Atomic Scale Analysis of InGaN Multi-Quantum Wells
- PDF / 3,145,067 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 39 Downloads / 189 Views
properties of the material through several mechanisms. It was recently shown by P. Perlin et al.[11 that strain induced piezoelectric fields are responsible for the emissions from InGaN quantum-wells (QW). Another explanation was given by Chichibu et al.[2] where emissions were suggested to arise from the recombination of excitons located at potential minima along the QW. This was supported by Kriger et al.[3] and by Grudowski et al.[4] whose photoluminescence measurements on InGaN QW showed a broadening of peaks. Thus, there is still some controversy about the emissions mechanisms of InGaN multi-quantum wells (MQW). Furthermore, the low miscibility of InN in GaN[5] and the large lattice mistmatch between these two counpounds (11 %) affect indirectly the emissions through phase separation[6], segregation or defect generation[7]. In this paper, we report on an atomic scale study of the microstructure of InGaN MQW using High-Resolution Electron Microscopy (HREM). Experiment Several InGaN/GaN MQW structures were grown by metal organic chemical vapor deposition (MOCVD) on 1 trm thick GaN layer using conventional (0001) oriented sapphire substrates. Details of the growth conditions have been published elsewhere[4]. The structure of sample 1 consists of five periods of 2.5 nm Ino.13 Gao. 87N separated by 50 nm In 0.03Ga 0.97N barriers. The structure of sample 2 consists of twenty periods of 2.5 nm In 0 .20 Ga 0 .80 N 357 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
separated by 50 nm GaN barriers. The TEM samples were prepared by standard procedure using ion-milling at liquid nitrogen temperature and examined using both Topcon 002B and Jeol ARM microscopes operating at 200 kV and 800 kV respectively. Before the observations, the samples were mounted on a gold covered grid and then dipped in a KOH (50%) solution for 5 rain. This etch step was aimed at removing damage created during the ion milling. For the numerical processing, HREM images were first digitized using a Kodak camera. An intensity profile for all bright spots ('blob') in the image is fitted using an apprpopriate twodimensional function so that the exact coordinates of its center is accurately determined with sub-pixel resolution. Since each bright spot in the image corresponds to either an atomic column or an electron pathway (depending on the imaging conditions), the displacement of the atomic positions away from their bulk values can be determined for each of the unit cells along any crystallographic direction. This allows to maping the strain distribution. Thickness and composition have the same effect on the projected potential. Since the projected potential can be
extracted and mapped from the intensity distribution within each unit cell, chemical fluctuations or thickness variations can be revealed. A detailed explanation of the procedure can be found in the original papers[8-9].
Result Fig. 1 is a conventional image of sample 1 (five QW) structure taken along the [1120] direction. Although the In content in the wells is onl
Data Loading...
