WO2019153431A1 - Preparation method for hot electron transistor in high frequency gallium nitride/graphene heterojunction - Google Patents

Abstract

The present invention relates to the technical field of semiconductor devices, and provides a preparation method for a hot electron transistor in a high frequency gallium nitride (GaN)/graphene heterojunction. The method comprises: first growing an III family nitride ternary alloy material on a GaN substrate to form a heterostructure as an emitter region and a first barrier layer, and growing an electrode on the heterojunction by means of a photoetching technique; insulating a device by means of an ion etching technique; transferring the graphene to the surface of the heterojunction as a base region and forming a base region electrode by means of the photoetching technique; finally, growing a GaN thin film as a second barrier layer on the graphene by means of PEALD, and forming a metal collector region on the surface. According to the present invention, the GaN is deposited on the graphene as a second barrier layer by means of PEALD, and the graphene is combined with a GaN-based wide bandgap semiconductor material so as to make good use of the advantages of the two material systems, significantly improve the performance of a hot electron transistor, and decrease the size of a device. The method has low thermal budget and small damages on the graphene, and effectively avoids the damage on the device caused during the production process.

Description

Method for preparing high frequency gallium nitride/graphene heterojunction thermal electron transistor Technical field

The invention relates to the technical field of semiconductor devices, in particular to a method for preparing a high frequency gallium nitride/graphene heterojunction thermal electron transistor.

Background technique

The research and application of GaN materials is the forefront and hotspot of global semiconductor research. It is a new semiconductor material for the development of microelectronic devices and optoelectronic devices. Together with semiconductor materials such as SiC and diamond, it is known as the first generation of Ge and Si. Third-generation semiconductor materials after semiconductor materials, second-generation GaAs, and InP compound semiconductor materials. It has a wide direct band gap, strong atomic bonds, high thermal conductivity, good chemical stability (nearly corroded by any acid) and strong anti-irradiation ability in optoelectronics, high temperature and high power devices and high There are broad prospects for the application of frequency microwave devices. However, there is still a lot of room for the development of GaN-based semiconductors. People have been looking for ways to solve the problems of "high-density dislocations, slow working speed, poor heat dissipation performance, high integration and difficult interconnection" of GaN semiconductor materials.

A two-dimensional electron gas formed by a gallium nitride heterojunction material such as GaN/AlGaN has a high electron density, which is advantageous for achieving high power, and a high electron mobility transistor (HEMT) made of an AlGaN/GaN heterojunction is used at a high temperature and high power. The aspect has very good application prospects and is currently a research hotspot at home and abroad.

The thickness of single-layer graphene is only 0.34 nm, which is the thinnest two-dimensional material discovered by humans so far. Because of its very good strength, flexibility, electrical conductivity, thermal conductivity, optical properties, it has made great progress in the fields of physics, materials science, electronic information, computer, aerospace and other fields. As one of the newest nanomaterials found to be the thinnest, strongest, and most conductive and thermally conductive, graphene is called "black gold" and is the "king of new materials." Scientists even predict that graphene will "completely change the 21st century." ". It is very possible to set off a revolutionary new technology and new industrial revolution that has swept the world.

Unlike conventional bipolar transistors that rely on electron and hole carriers to work, thermoelectric transistors rely on cold electrons (electrons that are in thermal equilibrium with the lattice) and hot electrons to work. Cold electrons provide the conductance of the different layers in the device, and the hot electrons carry the input information and magnify it in the device. The typical structure of such a device is very similar to that of a bipolar transistor, and also has an emitter region (E), a base region (B), and a collector region (C). There is a barrier between each side of the base region of the hot electron transistor connected to the emitter region and the collector region. The barrier function is to bind the cold electrons in their respective regions, and the hot electrons injected from the emitter region into the base region have A sufficiently large amount of energy passes through the barrier of the collector region, almost independent of the collector voltage, so the device has a very high output impedance R ₀ . Old-fashioned thermal electron transistors have not attracted much interest due to process and materials. Old-fashioned thermal electron transistors use metal as the base region, and since the mean free path of the hot electrons in the metal is very short, there is a problem of electron scattering. The obvious way is to reduce the thickness of the metal base, which will result in a large base resistance and a lack of metal tightness. The appearance of graphene has brought about a turning point in this problem. Graphene has metal-like properties. The carrier mobility of graphene at room temperature is about 15000 cm ² /(V·s), which is more than 10 for silicon materials. This guarantees a quasi-ballistic transport of hot electrons, while the single-layer graphene is very thin, only 0.34 nm. In addition, the electron mobility of graphene is less affected by temperature changes. At any temperature between 50 and 500 K, the electron mobility of single-layer graphene is around 15000 cm ² /(V·s), which ensures that the device is in poor condition. It can also work normally in the environment. The production of graphene is also mature in the process. High-quality graphene templates can be obtained by stripping and transfer, and related products are sold, so graphene is an ideal material to replace the metal base.

GaN is easy to form a mixed crystal with AlN, InN, etc., and the heterostructure formed by using it as a two-dimensional electron gas generated by the emitter will increase the carrier density, and the current transfer mechanism of the two-dimensional electron gas to the graphene can be effectively improved. The electron-emitting efficiency from the emitter to the base, together with the ultra-thin base of graphene, is beneficial for high-frequency, high-power devices. The corresponding base layer to the collector layer of the collector region also needs to be replaced with a thin layer of GaN, which minimizes the thermal electron backscattering of graphene to the barrier layer relative to the metal oxide, thereby increasing the current gain effect. In addition, GaN also has high thermal conductivity and stable chemical properties. Combined with graphene, it will greatly improve the performance of the device and the working ability in harsh environments. At present, there has been a report on the growth of nitride on the surface of graphene, so there is no technical problem in achieving ALD growth of gallium nitride on graphene.

ALD is a method of forming a deposited film by alternately passing a gas phase precursor pulse into the reactor and chemically adsorbing and reacting on the deposited substrate. Due to the self-limiting characteristics of ALD, the deposited film has the advantages of controllable thickness, good shape retention and uniformity. The graphene surface is chemically inert, and the growth of the film on the graphene surface requires activation treatment or growth at high temperatures. The use of PEALD to deposit a gallium nitride film on the surface of graphene has the following advantages: 1. The N source provided in PEALD is a Plasma gas, which does not require a very high temperature to form an active site on the surface of graphene to prevent graphene damage. 2. Due to the self-limiting characteristics of PEALD, the thickness of the grown GaN film can be controlled relatively accurately by the number of cycles, and the process is simple and reliable. 3. The international production of electronic devices tends to be nanoscale, and ALD deposition methods are gaining more and more attention. People are constantly making various technological improvements, and PEALD growth can better integrate with new technologies and continuously reduce device size.

Summary of the invention

The technical problem to be solved by the present invention is to provide a method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor, which can combine the advantages of graphene and gallium nitride to effectively improve the efficiency of the thermal electronic device. At the same time, the device size is reduced by PEALD technology.

The preparation method comprises the following steps:

(1) growing a group III nitride ternary alloy material on the GaN substrate to form a heterostructure as an emitter region and a first barrier layer, and growing the electrode by photolithography on the heterojunction;

(2) Insulating the device by ion etching;

(3) transferring the graphene to the surface of the heterostructure in step (1) as a base region, and forming a base electrode by photolithography;

(4) using PEALD to grow a GaN film on the graphene in step (3) as a second barrier layer, and forming a metal collector region on the surface of the GaN film to complete high-frequency gallium nitride/graphene heterojunction thermal electrons Preparation of transistors.

Wherein, in step (1), a heterogeneous structure is formed by depositing a thin film layer of a group III nitride ternary alloy material on the GaN substrate by MOCVD, and the deposited thin film layer has a thickness of 10 to 15 nm; in the step (1), the heterostructure is GaN. The interface with the group III nitride ternary alloy material serves as an emitter region, and the group III nitride ternary alloy material serves as the first barrier layer.

The emitter electrode of the emitter region in step (1) is one of Ti/Al/Ni/Au, Ti/Al/Mo/Au, Ti/Al/Ti/Au metal stack, and is formed by a lift-off process;

The specific process of the lift-off process is:

(1) spin-coating a photoresist on the surface of the grown heterojunction to define the position and shape of the electrode;

(2) removing the photoresist that needs to grow the position of the emitter electrode by using a developing technique;

(3) The metal is grown layer by layer by the electron beam evaporation method on the surface of the pattern obtained in the step (2), and then placed in acetone to remove the photoresist and the metal on the photoresist to obtain an emitter electrode.

The emitter electrode forming the emitter region in the step (1) is formed by a lift-off process and then rapidly annealed, and the annealing is performed in a vacuum environment or an inert gas. The annealing temperature is 800 to 900 ° C, and the annealing time is 0.5 to 2 minutes.

In step (2), the ion etching is low energy ion etching in a BCl ₃ /Cl ₂ environment, and the etching depth is 140-160 nm.

Step (3) Using the MOCVD growth transfer method, graphene is obtained on the surface of the heterojunction, and then the base electrode is formed on the surface of the graphene by a lift-off process, and the base electrode is a Ti/Pd/Au metal stack.

Step (3) The number of graphene layers obtained on the surface of the heterojunction is 1-4 layers by MOCVD growth transfer method.

Step (3) After forming the base electrode on the surface of the graphene, patterning is performed by photolithography and ion etching. In the photolithography, the MMA photoresist is spin-coated as a buffer layer, and then a positive photoresist is applied. Finally, the excess graphene is further removed by an ion etching machine and an oxygen plasma.

In step (4), a second GaN barrier layer is deposited on the surface of the graphene by using PEALD, and triethyl gallium and Ar/N ₂ /H ₂ are respectively used as a Ga source and an N source, and the second barrier layer has a thickness of 10-15 nm. Wherein the three gas volumes Ar:N ₂ :H _{2 in the} Ar/N ₂ /H ₂ mixed gas are 1:3:6.

In step (4), a metal collector region was prepared by a lift-off process on a GaN film, and annealed at 400 ° C for 5 minutes.

The beneficial effects of the above technical solution of the present invention are as follows:

The invention adjusts the structure of the thermoelectric device on the basis of the conventional thermoelectric device, adopts the heterojunction as the emitter region and uses the group III nitride ternary alloy material as the first barrier layer, and simultaneously in the graphene A GaN thin film was formed as a second barrier layer by a method using PEALD. This approach has the following three advantages:

1. Using a heterojunction as an emissive region and using a Group III nitride ternary alloy material as the first barrier layer can reduce the number of process steps;

2. The two-dimensional electron gas generated by using the heterojunction will increase the carrier density, and the current transfer mechanism of the two-dimensional electron gas to the graphene can effectively improve the electron emission efficiency from the emitter to the base;

3. The use of PEALD to grow GaN thin films on graphene combines the advantages of graphene high carrier mobility and GaN wide band gap to further improve the efficiency of the thermal electronic device, simplify the production process, and reduce the device size.

DRAWINGS

1 is a schematic view showing a process of preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to the present invention;

2 is a schematic view showing a specific preparation process in the embodiment of the present invention, wherein (a) is the step (1) operation, (b) is the step (2) operation, (c) is the step (3) operation, and (d) is Step (4) operation, (e) is the step (5) operation, (f) is the step (6) operation, and (g) is the step (7) operation;

3 is an energy band diagram of a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to the present invention, wherein (a) is in an equilibrium state and (b) is in a voltage-increasing state;

4 is a schematic view showing the structure of a high frequency gallium nitride/graphene heterojunction thermal electron transistor of the present invention.

Among them: 10-GaN substrate; 20-AlGaN film; 30-emitter electrode; 40-graphene film; 50-base electrode; 60-GaN film; 70-collector.

Detailed ways

The technical problems, the technical solutions, and the advantages of the present invention will be more clearly described in the following description.

The original thermoelectric transistor uses a heavily doped Si substrate as the base region, but the current density obtained is low, which limits the use of the thermoelectric device at high frequencies. At the same time, the old thermal electron transistor uses metal oxide as the second barrier layer, and its growth process is complicated and the performance is not satisfactory.

In order to overcome the above disadvantages, combined with the advantages of graphene and gallium nitride, the performance of the thermal electronic device is further improved. The present invention provides a method for preparing a high frequency gallium nitride/graphene heterojunction thermal electron transistor.

As shown in Figure 1, the general steps of the preparation method are as follows:

(1) growing a group III nitride ternary alloy material on the GaN substrate to form a heterostructure as an emitter region and a first barrier layer, and growing the electrode by photolithography on the heterojunction;

(2) Insulating the device by ion etching;

(3) transferring the graphene to the surface of the heterostructure in step (1) as a base region, and forming a base electrode by photolithography;

(4) using PEALD to grow a GaN film on the graphene in the step (3) as a second barrier layer, and forming a metal collector region on the surface of the GaN film to complete a high-frequency graphene/gallium nitride structure of the thermoelectric transistor preparation.

The structure of the produced hot electron transistor is shown in FIG.

In the specific operation process, the preparation is as follows:

(1) As shown in FIG. 2(a), a 1.2 μm GaN substrate 10 is provided, and a group III nitride ternary alloy material is grown on the GaN substrate 10 to form a heterostructure; specifically: MOCVD of the GaN substrate 10 at 1100 ° C The AlGaN thin film 20 epitaxially grown in a thickness of 10 to 15 nm forms a heterostructure in which the molar component concentration of Al is 25% to 30%.

(2) As shown in FIG. 2(b), the emitter electrode 30 is directly formed on the heterostructure obtained in the step (1); specifically:

The photoresist is spin-coated on the surface of the heterostructure grown in step (1) to define the position and shape of the emitter electrode 30, and the photoresist required to grow the position of the emitter electrode 30 is removed by a developing technique, and the electron beam evaporation method is used. Ti/Al/Ni/Au was grown layer by layer on the obtained pattern surface, and then placed in acetone to remove the photoresist and the metal on the photoresist to obtain an electrode. The product is then annealed in a vacuum environment or inert gas to form an ohmic contact with an annealing temperature of 800 to 900 ° C and an annealing time of 0.5 to 2 minutes.

(3) As shown in Figure 2(c), the product obtained in step (2) is subjected to low-energy ion etching; specifically:

The sample obtained in the step (2) is placed in a process gas environment containing Cl ₂ and BCl ₃ for low energy ion etching with an etching depth of 140-160 nm.

(4) As shown in Figure 2(d), transfer the graphene to the product obtained in step (3); specifically:

A single layer of graphene was grown on a copper sheet by MOCVD at a temperature of 1000 ° C, and the source was H ₂ and CH ₄ gas. The PMMA material is then spin coated onto the graphene film 40 grown on copper, and the PMMA is placed face up in the copper etchant for a period of time to completely dissolve the copper. The obtained PMMA/graphene film was transferred to 10% HCl for about 20 minutes. Finally, the film was rinsed with deionized water for a period of time and transferred to a patterned GaN/AlGaN wafer, and finally PMMA was removed to obtain a graphene film 40.

(5) as shown in Fig. 2(e), forming a base electrode on the surface of the graphene obtained in the step (4) by a lift-off technique, and patterning the graphene by photolithography and ion etching to perform device insulation; :

The base electrode 50 is formed on the surface of the graphene by a lift-off technique, which has been previously described and will not be repeated. The MMA material was then spin-coated on the surface of the graphene layer as a buffer layer, which was then patterned with a positive photoresist. After patterning, an ion etch machine was used to remove excess graphene and impact the surface with Plasma gas. Finally, the product was placed in an acetone solution to clean the surface of the graphene layer.

(6) As shown in Fig. 2(f), in the step (5), the surface of the graphene is deposited by PEALD as the second barrier layer by PEALD; specifically: firstly using UV/Ozone to the product in step (5) at 100 W power The surface of the graphene was activated by pretreatment for 30 s. The sample is then sent to the PEALD reaction chamber, and the temperature of the sample stage is raised to 200 ° C using triethyl gallium GaEt ₃ and Ar/N ₂ /H ₂ (1:3:6) as Ga source and N, respectively. The source was deposited for 100 cycles under the conditions of a growth parameter of 0.5 s GaEt ₃ dose/25 s purge/50 s plasma/15 s purge to obtain a GaN thin film 60 having a film thickness of about 10 to 15 nm.

(7) As shown in Fig. 2(g), the collector region 70 is formed by a lift-off technique on the GaN barrier layer deposited in the step (6), and annealed in an inert gas atmosphere for 4 minutes at a temperature of 400 °C. Finally, a high frequency gallium nitride/graphene heterojunction thermal electron transistor is obtained.

The energy band diagrams of the thermoelectric transistor obtained in the above process in the equilibrium state and the applied voltage state are respectively shown in Fig. 3 (a) and Fig. 3 (b).

The above is a preferred embodiment of the present invention, and it should be noted that those skilled in the art can also make several improvements and retouchings without departing from the principles of the present invention. It should also be considered as the scope of protection of the present invention.

Claims (10)

A method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor, characterized in that the steps are as follows: (1) growing a group III nitride ternary alloy material on the GaN substrate to form a heterostructure as an emitter region and a first barrier layer, and growing the electrode by photolithography on the heterojunction; (2) Insulating the device by ion etching; (3) transferring the graphene to the surface of the heterostructure in step (1) as a base region, and forming a base electrode by photolithography; (4) using PEALD to grow a GaN film on the graphene in the step (3) as a second barrier layer, and forming a metal collector region on the surface of the GaN film to complete a high-frequency graphene/gallium nitride structure of the thermoelectric transistor preparation. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 1, wherein in the step (1), a group III nitride ternary is deposited by MOCVD on a GaN substrate. The thin film layer of the alloy material forms a heterostructure, and the deposited thin film layer has a thickness of 10-15 nm; in the step (1), the heterostructure has an interface of GaN and a group III nitride ternary alloy material as an emitter region, and a group III nitride The ternary alloy material serves as the first barrier layer. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 1, wherein the emitter electrode of the emitter region in the step (1) is Ti/Al/Ni/Au, One of Ti/Al/Mo/Au, Ti/Al/Ti/Au metal laminates, formed by a lift-off process; The specific process of the lift-off process is: (1) spin-coating a photoresist on the surface of the grown heterojunction to define the position and shape of the electrode; (2) removing the photoresist that needs to grow the position of the emitter electrode by using a developing technique; (3) The metal is grown layer by layer by the electron beam evaporation method on the surface of the pattern obtained in the step (2), and then placed in acetone to remove the photoresist and the metal on the photoresist to obtain an emitter electrode. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 3, wherein the emitter electrode forming the emitter region in the step (1) is formed by a lift-off process Rapid annealing, annealing is carried out in a vacuum environment or in the protection of an inert gas, the annealing temperature is 800 to 900 ° C, and the annealing time is 0.5 to 2 minutes. The method for preparing a high frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 1, wherein the ion etching in the step (2) is a low energy ion in a BCl ₃ /Cl ₂ environment Etching, etching depth is 140-160nm. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 1, wherein the step (3) obtains graphene on the surface of the heterojunction by MOCVD growth and transfer. A base electrode is then formed on the surface of the graphene by a lift-off process, and the base electrode is a Ti/Pd/Au metal stack. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 6, wherein the step (3) adopts a MOCVD growth transfer method to obtain a graphene layer on the surface of the heterojunction. The number is 1 to 4 layers. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 1, wherein the step (3) is to form a base electrode on the surface of the graphene, and then pass through photolithography and ion etching. Graphene patterning is performed. In the photolithography, the MMA photoresist is spin-coated as a buffer layer, then a positive photoresist is applied, and finally the excess graphene is further removed by an ion etching machine and an oxygen plasma. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 1, wherein in the step (4), a second GaN barrier layer is deposited on the surface of the graphene by using PEALD. The trilayer gallium and Ar/N ₂ /H ₂ are respectively used as the Ga source and the N source, and the second barrier layer has a thickness of 10 to 15 nm, wherein the volume of the three gases in the Ar/N ₂ /H ₂ mixed gas is Ar: N ₂ :H ₂ is 1:3:6. The method for preparing a high-frequency gallium nitride/graphene heterojunction thermal electron transistor according to claim 1, wherein in the step (4), a metal collector region is prepared by a lift-off process on a GaN film. And annealed at 400 ° C for 5 minutes.

Images