RAPID THERMAL ANNEALING OF TiW SCHOTTKY CONTACTS ON GaAs

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RAPID THERMAL ANNEALING OF TiW SCHOTTKY CONTACTS ON GaAs
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  See discussions, stats, and author profiles for this publication at:https://www.researchgate.net/publication/224126633 Rapid Thermal Annealing of TiWSchottky Contacts on GaAs Conference Paper   in  Le Journal de Physique Colloques · October 1988 DOI: 10.1051/jphyscol:1988494 · Source: IEEE Xplore CITATION 1 READS 24 5 authors , including:Marleen Emma Van HoveIMEC International 229   PUBLICATIONS   1,903   CITATIONS   SEE PROFILE Walter L. De RaedtIMEC International 359   PUBLICATIONS   2,819   CITATIONS   SEE PROFILE Guisheng ZouTsinghua University 139   PUBLICATIONS   553   CITATIONS   SEE PROFILE All content following this page was uploaded by Marleen Emma Van Hove on 10 January 2017. The user has requested enhancement of the downloaded file.  JOURNAL  DE  PHYSIQUE Colloque C4 supplement  au  n°9 Tome  49 septembre  1988  C4-W5 R PID THERM L NNE LING  O TiW  SCHOTTKY CONT CTS  ON  Ga s M.  VAN  HOVE M. de  POTTER W. DE  RAEDT G. ZOU and M. VAN  ROSSUM IMEC vzw Kapeldreef 75 B-3030 Leuven Belgium Résumé  - La  stabilité  de  l'interface TiW/GaAs durant  le  recuit rapide  à des  températures allant  de 700°C  à  1050 o C  a été  étudiée avec  les  techniques SIMS, Auger, RBS, XRD,  TEM à  haute résolution, EDS,  et  des mesures  IV et CV.  Les mesures de contacts Schottky  ont  démontré une forte croissance  de hauteur  de  barrière, allant  de  0.70eV après déposition  à  0.95eV après  un  recuit  à  SSCC.  Les  résultats indiquent  une  augmentation artificielle  de la  hauteur  de  barrière  due au  dopage  de  type  p du  substrat par une diffusion  de Ti. Absiract  - The  stability  of the  TiW/GaAs interface during rapid thermal annealing  at  temperatures between 700 C and 1050°C has been investigated using SIMS, Auger, RBS, XRD, cross-sectional TEM, EDS,  IV and CV  analysis. Schottky contact measurements showed  a  steady increase  of the  barrier height with annealing temperature from  the  as-deposited value  of  0.70eV  to  0.95eV after annealing  at 950°C.  The  results  are  consistent with  an  artificial barrier height enhancement  due to  p-doping  of the substrate  by  indiffusing  Ti. INTRODUCTION Recently, there has been  an  increasing interest  in the  application  of  refractory gate metals  in a  self-aligned GaAs metal-semiconductor field- effect transistor (MESFET) technology.  The  processing requires the gate material  to maintain  a  good rectifying contact with low leakage current and high breakdown voltage when subjected  to  high temperature annealing (~900°C) necessary  to  activate  the n +  implant.  The  refractory metal/GaAs interface has  to be  mechanically, chemically and electrically stable. Most problems are related with peeling  off,  chemical reaction and interdiffusion. Although  TiW  and TiW-based alloys have been frequently considered  as  appropriate candidates /1,2,3/,  the  stability  of TiW  during rapid thermal annealing (RTA) has  not yet  been ascertained.  In this contribution  we  report  an  extensive study  of the  TiW/GaAs interface after  RTA. 2 EXPERIMENTS After degreasing  and in  situ  Ar  sputtercleaning, thin films (5Qnm-200nm)  of TiW  were deposited  on  Si-doped GaAs wafers  n ~  E17/cm 3 )  by dc  magnetron sputtering from  a  30%Ti/70%W compound targe^  The Ar pressure was optimized  to  obtain minimum film stress conditions.  The  composition  of  the deposited  TiW  films was determined  by RBS as  25%Ti/75%W.  The  samples were subjected  to RTA in  forming  gas  ambient  at 700°C-1050°C,  10s.  Electrical measurements were performed  on  90/imx90/im Schottky diodes patterned  by standard lithography. Alloyed AuGe/Ni  was  used  as  backside contact.  In  order  to  avoid diode leakage  at the edges about 200nm  of  GaAs was etched away  in  H2SO4:H202:H2O before measurement. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988494  C4 446 JOURN L DE PHYSIQUE Fig.1 High resolution cross-sectional T M image of the TiW/GaAs interface after annealing at 900°C, 10s. Depth cpr Sputter Titne ulin) Depth cp Sputter Tiulr ~uia) Fig.2 Auger depth profiles of Ti, W, Ga, As Fig.3 SIMS depth profiles of Ti, W, Ga and and of as-deposited and annealed 880°C, 10s) As of as-deposited and annealed 950°C, 10s) TiW TiW films on GaAs. films on GaAs.    RESULTS AND DISCUSSION All the films examined showed good adherence even after annealing at the highest temperatures. XRD analysis showed a clear W a-phase spectrum. After annealing, the peaks showed a reduction of the halfwidth, which can be attributed to grain size growth. High resolution cross-sectional TEM examination fig.1) showed good TiW/GaAs interface stability. However, local island growth of epitaxial phases induced by RTA were found. No clear morphologic structure in the TiW film could be detected by TEM, due to the very strong absorption of the electron beam by the metal. Auger fig.2), SIMS fig.3), RBS fig.4) and EDS show significant motion of the Ti resulting in surface accumulation as well as Ti diffusion into the GaAs substrate. The W7GaAs interface however remains stable for temperatures up to -1000°C. Forward and reverse current-voltage characteristics of as-deposited and annealed TiW Schottky contacts are shown in fig.5. They show a continuous increase in both the ideality factor and the Schottky barrier heights extracted from IV- and CV-data fig.6). Diodes annealed at 950°C have a barrier height as high as 0.95eV IV-value), which is considerably higher than the value of 0.70eV obtained for nonannealed diodes. Significant degradation of the diodes occurs after annealing at temperatures higher than 1000°C, coinciding with the onset of W/GaAs interdiffusion. Breakdown characteristics of as-deposited and annealed TiW/GaAs contacts are depicted in fig.7. Unan- nealed diodes show a soft turn-on of the breakdown, which is characteristic for Schottky diodes. After annealing the breakdown voltage increases and the characteristics become avalanche-like. Measurements at higher tem- peratures show an increase in breakdown voltage, which confirms avalanche being the breakdown mechanism. The RTA-induced properties of the contacts are consistent with the Shannon contact structure 4 metal/p+- GaAs/n-GaAs). The p+-formation is attributed to the indiffusion and activation of Ti which is known to have acceptor levels in GaAs 15 61 Due to the p+-layer formation a barrier height enhancement occurs. With increasing annealing temperature the p - layer becomes thicker and/or more highly doped. This explains the steady increase of the barrier height with annealing temperature. t is estimated that a 1.E18/cm9 p+-layer of 2002 thickness can account for the 250meV difference in the barrier height between the as-deposited and 950°C annealed diodes. The avalanche breakdown behaviour of the annealed diodes is consistent with the formation of a pf -layer between the metal and the n-GaAs substrate. Breakdown will be initiated by avalanche multiplication at the pf /n junction. Our observations show that rapid thermal annealed TiW/GaAs contacts, and Shannon contacts in general, can have advantageous properties for application in a self-aligned MESFET technology. ACKNOWLEDGMENTS The authors are pleased to thank M. Meuris for SIMS, H Bender for Auger, A. Demesmaecker for EDS and XRD and J. Vanhellemont for TEM analysis. REFERENCES I ohn, E., IEDM Techn. Dig. 1979) 775. /2/ Yokoyama, N., Mimura, T., Fukata, M. and Ishikawa, M.. International Solid-State Circ. Conf. Dig. of Techn. Papers 1981) 218. /3/ Geissberger A.E., Sadler R.A. Balzan M.L. and Crites J.W., J. Vac. Sci. Technol. B5 6) 1987) 1701. /4/ Schwartz, G.P.and Gaulieri, J. Electrochem. Soc. 133 1986) 1266. /5/ ~orhilov, .V., Marchukov,L.V. and Ergakov V.K., Sov. Phys. Semicond. 8 1974) 141. /6/ Ushakov, V.V. and Gippius A.A., Sov. Phys. Semicond. 16 1982) 1042.  C4 448 JOURN L DE PHYSIQUE 15 3 35 CHANNEL 7Sc 10s 25 35 CHANNEL Fig.6 Ideality factors and barrier heights for TiW/GaAs contacts annealed at different tempera- tures. CHANNEL Fig.4 RBS spectra of TiW/GaAs samples an- nealed at different temperatures. Fig.5 Forward and reverse current- voltage char- acteristics for TiW/GaAs contacts annealed at dif ferent temperatures. Breakdown Voltage V) Fig.7 Reverse leakage current versus reverse applied voltage of as-dCposited and annea1ed (95O0C. 10s) TiW/GaAs contacts. View publication statsView publication stats
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