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phys. stat. sol. (a) 202, No. 14, R161–R163 (2005) / DOI 10.1002/pssa.200521308

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X-ray absorption spectroscopic study on Ti/n-GaN

M. Senthil Kumar**, 1, V. Suresh Kumar1, K. Asokan*, 2, J. W. Chiou2, J. C. Jan2, W. F. Pong2, and J. Kumar1

1 Crystal Growth Centre, Anna University, Chennai-600 025, India 2 Department of Physics, Tamkang University, Tamsui, Taiwan

Received 15 September 2005, revised 11 October 2005, accepted 18 October 2005 Published online 21 October 2005

PACS 61.10.Ht, 73.40.Ns * Corresponding author: e-mail asokan@nsc.res.in Permanent address: Nuclear Science Centre, Aruna Asaf Ali Marg, New Delhi-110 067, India. ** Present address: Venture Business Laboratory, Nagoya University, Nagoya 464-8603, Japan.

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Gallium nitride has emerged as a most potential mate-rial for the fabrication of efficient blue light emitting di-odes (LEDs) and laser diodes [1]. In device fabrication, formation of high quality ohmic contacts is the most cri-tical step to achieve high performance. The high effi-ciency requires a low electrical resistance for the contact between current leads and the semiconductor. Considering the work function of contact metal and the electron affinity of the semiconductor, several metallization schemes for ohmic contacts to n-GaN have been proposed and investi-gated in the last few years [2–5]. In most schemes, the ni-trogen-containing layer at the interface appears to be re-sponsible for low resistivity and suggests the tunneling mechanism for the ohmic contact to n-GaN. Stable and practical ohmic contacts have been reported on n-GaN by using a metal bilayer of Ti/Al [6–8]. Thermal treatment always results in a much-reduced contact resistance, an-nealing at 500 °C being sufficient to achieve a specific contact resistance below 10–3 Ω cm2, while rapid thermal annealing at 900 °C may reduce the value to below 10–5 Ω cm2. The ohmic contact depends on Al diffusion throughout the Ti layer, intimate contact being made be-tween an Al/Ti intermetallic with a low work function and a clean GaN surface [9]. Most importantly, the higher an-

nealing temperature results in the formation of a TiN layer at the interface. This is of great significance because it im-plies creation of a high concentration of N vacancies in the GaN surface region, which act as donors and facilitate electron tunneling through any residual barrier. The large formation energy of VN in n-type material is presumably compensated here by the strength of the Ti–N bond in forming TiN. However, the reason for the low resistive ohmic contact obtained by metallization of the Ti layer in GaN is under debate due to the complication of the Ti/GaN interfacial properties. X-ray absorption near edge structure (XANES) has been effectively used for the study of the electronic structure of p-GaN layers [10] and Ni/Au/p-GaN contacts [11].

In this present study, we investigate 500 Å Ti/GaN epi-layers, as-deposited and rapidly thermally annealed at 900 °C for 30 s, by using X-ray diffraction (XRD) pattern, I–V characteristics, and XANES. The XRD measurement provides evidence for TiN formation after annealing and electrical properties show a linear behaviour in I–V meas-urements. XANES at Ti K- and Ti L3,2-edges are used to understand the electronic structures of as-deposited Ti/ n-GaN contacts and the phases at the Ti/n-GaN interface upon high temperature annealing.

Formation of low resistance and thermally stable ohmic con-tacts to GaN is of considerable importance for device applica-tions. Several metallization schemes for ohmic contacts to n-GaN with low contact resistance have been proposed andinvestigated by different techniques. We investigate 500 Å Ti/n-GaN contacts of as-deposited and rapid furnace annealedat 900 °C for 30 s, using X-ray diffraction pattern, I–V meas-

urements, and X-ray absorption near edge spectra at Ti K-and L3,2-edges and elucidate the mechanism responsible forthe high ohmic behaviour. These measurements indicate the formation of an interfacial Ti

xN layer and intermetallic alloys

of Ti and Ga at the Ti/n-GaN interface upon high temperatureannealing.

R162 M. Senthil Kumar et al.: X-ray absorption spectroscopic study on Ti/n-GaN

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a

stat

us

solid

i

ph

ysic

a

16 18 20 22 24 26 28

annealedas deposited

Ti 2N

(220

)

orsa

pphi

re(0

004)

GaN

(000

2)

TiN

(200

)

Ti 2N

(210

)

Inte

nsit

y(a

.u.)

θθθθ (degree)

Figure 1 X-ray diffraction patterns of 500 Å Ti/n-GaN pristine and 900 °C annealed sample.

Unintentionally doped 2 µm thick n-type GaN epitaxial layers grown on sapphire (0001) substrates by metal or-ganic chemical vapour deposition (MOCVD) technique have been used in this investigation. The mobility and car-rier concentration of the grown GaN layers are about 600 cm2/Vs and 6–7 × 1016 cm–3, respectively. The surface of the GaN layer was degreased with organic solvents and then etched with diluted HCl followed by rinsing in de-ionised water. Ti/n-GaN interface was established by evaporating high pure Ti metal using electron beam evapo-ration to a thickness of 500 Å with a background pressure of 1 × 10–6 Torr. Ti/n-GaN contacts were, then, subjected to high temperature annealing at 900 °C in high pure argon ambient for 30 s using rapid furnace annealing. These samples were characterized by X-ray diffraction, I–V char-acteristics for electrical properties and XANES for elec-tronic structures of phases. Figure 1 shows the XRD pat-terns, taken using Cu Kα radiation, of both as-deposited and annealed samples. The as-deposited Ti/n-GaN sample shows XRD peaks corresponding to the GaN epilayer, whereas the annealed Ti/n-GaN contact at 900 °C for 30 s shows new peaks corresponding to TiN and Ti2N in addi-tion to GaN. Thus, the XRD result indicates the formation of a Ti

xN layer at the Ti/n-GaN interface when subjected to

high temperature annealing for a short duration. Peaks of a reduced intensity corresponding to Ti–Ga intermetallic al-loys are observed in the θ range of 40° to 70° (not shown in the figure). The formation of interfacial layers at the Ti/n-GaN contact is highly complicated with different Ti–Ga–N compositional layers such as GaN/TiN/Ti2GaN/ Ti3GaN, however, an intimate contact is formed between TiN and GaN [10].

For I–V measurements, Ti/Al (250 Å/1500 Å) bilayer contacts were fabricated on GaN layers. The as-deposited contact gives a very low current (below 1 µA) with non-ohmic behavior indicating high resistive contacts. As evi-dent from Fig. 2, the I–V characteristics of the annealed contact at 900 °C is more linear without any rectifying na-ture and the contact resistance decreased by many orders of magnitude compared to the as-deposited sample. This re-

-3 -2 -1 0 1 2 3-2 5

-2 0

-1 5

-1 0

-5

0

5

1 0

1 5

2 0

2 5

Cur

rent

(mA

)

V o lta g e (V )

Figure 2 I–V characteristics of Al/Ti/GaN contact annealed at 900 °C for 30 s. sult is consistent with other reports found in literature [1]. Thus, the great improvement of I–V characteristics in case of annealed Ti/GaN contacts might be due to the formation of an interface containing TiN and other intermetallic al-loys.

In order to understand the local electronic structure of both as-deposited and annealed Ti/GaN contacts, the XANES spectra at Ti K- and Ti L3,2-edges were recorded at room temperature in the energy range of 4960–5040 eV and 450–470 eV, respectively, at the National Synchrotron Radiation Research Center (NSRRC) in HsinChu, Taiwan, operating with an electron energy of 1.5 GeV and a maxi-mum stored current of 200 mA. While the Ti K-edge spec-tra were obtained from the wiggler beam-line in fluores-cence yield mode using gas detectors, the Ti L3,2-edge spectra were obtained from HSGM using the sample cur-rent drain mode. The X-ray incident angle was about 45° to 50° with respect to the normal to the sample. For Ti L3,2-edge measurements, the samples were under a chamber pressure of better than 10–9 Torr. While the wiggler beam-line has a typical resolution of 2 eV, the HSGM beam-line has a typical energy resolution of 0.2 eV.

4960 4980 5000 5020 5040

as-deposited

900 oC

Ti K-edge

cba

E1D1C1

B1

A1

Abs

orpt

ion

Inte

nsit

y(a

.u.)

Energy (eV)

Figure 3 The XANES spectra at Ti K-edge of as-deposited and annealed Ti/GaN contacts.

phys. stat. sol. (a) 202, No. 14 (2005) / www.pss-rapid.com R163

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Rapid

Research Letter

Based on the band structure calculations on TiN, Sori-ano et al. have reported that the N 2p states are located mainly from –9 to –4 eV and Ti 3d character spread from –2 to 6 eV [12]. These 3d bands are split by crystal field effects, due to d-orbitals pointing in between the N ions, into low energy t2g and high energy eg sub-bands. The re-gion above 6 to 16 eV corresponds to Ti 4sp levels. Due to considerable Ti–Ga Ti–N mixing, there is significant weight of p character in the Ti 3d bands. Using the above theoretical results, the Ti K-edge spectra may be under-stood as follows: The pre-edge peak labeled as A1 corre-sponds to the 1s to 3d quadrupole transitions and these are forbidden and one needs to consider the near-neighbor ligand (i.e., N) with p characters. These spectral features change significantly when one compares the as-deposited sample and that annealed at 900 °C which indicates a change in the electronic structure. The lowest energy peak ‘a’ of the feature A1 is assigned to an excitonic transition within the forbidden band gap (Frenkel core exciton). The peaks ‘b’ and ‘c’ correspond to the t2g and eg bands that arise due to hybridization of Ti 3d and N 2p and Ga 3p states [13]. Apart from this we also observe feature B1 whose assignment is controversial since this is assigned both to defects or to the non-stoichiometric nature and also to charge transfer between Ti 3d ions to other cations mainly Ga. Prominent features C1 and D1 in the 900 °C an-nealed samples indicate change in the near neighbor envi-ronments of Ti ions that may arise due to the formation of Ti

xN, and intermetallic Ti–Ga alloys. This understanding

is consistent with XRD results. The reduction in the con-tact resistivites may be related to the formation of thin lay-ers of Ti

xN and Ti–Ga alloys.

To understand the nature of Ti 3d states, we have re-corded Ti L3,2-edges which directly provide information about Ti 3d electrons. The Ti L3,2-edge spectra of Ti/ n-GaN with Ti thickness of 500 Å are shown in Fig. 4. The spectral features A2 & C2 and B2 & D2 correspond to the t2g and eg bands, respectively [14]. Thus, it is evident from the 900 °C annealed samples that Ti reacts with GaN and the interface contains 3d bands hybridized with Ga and N ions. From the above XANES investigations, there is significant change in the conduction band after annealing especially at the Ti/n-GaN interface annealed at 900 °C due to the hy-bridization of Ti 3d with N 2p/Ga 3d states. There is for-mation of Ti

xN and possibly some Ti–Ga alloys which may

be responsible for the low contact resistance. It may be noted that Ti has a work function of 4.33 eV which matches the electron affinity of GaN. This matching helps to produce a Schottky contact when deposited on n-GaN [6]. The above findings are also consistent with the study carried out by Naono et al. using photoemission spectros-copy [15].

Our study on Ti/n-GaN epilayers, as-deposited and an-nealed at 900 °C for 30 s using XRD, I–V measurements, and XANES provides evidence for the formation of inter-facial layers of Ti

xN and Ti–GaN alloys which provide

very good ohmic contacts for device fabrications.

450 460 470

D 2

C 2

B 2A 2

T i L 3 , 2 - e d g e s

9 0 0 C

a s - d e p o s i t e d

T iN

Abs

orpt

ion

(arb

.uni

ts)

P h o t o n E n e r g y ( e V )

Figure 4 The XANES spectra at Ti L3,2-edges of as-deposited and annealed Ti/GaN contacts.

Acknowledgements The authors (W. F. P. and K. A.) thank the National Science Council, Taiwan, for financial support and the NSRRC staff for technical support in XANES measurements.

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