JP2958443B2 - n-type silicon carbide 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 silicon carbide semiconductor.

[0002]

2. Description of the Related Art Conventionally, as an n-type silicon carbide semiconductor, a silicon carbide semiconductor obtained by adding nitrogen (N) atoms as donor atoms is known (WJCho).
yke and L. Patrick, in Silicon Carbide-1973.edited b
y RCMarshall, JWFaust, and C.E.Ryan (University of
South Carolina, Colombia, South Carolina, 1974), p. 26
1 etc.).

[0003]

However, if this nitrogen atom is not added in a large amount, it cannot be made into an n-type silicon carbide semiconductor, and it is difficult to say that it is a suitable donor atom. Atomics have not been found, and the development of n-type silicon carbide semiconductors has not been advanced.

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, and 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 outermost four electrons of the silicon atom, and one covalent electron is lacking. However, it receives one electron from a silicon single crystal, maintains a diamond structure, and receives one electron. It is understood that the impurity atoms themselves become negatively charged and become p-type semiconductors.

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
The bond ionization potential (12.2396ev / model A1 in Table 1) is that of the bond between Si atoms (single bond) (11.0214ev / model B1 in table 1) and that of C atoms (single bond). (13.9334ev / 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 electron of the C atom is donated to the conduction band. (See FIG. 3).

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 a sigma orbit (single bond) and Max.of WFC (wa
ve function coefficient) is in the outermost occupied orbit of C atom (2pz), and LUMO (lowest unoccupied orbit / Low
est Unoccupied Molecular Orbital) also has a σ orbit, and the maximum value of the wave function coefficient (Max.of WFC / wave function co
(the maximum value of efficient) is evident from the fact that it is in the outermost non-occupied orbit of the Si atom.

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 and the C atom is negatively charged, and the Si atom becomes deficient in electrons and becomes an acceptor atom.

Therefore, SiC is considered to be an amphoteric intrinsic semiconductor from the calculation results. However, considering that the acceptor level of Si is shallow, the SiC semiconductor clearly shows the characteristics of the p-type semiconductor and has a large degree of compensation. It is presumed that this is the case, but in fact it exhibits such properties.

(2) In addition, the ab-initi
o MO calculation shows that C atom is negatively charged,
This indicates that it is a donorable atom. 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
While the ionization potential of C atoms is 11.256 eV, which is 8.151 eV, it is considered that the ionization potential of N atoms is much shallower than that of 14.53 eV.

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 between the C atom and the Si atom is about 3.1 eV, and the N atom is the same as the Si atom. Is about 6.4 eV, and SiC
The fact that the experimental value of the activation energy is about half of the value when N atoms are considered as donors can be understood by considering C atoms as donors, and it is considered that the experimental results are well explained.

The experimental results show that the N atom is n of SiC.
Although it is shown that the donor type is not suitable as a donor atom, from the theoretical considerations obtained by the inventors of the present invention, an n-type donor which is more suitable than the relationship between the electronegativity or the electron affinity of an atom and the ionization potential. An atom or a p-type acceptor atom can be selected.

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

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

Further, from the theoretical considerations obtained by the present inventors, Si atoms are acceptors in a SiC semiconductor,
Since C atoms are donors, an n-type SiC semiconductor can be formed by adding an impurity atom that can serve as a donor to C atoms.

That is, a donor atom suitable for producing an n-type SiC semiconductor has an electronegativity larger than that of C atoms and an ionization potential not much larger than that of C atoms. Si for semiconductors
It is only necessary to add, as a donor atom, a pentavalent or more atom that is less than that of a single bond between an atom and a C atom, more specifically, for example, that of methylsilane (H ₃ SiCH ₃ ) that is less than that of 12.296 eV It can be concluded that Se, S, Br and I are the promising candidates for the donor impurity meeting the conditions in FIG.

[0036]

According to the present invention, based on the findings obtained by the present inventors as described above, a silicon carbide semiconductor has an electronegativity greater than that of C atoms and an ionization potential. Is an n-type silicon carbide semiconductor in which a single bond of Si atom and C atom or less is added and pentavalent or more atoms are added as donor atoms.

Here, a preferred donor atom is S
e, S, Br and I.

[0038]

Embodiments of the present invention will be described below. Example 1 High purity (99.9999%) tetramethylsilane gas (CH ₃ ) ₄ Si
Then, a source gas obtained by adding a very small amount of high-purity (99.999%) hydrogen selenide gas (H ₂ Se) is diluted with high-purity (99.99999%) hydrogen gas (H ₂ ). A silicon carbide (SiC) semiconductor film was formed on a silicon single crystal substrate preheated to 900 ° C. or higher. The resistivity of the prepared thin film was measured by the four-terminal method and Hall measurement.
It was confirmed that a good n-type silicon carbide semiconductor film of 1.0 × 10 ⁻¹ Ωcm was formed.

Example 2 Trace amounts of trifluorosilane (SiHBr ₃ ) and trifluoroethane
A mixture of high-purity methane gas (CH ₄ ) diluted with (CH ₃ CBr ₃ ) and high-purity monosilane (SiH ₄ ) gas as a source gas is transferred to a microwave plasma CVD reactor,
Silicon carbide (SiC) on pre-heated silicon substrate
A semiconductor film was formed. The prepared thin film is subjected to the 4-terminal method and Ha
By the ll measurement, it was confirmed that a good n-type silicon carbide semiconductor film having a resistivity of about 1.5 × 10 ⁻³ Ωcm was formed.

[0040]

In summary, according to the present invention, it is possible to obtain a good n-type silicon carbide semiconductor by using atoms, which have not been considered heretofore, as donors based on the theoretical considerations obtained by the present inventors. Was.

[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.

────────────────────────────────────────────────── ─── Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C30B 28/00-35/00 H01L 21/203 H01L 21/205 H01L 31/00-31/20 CA (STN ) REGISTRY (STN)

Claims (2)

(57) [Claims] To 1. A silicon car bytes semiconductors, electronegativity larger than that of C atoms, and below it the ionization potential of the single bonds of Si atoms and C atoms, trace pentavalent or more atoms An n-type silicon carbide semiconductor characterized by being added as an impurity donor atom. 2. The n-type silicon carbide semiconductor according to claim 1, wherein the donor atom is selected from Se, S, Br and I.