Ge nanostructured air-channel photomixer based on SOI

Abstract

The present invention discloses a Ge nanoscale air trench optical mixer based on SOI, belonging to the technical field of semiconductor photodetectors and millimeter wave / terahertz devices. The device includes an SOI substrate composed of an Si substrate, an SiO2 layer, and a doped Si thin film; a lower electrode and a Ge thin film are disposed on the upper surface of the SOI substrate, an insulating sacrificial layer and an upper electrode are disposed above the Ge thin film, and the upper electrode is isolated from the Ge thin film and the SOI substrate through an isolation layer; a partial region of the insulating sacrificial layer is etched inward to form a nanoscale air trench. The device of the present invention can use amorphous / polycrystalline / single crystal Ge thin films, greatly reducing the requirements for the Ge thin film growth equipment; using SOI as the substrate can reduce the parasitic capacitance of the device; and using the doped Si thin film as the carrier collection layer and the optical waveguide layer can effectively reduce the thickness of the absorption layer Ge thin film while maintaining high light absorption rate and responsivity, and improve the device bandwidth.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor photoelectric detectors and millimeter wave / terahertz devices, and in particular relates to a Ge nanometer air channel optical mixer based on SOI. Background Art

[0002] With the continuous exploration of the electromagnetic wave field, millimeter wave and terahertz technology has been widely used in radar and communication. Optical mixing is an important way to generate millimeter wave and terahertz waves. By beating two laser beams with different frequencies to obtain the millimeter wave band light with the difference between the two frequencies, it is possible to generate millimeter wave and terahertz sources with high spectral purity, little temperature influence and on-chip integration, which are not limited by phase matching and Manley-Row relationship in optical mixing of nonlinear media. By adjusting the wavelength of the two beating laser beams, ultra-wide frequency tunability from GHz to THz can be provided. At present, photoconductive antennas, PIN photodiodes and single-carrier photodiodes (UTC-PD) are mainly used as optical mixers. However, the low output power and energy conversion efficiency in the high-frequency band are still the bottleneck problems faced by the two types of optical mixers, photoconductive antennas and photodiodes. At present, the power level of the terahertz source generated by optical mixing in the 1THz band is still at the μW level. Therefore, traditional all-solid-state semiconductor optical mixers face the bottleneck that it is difficult to improve bandwidth, responsivity and efficiency at the same time.

[0003] The ideal transport state of carriers in a photomixer should be scattering-free ballistic transport at a speed close to the speed of light. However, in traditional solid semiconductor photomixers, carriers are affected by the lattice scattering of the solid medium and cannot achieve ballistic transport at a speed close to the speed of light. Increasing the external electric field will increase the electron transport speed while also increasing the scattering, resulting in saturation of the carrier transport speed. Vacuum is an ideal medium for electron ballistic transport. In addition to traditional solid semiconductor optoelectronics, vacuum optoelectronic devices are also an important optoelectronic device. Compared with traditional semiconductor solid-state optoelectronic devices, vacuum optoelectronic devices have unique advantages in frequency, power, radiation resistance, high temperature resistance, and sensitivity due to the lack of lattice collisions and energy dissipation and low dark current. However, traditional vacuum optoelectronic devices use the external photoelectric effect to achieve field-assisted light emission of electrons on the surface of the photocathode, and generally have problems such as low emission current, high operating voltage, low responsiveness and quantum efficiency, and the need for a vacuum working environment, which leads to a large volume and is not easy to miniaturize and integrate.

[0004] Nano air channel diode is a new type of device, which combines the advantages of traditional semiconductor solid-state optoelectronic devices and vacuum optoelectronic devices. It can work in air without vacuum packaging environment by taking advantage of the fact that the length of its nanometer-scale air channel is less than the mean free path of electron scattering in air. It has the advantages of micro-integration and compatibility with semiconductor processes, and is one of the most promising solutions for realizing optical mixers. From 2015 to 2016, Myungji Kim, Siwapon Srisonphan and others successively reported Si / SiO2 / graphene nano vacuum photodetectors, which achieved response in the 325nm to 1064nm band, and found that the two-dimensional electron gas (2DEG) confined in the quantum well can produce carrier multiplication under large electric field impact ionization, effectively improving the response. However, on the one hand, the band gap of silicon is 1.1eV, and silicon-based nano vacuum photodetectors cannot operate in the fiber optic communication band of 1300-17000nm; on the other hand, these reports generally use silicon bulk materials as the light absorption region, and the transit time and RC time are too long to work as optical mixers in the millimeter wave and terahertz frequency bands. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention proposes a Ge nano-air channel optical mixer based on SOI, aiming to provide a new optical mixer that can work without a vacuum environment, has the advantages of large bandwidth, high responsiveness, high output power, and high conversion efficiency, is suitable for large-scale silicon photonic integration process, and can be mass-produced on large-area wafers.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] A Ge nanometer air channel optical mixer based on SOI, characterized in that: the optical mixer comprises an SOI substrate, a lower electrode, a Ge film, an isolation layer, a nanometer insulating sacrificial layer, and an upper electrode;

[0008] The SOI substrate comprises a lower Si substrate, a middle SiO2 layer, and an upper doped Si film; wherein the upper doped Si film serves as both a carrier collection layer and an optical waveguide layer;

[0009] The lower electrode and the Ge film are arranged on the upper surface of the SOI substrate, and there is no physical contact between the two;

[0010] A nano insulating sacrificial layer is disposed above the Ge film;

[0011] An isolation layer is arranged on a side of the Ge film and the nano insulating sacrificial layer away from the lower electrode;

[0012] An upper electrode is arranged above the nano insulating sacrificial layer and the isolation layer; the isolation layer is used to isolate the upper electrode from the Ge film and the SOI substrate;

[0013] A part of the nano insulating sacrificial layer is corroded inwardly on one side close to the lower electrode, and a nano air channel is formed on the outside of the nano insulating sacrificial layer between the Ge film and the upper electrode.

[0014] Furthermore, when the Ge film is used as a cathode, the Ge film is p-type doped and a negative bias is applied to the lower electrode; when the Ge film is used as an anode, the Ge film is n-type doped and a positive bias is applied to the lower electrode.

[0015] Furthermore, the Ge film is an amorphous Ge film, a polycrystalline Ge film or a single crystal Ge film, and its function is to absorb photons and convert them into electron-hole pairs.

[0016] Furthermore, the nano insulating sacrificial layer is made of insulating materials that can be wet-etched, such as SiO2, Al2O3, HfO2, Si3N4, AlN, etc.

[0017] Furthermore, the upper electrode and the lower electrode are made of conductive materials, such as metal, metalloid, transparent electrode, graphene, etc.

[0018] Furthermore, the isolation layer is made of insulating material.

[0019] Furthermore, the length of the nano-air channel is less than the scattering mean free path of electrons in the air; electrons can be transported ballistically in the nano-air channel without a vacuum environment, and the emission current can be increased and the operating voltage can be reduced.

[0020] The basic working principle of the Ge-based nano-air channel optical mixer based on SOI proposed in the present invention is:

[0021] Under the action of near-infrared light with a wavelength in the terahertz band and an external electric field, the Ge film absorbs photons and excites photoelectrons; or a high concentration of holes gathers on the surface of the Ge film, inducing the corresponding upper electrode to produce and emit two-dimensional electron gas. The emitted two-dimensional electron gas is ballistically transported in the nano-air channel, reaches the anode at a speed close to the speed of light, is absorbed, and generates high-frequency photocurrent. The high-frequency photocurrent is transmitted through the electrode to loads such as dipole antennas or coplanar waveguides to radiate millimeter waves or terahertz waves.

[0022] When the Ge film is n-type doped, the Ge film is used as the anode, the lower electrode is applied with a forward bias, and the upper electrode is applied with a negative bias; at this time, the Ge film is in a reverse depletion state, and a two-dimensional hole gas layer is formed on the upper surface of the Ge film near the nano-insulating sacrificial layer under a large voltage. After being illuminated by near-infrared light with a wavelength of about 1550nm and a wavelength difference in the terahertz band and an external electric field, the Ge film absorbs photons and generates a large number of electron-hole pairs in the depletion region. At the same time, a carrier multiplication effect occurs under a high electric field, which increases the number of holes exponentially. The high-concentration hole gas induces the upper electrode opposite the nano-air channel to produce a high-concentration two-dimensional electron gas. The Coulomb repulsion between the two-dimensional electron gases reduces the barrier for electron emission, making it easier for electrons to be emitted into the nano-air channel. The rapid ballistic transport is absorbed by the anode to generate high-frequency photocurrent, which can radiate millimeter-wave terahertz signals through the antenna.

[0023] When the Ge film is p-type doped, the Ge film is used as the cathode, the lower electrode is negatively biased, and the upper electrode is positively biased; at this time, the Ge film is in a reverse depletion state, and a two-dimensional electron gas layer is formed near the nano-insulating sacrificial layer on the upper surface of the semiconductor layer under a large voltage. After near-infrared light with a wavelength of about 1550nm and a wavelength difference in the terahertz band and an external electric field, the Ge film absorbs photons and generates a large number of electron-hole pairs in the depletion region. At the same time, a carrier multiplication effect occurs under a high electric field, which increases the number of electrons exponentially. The Coulomb repulsion between the two-dimensional electron gas reduces the barrier of electron emission, making it easier for electrons to be emitted into the nano-air channel. The rapid ballistic transport is absorbed by the anode to generate high-frequency photocurrent, which can radiate millimeter-wave terahertz signals through the antenna.

[0024] The advantages of the present invention are as follows:

[0025] 1. The device proposed in the present invention can use amorphous / polycrystalline / single crystal Ge thin film, and does not need to grow high-quality and complex multi-layer doping structure like traditional semiconductor PIN-PD and UTC-PD, which greatly reduces the requirements for Ge thin film growth equipment.

[0026] 2. The device proposed in the present invention uses SOI as the substrate, which can not only greatly reduce the parasitic capacitance of the device, but also use a non-absorbing doped Si film that can be used as a carrier collection layer and an optical waveguide layer at the same time. While maintaining high light absorption rate and responsiveness, it can effectively reduce the thickness of the absorption layer Ge film and improve the device bandwidth.

[0027] 3. The device proposed in the present invention uses SOI as a substrate and is suitable for large-scale silicon photonic integration technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1Schematic diagram of a single crystal Ge thin film device with edge-incident evanescent wave coupling structure based on SOI and with Si thin film as waveguide proposed by the present invention; wherein: 11, Si substrate; 12, SiO2 layer; 13, doped Si thin film; 14, lower electrode TiAu; 15, p-Ge thin film; 16, Si3N4 isolation layer; 17, SiO2 nano-insulating sacrificial layer; 18, upper electrode Au; 19, nano-air channel.

[0029] Figure 2 Schematic diagram of a normal-incident amorphous Ge thin film device based on SOI with a transparent electrode as an anode proposed by the present invention; wherein: 21, Si substrate; 22, SiO2 layer; 23, doped Si film; 24, lower electrode Au; 25, p-Ge film; 26, AIN isolation layer; 27, Al2O3 nano-insulating sacrificial layer; 28, upper electrode ITO; 29, nano-air channel.

[0030] Figure 3 Schematic diagram of a back-incident polycrystalline Ge thin film device based on SOI with metal as anode proposed by the present invention; wherein: 31, Si substrate; 32, SiO2 layer; 33, doped Si film; 34, lower electrode Al; 35, n-Ge film; 36, BN isolation layer; 37, HfO2 nano-insulating sacrificial layer; 38, upper electrode Al; 39, nano-air channel. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0032] Example 1

[0033] Single crystal Ge thin film device with edge-incident evanescent wave coupling structure using Si thin film as waveguide, such as Figure 1 As shown, the device includes an SOI substrate, a lower electrode TiAu, a single crystal p-Ge film, a Si3N4 isolation layer, a SiO2 nano-insulating sacrificial layer, and an upper electrode; wherein the SOI substrate is composed of a lower Si substrate, a middle SiO2 layer, and an upper low-doped Si film, and the Si film serves as a carrier collection layer and an optical waveguide layer at the same time; the lower electrode TiAu and the p-Ge film are respectively arranged on the left and right sides of the upper surface of the SOI substrate, and the thickness of the p-Ge film is 300nm; a SiO2 nano-insulating sacrificial layer with a thickness of 50nm is arranged above the p-Ge film; a Si3N4 isolation layer is arranged on the right side of the p-Ge film and the SiO2 nano-insulating sacrificial layer; an upper electrode Au is arranged above the SiO2 nano-insulating sacrificial layer and the Si3N4 isolation layer; the Si3N4 isolation layer isolates the upper electrode from the p-Ge film and the SOI substrate; a part of the left side of the nano-insulating sacrificial layer is corroded inward, and a nano-air channel is formed between the Ge film and the upper electrode Au and on the outside of the nano-insulating sacrificial layer.

[0034] The working process of the device is as follows: negative bias is applied to the lower electrode and positive bias is applied to the upper electrode; the p-type Ge film generates a depletion region under reverse bias, and an inversion layer is generated near its surface to form a two-dimensional electron gas; a 1540nm and 1560nm dual-wavelength combined laser is incident on the device along the Si waveguide, the depletion region absorbs photons to generate electron-hole pairs, and the photogenerated electrons drift to the interface between the SiO2 nano-insulating sacrificial layer and the Ge film to increase their concentration, and ionization collisions and multiplication occur in the process, and the Coulomb repulsion between electrons also increases; 2D The electrons at the edge of EG cross the surface potential barrier and enter the nano-air channel under the action of the internal electron Coulomb repulsion and the internal electric field, and the surface electrons at other positions are directly field-emitted into the nano-air channel under the large electric field, and quickly cross to the anode to be collected, forming a photocurrent; the frequency of the obtained photocurrent is equal to the frequency difference of the two incident light beams, and the changes in amplitude and phase are also synchronized with the amplitude and phase of the superimposed light field of the two incident light beams. The high-frequency photocurrent is transmitted through the electrode to the load such as the dipole antenna or the coplanar waveguide to radiate the terahertz wave, thereby realizing the generation of optical mixing terahertz signals. The device of this embodiment can generate 2.5THz terahertz waves.

[0035] Example 2

[0036] Normal-incidence amorphous Ge thin film devices with transparent electrodes as anodes, such as Figure 2 As shown, the device includes an SOI substrate, a lower electrode Au, an amorphous p-Ge film, an AIN isolation layer, an Al2O3 nano-insulating sacrificial layer, and an upper electrode ITO; wherein the SOI substrate is composed of a lower Si substrate, a middle SiO2 layer, and an upper doped Si film; the lower electrode Au and the p-Ge film are respectively arranged on the left and right sides of the upper surface of the SOI substrate, and the thickness of the p-Ge film is 300nm; an Al2O3 nano-insulating sacrificial layer with a thickness of 50nm is arranged above the p-Ge film; an AIN isolation layer is arranged on the right side of the p-Ge film and the Al2O3 nano-insulating sacrificial layer; an upper electrode ITO is arranged above the Al2O3 nano-insulating sacrificial layer and the AIN isolation layer; the AIN isolation layer isolates the upper electrode from the p-Ge film and the SOI substrate; a part of the left side of the nano-insulating sacrificial layer is corroded inward, and a nano-air channel is formed between the Ge film and the upper electrode and outside the nano-insulating sacrificial layer.

[0037] The working process of the device is as follows: negative bias is applied to the lower electrode and positive bias is applied to the upper electrode; the p-type Ge film generates a depletion region under reverse bias, and an inversion layer is generated near its surface to form a two-dimensional electron gas; the anode ITO is irradiated from top to bottom with a 1550nm and 1558nm dual-wavelength combined laser, and the incident light can pass through the upper electrode to make the Ge film absorb photons to generate electron-hole pairs, and the photogenerated electrons drift to the interface between the Al2O3 nano-insulating sacrificial layer and the Ge film to increase their concentration, and ionization collision and multiplication occur in this process, and the Coulomb repulsion between electrons also increases; due to the Coulomb repulsion, the electrons at the edge of the two-dimensional electron gas can easily overcome the potential barrier and be emitted into the nano-air channel, and at the same time, the electrons in the two-dimensional electron gas have a lower potential barrier under the action of an external electric field and tunnel into the nano-air channel. The electrons emitted into the nano-air channel are ballistically transported to the ITO transparent anode and collected, generating a photocurrent with an oscillation frequency of 1THz, which is radiated by the antenna to generate a terahertz wave with a frequency of 1THz.

[0038] Example 3

[0039] Back-incident polycrystalline Ge thin film devices with metal as anode, such as Figure 3 As shown, the device includes an SOI substrate, a lower electrode Al, a polycrystalline n-Ge film, a BN isolation layer, a HfO2 nano-insulating sacrificial layer, and an upper electrode Al; wherein the SOI substrate is composed of a lower Si substrate, a middle SiO2 layer, and an upper highly doped Si film; the lower electrode Al and the n-Ge film are respectively arranged on the left and right sides of the upper surface of the SOI substrate, and the thickness of the n-Ge film is 300nm; a 50nm thick HfO2 nano-insulating sacrificial layer is arranged above the n-Ge film; a BN isolation layer is arranged on the right side of the n-Ge film and the HfO2 nano-insulating sacrificial layer; an upper electrode Al is arranged above the HfO2 nano-insulating sacrificial layer and the BN isolation layer; the BN isolation layer isolates the upper electrode from the n-Ge film and the SOI substrate; a partial area on the left side of the nano-insulating sacrificial layer is corroded inward, and a nano-air channel is formed between the Ge film and the upper electrode and on the outside of the nano-insulating sacrificial layer.

[0040] The working process of the device is as follows: a positive bias is applied to the lower electrode and a positive bias is applied to the upper electrode; a depletion region is generated in the n-Ge film, and a dual-wavelength combined laser of 1550nm and 1550.8nm is used to irradiate the device from bottom to top. The n-Ge film absorbs photons to generate electron-hole pairs, and the light propagates to the metal cathode and is reflected, causing the Ge film to absorb twice. The generated photogenerated holes migrate to the interface between the n-Ge film and the HfO2 nano-insulating sacrificial layer under the action of the internal electric field to form a high-concentration two-dimensional hole gas, and induce a high-concentration two-dimensional electron gas on the HfO2 nano-insulating sacrificial layer and the Al side of the upper electrode. Due to the Coulomb repulsion, the electrons at the edge of the 2DEG are easily emitted into the nano-air channel. At the same time, the electrons in the two-dimensional electron gas have a lower potential barrier under the action of the external electric field and tunnel into the nano-air channel. The electrons emitted into the nano-air channel are ballistically transported and quickly reach the n-Ge anode and are collected, generating a terahertz wave with an oscillation frequency of 0.1THz.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A Ge nano-air channel optical mixer based on SOI, characterized in that: The optical mixer comprises an SOI substrate, a lower electrode, a Ge film, an isolation layer, a nano-insulating sacrificial layer, and an upper electrode; The SOI substrate comprises a lower Si substrate, a middle SiO2 layer, and an upper doped Si film; wherein the upper doped Si film serves as both a carrier collection layer and an optical waveguide layer; The lower electrode and the Ge film are arranged on the upper surface of the SOI substrate, and there is no physical contact between the two; An insulating sacrificial layer is disposed above the Ge film; An isolation layer is provided on a side of the Ge film and the insulating sacrificial layer away from the lower electrode; An upper electrode is arranged above the nano insulating sacrificial layer and the isolation layer; The isolation layer is used to isolate the upper electrode from the Ge film and the SOI substrate; A part of the nano insulating sacrificial layer is corroded inwardly on one side close to the lower electrode, and a nano air channel is formed on the outside of the nano insulating sacrificial layer between the Ge film and the upper electrode.

2. The SOI-based Ge nano-air channel optical mixer according to claim 1, characterized in that: When the Ge film is used as a cathode, the Ge film is p-type doped and a negative bias is applied to the lower electrode; when the Ge film is used as an anode, the Ge film is n-type doped and a positive bias is applied to the lower electrode.

3. A Ge nano-air channel optical mixer based on SOI as claimed in claim 1 or 2, characterized in that: The Ge thin film is an amorphous Ge thin film, a polycrystalline Ge thin film or a single crystal Ge thin film.

4. The SOI-based Ge nano-air channel optical mixer according to claim 3, characterized in that: The nano insulating sacrificial layer is made of insulating material that can be wet-etched.

5. The SOI-based Ge nano-air channel optical mixer according to claim 3, characterized in that: The upper electrode and the lower electrode are made of conductive material.

6. The SOI-based Ge nano-air channel optical mixer according to claim 3, characterized in that: The isolation layer is made of insulating material.

7. The SOI-based Ge nano-air channel optical mixer according to claim 3, characterized in that: The length of the nanometer air channel is smaller than the scattering mean free path of electrons in the air.

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