JPS6049681A - Semiconductor photodetector device - Google Patents

Abstract

PURPOSE:To increase ionization rate, and to reduce noises by making the discontinuous width of a conduction band between semiconductor layers constituting strain layer superlattice structure larger than that of a valence band. CONSTITUTION:An avalanche multiplication region 12 having strain layer superlattice structure in which N<-> type GaAs layers 12a and N<-> type InGaAs layers 12b are laminated alternately is formed on a P<+> type InP substrate 11. An N<-> type InGaAs optical absorption layer 13 and an N<+> type InP cap layer 14 are formed brought into contact with the strain layer superlattice structure 12. A semiconductor photodetector device is obtained by shaping an antireflection coating film 15, a surface protective film 16, an N type electrode 17 and a P type electrode 18. The discontinuous width of conduction bands among the layers 12a, 12b constituting the strain layer superlattice structure is made larger than that of a valence band at that time.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to a semiconductor light receiving device, and particularly to an avalanche photodiode in which an avalanche multiplication region has a strained layer superlattice structure to further reduce noise.

(b) Technical background In optical communications and other systems that use light as a medium for information signals, semiconductor photodetectors that convert optical signals into electrical signals are important and fundamental components, and many have already been put into practical use. has been done. Among these semiconductor light receiving devices, an avalanche photodiode (hereinafter abbreviated as APD), whose sensitivity is increased by multiplying the photocurrent by avalanche breakdown, is highly effective in improving the signal-to-noise ratio of a photodetector.

In optical fiber communication, which is a typical example of semiconductor photodetector applications, the material dispersion (based on the wavelength dependence of the refractive index) of quartz (SiO2)-based optical fibers used for optical transmission is extremely large at wavelengths around 1.3 [μm]. The sum of the material dispersion and the structural dispersion (based on the wavelength dependence of the propagation constant), that is, the intra-mode dispersion, is small at wavelengths of 163 to 1.55 Cμm.
).

Therefore, as a semiconductor photodetector for optical communication, the wavelength is 1 [μm].
There is a demand for an APD having excellent characteristics in the above band, particularly in the band of about 1.3 [μm] to 1.65 [μm].

(c) Prior Art and Problems Many proposals have already been made using germanium (Ge) or ■-■ group compound semiconductors as APDs targeting a wavelength band of 1 [μm] or more.

Among ■-■ group compound semiconductors, indium phosphorus (InP)
) Indium, gallium, arsenic (
InGaAs) or indium gallium arsenide
APDs using phosphorus (InGaAsP) mixed crystals have good light-receiving sensitivity in this wavelength band and have lower noise than Ge, which is currently in practical use.

Because it has the physical property of low dark current, it occupies an important position as a light receiving device corresponding to this wavelength band. FIG. 1 is a cross-sectional view showing a typical structure of an InP-InGaAs-based APD. In the figure, 1 is an n++InP substrate, 2 is an n-type InP buffer layer, and 3 is an n-type InGaAs substrate.
A light absorption layer, 4 is an n-type InP window layer, 5 is a p+ type region 6 formed in the InP window layer 4, is a p-type region forming a guard ring, 7 is a protective insulating film, 8 is an antireflection film,
9 indicates a p-side electrode, and 10 indicates an n-side electrode.
A depletion layer is formed between the pn junction, that is, the interface between the p+ type region 5 and the n type InP window layer 4, and this is the n type InP region.
It spreads to the GaAs light absorption layer 3, and in this light absorption layer 3, the input signal light excites electrons to the conduction band, thereby generating electron-hole pairs, with the electrons being transferred to the n-side electrode 10 and the holes being transferred to the p-side electrode 10. The holes drift toward the electrode 9, and in the n-type InP window layer 4, avalanche multiplication is performed using some of these holes as carriers. For this reason, the n-type InP window layer 4 is also called a multiplication layer or a multiplication region.

In the process of avalanche multiplication, there is statistical fluctuation in the number of collisions between carriers and atoms constituting the crystal lattice, which causes inherent shot noise. This noise is usually called multiplication noise.

3- In the process of avalanche multiplication, when the number of times electrons undergo impact ionization per unit length, that is, the ionization rate of electrons is α, and the ionization rate of holes is β, then the ionization rate ratio k (β/α or α/β ) is close to 1, the multiplication noise is large, and when the ionization rate ratio is large, the multiplication noise decreases.

In the conventional example of the InP--InGaAs-based APD described above, the ionization rate ratio = α/β is as small as about 2 to 3, and there are physical restrictions on the multiplication noise.

On the other hand, the introduction of superlattice structures has been proposed for various semiconductor devices, and for APDs, the multiplication layer is made of gallium.
Arsenic (GaAs) and gallium aluminum arsenic (
As a superlattice structure in which GaAIAs) are alternately stacked, the apparent ionization energy of carriers increases or decreases due to the discontinuity in the energy band that occurs at the heterojunction interface between the GaAs layer and the GaAlAs layer, and electrons and positive A structure that reduces multiplication noise by increasing the ionization rate ratio with holes has previously been known.

However, depending on the GaAs-GaAlAs semiconductor material, it is impossible to construct a light receiving device that can handle a wavelength band of 1.0 [μm] or more.
The present applicant previously filed Japanese Patent Application No. 38519-1985 for an APD in which a superlattice structure multiplication layer is formed using a P-based semiconductor material.
It is provided by the number.

FIG. 2 shows a nP and an I lattice-matched to it according to the invention.
FIG. 3 is a diagram showing a part of the energy diagram of a superlattice structure formed by n O, 53 Ga 2 O, and 47 As. The forbidden band width of the semiconductor layers used in this structure is approximately 1.35 (eV) for InP and approximately 0.75 (eV) for Ino53Qao and uAB, and there is a discontinuity in the conduction band at the heterojunction interface between both layers. △Ec * 0.2 (eV)
, the valence band discontinuity ΔEv is 0.4 (eV).

When a hole enters the InGaAs layer from the InP layer, it apparently gains extra energy by the energy difference ΔEv in the valence band, and the ionization rate increases.

For electrons, this effect is almost absent because the energy difference in the conduction band is small, and the ratio of ionization rates between the two is expanded by 1 β/α, thereby reducing multiplication noise.

However, the InP-InGaAs or InP of the invention
- In the superlattice structure formed by the combination of GaAsP,
Since the effect of energy discontinuity appears as an increase in the ionization rate of holes, the effect of expanding the ionization rate ratio is slightly smaller than that of increasing the ionization rate of electrons, and the ionization rate ratio is 8 to 1.
It has stopped at around 7. There is a need for an APD with a superlattice structure in which the ionization rate of electrons is increased by energy discontinuities to further reduce multiplication noise.

(d) Purpose of the Invention The present invention relates to a semiconductor photodetector, particularly an avalanche photodiode equipped with an avalanche multiplication region having a superlattice structure, by expanding the ionization rate ratio beyond the limitations of conventional physical properties and achieving low noise. The purpose of this invention is to provide a semiconductor photodetector.

(e) Structure of the Invention The object of the present invention is to provide a semiconductor light absorption region and a semiconductor avalanche multiplication region having a strained layer superlattice structure on a semiconductor substrate, thereby configuring the strained layer superlattice structure. This is achieved by a semiconductor photodetector in which the discontinuity width of the conduction band between semiconductor layers is smaller than the discontinuity width of the valence band.

In other words, while conventionally the semiconductor layer constituting the superlattice structure of the avalanche multiplication region is limited to a combination of semiconductor materials that are mismatched to the substrate crystal, in the present invention Applying the fact that high-quality crystals can be obtained in a superlattice structure even when using a certain semiconductor material, we have developed a strained layer superlattice (5tr
ained -1 ayer super lattice
) By forming an avalanche multiplication region with a structure, it is possible to realize an avalanche multiplication region in which the discontinuity width of the conduction band is large and the carrier ionization rate ratio is greatly expanded by increasing the electron ionization rate. This achieves low noise.

Note that it is practical to keep the lattice mismatch between the semiconductor layers constituting the strained layer superlattice structure to less than 4XIO'' when the superstructure is fully formed, and the lattice mismatch is 1×10
Depending on the combination of less than 100%, there will not be much difference from the conventional structure.

(f) Examples of the invention Hereinafter, the present invention will be explained in detail by way of examples and with reference to the drawings. Explain in detail.

FIGS. 3(a) and 3(b) are cross-sectional views showing the main manufacturing steps of an embodiment of the present invention.

FIG. 3(a) On a p1-type InP substrate 11 with a reference impurity concentration of about 1×1018 [σ-3] or more, each impurity concentration is 1×015
cIrL-3] or less
A strained layer superlattice structure 12 in which n″′-type InxGa and -xAs layers 12b are alternately laminated is formed by, for example, a vapor phase growth method.

The Inx Ga1-xAa layer 12b has x: 0.53
When the lattice constant matches that of the InP substrate 11,
It is not necessary to match the lattice constants, and the lattice constant of the GaAs layer 12a is smaller than that of the InP substrate 11, and is expanded compared to the GaAs bulk crystal, but the lattice constants do not match.

In this embodiment, a GaAs layer 12a and an InGaA layer 12a are used.
The thickness of the s layer 12b is about 30 to 40 (nm),
For example, the total thickness of the strained layer superlattice structure 12 is 1[/1f'n
] Strong.

=8− Furthermore, the impurity concentration is 1×10 in contact with the strained layer superlattice structure 12.
N-type b+ of about 15 [crn-3] or less 0.53
The GaO, 47As layer 13 is formed to a thickness of, for example, about 2 [μm], and then the impurity concentration is about 1×10 18 [cIn-3] or more (D n” type InP layer 14 is formed to a thickness of, for example, 0.4 [μm]).
micrometer].

Refer to FIG. 3(b) As shown in the figure, the semiconductor substrate is a p+ type InP substrate 11.
A non-reflection coating film 15 is formed on the surface of the n+ type InP layer 14, and a surface protection film 16 is formed on the mesa etched surface using silicon nitride (Si3N).
4) etc., and is in contact with the n+ type InP layer 14.
The side electrode 17 is formed using, for example, gold/germanium (AuGe), and the p-side electrode 18 in contact with the p+ type InP substrate 11 is formed using, for example, gold/zinc (AuZn).

FIG. 4 shows an energy diagram when a reverse bias voltage is applied to this embodiment described above. In FIG. 4, corresponding semiconductor layers are indicated by the same reference numerals as in FIGS. 3(a) and 3(b). The forbidden band width for GaAs is approximately 1.4
2 (eV) and about 0.75 (eV) for InGaAs, and the energy discontinuity at the heterojunction interface between the GaAs layer 12a and the InGaAs layer 12b means that the conduction band is ΔEc 0.47 (eV). ), the valence band is 0. in ΔEv.
20 (eV).

In the APD of this embodiment, electrons excited by optical energy in the n-type InGaAa light absorption layer 13 enter the strained layer superlattice structure 12, and when entering the InGaAs layer 12b from the GaAs layer 12a, the electrons enter the conduction band. Apparently, extra energy is obtained by the discontinuity width ΔEc: 0.47 CeV), and the ionization rate of electrons increases.

As for holes, the width of discontinuity in the valence band is small, and the ratio of ionization rates between the two is expanded by =α/β.

In the embodiments described above, for example, the electric field is 1 to 2×
105 [v/α], the ionization rate ratio -α/β is approximately 80, and the above-mentioned I
The ionization rate ratio of an APD having a multiplication region formed by an nP-InGaAs superlattice has been significantly increased compared to about Ki8 to Ki17.

The above explanation and examples are based on the wavelength band 1 to 1.65 [μm].
], but by selecting each semiconductor material according to the method of the present invention, similar effects can be obtained for APD's in other wavelength bands.

(g) As described in detail, in the present invention, the superlattice structure serving as the avalanche multiplication region of the semiconductor photodetector is replaced by a strained layer superlattice structure in which lattice matching is not performed between the semiconductor layers constituting the superlattice structure. By employing all combinations of large discontinuity widths in the conduction band, the ionization rate of electrons is increased and noise is reduced more than ever before, which is useful for optical information transmission, for example by increasing the relay spacing in optical fiber communications. It has an excellent effect on the advancement of the system as a medium.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing a conventional example of an APD, FIG. 2 is a diagram showing an example of an energy diagram of a conventional lattice-matched superlattice multiplication region, and FIGS. A sectional view showing the embodiment, and FIG. 4 is an energy diagram of the embodiment. 11- In the figure, 11 is a p-type 1nP substrate, 12 is an avalanche multiplication region with a strained layer superlattice structure, 12a is an n-type GaAs layer,
12 bll'j n-type InGaAs layer, 13 n-type I
nGaAs light absorption layer, 14 is n + type InP cap layer, 15 is anti-reflection coating film, 16 is surface protection film, 17ban
18 indicates a p-side electrode. 12- Rod 1 Figure 2 Figure 3 Figure 4 @

Claims (3)

[Claims] (1) A semiconductor substrate is provided with a semiconductor light absorption region and a semiconductor avalanche multiplication region having a strained layer superlattice structure, and conduction band discontinuity between semiconductor layers constituting the strained layer superlattice structure. A semiconductor light receiving device characterized in that the width is greater than the discontinuity width of a valence band. (2) The lattice constant mismatch between the semiconductor layers constituting the strained layer superlattice structure is 1×10−3 or more and 4×10−2
2. The semiconductor light receiving device according to claim 1, wherein the semiconductor light receiving device is less than 10%. (3) The semiconductor substrate is made of indium phosphide, and the strained layer superlattice structure is made of gallium arsenide and indium-gallium arsenide. Semiconductor photodetector.