The structure of InAs and InGaAs quantum dots on GaAs(001) grown in the Stranski-Krastanow growth mode was investigated using XSTM. This material system is in particular promising for optoelectronic components such as low-threshold semiconductor lasers in glass-fiber communications.
Here a variety of samples prepared using both MBE and MOCVD was studied. Besides the analysis of the appearance of the structures in the images, we also developed a technique to derive the local chemical composition based on the local lattice constant derived from the images [1].

The main results of these studies are:

• While top-view STM experiments of uncapped quantum dots show a pyramidal shape with a clear apex, XSTM experiments show a truncated pyramidal shape with a (001) top facet. This change in shape upon capping is related to strong strain-induced segregation [2].
• Such strong segregation effects during capping also lead to deviations of the nominal stoichiometry of the quantum dots, e.g. an InGaAs composition of nominally grown InAs quantum dots [1,3,4]. This segregation may also result in a reverse-cone shape of the most In-rich region [5].
• In the case of large and thus strongly strained quantum dots, nm-sized material vacancies, or nanovoids, may form due to a strain-induced spill-out of material during growth interruptions [6].
• The presence of a few percent of nitrogen leads to completely different structures, characterized by strong dissolution, almost spherical shapes even extending into the GaAs substrate, and a missing wetting layer [7].
• The wetting layer is usually also affected by segregation, leading to an extension of several nm and an exponential decrease of the In concentration in growth direction [5]. Only in the case of very fast growth an almost pure InAs wetting layer may develop [3,4].

Top-view STM image (left) and XSTM image (right) of uncapped and capped InAs quantum dots on GaAs(001) [2].

(a) XSTM image of an InGaAs quantum dot (dotted contour) with an In-rich core (dashed contour), and (b) local stochiometry derived from the atomic chain distance, or local lattice constant [1,5]

Nanovoids developing in highly strained quantum dots [6].

(a) Usual InAs quantum dots and (b) the influence of some percent nitrogen exposure during growth.



[1] Cross-sectional STM study of InAs quantum dots for laser devices, H. Eisele, O. Flebbe, T. Kalka, and M. Dähne-Prietsch, Surface and Interface Analysis 27, 537 (1999).
[2] Change of InAs/GaAs quantum dot shape and composition during capping, H. Eisele, A. Lenz, R. Heitz, R. Timm, M. Dähne, Y. Temko, T. Suzuki, and K. Jacobi, Journal of Applied Physics 104, 124301 (2008).
[3] Cross-sectional scanning tunneling microscopy of stacked InAs quantum dots, H. Eisele, O. Flebbe, T. Kalka, C. Preinesberger, F. Heinrichsdorff, A. Krost, D. Bimberg, and M. Dähne-Prietsch, Applied Physics Letters 75, 106 (1999).
[4] Atomic structure of stacked InAs quantum dots grown by metal-organic chemical-vapor deposition, O. Flebbe, H. Eisele, T. Kalka, F. Heinrichsdorff, A. Krost, D. Bimberg, and M. Dähne-Prietsch, Journal of Vacuum Science and Technology B 17, 1639 (1999).
[5] Reversed truncated cone composition distribution of In_0.8Ga_0.2As quantum dots overgrown by an In_0.1Ga_0.9As layer in a GaAs matrix, A. Lenz, R. Timm, H. Eisele, Ch. Hennig, S.K. Becker, R.L. Sellin, U.W. Pohl, D. Bimberg, and M. Dähne, Applied Physics Letters 81, 5150 (2002).
[6] Nanovoids in InGaAs/GaAs quantum dots observed by cross-sectional scanning tunneling microscopy, A. Lenz, H. Eisele, R. Timm, S.K. Becker, R.L. Sellin, U.W. Pohl, D. Bimberg, and M. Dähne, Applied Physics Letters 85, 3848 (2004).
[7] Nitrogen-induced intermixing of InAsN quantum dots with the GaAs matrix, L. Ivanova, H. Eisele, A. Lenz, R. Timm, M. Dähne, O. Schumann, L. Geelhaar, and H. Riechert, Applied Physics Letters 92, 203101 (2008).