Chabak, Kelson

Gordon, Roy Gerald

Lou, Xiabing

Ye, Peide

Zhou, Hongchao

Harvard University - DASH 

AlGaN/GaN MOSHEMT on Si Substratewith High on/off Ratio and High Off-stateBreakdown Voltage Enabled by AtomicLayer Epitaxial MgCaO as Gate DielectricThe Harvard community has made thisarticle openly available.  Please share  howthis access benefits you. Your story mattersCitation Zhou, Hong; Lou, Xiabing; Chabak, Kelson D.; Gordon, R.G.; Ye,Peide D. 2015. AlGaN/GaN MOSHEMT on Si Substrate with High on/off Ratio and High Off-state Breakdown Voltage Enabled by AtomicLayer Epitaxial MgCaO as Gate Dielectric. 46th IEEE SemiconductorInterface Specialists Conference, Arlington, VA, December 2-5, 2015.Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:29003619Terms of Use This article was downloaded from Harvard University’s DASHrepository, and is made available under the terms and conditionsapplicable to Open Access Policy Articles, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#OAPHong Zhou1, Xiabing Lou2, Kelson D. Chabak3, R. G. Gordon2, and Peide D. Ye1*1) School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47906, U.S.A2) Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, U.S.A.3) Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, OH 45433, U.S.A.*Tel: 1-765-494-7611, Fax: 1-765-496-7443, Email: yep@purdue.eduAlGaN/GaN high-electron-mobility-transistors (HEMTs) on Si substrates have attracted more and more attention in the area of high voltage power switches due to their lower-cost substrates, large substrate diametersand their ability to integrate with silicon processes [1-3]. Conventional Schottky gate HEMTs suffer from relatively high gate leakage currents which limit maximum forward gate bias swing and off-state performance. Metal-oxide-semiconductor HEMTs (MOSHEMTs) are proposed with a thin oxide layer in between gate and barrier to solve the aforementioned problems [4]. A good oxide must have a sufficiently large barrier height and high interface quality. In this work, we incorporate epitaxial Mg0.25Ca0.75O gate dielectric deposited by atomic layer deposition (ALD) into the GaN MOSHEMT process yielding improved device performance.Fig. 1 shows schematic view of an AlGaN/GaN MOSHEMT on a Si (111) substrate with sheet resistance (Rsh) ~ 450 ȍƑ. Device fabrication started with mesa isolation by Cl2/BCl3 etching to a depth of 150 nm. Then,Ohmic contacts were formed by depositing Ti/Al/Ni/Au (20/100/40/50 nm) followed by 775 °C rapid thermal anneal in N2 atmosphere, yielding a contact resistance (Rc) of 0.35 ȍāPP. A 4 nm epitaxial Mg0.25Ca0.75Odielectric capped with 2 nm of amorphous Al2O3 was then deposited by ALD. The growth temperature of MgCaO was 310 °C, using bis(N,N’-di-tert-butylacetamidinato)calcium, bis(N,N’-di-sec-butylacetamidinato) magnesium, and water vapor as precursors [5]. Single crystalline MgCaO offers an advantageous band offset, a good interface, andgood lattice matching to GaN alloys [6]. Finally, Ni/Au (30/50 nm) was deposited as the gate metal followed by a lift-off process. All of the lithography processes were carried out using a MJB3 mask aligner lithography system.'HYLFHVKDYHDJDWHZLGWK:RIȝPDQGJDWHOHQJWK/gRIDQGȝPFig. 3 shows the well-behaved DC output Ids-Vds characteristics of a GaN MOSHEMT. The device has anLg ȝPDQGVRXUFHWRGUDLQVSDFLQJ (Lsd) of 4.2 ȝm. Due to a 6 nm thick gate oxide, a high gate bias (Vgs) of 3 V can be applied, yielding a maximum drain current (Ids,max) of 700 mA/mm. Fig. 4 is the Ids-Vgs transfer characteristic measurement of the same device. Impressively, a high on/off ratio of 1010 is achieved with subthreshold swing (SS) of 65 mV/dec at Vds=5 V. Traditional HEMT devices are not able to have such a high on/off ratio because of their large gate leakage currents in the off-state. The oxide of the MOSHEMT suppresses this leakage, yielding large on/off ratios. In addition, benefiting from the lattice matching and good interface between MgCaO and GaN,[5] the GaN MOSHEMT also demonstrates a negligible hysteresis (50 mV) as shown in Fig. 5. Fig. 6 shows the Ids-Vgs and gm-Vgs plot at the linear region. Peak transconductance (gm,max) of 160 mS/mm and threshold voltage (VT) of -2.2 V are observed at Vds=5 V. The off-state breakdown/leakage characteristics of a MOSHEMT are shown in Fig.7. This device has a W/Lg ȝPȝPDQG/gs=Lgd ȝP.The device is operated at the pinch-off region with Vgs=-3.5 V and Vs=0 V.  It can be observed that the breakdown voltage is 150 V even with a short Lgd ȝP The breakdown voltage, which is a critical figure of merit for power switch, is expected to increase with the increase of Lgd and drain-gate region engineering. Scaling metrics of GaN MOSHEMTs are also studied as shown in Fig. 8 and Fig. 9. The Ids and gmax are found to increase when the Lg is scaled. SS and drain induced barrier lowering (DIBL) are found to be slightly influenced by the Lg,and VT shows roll-off behavior when Lg ȝPIn conclusion, we have demonstrated high performance AlGaN/GaN MOSHEMTs on Si substrate with high on/off ratio and high off-state breakdown voltage with epitaxial MgCaO gate dielectric. The lattice-matched MgCaO provides high quality interface and an appropriate electron barrier height, which makes it feasible to be applied to future GaN power switch applications.The work at Purdue is supported by AFOSR and the work at Harvard is supported by the ONR.AlGaN/GaN MOSHEMT on Si Substrate with High on/off Ratio and High Off-state Breakdown Voltage Enabled by Atomic Layer Epitaxial MgCaO as Gate DielectricReferences: [1] B. De Jaeger et al., Proc. ISPSD, pp. 49-52, 2012. [2] P. Moens et al., Proc. ISPSD, pp. 374-377, 2014. [3]N. Ikeda et al., Proc. of the IEEE, Vol. 98, No. 7, pp. 1151-1161, 2010. [4] P. D. Ye et al., Appl. Phys. Lett., vol. 86, pp. 063561, 2005. [5] X. B. Lou et al., CSW, 2015. [6] H. Zhou et al., DRC, 2015.0 2 4 6 8 100.00.30.60.91.21.51.8 DCId (A/mm) Vd (V)gate pulsed Id-Vd18.3% reduction(b)0 2 4 6 8 100.00.30.60.91.21.51.8 DCI (A/mm) V  (V)gate pulsed Id-Vd18.3% reduction(b)0 2 4 6 8 100.00.30.60.91.21.51.8 DCId (A/mm) Vd (V)gate pulsed Id-Vd18.3% reduction(b)Fig. 1 Schematic view of an AlGaN/GaN MOSHEMTFig. 2 Device fabrication process steps of AlGaN/GaN MOSHEMTs-4 -3 -2 -1 0 1 2 310-1110-910-710-510-310-1101I DS(A/mm)VGS (V)VDS=5 VHysteresisFig. 3 Output characteristics of an AlGaN/GaN MOSHEMT with Lg=1 ȝm and LSD=4.2 ȝm.Fig. 6 Ids-Vgs and gm-Vgs of the same device in the linear region plot.-4 -2 0010203040Capacitance (pF) V  (V) HEMT MISHEMT n(10cm -2)0246810-6 -4 -2 0 20.00.40.81.21.6 gm  (mS/mm)I d (A/mm) Vg (V)(b) MISHEMT0100200300400Vth=-3.1 V-4 -2 0 2 410-1110-910-710-510-310-1101VDS=5 VVDS=1 VI DS (A/mm)VGS (V)SS=65 mV/decDIBL=5 mV/VOn/off ratio~10100 10 20 30 400.00.20.40.60.8Gmax  (S/mm)IonGmaxLg (Pm)I on (A/mm)0.00.10.20.30 40 80 120 160 20010-1210-1010-810-610-410-2I OFF (A/mm)ID IGVDS (V)LG=1 PmLGD=1.6 PmVGS=-3.5 V0 10 20 30 40020406080VTVT  (V)SSDIBLLg (Pm)SS(mV/dec)and DIBL (mV/V)-2.7-2.4-2.1-1.8-1.5-1.2-4 -2 0 2 40.00.20.40.60.8VDS =5 VVDS =1 VVGS (V)0.00.10.20.3I DS (A/mm) GM  (S/mm)Fig. 4 Ids-Vgs transfer characteristics with on/off ratio of 1010 and low SS=65 mV/dec.Fig. 5 Ids-Vgs hysteresis measurement at Vds=5 V.Fig. 7 Three-terminal off-state breakdown measurement with Lg=1 ȝm and Lgd=1.6 ȝm.Fig. 8 Ion and Gmax scaling metrics of GaN MOSHEMTs.Fig. 9 SS, DIBL and VT scaling metrics of GaN MOSHEMTs.0.0 1.5 3.0 4.5 6.0 7.5 9.00.00.20.40.60.8I DS (A/mm)VGS:3 V to -4 VStep: -0.5 VVDS (V)

AlGaN/GaN MOSHEMT on Si Substrate with High on/off Ratio and High Off-state Breakdown Voltage Enabled by Atomic Layer Epitaxial MgCaO as Gate Dielectric

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AlGaN/GaN MOSHEMT on Si Substrate with High on/off Ratio and High Off-state Breakdown Voltage Enabled by Atomic Layer Epitaxial MgCaO as Gate Dielectric

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