INTRODUCTION

Its corporate literature states, "Ion Engineering Center Corp. (IECC) and Ion Engineering Institute Corporation (IERIC) were established in 1998 in New Kansai science city in Japan. Main projects are surface modification to improve corrosion and wear resistance of materials, high temperature SiC devices for combustion control systems, refractory coatings on C/C composites, hardness enhancement for plastics, and optosensors on the basis of inorganic and organic hetero-nano-systems. New challenges for micro-fabrication by ion implantation by using a vacuum arc evaporator and mimetic bio-materials synthesis with biocompatibility are proposed."

Professor Dmitriev gave a presentation of the status of SiC high temperature materials and electronics in the United States. Mr. Chris Clarke discussed microwave power devices formed of SiC, and Professor Michael Shur discussed the modeling and fabrication of GaN-based electronics.

RESULTS OF 1997 R&D ON COMBUSTION CONTROL SYSTEMS FOR ENERGY CONSERVATION

Objectives and Content of R&D

Demand for more advanced and optimal combustion control systems has been growing, from the viewpoint of both energy conservation and effective energy use. Realization of a control device operable at high temperatures is needed. The objectives of this R&D are to develop basic technologies for realizing a silicon carbide-based control device (hereinafter referred to as a "SiC device") that would meet the above-mentioned requirement, to manufacture the SiC device on a test basis, and to realize an energy-saving combustion control system technology incorporating the SiC device. More specifically, to achieve an advanced and optimal combustion control system, R&D is underway on the following basic technologies, with the aim of realizing a SiC device operable in high temperature environments:

• basic technology for manufacturing SiC wafer


• basic technology for p-n conductivity control


• basic technology for the SiC device


• combustion control system technology

THE OUTLINE OF 1997 R&D RESULTS

R&D on Basic Technology for SiC Device Fabrication

Crystallinity of commercially available SiC wafers has been characterized by X-ray topography. Dislocations in SiC wafers exist randomly. The dislocation density is about 1x10³~2x10³/cm², which is about 100 times higher than that of an Acheson 6H-SiC crystal. In addition, SiC wafers are warped, and radius of curvature for wafers is less than 18 m.

Ion Engineering Research Institute Corporation (IERI)

IERI has characterized the electric property of the thermal oxide layer by ion irradiation onto the Si face of 6H-SiC. The I-V characteristic indicates a high breakdown field and high resistivity of the oxide layer, and the C-V characteristic and DLTS measurement indicate residual damage by ion irradiation near the interface region of SiO₂/SiC.

4H-SiC Schottky rectifiers with vanadium-implanted guard rings have been fabricated by using highly resistive regions at the periphery of Schottky contacts. The reverse characteristics of 4H-SiC Schottky rectifiers with guard rings improve in comparison with the rectifiers without guard rings. A maximum breakdown voltage of 1630 V has been achieved. The breakdown field of the Schottky rectifier is very close to the ideal value of 4H-SiC.

IERI has confirmed a p-type layer synthesis by Sc⁺ implantation into an n-type SiC layer. As the solubility of Sc in SiC is very low, the annealing after Sc⁺ implantation causes the out-diffusion of Sc to get away from the surface of SiC. The C-V characteristic and SIMS measurement indicate that the electrical activation of implanted Sc in SiC is around 50%.

Technical Development for SiC Single Crystalline Wafer Process

Nippon Steel Corporation (NSC)

NSC has examined the validity of Rutherford backscattering spectrometry (RBS) to characterize the thickness of the damaged layer that should be removed in the final polishing process. Transmission electron microscope (TEM) observations and thermal oxidation followed by HF dip reveal that the RBS estimation of the thickness by the surface energy approximation is valid for relatively shallow damage (less than 70 nm). However, RBS is rather limited for a thick, damaged layer. The recovery of the ratio of the aligned to the random yields is favorable for the characterization of deep damage. RBS has proven to be a powerful tool, capable of characterizing the thickness and crystallinity of a thin surface-damaged layer.

NSC has also studied the possibility of chemomechanical polishing technology in the final polishing process. CMP technology for Si seems to be effective in reducing polishing induced damage. Although many problems still remain to be solved, progress in CMP technology for SiC could lead to the realization of a damage-free polishing technology in SiC wafer production.

The next subject in the slicing process has been to clarify problems for application to 2-inch crystals. Heavy wear of the diamond blades has been observed in 2-inch wafer slicing. It is necessary to prevent the diamond from wearing. More study should be undertaken.

R&D on SiC Sensor Devices

Matsushita Electrical Industrial Corporation

The HREELS measurement indicates that the 6H-SiC (0001) SiÖ 3xÖ 3 reconstruction is formed by additional Si atoms that have one dangling bond each. Monohydridation of the 6H-SiC (0001) SiÖ 3xÖ 3 is achieved by annealing it at 600° C in a hydrogen atmosphere. Surface structure controls epitaxial growth, in which the 6H-SiC (0001) SiÖ 3xÖ 3 surface is at a temperature of 1050° C. Heteroepitaxial CVD/MBE 3C-SiC films exhibit strong anisotropic electrical characteristics. Stacking faults observed with microtwins by TEM cause the strong anisotropic characteristics of the heteroepitaxial CVD/MBE 3C-SiC.

R&D on High Temperature and High-Speed SiC Devices

Sanyo Electric Corporation (SEC)

SEC has conducted evaluations at high temperature on the prototype of a 6H-SiC MESFET with an Au-gate and confirmed operation at temperatures as high as 400° C. At high temperatures, both drain current and transconductance (g_m) values drop to half of that at room temperature. High temperature characteristics for the Schottky gate diode indicate no significant reverse current leakage.

SEC has also developed a mesa-etching technology for the tapered structure of device edges in order to avoid broken gate wires. Then it has sought to improve element characteristics by producing a 4H-SiC MESFET prototype with a Pt-gate that diffuses very little Pt into the SiC at high temperature. As a result, SEC has achieved a g_m of 12.5 mS/mm, a pinch-off voltage of -7V, and a drain breakdown voltage of 140 V at room temperature using an element with a channel length of 1 m m, a channel width of 60 m m x 2, a channel thickness of 0.2 m m, and a source drain distance of 4 m m. SEC has also confirmed high temperature operation at up to 400° C for the 4H-SiC MESFET with a Pt-gate. Both the 6H- and 4H-SiC MESFETs indicate hysteresis characteristics during both heating and cooling that are probably due to RIE-caused changes in the SiC surface.

Finally, SEC has studied UV sensors, ion implanted nitrogen into p-type 6H-SiC as well as Al into n-type 6H-SiC. It has fabricated a narrow junction measuring just 0.2 m m depth and confirmed diode characteristics as part of an overall strategy aimed at integrating MESFETs and UV sensors.

Technical Developments on SiC Devices for Combustion System Control

Mitsubishi Electric Corporation

Mitsubishi has studied ion-implantation techniques for deep p-n junctions. It has implanted Al ions with a high energy of 1.0-7.0 MeV onto a 4H-SiC substrate heated to 1000° C for the first time. SIMS has been used to examine a Al depth profile where the Al was implanted to the depth of 3 microns with a dose density of 1.0 x 10¹⁹/cm³. RBS characterization indicates that the implantation causes irregularity of the SiC crystal. The irregularity of the SiC implanted at 1000° C decreases drastically compared to that obtained from room temperature implants.

Mitsubishi also has studied ohmic n-type contact electrodes formed of Ni. The electrodes have been deposited by electron-beam evaporation and annealed by rapid lamp annealing. Mitsubishi has also precisely characterized the contact resistance using TLM. The contact resistances are ~1.0 x 10^{-4W ×} cm² and 4.0 x 10^{-5 W ×} cm² for the carrier density of 8.0 x 10¹⁸/cm³ and 1.0 x 10¹⁹/cm³, respectively. The temperature of annealing must be more than 960° C to reduce contact resistance.

Mitsubishi has also studied treatments before epitaxial crystal growth. The surfaces of SiC substrates have been treated by several methods, such as polishing, RIE, thermal oxidation, HF etching, and, thermal etching, and have been observed by AFM. The use of RIE after polishing is most suitable for forming a flat surface. Thermal etching forms a surface with crystal steps at regular intervals. The best condition for thermal etching is an atmosphere of hydrogen, a substrate temperature of 1400° C, and 30 minutes etching time.

R&D on Combustion Control System

Ion Engineering Research Institute Corporation

Ion Engineering has measured the temperature and wavelength dependencies of SiC photosensors. The wavelength dependency measurement indicates that the SiC photosensor has a sensitivity between 200 and 400 nm and has a maximum sensitivity at 250 nm with a quantum efficiency of 0.8. This result shows that the sensitivity of the SiC photosensor is better than that of a Si photosensor for UV detection and that it holds its function above 200° C. However, the results indicate that the temperature and wavelength dependencies change from device to device. In order to stabilize its sensitivity, the manufacturing method of SiC devices needs to be refined.

A prototype of flame detector has been fabricated with the SiC photosensor evaluated above. The ignition of a butane gas flame triggers the functioning of the detection circuit. The detector picks up UV luminescence and sets off an alarm.

R&D ON FUNDAMENTAL STUDY FOR SiC SEMICONDUCTORs

Fundamental Study of Silicon Carbide by New Crystal Growth Method

Faculty of Engineering and Design, Kyoto Institute of Technology

The purpose of this research is to make high quality SiC epitaxial layers for devices having high breakdown voltages. For device applications, a thick epitaxial layer is required. In the conventional CVD method, growth rate of SiC is only about 3 m m/h. This study has used a CST method (Close Space Technique) that enables researchers to obtain thick epitaxial layers safely, simply, and more quickly.

In the CST method, an epitaxial layer is grown by sublimation. The method is basically the same as the conventional sublimation method for boule growth of SiC. One big difference in configuration is the distance between source and substrate. The SiC source and the substrate are closely spaced by thin graphite spacers. Using such a configuration, unwanted free carbon from the graphite wall is minimized, and growth is promoted under a quasi-equilibrium condition. Source materials include a 3C-SiC polycrystalline plate with high purity and Acheson and commercially available wafers with 3.5° and 8° off (0001) toward <1120>.

On the on-axis substrate, the surface morphology depends on the polarity of the substrate. Surface morphology of epitaxial layers grown on Si-face are smoother than the C-face. Though growth rate on the C-face is higher than on the Si-face, the morphology is rougher.

On the off-axis substrate, the surface becomes rather smooth compared with the on-axis substrate. Under a growth pressure of 760 Torr and a growth temperature between 2200-2400° C, growth rate is between 40 and ~200 m m/h. The epilayer on off-axis substrate has a mirror-like morphology. However, under microscope, a stripe-like morphology perpendicular to the <1120> off-direction appears. Researchers have also investigated surface morphology dependence on growth parameters such as temperature and distance between source and substrate. Photoluminescence measurement has confirmed that the epitaxial layers are of high quality.

SiC Thin Film Formation by Ion Beam Techniques

Osaka National Research Institute, AIST

Researchers have developed new methods for SiC thin film formation using ion beams: negative and positive ion beams deposition, negative ion beam self-sputtering, and vacuum arc plasma for its components. The results are summarized as following: Increasing the ion beam current allows for direct ion beam deposition of SiC in the negative and positive ion beams deposition technique. In the negative ion beam self-sputtering technique, RBS reveals no impurities in the deposited Si and SiC films. The initial stages of growth have also been observed, and multilayers have been formed using vacuum arc plasma.

Doping Technology by Nuclear Reaction Methods

Department of Nuclear Engineering, Kyoto University

Researchers have studied the neutron irradiation effects on 4H- and 6H-SiC crystals using electron spin resonance (ESR), optical absorption, resistivity, and Hall effect measurements. Neutron irradiations have been carried out using a research reactor (KUR), a fast source reactor (YAYOI), a fast neutron source (FNS), and an electron linac (Linac). Optical absorption and ESR spectra of 4H-SiC crystals irradiated by fast neutrons have been measured at liquid nitrogen temperature. A broad absorption band can be observed at about 780 nm in 4H-SiC. An ESR spectrum labeled Tl center is observed in fast neutron irradiated 4H-SiC. The origin of the 780-nm band and Tl center have been tentatively attributed to an electron-trapped silicon vacancy. The irradiation temperature effect for the production rate of the 780-nm absorption band has been investigated. Results show that the production rate at 15ēK is lower than the one at 360ēK. Isochronal annealing of neutron irradiated 4H-SiC shows that resistivity is annealed at two stages (373ēK, 523ēK) and that about 40% resistivity is recovered.

Characterization of SiC Single Crystals by Raman Spectroscopy

Osaka University

Study of basic properties of SiC at high temperatures is important for the development of SiC devices, and a number of electrical parameters for p-type materials should also be investigated. Establishing an optical method for characterizing thin top layers for the study of epitaxial and ion-implanted layers is an urgent problem. For these targets, researchers have performed mainly microscopic Raman studies and obtained the following results: Firstly, spectral profiles of optical phonon bands have been observed in detail up to 1200° C. Researchers have found that phonon-phonon interactions determine the profiles and that the effects of thermal carriers can be neglected. The results have been applied to assessment of temperature in SiC diodes in operation. By analyzing LO-phonon-plasmon coupled modes in heavily-doped n-type samples, Osaka University researchers have deduced temperature variation in carrier concentration and mobility. Secondly, researchers have observed a very weak carrier-concentration dependence for the coupled mode in p-type samples. The intensity of the inter-valence-band transition in the low frequency region and the Fano interference feature can be used as measures of hole concentration. Finally, a Raman microprobe system for ultraviolet laser excitation has been developed. Researchers have tested its performance using ion-implanted SiC samples and confirmed its superiority to systems with visible laser excitation for surface characterization.

Optical Characterization of SiC Crystals

Institute of Space and Astronautical Science (ISAS)

High temperature electronics is regarded as a key technology for space exploration. SiC is one of the most promising materials for high temperature electronic devices. The purpose of this study is to improve the crystalline quality of SiC wafers through the accurate analysis of impurities and defects in SiC by photoluminescence (PL) method. Deep-level PL has been investigated in 4H-SiC wafers utilizing below bandgap excitation. The excitation with a photon energy of 2.54 eV induces well-defined vanadium-related lines. Microscopic mappings of these emissions reveal that the intensities increase around macrodefects such as cracks and hexagonal defects, although their intensity patterns are substantially different. ISAS engineers suggest that the gettering effect of the macrodefects is responsible for microscopic intensity variations. The strain field draws impurities and point defects participating in the deep-level PL processes towards the macrodefects. Better understanding of the gettering effect is crucial for advanced device fabrications as is true for state-of-the-art silicon devices.

Study on Modification of SiC Electrical Properties Using a Pulsed Laser Irradiation Method

Nagoya Institute of Technology

A p-n junction has been formed in n-type SiC by Al doping at the surface region by controlling the dopant activation using the pulsed laser irradiation method. Similarly, a W ohmic electrode with a flat surface on n-type SiC for operation at high temperature has been formed. Furthermore, Nagoya Institute researchers have attempted to apply the CAICISS method to analyze crystallinity within thin surface layers of SiC.

The results of these investigations are shown as follows:

• CAICISS method is able to analyze the crystallinity of thin surface damaged layer induced by lapping SiC that was not observed by RBS measurement method.


• The formation of a W ohmic electrode with a flat morphology on SiC has been achieved using laser irradiation with a beam homogenizer.


• The laser irradiation method in SiC can be used to effectively modify electrical characteristics by changing the dopant profile and concentration through the use of controlled laser energy density and number of laser shots.

R&D on Study of Crystalline Surface Using STM and Silicide Electrode

Okayama University

4H- and 6H-SiC (0001) Si surfaces have been studied by scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), photoelectron spectroscopy (PES), etc., where clean specimen surfaces have been prepared by chemical treatment followed by heat treatment in an ultrahigh vacuum. Depending on the combination of chemical etchings and heat treatment temperatures, 1x1, Ö 3 x Ö 3, and 6Ö 3 x 6Ö 3 structures have been observed. Atomic structural models have been proposed for reconstructed surfaces. Au- and Cu-deposition processes have been studied on these cleaned surfaces. They have been found to form islands at the beginning of the deposition, and the island forming tendency has been found to be higher in Cu than Au.

R&D on Doping into 6H-SiC

Department of Electronic Science and Engineering, Kyoto University

The impurity incorporation mechanism in chemical vapor deposition (CVD) of SiC has systematically been investigated. Donor-type impurities such as N and P are more efficiently incorporated on a C-face whereas the doping efficiency of acceptor-type impurities such as Al and B is higher on a Si-face. Effects of substrate orientation and surface polarity on impurity incorporation have been revealed. The concentrations of N, Al, and B atoms drastically decrease with increasing growth temperature, which may be ascribed to the enhanced desorption of impurity atoms from a growing surface at high temperature. Al and B ion implantation into n-type SiC epilayers have been studied. A nearly perfect electrical activation ratio (>90%) could be attained by high temperature annealing at 1550-1600° C for Al and 1700° C for B ion implantation. Mesa p-n junction diodes formed by either Al or B ion implantation exhibit high blocking voltages of 950-1070V, which are 80-90% of the ideal value predicted in the diode structure. These diodes are operational even at a temperature of 400° C. B-implanted diodes show higher blocking voltages and lower leakage currents on average. However, one severe drawback of B-implanted diodes is their poor forward conduction.

References