Nitride semiconductor device and manufacturing method thereof

Abstract

The invention discloses a nitride semiconductor device and a manufacturing method thereof, and belongs to the technical field of compound semiconductor materials and devices. The nitride semiconductor device comprises an AlN / AlGaN heterojunction epitaxial layer, a secondary epitaxial layer, a passivation layer, a source electrode, a grid electrode and a drain electrode, the secondary epitaxial layer is a single crystal aluminum gallium nitride film, the chemical formula is AlxGa (1-x) N, and x is greater than or equal to 0 and less than or equal to 1; the AlN / AlGaN heterojunction epitaxial layer sequentially comprises a substrate, a buffer layer, an AlGaN channel layer and an AlN barrier layer from bottom to top; the conduction of the device is realized through the transmission of two-dimensional electron gas formed by the AlN / AlGaN heterojunction in a channel. In the manufacturing process of the nitride semiconductor device, a secondary epitaxial growth mode is adopted, the problem that an AlN material system is difficult to dope is solved, and the ohmic contact resistance of metal and a semiconductor material is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of compound semiconductor materials and devices, and in particular relates to a nitride semiconductor device and a manufacturing method thereof. Background Art

[0002] Compound semiconductor materials have developed rapidly in recent years and are increasingly widely used in lighting, optical communications, power semiconductor devices and wireless communications. Its main advantage is the wider bandgap. Compared with the bandgap width of silicon of 1.14eV, the bandgap widths of commonly used compound semiconductors are: gallium arsenide 1.43eV, indium phosphide 1.35eV, silicon carbide 3.25eV, gallium nitride 3.44eV, gallium oxide 4.8eV, diamond 5.5eV, aluminum nitride 6.2eV. Wide bandgap (bandgap width>2.3eV) semiconductors have broad application prospects in the development of high-temperature, high-frequency, high-power microwave devices, radiation-resistant devices, ultraviolet detectors, short-wave light-emitting diodes, etc., and are key electronic devices in the fields of wireless communications, national defense, new energy, and autonomous driving.

[0003] Aluminum nitride (AlN) is a new type of semiconductor material with advantages such as ultra-wide bandgap, high thermal conductivity, high breakdown voltage, high thermal stability, and high chemical stability. Aluminum nitride semiconductor materials have great potential in applications in high voltage, high power, and high temperature power electronics and optoelectronics. Although aluminum nitride has many advantages, there are also some challenges in practical applications, such as difficulty in doping, high cost, and difficulty in making ohmic contacts with metals. Aluminum gallium nitride (AlGaN) materials inherit the advantages of gallium nitride (GaN) and aluminum nitride (AlN) materials, with high electron mobility, high breakdown voltage, and good high-frequency performance. The bandgap can be adjusted between 3.4 and 6.2 eV by adjusting the Al:Ga ratio, and has good thermal stability, chemical stability, and excellent electrical and optical properties. Therefore, aluminum gallium nitride semiconductor materials are high-performance materials with broad application prospects, and have important applications in electronics, optoelectronics, power electronics, photoelectric switches, semiconductor lasers, solar cells, and other fields.

[0004] In the prior art, high electron mobility (HEMT) devices based on AlGaN / GaN heterojunctions have been widely studied, but there are fewer studies on the performance of devices based on AlN / AlGaN heterojunctions. The main reason is that the AlN material system is difficult to dope and the ohmic contact preparation with metal is difficult, which limits the research and development and industrial development of AlN / AlGaN heterojunctions in the semiconductor field. Summary of the invention

[0005] One of the purposes of the present invention is to provide a nitride semiconductor device to solve the problem that the AlN material system is difficult to dope and has poor ohmic contact with metal;

[0006] A second object of the present invention is to provide a method for manufacturing a nitride semiconductor device.

[0007] A nitride semiconductor device, comprising:

[0008] AlN / AlGaN heterojunction epitaxial layer, secondary epitaxial layer, passivation layer, source, gate, drain; the secondary epitaxial layer is a single crystal aluminum gallium nitride film with a chemical formula of Al x Ga (1-x) N, 0≤x≤1;

[0009] The conduction of the device is achieved through the transmission of two-dimensional electron gas (2DEG) formed by the AlN / AlGaN heterojunction in the channel.

[0010] Furthermore, the AlN / AlGaN heterojunction epitaxial layer includes, from bottom to top, a substrate, a buffer layer, an AlGaN channel layer, and an AlN barrier layer.

[0011] Furthermore, the material selected for the substrate includes silicon, sapphire, silicon nitride or diamond, and the substrate can be of any size, such as 2-8 inches (50-200nm); further, the material selected for the substrate is diamond, and the high thermal conductivity of the diamond material is utilized to improve the heat dissipation efficiency of the chip, so as to give full play to the potential performance advantages of semiconductor devices based on AlN / AlGaN heterojunction in high temperature, high pressure and high power applications.

[0012] Furthermore, the buffer layer includes one or more combinations of a stress absorption layer, a seed layer, a transition layer, and a superlattice layer; and further, the buffer layer is a single crystal AlN film. Providing a buffer layer between the substrate and the heterojunction epitaxial material can effectively eliminate or reduce defects such as dislocation, warping, and cracking caused by differences in lattice constants or thermal expansion coefficients.

[0013] Furthermore, the AlGaN channel layer is a single-crystal aluminum gallium nitride film with a thickness of 100-200 nm and a chemical formula of Al x Ga (1-x) N, 0.2≤x≤0.8, the bandgap width of the AlGaN channel layer is 3.96-5.64 eV.

[0014] Furthermore, the thickness of the AlN barrier layer is 2-30 nm, and the band gap width of the AlN barrier layer is 6.2 eV.

[0015] Furthermore, the AlN / AlGaN heterojunction epitaxial layer may include a cap layer with a thickness of 2-30 nm, and the cap layer includes a single crystal gallium nitride (GaN) film or a silicon nitride (SiN) film.

[0016] Furthermore, the secondary epitaxial layer is a single crystal aluminum gallium nitride film with Al component content gradually decreasing in the vertical direction upward. When x=0, the secondary epitaxial layer is a GaN film, and when x=1, the secondary epitaxial layer is an AlN film; preferably, the secondary epitaxial layer is a GaN film.

[0017] Furthermore, the single-crystalline aluminum gallium nitride film is an n-type doped single-crystalline aluminum gallium nitride film or a heavily doped n+ type single-crystalline aluminum gallium nitride film; silane (SiH4) is added during the material growth process for doping, wherein Si atoms replace Ga or Al atoms to form free electrons in the crystal to achieve doping of the single-crystalline aluminum gallium nitride; preferably, the secondary epitaxial layer is an n+GaN film with a doping concentration of 10 19 -10 21 / cm 3 .

[0018] Furthermore, the Al content in the secondary epitaxial layer is ≤ the Al content in the AlGaN channel layer. The lower the Al content, the smaller the bandgap width of the AlGaN material, and the easier it is to achieve doping.

[0019] Furthermore, the passivation layer covers the secondary epitaxial layer and the AlN barrier layer, and the selected materials include silicon nitride, silicon oxide or aluminum oxide, and the thickness is 10-100 nm.

[0020] Furthermore, the passivation layer under the source and drain is removed to contact the secondary epitaxial layer, and the metal materials selected for the source and drain include one or more combinations of Ti, Al, Ni, Au, and Ta.

[0021] Furthermore, the gate is covered on the passivation layer and is in contact with the AlN barrier layer, and the selected materials include one or more combinations of Pt, Ni, Au, and Ti.

[0022] Furthermore, the gate is close to a side of the source (ie, the distance between the gate and the source is smaller than the distance between the gate and the drain).

[0023] In a second aspect, a method for manufacturing a nitride semiconductor device comprises the following steps:

[0024] S1, fabricating an AlN / AlGaN heterojunction epitaxial layer;

[0025] S2. Fabricating a secondary epitaxial layer on the AlN / AlGaN heterojunction epitaxial layer:

[0026] S2-1, forming a dielectric layer on the AlN barrier layer as a mask layer in the secondary epitaxial layer preparation process, with a thickness of 10-500 nm, which is used to limit the growth of the epitaxial material film in the subsequent process, and the preparation method includes PECVD or LPCVD;

[0027] S2-2, etching the mask layer and the AlN barrier layer to form a secondary epitaxial layer growth region: including photolithography (resin coating, alignment, development, etc.), mask layer etching, AlN barrier layer etching, resist removal, and cleaning; the etching method may optionally include wet etching or dry etching; the etching depth is the thickness of the AlN barrier layer and the mask layer (i.e., stopping at the surface of the AlGaN channel layer), or etching into the AlGaN channel layer 5-100 nm;

[0028] For etching the AlN barrier layer, a low-damage etching method needs to be used to avoid damage to the AlN crystal and affect the two-dimensional electron gas on the heterojunction surface;

[0029] When wet etching is used, commonly used wet etching solutions include: HF, HCl, HNO3 or a combination; when dry etching is used, the etching gas is usually Cl2 and BCl3, or it can be a mixture of one or more etching gases such as H2 and CF4, CHF3, CH2F2 or C4F4. According to the requirements of the process formula, inert gas Ar or He can be added to achieve low-rate and low-damage etching.

[0030] S2-3. Secondary epitaxial layer growth: an aluminum gallium nitride thin film is prepared in the secondary epitaxial layer growth region by MOCVD, MBE (molecular beam epitaxy) or PVD (physical vapor deposition or sputtering); when the MOCVD method is used to grow a single crystal aluminum gallium nitride thin film, trimethylaluminum (TMAl, C3H9Al), trimethylgallium (TMG, C3H9Ga) and ammonia (NH3) are used as aluminum source, gallium source and nitrogen source respectively, and hydrogen is used as a carrier gas. The pressure, temperature and gas flow rate of the reactor are adjusted to grow single crystal aluminum gallium nitride (Al2O3) with different Al and Ga component contents. x Ga (1-x) N, 0≤x≤1) thin film, the secondary epitaxial layer can be a single crystal aluminum gallium nitride thin film with a gradient or non-gradient Al composition;

[0031] S2-4, removing the mask layer: using a wet process or a dry etching process; when dry etching, a low damage etching process is selected to reduce damage to the AlN barrier layer;

[0032] S3, forming a passivation layer on the secondary epitaxial layer and the AlN barrier layer;

[0033] S4, making source and drain, so that the metal materials of the source and drain form ohmic contact with the secondary epitaxial layer material:

[0034] S5. Making a gate that contacts the surface of the AlN barrier layer:

[0035] Photolithography is performed on the AlN barrier layer to expose the gate growth area, and then the passivation layer is etched to form a gate opening. After the etching is completed, metal deposition, stripping, degumming and cleaning are performed in the corresponding area to obtain a gate in contact with the AlN barrier layer.

[0036] Furthermore, the S1 further includes:

[0037] S1-1, forming a buffer layer on a substrate, wherein the forming method includes but is not limited to a MOCVD method;

[0038] S1-2, fabricating an AlGaN channel layer on the buffer layer, wherein the fabrication method includes but is not limited to a MOCVD method;

[0039] When using the MOCVD method: trimethylgallium (TMGa, C3H9Ga), trimethylaluminum (TMAl, C3H9Al) and ammonia (NH3) are used as gallium source, aluminum source and nitrogen source respectively, and hydrogen is used as a carrier gas. The pressure, temperature and gas flow rate of the reactor are adjusted to grow single crystal aluminum gallium nitrogen (AlGaN) with different Al and Ga component contents. x Ga (1-x) N, 0.2≤x≤0.8) film;

[0040] S1-3. Fabricate an AlN barrier layer on the AlGaN channel layer, wherein the fabrication method includes but is not limited to adopting a MOCVD method.

[0041] Furthermore, the S4 further includes:

[0042] S4-1, etching the passivation layer (including photolithography, etching, degumming, cleaning and other steps) to expose the source and drain regions, and depositing metal materials layer by layer on the source and drain regions by metal evaporation or sputtering to form source and drain electrodes;

[0043] S4-2, performing a metal stripping process to remove the photoresist and the metal layer on the photoresist;

[0044] S4-3, performing an annealing process in an RTA (rapid thermal annealing furnace) at a temperature of 700-900° C., annealing for 30-90 seconds in an argon or nitrogen atmosphere to achieve ohmic contact between the metal material of the source and drain and the secondary epitaxial layer.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] 1. The present invention provides a nitride semiconductor device, which greatly improves and optimizes the performance of the semiconductor device based on the AlN / AlGaN heterojunction, making it possible to apply it under high temperature, high pressure and high power conditions. In the process of manufacturing nitride semiconductor devices, the present invention adopts a secondary epitaxial growth method, which effectively solves the problem of doping difficulty of the AlN material system, reduces the source-drain ohmic contact resistance, and provides new possibilities for the development and application of the AlN material system in the semiconductor field.

[0047] 2. In the process of secondary epitaxial growth, the present invention can adopt a gradual structural growth to form a secondary epitaxial structure with a layer-by-layer decreasing Al content. The component at the bottom of the structure is the same or similar to the Al component of the channel layer, which is convenient for growth and conduction; the top becomes GaN or low-Al AlGaN, the bandgap width becomes smaller, and it is easy to form ohmic contact with the metal; heavy doping of GaN can be achieved to form n+GaN, further reducing the ohmic contact resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The present invention will be further described below in conjunction with the accompanying drawings.

[0049] Figure 1 It is a schematic diagram of the structure of the AlN / AlGaN heterojunction epitaxial layer of the present invention;

[0050] Figure 2 It is a schematic diagram of the energy band of the AlN / AlGaN heterojunction epitaxial layer of the present invention;

[0051] Figure 3 1 is a schematic diagram of the manufacturing process flow of the secondary epitaxial layer in Example 2 of the present invention, wherein (a) shows the manufacturing of a preparation mask layer on the AlN barrier layer 104 in the AlN / AlGaN heterojunction epitaxial layer structure; (b) shows the etching of the mask layer and the AlN barrier layer to form a secondary epitaxial layer growth region; (c) shows the growth of the secondary epitaxial layer in the secondary epitaxial layer growth region; (d) is a schematic diagram of the AlN / AlGaN heterojunction epitaxial layer structure containing the secondary epitaxial layer;

[0052] Figure 4 It is a schematic diagram of the structure of the secondary epitaxial layer having a gradient Al content in Example 2 of the present invention;

[0053] Figure 5 This is a schematic diagram of the device structure for making a passivation layer in a vertical direction in Example 3 of the present invention;

[0054] Figure 6 is a schematic diagram of the device structure after the source and drain are manufactured in Example 4 of the present invention;

[0055] Figure 7 is a schematic diagram of the device structure after the gate is manufactured in Example 5 of the present invention;

[0056] Figure 8 is a schematic diagram of the structure of the Schottky diode in Comparative Example 1 of the present invention;

[0057] Fig. 9 is a schematic diagram of the device structure in Comparative Example 2 of the present invention without a secondary epitaxial layer;

[0058] In the figure: 101 - substrate, 102 - buffer layer, 103 - AlGaN channel layer, 104 - AlN barrier layer, 105 - cap, 200 - secondary epitaxial layer, 201 - mask layer, 202 - secondary epitaxial layer growth region, 300 - passivation layer, 400 - source, 500 - drain, 600 - gate. DETAILED DESCRIPTION

[0059] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0060] Example 1

[0061] See also Figure 1 The AlN / AlGaN heterojunction epitaxial layer of the present invention includes, from bottom to top, a substrate 101, a buffer layer 102, an AlGaN channel layer 103, an AlN barrier layer 104, and a cap (Cap) 105 which may or may not be included;

[0062] The method for manufacturing the AlN / AlGaN heterojunction epitaxial layer comprises the following steps:

[0063] S1-1. Select a substrate. The substrate material can be any size of silicon, sapphire, silicon carbide or diamond. Diamond material is preferred as the substrate. The high thermal conductivity of diamond material is used to improve the heat dissipation efficiency of the chip, and give full play to the potential performance advantages of semiconductor devices based on AlN / AlGaN heterojunction in high temperature, high pressure and high power applications;

[0064] Since aluminum nitride (AlN) materials and substrate materials have different lattice constants and thermal expansion coefficients, the difference in lattice constants will cause lattice mismatch between substrate and epitaxy, resulting in dislocation defects; lattice mismatch will also generate stress in the material and cause wafer warp and crack. In addition, the growth of aluminum nitride epitaxial layers is usually carried out at a relatively high temperature (1000-2000°C), and will experience large temperature changes during the cooling process to room temperature (25-30°C). Due to the difference in thermal expansion coefficients between epitaxial materials and substrates, stress will be generated in the material when the temperature changes, resulting in warp and crack. When the stress caused by temperature changes exceeds the critical stress of the material, the stress caused by temperature changes will also cause dislocation defects. Taking silicon substrate as an example, the lattice constants of silicon and aluminum nitride crystals are 3.84A (crystal orientation is (111) silicon substrate) and 3.11A, respectively, and the lattice mismatch is 23%. The thermal expansion coefficients of silicon and aluminum nitride materials are 2.6×10 -6 K -1 and 4.5×10 -6 K -1 , the mismatch reaches 54%.

[0065] In order to eliminate or reduce defects such as dislocation, warping and cracking caused by differences in lattice constants and thermal expansion coefficients, a buffer layer is set between the substrate and the epitaxial layer, and the buffer layer is made on the substrate by MOCVD; the buffer layer includes a stress absorption layer, a seed layer, a transition layer, a superlattice layer, etc., or any combination thereof, which will not be described in detail here. In general, the buffer layer is a single crystal AlN film, which is prepared by MOCVD: trimethylaluminum (TMAl) and ammonia (NH3) are used as aluminum source and nitrogen source respectively, hydrogen is used as carrier gas, and the pressure, temperature and gas flow rate of the reactor are adjusted to prepare it.

[0066] S1-2, forming an AlGaN epitaxial layer on the buffer layer as a channel layer;

[0067] There are many methods for making AlGaN channel layer. The commonly used method is to use MOCVD method to prepare AlGaN channel layer:

[0068] Using trimethylgallium (TMGa, C3H9Ga), trimethylaluminum (TMAl, C3H9Al) and ammonia (NH3) as gallium source, aluminum source and nitrogen source respectively, and hydrogen as carrier gas, the pressure, temperature and gas flow rate of the reactor were adjusted to grow high-quality single crystal aluminum gallium nitrogen (AlGaN) with different Al and Ga component contents. x Ga (1-x) N, 0.2≤x≤0.8) film;

[0069] The thickness of the AlGaN epitaxial layer as the channel layer is 100-2000nm, and the aluminum component (X) in the AlGaN material is 0.2-0.8. The flow rate of the aluminum-containing gas source can be adjusted in the MOCVD reactor to obtain the required component. In order to obtain a higher aluminum component, the flow rate of the aluminum-containing gas source can be increased. In order to obtain a higher gallium component, the flow rate of the gallium-containing gas source can be increased. Different Al components of AlGaN correspond to different bandgap widths. For example, when x=0.5, its bandgap width is 4.8eV. When x=0 and x=1, the corresponding bandgap widths are 3.4eV (GaN) and 6.2eV (AlN), respectively.

[0070] S1-3, forming an AlN film on the AlGaN channel layer as a barrier layer, and adopting a MOCVD method to prepare an AlN barrier layer with a thickness of 2-30 nm.

[0071] S1-4. Optionally, a cap layer is formed in a vertical direction of the AlN barrier layer. The material of the cap layer may be GaN or SiN, and the thickness is 2-20 nm.

[0072] When GaN is used as the cap layer material, it is mainly produced by MOCVD method: trimethylgallium (TMGa, C3H9Ga) and ammonia (NH3) are used as gallium source and nitrogen source respectively, and hydrogen is used as carrier gas. The pressure, temperature and gas flow of the reactor are adjusted to grow high-quality single-crystal gallium nitride films.

[0073] When SiN is used as the cap layer material, it is mainly produced by an in-situ method, that is, it is carried out in the same reactor as the previous layer material to ensure the integrity of the entire epitaxial layer material surface.

[0074] The AlN / AlGaN heterojunction epitaxial layer obtained by the above-mentioned manufacturing method is an AlN / AlGaN heterostructure, wherein the bandgap width of AlN is 6.2eV, and the bandgap width of AlGaN is related to the Al component therein. When the Al component is between 0.2-0.8, the bandgap width of AlGaN is between 3.96-5.64eV. When the Al component is 0.5, its bandgap width is 4.8eV.

[0075] The energy band diagram of AlN / AlGaN heterostructure is shown in Figure 2As shown in the figure, because the bandgap widths of AlN and AlGaN are different, the bandgap width of AlN is higher than that of AlGaN, so that there is a band step difference at the bottom of the conduction band of the two. This band step difference of the conduction band will cause the energy band at the bottom of the conduction band to bend, and the energy band bending will form a two-dimensional potential well at the heterojunction interface. This two-dimensional potential well confines the electrons, and these electrons can only move two-dimensionally in the potential well along the plane parallel to the abrupt junction interface, thus forming a two-dimensional electron gas (2DEG). Therefore, in the AlN / AlGaN abrupt heterojunction structure, even if the AlN barrier layer is not doped, a higher surface density of 2DEG can be obtained by induction of a huge number of polarized positive charges.

[0076] Example 2

[0077] See also Figure 3 In this embodiment, a secondary epitaxial layer 200 is fabricated on the AlN / AlGaN heterojunction epitaxial layer without the cap 105:

[0078] S2-1, forming a mask layer 201 on the AlN barrier layer 104 in the AlN / AlGaN heterojunction epitaxial layer structure, such as Figure 3 (a) The structure shown in the figure;

[0079] The mask layer is also a dielectric layer, and the material includes silicon oxide or silicon nitride. As the mask layer in the secondary epitaxial layer manufacturing process, the thickness is 10-500nm. Its function is to limit the growth of epitaxial material film in the subsequent process. There are many kinds of manufacturing processes, and the commonly used methods include PECVD or LPCVD;

[0080] S2-2, etching the mask layer 201 and the AlN barrier layer 104 to form a secondary epitaxial layer growth region 202, such as Figure 3 (b)

[0081] The etching process includes photolithography (resin coating, alignment, development, etc.), mask layer etching, AlN barrier layer etching, resist stripping, and cleaning; the etching method may be wet etching or dry etching; the etching depth is the sum of the thickness of the AlN barrier layer and the mask layer (i.e., the etching stops at the surface of the AlGaN channel layer), or is etched into the AlGaN channel layer by 5-100 nm;

[0082] For etching the AlN barrier layer, a low-damage etching method needs to be used to avoid damage to the AlN crystal and affect the two-dimensional electron gas on the heterojunction surface;

[0083] When wet etching is used, commonly used wet etching solutions include: HF, HCl, HNO3 or a combination; when dry etching is used, the etching gas is usually Cl2 and BCl3, or it can be a mixture of one or more etching gases such as H2 and CF4, CHF3, CH2F2 or C4F4. According to the requirements of the process formula, inert gas Ar or He can be added to achieve low-rate and low-damage etching.

[0084] S2-3, growing the secondary epitaxial layer 200 in the secondary epitaxial layer growth region 202, such as Figure 3 As shown in (c), the material of the secondary epitaxial layer is single crystal aluminum gallium nitride, with the chemical formula Al x Ga (1-x) N, 0≤x≤1;

[0085] The aluminum gallium nitrogen thin film is prepared in the secondary epitaxial layer growth area by using MOCVD method, MBE (molecular beam epitaxy) method or PVD (physical vapor deposition or sputtering) method;

[0086] When using MOCVD to grow single-crystal AlGaN thin films, trimethylaluminum (TMAl, C3H9Al), trimethylgallium (TMG, C3H9Ga) and ammonia (NH3) are used as aluminum source, gallium source and nitrogen source respectively, and hydrogen is used as carrier gas. The pressure, temperature and gas flow rate of the reactor are adjusted to grow single-crystal AlGaN (AlGaN) with different Al and Ga component contents and high quality. x Ga (1-x) N, 0≤x≤1) film;

[0087] The Al component in the secondary grown AlGaN epitaxial material is ≤ the Al component of the AlGaN channel layer material. The lower the Al component, the smaller the bandgap width of the AlGaN material and the easier it is to dope. GaN material (x=0) can be selected as the secondary grown epitaxial material, at which time the material has the lowest bandgap width and is also easy to dope.

[0088] In the secondary growth Al x Ga (1-x) N material, it can be selectively doped with n-type, for example, when growing Al by MOCVD x Ga (1-x)N, in addition to trimethylaluminum (TMA, C3H9Al), trimethylgallium (TMG, C3H9Ga) and NH3, a certain flow of silane (SiH4) is added to the reaction chamber, in which Si atoms replace Ga and Al atoms, forming free electrons in the crystal, and realizing n-type doping of aluminum gallium nitrogen single crystal film. Similarly, when x=0, it is n-type doped GaN crystal material, and when x=1, it is n-type doped AlN crystal material. In order to achieve excellent ohmic contact, AlGaN or GaN materials can be highly doped to form n+AlGaN or n+GaN, with a doping concentration between 10 19 -10 21 / cm 3 between.

[0089] S2-4, removing the mask layer 201, and obtaining a structure including a secondary epitaxial layer 200, such as Figure 3 (d)

[0090] The mask layer can be removed by a wet process or a dry etching process; when dry etching is used, a low damage etching process is selected to reduce damage to the AlN barrier layer.

[0091] A secondary epitaxial layer is fabricated on the AlN / AlGaN heterojunction epitaxial layer, and a gradual growth process can be implemented, that is, the Al component x in the secondary epitaxial layer changes from large to small from bottom to top in the vertical direction of the contact area between the secondary epitaxial layer and the AlGaN channel layer, that is, the material of the secondary epitaxial layer and the material of the AlGaN channel layer gradually form a secondary epitaxial layer with low Al content or no Al content from the same Al content;

[0092] by Figure 4 For example, when the Al content in the AlGaN channel layer material is 0.5, that is, the chemical formula of the AlGaN channel layer is Al 0.5 Ga 0.5 N, a gradient growth process is used for the secondary epitaxial layer, and Al with an Al content of 0.3-0.4 is grown in sequence. 0.3- 0.4 Ga 0.6-0.7 N layer, Al content of 0.2-0.3 0.2-0.3 Ga 0.7-0.8 N layer, Al content of 0-0.1 0-0.1 Ga 0.9-1 N layers;

[0093] From the bottom to the top of the contact surface, the Al content in the secondary epitaxial layer gradually decreases until it reaches zero. Figure 4 The secondary epitaxial layer is shown as a three-layer gradient structure, and the actual number of layers can be increased according to the manufacturing process.

[0094] Similarly, in each step of the growth process of the secondary epitaxial layer, silane (SiH4) is added to achieve n-type doping to achieve better ohmic contact performance.

[0095] The use of the above-mentioned gradient structure provides multiple advantages: 1) The component at the bottom is the same or similar to the Al component of the channel layer, which is convenient for growth and conduction; 2) The top becomes GaN or low-Al AlGaN, the bandgap width becomes smaller, and it is easy to form ohmic contact with metal; 3) GaN can be heavily doped to form n+GaN, further reducing the ohmic contact resistance.

[0096] Example 3

[0097] See also Figure 5 In this embodiment, a passivation layer 300 is formed in the vertical direction of the AlN / AlGaN heterojunction epitaxial layer containing the secondary epitaxial layer 200. The material of the passivation layer includes silicon nitride, silicon oxide, and aluminum oxide, and the thickness is 10-100 nm.

[0098] Example 4

[0099] See also Figure 6 In this embodiment, after the passivation layer 300 is fabricated, the source electrode 400 and the drain electrode 500 are fabricated so that an ohmic contact is formed between the metal materials of the source electrode and the drain electrode and the secondary epitaxial layer material:

[0100] S4-1, etching the passivation layer (including photolithography, etching, degumming, cleaning and other steps) to expose the source and drain regions, applying glue, aligning, exposing and developing through photolithography, and then depositing metal materials layer by layer on the source and drain regions through metal evaporation or sputtering to form the source and drain; the metal materials of the source and drain include Ti, Al, Ni, Au, Ta, etc., which can be a metal or a combination of multiple (2-6) metals, and are deposited layer by layer on the secondary epitaxial layer through metal evaporation; for example, the metal layer can be composed of 4 metal layers: Ti, Al, Ti, Au, with thickness ranges of 2-25nm, 30-300nm, 20-100nm, 50-500nm respectively. For example, Ti 25nm, Al 50nm, Ti 100nm, Au 250nm can be selected respectively, with a total thickness of 450nm; the metal layer can also be other combinations and thicknesses.

[0101] S4-2, performing a metal lift-off process to remove the photoresist and the metal layer on the photoresist;

[0102] S4-3, performing an annealing process in an RTA (rapid thermal annealing furnace) at a temperature of 700-900° C., annealing for 30-90 seconds in an argon or nitrogen atmosphere to achieve ohmic contact between the metal material of the source and drain and the secondary epitaxial layer material.

[0103] Example 5

[0104] See also Figure 7 In this embodiment, after the source 400 and the drain 500 are fabricated, a gate 600 is fabricated to form a Schottky contact with the ALN barrier layer 104:

[0105] Photolithography (including glue coating, alignment, exposure, and development) is performed on the AlN barrier layer to expose the gate growth area, and then the passivation layer is etched to form a gate opening. After the etching is completed, metal deposition, metal stripping, glue removal, and cleaning are performed in the corresponding area to obtain a gate in contact with the AlN barrier layer; the metal material of the gate is generally selected from metals with high work functions, such as Pt, Ni, Au, and Ti. Their work functions are 5.65eV, 5.15eV, 5.1eV, and 4.33eV, respectively. The purpose is to form a Schottky contact with the AlN barrier layer. The metal layer can be composed of one layer of metal or 2-4 layers of metal, such as Pt / Au or Ni / Au, with a thickness range of 10-50nm and 50-250nm; the manufacturing method is usually to deposit metal layer by layer on the AlN barrier layer by evaporation or sputtering.

[0106] The structural schematic diagram of the nitride semiconductor device finally manufactured through Examples 1 to 5 is as follows Figure 7 As shown; it should be pointed out that Figure 5 and Figure 6 In the process of manufacturing the source, drain and gate shown in the figure, the epitaxial layer does not include Figure 1 When the epitaxial layer includes a capping layer, such as a GaN or SiN capping layer, the manufacturing process of the source, drain and gate needs to be adjusted accordingly, which will not be described in detail here.

[0107] Comparative Example 1

[0108] See also Figure 8 In this comparative example, a Schottky diode (SBD) structure is fabricated on the AlN / AlGaN heterojunction epitaxial layer including the cap 105:

[0109] (1) forming a cap layer in the vertical direction of the AlN barrier layer. The material of the cap layer can be GaN or SiN with a thickness of 2-20 nm;

[0110] (2) Making the anode: After coating, aligning / exposure, and developing the anode area, the metal material is deposited layer by layer on the anode area of ​​the AlN barrier layer by metal evaporation or sputtering; the anode metal layer generally uses metals with high work functions, such as Pt, Ni, Au, and Ti. Their work functions are 5.65eV, 5.15eV, 5.1eV, and 4.33eV, respectively. The purpose is to form a Schottky contact with the aluminum nitride epitaxial layer. The metal layer can be composed of one metal layer or 2-4 metal layers, such as Pt / Au or Ni / Au, with a thickness range of 10-50nm and 50-250nm.

[0111] (3) Making the cathode: After coating, aligning / exposure, and developing in the cathode area, the metal material is deposited layer by layer on the cathode area of ​​the AlN barrier layer by metal evaporation or sputtering; the cathode metal layer is generally composed of a combination of several metals that are annealed at high temperature to form an alloy to reduce resistance. These metals include Ti, Al, Ni, Au, Ta, or others, and can be a single metal or a combination of multiple (2-6) metals. For example, the metal layer can be composed of 4 metal layers: Ti, Al, Ti, Au, with thicknesses ranging from 2-25nm, 30-300nm, 20-100nm, and 50-500nm, respectively. For example, Ti 25nm, Al 50nm, Ti 100nm, and Au 250nm can be selected respectively, with a total thickness of 450nm. The metal layer can also be other combinations and thicknesses. The annealing process is carried out in an RTA (rapid thermal annealing furnace), and the temperature is generally between 700-900°C, usually around 800°C. This is done in an argon or nitrogen environment for 30-90 seconds in order to form an ohmic contact to reduce resistance.

[0112] Comparative Example 2

[0113] See also Fig. 9 In this comparative example, the source, drain and gate are directly fabricated on the AlN / AlGaN heterojunction epitaxial layer including the cap 105:

[0114] (1) forming a cap layer in the vertical direction of the AlN barrier layer. The material of the cap layer can be GaN or SiN with a thickness of 2-20 nm;

[0115] (2) Making the gate: After coating, aligning / exposure, and developing the gate area, the metal material is deposited layer by layer on the gate area of ​​the AlN barrier layer by metal evaporation or sputtering; the gate metal layer generally uses metals with high work functions, such as Pt, Ni, Au, and Ti. Their work functions are 5.65eV, 5.15eV, 5.1eV, and 4.33eV, respectively. The purpose is to form a Schottky contact with the aluminum nitride epitaxial layer. The metal layer can be composed of one metal layer or 2-4 metal layers, such as Pt / Au or Ni / Au, with a thickness range of 10-50nm and 50-250nm.

[0116] (3) Making the source and drain: After the source and drain are coated, aligned / exposed, and developed, the metal material is deposited layer by layer on the source and drain regions of the AlN barrier layer by metal evaporation or sputtering; the source and drain metal layers are generally composed of a combination of several metals and annealed at high temperature to form an alloy to reduce resistance. These metals include Ti, Al, Ni, Au, Ta, or others, which can be a single metal or a combination of multiple (2-6) metals. For example, the metal layer can be composed of 4 metal layers: Ti, Al, Ti, Au, with thicknesses ranging from 2-25nm, 30-300nm, 20-100nm, and 50-500nm, respectively. For example, Ti25nm, Al 50nm, Ti 100nm, and Au 250nm can be selected respectively, with a total thickness of 450nm. The metal layer can also be other combinations and thicknesses. The annealing process is carried out in an RTA (rapid thermal annealing furnace), and the temperature is generally between 700-900℃, usually around 800℃. This is done in an argon or nitrogen environment for 30-90 seconds in order to form an ohmic contact to reduce resistance.

[0117] In Comparative Examples 1 and 2, in order to achieve good conduction, the cathode metal in the SBD device or the source and drain metal in the HEMT device need to form good ohmic contact with the semiconductor material.

[0118] There are two prerequisites for the formation of ohmic contact: a. There is a low barrier height between the metal and the semiconductor; b. The semiconductor is doped with high concentrations of impurities.

[0119] AlN has a very strong bond energy, with a bond energy of up to 11.52eV. In comparison, the bond energies of GaN and InN are 8.92 and 7.72eV respectively. Therefore, it is difficult to dope single-crystal AlN materials directly by ion implantation. Since AlN is an ultra-wide bandgap semiconductor material with a bandgap width of up to 6.2eV, there is a large barrier height between the metal and the aluminum nitride semiconductor.

[0120] Therefore, it is difficult to form a good ohmic contact using the existing methods (Comparative Examples 1 and 2), resulting in a large on-resistance and poor on-performance of the device.

[0121] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0122] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A nitride semiconductor device, characterized in that: include: AlN / AlGaN heterojunction epitaxial layer, secondary epitaxial layer, passivation layer, source, gate, drain; the secondary epitaxial layer is a single crystal aluminum gallium nitride film with a chemical formula of Al x Ga (1-x) N, 0≤x≤1; The AlN / AlGaN heterojunction epitaxial layer includes a substrate, a buffer layer, an AlGaN channel layer, and an AlN barrier layer in order from bottom to top; The conduction of the device is achieved through the transmission of two-dimensional electron gas formed by the AlN / AlGaN heterojunction in the channel.

2. A nitride semiconductor device according to claim 1, characterized in that: The secondary epitaxial layer is a single crystal aluminum gallium nitride thin film with Al component content gradually decreasing in the vertical direction upward; And / or, the secondary epitaxial layer is an n-type doped single crystal aluminum gallium nitride film or a heavily doped n+ type doped single crystal aluminum gallium nitride film.

3. A nitride semiconductor device according to claim 1, characterized in that: The Al content in the secondary epitaxial layer is less than or equal to the Al content in the AlGaN channel layer.

4. The nitride semiconductor device according to claim 1, characterized in that: The AlGaN channel layer is a single-crystal aluminum gallium nitride film with a thickness of 100-200 nm and a chemical formula of Al x Ga (1-x) N, 0.2≤x≤0.8, the bandgap width of the AlGaN channel layer is 3.96-5.64 eV.

5. The nitride semiconductor device according to claim 1, characterized in that: The passivation layer covers the secondary epitaxial layer and the AlN barrier layer, and the selected materials include silicon nitride, silicon oxide or aluminum oxide, and the thickness is 10-100nm.

6. The nitride semiconductor device according to claim 1, characterized in that: The passivation layer under the source and drain is removed and contacts the secondary epitaxial layer. The metal materials selected for the source and drain include one or more combinations of Ti, Al, Ni, Au, and Ta.

7. The nitride semiconductor device according to claim 1, characterized in that: The gate is covered on the passivation layer and is in contact with the AlN barrier layer. The selected materials include one or more combinations of Pt, Ni, Au, and Ti.

8. A method for preparing a nitride semiconductor device as claimed in claim 1, characterized in that: The following steps are involved: S1, fabricating an AlN / AlGaN heterojunction epitaxial layer; S2. Fabricating a secondary epitaxial layer on the AlN / AlGaN heterojunction epitaxial layer: S2-1, forming a mask layer on the AlN barrier layer, with a thickness of 10-500 nm; S2-2, etching the mask layer and the AlN barrier layer to form a secondary epitaxial layer growth region; S2-3, secondary epitaxial layer growth, forming a single crystal aluminum gallium nitrogen thin film in the secondary epitaxial layer growth region, the forming method including but not limited to MOCVD method; S2-4, removing the mask layer; S3, forming a passivation layer on the secondary epitaxial layer and the AlN barrier layer; S4, making source and drain, so that the metal materials of the source and drain form ohmic contact with the secondary epitaxial layer: S5. Fabricate a gate that contacts the AlN barrier layer.

9. The method for preparing a nitride semiconductor device according to claim 8, characterized in that: The S1 further comprises: S1-1, forming a buffer layer on a substrate, wherein the forming method includes but is not limited to a MOCVD method; S1-2, fabricating an AlGaN channel layer on the buffer layer, wherein the fabrication method includes but is not limited to a MOCVD method; S1-3. Fabricate an AlN barrier layer on the AlGaN channel layer, wherein the fabrication method includes but is not limited to a MOCVD method.

10. The method for preparing a nitride semiconductor device according to claim 8, characterized in that: The S4 further comprises: S4-1, etching the passivation layer to expose the source and drain regions, applying glue, aligning, exposing, and developing through a photolithography process, and then depositing metal materials layer by layer on the source and drain regions through metal evaporation or sputtering to form source and drain electrodes; S4-2, performing a metal stripping process to remove the photoresist and the metal layer on the photoresist; S4-3, performing an annealing process in RTA at a temperature of 700-900° C. for 30-90 seconds in an argon or nitrogen atmosphere to achieve ohmic contact between the metal material of the source and drain and the secondary epitaxial layer.

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