JP4196933B2 - Method for evaluating the electrical resistance of boron-doped diamond and boron-doped diamond - Google Patents

Description

  The present invention relates to a method for evaluating electrical resistance of boron-doped p-type diamond and boron-doped p-type diamond. Natural diamonds with high transparency and high refraction are used in jewelry, but the present invention does not target such diamonds. Diamond has been used for a long time as a cutting tool or an abrasive material because of its high hardness. It is an insulating diamond that is particulate or coated. Insulator diamonds can be natural, high-pressure, or vapor-phase. Diamonds as cutting tools and abrasives have a rich track record.

  The present invention is not such, and has a higher electrical conductivity and is intended for use as an electrode or a semiconductor element. It is made by vapor phase synthesis and often takes the form of a thin film formed on a base substrate. Due to the high impurity concentration, it is no longer transparent but has a dark appearance.

  Diamond is promising as a new semiconductor electronic material because it has a wide band gap of 5.5 eV, a large carrier mobility and withstands high temperatures. There is an application as an electrode utilizing conductivity. Also, there is an application as a semiconductor active device having a pn junction. Recently, attention has been focused on the use of diamond as an electron emission source in place of an electron emission source using a W filament. When a strong electric field is formed in the vicinity of diamond, electrons are ejected from the diamond by the action of the electric field, so that it can be used as an electron source. In order to apply a strong electric field, it is desirable that diamond has a low resistance.

Since it is considered as a semiconductor material, the possibility of p-type diamond and n-type diamond is considered. If both are not possible, it will be difficult to use as a semiconductor element material. A p-type diamond can be obtained by doping with boron (B) from group 3. However, since boron has a diameter larger than that of carbon in diamond, it is difficult to dope with high concentration, and since the activation rate is low, the hole concentration is usually about p = 10 ¹⁶ cm ⁻³ . What impurity (acceptor) levels are created by boron doping? It seems that this is not well understood.

  Originally, research for semiconductor diamond was preceded by boron-doped p-type diamond. However, n-type diamond is indispensable for making a semiconductor element. Therefore, research on n-type diamond is more active than p-type. The n-type diamond should be obtained by doping group 5 nitrogen (N), phosphorus (P), and arsenic (As), but there are doubts. P and As doped ones have not been tried much. Nitrogen (N) -doped diamond is produced, but it is not clear what donor level nitrogen produces.

  Nitrogen is considered to have deep levels and not many free carriers. In addition, Li and Na are said to be donors. There is currently no excellent n-type diamond. However, since the n-type is fundamental to any semiconductor material, a high-quality n-type must be realized. As a result, research on n-type diamonds is currently underway.

  The present invention is directed to boron-doped p-type diamond, not n-type. Boron has a larger effective radius than carbon and is difficult to enter into diamond crystals. Therefore, p-type and low-resistance ones are still difficult to obtain. Therefore, a method called co-doping (co-doping) is also attempted, in which nitrogen is doped together and boron is doped at a higher concentration while preserving the crystal structure. In order to force p-type boron into the diamond crystal, nitrogen of n-type impurities is doped together.

There are various theories, but there is also a theory that stress is relieved because a bond of -B-N- is formed in diamond.
There is a theory that since the p-type and n-type compensate for each other, the difference between the boron concentration and the nitrogen concentration gives the hole concentration, so the boron concentration must be greater than the nitrogen concentration.
There is also the theory that nitrogen donors are inert and do not interfere with hole generation.

  There seems to be no fixed view on such a code group. This is probably because it is difficult to obtain a direct knowledge of what level nitrogen produces and what kind of acceptor level boron forms in the diamond band structure.

Japanese Patent Laid-Open No. 4-266020 "Semiconductor Diamond"

Patent Document 1 proposes a co-doping diamond that contains both nitrogen and boron at a concentration of 1000 ppm or more. As for the p-type doped with boron, only p = 10 ¹⁶ cm ⁻³ has been obtained even if boron is doped at 1000 ppm. When boron was further doped, the crystal structure collapsed, but by doping together with nitrogen, boron could be doped at a concentration of 1000 ppm or more while maintaining the crystal structure. That required simultaneous doping of more than 1000 ppm of nitrogen. If approximately the same amount of nitrogen and boron is doped, the carriers emitted from the donor and the acceptor will cancel each other and free carriers should be reduced.

However, Cited Document 1 states that the level created by nitrogen is deep in the forbidden band, but rather at a height close to the valence band, so it does not serve as a donor. Therefore, only boron holes are emitted, and it is claimed that a high carrier concentration of p = 10 ¹⁸ cm ⁻³ was obtained as the hole concentration. There is doubt that nitrogen is not a donor in diamond.

Japanese Patent Laid-Open No. 6-144993 “Boron Doped Diamond”

Patent Document 2 compares diamonds made by high pressure synthesis with diamonds made by CVD. Even if p type with the same boron concentration is made by CVD, the resistance is lower and it is used as an electrode for switch electrodes and chemical sensors. It is preferred. Until now, diamond doped with 1000 ppm or more of boron could not be made. The present invention claims that diamond containing 1000 to 4500 ppm of boron could be made by CVD. Diamond with a boron concentration of 1800 ppm by CVD is said to have a sufficiently low resistivity of about 1000 Ωcm. And diamonds containing 1000 ppm to 4500 ppm boron can be made, which states that the resistance is low.

Japanese Patent Laid-Open No. 9-13188 “Diamond Electrode”

Patent Document 3 proposes an electrode obtained by providing a diamond electrode on a carbon electrode (graphite) by a CVD method and further chemically modifying such as hydrogenation, oxidation, or halogenation. It states that chemical modification prevents overvoltage when used as an electrode for electrolysis of sodium chloride, increases current efficiency, and reduces electrode consumption.

Japanese Patent Application Laid-Open No. 2004-31022 “Conductive Diamond and Method for Forming Conductive Diamond”

  Patent Document 4 states that a diamond film containing boron and nitrogen at a high concentration of 10000 ppm or more is formed on a Si substrate by simultaneously doping boron and nitrogen by CVD, and is p-type and low resistance. . This is also a co-doping diamond. It is claimed that the p-type can be obtained by making the boron concentration 10,000 ppm or more higher than the nitrogen concentration. He says that the boron structure cannot be doped with boron at a high concentration due to the collapse of the diamond structure. Therefore, it is better to co-dope nitrogen and boron as in Patent Document 1, but when the nitrogen concentration is equal to the boron concentration as in Patent Document 1, it is stated that the resistance will not be reduced as a p-type by canceling. .

Patent Document 4 denies the opinion that nitrogen does not interfere with the boron acceptor and argues that the nitrogen concentration should not be the same as the boron concentration because the nitrogen donor emits electrons and counteracts the boron holes. Therefore, both nitrogen and boron are doped at a high concentration of 10,000 ppm or more. However, by making the boron concentration higher than the nitrogen concentration by 10,000 ppm or more, boron corresponding to the concentration difference generates holes without canceling out all of the boron. He says he will be a p-type diamond. For example, it is stated that a low-resistance diamond film having a resistivity of 10 ⁻³ Ωcm was obtained when the boron concentration was 50600 ppm, the nitrogen concentration was 13600 ppm, and the concentration difference was 37000 ppm.

R. Kalish, A .; Reznik, C.I. Uzan-Sagiy, & C.I. Cytmann, “Is Sulfur a donor in diamond?”, Appl. Phys. Lett. , Vol. 76. No. 6, p757-759 (2000)

Non-Patent Document 1 is searching for a material for forming an appropriate donor for producing n-type diamond. It has been said that all the candidates for donors in diamond such as Li, Na, N, P, and As have difficulty so far. There is a paper that N makes a level of 1.7 eV measured from the bottom of the conduction band and P makes a level 0.5 eV below the bottom of the conduction band, and then P should be a suitable n-type impurity. Argues that P actually does not go into the diamond and is not useful because the hole mobility is low.

  Non-Patent Document 1 examined the sulfur-doped diamond of vapor phase synthesis because there is an opinion that sulfur (S) forms a donor at 0.37 eV below the conduction band. The Hall measurement showed that the carrier was not an electron but a hole. Therefore, we conclude that sulfur-doped diamond is p-type. Furthermore, it is said that sulfur-doped diamond is similar in temperature / resistance, temperature / mobility characteristics to boron-doped diamond. Thus, when SIMS investigated what elements were actually contained, it was concluded that boron was contained in a larger amount than sulfur and the p-type nature was due to boron. He says that unintentionally contained boron works as p-type. It states that a material that does not contain boron and is purely doped with only sulfur has high resistance and cannot perform Hall measurement. That is, Non-Patent Document 1 insists that sulfur does not become n-type or p-type in diamond.

Yung-Hsin Chen & Chen-Ti Hu, “Defect structure and electron field-emission properties of boron-doped diamond films”, Appl. Phys. Lett. Vol. 75 No. 18, p2857-2859, 1999

Non-Patent Document 2 investigates how much boron content can be increased when boron-doped diamond is used as a field-induced electron emission source. A sample having a low boron concentration may have a small electric field strength and a large amount of electrons necessary for electron emission, but a sample having a high boron concentration requires a strong electric field and has a low amount of emitted electrons. Increasing the boron source concentration (B (OCH ₃ ) ₃ ) in the raw material increases the proportion of boron incorporated into the diamond, but has an upper limit. When the upper limit was measured by SIMS, the upper limit concentration was B = 5 × 10 ²¹ cm ⁻³ . Even if the boron source concentration is increased, no further boron enters the diamond.

When the peak of 1280 cm ⁻¹ corresponding to the BC bond is examined in the IR absorption spectrum, the highest concentration of boron substituted for the carbon site is B = 5 × 10 ²⁰ cm ^−3. I know. It is only 1/10 of the upper limit of B concentration in SIMS measurement. This means that boron doped to 5 × 10 ²⁰ cm ⁻³ or more is not at a lattice point.

  In boron-doped diamond, there is a parameter governing resistivity other than boron. It is an object of the present invention to clarify this. It is another object of the present invention to clarify a method of effectively reducing the resistance although the resistance does not decrease easily even when boron is doped at a high concentration. It is another object to provide a method for measuring the resistivity of non-destructive and highly boron doped diamond. It is a further object of the present invention to provide a highly doped boron, high conductivity diamond.

In the present invention, a boron concentration is set to 6000 ppm or more without using a co-doping containing nitrogen, and an impurity level Q related to boron and hydrogen of 2 eV from the top of the valence band is formed in the forbidden band. The resistivity of the p-type diamond is lowered by increasing the level Q. If the level Q at 2 eV in the forbidden band is large, the conductivity becomes high. That is, the level Q can be increased to increase the conductivity. Even at the same boron concentration b, the level Q increases when the hydrogen concentration x is low. In other words, high conductivity p-type diamond can be obtained if hydrogen is not taken in when doping with high concentration boron. When diamond is produced by CVD, hydrocarbons (such as methane) contain hydrogen. Therefore, hydrogen is easily contained in CVD diamond. Since hydrogen is also used as a carrier gas when producing diamond by plasma CVD, it is unintentionally contained in diamond. By reducing the hydrogen, a diamond having excellent conductivity can be obtained. In order to reduce hydrogen, the dilution rate may be lowered when the source gas is diluted with the carrier gas.

  The present invention can also provide a non-destructive method for measuring electrical resistance. For the peak 1 of the first energy level (S) corresponding to the acceptor formed at the top of the valence band, the ratio of the peak 2 of the second energy level (Q) that can be as high as 2 eV in the forbidden band is x = peak 2 / Peak 1 = Q / S, and the resistivity is evaluated by the value of x. Even at the same boron concentration b, the resistivity is lower as the peak ratio x is larger (Q is dominant). The smaller the peak ratio x (Q is inferior), the higher the resistivity. From such properties, the electrical resistance of diamond can be evaluated.

  When the boron concentration is 7000 ppm, the resistivity y is given by y = exp (−4.5x + 0.6).

  When the boron concentration is 70000 ppm, the resistivity y is given by y = exp (−4.5x−0.3).

When the boron concentration is 6000 ppm or more, the conductivity σ ₁ associated with hydrogen is σ ₁ = (b−5000) ^0.258 exp (+ 4.5x−2.56)

Or σ ₁ = (b-5500) ^0.239 exp (+ 4.5x−2.35)

Or
σ ₁ = (b−6000) ^0.216 exp (+ 4.5x−2.09)

Given by. It depends on whether the critical value is 5000 ppm, 5500 ppm, or 6000 ppm. The present invention is applied to a material of 6000 ppm or more. However, if it is 6000 ppm or more, a value that does not change much can be obtained by calculation using any equation.

  An S-level near the valence band, entering the 2 eV forbidden band (forbidden band width is 5.5 eV) The electron density of the Q level is an X-ray source that continuously contains X-rays in the energy band of 180 eV to 200 eV The absorption spectrum can be obtained by applying X-rays of continuous wavelength to the sample using, and can be measured from the magnitude of the absorption. 1 and 2 are X-ray absorption spectra of two diamond samples, which will be described later. 1s electrons are excited to the S level or Q level by X-rays. Since the energy difference between the 1s orbit and the forbidden band is 180 eV to 200 eV, an X-ray source that emits a continuous wavelength around that is required. Here, the Fermi level is set to 0 eV. Since a large amount of boron is doped, there are a large number of acceptors, and the Fermi level is almost the same as the top of the valence band. Therefore, the reference 0 eV is the Fermi level, but it can be considered as the top of the valence band. The second peak Q is an impurity level formed by boron and has an effect of increasing the conductivity. It is dominant when the amount of hydrogen is small, but it becomes inferior when the amount of hydrogen increases (FIG. 2). Hydrogen disappears at 20000 ppm or more. The higher the peak ratio x, the higher the conductivity. For this purpose, the hydrogen content may be reduced.

  In order to make diamond with high conductivity, the present invention proposes not only to increase the boron concentration but also to decrease the hydrogen concentration in order to form the second energy level Q and increase the peak. When the hydrogen concentration is lowered, the second level Q is strengthened and the conductivity is improved. In addition, boron is stabilized by lowering the hydrogen concentration, so that boron can be doped at a higher concentration.

  Furthermore, when the boron concentration is 6000 ppm or more, the electrical resistance (conductivity) of diamond can be evaluated by the present invention by knowing the boron concentration b and the peak ratio x = Q / S. It is useful because it is a non-destructive inspection. However, direct observation of electronic density of states is required for this purpose. An X-ray source with continuous energy is required. Such convenient x-ray sources are currently few but can be manufactured for any application. If such a light source is made, it is not difficult to implement the present invention.

  The present invention can be applied to all boron-doped p-type diamonds, which can be applied to diamonds produced by a high pressure synthesis method or diamonds produced by a vapor phase synthesis method using CVD. There are various CVD methods such as a filament CVD method and a plasma CVD method. The amount of hydrogen varies depending on the synthesis method, but in any case, excess hydrogen increases resistance. Various substrates such as diamond, Si, GaAs, InP, and sapphire can be used as the base substrate.

  In the case of high pressure synthesis, the raw materials are carbon powder and a catalyst.

  In the case of vapor phase synthesis, a carbon raw material obtained by diluting hydrocarbons such as methane, ethyl alcohol, and acetone with hydrogen is used. Since the raw material contains hydrogen and the carrier gas is also hydrogen, a large amount of hydrogen is unintentionally included in diamond.

The boron source is B ₂ H ₆ , B ₂ O ₃ , B ₂ (CH ₃ ) ₃ , B (OCH ₃ ) ₃ or the like. These can be used by diluting with hydrogen or argon.

The most excellent method is to form diamond on a diamond substrate using hydrogen diluted methane (CH ₄ ) and argon diluted B (OCH ₃ ) ₃ by microwave plasma CVD.

  There are not many techniques for directly observing the energy level, but X-ray absorption spectroscopy (XAFS method: X-) that can measure an absorption spectrum using a continuous wavelength X-ray source including an energy band of 180 eV to 200 eV. ray abstraction fine structure) is suitable.

  Alternatively, the electron density of state can be directly measured by electron energy loss spectroscopy (EELS) in which an electron energy loss spectrum is measured by applying an electron beam of 195 eV or more.

The first energy level S near the top of the valence band (near 0 eV) is a shallow acceptor level formed by boron. It forms an electron density that is independent of the hydrogen content. The second level Q that can be 2 eV from the valence band is induced by boron. Boron is not generated at the crystal site because boron is generated for the first time exceeding 6000 ppm, but it is considered to be a level generated by jumping out of the carbon site and distorting the carbon lattice. Although it seems to be a carbon vacancy, it is not clear why the vacancy generates holes. Also, why is it inactive when hydrogen enters because it is a vacancy? That is not clear. However, the present invention claims that the second level Q is produced above 6000 ppm boron, which is useful for conduction.
The common sense is that the first level S contributes to electrical conduction, but the present invention is not, and the second level Q is considered to play a major role in electrical conduction.

  A boron-doped diamond thin film was formed on an insulating single crystal diamond substrate by plasma CVD. There are 10 samples A to J with different conditions. Samples A to H are examples of the present invention. Samples I and J are comparative examples. About each sample, hydrogen amount (H), boron amount (B), resistivity, etc. were measured. The results are summarized in Table 1.

1. Hydrogen amount and boron amount Boron amount and hydrogen amount were measured by secondary ion mass spectrometry (SIMS). In this method, the number of constituent elements on the surface of the sample is directly examined by applying an accelerated ion beam to the surface of the sample, causing the surface atoms to jump out by impact, mass analysis, and counting the number. Since the number of constituent elements is counted while etching the surface, the distribution of constituent elements (base material, impurities) can be known in the depth direction. Since it is a boron-doped diamond, it is natural to measure the amount of boron, but the idea of measuring the amount of hydrogen is novel. This is necessary because it supports the claim of the present invention that the relevant level of hydrogen is formed in a forbidden band.

  Samples A to D have a boron content of 7000 ppm. Samples E to H have a boron content of 70000 ppm. First, the literature related to the prior art was explained. However, it is rare that such a large amount of boron enters itself. Samples I and J have a boron content of 5000 ppm. As such, the boron content includes three types.

  There is a greater variation in the amount of hydrogen. When diamond is produced by plasma CVD, the carrier gas can be hydrogen, nitrogen, Ar, or the like. However, when hydrogen is used as the carrier gas, hydrogen is naturally contained as an impurity in the diamond. Samples A, E, and I have a very low hydrogen content of 10 ppm. Sample B has a low hydrogen concentration of 80 ppm and sample F of 100 ppm. Sample C is 1200 ppm, sample G is 900 ppm, and sample J is 1000 ppm, which is a considerably large amount of hydrogen. Sample D had a maximum hydrogen concentration at 9000 ppm and sample H at 15000 ppm. Since the present invention claims that the level cannot be set in the forbidden band due to the difference in hydrogen concentration, the hydrogen concentration is an important parameter in the present invention.

2. Resistivity
The sheet resistance is measured by Hall measurement. Since the thickness d of the thin film of the sample can be found by SIMS measurement, the resistivity is obtained by multiplying the sheet resistance by the thickness. The above-mentioned Patent Document 2 (Japanese Patent Laid-Open No. 6-144993) is CVD diamond and has a boron content of 500 to 1800 ppm and a resistivity of about 1000 Ωcm. In the present invention, the amount of boron is higher than that of Patent Document 2, but the resistivity of the thin film is about 1/10 to 1/1000 Ωcm, and it can be seen that the resistance is much lower than that of Patent Document 2. Sample E has a minimum resistivity of 5 × 10 ⁻⁴ Ωcm. Samples A, B, F, and G have extremely low resistivity and are in the range of 10 ⁻² to 10 ⁻³ Ωcm. Samples C, D, and H have a slightly higher resistivity and are in the range of 10 ⁻¹ to 10 ⁻² Ωcm. Samples I and J are comparative examples, but the resistivity is about 10 ⁻¹ Ωcm.

3. Peak intensity ratio The peak intensity ratio is an impurity at a height of about 2 eV from the top of the valence band in the middle of the forbidden band and the first peak S corresponding to the density of electronic states by the acceptor that can be in the immediate vicinity of the valence band. Is the ratio Q / S of the height of the second peak Q made by It would be accurate to obtain the ratio by integrating the area of the peak, but since it is not easy, measure the peak height and take the ratio. It is a new claim of the present invention that the change in the ratio value Q / S has a strong relationship with the change in the hydrogen concentration and resistivity, which dominates the resistance.

Of the samples A, B, C, and D having a boron content of 7000 ppm, the sample A has the lowest resistivity of 2 × 10 ⁻³ Ωcm. Sample B is twice that, Sample C is 16 times, and Sample D is about 50 times that of Sample A. What does such a difference come from? It can be associated with an increase in hydrogen content. The amount of hydrogen in sample A is 10 ppm, sample B is 8 times, sample C is 120 times, and sample D is 900 times. The resistance increases as the amount of hydrogen increases. However, the resistivity does not simply increase in proportion to the amount of hydrogen. In addition, the amount of hydrogen can only be measured by SIMS and is not easily understood. The resistivity can also be discussed by the ratio Q / S of the second peak Q to the first peak S.

  Sample A has a large second peak Q and a Q / S of 1.5. In that case, the resistivity is the lowest. In the sample B, the second peak Q is slightly lowered and the Q / S is 1.3. I believe that this increases the resistivity by a factor of two. Sample C has a lower second peak and a ratio Q / S of 0.9. This raises the resistance of the sample C by 16 times that of the sample A. Sample D has a lower second peak Q and a Q / S of 0.75. It is considered that this increases the resistance of the sample D to 50 times that of the sample A. Although it is not directly proportional, it seems that the size of the second peak increases the conductivity.

The same tendency is observed for samples E, F, G, and H having a boron content of 70,000 ppm. Sample E, which has the minimum amount of hydrogen at 10 ppm, has a resistivity of 5 × 10 ⁻⁴ Ωcm, indicating the minimum value among the 10 samples. It is inferred that this is because the amount of hydrogen is minimal and the amount of boron is large. Sample F having a hydrogen content of 100 ppm has a resistivity of 2.5 × 10 ⁻³ Ωcm, which is five times that of sample E. Since sample G has a hydrogen content of 900 ppm, it contains 90 times as much hydrogen as sample E. Since the resistivity is 6 × 10 ⁻³ Ωcm, the resistivity is 12 times that of the sample E. Sample H has a hydrogen content of 15000 ppm, 1500 times that of sample E. The resistivity of sample H is as high as 3 × 10 ⁻² Ωcm, which is 60 times that of sample E. This is also due to the large amount of hydrogen. Let's look at the relationship between resistivity and peak height ratio Q / S.

  Sample E has a peak ratio of 1.6, the largest of the four samples at 70000 ppm. Sample F has a peak ratio Q / S of 1.2, which is a little low. As hydrogen is reduced, the peak ratio is also reduced. The resistivity increased 5 times. Sample G has a peak ratio Q / S of 1.0. The peak ratio has decreased and the resistivity has increased 12 times. There is a tendency similar to what we have seen so far. In Sample H, the peak ratio Q / S is 0.8, and the second peak is further lowered. It can be seen that the resistivity increases 60 times and the relationship between the peak ratio and the resistivity is not directly proportional but has a strong relationship.

  Samples I and J are comparative examples, but the resistivity does not change much in the order of 0.1 Ωcm, although the hydrogen content is greatly different such as 10 ppm and 1000 ppm. In addition, the second peak Q in the forbidden band has disappeared. It appears where the peak ratio is 0 (Q / S = 0). That is, Q = 0. In the case of a sample in which the second peak does not appear in the forbidden band, the above-described property that the resistivity changes depending on the amount of hydrogen seems to be lost.

  This means that when the boron content is the same, the resistivity is greatly changed by the hydrogen content, but rather by the second peak. It can be inferred that the second peak reduces the resistivity.

  When will the second peak appear? This is the next problem, which appears in Samples A to D having a boron content of 7000 ppm, and in Samples I and J having a boron content of 5000 ppm, there is no second peak even if the hydrogen content is 10 ppm or 1000 ppm. The appearance of the second peak is estimated to be about 6000 ppm at the boundary. Therefore, the present invention can be applied to a diamond containing boron of 6000 ppm or more.

  The peak intensity ratio Q / S (x) and resistivity (y) were expressed in a semilogarithmic graph for samples A to D when the boron content was 7000 ppm and samples E to H when the boron content was 70000 ppm. Is FIG. Since the change in resistivity is extremely large, the scale is logarithmic, and the change in peak ratio Q / S is about 0 to 1.6, so the scale is used as it is. When these points are approximated by straight lines, they appear as two straight lines drawn downward to the right.

When boron content is 7000ppm
y = exp (−4.5x + 0.6) (1)

When boron content is 70000ppm
y = exp (−4.5x−0.3) (2)

The approximate expression is obtained. Since there is only this data, what happens when the boron content is other than 7000ppm and 70000ppm? I don't know from here. Then it seems to be good to take a lot of data, but it has a difficult problem. As described above, the measurement method for directly knowing the density of electronic states is the XAFS method (X-ray absorption fine structure) for obtaining the electronic state from the absorption spectrum by applying white (continuous) X-rays. It is about. White X-rays are X-rays of uniform power with continuous wavelengths, but there are not many such ideal light sources. A light source that utilizes the transition between excited states and ground states of atoms and molecules produces an electromagnetic wave with a specific wavelength corresponding to the energy difference between the states. There are X-ray sources that emit intense X-rays at isolated wavelengths. However, few X-ray light sources emit strong X-rays at continuous wavelengths. In order to emit continuous energy electromagnetic waves, transition between excited states of atoms and molecules must be avoided.

  For continuous X-rays, it is conceivable to use synchrotron radiation (SR), for example. It is an X-ray source utilizing the property that when an electron beam accelerated near the speed of light is bent by a magnet, an electromagnetic wave is generated by bremsstrahlung in the tangential direction of the bending. The power is not uniform but is generally uniform and has a continuous spectrum. In addition, the intensity is large and suitable for XAFS measurement. The SR optical device has higher radiant energy as the device is larger. The SR optical device in Nishiharima emits strong continuous X-rays including a considerable wavelength range, but the X-ray energy is too high. The difference between the energy of the electron orbit (1s electrons) at the lowest energy of diamond and the energy of the valence band and the conduction band is about 190 eV (6.5 nm). It is soft X-ray and has low energy. The Nishi-Harima SR device accelerates the electron beam to 8 GeV and thus emits high-energy continuous X-rays, but is unlikely to be a continuous low-energy X-ray light source of about 200 eV. Therefore, it is very difficult to directly measure the electronic density of diamond due to the limitation of the light source. Since the light source which emits such a convenient low-energy continuous light is very limited and there are many users who wish to use it, it takes a long time to wait for the light source to be free. Finally there may be no chance. Even if samples with various amounts of boron can be made, it is difficult to measure the density of electronic states and the data is limited.

Consider interpolation using only the above data. The resistivity has a dependency on the boron concentration b and a dependency on the peak ratio x. At a boron concentration of 5000 ppm, there is no dependency of the peak ratio x and high resistivity is shown, and at 7000 ppm and 70000 ppm, there is a dependency of the peak ratio x and low resistivity is shown. Since the resistivity y is difficult to understand, consider the reciprocal conductivity σ. The definition is σ = 1 / y. Now consider the conductivity dependent on the amount of hydrogen and let it be σ ₁ . The conductivity component not dependent on the hydrogen concentration will be described later.

When boron content is 7000ppm
σ ₁ = exp (+ 4.5x−0.6) (3)

When boron content is 70000ppm
σ ₁ = exp (+ 4.5x + 0.3) (4)

It becomes. The critical boron concentration is between 5000 and 7000 ppm.

(A) When the critical boron concentration is 5000 ppm Assuming that the critical boron concentration is 5000 ppm, let us assume that the conductivity σ ₁ increases from the critical boron concentration by the 1 / mth power of the difference value. This is considered to be a kind of phase transition. If so, it is presumed that there will be a discontinuous change at the critical point. The simplest non-continuous change is the multiple root approximation. Consider a unified formula with such an estimate. Then, the difference in boron concentration is 70000−5000 = 65000 ppm and 7000−5000 = 2000 ppm, and the value of the ratio of the expressions (3) and (4) is exp (0.3 + 0.6) = exp (0.9) = 2.459, so

(65000/2000) ^{1 / m} = 2.459 (5)

It becomes. The value in the left parenthesis is 32.5. Since log 32.5 = 1.511 and log 2.459 = 0.3907,

      m = 3.87 (6)

Or
1 / m = 0.258 (7)
And so on. Using this, we get a unified formula for σ ₁ .

σ ₁ = (b−5000) ^0.258 exp (+ 4.5x−2.56) (8)

Can be obtained. The constant in exp () is −2.56, which is determined from the continuous condition at b = 7000 ppm. b represents the boron (B) concentration in ppm. In this case, the critical value at which the dependence on hydrogen concentration appears is boron concentration b = 5000 ppm, but since there is a critical value between 5000 ppm and 7000 ppm, there is a possibility that it is different. The case where the critical values of boron concentration are 5500 ppm and 6000 ppm will be described below.

(B) When the critical boron concentration is 5500 ppm Assuming that the critical boron concentration is 5500 ppm, as in the previous example, it is assumed that the conductivity σ ₁ increases from the critical boron concentration by the 1 / mth power of the difference value. To do. Then, the difference in boron concentration is 70000-5500 = 64500 ppm and 7000-5500 = 1500 ppm. exp (0.3 + 0.6) = exp (0.9) = 2.459.
(64500/1500) ^{1 / m} = 2.459 (9)
It becomes. 43 in the left parenthesis. Since log 43 = 1.633 and log 2.459 = 0.3907,
m = 4.18 (10)
Or
1 / m = 0.239 (11)
And so on. Using this, we get a unified second equation for σ ₁ .
σ ₁ = (b-5500) ^0.239 exp (+ 4.5x−2.35)
(12)
Can be obtained.

(C) When the critical boron concentration is 6000 ppm Assuming that the critical boron concentration is 6000 ppm, as in the previous example, it is assumed that the conductivity σ ₁ increases from the critical boron concentration by the 1 / mth power of the difference value. To do. Then, the difference in boron concentration is 70000-6000 = 64000 ppm and 7000-6000 = 1000 ppm. exp (0.3 + 0.6) = exp (0.9) = 2.459.
(64000/1000) ^{1 / m} = 2.459 (13)
Should be. The number in the left parenthesis is 64. Since log64 = 1.806 and log2.459 = 0.3907,
m = 4.62 (14)
Or
1 / m = 0.216 (15)
And so on. Using this, we get a unified second equation for σ ₁ .
σ ₁ = (b−6000) ^0.216 exp (+ 4.5x−2.09) (16)
Can be obtained.

(8), (12), and (16) have the critical values set to 5000 ppm, 5500 ppm, and 6000 ppm, and the appearance is different. But there is not much change. From these equations, the conductivity of diamond having a boron concentration of 7000 ppm to 100000 ppm can be obtained from the peak ratio x obtained from the XAFS data.

This is a portion that depends on the hydrogen concentration x, but there are also portions that do not depend on the hydrogen concentration. Let it be σ ₀ (b). Samples I and J having a boron concentration of 5000 ppm contain only that portion. So the overall conductivity σ is

σ = σ ₀ (b) + σ ₁ (b, x) (17)

Can be expressed as follows. Put simply, the first term is the conduction by peak 1 (acceptor level), and the second term is the conduction by peak 2. x is the value Q / S of the ratio of peak 1 (S) divided by the height of peak 2 (Q), but peak 1 is a standard for normalization, and the action of peak 1 is included in the latter section. That is not to say. In samples I and J (comparative examples), only σ ₀ (b) is used, so the conductivity is low. Since A to H of the embodiment have the contribution of σ ₁ (b, x) in addition to σ ₀ (b), the conductivity is increased. This is not due to the acceptor level (peak 1, S level), but due to the Q level due to impurities formed at 2 eV from the top of the valence band. It disappears easily due to hydrogen contamination.

The plasma CVD diamond using hydrogen as a carrier gas, which has been tried conventionally, uses hydrogen as a carrier gas, so that hydrogen is mixed into the diamond. The conventional method was indifferent to the mixed hydrogen. However, according to the view of the present inventor, the hydrogen is an important factor determining the conductivity. By measuring the amount of hydrogen and reducing the amount of hydrogen, the Q level can be activated and the conductivity can be increased. Such knowledge is first discovered by the present invention. In order to increase the conductivity, it is only necessary to suppress the mixing of hydrogen, and a clear guide can be obtained.
Further, from the above equation, the conductivity of the sample can be obtained by XAFS with the boron concentration b known. It is preferable because it is a non-destructive inspection.

  High conductivity diamond can be obtained by reducing the amount of hydrogen contained in the boron-doped p-type plasma CVD diamond as much as possible. If a large amount of boron is included, the number of holes increases and the conductivity increases. This is not the case. Increasing the boron content destroys the diamond structure. Boron has a larger atomic radius than carbon in the diamond, and excess boron acts to destroy the diamond structure. Therefore, as disclosed in Patent Document 2, there was a case where the upper limit of the content of boron was 1000 ppm, which was expanded to 1000 ppm to 4500 ppm by some device. The present invention has a high doping of 7000 ppm and 70000 ppm above it.

  There is also a technique as described in Patent Document 1 in which the boron concentration is increased to 1000 ppm by co-doping with nitrogen. Since it is co-doped, holes do not increase excessively and conductivity does not increase. Patent Document 4 that substantially increases the hole concentration by co-doping so that boron is 10000 ppm higher than nitrogen increases the carrier concentration with the help of nitrogen. Non-Patent Document 2 says that a vacancy level having a strong ESR signal (high paramagnetic susceptibility) is generated around 2 eV, but it disappears when boron is added excessively. Therefore, it is said that this level will not appear unless the boron content is close to zero.

The present invention also claims the appearance of an impurity level Q around 2 eV from the top of the valence band. 2eV is common, but the essence is different. The vacancy level of Non-Patent Document 2 appears when boron is almost zero, and disappears when the boron doping amount is increased. The 2 eV Q level of the present invention is not so, and the boron concentration appears at a high concentration of about 6000 ppm or more. Moreover, it disappears due to excess hydrogen. More importantly, the Q level due to this 2 eV impurity generates holes and contributes to increasing the conductivity.
According to the knowledge of the present invention, a large Q level can be created by reducing the hydrogen content, and the Q level increases the conductivity. When hydrogen is used as a carrier gas in the plasma CVD method, hydrogen is included, but p-type diamond with high conductivity can be made if hydrogen is prevented from being taken into diamond as much as possible. It is. This is greatly different from Patent Documents 1, 2, 3, and 4 that only consider increasing the boron concentration in order to increase the conductivity.

  The present invention makes it possible to measure the electrical resistance of a diamond sample by XAFS. The conventional method of measuring electrical resistance, such as a four-terminal method, in which an electrode is placed on the surface of a sample is a destructive inspection, and there are cases where the electrode cannot always be placed ohmic. In the case of diamond, the measurement of conductivity is often not easy. The present invention makes it possible to measure conductivity and resistivity non-destructively with XAFS. As described above, there are still few continuous X-ray light sources of about 200 eV in the world, but this is not the case. Low energy continuous x-ray sources are easier to manufacture, install and maintain because they require smaller equipment than existing high x-ray sources. Many may be installed if necessary. In such a case, a method capable of non-destructively measuring the conductivity and resistivity by the X-ray absorption spectrum as in the present invention may be advantageous in terms of cost and labor.

According to an embodiment of the present invention, an electron state density diagram of an energy band including a valence band, a forbidden band, and a conduction band of Sample A having a hydrogen content of 10 ppm and a boron content of 7000 ppm, with the top of the valence band being 0 V. . There are an electronic state peak 1 (S) due to an acceptor in the vicinity of the valence band and an electronic state peak 2 (Q) due to an unknown impurity level at a height of 2 eV from the valence band. ) Is dominant, and the peak ratio x defined as the ratio of peak 2 / peak 1 is greater than 1.

According to an embodiment of the present invention, the density of states of an electron in an energy band including a valence band, a forbidden band, and a conduction band of Sample D having a hydrogen content of 9000 ppm and a boron content of 7000 ppm, where the top of the valence band is 0 V . There are an electronic state peak 1 (S) due to an acceptor in the vicinity of the valence band and an electronic state peak 2 (Q) due to an unknown impurity level at a height of 2 eV from the valence band. ) Is inferior, indicating that the peak ratio x defined as the ratio of peak 2 / peak 1 is less than 1.

The peak ratio x defined as x = peak 2 / peak 1 and the resistivity (Ωcm) of the samples according to the eight examples are plotted on a semi-logarithmic graph paper, and the plot points of four samples of boron 7000 ppm are The graph which shows that it can approximate with one right-falling straight line, and the plot point of four samples of boron 70000 ppm can be approximated with the straight-falling straight line parallel to the upper straight line.

Claims (10)

Created by high pressure synthesis method or CVD method using carbon raw material or hydrocarbon raw material and boron raw material without using nitrogen raw material, and containing 100% or less even if nitrogen is not contained or not, boron concentration is 6000ppm or more and hydrogen concentration is 20000ppm or less A boron-doped diamond characterized by having a first energy level (S) in the vicinity of 0 eV with the upper end of the valence band as a reference and a second energy level (Q) in the vicinity of 2 eV . When the hydrogen concentration is the same, the resistivity is controlled by controlling the hydrogen concentration by utilizing the fact that the state density of the second energy level increases and the resistivity decreases when the hydrogen concentration is low. Item 3. A boron-doped diamond according to Item 1. Created by high pressure synthesis method or CVD method using carbon raw material or hydrocarbon raw material and boron raw material without using nitrogen raw material, and containing 100% or less even if nitrogen is not contained or not, boron concentration is 6000ppm or more and hydrogen concentration is 20000ppm or less In the boron-doped diamond having the first energy level (S) near 0 eV and the second energy level (Q) near 2 eV with respect to the upper end of the valence band, the first electrons The electronic state density of the energy level (S) and the electronic state density of the second energy level (Q) are measured, and the second energy level (Q) is measured at the peak height of the first energy level (S). Of resistivity of boron-doped diamond, wherein the resistivity of diamond is calculated from a value x (= peak 2 / peak 1) obtained by dividing the peak height of γ and the boron concentration b Law. Resistivity y from the value x (= Q / S) of the ratio of the electron state density of the second energy level (Q) divided by the electron state density of the first energy level (S) when the boron concentration is 7000 ppm (Ωcm)
y = exp (−4.5x + 0.6)
The electrical resistance evaluation method for boron-doped diamond according to claim 3, wherein the electrical resistance is calculated by: The resistivity y from the ratio x (= Q / S) of the boron concentration of 70,000 ppm, which is the electronic state density of the first energy level (S) divided by the electronic state density of the second energy level (Q). (Ωcm)
y = exp (−4.5x−0.3)
The electrical resistance evaluation method for boron-doped diamond according to claim 3, wherein the electrical resistance is calculated by: The boron concentration b is 6000 ppm or more, the hydrogen concentration is 20000 ppm or less, and the ratio of the second peak (Q) and the first peak (S) of the measured value of the electronic state density is x, and the conductivity σ (= 1 / Y), the portion σ ₁ depending on the hydrogen concentration is σ ₁ = (b−5000) ^0.258 exp (+ 4.5x−2.56)
The electrical resistance evaluation method for boron-doped diamond according to claim 3, wherein the electrical resistance is calculated by: The boron concentration b is 6000 ppm or more, the hydrogen concentration is 20000 ppm or less, and the ratio of the second peak (Q) and the first peak (S) of the measured value of the electronic state density is x, and the conductivity σ (= 1 / Y), the portion σ ₁ depending on the hydrogen concentration is σ ₁ = (b-5500) ^0.239 exp (+ 4.5x−2.35)
The electrical resistance evaluation method for boron-doped diamond according to claim 3, wherein the electrical resistance is calculated by: The boron concentration b is 6000 ppm or more, the hydrogen concentration is 20000 ppm or less, and the ratio of the second peak (Q) and the first peak (S) of the measured value of the electronic state density is x, and the conductivity σ (= 1 / Y), the portion σ ₁ depending on the hydrogen concentration is σ ₁ = (b−6000) ^0.216 exp (+ 4.5x−2.09)
The electrical resistance evaluation method for boron-doped diamond according to claim 3, wherein the electrical resistance is calculated by: Using an apparatus that generates continuous X-rays including an energy range of 180 eV to 195 eV, X-rays having a continuous wavelength are applied to the diamond sample, and the electronic state density of the first energy level (S) and the second energy level from the absorption spectrum 4. The method for evaluating the electrical resistance of boron-doped diamond according to claim 3, wherein an electronic density of states (Q) is measured. It is characterized in that an electron beam of 195 eV or more is applied to a diamond sample, and the electronic state density of the first energy level (S) and the electronic state density of the second energy level (Q) are measured from the electron energy loss spectrum. The method for evaluating electrical resistance of boron-doped diamond according to claim 3.

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