X. H. Pan

Characterization of amorphous Si-Zn-Sn-O thin films and applications in thin-film transistors

Amorphous silicon-zinc-tin-oxide (α-SZTO) thin films were prepared, and their properties were investigated physically and electrically, with an emphasis on the Si effects. An appropriate Si content in the matrix can not only achieve stable and dense films, but also suppress the formation of oxygen vacancies efficiently, due to its high oxygen bonding ability. Thin film transistors (TFTs) with α-SZTO active channel layers exhibited a field-effect mobility of around 1u2009cm2V−1s−1, an on/off current ratio of 107, and a subthreshold swing of 0.863u2009V/decade with a good long-term stability. The α-SZTO TFT is a potential candidate for electronic applications.

Instability Induced by Ultraviolet Light in ZnO Thin-Film Transistors

We investigate the electrical properties of ZnO thin-film transistors (TFTs) under the ultraviolet (UV) light illumination to clarify the device reliability. The ON/OFF current ratio of ZnO TFTs shows an evident reduction due to the creation of UV photo-induced current. This behavior degradation becomes severe for devices exposed to a short-wavelength UV light. The persistent photoconductivity is observed in ZnO TFTs under the UV illumination with a much slower decay for the ON-state than the OFF-state, which is attributed to the reduced recombination under a positive gate bias. The UV light instabilities of ZnO TFTs can be explained on the basis of UV photoexcited oxygen vacancies. A feasible mechanism model is proposed in this paper. These observations and analytical model not only provide useful physical insight into the device behaviors, but also offer basic design guideline for oxide TFTs.

Water assisted oxygen absorption on the instability of amorphous InAlZnO thin-film transistors

Stability of amorphous InAlZnO thin-film transistors was investigated under vacuum conditions and an oxygen atmosphere with different rates of relative humidity. The mechanism of water assisted oxygen absorption on the back channel is proposed to explain the phenomenon that wet oxygen leads to a larger positive threshold voltage (Vth) shift than dry oxygen.

The role of band alignment in p-type conductivity of Na-doped ZnMgO: Polar versus non-polar

We investigate the electrical properties of polar and non-polar ZnMgO:Na films that have been fabricated on c-plane and r-plane sapphire substrates using intervened ZnO layers (10–30u2009nm thick) by pulsed laser deposition. Hall-effect measurements indicate that the a-plane ZnMgO:Na film exhibits p-type conductivity with a carrier concentration of about 3.5u2009×u20091016u2009cm−3, while the polar film shows a compensatory conductivity. Meanwhile, the dependence of the band alignment on the orientation of the ZnMgO/ZnO heterojunctions has been investigated using photoelectron spectroscopy. The heterojunctions form in the type-I straddling alignment with valence band offsets of 0.07 (0.02) eV for the (non-)polar heterojunction. The difference in valence band offsets is primarily attributed to the spontaneous polarization effect. We propose that the smaller valence band offsets and larger conduction band offsets would reduce the NaZn acceptor level and enhance the relative intrinsic donor levels. Such effects consequently...

Structure and optical properties of a-plane ZnO/Zn0.9Mg0.1O multiple quantum wells grown on r-plane sapphire substrates by pulsed laser deposition

A series of 10-period ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with well widths varying from 2.2 to 5.6u2009nm have been grown on r-plane sapphire substrates by pulsed laser deposition. A good periodic structure with clear interfaces was observed by transmission electron microscopy. In a-plane ZnO/Zn0.9Mg0.1O MQWs, the luminescence was dominated by localized exciton emissions at low temperatures, while the free exciton (FE) transition was dominating emissions at temperatures above 100u2009K. The thermal quenching behavior of exciton emission has been analyzed. A rate equation assuming two nonradiative recombination channels is used to describe the quenching of the transitions observed. Moreover, the FE emission energy in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum confinement effect.

Ferromagnetic enhancement and magnetic anisotropy in nonpolar-oriented (Mn, Na)-codoped ZnO thin films

High-resistive Zn0.95Mn0.05O and weak p-type nonpolar-oriented Zn0.94Mn0.05Na0.01O thins films were grown on quartz by pulsed laser deposition. Both samples exhibit room temperature ferromagnetism while with Mn-Na codoping, the saturation magnetic moment is greatly enhanced. It is revealed that the doped Mn impurities are substitutionally incorporated into the ZnO host. Magnetic anisotropy was also observed in the Zn0.94Mn0.05Na0.01O film, which is the indication for intrinsic ferromagnetism. The first-principles calculations reveal that codoping of Na in Zn0.94Mn0.05Na0.01O changes the antiferromagnetic interaction to ferromagnetic due to the hybridization between spin-split delocalized Mn 3d and shallow acceptor states of Na 2p, thereby enhancing the ferromagnetism.

Orientation dependent band alignment for p-NiO/n-ZnO heterojunctions

Nonpolor a-plane and polar c-plane ZnO thin films were prepared on r-plane sapphire and quartz substrates, respectively. The electronic structure of the interface between subsequently fabricated NiO/ZnO heterojunctions has been investigated by x-ray photoelectron spectroscopy measurements and the band offsets are determined together with information yielded from UV-vis transition spectra. It is found that a type-II band alignment forms at the interface for both the samples. The revealed ZnO-orientation dependent band offsets are analyzed and are attributed mainly due to the variations in internal electric field arose from spontaneous polarization effect. The accurate determination of the band alignment is important for the design and application of NiO/ZnO based hybrid devices.

Non-polar p-type Zn0.94Mn0.05Na0.01O texture: Growth mechanism and codoping effect

The microstructure and crystal orientations of polycrystalline films crucially affect the properties and performance of the films. Controlling preferred orientations (PO) and related film morphology are necessary to obtain the desirable properties. In this paper, we demonstrate a rational and effective route toward the realization of non-polar p-type ZnO thin film with surface texture on quartz substrate through Mn-Na codoping. It is uncovered experimentally and theoretically that mono-doping of Mn creates opportunity to realize PO from polar (c-axis) to non-polar ((101¯0), (101¯1), and (112¯0)) changing. With Mn-Na codoping, an acute modulation of the growth behavior and electrical conductivity of the film have been revealed, leading to weak p-type non-polar Zn0.94Mn0.05Na0.01O (ZMNO) texture. The dominant mechanism for the non-polar self-texture in the current paper is deliberately elucidated as resulting from the interplane surface diffusion with the cooperative effect of impurity dopants. The ZMNO fil...

Stability of amorphous InAlZnO thin-film transistors

The stability of thin-film transistors (TFTs) with amorphous InAlZnO (a-IAZO) thin films as the channel layers was investigated. The devices annealed at 300u2009°C had a large threshold voltage (Vth) shift under gate voltage sweep, while the devices annealed at 400u2009°C were quite stable. The S value of the transfer characteristic curve was effectively reduced after 400u2009°C annealing as compared to 300u2009°C annealing. X-ray photo-electron spectroscopy results also showed oxygen deficiencies decreased as the annealing temperature increased. The improvement of TFTs stability might attribute to the reduction of trap states related to oxygen deficiencies. The 400u2009°C annealed a-IAZO TFTs exhibited small positive shift of threshold voltages under bias stress conditions, suggesting the a-IAZO might be a promising candidate for application in TFTs.

The role of beryllium in the band structure of MgZnO: Lifting the valence band maximum

We investigate the effect of Be on the valence band maximum (VBM) of MgZnO by measuring the band offsets of MgxZn1−xO/BexMgyZn1−x−yO heterojunctions using X-ray photoelectron spectroscopy measurements. MgxZn1−xO and BexMgyZn1−x−yO films have been grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The valence band offset ( ΔEV) of Mg0.15Zn0.85O ( Egu2009=u20093.62u2009eV)/Be0.005Mg0.19Zn0.805O ( Egu2009=u20093.73u2009eV) heterojunction is 0.01u2009eV and Be0.005Mg0.19Zn0.805O has a lower VBM. The increased Mg composition is the main factor for the reduction of VBM. The VBM of MgxZn1−xO is lower by 0.03u2009eV with the enlargement of Eg from 3.62u2009eV to 3.73u2009eV by increasing Mg composition. Considering the effect of increased Mg composition, it is concluded that the little amount of Be makes the VBM go up by 0.02u2009eV when the Eg of the alloy is 3.73u2009eV. The ΔEV of Mg0.11Zn0.89O ( Egu2009=u20093.56u2009eV)/Be0.007Mg0.12Zn0.873O ( Egu2009=u20093.56u2009eV) heterojunction is calculated to be 0.03u2009eV and Be0.007Mg0.12Zn0.873O has a higher ...