NiGeW ohmic contacts on GaAs heterostructure epitaxial layers

NiGeW ohmic contacts on GaAs heterostructure epitaxial layers

ELSEVIER ThinSolidPilms290-291 (1996) 493-496 NiGeW ohmic contacts on GaAs heterostructure epitaxial layers LH. Huang a S. Tehrani a M, Durlam a M,J...

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ELSEVIER

ThinSolidPilms290-291 (1996) 493-496

NiGeW ohmic contacts on GaAs heterostructure epitaxial layers LH. Huang a S. Tehrani a M, Durlam a M,J, Martinezb E. Schirmann n,N. Cody b ' Motorola Inc,, PCRL 210{) E. ElliotRoad, Tempe. AZ85284, USA b Motorola Inc., CST, 3P,.e, 2100 E. Elliot Road, Tempe, .4Z85284, USA

Abstract NiOeW has been succ¢ssfullyimplementedas a contact materialon GnAsMESFETsin n productionenvironment,However,when idenlical contacts were used on heterostmctur¢field effect transistors (HFETs), a strong interactionwas observed bctw~n NiGeWand GaAs/AIGaAs epitaxial layers after annealing, AlthouBhNiOeW was considered a refractorycontact and was not expected to react with the substrate, we detected defects at the GaAs/AlOaAs interfaceby cross.scctioml transmissionelectron microscopy.These clectrlcallyactivcdefects degrade the HFET performance significantly, As a result, the HFET epitaxial stm,'tum had to be re-optimized in order to minimize the observed adverse effe.ct, geyword~: OaAs;OaAs/AIGaAsepitaxy;NiOeWohmiccontacts;NiOeAuohmiccontacts

1. Introduction NiGeAu is the most commonly used ohmic contact metal on n-type HaAs. However, its thermal stability is not yet satisfactory [ 1]. This is primarily because of the formation, during alloying, of Au-eontaining inter-metallic compounds, such as ~-AuGa, having low melting temperatures [ 2]. Consequenfly, in the last decad*, significant efforts have been devoted to the investigation of Au-free ohmic contacts [35], Of these studies NiGeW has been shown to be the most promising alternative to NiGeAu since excellent contact resistances ( <0,1 fl mm) were achieved on HaAs MHS. bETs [ 3 ]~ Also, due to its excellent thermal stability~NiGeW is compatible with high.temperatur~ ( - 5 5 0 °C) AI.-Cu multi-level interconnects. Them is minimal interaction between NiGeW and HaAs after alloying at ~ 550 °C for 60 s, Fig, I shows the NiGeW ohmic contact of a MESFET, From this picture we can see that the penetration depth of ohmic metal constituents into the HaAs substrate is approximately 20 nm and no microstructural defects were detected underneath the ohmic metal, There arc voids formed at the NiGo/W and NiGe/GaAs interfaces and also along the grain boundaries in the NiGe layer, We believe the voids are formed by He diffusion into the substrate, Although the penetration of NiG~,W into the HaAs sub. strate is apparently minimal,diffusion at the' atomic level can reach as dnep as ~ 120 nm, This effect is especially significant when diffusion barriers, such as heterojunetions, am 0040.6090196t$15,[email protected] 1996HlsevierScienceS.A,Allrishlsreserved, Pn $o040-609o ( 96 )09035-9

Fig. I, cross.sectional'FEM[ mpl.,so[NK]eWon HaAs+Thisistheenlar~r~ pictnmof tl~ohmicmelMo~ a (~aAsMESI~.T,Voids~longthe NiGe/W end NiGe/GnAsi~terfe~mare clearlyvisible, present, The impact of this effect, when NiGeW is substituted for NiOeAu on HFETs without optimization, is a dmslle increase in the ohmic contact resistance and HFET series resistance. This report explains Ihe observed interactions of NiGcW with the epitaxial layers in F1FETs, the effects of these interactions on HFET performance, and the method to mitigate these effecls.

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J,H, Huang et at, / ThinSolid Rlmx 290-291 (t996) 49.~..496

Experimental

3. Results and discussion 3.1. EffectofN + GaAsthickness

The epitaxial structure used in this project is shown in Fig, 2, The epitaxial layers were grown by molecular beam epitaxy (MBE) on semi.insulating GaAs substrates. The composition and thickness of each layer is also indicated in the figure. The NiGeW ohmic metal was d.c,-sputter deposited and defined with a dielectric assisted lift-off method. Ni thickness was ,'. 200 A, Ge ~ 200/~ and W 1500/lt. The alloy t~m. perature was typically 550 °C and the anneal time tutin [ 3]. Wafers with different N + GaAs thicknesses were evalu. ated to assess the effect of N ÷ GaAs thickness on a NiGeW contact resistance. The effects of metal/substrata interaction on device performance were evaluated with the transmission line (TLM) patterns having various metal spacings. The lateral diffusion of the defects generated by the interaction was also estimated with this method. Cross-sectional transmission electron microscopy ( X T E M ) was employed to help characteriz¢ the defect structure.

Fig. 3 shows a plot of the ohmic contact resistance as a function of the N + GaAs thickness. It is obvious that the contact resistance increases as the N* thickness decreases. This is not the case when a NiGeAu ohmic metal is used. Figs. 4 and 5 compare the XTEM images of NiGeAu on 50 nm N + GaAs and NiGeW on 100 nm N + GaAs, The reaction between NiGeAu and the epitaxial layers ranchos deep into the A I G ~ s layer, whereas for NiGeW, there is a layer of defects along the N + GaAs and ?dAn or AIGi~,s interface,

la the case of NiGeW on 50 ~m N + GaAs, the defects were also observed in the InOaAs channel. These defects were

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FiB,3. ?.!iQcWohmiccontactresistanceaso functionofN + GaAsthi¢k~s;, The data for MESFETis equivalentto that for a N+ O~s thicknessof ~20grim.

Pig 5. Cross,sectional'l'IZMimageof •GeW ohmic on tO0 am N ÷ G~ks. A layerofdefectsdirectly underthe ohndametalcon be seento accumulate

alongthe AIAs/AIGaAsinterface.I~ f'~ctscanalsobe foundin the lnGuAs chmmlbut withlowerdensity,

J,ll,liuang etat I T~in SolidFilms 290-291 (I996) 493-496

identified, through electron diffraction, as possible Oa precipitates. It is well known for NiGeAu ohmic contacts that two intermetallic compounds form during annealing. Ni and Ge react with As and form NiGeAs; while Au reacts with Ga and forms AuGa [6]. For NiOeW ohmic contacts, it is believed that the same NiGeAsphase is formed,It is also believedthat Ni and G¢ diffuse into the subslzateto form NiGeAs in the substrata leaving behind the voids obsexvedin Fig, 1. Excess Ga is generated as a result of this reaction, However,Ga can only diffuse into the epitaxial layers since W is known as a diffusion barrier. Ga then accumulatesat the GaAs-AIAs or AIGaAs heterointefface and beyond. In a MESFET with NiGcW contacts, the GaAs substrata acts as a sink for Ga diffusion, therefore no Ga precipitates are observed. However, in a heterojunctionfieldeffect transistor (HFET), since the heterointerfaces are known to be diffusion barriers, Ga will gather at these interfaces and forra precipitateswhen its concentrationexceeds the solubility limit. This is consistent with the fact that defects form only under the ohmic metal and extend laterally along the N + GaAs and AlAs heterointefface, Thus the thicker the N + GaAs layer the less the likelihoodGa will precipitatealongthe OaAs-AlO~.s interface because either the average Ga concentration will be lower than the solubility for precipitation or Ga will have a longer diffusion distance to reach the beterointerfaee, Ga and its precipitates at the N +GaAs/AlGa.As interface have to be electrically charged since they affect the contact resistance and also the/-V characteristicsof the contacts,We postulate that there is a depletion region associated with Ga and these precipitates. When most of the N+ GaAs layer is depleted by these electrically active defects, a high contact ~esistanceis measured, as confu'medby Fig. 3, When the N ÷ OaAs was inereas;edto 100 tan, good ohmic contact resistance, i.e.O. I ~ ram, was obtained. In this case,the depletion

layer formed by the precipitates at the N* GaAs/AIGaAs interface will not restrict the current flow through the N ÷ GaAs layer,

3.2. Lateral dif/usion of defects along the A~s/AlGaA$ interlace In an HFET, the electrons flow from the N* GaAs region through the AlAs and AIGaAslayer to the InGaAs channel. Fig. 6 compares the i-V characteristics of HFETs with NiGeAu and NiGeW ohmic contacts, Significantdegradation in the HFET performancecan be clearly seen. The N + GaAs contact layer in this case ie 100 am and the spacing between the edge of the ohmic metal to the end of the N ÷, as defined in the inset in this figure, is 2 p,m. When this spacing was reduced below 2 p,m, a non-linear i-V characteristicbelow the knee voltage was observed, although a good ohmic contact to the N + GaAs layer was measured. These evidences indicatethat the current flow to the channel is restrictedby a potential barrier.

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0mla VO~ISN~'dtl Fig.6, [-Vetltvcs for ] × 200 p,m=HFBTswilhN|QeAtland NIGeWohmic contacts. The sFaciag from the edge of the ohmic metal to N~"GaAsedge is 2 p.mas shown in the insetabovethe I-V curves, The performanceof the HFETswithNiC,eW in1hisca~ Is se~elely~gm~l, Thegatevollabestart;

at0 V with- 0.25Vpersteptowudpinch-off, The potential barrier is believed to be due to the lateral diffusion of electrically active Ga beyond the edge of the ohmic metal. The lateral diffusion of Ga was measured to be about 2.5 tun when annealed at 550 °C for 60 s. The diffusi,aity of Ga was then estimated to be -- 2.6 x O- t, cm2 s- t assuming a square-root dependence of the diffusion length on the product of diffusivity and time. This diffusivity is signi~cantly greater than the diffusion of Ga in the GaAs lattice [7].

4, Conclusions We have investigated the interaction of a NiGeW ohmic contact with hetarostruetare interfaces. Although the apparent penetration depth of NiGeW is oxound20 nm, its interaction with the epitaxial layers, and thus the impact on the electricalperformanceof lCFETs,is far deeper,However,this adverse effect can be mitigated by increasing the N ÷ GaAs contact layer thickness to ~ 100 nm and the spacing between the N + GaAs edge and the NiGeW edge to greater than 2.5 ~m. tt is also proposed that the interaction of NiGeW with the substratacauses the diffusionof Ga into the substrata;Ga then accumulatesat the beterointeffaceand forms a depletion layer. The formation of this depletion layer significantly degrades the series resistance and other performance charaeteristicsof HFETs,

Admuwle¢lgements The authors would like to acknowledge the support from both the CS-! and PCRL process teams for processing the wafers used in this report, Helpful discussions and managemeat support from ]. Gilbert, WJ. Ooms and H. Goronkin are much appreciated. K, Nordquist has contributed signifi~

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Zll. tlaang et el, / Thin3olid Films290-291 (1996)493-496

cantly in verifying the validity of device layout change using e-beam lithography.

Re~erenccs [ 1] N. Breslau,J.B. Gann andJ.L Staples.Solld.ftate Electron,,10 ( 1967) 38l, [2] M, Murakami,K,D,Childs,J,M,BakerandA. Co.lIegm,Mierostrecture Studies of AoGeNi ohmic canteens to N- type G ~ , Y. Yac, $ci, Technol., B4 (1986) 903,

[3] J. Cho, W. Crania, S. ~ingbeil, J. Jotmso~and G, Hansel]~low contact resistanceand thermallystableNIGeWohmic~atacls to N-typeGaAs, Motorola SPS Corn Techno/oSy TechnicalRepots #02, Tempe, AZ, Septereber21,1992. [4] For example,M. Muzaka~, W,H, Price, Y.-C. Shih, N. Breslau, K.D. Chi]dsandCC. Padrs,Thermallystableehreiccontactston.typeG~As'. MoGeInWcontact metal,.t, AppL Phys.,62 (1987) 3295, 15] Par example,L,C. Wang,S,S, Lau, E, K Hsiehand J.R. Velebir,Low. resistance nonspikingohmic contact for AIGaAs/GaAshigh electron reobilLtytransistors usingGe/Pd [email protected], P~Fs,Let.,,,54 (1989) 2677. 16t R.E. Williams, Modern GaAs Processing Merhodx, Ar.:ee House, Boston, MA, 1990,Chapter 1[, [7] SM, Sze, Physics of Semiconductor Devices, 2ad F.dilian, WileyletersciencePublication,New York, 1981,p.68.