Study of interface reactions between Ti/Al/Ni/Aumetallization and AlGaN/GaN heterostructures
Research Article
Wojciech Macherzyński∗, Bogdan Paszkiewicz
Wroclaw University of Technology, Faculty of Microsystem Electronics and Photonics,11/17 Janiszewskiego Street, 50-372 Wrocław, Poland
Received 21 March 2012; accepted 4 November 2012
Abstract: The electrical characteristics, and the range of interface metal-semiconductor reactions of Ti/Al/Ni/Aumetallization with AlGaN/GaN heterostructures at various annealing temperatures ranging from 715◦C to865◦C, have been investigated. The relation between the depth of the interface solid state reaction andthe current-voltage (I-V) characteristics of the ohmic contact, have been studied. It was observed, thatthe transition from nonlinear to linear I-V behaviour occurred after the annealing at 805◦C. The structuralchanges in AlGaN/GaN heterostructures beneath the metallic contact after the thermal treatment, wereinvestigated. After removing the metallization by etching, the atomic force microscope profiles and scanningelectron microscope images, were studied to define the depth to which the interfacial solid state reactionsbetween the metallization and the semiconductor structure take place. It was observed, that the changesin the heterostructures, caused by the interface m-s reactions, were observed up to a depth of 180 nm at865◦C. In the worst case, this could result in the complete removal of the two-dimensional electron gasunder the metallization of the ohmic contacts. To study the influence of the annealing process parameterson the properties of the two-dimensional electron gas, the van der Pauw Hall mobility measurement wasperformed.
AlGaN/GaN heterostructure based sensors andfield-effect transistors have been an area of intenseinterest for high temperature, high power, and highfrequency electronic device applications [1–4]. The∗E-mail: [email protected]
AlGaN/GaN heterostructure field effect transistors(HFET) are capable to handle higher current densitiesthan other III-V HFETs due to higher two-dimensionalelectron gas (2DEG) density (1013 cm−2 or higher)accumulated at the AlGaN/GaN interface [5, 6]. Manyefforts have been dedicated to the development offabrication processes of the nitride devices. Widebandgap semiconductors are able to withstand the harshenvironment and high temperature, thus, AlGaN/GaNdevices could find many applications in such fields258
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as: the military, aerospace, automotive and petroleumindustries, engine monitoring, flame detection, and solarUV detection [7, 8]. However, the thermal stability ofAlGaN/GaN heterostructures and their chemical inertnessalso engender difficulties in ohmic contact formation. Highannealing temperatures, usually over 800◦C, are requiredto established good ohmic contact performance [9–15].On the other hand, so high annealing temperature causeschanges at the heterostructure and metal-semiconductorinterface, which in turn leads to alteration of 2DEGparameter - carrier mobility. Low resistance ohmiccontacts to AlGaN/GaN are of great importance becausean improvement of their electrical properties wouldlead to enhancement of the device performance. Thefabrication of low resistance ohmic contacts is difficultbecause of the relatively high work functions of a largenumber of various metals in comparison with the electronaffinity of AlxGa1−xN materials [16]. Aside from the lowresistance requirement, the ohmic contacts to AlGaN/GaNheterostructures have to meet additional requirements,such as thermal and chemical stability, when they arededicated to operating in extremely harsh conditions.In this paper we report on the electrical properties andinterface reaction between Ti/Al/Ni/Au metallizationand AlGaN/GaN heterostructures. After etching offTi/Al/Ni/Au metallization, the authors deduced fromscanning electron microscopy (SEM) images and atomicforce microscopy (AFM) profiles that microstructuralchanges had been introduced at the metal-semiconductorinterface after thermal annealing. To study the influenceof the annealing process parameters on the propertiesof AlGaN/GaN heterostructure, in particular on thetwo-dimensional electron gas (2DEG), an indirect methodusing the Hall mobility measurement in the van der Pauwgeometry was employed [17].The Ti/Al based ohmic contact is one of the most prevalentmetallization schemes of ohmic contact in AlGaN/GaNheterostructures [9–15]. A titanium layer is essentialas, at elevated temperatures, the Ti participates in thereaction with nitrides at the interface and forms TiN[13–15]. This reaction extracts nitrogen and generatesN-vacancies. N-vacancies act as n-type dopants andcreate a highly doped layer underneath the metallization,leading to low-contact resistance of the Ti/Al based ohmiccontact. The aluminium is the layer which is responsiblefor the formation of the ohmic contact to AlGaN/GaNheterostructures. A gold layer is commonly adopted asthe outer layer to minimize the oxidation of Ti/Al metalsduring the rapid thermal annealing process. In addition,a Ni barrier layer has to be employed to minimizethe diffusion of the upper Au layer downward at theTi/Al/Ni/Au ohmic stack metallization.
2. Experimental details
The AlGaN/GaN heterostructure applied in this study,consisted of Al0.3Ga0.7N(25 nm)/GaN(1000 nm) grown bymetalorganic vapor phase epitaxy (MOVPE) on sapphiresubstrate. For the research purposes, two main groupsof samples have been prepared, the first one with aphotolithographic mask designed for I-V measurements,and the second one without a photolithographic maskdedicated for the depth AFM profiles and SEMimages. The sheet carrier concentration and themobility of electrons in the channel were 9x1012 cm−3and 1769 cm2/Vs, respectively, as determined by Hallmeasurements. Prior to metal deposition, the native oxide(Ga2O3) was removed from surface by etching in HCl:H2O(1:1) solution, followed by a deionised water rinsing anddrying in N2 flow. Then, the samples were immediatelyloaded into the vacuum chamber of an evaporationsystem. The metallic contact, consisting of Ti/Al/Ni/Au(20/100/40/150 nm), was deposited on the substrateunder vacuum conditions with a base pressure lowerthan 10−6 mbar. The Ti and Ni layers were depositedby using an electron beam evaporator in contrary toAl and Au metallic layer, which were deposited by aresistance evaporator. The Ti/Al/Ni/Au multilayer ohmicmetallizations were annealed at various temperatures inthe rapid thermal annealing (RTA) system. The rangeof temperature of thermal annealing was selected on thebasis of the authors’ previous study [18]. The temperatureof each annealing process was changed over the rangefrom 715◦C to 865◦C and the annealing time of 60 secondswas kept for all samples. The influence of the annealingprocess on the electrical contact properties was studiedby evaluation of its I-V characteristic. To study theinfluence of the annealing process parameters on theproperties of the heterostructure, in particular on the 2dimensional electron gas, an indirect method using the vander Pauw Hall mobility measurement was employed. Theatomic force microscopy profiles and scanning electronmicroscopy (SEM) images were used to define howdeep the interface metal-semiconductor (m-s) solid statereactions reach. In order to assess the depth of thereactions occurring during the thermal treatment betweenthe metallization and the semiconductor structure, themetals forming the ohmic contact (Ti, Al, Ni, Au) wereselectively etched. After the etching of the metallayers, the SEM images of topography of the surfacesunderneath the contact area were taken and AFMprofiles were measured. This procedure allowed us toobserve the changes of the metal-semiconductor interface.The samples dedicated for AFM profiles and for someSEM images, were etched to remove the multilayer259
Study of interface reactions between Ti/Al/Ni/Au metallization and AlGaN/GaN heterostructures
metallization in the following steps: (a) Au metal layerwas etched in iodine-potassium iodide solution, (b) Ni wasetched in ferrous chloride, (c) Al was etched at 45◦C inthe solution of H3PO4/HNO3/CH2COOH/H2O (85:5:5:5),(d) Ti layer was etched in perhydrol warmed to 65◦C. Toinvestigate the I-V characteristics of metal-semiconductorcontacts, two contacts from the test structure intendedfor the linear transfer length method, were used for themeasurement.3. Results and discussionFigure 1 shows the current-voltage (I-V) characteristicsbetween two contact pads for the samples annealed atvarious temperatures. The linear ohmic I-V characteristicswere obtained after annealing at the temperature of 805◦Cand higher. The as-deposited (not annealed) sampleshowed a rectifying character. The van der Pauw Hallmobility measurement showed that the sheet density ofcarriers beneath the metal contacts remained on the samelevel of 9×1012 cm−3, however, the Hall mobility of thecarriers decreased from 1769 cm2/Vs for the non annealedsamples, to 506 cm2/Vs for the samples annealed at thehighest (865◦C) temperature (Tab 1). This resulted in asignificant increase in sheet resistance of the AlGaN/GaNheterostructures underneath the metal contact.
Figure 1. I-V characteristics of the contacts annealed at differenttemperatures.
The thermal annealing in an elevated temperatureaffected the whole structure and also the 2DEG justunderneath the metal contacts, which led to thedegradation of the heterostructure up to the completedeteriorating of AlGaN/GaN properties after annealingat temperatures above 835◦C (the Hall mobility decreasedbelow 538 cm2/Vs). The process of the degradation canbe explained by the interface m-s solid state reaction
Table 1. Influence of thermal annealing (t = 60 s) of metal contactson the electrical parameters of AlGaN/GaN heterostructuresunderneath the metal contacts.Temperatures of thermal Hall mobility I-Vannealing [◦C] [ cm2Vs ] characteristicsNon annealed 1769 Nonlinear715 1562 Nonlinear745 1411 Nonlinear775 1379 Nonlinear805 1447 Linear835 538 Linear865 506 Linear
of chemical elements between the metal contact layersand AlGaN/GaN heterostructures (Fig. 2). This couldlead to the consumption of AlGaN layer into the GaNlayer. What is important, is the critical AlGaN thicknessof 3 nm required to enable 2DEG accumulation at theAlGaN/GaN interface [19]. To confirm the structuralchanges at the m-s junction during the thermal treatment(as a result of the reactions occurring in the bulksubstrate), and to study the influence of the reactionson the structure of m-s ohmic junction to AlGaN/GaNheterostructure, we have observed the microstructuralchanges after the thermal treatment at various annealingtemperatures ranging from 715◦C to 865◦C. The SEMimages of surface topography after etching off themetallization are shown in Fig. 3a-f.
Figure 2. Schematic drawing of the possible location of metalcontact in relation to 2DEG at the GaN(cap)/AlGaN/GaNheterostructure a) non-annealed, b) after thermalannealing up to 805◦C (60s) - 2DEG has been preserved,c) after thermal annealing at a temperature higher than835◦C (60s) - 2DEG is completely destructed.
In order to analyse the depth of the reactions occurringin the bulk during the thermal treatment on the m-sinterface, the AFM topography profiles of the surface afteretching off the metallization, were observed (Fig. 4a-f).Level 0 nm in the Fig. 4 represents the location of themetal-semicondutor interface. To facilitate the analysis,the Fig. 4 also shows the location of 2DEG (the thicknessof AlGaN layer was 25 nm).On the basis of the analysis of SEM images(Fig. 3a-b) and AFM profiles (Fig. 4a-b) of the samplesannealed at 715◦C and 745◦C, we were not able to260
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Figure 3. SEM topography images of the surface of AlGaN/GaN heterostructures after etching off the ohmic Ti/Al/Ni/Au contacts annealed atvarious temperatures: a) 715◦C, b) 745◦C, c) 775◦C, d) 805◦C, e) 835◦C and f) 865◦C.
observe any changes in surface microstructure of theAlGaN/GaN heterostructure (underneath metallization).The topography of this surface was the same as thetopography of a non-annealed heterostructure (not shownhere). At 775◦C annealing, we observed the beginningof changes in the microstructure, which confirms that theinterface metal-semiconductor solid state reactions begin.The depth of the reactions was up to 10 nm for thistemperature (Fig. 4c). Presenting our results, we havechosen the profiles where the changes were the deepest.After annealing at 805◦C, numerous changes could be
observed up to the depth of 20 nm. But what is much moreimportant, for the first time, we have observed the changesin the heterostructures exceeding the depth of 25 nm i.e.at the level where two-dimensional electron gas occurs.We believe, that this could be the key for linearizationof I-V characteristics (Fig. 1). These deep m-s solidstate reaction products penetrated through the AlGaN andestablished electrical connections with the 2DEG alongthe AlGaN/GaN interface. However, further increase ofthermal annealing temperature above 835◦C, extends thesize and distribution of the solid state reaction at the261
Study of interface reactions between Ti/Al/Ni/Au metallization and AlGaN/GaN heterostructures
Figure 4. Representative AFM topography profiles of the surface of AlGaN/GaN heterostructures after etching off the ohmic Ti/Al/Ni/Au contactsannealed at various temperatures: a) 715◦C, b) 745◦C, c) 775◦C, d) 805◦C, e) 835◦C and f) 865◦C.
interface (Fig. 3e-f, Fig. 4e-f), which severely disturbs theformation of 2DEG due to the extensive consumption ofthe AlGaN/GaN interface. For these two temperatures,i.e. 835◦C and 865◦C, the electron mobility falls below538 cm2/Vs (Tab. 1), showing a complete degradationof the heterostructure underneath the metallization. Webelieve, that the depth of interface m-s solid statereaction is the major factor which enables and controlsthe mechanism of improvement and degradation of ohmicTi/Al/Ni/Au contact to AlGaN/GaN heterostructures.
4. Conclusions
Current-Voltage measurements, SEM images, AFMtopography profiles, and Hall mobility measurementwere used to study the interface m-s solid statereactions of Ti/Al/Ni/Au ohmic contact metallizationon AlGaN/GaN heterostructures as a function of theannealing temperature. The microstructural changes atthe m-s interface, after etching off the metallizations,at various annealing temperatures ranging from 715◦C262
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to 865◦C, were investigated. Emphasis was givento the investigation of the depth of interfacial m-sreaction in the heterostructure under metallization, to findthe differences between the structures with linear andnonlinear I-V characteristics. The present study revealeda close relationship between the evolution of the electricalperformance and the solid state reactions occurring at themetal-semiconductor interface. The first changes in themicrostructure of AlGaN/GaN heterostructures have beenobserved at 775◦C. The depth of reaction was up to 10 nmat this temperature, but the I-V characteristics were stillnon-linear. After annealing at 805◦C, we could observenumerous changes up to the depth of 20 nm and we haveobserved a small number of changes in heterostructureswhich exceeded the depth of 25 nm i.e. the level atwhich the two-dimensional electron gas occurs. Webelieve, that this could be the key for linearization ofI-V characteristics. The m-s solid state reaction productspenetrated up to the depth of 25 nm through the AlGaNand could establish the electrical connections with the2DEG along the AlGaN/GaN interface. However, furtherincrease of thermal annealing temperature above 835◦C,extends the size and distribution of the solid statereaction at the interface, which severely disturbs theformation of 2DEG due to the extensive consumption ofthe AlGaN/GaN interface. For these two temperaturesi.e. 835◦C and 865◦C, the electron mobility falls below538 cm2/Vs, showing a complete degradation of theheterostructure underneath the metallization. We believe,that the depth of the interface m-s solid state reaction isthe major factor enabling and controlling the mechanismof improvement and degradation of ohmic Ti/Al/Ni/Aucontact to AlGaN/GaN heterostructures.AcknowledgmentsThis work was co-financed by European Union withinEuropean Regional Development Fund, through grantInnovative Economy (POIG.01.01.02-00-008/08-04),National Centre for Science under the grant no. NN515 495740, R02 018 02, by Wroclaw University ofTechnology statutory grants no. S10019, B10010 andSlovak-Polish International Cooperation Program no.
SK-PL-0017-09. Wojciech Macherzyński benefited froma fellowship co-financed by European Union withinEuropean Social Fund.References
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