ong Qi

Influence of different dielectrics on the first layer grain sizes and its effect on the mobility of pentacene-based thin-film transistors

Pentacene thin-film transistors using different dielectrics including thermal oxide SiO2 and a series of bilayered dielectrics that are polymethyl methacrylate/SiO2, polycarbonate/SiO2, and polystyrene/SiO2 were fabricated. Atomic force microscopic measurements indicated that grain sizes of the submonolayer films were drastically different with the various dielectrics. Dielectrics with lower surface energy lead to larger grain sizes at the first monolayer and consequently higher mobility of the fabricated transistors. The correlation between the mobility and the grain sizes at the first monolayer of the pentacene films could be explained by utilizing a microscopic mobility model relating to the grains and grain boundaries.

Direct observation and measurement of mobile charge carriers in a monolayer organic semiconductor on a dielectric substrate.

We report the electrical characterization of a single layer of an organic semiconductor grown on a dielectric surface. The dynamic response of the charge carriers in the monolayer film of pentacene was characterized through the electrostatic interactions between an electric force microscope (EFM) probe and pentacene islands of various sizes. These islands were formed in situ by segmenting a coalesced pentacene monolayer into separated regions. The size-dependent dielectric responses of the pentacene islands suggest that mobile charges exist in the organic monolayer. Local capacitance spectroscopy revealed that the charge carriers in the p-type pentacene monolayer could be depleted at high bias voltages, enabling a further determination of the charge-carrier concentration in the organic semiconductor ultrathin film.

Effect of poling process on resistive switching in Au/BiFeO3/SrRuO3 structures

We studied the resistive switching of Au/BiFeO3/SrRuO3 structures. Their current–voltage characteristics complied with space-charge-limited (SCL) conduction. A full cycle of the poling process enlarged the I–V hysteresis and increased the transition voltage from Ohmic to SCL for an I–V sweeping bias less than the coercive voltage. A cycle of poling could increase the resistance switching ratio under low bias by 20 times over that of a virgin sample without poling. Measurements of thermally stimulated current showed a difference between the sample in the pristine state and that under the polarization state, indicating a relationship between the trap filling status and the resistive switching behavior.

Enhancement of Electrical Conductance for Pentacene Thin Film Transistor by Controlling an Initial Layer-by-Layer Growth Mode Directly on SiO2 Insulator

Initial nucleation and growth of pentacene films on various pre-cleaning treated SiO2 gate insulators were systematically examined by atomic force microscope. The performance of fabricated pentacene thin film transistor devices was found to be highly related to the initial film growth modes. In contrast to the film in the three-dimensional island-like growth mode on SiO2 under an organic cleaning process, a layer-by-layer initial growth mode occurred on the SiO2 insulator cleaned with ammonia solution which has shown much improved electrical properties of the thin film transistors. Field effect mobility of the thin film transistor devices could be achieved as high as 1.0 cm2 V-1 s-1 on the bared SiO2/Si substrate and the on/off ratio was over 106. The enhanced electrical conductance was further confirmed by an electrostatic force microscopic observation of quantized electrical potentials via charge-injection to the submonolayer pentacene islands with layer-by-layer growth mode.

An exploration of slab-coupled semiconductor lasers

To obtain a high-power and efficient single-mode laser, a new laser called the slab coupled optical waveguide laser (SCOWL) has been developed. We have simulated its structure and grown the chip with this structure by low-pressure metal organic chemical vapor deposition. We have also produced the broad-area SCOWL and compared it with the traditional structure laser in terms of some performances. This work lays the foundation for further research of ridged lasers with the same structure.

Asymmetric wide-coupled waveguide 980nm laser diode with high power and low vertical divergence angle

A novel asymmetric wide-coupled waveguide structure is put forward. We optimize the thickness of N-type waveguide and cladding through theoretical calculation of this structure, and adopt LP-MOCVD to grow the designed semiconductor lasers. The fabricated 980nm laser with 1200μm cavity length has a threshold current of 590mA, slope efficiency of 0.96W/A. When the operating current is 2.6A, the output power is 2000mW and the far field divergence angle is 16.1° (vertical) by 10.2° (horizontal). Experimental results indicate that the novel asymmetric wide-coupled waveguide structure can achieve high power output, effectively reduce the vertical far field divergence angle and improve the beam quality of the device.