CN113644150B - A high gain photodetector - Google Patents

Abstract

The invention discloses a high-gain photoelectric detector, which comprises an emitter, a base, an absorption layer and a collector which are sequentially arranged from bottom to top, and also comprises a periodic quantum structure layer, wherein the periodic quantum structure layer is arranged between the emitter and the collector, or is arranged in the base, or is arranged in the emitter; the periodic quantum structure layer comprises a repeating structure with a plurality of periods, and each period contains a semiconductor heterojunction. The periodic quantum structure of the invention can block the diffusion of holes of the base electrode to the emitter electrode, improve the electron injection ratio of the emitter junction and the amplification factor of the transistor, and can bind the holes with larger effective mass in the multilayer quantum structure to reduce the recombination loss, and ensure that the service life of the holes in the device exceeds the transport time of electrons, thereby generating the superimposed photoconductive gain in the photoelectric transistor.

Description

A high gain photodetector

Technical Field

The present invention relates to the technical field of semiconductor photodetectors, and more particularly to a high-gain photodetector.

Background Art

Photodetectors are widely used in optical communications, imaging, environmental monitoring, assisted driving, and various experimental tests. For photodetectors, responsivity is a very important indicator. High-responsivity semiconductor photodetectors with internal gain can achieve high-sensitivity detection of weak light signals at room temperature and low bias voltage, and can eliminate the need for additional signal amplifiers, making the photodetection system more compact, lightweight, and low-cost. The n-p-n phototransistor is a commonly used semiconductor photodetector with internal gain. The principle of its photoelectric gain is that when the base region is suspended, the holes generated by light absorption in the collector junction will drift to the base under the action of the built-in field and the external electric field to accumulate, resulting in a lower emitter junction barrier, causing emitter electron injection, and thus generating a light-induced current with internal gain.

On this basis, another commonly used improved structure is the heterojunction phototransistor, which uses emitter materials with a wider bandgap to design the emitter junction into a heterojunction, thereby blocking the diffusion of holes and improving the electron injection ratio and transistor amplification factor.

However, current phototransistors and heterojunction phototransistors still have shortcomings. For example, for weak light signals, or even single-photon incident conditions, the number of photogenerated holes is extremely limited, and it is difficult to have a significant impact on the emission junction barrier height, thus limiting the phototransistor's detection of extremely weak light. Other common internal gain mechanisms of detectors also have their own limitations. For example, avalanche photodetectors require a very low material defect density to avoid premature breakdown, and require a higher operating voltage; photoconductive detectors have a high dark current, and generally have an uncontrollable gain process involving trap states, which will seriously degrade the detector's response speed. As a result, although the device can achieve high responsiveness, the response speed is generally in the order of milliseconds or even seconds, and the scope of application is severely limited.

Summary of the invention

The present invention aims to overcome at least one defect (shortcoming) of the above-mentioned prior art and provide a high-gain photodetector having the effects of obtaining low dark current, higher responsiveness, extremely weak light detection capability, and high-speed response.

The technical solution adopted by the present invention is:

A high-gain photodetector comprises an emitter, a base, an absorption layer and a collector arranged in sequence from bottom to top, and further comprises a periodic quantum structure layer, wherein the periodic quantum structure layer is arranged between the emitter and the collector, or arranged inside the base, or arranged inside the emitter;

The periodic quantum structure layer includes a repeated structure of multiple periods, and each period contains a semiconductor heterostructure, thereby forming multiple valence band steps and corresponding hole potential wells and potential barriers. Among them, the semiconductor junction formed by two adjacent heterogeneous materials is a heterojunction.

In this technical solution, the emitter, base, absorption layer and collector constitute the main device structure of the phototransistor. By adding a periodic quantum structure layer to the main device structure, the electron injection ratio and the device amplification factor are improved, and a superimposed photoconductivity gain is provided to obtain a higher responsiveness. At the same time, since this superimposed photoconductivity gain comes from the periodic quantum structure rather than the trap state, the hole lifetime can be regulated by controlling its structural parameters, thereby achieving high gain while ensuring the response speed of the device. Specifically, the periodic quantum structure layer uses a periodic quantum structure to form a series of valence band hole barriers to block the diffusion of holes from the base to the emitter, and there is no conduction band electron barrier or the thickness of the electron barrier in the structure allows electrons with a smaller effective mass to tunnel directly through, so it can not affect the diffusion of electrons from the emitter to the base, thereby improving the emitter electron injection ratio of the phototransistor and improving the transistor amplification factor. In addition, since the periodic quantum structure includes multiple valence band steps, the diffusion barrier effect on holes will be better than the heterojunction phototransistor structure with a single valence band step, that is, it can provide a higher electron injection ratio and amplification factor than the heterojunction phototransistor.

Furthermore, while the periodic quantum structure blocks the diffusion of base holes to the emitter, it will also bind some of the photogenerated holes in the hole potential well of the device structure. The lifetime of the holes in the potential well will be affected by escape mechanisms such as thermal emission and tunneling, and the probability of escaping the quantum well depends on the depth of the hole potential well and the thickness of the hole barrier. The depth of the hole potential well (the difference in the valence band order) can be adjusted by selecting different heterogeneous semiconductor materials, and the thickness of the hole barrier can be directly controlled by the material thickness of each layer in the periodic structure. Therefore, changing these structural parameters can regulate the hole lifetime, so that the hole lifetime exceeds the electron transit time, obtain superimposed photoconductivity gain, and further improve the detector's responsiveness.

Since the hole lifetime in the periodic quantum structure can be controlled by structural parameters, the reduction in response speed caused by the long hole lifetime can be avoided, and high-speed response can be obtained while achieving high photoelectric gain.

Among them, the photoconductivity gain mechanism of the present invention is: through the periodic quantum structure, a series of hole potential wells are formed to obtain the hole confinement effect, so that the hole lifetime is longer than the time required for electron transit. In this way, after the photogenerated electrons transit to the electrode and are collected, the holes are still bound in the quantum structure, so that the cathode continues to emit electrons to maintain electrical neutrality. After multiple cycles, the photoelectron current collected by the phototransistor is much higher than the photoelectron current directly collected for the first time, thereby forming a photocurrent gain effect, wherein the gain multiple is the ratio of the hole lifetime to the electron transit time.

Furthermore, the periodic quantum structure layer may be between the emitter and the collector, or may be embedded inside the base or inside the emitter.

Furthermore, the hole lifetime can be controlled by the depth of the hole potential well in the periodic structure (the size of the valence band step, which depends on the material used) and the thickness of each layer, thereby achieving regulation of the detector response speed and achieving high-speed response while obtaining high gain.

Furthermore, the periodic quantum structure layer contains a hole potential well layer and a hole barrier layer formed by the valence band steps of the heterostructure. Furthermore, the depth of the hole potential well is higher than 3kT, where k is the Boltzmann constant and T is the operating temperature. When the potential well depth is less than 3kT, holes can easily escape from the potential well layer through thermal emission, resulting in the failure of the hole confinement effect of the periodic quantum structure.

The hole potential well formed in the quantum structure is obtained through the valence band order of the heterostructure, so it has a strong binding effect on holes, but there may be no potential well for electrons, or there may be an electron potential well but the electrons can escape from the electron potential well through tunneling by adjusting the thickness, that is, the effective mass of the electron is smaller than that of the hole, making tunneling more likely to occur, and thus the binding effect on the electrons is very weak.

In another embodiment, there are both electron potential well layers and electron potential barrier layers in the periodic quantum structure layer, and the probability of electrons escaping from the electron potential well layer is higher than the probability of holes escaping from the hole potential well layer, that is, the electron potential well has a relatively weak binding effect on electrons, so that the time required for electron crossing is still less than the hole lifetime. Therefore, a high gain effect can also be obtained.

Specifically, if there are electron potential wells and electron potential barriers in the periodic quantum structure layer, the thickness of the electron potential barrier layer in the periodic quantum structure should not exceed the de Broglie wavelength of the electrons in the electron potential well layer; when the thickness of the periodic quantum structure is shorter than the electron wavelength, the electrons can tunnel through and the blocking effect is weak, so that the binding effect of the entire periodic quantum structure layer on holes will be stronger than the binding effect on electrons, so that the time required for electron crossing is less than the hole lifetime.

Furthermore, the period number of the periodic quantum structure layer is ≥ 3. If the period number is too small, the hole binding effect will be weakened, the photoconductivity gain will be insignificant, and the electron injection ratio and transistor amplification factor will also be reduced.

In one embodiment, the periodic quantum structure layer is a quantum well layer.

In another embodiment, the periodic quantum structure layer is a superlattice layer.

The quantum well layer or superlattice layer forms a series of valence band steps, which play the role of hole potential wells, so that holes are bound in the potential wells, thereby extending the hole lifetime t1 and exceeding the electron transit time t2 to form the effect of photoconductive gain. Specifically, the size of photoconductive gain is t1/t2.

Furthermore, the periodic quantum structure of the present technical solution can also be a structure that can form a hole confinement effect, such as a structure that has only hole potential wells but no electron potential wells.

Furthermore, the periodic quantum structure is one of an AlGaN/GaN superlattice structure, an AlGaN/GaN multiple quantum well structure, an AlGaAs/GaAs superlattice structure, and an AlGaAs/GaAs multiple quantum well structure.

In one embodiment, it also includes upper and lower contact electrodes.

In one embodiment, an ohmic contact is formed between the upper contact electrode and the collector electrode, and between the lower contact electrode and the emitter electrode, or the height of the contact barrier is lower than 0.5 eV, so as to avoid reducing the photocurrent of the device.

Furthermore, the lower contact electrode and the upper contact electrode are Ti/Al/Ni/Au or Au/AuGeNi alloy.

In one embodiment, the emitter and the collector are made of n-type conductive semiconductor material, and the electron concentration is higher than 5×10 ¹⁶ cm ⁻³ .

Furthermore, the emitter is n-type AlGaN or n-type AlGaAs or n-type GaN. The collector is n-type GaN or n-type GaAs. If the electron concentration of the emitter is too low, the photocurrent will be reduced.

In one embodiment, the base is a p-type conductive semiconductor material with a hole concentration higher than 1×10 ¹⁶ cm ⁻³ .

Among them, too low hole concentration in the base region will also cause the base region to punch through early and increase the dark current.

Furthermore, the base is a p-type GaN layer.

In one embodiment, the absorption layer is an intrinsic or unintentionally doped semiconductor material, and the hole or electron concentration is less than 5×10 ¹⁷ cm ⁻³ .

Among them, excessively high carrier concentration in the absorption layer will lead to a decrease in the width of the depletion layer, a decrease in the efficiency of collecting photogenerated carriers, and a degradation in the detector responsivity.

Furthermore, the absorption layer is unintentionally doped GaN or unintentionally doped GaAs.

Compared with the prior art, the present invention has the following beneficial effects:

This technical solution adds a periodic quantum structure layer to the device structure, so that during the transport/migration of photogenerated electron holes, the holes are bound, so that more electrons are injected into the electrode to form current gain and obtain a high gain effect.

This technical solution achieves the effect of high photoelectric gain without introducing trap states, degrading the response speed of the device, and with controllable speed.

The present invention achieves blocking of holes from diffusing from the base to the emitter without affecting the diffusion of electrons from the emitter to the base by adding a periodic quantum structure layer, thereby improving the electron injection ratio of the phototransistor and improving the transistor amplification factor.

The present invention introduces a superimposed speed-controllable photoconductivity gain on the basis of the phototransistor, which can not only make up for the deficiency of the weak light response of the phototransistor, but also further improve the overall gain; it has the beneficial effect of realizing high-speed and extremely high-response photoelectric detection under low bias voltage without demanding on the quality of the material crystal.

The present invention forms a series of hole potential wells and potential barriers by introducing a periodic quantum structure to regulate the transport of photogenerated electrons and holes, thereby superimposing photoconductivity gain on the light-induced mechanism of the phototransistor. On the one hand, the periodic quantum structure of the present invention can block the diffusion of base holes to the emitter, improve the electron injection ratio of the emitter junction and the amplification factor of the transistor, and on the other hand, it can bind holes with larger effective mass in the multi-layer quantum structure to reduce the recombination loss, and make the hole lifetime in the device exceed the transport time of the electron, thereby generating superimposed photoconductivity gain in the phototransistor. In addition, in the device structure provided by the present invention, the hole lifetime can be controlled by the material selection and thickness ratio of the periodic structure, thereby realizing the regulation of the detector response speed, and achieving high-speed response while obtaining high photoelectric gain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of Embodiment 1 of the present invention.

2 is a band simulation result of the periodic quantum structure (AlGaN/GaN superlattice) in Example 1 of the present invention, and a schematic diagram of the process of conduction band electrons and valence band holes escaping the potential well through thermal emission (TE) and tunneling (T).

FIG3 is a schematic diagram of the structure of embodiment 4 of the present invention.

FIG. 4 is a schematic diagram of the structure of Embodiment 6 of the present invention.

FIG. 5 is a test result of the light-dark current characteristics of the photodetector in Example 1 of the present invention.

FIG. 6 is a test result of the response speed of the photodetector in Example 1 of the present invention.

DETAILED DESCRIPTION

The drawings of the present invention are only for illustrative purposes and should not be construed as limiting the present invention. In order to better illustrate the following embodiments, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; it is understandable to those skilled in the art that some well-known structures and their descriptions in the drawings may be omitted.

Example 1

As shown in FIG. 1 , a GaN-based high-gain photodetector structure mainly includes an emitter layer 201 , a periodic quantum structure layer 202 , a base layer 203 , an absorption layer 207 and a collector layer 204 arranged in sequence from bottom to top.

In this embodiment, lower contact electrodes 205 are disposed on both sides of the upper surface of the emitter layer 201 , and upper contact electrodes 206 are disposed on both sides of the upper surface of the absorption layer 205 .

The emitter layer 201 is made of n-type AlGaN, with an Al component of 20%, an electron concentration of 2×10 ¹⁸ cm ^-3 , and a thickness of 300 nm. The preparation method is not limited. Specifically, a substrate layer and/or a buffer layer is provided below the emitter layer.

The periodic quantum structure layer 202 is an AlGaN/GaN superlattice structure, in which the Al component is 20%, the number of periods is 20, and the thickness of the AlGaN layer in each period is 2 nm, and the thickness of the GaN layer is 4 nm.

The base layer 203 is a p-type GaN layer, with a hole concentration of 1×10 ¹⁸ cm ⁻³ and a thickness of 100 nm. The preparation method and doping method are not limited.

The collector layer 204 is made of n-type GaN, has an electron concentration of 5×10 ¹⁷ cm ⁻³ , and is 120 nm thick.

The lower contact electrode 205 and the upper contact electrode 206 are both Ti/Al/Ni/Au, with thicknesses of 15/80/20/60 nm.

The absorption layer 207 is non-intentionally doped GaN and has a thickness of 150 nm.

FIG2 is a schematic diagram showing the energy band simulation results of the AlGaN/GaN superlattice layer in this embodiment and the process of conduction band electrons/valence band holes escaping from the electron potential well/hole potential well through thermal emission (TE) and tunneling (T).

Due to the influence of the quantum confined stark effect (QCSE), electrons and holes will be separated in space, and the overlap of wave functions will be reduced, resulting in a very low recombination rate between electrons and holes, so the radiative recombination lifetime and non-radiative recombination lifetime will be very large. At this time, the carrier lifetime τ will be mainly determined by the escape mechanism, that is: Among them, the carrier tunneling lifetime can be written as the carrier rate according to the WKB approximation theory in quantum mechanics: and tunneling probability (T(E _n )):

The thermal emission lifetime (τ _TE ) depends mainly on the effective barrier height:

Where _Lw represents the thickness of the quantum well, represents the larger value of the potential energy of the electron and hole barriers, and En _is the subband energy level height of the AlGaN quantum well, which can be written as:

By using the energy band shape obtained by simulation into the above three formulas for calculation, it can be obtained that the escape lifetimes of electrons and holes are 1.4× ^10-11 s and 8.3× ^10-8 s, respectively. The calculation results show that the escape lifetime of electrons in the quantum structure of this embodiment is much shorter than that of holes, that is, electrons are more likely to escape from the quantum well. Further, the equivalent mobility and drift velocity of electrons in the periodic structure can be calculated through the above process, thereby calculating the transit time of electrons. The calculated result of the electron transit time in this structure is approximately 4.9× ^10-9 s, so the superposition photoconductivity gain generated is approximately 17 times.

Since the hole lifetime in this structure is calculated to be approximately 80ns, the device can achieve a high-speed response at the ns level while achieving higher gain.

Example 2

The difference between this embodiment and the first embodiment is that the material of the emitter layer is n-type GaN, the periodic quantum structure layer is a multi-quantum well of AlGaN (6nm)/GaN (3nm), and the number of periods is 10.

Example 3

The present embodiment is different from the first embodiment in that: in the AlGaN/GaN superlattice layer of the periodic quantum structure layer, the Al component of AlGaN is 15%, the thickness of AlGaN in each period is 3 nm, and the thickness of GaN is 4 nm.

Example 4

As shown in FIG3 , a GaN-based high-gain photodetector structure mainly includes an emitter layer 301, a base layer 3031, a periodic quantum structure layer 302, a base layer 3032, an absorption layer 307 and a collector layer 304, which are arranged in sequence from bottom to top. Lower contact electrodes 305 are arranged on both sides of the upper surface of the emitter layer 301, and upper contact electrodes 306 are arranged on both sides of the upper surface of the absorption layer 305.

The difference between this embodiment and embodiment 1 is that: in this embodiment, the base layer is divided into a base layer 3031 and a base layer 3032, and the periodic quantum structure layer 302 is located in the base layer, that is, between the base layer 3031 and the base layer 3032, as shown in FIG. 3 .

Example 5

The difference between this embodiment and embodiment 1 is that the periodic quantum structure layer is composed of 15 layers of AlGaN/GaN repeatedly overlapped, wherein the thickness of GaN in each period is 4nm, and AlGaN includes 3 thicknesses: wherein, from top to bottom, in the first 5 periods of the periodic quantum structure layer, the thickness of each AlGaN layer is 1.5nm, in the middle five periods, the thickness of each AlGaN layer is 2.5nm, and in the bottom five periods, the thickness of each AlGaN layer is 4nm. In the periodic quantum structure thus set, the hole barrier layer is an AlGaN layer, and the closer the hole barrier to the base region, the thinner it is, which is conducive to more holes diffusing into the periodic quantum structure and improving the photoconductivity gain effect; and the closer the hole barrier to the emitter, the thicker it is, which is conducive to blocking the diffusion of holes into the emitter and maintaining a higher electron injection ratio and transistor amplification factor.

Example 6

As shown in FIG4 , this embodiment shows a GaAs-based high-gain photodetector, which mainly includes an emitter layer 401, a periodic quantum structure layer 402, a base layer 403, an absorption layer 407 and a collector layer 404 arranged in sequence from bottom to top. Lower contact electrodes 405 are arranged on both sides of the upper surface of the emitter layer 401, and upper contact electrodes 406 are arranged on both sides of the upper surface of the absorption layer 405.

The material of the emitter layer 401 is n-type AlGaAs, with an Al composition of 20%, an electron concentration of 2×10 ¹⁸ cm ⁻³ , and a thickness of 300 nm.

The periodic quantum structure layer 402 is an AlGaAs/GaAs superlattice structure, in which the Al component is 20%, the number of periods is 20, the thickness of the AlGaN layer in each period is 4 nm, and the thickness of the GaN layer is 6 nm.

The base 403 is a p-type GaN layer with a hole concentration of 1×10 ¹⁸ cm ⁻³ and a thickness of 100 nm.

The collector electrode 404 is made of n-type GaAs, has an electron concentration of 5×10 ¹⁷ cm ⁻³ , and a thickness of 120 nm.

The lower contact electrode 405 and the upper contact electrode 406 are both Au/AuGeNi alloy.

The absorption layer 407 is non-intentionally doped GaAs with a thickness of 200 nm.

Example 7

In this example, the photoelectric detector prepared in Example 1 is used to characterize the detector characteristics.

As shown in FIG5 , the light and dark current test results of the photodetector described in Example 1 are shown. At a low bias voltage of 10 V, the photocurrent gain of the detector is as high as 1.3×10 ⁵ . The gain of the photodetector without the AlGaN/GaN superlattice in Example 1 is 6.5×10 ⁴ under the same conditions. Obviously, by introducing the periodic quantum structure, the photoelectric gain of the device is increased by about 20 times, which is roughly consistent with the calculation results in Example 1.

As shown in FIG6 , the response speed test result of the device prepared in Example 1 is shown. The measured rise time and fall time of the detector are 2.4ns and 500ns respectively, which are much faster than the response speed of common photoconductive detectors exceeding the ms level. It can be seen that the solution provided by the present invention can achieve a very fast response speed while improving the device responsiveness.

Similarly, after the photodetectors prepared in Examples 2 to 6 were tested for light and dark current and response speed, the present embodiment obtained the same effects of improved photoelectric gain and high response speed as in Example 1. Therefore, the photodetector obtained in the present invention has the effects of high photoelectric gain and high response speed.

Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the technical solution of the present invention, and are not intended to limit the specific implementation methods of the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the claims of the present invention shall be included in the protection scope of the claims of the present invention.

Claims (7)

1. A high-gain photodetector, characterized in that it comprises an emitter, a base, an absorption layer and a collector arranged in sequence from bottom to top, and further comprises a periodic quantum structure layer, wherein the periodic quantum structure layer is arranged between the emitter and the collector, or arranged inside the base, or arranged inside the emitter; The periodic quantum structure layer includes a repeated structure of multiple periods, and each period contains a semiconductor heterojunction; The periodic quantum structure layer contains a series of hole potential well layers and hole potential barrier layers formed by the heterojunction valence band steps; A series of electron potential well layers and electron potential barrier layers formed by heterojunction conduction band steps exist in the periodic quantum structure layer, and the probability of electrons escaping from the electron potential well layers is higher than the probability of holes escaping from the hole potential well layers; The periodic quantum structure is one of an AlGaN/GaN superlattice structure, an AlGaN/GaN multiple quantum well structure, an AlGaAs/GaAs superlattice structure, and an AlGaAs/GaAs multiple quantum well structure; The depth of the hole potential well in the periodic quantum structure is higher than 3kT, where k is the Boltzmann constant and T is the operating temperature; the thickness of a single hole barrier layer is not less than 0.5 nm. 2 . The high-gain photodetector according to claim 1 , wherein the number of periods of the periodic quantum structure layer is ≥3. 3. A high-gain photodetector according to claim 1, characterized in that it also includes upper and lower contact electrodes; Ohmic contacts are formed between the upper contact electrode and the collector electrode, and between the lower contact electrode and the emitter electrode, or the height of the contact barrier is lower than 0.5 eV. 4. A high-gain photodetector according to claim 1, characterized in that the emitter and collector are made of n-type conductive semiconductor materials, and the electron concentration is higher than 5×10 ¹⁶ cm ^-3 . 5. The high-gain photodetector according to claim 1, wherein the base is a p-type conductive semiconductor material, and the hole concentration is higher than 1×10 ¹⁶ cm ^-3 . 6. A high-gain photodetector according to claim 1, characterized in that the absorption layer is an intrinsic semiconductor or an unintentionally doped semiconductor, and the concentrations of electrons and holes are both lower than 5×10 ¹⁷ cm ^-3 . 7 . The high-gain photodetector according to claim 1 , wherein the periodic quantum structure layer is a quantum well layer or a superlattice layer.