RAPID THERMAL ANNEALING OF TiW SCHOTTKY CONTACTS ON GaAs

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RAPID THERMAL ANNEALING OF TiW SCHOTTKY CONTACTS ON GaAs

M. van Hove, M. de Potter, W. de Raedt, G. Zou, M. van Rossum

To cite this version:

M. van Hove, M. de Potter, W. de Raedt, G. Zou, M. van Rossum. RAPID THERMAL ANNEALING

OF TiW SCHOTTKY CONTACTS ON GaAs. Journal de Physique Colloques, 1988, 49 (C4), pp.C4-

445-C4-448. �10.1051/jphyscol:1988494�. �jpa-00227992�

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JOURNAL DE PHYSIQUE

Colloque C4, supplement au n°9, Tome 49, septembre 1988 C4-W5

RAPID THERMAL ANNEALING OF TiW SCHOTTKY CONTACTS ON GaAs

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 1050oC 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 T E M , 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 T i .

1 I N T R O D U C T I O N

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 E X P E R I M E N T S

After degreasing and in situ Ar sputtercleaning, thin films (5Qnm-200nm) of TiW were deposited on Si-doped GaAs wafers (n ~ E17/cm3) 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

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C4-446 JOURNAL

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PHYSIQUE

Fig.1

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High resolution cross-sectional TEM 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

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SIMS depth profiles of Ti, W, Ga and and

0

of as-deposited and annealed (880°C, 10s) As of as-deposited and annealed (950°C, 10s) T i W

T i W films on GaAs. films on GaAs.

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3 RESULTS A N D 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 t o grain size growth. High resolution cross-sectional T E M 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 i n the T i W film could be detected by TEM, due t o 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 T i resulting i n surface accumulation as well as T i diffusion into the GaAs substrate. The W7GaAs interface however remains stable for temperatures up t o -1000°C.

Forward and reverse current-voltage characteristics of as-deposited and annealed T i W Schottky contacts are shown i n fig.5. They show a continuous increase i n 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 o f as-deposited and annealed TiW/GaAs contacts are depicted i n fig.7. Unan- nealed diodes show a soft turn-on o f the breakdown, which is characteristic for Schottky diodes. After annealing the breakdown voltage increases and the characteristics become avalanche-like. Measurements at higher temperatures show an increase i n 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 t o the indiffusion and activation o f T i which is known t o have acceptor levels i n GaAs 15.61. Due t o the p+-layer formation a barrier height enhancement occurs. W i t h increasing annealing temperature the p+- layer becomes thicker and/or more highly doped. This explains the steady increase o f the barrier height with annealing temperature. It is estimated that a 1.E18/cm9 p+-layer of 2 0 0 2 thickness can account for the 250meV difference i n 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 i n a self-aligned MESFET technology.

ACKNOWLEDGMENTS

The authors are pleased t o thank M. Meuris for SIMS, H. Bender for Auger, A. Demesmaecker for EDS and XRD and J. Vanhellemont for T E M analysis.

REFERENCES

/I/ Kohn, 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/ ~ o r h i l o v , B.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.

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C4-448 JOURNAL D E PHYSIQUE

150 0 300 0 350 0

CHANNEL

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s 7 S c , 1 0 s

2 5 0 0 350 0

CHANNEL

Fig.6

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Ideality factors and barrier heights for TiW/GaAs contacts annealed at different temperatures.

C H A N N E L

Fig.4

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RBS spectra o f TiW/GaAs samples annealed at different temperatures.

Fig.5

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Forward and reverse current- voltage characteristics for TiW/GaAs contacts annealed at different temperatures.

Breakdown Voltage (V)

Fig.7

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Reverse leakage current versus reverse applied voltage o f as-dCposited and annea1ed'(95O0C.

10s) TiW/GaAs contacts.