of the light receiving portion 6 inside the contact layer 15 and the electrode 8a, a reflection is formed of a dielectric film 16 made of silicon oxide and a metal electrode 8b such as Au whose periphery is in contact with the ohmic electrode 8a. A mirror structure is formed.

In the PIN photodiode of this embodiment, even if the incident light is radiated from the upper side of the element to the entire area, the lens portion 11 concentrates on the depletion layer 30 at the boundary of the light absorption layer 3. Light is incident on the depletion layer 30, and the light reflection layer 12 provided under the buffer layer 2 reflects incident light in a region outside the depletion layer 30, so that an electron-hole pair is formed in a region away from the depletion layer 30. Can be suppressed. Therefore, the amount of holes generated in a region apart from the depletion layer reaching the depletion layer by diffusion and significantly contributing to the photocurrent can be significantly reduced, and as a result, as shown by a broken line A in FIG. Becomes steep, and high-speed response is possible to incident light that is high-intensity-modulated at high speed. Moreover, in this embodiment, since the reflecting mirror structure is formed below the p-type diffusion layer 6, the light (about 50%) which is transmitted without being absorbed by the depletion layer 30 is reflected and depleted again. A part of the light that has passed through the layer 30 and is not absorbed, reaches the light reflection layer 12, is reflected, and by repeating this, it is gradually absorbed and contributes to the photocurrent. The amount of light absorbed is significantly increased, the output is increased, and the sensitivity of the device is improved.

Further, in this embodiment, the ohmic electrode 8
Between the a and the surface of the p-type diffusion layer 6, a contact layer 16 made of a semiconductor having a band gap smaller than that of the first cap layer 4.
Since the contact resistance of the ohmic electrode 8a is reduced, the cutoff frequency of the element determined by the CR time constant is increased, and the response of the element is improved. Further, in the above-described embodiment, the light reflection layer 9 is a semiconductor multilayer film (InP (film thickness 50
Å) / InGaAsP (thickness 30 Å) is laminated in 20 pairs), the process consistency is good,
The diode having the light reflection layer 12 can be manufactured at low cost. Although the light reflecting layer 12 is formed under the buffer layer 2 in the above-mentioned embodiment, the light reflecting layer 12 may be formed directly on the surface (lower surface in the figure) of the substrate 1.

Next, an example of the manufacturing process of the PIN photodiode will be described. First, M on the substrate
The n-type or non-doped InP buffer layer 2 is formed by the OVPE method (organic metal vapor phase epitaxial growth method).
And InP (film thickness 50Å) / InGaAsP (film thickness 30Å)
Light reflection layer 12 made of a semiconductor multilayer film in which 20 pairs of
And the n-type InP second cap layer 13 are sequentially grown. Then, plasma CV is formed on the surface of the cap layer 13.
The silicon nitride film 20 is formed by the D method, the opening 20a is formed in the silicon nitride film 20, and the opening 20a is used as a mask, and the center of the cap layer 13 and the light reflection layer 12 (the portion corresponding to the lens portion 11) is formed. Then, S (sulfur) is selectively diffused to partially mix the light reflection layer 12 with a mixed crystal. Then, the mixed crystal semiconductor multilayer film of the light-reflecting layer 12 transmits light, and the second incident window portion 12a is formed in the center of the light-reflecting layer 12 (see FIG. 1).

Next, after removing the silicon nitride film 20 on the surface of the second cap layer 13 serving as a mask with a hydrofluoric acid-based etchant, the surface of the second cap layer 13 is MO-coated.
An n-type InP layer 14, an n-type or non-doped InGaAs light absorption layer 3, a similar n-type or non-doped InP first cap layer 4, and an InGaAs layer 15 for a contact layer are sequentially grown by the VPE method. Then, a silicon nitride film 21 is formed on the surface of the contact layer InGaAs layer 15 by a plasma CVD method, an opening 21a is formed in the silicon nitride film 4, and this is used as a mask, and ZnP ₂ is used as a diffusion source. Zn, which is a p-type impurity, is diffused on the substrate surface by
s The p-type diffusion layer 6 is formed so as to reach the interface with the light absorption layer 3 (see FIG. 2).

Then, InGaAs for the contact layer is formed.
The silicon nitride film 21 on the surface of the layer 15 is removed, the ring-shaped contact layer 15 is formed by selective etching so that the InGaAs layer remains only on the peripheral portion of the surface of the p-type diffusion layer 6, and then the plasma CVD method is performed on the surface. Thus, a silicon nitride film is formed, the silicon nitride film outside the surface of the p-type diffusion layer 6 is removed, and the surface protective film 7 is formed. Then Au / Z
After the n layer is entirely vapor-deposited on the surface side of the substrate, selective etching is performed so that the Au / Zn layer remains only on the contact layer 15 to form a ring-shaped ohmic electrode 8a, and then the ohmic electrode 8a is formed. A dielectric film 16 made of silicon oxide is formed on the inside of the
On p. 6, a p-side electrode 8b made of Au that contacts the ohmic electrode 8a is formed. An n-side electrode 9 made of a metal such as AuGe alloy is formed on the back surface of the InP substrate 1 having a lens portion 11 on the back surface (upper surface in the figure), and a light incident opening 9a is formed corresponding to the lens portion 11 at the center thereof. After opening
An antireflection film 10 made of silicon nitride having a film thickness controlled so as to have a minimum reflectance is formed inside thereof (FIG. 3).

In the PIN photodiode of the above-mentioned embodiment, a reflecting mirror structure consisting of three layers of semiconductor-dielectric-metal is formed on the surface of the p-type diffusion layer 6 and comes from the p-type diffusion layer 6 side. Light will be reflected here. In the above structure, the thickness of the dielectric film 16 is set to n, the refractive index of the dielectric film is set to n,
When the wavelength of the light to be detected is λ, λ / 4n (2m
The reflectivity can be maximized by multiplying by +1 (where m is a positive integer). Incidentally, when the dielectric film 16 is made of silicon oxide and the detection wavelength band is 1.3 μm, when the thickness of the dielectric film 16 is set to 2140Å, the reflectance is 9
7% is obtained. In addition, in the above-described embodiment, the case where the invention is applied to the PIN photodiode using InP as a substrate has been described, but the invention can be applied to a light receiving diode using GaAs as a substrate and other optical semiconductor devices.

[0019]

As described above, according to the present invention, a light absorption layer formed of a first conductive type semiconductor layer formed on a semiconductor substrate with a buffer layer interposed between the light absorption layer and the light absorption layer is more forbidden than the light absorption layer. A cap layer made of a semiconductor layer of the first conductivity type having a large band width is formed, 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 electrodes are in contact with each other and a window through which light can be incident is formed on the back surface of the semiconductor substrate, an outer side of a diffusion region is covered between the light absorption layer and the buffer layer. Since the light reflection layer made of the semiconductor multilayer film is formed in the light reflection layer, the light reflection layer suppresses light from entering the light absorption layer outside the depletion layer as the light receiving portion,
Since it is possible to prevent the generation of electron-hole pairs in the region away from the depletion layer, the photocurrent waveform has a steep falling edge, and a PIN photodiode capable of high-speed response can be obtained.

Further, since the reflecting mirror structure composed of the dielectric film and the metal film is formed on the surface side of the substrate, light reaching the surface of the substrate without being absorbed even if it enters and passes through the depletion layer. Is about 50% of the incident amount, but the unabsorbed light is reflected by the substrate surface and passes through the depletion layer again, and a part of the unabsorbed light reaches the light reflection layer and is reflected. By repeating this, the amount of light absorbed is significantly increased as compared with the conventional device, and the sensitivity of the device is also improved.

Further, since the lens structure is integrally formed in the entrance window portion on the back surface of the substrate where the light is incident, the light absorption layer outside the depletion layer can be formed without increasing the area of the depletion layer as the light receiving portion. It is possible to prevent the delay of the trailing edge of the photocurrent waveform by reducing the ratio of the electron-hole pairs generated in 1) and further improve the response of the device. Further, since the contact layer made of the second conductive type semiconductor having a band gap smaller than that of the cap layer is formed between the electrode of the light receiving element and the surface of the diffusion region, the ohmic contact is directly formed on the cap layer. It is possible to form an electrode having a smaller contact resistance than that of a conventional element in which the electrodes are in contact, increase the cutoff frequency of the element determined by the CR time constant, and further improve the responsiveness of the element.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the steps of an example manufacturing process when the present invention is applied to a PIN photodiode using InP as a substrate.

FIG. 2 is a cross-sectional view showing the steps of a manufacturing process of an example when the present invention is applied to a PIN photodiode using InP as a substrate.

FIG. 3 is a cross-sectional view showing the structure of an example when the present invention is applied to a PIN photodiode using InP as a substrate.

FIG. 4 is a sectional view showing an example of a conventional PIN photodiode.

FIG. 5 is a waveform diagram showing a change in incident light on a PIN photodiode.

FIG. 6 is a waveform diagram showing changes in photocurrent in the conventional PIN photodiode and the present invention.

[Explanation of symbols]

 1 substrate 3 light absorption layer 4 cap layer 6 p-type diffusion layer 7 surface protection film 12 light reflection film 16 contact layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyuki Kanda 3-17-35, Shinsōnan, Toda City, Saitama Prefecture Japan Mining Co., Ltd.

Claims (4)

[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 formed of a semiconductor layer of the first conductivity type. And a second conductive type diffusion region is formed in a part of the cap layer, and an electrode is brought into contact with the surface of the diffusion region.
In a semiconductor light receiving element in which a window through which light can enter is formed on the back surface of the semiconductor substrate, a light formed of a semiconductor multilayer film is provided between the light absorption layer and the buffer layer so as to cover the outside of the diffusion region. A semiconductor light-receiving element comprising a reflective layer. 2. The semiconductor light receiving element according to claim 1, wherein a reflecting mirror structure including a dielectric film and a metal film is formed on the front surface side of the substrate. 3. The semiconductor light receiving element according to claim 1, wherein a lens structure is integrally formed in an incident window portion on the back surface of the substrate on which light is incident. 4. A contact layer made of a second conductive type semiconductor having a band gap smaller than that of the cap layer is formed between the electrode and the surface of the diffusion region. The semiconductor light receiving element according to claim 1, claim 2 or claim 3.