https://doi.org/10.1595/205651318X696639 Johnson Matthey Technol. Rev., 2018, 62, (2), 211–230 www.technology.matthey.com Inter-Diffusion of Iridium, Platinum, Palladium and Rhodium with Germanium Improved materials for the next generation of electronic devices Adrian Habanyama* platinum, palladium and rhodium. Our approach Department of Physics, Copperbelt University, was essentially twofold. Firstly, conventional thin PO Box 21692, Jambo Drive, Riverside, Kitwe film couples were used to study the sequence 10101, Zambia; Department of Physics, of phase formation in the germanide systems. University of Cape Town, Rondebosch 7700, Conventional thin film couples were also used to South Africa identify and monitor the dominant diffusing species during the formation of some of the germanides *Email: [email protected] as these can influence the thermal stability of a device. Secondly, we observed and analysed Craig M. Comrie several aspects of the lateral diffusion reactions in iThemba LABS, National Research Foundation, these four systems, including activation energies PO Box 722, Somerset West 7129 South and diffusion mechanisms. Lateral diffusion Africa; Department of Physics, University of couples were prepared by the deposition of thick Cape Town, Rondebosch 7700, South Africa rectangular islands of one material on to thin films of another material. Rutherford backscattering spectrometry (RBS) and microprobe-Rutherford The down-scaling of nanoelectronic devices to backscattering spectrometry (µRBS) were ever smaller dimensions and greater performance used to analyse several aspects of the thin film has pushed silicon-based devices to their physical and lateral diffusion interactions respectively. limits. Much effort is currently being invested in X-ray diffraction (XRD) and scanning electron research to examine the feasibility of replacing microscopy (SEM) were also employed. Si by a higher mobility semiconductor, such as germanium, in niche high-performance metal 1. Introduction oxide semiconductor (MOS) devices. Before Ge can be adopted in industry, a suitable contact Ge has several attractive properties such as high material for the active areas of a transistor must mobility of charge carriers and very low carrier be identified. It is proposed that platinum group freeze-out temperatures (1–4). There is currently metal (pgm) germanides be used for this purpose, much research on high mobility semiconductors, in a similar manner as metal silicides are used such as Ge, with the view of using them to replace in Si technology. Implementation of Ge-based Si in niche high-performance MOS devices (5–7). technology requires a thorough understanding of Before Ge can be adopted by industry a suitable the solid-state interactions in metal/Ge systems contact material to the active areas (source, drain in order to foresee and avoid problems that may and gate) of a transistor must be identified. It is be encountered during integration. We present a proposed that pgm germanides be used for this systematic study of the solid-state interactions in purpose, similar to the manner in which metal germanide systems of four of the pgms: iridium, silicides are used in present Si technology (6, 8). 211 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696639 Johnson Matthey Technol. Rev., 2018, 62, (2) Our work investigates the solid-state interactions (23) reported a scanning tunnelling microscopy in germanide systems of four of the pgms: Ir, Pt, (STM) and spectroscopy study of the formation of Pd and Rh, in thin film and lateral diffusion couples. Pt/germanide phases on Ge (111). This study gave In the design of transistors the contact material a demonstration of the structural dependence of should be stable over a wide temperature range. electronic properties in the Pt-Ge system. Schottky Conventional thin film couples are well suited for barrier diodes have been used in many applications investigating the phase formation sequence and such as gates for metal semiconductor field-effect temperature stability of the phases of a system. We transistors (MESFET), solar cells and detectors have also used thin film couples to identify some (24–27). A reduction of the PtGe/Ge electron of the dominant diffusing species during the phase Schottky barrier height by rapid thermal diffusion formation. For device integrity it is important to of phosphorus was reported by Henkel et al. (28). identify the dominant diffusing species during the The results showed that rapid thermal diffusion formation of the respective germanides as this can from a solid diffusion doping source was effective in influence their thermal stability. reducing Schottky barrier heights of Pt germanide The samples used for studying lateral diffusion Schottky barrier diodes on Ge. Chawanda et al. reactions were composed of a thick island of one (29) investigated the change in the current-voltage material on top of a much thinner film of another (I–V) electrical properties of Pt Schottky contacts material. Upon annealing the island material on Ge (100) at different annealing temperatures. would react with the underlying film through Their results showed that the as-deposited barrier vertical diffusion, going through a sequence of heights had values that were near the band gap phases until the most island-material rich phase of Ge for Pt/Ge (100) Schottky diodes resulting in is formed. Since no further vertical reaction with good Schottky source/drain contact materials in the underlying film is possible, the most island- p-channel Ge-MESFETs for the hole injection from material rich phase may then grow laterally until source into inverted p-channel (30). Chawanda it attains a critical width, after which other phases et al. (31) also studied the electrical properties appear and grow simultaneously. This is a case of of Pd Schottky contacts with Ge (100). I–V and multiple phase formation as would be found in bulk capacitance-voltage (C–V) measurements were diffusion couples. Lateral diffusion couples thus performed under various annealing conditions. provide the transition between thin film and bulk Only one Pd germanide phase, PdGe, was formed. behaviour. A hole trap at 0.33 eV above the valence band was Since the island material is abundant for diffusion observed after annealing at 300°C. In another in the lateral diffusion couples, phase formation and study Chawanda et al. (32) investigated the change reaction kinetics can be studied to a greater extent in the electrical properties of Ir Schottky barrier than in thin film planar structures. Lateral diffusion diodes on Ge (100). Electrical characterisation of structures can be used to simulate bulk diffusion these contacts using I–V and C–V measurements couples because phase formation could extend to was performed under various annealing conditions. lengths of around 100 µm (9). In kinetic studies Thermal stability of the Ir/Ge (100) sample was of thin film planar structures the diffusion lengths observed up to an annealing temperature of are typically less than 0.5 µm. One can therefore 500°C. The results also showed that the onset study the transition from thin film to bulk diffusion temperature for agglomeration (binding of primary couple behaviour. The study of lateral diffusion particles leading to phase formation) in 20 nm Ir/ couples is particularly well suited for dealing with Ge (100) samples occurs between 600–700°C. the challenges of achieving the required lateral Gaudet et al. (7) carried out a systematic study abruptness of semiconductor junctions. Excessive of thermally induced reactions of 20 transition diffusion of the substrate element, in this case metals with Ge substrates. They monitored Ge, during germanide formation could result in metal/Ge reactions in situ during ramp anneals overgrowth and bridging in devices (10). at 3°C s–1 using time-resolved XRD and diffusion Various early techniques were developed to light scattering. They also carried out resistance study lateral diffusion couples (11–20). In later measurements. Their results showed that the pgms studies, µRBS was used (9, 21, 22). The major Pd and Pt were among the six most promising advantage of this technique is its ability to give candidates for microelectronic applications, the depth information. other candidates being nickel, cobalt, copper and Some previous work has been carried out in the iron. Ni is the most used metal for reducing contact research field of pgm/Ge junctions. Saedi et al. resistance. An example of previous research in the 212 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696639 Johnson Matthey Technol. Rev., 2018, 62, (2) area of Ni/Ge junctions is the work reported by plane in monitoring the direction of flow of atoms. A Peng et al. (33) on the I–V characteristics of Ni/ thin layer of Ti acted as an inert marker interposed Ge (100) Schottky diodes and the Ni germanide between coupling layers of metal and Ge; the Ge induced strain after subjecting the Schottky layer being at the surface of the sample. Upon contacts to rapid thermal annealing in the annealing of this structure both Ge and metal atoms temperature range of 300–600°C. Their results could have diffused across the marker at different showed that the orthorhombic structure of NiGe rates causing it to shift in position towards the induces epitaxial tensile strain on Ge substrates dominant diffusing species. The marker Ti signal due to the difference in lattice constants. They also was monitored by RBS for different annealing suggested that the increase in barrier height with times. The dominant diffusing species (DDS) during increasing annealing temperature may have been phase formation was determined by observing the due to the conduction band edge shift by the strain relative shift of the marker. after the germanidation process. The lateral diffusion couples were also prepared Hallstedt et al. (34) studied the phase by electron beam evaporation at a base pressure in transformation and sheet resistance of Ni on the low 10–5 Pa range.