INTRODUCTION

In its pamphlet "Development of Radiation-Resistant Semiconductor Devices for Space Applications," JAERI states, "Artificial satellites in the Van Allen Belt are exposed to energetic radiation. These satellites are equipped with many varieties of semiconductor devices, which are very sensitive to different types of radiation. Thus, the mission life and reliability of satellites are limited by the radiation damage inflicted on the devices. Using JAERI's ion beam irradiation facility (TIARA) adapted for simulating the radiation environment of space, we have been studying radiation effects on semiconductor devices for space application and developing new radiation resistant semiconductors."

Dr. Hagiwara, the Director General of the institute, indicated that JAERI has been engaged in the development of space and radiation hard high temperature devices. JAERI is a semi-national laboratory, a 92% government funded facility that is open for the use by industrial and small business partners.

WHY SiC?

For space applications, the United States seems to be ahead in the high temperature radiation hard electronics that will be needed. SiC is being pursued to demonstrate 100 times the alpha ray resistance that Si presently is capable of. SiC will find applications in total dose tolerance and single upset event resistance with major targets being the high temperature and high power device area. The institute estimates and demonstrates the technical potential of SiC and encourages private sectors to adopt this technology. Japan Science and Technology Corporation (JST) acts as a liaison for commercialization of inventions from the government sector. When private companies wish to use JAERI inventions, collaboration occurs. JAERI also collaborates with the electrotechnical laboratory under MITI. There is no collaboration with the United States at the moment. Some work in cooperation with the University of Erlangen in Germany is in process. Silicon-based device technologies lack the radiation hard qualities required of space electronics.

Takasaki Radiation Chemistry Research Establishment

Various R&D activities of radiation applications are conducted using large Co-60 irradiation facilities and accelerators for electron and ion beams. Through these activities, many useful results have been produced in such fields as environmental conservation and upgrading of polymers using electron beams and gamma rays. Further R&D of advanced radiation technology is in progress by fully utilizing the ion beam irradiation facility.

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

WHAT IS THE CRYSTAL ORIENTATION OF SiC, GaN, AND AlN GROWTH?

Mostly a 6H (0001) orientation on the Si face is needed for the substrates to grow nitrides.

ARE THERE ANY RADIATION RESEARCH STUDIES ON THE RADIATION RESISTANCE OF NITRIDES?

Radiation hardness is a new area, but the bond strengths of nitrides are very high, which provides both wide bandgap and radiation hard properties. Dr. John Zolper has performed ion implantation studies at Sandia. Some implantation studies for making p-n junctions in nitrides at the Russian Ioffe Institute have been reported, but p-type doping is difficult at present.

Dr. Nashiyama gave an excellent presentation of the ongoing radiation studies in semiconductor devices being performed at the JAERI. Interest is focused on the radiation effects in solar cells, radiation effects on MOS devices, and the observation of single event upsets using focused ion beams. Of particular attention was the demonstration of the radiation tolerance of SiC devices such as MOS structures, SRAM/flip flop, and radiation testing using both single event upset and total dose tolerance.

INDIVIDUAL RESEARCH

Dr. Itoh

Dr. Itoh, senior scientist, is working on the radiation effects in SiC to study the following:

• electrical effects


• defects structure and annealing


• ion implantation including hot implantation and co-implantation

He has studied co-implantation in collaboration with University of Erlangen. Researchers have examined the effects of carbon or silicon co-implants in the modification of impurity incorporation and lattice annealing.

Co-implantation

Dr. Itoh has added carbon implantation to boron or aluminum implants in 4H-SiC. Implant annealing was performed at 1700ºC under an argon atmosphere.

Activation of aluminum and boron implants improve when additional carbon is used. The optimum dose of co-implanted carbon is 1e 18 cm^-3 for a 5e 18 cm^-3 aluminum or boron dose.

Hot implantation (800°C) also improves aluminum activation.

K. Ohshima

Hot implantation (1200°C) of phosphorus is being investigated primarily because this impurity can be created in SiC by neutron transmutation and because it provides exceptionally uniform doping. Activation of hot-implanted phosphorus atoms has been shown with up to 5e 19 cm^-3 doping using 1400°C anneals for 20 min.

For severe environments such as space MOSFETs are being investigated. SiC MOSFETs are formed in 6H- SiC epitaxial layers on SiC substrates. Using 1200°C implants, no further annealing is needed. Gate oxides have been grown wet at 1100°C for 1hr. Using a gate length of 10 microns and 200 µm wide, transistors show V_th of 3.41 V. Gamma-ray and ion irradiation studies will be performed on these devices.

Mr. Yoshikawa

Mr. Yoshikawa conducts 6H-SiC MOS structure studies. This work investigates the oxide trapped charges in SiC MOSCAPs with the intent of employing SiC MOS devices in radiation hard environments. An oxide has been formed at a depth of 100 nm on 6H-SiC, doped at 5e 17 using 1 hr, 1100°C wet oxidation. The oxide has been slant etched using the gradual dipping technique into a buffered HF etch solution, and an array of aluminum dots has been patterned on the wafer surface with oxide thicknesses varying from 10 nm to 100 nm. Devices exposed to gamma radiation (190 K Gy) reveal the presence of trapped charges by subsequent CV measurements near the SiO₂ -SiC interface and at a distance of 40 nm from the interface.

Dr. Kojima

To provide large area 3C SiC substrates for devices, Dr. Kojima is growing 3C SiC on Si substrates using low pressure CVD and using propane, silane and TMA for aluminum doping of p-type films. He also uses (100-200 Torr) van der Waals forces to reduce stress at the silicon SiC interface during growth. He has grown films as thick as 20 microns in an attempt to reduce defects in the film.

An extensive laboratory tour to view the ion implant machines and growth equipment followed the discussions.

References