JPH0258792B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a semiconductor photodetecting element having a mesa-type heterojunction. In the field of semiconductor photodetectors, one of the important issues is to increase quantum efficiency and improve voltage-current characteristics. One way to solve this problem is to reduce dark current by using a semiconductor layer with a narrow bandgap for light reception and forming the P-n junction in a semiconductor layer with a wider bandgap. There is an element that attempts to do so. FIG. 1 is a cross-sectional view of such a conventional mesa structure diode having a heterojunction. A first semiconductor layer P ⁺ -InP formed on the P + -INP substrate 10 A second semiconductor layer n ^- -InP formed on the P ⁺ -InP layer 11
Form layer 12 and further form a third semiconductor layer n ^- -
It has a structure in which an InGaAs layer 13 is formed, 15 is a ring electrode on the light receiving window side, 16 is the other electrode,
Each is shown schematically. In such a conventional structure, the depletion layer (the front surface of which is indicated by a broken line 14) is curved near the mesa wall, and this portion forms a third layer having a narrow forbidden band width.
As a result, the voltage-current characteristics, especially the breakdown characteristics, are deteriorated. The present invention was made in order to eliminate such drawbacks of the conventional structure and reduce microplasma with low dark current _.
A second semiconductor layer exhibiting a second conductivity type opposite to the first conductivity type and having an impurity concentration of N ₂ is laminated on the first semiconductor layer, and further on this second semiconductor layer. exhibiting a second conductivity type and having a forbidden band width narrower than at least the semiconductor constituting the second semiconductor layer;
A mesa-type semiconductor photodetecting element having a structure in which a third semiconductor layer made of a semiconductor is laminated, wherein the interface between the second semiconductor layer and the third semiconductor layer is located at the center of the mesa. It is formed so as to protrude toward the second semiconductor layer side, and furthermore, the first semiconductor layer
An effective length d ₁ from the interface between the semiconductor layer and the second semiconductor layer to the protruding interface, and a thickness d ₂ of the second semiconductor layer other than the protrusion.

【formula】

[Formula] (where q is the electron charge, ξ ₂ is the dielectric constant of the second semiconductor layer, and V is the breakdown voltage determined by the second semiconductor layer.) A semiconductor photodetecting element is provided. The breakdown voltage V can be expressed by the following relationship: V60×(Eg ² /1.1) ^3/2 ×(N ² /10 ¹⁶ ) ^−3/4 . Here, Eg ² indicates the forbidden band width of the second semiconductor layer. In the present invention d1
and used as a limiting criterion for d2.

[Equation] represents the width of the depletion layer at the time of breakdown, and if d1 is larger than that, the depletion layer will not reach the second semiconductor layer that should receive light in the protrusion, which is disadvantageous. Also d2

If the equation below is satisfied, the front of the depletion layer will completely enter the third semiconductor layer having a narrow forbidden band near the mesa wall surface at the time of breakdown, and the advantages of the present invention will no longer be obtained. With this structure, the curved part of the depletion layer near the mesa wall remains in the second semiconductor layer with a wide forbidden band width, and only the flat part at the front of the depletion layer remains in the third semiconductor layer with a narrow forbidden band width. It can be spread into layers. This makes it possible to improve the dark current characteristics, which was a drawback of the conventional structure, and not only that, but also as can be seen from the equation giving V above, a semiconductor layer with a narrow bandgap is a semiconductor layer with a wide bandgap. Taking advantage of the fact that breakdown occurs at a lower voltage than that of the third semiconductor layer, breakdown occurs only in the protrusion of the third semiconductor layer in the center of the mesa, thereby preventing breakdown in areas other than the light receiving area. In addition, it reduces microplasma due to electric field concentration at the curved part of the depletion layer near the mesa wall, and also improves operational stability because the breakdown part light-receiving part is not exposed to the surface unlike conventional structures. However, instability due to surface current, which is a drawback of conventional mesa structures, is also reduced. In addition, a highly multiplied avalanche photo diode can be obtained by reducing dark current and reducing microplasma. Here the third
There is no need to particularly limit the shape of the semiconductor layer, the mesa angle, etc. There is no clear limit on the upper limit of d2 as it is determined by manufacturing technology.

[Formula] degree should be the standard. Further, in the present invention, the difference in forbidden band width between the second semiconductor layer and the third semiconductor layer is utilized. If for some reason there is not a large difference between the forbidden band widths of the two, the effect can be further improved by making the impurity concentration of the third semiconductor layer higher than that of the second semiconductor layer. . Next, one embodiment of the present invention will be specifically described. FIG. 2 is a sectional view of a semiconductor photodetecting element to which the present invention is applied. A P ^{+ -InP} layer (N11×10 ¹⁹
(cm ^-3 )) 21 was grown to a thickness of 3 μm to form a buffer layer, and then an n ^- -InP layer (N21.2×10 ¹⁶ (cm ^-3 )) 22 was grown using the liquid phase or vapor phase growth method in the same manner as 21. 4μ
After growing to a thickness of m and going through processes such as photoresist treatment, a (concavity) with a diameter of 100 μm and a depth of 2.5 μm is formed.
Then, by liquid phase or vapor phase growth method, n ^- -
An InGaAs layer 23 (with impurity concentration of 1.5×10 ¹⁶ (cm ^-3 )) was grown, aligned so that the (concave portion) was at the center of the mesa, and a mesa structure with a diameter of 200 μm was formed by etching. Through the above steps, the n ^- -InGaAs layer protrudes toward the n ^- -InP layer at the center of the mesa shape. Ohmic electrode 2 on the n side
5 used Au-Ge as a ring electrode. The ohmic electrode 26 on the P side was made of Au--Zn. The reason why a window is provided in the ohmic electrode 26 is that the quantum efficiency will not be good if light enters from the InP side when surface recombination is taken into account. Due to this structure, only the central part of the mesa of the depletion layer 24
The curved part of the depletion layer near the mesa wall remains in the InP layer, which improves dark current characteristics and reduces microplasma compared to the conventional mesa structure.
It showed harder breakdown characteristics. These improvements also improved the multiplication factor and operational stability. Here, for InP with a concentration of 1.2×10 ¹⁶ (cm. ³ )

Calculating the value of [Formula], it is approximately 3 μm, but if d1 is set to a value just below 3 μm, the depletion layer
Since the InGaAs layer cannot be wide enough, the quantum efficiency will be low, and conversely, if d1 is close to 0, the depletion layer will be
Dark current increases because it spreads in the InGaAs layer at a low voltage. Further, as an improvement to the above embodiment, an InGaAsP layer (not shown) is further provided on the InGaAs layer 23,
When the window layer was used, even if the light was incident from the InGaAsP side, the problem of surface recombination was improved and the quantum efficiency was significantly improved. FIG. 3 is also a sectional view of a semiconductor photodetecting element to which the present invention is applied. After forming a mesa structure similar to the embodiment shown in FIG. 2, CD diffusion is performed on the mesa wall surface to form a P ⁺ -InP layer 31.
A mesa structure having a diameter of 150 μm was formed by etching only the InGaAs layer 23, which was formed to a thickness of 10 μm and aligned so that the protrusion was at the center of the mesa. Due to this structure, the depletion layer stretches as shown by the broken line 24, and breakdown occurs only at the protruding part of the InGaAs layer 23, and the curved part at the end of the depletion layer 24 remains in the InP layer 22. Therefore, in the embodiment shown in FIG. This structure has the same effect as the above structure, but since the curved portion of the depletion layer spreads along the InP surface 32 that appears by etching, this structure has the advantage that a thick n ^- -InP layer 22 is not required compared to the above structure. There is. Here, the conductivity types are reversed so that the first semiconductor layer is composed of a P ⁺ -InP layer, the second semiconductor layer is composed of a P ^- -InP layer, and the third semiconductor layer is composed of a P ^- - InP layer.
It goes without saying that the present invention is also effective when constructed of InGaAs. However, since the ionization rates of holes and electrons differ depending on the semiconductor, it is necessary to cause avalanche in the one with the higher ionization rate to reduce noise. In the case of the example, since the avalanche region is provided in InP, the ionization rate of holes is higher than that of electrons in the case of InP. Therefore, the combination of the example, that is, the first semiconductor layer is P ⁺ -InP
The noise characteristics are better when the second semiconductor layer is made of n ^-- InP and the third semiconductor is made of n-InGaAs. Above, we have described an example in which InP is used as a semiconductor with a wide forbidden band width, and InGaAs is used as a semiconductor with a narrow forbidden band width. Combination using InGaAsP A combination of GaAlPsb for the first and second semiconductor layers, GaSb for the third semiconductor layer, a combination of GaAlAsSb for the first and second semiconductor layers, GaSb for the third semiconductor layer, and a combination of GaAlAsSb for the first and second semiconductor layers and GaSb for the third semiconductor layer. 1.InP or second semiconductor layer
It is also effective for the combination of GaSb and InGaAsSb for the third semiconductor layer, and in each combination, the conductivity type can function satisfactorily even if the first conductivity type or the second conductivity type is P type, unless noise characteristics are considered. , dark current characteristics, operational stability, multiplication factor, etc. can be improved. As described above, the present invention provides a structure in which dark current characteristics, operational stability, multiplication factor, etc. can be improved by making a semiconductor layer with a narrow bandgap protrude into a semiconductor with a wide bandgap in a semiconductor photodetector having a mesa-type heterojunction. did.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a semiconductor photodetecting element having a known mesa-type heterostructure, and FIGS. 2 and 3 are sectional views of a photodetecting element to which the present invention is applied.
The symbols in the diagram indicate the following. 10...P ⁺ -INP substrate, 11...P ⁺ -InP buffer layer, 12...n ^- -InP layer, 13...n ^- -
InGaAs layer, 14... Front of depletion layer before breakdown, 15... Ring electrode using Au-Ge, 16... Electrode using Au-Zn, 20... P ⁺
-InP substrate, 21...P ⁺ -InP buffer layer, 2
2...n ^- -InP layer, 23...n ^- -InGaAs layer, 2
4... Front of depletion layer at breakdown, 2
5...Ring electrode using Au-Ge, 26...
Electrode with a light receiving window using Au-Zn, 31...
P ⁺ -InP layer due to Cd diffusion, 32...InP surface appeared by etching InGaAs.

Claims (1)

[Claims] 1. On a first semiconductor layer exhibiting a first conductivity type and having an impurity concentration of N ₁ , a semiconductor layer exhibiting a second conductivity type opposite to the first conductivity type and having an impurity concentration of N ₂ A third semiconductor layer having a second conductivity type and having a bandgap narrower than at least the semiconductor constituting the second semiconductor layer is formed on the second semiconductor layer. A mesa-type semiconductor photodetecting element having a structure in which semiconductor layers are stacked, wherein an interface between the second semiconductor layer and the third semiconductor layer protrudes toward the second semiconductor layer in the center of the mesa. The effective length d ₁ from the interface between the first semiconductor layer and the second semiconductor layer to the protruding interface and the length of the second semiconductor layer other than the protrusion are The thickness _{d 2} and ) A semiconductor photodetecting element characterized by: