JP4345437B2 - Method for producing n-type semiconductor diamond and n-type semiconductor diamond - Google Patents

Description

  The present invention relates to a method for producing an n-type semiconductor diamond and an n-type semiconductor diamond. More specifically, in a diamond vapor phase synthesis method on a substrate using a microwave plasma CVD apparatus, Li and N are introduced into the apparatus as impurity elements during synthesis, and impurity doping is performed simultaneously with synthesis of diamond. The present invention relates to a method for producing n-type semiconductor diamond.

Diamond is composed of carbon, which is an IVb group element of the same group as silicon (Si), which is widely used as a semiconductor material, and has a crystal structure similar to that of Si, and can be viewed as a semiconductor material. Diamond as a semiconductor material has a very large band gap of 5.5 eV, and carrier mobility is as high as 2000 cm ² / Vs at room temperature for both electrons and holes. Further, the dielectric constant is as small as 5.7, and the breakdown electric field is as large as 5 × 10 ⁶ V / cm. Furthermore, it has a rare characteristic of negative electron affinity in which the vacuum level exists below the lower end of the conduction band.

  Thus, diamond has excellent semiconductor characteristics, and therefore, an environment-resistant device that operates even in a high-temperature environment or space environment, a power device that can operate at high frequency and high output, a light-emitting device that can emit ultraviolet light, or Applications as semiconductor device materials such as electron-emitting devices that can be driven at low voltage are expected.

  In order to use a semiconductor material as a semiconductor device, p-type and n-type conductivity control is required. Such control is performed by doping the semiconductor material with impurities. For example, in the case of Si, if a silicon single crystal is doped with phosphorus, it becomes n-type and if boron is doped, it becomes p-type.

  Typical doping methods for adding such impurities include (a) a method of doping by adding an impurity element during crystal growth, (b) a thermal diffusion method of doping impurities by diffusion from the crystal surface, C) An ion implantation method in which accelerated impurity ions are implanted from the crystal surface. In order to apply the thermal diffusion method to diamond, it is difficult to realize the bond radius and bond energy between carbons. In addition, the ion implantation method requires an expensive ion implantation apparatus, and requires an annealing process for recovering defects after ion implantation and electrically activating impurities. For this reason, the (b) doping method having a relatively small number of steps is mainly used in experiments for producing semiconductor diamond.

  In the vapor phase synthesis of diamond, for example, hydrogen gas and methane gas are introduced into a synthesis furnace as raw materials, and after maintaining a suitable pressure, these gases are activated and activated species including carbon are deposited on a substrate. This is a method for growing diamond. There are various methods for activating the source gas, but currently widely used are a microwave plasma CVD (MPCVD) method and a hot filament CVD (HFCVD) method that allow a relatively simple apparatus configuration.

  Among these, the MPCVD method excites the source gas by non-polar discharge, so that high quality diamond can be synthesized since there is no mixing of electrode materials. Further, in the HFCVD method, for example, a tungsten wire is energized and heated, and the source gas is excited by radiated thermoelectrons, so that tungsten is slightly mixed in the diamond, but diamond can be grown on a substrate of any shape. . For the synthesis of semiconductor diamond, the MPCVD method is used which contains less unintended impurities.

For example, boron-doped p-type semiconductor diamond can be produced by introducing hydrogen gas, methane gas, and diborane (B ₂ H ₆ ) gas into the synthesis apparatus by MPCVD. The phosphorus-doped n-type semiconductor diamond can be produced by introducing hydrogen gas, methane gas, and phosphine (PH ₃ ) gas into the synthesis apparatus by MPCVD.

As described above, p-type and n-type semiconductor diamond can be synthesized by the MPCVD method. However, the resistivity of phosphorus-doped n-type semiconductor diamond is as high as about 10 ⁴ Ωcm. In addition, since the covalent bond radius of carbon of diamond is 0.077 nm, and the covalent bond radius of phosphorus (P) is 0.106 nm, the phosphorus-doped n-type semiconductor diamond grows to a thickness of about 10 μm. Then, the diamond will crack. This is not much different in sulfur (S) -doped n-type semiconductor diamond. For these reasons, search for dopants for forming n-type semiconductor diamond has been underway.

  In addition to P and S, lithium (Li) can be used as a dopant for forming the n-type semiconductor diamond. Li is mixed between lattices in the diamond crystal and becomes n-type. However, since the ionic radius of Li is 0.060 nm, it is promising in that cracks are unlikely to occur.

  For example, Japanese Laid-Open Patent Application No. 03-205398 discloses a method for synthesizing diamond doped with Li by using a simple substance of Li or a Li compound (lithium oxide, lithium hydroxide, lithium chloride, lithium ethylate, etc.). A method is disclosed in which a liquid organic compound (acetone, methanol, ethanol, acetaldehyde, or the like) is introduced into a CVD apparatus by gasification with a liquid vaporizer. However, in this method, Li alone or Li compound and liquid organic compound are easily separated, and only the liquid organic compound is vaporized, and Li alone or Li compound remains in the liquid vaporizer as an unvaporized residue. Therefore, there is a problem in supplying Li stably.

  Japanese Patent Application Laid-Open No. 04-175295 discloses a method of heating and evaporating a Li source (Li simple substance, lithium oxide, lithium hydroxide, lithium chloride, lithium fluoride, etc.) and introducing it into a CVD apparatus. However, since such a Li source is difficult to gasify, it is difficult to introduce a precisely controlled amount of Li into the CVD apparatus.

Furthermore, Japanese Patent Application Laid-Open No. 11-054443 discloses a method for introducing a vapor of a Li compound solution (such as an ethanol solution or an aqueous solution of lithium acetate). However, even in this method, the Li compound is easily generated as a residue, and there is still a problem in the stable supply of Li. Japanese Patent Laid-Open No. 7-106266 discloses a method of doping Li into diamond using ECR plasma.
Japanese Patent Laid-Open No. 03-205398 Japanese Patent Laid-Open No. 04-175295 Japanese Patent Laid-Open No. 11-054443 Japanese Patent Application Laid-Open No. 07-106266

  Thus, although Li-doped diamond is promising as an n-type semiconductor diamond, there is a problem that a simple, safe and reliable method for stably introducing Li into a CVD apparatus has not been realized. In the gas phase synthesis of diamond, in view of avoiding as much as possible the mixing of unintended elements into the CVD apparatus, the Li source is composed of at most four kinds of elements of Li, C, H, and O. It is desirable.

  Further, the conventional Li-doped diamond has a problem that Li moves around in the diamond crystal and stable electric characteristics cannot be obtained. Furthermore, depending on the synthesis conditions, there is a problem that Li is combined with H in the diamond and becomes electrically inactive.

  The present invention has been made in order to solve the above-mentioned problems, and a simple, safe and reliable method for stably introducing Li into a CVD apparatus in order to synthesize n-type semiconductor diamond. It is another object of the present invention to provide a technique for obtaining stable electrical characteristics and an n-type semiconductor diamond.

  The method for producing n-type semiconductor diamond according to the present invention comprises adding Li and N sublimated from Li (dpm) as impurity elements during synthesis in a diamond gas phase synthesis method on a substrate using an MPCVD apparatus. However, diamond is synthesized at a pressure of 2.7 kPa or more and a temperature of 700 ° C. or more.

  According to the method for producing an n-type semiconductor diamond of the present invention, a high-quality n-type semiconductor diamond doped with Li and N can be produced. Since such n-type semiconductor diamond has excellent semiconductor characteristics, it is an environment-resistant device that operates in a high-temperature environment or space environment, a power device that can operate at a high frequency and a high output, and light emission that can emit ultraviolet light. It can be applied as a material for a semiconductor device such as a device or an electron emission device that can be driven at a low voltage.

  The inventors of the present invention have described a safe and reliable method for stably introducing Li into a CVD apparatus in order to synthesize n-type semiconductor diamond, and a synthesis method in which Li-doped n-type semiconductor diamond has stable electrical characteristics. We conducted intensive research.

As a result, it has been found that in the conventional synthesis method, nitrogen gas may be added during synthesis in order to fix Li that moves around in the diamond crystal. Li and N are easily bonded to each other so that lithium nitride (Li ₃ N) exists as a stable Li nitrogen compound. The inventor examined the application of such properties of Li and N to n-type doping by the MPCVD method.

  As a result, if Li and N with sublimated Li (dpm) were added during synthesis and diamond was synthesized at a pressure of 2.7 kPa or higher and a temperature of 700 ° C. or higher, Li and N paired, It has been found that as a result of N being in the carbon substitution position of the diamond crystal and Li being in the interstitial position in the vicinity of N, and fixing Li with N, stable electrical characteristics can be obtained.

  It has been found that the method of using Li (dpm) as the Li source, sublimating the Li (dpm) and introducing it into the MPCVD apparatus is a simple, safe and reliable method for stably introducing Li.

Li (dpm) is commercially available as an MOCVD material for a ferroelectric thin film, and can be easily obtained. According to MSDS (Material Safety Data Sheet), Li (dpm) itself does not fall under the category of hazard and is a relatively safe substance. Moreover, since it sublimes even at a temperature of 280 ° C. or lower, which is the melting point of Li (dpm), it is not necessary to devise any special device in the heating device. Furthermore, most importantly, Li (dpm) is composed of Li, C, H, and O, as shown by LiC ₁₁ H ₁₉ O ₂ , so that Li (dpm) If it is used as a Li source, there is no concern of unnecessary impurities being doped. The present inventors have found an advantage of using Li (dpm) as a Li source for n-type semiconductor diamond synthesis.

  Li (dpm) supported on porous ceramic grains is better as a stable source of Li. Since the surface area of Li (dpm) hardly changes during use because it is supported on the porous ceramic grains, the temporal variation in the sublimation amount of Li becomes very small, and the Li Stable supply becomes possible. Li (dpm) supported on porous ceramic grains is commercially available and can be easily obtained.

  Depending on the synthesis conditions, Li may combine with H in the diamond crystal and become electrically inactive. However, if the pressure in the MPCVD apparatus is set to 2.7 kPa or more and the temperature of the base material is 700 ° C. or more, Li and N are paired. Therefore, Li and H are combined to be electrically inactive. I found out that there was no such thing.

The N raw material is not limited to nitrogen gas, and can be used as long as it is composed of four elements of N, C, H, and O at most. As such a raw material, ammonia (NH ₃ ) is one of materials that can be suitably used.

As described above, according to the method for producing an n-type semiconductor diamond of the present invention, a low-resistance n-type semiconductor diamond containing Li and N and having a resistivity of 10 ³ Ωcm or less can be obtained.

  FIG. 1 is a schematic diagram showing an example of an MPCVD apparatus used for carrying out the present invention. First, the diamond single crystal substrate 1 is set on the substrate holder 3 in the vacuum chamber 2 made of SUS. As the diamond single crystal substrate, Ib type diamond synthesized at high temperature and high pressure was used. The size is 2 mm × 2 mm × 0.3 mm. The plane orientation of the 2 mm × 2 mm plane was {100}.

After setting the diamond single crystal substrate, the inside of the vacuum chamber 2 is preliminarily evacuated to 1.3 × 10 ⁻⁴ Pa (10 ⁻⁶ torr) from the vacuum exhaust port 4. Next, the raw material is introduced into the vacuum chamber and kept at a predetermined pressure. The raw materials are H ₂ , CH ₄ , N ₂ , and Li (dpm). H ₂ , CH ₄ , and N ₂ are each adjusted to a predetermined flow rate by the mass flow controllers 8, 9, and 10 from the respective cylinders 5, 6, and 7, and are introduced into the vacuum chamber 2 from the gas inlet 11. . N ₂ was diluted with H _{2 at} 1%.

Li (dpm) is a Li (dpm) 12 which is supported on a powder or a porous ceramic grains placed in a sublimation apparatus 13, heated, sublimated, _{H 2} was adjusted to a predetermined flow rate by the mass flow controller 15 than _{H 2} gas cylinder 14 It introduce | transduces in a vacuum chamber from the gas inlet 11 with carrier gas. The H ₂ carrier gas is heated to the sublimation temperature of Li (dpm) by the heater 16 and then introduced into the sublimation apparatus 13. The sublimation device 13 can arbitrarily adjust the internal pressure by the pressure adjustment valve 17. The gas piping from the sublimation device 13 to the vacuum chamber 2 can be controlled in temperature according to the sublimation temperature.

  Then, a microwave 19 of 2.45 GHz is introduced into the vacuum chamber 2 through a quartz window 18 for microwave introduction provided on the upper portion of the vacuum chamber 2, and a source gas plasma 20 is introduced into the upper portion of the diamond single crystal substrate 1. generate.

  Thereby, the temperature of the diamond single crystal substrate 1 rises and reaches a predetermined temperature. After reaching the predetermined temperature, the temperature of the diamond single crystal substrate is controlled by adjusting the power of the microwave. The raw material active species generated by the plasma 20 reaches the diamond single crystal substrate, and diamond doped with Li and N is epitaxially grown on the diamond single crystal substrate 1.

  The diamond thus prepared and doped with Li and N was evaluated. First, sample No. 1 was determined by scanning electron microscope (SEM), Raman spectroscopic analysis, reflection high-energy electron diffraction (RHEED). 1 to 6 were confirmed to be high-quality epitaxially grown diamonds. Next, as an electrical property evaluation, Hall effect measurement by the van der Pauw method was performed, and the carrier type and the resistivity at room temperature (27 ° C.) were measured. Moreover, the measurement of the density | concentration and film thickness of Li and N in a diamond crystal was performed by secondary ion mass spectrometry (SIMS). Further, for H in the diamond crystal, the secondary ion count rate was measured by SIMS.

  In the Hall effect measurement, the electrode was formed as follows. First, an area of 200 μm in diameter at the four corners of the surface of the epitaxial layer of diamond doped with Li and N by Ar ion implantation is graphitized, and an electron beam of Ti, Pt, and Au is sequentially deposited into the graphitized area by 100 nm each. The ohmic contact was formed by vapor deposition and annealing at 400 ° C. for 20 minutes.

The synthesis conditions are shown in Table 1, and the evaluation results are shown in Table 2. Sample No. In Nos. 1, 2, 5 and 6, Li (dpm) powder was used. In 3 and 4, Li (dpm) supported on porous ceramic grains was used. In Table 1, H ₂ , CH ₄ , N ₂ (H ₂ dilution 1%), and H ₂ carrier gas indicate respective gas flow rates, and the unit is sccm (Standard cc / min).

From Tables 1 and 2, No. In Nos. 1 to 4, it was confirmed that Li and N were electrically activated to form a low-resistance n-type semiconductor diamond. The count rate of H in these diamonds is 4 × 10 ³ cps, which is basically the same as the count rate of high temperature and high pressure synthetic diamond single crystals containing no H. Samples 1 to 4 are considered not to contain H. In contrast, no. The samples 5 and 6 are mixed with H, and Li and H are combined to become electrically inactive. Therefore, it is considered that the resistivity is increased.

  Since the n-type semiconductor diamond of the present invention has excellent semiconductor characteristics, it is an environment-resistant device that operates even in a high-temperature environment or in a space environment, a power device that can operate at high frequency and high power, and light emission that can emit ultraviolet light. It can be applied as a material for a semiconductor device such as a device or an electron emission device that can be driven at a low voltage.

It is a schematic diagram which shows an example of the microwave plasma CVD apparatus used for implementation of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Vacuum chamber 3 Substrate holder 4 Vacuum exhaust port 5, 6, 7, 14 Cylinder 8, 9, 10, 15 Mass flow controller 11 Gas inlet 12 Li (dpm)
13 Sublimation device 16 Heater 17 Pressure adjustment valve 18 Quartz window 19 Microwave 20 Plasma

Claims (1)

  In a gas phase synthesis method of diamond on a substrate using a microwave plasma CVD apparatus, while adding Li and N sublimated Li (dpm) as impurity elements during synthesis, a pressure of 2.7 kPa or more A method for producing an n-type semiconductor diamond, comprising synthesizing diamond at a temperature of 700 ° C. or higher.

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