The ternary compound Fe3Ga2 − xAsx: a promising candidate for epitaxial and thermodynamically stable contacts on GaAs

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ELSEVIER

ternary compound Fe3Ga2_xASx: a promising candidate for epitaxial and thermodynamically stable contacts on GaAs S. D~putier"'*, R. Gu~rin% B. L6pine b, A. Guivarc'h b, G. J 6 z ~ q u e l b La~u+r~atoirede Chimie du Sol(de et lnorganique Moi&'ulaire, UMR CNRS 6511, Campus de Beaulieu, F-35042 Rennes, Fram:e %~MRCNRS.Universit~ 6627 'PALMS: EquitY'de Physique ties Surfaces et btterfaces, Campus de Bemdieu, 1.35042 Rennes, France

~!id state phase equilibria in the ternary Fe-Ga-As pha~ diagram have been esta!iished at 600+12using X-ray diffraction, ~anning electron microcopy and EDX microprobe analysis as experimental techniques. The existence of a ternary phase, expres~d by the general chemical formula Fe~Ga:+,As, (0.20 ~; x ~ 1.17.5), which crystallizes in hexagonal symmetry and derives structurally from the NiAs4ype structure, has been evidenced. The original feature of the diagram is the ta:currence of a ~i~ line t ~ c ~ n Ihi~ ternary pha~ and the semiconductor OaAs. It is the first time that such a tenmry phase M,(Ga,As)~ (M + ~.r~n+i~ion metal) has been found to be i,~ thermodynamic equilibrium with OaAs. Based on hexagonal-pseudocubie c~sta!lographic considerations, this pha~ F¢~Gaz+~As~ pre~nts real possibilities to be epitaxied on GaAs+ Therefore, this ,~om~und a ~ a r ~ as a unique cared(date to obtain a quasi.ideal metal/GaAs contact. I ~ t h thermodynamically stable and ~iagl~oc~+lallin¢ For thi+~ pttrpos~+ we. have studied the ~lid ~tate interdiffusion~ in epitaxial (70 nm Fe)/OaA.,~(H)l) h~l~ro~tru~:tttr~ grown at room teml~rature and am~ealed at 4~0+C i|t ultra°high vacuum ct~ndi|ions. The epitaxial growth of the FeiGn+:o ....~A,~ compound oil GaA~IHI1) ha~ [~en proved but+ G)r an annealing tcm~ratur¢ of ~°0C,$ ° the binary ~om~mnd Pc+As, whi~:h is not found in thermodynamic equilibrium with GaAs, coexists as patches at the interface. Thi~ i~d~at~ that aanqaling at higher tcmpcraturq ~+houldI~ad to a qu+~ioidea! Fe~GaAs/GaAs(001) co~ttact, ¢) 1997 Elsevier

I~wo,n#: Solid state phase equilibfla: X°rav diffraction: Thin-film: Metallization contacts

1. | n l ~ u ~ l o n The fabrication of stable and epitaxial contacts to I!I°V compound ~miconductors is a formidable thai. lenge to obtain 'ideal' metallizations for modern corn° pone~ts t~chnology (for recent general p~'rs, see [1~3])~ U p to now, the growth by molecular beam epita~y (MBE) of three classes of metallic or + mtmctaltm m,aterml+ (mmn+y + on GaAs) has ~ e n studied: the transition°metal gallides aM aluminides

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like NiGa [4] and NiAl [5], the rare-earth monoarsenides such as YbAs [6] and ErAs [7] and the Feobased intermetallic compounds Fe~(AI,Si) [8,9]. However, in silicon technology, to obtain metal/St contacts (M/St), another procedure has been extensively siudied with success: the formation by ~Jlid phase interdiffusions of epitaxied and thermodynami. catly stable disilicides/silicon interfaces such as C o S i j S i , NiSi:/Si and C r S i j S i [10,11]. This ap+ proach was tested in the case of GaAs, too. After depositing a metal thin film onto the semiconductor substrate by MBE and after annealings at various teml~ratures, the metal/GaAs interdiffusion couples

S. Ddputieret al. / Jo.,mal of AIIoys and Compmmds 262-263 (199D 416-422

(M/GaAs) have led to the formation of ternary M~GayAs~ phases, such as Ni3GaAs, Co2GaAs and Pd~2GasAs 2 [12]. Unfortunately, up to now, no ternary compound has presented all the required properties for ideal M/GaAs contacts, since these compounds are not at the same time epitaxied and thermodynamically stable with the substrate. In fact, at higher annealing temperatures, the final step of the interaction (M/GaAs) is always a mixture of binaries (e.g. NiGa and NiAs2 in the Ni/GaAs case) and not a single-crystalline and single-phase film. For the last 10 years, we have largely contributed to the metallurgy of the M/GaAS contacts by simultaneously studying the experimental ternary phase diagrams M-Ga-As and the solid state interdiffusions in M/GaAs structures, for example in the three s~stems Rh-Ga-As [13], Ni-Ga-As [14-16] and Er-Ga-As [17,18]. As previously proposed by Williams et al. [19], Beyers et al. [20] and Sands [12,21], these different studies confirm that the experimental determination of the M-Ga-As phase diagram is absolutely necessary to describe and understand accurately the solid-phase interdiffusions in the M/GaAs contacts. In the same way, the Fe/GaAs interdiffusion couFlc:~ have recently been studied under standard experimental conditions: after annealings for 1 h at 500°C, formation of the binary compounds Fe3Ga and Fe, As has been observed [22-24]. To our knowledge, no equivalent study has been published concerning the phase formation in Fe/GaAs contacts, fabricated under ultra~high vacuum (UHV) conditions for the GaAs surface preparation, iron deposition and annealing treatments. The present paper deals with the solid state inter. diffusions of an iron thin film deposited on GaAs, To ra'~ionalize the different steps of the interaction, the experimental determination of the bulk equilibrium phase diagram Fe-Ga-As has first been established.

2. The experimental Fe-Ga.As phase diagram and the Fe~Ga2_~Asx ternary phase Although the bulk Fe-Ga-As phase diagram has not yet been reported in detail, it is generally considered as closely related to the Ni-Ga.As one [I], owing to the fact that iron and nickel both have similar elec. tronegativity factors and metallic radii. In both diagrams, neither iron nor nickel metal can be found in thermodynamic equilibrium with GaAs since, in the upper part of these diagrams, the occurrence of ternary phases with the general chemical formula M~Ga~=,,Ass (M ~ Fe, Ni) has been proved. Indeed the Fe3Ga2o~As x ternary phase, chemically and structurally similar to the Ni3Ga2~As Xone [14], has been observed in Fe.doped GaAs [25] and synthesized as a ternary oulk compound [26-28]. However, as

417

previously mentioned for the Ni-Ga-As diagram, the ternary Fe3Ga2_xAs x phase would not be found in thermodynamic equilibrium with GaAs since, in the case of nickel, a tie line connects the binary p h ~ NiGa and NiAs2, inhibiting the existence of a Ni3Ga2_xAsx-GaAs tie line. Moreover, it may be considered that, in the case of iron, the binary phases Fe~Ga v and Fe~As, are more !ikely to be in thermodynamic equilibriun~ with GaAs. Firstly, in order to check this assumption, theoretical calculations have been made. In the absence of true experimental thermodynamic data, the standard formation enthalpies for the Fe-Ga and Fe-As binary systems have been calculated using the 'semi-cmpiricar Miedema model [29-31]. This has enabled us to elaborate a theoretical Fe-Ga-As ternary diagram using the simplifications proposed by Schmidt-Fetzer [32]. Astonishingly, this theoretical diagram (not presented here) differs significantly from the Ni-Oa-As one, because of the lack of tie lines between the Ga-containing binary compounds Fe~Gas or Fe3Oa4 and the As-containing ones FeAs2 and/or FeAs. This suggests that, in contrast to the ternary Ni3Oa:_xAs~ phase, there is a possibility for the Fe3Oa2_xAs x one (and in particular Fe3GaAs which is stoichiometric in As and Ga atoms) to be in thermodynamic equilibrium with GaAs. This ternary phase F%Ga:oxAs ~ has been mentioned in the literature as existing over a broad homo~ geneity range corresponding to 0.21 g x g 1.125 [26,27]. This phase has been indexed in hexagonal symmetry and exhibits a partly filled NiAsotype strUCo tare. However, since the structure has been found to be fully disordered for 0.85 < x g 1.125, a structural change occurs at x ~ 0.85 down to 0.21, to a more ordered hexagonal structure which needs a doubling of the a unit cell parameter [26,27]. The magnetic structures of Fe~GaAs and Fe~GatTAs,~ have tee cently been discussed [33,34]. To establish the experimental Fe~Oa-As ternary diagram, numerous samples with appropriate atomic compositions have been prepared by direct combinao tion of the elements. The starting materials were either powders (iron and amorphous t/-As) or ingots (gallium), all with a minimum purity of 99.99%. The samples were intimately mixed and placed in silica tubes which were evacuated to 10=3 torr, sealed under vacuum and placed in a resistance furnace for 24 h at 600°C. Numerous grindings, cold pressings and re-annealings at the same temperature for a long time were needed to reach thermodynamic equilibrium. Finally, samples were quenched in ice water. Then, they were analyzed using a powder diffractometcr (CPS 120 INEL) equipped with a position-sensitive detector covering 1200 in 20. Backscattered electron imaging was done with a Jeol JSM-35C scanning

4tg

S, Dilmtier ¢t aL / J.umai ¢ffAUoys m~MComl~m~ds 202~263 (1~7) 416 ~d22

electron micro~ope as well as standardless electron microprobe analysis. Fi!,. I shows the experimental ternary phase dia~am deduced from about 30 samples. Our results confirm the broad homogeneity range of the ternary F¢3Ga:_~As~ phase (0.20 _
the hexagonal superstructure, using the Rietveld method, is currently in progress in order to determine exactly the origin of this superlattice. However, the original feature of the experimental Fe-Ga-As ternary phase diagram is that the ternary Fe3Ga:- xASx phase is in thermodynamic equilibrium with GaAs. Indeed, as presented in Fig. 2, the X-ray diffraction patterns for the initial atomic compositions 'Fe:GatAs t' and 'Fe2Gat.4As0.~' (labelled samples no. 2 and 5, respectively, in Fig. 1) show a mixture of the binary compound GaAs and of the ternary phases Fe3GaAs and Fe3Gat.aAso.4, respectively. These results, which have been confirmed by SEM studies (not presented here), prove that the Fe~Ga.,_~As, ternary phase is truly in thermodynamic equilibrium with GaAs at 600°C and probably al~ at higher temperatures, As shown previously for the solid state M/GaAs interdiffusions (M = Ni, Rh), the average atomic composition of the reacted layer remains on the vertical line connecting M to GaAs.

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419

S. IX:p.tier et al. / Jounml o[A!loys and Compounds 262-263 (1997) 416-422

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This result involves the occurrence of equiatomic ternary phases e.g. Ni3GaAs, during the first steps of the interaction. In the same manner, in the case of

the Fe/GaAs system, one might expect that, as long as the interfacial region may be considered as a closed system, the solid state interdiffusions in an

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3: Ddputieret al. / Journal (~t'Alloysand C~mtlnmnds 202-203 ¢!997) 410-422

Fe/GaAs contact lead m a thermodynamically stable F e ~ G ~ / G a A s heterostructure which must now be the final step of the interaction. After these thermo~namic considerations, it is inte~sting to have a look at the possibilities of epitaxial growth of the hexagonal Fe3Ga,_,,As~ phases on Ga.&s. As has previously been demonstrated, hexagonal p h ~ s can ~ considered to be pseudocubic becaus¢ of their c / a ratio which is close to 43/~/2 = !.2~ (here for the Fe3Ga2_~As~ solid solmion, we found !.23 g c / a ~ 1.274). Such pseudocubic phases were previously encountered in the studies of Ni/GaAs [15,16], Ni/AIAs [35,~] and Ni/GaSb [37,38] interdiffusions. The accurate study of the epitaxy of hexagonal pseudocubic binary Ni :Ga.~ (c/a -1.206) on GaAg@l) and (111), carried out by transtallion electron microscopy [39,40], gave a good illustration of this mechanism. For ~ t h substrate orientations, the relationships [100](001)hex.// [ll0glll)cubic {1} were observed between Ni:Ga~ and GaAs. In the ca,~ of (001) substrates, this implies a small a angle between the (101) plane of the hexagonal pha~s and the (001) plane of the GaAs substrate (Fig. 3). This c~ angle is a typical feature of each he~gonal ~¢udocubic pham beea,J~e it is directly related to the c / a ratio. The a n g l e , has been found to be, equal to ~ ~ =0,50 and + 2° for Ni:Ga~ and Ni~GaAs on GaAg001), resp¢ctively, ~cau,~ of the cubic symmetry of OaAs, four variants can exist, corr, sponding to the four < ! ! ! > axes of GaAs. leading to the formation of slightly twinned domains. In th¢ cam of the Fe~Oa~ :~A~ pha~ on GaAg(}OI), although the corresponding ~ angles are very small (t),2° _~ o g 1,1~9, a twinned structur, of the epitaxial r¢a¢t¢d layer is obviously probable, with the appearo ante of a momic structure.

3, ~lld scale |ut¢rdlt~astons in the Fe/GaAs(~l) s | ~ u r c s at 4$0"C under UHV ¢omlitlons The ~mples were prepared in situ in a RIBER 234~1 molecular ~ a m epitaxy (MBE) system con° netted by. a UHV m~utrack system to an analysis chamber, A 5(~)-nm thick undo~d GaAs buffer layer was first ~rown on n ~otyp¢ GaAs(tN)I) substrates using standard MBE, R ~ m temperature Fe depositions, abooi 70 nm thick, were made in the analysts churne r with a deposition rate equal to 9 ~/min using a highotem;~rattt~ effusion c¢II with an alumina -ctuo ~ibi~:, |n the presto wotL the vam#es were annealed in situ for IS rain at 45(P'C, ~t tem~rature which M ~ a total reacti~ of the Fe thin film with the subs;irate without GaAs decompositiom ~e chamber is equip~d with a i0 keV high energy electron diffraction for in situ

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characterization. X-ray diffraction (XRD) patterns were drawn ex situ using a (0-2 0) powder diffraction set-up. Fig. 4a shows the RHEED diffraction patterns with the electron beam along the [011] direction of GaAs obtained from the starting (2 x 4) GaAs surface. After a 70-rm thick deposition of Fe at room temperature, the RHEED pattern (Fig. 4b) with a (1 x I) reconstruction indicates that the Fe film grows as a single-crystal a-Fe with a (100) surface and with the in-plane < 100 > axes of the Fe and GaAs aligned. This was expected, based on the fact that the GaAs lattice constant is almost exactly twice that of a-Fe ( A a / a ~ + 1.4%) and on the Fe growth on GaAs(ll0) and (O01L previously reported [41A2], Since the RHEED patter)) does not fi)rm continuous streaks, we conclude that the Fe surface is not perfectly flat, During the temperature increase, an evolution of the diagram ~curs till 3(R}°C,with progre~iv¢ occurrence of continuous streaks and of lines I / 2 leading to the diagram given in Fig. 4c at 450°C This last result suggests the formation of an epitaxial single-cr#talline reacted layer by ~lid state interdiffusion, with a surface flatter than the F¢ one. The XRD pattern of the unannealed samples, shown in Fig. fin, confirms the epitaxial growth of Fe on GaAs. After a UHV annealing for 15 rain at 450°C (Fig. 5b). four extra ~ a k s occur, which are added to the OaAs ones. The I~aks at 20 ~, 30.7° (not shown) and 03.70 correspond to the I01 and 202 reflections of the ternary hexagonal phase Fe~Ga:_~As~. The XRD measurements indicate a music structure with a full-width at half height of the 101 reflection equal up to 0.8°. The two other ~aks at 20 ~ 35,20° and 74.4ff' are denhfied ~ the 110 and 220 reflections of the te~ragonal compound Fe,,As presumably I~ated at the interfa~ [22-24]. The lack of other reflections of Fe~Ga~_~Ass and Fe2As shows that the two compounds are at least strongly textured with the fob

S. ~ t i e r et al. ] Joumalof AUoysand Compounds262-263 0997) 416-422

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4. C o n c l u d i n g

Fig. 4. RHEED patterns along [I-10] azimuth of the GaAs subsIrate taken (a) on the starting (2 x 4) GaAs(001) surhce, (b) after a 711 nm Fe del~sifion and (c) after a 15-min UIW annealing at 450°C.

lowing relationships with the substrate: (101)Fe~ Ga2+~As,,//(001)GaAs and ( I I 0 ) F e ~ A s / / (001K3aAs. In fact, the RHEED picture (which points out an ordering in the surface plane) azd the XRD powder patterns reveal an epitaxial growth of Fe3Ga2+~As,, on GaAs(001). Moreover, the structural relationship (1} expected for pseudocubic compcunds on GaAs, was recently confirmed by transmission electronic microscopy.

remarks

By working under UHV conditions, the ,solid state interdiffusions at 450°C in Fe/GaAs contacts lead to epitaxial (Fe3Gal.,As,. 2 + Fe2As)/GaAs(001) heto erostructures. The mix,,re of binary compounds Fe3Ga and Fe2As previously observed after annealing at 600°C of a (polycrystalline Fe)/GaAs(001) contact [22-24] correspond to a previous step of the interdiffusion process which is probably due to the pre~nce of impurities in the studied structures. The fact that the stoichiometric compositio. Fe:~GaAs of the hexagonal ternary phase was not observed in the Fe/GaAs interdiffusions is not really surprising because of the presence of Fe:As+ ~ i s implies that the composition of the ternary phase obtained at 450°C is Ga-rich. Our work indicates that the UHV anneal at 450°C is not sufficient to reach the final step of the interaction Fe~GaAs/GaAs be+ cause of the stability of Fe~As on GaAs at this temperature. However, ~i,¢e Fe~As is not in equilibrium with GaAs as shown in Fig. l+ a slightly higher annealing temperature should be necessary for the occurrence of Fe~GaAs, ~ith eventually an inert cap to avoid the As escape (for instance an SigN4 film was previously used for Er/GaAs study [18]). In conclusion, this set of experimental data shows the epitaxial growth of the Fe~Gae o,,As~ compound by solid phase epitaxy on GaAs(001) at 450°C but at this temperature Fe2As is stable on GaAs and the fabrication of quasi-ideal single-crystalline and single-phase Fe3GaAs/GaAs(001) contacts will re+ quire UHV annealing treatments at higher temperatures. Work is now in progress to this end.

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