Ultra-High Implant Activation Efficiency in GaN Using Novel High Temperature RTP System
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Sekhar **, J. C. Zolper ***, D. J. Rieger ****, J. Han ****, T. J. Drummond ****, R. J. Shul **** and R. G. Wilson ***** *
Department of Materials Science and Engineering, University of Florida, Gainesville, FL
32611 USA ** Micropyretics Heaters international, Inc. Cincinnati, OH 45212 USA *** Office of Naval Research, Arlington, VA 22217 USA **** Sandia National Laboratories, Albuquerque, NM 87185 USA ***** Consultant, Stevenson Ranch, CA 91381 USA
ABSTRACT Si' implant activation efficiencies above 90%, even at doses of 5x10 15 cm 2, have been achieved in GaN by RTP at 1400-1500 'C for 10 secs. The annealing system utilizes with MoSi 2 heating elements capable of operation up to 1900 'C, producing high heating and cooling rates (up to 100 °C . s-). Unencapsulated GaN show severe surface pitting at 1300 'C, and complete loss of the film by evaporation at 1400 'C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AIN and 3.4 eV for InN). Encapsulation with either rf-magnetron reactively sputtered or MOMBE-grown AIN thin films provide protection against GaN surface degradation up to 1400 'C, where peak electron concentrations of -5x 1020 cm-3 can be achieved in Si-implanted GaN. SIMS profiling showed little measurable redistribution of Si, suggesting Dsi < 10-13 cm 2 . s-1 at 1400 'C . The implant activation efficiency decreases at higher temperatures, which may result from SiGa to SiN site switching and resultant self-compensation. Introduction Ion implantation is an enabling technology for fabrication of GaN-based ultra-high power thrysistors, Junction field-effect transistors (JFETs) and heterostructure field effect transistors
(HFETs)( 1'"1). In particular selective area implantation can be used to reduce transistor access resistance by creating highly doped contact regions. Most published device characteristics show evidence of relatively high access resistances (1-14). To date GaN JFETs formed entirely by implantation into undoped material(2 ) and GaN light emitting diodes (LEDs) formed by Mg+ implantation into n-type epi layers (5) are the only devices fabricated using implant doping. Past work has shown that in compound semiconductors the annealing temperature required for implant activation is a fairly high percentage of the melting temperature of the material, and 6 is also a function of the implant dose( ' 17). In GaN it has been observed that low dose (•< 5 X1014 cm2) implants anneal poorly up to 1100 °C, leaving a coarse network of extended defects, while high dose ( 2x10 15 cm 2 ) implants may lead to amorphization. Amorphous layers recrystallize in the range 800-1000 'C to form defective polycrystalline material(ls8 . Quite good activation9 efficiencies have been obtained for n-type implanted dopants in spite of high residual damage(' 21). It is also clear that annealing temperatures above 1300 'C are desirable for optimal electrical properties in the implanted layers' 9' 20) The equilibrium N2 pressure over G
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