Electric field effect in amorphous semiconductor films assembled from transition-metal-encapsulating Si clusters
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Electric field effect in amorphous semiconductor films assembled from transition-metalencapsulating Si clusters N. Uchida 1, T. Miyazaki2, Y. Matsushita1, 3, K. Sameshima1, 3, and T. Kanayama 3, 4 1
Nanodevice Innovation Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan 2 Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology,, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 3 Institute of Applied Physics and Doctoral Program in Applied Physics, University of Tsukuba, 1-1-1 Tenoudai, Tsukuba, Ibaraki 305-8573, Japan 4 National Institute of Advanced Industrial Science and Technology,, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ABSTRACT We synthesized amorphous semiconductor films composed of Mo-encapsulating Si clusters (MoSin : n~10) on solid substrates. The MoSi10 films had Si networks similar to hydrogenated amorphous Si and an optical gap of 1.5 eV. Electron spin resonance signals were not observed in the films indicating that dangling bonds of Si were terminated by Mo atoms. We fabricated thin-film-transistors using the MoSi10 film as a channel material. The electric field effect of the film was clearly observed. This suggests that the density of mid-gap states in the film is low enough for the field effect to occur. INTRODUCTION Carrier transport properties of amorphous semiconductors are sensitive to structural disorders. Hydrogenated amorphous Si (a-Si:H) is useful for a wide range of applications, e.g., thin film transistors (TFT), sensors, and solar cells, however structural disorders are present in the Si sp3 bonding network in a-Si:H [1]. As an alternative way to compose Si-based amorphous semiconductor films with reduced disorders, we have demonstrated the synthesis of amorphous films by using deposition of transition metal (M= Mo, Nb and W) encapsulating Si clusters, MSin, where n=7ā16 on solid substrates [2, 3]. The MSin and the hydrogenated clusters (MSinHx ) were actually synthesized through the reaction between the M vapor and the silane (SiH4) gas [4, -7]. The Si cage structures were observed for the MSin (nĀ10) and stabilized by covalent bonding between the M atoms and the Si cages. We confirmed for M=W by X-ray absorption spectroscopy that the MSin films were actually composed of random arrangement of unit MSin clusters with n= 8 ā 10 [8]. The MSin films have tunable optical gap (Eog) of 0.4 -1.8 eV by changing unit clusters. Raman spectra show that MSin films have amorphous Si (a-Si) networks, but do not contain hydrogen. The M atoms stabilize a-Si networks and terminate the Si dangling bonds due to formation of MSin clusters as unit structures. The aspect of the cluster-assembled material was supported by ab-initio structural modeling of the MSin film [9].While the MSin film has amorphous Si networks, we observed superior properties owing to a reduction in electronic disorder in Si networks as a result of the MSin clusters being used as the buildig blocks. For example,
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