In this study, titanium nitride (TiN) is synthesized using reactive sputtering for a self-aligned gate process. The Schottky barrier height of the TiN on n-GaN is around 0.5 to 0.6 eV and remains virtually constant with varying nitrogen ratios. As compared with the conventional Ni electrode, the TiN electrode presents a lower turn-on voltage, while its reverse leakage current is comparable with that of Ni. The results of annealing evaluation at different temperatures and duration times show that the TiN/W/Au gate stack can withstand the ohmic annealing process at 800°C for 1 or 3 min. Finally, the self-aligned TiN-gated AlGaN/GaN heterostructure field-effect transistors are obtained with good pinch-off characteristics

Ao, Jin-Ping

Jiang, Ying

Li, Liuan

Nakamura, Ryosuke

Wang, Qingpeng

English

Tokushima University Institutional Repository

Li et al. Nanoscale Research Letters 2014, 9:590http://www.nanoscalereslett.com/content/9/1/590NANO EXPRESS Open AccessSynthesis of titanium nitride for self-aligned gateAlGaN/GaN heterostructure field-effect transistorsLiuan Li, Ryosuke Nakamura, Qingpeng Wang, Ying Jiang and Jin-Ping Ao*AbstractIn this study, titanium nitride (TiN) is synthesized using reactive sputtering for a self-aligned gate process. TheSchottky barrier height of the TiN on n-GaN is around 0.5 to 0.6 eV and remains virtually constant with varyingnitrogen ratios. As compared with the conventional Ni electrode, the TiN electrode presents a lower turn-onvoltage, while its reverse leakage current is comparable with that of Ni. The results of annealing evaluation atdifferent temperatures and duration times show that the TiN/W/Au gate stack can withstand the ohmic annealingprocess at 800°C for 1 or 3 min. Finally, the self-aligned TiN-gated AlGaN/GaN heterostructure field-effect transistorsare obtained with good pinch-off characteristics.Keywords: Titanium nitride; Self-aligned gate; AlGaN/GaN heterostructure field-effect transistorsPACS: 73.40.Kp; 77.84.Bw; 73.40.QvBackgroundThe AlGaN/GaN heterostructure field-effect transistors(HFETs) are excellent candidates for high-power andhigh-frequency electronic devices [1,2]. To achieve ahigh-temperature performance, it is very desirable toproduce a gate contact with a large Schottky barrierheight (SBH) and an excellent thermal stability. In high-frequency applications, a self-aligned gate (SAG) processis proposed to minimize the source-to-gate and drain-to-gate distances for smaller access resistance, in whicha T-shaped Schottky gate is fabricated first and thenused as a mask directly for ohmic metal evaporation.Then, the Schottky gate and the ohmic electrodes areannealed simultaneously to obtain ohmic contacts [3].An important technology to form the SAG structure isthe Schottky gate which can withstand the ohmic an-nealing process, because the optimized ohmic contactannealing temperature of the Ti-based multilayers onGaN-based materials is usually around 800°C to 850°C[4,5]. Therefore, the Schottky gate must be able to with-stand such a high temperature during the source-drainohmic contact annealing process.In previous studies, we have evaluated the electricalperformance of Schottky contacts produced using* Correspondence: jpao@ee.tokushima-u.ac.jpInstitute of Technology and Science, The University of Tokushima, Tokushima770-8506, Japan© 2014 Li et al.; licensee Springer. This is an OpAttribution License (http://creativecommons.orin any medium, provided the original work is pdifferent kinds of refractory metal nitrides such as titan-ium nitride (TiN), MoN, TaN, MoSiN, WTiN, ZrN, andHfN on GaN by reactive sputtering in an ambient of Arand N2 mixture sputtering gas [6-8]. Considering the ad-hesion on GaN, sheet resistivity, reverse leakage current,SBH, and thermal stability of these devices, we regardTiN as the suitable material for the Schottky electrode.It can be obtained easily by reactive sputtering with ni-trogen as the reactive gas and shows a relatively smallerresistivity, good adhesion, and less leakage current onthe GaN Schottky contact.Herein, we obtain titanium nitride (TiN) by reactivesputtering using different N2/Ar sputtering gas ratios.The annealing evaluation results demonstrate that theTiN/W/Au gate on AlGaN/GaN HFETs can withstandthe 800°C annealing temperature for 1 or 3 min. Finally,the TiN-gated AlGaN/GaN HFETs fabricated with a self-aligned process are obtained.MethodsTo evaluate the effect of the precursor composition, wedeposited TiN films on Si-doped n-GaN (3 × 1017 cm−3dopant density, 1-μm thick) using different N2/Ar sput-tering gas ratios (nitrogen percentage) of 0:18 sccm(0%), 1:17 sccm (5%), 3:15 sccm (15%), 7:11 sccm (40%),11:7 sccm (60%), and 15:3 sccm (85%). To maintain auniform current flow between the ohmic contact anden Access article distributed under the terms of the Creative Commonsg/licenses/by/4.0), which permits unrestricted use, distribution, and reproductionroperly credited.Figure 1 AFM images of the TiN films deposited with various nitrogen percentages. (a) 0%, (b) 15%, (c) 40%, and (d) 85%.Figure 2 I-V characteristics and linear plot. The I-V characteristicsof the TiN Schottky diodes deposited with different N2/Ar ratios (a)and linear plot in the forward region (b).Li et al. Nanoscale Research Letters 2014, 9:590 Page 2 of 5http://www.nanoscalereslett.com/content/9/1/590the Schottky contact, a circular Schottky pattern with adiameter of 166 μm was adopted. The ohmic contact wasplaced on the same side as the Schottky contact with aseparating distance of 15 μm to simplify the process. Astandard lift-off technology was used to form both theohmic and the Schottky contacts. Ohmic contact wasformed using a Ti/Al/Ti/Au (50/200/40/40 nm) multilayerstructure, a fixed structure which is being adopted in ourprocess system, and annealed at 800°C for 1 min. Prior tothe TiN deposition process, the sample surfaces werecleaned by O2 plasma ashing and immersion in a dilutedHCl (HCl:H2O = 1:1) solution for 5 min to remove anyoxide layer that developed after the lithography process.The direct current (DC) sputtering power was fixed at 75W with a chamber pressure of 0.14 Pa during the reactivesputtering.To fabricate the TiN-gated devices by gate-first process,we firstly optimized the T-gate fabrication process on a sili-con substrate using a three-layer e-beam resist technique[9]. Then, the self-aligned gate HFETs were obtained onAlGaN/GaN wafer with the optimized conditions. For thethree-layer e-beam resist technique, the bottom resist layerwas a ZEP520A (500 nm) layer, followed by a LOR 5B (800nm) resist. The upper layer was also a ZEP520A (500 nm)layer. At first, the upper layer was exposed by the electronbeam and developed. Then, the LOR layer was developedusing a specific developer without exposure. Finally, thebottom layer was exposed by the electron beam and devel-oped again. The stack structure of TiN/W/Au (200/50/200nm) was then deposited to form a T-gate electrode. Thedrain and source regions, including the gate and the accessregions, were exposed and developed using lithographytechnology. Ti/Al/Ti/Au multilayers with thickness of 30/120/40/40 nm were formed by lift-off technology for thedrain and source contact, where the T-shaped Schottkygate was used as the mask directly. The Schottky gate andTable 1 Comparison of Schottky properties of GaN diodeswith TiN and Ni electrodesMetal Work function (eV) SBH (eV) Ideality factor ReferenceTiN 4.7 0.59 1.07 This workNi 5.15 0.87 1.23 This workTiN 4.7 0.67 1.04 [11]Ni 5.15 0.88 to 0.93 1.10 to 1.20 [12,13]Figure 4 Current-voltage (a) and gate leakage current (b)characteristics of the devices annealed at different times.Li et al. Nanoscale Research Letters 2014, 9:590 Page 3 of 5http://www.nanoscalereslett.com/content/9/1/590the ohmic electrodes were annealed simultaneously at 800°C for 1 min to obtain ohmic contacts.Results and discussionThe typical atomic force microscope (AFM) images of theTiN samples were recorded in tapping mode in an area of20 × 20 μm2 (Figure 1). Amorphous surface morphologiesare represented in all the samples. The root-mean-squaresurface roughness are about 2.29,1.73, 0.38, 0.48, 0.53, and0.71 nm for the films with nitrogen percentage of 0%, 5%,15%, 40%, 60%, and 85%, respectively, which indicates thatthe films deposited with a mediate nitrogen content showa relatively smoother surface. The actual nitrogen contentsof the samples determined from the X-ray photoelectronspectroscopy (XPS) data are 80.49%, 84.50%, 87.20%,85.62%, and 82.06% for the samples with nitrogen depos-ition gas percentages of 5%, 15%, 40%, 60%, and 85%,respectively. These results show that all of the TiN filmsare Ti-rich with similar actual nitrogen contents, exceptthat the sample grown with 5% N2 has a slightly lower ac-tual nitrogen content than the rest.The current-voltage (I-V) characteristics of the samplewith a pure Ti contact show an ohmic-like contact witha reverse leakage current quite similar to the forwardcurrent. The I-V curves of the other TiN contact sam-ples are comparable to each other, showing good rectifi-cation characteristics and a reverse leakage current ofapproximately 10−5 A (Figure 2a). The average SBH cal-culated from the thermionic emission theory for thesamples created with different N2/Ar sputtering gas ratiois around 0.5 to 0.6 eV, with ideality factors that rangefrom 1.05 ~ 1.4 [10]. Examination of the AFM and XPSimplies that the possession of similar film structure andcomposition causes the work function and SBH of theFigure 3 Schematics of the cross-sectional view of the conventional (TiN to be essentially independent of the N2/Ar sputter-ing gas ratio [10].For comparison, the forward region of the I-V charac-teristics of the Schottky diodes with Ni and TiN contactsare plotted linearly in Figure 2b. A 10-nm TiN film wasformed by DC reactive sputtering in N2/Ar (3:15 sccm)mixed gas atmosphere with an average sputtering rate ofabout 0.6 nm/min. A Ni/Au (10/10 nm) electrode withan average sputtering rate of about 1.0 nm/min was pre-pared by radiofrequency (RF) sputtering in Ar (30 sccm)gas atmosphere under a pressure of 0.14 Pa and a powerof 10 W. The SBH (ideality factor) of the TiN and Nidiode is around 0.59 eV (1.07) and 0.87 eV (1.23), re-spectively. These results are comparable with those ofprevious reports as shown in Table 1 [11-13]. The turn-on voltage, which is extracted by linear fitting the for-ward region (Figure 2b), is about 0.38 and 0.75 V forTiN and Ni electrodes, respectively. Generally, the Nihas a higher work function (5.15 eV) than TiN (about4.7 eV). The lower SBH and turn-on voltage are attrib-uted to the lower work function of TiN [14].Beside, the reverse current leakage of Ni is just about1 ~ 2 orders of magnitude lower than that of TiN diode.As compared with the value calculated from the thermi-onic emission theory using the Schottky barrier heightof 0.59 and 0.87 eV for TiN and Ni diode, respectively,a) and self-align gate (b) HFETs.Figure 5 SEM pictures of the T-gate on silicon (a) and the AlGaN/GaN HFET device (b). S, source; G, gate; D, drain.Figure 6 Current-voltage characteristics (a) and transfercharacteristics (b) of the self-aligned gated AlGaN/GaN HFETs.The blue line is the Gm, and the black one is the drain current.Li et al. Nanoscale Research Letters 2014, 9:590 Page 4 of 5http://www.nanoscalereslett.com/content/9/1/590the measured leakage current of Ni is much higher thanthe calculated one, while those currents are in the samelevel for TiN. The obvious increase of leakage current ofthe Ni diode is attributed to the higher interface states(as compared with TiN) existing at the Ni/GaN interfacecaused by the interaction, leading to a defect-assisted tun-neling effect [15].As an evaluation method for the gate-first process, wefabricated AlGaN/GaN HFETs with ohmic metals whichwere deposited first followed by the deposition of gate elec-trodes. The deposited ohmic and gate electrodes were thenannealed together by changing the annealing temperatureand annealing time. The schematic of the cross-sectionalview of the HFETs for evaluation is shown in Figure 3a.The device fabrication started with the isolation of themesa, which was formed by inductively coupled plasma(ICP) with an etching depth of 100 nm. The following pro-cesses were similar with the fabrication of Schottky diodementioned above. The TiN film (about 200 nm) wasformed under a N2/Ar sputtering gas with ratio (nitrogenpercentage) of 3:15 (15%). After that, a cap layer of W/Au(30/70 nm) was deposited on the TiN layer in Ar ambientto reduce the gate resistance. The gate length and gate-source/drain spacing of the TiN/W/Au-gated AlGaN/GaNHFET were 3 and 3 μm, respectively.It is noticed that an appropriate ohmic contact resist-ance of about 0.7 Ω mm can be obtained at 800°C undera mediate time of 1 or 3 min, and the sheet resistance ofthe gate film accreted slightly from about 0.47 to 0.67Ω/□ when prolonging the annealing time to 10 min [16].Figure 4a shows the I-V characteristics of HFETs for dif-ferent annealing times (0.5, 1, 3, and 10 min) at 800°C.We notice that all the HFETs annealed at different timesoperated very well with the maximum drain current ob-tained for the samples annealed after 1 or 3 min (Figure 4a).Therefore, the TiN electrode can withstand the ohmic an-nealing temperature for 1 to 3 min. However, as shown inFigure 4b, the gate reverse leakage current increased fromapproximately 10−5 to approximately 10−4 A with prolongannealing. This increase in gate leakage can be contributedby the disappearance of the natural oxide on the AlGaNlayer and the interface reaction between the non-nitridizedTi in the gate metal and the AlGaN layer, which can resultin the increase of the defect-assisted tunneling leakagecurrent.The cross-sectional view of the TiN-gated devices fab-ricated by the gate-first process is shown in Figure 3b.Figure 5a is the scanning electron micrograph (SEM)picture of the T-gate on silicon substrate, with a cross-sectional foot width of about 500 nm and a head widthof about 2 μm. The AlGaN/GaN HFETs with SAG struc-ture is also shown in Figure 5b, with a cross-sectionalfoot width of about 500 nm and a head width of about 4μm. The width of the upper layer was enlarged slightlydue to the changed conditions on the AlGaN/GaN HFETssubstrate.The I-V characteristics of HFETs show that the devicewith SAG structure can operate very well when the gatevoltage is swept from −4 to 1 V, with a threshold voltageof about −4 V and a maximum drain current density of540 mA/mm (Figure 6a). The transfer characteristic ofLi et al. Nanoscale Research Letters 2014, 9:590 Page 5 of 5http://www.nanoscalereslett.com/content/9/1/590the device (Figure 6b) shows that the transconductanceis about 160 mS/mm. However, the surface of the elec-trode after annealing is rough due to the ball-up of theAu layer. More careful optimization is required to im-prove the morphology.ConclusionsWe deposited TiN films with different N2/Ar sputteringgas ratios for self-aligned gate process. All of the samplesshowed good rectifying properties with an SBH of 0.5 to0.6 eV. As compared with the conventional Ni electrode,it presented a lower turn-on voltage while the reverseleakage currents are comparable with Ni. The effects ofannealing temperature and time on the electrical proper-ties of TiN/W/Au-gate AlGaN/GaN HFETs have beeninvestigated. It is demonstrated that the TiN/W/Au elec-trode could withstand the ohmic annealing process at800°C for 1 or 3 min. Then, AlGaN/GaN HFETs withTiN gate were obtained with a self-aligned process,showing an excellent operation with a threshold voltageof about −4 V and a maximum drain current density of540 mA/mm.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsLL deposited TiN on GaN and evaluated its characterizations and drafted themanuscript. RN fabricated the devices with the gate-first process. JPAplanned the experiments and agreed with the paper's publication. QW andYJ assisted in the data analysis. All authors read and approved the finalmanuscript.AcknowledgementsThis work is partly supported by the Grant-in-Aid for Scientific Research(No. 22560332) from the Ministry of Education, Culture, Sports, Science andTechnology of Japan. The authors would like to thank Mr. Yunbo Liu fromSAE magnetic Ltd. for supporting in the measurements and analysis of theatomic force microscope images.Received: 30 June 2014 Accepted: 16 October 2014Published: 28 October 2014References1. Keller S, Wu YF, Parish G, Zhang N, Xu JJ, Keller BP, DenBaars SP, Mishra UK:Gallium nitride based high power heterojunction field effect transistors:process development and present status at UCSB. IEEE Trans ElectronDevices 2001, 48(3):552–559.2. Mishra UK, Shen L, Kazior TE, Wu YF: GaN-based RF power devices andamplifiers. Proc IEEE 2008, 96(2):287–305.3. Kumar V, Basu A, Kim DH, Adesida I: Self-aligned AlGaN/GaN highelectron mobility transistors with 0.18 μm gate-length. Electron Lett 2008,44(22):1323–1325.4. Iucolano F, Roccaforte F, Alberti A, Bongiorno C, Franco SD, Raineri V:Temperature dependence of the specific resistance in Ti/Al/Ni/Aucontacts on n-type GaN. J Appl Phys 2006, 100(12):123706.5. Mohammed FM, Wang L, Adesida I: Ultralow resistance Si-containing Ti/Al/Mo/Au Ohmic contacts with large processing window for AlGaN/GaNheterostructures. Appl Phys Lett 2006, 88(21):212107.6. Ao J-P, Suzuki A, Sawada K, Shinkai S, Naoi Y, Ohno Y: Schottky contacts ofrefractory metal nitrides on gallium nitride using reactive sputtering.Vacuum 2010, 84(12):1439–1443.7. Ao J-P, Naoi Y, Ohno Y: Thermally stable TiN Schottky contact on AlGaN/GaN heterostructure. Vacuum 2013, 87(1):150–154.8. Ao J-P, Shiraishi T, Li L, Kishi A, Ohno Y: Synthesis and application of metalnitrides as Schottky electrodes for gallium nitride electron devices. SciAdv Mater 2014, 6(7):1645–1649.9. Kim SC, Lim B, Lee HS, Shin D-H, Kim SK, Park HC, Rhee JK: Sub-100 nmT-gate fabrication using a positive resist ZEP520/P(MMA-MAA)/PMMAtrilayer by double exposure at 50 kV e-beam lithography. Mater Sci SemiProc 2004, 7(1–2):7–11.10. Li LA, Kishi A, Shiraishi T, Jiang Y, Wang QP, Ao J-P: Electrical properties ofTiN on gallium nitride grown using different deposition conditions andannealing. J Vac Sci Technol A 2014, 32(2):02B116.11. Lin YC, Chang CH, Li FM, Hsu LH, Chang EY: Evaluation of TiN/Cugate metal scheme for AlGaN/GaN high-electron-mobility transistorapplication. Appl Phys Express 2013, 6:091003.12. Liu QZ, Yu LS, Lau SS, Deng F, Redwing JM: Ni and Ni silicide Schottkycontacts on n-GaN. J Appl Phys 1998, 84:881–886.13. Chen GL, Chang FC, Shen KC, Ou J, Chen WH, Lee MC, Chen WK, Jou MJ,Huang CN: Thermal stability study of Ni/Ta/n-GaN Schottky contacts.Appl Phys Lett 2002, 80:595–597.14. Westlinder J, Schram T, Pantisano L, Cartier E, Kerber A, Lujan GS, Olsson J,Groeseneken G: On the thermal stability of atomic layer deposited TiNas gate electrode in MOS devices. IEEE Electron Device Lett 2003,24(9):550–552.15. Saadaoui S, Salem M, Gassoumi M, Maaref H, Gaquière C: Electricalcharacterization of (Ni/Au)/Al0.25Ga0.75N/GaN/SiC Schottky barrier diode.J Appl Phys 2011, 110(1):013701.16. Li LA, Kishi A, Shiraishi T, Jiang Y, Wang QP, Ao J-P, Ohno Y: Evaluation of agate-first process for AlGaN/GaN heterostructure field-effect transistors.Jpn J Appl Phys 2013, 52(11):11NH01.doi:10.1186/1556-276X-9-590Cite this article as: Li et al.: Synthesis of titanium nitride for self-alignedgate AlGaN/GaN heterostructure field-effect transistors. Nanoscale ResearchLetters 2014 9:590.Submit your manuscript to a journal and benefi t from:7 Convenient online submission7 Rigorous peer review7 Immediate publication on acceptance7 Open access: articles freely available online7 High visibility within the fi eld7 Retaining the copyright to your article    Submit your next manuscript at 7 springeropen.com

Synthesis of titanium nitride for self-aligned gate AlGaN/GaN heterostructure field-effect transistors

https://repo.lib.tokushima-u.ac.jp/files/public/11/115610/20210122110057633135/nrl_9_590.pdf

Synthesis of titanium nitride for self-aligned gate AlGaN/GaN heterostructure field-effect transistors

Abstract

Similar works

Full text

Available Versions

Tokushima University Institutional Repository