Wenjun Liu

Band alignment of HfO2/multilayer MoS2 interface determined by x-ray photoelectron spectroscopy: Effect of CHF3 treatment

The energy band alignment between HfO2/multilayer (ML)-MoS2 was characterized using high-resolution x-ray photoelectron spectroscopy. The HfO2 was deposited using an atomic layer deposition tool, and ML-MoS2 was grown by chemical vapor deposition. A valence band offset (VBO) of 1.98u2009eV and a conduction band offset (CBO) of 2.72u2009eV were obtained for the HfO2/ML-MoS2 interface without any treatment. With CHF3 plasma treatment, a VBO and a CBO across the HfO2/ML-MoS2 interface were found to be 2.47u2009eV and 2.23u2009eV, respectively. The band alignment difference is believed to be dominated by the down-shift in the core level of Hf 4d and up-shift in the core level of Mo 3d, or the interface dipoles, which caused by the interfacial layer in rich of F.

Influence of ZnO buffer layers on the optoelectronic properties in Ga-doped ZnO thin films prepared by RF magnetron sputtering on PET substrates

The influence of ZnO buffer layers on the optoelectronic properties in Ga-doped ZnO (GZO) thin films deposited on polyethylene terephthalate (PET) substrates by RF magnetron sputtering was investigated. The resistivity in GZO/ZnO bilayer films decreases significantly more than one order of magnitude than that in GZO film without a ZnO buffer layer. X-ray diffraction results show that the significant improvement of electrical properties is not related to the crystalline quality. Based on X-ray photoelectron spectroscopy analysis, it is suggested that the decrease in resistivity after the introduction of ZnO buffer layers is ascribed to the restraint of diffusion of moisture and gas from PET substrates to GZO thin films. The moisture and gas diffused into GZO films will be absorbed on the films’ surface in the form of negatively charged oxygen species acting as acceptors. Fifty nm buffer layer is thick enough to achieve the best effect, with further increase of the ZnO buffer layer thickness, the resistivity in the samples will increase due to the parallel effect of ZnO and GZO layers. The introduction of ZnO buffer layers has no obvious influence on the average transmittance in the visible range which isxa0~90xa0% high for all samples.

Influence of hydrogen annealing on structure and optoelectronic properties in Al doped ZnO thin films

Abstract Al doped ZnO (AZO) thin films are of significant interest for flat panel displays, solar cells, etc. In this regard, AZO films were prepared in Ar atmosphere by radio frequency magnetron sputtering with an AZO (2 wt-%Al2O3) ceramic target at room temperature. To investigate the influence of hydrogen related defects on the structure and optoelectronic properties in AZO films, the prepared films were annealed in Ar+H2 ambient at different temperatures (100–500°C). The results show that the films’ crystallinity becomes better and the resistivity decreases with the increase in annealing temperature. The lowest resistivity of 2·79×10−3 Ω cm is obtained after 500°C hydrogen annealing, which decreases by about four orders of magnitude compared to the resistivity of as deposited film. The improvement of electrical properties is attributed to the formation of H related defects and desorption of weakly bonded oxygen species near grain boundaries in AZO films after H2 annealing treatment, and the corresponding physical mechanism was discussed. It is proved that hydrogen annealing is an effective method for the improvement of electrical properties in AZO thin films. The optical bandgap energy of the films obviously increases with the increase in annealing temperature due to Burstein–Moss effect.

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.

UV absorption characteristics and element composition of (200) and (111) orientation cubic MgZnO thin films deposited at different temperature by PLD method

Cubic structure MgZnO thin films were made on fused quartz substrate at different temperature by PLD method, the preferred orientation of MgZnO thin films changed from (200) to (111) when growth temperature increased because of different substrate surface situation. MgZnO thin film deposited more along (200) orientation at 300xa0°C with narrower band gap because it contain more Zn atoms in MgZnO lattice than the ones deposited at other temperature, and the UV absorption edge of which is located in longer wavelength position. MgZnO thin film deposited more along (111) orientation at 700xa0°C with better crystal quality than the ones deposited at other temperature, but the band gap of this sample is wider than the other ones though it also contain more Zn atoms in MgZnO lattice as the one deposited at 300xa0°C, which is reason from the function of much lower grain boundary density in this MgZnO thin film.

Influence of H2 introduction on properties in Al-doped ZnO thin films prepared by RF magnetron sputtering at room temperature

Al-doped ZnO (AZO) thin films were prepared by RF magnetron sputtering on quartz substrates at room temperature in different Arxa0+xa0H2 ambient. The influence of H2 flow ratio on the structure and optoelectronic properties in AZO films was investigated. The prepared films are hexagonal wurtzite structure with c-axis preferred orientation, and the intensity of (002) peak decreases with the increase of H2 flow ratio. The resistivity significantly decreases with increasing the H2 flow ratio to 1.0xa0% by almost four orders of magnitude. X-Ray photoelectron spectroscopy and X-ray diffraction measurements exhibit that the effectiveness of Al doping in the substitutional positions is not influenced by H2 addition. We suggest that there exist a large number of acceptors in the films, the introduced H2 will passivate the acceptors, which raises both carrier concentration and Hall mobility. The increase of carrier concentration consequently induces the blue shift of optical absorption edge according to the Burstein-Moss effect.

Growth and Characteristics of High Quality (200) and (111) Orientations Cubic Structure MgZnO Thin Films by Pulse Laser Deposition (PLD) Method

High quality (200) and (111) orientations cubic MgZnO thin films were made on (200) and (111) orientations MgO substrates separately under different condition with higher and lower migration energy of reactive atoms separately. The crystal quality of (111) orientation MgZnO thin film is higher than (200) one because of the stronger horizontal migration of atoms on (111) surface under high temperature condition, the surface of (200) orientation MgZnO thin film is smoother than (111) orientation one because of lower vertical growth speed of (200) MgZnO grains. The band gap of (111) orientation MgZnO thin film is smaller than (200) one because of more Zn atoms in (111) orientation MgZnO lattice than that in (200) ones. This paper gives an effective method to improve crystal quality of different orientation MgZnO thin film under different condition, which is meaningful in application of cubic MgZnO in different areas.

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 3.32u2009eV and a conduction band offset (CBO) of 1.12u2009eV 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 3.54u2009eV and 1.34u2009eV, 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.