JPH06120556A - Photodetector - Google Patents

Abstract

PURPOSE:To realize a photodetector, provided with a small excess multiplying noise factor F, a large S/N ratio, high quantum efficiency same as an APD so far and sensibility in a desired wave-length band. CONSTITUTION:A photodetector is provided with a transforming layer 24, formed on a III-V compound semiconductor 1, a solid layer 2, laminated on the transforming layer 24 and mainly constituted of selenium, and voltage impressing means 3, 4, generating an electric field on the solid layer mainly constituted of selenium, while avalanche multiplication is caused by pouring a positive hole 6 generated in the compound semiconductor 1 into the solid layer 2. According to this method, a photodetector, provided with a large excess multiplying noise factor F and a high sensibility in a desired wave length band, can be realized by the large coefficient of ionization rate of selenium, the prevention of recombination effected by the transforming layer 24 and the high quantum efficiency of the III-V compound semiconductor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light receiving element, and more particularly to a light receiving element which converts an optical signal into an electrical signal by avalanche multiplication.

[0002]

2. Description of the Related Art When light is absorbed in a semiconductor, electrons or holes which are conductive carriers are formed in the semiconductor. The so-called light receiving element takes out the electric current by the carrier to the outside and converts it into an electric signal. Among them, avalanche photodiodes (hereinafter referred to as “AP”), which have avalanche multiplication (avalanche multiplication) by collision ionization of generated conductive carriers in a high electric field region formed in the device,
It is called D) and has high sensitivity and high-speed response. In particular, I
APDs that use compound semiconductors such as nGaAs and InGaAsP for the light absorption layer can be used in the light receiving element for optical communication because they can operate in the long wavelength (1.0 to 1.6 μm) band where the transmission loss of the optical fiber is small. There is. Fig. 2 shows a cross-sectional view of a conventional APD, which is an example of this (Hiroaki Ando et al., AIE Journal of Quantum Electronics Volume QE-17 No. 2, No. 25).
Pp. 0-254 (1981) ", Hiroaki A
ndo et al. , IEEE Journal
ofQuantum Electronics, Vo
lume QE-17, Number 2, p. 25
0-254 (1981) ”), which is formed on the InP substrates 11 and 12 which are III-V group compound semiconductors by III-.
It is an APD obtained by growing InGaAs 13, which is a group V compound semiconductor, and is a light receiving element in the long wavelength band of wavelength 1.3 to 1.6 μm. The light 5 incident from the upper surface of the element is absorbed by the n-InGaAs layer 13 having a small band gap, and n-
Electron-hole pairs are formed in the InGaAs layer 13. On the other hand, the Au-Zn layer 17, which is an ohmic electrode, and Au-Ge-
By the reverse bias applied between the Ni layers 18, n-InP
A high electric field region is formed at the pn junction of the layer 14 and the p (+)-InP region 15 (which indicates that the concentration is high ((+); the same applies hereinafter). Holes of the generated electron-hole pairs are injected into this high electric field region and avalanche multiplication is performed. In
There is also an APD that uses InGaAsP instead of GaAs (Katsuhiko et al., Applied Physics Letters, Vol. 3, No. 3, page 251, 1979, "Katsuh.
iko Nishida et al, Applie
d Physics Letters, Volume
35, Number 3, p. 251 (197
9) ”).

In the compound semiconductor APD, the light absorption coefficient of the compound semiconductor is large, high quantum efficiency is obtained, and the band structure such as the band gap is changed by using a mixed crystal or heterojunction in which different compound semiconductors are mixed, It has the feature that the operating wavelength band can be freely designed. In particular, with the compound semiconductor APD, an APD having sensitivity in a long wavelength band of 1.0 to 1.6 μm, which is important in optical communication, can be obtained as in the above-mentioned conventional example.

[0004]

The over-multiplication noise factor F is an important parameter indicating the performance of the APD. This shows the degree of noise added to the normal noise multiplication due to the statistical fluctuation that occurs in the avalanche multiplication process. The smaller the value of F, the smaller the added noise, and the APD having a higher S / N ratio and high sensitivity can be obtained. Excess multiplication noise factor F when holes are injected into a high electric field region
Is represented by the following McIntyre formula. F = M {1- (1-1 / k) (1-1 / M) ² } where k is the ratio β / α of the ionization coefficient β of holes and the ionization coefficient α of electrons. The ionization coefficients α and β are the number of times that electrons and holes cause impact ionization while traveling a unit distance.
M is a multiplication factor in avalanche multiplication and is a function of the ionization coefficient and the thickness of the multiplication region. From the above equation, it can be seen that F becomes larger as the ionization coefficient ratio k becomes smaller. This can be explained as follows. k to 1, that is, when the collision ionization of electrons and holes occurs to the same degree, the electrons of the electron-hole pairs generated by the collision ionization of the injected holes are accelerated in the opposite direction and collision ionization is performed again. cause. For this reason, the multiplication time becomes long and the statistical fluctuation in the multiplication process becomes large, resulting in a large generation of excessive multiplication noise. On the contrary, when k >> 1, that is, the impact ionization of electrons is smaller than the impact ionization of holes, the multiplication process is completed promptly and there is little noise generation because there is no ionization by electrons.

In the conventional APD shown in FIG. 2, InP14
Since the avalanche multiplication is performed therein, the S / N ratio is determined by the ionization coefficient ratio k of InP14. However, the k of InP is as small as about 2, which causes a problem that the excessive multiplication noise coefficient F increases and the S / N ratio of APD decreases. The same is true when a compound semiconductor other than InP is used, and since the ionization coefficient ratio k of the compound semiconductor is as small as about 2, a sufficient A / N ratio can be obtained in the conventional APD that uses avalanche multiplication in the compound semiconductor. There was a problem that can not be. The object of the present invention is to solve the above-mentioned conventional problems, to have a small excess multiplication noise factor F and a large S / N ratio,
Moreover, at the same time, the quantum efficiency is high as in the conventional APD,
It is to realize a light receiving element having a sensitivity in a desired wavelength band.

[0006]

In order to achieve the above object, a light receiving element according to the present invention is mainly composed of a compound semiconductor as a light absorption layer and selenium laminated on the compound semiconductor via a modified layer. It has an electric field applying means for generating a high electric field in the solid layer and the solid layer mainly containing selenium.
The modified layer is composed of a compound of a group III element and chalcogen and a compound of a group V element and chalcogen (in the present specification, selenium, tellurium and sulfur are referred to as chalcogen) alone or both, and a preferable embodiment is a modified layer. Has a thickness of 1 atomic layer or more and several atomic layers or less. The compound semiconductor is made of indium gallium arsenide (InGaAs) or indium gallium arsenide phosphide (In) as in the conventional APD.
GaAsP) gallium arsenide antimony (GaAsS)
b), aluminum gallium arsenide antimony (AlGa
Group III-V compounds such as AsSb) are used. Further, depending on the type of the compound semiconductor, a material such as tellurium which has an effect of reducing the band gap is added to the selenium layer near the junction with the compound semiconductor to reduce the barrier. Further, a substance that improves electrical properties such as arsenic and thermal stability of the film is added to the solid layer containing selenium as a main component.

[0007]

1 is a block diagram of a light receiving element of the present invention for explaining the principle of the present invention. A solid layer 2 containing selenium as a main component is deposited on a compound semiconductor 1 which is a light absorbing layer via a modified layer 24, and electrodes 3 and 4 are provided on the compound semiconductor 1 and the solid layer 2, respectively. By applying a bias voltage between the electrodes 3 and 4, a high electric field (E) 7 is generated in the solid layer 2 mainly composed of selenium. Light from the outside 5
Are absorbed by the compound semiconductor 1 to generate electron and hole current carriers. Due to the high electric field (E) 7, the holes 6 of the current carrier pass through the modified layer 24 and are injected into the solid layer 2 mainly composed of selenium, and avalanche multiplication is performed.

In the above light-receiving element, the ionization coefficient of holes of selenium is remarkably larger than that of electrons, and the ionization coefficient ratio k is 10 to 100, which is several times or more that of conventional compound semiconductors. Therefore, according to the light receiving element of the present invention, a highly sensitive APD having a small excess multiplication noise coefficient F and a high S / N ratio can be obtained. In addition, in the light receiving element according to the present invention, since the compound semiconductor layer absorbs light, the quantum efficiency is high, and the band structure such as the band gap is changed by using a mixed crystal or a heterojunction in which different compound semiconductors are mixed, It is possible to design an element having sensitivity in the wavelength band of. Further, since the band gap of selenium is as large as 2 eV, there is a feature that a high electric field can be applied without the generation of leak current due to tunneling.

In order to obtain good operation in the light receiving device of the present invention, it is necessary that the interface state density at the interface between the compound semiconductor layer and the selenium layer is small. When a high-density interface state exists at the interface, holes are recombined with electrons through the interface state before being injected into the selenium layer, which causes a problem of lowering quantum efficiency. In particular, when a natural oxide film of a compound semiconductor is present at the interface, the interface state density increases and good operation cannot be obtained. However, if a modified layer is formed,
A very good bond with few interface states can be obtained. Therefore, in this case, there is almost no problem due to recombination of holes. Generally, there is a discontinuity at the top of the valence band at the heterojunction interface between different kinds of semiconductors, which may serve as a potential barrier for holes. However, as a result of evaluating the light receiving element of the present invention, when GaAs is used as the compound semiconductor, GaA
It was found that there was almost no barrier to holes seen from the GaAs side at the junction interface between s and selenium. Also, GaAs
Other compound semiconductors also have small barriers. Therefore, in the light receiving element of the present invention, the holes generated in the compound semiconductor are injected into the selenium layer which is the avalanche region without being blocked by the barrier. When the barrier generated at the interface of the compound semiconductor becomes a problem, a material having an effect of reducing the band gap such as tellurium may be added to the selenium layer near the junction with the compound semiconductor to reduce the barrier.

As the selenium layer used in the multiplication region, an amorphous or microcrystalline film is usually used. In this case, however, it is easy to obtain a uniform film, and local breakdown due to electric field concentration is unlikely to occur. Further, depending on the film forming conditions, it may be in a single crystal form. In this case, the thermal deterioration of the characteristics is smaller than that of the amorphous or microcrystalline film, and the mobility of the current carrier is increased, so that the device can operate at higher speed. Also, instead of using a pure selenium film, other materials such as arsenic may be included as an accessory component to improve the electrical properties and thermal stability of the film.

[0011]

Embodiments of the light receiving element of the present invention will now be described with reference to the drawings together with a manufacturing method. FIG. 3 shows an essential part of a manufacturing process of one embodiment of a light receiving element (APD) according to the present invention. The n-InP layer 22 was grown as a buffer layer on the n (+)-InP substrate 21 by the molecular beam epitaxy method, and then the n-InGaAs layer 23 was grown (FIG. 3A). n
Mole fraction -InGaAs layer 23, i.e. In _x Ga ₁ _ _x
When expressed as As, x was 0.53. In this case I
The nGaAs 23 lattice-matches the InP 21 of the substrate. Next, selenium was reacted with the surface of the InGaAs layer 23 to form a modified layer 24 made of selenide, and then the amorphous selenium layer 2 was formed on the metamorphic layer 24 (FIG. 3).
(B)). Subsequently, the substrate temperature was raised to room temperature and the amorphous selenium layer 24 was deposited. Next, A on the selenium layer 2
An electrode 3 made of u and an electrode 4 made of Au—Ge—Ni were formed on the back surface of the n (+)-InP substrate 21. Although the molecular beam epitaxy method is used in the above embodiment, chemical vapor deposition method, liquid phase growth method or the like may be used. In this embodiment, the surface of InGaAs 23 is irradiated with a selenium molecular beam at a substrate temperature of 300 ° C. in a vacuum. Although the modified layer was formed by the method, when the selenium molecular beam is irradiated at a substrate temperature of 200 ° C. or higher, a modified layer of selenide having a thickness of several atomic layers or less is formed on the surface, and excess selenium is evaporated. On the other hand, if a selenium molecular beam is irradiated at room temperature, a thick selenium film can be deposited. When the substrate temperature is room temperature, the film becomes amorphous,
It is also possible to form a single crystal selenium film by irradiating a selenium molecular beam at a substrate temperature of about 70 to 80 ° C. In the above, the selenium molecular beam irradiation is performed as the treatment before the selenium layer deposition, but the surface may be treated by the following method.

After the surface of the compound semiconductor is treated with a solution containing chalcogen such as ammonium sulfide to form a modified layer on the surface, a solid layer containing selenium as a main component is deposited in a vacuum. In this case, the solution has a function of removing the natural oxide film and a function of forming a modified layer on the clean surface thus obtained. After the compound semiconductor layer is formed, the natural oxide film on the surface of the compound semiconductor layer is removed to form a clean surface, and the surface of the compound semiconductor layer is reacted with chalcogen to form a modified layer. After forming the compound semiconductor layer, the natural oxide film on the surface of the compound semiconductor layer is removed to form a clean surface, and a solid layer mainly containing selenium is directly deposited. Also in this case, a modified layer is formed at the interface between selenium and the compound semiconductor. In any of the above methods, since the modified layer is formed on the surface from which the natural oxide film is substantially removed, the natural oxide film formed by the molecular beam epitaxy method on the surface where substantially no natural oxide film is present as in this example. The same good interface as when the modified layer is formed is created.

In order to operate the light receiving element according to this embodiment, first, a negative bias is applied to the electrode 3 with respect to the electrode 4 to generate a high electric field in the selenium layer 2. Light having a wavelength of 1.0 to 1.6 μm incident from the lower surface of the n (+)-InP substrate 21 passes through the InP substrate 21 and the buffer layer 22, and is absorbed by the n-InGaAs layer 23 having a band gap smaller than InP. . Of the electron-hole pairs formed at that time, holes are injected into the selenium layer 2 and avalanche multiplication is performed in a high electric field.
As a result, the optical signal is extracted as an electrical signal to the outside. In the light receiving element of this embodiment, since the avalanche multiplication is performed in the selenium layer, the excess multiplication noise coefficient F is small and a high S / N ratio can be obtained. Further, since the metamorphic layer 24 made of selenide is formed by reacting selenium on the surface of the n-InGaAs layer before forming the selenium layer, the generation of harmful interface states that cause recombination of electron-hole pairs occurs. It hardly occurs at the interface between the layer and the n-InGaAs layer. Therefore, it is possible to obtain an APD with high conversion efficiency of a light signal. When the selenium layer 2 is made amorphous, the addition of arsenic suppresses the change in characteristics due to the crystallization of the film and improves the heat resistance.

In the embodiment described above, InGaAs is used as the light absorption layer, but other compound semiconductors such as InGaAsP, GaAsSb, AlGaA are used.
sSb or the like may be used, and the operating wavelength band can be changed depending on the difference in band gap. The type of substrate used is selected in consideration of the lattice constant of the compound semiconductor used as the absorption layer. Further, if a material whose band gap is larger than the energy corresponding to the wavelength of light to be detected is used for the substrate, light can be incident from the back surface of the substrate as in this embodiment. However, if a material capable of transmitting the light to be detected is used for the electrode 3, the light can be incident from the upper surface. When the energy of light is 2 eV or less (wavelength is 0.6 μm or more), the light reaches the absorption layer made of a compound semiconductor without being absorbed by the selenium layer. Further, there is no limitation on the band gap of the substrate material used in this case.

[0015]

According to the present invention, the S / N ratio is large because the excess multiplication noise coefficient is small, and in particular, by providing the modified layer, carrier recombination due to the interface state does not occur and high quantum efficiency and high sensitivity are achieved. The avalanche photodiode of is obtained. Further, it is possible to change the operating wavelength band by changing the material of the light absorption region. Especially important for optical communication
It is possible to obtain a high-sensitivity, high-speed avalanche photodiode that has sensitivity in the wavelength band of 1.6 μm and is particularly useful as a light-receiving element for optical communication.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a light receiving element of the present invention for explaining the principle of the present invention.

FIG. 2 is a cross-sectional view of a conventional avalanche photodiode.

FIG. 3 shows a structure and a manufacturing process of an embodiment of a light receiving element according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Compound semiconductor, 2 ... Solid layer mainly containing selenium, 3, 4 ... Electrode, 5 ... Incident light, 6 ... Symbol representing a hole, 7 ... Symbol representing an electric field, 11, 21 ... n (+)- InP substrate, 12, 22 ... n-InP layer, 13,23 ... n-InGaAs layer, 14 ... n-InP layer, 15 ... p (+)-InP region, 16 ... SiO ₂ film, 17 ... Au-Zn layer , 18 ... Au-Ge-Ni layer, 24 ... Modified layer.

Claims (6)

[Claims] 1. A modified layer formed on a compound semiconductor,
A selenium-based solid layer laminated on the modified layer, and a voltage applying means for generating an electric field in the selenium-based solid layer, and a current carrier generated by absorption of light in the compound semiconductor. Is injected into the solid layer containing selenium as a main component and avalanche multiplication is performed. 2. The light-receiving element according to claim 1, wherein the thickness of the modified layer is at least one atomic layer and several atomic layers or less at the interface between the compound semiconductor and the solid layer mainly containing selenium. 3. The light receiving element according to claim 1, wherein the compound semiconductor is a III-V group compound semiconductor. 4. The compound semiconductor layer comprises indium gallium arsenide (InGaAs) or indium gallium arsenide phosphide (InGaAsP) gallium arsenide antimony (G).
3. The light receiving element according to claim 1, wherein the light receiving element is selected from the group consisting of aAsSb) and aluminum gallium arsenide antimony (AlGaAsSb). 5. The light-receiving element according to claim 1, wherein tellurium is added to the solid layer containing selenium as a main component. 6. The light-receiving element according to claim 1, wherein arsenic is added to the solid layer containing selenium as a main component.