JPH03232279A - Photosensor - Google Patents

Abstract

PURPOSE:To prevent a photosensor from deteriorating in characteristics by a method wherein a reflective section comparatively excellent in reflectivity to an optical signal is formed on a side opposite to a photodetective surface interposing a photoelectric conversion region between them. CONSTITUTION:A PIN photodiode is formed in such a manner that an Si film is epitaxially grown on a sapphire substrate 4, N-type impurity is implanted into the film concerned high in concentration, implanted impurities are diffused to form an N<+> layer 5, an I layer 6 is epitaxially grown, P-type impurities are injected into the upper part of the layer 6, and the implanted impurities are diffused to form a P<+> layer 7. Signal lead-out electrodes 8 and 9 are formed so as to be connected to the N<+> layer 5 and the P<+> layer 7 respectively, and then a metal film 10 is attached to the rear side of the sapphire substrate 4, light incident on the P<+> layer 7 passes through the I layer 6 and the N<+> layer 5, attenuating in energy, reaches the metal film 10 penetrating through the sapphire substrate 4 without being attenuated, the reflected light is made to penetrate through the sapphire substrate 4 again and to advance toward the N<+> layer 5, the I layer 6, and the P<+> layer 7 attenuating in energy. In result, an photosensor of this design can be elongated in absorption length without changing a photoelectric conversion region in physical length and prevented from deteriorating in characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical sensor that converts an optical signal incident on a light receiving surface into an electrical signal in a photoelectric conversion region and outputs the electrical signal.

[Conventional technology]

Conventional optical sensors include those that utilize photodiodes and those that utilize photoconductive elements made of PbS or the like.

FIG. 6 is a cross-sectional view showing an optical sensor using a photodiode. This optical sensor has an N
It has a three-layer structure: layer 1, layer 1 2, which is a neutral region formed on top of layer 1, and layer 3, which is a neutral region formed on layer 2.

When this optical sensor is used to absorb energy from light in a long wavelength range, the absorption length must be increased, so the distance of the depletion layer must be increased. For example, at an operating wavelength of 800 to 900 nm for a PIN photodiode, an absorption length of 30 to 50 μm is required, so that distance must be depleted.

In addition, optical sensors using PbS elements have a PbS thin film formed on a substrate such as glass or quartz.
An Au electrode is deposited on the thin film. A PbS thin film is produced on a substrate using a chemical precipitation method, and is then manufactured into a product through processes such as activation of an oxide film, vapor deposition of an Au electrode, and division/assembly.

When using this optical sensor to absorb energy from light in the long wavelength range, the absorption length must be made long, so PbS is used.
It is necessary to increase the film thickness of the element.

[Problem to be solved by the invention]

However, conventional optical sensors using photodiodes have the disadvantage that the distance of the depletion layer needs to be long in order to use light in a long wavelength range, resulting in poor high-speed performance.

Incidentally, the operating speed of the optical sensor is determined by the CR time constant and the carrier transit time, but the speed is not limited much by the component contributing to the cutoff frequency due to the CR time constant. For example, in the case of a silicon PIN photodiode with a receiving diameter of 250 μm,
The cut-off frequency contribution due to the CR time constant is 18 GHz, and the speed is not limited by this factor. On the other hand, the speed limit due to the component contributing to the cut-off frequency due to the transit time is large; for example, when the width of the depletion layer becomes 30 μm, the component contributing to the cut-off frequency due to the transit time decreases to 1.5 GHz. In this case, in order to restore high speed performance, the operating voltage must be increased until the saturation speed is reached. When the carriers are electrons, an electric field strength of 1×10'V/cln is required to increase the drift velocity to the saturation velocity region, and if the width of the depletion layer is 30 μm, the applied voltage is 30 μm.
It becomes high as V.

In addition, according to optical sensors using PbS elements, it is necessary to increase the film thickness of the PbS element in order to use light in the long wavelength range, but if the film thickness is increased, it becomes difficult to manage dark resistance. However, there are disadvantages in that it may cause problems such as oxidation and deterioration of other characteristics.

Therefore, the present invention aims to solve the above-mentioned drawbacks.

[Means to solve the problem]

In order to achieve the above-mentioned problems, the present invention provides an optical sensor that converts an optical signal incident on a light-receiving surface into an electrical signal in a photoelectric conversion region and outputs the electrical signal, the sensor being on the opposite side of the light-receiving surface with the photoelectric conversion region sandwiched therebetween. A reflecting portion having a relatively good reflectance for the optical signal is formed on the substrate.

[Effect]

Since the present invention is configured as described above, the light incident from the light receiving surface is reflected by the action of the reflecting portions formed on the opposite side of the light receiving surface, sandwiching the photoelectric conversion region, and the light enters the photoelectric conversion region again. pass. Therefore, the physical length of the photoelectric conversion region does not change, but the absorption length increases.

〔Example〕

Embodiments of the optical sensor according to the present invention will be described below with reference to the accompanying drawings. In the description, the same elements are denoted by the same reference numerals, and redundant description will be omitted.

FIG. 1 is a sectional view showing the structure of a PIN photodiode according to this embodiment. At an operating wavelength of 800 to 900 nm for a PIN photodiode, the absorption length is 30 to 900 nm.
Since 50 μm is required, that distance must be depleted. The present invention increases the light absorption length without changing the physical length of the depletion layer. The configuration will be explained below.

First, an SL film is epitaxially grown on the sapphire substrate 4 using SO8 technology. Next, an n-type impurity is implanted at a high concentration onto this St model film, and the implanted region is diffused to form an N 2 layer 5. Further, a first layer 6 on the N+ layer is epitaxially grown, and a p-type impurity is implanted into the top of the first layer 6. The photoelectric conversion region is formed by a part of one layer 6. Further, by diffusing this implanted region, a P layer 7 is formed. In this case, the upper surface of the P layer 7 is the light-receiving surface. Note that the signal extraction electrode 8.9 is N+
Formed to connect to layer 5 and P+ layer 7.

Next, a metal film (reflection part) 1 is placed on the back surface of the sapphire substrate 4.
Attach 0. This metal film 10 is disposed at the rear of one layer 6 with respect to the direction of light incidence, and has a high reflectance with respect to the incident light. The reflecting portion may be formed on the side opposite to the light receiving surface across the photoelectric conversion region, and may reflect incident light.

Therefore, the material is not limited to metal. For example, if infrared light is used as the incident light, plastic can be used as the reflective part.

According to this embodiment, as shown in FIG.
It passes through layer 5 and reaches sapphire substrate 4 . Since the sapphire substrate 4 has a high light transmittance, the incident light passes through the sapphire substrate 4 without attenuating its energy and reaches the metal film 10. Since the metal film 10 has a relatively high reflectance for incident light, the light that reaches the metal film 10 is reflected. Since the light reflected by the metal film 10 passes through the sapphire substrate 4 again, it travels to the N layer 5, the first layer 6, and the P layer 7 while its energy is attenuated.

What is important here is that the incident light passes through the photoelectric conversion region formed in one layer 6 twice, and its energy is absorbed twice. Therefore, since the absorption length can be increased without changing the physical length of the photoelectric conversion region, there is no need to increase the operating voltage even when receiving light in a long wavelength range.

Specifically, in this embodiment, since the absorption length can be effectively doubled, the physical length of the photoelectric conversion region can be approximately halved compared to conventional photosensors. For example, P.I.
When the operating wavelength of the N photodiode is 800 nm, a conventional PIN photodiode would have to be depleted over a length of 30 μm, but according to the present invention, the length must be depleted by 15 μm.
That's enough.

FIG. 3 is a cross-sectional view showing the structure of a PIN photodiode according to another embodiment. The difference from the above embodiment is that a sapphire substrate is not used, and an opening is etched on the back side of the first layer 6, and the N layer 5 and metal film 10 are formed in this opening. be. Note that the electrodes are omitted because they have the same structure. In this case, there is no need to use SO8 technology, and the same effect can be obtained by using the conventional structure as is.

According to the optical sensor according to the above embodiment, for example, 250 μm
It has been confirmed that the cutoff frequency can be set to 3 GHz when the receiving diameter is . Furthermore, since the operating voltage can be lowered, it is suitable for a wide range of applications such as optical communications.

FIG. 4 is a sectional view showing the structure of a PbS photoconductive detector according to another embodiment of the present invention. Pb shown in figure (a)
In the S photoconductive type detector, the PbS element has a sensitivity of 1 to 3.
．． A PbS thin film (including a photoelectric conversion region) 12 is formed by chemical precipitation on a high-purity quartz substrate 11 such as sapphire that has good transmittance within 5 μm, and an Au electrode 13 is deposited on this PbS thin film 12. There is. A light-receiving surface is formed inside the Au electrode 13. An AI is placed on the opposite side of the light-receiving surface across the high-purity quartz substrate 11.

A metal film (reflection portion) 10 made of Au or the like is formed.

This metal film 10 may be made of any material that has a high reflectance within the range of 1 to 3.5 μm. The PbS photoconductive type detector shown in FIG.
is formed, and a PbS thin film 12 is formed thereon. The PbS photoconductive detector shown in FIG. 2C uses an insulating reflective substrate (reflecting portion) 16 as a substrate, and a PbS thin film 12 is formed on the insulating reflective substrate 16. This insulating reflective substrate 16 may be made of any material that has a high reflectance in the range of 1 to 3.5 μm.

FIG. 5 is a graph comparing the spectral characteristics of the PbS photoconductive detector according to the above-mentioned another embodiment and the PbS photoconductive detector according to the prior art. The PbS photoconductive detector according to the example has a sensitivity of 2.5 μm and 3
It can be seen that there is an increase of 50% at 0% and 3 μm. In this way, the sensitivity in the long wavelength range can be increased without changing the film thickness, so the characteristics (S/N, dark resistance; response characteristics)
It is possible to increase the sensitivity in the long wavelength range while maintaining the

Note that the present invention is not limited to the above embodiments.

For example, the material, shape, electrode position, size, etc. of the photodiode and photoconductive detector are arbitrary, and appropriate ones are selected depending on the conditions of use. The reflective portion can also be formed by performing back etching on the back surface of the substrate and plating a metal film on the etched area.

Furthermore, although sapphire is used as the reinforcing substrate in the first embodiment, a material with high transmittance can be used instead.

Further, although a semiconductor made of a single element such as Si is used in this embodiment, it is not limited to a single element semiconductor, and a compound semiconductor such as GaAs5 I nP can be used.

In addition, since the optical sensor according to the present invention only needs to have a photoelectric conversion region, it has a structure in which a reflective part is directly formed in one layer and does not have an N layer, and PbO and Cd55CdS instead of PbS.
A structure using e, etc. may also be used.

〔Effect of the invention〕

Since this invention is configured as explained above,
The absorption length can be increased without changing the physical length of the photoelectric conversion region.

For example, an optical sensor using a photodiode can improve speed without increasing the operating voltage. Also, an optical sensor using a photoconductive element does not cause deterioration of characteristics.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the structure of a photodiode according to an embodiment of the present invention, FIG. 2 is a graph showing absorption characteristics in Si, and FIG. 3 is a diagram showing the structure of a photodiode according to another embodiment of the present invention. The cross-sectional view shown in FIG. 4 is P according to another embodiment of the present invention.
A cross-sectional view showing the structure of a bS photoconductive type detector, FIG. 5 shows a PbS photoconductive type detector according to an embodiment and a PbS according to the prior art.
FIG. 6 is a graph comparing the spectral responsivity of photoconductive detectors, and is a cross-sectional view showing the structure of a photodiode according to the prior art. 1.5...N layer, 2...1 layer, 3...P layer,
4... Sapphire substrate, 6... 1 layer (second conductivity type region), 7... P+ layer (first conductivity type region), 8.9...
- Electrode, 10... Metal film (reflective film), 11... High purity quartz substrate, 12... PbS thin film, 13... Au electrode, 15... Silicon nitride film, 16... Insulation reflective substrate. Figure 2 Example of implementation of the world Figure 3

Claims (1)

[Claims] An optical sensor that converts an optical signal incident on a light receiving surface into an electrical signal in a photoelectric conversion region and outputs the electrical signal, the optical sensor having a reflectance for the optical signal on the opposite side of the light receiving surface sandwiching the photoelectric conversion region. An optical sensor that has a relatively high reflective area.