1.School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, Suzhou 215123, China 2.Suzhou Institue of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China 3.Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China 4.Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
Abstract:AlGaN is a key material for deep ultraviolet optoelectronic and electronic devices. With the increase of the Al composition ratio, the phase separation on the surface, caused by small-scale compositional fluctuations, is prone to affecting the performance of the device. In order to explore the mechanism of the phase separation on a nanoscale, the AlGaN wafers with different quantities of Al compositions are investigated by the confocal photoluminescence spectroscopy and the single-pass Kelvin force probe microscopy. The composition ratios of Al for the three samples are about 0.3, 0.5, and 0.7, respectively. The single-pass Kelvin force probe microscopy based on dual-frequency phase-locking is used to obtain high spatially resolved (about 10 nm) surface potential images. In the area where the phase separation phenomenon is obvious in the photoluminescence spectrum, the sharp change of the surface potential can be observed at the irregular steps and the edges of the surface pits. The potential changes can be ascribed to the inhomogeneous composition distribution. In the area where the topography turns into step flow, the surface pits shrink and merge. No obvious surface potential domain boundaries appear at the steps nor on the edges of the surface pits. Meanwhile, the phase separation phenomenon in the photoluminescence spectrum almost disappears. Our experiments show that the steps and the edges of the surface pits on AlGaN surfaces are main reasons for small-scale compositional fluctuations and the phase separation in the spectrum. Combining with in-situ confocal photoluminescence spectra, high spatially resolved surface potential image by single-pass Kelvin force probe microscopy is an effective method to characterize the phase separation on AlGaN surface on a nanoscale. Keywords:AlGaN/ phase separation/ Kelvin force probe microscopy/ photoluminescence spectroscopy
Al0.5Ga0.5N样品的Al组分比例约为0.5. 该样品的表面性质不太均匀, 测量表面不同位置时, 出现了两种典型的荧光光谱, 分别如图2(a)和图3(a)所示, 其对应的表面形貌和表面电势像如图2(c)—(f)和图3(c)—(f)所示. 图 3 Al0.5Ga0.5N样品表面没有明显相分离现象的区域 (a) 该区域的典型荧光光谱; (b)上图和下图分别为形貌和表面电势的剖面图, 对应图(e)中标记1的位置和图(f)中标记2的位置; (c) 和 (d) 扫描尺寸为10 μm时的表面形貌像及对应表面电势像; (e)和(f) 扫描尺寸为3 μm时的表面形貌像及对应表面电势像; 图(e)中白色横线标记1和图(f)中标记2对应同一位置 Figure3. The area without phase separation phenomenon on the Al0.5Ga0.5N sample surface. (a) A typical photoluminescence spectrum of the area. (b) Profiles of the topography and the surface potential shown in the plot 1 and 2, respectively. The profile of the topography is extracted from mark 1 in panel (e). The profile of the surface potential is extracted from the mark 2 in panel (f). (c) and (d) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 10 μm. (e) and (f) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 3 μm. The white lines marked by 1 in panel (e) and 2 in panel (f) are picked at the same position.
Al组分为0.3的Al0.3Ga0.7N样品和Al组分为0.7的Al0.7Ga0.3N样品表面形态比较均匀, 其典型的荧光光谱、表面形貌和表面电势像分别显示在图4和图5中. 图 4 Al0.3Ga0.7N样品表面 (a) 该区域的典型荧光光谱; (b)上图和下图分别为形貌和表面电势的剖面图, 对应图(e)中标记1的位置和图(f)中标记2的位置; (c)和(d)扫描尺寸为10 μm时的表面形貌像及对应表面电势像; (e)和(f)扫描尺寸为3 μm时的表面形貌像及对应表面电势像; 图(e) 中白色横线标记1和图(f)中标记2对应同一位置 Figure4. The area on the Al0.3Ga0.7N sample surface. (a) A typical photoluminescence spectrum of the area. (b) Profiles of the topography and the surface potential shown in the plot 1 and 2, respectively. The profile of the topography is extracted from mark 1 in panel (e). The profile of the surface potential is extracted from the mark 2 in panel (f). (c) and (d) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 10 μm. (e) and (f) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 3 μm. The white lines marked by 1 in panel (e) and 2 in panel (f) are picked at the same position.
图 5 Al0.7Ga0.3N样品表面 (a) 该区域的典型荧光光谱; (b)上图和下图分别为形貌和表面电势的剖面图, 对应图(e)中标记1的位置和图(f)中标记2的位置; (c)和(d)扫描尺寸为10 μm的表面形貌像及对应表面电势像; (e) 和 (f)扫描尺寸为3 μm时的表面形貌像及对应表面电势像; 图(e) 中白色横线标记1和图(f)中标记2对应同一位置 Figure5. The area on the Al0.7Ga0.3N sample surface. (a) A typical photoluminescence spectrum of the area. (b) profiles of the topography and the surface potential shown in the plot 1 and 2, respectively. The profile of the topography is extracted from mark 1 in panel (e). the profile of the surface potential is extracted from the mark 2 in panel (f). (c) and (d) the topography image and the surface potential image, respectively, obtained at the same area with a scan size of 10 μm. (e) and (f) the topography image and the surface potential image, respectively, obtained at the same area with a scan size of 3 μm. The white lines marked by 1 in panel (e) and 2 in panel (f) are picked at the same position.