007) 4369–4372www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Improved DC performance of AlGaN/GaN high electron mobilitytransistors using hafnium oxide for surface passivation

Chang Liu, Eng Fong Chor ⁎, Leng Seow Tan

Centre for Optoelectronics, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore

Available online 28 August 2006

Abstract

Improved DC performance of AlGaN/GaN high electron mobility transistors (HEMTs) have been demonstrated using reactive-sputteredhafnium oxide (HfO2) thin film as the surface passivation layer. Hall data indicate a significant increase in the product of sheet carrierconcentration (ns) and electron mobility (μn) in the HfO2-passivated HEMTs, compared to the unpassivated HEMTs. This improvement in electroncarrier characteristics gives rise to a 22% higher IDmax and an 18% higher gmmax in HEMTs with HfO2 passivation relative to the unpassivateddevices. On the other hand, Igleak of the HEMTs decreases by nearly one order of magnitude when HfO2 passivation is applied. In addition, draincurrent is measured in the subthreshold regime. Compared to the unpassivated HEMTs, HfO2-passivated HEMTs exhibit a much smaller off-stateID, indicating better turn-off characteristics.© 2006 Elsevier B.V. All rights reserved.

Keywords: Gallium nitride; HEMT; Hafnium oxide; Passivation

1. Introduction

GaN electronic devices such as high electron mobilitytransistors (HEMTs) have been intensively investigated forhigh-frequency, high-power military applications and wirelesscommunications, mainly due to their high-breakdown electricalfield and excellent high field electron transport properties. Inaddition, the presence of strong spontaneous and piezoelectricpolarization in the AlGaN/GaN heterostructures yields a highelectron sheet density on the order of 1013 cm−2 for the two-dimensional electron gas (2DEG) [1]. However, applications ofthese AlGaN/GaNHEMTs are largely limited by surface trappingeffects through drain current collapse [2]. RF power obtained ismuch lower than that expected from the device DC characteristics[3–6] owing to the electron trapping states at the active surfacearea, and this device degradation results in a significant decrease inthe output power, as well as the power-added efficiency (PAE) [7].

Surface passivation technique using various dielectrics hasbeen found to effectively relieve the current collapse issue byreducing the density of surface states nowadays. Besides the

⁎ Corresponding author. Tel./fax: +65 68745251.E-mail address: elecef@nus.edu.sg (E.F. Chor).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.07.103

conventional passivation dielectrics SiO2 and Si3N4 [8–13],passivation properties of new materials like SiON [14], MgO[15–17] and Sc2O3 [15–18] have also been studied. Recently,Park et al. has reported using Al2O3–HfO2 laminated dielectricsprepared by plasma-enhanced atomic-layer deposition (PEALD)for metal-oxide semiconductor (MOS)-HEMTapplications [19].In this paper, we demonstrate that HfO2 thin film formed byreactive sputtering can also effectively act as the surface passi-vation layer on AlGaN/GaN HEMTs to improve their DC per-formance. Widely explored for use in future generation Si-basedMOSFETs as the gate dielectric, HfO2 has many desirableproperties, such as a relatively large bandgap (5.65 eV) [20] anda high dielectric constant (∼20) [21–23]. In addition, sputteredHfO2 does not contain hydrogen and may have advantages overPECVD-prepared SiO2 or Si3N4, as atomic hydrogen incorpo-rated during the PECVD process could diffuse into GaN or gatemetal over extended periods of device operation and degrade theperformance and reliability of HEMTs [24].

2. Experimental procedures

The AlGaN/GaN heterostructures were grown by molecularbeam epitaxy (MBE) on 2-in.-diameter sapphire substrates. A400-nm AlN transition layer was followed by a 1500-nm GaN

Fig. 1. (a) Typical output characteristics of HfO2-passivated and unpassivatedAlGaN/GaN HEMTs with a gate length of 1.5 μm. The gate is biased from +1 to−4 V in steps of −1 V. Good ID–VDS pinch-off and saturation characteristics areobserved for both HEMTs investigated. (b) Extrinsic transconductance gm as afunction of gate voltage VGS of the HfO2-passivated and unpassivated AlGaN/GaN HEMTs. Drain bias is +7 V. A negative shift in the threshold voltage, Vth,of the HfO2-passivated HEMTs is observed, corresponding to the larger channelcarrier concentration.

Fig. 2. Two-terminal gate leakage current, Igleak, measured on Schottky diodeswith an active area of 100×50 μm2. AlGaN/GaN HEMTs with HfO2 passivationshow a decreased gate leakage current, indicating a higher breakdown voltagefor the drain-to-gate regions.

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buffer, a 5-nm undoped Al0.15Ga0.85N spacer, a 5-nm silicon-doped Al0.15Ga0.85N layer (n≈9×1018 cm− 3), a 15-nmundoped Al0.15Ga0.85N barrier and a 2-nm undoped GaN caplayer. Device isolation was achieved by mesa etching using Cl2/Ar inductively coupled plasma (ICP) system with an etchingrate of 100 nm/min for GaN. Ohmic contact was formed byevaporating Ti/Al/Pd/Au with a subsequent annealing at 750 °Cfor 30 s in N2 ambient. The measured specific contact resistanceρc was 3×10−7 Ω·cm2 using linear transmission-line method(LTLM) structures at room temperature. Schottky gate metal-lization, consisting of Ni/Au layers, was patterned by opticallithography using “turn-over” bake. The surfaces of HEMTswere then passivated by 30 nm HfO2 deposited by reactivesputtering at 200 °C, using elemental Hf target sputtered in O2

atmosphere, followed by the opening of contact windows in thepassivation layers using BCl3-based plasma. DC measurementswere done on both HfO2-passivated and unpassivated HEMTsusing a HP4156A Semiconductor Parameter Analyzer.

3. Results and discussions

3.1. Hall effect measurements

Room-temperature Hall measurements were performed onstandard van der Pauw patterns on both HfO2-passivated andunpassivated AlGaN/GaN HEMTs. It has been found that theunpassivated HEMTs yielded a sheet carrier concentrationns=1.01×10

13 cm−2, which was relatively low for the HEMTstructure with an intentional Si doping (n≈9×1018 cm−3). Asfor the HfO2-passivated HEMTs, ns was increased to1.92×1013 cm−2, almost twice that of the unpassivatedcounterpart. This indicates that the electron trapping by thesurface states at the GaN surface has been alleviatedsubstantially by HfO2 passivation [11]. On the other hand, theelectron carrier mobility, μn, decreased from 719 to 474 cm2/V·s after HfO2 passivation has been applied, probably due to anenhanced electron–electron Coulomb scattering in the 2-DEGchannel with a higher electron sheet concentration. On a whole,

Fig. 3. Subthreshold drain-current (Ith) measured on HfO2-passivated andunpassivated AlGaN/GaN HEMTs. The gate is biased at −6 V to fully turn offthe conducing channel. HfO2-passivated HEMTs show a lower Ith (by at least anorder of magnitude), indicating better turn-off characteristics as well as a higherdrain-to-source breakdown voltage.

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better electron carrier characteristics have been obtained interms of a 27% larger product of ns·μn in the conductingchannel for HEMTs with HfO2 passivation relative to theunpassivated HEMTs, indicating better device performance forHfO2-passivated HEMTs.

3.2. DC characteristics

Fig. 1(a) and (b) shows the typical DC output and transfercharacteristics of HfO2-passivated and unpassivated AlGaN/GaN HEMTs, respectively, with a gate length of 1.5 μm. ID–VDS characteristics with the gate biased from +1 V to −4 V insteps of −1 V are shown in Fig. 1(a). Both types of HEMTsexhibited good ID–VDS pinch-off and saturation characteristics,indicating a reasonable good interface between HfO2 dielectricand the GaN surface [25]. The maximum drain current density,IDmax, of the HfO2-passivated HEMTs is about 22% higherrelative to the unpassivated HEMTs, increasing from 586 to718 mA/mm. This improvement of device performancecorresponds well to a larger product of ns·μn in the 2-DEGchannel of the HfO2-passivated HEMT as indicated by Halldata. On the other hand, the self-heating problems were seen atlarger drain current due to the poor thermal conductivity of thesapphire substrates.

Similarly, as seen from Fig. 1(b), the peak extrinsic trans-conductance, gmmax, also increased from 97 mS/mm for theunpassivated HEMTs to 115 mS/mm of the HEMTs with HfO2

passivation. This clearly demonstrates that a better gate controlhas been resulted from passivation of the surface states. Inaddition, we observed a negative shift in the threshold voltage,Vth, of the HfO2-passivated HEMTs compared to the unpassi-vated devices. This corresponds to the higher carrier concentra-tion in the 2-DEG channel originated from surface passivation,as shown from Hall data. It should also be mentioned that thegate voltage corresponding to gmmax has changed slightly to amore negative value after HfO2 passivation.

Two-terminal gate leakage current, Igleak, of the unpassivatedand HfO2-passivated HEMTs was investigated on Schottkydiodes processed simultaneously with the HEMT structureswith an active area of 100×50 μm2. As seen from the I–Vcharacteristics in Fig. 2, HfO2-passivated HEMTs exhibit adecreased Igleak, by nearly one order of magnitude relative to theunpassivated HEMTs. More specifically, at Vgs=−10 V, thegate current density decreased from 0.22 to 0.029 mA/mm. Thisshows that HEMTs with HfO2 passivation layer have a highergate-to-drain breakdown voltage, compared to the unpassivateddevices. The surface passivation effects on the gate leakageobserved for HfO2 dielectric are believed to be related to thedensity of interface states between the passivation layer and theunderlying GaN surface [15].

To investigate the device pinch-off characteristics, as well asthe drain-to-source breakdown voltage of AlGaN/GaN HEMTs,subthreshold drain-current, Ith, has been measured on thedevices with and without passivation, as shown in Fig. 3. Thegate was biased at −6 V to fully turn off the conducting channel.A trend similar to that observed in Fig. 2 is seen in Fig. 3 withregard to the effect of HfO2 passivation. A clear decrease by

more than one order of magnitude in the off-state drain currentat a drain bias up to 15 V was observed in Fig. 3, indicating ahigher drain-to-source breakdown voltage. Although completebreakdown measurements were not done in the gate leakage andthe subthreshold measurements, we could still conclude thathigher breakdown voltages for both the drain-to-gate and drain-to-source regions are expected for HEMTs with HfO2

passivation.

4. Conclusion

The effects of surface passivation on DC performance ofAlGaN/GaN HEMTs have been investigated using the reactive-sputtered HfO2 dielectrics. Hall effect measurements have shownan 27% increase in the product of sheet carrier concentration andelectron mobility in HEMTs with HfO2 passivation relative to theunpassivated HEMTs. Accordingly, an increase in IDmax and gmmax

has been observed in the HfO2-pasivated HEMTs compared to theunpassivated HEMTs. As for the gate leakage, HfO2-passivateddevices exhibit a lower Igleak in comparison to unpassivatedHEMTs.A similar trend is observed for subthreshold drain current-HEMTs with HfO2 passivation yield a lower Ith, by more than anorder of magnitude. Thus, better turn-off characteristics and higherbreakdown voltages could be expected for the HfO2-passivatedHEMTs compared to the unpassivated HEMTs.

Acknowledgements

The authors acknowledge Associate Professor B. J. Cho andMr. O Yan Wai Linn for their assistance in growing the HfO2

dielectrics. The scholarship provided to Chang Liu to pursue hisPhD studies by the National University of Singapore (NUS) isalso gratefully acknowledged. The current work is supported byan NUS Research Grant WBS: R-263-000-278-112.

References

[1] W.S. Tan, P.A. Houston, P.J. Parbrook, G. Hill, R.J. Airey, J. Phys. D:Appl. Phys. 35 (2002) 595.

[2] S. Arulkumaran, T. Egawa, H. Ishikawa, T. Jimbo, Appl. Phys. Lett. 81(2002) 3073.

[3] K.K. Chu, J.A. Smart, J.R. Shealy, L.F. Eastman, Proceedings of State-of-The-Art Program on Compound Semiconductors, SOTAPOCS XX/X,Electrochem. Soc., Pennington, NJ, 1998, p. M.3.

[4] B.M. Green, K.K. Chu, E.M. Chumbes, J.A. Smart, J.R. Shealy, L.F.Eastman, IEEE Electron Device Lett. 21 (2000) 268.

[5] S.C. Binari, W. Kruppa, H.B. Dietrich, G. Kelner, A.E. Wickenden, J.A.Freitas Jr., Solid-State Electron. 41 (1997) 1549.

[6] L.F. Eastman, V. Tilak, J. Smart, B.M. Green, E.M. Chumbes, R. Dimitrov,H. Kim, O.S. Ambacher, N. Weimann, T. Prunty, M. Murphy, W.J. Schaff,J.R. Shealy, IEEE Trans. Electron Devices 48 (2001) 479.

[7] S.C. Binari, K. Ikossi, J.A. Roussos, W. Kruppa, D. Park, H.B. Dietrich,D.D. Koleske, A.E. Wickenden, R.L. Henry, IEEE Trans. ElectronDevices 48 (2001) 465.

[8] M.A. Khan, X. Hu, G. Simin, A. Lunev, J. Yang, R. Gaska, M.S. Shur,IEEE Trans. Electron Devices 21 (2000) 63.

[9] R. Vetury, N.Q. Zhang, S. Keller, U.K. Mishra, IEEE Trans. ElectronDevices 48 (2001) 560.

[10] A.V. Vertiatchikh, L.F. Eastman, IEEEElectronDevice Lett. 24 (2003) 535.[11] J. Bernat, P. Javorka, A. Fox, M. Marso, H. Luth, P. Kordos, Solid-State

Electron. 47 (2003) 2097.

4372 C. Liu et al. / Thin Solid Films 515 (2007) 4369–4372

[12] P. Javorka, J. Bernat, A. Fox, M. Marso, H. Luth, P. Kordos, Electron. Lett.39 (2003) 1155.

[13] W. Lu, V. Kumar, R. Schwindt, E. Piner, I. Adesida, Solid-State Electron.46 (2002) 1441.

[14] S. Arulkumaran, T. Egawa, H. Ishikawa, T. Jimbo, Y. Sano, Appl. Phys.Lett. 84 (2004) 613.

[15] B. Luo, R.M.Mehandru, Jihyun Kim, F. Ren, B.P. Gila, A.H. Onstine, C.R.Abernathy, S.J. Pearton, R.C. Fitch, J. Gillespie, R. Dellmer, T. Jenkins,J. Sewell, D. Via, A. Crespo, Solid-State Electron. 46 (2002) 2185.

[16] J.K. Gillespie, R.C. Fitch, J. Sewell, R. Dettmer, G.D. Via, A. Crespo, T.J.Jenkins, B. Luo, R.Mehandru, J. Kim, F. Ren, B.P. Gila, A.H. Onstine, C.R.Abernathy, S.J. Pearton, IEEE Electron Device Lett. 23 (2002) 505.

[17] B. Luo, J.W. Johnson, B.P. Gila, A. Onstine, C.R. Abernathy, F. Ren, S.J.Pearton, A.G. Baca, A.M. Dabiran, A.M. Wowchack, P.P. Chow, Solid-State Electron. 46 (2002) 467.

[18] B. Luo, R. Mehandru, Jihyun Kim, F. Ren, B.P. Gila, A.H. Onstine, C.R.Abernathy, S.J. Pearton, D. Gotthold, R. Birkhahn, B. Peres, R.C. Fitch, N.

Moser, J.K. Gillespie, G.H. Jessen, T.J. Jenkins, M.J. Yannuzi, G.D. Via,A. Crespo, Solid-State Electron. 47 (2003) 1781.

[19] K.Y. Park, H.I. Cho, H.C. Choi, Y.H. Bae, C.S. Lee, J.L. Lee, J.H. Lee,Jpn. J. Appl. Phys. 43 (2004) L 1433.

[20] J. Robertson, J. Vac. Sci. Technol., B 18 (2000) 1785.[21] C.T. Kuo, R. Kwor, K.M. Jones, Thin Solid Films 213 (1992) 257.[22] B.H. Lee, L. Kang, R. Nieh, W.J. Qi, J.C. Lee, Appl. Phys. Lett. 76 (2000)

1926.[23] L. Kang, B.H. Lee, W.J. Qi, Y. Jeon, R. Nieh, S. Gopalan, K. Onishiand,

J.C. Lee, IEEE Electron Device Lett. 4 (2000) 181.[24] B. Luo, R. Mehandru, J. Kim, F. Ren, B.P. Gila, A.H. Onstine, C.R.

Abernathy, S.J. Pearton, R. Fitch, J. Gillespie, T. Jenkins, J. Sewell, D. Via,A. Crespo, Y. Irokawa, J. Electrochem. Soc. 149 (2002) G613.

[25] M. Ochiai, M. Akita, Y. Ohno, S. Kishimoto, K. Maezawa, T. Mizutani,Jpn. J. Appl. Phys. 42 (2003) 2278.