Preparation Of Non-polar ZnO-based Film And Investigation On Na Doping And ZnMgO/ZnO Multiple Quantum Wells

Zinc Oxide (ZnO), as a novel II-VI compound semiconductor with direct band gap of3.37eV and large excition binding energy of60meV, has been considered one of the most promising materials for short wavelength optoeletronic devices such as light-emitting diodes (LEDs) and laser diodes (LDs). However, with polar c-axis as the natural growth direction, ZnO suffers build-in electric fields along [0001] direction due to strong spontaneous and piezoelectric polarization, which will cause a decrease in the internal quantum efficiency of LEDs. The way to remove the build-in electric fields is to grow ZnO film along the direction perpendicular to the c-axis, called "non-polar" ZnO film. In this work, we performed systematic study of growth and properties of non-poplar ZnO films, alloys and multiple quantum wells (MQWs). We investigate the growth and characterization of Na-doped non-poplar ZnO films in attempt to obtain p-type non-poplar ZnO films. The main work included:1. Non-polar m-plane ZnO films were deposited on m-plane sapphire substrates by pulsed laser deposition. The effects of growth temperature and oxygen pressure on the structural, electrical and optical properties were systematic studied. All films we prepared were grown along m-plane direction without any polar and semipolar components. We also prepared a-plane ZnO films on r-plane sapphire substrates. The full width at half maximum (FWHM) and surface roughness results were0.47℃and1.7mm respectively, indicating better crysatal quality than the m-plane ZnO film grown on m-plane sapphire substrate.2. Non-polar Na-doped ZnO films were deposited on r-plane sapphire substrates by pulsed laser deposition, and the p-type conduction non-polar ZnO films were obtained. The effects of growth temperature and oxygen pressure and Na doping concentration on the electrical properties of the films were systematic investigated. The minimum resistivity was102Ωcm, with hole mobililty of1.41cm2/Vs and hole concentration of5.19x1016cm-3, We prepared ZnO-based p-n homojunction grown along a-plane direction. The obvious rectify effect of the p-n homojunction detected by I-V curve confirms the p-type conductivity of the Na-doped non-polar a-plane ZnO film.3. The band gap modulation of non-polar ZnO film was achieved by preparing high quality non-polar ZnMgO and ZnCdO films. The effects of growth temperature and oxygen pressure on the structural and properties of the non-polar ZnMgO films were investigated. The minimum FWHM of0.53°and surface roughness of1.54nm for the non-polar ZnMgO films was obtained at550℃and1Pa, corresponding to the resistivity was1.51Ωcm, with electron mobililty of7.74cm2/Vs and carrier concentration of1.88x1018cm-3. By changing the oxygen pressure, the maximum Cd content of13%in the non-polar ZnCdO film can be achieved without any second phases and the band gap can be modulated from3.01to3.30eV. The film absorbed13%Cd will mainly grow along [0001] direction. By introducing7.2%Cd, the film was unique non-polar a-plane direction.4. A series of10-period ZnO/ZnMgO multiple quantum wells (MQWs) with well widths varying from2.2to5.6nm have been grown on r-plane sapphire substrates by pulsed laser deposition. XRD reveal the MQWs grow along a-plane (1120) direction. A good periodic structure with clear interfaces was observed by transmission electron microscopy (TEM). The systematic blueshift for the emission energy in the MQWs behavior was observed at room and low temperature. The polar ZnO/ZnMgO MQWs fabricated on c-plane sapphire substrates with the same well width were also discussed. Low temperature photoluminescence (PL) is investigated to reveal that the non-polar MQWs exhibit confinement but no indication of quantum confined Stark (QCS) effect when well width is larger than5nm, contrary to what is observed in polar MQWs, and the non-polar MQWs have higher injection effect.