Fabrication Of N-ZnO/i-ZnO/p-GaN Heterostructure LEDs By PLD

In this dissertation, we focused on the effects of semi-insulating i-ZnO layer on the electroluminescence of p-GaN/i-ZnO/n-ZnO heterostructural light emitting diodes (LEDs) fabricated by pulsed laser deposition (PLD) technique. Based on this, a new approach to white light emitting diodes was proposed.In order to investigate effects of system parameters on the quality of ZnO films, a series of ZnO films were deposited on fused silica substrates at various substrate temperatures and oxygen pressures by PLD. X-ray diffraction (XRD), Raman spectra and photoluminescence (PL) spectra were utilized to analyze the effects of system parameters on the crystalline and optical qualities of ZnO films. The results indicated that substrate temperature and oxygen pressure have substantial effects on the qualities of ZnO films. When the oxygen pressure is above 80 mTorr, wurtzite ZnO films with high c-orientation were obtained. The optimal substrate temperature and oxygen pressure for depositing ZnO films with high crystalline and optical properties were 500°C and 160 mTorr, respectively.Based on the investigation of ZnO films, we fabricated p-GaN/n-ZnO and p-GaN/i-ZnO/n-ZnO heterostructural LEDs. Electrodes on p-GaN and n-ZnO were fabricated by thermal evaporating Ni/Au alloy and In, respectively. Current-Voltage (I-V) measurement indicates that very good ohmic contacts were achieved for both electrodes and the LEDs exhibited typical rectifying behavior. Under forward-bias,electroluminescence (EL) was observed. EL spectra of p-GaN/n-ZnO heterojunctions were dominated by the blue-violet emission peaking 410 nm. This emission originated from p-GaN, since the concentration and mobility of holes from p-GaN were both relatively lower compared with those of electrons form n-ZnO. Thereby, most carriers recombined in the p-GaN region. EL spectra of p-GaN/i-ZnO/n-ZnO heterojunctions were composed of two parts: blue-violet emission from p-GaN at 410 nm and deep-level yellow emission band from i-ZnO. This is because that insertion of i-ZnO layer between p-GaN and n-ZnO transferred a portion of carrier recombination from p-GaN to i-ZnO. By varying the thickness of i-ZnO layer, the intensity ratios of yellow to blue-violet emissions could be controlled. Based on this phenomenon, a new method to fabricate phosphor-free white LEDs was proposed.