Preparation Of Non-polar ZnO-based Films And Investigation On ZnO/ZnMgO Hetero-structures

ZnO has been considered as a promising material for the potential applications in optoelectric devices owing to its unique properties, such as wide direct bad gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature. By taking advantage of band gap engineering to create barrier layers in heterostructures, multiple quantum wells (MQWs) are of important to optimize the performance of ZnO-based optoelectronic devices. Unfortunately, with polar c-axis as the natural growth direction, ZnO usually suffers from the internal electric fields induced by spontaneous polarization and piezoelectric polarization. The internal electric fields will decrease the internal quantum efficiency and increase the emission wavelength of MQWs, which will make negative effects on the performance of the polar ZnO-based devices. In order to eliminate the polarization effects, we focus on the research of non-polar a-plane ZnO-based films and ZnO/Zn1-xMgxO hetero-structures. Our work mainly includes the fabrication of high quality and band gap tunable a-plane Zn1-xMgxO films, the measurement of band offsets in a-plane ZnO/Zn1-xMgxO heterojunctions, the investigation of a-plane ZnO/Zn1-xMgxO MQWs and the improvement of a-plane ZnO crystal quality by using GaN buffer. The details are as follows:1. After the optimization of growth parameters, a series of non-polar a-plane Zn1-xMgxO alloy films with different Mg contents were grown on r-sapphire substrates by plasma-assisted molecular beam epitaxy (P-MBE). The quality of the films is evidenced by X-ray diffraction (XRD) rocking curves full-width at half-maximum (FWHM) of 1300-2000 arcsec for the (1120) reflection and (1011) reflection. All the films exhibit atomically smooth surface with the root mean square (RMS) surface roughness less than 1 nm. Alloying with Mg is found to widen the band gap energy of the ZnO. Room-temperature photoluminescence (PL) peak shifts monotonously from 3.29 eV to 3.63 eV as Mg content increases from 0 to 16%. Non-polar Zn1-xMgxO thin film is much easier to obtain pure a-plane single crystal orientation when Mg content is high.2. The band offsets of the non-polar a-plane ZnO/Zn1-xMgxO heterojunctions with different Mg contents were measured by X-ray photoelectron spectroscopy (XPS). All the non-polar heterojunctions are revealed to have a type-I band alignment and the widening of band gap is mainly attributed to conduction band offset. Compared to the polar ZnO/Zn1-xMgxO heterojunction, non-polar heterojunctions have smaller △EV and larger △EC, which can be utilized to facilitate p-type doping.3. Non-polar a-plane ZnO/Zn1-xMgxO MQWs were prepared on r-sapphire substrates by P-MBE. Pure a-plane single crystal orientation, smooth surface and sharp interfaces have been achieved. Quantum confinement effect is observed at low temperature as well as at room temperature. Temperature-dependent PL spectra reveal the exciton localization effect in MQWs, which results from the "potential minima" in well layers induced by interface fluctuations. In addition, the properties of a-plane MQWs with different well widths and barrier heights have been studied. As well width decreases or Mg content in the barriers increases, dominant PL emission exhibits monotonous blue shift due to quantum confinement effect. Compared to the polar ZnO/Zn1-xMgxO MQWs, non-polar MQWs can effectively avoid the quantum confined Stark effect. Moreover, increasing the Mg content in the barriers can improve carrier confinement efficiency of MQWs.4. Non-polar a-plane ZnO films were grown on r-sapphire substrates using GaN buffer grown in situ by the combination of L-MBE and P-MBE. The film has a mobility of 39.8 cm2V-1s-1 and the FWHM of (1010) ω-rocking curve is only 159 arc sec, indicating the crystal quality is improved significantly by the growth of GaN buffer. Moreover, the strong UV emission and negligible deep level emission in PL spectrum imply that the ZnO film is of high optical quality.