Ge/Si(001) heterostructures with dense arrays of Ge quantum dots: morphology, defects, photo-emf spectra and terahertz conductivity

Abstract

Issues of Ge hut cluster array formation and growth at low temperatures on the Ge/Si(001) wetting layer are discussed on the basis of explorations performed by high resolution STM and in-situ RHEED. Dynamics of the RHEED patterns in the process of Ge hut array formation is investigated at low and high temperatures of Ge deposition. Different dynamics of RHEED patterns during the deposition of Ge atoms in different growth modes is observed, which reflects the difference in adatom mobility and their ‘condensation’ fluxes from Ge 2D gas on the surface for different modes, which in turn control the nucleation rates and densities of Ge clusters. Data of HRTEM studies of multilayer Ge/Si heterostructures are presented with the focus on low-temperature formation of perfect films.Heteroepitaxial Si p–i–n-diodes with multilayer stacks of Ge/Si(001) quantum dot dense arrays built in intrinsic domains have been investigated and found to exhibit the photo-emf in a wide spectral range from 0.8 to 5 μm. An effect of wide-band irradiation by infrared light on the photo-emf spectra has been observed. Photo-emf in different spectral ranges has been found to be differently affected by the wide-band irradiation. A significant increase in photo-emf is observed in the fundamental absorption range under the wide-band irradiation. The observed phenomena are explained in terms of positive and neutral charge states of the quantum dot layers and the Coulomb potential of the quantum dot ensemble. A new design of quantum dot infrared photodetectors is proposed.By using a coherent source spectrometer, first measurements of terahertz dynamical conductivity (absorptivity) spectra of Ge/Si(001) heterostructures were performed at frequencies ranged from 0.3 to 1.2 THz in the temperature interval from 300 to 5 K. The effective dynamical conductivity of the heterostructures with Ge quantum dots has been discovered to be significantly higher than that of the structure with the same amount of bulk germanium (not organized in an array of quantum dots). The excess conductivity is not observed in the structures with the Ge coverage less than 8 Å. When a Ge/Si(001) sample is cooled down the conductivity of the heterostructure decreases.

On atomic structure of Ge huts growing on the Ge/Si(001) wetting layer

“…4,6,15 The explanation might be as follows: Pyramids of the first stable height (2 ML) are often observed at low Ge coverages (less or around 6Å); 16,18 numerous 3, 4 and 5-ML pyramids are also observed at Ge coverages some less or about 6Å (Fig. 7) when the clusters are small enough and the distances between them are large in comparison with their dimensions.…”

“…Some pyramids, probably those which nucleated at early stages and had enough time to grow, succeed to reach the stable heights of 10, 14 or 18 ML over WL being sufficiently large to successfully compete with other clusters even at intermediate heights-between 6 and 10 ML, 10 and 14 ML, etc.-when the effect of minor asymmetry is negligible and they become virtually stable; these pyramids remain in arrays and are observed as residual fraction in amount of around 10 % of the total number density of huts or as array defects. 4,6,15,16,33 Notice also, that at high temperatures, when the cluster number density is small and the Ge dimer mobility on WL is high, pyramids become more stable than wedges which for some reason do not nucleate in the arrays. 19 Pyramids can grow even being metastable at intermediate heights in these conditions as they do not experience rivalry for Ge with their counterparts.…”

“…23 We have demonstrated that both pyramids and wedges grow in height conserving the width and the atomic structure of their apexes (the topmost (001) terraces). 4,6,17 This phenomenon also requires comprehension and adequate description: up to date, we have only hinted at such description in our recent articles. [15][16][17] An aim of this article is to propose physical processes on WL and cluster facets, and their structural schemes, on the basis of the previously performed STM studies, which could give adequate interpretation on atomic level to the experimentally observed evolution of each species of hut clusters during their growth setting the direction of thought for explanation of the presently incomprehensible phenomenon of simultaneous appearance of two types of hut nuclei on Ge/Si(001) WL which give rise to two mutually independent species of huts.…”

“…Ge "hut" clusters or small self-assembled Ge/Si(001) clusters faceted by the {105} planes and coherent with the substrate lattice (pyramids with square bases and wedges with rectangular bases elongated in one of the <100> directions), [1][2][3][4] which form on the Ge wetting layer (WL) at low temperatures of Si substrates ( 600℃) in the process of ultrahigh-vacuum molecular-beam epitaxy (UHV MBE) or-probably with some peculiarities due to hydrogenation of WL-in the process of chemical vapor deposition, have attracted an interest of researchers for more than twenty years since their discovery by Mo et al in 1990 1 , because of both their potential practical importance for development of Si-based monolithic optoelectronic devices 5,6 and convenience and simplicity of their usage as model objects for investigation of the Stranski-Krastanow growth of heteroepitaxial structures.…”

“…Our recent experimental explorations 6,[15][16][17][18][19]21 carried out on atomic level by high-resolution STM have demonstrated fine details of hut nucleation and its evolution during the growth which however have not been sufficiently interpreted thus far and presented in terms of structural schematics and drawings which would be helpful for further theoretical calculations and numerical simulations. For example, we have discovered the phenomenon of simultaneous appearance of two types of nuclei 15,19 which are composed by 16 dimers and different only in symmetry-a separate nucleus for each species of huts-on tops of Ge WL M × N patches 22 of 4-monolayer 18 (ML) height (Fig.…”