Ti/Al/Ti/Au and Ti/Al/Pd/Au contacts to AlGaN/GaN have been investigated to ascertain the effect of annealing temperature on the structural evolution of the contacts. Ti/Al/Ti/Au contacts become ohmic after rapid thermal annealing at 750°C or higher, corresponding to the formation of an interfacial TiN phase, with inclusions penetrating through the AlGaN layer observed after annealing at 950°C. The Pd layer is shown to be more efficient at inhibiting diffusion of Au to the interface than Ti. Ohmic behaviour was not seen with the Ti/Al/Pd/Au scheme. Either the presence of Au at the interface may improve ohmic behaviour, or the Ti:Al ratio is insufficient in this scheme

Birbeck, J.C.

Brown, Paul D.

Fay, Mike W.

Harrison, Ian

Hughes, B.T.

Martin, T.

Uren, M.J.

Nottingham eTheses

Fay, Mike W. and Harrison, Ian and Birbeck, J.C. and Hughes, B.T. and Uren, M.J. and Martin, T. and Brown, Paul D. (2001) Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts to AlGaN/GaN. In: Electron microscopy and analysis 2001: proceedings of the Institute of Physics Electron Microscopy and Analysis Group Conference, University of Dundee, 5-7 September. Institute of Physics conference series (168). IOP Publishing Ltd, Bristol, pp. 497-500. ISBN 9780750308120 Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/1468/1/Inst._Phys._Conf._Ser._168_%282001%29_pp_497-500.pdfCopyright and reuse: The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions.This article is made available under the University of Nottingham End User licence and may be reused according to the conditions of the licence.  For more details see: http://eprints.nottingham.ac.uk/end_user_agreement.pdfA note on versions: The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.For more information, please contact eprints@nottingham.ac.uk   Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts on AlGaN/GaN  M W Fay, I Harrison1, J C Birbeck2, B T Hughes2, M J Uren2, T Martin2 and P D Brown  School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham NG7 2RD 1School of Electrical and Electronic Engineering, University of Nottingham, University Park, Nottingham NG7 2RD 2DERA, MMS Dept, PA 125, St Andrews Rd, Malvern, Worcs WR14 3PS  ABSTRACT: Ti/Al/Ti/Au and Ti/Al/Pd/Au contacts to AlGaN/GaN have been investigated to ascertain the effect of annealing temperature on the structural evolution of the contacts. Ti/Al/Ti/Au contacts become ohmic after rapid thermal annealing at 750°C or higher, corresponding to the formation of an interfacial TiN phase, with inclusions penetrating through the AlGaN layer observed after annealing at 950°C. The Pd layer is shown to be more efficient at inhibiting diffusion of Au to the interface than Ti. Ohmic behaviour was not seen with the Ti/Al/Pd/Au scheme. Either the presence of Au at the interface may improve ohmic behaviour, or the Ti:Al ratio is insufficient in this scheme.   1. INTRODUCTION  AlGaN/GaN heterostructure field effect transistors (HFETs) are attracting attention as high power, high frequency devices. The performance of these devices is limited by the need for reproducible ohmic contacts with low resistance and good thermal stability. It is also desirable to have contacts with a flat surface morphology for processing.  Contacts containing TiAl have become the conventional scheme for n-type GaN and AlGaN. TiAl contacts are believed to become ohmic by the formation of a Ti-nitride interfacial phase during annealing, which depletes N from the surface of the GaN to produce a highly n-type doped layer (Lin et al 1994, Ruvimov et al 1998). In view of the ease with which TiAl layers oxidise, multilayer schemes with Au as the surface layer and materials such as Ni, Pd or Ti acting as a barrier layer between the Au and TiAl layers have been used to obtain improved contacts (Cai et al 1998, Fan et al 1996, Mohammed et al 1996). However, a full understanding of the effect of annealing temperature on the structure of these multilayers and the resultant electrical behaviour is still required. In this work, the effect of annealing temperature on a TiAlTiAu contact to AlGaN/GaN is analysed, and compared to the structure of a TiAlPdAu contact.  2. EXPERIMENTAL    7KH +)(7 ZDIHU VWUXFWXUH FRQVLVWHG RI QP RI $O*D1 a RQ  ȝP RI *D1, grown on an (0001) sapphire substrate by metal organic chemical vapour deposition (MOCVD). Onto this was deposited the device contact structures, either 20nm Ti / 100nm Al / 60nm Ti / 300nm Au, or 10nm Ti / 220nm Al / 200nm Pd / 200nm Au. The TiAlTiAu contact was cleaved into separate sections to be rapid thermal annealed in flowing N2 at a range of temperatures from Published under licence in IOP Conference Series: by IOP Publishing Ltd. Inst. Phys. Conf. Ser. 168 (2001) pp 497-500 Electron Microscopy & Analysis Conference, EMAG 2001, Dundee 650°C to 950°C. The TiAlPdAu sample was rapid thermal annealed at 950°C. Cross sectional samples were prepared for conventional and high-resolution transmission electron microscopy, energy filtered TEM (EFTEM) and Energy Dispersive X-ray (EDX) analysis.  3. RESULTS AND DISCUSSION  Visual inspection showed that the colour of both TiAlTiAu and TiAlPdAu contact schemes altered from gold to silver-grey after annealing. Transmission Line Method (TLM) measurements showed that the TiAlTiAu contacts became ohmic after annealing at 750°C or higher, while the TiAlPdAu contact did not become ohmic at anneals up to 950°C.  Bright field imaging of the annealed TiAlTiAu contacts (Fig 1a-1c) showed that the layered material in the contact had mixed, resulting in an Au/intermetallic grain structure with grain sizes of up to 200 nm. The TiAlTiAu sample annealed at 850°C (Fig 1b) showed a thin interfacial region comprising small grains approximately 5-10nm wide, which was identified by EFTEM as being TiN (Fay et al 2001). This interfacial phase could not be observed with the TiAlTiAu sample annealed at 650°C (Fig 1a). The TiAlTiAu sample annealed at 950°C showed large inclusions penetrating through the AlGaN layer into the GaN layer below, up to a depth of 100 nm (Fig 1c). There appears to be an association between the size and depth of these inclusions and the presence of extended dislocations. A layer of small grains 10-20 nm wide was again identified between these inclusions and the contact metal. Bright field TEM of the TiAlPdAu sample annealed at 950°C (Fig. 1d) shows a greatly reduced intensity of inclusions compared to that seen in the TiAlTiAu sample annealed at the same temperature, despite a similar defect structure in the GaN/AlGaN layer. The structure of the contact is that of large Au and intermetallic grains in the bulk of the contact, with smaller grains 10-20 nm wide near an 50nm 50nm 50nm a b 50nm GaN AlGaN contact d c Fig. 1 ± Bright field images of contacts to AlGaN/GaN a)-c) TiAlTiAu annealed at 650, 850 and 950°C respectively; d) TiAlPdAu annealed at 950°C interfacial region ~5 nm thick. It was also observed that the surface morphology of this sample is smoother than with TiAlTiAu contacts annealed at the same temperature.   High resolution TEM performed on the TiAlTiAu sample annealed at 950°C demonstrated that the inclusions were crystalline, with a dominant lattice spacing of 0.25nm, consistent with {111} TiN. An HRTEM image, taken down the <11-20> axis shows the integrity of the AlGaN/GaN 2DEG interface appears to be unaffected beyond a few nm of the inclusion (Fig 2). The region of the inclusion in this image shows moiré fringes due to the superposition of the inclusion material with the AlGaN/GaN matrix.  An EDX line profile taken across the TiAlTiAu sample annealed at 950°C, from the GaN to the contact crossing an inclusion, shown in Fig. 3. It can be seen clearly that the Ga has been completely displaced in the region of the inclusion, with an Al and Au rich layer separating the TiN of the inclusion from the GaN layer.  The layer of small grains between Fig. 2 HRTEM image of TiAlTiAu sample annealed at 950°C, showing the location of the 2DEG layer between the AlGaN and GaN (arrowed) and a TiN inclusion.  Fig. 3 EDX linescans taken across an inclusion within a TiAlTiAu sample annealed at 950°C, along the arrow indicated in the secondary electron image. The inclusion can be seen to be rich in Ti, with the Ga completely displaced. An Al/Au layer can be seen between the inclusion and GaN layer. The contact adjacent to this region can be seen to be rich in Al and Au.  SEGaAlTiAuGaN Inclusion Contact GaN Contact 100nm the inclusion and contact was also identified by EDX spot spectra (not shown) as being a Ti-rich layer, most likely TiN, as there is no corresponding peak in Al or Au. The contact grains are also separated from the inclusions by a diffuse region richer in Al and Au. Analysis of the TiAlTiAu sample annealed at 950°C concludes that the inclusions are TiN, which is a semi-metal with a low work function, and would therefore satisfy the requirement for ohmic contacting to n-type GaN. However, despite the significant change in interfacial morphology observed between TiAlTiAu samples annealed at 850°C and 950°C, the electrical behaviour showed only a slight improvement. This indicates that it is the formation of the interfacial TiN phase that is the key to ohmic behaviour in this contact. The TiAlPdAu contact annealed at 950°C showed a much lower density of inclusions than the TiAlTiAu sample. It is believed that the Al/Au diffusion front is essential to the formation of the inclusions, indicating that the Pd layer is more effective than the second Ti layer at inhibiting the diffusion of Au to the interfacial region. However, the lack of ohmic behaviour in the TiAlPdAu contact suggests that this sample contained inadequate activities of Ti at the interfacing to form adequate levels of interfacial TiN, which is necessary for the formation of an ohmic contact in these schemes.  4. CONCLUSIONS  Rapid thermal annealing temperatures of 750°C or higher are required to establish ohmic behaviour from TiAlTiAu contact schemes, corresponding to the formation of an interfacial TiN phase. At anneals of 950°C, large inclusions are formed that consume the AlGaN layer, but do not significantly affect the electrical characteristics of the layer. The TiAlPdAu scheme investigated produces a superior contact morphology, but does not become ohmic. The Pd is more effective at inhibiting the diffusion of Au to the contact/AlGaN interfacial region than the Ti barrier.   ACKNOWLEDGEMENTS   MWF wishes to acknowledge EPSRC/DERA for funding under contract number GR/M87078, and also to thank Alex Bright and Colin Humphreys of Cambridge University, and Alessandro Benedetti, Hira Media, David Norris and Tony Cullis of Sheffield University for all their assistance. The DERA Malvern contribution was carried out as part of Technology Groups 07 and 09 of the UK MOD Corporate Research Programme.   REFERENCES  Cai S J, Li R, Chen Y L, Wong L, Wu W G, Thomas S G and Wang K L 1998, Electron. Lett. 34, 2354 Fan Z F, Mohammad S N, Kim W, Aktas O, Botchkarev A E and Morkoç H 1996, Appl. Phys. Lett. 68, 1672 Fay MF, Harrison I, Birbeck JC, Hughes BT, Uren MJ, Martin T and Brown PD 2001, Microsc. Semicond. Mater. Conf. XII, 25-29 March,  Oxford to be published in the Institute of Physics Conference series Lin M E, Ma Z, Huang F Y, Fan Z F, Allen L H and Morkoç H 1994 Appl. Phys. Lett. 64, 1003. Mohammad S N, Fan Z F, Salvador A, Aktas O, Botchkarev A E, Kim W and Morkoç H 1996, Appl. Phys. Lett. 69 1420 Ruvimov S, Liliental-Weber Z, Washburn J, Qiao D, Lau SS and Chu P K 1998, Appl. Phys. Lett. 73, 2582    

Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts to AlGaN/GaN

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Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts to AlGaN/GaN

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