JPH04241473A - Avalanche photo diode - Google Patents

Abstract

PURPOSE:To obtain an avalanche photo diode having light reception sensitivity in a 1.55mum band and having low noise and high speed response characteristics with high ionization rate of alpha/beta. CONSTITUTION:An end face incident type avalanche photo diode comprises a superlattice of an In0.52Al0.48As/In1-xGaxAsyP1-y system which is lattice-aligned with InP used as an avalanche multiplier, layer, wherein a p<->-type In0.53Ga0.47As light absorbing layer 13 is made into a thin film up to 0.6mum and a part of the n<->-type superlattice multiplier layer 15 as thick as 1.0mum and an n<+>-type InP cap layer 18 are made into a part of a rib waveguide path with a mesa formed where depth extends to an upper part of a p<+>-type InP field lower layer 7 and width is 10mum. Its ionization ratio is approximately up to 10, and noise reduction is performed wherein excessive noise index is up to 0.3 while wide band characteristics of approximately 10GHz can be obtained. External quantum efficiency is 60 to 70% with respect to wavelength of 1.55mum.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an avalanche photodiode (APD) having high speed and low noise characteristics.

0002

2. Description of the Related Art In order to construct a high-speed, large-capacity optical communication system, a semiconductor light-receiving element having ultra-high speed, low noise, and high sensitivity characteristics is essential. For this reason, InP, which can be applied to the low-loss wavelength range of 1.0 to 1.6 μm for silica-based fibers, has recently been developed.
/InGaAs avalanche photodiode (AP
Research into increasing the speed and sensitivity of D) is active. In this InP/InGaAs APD, the gain bandwidth (GB) and 75GH
z speed has been realized. However, in this device structure, the ionization rate ratio β/α of InP, which is the avalanche multiplication layer, is as small as ~2 (α: electron ionization rate, β: hole ionization rate); (The smaller the rate ratio is, the larger it is) is as large as ~0.7, and there is a limit to lower noise and higher sensitivity. This is also true when other bulk III-V compound semiconductors are used in the avalanche multiplication layer, and in order to achieve low noise and high GB stacking (high-speed response characteristics), the ionization rate ratio α/ It is necessary to artificially increase β.

[0003] Therefore, F. Capasso
) etc. are Applied Physics Letters (Appl.
Phys. Lett. ), Volume 40(1), p. 38-4
0. In 1982, he proposed a structure in which the ionization rate ratio α/β was artificially increased by utilizing the conduction band energy discontinuity ΔEc due to the superlattice for electron impact ionization.
Increase in the ionization rate ratio α/β in the aAs/GaAlAs superlattice (~8 in the superlattice layer compared to ~2 in bulk GaAs)
It was confirmed. Furthermore, Kagawa et al., Applied Physics Letters (Appl. Phys. Lett.), p.
．． 1895-1897, volume (18), 1990, a similar structure was formed using an InGaAs/InAlAs superlattice having light receiving sensitivity in the wavelength band of 1.3 to 1.6 μm used for long-distance optical communications. Again, the ionization rate ratio α/β
(~10 in the superlattice layer versus ~2 in bulk InGaAs) was observed. FIG. 3 shows an energy band diagram when the bias is applied. n- used in this structure
In the superlattice multiplier layer consisting of the type InAlAs obstacle layer 31/n- type InGaAs well layer 32, the conduction band discontinuity amount ΔE
c33 is 0.5 eV, which is larger than the valence band discontinuity amount ΔEv 34 of 0.2 eV, and when the electron 391 enters the well layer 32, the energy acquired by the band discontinuity is greater than that of the hole 392. As a result, the electron ionization rate increases and the ionization rate ratio α/β increases. Furthermore, light 39 with a wavelength of 1.55 μm incident from the substrate surface
3 is a p-type In0.53Ga0.4 with a thickness of 1.7 μm.
By absorbing ~73% in the 7As light absorption layer 36, InAlAs/InGa has a large electron ionization rate ratio α/β.
Almost pure electron injection into the As superlattice is achieved, and noise reduction is achieved by increasing the ionization rate ratio α/β.

0004

[Problems to be Solved by the Invention] However, in the avalanche photodiode with this structure, the light absorption layer 36 is as thick as 1.7 μm in order to obtain sufficient quantum efficiency, and this layer is generated by a superlattice avalanche multiplication layer. Due to the time delay in which the multiplied secondary holes travel, the frequency response band of the device changes.
The effect of increasing the GB product by increasing the ionization rate ratio α/β (If there is no trapping effect of holes in the superlattice well layer, the GB product is 100
It has the disadvantage that it does not become as large as expected depending on the degree of

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to realize an avalanche photodiode which has light receiving sensitivity in the wavelength band of 1.55 μm, has a high ionization ratio α/β, low noise, and has high-speed response characteristics.

[0006]

Means for Solving the Problems The avalanche photodiode of the present invention has the following features.

A photodiode having a structure in which a semiconductor superlattice layer is an avalanche multiplication layer is characterized in that the superlattice avalanche multiplication layer forms a part of a waveguide structure and is of an end-incident type.

Alternatively, the superlattice avalanche multiplication layer is
In(AlzGa1-z)As(z=0-1), I
n1-x Gax Asy P1-y (x=0~1,
It is characterized by being configured by a combination of y=0 to 1).

[0009]

[Operation] The present invention has improved characteristics compared to the conventional one by the above-mentioned structure. FIG. 1 is a structural perspective view showing an example of an avalanche photodiode, and FIG. 2 is a diagram showing the dependence of the response cutoff frequency on the multiplication factor due to transit time limitations of the present invention and the conventional structure.

In FIG. 2, the dotted line indicates the dependence of the response cutoff frequency on the multiplication factor due to transit time limitations of the conventional structure (light absorption/multiplication separation type = SAM type), and the straight line indicates the dependence of the response cut-off frequency on the transit time limit of the waveguide structure of the present invention. In both cases, the calculation results are obtained when there is no hole trap in the superlattice well layer.

The operation of the present invention will be explained using these figures. In the conventional front-illuminated structure (the one in FIG. 3), the light absorption layer 36 is as thick as 1.7 μm in order to obtain sufficient quantum efficiency, and this layer is used as the multiplication layer 2 generated by the superlattice avalanche multiplication layer.
Due to the time delay for the next hole to travel, the frequency response band of the device changes due to an increase in the ionization rate ratio α/β, which increases the GB product (if there is no hole trapping effect in the superlattice well layer, the GB
It has the disadvantage that it does not become as large as expected due to the product (approximately 100).

This is shown in the dependence of the response cutoff frequency on the multiplication factor shown by the dotted line in FIG. This dotted line indicates a superlattice layer thickness of 1.
0 μm, the ionization rate of the superlattice is that electron α is bulk InGa
The light absorption/multiplication separation type (SA
These are the calculation results for the M type) front-incidence structure. From now on, GB
When the product is 70 and the multiplication factor M is 10, the cutoff frequency fc is 6G.
Hz, and the band at low multiplication factor (M < 7) is at most 7 GHz.
It turns out that it is z. GB product 100 predicted from Emmons's theoretical formula when the ionization rate ratio α/β is about 14
The reason for the smaller value is the above-mentioned reason, namely,
This is due to the fact that the light absorption layer is as thick as 1.7 μm, and there is a time delay in the travel of multiplied secondary holes generated in the superlattice avalanche multiplication layer through this layer.

[0013] This means that the light absorption layer thickness shown by the solid line is 0.6
This becomes clear when compared with calculation results assuming that the thickness is μm and other parameters are the same as in the conventional example. The light absorption layer is 0.6
When the thickness is reduced to μm, the transit time delay of multiplied secondary holes can be reduced, and when the GB product is ~100 and the multiplication factor M is 10, the cutoff frequency fc is 9 GHz, and the band at low multiplication factors (M < 6) is also reduced. 1
A wide band of 0 GHz or more is possible. However, in the conventional front-illuminated structure, when the light absorption layer is made as thin as 0.6 μm, the quantum efficiency at a wavelength of 1.55 μm is extremely reduced to ~15%, and broadband characteristics and high sensitivity characteristics cannot be satisfied at the same time.

On the other hand, in the structure of the present invention shown in FIG. 1, the cap layer 18 and a part of the superlattice avalanche multiplication layer 15 are etched away to the top of the p+ type InP field drop layer 14. By forming a mesa with a width of 20 to 30 μm or less, a rib waveguide is formed by this mesa, the light absorption layer 13, and the wide gap electric field drop layer 14. When light enters such a waveguide structure from the end face, the light absorption layer becomes 0.
Even if the waveguide is as thin as about 6 μm, sufficient light absorption can be obtained by making the waveguide length several hundred μm, and the junction capacitance can be reduced by narrowing the mesa width, so broadband characteristics and high sensitivity characteristics can be achieved at the same time. Satisfied.

As a result of the above-mentioned effects, the structure of the present invention allows the frequency response band of the device to change to the ionization rate ratio due to the time delay in which the multiplied secondary holes generated in the superlattice avalanche multiplication layer move through the light absorption layer. Effect of increasing GB product due to increase of α/β (If there is no trapping effect of holes in the superlattice well layer, G
B product (approximately 100) can overcome the drawback that it is not as large as expected.

[0016] As an avalanche photodiode having a waveguide structure, ITriple E, Electron Device Letters (IEEE, ELECTRON DE
VICE LETTERS), EDL-7 volume, PP.
330-332, a Si/SiGe-based light receiving element was reported in 1986, but in this structure, the light absorption layer is S
In the iGe short-period strain layer superlattice, avalanche multiplication layers are each made of Si. This is because the light absorption layer is made of bulk InGa.
In As, the avalanche multiplication layer is a superlattice (InAlAs/I
This is different from the present invention, which is composed of a
While it has a light receiving sensitivity at 5 μm, the conventional SiGe type has a light receiving sensitivity only for light having a wavelength shorter than 1.3 μm due to the absorption edge of this system.

Therefore, by using the semiconductor shown in the embodiment of the present invention, it is possible to have light receiving sensitivity in the wavelength band of 1.55 μm and simultaneously satisfy low noise characteristics and high-speed response characteristics by improving the ionization rate ratio α/β. An avalanche photodiode can be realized.

[0018]

[Example] As an example of the present invention, In0.52Al0.48As/In1-x which is lattice matched to InP will be described below.
This will be explained using a Gax Asy P1-y based superlattice avalanche photodiode.

An embodiment of the avalanche photodiode of the present invention shown in FIG. 1 was manufactured by the following steps.

A p-type InP buffer layer 12 is formed on a p+-type InP substrate 11 with a thickness of 0.5 μm and a carrier concentration of ~2×1.
015cm-3 p-type In0.53Ga0.47A
The light absorption layer 13 has a thickness of ~0.6 μm and a carrier concentration of ~1
x1017 cm-3 p-type InP electric field drop layer 14.
2μm thick, carrier concentration ~1×1015cm−3 n
- type In0.52Al0.48As failure layer 16/In
1-x Gax Asy P1-y The superlattice avalanche multiplication layer 15 consisting of the well layer 17 is made 1.0 μm thick, and the n+ type InP cap layer 18 with a carrier concentration of ~5×10 18 cm −3 is made 1 μm thick. Growth is performed using linear vapor phase epitaxy (CBE).

This superlattice layer 15 is made of In0.52Al0.48As16 with a thickness of 400A (angstroms) and In1-xGa with a thickness of 200A (angstroms).
It has a structure in which x Asy P1-y 17 are alternately stacked in 17 periods. In1-x Gax Asy P1-y
In order to suppress the trapping of holes into the superlattice well layer, the composition Y of is determined by a composition region in which the valence band discontinuity amount ΔEv between In0.52Al0.48As is 0 to 0.15 eV. Decided.

Next, using ordinary photolithography and wet etching techniques, a mesa with a width of 10 μm and a depth up to the top of the p+ type InP field drop layer 14 is formed.
As part of the rib waveguide, a SiN insulating protective film 1 is applied to the entire surface.
form 9. Next, a polyimide insulating buried layer 112 is formed on the side of the mesa, and a region for forming an n-side electrode pad with a diameter of about 60 μmφ is formed thereon. Then, an n-side electrode 110 is formed of AuGeNi/TiPt, and the back surface is polished. After that, the p-side electrode 111 was formed of AuZn. The element was cleaved to a length of 250 μm, and a non-reflective coating film 113 made of SiN was formed on the cleaved surface. With the above steps, the avalanche photodiode of this example was completed. In this element, light is detected by entering the light into the light source 114 from the end surface.

[0023] If the mesa width is 30 μm or less, the capacitance is sufficiently small and high-speed response can be achieved, and if the waveguide length is about several 100 μm, sufficient light absorption can be obtained.

[0024]

Effects of the Invention In the structure of the above embodiment, the electron ionization rate is about 2.0 of that of bulk In0.53Ga0.47As.
On the other hand, the hole ionization rate is about 1/3 times smaller than that of bulk InGaAs, and the ionization rate ratio is about 10, which is increased compared to 2 for bulk InGaAs. The noise was also reduced to an index of ~0.3. Moreover, the frequency response characteristics of the InGaA of the present invention
Since the S-layer optical absorption layer has a waveguide structure with a thickness of 0.6 μm and a superlattice layer thickness of 1.0 μm, the capacitance is sufficiently small at 0.2 pF, which limits the transit time, and the cutoff frequency shown by the solid line in FIG. A result almost equal to the multiplication factor dependence was obtained. Regarding quantum efficiency, by performing butt coupling using a spherical fiber at the end face, the coupling loss can be reduced from -1.5 to -1 per end face.
2 dB (this value can be further reduced by rigorous design of the waveguide), and since the internal absorption efficiency is approximately 100%, an external quantum efficiency of 60 to 70% can be obtained. In addition, in the rib superlattice region, the In1-x Ga of the well layer
Since x Asy P1-y does not absorb light with a wavelength of 1.55 μm, the light seeping out from the InGaAs waveguide layer is not absorbed here and becomes a mixed injection of electrons and holes into the superlattice, and the noise characteristics are It doesn't get worse.

From this, according to the present invention, the wavelength 1.55
It is possible to realize an avalanche photodiode that has light receiving sensitivity in the μm band, has a high ionization rate ratio α/β, low noise, and has high-speed response characteristics, which is highly effective.

[Brief explanation of the drawing]

FIG. 1 is a structural perspective view showing an embodiment of an avalanche photodiode of the present invention.

FIG. 2 is a diagram showing the dependence of the response cutoff frequency on the multiplication factor due to transit time limitations of elements of the present invention and a conventional structure.

FIG. 3 is a diagram illustrating the structure of a conventional avalanche photodiode.

[Explanation of symbols]

11 p+ type semiconductor substrate 12 p type buffer layer 13 p− type light absorption layer 14 p type wide gap electric field drop layer 15 n−
type superlattice avalanche multiplication layer 16 semiconductor barrier layer 17 semiconductor well layer 18 n+ type cap layer 19 insulating protective film 110 n side electrode 111 p side electrode 112 insulating buried layer 113 anti-reflection coating film 114 incident light 31 n- type In0. 52Al0.48As barrier layer 32 n- type In0.53Ga0.47As well layer 33 conduction band discontinuity amount ΔEc 34 valence band discontinuity amount ΔEv 35 p+ type In0.53Ga0.47As electric field drop layer 36 p- type In0.53Ga0. 47As light absorption layer 37 p+ type In0.52Al0.48As
Cap layer 38 n+ type In0.52Al0.48
As buffer layer 391 Electrons 392 Holes 393 Incident light

Claims (2)

[Claims] 1. An avalanche photo comprising a semiconductor light absorption layer and a semiconductor superlattice avalanche multiplication layer, the superlattice avalanche multiplication layer forming part of a waveguide structure and being of an end-incident type. diode. 2. The superlattice avalanche multiplication layer is made of In(
Alz Ga1-z ) As (Z=0~1) and In1-
x Gax Asy P1-y (x=0~1, y=0
2. The avalanche photodiode according to claim 1, wherein the avalanche photodiode comprises a superlattice formed by a combination of the above.