Electron microscopy analysis by visualization of relationship between composition distribution and charge distribution in quantum well of GaN system

采用电子全息技术测量了InGaN/GaN量子阱结构中InGaN层的压电电场。这种压电电场是由InGaN层中沿c轴的晶格应变引起的。根据维加德定律，从组成中计算晶格常数，然后估计压电极化。我们从一片晶圆上制备了不同厚度的样品。对样品进行电子全息照相，测得内部电场值为0.7±0.2 MV/cm。该值对应于理论计算估计的In组成4.1±1.3%。另一方面，通过Z-contrast STEM观测，我们得到In的组成为4.8±1.6%，与电子全息图得到的结果一致。对于AlGaN/GaN量子阱结构，我们建立了样品制备技术，即先用楔形抛光法对样品进行减薄，再用低压氩离子铣削，使样品厚度达到50-100 nm。该厚度足够薄，可以通过HRTEM、Z-contrast STEM和电子全息术观察。从这些综合研究中发现，AlGaN/GaN量子阱中AlGaN的压电电场要比InGaN/GaN量子阱中的InGaN小得多，这表明最小化样品表面损伤层和精确测量厚度是必不可少的。

Piezo electric field of InGaN layer in InGaN/GaN quantum well structure was measured by electron holography. This piezo electric field was caused by lattice strain along c-axis in InGaN layer. We calculated lattice constant from In composition according to Vegards law, and then estimated piezo-polarization. We prepared some samples with various thickness from one wafer. Electron holography was carried out for those samples and internal electric field value evaluated was 0.7±0.2 MV/cm. This value corresponds to the In composition 4.1±1.3% estimated from the theoretical calculation. On the other hand, Z-contrast STEM observation allowed us to evaluate the In composition 4.8±1.6%, which agree to the value obtained by electron holography.As for AlGaN/GaN quantum well structure, we established the sample preparation technique, e.g., Samples were thinned by wedge-polishing method followed by Ar-ion milling with low voltage, so that the sample thickness was 50-100 nm. This thickness was enough thin to be observed by HRTEM and Z-contrast STEM, and electron holography. From these comprehensive studies, it was found that piezo electric field of AlGaN in AlGaN/GaN quantum well is considerably small rather than InGaN in InGaN/GaN quantum well, suggesting that minimization of damage layer on the sample surfaces and precise thickness measurement are indispensable.