JPH08316172A - Sic semiconductor device - Google Patents

Abstract

PURPOSE: To obtain a SiC semiconductor device having a low ON-voltage by providing an ohmic electrode having a low contact resistance for the device which is made of an alloy of Al and metal having a lower work function than that of Al. CONSTITUTION: An epitxial wafer having an n-doped epitaxial layer 2 formed on an n-type Sic substrate 1 is used. A back electrode 4 is formed on the back face of the substrate 1 by the DC sputtering method using an Al-Mg alloy target and heat-treated. On the front face of the layer 2 Au is deposited at room temp. to form a Schottky electrode 3. On its surface a compact alumina oxide film is formed in the atmospheric air and Al which reveals a good etch resistance and metal Mg having a low work function are alloyed to form a low-work function alloy. Especially Mg is the most suited material which is at a comparatively low cost and easy to handle in addition to its low work function.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to silicon carbide (hereinafter referred to as SiC
Abbreviated as “)”.

[0002]

2. Description of the Related Art SiC, which is a wide-gap semiconductor, is
Since the energy gap is larger than that of silicon, high-temperature operation up to about 600 ° C is possible, and the high dielectric breakdown strength allows not only a high breakdown voltage semiconductor device but also the impurity concentration of each layer constituting the semiconductor device. High
It is a promising material for improving the characteristics of a power semiconductor device, in particular, because it can be made thin and the loss in the ON state can be reduced. Further, SiC has an advantage that a stable silicon oxide film can be obtained by thermal oxidation like silicon, and the main process technology of silicon semiconductor can be diverted.

[0003]

In manufacturing a semiconductor device such as a Schottky diode or MOSFET using SiC, it is necessary to form an ohmic electrode. Conventionally, some methods have been tried, but all of them have problems to be solved in practical use. For example, if an aluminum (hereinafter referred to as Al) electrode that is most commonly used in silicon semiconductors is provided on the surface of n-type SiC, the electrode becomes a Schottky electrode instead of an ohmic electrode. Conventionally, nickel (hereinafter referred to as Ni) has been used as an electrode metal provided on the surface of n-type SiC, but it requires heat treatment at 1000 ° C. or higher, and an ohmic electrode cannot be easily obtained. Moreover, in the case of Ni, 1
Since the silicide of Ni is formed by the heat treatment at 000 ° C. or more and the depth thereof becomes 1 μm or more, the contact resistance becomes large unless the depth of the n-type high concentration doped layer thereunder is sufficiently deep. There is also.

In view of the above problems, an object of the present invention is to provide an SiC semiconductor device having a low on-voltage by making it possible to easily obtain an ohmic electrode having a low contact resistance.

[0005]

A metal forming an ohmic contact with n-type SiC preferably has a work function as low as possible. FIG. 2 shows the energy band when n-type SiC is brought into contact with a metal. In FIG. 2A, the work function of metal is φ _m , the work function of SiC is φ _s ,
Let χ be the electron affinity of SiC.

Now, when the metal and SiC are brought into contact with each other, as shown in FIG.
As shown in (b), n
The energy band of type SiC bends. At this time,
An ohmic contact is obtained when the relationship of φ _s >χ> φ _m holds. The work function φ _{s of} SiC is 4.5 eV [Handbook of Physical Properties of Surface, 198
7 years, published by Maruzen], but the difference between φ _s and χ depends on the type and concentration of n-type impurities. It is considered to be about 0.3 eV. From this, in the case of n-type SiC, it is desirable that the work function φ _{m be} a metal having a work function φ _m of about 4.2 eV or less, and it is desirable that the work function φ _m be as low as possible.

Table 1 shows metals having a low work function [Surface Property Engineering Handbook, published by Maruzen in 1987]. As shown in this table, most of the metals having a low work function are alkali metals, alkaline earth metals or rare earth metals, and generally they are easily oxidized in air, so they cannot be put to practical use as they are.

[0008]

[Table 1]

In order to achieve the above object, therefore, in the silicon carbide semiconductor device of the present invention, an alloy of aluminum and a metal M having a work function smaller than that of aluminum (Al
-M alloy). Then, it is assumed that M is either Mg or Mn.

[0010]

As described above, aluminum and, for example, Mg,
A metal M having a smaller work function than aluminum, such as Mn
If an electrode made of an alloy (Al-M alloy) with
An ohmic contact with the type SiC is easily formed.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of a Schottky diode according to the first embodiment of the present invention. 6H-type impurity concentration 5 × 10 ¹⁸ cm ⁻³ , nitrogen-doped impurity concentration 1 × 10 ¹⁶ cm ^{3 on} n-type SiC substrate 1 having a thickness of 400 μm
^-3 , using an epitaxial wafer on which an epitaxial layer 2 having a thickness of 5 μm is formed, an n-type SiC substrate 1
A back electrode 4 having a thickness of 5 μm was formed by DC sputtering using a target of Al—Mg alloy (Mg 30% by weight) on the back surface of the above. Subsequently, heat treatment was performed at about 200 ° C. Next, gold (Au) was deposited on the surface of the epitaxial layer 2 at room temperature to form a Schottky electrode 3.

The current-voltage characteristics of this Schottky diode were measured, and it was confirmed that the Schottky diode exhibited excellent diode characteristics in the range of room temperature to 300 ° C., and the back electrode 4 was in ohmic contact. Thus, Al-
The ohmic electrode of the Mg alloy can be easily formed without the need for high-temperature heat treatment such as that of conventional Ni, and a sufficiently low contact resistance can be obtained without making the depth of the high-concentration doped layer under the ohmic electrode so deep. .

Al itself has a work function of 4.28 eV as a simple substance and is a material having a relatively low work function. However, as it is, an excellent ohmic junction with n-type SiC cannot be obtained. On the other hand, Al forms a fine alumina oxide film on its surface in the air and exhibits good corrosion resistance. In the present invention, this Al is alloyed with a metal having a low work function to obtain an alloy having a good work resistance and a low work function. In particular, Mg is the most practical material because it has a low work function of 3.66 eV, is relatively inexpensive, and is easy to handle. (The JIS defines that a material having a Mg content of 5% by weight or less is defined as a corrosion resistant material.) The higher the Mg content, the lower the contact resistance and the lower the corrosion resistance. Although there is a tendency, a material having a high content could be put to practical use by providing an appropriate protective layer and sealing.

Further, an n-type layer in which nitrogen was added at 1 × 10 ¹⁹ cm ⁻³ was formed on a 6H-type SiC single crystal substrate by an epitaxial growth method. Al-Mg is formed on the surface of the n-type layer.
An electrode having a thickness of 5 μm was formed by a DC sputtering method using an alloy (Mg 20 wt%) target. The composition of the electrode film at this time was almost the same as the composition of the target. Moreover, the contact resistivity was about 8 × 10 ⁻⁴ Ωcm ² , and good ohmic characteristics were obtained.

Similarly, an n-type layer in which nitrogen was added at 1 × 10 ¹⁸ cm ^-3 was formed on a 6H type SiC single crystal substrate by an epitaxial growth method. Al-M on the surface of this n-type layer
DC using g alloy (30 wt% Mg) target
An electrode having a thickness of 5 μm was formed by the sputtering method. At this time, the composition of the electrode film was almost the same as the composition of the target. Moreover, the contact resistivity was about 4 × 10 ⁻⁴ Ωcm ² , and good ohmic characteristics were obtained.

Further, an n-type layer in which nitrogen was added at 1 × 10 ¹⁸ cm ^-3 was formed on a 6H type SiC single crystal substrate by an epitaxial growth method. Al-Mn is formed on the surface of the n-type layer.
An electrode having a thickness of 5 μm was formed by a DC sputtering method using an alloy (Mn 50% by weight) target. The composition of the electrode film at this time was almost the same as the composition of the target. Moreover, the contact resistivity was about 1 × 10 ⁻³ Ωcm ² , and good ohmic characteristics were obtained.

Although the Schottky diode is used as an example, it goes without saying that the present invention can be applied to other semiconductor devices such as bipolar transistors and MOSFETs. Further, although SiC has a plurality of crystal forms, each of which has different electrical characteristics, 6H-type SiC is mainly studied at present because of its ease of preparation. Although discussion has been made on 6H-type SiC in the above discussion, the effectiveness of the present invention is the same for other crystal forms (3H-type, 4H-type, etc.) and is not limited to 6H-type.

[0018]

As described above, according to the present invention,
By using an alloy of aluminum and a metal M having a work function smaller than that of aluminum (for example, Al-Mg, Al-Mn alloy), an ohmic electrode having a small contact resistance can be easily formed on the n-type silicon carbide, and the on-voltage can be increased. A silicon carbide semiconductor device having a low power consumption can be easily manufactured. Therefore, the present invention greatly contributes to the development of a power semiconductor device using silicon carbide.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a SiC Schottky diode.

2A is an energy band diagram before contact between a metal and n-type SiC, and FIG. 2B is an energy band diagram after contact.

[Explanation of symbols]

 1 SiC substrate 2 SiC epitaxial layer 3 Schottky electrode 4 Backside electrode

Claims (2)

[Claims] 1. A silicon carbide semiconductor device comprising an ohmic electrode made of an alloy of aluminum and a metal M having a work function smaller than that of aluminum (Al-M alloy). 2. The silicon carbide semiconductor device according to claim 1, wherein M is either Mg or Mn.