| Summary: | 碩士 === 國立臺灣大學 === 電子工程學研究所 === 97 === Self-assembled Ge dots have been extensively investigated because of its potential applications. Recently many reports discussed on the control of Ge dot position which is necessary for most electronic applications. One approach to fabricate ordered Ge dots is to integrate lithography and self-assembly growth techniques. In this study, we demonstrated well-aligned Ge dots with uniform size distribution on Si (001) with two-dimension hole array. The influence of the period, the depth, and the shape of the hole array on alignment of Ge dots was investigated.
All samples in this work were grown by UHVCVD system at 600℃. The Si (001) templates were fabricated by e-beam lithography and reactive ion etching (RIE). Before the growth of Ge dots, a Si buffer layer was grown to eliminate the roughness introduced during etching. The dimension and surface morphology were investigated by scanning electron microscope (SEM) and tapping mode atomic force microscope (AFM). The SEM images of 2D hole array (circle, square, and diamond) fabricated by e-beam lithography and RIE. The dimensions of these patterns are about 200nm. All of these patterns are along the [110] direction. The AFM images after growth of the Ge dots on circle-shaped hole array. The hole depth is ~20nm and the period is 300nm, 350nm, and 400 nm, respectively. As it can be seen the dome-shaped Ge dots nucleate at the intersection of {110} planes inside the hole array. Besides, the degree of alignment of dots increases with decreasing the period. Here we defined the dots located at the intersection of {110} planes as the “aligned dots” (we ignored the pyramid-shaped dots on ridges and discussed it later). The relation between alignment percentage (number of aligned dots/number of all dots) and the period of hole array is linear. The highest alignment percentage is 75% of the 300nm-period sample. The AFM images of the samples with same period of hole array (300nm) but different hole depth (4nm, 23nm, and 31nm). The alignment percentage also increases linearly with the hole depth. The highest alignment percentage is 97% of the 31nm-depth sample.
Although the 2D well-aligned dome-shaped Ge dots has been achieved, the size distribution is bimodal if we take the pyramid-shaped dots on ridges into account. The AFM images of the samples with different hole shape (circle, square, and diamond). For the circle case, there are pyramid-shaped dots on ridges and the size distribution is bimodal. For the square case, no pyramid-shaped dots observed on ridges. However, the dots inside the hole are irregular rather than rounded dots. As for the diamond case, there are no pyramid-shaped dots on ridges and the dots inside the hole are dome-shaped. The size distribution of Ge dots of the sample with 300nm-period, 31nm-depth, and diamond-shaped hole array. The average diameter is 85nm and the standard deviation is 3.5nm. Besides, the dot density is ~4×109/cm2 which is comparable to the Ge dots grown on blanket Si substrate.
Alignment of Ge dots on Si (001) substrate with two-dimension hole array was achieved by modulating the period and depth of the hole array. The size uniformity of dots can be improved by diamond-shaped pattern. The average diameter and areal density is 85nm and 4×109/cm2, respectively.
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