JP4241174B2 - Low resistance n-type semiconductor diamond - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low resistance n-type semiconductor diamond doped with both lithium and phosphorus. More specifically, the present invention relates to a low-resistance n-type semiconductor diamond in which both lithium atom concentration C _Li and phosphorus atom concentration C _P in diamond are 10 ¹⁷ cm ⁻³ or more and C _Li ≦ C _P.
[0002]
[Prior art]
Conventionally, research for utilizing diamond as a semiconductor material and applying it as a semiconductor device has been advanced.
[0003]
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 / V · s 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.
[0004]
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.
[0005]
In order to use diamond as a semiconductor device material, it is necessary to control p-type and n-type conductivity. The p-type semiconductor diamond can be obtained by adding boron as an impurity so as to replace the carbon atom of the diamond crystal. P-type semiconductor diamond also exists in nature, and can be synthesized relatively easily by introducing a gas containing boron atoms into a source gas by a chemical vapor synthesis (CVD) method.
[0006]
On the other hand, n-type semiconductor diamond does not exist in nature and has been considered difficult to synthesize until now. However, in recent years, the synthesis conditions have been optimized in microwave plasma CVD using phosphorus and sulfur as dopants. As a result, single crystal n-type semiconductor diamond having relatively good crystallinity is obtained. Further, a pn junction is formed by combining these n-type semiconductor diamond doped with phosphorus or sulfur and a p-type semiconductor diamond doped with boron, and an ultraviolet light-emitting device has been prototyped.
[0007]
However, even in the n-type semiconductor diamond doped with phosphorus or sulfur having the best performance among the single crystal n-type semiconductor diamonds with good crystallinity currently realized, the resistivity at room temperature is about 10 ⁴ Ω · cm, which is boron. Compared with the p-type doped with n-type, it has a high resistance of about 3 to 5 digits. Compared with other semiconductor materials, it is no exaggeration to say that it is an insulator. The activation energy obtained from the temperature dependence of the carrier concentration of these n-type semiconductor diamonds is about 0.6 eV for phosphorus doped and about 0.4 eV for sulfur doped. Therefore, it is difficult to stably use devices using these n-type semiconductor diamonds over a wide temperature range.
[0008]
Nitrogen is another dopant that has been confirmed to have n-type semiconductor properties in addition to phosphorus and sulfur. Although diamond containing nitrogen is naturally present and n-type has been confirmed, the activation energy of nitrogen-doped diamond is about 1.7 eV, and the resistivity at room temperature is 10 ¹⁰ Ω. -It is an insulator with cm or more.
[0009]
Furthermore, it is known that when lithium is added, n-type semiconductor characteristics are exhibited. However, lithium is not doped in the substitution position with the carbon atoms constituting the diamond like phosphorus, sulfur and nitrogen, but is doped in the interstitial positions of the carbon atoms. Regarding the method of doping lithium, for example, JP-A-3-205398, JP-A-4-174517, JP-A-4-348514, JP-A-7-106266, JP-A-11-54443, etc. Various methods have been proposed.
[0010]
The activation energy of the n-type semiconductor diamond doped with lithium is estimated to be 0.1 to 0.3 eV, and is expected to be a donor having a relatively shallow level and at the same time having a low resistance. However, there is a problem that lithium existing between diamond lattices is easily diffused and unstable, and the electric characteristics of the n-type semiconductor diamond doped with lithium are not stable.
[0011]
[Patent Document 1]
Japanese Patent Laid-Open No. 03-205398 [Patent Document 2]
JP 04-174517 A [Patent Document 3]
Japanese Patent Application Laid-Open No. 04-348514 [Patent Document 4]
Japanese Patent Laid-Open No. 07-106266 [Patent Document 5]
Japanese Patent Laid-Open No. 11-054443
[Problems to be solved by the invention]
The present invention has been made to solve the above problems. That is, the present invention provides high resistivity at room temperature due to the deep impurity level of phosphorus, which is a substitution donor, and instability of electrical characteristics of n-type semiconductor diamond due to the fact that lithium, which is an interstitial donor, is not stable in diamond. An object is to provide a low-resistance n-type semiconductor diamond which is simultaneously solved.
[0013]
[Means for Solving the Problems]
In the low resistance n-type semiconductor diamond of the present invention, the lithium atom concentration C _Li and the phosphorus atom concentration C _P are both 10 ¹⁷ cm ⁻³ or more and C _Li ≦ C _P. The lithium atom is preferably doped in the interstitial position of the carbon atoms constituting the diamond, the phosphorus atom is preferably doped in the carbon atom and the substitution position, and the lithium atom and the phosphorus atom are adjacent to each other. It is further preferable to have
[0014]
Furthermore, it is preferable that the center distance between the lithium atom and the phosphorus atom is 0.190 nm or more and 0.200 nm or less. These low resistance n-type semiconductor diamonds are preferably single crystal diamonds. The activation energy of the donor is preferably 0.1 eV or less and the resistivity is preferably 10 ³ Ω · cm or less.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
In order to obtain a low-resistance n-type semiconductor diamond with good crystallinity, the inventor doped 10 ¹⁷ cm ⁻³ or more of lithium and phosphorus simultaneously in the diamond, and the lithium atom concentration C _Li and phosphorus in the diamond It has been found that the atomic concentration C _P may be C _Li ≦ C _P. In this way, it has been found that the problems associated with doping lithium or phosphorus alone can be solved.
[0016]
When both lithium and phosphorus are less than 10 ¹⁷ cm ⁻³ , n-type electrical characteristics cannot be obtained. In addition, when the lithium atom concentration is less than 10 ¹⁷ cm ⁻³ and the phosphorus atom concentration is 10 ¹⁷ cm ⁻³ or more, the same n-type semiconductor characteristics as when doped with phosphorus alone are obtained. Conversely, when the lithium atom concentration is 10 ¹⁷ cm ⁻³ or more and the phosphorus atom concentration is less than 10 ¹⁷ cm ⁻³ , the same electrical characteristics as when doped with lithium alone are obtained, and it is not stable. Furthermore, although both lithium and phosphorus are 10 ¹⁷ cm ⁻³ or more, when the lithium atom concentration exceeds the phosphorus atom concentration (C _Li > C _P ), the electrical characteristics are also unstable.
[0017]
When only lithium is doped, the doped lithium moves between the diamond lattices as described above and is unstable. However, if phosphorus is doped at the same time with lithium as the carbon atoms constituting the diamond, the interstitial sites are doped. It has been found that a low resistance n-type semiconductor diamond having good crystallinity and electrically stable can be obtained without movement of doped lithium. When lithium and phosphorus are doped in this way, lithium atoms are fixed by phosphorus atoms, so that the electrical characteristics are stabilized and the activation energy of carriers can be greatly reduced.
[0018]
The distance between the centers of lithium atoms and phosphorus atoms is preferably 0.190 nm or more and 0.200 nm or less. If it is less than 0.190 nm or exceeds 0.200 nm, it becomes difficult to dope lithium and phosphorus simultaneously.
[0019]
Here, in order to predict the activation energy of the n-type semiconductor diamond, a simulation based on first-principles calculation was performed for the case where lithium interstitial doping and phosphorus substitution doping were combined. The first-principles calculation also simulated the position and crystal stability of lithium and phosphorus atoms in diamond.
[0020]
As a result, it was found that the structure in which the phosphorous atom existing at the substitution position and the lithium atom existing at the interstitial position are closer to each other than the case where they are separated from each other is superior in crystal stability. . The activation energy in this case was calculated to be 0.10 eV or less. Further, the distance between the lithium atom and the phosphorus atom having the optimum structure was 0.195 nm.
[0021]
When the distance between the lithium atom and the phosphorus atom is less than 0.190 nm, the crystal state is unstable, that is, it is difficult for the lithium atom and the phosphorus atom to be doped simultaneously in the diamond by the technique of doping during CVD. Further, it was found that when the length is longer than 0.200 nm, the activation energy is larger than 0.10 eV, and the temperature dependency of the resistivity is increased.
[0022]
As described above, according to the present invention, a low-resistance n-type semiconductor single crystal diamond doped with lithium and phosphorus simultaneously can be obtained.
[0023]
【Example】
Example 1
A IIa diamond single crystal substrate synthesized at high temperature and high pressure was prepared. The size was 2 mm × 2 mm × 0.3 mm, and the surface orientation of the upper and lower surfaces of 2 mm square was {111}. A non-doped diamond layer was formed on this diamond single crystal substrate by microwave plasma CVD, and a diamond layer doped with lithium and phosphorus was further formed thereon. Two types of samples were prepared: a sample for measuring the impurity concentration in the doped layer and a sample for measuring electrical characteristics. The synthesis conditions of the non-doped diamond layer and the doped diamond layer are shown below.
[0024]
(Non-doped diamond layer)
Hydrogen flow rate: 500sccm
Methane (CH ₄ ) flow rate: 1.0 sccm
Diamond substrate temperature: 950 ° C
Pressure: 13.3 kPa (100 torr)
Film thickness: about 2μm
[0025]
(Dope diamond layer)
Hydrogen flow rate: 1000sccm
Methane (CH ₄ ) flow rate: 1.0 sccm
Phosphine (PH ₃ ) (hydrogen dilution 1000 ppm) Flow rate: 1.0 sccm
Lithium tert-butoxide ((CH ₃ ) ₃ COLi) (carrier gas H ₂ )
Flow rate: 10sccm
Diamond substrate temperature: 900 ° C
Pressure: 13.3 kPa (100 torr)
Film thickness: about 1μm
[0026]
After the synthesis, the diamond single crystal substrate was taken out, and the portion grown on the single crystal substrate was measured by a scanning electron microscope (SEM), Raman spectroscopic analysis, and reflection high-energy electron diffraction (RHEED). As a result, it was confirmed that the portion grown on the diamond single crystal substrate was an epitaxially grown diamond single crystal, and a doped diamond layer was formed on the non-doped diamond layer. This epitaxially grown diamond single crystal had no cracks.
[0027]
Further, when a doped diamond layer of the obtained epitaxial single-crystal diamond was secondary ion mass spectrometry (SIMS), lithium ^7.2x10 18 ^{cm -3,} phosphorus is ^9.0x10 18 ^{cm -3} detection. Furthermore, the following analysis was performed in order to identify the mixing position of lithium atoms and phosphorus atoms in the epitaxial diamond single crystal.
[0028]
Lithium and phosphorus lattices by channeling measurements for Rutherford backscattering analysis (RBS), particle-induced X-ray emission analysis (PIXE), and nuclear reaction analysis (NRA). The substitution rate was determined. As a result, it was found that the lattice substitution rate of lithium was 5% or less, and the lattice substitution rate of phosphorus was 95% or more. From this result, it was found that most of the lithium in the diamond exists between the lattices, and phosphorus exists in the substitution position.
[0029]
Next, the electrical characteristics of the epitaxial diamond single crystal formed in the present invention were measured. By oxidizing the surface of the diamond single crystal, the surface is oxygen-terminated, and after removing the electrically conductive layer on the diamond surface, ohmic contact electrodes are formed at the four corners of the diamond single crystal, and holes are formed by van der Pauw method. The effect was measured. As a result, the doped diamond layer is n-type, the activation energy is 0.09 eV, the resistivity at room temperature (300 K) is 2.3 Ωcm, and it is confirmed that diamond is a low-resistance n-type semiconductor. did.
[0030]
During the measurement of the Hall effect, the temperature of the diamond single crystal was raised to 873 K, but its electrical characteristics were very stable. This indicates that the lithium atom existing between the lattices is fixed by the phosphorus atom existing at the substitution position.
[0031]
From the above results, it is confirmed that the diamond in which the lithium atom concentration C _Li and the phosphorus atom concentration C _P are both 10 ¹⁷ cm ⁻³ or more and C _Li ≦ C _P is a low-resistance n-type semiconductor diamond. did it. In addition, it was found that most of the lithium atoms in the low resistance n-type semiconductor diamond prepared were present at the interstitial positions of the carbon atoms constituting the diamond, and most of the phosphorus atoms were present at the carbon atom substitution positions. . Furthermore, the electrical characteristics of the prepared low resistance n-type semiconductor diamond were very stable. From these results, it was predicted that the lithium atom and the phosphorus atom are in the close state as predicted by the first principle calculation, and the interatomic distance is 0.190 nm or more and 0.200 nm or less.
[0032]
Comparative Example 1
A doped diamond layer was formed on the non-doped diamond layer in the same manner as in Example 1 except that the synthesis conditions of the doped diamond layer were as follows.
[0033]
(Dope diamond layer)
Hydrogen flow rate: 1000sccm
Methane (CH ₄ ) flow rate: 1.0 sccm
Phosphine (PH ₃ ) (hydrogen dilution 10 ppm) Flow rate: 1.0 sccm
Lithium tert-butoxide ((CH ₃ ) ₃ COLi) (carrier gas H ₂ )
Flow rate: 1.0sccm
Diamond substrate temperature: 900 ° C
Pressure: 13.3 kPa (100 torr)
Film thickness: about 1μm
[0034]
When the prepared sample was subjected to impurity concentration measurement by SIMS in the same manner as in Example 1, both the lithium atom concentration and the phosphorus atom concentration were less than the measurement limit and less than 10 ¹⁷ cm ⁻³ . The resistivity at room temperature was 10 ⁶ Ω · cm, which was very high resistance.
[0035]
Comparative Example 2
A doped diamond layer was formed on the non-doped diamond layer in the same manner as in Example 1 except that the synthesis conditions of the doped diamond layer were as follows.
[0036]
(Dope diamond layer)
Hydrogen flow rate: 1000sccm
Methane (CH ₄ ) flow rate: 1.0 sccm
Phosphine (PH ₃ ) (hydrogen dilution 1000 ppm) Flow rate: 1.0 sccm
Lithium tert-butoxide ((CH ₃ ) ₃ COLi) (carrier gas H ₂ )
Flow rate: 1.0sccm
Diamond substrate temperature: 900 ° C
Pressure: 13.3 kPa (100 torr)
Film thickness: about 1μm
[0037]
The prepared sample was subjected to impurity concentration measurement by SIMS in the same manner as in Example 1. As a result, the lithium atom concentration was less than 10 ¹⁷ cm ^−3, which is below the measurement limit, but the phosphorus atom concentration was 6.5 × 10 ¹⁸ cm. ^-3 . In addition, the Hall effect measurement was performed and it was confirmed that the conductivity type of the doped layer was n-type, but the resistivity at room temperature was as high as 10 ⁴ Ω · cm, and the activation energy of the carrier was 0.63 eV. there were.
[0038]
Comparative Example 3
A doped diamond layer was formed on the non-doped diamond layer in the same manner as in Example 1 except that the synthesis conditions of the doped diamond layer were as follows.
[0039]
(Dope diamond layer)
Hydrogen flow rate: 1000sccm
Methane (CH ₄ ) flow rate: 1.0 sccm
Phosphine (PH ₃ ) (hydrogen dilution 10 ppm) Flow rate: 1.0 sccm
Lithium tert-butoxide ((CH ₃ ) ₃ COLi) (carrier gas H ₂ )
Flow rate: 10.0sccm
Diamond substrate temperature: 900 ° C
Pressure: 13.3 kPa (100 torr)
Film thickness: about 1μm
[0040]
When the impurity concentration measurement by SIMS was performed on the prepared sample in the same manner as in Example 1, the lithium atom concentration was 7.4 × 10 ¹⁸ cm ⁻³ , but the phosphorus atom concentration was 10 ¹⁷ cm ⁻³ below the measurement limit. Was less than. In addition, the Hall effect measurement was performed, but the measurement was not possible due to unstable electrical characteristics.
[0041]
Comparative Example 4
A doped diamond layer was formed on the non-doped diamond layer in the same manner as in Example 1 except that the synthesis conditions of the doped diamond layer were as follows.
[0042]
(Dope diamond layer)
Hydrogen flow rate: 1000sccm
Methane (CH ₄ ) flow rate: 1.0 sccm
Phosphine (PH ₃ ) (hydrogen dilution 1000 ppm) Flow rate: 1.0 sccm
Lithium tert-butoxide ((CH ₃ ) ₃ COLi) (carrier gas H ₂ )
Flow rate: 30.0sccm
Diamond substrate temperature: 900 ° C
Pressure: 13.3 kPa (100 torr)
Film thickness: about 1μm
[0043]
The prepared sample was subjected to impurity concentration measurement by SIMS in the same manner as in Example 1. As a result, the lithium atom concentration was 2.1 × 10 ¹⁹ cm ⁻³ and the phosphorus atom concentration was 8.6 × 10 ¹⁸ cm ⁻³ . Although the Hall effect measurement was performed, the measurement was not possible due to unstable electrical characteristics.
[0044]
【The invention's effect】
According to the present invention, both lithium atoms and phosphorus atoms are doped to 10 ¹⁷ cm ⁻³ or more in diamond, and the lithium atom concentration C _Li and the phosphorus atom concentration C _P are set to C _Li ≦ C _P. It is possible to obtain an n-type semiconductor diamond having a low resistance which is not present in the present invention. By using such a low resistance n-type semiconductor diamond, 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, a light-emitting device that can emit ultraviolet light, or Semiconductor devices such as electron-emitting devices that can be driven at a low voltage can be put to practical use.

Claims (4)

A lithium atom concentration _{C Li} and a phosphorus atom concentration _{C P} is at both ¹⁰ 17 ^{cm -3} or more _and I _{C Li} ≦ _{C P} der, resistivity Oh in the following single crystal diamond 10 ³ Ω · cm A low-resistance n-type semiconductor diamond.   2. The low resistance n-type semiconductor diamond according to claim 1, wherein lithium atoms are doped at interstitial positions of carbon atoms constituting the diamond, and phosphorus atoms are doped at substitution positions with the carbon atoms.   The low resistance n-type semiconductor diamond according to claim 2, wherein the lithium atom and the phosphorus atom are adjacent to each other. 4. The low resistance n-type according to claim 1, wherein the carrier activation energy obtained from the temperature dependence of the carrier concentration obtained by measuring the Hall effect is 0.1 eV or less. 5. Semiconductor diamond.