Ohmic connection electrodes for p-type semiconductor diamonds

Abstract: In an ohmic contact electrode for the p-type semicon­ ductor diamond, the electrode is formed of metals or metallic compounds containing boron on a p-type semiconduc­ tor diamond, so as to obtain a decreased contact resistance.

Description: BACKGROUND OF THE INVENTION Filed of the Invention This invention relates to ohmic connection electrodes for p-type semiconductor diamonds. Description of the Prior Art The semiconductor diamond is now drawing an industrial attention as a new material for semiconductor devices such as diodes, transistors, sensors, or the like. Although the diamond is widely known as an insulating substance, the one discussed herein is the semiconductor diamond having a low resistivity. The diamond has broad forbidden band width (5.5 eV) as well as a large mobility of carriers (2000 cm²/Vs), and is stable thermally and chemically. For these reasons, the semiconductor diamond is keenly expected as a material for environmentally withstanding, high-speed power-use devices or blue light emitting elements. The semiconductor diamond is available in the forms of a natural bulk, high-pressure synthetic bulk, and vapor-phase synthetic film. The p-type diamond can be obtained by doping boron (B). On the other hand, the n-type diamond, though it may be obtained by doping phosphorus (P) and lithium (Li), has a high resistivity. And an n-type diamond with a low resistivity has not yet been developed. Although bipolar devices are not involved, the devices that have already come out in prototypes on the basis of the semiconductor diamond include at present Schottky diodes, making use of Schottky junction with tungsten (W) and a p-type diamond, and several types of unipolar transistors. In manufacturing semiconductor devices, it is extremely important to form such electrodes that allow ohmic connection. The ohmic connection electrode means an electrode the current-voltage characteristics through which is symmetric between the forward and backward directions in accordance with the Ohm's law. Moreover, it is preferable that the contact resistance thereof be as low as possible, where the contact resistance is defined as a voltage to be applied to make flow of a unit current through a unit contact surface. The unit thereof is Ωcm². It is common knowledge that an electrode capable of ohmic connection can be obtained by forming some metals such as Ti, AuTi, and AuTa through the method of metallizing on a p-type diamond, see for instance IEEE EDL-9(1988) July, No.7, pgs 350-351. The technique of forming ohmic connections is an important technique in manufacturing semiconductor devices. However, the ohmic connections now available to semiconductor diamonds using Ti, AuTa, and the like involve a large contact resistance as much as more than 1 Ωcm². In any device manufactured using the conventional ohmic connection with the semiconductor diamond, even if a voltage is applied to, there occurs a remarkable voltage drop across such an ohmic connection electrode. This results in a drop of an effective voltage applied to the device, whereby good characteristics of the semiconductor diamond connection can not be sufficiently utilized. Furthermore, the device suffers from a serious drawback that heat is generated at an ohmic connection electrode. In manufacturing any electronic devices, contact resistances less than 10⁻⁴Ωcm² are required for that of ohmic connections, and far lower contact resistances are demanded for high-speed, high frequency devices. To enable the semiconductor diamond to be utilized as a material for semiconductor devices, a low-resistance ohmic connection is essential. SUMMARY OF THE INVENTION An essential object of the present invention is therefore to provide a p-type semiconductor diamond ohmic connection electrode having a low contact resistance. In order to accomplish the above object, there is provided an ohmic connection electrode for the p-type semiconductor diamond in accordance with this invention is such that the electrode is formed of metals or metallic compounds containing boron on a p-type semiconductor diamond. The electrode material for containing boron is at discretion selectable from Au, Pt, Al, Ag, Ni, Ti, Ta, Cr, Zr, TiSi₂, NiSi, PtSi, or the like. As the diamond this may be effectively used in any form of a natural bulk, a high-pressure synthetic bulk, and a vapor-phase synthetic film. This invention provides an ohmic contact electrode having a low resistance by forming metals or metallic compounds containing boron wherein the amount of boron in the metal or metal compound is more than 0.01 at % into electrodes on p-type semiconductor diamonds. The above features and advantage have been found by the inventors of the present invention through various experiments, it has not yet been clarified by the boron involved therein serves to obtain a low resistance, but it may be deducted as follows. When a metal or metallic compound containing boron is formed into an electrode on a p-type semiconductor diamond, the boron is activated by the heating of the substrate in forming electrodes, the radiation heat in forming electrodes, the annealing effect after forming electrodes, or the like, so that the boron will be diffused into the diamond. The boron acts in a diamond as effective acceptors, as is well known to those skilled in the art. The region into which the boron is diffused, therefore, forms a p-type layer (p+ layer) doped to a high-level concentration. Whereas the diamond is originally of a p-type, this causes the diamond to be a p-type semiconductor having a much high concentration than the original average one. As an actual junction between a metal and a semiconductor is in general of the Schottky type, the depletion layer width, d, in the case of p-type semiconductors, is given by the following formula: Where, ε s represents the dielectric constant of the semiconductor, ε₀ does the dielectric constant of vacuum, and e the elementary quantity of electric charge, N a the acceptor density of the semiconductor, V a the diffusion potential, and V the applied voltage. The band structure in the vicinity of the depletion layer is shown in Fig. 1. A Fermi level 3 of the semiconductor and a Fermi level 1 of the outer metal lie on a line. The semiconductor includes a conduction band 2 and a valence band 4, where, the semiconductor being of a p-type, the valence band 4 and the Fermi level 3 are close to each other. As the acceptor density gets higher in proximity of a boundary 5, the band bend sharply downward. Holes, which are collected in an upward bent portion 6 of the valence band, can go through the barrier into the metal due to the tunneling effect. All the acceptors in the boundary between the metal and semiconductor capture electrons, being negatively charged to e. Such acceptors are distributed almost uniformly in the proximity of the boundary and hence the Gauss' equation for electric fields and charges can be integrated into the formula (1). As is apparent from the formula (1), the higher the acceptor density of the semiconductor, the less the depletion layer width d. Therefore, when a p+ layer is formed on the diamond surface by the diffusion of boron, as in the method of electrode formation of this invention, the depletion layer is remarkably thinned. For this reason, the holes can pass through the depletion layer due to the tunneling effect without jumping over the barrier existing in the interface between the metal or metallic compound and the diamond. The contact between the metal or metallic compound and the diamond is accompanied by a low resistance due to the fact that the holes can easily pass through a thin depletion layer. This invention makes it possible to form an ohmic contact having a low contact resistance on a p-type semiconductor diamond with the aid of the aforementioned mechanism. BRIEF EXPLANATION OF THE DRAWINGS Fig. 1 is a schematic diagram showing a band structure of the semiconductor and diamond when electrodes are formed of metals or metallic compounds containing boron on a p-type semiconductor diamond; and Fig. 2 is a plan view of the electrode patterns used in the embodiments of the present invention to determine the contact resistances between the semiconductor diamond and electrodes, where the shaded portions show the electrodes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] Now the method of electrode formation in accordance with the invention will be described more specifically by way of embodiments thereof. To examine which types of diamonds this invention are effective for, electrodes in accordance with the method of the present invention were formed on the following three types of diamonds, then the contact resistances and ohmic characteristics of the respective electrodes were measured. a: Natural II b type of diamond (P-type with B doped.) b: High-pressure synthetic II b type of diamond c: B-doped p-type film type of diamond subjected to a homo-epitaxial growth in the way of vapor-phase synthesis on a high-pressure synthetic I b type of diamond. Among them, the vapor-phase synthetic film was formed using CH₄, H₂, and B₂H₆ gases by the microwave plasma CVD method. The electrodes were formed by vacuum evaporation or sputtering method. On this experiment, the electrode patterns as shown in Fig. 2 were formed on the diamonds using a metal mask. Four electrodes 10, 11, 12 and 13 were formed, each one 1.5 mm long by 0.2 mm wide, in parallel with each other and spaced at the intervals of 0.2 mm. 0.4 mm. and 0.6 mm. The ohmic characteristics of the electrodes were measured and decided from the current-voltage characteristics between adjacent electrodes out of the four electrodes for a voltage range of -10 to +10 V. The contact resistances of the electrodes were determined using Transmission Line Model based on the resistances between two adjacent electrodes as shown in Fig. 2 and the electrode intervals thereof. Table 1 lists the results from Embodiment 1. The three types of diamonds before described were used here. The electrodes were formed by a vacuum evaporation method at a substrate temperature of 400 °C using AuB containing 0.5 at % of B, where the thickness of the AuB was approximately 0.2 to 0.3 µm. As is understood from the Table 1, the AuB has proved to be ohmic to any one of the three types of diamonds, wherein the contact resistances thereof have resulted in values of the order of 10⁻³ to 10⁻⁵Ωcm², 2 to 5 orders of magnitude lower than their conventional counterparts. It may be effective to add, annealing to an adequate degree after the electrodes are formed. Although the temperature and time duration for annealing differ in their optimum values depending on the electrode materials used, a temperature of approximately a few hundred C° to 1000 °C is required. In the case where the temperature exceeds over 600 °C, however, it is desirable to carry out the annealing in inert gas, N₂, H₂ or vacuum owing to that the presence of O₂ causes oxidation or graphitization of a diamond to be easily progressed. Instead of the annealing, it is also effective to increase temperature to an adequate degree during forming of the electrodes. [Embodiment 2] This invention is characterized by using metals or metallic compounds containing B for electrodes. Suitable concentrations of B were examined. With the electrodes made of AuB, the contact resistances thereof were measured with various concentrations of B. The results of the measurement are shown in Table 2. Concentration of Boron (B) vs. Contact Resistances of Electrodes Concentration of B (AuB) Contact Resistance (Ωcm²) 0.01 at % 6x10⁻³ 0.1 at % 3x10⁻⁴ 0.5 at % 2x10⁻⁵ 1.0 at % 2x10⁻⁵ 5.0 at % 8x10⁻⁵ 10.0 at % 5x10⁻⁴ The diamond here used in a p-type diamond of the vapor-phase synthetic type have a resistivity of approximately 50 Ωcm, and a thickness of 2 µm. The AuB was formed by the vacuum evaporation method without heating the substrate, and subjected to an annealing at 800 °C for five minutes in an atmosphere of N₂ + H₂ (H₂ : 10%). It is understood from the Table 2 that the contact resistance is first decreased with the increasing of concentration of B and reaches a minimum value at a concentration of B of approximately 1 at %, thereafter in turn being increased. In this case, if the concentration of B involved become excessively high, the resistance of the electrode itself is increased, thus being undesirable. A value of the concentration of B of 0.1 at % or so is practically sufficient as a material of the electrodes. [Embodiment 3] To see what materials is suitable for the electrodes, a number of the electrodes as shown in Fig. 2 were prepared using various materials the contact resistances of electrodes were measured. The primary components of the electrode materials were metals and metallic silicides, i.e. specifically Au, Pt, Al, Ag, Ni, Ti, Ta, Cr, Zr, TiSi₂, NiSI, and PtSi, whereto boron (B) was added. The electrodes here were made by the vacuum evaporation or sputtering depending on the kinds of the material used. The diamond used was a vapor-phase synthetic p-type diamond like Embodiment 2. The concentration of B used within the electrode materials was in a range of 0.1 to 0.5 at %. The results of the measurement for various electrode materials are shown in Table 3. The measurement was performed with different types of simple-substance metals, alloys, silicides, and so forth, all the materials showed the ohmic contacts with a low contact resistance. The above result proves the tunneling effect of the holes, as mentioned previously, due to the generation of high-concentration acceptors in the boundary of the metal semiconductor. Electrode Materials vs. Contact Resistances Electrodes Contact resistance (Ωcm²) Material Formation method AuB Vacuum evaporation 3x10⁻⁵ PtB Vacuum evaporation 2x10⁻⁵ AlB Vacuum evaporation 1x10⁻⁴ AgB Vacuum evaporation 2x10⁻⁴ NiB Vacuum evaporation 5x10⁻⁵ TiB Vacuum evaporation 2x10⁻⁵ TaB Vacuum evaporation 3x10⁻⁵ CrB Vacuum evaporation 1x10⁻⁴ ZrB Vacuum evaporation 7x10⁻⁵ TiSi₂B Sputtering 3x10⁻⁵ NiSiB Sputtering 4x10⁻⁵ PtSiB Sputtering 2x10⁻⁵ As described hereinabove, it is possible to obtain an ohmic contact with a low contact resistance by forming electrodes out of metals or metallic compounds containing boron on a semiconductor diamond. The art of forming the contact electrodes is indispensable in manufacturing the semiconductor diamond devices and therefore the electrodes according to the present invention are effective in forming any types of electronic device.

Claims: An ohmic contact electrode for a p-type semiconductor diamond characterized by being formed of metals or metallic compounds containing boron on said p-type semiconductor diamond, wherein the amount of B in the metal or metal compound is more than 0.01 at %. The electrode according to claim 1, wherein the amount of B in the metal or metal compound is more than 0.01 at % inclusive and less than 10 at % inclusive. The electrode according to claims 1 or 2, wherein the metal or metal compound is composed of at least one sort of composition of Au, Pt, Al, Ag, Ni, Ti, Ta, Cr, Zr, TiSi₂, NiSi and PtSi. A method for forming The electrode according to claims 1 to 3, the temperature at the time of forming the electrode or at the time of annealing thereof is 300 to 1000 C°.

Inventors: FUJITA NOBUHIKO C O ITAMI WORK, ISHII MASAYUKI C O ITAMI WORKS, KIMOTO TUNENOBU C O ITAMI WORK, NAKAGAMA SHOJI C O ITAMI WORKS, TOMIKAWA TADASHI C O ITAMI WOR, FUJITA, NOBUHIKO, ISHII, MASAYUKI, KIMOTO, TUNENOBU, NAKAGAMA, SHOJI, TOMIKAWA, TADASHI, FUJITA, NOBUHIKO, C/O ITAMI WORKS, ISHII, MASAYUKI, C/O ITAMI WORKS, KIMOTO, TUNENOBU, C/O ITAMI WORKS, NAKAGAMA, SHOJI, C/O ITAMI WORKS, TOMIKAWA, TADASHI, C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: H01L29
IPC codes: H01L29, H01L21