Structural and Electrical Characterization of Undoped Poly-Si Oxides
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We evaluated structural and electrical characteristics of undoped poly-Si oxide films. Poly-Si films made by solid phase crystallization at 600-900"C from undoped amorphous Si films were oxidized to form oxide layers of 140nm thickness. We observed protuberances on the surface of poly-Si layers after oxidation. Poly-Si oxide layers also generated protuberances above the protuberances of poly-Si films. The number of protuberances per unit area is larger inthe case of high temperature crystallization. The measurement of current through the poly-Si oxide films shows that the conductivity of poly-Si oxide films depends on crystallization temperature of poly-Si films in the case of positive gate bias. When the gate is biased negatively, current through the poly-Si oxide films remained almost constant regardless of crystallization temperature. We find that poly-Si crystallized at lower temperatures offers polySi oxide films of lower leakage current in the case of electron injection from undoped poly-Si layers. The lower leakage current is due to highness of energy barrier for electron at undoped poly-Si/poly-Si oxide interface. INTRODUCTION
Thin film transistors(TFTs) using undoped polycrystalline Si(poly-Si) have been applied to Liquid Crystal Displays in the aim to reduce pixel size and integrate driving circuits on the TFT array substrate[l]. For the gate insulator of poly-Si TFTs, oxides of undoped polySi offer low interface state films. Thus far, there have been few reports discussing the
conductivity of undoped poly-Si oxide in detail yet. It is important to study the mechanism of current through undoped poly-Si oxide film from the viewpoints of high voltage driving of
liquid crystals and reducing oxide thickness which results inlarger on-current of TFT. Oxide films of doped poly-Si are being applied to some VLSI devices and their structural and electrical characteristics have been investigated. The fact that doped poly-Si oxide
films provide higher leakage current than single crystal Si oxide films is attributed to the nonuniformity in poly-Si oxide film thickness and to the asperities at the poly-Si/oxide interface [2,3]. The structural characteristics of undoped poly-Si oxides have been also investigated and roughness on the poly-Si surface and oxide surfaces were reported, but not quantitatively[4,5]. In this paper, we performed structural and electrical characterization of undoped poly-Si oxides quantitatively. EXPERIMENTAL Undoped poly-Si films of thickness 140nm were prepared by solid phase crystallization (SPC) of amorphous Si films on quartz substrates. The amorphous Si films were deposited in a conventional Low Pressure Chemical Vapor Deposition(LPCVD) reactor using Si 2 H6 gas at
485°C. The amorphous Si films were annealed at 600, 620, 650 and 900"C in nitrogen atmosphere and then they were oxidized at 1100*C in dry oxygen atmosphere to form 140nm thick poly-Si oxide films. To observe the structure of undoped poly-Si/poly-Si oxide interface, we removed oxide using buffered HF. Grain size of poly-Si
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