CN107403848B - A back-illuminated cascaded multiplied avalanche photodiode - Google Patents

Abstract

The invention discloses a back-illuminated cascade multiplication avalanche photodiode, which sequentially comprises a buffer layer and n-type doped Al from bottom to top on a substrate _x Ga _1‑x The device comprises an N layer, an i-type periodical cascade multiplication layer, an i-type intrinsic absorption layer and a p-type electrode layer, wherein an optical coupling convergence structure is arranged around the table top of the i-type intrinsic absorption layer. The invention effectively solves the problem of low effective absorption rate of the periodic cascade multiplication structure under back incidence through the optical coupling convergence structure, reduces the size of the device table top and reduces the body leakage current. The invention provides a solution for effectively working the periodic cascade multiplication avalanche photodiode under back irradiation, is suitable for large-scale array integration, is suitable for each wave band of ultraviolet, visible and near infrared, and can be widely applied to the fields of weak light detection imaging and even single photon detection imaging.

Description

A back-illuminated cascaded multiplied avalanche photodiode

technical field

The invention relates to the field of photoelectric detection and single photon detection, in particular to a back-illuminated cascaded multiplied avalanche photodiode.

Background technique

High-sensitivity, low-noise weak light detection has great application value in the fields of secure communication, early warning and guidance, fire early warning, atmospheric environment monitoring, biological detection, deep space detection, etc., and is one of the main directions for the development of photoelectric detection technology. Common high-sensitivity ultraviolet-visible-near-infrared detectors for weak light detection mainly include photomultiplier tubes (PMTs) and semiconductor avalanche detectors (APDs). Compared with PMT, APD has high quantum efficiency, low operating voltage, high reliability, and is easy to fabricate a large-scale high-resolution detector array. Although the most mature Si-based avalanche detector has good performance in the visible band, it is limited by the characteristics of the Si material itself (for example, the band gap is much lower than the energy of the ultraviolet, the responsivity of ultraviolet photons is low, and filters must be used to block Visible light, infrared light, and ultraviolet light are easy to age), which limits the performance of its weak light ultraviolet detection. Therefore, researchers began to study APDs based on third-generation wide-bandgap semiconductor materials such as GaN and SiC to overcome the above-mentioned shortcomings. Among them, the nitride APD has the ability to work at room temperature with low dark current, no response to visible light, high field strength, stable physical and chemical properties, strong resistance to ultraviolet-visible radiation, and stability in the ultraviolet-visible band It is one of the most promising "single photon" level weak light detectors. For the near-infrared band, InGaAs detectors are often used, but the dark noise is large and it needs to work under cooling conditions. At the same time, the above-mentioned APDs all need to work in Geiger mode for weak light detection, which requires complex quenching circuits.

The publication date is July 6, 2016, and the Chinese invention patent document with the publication number CN105742387A discloses a semiconductor structure, which proposes a superlattice multiplication region, which has the advantage of high-gain linear multiplication, and can be detected in a linear mode Dim light, no need to work in Geiger mode. However, unlike traditional pin-APDs, which can achieve electron/hole injection to achieve bipolar ionization under front/back illumination, periodic cascaded structure APDs can achieve effective multiplication of electrons under normal incident electron injection. However, under the back-illuminated hole injection, due to the energy band structure of the multiplication region, the ratio k of the ionization coefficient of the hole to the electron is only 0.05, and the holes are basically unable to achieve collision ionization in the cascade structure to produce a multiplication effect. At the same time, the back-incident light will be absorbed when passing through the multiplication region, and cannot reach the absorption layer to form an effective multiplication. Based on the above two points, it is proved that the existing cascade structure is not suitable for the back-illuminated type. It is necessary to introduce an optical coupling structure to adjust the propagation direction of the incident light so that it changes from vertical propagation to lateral in-plane propagation, so that the light in a large area other than the mesa can be directly concentrated to the absorption layer without going through the multiplication region.

Contents of the invention

In order to overcome the above-mentioned technical defects, the present invention proposes a back-illuminated cascade structure avalanche photodiode, which introduces the periodic cascade structure multiplication region into the back-illuminated APD structure that is easy to integrate, so that the APD device can achieve linearity like a PMT. Mode high gain; at the same time, the grating coupling structure is used to make the back-illuminated light effectively reach the main absorption area, which solves the problem of low effective light absorption under the back-illuminated type. As a result, its ability to absorb photons and monopole multiplication of electrons has been greatly improved.

Technical scheme of the present invention is as follows:

A back-illuminated cascaded multiplication avalanche photodiode is characterized in that: a buffer layer is arranged on the substrate, an n-type doped _{AlxGa1 -xN} layer is arranged on the buffer layer, and an n-type doped _AlxGa A protruding mesa is arranged on the _1-x N layer, and an i-type periodic cascaded multiplication layer, an i-type intrinsic absorption layer, and a p-type electrode layer are sequentially arranged on the protruding mesa from bottom to top; the surrounding of the protruding mesa is arranged There is an optical coupling and converging structure; an n-type ohmic contact layer is deposited on the sunken mesa of the n-type doped Al _x Ga _1-x N layer, and a p-type ohmic contact layer is deposited on the p-type electrode layer; by adjusting the n-type doped The x of the Al component in the doped _{AlxGa1 -} xN layer makes the absorption rate of the n-type doped _{AlxGa1 -xN} layer to the detection band corresponding to the i-type intrinsic absorption layer in the photodiode less than 20%.

The i-type periodic cascade multiplication layer is a superlattice of group III nitride, and its material can be In _y Ga _1-y N or Aly _{Ga 1-y} N, etc., wherein 0≤y≤1, and the i-type periodic The thickness of the cascade multiplication layer is 0.001~1 μm.

The material of the i-type intrinsic absorption layer may be In _z Ga _1-z N or Al _z Ga _1-z N, etc., wherein 0≤z≤1, and the thickness of the i-type intrinsic absorption layer is 0.001-1 μm.

The optical coupling and converging structure is composed of a grating structure, including a dielectric layer deposited on the sunken mesa of the n-type doped _{AlxGa1 -xN} layer and a grating formed by etching on the dielectric layer; the optical coupling and converging The material of the structure may be Au, Ag, SiN, or SiO ₂ ; the vertical height of the light coupling and concentrating structure corresponds to the i-type periodic cascade multiplication layer. The line width of the optical coupling and convergence structure is s , the period is p , and the thickness is h1 , the value of the period p is one tenth to ten tenths of the detection wavelength, and the value of the line width s is one tenth of the detection wavelength From one to ten tenths, the thickness h1 is not less than 0.0096 times the square root of the detection wavelength in microns.

Working process and principle of the present invention are as follows:

The incident light is incident from the back of the substrate, and after passing through the n-type doped Al _x Ga _1-x N layer, a small part of the light in the middle enters the APD mesa, and nearly 90% of the incident light is absorbed by the i-type periodic cascade multiplication layer, only the non- to 10% to reach the i-type intrinsic absorption layer to generate photogenerated carriers; most of the light around the APD table is transformed into a transverse propagation mode by the grating coupling structure, and directly converges to the i-type intrinsic absorption layer to generate photogenerated carriers In this way, from the perspective of the overall effect, most of the incident light can be absorbed by the i-type intrinsic absorption layer to generate photogenerated carriers, and then transported downward to the i-type periodic cascade multiplication layer to form an effective multiplication and generate high gain. Since the stack cascaded multiplication structure can produce an effect similar to that of PMT single-polarization multiplication, it can not only greatly improve the linear multiplication capability, reduce excess noise, but also work in a stable voltage mode, abandoning the traditional APD single-photon detector Geiger The mode works, so that large-scale array integration can be easily formed for weak light detection imaging or even single photon detection imaging.

The advantages of the present invention are:

1. On the basis of the advantages of back-illuminated detectors suitable for the integration of indium column interconnected arrays, the nitride stacked multiplication region is introduced into the back-illuminated APD device structure, and the periodic stack structure energy band regulation is introduced to produce a cascade multiplication effect. Making the multiplication region has advantages similar to PMT single-polarization multiplication. Compared with the traditional back-illuminated PIN structure, it has a higher electron ionization rate and a larger electron-hole ionization ratio, which can not only greatly improve the linear multiplication capability, reduce excess noise, but also work in a voltage-stabilizing mode, eliminating the need for The way traditional APD single photon detectors work in Geiger mode.

2. By designing the optical coupling structure in the back-illuminated stacked APD absorption area, the propagation direction of the incident light is adjusted from vertical propagation to lateral propagation, and converged to the i-type intrinsic absorption layer, so that it bypasses the large impact on light. The lossy stack-type multiplication region greatly improves the absorption and utilization of photons, thereby effectively generating photogenerated carriers, achieving efficient multiplication and efficient absorption at the same time.

3. The present invention proposes that the back-firing stacked APD structure is applicable to various wavelength bands of ultraviolet, visible, and near-infrared.

Description of drawings

Fig. 1 is a structural schematic diagram of the present invention.

Fig. 2 is a schematic structural diagram of the present invention for the ultraviolet band.

Fig. 3 is a schematic structural diagram of the present invention for the visible waveband.

Fig. 4 is a schematic structural diagram of the present invention for the near-infrared band.

Wherein, reference numerals are: 1-substrate and buffer layer, 2-n-type doped Al _x Ga _1-x N layer, 3-i-type periodic cascade multiplication layer, 4-i-type intrinsic absorption layer, 5-type -p-type electrode layer, 6-dielectric layer, 7-grating, 8-n-type ohmic contact layer, 9-p-type ohmic contact layer, 10-SiN _m dielectric layer, 11-air groove grating, 12-SiO ₂ dielectric layer , 13-Si ₃ N ₄ grating, 14-Ag reflective layer, 15-Au grating, 16-passivation layer.

Detailed ways

As shown in Figure 1, a back-illuminated cascaded multiplied avalanche photodiode is provided with a buffer layer on the substrate, and the buffer layer is provided with an n-type doped _{AlxGa1 -xN} layer 2, n-type doped A protruding mesa is arranged on the _{AlxGa1 -xN} layer 2, and an i-type periodic cascaded multiplication layer 3, an i-type intrinsic absorption layer 4 and a p-type electrode layer 5 are sequentially arranged on the protruding mesa from bottom to top; A light coupling and converging structure is arranged around the protruding mesa; an n-type ohmic contact layer 8 is deposited on the sunken mesa of the n-type doped _{AlxGa1 -xN} layer 2, and a p-type electrode layer 5 is deposited on the p-type electrode layer 5 type ohmic contact layer 9; by adjusting the x of the Al composition in the n-type doped _{AlxGa1 -} xN layer 2, the n-type doped _{AlxGa1 -xN} layer 2 has a positive effect on the i-type intrinsic The absorption rate of the detection band corresponding to the absorption layer 4 is less than 20%.

The light coupling and converging structure is composed of a grating structure, including a dielectric layer 6 deposited on the sunken mesa of the n-type doped _{AlxGa1 -xN} layer 2 and a grating 7 formed by etching on the dielectric layer 6 .

Example 1

The back-illuminated cascaded multiplied avalanche photodiode for the ultraviolet band specifically refers to a back-illuminated cascaded multiplied avalanche photodiode with a dielectric light coupling and convergence structure for a detection wavelength of 350nm. The structure is sequentially substrate and buffer layer 1, n-type doped _{AlxGa1 -} xN layer 2, i-type periodic cascade multiplication layer 3, i-type intrinsic absorption layer 4 and p-type electrode layer 5. An optical coupling and converging structure layer is prepared outside the 4 mesas of the i-type intrinsic absorption layer.

As shown in Figure 2, in this embodiment, an AlN template buffer layer is grown on a sapphire substrate, an n-type doped _{AlxGa1 -xN} layer 2 is grown on it, the composition x=0.2, and the thickness is 200nm, to ensure n The doped Al _x Ga _1-x N layer 2 absorbs less than 10% for a detection wavelength of 350 nm, and the doping concentration is 1×10 ¹⁸ cm ^-3 . The material of the i-type periodic cascaded multiplication layer 3 is AlN/Al _0.2 Ga _0.8 N periodic stacked thin film, the film thickness is 10nm/10nm, 20 periods in total. The i-type intrinsic absorption layer 4 is a GaN layer with a thickness of 200 nm. The p-type electrode layer 5 is a GaN layer with a thickness of 200 nm and a doping concentration of 2×10 ¹⁷ cm ⁻³ . Etch the device mesa in the epitaxial structure, the diameter of the mesa is 5um, and the height of the device mesa is 900nm, and then respectively complete the ohmic contact of the n-type electrode layer and the p-type electrode layer 5 to form the n-type ohmic contact layer 8 and the p-type ohmic contact layer 9.

The optical coupling and converging structure is a dielectric grating structure. Photolithographically deposit a SiN _m dielectric layer 10 around the mesa, with a thickness of 700nm, so that it covers the i-type intrinsic absorption layer 4; then etch a ring grating with a groove thickness of 200nm, and the width of the ring grating corresponding to the air groove 11 is 87.5nm, the period is 175nm, and when the ring grating period is 20, the light concentration is enhanced by 5 times.

Example 2

The back-illuminated cascaded multiplied avalanche photodiode for the visible band specifically refers to a back-illuminated cascaded multiplied avalanche photodiode with two dielectric optical coupling and convergence structures for a detection wavelength of 500nm. The structure is sequentially substrate and buffer layer 1, n-type doped _{AlxGa1 -} xN layer 2, i-type periodic cascade multiplication layer 3, i-type intrinsic absorption layer 4 and p-type electrode layer 5. An optical coupling and converging structure layer is prepared outside the 4 mesas of the i-type intrinsic absorption layer.

As shown in Figure 3, in this embodiment, an AlN template buffer layer is grown on a sapphire substrate, an n-type doped _{AlxGa1 -xN} layer 2 is grown on it, the composition x=0, and the thickness is 1000nm, which can ensure n The doped Al _x Ga _1-x N layer 2 absorbs less than 5% for a detection wavelength of 500nm, and the doping concentration is 1×10 ¹⁸ cm ^-3 . The material of the i-type periodic cascaded multiplication layer 3 is AlN/In _0.2 Ga _0.8 N periodic stacked thin film, the film thickness is 10nm/10nm, 20 periods in total. The i-type intrinsic absorption layer 4 is an In _z Ga _1-z N layer, where z=0.33, and a thickness of 200 nm. The p-type electrode layer is a GaN layer with a thickness of 200 nm and a doping concentration of 2×10 ¹⁷ cm ⁻³ . Etch the device mesa in the epitaxial structure, the diameter of the mesa is 5um, and the height of the device mesa is 900nm, and then respectively complete the ohmic contact of the n-type electrode layer and the p-type electrode layer 5 to form the n-type ohmic contact layer 8 and the p-type ohmic contact layer 9.

The optical coupling and converging structure is a dielectric grating structure. A SiO ₂ dielectric layer 12 is deposited around the mesa by lithography SOG spin-on-glass method, with a thickness of 700 nm, so that it covers the i-type intrinsic absorption layer 4 . Afterwards, a grating grooved with a thickness of 200nm is etched, the groove width of the grating is 125nm, the period is 250nm, and the number of periods is 20. Si ₃ N ₄ is deposited and filled at the groove to form a Si ₃ N ₄ grating 13 with a thickness of 200 nm. After that, Ag is deposited on the Si ₃ N ₄ grating 13 to form an Ag reflective layer 14 with a thickness of 100 nm. For the detection wavelength of 500nm, the 20-period ring grating enhances the light concentration by 8 times.

Example 3

The back-illuminated cascaded multiplied avalanche photodiode for the near-infrared band specifically refers to a back-illuminated cascaded multiplied avalanche photodiode with a metal grating optical coupling convergence structure for the 1550 nm communication band. The structure is sequentially substrate and buffer layer 1, n-type doped _{AlxGa1 -} xN layer 2, i-type periodic cascade multiplication layer 3, i-type intrinsic absorption layer 4 and p-type electrode layer 5. An optical coupling and converging structure layer is prepared outside the 4 mesas of the i-type intrinsic absorption layer.

As shown in Figure 4, in this embodiment, an AlN template buffer layer is grown on a sapphire substrate, an n-type doped _{AlxGa1 -xN} layer 2 is grown on it, the composition x=0, and the thickness is 1000nm, which can ensure n The doped Al _x Ga _1-x N layer 2 absorbs less than 5% for a detection wavelength of 1550 nm, and the doping concentration is 1×10 ¹⁸ cm ^-3 . The material of the i-type periodic cascaded multiplication layer 3 is AlN/GaN periodic stacked thin film, the thickness of which is 10nm/10nm, 20 periods in total. The i-type intrinsic absorption layer 4 is an In _z Ga _1-z N layer, where z=0.94, and a thickness of 200 nm. The p-type electrode layer is a GaN layer with a thickness of 200 nm and a doping concentration of 2×10 ¹⁷ cm ⁻³ . The device mesa is etched in the epitaxial structure, the height of the device mesa is 900nm, and the diameter of the mesa is 10um. After that, the ohmic contact of the n-type electrode layer and the p-type electrode layer 5 is respectively completed to form the n-type ohmic contact layer 8 and the p-type ohmic contact layer. 9.

The optical coupling and converging structure is a dielectric grating structure. Deposit a SiO ₂ dielectric layer with a thickness of 700nm around the mesa by lithography SOG spin-coated glass, so that it covers the i-type intrinsic absorption layer 4; then etch a grating with a thickness of 200nm and a groove width of 387.5nm, the period is 775nm, and the period number is 20. The filling metal Au is deposited on the groove with a thickness of 200 nm to form the Au grating 15 . For the detection wavelength of 1550nm and the 20-period ring grating, the electric field intensity convergence is enhanced by 16 times. A passivation layer 16 is also deposited on the upper part.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (6)

1. a back-illuminated cascaded multiplication avalanche photodiode is characterized in that: a buffer layer is provided on the substrate, and the buffer layer is provided with n-type doped Al _x Ga _1-x N layer, n-type doped Al _x Ga _1-x N layer is provided with a protruding mesa, and the protruding mesa is provided with an i-type periodic cascade multiplication layer, an i-type intrinsic absorption layer and a p-type electrode layer in sequence from bottom to top; the protruding mesa An optical coupling converging structure is arranged around; an n-type ohmic contact layer is deposited on the sunken mesa of the n-type doped _{AlxGa1 -xN} layer, and a p-type ohmic contact layer is deposited on the p-type electrode layer; by adjusting n The x of the Al component in the Al _x Ga _1-x N layer doped with type doping, so that the absorption rate of the detection band corresponding to the i-type intrinsic absorption layer in the photodiode by the n-type doped Al _x Ga _1-x N layer is less than 20% ; The optical coupling and converging structure is designed in the back-illuminated stacked APD absorption region to adjust the propagation direction of the incident light from vertical propagation to lateral propagation, converging to the i-type intrinsic absorption layer, so that it bypasses the A stacked multiplication region that is lossy to light is created, thereby improving the ability to absorb and utilize photons, generating photogenerated carriers, and achieving multiplication and absorption at the same time. 2. The back-illuminated cascaded multiplied avalanche photodiode according to claim 1, characterized in that: the i-type periodic cascaded multiplied layer is a superlattice of Group III nitride, and the material is In _y Ga _1-y N or Al _y Ga _1-y N, where 0≤y≤1, and the thickness of the i-type periodic cascade multiplication layer is 0.001~1 μm. 3. The back-illuminated cascaded multiplied avalanche photodiode according to claim 1, characterized in that: the i-type intrinsic absorption layer is made of In _z Ga _1-z N or Al _z Ga _1-z N, Where 0≤z≤1, the thickness of the i-type intrinsic absorption layer is 0.001~1 μm. 4. The back-illuminated cascaded multiplied avalanche photodiode according to claim 1, characterized in that: the light coupling and converging structure is composed of a grating structure, including a layer deposited on an n-type doped _{AlxGa1 -xN} layer The dielectric layer on the sunken mesa and the grating formed by etching on the dielectric layer; the vertical height of the light coupling and converging structure corresponds to the i-type periodic cascade multiplication layer. 5 . The back-illuminated cascaded multiplied avalanche photodiode according to claim 4 , wherein the dielectric layer is made of Au, Ag, SiN, or SiO ₂ . 6. The back-illuminated cascaded multiplied avalanche photodiode according to claim 1 or 4, characterized in that: the line width of the optical coupling and convergence structure is s , the period is p , and the thickness is h1 , and the value of the period p is One-tenth to tenths of the detection wavelength, the value of the line width s is one-tenth to tenths of the detection wavelength, and the thickness h1 is greater than or equal to 0.0096 times the square root of the detection wavelength in microns.

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