Preparation method of bimetallic waveguide device for GaN terahertz quantum cascade laser

Abstract

The invention discloses a preparation method of a bimetallic waveguide device for a GaN terahertz quantum cascade laser, and the method comprises the steps: providing an epitaxial wafer of a silicon substrate GaN-based terahertz quantum cascade laser, and carrying out the etching of the epitaxial wafer till a silicon substrate, and forming a bar structure; forming a first ohmic contact layer on the surface of the bar; evaporating bonding metal on the surface of the receiving substrate; bonding the epitaxial wafer with the substrate; removing the silicon substrate by adopting a wet etching mode; removing the buffer layer through an inductive coupling plasma etching mode, exposing the bottom n-GaN layer, and forming a second ohmic contact layer; and cleavage is carried out on the bar to obtain the bimetallic waveguide device. According to the method, the tensile strain of the Si-based epitaxial layer is released in advance by etching the epitaxial layer to the growth substrate before wafer bonding, so that the purpose of releasing the stress is achieved, cracks of the epitaxial layer caused by release of the tensile stress after the substrate is removed are avoided, and improvement of the device manufacturing yield is facilitated.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a double-metal waveguide device for a GaN terahertz quantum cascade laser, and belongs to the technical field of semiconductor devices. BACKGROUND

[0002] The device structure of a GaN-based quantum cascade laser (GaN-QCL) can be generally divided into a single-metal semi-insulating surface plasmon waveguide (SI-SPWs) quantum cascade laser (single-metal waveguide QCL) and a double-metal waveguide (DMWs) quantum cascade laser (double-metal waveguide QCL). The single-metal waveguide QCL has a simple structure and is easy to design and process, and is fully compatible with the current microelectronic process, but has poor functional expandability, is difficult to realize multi-band and multi-functional collaborative output, and has a gain saturation problem. A single active region may be saturated or thermally broken down under high-power injection. The double-metal waveguide QCL can realize collaborative emission of different wavelengths, gain mechanisms or modulation mechanisms compared with the single-metal waveguide QCL, and has smaller loss and a higher optical field restriction factor, which is beneficial to reducing the threshold gain of the laser. However, the double-metal waveguide QCL structure is more complex, and higher requirements are put forward for the manufacturing process and cost control. Si substrate is the most easily obtained growth substrate of nitride thin film, and has the characteristics of low cost and easy removal, and is the best choice for epitaxial growth of double-waveguide GaN-QCL. However, in the preparation process of the double-waveguide GaN-QCL structure, the epitaxial layer needs to be transferred to the receiving substrate by the method of metal bonding, and the Si substrate needs to be removed. Considering the efficiency of removing the substrate, a wet etching method is generally used, that is, an HNA solution (nitric acid, glacial acetic acid, hydrofluoric acid) is used. Because of the lattice mismatch and thermal mismatch between Si and GaN, the Si-based III-V nitride epitaxial wafer shows a concave curvature, so a low-melting-point metal In or Sn with good fluidity is often added in the wafer bonding process, and a solid-liquid interdiffusion bonding (SLID) is used, for example, Au-In, Au-Sn or In-In bonding. The ohmic contact of n-GaN is generally Ti / Al / Ni / Au. The metals Al, In and Sn are inevitably corroded by the etching solution in the process of wet etching the Si substrate, which leads to a decrease in the reliability of the device and even a process failure. In addition, because the III-V nitride epitaxial layer on the Si substrate is in a state of tensile strain, cracks appear in the epitaxial layer after the substrate is removed due to the release of stress, which affects the yield of device preparation. SUMMARY

[0003] The application aims to provide a preparation method of a double-metal waveguide device for a GaN terahertz quantum cascade laser.

[0004] The object of the present application is achieved by the following technical solutions. A preparation method of a bimetal waveguide device for a GaN terahertz quantum cascade laser, comprising the following steps: (1) forming a bar structure by etching an epitaxial wafer until a Si substrate on a GaN-terahertz quantum cascade laser epitaxial wafer, wherein the structure of the GaN-terahertz quantum cascade laser epitaxial wafer comprises a Si substrate, a buffer layer, a bottom n-GaN layer, a terahertz quantum cascade laser structure and a top n-GaN layer in sequence; (2) forming a first ohmic contact layer on the surface of the bar; (3) evaporating a bonding metal on the surface of a receiving substrate; (4) bonding the epitaxial wafer with the first ohmic contact layer and the substrate with the bonding metal; (5) removing the Si substrate by wet etching; (6) removing the buffer layer by inductively coupled plasma etching to expose the bottom n-GaN layer; (7) forming a second ohmic contact layer on the surface of the exposed bottom n-GaN layer by evaporation; (8) cleaving the bar along a direction perpendicular to the bar to obtain a bimetal waveguide device of a GaN terahertz quantum cascade laser.

[0005] Preferably, in the GaN-terahertz quantum cascade laser epitaxial wafer, the thickness of the buffer layer is 0.5-1.5 μm, the thickness of the bottom n-GaN is 0.5-2 μm, the doping concentration is 1*10 19 -1*10 20 / cm 3 , the thickness of the terahertz quantum cascade laser structure is 1-10 μm, the thickness of the top n-GaN is 0.5-2 μm, and the doping concentration is 5*10 18 -1*10 20 / cm 3 .

[0006] Preferably, the first ohmic contact layer is Cr / Pt / Au, and the thickness is 10-50 / 100-150 / 100-200 nm.

[0007] Preferably, the receiving substrate is a GaAs substrate, a SiC substrate or a sapphire substrate.

[0008] Preferably, the bonding metal is Ti / Au with thickness of 10-50 / 100-200 nm, or Cr / Au with thickness of 10-50 / 100-200 nm, or Cr / Pt / Au with thickness of 10-50 / 100-150 / 100-200 nm.

[0009] Preferably, the etching solution used in the wet etching is KOH solution or NaOH solution with concentration of 15-50%.

[0010] Preferably, the second ohmic contact layer is Cr / Pt / Au or Ti / Al / Ni / Au.

[0011] Preferably, the width of the bar after cleavage is 30-400 μm, and the length is 0.5-2 mm.

[0012] The present application provides a preparation method of a double-metal waveguide device for a GaN terahertz quantum cascade laser. The method releases the tensile strain of the Si-based epitaxial layer in advance by etching through the epitaxial layer to the growth substrate before wafer bonding, so as to achieve the purpose of stress release and avoid the occurrence of cracks in the epitaxial layer due to the release of tensile stress after the substrate is removed. At the same time, the stress is released in advance, which reduces the curvature of the epitaxial wafer, and Au-Au can be used as the bonding material, avoiding the use of low-melting-point metals such as Sn and In in the bonding process, reducing the damage of the etching solution to the bonding layer metal, and being beneficial to improve the yield of the double-waveguide GaN-QCL device preparation and the performance of the device. In addition, Cr / Pt / Au is used instead of Ti / Al / Ni / Au as the first ohmic contact layer, which reduces the damage of the etching solution to the ohmic contact layer, and is beneficial to improve the electrical performance of the device. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 The present application provides a preparation process of a double-metal waveguide device for a GaN terahertz quantum cascade laser, wherein: ① is a schematic diagram of the provided GaN-terahertz quantum cascade laser epitaxial wafer structure; ② is a schematic diagram of the bar structure formed after the provided epitaxial wafer is etched through; ③ is a schematic diagram of the structure after the first ohmic contact layer is formed on the top n-GaN surface; ④ is a schematic diagram of the receiving substrate structure with bonding metal on the surface; ⑤ is a schematic diagram of the structure after the epitaxial wafer and the receiving substrate are bonded; ⑥ is a schematic diagram of the device structure after the Si substrate is removed by wet etching; ⑦ is a schematic diagram of the device structure after the buffer layer is removed by dry etching; ⑧ This is a schematic diagram of the structure after the second ohmic contact layer is formed on the bottom n-GaN surface.

[0014] Figure 2 This is a flowchart illustrating the fabrication process of the bimetallic waveguide device for the GaN terahertz quantum cascade laser of the present invention.

[0015] Figure 3 This is a schematic diagram of the cleavage direction of the bimetallic waveguide device of the GaN terahertz quantum cascade laser of the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example 1: Fabrication method of bimetallic waveguide device for GaN terahertz quantum cascade laser. (1) such as Figure 1 and 2 As shown, a GaN-terahertz quantum cascade laser epitaxial wafer was selected. Its structure consists of a Si substrate, a 0.5-1.5 μm buffer layer, and a 0.5-2 μm bottom n-GaN layer, with a doping concentration of 1*10⁻⁶. 19 -1*10 20 / cm 3 The terahertz quantum cascade laser structure has a 1-10 μm core and a top n-GaN core of 0.5-2 μm, with a doping concentration of 5*10⁻⁶. 18 -1*10 20 / cm 3 .

[0018] (2) The entire epitaxial layer is etched through the Si substrate by photolithography and etching to form a bar structure. The etching mask can be photoresist or SiO2, etc.; the etching method is inductively coupled plasma etching, the etching gas is Cl / BCl3, the gas flow rate is 30-60 / 5-15 sccm, the etching rate is 200-400 nm / min, and the etching depth is controlled to be 1-10 μm.

[0019] (3) A first ohmic contact layer is formed on the surface of the bar strip by photolithography and evaporation. The ohmic contact structure can be Cr / Pt / Au with a thickness of 10-50 / 100-150 / 100-200nm.

[0020] (4) Provide a receiving substrate, which can be selected from GaAs substrate, SiC substrate or sapphire substrate. A bonding metal is deposited on the surface of the substrate. The bonding metal can be Ti / Au, Cr / Au, Cr / Pt / Au, etc. The thickness of the former two is 10-50 / 100-200nm, and the thickness of the latter is 10-50 / 100-150 / 100-200nm.

[0021] (5) The epitaxial wafer with the first ohmic contact layer and the substrate with the bonding metal are bonded by heating and pressurizing, with a bonding temperature of 300-400℃, 300-500N, and 4-8h.

[0022] (6) The Si substrate is removed by wet etching using KOH solution.

[0023] (7) The buffer layer was removed by inductively coupled plasma etching to expose n-GaN.

[0024] (8) A second ohmic contact layer is formed at the other end of the bar by photolithography and evaporation. The ohmic contact structure can be Cr / Pt / Au or Ti / Al / Ni / Au. When Cr / Pt / Au is selected, the thickness is 10-50 / 100-150 / 100-200nm.

[0025] (9) such as Figure 3 As shown, the bar strip is cleaved along the direction perpendicular to the bar strip to obtain a bimetallic waveguide device for a GaN terahertz quantum cascade laser.

[0026] Those skilled in the art should understand that Figure 1 and Figure 3 To demonstrate the device fabrication process, only two parallel bars and the etching groove between them are shown. In the actual fabrication process, the device includes multiple parallel bars.

[0027] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for fabricating a bimetallic waveguide device for a GaN terahertz quantum cascade laser, comprising the following steps: (1) On the epitaxial wafer of GaN-terahertz quantum cascade laser, the epitaxial wafer is etched through to the Si substrate to form a bar structure, wherein the structure of the epitaxial wafer of GaN-terahertz quantum cascade laser includes, in sequence, a Si substrate, a buffer layer, a bottom n-GaN layer, a terahertz quantum cascade laser structure and a top n-GaN layer. (2) A first ohmic contact layer is formed on the surface of the bar strip; (3) Deposit bonding metal on the surface of the receiving substrate; (4) Bond the epitaxial wafer with the first ohmic contact layer to the substrate with the bonding metal; (5) The Si substrate is removed by wet etching; (6) The buffer layer is removed by inductively coupled plasma etching to expose the bottom n-GaN layer; (7) A second ohmic contact layer is formed on the exposed bottom n-GaN layer surface; (8) Cleave the bar along the direction perpendicular to the bar to obtain the bimetallic waveguide device for GaN terahertz quantum cascade laser.

2. The method for fabricating the bimetallic waveguide device of the GaN terahertz quantum cascade laser according to claim 1, characterized in that, In the epitaxial wafer of the GaN-terahertz quantum cascade laser, the buffer layer thickness is 0.5-1.5 μm, the bottom n-GaN thickness is 0.5-2 μm, and the doping concentration is 1*10⁻⁶. 19 -1*10 20 / cm 3 The QCL structure has a thickness of 1-10 μm, the top n-GaN thickness is 0.5-2 μm, and the doping concentration is 5*10⁻⁶. 18 -1*10 20 / cm 3 .

3. The method for fabricating the bimetallic waveguide device of the GaN terahertz quantum cascade laser according to claim 2, characterized in that, The first ohmic contact layer is Cr / Pt / Au with a thickness of 10-50 / 100-150 / 100-200nm.

4. The method for fabricating the bimetallic waveguide device of the GaN terahertz quantum cascade laser according to claim 3, characterized in that, The receiving substrate is a GaAs substrate, a SiC substrate, or a sapphire substrate.

5. The method for fabricating the bimetallic waveguide device of the GaN terahertz quantum cascade laser according to claim 4, characterized in that, The bonding metal is Ti / Au with a thickness of 10-50 / 100-200 nm, or Cr / Au with a thickness of 10-50 / 100-200 nm, or Cr / Pt / Au with a thickness of 10-50 / 100-150 / 100-200 nm.

6. The method for fabricating the bimetallic waveguide device of the GaN terahertz quantum cascade laser according to claim 5, characterized in that, The wet etching process uses KOH or NaOH solution with a concentration of 15-50%.

7. The method for fabricating the bimetallic waveguide device of the GaN terahertz quantum cascade laser according to claim 6, characterized in that, The second ohmic contact layer is Cr / Pt / Au or Ti / Al / Ni / Au.

8. The method for fabricating the bimetallic waveguide device of the GaN terahertz quantum cascade laser according to claim 7, characterized in that, The width of the cleaved bar is 30-400μm and the length is 0.5-2mm.

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