Jian Z. Chen

Two dimensional electron gases in polycrystalline MgZnO/ZnO heterostructures grown by rf-sputtering process

This paper reports the formation of two-dimensional electron gas (2DEG) in rf-sputtered defective polycrystalline MgZnO/ZnO heterostructure via the screening of grain boundary potential by polarization-induced charges. As the MgZnO thickness increases, the sheet resistance reduces rapidly and then saturates. The enhancement of the interfacial polarization effect becomes stronger, corresponding to a larger amount of resistance reduction, when the Mg content in the cap layer increases. Monte Carlo method by including grain boundary scattering effect as well as 2D finite-element-method Poisson and drift-diffusion solver is applied to analyze the polycrystalline heterostructure. The experimental and Monte Carlo simulation results show good agreement. From low temperature Hall measurement, the carrier density and mobility are both independent of temperature, indicating the formation of 2DEG with roughness scattering at the MgZnO/ZnO interface.

Mobility Enhancement of Polycrystalline MgZnO/ZnO Thin Film Layers With Modulation Doping and Polarization Effects

ZnO has shown great promise for application in optoelectronic devices, in which the modulation of conductivity is crucial to device performance. In this paper, we have applied the Monte Carlo method to analyze the mobility of single-crystalline and polycrystalline MgZnO/ZnO heterostructure thin film layers. The effects of grain boundary scattering and ionized impurity scattering, as well as phonon scattering, are considered. Our studies show that, with careful design of modulation doping that considers the effects of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed to improve the mobility of the ZnO layer by at least one order of magnitude. Simulation results are also confirmed by our experimental work, which shows that the polarization effect does play an important role in attracting carriers and increasing the mobility.

Negative bias temperature instability of Rf-sputtered Mg0.05Zn0.95O thin film transistors with MgO gate dielectrics

This study investigated the negative bias temperature instability of rf-sputtered bottom-gate Mg0.05Zn0.95O active layer thin-film transistors (TFTs) annealed at 200 °C for 5 h and 350 °C for 30 min. The TFT devices initially exhibited similar electrical characteristics. When TFTs were stressed at room temperature, the TFTs (in both annealing conditions) showed typical negative shift of threshold voltage (ΔVth). When TFTs were stressed at 80 °C, a turnaround of ΔVth appeared for the 200 °C-annealed TFTs; the Vth shifted toward a negative direction initially and consequently turned positive. The turnaround phenomenon should result from the competing charge trapping and defect creation. On the other hand, the 350 °C-annealed TFTs still showed typical negative shift of threshold voltage when stressed at 80 °C.

The Electromechanical Characteristics of ZnO Grown on Poly(ethylene terephthalate) Substrates

We investigate the electromechanical properties of ZnO grown on polyethylene terephthalatePET and compare the results with those obtained from the counterparts on glass substrates. The elastic moduli of ZnO on PET measured by a microstretcher are very close to those of ZnO grown on glass substrates measured by nanoindentation: The moduli decrease then increase as the O2/Ar ratio increases. Films start to peel off from the plastic substrates when oxygen content reaches 50%. The in situ measurement of the ZnO resistance under uniaxial tensile stretching is influenced by the induced piezoelectric voltage, leading to a reduction in the electrical resistance for highly resistive ZnO films. The trend of the preferred orientation in relation to the oxygen content of the sputtering atmosphere is the same for ZnO films grown on either PET or glass substrates. The optical bandgap Eg of the ZnO films on both substrates exhibits crystal orientation dependence, varying from 3.18 eV with 002 preferred orientation to 3.25 eV, with a large number of 100 and 101 oriented crystals.

Electromechanical Stability of Flexible Nanocrystalline-Silicon Thin-Film Transistors

We have demonstrated inverted staggered bottom-gate back-channel-passivated hydrogenated nanocrystalline-silicon thin-film transistors (TFTs) on colorless polyimide foil substrates. Their electrical performance and stability have been investigated under mechanical flexing. The electron field-effect mobilities and threshold voltages of these TFTs increased as the applied tensile strain increased, but their electrical stability deteriorated under mechanical strain.

Mobility Study of Polycrystalline MgZnO/ZnO Thin Film Layers with Monte Carlo Method

The study of transparent conducting oxide (TCO) has become an important area due to the applications of lighting and display technology. Therefore, high mobility and conductivity TCO materials would be a key issue to the industry. In this paper, we have applied the Monte Carlo method to analyze the mobility of single and poly-crystalline MgZnO/ZnO thin film layer. The effects of grain boundary scattering, ionized impurity scattering as well as phonon scattering have been included in our program. The grain boundary potential size and carrier screening effect has been analyzed with our developed 2D Poisson and drift-diffusion solver. With a careful design of modulation doping and including the effect of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed and thus the mobility of the ZnO layer can be improved.

Temperature and humidity effects on the stability of on-plastic a-Si:H thin film transistors with various conduction channel layer thicknesses

Stability is an important issue for the application of TFTs. In this paper, we present the effects of humidity and temperature on the stability of inverted-staggered back-channel-cut aSi:H TFTs with various conduction channel layer thickness. We evaluated the stability of onplastic TFTs of different conduction layer thicknesses made at a process temperature of 150oC on 51-μm thick Kapton polyimide foil substrates.. With conduction channel layer thickness of 50nm, humidity reversibly varies the characteristics of TFTs, but TFTs of conduction layer thickness greater than 100 nm is pretty immune to the humidity change. The temperature dependent stability and characteristics of TFTs were analyzed from 20C to 60C. Rising temperature from 20C to 56C, the threshold voltage (Vt) drops about 2 volts; on-off current ratio decreases by one order of magnitude mainly due to thermally excited carriers in the off-region.