Morphological investigations and classification of Ge hut clusters forming the arrays of quantum dots on the Si(001) surface at low temperatures in the process of the ultrahigh vacuum molecular beam epitaxy have been carried out using in situ scanning tunnelling microscopy. Two main species of Ge hut clusters composing the arrays-pyramidal and wedge-shaped ones-have been found to have different atomic structures. The inference is made that shape transitions between pyramids and wedges are impossible. The nucleation probabilities of pyramids and wedges equal 1/2 at the initial stage of the array formation. The wedges become the dominating species as the amount of the deposited germanium is increased. A fraction and a density of the pyramids in the arrays are rapidly decreased with the growth of Ge coverage.The derivative types of the clusters-obelisks (or truncated wedges) and accreted wedges-have been revealed and investigated for the first time, they have been found to start dominating at high Ge coverages. The obelisks originate from the wedges as a result of their height limitation and further growth of trapezoid facets. The apexes of the obelisks are formed by sets of the parallel (001) ridges.The uniformity of the cluster arrays have been evidenced to be controlled by the length distribution of the wedge-like clusters. At low growth temperatures (360 • C) nucleation of new clusters is observed during the array growth at all values of Ge coverage except for a particular point at which the arrays are more uniform than at higher or lower coverages. At higher temperatures (530 • C) cluster nucleation has not been observed after the initial stage of the array formation.
A direct observation of nucleation of Ge hut clusters formed by ultrahigh vacuum molecular beam epitaxy is reported for the first time. The nuclei of the pyramidal and wedge-like clusters have been observed on the wetting layer blocks and found to have different structures. The growth of the clusters of both species goes on following different scenarios: Formation of the second atomic layer of the wedge-like cluster results in rearrangement of its first layer. Its ridge structure does not replicate the structure of the nucleus. The pyramidal cluster grows without phase transitions. The structure of its vertex copies the structure of the nucleus. The wedge-like clusters contain point defects on the triangular faces and have preferential directions of growth along the ridges. The derived structure of the {105} facet corresponds to the PD model. The critical epinucleation phenomenon may be responsible for hut formation.Comment: 6 pages, 6 figures; Fig. 4e has been improved; additional explanations have been added. To appear in Phys. Rev.
We report a direct observation of Ge hut nucleation on Si(001) during UHV molecular beam epitaxy at 360°C. Nuclei of pyramids and wedges were observed on the wetting layer (WL) (M × N) patches starting from the coverage of 5.1 Å and found to have different structures. Atomic models of nuclei of both hut species have been built as well as models of the growing clusters. The growth of huts of each species has been demonstrated to follow generic scenarios. The formation of the second atomic layer of a wedge results in rearrangement of its first layer. Its ridge structure does not repeat the nucleus. A pyramid grows without phase transitions. A structure of its vertex copies the nucleus. Transitions between hut species turned out to be impossible. The wedges contain point defects in the upper corners of the triangular faces and have preferential growth directions along the ridges. The derived structure of the {105} facet follows the paired dimer model. Further growth of hut arrays results in domination of wedges, and the density of pyramids exponentially drops. The second generation of huts arises at coverages >10 Å; new huts occupy the whole WL at coverages ~14 Å. Nanocrystalline Ge 2D layer begins forming at coverages >14 Å.
We report a direct STM observation of Ge hut array nucleation on the Si(001) surface during ultrahigh vacuum molecular-beam epitaxy at 360C. Nuclei of pyramids and wedges have been observed on the wetting layer MxN patches starting from the coverage of about 5.1 \r{A} (~3.6 ML). Further development of hut arrays consists in simultaneous growth of the formerly appeared clusters and nucleation of new ones resulting in gradual rise of hut number density with increasing surface coverage. Huts nucleate reconstructing the patch surface from the usual c(4x2) or p(2x2) structure to one of two recently described formations composed by epitaxially oriented Ge dimer pairs and chains of four dimers.Comment: Extended discussion; to appear in J. Appl. Phys., 201
Issues of morphology, nucleation, and growth of Ge cluster arrays deposited by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface are considered. Difference in nucleation of quantum dots during Ge deposition at low (≲600°C) and high (≳600°C) temperatures is studied by high resolution scanning tunneling microscopy. The atomic models of growth of both species of Ge huts--pyramids and wedges-- are proposed. The growth cycle of Ge QD arrays at low temperatures is explored. A problem of lowering of the array formation temperature is discussed with the focus on CMOS compatibility of the entire process; a special attention is paid upon approaches to reduction of treatment temperature during the Si(001) surface pre-growth cleaning, which is at once a key and the highest-temperature phase of the Ge/Si(001) quantum dot dense array formation process. The temperature of the Si clean surface preparation, the final high-temperature step of which is, as a rule, carried out directly in the MBE chamber just before the structure deposition, determines the compatibility of formation process of Ge-QD-array based devices with the CMOS manufacturing cycle. Silicon surface hydrogenation at the final stage of its wet chemical etching during the preliminary cleaning is proposed as a possible way of efficient reduction of the Si wafer pre-growth annealing temperature.
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