| Summary: | 碩士 === 中原大學 === 電子工程學系 === 88 === This study aims essentially at recognizing whether the Si-doped multi-layers can improve the quality of GaN epitaxial films effectively. It is widely reported that slight Si-doping improves the electrical quality of GaN films effectively; i.e., it enhances the mobility and reduces the background electron concentration. However, Si-doping will enhance the yellow band intensity, which represents deep level defects. To obtain high-quality epitaxial layer simultaneously with good electrical and optical quality, a practical solution is to grow slightly Si-doped GaN layer first and then to grow an undoped GaN layer.
Each layer of our multi-layer structure involves two sub-layers. First, a 100Ǻ undoped GaN sub-layer is grown. Then a 100Ǻ Si-doped GaN layer is grown as another sub-layer. These two sub-layers form fundamental period with the thickness of 200Ǻ for our multi-layer structure. The use of the multi-layer structure given above is possible to have the same effect as a multi-buffer-layer structure, which can continuously improve the quality of GaN film. Each slightly Si-doped sub-layer plays the role of a buffer, which can improve the quality of the overgrowth GaN sub-layer. This defect reduction process occurs repeatedly through the whole multi-layer structure. Consequently, the final top layer of undoped GaN will exhibit the best film quality.
During our experiments, we have grown by MOCVD, a series of samples with the multi-layer structure of different numbers of period ( the numbers used are 1, 7, 10, 20, 30, 40 ). In the C-V measurement, we have found that donor concentration decreases from 2.82 x 1016 cm-3 to 3.8 x 1015 cm-3. Through the Atomic Force Microscopy (AFM) observations, we have found that the density of dark spots, which are corresponding to dislocations, decreases from 1.26 x 109 cm2 to 2.5 x 108 cm2. In the photoluminescence spectra, the intensity ratio ( INBE / IYL ) of near band-edge emission to yellow luminescence raises from 20 to 120. Further from PL transient measurement performed at the wavelength of near band-edge emission, we found that the lifetime raises from 42ps to 55ps.
All the results demonstrated that Si-doped GaN multi-layer structure can be used to reduce the densities of dislocations and other defects effectively, and obviously improve the electrical and optical qualities of the top undoped GaN epitaxial films.
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