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' Physikalisches Institut EP III, Am Hubland, 97074 Wiirzburg, Germany bon leave from loffe Institute, Polytechnicheskaya 26, St. Petersburg, Russia CPhysikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany ABSTRACT Up to 10ll cm2 InAs quantum dots (QD) can be grown on Silicon(001) by molecular beam epitaxy. This very new material system is on the one hand interesting with regard to the integration of optoelectronics with silicon technology on the other hand it offers new insight into the formation of QDs. We report on RHEED, TEM and Raman studies about (in-) coherence of the QDs and on an according to our knowledge so far unknown dewetting transition in this material system. The results are being discussed on the basis of a thermodynamic model, assuming a liquid-like behavior of a strained adlayer. INTRODUCTION The unique physical and chemical properties of silicon have led to its key position in microelectronics. Nevertheless, silicon as a material for optoelectronic devices is still a great challenge due to its indirect bandgap. Several new methods are being investigated at the moment with the goal to produce light emitting devices on the basis of silicon, such as porous silicon [1], silicon nanoparticles [2] and Erbium doping of silicon [3]. In the case of III/V semiconductors numerous studies on the self-organized formation of nanoparticles (quantum dots, QD) due to heteroepitaxial strain [4,5] have finally resulted in the fabrication of laser diodes based on e.g. InAs/GaAs [6]. Recently, the self-organized growth of InAs QDs embedded in a silicon matrix, has proofed very promising due to showing an intense photoluminescence peak at a wavelength of 1.3 ýtm

[7]. In addition, the investigation of QD growth on Silicon substrates terminated in various ways is believed to give new insight into the physics of self-organization of semiconductor nanostructures. In a former publication we report on the possibility of growing up to 101 cm-2 InAs QDs, problems of the formation of large Indium clusters between the InAs QDs and a study on the embedding of the QDs in a silicon matrix [8]. For a later application, knowledge on the QD size distribution and ways of improving their uniformity is essential together with knowledge on the defect structure and strain state in the QDs which we determined by AFM, RHEED, TEM and Raman measurements. EXPERIMENT The InAs QDs were grown in a Riber 2300 MBE system. 2 inch (001) silicon wafers with and without a 20 tilt towards (110) were chemically cleaned by a modified RCA-etch and finally Hydrogen-passivated with a short dip into diluted HF. Mounted on an In-free molybdenum holder the samples were brought into UHV environment within 5 minutes after the final HFdip. Residual contamination on top of the H-passivation was removed by heating the samples to 200'C for 10 minutes before transferring them into the III/V growth chamber where the Hpassivation was desorbed by heating shortly up to 450'C. Growth of InAs was performed at 370'C and a V/Ill ratio of 3 (beam