JP2978572B2 - Semiconductor light receiving element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor photodetector, and more particularly to an avalanche multiplication semiconductor photodetector excellent in low noise and high speed response.

[0002]

2. Description of the Related Art Conventionally, an InP lattice-matched on an InP substrate has been used as a semiconductor light receiving element for optical communication in the wavelength band of 1 to 1.6 μm.
_0.53 Ga _0.47 As layer (hereinafter abbreviated as InGaAs layer)
A PIN-type semiconductor light-receiving element, an avalanche multiplication type semiconductor light-receiving element, and the like, each having a light absorption layer, are known. Above all, avalanche multiplication type semiconductor light receiving elements have been put to practical use for long-distance optical communication in that they have an internal gain effect due to avalanche multiplication and high-speed response.

In the avalanche multiplication type semiconductor light receiving device, it is known that noise and high speed, which are important factors in device characteristics, are governed by a random ionization process of a carrier in a multiplication process.

That is, since the ionization process itself behaves as shot noise and further participates in the multiplication rise time, it is ideally desirable that the ionization process does not last long. For this purpose, it is desirable that there is a difference between the electron ionization rate (α) and the hole ionization rate (β) in the multiplication region. However, the ionization rate ratio (α / β or β /
α) is determined by the material properties, and the above-mentioned InGa
In an As-based avalanche multiplication semiconductor photodetector, I
At most β / α〜2 in order to use the nP layer as a multiplication region. This is a Si-based device α / β having low noise characteristics.
There is a great difference from ２０20, and a breakthrough material technology has been required to realize lower noise and faster response characteristics.

On the other hand, F. Capaso (F. Capa)
et al. propose that the ionization rate ratio can be artificially controlled by applying a superlattice structure having a large band discontinuity to the avalanche multiplication layer (Applied Physics Letters (Appl. Phys. Let
t. ), 40, pp 38-40 (1982)). The basic principle is that the photocarrier generated by light absorption travels through the superlattice avalanche multiplication layer, thereby taking in large band discontinuity at the superlattice interface as energy and promoting ionization of one carrier. .

For the wavelength band for optical communication (1 to 1.6 μm band), K. Brennan reported In.
_0.53 Al _0.47 As (hereinafter abbreviated as InAlAs) / I
By applying the nGaAs superlattice system as a multiplication layer, it has been theoretically speculated that the ionization rate ratio is increased (α / β し て 20) by using the large conduction band discontinuity (I / E). -Transaction on Electron Devices (IEEE. Trans. Elec)
tron. Devices, ED-33, pp1502
-1510 (1986)). Experimentally, Makita et al. Exaggerated that the ionization rate ratio α / β was exaggerated to 100 or more (Proceeding 16th International Symposium Proc. 16th Int. Symp.
nd Related Compounds, Karu
Izawa, pp-907 (1989)) and Kagawa et al. reported basic performance in a more practical structure in which a light absorption layer and an avalanche multiplication layer were separated (Applied Physics Letters (Appl. Phys.)).
s. Lett. , 57, pp 1895-1897 (19
90)). An avalanche multiplication type light receiving element using such an InAlAs / InGaAs superlattice system is a conventional InAlAs / InGaAs superlattice.
The GaAs-based avalanche multiplication type light receiving element is expected to be a device that far surpasses the characteristics.

[0007]

FIG. 5 shows a device structure diagram of an avalanche multiplication type semiconductor light receiving device in which a light absorption layer and a superlattice multiplication layer proposed by Kagawa et al. The distribution map is shown. Basically, n ⁻ on the InP substrate 1
-InAlAs / InGaAs superlattice multiplication layer 4, p ⁺ -
An InGaAs electric field lowering layer 5 and ap ⁻ -InGaAs light absorbing layer 6 are provided.

The principle of operation is that electrons in the photocarriers generated in the light absorption layer travel and are injected into the electric field drop layer and the avalanche multiplication layer by the drift electric field. Where I
Since the nAlAs / InGaAs superlattice structure is a material system having a large conduction band discontinuity, traveling electrons effectively acquire energy at the heterointerface and easily reach ionization. Therefore, it is possible to obtain a good multiplication phenomenon in which the ionization of electrons is exaggerated.

An important factor in obtaining good characteristics in the device structure is to realize an appropriate electric field intensity distribution. The electric field intensity distribution at the time of voltage application in FIG. 5 is shown. As shown in the figure, a strong electric field sufficient to cause ionization (up to 400) is applied to the avalanche multiplication layer via the p ⁺ InGaAs electric field drop layer.
KV / cm) on the light absorbing layer side, a sufficiently low electric field (<150K) to drift carriers and prevent tunnel breakdown.
V / cm). Therefore, it is necessary to strictly control the layer thickness and the carrier concentration of the p ⁺ -field drop layer.

By the way, the above structure is obtained by Kagawa et al. Using the molecular beam epitaxy method (MBE method), and the P ⁺ field-drop layer is a Be-doped InGaAs layer having a thickness of 140 A (angstrom). . However,
It is inevitable that Be undergoes self-diffusion during the crystal growth process, so that the p ⁺ region effectively expands. In particular, in the above structure, the electric field drop region and the light absorption region are InGaAs layers having a relatively narrow energy gap (up to 0.78 ev), and it is expected that tunnel breakdown will easily occur. For this reason, in the structure described above, unless self-diffusion is suppressed, it is clear that a competition state occurs between the avalanche breakdown in the superlattice multiplication layer and the tunnel breakdown induced by self-diffusion. There is a problem in element characteristics due to excessive deterioration.

An object of the present invention is to solve these problems and to provide an avalanche multiplication type semiconductor light receiving element having low noise and high speed response characteristics.

[0012]

The semiconductor light receiving device of the present invention comprises a semiconductor substrate having a basic structure comprising an avalanche multiplication layer having a superlattice structure, an electric field drop layer, a light absorption layer, and a light window layer. to the electron injection type semiconductor light-receiving element, the semiconductor layer is InP or InG of forming the electric field drop layer
aAsP, and the semiconductor layer forming the light absorbing layer is InG
aAs .

Further, in the aforementioned semiconductor light receiving element,
The electron affinity of the semiconductor layer forming the electric field drop layer_D,light
The electron affinity of the semiconductor layer forming the absorption layer_APlace
If χ_A> Χ_I1> Χ_D  Electron affinity that satisfies the relationship_I1Semiconductor layer having
Is interposed in the boundary region.

Furthermore, the semiconductor layer forming the light absorption layer
E the energy gap_A, Electron affinity_A, Light window layer
The energy gap of the semiconductor layer forming_W, Electronic
Affinity_WAnd_A+ E_A<Χ_I2+ E_I2<Χ_W+ E_W  Energy gap E that satisfies the relationship_I2, Electron affinity
Force_I2A semiconductor layer having
It is characterized by things.

[0015]

The P-doped species inevitably causes self-diffusion as pointed out in the structure of the prior art. Therefore, an element structure that does not easily cause tunnel breakdown even if self-diffusion occurs is required. For this reason, in the present invention, the p ⁺
It is formed in a semiconductor layer having a larger energy gap than aAs. Here, it is generally known that the tunnel breakdown voltage is proportional to the 3/2 power of the energy gap, and widening the energy gap in the p ⁺ electric field drop region becomes an effective structure for suppressing the tunnel breakdown.

Although the structure having the p ⁺ wide gap electric field drop layer described above is effective in suppressing tunnel breakdown, there is room for improvement in the carrier traveling process.

FIG. 2B shows a band structure diagram (first example) in the case of the first aspect of the present invention. Occurred in the light absorbing layer,
When the electrons travel by the drift electric field, the conduction band discontinuity ΔE _c is felt due to the existence of the p ⁺ electric field drop layer having the wide energy gap. This causes energy loss and pile-up of electrons, which is a problem in obtaining a high-speed response. Therefore, in the present invention, as shown in FIG. 2 (a), a structure is provided in which an intermediate layer 1 satisfies claim 2 is interposed at the boundary between the electric field lowering layer and the light absorbing layer. According to this structure, the electrons sense the conduction band discontinuity in a stepwise manner (△ E _c1 , ΔE _c2 ), so that energy loss, pile-up, and the like are reduced. Therefore, low noise and high-speed response can be achieved at the same time.

Further, even when holes generated by light absorption, multiplication, etc. travel, as shown in FIG. 2B, the valence band discontinuity ΔE _v in the light window layer and the light absorption layer is reduced. You will feel it. In particular, due to the device structure, a high electric field cannot be applied to the hetero interface between the optical window layer and the light absorbing layer, and the hole mass is large. Is a problem. Therefore, in the present invention, as shown in FIG. 2A, a structure in which an intermediate layer 2 that satisfies claim 3 is interposed at the boundary between the light window layer and the light absorbing layer is added. With this structure, holes feel valence band discontinuity in a stepwise manner (ΔE _v1 , ΔE _v2 ), so that energy loss, pile-up, etc. are alleviated.

By using the intermediate layers 1 and 2 together, energy loss and pileup of both electrons and holes can be simultaneously improved, and a higher speed response can be achieved. (Figure 2
(In the case of the second example of (a))

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a structural sectional view of an avalanche multiplication type semiconductor light receiving device formed according to an embodiment of the present invention.

The structure of this embodiment is such that an n-buffer layer (n ⁺ -InP buffer layer 11, n
⁺ The -InGaAs buffer layer, n ⁺ type InAlAs buffer layer ^13), n - -InAlAs / InGaAs superlattice multiplication layer 14, p ⁺ -InP electric field drop layer 15, p ^- -
InGaAsP intermediate layer 16, p ^-- InGaAs light absorbing layer 17, p ^-- InGaAsP intermediate layer 18, p ⁺ -In
P optical window layer 19, p ⁺ -InGaAs electrode contact layer 2
0 are sequentially stacked. Here, the electrons generated by the light absorption travel and are injected into the avalanche multiplication layer, and the ionization of the electrons is enhanced by utilizing the large conduction band discontinuity of the superlattice structure. Therefore, an avalanche multiplication type semiconductor light receiving element having a good ionization ratio and good characteristics can be obtained.

Here, the p ⁺ -InP electric field drop layer 15 is based on claim 1, and by controlling the carrier concentration and the layer thickness, the electric field drop amount is up to 250 kV / cm. The P doping species can be various materials such as Be, Zn, and Mg. The important thing is that even if self-diffusion occurs, it does not diffuse out of the electric field drop layer. P doping is performed. This alleviates the effect of tunnel breakdown due to self-diffusion. In this embodiment, the semiconductor layer having a wide energy gap is In
P (energy gap 1.35 ev) is used, but InGaAsP, InAlGaAs, InAl
It is clear that As has the same effect.

The p ^-- InGaAsP intermediate layers 16 and 18 are in accordance with the second and third aspects of the present invention, respectively, and as described in the section of action, energy loss of electrons and holes,
Pile-up is reduced. Therefore, a faster response operation is possible. In this embodiment, an InGaAsP semiconductor layer is used as a material for the intermediate layer.
It is obvious that GaAs has the same effect.

FIG. 3 shows the multiplication and dark current characteristics of the element according to this embodiment together with the conventional example (element of FIG. 5).
As a result, the tunnel current component is suppressed by widening the energy gap of the electric field dropping layer, so that the overall dark current is reduced. For this reason, the dark current level at the multiplication factor M = 10 is reduced to 1/5 as compared with the conventional device, and the maximum multiplication factor M _max ３０30 is increased as compared with the conventional device.

FIG. 4 shows an example of measurement of the multiplication factor and the cutoff frequency of the device of the present invention. Here, (a) in the figure is the case of the device according to the embodiment shown in FIG. 1 (second example), and (b) is p ⁻
This is a case of a device having a structure without the InGaAsP intermediate layers 16 and 18 (first example). This clearly shows that the insertion of the intermediate layer improves the cut-off frequency and achieves a high-speed response.

[0025]

As described above, the avalanche multiplication type semiconductor light receiving device using the superlattice structure according to the present invention as a multiplication layer has a wide energy gap of the electric field drop layer or the light absorption layer and the electric field drop layer, By providing an intermediate layer between the window layers, good multiplication characteristics can be obtained. Therefore, an avalanche multiplication type semiconductor light receiving element excellent in low noise and high speed response can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a superlattice avalanche multiplication type light receiving element according to an embodiment of the present invention and a diagram for explaining an electric field temperature.

FIG. 2 is a diagram showing a band structure of a device according to an example of the present invention.

FIG. 3 is a diagram showing dark current and multiplication characteristics of an element according to the present invention and an element according to a conventional example.

FIG. 4 is a diagram showing a relationship between a multiplication factor and a cutoff frequency of an element according to the present invention.

FIG. 5 is a structural cross-sectional view of an element of a conventional example and a diagram for explaining electric field strength.

[Explanation of symbols]

1 n ⁺ -InP substrate 2 n ⁺ -InAlAs / InGaAs superlattice buffer layer 3 n ⁺ type InAlAs buffer layer 4 n ^- -InAlAs / InGaAs superlattice multiplication layer 5 p ⁺ -InGaAs electric field drop layer 6 p ^- -InGaAs light Absorption layer 7 p ⁺ -InGaAs layer 8 p ⁺ -InAlAs optical window layer 9 p ⁺ -InGaAs electrode contact layer 10 incident light 11 n ⁺ -InP buffer layer 12 n ⁺ -InGaAs buffer layer 13 n ⁺ -InAlAs buffer layer 14 n ^- type InAlAs / InGaAs superlattice multiplication layer 15 p ⁺ -InP electric field drop layer 16 p ^- -InGaAsP intermediate layer 17 p ^- -InGaAs light-absorbing layer 18 p ^- -InGaAsP intermediate layer 19 p ⁺ -InP light window layer 20 p ⁺ -InGaAs electrode contact layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Isao Watanabe, Inventor 5-7-1 Shiba, Minato-ku, Tokyo Inside NEC Corporation (72) Inventor Toshitaka Toritaka 5-7-1 Shiba, Minato-ku, Tokyo Japan (56) References JP-A-2-90575 (JP, A) JP-A-2-298082 (JP, A) JP-A-4-206577 (JP, A)

Claims (4)

(57) [Claims] 1. An electron injection type semiconductor light receiving element having a basic structure of an avalanche multiplication layer having a superlattice structure, an electric field drop layer, a light absorption layer, and an optical window layer on a semiconductor substrate. The semiconductor layer forming the electric field drop layer is InP
Or InGaAsP, wherein the semiconductor layer forming the light absorbing layer is InGaAs . 2. A semiconductor light receiving device having a basic structure of an avalanche multiplication layer having a superlattice structure, an electric field drop layer, a light absorption layer, and an optical window layer on a semiconductor substrate. The semiconductor layer to be formed is p ⁺ -InP or p ⁺
-InGaAsP, and the semiconductor layer forming the light absorption layer is p ^-- InGaAs . 3. A semiconductor according to claim 1 or claim 2.
A light receiving element, wherein a semiconductor layer forming the electric field drop layer
The electron affinity chi _D, conductive semiconductor layer forming the light absorbing layer
When the child affinity is χ _A , the relationship χ _A > χ _I1 > χ _D
A semiconductor layer having an electron affinity χ _I1 that satisfies
That it was interposed in the boundary region between the electric field drop layer and the light absorption layer.
Characteristic semiconductor light receiving element. 4. A semiconductor light receiving element having a basic structure of an avalanche multiplication layer having a superlattice structure, an electric field drop layer, a light absorption layer, and an optical window layer on a semiconductor substrate, wherein
Energy gap E _{D of} the semiconductor layer forming the field lowering layer
Is the energy gap E of the semiconductor layer forming the light absorbing layer
_{A of} the semiconductor layer that is larger than _A and forms the light absorbing layer.
The energy window is E _A , the electron affinity is χ _A , and the optical window
The energy gap of the semiconductor layer forming the layer is E _V ,
When the child affinity is χ _V , χ _A + E _A <χ _I2 + E _I2 <
energy gap E, such as to satisfy the relation of χ _V + E _V
I2, a semiconductor layer having an electron affinity chi _I2, and the light absorbing layer
A semiconductor light receiving element, which is interposed in a boundary region with a light window layer .