P. J. Cao

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.

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.

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.