Ultra-wide forbidden band MIS diode and preparation method thereof

Abstract

The invention provides an ultra-wide forbidden band MIS diode and a preparation method thereof, and belongs to the technical field of semiconductors. The ultra-wide forbidden band MIS diode comprises a cathode, a substrate, a Ga2O3 epitaxial layer, a dielectric layer, an anode and an antireflection film, wherein the substrate is laid on the cathode; the Ga2O3 epitaxial layer is laid on the substrate, the n-type doping concentration of the Ga2O3 epitaxial layer is lower than that of the substrate, a plurality of grooves are formed in the Ga2O3 epitaxial layer, and an n + region is formed on the edge of the top of each groove; the dielectric layer is laid on the Ga2O3 epitaxial layer and the inner wall of each groove; the anodes are laid on the dielectric layer, are filled in the grooves at the same time, and penetrate through the dielectric layer to be in contact with the n + region; the antireflection film is laid on the side wall of the Ga2O3 epitaxial layer, and the vertical projection of the antireflection film covers the side wall of each groove. The problem that avalanche breakdown cannot occur due to the fact that gallium oxide lacks a P-type material can be solved, and the conductivity modulation effect is easily achieved.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor technology, and particularly relates to an ultra-wide bandgap MIS diode and a preparation method thereof. Background Art

[0002] As a key component in power electronic circuits, power diodes play an indispensable role in rectification, energy transfer, and circuit protection. During the design and maintenance of high-voltage circuits, dedicated discharge circuits or discharge devices are often required. Specifically, when a high-voltage circuit operates, a large amount of charge and energy will accumulate. After the circuit is disconnected, this energy will not dissipate immediately but will be temporarily stored in energy storage components in the circuit (such as capacitors, inductors, etc.). Therefore, designing a solution that can effectively discharge this energy is the key to ensuring system safety. Using a power diode can provide a path for the sudden voltage and current generated during the state transition of the energy storage component, thereby achieving rapid energy discharge and avoiding potential safety hazards.

[0003] Gallium oxide (Ga 2 O 3 ) material has become an ideal choice in the manufacturing of power devices due to its unique physical and chemical properties. Its wide bandgap characteristics, high breakdown voltage, and relatively simple preparation process make Ga 2 O 3 material have the potential to manufacture high-efficiency, reliable, and low-cost power devices. With the continuous progress of technology and the further reduction of production costs, Ga 2 O 3 is expected to be widely used in the field of power electronics, thereby promoting the innovation and development of related industries.

[0004] However, there are still significant challenges in p-type doping of Ga 2 O 3 material. Factors such as its flat valence band, large effective mass, easy formation of self-trapped holes, and the self-compensation effect of unintentional doping donor defects make p-type doping very difficult. The lack of p-type material in Ga 2 O 3 diode makes it difficult to achieve the conductivity modulation effect. Summary of the Invention

[0005] In view of this, the present invention provides an ultra-wide bandgap MIS diode and a preparation method thereof, which can overcome the problem that gallium oxide lacks p-type material and cannot undergo avalanche breakdown. The device can quickly discharge the residual energy of the high-voltage circuit and is easy to achieve the conductivity modulation effect.

[0006] To achieve the above object, the present invention adopts the following technical solutions: On the one hand, an ultra-wide bandgap MIS diode is provided, including a cathode, a substrate, Ga2 O 3 An epitaxial layer, a dielectric layer, an anode, and an antireflection film. The substrate is disposed on one side of the cathode and has a thickness of 150 - 650 μm. The substrate is made of n-type doped Ga 2 O 3 material, and the concentration of n-type doping is 1e18 - 1e19 cm -3 ; The Ga 2 O 3 epitaxial layer is disposed on the side of the substrate away from the cathode and is n-type doped. The thickness of the Ga 2 O 3 epitaxial layer is 10 - 100 μm, and the concentration of n-type doping is 5e14 - 1e17 cm -3 . The top of the Ga 2 O 3 epitaxial layer is recessed downward to form a plurality of grooves, and an n+ region is formed at the top edge of each groove; The dielectric layer is disposed on the side of the Ga 2 O 3 epitaxial layer away from the substrate, and the inner wall of each groove; The anode is disposed on the side of the dielectric layer away from the Ga 2 O 3 epitaxial layer, and at the same time fills each groove. The anode penetrates the dielectric layer and contacts the n+ region; The antireflection film is disposed on the side wall of the Ga 2 O 3 epitaxial layer parallel to the inner side wall of the groove, and the vertical projection of the antireflection film covers the side wall of each groove.

[0007] Preferably, a metal layer is disposed on the side of each n+ region close to the dielectric layer, and the anode penetrates the dielectric layer and contacts the metal layer.

[0008] Preferably, the cross-section of each groove is circular or polygonal.

[0009] Preferably, the groove is provided with at least one level of steps.

[0010] Preferably, the material for preparing the cathode includes Ti, Al, or Ti / Au.

[0011] Preferably, the material for preparing the n+ region includes Si, Ge, or Sn.

[0012] Preferably, the material for preparing the metal layer includes Ni, Ni / Au, or W / Au.

[0013] Preferably, the material for preparing the dielectric layer includes SiO 2 , Al 2 O 3 or at least one of SiN.

[0014] Preferably, the material for preparing the anode includes Ti, Al, or Ti / Au.

[0015] On the other hand, the present invention provides a method for preparing the ultra-wide bandgap MIS diode, including the following steps: S1. Epitaxially form the Ga 2 O 3 epitaxial layer on the substrate, and etch downward from the top of the Ga 2 O 3 epitaxial layer to form a plurality of trenches; S2. Perform n-type ion implantation on the top edge of the trench to form an n+ region; S3. Deposit on both the Ga 2 O 3 epitaxial layer and the sidewalls of the trenches to form a dielectric layer, the dielectric layer exposing the n+ region, and deposit and form an anode in the dielectric layer and the trenches; S4. Deposit and form a cathode on the side of the substrate away from the Ga 2 O 3 epitaxial layer, and deposit an antireflection film on one side of the Ga 2 O 3 epitaxial layer parallel to the sidewalls of the trenches.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention adds the antireflection film on the Ga 2 O 3 epitaxial layer. The antireflection film helps more laser beams enter the interior of the Ga 2 O 3 epitaxial layer material by reducing the reflection loss of light. It can effectively reduce the reflection occurring on the material surface, making the laser beam more easily penetrate the wafer surface and reach the gallium oxide layer. In the forward conduction state of the diode device, when light is applied, the semiconductor material is irradiated by high-energy laser light, and some valence electrons obtain sufficient energy to break free from the bondage of covalent bonds and become free electrons, thereby generating photo-generated carriers. The carrier concentration in the Ga 2 O 3 epitaxial layer increases, which can reduce the on-resistance and on-loss, and the photo-injection enhances the on-current of the device. In the reverse blocking state of the device, when light is applied, the Ga 2 O 3Photo-generated carriers are generated inside the epitaxial layer. The electrons and holes generated by photons are rapidly separated under the action of a high reverse electric field, and at the same time, they perform accelerated motion under the action of the electric field and obtain a large kinetic energy. When the carriers collide with atoms, there is a probability of generating new electron-hole pairs. At this time, it will enter a positive cycle, repeating the accelerated motion of the electron-hole pairs, and the device current will increase rapidly, causing the device to quickly change from a high impedance to a low impedance, realizing conductance regulation. This process corresponds to the situation where the load in the circuit of the diode is open. When the load is open, the energy in the inductor needs to be released through the diode. At this time, the diode will show a high voltage and large current situation. The traditional energy release is through the PN junction to generate a large number of carriers for conduction through avalanche. At this time, the voltage is equivalent to or higher than the avalanche breakdown voltage. However, the present invention conducts electricity by directly generating carriers through laser irradiation, without having to reach the avalanche breakdown voltage, which can overcome the problem that gallium oxide lacks P-type materials and cannot undergo avalanche breakdown. At this time, the device changes from a high impedance to a low impedance through photo-injected conductance regulation, thereby absorbing a large amount of transient voltage energy and converting it into heat energy to be released, and can quickly discharge the residual energy of the high-voltage circuit.

[0017] (2) The coverage range of the antireflection film of the present invention exceeds the trench depth, which means that it not only acts on the surface of the wafer, but also ensures that the area around the trench can effectively receive laser irradiation, thereby further ensuring that the generation area of photo-generated carriers covers as much as possible the working area of the entire device.

[0018] (3) The present invention adds the metal layer, and the metal layer forms an ohmic contact with the Ga 2 O 3 semiconductor. The n+ region can further improve the ohmic contact performance, and thus improve the device conduction performance. Brief Description of the Drawings

[0019] Figure 1 is a cross-sectional view of an ultra-wide bandgap MIS diode provided by an embodiment of the present invention; Figure 2 is a cross-sectional view of another ultra-wide bandgap MIS diode provided by an embodiment of the present invention; Figure 3 is a cross-sectional view of a substrate and a Ga 2 O 3 epitaxial layer provided by an embodiment of the present invention; Figure 4 is a top view of a Ga 2 O 3 epitaxial layer with a rectangular cross-section of the trench provided by an embodiment of the present invention; Figure 5 is a top view of a Ga 2 O 3Top view of the epitaxial layer.

[0020] Reference numerals: ultra-wide bandgap MIS diode 100, cathode 1, substrate 2, Ga 2 O 3 epitaxial layer 3, trench 31, n+-region 4, dielectric layer 5, anode 6, antireflection film 7, metal layer 8, step 9. Detailed implementation manners

[0021] The present invention will be further described in detail below in conjunction with specific embodiments, so that those skilled in the art can understand the present invention more clearly.

[0022] As a key component in power electronic circuits, power diodes play an indispensable role in rectification, energy transmission, and circuit protection. During the design and maintenance of high-voltage circuits, dedicated discharge circuits or discharge devices are often required. Specifically, when a high-voltage circuit is operating, a large amount of charge and energy will accumulate. After the circuit is disconnected, this energy will not dissipate immediately but will be temporarily stored in the energy storage components (such as capacitors, inductors, etc.) in the circuit. Therefore, designing a scheme that can effectively discharge this energy is the key to ensuring system safety. By using power diodes, a path can be provided for the sudden voltage and current generated during the state transition of the energy storage component, thereby realizing the rapid discharge of energy and avoiding potential safety hazards.

[0023] Gallium oxide (Ga 2 O 3 ) material has become an ideal choice in the manufacturing of power devices due to its unique physical and chemical properties. The wide bandgap characteristic, high breakdown voltage, and relatively simple manufacturing process enable Ga 2 O 3 material to have the potential to manufacture high-efficiency, reliable, and low-cost power devices. With the continuous progress of technology and the further reduction of production costs, Ga 2 O 3 is expected to be widely used in the field of power electronics, thus promoting the innovation and development of related industries.

[0024] However, there are still significant challenges in p-type doping of Ga 2 O 3 material. Factors such as the flat valence band, large effective mass, easy formation of self-trapped holes, and the self-compensation effect of unintentionally doped donor defects make p-type doping very difficult. And the lack of p-type material in Ga 2 O 3 diodes makes it difficult to achieve the conductivity modulation effect.

[0025] To solve the above technical problems, in combination with Figures 1-5, the present invention provides an ultra-wide bandgap MIS diode 100, comprising a cathode 1, a substrate 2, Ga 2 O 3 epitaxial layer 3, a dielectric layer 5, an anode 6, and an antireflection film 7. The substrate 2 is disposed on one side of the cathode 1 and has a thickness of 150 - 650 μm. The substrate 2 is made of an n-type doped Ga 2 O 3 material, and the n-type doping concentration is 1e18 - 1e19 cm -3 ; The Ga 2 O 3 epitaxial layer 3 is disposed on the side of the substrate 2 away from the cathode 1 and is n-type doped. The thickness of the Ga 2 O 3 epitaxial layer 3 is 10 - 100 μm, and the n-type doping concentration is 5e14 - 1e17 cm -3 . The top of the Ga 2 O 3 epitaxial layer 3 is recessed downward to form a plurality of grooves 31, and an n+ region 4 is formed at the top edge of each groove 31; The dielectric layer 5 is disposed on the side of the Ga 2 O 3 epitaxial layer 3 away from the substrate 2 and the inner walls of each groove 31; The anode 6 is disposed on the side of the dielectric layer 5 away from the Ga 2 O 3 epitaxial layer 3, and at the same time fills each groove 31. The anode 6 penetrates through the dielectric layer 5 and contacts the n+ region 4; The antireflection film 7 is disposed on the sidewall of the Ga 2 O 3 epitaxial layer 3 parallel to the inner sidewall of the groove 31, and the vertical projection of the antireflection film 7 covers the sidewalls of each groove 31. In some embodiments, the antireflection film is made of MgF 2 , AlF 2 , Al 2 O 3 , HfO 2 in a combination of two or more, and a multi-layer film design with alternating high and low refractive indices is used to approximate the ideal refractive index. The thickness of the antireflection film is adjusted by a coefficient to match the laser wavelength.

[0026] In the above technical solution, the cathode 1 is a key part of the diode structure and is located at the bottommost layer of the entire structure. The main function of the cathode 1 is to provide an inlet for electrons or current for the diode. The substrate 2 is located above the cathode 1 and is usually made of an n-type doped Ga 2 O 3 material with a thickness of 150 - 650 μm. n-type Ga 2 O 3The substrate provides a stable support structure, and its n-doping concentration (1e18 - 1e19 cm -3 -3) controls the conductivity of the substrate 2. The thickness of the substrate 2 is selectable and adjustable to adapt to different current density and thermal management requirements. The Ga 2 O 3 is a ultra-wide bandgap material with a large bandgap width, which enables it to exhibit excellent performance under conditions such as high temperature, high voltage, and high frequency. In the present invention, the thickness of the Ga 2 O 3 epitaxial layer 3 is 10 - 100 μm, and the n-type doping concentration is 5e14 - 1e17 cm -3 -3, and the formed n-type epitaxial layer provides the main functional region of the device. At the top of this epitaxial layer, a plurality of trenches 31 are formed by an etching process, and these trenches 31 are the key positions for the generation and concentration of photo-generated carriers. The n+ region 4 is formed at the top edge of each trench 31. The n+ region 4 is formed by ion implantation technology and has a high carrier concentration (compared with the n-type region in the Ga 2 O 3 epitaxial layer 3). These n+ regions 4 form the part in contact with the anode 6 electrode, thus realizing effective current transmission. The dielectric layer 5 is located on the side of the Ga 2 O 3 epitaxial layer 3 away from the substrate 2 and covers the inner walls of the trenches 31. It plays a role of isolation and protection in the device, and at the same time exposes the n+ region 4, ensuring the effective transmission of light and current. The anode 6 is located above the dielectric layer 5 and fills the trenches 31. The anode 6 is in contact with the n+ region 4, ensuring good electrical conductivity. The function of the anode 6 is to provide an outlet for the forward current and ensure the forward conduction of the diode. The antireflection film 7 is provided on the side walls of the Ga 2 O 3 epitaxial layer 3 parallel to the inner walls of the trenches 31, which can significantly reduce the reflection loss of light. The function of the antireflection film 7 is to increase the transmittance of light entering the Ga 2 O 3 material, especially in the case of laser irradiation, and can improve the photo-generated carrier generation efficiency of the device.

[0027] First, in the present invention, the antireflection film 7 is added on the Ga 2 O 3 epitaxial layer 3. The antireflection film 7 helps more laser beams enter the interior of the Ga 2 O 3 epitaxial layer 3 material by reducing the reflection loss of light. It can effectively reduce the reflection occurring on the material surface, making the laser beam more easily penetrate the wafer surface and reach the Ga 2 O3 Epitaxial layer 3. In the forward conduction state of the diode device, when light is applied, the semiconductor material is irradiated by high-energy laser, and some valence electrons obtain sufficient energy to break free from the bondage of covalent bonds and become free electrons, thus generating photo-generated carriers. The 2 O 3 carrier concentration in the epitaxial layer 3 increases, which can reduce the on-resistance and conduction loss, and the optical injection enhances the conduction current of the device. In the reverse blocking state of the device, when light is applied, the 2 O 3 photo-generated carriers are generated inside the epitaxial layer 3. The electrons and holes generated by photons are quickly separated under the action of a high reverse electric field, and at the same time, they are accelerated under the action of the electric field and obtain greater kinetic energy. When the carriers collide with atoms, there is a probability of generating new electron-hole pairs. At this time, it will enter a positive cycle and repeat the accelerated movement of the electron-hole pairs, and the device current increases rapidly, making the device quickly change from a high impedance to a low impedance, realizing conductance regulation. This process corresponds to the situation where the load in the circuit of the diode is open. When the load is open, the energy in the inductor needs to be released through the diode, and at this time, the diode will show a high voltage and large current situation. The traditional energy release is through the avalanche of the PN junction to generate a large number of carriers for conduction, and the voltage at this time is equivalent to or higher than the avalanche breakdown voltage. However, the present invention conducts electricity by directly generating carriers through laser irradiation, and it does not need to reach the avalanche breakdown voltage, which can overcome the problem that gallium oxide lacks P-type material and cannot undergo avalanche breakdown. At this time, the device can quickly discharge the residual energy of the high-voltage circuit.

[0028] Secondly, the coverage range of the anti-reflection film 7 of the present invention exceeds the depth of the trench 31, which means that it not only acts on the surface of the wafer, but also ensures that the area around the trench 31 can effectively receive laser irradiation, thereby further ensuring that the generation area of photo-generated carriers covers as much as possible the working area of the entire device.

[0029] Furthermore, a metal layer 8 is disposed on one side of each n+ region 4 close to the dielectric layer 5, the anode 6 penetrates through the dielectric layer 5 and contacts the metal layer 8, and the metal layer 8 is disposed on one side of each n+ region 4 close to the dielectric layer 5. The core objective of this design is to provide a low-resistance ohmic contact to optimize the conductive performance of the diode. Generally, the n+ regions 4 are formed by ion implantation and have a higher electron concentration than the 2 O 3 epitaxial layer 3. These n+ regions 4 are the key parts of the device in contact with the anode 6. In order to ensure the smooth flow of current, a good ohmic contact needs to be formed between the n+ regions 4 and the anode 6. The metal layer 8, the dielectric layer 5 and the gallium oxide material form a MIS structure. In the reverse state of the device, due to the metal layer 8 and the 2O 3 There is a work function difference between the epitaxial layers 3, so a depletion region is formed around the trench 31, blocking the conductive channel, and thus the device is not conducting. In some embodiments, the material for preparing the metal layer 8 includes Ni, Ni / Au or W / Au. These materials have good electrical conductivity and can form a stable ohmic contact with the Ga 2 O 3 epitaxial layer 3. Through the design of the metal layer 8, the contact between the n+ region 4 and the anode 6 is more reliable, and the current transmission is more efficient.

[0030] Furthermore, the cross-section of each trench 31 is circular or polygonal. The main advantage of the trench 31 with a circular cross-section is that the electric field and heat distribution are uniform, which is suitable for high-voltage and high-frequency applications; while the trench 31 with a polygonal cross-section improves the overall performance and mechanical stability of the device by increasing the effective contact area and optimizing the current distribution. Selecting a suitable shape of the trench 31 according to the specific application scenario can significantly improve the performance and reliability of the device.

[0031] Furthermore, the trench 31 is provided with at least one level of step 9. The setting of the step 9 can make the thickness of the dielectric layer 5 on the surface of the trench 31 more uniform, preventing premature breakdown at the thinner part of the dielectric layer 5. At the same time, the downward current path is larger, and the forward conduction characteristic is better.

[0032] Furthermore, the material for preparing the cathode 1 includes Ti, Al or Ti / Au, which can meet the requirements of high performance, long life and reliability on the basis of ensuring excellent electrical conductivity, corrosion resistance, thermal management and mechanical strength. Different material combinations can be selected according to the specific application scenario, so as to improve the device performance while ensuring production efficiency and cost control.

[0033] Furthermore, the material for preparing the n+ region 4 includes Si, Ge or Sn, and these materials can bring significant effects in improving the electrical conductivity, response speed, efficiency, etc. of the electronic device.

[0034] Furthermore, the material for preparing the dielectric layer 5 includes SiO 2 、Al 2 O 3 or at least one of SiN. Furthermore, the material for preparing the anode 6 includes Ti, Al or Ti / Au.

[0035] The present invention also provides a preparation method of the ultra-wide bandgap MIS diode 100 as described above, including the following steps: S1. Epitaxially form the Ga 2 O 3 epitaxial layer 3 on the substrate 2, and on the Ga2 O 3 Etch downward from the top of the epitaxial layer 3 to form a plurality of grooves 31; S2. Perform n-type ion implantation on the top edge of the groove 31 to form an n+ region 4; S3. Deposit on both the Ga 2 O 3 Deposit on both the sidewalls of the epitaxial layer 3 and the groove 31 to form a dielectric layer 5. The dielectric layer 5 exposes the n+ region 4. Deposit to form an anode 6 in the dielectric layer 5 and the groove 31; S4. Deposit to form a cathode 1 on the side of the substrate 2 away from the Ga 2 O 3 epitaxial layer 3, and deposit an antireflection film 7 on one side of the Ga 2 O 3 epitaxial layer 3 parallel to the sidewall of the groove 31.

[0036] The present invention conducts electricity by directly generating carriers through laser irradiation, without having to reach the avalanche breakdown voltage, and can overcome the problem that gallium oxide lacks P-type materials and cannot undergo avalanche breakdown. At this time, the device can quickly discharge the residual energy of the high-voltage circuit.

[0037] In the present invention, substances not specifically described are existing substances and can be directly purchased on the market.

[0038] The above are only the preferred embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An ultra-wide bandgap MIS diode, characterized in that: include: cathode; The substrate is laid on one side of the cathode and has a thickness of 150-650 μm. The substrate is made of n-type doped Ga2O3 material, and the concentration of n-type doping is 1e18-1e19 cm -3 ; The Ga2O3 epitaxial layer is laid on the side of the substrate away from the cathode and is n-type doped. The thickness of the Ga2O3 epitaxial layer is 10-100 μm, and the concentration of n-type doping is 5e14-1e17 cm -3 The top of the Ga2O3 epitaxial layer is recessed downward to form a plurality of grooves, and an n+ region is formed at the top edge of each groove; A dielectric layer is applied on a side of the Ga2O3 epitaxial layer away from the substrate and on the inner wall of each trench; an anode, which is laid on a side of the dielectric layer away from the Ga2O3 epitaxial layer and filled in each groove, and the anode penetrates the dielectric layer and contacts the n+ region; and An anti-reflection film is applied on the sidewalls of the Ga2O3 epitaxial layer parallel to the inner sidewalls of the grooves, and the vertical projection of the anti-reflection film covers the sidewalls of each groove.

2. The ultra-wide bandgap MIS diode according to claim 1, characterized in that: A metal layer is applied on a side of each n+ region close to the dielectric layer, and the anode penetrates through the dielectric layer and contacts the metal layer.

3. The ultra-wide bandgap MIS diode according to claim 1, characterized in that: The cross section of each groove is circular or polygonal.

4. The ultra-wide bandgap MIS diode according to claim 1, characterized in that: The groove is provided with at least one step.

5. The ultra-wide bandgap MIS diode according to claim 1, characterized in that: The cathode is made of materials including Ti, Al or Ti / Au.

6. The ultra-wide bandgap MIS diode according to claim 1, characterized in that: The material used to make the n+ region includes Si, Ge or Sn.

7. The ultra-wide bandgap MIS diode according to claim 2, characterized in that: The material for preparing the metal layer includes Ni, Ni / Au or W / Au.

8. The ultra-wide bandgap MIS diode according to claim 1, characterized in that: The material used to prepare the dielectric layer includes at least one of SiO2, Al2O3 or SiN.

9. The ultra-wide bandgap MIS diode according to claim 1, characterized in that: The material used to prepare the anode includes Ti, Al or Ti / Au.

10. The method for preparing an ultra-wide bandgap MIS diode according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1, epitaxially forming the Ga2O3 epitaxial layer on the substrate, and etching downwards on the top of the Ga2O3 epitaxial layer to form a plurality of grooves; S2, performing n-type ion implantation at the top edge of the trench to form an n+ region; S3, depositing on the Ga2O3 epitaxial layer and the sidewalls of the trench to form a dielectric layer, wherein the dielectric layer exposes the n+ region, and depositing in the dielectric layer and the trench to form an anode; S4. Deposit a cathode on a side of the substrate away from the Ga2O3 epitaxial layer, and apply an anti-reflection film on a side of the Ga2O3 epitaxial layer parallel to the side wall of the trench.

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