| Summary: | 碩士 === 國立彰化師範大學 === 電子工程學系 === 102 === We present the study of crack-free AlGaN/GaN HEMTs on Si substrate grown by metalorganic chemical vapor deposition with the presence of two-dimensional electron gas (2DEG). In this study, three AlxGa1-xN /GaN HEMTs on Si with similar structure are analyzed and marked A, B and C. In addition, we detect the behavior of 2DEG by employing a series of optical and electrical measurements. The band gap energy of GaN and AlxGa1-xN are also identified based on the reflectance (R) and photoluminescence (PL) spectra. Therefore, the Al compositions are determined to be 25, 30 and 30% for sample A, B and C, respectively. In addition, crack-free GaN on Si is confirmed by the images of SEM for sample A, B and C. According to reciprocal space mapping of X-ray diffraction for sample A to C, the thin top AlGaN layer are observed to be fully strained on the thick GaN buffer layers where the graded AlxGa1-xN layers are fully relaxed. The fitted strength of electric field at the top AlGaN layer through electrolyte electro-reflectance (EER) spectra is about 731, 979 and 655 kV/cm for sample A to C, respectively. The sheet 2DEG concentration measured using EER technique and Hall measurement are both close to 1.1×1013, 1.4×1013 and 1.4×1013 cm-2 which are in excellent agreement with the theoretical value under specific AlGaN thickness and Al composition (~1.1×1013 cm-2 for sample A; ~1.4×1013 cm-2 for sample B and C). In addition, PL spectra at room temperature prove the epitaxy quality for GaN layer is great for sample A to C owing to its narrow full widths at half maximum.
In addition, we develop an integrated and automatically measuring system for optical characteristics measurement. The system is named after “OSM”, which pronounce like the word “awesome”. Most of the measurements conducted in this study are achieved by this system, including R, EER, PL measurements etc.
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