JPH0429376A - Ohmic electrode of n-type semiconductor cubic boron nitride - Google Patents

Abstract

PURPOSE:To obtain an ohmic connection for an n-type semiconductor boron nitride by forming an electrode by a metal or an alloy which includes silicon (Si) or sulfuric (S) on an n-type semiconductor cubic boron nitride. CONSTITUTION:An electrode is formed on an n-type semiconductor cubic boron nitride by a metal or an alloy which includes Si or S. In this case, the Si or S within the electrode is activated due to heating of a substrate when forming an electrode, radiant heat when performing deposition of electrode, or annealing after forming the electrode, thus resulting in diffusion into an adjacent cubic boron nitride. An electrode material as a mother body including the Si or S is Cr, W, Mn, Co, Ni, Pt, Ag, Zn, WC, MnZn, Ni2B, PtPd, AgAl etc., thus obtaining ohmic connection electrode.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an ohmic connection electrode of n-type semiconductor cubic boron nitride.

[Conventional technology]

The semiconductor cubic boron nitride is used in diodes, transistors,
It is attracting attention as a new material for semiconductor devices such as sensors. Cubic boron nitride is widely known as an insulator, but here we focus on the semiconductor cubic boron nitride, which has a low resistivity. Cubic boron nitride has a wide forbidden band width (7.0 eV), has a high temperature limit (1300°C), and is chemically stable. Therefore, there are strong expectations for semiconductor cubic boron nitride as a material with excellent environmental resistance, power devices, or blue light emitting elements. Cubic boron nitride does not occur naturally and can be formed by high pressure synthesis. In recent years, formation of thin films by vapor phase synthesis has also been reported. P-type cubic boron nitride is obtained by doping with Be. On the other hand, n-type cubic boron nitride can be obtained by doping with sulfur (S) and silicon (Si). At present, pn junction diodes are being prototyped by using a high-pressure synthesis method to continuously grow n-type cubic boron nitride using p-type cubic boron nitride as a seed crystal. This pn junction diode exhibits diode characteristics up to 650°C. Furthermore, it has been reported that when current is injected, it emits ultraviolet to blue light around 340 nm. When making semiconductor devices, it is extremely important to form electrodes that can make ohmic connections. An ohmic connection electrode means an electrode whose current and voltage characteristics passing through the electrode are symmetrical in the forward and reverse directions according to Ohm's law. Also, it is better to have as little contact resistance as possible. Contact resistance is defined as the voltage that must be applied for a unit current to flow through a unit contact surface. The unit is Ω·Cm2. The connection between metal and semiconductor is either ohmic connection or Schottky connection. When a semiconductor and a metal are bonded, the band forms a strain barrier upwards, so in many cases it becomes Schottky and is difficult to become ohmic. There is still no excellent n-type cubic boron nitride electrode. Conventionally, silver (Ag) electrodes, silver paste, etc. have been used (■R, H, Wentorf, Jr.: J,
of CheIl, Phys, Vol, 3G (196
2) +990■O, MIshlma, etc.:5cl
ence Vol. 238 (1987) 181), and there is currently no report that an ohmic connection has been obtained.

[Problem to be solved by the invention]

Ohmic connection formation technology is an essential and important technology in manufacturing semiconductor devices. Furthermore, once an ohmic connection is obtained, it is desirable that the contact resistance of the ohmic connection be as small as possible. The contact resistance used for ordinary electronic devices is lo-2Ω・0m2 or less, and for high-speed, high-frequency devices, it is even smaller than 10-2Ω.
A contact resistance of 4Ω·Cm2 or less is required. Is the connection you created not ohmic but sill? 7) If it is key, even if a current is attempted to flow through the fabricated device, the current cannot be effectively injected due to the Schottky barrier in the connection electrode portion, resulting in low current injection efficiency into the device. Furthermore, since the resistance at the connection electrode portion is also large, the voltage drop is significant, and the effective voltage applied to the device becomes small. For these reasons, sufficient characteristics as a device cannot be obtained. Furthermore, the problem of heat generation at the contact electrode portion also poses a major difficulty. Therefore, in order to utilize semiconductor cubic boron nitride as a material for semiconductor devices, it is essential to form ohmic connection electrodes. The object of the present invention is to form an ohmic connection electrode on an n-type semiconductor cubic boron nitride.

[Means to solve the problem]

The ohmic electrode of the n-type semiconductor cubic boron nitride of the present invention is an electrode formed of a metal or alloy containing Sl or S on the n-type semiconductor cubic boron nitride. The electrode material as a matrix containing Sl or S is Or, w
lMn, Co, Nl, PtN AglZnlWClM
nZn1NIQB, PtPdlAgAj, etc. are arbitrary.

[For works]

In the present invention, since a metal or alloy containing Sl or S is formed as an electrode on the n-type cubic boron nitride,
An ohmic connection electrode is obtained. This was discovered by the inventor through repeated experiments. Although it is not yet clear why metals or alloys containing Sl and S become ohmic, it is inferred as follows. A metal or alloy containing Sl or S is formed as an electrode on n-type cubic boron nitride. In this case, Sl or S in the electrode is activated by substrate heating during electrode formation, radiant heat during electrode deposition, or annealing after electrode formation. It is thought that it then diffuses into the adjacent cubic boron nitride. Sl and S act as effective donors in cubic boron nitride. This is well known. Therefore, in the region where Sl and S are diffused, there is a highly doped n layer (n
layers) are formed. Although this cubic boron nitride is originally n-type, it becomes n-type at a much higher doping concentration than this average doping concentration. It is inferred that the high concentration doping reduces the thickness of the barrier layer and makes it ohmic due to tunneling current. In an ideal case, whether the metal-semiconductor junction is ohmic or Schottky is determined by the metal's work function (pass) and the semiconductor's electron affinity (X-), where the metal's work function is at the Fermi level. It is the energy required to extract a certain electron to an infinite distance.The electron affinity of a semiconductor is the energy required to extract an electron at the bottom of the conduction band to an infinite distance.In the case of an n-type semiconductor, metal,
The barrier height φ8 of the semiconductor junction is expressed as φ8 = φ, -
Toki - becomes. Since φ8 is the height of the barrier, if it is positive, electrons are blocked by it, making it Schottky; if it is negative, there is no barrier, making it ohmic. From this equation, it appears that if a metal with a low work function is selected for the electrode, an ohmic connection can be obtained. However, in an actual metal-semiconductor junction, defects exist at the metal-semiconductor junction interface, forming a localized level (φ.: also called a surface level) near the center of the band gap. If there are many defects, the Fermi level of the semiconductor will be this φ. pinned to. As a result, the barrier height φ8 is expressed as φB=Eg−φ0 (2), where Eg is the band gap of the semiconductor, and becomes almost independent of the type of metal. is measured from the top of the valence band. This is because electrons from the metal can freely flow into the localized level and occupy this level, so the top of the localized level matches the Fermi surface of the metal, and the Fermi level of the semiconductor also matches this. It is. Naturally, since φ6>0, the metal/semiconductor junction becomes a Schottky. This phenomenon is known as the pinning effect, and is particularly noticeable in ■-v group compound semiconductors. Also,
Usually φ. is about 1/3 to 172 of E6. In the case of cubic boron nitride as well, many types of surface defects may exist because it is not a single element material. In addition, the current cubic boron nitride crystal is not perfect and has many defects, so it is thought that there are many surface defects and a pinning effect occurs. A band diagram of a Schottky metal LIn semiconductor junction is shown in FIG. The Fermi level 2 of the semiconductor is continuous with the Fermi level 1 of the outer metal. A semiconductor has a conduction band and a valence band, and 3 is the lower end of the conduction band and 4 is the upper end of the valence band. Since it is an n-type, the lower end 3 of the conduction band and the Fermi level 2 are close. At boundary 5, the Fermi level is a localized level φ. This creates a barrier with a height of φ8. Near boundary 5, the band bends strongly upwards due to the high donor density. Electrons are accumulated in the downward curved portion 6 of the conduction band, but they can escape into the metal due to the tunnel effect. All donors at the metal-semiconductor boundary emit electrons, so they have a positive charge of e. Since this is distributed almost uniformly near the boundary, this portion becomes a depletion layer without electrons. This thickness can be calculated as follows. By integrating Gauss's equation regarding electric field and charge, the following equation holds true. d=[2・−・to・(φn V ) / (e−N
d):) ”” (3) d: Depletion layer width ε6: Semiconductor relative dielectric constant ε0 = Permittivity in vacuum V: Applied voltage e: Charge position Nd: Semiconductor donor density Until now, φ8 is potential and energy Hereafter, this will be treated as the voltage value divided by the charge element e. As can be seen from this equation, as the donor density in the semiconductor increases, the depletion layer width d decreases. This contact has low resistance because electrons can easily pass through the thin depletion layer. Let me explain in more detail. Since the current I per unit area is a tunnel current, ■: A T 2exp (-2f (8r2me
U, /h2)"' dx) (4) can be written as, where U is the potential of the conduction electrons measured from the bottom of the conduction band. It is given by the Gaussian equation % Equation % (5) A= 4πeilek2/h3 (6) where me is the electron effective material poison in the semiconductor, h is a blank constant,
k is Boltzmann's constant. For free electrons, A is +2
OA/cm2K". Integrating over X, I = AT2expl - (φa V) /Voo
) (7) However, Voo” h Nd”’ / 4 π(to!gme
)”” (8), Nd= 10”cm-3
And 1 is the free electron mass, ! If 1=1, VOO becomes 0.5GV. Although the definition of φ8 was described above, it is thought that φ8 itself decreases when n-type impurities are doped at a high concentration. This is because the local material level increases and φ0 increases. Eg is 7eV, but φ8 drops to about 6 to 3V. Of course, this assumes that the doping concentration is extremely large near the boundary. If VOO is 0.5 V, it can be considered almost ohmic from (6). However, the voltage is positive in the direction from the semiconductor side to the metal electrode. The resistance R is obtained by partially differentiating the current with respect to the voltage, and assuming ■=0. R6 at this time can be expressed by the following formula. Rc=(Voo/AT2)exp(φn/Voo)
(9) If Nd:=10"cm-', the mass of the free electron is 1, Co" 1,T:
If it is 300, Vo. =0.56V, and if the previous coefficient is also included in exp, Rc=expl(φB10.5
G)-111i, 77) (10)
When φ8 is 6v, Rc becomes 10-3ΩC1!127v to 10-2ΩCm2. If we assume that Nd==1020cm-3, vo
o is 0. ! 8V, Rc=expl(φn10.18) 17.91)
(+1) For example, if φ8 is 3v, R6
is approximately 1O-1ΩCTfI2. If φ8 is 2.5V, Ro will be about 1O-2Ωcm2. Doping the n-type impurity at a high concentration in this way reduces the thickness d of the depletion layer. Also, due to high concentration doping, the localized level on the surface increases φ. It also has the effect of increasing φ8 and decreasing φ8. I wrote earlier that this is about 1/2 to 273 of the band gap (7 eV), but it is thought that this may become even smaller. By performing high concentration doping in this manner, it is expected that the higher the donor density, the less the thickness of the depletion layer, the lower φB, the easier the tunnel current will be, the lower Ro, and the resulting ohmic state. But nothing is clear yet. Electrons may be conducting hopping in localized levels. Although such an effect can be expected if the impurity is doped at a high concentration, an increase in the density of the impurity increases the resistance of the electrode, which is rather undesirable. There may be an appropriate concentration range. The present invention makes it possible to utilize such a mechanism to form ohmic connections on n-type cubic boron nitride.

[Example 1] The ohmic electrode of the present invention will be explained in more detail with reference to an example. In order to see what kind of electrode material the electrode structure of the present invention is suitable for, electrodes were made using different materials and their ohmic properties and contact resistance were evaluated. The main component of the electrode is a metal or a metal compound. Specifically, Cr1W, Mn N Co1 old, pt, Ag1Zn
1We, MnZn1N12B, PtPd, kgkl
Add Sl or S to. Cubic boron nitride was synthesized under high pressure. The electrodes were formed using vacuum evaporation or sputtering, depending on the material, at a base material temperature of 400°C. In addition, the Sl concentration and S concentration in the electrode material are 0.1% to 0.5%.
% was used. At this time, an electrode pattern shown in FIG. 2 was formed on the cubic boron nitride using a metal mask. The thickness of the electrode is 0.
It is about 2 to 0.3 μm. There are four M poles. The length is 0.8 mm and the width is 0.0 L mm, respectively. They are lined up in parallel, and the interval is (1.1mm 10.2mm
, 0.3mm. The ohmic nature of the electrodes changes the current-voltage characteristics between any of these electrodes from -10V to +
The determination was made by measuring within the IOV range. For electrodes with ohmic characteristics, please refer to the electrode pattern 2 shown in Figure 2.
The contact resistance was determined from the resistance between the electrodes and the electrode distance using a transmission line model. Table 1 summarizes the results when Sl was added, and Table 2 summarizes the results when S was added. Table 2 Current-voltage characteristics of electrodes including S1, contact resistance Table 2 Current-voltage characteristics of electrodes including S, contact resistance Tests were conducted on various metal materials and metal compounds, but as can be seen from Tables 1 and 2, all materials for,
Although the contact resistance was on the order of 10-2 to 10-'Ωcm2, ohmic characteristics could be obtained. This supports the assumption that electron tunneling occurs due to the generation of a high concentration of donors at the metal/semiconductor boundary, as mentioned earlier. At this time, it is effective to perform appropriate annealing after electrode formation. This is because Si20 contained in the metal electrode must be activated and diffused into the semiconductor. The optimum temperature and time for annealing vary depending on the electrode material used, but a temperature of about 300°C to 1800°C is preferable. However, depending on the electrode material, oxidation of the electrode progresses during annealing, so annealing in an inert gas, N2, N2, or vacuum is desired. In addition, when the temperature is raised to 1000°C or higher, the presence of 0□ will cause the oxidation of cubic boron nitride and the formation of boron nitride, so annealing in an inert gas, N11lH2, or vacuum is still preferable. . Furthermore, instead of this annealing, it is also effective to appropriately raise the temperature during electrode formation. This temperature is preferably 300°C to 1800°C, although it depends on the electrode material.

Embodiment 2 The present invention is characterized in that a metal or metal compound containing Sl or S is used for the electrode. We investigated what concentrations of Sl and S are appropriate. The electrodes are Pt5l (Pt containing some Sl, atomic ratio does not mean 1:1), PtS (Pt
The atomic ratio does not mean 1:1), but the ohmic properties and contact resistance were evaluated by changing the concentrations of Sl and S. The results are shown in Tables 3 and 4. As in Example 1, the cubic boron nitride produced by high-pressure synthesis was used. The electrodes were formed by vacuum evaporation without heating the substrate, and were 8QO' in an N2+N2 (N2: 10%) atmosphere.
C11 (1 minute annealing was performed. From Tables 3 and 4, ohmic was obtained in all S I m degree and 5 tlA degree regions. Also, contact resistance decreases with srtM degree and S concentration. It can be seen that the S concentration reaches the minimum value at about several percent and then increases after that.It is also undesirable to increase the Sl or Si/,1 degree excessively because it increases the resistance of the electrode itself. .Practically speaking, the electrode material concentration is 81 and the S concentration is 0.1.
% is sufficient.

【Effect of the invention】

As explained above, by forming a metal or alloy containing 51 or S as an electrode on the n-type semiconductor cubic boron nitride, an ohmic connection can be obtained with the n-type semiconductor boron nitride. This technique is essential for manufacturing semiconductor cubic boron nitride devices, and is therefore effective in forming all kinds of devices.

[Brief explanation of drawings]

Figure 1 shows the n-type semiconductor cubic boron nitride with Sl or S.
A band diagram when a highly doped layer is formed near the metal/Φ semiconductor interface when a metal or alloy containing the metal or alloy is formed as an electrode. FIG. 2 is a plan view of the electrode pattern used to measure the contact resistance between the semiconductor cubic boron nitride and the electrode. The shaded area is the electrode. l am...Fermi level 2 of electrode...Fermi level 3 of semiconductor...Lower end of conduction band of semiconductor 4φ...Upper end of valence band of semiconductor 5...Electrode/semiconductor junction Interface Inventor Yui Tomikawa Yosui Nobufuji Mototsune 1) Junhiko Diagram

Claims (1)

[Claims] An ohmic electrode of n-type semiconductor cubic boron nitride, characterized in that the electrode is formed on n-type semiconductor cubic boron nitride using a metal or alloy containing silicon (Si) or sulfur (S).