One-dimensional ordering of Ge nanoclusters along atomically straight steps of Si(111)

Abstract

Ge nanostructures grown by molecular beam epitaxy on a vicinal Si(111) surface with atomically well-defined steps are studied by means of scanning tunneling microscopy and spectroscopy. When the substrate temperature during deposition is around 250°C, Ge nanoclusters of diameters less than 2.0nm form a one-dimensional array of the periodicity 2.7nm along each step. This self-organization is due to preferential nucleation of Ge on the unfaulted 7×7 half-unit cells at the upper step edges. Scanning tunneling spectroscopy reveals localized electronic states of the nanoclusters.

Low-flux elucidation of initial growth of Ge clusters deposited onSi(111)−7×7observed by scanning tunneling microscopy

“…[1][2][3][4][5][6][7][8][9][10] While Ge nanostructures have been widely studied on Si͑001͒ surfaces, 1-3 there has been growing interest for Ge nanostructures on a Si͑111͒-7 ϫ 7 reconstructed surface with dimer rows, adatoms, and stacking fault ͑DAS͒ explained by the DAS model. 11 Many reports of Ge growth on Si͑111͒-7 ϫ 7 surfaces presented thermodynamic fluctuations on morphology of nanostructures grown at different temperatures and fluxes; [6][7][8][9][10][12][13][14][15][16][17][18][19] the morphology delicately changes with growth conditions.…”

Self-assembly of periodic nanoclusters of Si and Ge along atomically straight steps of a vicinal Si(111)

“…Thus, we choose the annealing time of 10 h for this sample ͑ Ϸ 1°and Ϸ 3°͒ to obtain the most extended straight-step regions. [22][23][24] The single steps in the straight-step regions obtained with this annealing process have an identical atomic structure. A STM image of this structure is shown in Fig.…”

“…24 Figure 4 shows STS spectra of the Ge nanoclusters at the step edge ͑solid line͒ and on the terrace ͑dashed͒ along with that of the intact 7 ϫ 7 structure on the same terrace ͑dotted͒. The clean terrace spectrum exhibits the well-known features characteristic to the STS spectrum of the 7 ϫ 7 surface at room temperature: 33 a significant density of states ͑DOS͒ at the sample Fermi level ͑sample bias voltage V s Ϸ 0 V͒ and two broad maxima at V s Ϸ −0.6 V and Ϸ + 0.5 V. The two spectra of the Ge nanoclusters are very similar to each other but distinctly different from the clean 7 ϫ 7 spectrum.…”

“…23,24 At the very beginning of Si and Ge growth ͑b Յ w Si,Ge Յ 7b͒, a characteristic atomic wire structure is formed as an extension of the step edges at 300°C Յ T s Յ 400°C for Si and at 250°C Յ T s Յ 400°C for Ge. Figure 2͑c͒ shows a typical STM image of such wire ͑w Si =7b and T s = 400°C͒.…”

“…32 We propose that the observed differences are due to the intermixing of deposited Ge atoms and substrate Si atoms. 24 In the case of such an elevated temperature ͑T s Ն 250°C͒ and a small step-step distance ͑around 24 nm for = 0.75°͒, most of Ge atoms adsorbed on the substrate will migrate on the 7 ϫ 7 terraces and can reach the step edges. However, according to Wang et al, 31 part of adsorbed Ge atoms can substitute Si adatoms in the 7 ϫ 7 structure and are trapped in the adatom sites S 4 .…”