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

用电子全息术测量了InGaN/GaN量子阱结构中InGaN层的压电场。这种压电场是由InGaN层中沿c轴方向的晶格应变引起的。根据Vegards定律计算了In组分的晶格常数，进而估算了压电极化。我们从一个晶片上制备了一些不同厚度的样品。对样品进行了电子全息照相，测得其内部电场为0.7±0.2 mV/cm。这一值与理论计算估算的In组分4.1±1.3%相对应。对于AlGaN/GaN量子阱结构，我们建立了样品的制备工艺，即先用楔形抛光方法减薄样品，然后用低电压的Ar离子球磨，使样品厚度达到50-100 nm。这一厚度足够薄，可以用HRTEM和Z-对比度STEM以及电子全息术观察到。综合研究发现，AlGaN/GaN量子阱中AlGaN的压电场比InGaN/GaN中的InGaN的压电场要小得多，这表明样品表面损伤层的最小化和精确的厚度测量是必不可少的。

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.