Patent Description:
An avalanche photodiode (APD) is a type of photodiode (PD) utilizing a phenomenon called avalanche multiplication in which photocurrent is amplified by an intense electric field within a semiconductor layer, and is a light receiving element capable of achieving a higher receiving sensitivity compared to a typical photodiode due to its high photoelectric conversion efficiency, which is widely applied to an image sensor and optical communication. In particular, in optical communication applications, as compared to an optical receiver using a typical PD, in the case of an optical receiver using an APD (APD optical receiver), a transmission distance of an optical signal can be extended, so that the APD optical receiver is widely used in metro-access applications.

Examples of a trade-off that dominates receiving sensitivity performance of the APD optical receiver in optical communications include a trade-off between a gain and a bandwidth of the APD and a trade-off between a gain and an excessive noise of the APD. The trade-off between a gain and a bandwidth is often known as a gain-bandwidth product (GBP) of the APD (NPL <NUM>). An operating band of the APD decreases along with an increased gain of the APD, and thus improvement in reception sensitivity as an APD optical receiver saturates when the operating band becomes insufficient for a modulation rate applied to the APD.

In addition, noise current also rises along with the increased gain of the APD (NPL <NUM>). This excessive noise specific to the APD is caused by an amplification process of carriers in a multiplication layer that amplifies photocurrent inside the APD, and an extent of noise is determined by a ratio of an electron collision ionization rate to a hole collision ionization rate (ionization rate ratio) specific to a material of the multiplication layer. Accordingly, in order to obtain characteristics of a high gain and low noise in the APD, it is necessary to carefully select the material of the multiplication layer.

For example, for an APD composed of a Group III to V compound semiconductor, InP and InAlAs have been used as the multiplication layer due to a constraint that InP and InAlAs are lattice-matched with an InP substrate and an InGaAs light absorbing layer. In particular, InAlAs is widely used as the material of the multiplication layer of an APD having a high speed and a high sensitivity because InAlAs has a smaller ionization rate ratio compared to InP. Furthermore, in recent years, a material of the multiplication layer exhibiting an ionization rate ratio further smaller than that of InAlAs and a low noise property, such as InAlAsSb or Si, has also been developed (NPLs <NUM>, <NUM>).

In the meantime, the APD is an element that operates by applying a high voltage as compared to the typical PD and inducing a high electric field concentrically in the multiplication layer. Thus, it is generally difficult to achieve a normal operation and to ensure long-term reliability, as compared to the typical PD.

In terms of reliability, it is a requirement that no electric field be generated on the side surface of the element of the APD (NPL <NUM>). Due to this, the APD has a structure called an "electric field constriction structure" in which a process technique such as selective diffusion, ion implantation, or the like is used to induce a high electric field intensity in the center of an element while the electric field on the side surface of the element is kept weak.

On the other hand, when this electric field constriction structure is used, electric field concentration may occur at the end of the constriction portion to generate multiplication locally in the APD. This phenomenon is often referred to as "edge breakdown" (NPL <NUM>, <FIG>). When edge breakdown occurs, the breakdown occurs at a voltage lower than that when the gain of the APD in the multiplication layer becomes sufficiently large, and thus, even if a material with low excessive noise is used as the multiplication layer, APD operation cannot be performed with a large gain.

In this way, in order to achieve an APD having high performance and high reliability, it is necessary to use a multiplication layer material with low noise and to employ an electric field constriction structure; however, there is a problem in that edge breakdown occurs due to the employment of the electric field constriction structure, as a result of which the APD cannot operate in a high gain state and it is difficult to heighten the sensitivity of an APD receiver.

Further background is disclosed in <CIT> "Avalanche photodiode".

As described in the Background Art, in the APD for optical communications, it is important to select a material of the multiplication layer such that excessive noise is reduced even in a high gain state. On the other hand, in order to increase the reliability of the APD, it is necessary to employ the electric field constriction structure such that no electric field is applied to the side surface of the element.

However, in the APD in the related art, when the electric field constriction structure is employed, edge breakdown makes it impossible to sufficiently increase a multiplication rate in the APD, and thus, even when a multiplication layer with low excessive noise is applied, it is impossible to sufficiently improve a reception sensitivity as the APD receiver, which is a problem.

In view of such a problem, an object of the present invention is to provide an element structure that can operate in a high gain state while having high reliability and low noise property in an avalanche photodiode.

An aspect of the present invention includes the following configurations in order to achieve the object as described above.

An avalanche photodiode as defined by the subject-matter of claim <NUM>.

The avalanche photodiode according to Configuration <NUM>, wherein
the electric field relaxation layer has a film thickness of <NUM> or greater, and is doped in a concentration range of <NUM>×<NUM><NUM> cm-<NUM> to <NUM>×<NUM><NUM> cm-<NUM>.

The avalanche photodiode according to Configuration <NUM>, wherein
an electric field intensity of the electric field relaxation layer is <NUM> kV/cm or less in an operating state.

The avalanche photodiode according to Configuration <NUM>, wherein
an area of the electric field relaxation layer is an area of the multiplication layer or less and greater than an area of the first semiconductor contact layer.

The avalanche photodiode according to any one of Configurations <NUM> to <NUM>, the avalanche photodiode having
a backside illumination structure in which light to be detected is incident from a back surface side of a substrate.

The avalanche photodiode having the backside illumination structure according to Configuration <NUM>, the avalanche photodiode having
a structure in which a mirror is disposed on a front surface side of the substrate to reflect light transmitted through the light absorbing layer and the reflected light is again incident on the light absorbing layer.

As described above, according to the present invention, it is possible to provide an avalanche photodiode that can operate in a high gain state while having high reliability and low noise property.

A structure of a first embodiment of the present invention will be described with reference to <FIG>. The first embodiment is a basic structure in the present invention.

<FIG> is a cross-sectional view of a substrate of an avalanche photodiode (APD) <NUM> for describing the first embodiment. In the first embodiment, the APD <NUM> has a layered structure in which a P-type InP contact layer <NUM>, an undoped InGaAs light absorbing layer <NUM>, a P-type InAlAs electric field control layer <NUM>, an InAlAs multiplication layer <NUM>, an N-type InAlAs electric field control layer <NUM>, an InP electric field relaxation layer <NUM>, and an InAlAs cap layer <NUM> are layered in this order on an InP substrate <NUM>.

In addition, local doping is performed by a Si ion implantation method to form an N-type contact region (layer) <NUM> in a center of the InAlAs cap layer <NUM> which is the top layer, and the N-type contact region <NUM> is connected to a metal electrode <NUM>, to which a voltage is applied. The P-type InP contact layer <NUM> which is a lower layer is also connected to a metal electrode <NUM> at an appropriate location, to which a voltage is applied.

Generally, because a photodiode usually operates in a reverse-biased state, the metal electrode <NUM> connected to the N-type contact region <NUM> is biased to a high potential and the metal electrode <NUM> connected to the P-type InP contact layer <NUM> is biased to a low potential to generate an electric field in the layered structure.

Because the N-type contact region <NUM> is selectively doped in the cap layer <NUM>, an area of the contact region is smaller than an area of the multiplication layer <NUM> or the like and forms an electric field constriction structure.

It applies to the following embodiments that the same operation can be performed by reversing the polarity of an operating voltage even when the semiconductor type (P/N polarity) is reversed. Thus, in order to form the electric field constriction structure in the avalanche photodiode, an area of either of the two contact layers, that is, the first and second contact layers having different semiconductor types (P/N polarity), needs to be smaller than the area of the multiplication layer or the like between both the contact layers, and generality is not lost even if a contact layer having a smaller area is used as the first contact layer.

It is also applicable to the following embodiments that in general, an incident direction of light received by a photodiode may be any direction as long as the light can reach the light absorbing layer to generate photocarriers, and the incident direction of light is not limited.

For example, in a backside illumination structure, the avalanche photodiode can have a structure in which light to be detected is incident vertically upward from the back surface side (bottom) of the substrate to the substrate surface. In the backside illumination structure, it is possible to increase a detection efficiency by a structure in which a mirror is disposed on the front surface side of the substrate to reflect light transmitted through the light absorbing layer, and the reflected light is incident again on the light absorbing layer. It is also possible to make a structure in which light incident in a substrate in-plane direction (e.g., in a direction perpendicular to the sheet surface of the drawing) is received from other optical circuits, optical waveguides, optical fibers, or the like provided in the substrate.

An operation of a light receiving element according to the first embodiment in <FIG> will be described. When a reverse bias voltage is applied to the APD <NUM>, an electric field intensity of the InAlAs multiplication layer <NUM> first increases, and at the same time, depletion of the N-type and P-type InAlAs electric field control layers <NUM>, <NUM> disposed above and below the InAlAs multiplication layer <NUM> proceeds. After the N-type electric field control layer <NUM> has been depleted, an electric field intensity of the InP electric field relaxation layer <NUM> increases for a further applied voltage. Similarly, after the P-type electric field control layer <NUM> has been depleted, an electric field intensity of the InGaAs light absorbing layer <NUM> increases for a further applied voltage.

In order for the APD <NUM> to operate as a light receiving element, a photocarrier generated in the InGaAs light absorbing layer <NUM> needs to obtain a drift component by the electric field in the InGaAs light absorbing layer <NUM> to be injected to the multiplication layer <NUM>. The injection depletes the P-type electric field control layer <NUM> and generates an electric field in the InGaAs light absorbing layer <NUM>. The applied reverse bias voltage at this time is an ON voltage (Von) in this APD.

After the P-type and N-type electric field control layers <NUM>, <NUM> have been depleted, the electric field intensities continue to increase in all layers of the absorbing layer <NUM>, the multiplication layer <NUM>, and the electric field relaxation layer <NUM> for a further applied voltage. Basically, an applied voltage at which the electric field intensity of the multiplication layer <NUM> becomes high to an extent that a gain of the APD <NUM> is sufficiently increased is a breakdown voltage (Vb) of the APD <NUM>.

However, in the case of the first embodiment, electric field concentration occurs in a portion corresponding to an edge of the N-type contact layer <NUM> (portions surrounded by dotted lines in <FIG>). This locally heightening electric field is referred to as an edge electric field. When the edge electric field extends to the multiplication layer <NUM>, a multiplication rate locally increases in the multiplication layer <NUM> or a local tunnel current due to the edge electric field occurs, whereby a dark current (current flowing even in the absence of light input) rises suddenly at a voltage lower than that at which a sufficient gain is obtained as the APD, leading to breakdown.

In the first embodiment, by providing the electric field relaxation layer <NUM> between the multiplication layer <NUM> and the N-type contact layer <NUM> as illustrated in <FIG>, it is possible to spatially separate a portion having the largest edge electric field near the N-type contact layer <NUM> and the multiplication layer <NUM>. Thus, it is possible to suppress local increase of an electric field in the multiplication layer <NUM>.

In addition, by providing the N-type electric field control layer <NUM>, even at the operating voltage of the APD <NUM>, it is possible to reduce the electric field intensity of the entire electric field relaxation layer <NUM>. In this way, although the edge electric field is inevitably generated in the electric field relaxation layer <NUM>, the intensity of the edge electric field can be reduced to an extent that local tunneling current and local multiplication are not generated in the electric field relaxation layer <NUM>.

In order to suppress an influence of the edge electric field caused by the N-type contact layer <NUM> on the multiplication layer <NUM>, the electric field relaxation layer <NUM> needs to have a certain thickness. Eventually, the thickness of the electric field relaxation layer <NUM> is designed depending on a desired band. The film thickness is desirably <NUM> or greater, and the electric field relaxation layer <NUM> is desirably depleted at the operating voltage. In addition, when the film thickness is greater, the effect of relaxation of the edge electric field is more expected, and the greater film thickness contributes to reduction in element capacity. On the other hand, a traveling delay of a carrier increases conversely, which limits the operating speed of the element. Accordingly, the maximum film thickness is designed depending on a desired operating speed.

According to the above-described principle, with the configuration of the first embodiment, it is possible to suppress edge breakdown even in a state where the gain of the APD is large, so that the sensitivity of the APD optical receiver can be improved.

A structure of a second embodiment will be described with reference to <FIG> is a cross-sectional view of a substrate of an avalanche photodiode (APD) <NUM> illustrating the second embodiment. In the second embodiment, generally, layering is performed in a reverse order to that of the first embodiment. Here, in the APD <NUM>, an N-type contact region <NUM> is formed by growing an InP cap layer <NUM> on an N-type InP substrate <NUM>, and doping the center portion thereof locally by an Si ion implantation method. The N-type contact region <NUM> is connected to an electrode <NUM> via the N-type InP substrate <NUM>.

In the APD <NUM>, then, an N-type InAlAs electric field relaxation layer <NUM>, an N-type InAlAs electric field control layer <NUM>, an InAlAs multiplication layer <NUM>, a P-type InAlAs electric field control layer <NUM>, an InGaAs light absorbing layer <NUM>, and a P-type InP contact layer <NUM> are grown in this order and layered in a mesa structure, and the P-type InP contact layer <NUM> is connected to an electrode <NUM>.

An operation principle of a light receiving element according to the second embodiment is basically the same as that of the first embodiment except that a direction of the electric field is reversed in response to the order of the layers opposite to that of the first embodiment. When a voltage is applied to the APD <NUM>, an electric field intensity of the InAlAs multiplication layer <NUM> first increases, and at the same time, depletion of the N-type and P-type InAlAs electric field control layers <NUM>, <NUM> disposed above and below the InAlAs multiplication layer <NUM> proceeds. After the P-type electric field control layer <NUM> has been depleted, an electric field intensity of the InGaAs light absorbing layer <NUM> increases for a further applied voltage.

On the other hand, after the N-type electric field control layer <NUM> has been depleted, depletion of the N-type electric field relaxation layer <NUM> subsequently proceeds. Due to this, after the P-type and N-type electric field control layers have been depleted, the electric field intensities of the InAlAs multiplication layer <NUM> and the InGaAs absorbing layer <NUM> continue to increase for voltage application, but increase in the electric field intensity of the N-type InAlAs electric field relaxation layer <NUM> becomes very gentle for an applied voltage.

Accordingly, even when an edge electric field caused by the N-type contact layer <NUM> occurs, the electric field intensity of the N-type electric field relaxation layer <NUM> is suppressed to a very small level, and thus the edge electric field does not affect the electric field relaxation layer <NUM>.

Here, an impurity doping concentration of the electric field relaxation layer <NUM> needs to be carefully designed. When the concentration is too high, the electric field relaxation layer <NUM> is not depleted even at the operating voltage of the APD <NUM>, and an effect of electric field constriction on the inside of the element by the N-type contact layer <NUM> is not exhibited. On the other hand, when the concentration is too low, the N-type electric field relaxation layer <NUM> is immediately depleted for an applied voltage after depletion of the N-type electric field control layer <NUM>, and the electric field intensity within the electric field relaxation layer <NUM> readily increases.

In order to mitigate the edge electric field while the effect of the concentration described above is avoided and the APD operation is properly ensured, the impurity concentration only needs to be about <NUM>×<NUM><NUM> cm-<NUM> to <NUM>×<NUM><NUM> cm-<NUM>, and the electric field intensity of the electric field relaxation layer <NUM> at a desired operating voltage of the APD <NUM> only needs to be <NUM> kV/cm.

According to the above-described principle, with the configuration of the second embodiment, it is possible to suppress edge breakdown even in a state where the gain of the APD is large, so that the sensitivity of the APD optical receiver can be improved.

A structure of a first example will be described with reference to <FIG> is a cross-sectional view of a substrate of an avalanche photodiode (APD) <NUM> illustrating the first example. In the first example, unlike the first and second embodiments, the APD <NUM> is formed based on silicon (Si) rather than a compound semiconductor, and germanium (Ge) is used for a light absorbing layer <NUM>.

In the APD <NUM> in <FIG>, an N-type Si contact layer <NUM>, an Si multiplication layer <NUM>, a P-type Si electric field control layer <NUM>, an Si electric field relaxation layer <NUM>, a Ge light absorbing layer <NUM>, and a P-Ge contact layer <NUM> are grown in this order on a Si substrate <NUM> and layered in a mesa structure, and the P-Ge contact layer <NUM> is connected to an electrode <NUM>.

An operation principle of a light receiving element according to the first example is basically the same as that of the first embodiment except for a direction of an electric field and a structure of each layer. When a voltage is applied to the APD <NUM>, an electric field intensity of the Si multiplication layer <NUM> first increases, and at the same time, depletion of the P-type Si electric field control layer <NUM> disposed as the upper layer thereof proceeds. After the P-type Si electric field control layer <NUM> has been depleted, electric field intensities of the Si electric field relaxation layer <NUM> and the Ge light absorbing layer <NUM> increase for a further applied voltage.

Accordingly, even when an edge electric field caused by the Ge absorbing layer <NUM> occurs, the Si multiplication layer <NUM> is not affected by the edge electric field, and an electric field intensity of the Si electric field relaxation layer <NUM> is suppressed to a very small level, so that the edge electric field does not affect the electric field relaxation layer.

According to the above-described principle, with the configuration of the first example, it is possible to suppress edge breakdown even in a state where the gain of the APD is large, so that the sensitivity of the APD optical receiver can be improved.

A structure of a second example will be described with reference to <FIG> is a cross-sectional view of a substrate of an avalanche photodiode (APD) <NUM> illustrating the second example. In the second example, generally, the PN polarity is reversed without changing the order of layers of the first embodiment, a direction of the electric field is opposite to that of the first embodiment (<FIG>), and the layer configuration is a so-called multi-stage mesa structure.

In the APD <NUM> in <FIG>, an N-type InP contact layer <NUM>, a InGaAs light absorbing layer <NUM>, an N-type InP electric field control layer <NUM>, an InP multiplication layer <NUM>, a P-type InP electric field control layer <NUM>, an InAlAs electric field relaxation layer <NUM>, and an InAlAs cap layer <NUM> are grown in this order on an InP substrate <NUM> and layered in a mesa structure.

After crystal growth, a doping region is selectively formed in the center portion of the InAlAs cap layer <NUM> by a Zn selective diffusion method to make a P-type contact layer <NUM> which is connected to an electrode <NUM>. In addition, etching is performed up to the P-type InP electric field control layer <NUM> by typical wet etching to form a two-step mesa.

An operation principle of a light receiving element according to the second example in <FIG> will be described. When a voltage is applied to the APD <NUM> of the present embodiment, an electric field intensity of the InP multiplication layer <NUM> first increases, and at the same time, depletion of the P-type and N-type InP electric field control layers <NUM>, <NUM> disposed as the upper and lower layers thereof proceeds. After the P-type electric field control layer <NUM> has been depleted, electric field intensities of the InAlAs electric field relaxation layer <NUM> and the InGaAs light absorbing layer <NUM> increase for a further applied voltage.

Similarly to the above embodiments, the electric field intensity of the InAlAs electric field relaxation layer <NUM> is kept small to suppress edge breakdown of the electric field relaxation layer <NUM> itself, and at the same time, the P-type contact layer <NUM> and the InP multiplication layer <NUM> are spatially separated to suppress edge breakdown at the multiplication layer <NUM>. Furthermore, the electric field relaxation layer <NUM> is composed of InAlAs having a large band gap (<NUM> eV), and thus an effect of edge breakdown in the InAlAs electric field relaxation layer <NUM> can be eliminated.

In addition, in the present APD <NUM>, the P-type InP electric field control layer <NUM> is not present above a peripheral portion of the InP multiplication layer <NUM>. Due to this, in the APD <NUM> of the second example, even if a voltage is applied, the electric field does not rise around or on the side surface of a lower mesa including the multiplication layer <NUM> and the absorbing layer <NUM> in principle. As a result, side dark current causing dark current in the light receiving element can be suppressed.

According to the above-described principle, with the configuration of the second example, it is possible to suppress edge breakdown even in a state where the gain of the APD is large, and to reduce the side dark current. As a result, it is possible to improve the sensitivity of the APD optical receiver and the reliability thereof.

A structure of a third example will be described with reference to <FIG> is a cross-sectional view of a substrate of an avalanche photodiode (APD) <NUM> illustrating the third example. The third example is an embodiment in which the P-type InP electric field control layer is removed in the second example to simplify the configuration.

An operation principle of a light receiving element according to the third example will be described. When a voltage is applied to the APD <NUM> of the third example in <FIG>, an electric field intensity of an InP multiplication layer <NUM> first increases, and at the same time, an electric field intensity of an InAlAs electric field relaxation layer <NUM> also increases. In addition, depletion of an N-type InP electric field control layer <NUM> disposed as a lower layer thereof proceeds. After the N-type electric field control layer <NUM> has been depleted, an electric field intensity of an InGaAs light absorbing layer <NUM> increases for a further applied voltage.

In the case of the third example, the electric field intensity of the InGaAs light absorbing layer <NUM> is kept small at an operating voltage of the APD <NUM>, but an electric field intensity of the electric field relaxation layer <NUM> increases similarly to that of the multiplication layer <NUM>. However, when a material having a band gap energy higher than that of the multiplication layer <NUM> (e.g., <NUM> eV or greater) is intentionally used as a semiconductor material constituting the electric field relaxation layer <NUM>, the electric field relaxation layer <NUM> can be configured to have a higher breakdown voltage than that of the multiplication layer even with the same electric field intensity.

As a result, even if an edge electric field caused by the P-type contact layer <NUM> occurs, it is possible to suppress edge breakdown in the multiplication layer <NUM> while eliminating an influence of the edge electric field in the InAlAs electric field relaxation layer <NUM>.

In addition, as can be seen in <FIG>, in the third example, the InAlAs electric field relaxation layer <NUM> is set to have an area smaller than that of the InP multiplication layer <NUM> and greater than that of the P-type contact region <NUM>. As a result, even in the operating voltage of the APD <NUM>, no high electric filed occurs on the side surfaces of the electric field relaxation layer <NUM> and the multiplication layer <NUM>, so that the side dark current can be reduced and element degradation from the side surface can be suppressed.

According to the above-described principle, with the configuration of the third example, it is possible to suppress edge breakdown even in a state where the gain of the APD is great, so that the side dark current can be reduced. As a result, it is possible to improve the sensitivity of the APD optical receiver and the reliability thereof.

Claim 1:
An avalanche photodiode comprising:
a first semiconductor contact layer (<NUM>), formed in a center of a cap layer (<NUM>) by local doping, in contact with a first electrode (<NUM>);
a second semiconductor contact layer (<NUM>) in contact with a second electrode (<NUM>); and
a multiplication layer (<NUM>) formed between the first semiconductor contact layer (<NUM>) and the second semiconductor contact layer (<NUM>), wherein the first semiconductor contact layer (<NUM>) has smaller area than an area of the multiplication layer (<NUM>);
a N-type InAlAs electric field control layer (<NUM>) and a P-type InAlAs electric field control layer (<NUM>) formed above and below and adjacent to the multiplication layer (<NUM>);
a light absorbing layer (<NUM>) formed between and adjacent to the electric field control layer (<NUM>) and the second semiconductor contact layer (<NUM>); and,
an electric field relaxation layer (<NUM>) formed between and adjacent to the first semiconductor contact layer (<NUM>) and the N-type electric field control layer (<NUM>) and configured to be depleted at an operating voltage between the first semiconductor contact layer (<NUM>) and the multiplication layer (<NUM>),
wherein
the first semiconductor contact layer (<NUM>) consists of N-type InAlAs,
the second semiconductor contact layer (<NUM>) consists of P-type InP,
the multiplication layer (<NUM>) consists of InAlAs,
the light absorbing layer (<NUM>) consists of undoped InGaAs,
the cap layer (<NUM>) consists of InAlAs, and
the electric field relaxation layer (<NUM>) consists of InP.