Tomotaka Matsumoto

Hydrogen effusion from hydrogenated amorphous silicon caused by the deposition of a silicon nitride overlayer

The effect of silicon nitride (SiN) deposition on hydrogenated amorphous silicon (a‐Si:H) has been investigated to find the origin of the difference of a‐Si:H/SiN interface properties caused by the order of deposition. Sheet conductance of the on‐state in inverted staggered (a‐Si:H on SiN) thin‐film transistors (TFTs) increases gradually with the substrate temperature (Tsub) of SiN, but decreases rapidly with the Tsub of SiN in staggered TFTs (SiN on a‐Si:H). Photoluminescence experiments indicated that the degradation in staggered TFTs was due to the creation of defects in a‐Si:H by the deposition of the SiN overlayer. It was shown by Fourier transform infrared attenuated total reflection that the origin of the defects was hydrogen effusion from a‐Si:H.

Study of silicon‐hydrogen bonds at an amorphous silicon/silicon nitride interface using infrared attenuated total reflection spectroscopy

Silicon‐hydrogen bonding structures at a hydrogenated amorphous silicon (a‐Si:H)/silicon nitride (SiN) interface have been investigated using Fourier transform infrared attenuated total reflection (FTIR‐ATR). Depositing a SiN overlayer markedly decreased the higher hydrides, which consist of SiHn (n=2,3) bonds, on the a‐Si:H surface. The low density of higher hydrides at the resulting SiN‐on‐a‐Si:H interface may be due to plasma‐enhanced extraction or a transfer of the growing surface. By contrast, at an a‐Si:H‐on‐SiN interface, the higher hydrides density is about 8.1×1014 cm−2. We believe this large amount of hydrogen at the a‐Si:H‐on‐SiN interface relaxes strained bonds at the interface. In both the SiN‐on‐a‐Si:H interface and the a‐Si:H‐on‐SiN interface, hydrogen is implanted in the underlayer during the deposition of the overlayer. Our results indicate the structure of underlayer near the interface is strongly affected by the deposition of the overlayer.

Crystallization at initial stage of low‐temperature polycrystalline silicon growth using ZnS buffer layer with 〈111〉 preferred orientation

We developed a low‐temperature growth technique for polycrystalline silicon (poly‐Si). When Si is deposited on glass substrates at 450 °C, it crystallizes as thickness increases, but 10‐nm‐thick layers of Si are mainly amorphous. Use of a ZnS buffer layer with 〈111〉 preferred orientation facilities crystallization of Si during the initial growth stages. The preferred orientation of poly‐Si on glass substrates is 〈110〉, while that of poly‐Si on the ZnS buffer layer is 〈111〉. This is probably due to local epitaxial growth on polycrystalline ZnS grains with 〈111〉 preferred orientation. Raman spectroscopy showed that the ZnS buffer layer significantly improved the crystallinity of 25‐nm‐thick Si layers.

Threshold voltage shift of amorphous silicon thin‐film transistors by step doping

The threshold voltage (VT) shift of hydrogenated amorphous silicon thin‐film transistors (a‐Si:H TFTs) by boron doping has been investigated. In TFTs with a uniformly doped structure (SiN/B‐doped a‐Si:H), VT shifts to a positive voltage by boron doping, while the field‐effect mobility decreases markedly. By using a step‐doped structure (SiN/undoped a‐Si:H/B‐doped a‐Si:H), the degradation of the field‐effect mobility by boron doping becomes less than that of a uniformly doped TFT with the same VT shift, and a VT shift of 3.5 V was obtained without degradation of the field‐effect mobility.

Step doping in hydrogenated amorphous silicon thin‐film transistors for threshold voltage shifts

We have developed a way to control the threshold voltage (VT ) in hydrogenated amorphous silicon thin‐film transistors (a‐Si:H TFTs) by boron doping. The field‐effect mobility of uniformly boron‐doped TFTs decreases rapidly with increased doping ratios. To keep the field‐effect mobility from decreasing, we developed step‐doped TFTs (SiN/undoped a‐Si:H/boron‐doped a‐Si:H). With this structure, the decrease in the field‐effect mobility by boron doping was lessened. The transfer characteristics calculated using a one‐dimensional model were in good agreement with measurements below a doping ratio of 80 ppm. The difference between measurements and calculations for higher doping ratio was decreased using a new step‐doped structure with the boron‐doped layer under n+a‐Si:H removed.