Si/Ge and III-V Light-Emitting Nanostructures


Quantum Dots - Growth and HREM Imaging

P. Werner, R. Hillebrand, G. Cirlin, and V. Talalaev


Semiconductor "quantum dots", which refer to crystalline particles in the nano-meter range, are grown from semiconductor materials such as InAs or SiGe alloys. Quantum dots enable studies of, e.g., transport properties of single electrons and quantum structures at the limit of zero dimensions. The special properties allow the development of new device concepts such as large-gain, low threshold quantum lasers. Fig. 1 illustrates the process of self-organization of the InAs pyramids, which is revealed in the cross-section TEM image (bottom).




Characterization of quantum dots by image analysis


At the MPI a group is dealing with the characterization of such complex quantum dot structures, grown by techniques such as MBE and MOCVD. The goal is a better understanding between growth processes and structures, on the one side, and their electrical and optical properties on the other side. Fig. 2 shows the layer stacking sequence of a quantum dot laser. It can be studied by HREM with high lateral resolution and chemical sensitivity. At the right hand side the HREM micrograph of a Bragg reflector is given. The local indium profile can be extracted by Fourier-based image processing techniques to characterize the active QD zone.




Image processing of Al,GaAs micrographs via neural networks


For opto-electronic applications, the study and control of the interface quality of layer systems of Al,GaAs type is necessary. The quantitative study of semiconductor hetero structures by high resolution electron microscopy requires the design and application of appropriate image processing software. In Fig. 3 the application of a novel, neural network-based image processing packige is illustrated. Neural networks (ANNs) are trained on the basis of simulated images with varying compositional and imaging parameters. Including the influence of amorphous noise, the ANNs learn to determine the local specimen thickness and the local composition, applied image cell by image cell.





Fig. 4 shows the HREM image of an Al,GaAs Bragg reflector. These specific layer systems of AlAs and GaAs are produced to cover quantum dot lasers. The electron micrograph is analysed cellwise by neural networks, which have been trained on the basis of simulated images. This method includes the amorphous films at the outer faces of the specimen, which result from ion beam thinning. The thickness of these films is estimated by comparing the diffraction intensity inside the crystalline reflections to that caused by the amorphous background.

The pseudocolor scale of Fig. 5 represents the local chemical composition. It proves a good quality of the interfaces with nearly binary composition regions. The height of each of the colored colums is a measure for the crystalline specimen thickness. It is obvious that the layers of AlAs (blue) are more stable to ion bombardment, whereas the GaAs layers (red) form 'valleys'. The surface roughness as well as the influence of amorphous films could be reduced by chemical post-processing of the EM specimens.






This work was supported by the Volkswagen-Stiftung under contract number I/71108.



U. W. Pohl, K. Pötschke, A. Schliwa, F. Guffarth, and D. Bimberg
N. D. Zakharov and P. Werner, M. B. Lifshits and V. A. Shchukin,D. E. Jesson
Evolution of a multimodal distribution of self-organized InAs/GaAs quantum dots
Phys.Rev. B 72, 245332  2005


Dubrovskij, V. G., G. E. Cirlin, Y. G. Musikhin, A. A. Tonkikh, N. K. Polykov, V. A. Egorov, A. F. Tsatsulnikov, N. A. Krizhanovskaya, V. M. Ustinov, and P. Werner: Effect of growth kinetics on the structural and optical properties of quantum dot ensembles. Journal of Crystal Growth 267 (1-2), 47-59 (2004).


Hanke, M., D. Grigoriev, M. Schmidbauer, P. Schäfer, R. Köhler, U. W. Pohl, R. L. Sellin, D. Bimberg, N. D. Zakharov, and P. Werner: Diffuse X-ray scattering of InGaAs/GaAs quantum dots. Physica E 21 (2-4), 684-688 (2004).


Pötschke, K., L. Müller-Kirsch, R. Heitz, R. L. Sellin, U. W. Pohl, D. Bimberg, N. D. Zakharov, and P. Werner: Ripening of self-organized InAs quantum dots. Physica E 21 (2-4), 606-610 (2004).


Cirlin, G. E., V. G. Talalaev, V. A. Egorov, N. D. Zakharov, P. Werner, N. N. Ledentsov, and V. M. Ustinov:Nanostructures formed by sub- and close-to-critical Ge inclusions in a Si matrix. Physica E 17, 131-133 (2003).


Cirlin, G. E., N. D. Zakharov, V. A. Egorov, P. Werner, V. M. Ustinov, and N. N. Ledentsov: Mechanism of germanium nanoinclusions formation in a silicon matrix during sub- monolayer MBE. Thin Solid Films 428, 156-159 (2003).


Dubrovskii, V. G., V. M. Ustinov, A. A. Tonkikh, V. A. Egorov, G. E. Cirlin, and P. Werner: Temperature dependence of a morphology of the nanocluster ensembles in the Ge/Si(100) system. Physical Technical Letters 29, 41-48 (2003).


Sellin, R. L., I. N. Kaiander, D. Ouyang, T. Kettler, U. W. Pohl, D. Bimberg, N. D. Za- kharov, and P. Werner: Alternative-precursor metalorganic chemical vapor deposition of self-organized InGaAs/GaAs quantum dots and quantum-dot lasers. Applied Physics Letters 82, 841-843 (2003).


Tonkikh, A. A., V. G. Dubrovskii, G. E. Cirlin, V. A. Egorov, V. M. Ustinov, and P. Werner: Temperature dependence of the quantum dot lateral size in the Ge/Si(100) system. Physica Status Solidi B 236, R1-R3 (2003).


Tonkikh, A. A., V. G. Talalaev, N. D. Zakharov, G. E. Cirlin, V. M. Ustinov, and P. Werner: The influence of a post growth annealing on a structural and optical properties of Ge/Si mulilayer heterostructures. Physical Technical Letters 29, 78-85 (2003).


Wakayama, Y., L. V. Sokolov, N. D. Zakharov, P. Werner, and U. Gösele: Stabilization and fine control of Ge dot structure on Si (100) by C cover layer. Journal of Applied Physics 93, 765-767 (2003).


N.D. Zakharov, V.G. Talalaev, P. Werner, A.A. Tonkikh, G.E, Cirlin: Room-temperature light emission from a highly strained Si/Ge superlattice. Appl. Phys. Lett. 83, 3084-3086 (2003).


Cirlin, G.E., Talalaev, V.G., Zakharov, N.D., Egorov, V.A., and Werner, P., Room temperature superlinear power dependence of photoluminescence from defect-free Si/Ge quantum dot multilayer structures.
phys.stat.sol. (b) 232, R1-R3 (2002).


K. Scheerschmidt, P. Werner: Characterization of structure and composition of quantum dots by transmission electron microscopy, in: Nano-Optoelectronics, Concepts, Physics and Devices, M. Grundmann (ed.) Springer, Heidelberg, 2002.

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