JP2007191356A - Nitrogen-doped diamond film and its producing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an n-type semiconductor diamond film doped with highly concentrated nitrogen atoms using a microwave plasma CVD method. <P>SOLUTION: A mixed gas where one or more kinds of gases selected from among methylamine, dimethylamine and trimethylamine are diluted with hydrogen or a mixed gas where one or more kinds of gases selected from among methylamine, dimethylamine and trimethylamine, a hydrocarbon gas and a hydrogen gas are mixed is used as a raw material gas. A nitrogen-doped diamond film containing nitrogen atoms of 10<SP>20</SP>cm<SP>-3</SP>or more on a substrate surface is formed by the microwave plasma chemical vapor deposition method at gas pressure of 80 Torr (10,664 Pa) or more. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a high concentration nitrogen-doped diamond film.

  Diamond is excellent in heat resistance, has a large band gap (5.5 eV), and is usually an insulator, but can be made into a semiconductor by impurity doping. In addition, it has excellent electrical characteristics such as high dielectric breakdown voltage and saturation drift velocity and low dielectric constant. Due to such characteristics, diamond is expected as an electronic device or sensor material for high temperature, high frequency, and high electric field.

  Single crystals of diamond can be obtained by mining natural diamonds or artificially synthesizing them under high temperature and high pressure conditions (these are called bulk diamonds). The size of the surface is a few millimeters, and the price is extremely high. For this reason, the industrial utility value is limited to specific fields such as abrasive grains for polishing or cutting edges for precision cutting.

  On the other hand, the diamond film can be synthesized using, for example, a mixed gas of hydrocarbon and hydrogen as a raw material. As a vapor phase synthesis method of diamond, for example, as described in Patent Document 1 or Patent Document 2, a synthesis method using a microwave CVD (Chemical Vapor Deposition) method is known. In addition, a high-frequency plasma CVD method, a hot filament CVD method, a direct current plasma CVD method, a plasma jet method, a combustion method, a thermal CVD method, and the like are known.

For example, when diborane (B ₂ H ₆ ) is added to the source gas, B (boron) atoms are taken into the diamond film, and the diamond film becomes a p-type semiconductor and becomes conductive. The B atom as the dopant forms an acceptor level at 0.37 eV on the higher energy side of the valence band of diamond. When the B concentration (for example, B ₂ H ₆ concentration) of the source gas is increased, B atoms are taken into the diamond up to about 10 ²¹ cm ⁻³ , and the diamond exhibits metallic conductivity.

  On the other hand, it is not easy to make diamond an n-type semiconductor with high conductivity. According to the periodic table, if N (nitrogen) atoms are doped into diamond composed of C (carbon) atoms, an n-type semiconductor should be formed. However, in reality, N atoms have a lower energy side than the conduction band of diamond. A donor level is formed at .14 eV, and at room temperature (0.025 eV), it is impossible to thermally excite electrons from the donor level to the conduction band, and N-doped diamond is an electrical insulator. .

In Patent Document 3, a diamond-doped diamond layer (intervening layer) is formed between a gate electrode and an operation layer so that a field effect transistor using diamond is suitable for high temperature, high output, and high frequency. In the disclosed example, a method for producing a nitrogen-doped diamond film using N ₂ gas as an N source is described.

  Further, in the semiconductor element described in Patent Document 4, an intervening layer made of diamond doped with nitrogen atoms is provided between the operating layer mainly composed of p-type diamond and the gate electrode.

Patent Document 5 proposes a conventional technique of doping a diamond film with N atoms of 10 ¹⁹ cm ⁻³ or more, and in the embodiment, NH ₃ is used as an N source. In addition, a relatively low gas pressure of 40 Torr (5332 Pa) is used as the gas pressure during synthesis.

Japanese Examined Patent Publication No.59-027754 Japanese Patent Publication No. 61-003320 Japanese Patent Laid-Open No. 5-29610 JP-A-6-209014 Japanese Patent Laid-Open No. 7-69794

  However, the conventional techniques described above have the following problems.

According to the manufacturing method described in the example of Patent Document 3, the nitrogen concentration in the diamond film is 5 × 10 ¹⁹ cm ⁻³ , the concentration is low, and the diamond film does not become a semiconductor. Further, in claim 1 of the same document, there is a description of an intervening layer made of diamond containing nitrogen of 10 ¹⁵ to 10 ²¹ cm ⁻³ as a nitrogen concentration, but actually disclosed a high concentration nitrogen-doped diamond. There is no experimental example, and only a field effect transistor having a structure in which an intervening layer made of diamond containing a nitrogen dopant at a concentration of 10 ¹⁵ to 10 ²¹ cm ⁻³ is sandwiched between a gate electrode and an operating layer is proposed. It is.

Moreover, in the Example of patent document 4, the description about the intervening area | region which doped about 10 < ¹⁹ > -10 < ²¹ > cm ^<-3 > nitrogen atoms to a part of p-type diamond layer by ion implantation of nitrogen atoms is described. In addition, there is described an intervening layer that is made intrinsic by introducing nitrogen atoms into a p-type diamond layer by NH ₃ (ammonia) plasma irradiation. However, the method described in this document is different from the manufacturing method using the microwave plasma CVD method used in the present invention. Moreover, although the Example of the same document describes formation of an n-type diamond layer having a nitrogen atom concentration of 10 ¹⁹ cm ⁻³ , the nitrogen atom concentration is low.

  In addition, the present inventors synthesized a diamond film doped with N atoms according to the prior art of Patent Document 5, but the resistivity was reduced, but the Raman spectrum was measured. It was found that the component was contained in a large amount. Therefore, it was concluded that the decrease in resistivity by the prior art described in Patent Document 5 is actually due to graphite.

The problem with conventional high-concentration N atom doping in diamond is that, for example, as the N ₂ gas concentration in the source gas increases, the crystallinity of the diamond itself decreases, and when N ₂ gas is added at several percent, It was only possible to use crystalline diamond. Therefore, there is a need for a technique that does not decrease the crystallinity of the base diamond while doping N atoms at a high concentration.

  The present invention has been made in view of such problems, and an object thereof is to provide a method for producing an n-type semiconductor diamond film doped with a high concentration of nitrogen atoms using a microwave plasma CVD method. .

  In the method for producing a nitrogen-doped diamond film according to the present invention, one or more gases selected from methylamine, dimethylamine, and trimethylamine (hereinafter sometimes referred to as methylamine-based materials) are diluted with hydrogen. Or a mixed gas of one or more gases selected from methylamine, dimethylamine, and trimethylamine, a hydrocarbon, and hydrogen as a raw material gas, and a gas pressure of 80 Torr (10664 Pa) or higher. A diamond film doped with nitrogen is formed on a substrate by a microwave plasma chemical vapor deposition method.

According to the present invention, the diamond film can be manufactured so that the nitrogen atom concentration in the diamond film is 10 ^{20 to} 5 × 10 ²¹ cm ⁻³ .

The value of the (N / C) ratio in the raw material gas needs to be 10 ^{5 to} 10 ⁶ ppm which greatly exceeds the conventional common sense value.

  The substrate temperature during diamond film formation needs to be 700 to 1200 ° C., and more preferably 900 to 1100 ° C.

  The gas pressure is preferably 760 Torr (101308 Pa) or less.

The diamond film manufactured by the manufacturing method includes nitrogen atoms of 10 ²⁰ cm ⁻³ or more in the film.

  According to the present invention, high-concentration nitrogen is obtained by microwave plasma CVD using high-pressure plasma using one or more gases selected from methylamine, dimethylamine, and trimethylamine as a nitrogen source. A diamond film containing atoms can be produced. This makes it possible to produce an n-type semiconductor diamond film that has been difficult in the past.

Next, the present invention will be described in more detail. According to the study by the present inventors, there is only a possibility that N atoms having a small atomic radius are suitable n-type dopants for diamond, and the diamond film contains N atoms having a content of 10 ²⁰ cm ⁻³ or more. Thus, it was estimated that the donor level significantly increased from 1.4 eV below the conduction band and approached the conduction band. In this case, the electron density in diamond is 10 ¹⁸ cm ⁻³ or more, and diamond becomes an n-type semiconductor. When the diamond film contains only N atoms of less than 10 ²⁰ cm ⁻³ , the donor level remains in the vicinity of 1.4 eV below the conduction band, and the diamond film has a high resistivity of 10 ⁴ Ω · cm or more. It becomes an insulator.

In order to introduce high-concentration N atoms having a content of 10 ²⁰ cm ⁻³ or more into diamond, methylamine ((CH ₃ ) NH ₂ ), dimethylamine ((CH ₃ ) ₂ NH), and trimethylamine (( A gas obtained by diluting one or more gases selected from CH ₃ ) ₃ N) with hydrogen, or one or two or more gases selected from methylamine, dimethylamine, and trimethylamine and carbonization It has been found that this can be achieved by using a mixed gas of hydrogen and hydrogen as a raw material gas and synthesizing using a microwave plasma CVD method under a gas pressure of 80 Torr (10664 Pa) or higher.

In addition, when substances other than methylamine, dimethylamine, and trimethylamine are used as the N source (for example, nitrogen gas, ammonia, urea, etc.), the diamond film contains a large amount of graphite components, and the quality of the diamond film deteriorates. Then, it was confirmed that N doping of 10 ²⁰ cm ⁻³ or more is difficult.

  The mixing ratio (N / C) of N and C in the raw material gas is preferably as large as possible. The maximum value of (N / C) is (N / C) = 1000000 ppm when methylamine is diluted with hydrogen. Further, when a gas obtained by mixing a methylamine-based material, a hydrocarbon gas, and hydrogen is used as a raw material gas, (N / C) ≧ 100,000 ppm is necessary. When (N / C) <100,000 ppm, the number of N atoms taken into the diamond film is reduced, and the diamond film has a high resistance. Only one type of methylamine-based material may be used, or two or three types may be combined, and the N / C atomic ratio can be adjusted according to the hydrocarbon used.

  The substrate temperature during diamond film formation needs to be 700 to 1200 ° C., and more preferably 900 to 1100 ° C.

In the synthesis of the N-doped diamond film, it was found that when the source gas pressure is 80 Torr (1064 Pa) or less, the diamond film deteriorates due to N doping. The upper limit of the raw material gas pressure is 760 Torr (101308 Pa). On the other hand, if the gas pressure is higher than 760 Torr (101308 Pa), it is necessary to input a high-power microwave for plasma generation, and it is difficult to control the substrate temperature, which increases technical difficulties. More preferably, the raw material gas pressure is 80 to 200 Torr (10664 to 26660 Pa). Normally, CVD synthesis of diamond film is performed at a substrate temperature of 700 to 1200 ° C. Therefore, if the source gas pressure is 80 Torr (10664 Pa) or higher, a microwave of 50 kW or higher is put into the reaction chamber, depending on the scale of the reaction vessel. There must be. That is, it has been found that if diamond is synthesized by plasma with high gas pressure and high power, the N atom doping concentration becomes 10 ²⁰ cm ⁻³ or more and the crystallinity of the base diamond is not lost.

  The hydrocarbon used for the raw material gas is, for example, methane, but in addition, ethane, acetylene, propane, ethylene, methanol, ethanol, acetone, coke gas, or the like may be used.

  When N atoms are so thinly doped in diamond that there is little interaction with other N atoms, the donor level is formed on the low energy side of 1.4 eV from the valence band as described above. The reason why the donor level having a low energy is formed in this way is that the N atom substitutionally doped with the C atom in the diamond shifts from the C atom position, thereby stabilizing the lattice energy. It is said to be due to the Jahn-Teller effect.

On the other hand, when N atoms are contained in diamond at 10 ²⁰ cm ⁻³ or more as in the present invention, on average, one N atom in a cube in which 10 C (carbon) atoms are arranged on average. Since the distance between N atoms is small, a strong electronic interaction occurs between the atoms, the position of the N atom is almost the same as the C atom of diamond, and the yarn-teller effect disappears.

As described above, in the present invention, in order to introduce N atoms of 10 ²⁰ cm ⁻³ or more into the diamond lattice, one or more gases selected from methylamine, dimethylamine, and trimethylamine as the N source are used. And use plasma with high gas pressure and high power. That is, a high energy chemical reaction occurs in the plasma. As a result, the N doping concentration in the diamond film is high, and the diamond film grown on the substrate surface is of high quality and contains almost no graphite component.

The reason why such a high-quality N-doped film was synthesized is that molecular species (· CH ₃ ) generated by decomposing methylamine, dimethylamine, and trimethylamine in addition to using plasma with high gas pressure and high power. However, it is thought that it contributes to the improvement of diamond crystallinity.

When the hole density of the diamond film synthesized under the above conditions was measured and the electron density in the diamond film was determined, it was found to be 10 ¹⁸ cm ⁻³ or more. That is, it was concluded that the diamond film synthesized according to the present invention is a highly conductive n-type semiconductor.

The upper limit of the nitrogen atom content in the diamond film is 5 × 10 ²¹ cm ⁻³ . When the nitrogen atom content is larger than 5 × 10 ²¹ cm ⁻³ , the crystal structure of diamond is not maintained (in carbon, the carbon atom density is −10 ²³ cm ⁻³ ). Moreover, it is preferably 10 ^{20 to} 10 ²¹ cm ⁻³ . In actual measurement, the electron density in the diamond film at room temperature was 10 ^{18 to} 10 ²¹ cm ⁻³ .

  Hereinafter, the effect of the Example of this invention is demonstrated compared with the comparative example which remove | deviates from the range of this invention.

First, as an example of the present invention, a nitrogen-doped diamond film was synthesized under the following conditions. A CVD apparatus using a 915 MHz microwave was used for forming the diamond film. The substrate used was a polycrystalline Si (silicon) having a thickness of 5 mm and a diameter of 6 inches (150 mm) coated with a polycrystalline undoped diamond film having a thickness of 20 μm. The microwave output and the gas pressure were adjusted so that the substrate temperature was 900 ° C., the microwave output was 60 kW, and the gas pressure was 100 Torr (13330 Pa). As a source gas, a mixed gas of methylamine material, methane (CH ₄ ), and hydrogen (H ₂ ) was used, and the total gas flow rate was 2000 sccm. In this example, polycrystalline undoped diamond was used as the substrate, but diamond other than polycrystalline undoped may be used. In addition, Si, SiC (silicon carbide), SiO ₂ (silicon dioxide), CBN (cubic boron nitride) ), Mo (molybdenum), W (tungsten), Ir (iridium), Pt (platinum), Al ₂ O ₃ (alumina, sapphire), ceramics, refractory metal, and the like.

  The synthesis was performed for 20 hours, and a diamond film having a thickness of about 2 μm was synthesized. Table 1 shows the N atom content (determined by SIM: Secondary Ion Mass Spectrometer measurement) contained in the diamond film synthesized under various conditions, the electrical resistivity, and the electron density obtained by hole measurement. The diamond film doped with N atoms at a high concentration exhibited metallic electrical conductivity (low resistance and increased electrical resistance with increasing temperature).

  As a comparative example of this example, nitrogen gas was used as the N source, but only a low-quality diamond film containing graphite could be synthesized. Furthermore, a diamond film was synthesized using a methylamine-based material as an N source, and the gas pressure was set at 70 Torr (9331 Pa). Other synthesis conditions are the same as in Example 1. In this case, only a low-quality diamond film containing graphite was synthesized.

  Further, as an example of the present invention, a nitrogen-doped diamond film was synthesized with a gas pressure of 80 Torr (10664 Pa) in Example 8 and a gas pressure of 120 Torr (15996 Pa) in Example 9. Other synthesis conditions are the same as in Example 1. Table 2 shows the N atom content, electrical resistivity, and electron density contained in the diamond film for these examples.

  Furthermore, as an example of the present invention, a nitrogen-doped diamond film was synthesized when methylamine, dimethylamine, or trimethylamine was diluted with hydrogen as a source gas, and when trimethylamine and methylamine were diluted with hydrogen. . The synthesis conditions are the same as in Example 1 except for the conditions for the source gas. Table 3 shows the N atom content, electrical resistivity, and electron density contained in the diamond film for these examples. In the above embodiment, methane is used as the hydrocarbon, but other hydrocarbons described above may be used, and may be used in combination as appropriate.

The present invention is suitably used for electronic devices such as transistors, diodes, and various sensors, or optical recording.

Claims (6)

A mixed gas obtained by diluting one or more gases selected from methylamine, dimethylamine, and trimethylamine with hydrogen, or one or more gases selected from methylamine, dimethylamine, and trimethylamine; Using a mixed gas of hydrocarbon and hydrogen as a source gas and forming a diamond film doped with nitrogen on a substrate by microwave plasma chemical vapor deposition under a gas pressure of 80 Torr (10664 Pa) or higher. A method for producing a nitrogen-doped diamond film characterized by 2. The method for producing a nitrogen-doped diamond film according to claim 1, wherein a nitrogen atom concentration in the diamond film is 10 ^{20 to} 5 × 10 ²¹ cm ⁻³ . 3. The method for producing a nitrogen-doped diamond film according to claim 1, wherein a value of the (N / C) ratio in the source gas is 10 ^{5 to} 10 ⁶ ppm. 4. The method for producing a nitrogen-doped diamond film according to claim 1, wherein the substrate temperature during the formation of the diamond film is 700 to 1200 ° C. 5. 5. The method for producing a nitrogen-doped diamond film according to claim 1, wherein the gas pressure is 760 Torr (101308 Pa) or less. A nitrogen-doped diamond film produced by the method according to claim 1, wherein the film contains nitrogen atoms of 10 ²⁰ cm ⁻³ or more.