JPH05251725A - Silicon semiconductor light-receiving element and its manufacture - Google Patents

Abstract

(57) [Summary] [Purpose] Since silicon has a bandgap of 1.1 eV, a photodetector using a silicon semiconductor has the highest sensitivity in the infrared region. It has low sensitivity to visible light. To provide a silicon semiconductor light receiving element which has high sensitivity to visible light without being restricted by the narrow bandgap. [Configuration] Silicon is anodized in hydrofluoric acid. A large number of holes grow in the direction perpendicular to the surface by anodic oxidation. Due to the holes, the silicon is divided into columns and electrons are confined two-dimensionally. Therefore, the band gap of silicon is effectively widened. The sensitivity extends in the visible light range. The electron mobility does not decrease because the current flow is perpendicular to the plane and the resistance does not increase in this direction. The sensitive region can be expanded toward a shorter wavelength range without impairing the high speed property.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon semiconductor light receiving element having high sensitivity in the visible light region and a method for manufacturing the same.

[0002]

2. Description of the Related Art Semiconductor materials are used as materials for converting light energy and light signals into electrical energy and signals. Among them, silicon semiconductors are cheap and can be made large in area. Photoelectric conversion elements using silicon such as photodiodes, phototransistors, and solar cells are widely manufactured and used. In the case of energy conversion, when light exceeding the band gap is incident on the vicinity of the pn junction, the light excites electron-hole pairs, which generate an electric current due to an internal electric field, resulting in light energy. In the case of signal conversion, a reverse bias is applied to the pn junction, and when light exceeding the band gap is incident near the pn junction, electron-hole pairs are excited across the band gap, and this causes reverse bias to cause electrons to reach the n-pole. Then, holes flow to the p-pole and become a current. In photoelectric conversion by semiconductors, electron-hole pairs are excited over the band gap, so that the wavelength of light that can be detected is limited. In other words, it is highly sensitive to light with energy near the band gap,
If it deviates from this, the sensitivity will decrease. It is not sensitive to light of lower energy and less sensitive to light of energy much larger than the bandgap.

FIG. 2 shows an example of a conventional silicon photodiode. The i layer 2 is stacked on the n-type layer 1. A p-type region 3 is selectively formed in the center of i layer 2. This is made by impurity diffusion, ion implantation, etc. Further, a SiO ₂ layer 4 which is an insulator is provided in a ring shape, and a ring-shaped p-side electrode 5 is formed thereon. This is in ohmic contact with the p-type region 3. The portion where the light enters the p-type region 3 is an opening. This portion is covered with the antireflection film 6. The light enters through the opening on the upper side, and in the vicinity of the boundary between the p-type layer 3 and the i-layer 2, electron-hole pairs are generated and disappear. The n-type layer 1 shown below may be an n-type substrate or a thin film grown on the n-type substrate. An n-side electrode is formed under the n-type layer, but this is not shown. The material silicon is a single crystal with almost no defects. There are various types of photodiodes, and there are various types in which light is incident from the substrate. The one shown in FIG. 2 is the most common type of photodiode. When a reverse bias is applied between the electrodes and light is irradiated, the light enters the p-type region and the i-layer through the central opening to generate electron-hole pairs, and a current flows due to the reverse bias.

FIG. 4 shows another type of photodiode according to the conventional example. This is a structure in which an n ⁺ type layer 7 and an n type layer 8 are laminated. The n ⁺ type layer 7 may be a substrate or a thin film grown on the substrate. A ring-shaped insulator SiO ₂ film 9 is formed on the peripheral portion of the n-type layer 8.
A semitransparent metal thin film 10 is provided in the central portion. A ring-shaped metal electrode 11 is provided on these. There is an antireflection film 12 in the central opening. In this, the semitransparent metal film 10 and the n-type layer 8 are in Schottky junction. Here, the diode characteristic occurs. When light enters the n-type layer and generates electron-hole pairs, a current flows due to the reverse bias voltage. Of course, when impurities are added to create an electron level in the band gap, a transition from this level occurs, so that it is possible to have a finite sensitivity even to light with a lower energy. However, even so, the sensitivity is extremely low for light having an energy far away from the band gap energy.

[0005]

However, the forbidden band width of a silicon semiconductor is 1.12 eV, which is small. This is 1.1 μm when converted to the wavelength of light. Therefore, the wavelength of the silicon semiconductor having high sensitivity is in the infrared region. In order to detect light having a shorter wavelength and to convert light having a shorter wavelength into energy, it is necessary to use a semiconductor having a wider band gap. For example, GaAs is 1.4
Has a band gap of. Since this is a direct transition type, it can have high sensitivity to light. However, the sensitivity to visible light is not so good. In addition, since GaAs semiconductor is an expensive material, it becomes an expensive element when used as a photodiode or a solar cell. Of course, GaAs is often used when high speed is important. This utilizes the property that the electron mobility is larger than the band gap. It is an object of the present invention to provide a light-receiving element having high sensitivity to light having a shorter wavelength and a manufacturing method thereof.

[0006]

A silicon semiconductor light receiving element of the present invention comprises a porous silicon semiconductor layer, a p region, and an n region.
A region, a pn junction is provided, and an electrode connected to the p region and the n region of porous silicon is formed on the silicon semiconductor,
In addition, a light receiving portion for guiding light to the pn junction is provided. In the method for manufacturing a light-receiving element of the present invention, an n-type or p-type single crystal silicon is anodized in hydrofluoric acid to partially make it porous, and a p region or an n region is formed on this silicon substrate by a diffusion method. It is characterized in that it is formed, a ring-shaped insulator and a ring-shaped electrode are provided thereon, and the other electrode is provided on the silicon substrate. Another method of manufacturing a light-receiving element of the present invention is to epitaxially grow a p-type layer or an n-type layer on an n-type or p-type substrate, and then anodize in hydrofluoric acid to make a part of silicon porous. , And an n-side electrode and a p-side electrode are formed on each of them.

[0007]

The single crystal silicon has a isotropic energy-band structure because it becomes a Bloch wave because the wave function can spread three-dimensionally. It has a narrow and constant band gap as described above. However, it has been suggested in recent years that silicon made porous by anodic oxidation may have a wider band gap. L.
T. Canham, "Silicon quantum wire array fabrication b
y electrochemicaland chemical dissolution of wafer
s ", Appl.Phys.Lett.57, (10), p1046, (1990) This is when a silicon single crystal wafer is oxidized in a 40% hydrofluoric acid solution using the wafer as an anode and the Pt electrode as a cathode. It is said that the surface of the wafer is porous and the band structure is different.Since glass and SiO ₂ are corroded by hydrofluoric acid, Si is not corroded.This is a condition that enables photolithography of Si. However, it has been found that Si is corroded in the direction perpendicular to the plane when Si is used as an anode in hydrofluoric acid, and many holes grow in the direction perpendicular to the plane. As the anodization progresses, the depth and diameter of the holes increase, and when the porosity increases to about 80%, adjacent holes come into contact with each other. The columnar Si has a thickness of about several nm. If that, the running possible region of the electron is perpendicular linear to the surface. This is the is the quantum wires, Canham alleges.

In the bulk single crystal, the electrons are in the Bloch state, and the wave function is three-dimensionally spread freely. If this is limited to a two-dimensional spread or a one-dimensional spread, various new properties will appear. Such attempts are mainly based on physical interests, but in some cases they are actually used in industry. For example, in the case where the Si-doped AlGaAs layer is provided on the non-doped GaAs layer, the latter electrons enter the non-doped GaAs layer and become a two-dimensional electron gas there. This is because the electron affinity of AlGaAs is larger and the band gap of AlGaAs is larger. These thin films are made by the molecular beam epitaxy method. Since the electron mobility is extremely high, F
When used as an ET, it can be a high-speed device. This is a two-dimensional electron gas because it can move freely in the direction parallel to the plane. The molecular beam epitaxy method is most suitable for producing such a material. On the contrary, it must have been possible to produce such a material without the molecular beam epitaxy method. This is because it is necessary to epitaxially grow a thin thin film having an order close to the atomic spacing. In order for the quantum effect to appear, a well-type potential that can confine electrons in a narrow range must be formed. For example, a thin film of GaAs and AlGaAs is alternately grown to form a large number of layers. This is a well-type potential arranged in the plane direction.

In order to confine one-dimensionally, the above structure must be formed not only in the plane direction but also in the direction parallel to the plane. For example, there is one in which thin films of different components are laminated, and this is obliquely etched to further laminate thin films of different components. In this case, a part where electrons are localized is generated at a part corresponding to an intersection of layers having a small band gap. Since this has a one-dimensional spread, the electrons move in a linear direction. Since this spread is on the order of Å, the quantum effect occurs. Such a thing is sometimes called a quantum wire. In the well-type potential, the energy of the electron is discretized, but the energy of the lowest level is not zero due to the zero-point oscillation. The width of the potential is d
Then, since the wave function of the lowest level has the wavelength λ = 2d, the wave number k = 2π / λ = π / d. Thus the width direction of the kinetic energy - is h ² / 8md ^2. This is the lowest level energy. The higher level energies form discrete levels above this. Although movement is restricted in the direction orthogonal to the quantum wire, this can create a normal band because electrons can move freely in the direction of the wire. Then, the quantum effect can form a state having a level above the band edge. Two
The behavior of electrons trapped in one dimension and one dimension has been studied by examining the response by applying a magnetic field or irradiating light. It has been said that quantum wires cannot be produced without epitaxial growth by molecular beam epitaxy. But S
It is argued that this is a quantum wire, since the i made porous by anodic oxidation is localized in the direction perpendicular to the plane.

Canham has investigated photoluminescence at 4.2K for anodized p ⁻ and p ⁺ Si wafers. These are 1.43eV, 1.38e
A peak is shown for an energy of about V. Since the original band gap of Si is 1.1 eV, this means that the band gap is effectively expanded by the quantum effect. It is also observed that the peak energy of photoluminescence increases in proportion to the time of anodic oxidation. This means that quantum wires could be formed by means other than molecular beam epitaxy. However, to be a quantum wire, there are conditions such as that the wire extends one-dimensionally and that the holes are not complicated and are parallel and orderly. Photoluminescence is a measurement of the intensity of light emitted by irradiating light having a high energy for each wavelength. Light may be emitted at the impurity level or may be emitted due to bandgap transition. There are various light emission mechanisms. In this case, it is not necessary to attach an electrode, and the band gap can be easily examined. However, there remains the question of whether the photoluminescence peak shift corresponds to the widening of the bandgap or the emission due to impurities. This may be the case if the holes are evenly formed perpendicular to the plane.

V. Lehmann et al, "Porous silicon format
ion: A quantum wire effect ", Appl.Phys.Lett.58, (8),
p856, (1991) This is a thin hydrofluoric acid solution in which Si is used as an anode and Pt is used as a cathode to pass an electric current to make Si porous. It is said that it is better to use thin hydrofluoric acid. This paper also proposes a hypothesis for the question of why Si is corroded in hydrofluoric acid solution. The hydroxyl group OH ^- is attracted toward Si, which is the anode, by the action of the electric field. Here, oxygen is partially oxidized and becomes a gas and is eliminated. Hydrogen remains on the surface of Si and covers it. Originally, a large number of dangling bonds existed on the outermost surface of Si, but some of these bonds were broken to bond with hydrogen. Si-H
Bonding occurs. This can be done two times for one Si. There are holes inside Si, which move about freely. This reaches the outermost surface and breaks the Si—H bond. Since free F exists in this vicinity, F binds to Si instead of H. Si-F bonds can be formed. This can also be done for one Si.
Further, the Si-Si bond is replaced with H. Four Fs are bonded to Si on the outermost surface, and this Si is SiF.
It becomes ₄ and leaves. In this way, Si is corroded by using H and F as intermediaries. This phenomenon occurs because H is closely attached to the Si surface.

Therefore, Lehmann argues that such a phenomenon does not occur unless a solution having a high OH ⁻ concentration and a high pH and a low hydrofluoric acid concentration is used. He said that the current density must be low to some extent. This hypothesis is also questionable. However, it seems certain that anodic oxidation will cause holes to grow perpendicular to the plane. This differs greatly from activated carbon, which is usually known as a porous material, in this respect. This paper measures light transmission, not photoluminescence. In semiconductors, light with energy smaller than the band gap is transmitted without being absorbed. The energy at the absorption edge almost corresponds to the band gap. Therefore, the band gap can be known from the transmittance. According to this, in the case where the surface of p ⁺ type Si is anodized to be porous, the absorption edge is increased to 1.24 eV. The surface of p-type Si is anodized,
The absorption edge is 1.76 eV. This is evidence that the bandgap has expanded to this point.

N. Koshida et al, "efficient Visible Pho
toluminescence from Porous silicon ", Jpn.J.Appl.Phy
s.30, 7B, pL1221-L1223 (1991) This also anodizes Si in hydrofluoric acid. It measures photoluminescence at room temperature. In the case of anodized porous p-type Si, the photoluminescence peak is 700 nm. In case of n-type Si, the peak is 650n
m. The fact that the photoluminescence peak has shifted to the lower wavelength side means that the bandgap has expanded. Si is an indirect transition type semiconductor,
The intensity of photoluminescence is also large. Thus anodized to porous the Si electron is localized in the direction parallel to the surface energy - increases than the band edge by h ² / 8md ² as described above. Since it is localized in two directions, h ^{2 is} eventually
This means that the energy becomes higher by / 4 md ² . m is the effective mass of the electron. The energy of the conduction band is increased by this amount, but the energy of the valence band is decreased by the same formula, so that the expansion of the band gap is the sum of these two. However, m is the effective mass at each band edge.

Although there are some papers that the band gap is effectively widened because Si is anodized and made porous to form quantum wires, no one has positively utilized this. The absorption coefficient measurement is only a very passive bandgap measurement. Is such a high porosity still a semiconductor, and why is it not amorphous? Not clear. A semiconductor has a band gap, but it is unclear whether a semiconductor with a high porosity such that Bloch waves cannot exist is a semiconductor. Photoluminescence is a passive measuring method as well as absorption. This should also reflect impurity levels and other levels. Is there a band gap, or is there a conduction band or valence band in the first place? Amorphous Si
I feel the question that was submitted when was first introduced. Moreover, this is still out of the scope of physical property interest, and its application is not considered. The present inventor has succeeded in producing a Si light receiving element having a wide bandgap sensitivity in the visible light region by positively utilizing this phenomenon. Anodization has the advantage that the sensitivity is extended to the visible light range because a large number of parallel holes are formed in a one-dimensional order. This is because the energy at the edge of the conduction band increases and the energy at the edge of the valence band decreases as described above, so that the band gap is effectively widened.

Of course, since the electrons and holes are one-dimensionally localized, the mobility should be significantly reduced. But that is okay. Mobility decreases in the direction parallel to the plane. However, the mobility hardly decreases in the direction perpendicular to the plane, that is, in the thickness direction. In the case of a light receiving element, the electrodes are provided on both sides of the surface, so that the current flows in the direction orthogonal to the surface.
The mobility in the direction of current flow does not decrease. Therefore, this light receiving element is comparable to the conventional one in terms of mobility. This is different from the case where FETs, bipolar transistors, etc. are made. If FET is this S
If it is made with i, the current flow is parallel to the plane, so the mobility is significantly reduced, and it is very useless. It will not be able to respond at high speed and will have low transconductance. Since the present invention is applied as a light receiving element in which a current flows in a direction orthogonal to the plane, there is no reduction in mobility. It is a highly anisotropic material, but the best direction can be used. Although it is worrisome whether the pn junction can be formed, the pn junction is not destroyed because it is anodized after the pn junction is formed.

What has been described above is the anodized porous S.
If i is used as a material, it may be expected.
However, the light receiving element of the present invention has another great advantage. That is, the surface easily reaches the pn junction region where electron-hole pairs are easily generated. This improves quantum efficiency. Extremely high porosity (70% -80%
%), Most of the vertically incident light travels in the air and reaches the pn junction region. Therefore, there is little attenuation.
For example, the conventional light receiving element has a drawback that it is not very efficient because it is attenuated until it reaches the pn junction after entering the p region. Also in this respect, the light receiving element of the present invention is advantageous.
In other words, it means higher sensitivity.

[0017]

1 shows the structure of a silicon photodiode according to an embodiment of the present invention. This is a case where the present invention is applied to the light receiving element having the structure shown in FIG. 2, and the entire structure is almost the same. The i layer 2 is stacked on the n-type layer 1. A p-type region 3 is selectively formed in the center of i layer 2. This is made by impurity diffusion, ion implantation, etc. The upper part of the p-type region is made porous (porous layer 15) by anodic oxidation. This is obtained by anodizing silicon in hydrofluoric acid with Pt as a cathode. Here, only the p-type region is selectively made porous, but the i layer may be similarly anodized to be porous. P as in this example
When the porous region is limited to the mold region, it is possible to prevent a decrease in material strength due to the porosity. Further, the thickness of the porous layer may be further increased to make the region up to the p-i junction porous. If the porosity is limited to a part of the p-type region, an element having different band gaps in the thickness direction is made, so that the wavelength range with high sensitivity can be widened.

Further, a SiO ₂ layer 4 which is an insulator is provided in a ring shape, and a ring-shaped p-side electrode 5 is formed thereon. This is in ohmic contact with the p-type region 3. The part where the light enters the p-type region 3 is an opening. This portion is covered with the antireflection film 6. The light enters through the opening on the upper side, and in the vicinity of the boundary between the p-type layer 3 and the i-layer 2, electron-hole pairs are generated and disappear. N-type layer 1 shown below
May be an n-type substrate or a thin film grown on the n-type substrate. An n-side electrode is formed under the n-type layer, but this is not shown. The material silicon is originally a single crystal, but the anodized porous region does not have a regular lattice structure in the direction parallel to the plane. . There are various types of photodiodes, and there are various types in which light is incident from the substrate. When a reverse bias is applied between the electrodes and light is irradiated, the light is incident on the p-type region and the i-layer through the central opening to generate electron-hole pairs. This produces a current due to reverse bias. In this way light can be converted into electrical signals. The present invention can be equally applied to both the light receiving element and the solar cell. This point is no different from the conventional example.

FIG. 3 shows an example using a Schottky junction. This is an application of the present invention to the structure of FIG. This light receiving element has a structure in which an n ⁺ type layer 7 and an n type layer 8 are stacked. The n ⁺ type layer 7 may be a substrate or a thin film grown on the substrate. The upper part of the n-type layer is the anodized porous layer 16. The band gap of this part is expanding. A ring-shaped insulator SiO ₂ film 9 is formed on the peripheral portion of the n-type layer 8.
A semitransparent metal thin film 10 is provided in the central portion. A ring-shaped metal electrode 11 is provided on these. There is an antireflection film 12 in the central opening. In this, the semitransparent metal film 10 and the n-type layer 8 are in Schottky junction. Here, the diode characteristic occurs. When light enters the n-type layer and generates electron-hole pairs, a current flows due to the reverse bias voltage. Since the band gap is widened by the porous layer, there is sensitivity up to the higher energy of light. In this way, it becomes quite sensitive to visible light.

The light receiving element according to the conventional example of FIG.
Although it had the highest sensitivity at m to 900 nm, the light receiving element of the present invention has the highest sensitivity at 600 to 700 nm. 20
This means that the maximum sensitivity region of about 0 nm could be shifted to the low wavelength side. This is the case of a light receiving element, but when it is applied to a solar cell, visible light can be more effectively converted into electric energy. Further, since the efficiency can be increased by making it porous, a more excellent solar cell can be obtained.

[0021]

In the light receiving element of the present invention, the light receiving sensitivity is shifted to the shorter wavelength side because the light receiving portion is made of silicon which is anodized and becomes porous to effectively widen the band gap. Therefore, a light receiving element having maximum sensitivity to visible light can be obtained. Since it can be made of silicon, which is a cheaper material than GaAs, it is possible to obtain a cheaper light receiving element for visible light. In addition, a solar cell is more useful because it can more efficiently convert visible light into electric energy. It can be widely used as a light receiving element such as a photodiode, a phototransistor, and a solar cell. The properties of anodized porous silicon have been studied so far by changes in the absorption edge of light and changes in photoluminescence, but there was no way to positively utilize this.
The present invention for the first time gives anodized porous silicon practical applications. When used for a transistor or a FET, the current flow becomes parallel to the surface, so the mobility is extremely small and it is close to an insulator, so it would be useless. However, since the present invention is a light receiving element in which a current flows in a direction perpendicular to the surface, the mobility does not decrease. In this respect, the present invention can be said to take advantage of the features of anodized porous silicon. Further, since the effective surface area is increased, the conversion efficiency is increased, which is an advantageous photoelectric conversion element.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a photodiode according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view of a conventional photodiode.

FIG. 3 is a schematic cross-sectional view of a photodiode according to another embodiment of the present invention using Schottky junction.

FIG. 4 is a schematic sectional view of a conventional photodiode using Schottky junction.

[Explanation of symbols]

1 n-type layer 2 i-layer 3 p-type region 4 SiO ₂ film 5 metal electrode 6 antireflection film 7 n ⁺ type layer 8 n-type layer 9 SiO ₂ film 10 semitransparent metal thin film 11 metal electrode 12 antireflection film 15 porous Layer 16 Porous layer

Claims (4)

[Claims] 1. A silicon semiconductor layer having a p-region, an n-region, an i-region and a pn junction, an electrode ohmic-connected to the p-region and the n-region, and light reception for guiding light to the pn junction. A silicon semiconductor light-receiving element having a portion, wherein the surface of the silicon semiconductor layer is anodized to be porous. 2. A silicon semiconductor layer having a p region or an n region in a silicon semiconductor layer, an electrode Schottky-connected to the p region or the n region, an electrode ohmic-connected, and a p region or A light-receiving element provided with a light-receiving portion for guiding light to the n region, wherein the surface of the silicon semiconductor layer is anodized and made porous. 3. A p-type region, an n-type region, and
An i region and a pn junction are provided, the surface of the silicon semiconductor is anodized in hydrofluoric acid to make it porous, and an electrode ohmic-connected to the p region and the n region is formed for the silicon semiconductor. And a method for manufacturing a silicon semiconductor light receiving element. 4. A silicon semiconductor layer is provided with a p region or an n region, the silicon semiconductor is anodized in hydrofluoric acid to make it porous, and an electrode and an ohmic contact which are Schottky connected to the p region or the n region. A method for manufacturing a silicon semiconductor light receiving element, characterized in that electrodes to be connected are formed.