Journal of Crystal Growth 237–239 (2002) 1176–1179

On the capacitance–voltage characteristics of Al/BaTiO3/GaNMFS structures

M. Senthil Kumar*, R.R. Sumathi, N.V. Giridharan, R. Jayavel, J. Kumar

Crystal Growth Centre, Anna University, Chennai 600 025, India

Abstract

Ametal–insulator–semiconductor device structure has been successfully grown on GaN by using ferroelectric BaTiO3

(BTO) as an insulating layer. The GaN and BTO films of the GaN metal–ferroelectric–semiconductor (MFS) structures

have been characterised using high-resolution X-ray diffraction, wide-angle X-ray diffraction and energy dispersive

X-ray analysis. The electrical properties of fabricated Al/BaTiO3/GaNMFS structures have been characterised by high-

frequency capacitance–voltage (C–V) measurement. C–V characteristics are remarkably improved for GaNMFS with a

ferroelectric insulator when compared to other traditional oxide insulators. The fabricated GaN MFS structures

approach inversion just for the applied bias of 5V, which is mainly due to the high dielectric constant and large

polarisation field of the gate ferroelectric layer. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 81.05.Ea; 77.84.Dy; 82.80.Pv; 85.30.Tv

Keywords: A1. X-ray diffraction; B1. Gallium compounds

1. Introduction

Gallium nitride (GaN) has been the material foractive research from the last decade due to therecent establishment of high efficiency blue/violetlight emitting diodes (LEDs) and blue laser diodes(LDs) [1,2]. Also, GaN-based electronic devicescould be operated in a high temperature environ-ment with high power conditions because of wideband-gap nature as well as high thermal conduc-tivity [3–6]. The very strong chemical bonds ofGaN compounds make them very stable, whichdiminish the degradation problems of the fabri-cated devices. p-Type doping of GaN has now

practically matured and the GaN-based LED andLD device structures are becoming well estab-lished. However, GaN technology is yet to bedeveloped for the fabrication of practical MES-FETs and MOSFETs. The traditional GaN MISstructures are fabricated with conventional oxideslike SiO2 [7,8], Si3N4 [9] and Ga2O3 (Gd2O3) [10] asinsulators. GaN metal–insulator–semiconductor(MIS) devices with these conventional oxides givegood inversion characteristics only under the high-applied gate voltage, which is incompatible withmany other electronic devices. In the present work,BaTiO3 ferroelectric oxide has been applied as gatematerial to overcome this problem because of largepolarisation and high dielectric constant of ferro-electric gate material.

The concept of metal–ferroelectric–semiconduc-tor (MFS) was first proposed by Wu in the 70s

*Corresponding author. Tel.:/fax: +91-44-235-27-74.

E-mail address: m.senthilkumar@mailcity.com

(M. Senthil Kumar).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 1 5 2 - 2

[11]. Development of MFS structures on GaN isthe important field of current research in order torealise practical MFSFETs. We have fabricatedGaN MFS using ferroelectric BaTiO3 instead ofother conventional oxides for the first time andthe capacitance–voltage (C–V) characteristics ofAl/BaTiO3/GaN MFS structure is discussed.

2. Experimental procedure

MOCVD grown 2 mm thick unintentionallydoped n-type GaN epitaxial layers on sapphiresubstrate (0 0 0 1) were taken for the experiment.The mobility and carrier density of GaN layersare 600 cm2/V s and 1–2� 1017 cm�3, respectively.After degreasing the GaN layer surface with TCE,acetone and methanol, the samples were etched inNH4OH:H2O solution followed by rinsing inMillipore water. BaTiO3 (BTO) thin film wasdeposited on the cleaned GaN surface at roomtemperature using the sol–gel process for athickness of E100 nm. The precursors used forBTO deposition are barium acetate (99.9%,Sigma-Aldrich, UK) and titanium (IV) butoxide(99%, Johnson Mathey Co., UK). Glacial aceticacid and 2-methoxy ethanol were used as thesolvents. Initially, barium acetate was dissolved inglacial acetic acid at room temperature and it hadto be refluxed in a reflex condenser at a tempera-ture of about 353K for 4 h to obtain a clearsolution. After getting a clear solution, titanium(IV) butoxide was slowly added in the proper moleratio into it and the solution was stirred to get thehomogeneous mixing of the precursor. The visc-osity of the solution was varied by the addition of2-methoxy ethanol for a desired state. The BaTiO3

films were coated on GaN surface using spin-coating method with rotation rate of 2300 rpm.The coated films were annealed at 673K for 30minto remove the remaining solvent elements presentin the films.

Gate electrodes were fabricated by depositingaluminium (2000 (A) through a metal mask of holesof area 7.85� 10�3 cm2 on BaTiO3 films usingthermal evaporation with a background pressureof 2� 10�6 Torr. Bottom electrodes for the GaNlayer were made with aluminium (Al) of thickness

1500 (A followed by 6001C annealing in nitrogenambient.

The GaN layer and BTO films in the MFSstructures were characterised through high resolu-tion X-ray diffraction (HRXRD) and wide angleX-ray diffraction (XRD), respectively. The elec-trical properties of Al/BTO/GaN MFS structurewere analysed using capacitance–voltage (C–V)characterisation.

3. Results and discussion

3.1. XRD analysis of GaN layer and BaTiO3 film

XRD pattern of BTO films deposited directly onGaN layer have been recorded in the 2y scanningmode using Cu Ka radiation as shown in Fig. 1.The results reveal that the BTO films are crystal-lised along the /1 1 0S, /2 0 1S and /2 1 3Sorientations, among which /1 1 0S peak is stron-gest. The observed XRD data is well matched withthe ASTM data for BTO.

Typical HRXRD peak of GaN (0 0 0 2) grownon sapphire is shown in Fig. 2. The full-width athalf-maximum (FWHM) of the curve is measuredas 272 s, which indicates the high quality of theGaN layer.

3.2. EDX analysis of BTO films

Composition analysis of BTO films was per-formed by using EDX measurements. It has been

Fig. 1. A typical HRXRD spectrum of GaN grown on

sapphire.

M. Senthil Kumar et al. / Journal of Crystal Growth 237–239 (2002) 1176–1179 1177

observed that the coated BTO films consist of Ba,Ti and O with composition 1:1:3, which corre-sponds well with the proportion of the coatingsolution taken for the experiment. This resultdeclares the perfect composition control of sol–geltechnique for the deposition of BTO films.

3.3. High-frequency C–V characteristics of Al/

BaTiO3/GaN MFS structures

In MIS C–V characteristics, the applied biasvoltage is distributed between gate insulator anddepletion region. The MIS capacitance is nothingbut the insulator capacitance and depletioncapacitance in series. Since the depletion capaci-tance is constant for a specific semiconductor, theonly parameter, which could be altered to mini-mise the operating voltage of a MIS device, is theinsulator capacitance and it has to be increased.When ferroelectric materials replace the oxideinsulators (i.e. MFS), the insulator capacitance ishighly increased due to the high dielectric constantof ferroelectrics. Moreover, when negative bias isapplied on the MFS structure, the ferroelectric ispoled negative and a large polarization field on theorder of 106V/cm much larger than externalapplied electric field (on the order of 10�4–10�5 V/cm) is produced at ferroelectric/GaN inter-

face. Consequently, it depletes carriers in the GaNlayer under a small external bias and thendecreases the value of MOS capacitance morequickly [12].

High-frequency (1MHz) capacitance–voltage(C–V) measurements have been carried out onthe fabricated GaN MFS structures using BOON-TON 7200 capacitance meter. Fig. 3 shows theclockwise injection type hysteresis curve of Al/BaTiO3/GaN MFS structures. Well-defined accu-mulation, depletion and inversion characteristicsof the MFS structures have been observed withinthe applied gate bias of 5V, which is compatiblewith other electronic devices. The C–V character-istics show a negligible hysteresis width. The activeGaN layer reaches the inversion condition by aminimum applied gate voltage of 5V, which is verymuch less than other conventional GaN MISstructures. Recently Li et al. have reported goodGaN MIS characteristics under low applied biasusing Pb(Zr0.53Ti0.47)O3, a ferroelectric oxide [12].The improvement in MFS inversion behaviour ismainly due to the large polarisation field of BTOferroelectric layer at the BTO/GaN interface. Thehigher dielectric constant value of BTO layer alsoplays an important role in improving the inversionbehaviour by dropping a large part of applied gatevoltage across the active GaN layer. Fabricated

Fig. 2. XRD spectrum of BTO film grown on GaN layer by

sol–gel technique.

Fig. 3. 1MHz C–V characteristics of Al/BaTiO3/GaN MFS

structures.

M. Senthil Kumar et al. / Journal of Crystal Growth 237–239 (2002) 1176–11791178

MFS structures are very stable in the forward biasof 5V. The shift in C–V curve towards the negativebias side indicates the presence of positive fixedcharges at the BTO/GaN interface.

4. Conclusion

GaN based metal–ferroelectric–semiconductor(MFS) structures have been developed onMOCVD grown unintentionally doped n-GaNby using BTO ferroelectric material as an insulat-ing layer. BTO films were coated from organicprecursors using the sol–gel technique. XRD andEDX measurements confirm the formation ofpolycrystalline BTO films with proper elementalcompositions. The C–V measurement shows thatthe GaN MFS can reach inversion just for theapplied bias of 5V, which is comparatively veryless than other conventional gate oxides. Theseresults nominate the ferroelectric BTO as apromising candidate for the successful realisationof practical GaN-based MFSFETs.

Acknowledgements

One of the authors (Senthil Kumar) gratefullyacknowledges the Council of Scientific and

Industrial Research (CSIR), Govt. of India, forthe award of Senior Research Fellowship. Authorsthank the Department of Science and Technologyfor the financial support.

References

[1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64

(1994) 28.

[2] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer,

Verlag, 1997.

[3] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov,

M. Burns, J. Appl. Phys. 76 (1994) 1363.

[4] Q. Chen, J.W. Yang, R. Gaska, M. Asif khan, M.S. Schur,

IEEE Electron Device Lett. 19 (1998) 44.

[5] M. Trivedi, K. Shenai, J. Appl. Phys. 85 (1999) 6889.

[6] K.J. Schoen, J.M. Woodall, J.A. Cooper, M.R. Melloch,

IEEE Trans. Electron. Devices 45 (1998) 1595.

[7] M. Sawada, T. Sawada, Y. Yamagata, K. Imai, H.

Kumura, M. Yoshino, K. Lizuka, H. Tomozawa, Proceed-

ings of the Second International Conference on Nitride

Semiconductors, Tokushima, 1997, p. 482.

[8] H.C. Casey, G. Gfoutain, R.G. Alley, B.P. Keller, S.P.

Den Barrs, Appl. Phys. Lett. 68 (1996) 1850.

[9] S. Arulkumaran, T. Egawa, H. Ishikawa, T. Jimbo, M.

Umeno, Appl. Phys. Lett. 73 (1998) 809.

[10] F. Ren, M. Hong, S.N.G. Chu, M.A. Marcus, M.J.

Schurman, A. Baca, S.J. Pearton, C.R. Abernathy, Appl.

Phys. Lett. 73 (1998) 3893.

[11] S.Y. Wu, IEEE Trans. Electron Devices ED-21 (1973) 499.

[12] W.P. Li, R. Zhang, Y.G. Zhou, J. Yin, H.M. Bu, Z.Y.

Luo, B. Shen, Y. Shi, R.L. Jiang, S.L. Gu, Z.G. Liu, Y.D.

Zheng, Z.C. Huang, Appl. Phys. Lett. 75 (1999) 416.

M. Senthil Kumar et al. / Journal of Crystal Growth 237–239 (2002) 1176–1179 1179