Silicon–Germanium Nanostructures with Quantum Dots: Formation
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Semiconductors, Vol. 34, No. 11, 2000, pp. 12291247. Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 11, 2000, pp. 12811299.Original Russian Text Copyright 2000 by Pchelyakov, Bolkhovityanov, Dvurechenski
, Sokolov, Nikiforov, Yakimov, Voigtlnder.
SiliconGermanium Nanostructures with Quantum Dots: Formation Mechanisms and Electrical Properties
O. P. Pchelyakov*, Yu. B. Bolkhovityanov*, A. V. Dvurechenski*, L. V. Sokolov*,A. I. Nikiforov*, A. I. Yakimov*, and B. Voigtlnder**
* Institute of Semiconductor Physics, Siberian Division, Russian Academy of Sciences,pr. Akademika Lavrenteva 13, Novosibirsk, 630090 Russia
** Research Center, Yuelich, GermanySubmitted April 17, 2000; accepted for publication May 10, 2000
AbstractThe generally accepted notions about the formation mechanisms for germanium islands withnanometer-scale sizes in a Ge-on-Si system are reviewed on the basis of analysis of recent publications. Thepresence of elastic strains in the epilayers and in the three-dimensional Ge islands on Si is a key factor that notonly initiates a morphological transition from a planar film to an island-containing film (the StranskiKrastanovmechanism) but also influences the subsequent stages of the islands evolution, including their shape, size, andspatial distribution. In many cases, this factor modifies appreciably the classical mechanisms of phase-forma-tion and their sequence up to the quasi-equilibrium coexistence of three-dimensional Ge nanoislands at the sur-face of the Si substrate. The methods for improving the degree of the ordering of nanoislands to attain the small-est possible sizes and large density of areal distribution of these islands are discussed. The published data onoptical absorption in the multilayered GeSi systems with quantum dots are considered; these data are indica-tive of an anomalously large cross section of intraband absorption, which makes this class of nanostructurespromising for the development of photodetectors of the infrared region of the spectrum. The results of originalstudies of electrical and optical properties of heterostructures that involve Ge quantum dots and are synthesizedby molecular-beam epitaxy on the Si substrates are reported. 2000 MAIK Nauka/Interperiodica.1. INTRODUCTION
Nanostructures based on the germanium-on-siliconheterosystem attract the attention of technologistsowing to significant progress in the development ofnew quantum-effect devices in spite of a 4% differencebetween the lattice parameters of Ge and Si. We witnessthe appearance of silicongermanium light-emittingand photodetecting devices that make silicon technol-ogy quite competitive with those of conventional opto-electronic materials, such as the IIIV compounds. In recent years, the potential applications of theGeSi-based semiconductor materials containing thenanometer-sized Ge clusters (quantum dots) embeddedin the Si matrix have become apparent. Interest in theGe and Si nanoclusters is related to the following cir-cumstances: (i) progress in the development of technol-ogy for producing a Ge-nanocluster array that is fairlyuniform in size; (ii) the sizes of nanoclusters have beenreduced to the values that ensure the manifestation ofthe size-quantization effects and the electronelectroninteraction up to room temperature; and (iii) compati-bility of the developed methods with existing silicontechnology for the production of discrete devices andcircuits. Such designs that have been considered exoticuntil recently may bring about an actual revolution insilicon integration technology.
A steady increase in the number of publicationsworldwide devoted to low-dimensional heterostruc-1063-7826/00/3411- $20.00 21229tures is indicative of the growing interest in these struc-tures. Figure 1 shows the histograms for the number ofannual publications that include the keywords quan-tum wells and quantum dots. In the latter case, asteady increase in the annual number of relevant publi-cations is observed. Starting in 1992, changes becameevident in the technology producing the structures withquantum dots. Before that time, the main method forforming such structures was photolithography, with theconstraint on the minimum sizes inherent in thismethod. Manifestation of the effect of ordering in thearray of nanometer-sized islands in the GeSi andInAsGaAs heterosystems made it possible to obtainquantum dots that had no defects, had the smallest pos-sible size (10100 nm), and had the density of10101011 cm2, which made the atomlike characteris-tics of these systems more pronounced in the relevantelectronic and optical spectra. The development ofstudies in this field is illustrated in Fig. 2 by histogramsof the annual number of publications devoted to aGeSi system. It is in this system that the arrays ofislands were first used to observe the one-electroneffects . Later, most studies of electronic propertiesof quantum dots (QDs) were based on IIIV com-pounds. This was caused by the following factors:(a) progress in the technology of heteroepitaxy forIIIV compounds; (b) the possibility of producing theheterostructures of type I (the offsets of the conduction000 MAIK Nauka/Interperiodica
and valence bands are of opposite signs), which isimportant for the optical properties of these systems;and (c) the small value of the charge-carrier effectivemass, which ensured the manifestation of size-quanti-zation effects for islands of a relatively large size. Thefirst studies of QDs in IIIV compounds were per-formed on the basis of InAsGaAs structures [7, 8].
A transition from the layer-by-layer [i.e., two-dimensional (2D)] growth of the film to the formationof three-dimensional (3D) islands (the StranskiKrast-anov mechanism) has been studied for a long time inthe germanium-on-silicon heterosystem. The first pub-lication devoted to this heterosystem, in which theobservation of pseudomorphous Ge stripes (referred tonow as quantum wires) that follow the outlines of stepsand of the nanometer-sized islands (currently, quantumdots) was reported, was apparently that of the study performed at the Institute of Semiconductor Physics(Siberian Division, Academy of Sciences of USSR) asfar back as 1974. At relatively low temperatures of syn-thesis, such islands do not contain the misfit disloca-tions even if the thickness of the islands exceeds appre-ciably the critical value, which was demonstrated mostclearly in studies of the GeSi  and InGaAsGaAs systems. Following these publications, a sharpincrease in the study of the mechanisms of formation ofstrained islands and the special features of their order-ing set in, because an opportunity arose to form 3Dobjects that have no defects (no misfit dislocations),have nanometer-scale sizes, and may find practicalapplications in nanoelectronics.
The objective of this review was to analyze thedevelopment and current ideas about the mechanismsof ordering of the QD ensembles in the course of hete-roepitaxy. This has been the subject of a number ofreviews . However, without laying claim to
091 92 93 94 95 96 97 98
Number of publications
Fig. 1. The number of annual publications according to thedata of the Materials Science Citation Index (MSCI) for19901998. Search was based on the keywords quantumwells and quantum dots.completeness, as concerns a review of all heterosys-tems, we have attempted to outline the generallyaccepted concepts of the Ge-on-Si system and to sup-plement these with an analysis of the newest data,including the results of our experiments with synthesisof a GeSi heterosystem with QDs, and the study of theelectronic and optical properties of this heterosystem.
In Section 2, we consider the driving forces andmain mechanisms of evolution and ordering of nanoob-jects in heterosystems with a large lattice mismatch inthe course of molecular-beam epitaxy (MBE) and heattreatment. In Section 3, we analyze the experimentalobservations of cluster formation and self-organizationfor GeSi nanostructures at the silicon surface and dis-cuss the feasible methods for enhancing the ordering,reducing the sizes, and increasing the density of thegermanium QDs. In Section 4, we summarize the orig-inal results of our studies of the electronic and opticalproperties of heterostructures and multilayered compo-sitions with Ge QDs.
2. BASIC PREMISES
It is possible to distinguish between the stages ofnucleation and further development in the formation of3D islands. The main pattern in the nucleation ofislands in an epitaxial heterosystem is governed by thebalance between the surface energies of the film andsubstrate and also between the energy of the filmsub-strate interface and internal energy of the island bulk.The free energy of a newly formed island nucleus at thesubstrate surface may be expressed as the sum of threeterms ; i.e.,
G V s Ei V h/l,( ).+ +=
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Number of publications
A. I. Yakimov et al.,Philos. Mag. B 65
Fig. 2. Selection (from the complex quantum dots, seeFig. 1) of publications devoted to the island growth of Ge(GeSi) on Si and also to the properties of these entities (forthe year 1999, the data are given for 10 months; the datarefer to the most important journals).SEMICONDUCTORS Vol. 34 No. 11 2000
SILICONGERMANIUM NANOSTRUCTURES 1231
Here, the first term accounts for the formation energy ofa new nucleus with the volume V, with standing forthe thermodynamic force for crystallization in the eventof supersaturation.