Microelectronics Reliability 51 (2011) 1330–1336

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Microelectronics Reliability

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Investigation on the effect of annealing process parameters on AuGeNi ohmiccontact to n-GaAs using microstructural characteristics

S. Tahamtan ⇑, A. Goodarzi, S.P. Abbasi, A. Hodaei, M.S. Zabihi, J. SabbaghzadehIranian National Center for Laser Science and Technology (INLC), P.O. Box 14665-576, Tehran, Iran

a r t i c l e i n f o

Article history:Received 4 November 2010Received in revised form 14 February 2011Accepted 28 March 2011Available online 22 April 2011

0026-2714/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.microrel.2011.03.039

⇑ Corresponding author.E-mail addresses: tahamtan@inlc.ir, tahamtan59@

a b s t r a c t

The electrical and structural properties of AuGeNi ohmic contact to n-GaAs have been studied. A combi-nation of EDX and X-ray diffraction analysis was used to examine the reactions between AuGeNi-basedmetallization and GaAs. Scanning Tunneling Microscope (STM) was used to study surface morphologyand surface roughness. By the use of Rapid Thermal Annealing (RTA), contact resistivity as low as5.5 � 10�8 X cm2 have been obtained. The minimum in the contact resistivity coincides with the forma-tion of AuGa and NiAs phases. On the other hand, poor thermal stability after contact formation was con-cluded to be due to the formation of low melting point AuGa phases. Formation of dark particles,recognized as GeNi particles, in different distributions and shapes after annealing, was found to be essen-tial for low contact resistivity. Correlation between GeNi particles distribution and contact resistivity wasfound and introduced as d/k parameter. It was found that the lower the size of these particles (d) as wellas the larger the contact area over which they are distributed (k) leading to the better contact resistivity.

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1. Introduction

The investigation of metal–semiconductor contacts is a veryimportant task from both practical and theoretical point of view[1]. All electronic devices rely heavily on the quality of their ohmiccontacts in order to fulfill their ultimate performance potential [2].Gallium arsenide semiconductor compound has intrinsic electricalproperties superior to silicon including higher electron mobility,direct energy gap and lower power dissipation [1,3–5]. The poorproperties of GaAs such as high defect density and poor mechanicalproperties are due to the fact that GaAs is composed of two ele-ments and the control of chemical stoichiometry of Ga and As isextremely difficult. The difficulty to control, also, the concentrationof electrically active donors and acceptors is caused by the exis-tence of two different vacancy sites of Ga and As. The conventionalohmic contact formation involves deposition of a contact metal onGaAs and subsequent annealing at elevated temperatures. Thisprocess will reduce electrical voltage drop at the contact metaland GaAs interface. Chemical reaction at interface involves processparameters i.e., metal selection, annealing temperature, annealingtime and atmosphere [1]. The development of a reliable, low resis-tance ohmic contact to n-type GaAs has been a problem for a longtime in the semiconductor industry that used for fabrication ofelectronics and optoelectronics devices [6]. Among the variousmetallization employed in the fabrication of GaAs semiconductors

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yahoo.ca (S. Tahamtan).

to produce low resistance ohmic contacts, those based on AuGeNiremain the most common [3,5,7–11]. Ni enhances the diffusion ofGe and Au enhances the out-diffusion of Ga. As a result, Ge atomsgo substitutionally in Ga sites. This helps to create a thin n-typehigh-doped layer on the GaAs surface responsible for a high sym-metrical conduction due to tunneling across the schottky barrierwith a low harrier height [3,12].

Temperature is a main factor affects the ohmic contact resis-tance. The optimum temperatures and the lowest contact resis-tance values are influenced by GaAs substrate surface cleaningprocedure and the deposition sequence/layer thickness [13].

Although good ohmic contacts can be achieved with thisAuGeNi metallization, the resultant topography is very uneven[5,14,15]. The uneven topography is closely related to the meltingof the contact metallization which occurs in the temperatureregime normally employed for alloying [14]. The uneven topogra-phy of the surface affects the character of interactions betweenphases in metal–semiconductor contacts, i.e., on the electrophysi-cal properties [16].

Viewing the top surface with a light or a scanning electronmicroscope reveals dark regions in cluster form (GeNi particles).GeNi in the form of clusters or nodules are responsible for contactformation, and that the current flows into the semiconductornonuniformly [12,17].

The nodular or cluster form of GeNi particles on the surfaceafter ohmic contact formation has been clearly identified by SEMand its effects on the contact resistivity has been predicted. Contactresistivity becomes lower by increasing the concentrations of thesenodules [12].

S. Tahamtan et al. / Microelectronics Reliability 51 (2011) 1330–1336 1331

NiGeAu ohmic contact as a function of alloying temperature onInGaAs was investigated by E. Lan. It was found that the optimumRTA condition obtained by annealing at 460 �C for 60 s for n+ con-tact resistance and 420 �C for 30 s for on-resistance [18].

A comparative study was presented by M.S. Islam et al. on con-ventional alloyed Au/Ge/Au/Ni/Au, Au–Ge/Ni and Ni/Au–Ge/Niohmic contacts to n-GaAs metallization. It was found that contactresistivity as low as 1.90 � 10�5 can be obtained with Au–Ge(150nm)/Ni (16 nm) metallization by annealing at 420 �C for 150 s [19].

There are many combinations of process parameters to fabri-cate low resistance ohmic contacts. In order to find a processwindow which routinely yields low contact resistance, the micro-structural characteristics should be correlated with the electricalproperties.

On the basis of this information several annealing schedules aredevised and the subsequent contact resistance and surface rough-ness was examined. Quantitative analysis of dark particles on thesurface of samples was performed to understand the relationshipbetween contact resistance and distribution of particles. By exam-ining the microstructure of annealed samples, optimum conditionsfor annealing were reported.

2. Materials and methods

The semiconductor wafers used in this study were 120 lm pol-ished n-type GaAs (1 0 0) oriented wafers with Si-doping concen-tration of 1018 cm�3. Prior to metal deposition, GaAs wasdegreased employing a cleaning process using chloroform, 2-pro-panol, methanol and ethanol.

AuGeNi thin film was deposited by thermal evaporation of theeutectic alloy AuGeNi (23% wt) + Au (77% wt) at 3.5 � 10�4 Pachamber pressure. Initial and final temperature for coating was170 �C and 190 �C, respectively. The resulting AuGeNi and Au filmthickness was 250 Å and 500 Å, respectively. The lithography of thecontact pads was formed by the lift-off process to fabricate Trans-mission Line Method (TLM).

Poor topography which results from a typical annealing of anAuGeNi-based contact on GaAs is largely due to the initial inhomo-geneity of GaAs-metal interface. A pre-anneal is sufficient to re-move this inhomogeneity [14]. Therefore, after deposition wascompleted, ohmic contact was formed by a rapid thermal anneal-ing method in an N2/H2 atmosphere, according to Table 1.

The electrical measurements involved both current voltage(I–V) and ohmic contact resistance analysis. Contact resistivitywas measured using current voltage (I–V) diagrams at roomtemperature with TLM by two-point method. I–V measurementswere performed by Keithley 2361 using a standard probe station

Table 1Rapid thermal annealing conditions used in this study.

Sample Cycle 1 Cycle 2

Temperature(�C)

Time(S)

Temperature(�C)

Time(S)

1 360 602 380 603 400 604 450 405 450 606 450 807 240 20 450 2408 450 4809 500 6010 550 6011 600 6012 650 6013 700 60

measurement technique and a computer controlled auto-prober.The size of each contact was 250 lm � 250 lm. Mean and stan-dard deviations were subsequently extracted. EDX analyzing wasused to determine the intermixing of metals and the semiconduc-tor. XRD was used to study metallurgical reactions occurring dur-ing annealing process. Samples were investigated by RutherfordBackscattering (RBS) using channeling method. The RBS spectrawere obtained using helium beam with 2 MeV Van de Graafaccelerator.

Roughness measurements and surface morphology of samplesbefore and after annealing were studied by Scanning TunnelingMicroscope (STM).

Roughness measurements were evaluated quantitatively by Ra

parameter. Ra roughness is defined as the average deviation ofpeaks and valleys from the mean plane [20].

Measurements of dark particles size (d) and particles distance(k), observed after annealing on the surface of the specimens, wereperformed using Image analysis technique on SEM figures. Darkparticles size (d) is equal to equivalent diameter of a circle whosearea is equal to the average area of these particles defined accord-ing to Eq. (1), where A is the area and n is the number of measuredparticles.

d ¼ ð1=nÞXn

k¼1

ffiffiffiffiffiffi4Ap

rð1Þ

Measurements of dark particles size and particles distance wereperformed on at least five SEM figures.

3. Results and discussion

Fig. 1 shows the current–voltage characteristics of AuGeNi con-tacts after rapid thermal annealing at different temperatures. Fig. 2shows the result of contact resistivity measurements of AuGeNicontacts to n-GaAs as-deposited and subsequent RTP accordingto Table 1 (in constant annealing time, 60 s) versus annealingtemperature.

Ohmic behavior was observed at 360 �C. However, contactresistivity is too high (1 � 10�1 X cm2) to be used as an ohmic con-tact. The lowest contact resistivity was obtained by annealingAuGeNi ohmic contact at 550 �C for 1 min (sample 10) and main-tains almost the same level up to the annealing temperature of650 �C (sample 12). By increasing temperature from 650 �C, themeasured total resistivity increases, which may be related to thesurface deterioration as well as defect density of samples. Defectdensity decreases carrier mobility and therefore increases contactresistivity. In order to investigate the mechanism responsible forgood ohmic characteristics, XRD measurements were performedon AuGeNi sample as a function of annealing temperature. Fig. 3

Fig. 1. Current–voltage characteristics of AuGeNi contacts after rapid thermalannealing at different temperatures.

Fig. 2. Measured contact resistivity versus temperature of annealing.

Fig. 3. X-ray spectra for Au/Ge/Ni/GaAs system (s: GaAs, �: Au, �:Ni3Ge, d: NiAs, ¤: AuGa) in (a) as-deposited state and annealing temperature of (b) 360 �C, (c) 450 �C,(d) 550 �C.

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shows a series of X-ray diffraction patterns obtained for AuGeNimetallization. Comparing X-ray spectra of annealed samplesfrom that associated with as-deposited sample reveals somechanges. Before annealing, X-ray diffraction peaks correspondingto Au and GaAs were observed. No compound formation or sub-stantial mixing between the layers occurred in the as-depositedsample, suggesting that interdiffusion between layers was negligi-ble at room temperature (Fig. 3a). After annealing at higher

temperatures, Au, Ni and Ge started to react with GaAs and newcompounds were formed. Intensities of diffraction peaks corre-sponding to Au were decreased by increasing annealing tempera-ture. Intensities of Ni peaks were too low to be observed. Afterannealing at 360 �C (sample 1), Au peaks significantly decreasedand a new peak corresponding to Ni3Ge was detected (Fig. 3b). Inthe case of sample 5 (annealing temperature = 450 �C), diffractionpattern was not significantly changed but peak intensities

S. Tahamtan et al. / Microelectronics Reliability 51 (2011) 1330–1336 1333

decreased (Fig. 3c). In the case of sample 10 (annealing tempera-ture = 550 �C), new peaks corresponding to NiAs and AuGa weredetected (Fig. 3d).

Fig. 4 shows the relative volume fractions of these compoundswhich were calculated from the measured X-ray diffraction inten-sities in annealing temperature range 300–650 �C. As seen in thisfigure, Au decreases by increasing annealing temperature to500 �C. Ni and Ge formed Ni3Ge compounds at temperatures below550 �C, whereas NiAs compound was formed at 450 �C. The resultsindicate that upon heating, Ni and Ge layers first interdiffused toform Ni3Ge compound, and then Ga and As atoms diffused outfrom the substrate to form compounds with Au, Ni and Ge at450 �C (Figs. 3 and 4).

Fig. 5 shows RBS spectrum of Au obtained for different anneal-ing temperatures. Results of RBS spectrum are in good agreementwith Figs. 3 and 4. As seen in Fig. 5, Au penetration depth is higherat high annealing temperatures leading to the formation of newcompounds (AuGa) which was not detected at lower annealingtemperatures.

Contact resistivity values decrease monotonically with increas-ing total area of GaAs interface covered by NiAs [21,22]. ComparingFigs. 2 and 4 show that formation of NiAs and AuGa compounds areessential for low contact resistivity. Formation of AuGa produces

Fig. 4. Volume fractions of Au and Ni phases formed in AuGeNi contacts afterannealing at various temperatures calculated from the measured X-ray diffractionintensities.

Fig. 5. RBS spectra of Au in AuGeNi/GaAs at different annealing temperatures.

Ga vacancies below the contact, which generates free electronsby incorporation of Ge into the Ga vacancies. Thus, the increase

Fig. 6. SEM micrograph of top alloyed AuGeNi contact surface after annealing at: (a)550 �C for 1 min and (b) 650 �C for 1 min.

Fig. 7. Variations of contact resistivity with annealing temperature showing threedifferent microstructure morphologies.

Fig. 8. Contact resistivity versus d/k parameter for AuGeNi contacts.

Fig. 9. Cross section SEM image of inward growth of GeNi rich particles (700 �C).

Fig. 10. STM photograph of (a) as-deposited sample and anneale

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of the carrier concentration below the contact makes the contactresistivity reduction [4].

As shown in SEM micrograph of Fig. 6, surface of contact afteralloying reveals distinct circular dark particles (Fig. 6a) or coarsedark particles (Fig. 6b). These particles were a mixture of circularand elongated shapes but different in size. These particles havebeen identified by EDX analysis to be rich in Ge and Ni and Au defi-cient. It was observed that contact resistivity is somewhat corre-lated to the size of these particles and their distribution. Smallparticles produce a lower contact resistivity. These results suggestthat these particles are responsible for the contact formation andconsequently the current flow into the semiconductor.

The variation of contact resistivity with alloy temperature forAuGeNi was found to depend on the presence of these regions.Fig. 7 indicates the variation of contact resistivity with alloy tem-perature showing three different microstructure morphologies:sparse circular dark particles, dense circular dark particles andcoarse dark particles. Fig. 8 shows the combination effects of par-ticle size (d) and particle distance (k) introduced as d/k parameterversus contact resistivity. As seen in this figure, as d/k decreases(from d/k = 2) contact resistivity increases. There is a critical rangefor d/k which provides low contact resistivity. Comparing Figs. 7and 8 show that the best contact resistivity can be obtained bythe larger the contact area over which these particles are distrib-uted. Increasing from d/k = 2 cause contact resistivity to increasedue to coarsening and inward growth of these particles (Fig. 9).This kind of growth caused the interface to become irregular lead-ing to subsequent rise in contact resistivity.

Fig. 10 shows the STM photograph of as-deposited sample(Fig. 10a) and annealed samples at 450 �C (Fig. 10b), 500 �C(Fig. 10c), 550 �C (Fig. 10d) and 600 �C (Fig. 10e). As seen in this fig-ure, in comparison to as-deposited samples, surface morphologyimproved after annealing at 450 �C, 500 �C and 550 �C while inthe case of annealed sample at 600 �C, surface morphology deteri-orated. Results of STM measurements and Ra values that summa-rized in Fig. 11 and Table 2, respectively, confirmed thisphotograph. As seen in Table 2, the best surface morphology wasobtained in the case of annealed sample at 450 �C.

Comparing Figs. 2 and 11 show that good ohmic contact cannotbe achieved without scarifying surface morphology. In fact, AuGacompound has a negative effect on the thermal stability of AuGeNicontacts after contact formation and deteriorates surface morphol-ogy of AuGeNi contacts. AuGa phases cannot be removed withoutsacrificing contact resistivity.

d samples at (b) 450 �C (c) 500 �C (d) 550 �C and (e) 600 �C.

Fig. 11. Surface profile at different annealing temperatures.

Table 2Results of STM roughness measurements.

Samplea Cycle 1 Cycle 2 Ra (lm)

Temperature(�C)

Time(S)

Temperature(�C)

Time(S)

As-deposited – – – – 0.145 240 20 450 60 0.039 240 20 500 60 0.0610 240 20 550 60 0.1211 240 20 600 60 0.17

a Sample numbering is according to Table 1.

Fig. 13. Change of the contact resistivity of AuGeNi contact during isothermalannealing at 450 �C.

S. Tahamtan et al. / Microelectronics Reliability 51 (2011) 1330–1336 1335

Based on the above observations it can be said that before450 �C, Ge diffuses towards the contact surface and Ni3Ge forma-tion encouraged. After 450 �C, Ge diffuses inward. Some Ge atomsreached the substrate interface while some are captured by formedNiAs (according to Figs. 4 and 12). As Ge reached to the interface, itstarts to dope the GaAs by placing on the vacant sites. Therefore,the dopant concentration will increase leading to decrease incontact resistivity. As temperature increases, diffusion ability ofGe will increase leading to more reduction in contact resistivity

Fig. 12. SEM line scan analysis of Ge distribution as a function of depth at twoannealing temperatures.

(meanwhile the distribution of dark particles are regular accordingto Fig. 8). After 650 �C partial melting of AuGa as well as defectdensities will cause deboning and increase in contact resistivity(in this stage distribution of dark particles are irregular and theirsize increases according to Fig. 8).

It can be concluded that contact resistivity in temperature range550–650 �C is the lowest. In this temperature range the topographyis poor. Below 550 �C, contact resistivity increases but topographyimproves. It should be noted that depend on the next applicationthe suitable annealing temperature should be selected in both con-tact resistivity and surface roughness point of view.

Another parameter in annealing process that affects the contactresistivity is time. Results of XRD analysis showed that a smallamount of unreacted Au was detected in the contacts due toincompletion of Au/GaAs reaction. In the case of sample 5 and dur-ing subsequent annealing, unreacted Au continued to react withGaAs, forming AuGa phases. Consequently, total area of AuGa/GaAsinterfaces increased and the percentage of GaAs interface coveredby NiAs decreased. This causes an increase of the contact resistivityvalues. Fig. 13 shows contact resistivity of AuGeNi contacts, whichwere prepared by annealing at 240 �C for 20 s and were subse-quently annealed at 450 �C (samples 4–8). As is seen, contact resis-tivity increases after annealing at 60 s due to the reason describedabove. Also, increasing annealing time led to the coarsening andirregular shape of dark particles, as seen in Fig. 13, leading to moreincrease in contact resistivity.

4. Conclusions

AuGeNi ohmic contact to n-GaAs at different annealing temper-atures and annealing time was interpreted using the microstruc-tural observations by SEM, XRD measurements STM and Imageanalysis technique. The lowest contact resistivity of 5.5 � 10�8

X cm2 obtained at 550 �C. It was found that good ohmic contactis due to the formation of the interfacial compounds, NiAs andAuGa.

Formation of dark particles after annealing identified as Ge andNi rich, affected the contact resistivity. A correlation between sizeof particles and particle distribution with contact resistivity werefound as d/k parameter. It was found that increasing from d/k = 2

1336 S. Tahamtan et al. / Microelectronics Reliability 51 (2011) 1330–1336

cause contact resistivity to increase due to coarsening and inwardgrowth of these particles.

Acknowledgments

Authors would like to thanks Mr. H. Nakhaei Motlagh, B. Sabr-louy Kolian, K. Sasani and A. Parnia Baran because of cooperationin this work.

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