Patent Application
20130299936
1. Field of the Invention
The present invention relates to an avalanche photodiode used in optical fiber communications, and a method for manufacturing the same.
2. Background Art
An avalanche diode provides a light-absorbing layer and an avalanche multiplying layer. When a light enters into a light-absorbing layer, electron-hole pairs occur. When they become carriers and reach the avalanche multiplying layer, carrier multiplication occurs like an avalanche. Since the incident light can be amplified and taken out as signals thereby, avalanche diodes are frequently used in long-distance optical communications or the like that receive weak optical signals.
In order to cause avalanche multiplying, a high electric field must be applied to the avalanche multiplying layer. However, if a high electric field is applied to the light-absorbing layer, a tunnel breakdown is generated on the light-absorbing layer. Therefore, an electric field controlling layer to control the distribution of the electric field so that a high electric field is applied only to the avalanche multiplying layer is provided between the avalanche multiplying layer and the light-absorbing layer. In general, the electric field strength in the avalanche multiplying layer is preferably 600 kV/cm or higher, and the electric field strength in the light-absorbing layer is preferably 200 kV/cm or lower.
Depending on the material of the avalanche multiplying layer, which of generated electrons and holes are easily multiplied differ. Since low noise and quick response are desired in the use of high-speed optical communications, the materials that easily multiply electrons are frequently used for the avalanche multiplying layer. In these cases, the electric field controlling layer is made to be p-type, and for its dopant, Zn or Be easily doped as their dopant is frequently used.
On the upper portion of the light-absorbing layer, window layers transmitting light are frequently laminated. The window layers consist of materials having a wide band gap for lowering dark current. On a part of the window layer, a p-type region for electrical contacting is formed (for example, refer to Japanese Patent No. 4166560).
Be or Zn doped into the electric field controlling layer has a high diffusion constant due to heat. For this reason, these diffuse into un-doped layers at both ends of the electric field controlling layer due to the heat treatment during the process, and highly affect the characteristics of the avalanche photodiode. Therefore, the thermal history during the process had to be as low as possible. The heat treatment temperature during the process is generally 450 to 540° C. (for example, refer to Japanese Patent Laid-Open No. 2-20074 and Japanese Patent No. 4103885).
As a p-type dopant having a low diffusion constant, carbon has been known (for example, refer to National Publication of International Patent Application No. 2005-516414, European Patent Application Laid-Open No. 2073277 and Japanese Patent Laid-Open No. 2011-243675). Therefore, for elongating the heat treatment time, the usage of carbon has been considered as the dopant of electric field controlling layer. However, doping itself of carbon is more difficult than that of Be or Zn. Consequently, for succeeding the doping concentration required in the electric field controlling layer, the growing temperature of the electric field controlling layer must be lowered than that of other layers, such as the light-absorbing layer or the like. Thereby, unnecessary elements such as hydrogen have been taken in the electric field controlling layer. Since the unnecessary elements easily move during the operation of the avalanche photodiode and affect the activation of carriers, they have been the cause of age variation in characteristics such as breakdown voltages.
In addition, in the conventional avalanche photodiode, since the discontinuity of the energy level of a valence band and conduction band in the interface between the window layer and the light-absorbing layer was large and the movement of carriers was inhibited, there was a problem of poor high-speed response.
In view of the above-described problems, an object of the present invention is to provide an avalanche photodiode and a method for manufacturing the same which can improve the high-speed response and reduce the time variation of the characteristics.
According to the present invention, an avalanche photodiode comprises: a substrate; an avalanche multiplying layer, a p-type electric field controlling layer, a light-absorbing layer, and a window layer sequentially laminated on the substrate; and a p-type region on parts of the window layer and the light-absorbing layer. Carbon is used as a dopant of the electric field controlling layer. Zn is used as a dopant of the p-type region. A bottom face of the p-type region is below an interface between the light-absorbing layer and the window layer.
The present invention makes it possible to improve the high-speed response and reduce the time variation of the characteristics.
Other and further objects, features and advantages of the invention will appear more fully from the following description.
An avalanche photodiode and a method for manufacturing the same according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted.
The carrier concentration of the n-type AlInAs buffer layer 2 is 5×1018 cm−3 or lower, and the layer thickness is 0.1 to 1 μm. The carrier concentration of the AlInAs avalanche multiplying layer 3 is 0.1×1015 to 8×1015 cm−3, and the layer thickness is 0.05 to 0.5 μm. The carrier concentration of the p-type AlInAs electric field controlling layer 4 is 2×1017 to 2×1018 cm−3, and the layer thickness is 0.01 to 0.2 μm. The layer thickness of the un-doped light-absorbing layer 5 is 0.5 to 2.5 μm. The window layer 6 is un-doped, or doped in n-type, and the carrier concentration is 3×1016 cm3 or lower, and the layer thickness is 0.5 to 2 μm.
A p-type region 7 is provided on parts of the window layer 6 and the un-doped light-absorbing layer 5. As the dopant of the p-type region 7, Zn is used. Zn diffusion reaches to the un-doped light-absorbing layer 5, and the bottom face of the p-type region 7 is below the interface between the un-doped light-absorbing layer 5 and the window layer 6.
An InGaAs contact layer 8 is provided on the p-type region 7, and a P-side electrode 9 is provided so as to contact the InGaAs contact layer 8. On the region other than the InGaAs contact layer 8, the upper face of the window layer 6 is covered with SiN film 10, which is the passivation film and the reflection preventing film. On the back face of the n-type InP substrate 1, an n-side electrode 11 is provided.
Next, a method for manufacturing the avalanche photodiode according to the present embodiment will be described. By the MOCVD (Metal Organic Chemical Vapor Deposition) method or the MBE (Molecular Beam Epitaxy) method, an n-type AlInAs buffer layer 2, an AlInAs avalanche multiplying layer 3, a p-type AlInAs electric field controlling layer 4, an un-doped light-absorbing layer 5, a window layer 6, and an InGaAs contact layer 8 are sequentially formed on an n-type InP substrate 1.
Next, Zn is diffused by a selective thermal diffusing method using an insulating film having a circular hole as a mask (solid diffusion of Zn), and the p-type region 7 is formed on parts of the window layer 6 and the un-doped light-absorbing layer 5.
Next, the InGaAs contact layer 8 on the p-type region 7 is etched so as to leave a ring having an approximately 5 μm in width. Then, a SiN film 10 is formed. The thickness d of the SiN film 10 is adjusted so as to be nearly d=λ/4/n to form a reflection preventing film, where n is a refractive index, and X, is a wavelength of an incident light.
Next, a part of the SiN film 10 on the InGaAs contact layer 8 is removed. A p-side electrode 9 is formed by patterning so as to contact with the InGaAs contact layer 8. Thereafter, the back face of the n-type InP substrate 1 is polished to form the n-side electrode 11.
Subsequently, the operation of the avalanche photodiode according to the present embodiment will be described. When a light is incident to the un-doped light-absorbing layer 5 in the condition where a plus voltage is applied to the n-side electrode 11 and a minus voltage is applied to the p-side electrode 9, electrons and holes are generated. Since the generated electrons are moved to the side of the n-type InP substrate 1, they reach the AlInAs avalanche multiplying layer 3 after passing through the p-type AlInAs electric field controlling layer 4. The operations where an electric field sufficiently high to cause multiplication is applied to the AlInAs avalanche multiplying layer 3, the entered electrons create electron-hole pairs, and further generated electrons create other electron-hole pairs are repeated to multiply signals.
Next, the effect of the present embodiment will be described by comparing with a comparative example.
In a part of Zn reached to the un-doped light-absorbing layer 5, the diffusion speed becomes exponentially high and passes through the un-doped light-absorbing layer 5, and some reaches to the vicinity of the p-type AlInAs electric field controlling layer 4. In the comparative example, the constituents having a high Zn diffusion speed are reciprocally diffused with Be or Zn which is the dopant of the p-type AlInAs electric field controlling layer 4. Therefore, the carrier concentration of the p-type AlInAs electric field controlling layer 4 is markedly lowered, and desired electric field distribution cannot be obtained, and the device may not be able to operate as the avalanche photodiode. On the other hand, in the present embodiment, since carbon is used as the dopant of the p-type AlInAs electric field controlling layer 4, the lowering of the carrier concentration of the p-type AlInAs electric field controlling layer 4 due to interdiffusion does not occur.
Also in the comparative example, for preventing thermal diffusion from the p-type AlInAs electric field controlling layer 4 of Be or Zn having a high diffusion constant, the Zn diffusing time cannot be increased. In the present embodiment, on the other hand, since carbon is used as the dopant of the p-type AlInAs electric field controlling layer 4, the Zn diffusing time can be belonged. Thereby, the thermal history during the process can be increased, and the unnecessary elements such as hydrogen in the p-type AlInAs electric field controlling layer 4 using carbon can be thermally diffused and removed. Furthermore, by reacting the unnecessary elements with the constituent having high Zn diffusion speed, the concentration can be lowered. As a result, the movement of the unnecessary elements at the usage of the avalanche photodiode can be prevented, and the time variation of the characteristics can be lowered.
In addition, if the impurity concentration in the p-type AlInAs electric field controlling layer 4 is lower than 2×1017 cm−3, the layer thickness of the p-type AlInAs electric field controlling layer 4 must be thicker than 0.2 μm to obtain the required relaxation, and the running time of the carriers is increased and the high-speed responsivity becomes poor. It is therefore preferable that the impurity concentration of the p-type AlInAs electric field controlling layer 4 is at least 2×1017 cm−3.
In addition, if the carrier concentration of the p-type AlInAs electric field controlling layer 4 is high, the growing temperature is lowered, and the unnecessary elements such as hydrogen increase. Furthermore, if the carrier concentration is higher than 2×1018cm−3, the layer thickness of the p-type AlInAs electric field controlling layer 4 becomes thinner than 10 nm, and problems in layer thickness control occur. Therefore, the impurity concentration of the p-type AlInAs electric field controlling layer 4 preferably does not exceed 2×1018cm−3.
The constituents having the fast Zn diffusion speed increase when the diffusion temperature is high. Therefore, for reducing the time variation of characteristics, it is preferable that the temperature in the solid-phase diffusion of Zn is higher than 540° C.
InP is used in the substrate, and InP or AlInAs is used in the n-type buffer layer. AlInAs or AIAsSb is used in the avalanche multiplying layer. AlInAs, AlGaInAs, InGaAsP, or InP is used in the electric field control layer. InGaAs or InGaAsP is used in the light-absorbing layer. InP, InGaAsP, AlGaInAs, AlInAs or the like are used in the window layer. However, any materials can be used if the required characteristics in each layer can be obtained, and these materials do not limit the range of the invention. In addition, the n-type buffer layer can be made as the contact layer such as InGaAs; and the substrate can be a semi-insulating substrate such as the Fe-doped substrate.
By the buried semiconductor layer 12, the exposure of the un-doped light-absorbing layer 5 having the narrow band gap is prevented, and the element reliability can be improved. In addition, since the un-doped window layer 6 exists between the p-type region 7 and the buried semiconductor layer 12, the leak current is not increased.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
The entire disclosure of Japanese Patent Application No. 2012-108577, filed on May 10, 2012, including specification, claims, drawings, and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-108577 | May 2012 | JP | national |
