We report an In0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistor with a peak Hall mobility of 8300 cm(2)/Vs at a carrier density of 2 x 10(12) cm(-2). Comparison of split capacitance-voltage (CV) and Hall Effect measurements for the extracted electron mobility have shown that the split-CV can lead to an overestimation of the channel carrier concentration and a corresponding underestimation of electron mobility. An analysis of the electron density dependence versus gate voltage allows quantifying the inaccuracy of the split-CV technique. Finally, the analysis supported by multi-channel conduction simulations indicates presence of carriers spill over into the top InP barrier layer at high gate voltages. (C) 2011 American Institute of Physics. (doi: 10.1063/1.3665033

Bersuker, G.

Ghibaudo, G.

Goel, N.

Huang, J.

Hurley, Paul K.

Kirsch, P.

Negara, Muhammad A.

Veksler, D.

Applied Physics Letters

Irish Universities

Analysis of effective mobility and hall effect mobility in high-k based In0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistors

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Title Analysis of effective mobility and hall effect mobility in high-k basedIn0.75Ga0.25As metal-oxide-semiconductor high-electron-mobilitytransistorsAuthor(s) Negara, Muhammad A.; Veksler, D.; Huang, J.; Ghibaudo, G.; Hurley,Paul K.; Bersuker, G.; Goel, N.; Kirsch, P.Publication date 2011Original citation Negara, M. A., Veksler, D., Huang, J., Ghibaudo, G., Hurley, P. K.,Bersuker, G., Goel, N. and Kirsch, P. (2011) 'Analysis of effectivemobility and hall effect mobility in high-k based In0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistors', AppliedPhysics Letters, 99(23), pp. 232101. doi: 10.1063/1.3665033Type of publication Article (peer-reviewed)Link to publisher'sversionhttp://aip.scitation.org/doi/abs/10.1063/1.3665033http://dx.doi.org/10.1063/1.3665033Access to the full text of the published version may require asubscription.Rights © 2011 American Institute of Physics.This article may bedownloaded for personal use only. Any other use requires priorpermission of the author and AIP Publishing. The following articleappeared in Negara, M. A., Veksler, D., Huang, J., Ghibaudo, G.,Hurley, P. K., Bersuker, G., Goel, N. and Kirsch, P. (2011) 'Analysisof effective mobility and hall effect mobility in high-k basedIn0.75Ga0.25As metal-oxide-semiconductor high-electron-mobilitytransistors', Applied Physics Letters, 99(23), pp. 232101 and may befound at http://aip.scitation.org/doi/abs/10.1063/1.3665033Item downloadedfromhttp://hdl.handle.net/10468/4309Downloaded on 2018-08-23T18:46:35ZAnalysis of effective mobility and hall effect mobility in high-k based In0.75Ga0.25Asmetal-oxide-semiconductor high-electron-mobility transistorsM. A. Negara, , D. Veksler, J. Huang, G. Ghibaudo, P. K. Hurley, G. Bersuker, N. Goel, and P. KirschCitation: Appl. Phys. Lett. 99, 232101 (2011); doi: 10.1063/1.3665033View online: http://dx.doi.org/10.1063/1.3665033View Table of Contents: http://aip.scitation.org/toc/apl/99/23Published by the American Institute of PhysicsAnalysis of effective mobility and hall effect mobility in high-k basedIn0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistorsM. A. Negara,1,2,3,a) D. Veksler,1 J. Huang,1 G. Ghibaudo,2 P. K. Hurley,3 G. Bersuker,1N. Goel,1 and P. Kirsch11SEMATECH, 257 Fuller Road, Albany, New York 12203 and 2706 Montopolis Drive, Austin,Texas 78741, USA2IMEP, ENSERG, BP 257, 38016 Grenoble, France3Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland(Received 13 July 2011; accepted 9 November 2011; published online 5 December 2011)We report an In0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistor with apeak Hall mobility of 8300 cm2/Vs at a carrier density of 2 1012 cm2. Comparison of splitcapacitance-voltage (CV) and Hall Effect measurements for the extracted electron mobility haveshown that the split-CV can lead to an overestimation of the channel carrier concentration and acorresponding underestimation of electron mobility. An analysis of the electron density dependenceversus gate voltage allows quantifying the inaccuracy of the split-CV technique. Finally, theanalysis supported by multi-channel conduction simulations indicates presence of carriers spillover into the top InP barrier layer at high gate voltages. VC 2011 American Institute of Physics.[doi:10.1063/1.3665033]In recent years, significant research efforts have focusedon exploring the use of high mobility materials such as III-V, Ge, and graphene as replacements for Si channels infuture complementary metal-oxide-semiconductor (CMOS)technology nodes. In the case of III-V channel materialsthere has been a growing interest in quantum well field-effect transistors (QW-FETs) or metal-oxide-semiconductorhigh-electron-mobility transistors (MOSHEMTs) due to thehigh intrinsic electron mobility of III-V materials.1–5 The useof In0.7Ga0.3As MOSFETs with an Al2O3 gate oxide andInP/In0.52Al0.48As double-barrier layer structures have beenreported with a peak channel mobility of 4729 cm2/Vs at adensity of 1.5 1012 cm2 and capacitance equivalent thick-ness (CET) 3 nm.1 High-k InGaAs MOSFETs employing aflat band architecture, with a GaAs/AlGaAs barrier layer andbottom Si-d doping, exhibited a peak mobility reaching 5500cm2/Vs, at density of 23 1012 cm2 for a CET around5 nm.2 The electron mobilities reported were extracted usingthe split capacitance-voltage (CV) technique,6 whichassumes that the integral of the gate-to-channel capacitance(Cgc) yields the charge density contributing to conduction.However, for device structures that exhibit high interfacestate densities (Dit), the interface states can affect the calcu-lated mobility,7 which must be taken into account whenusing the split CV method. When the energies of interfacedefects in the high-k/InGaAs system align with the InGaAsconduction band, the evaluation of true effective mobility(leff) from the split CV is further complicated.8,9 As the Hallvoltage is developed by mobile charges only, the Hall Effecttechnique can yield values for the conduction charge densityand the carrier mobility that are not impacted by the interfacestates/defects. This study applied both techniques to extractand compare the carrier density and mobility values deter-mined using the split CV method and the gated Hall barapproach for InGaAs MOSHEMT devices.Lattice-matched buffer layers of InAlAs were grown ona semi-insulating InP substrate using molecular beam epi-taxy. The channel was 10 nm In0.75Ga0.25As. Carriers in thechannel were separated from the dopants to reduce Coulombscattering due to ionized impurities. This was achieved byplacing a 3 1012 cm2 silicon delta-doped layer into thelower InAlAs barrier and by introducing an undoped spacerlayer between the delta-doped layer and the channel. Thechannel was then buried using a 2 nm thin undoped largeband gap InP barrier layer and thick nþ InGaAs cappinglayer. After growth, an active region was patterned, the nþcapping layer was etched out in the active region, and 1 nmAl2O3/5 nm ZrO2/TiN films were deposited by atomic layerdeposition (ALD). The use of Al2O3/high-k bilayers hasbeen shown to reduce gate leakage and interface defect den-sity in In0.53Ga0.47As MOS systems.10 A high resolutioncross-sectional transmission electron microscopy (HRTEM)image through the gate stack region of the high-kIn0.75Ga0.25As MOSHEMT is shown in the inset to Fig. 1.The nþ InGaAs cap remained in the source-drain junctions,and a metal was used for making ohmic contacts. After de-vice lithography, Hall bar-type MOSHEMTs with a gatelength (L)¼ 170 lm and width (W)¼ 20 lm were used forelectrical measurements. A long channel device was chosento minimize the effect of series resistance.Fig. 1 shows the temperature dependence of the MOSH-EMT transfer characteristics at a drain voltage(Vds)¼ 50mV over a range of temperatures (77K to 300K).The gate leakage current was found to be less than 3 109A at a gate voltage (Vg)¼ 1V, which is negligible comparedto the drain current (Id). The drain current for Vg> 0 isreduced at higher temperatures, and the influence of temper-ature on the drain current tends to diminish as the gate volt-age increases. This effect could be associated with thecarriers spilling over into the InP barrier layer, which ismore effective at elevated temperatures, and preventing thechannel carrier concentration from increasing at higher gatevoltages. Such population of the barrier layer may result ina)Author to whom correspondence should be addressed. Electronic mail:adi.negara@tyndall.ie.0003-6951/2011/99(23)/232101/3/$30.00 VC 2011 American Institute of Physics99, 232101-1APPLIED PHYSICS LETTERS 99, 232101 (2011)the formation of a parallel channel, and is considered later inthis letter.The off-current (Ioff) is significantly reduced at lowertemperatures, whereas Igate is much less than Ioff at all temper-atures. At Vg¼1V, the activation energy of 0.14 eV wasextracted, similar to that reported for trap assisted tunneling(TAT) in an InP film,11 suggesting a defect-related conductionthrough the InP barrier layer may be the source of the high Ioffcurrent. From the high resolution TEM inset in Figure 1, nointerfacial layer between the Al2O3 and III-V substrate orbetween TiN and ZrO2 was observed within the resolutionlimits. However, Dit in the lower band gap region is on theorder of 1013 cm2eV1 (not shown), which degrades the sub-threshold slope (SS) and could also contribute to the Ioff.To delineate contributions from gate stack defects and/or carrier spill over into the barrier layer to electron mobility,the leff is compared to the Hall mobility (lHall) extractedusing the Hall Effect measurement at room temperature.12,13For the Hall Effect measurements, a magnetic field (B) up to0.58 T was applied. A linear relationship between VHall andB for various Vg was observed, and the Hall carrier density(nHall) values were extracted. The lHall values deduced fromthe conductance measurements along the channel using theextracted nHall are presented in Fig. 2.In agreement with previous reports in Refs. 1, 4, and 5,high-k/In0.75Ga0.25As devices with an InP barrier layer ex-hibit a high peak channel mobility (leff 5300 cm2/Vs andlHall 8300 cm2/Vs at room temperature), mainly due to:(1) charge transport occurring primarily at the epitaxial InP/In0.75Ga0.25As interface as opposed to an high-k/InxGa1-xAsinterface, (2) better interface passivation using a compositehigh-k/Al2O3 bilayer structure10,14 and the use of an undopedInP barrier, and (3) separation of the channel carriers fromthe scattering centers in the high-k dielectric and/or along thehigh-k/III-V interface due to the barrier layer (InP) with alarge band gap.The maximum difference between lHall and leff (asmeasured by the split CV method) is typically less than10%-20% for Si MOSFETs,12 while the measurements onour devices exhibited a much greater difference (56%).This could be related to a significant capacitance contribu-tion to the measured Cgc from immobile charges when usingthe split CV technique and/or to the presence of parallel con-duction channels in the In0.75Ga0.25As channel and the InPcapping layer. Considering first the case of mobile andimmobile charges, the carrier density extracted from the splitCV measurements consists of the total charges: in the chan-nel, barrier layers, and charge trapping sites at the high-k/InPinterface. The split CV method, when employed at roomtemperature, can lead to an overestimation of the channelcarrier density, which, in turn, leads to an underestimation ofthe extracted electron mobility values.Considering next the case of parallel transport, at highergate voltages carriers can spill into the low mobility InP bar-rier layer, the parallel conduction channels should then betaken into account on the mobility extraction. The differencein the measured lHall and leff mobilities, described by theEqs. (1) and (2), respectively, can be understood whenmulti-channel transport is assumed.15 With two paralleltransport channels (m¼ 2), the higher mobility layer (anintended channel¼ l1) dominates the Hall mobility due tothe l2 dependency in the numerator of Eq. (1). The unin-tended parallel channel (a conducting layer with relativelylower mobility¼ l2) provides only a small contribution tothe total lHall compared to leff.lHall¼Pmi¼1liriPmi¼1ri; form¼2 lHall¼n1l21þn2l22n1l1þn2l2; (1)leff ¼Pmi¼1liniPmi¼1ni; for m¼ 2 leff ¼n1l1þn2l2n1þn2 ; (2)nHall ¼ IdLWqlHallVds; (3)where m¼ the number of parallel transport channels, n¼ thenumber of carriers, and r¼ qnl represents the conductivityFIG. 1. (Color online) Typical transfer characteristics of high-k/In0.75Ga0.25As MOSHEMTs for L¼ 170lm and W¼ 20lm for varioustemperatures and Vds¼ 50mV with a TEM image of the gate stack regionshown in the inset.FIG. 2. (Color online) Comparison of lHall and leff (f¼ 1MHz) versus thecorresponding carrier density (n, nHall) at room temperature for a high-k/In0.75Ga0.25As MOSHEMT with 2 nm InP top barrier layer, L¼ 170lm,W¼ 20lm, Vg¼1V to 1V range, and Vds¼ 50mV.232101-2 Negara et al. Appl. Phys. Lett. 99, 232101 (2011)inside each device layer, Id¼ drain current, L¼ devicelength, W¼ device width, and Vds¼ drain source voltage.The carrier density extracted by these two methods isshown in Fig. 3(a). For the same Vg, the maximum channelcarrier density obtained from the Hall Effect measurement is3.4 1012 cm2 compared to 8 1012 cm2 obtained usingthe split CV technique. Earlier, we hypothesized that athigher gate voltages, the channel confinement is lost and thatcarriers can spill into the low mobility InP barrier layer overthe channel region. This suggestion finds support in the nand nHall dependencies on the gate voltage, Fig. 3(a). A pla-teau in the nHall-Vg plot observed at higher gate voltagesmight be caused by screening of the channel when chargesspill over into the barrier layer. These charges may form aparallel channel for carrier transport.When there are trapped charges (m¼ 3), an additionalterm of n3 is added to Eqs. (1) and (2) with a mobility l3¼ 0.From inspection of Eqs. (1) and (2), this has no impact on lhall,due to the product of n3 and u3. However, it will reduce leff asthere is an additional term in the denominator of Eq. (2). Theinfluence of the trapped charges should be detected experimen-tally as a shift in the electron density versus vs Vg characteris-tics obtained from the split CV approach when compared to theelectron density obtained from the Hall mobility and Eq. (3). Ashift (0.2V) is observed experimentally in Fig. 3(a) corre-sponding to a charge density of 1.23 1012 cm2. This alone isnot sufficient to explain the difference in the electron densityfrom split CV (3.4 1012 cm2) and from the Hall measure-ments (8.0 1012 cm2).Eqs. (1)–(3) and Fig. 3(b) illustrate the nHall vs. Vg char-acteristics solved using classical Poisson equation with aconstant InP/In0.75Ga0.25As barrier offset¼ 0.2 eV for threecases of l2¼ l1¼ 7000 cm2/Vs (comparable to single chan-nel), l2¼ 1/4 l1 and l2¼ 1/10 l1 in bilayer semiconduc-tors.15 The nHall characteristics in Fig. 3(a) compared to thesimulations in Fig. 3(b) suggest carrier spill over into the InPbarrier does occurs in the devices for Vg> 0 and that thetotal measured nHall is dominated by the high mobility chan-nel with l1 l2.Comparison of electron density and mobility obtainedfrom the gated Hall effect measurements versus split-CV indi-cates that the split CV method results in significant (by a fac-tor of x2) overestimation of the channel carrier concentrationand a corresponding underestimation of the electron mobility.Gated channel devices designed for Hall Effect measurementsallow for more confident extraction of the electron densityand mobility. It was also confirmed that the real space transferof charge carriers from the channel into the top barrier takesplace at Vg> 0V (Figs. 3(a) and 3(b)). However, even withparallel conduction channels, the average Hall effect mobilityis still dominated by the high mobility channel. The spill overmay be suppressed by choosing the barrier layers with thehigher band offset. These findings should be considered in thedesign of the III-V quantum well structure and the gate stack.This work is supported by the Irish Research Councilfor Science, Engineering and Technology (IRCSET)-MarieCurie International Mobility Fellowship in Science, Engi-neering and Technology, and Science Foundation Irelandunder Grant No. 08/US/I1546.1H. Zhao, Y. T. Chen, J. H. Yum, Y. Wang, F. Zhou, F. Xue, and J. C. Lee,Appl. Phys. Lett. 96, 102101 (2010).2M. Passlack, P. Zurcher, K. Rajagopalan, R. Droopad, J. Abrokwah, M.Tutt, Y. B. Park, E. Johnson, O. Hartin, A. Zlotnicka et al., Tech. Dig. -Int. 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Nolan, E.Oconnor, K. Cherkaoui, S. B. Newcomb, F. Crupi, and P. K. Hurley, Appl.Phys. Lett. 97, 052904 (2010).11A. Tosi, A. D. Mora, F. Zappa, and S. Cova, J. Mod. Opt. 56, 299 (2009).12D. K. Schroder, Semiconductor Material and Device Characterization(Wiley, New York, 2006).13D. Shahrjerdi, J. Nah, B. Hekmatshoar, T. Akyol, M. Ramon, E. Tutuc,and S. K. Banerjee, Appl. Phys. Lett. 97, 213506 (2010).14H. Zhao, Y. Chen, J. Yum, Y. Wang, and J. C. Lee, in Proceedings of the67th IEEE Device Research Conference (IEEE, New York, 2009), p. 89.15G. Ghibaudo and G. Kamarinos, Rev. Phys. Appl. 17, 133 (1982).FIG. 3. (a) (Color online) Comparison of Hall channel carrier density (nHall)and split CV carrier density (n) at room temperature in a high-k/In0.75Ga0.25AsMOSHEMT with L¼ 170lm and W¼ 20lm and Vds¼ 50mV (b) SimulatednHall with a constant InP/In0.75Ga0.25As barrier offset¼ 0.2 eV for three casesof l2¼l1¼ 7000 cm2/Vs (comparable to single channel), l2¼ 1/4l1 andl2¼ 1/10l1 in bilayer semiconductors (see Ref. 15).232101-3 Negara et al. Appl. Phys. Lett. 99, 232101 (2011)

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Analysis of effective mobility and hall effect mobility in high-k based In0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistors

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