Huijin Li

Temperature dependence of the Raman spectra of single-wall carbon nanotubes

Raman spectra of single-wall carbon nanotubes (SWCNTs) were measured at different temperatures by varying the incident laser power. The elevated temperature of the SWCNTs and multiwall carbon nanotubes (MWCNTs) is confirmed to be due to the presence of impurities, defects, and disorder. The temperature coefficient of the frequency of the C–C stretching mode E2g (GM) and that of the radial breathing mode in the SWCNT were determined to be ∼−0.038 and ∼−0.013 cm−1/K, respectively. It is found that the temperature coefficient of the GM in the SWCNT is larger than that of the MWCNT, highly oriented pyrolytic graphite, and the graphite. This is attributed to the structural characteristic of the SWCNT—a single tubular carbon sheet with smaller diameter.

Raman spectroscopy of nanocrystalline GaN synthesized by arc plasma

We report on a Raman study of nanocrystalline GaN with the wurtzite structure synthesized by arc plasma method. Resonant Raman scattering is observed using 514.5 nm (2.41 eV) laser excitation, which is near the band gap of the “yellow band” (2.2–2.3 eV). Under such near-resonant excitation, new Raman bands unexpected in an ideal wurtzite GaN crystal were found. The transverse optical modes of A1 (531 cm−1) and E1 (560 cm−1), and the nonpolar modes of E2 [567 cm−1 (high) and 143 cm−1 (low)] normally observed in bulk crystals, were recorded and were observed to be resonantly enhanced. Two new bands (680 and 344 cm−1) were assigned to the inactive optical phonon modes B1 (high) and B1 (low), respectively. A broadband centered at 710 cm−1 was attributed to surface modes of the nanocrystals, providing good agreement with the calculated result based on Frohlich theory. As a result of this study, Raman scattering of GaN nanocrystals has been characterized.

Investigation of c-axis-aligned crystalline gadolinium doped aluminum-zinc-oxide films sputtered at room-temperature

The c-axis-aligned crystalline (CAAC) rare earth gadolinium doped aluminum-zinc-oxide (Gd-AZO) thin films sputtered at room temperature are investigated in this work. It is found that the polycrystalline AZO is restructured into CAAC Gd-AZO through gadolinium doping. The X-ray diffraction spectrum and high-resolution transmission electron microscopy images indicate the (002) crystalline orientation of the local Gd-AZO grains. The film-formation mechanism of room-temperature sputtered CAAC Gd-AZO thin films is analyzed. Bottom gate oxide thin film transistors with a Gd-AZO active layer are fabricated by low temperature processes. The devices show preferable electrical properties, such as good I-V characteristics, high uniformity, and excellent bias stress stability.

Effects of channel structure consisting of ZnO/Al2O3 multilayers on thin-film transistors fabricated by atomic layer deposition

By applying a novel active layer comprising ZnO/Al2O3 multilayers, we have successfully fabricated fully transparent high-performance thin-film transistors (TFTs) with a bottom gate structure by atomic layer deposition (ALD) at low temperature. The effects of various ZnO/Al2O3 multilayers were studied to improve the morphological and electrical properties of the devices. We found that the ZnO/Al2O3 multilayers have a significant impact on the performance of the TFTs, and that the TFTs with the ZnO/15-cycle Al2O3/ZnO structure exhibit superior performance with a low threshold voltage (V TH) of 0.9 V, a high saturation mobility (μsat) of 145 cm2 V−1 s−1, a steep subthreshold swing (SS) of 162 mV/decade, and a high I on/I off ratio of 3.15 × 108. The enhanced electrical properties were explained by the improved crystalline nature of the channel layer and the passivation effect of the Al2O3 layer.

Characteristic research of zinc oxide based thin film transistor by ALD technology

Zinc oxide is a well-known wide band gap semiconductor material which can be applied to thin film transistors. Al-doped ZnO (AZO) has a better electrical conductivity than Zinc oxide at the same time with good photoelectric properties. In this paper, the method of atomic layer deposition (ALD) was used to prepare the ZnO and Al:ZnO (AZO) thin films as the active layers on silicon substrates at 100 °C. TEM and SEM were used to compare the characters of these films. While, transfer characteristic was an important basis for measuring the electrical characteristics of the devices with different active layers before and after annealing. The unannealed three-layers AZO-based thin film transistor (TFT) exhibits saturation mobility (µsat) of 5.97cm2V−1S−1, a lower subthreshold swing (SS) of 188mV/decade and a high Ion/Ioff ratio of 1.47 × 108.