Amorphous Silicon Thin-Film Transistors Modified by Doping and Plasma Treatment of the Nitride
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AMORPHOUS SILICON THIN-FILM TRANSISTORS MODIFIED BY DOPING AND PLASMA TREATMENT OF THE NITRIDE NORBERT NICKEL, ROSARI SALEH, WALTHER FUHS AND HELMUT MELL Fachbereich Physik und Wissenschaftliches Zentrum ffir Materialwissenschaften Philipps-Universitit Marburg, Renthof 5, F.R. Germany
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ABSTRACT Thin-film transistors (TFTs) were prepared by the glow-discharge deposition of amorphous silicon nitride (a-SiNx1:H) and amorphous silicon (a-Si:H). The properties of these TFTs were varied in two ways: a) doping of the amorphous silicon film with phosphine or diborane and b) exposure of the a-SiNx:H film to an oxygen plasma prior to the deposition of the a-Si:H layer. The TFTs are characterized by measurements of the transfer characteristic, ISD(VG), and of the effective density of interface states, N.(E), using a transient current spectroscopy (TCS). The dependence of Ni(E) on the Fermi-level position in the a-Si:H film suggests that for EC-EF > 0.6eV this quantity is mainly determined by interface related defect states whereas for EC-EF < 0.6eV it is determined by doping-induced defect states. The exposure to the oxygen plasma results in a reduction of N. in both the upper and lower half of the gap and in an improvement of the characteristic, in particular in p-channel TFTs. These changes are discussed in terms of the chemical-equilibrium or defect-pool concept. INTRODUCTION It is still unclear to what extent the transfer characteristic, ISD(VG), of thin-film transistors made from a-Si:H and a-SiNx:H is determined by bulk states and by interface states [1,2]. The same is also true for the effective density of interface states, Ni(E), which has been determined by a transient current spectroscopy (TCS) [3]. To obtain further experimental information, which may help to answer this question, we have modified the properties of TFT structures in two ways a) doping of the a-Si:H layer with phosphine or diborane, respectively, and b) exposure of the nitride layer to an oxygen plasma prior to the deposition of the amorphous silicon layer. EXPERIMENTAL DETAILS The amorphous thin-film transistors (TFTs) used in this study were prepared by rf glow-discharge decomposition. In all samples the gate insulator was silicon nitride (a-Si1 x NX:H), 0.3pm thick. This film was deposited from a 15:1 gas mixture of ammonia and silane at a substrate temperature of 575K. The a-Si:H film, typically 0.3Arm thick, was deposited from undiluted silane. Phosphorus- and boron-doping was accomplished by premixing the SiHl4 with various volume parts of PH 3 and B 2H 6, respectively, using parameters known to lead to films of high quality (Ts = 520K, pd = 0.1mbar, Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
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P = 5OmWcm- 2). In addition to homogeneously doped TFTs we also prepared a set of channel-doped TFTs: After depositing a 20nm thick doped a-Si:H layer we stopped the deposition and then continued with pure Sill4. For comparison we also prepared an undoped TFT where the deposition of the undoped a-Si:H was interrupted
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