Ohmic contacts based on Pd/Ir/Au metallisations have been formed on highly doped (5 Ã 1019 cmÃ3) p-type GaAsSb. Typical hole mobi- lity in the ...
Ohmic contacts based on Pd=Ir=Au metallisations have been formed on highly doped (5 1019 cm3) p-type GaAsSb. Typical hole mobility in the MOCVD-grown p-GaAsSb was measured to be 50 cm2=Vs. Electrical characteristics of ohmic contacts were measured utilising the transfer length method. The ohmic contact exhibited specific contact resistivity less than 108 O-cm2 and transfer length less than 100 nm. X-ray photoelectron spectroscopy study was also utilised to investigate the interactions between the multilayer metallisations and the ternary GaAsSb compound semiconductor.
Introduction: Gallium arsenide antimonide (GaAsSb) is attracting attention for applications in optoelectronic devices such as double heterojunction bipolar transistors (DHBTs) and photodetectors [1–3]. Base access resistance is one of the most important parasitic elements determining device performance, especially for super-scaled DHBTs the base width of which is in the deep-submicron range [4]. Therefore, it is very important to realise high-performance ohmic contacts on p-GaAsSb utilised for the base layers of DHBTs for high-speed and high-power applications. Extensive research efforts have been conducted to realise high-performance p-type ohmic contacts on base layers of DHBTs (especially in InP=InGaAs). However, there is paucity of work on ohmic contacts on p-GaAsSb [3]. It is indeed important to investigate the properties of p-ohmic contacts on GaAsSb compounds. Iridium has been demonstrated as a suitable metal for high temperature contacts on various III-V semiconductor materials. For instance, it has been utilised for Schottky contacts on n-type GaAs and GaN materials because of its superior performance in high-temperature environments [5, 6]. Palladium has been investigated for thermallystable ohmic contacts on p-InGaAs [7]. Pd=Ir=Au contacts on p-InGaAs have been studied by Jang et al. and found potential as a thermallystable ohmic metallisation [8]. In this Letter, Pd=Ir=Au and Pd=Au contacts are formed on p-type GaAsSb and their electrical properties characterised and compared. An X-ray photoelectron spectroscopy (XPS) study was also carried out to investigate the interactions between the multilayer metallisations and GaAsSb. Experiments: Carbon-doped GaAsSb (5 1019) with a thickness of 500 nm was grown on InP substrates utilising metal organic chemical vapour deposition (MOCVD). Hall measurements were carried out at room temperature and the resulting hole mobility was measured to be as high as 50 cm2=V s despite the high carbon doping concentration. The fabrication process of TLM patterns to measure the electrical characteristics of the ohmic contacts is as follows. A 50 nm-thick SiO2 film was first deposited on the GaAsSb sample using plasma-enhanced chemical vapour deposition. Optical lithography and reactive ion etching were utilised to fabricate SiO2 mesa etch mask. The etching of the GaAsSb was performed using inductivelycoupled-plasma reactive-ion-etching in a Cl2-based chemistry with SiO2 etch mask. The GaAsSb layer was fully etched to expose InP substrate to ensure complete electrical isolation and to restrict electrical current paths. Optical lithography was then performed to delineate TLM patterns. Multilayer metallisation schemes including Pd=Au and Pd=Ir=Au contacts were deposited by electron beam evaporation and lifted-off. Pd was selected for the bottom metallisation because it reacts with semiconductors easily. Iridium, with its high work function, is utilised as the metal on the Pd layer to achieve good ohmic contact resistance under high-temperature stress. Au was used as a capping layer to reduce metal sheet resistance and to protect the ohmic contact metallisation from oxidation. The electrical characteristics of Pd=Ir=Au (5=15=150 nm) ohmic contacts on p-GaAsSb are shown in Fig. 1. The gap spacing of the TLM patterns ranged from 2 to 10 mm. The resulting ohmic contact properties were excellent. The sheet resistance was calculated to be 66 O=u. The specific contact resistivity was 5.0 109 O-cm2 and the contact resistance was 0.06 O-mm. These ohmic contact characteristics are better than those obtained for p-InGaAs with the same metallisation scheme where the specific ohmic contact resistance was of the order of 106 O-cm2 [8]. The superior ohmic contact characteristics can be
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J.H. Jang, H.K. Cho, J.W. Bae, N. Pan and I. Adesida
ascribed to the lower Schottky barrier for holes, fBp, on GaAsxSb1x layer (0.2 eV for x ¼ 0 and 0.5 eV for x ¼ 1) compared to that on p-InGaAs (0.45 eV) considering that the Hall mobility in p-GaAsSb was much lower than that of p-InGaAs due to alloy scattering effects [9]. In addition to these properties, the transfer length of this ohmic contact is less than 0.1 mm. In the super-scale HBTs, the width of base layer should be larger than the transfer length of base ohmic contacts so that it determines the scaling limit of the base-width.
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Fig. 1 Ohmic characteristics of Pd=Ir=Au (5=15=150 nm) metallisation annealed at 250C for 30 s in RTA chamber 10-4
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Low-resistance Pd=Ir=Au ohmic contacts on p-GaAsSb
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Fig. 2 Temperature dependence of specific ohmic contact resistivities of Pd=Au and Pd=Ir=Au with thin Au capping layers Annealing time set to 30 s for both ohmic metallisation schemes
To study the material and electrical characteristics of the ohmic contacts, two metallisation schemes, Pd=Au (5=35 nm) and Pd=Ir=Au (5=15=20 nm) with thin Au capping layers were fabricated. The electrical characteristics are compared in Fig. 2. These data show that ohmic contact resistivities are much higher than the results obtained with thicker Au capping layer (150 nm) as shown in Fig. 1. This is due to the increased sheet resistance of the thinner metallisations. It is also noted that, although, the ohmic contact characteristics are essentially the same for both Pd=Au and Pd=Ir=Au in as-deposited condition, they however exhibited different characteristics when they were annealed at high temperatures. Pd=Au contact deteriorated above the annealing temperature of 350 C while Pd=Ir=Au maintained good ohmic characteristics at temperatures under 500 C for 30 s. It clearly shows that the thermal properties of ohmic contacts were improved by inserting the Ir layer between the Pd and Au capping layers. Pd=Ir=Au contacts exhibited a minimum specific ohmic contact resistivity when thermal annealing was carried out at 300 C for 30 s while Pd=Au contact properties were degraded. For ohmic metallisation schemes with 150 nm Au layer, the minimum contact resistivity was measured when it was annealed at 250 C for 30 s. The optimum annealing temperature and time depended on the thickness of Au layer. To study the interactions of the metallisations, an XPS study was carried out. As-deposited and annealed Pd=Ir=Au (5=15=20 nm) samples were ˚ =min. studied. The sample was sputter thinned at the rate of 20 A Atomic percentages were calculated from the XPS data and plotted in Figs. 3a and b for the as-deposited sample and the annealed sample at 400 C for 30 s, respectively. As shown in Fig. 3, Au diffusion into the semiconductor layer was minimal at this temperature and out-diffusion of Ga, As and Sb were also minimal. The only differences between Figs. 3a and b are in the profiles of Pd and Ir. Pd diffused into the semiconductor layer while Ir diffused in very slightly. The improved ohmic characteristics measured with the sample shown in Fig. 3 can be ascribed to the Pd diffusion and interaction with GaAsSb compounds.
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Although it is not shown here, an XPS study of the Pd=Au sample that was annealed at 400 C for 30 s showed that Au penetrated all the way through the Pd layer far into the GaAsSb. The degradation of the ohmic contact resistance at high temperatures, as shown in Fig. 2 for Pd=Au, is ascribed to the excessive Au diffusion into the semiconductor layer as confirmed by the XPS study. 100
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N. Pan (Microlink Devices, 6457 Howard Street, Niles, IL 60714, USA) J.H. Jang: Also with Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), South Korea
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# IEE 2004 Electronics Letters online no: 20046658 doi: 10.1049/el:20046658
J.H. Jang, H.K. Cho, J.W. Bae and I. Adesida (Micro and Nanotechnology Laboratory and Department of Electrical and Computer Engineering, University of Illinois at Urbana Champaign, 208 N. Wright St., Urbana, IL 61801, USA)
As Au Ga Ir Pd Sb
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Acknowledgment: This work was supported by ONR Grant N0001401-1-1000.
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a As-deposited Pd=Ir=Au (5=15=20 nm) b Annealed Pd=Ir=Au (5=15=20 nm) at 300 C for 30 s
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Conclusion: Pd=Ir=Au ohmic metallisation on p-GaAsSb exhibited a very low ohmic contact resistance with a better thermal stability than Pd=Au metallisation. Ohmic contact formation on GaAsSb using Pd=Ir=Au was found to be superior to that obtained for p-InGaAs [10]. The diffusion of Pd and Ir into GaAsSb improved the ohmic properties of Pd=Ir=Au after thermal stress.
McDermott, B.T., et al.: ‘Growth and doping of GaAsSb via metalorganic chemical vapor deposition for InP heterojunction bipolar transistors’, Appl. Phys. Lett., 1996, 68, (10), pp. 1386–1388 Xu, X.G., et al.: ‘Metalorganic vapor phase epitaxy of high-quality GaAs0.5Sb0.5 and its application to heterostructure bipolar transistor’, Appl. Phys. Lett., 1999, 74, (7), pp. 976–978 Dvorak, M.W., et al.: ‘300 GHz InP=GaAsSb=InP double HBTs with high current capability and BVCEO 6V’, IEEE Electron Device Lett., 2001, 22, (8), pp. 361–363 Rodwell, M.J.W., et al.: ‘Submicron scaling of HBTs’, IEEE Trans. Electron Devices, 2001, 48, (11), pp. 2606–2624 Lanlinsky, T., et al.: ‘High-temperature stable Ir-Al=n-GaAs Schottky diodes: effect of the barrier height controlling’, J. Vac. Sci. Technol. B, 1996, 14, (2), pp. 657–661 Kumar, V., et al.: ‘Characterisation of iridium Schottky contacts on n-AlxGa1xN’, Electron. Lett., 2003, 39, (9), pp. 747–748 Ressel, K., et al.: ‘Non-alloyed ohmic contacts for pþ-type InGaAs base layer in HBTs’, Electron. Lett., 1992, 28, (24), pp. 2237–2238 Jang, J.H., Kim, S., and Adesida, I.: ‘Electrical characteristics of Ir=Au and Pd=Ir=Au ohmic contacts on p-InGaAs’, Electron. Lett., 2004, 40, (1), pp. 77–78 Bolognesi, C.R., et al.: ‘InP=GaAsSb=InP double HBTs: a new alternative for InP-based DHBTs’, IEEE Trans. Electron Devices, 2001, 48, (11), pp. 2631–2639
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