JP2952342B2 - n-type diamond semiconductor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an n-type diamond semiconductor.

[0002]

2. Description of the Related Art Conventionally, the development of a diamond semiconductor having a diamond structure of a tetravalent carbon crystal has been delayed compared to a silicon semiconductor having a diamond structure of the same tetravalent silicon crystal.

[0003]

This is due to the fact that suitable impurities, especially donor atoms, have not been found in diamond semiconductors.

A conventional theory used for finding a donor atom or an acceptor atom will be described with respect to a silicon semiconductor. An n-type silicon semiconductor is a single crystal of silicon in which pentavalent phosphorus (P) or arsenic ( It is known that it can be obtained by adding As) atoms as trace impurities.

[0005] As shown in the conceptual diagram of FIG.
Some of the silicon atoms in the diamond structure of the tetravalent silicon crystal are replaced by pentavalent phosphorus (P) or arsenic (As) atoms and covalently bond with the four outermost electrons of the pentavalent atom It is understood that the diamond structure is maintained, and the remaining one electron supplies electrons to the silicon crystal, and the impurity atoms themselves become positively charged and become an n-type semiconductor.

The positively charged impurity atoms that supply the electrons are called donor atoms. The electrons supplied to the silicon single crystal have only a small energy (kT of about room temperature).
Is released from the bond from the silicon atom, and can move to the next silicon atom, and becomes a so-called conduction electron.

It is known that a p-type silicon semiconductor can be obtained by adding trivalent boron (B), aluminum (Al), or gallium (Ga) atoms as trace impurities to a single crystal of silicon. .

In this case, as shown in the conceptual view of FIG. 1C, trivalent atoms are partially substituted for silicon atoms in the diamond structure of the tetravalent silicon crystal, and trivalent atoms are adjacent to tetravalent atoms. Is covalently bonded to the four outermost electrons of the silicon atom, and one covalent electron is lacking, but receives one electron from the single crystal of silicon to form a covalent bond and maintain the diamond structure, It is understood that the impurity atoms themselves are negatively charged by the amount of receiving one electron and become a p-type semiconductor.

The impurity atoms that receive the electrons are called acceptor atoms, but holes are formed in the silicon single crystal to which the electrons are supplied, and in this case, only a small amount of energy (kT at room temperature or so) is generated. It is understood that the electrons released from the binding of the adjacent silicon atoms are supplied to this hole, and the hole can move to the next atom.

According to the consideration based on such a conventional theory, when a tetravalent carbon (C) atom is added as a small amount of impurity to a single crystal of silicon, a single bond of SiC is generated.
d) is a covalent bond composed of tetravalent Si atoms and C atoms, and both atoms were considered to be neutral.

On the other hand, tetravalent carbon (C) is added to a silicon single crystal.
In the case where atoms are added as trace impurities, the inventors of the present invention have changed the characteristics of the bond between silicon and carbon to ab-initio.
The calculation was performed using MO (Molecular Obital / Molecular Orbital Method / RHF631G).

The model of methylsilane (A1
/ CH ₃ SiH ₃ ) and bonding (sin) between Si atoms for comparison.
disilane (B1 / Si ₂ H ₆ ) and C
Ethane (C1 / C ₂ H ₆ ) is also shown in FIG. 2 as a model of a single bond, and the summary of the calculation results is shown in Tables 1 and 3 below.

[0013]

[Table 1]

According to the above calculation results, neither the Si atom nor the C atom is neutral, and the Si atom is positive (0.2382 / net atomic cha
rge), the C atom was negatively charged (−0.2382 / net atomic charge).

According to these results, the Si atom is positively charged and is considered to be a donor. However, the calculation result of the ab-initio MO of the present inventors shows that the SiC bond (single
bond) The ionization potential (12.2396ev / model A1 in Table 1) is the ionization potential (11.0214ev / model B1 in Table 1) of the bond between Si atoms (single bond) and that of the bond (single bond) between C atoms. 13.9334ev / indicates an intermediate value from the model C1) in Table 1.

This is because the outermost orbits of the Si atom and the C atom overlap, and the valence electrons of the outermost orbital of the Si atom overlap with the outermost orbital of the C atom due to the difference in electron affinity. It indicates that it exists slightly biased.

That is, if the Si shell and the C atom are neglected because their inner shells are shielded, they are considered to have a structure having four electrons in the outermost shell with a +4 charge at the center. Is the same as +4, but the outermost orbital radii of Si and C atoms are 3p for Si atoms and 2p for C atoms, and the outermost orbital radii of C atoms are much smaller. As shown in the figure, the overlapped valence electrons are attracted and biased toward the C atom, and as a result,
The result is that atoms are positively charged and C atoms are negatively charged.

Therefore, considering the electronic structure of the electron-excess C atom, the ionization potential of the C atom in the case where the outermost electron is bonded to the Si atom as compared with the case where only four carbon atoms are used alone is considered. Is smaller than the ionization potential Vip of carbon atoms by the amount of excess electrons, and it is considered that electrons can be easily emitted (see FIG. 3).

In the case of the Si-deficient Si atom bonded to the C atom, the first ionization potential of the Si atom is lower than that of the case where the outermost electrons are four silicon atoms alone. The value becomes closer to the second ionization potential than the ionization potential Vip of the silicon atom by the amount of, and it is considered that it is difficult to emit electrons.

Considering this fact, it is considered that the electron-rich C atom is a donor atom, and that the outermost shell electron of the C atom is donated to the conduction band (FIG. 3). reference).

It is considered that the Si atom becomes short of electrons, indicating that it is an acceptor atom.

This can be seen from the result of MO calculation in Table 1, which shows that HOMO (Highest Occupied Molecular
Orbital) is the σ orbit (single bond), the maximum value of the wave function coefficient (Max.of WFC / wave function coefficient) is in the outermost occupied orbital of C atom (2pz), and
LUMO (Lowest Unoccupied Molecular Or
bital) is also the σ orbit, and the maximum value of the wave function coefficient (Max.of
The maximum value of WFC / wave function coefficient) is Si
It is clear from the fact that the atom is in the outermost unoccupied orbit.

Further, as shown in Table 1, the calculation results of the double bond (double bond / model A2 in Table 1) and the conjugate (conjugated double bond / model A3 in Table 1) of HOMO (π orbital) are also shown in Table 1. Maximum value of the wave function coefficient (Max.of
The maximum value of the WFC / wave function coefficient is in the outermost occupied orbit of the C atom, and the maximum value of the wave function coefficient of LUMO (π orbit) (Max.of WFC / wave function coe
fficient) indicates that the Si atom is in the outermost unoccupied orbit.

The following conclusions can be drawn from the above calculation results of the characteristics of the bond between Si atoms and C atoms. (1) According to the conventional theory described above, S in the SiC semiconductor crystal
Although both the i atom and the C atom were considered to be neutral, the carbon in the silicon single crystal is a donor atom, the C atom is a donor atom in the SiC semiconductor crystal, and the Si atom is an acceptor atom. That is, the Si atom is positively charged, the C atom is negatively charged, and the Si atom becomes an electron poor and becomes an acceptor atom.

Accordingly, SiC is considered to be an amphoteric intrinsic semiconductor. However, considering that the acceptor level of Si is shallow, the SiC semiconductor more clearly shows the characteristics of the p-type semiconductor and has a high degree of compensation. It is presumed to be waxy, but in fact exhibits such properties.

(2) In addition, the ab-initi
o MO calculations show that C atoms are negatively charged but are donor atoms. Considering the difference in electronegativity or electron affinity between C atoms and Si atoms, this calculation result is an extremely natural result as is apparent from FIG.

On the other hand, the activation energy of the nitrogen donor of β-SiC doped with nitrogen is 34-38 meV, whereas
Experimental results have reported that the activation energy of an unknown donor of non-doped n-type β-SiC is 18-25 meV.

According to this, the experimental value of the activation energy of SiC is about half of the value when a nitrogen atom is considered as a donor, but these experimental results are, as shown by the calculation results of the present inventors, the C atom. Can be explained by considering that is a donor.

That is, the ionization potential of the Si atom is
In contrast to 8.151 eV, the ionization potential of C atoms is 11.256 eV, which is much smaller than the ionization potential of N atoms of 14.53 eV, and the donor level is considered to be much shallower than that of N atoms.

Assuming that the activation energy is proportional to the difference between the ionization potential of the donor atom and the ionization potential of the Si atom, the difference in ionization potential of the C atom is about 3.1 eV, and the difference in the ionization potential of the N atom is about 6.4. eV, and the experimental value of the activation energy of SiC is about half of the value when N atoms are considered as donors. Thus, it can be understood that C atoms are considered as donors.
It can be said that the experimental results are well explained.

The experimental results show that the N atom is n of SiC.
Although it is shown that it is unsuitable as a donor atom of the type, from the theoretical considerations obtained by the inventors of the present invention, a plot of the electronegativity or electron affinity of the atom and the ionization potential of the appropriate n-type A donor atom and a p-type acceptor atom can be selected.

FIG. 5 shows a plot of the electronegativity of various atoms and the ionization potential. For example, a p-type acceptor atom suitable for a Si atom has a lower electronegativity than a Si atom. In addition, an atom having an ionization potential smaller than that of a Si atom, that is, an atom existing at the upper left of the Si atom in the figure is a corresponding acceptor atom.

An n-type donor atom suitable for a Si atom is an atom whose electronegativity is larger than that of a Si atom and whose ionization potential is not much larger than that of the Si atom, that is, the right side of the Si atom in the figure. This indicates that atoms existing in a portion that is not too low are the corresponding donor atoms.

Further, from the theoretical considerations obtained by the present inventors, a p-type diamond semiconductor can be produced by introducing Si atoms as impurities into the diamond semiconductor.

According to the theory obtained by the present inventors, C
If an impurity atom serving as a donor atom is added to an atom, an n-type diamond semiconductor which is considered to be difficult to produce at present is produced.

That is, a donor atom suitable for producing an n-type diamond semiconductor has an electronegativity larger than that of a C atom and has an ionization potential not much larger than that of a C atom. A pentavalent or higher atom that is less than that of a single bond between C atoms constituting the diamond semiconductor, more specifically, that of ethane (C ₂ H ₆ ) that is less than that (13.831 eV), It can be concluded that the promising candidates for impurities are Se, S, Br and I in FIG.

[0037]

According to the present invention, based on the findings obtained by the inventors of the present invention, the electronegativity is larger than that of C atoms, and the ionization potential is lower than that of C atoms. The present invention proposes an n-type diamond semiconductor in which a pentavalent or higher valent atom is added to a diamond semiconductor as a donor atom.

Here, a preferred donor atom is B
r and I.

[0039]

Reference Example Hereinafter, a reference example of the present invention will be described. Reference Example 1 A small amount of high-purity selenium dioxide (SeO ₂ ) was added to ethanol (C ₂ H
₅ OH), and this is dissolved in acetone (CH ₃ ) ₂ CO ₂ as an organic solvent.
Dilute the source gas diluted with high-purity (99.99999%) hydrogen gas (H ₂ ), transport it into a hot filament CVD reactor, and preheat it to 900 ° C or higher. Was formed into a film. It was confirmed that a good n-type diamond semiconductor film having a resistivity of about 1.0 × 10 ⁻² Ωcm was prepared from the prepared artificial diamond thin film by a four-terminal method and Hall measurement.

Reference Example 2 High-purity methane gas (CH ₄ ) was mixed with high-purity hydrogen selenide (H
₂ Se) a very small amount (0.02 mol%) added source gas, diluted with hydrogen gas of high purity _{(99.99999%) (H 2)} (0.05%),
Microwave (2.45GHz) Plasma CVD reactor (50 to 1000
(50mms ^-1 ) and preheated (850 ° C)
A diamond semiconductor film was formed on the silicon single crystal substrate (<100>) and on the β-SiC substrate. The resistivity of the prepared diamond thin film was about 2.
It was confirmed that a good n-type diamond semiconductor film of 5 × 10 ² Ωcm was formed.

Reference Example 3 High purity selenium dioxide (SeO ₂ ) and ethanol (C ₂ H ₅ OH)
And a source gas diluted with acetone ((CH ₃ ) ₂ CO) is diluted with high-purity (99.99999%) hydrogen gas (2 vol.
%), Transferred (50 SCCM) to a hot filament CVD reactor (100 Torr), and formed a film on a preheated (850 ° C.) silicon single crystal substrate (<100>). By the four-terminal method and Hall measurement of the formed thin film, it was confirmed that a good n-type diamond semiconductor film having a resistivity of about 4.5 × 10 ² Ωcm was formed.

[0042]

In summary, according to the present invention, based on the theoretical considerations obtained by the inventors of the present invention, a good n-type diamond semiconductor could be obtained using atoms, which could not be considered heretofore, as donors.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of an n-type or p-type silicon semiconductor based on a conventional theory, wherein (a) is a state in which impurities are not doped, (b) is a state in which P atoms are doped to be n-type,
(C) is a state in which Al atoms are doped to be p-type.

FIG. 2 is a diagram showing a molecular model of SiC used for calculation.

FIG. 3 is a diagram showing energy levels of HOMO and LUMO.

FIG. 4 is a diagram showing a distribution state of valence charges in a Si—C bond;

FIG. 5 is a diagram plotting the electronegativity and ionization potential of various atoms.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C30B 29/04 CA (STN)

Claims (1)

(57) [Claims] 1. A diamond semiconductor having an electronegativity greater than that of a C atom, an ionization potential lower than that of a single bond between C atoms, and a pentavalent or higher Br atom.
Or n added with I as a donor atom
Type diamond semiconductor.