Chemical Vapor Deposited Tungsten Film on Molecular Layer Epitaxially-Grown GaAs and its Application to Low Resistivity
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for such thin layered structures. Therefore, low resistivity metal/semiconductor contact formed at low temperature with atomically flat interface has been urgently required. W/GaAs contacts were found stable up to 5000 C. This temperature is higher than that used for selective regrowth with MLE for the 10nm channel GaAs static induction transistor (ST [1]. Whereas sputtering was commonly used for W deposition, it results in serious generation of defects in thin active semiconductor layers. In this paper, novel CVD W suitable for ultra-thin device fabrication is shown. The contact resistance in W/GaAs isshown as a fuinction of surface stoichiometry by using the TLM on heavily doped GaAs grown by MLE. Barrier height of W/GaAs is studied in reference for lowering the contact resistance. The W/GaAs interface and the impurity profiles in MLE GaAs layers are measured by secondary ion mass spectrometry (SIMS). Interface structure and crystalline properties are observed by HRTEM. Rutherford backscattering spectroscopy (RBS) is used to evaluate the structural properties of W/GaAs interface. EXPERIMENTS The precursor for the W CVD on GaAs used is W(CO)6. The W layers were deposited in the UHV MLE reactor [2]. Prior to the W CVD, oxides were removed in-situ from GaAs surface in the deposition chamber by exposing AsH 3 at below 480'C. Oxides are chemically reduced with this process rather than physically evaporated [3]. Immediately after this process, W(CO) 6 was introduced continuously at 360-400°C with the 175 Mat. Res. Soc. Symp. Proc. Vol. 573 ©1999 Materials Research Society
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Fig.2 Cross sectional HRTEM image of W/GaAs deposited on MLE-grown GaAs doped with Te with the concentration of about l&cm". Low angle Ar+ ion milling was applied for the sample preparation.
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range of temperature 360-400TC. Deposition rate was about 3 A/mmn on GaAs, like reported for pyrolytic decomposition[5]. However, we can not exclude some photolytic reaction [6]. Although the GaAs substrates were not intentionally ilhunimted, the light of the halogen heater lamp contained near-UV wavelengths. The W layers on GaAs observed with Nomarski and SEM microscopes appeared miror-like. AFM observation also shows extremely flat surface with the maximum roughness of about 2nm. The layers on SiN had a grain structure, similar to the one reported previously [5]. The difference in layer morphology between patterned SiN/GaAs edge in plane suggests a catalytic properties of the GaAs clean swface. Figure 1 shows the TOF-SIMS depth profile of W/GaAs interface. W/GaAs interface was clearly separated, and the mixed layer in the irterface was estimated less than 2m. But stange profile was seen on the surface of WGa, As and 0 piled- up. In addition, 0 and C yields look to be reduced in W layer. It is possible to explain these results by a surface unstability or a different matrix effect Fig.2 shows the cross sectional HRTEM •- WIG.As ( SI doped) image of the present W/GaAs structure. It is Ssurface treated in shown that the atomically flat interface
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