Platelet Inversion Domains Induced by Mg-doping in ELOG AlGaN Films
- PDF / 2,641,621 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 7 Downloads / 163 Views
Y5.21.1
Platelet Inversion Domains Induced by Mg-doping in ELOG AlGaN Films R. Liu and F. A. Ponce Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287 D. Cherns H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK H. Amano and I. Akasaki Department of Materials Science and Engineering, Meijo University, Nagoya 468, Japan ABSTRACT We have studied the microstructure of heavily Mg-doped Al0.03Ga0.97N films grown by metal-organic vapor phase epitaxy in the lateral overgrowth mode (ELOG). A new type of defects with a platelet shape has been observed. According to TEM analysis, these defects are embedded in the overgrowth regions. The platelet is normal to the ELOG stripe direction [1100]AlGaN, forming trapezoidal trenches on the film surface. The thickness of the platelet is about 100nm. We have identified these defects as inversion domains using convergent beam electron diffraction and HR-TEM. Mg segregation at the coalescence boundaries between ELOG islands is believed to result in the formation of the defects. INTRODUCTION Mg is commonly used as p-type dopant in III-nitride semiconductors. In attempts to achieve improved conductivity, doping levels have sometimes exceeded the solid solubility limit. As a result, a variety of extended defects related to Mg impurities have been observed [1-5]. Inversion of growth polarity due to Mg doping was reported for molecular beam epitaxy of GaN, and it was believed to originate from Mg segregation at the faceted interface [1]. Pyramid defects due to Mg doping have also been observed by several groups in materials grown under different conditions [2-5], and some of the results suggest these defects are inversion domains with Mg-rich boundaries [4]. Theoretical calculations also indicate that Mg segregation at domain boundaries can produce stable inversion domain structures in GaN [6]. In this work, microstructure of a new defect induced by Mg impurities has been studied by transmission electron microscopy. The formation mechanism is discussed by taking into consideration the influence of Mg segregation at lateral overgrowth coalescence boundaries. EXPERIMENTAL The sample studied is a 5µm thick Al0.03Ga0.97N film doped with ~1020 cm-3 Mg. The film was grown on groove-patterned sapphire (0001) substrate by metal-organic vapor phase epitaxy at 1100oC using low temperature AlN as a nucleation layer [5]. Figure 1 shows a photograph of the film surface. Epitaxial lateral overgrowth (ELOG) was achieved by using the groove-patterned substrate. The grooves are oriented along sapphire [1120] direction, which is parallel to AlGaN [1100]. The width of a period of ELOG stripe is about 12µm. TEM analysis was performed using a JEOL 4000FX
Y5.21.2
microscope operated at 400kV with a resolution limit ~1.7Å. Convergent beam electron diffraction (CBED) study of the film polarity was done using a Philip CM200 FEG microscope operated at 200kV. TEM foil specimens were prepared by mechanical polishing followed by Ar+ ion-milling at 4.5keV. RESULTS AND
Data Loading...
