TRANSMISSION ELECTRON MICROSCOPY OF QUANTUM STRUCTURES
Wolfgang Neumann, Reinhard Schneider, Holm Kirmse, and Irmela Hähnert
Humboldt University of Berlin, Institute of Physics/Crystallography, Invalidenstr. 110, D-10115 Berlin, Germany, neumann@physik.hu-berlin.de
Keywords: quantum structures,
self-organization, transmission electron microscopy,
high-resolution imaging, analytical electron microscopy
For semiconducting materials promising new electronic and optical properties can be attained by scaling down the dimensions of active regions at about 2-20 nm in one, two or even three directions. Then the movement of charge carriers (electrons, holes) is confined at a scale which can be compared with the electron wavelength (or the confinement is even smaller) and quantum-physical phenomena become important. Such quantum structures, depending on the degree of confinement, are quantum wells (QWs), quantum wires (QWIs) or quantum dots (QDs). Their materials properties are essentially affected by the perfection of the structural details, including size, shape, arrangement, crystal morphology, and chemical composition. Owing to their small dimensions the different techniques of transmission electron microscopy (TEM) are needed to characterize both the microstructure and the microchemistry down to the atomic scale.
Within this field the potential applicability of the various TEM methods, including diffraction-contrast as well as high-resolution imaging and analytical TEM, viz. energy-dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS), is demonstrated by examples of investigations on QWI and QD structures of II-VI and III-V compound semiconductors. In detail, (In, Ga)As quantum wires on V-grooved (001) InP substrates grown by MOCVD and (In,Ga)As quantum dots on (100) GaAs and MBE grown CdSe quantum dots on ZnSe were elucidated in plan-view and cross-section. The TEM studies were carried out on two TEM/STEMs at 200 kV, a HITACHI H 8110 equipped with a EDX system (KEVEX) and a PHILIPS CM 20 FEG combined with a TRACOR EDX system and, in addition, a GATAN imaging filter (GIF) enabling both EELS as well as energy-filtered TEM (EFTEM).
As a relatively simple QWR configuration, a structure was investigated which consists of an about 10 nm thick (In, Ga)As layer directly deposited on InP. TEM imaging clearly exhibits a crescent-like wire shape and its width amounts to about 240 nm, whereas it extends ca. 15 nm in growth direction. By means of EFTEM the QWI was distinctly visualized by the Ga enrichment in the wire region. In the case of another (In,Ga)As QWI structure, also grown on InP but embedded in between (In,Al)As layers, a lot of defects, in particular microtwins, were found. With respect to the chemical composition, for the wire EDXS line profile measurements showed that Ga is present in a zone of about 15-20 nm in extension, whereas it extends ca. 10 nm perpendicularly to the (In,Ga)As side walls. Moreover, in the QWI region an additional In enrichment was detected.
As one example of III-V QDs, findings are discussed of (In,Ga)As QDs grown on a GaAs substrate. Here, the presence of a wetting layer clearly indicates the Stranski-Krastanov growth of the self-organized dots. Besides high-resolution imaging (HRTEM) of the structural peculiarities the element distribution in the QDs and wetting layer regions was imaged by EFTEM at nearly atomic resolution. Measuring the thickness of the (In,Ga)As layer in the Ga map yields about 0.7 nm, whereas the height of one single dot is, e.g., ca. 5 nm. For the II-VI system CdSe on ZnSe, the existence of CdSe QDs was proved by the expansion of the crystal lattice in the dot region via quantitative HRTEM as well as by EDXS line profiles. Plan-view images show the coexistence of two classes of QDs with an average lateral size of 10 nm (area density 100 µm-2) and 10 - 50 nm (20 µm-2), respectively. The shape of the larger entities is pyramid-like, which is in agreement with the results of image contrast simulations carried out for HRTEM in cross-section and plan-view diffraction contrast imaging.