JPH06260679A - Avalanche photodiode - Google Patents

Abstract

PURPOSE:To provide an APD having a high speed and high sensitivity to be applied to a Gb/s band optical communications. CONSTITUTION:A thickness (d1, d2, d3,...) of a barrier layer for constituting a superlattice multiplying layer of an APD is so set as to be sequentially varied (d1>d2>d3...) along a moving direction of main carrier (electrons). Thus, an ionization rate ratio can be improved as compared with that of a conventional element. As a result, a gain bandwidth product of an important index of acceleration of APD can be improved, and excess noises of an important index of high sensitivity of the APD can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an avalanche photodiode, more specifically, an avalanche photodiode having a multiplication layer composed of a superlattice type multiplication layer in which two or more kinds of semiconductor films having different band gaps are laminated. The present invention relates to a diode (hereinafter abbreviated as APD).

[0002]

2. Description of the Related Art In recent years, aiming at higher speed and higher sensitivity of APDs, development of APDs having an excellent ionization ratio as compared with conventional APDs having a bulk type multiplication layer has been promoted. This is because, as is well known, the ratio of the ionization rate of electrons as carriers and the ionization rate of holes as carriers in the multiplication layer, that is, the ionization rate ratio is improved to speed up APD. This is because it is possible to improve the gain band product, which is an important index, and to reduce excess noise, which is an important index for increasing the sensitivity of the APD.

A typical example is an APD having a superlattice multiplication layer. The superlattice multiplication layer is an APD that utilizes the difference in band discontinuity value at the hetero interface in order to improve the ionization ratio. That is, by combining materials having different discontinuity values in the conduction band in which electrons travel and discontinuity values in the valence band in which holes travel, ionization of carriers traveling in a band with a large band discontinuity value is achieved. Rate is increased preferentially to improve the ionization rate ratio. References regarding this type of technology include, for example, T. Kawagawa et al., Third Optoelectronics Conference Technical
Digest Page 194 1990 (Third
Optoelectronics Conference
eTechnical Digest, p194,1
990).

[0004]

In the above-described APD having the conventionally known superlattice multiplication layer, the film thickness of the plurality of semiconductor films (barrier layers) having a large bandgap constituting the superlattice multiplication layer is Generally, the film thickness is not considered from the viewpoint of improving the ionization rate ratio. for that reason,
As described above, the ionization rate ratio in the superlattice multiplication layer of the conventional superlattice APD is mainly determined by the difference in band discontinuity value at the hetero interface, while the selection of the semiconductor material forming the multiplication layer is selected. There is a limit to setting the band discontinuity value due to wavelength sensitivity characteristics and the like, and it is not possible to arbitrarily set the band discontinuity value. Therefore, there is a limit to the improvement of the ionization ratio.

An object of the present invention is AP having a superlattice multiplication layer.
An object of the present invention is to provide an APD that further improves the ionization rate ratio of D and enables higher speed and higher sensitivity at the same time.

[0006]

In order to achieve the above object, the present invention multiplies a superlattice in which a barrier layer of a semiconductor film having a large band gap and a well layer of a semiconductor film having a small band gap are alternately laminated. In the APD having layers, the thickness of the barrier layer is sequentially increased or decreased in accordance with the carrier moving direction. This means that, when focusing on each well layer forming the superlattice, the band discontinuity value of the valence band for holes and the band discontinuity of the conduction band for electrons formed by the hetero interfaces on both sides of the specific well layer. Compared with the value, the thickness of the barrier layer passing immediately before the carriers corresponding to the large band discontinuity value is injected into each well layer, the carrier corresponding to the small band discontinuity value in the well layer. The barrier layer is set so that it becomes thicker than the film thickness of the barrier layer that passes immediately before the implantation.

In particular, an APD in which the light absorption layer and the multiplication layer are separated
In the above configuration, the thickness of the barrier layer forming the multiplication layer is gradually reduced along the traveling direction of the photocarriers injected from the light absorption layer into the multiplication layer. In the following description, for the sake of simplicity, a case where the semiconductor layer forming the well layer and the barrier layer is of a single type will be described, but it may be formed of a plurality of types.

[0008]

Next, the principle of the present invention will be described with reference to FIGS. 1 and 2 show the APD of the present invention and a conventional APD, respectively.
It is a figure which shows the band structure of the superlattice multiplication layer of FIG. In the figure, the vertical axis and the horizontal axis represent the energy level and the film thickness, respectively. In both cases, the multiplication layer is formed by alternately stacking two types of layers (barrier layers and well layers) having large and small band gaps. Further, in the examples shown in the figures, the band discontinuity values are all shown in the case where the band discontinuity value ΔEc of the conduction band Ec for electrons is larger than the band discontinuity value ΔEv of the valence band Ev for holes. There is. That is, in both cases, the ionization rate of electrons is made larger than that of holes to improve the ionization rate ratio.

In order to improve the ionization rate ratio in the superlattice multiplication layer, it is effective to selectively raise the energy of one carrier of electrons or holes higher than the energy of the other.
The film thicknesses of the well layer and the barrier layer of the conventional ADP (FIG. 2) are constant at do and d, respectively. By making the band discontinuity value ΔEc of the conduction band for electrons larger than the band discontinuity value ΔEv of the valence band for holes, the ionization rate of electrons is further increased as compared with the ionization rate of holes. Therefore, the improvement of the ionization rate is achieved.

On the other hand, the thickness of the well layer and the barrier layer of the ADP of the present invention (FIG. 1) is a constant do, but the barrier layer thickness is d1> from left to right. d2> d3
It is becoming thinner sequentially. That is, the thickness of the carrier becomes gradually thinner along the traveling direction of the electron. Focusing on a pair of specific well layers a, the film thickness d1 of the barrier layer passing immediately before the carriers (electrons) corresponding to the large band discontinuity ΔEc are injected into the well layer a has a small band discontinuity Δ.
The thickness of the barrier layer is set so that it becomes thicker than the thickness d2 of the barrier layer through which carriers (holes) corresponding to Ev pass immediately before being injected into the well layer a. Therefore, the energy of electrons injected into the well layer a is larger than the energy of holes injected into the well layer a. As a result, the ionization rate of electrons can be made higher than that of holes, and the ionization rate ratio can be increased. In other words, the ionization ratio is controlled by changing the film thickness of the layer having a large band gap, that is, the region (barrier layer) that accelerates carriers, with respect to electrons and holes.

In the APD in which the light absorption layer and the multiplication layer are separated from each other, in the example shown in FIG. 1, the barrier layer thickness is gradually reduced from the left to the right, so that it is injected into the multiplication layer. Carriers travel in the superlattice layer from left to right.
Therefore, the light absorption layer is arranged on the left side of this figure. Further, the change in the film thickness of the barrier layer can be expressed as that the film thickness of the well layer and the barrier layer become thinner successively along the traveling direction of electrons having a higher ionization rate in the conventional example in which the film thicknesses are constant.

[0012]

FIG. 3 is a diagram showing the cross-sectional structure of one embodiment of the APD according to the present invention. The present embodiment is a mesa-type APD in which a light absorption layer having a superlattice multiplication layer 5, an electric field relaxation layer 6 and a light absorption layer 7 in a depletion region and a multiplication layer are separated. Light is incident from the back surface. The energy band structure of the superlattice multiplication layer 5 of this example is similar to the energy band structure shown in FIG. The structure of each layer is as follows. In the parentheses (), d represents the layer thickness, and N and P represent the impurity concentration.

N-InP substrate (d = 150 μm, N = 2
X10 ¹⁸ / cm ³ ) 3 plane, N-InAlAs buffer layer (d = 1 μm, N = 2 × 10 ¹⁸ / cm ³ ) 4, undoped-superlattice multiplication layer (d = 0.31 μm, N < 1 x
10 ¹⁵ / cm ³ ) 5, P-InGaAlAs electric field relaxation layer (d = 0.2 μm, P = 1.3 × 10 ¹⁷ / cm ³ ) 6,
P-InGaAs light absorption layer (d = 1.2 μm, P = 2 ×
10 ¹⁵ / cm ³ ) 7, P-InAlAs buffer layer (d
= 1 μm, P = 2 × 10 ¹⁸ / cm ³ ) 8 and P-InG
aAs contact layer (d = 0.2 μm, P = 2 × 10 ¹⁹
/ Cm ³ ) 9 are sequentially laminated. The crystals of the layers 4 to 9 are all mixed crystals lattice-matched to the InP substrate 1. The side surface of each layer and the exposed surface of the substrate are covered with a polyimide passivation film 10. The N-InP substrate 3 and the P-InGaAs contact layer 9 each have N.
An electrode 1 and a P electrode 2 are provided. The junction diameter of the ADP element is 50 μm.

A molecular beam epitaxy method was used for crystal growth in this example, and wet etching with a Br-based solution was used for forming the mesa shape. For the electrodes, Au / Pt / Ti formed by vacuum vapor deposition was used for both P-type and N-type.

The superlattice multiplication layer 5 is made of InGa having a thickness of 5 nm.
Eight As well layers and thicknesses of 46, 42 and 3 respectively
InA of 8, 34, 30, 26, 22, 18, 14 nm
The 1As barrier layer was alternately laminated with the well layers in this order from the light absorption layer 7 side. The total thickness of the superlattice multiplication layer 5 is 0.31μ
m.

For comparison, an element having a conventional superlattice multiplication layer was also manufactured. In the superlattice structure of the conventional device, the film thickness of the well layer was 5 nm, the film thickness of the barrier layer was 30 nm, and the total film thickness of the superlattice multiplication layer was 0.31 μm. In other words, the barrier layer is an element having an average value of the layer thickness of the well layer of the present invention, and other parameters are the same as those of the present embodiment of the present invention. AD
The bonding diameter of the P element is 50 μm.

The characteristics of the above-described embodiment of the present invention and the conventional APD prototyped for the above comparison are shown below. Gain bandwidth product,
The dark current at an ionization ratio (α / β) and a multiplication factor M = 10 is 150 for the APD of the present invention and that of the conventional APD element.
GHz, 80 GHz, 6, 3, 1 μA and 0.5 μA. From this result, the effect of the present invention is clear. The ionization rate ratio aimed at by the present invention is improved by a factor of 2 as compared with the conventional example, and the gain band product is also improved by a factor of 2 by reflecting this. Dark current is inferior to conventional devices, but 1
Considering the application to a receiver of a high-speed optical communication system of about 0 Gb / s, it is estimated that the influence of this difference in dark current on the receiving sensitivity is 0.2 dB or less, and there is no practical problem. Rather, the APD of the present invention achieves a large ionization rate ratio, and the effect of lowering excess noise due to it is sufficiently compensated for the effect of increasing dark current, and the 10 Gb / s system is superior to the conventional example in receiving. Sensitivity characteristics can be achieved.

[0018]

The APD of the present invention can improve the ionization ratio as compared with the conventional APD having a superlattice multiplication layer. As a result, compared to the conventional superlattice type APD, the APD
Gain band product improvement and AP
D It is possible to reduce excess noise, which is an important index for increasing sensitivity.

[Brief description of drawings]

FIG. 1 is a diagram showing a band structure of a superlattice multiplication layer of an APD according to an embodiment of the present invention.

FIG. 2 is a diagram showing a superlattice multiplication layer band structure of a conventional superlattice APD.

FIG. 3 is a diagram showing a cross-sectional structure in one embodiment of the APD according to the present invention.

[Explanation of symbols]

 1: N electrode 2: P electrode 3: N-InP substrate 4: N-InAlAs buffer layer 5: And-super-superlattice multiplication layer 6: P-InGaAlAs electric field relaxation layer 7: P-InGaAs light absorption layer 8: P-InAlAs buffer layer 9: P-InGaAs contact layer 10: Polyimide passivation film

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Shigehisa Tanaka 1-280, Higashi Koikeku, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Chiaki Nozu 3681 Hayano, Mobara-shi, Chiba Hitachi Device Engineering Co., Ltd. (72) Inventor Tadao Kaneko 1-280, Higashi Koigokubo, Kokubunji City, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd.

Claims (3)

[Claims] 1. In an APD having a multiplication layer of a superlattice in which a barrier layer of a semiconductor film having a large bandgap and a well layer of a semiconductor film having a small bandgap are alternately laminated, the thickness of the barrier layer is a carrier layer. An avalanche photodiode having a structure in which the thickness gradually increases or decreases in accordance with the moving direction. 2. An avalanche photodiode having the multiplication layer according to claim 1 and a light absorption layer separated from the multiplication layer, of carriers generated by absorbing signal light in the light absorption layer. An avalanche photodiode, characterized in that a semiconductor film having a large bandgap forming the multiplication layer is gradually thinned along a traveling direction of carriers injected into the multiplication layer. 3. In an APD having a superlattice multiplication layer in which a barrier layer of a semiconductor film having a large band gap and a well layer of a semiconductor film having a small band gap are alternately laminated, a hetero interface on each side of each well layer is formed. By comparing the band discontinuity value of the valence band with respect to the holes to be formed and the band discontinuity value of the conduction band with respect to the electrons, the carriers corresponding to the large band discontinuity value pass just before being injected into each well layer. The thickness of the barrier layer is set so that it becomes thicker than the thickness of the barrier layer passing immediately before the carriers corresponding to the small band discontinuity are injected into the well layer. An avalanche photodiode.