Materials Science and Engineering A422(2006)
316–320
The thermodynamic analysis of the driving force for
the Ni/GaN interfacial reaction
Mei Li a,Changrong Li a,∗,Fuming Wang b,Weijing Zhang a
a School of Materials Science and Engineering,University of Science and Technology Beijing,Beijing100083,PR China
b School of Metallurgical and Ecological Engineering,University of Science and Technology Beijing,Beijing100083,PR China
Received25November2005;accepted14February2006
Abstract
Based on the optimized thermodynamic data from literature and the phase equilibrium calculation,the contact interfacial reactions and the phase formation sequences of Ni/GaN joints were analyzed theoretically under the reported experimental conditions.The phase formation sequences and the apparent spatial phase sequences were obtained.It was revealed that the phase formation sequences
and the apparent spatial phase sequences were dominated by many factors,such as the temperature range,time period and the amounts of the original layers of the diffusion couples.
©2006Elsevier B.V.All rights reserved.
Keywords:Ga–N–Ni;Interfacial reaction;Phase formation sequence
1.Introduction
Recently,GaN-based materials have been intensively stud-ied for optoelectronic devices as well as high-temperature and high-power electronic devices.Visible light-emitting diodes[1], ultraviolet(UV)detectors[2]and high-electro-mobility tran-sistors(HEMTs)[3]have been demonstrated.Fabrication of electrical contacts to theses devices requires the deposition of metals on semiconductors with subsequent annealing.Interfa-cial reactions between metals and GaN are important since the electrical properties of such contacts are influenced by the phases formed directly on the GaN surface because of process annealing or high-temperature service.
As device performance improves,studies on metal/GaN inter-faces have attracted much attention[4].Since Ni has been used in Ni/Au ohmic contacts to GaN[5]and has been examined as a Sc
hottky barrier to n-type GaN[4,6–8],an understanding of the metallurgy of the Ni/GaN system is clearly needed.
The knowledge of the phase equilibria is of great help to understand the interfacial phenomena of the metal/compound-semiconductor contact.If the system can be described thermo-dynamically,the possible phase sequences can be predicted and the experimental results can be analyzed.
∗Corresponding author.Tel.:+861062333607;fax:+861062333772.
E-mail address:crli@mater.ustb.edu(C.Li).
In this paper,on the basis of the thermodynamic parameters from literature and the phase equilibrium calculation,the inter-facial reaction sequences of the Ni/GaN joints were analyzed theoretically.
2.Literature results
2.1.Thermodynamics of the Ga–N–Ni ternary system
Grobner et al.[9]prepared the ternary samples from Ni and GaN powders in various ratios,pressed to pellets and annealed between773and973K for24h.Phase analyses were carried out by X-ray diffraction
(XRD).Experimental results show that the reactions are sluggish,and no ternary compound was reported in the Ga–N–Ni ternary system.Based on their experiments and the data from other literature,Grobner et al.assessed thermody-namically the Ga–N–Ni ternary system.The isothermal section of Ga–N–Ni system at773K[9]was shown in Fig.1.
2.2.Literature experimental results of interfacial reactions
Ni/GaN interfacial reactions were studied by many investi-gators.Guo et al.[7]studied the thermal stability of Ni Schottky contacts on n-type GaN.Phase identification was performed primarily by XRD.According to these investigators,Ni3Ga4 was identified to be present along with Ni in the as-deposited
0921-5093/$–see front matter©2006Elsevier B.V.All rights reserved. doi:10.1016/j.msea.2006.02.017
M.Li et al./Materials Science and Engineering A422(2006)316–320
317
Fig.1.The isothermal section of the Ga–N–Ni system at773K.
film.However,no intermixing between Ni and Ga was observed in the corresponding SIMS(secondary-ion mass spectroscopy) spectra.On subsequent annealing in a N2atmosphere at473 and673K,nickel nitrides,Ni3N and Ni4N,were identified by XRD[7].However,Venugopalan et al.[10,11],Liu et al.[6] and Bermudez et al.[12]observed that a pronounced interfacial reaction occurred upon annealing above873K.Grobner et al.
[9]suggested that the Ni3N phase starts to form under a pressure of104bar.However,this is an external pressure and the internal pressure,exerted by the reaction,cannot exceed100bar.Conse-quently,no Ni3N was observed by XRD,the only solid reaction product was Ni3Ga.Venugopalan et al.[10]investigated the interfacial reactions between thin nickelfilms and GaN.The Table1
Annealing conditions and the corresponding reaction products in Ni/GaN joints from Ref.[10]
Temperature(K)Time Atmosphere Ni–Ga phase in
contact with GaN 67310min N2Ni
10min95%N2+5%H2Ni
8731h N2fcc
1h Ar fcc
17h N2fcc
7days Sealed in vacuum fcc
10231h N2fcc or Ni3Ga
1h Ar fcc or Ni3Ga
4h N2fcc or Ni3Ga
7days Sealed in vacuum fcc or Ni3Ga 117310min N2NiGa
10min Ar NiGa
30min Ar NiGa thin Nifilms on GaN were annealed at temperatures between 673and1173K in N2or Ar
and forming gas,and were analyzed using glancing angle XRD and Auger depth profiling.Thefirst indication of an interfacial reaction was found after annealing at 873K for1h,showing that Ga was dissolved in the face-centered cubic Nifilm.The extent of dissolution increased with contin-ued annealing.After annealing at1023K for1h in either N2or Ar,greater intermixing occurred.The different annealing condi-tions and the corresponding reaction products for Ni/GaN joints [10]were shown in Table1.A trend of increasing Ga content in the reactedfilms was observed with increasing temperature.
3.Driving force description
Driving force is the affinity between reacting chemical species in a certain process,for which the process extent is usually represented by the internal variableξ.In the Thermo-Calc software which is based on Gibbs energy minimization for equilibrium systems,the driving force for a phase␣is always evaluated by[13]:
D␥=−
∂G
∂ξ
T,P,N i
=−
∂G
∂N␥
T,P,N i
The driving force for a stable equilibrated phase is zero,whilst for an unstable phase is negative.Any phase with positive driving force will precipitate from the meta-stable equilibrated system.
In the Thermo-Calc software,there are three types of the defined status for a phase during the equilibrium calculation: ENTERED,SUSPEND and DORMANT.ENTERED or SUS-PEND means that the phase is concluded or excluded in the calculation respectively,while DORMANT means that the phase is not considered in the calculation but its driving force for pre-cipitation is calculated.
Based on the thermodynamic data[9]and the experimental data[9–11],the phase formation sequences were predicted for the Ni/GaN joints using Thermo-Calc software.
4.Driving force analysis
For the Ni/GaN joint,the initial status was set as ENTERED for fcc(Ni)and GaN phases and DORMANT for other phases. Above873K,the reaction at the Ni/GaN interface starts by the diffusion and dissolution of Ga in Ni[10],resulting in the decom-position of GaN.So,there are a little N2between Ni and GaN and the status of gas was changed to be ENTERED.The con-centration of Ga in fcc(Ni)increases with the diffusion of Ga. The starting point of the calculation was supposed to be at the Ni side,x(Ni)=0.99and x(Ga)=0.01for the molar fractions of Ni and Ga,respectively.The driving force was calculated as the concentration of Ga in fcc(Ni)increases with step0.01while the concentration of Ni in fcc(Ni)decreases with the same step.No positive driving force was obtained until x(Ni)=0.8,x(Ga)=0.2, when the driving force for Ni3Ga precipitation is positive.The calculation results were shown in Table2.
In Table2,the third and the fourth columns are respectively the calculated driving forces for the formation of new precipi-
318M.Li et al./Materials Science and Engineering A422(2006)316–320
Table2
Calculated driving forces at900K for the Ni/GaN interface at x(Ni)=0.8and x(Ga)=0.2
Phase Status Driving force(J/mol)Mole fraction fcc ENTERED0.00000000  1.00000000 Gas ENTERED0.000000000.00000000 GaN ENTERED−3.563620350.00000000 Ni3Ga DORMANT  5.24105204×10−2
NiGa DORMANT−2.80925178×10−1
Ni5Ga3DORMANT−3.31288701×10−1
Ni3Ga2DORMANT−3.68852545×10−1
Liq DORMANT−9.57188872×10−1
Ni3Ga4DORMANT−1.91467142
Ni2Ga3DORMANT−2.29357083
Ni3N DORMANT−3.22653309
NiGa4DORMANT−6.09998523
Ni4N DORMANT−9.42599631
tates and the calculated mole fractions of the old phases both in the meta-stable equilibrium system.It is shown in Table2that the driving force for the formation of Ni3Ga phase is positive and maximum.The Ni3Ga phase will form at the interface between Ni/GaN.
After Ni3Ga precipitation,two new interfaces,Ni/Ni3Ga and Ni3Ga/GaN,will be formed.Thus,the Ni3Ga phase was put into the equilibrium calculation and its status was changed to be the ENTERED.And the status of GaN and fcc(Ni)phases were changed to be DORMANT for driving forces calculation at interfaces Ni/Ni3Ga and Ni3Ga/GaN,respectively.The driving forces at900K for the phase formations at interfaces Ni/Ni3Ga and Ni3Ga/GaN were calculated.Because the diffusion of N2 generated by the reaction of Ni and GaN is slow[9],there are a little N2between Ni3Ga and GaN.Therefore,the status of gas was changed to be ENTERED.The calculated driving forces at900K for the Ni3Ga/GaN interface were shown in Table3. Those for Ni/Ni3Ga were not listed since no positive driving force was obtained.
It is shown in Table3that the driving force of the forma-tion of NiGa phase is positive and maximum.The NiGa phase turns to precipitate at the interface Ni3Ga/GaN.Again two new interfaces,Ni3Ga/NiGa and NiGa/GaN,will be formed.Thus, Table3
Calculated driving forces at900K for the Ni3Ga/GaN interface at x(Ni)=0.74 and x(Ga)=0.25
Phase Status Driving force(J/mol)Mole fraction
Ni3Ga ENTERED0.000000009.86666667×10−1 GaN ENTERED0.00000000  6.66666661×10−3 Gas ENTERED0.00000000  6.66666673×10−3 NiGa DORMANT  1.33928839
Ni3Ga4DORMANT  1.09623592
Ni2Ga3DORMANT  1.01446718
fcc DORMANT  1.00452255
Liq DORMANT9.15408106×10−1
Ni3Ga2DORMANT8.15408497×10−1
Ni5Ga3DORMANT  4.48206499×10−1
NiGa4DORMANT−8.87318187×10−1
Ni3N DORMANT−5.05269039
Ni4N DORMANT−1.13824984×10Table4
Calculated driving forces at900K for the Ni3Ga/NiGa interface at x(Ni)=0.74 and x(Ga)=0.25
Phase Status Driving force(J/mol)Mole fraction
Ni3Ga ENTERED0.000000009.77038187×10−1 NiGa ENTERED0.00000000  1.29618134×10−2 Gas ENTERED0.00000000  1.00000000×10−2 Ni3Ga2DORMANT  1.66553498×10−1
Ni5Ga3DORMANT  1.36917552×10−1
fcc DORMANT−1.03194150×10−2
Liq DORMANT−7.13073270×10−1
Ni3Ga4DORMANT−8.06239946×10−1
Ni2Ga3DORMANT−9.74409372×10−1
GaN DORMANT−2.12028420
NiGa4DORMANT−3.99706835
Ni3N DORMANT−4.00061168
Ni4N DORMANT−1.02516804×10
the NiGa phase was put into the equilibrium calculation and its status was changed to be the ENTERED.The status of GaN and Ni3Ga phases were changed to be DORMANT for the driv-ing forces calculation at interfaces Ni3Ga/NiGa and NiGa/GaN, respectively.The driving forces at900K for the phase forma-tions at interfaces Ni3Ga/NiGa and NiGa/GaN were calculated. The calculated driving forces at900K for the Ni3Ga/NiGa inter-face were shown in Table4.Those for NiGa/GaN were not listed since no positive driving force was obtained.
It is shown in Table4that the driving force of the Ni3Ga2 phase is positive and maximum.So the Ni3Ga2phase turns to precipitate at the interface Ni3Ga/NiGa.Again two new inter-faces,Ni3Ga/Ni3Ga2and Ni3Ga2/NiGa,will be formed.Thus, the Ni3Ga2phase was put into the equilibrium calculation and its status was changed to be the ENTERED.The status of NiGa and Ni3Ga phases were changed to be DORMANT for the driving forces calculation at interfaces Ni3Ga/Ni3Ga2and Ni3Ga2/NiGa,respectively.The driving forces at900K for the phase formations at interfaces Ni3Ga/Ni3Ga2and Ni3Ga2/NiGa were calculated.The calculated driving forces at900K for th
e Ni3Ga/Ni3Ga2interface were shown in Table5.The calculated driving forces for the Ni3Ga2/NiGa interface were not listed since no positive driving force was obtained.
Table5
Calculated driving forces at900K for the Ni3Ga/Ni3Ga2interface at x(Ni)=0.74 and x(Ga)=0.25
Phase Status Driving force(J/mol)Mole fraction
Ni3Ga2ENTERED0.00000000  1.66666667×10−2 Ni3Ga ENTERED0.000000009.73333333×10−1 Gas ENTERED0.00000000  1.00000000×10−2 Ni5Ga3DORMANT  3.67475387×10−3
fcc DORMANT−7.12130939×10−2
NiGa DORMANT−1.89516858×10−1
Liq DORMANT−9.18473903
Ni3Ga4DORMANT−1.06838740
Ni2Ga3DORMANT−1.36303420
GaN DORMANT−2.53666795
NiGa4DORMANT−3.79241981
Ni3N DORMANT−4.60776451
Ni4N DORMANT−1.00296088×10
M.Li et al./Materials Science and Engineering A 422(2006)316–320
319
Fig.2.The phase formation sequence at the Ni/GaN couple at 900K.
It is shown in Table 5that the driving force of the Ni 5Ga 3phase is positive and maximum.The Ni 5Ga 3phase turns to precipitate at the interface Ni 3Ga/Ni 3Ga 2.Again two new inter-faces,Ni 3Ga/Ni 5Ga 3and Ni 3Ga 2/Ni 5Ga 3,will be formed.The local equilibrium states are reached at the interfaces between Ni/Ni 3Ga,Ni 3Ga/Ni 5Ga 3,Ni 5Ga 3/Ni 3Ga 2,Ni 3Ga 2/NiGa and NiGa/GaN,respectively.
The above-mentioned phase formation sequence is illustrated in Fig.2.As for the Ni/GaN joint at 900K,the newly formed phases in the phase formation sequence are Ni 3Ga at the Ni/GaN interface,NiGa at the Ni 3Ga/GaN interface,Ni 3Ga 2at the Ni 3Ga/NiGa interface and Ni 5Ga 3at the Ni 3Ga/Ni 3Ga 2inter-face.The apparent spatial phase sequence is the bottom line in Fig.2,Ni/Ni 3Ga/Ni 5Ga 3/Ni 3Ga 2/NiGa/GaN.The
apparent
Fig.3.The isothermal section of the Ga–N–Ni system at 900K and the phase sequence of the Ni/GaN couple.
spatial phase sequence is drawn in the isothermal section of Ga–N–Ni system in Fig.3.5.Discussion and summary
Based on the thermodynamic data and the driving force analy-sis,the phase formation sequences were analyzed for the Ni/GaN joints.The apparent spatial phase sequence was obtained for the Ni/GaN joint at Ni/Ni 3Ga/Ni 5Ga 3/Ni 3Ga 2/NiGa/GaN.
Since the reactions between Ni and GaN are slow [9],the spa-tial phase sequence is different at different temperature range and different time period.For example,the spatial phase sequence is Ni/Ni 3Ga/GaN at 1023K,4h as shown in the dash line box of Fig.2,while the spatial phase sequence is Ni/Ni 3Ga/NiGa/GaN at 1173K,10min as shown in the dot dash line box of Fig.2.So the spatial phase sequences depend on the temperature and time.
The spatial phase sequences depend on not only the tem-perature and time,but also the amounts of Ni and GaN.If the Ni forms thin-film,the spatial phase sequence is Ni 3Ga/Ni 5Ga 3/Ni 3Ga 2/NiGa/GaN after the thin-film Ni is con-sumed.If the Ni film is very thin,the reaction between Ni 3Ga a
nd NiGa may not occur.
These theoretical analyses explain the experimental observa-tions well.The different phase formation sequences and hence the different apparent spatial phase sequences are mainly dom-inated by the temperature,time and the relative amounts of the diffusion couples.Acknowledgements
The authors would like to express their appreciation to the Royal Institute of Technology Sweden for supplying the Thermo-Calc software.This work was supported by the National Natural Science Foundation of China (No.50371008)and the National Educational Committee for the Doctorate program (No.20030008016).References
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