SiC Developers on the Cutting Edge

Jo Ann McDonald Reminiscent of the early days of GaAs, when the fledglings were first trying

their new commercial wings, the silicon carbide (SIC) community is just starting to define itself. Well ahead of the fullerenes, more

focused than HTSCs, and trying hard not to repeat the painfully slow development timeframe GaAs

experienced, I continue our ongoing reportage of the evolution

of the SiC industry.

A s another of the wide band- gap materials in pursuit of the elusive "blue laser," SiC

can be generally grouped with CVD diamond and nitride development, among other related compounds. As device development begins to super- sede materials research, these various thin film technologies will likely pursue very different niche applica- tions. However, unlike GaAs and its related compounds, SiC is interest- ingly compatible with advanced sili- con processes. We cont inue our coverage of SiC by taking a look at who's who in SiC research, who's leading in device development, and what steps the community is taking to further organize itself.

By definition the SiC "community" is very international, with no one country in a clear leadership position anymore, considered by many to be an especially healthy sign in these days of stepped-up international co- opera t ion . The field is reaching critical mass fast and some very important systems companies such as Westinghouse and General Elec- tric in the US, Nippon Steel in Japan, Siemens in Germany, and Thomson in France are already considered major players.

The primary catalysts at this stage of development come from a variety of sectors. Key universities include Nor th Carol ina State Universi ty (NCSU), Howard University, Case

The poster session at I C S C R M 93.

Western Reserve University, Cornell, John Hopkins, and the University of Illinois in the US, Kyoto University in Japan, University of Erlangen in Germany, Linkoping in Sweden, and the Ioffe Institute in Russia, among a very long list of prestigious academic centres. Historically, the Russians have clearly contributed the most, and as those scientists move from their large institutes to more entre- preneurial pursuits, we see leading Russian developers at various com- panies all over the world. We also look forward to a strong entrepre- neurial effort within Russia itself.

That relatively rapid cross-polleniza- tion bodes well for pushing the traditional timelines it takes for any new technology to gain commercial viability.

The military has been the tradi- tional driver, obviously in the former Soviet Union. In the US, the Office of Naval R e s e a r c h , Army Research Labs, Wright Patterson representing the Air Force, NASA Lewis, and DoD's ARPA have been the agencies most active in SiC R&D.

In addition to the various large company R&D sectors, two entrepre- neurial companies seem to have the

Page 37 Vol 7 No 1

inside track on providing that neces- sary "merchant foundry" ingredient all technologies must inevitably have if they are ever to realize true commercial volume production. Cree Research (Durham. North Carolina) was the first, but Advanced Technol- ogy Mater ia ls ( A T M I , Danbury , Connecticut) is the new shining hope lbr providing that critical "second source" everyone's looking for at this stage. Cree was featured in our first SiC coverage in Vol. 6 Issue 5, October last year, and ATMI will be the subject of in-depth coverage next issue.

Although not as Ii~r along in device development as Cree at this stage, ATM! is an extremely good prospect for the kind of company people like to work with, tbr it is concentrating on both CVD diamond device develop- ment as well as SiC, and the thin fihn expertise helps both pursuits. Like Cree, ATMI recently went public on the Nasdaq stock exchange (symbol "ATMI") and in its prospectus sun> rnary, the company states that they believe diamond and silicon carbide devices, which they collectively classi- fy as "diamond semiconductors," will offer "significant advantages over today 's silicon devices due to the high power and high temperature performance characteristics of these materials." We'll look forward to hearing from Duncan Brown, Vice President of ATMI, in our next issue, and to getting an update on ATMI ' s diamond progress along with details about their SiC pursuits. Their lineup of c o r p o r a t e aff i l ia t ions is most impressive.

"Ice Cream" '93 and '95 As repor t ed last issue, the SiC catalysts held their 5th "sometimes annual meeting" in Washington, DC in early November. The event is now official ly called I C S C R M ,ahich stands for International Conference on SiC and Related Materials. Some- one fl'om outside the meeting, upon seeing my badge, asked "what 's with the Ice Cream?" Not a bad ~av to remember the otherwise non-acro- nym. The event attracted over 400 people, a record a t tendance and extremely impressive, especially' when looking at who the attendees were. Following the meeting, the organizing

Page 38 Vol 7 No t

SiC R&D at Howard University.

committee elected to wait until the fall of 1995 for their next meeting, which will be held in Japan, and hosted by Professor Matsunami of Kyoto Uni- versity, a recognized leader in the field.

The Japanese are obviously very excited about SiC. On the 18th of November, just after the US event, Japan 's MITI hosted their 2nd annual meeting of the Japan Society of SiC and Related Wide Band-Gap Semi- conductors.

Device Progress Howard University's Mike Spencer, chairman of the technical committee for ICSCRM '93, summarized the overall progress reported in Washing- ton DC, first from the device perspec- tive. "I think we got some very good results in the microwave area from both Westinghouse and Cree, in terms of standard microwave transistors. The power MESFET transistor looks like it's going to be a good SiC product, and we've now seen devices m a d e out o f bo th 6H and 4H materials. The 4H materials show the greater promise because of the higher electron mobility, but the 6H produc t s at this stage are more advanced."

The Westinghouse group reported an output power of 1.8 watts from a device with a 1.9-2 mm periphery, operating at a frequency of 1 GHz, with an associated gain of 9.5 dB. Of note is that the RF power density of

0.94 W/ram is already superior to values obtained with similar GaAs devices. Those similar GaAs values are 0.5 - 0.6 W/ram, indicating a real area where SiC can be competitive. SiC is predicted to be about a factor of 10 better than silicon and is already showing approximately a factor of 2 better.

"Cree provided some very nice n u m b e r s on t he i r 4H dev ices , whereas Westinghouse concentrated on repor t ing on their 6H work because they are really more of a production device than the 4H just coming out of the laboratories," notes Spencer. "The most important feature about these SiC devices is the fact that, at a given frequency, the power density is going to be significantly higher than GaAs devices."

SiC has extremely good thermal conductivity so the heat gets taken out of the device more easily. It also has a higher breakdown field so more voltage can be applied across the device. The combination of getting the heat out more efficiently and applying more voltage means it can operate at a higher power density than GaAs or Si which makes it a very good candidate for a microwave device. "The other devices that seem to be moving on quite nicely are the SiC MOS (Metal Oxide Semiconduct- ing) devices which are useful as power switches and as potential logic ele- ments. General Electric has been doing a lot of nice work on some of the MOS processing techniques and

they 've been doing work on ion implantation to produce MOS de- vices," Spencer notes.

The question of getting an IM- PATT diode to work in SiC remains elusive. To Spencer's knowledge, that still hasn' t been done. "Just like in a microwave transistor, for all the same reasons, SiC is an excellent candidate for making these I M P A T T diodes which are two terminal devices and have applications as oscillators."

Materials Progress and Bottlenecks The big question on everybody's mind is how to improve the quality of substrates . Reminiscent of G a A s substrate development, SiC boules are full of defect densities, called "micropipes." Mike Spencer, who heads the Materials Science Center of Excellence (MSCE) at Howard University, which is a leader in the SiC substrate field and one of the first to work with the Russian scientists, sums up the current status. "The whole question is how do you get good substrates in large areas. Typi- cally, the field has been looking pr imar i ly at 6H, and now most people think that the polytype of interest will be 4H. The question is, how do you grow large areas of these two polytypes?" Spencer holds a slightly different view, and certainly one worth keeping in mind. "I think 3C is the one that holds the best prospect, but I think you probably should concentrate on 6H and 4H for now, for 3C has not been demon- strated to be done in large areas. In the short term, most people would say 6H is the technology for right now, and we think just around the corner is 4H." The materials people are going to have to deal with a lot o f micropipes before SiC can become a competitive technology.

There's a prevailing theory that the presence of micropipes is strongly related to the presence of disloca- tions, and that in materials with large Burgess vectors, like 6H, a micropipe can form in regions of high stress. "That was a view that was presented. That 's a point of discussion. 1 don' t think that can be called an accepted view," says Spencer,

The group reporting from NASA Lewis demonstrated pretty conclu- sively at the Washington meeting

Page 39 l l l I ~ ¥ ~tlVol 7 No 1

that micropipes have definitely been demonst ra ted to serve unsatisfactory functions, like reducing the break- down voltage, and will have to be elmainatcd.

What to Expect Next It 's speculated that we may next see conflict ing s ta tements as materials makers claim they ' re making pro- gress, which could become a difficult ptoblem in itself. One proposed way of eliminating micropipes is through growth along the a-axis direction. "That was investigated at the con- ference but it's not certain that a-axis growth doesn ' t bring along enough problems of its own to be considered a solution. One especially interesting group of papers was presented about growing bulk crystals along the a-axis and growing epitaxial layers on the a- axis," says Spencer, Wr}'I+O thinks the substrates remain the biggest pro- blem.

One of the special success stories reported was that of the N A S A Lewis group, where they showed an ability to control doping over a very wide range, between 10 ~5 and 1() ~'+. "That

was a nice result, and the Japanese presented a number o f papers on unders tanding the growth mechar~- isms OUt Of Professor Matsunami ' s group at Kyoto ." says Spencer.

Time for a Business Opportunities Workshop? Likc G a A s before them, SIC', its related compounds , and d iamonds are a l r eady fo l lowing a fami l ia r development course. This time. the overa l l goal is to c o m p r e s s the d e v e l o p m e n t t imeline. No longer need five years stretch out to ten, and ten to twenty+ SiC has already suffered a delay of many years due to the xirtual "non-communica t ion" that occurred betv~een the Russian and Western scientists. Now that scientists and their colleagucs m other countries arc c losch collaborating, SiC should realize commercial quality substrates and devices morc quickly than usual.

Nc,,cr before has there been such at blending of international expertise. If evety.onc c o n t i n u e s to w o r k closel~ and opcnl 3. progress could be signifi-

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3d IPRMAI).3d

March 28-31, 1994 Fess Parker's Red Lion Inn, Santa Barbara, California, USA

• Optoelectronics: Advances in lasers, detectors, optical amplifiers, and waveguide based devices, quantum structures, optoelectronic and photonic integrated circuits. New devices for optical switching, networking and signal processing. Solar Cells.

• Electron Devices: FETs and HBTs, Millimctre wave and microwave circuits. Pseudomorphic and lattice matched devices. Lm~ noise an power characteristics.

t st ,tg~et,

• Processing: New Schottky and ohmic contact~, l_ow damage dielectric deposition. Passivation and reliabili D ~ssuc.,. Selective, wet and dry etching. Degradation in quantum structures. Processing related to intelligent manufacturing.

• Epitaxy: MOCVD, MBE and related techniques. Growth of quantum structures and lattice mismatched materials. Selective epitaxy and growth on structured substrates. Epitaxial la~.er characterization.

• Bulk: Advances in crystal growth technology'. Characterization of optical, chemical and electrical propcrtics. Mcthod for in-situ and postgrowth defect and doping control. Wafer thermal stability and characteristics.

• The conference will include invited and contributed oral and poster presentations, short courses and an industrial exhibn.

General enquiries contact: Susan Exans, 1EEE LEOS. td I'av l l] (90~) 5(+2-3896 1571.

Conference Chair: John Bowers Dept of Elec. & Comp Eng. Unixersity of California Santa Barbara. CA c)3106

( 'onferencc ( o - ( 'hall: Evelyn Hu Dept. of Elec. & Comp Eng. Unixersit,, of California Santa Barbara. ( ' \ 0z;106

Local Arrangenlents: Steven DenBaars Dept of Elec & Comp Eng. University of CalilBrnia Santa Barbara. ( 'A 03106

Program ('halt: Robert Nahor~ Bcllcore 331 Newmun Sprillg:-, Road Red Bank. NJ 07701-7!)2t~

Sponsored hv IEEE Lasers aml E/ectro-Optic.s Society and IEEE Electron l)cvicu.v Socict~

For Information Circle 219 Page 40 Ivo, 7 N o l