VERY LOW RESISTIVITY AuMn GATE OHMIC CONTACTS FOR GaInAs DIFFUSED JFETs

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VERY LOW RESISTIVITY AuMn GATE OHMIC CONTACTS FOR GaInAs DIFFUSED JFETs

P. Hallali, P. Blanconnier, L. Bricard, J.-C. Renaud

To cite this version:

P. Hallali, P. Blanconnier, L. Bricard, J.-C. Renaud. VERY LOW RESISTIVITY AuMn GATE

OHMIC CONTACTS FOR GaInAs DIFFUSED JFETs. Journal de Physique Colloques, 1988, 49

(C4), pp.C4-453-C4-456. �10.1051/jphyscol:1988496�. �jpa-00227994�

JOURNAL DE PHYSIQUE

Colloque C4, supplément au n°9, Tome 49, septembre 1988 C*-*53

VERY LOW RESISTIVITY AuMn GATE OHMIC CONTACTS FOR GalnAs DIFFUSED JFETs

P.E. HALLALI, P. BLANCONNIER, L. BRICARD and J.C. RENAUD

Centre National d'Etudes des Télécommunications, Laboratoire de Bagneux, 196, Av. Henri Ravera, F-92220 Bagneux, France

RESUME - Tant pour les transistors à effet de champ à jonction que pour les transistors bipolaires à hétérojonction de la famille GalnAs/InP, l'obtention de contacts ohmiques de type P à faible résistivité est une étape particulièrement critique en raison de la forte hauteur de barrière Schottky sur ces matériaux. La réalisation d'un surdopage p+ par diffusion de Zn en boîte semi-fermée ainsi que l'utilisation de l'alliage MnAu ont permis de résoudre ces problèmes : une résistivité de contact aussi faible que 10~7 H cm2 a en effet pu être obtenue.

ABSTRACT - For GalnAs/InP junction field effect t r a n s i s t o r s as well as heterojunction b i p o l a r t r a n s i s t o r s , t h e achievement of very low r e s i s t i v i t y P type ohmic contact i s a very c r i t i c a l s t e p because t h e Schottky b a r r i e r h e i g h t on t h e s e materials i s q u i t e high. The realization of a highly doped P+ layer by Zn diffusion in a semi-closed box and t h e u s e of MnAu alloy contact have allowed t o solve t h e s e difficulties : in fact, a contact r e s i s t i v i t y as low a s 10~7 D. cm2 h a s been obtained.

1 - INTRODUCTION

GalnAs and AlGalnAs materials, l a t t i c e matched to InP, a r e c u r r e n t l y of f i r s t importance for optoelectronic devices in t h e 1.3 - 1,55 um wavelength range / l / . Furthermore, t h e combination of a high electron mobility and good q u a l i t y h e t e r o j u n c t i o n s p r e d e s t i n a t e them for high frequency systems /2/. To t a k e advantage of t h e s e p o t e n t i a l i t i e s , i t i s e s s e n t i a l t o minimize a l l t h e p a r a s i t i c RC components and so t o reduce t h e specific contact r e s i s t a n c e s . Thus, for i n s t a n c e , microwave power and noise behaviour of field effect t r a n s i s t o r s (FETs) a r e s t r o n g l y dependent on t h e gate r e s i s t a n c e ; for junction FETs, t h i s i s p a r t i c u l a r l y c r i t i c a l : b e s i d e s t h e influence of t h e gate metal r e s i s t a n c e (as for MESFETs), t h e contact r e s i s t a n c e and t h e spreading r e s i s t a n c e of t h e gate layer a r e very important. The s i t u a t i o n i s equivalent f o r heterojunction bipolar t r a n s i s t o r s (HBTs) for which t h e base r e s i s t a n c e i s a key point.

For (GaInAs)i-z(AlInAs)z, a Fermi level pinning h a s been found a t about (0.5+0.3z) eV above t h e valence band, due t o surface s t a t e s of t h e metal semiconductor interface.

This implies b a r r i e r h e i g h t s on GalnAs close t o 0.2 eV for * gn and 0.55 eV f o r •pQ-p.

Consequently, while extremely low specific contact r e s i s t a n c e s a r e achievable on n (Al)GalnAs, good ohmic contact on p (Al)GalnAs a r e very difficult to obtain.

The AuZn alloy i s c u r r e n t l y used t o contact p - t y p e Ga(Al)InAs / 3 / . However, Zn outdiffusion during t h e alloying s t e p g e n e r a t e s an increase of t h e metal r e s i s t a n c e . Besides, for JFETs as well as HBTs, t h e contact must be shallow (< 1000 A ) in order t o maintain t h e q u a l i t y of t h e underlying pn junction. The non alloyed TiAu metal i s also very often used /4/ b u t t h e best r e s i s t i v i t y obtained with t h i s metal (=*2.10-6 iQ cm2) i s not a q u i t e s a t i s f a c t o r y v a l u e f o r high performances microwave t r a n s i s t o r s .

This p a p e r p r e s e n t s t h e technique we have developed t o obtain reproducible low r e s i s t i v i t y p type ohmic c o n t a c t s : i t i s based on t h e use of MnAu alloyed on a Zn diffused (Al)GalnAs layer. The performance of t h e contact will be described in terms of t h e d.c. contact r e s i s t a n c e and two devices (JFETs and HBTs) using t h i s process will be characterized.

2 - EXPERIMENTAL PROCEDURE

The (GaInAs)i-z(AlInAs)z l a y e r s used in t h i s work have been grown by Molecular Beam Epitaxy on semi-insulating InP / 5 / . The samples provided for evaluating t h e contact

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988496

JOURNAL DE PHYSIQUE

resistivity by the transmission line model (TLM) 161, are non intentionally doped. The JFET and HBT structures depicted i n figure 1 a r e detailed elsewhere /7,8/. In all cases, the process can be divided in three main p a r t s :

a) Zn diffusion : it occurs in a semi-closed quartz box /9,10/, with Znbs2 a s diffusion source under a flow of purified Hz. The diffusion temperature is 500 C f o r GaInAs and 480'C f o r AlGaInAs.

b) 4 % Mn 96 % Au deposition : t h e metals a r e deposited using electron beam evaporation with a background pressure of 2.10-7 Torr. Deposition rates of 5 A/s for Mn and 10 A / s f o r Au are regulated by a controller. The Mn is deposited first. The proportion of Mn optimized for GaAs /11/ has been kept for (AlfGaInAs.

c) Annealing : t h e samples a r e annealed i n a conventional furnace under a flow of Ar-HZ gas. They are heated till they reach t h e chosen temperature and then immediately transfered i n t h e cold part of t h e furnace. With such an annealing technique, a temperature of 300'C is reached with a thermal time constant of 25 s.

Zn diffused

InGaAs : Si AuGeNi .4uMn

/

S.I. InP

Figure 1 : JFET and HBT structures

S . I . InP

P

3

-

The Zn diffusion has been investigated f o r different compositions of (Ga1nA~)~-, (AlInAs),. Figure 2a illustrates t h e Zn diffusion speed f o r z = 0 and z = 0.4 ; a high surface concentration is obtained : 4.1019 cm-3 f o r GaInAs (figure 2b) which is essential f o r reducing t h e p contact resistivity. Besides, t h e reproducibility of t h e technique and the abrupt junction profile give a very good control of diffused depths.

Finally, the high quality of these p-n junctions has allowed t o fabricate GaInAs JFETs with low gate leakage currents (Ig < 25 nA a t Vg = - 5 V), showing thus t h e under-layer is not damaged during t h e diffusion process.

Figure 2 : a) Zn diffusion speed f o r z=0 and z=0.4 b) Zn Electrical profile ( POLARON )

Several experiments with TLM stripes have been performed f o r determining t h e minimum of contact resistivity a s a function of the annealing temperature (Fig. 3). For a s deposited contacts, TLM evaluation gives specific resistances close t o 10-5 R cm2 f o r GaInAs and 10-4 R cm2 for (GaInAs)0.6(A11nAs)0.4. The best contact resistivities Rc a r e obtained after an annealing a t 290'C ; Rc i s lower than 10-7 f2 cm2 f o r GaInAs and Rc = R cm2 for z = 0.4. These values, which a r e particularly reproducible, are t h e lowest ever reported on these types of materials.

Figure 3 : Variation of t h e specific contact resistivity with the Zn doping level

Figure 3 illustrates also the interest to "over-dope" p type layers by a Zn diffusion : with a p doping of 2.1017 cm-3, t h e contact resistivity is not better than 3.10-5 R cm2.

The variation of Rc with the doping level is shown in Figure 4a. Such a behaviour can be expected referring t o two main effects (figure 4b):

- t h e high doping level due t o the diffusion process reduces the barrier width and makes easier the conduction by tunneling between the metal and the semiconductor.

-

E~

Ev

Zn DIFFUSION

/ '

Figure 4 : a) variation of t h e contact resistivity with the Zn concentration

b) Schematic valence band diagram a t the equilibrium, after Zn diffusion and a f t e r MnAu alloying

C4-456 JOURNAL

DE

These two characteristics have been exploited for t h e processing of JFETs and HBTs.

Even if t h e geometries of the devices are quite large : gate length of 2.5 pm f o r the JFET and emitter width of 10 pm f o r the HBT, their frequency b e h a v i o x is excellent :

a maximum oscillation frequency of 14 GHz is reached f o r t h e JFET (which is t h e best value reported f o r diffused JFETs) and a transition frequency of 1.7 GHz f o r t h e HBT 1121.

4 - CONCLUSION

MnAu alloyed contacts with excellent ohmic characteristics have been fabricated on p+

diffused (A1)Ga.InAs ; t h e technique presented here is an efficient method for obtaining very low contact resistivities required f o r high performances JFETs and HBTs a s well as lasers and photodiodes. The lowest contact resistivity, 5 10-7 l2 cm2, is found t o occur with an annealing temperature, near 290 'C. This resistance value is t h e lowest ever reported for contact t o p-type GaInAs.

REFERENCES:

111 J.C. Renaud, F. Lugiez, S. Vuye, M. Allovon, L. Nguyen, A-Scavennec, B. Bourdon ECOC 1888 - Brighton

/2/ A. Scavennec

ESSDERC 1988 - Montpellier

-

IEEE Transactions on Electron Devices, Vol ED-32 NO6 June 1985 141 R. Kaumanns, N. Grote, H.G. Bach, F. Fidorra

GaAs and Related Compounds 1987

-

/5/ J.P. Praseuth, M. Quillec, M. Allovon, M.C. Joncour, J.M. G&rard,P. H6noc GaAs and Related Compounds 1987 - Heraklion

/6/ H.H. Berger

Solid-State Electronics 15, 145, 1972

/7/ L. Nguyen, M. Allovon, P. Blanconnier, E. Caquot, A. Scavennec,B. Bourdon ESSDERC 1987 - Bologna pp 943

/8/ J.L. Pelouard, P. Hesto, J.P. Praseuth, L. Goldstein

IEEE Electron Device Letters, V. EDL 7, N O 7, Sept. 1986 pp 516 191 K. Kazmiersky, A.M. Huber, G. Morillot, B. De Cr6moux

Jap. J. Appl. Phys 23 (1984) p 628

/lo/

To be published in Journal of the Electrochemical Society /11/ C. Dubon-Chevallier, M. Gauneau, J.F. Bresse, A. Izrael, D. Ankri

Journal of Applied Physics - V59 - 1 June 1986 - pp 3783 1121 J.L. Pelouard, J.P. Praseuth, P.E. Hallali, G. Tremblay

Unpublished