Y.X. Zeng

C-Axis oriented crystalline IGZO thin-film transistors by magnetron sputtering

We demonstrate the direct formation of c-axis oriented crystalline IGZO thin films at room temperature by magnetron sputtering. The influence of processing parameters such as oxygen partial pressure, post-annealing temperature and channel thickness on the electrical performance of IGZO films and thin-film transistors (TFTs) was intensively investigated. The as-deposited crystalline IGZO TFTs exhibited a mobility of 4.49 cm2 V−1 s−1 and an on/off ratio of 2.08 × 107. For the annealed device, a high mobility of 10.51 cm2 V−1 s−1, a subthreshold swing of 0.672 V decade−1, a threshold voltage of 0.38 V, as well as an on/off current ratio of ∼108 are achieved with an annealing temperature of 400 °C. These results present a significant step towards the development of high-performance TFTs using oriented crystalline IGZO.

Effect of laser energy on the crystal structure and UV response characteristics of mixed-phase MgZnO thin films deposited by PLD and the fabrication of high signal/noise ratio solar-blind UV detector based on mix-phase MgZnO at lower voltage

MgZnO thin films (with Mg0.4Zn0.6O as the target) were fabricated on fused quartz substrates employing PLD method under different laser energy densities. Cubic structured MgZnO thin films were made at laser energy densities of 20 J cm−2 and 22 J cm−2, whilst the MgZnO thin films were deposited along both cubic and hexagonal structures under higher laser energy density condition. When a higher energy density laser was focused on the MgZnO target during the deposition process, the MgZnO thin film was found to be deposited with a more hexagonal structure and a higher Zn composition. There are two response peaks located in the solar-blind UV and visible-blind UV regions within the UV response spectrum of the mixed-phase MgZnO based detector deposited at laser energy over 24 J cm−2. When the deposition laser energy density increased from 24 to 30 J cm−2, the maximum UV responsivity of the mixed-phase MgZnO-based detector increased from 0.06 A W−1 to 1 A W−1 at 40 V bias voltage, and the visible-blind UV response peak of the mix-phase MgZnO based detector was also higher due to the MgZnO adopting a more hexagonal structure; furthermore, a higher internal gain is obtained, which can be attributed to a higher density of interfaces between the MgZnO grains of different structures in the mixed-phase MgZnO thin film. At 5 V bias voltage, the Ilight(230 nm)/Idark ratio of the UV detector based on mix-phase MgZnO deposited at 24 J cm−2 reached 500, and the Ilight(290 nm)/Idark ratio of the UV detector based on mix-phase MgZnO deposited at 26 J cm−2 reached 1100. Therefore, the UV-detector based on mixed-phase MgZnO thin films is sensitive to solar-blind and visible-blind UV light with strong background noise (on Earth) when the boundaries between (111) an (200) cubic MgZnO makes obvious function in decreasing the dark current of the detectors at lower bias voltage.

Study of H-related defects in Ga-doped ZnO thin films deposited by RF magnetron sputtering in Ar+H2 ambient

Abstract H-related defects have been investigated in Ga-doped ZnO thin films deposited by RF magnetron sputtering at room temperature in Ar+H2 ambient. When the flow ratio of H2/(Ar+H2) increases from 0 to 4%, the resistivity significantly decreases from 1·94 × 10−2 to 5·69 × 10−4 Ω cm. X-ray diffraction and X-ray photoelectron spectroscopy results show that it should not be ascribed to the films’ crystalline quality, the chemical states and substitutional situation of Ga and Zn. It is suggested that there are a large number of acceptors in the films, the major role of H is to passivate the acceptors but H-donors themselves do not play a significant role. These acceptor-like defects are located at grain boundaries (dangling bonds) and in bulk (VZn and/or GaZn–VZn). Post-growth annealing experiment and optical transmittance results exhibit that the passivated acceptors are mainly at grain boundaries rather than in bulk.

Band alignment of ZnO/multilayer MoS2 interface determined by x-ray photoelectron spectroscopy

The energy band alignment between ZnO and multilayer (ML)-MoS2 was characterized using high-resolution x-ray photoelectron spectroscopy. The ZnO film was deposited using an atomic layer deposition tool, and ML-MoS2 was grown by chemical vapor deposition. A valence band offset (VBO) of 3.32 eV and a conduction band offset (CBO) of 1.12 eV were obtained for the ZnO/ML-MoS2 interface without any treatment. With CHF3 plasma treatment, a VBO and a CBO across the ZnO/ML-MoS2 interface were found to be 3.54 eV and 1.34 eV, respectively. With the CHF3 plasma treatment, the band alignment of the ZnO/ML-MoS2 interface has been changed from type II or staggered band alignment to type III or misaligned one, which favors the electron-hole pair separation. The band alignment difference is believed to be dominated by the down-shift in the core level of Zn 2p or the interface dipoles, which is caused by the interfacial layer rich in F.

Effect of O2 partial pressure on optical and electrical properties in Al doped ZnO thin films

Abstract Al doped ZnO (AZO) thin films were synthesised on single crystal silicon and quartz glass substrates by pulsed laser deposition method at different O2 partial pressure. The structure, composition, optical and electrical properties in AZO thin films were investigated. With the increase in O2 partial pressure, the crystalline quality of AZO thin films becomes poor, and the Al content in the films decreases from 6·3 to 4·3%. The former leads to the enhancement of grain boundary scattering, consequently Hall mobility will decrease. While the latter results in the decrease of carrier concentration. As a result, the electrical resistivity in AZO thin films increases from 2·18×10−4 to 13·40×10−4 Ω cm with increasing the O2 partial pressure. The increase of carrier concentration induces the widening of optical band gap due to the Burstein-Moss effect with the increase of O2 partial pressure. For AZO thin films grown by PLD method, the control of O2 partial pressure is a simple method to adjust the electrical resistivity, optical band gap and UV emission in the films.

Effect of oxygen flowrate on optical and electrical properties in Al doped ZnO thin films

Abstract Al doped ZnO (AZO) thin films were synthesised on quartz glass substrates by pulsed laser deposition (PLD) method at different oxygen flowrates. The structure, composition, optical and electrical properties in the films were investigated. The result of surface profiler shows that with the increase in oxygen flowrate from 10 to 70 sccm, the films’ thickness increases from 526 to 706 nm. X-ray diffraction results show that the crystalline quality of the films is improved with the increase in oxygen flowrate; energy dispersive spectroscopy results exhibit that the Al content in the films increases from 4·32 to 6·45% with the increase in oxygen flowrate. The former leads to the weakening of grain boundary scattering; consequently, Hall mobility will increase, and the latter results in the increase in carrier concentration. As a result, the electrical resistivity in AZO films decreases from 10·92×10−3 to 2·34×10−3 Ω cm with increasing the oxygen flowrate. Moreover, the increase in carrier concentration induces the slight widening of optical band gap due to the Burstein–Moss effect with the increase in oxygen flowrate. For AZO thin films grown by the PLD method, the control of oxygen flowrate is a simple method to obtain adjustable electrical resistivity, optical band gap and ultraviolet emission.