JP4043555B2 - Waveguide type photodetector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a waveguide type photodetector having an element structure capable of high-speed operation and reducing dark current.
[0002]
[Related background]
The waveguide type photodetector generally has an element structure in which a p-type conductive layer and an n-type conductive layer are arranged above and below a light absorption layer having a low carrier concentration to form a PN junction. Then, by applying a reverse voltage between the p-type conductive layer and the n-type conductive layer, the light absorption layer is depleted, and by using a high electric field generated in the depletion layer, from the end face of the light absorption layer It acts to photoelectrically convert the incident optical signal.
[0003]
By the way, the response (response speed) of the photodetector is determined by the CR time constant resulting from the element structure and the travel time of carriers excited by the optical signal. However, the travel time of the excited carriers can be reduced by narrowing the width of the light absorption layer. For example, by reducing the width to 1 μm or less, it can be shortened to a level that can be substantially ignored. Therefore, it can be said that the response depends solely on the CR time constant.
[0004]
[Problems to be solved by the invention]
Now, most of the capacitance component C involved in the CR time constant that affects the response of the photodetector is the capacitance at the PN junction. This capacitance is inversely proportional to the thickness of the PN junction and also proportional to its area. Therefore, in order to suppress the capacitance C, reduce the time constant, and increase the response, for example, the area of the light absorption layer, which is the area of the PN junction, may be reduced. Therefore, conventionally, the PN junction portion including the light absorption layer has a mesa stripe structure, and the width and length of the stripe are shortened.
[0005]
FIG. 1 shows an element structure of a conventional general waveguide type photodetector based on such a viewpoint. That is, the photodetector includes, for example, an n-GaInAsP light guide layer 2 as an n-type conductive layer, an i-GaInAs (P) light absorption layer 3, and a p-type as a p-type conductive layer on an n-InP substrate 1. A GaInAsP light guide layer 4 is laminated in order to form a PN junction sandwiching the light absorption layer 3, and a p-InP cladding layer 5, a p ⁺ -GaInAs (P) layer 6, and a metal layer 7 are sequentially laminated thereon. And having a multilayered film structure. Then, the laminated body is etched into a stripe shape to form a mesa, and an element structure is provided in which a dielectric film 8 is provided on the side surface of the ridge of the mesa.
[0006]
Here, the dielectric film 8 is provided for the purpose of preventing a surface leakage current passing through the side surface of the ridge. However, even if the dielectric film 8 is provided to protect the side surface of the ridge, a leakage current path with an unintended surface state is formed, or a depletion layer from the light absorption layer 3 extends due to charging of the dielectric film 8. Inadvertent leakage current paths may be formed. Such charging of the dielectric film 8 and the occurrence of a leakage current through the path cause an increase in dark current and hinders the response.
[0007]
The present invention has been made in consideration of such circumstances, and its object is to provide a waveguide-type photodetector having a simple element structure capable of high-speed operation and suppressing generation of dark current due to leakage current. Is to provide.
[0008]
[Means for Solving the Problems]
In order to achieve the above-described object, the present invention provides a waveguide type photodetector having a mesa stripe structure in which a light absorption layer having a low carrier concentration is disposed between a p-type conductive layer and an n-type conductive layer to form a PN junction. the dependency, and element structure of the protective layer buried in the side surface of the mesa stripe with the light absorbing layer, in particular, that the protective layer was a dielectric film covering the p-type layer and the p-type layer It is characterized by. That is, according to the present invention, a protective layer composed of a p-type layer and a dielectric film is provided on the side surface of the ridge including the light absorption layer, thereby making it difficult to generate an unintended surface level and depletion layer from the light absorption layer. As an element structure that is difficult to extend, the generation of leakage current is suppressed.
A waveguide-type photodetector having a mesa stripe structure in which a light absorption layer having a low carrier concentration is disposed between a p-type conductive layer and an n-type conductive layer to form a PN junction, and the mesa having the light absorption layer is provided. A waveguide-type photodetector comprising a protective layer embedded in a side surface of a stripe , wherein the protective layer is composed of a p-type layer and a dielectric film covering the p-type layer .
[0009]
In a specific embodiment, the p-type layer of the protective layer is composed of p − InP formed by metal organic chemical vapor deposition .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a waveguide type photodetector according to an embodiment of the present invention will be described with reference to the drawings.
2 to 4 show element structures of the waveguide type photodetectors according to the first to third embodiments of the present invention, respectively. The waveguide type photodetectors according to these embodiments are basically the same as the conventional photodetector shown in FIG. 1, for example, an n-GaInAsP light guide layer 2 on the n-InP substrate 1, i. -GaInAs (P) light absorption layer 3 and p-GaInAsP light guide layer 4 are laminated in this order, and the light absorption layer 3 is sandwiched between an n-type conductive layer and a p-type conductive layer to form a PN junction. A p-InP clad layer 5, a p ⁺ -GaInAs (P) layer 6, and a metal layer 7 are laminated in this order, and then the multilayer film stack is etched into a stripe shape to form a mesa. It has a basic element structure in which the area of (PN junction) is reduced.
[0011]
Thus, in the photodetector according to the first embodiment, the semiconductor protective layer (p-type layer) 11 made of p-InP is formed on the ridge side surface of the mesa stripe as shown in FIG. By forming the film 8, an element structure in which the semiconductor protective layer 11 is embedded in the side surface of the ridge of the light absorption layer 3 is characterized. Incidentally, the growth of the semiconductor protective layer (p-InP) 11 is performed using a metal organic chemical vapor deposition method (MOCVD method). Then, the semiconductor layer 11 is etched off at a portion 2 to 4 μm wider than the width of the mesa stripe, and then the dielectric film 8 covering the whole is formed to protect the ridge side surface.
[0012]
Thus, according to the waveguide type photodetector having the element structure as shown in FIG. 2, the semiconductor protective layer 11 made of p-InP is formed on the side surface of the ridge of the light absorption layer 3, so that the ridge of the light absorption layer 3 is formed. Undesirable surface levels are unlikely to occur on the side surfaces, and leakage current is difficult to flow. In addition, since the dielectric film 8 is separated from the ridge side surface of the light absorption layer 3 through the semiconductor protective layer 11, the dielectric film 8 is difficult to be charged. Even if the dielectric film 8 is charged, since the dielectric film 8 is separated from the ridge side surface, the depletion layer from the light absorption layer 3 does not extend along the ridge side surface. Accordingly, it is possible to suppress the generation of unintentional leakage current, reduce the dark current, and improve the response. Therefore, by adopting a simple device structure in which the semiconductor protective layer 11 is provided on the ridge side surface of the mesa stripe including the light absorption layer 3, the waveguide type photodetector can be operated at high speed, and the dark current is effectively reduced. It is possible to increase the sensitivity while suppressing the pressure to a minimum.
[0013]
Further, in the photodetector according to the second embodiment shown in FIG. 3, si (semi-insulating) -InP is provided on the ridge side surface of the stripe including the light absorption layer 3 instead of the semiconductor protective layer 11 made of p-InP. An element structure in which a semiconductor protective layer 12 made of is embedded is formed.
Even in the waveguide type photodetector having such an element structure, as in the photodetector shown in the previous embodiment, generation of unintended surface levels on the ridge side surface of the light absorption layer 3 is suppressed, and Since the dielectric film 8 can be prevented from being charged, it is possible to increase the operation speed and reduce the dark current. In particular, when the semiconductor protective layer 12 made of si-InP is embedded, the semiconductor protective layer 12 need not be etched off, and the manufacturing process can be simplified. That is, as shown in FIG. 3, since the semiconductor protective layer 12 embedded in the side surface of the ridge is semi-insulating, it is not necessary to etch off the skirt 12a of the semiconductor protective layer 12. In addition, since the semiconductor protective layer 12 itself can effectively protect the ridge side surface of the stripe, the formation of the dielectric film 8 can be omitted.
[0014]
As shown in the third embodiment of the present invention in FIG. 4, prior to embedding the semiconductor protective layer 13 made of InP on the ridge side surface of the light absorption layer 3, for example, it is made of GaP or GaInP having a large energy gap Eg. An element structure in which the semiconductor layer 14 is embedded in a thickness equal to or less than the critical film thickness may be employed. If such a device structure provided with a protective layer having a multilayer structure in which the semiconductor layer 14 made of GaP or GaInP and having a large energy gap Eg is embedded together with the semiconductor protective layer 13 made of InP is provided, the depletion occurs along the interface. Even if a layer is formed, the semiconductor layer 14 made of GaP or GaInP has a wide energy gap, so that a tunnel current does not flow even when a high electric field is applied to the depletion layer. Therefore, it is possible to effectively suppress the generation of leakage current, and it is possible to realize a photodetector capable of high-speed operation with reduced dark current.
[0015]
That is, according to the waveguide type photodetector of the element structure according to the present invention, the dielectric film 8 that protects the ridge side surface of the mesa stripe including the light absorption layer 3 is prevented from being charged, and the surface level of the unintentional surface state is prevented. Since generation | occurrence | production is suppressed, it can prevent effectively that the depletion layer from the light absorption layer 3 extends along a ridge side surface. As a result, it is possible to suppress the generation of leakage current due to the tunnel current through the depletion layer, and to achieve high speed operation and high sensitivity.
[0016]
In addition, this invention is not limited to each embodiment mentioned above. For example, the same effect can be achieved not only when the stripe including the light absorption layer 3 is formed in a forward mesa shape but also when formed in a reverse mesa shape. The width of the mesa stripe and the like may be determined according to the specification. In short, the present invention can be implemented with various modifications without departing from the gist thereof.
[0017]
【The invention's effect】
As described above, the waveguide photodetector according to the present invention has a mesa stripe side surface in which a light absorption layer having a low carrier concentration is disposed between a p-type conductive layer and an n-type conductive layer to form a PN junction. In addition, an element structure in which a p-type conductive semiconductor layer, a semi-insulating semiconductor layer, or a multilayer semiconductor protective layer including a semiconductor layer having a large energy gap and a semi-insulating semiconductor layer is embedded is formed. Therefore, according to the present invention, it is possible to suppress the generation of a leakage current involving an unintended surface level, to prevent the dielectric film that protects the ridge side surface from being charged, and to prevent the generation of a depletion layer along the ridge side. be able to. As a result, it is possible to easily realize a photodetector that suppresses dark current and achieves high speed operation.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic element structure of a conventional general waveguide type photodetector.
FIG. 2 is a diagram showing a schematic element structure of a waveguide type photodetector according to the first embodiment of the present invention.
FIG. 3 is a diagram showing a schematic element structure of a waveguide type photodetector according to a second embodiment of the present invention.
FIG. 4 is a diagram showing a schematic element structure of a waveguide type photodetector according to a third embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 n-InP board | substrate 2 n-GaInAsP light guide layer 3 i-GaInAs (P) light absorption layer 4 p-GaInAsP light guide layer 5 p-InP clad layer 6 p ⁺ -GaInAs (P) layer 7 Metal layer 11 Semiconductor protection Layer (p-InP)
12 Semiconductor protective layer (si-InP)
13 Semiconductor protective layer (si-InP)
14 Semiconductor layer with large energy gap (GaP and GaInP)

Claims (2)

A waveguide-type photodetector having a mesa stripe structure in which a light absorption layer having a low carrier concentration is disposed between a p-type conductive layer and an n-type conductive layer to form a PN junction, and the mesa having the light absorption layer is provided. A waveguide-type photodetector comprising a protective layer embedded in a side surface of a stripe , wherein the protective layer is composed of a p-type layer and a dielectric film covering the p-type layer . The p-type layer is a p-type formed by metalorganic vapor phase epitaxy. - The waveguide type photodetector according to claim 1, which is made of InP.

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