JP2003023174A - Avalanche photodiode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an avalanche photodiode(APD) that can solve the problem that an excessive electric current flows to the conventional APD and a p-n junction in the conventional APD may be broken in the worst case when the intensity of input signal light is high, because the conventional APD makes electric current multiplying actions, and the high-frequency responsiveness, multiplication factor, noise producing amount of the conventional ADP are not able to be adjusted independently, because the conventional APD is a two- pole terminal device. SOLUTION: This APD is constituted in a mesa structure in which the side face of a multiplying layer or a light absorbing layer from which current leakage frequently occurs when a high voltage is impressed upon the layer is prevented from being exposed on the side face of the structure by installing a third electrode to the portion of the light absorbing layer. In the structure, in addition, the high-frequency responsiveness and multiplication factor of the APD can be adjusted independently.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photodetector for optical communication and a structure and manufacturing method of an avalanche photodiode (hereinafter abbreviated as APD) which is a detector for measuring an optical signal.

[0002]

2. Description of the Related Art Optical communication systems using near-infrared light with a wavelength of around 1.3 μm to 1.56 μm have become the backbone of data communication systems with the spread of the Internet. Until now, what has been required for optical communication has been a long-distance, large-capacity system capable of performing wide-bandwidth communication between long-distance locations. However, not only in such a usage pattern for connecting cities, but also as a communication network for an apartment house, an office building, etc., the advantage of broadband provided by optical communication has been desired. Here, it is assumed that a local communication network exists in each place and a backbone network connects these regional networks. There are proposals for communication systems that use wireless other than optical communication or use conventional coaxial cables for the newly requested regional network systems for short distances and a large number of users. It is expected to form forms of mutual complement rather than exclusion. Here, optical communication is the same medium as the core system, and is used in the part closer to the base station by taking advantage of its characteristics such as wide band, high signal quality against external noise, and low noise generation. Is expected to be done. In fact
Optical communication has an example of use in a narrower area (LAN), and is being standardized as a Gigabit Ethernet.

Due to the popularization of such optical communication, the requirements for system parts are such that the cost of individual parts can be reduced and the number of system constituent parts can be reduced.
It has high performance and high reliability because it can be used for a long time. To achieve this, instead of preparing parts and modules for each system, we will promote standardization of standards and have a wide operating range so that they can function properly in various situations. It seems that you can do it.

There is a light receiving element as an important active component of optical communication. In this case, high sensitivity, wide band and low noise are realized at the same time, and reliability such as temperature as an external environment and little deterioration with time due to operating current is high. If you have sex,
A single component can be used for various systems, and the above-mentioned object can be achieved. However, the widely used PIN type light receiving element is inferior in high sensitivity because it does not have a self-amplifying action.

As APDs capable of high-speed response have been demanded, a superlattice has been developed as a carrier multiplication layer particularly for the purpose of reducing excess noise (Applied Physics Vol. 55, No. 10). 1989 9
Page 93). In this report, a superlattice layer made of InAlAs / InGaAsP is used as a multiplication layer in order to increase the ratio α / β of the ionization rates of electrons and holes (denoted by α and β, respectively). In this superlattice layer, the valence band discontinuity is small and the conduction band discontinuity is large, so that α becomes large and the ratio α / β can be made large, which is effective in reducing noise and speeding up. . Furthermore, in order to achieve a high-speed response, it is necessary to reduce the inherent capacitance of the element. For example, in the above paper, the light receiving portion is formed in an island shape with a diameter of 100 μm, and the multilayer film in other regions is formed. It is removed by wet etching. Further, in order to achieve a high speed response, for example, the diameter is set to 40 μm.
However, when such an island structure (hereinafter, this structure is referred to as a mesa or a mesa structure) is formed, the pn junction is exposed on the sidewall of the island structure. Usually, this exposed portion is covered with an oxide film or a nitride film of polyimide or silicon to shield it from the atmosphere and protect it.

[0006]

However, especially in the avalanche photodiode, since there is a current multiplication effect, the current flows too much when the intensity of the input signal light is large, and the pn junction in the APD is worst in the worst case. Destroyed.
Optical fiber network, especially Gbit E
in the the 10Gbit Ether from a short distance,
Standards corresponding to various transmission distances up to a long distance will be lined up (Nikkei Electronics 2000.6.19)
no. 772 p65). In this case, not only the distance to the connection destination but also the intensity of the signal light at the detection location changes according to the situation in the middle, and it takes extra time and effort to perform those settings on site. If the optical signal is strong, A
The PD may be destroyed.

In photonic communication, the destination of signal light is changed by an optical switch using a mirror without converting the optical signal into an electric signal at a site such as a router. Also in this case, the light reception level fluctuates greatly due to a change in the combination of the sender and the receiver. At this time, it is difficult to adjust to the optimum operating condition of the APD with respect to the fluctuation of the received light intensity. That is, since the APD is a device with a bipolar terminal, it is impossible to independently adjust the high frequency response, the multiplication factor, the amount of noise generated, and the like.

[0008]

In order to solve the above-mentioned problems, the present invention provides a third electrode at the portion of the light absorption layer, and a high electric field is applied to the mesa structure to reduce the frequency of current leakage. A structure that prevents the mesa side surfaces of the multiplication layer and the light absorption layer, which are high parts, from being exposed is provided, and a structure in which the high-frequency response and the multiplication factor can be independently adjusted is provided. That is, the basic semiconductor laminated structure and mesa structure are the same as those of the conventional one, but a new electrode is provided so as to cover a part of the mesa side wall portion, and the other mesa side wall portions are transformed into semi-insulating properties. .

In this way, by installing the control electrode and protecting the heel portion of other high electric fields, it is possible to avoid a reduction in high frequency characteristics and an increase in noise, and also to endure a strong optical signal input, and to be reliable. An avalanche photodiode with high performance is realized.

When the third electrode is formed and a depletion layer is generated under that portion, for example, a pn junction,
Either a Schottky junction or a structure such as a metal-dielectric-semiconductor structure (MIS structure) is provided. By forming a depletion layer in the vicinity of the surface of the mesa portion by this treatment, there is an effect that carriers do not reach the surface even toward the side wall of the mesa. With such a configuration, it is possible to provide an avalanche photodiode that is not easily broken by an optical input having high intensity.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention according to claim 1 is based on InP.
It relates to an element having a structure composed of at least the following layers on a substrate. In other words, In, which has the same type as the InP substrate and has the same high conductivity,
A P buffer layer is first formed, and on the InP substrate, 0.4 is formed on the InP substrate with high purity in which impurities are not intentionally added.
% Lattice-matched ternary InAlAs layer, and also quaternary InAl lattice-matched to InP within 0.4% and having an energy gap larger than 0.85 eV.
GaAsP layers, both having a thickness of 5 nm to 20 nm
Has a carrier multiplication layer consisting of a superlattice in which layers in the range of 10 are alternately laminated for 10 cycles or more, and further has a composition lattice-matched to InP that absorbs light to generate electron-hole pairs, The InGaAs light absorption layer, which has a different conductivity from the substrate, and the multi-layered crystal that has the electrode layer that is opposite to the substrate and has a high conductivity on top of that has the effect of actually receiving this semiconductor multilayer structure. The avalanche photodiode is characterized in that the region is formed in the shape of a trapezoidal island (mesa). Note that an electrode using a metal is provided in contact with an electrode layer which is opposite to the substrate and has high conductivity, and this is referred to as an upper surface electrode here.

Here, as described in claim 2, the light receiving portion formed in a mesa shape, the vicinity of the exposed side surface of the multiplication layer (depending on the size of the element and the concentration of added impurities, It is possible to control the original signal current flowing inside the device by providing one or a plurality of independent third electrodes (defined as regions having a depth of at most 2 μm inside). In order to generate the depletion layer near the side surface, which is the function required by this claim 2, the interface structure as claimed in claims 3 to 5 is actually used. These are specific to the third electrode. It becomes a simple structure.

With the provision of the third electrode as described in claim 6, at least the upper plane of the mesa and, if possible, all the surfaces except the side wall of the mesa are covered with a silicon oxide film or a nitride film. That is, the signal current flowing through the element is less likely to be affected by all of the sidewalls of the element, and as a result, the dark current can be reduced.

As described in claim 7, the upper surface electrode and the side wall of the mesa are insulated by, for example, introducing an appropriate lattice defect, that is, a vacancy mainly of a group III atom, thereby eliminating the influence of the surface on the signal current and reducing the darkness. The current can be reduced. As a method therefor, a silicon oxide film is attached to the side wall of the mesa, and a heat treatment is performed at a high temperature covering the silicon nitride film at other portions, and the group III element in the crystal diffuses outward into the silicon oxide film, resulting in , The resistance is increased only near the side wall of the mesa. Further, except for a part of the side wall of the mesa, the silicon nitride film is covered and a hole is formed by dry etching to similarly insulate the upper electrode and the third electrode, so that the signal current is not influenced by the side surface of the mesa. You can

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the structure of the basic avalanche photodiode described in claim 1. n-type InP
Using a gas source molecular beam epitaxial growth method on a crystalline substrate (101), silicon-doped n-type InP is sequentially formed.
Buffer layer (102), non-doped InAlAs / In
GaAsP superlattice layer (13 nm each, 13 periods) (10
3), p-type InP electric field relaxation layer (104), p-type InGa
As light absorption layer (105), p-type InP window layer (106),
The p-type InGaAs electrode layer (107) is grown in this order.
A mesa structure is formed on this wafer by using photolithography and wet etching. In this embodiment, a silicon nitride film is used as a mask for forming a mesa structure by wet etching.

FIG. 2a shows the structure of an avalanche photodiode provided with a third electrode as defined in claim 2.
A nitride film is attached to the crystal-grown wafer, and the nitride film is first etched so that it remains in the shape of a mesa. The nitride film is used as a mask to form mesas on the crystal. Although the position of the third electrode is shown in FIG. 2a, the nitride film for the purpose of blocking and protecting from the atmosphere is not drawn here. FIG. 2b is an enlarged view of the part related to the backbone of the present invention in FIG. 2a, and the nitride film is drawn here. An element forming process similar to the normal process was performed except for the process of providing the third electrode.

The function of the avalanche photodiode manufactured in this embodiment will be described above. In FIG. 2a, a voltage is applied between the top electrode and the avalanche photodiode provided with the third electrode described in claim 2,
The situation is shown by generating a depletion layer near the side surface. Carriers are discharged in the depletion layer, and a relatively strong electric field exists therein. For example, when carriers are generated by photoexcitation in this region, the carriers are promptly moved to both electrodes. In an avalanche photodiode that is actually operating, the upper electrode (209) and the lower electrode (208) are applied so as to sandwich the pn junction, and the carriers generated in the light absorption layer (205) are extracted as an optical signal current. However,
When such a third electrode is provided and a voltage is applied in the direction in which the depletion layer is generated as in the previous example, part of the carriers generated by light is carried away by the third electrode,
The current drawn from the bottom electrode (208) is reduced.
In an ordinary avalanche photodiode, when such a phenomenon occurs, it means that the sensitivity is lowered, that is, the performance is lowered. However, since the current flowing through the pn junction is reduced under a very strong optical signal, the destruction of the pn junction is reduced. Is effective. G
In the case of bit Ether and 10 Gbit Ether, there is a large variation in the intensity of the input signal light at each connection point, but the signal current reduction effect of this third electrode extends the life of the avalanche photodiode, improves reliability, and increases the output current range. Since the width becomes narrower, the retrofit circuit can be simplified. It also includes the fact that the multiplication of the current can be controlled by the voltage applied to the third electrode. Also, when the photocurrent increases,
Although the frequency band is narrowed from a certain point and the high frequency characteristic is deteriorated, the bandwidth can be improved by reducing the signal current.

[0019]

As described above, according to the present invention, in the mesa structure avalanche photodiode, by extracting a part of the carriers generated from the light absorption layer, the conversion efficiency of light-current is changed and the multiplication factor is increased. Can be controlled.
Also, reliability is improved by reducing the current passing through the pn junction.

[Brief description of drawings]

FIG. 1A is a diagram showing an avalanche photodiode structure according to an embodiment of the present invention. FIG. 1B is a partially enlarged view of FIG. 1A.

FIG. 2 is a characteristic diagram showing an effect of reducing photocurrent according to an embodiment of the present invention.

FIG. 3 is a diagram showing a conventional avalanche photodiode structure.

[Explanation of symbols]

101 n + InP substrate 102 n + -InP buffer layer 103 InGaAsP superlattice multiplication layer 104 p-InP electric field relaxation layer 105 p-InGaAs light absorption layer 106 p-InP surface recombination prevention layer 107 p-InGaAs contact layer 108 Bottom electrode 109 top electrode 110 Third electrode 111 Nitride film

Claims (7)

[Claims] 1. An avalanche photodiode having a laminated structure on an InP substrate, wherein the laminated structure is an InP buffer layer having the same conductivity as the InP substrate, and is intentionally formed above the buffer layer. And an InAlAs layer grown on the InP substrate with a lattice constant deviation of 0.4% or less, and on the InP substrate with a lattice constant deviation of 0.4% or less and an energy gap. Is greater than 0.8 eV InGaAsP
A carrier multiplication layer composed of a superlattice in which layers are alternately laminated, and a composition lattice-matched to InP formed above the carrier multiplication layer and absorbing light to generate electron-hole pairs,
An avalanche photodiode having a layer having a main carrier having a polarity different from that of the InP substrate, and the light receiving region having the laminated structure is formed in an island (mesa) shape having a trapezoidal cross section. 2. The amount of current generated by an optical signal is controlled by providing one or a plurality of independent third electrodes on the side surface of the light absorption layer of the light-receiving portion formed in a mesa shape. The avalanche photodiode according to claim 1. 3. The electrode and InGaAs in the third electrode section
The avalanche photodiode according to claim 2, wherein a Schottky junction is formed at the interface with the light absorption layer. 4. The electrode and InGaAs in the third electrode portion.
The avalanche photodiode according to claim 2, wherein a pn junction is formed by providing a thin film semiconductor of a conductivity type different from that of the light absorption layer at an interface with the light absorption layer. 5. The electrode and InGaAs in the third electrode portion
The avalanche photodiode according to claim 2, wherein a thin film dielectric is provided at an interface with the light absorption layer to form a metal-insulator-semiconductor junction. 6. At least the upper plane of the mesa and, if possible, all surfaces other than the side walls of the mesa are covered with a silicon oxide film or a nitride film, and a third electrode is provided on the side surface of the mesa. The avalanche photodiode according to claim 3. 7. A high resistance between at least a semiconductor portion which is in ohmic contact with an electrode formed on the upper surface of the mesa and which is doped with a high concentration of impurities, and a side surface portion of the mesa in a light receiving region portion adjacent to the semiconductor portion. 7. A portion or a void for the purpose of reducing leakage current is provided.
The avalanche photodiode according to any one of 1.